Table of contents

Cloud physics has for a long time been an important segment of atmospheric science. It is common knowledge that clouds are crucial for our understanding of weather and climate. Clouds are also interesting by themselves (not to mention that they are beautiful). Complexity is hidden behind the common picture of these beautiful and interesting objects. The typical school textbook definition that a cloud is 'a set of droplets or particles suspended in the atmosphere' is not adequate. Clouds are complicated phenomena in which dynamics, turbulence, microphysics, thermodynamics and radiative transfer interact on a wide range of scales, from sub-micron to kilometres. Some of these interactions are subtle and others are more straightforward. Large and small-scale motions lead to activation of cloud condensation nuclei, condensational growth and collisions; small changes in composition and concentration of atmospheric aerosol lead to significant differences in radiative properties of the clouds and influence rainfall formation. It is justified to look at a cloud as a composite, nonlinear system which involves many interactions and feedback. This system is actively linked into a web of atmospheric, oceanic and even cosmic interactions.

Due to the complexity of the cloud system, present-day descriptions of clouds suffer from simplifications, inadequate parameterizations, and omissions. Sometimes the most fundamental physics hidden behind these simplifications and parameterizations is not known, and a wide scope of view can sometimes prevent a 'microscopic', deep insight into the detail. Only the expertise offered by scientists focused on particular elementary processes involved in this complicated pattern of interactions allows us to shape elements of the puzzle from which a general picture of clouds can be created. To be useful, every element of the puzzle must be shaped precisely. This often creates problems in communication between the sciences responsible for shaping elements of the puzzle, and those which combine them. Scales, assumptions and the conditions used in order to describe a particular single process of interest must be consistent with the conditions in clouds.

The papers in this focus issue of New Journal of Physics collectively demonstrate (i) the variation in scientific approaches towards investigating cloud processes, (ii) the various stages of shaping elements of the puzzle, and (iii) some attempts to put the pieces together. These papers present just a small subset of loosely arranged elements in an initial stage of puzzle creation.

Addressed by this issue is one of the important problems in our understanding of cloud processes—the interaction between cloud particles and turbulence. There is currently a gap between the cloud physics community and scientists working in wind tunnels, on turbulence theory and particle interactions. This collection is intended to narrow this gap by bringing together work by theoreticians, modelers, laboratory experimentalists and those who measure and observe actual processes in clouds. It forms a collage of contributions showing various approaches to cloud processes including:

• theoretical works with possible applications to clouds (Bistagnino and Boffetta, Gustavsson et al), • an attempt to construct a phenomenological description of clouds and rain (Lovejoy and Schertzer), • simplified models designed to parameterize turbulence micro- and macro-effects (Celani et al, Derevyanko et al), • focused theoretical research aimed at particular cloud processes (Ayala et al, parts I and II, Wang et al), • laboratory and modeling studies of complex cloud processes (Malinowski et al).

This collage is far from being complete but, hopefully, should give the reader a representative impression of the current state of knowledge in the field. We hope it will be useful to all scientists whose work is inspired by cloud processes.

Focus on Cloud Physics Contents

The development of ice in a cumulus cloud over southwest England Yahui Huang, Alan M Blyth, Philip R A Brown, Tom W Choularton, Paul Connolly, Alan M Gadian, Hazel Jones, John Latham, Zhiqiang Cui and Ken Carslaw

The equivalent size of cloud condensation nuclei Antonio Celani, Andrea Mazzino and Marco Tizzi

Laboratory and modeling studies of cloud–clear air interfacial mixing: anisotropy of small-scale turbulence due to evaporative cooling Szymon P Malinowski, Miroslaw Andrejczuk, Wojciech W Grabowski, Piotr Korczyk, Tomasz A Kowalewski and Piotr K Smolarkiewicz

Evolution of non-uniformly seeded warm clouds in idealized turbulent conditions Stanislav Derevyanko, Gregory Falkovich and Sergei Turitsyn

Lagrangian statistics in two-dimensional free turbulent convection A Bistagnino and G Boffetta

Turbulence, raindrops and the l^1/2 number density law S Lovejoy and D Schertzer

Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 2. Theory and parameterization Orlando Ayala, Bogdan Rosa and Lian-Ping Wang

Effects of turbulence on the geometric collision rate of sedimenting droplets. Part 1. Results from direct numerical simulation Orlando Ayala, Bogdan Rosa, Lian-Ping Wang and Wojciech W Grabowski

Collisions of particles advected in random flows K Gustavsson, B Mehlig and M Wilkinson

Turbulent collision efficiency of heavy particles relevant to cloud droplets Lian-Ping Wang, Orlando Ayala, Bogdan Rosa and Wojciech W Grabowski

The role of turbulence in the growth process of cloud droplets in the condensation phase is investigated by means of direct numerical simulations in two dimensions. Both curvature effect and solute effect are incorporated in the model for the evolution of cloud droplet radii. The latter effect is allowed because of the explicit description of cloud condensation nuclei (CCN) dynamics. Spectrum broadening appears as a spectacular effect of turbulence. In order to give a quantitative measure of the importance of turbulence in the growth process, we introduce the concept of equivalent CCN radius. The latter naturally emerges as a sort of parametrization of turbulence effects to be incorporated within a classical description of the condensation phase. In that context, it is the minimal CCN radius required to produce spontaneous condensation of cloud droplets. The equivalent CCN size behaves with the dissipation rate ε as ε^−1/3, thus becoming smaller and smaller as turbulence intensity increases.

Small-scale mixing between cloudy air and unsaturated clear air is investigated in numerical simulations and in a laboratory cloud chamber. Despite substantial differences in physical conditions and some differences in resolved scales of motion, results of both studies indicate that small-scale turbulence generated through cloud–clear air interfacial mixing is highly anisotropic. For velocity fluctuations, numerical simulations and cloud chamber observations demonstrate that the vertical velocity variance is up to a factor of two larger than the horizontal velocity variance. The Taylor microscales calculated separately for the horizontal and vertical directions also indicate anisotropy of turbulent eddies. This anisotropy is attributed to production of turbulent kinetic energy (TKE) by buoyancy forces due to evaporative cooling of cloud droplets at the cloud–clear air interface. Numerical simulations quantify the effects of buoyancy oscillations relative to the values expected from adiabatic and isobaric mixing, standardly assumed in cloud physics. The buoyancy oscillations result from microscale transport of liquid water due to the gravitational sedimentation of cloud droplets. In the particular modeling setup considered here, these oscillations contribute to about a fifth of the total TKE production.

We present a mean-field model of cloud evolution that describes droplet growth due to condensation and collisions and droplet loss due to fallout. The model accounts for the effects of cloud turbulence both in a large-scale turbulent mixing and in a microphysical enhancement of condensation and collisions. The model allows for an effective numerical simulation by a scheme that is conservative in water mass and keeps accurate count of the number of droplets. We first study the homogeneous situation and determine how the rain-initiation time depends on the concentration of cloud condensation nuclei (CCN) and turbulence level. We then consider clouds with an inhomogeneous concentration of CCN and evaluate how the rain initiation time and the effective optical depth vary in space and time. We argue that over-seeding even a part of a cloud by small hygroscopic nuclei, one can substantially delay the onset and increase the amount of precipitation.

