Planar hall effect in ferromagnets

The planar Hall effect in a ferromagnetic conductor is considered within a simple two-liquid hydro-dynamic model. It is shown that, even in the simple case of an isotropic Fermi surface in the absence of thermal spread, the magnitude of the Hall effect is comparable to that in semiconductors because of the presence of two groups of conduction electrons with their spins parallel and perpendicular to the quantization axis, respectively. In addition to the planar Hall field, a spin flux parallel to this field arises, with the consequence that the extent of spin polarization of the conduction electrons varies along the Hall field direction (planar spin Hall effect).     

Spin-orbit coupling effect on quantum hall ferromagnets with vanishing zeeman energy

We present the phase diagram of a ferromagnetic nu = 2N+1 quantum Hall liquid in a narrow quantum well with vanishing single-particle Zeeman splitting, varepsilon(Z), and a pronounced spin-orbit coupling. Upon decreasing varepsilon(Z) the spin-polarization field of a liquid takes, first, the easy-axis configuration, followed by the formation of a helical state which affects the transport and NMR properties of a liquid and the form of topological defects in it.     

Spin waves in quantum ferromagnets

Low-temperature properties of Heisenberg quantum ferromagnets (spin waves) are derived within aconfiguration space formalism. Most of the work is done without explicitly assuming translational invariance. We provide a general criterion, classical domination, to decide about the nature and uniqueness of ground states for a large class of quantum ferromagnets. We also analyze and clarify the Dyson formalism and indicate why an energy gap between the physical ground state and the improper (unphysical) states does not exist. This is of particular relevance to the kinematical interaction. Using reflection positivity we provide upper and lower bounds to the contribution of the dynamical interaction to the free energy. In a certain approximation, these bounds imply that the dynamical interaction may be dropped if the inverse temperature and the spin quantum numberS are large enough.Suported in part by FAPESP.Supported in part by CAPES.Partial support by FAPESP and CNPq.     

Half-polarized quantum hall states

We report a theoretical analysis of the half-polarized quantum Hall states observed in a recent experiment. Our numerical results indicate that the ground state energy of the quantum Hall nu = 2 / 3 and nu = 2 / 5 states versus spin polarization has a downward cusp at half the maximal spin polarization. We map the two-component fermion system onto a system of excitons and describe the ground state as a liquid state of excitons with nonzero values of exciton angular momentum.     

Interlayer tunneling in double-layer quantum hall pseudoferromagnets

We show that the interlayer tunneling I-V in double-layer quantum Hall states displays a rich behavior which depends on the relative magnitude of sample size, voltage length scale, current screening, disorder, and thermal lengths. For weak tunneling, we predict a negative differential conductance of a power-law shape crossing over to a sharp zero-bias peak. An in-plane magnetic field splits this zero-bias peak, leading instead to a "derivative" feature at V(B)(B(parallel)) = 2 pi Planck's over 2 pi upsilon B(parallel)d/e phi(0), which gives a direct measurement of the dispersion of the Goldstone mode corresponding to the spontaneous symmetry breaking of the double-layer Hall state.     

Fractional quantum hall effect in infinite-layer systems

Stacked two dimensional electron systems in transverse magnetic fields exhibit three dimensional fractional quantum Hall phases. We analyze the simplest such phases and find novel bulk properties, e.g. , irrational braiding. These phases host "one and a half" dimensional surface phases in which motion in one direction is chiral. We offer a general analysis of conduction in the latter by combining sum rule and renormalization group arguments, and find that when interlayer tunneling is marginal or irrelevant they are chiral semimetals that conduct only at T > 0 or with disorder.     

Vertical hall effect in GaAs quantum wells

Applying orthogonal in-plane electric and magnetic fields in a 2D system leads to the development of a Hall voltage across the width of the quantum well when the cyclotron orbit is greater than the well width. Tang and Butcher [1] have calculated the developed Hall voltage for a parabolic quantum well where they find that the Hall voltage is dependent on the frequency associated with the harmonic potential in the well. The limitation of this model is that it does not enable one to determine the well width dependence of the Hall Voltage, nor is it a particularly good model for a quantum well. It is also difficult to compare their model with the bulk result which would apply at large well widths. In this work we present a model calculation which considers a square quantum well and hence is able to predict the well width dependence of the Hall Voltage and compare the large well width case to the bulk result. An electro-optic probing method previously used to measure bulk Hall voltages [2] is shown to be capable of measuring the Hall 'voltage across a quantum well, and therefore can be used to confirm the prediction of the model presented here.     

