Energy spectrum of holes in Sb<Subscript>2</Subscript>Te<Subscript>2.9</Subscript>Se<Subscript>0.1</Subscript> solid solution according to the data on the transfer phenomena

The temperature dependence of the Hall coefficient of a single crystal of the p-Sb₂Te_2.9Se_0.1 solid solution grown by the Czochralski technique is studied in the temperature range 77–450 K. The data on the Hall coefficient of the p-Sb₂Te_2.9Se_0.1 are analyzed in combination with the data on the Seebeck and Nernst–Ettingshausen effects and the electrical conductivity with allowance for interband scattering. From an analysis of the temperature dependences of the four kinetic coefficients, it follows that, at T < 200 K, the experimental data are qualitatively and quantitatively described in terms of the one-band model. At higher temperatures, a complex structure of the valence band and the participation of the second-kind additional carriers (heavy holes) in the kinetic phenomena should be taken into account. It is shown that the calculations of the temperature dependences of the Seebeck and Hall coefficients performed in the two-band model agree with the experimental data with inclusion of the interband scattering when using the following parameters: effective masses of the density of states of light holes m_d1^*≈ 0.5m₀ (m₀ is the free electron mass) and heavy holes m_d2^*≈ 1.4m₀, the energy gap between the main and the additional extremes of the valence band ΔE_v ≈ 0.14 eV that is weakly dependent on temperature.     

Electronic structure of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>: Ab initio simulations and comparison with experiment

Al₂O₃ films 150 Å thick are deposited on silicon by the ALD technique, and their x-ray (XPS) and ultraviolet (UPS) photoelectron spectra of the valence band are investigated. The electronic band structure of corundum (α-Al₂O₃) is calculated by the ab initio density functional method and compared with experimental results. The α-Al₂O₃ valence band consists of two subbands separated with an ionic gap. The lower band is mainly formed by oxygen 2s states. The upper band is formed by oxygen 2p states with a contribution of aluminum 3s and 3p states. A strong anisotropy of the effective mass is observed for holes: m _h⊥ ^* ≈ 6.3m ₀ and m _h‖ ^* ≈ 0.36m ₀. The effective electron mass is independent of the direction m _e‖ ^* ≈ m _e⊥ ^* ≈ 0.4m ₀.     

The atomic and electron structure of ZrO2

The atomic structure of amorphous and crystalline zirconium dioxide (ZrO₂) films is studied using X-ray diffraction and extended X-ray absorption fine structure techniques. The electron structure of ZrO₂ is experimentally determined using X-ray and UV photoelectron spectroscopy, and the electron energy band structure is theoretically calculated using electron density functional method. According to these data, the valence band of ZrO₂ consists of three subbands separated by an ionic gap. The upper subband is formed by the O2p states and Zr4d states; the medium subband is formed by the O2s states; and the narrow lower subband is formed predominantly by the Zr4p states. The bandgap width in amorphous ZrO₂, as determined using the electron energy loss spectroscopy data, amounts to 4.7 eV. The electron band structure calculations performed for a cubic ZrO₂ phase point to the existence of both light (0.3m ₀) and heavy (3.5m ₀) holes, where m ₀ is the free electron mass. The effective masses of band electrons in ZrO₂ fall within (0.6–2.0)m ₀.     

Electron and hole injection in metal-oxide-nitride-oxide-silicon structures

The kinetics of electron and hole accumulation in metal-oxide-nitride-oxide-semiconductor structures is studied. Experimental data are compared with a theoretical model that takes into account tunnel injection, electron and hole capture by traps in amorphous silicon nitride SiN_x, and trap ionization. Agreement between experimental and calculated data is obtained for the bandgap width E _g = 8.0 eV of amorphous SiO₂, which corresponds to the barrier for holes Φ_h = 3.8 eV at the Si/SiO₂ interface. The tunneling effective masses for holes in SiO₂ and SiN_x are estimated at m _h ^* ≈ (0.4–0.5)m ₀. The parameters of electron and hole traps in SiN_x are determined within the phonon-coupled trap model: the optical energy W _opt = 2.6 eV and the thermal energy W _T = 1.3 eV.     

