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Circularly polarized laser pulses that excite electron–hole pairs across the band gap of (III,Mn)V ferromagnetic semiconductors can be used to manipulate the magnetization dynamics. The spin of the optically injected photocarriers is transferred to the collective magnetization, acting as a non-conservative driving force which can either deliver or extract energy from the collective magnetic degree of freedom.

There is a surprising lack of symmetry in Maxwell's equations of electromagnetism. Electric fields are created by electric charges, while magnetic fields are produced by the movement of electric charges. Magnetic charges are completely absent from the theory, not because Maxwell forgot to put them in, but because isolated magnetic charges do not seem to exist in the real world.

We report on a microscopic theory of the Skyrmion states which occur in the quantum Hall regime. The theory is based on the identification of Skyrmion states in this system with zero-energy eigenstates of a hard-core model Hamiltonian. We find that for $N_{\phi}$ orbital states in a Landau level, a set of Skyrmions states with orbital degeneracy $N_{\phi}-K$ and spin quantum number $S = N/2 -K$ exists for each nonnegative integer $K$. The energetic ordering of states with different $K$ depends on the interaction potential., Comment: 4 pages, uuencoded LaTeX and eps files

We investigate transresistance effects in electron-hole double layer systems with an excitonic condensate. Our theory is based on the use of a minimum dissipation premise to fix the current carried by the condensate. We find that the drag resistance jumps discontinuously at the condensation temperature and diverges as the temperature approaches zero., 12 pages, 1 Figure, .eps file attached

The ground state of a two-dimensional electron gas at Landau level filling factors near $\nu =1$ is a Skyrme crystal with long range order in the positions and orientations of the topologically and electrically charged elementary excitations of the $\nu=1$ ferromagnetic ground state. The lowest energy Skyrme crystal is a square lattice with opposing postures for topological excitations on opposite sublattices. The filling factor dependence of the electron spin-polarization, calculated for the square lattice Skyrme crystal, is in excellent agreement with recent experiments., Comment: 3 pages, latex, 3 figures available upon request from brey@marlene.fmc.uam.es

The two-dimensional interacting electron gas at Landau level filling factor $\nu =1$ and temperature $T=0$ is a strong ferromagnet; all spins are completely aligned by arbitrarily weak Zeeman coupling. We report on a theoretical study of its thermodynamic properties using a many-body perturbation theory approach and concentrating on the recently measured temperature dependence of the spin magnetization. We discuss the interplay of collective and single-particle aspects of the physics and the opportunities for progress in our understanding of itinerant electron ferromagnetism presented by quantum Hall ferromagnets., Comment: REVTex, 10 pages, 3 uuencoded, compressed and tarred PostScript figures appended

We report on a Hartree-Fock approximation study of the meron pseudospin-texture excitations of the broken-symmetry incompressible ground states of double-layer quantum Hall states at $\nu =1$. We have obtained results for meron core energies and core sizes. We use these to estimate the charge gap which determines the activation energy for dissipation in the quantum Hall effect., Comment: 12 pages, RevTex 3.0, 2 figures available upon request

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We report on a study of the thermodynamics of the Ginzburg-Landau model for two-dimensional type-II superconductors near the transition from the normal state to the Abrikosov-lattice state. We couch our analysis in terms of the participation-ratio entropy, $s(P)$, which expresses the volume in order-parameter-space with a given participation-ratio for the local superfluid density. $s(P)$ completely determines the thermodynamics of the system. We report on results for $s(P)$ obtained analytically using perturbation expansion methods and numerically using Monte Carlo simulation methods and discuss the weak first-order phase transition which occurs in this system in terms of the properties of $s(P)$., 12 pages (RevTeX 3.0) and 2 figures, available upon request, IUCM94-016

