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Motivated by recent experiments on Bi$_3$Mn$_4$O$_{12}$(NO$_3$), and a broader interest arising from numerical work on the honeycomb lattice Hubbard model, we have studied the effect of a magnetic field on honeycomb lattice spin models with quantum paramagnetic ground states. For a model with frustrating second-neighbor exchange, $J_2$, we use a Lindemann-like criterion within spin wave theory to show that N\'eel order melts beyond a critical $J_2$. The critical $J_2$ increases with a magnetic field, implying the existence of a field-induced paramagnet-N\'eel transition over a range of $J_2$. We also study bilayer model using a spin-$S$ generalization of bond operator mean field theory. We show that there is a N\'eel-dimer transition for various spin values with increasing bilayer coupling, and that the resulting interlayer dimer state undergoes a field induced transition into a state with transverse N\'eel order. Finally, we study a spin-3/2 model which interpolates between the Heisenberg model and the Affleck-Kennedy-Lieb-Tasaki (AKLT) parent Hamiltonian. Using exact diagonalization, we compute the fidelity susceptibility to locate the Neel-AKLT quantum critical point, obtain the spin gap of the AKLT parent Hamiltonian, and argue that AKLT state also undergoes field-induced Neel ordering., Comment: 8 pages, revised longer version of arXiv:1012.0316. Corrected factor of 2 error in Eq.[16], replotted Fig.[4] and revised the critical $J_c/J_1$ needed to stabilize interlayer dimer state. We thank S. V. Isakov for discussions which uncovered this error

Quantum spin Hall (QSH) state of matter is usually considered to be protected by time-reversal (TR) symmetry. We investigate the fate of the QSH effect in the presence of the Rashba spin-orbit coupling and an exchange field, which break both inversion and TR symmetries. It is found that the QSH state characterized by nonzero spin Chern numbers $C_{\pm}=\pm 1$ persists when the TR symmetry is broken. A topological phase transition from the TR symmetry-broken QSH phase to a quantum anomalous Hall phase occurs at a critical exchange field, where the bulk band gap just closes. It is also shown that the transition from the TR symmetry-broken QSH phase to an ordinary insulator state can not happen without closing the band gap., Comment: 5 pages, 5 figures

We numerically study the disorder effect on the thermoelectric and thermal transport for bilayer graphene under a strong perpendicular magnetic field. In the unbiased case, we find that the thermoelectric transport has similar properties as in the monolayer graphene, i.e., the Nernst signal has a peak at the central Landau level (LL) with the value of the order of $k_B/e$ and changes sign near other LLs while the thermopower has an opposite behavior. We attribute this to the coexistence of particle and hole LLs around the Dirac point. When a finite interlayer bias is applied and a band gap is opened, it is found that the transport properties are consistent with those of a band insulator. We further study the thermal transport from electronic origins and verify the validity of the generalized Weidemann-Franz law., Comment: 7 pages, 5 figures

Recent proposals of topological flat band (TFB) models have provided a new route to realize the fractional quantum Hall effect (FQHE) without Landau levels. We study hard-core bosons with short-range interactions in two representative TFB models, one of which is the well known Haldane model (but with different parameters). We demonstrate that FQHE states emerge with signatures of even number of quasi-degenerate ground states on a torus and a robust spectrum gap separating these states from higher energy spectrum. We also establish quantum phase diagrams for the filling factor 1/2 and illustrate quantum phase transitions to other competing symmetry-breaking phases., Comment: 4 pages, 6 figures

In this paper, the density-matrix renormalization group method is employed to investigate the fractional quantum Hall effect at filling fractions $\nu=1/3$ and 5/2. We first present benchmark results at both filling fractions for large system sizes to show the accuracy as well as the capability of the numerical algorithm. Furthermore, we show that by keeping a large number of basis states, one can also obtain accurate entanglement spectrum at $\nu=5/2$ for large system with electron number up to $N_e=34$, much larger than systems previously studied. Based on a finite-size scaling analysis, we demonstrate that the entanglement gap defined by Li and Haldane is finite in the thermodynamic limit, which characterizes the topological order of the FQHE state., Comment: 6 pages, minor changes

