arXiv subjects

Hiroshi Fukuyama

Publications and source records attributed to Hiroshi Fukuyama.

At least 19 recordsLinked to original sources

arXiv2026

On the Novel Superfluidity in the Second Layer of $^4$He on Graphite

Evidence for a new type of superfluid phase in second-layer $^4$He on graphite has been obtained from simultaneous measurements of torsional-oscillator response and heat-capacity on exactly the same sample down to 30 mK, which resolve substrate-related uncertainties in the previous studies. The new phase hosts both superfluidity and enhanced viscoelasticity, and is stable over a finite density interval, strongly supporting the proposed superfluid liquid-crystal hypothesis. A random-Josephson-network analysis shows that the widely reported log-$T$ dependence of the superfluid density is likely due to substrate imperfections.

cond-mat.other↗

arXiv2025

Growth of uniform helium submonolayers adsorbed on single-surface graphite observed by surface X-ray diffraction

We observed surface X-ray diffraction from He-4 submonolayers adsorbed on a single-surface graphite using synchrotron X-rays. Time evolutions of scattering intensities along the crystal truncation rod (CTR) were observed even after reaching the base low temperature in a selected condition of sample preparation. Our simulations for CTR scatterings based on the random double-layer model, in which helium atoms are distributed randomly in the first and second layers with a certain occupancy ratio, can consistently explain the observed intensity changes. These results support the scenario that He atoms are stratified initially as a nonequilibrium state and then relaxed into a uniform monolayer by surface diffusion, where the relaxation process was observed as a decrease in CTR scattering intensity. The observed time constant was, however, much longer than those estimated from quantum and thermal surface diffusions. This implies homogeneous processes in surface diffusions were strongly suppressed by local potentials in such as atomic steps or microcrystalline boundaries.

cond-mat.mtrl-sci↗

arXiv2022

Simulations of Surface X-ray Diffraction from a Monolayer 4He Film Adsorbed on Graphite

We carried out simulations of crystal truncation rod (CTR) scatterings, i.e., one of the surface X-ray diffraction techniques with atomic resolution, from a monolayer He film adsorbed on graphite. Our simulations reveal that the 00L rod scatterings from the He monolayer exhibit notable intensity modifications for those from a graphite surface in the ranges of approximately L = 0.6 - 1.7 and L = 2.2 - 3.5. The height of the He monolayer from the graphite surface largely affects the CTR scattering profiles, indicating that CTR scatterings have enough sensitivities to determine the surface structure of the various phases in the He layer. In particular, in the incommensurate solid phase, our preliminary experimental data show the intensity modulations that are expected from the present simulations.

cond-mat.other↗

arXiv2022

Superconducting zinc heat switch for continuous nuclear demagnetization refrigerator and sub-mK experiments

We have developed and tested a zinc superconducting heat switch suitable for magnetic refrigeration and calorimetric experiments at sub-millikevin (sub-mK) temperatures. The specific application here is an adiabatic demagnetization refrigerator with two PrNi$_{5}$ nuclear stages, which can keep a temperature of 0.8 mK continuously, (CNDR) proposed by Toda et al. (J. Phys.: Conf. Ser. 969, 012093 (2018). The switch consists of six high-purity zinc foils of 0.25 mm thick which contact seven silver foils by diffusive bonding. The silver foils are electron beam welded to silver rods that are thermal links to other components. The choice of the thin zinc foils is due to reduce the magnetic latent heat on switching and the contact thermal resistance under a constraint on the aspect ratio of the switch element. The measured thermal conductance of the whole switch assembly in the normal (closed) state, $K_\mathrm{closed}$, agrees very well down to 70 mK with the value estimated from the residual electrical resistance 114 n$\mathrmΩ$ at 4.2 K, indicating the validity of the Wiedemann-Franz law for zinc. The measured thermal conductance in the superconducting (open) state, $K_\mathrm{open}$, follows nicely the prediction from the BCS theory and approaches the value expected from the Debye model for thermal phonons near 70 mK. The heat leak through the HSW from the higher temperature side of 30 mK at most is estimated to be less than 0.5 nW, which is much lower than the expected cooling power ($= 10$ nW) of the CNDR at 0.8 mK . The switching ratio $K_\mathrm{closed}/K_\mathrm{open}$ extrapolated to 30 mK, is as high as 5$\times10^4$. All the test results meet the requirements for the CNDR and, for example, heat capacity measurements at sub-mK.

physics.ins-det↗

arXiv2021

Construction of Continuous Magnetic Cooling Apparatus with Zinc Soldered PrNi$_5$ Nuclear Stages