Spin-wave relaxation in a quantum hall ferromagnet

We study spin wave relaxation in quantum Hall ferromagnet regimes. Spin-orbit coupling is considered as a factor determining spin nonconservation, and external random potential as a cause of energy dissipation making spin-flip processes irreversible. We compare this relaxation mechanism with other relaxation channels existing in a quantum Hall ferromagnet. The article is published in the original.     

Nonabelions in the fractional quantum hall effect

Applications of conformal field theory to the theory of fractional quantum Hall systems are discussed. In particular, Laughlin's wave function and its cousins are interpreted as conformal blocks in certain rational conformal field theories. Using this point of view a hamiltonian is constructed for electrons for which the ground state is known exactly and whose quasihole excitations have nonabelian statistics; we term these objects “nonabelions”. It is argued that universality classes of fractional quantum Hall systems can be characterized by the quantum numbers and statistics of their excitations. The relation between the order parameter in the fractional quantum Hall effect and the chiral algebra in rational conformal field theory is stressed, and new order parameters for several states are given.     

Anomalous magnetotransport in narrow quantum hall conductors

We present the anomalous magnetotransport observed in a narrow high mobility Hall device; both the peaks and minima in the Shubnikov de Haas oscillations of the longitudinal resistance appear in the quantized plateau region of the Hall resistance. We interpret this feature as a consequence of an inhomogeneous distribution of two dimensional electron gas and provide a quantitative view with a model based on the edge current picture.     

Fluctuation-driven quantum phase transitions in clean itinerant ferromagnets

The quantum phase transition in clean itinerant ferromagnets is analyzed. It is shown that soft particle-hole modes invalidate Hertz's mean-field theory for d< or =3. A renormalized mean-field theory predicts a fluctuation-induced first order transition for 1相似文献   

On the fractional quantum hall effect

We show in the Hartree-Fock approximation that the formation of a two dimensional electron lattice allows for a natural explanation of the anomalous fractional quantum Hall effect. Landau levels are broadened and split in a number of bands in such a way that if the number of electrons per unit cell is a half odd integer the Fermi energy is in a gap for an odd filling fraction denominator, and at the center of a band if the denominator is even.     

Hydrodynamics of the quantum hall smectics

We propose a dynamical theory of the stripe phase arising in a two-dimensional electron liquid near half-integral fillings of high Landau levels. The system is modeled as a novel type of a smectic liquid crystal with Lorentz force dominated dynamics. We calculate the structure factor, the dispersion relation of the collective modes, and their intrinsic attenuation rate. We show that thermal fluctuations cause a strong power-law renormalization of the elastic and dissipative parameters familiar from the conventional smectics but with different dynamical scaling exponents.     

Probing the plateau-insulator quantum phase transition in the quantum hall regime

We report quantum Hall experiments on the plateau-insulator transition in a low mobility In(0.53)Ga(0.47)As/InP heterostructure. The data for the longitudinal resistance rho(xx) follow an exponential law and we extract a critical exponent kappa = 0.55+/-0. 05 which is slightly different from the established value kappa = 0. 42+/-0.04 for the plateau transitions. Upon correction for inhomogeneity effects, which cause the critical conductance sigma(*)(xx) to depend marginally on temperature, our data indicate that the plateau-plateau and plateau-insulator transitions are in the same universality class.     

Theory of interlayer tunneling in bilayer quantum Hall ferromagnets

Spielman et al. [Phys. Rev. Lett. 84, 5808 (2000] recently observed a large and sharp Josephson-like zero-bias peak in the tunnel conductance of a bilayer system in a quantum Hall ferromagnet state. We argue that disorder-induced topological defects in the pseudospin order parameter limit the peak size and destroy the predicted Josephson effect. We predict that the peak would be split and shifted by an in-plane magnetic field in a way that maps the dispersion relation of the ferromagnet's Goldstone mode. We also predict resonant structures in the dc I-V characteristic under bias by an ac electric field.