It has been well-known that topological phenomena with fractional excitations, i.e., the fractional quantum Hall effect (FQHE) \cite{Tsui1982} will emerge when electrons move in Landau levels. In this letter, we report the discovery of the FQHE in the absence of Landau levels in an interacting fermion model. The non-interacting part of our Hamiltonian is the recently proposed topologically nontrivial flat band model on the checkerboard lattice \cite{sun}. In the presence of nearest-neighboring repulsion ($U$), we find that at 1/3 filling, the Fermi-liquid state is unstable towards FQHE. At 1/5 filling, however, a next-nearest-neighboring repulsion is needed for the occurrence of the 1/5 FQHE when $U$ is not too strong. We demonstrate the characteristic features of these novel states and determine the phase diagram correspondingly., Comment: 6 pages and 4 figures

Motivated by recent experiments on the spin-3/2 frustrated bilayer honeycomb antiferromagnet Bi$_3$Mn$_4$O$_{12}$(NO$_3$), we study the spin-S Heisenberg model on the honeycomb lattice with various additional exchange interactions which frustrate N\'eel order. Using spin wave theory, exact diagonalization, and bond operator theory, we consider the effects of (i) second-neighbor exchange, (ii) biquadratic exchange for $S = 3/2$ which leads to an AKLT valence bond solid, and (iii) bilayer coupling which leads to an interlayer dimer solid. We show that the resulting paramagnetic states undergo a transition to N\'eel order beyond a critical magnetic field. We discuss experimental implications for Bi$_3$Mn$_4$O$_{12}$(NO$_3$)., Comment: 5 pages, 4 figures; 2 page supplementary material with 1 figure; Longer, updated version: arXiv:1104.0691

We study a spin-1/2 system with Heisenberg plus ring exchanges on a four-leg triangular ladder using the density matrix renormalization group and Gutzwiller variational wave functions. Near an isotropic lattice regime, for moderate to large ring exchanges we find a spin Bose-metal phase with a spinon Fermi sea consisting of three partially filled bands. Going away from the triangular towards the square lattice regime, we find a staggered dimer phase with dimers in the transverse direction, while for small ring exchanges the system is in a featureless rung phase. We also discuss parent states and a possible phase diagram in two dimensions., Comment: 4 pages, 5 figures, v3 is the print version

We present evidence for an exotic gapless insulating phase of hard-core bosons on multi-leg ladders with a density commensurate with the number of legs. In particular, we study in detail a model of bosons moving with direct hopping and frustrating ring exchange on a 3-leg ladder at $\nu=1/3$ filling. For sufficiently large ring exchange, the system is insulating along the ladder but has two gapless modes and power law transverse density correlations at incommensurate wave vectors. We propose a determinantal wave function for this phase and find excellent comparison between variational Monte Carlo and density matrix renormalization group calculations on the model Hamiltonian, thus providing strong evidence for the existence of this exotic phase. Finally, we discuss extensions of our results to other $N$-leg systems and to $N$-layer two-dimensional structures., Comment: 5 pages, 4 figures; v3 is the print version; supplemental material attached

Preparations for the anapole measurement in Fr indicate the possibility of performing a similar measurement in a chain of Rb. The sensitivity analysis based on a single nucleon model shows the potential for placing strong limits on the nucleon weak interaction parameters. There are values of the magnetic fields at much lower values than found before that are insensitive to first order changes in the field. The anapole moment effect in Rb corresponds to an equivalent electric field that is eighty times smaller than Fr, but the stability of the isotopes and the current performance of the dipole trap in the apparatus, presented here, are encouraging for pursuing the measurment., Comment: 16 pages, 6 figures. Accepted for publication in the J. Phys. B