We report design details of the whole assembly of a compact and continuous nuclear demagnetization refrigerator (CNDR) with two PrNi$_5$ nuclear stages, which can keep temperature below 1 mK continuously, and test results of a new thermal contact method for the PrNi$_5$ stage using Zn soldering rather than Cd soldering. By measuring a residual electrical resistance of a short test piece, the thermal contact resistivity between the PrNi$_5$ rod and an Ag wires thermal link was estimated as $1.8\pm0.1 \times 10^{-4} T^{-1}~\rm{Km^2W^{-1}}$. Based on this value and 2D numerical and 1D analytical thermal simulations, the largest possible temperature gradient throughout the nuclear stage was calculated to be negligibly small ($\leq$ 2 %) at 1 mK under a 10 nW heat leak, the expected cooling power of the CNDR.

physics.ins-det↗

arXiv2021

A Simple Experimental Setup for Simultaneous Superfluid-response and Heat-capacity Measurements for Helium in Confined Geometries

Torsional oscillator (TO) is an experimental technique which is widely used to investigate superfluid responses in helium systems confined in porous materials or adsorbed on substrates. In these systems, heat capacity (HC) is also an important quantity to study the local thermodynamic properties. We have developed a simple method to incorporate the capability of HC measurement into an existing TO without modifying the TO itself. By inserting a rigid thermal isolation support made of alumina and a weak thermal link made of fine copper wires between a standard TO and the mixing chamber of a dilution refrigerator in parallel, we were able to carry out simultaneous TO and HC measurements on exactly the same helium sample, i.e., four atomic layers of $^4$He adsorbed on graphite, with good accuracies down to 30 mK. The data reproduced very well the previous workers' results obtained independently using setups optimized for individual measurements. This method is conveniently applicable to a variety of experiments where careful comparisons between results of TO and HC measurements are crucial. We describe how to design the thermal isolation support and the weak thermal link to manage conflicting requirements in the two techniques.

cond-mat.other↗

arXiv2021

Structural Study of Adsorbed Helium Films: New Approach with Synchrotron Radiation X-rays

A few atomic layers of helium adsorbed on graphite have been attracting much attention as one of the ideal quantum systems in two dimension. Although previous reports on neutron diffraction have shown fundamental structural information in these systems, there still remain many open questions. Here, we propose surface crystal truncation rod (CTR) scatterings using synchrotron radiation X-rays as a promising method to reveal surface and interface structures of helium films on graphite at temperatures below 2 K, based on the preliminary experimental results on a monolayer of He-4 on a thin graphite. Our estimation on heat generation by X-ray irradiations also suggests that CTR scatterings are applicable to even at system temperatures near 100 mK.

cond-mat.other↗

arXiv2020

Superconducting Niobium Calorimeter for Studies of Adsorbed Helium Monolayers

We developed a calorimeter with a vacuum container made of superconducting niobium (Nb) to study monolayers of helium adsorbed on graphite which are prototypical two-dimensional quantum matters below 1 K. Nb was chosen because of its small specific heat in the superconducting state. It is crucially important to reduce the addendum heat capacity ($C_{\rm{ad}}$) when the specific surface area of substrate is small. Here we show details of design, construction and results of $C_{\rm{ad}}$ measurements of the Nb calorimeter down to 40 mK. The measured $C_{\rm{ad}}$ was sufficiently small so that we can use it for heat capacity measurements on helium monolayers in a wide temperature range below 1 K. We found a relatively large excess heat capacity in $C_{\rm{ad}}$, which was successfully attributed to atomic tunneling of hydrogen (H) and deuterium (D) between trap centers near oxygen or nitrogen impurities in Nb. The tunnel frequencies of H and D deduced by fitting the data to the tunneling model are consistent with the previous experiments on Nb doped with H or D.

cond-mat.other↗

arXiv2020

Negative resistance state in superconducting NbSe$_2$ induced by surface acoustic waves

We report a negative resistance, namely, a voltage drop along the opposite direction of a current flow, in the superconducting gap of NbSe$_2$ thin films under the irradiation of surface acoustic waves (SAWs). The amplitude of the negative resistance becomes larger by increasing the SAW power and decreasing temperature. As one possible scenario, we propose that soliton-antisoliton pairs in the charge density wave of NbSe$_2$ modulated by the SAW serve as a time-dependent capacitance in the superconducting state, leading to the dc negative resistance. The present experimental result would provide a previously unexplored way to examine nonequilibrium manipulation of the superconductivity.

cond-mat.mes-hall↗

arXiv2019

Performances of a Compact Shielded Superconducting Magnet for Continuous Nuclear Demagnetization Refrigerator

