Theoretical Calculation of Drag Component of Tensor M in Quantizing Magnetic Field

All the previous homogeneous theories result in M_ΧΧ = 0 which are at variance with the experimental data. It is for this reason, previous theories have consistently used experimental value of M_ΧΧ in calculating thermopower tensor S. The inhomogeneous and non-equilibrium formalism proposed by Dyakin et al. was generalized to including both transverse and longitudinal modes of electron-phonon interaction and considering the broadening of the Landau level. The resulting formula gives theoretical values of M_ΧΧ for GaAs/AlGaAs heterojunctions which are in accord with experimental ,data at oscillation phases but (λ_p/)^l/2 times smaller than experimental data for T = 1.275 K, 2.937 K, 5.005 K and C = 0.20 where λ_p and are the phonon mean free path and effective thickness of the heterojunctions. By means of the semi-empirical formula of MzZ with extra factor (λ_p/)^l/2 in, phonon drag magnetothermopower can be calculated from theoretical values of M_yΧ and M_ΧΧ.     

Thermoelectric Coefficients of Silicon MOSFETS in Quantizing Magnetic Field

The phonon drag and electron diffusion contribution to the tensor M which determines 3 the heat flux U = M·E is calculated for a silicon MOSFETS in a perpendicular magnetic field B. We used nearly the same theoretical formalism as Ref [6], but improvements are made in several respects. First of all the dielectric function of Fermi-Thomas approximation which has been proved to result in overscreening of the interaction is replaced by rigorous Lindhard-type dielectric function to take account of the screening between electrons and phonons. Secondly the contributions of localized electrons are separated from those of the free state electrons which are the only part that contributes to both conductivity tensor and magnetothermopower tensor. The calculated M_yx and S_xx reveal magneto-oscillationsoriginating Gom oscillations in the density of states at the Fermi level. At T = 5.02 K, our new results show that the diffusion components of thermopower are negligibly small compared with those due to phonon drag. All the theoretical values of M_yx, S_xx and S_yx are in accordance with the experimental data better than previous theoretical results.     

Quenching of the quantum Hall effect in multilayered epitaxial graphene: the role of undoped planes

We propose a mechanism for the quenching of the Shubnikov-de Haas oscillations and the quantum Hall effect observed in epitaxial graphene. Experimental data show that the scattering time of the conduction electron is magnetic field dependent and of the order of the cyclotron orbit period, i.e., it can be much smaller than the zero field scattering time. Our scenario involves the extraordinary graphene n=0 Landau level of the uncharged layers which is pinned at the Fermi level. We find that the coupling between this n=0 Landau level and the conducting states of the doped plane leads to a scattering mechanism having the right magnitude to explain the experimental data.     

Real-space observation of drift States in a two-dimensional electron system at high magnetic fields

The local density of states of the adsorbate-induced two-dimensional electron system is studied in magnetic fields up to B=6 T. Landau quantization is observed and drift states with a width of about the magnetic length are found in agreement with theoretical predictions. At the tails of the Landau levels the states form closed paths indicating localization. These states show the expected energy dependence. A multifractal analysis applied to the data results in a nice parabolic shape of the characteristic f(alpha) spectra, but we find only a slight displacement of the origin from alpha=2.0 for the states in the center of the Landau level.     

Low-lying exciton levels in a strong magnetic field

The behavior of the n = 2 and n = 3 exciton levels in a strong magnetic field is analyzed for the case in which the spacing between the Coulomb levels is much greater than that between Landau levels. The results are compared with experimental data on magnetooptical absorption in Cu₂O. The observed splitting of these levels can be attributed to the appearance of a Landau series above the Coulomb levels.Translated from Izvestiya VUZ. Fizika, No. 2, pp. 7–11, February, 1970.     

Quantum-Hall activation gaps in graphene

We have measured the quantum-Hall activation gaps in graphene at filling factors nu=2 and nu=6 for magnetic fields up to 32 T and temperatures from 4 to 300 K. The nu=6 gap can be described by thermal excitation to broadened Landau levels with a width of 400 K. In contrast, the gap measured at nu=2 is strongly temperature and field dependent and approaches the expected value for sharp Landau levels for fields B>20 T and temperatures T>100 K. We explain this surprising behavior by a narrowing of the lowest Landau level.     

