Publications

*Tomoaki Ota, Masayuki Hashisaka, Koji Muraki, and Toshimasa Fujisawa,
Negative and positive cross-correlations of current noises in quantum Hall edge channels at bulk filling factor v =1,
Journal of Physics: Condensed Matter 29, 225302 (2017).

[Summary] Cross-correlation noise in electrical currents generated from a series connection of two quantum point contacts (QPCs), the injector and the detector, is described for investigating energy relaxation in quantum Hall edge channels at bulk filling factor v =1. We address the importance of tuning the energy bias across the detector for this purpose. For a long channel with a macroscopic floating ohmic contact that thermalizes the electrons, the cross-correlation turns from negative values to the maximally positive value (identical noise in the two currents) by tuning the effective energy bias to zero. This can be understood by considering competition between the low-frequency charge fluctuation generated at the injector, which contributes positive correlation, and the partition noise at the detector, which gives negative correlation. Strikingly, even for a short channel without intentional thermalization, significantly large positive correlation is observed in contrast to negative values expected for coherent transport between the two QPCs.

*Masayuki Hashisaka, Naoaki Hiyama, Takafumi Akiho, Koji Muraki, *Toshimasa Fujisawa,
Waveform measurement of charge- and spin-density wavepackets in a chiral Tomonaga–Luttinger liquid,
Nature Physics 13, 559-562 (2017).

[Summary] In contrast to a free-electron system, a Tomonaga–Luttinger (TL) liquid in a one-dimensional (1D) electron system hosts charge and spin excitations as independent entities. When an electron is injected into a TL liquid, it transforms into charge- and spin-density wavepackets that propagate at different group velocities and move away from each other. This process, known as spin–charge separation, is the hallmark of TL physics. While spin–charge separation has been probed in momentum- or frequency-domain measurements in various 1D systems, waveforms of separated excitations, which are a direct manifestation of the TL behaviour, have been long awaited to be measured. Here, we present a waveform measurement for the pseudospin–charge separation process in a chiral TL liquid comprising quantum Hall edge channels. The charge- and pseudospin-density waveforms are captured by utilizing a spin-resolved sampling scope that records the spin-up or -down component of the excitations. This experimental technique provides full information for time evolution of the 1D electron system, including not only propagation of TL eigenmodes but also their decay in a practical device14.

Kazuhisa Washio, Ryo Nakazawa, Masayuki Hashisaka, Koji Muraki, Y. Tokura, and Toshimasa Fujisawa,
Long-lived binary tunneling spectrum in the quantum Hall Tomonaga-Luttinger liquid,
Physical Review B 93, 075304 (2016).

[Summary] 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.

D. Imanaka, S. Sharmin, Masayuki Hashisaka, Koji Muraki, and *Toshimasa Fujisawa,
Exchange-induced spin blockade in a two-electron double quantum dot,
Physical Review Letters 115, 176802 (2015).

[Summary] We have experimentally identified the exchange-induced spin blockade in a GaAs double quantum dot. The transport is suppressed only when the eigenstates are well-defined singlet and triplet states, and thus sensitive to dynamic nuclear-spin polarization that causes singlet-triplet mixing. This gives rise to unusual current spectra, such as a sharp current dip and an asymmetric current profile near the triplet resonance of a double quantum dot. Numerical simulations suggest that the current dip is a signature of identical nuclear-spin polarization in the two dots, which is attractive for coherent spin manipulations in a material with nuclear spins.

J. C. H. Chen, Y. Sato, R. Kosaka, Masayuki Hashisaka, Koji Muraki, and Toshimasa Fujisawa,
Enhanced electron-phonon coupling for a semiconductor charge qubit in a surface phonon cavity,
Scientific Reports 5, 15176 (2015).

[Summary] Electron-phonon coupling is a major decoherence mechanism, which often causes scattering and energy dissipation in semiconductor electronic systems. However, this electron-phonon coupling may be used in a positive way for reaching the strong or ultra-strong coupling regime in an acoustic version of the cavity quantum electrodynamic system. Here we propose and demonstrate a phonon cavity for surface acoustic waves, which is made of periodic metal fingers that constitute Bragg reflectors on a GaAs/AlGaAs heterostructure. Phonon band gap and cavity phonon modes are identified by frequency, time and spatially resolved measurements of the piezoelectric potential. Tunneling spectroscopy on a double quantum dot indicates the enhancement of phonon assisted transitions in a charge qubit. This encourages studying of acoustic cavity quantum electrodynamics with surface phonons.

Naoaki Hiyama, *Masayuki Hashisaka, and Toshimasa Fujisawa,
An edge magnetoplasmon Mach-Zehnder interferometer,
Applied Physics Letters 107, 143101 (2015).

[Summary] We report an edge-magnetoplasmon (EMP) Mach-Zehnder (MZ) interferometer in a quantum Hall system. The MZ interferometer, which is based on the interference of two EMP beams traveling in chiral one-dimensional edge channels, is constructed by tailoring edge channels with functional devices such as splitters and delay lines. We measured 1 GHz EMP beams transmitted through the interferometer while tuning the phase evolution along two interference paths using tunable delay lines. Clear interference patterns as a function of the phase difference ensure the MZ interference. Moreover, the MZ interferometry is applied to evaluate the EMP transport through an attenuator interposed in one of the paths. This technique will be useful for investigating the functionalities of devices in plasmonics.