We have successfully developed and tested a compact shielded superconducting (SSC) magnet with a FeCoV magnetic shield. This was developed for the PrNi$_5$ based nuclear demagnetization refrigerator which can keep temperatures below 1 mK continuously (CNDR) [Toda $\it{et~al}$., J. Phys.: Conf. Ser. $\bf{969}$, 012093 (2018)]. The clear bore diameter, outer diameter, and total length of the SSC magnet are 22, 42 and 169 mm, respectively, and it produces the maximum field of 1.38 T at an electric current of 6 A. In order to realize both the compactness and the high shielding performance, we carefully chose material and optimized design of the magnetic shield by numerical simulations of the field distribution based on measured magnetization curves of several candidate materials with high permeability. We also measured a heat generated by sweeping the SSC magnet in vacuum to be 230 mJ per field cycle. This value agrees very well with an estimation from the measured magnetic hysteresis of the superconducting wire used to wind the magnet.

physics.ins-det↗

arXiv2019

Hexagonal Nanopits with the Zigzag Edge State on Graphite Surfaces Synthesized by Hydrogen-Plasma Etching

We studied, by scanning tunneling microscopy, the morphology of nanopits of monolayer depth created at graphite surfaces by hydrogen plasma etching under various conditions such as H$_2$ pressure, temperature, etching time, and RF power of the plasma generation. In addition to the known pressure-induced transition of the nanopit morphology, we found a sharp temperature-induced transition from many small rather round nanopits of ~150 nm size to few large hexagonal ones of 300-600 nm within a narrow temperature range. The remote and direct plasma modes switching mechanism, which was proposed to explain the pressure-induced transition, is not directly applicable to this newly found transition. Scanning tunneling spectroscopy (STS) measurements of edges of the hexagonal nanopits fabricated at graphite surfaces by this method show clear signatures of the peculiar electronic state localized at the zigzag edge (edge state), i.e., a prominent peak near the Fermi energy accompanied by suppressions on either side in the local density of states. These observations indicate that the hexagonal nanopits consist of a high density of zigzag edges. The STS data also revealed a domain structure of the edge state in which the electronic state varies over a length scale of ~3 nm along the edge. The present study will pave the way for microscopic understanding of the anisotropic etching mechanism and of spin polarization in zigzag nanoribbons which are promising key elements for future graphene nanoelectronics.

cond-mat.mes-hall↗

arXiv2018

STM/S Observations of Graphene on SiC(0001) Etched by H-plasma

Monolayer graphene epitaxially grown on SiC(0001) was etched by H-plasma and studied by scanning tunneling microscopy and spectroscopy. The etching created partly hexagonal nanopits of monatomic depth as well as elevated regions with a height of about 0.12 nm which are stable at $T$ = 78 K. The symmetric tunnel spectrum about the Femi energy and the absence of a $6\times6$ corrugation on the elevated regions suggest that in these regions the carbon buffer layer is decoupled from the SiC substrate and quasi-free-standing bilayer graphene appears at originally monolayer graphene on the buffer layer. This is a result of passivation of the SiC substrate by intercalated hydrogen as in previous reports for graphene on SiC(0001) heat treated in atomic hydrogen.

cond-mat.mes-hall↗

arXiv2018

The Role of Substrate Roughness in Superfluid Film Flow Velocity

It is known that the apparent film flow rate $j_0$ of superfluid $^4$He increases significantly when the container wall is contaminated by a thin layer of solid air. However, its microscopic mechanism has not yet been clarified enough. We have measured $j_0$ under largely different conditions for the container wall in terms of surface area (0.77-6.15 m$^2$) and surface morphology using silver fine powders (particle size: $0.10$ μm) and porous glass (pore size: 0.5, 1 μm). We could increase $j_0$ by more than two orders of magnitude compared to non-treated smooth glass walls, where liquid helium flows down from the bottom of container as a continuous stream rather than discrete drips. By modeling the surface morphology, we estimated the effective perimeter of container $L_{\mathrm{eff}}$ and calculated the flow rate $j~(= j_0L_0/L_{\mathrm{eff}})$, where $L_0$ is the apparent perimeter without considering the microscopic surface structures. The resultant $j$ values for the various containers are constant each other within a factor of four, suggesting that the enhancement of $L_{\mathrm{eff}}$ plays a major role to change $j_0$ to such a huge extent and that the superfluid critical velocity, $v_{\mathrm{c}}$, does not change appreciably. The measured temperature dependence of $j$ revealed that $v_{\mathrm{c}}$ values in our experiments are determined by the vortex depinning model of Schwarz (Phys. Rev. B $\textbf{31}$, 5782 (1986)) with several nm size pinning sites.

cond-mat.other↗

arXiv2018

Specific heat, thermal conductivity, and magnetic susceptibility of cyanate ester resins --- An alternative to commonly used epoxy resins