We numerically study the thermoelectric transports of Dirac fermions in graphene in the presence of a strong magnetic field and disorder. We find that the thermoelectric transport coefficients demonstrate universal behavior depending on the ratio between the temperature and the width of the disorder-broadened Landau levels(LLs). The transverse thermoelectric conductivity $\alpha_{xy}$ reaches a universal quantum value at the center of each LL in the high temperature regime, and it has a linear temperature dependence at low temperatures. The calculated Nernst signal has a peak at the central LL with heights of the order of $k_B/e$, and changes sign near other LLs, while the thermopower has an opposite behavior, in good agreement with experimental data. The validity of the generalized Mott relation between the thermoelectric and electrical transport coefficients is verified in a wide range of temperatures., Comment: 4 pages, 4 figures, published version

We report on the study of the fractional quantum Hall effect at the filling factor 5/2 using exact diagonalization method with torus geometry. The particle-hole symmetry breaking effect is considered using an additional three-body interaction. Both Pfaffian and anti-Pfaffian states can be the ground state depending on the sign of the three-body interaction. The results of the low-energy spectrum, the wave function overlap, and the particle-hole parity evolution, have shown the clear evidence of a direct sharp transition (possibly first-order) from the Pfaffian to the anti-Pfaffian state at the Coulomb point. A quantum phase diagram is established, where one finds further transitions from the Pfaffian or anti-Pfaffian state to the nearby compressible phases induced by a change of the pseudopotential., Comment: 4 pages, 4 figures

We numerically study the quantum Hall effect in biased bilayer graphene based on a tight-binding model in the presence of disorder. Integer quantum Hall plateaus with quantized conductivity $\sigma_{xy}=\nu e^2/h$ (where $\nu$ is any integer) are observed around the band center due to the split of the valley degeneracy by an opposite voltage bias added to the two layers. The central ($n=0$) Dirac Landau level is also split, which leads to a pronounced $\nu=0$ plateau. This is consistent with the opening of a sizable gap between the valence and conduction bands. The exact spectrum in an open system further reveals that there are no conducting edge states near zero energy, indicating an insulator state with zero conductance. Consequently, the resistivity should diverge at Dirac point. Interestingly, the $\nu=0$ insulating state can be destroyed by disorder scattering with intermediate strength, where a metallic region is observed near zero energy. In the strong disorder regime, the Hall plateaus with nonzero $\nu$ are destroyed due to the float-up of extended levels toward the band center and higher plateaus disappear first., Comment: 5 pages, 5 figures

Recent experiments on triangular lattice organic Mott insulators have found evidence for a 2D spin liquid in proximity to the metal-insulator transition. A Gutzwiller wavefunction study of the triangular lattice Heisenberg model with appropriate four-spin ring exchanges has found that the projected spinon Fermi sea state has a low variational energy. This wavefunction, together with a slave particle gauge theory, suggests that such spin liquid possesses spin correlations that are singular along surfaces in momentum space ("Bose surfaces"). Signatures of this state, which we refer to as a "Spin Bose-Metal" (SBM), are expected to be manifest in quasi-1D ladder systems: The discrete transverse momenta cut through the 2D Bose surface leading to a distinct pattern of 1D gapless modes. Here we search for a quasi-1D descendant of the triangular lattice SBM state by exploring the Heisenberg plus ring model on a two-leg strip (zigzag chain). Using DMRG, variational wavefunctions, and a Bosonization analysis, we map out the full phase diagram. Without ring exchange the model is equivalent to the J_1 - J_2 Heisenberg chain, and we find the expected Bethe-chain and dimerized phases. Remarkably, moderate ring exchange reveals a new gapless phase over a large swath of the phase diagram. Spin and dimer correlations possess particular singular wavevectors and allow us to identify this phase as the hoped for quasi-1D descendant SBM state. We derive a low energy theory and find three gapless modes and one Luttinger parameter controlling all power laws. Potential instabilities out of the zigzag SBM give rise to other interesting phases such as a period-3 VBS or a period-4 Chirality order, which we discover in the DMRG; we also find an interesting SBM state with partial ferromagnetism., Comment: 30 pages, 23 figures