At strong magnetic fields double-layer two-dimensional-electron-gas systems can form an unusual broken symmetry state with spontaneous inter-layer phase coherence. In this paper we explore the rich variety of quantum and finite-temperature phase transitions associated with this broken symmetry. We describe the system using a pseudospin language in which the layer degree-of-freedom is mapped to a fictional spin 1/2 degree-of-freedom. With this mapping the spontaneous symmetry breaking is equivalent to that of a spin 1/2 easy-plane ferromagnet. In this language spin-textures can carry a charge. In particular, vortices carry e/2 electrical charge and vortex-antivortex pairs can be neutral or carry charge e. We derive an effective low-energy action and use it to discuss the charged and collective neutral excitations of the system. We have obtained the parameters of the Landau-Ginzburg functional from first-principles estimates and from finite-size exact diagonalization studies. We use these results to estimate the dependence of the critical temperature for the Kosterlitz-Thouless phase transition on layer separation., 56 pages, 19 figures available upon request at klemme@gibbs.physics.indiana.edu. RevTex 3.0. IUCM94-009

Double layer quantum Hall systems have interesting properties associated with interlayer correlations. At $\nu =1/m$ where $m$ is an odd integer they exhibit spontaneous symmetry breaking equivalent to that of spin $1/2$ easy-plane ferromagnets, with the layer degree of freedom playing the role of spin. We explore the rich variety of quantum and finite temperature phase transitions in these systems. In particular, we show that a magnetic field oriented parallel to the layers induces a highly collective commensurate-incommensurate phase transition in the magnetic order., Comment: 4 pages, REVTEX 3.0, IUCM93-013, 1 FIGURE, hardcopy available from: dklemme@indiana.edu

We study the collective excitation spectra of double-layer quantum-Hall systems using the single mode approximation. The double-layer in-phase density excitations are similar to those of a single-layer system. For out-of-phase density excitations, however, both inter-Landau-level and intra-Landau-level double-layer modes have finite dipole oscillator strengths. The oscillator strengths at long wavelengths for the latter transitions are shifted upward by interactions by identical amounts proportional to the interlayer Coulomb coupling. The intra-Landau-level out-of-phase mode has a gap when the ground state is incompressible except in the presence of spontaneous inter-layer coherence. We compare our results with predictions based on the Chern-Simons-Landau-Ginzburg theory for double-layer quantum Hall systems., Comment: RevTeX, 21 pages

In strong perpendicular magnetic fields double-quantum-well systems can sometimes occur in unusual broken symmetry states which have interwell phase coherence in the absence of interwell hopping. When hopping is present in such systems and the magnetic field is tilted away from the normal to the quantum well planes, a related soliton-lattice state can occur which has kinks in the dependence of the relative phase between electrons in opposite layers on the coordinate perpendicular to the in-plane component of the magnetic field. In this article we evaluate the collective modes of this soliton-lattice state in the generalized random-phase aproximation. We find that, in addition to the Goldstone modes associated with the broken translational symmetry of the soliton-lattice state, higher energy collective modes occur which are closely related to the Goldstone modes present in the spontaneously phase-coherent state. We study the evolution of these collective modes as a function of the strength of the in-plane magnetic field and comment on the possibility of using the in-plane field to generate a finite wave probe of the spontaneously phase-coherent state., Comment: REVTEX, 37 pages (text) and 15 uuencoded postscript figures

Chirally stacked neutral N-layer graphene systems with N ≥ 2 exhibit spontaneous inversion symmetry breaking and interaction induced charge gaps due to interlayer exchange interactions. A variety of distinct broken symmetry states are distinguished by their charge, spin, and valley Hall conductivities, by their orbital magnetizations, and by their edge state properties. In the spinless case, quantum anomalous and valley Hall states are favored by a weak magnetic field and by an electric field between the graphene layers, respectively. At interfaces between different phases one dimensional gapless modes exhibit novel Luttinger liquid behaviors.

