Journal of Low Temperature Physics最新文献

This study extends the applicability of epoxy composites with acid-treated carbon nanotubes (CNTs), previously utilized at 77 K, to 4.2 K, and confirms that the thermal management and operational electrical stability of the filler material are maintained and developed in cryogenic environments. We investigate conduction-cooled NbTi coils impregnated with an acid-treated multi-walled carbon nanotube (MWCNT)/epoxy composite at 4.2 K. The matrix is LOCTITE Stycast 2850 FT with Catalyst 23 LV, and the MWCNTs (outer diameter 6–9 nm) were functionalized using a sulfuric/nitric acid (H₂SO₄/HNO₃) mixture to improve the dispersion and interfacial coupling. SEM confirms uniform dispersion of the treated MWCNTs in the cured matrix. Under conduction cooling at 4.2 K, the composite-impregnated coil cools down faster and has higher minimum quench energy (MQE) across 0.6–0.9 ({I}_{c})(energy defined as (E={I}^{2}{R}_{text{heater}}t)), a shorter post-quench re-cooling period, and stable persistent current operation, evidence of the enhanced operational electrical stability of the coil. Transient laser-flash measurements on the composite at 20.7–30.0 °C show a modest increase in through-thickness thermal conductivity, consistent with improved heat spreading. These results demonstrate improved cool-down efficiency, higher MQE, faster post-quench re-cooling, and stable persistent current operation—evidence of enhanced operational electrical stability. In future, we plan to measure the intrinsic low-temperature material properties [(k(T)),({c}_{p}(T)), dielectric metrics], together with multi-loading (0–1.0 wt% MWCNTs) testing.

The present experimental study investigates the magnetic ground state and spin fluctuation behavior of Ni₉₂TM₈ (TM = Cr, Nb) alloys through detailed magnetization measurements and theoretical analysis using Self-Consistent Renormalization (SCR) and Takahashi’s spin fluctuation theories. The alloys were synthesized via vacuum arc melting and subsequently characterized using energy-dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM), confirming single-phase formation with compositional homogeneity. Temperature and field dependent magnetization (M–T and M–H) measurements reveal second-order ferromagnetic transitions with Curie temperatures of 168 K (Ni₉₂Cr₈) and 173 K (Ni₉₂Nb₈). Both alloys exhibit spontaneous magnetic moments (0.18 μ_B/f.u and 0.21 μ_B/f.u) and large Rhodes–Wohlfarth ratios, characteristic of weak itinerant ferromagnetism (WIFM). The temperature dependence of spontaneous magnetization follows M² ∝ T² at low temperatures and M² ∝ T^4/3 near T_C, in agreement with SCR predictions. Analysis based on Takahashi’s theory yields spin fluctuation parameters (T₀, T_A, F₁₀) consistent with values reported for classical WIFMs such as ZrZn₂ and Ni₃Al, confirming dominant spin fluctuation effects. Deviations from linearity in Arrott plots near T_C and the linearity of M⁴ vs H/M plots substantiate the itinerant nature of magnetism. The findings elucidate Ni₉₂Cr₈ and Ni₉₂Nb₈ within the weak itinerant regime, governed by delocalized electron magnetism and strong spin fluctuations, offering potential for spintronic and quantum phase transition studies.

We study the weak-field magnetic susceptibility of a two-dimensional electron gas for different types of confinement potentials. The susceptibility is strongly dependent on the boundary confinement, and removal of the boundary results in a singularity. The cases of spatially isotropic parabolic, anisotropic, and Gaussian confinement potential are discussed in detail. The effect of spin–orbit interaction via Rashba type of coupling is analyzed in detail for all the potentials mentioned above. We show that a field-dependent susceptibility emerges when the confinement is Gaussian, in contrast to the canonical case of a field-independent susceptibility. The field-dependent weak-field susceptibility arises, even without Rashba spin–orbit coupling or many-body interactions. This distinguishes our result from conventional systems where such behavior typically relies on disorder, spin–orbit effects, or strong correlations. The combination of smooth confinement and magnetic field effects in our model leads to field-dependent behavior in a simple single-particle Hamiltonian, making it potentially amenable to experimental realization in semiconductor quantum wells and nanostructures. Our findings challenge the conventional understanding of susceptibility in weak-field regimes and open new avenues for exploration in quantum and nanoscale systems. We also show that the weak-field susceptibility is independent of the anisotropy parameter as well as the spin–orbit coupling for the anisotropic confinement model. For all the other models, the susceptibility vanishes for large spin–orbit coupling.