In low temperature experiments, resins have many applications as glues or thermal and electrical insulators. Cyanate ester resins (CEs) are a high-temperature compatible thermoset resin whose glass-transition temperature $T_g$ is ~300 $^\circ$C. Recently, we found that CEs also withstand low temperatures without microcracking by measuring $^4$He permeability. Here, we measured specific heat C, thermal conductivity κ, and magnetic susceptibility $χ$ of different kinds of CEs in the wide temperature range from room temperature to 0.5 K for C and 2 K for other two. The thermal properties, C and κ, of different kinds of CEs are surprisingly coincident with each other. We discuss chemical structures and crystallinity of CEs and their blends based on the measured thermal properties. Compared to Stycast 1266, a commonly-used epoxy resin in low temperature experiments, C of CEs is larger by a factor of 3 (<= 30 K), κis lower by a factor of 4 (<= 10 K), indicating the small thermal diffusivity. The χvalues are as small as Stycast 1266, indicative of their high purity. Our results show that cyanate esters are a new option for cryogenic resins with thermal insulative properties in/for low temperature experiments.

cond-mat.mtrl-sci↗

arXiv2018

Pyrolytic Graphite Sheet, a New Adsorption Substrate for Superfluid Thin Films

We have measured surface morphology and gas adsorption characteristics of uncompressed pyrolytic graphite sheet (uPGS) which is a candidate substrate for AC and DC superflow experiments on monolayers of 4He below T = 1 K. The PGS is a mass-produced thin graphite sheet with various thicknesses between 10 and 100 μm. We employed a variety of measuring techniques such as imagings with optical microscope, SEM and STM, Raman spectroscopy, and adsorption isotherm. PGS has smooth and atomically-flat external surfaces with high crystallinity. Although the specific surface area (<0.1 m$^2$/g) is rather small, by making use of its smooth external surface, the thinnest uPGS of 10 μm thick is found to be suitable for the superflow experiments on the strictly two-dimensional helium systems.

cond-mat.mtrl-sci↗

arXiv2017

Low temperature transport properties of pyrolytic graphite sheet

We have made thermal and electrical transport measurements of uncompressed pyrolytic graphite sheet (uPGS), a mass-produced thin graphite sheet with various thicknesses between 10 and 100 μm, at temperatures between 2 and 300 K. Compared to exfoliated graphite sheets like Grafoil, uPGS has much higher conductivities by an order of magnitude because of its high crystallinity confirmed by X-ray diffraction and Raman spectroscopy. This material is advantageous as a thermal link of light weight in a wide temperature range particularly above 60 K where the thermal conductivity is much higher than common thermal conductors such as copper and aluminum alloys. We also found a general relationship between thermal and electrical conductivities in graphite-based materials which have highly anisotropic conductivities. This would be useful to estimate thermal conductance of a cryogenic part made of these materials from its electrical conductance more easily measurable at low temperature.

cond-mat.mtrl-sci↗

arXiv2016

Possible quantum liquid crystal phases of helium monolayers

The second-layer phase diagrams of $^4$He and $^3$He adsorbed on graphite are investigated. Intrinsically rounded specific-heat anomalies are observed at 1.4 and 0.9 K, respectively, over extended density regions in between the liquid and incommensurate solid phases. They are identified to anomalies associated with the Kosterlitz-Thouless-Halperin-Nelson-Young type two-dimensional melting. The prospected low temperature phase (C2 phase) is a commensurate phase or a $\textit{quantum hexatic}$ phase with quasi-bond-orientational order, both containing $\textit{zero}$-$\textit{point}$ defectons. In either case, this would be the first atomic realization of the $\textit{quantum liquid crystal}$, a new state of matter. From the large enhancement of the melting temperature over $^3$He, we propose to assign the observed anomaly of $^4$He-C2 phase at 1.4 K to the hypothetical supersolid or superhexatic transition.

cond-mat.other↗

arXiv2012

Preliminary Heat Capacity and Vapor Pressure Measurements of 2D 4He on ZYX Graphite

We report preliminary heat capacity and vapor pressure measurements of the first and second layers of 4He adsorbed on ZYX graphite. ZYX is known to have much better crystallinity than Grafoil, the most commonly-used exfoliated graphite substrate, such as a ten-times larger platelet size. This allows us to distinguish different phases in 2D helium-4 much more clearly and may provide qualitatively different insights into this system. We found a significantly asymmetric density-dependence of the heat-capacity peak associated with the 1/3 phase formation comparing with that obtained with Grafoil. The 2nd-layer promotion density is determined as 11.8+-0.3 nm-2 from the heat-capacity measurement of low density samples in the 2nd layer and vapor pressure measurement.

cond-mat.other↗