We report on a high temperature perturbation expansion study of the superfluid-density spatial correlation function of a Ginzburg-Landau-model superconducting film in a magnetic field. We have derived a closed form which expresses the contribution to the correlation function from each graph of the perturbation theory in terms of the number of Euler paths around appropriate subgraphs. We have enumerated all graphs appearing out to 10-th order in the expansion and have evaluated their contributions to the correlation function. Low temperature correlation functions, obtained using Pad\'{e} approximants, are in good agreement with Monte Carlo simulation results and show that the vortex-liquid becomes strongly correlated at temperatures well above the vortex solidification temperature., Comment: 18 pages (RevTeX 3.0) and 4 figures, available upon request, IUCM93-016

We consider the magnetic field dependence of the chemical potential for parabolically confined quantum dots in a strong magnetic field. Approximate expressions based on the notion that the size of a dot is determined by a competition between confinement and interaction energies are shown to be consistent with exact diagonalization studies for small quantum dots. Fine structure is present in the magnetic field dependence which cannot be explained without a full many-body description and is associated with ground-state level crossings as a function of confinement strength or Zeeman interaction strength. Some of this fine structure is associated with precursors of the bulk incompressible states responsible for the fractional quantum Hall effect., Comment: 11 pages, 3 figures (available from yang@nrcphy1.phy.nrc.ca). Revtex 3.0. (IUCM93-010)

The longitudinal and Hall conductivities and resistivities are compared for two-dimensional (2D) square arrays of quantum dots and antidots in 2D electronic systems (ES's). The effects on the magnetotransport from the bound and coherent collimated states are explained. The energy eigenvalue spectra are obtained as a function of the magnetic field. The wave-vector dispersion of the energy eigenvalues is also calculated for the two systems. For antidots, there is a threshold magnetic field above which the system behaves like a homogeneous 2D ES. This threshold and the quenching of the Hall effect for antidots and the insulating behavior and Aharonov-Bohm quantum oscillations in a quantum-dot array are the main findings in this paper.

Measurements of frictional drag between adjacent electron gases in double quantum wells provide strong evidence for phonon-mediated electron-electron interactions. These interactions, which dominate the contribution from simple Coulomb scattering at layer spacings larger than a few hundred Å, are between 20 and 100 times stronger than expected for sequential emission and absorption of real phonons. The observed range of the interactions, substantially smaller than the acoustic phonon mean free path, points to a novel electron scattering mechanism involving virtual-phonon exchange.

We derive and evaluate expressions for the frictional Coulomb drag between disordered two-dimensional electron gas layers. Our derivation is based on the memory-function formalism and the expression for the drag reduces to previously known results in the ballistic limit. We find that Coulomb drag is appreciably enhanced by disorder at low temperatures when the mean-free-path within a layer is comparable to or shorter than the layer separation. In high mobility two-dimensional electron gas systems, where the drag has been studied experimentally, the effect of disorder on the drag is negligible at attainable temperatures. We predict that an enhancement due to disorder and a crossover in the temperature-dependence and layer-separation dependence will be observable at low temperatures in moderate and low mobility samples., 17 pages, revtex, iucm93-004

In this article we discuss the ground state of a parabolically confined quantum dots in the limit of very strong magnetic fields where the electron system is completely spin-polarized and all electrons are in the lowest Landau level. Without electron-electron interactions the ground state is a single Slater determinant corresponding to a droplet centered on the minimum of the confinement potential and occupying the minimum area allowed by the Pauli exclusion principle. Electron-electron interactions favor droplets of larger area. We derive exact criteria for the stability of the maximum density droplet against edge excitations and against the introduction of holes in the interior of the droplet. The possibility of obtaining exact results in the strong magnetic field is related to important simplifications associated with broken time-reversal symmetry in a strong magnetic field., Comment: 17 pages, 5 figures (not included), RevTeX 3.0. (UCF-CM-93-002)

This paper reviews our study of the interaction of phonons with two-dimensional electron gases (2DEG) using a direct probe of electron-electron (e-e) scattering rates. We find evidence for an e-e scattering process whose temperature and distance dependence strongly suggest a phonon-mediated interaction. The magnitude of the scattering rates, however, are much greater than can be accounted for by real phonon exchange. Instead, we argue that we have directly detected scattering due to the exchange of virtual phonons.

A technique for injecting electrons into the surface layers of materials has now been applied to the most mysterious of superconducting compounds — the copper oxides.

