Akira Fukuda

Bio: Akira Fukuda is an academic researcher from Hyogo College of Medicine. The author has contributed to research in topics: Quantum Hall effect & Magnetic field. The author has an hindex of 6, co-authored 62 publications receiving 146 citations. Previous affiliations of Akira Fukuda include Kyoto University.

Magnetotransport study of the canted antiferromagnetic phase in bilayer ν = 2 quantum Hall state

Abstract: Magnetotransport properties are investigated in the bilayer quantum Hall state at the total filling factor $\ensuremath{ u}=2$. We measured the activation energy elaborately as a function of the total electron density and the density difference between the two layers. Our experimental data demonstrate clearly the emergence of the canted antiferromagnetic (CAF) phase between the ferromagnetic phase and the spin-singlet phase. The stability of the CAF phase is discussed by the comparison between experimental results and theoretical calculations using a Hartree-Fock approximation and an exact diagonalization study. The data reveal also an intrinsic structure of the CAF phase divided into two regions according to the dominancy between the intralayer and interlayer correlations.

The second order recombination and spin-exchange relaxation of atomic deuterium at low temperatures in a low magnetic field

Abstract: We measured the 2nd order recombination rates and spin-exchange relaxation of atomic deuterium (D) in a4He coated sample cell, using the hyperfine resonance of β(F = 1/2, mF= −1/2) − δ(F = 3/2, mF= −1/2) transition in a low magnetic field (3.9 mT) at temperatures between 0.6 K and 1.2 K. At lower temperatures below 0.9 K, the density decay of D atoms was dominated by D-D recombination on the liquid He surface. We found that the surface recombination cross length was 1DD= (5.5 ± 1.3) × 10−9cm and the adsorption energy of D on4He surface was ea= 3.97 ± 0.07 K. Compared with prior measurements at high magnetic fields by other groups, 1DDat low field was orders of magnitude smaller than what was expected when the scaling of 1/B2dependence of the direct recombination mechanism was used, and in addition, eawas significantly larger. This was attributed to the onset of the resonant recombination mechanism for the D-D surface recombination at high fields. Above 0.9 K, D-D volume recombination and recombination of D with hydrogen impurity became dominant processes of the density decay of D. The transverse relaxation times were measured and we determined the D-D spin-exchange relaxation rates, GDD= (1.4 ± 0.6) × 10−10cm3sec−1. It was smaller than theoretical calculations.

Pseudospin Soliton in the ν = 1 Bilayer Quantum Hall State

Abstract: We investigate a domain structure of pseudospins, a soliton lattice in the bilayer quantum Hall state at total Landau level filling factor $\ensuremath{ u}=1$, in a tilted magnetic field, where the pseudospin represents the layer degree of freedom. An anomalous peak in the magnetoresistance ${R}_{xx}$ appears at the transition point between the commensurate and incommensurate phases. The ${R}_{xx}$ at the peak is highly anisotropic for the angle between the in-plain magnetic field ${B}_{\ensuremath{\parallel}}$ and the current, and indicates a formation of the soliton lattice aligned parallel to ${B}_{\ensuremath{\parallel}}$. The temperature dependence of the ${R}_{xx}$ peak reveals that the dissipation is caused by thermal fluctuations of pseudospin solitons. We also study a phase diagram of the bilayer $\ensuremath{ u}=1$ system, and the effects of density imbalance between the two layers.

Relationship between conductance fluctuation and weak localization in graphene

Abstract: The relationship between the universal conductance fluctuation and the weak localization effect in monolayer graphene is investigated. By comparing experimental results with the predictions of the weak localization theory for graphene, we find that the ratio of the elastic intervalley scattering time to the inelastic dephasing time varies in accordance with the conductance fluctuation; this is a clear evidence connecting the universal conductance fluctuation with the weak localization effect. We also find a series of scattering lengths that are related to the phase shifts caused by magnetic flux by Fourier analysis.

Practical design for improving the sensitivity to search for dark matter axions with Rydberg atoms

Abstract: A microwave single-photon detector was developed with highly-excited alkaline Rydberg-atoms in a cooled resonant cavity to search for dark matter axions. This detector belongs to a microwave single-photon counter, thus being free from the standard quantum limit (SQL). High sensitivity of the present detector system was demonstrated by measuring the thermal blackbody radiations in the cavity at temperatures as low as 70 mK where the sensitivity is below the SQL. The detection sensitivity of the present system is mainly limited by stray electric fields present in the detection region. Practical design of a new experimental scheme with a guiding electric field through the atomic-beam trajectory is here presented and discussed to avoid the effect of stray electric field and thus to improve the detection sensitivity.

