Table of contents

We have studied the effect of pressure on the electrical resistivity of a high-quality single crystal CeAgSb₂ which has a small net ferromagnetic moment of 0.4μ_B/Ce. The magnetic ordering temperature T_ord = 9.7 K decreases with increasing pressure p and disappears at a critical pressure p_c ≃ 3.3 GPa. The residual resistivity, which is close to zero up to 3 GPa, increases steeply above 3 GPa, reaching 55μΩ cm at p_c. A huge residual resistivity is found to appear when the magnetic order disappears.

The magnetocaloric effect in high spin Mn₁₂ 2Cl-benzoate clusters was studied. We observed a high entropy change around the blocking temperature of the magnetization. Theoretical simulations indicate the shift of the entropy change with the sweeping rate of the magnetic field.

We reduce the problem of the integer quantum Hall transition (QHT) to a random rotation of an N-dimensional vector by using an su(N) algebra, where only N specially selected generators of the algebra are nonzero. The group-theoretical structure revealed in this way allows us to obtain a new series of conservation laws for the equation describing the electron density evolution in the lowest Landau level. The resulting formalism is particularly well suited to numerical simulations, allowing us to obtain the critical exponent ν numerically in a very simple way. We also suggest that if the number of nonzero generators is much less than N, the same model, in a certain intermediate time interval, describes percolating properties of a random incompressible steady two-dimensional flow. In other words, the QHT in a very smooth random potential inherits certain properties of percolation.

Randomly distributed small holes of various subwavelength sizes were fabricated in a thin gold film. We study the optical near-field transmission of the film. In the wavelength spectrum from 350 to 650 nm, a number of strongly enhanced transmission peaks were observed. These transmission peaks can only be observed in the near field. We attribute the new phenomenon to the surface plasmon coupling inside the holes and between the surfaces on the two sides of the thin film.

The phenomenon of electron tunnelling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunnelling (SDT) in magnetic tunnel junctions (MTJs) has recently aroused enormous interest and has developed in a vigorous field of research. The large tunnelling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible applications in non-volatile random-access memories and next-generation magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. In this review article we present an overview of this field of research. We discuss various factors that control the spin polarization and magnetoresistance in MTJs. Starting from early experiments on SDT and their interpretation, we consider thereafter recent experiments and models which highlight the role of the electronic structure of the ferromagnets, the insulating layer, and the ferromagnet/insulator interfaces. We also discuss the role of disorder in the barrier and in the ferromagnetic electrodes and their influence on TMR.

The object of this paper is to review the electronic conductance in double quantum well systems. These are quantum well structures in which electrons are confined in the z direction by large band gap material barrier layers, yet form a free two-dimensional Fermi gas within the sandwiched low band gap material layers in the x–y plane. Aspects related to the conductance in addition to the research progress made since the inception of such systems are included. While the review focuses on the tunnelling conductance properties of double quantum well devices, the longitudinal conductance is also discussed. Double quantum well systems are a more recent generation of structures whose precursors are the well known double-barrier resonant tunnelling systems. Thus, they have electronic signatures such as negative differential resistance, in addition to resonant tunnelling, whose behaviours depend on the wavefunction coupling between the quantum wells. As such, the barrier which separates the quantum wells can be tailored in order to provide better control of the device's electronic properties over their single well ancestors.

We present a review of field-emission properties of carbon protrusions standing on a metallic substrate. The transfer-matrix technique used for the scattering calculations takes account of three-dimensional aspects of the potential energy. The structures considered consist of a single carbon atom, half of a C₆₀ molecule on a flat metallic substrate or on top of a metallic cylinder, open and closed (5, 5) nanotubes and the (5, 5)@(10, 10)@(15, 15) multi-wall structure. The results indicate that enhanced emission is obtained from the protrusions having the largest aspect ratio, (quasi-)localized states or allowing for the highest field penetration (due to an open rather than closed configuration, a reduced polarizability or because of adsorbed/bonded species). From the comparison between the total-energy distributions, it is found that the half C₆₀ molecule has a (quasi-)localized state, which gives rise to a resonance in the emission (also when present at the apex of the closed (5, 5) nanotube). The results show that the field enhancement factor, as derived from a Fowler–Nordheim analysis of our data, is only an indicator for the quality of the emitter and does not justify all differences in the rates of emission.