We study the S=1 square lattice Heisenberg antiferromagnet with spatially anisotropic nearest neighbor couplings $J_{1x}$, $J_{1y}$ frustrated by a next-nearest neighbor coupling $J_{2}$ numerically using the density-matrix renormalization group (DMRG) method and analytically employing the Schwinger-Boson mean-field theory (SBMFT). Up to relatively strong values of the anisotropy, within both methods we find quantum fluctuations to stabilize the N\'{e}el ordered state above the classically stable region. Whereas SBMFT suggests a fluctuation-induced first order transition between the N\'{e}el state and a stripe antiferromagnet for $1/3\leq J_{1x}/J_{1y}\leq 1$ and an intermediate paramagnetic region opening only for very strong anisotropy, the DMRG results clearly demonstrate that the two magnetically ordered phases are separated by a quantum disordered region for all values of the anisotropy with the remarkable implication that the quantum paramagnetic phase of the spatially isotropic $J_{1}$-$J_{2}$ model is continuously connected to the limit of decoupled Haldane spin chains. Our findings indicate that for S=1 quantum fluctuations in strongly frustrated antiferromagnets are crucial and not correctly treated on the semiclassical level., Comment: 10 pages, 10 figures

We present two-color modulation transfer spectroscopy as a tool for precision studies of atomic properties of excited states. The bi-colored technique addresses a narrow set of velocity groups of a thermal atomic vapour using a two-step transition to "burn a hole" in the velocity distribution. The resulting spectrum presents sub-Doppler linewidths, good signal to noise ratio and the trademark sidebands that work as an in situ ruler for the energy spacing between atomic resonances. The spectra obtained can be used for different applications such as measurements of energy splittings or stabilization of laser frequencies to excited atomic transitions., Comment: 14 pages, 13 figures, to be published in the Canadian Journal of Physics

We present lifetime measurements of the $5D_{3/2}$ and $5D_{5/2}$ states of rubidium using the time correlated single photon counting method. We perform the experiment in a magneto-optical trap of $^{87}$Rb atoms using a two-step excitation with the trap laser at 780 nm as the first step. We record the 761.9 nm fluorescence from the decay of the $5D_{3/2}$ state to the $5P_{1/2}$ state, and measure the lifetime of the $5D_{3/2}$ state $\tau=246.3(1.6)$ ns. We record the 420.2 nm fluorescence from the cascade decay of the $5D_{5/2}$ state to the $5S_{1/2}$ state through the $6P_{3/2}$ state, and extract the lifetime of the $5D_{5/2}$ state ${\tau}=238.5(2.3)$ ns., Comment: 9 pages, 6 figures

We numerically study the quantum Hall effect (QHE) in bilayer graphene based on tight-binding model in the presence of disorder. Two distinct QHE regimes are identified in the full energy band separated by a critical region with non-quantized Hall Effect. The Hall conductivity around the band center (Dirac point) shows an anomalous quantization proportional to the valley degeneracy, but the $\nu=0$ plateau is markedly absent, which is in agreement with experimental observation. In the presence of disorder, the Hall plateaus can be destroyed through the float-up of extended levels toward the band center and higher plateaus disappear first. The central two plateaus around the band center are most robust against disorder scattering, which is separated by a small critical region in between near the Dirac point. The longitudinal conductance around the Dirac point is shown to be nearly a constant in a range of disorder strength, till the last two QHE plateaus completely collapse., Comment: 6 pages, 6 figures

We numerically demonstrate that a supersolid phase exists in a frustrated hard-core boson system on a triangular lattice over a wide range of interaction strength. In the infinite repulsion (Ising) limit, we establish a mapping to the same problem with unfrustrated hopping, which connects the supersolid to the known results in that case. The weak superfluidity can be destroyed or strongly enhanced by a next nearest neighbor hopping term, which provides valuable information for experimental realization of a supersolid phase on optical lattice., Comment: 4 pages, 4 figures