Dark matter axions revisited

Abstract: We study for what specific values of the theoretical parameters the axion can form the totality of cold dark matter. We examine the allowed axion parameter region in the light of recent data collected by the WMAP5 mission plus baryon acoustic oscillations and supernovae, and assume an inflationary scenario and standard cosmology. We also upgrade the treatment of anharmonicities in the axion potential, which we find important in certain cases. If the Peccei-Quinn symmetry is restored after inflation, we recover the usual relation between axion mass and density, so that an axion mass ${m}_{a}=(85\ifmmode\pm\else\textpm\fi{}3)\text{ }\text{ }\ensuremath{\mu}\mathrm{eV}$ makes the axion 100% of the cold dark matter. If the Peccei-Quinn symmetry is broken during inflation, the axion can instead be 100% of the cold dark matter for ${m}_{a}l15\text{ }\text{ }\mathrm{meV}$ provided a specific value of the initial misalignment angle ${\ensuremath{\theta}}_{i}$ is chosen in correspondence to a given value of its mass ${m}_{a}$. Large values of the Peccei-Quinn symmetry breaking scale correspond to small, perhaps uncomfortably small, values of the initial misalignment angle ${\ensuremath{\theta}}_{i}$.

Transition between electron localisation and antilocalisation in graphene

Abstract: The wave nature of electrons in low-dimensional structures manifests itself in conventional electrical measurements as a quantum correction to the classical conductance. This correction comes from the interference of scattered electrons which results in electron localisation and therefore a decrease of the conductance. In graphene, where the charge carriers are chiral and have an additional (Berry) phase of \pi, the quantum interference is expected to lead to anti-localisation: an increase of the conductance accompanied by negative magnetoconductance (a decrease of conductance in magnetic field). Here we observe such negative magnetoconductance which is a direct consequence of the chirality of electrons in graphene. We show that graphene is a unique two-dimensional material in that, depending on experimental conditions, it can demonstrate both localisation and anti-localisation effects. We also show that quantum interference in graphene can survive at unusually high temperatures, up to T~200 K.

New experimental approaches in the search for axion-like particles

Abstract: Axions and other very light axion-like particles appear in many extensions of the Standard Model, and are leading candidates to compose part or all of the missing matter of the Universe. They also appear in models of inflation, dark radiation, or even dark energy, and could solve some long-standing astrophysical anomalies. The physics case of these particles has been considerably developed in recent years, and there are now useful guidelines and powerful motivations to attempt experimental detection. Admittedly, the lack of positive signal of new physics at the high energy frontier, and in underground detectors searching for weakly interacting massive particles, is also contributing to the increase of the interest in axion searches. The experimental landscape is rapidly evolving, with many novel detection concepts and new experiments being proposed lately. An updated account of those initiatives is lacking in the literature. In this review we attempt to provide such a review. We will focus on the new experimental approaches and their complementarity, but will also review the most relevant recent results from the consolidated strategies and the prospects of new generation experiments under consideration in the field. We will also briefly review the latest developments of the theory, cosmology and astrophysics of axions and we will discuss the prospects to probe a large fraction of relevant parameter space in the coming decade.

Two-Dimensional Materials in Biosensing and Healthcare: From In Vitro Diagnostics to Optogenetics and Beyond

Abstract: Since the isolation of graphene in 2004, there has been an exponentially growing number of reports on layered two-dimensional (2D) materials for applications ranging from protective coatings to biochemical sensing. Due to the exceptional, and often tunable, electrical, optical, electrochemical, and physical properties of these materials, they can serve as the active sensing element or a supporting substrate for diverse healthcare applications. In this review, we provide a survey of the recent reports on the applications of 2D materials in biosensing and other emerging healthcare areas, ranging from wearable technologies to optogenetics to neural interfacing. Specifically, this review provides (i) a holistic evaluation of relevant material properties across a wide range of 2D systems, (ii) a comparison of 2D material-based biosensors to the state-of-the-art, (iii) relevant material synthesis approaches specifically reported for healthcare applications, and (iv) the technological considerations to facilitate mass production and commercialization.