Structural transformation-induced magnetic ordering in degenerate magnetic semiconductors

Electrical properties and molar specific heats have been measured in the temperature range 2–300 K of Bridgman-grown Sn_1–x Cr _x Te crystals. In addition to the paramagnetic-to-ferro or ferrimagnetic transition temperatureT _c (= 150–300 K), in this magnetic system there is another characteristic temperatureT _AHE (= 4–5 K), above which the anomalous Hall effects vanish and around which the specific heats show a small peak, indicating an extra magnetic transition. The low-temperature transition is considered as induced by a cubic-to-rhombohedral structural phase transition of the host SnTe crystal occurring at a critical temperature ofT _s (= 80–100 K). Data on the isothermal annealing of these crystals are also presented.     

Hall effect and magnetoresistance in extrinsic piezoelectric semiconductors

The Hall effect and transverse magnetoresistance in extrinsic piezoelectric semiconductors such as InSb are investigated according to the scattering processes of carriers in semiconductors. These scattering processes contain the acoustic phonon scattering, the piezoelectric scattering, and the ionized-impurity scattering. The energy band structure of carriers in semiconductors is assumed to be nonparabolic. Results show that Hall angle, Hall coefficient, and transverse magnetoresistance depend strongly on the dc magnetic field and carrier density due to the energy-dependent relaxation time. Comparison with experimental data is made. It is also found that the magnetoresistance in degenerate InSb oscillates with the dc magnetic field due to the scattering of carriers with impurities in semiconductors.     

Influence of energy bands on the Hall effect in degenerate semiconductors

The influence of energy bands on the Hall effect and transverse magnetoresistance has been investigated according to the scattering processes of carriers in degenerate semiconductors such as InSb. Results show that the Hall angle, Hall coefficient, and transverse magnetoresistance depend on the dc magnetic field for both parabolic and nonparabolic band structures of semiconductors and also depend on the scattering processes of carriers in semiconductors due to the energy-dependent relaxation time. From our numerical analysis for the Hall effect, it is shown that the conduction electrons in degenerate semiconductors play a major role for the carrier transport phenomenon. By comparing with experimental data of the transverse magnetoresistance, it shows that the nonparabolic band model is better in agreement with the experimental work than the parabolic band model of semiconductors.     

Quantum oscillations in the high-Tc cuprates

We review recent progress in the study of quantum oscillations as a tool for uniquely probing low-energy electronic excitations in high-T(c) cuprate superconductors. Quantum oscillations in the underdoped cuprates reveal that a close correspondence with Landau Fermi-liquid behaviour persists in the accessed regions of the phase diagram, where small pockets are observed. Quantum oscillation results are viewed in the context of momentum-resolved probes such as photoemission, and evidence examined from complementary experiments for potential explanations for the transformation from a large Fermi surface into small sections. Indications from quantum oscillation measurements of a low-energy Fermi surface instability at low dopings under the superconducting dome at the metal-insulator transition are reviewed, and potential implications for enhanced superconducting temperatures are discussed.     

Friedel type oscillations in problem of skin effect in degenerate plasma

It is shown that Friedel type oscillations accompany the skin effect in degenerate metal plasma. The nature of Friedel oscillations is related to features of the degenerate Fermi distribution function, which sharply vanishes immediately behind the Fermi surface. This circumstance accounts for the appearance of Friedel type oscillations in anomalous skin effect.     

