arXiv subjects

Kimitoshi Kono

Publications and source records attributed to Kimitoshi Kono.

At least 19 recordsLinked to original sources

arXiv2024

Influence of Magnetic Field on Surface Andreev Bound States in Superfluid $^3$He-B Studied by Mobility of Electron Bubble

The B phase of superfluid $^\textrm 3$He ($^\textrm 3$He-B) is topologically nontrivial and the surface Andreev bound states formed on a surface are conceived as Majorana fermions. In a magnetic field, the surface Andreev bound states acquire a Zeeman gap. How the Zeeman gap opens when a magnetic field is applied is intimately related to how the topological properties are lost. In this article, we study the mobility of an electron bubble trapped under a free surface of $^\textrm 3$He-B in a magnetic field of 0.25 T to examine the influence of the magnetic field on the surface Andreev bound states. We observe experimentally and theoretically that, with decreasing temperature, the mobility at 0.25 T increases steeper than that in zero magnetic field when the thermal energy is comparable to the Zeeman energy, while the mobility is slightly smaller than that in zero magnetic field at higher temperatures. These features are understood by the opening of the Zeeman gap, the resulting change in the density of states within the bulk superfluid gap, and the distortion of the bulk superfluid gap by the magnetic field.

cond-mat.other↗

arXiv2024

Contribution of Andreev reflection to the mobility of surface state electrons on superfluid $^3$He-B

The mobility of the Wigner solid on the superfluid $^3$He is determined by the momentum transfer from the scattered $^3$He quasiparticles at the free surface. The scattering process of the quasiparticles is classified into the normal reflection and the Andreev retroreflection. Since the quasiparticles nearly conserve the momentum in the process of the Andreev retroreflection at the free surface, the Andreev reflected quasiparticles do not produce a resistive force to the Wigner solid. In this report, we have analytically calculated the contribution of the Andreev retroreflection to the mobility of the Wigner solid on superfluid $^3$He-B by employing a realistic model order parameter with the free surface. The Andreev retroreflection is lacked for quasiparticles with energy above the bulk energy gap under the model order parameter. Then, the Andreev retroreflection does not contribute to a rise in the mobility of the Wigner solid on the superfluid $^3$He-B. The present model calculation is in good agreement with the previous experimental observation. We have also discussed the Andreev retroreflection under a self-consistently calculated order parameter.

cond-mat.other↗

arXiv2021

Visualization of Oscillatory Electron Dynamics on the Surface of Liquid Helium

We have measured signals induced in 5 Corbino electrodes by spontaneous oscillations of 2D surface electrons on liquid helium at $\sim$0.3\,K, with a perpendicular magnetic field and microwave radiation. Analysis using multi-scale, time-resolved, methods yields results consistent with magnetoplasmons modulated by slow surface gravity waves, with the latter requiring consideration of the 3rd dimension. Calculation of phase differences and phase coherences between signals from differently-positioned pairs of electrodes enables reconstruction of the electron dynamics on the helium surface.

cond-mat.other↗

arXiv2020

Memory device employing hysteretic properties of a tungsten filament in superfluid helium-4

A tungsten filament immersed in superfluid helium has strong hysteretic $I$-$V$ characteristics. By increasing the applied voltage, a remarkable current drop occurs at a transition voltage, at which the filament enters a non-ohmic hot state and becomes covered with a helium gas sheath. The return to an ohmic state occurs at a lower voltage because of the poor heat conduction of the gas sheath. Hence, the $I$-$V$ characteristic is strongly hysteretic. The stable hysteresis window was employed to fabricate a novel memory device, which demonstrated fast switching and stable reading.

