arXiv subjects

T. Fujisawa

Publications and source records attributed to T. Fujisawa.

At least 19 recordsLinked to original sources

arXiv2016

Long-lived binary tunneling spectrum in a quantum-Hall Tomonaga-Luttinger liquid

The existence of long-lived non-equilibrium states without showing thermalization, which has previously been demonstrated in time evolution of ultracold atoms, suggests the possibility of their spatial analogue in transport behavior of interacting electrons in solid-state systems. Here we report long-lived non-equilibrium states in one-dimensional edge channels in the integer quantum Hall regime. An indirect heating scheme in a counterpropagating configuration is employed to generate a non-trivial binary spectrum consisting of high- and low-temperature components. This unusual spectrum is sustained even after travelling 5 - 10 μm, much longer than the length for electronic relaxation (about 0.1 μm), without showing significant thermalization. This observation is consistent with the integrable model of Tomonaga-Luttinger liquid. The long-lived spectrum implies that the system is well described by non-interacting plasmons, which are attractive for carrying information for a long distance.

cond-mat.mes-hall↗

arXiv2014

Time-resolved charge fractionalization in inhomogeneous Luttinger liquids

The recent observation of charge fractionalization in single Tomanga-Luttinger liquids (TLLs) [Kamata et al., Nature Nanotech., 9 177 (2014)] opens new routes for a systematic investigation of this exotic quantum phenomenon. In this Letter we perform measurements on two adjacent TLLs and put forward an accurate theoretical framework to address the experiments. The theory is based on the plasmon scattering approach and can deal with injected charge pulses of arbitrary shape in TLL regions. We accurately reproduce and interpret the time-resolved multiple fractionalization events in both single and double TLLs. The effect of inter-correlations between the two TLLs is also discussed.

cond-mat.mes-hall↗

arXiv2012

Magnetic-Field Dependence of Tunnel Couplings in Carbon Nanotube Quantum Dots

By means of sequential and cotunneling spectroscopy, we study the tunnel couplings between metallic leads and individual levels in a carbon nanotube quantum dot. The levels are ordered in shells consisting of two doublets with strong- and weak-tunnel couplings, leading to gate-dependent level renormalization. By comparison to a one- and two-shell model, this is shown to be a consequence of disorder-induced valley mixing in the nanotube. Moreover, a parallel magnetic field is shown to reduce this mixing and thus suppress the effects of tunnel renormalization.

cond-mat.str-el↗

arXiv2012

Plasmon transport in graphene investigated by time-resolved measurement

Plasmons, which are collective charge oscillations, offer the potential to use optical signals in nano-scale electric circuits. Recently, plasmonics using graphene have attracted interest, particularly because of the tunable plasmon frequency through the carrier density $n$. However, the $n$ dependence of the plasmon velocity is weak ($\propto n^{1/4}$) and it is difficult to tune the frequency over orders of magnitude. Here, we demonstrate that the velocity of plasmons in graphene can be changed over two orders of magnitude by applying a magnetic field $B$ and by the presence/absence of a gate; at high $B$, edge magnetoplasmons (EMPs), which are plasmons localized at the sample edge, are formed and their velocity depends on $B$ and the gate screening effect. The wide range tunability of the velocity and the observed low-loss plasmon transport encourage designing graphene nanostructures for plasmonics applications.

cond-mat.mes-hall↗

arXiv2010

Intrinsic gap and exciton condensation in the nu_T=1 bilayer system

We investigate the quasiparticle excitation of the bilayer quantum Hall (QH) system at total filling factor $ν_{\mathrm{T}} = 1$ in the limit of negligible interlayer tunneling under tilted magnetic field. We show that the intrinsic quasiparticle excitation is of purely pseudospin origin and solely governed by the inter- and intra-layer electron interactions. A model based on exciton formation successfully explains the quantitative behavior of the quasiparticle excitation gap, demonstrating the existence of a link between the excitonic QH state and the composite fermion liquid. Our results provide a new insight into the nature of the phase transition between the two states.

cond-mat.str-el↗

arXiv2009

Fano-Kondo interplay in a side-coupled double quantum dot

We investigate low-temperature transport characteristics of a side-coupled double quantum dot where only one of the dots is directly connected to the leads. We observe Fano resonances, which arise from interference between discrete levels in one dot and the Kondo effect, or cotunneling in general, in the other dot, playing the role of a continuum. The Kondo resonance is partially suppressed by destructive Fano interference, reflecting novel Fano-Kondo competition. We also present a theoretical calculation based on the tight-binding model with slave boson mean field approximation, which qualitatively reproduces the experimental findings.

