Showing papers in "Journal of Low Temperature Physics in 2019"

LiteBIRD: A Satellite for the Studies of B-Mode Polarization and Inflation from Cosmic Background Radiation Detection

Abstract: LiteBIRD is a candidate satellite for a strategic large mission of JAXA. With its expected launch in the middle of the 2020s with a H3 rocket, LiteBIRD plans to map the polarization of the cosmic microwave background radiation over the full sky with unprecedented precision. The full success of LiteBIRD is to achieve $\delta r < 0.001$ , where $\delta r$ is the total error on the tensor-to-scalar ratio r. The required angular coverage corresponds to $2 \le \ell \le 200$ , where $\ell $ is the multipole moment. This allows us to test well-motivated cosmic inflation models. Full-sky surveys for 3 years at a Lagrangian point L2 will be carried out for 15 frequency bands between 34 and 448 GHz with two telescopes to achieve the total sensitivity of 2.5 $\upmu $ K arcmin with a typical angular resolution of 0.5$^\circ $ at 150 GHz. Each telescope is equipped with a half-wave plate system for polarization signal modulation and a focal plane filled with polarization-sensitive TES bolometers. A cryogenic system provides a 100 mK base temperature for the focal planes and 2 K and 5 K stages for optical components.

Patterned Supersolids in Dipolar Bose Systems

Abstract: We study by means of first-principle quantum Monte Carlo simulations the ground state phase diagram of a system of dipolar bosons with aligned dipole moments, and with the inclusion of a two-body repulsive potential of varying range. The system is shown to display a supersolid phase in a relatively broad region of the phase diagram, featuring different crystalline patterns depending on the density and on the range of the repulsive part of the interaction (scattering length). The supersolid phase is sandwiched between a classical crystal of parallel filaments and a homogeneous superfluid phase. We show that a “roton” minimum appears in the elementary excitation spectrum of the superfluid as the system approaches crystallization. The predictions of this study are in quantitative agreement with recent experimental results.

Electrical Detection of Magnetic Skyrmions

Abstract: Magnetic skyrmions are vortex-like spin textures that possess small size, nontrivial topology and high mobility which make them great promise as data carriers for high-density, high-velocity, and low-energy-consumption memory devices. In order to achieve this purpose, it is necessary to transfer the magnetic signal of a single skyrmion into an electrical signal. Here, we give an overview of recent progress in the active research field. Skyrmion and the corresponding memory device are firstly introduced, which is followed by the experimental achievements to electrical detection of magnetic skyrmion in both bulk and nanostructured materials. Finally, we discussed the electrical signal of other localized magnetic structures, which remains unexplored.

Thermodynamic Properties of a GaAs Quantum Dot with an Effective-Parabolic Potential: Theory and Simulation

Abstract: In this work, we have studied the thermodynamic properties of a GaAs quantum dot (QD) with an effective-parabolic potential. We have analytically obtained entropy, heat capacity and average energy of the QD in the presence of a magnetic field and its interaction with the electron spin using the canonical ensemble approach. According to the results, it is found that the entropy is an increasing function of temperature. At low temperatures, the entropy increases monotonically with increasing the temperature for all values of the magnetic field and it is independent of the magnetic field. But, the entropy depends on the magnetic field at high temperatures. The entropy is also decreased with increasing magnetic field. The average energy of the system is increased by enhancing temperature and magnetic field. At low temperatures, the average energy appears weakly dependent on magnetic field. It is nearly independent of magnetic field. The heat capacity increases with enhancing temperatures for all considered values of magnetic fields, and it approaches a saturation value at high temperatures. The heat capacity decreases with increasing the magnetic field for all considered values of temperatures. There is an interesting behavior in heat capacity as a function of magnetic field. The heat capacity does not change much at high temperatures like T = 200 K and 300 K. However, at relatively lower temperatures like T = 100 K, the heat capacity reduces with increasing magnetic field. We have also calculated the magnetization of the system. The magnitude of the magnetization $$ \left( {\left| M \right|} \right) $$ increases with the magnetic field and the confinement range.

