Exact transport properties of degenerate,weakly interacting,and spin-polarized fermions

Exact results for the transport coefficients of a highly degenerate spin-polarized Fermi system are presented. The case of weakly interacting particles, such as dilute³He in⁴He or D ↓, is treated. The results are compared to the variational treatment and corrections as large as 8% are found. Detailed behavior of the polarization of an ideal Fermi gas as a function of temperature and magnetic field is discussed as a model for dilute³He in⁴He. Limits of applicability of the formulas are given. We find that at appropriate fields and temperatures, the two spin species may be considered to be a mixture of a degenerate and a classical gas.     

Extracting Electron Densities in N-Type GaAs From Raman Spectra: Theory

In this paper, we present the theory for calculating Raman line shapes as functions of the Fermi energy and finite temperatures in zinc blende, n-type GaAs for donor densities between 10¹⁶ cm⁻³ and 10¹⁹ cm⁻³. Compared to other theories, this theory is unique in two respects: 1) the many-body effects are treated self-consistently and 2) the theory is valid at room temperature for arbitrary values of the ratio R = (Q²/α), where Q is the magnitude of the normalized wave vector and α is the normalized frequency used in the Raman measurements. These calculations solve the charge neutrality equation self-consistently for a two-band model of GaAs at 300 K that includes the effects of high carrier concentrations and dopant densities on the perturbed densities of states used to calculate the Fermi energy as a function of temperature. The results are then applied to obtain the carrier concentrations from Fermi energies in the context of line shapes in Raman spectra due to the coupling between longitudinal optical phonons and plasmons. Raman measurements have been proposed as a non-destructive method for wafer acceptance tests of carrier density in semiconductor epilayers. The interpretation of Raman spectra to determine the majority electron density in n-type semiconductors requires an interdisciplinary effort involving experiments, theory, and computer-based simulations and visualizations of the theoretical calculations.     

Fermi Arcs and Pseudogap in Cuprate Superconductors

Earlier, we have proposed [arXiv:1110.0227] the model of HTSC electronic structure modification under doping. In this model, doping by localized charges plays the key role, being responsible for local closing of the gap ??_ct between excitonic-like 3d¹⁰L^? state of cation and electronic 3d⁹L state of anion and formation (at a certain dopant concentration) of the percolation cluster with the Fermi surface located in the cation-anion band of peculiar nature. This electronic structure is favorable for the formation of diatomic negative-U centers (NUCs) and realization of an unusual mechanism of electron?Celectron interaction. Here, the nature of normal state of cuprates as well as the mechanism of pseudogap and Fermi arcs formation are considered in the framework of this model by example YBCO.     

Deviations from Matthiessen's rule and longitudinal magnetoresistance in copper

Deviations from Matthiessen's rule (MR) due to phonons inCuAu andCuNi alloys were measured as a function of temperature between 4°K and 300°C. They can amount to 40% of the residual resistivity of the doped samples. A second group of measurements concerns deviations from MR that occur inCuNi during a neutron irradiation at 4.6°K. It turned out that there are two essential reasons for these deviations. The first one is the influence of the defects on the temperature-dependent part of the electrical resistivity. It plays an important role mainly at low temperatures. We found experimentally and theoretically aT ⁵ dependence for this additional resistivity at low temperatures. The second reason is the existence of anisotropies in the Fermi surface of Cu and in the scattering potential of the defects. These deviations can be described, as suggested by J. M. Ziman, by a two-band model for the Fermi surface. Furthermore, we could show that the enhanced resistivity increase in theCuNi alloys during neutron irradiation at 4.6°K, which was generally interpreted as an enhanced defect production, is essentially caused by deviations from MR. Measurements of the longitudinal magnetoresistance of the doped copper at 4.2°K were very useful for discriminating between the different causes of deviations from MR.     

