Journal of Low Temperature Physics最新文献

The evolution of a vortex line following the binormal flow equation (i.e. with a velocity proportional to the local curvature in the direction of the binormal vector) has been postulated as an approximation for the evolution of vortex filaments in both the Euler system for inviscid incompressible fluids and the Gross–Pitaevskii equation in superfluids. We address the issue of whether this is a suitable approximation or not and its degree of validity by using rigorous mathematical methods and direct numerical simulations. More specifically, we show that as the vortex core thickness goes to zero, the vortex core moves (at leading order and for long periods of time) with a velocity proportional to its local curvature and the binormal vector to the curve. The main idea of our analysis lies in a reformulation of the Gross–Pitaevskii equation in terms of associated velocity and vorticity fields that resemble the Euler system written in terms of vorticity in its weak form. We also present full numerical simulations aimed to compare Gross–Pitaevskii and binormal flow in various physical situations of interest such as the periodic evolution of deformed vortex rings and the reconnection of vortex filaments.

We describe the design and performance of the cryostat and the multi-stage sub-K single-shot sorption cooler for the MIllimeter Sardinia Radio Telescope Receiver based on Array of Lumped elements kids (MISTRAL) experiment. MISTRAL is a W-band (77 - 103 GHz) Ti/Al bi-layer Lumped Elements Kinetic Inductance Detectors (LEKIDs) camera working at the Gregorian focus of the 64 m aperture Sardinia Radio Telescope (SRT), located in Sardinia (Italy). The cryogenic system, based on a 1.5 W at 4.2 K Pulse Tube (PT) cryocooler, provides the 4 K base temperature for the sub-K refrigerator, and cools down the cold optics and the filters chain of the instrument. The sub-K sorption cooler consists of two intermediate stages, (^{4})He and (^{3})He sorption refrigerators that allow to reduce the heat load on the ultra-cold head, and a twin stage of (^{3})He sorption refrigerator providing the 0.2 K operation temperature for the 415-pixel array of LEKIDs. MISTRAL experiment was installed at SRT in May 2023, the technical commissioning started in June 2023. We will show the performance of the system in the laboratory.

In the context of axion search in the BabyIAXO and IAXO helioscopes, various types of cryogenic detectors are investigated as high energy resolution, low energy threshold alternatives for the standard micromegas X-ray detectors. The setup presented in this paper comprising metallic magnetic calorimeter (MMC) X-ray detectors, a cryogenic local muon veto read out by an MMC, and internal and external lead shields, aims at establishing the background level that can be reached with MMCs in an above-ground experimental site. The low background setup is described in detail, and first observations from a low-temperature run are presented.

The low-temperature T-linear resistivity is a generic feature in the strange metal phase of overdoped cuprate superconductors; however, its origin is still not well understood. Based on the t–J model and the full charge-spin recombination scheme, the temperature dependence of electrical resistivity in the strange metal phase of the optimally doped and overdoped cuprates is studied. It is shown that at the optimal doping, the resistivity develops a linear-in-temperature behavior, while in the overdoped regime, the resistivity exhibits a nonlinear behavior which contains T-linear and T-quadratic components. The T-linear resistivity is thought to be dominated by isotropic inelastic scattering in the nodal region of the Fermi surface, where the most quasiparticle spectrum weight is assembled at around the tips of Fermi arcs.

Mechanical objects have been widely used at low temperatures for decades, for various applications; from quantum fluids sensing with vibrating wires or tuning forks, to torsional oscillators for the study of mechanical properties of glasses, and finally micro and nano-mechanical objects with the advent of clean room technologies. These small structures opened up new possibilities to experimentalists, thanks to their small size. We report on the characterization of purely metallic goalpost nano-mechanical structures, which are employed today for both quantum fluids studies (especially quantum turbulence in (^4)He, (^3)He) and intrinsic friction studies (Two-level-systems unraveling). Extending existing literature, we demonstrate the analytic modeling of the resonances, in good agreement with numerical simulations, for both first and second mechanical modes. Especially, the impact of the curvature of the whole structure (and therefore, in-built surface stress) is analyzed, together with nonlinear properties. We demonstrate that these are of geometrical origin and device-dependent. Motion and forces are expressed in meters and Newtons experienced at the level of the goalpost’s paddle, for any magnitude or curvature, which is of particular importance for quantum fluids and solids studies.