Developing a theoretical framework to access quantum phases of itinerant bosons or fermions in two dimensions (2D) that exhibit singular structure along surfaces in momentum space but have no quasi-particle description remains as a central challenge in the field of strongly correlated physics. In this paper we propose that distinctive signatures of such 2D strongly correlated phases will be manifest in quasi-one-dimensional "N-leg ladder" systems. Characteristic of each parent 2D quantum liquid would be a precise pattern of 1D gapless modes on the N-leg ladder. These signatures could be potentially exploited to approach the 2D phases from controlled numerical and analytical studies in quasi-1D. As a first step we explore itinerant boson models with a frustrating ring exchange interaction on the 2-leg ladder, searching for signatures of the recently proposed two-dimensional d-wave correlated Bose liquid (DBL) phase. A combination of exact diagonalization, density matrix renormalization group, variational Monte Carlo, and bosonization analysis of a quasi-1D gauge theory, all provide compelling evidence for the existence of a new strong-coupling phase of bosons on the 2-leg ladder which can be understood as a descendant of the two-dimensional DBL. We suggest several generalizations to quantum spin and electron Hamiltonians on ladders which could likewise reveal fingerprints of such 2D non-Fermi liquid phases., Comment: 22 pages, 15 figures

We numerically study the spin-1/2 antiferromagnetic Heisenberg model on the kagom\'{e} lattice using the density-matrix renormalization group (DMRG) method. We find that the ground state is a magnetically disordered spin liquid, characterized by an exponential decay of spin-spin correlation function in real space and a magnetic structure factor showing system-size independent peaks at commersurate antiferromangetic wavevectors. We obtain a spin triplet excitation gap $\Delta E(S=1)=0.055\pm 0.005$ by extrapolation based on the large size results, and confirm the presence of gapless singlet excitations. The physical nature of such an exotic spin liquid is also discussed., Comment: 5 pages, 5 figures and 1 table

We report on numerical study of the Dirac fermions in partially filled N=3 Landau level (LL) in graphene. At half-filling, the equal-time density-density correlation function displays sharp peaks at nonzero wavevectors $\pm {\bf q^{*}}$. Finite-size scaling shows that the peak value grows with electron number and diverges in the thermodynamic limit, which suggests an instability toward a charge density wave. A symmetry broken stripe phase is formed at large system size limit, which is robust against purturbation from disorder scattering. Such a quantum phase is experimentally observable through transport measurements. Associated with the special wavefunctions of the Dirac LL, both stripe and bubble phases become possible candidates for the ground state of the Dirac fermions in graphene with lower filling factors in the N=3 LL., Comment: Contains are slightly changed. Journal reference and DOI are added

We study the competition between the long-range Coulomb interaction, disorder scattering, and lattice effects in the integer quantum Hall effect (IQHE) in graphene. By direct transport calculations, both $\nu=1$ and $\nu=3$ IQHE states are revealed in the lowest two Dirac Landau levels. However, the critical disorder strength above which the $\nu=3$ IQHE is destroyed is much smaller than that for the $\nu=1$ IQHE, which may explain the absence of a $\nu=3$ plateau in recent experiments. While the excitation spectrum in the IQHE phase is gapless within numerical finite-size analysis, we do find and determine a mobility gap, which characterizes the energy scale of the stability of the IQHE. Furthermore, we demonstrate that the $\nu=1$ IQHE state is a Dirac valley and sublattice polarized Ising pseudospin ferromagnet, while the $\nu=3$ state is an $xy$ plane polarized pseudospin ferromagnet., Comment: 5 pages, 5 figures

