Journal of Low Temperature Physics最新文献

The mobility of the Wigner solid on the free surface of superfluid (^3)He is determined by the momentum transfer from the scattered (^3)He quasiparticles at the free surface. The scattering process of the quasiparticles is classified into the normal reflection and the Andreev retroreflection. Since the quasiparticles nearly conserve the momentum in the process of the Andreev retroreflection at the free surface, the Andreev-reflected quasiparticles do not produce a resistive force to the Wigner solid. In this report, we have analytically calculated the contribution of the Andreev retroreflection to the mobility of the Wigner solid on superfluid (^3)He-B by employing a realistic model order parameter with the free surface. The Andreev retroreflection is absent for quasiparticles with energies above the bulk energy gap under the model order parameter. Then, the Andreev retroreflection does not contribute to a rise in the mobility of the Wigner solid on the superfluid (^3)He-B. The present model calculation is in good agreement with the previous experimental observation. We have also discussed the Andreev retroreflection under a self-consistently calculated order parameter.

The analysis in the present paper is based on the most known concept introduced by the brilliant physicist Alexander Andreev: Andreev bound states in a normal metal sandwiched between two superconductors. The paper presents results of direct calculations of ab initio expressions for the currents in a long ballistic SNS junction. The expressions are expanded in 1/L (L is the thickness of the normal layer). The main contribution (propto 1/L) to the current agrees with the results obtained in the past, but the analysis suggests a new physical picture of the charge transport through the junction free from the problem with the charge conservation law. The saw-tooth current-phase relation at (T=0) directly follows from the Galilean invariance of the Bogolyubov–de Gennes equations proved in the paper. The proof is valid for any variation of the energy gap in space if the Andreev reflection is the only scattering process. The respective roles of the contributions of bound and continuum states to the current are clarified. They depend on the junction dimensionality.

In this article in memory of academician A.F. Andreev, we consider the results of experiments in superfluid (^{3})He-B at ultralow temperatures. These experiments directly demonstrate the existence of gapless Majorana quasiparticles, which appear as Andreev bound states at the boundary of superfluid (^3)He-B (Silaev and Volovik in JETP 119:1042, 2014). Precision measurements of the heat capacity of superfluid (^3)He-B showed that at a temperature of 0.12 mK, Majorana fermions can account for up to (50%) of the heat capacity of (^3)He-B in our experimental conditions.

At SRON, we have been developing X-ray TES micro-calorimeters as backup technology for the X-ray Integral Field Unit (X-IFU) of the Athena mission, demonstrating excellent resolving powers both under DC and AC bias. We also developed a frequency-domain multiplexing (FDM) readout technology, where each TES is coupled to a superconducting band-pass LC resonator and AC biased at MHz frequencies through a common readout line. The TES signals are summed at the input of a superconducting quantum interference device (SQUID), which performs a first amplification at cryogenic stage. Custom analog front-end electronics and digital boards take care of further amplifying the signals at room temperature and of the modulation/demodulation of the TES signals and bias carrier, respectively. We report on the most recent developments on our FDM technology, which involves a two-channel demonstration with a total of 70 pixels with a summed energy resolution of 2.34 ± 0.02 eV at 5.9 keV without spectral performance degradation with respect to single-channel operation. Moreover, we discuss prospects towards the scaling-up to a larger multiplexing factor up to 78 pixels per channel in a 1–6 MHz readout bandwidth.

The B-BOP detector is composed of polarization sensitive bolometer arrays in the 70–350 µm spectral range with a goal sensitivity around one attoWatt/sqrt (Hz). A polarized infrared calibration set-up is required to evaluate its performances. The calibration source is based on 6-ring-shaped, concentric blackbody-like emitters that can be individually controlled. The total emitting surface is therefore tunable from 9 up to 1200 mm². The temperature is tunable from 1 to 40 K without disturbing the dilution cryostat and the working temperature of the detector arrays. The present paper describes the source, the rotating polarizer at low temperature, the modulation means, and the complete optical tube leading polarized infrared photons for characterizing detectors down to femtowatt level. Details of the design and realization are discussed.

Starting from the Bogolubov–de Gennes equations for superconductor with an arbitrary anisotropic Fermi surface we derive the Andreev-type theory accounting for the quasiparticle trajectory interference and corresponding nonquasiclassical quantization effects. The resulting Andreev equations are applied for the analysis of the subgap quasiparticle states localized at the pinned vortex cores. The normal reflection of electrons and holes at the defects is shown to result in the qualitative transformation of the subgap spectra and formation of new types of bound quasiparticle states. We focus on the effect of the Fermi surface anisotropy on the quasiparticle spectral properties including opening of soft and hard minigaps in the spectrum and spatial behavior of the local density of states around pinned vortex cores.

The cooling capacity of a typical dilution refrigerator is mainly determined by the circulating flow of the He-3 in the system, which is usually realized by the external circulation pump. In order to increase the concentration of circulating He-3, the still often take the orifice structure to limit the influence of the superfluid helium film, and this structure will also have a certain impact on the flow rate of He-3. This paper summarizes the flow rate models of each part of the orifice structure, and establishes a model related to the influence of pumping speed and the diameter of orifice on the flow rate of circulating He-3. The relationship between pumping speed and flow rate is developed and the relation is divided into three phases for detailed analysis. It is found that the flow rate and the concentration of circulating He-3 increase first and then decrease with the increasing diameter of orifice at a certain pumping speed, and the possible reasons for this phenomenon are analyzed. The results indicate that there is an optimal orifice diameter at a determined pumping speed.

We report progress on the development of x-ray microcalorimeter thermal kinetic inductance detector (TKID) arrays, where each TKID is an independent pixel. Our goal in developing this detector technology is to arrive at high quantum efficiency, high fill factor, large-format, moderate energy resolution x-ray detector array which can be readily scaled to tens of kilo-pixels, to be used as an x-ray imaging spectrograph for astronomy and metrology applications. We discuss the evolution of the design, how it has been driven by fabrication related constraints, and the resulting impacts on detector performance.

Recent reports of room-temperature, ambient pressure superconductivity in copper-substituted lead phosphate apatite, commonly referred to as LK99, have prompted numerous theoretical and experimental studies into its properties. As the electron–phonon interaction is a common mechanism for superconductivity, the electron–phonon coupling strength is an important quantity to compute for LK99. In this work, we compare the electron–phonon coupling strength among the proposed compositions of LK99. The results of our study are in alignment with the conclusion that LK99 is a candidate for low-temperature, not room-temperature, superconductivity if electron–phonon interaction is to serve as the mechanism.