The benefit of using in situ ultrahigh vacuum ferromagnetic resonance (FMR) to study exchange coupled magnetic films is demonstrated. Structurally well defined trilayer systems consisting of two ultrathin magnetic films (Ni or Co) separated by a non-magnetic Cu spacer layer are examined. The stepwise construction of the sample reveals how the single uniform resonance mode of the bottom magnetic layer is influenced upon depositing the second layer on top. The coupling leads to an acoustical and an optical mode. The positions, intensities and linewidths of these modes are compared to theory which uses a continuum approach in the framework of the Landau–Lifshitz equation of motion. We discuss how to determine the coupling strength in absolute units which for our trilayers ranges from a few to about 100μeV/atom and then systematically investigate the parameters that influence this coupling. The effect of the spacer thickness and its roughness is studied in detail with the help of scanning tunnelling microscopy to obtain a realistic and quantitative picture of the spacer morphology. In a next step we investigate the influence of a non-magnetic overlayer usually used as a protection layer. For small coupling strengths such a capping layer can change the sign of the coupling. The temperature dependence of the coupling in the framework of existing theoretical models is discussed. Moreover, the resonance method is used to address via the linewidth the dynamical response of the magnetic layers upon being coupled. It is demonstrated that 2D spin fluctuations originating from one layer influence the damping properties within the other. To support the results obtained from the FMR, the measurements are complemented by magneto-optic Kerr effect experiments on the same systems.

We employ the method of phase-modulated KrF excimer pulsed laser interference crystallization (LIC) to fabricate nanocrystalline silicon with a two-dimensional (2D) patterned distribution within an ultrathin a-Si:H single layer. A local phase transition occurs in the ultrathin a-Si:H film upon LIC with the appropriate energy density. The results from atomic force microscopy, Raman scattering spectroscopy, planar and cross-sectional transmission electron microscopy and scanning electron microscopy demonstrate that Si nanocrystallites are formed within the initial a-Si:H single layer, selectively located in disc-shaped regions with diameters of 250 nm and patterned with the same 2D periodicity of 2.0 μm as the phase-shifting grating.

The effect of N-doping on the microstructure and coercivity of the 'free' Fe layer in Fe/insulator/Fe trilayers has been examined. It was found that N-doping leads to a magnetic softening of the Fe layer and a corresponding reduction in the grain size. Hard/soft spin-valve trilayers, showing good independent layer reversal, were obtained using N-doped and undoped Fe layers. Ferromagnetic interlayer coupling was found in these trilayers that could be well described by a Néel coupling mechanism. Nonuniform reversal of the harder Fe layer, once incorporated in the trilayer, was also observed and could be reproduced using a simple model in which local variations in the interlayer coupling energy are considered. Such variations are likely to be caused by structural inhomogeneity in the films. N-doping is potentially important as a method for tailoring the coercivity of the 'free' layer in spin-valves comprising high-polarization magnetic materials.

Epitaxial Fe_0.82Ni_0.18/V(001) superlattices grown by dc magnetron sputtering on MgO(001) substrate were investigated structurally by means of x-ray diffraction, Rutherford backscattering spectrometry and transmission electron microscopy. The results show that samples grown around 150^°C have high interface sharpness with low interdiffusion and good crystallographic quality.

The research into new materials with good thermoelectric properties has revealed new compounds consisting of metallic elements (Bando Y, Suemitsu T, Takagi K, Tokushima H, Echizen Y, Katoh K, Umeo K, Maeda Y and Takabatake T 2000 J. Alloys Compounds313 1–6, Ghelani N, Loo S, Chung D, Sportouch S, Nardi S, Kanatzidis M, Hogan T and Nolas G 2000 Mater. Res. Soc.626 Z8.6.1). The half-Heusler compound ZrNiSn, in particular, shows promising thermoelectric properties and has been studied by many scientists during recent years (Uher C, Hu S, Yang J, Meisner G P and Morelli D T 1997 Proc. ICT'97: 16th Int. Conf. on Thermoelectrics pp 485–8, Romaka L P, Stadnyk Yu V, Goryn A M, Gorelenko Yu K and Skolozdra R V 1997 Proc. ICT'97: 16th Int. Conf. on Thermoelectrics pp 516–19, Hohl H, Ramirez A P, Goldmann C, Ernst G, Wölfing B and Bucher E 1998 J. Phys.: Condens. Matter11 1697–709, Oestreich J, Käfer W, Richardt F, Probst U and Bucher E 1999 Proc. 5th European Workshop on Thermoelectrics pp 192–5). In an effort to find new thermoelectric materials, the half-Heusler compounds of the groups ScM^VIIISb and YM^VIIISb (M^VIII = Ni, Pd, Pt) were synthesized by arc melting and the thermoelectric properties were examined by standard characterization methods. Doping experiments showed that it is possible to change the electrical properties of the compounds while retaining the half-Heusler structure. Within the two groups, YPtSb showed the best thermoelectrical properties. At a temperature of 400 K the electrical conductivity of YPtSb is 748Ω⁻¹ cm⁻¹ and the Seebeck coefficient is 116.3μV K⁻¹. The thermal conductivity at 400 K extrapolated using the Wiedemann–Franz law is 2.87 W K⁻¹ m⁻¹. This leads to a dimensionless figure of merit of 0.14.

