arXiv subjects

Y. Hirayama

Publications and source records attributed to Y. Hirayama.

At least 73 records · Page 4Linked to original sources

arXiv2009

Superdeformation in Asymmetric N$>$Z Nucleus $^{40}$Ar

A rotational band with five $γ$-ray transitions ranging from 2$^{+}$ to 12$^{+}$ states was identified in $^{40}$Ar. This band is linked through $γ$ transitions from the excited 2$^{+}$, 4$^{+}$ and 6$^{+}$ levels to the low-lying states; this determines the excitation energy and the spin-parity of the band. The deduced transition quadrupole moment of 1.45$^{+0.49}_{-0.31} eb$ indicates that the band has a superdeformed shape. The nature of the band is revealed by cranked Hartree--Fock--Bogoliubov calculations and a multiparticle--multihole configuration is assigned to the band.

nucl-ex↗

arXiv2009

Microwave magnetoplasmon absorption by a 2DEG stripe

Microwave absorption by a high mobility 2DEG has been investigated experimentally using sensitive Electron Paramagnetic Resonance cavity technique. It is found that MW absorption spectra are chiefly governed by confined magnetoplasmon excitations in a 2DEG stripe. Spectra of the 2D magnetoplasmons are studied as a function of magnetic field, MW frequency and carrier density. The electron concentration is tuned by illumination and monitored using optical photoluminescence technique.

cond-mat.mes-hall↗

arXiv2008

The Quantum Hall Transition in Real Space: From Localized to Extended States

Using scanning tunneling spectroscopy in ultra-high vacuum at low temperature (T = 0.3 K) and high magnetic fields (B < 12 T), we directly probe electronic wave functions across an integer quantum Hall transition. In accordance with theoretical predictions, we observe the evolution from localized drift states in the insulating phases to branched extended drift states at the quantum critical point. The observed microscopic behavior close to the extended state indicates points of localized quantum tunneling, which are considered to be decisive for a quantitative description of the transition.

cond-mat.mes-hall↗

arXiv2008

Interplay between one-dimensional confinement and crystallographic anisotropy in ballistic hole quantum wires

We study the Zeeman splitting in induced ballistic 1D quantum wires aligned along the [233] and [011] axes of a high mobility (311)A undoped heterostructure. Our data shows that the g-factor anisotropy for magnetic fields applied along the high symmetry [011] direction can be explained by the 1D confinement only. However when the magnetic field is along [233] there is an interplay between the 1D confinement and 2D crystal anisotropy. This is highlighted for the [233] wire by an unusual non-monotonic behavior of the g-factor as the wire is made narrower.

cond-mat.mes-hall↗

arXiv2008

Electric-Field-Induced Nuclear Spin Resonance Mediated by Oscillating Electron Spin Domains in GaAs-Based Semiconductors

We demonstrate an alternative nuclear spin resonance using radio frequency (RF) electric field (nuclear electric resonance: NER) instead of magnetic field. The NER is based on the electronic control of electron spins forming a domain structure. The RF electric field applied to a gate excites spatial oscillations of the domain walls and thus temporal oscillations of the hyperfine field to nuclear spins. The RF power and burst duration dependence of the NER spectrum provides insight into the interplay between nuclear spins and the oscillating domain walls.

cond-mat.mes-hall↗

arXiv2008

A gate-defined silicon quantum dot molecule

We report electron transport measurements of a silicon double dot formed in multi-gated metal-oxide-semiconductor structures with a 15-nm-thick silicon-on-insulator layer. Tunable tunnel coupling enables us to observe an excitation spectrum in weakly coupled dots and an energy level anticrossing in strongly coupled ones. Such a quantum dot molecule with both charge and energy quantization provides the essential prerequisite for future implementation of silicon-based quantum computations.

