
I am interested in quantum trasnport and charge dynamics in twodimensional systems.
I mainly use timeresolved transport measurements in the GHz range for investigating plasmon transport in semiconductors and graphene.
Very recently, I have started THz spectroscopy for graphene plasmons.
Achievements


Active Spatial Control of Graphene Plasmons
(by THz spectroscopy)
Graphene offers the possibility for actively controlling plasmon confinement and propagation by tailoring its spatial conductivity.
Here we demonstrate full electrical control of plasmon reflection in graphene at electronic boundaries induced by a transparent patterned zinc oxide gate. This approach enables plasmons to be confined to desired regions.
Commun. Mater. 1, 7 (2020).
Related work: ACS Photon. 6, 947 (2019).



Charge Fractionalization in Artificial TomonagaLuttinger Liquids
(by highfrequency transport measurement)
We investigate charge fractionalizations in artificial TomonagaLuttinger liquids (TLLs) composed of two capacitively coupled quantum Hall edge channels (ECs) in graphene.
Using timeresolved transport measurements, we show that the fractionalization ratio and the TLL mode velocity vary with the interaction strength, following a unique function predicted by the TLL theory.
Phys. Rev. B 96, 081101(R) (2017).
Related work: Nature Nanotechnol. 9, 177 (2014).



Shot Noise Generated by Graphene pn Junctions in the Quantum Hall Effect Regime
(by shot noise measurement)
Graphene offers a unique system to investigate transport of Dirac Fermions at pn junctions.
In a magnetic field, combination of quantum Hall physics and the characteristic transport across pn junctions leads to a mixing of electronlike and holelike modes and their subsequent partitioning. By measuring the shot noise generated by the mixing and partitioning process, we show that a graphene pn junction could be used as an electronic beam splitter.
Nat. Commun. 6, 8068 (2015).



Edge Magnetoplasmons and Their Decay in Graphene
(by highfrequency transport measurement)
We investigate resonant edge magnetoplasmons (EMPs) and their decay in graphene by highfrequency (upto 50 GHz) electronic measurements. By frequencydomain measurement of EMP resonances in disk shaped graphene, we determine the dispersion relation of EMPs in graphene.
By timedomain measurement of the time evolution of EMP wavepackets, we identify extrinsic dissipation mechanisms.
Phys. Rev. Lett. 113, 266601 (2014).



Tunable Edge Magnetoplasmons in Graphene
(by highfrequency transport measurement)
Graphene plasmonics have attracted interest, particularly because of the tunable plasmon dispersion. However, the carrier density dependence of the dispersion is weak (proportional to n1/4) and it is difficult to tune the frequency over orders of magnitude. Here, we demonstrate that the plasmon velocity can be changed over two orders of magnitude by applying a magnetic field and by screening the plasmon electric field with a gate metal.
Nat. Commun. 4, 1363 (2013).
Related work: Phys. Rev. B 84, 045314 (2011).



Unraveling the Spin Polarization of the nu = 5/2 Fractional Quantum Hall State
(by resistivelydetected NMR)
The fractional quantum Hall (FQH) effect at filling factor ƒË = 5/2 has recently come under close scrutiny, as it may possess quasiparticle excitations obeying nonabelian statistics, a property sought for topologically protected quantum operations. Yet, its microscopic origin remains unidentified. Here we report direct measurements of the electron spin polarization of the nu = 5/2 FQH state. We find the system to be fully polarized, which unambiguously rules out the mostlikely abelian contender and thus lends strong support for the nu = 5/2 state being nonabelian.
Science 335, 828 (2012).



Electric Field Induced Nuclear Spin Resonance in GaAsBased Semiconductors
(by resistivelydetected NMR)
We demonstrate an alternative nuclear spin resonance using a radio frequency (rf) electric field instead of a magnetic field. It 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.
Phys. Rev. Lett. 101, 137602 (2008).



SubbandLandauLevel Coupling in Bilayer Quantum Hall States
(by transport measurement)
We study effects of tilted magnetic fields on energy levels in a doublequantumwell (DQW) system, focusing on the coupling of subbands and Landau levels (LLs). The subbandLL coupling induces anticrossings between LLs directly manifested in the magnetoresistance.
We also find that when the DQW potential is asymmetric, LL coupling occurs even within a subband.
Phys. Rev. B 77, 155324 (2008).



NMR Evidence for Spin Canting in a Bilayer nu=2 Quantum Hall System
(by resistivelydetected NMR)
We investigate electron spin states in the bilayer quantum Hall system at total Landau level filling factor nu=2 by resistivelydetected NMR. The measured Knight shift reveals continuous variation of the outofplane electronic spin polarization between nearly full and zero as a function of density imbalance. Nuclear spin relaxation measurements indicate a concurrent development of an inplane spin component. These results provide direct information on the spin configuration in this system and comprise strong evidence for the spin canting.
Phys. Rev. Lett. 99, 076805 (2007).



AllElectrical Nuclear Magnetic Resonance
(by resistivelydetected NMR)
We study the nuclear spin population in a GaAs quantum well structure and demonstrate its initialization using an allelectrical nuclear magnetic resonance (NMR) device. In this device, nuclear spins are dynamically polarized in a submicron scale region defined by split gates. We find that nuclear spin populations are determined by electron spin configurations.
Appl. Phys. Lett. 90, 102118 (2007).
Related work: Appl. Phys. Lett. 91, 193101 (2007).



LowFrequency Spin Dynamics in a Canted Antiferromagnet
(by resistivelydetected NMR)
Resistively detected nuclear spin relaxation measurements in closely separated twodimensional electron systems reveal strong lowfrequency electronspin fluctuations in the quantum Hall regime.
Our data demonstrate the realization of a twodimensional system with planar broken symmetry, in which fluctuations do not freeze out when approaching the zero temperature limit.
Science 313, 329 (2006).



Spin Degree of Freedom in the nu=1 Bilayer Electron System Investigated by Nuclear Spin Relaxation
(by resistivelydetected NMR)
We investigate electron spin fluctuations around nu=1 in a bilayer quantum Hall (QH) system.
The measured 1/T1 is found to increase gradually from the QH state at low fields through a phase transition to the compressible state at high fields.
Our result demonstrates that, as opposed to common assumption, the electron spin degree of freedom is not completely frozen in the nu=1 QH state.
Phys. Rev. Lett. 94, 096802 (2005).
Related works:
Phys. Rev. Lett. 100, 106803 (2008).
Phys. Rev. Lett. 104, 056802 (2010).



Phase Diagram of Interacting Composite Fermions in the Bilayer nu=2/3 Quantum Hall Effect
(by transport measurement)
We study the phase diagram of composite fermions (CFs) in the presence of spin and pseudospin degrees of freedom in the bilayer nu=2/3 quantum Hall (QH) state.
While a noninteracting CF model provides a qualitative account of the phase diagram, the observed renormalization of tunneling gap and a nonQH state at high densities are the manifestations of interactions between CFs.
Phys. Rev. Lett. 89, 116802 (2002).
Related work: Phys. Rev. B 69, 155319 (2004).



Doubly Enhanced Skyrmions in a Bilayer Quantum Hall State
(by transport measurement)
We compare skyrmion excitations in the bilayer quantum Hall (QH) state at the Landaulevel filling factor nu=2 and in the monolayer QH state at nu=1.
The observed number of flipped spins, N_spin, is 14 in the bilayer QH state, while N_spin=7 in the monolayer QH state.
The difference is interpreted to be due to the interlayer exchange interaction.
J. Phys. Soc. Jpn. 69, 3178 (2000).