The structural and magnetic properties of ZnFe₂O₄ nanoparticles embedded in a nonmagnetic ZnO matrix are presented. X-ray diffractograms and transmission electron microscopy images showed that the resulting samples are composed of crystalline ferrite nanoparticles with average crystallite size ⟨D⟩ = 23.4 ± 0.9 nm, uniformly dispersed within the ZnO matrix. Magnetization data indicated a superparamagnetic-like behaviour from room temperature down to T_M ∼ 20 K, where a transition to a frozen state is observed. The M(H) curves displayed nearly zero coercive field down to T_M, where a sharp increase in the H_C value is observed. The measured saturation magnetization M_S values at 200 and 2 K were M_S = 0.028(3) and 0.134(7) μ_B/f.u. ZnFe₂O₄, respectively, showing the existence of small amounts of non-compensated atomic moments. Mössbauer measurements at low temperatures confirmed the transition to a magnetically ordered state for T < 25 K, where two magnetically split sextets develop. Whereas these two sextets show strong overlap due to the similar hyperfine fields, in-field Mössbauer spectra clearly showed two different Fe³⁺ sites, demonstrating that the sample is ferrimagnetically ordered. The two spinel sites are found to behave differently under an external field of 12 T: whereas the moments located at A sites show a perfect alignment with the external field, spins at B sites are canted by an angle α_B = 49^° ± 2^°. We discuss the significance of this particle structure for the observed magnetic behaviour.

The magnetic properties of the compounds Pr_1−xDy_xMn₂Ge₂ (0 ≤ x ≤ 1) and Ce_1−xDy_xMn₂Ge₂ (0 ≤ x ≤ 1) were investigated by means of temperature-dependent DC magnetization measurements in low external magnetic fields. The substitution of Dy for Pr or Ce leads to a linear decrease in the lattice parameters. Below about 330 K, the interlayer magnetic coupling in the Mn sublattice is ferromagnetic for Pr-rich and Ce-rich compounds and antiferromagnetic for Dy-rich compounds. At low temperatures, the Dy sublattice also orders and reconfigures the ordering in the Mn sublattice. This leads to ferrimagnetic ordering for Dy-rich compounds.

Manganese tricyanomethanide, Mn[C(CN)₃]₂, crystallizes in an orthorhombic lattice consisting of two interpenetrating three-dimensional rutile-like networks. In each network, the tridentate C(CN)₃⁻ anion gives rise to superexchange interactions between the Mn²⁺ ions (S = 5/2) that can be mapped onto the 'row model' for partially frustrated triangular magnets. We present heat capacity measurements that reveal a phase transition at T_N = 1.18 K, indicative of magnetic ordering. From magnetic-field dependent heat capacity data a saturation field H_sat = 42 kOe is estimated. The zero-field magnetically ordered structure was solved from neutron powder diffraction data taken between 0.04 and 1.2 K. It consists of an incommensurate spiral with a temperature independent propagation vector Q = [2Q00] = [±0.62200], where different signs relate to the two different networks. This corresponds to [±0.311 ± 0.3110] in a quasi-hexagonal representation. The ordered moment μ = 3.3μ_B is about two-thirds of the full Mn²⁺ moment. From the values of T_N and Q, the exchange parameters J/k_B = 0.15 K and J'/J = 0.749 are estimated. The magnetic-field dependence of the intensity of the (2Q00) Bragg reflection, measured for external fields H || Q, indicates the presence of three different magnetic phases. We associate them with the incommensurate spiral (H < 13.5 kOe), an intermediate 'up–up–down' phase (13.5 kOe < H < 16 kOe) and the '2–1' spin-flop-like magnetic structure (H > 16 kOe) proposed for related compounds. For increasing fields, Q continuously approaches the value 1/3, corresponding to the commensurate magnetic structure of the fully frustrated triangular lattice. This value is reached at H^* = 19 kOe. At this point, the field dependence reverses and Q adopts a value of 0.327 at 26 kOe, the highest field applied in the experiment. Except for H^*, the magnetic ordering is incommensurate in all three field dependent magnetic phases of Mn[C(CN)₃]₂.

