arXiv subjects

Kentarou Sawano

Publications and source records attributed to Kentarou Sawano.

11 recordsLinked to original sources

arXiv2020

Large, tunable valley splitting and single-spin relaxation mechanisms in a Si/Si$_x$Ge$_{1-x}$ quantum dot

Valley splitting is a key figure of silicon-based spin qubits. Quantum dots in Si/SiGe heterostructures reportedly suffer from a relatively low valley splitting, limiting the operation temperature and the scalability of such qubit devices. Here, we demonstrate a robust and large valley splitting exceeding 200 $μ$eV in a gate-defined single quantum dot, hosted in molecular-beam epitaxy-grown $^{28}$Si/SiGe. The valley splitting is monotonically and reproducibly tunable up to 15 % by gate voltages, originating from a 6 nm lateral displacement of the quantum dot. We observe static spin relaxation times $T_1>1$ s at low magnetic fields in our device containing an integrated nanomagnet. At higher magnetic fields, $T_1$ is limited by the valley hotspot and by phonon noise coupling to intrinsic and artificial spin-orbit coupling, including phonon bottlenecking.

cond-mat.mes-hall↗

arXiv2019

Low-frequency spin qubit detuning noise in highly purified $^{28}$Si/SiGe

The manipulation fidelity of a single electron qubit gate-confined in a $^{28}$Si/SiGe quantum dot has recently been drastically improved by nuclear isotope purification. Here, we identify the dominant source for low-frequency qubit detuning noise in a device with an embedded nanomagnet, a remaining $^{29}$Si concentration of only 60$\,$ppm in the strained $^{28}$Si quantum well layer and a spin echo decay time $T_2^{\text{echo}}=128\,μ$s. The power spectral density (PSD) of the charge noise explains both the observed transition of a $1/f^2$- to a $1/f$-dependence of the detuning noise PSD as well as the observation of a decreasing time-ensemble spin dephasing time from $T_2^* \approx 20\,μ$s with increasing measurement time over several hours. Despite their strong hyperfine contact interaction, the few $^{73}$Ge nuclei overlapping with the quantum dot in the barrier do not limit $T_2^*$, as their dynamics is frozen on a few hours measurement scale. We conclude that charge noise and the design of the gradient magnetic field is the key to further improve the qubit fidelity.

quant-ph↗

arXiv2017

Room-temperature spin transport in n-Ge probed by four-terminal nonlocal measurements

We demonsrtate electrical spin injection and detection in $n$-type Ge ($n$-Ge) at room temperature using four-terminal nonlocal spin-valve and Hanle-effect measurements in lateral spin-valve (LSV) devices with Heusler-alloy Schottky tunnel contacts. The spin diffusion length ($λ$$_{\rm Ge}$) of the Ge layer used ($n \sim$ 1 $\times$ 10$^{19}$ cm$^{-3}$) at 296 K is estimated to be $\sim$ 0.44 $\pm$ 0.02 $μ$m. Room-temperature spin signals can be observed reproducibly at the low bias voltage range ($\le$ 0.7 V) for LSVs with relatively low resistance-area product ($RA$) values ($\le$ 1 k$Ω$$μ$m$^{2}$). This means that the Schottky tunnel contacts used here are more suitable than ferromagnet/MgO tunnel contacts ($RA \ge$ 100 k$Ω$$μ$m$^{2}$) for developing Ge spintronic applications.

cond-mat.mtrl-sci↗

arXiv2014

Cubic Rashba spin-orbit interaction of two-dimensional hole gas in strained-Ge/SiGe quantum well

The spin-orbit interaction (SOI) of the two-dimensional hole gas in the inversion symmetric semiconductor Ge is studied in a strained-Ge/SiGe quantum well structure. We observed weak anti-localization (WAL) in the magnetoconductivity measurement, revealing that the WAL feature can be fully described by the k-cubic Rashba SOI theory. Furthermore, we demonstrated electric field control of the Rashba SOI. Our findings reveal that the heavy hole (HH) in strained-Ge is a purely cubic-Rashba-system, which is consistent with the spin angular momentum mj = +-3/2 nature of the HH wave function.

cond-mat.mes-hall↗

arXiv2014

Exchange-driven magnetoresistance in silicon facilitated by electrical spin injection

