Table of contents

Several theories have been proposed for the synthesis of prebiotic molecules. This letter shows that the structure of supercritical water, or high-pressure water, could trigger prebiotic synthesis and the origin of life deep in the oceans, in hydrothermal vent systems. Dimer geometries of high-pressure water may have a point of symmetry and a zero dipole moment. Consequently, simple apolar molecules found in submarine hydrothermal vent systems will dissolve in the apolar environment provided by the apolar form of the water dimer. Apolar water could be the medium which helps precursor molecules to concentrate and react more efficiently. The formation of prebiotic molecules could thus be linked to the structure of the water inside chimney nanochannels and cavities where hydrothermal piezochemistry and shock wave chemistry could occur.

Analytical expressions are derived as a first-order approximation for the elastic displacement fields, including both phonon- and phason-type components, induced by spherical inclusions in icosahedral quasicrystals (IQCs). The phonon-type component obtained is the same as that for a spherical inclusion in conventional elastically isotropic crystals, while the expression for the phason-type displacement, which decreases with increasing distance r to the sphere centre as r⁻¹, is derived for the first time. Three concrete cases are discussed. The phonon part of the analytical expressions has been used to simulate the black–white double-lobe contrasts in the x-ray topograph (XRT) images of the strain field around the pores in the AlPdMn IQCs. The analytical expression for the phason-type displacement is used to simulate the XRT images of the strain field around spherical inclusions having isomorphic structures. The striking resemblance between experimental and simulated images suggests that such inclusions are surrounded by antiphase boundaries with a phason-type displacement vector, being particular structural defects in IQCs.

The transition temperature (T_c) for δ-Pu has been calculated for the first time. A Monte Carlo method is employed for this purpose and the effective cluster interactions are obtained from first-principles calculations incorporated with the Connolly–Williams and generalized perturbation methods. It is found that at T_c ∼ 548 K, δ-Pu undergoes transformation from a disordered magnetic state to a structure with an antiferromagnetic spin alignment that is mechanically unstable with respect to tetragonal distortion. The calculated transition temperature is in good agreement with the temperature measured at the γ → δ transition (593 K).

Charge transport of a two-dimensional electron gas in the presence of a magnetic field is studied by means of the Keldysh–Green function formalism and the tight-binding method. We evaluate the spatial distributions of persistent (equilibrium) and transport (nonequilibrium) currents, and give a vivid picture of their profiles. In the quantum Hall regime, we find exact conductance quantization both for persistent currents and for transport currents, even in the presence of impurity scattering centres and moderate disorder.

This letter discusses magnon-mediated superconductivity in ferromagnetic metals. The mechanism explains in a natural way the fact that the superconductivity in UGe₂, ZrZn₂ and URhGe is apparently confined to the ferromagnetic phase. The order parameter is a spin anti-parallel component of a spin-1 triplet with zero spin projection. The transverse spin fluctuations are pair forming and the longitudinal ones are pair breaking. The competition between magnons and paramagnons explains the existence of two successive quantum phase transitions in UGe₂, from ferromagnetism to ferromagnetic superconductivity, and at higher pressure to paramagnetism. The maximum T_SC results from the suppression of the paramagnon contribution. To form a Cooper pair an electron transfers from one Fermi surface to the other. As a result, the onset of superconductivity leads to the appearance of two Fermi surfaces in each of the spin up and spin down momentum distribution functions. This fact explains the linear temperature dependence at low temperature of the specific heat, and the experimental results for UGe₂.

Empirically, the coercive field needed to reverse the polarization in a ferroelectric increases with decreasing film thickness. For ferroelectric films of 100 µm to 100 nm in thickness the coercive field has been successfully described by a semi-empirical scaling law. Accounting for depolarization corrections, we show that this scaling behaviour is consistent with field measurements of ultrathin ferroelectric capacitors down to one nanometre in film thickness. Our results also indicate that the minimum film thickness, determined by a polarization instability, can be tuned by the choice of electrodes, and recommendations for next-generation ferroelectric devices are discussed.

