Quantum Interference and Surface States Effects in Bismuth Nanowires

We report the observation of Aharonov-Bohm (AB) oscillations for single Bi nanowires with diameter d< 80 nm. The single nanowire samples with glass coating were prepared by the Ulitovsky technique; they were cylindrical single crystals with (10 $\bar{1}$ 1) orientation along the wire axis. The surface of Bi nanowire supports surface states which give rise to a significant population of charge carriers with high effective mass that form a highly conducting tube around the nanowire. The oscillations of longitudinal magnetoresistance (MR) of Bi nanowires with two periods ΔB ₁ and ΔB ₂ proportional to Φ₀ and Φ₀/2 were observed, where Φ₀=h/e is the flux quantum. From B～ 8 T down to B = 0 the extremums of Φ₀/2 oscillations are shifted up to 3π at B = 0 which is the manifestation of Berry phase shift due to electron moving in a nonuniform magnetic field. A derivative of MR was measured at various inclined angles. The observed angle variation of the periods is not in agreement with the theoretical dependence Δ(α)=Δ(0)/cos?α of the size effect oscillations of the “flux quantization” type. Moreover, the equidistant oscillations of MR exist in transverse magnetic fields under certain rotation angles. An interpretation of the MR oscillations is presented.     

Laser-Induced Crystallization of a Si1−x Ge x Microstructure

Laser-induced crystallization of SiGe was investigated using a crystallization technique of pulsed excimer KrF laser irradiation on a-SiGe films that were prepared by plasma-enhanced chemical vapor deposition on quartz. The crystallized SiGe sample was investigated by scanning electronic microscopy (SEM); the SiGe microcrystals are 0.5 m in size and embedded in the a-SiGe:H matrix. Strong photoluminescence with two peaks at 720 and 750 nm was observed at room temperature in the crystallized film, whereas the uncrystallized a-SiGe:H films emit do not emit light in the visible range. This indicates that laser-induced crystallization can be used to improve the luminescence efficiency for Si-based materials.     

Non-substitutional Sn Defects in Ge1−x Sn x Alloys for Opto- and Nanoelectronics

Important technological applications are envisaged for Ge_1?x Sn _x alloys. They provide a route to obtain direct-gap group IV materials, tuneable by concentration. Therefore, these alloys are ideal candidates for optoelectronic devices, highly compatible with Si integrated circuits. Contrary to other binary alloys with group IV elements, homogeneous Ge_1?x Sn _x alloys, as required for device applications, have proven difficult to form above a certain temperature-dependent critical Sn concentration. Through a detailed ab-initio local defect study, and the proposal of a statistical model for the formation of these alloys, we predicted that a new type of Sn defect (β-Sn), consisting of a single Sn atom in the centre of a Ge divacancy, might be formed. The environment of this defect relaxes towards a cubic octahedral configuration, facilitating the nucleation of white tin and its segregation, as found in amorphous samples. We confirmed that Sn would enter substitutionally in the Ge lattice, but above a temperature-dependent critical concentration, non-substitutional β-Sn defects should be formed, consistent with experimental observations. In this paper we introduce a two-site substitutional equivalent for the non-substitutional β-Sn defect in Ge, as needed in order to be able to include β-Sn in electronic structure calculations with effective-field electronic models for disorder, like the Virtual Crystal Approximation (VCA). The equivalent substitutional model is derived in order to take into account the different symmetries in the immediate environment of the substitutional α and non-substitutional β defect sites and their effect on the electronic structure.     

Quantum Transport and Cyclotron Resonance Study of Ge/SiGe Quantum Wells in High Magnetic Fields

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si_1?x Ge _x quantum wells, using pulsed high magnetic fields up to 50 T. Fine quantum oscillations were observed in ρ _xx . Reflecting the complex Landau level structure in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consists of several peaks. Cyclotron resonance was measured at photon energies between 10 and 17 meV. Two well-defined resonance peaks were observed in two samples with different x, resulting in different strains. A large non-parabolicity and large strain dependence of the effective masses were observed.     

Crystallization kinetics in glassy Ge20Se80−x In x alloys

Crystallization kinetics is studied in glassy Ge₂₀Se_80–xIn_x (0 x 20) using isothermal annealing at temperatures between the glass transition and melting. D.c. conductivity is taken as a parameter to estimate the extent of crystallization (). The activation energy of crystallization (E) and the order parameter (n) are calculated by fitting the values of in the Avrami equations of isothermal crystallization. The results indicate that E is highly composition-dependent, which is explained in terms of the stable phases in the Ge-Se-In system.     

High-Pressure Studies on Superconductivity in LaFeAsO1−x F x and SmFeAsO1−x F x

The pressure dependence on the superconducting transition temperature (T _c ) was investigated for the iron-based superconductors LaFeAsO_1−x F _x and SmFeAsO_1−x F _x . The T _c ’s increase largely for LaFeAsO_1−x F _x with a small increase of pressure, while a sharp decrease of T _c was observed for SmFeAsO_1−x F _x . The electrical resistivity measurements reveal pressure-induced superconductivity for undoped LaFeAsO and SmFeAsO. These pressure effects seem to be related to an anisotropic decrease of the lattice constants under high pressure from the x-ray diffraction measurements up to 10 GPa for the LaFeAsO_1−x F _x system.     

