Journal of Low Temperature Physics最新文献

Liquid helium cryogenic system is crucial for achieving low-temperature superconductivity in particle accelerator and controllable nuclear fusion devices. However, the heat conductivity of copper in the 4K region is 400–800 W m⁻¹ K⁻¹, which limits the performance of superconductivity system. The application of helium-based oscillating heat pipe (OHP) promotes this deficiency mitigation, with a maximum effective thermal conductivity (ETC) ranging from 4000 to 16,000 W m⁻¹ K⁻¹. Although numerous scholars have experimentally observed the maximum efficiency point of OHP, but its underlying mechanism remains unclear. In this study, a test rig for measuring the heat transfer performance and dynamic parameters of helium-based OHP in the 4K region was constructed. A numerical simulation method for the gas–liquid two-phase unsteady flow process in the OHP was established. The amplitude and period distribution of dynamic pressure fluctuations in OHP were analyzed. The correlation between its pressure fluctuations and heat transfer process was explored. Finally, the mechanism of the maximum efficiency point was revealed with the oscillating characteristics for helium-based OHP in the 4K region.

In this article, a novel DC-biased interface for multi-channel superconducting computers was designed, fabricated, and tested. Conventional interfaces for Josephson-CMOS memory rely on Josephson latching drivers (JLDs) or SQUID (Superconducting Quantum Interference Device) stacks to convert weak signals. However, SQUID stacks achieve high frequencies (tens of GHz) but produce only a few millivolts of output and occupy large areas, while JLDs provide higher output voltages (tens of millivolts) but require AC bias. To address these limitations, an interface based on SiGe BiCMOS (Silicon-Germanium Bipolar CMOS) technology was proposed, integrating the functions of JLDs and CMOS amplifiers into a single chip. Fabricated using a 130 nm SiGe BiCMOS process, the interface converts 200 µV to 1.2 V with a power consumption of only 386 µW per channel at 4.2 K. Low-frequency measurements demonstrated 21-channel signal conversion without the need for clock synchronization or additional amplifiers, significantly simplifying the cryogenic system. The proposed interface features key advantages, including DC bias, high gain, and asynchronous operation, making it a practical solution for superconductor–semiconductor signal conversion. While its maximum speed is currently limited, this interface represents a promising step toward scalable, energy-efficient multi-channel superconducting computers.

We observed surface X-ray diffraction from ⁴He 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.

We used the method of electron spin resonance (ESR) to investigate the temperature-dependent recombination rate of H atoms in solid molecular hydrogen deuteride (HD). A 1.5 (mu)m thick solid molecular HD film was deposited at a rate of 2 monolayer/s, onto a gold surface maintained at T=1.5 K. H and D atoms were accumulated in the film by maintaining radio-frequency electric discharge above the film for 19 days. After further storage of the sample for 48 h, at T < 1 K, the D atom signal vanished. The concentration of H atoms was monitored as the sample was warmed stepwise from 1.1 K to 2.8 K. The recombination rate of H atoms in solid HD was found to be proportional to temperature in this range.

We have performed numerical calculations of the structure of the single-quantum vortex in superfluid (^3)He-A. The GPU-accelerated large-scale numerical simulation is performed in the Ginzburg-Landau model and resolves length scales of both coherence-length-sized hard core and dipolar-length-sized soft core of the vortex. The calculations support previously suggested qualitative structure of the vortex, recently named as eccentric fractional skyrmion, and provide numerical values for the vortex energy, sizes and locations of the hard and soft cores and highly-asymmetric flow profile of the vortex.

We investigate the properties of liquid (^4)He in both the normal and superfluid phases using path-integral Monte Carlo simulations and recently developed ab initio potentials that incorporate pair, three-body, and four-body interactions. By focusing on the energy per particle as a representative observable, we use a perturbative approach to quantify the individual contributions of the many-body potentials and systematically propagate their associated uncertainties. Our findings indicate that the three-body and four-body potentials contribute to the total energy by approximately 4% and 0.5%, respectively. However, the primary limitation in achieving highly accurate first principles calculations arises from the uncertainty in the four-body potential, which currently dominates the propagated uncertainty. In addition to the energy per particle, we analyze other key observables, including the superfluid fraction, condensed fraction, and pair distribution function, all of which demonstrate excellent agreement with experimental measurements.

In the framework of the Gross-Pitaevskii theory, based on the double-parabola approximation method, we successfully found the expressions for dispersion relations of three NG modes of two immiscible Bose-Einstein condensates restricted by a hard wall (two phonons and a ripplon). For the first condensate component, which is not confined by a hard wall, we found one phonon and one ripplon. Meanwhile, for the second component confined by a hard wall, we found only one phonon (the dispersion relation depends on the position of the hard wall), and this manifests explicitly the finite-size effect.

The paper presents analytic expressions of the Helmholtz free energy, the crystal parameters, the nearest neighbor distance between two atoms, the isothermal compressibility, the thermal expansion coefficient, the heat capacity at constant volume, the Gruneisen parameter, the isothermal elastic modulus, the Young modulus, the mean squared displacement and the Debye–Waller factor for FCC monoatomic crystal builded by the statistical moment method. The paper performs numerical calculation of the obtained theoretical results for FCC quantum crystals ³He and ⁴He using the Lennard–Jones (6–12) pair interaction potential and the coordination sphere method. Some calculated results are compared with the experimental data and other calculations.

We propose a multi-spin Ising model to report a computational study of a mixed spin-1 and spin-1/2 ferrimagnetic nanoisland. Employing the finite cluster approximation (FCA), we thoroughly examine how the four-spin interaction, next-nearest neighbor interaction, and crystal field affect the phase diagrams and the magnetic properties including magnetizations, internal energy, and specific heat. The exact calculations derived from the Hamiltonian at the ground state (T = 0) reveal the existence of a critical equation, J′ + J₄ = − 4J_s delimiting two distinct ground states (I) and (II). Phase diagrams analysis shows that the transition temperature T_c trends toward a tricritical point as the four-spin interaction J₄ varies, without evidence of compensation phenomena; while the next-nearest neighbor interaction J' leads to the manifestation of compensation points only if J′ surpasses a J_s-dependent threshold value. Interestingly, when the crystal field D_S is inserted, one, two, or three compensation points may emerge according to the strength of J′.