Continuous wave room-temperature output power of ∼ 3 W for edge emitters and of 1.2 mW for vertical-cavity surface-emitting lasers is realized for GaAs-based devices using InAs quantum dots (QDs) operating at 1.3 µm. Characteristic temperatures up to 170 K below 330 K are realized. Simultaneously, differential efficiency exceeds 80% for these devices. Lasers emitting up to 12 W at 1140–1160 nm are useful as pump sources for Tm³⁺-doped fibres for frequency up-conversion to 470 nm. Both types of lasers show transparency current densities of 6 A cm⁻² per dot layer, η_int = 98% and α_i around 1.5 cm⁻¹. Long operation lifetimes (above 3000 h at 50 °C heatsink temperature at 1.5 W CW) and improved radiation hardness as compared to quantum well (QW) devices are manifested. Cut-off frequencies of about 10 GHz at 1100 nm and 6 GHz at 1300 nm and low α factors resulting in reduced filamentation and improved M² values in single-mode operation are realized. Quantum dot semiconductor optical amplifiers (QD SOAs) demonstrate gain recovery times of 120–140 fs, 4–7 times faster than bulk/QW SOAs. The breakthrough became possible due to the development of self-organized growth in QD technology.

An ab initio total energy calculation has been performed for electronic excited states in diamond and rhombohedral graphite by the full-potential linearized augmented plane wave method within the framework of the local density approximation (LDA). First, calculations for the core-excited state in diamond have been performed to show that the ab initio calculations based on the LDA describe the wavefunctions in the electronic excited states as well as in the ground state quite well. Fairly good coincidence with both experimental data and theoretical prediction has been obtained for the lattice relaxation of the core exciton state. The results of the core exciton state are compared with nitrogen-doped diamond. Next, the structural stability of rhombohedral graphite has been investigated to examine the possibility of the transition into the diamond structure through electronic excited states. While maintaining the rhombohedral symmetry, rhombohedral graphite can be spontaneously transformed to cubic diamond. Total energy in the rhombohedral structure has been calculated as a function of cell volume V, c/a ratio and bond length between layers R. The adiabatic potential energy surfaces for the transition from rhombohedral graphite to diamond in the states after core excitation have been investigated. In core exciton state, the graphite structure is more stable than the diamond. In the valence hole state after the Auger decay process, in contrast, the graphite structure is remarkably unstable compared with the diamond. The conversion into diamond from graphite can be induced spontaneously even at room temperatures due to excited holes. The induced holes decrease the stable interlayer bond length, which can lower the activation energy for buckling displacement of the hexagonal bonds, and the activation energy becomes zero by increasing the concentration of holes up to 0.1/C atom. These results predict that diamond synthesis is possible by a core excitation through the Auger decay process.

In a recent paper (Acremann et al 2000 Science290 492) the precessional trajectory of the magnetization vector was imaged with spatial resolution as a function of the time elapsed after a magnetic field pulse was applied. The most surprising observations—the reversal of the magnetic excitation upon reflection from the boundary and the spatial non-uniformities of the precessional mode—have remained unaccounted for so far. Here we present a 'back of the envelope' model of the precessional motion that is analytical, free of adjustable parameters, and that reproduces all the essential experimental features, including the behaviour of the dynamical magnetization at boundaries.

A many-body theory of paramagnetic Kondo insulators is described, focusing specifically on single-particle dynamics, scattering rates, dc transport and optical conductivities. This is achieved by development of a non-perturbative local moment approach to the symmetric periodic Anderson model within the framework of dynamical mean-field theory. Our natural focus is the strong-coupling, Kondo lattice regime, in particular the resultant 'universal' scaling behaviour in terms of the single, exponentially small low-energy scale characteristic of the problem. Dynamics/transport on all relevant (ω, T)-scales are considered, from the gapped/activated behaviour characteristic of the low-temperature insulator through to explicit connection to single-impurity physics at high ω and/or T; and for optical conductivities emphasis is given to the nature of the optical gap, the temperature scale responsible for its destruction and the consequent clear distinction between indirect and direct gap scales. Using scaling, explicit comparison is also made to experimental results for dc transport and optical conductivities of Ce₃Bi₄Pt₃, SmB₆ and YbB₁₂. Good agreement is found, even quantitatively; and a mutually consistent picture of transport and optics results.