We discuss single-particle and two-particle statistics in two-dimensional turbulent convection in the Bolgiano–Oboukhov regime by means of high-resolution direct numerical simulations. Relative separation of two particles is found to be described well by a generalization of the Richardson diffusion model. Single-particle velocity structure functions are dominated by large-scale eddies and therefore a careful analysis based on 'exit-time' statistics is necessary to identify turbulent contributions. Because the velocity field is not intermittent, small-scale acceleration statistics is found to be in good agreement with simple dimensional predictions.

Using a unique data set of three-dimensional drop positions and masses (the HYDROP experiment), we show that the distribution of liquid water in rain displays a sharp transition between large scales which follow a passive scalar-like Corrsin–Obukhov (k^-5/3) spectrum and a small-scale statistically homogeneous white noise regime. We argue that the transition scale l_c is the critical scale where the mean Stokes number (= drop inertial time/turbulent eddy time) St_l is unity. For five storms, we found l_c in the range 45–75 cm with the corresponding dissipation scale St_η in the range 200–300. Since the mean interdrop distance was significantly smaller (≈ 10 cm) than l_c we infer that rain consists of 'patches' whose mean liquid water content is determined by turbulence with each patch being statistically homogeneous. For l>l_c, we have St_l<1 and due to the observed statistical homogeneity for l<l_c, we argue that we can use Maxey's relations between drop and wind velocities at coarse grained resolution l_c. From this, we derive equations for the number and mass densities (n and ρ) and their variance fluxes (ψ and χ). By showing that χ is dissipated at small scales (with l_{ρ, diss}≈l_c) and ψ over a wide range, we conclude that ρ should indeed follow Corrsin–Obukhov k^-5/3 spectra but that n should instead follow a k^-2 spectrum corresponding to fluctuations scaling as Δρ∝l^1/3 and Δn∝l^1/2. While the Corrsin–Obukhov law has never been observed in rain before, its discovery is perhaps not surprising; in contrast the Δn≈l^1/2 number density law is quite new. The key difference between the Δρ, Δn laws is the fact that the microphysics (coalescence, breakup) conserves drop mass, but not numbers of particles. This implies that the timescale for the transfer of the density variance flux χ is determined by the strongly scale-dependent turbulent velocity whereas the timescale for the transfer of the number variance flux ψ is determined by the weakly scale-dependent drop coalescence speed. We argue that the l^1/2 law may also hold (although in a slightly different form) for cloud drops. Because they are consequences of symmetries, we expect the l^1/3, l^1/2 laws to be robust. Since the large-scale turbulence determines the n and ρ fields which are the 0th and 1st moments of the drop-size distribution, they constrain the microphysics: dimensional analysis shows that the cumulative probability distribution of nondimensional drop mass should be a universal function dependent only on scale; we confirm this empirically. The combination of number and mass density laws can be used to develop stochastic compound multifractal Poisson processes which are useful new tools for studying and modelling rain. We discuss the implications of this for the rain rate statistics including a simplified model, which can explain the observed rain rate spectra.

The effect of air turbulence on the geometric collision kernel of cloud droplets can be predicted if the effects of air turbulence on two kinematic pair statistics can be modeled. The first is the average radial relative velocity and the second is the radial distribution function (RDF). A survey of the literature shows that no theory is available for predicting the radial relative velocity of finite-inertia sedimenting droplets in a turbulent flow. In this paper, a theory for the radial relative velocity is developed, using a statistical approach assuming that gravitational sedimentation dominates the relative motion of droplets before collision. In the weak-inertia limit, the theory reveals a new term making a positive contribution to the radial relative velocity resulting from a coupling between sedimentation and air turbulence on the motion of finite-inertia droplets. The theory is compared to the direct numerical simulations (DNS) results in part 1, showing a reasonable agreement with the DNS data for bidisperse cloud droplets. For droplets larger than 30 μm in radius, a nonlinear drag (NLD) can also be included in the theory in terms of an effective inertial response time and an effective terminal velocity. In addition, an empirical model is developed to quantify the RDF. This, together with the theory for radial relative velocity, provides a parameterization for the turbulent geometric collision kernel. Using this integrated model, we find that turbulence could triple the geometric collision kernel, relative to the stagnant air case, for a droplet pair of 10 and 20 μm sedimenting through a cumulus cloud at R_λ=2×10⁴ and =600 cm² s⁻³. For the self-collisions of 20 μm droplets, the collision kernel depends sensitively on the flow dissipation rate.

There have been relatively few studies of turbulent collision rate of sedimenting droplets in the context of cloud physics, for which both the gravitational settling and inertial effects must be simultaneously considered. In this study, direct numerical simulations (DNS) were used to study the geometric collision rates of cloud droplets. Both Stokes drag law and a nonlinear drag law were considered, but the droplet–droplet local aerodynamic interactions were not included. Typical droplet and turbulence parameters of convective clouds were used to determine the flow dissipation rate , characteristic Stokes numbers, and the nondimensional terminal velocities. DNS results from a large number of runs covering the range from 10 to 400 cm² s^{− 3} and droplet sizes from 10 to 60 μm in radius are presented. These results show that air turbulence can increase the geometric collision kernel by up to 47%, relative to geometric collision by differential sedimentation. This is due to both a moderate enhancement of the radial relative velocity between droplets and a moderate level of pair nonuniform concentration due to local droplet clustering. The turbulence enhancements increase with the flow dissipation rate and flow Reynolds number. Comparisons with related DNS studies show that our results confirm and extend the previous findings. The mean settling velocity of droplets in a turbulent flow was also obtained, showing that a maximum increase relative to the terminal velocity occurs for 20 μm cloud droplets. This agrees with a previous theory based on simple vortex flows and confirms the importance of a new nondimensional parameter τ_p³g²/ν for sedimenting droplets, where τ_p is the droplet inertial response time, g is the gravitational acceleration and ν is the air kinematic viscosity. Limitations of DNS and future directions are also noted.

We consider collisions of particles advected in a fluid. As already pointed out by Smoluchowski (1917 Z. Phys. Chem. 92 129–68), macroscopic motion of the fluid can significantly enhance the frequency of collisions between the suspended particles. This effect was invoked by Saffman and Turner (1956 J. Fluid Mech. 1 16–30) to estimate collision rates of small water droplets in turbulent rain clouds, the macroscopic motion being caused by turbulence. Here, we show that the Saffman–Turner theory is unsatisfactory because it describes an initial transient only. The reason for this failure is that the local flow in the vicinity of a particle is treated as if it were a steady hyperbolic flow, whereas, in reality, it must fluctuate. We derive exact expressions for the steady-state collision rate for particles suspended in rapidly fluctuating random flows and compute how this steady state is approached. For incompressible flows, the Saffman–Turner expression is an upper bound.