cond-mat.mes-hall↗

arXiv2009

Single-Electron Tunneling and the Fluctuation Theorem

Experiments on the direction-resolved full-counting statistics of single-electron tunneling allow testing the fundamentally important Fluctuation Theorem (FT). At the same time, the FT provides a frame for analyzing such data. Here we consider tunneling through a double quantum dot system which is coupled capacitively to a quantum point contact (QPC) detector. Fluctuations of the environment, including the shot noise of the QPC, lead to an enhancement of the effective temperature in the FT. We provide a quantitative explanation of this effect; in addition we discuss the influence of the finite detector bandwidth on the measurements.

cond-mat.mes-hall↗

arXiv2009

Bidirectional Single-Electron Counting and the Fluctuation Theorem

We investigate theoretically and experimentally the full counting statistics of bidirectional single-electron tunneling through a double quantum dot in a GaAs/GaAlAs heterostructure and compare with predictions of the fluctuation theorem (FT) for Markovian stochastic processes. We observe that the quantum point contact electrometer used to study the transport induces nonequilibrium shot noise and dot-level fluctuations and strongly modifies the tunneling statistics. As a result, the FT appears to be violated. We show that it is satisfied if the back-action of the electrometer is taken into account, and we provide a quantitative estimate of this effect.

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

A triple quantum dot in a single wall carbon nanotube

A top-gated single wall carbon nanotube is used to define three coupled quantum dots in series between two electrodes. The additional electron number on each quantum dot is controlled by top-gate voltages allowing for current measurements of single, double and triple quantum dot stability diagrams. Simulations using a capacitor model including tunnel coupling between neighboring dots captures the observed behavior with good agreement. Furthermore, anti-crossings between indirectly coupled levels and higher order cotunneling are discussed.

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

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

Many-body excitations in tunneling current spectra of a few-electron quantum dot

Inherent asymmetry in the tunneling barriers of few-electron quantum dots induces intrinsically different tunneling currents for forward and reverse source-drain biases in the non-linear transport regime. Here we show that in addition to spin selection rules, overlap matrix elements between many-body states are crucial for the correct description of tunneling transmission through quantum dots at large magnetic fields. Signatures of excited (N-1)-electron states in the transport process through the N-electron system are clearly identified in the measured transconductances. Our analysis clearly confirms the validity of single-electron quantum transport theory in quantum dots.

cond-mat.mes-hall↗

arXiv2006

Surface Acoustic Wave induced Transport in a Double Quantum Dot

We report on non-adiabatic transport through a double quantum dot under irradiation of surface acoustic waves generated on-chip. At low excitation powers, absorption and emission of single and multiple phonons is observed. At higher power, sequential phonon assisted tunneling processes excite the double dot in a highly non-equilibrium state. The present system is attractive for studying electron-phonon interaction with piezoelectric coupling.

cond-mat.mes-hall↗

arXiv2005

Pauli spin blockade in cotunneling transport through a double quantum dot

We study spin correlation in a double quantum dot containing a few electrons in each dot (around 10). Clear current rectification with negative differential conductance is observed in the cotunneling regime, which is well explained by a Pauli spin blockade. The current feature is discussed in connection with exchange singlet-triplet splitting based on two simple models, one of which takes coherent interdot coupling and the other incoherent interdot coupling into account.

cond-mat.mes-hall↗

arXiv2005

Electrical pump-and-probe study of spin singlet-triplet relaxation in a quantum dot

Spin relaxation from a triplet excited state to a singlet ground state in a semiconductor quantum dot is studied by employing an electrical pump-and-probe method. Spin relaxation occurs via cotunneling when the tunneling rate is relatively large, confirmed by a characteristic square dependence of the relaxation rate on the tunneling rate. When cotunneling is suppressed by reducing the tunneling rate, the intrinsic spin relaxation is dominated by spin-orbit interaction. We discuss a selection rule of the spin-orbit interaction based on the observed double-exponential decay of the triplet state.

cond-mat.mes-hall↗

arXiv2004

Background charge fluctuation in a GaAs quantum dot device

We investigate background charge fluctuation in a GaAs quantum dot device by measuring 1/f noise in the single-electron tunneling current through the dot. The current noise is understood as fluctuations of the confinement potential and tunneling barriers. The estimated potential fluctuation increases almost linearly with temperature, which is consistent with a simple model of the 1/f noise. We find that the fluctuation increases very slightly when electrons are injected into excited states of the quantum dot.

cond-mat.mes-hall↗