An efficient algorithm, based on the Frank–Lobb reduction scheme, for calculating the equivalent dielectric properties of very large random resistor–capacitor (R–C) networks has been developed. It has been used to investigate the network size and composition dependence of dielectric properties and their statistical variability. The dielectric properties of 256 samples of random networks containing: 512, 2048, 8192 and 32 768 components distributed randomly in the ratios 60% R–40% C, 50% R–50% C and 40% R–60% C have been computed. It has been found that these properties exhibit the anomalous power law dependences on frequency known as the 'universal dielectric response' (UDR). Attention is drawn to the contrast between frequency ranges across which percolation determines dielectric response, where considerable variability is found amongst the samples, and those across which power laws define response where very little variability is found between samples. It is concluded that the power law UDRs are emergent properties of large random R–C networks.

The energy difference ΔE between the spin-allowed and spin-forbidden states of Tb³⁺ in crystals is studied. The environmental factor h_e representing the character of the host is redefined by using the chemical band of complex crystals. The relationship between h_e and ΔE is found to be a linear relation. The results show that the energy difference between the spin-forbidden and spin-allowed states for Tb³⁺ ions in crystals can be predicted from the environmental factor.

We investigate the properties of guide modes localized at the interfaces of photonic crystal (PC) heterostructures which are composed of two semi-infinite two-dimensional PCs consisting of non-circular air cylinders with different rotating angles embedded in a homogeneous host dielectric. Photonic band gap structures are calculated with the use of the plane-wave expansion method in combination with a supercell technique. We consider various configurations, for instance, rectangular (square) lattice–rectangular (square) air cylinders, and different rotating angles of the cylinders in the lattices on either side of the interface of a heterostructure. We find that the absolute gap width and the number of guide modes strongly depend on geometric and physical parameters of the heterostructures. It is anticipated that the guide modes in such heterostructures can be engineered by adjusting parameters.

The atomic and electronic properties of amorphous unhydrogenated (a-SiC) and hydrogenated (a-SiC:H) silicon carbides are studied using an sp³s^⋆ tight-binding force model with molecular dynamics simulations. The parameters of a repulsive pairwise potential are determined from ab initio pseudopotential calculations. Both carbides are generated from dilute vapours condensed from high temperature, with post-annealing at low temperature for a-SiC:H. A plausible model for the inter-atomic correlations and electronic states in a-SiC:H is suggested. According to this model, the formation of the amorphous network is weakly sensitive to the presence of hydrogen. Hydrogen passivates effectively only the weak bonds of threefold-coordinated atoms. Chemical ordering is very much affected by the cooling rate and the structure of the high-temperature vapour. The as-computed characteristics are in rather good agreement with the results for a-SiC and a-Si:H from ab initio calculations.

Recent experiments on Ir under pressure (Cerenius and Dubrovinsky 2000 J. Alloys Compounds306 26) show a transition to a superlattice structure comprising 14 atomic layers. This observation has implications for high-pressure applications since Ir, with its high bulk modulus and high thermal stability, is ideally suited for use as a gasket for high-temperature, high-pressure diamond anvil cell experiments. We perform first-principles total energy calculations to study the crystal phases and defect structures of Ir under pressure. We have extended the bond-orientation model (Chetty and Weinert 1997 Phys. Rev. B 56 10844) to compute all of the ∼ 2^N defect structures as a function of atomic volume. We find Ir in the FCC structure to be extremely stable for pressures up to about 60 GPa. We also calculate the stacking fault energies of Ir.

DC conductivity and dielectric relaxation measurements are exploited to study the influence of La substitution on the dielectric properties and oxygen-ion transportation in PbMoO₄ samples. The DC conductivity of La-doped samples is about 10⁻³  S cm⁻¹ around 1073 K. A dielectric loss peak with activation energy of 0.6–0.8 eV is observed in the temperature spectrum as well as in the frequency spectrum for all La-doped PbMoO₄ samples. With increasing La doping content, this peak becomes higher and shifts to higher temperature or lower frequency, and the activation energy becomes larger. It is suggested that this dielectric loss peak is associated with the short-distance diffusion of oxygen ions (or oxygen vacancies) between the 16f and 8e sites of the scheelite structure type with I4₁/a symmetry.