We discuss the interplay of disorder and interactions in determining the spectrum of inter-Landau-level excitations of a two-dimensional electron gas in a strong magnetic field. Our work is based in part on some new exact sum rules for the inter-Landau-level response functions of noninteracting electrons in a strong magnetic field and in part on perturbative approximations for interaction and disorder. We emphasize the importance of including vertex corrections in the response functions which are consistent with the approximation that has been used for the self-energy. In the absence of disorder or in the absence of interactions our perturbative approximations reduce to those used by earlier workers for those limits.

We present model calculations of far-infrared absorption by the combined system of a grating coupler plus a laterally modulated two-dimensional electron gas. The response of the electron gas to the optical fields has been determined in a companion paper and the grating is described as a planar sheet with alternating strips of different conductivity. By varying parameters for the strength and period of the modulation the fractional change of transmission through the system is examined in different response regimes. We find that a single peak can dominate the spectra in certain limits although the physical origin of this peak varies

We have evaluated within the random-phase approximation the density-response function of laterally modulated two-dimensional electron systems, examining both single-particle and collective behavior. By varying model parameters we can produce crossovers between different response characteristics. When the lateral period is long compared to the Fermi wavelength, the excitation spectrum evolves as the modulation strength is increased from a regime of plasmon bands with small gaps to a regime of geometrically confined plasmons

We report on a study of the electronic structure of two parallel two-dimensional layer systems in tilted magnetic fields. Magnetic oscillations are studied by calculating the magnetization of the system as a function of field strength and tilt angle. The calculation is fully quantum mechanical and comparisons with semiclassical approximations are discussed. We find that a crossover occurs as a function of field strength between a region at weak perpendicular fields where semiclassical approximations are accurate and a region at stronger perpendicular fields where the probability of magnetic breakdown becomes large and semiclassical approximations fail

We evaluate the sum of particle-particle ladder diagrams for a two-dimensional (2D) electron gas in the presence of an external magnetic field. The evaluation depends on the observation that the quantized center-of-mass kinetic energy is conserved in electron-electron scattering events. For a delta-function interaction scattering channels with different center-of-mass kinetic energy separate and the integral equation reduces to an algebraic equation which can be solved explicitly. Divergences associated with superconducting instabilities are identified. In the weak-field limit a connection is established between the different scattering channels and different solutions of the linearized Ginzburg-Landau equations.

The single-particle self-energy in a Landau quantized two-dimensional electron gas has been evaluated in magnetic fields corresponding to arbitrarily large integer filling factors using the random-phase approximation. Quasiparticle energy separations have been determined for Landau levels near the Fermi level and comparisons are made with values determined from recent activation energy measurements on integral quantum Hall plateaus. Conclusions are drawn concerning the field strengths required for the validity of commonly adopted strong-field approximations. A series of transitions characterized by jumps in the spin polarization is predicted to occur near odd filling factors for sufficiently low densities.

The competition between liquid and solid states of strongly correlated electron systems occurs in a novel way in a strong magnetic field. The occurrence of the incompressible states responsible for the fractional quantum Hall effect results in causes the liquid state to be reentrant with increasing magnetic field. A number of experimental anomolies have been observed near the Landau level filling factors where the transition is expected. We discuss the possibility of using dynamic probes of the electron system to provide conclusive evidence that these anomolies are associated with crossing the liquid-solid phase boundary.

We show that a quantum dot in the fractional Hall regime exhibits mesoscopic magnetic oscillations with a period which is a multiple of the period for free electrons. Our calculations are performed for parabolic quantum dots with hard-core electron-electron interactions and are exact in the strong field limit for $k_B T$ smaller than the fractional Hall gap. Explicit expressions are given for the temperature dependence of the amplitude of the oscillations., Comment: 11 pages, IUCM-004, plain tex