The spin-flip cross-sections of two samples each of Fe₉₀Zr₁₀ and Fe₉₂Zr₈ produced in two different laboratories were measured using the polarized neutron spectrometers IN20 and D7 at the Institut Laue-Langevin, Grenoble. All the samples have non-zero spin-flip cross-sections, with peaks at Q ∼ 3.1Å⁻¹, coinciding with the first maximum in the structure factor S(Q), on a diffuse Q-dependent 'background'. The features change little on cooling to 2 K and persist above the Curie temperature. The samples are therefore all non-collinear ferromagnets, with significant ferromagnetic correlations between the non-collinear components of the moments. The compositions of the samples were shown to be correct to ≤ 0.1at.% by magnetic susceptibility measurements. The 'background' was found to be significantly larger in the spin-flip cross-sections of one set of the samples. Careful examination of neutron diffraction data measured using the LAD diffractometer, Rutherford Appleton Laboratory, showed that these samples had barely detectable impurities of α-Fe. The critical re-evaluation of previous data is therefore necessary as experimental results are evidently influenced by very low levels of crystalline impurities that are difficult to detect.

We consider the interaction of the acoustic phonons localized at the free surface (surface phonons) in semi-infinite metallic superlattices with electrons and the resulting surface phonon attenuation rates. We assume a semi-infinite Kronig–Penney model for the electrons, and the acoustic surface phonons in the superlattices are described through the continuum elasticity theory. The deformation-potential coupling is used to calculate the attenuation rates of the surface phonons in the superlattice. The results are compared with the experimental damping rates of the localized vibrational modes at the surface of Al/Ag superlattices.

We present the results of a first-principles study on BaF₂ in its stable (cubic) and high-pressure phases. A linear combination of atomic orbitals approach in the framework of density functional theory is employed for total energy calculations in cubic, orthorhombic and hexagonal phases of BaF₂. A fitting of the energy surface to the equation of state yields the lattice constant and the bulk modulus of these phases at zero pressure which are in good agreement with the corresponding experimental values. Analysis of band structure determines the high-pressure phases to be direct-gap materials and no metallization of BaF₂ is predicted to occur for pressures up to 50 GPa. Furthermore, several peaks observed in the spectroscopic experiments have been identified with interband transitions in the cubic BaF₂. The calculated mean value of the refractive index is found to increase in going from the cubic to orthorhombic to hexagonal phases of BaF₂.

SrAl₁₂O₁₉:Pr phosphors have been investigated under ultraviolet/vacuum ultraviolet (5–20 eV) synchrotron radiation at room and near liquid He temperatures. The excitation of ¹S ₀ →¹I ₆ and ³P ₀ →³H ₄ emission of Pr³⁺ in SrAl₁₂O₁₉ occurs in a 1 eV wide region originating from the interconfiguration 4f² → 4f5d transitions with threshold (onset of the transitions) at 6.0 eV at room temperature and at 6.15 eV at 14 K. A comparison of the excitation spectra of Pr³⁺ and known data for Ce³⁺ in SrAl₁₂O₁₉ reveals a shift of 1.52 eV between 5d states of the ions. The next threshold near 7.5 eV corresponds to the transitions from valence to conduction band. The luminescence from the ³P ₀ level is excited efficiently at low temperature while ¹S ₀ luminescence is absent under band-to-band excitation. To explain this, a mechanism involving valence hole trapping and energy transfer from excitonic states to Pr³⁺ is proposed. At excitation near the maximum of 4f² → 4f5d transitions (6.4 eV), the ¹S ₀ →¹I ₆ luminescence exhibits exponential decay with constant τ = 330 ± 10 ns at 300 K and τ ≈ 400 ns at 14 K. At pre-threshold excitation (E < 6.0 eV) a fast component τ ≈ 10 ns originating from excitonic emission prevails.

A tight-binding model is developed to describe the electron–phonon coupling in atomic wires under an applied voltage and to model their inelastic current–voltage spectroscopy. Particular longitudinal phonons are found to have greatly enhanced coupling to the electronic states of the system. This leads to a large drop in differential conductance at threshold energies associated with these phonons. It is found that with increasing tension these energies decrease, while the size of the conductance drops increases, in agreement with experiment.