Absence of Superfluidity in 2D Dipolar Bose Striped Crystals

Abstract: We present the results of computer simulations at low temperature of a two-dimensional system of dipolar bosons, with dipole moments aligned at an arbitrary angle with respect to the direction perpendicular to the plane. The phase diagram includes a homogeneous superfluid phase, as well as triangular and striped crystalline phases, as the particle density and the tilt angle are varied. In the striped solid, no phase coherence among stripes and consequently no “supersolid” phase are found, in disagreement with recent theoretical predictions.

Magnetic Susceptibility of Topological Semimetals

Abstract: We give a review of theoretical and experimental results concerning the magnetic susceptibility of the Weyl, Dirac, and nodal-line semimetals. In particular, dependences of the susceptibility on the chemical potential, temperature, and magnitude of the magnetic field are discussed. The presented results show that the specific features of the magnetic susceptibility can serve as a hallmark of the topological semimetals, and hence, magnetic measurements can be useful in investigating these materials.

Equilibrium of Methane and Carbon Dioxide Hydrates Below the Freezing Point of Water: Literature Review and Modeling

Abstract: This work presents a review of literature data of methane and carbon dioxide hydrate equilibrium at low temperatures. Constants of Arrhenius-type equation accurately determined for the mentioned lines which allow calculating the hydrate equilibrium pressure at any temperature below the quadruple point for both systems contain ice or supercooled water. Through intersection analysis, new accurate quadruple points were determined. Interpretations based on flash calculations by high accurate equations of states shown enthalpies of clathrate formation/dissociation, for equilibrium below quadruple point, lead to the similarity of Clapeyron and Clausius–Clapeyron approaches. Based on equality of equilibrium conditions at the quadruple point, new hydration numbers were calculated. Gamma–phi approach through high accurate equations of states of GERG-2008 and CG for the prediction of VHIw three-phase equilibrium line was evaluated. Commercial packages of Multiflash and PVTsim and open-source codes of CSMGem and CSMHYD were used to model the phenomena.

On the Chemical Potential of Ideal Fermi and Bose Gases

Abstract: Knowledge of the chemical potential is essential in application of the Fermi–Dirac and the Bose–Einstein distribution functions for the calculation of properties of quantum gases. We give expressions for the chemical potential of ideal Fermi and Bose gases in 1, 2 and 3 dimensions in terms of inverse polylogarithm functions. We provide Mathematica functions for these chemical potentials together with low- and high-temperature series expansions. In the 3d Bose case we give also expansions about $$T_{{{{\mathrm {B}}}}}$$ . The Mathematica routines for the series allow calculation to arbitrary order.

Spin, Orbital, Weyl and Other Glasses in Topological Superfluids.

Abstract: One of the most spectacular discoveries made in superfluid $$^3$$ He confined in a nanostructured material like aerogel or nafen was the observation of the destruction of the long-range orientational order by a weak random anisotropy. The quenched random anisotropy provided by the confining material strands produces several different glass states resolved in NMR experiments in the chiral superfluid $$^3$$ He-A and in the time-reversal-invariant polar phase. The smooth textures of spin and orbital order parameters in these glasses can be characterized in terms of the randomly distributed topological charges, which describe skyrmions, spin vortices and hopfions. In addition, in these skyrmion glasses the momentum-space topological invariants are randomly distributed in space. The Chern mosaic, Weyl glass, torsion glass and other exotic topological states are examples of close connections between the real-space and momentum-space topologies in superfluid $$^3$$ He phases in aerogel.

Mass Flux Experiments in Solid $$^4$$ 4 He: Some History, Recent Work and the Current Status

Abstract: Measurements that document a number of the characteristics of $$^{4}\hbox {He}$$ mass flux through a cell filled with solid $$^{4}\hbox {He}$$ carried out at the University of Massachusetts and elsewhere are reviewed. The mass flux is found to be a universal function of temperature, to have nonlinear behavior as a function of the driving chemical potential as might be expected for a Luttinger liquid and the flux can be extinguished at a concentration-dependent temperature by the presence of $$^{3}\hbox {He}$$ . Strong evidence indicates that the flux is carried by the cores of dislocations present in the solid. Some contact with the history of quantum fluids and relevant early work in solid helium is made as is contact with theory related to the notion of a supersolid and mass flux in $$^{4}\hbox {He}$$ .