The collision efficiency of sedimenting cloud droplets in a turbulent air flow is a key input parameter in predicting the growth of cloud droplets by collision–coalescence. In this study, turbulent collision efficiency was directly computed, using a hybrid direct numerical simulation (HDNS) approach (Ayala et al 2007 J. Comput. Phys.225 51–73). The HDNS results show that air turbulence enhances the collision efficiency partly due to the fact that aerodynamic interactions (AIs) become less effective in reducing the relative motion of droplets in the presence of background air turbulence. The level of increase in the collision efficiency depends on the flow dissipation rate and the droplet size ratio. For example, the collision efficiency between droplets of 18 and 20 μm in radii is increased by air turbulence (relative to the stagnant air case) by a factor of 4 and 1.6 at dissipation rates of 400 and 100 cm² s⁻³, respectively. The collision efficiency for self-collisions in a bidisperse turbulent suspension can be larger than one. Such an increase in self-collisions is related to the far-field many-body AI and depends on the volumetric concentration of droplets. The total turbulent enhancement agrees qualitatively with previous results, but differs on a quantitative level. In the case of cross-size collisions between 18 and 20 μm droplets, the total turbulent enhancement can be a factor of 7 and 2 at dissipation rates of 400 and 100 cm² s⁻³, respectively. For intermediate size ratios (0.2<a₂/a₁<0.8), the overall enhancement typically falls below 2. Scaling arguments show that the overall enhancement factor tends to peak at the two limiting cases of a₂/a₁→1 and a₂/a₁→0.

Interest in energetic heavy ions is rapidly increasing in the field of biomedicine. Heavy ions are normally excluded from radiation protection, because they are not normally experienced by humans on Earth. However, knowledge of heavy ion biophysics is necessary in two fields: charged particle cancer therapy (hadrontherapy), and radiation protection in space missions.

The possibility to cure tumours using accelerated heavy charged particles was first tested in Berkeley in the sixties, but results were not satisfactory. However, about 15 years ago therapy with carbon ions was resumed first in Japan and then in Europe. Heavy ions are preferable to photons for both physical and biological characteristics: the Bragg peak and limited lateral diffusion ensure a conformal dose distribution, while the high relative biological effectiveness and low oxygen enhancement ration in the Bragg peak region make the beam very effective in treating radioresistant and hypoxic tumours. Recent results coming from the National Institute of Radiological Sciences in Chiba (see the paper by Dr Tsujii and co-workers in this issue) and GSI (Germany) provide strong clinical evidence that heavy ions are indeed an extremely effective weapon in the fight against cancer. However, more research is needed in the field, especially on optimization of the treatment planning and risk of late effects in normal tissue, including secondary cancers.

On the other hand, high-energy heavy ions are present in galactic cosmic radiation and, although they are rare as compared to protons, they give a major contribution in terms of equivalent dose to the crews of manned space exploratory-class missions. Exploration of the Solar System is now the main goal of the space program, and the risk caused by exposure to galactic cosmic radiation is considered a serious hindrance toward this goal, because of the high uncertainty on late effects of energetic heavy nuclei, and the lack of effective countermeasures. Risks include carcinogenesis, late degenerative tissue effects (including damage to the central nervous system), and hereditary effects. For these studies, microbeams represent an essential tool, considering that in space each cell in the human body will not experience more than one heavy-ion traversal. Both NASA and ESA are investing important resources in ground-based space radiation research programs, to reduce risk uncertainty and to develop countermeasures.

For both cancer therapy and space radiation protection a better understanding of the effects of energetic heavy ions is needed. Physics should be improved, especially the measurements of nuclear fragmentation cross-sections, and the transport calculations. Biological effects need to be studied in greater detail, and clearly only understanding the mechanisms of heavy-ion induced biological damage will reduce the uncertainty on late effects in humans.

This focus issue of New Journal of Physics aims to provide the state-of-the-art of the biophysics of energetic heavy ions and to highlight the areas where more research is urgently needed for therapy and the space program.

Focus on Heavy Ions in Biophysics and Medical Physics Contents

Research needed for improving heavy-ion therapyG Kraft and S D Kraft

Ionization and excitation cross sections for the interaction of HZE particles in liquid water and application to Monte Carlo simulation of radiation tracksIanik Plante and Francis A Cucinotta

Dose calculations at high altitudes and in deep space with GEANT4 using BIC and JQMD models for nucleus-nucleus reactions L Sihver, D Matthiä, T Koi and D Mancusi

Heavy ion microprobes: a unique tool for bystander research and other radiobiological applications K O Voss, C Fournier and G Taucher-Scholz

Heavy ions light flashes and brain functions: recent observations at accelerators and in spaceflight L Narici

Clinical advantages of carbon-ion radiotherapy Hirohiko Tsujii, Tadashi Kamada, Masayuki Baba, Hiroshi Tsuji, Hirotoshi Kato, Shingo Kato, Shigeru Yamada, Shigeo Yasuda, Takeshi Yanagi, Hiroyuki Kato, Ryusuke Hara, Naotaka Yamamoto and Junetsu Mizoe

Heavy-ion effects: from track structure to DNA and chromosome damage F Ballarini, D Alloni, A Facoetti and A Ottolenghi

Shielding experiments with high-energy heavy ions for spaceflight applications C Zeitlin, S Guetersloh, L Heilbronn, J Miller, N Elkhayari, A Empl, M LeBourgeois, B W Mayes, L Pinsky, M Christl and E Kuznetsov

Heavy charged particles in radiation biology and biophysics H Nikjoo, S Uehara, D Emfietzoglou and A Brahme

Impact of track structure calculations on biological treatment planning in ion radiotherapy Thilo Elsässer, Richard Cunrath, Michael Krämer and Michael Scholz

The physical basis for the biological action of heavy ions Jürgen Kiefer

Secondary beam fragments produced by 200 MeV u⁻¹¹²C ions in water and their dose contributions in carbon ion radiotherapy K Gunzert-Marx, H Iwase, D Schardt and R S Simon

The risk assessment for low doses of high linear energy transfer (LET) radiation has been challenged by a growing body of experimental evidence showing that non-irradiated bystander cells can receive signals from irradiated cells to elicit a variety of cellular responses. These may be significant for radiation protection but also for radiation therapy using heavy ions. Charged particle microbeams for radiobiological application provide a unique means to address these issues by allowing the precise irradiation of single cells with a counted numbers of ions. Here, we focus specifically on heavy ion microbeam facilities currently in use for biological purposes, describing their technical features and biological results. Typically, ion species up to argon are used for targeted biological irradiation at the vertically collimated microbeam at JAEA (Takasaki, Japan). At the SNAKE microprobe in Munich, mostly oxygen ions have been used in a horizontal focused beam line for cell targeting. At GSI (Darmstadt), a horizontal microprobe with a focused beam for defined targeting using ion species up to uranium is operational. The visualization of DNA damage response proteins relocalizing to defined sites of ion traversal has been accomplished at the three heavy ion microbeam facilities described above and is used to study mechanistic aspects of heavy ion effects. However, bystander studies have constituted the main focus of biological applications. While for cell inactivation and effects on cell cycle progression a response of non-targeted cells has been described at JAEA and GSI, respectively, in part controversial results have been obtained for the induction of DNA damage measured by double-strand formation or at the cytogenetic level. The results emphasize the influence of the cellular environment, and standardization of experimental conditions for cellular studies at different facilities as well as the investigation of bystander effects in tissue will be the aims of future research. At present, the most important conclusion of radiobiology studies at heavy ion microbeams is that bystander responses are not accentuated for increasing ionizing density radiation.