It is shown that quantum entanglement in condensed matter can be observed with scattering experiments if the energy resolution of the experiments enables a clear separation between the elastic scattering and the scattering from the excitations in the system. These conditions are not satisfied in recent deep inelastic neutron scattering experiments from hydrogen-containing systems that have been interpreted as showing the existence of quantum entanglement for short times in, for example, water at room temperature. It is shown that the theory put forward to explain these experiments is inconsistent with the first-moment sum rule for the Van Hove scattering function and we suggest that the theory is incorrect. The experiments were performed using the unique EVS spectrometer at ISIS and suggestions are made about how the data and their interpretation should be re-examined.

Measurements of differential current–voltage (I–V) characteristics of point contacts between Nb and the charge density wave (CDW) conductor NbSe₃ formed along the conducting chain direction are reported. Below the superconducting transition of Nb, we have clearly observed Andreev reflection of the gapless electrons of NbSe₃. Analysis of the spectra obtained indicates that when the energy of injected particles exceeds the superconducting energy gap, the superconductivity near the S–CDW interface is suppressed because of non-equilibrium effects.

Comprehensive measurements of electron spin resonance were carried out on a La_0.7Ca_0.3MnO₃ single crystal over a wide temperature range covering the ferromagnetic as well as the paramagnetic phases. Our analysis of the asymmetric lineshapes indicates that the phase segregation of good and poor conducting regions persists far above the ferromagnetic–paramagnetic phase transition temperature.

We have observed that the conductivity σ(T) for the Al₇₀Pd_22.5Re_7.5 quasicrystal, with a resistivity ratio r = R(4.2 K)/R(300 K) = 13.2, obeys the variable-range hopping law, σ(T) = σ₀/exp [(T₀/T)]^μ, in the temperature range between 64 mK and 1.6 K. The hopping exponent μ is extracted to be 0.23, close to the Mott exponent of 1/4, and T₀ is 3.5 K. This insulating behaviour is consistent with the prediction of a previously determined scaling law that bulk Al₇₀Pd_22.5Re_7.5 samples having r ≥ 12.8 will be insulating. Large positive magnetoresistances (MRs) were observed in this sample. The percentage change in the MR = ΔR(B, T)/R(0, T), as high as 185% at T = 0.11 K and B = 17 T, is the largest value ever reported in Al–Pd–Re QCs, to our knowledge. The difficulties using existing MR theories to explain the MR data for this insulating sample near the metal–insulator transition are discussed.

The thermopower of two-dimensional parabolic quantum wires and quantum contacts in magnetic field is investigated. We obtain a convenient analytic formula for the thermopower of these structures. The temperature dependence of the thermopower is studied and the influence of the magnetic field on the thermopower is examined. Oscillations in the thermopower are investigated.

A theoretical analysis of formation and symmetry transformations is presented for Wigner molecules with N = 2,..., 20 electrons confined in quantum dots at high magnetic fields. Using the unrestricted Hartree–Fock method with the multicentre Gaussian basis, we have found that Wigner molecules with N ≥ 6 abruptly change their shape and symmetry with an associated jump in the first derivative of the ground-state energy, i.e. they undergo phase transitions. In particular, the phases of the Wigner molecules obtained just after emerging from the maximum-density droplet (MDD) phase possess a different symmetry from that formed at a high magnetic field. We show that the properties of the electron–electron interaction energy demonstrate very well both the breakdown of the MDD and the quasi-classical character of the Wigner molecule in the high magnetic field. Possible mechanisms of the MDD decay are discussed.

The spin correlation (SC) function on the main chain of a theoretical model for organic ferromagnets is studied by using a density matrix renormalization group method with both periodic boundary conditions and open boundary conditions (OBCs). It is found that the chain–radical interaction, the on-site electron–electron repulsion, and the electron–phonon coupling affect the SC greatly and play important roles in the stability of the ferromagnetic state. There exist some types of 'weak' OBC, diminishing the alternation of the SC.