Niobium Nitride Thin Films for Very Low Temperature Resistive Thermometry

Abstract: We investigate thin-film resistive thermometry based on metal-to-insulator transition (niobium nitride) materials down to very low temperature. The variation of the NbN thermometer resistance has been calibrated versus temperature and magnetic field. High sensitivity in temperature variation detection is demonstrated through efficient temperature coefficient of resistance. The nitrogen content of the niobium nitride thin films can be tuned to adjust the optimal working temperature range. In the present experiment, we show the versatility of the NbN thin-film technology through applications in very different low-temperature use cases. We demonstrate that thin-film resistive thermometry can be extended to temperatures below 30 mK with low electrical impedance.

UBe 13 and U 1−x Th x Be 13 : Unconventional Superconductors

Abstract: UBe13 was the second discovered heavy fermion superconductor, and numerous pieces of evidence exist that imply that it is an unconventional (non-BCS s-wave) superconductor. Exhibiting even more signs of unconventional superconductivity, Th-doped UBe13 is perhaps the most puzzling of any of the unconventional superconductors. This review considers both the parent, undoped compound as well as the more interesting U1−xThxBe13. After summarizing the rather thorough characterization—which because of the interest in these compounds, has continued from their discovery in 1983 and 1984 to date—these properties are compared with a recent ‘template’ for determining whether a superconductor is unconventional. Finally, further experiments are suggested.

Nanomechanical Resonators for Cryogenic Research

Abstract: Suspended aluminum nanoelectromechanical resonators have been fabricated, and the manufacturing process is described in this work Device motion is driven and detected with a magnetomotive method The resonance response has been measured at 42 K temperature in vacuum and low-pressure $$^4\hbox {He}$$ gas At low oscillation amplitudes, the resonance response is linear, producing Lorentzian line shapes, and Q values up to 4400 have been achieved At higher oscillation amplitudes, the devices show nonlinear Duffing-like behavior The devices are found to be extremely sensitive to pressure in $$^4\hbox {He}$$ gas Such device is a promising tool for studying properties of superfluid helium

Operation of a Latching, Low-Loss, Wideband Microwave Phase-Change Switch Below 1 K

Abstract: We report on the design, fabrication, and demonstration of the operation of a latching (nonvolatile) low-loss microwave switch at 4.2 K and 40 mK using the phase-change material germanium telluride (GeTe) as the switching element. The single-pole double-throw (SPDT) series–shunt switch has a single RF input and two selectable RF outputs. An insertion loss of less than 1 dB from DC to 10 GHz was demonstrated, with virtually identical performance across 5000 switching cycles at 40 mK. We have also characterized the resistance of the GeTe thin-film material used in the fabrication of the SPDT switch from room temperature down to 11 mK. While bulk GeTe is known to become superconducting below 1 K, these GeTe films showed no detectable superconducting transition, resulting in a switch with finite on-state DC resistance. Given the wide range of phase-change material candidates reported in the literature and prior evidence of superconductivity, this demonstration paves the way for future development of a near-ideal switch which could have zero on-state DC resistance at cryogenic temperatures.

Temperature Effects on the First Excited State of the Polaron in an Asymmetric Quantum Pseudodot Under Magnetic Field

Abstract: Using the variational method of the Pekar type, we investigate the first excited state energy of the polaron in an asymmetric quantum pseudodot under the magnetic field. Temperature effects on the polaron are calculated by employing the quantum statistical theory, and the influences of the chemical potential, the zero point of pseudo-harmonic potential (PHP), the cyclotron frequency, the electron–phonon coupling strength and the transverse and the longitudinal effective confinement lengths are taken into account. The results show that the first excited state energy decreases (increases) when the temperature is increased at lower (higher) temperature region. And it is an increasing function of the chemical potential, the zero point of PHP, the cyclotron frequency and the electron–phonon coupling strength. Simultaneously, it is a decreasing one of the transverse and the longitudinal effective confinement lengths.

A Brief Review of Dilution Refrigerator Development for Space Applications

Abstract: The open-cycle dilution refrigerator (OCDR) has been successfully applied on the Planck mission because it doesn’t need a magnetic field and can continuously provide cooling power. However, the short lifetime and small cooling power have made OCDR unable to meet the increasing requirements of future space missions. The closed-cycle dilution refrigerator currently under study provides a promising method to improve the performances of OCDR. This paper briefly reviews the basic structure and working principle of dilution refrigerators and introduces the development of dilution refrigerators for space applications, especially for Planck missions. Some optimized details of closed-cycle space dilution refrigerators are described, and their advantages and problems are summarized.