Interactions between ionizing radiation in space and brain functions, and the related risk assessments, are among the major concerns when programming long permanence in space, especially when outside the protective shield of the Earth's magnetosphere. The light flashes (LF) observed by astronauts in space, mostly when dark adapted, are an example of these interactions; investigations in space and on the ground showed that these effects can originate with the action of ionizing radiation in the eye. Recent findings from ALTEA, an interdisciplinary and multiapproach program devoted to the study of different aspects of the radiation–brain functions interaction, are presented in this paper. These include: (i) study of radiation passing through the astronauts' eyes in the International Space Station (≈20 ions min⁻¹, excluding H and fast and very slow He), measured in conjunction with reporting of the perception of LF; (ii) preliminary electrophysiological evidence of these events in astronauts and in patients during heavy ion therapy; and (iii) in vitro results showing the radiation driven activation of rhodopsin at the start of the phototransduction cascade in the process of vision. These results are in agreement with our previous work on mice. A brief but complete summary of the earlier works is also reported to permit a discussion of the results.

Carbon-ion radiotherapy (C-ion RT) possesses physical and biological advantages. It was started at NIRS in 1994 using the Heavy Ion Medical Accelerator in Chiba (HIMAC); since then more than 50 protocol studies have been conducted on almost 4000 patients with a variety of tumors. Clinical experiences have demonstrated that C-ion RT is effective in such regions as the head and neck, skull base, lung, liver, prostate, bone and soft tissues, and pelvic recurrence of rectal cancer, as well as for histological types including adenocarcinoma, adenoid cystic carcinoma, malignant melanoma and various types of sarcomas, against which photon therapy could be less effective. Furthermore, when compared with photon and proton RT, a significant reduction of overall treatment time and fractions has been accomplished without enhancing toxicities. Currently, the number of irradiation sessions per patient averages 13 fractions spread over approximately three weeks. This means that in a carbon therapy facility a larger number of patients than is possible with other modalities can be treated over the same period of time.

The use of carbon ions for the treatment of certain tumour types, especially radioresistant tumours, is becoming more frequent due to the carbon-ion dose localization and high relative biological effectiveness (RBE) in the Bragg peak region. Human beings can also be exposed to heavy ions in space, since galactic cosmic rays are a mixed field consisting of not only high-energy protons and He ions, but also heavier ions including iron. Due to their high linear energy transfer (LET), heavy ions have peculiar track structures, characterized by a high level of energy deposition clustering. Furthermore, high-energy ions produce energetic secondary electrons ('delta rays') which can give rise to energy depositions several micrometres away from the core of the primary particle track. Also in view of hadron therapy and space radiation applications, it is therefore important to characterize heavy-ion tracks from a physical and biophysical point of view. In this framework, herein we will discuss the main physical features of heavy-ion track structure, as well as heavy-ion-induced DNA double-strand breaks, which are regarded as one of the most important initial radiobiological lesions and chromosome aberrations, which are correlated both with cell death and with cell conversion to malignancy.

Mitigation of radiation exposures received by astronauts on deep-space missions must be considered in the design of future spacecraft. The galactic cosmic rays (GCR) include high-energy heavy ions, many of which have ranges that exceed the depth of shielding that can be launched in realistic scenarios. Some of these ions are highly ionizing (producing a high dose per particle) and for some biological endpoints are more damaging per unit dose than sparsely ionizing radiation. The principal physical mechanism by which the dose and dose equivalent delivered by these particles can be reduced is nuclear fragmentation, the result of inelastic collisions between nuclei in the hull of the spacecraft and/or other materials. These interactions break the incident ions into lighter, less ionizing and less biologically effective particles. We have previously reported the tests of shielding effectiveness using many materials in a 1 GeV nucleon⁻¹⁵⁶Fe beam, and also reported results using a single polyethylene (CH₂) target in a variety of beam ions and energies up to 1 GeV nucleon⁻¹. An important, but tentative, conclusion of those studies was that the average behavior of heavy ions in the GCR would be better simulated by heavy beams at energies above 1 GeV nucleon⁻¹. Following up on that work, we report new results using beams of ¹²C, ²⁸Si and ⁵⁶Fe, each at three energies, 3, 5 and 10 GeV nucleon⁻¹, on carbon, polyethylene, aluminium and iron targets.

Ionizing radiations induce a variety of molecular and cellular types of damage in mammalian cells as a result of energy deposition by the radiation track. In general, tracks are divided into two classes of sparsely ionizing ones such as electron tracks and densely ionizing tracks such as heavy ions. The paper discusses various aspects and differences between the two types of radiations and their efficacies in radiation therapy. Biophysical studies of radiation tracks have provided much of the insight in mechanistic understanding of the relationship between the initial physical events and observed biological responses. Therefore, development of Monte Carlo track-structure techniques and codes are paramount for the progress of the field. In this paper, we report for the first time the latest development for the simulation of proton tracks up to 200 MeV similar to beam energies in proton radiotherapy and space radiation. Vital to the development of the models for ion tracks is the accurate simulation of electron tracks cross sections in liquid water. In this paper, we report the development of electron track cross sections in liquid water using a new dielectric model of low-energy electrons accurate to nearly 10% down to 100 eV.

Treatment planning for ion therapy requires precise knowledge about the biological effectiveness of particle beams, which is strongly determined by the microscopic radial energy deposition around individual ion tracks. We analyse different amorphous track structure models based on simple analytical formulae as well as on radial dose distributions derived by means of Monte Carlo simulations. Moreover, these track structure representations are used as input for the local effect model (LEM) in order to determine their impact on the relative biological effectiveness (RBE) of cell inactivation. It demonstrates the relevance of the inner part of the ion track with a radius of the order of a few nanometres. We show that simple analytical formulae for the radial dose distributions give good results for the prediction of cell inactivation. However, they strongly depend on the assumptions about the local dose in the track core. Additionally, we discuss the interdependence of track structure calculations with other model constituents such as target size and the choice of the biological input data for conventional photon irradiation.