Dimensional-Crossover of 3He Gas Formed in One-Dimensional Nanometer Tunnel

No Heading We have measured the heat capacity of ³He adsorbed on one-dimensional (1D) nanopores of FSM-16 with different pore-diameters. Dilute ³He atoms on more than one layers of ⁴ He preplated nanotunnels with 22 Å in diameter show a Schottky type peak around 0.25 K, which is similar to our recent report for 28 Å nanotunnels. This behavior of heat capacity is explained as the crossover from 1D to 2D motion of Fermi gas in a confined geometry. The pore-size effect on the temperature and magnitude of the peak is quantitatively reproduced by a simple calculation for non-interacting Fermi gas in the adsorption potential from the substrate.PACS numbers: 67.55.Cx, 67.70.+n, 68.65.–k     

Requirements imposed on the electrical properties of the absorber layer in CdTe-based solar cells

The requirements for minimizing the electric losses in the CdTe layer in CdS/CdTe thin-film solar cells are discussed. It is shown that for achieving the total absorption of the radiation and to avoid electrical losses, the separation between Fermi level and the valance band should not be more than 0.3 eV. In order to fix the Fermi level near the valence band, it is necessary to dope with acceptor impurities which can introduce shallow level in the band gap with a concentration considerably exceeding the concentration of native impurities and defects (10¹⁵–10¹⁶ cm⁻³). Taking into account the fact that the location of the Fermi level in the bandgap of a semiconductor depends on the degree of compensation, the energy of ionization of the impurity should not be greater than 0.05–0.15 eV.

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Calorimetric Study of the Desorption and Phases of 3He And 4He Adsorbed on Two Layers of H2-plated Graphite Above 0.2K

We have conducted new heat capacity studies of films of ³ He and ⁴ He adsorbed on 2.15 layers of hydrogen plating exfoliated graphite for temperatures T > 0.2K and coverages (n) over the entire first layer and beginning of the second layer. Low-density ³ He films show two-dimensional Fermi fluid behavior with no liquid-vapor condensation below 0.9K. We have mapped the n-T melting tine and estimated monolayer completion density using heat capacity isotherms. By fitting ³ He films' desorption specific heats to a theoretical model, we have calculated the ³ He binding energy (21.3K to 21. 7K) and compare it to a previous measurement for ⁴ He films on the same substrate (25.1K to 25.5K).     

Acoustics of a “Dirty” Fermi Liquid: 3He in Aerogel

An acoustic cavity containing ³He in 98% porous silica aerogel was used to investigate the effects of impurity scattering in a Fermi liquid. The pressure and temperature dependence of the sound attenuation in the normal Fermi liquid was extracted from the cavity response. The attenuation of sound displays behavior very different from the bulk owing to strong elastic scattering of quasi-particles by the silica strands. Using a visco-elastic model of the Fermi liquid, we find a mean free path restricted to 340 nm. Information on the sound velocity is inferred from the pressure dependence of the oscillation period of the cavity response. The data can be accounted for by a Biot model of the ³He liquid in the porous aerogel.     

Phonon-limited mean free path in the Sondheimer oscillation of aluminum

The phonon-limited part of the electron mean free path l _Hp ,in aluminum has been investigated by the Sondheimer size effect for a magnetic field along the three principal crystallographic orientations in the temperature range 1.8–12 K. It is found that l _Hp decreases with temperature approximately as T ^–3 and varies from 1.4 × 10^–3 to 1.3 × 10^–2 cm, with the orbits responsible for the oscillations at 20 K. The results are in agreement with a calculation based on the 4-OPW Fermi surface model and the anisotropic electron-phonon scattering time given by Leung et al.Work supported in part by a Grant-in Aid for Scientific Research from the Ministry of Education.     

Superconductivity,normal state electrical and structural properties of Nb/Al2O3 cermet films

Nb/Al₂O₃ cermet films were obtained by electron-gun evaporation from two independent sources. Films containing 71 wt.% Nb (particle size about 20 Å) have an activated type of conduction mechanism due to the hopping of the charge carriers between localized states. The activation energy decreases continously from 8.8 x 10^?3 eV (300°K) to 1 x 10^?6 eV (1.9°K), which indicates that the localized states lie close to the Fermi level of the system. These states can be associated with small Nb particles (below the resolution power of the electron microscope—10 Å) in the alumina matrix. Films containing more than 85wt.%Nb (mean particle size 32 Å) have a metallic-type conduction mechanism and show superconductive properties. The transition temperature is reduced to 4.5—5.2°K compared with the Nb bulk value of 9.2°K; this is presumably due to a decrease of the electronic interaction in the surface layer of the Nb particles.     