Transport Properties of a Quasi-1D Wigner Solid on Liquid Helium Confined in a Microchannel with Periodic Potential

Abstract: We present transport measurements in a quasi-1D system of surface electrons on liquid helium confined in a 101- $$\upmu $$ m-long and 5- $$\upmu $$ m-wide microchannel where an electrostatic potential with periodicity of 1- $$\upmu $$ m along the channel is introduced. In particular, we investigate the influence of such a potential on the nonlinear transport properties of a quasi-1D Wigner solid (WS) by varying the amplitude of the periodic potential in a wide range. At zero and small values of amplitude, the quasi-1D WS in the microchannel shows expected features such as the Bragg–Cherenkov scattering of ripplons and re-entrant melting. As the amplitude of the potential increases, the above features are strongly suppressed. This behavior suggests the loss of long-range positional order in the electron system, which is reminiscent of re-entrant melting behavior due to lateral confinement of the WS in the channel.

Peculiarities of Spherically Symmetric Counterflow

Abstract: Thermal counterflow in superfluid $$^4$$ He (He II) was studied numerically using the vortex filament model in a spherically symmetric geometry (as resulting from a point heat source). It is found that for the range of temperatures and velocities studied, turbulent tangle of the quantised vortices develops only for sufficiently low temperatures, hinting at the existence of a critical temperature, and only for velocities bounded from above (and presumably from below). A velocity–temperature phase diagram is presented. A simple physical model is proposed that qualitatively explains both observations.

Magnetic, Magnetocaloric and Correlation with Critical Behavior in Pr 0.8 Sr 0.2 MnO 3 Compound Prepared via Solid-State Reaction

Abstract: In this study, the critical behavior and magnetic and magnetocaloric properties in a polycrystalline Pr0.8Sr0.2MnO3 sample synthesized by solid-state technique were investigated in detail. By analyzing the temperature and the field dependence of magnetization, we have demonstrated that this compound exhibits a clear second-order magnetic phase transition around the Curie temperature estimated at TC = 161 K. Refined values of the critical exponents β, γ and δ determined by the modified Arrott plots, the critical isotherm and Kouvel–Fisher analysis show that our compound is described by the 3D Ising model. The reliability of these critical exponents was further confirmed using the universal scaling hypothesis. Under an applied magnetic field of 5 T, the calculated value of the maximum of the magnetic entropy change is estimated at 1.740 J K−1 kg−1 and the relative cooling power (RCP) turns out to be 134.46 J kg−1. Moreover, the critical exponents were evaluated from RCP results which confirm the correlation between the critical behavior and magnetocaloric results in the Pr0.8Sr0.2MnO3 compound.

Superconductivity of Organic Charge-Transfer Salts

Abstract: Forty years after the discovery of the first organic superconductor, the nature of the superconducting state in these materials is still not fully understood. Here, I present an overview on the historical developments and current knowledge on this topic for the quasi-one- and quasi-two-dimensional (2D) organic charge-transfer salts. Thereby, I focus on the prototype materials based on the donor molecules tetramethyltetraselenafulvalene (TMTSF) and bisethylenedithio-tetrathiafulvalene (BEDT-TTF or ET for short). 2D organic superconductors based on the latter molecule are found to show Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) states at high magnetic fields and low temperatures. Thermodynamic and nuclear magnetic resonance data give robust evidence for the existence of this FFLO state with modulated order parameter.

The Use of Second Sound in Investigations of Quantum Turbulence in He II

Abstract: Starting from a historic perspective, we discuss the use of second sound in experimental investigations of quantized vorticity and quantum turbulence in the two-fluid temperature regime of superfluid $$^4$$ He. Starting with the theoretical prediction of second sound by Tisza and Landau and its experimental discovery by Peshkov, we briefly review the pioneering experiments of Hall, Vinen and others that have contributed in an essential way to our current understanding of quantum turbulence in superfluid $$^4$$ He, with an emphasis on relevant research performed over the last two decades in our laboratory in Prague. We then propose further dedicated experiments where second sound can be used both as a generator and detector of quantum turbulence.