Impurity tin atoms in glassy As x S1 − x and As x Se1 − x

¹¹⁹Sn Mössbauer emission spectroscopy data demonstrate that the impurity tin atoms resulting from the radioactive decay of the ¹¹⁹Sb isotope in the structure of glassy As _x S_{1 ? x} and As _x Se_{1 ? x} alloys reside on the arsenic site and act as two-electron amphoteric centers with a negative correlation energy: tetravalent six-coordinate tin acts as a singly ionized donor, and divalent three-coordinate tin, as a singly ionized acceptor.     

Quantum Criticality and Scaling of the Magnetic Response in CeCu6−x Au x

The heavy-fermion system CeCu_6?x Au _x which exhibits antiferromagnetic order for Au concentrations x>0.1, can be easily tuned to a quantum phase transition, either by chemical composition, i.e., Au concentration, or by an external magnetic field. We present extensive inelastic neutron scattering measurements in order to study the momentum and energy dependence of the critical spin fluctuations for concentration- and magnetic field-tuned CeCu_6?x Au _x . The quasi-2D fluctuations, observed for x=0.1 at the magnetic instability, are already present in pure CeCu₆ and persist to x=0.2, i.e., well into the magnetically ordered regime. This is a strong hint that disorder does not play a major role in this system. The magnetic response for quantum critical CeCu_6?x Au _x exhibits critical slowing down and obeys scaling relations that depend on the tuning parameter. While the E/T scaling of the dynamic susceptibility in concentration-tuned CeCu_5.9Au_0.1 is an indication for local quantum criticality, the E/T ^3/2 scaling in field-tuned CeCu_5.8Au_0.2 points to a conventional spin-density-wave instability.     

Homogeneous Coexistence in CaFe1−x Co x AsF and Phase Segregation in LaFeAsO1−x F x Studied via NMR

In iron-based pnictides, one of the interesting topics is homogeneous coexistence or phase segregation at the boundary between antiferromagnetic (AF) and superconducting (SC) phases. We addressed this problem on a microscopic level by means of ⁷⁵As NMR measurements in LaFeAsO_1?x F _x (x=0.026) (La1111), and CaFe_1?x Co _x AsF (x=0.06) (Ca1111) having an intermediate electronic phase diagram between Ba(Fe_1?x Co _x )₂As₂ and the La1111 series. NMR spectra for 6 % Co-doped Ca1111 samples were very similar to those for the undoped samples even below T _c , suggesting homogeneous coexistence of the AF and SC states. For 2.6 % F-doped La1111 samples, AF and paramagnetic (PM) domains coexist at ambient pressure, and AF and SC domains coexist at 3.0 GPa. The coexistence of domains is explained by considering a SC dome separated from the AF phase in the phase diagram. The homogeneous coexistence support S _±-wave superconductivity, whereas separation of the AF and SC phases gives credence to S ₊₊-wave superconductivity.     

Growth of perfect-crystal Si-Si1−x Gex-(Ge2)1−x (InP)x structures from the liquid phase

Structures comprising Si-Si_1−x Ge_x-(Ge₂)_1−x (InP)_x with an intermediate Si_1−x Ge_x buffer layer were grown on silicon substrates. Morphological examinations, scanning patterns and diffraction spectra, and also the electrophysical and luminescence properties of the heterostructures were used to show that the crystal perfection of these structures depends on the choice of liquid-phase epitaxy conditions. Pis’ma Zh. Tekh. Fiz. 25, 37–40 (December 26, 1999)     

Rashba Effect in IV–VI Semiconductor Quantum Wells

The spin–orbit split quantized states for electrons confined in IV–VI lead-salt asymmetric quantum wells (QWs), grown on both (100) and (111) substrates, are obtained analytically within the envelope function approximation with a k p four-band model for the bulk. Nonparabolicity, anisotropy, and break of valley degeneracy are included. In (111) QWs, it is shown that for the valley along the growth direction the Rashba splitting is like the one in III–V QWs, i.e., isotropic and linear in k (for k 0). For the oblique valleys, contrary to the III–V case, a Rashba effect is obtained that is anisotropic and described by an effective magnetic field with nonzero component along the growth direction. Specific results for PbTe/PbEuTe asymmetric QWs with large Rashba splittings are shown.     

Current-voltage characteristics of Si/Si1 − x Ge x heterodiodes fabricated by direct bonding

We have studied the current-voltage (I–U) characteristics of Si/Si_{1 ? x} Ge _x (0.02 < x < 0.15) heterodiodes fabricated by direct bonding of (111)-oriented n-type single crystal silicon wafers with p-type Si_{1 ? x} Ge _x wafers of the same orientation containing 2–15 at % Ge. An increase in the germanium concentration N _Ge in Si_{1 ? x} Ge _x crystals is accompanied by a growth in the density of crystal lattice defects, which leads to a decrease in the minority carrier lifetime in the base of the heterodiode and an increase in the recombination component of the forward current and in the differential resistance (slope) of the I–U curve. However, for all samples with N _Ge ≤ 15 at %, the I–U curves of Si/Si_{1 ? x} Ge _x heterodiodes are satisfactory in the entire range of current densities (1 mA/cm²–200 A/cm²). This result shows good prospects for using direct bonding technology in the fabrication of Si/Si_{1 ? x} Ge _x heterostructures.     

