Dependence of Quantized Hall Effect Breakdown Voltage on Magnetic Field and Current

When large currents are passed through a high-quality quantized Hall resistance device the voltage drop along the device is observed to assume discrete, quantized states if the voltage is plotted versus the magnetic field. These quantized dissipative voltage states are interpreted as occurring when electrons are excited to higher Landau levels and then return to the original Landau level. The quantization is found to be, in general, both a function of magnetic field and current. Consequently, it can be more difficult to verify and determine dissipative voltage quantization than previously suspected.     

Irradiated bilayer graphene

We describe the gated bilayer graphene system when it is subjected to intense terahertz frequency electromagnetic radiation. We examine the electron band structure and density of states via exact diagonalization methods within Floquet theory. We find that dynamical states are induced which lead to modification of the band structure. We first examine the situation where there is no external magnetic field. In the unbiased case, dynamical gaps appear in the spectrum which manifest as dips in the density of states. For finite inter-layer bias (where a static gap is present in the band structure of unirradiated bilayer graphene), dynamical states may be induced in the static gap. These states can show a high degree of valley polarization. When the system is placed in a strong magnetic field, the radiation induces coupling between the Landau levels which allows dynamical levels to exist. For strong fields, this means the Landau levels are smeared to form a near-continuum of states.     

Quantized dissipation of the quantum Hall effect at high currents

Quantized dissipative voltage states are observed when large currents are passed through a high-quality quantized Hall resistance device. These dissipative states are interpreted as occurring when electrons are excited to higher Landau levels and then return to the original Landau level. The author shows that the quantization is more complicated than previously thought. For example, the quantization can be a function of magnetic field. Therefore, the dissipative voltage quantization can, in general, be difficult to verify and determine     

Graphene Acoustic Phonon‐Mediated Pseudo‐Landau Levels Tailoring Probed by Scanning Tunneling Spectroscopy

Graphene has attracted great interests in various areas including optoelectronics, spintronics, and nanomechanics due to its unique electronic structure, a linear dispersion with a zero bandgap around the Dirac point. Shifts of Dirac cones in graphene creates pseudo‐magnetic field, which generates an energy gap and brings a zero‐magnetic‐field analogue of the quantum Hall effect. Recent studies have demonstrated that graphene pseudo‐magnetic effects can be generated by vacancy defects, atom adsorption, zigzag or armchair edges, and external strain. Here, a larger than 100 T pseudo‐magnetic field is reported that generated on the step area of graphene; and with the ultrahigh vacuum scanning tunneling microscopy, the observed Landau levels can be effectively tailored by graphene phonons. The zero pseudo‐Landau level is suppressed due to the phonon‐mediated inelastic tunneling, and this is observed by the scanning tunneling spectroscopy spectrum and confirmed by the Vienna ab initio simulation package calculation, where graphene phonons modulate the flow of tunneling electrons and further mediate pseudo‐Landau levels. These observations demonstrate a viable approach for the control of pseudo‐Landau levels, which tailors the electronic structure of graphene, and further ignites applications in graphene valley electronics.     

Coupling of surface and lowest Landau level states in a rectangular graphene dot

A rectangular graphene dot with two zigzag edges and two armchair edges have electronic states in the presence of a magnetic field that are localized on the zigzag edges with non zero values of the wavefunction inside the dot. We have investigated the dependence of these wavefunctions on the size of the dot, and explain the physical origin of them in terms of surface and the lowest Landau level (LLL) states of infinitely long nanoribbons. We find that the armchair edges play a crucial role by coupling the surface and LLL states.     

Graphene nanoribbons in criss-crossed electric and magnetic fields

Graphene nanoribbons (GNRs) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron-transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry E(k)=E(-k) of the sub-band dispersion. When applied together, they reverse the dispersion parity to be odd, which gives E(e,k)=-E(h,-k), and mix the electron and hole sub-bands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range. The broken time-reversal symmetry provides dichroism in the absorption of the circularly polarized light. As a consequence, one can observe electrically enhanced Faraday rotation, since the edges of the ribbon provide formation of the substantial density of states.     