We use electrical spin injection to probe exchange interactions in phosphorus doped silicon (Si:P). The detection is enabled by a magnetoresistance effect that demonstrates the efficiency of exchange in imprinting spin information from the magnetic lead onto the localized moments in the Si:P region. A unique Lorentzian-shaped signal existing only at low temperatures ($\lesssim 25 K$) is observed experimentally and analyzed theoretically in electrical Hanle effect measurement. It stems from spin-dependent scattering of electrons by neutral impurities in Si:P. The shape of this signal is not directly related to spin relaxation but to exchange interaction between spin-polarized electrons that are localized on adjacent impurities.

cond-mat.mtrl-sci↗

arXiv2013

In-plane magnetic field dependence of cyclotron relaxation time in a Si two-dimensional electron system

Cyclotron resonance of two-dimensional electrons is studied for a high-mobility Si/SiGe quantum well in the presence of an in-plane magnetic field, which induces spin polarization. The relaxation time $τ_{CR}$ shows a negative in-plane magnetic field dependence, which is similar to that of the transport scattering time $τ_t$ obtained from dc resistivity. The resonance magnetic field shows an unexpected negative shift with increasing in-plane magnetic field.

cond-mat.mes-hall↗

arXiv2011

Metallic Behavior of Cyclotron Relaxation Time in Two-Dimensional Systems

Cyclotron resonance of two-dimensional electrons is studied at low temperatures down to 0.4 K for a high-mobility Si/SiGe quantum well which exhibits a metallic temperature dependence of dc resistivity $ρ$. The relaxation time $τ_{\rm CR}$ shows a negative temperature dependence, which is similar to that of the transport scattering time $τ_t$ obtained from $ρ$. The ratio $τ_{\rm CR}/τ_t$ at 0.4 K increases as the electron density $N_s$ decreases, and exceeds unity when $N_s$ approaches the critical density for the metal-insulator transition.

cond-mat.mes-hall↗

arXiv2010

Excitonic Aharonov-Bohm Effect in Isotopically Pure 70Ge/Si Type-II Quantum Dots

We report on a magneto-photoluminescence study of isotopically pure 70Ge/Si self-assembled type-II quantum dots. Oscillatory behaviors attributed to the Aharonov-Bohm effect are simultaneously observed for the emission energy and intensity of excitons subject to an increasing magnetic field. When the magnetic flux penetrates through the ring-like trajectory of an electron moving around each quantum dot, the ground state of an exciton experiences a change in its angular momentum. Our results provide the experimental evidence for the phase coherence of a localized electron wave function in group-IV Ge/Si self-assembled quantum structures.

cond-mat.mes-hall↗

arXiv2009

Well-width dependence of valley splitting in Si/SiGe quantum wells

The valley splitting in Si two-dimensional electron systems is studied using Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps for 4 and 5.3 nm QWs, obtained from the temperature dependence of the longitudinal resistivity at the Landau level filling factor $ν=1$, are much larger than those for 10 and 20 nm QWs. This is consistent with the well-width dependence of the bare valley splitting estimated from the comparison with the Zeeman splitting in the Shubnikov-de Haas oscillations.

cond-mat.mes-hall↗

arXiv2009

Insulating Phases Induced by Crossing of Partially Filled Landau Levels in a Si Quantum Well

We study magnetotransport in a high mobility Si two-dimensional electron system by in situ tilting of the sample relative to the magnetic field. A pronounced dip in the longitudinal resistivity is observed during the Landau level crossing process for noninteger filling factors. Together with a Hall resistivity change which exhibits the particle-hole symmetry, this indicates that electrons or holes in the relevant Landau levels become localized at the coincidence where the pseudospin-unpolarized state is expected to be stable.

cond-mat.mes-hall↗

arXiv2008

Electronic Transport Properties of the Ising Quantum Hall Ferromagnet in a Si Quantum Well

Magnetotransport properties are investigated for a high mobility Si two dimensional electron systems in the vicinity of a Landau level crossing point. At low temperatures, the resistance peak having a strong anisotropy shows large hysteresis which is attributed to Ising quantum Hall ferromagnetism. The peak is split into two peaks in the paramagnetic regime. A mean field calculation for the peak positions indicates that electron scattering is strong when the pseudospin is partially polarized. We also study the current-voltage characteristics which exhibit a wide voltage plateau.

cond-mat.mes-hall↗