Surface quantum oscillations in (100) inversion layers under uniaxial stress

Surface quantum oscillations have been measured with uniaxially stressed (100) n-type inversion layers. A relation between mechanical stress and cyclotron mass m_c has been observed. In the quantum limit the two-fold valley degeneracy is lifted by about 1 meV with compression.     

Weak localization with a specific role of t symmetry (2D and 3D holes in tellurium)

The anomalous magnetoresistance in crystalline tellurium is analyzed for different p-type carrier dimensions: a bulk sample, size-quantized accumulation layers on different tellurium crystallographic surfaces, and tellurium clusters (tellurium embedded in a dielectric opal matrix). It is shown that the effect can be interpreted in all cases in terms of the theory of weak localization of noninteracting particles with inclusion of the specific features of the tellurium band spectrum, namely, fully lifted spin degeneracy, trigonal spectrum distortion, and a specific role played by the t symmetry in inter-valley scattering. The differences observed among the various manifestations of the weak localization effect are determined by the hole wave function phase-relaxation channel which is dominant in a particular case. A case is discussed where the time characterizing the inter-valley transition probability becomes comparable to the momentum relaxation time. Fiz. Tverd. Tela (St. Petersburg) 41, 879–881 (May 1999)     

Extension of Yafet's theory of spin relaxation to ferromagnets

By making use of Kramer's degeneracy of the electronic states in a nonmagnetic material, Yafet has derived an expression for the spin relaxation time T₁ due to scattering of electrons at phonons which involves the properties of electronic and phononic states and the matrix elements for the scattering. It is shown that an analogous expression for T₁ can be derived for ferromagnets (where Kramer's degeneracy does not hold) when taking into account the conservation of the total number of electrons. This expression can be used as a starting point for the ab initio calculation of T₁, and this quantity is required for an interpretation of the ultrafast demagnetization of ferromagnets after excitation with a femtosecond laser pulse.     

Self-consistent calculations of electric subbands in p-type GaAlAs-GaAs heterojunctions

The calculations in the Hartree-approximation indicate that only the heavy hole subband is occupied for carrier concentrations < 4×10¹²/cm² and that the spin degeneracy is lifted for finite wave vectors. Without magnetic field, two effective masses of 0.16 m_o and 0.61 m_o are deduced. A calculation of the Landau levels predicts a three-line cyclotron resonance absorption spectrum at high magnetic fields, which partially agrees with experimental data.     

Nuclear Spin relaxation mediated by Fermi-edge electrons in n-type GaAs

A method based on the optical orientation technique was developed to measure the nuclear-spin lattice relaxation time T ₁ in semiconductors. It was applied to bulk n-type GaAs, where T ₁ was measured after switching off the optical excitation in magnetic fields from 400 to 1200 G at low (< 30 K) temperatures. The spin-lattice relaxation of nuclei in the studied sample with n _D = 9 × 10¹⁶ cm^?3 was found to be determined by hyperfine scattering of itinerant electrons (Korringa mechanism) which predicts invariability of T ₁ with the change in magnetic field and linear dependence of the relaxation rate on temperature. This result extends the experimentally verified applicability of the Korringa relaxation law in degenerate semiconductors, previously studied in strong magnetic fields (several Tesla), to the moderate field range.     

Electronic Raman scattering from inter-valence band transition in photo-excited GaP and GaAs1−xPx crystals

Raman scattering from photo-created free carriers in undoped GaP and GaAs_1?xP_x (x = 0.85, 0.73 and 0.66) under high excitation intensity has been studied. Two new Raman bands have been observed and assigned to electronic transitions from the split-off hole band to the heavy hole band and from the light hole band to the heavy hole band. The spin-orbit splitting energies in these crystals have been determined from the analysis of observed Raman bands, and compared with other experimental values.     