cond-mat.mes-hall↗

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↗

arXiv2008

Pauli-Spin-Blockade Transport through a Silicon Double Quantum Dot

We present measurements of resonant tunneling through discrete energy levels of a silicon double quantum dot formed in a thin silicon-on-insulator layer. In the absence of piezoelectric phonon coupling, spontaneous phonon emission with deformation-potential coupling accounts for inelastic tunneling through the ground states of the two dots. Such transport measurements enable us to observe a Pauli spin blockade due to effective two-electron spin-triplet correlations, evident in a distinct bias-polarity dependence of resonant tunneling through the ground states. The blockade is lifted by the excited-state resonance by virtue of efficient phonon emission between the ground states. Our experiment demonstrates considerable potential for investigating silicon-based spin dynamics and spin-based quantum information processing.

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.

cond-mat.mes-hall↗

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

Anomalous Resistance Ridges Along Filling Factor $ν= 4i$

We report anomalous structure in the magnetoresistance of SiO$_2$/Si(100)/SiO$_2$ quantum wells. When Landau levels of opposite valleys are driven through coincidence at the Fermi level, the longitudinal resistance displays elevations at filling factors that are integer multiples of 4 $(ν=4i)$ accompanied by suppression on either side of $ν=4i$. This persists when either magnetic field or valley splitting is swept leading to resistance ridges running along $ν=4i$. The range of field over which they are observed points to the role of spin degeneracy, which is directly confirmed by their disappearance under in-plane magnetic field. The data suggest a new type of many-body effect due to the combined degeneracy of valley and spin.

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

Nuclear Spins in a Nanoscale Device for Quantum Information Processing

Coherent oscillations between any two levels from four nuclear spin states of I=3/2 have been demonstrated in a nanometre-scale NMR semiconductor device, where nuclear spins are all-electrically controlled. Using this device, we discuss quantum logic operations on two fictitious qubits of the I=3/2 system, and propose a quantum state tomography scheme based on the measurement of longitudinal magnetization, $M_z$.

quant-ph↗

arXiv2006

Ballistic transport in induced one-dimensional hole systems

We have fabricated and studied a ballistic one-dimensional p-type quantum wire using an undoped AlGaAs/GaAs heterostructure. The absence of modulation doping eliminates remote ionized impurity scattering and allows high mobilities to be achieved over a wide range of hole densities, and in particular, at very low densities where carrier-carrier interactions are strongest. The device exhibits clear quantized conductance plateaus with highly stable gate characteristics. These devices provide opportunities for studying spin-orbit coupling and interaction effects in mesoscopic hole systems in the strong interaction regime where rs > 10.

cond-mat.mes-hall↗

arXiv2006

Valley Polarization in Si(100) at Zero Magnetic Field

The valley splitting, which lifts the degeneracy of the lowest two valley states in a SiO$_2$/(100)Si/SiO$_2$ quantum well is examined through transport measurements. We demonstrate that the valley splitting can be observed directly as a step in the conductance defining a boundary between valley-unpolarized and polarized regions. This persists to well above liquid helium temperature and shows no dependence on magnetic field, indicating that single-particle valley splitting and valley-polarization exist in (100) silicon even at zero magnetic field.

cond-mat.mes-hall↗

arXiv2006

Nanometre-scale nuclear-spin device for quantum information processing

We have developed semiconductor point contact devices in which nuclear spins in a nanoscale region are coherently controlled by all-electrical methods. Different from the standard nuclear-magnetic resonance technique, the longitudinal magnetization of nuclear spins is directly detected by measuring resistance, resulting in ultra-sensitive detection of the microscopic quantity of nuclear spins. All possible coherent oscillations have been successfully demonstrated between two levels from four nuclear spin states of I = 3/2 nuclei. Quantum information processing is discussed based on two fictitious qubits of an I = 3/2 system and methods are described for performing arbitrary logical gates both on one and two qubits. A scheme for quantum state tomography based on Mz-detection is also proposed. As the starting point of quantum manipulations, we have experimentally prepared the effective pure states for the I = 3/2 nuclear spin system.

quant-ph↗