Bismuth deformation potentials calculated from SdH periods under static strain and from magnetoacoustic attenuation

We describe the non-parabolicity of the electron dispersion in bismuth by the Lax model, which replaces the energy Eby E(1+E/E_G), E_G being the L-point energy gap. It is assumed that the effect of small strains can be accounted for solely by small changes of the electron and hole Fermi energies, dE_F = D_jke_jk,where D_jk and e_jk denote deformation potentials and strains. With this assumption we show that the deformation potentials come out the same whether the dispersion relation is non-parabolic or parabolic. This finding we use in a re-evaluation of the deformation potentials obtained from SdH-measurements under static strain. We further make a mass data correction of deformation potentials obtained from magnetoacoustic attenuation. The two sets of values so obtained are in excellent agreement. This allows us to improve the accuracy, and we recommend to use the following values (unit eV): for electrons: D₁₁ = 2.74 ± 0.50, D₂₂ = –7.38 ± 0.56, D₃₃ = 2.17 ±0.25, D₂₃ = –1.85 ± 0.44and for holes: D₁₁ = –1.06 ± 0.27, D₃₃ = –1.06 ± 0.09     

Nonlinear response of electron-phonon interaction inn-type nondegenerate piezoelectric semiconductors

The nonlinear response of the electron-phonon interaction in nondegenerate piezoelectric semiconductors such asn-type InSb in the presence of a dc magnetic fieldB directed along the propagation of acoustic waves has been studied by using a quantum mechanical treatment. The effect of electron scattering in solids has been taken into consideration, so the electron relaxation time cannot be neglected. The nonlinear nature of the energy band is corrected for, using the Heisenberg equation of motion. It is found that the nonlinear response is proportional to a nonlinear longitudinal conductivity tensor _zzz when acoustic waves propagate parallel to the [111] direction for both piezoelectric and deformation-potential couplings. Numerical calculations for _zzz of n-type InSb at low temperatures are presented. Results show that the nonlinear response decreases rapidly with the sound frequency, and decreases slowly with the dc magnetic field and temperature. Therefore, the electron relaxation time and the nonparabolicity of energy bands play important roles for the electron-phonon interaction due to the piezoelectric and deformation-potential couplings in the microwave region.     

Quantum oscillations in magnetically doped colloidal nanocrystals

Progress in the synthesis of colloidal quantum dots has recently provided access to entirely new forms of diluted magnetic semiconductors, some of which may find use in quantum computation. The usefulness of a spin qubit is defined by its Rabi frequency, which determines the operation time, and its coherence time, which sets the error correction window. However, the spin dynamics of magnetic impurity ions in colloidal doped quantum dots remain entirely unexplored. Here, we use pulsed electron paramagnetic resonance spectroscopy to demonstrate long spin coherence times of ～0.9 μs in colloidal ZnO quantum dots containing the paramagnetic dopant Mn(2+), as well as Rabi oscillations with frequencies ranging between 2 and 20 MHz depending on microwave power. We also observe electron spin echo envelope modulations of the Mn(2+) signal due to hyperfine coupling with protons outside the quantum dots, a situation unique to the colloidal form of quantum dots, and not observed to date.     

Torsional oscillations of a finite inhomogeneous piezoelectric cylindrical shell

Torsional wave motion of a finite right circular cylindrical shell of inhomogeneous piezoelectric material of (622) crystal class under time dependent electric potential on the boundary is investigated. The inhomogeneity is restricted to the variations of density and other physical constants of the medium as expf((r)) where f is a suitable function of the radial distance. Two related problems, investigated by the author, in the papers [1, 2] are shown to be particular cases of the present problem.     

Quantum oscillations of the superconducting transition temperature in Al-SiO systems

We report measurements on the superconducting transition temperature T _cand the normal resistance R _nof quench-condensed pure and granular aluminum films. The changes of the superconducting transition temperature are characteristically dependent on the coating film thickness and are inversely dependent on the Al film thickness. The observed oscillations of the transition temperature are interpreted in the framework of the theory of quantum oscillations developed by Kagan and Dubovskii.This work was supported in part by the Deutsche Forschungsgemeinschaft.     