*Masayuki Hashisaka, Tomoaki Ota, Koji Muraki, and Toshimasa Fujisawa,
Shot-noise evidence of fractional quasiparticle creation in a local fractional quantum Hall state,
Physical Review Letters 114, 056802/1-5 (2015).

[Summary] We experimentally identify fractional quasiparticle creation in a tunneling process through a local fractional quantum Hall (FQH) state. The local FQH state is prepared in a low-density region near a quantum point contact in an integer quantum Hall (IQH) system. Shot-noise measurements reveal a clear transition from elementary-charge tunneling at low bias to fractional-charge tunneling at high bias. The fractional shot noise is proportional to T1(1−T1) over a wide range of T1, where T1 is the transmission probability of the IQH edge channel. This binomial distribution indicates that fractional quasiparticles emerge from the IQH state to be transmitted through the local FQH state. The study of this tunneling process enables us to elucidate the dynamics of Laughlin quasiparticles in FQH systems.

*Norio Kumada, Preden Roulleau, Benoit Roche, Masayuki Hashisaka, Hiroki Hibino, Ivana Petković, and Christian Glattli,
Resonant edge magnetoplasmons and their decay in graphene,
Physical Review Letters 113, 266601 (2014).

[Summary] We investigate resonant edge magnetoplasmons (EMPs) and their decay in graphene by high-frequency electronic measurements. From EMP resonances in disk shaped graphene, we show that the dispersion relation of EMPs is nonlinear due to interactions, giving rise to the intrinsic decay of EMP wave packets. We also identify extrinsic dissipation mechanisms due to interaction with localized states in bulk graphene from the decay time of EMP wave packets. We indicate that, owing to the linear band structure and the sharp edge potential, EMP dissipation in graphene can be lower than that in GaAs systems.

Masakazu Yamagishi, Narii Watase, Masayuki Hashisaka, Koji Muraki, and *Toshimasa Fujisawa,
Spin-dependent tunneling rates for electrostatically defined GaAs quantum dot,
Physical Review B 90, 035306 (2014).

[Summary] The tunneling rates for spin-up and -down electrons are investigated for a GaAs quantum dot in an in-plane magnetic field by using a real-time single-electron counting scheme with a nearby charge detector. An extremely small spin-polarized current on the order of attoamperes is analyzed with the spin and energy dependences of the tunneling rates. Fully spin-polarized current is obtained when only a spin-up Zeeman sublevel is located in the transport window. When both Zeeman sublevels are allowed to contribute to the transport, we find that the tunneling rate for spin-up electrons is considerably higher than that for spin-down electrons. This partially spin-polarized current can be explained by the exchange-enhanced spin splitting in low-density regions near the tunneling barriers.

*Norio Kumada, Romain Dubourget, Ken’ichi Sasaki, Shinichi Tanabe, Hiroki Hibino, Hiroshi Kamata, Masayuki Hashisaka, Koji Muraki, Toshimasa Fujisawa,
Plasmon transport and its guiding in graphene,
New Journal of Physics 16, 063055 (2014).

[Summary] Transport of plasmons in graphene has been investigated by time-resolved electrical measurements. We demonstrate that the velocity v (or the refractive index ∝ −v^1) and the characteristic impedance Z of the plasmon mode can be tuned through the carrier density. By exploiting the Z tunability, we present a gate-defined plasmonic waveguide. An important advantage of the gate-defined waveguide is dynamical switching of guiding characteristics with the gate voltages. One can tailor the patterns of gate electrodes to define two output waveguides branching off from a source waveguide, and the output waveguide can be switched by changing the gate voltages. Indeed, we show the routing in a Y-shaped channel: the path for the plasmon transmission can be selected by tuning Z of each path. These results can be well reproduced by simulation, encouraging the design of graphene-based plasmonic devices.

*Masayuki Hashisaka, Tomoaki Ota, Masakazu Yamagishi, Toshimasa Fujisawa and Koji Muraki,
Cross-correlation measurement of quantum shot noise using homemade transimpedance amplifiers,
Review of Scientific Instruments 85, 054704 (2014).

[Summary] We report a cross-correlation measurement system, based on a new approach, which can be used to measure shot noise in a mesoscopic conductor at milliKelvin temperatures. In contrast to other measurement systems in which high-speed low-noise voltage amplifiers are commonly used, our system employs homemade transimpedance amplifiers (TAs). The low input impedance of the TAs signifi-cantly reduces the crosstalk caused by unavoidable parasitic capacitance between wires. The TAs are designed to have a flat gain over a frequency band from 2 kHz to 1 MHz. Low-noise performance is attained by installing the TAs at a 4 K stage of a dilution refrigerator. Our system thus fulfills the technical requirements for cross-correlation measurements: low noise floor, high frequency band, and negligible crosstalk between two signal lines. Using our system, shot noise generated at a quantum point contact embedded in a quantum Hall system is measured. The good agreement between the obtained shot-noise data and theoretical predictions demonstrates the accuracy of the measurements.