Influence of screening on deformation potential and piezoelectric scattering in doped semiconductors

Mobility relations are derived for deformation potential and piezoelectric scattering accounting for the screening by free charge carriers. Estimations of the screening influence on the carrier mobility in n-type GaN and p-type GaAs are made. It is shown that particularly in p-type GaAs the screening effect cannot be neglected in analysing the hole mobility even at high carrier concentrations.     

Transient InSb spin-flip laser - a measurement of T1

A double pulse spin-flip laser technique has been used to evaluate the InSb conduction electron spin-relaxation time T₁; a value of 60 ± 20 nsec (n = 1.2 × 10¹⁶cm^?3, H = 60 kG, T = 20 K)is obtained. The effects of electron heating by free carrier absorption are measured and an energy relaxation time of 20–40 nsec is obtained. A calculation of T₁ based on ionized impurity scattering in the quantum limit regime is in order of magnitude agreement with the experiment.     

Pulsed ESR on the organic ion radical salt: 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-[TCNQ]2

We present firstT _1e ^?1 andT _2e ^?1 measurements on the organic ion radical salt 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)₂ as function of temperature and of orientation. The electronic spin diffusion constant could be determined directly by the electron spin echo field gradient technique:D (300 K)=0.03±0.02 cm²/sec. Pulsed ESR experiments have — in comparison to conventional cw-ESR — the advantage to monitor viaT _1e ^?1 andT _2e ^?1 the spectral density of dynamical processes at different frequencies. This is shown in a general manner on 3,3′-diethyl-4,4′-dimethyl-2,2′-thiazolocyanine-(TCNQ)₂. Between 300 and 60 K,T _1e ^?1 andT _2e ^?1 are close in amplitude and have a similar temperature dependence. At 60 K their degeneracy is lifted, yielding a quantitative value for the effective spin exchange between localized spinsτ _ex ^?1 sec^?1 and via the absolute value of the relaxation an average distance of the localized centers of about 12 Å. The dynamical data as evaluated above cannot be correlated with the conductivity, clearly indicating that the conduction electrons are a minority, not being monitored by the ESR-experiments.     

Preparation and properties of co-evaporated bismuth telluride [Bi2Te3] thin films

Thin films of bismuth telluride have been prepared by the reactive evaporation method. Film properties, such as conductivity, Hall effect, and thermoelectric power were studied in the temperature range from liquid nitrogen to 350 K. The films prepared were of n-type with a carrier concentration of 1.25 x 10²⁰ at room temperature. The temperature dependence of the Hall mobility is found to be T^?1.8 indicating lattice scattering.     

Relaxation function of a Heisenberg paramagnet at large wavevectors

An approach based on the Mori's theory to calculate the Kubo relaxation function is presented in order to explain the structure of neutron scattering cross section at large wavevectors for Heisenberg paramagnets. The limits of validity of the previous approximate theories are discussed. A low-frequency expansion for the third order memory function is performed: this corresponds to considering a more consistent long-time approximation for the second order memory function. The relaxation function has been computed for RbMnF₃ at T?T_N and T = 1.125T_N.     

Semiconducting properties of pure and Mn-doped chromium disilicides

The phase analysis was carried out for the system (l?x) CrSi₂ + xMnSi₂ in the range 0?x?0.5 by X-ray technique. The solid solution Cr_1?xMn_xSi₂ was identified in the composition range 0?x?0.225, where the added Mn-atoms occupied substitutionally the Cr-atom sites in the CrSi₂ structure. Resistivity, Hall coefficient as well as thermoelectric power were measured as functions of temperature in the range, 80–1200 K and composition x in the single phase region, 0 ?x?0.225. The pure CrSi₂ (x = 0) was a p-type degenerate semiconductor, whose hole concentration was determined to be 7.7 × 10²⁰ cm^?3 at room temperature. Mn-atoms introduced in the CrSi₂ crystal were found to act as donors. The forbidden energy gap was determined to be 0.30 eV from the Hall-data in the intrinsic region. With increasing x, a conversion from p- into n-type semiconductor took place in Cr_1?xMn_xSi_y. From the analysis of Hall- as well as resistivity-data, the mobility ratio b was obtained as a function of composition x. It was revealed that b increased with increasing x from 0.01 for x = 0 to 0.12 for x = 0.182. The electron-hole concentration product could be expressed as $\text{np = 1.2 × 10}{}^{35}\text{T}{}^{35}\text{exp}\text{(?}\text{3480}\text{T}\text{)}$, and the hole mobility as $\text{μ}{}_{\mathrm{p}}\text{= 7.0 × 10}{}^4\text{T}{}^{\mathrm{<mtext>?</mtext><mtext>3</mtext><mtext>2</mtext>}}$ in the acoustic scattering region. The effective mass of hole was found out to be 3.2 m₀ and independent of x, whereas that of electron varied from 20.2 m₀ for x = 0 to 7.5 m₀ for x = 0.182. When these parameters are used, the theoretical temperature variation of the thermoelectric power curve was found out to be in good agreement with the measurement.     