physics.app-ph↗

arXiv2018

Review: Observation of Majorana Bound States at a Free Surface of $^3$He-B

The $p$-wave superfluid $^3$He is a textbook example of topological superfluids. Among its multiple superfluid phases, the B phase ($^3$He-B) is known as a topological state protected by time-reversal symmetry. One of the important topological features of $^3$He-B is the formation of bound states at its surface. Notably, such surface Andreev bound states are predicted to be Majorana fermions, i.e., their antiparticles are identical to their particles. Because of the well-elucidated bulk properties of the superfluid $^3$He owing to its cleanliness, $^3$He-B provides an ideal platform to pursue Majorana fermions in condensed matter systems. In this article, we review recent investigations of surface Andreev bound states by the mobility of ions trapped below a free surface of $^3$He-B.The free surface is an ideal surface providing a specular boundary condition; The surface Andreev bound states formed there are expected to be Majorana fermions with a well-defined energy spectrum. We show that the temperature and depth dependences of the experimentally obtained mobility of negative ions [H. Ikegami {\it et al.,} {\it J. Phys. Soc. Jpn.} {\bf 82} 124607 (2013)] are {\it quantitatively} reproduced by a theoretical study that includes scattering of the surface Andreev bound states [Y. Tsutsumi, {\it Phys. Rev. Lett.} {\bf 118} 145301 (2017)].This quantitative agreement unambiguously demonstrates the experimental detection of surface Andreev bound states.We also discuss the future prospects of the Majorana physics in $^3$He-B.

cond-mat.other↗

arXiv2018

Can warmer than room temperature electrons levitate above a liquid helium surface ?

We address the problem of overheating of electrons trapped on the liquid helium surface by cyclotron resonance excitation. Previous experiments, suggest that electrons can be heated to temperatures up to 1000K more than three order of magnitude higher than the temperature of the helium bath in the sub-Kelvin range. In this work we attempt to discriminate between a redistribution of thermal origin and other out-of equilibrium mechanisms that would not require so high temperatures like resonant photo-galvanic effects, or negative mobilities. We argue that for a heating scenario the direction of the electron flow under cyclotron resonance can be controlled by the shape of the initial electron density profile, with a dependence that can be modeled accurately within the Poisson-Boltzmann theory framework. This provides an self consistency-check to probe if the redistribution is indeed consistent with a thermal origin. We find that while our experimental results are consistent with the Poisson-Boltzmann theoretical dependence but some deviations suggest that other physical mechanisms can also provide a measurable contribution. Analyzing our results with the heating model we find that the electron temperatures increases with electron density under the same microwave irradiation conditions. This unexpected density dependence calls for a microscopic treatment of the energy relaxation of overheated electrons.

cond-mat.mes-hall↗

arXiv2017

Anomalous Quasiparticle Reflection from the Surface of a $^3$He-$^4$He Dilute Solution

A free surface of a dilute $^3$He-$^4$He liquid mixture is a unique system where two Fermi liquids with distinct dimensions coexist: a three-dimensional (3D) $^3$He Fermi liquid in bulk and a two-dimensional (2D) $^3$He Fermi liquid at the surface. To investigate a novel effect generated by the interaction between the two Fermi liquids, mobility of a Wigner crystal of electrons formed on the free surface of the mixture is studied. An anomalous enhancement of the mobility, compared with the case where the 3D and 2D systems do not interact with each other, is observed. The enhancement is explained by non-trivial reflection of 3D quasiparticles from the surface covered with the 2D $^3$He system.

cond-mat.other↗

arXiv2017

Effect of Rotation on Elastic Moduli of Solid $^4$He

We report measurements of elastic moduli of hcp solid $^4$He down to 15 mK when the samples are rotated unidirectionally. Recent investigations have revealed that the elastic behavior of solid $^4$He is dominated by gliding of dislocations and pinning of them by $^3$He impurities, which move in the solid like Bloch waves (impuritons). Motivated by the recent controversy of torsional oscillator studies, we have preformed direct measurements of shear and Young's moduli of annular solid $^4$He using pairs of quarter-circle shape piezoelectric transducers (PZTs) while the whole apparatus is rotated with angular velocity $Ω$ up to 4 rad/s. We have found that shear modulus $μ$ is suppressed by rotation below 80 mK, when shear strain applied by PZT exceeds a critical value, above which $μ$ decreases because the shear strain unbinds dislocations from $^3$He impurities. The rotation - induced decrement of $μ$ at $Ω= 4$ rad/s is about 14.7 (12.3) % of the total change of temperature dependent $μ$ for solid samples of pressure 3.6 (5.4) MPa. The decrements indicate that the probability of pinning of $^3$He on dislocation segment, $G$, decreases by several orders of magnitude. We propose that the motion of $^3$He impuritons under rotation becomes strongly anisotropic by the Coriolis force, resulting a decrease in $G$ for dislocation lines aligning parallel to the rotation axis.