On the susceptibility exponent of random anisotropy magnets

The temperature dependence of the non-linear susceptibility ≈₂(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature T_f within the framework of the correlated molecular field theory. For the effective susceptibility exponent λ_s(T) = (T?T_f)≈₂d^-1_≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data.     

Fermi surface studies in the low- and high-T c phase of the organic superconductor β-(BEDT-TTF)2I3

We report de Haas-van Alphen (dHvA) measurements of the high-T _c modification (T _c = 7 - 8 K) and Shubnikov-de Haas results of the low-T _c modification (Tc ≈ 1:4 K) of the organic superconductor β-(BEDT-TTF)₂I₃. In the high-T _c phase we find a single closed two-dimensional orbit with a dHvA frequency F ₀ = (3815±10) T and a 1/cosθ dependence as expected for a quasi two-dimensional system (where θ is the angle between the magnetic field and the direction normal to the conducting planes). The effective cyclotron mass is m _c = (4:2±0:2)m _e. The dHvA signal shows a beating pattern caused by the three-dimensional warping of the Fermi cylinder. The inter-plane transfer integral estimated from the maximum beating frequency ΔF is t _⊥/εF ≈ 1/175. At the angles where the beating disappears, i.e., ΔF ≈ 0, the oscillations have a harmonic content which is much larger than expected from the Lifshitz-Kosevich formula. In the low-T _c phase weak SdH oscillations with a frequency of F _SdH ≈ 110 T and an effective cyclotron mass m _c ≈ 1:0m _e is found. These results suggest a possible reconstruction of the Fermi surface in the low-T _c phase.     

Gravitational stability of cold Bose star

The maximum mass of cold Bose star that can sustain its own gravity is determined to be ≈0.6/Gm, wherem is the mass of boson. This is different from the Oppenheimer-Volkoff mass of ≈0.4/G ^3/2 m _N ² for the neutron star and may facilitate formation of smaller black holes made of cold invisible axions in the axion dominated universe.     

Conduction electron distributions in europium and gadolinium by soft x-ray spectroscopy

A study of the 5d6s conduction band in Eu and Gd by soft X-ray spectroscopy is presented. Our observation that the density of occupied d states is lower for Eu than for Gd is supported by band structure calculations. For Gd, the convolution integrals between the 3p_3/2 inner level Lorentzian distribution and the A.P.W. densities of states are compared with our experimental M_m spectra. The position of the occupied 4f states with respect to the Fermi level is determined, compared with X-ray photoemission data and discussed for both metals.     

Mechanisms of low-temperature high-frequency conductivity in systems with a dense array of Ge<Subscript>0.7</Subscript>Si<Subscript>0.3</Subscript> quantum dots in silicon

High-frequency (HF) conductivity in systems with a dense (with a density of n = 3 × 10¹¹ cm^?2) array of self-organized Ge_0.7Si_0.3 quantum dots in silicon with different boron concentrations n_B is determined by acoustic methods. The measurements of the absorption coefficient and the velocity of surface acoustic waves (SAWs) with frequencies of 30–300 MHz that interact with holes localized in quantum dots are carried out in magnetic fields of up to 18 T in the temperature interval from 1 to 20 K. Using one of the samples (n_B = 8.2 × 10¹¹ cm^?2), it is shown that, at temperatures T ≤ 4 K, the HF conductivity is realized by the hopping of holes between the states localized in different quantum dots and can be explained within a two-site model in the case of

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, where ω is the SAW frequency and τ₀ is the relaxation time of the populations of the sites (quantum dots). For T > 7 K, the HF conductivity has an activation character associated with the diffusion over the states at the mobility threshold. In the interval 4 K < T < 7 K, the HF conductivity is determined by a combination of the hopping and activation mechanisms. The contributions of these mechanisms are distinguished; it is found that the temperature dependence of the hopping HF conductivity approaches saturation at T* ≈ 4.5 K, which points to a τ₀ ≤ 1. A value of τ₀(T*) ≈ 5 × 10^?9 s is determined from the condition ωτ₀(T*) ≈ 1.