We study experimentally the dynamical and decay properties of the stimulated nutation echo (SNE) in a two-level spin system, the signal of which allows the observation timescale of the driven coherence relaxation to be extended. This signal appears in the transient response of the system to the second pulse at time τ₁ from its start and coinciding with the duration of the first pulse. The information about the first pulse duration is imprinted into the population difference of the inhomogeneously broadened ensemble of the two-level absorbers. The decay of the SNE signal has two contributions. One originates from the population decay during the time τ between the two pulses. Another is caused by the coherence loss during the excitation by the first pulse and the reading time of the second pulse. Experimental results on the decay properties induced by these mechanisms are presented for the first time. We investigate the dependence of these decay rates on the pulse intensity and we examine its relationship with the anomalous (non-Bloch) decay of other coherent transients in solids.

(1 − x)La(Mg_1/2Ti_1/2)O₃–xNd(Mg_1/2Ti_1/2)O₃  [0 ≤ x ≤ 1] ceramics were prepared from powders obtained by a non-conventional chemical route based on the Pechini method. La(Mg_1/2Ti_1/2)O₃ and Nd(Mg_1/2Ti_1/2)O₃ were found to form perovskite solid solutions in the whole compositional range. The structure of all the compositions was refined using the P2₁/n space group, which allows account of B-site ordering. The relative permittivity and temperature coefficient of the resonant frequency measured in the gigahertz range were analysed with respect to composition of the system. The non-monotonic dependence of the microwave quality factor on x was described qualitatively with the damped classical oscillator model and assuming 'two-mode' behaviour of the A ↔ BO₆ vibration mode in the solid solutions.

We investigate transport properties of a trilayer made of a d-wave superconductor connected to two ferromagnetic electrodes. Using Keldysh formalism we show that crossed Andreev reflection and elastic cotunnelling exist also with d-wave superconductors. Their properties are controlled by the existence of zero-energy states due to the anisotropy of the d-wave pair potential.

Diffuse scattering (DS) due to the polar nanoregions in perovskite relaxor ferroelectric Pb(Zm_1/3Nb_2/3)O₃ with 8% PbTiO₃ was investigated by cold neutron scattering with high energy resolution (8 GHz). The anisotropy of DS near the (100) Bragg reflection cannot be explained merely by the polar character of the inhomogeneous displacement field. The occurrence of 'soft' [110]-type directions suggests a close relation to the [110]-type ridges seen in x-ray scattering on Pb(Mg_1/3Nb_2/3)O₃. Transverse (quasishear) acoustic phonons with wavevectors near the half maximum of DS density in the 'soft' [110] direction have anomalously large damping but their integrated intensity is small in comparison with the intensity of DS. Since no quasielastic width of the DS was found in this experiment, it is concluded that the characteristic relaxation frequency associated with this DS is less than 1–2 GHz even at 500 K. Results are compared with predictions based on recently proposed models for DS in perovskite relaxors.

A consistent calculation of resonant inelastic (Raman) scattering amplitudes for relatively large quantum dots, which takes account of valence band mixing, the discrete character of the spectrum in intermediate and final states, and interference effects, is presented. Raman peaks in charge and spin channels are compared with multipole strengths and with the density of energy levels in final states. A qualitative comparison with the available experimental results is given.

Giant magnetoresistivity (GMR) at different temperatures as well as magnetization processes were studied in amorphous granular composites (Co₄₁Fe₃₉B₂₀)_x (SiO_n)_100−x over a wide concentration range (20 ≤ x, at. % ≤ 59). Magnetoresistivity of the composites with large dielectric content (below a percolation threshold) is enhanced on cooling down to 77 K and reaches 7% in (Co₄₁Fe₃₉B₂₀)₃₀ (SiO_n)₇₀. The observed GMR enhancement directly correlates with the rise of the relative change of the composites magnetization ([m₇₇ − m₃₀₀ ]/m₃₀₀) occurring on temperature reduction. The temperature dependence of the composite's GMR exhibits a clear maximum in the vicinity of 77 K if a wider temperature range (2–300 K) is considered. The difference between the GMR values measured at 77 and 2 K is one order of magnitude. Strong Coulomb blockade is observed in this composite at low temperature.