arXiv subjects

Z. F. Ezawa

Publications and source records attributed to Z. F. Ezawa.

At least 19 recordsLinked to original sources

arXiv2015

Dicke Model for Quantum Hall Systems

Quantum Hall (QH) systems consist of many-body electron and nuclear spins. They are coupled so weakly through the hyperfine interaction that normally electron spin dynamics are scarcely affected by the nuclear spins. The dynamics of the QH systems, however, may drastically change when the nuclear spins interact with low-energy collective excitation modes of the electron spins. We theoretically investigate the nuclear-electron spin interaction in the QH systems as hybrid quantum systems driven by the hyperfine interaction. In particular, we study the interaction between the nuclear spins and the Nambu-Goldstone (NG) mode with the linear dispersion relation associated with the U(1) spin rotational symmetry breaking. We show that such an interaction is described as nuclear spins collectively coupled to the NG mode, and can be effectively described by the Dicke model. Based on the model we suggest that various collective spin phenomena realized in quantum optical systems also emerge in the QH systems.

cond-mat.mes-hall↗

arXiv2013

The Study of Goldstone Modes in $ν$=2 Bilayer Quantum Hall Systems

At the filling factor $ν$=2, the bilayer quantum Hall system has three phases, the spin-ferromagnet phase, the spin singlet phase and the canted antiferromagnet (CAF) phase, depending on the relative strength between the Zeeman energy and interlayer tunneling energy. We present a systematic method to derive the effective Hamiltonian for the Goldstone modes in these three phases. We then investigate the dispersion relations and the coherence lengths of the Goldstone modes. To explore a possible emergence of the interlayer phase coherence, we analyze the dispersion relations in the zero tunneling energy limit. We find one gapless mode with the linear dispersion relation in the CAF phase.

cond-mat.mes-hall↗

arXiv2010

Anisotropy of Magnetoresistance Hysteresis around the $ν=2/3$ Quantum Hall State in Tilted Magnetic Field

We present an anisotropy of the hysteretic transport around the spin transition point at Landau level filling factor $ν=2/3$ in tilted magnetic field. When the direction of the in-plane component of the magnetic field $B_{\parallel}$ is normal to the probe current $I$, a strong hysteretic transport due to the current-induced nuclear spin polarization occurs. When $B_{\parallel}$ is parallel to $I$, on the other hand, the hysteresis almost disappears. We also demonstrate that the nuclear spin-lattice relaxation rate $T_{1}^{-1}$ at the transition point increases with decreasing angle between the directions of $B_{\parallel}$ and $I$. These results suggest that the morphology of electron spin domains around $ν=2/3$ is affected by the current direction.

cond-mat.str-el↗

arXiv2007

Pseudospin Soliton in the $ν=1$ Bilayer Quantum Hall State

We investigate a domain structure of pseudospins, a soliton lattice in the bilayer quantum Hall state at total Landau level filling factor $ν=1$, in a tilted magnetic field, where the pseudospin represents the layer degree of freedom. An anomalous peak in the magnetoresistance $R_{xx}$ appears at the transition point between the commensurate and incommensurate phases. The $R_{xx}$ at the peak is highly anisotropic for the angle between the in-plain magnetic field $B_\parallel $ and the current, and indicates a formation of the soliton lattice aligned parallel to $B_\parallel $. Temperature dependence of the $R_{xx}$ peak reveals that the dissipation is caused by thermal fluctuations of pseudospin solitons. We construct a phase diagram of the bilayer $ν=1$ system as a function of $B_\parallel$ and the total electron density. We also study effects of density imbalance between the two layers.

cond-mat.mes-hall↗

arXiv2007

Anomalous Hall Resistance in Bilayer Quantum Hall Systems

We present a microscopic theory of the Hall current in the bilayer quantum Hall system on the basis of noncommutative geometry. By analyzing the Heisenberg equation of motion and the continuity equation of charge, we demonstrate the emergence of the phase current in a system where the interlayer phase coherence develops spontaneously. The phase current arranges itself to minimize the total energy of the system, as induces certain anomalous behaviors in the Hall current in the counterflow geometry and also in the drag experiment. They explain the recent experimental data for anomalous Hall resistances due to Kellogg et al. [M. Kellogg, I.B. Spielman, J.P. Eisenstein, L.N. Pfeiffer and K.W. West, Phys. Rev. Lett. \textbf{88} (2002) 126804; M. Kellogg, J.P. Eisenstein, L.N. Pfeiffer and K.W. West, Phys. Rev. Lett. \textbf{93} (2004) 036801] and Tutuc et al. [E. Tutuc, M. Shayegan and D.A. Huse, Phys. Rev. Lett. \textbf{93} (2004) 036802] at $ν=1$.

cond-mat.mes-hall↗

arXiv2006

Magnetotransport Study of the Canted Antiferromagnetic Phase in Bilayer $ν=2$ Quantum Hall State

Magnetotransport properties are investigated in the bilayer quantum Hall state at the total filling factor $ν=2$. We measured the activation energy elaborately as a function of the total electron density and the density difference between the two layers. Our experimental data demonstrate clearly the emergence of the canted antiferromagnetic (CAF) phase between the ferromagnetic phase and the spin-singlet phase. The stability of the CAF phase is discussed by the comparison between experimental results and theoretical calculations using a Hartree-Fock approximation and an exact diagonalization study. The data reveal also an intrinsic structure of the CAF phase divided into two regions according to the dominancy between the intralayer and interlayer correlations.

cond-mat.mes-hall↗

arXiv2005

Microscopic Theory of Skyrmions in Quantum Hall Ferromagnets

We present a microscopic theory of skyrmions in the monolayer quantum Hall ferromagnet. It is a peculiar feature of the system that the number density and the spin density are entangled intrinsically as dictated by the W$%_{\infty}$ algebra. The skyrmion and antiskyrmion states are constructed as W$_{\infty }$-rotated states of the hole-excited and electron-excited states, respectively. They are spin textures accompanied with density modulation that decreases the Coulomb energy. We calculate their excitation energy as a function of the Zeeman gap and compared the result with experimental data.

cond-mat.mes-hall↗

arXiv2005

Topological Solitons in Noncommutative Plane and Quantum Hall Skyrmions

We analyze topological solitons in the noncommutative plane by taking a concrete instance of the quantum Hall system with the SU(N) symmetry, where a soliton is identified with a skyrmion. It is shown that a topological soliton induces an excitation of the electron number density from the ground-state value around it. When a judicious choice of the topological charge density $J_{0}(\mathbf{x})$ is made, it acquires a physical reality as the electron density excitation $Δρ^{\text{cl}}(\mathbf{x})$ around a topological soliton, $Δρ^{\text{cl}}(\mathbf{x})=-J_{0}(% \mathbf{x})$. Hence a noncommutative soliton carries necessarily the electric charge proportional to its topological charge. A field-theoretical state is constructed for a soliton state irrespectively of the Hamiltonian. In general it involves an infinitely many parameters. They are fixed by minimizing its energy once the Hamiltonian is chosen. We study explicitly the cases where the system is governed by the hard-core interaction and by the noncommutative CP$^{N-1}$ model, where all these parameters are determined analytically and the soliton excitation energy is obtained.

hep-th↗

arXiv2005

Ground-State Structure in $ν=2$ Bilayer Quantum Hall Systems

We investigate the ground-state structure of the bilayer quantum Hall system at the filling factor $ν=2$. Making an exact analysis of the ground state in the SU(4)-invariant limit, we include all other interactions as small perturbation. We carry out analytic calculations and construct phase diagrams for nonzero values of the Zeeman, tunneling and bias interactions. In particular we examine carefully how the phase transition occurs by applying the bias voltage and inducing a density imbalance between the two layers. We compare our theoretical result with the experimental data due to Sawada et al. based on the phase diagram in the $σ_{0}$-$ρ_{0}$ plane, where $ρ_{0}$ and $σ_{0}$ are the total electron density and the density difference between the two layers, respectively.

cond-mat.str-el↗

arXiv2004

SU(4) Skyrmions and Activation Energy Anomaly in Bilayer Quantum Hall Systems

The bilayer QH system has four energy levels in the lowest Landau level, corresponding to the layer and spin degrees of freedom. We investigate the system in the regime where all four levels are nearly degenerate and equally active. The underlying group structure is SU(4). At $ν=1$ the QH state is a charge-transferable state between the two layers and the SU(4) isospin coherence develops spontaneously. Quasiparticles are isospin textures to be identified with SU(4) skyrmions. The skyrmion energy consists of the Coulomb energy, the Zeeman energy and the pseudo-Zeeman energy. The Coulomb energy consists of the self-energy, the capacitance energy and the exchange energy. At the balanced point only pseudospins are excited unless the tunneling gap is too large. Then, the SU(4) skyrmion evolves continuously from the pseudospin-skyrmion limit into the spin-skyrmion limit as the system is transformed from the balanced point to the monolayer point by controlling the bias voltage. Our theoretical result explains quite well the experimental data due to Murphy et al. and Sawada et al. on the activation energy anomaly induced by applying parallel magnetic field.

cond-mat.mes-hall↗

arXiv2004

Simultaneous Excitation of Spins and Pseudospins in the Bilayer $ν=1$ Quantum Hall State

The tilting angular dependence of the energy gap was measured in the bilayer quantum Hall state at the Landau level filling $ν=1$ by changing the density imbalance between the two layers. The observed gap behavior shows a continuous transformation from the bilayer balanced density state to the monolayer state. Even a sample with 33 K tunneling gap shows the same activation energy anomaly reported by Murphy {\it et al.}. We discuss a possible relation between our experimental results and the quantum Hall ferromagnet of spins and pseudospins.

cond-mat.mes-hall↗

arXiv2002

Grassmannian Fields and Doubly Enhanced Skyrmions in Bilayer Quantum Hall system at $ν=2$

At the filling factor $ν=2$ the bilayer quantum Hall system has three phases, the ferromagnetic phase (spin phase), the spin singlet phase (ppin phase) and the canted antiferromagnetic phase. We analyze soft waves and quasiparticle excitations in the spin and ppin phases. It is shown that the dynamic field is the Grassmannian G$_{4,2}$ field carrying four complex degrees of freedom. In each phase there are four complex soft waves (pseudo-Goldstone modes) and one kind of skyrmion excitations (G$_{4,2}$ skyrmions) flipping either spins or pseudospins coherently. An intriguing property is that a quasiparticle is a G$_{4,2}$ skyrmion essentially consisting of two CP$^{3}$ skyrmions and thus possesses charge $2e$.

cond-mat.mes-hall↗

arXiv2002

Noncommutative Geometry, Extended W(infty) Algebra and Grassmannian Solitons in Multicomponent Quantum Hall Systems

Noncommutative geometry governs the physics of quantum Hall (QH) effects. We introduce the Weyl ordering of the second quantized density operator to explore the dynamics of electrons in the lowest Landau level. We analyze QH systems made of $N$-component electrons at the integer filling factor $ν=k\leq N$. The basic algebra is the SU(N)-extended W$_{\infty}$. A specific feature is that noncommutative geometry leads to a spontaneous development of SU(N) quantum coherence by generating the exchange Coulomb interaction. The effective Hamiltonian is the Grassmannian $G_{N,k}$ sigma model, and the dynamical field is the Grassmannian $G_{N,k}$ field, describing $k(N-k)$ complex Goldstone modes and one kind of topological solitons (Grassmannian solitons).

hep-th↗

arXiv2002

Exact Symmetries of Electron Interactions in the Lowest Landau Level

Considering the system of interacting electrons in the lowest Landau level we show that the corresponding four-fermion Hamiltonian is invariant with respect to the local area-preserving transformations. Testing a certain class of interaction potentials, we find that this symmetry is universal with respect to a concrete type of potentials.

cond-mat.mes-hall↗

arXiv2001

Phase Transitions in the Bilayer $ν=2/3$ Quantum Hall Effect

We measured the magnetoresistance of bilayer quantum Hall (QH) effects at the fractional filling factor $ν=2/3$ by changing the total electron density and the density difference between two layers. Three different QH states were separated by two types of phase transition: One is the spin transition and the other is the pseudospin transition. In addition, two different hystereses were detected, one of which is specific to bilayer systems. The phase transitions and the hystereses are described well by a composite fermion model extended to a bilayer system.

cond-mat.mes-hall↗

arXiv2001

Interlayer Exchange Interactions, SU(4) Soft Waves and Skyrmions in Bilayer Quantum Hall Ferromagnets

The Coulomb exchange interaction is the driving force for quantum coherence in quantum Hall systems. We construct a microscopic Landau-site Hamiltonian for the exchange interaction in bilayer quantum Hall ferromagnets, which is characterized by the SU(4) isospin structure. By taking a continuous limit, the Hamiltonian gives rise to the SU(4) nonlinear sigma model in the von-Neumann-lattice formulation. The ground-state energy is evaluated at filling factors $ν=1,2,3,4$. It is shown at $ν=1$ that there are 3 independent soft waves, where only one soft wave is responsible for the coherent tunneling of electrons between the two layers. It is also shown at $ν=1$ that there are 3 independent skyrmion states apart from the translational degree of freedom. They are CP$^{3}$ skyrmions enjoying the spin-charge entanglement confined within the \LLL.

cond-mat.mes-hall↗

arXiv2000

Doubly Enhanced Skyrmions in $ν=2$ Bilayer Quantum Hall States

By tilting the samples in the magnetic field, we measured and compared the Skyrmion excitations in the bilayer quantum Hall (QH) state at the Landau-level filling factor $ν=2$ and in the monolayer QH state at $ν=1$. The observed number of flipped spins is $N_s=14$ in the bilayer system with a large tunneling gap, and $N_s=7$ in the bilayer system with a small tunneling gap, while it is $N_s=7$ in the monolayer system. The difference is interpreted due to the interlayer exchange interaction. Moreover, we have observed seemingly preferred numbers $N_s=14,7,1$ for the flipped spins by tilting bilayer samples.

cond-mat.mes-hall↗

arXiv1999

Thermal and Tunneling Pair Creation of Quasiparticles in Quantum Hall Systems

We make a semiclassical analysis of thermal pair creations of quasiparticles at various filling factors in quantum Hall systems. It is argued that the gap energy is reduced considerably by the Coulomb potential made by impurities. It is also shown that a tunneling process becomes important at low temperature and at strong magnetic field. We fit typical experimental data excellently based on our semiclassical results of the gap energy.

cond-mat.mes-hall↗