Electronic and optical properties of monolayer and bilayer graphene

The electronic and optical properties of monolayer and bilayer graphene are investigated to verify the effects of interlayer interactions and external magnetic field. Monolayer graphene exhibits linear bands in the low-energy region. Then the interlayer interactions in bilayers change these bands into two pairs of parabolic bands, where the lower pair is slightly overlapped and the occupied states are asymmetric with respect to the unoccupied ones. The characteristics of zero-field electronic structures are directly reflected in the Landau levels. In monolayer and bilayer graphene, these levels can be classified into one and two groups, respectively. With respect to the optical transitions between the Landau levels, bilayer graphene possesses much richer spectral features in comparison with monolayers, such as four kinds of absorption channels and double-peaked absorption lines. The explicit wave functions can further elucidate the frequency-dependent absorption rates and the complex optical selection rules. These numerical calculations would be useful in identifying the optical measurements on graphene layers.     

Localized spin wave states of superfluid3He in a magnetic field gradient

It is shown that in the presence of a magnetic field gradient, spin wave states in superfluid³He split into discrete levels, just as for a charged particle in an electric field near the surface. The energy levels and the intensity of the magnetic absorption due to localized states are determined for the simplest cases in superfluid³He A and B. The localized states will provide a unique way to determine the spin wave velocity and spin diffusion constant in superfluid³He.Work supported by National Science Foundation under Grant No. DMR76-17702.     

Magnetization of a two-dimensional electron gas

The oscillations of magnetization of a two-dimensional free electron gas with field are derived first in the ideal situationT=0 and sharp Landau levels without electron spin and then for finiteT and for broadened levels. Explicit results are obtained in the limiting approximations that the broadening (or kT) is small or large compared with the separation of Landau levels, and the special situations of a field-independent Fermi energy and a field-independent electron density are discussed. The modifications introduced by taking account of electron spin are then considered, and finally the steady magnetic susceptibility superimposed on the oscillations is discussed.     

Mechanism of the photoelectromagnetic effect in two-dimensional systems in quantizing magnetic fields

A new mechanism of the photovoltaic effect in a terahertz frequency range is proposed that is capable of explaining the features of terahertz photocurrents observed previously in asymmetric InAs quantum wells and double GaAs quantum wells in an external tilted magnetic field. It is shown that, in the asymmetric semiconductor heterostructures occurring in a tilted magnetic field, an edge photocurrent is generated due to the absorption of radiation on the electron transitions between Landau levels. A new approach to the creation of highly sensitive terahertz photodetectors based on asymmetric semiconductor heterostructures is proposed.     

Anisotropic Quantum Hall Liquids at Intermediate Magnetic Fields

The observation of significant magnetoresistance anisotropy of a two-dimensional electronic system in very clean GaAs/AlGaAs heterostructure samples in presence of moderately large perpendicular magnetic fields is a striking example of novel anisotropic behavior in the quantum Hall regime. Anisotropy appears to be the strongest at quantum Hall even-denominator filled states for filling factors ν>4 where several Landau levels are occupied. A possible explanation of these findings is due to the existence of charge density waves that are known to cause interesting phase transitions at high Landau levels. An alternative explanation of this phenomenon is to argue that the strongly correlated electronic system has stabilized in an orientationally ordered anisotropic quantum Hall liquid state. Quantum Monte Carlo calculations with a translationally invariant wave function in which rotation symmetry is broken indicate that this might be the case.     

Magnetopolarons on the surface of helium films

The energy spectrum of the surface electrons on liquid helium films in the presence of a perpendicular applied magnetic field is determined in different approaches within second order perturbation theory. The influence of the electron-ripplon interaction on the Landau levels are studied for arbitrary strength of the magnetic field and several values of the coupling constant. Current theories of the polaron ground-state and cyclotron resonance are compared to the exact weak-coupling results obtained. We found a shift down in the cyclotron resonance frequency due to polaron effects.     