In this study, polycrystalline samples of La_0.7Ca_0.3-xDy_xMnO₃ (x = 0, 0.15) were synthesized via the solid-state reaction method. Their structures, magnetic properties, magnetocaloric effects, and critical behaviors associated with phase transitions were systematically investigated. All samples exhibited structures belonging to the Pbnm space group, characterized by precise compositions and good single-phase. The samples underwent paramagnetic-ferromagnetic (PM-FM) phase transitions at Curie temperatures (T_C) of approximately 244 K for x = 0 and 132 K for x = 0.15. The incorporation of Dy significantly broadened the half height wide temperature range (ΔT_FWHM) from 39.36 K (x = 0) to 121.92 K (x = 0.15). Consequently, the relative cooling capacity (RCP) of the samples was markedly increased, rising from 369.76 J·kg⁻¹ (x = 0) to 721.09 J·kg⁻¹ (x = 0.15). Furthermore, upon doping with x = 0.15, the phase transition type shifted from the first-order phase transition (FOPT) of the parent phase to a second-order phase transition (SOPT). This shift is attributed to the substitution of some Ca²⁺ ions by Dy³⁺, which weakened the double-exchange interaction and altered the phase transition type. Analysis of the critical behavior using the Kouvel-Fisher (K-F) and Modified Arrott plot (MAP) methods revealed that the critical features of the phase transition in La_0.7Ca_0.15Dy_0.15MnO₃ are better described by a Mean-Field Model with long-range ordering. Therefore, this study not only enriches our understanding of the physical properties of this class of materials but also enhances their potential for magnetic refrigeration (MR) applications.

The morphology of rotating viscous classical liquid droplets has been extensively studied and is well understood. However, our understanding of rotating superfluid droplets remains limited. For instance, superfluid (^4)He (He II) can carry angular momentum through two distinct mechanisms: the formation of an array of quantized vortex lines, which induce flows resembling solid-body rotation, and surface-traveling deformation modes associated with irrotational internal flows. These two mechanisms can result in significantly different droplet morphologies, and it remains unclear how the injected angular momentum is partitioned between them. To investigate this complex problem experimentally, one must first levitate an isolated He II droplet using techniques such as magnetic levitation. However, an outstanding challenge lies in effectively injecting angular momentum into the levitated droplet. In this paper, we describe a magneto-optical cryostat system designed to levitate He II droplets and present the design of a time-dependent, non-axially symmetric electric driving system. Based on our numerical simulations, this system should enable controlled angular momentum injection into the droplet. This study lays the foundation for future investigations into the morphology of rotating He II droplets.

Monte Carlo simulations represent a useful tool to predict and understand the behavior of X-ray detectors in space and on ground. We made use of the Geant4 software to simulate the performances of several TES detectors. We investigated the performances of the X-IFU CryoAC, a large area TES-based silicon detector in a laboratory environment, and its response to the ground level flux of cosmic muons. We were able to predict the background of the Athena X-IFU instrument (ESA) in the L1 environment for an equivalent time of (sim)100 ks and used the code to investigate the dependence of such a background on possible changes in the geometry such as pixel layout and size. We also compared the results with the ones obtained for the Line Emission Mapper (LEM), a probe concept proposed to NASA that uses a different TES array optimized for higher spectral resolution of lower energy photons, identifying issues with the detector design and indicating possible solutions.

When the frequencies (omega _A) and (omega _B) for trapping A- and B-species, respectively, in a binary Bose–Einstein condensates are tuned so that the mixture is in a special status, then all the parameters would fulfill a special relation. It implies that in this case the unknown parameter can be determined by the known parameters. In this paper, (omega _A) and (omega _B), which can be accurately controlled, are tuned so that the two clouds of this system either overlap completely with each other or one cloud just disappears from the center. In each case, based on the analytical solution of the coupled Gross–Pitaevskii equations which is obtained under the Thomas–Fermi approximation, two formulae relating to the parameters have been derived. When the two intraspecies interactions have been known, the interspecies interaction can be thereby determined via these formulae.

We study the onset of unconventional pair superfluidity in the t-J model on the Creutz lattice, which shows strictly flat bands in the noninteracting regime, using renormalized mean-field theory. Our study reveals the competition and coexistence between intrachain electron pairs and interchain electron pairs with varying antiferromagnetic interaction strengths at zero temperature. We observe that interchain pairs persist under strong interchain antiferromagnetic interaction but are suppressed by intrachain antiferromagnetic interaction. Interestingly, as hole-doping increases, we find that intrachain and interchain pairs exhibit distinct properties: The interchain pairs display a dome-like shape reminiscent of the superconducting dome observed in high-(T_{c}) superconductors. In contrast, the intrachain pairs’ gap increases smoothly with the hole-doping level. Furthermore, we find that the interchain pairs are more robust than intrachain pairs under thermal fluctuations; their critical temperature is higher than that of intrachain pairs. It is implementable to simulate and control strong electron correlation behavior on the Creutz lattice in ultracold atoms experiment or other artificial structures. Our predictions are verifiable and promote the understanding of flat band superconductivity.