Spin relaxation of conduction electrons in highly-doped n-type germanium at low temperature

We present a theory of the spin relaxation time of the conduction electrons in highly-doped n-type germanium at liquid helium temperature. The theory is compared with some of our measurements and the experimental data available in the literature on As-doped germanium. The observed linewidth at T = 10 K is accounted for in the whole metallic concentration range (N_D > 3 × 10¹⁷cm^?3).In the lower concentration range (3 × 10¹⁷ < N_D < 10¹⁸cm^?3), the relevant mechanism is the random jumping of the g factor upon intervalley scattering. The agreement with experiment is good without any adjustable parameter.In the higher concentration range (N_D > 10¹⁸cm^?3), the dominant process for the linewidth is the spin-flip scattering by ionized donors (Elliott process); the usual theory is shown to be insufficient and the greater effectiveness of scattering by the localized part of the donor potentials is pointed out. The calculated linewidth is related to the intervalley scattering time T_iv. The agreement with experiment is good and predictions are given for the linewidth in the case of other shallow donors in the same concentration range.     

Coexistence of the superconducting condensate and the quasi-stationary states of hole pairs

The formation of bound states in the case of a direct Coulomb repulsion between two holes whose reciprocal effective mass tensor has principal values of opposite signs is considered as a possible mechanism of high-T _C superconductivity. The study of the specific features of the scattering amplitude shows that, under certain conditions, in addition to the quasi-stationary states, states with a negative attenuation are possible, which corresponds to the tendency toward the formation of a hole pair condensate. The coexistence of the quasi-stationary states and the condensate qualitatively agrees with the phase diagram of p-type doped high-T _C superconducting cuprates.     

The influence of AlN interlayers on the microstructural and electrical properties of p-type AlGaN/GaN superlattices grown on GaN/sapphire templates

AlN with different thicknesses were grown as interlayers (ILs) between GaN and p-type Al_0.15Ga_0.85N/GaN superlattices (SLs) by metal organic vapor phase epitaxy (MOVPE). It was found that the edge-type threading dislocation density (TDD) increased gradually from the minimum of 2.5×10⁹?cm^?2 without AlN IL to the maximum of 1×10¹⁰?cm^?2 at an AlN thickness of 20 nm, while the screw-type TDD remained almost unchanged due to the interface-related TD suppression and regeneration mechanism. We obtained that the edge-type dislocations acted as acceptors in p-type Al _x Ga_1?x N/GaN SLs, through the comparison of the edge-type TDD and hole concentration with different thicknesses of AlN IL. The Mg activation energy was significantly decreased from 153 to 70?meV with a?10-nm AlN IL, which was attributed to the strain modulation between AlGaN barrier and GaN well. The large activation efficiency, together with the TDs, led to the enhanced hole concentration. The variation trend of Hall mobility was also observed, which originated from the scattering at TDs.     