Circularly polarized electroluminescence of quantum-size InGaAs/GaAs heterostructures with ferromagnetic metal-GaAs Schottky contacts

Electroluminescence (EL) of InGaAs/GaAs heterostructures with quantum wells and ferromagnetic metal (Co, Ni)/GaAs Schottky contacts has been studied in magnetic fields up to 10 T at a temperature of 1.5 K. The EL line corresponding to the recombination of electrons with injected holes exhibits splitting into components corresponding to the Landau levels in the applied magnetic field and shows circular polarization that significantly exceeds the level typical of such structures with nonmagnetic (Au/GaAs) contacts. The degree of circular polarization (P _EL) exhibits a nonmonotonic dependence on the applied magnetic field and is correlated with the filling of Landau levels. The maximum degree of circular polarization reached in the heterostructures studied is P _EL = 40%.     

The ion density distribution in a magnetized plasma sheath

The ion density distribution of plasma sheath in an oblique magnetic field is investigated with a fluid model. We performed numerical simulations of the sheath. The results reveal that the magnetic field has significant effects on the plasma sheath, including ion density distribution and space charge density distribution. Two cases of ion incidence are considered here. Under suitable conditions, Lorentz force induces fluctuations in the ion density. And the magnetic field parallel to the board is responsible for these changes. The action states of ions are more complicated while the ions enter the sheath with an oblique incidence angle. Ions could gather in some regions, so that it leads to small peaks of the density curve. Also the space charge density in such regions is slightly higher.     

Pressure-Induced Quantum Limit in a Q1D System in High Magnetic Fields

The quasi-one-dimensional (Q1D) materials, (Per)₂M(mnt)₂, have transition temperatures of 12 and 8 K for M = Au and Pt, respectively, as samples are cooled from a metallic state to an insulating, charge density wave (CDW) ground state. In these proceedings, we present results regarding the effect of quasi-hydrostatic pressure and high magnetic fields on the CDW and field-induced density wave (FIDW) states of these materials. With pressures greater than 4 kbar, Shubnikov de Haas oscillations appear in the M = Au compound, which reach the n=1 Landau level before the limit of the applied magnetic field. While lower field (B < 20 T) results are similar for M = Pt, the FIDW state persists in higher fields, even under 5.3 kbar of pressure.     

The intermediate state in superconducting mercury

Using the magneto-optic technique, an extensive study has been made of the intermediate state in single crystals of superconducting mercury. A regular laminar structure, closely approaching that envisaged by Landau, was realized in the thinnest sample examined (47 Μm). The field dependence of the lamina periodicity displays excellent agreement with the Landau model and the deduced surface energy is in good agreement with the Ginzburg-Landau theory. At applied magnetic fields very close to the critical one, the stable magnetic structure consists of isolated superconducting filaments. A number of observations are reported on their growth and stability.     

Possible mechanisms of the external magnetic field influence on the exchange coupling

Possible mechanisms of the external magnetic field influence on exchange coupling in ultrathin M/N/M film structures are discussed. It is shown that the most reasonable mechanism is due to variation of the translational motion of spacer electrons in the external magnetic field. In this case in-plane components of the electron momentum should be quantized according to Landau levels. The most drastic effect should be observed in the case when magnetic field is perpendicular to the film plane. Comparison of calculations according to the proposed model with the experimental data is presented, showing very good correlation for the cases of presence and absence of external magnetic field.     

基于Landau二级相变理论的磁热效应分析

将Landau二级相变理论应用于二级磁相变材料的磁热效应,建立了磁熵变与磁场的直接关系表达的理论模型。以La0.7Sr0.3MnO3为例,在居里温度附近利用该理论模型和麦克斯韦关系式计算了磁熵变ΔSM,并进行了对比。结果表明基于Landau理论的计算结果与利用传统方法的计算结果相符合。而根据Landau平均场理论,二级磁相变材料中居里温度TC和磁熵变ΔSM最大的温度Tpeak不一致,但在居里温度附近ΔSM与磁场的相关性ΔSM=kH^n表达的指数为n=2/3。     

Magnetic Properties of a Multielectron Bubble in Liquid Helium

No Heading Multielectron bubbles in liquid helium contain a spherical two-dimensional shell of electrons at the bubble surface. We investigate the properties of this electron gas when the bubble is placed in a homogeneous magnetic field. Results are reported for the Fermi level and the magnetization of the bubble. At high field, we find the formation of Landau bands rather than Landau levels.PACS numbers: 73.20.Qt; 73.20.–r; 64.70.Dv; 68.35.Ja; 47.55.Dz