The physical properties of heavy ions determine to some extent their biological action. The spatial pattern of energy deposition is different from that by sparsely ionizing radiations like x- or γ-rays. The interaction with atomic electrons may lead to large local energy depositions in sensitive targets. Dose is essentially determined by particle fluence and the linear energy transfer (LET) of the projectile. The biological action depends on the number of targets hit by the particle and the probability to cause an effect by a single traversal which is called the 'action cross section'. Action cross-section increases with LET approaching the geometrical cross section of sensitive targets. A detailed analysis shows that ion tracks have a certain structure produced by the action of more energetic far-reaching electrons liberated by the initial interactions of the incoming ion. Track structure is analysed using a simple model based on classical collision dynamics describing at least the essential features. It is shown that very high energies are deposited in the track centre which may lead to large local concentrations of initial lesions. The radial extension of tracks can lead to damage in sensitive targets even if they are not directly traversed. Track structure analysis can account for this in a qualitative way but the quantitative agreement is still poor. The models are not able to explain the higher relative biological effectiveness (RBE) of heavy ions compared to x- or γ-rays. Biological factors such as lesion clustering and repair processes have additionally been taken into account. This can be done by `phenomenological models' incorporating track structure analysis and assumptions on the dependence of biological effectiveness on local energy density.

For applications in heavy-ion radiotherapy, the emission of secondary fragments from 200 MeV u⁻¹ carbon ions was investigated using a 12.78 cm thick water absorber as a tissue-equivalent beam stopping target. Secondary light particles (n, p, d, t, ³He and ⁴He) produced by nuclear fragmentation and emerging from the target in forward direction were detected with a ΔE−E-telescope consisting of NE102 and BaF₂ scintillation detectors. Energy spectra of the fragments at angles of 0°, 5°, 10°, 20° and 30° to the beam axis were obtained from time-of-flight measurements. They show a broad maximum at about half of the projectiles energy per nucleon, the shape at high energies is exponential and extends up to the projectiles initial energy per nucleon—for neutrons and protons to about twice the energy of the projectile per nucleon. Comparison of the experimental data with calculations performed with the Monte-Carlo code (partide and heavy ion transport code system (PHITS)) shows fairly good agreement for neutrons, protons and deuterons, but some deviations for tritons and helium fragments. The neutron dose in patient treatments with carbon ions was estimated to be 8 mGy per treatment-Gy based on the measured neutron yield.

We introduce electron energy-gain spectroscopy as a tool to yield information on local optical excitations of nanostructured systems using transmission electron microscopes equipped with external optical illumination. The new spectroscopy combines the superb spatial resolution of electron microscopes with unprecedented energy resolution below the milli-electron-volt level, only limited by the bandwidth of the external light. The analysis of energy gain events should reveal hyperfine details in the optical response of individual nanostructures (e.g. plasmons in nanoparticles). Our conclusions rely on a general formalism capable of describing light absorption by fast electrons moving in vacuum near an illuminated nanostructure, thus paving the way towards new light-assisted electron- and ion-acceleration schemes. Energy gain probabilities are shown to be comparable to those observed in energy loss experiments for reasonable illumination intensity.

q-breathers (QBs) are exact time-periodic solutions of extended nonlinear systems continued from the normal modes of the corresponding linearized system. They are localized in the space of normal modes. The existence of these solutions in a weakly anharmonic atomic chain explained essential features of the Fermi–Pasta–Ulam (FPU) paradox. We study QBs in one-, two- and three-dimensional discrete nonlinear Schrödinger (DNLS) lattices—theoretical playgrounds for light propagation in nonlinear optical waveguide networks, and the dynamics of cold atoms in optical lattices. We prove the existence of these solutions for weak nonlinearity. We find that the localization of QBs is controlled by a single parameter which depends on the norm density, nonlinearity strength and seed wave vector. At a critical value of that parameter QBs delocalize via resonances, signaling a breakdown of the normal mode picture and a transition into the strong mode–mode interaction regime. In particular, this breakdown takes place at one of the edges of the normal mode spectrum, and in a singular way also in the center of that spectrum. A stability analysis of QBs supplements these findings. We relate our findings to previous studies of time-periodic multibreather standing waves. For three-dimensional lattices, we find QB vortices which violate time reversal symmetry and generate a vortex ring flow of energy in normal mode space.

Quantum metrology promises greater sensitivity for optical phase measurements than could ever be achieved classically. Here, we present a theory of the phase sensitivity for the general case where the detection probability is given by an N photon interference fringe. We find that the phase sensitivity has a complicated dependence on both the intrinsic efficiency of detection η and the interference fringe visibility V. Most importantly, the phase that gives maximum phase sensitivity is in general not the same as the phase at which the slope of the interference fringe is a maximum, as has previously been assumed. We determine the parameter range where quantum enhanced sensitivity can be achieved. In order to illustrate these theoretical results, we perform a four-photon experiment with η=3/4 and V=82±6% (an extension of our previous work (Nagata et al 2007 Science 316 726)) and find a phase sensitivity 1.3 times greater than the standard quantum limit at a phase different to that which gives maximum slope of the interference fringe.

We study cold atomic gases in a disorder potential and analyse the correlations between different systems subjected to the same disorder landscape. Such independent copies with the same disorder landscape are known as replicas. While, in general, these are not accessible experimentally in condensed matter systems, they can be realized using standard tools for controlling cold atomic gases in an optical lattice. Of special interest is the overlap function which represents a natural order parameter for disordered systems and is a correlation function between the atoms of two independent replicas with the same disorder. We demonstrate an efficient measurement scheme for the determination of this disorder-induced correlation function. As an application, we focus on the disordered Bose–Hubbard model and determine the overlap function within the perturbation theory and a numerical analysis. We find that the measurement of the overlap function allows for the identification of the Bose-glass phase in certain parameter regimes.

The magnetoresistance of a quasi-ballistic Aharonov–Bohm (AB) ring defined in the two-dimensional electron gas (2DEG) of an InP/In_0.8Ga_0.2As quantum well is studied. The ring is connected to an Al contact on one side and to a 2DEG reservoir at the other side. Two distinct magnetic field regimes can be identified. At magnetic field values where time-reversal symmetry (TRS) is broken, AB oscillations are observed. Besides oscillations with h/e periodicity, we also observe higher harmonics with h/2e and h/3e periods. In the low-magnetic field range, where TRS is preserved, the AB oscillations are alternately dominated by the h/e or h/2e component, depending on the bias voltage. Although Al is superconducting at these low magnetic fields, no evidence is found that the observed AB oscillations are related to the proximity of the superconductor. The bias voltage dependence is qualitatively described in terms of a 1D scattering model.

