Low Temperature Physics

Fluids in porous media are commonly studied with analytical or simulation methods, usually assuming that the host medium is rigid. By evaluating the substrate’s response (relaxation) to the presence of the fluid we assess the error inherent in that assumption. One application is a determination of the ground state of 3He in slit and cylindrical pores. With the relaxation, there results a much stronger cohesion than would be found for a rigid host. Similar increased binding effects of relaxation are found for classical fluids confined within slit pores or nanotube bundles.

From measurements of the magnetic penetration depth, $\lambda(T)$, from 1.6 K to $T_c$ in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, $N_s(E)$ at T=0 by using a simple model. In this framework, the flat behavior of $\lambda^{-2}(T)$ at low $T$ implies $N_s(E)$ is small, possibly gapped, at low energies. The upward curvature in $\lambda^{-2}(T)$ near $T_c$ seen in overdoped films implies that superfluid comes from an anomalously small energy band within about $3k_BT_c$ of the Fermi surface.

Thermoelectric generation using the anomalous Nernst effect (ANE) has great potential for application in energy harvesting technology because the transverse geometry of the Nernst effect should enable efficient, large-area and flexible coverage of a heat source. For such applications to be viable, substantial improvements will be necessary not only for their performance but also for the associated material costs, safety and stability. In terms of the electronic structure, the anomalous Nernst effect (ANE) originates from the Berry curvature of the conduction electrons near the Fermi energy 1,2. To design a large Berry curvature, several approaches have been considered using nodal points and lines in momentum space 3-10. Here we perform a high-throughput computational search and find that 25 percent doping of aluminium and gallium in alpha iron, a naturally abundant and low-cost element, dramatically enhances the ANE by a factor of more than ten, reaching about 4 and 6 microvolts per kelvin at room temperature, respectively, close to the highest value reported so far. The comparison between experiment and theory indicates that the Fermi energy tuning to the nodal web-a flat band structure made of interconnected nodal lines-is the key for the strong enhancement in the transverse thermoelectric coefficient, reaching a value of about 5 amperes per kelvin per metre with a logarithmic temperature dependence. We have also succeeded in fabricating thin films that exhibit a large ANE at zero field, which could be suitable for designing low-cost, flexible microelectronic thermoelectric generators 11-13. Thermoelectricity, the conversion of heat current into electric energy, provides a key technology for versatile energy harvesting and for heat flow sensors. There is a rapidly growing demand for novel energy-harvesting technology to power Internet of Things sensors and wearable devices, particularly in the form of flexible, durable micro-thermoelectric generators (μ-TEGs). So far, the technology has relied on the longitudinal thermoelectric response known as the Seebeck effect 11-15. Recently, its transverse counterpart in ferromagnets, the anomalous Nernst effect (ANE), has gained increasing attention, as it has a number of potential benefits 6-9,16-18. For example, the transverse geometry of the Nernst effect enables efficient, large-area and flexible coverage of a curved heat source (Fig. 1a, left). It substantially reduces the number of production processes as well as the contact resistance of a thermopile, compared with a conventional thermoelectric device (Fig. 1a, right). In addition, the transverse geometry is hypothetically better suited for thermoelectric conversion as the Ettingshausen heat current should support the Nernst voltage, whereas the Peltier heat current may suppress the Seebeck voltage 18. However, the ANE is too small compared with the Seebeck effect for any thermoelectric application. Thus, it is important to develop an approach to design a new class of materials that exhibit a large ANE at zero field. Moreover, as the use of rare and toxic elements would pose a barrier for applications , low-cost and safe elements should be used for thermoelectric materials. Here we introduce two iron-based cubic compounds Fe 3 X (where X is Ga or Al) as materials suitable for designing such low-cost, flexible μ-TEGs, in particular by using their thin-film forms. As detailed in Methods, our successful fabrication of their thin films enables us to obtain a large ANE of about 4 μV K −1 for Fe 3 Ga and about 2 μV K −1 for Fe 3 Al at room temperature using an in-plane temperature gradient (Extended Data Fig. 3a). In addition, our films of Fe 3 Ga and Fe 3 Al have in-plane magnetization with a coercivity B c of about 40 Oe and 20 Oe, respectively, and exhibit a spontaneous ANE at zero field (Fig. 1c) for the out-of-plane temperature gradient. This enables us to design a much simpler μ-TEG than the conventional Seebeck analogues (Fig. 1a, Extended Data Fig. 4) 16. Moreover, the specific power generation capacity Γ P = P max /(A(∆T) 2) is estimated to be of the same order as or potentially greater than that of conventional μ-TEGs (Methods), where P max , A and ∆T are the maximum power, the area of the thermopile device and the temperature difference, respectively 11-13. In the following, we discuss