Negative U centers, percolation, and the insulator-metal transition in high-T c superconductors

The mechanism of formation of ?U centers in high-T _c superconductors (HTSCs) is considered. It is shown that the transition from the insulator to the metallic state on doping passes through a certain range of dopant concentrations in which it becomes possible for local transitions of singlet electron pairs to occur from oxygen ions to two neighboring cations (a ?U center), while single-electron transitions are still forbidden. Conduction arises in such systems at a concentration of ?U centers exceeding the percolation threshold for the orbitals of singlet hole pairs. A phase diagram constructed on the basis of the proposed model for the HTSC compounds of the Ln-214 class is in complete agreement with experiment. The mechanisms of formation and relaxation of free hole carriers are considered. It is shown that a distinctive feature of the normal state of HTSCs is the dominant contribution of electron-electron scattering to the charge carrier relaxation processes. It is concluded from the analysis presented that HTSCs comprise a special class of solids in which a nonstandard mechanism of superconductivity, different from the BCS mechanism, is realized.     

Superconductivity of In-doped Sn0.62Pb0.33Ge0.05Te alloys

A study of the Hall and Seebeck coefficients and of resistivity has been carried out on an Sn_0.62Pb_0.33Ge_0.05Te alloy doped by 5 and 10 at. % In. A superconducting transition with the maximum critical temperature T _C～4 K has been discovered in samples with hole concentrations p≥1×10²¹ cm^?3. The dependence of T _C on hole concentration has been established to be of a threshold nature. The onset of superconductivity is accompanied practically simultaneously by a growth of the resistivity and a sharp drop of the Seebeck coefficient. These features in the experimental data indicate the existence of a band of In resonance states within the allowed valence-band spectrum and strong resonance hole scattering to impurity states. The threshold character of the T _C(p) dependence is connected with the holes filling the resonance states. A positive correlation between the resonance scattering intensity and the critical temperature is observed.     

Magnetic moment relaxation of a shallow acceptor center in heavily doped silicon

Results of studying the temperature dependence of the residual polarization of negative muons in crystalline silicon with germanium (9×10 ¹⁹ cm ^?3 ) and boron (4.1×10 ¹⁸ , 1.34×10 ¹⁹ , and 4.9×10 ¹⁹ cm ^?3 ) impurities are presented. It is found that, similarly to n-and p-type silicon samples with impurity concentrations up to ～10 ¹⁷ cm ^?3 , the relaxation rate ν of the magnetic moment of a _μ Al acceptor in silicon with a high impurity concentration of germanium (9×10 ¹⁹ cm ^?3 ) depends on temperature as ν～T ^q , q≈3 at T=(5–30) K. An increase in the absolute value of the relaxation rate and a weakening of its temperature dependence are observed in samples of degenerate silicon in the given temperature range. Based on the experimental data obtained, the conclusion is made that the spin-exchange scattering of free charge carriers makes a significant contribution to the magnetic moment relaxation of a shallow acceptor center in degenerate silicon at T?30 K. Estimates are obtained for the effective cross section of the spin-exchange scattering of holes (σ _h ) and electrons (σ _e ) from an Al acceptor center in Si: σ _h ～10^?13 cm² and σ _e ～8×10^?15 cm² at the acceptor (donor) impurity concentration n _a (n _d )～4×10¹⁸ cm^?3.     

Conductivity relaxation in Ag+ ion conducting PEO–PMMA–PEG polymer blends

In this paper, the electrical conductivity and relaxation studies in two different temperature regions (T?<?T _m and T?>?T _m) on plasticized PEO–PMMA blend polymer electrolyte system with AgNO₃ salt are reported. The polymer electrolyte system has been prepared by solution cast technique and characterized by X-ray diffraction and differential scanning calorimetry. The conduction and relaxation processes at various temperatures have been investigated in the framework of modulus formalism. The distribution of relaxation times is discussed using Argand plot. Variation in ionic conductivity of polymer blends is discussed with the increase of PEO as well as with temperature.     

n-Si/SiGe quantum cascade structures for THz emission

In this work we report on modeling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (X_z) and in-plane (X_xy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures.