arXiv subjects

A. Sawada

Publications and source records attributed to A. Sawada.

11 recordsLinked to original sources

arXiv2013

Spin blocker made of semiconductor double quantum well using the Rashba effect

We propose a lateral spin-blockade device that uses an InGaAs/InAlAs double quantum well (DQW), where the values of the Rashba spin-orbit parameter $α_{\rm R}$ are opposite in sign but equal in magnitude between the constituent quantum wells (QW). By tuning the channel length of DQW and the magnitude of the externally applied in-plane magnetic field, one can block the transmission of one spin (e.g., spin-down) component, leading to a spin-polarized current. Such a spin-blocking effect, brought about by wave vector matching of the spin-split Fermi surfaces between the two QWs, paves the way for a new scheme of spin-polarized electric current generation for future spintronics applications based on semiconductor band engineering.

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

Modulation of bilayer quantum Hall states by tilted-field-induced subband-Landau-level coupling

We study effects of tilted magnetic fields on energy levels in a double-quantum-well (DQW) system, focusing on the coupling of subbands and Landau levels (LLs). The subband-LL coupling induces anticrossings between LLs, manifested directly in the magnetoresistance. The anticrossing gap becomes larger than the spin splitting at the tilting angle $θ\sim 20^\circ $ and larger than the cyclotron energy at $θ\sim 50^\circ $, demonstrating that the subband-LL coupling exerts a strong influence on quantum Hall states even in at a relatively small $θ$ and plays a dominant role for larger $θ$. We also find that when the DQW potential is asymmetric, LL coupling occurs even within a subband. Calculations including higher-order coupling reproduce the experimental results quantitatively well.

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↗

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↗

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↗

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↗

arXiv1998

Interlayer Coherence in the $ν=1$ and $ν=2$ Bilayer Quantum Hall States

We have measured the Hall-plateau width and the activation energy of the bilayer quantum Hall (BLQH) states at the Landau-level filling factor $ν=1$ and 2 by tilting the sample and simultaneously changing the electron density in each quantum well. The phase transition between the commensurate and incommensurate states are confirmed at $ν=1$ and discovered at $ν=2$. In particular, three different $ν=2$ BLQH states are identified; the compound state, the coherent commensurate state, and the coherent incommensurate state.

cond-mat.mes-hall↗

arXiv1998

Phase Transition in ν=2 Bilayer Quantum Hall State

The Hall-plateau width and the activation energy were measured in the bilayer quantum Hall state at filling factor ν=2, 1 and 2/3, by changing the total electron density and the density ratio in the two quantum wells. Their behavior are remarkably different from one to another. The ν=1 state is found stable over all measured range of the density difference, while the ν=2/3$ state is stable only around the balanced point. The ν=2 state, on the other hand, shows a phase transition between these two types of the states as the electron density is changed.

cond-mat.mes-hall↗

arXiv1997

Anomalous Stability of nu=1 Bilayer Quantum Hall State

We have studied the fractional and integer quantum Hall (QH) effects in a high-mobility double-layer two-dimensional electron system. We have compared the "stability" of the QH state in balanced and unbalanced double quantum wells. The behavior of the n=1 QH state is found to be strikingly different from all others. It is anomalously stable, though all other states decay, as the electron density is made unbalanced between the two quantum wells. We interpret the peculiar features of the nu=1 state as the consequences of the interlayer quantum coherence developed spontaneously on the basis of the composite-boson picture.

cond-mat.mes-hall↗