Dielectric Loss of Boron-Based Dielectrics on Niobium Resonators

Abstract: Advanced solid-state quantum bits (qubits) are likely to require a variety of dielectrics for wiring crossovers, substrates, and Josephson junctions. Microwave superconducting resonators are an excellent tool for measuring the internal dielectric loss of materials. We report the dielectric loss of boron-based dielectric films using a microwave coplanar waveguide (CPW) resonator with heterostructure geometry. Power-dependent internal quality factors of magnetron-sputtered boron carbide ( $$\hbox {B}_{{4}}\hbox {C}$$ ) and boron nitride (BN) were measured and are compared to silicon oxide ( $$\hbox {SiO}_{{2}}$$ ), a common material used in wiring crossovers. The internal dielectric loss due to two-level systems for $$\hbox {B}_{{4}}\hbox {C}$$ , and BN is less than silicon dioxide ( $$\hbox {SiO}_{{2}}$$ ), which demonstrates the existence of low-loss sputtered materials. We also found that niobium (Nb) CPW resonators suffer a decrease in internal quality factor after deposition of $$\hbox {B}_{{4}}\hbox {C}$$ at temperatures above 150 $$^{\circ }\hbox {C}$$ . This result is consistent with the idea that the oxidation of the surface of the superconducting metal can contribute to loss in a device.

Plastic Deformation and Creep in Solid Helium

Abstract: Helium crystals are quantum solids, with unusual mechanical properties. Quantum zero point motion prevents helium from freezing, unless pressure is applied, and helium crystals are extremely compressible, with elastic constants orders of magnitude smaller than those of conventional solids. In such quantum solids, tunneling allows atomic exchange and defects may move easily at low temperatures. The unusual mobility of dislocations and isotopic impurities in $$^4$$ He crystals can reduce their shear modulus at small strains by as much as 90%. For large strains, solid helium crystals are extraordinarily soft and ductile near their melting points, flowing under millibar stresses. At low strain rates, this high-temperature creep is thermally activated and involves diffusion of vacancies, which allow dislocations to move via climb. At low temperatures, these processes freeze out and helium crystals are much less ductile. Deformation proceeds via sudden slip events—dislocation avalanches—with a wide range of sizes and timescales. In this paper, we review experiments on plastic deformation and flow in solid $$^4$$ He and $$^3$$ He during the past 50 years, and discuss the plastic deformation mechanisms in solid helium that have been identified from these experiments.

Microscopic Tunneling Model of Nb–AlN–NbN Josephson Flux-Flow Oscillator

Abstract: Since the very first experimental realization of a Josephson flux-flow oscillator (FFO), its theoretical description has been limited by the phenomenological perturbed sine-Gordon equation (PSGE). While PSGE can qualitatively describe the topological excitations in Josephson junctions that are sine-Gordon solitons or fluxons, it is unable to capture essential physical phenomena of a realistic system such as the coupling between tunnel currents and electromagnetic radiation. Furthermore, PSGE neglects any dependence on energy gaps of superconductors and makes no distinction between symmetric and asymmetric junctions: those made of two identical or two different superconducting materials. It was not until recently when it became possible to calculate properties of FFO by taking into account information about energy gaps of superconductors (Gulevich et al. in Phys Rev B 96:024215, 2017). Such approach is based on the microscopic tunneling theory and has been shown to describe essential features of symmetric Nb–AlO $$_\mathrm{x}$$ –Nb junctions. Here, we extend this approach to asymmetric Nb–AlN–NbN junctions and compare the calculated current–voltage characteristics to our experimental results.

Magneto-Oscillations and Anomalous Current States in a Photoexcited Electron Gas on Liquid Helium

Abstract: The paper reviews a novel class of phenomena observed recently in the two-dimensional (2D) electron system formed on the free surface of liquid helium in the presence of a magnetic field directed normally and exposed to microwave radiation. The distinctive feature of these nonequilibrium phenomena is magnetoconductivity oscillations induced by inter-subband (out-of-plane) and intra-subband (in-plane) microwave excitations. The conductivity magneto-oscillations induced by intra-subband excitation are similar to remarkable microwave-induced resistance oscillations (MIRO) reported for semiconductor heterostructures. Investigations of microwave-induced conductivity oscillations on liquid helium helped with understanding of the origin of MIRO. Much stronger microwave-induced conductivity oscillations were observed and well described theoretically for resonant inter-subband microwave excitation. At strong powers, such excitation leads to zero-resistance states, the in-plane redistribution of electrons, self-generated audio-frequency oscillations, and incompressible states. These phenomena are caused by unusual current states of the 2D electron system formed under resonant microwave excitation.