We present a topological description of quantum spin Hall effect (QSHE) in a two-dimensional electron system on honeycomb lattice with both intrinsic and Rashba spin-orbit couplings. We show that the topology of the band insulator can be characterized by a $2\times 2$ traceless matrix of first Chern integers. The nontrivial QSHE phase is identified by the nonzero diagonal matrix elements of the Chern number matrix (CNM). A spin Chern number is derived from the CNM, which is conserved in the presence of finite disorder scattering and spin nonconserving Rashba coupling. By using the Laughlin's gedanken experiment, we numerically calculate the spin polarization and spin transfer rate of the conducting edge states, and determine a phase diagram for the QSHE., Comment: 4 pages and 4 figures

We numerically study the interplay of band structure, topological invariant and disorder effect in two-dimensional electron system of graphene in a magnetic field. Two \emph{distinct} quantum Hall effect (QHE) regimes exist in the energy band with the unconventional "half-integer" QHE appearing near the band center, consistent with the experimental observation. The latter is more robust against disorder scattering than the conventional QHE states near the band edges. The phase diagram for the unconventional QHE is obtained where the destruction of the Hall plateaus at strong disorder is through the float-up of extended levels toward band center and higher plateaus always disappear first. We further predict a new insulating phase between $\nu =\pm 2$ QHE states at the band center, which may explain the experimentally observed resistance discontinuity near zero gate voltage., Comment: 4 pages, 4 figures

Based upon spin-lattice interaction, we propose a theoretical model for the phonon Hall effect in paramagnetic dielectrics. The thermal Hall conductivity is calculated by using the Kubo formula. Our theory reproduces the essential experimental features of the phonon Hall effect discovered recently in ionic dielectric Tb$_3$Ga$_5$O$_{12}$, including the sign, magnitude and linear magnetic field dependence of the thermal Hall conductivity., Comment: 5 pages, 3 figures

Based on exact diagonalization and density matrix renormalization group (DMRG) method, we show that an anisotropic triangle lattice Heisenberg spin model has three distinct quantum phases. In particular, a spin-liquid phase is present in the weak interchain coupling regime, which is characterized by an exponentially-decaying spin correlator along the weaker coupling direction and a quasi-one-dimensional spin structure factor along the chain direction, in agreement with experiments on the Cs2CuCl4 compounds. In the obtained phase diagram, the spin liquid phase is found to persist up to a relatively large critical anisotropic coupling ratio J'/J=0.78, which is stabilized by strong quantum fluctuations, with a parity symmetry distinct from two magnetic ordered states in the stronger coupling regime., Comment: 4 pages, 3 figures; revised version

We present a numerical study of the spin Hall effect in a two-dimensional hole gas (2DHG) system in the presence of disorder. We find that the spin Hall conductance (SHC), extrapolated to the thermodynamic limit, remains finite in a wide range of disorder strengths for a closed system on torus. But there is no intrinsic spin Hall accumulation as induced by an external electric field once the disorder is turned on. The latter is examined by performing a Laughlin's Gedanken gauge experiment numerically with the adiabatical insertion of a flux quantum in a belt-shaped sample, in which the absence of level crossing is found under the disorder effect. Without disorder, on the other hand, energy levels do cross each other, which results in an oscillating spin-density-modulation at the sample boundary after the insertion of one flux quantum in the belt-shaped system. But the corresponding net spin transfer is only about one order of magnitude smaller than what is expected from the bulk SHC. These apparently contradictory results can be attributed to the violation of the spin conservation law in such a system. We also briefly address the dissipative Fermi surface contribution to spin polarization, which may be relevant to experimental measurements., Comment: 9 pages, 11 figures; final version to appear in Phys. Rev. B

The spin Hall effect in a two-dimensional electron system on honeycomb lattice with both intrinsic and Rashba spin-orbit couplings is studied numerically. Integer quantized spin Hall conductance is obtained at zero Rashba coupling limit when electron Fermi energy lies in the energy gap created by the intrinsic spin-orbit coupling, in agreement with recent theoretical prediction. While nonzero Rashba coupling destroys electron spin conservation, the spin Hall conductance is found to remain near the quantized value, being insensitive to disorder scattering, until the energy gap collapses with increasing the Rashba coupling. We further show that the charge transport through counterpropagating spin-polarized edge channels is well quantized, which is associated with a topological invariant of the system., Comment: 5 pages, 4 figures [(updated journal reference) + (minor revision)]