The Wall Potential in the Pores of Vycor Glass Experienced by 3He Quasiparticles in a Dilute 3He-4He Solution

We have studied the Fermi fluid, ³He in ³He-⁴He solution, confined within the narrow channels of Vycor glass. Continuous wave NMR signals were observed in two almost identical coils, one containing bulk solution and the other the solution in Vycor glass. Three different solutions (0.1, 1.6, and 6%) have been studied over the temperature range 3mK to above 1K. The measurements relate to two exclusion effects of ³He from Vycor glass. One concerns potential energy, and we demonstrate the partial exclusion of ³He independent of concentration as the temperature is lowered from high temperatures down to around 200mK, thus giving an experimental measure of the van der Waals forces between the wall and the helium. The second effect is the additional quantum exclusion which arises from the influence of quantum wire confinement on the kinetic energy, as the temperature is progressively lowered below 50mK in the very dilute (0.1%) solution.     

A model for the temperature dependence of the electronic mean free path in the radiofrequency size effect

A model is proposed which takes into account the effect of the finite skin depth in a determination of the phonon-limited electronic mean free path (EMFP) by means of the radiofrequency-size-effect amplitude. The model results in a temperature dependence of the inverse EMFP which obeys neither aT ⁿ nor anaT ³+bT⁵ law, in general. Numerical examples show the pureT ³ behavior usually expected is reasonable for small Fermi radii of curvature, but that more complicated behavior can be expected for Fermi surfaces having nearly free electron curvature. Existing data from the lens of Cd, obeying an apparentT ⁵ law, are discussed in terms of the present model.     

The de Haas-van Alphen effect in Sb(Sn) and Sb(Te) alloys

Measurements have been made of the de Haas-van Alphen effect in Sb(Sn) alloys containing up to 0.58 at % Sn and Sb(Te) alloys with up to 0.26 at % Te. The maximum electron period in the bisectrix-trigonal plane increased from 14.66 × 10^–7 G^–1 in pure Sb to 68.0 × 10^–7 G^–1 in the most concentrated Sn-doped sample, whereas the maximum hole period decreased from 16.33 × 10^–7 to 7.4 × 10^–7 G^–1 . The Te doping had the opposite effect—increasing the number of electrons and decreasing the number of holes. The results of measurements of effective mass and Fermi surface area are found to be consistent with a rigid-band model of these dilute alloys. Both electron and hole bands are strongly nonparabolic and a two-band model is used to estimate that the gap below the electron pocket at L is 110 ± 25 meV. The Fermi levels of electrons and holes in Sb are estimated to be 150 ± 10 and 180 ± 40 meV, respectively. It is predicted that the electron pockets will be completely emptied for an alloy with 0.78 ± 0.07 at % Sn. Pseudopotential calculations suggest that the rigid-band model is a reasonable approximation in these dilute alloys.     

Fermi surface changes and electron scattering anisotropy in the potassium-rubidium alloy system

The de Haas-van Alphen effect has been used to study the changes in Fermi surface (FS) dimensions and the electron scattering in dilute K(Rb) and Rb(K) alloys. Small discrepancies, around 10%, have been found between the observed FS cross-section changes and those calculated from a linear interpolation of the lattice constant between the two host values, the cross-section changing more rapidly than expected in each case: these discrepancies appear to reflect rather accurately a real nonlinearity in the lattice constant change with solute concentration. There are also measurable changes in the FS anisotropy, which are described well by a combination ofs andp phase shifts in K(Rb) and bys andd in Rb(K). The electron scattering has a significant forward-scattering component, the ratiox _ρ/x ₀ between the resistivity temperature and the average Dingle temperature being 0.63 and 0.59 for K(Rb) and Rb(K), respectively. The scattering rate anisotropy in Rb(K) is about 7%, with the maximum rate at 〈110〉: this can be fitted rather well by a combination ofs andp phase shifts.     

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1−x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb_1–x Sn _x Te single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (H _uql) for the metallic state of Pb_1–x Sn _x Te(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>H _uql the equationE _F = const is valid instead of the equationn = const which is usual for degenerate semiconductors (E _F is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb_1–x Sn _x Te energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (10⁵ sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.     