We present the characterization of a decanedithiol monolayer using a technique which is based on micro-transfer printing. This technique allows us to contact molecular monolayers on the scale of 100 μm² without producing electrical shorts between the metallic contacts. This opens the possibility to characterize the behavior of metal–molecule contacts on a large ensemble. Electrical measurements of the decanes show that two main transport mechanisms dominate conduction through this monolayer, tunneling between the electrodes and transport through electric states of the molecules. In our setup, tunneling is temperature dependent, because a length change of the molecules causes changes of the effective tunneling width.

Here we study how to control the dynamics of excitable systems by using the phase control technique. Excitable systems are relevant in neuronal dynamics and therefore this method might have important applications. We use the periodically driven FitzHugh–Nagumo (FHN) model, which displays both spiking and non-spiking behaviours in chaotic or periodic regimes. The phase control technique consists of applying a harmonic perturbation with a suitable phase ϕ that we adjust in search of different behaviours of the FHN dynamics. We compare our numerical results with experimental measurements performed on an electronic circuit and find good agreement between them. This method might be useful for a better understanding of excitable systems and different phenomena in neuronal dynamics.

Using an exact functional method, within the framework of the gradient expansion for the Liouville effective action, we show that the kinetic term for the Liouville field is not renormalized.

An experimental facility named HiRadMat, will be constructed at CERN to study the impact of the 450 GeV c⁻¹ proton beam generated by the Super Proton Synchrotron (SPS) on solid targets. This is designed to study damage caused to the equipment including absorbers, collimators and others in case of an accidental release of the beam energy. This paper presents two-dimensional numerical simulations of target behavior irradiated by the SPS beam. These numerical simulations have shown that the target will be completely destroyed in such an accident, thereby generating high energy density (HED) matter. This study therefore suggests that this facility may also be used for carrying out dedicated experiments to study HED states in matter.

We demonstrate that two remote qubits can be entangled through an optically active intermediary even if the coupling strengths between mediator and qubits are different. This is true for a broad class of interactions. We consider two contrasting scenarios. Firstly, we extend the analysis of a previously studied gate operation which relies on pulsed, dynamical control of the optical state and which may be performed quickly. We show that remote spins can be entangled in this case even when the intermediary coupling strengths are unequal. Secondly, we propose an alternative adiabatic control procedure, and find that the system requirements become even less restrictive in this case. The scheme could be tested immediately in a range of systems including molecules, quantum dots, or defects in crystals.

Rabi oscillations are coherent transitions in a quantum two-level system under the influence of a resonant drive, with a much lower frequency dependent on the perturbation amplitude. These serve as one of the signatures of quantum coherent evolution in mesoscopic systems. It was shown recently (Grønbech-Jensen N and Cirillo M 2005 Phys. Rev. Lett.95 067001) that in phase qubits (current-biased Josephson junctions) this effect can be mimicked by classical oscillations arising due to the anharmonicity of the effective potential. Nevertheless, we find qualitative differences between the classical and quantum effects. Firstly, while the quantum Rabi oscillations can be produced by the subharmonics of the resonant frequency ω₁₀ (multiphoton processes), the classical effect also exists when the system is excited at the overtones, nω₁₀. Secondly, the shape of the resonance is, in the classical case, characteristically asymmetric, whereas quantum resonances are described by symmetric Lorentzians. Thirdly, the anharmonicity of the potential results in the negative shift of the resonant frequency in the classical case, in contrast to the positive Bloch–Siegert shift in the quantum case. We show that in the relevant range of parameters these features allow us to distinguish confidently the bona fide Rabi oscillations from their classical Doppelgänger.

The dc response of one-dimensional (1D) arrays of superconductor/insulator/superconductor (SIS) junctions under radio-frequency (RF) irradiation has been experimentally studied. The zero-bias resistance as a function of RF amplitude in the low-frequency regime shows an oscillatory behaviour, and can be explained using the phase fluctuation model for Cooper-pair tunnelling. It follows that an effective classical detector theory is applicable and the oscillation period can be compared with the superconducting gap. The results suggest a primary and direct RF detection scheme using 1D SIS junction arrays.

Thin films of Co/Ru(0001) are known to exhibit an unusual spin reorientation transition (SRT) coupled to the completion of Co atomic layers for Co thicknesses under four layers. By means of spin-polarized low-energy electron microscopy, we follow in real space the magnetization orientation during the growth of atomically thick capping layers on Co/Ru(0001). Capping with noble metal (Cu, Ag and Au) elements modifies the SRT depending on the Co and overlayer thickness and on the overlayer material, resulting in an expanded range of structures with high perpendicular magnetic anisotropy. The origin of the SRT can be explained in terms of ab initio calculations of the layer-resolved contributions to the magnetic anisotropy energy. Besides the changes in the SRT introduced by the capping, a quantitative enhancement of the magnetic anisotropy is identified. A detailed analysis of the interplay between strain and purely electronic effects allows us to identify the conditions that lead to a high perpendicular magnetic anisotropy in thin hcp Co films.

We experimentally investigate the interaction between one and two atoms and the field of a high-finesse optical resonator. Laser-cooled caesium atoms are transported into the cavity using an optical dipole trap. We monitor the interaction dynamics of a single atom strongly coupled to the resonator mode for several hundred milliseconds by observing the cavity transmission. Moreover, we investigate the position-dependent coupling of one and two atoms by shuttling them through the cavity mode. We demonstrate an alternative method, which suppresses heating effects, to analyse the atom–field interaction by retrieving the atom from the cavity and by measuring its final state.

Colloidal suspensions of small silicon nanoparticles (diameter around 2 nm) with fast and efficient ultraviolet–blue photoluminescence (PL) band are fabricated by enhanced electrochemical etching of Si wafers. The detailed study of photoluminescence excitation spectra in a wide range of excitation photon energies (270–420 nm) reveals specific behavior of the Stokes shift of the fast PL band that agrees well with theoretical calculation of optical transitions in small silicon nanocrystals and is distinct from emission of silicon dioxide defects.

This work is devoted to the analysis of the orbital ordering patterns and associated interatomic magnetic interactions in the centrosymmetric monoclinic structures of BiMnO₃, which have been recently determined experimentally. First, we set up an effective lattice fermion model for the manganese 3d bands and derive parameters of this model from first-principles electronic structure calculations. Then, we solve this model in terms of the mean-field Hartree–Fock method and obtain parameters of interatomic magnetic interactions between Mn ions. We argue that although the nearest-neighbor interactions favor the ferromagnetism, they compete with the longer range antiferromagnetic (AFM) interactions, the existence of which is directly related to the peculiar geometry of the orbital ordering pattern realized in BiMnO₃ below 474 K. These AFM interactions favor an AFM alignment, which breaks the inversion symmetry. The formation of the AFM phase is additionally assisted by the orbital degrees of freedom, which tend to adjust the nearest-neighbor magnetic interactions in the direction which further stabilizes this phase. We propose that the multiferroelectric behavior, which was observed in BiMnO₃, may be related to the emergence of the AFM phase under certain conditions.

