Classical percolation fingerprints in the high-temperature regime of the integer quantum Hall effect

We have performed magnetotransport experiments in the high-temperature regime (up to 50 K) of the integer quantum Hall effect for two-dimensional electron gases in semiconducting heterostructures. While the magnetic field dependence of the classical Hall law presents no anomaly at high temperatures, we find a breakdown of the Drude-Lorentz law for the longitudinal conductance beyond a crossover magnetic field B_c ~ 1 T, which turns out to be correlated with the onset of the integer quantum Hall effect at low temperatures. We show that the high magnetic field regime at B > B_c can be understood in terms of classical percolative transport in a smooth disordered potential. From the temperature dependence of the peak longitudinal conductance, we extract scaling exponents which are in good agreement with the theoretically expected values. We also prove that inelastic scattering on phonons is responsible for dissipation in a wide temperature range going from 1 to 50 K at high magnetic fields.Comment: 14 pages + 8 Figure

Magnetic Quantum Oscillations of the Longitudinal Conductivity σzz\sigma_{zz} in Quasi two-dimensional Metals

We derive an analytical expression for the longitudinal magnetoconductivity $\sigma_{zz}$ in layered conductors in presence of a quantizing magnetic field perpendicular to the layers and for short-range in-plane impurity scattering in frame of the quantum transport theory. Our derivation points out quite unusual temperature and magnetic field dependences for Shubnikov-de Haas oscillations in the two-dimensional limit, i.e. $\hbar \omega_{c} \gg 4 \pi t$, where $t$ is the interlayer hopping integral for electrons, and $\omega_{c}$ the cyclotron frequency. In particular, when $\hbar \omega_{c} \gg 4 \pi t$ and $\hbar \omega_{c} \geq 2 \pi \Gamma_{\mu}$ (here $\Gamma_{\mu}$ is the value of the imaginary part of the impurity self-energy at the chemical potential $\mu$), a pseudo-gap centered on integer values of $\mu/\hbar\omega_{c}$ appears in the zero-temperature magnetoconductivity function $\sigma_{zz}(\mu/\hbar\omega_{c})$. At low temperatures, this high-field regime is characterized by a thermally activated behavior of the conductivity minima (when chemical potential $\mu$ lies between Landau levels) in correspondence with the recent observation in the organic conductor $\beta''\text{-(BEDT-TTF)}_{2}\text{SF}_{5}\text{CH}_{2}\text{CF}_{2}\text{SO}_ {3}$.Comment: 16 pages, 4 figures, to be published in Phys. Rev.

Transport of Dirac quasiparticles in graphene: Hall and optical conductivities

The analytical expressions for both diagonal and off-diagonal ac and dc conductivities of graphene placed in an external magnetic field are derived. These conductivities exhibit rather unusual behavior as functions of frequency, chemical potential and applied field which is caused by the fact that the quasiparticle excitations in graphene are Dirac-like. One of the most striking effects observed in graphene is the odd integer quantum Hall effect. We argue that it is caused by the anomalous properties of the Dirac quasiparticles from the lowest Landau level. Other quantities such as Hall angle and Nernst signal also exhibit rather unusual behavior, in particular when there is an excitonic gap in the spectrum of the Dirac quasiparticle excitations.Comment: 25 pages, RevTeX4, 8 EPS figures; final version published in PR

Theory of the Shubnikov-de Haas effect in quasi-two-dimensional metals

The Shubnikov - de Haas effect in quasi-two-dimensional normal metals is studied. The interlayer conductivity is calculated using the Kubo formula. The electron scattering on short-range is considered in the self-consistent Born approximation. The result obtained differs from that derived from the Boltzmann transport equation. This difference is shown to be a general feature of conductivity in magnetic field. A detailed description of the two new qualitative effects -- the field-dependent phase shift of beats and of the slow oscillations of conductivity is provided. The results obtained are applicable to strongly anisotropic organic metals and to other quasi-two-dimensional compounds.Comment: 10 page

Crystal structure, Fermi surface calculations and Shubnikov-de Haas oscillations spectrum of the organic metal θ\theta-(BETS)4_4HgBr4_4(C6_6H5_5Cl) at low temperature