The influence of a magnetic field on superconductivity is usually described either phenomenologically, using Ginzburg-Landau theory, or semiclassically using Gor'kov theory. In this article we discuss the influence of magnetic fields on the mean-field theory of the superconducting instability from a completely quantum mechanical point of view. The suppression of superconductivity by an external magnetic field is seen in this more physically direct picture to be due to the impossibility, in quantum mechanics, of precisely specifying both the center-of-mass state of a pair and the individual electron kinetic energies. We also discuss the possibility of novel aspects of superconductivity at extremely strong magnetic fields where recent work has shown that the transition temperature may be enhanced rather than suppressed by a magnetic field and where a quantum treatment is essential., Comment: 14 pages, 4 figures upon request, revtex version 3,IUCM92-003

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The dispersion relation for magnetoplasmons propagating perpendicular to the magnetic field is investigated for a quasi-three-dimensional electron system. The magnetic field is in the plane of a \ensuremath{\simeq}100-nm-wide electron slab which is confined by a wide ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As parabolic quantum well. A minimum is found in the magnetoplasma dispersion relation indicating the existence of a magnetoroton excitation for wave vector q about twice the inverse magnetic length. Calculations of the dispersion relation within the single-mode approximation are in good qualitative agreement with the measurements.

At strong magnetic fields, Landau quantization invalidates the semiclassical approximations which underly the Ginzburg-Landau (GL) theory of the mixed states of type-II superconductors. We have solved the {ital microscopic} mean-field equations for the case of a two-dimensional electron system in the strong magnetic-field limit. For delta-function attractive interactions there exist {ital n}+1 pairing channels in the {ital n}th Landau level. For {ital n}{gt}0, two channels share the maximum {ital T}{sub {ital c}}, and the order parameter differs markedly from expectations based on GL theory.

We present numerical evidence in support of the view that the physical origin of the fractional quantum Hall effect at \ensuremath{\nu}=n/(2n+1) is associated with the fact that this is the largest filling factor at which an n-component system of electrons can avoid having any pair of electrons occur in a state of relative angular momentum 1. Our calculations, for n=2, are based on a model in which the Hilbert space is truncated to the two lowest Landau levels and the Landau-level separation, \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$, is a parameter of the model. For \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$=0 and short-range repulsive interactions, a zero-energy incompressible ground state occurs at \ensuremath{\nu}=2/5. Our numerical results suggest that, in agreement with arguments advanced by J. K. Jain, this incompressibility survives to the strong-field limit \ensuremath{\Elzxh}${\mathrm{\ensuremath{\omega}}}_{\mathit{c}}$=\ensuremath{\infty}.

The spin-polarization of nuclei near a two-dimensional electron gas (2D EG) may be relaxed by spin-flip excitations of the electron system. The spectrum of low-energy electronic spin-flip excitations depends on the disorder broadening of Landau levels and on the interaction enhancement of the Landau-level spin splitting. Disorder vertex corrections to the spin-flip response function capture the fact that the nuclear relaxation rate depends on local rather than thermodynamic Landau-level broadening, while interaction vertex corrections can strongly enhance the relaxation rate. We illustrate these effects by summing the disorder and interaction ladder diagrams for the spin-flip response function in the strong-magnetic-field limit. Our approach is also able to describe the effect of disorder on the spin-wave collective modes of a spin-polarized 2D EG.

The quantum Hall effect [1] occurs in two-dimensional electronic systems (2 DES) [2] in strong perpendicular magnetic fields. It differs from many of the phenomena which are being discussed at this ASI in that it does not depend directly on quantum coherence for its occurrence. Quantum coherence plays an important supporting role rather that a starring role in this drama. At low temperatures and strong magnetic fields the magnetoresistance of two-dimensional (2D) metals becomes extremely small and the Hall resistance nearly constant over certain ranges of carrier density and magnetic field strength. Although this effect seemed quite mysterious at the time of its discovery, the principle elements of the physics which is responsible are now understood. In these notes I hope to explain these elements and to point out some aspects of our current theoretical picture of the phenomenon which seem to be incomplete.

