Journal of Low Temperature Physics最新文献

Superfluid liquid ⁴He consists of superfluid and normal components. The superfluid component has zero viscosity and zero entropy, therefore flows without dissipation. The normal component carries heat and can produce a high velocity flow which leads to very high thermal conductivity. In neutron scattering and muon spectroscopy experiments with powder samples, ⁴He gas is often used as an exchange medium to thermalise the sample. Samples are usually cooled using a dilution refrigerator with installed capillaries which admit ⁴He into the experimental volume. Below 2 K the inner surface of the capillary is covered with a superfluid ⁴He film, which can thermally link stages of the dilution refrigerator with a catastrophic effect on its performance. In our experiment, we utilised a 0.1% mixture of ³He in ⁴He to disrupt the superfluid helium film flow and, as a result, significantly reduced the parasitic thermal flow to the experimental can. The addition of a small amount of ³He into ⁴He allows a significant improvement of thermal performance when using this technique in neutron scattering experiments at very low temperatures.

We introduce a generalized two-mode vacuum state (GTMVS) that bridges the gap between the two-mode squeezed vacuum state (TMSVS) and the normal two-mode vacuum state (NTMVS). This state is generated by applying a bosonic exponential quadratic operator (BEQO) to the NTMVS, whose properties are tunable via two parameters: λ (squeezing strength) and θ (beam splitter-like mixing). Fidelity analysis confirms its continuous transition between these limiting cases. While GTMVS exhibits weaker yet tunable quadrature squeezing compared to TMSVS, it retains strong photon correlations and displays super-Poissonian statistics. Entanglement, quantified by linear entropy, is widely adjustable with tunable near-maximal values or degraded near-separability, extending beyond the static entanglement range of TMSVS. The GTMVS provides a framework for quantum correlation manipulation, with potential applications in quantum metrology and sensing where tunable nonclassicality and noise resilience are critical.

The topological semimetal YPtBi has attracted considerable attention, owing to its novel superconducting and normal state properties. A strong band inversion from spin-orbit coupling allows the existence of (j=3/2) quasi-particles near the Fermi level, which form Cooper pairs with angular momentum potentially higher than single or triplet states. In this report, we present high-pressure magnetotransport and Shubnikov-de Haas effect measurements on high-quality YPtBi up to (P = 2.08) GPa. As a function of pressure, we observe a trend toward more insulating resistivity at low temperatures concomitant with a suppression of quantum oscillation amplitude. Together with a decrease of the upper critical field and significant increase in the Dingle temperature, the pressure-induced changes point to a weakening of the band inversion and potential tuning of the topological nature of YPtBi, suggesting pressure as a useful tool for understanding the nature of topology in other related half-Heusler compounds.

In this work, we propose a procedure to obtain the voltage vs current (I–V) curves by means of AC transport measurements, of particular interest for single crystals mounted on settings in which its thermal coupling to the environment is week, and Joule dissipation can cause unwanted thermal decoupling from the thermometer. We present a simple model that accounts for the overheating of a sample in this type of experimental setup and propose two experimental protocols in which thermal oscillations and gradients on the measured region of the sample can be neglected and the overall temperature shifts can be accounted for and corrected. As an example, we present preliminary results obtained using one of these protocols in a Ba(Fe(_{0.81})Co(_{0.09}))(_2)As(_2) single crystal, mounted on a strain-cell, demonstrating that we are able to obtain reliable I–V curves in the vicinity of a vortex-glass transition that exhibit the expected critical behavior. Both the model and the proposed procedure can be used in a broad spectrum of experimental situations far away the presented example.

In a mixture of two Bose–Einstein condensates, the interactions can be tuned such that self-bound objects called quantum droplets appear. Whereas the ground states of such quantum droplets at finite temperature have been studied for three- and one-dimensional configurations, the possible fragmentation of these droplets has so far not been considered in these studies. In this paper, we show that droplets can lower their free energy by splitting or fragmenting in a combination of multiple smaller droplets and/or a gas. Three-dimensional droplets will split when the interspecies interaction strength is considerably stronger than the intraspecies interaction strength, and the number of atoms is of the same order as the minimum number of atoms necessary to form a droplet. One-dimensional droplets will fragment as long as the intraspecies and interspecies interactions strength do not vary too much in strength and the density is not to big compared with the scattering length. If the temperature rises, 1D droplets will split by expelling atoms, forming a gas of predominantly free atoms and pairs of atoms. These pairs remain present in the system up to considerably high temperatures compared to the transition temperature. Our results provide important insights on the stability of these droplets.