Spin Seebeck Effect in a Multiple Quantum Dot Molecule with Spin-Dependent Interdot Coupling

Abstract: We study the spin Seebeck effect in a circularly connected triple quantum dot (TQD) structure taking the spin-dependent interdot coupling and magnetic flux into consideration. Particular attention is paid on the generation and manipulation of the 100% spin-polarized and pure spin thermopowers, which denote the arisen spin voltage in response of an infinitely small temperature gradient applied across the system. This can be realized by adjusting the peaks’ positions in the spin-up and spin-down thermopowers with the help of the spin polarization of the interdot couplings. At low temperature, a large value of pure spin thermopower is obtained even under very weak spin polarization of the interdot coupling. Strong spin polarization of it is favorable for 100% spin-polarized thermopower whose magnitude can reach as large as that of the charge one. We also find that a sign change of the considered two quantities can be realized by adjusting the magnetic flux penetrating through the TQDs. The present results could be useful in designing high-efficiency pure spin energy conversion and spin filter devices.

Statistical Properties of Small Particle Trajectories in a Fully Developed Turbulent State in He-II

Abstract: Lagrangian trajectories of small particles in a fully developed turbulent state are studied in a rectangular duct. A plate heater is attached on the bottom to generate the thermal counter flow. The bath temperature is changed from 1.7 to 2.1 K and is controlled within 0.1 mK. Small particles made of solid hydrogen are visualized by high-speed camera, and their trajectories are recorded. Their motions indicate complex features depending not only on bath temperature and heater power, but also on particle size. In this study, the Hurst exponent is defined by $$|\mathbf x (t+\tau )-\mathbf x (t)| \propto \tau ^H$$ , where $$\mathbf x (t)$$ denotes the particle position at time t. It was found that there is a characteristic timescale $$\tau _0$$ . For small time separation, $$\tau \le \tau _0$$ , the exponent H is small. However, for large time separation, $$\tau _0 \ll \tau $$ , H is nearly 1.

Ground-State Properties of a Dilute Two-Dimensional Bose Gas

Abstract: We revisit the problem of the calculation of zero-temperature properties for the dilute two-dimensional Bose gas. By using Popov’s hydrodynamic approach and perturbation theory on the two-loop level, we recover not only the known expansion for the ground-state energy but also calculate for the first time the condensate density and Tan’s contact.

Disk Resonator Design for Kinetic Inductance Detectors

Abstract: We propose disk resonators for use as kinetic inductance detectors. Benefits and drawbacks of this design are discussed with consideration for potential detector applications. We have conducted electromagnetic simulations of the resonator geometry and will use them to examine the disk resonator properties. We will compare several schemes for coupling these resonators to feed lines and review the effect of a large kinetic inductance on the design equations for the resonant frequency. Additionally, a strategy to reduce the resonator metal volume by meshing is considered, and its effects on the resonator’s current distribution are shown. We find that meshing can significantly alter the resonant frequency of the disk. We conclude by discussing an example disk resonator, gamma-ray microcalorimeter detector design.

The Effect of Coulomb Impurity Potential on the Coherence Time of RbCl Quantum Pseudodot Qubit

Abstract: We have considered the pseudoharmonic potential (PHP) for a RbCl quantum pseudodot (QPD) qubit with a hydrogen-like impurity at the center. By employing the Pekar variational method and the Fermi Golden Rule, we study the properties of the coherence time of an electron strongly interacting with longitudinal optical phonon in a RbCl QPD qubit. The coherence time changing with the Coulombic impurity potential strength, the two-dimensional chemical potential of the electron gas, the PHP zero point and the polaron radius is theoretically investigated. The obtained results indicate that ① the coherence time of RbCl QPD qubit decreases with increasing Coulombic impurity potential strength, ② the coherence time is a decaying function of the two-dimensional chemical potential of the electron gas and the PHP zero point, ③ the smaller the polaron radius is, the smaller the coherence time is.