Journal of Low Temperature Physics最新文献

Phase-sensitive parametric devices facilitate quadrature-selective amplification with the possibility for sub-quantum-limited noise performance. In this work, we investigate the operation of a SQUID-based Josephson parametric amplifier (JPA), comparing its gain and noise performance in the phase-preserving and phase-sensitive regimes. The device, fabricated using VTT SWAPS technology, is driven in a three-wave mixing configuration and characterized in a reflection-based measurement setup at millikelvin temperatures. To directly probe the noise performance at low JPA gains, we employ a cascaded amplification scheme in which a traveling-wave parametric amplifier (TWPA) provides low-noise preamplification of the JPA output. In a phase-preserving operation, the JPA exhibits near-quantum-limited performance with a system noise temperature of (351pm 53) mK at 6 GHz. In contrast, phase-sensitive operation yields a minimum system noise temperature of (94pm 12) mK, well below the standard quantum limit of 288 mK. Our results demonstrate that a JPA–TWPA amplifier cascade opens the door to direct, high-fidelity probing of quantum devices without the need for background noise subtraction.

The readout system with a high multiplexing ratio has become a bottleneck limiting the application of large-scale transition-edge sensor (TES) detector arrays. In recent years, the microwave superconducting quantum interference device (SQUID) multiplexer has emerged as a key technology for effectively reading large-scale cryogenic detector arrays. Currently, the microwave SQUID multiplexer is being adopted by an increasing number of experiments due to its capability of achieving a multiplexing ratio of 2000:1 within the readout bandwidth. In this study, we developed a 32-channel microwave SQUID multiplexer prototype. And we measured eight channels of the prototype. The measured equivalent noise current of the prototype reached 42 pA/(sqrt{Hz}).

The energy-dependent electrical conductivity in spin-1 chiral fermion systems with disorder is studied using the self-consistent Born approximation. A distinct cusp-like feature appears at an energy different from the band-crossing point, arising from the multifold band-crossing structure formed by the Dirac and trivial bands. By decomposing the conductivity into band-resolved intraband and interband contributions, we clarify the microscopic origin of the cusp structure previously identified in spin-1 chiral fermion systems. These findings demonstrate the critical role of multifold band crossings and disorder-induced broadening of energy levels in determining the transport properties, offering theoretical insight into the unconventional conductivity behavior observed in topological semimetals hosting spin-1 chiral fermions.

Argon belongs to the class of rare and molecular gases that exhibit triple-point wetting, a regime in which adsorbed solid films remain only a few monolayers thick under thermodynamic equilibrium well below the triple point. Here we investigate the behavior of such films under nonequilibrium conditions, with particular emphasis on temperature gradients imposed within the sample cell. We show that these gradients can drive the formation of significantly thicker films than predicted by triple-point wetting theory. Our findings are motivated by, and possibly offer insight into, recent experiments with electron qubits above solid neon films, where film thicknesses substantially exceed equilibrium expectations.

Single crystals of La_xFe_1−xSe_0.5Te_0.5 (x = 0, 0.02, 0.04, 0.06, 0.08) were grown using the self-flux method. A systematic investigation of the effects of La addition revealed changes in the lattice parameters, with the lattice parameter a decreasing and the lattice parameter c showing a slight increasing tendency. The superconducting transition temperature showed no significant dependence on La doping. However, when x reached 0.04, the critical current density derived from magnetization loops using the Bean model increased. The dominant pinning mechanism induced by La in FeSe_0.5Te_0.5 was identified as normal point pinning at low magnetic fields. At the same time, machine learning was employed to quantitatively model the composition–property relationship between La doping and superconducting and magnetic properties; the results indicate that this method is effective in capturing experimental trends. This work demonstrates that rare earth doping could be a viable strategy for optimizing flux pinning in iron-based superconductors.

We report the design, fabrication, microwave characterization, and room-temperature ESR demonstration of a compact planar loop-gap resonator operating near 12 GHz. The resonator was fabricated by etching a 20-µm-thick copper layer on a glass-composite substrate and consisted of a 4.2-mm-diameter circular conductor with a 0.15-mm gap. Non-contact coupling to a stripline was characterized by varying the relative lateral and vertical positions of the resonator. At the position giving the largest resonance dip, the resonance frequency and loaded Q-factor were 12.2 GHz and 67.8, respectively, corresponding to a bandwidth of approximately 180 MHz. A first-order lumped RLC description was used to interpret the measured resonance characteristics. Room-temperature continuous-wave ESR measurements of DPPH demonstrated spin detection near 12.7 GHz with a signal-to-noise ratio of approximately 20. The present results demonstrate a simple planar fabrication and coupling approach for compact ESR resonators.

The band-degeneracy points in the Brillouin zones of chiral crystals exist in multiple avatars, with the high-symmetry points being able to host multifold nodes of distinct characters. A class of such crystals, assisted by the spin-orbit coupling, harbours fourfold degeneracy in the form of Rarita–Schwinger–Weyl node (RSWN) at the (Gamma )-point. Our aim is to explore the nature of longitudinal magnetoconductivity, arising from applying collinear electric and magnetic fields, for such systems. Adjusting the chemical potential to lie near the intrinsic energy-location of the RSWN, the multifold nature of the RSWN is revealed by an interplay of intraband and interband scatterings, which would not arise in twofold degeneracies like the conventional Weyl nodes. The current study fills up the much-needed gap in obtaining the linear response from an exact computation, rather than the insufficient relaxation-time approximation employed earlier.

The zero-point energy of a trapped ultracold Fermi gas at unitarity is investigated in relation to the combined effects of the Heisenberg uncertainty principle and the Pauli principle. This lowest allowed quantum state is a superfluid state which has been studied extensively both experimentally and theoretically. The method used for the current investigation is based on a recent series of papers that proposed microscopic dynamics based on normal modes to describe superfluidity instead of real-space Cooper pairs. This approach yielded excellent agreement with experimental data for multiple properties and allowed the microscopic behavior underlying these results as well as the basis of universal behavior to be analyzed in detail using the group theoretic basis of this general N-body approach. This microscopic picture is now used to elucidate the roles played by the uncertainty principle and the Pauli principle in determining the energy and character of the lowest allowed quantum state. The squeezing of the Heisenberg uncertainty principle, a well-known phenomenon in other systems, is proposed to play a role in the creation of superfluidity. The suppression of the Pauli principle is introduced as a new phenomenon.

We present an exact solution of the quantum kinetic equation of a weakly-interacting two-component unpolarized Fermi gas in the crossover from the degenerate Fermi-liquid regime to the classical Boltzmann gas. We construct families of orthogonal polynomials tailored to each angular momentum channel, enabling a fast and controlled decomposition of the phase-space distribution. This approach yields accurate predictions for the shear viscosity, thermal diffusivity, and spin diffusivity to leading order in the scattering length. We demonstrate that the commonly used variational approximation fails at low temperature—by up to 25%. Our method provides a numerically efficient framework for benchmarking transport in strongly correlated regimes and for simulating the kinetics of quantum gases beyond hydrodynamics.