We study the behavior of two-dimensional electron gas in the fractional quantum Hall regime in the presence of finite layer thickness and correlated disordered potential. Generalizing the Chern number calculation to many-body systems, we determine the mobility gaps of fractional quantum Hall states based on the distribution of Chern numbers in a microscopic model. We find excellent agreement between experimentally measured activation gaps and our calculated mobility gaps, when combining the effects of both disordered potential and layer thickness. We clarify the difference between mobility gap and spectral gap of fractional quantum Hall states and explain the disorder-driven collapse of the gap and the subsequent transitions from the fractional quantum Hall states to insulator., Comment: 13 pages, 8 figures

We study the interplay of Mott localization, geometric frustration, and superfluidity for hard-core bosons with nearest-neighbor repulsion on the triangular lattice. For this model at half-filling, we demonstrate that superfluidity survives for arbitrarily large repulsion, and that diagonal solid order emerges in the strongly correlated regime from an order-by-disorder mechanism. This is thus an unusual example of a stable supersolid phase of hard-core lattice bosons at a commensurate filling., Comment: 4 pages, 2 figures; finite-size scaling discussion added

Based on numerical study of the Rashba model, we show that the spin Hall conductance remains finite in the presence of disorder up to a characteristic length scale, beyond which it vanishes exponentially with the system size. We further perform a Laughlin's gauge experiment numerically and find that all energy levels cannot cross each other during an adiabatic insertion of the flux in accordance with the general level-repulsion rule. It results in zero spin transfer between two edges of the sample as each state always evolves back after the insertion of one flux quantum, in contrast to the quantum Hall effect. It implies that the topological spin Hall effect vanishes with the turn-on of disorder., Comment: 4 pages, 4 figures final version

We present first numerical studies of the disorder effect on the recently proposed intrinsic spin Hall conductance in a three dimensional (3D) lattice Luttinger model. The results show that the spin Hall conductance remains finite in a wide range of disorder strength, with large fluctuations. The disorder-configuration-averaged spin Hall conductance monotonically decreases with the increase of disorder strength and vanishes before the Anderson localization takes place. The finite-size effect is also discussed., Comment: 4 pages, 4 figures; the final version appearing in PRL

Using the four-terminal Landauer-B\"{u}ttiker formula and Green's function approach, we calculate numerically the spin-Hall conductance in a two-dimensional junction system with the Rashba spin-orbit (SO) coupling and disorder. We find that the spin-Hall conductance can be much greater or smaller than the universal value $e/8\pi$, depending on the magnitude of the SO coupling, the electron Fermi energy and the disorder strength. The spin-Hall conductance does not vanish with increasing sample size for a wide range of disorder strength. Our numerical calculation reveals that a nonzero SO coupling can induce electron delocalization for disorder strength smaller than a critical value, and the nonvanishing spin-Hall effect appears mainly in the metallic regime., Comment: 5 pages, 4 figures

Based on exact numerical calculations, we show that the generalized Kagome spin model in the easy axis limit exhibits a spin liquid, topologically degenerate ground state over a broad range of phase space. We present an (to our knowledge the first) explicit calculation of the gap (and dispersion) of ``vison'' excitations, and exponentially decaying spin and vison 2-point correlators, hallmarks of deconfined, fractionalized and gapped spinons. The region of the spin liquid phase includes a point at which the model is equivalent to a Heisenberg model with purely two-spin interactions. Beyond this range, a negative ``potential'' term tunes a first order transition to a magnetic ordered state. The nature of the phase transition is also discussed in light of the low energy spectrum. These results greatly expand the results and range of a previous study of this model in the vicinity of an exactly soluble point., Comment: 4 pages, 5 figures