The specific heat, and dc and ac magnetic susceptibility are reported for a large single crystal of PrOs4 Sb12 and, after grinding, its powder. The physical properties of the crystal are typical of the majority of reported PrOs4 Sb12 samples. The room temperature effective paramagnetic moment of the crystal was consistent with the Pr3+ ionic configuration and full (or nearly full) occupancy of the Pr sublattice. The crystal showed two distinct anomalies in the specific heat and an overall discontinuity in C/T of approximately 1000 mJ K−2 mol−1 . The upper transition (at Tc1 ) was characteristically rounded. The anomaly at Tc2 was very sharp, consistent with a good quality for the crystal. We observed a shoulder in χ and two peaks in χ below Tc1 . However, there were no signatures in χ of the lower temperature transition. Grinding to powder size smaller than 50 μm completely suppresses the upper superconducting transition in both the specific heat and magnetic susceptibility. It also strongly reduces C/Tc at Tc2 . Stress annealing brings back some of this lost C/Tc but does not restore the upper temperature transition. Possible explanations of the existence of two superconducting specific heat anomalies for single crystals are discussed.

Clean superconductors with weakly coupled conducting planes have been suggested as promising candidates for observing the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. We consider here a layered superconductor in a magnetic field of arbitrary orientation with respect to the conducting plane. In this case there is competition of spin-pair-breaking and orbital-pair-breaking effects. In previous work, phase boundaries characterized by Landau quantum numbers n > 0 have been predicted. Here, we calculate the actual structure of the stable states below Hc2 by minimizing the free energy. We find several new order parameter structures differing from both the traditional Abrikosov and FFLO solutions. Some interesting unsolved questions appear in the limit of large n.

The magnetic field dependence of the resistance of bismuth thin films (100-700 A˚ thick) at low temperatures (1.5-77 K) are analyzed in the conceptual framework of quantum corrections to the conductivity due to weak localization and electron interaction effects. It is shown that the diversity and variability of the magnetoresistance curves in a parallel field upon variations of the thickness and temperature are due to the fact that the spin-orbit interaction time τso increases with increasing field, altering the relationship between τso and the phase relaxation time τφ. This result supports the hypothesis that the strong spin-orbit interaction manifested in the surface scattering of electrons is due to the existence of a potential gradient near the metal surface, and a parallel magnetic field alters the orientation of the spins, accompanied by a decrease of the rate of spin-orbit processes.

We present results of a hole burning study with thermal cycling and waiting time spectral diffusion experiments on a modified cytochrome - c protein in its native as well as in its denatured state. The experiments show features which seem to be characteristic for the protein state of matter and its associated dynamics at low temperature. The properties responsible for the observed patterns are organisation paired with randomness and, in addition, the finite size which gives rise to surface and solvent effects. We discuss some general model approaches which might serve as guide lines for understanding these features.

Experimental results of the thermal conductivity (k(T)) of nanostructured gAs 2 S 3 during cooling and heating processes within the temperature range from 2.5 to 100 K have been analysed. The paper has considered thermal conductivity is weakly temperature k(T) dependent from 2.5 to 100 K showing a plateau in region from 3.6 to 10. 7 K during both cooling and heating regimes. This paper is the first attempt to consider the k(T) hysteresis above plateau while heating in the range of temperature from 11 to 60 K. The results obtained have not been reported yet in the scientific literature. Differential curve Δk(T) of k(T) (heating k(T) curve minus cooling k(T) curve) possesses a complex asymmetric peak in the energy range from 1 to 10 meV. Δk(T) reproduces the density of states in a g(ω)/ω 2 representation estimated from a boson peak experimentally obtained by Raman measurement within the range of low and room temperatures. Theoretical and experimental spectroscopic studies have confirmed a glassy structure of gAs 2 S 3 in cluster approximation. The origin of the low-frequency excitations resulted from a rich variety of vibrational properties. The nanocluster vibrations can be created by disorder on atomic scale.