The properties of superconductors can to a certain extent be understood within a framework of band theory, similar to the behavior of electrons in solids. However, as Atkinson and McDonald explain on page 57, the response of superconductors to disorder can differ widely from that expected from band theory. Conventional superconductors do not form bound states at defects, whereas unconventional superconductors do. Hudson et al. (page 88) have imaged resonances as a result of such bound states in such unconventional superconductors. Such direct imaging studies should improve our understanding of fundamental mechanisms underlying high-temperature superconductivity.

Electrons in a Fermi liquid can be regarded as non-interacting particles (quasiparticles) in an effective potential that is a consequence of interactions between electrons. In many-body theory the effective potential is known as the electronic self-energy. When the electronic system has a broken symmetry, the effective potential can have different symmetries than the potential term that appears in the single particle Hamiltonian. For example, the effective potential of a superconductor includes terms that change the electron particle number and the effective potential of a ferromagnet includes terms that change the electron spin. When the electron system is not in equilibrium, the effective potential is altered. I will present a view of spin-transfer phenomena in ferromagnetic metals in which current-induced spin torques arise from changes in the ferromagnet's spin-dependent effective potential when the quasiparticle system is held out of equilibrium by applying a bias potential. In this view spin-transfer is an example of a more general set of phenomena and is not fundamentally associated with an approximate conservation law, for example the approximate conservation of total spin angular momentum. I will illustrate this point of view by discussing other examples of spin-transfer like phenomena, including transport anomalies that have been observed in bilayer quantum Hall systems, current-driven magnetization changes in antiferromagnetic metals, and current-drive bond-weakening in molecular electronics. Note from Publisher: This article contains the abstract only.

According to the Kubo formula, the static uniform electric fields in a metal or semiconductor can induce coherence between band states far from the Fermi energy. This interband coherence response appears to be at odds with the normal view that transport is a Fermi-energy property, and is relatively unfamiliar since it makes a negligibly small contribution to the most commonly studied transport coefficients such as the longitudinal conductivity. It has recently been argued that interband coherence response can dominate the anomalous Hall conductivity of ferromagnetic metals and semiconductors and the spin-Hall conductivity of paramagnetic materials. I will review recent theoretical ideas related to the charge Hall conductivity of ferromagnetic materials and the spin Hall conductivity of paramagnetic materials and their relation to recent and future experiments. Note from Publisher: This article contains the abstract only.

Les niveaux donneurs et accepteurs de ces magnetoplasmas bidimensionnels a dopage δ sont etudies dans la limite du champ magnetique fort. On traite de facon auto-coherente la diffusion due au desordre et aux impuretes, en incluant les effets d'ecrantages non lineaires. Ces resultats peuvent apporter une interpretation a de nouvelles caracteristiques observees par magnetospectroscopie des impuretes peu profondes dans un gaz d'electrons bidimensionnel

An anyon is a particle in 2+1 dimensions obeying fractional statistics' and may be viewed as a hard-core boson (or a fermion) to which a flux tube is attached. The statistical interaction is highly nontrivial and it has so far resisted full solution. Some progress can be made using a mean-field theory of statistics in which the sta- tistical flux is replaced by a uniform magnetic field 8,&. This idea was first put to practical use in the theory of the fractional quantum Hall effect (FQHE), where fermions in an external field 8,", can be mapped onto bo- sons in zero mean field (B,tt+8", =0). The ordering associated with the FQHE gap is revealed in the form of algebraic of-diagonal long-range order (ODLRO) for the composite particles (bosons plus flux tubes). More recently this idea has been greatly expanded and extend- ed by Laughlin ' to the case of semions (particles with statistical angle 8/tr = 1/2) for which the mean-field theory yields fermions filling the two lowest Landau lev- els. Laughlin pointed out the remarkable fact that pairs of semions can condense to form a Bose superfluid. The mean-field approximation has several peculiari- ties ' since the purely quantum Aharonov-Bohm phase is replaced by a classical Lorentz force and the particles see a preferred (quantum) length scale I— = (Ac/eB, ff) ' This turns out not to be a problem in the FQHE since the incompressible Laughlin state has a gap and its den- sity is pinned at precisely this same length scale due to the (physical) external field. However, mean-field theory for semions incorrectly suggests the existence of an excitation gap analogous to that in the integer Hall effect. Augmenting the mean-field theory with fluctua- tions at the RPA level'' restores the linearly dispersing density mode expected in a superfluid. A local increase in density is accompanied by a compensating increase in the local B,g such that the lowest two Landau levels can remain filled and there is no preferred length scale. ' The purpose of this paper is to present an exactly solu- ble model of anyons which sheds considerable light on all these points and which totally bypasses difficulties in- herent in the mean-field solutions. The model differs in one (highly) nontrivial way from the usual one in that the particles have an attractive hard-core interaction. (The effect of the hard-core repulsion that is usually con- sidered is to prevent intersection of the boson world lines and hence make the homotopy class of their braiding well defined. ') The present model was inspired by recent applications of supersymmetry and the Atiyah-Singer in- dex theorem to particles in an arbitrary magnetic field. " We will prove below that the index theorem can be ex- tended to the nontrivial many-body case where the flux is not axed in time but is carried on the particles them selves; i.e., for anyons. Consider the pair of Hamiltoni- ans .'V

