Norio Kumada

Plasmon transport in graphene investigated by time-resolved electrical measurements

Plasmons, which are collective charge oscillations, could provide a means of confining electromagnetic field to nanoscale structures. Recently, plasmonics using graphene have attracted interest, particularly because of the tunable plasmon dispersion, which will be useful for tunable frequency in cavity applications. 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, by exploiting electronic excitation and detection, we carry out time-resolved measurements of a charge pulse travelling in a plasmon mode in graphene corresponding to the gigahertz range. We demonstrate that the plasmon velocity can be changed over two orders of magnitude by applying a magnetic field B and by screening the plasmon electric field with a gate metal; at high B, edge magnetoplasmons, which are plasmons localized at the sample edge, are formed and their velocity depends on B, n and the gate screening effect.

Coherent zero-state and π-state in an exciton–polariton condensate array

The effect of quantum statistics in quantum gases and liquids results in observable collective properties among many-particle systems. One prime example is Bose–Einstein condensation, whose onset in a quantum liquid leads to phenomena such as superfluidity and superconductivity. A Bose–Einstein condensate is generally defined as a macroscopic occupation of a single-particle quantum state, a phenomenon technically referred to as off-diagonal long-range order due to non-vanishing off-diagonal components of the single-particle density matrix. The wavefunction of the condensate is an order parameter whose phase is essential in characterizing the coherence and superfluid phenomena. The long-range spatial coherence leads to the existence of phase-locked multiple condensates in an array of superfluid helium, superconducting Josephson junctions or atomic Bose–Einstein condensates. Under certain circumstances, a quantum phase difference of π is predicted to develop among weakly coupled Josephson junctions. Such a meta-stable π-state was discovered in a weak link of superfluid 3He, which is characterized by a ‘p-wave’ order parameter. The possible existence of such a π-state in weakly coupled atomic Bose–Einstein condensates has also been proposed, but remains undiscovered. Here we report the observation of spontaneous build-up of in-phase (‘zero-state’) and antiphase (‘π-state’) ‘superfluid’ states in a solid-state system; an array of exciton–polariton condensates connected by weak periodic potential barriers within a semiconductor microcavity. These in-phase and antiphase states reflect the band structure of the one-dimensional polariton array and the dynamic characteristics of metastable exciton–polariton condensates.

Unraveling the Spin Polarization of the ν = 5/2 Fractional Quantum Hall State

Toward Quantum Computing Quantum computers are expected to be able to tackle problems that would take classical computers many lifetimes to solve. Nonabelian states of matter can store quantum information in their topology, making them immune to environmental perturbations. A physical system expected to possess this unusual property is the fractional quantum Hall state at filling factor ν = 5/2. Tiemann et al. (p. 828, published online 26 January) used nuclear magnetic resonance to measure the polarization of the 5/2 state and found that it is fully polarized—a finding consistent with a nonabelian state—keeping hopes for topological fault-tolerant quantum computing alive. Nuclear magnetic resonance shows that an exotic state of matter may have the properties necessary for error-free quantum computing. The fractional quantum Hall (FQH) effect at filling factor ν = 5/2 has recently come under close scrutiny, as its ground state may possess quasi-particle excitations obeying nonabelian statistics, a property sought for topologically protected quantum operations. However, its microscopic origin remains unknown, and candidate model wave functions include those with undesirable abelian statistics. We report direct measurements of the electron spin polarization of the ν = 5/2 FQH state using resistively detected nuclear magnetic resonance. We find the system to be fully polarized, which unambiguously rules out the most likely abelian contender and lends strong support for the ν = 5/2 state being nonabelian. Our measurements reveal an intrinsically different nature of interaction in the first excited Landau level underlying the physics at ν = 5/2.

Spin Degree of Freedom in the nu = 1 Bilayer Electron System Investigated by Nuclear Spin Relaxation

The nuclear-spin-relaxation rate 1/T(1) has been measured in a bilayer electron system at and around total Landau level filling factor nu=1. The measured 1/T(1), which probes electron spin fluctuations, is found to increase gradually from the quantum Hall (QH) state at low fields through a phase transition to the compressible state at high fields. Furthermore, 1/T(1) in the QH state shows a noticeable increase away from nu=1. These results demonstrate that, as opposed to common assumption, the electron spin degree of freedom is not completely frozen either in the QH or the compressible states.

INTERLAYER COHERENCE IN UPSILON =1 AND UPSILON =2 BILAYER QUANTUM HALL STATES

We have measured the Hall-plateau width and the activation energy of the bilayer quantum Hall (BLQH) states at the Landau-level filling factor

n^+-GaAs Back-Gated Double-Quantum-Well Structures with Full Density Control

\nu=1

Interlayer charge transfer in bilayer quantum Hall states at various filling factors

and 2 by tilting the sample and simultaneously changing the electron density in each quantum well. The phase transition between the commensurate and incommensurate states are confirmed at

NMR evidence for spin canting in a bilayer nu=2 quantum hall system.

\nu =1

Distributed-element circuit model of edge magnetoplasmon transport

and discovered at

Nuclear spin population and its control toward initialization using an all-electrical submicron scale nuclear magnetic resonance device

\nu =2