Journal of Low Temperature Physics最新文献

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.

Cosmic microwave background (CMB) experiments have deployed focal planes with (mathcal {O}(10^{4})) transition edge sensor (TES) bolometers cooled to sub-Kelvin temperatures by multiplexing the readout of many TES channels onto a single pair of wires. Digital Frequency-domain Multiplexing (DfMux) is a multiplexing technique used in many CMB polarization experiments, such as the Simons Array, SPT-3 G, and EBEX. The DfMux system studied here uses LC filters with resonant frequencies ranging from 1.5 to 4.5 MHz connected to an array of TESs. Each detector has an amplitude-modulated carrier tone at the resonant frequency of its accompanying LC resonator. The signal is recovered via quadrature demodulation where the in-phase (I) component of the demodulated current is in phase with the complex admittance of the circuit and the quadrature (Q) component is orthogonal to I. Observed excess current noise in the Q component is consistent with fluctuations in the resonant frequency. This noise has been shown to be non-orthogonal to the phase of the detector’s responsivity. We present a detailed analysis of the phase of responsivity of the TES and noise sources in our DfMux readout system. Further, we investigate how modifications to the TES operating resistance and bias frequency can affect the phase of noise relative to the phase of the detector responsivity, using data from Simons Array to evaluate our predictions. We find that both the phase of responsivity and phase of noise are functions of the two tuning parameters, which can be purposefully selected to maximize signal-to-noise (SNR) ratio.

We established a design method for a Magic-T with a single-layer dielectric/metal structure suitable for both wideband and multi-element applications for millimeter and submillimeter wave imaging observations. The design method was applied to a Magic-T with a coupled-line, stubs, and single-stage impedance transformers in a frequency-scaled model (6–14 GHz) that is relatively easy to demonstrate through manufacturing and evaluation. The major problem is that using the conventional perfect matching condition for a coupled-line alone produces an impractically large width coplanar coupled-line (CPCL) to satisfy the desired bandwidth ratio. In our study, by removing this constraint and optimizing impedances utilizing a circuit simulator with high computation speed, we found a solution with a (sim) 180 μm wide CPCL, which is approximately an order of magnitude smaller than the conventional analytical solution. Furthermore, considering the effect of transition discontinuities in the transmission lines, we optimized the line length and obtained a design solution with return loss < − 20 dB, amplitude imbalance < 0.1 dB, and phase imbalance < 0.5(^{circ }) from 6.1 to 14.1 GHz.

We present advancements in a finite element method for computing the physical properties of magnetic microcalorimeters (MMCs). Utilizing the COMSOL package, we conducted 3D simulations of a meander-shaped Nb coil with a realistic geometry. The resulting magnetic field distribution showed good agreement with a previous 2D simulation and revealed non-negligible differences at the side edge of the sensor material. Employing the simulation results, we calculated the MMC properties and compared them with previous measurements. Our calculated values closely align with the measured values for the sensor magnetization, the pulse heights from alpha detection, and the coil inductance.

The interplay between electron–electron interactions and weak localization (or anti-localization) phenomena in two-dimensional systems can significantly enhance the superconducting transition temperature. We develop the theory of quantum fluctuations within such multifractally enhanced superconducting states in thin films. In conditions of weak disorder, we employ the Finkel’stein nonlinear sigma model to derive an effective action for the superconducting order parameter and the quasiclassical Green’s function, meticulously accounting for the influence of quantum fluctuations. This effective action, applicable for interactions of any strength, reveals the critical role of well-known collective modes in a dirty superconductor, and its saddle-point analysis leads to modified Usadel and gap equations. These equations comprehensively incorporate the renormalizations stemming from the interplay between interactions and disorder, resulting in the non-trivial energy dependence of the gap function. Notably, our analysis establishes a direct relation between the self-consistent gap equation at the superconducting transition temperature and the known renormalization group equations for interaction parameters in the normal state.

ARIADNE (Axion Resonant InterAction Detection Experiment) is a table-top experiment that intends to search for QCD axions from exotic spin-dependent interactions mediated by axion between nuclei at sub-mm range. This experiment includes a non-magnetic mass to source the axion field, and a dense ensemble of hyper-polarized ³He nuclei to detect the axion field with nuclear-magnetic-resonance (NMR)-based method. The expected NMR signal from the interaction could be easily buried in the noise spectrum of the magnetometer, especially in a frequency range (~ 100 Hz) where the interaction signal is supposed to exist. In this work, we report optimization of SQUID gradiometer for ARIADNE including noise spectrum measurement.

The importance and non-trivial properties of superconductor normal metal interfaces were discovered by Alexander Fyodorovich Andreev more than 60 years ago. Only much later, these hybrids have found wide interest in applications such as thermometry and refrigeration, electrical metrology, and quantum circuit engineering. Here we discuss the central properties of such interfaces and describe some of the most prominent and recent applications of them.

Characterization of the magnetic fields at different scales in the Universe is a new frontier for submillimeter astronomy. Polarimetric measurements between 50 and 500 µm are the golden path for this research. We develop, in the prospect of space observatories, all-silicon 50 mK bolometer arrays with polarimetric capabilities in the pixel. Here, we present the first results of the new detectors: performances of thermal sensors, optical absorption and polarimetry.