We studied spatially-resolved measurements of dynamic nuclear spin polarization (DNP) in the (nu =2/3) fractional quantum Hall state. To avoid the influence of quantum Hall edge effects and to perform spatially-resolved measurements of bulk DNP alone, we fabricated a device with a multiply-connected Corbino geometry from a high-mobility GaAs/AlGaAs two-dimensional electron system wafer. We simultaneously measured six sets of resistances to track their time evolution during the depolarization, DNP, and relaxation processes. DNP was induced using large alternating or direct currents, and the polarization was carried out in the polarized phase. Under AC DNP, only monotonic changes in resistance were observed during the DNP process, whereas under DC DNP, non-monotonic behavior was detected, first decreasing (increasing) and then turning into increasing (decreasing) stage a few thousand seconds after starting the DNP, and showing random fluctuations in the latter stage. The results are consistent with our previous measurements performed in a Hall bar. Comparison of the resistances measured with different pairs of electrodes reveals that the resistance induced by domain wall mainly increase around the area close to the inner (source) electrode, with the area being smaller for DC DNP.

The quantum mechanics of the Josephson effect is the core ingredient for quantum technologies with superconducting circuits. A new avenue was recently opened in this field by predicting that the Josephson quantum mechanics in the odd parity sector, when a quasiparticle is trapped in an Andreev bound state, is fundamentally different from the conventional one in the even sector. The focus was then on a Josephson junction surrounded by an electromagnetic environment formed of a collection of bosonic modes, including the case of an ohmic environment. Here we consider the distinct case of a superconducting qubit made of a single Josephson junction whose environment reduces to a capacitance. We find a novel structure for the low-lying discrete states in the odd sector, which is altogether different from the one that appears in the even sector. Our study of the bound-state spectrum ranges from the Coulomb-dominated (Cooper pair box) to the Josephson-dominated (transmon) regime. Our prediction could be tested in forthcoming experiments with superconductor/semiconductor/superconductor junctions, which have been studied intensively in recent years, both using nanowires as well as two-dimensional electron gases.

We numerically investigate finite-momentum superconductivity in noncentrosymmetric metallic altermagnets with d-wave spin splitting and strong Rashba-type spin-orbit coupling. Focusing on a stripe phase in which Cooper pairs acquire multiple center-of-mass momenta, we construct phase diagrams that reveal phase boundaries between the stripe phase and a helical phase characterized by a single center-of-mass momentum. Our results show that the stripe phase emerges at low temperatures and exhibits a reentrant behavior as a function of the strength of the altermagnetic splitting. We further analyze the stripe phase within a linearized gap equation, and uncover the mechanism of the pairing formation unique to the stripe phase. This mechanism originates from the anisotropic deformation of the Fermi surfaces induced by the altermagnetic splitting, highlighting the intriguing interplay between the spin-orbit coupling and the altermagnets.

Superconducting resonator parametric amplifiers are potentially important components for a wide variety of fundamental physics experiments and utilitarian applications. We propose and realise an operating scheme that achieves amplification through the use of non-degenerate pumps, which addresses two key challenges in the design of parametric amplifiers: non-continuous gain across the amplification band and pump tone removal. We have experimentally demonstrated the non-degenerate pumping scheme using a half-wave resonator amplifier based on NbN thin-film, and measured a peak gain of 26 dB and 3-dB bandwidth of 0.5 MHz. The two non-degenerate pump tones were positioned (sim 10) bandwidths above and below the frequency at which peak gain occurs. We have found the non-degenerate pumping scheme to be more stable compared to the usual degenerate pumping scheme in terms of gain drift over time, by a factor of 4. This scheme also retains the usual flexibility of NbN resonator parametric amplifiers in terms of reliable amplification in a (sim 4) K environment, and is suitable for cross-harmonic amplification. The use of pump tones at different frequencies allows phase-sensitive amplification when the signal tone is degenerate with the idler tone. A gain of 23 dB and squeezing ratio of 6 dB were measured.

We examine the (S=7) Heisenberg antiferromagnet in one dimension by numerical-diagonalization method. This system reveals nonzero energy gap above the unique ground state in its spin excitation, namely the Haldane gap; its amplitude is extremely small. We have carried out our numerical-diagonalization calculations based on the Lanczos algorithm applied to finite-size clusters up to 12 sites as highly parallelized computations. We successfully estimate the (S=7) Haldane gap directly from our data under the twisted boundary condition. We successfully confirm the agreement between the asymptotic behavior determined by the systems up to (S=6) and our present result that is presently estimated in the (S=7) case. Our results deepen our understanding concerning quantum spin systems and quantum magnets.