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We investigate the nonequilibrium quantum heat transfer in a quantum thermal transistor, constructed by a triangle-coupled spin-boson system in a three-terminal setup. By exploiting the nonequilibrium noninteracting blip approximation approach combined with full counting statistics, we obtain the steady-state thermal transport, such as heat currents. We identify the giant heat amplification feature in a strong coupling regime, which results from the negative differential thermal conductance with respect to the gate temperature. Analysis shows that the strong coupling between the gate qubit and corresponding gate thermal bath plays the crucial role in exhibiting these far-from-equilibrium features. These results would have potential implications in designing efficient quantum thermal transistors in the future.

We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of $(\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x})\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ ($0.18\ensuremath{\le}x\ensuremath{\le}1$). The critical current density ${J}_{c}$ reaches maximum in the underdoped sample $x=0.26$ and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier ${U}_{0}$ sharply decreases in the overdoped sample $x=0.70$. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples $x=0.38$, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in ${T}_{c}$ become small in the samples $x=0.43$ and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the $(\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x})\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ crystals: (i) strong \ensuremath{\delta}l pinning, which results from the fluctuations in the mean free path $l$ and $\ensuremath{\delta}{T}_{c}$ pinning from the spatial variations in ${T}_{c}$ in the underdoped regime, and (ii) weak $\ensuremath{\delta}{T}_{c}$ pinning in the optimally doped and overdoped regime.

We demonstrate substantial enhancement in the energy product of MnBi-based magnets by forming robust ferromagnetic exchange coupling between a MnBi layer and a thin CoFe layer in a unique perpendicular coupling configuration, which provides increased resistance to magnetization reversal. The measured nominal energy product of $172\phantom{\rule{0.16em}{0ex}}\mathrm{kJ}/{\mathrm{m}}^{3}$ at room temperature is the largest value experimentally attained for permanent magnets free of expensive raw materials. Our finding shows that exchange-coupled MnBi/CoFe magnets are a viable option for pursuing rare-earth-free magnets with energy products approaching those containing rare-earth elements.

The quantum phase transition (QPT) of the one-dimensional (1D) quantum compass model in a transverse magnetic field is studied in this paper. An exact solution is obtained by using an extended Jordan and Wigner transformation to the pseudospin operators. The fidelity susceptibility, the concurrence, the block-block entanglement entropy, and the pseudospin correlation functions are calculated with antiperiodic boundary conditions. The QPT driven by the transverse-field only emerges at zero field and is of the second order. Several critical exponents obtained by finite-size scaling analysis are the same as those in the 1D transverse-field Ising model, suggesting the same universality class. A logarithmic divergence of the entanglement entropy of a block at the quantum critical point is also observed. From the calculated coefficient connected to the central charge of the conformal field theory, it is suggested that the block entanglement depends crucially on the detailed topological structure of a system.

Quantum phase transition in the one-dimensional period-two and uniform quantum compass model are studied by using the pseudo-spin transformation method and the trace map method. The exact solutions are presented, the fidelity, the nearest-neighbor pseudo-spin entanglement, spin and pseudo-spin correlation functions are then calculated. At the critical point, the fidelity and its susceptibility change substantially, the gap of pseudo-spin concurrence is observed, which scales as $1/N$ (N is system size). The spin correlation functions show smooth behavior around the critical point. In the period-two chain, the pseudo-spin correlation functions exhibit a oscillating behavior, which is absent in the unform chain. The divergent correlation length at the critical point is demonstrated in the general trend for both cases., Comment: 5 pages, 6 figures

The heat capacities C(T) of high-purity polycrystalline pellet and powder samples of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ are reported for the temperature T in the range 0.4 to 400 K. In addition to the feature associated with the onset of superconductivity at ${\mathit{T}}_{\mathit{c}}$\ensuremath{\simeq}91 K, two clear anomalies in C(T) for the pellet sample were observed near 74 and 330 K; the origins of the latter two anomalies are unknown, but the temperatures at which they occur are similar to those at which anomalies are seen in the temperature-dependent magnetization M(T). The feature in M(T) at \ensuremath{\simeq}70 K increases linearly with field H at a rate of 0.30 K/kG for 20 kG\ensuremath{\le}H\ensuremath{\le}40 kG. The anomaly in C(T) at \ensuremath{\simeq}330 K observed for the pellet was not observed for the powder sample, which is consistent with the lack of an anomaly in M(T) near this temperature for the powder sample. For the pellet sample, the electronic entropy near ${\mathit{T}}_{\mathit{c}}$, the contribution of superconducting fluctuations to C(T) near ${\mathit{T}}_{\mathit{c}}$, and the influence of a 70-kG magnetic field on C(T) near ${\mathit{T}}_{\mathit{c}}$ and below 10 K are analyzed and discussed.Various features of the C(T) for the pellet sample such as the magnitude of C(T) and the derived Debye temperature and heat capacity jump at ${\mathit{T}}_{\mathit{c}}$, as well as the size and shape of the C(T) anomalies at \ensuremath{\simeq}74 and 330 K, were found to depend on the thermal- and/or magnetic-field treatment history of the sample. Differential-thermal-analysis measurements quantitatively determined the ${\mathrm{BaCuO}}_{2}$ impurity content in our batch of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ to be 0.3(1) wt.%. Coupled with the analysis of the low-T C(T) measurements, we conclude that the linear C(T) coefficient \ensuremath{\gamma}(0) associated with the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ phase in the pellet sample is \ensuremath{\simeq}4.0 mJ/mol ${\mathrm{K}}^{2}$. It is suggested that some fraction of \ensuremath{\gamma}(0) could arise from thermal excitation of antiferromagnetic spin waves in the Cu-O chains of the structure.

Heat-capacity {ital C}({ital T}) data on the title materials, as well as on CuO for comparison, were obtained with use of a differential scanning calorimeter to a precision of {congruent}0.1--1%. The measurements were carried out (i) to characterize the thermal anomaly at the tetragonal-orthorhombic transition temperature {ital T}{sub 0} of single-crystal La{sub 2}CuO{sub 4} and polycrystalline La{sub 2{minus}{ital x}}Sr{sub {ital x}}CuO{sub 4} (0{le}{ital x}{le}0.20) and (ii) to search for thermal anomalies at the Neel temperature {ital T}{sub {ital N}} of La{sub 2}CuO{sub 4} ({congruent}300 K). We find a cusp-shaped anomaly with a peak at 523 K ({approx}{ital T}{sub 0}) for a single crystal of La{sub 2}CuO{sub 4} with {ital T}{sub {ital N}}=304 K and find the transition to be second order, consistent with previous neutron- and x-ray-diffraction measurements. {ital T}{sub {ital N}} was determined from measurements of the anisotropic magnetic susceptibility on the same crystal, which are also presented. The {ital C}({ital T}) anomaly at {ital T}{sub 0} is smeared out somewhat in polycrystalline La{sub 2}CuO{sub 4}.