arXiv subjects

N. Kumada

Publications and source records attributed to N. Kumada.

30 records · Page 2Linked to original sources

arXiv2008

Boundary-mediated electron-electron interactions in quantum point contacts

An unusual increase of the conductance with temperature is observed in clean quantum point contacts for conductances larger than 2e^2/h. At the same time a positive magnetoresistance arises at high temperatures. A model accounting for electron-electron interactions mediated by bound- aries (scattering on Friedel oscillations) qualitatively describes the observation. It is supported by numerical simulation at zero magnetic field.

cond-mat.mes-hall↗

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

Decoherence of nuclear spins due to direct dipole-dipole interactions probed by resistively detected nuclear magnetic resonance

We study decoherence of nuclear spins in a GaAs quantum well structure using resistively detected nuclear magnetic resonance. The transverse decoherence time T2 of 75As nuclei is estimated from Rabi-type coherent oscillations as well as by using spin-echo techniques. By analyzing T2 obtained by decoupling techniques, we extract the role of dipole-dipole interactions as sources of decoherence in GaAs. Under the condition that the device is tilted in an external magnetic field, we exhibit enhanced decoherence induced by the change in strength of the direct dipole-dipole interactions between first nearest-neighbor nuclei. The results agree well with simple numerical calculations.

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.

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arXiv2007

NMR Evidence for Spin Canting in a Bilayer nu = 2 Quantum Hall System

We investigate the electron spin states in the bilayer quantum Hall system at total Landau level filling factor nu =2 exploiting current-pumped and resistively detected NMR. The measured Knight shift, KS, of 75As nuclei reveals continuous variation of the out-of-plane electronic spin polarization between nearly full and zero as a function of density imbalance. Nuclear spin relaxation measurements indicate a concurrent development of an in-plane spin component. These results provide direct information on the spin configuration in this system and comprise strong evidence for the spin canting suggested by previous experiments.

cond-mat.mes-hall↗

arXiv2007

Nuclear spin population and its control toward initialization using an all-electrical sub-micron scale nuclear magnetic resonance device

We study the nuclear spin population in a GaAs quantum well structure and demonstrate its initialization using an all-electrical nuclear magnetic resonance (NMR) device. In our device, nuclear spins are dynamically polarized in a sub-micron scale region defined by split gates. The nuclear spin populations under various polarization conditions are estimated from resistively-detected pulsed NMR spectra. We find that nuclear spin populations are determined by electron spin configurations. By applying radio frequency pulses to the strongly polarized nuclear spins, we demonstrate the creation of two-qubit effective pure states, which is a crucial step toward NMR quantum computation.

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.

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arXiv2004

Spin Degree of Freedom in the nu =1 Bilayer Electron System Investigated via Nuclear Spin Relaxation

The nuclear-spin-relaxation rate 1/T_1 has been measured in a bilayer electron system at and around total Landau level filling factor nu =1. The measured 1/T_1, which probes electron spin fluctuations, is found to increase gradually from the quantum Hall (QH) state at low fields through a phase transition to the compressible state at high fields. Furthermore, 1/T_1 in the QH state shows a small but noticeable increase away from nu =1. These results demonstrate that, as opposed to common assumption, the electron spin degree of freedom is completely frozen neither in the QH nor compressible states.

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.

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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.

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