cond-mat.other↗

arXiv2017

Bistable Transport Properties of a Quasi-One-Dimensional Wigner Solid on Liquid Helium under Continuous Driving

We investigate low-frequency fluctuations in the transport characteristics of a quasi-1D Wigner solid (WS) moving along a liquid helium substrate in response to a sinusoidal driving voltage. The fluctuations occur between distinct transport modes in which the decoupling of the WS from ripplonic polarons (or 'dimple lattice', DL) formed on the helium surface does, or does not, occur during each ac cycle. We demonstrate that a Gaussian-like distribution in the decoupling threshold force gives rise to this bistability, as the low-frequency switching occurs when the probability of decoupling during each ac cycle is small but finite. We attribute the distribution in the decoupling threshold force to the range of structural configurations allowed for the quasi-1D electron lattice, which influences the strength of the WS-DL coupling. Hence, the switching rate between the ac transport modes is extremely sensitive to the microscopic properties of the electron solid.

cond-mat.str-el↗

arXiv2017

Superfluid-like TO Responses in Rotating Solid Helium

The irrotational nature of superfluid helium was discovered through its decoupling from the container under rotation. Similarly, the resonant period drop of a torsional oscillator (TO) containing solid helium was first interpreted as the decoupling of solid from the TO and appearance of supersolid. However, the resonant period can be changed by mechanisms other than supersolid, such as the elastic stiffening of solid helium that is widely accepted as the reason for the TO response. To demonstrate the irrotational nature more directly, the previous experiments superimposed the dc rotation onto the TO and revealed strong suppression on the TO response without affecting the shear modulus. This result is inconsistent with the simple temperature-dependent elasticity model and supports the supersolid scenario. Here, we re-examine the rotational effect on solid helium with a two-frequency rigid TO to clarify the conflicting observations. Surprisingly, most of the result of previous rotation experiments were not reproduced. Instead, we found a very interesting superfluid-like irrotational response that cannot be explained by elastic models.

cond-mat.other↗

arXiv2016

Structural Order and Melting of a Quasi-One-Dimensional Electron System

We investigate the influence of confinement on the positional order of a quasi-1D electron system trapped on the surface of liquid helium. We find evidence that the melting of the Wigner solid (WS) depends on the confinement strength, as well as electron density and temperature. A reentrant solid-liquid-solid transition is observed for increasing electron density under constant electrostatic confinement. As the electron row number $N_y$ changes, varying commensurability results in a modulation of the WS order, even when $N_y$ is large (several tens). This is confirmed by Monte Carlo simulations.

cond-mat.mes-hall↗

arXiv2016

Bound-bound transitions in the emission spectra of Ba$^{+}$--He excimer

We present an experimental and theoretical study of the emission and absorption spectra of the Ba$^{+}$ ions and Ba$^{+\ast}$He excimer quasimolecules in the cryogenic Ba--He plasma. We observe several new spectral features in the emission spectrum which we assign to the electronic transitions between bound states of the excimer correlating to the 6$^{2}P_{3/2}$ and 5$^{2}D_{3/2,5/2}$ states of Ba$^{+}$. The resulting Ba$^{+}$(5$^{2}D_{J}$)He is a metastable electronically excited complex with orbital angular momentum $L$=2, thus expanding the family of known metal--helium quasimolecules. It might be suitable for high-resolution spectroscopic studies and for the search for new polyatomic exciplex structures.