Magneto-optical studies of GaAs/AlAs quantum wells

We present a magnetoreflectivity study of three GaAs/AlAs multiple quantum wells with widths 75, 100 and 150 Å. At T = 5K, the reflectivity spectra exhibit features associated with the excitons as well as interband Landau transitions. The slopes of these transitions imply that the electrons are confined in the GaAs layers. In addition, experimental values for the exciton binding energies are determined from the zero field intercepts of the Landau transitions.     

Phonon dispersion in Xe

The predictions of the Mie-Lennard-Jones potentials for phonon dispersion in solid Xe at 10°K and at 77°K are compared with the results of recent measurements. The dynamics of the crystal are described by the first order self-consistent phonon theory for the imaginary part of the one phonon Green's function. The fit obtained is quite poor though we point out that no conclusion can be drawn about the size of three-body forces. We conclude that improved potentials are needed to account for the experimental data.     

Phonon focusing in [001] germanium

Phonon focusing in [001]-oriented Ge crystals has been studied at temperatures near 2.0K using electron-beam scanning for phonon generation and thin-film bolometers with an effective area of 10 m×10 m or smaller for phonon detection. The thickness of the Ge crystals was 2 or 3 mm. In our experiments the angular resolution appeared to be dominated by effective diameter of the local region heated by the beam and acting as phonon source. Whereas metallic overlay films did not appreciably effect the angular resolution, a distinct increase in resolution due to high-frequency beam modulation has been observed which can be understood in terms of the thermal skin effect. From a comparison between theoretical calculations and our experimental results the effective diameter of the phonon source has been found to be 20–30 m. Our focusing images show appreciable deviations from theoretical calculations in the low-frequency limit. However, excellent agreement is obtained, if dispersion is included and if the dominant phonon frequencies are place in the range 300–400 GHz.     

Evolution of the 7/2 fractional quantum Hall state in two-subband systems

We report the evolution of the fractional quantum Hall state (FQHS) at a total Landau level (LL) filling factor of ν=7/2 in wide GaAs quantum wells in which electrons occupy two electric subbands. The data reveal subtle and distinct evolutions as a function of density, magnetic field tilt angle, or symmetry of the charge distribution. At intermediate tilt angles, for example, we observe a strengthening of the ν=7/2 FQHS. Moreover, in a well with asymmetric change distribution, there is a developing FQHS when the LL filling factor of the symmetric subband ν(S) equals 5/2 while the antisymmetric subband has a filling factor of 1<ν(A)<2.     

Coherent Acoustic Phonon and Its Chirping in Dirac Semimetal Cd_3As_2

Ultrafast optical spectroscopy of a single crystal of a Dirac semimetal Cd_3As_(2 )is carried out.An acoustic phonon(AP)mode with central frequency f=0.037 THz(i.e.,1.23 cm~(-1)or 0.153 meV)is unambiguously generated and detected,which we attribute to laser-induced thermal strain.An AP chirping(i.e.,variation of the phonon frequency)is clearly detected,which is ascribed to heat capacity variation with time.By comparing our experimental results and the theoretical model,we obtain a chirping time constant,which is 31.2 ps at 6 K and 19.8 ps at 300 K,respectively.Significantly,we identify an asymmetry in the AP frequency domain peak and find that it is caused by the chirping,instead of a Fano resonance.Moreover,we experimentally demonstrate that the central frequency of AP is extremely stable with varying laser fluence,as well as temperature,which endows Cd_3As_2application potentials in thermoelectric devices.     

ASYMMETRY PARAMETERS AND DIFFERENTIAL CROSSSECTIONS FOR PHOTOIONIZATION-EXCITATION OF HELIUM

Asymmetry parameterβ2 for photoionization of helium leaving the He+ ion in the n = 2 level and differential cross sections(DCSs) in the n = 2, 3, 4 levels with photon emission angle θ=90°are provided at photon energies 69－76.8 eV employing the R-matrix method with a 20-term target representation, in which five polarization orbitals 6l are included. The asymmetry parameter β2 and the DCS, in the n=2 level in the region of 69－73eV, are in good agreement with available experimental and theoretical results. Above the n = 3 threshold, we present new theoretical results along with measurements. No theoretical and experimental results can be used to compare with the present calculations of the DCSs for photoionization into the He+ ion in the n = 3, 4 levels.     