Using the closed-time path integral approach, we nonperturbatively study inelastic tunneling of electrons via magnetic impurities in the barrier accompanied by phonon emission in a magnetic tunnel junction. The spectrum density of phonon emission is found to show a power-law infrared singularity $\sim\omega^{-(1-g)}$ with $g$ the dimensionless electron-phonon coupling. As a consequence, the tunneling conductance $G(V)$ increases with bias voltage $| V|$ as $G(V)-G(0)\sim| V|^{2g}$, exhibiting a discontinuity in slope at V=0 for $g\le 0.5$. This theory can reproduce both cusp-like and non-cusp-like feature of the zero-bias anomaly of tunneling resistance and magnetoresistance widely observed in experiments.

We study electron tunneling through two small ferromagnetic dots. Quantum charge fluctuations and interdot coupling make each Coulomb peak of the conductance at zero interdot coupling split across. The interdot tunnel coupling is determined by the relative orientation of magnetizations of the two dots, leading to different splitting energies of the Coulomb peaks in parallel and antiparallel magnetization alignments. As a result, a very large tunneling magnetoresistance occurs near the Coulomb peaks, and its sign may be either positive or negative., Comment: submitted to Phys. Rev. B

We present a phase diagram for a double quantum well bilayer electron gas in the quantum Hall regime at total filling factor $\nu =1$, based on exact numerical calculations of the topological Chern number matrix and the (inter-layer) superfluid density. We find three phases: a quantized Hall state with pseudo-spin superfluidity, a quantized Hall state with pseudo-spin ``gauge-glass'' order, and a decoupled composite Fermi liquid. Comparison with experiments provides a consistent explanation of the observed quantum Hall plateau, Hall drag plateau and vanishing Hall drag resistance, as well as the zero-bias conductance peak effect, and suggests some interesting points to pursue experimentally., Comment: 4 pages with 4 figures

We study the nu=1/3 quantum Hall state in presence of the random disorder. We calculate the topologically invariant Chern number, which is the only quantity known at present to unambiguously distinguish between insulating and current carrying states in an interacting system. The mobility gap can be determined numerically this way, which is found to agree with experimental value semiquantitatively. As the disorder strength increases towards a critical value, both the mobility gap and plateau width narrow continuously and ultimately collapse leading to an insulating phase., Comment: 4 pages with 4 figures

We study the effects of disorder on the quantum Hall stripe phases in half-filled high Landau levels using exact numerical diagonalization. We show that, in the presence of weak disorder, a compressible, striped charge density wave, becomes the true ground state. The projected electron density profile resembles that of a smectic liquid. With increasing disorder strength W, we find that there exists a critical value, W_c \sim 0.12 e^2/\epsilon l, where a transition/crossover to an isotropic phase with strong local electron density fluctuations takes place. The many-body density of states are qualitatively distinguishable in these two phases and help elucidate the nature of the transition., Comment: 4 pages, 4 figures

We present a model calculation of the spectral function of an electron in a superconducting resonating valence bond (RVB) state. The RVB state, described by the phase-string mean field theory is characterized by three important features: (i) spin-charge separation, (ii) short range antiferromagnetic correlations, and (iii) holon condensation. The results of our calculation are in good agreement with data obtained from Angle Resolved Photoemission Spectroscopy (ARPES) in superconducting Bi 2212 at optimal doping concentration., Comment: 4 pages, 3 figures

In the presence of nonlocal phase shift effects, a quasiparticle can remain topologically stable even in a spin-charge separation state due to the confinement effect introduced by the phase shifts at finite doping. True deconfinement only happens in the zero-doping limit where a bare hole can lose its integrity and decay into holon and spinon elementary excitations. The Fermi surface structure is completely different in these two cases, from a large band-structure-like one to four Fermi points in one-hole case, and we argue that the so-called underdoped regime actually corresponds to a situation in between., Comment: 4 pages, 2 figures, presented in M2S-HTSC-VI conference (2000)