Magnetocaloric Effect in La1−x Cd x MnO3

Calculations of the magnetocaloric effect for La_1?x Cd _x MnO₃ (LSCM) upon 0.05 T magnetic field variation have been carried out. It is found that magnetic entropy change distribution of the LSCM is much more uniform than that of gadolinium. This feature is desirable for an Ericson-cycle magnetic refrigerator. Furthermore, at different concentrations of Cd, the temperature range between 150 K and room temperature can be covered using the La_1?x Cd _x MnO₃ system. Therefore, the LSCM system is beneficial for manipulating magnetocaloric refrigeration that occurs in various temperatures.     

Inhomogeneous Superconductivity in BaPb1−x Bi x O3

Transport and magnetization measurements of single crystas of BaPb_1?x Bi _x O₃, for Bi compositions spanning the superconducting phase diagram, show signatures of granular and inhomogeneous superconductivity, possibly associated with the nanoscale structural phase separation found for compositions around optimal doping. The material exhibits a field-tuned superconductor-to-insulator transition for Bi compositions 0.24≤x≤0.28. For optimally doped samples (x≈0.25), the magnetoresistance curves show scaling, suggesting some kind of critical behavior.     

Electrical properties and phase transition of Ge1−x Sn x Se2.5 thin films

The Ge_1–x Sn _x Se_2.5 system was prepared by melting the correct ratio of high purity elements in quartz evacuated ampoules followed by quenching in ice. It was found that, within the Ge_1–x Sn _x Se_2.5 system, a glassy state can be formed when 0 x 0.4. On increasing x to 0.6 a glassy state could not be obtained, as is confirmed by X-ray diffraction. Differential thermal analysis (DTA) was carried out to study the effect of composition on the stability of amorphous phase. Ge_1–x Sn _x Se_2.5 (where 0 x 0.6) thin films have been prepared by the thermal evaporation technique. The electrical conductivity of the thin films have been studied as a function of composition and film thickness.     

Fe-Site Doping and Spin Effects on Phonon and Superconductivity in SrFe1−x Co x AsF (x=0,0.125)

First-principles calculations are performed for phonon spectra, phonon density of states and electron-phonon coupling constants of SrFe_1?x Co _x AsF (x=0,0.125) with tetragonal nonmagnetic (NM) and orthorhombic striped antiferromagnetic (SAF) structures. It is found that Co-doping in Fe-site harden phonons in NM structure and soften phonons in SAF structure, and spin interaction makes phonons soften in evidence. Electron-phonon coupling is enhanced by both doping and spin interaction in Fe-site, but does not large enough to produce experimental superconductivity, which rules out phonon mechanism superconductivity in SrFe_1?x Co _x AsF.     

Pb1−x Bi x /Bi/Pb1−x Bi x Josephson junction

A superconductor/semimetal/superconductor (S/SM/S) Josephson junction has been developed. We have used an alloy of Pb_1–x Bi _x (0x 0.6) as the superconductor and Bi as the semimetal. By irradiating at X-band microwave of 10 GHz, Shapiro steps were observed for various bismuth barrier thicknesses inÅ and bismuth weight ratiosx. Finally, we obtained the empirical relationship for barrier thickness, below which microwaves could be detected for various bismuth weight ratiosx at the temperature of 4.2 K.     

Lattice-matched epitaxial Pb1 − x Mn x Se/PbSe1 − x S x heterojunctions

This paper examines the possibility of producing lattice-matched p-n heterojunctions based on epitaxial n-Pb_{1 ? x} Mn _x Se (x = 0.02) and p-PbSe_{1 ? x} S _x (x = 0.04) films. The heterojunctions have been grown by molecular beam epitaxy in a single processing cycle, without breaking the vacuum, using a compensating Se vapor source in the growth process. Optimal conditions have been found for the growth of structurally perfect (W _1/2 = 90″-100″) epitaxial films and fabrication of lattice-matched heterojunctions based on such films, photosensitive in the IR spectral region.     

Non-Fermi-Liquid Behavior in CePt1+xSi1−x and CePtSi1−yGey

The heavy-fermion system CePtSi is located close to a magnetic instability. In order to study the influence of composition on the physical properties of CePtSi we prepared slightly off-stoichiometric CePt_1+xSi_1–x polycrystals. CePt_1+xSi_1–x with x=–0.04 exhibits typical non-Fermi-liquid behavior (C/TlnT, T^–0.21, T^1.5). A scaling analysis of the thermodynamic quantities at magnetic fields up to 10T and temperatures up to 6K points to a quantum phase transition described by some unknown non-Gaussian fixed point. In contrast substitution of Si by Ge leads to a quantum phase transition involving weak-coupling spin fluctuations for CePtSi_0.89Ge_0.11, which can be described within the framework of the self-consistent renormalization theory.