Effect of nonparabolicity on longitudinal conductivity of degenerate semiconductors for transverse ultrasound in strong magnetic fields

The effect of nonparabolicity on the linear longitudinal conductivity of degenerate semiconductors such asn-type InSb for transverse ultrasound has been investigated by using a quantum treatment which is valid at high frequencies and in strong magnetic fields. Results show that both the real and imaginary parts of the longitudinal conductivity depend on the sound frequency and oscillate with the dc magnetic field due to the nonlinear effect of the energy eigenvalue equation in the nonparabolicity. In the case of parabolic band structure, however, the real part of the longitudinal conductivity vanishes and no oscillations appear in the imaginary part.Partially supported by the National Science Council of China.     

Linearized magnetoacoustic sensor of angular displacements

The design of a quasilinear sensor of angular displacements based on acoustic resonance in an antiferromagnetic single crystal was considered. The frequency versus angle dependence of a crystalline alpha-Fe(2)O(3) acoustic resonator placed in a rotating inhomogeneous bias field was studied experimentally. A linearization of this dependence was obtained, respectively, for frequency and angular operation ranges equal to Deltaf/f(max)=14% and Deltatheta=140.     

Quantum 1/f effect in resonant biochemical piezoelectric and MEMS sensors

Piezoelectric sensors used for the detection of chemical agents and as electronic nose instruments are based on bulk and surface acoustic wave resonators. Adsorption of gas molecules on the surface of the polymer coating is detected by a reduction of the resonance frequency of the quartz disk, subject also to fundamental quantum 1/f frequency fluctuations. The quantum 1/f limit of detection is given by the quantum 1/f formula for quartz resonators. Therefore, for quantum 1/f optimization and for calculation and improvement of the fundamental sensitivity limits, we must avoid closeness of the crystal size to the phonon coherence length, which corresponds to the maximum error and minimal sensitivity situation, as shown here. Adsorbed masses below the pg range can be detected. Microelectromechanical system (MEMS) resonators have provided a possibility for the nanominiaturization of these sensors. Essential for integrated nanotechnology, these resonant silicon bars (fingers) are excited magnetically or electrically through external applied forces, since they are not piezoelectric or magnetostrictive. The application of the quantum 1/f theory to these systems is published here for the first time. It provides simple formulas that yield much lower quantum 1/f frequency fluctuations for magnetic excitation, in comparison with electrostatically driven MEMS resonators.     

Quantum oscillations of the differential resistance of superconductor-two-dimensional electron gas contacts

Oscillations of the differential resistance of a superconductor-two-dimensional electron gas contact were observed at low temperatures. These oscillations reflect the quantum nature of the electric charge transport perpendicular to the two-dimensional electron gas. Pis’ma Zh. Tekh. Fiz. 24, 56–61 (April 26, 1998)     

Quantum oscillations and reversible and irreversible superconducting phase transitions in indium microcylinders

Using a sensitive mutual inductance method, measurements of first- and second-order phase transitions in indium microcylinders (2.8–7.6 µm in diameter) have been made. Quantum oscillations in the supercooling field have been observed. The values of _SC (T/T _c )=₁(T/T _c ) derived fromH _SC =H _C3 have been corrected for both the temperature and the size dependence in the relationship betweenH _c2 andH _c3 . The results for ₁(T/T _c ) from the 7.6-µm sample then agree reasonably well with the temperature dependence of ₁ observed for niobium. However, for the smallest cylinders anomalously low values of _SC (T/T _c ) were obtained nearT _c . Very close to the transition temperature second-order (nonhysteretic) phase transitions have been observed and are interpreted in terms of the Ginzburg-Landau theory.     

Quantum oscillations and ferromagnetic hysteresis observed in iron filled multiwall carbon nanotubes

We report on the electrical transport properties of single multiwall carbon nanotubes with and without an iron filling as a function of temperature and magnetic field. For the iron filled nanotubes the magnetoresistance shows a magnetic behavior induced by iron, which can be explained by taking into account a contribution of s-d hybridization. In particular, ferromagnetic-like hysteresis loops were observed up to 50 K for the iron filled multiwall carbon nanotubes. The magnetoresistance shows quantum interference phenomena such as universal conductance fluctuations and weak localization effects.