The last years have seen the first cryogenic detectors to be proposed for usage on balloon-borne missions. In such missions, the instrument will be exposed to the high radiation environment of the upper atmosphere. This radiation can induce a significant background to the measurements, something which can be mitigated through the use of an anti-coincidence shield. For hard X-ray and gamma-ray detectors such a shield typically consists of photomultiplier tubes or, more recently, silicon photomultipliers coupled to scintillators placed around the detector. When using cryogenic detectors, the shield can be placed around the entire cryostat which will make it large, heavy and expensive. For the ASCENT (A SuperConducting ENergetic X-ray Telescope) mission, which uses Transition Edge Sensor microcalorimeter detectors, it was therefore considered to instead place the shield inside. This comes with the challenge of operating it at cryogenic temperatures. For this purpose, we tested the performance of 2 different types of GAGG:Ce scintillators down to 15 mK for the first time. Although significant variations of both the decay time and the light yield were found when varying the temperature, at 4 K its performance was found to be similar to that at room temperature. Furthermore, unexpected behavior around 2 K was found for both types of GAGG:Ce, leading to more in-depth studies around these temperatures. Overall, the studies show that the combination of materials will allow to produce a functional anti-coincidence shield at several Kelvin.

We report preliminary results on spherical thermal counterflow generated by a small central heater in an open geometry, an open bath of superfluid He II, as closed cell experiments could have introduced artifacts such as overheating and boundary-induced flows. In order to eliminate them, we measure second sound attenuation in a plane-parallel resonator. Our results are at variance with the previous experiments in closed spherical cavity that showed plateau in the steady-state vortex line density and its inverse time decay, neither of which is observed presently. We find that in open geometry the vortex line density L increases steadily with counterflow velocity (v_textrm{ns}), displaying a crossover between (L propto v_textrm{ns}^2) typical for counterflow and (L propto v_textrm{ns}^{3/2}), characteristic for the quasi-classical scaling.

This work presents a theoretical investigation of the smearing of the 0–(pi) phase transition in a superconductor–quantum dot–superconductor system using the Keldysh Green’s function formalism within the equation of motion approach. The analysis focuses on the behavior of the Josephson current under the influence of thermal energy and on-site Coulomb interaction. Our results reveal that as thermal energy increases, the sharp 0–(pi) transition gradually smoothens, ultimately yielding a fully sinusoidal current-phase relationship.

A superconducting device is proposed for experimentally investigating whether an Abrikosov vortex can be modeled as a quantum mechanical quasiparticle. The design process of a type-II superconducting device capable of reliably pinning a single Abrikosov vortex is presented, creating a particle-in-a-box-like system. The proposed device consists of a cylindrically symmetric Nb film, 30 nm in diameter and 5 nm thick, with a 14 nm diameter artificial pinning center at its center. Time-dependent Ginzburg–Landau simulations indicate robust single-vortex pinning under an applied field of 6 T. The presumed quantized energy levels and associated quantum wavefunctions of the vortex quasiparticle are obtained by numerically solving the two-dimensional time-independent Schrödinger equation for this system. It is shown that distinguishing the ground and first excited states is experimentally feasible. Beyond fundamental physics studies, the application of the proposed device in cryogenic memory technology and quantum computing warrant further exploration.

Perovskite-type manganites Y_1-xCa_xFMnO₃ in the composition range of x = 0.2 of orthorhombic crystal structure was synthesized through solid-state route. XRD cast-off for checking phase configuration using XRD and with support of freely available FullProf program. Different structural parameters were measured after refinement of data. Impedance spectroscopy was useful to discover response of diverse electrical characteristics like conduction mechanism, electrical conductivity, different relaxation processes, and capacitance of probed sample under the variation of temperature and frequency. Electrical constraints of grain boundaries specify a deviation in electrical transport phenomena about 110 K termed as T_MI. Different electrical parameters were evaluated in terms of temperature and frequency effects on conduction mechanism through hopping, having double exchange through Mn³⁺ and Mn⁴⁺. Contrary to the typical investigation of Arrhenius behavior, the conduction processes of the charge carriers in the system have been investigated using conduction models of small-polaron hopping (SPH) and variable range hopping (VRH) models in a wide range of temperature and applied frequency.

We propose the parallel segmentation of the sensitive area of the optical transition edge sensor. The aim of parallel segmentation is to enhance the timing properties of the TES particularly in its rising edge by confining each heat diffusion event inside a segment and hence reducing the time required for the rise in resistance. All the segments are operated by a single voltage bias. We select iridium as its material and fabricated a two-segment device using photolithography and dry etching. The static characteristics were compared with a non-segmented device. We demonstrated the photon number resolving capability at the wavelength of 1547 nm and the parallel-segmented device exhibited an energy resolution of ΔE = 0.658 eV in FWHM using a SQUID readout. The electro-thermal simulations by finite difference method were performed to compare the timing properties among the different geometries including parallel-segmented structure.