We show that a two-dimensional electron-hole fluid in a strong perpendicular magnetic field has a quantized Hall conductance equal to ${\mathit{e}}^{2}$${\ensuremath{\nu}}_{\mathit{c}}$/h at certain values of ${\ensuremath{\nu}}_{\mathit{c}}$, where ${\ensuremath{\nu}}_{\mathit{c}}$=${\ensuremath{\nu}}_{\mathit{e}}$-${\ensuremath{\nu}}_{\mathit{h}}$ and ${\ensuremath{\nu}}_{\mathit{e}}$ and ${\ensuremath{\nu}}_{\mathit{h}}$ are the electron and hole filling factors. The ground states responsible for the quantized Hall effect consist of an incompressible fluid of excess charges and a Bose-condensed gas of excitons which do not interact either with each other or with the excess charge fluid. These results follow from an exact mapping between the electron-hole system and a two-component electron system.

We study the phases of a multiple-quantum-well system of layered two-dimensional electrons in a strong perpendicular magnetic field. Solid phases are found at small fillings of the lowest Landau level which are the hexagonal-close-packed lattice at large interlayer separations and the body-centered-tetragonal lattice at small separations. We find that a sequence of incompressible-Laughlin-liquid-like states occurs at larger fillings. The energies and correlation functions of these states have been calculated by using Monte Carlo simulations. We also discuss the quasiparticle and collective excitations of the liquid states. We find that the electric charge carried by a quasiparticle can have a contribution in one layer which is an irrational fraction of the electron charge. It should be possible to observe the transitions between different Laughlin-like phases experimentally. Transport and microwave experiments are particularly promising in this regard.

We develop a semiclassical theory for the magnetoresistance oscillations recently observed in two-dimensional systems with periodically modulated potentials. We show that these oscillations may be thought of as oscillations in the probability of magnetic breakdown. Our theory demonstrates a connection between the magnitude of the oscillations and the size of the positive magnetoresistance at weak magnetic fields.

Presentation d'un nouveau type de mode collectif qui resulte du couplage des plasmons aux excitations transverses par la force de Lorentz. Il emerge au-dessus du continuum des excitations particule-trou dans le niveau de Landau quand ce dernier est pratiquement a demi-rempli

It is shown that the unitary transformation used as the basis of a t/U expansion for the Hubbard model in an earlier Brief Report is not equivalent to the canonical perturbation expansion (CPE) discussed in the preceding Comment. The CPE is valid only to leading order in t/U. More evidence is offered in support of our suggestion that higher-order terms in the t/U expansion tend to favor spin-liquid states over antiferromagnetically ordered states.

La dispersion du magnetoplasmon de trou des heterostructures GaAs/Al x Ga 1−x As est calculee par l'approximation de Hartree-Fock dependante du temps. Les positions des resonances de cyclotron sont nettement deplacees par les interactions trou-trou. La dispersion du magnetoplasmon de trou augmente de facon monotone avec le vecteur d'onde