     

Magnetotransport of the low-carrier density one-dimensional S=1/2 antiferromagnet Yb4As3

The transport properties of the semimetallic quasi-one-dimensional S=1/2 antiferromagnet Yb₄As₃ have been studied by performing low-temperature (T≥0.02 K) and high magneticfield (B≤60 T) measurements of the electrical resistivity ρ(T, B). For T ≿ 2 K a ‘heavy-fermion’-like behavior Δρ(T)=AT ² with huge and nearly field-independent coefficient A ≈ 3 μΩ cm/K² is observed, whereas at lower temperatures ρ(T) deviates from this behavior and slightly increases to the lowest T. In B>0 and T ≾ 6 K the resistivity shows an anomalous magnetic-history dependence together with an unusual relaxation behavior. In the isothermal resistivity Shubnikov-de Haas (SdH) oscillations, arising from a low-density system of mobile As-4p holes, with a frequency of 25 T have been recorded. From the T- and B-dependence of the SdH oscillations an effective carrier mass of (0.275±0.005)m ₀ and a charge-carrier mean-free path of 215 ? are determined. Furthermore, in B≥15 T, the system is near the quantum limit and spin-splitting effects are observed.     

A tunneling spectroscopy study of the temperature dependence of the forbidden band in Bi2Te3 and Sb2Te3

Tunneling measurements of dI/dV, d ² I/dV ², and d ³ I/dV ³ were formed along the C ₃ axis (normally to layers) for Bi₂Te₃ and Sb₂Te₃ layered semiconductors in the temperature range 4.2<T>29 5 K. Temperature dependences of the forbidden band energy E _g were obtained. The forbidden band energy in Bi₂Te₃ was 0.20 eV at room temperature and increased to 0.24 eV at T=4.2 K. The E _g value for Sb₂Te₃ was 0.25 eV at 295 K and 0.26 eV at 4.2 K. The distance between the top of the higher valence band of light holes and the top of the valence band of heavy holes situated lower was found to be ΔE _V≈19 meV in Bi₂Te₃; this distance was independent of temperature. The conduction bands of Bi₂Te₃ and Sb₂Te₃ each contain two extrema with distances between them of ΔE _c≈25 and 30 meV, respectively.     

Effective electron mass in high-mobility SiGe/Si/SiGe quantum wells

The effective mass m* of the electrons confined in high-mobility SiGe/Si/SiGe quantum wells has been measured by the analysis of the temperature dependence of the Shubnikov-de Haas oscillations. In the accessible range of electron densities, n _s , the effective mass has been found to grow with decreasing n _s , obeying the relation m*/m _b = n _s /(n _s ? n _c ), where m _b is the electron band mass and n _c ≈ 0.54 × 10¹¹ cm^?2. In samples with maximum mobilities ranging between 90 and 220 m²/(V s), the dependence of the effective mass on the electron density has been found to be identical suggesting that the effective mass is disorder-independent, at least in the most perfect samples.     

Thermal production of superheavy magnetic monopoles in the early universe

Starting with an initial abundance of zero, we calculate the thermal production of magnetic monopoles by using the monopole-antimonopole annihilation cross section and detailed balance. The final abundance of monopoles depends exponentially upon m/T_i; m is the monopole mass ～10¹⁶ GeV and T_i is the temperature when production commences. For m/T_i ≈35–55 the present monopole to photon ratio is nM/n_γ≈10^?23?10^?38, less than the bound derived from the present mass density, but possibly large enough to be detected. This calculation provides a lower bound on the monopole abundance for a given m/T_i.     

Depinning by thermal fluctuations of the charge density wave in Peierls Fröhlich state

It is shown that the thermal fluctuations depin the charge density wave of the Peierls Fröhlich state pinned by impurities at absolute zero temperature. The critical temperature, T_d, of this depinning is estimated as $\text{T}{}_{\mathrm{d}}\text{= 0.55√}\text{m}{}^1\text{mγ}{}_0$ where m¹ and γ₀ are the mass of the collective mode and the pinning frequency at T = 0.     

