Journal of Low Temperature Physics最新文献

Coaxial cables are widely used in radiofrequency and microwave cryogenic setups for condensed matter and quantum experiments. Since the inner conductor of coaxes is often in good thermal contact with the sample to be measured, it is desirable to know the phononic heat channeled by the inner conductor. Although cryogenic attenuators are widely used to thermalize the inner conductor of coaxes, to our knowledge, quantitative information is not available. We present data on the effectiveness of three commercially available 0 dB attenuators as thermal heatsinks. In particular, we measured the temperature of the inner pin of several 0 dB attenuators under a heat load. This information will aid in designing and carefully controlling the thermal environment of samples in high-frequency experiments mounted in dilution refrigerators and on nuclear demagnetization stages.

Rare earth sesquisulfides ({alpha })-({R_2})S(_3) (R = Dy, Sm) possess an orthorhombic crystal structure having two crystallographically inequivalent rare earth sites, R1 and R2. The compound (alpha )-(textrm{Dy}_2textrm{S}_3) exhibits successive antiferromagnetic transitions at (T_text {N1} = 11.4) K and (T_text {N2} = 6.4) K, while ({alpha })-Sm(_2)S(_3) shows successive weak-ferromagnetic transitions at (T_text {C1} = 3.6) K and (T_text {C2} = 1.8) K. Furthermore, they are fascinating because they exhibit a very large increase and recovery in electrical resistivity within a narrow temperature range just above (T_text {N2}) and (T_text {C1}), respectively. In this study, the AC magnetic susceptibility was measured near (T_text {N1}) and (T_text {N2}) using ({alpha })-Dy(_2)S(_3) single crystals, and near (T_text {C1}) using ({alpha })-Sm(_2)S(_3) single crystals. The imaginary part of AC susceptibility for each compound exhibited sharp peak near (T_text {N2}) or (T_text {C1}), respectively, clearly indicating a phase delay in the magnetic moment motion within each temperature range, where it could not follow the changes in the AC magnetic field. Furthermore, it was found that in ({alpha })-Dy(_2)S(_3), the peak height increases with increasing AC magnetic field frequency, whereas in ({alpha })-Sm(_2)S(_3), it decreases, reflecting the difference in magnetic ordering between the two compounds.

Superconducting vortices have a normal core and are pinned at imperfections, facilitating large current flow. Applications such as high-field solenoids or superconducting motors rarely use pure materials, as these are brittle, and instead employ superconductors embedded in ductile matrices (e.g., Cu or Ag). Processing superconductors into grains and then embedding in wires can significantly affect their properties, which remain less explored than in pure materials. In particular, the superconducting gap, relevant for vortex pinning, has been little studied in wires. Here, we determine the gap as a function of temperature and magnetic field in NbTi and MgB(_2) wires using scanning tunneling microscopy. We find strong gap inhomogeneity, with (Delta _{NbTi}=0.9pm 0.6) mV and (Delta _{MgB_2}=1.8pm 0.2) mV. The temperature dependence follows BCS theory. Under magnetic field, the gap decreases approximately linearly, deviating from the usual (left( 1-frac{H}{H_{c2}}right) ^2) behavior. We attribute this deviation to gap inhomogeneity arising possibly due to processing the materials into a wire. Our work shows that gap structure studies could complement efforts to improve superconducting properties of wires.

We present a theoretical study of spin-dependent quantum interference and persistent spin currents in tunnel-coupled double quantum rings in the presence of Rashba spin–orbit interaction and an external magnetic field. Within the effective-mass framework, the electronic spectrum is obtained by numerical diagonalization, and the influence of magnetic flux and spin–orbit coupling on the energy levels is analyzed. The results indicate that the energy spectrum exhibits oscillatory behavior as a function of magnetic flux, characteristic of the Aharonov–Bohm effect. Tunnel coupling leads to the formation of bonding and antibonding states, while Rashba spin–orbit interaction introduces spin-dependent modifications, including level splitting and anticrossings. The ground state energy shows periodic dependence on the magnetic flux, reflecting quantum interference effects. Furthermore, a finite equilibrium persistent spin current is obtained, whose behavior is influenced by both tunnel coupling and spin–orbit interaction. The interplay between these effects suggests that the system parameters can be tuned by external fields. These findings contribute to the understanding of spin-dependent quantum transport in mesoscopic ring structures and may be relevant for future studies of spin-related phenomena.

In this study, we report direct force measurements of the temperature-dependent macroscopic phonon angular momentum, using a fiber-optic interferometer and a torsional crystal oscillator. An oscillating magnetic field was applied to an insulating ferromagnet attached to a single-crystal silicon double-torsional oscillator. By the Einstein-de Haas effect, oscillator displacement measurements between low temperatures and those closer to the Debye temperature allow observation of the changing phonon angular momentum. A force change of approximately 60 nN was detected between 77 K and 300 K for a 0.3 mm(^3) MgZn ferrite sample, in fair agreement with theoretical predictions. Our oscillator, with a thermal noise limit on the order of 10(^{-12}) N/(sqrt{text {Hz}}), allows the possibility of high-accuracy detection. Competing effects were minimized; for example, induced eddy current momentum can overwhelm the phonon effect for metallic ferromagnets, and careful temperature-dependent force calibrations were required.