The organic metal \theta$-(BETS)$_4$HgBr$_4$(C$_6$H$_5$Cl) is known to undergo a phase transition as the temperature is lowered down to about 240 K. X-ray data obtained at 200 K indicate a corresponding modification of the crystal structure, the symmetry of which is lowered from quadratic to monoclinic. In addition, two different types of cation layers are observed in the unit cell. The Fermi surface (FS), which can be regarded as a network of compensated electron and hole orbits according to band structure calculations at room temperature, turns to a set of two alternating linear chains of orbits at low temperature. The field and temperature dependence of the Shubnikov-de Haas oscillations spectrum have been studied up to 54 T. Eight frequencies are observed which, in any case, points to a FS much more complex than predicted by band structure calculations at room temperature, even though some of the observed Fourier components might be ascribed to magnetic breakdown or frequency mixing. The obtained spectrum could result from either an interaction between the FS's linked to each of the two cation layers or to an eventual additional phase transition in the temperature range below 200 K.Comment: accepted for publication in Solid State Science

Monotonic growth of interlayer magnetoresistance in strong magnetic field in very anisotropic layered metals

It is shown, that the monotonic part of interlayer electronic conductivity strongly decreases in high magnetic field perpendicular to the conducting layers. We consider only the coherent interlayer tunnelling, and the obtained result strongly contradicts the standard theory. This effect appears in very anisotropic layered quasi-two-dimensional metals, when the interlayer transfer integral is less than the Landau level separation.Comment: 4 pages, no figure

Theory of de Haas-van Alphen Effect in Type-II Superconductors

Theory of quasiparticle spectra and the de Haas-van Alphen (dHvA) oscillation in type-II superconductors are developed based on the Bogoliubov-de Gennes equations for vortex-lattice states. As the pair potential grows through the superconducting transition, each degenerate Landau level in the normal state splits into quasiparticle bands in the magnetic Brillouin zone. This brings Landau-level broadening, which in turn leads to the extra dHvA oscillation damping in the vortex state. We perform extensive numerical calculations for three-dimensional systems with various gap structures. It is thereby shown that (i) this Landau-level broadening is directly connected with the average gap at H=0 along each Fermi-surface orbit perpendicular to the field H; (ii) the extra dHvA oscillation attenuation is caused by the broadening around each extremal orbit. These results imply that the dHvA experiment can be a unique probe to detect band- and/or angle-dependent gap amplitudes. We derive an analytic expression for the extra damping based on the second-order perturbation with respect to the pair potential for the Luttinger-Ward thermodynamic potential. This formula reproduces all our numerical results excellently, and is used to estimate band-specific gap amplitudes from available data on NbSe_2, Nb_3Sn, and YNi_2B_2C. The obtained value for YNi_2B_2C is fairly different from the one through a specific-heat measurement, indicating presence of gap anisotropy in this material. C programs to solve the two-dimensional Bogoliubov-de Gennes equations are available at http://phys.sci.hokudai.ac.jp/~kita/index-e.html .Comment: 16 pages, 11 figure

Tomographic Probability Representation for States of Charge moving in Varying Field

The coherent and Fock states of a charge moving in varying homogeneous magnetic field are studied in the tomographic probability representation of quantum mechanics. The states are expressed in terms of quantum tomograms. The coherent states tomograms are shown to be described by normal distributions with varying dispersions and means. The Fock state tomograms are given in the form of probability distributions described by multivariable Hermite polynomials with time-dependent arguments.Comment: 12 pages, submitted to "Optics and Spectroscopy

Magnetometry of low-dimensional electron and hole systems

Copyright © 2009 Institute of PhysicsThe high-magnetic-field, low-temperature magnetic properties of low-dimensional electron and hole systems reveal a wealth of fundamental information. Quantum oscillations of the thermodynamic equilibrium magnetization yield the total density of states, a central quantity in understanding the quantum Hall effect in 2D systems. The magnetization arising from non-equilibrium circulating currents reveals details, not accessible with traditional measurements, of the vanishingly small longitudinal resistance in the quantum Hall regime. We review how the technique of magnetometry has been applied to these systems, the most important discoveries that have been made, and their theoretical significance

Fetal adverse drug reactions with biologic Disease Modifying Anti-Rheumatic Drugs used during pregnancy: Insights from a World Health Organization pharmacovigilance database study (VigiBase (R))

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