Comparative study of electronic structure of new superconductors (Sr, Ca)Pd2As2 and related compound BaPd2As2

This paper presents the comparative study of LDA calculated electronic structure of new isostructural to iron based systems superconductors (Sr, Ca)Pd₂As₂ with T _c about 1 K and similar but structurally different system BaPd₂As₂. Despite chemical formula looks similar to iron superconductors and even main structural motif is the same—layers of Fe square lattices, electronic structure of (Sr, Ca)Pd₂As₂ and BaPd₂As₂ differs from Fe (As, Se)-HTSC completely. All these systems have essentially three dimensional Fermi surfaces in contrast to Fe (As, Se) materials. The Fermi level is crossed by low intensive tails of Pd-4d and As-4p states. However, (Sr, Ca)Pd₂As₂ and BaPd₂As₂ materials have rather well developed peaks of Pd-4d (x ² ? y ²) band. Thus, by doping of about 2 holes per unit cell one can increase density of states at the Fermi level by a factor about 2.5. Since experimentally these compounds were found to be simple BCS superconductors the hole doping may considerably increase T _c . LDA calculated total densities of states at the Fermi level for stoichiometric systems perfectly agree with experimental estimates signifying rather small role of electronic correlations.     

Chirality and its breaking in a new theory of hadrons

We show that in Anisotropic Chromo-Dynamics (ACD), a new approach to the dynamics of coloured, confined quarks, the π-meson is a qq?-bound state very close to the Nambu-Goldstone boson of a spontaneously broken chiral symmetry. We calculate the “current” quark masses and obtain m_u⁽⁰⁾ ≈ m_d⁽⁰⁾ ≈ 18 MeV, and m_s⁽⁰⁾ ≈ 123 Mev, in disagreement with the usual “strong PCAC” Ansatz.     

Refractive index, oscillator parameters and temperature-tuned energy band gap of Tl4In3GaS8-layered single crystals

Transmission and reflection measurements in the wavelength region 450-1100 nm were carried out on Tl₄In₃GaS₈-layered single crystals. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.32 and 2.52 eV, respectively. The rate of change of the indirect band gap with temperature dE_gi/dT=-6.0×10⁻⁴ eV/K was determined from transmission measurements in the temperature range of 10-300 K. The absolute zero value of the band gap energy was obtained as E_gi(0)=2.44 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index were found to be 4.87 eV, 26.77 eV, 8.48×10¹³ m⁻² and 2.55, respectively.     

Stimulated recombination from the defect level in doped lead telluride

The relaxation processes of the photoexcited carriers from the defect level in the band gap to the valence band states were investigated in Na and Tl doped p-type PbTe single crystals at T = 77 K. The observed photosignal oscillations were proved to be induced by stimulated recombination of photoexcited carriers from the defect level E_d ≈ 50 meV above the top of the valence band. Non-equilibrium carrier inversion population was produced by impulses of a TEA CO₂-laser. The observed stimulated recombination may presumably be used for designing IR semiconductor lasers operating in the wavelength range of λ ∼ 25 μm at T = 77 K.     

Electric field gradients and band structure for V3Si

The electric field gradient (e.f.g.) tensor measured for V₃Si is analysed in terms of the electronic band structure. This crystal undergoes a martensitic transformation at T_m = 21.3°K. Above T_m the crystal is cubic and the point symmetry of the vanadium atoms is tetragonal. Below T_m the crystal becomes tetragonal and nuclei which sit on chains orthogonal to the axis of symmetry “see” orthorhombic neighbourhood. The experimental data include: (i) The value of the axial part of the e.f.g. (q) measured in the cubic phase, (ii) The asymmetry part of the e.f.g. tensor measured in a single crystal below T_m (iii) The difference of q measured for orthogonal chains in the tetragonal phase. All this data could not be explained by the well-known model of Watson-Gossard and Yefet. Here, a full analysis is given, in terms of an approximate LCAO band structure calculation. It is found that assuming the inaccuracies of the calculated band structure are at most 50 m Ry, the peak in the density of states which “falls” on E_F is X₃ (δ₂ + π). In this case all the experimental data may be explained to within an accuracy of a factor of 2 to 3.