Journal of Low Temperature Physics最新文献

Layered transition metal dichalcogenides, such as MoS₂, are promising platforms for exploring electric-field-controlled phenomena. While gate-induced superconductivity has been extensively studied in single-crystalline MoS₂ flakes, the behavior of bulk polycrystalline MoS₂ under similar conditions remains unexplored. In this study, we investigate electric-field-induced phase transitions in bulk polycrystalline MoS₂ using an ionic liquid (IL) gate, focusing on the role of electrode configuration and contact proximity in transport properties, and on the emergence of superconductivity in bulk samples. Three different electrode configurations were employed to examine the impact of contact geometry. In setups with distant contacts, the samples exhibited insulating behavior down to 1.8 K, whereas in the configuration with contacts placed close to the gate electrode, a clear metal–insulator transition and the onset of superconductivity were observed, with a maximum T_c of 4.2 K. Analysis revealed that the contact resistance strongly depends on the distance between the gate electrode and the voltage/current leads, differing by nearly an order of magnitude between contacts located near and far from the gate. The critical apparent sheet resistance at the metal–insulator transition was estimated to be ~ 50 Ω, much lower than the quantum resistance, likely due to penetration of the IL into the granular polycrystalline structure. These findings highlight the crucial role of electrode configuration in IL gating of bulk materials and demonstrate that electrostatic doping can induce superconductivity in polycrystalline systems, extending the scope of gate-controlled quantum phenomena to materials for which high-quality single crystals are difficult to obtain.

In photonic quantum computing and quantum information processing, photon number resolution is crucial for generating highly non-classical quantum states. Superconducting transition-edge sensors (TESs) are considered one of the most effective photon number-resolving detectors for this purpose. However, the current timing properties of TESs, such as timing jitter (a few ns) and recovery time (several hundred ns), are insufficient, limiting the operational speed of quantum applications. To address the potential for improving TES timing performance, we have proposed an improved calculation to derive TES timing jitter. Our calculations including the effect of the SQUID noise indicate that expanding the electrical bandwidth of the external voltage amplifier and optimizing the inductance to match that bandwidth, can significantly improve timing jitter. According to our model, timing jitter 1 ns FWHM is achievable with an input inductance of 0.35 nH and an electrical bandwidth of 1 GHz. Additionally, we have shown that with our TES and SQUID parameters, the lowest achievable timing jitter is approximately 0.71 ns FWHM. Achieving an even lower jitter would require a redesign aimed at further reducing the SQUID noise.

Using Gross–Pitaevskii theory within the double-parabola approximation framework, we investigate wetting phase transitions in three-component Bose–Einstein condensates. A comprehensive mathematical framework is developed for interfacial phenomena, including explicit expressions for interfacial tensions, binding potentials, and phase transition criteria. Our analysis reveals both critical and first-order wetting transitions, with tricritical points where transition lines intersect. Detailed phase diagrams are constructed in immiscibility parameter space, and precise conditions for complete versus partial wetting are identified. Theoretical predictions are supported by numerical Gross–Pitaevskii solutions and provide specific guidance for experimental verification in ultracold atomic gases. The work addresses fundamental aspects of multicomponent wetting phenomena and establishes theoretical foundations for quantum phase transition studies.

Transition edge sensors (TESs) have emerged as highly sensitive radiation detectors with applications ranging from astrophysics to materials science. In the framework of the Athena X-ray space telescope, we have developed Mo/Au-based TES devices designed to meet the ESA specifications for the X-IFU instrument. The detectors were fabricated using a Mo/Au/Au trilayer process and characterized in a dilution refrigerator using a SQUID readout. From I to V curves and complex impedance measurements, we extracted the electrothermal parameters of the TES, which are consistent with theoretical models. Noise spectra were analyzed to estimate the energy resolution, and an excess noise factor was obtained from the comparison of theoretical and experimental data. A spectral resolution below 4 eV was achieved for 5.9 keV X-rays from a ⁵⁵Fe source.

We study an ideal Bose gas confined by a D-dimensional power-law potential within the framework of the Dunkl formalism. By analyzing the combined effects of spatial dimensionality and trap geometry, we derive universal expressions for the thermodynamic quantities, which depend solely on a single parameter. This reduction reveals the existence of universality classes that apply to any power-law potential, regardless of its specific form. Furthermore, we demonstrate that the thermodynamic consistency of the Dunkl formalism requires the Wigner parameter to lie within the interval [0, 2]. This finding extends previous results obtained for harmonically trapped ideal Bose gases and establishes that these bounds hold for arbitrary regular potentials in any dimension.

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.