Experimental-theoretical Landau energy level data and its use in making p-Ge laser transition assignments

Landau energy levels are determined from cyclotron resonance absorption data and compared to theoretical calculations. Energy level diagrams are presented to graphically display the nonequal energy level spacings and identify absorption transitions. This information is then used to make transition assignments responsible for the p-Ge laser signals seen by V. N. Shastin et al. and C. Kremser et al. This work suggests that the p-Ge laser yields signals via the following transitions: (a) the expected cyclotron transitions for n=1, (b) harmonic cyclotron transitions for n=2, 3, (c) light-to-heavy hole transitions and (d) light hole to acceptor level transitions. The material of this paper should be of aid in the study of the pumping-oscillation cycle of the p-Ge laser.     

Landau damping of collective mode in a quasi-two-dimensional repulsive Bose-Einstein condensate

We investigate the Landau damping of the collective mode in a quasi-two-dimension repulsive Bose-Einstein condensate by using the self-consistent time-dependent Hatree-Fock-Bogoliubov approximation and a complete and orthogonal eigenfunction set for the elementary excitation of the system.We calculate the three-mode coupling matrix element between the collective mode and the thermal excited quasi-particles and the Landau damping rate of the collective mode.We discuss the dependence of the Landau damping on temperature,on atom number in the condensate,on transverse trapping frequency and on the length of the condensate.The energy width of the collective mode is taken into account in our calculation.With little approximation,our theoretic calculation results agree well with the experimental ones and are helpful for deducing the damping mechanics and the inter-particle interaction.     

Phonon anomaly in high-pressure Zn

The equation of states and phonon dispersions of hexagonal zinc have been calculated by the plane-wave pseudopotential method within the generalized-gradient approximation. Weak discontinuities are found in the pressure-volume relation as well as the c/a-volume curve. Phonon dispersions of Zn under pressure have been obtained with a direct method and the results are consistent with the neutron scattering data. At V/V0 approximately 0.88, the calculated frequencies of the acoustic phonons near the zone center softened substantially as a result of an electronic topological transition. The theoretical result is consistent with the observed anomaly in the Lam-Mossbauer factor at low temperature.     

Phonon eigenvectors in Si determined by inelastic neutron scattering

We have determined the eigenvectors of longitudinal phonons with wavevectors in the direction in Si at 12 K from inelastic neutron scattering intensities. The eigenvectors obtained from different model and quantum-mechanical calculations are at variance. Comparison of experimental and theoretical results shows that of the various theoretical predictions the ones from the bond-charge model are in best agreement with experiment. Internal-strain constants from these models and from experiment are compared.     

Fractional quantum Hall States of Dirac electrons in graphene

We have investigated the fractional quantum Hall states of Dirac electrons in a graphene layer in different Landau levels. The relativistic nature of the energy dispersion relation of electrons in graphene significantly modifies the interelectron interactions. This results in a specific dependence of the ground state energy and the energy gaps for electrons on the Landau-level index. For the valley-polarized states, i.e., at nu=1/m, m being an odd integer, the energy gaps have the largest values in the n=1 Landau level. For the valley-unpolarized states, e.g., for the 2/3 state, the energy gaps are suppressed for n=1 as compared to those at n=0. For both n=1 and n=0, the ground state of the 2/3 system is fully valley-unpolarized.     

Coulomb drag as a probe of the nature of compressible States in a magnetic field

Magnetodrag reveals the nature of compressible states and the underlying interplay of disorder and interactions. At nu=3/2 clear T(4/3) dependence is observed, which signifies the metallic nature of the N=0 Landau level. In contrast, drag in higher Landau levels reveals an additional contribution, which anomalously grows with decreasing T before turning to zero following a thermal activation law. The anomalous drag is discussed in terms of electron-hole asymmetry arising from disorder and localization, and the crossover to normal drag at high fields as due to screening of disorder.     

Microscopic calculation of the magnetic field of neutron stars

Based on the Dirac equation describing an electron moving in a uniform and cylindrically symmetric magnetic field which may be the result of the self-consistent mean field of the electrons themselves in a neutron star, we have obtained the eigen solutions and the orbital magnetic moments of electrons in which each eigen orbital can be calculated. From the eigen energy spectrum we find that the lowest energy level is the highly degenerate orbitals with the quantum numbers pZ=0, n=0, and m≥0. At the ground state, the electrons fill the lowest eigen states to form many Landau magnetic cells and each cell is a circular disk with the radius λfree and the thickness λe, where λfree is the electron mean free path determined by Coulomb cross section and electron density and λe is the electron Compton wavelength. The magnetic moment of each cell and the number of cells in the neutron star are calculated, from which the total magnetic moment and magnetic field of the neutron star can be calculated. The results are compared with the observational data and the agreement is reasonable.