Structure Properties of the 3He-4He mixture at T = 0 K

The spatial structure properties of³ He-⁴ He mixtures at T = 0 K are investigated using the hypernetted-chain formalism. The variational wave function used to describe the ground-state of the mixture is a simple generalization of the trial wave functions for pure phases and contains two- and three-body correlations. The elementary diagrams are taken into account by means of an extension of the scaling approximation to the mixtures. The two-body distribution (g(^,)(r)) and the structure functions (S(^,)(k)) together with the different spin-spin distribution functions of the ³He component in the mixture are analyzed for several concentrations of³ He. Two sum-rules, for the direct and the exchange part of the g ^{(3, 3)}(r), are used to ascertain the importance of the full treatment of the Fermi statistics in the calculation. The statistical correlations are found responsible for the main differences between the several components of the distribution function. Due to its low concentration, ³He behaves as a quasi-free Fermi gas, as far as the statistical correlations are concerned, although it is strongly correlated with the ⁴He atoms through the interatomic potential.     

The separation effect of radiation-induced defects in nickel

Pure nickel, the model material for austenitic steels used in reactors, with an electrical resistivity ratio of ρ_{300 K}/ρ_{4.2 K} ～ 300 has been investigated under electron and neutron irradiation at T _IRR ～ 320–340 K. Three of its states have been subjected to irradiation: a recrystallized state at 873 K, a deformed one to 90%, and an annealed one at 450 K after deformation to remove deformation-induced vacancies. It is has been experimentally shown that neutron and electron irradiation of the deformed nickel results in the separation of radiation-induced defects. This separation occurs because a significant portion of the radiation-induced interstitial atoms is captured by dislocation sinks and does not participate in recombination with vacancies. As a result, the concentration of accumulated vacancies in the deformed nickel can exceed their concentration in the annealed nickel and be almost twice as high. At higher doses of neutron irradiation, above 10¹⁸ cm^?2, separation does not occur, since vacancy sinks in the form of clusters are more powerful than dislocation sinks.     

Electrical Properties of Amorphous S<Subscript>0.60</Subscript>Ge<Subscript>0.40</Subscript>:H<Subscript>x</Subscript> Films

Amorphous Si_0.60Ge_0.40:H_x films containing 1.7, 3.9, 7.1, 12.1, and 17.3 at % H are prepared by magnetron sputtering at different hydrogen partial pressures, and their electrical conductivity is measured from 80 to 350 K. The conductivity of the films exhibits Arrhenius behavior in the range 250–350 K and satisfies the relation log(σT ^1/2) ∼ T^−1/4 between 80 and 250 K. The conductivity data are used to evaluate the electron localization radius, hop distance, and activation energy of hopping conduction at 80 K; the electron mobility at the Fermi level and in the conduction band; and the 300-K activation energy of conduction.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 7, 2005, pp. 787–791.Original Russian Text Copyright © 2005 by Najafov, Isakov.     

Two‐Dimensional Mott Insulators in SrVO3 Ultrathin Films

Strongly correlated oxides that undergo a metal‐insulator transition (MIT) are a subject of great current interest for their potential application to future electronics as switches and sensors. Recent advances in thin film technology have opened up new avenues to tailor MIT for novel devices beyond conventional CMOS scaling. Here, dimensional‐crossover‐driven MITs are demonstrated in high‐quality epitaxial SrVO₃ (SVO) thin films grown by a pulsed electron‐beam deposition technique. Thick SVO films (∼25 nm) exhibit metallic behavior with the electrical resistivity following the T² law corresponding to a Fermi liquid system. A temperature driven MIT is induced in SVO ultrathin films with thicknesses below 6.5 nm. The transition temperature T_MIT is at 50 K for the 6.5 nm film, 120 K for the 5.7 nm film and 205 K for the 3 nm film. The emergence of the observed MIT can be attributed to the dimensional crossover from a three‐dimensional metal to a two‐dimensional Mott insulator, as the resulting reduction in the effective bandwidth W opens a band gap at the Fermi level. The magneto‐transport study of the SVO ultrathin films also confirm the observed MIT is due to the electron‐electron interactions other than disorder‐induced localization.