Journal of Low Temperature Physics最新文献

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.

A Super-Conducting ENergetic x-ray Telescope (ASCENT) is a concept for a future balloon-borne high-energy X-ray telescope in the energy range 60–85 keV to study gamma-ray emissions of 67.87 keV and 78.32 keV from the radioactive isotope ⁴⁴Ti. For the focal plane instrumentation, ASCENT will use Mo-Cu/Mo-Au bilayer transition edge sensor (TES) microcalorimeter gamma-ray detectors with tin (Sn) absorbers. ‘Spectrometer to Leverage Extensive Development of Gamma-ray TESs for Huge Arrays using Microwave Multiplexed Enabled Readout’ (SLEDGEHAMMER), a detector development project at the National Institute of Standards and Technology, acts as the basis for the detector arrays for ASCENT. SLEDGEHAMMER has tin (Sn) absorbers attached to the SU-8 epoxy posts, lithographically placed on the detectors, but we are also considering other geometries for the chips where the absorbers are attached to the chips separated from the TESs, which could help to avoid parallel path for a current flow around the detectors with these BiSn sphere attachments. In this work, we are reporting on developing a method to attach Tin (Sn) absorbers to the transition edge sensors (TESs) with 0.2 mm diameter BiSn solder spheres replacing epoxy. The goal is to improve the thermal conductivity between the absorbers and the TESs compared to what was achieved using epoxy, potentially reducing the presence of an athermal component in the tails of signal pulses. We describe our efforts toward finding optimal temperature and pressure conditions for making this contact and the progress toward contact resistance measurements of these joints.

The IV-VI compound semiconductors comprising binary and ternary tellurides exhibit an elevated effective g factor ((g_textrm{eff})) and reduced effective mass ((m^{*})) as a consequence of potent spin-orbit (s.o) interaction. In the ternary compound with magnetic impurity, the observations of high (g_textrm{eff}) and corresponding low (m^{*}) are not only attributed to s.o interaction, but also s/p-d/f hybridization. However, as compared to the exchange interaction, the s.o interaction is the strongest and most dominating in these diluted magnetic semiconductors. The strength of the s.o coupling is manifested by the presence of an elevated (g_textrm{eff}) and low (m^{*}) in the system. In this correspondence, we measure these parameters in (Pb_{1-x}Sn_{x}Te), with Sn as the non-magnetic impurity for different carriers at fixed temperature; T = 1.5K. Here, we formulate an effective equation within the framework of multi-band (vec {k}cdot vec {pi }) theory, incorporating the effect of s.o interaction and a magnetic field. We derive expressions for the (g_textrm{eff}) and (m^{*}) through Green’s function expansion method by considering the above interactions with Sn impurity and assuming the band inversion model relevant to the (Pb_{1-x}Sn_{x}Te) system. Finally, we extensively examine these parameters and their respective anisotropies in (Pb_{1-x}Sn_{x}Te) as functions of carrier concentration and impurity levels at (T=1.5 K).We introduce a remarkably high effective g factor, specifically (g_textrm{eff}=4280), for n(-Pb_{1-x}Sn_{x}Te) and (g_textrm{eff}=3680), for p(-Pb_{1-x}Sn_{x}Te). These values exhibit significant anisotropies, accompanied by correspondingly low effective masses, with (m_{c}^{*}=0.0023m_{0}) and (m_{v}^{*}=0.0024m_{0}) at (x=0.36) within the concentration range of (10^{17}) to (10^{18} {text{cm}}^{{ - 3}}). The existence of high (g_textrm{eff}) and low (m^{*}) in strongly s.o interacting systems like PbTe, and its alloy (Pb_{1-x}Sn_{x}Te) qualify them to be used in spin-orbit physics apart from the field of thermoelectronics.

The study of critical magnetic properties of La_0.9Bi_0.1MnO₃ is reported and discussed. The sample exhibits rhombohedral perovskite crystal structure with trivial lattice structural distortions, surprisingly maintaining the proper stoichiometry and composition as evidenced by energy-dispersive X-ray spectroscopy. The critical behavior observed in the magnetization measurements is studied using different models with proper data analysis attributed to the magnetic frustrations arising due to the inhomogeneities. The zero-field-cooled magnetization measurements below T < T_K exhibit frozen ferromagnetic inhomogeneous spin agglomerates. The observed low-temperature-dependent field-cooled magnetization measurement has been approximated by considering the quadratic and non-quadratic dispersion law incorporating corrections to the Bloch’s relation. The exponents like γ and β obtained from critical scaling analysis further confirms the inhomogeneities in magnetic behavior and doesn’t correspond to any of the well-known universality class such as 3D Ising, 3D Heisenberg and mean-field models.

We report on low-frequency measurements of few electrons floating on superfluid helium using a bespoke cryogenic cascode amplifier circuit built with off-the-shelf GaAs high-electron-mobility transistors (HEMTs). We integrate this circuit with a charge-coupled device (CCD) to transport the electrons on helium and characterize its performance. We show that this circuit has a signal-to-noise ratio (SNR) of (thicksim) 2(frac{e}{sqrt{Hz}}) at 102 kHz, an order of magnitude improvement from previous implementations, and provides a compelling alternative to few electron sensing with high-frequency resonators.