Food is an essential part of civilization, with a scope that ranges from the biological to the economic and cultural levels. Here, we study the statistics of ingredients and recipes taken from Brazilian, British, French and Medieval cookery books. We find universal distributions with scale invariant behaviour. We propose a copy-mutate process to model culinary evolution that fits our empirical data very well. We find a cultural 'founder effect' produced by the non-equilibrium dynamics of the model. Both the invariant and idiosyncratic aspects of culture are accounted for by our model, which may have applications in other kinds of evolutionary processes.

The dissipation of energy following the impact of multiply charged ions on a polycrystalline metal surface was studied using thin film metal–insulator–metal junctions as targets. The ions hit the top Ag layer of a Ag–AlO_x–Al junction, where they excite electrons and holes. A substantial fraction of these charge carriers is transported across the insulating barrier and can be detected as an internal current in the bottom Al layer. The effects of potential and kinetic energies on this tunneling yield are investigated separately by varying the charge state of the Ar projectile ions from 2+ to 8+ for kinetic energies in the range from 1 to 12 keV. Per impinging ion yields of typically 0.1–1 electrons are measured within the thin film tunnel junction. The tunneling yield is found to scale linearly with the potential energy of the projectile. In addition, the tunneling yield shows a strong dependence on the internal barrier height which can be modified by an external bias voltage.

Parametric down-conversion (PDC) sources can be used as triggered single photon sources in quantum key distribution (QKD). Recently, there have been several practical proposals of the decoy state QKD with triggering PDC sources. In this paper, we generalize the passive decoy state idea, originally proposed by Mauerer and Silberhorn. The generalized passive decoy state idea can be applied to cases where either threshold detectors or photon-number-resolving detectors are used. The decoy state protocol proposed by Adachi, Yamamoto, Koashi and Imoto (AYKI) can be treated as a special case of the generalized passive decoy state method. By simulating a recent PDC experiment, we compare various practical decoy state protocols with the infinite decoy state protocol and also compare the cases using threshold detectors and photon-number-resolving detectors. Our simulation result shows that with the AYKI protocol, one can achieve a key generation rate that is close to the theoretical limit of an infinite decoy state protocol. Furthermore, our simulation result shows that a photon-number-resolving detector appears to be not very useful for improving QKD performance in this case. Although our analysis is focused on QKD with PDC sources, we emphasize that it can also be applied to QKD setups with other triggered single photon sources.

200 keV hydrogen implantation was performed in order to investigate the formation of electrically active hydrogen-related defects in Al-doped 4H-SiC epitaxial layers. Samples were annealed in the 100–1200 °C temperature range for 15 min and capacitance–voltage (C–V) profiling was employed to study the passivating effects of hydrogen. Eight electrically active hole traps have been found by means of deep level transient spectroscopy (DLTS) and the study of their thermal stability, as well as their depth profiles, revealed the possible formation of an electrically active hydrogen-related center.

Excitonic transitions offer a possible route to ultrafast optical spin manipulation in coupled nanostructures. We perform here a detailed study of the three principal exciton-mediated decoherence channels for optically controlled electron spin qubits in coupled quantum dots: radiative decay of the excitonic state, exciton–phonon interactions, and Landau–Zener transitions between laser-dressed states. We consider a scheme for producing an entangling controlled-phase gate on a pair of coupled spins which, in its simplest dynamic form, renders the system subject to fast decoherence rates associated with exciton creation during the gating operation. In contrast, we show that an adiabatic approach employing off-resonant laser excitation allows us to suppress all sources of decoherence simultaneously, significantly increasing the fidelity of operations at only a relatively small gating time cost. We find that controlled-phase gates accurate to one part in 10² can realistically be achieved with the adiabatic approach, whereas the conventional dynamic approach does not appear to support a fidelity suitable for scalable quantum computation. Our predictions could be demonstrated experimentally in the near future.

We discuss how adiabatic potentials can be used to create addressable lattices on a subwavelength scale, which can be used as a tool for local operations and readout within a lattice substructure, while taking advantage of the faster timescales and higher energy and temperature scales determined by the shorter lattice spacing. For alkaline-earth-like atoms with nonzero nuclear spin, these potentials can be made state-dependent, for which we give specific examples with ¹⁷¹Yb atoms. We discuss in detail the limitations in generating the lattice potentials, in particular non-adiabatic losses, and show that the loss rates can always be made exponentially small by increasing the laser power.

Quantum projection noise will soon limit the best achievable precision of optical atomic clocks based on lattice-confined neutral atoms. Squeezing the collective atomic pseudo-spin via measurement of the clock state populations during Ramsey interrogation suppresses the projection noise. We show here that the lattice laser field can be used to perform ideal quantum non-demolition measurements without clock shifts or decoherence and explore the feasibility of such an approach in theory with the lattice field confined in a ring-resonator. Detection of the motional sideband due to the atomic vibration in the lattice wells can yield signal sizes a hundredfold above the projection noise limit.

We are interested in the problem of characterizing the correlations that arise when performing local measurements on separate quantum systems. In a previous work (Navascués et al 2007 Phys. Rev. Lett. 98 010401), we introduced an infinite hierarchy of conditions necessarily satisfied by any set of quantum correlations. Each of these conditions could be tested using semidefinite programming. We present here new results concerning this hierarchy. We prove in particular that it is complete, in the sense that any set of correlations satisfying every condition in the hierarchy has a quantum representation in terms of commuting measurements. Although our tests are conceived to rule out non-quantum correlations, and can in principle certify that a set of correlations is quantum only in the asymptotic limit where all tests are satisfied, we show that in some cases it is possible to conclude that a given set of correlations is quantum after performing only a finite number of tests. We provide a criterion to detect when such a situation arises, and we explain how to reconstruct the quantum states and measurement operators reproducing the given correlations. Finally, we present several applications of our approach. We use it in particular to bound the quantum violation of various Bell inequalities.

We report measurements on the angular radiation patterns from dye molecules embedded in a polymethyl methacrylate thin film spin-coated on a thin silver film. We systematically studied the influence of the thickness of the silver films and the thickness of the dielectric layers on the radiation pattern. We present the detailed radiation patterns over a large angular range showing highly polarized fluorescence emission coupled into the surface plasmon modes or waveguide modes. We also studied the influence of the polarization of the excitation beam on the radiation patterns. The experimental data are compared with numerical simulations using an asymptotic approach based on the Lorentz reciprocity theorem.