With nonuniform crossed electric and magnetic fields inside a long nonconducting cylinder, we show that a non-null nonclassical moment of inertia (at zero temperature) and revival time can be obtained for s-wave from the interaction of these fields with the induced electric dipole moment of a neutral particle combined with rotation.

Cryogenic Rayleigh–Bénard convection (RBC) at very high Rayleigh numbers (Ra) serves as a key system for understanding buoyancy-driven industrial and large-scale natural flows and for testing theories of turbulent convective heat transport. The central measured response is the Nusselt number (Nu), which quantifies the convective heat transfer efficiency, i.e., the enhancement of heat transport relative to the purely conductive state. Cryogenic helium experiments allow one to reach extremely high Ra under well-controlled laboratory conditions; however, interpretation of the resulting Nu(Ra) scalings remains sensitive to non-Oberbeck–Boussinesq (NOB) effects, experimental uncertainties, and corrections that must be applied to raw data. We present an analysis of experimental uncertainties and data correction procedures applicable to cryogenic RBC experiments, specifically to those performed in the present, newly built cylindrical RBC cell in Brno. For a representative set of (^{4})He working points in the p–T diagram, we examine how the evaluated Nu(Ra) dependence is affected by measurement uncertainties, corrections for the adiabatic temperature gradient, parasitic heat leaks, finite thermal conductivity of the plates and sidewalls, and by the choice of thermophysical property database used to determine the relevant properties of (^{4})He at the selected working points. The analysis shows that uncertainties and offset corrections associated with the temperature difference across the cell have the strongest local impact on the evaluated Nu(Ra) dependence, while sidewall corrections and the choice of thermophysical property database mainly affect the global scaling behavior. Our study highlights the necessity of rigorous uncertainty and correction analysis when assessing experimental evidence for a transition to the ultimate regime of RBC, since apparent changes in scaling may also arise from NOB effects and experimental imperfections.

The hot end terminal section of a pulse tube refrigerator fundamentally shapes its internal energy transport and irreversibility. In this work, the roles of the terminal compressible volume and the high resistance structure are decoupled using three 2D axisymmetric CFD models. Under no-load conditions, the inertance pulse tube refrigerator achieves effective cooling to 84 K, whereas the model with only the high resistance structure or only the terminal compressible volume shows much poorer cooling performance. Detailed thermodynamic analyses are performed for the pulse tube, regenerator, and cold heat exchanger, with emphasis on cooling behavior, pressure–volume (PV) power and enthalpy flow conversion, and entropy generation. The results show that the main role of terminal resistance is to reduce the losses associated with viscous dissipation in the pulse tube and the pressure drop losses in the regenerator. In contrast, the terminal compressible volume mainly helps maintain stable periodic oscillating flow in the pulse tube and provides a certain phase adjustment effect, thereby improving PV power transmission. Proper matching between these two effects can effectively reduce the irreversible losses of the system and improve energy utilization efficiency. In comparison, the continuous phase shifting capability of the inertance tube plays a more critical role in increasing the transmissible PV power of the system. The calculation results at a cold-end temperature of 120 K further show that the present study can provide theoretical guidance for the optimal design of the phase shifter and the improvement of cooling performance in inertance pulse tube refrigerators.

This work investigates the effect of low-level Fe incorporation (x ≤ 0.009) in YBa₂Cu₃O_7−δ, nominally at the Y site, on the structural and superconducting properties. X-ray diffraction confirms preservation of the orthorhombic phase with subtle changes in lattice parameters, suggesting local distortions and modifications in oxygen ordering. Resistivity measurements reveal a two-stage superconducting transition associated with intragranular pairing and intergranular coherence, consistent with weak-link behavior in polycrystalline samples. A slight maximum in characteristic temperatures is observed at x ≈ 0.2–0.3%, followed by a decrease at higher Fe content. Magnetic measurements show a reduction of the diamagnetic response and irreversibility temperature with doping. The results indicate competition between weakened intergranular coupling and enhanced intragranular vortex pinning induced by Fe-related defects, highlighting a low-doping regime for tuning superconducting performance.

Superconductors in nano-sized shape are investigated. Superconductor and also electrons which form superconductivity have quantum nature. This means when size of superconductor is small, there appear quantum effects. The critical temperature (T_c) becomes higher than that of a bulk superconductor. In the extremely dirty nano-sized superconductor, the critical temperature becomes much higher than that of a clean nano-sized superconductor. This is because a part of superconducting electrons concentrates in a localized state and has a high density of states. However, if the superconducting state remains localized below (T_c), resistivity may not be zero just below (T_c). Therefore we investigate how this localized state of superconductivity develops with decreasing temperature below (T_c). We show this localized state extends to whole superconductor just below (T_c).