physics.atom-ph↗

arXiv2016

Stick-Slip Motion of the Wigner Solid on Liquid Helium

We present time-resolved transport measurements of a Wigner solid (WS) on the surface of liquid Helium confined in a micron-scale channel. At rest, the WS is `dressed' by a cloud of quantised capillary waves (ripplons). Under a driving force, we find that repeated WS-ripplon decoupling leads to stick-slip current oscillations, the frequency of which can be tuned by adjusting the temperature, pressing electric field, or electron density. The WS on liquid He is a promising system for the study of polaron-like decoupling dynamics.

cond-mat.mes-hall↗

arXiv2015

Electronic magnetization of a quantum point contact measured by nuclear magnetic resonance

We report an electronic magnetization measurement of a quantum point contact (QPC) based on nuclear magnetic resonance (NMR) spectroscopy. We find that NMR signals can be detected by measuring the QPC conductance under in-plane magnetic fields. This makes it possible to measure, from Knight shifts of the NMR spectra, the electronic magnetization of a QPC containing only a few electron spins. The magnetization changes smoothly with the QPC potential barrier height and peaks at the conductance plateau of 0.5 $\times$ $2e^2/h$. The observed features are well captured by a model calculation assuming a smooth potential barrier, supporting a no bound state origin of the 0.7 structure.

cond-mat.mes-hall↗

arXiv2015

An incompressible state of a photo-excited electron gas

Two dimensional electrons in a magnetic field can form new states of matter characterized by topological properties and strong electronic correlations as displayed in the integer and fractional quantum Hall states. In these states the electron liquid displays several spectacular characteristics which manifest themselves in transport experiments with the quantization of the Hall resistance and a vanishing longitudinal conductivity or in thermodynamic equilibrium when the electron fluid becomes incompressible. Several experiments have reported that dissipation-less transport can be achieved even at weak, non-quantizing magnetic fields when the electrons absorb photons at specific energies related to their cyclotron frequency. Compressibility measurements on electrons on liquid helium demonstrate the formation of an incompressible electronic state under these resonant excitation conditions.

cond-mat.mes-hall↗

arXiv2013

First Study of Intersubband Absorption in Electrons on Helium under Quantizing Magnetic Fields

We present the first measurements of inter-subband absorption of microwaves in surface electrons on liquid helium subjected to perpendicular magnetic field B. In quantizing B, the power absorption shows intermittent regions of enhanced and suppressed absorption. This behavior is caused by strong variation of the excited-electron decay rate with B. Particularly, fast decay due to elastic scattering provides condition for strong absorption and overheating of the electron system, while slow decay due to inelastic scattering limits absorption and causes its saturation. An unexpected feature is the strong suppression of absorption at magnetic fields where the inter-subband energy splitting is a multiple number of the cyclotron energy.

cond-mat.mes-hall↗

arXiv2013

Resistive detection of nuclear spins in a single quantum dot under Kondo effect regime

We study dynamic polarization and resistive detection of nuclear spins in a semiconductor quantum dot (QD) under the Kondo effect regime. We find that the differential conductance spectra of the QD exhibit hysteresis under the Kondo effect regime in magnetic fields. Relevance of nuclear spins to the hysteresis is confirmed by the detection of nuclear magnetic resonance signals by monitoring the differential conductance. We attribute the origin of the hysteresis to the dynamic nuclear spin polarization (DNP) induced in the QD. Using the DNP, we demonstrate nuclear spin relaxation rate measurements in the QD under the Kondo effect regime.

cond-mat.mes-hall↗

arXiv2012

Resonant Photovoltaic Effect in Surface-State Electrons on Liquid Helium

We observed an ultra-strong photovoltaic effect induced by resonant intersubband absorption of microwaves in a two-dimensional electrons system on the surface of liquid helium. The effect emerges in the regime of microwave-induced vanishing of dissipative conductance reported previously [D. Konstantinov and K. Kono: Phys. Rev. Lett. 105 (2010) 226801)] and is characterized by a nonequilibrium spatial distribution of electrons in the confining electrostatic potential. The electrostatic energy acquired by an electron exceeds other relevant energies by several orders of magnitude.

cond-mat.mes-hall↗