Neutral particles can be guided and focused using electric field gradients that focus in one transverse direction and defocus in the other, alternating between the two directions. Such a guide is suitable for transporting particles that are attracted to strong electric fields, which cannot be guided using static fields. Particles are only transmitted if their initial positions and transverse speeds lie within the guide's phase-space acceptance. Nonlinear forces are always present in the guide and can severely reduce this acceptance. We consider the effects of the two most important nonlinear forces, a term in the force that is cubic in the off-axis displacement, and a nonlinear term which couples together the two transverse motions. We use approximate analytical techniques, along with numerical methods, to calculate the influence of these nonlinear forces on the particle trajectories and on the phase-space acceptance. The cubic term alters the focusing and defocusing powers, leading either to an increase or a decrease of the acceptance depending on its sign. We find an approximate analytical result for the phase-space acceptance including this cubic term. Using a perturbation method we show how the coupling term leads to slow changes in the amplitudes of the transverse oscillations. This term reduces the acceptance when it reduces the focusing power, but has little influence when it increases that power. It is not possible to eliminate both nonlinear terms, but one can be made small at the expense of the other. We show how to choose the guide parameters so that the acceptance is optimized.

Increasing recent empirical evidence indicates the extensive existence of heavy tails in the inter-event time distributions of various human behaviors. Based on the queuing theory, the Barabási model and its variations suggest the highest-priority-first protocol to be a potential origin of those heavy tails. However, some human activity patterns, also displaying heavy-tailed temporal statistics, could not be explained by a task-based mechanism. In this paper, different from the mainstream, we propose an interest-based model. Both the simulation and analysis indicate a power-law inter-event time distribution with exponent −1, which is in accordance with some empirical observations in human-initiated systems.

The effect on flavour oscillations of simple expanding background space–times, motivated by some D-particle foam models, is calculated for a toy model of bosons with flavour degrees of freedom. The presence of D-particle defects in the space–time, which can interact non-trivially (via 'particle capture') with flavoured particles in a flavour non-preserving way, generates mixing in the effective field theory of low-energy string excitations. Moreover, the recoil of the D-particle defect during the capture/scattering process implies Lorentz violation, which, however, may be averaged to zero in isotropic D-particle populations, but implies non-trivial effects in correlators. Both features imply that the flavoured mixed state sees a non-trivial flavour (Fock-space) vacuum of a type introduced earlier by Blasone and Vitiello in a generic context of theories with mixing. We discuss the orthogonality of the flavour vacua to the usual Fock vacua and the effect on flavour oscillations in these backgrounds. Furthermore, we analyse the equation of state of the flavour vacuum, and find that, for slow expansion rates induced by D-particle recoil, it is equivalent to that of a cosmological constant. Some estimates of these novel non-perturbative contributions to the vacuum energy are made. The contribution vanishes if the mass difference and the mixing angle of the flavoured states vanish.

Effects of a periodic driving field on Landau–Zener (LZ) processes are studied using a nonlinear two-mode model that describes the mean-field dynamics of a many-body system. A variety of different dynamical phenomena in different parameter regimes of the driving field are discussed and analyzed. These include shifted, weakened, or enhanced phase dependence of nonlinear LZ (NLZ) processes, nonlinearity-enhanced population transfer in the adiabatic limit and Hamiltonian chaos on the mean-field level. The emphasis of this work is based on how the impact of a periodic driving field on LZ processes with self-interaction differs from those without self-interaction. Apart from gaining knowledge of driven NLZ processes, our findings can be used to gauge the strength of nonlinearity and for efficient manipulation of the mean-field dynamics of many-body systems.

We present a numerical study on ground state properties of a one-dimensional (1D) general Hubbard model (GHM) with particle-assisted tunnelling rates and repulsive on-site interaction (positive-U), which describes fermionic atoms in an anisotropic optical lattice near a wide Feshbach resonance. For our calculation, we utilize the time evolving block decimation (TEBD) algorithm, which is an extension of the density matrix renormalization group and provides a well-controlled method for 1D systems. We show that the positive-U GHM, when hole-doped from half-filling, exhibits a phase with coexistence of quasi-long-range superfluid and charge-density-wave orders. This feature is different from the property of the conventional Hubbard model with positive-U, indicating the particle-assisted tunnelling mechanism in GHM brings in qualitatively new physics.

We evaluate the realization of a novel geometry of a guided atom interferometer based on a high-temperature superconducting microstructure. The interferometer-type structure is obtained with a guiding potential realized by two current carrying superconducting wires in combination with a closed superconducting loop sustaining a persistent current. We present the layout and realization of our superconducting atom chip. By employing simulations, we discuss the critical parameters of the interferometer guide in particular near the splitting regions of the matter waves. Based on measurements of the relevant chip properties, we discuss the application of a compact and reliable on-chip atom interferometer.

We have performed soft x-ray angle-resolved photoemission spectroscopy (ARPES) for the electron-doped high-temperature superconducting cuprates (HTSCs) Nd_2-xCe_xCuO₄ (x=0.075 and 0.15) in order to disclose their genuine bulk electronic structures. Our soft x-ray ARPES has revealed that the bulk-derived nodal 'kink' (the abrupt change of the electron velocity in quasiparticle dispersions), being absent in the low-energy ARPES, is clearly observable for the electron-doped superconducting Nd_1.85Ce_0.15CuO₄. This result confirms that the antiferromagnetic spin correlation is suppressed and the electron–phonon interactions are prevailing in the bulk of the electron-doped superconducting Nd_{2- x}Ce_xCuO₄ at the optimal doping level. The nodal 'kink' behavior is equivalent to that observed in the hole-doped HTSCs. The anisotropic strong electron–phonon interactions seem to be inherent in the HTSCs irrespective of the type of doped carriers.

We identify a newly found Z(4430) with the first radial excitation of the tetraquark basic supermultiplet to which X(3872) and X(3876) belong. Experimental predictions following from this hypothesis are spelled out.

We present a continuous-wave dual-crystal optical parametric oscillator that generates signal waves of about 1.4 μm wavelength with a tunable difference frequency Δν between 0.64 and 5.3 THz. A model of the parametric gain in such a system indicates that the lowest reachable value for Δν depends on the refractive index dispersion of the nonlinear material used. As a proof of principle, we demonstrate the generation of terahertz radiation by photomixing the signal waves. Frequencies between 0.64 and 0.85 THz are created and detected coherently with ion-irradiated InGaAs interdigitated photomixers.

Magnetic field-induced twin boundary motion leading to large magnetic field-induced strain of ∼1.0% was established in polycrystalline Ni_50.9Mn_27.1Ga_22.0 (at.%) fibres at room temperature (∼60–100 μm in diameter and ∼3 mm in length). The fibres' grains are as large as the fibre diameter and of random orientation. At room temperature, a ferromagnetic 5M martensite is found. Magnetic field-induced twin boundary motion was indicated by magnetic measurements and validated by electron backscatter diffraction (EBSD). The application of a magnetic field shifts the equilibrium temperature of martensite and austenite by ∼0.4 K T⁻¹, which agrees with calculations using the Clapeyron–Clausius approach.

We use lithographically induced strain relaxation to periodically modulate the magnetic anisotropy in a single (Ga,Mn)As layer. This results in a lateral magnetoresistance device where two non-volatile magnetic states exist at zero external magnetic field with resistances resulting from the orientation of two lithographically defined regions in a single and continuous layer.