The presence of a quantum-critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T . This is reflected, e.g., in the entropy landscape SðT,rÞ in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of ∂S∕∂B near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), ð∂T ∕∂BÞ S and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu 2þ -containing coordination polymer, which is a very good realization of a spin-½ antiferromagnetic Heisenberg chain-one of the simplest quantum-critical systems.quantum criticality | quantum magnetism | low-dimensional spin systems | magnetothermal effect T he magnetocaloric effect (MCE), i.e., a temperature change in response to an adiabatic change of the magnetic field, has been widely used for refrigeration. Although up until now applications have focused on cryogenic temperatures (1-3), possible extensions to room temperature have been discussed (4). The MCE is an intrinsic property of all magnetic materials in which the entropy S changes with magnetic field B. Paramagnetic salts have been the materials of choice for low-temperature refrigeration (1), including space applications (5-7), with an area of operation ranging from about one or two degrees Kelvin down to some hundredths or even thousandths degree Kelvin. Owing to their large ΔS∕ΔB values, the ease of operation, and the applicability under microgravity conditions, paramagnets have matured to a valuable alternative to 3 He-4 He dilution refrigerators, the standard cooling technology for reaching sub-Kelvin temperatures.A large MCE also characterizes a distinctly different class of materials, where the low-temperature properties are governed by pronounced quantum many-body effects. These materials exhibit a B-induced quantum-critical point (QCP)-a zero-temperature phase transition-and the MCE has been used to study their quantum criticality (8)(9)(10)(11)(12)(13)(14) or to determine their B-T phase diagrams (15)(16)(17)(18)(19). The aim of the present work is to provide an accurate determination of the enhanced MCE upon approaching a B-induced QCP both as a function of B and T and to explore the potential of this effect for magnetic cooling.Materials in the vicinity of a QCP have been of particular current interest, as their properties reflect critical behavior arising from quantum fluctuations instead of thermal fluctuations that govern classical critical points (20). Prominent examples of findings made here include the intriguing low-temperature behaviors encountered in some heavy-fermion metals, itinerant transition metal magnets (21 and references cited therein, 22), or magnetic insulators (23, 24) and the ...

We present a series of measurements examining the penetration force required to push a flat plate vertically through a dense granular medium, focusing in particular on the effects of the bottom boundary of the vessel containing the medium. Our data demonstrate that the penetration force near the bottom is strongly affected by the surface properties of the bottom boundary, even many grain diameters above the bottom. Furthermore, the data indicate an intrinsic length scale for the interaction of the penetrating plate with the vessel bottom via the medium. This length scale, which corresponds to the extent of local jamming induced by the penetrating plate, has a square root dependence both upon the plate radius and the ambient granular stress near the bottom boundary, but it is independent of penetration velocity and grain diameter.

The normalized in-plane magnetoconductivity of the dilute strongly interacting system of electrons in silicon metal-oxide-semiconductor field effect transistors scales with B / T for low densities in the insulating phase. Pronounced deviations occur at higher metalliclike densities, where an energy scale k B ⌬ emerges which is not associated with either magnetic field or thermal effects. While the energy scale k B ⌬ vanishes at n 0 approaching from the metallic side, we do not find a finite ⌬ which goes to zero approaching n 0 from the insulating side. DOI: 10.1103/PhysRevB.71.033312 PACS number͑s͒: 73.40.Qv, 71.30.ϩh, 73.50.Jt According to well established theory, no metallic state can exist in two dimensions for noninteracting 1 or weakly interacting 2 electrons ͑or holes͒ in zero magnetic field in the limit of zero temperature. In dilute two-dimensional systems where the interactions are known to be strong, however, experimental studies have revealed an unexpected decrease in the resistance as the temperature is lowered, behavior that is generally a characteristic of metals. This metallic behavior has been observed down to the lowest accessible temperatures at electron 3,4 and hole 5-7 densities above some critical density n c ͑or p c ͒. For densities down to approximately 1.5n c , the temperature dependence has been attributed to electron-electron scattering in the ballistic limit ͑k B T ӷប/ ͒, confirming the importance of strong e-e interactions in determining the behavior of these systems.8 However, the behavior observed at lower densities and the nature of the apparent metal-insulator transition are still not understood. 9 A very unusual characteristic of dilute, strongly interacting electron ͑hole͒ systems is their strong response to an in-plane magnetic field: the resistivity increases dramatically with increasing field and saturates to a value above a characteristic magnetic field that depends on density and temperature.9 From an analysis of the temperature and density dependence of the magnetoconductance of silicon metal-oxide-semiconductor field effect transistors ͑MOSFETs͒, Vitkalov et al. 10 have identified an energy scale ͑k B ⌬͒ which extrapolates to zero at a finite density n 0 in the vicinity of n c ; this was attributed to an increase in the magnetic susceptibility ϰ ͑g * m * ͒ and the approach to a zero temperature quantum phase transition at n 0 ͑here g* and m* are the renormalized Lande-g factor and effective mass, respectively͒. Shashkin et al. 11 found similar results; moreover, these authors have claimed that the sharp increase in the susceptibility is associated with an increase in the effective mass while the g value remains essentially constant as the electron density approaches n c . 12 These findings suggest critical behavior and the approach to a quantum phase transition where an energy scale k B ⌬ vanishes at n 0 approaching from the metallic side. The question thus arises whether there exists an energy scale that goes to zero approaching n 0 from the insulating side, and whet...

Penetration by an object through a dense granular medium (for example, by a finger pushing slowly into the sand on a beach) presents an interesting physics problem that is closely related to issues of practical importance in soil science. Here we measure the penetration-resistance force for an object approaching the solid bottom boundary of a granular sample--analogous to the finger approaching a flat rock buried in the beach. We find that the penetration resistance near the boundary increases exponentially, which demonstrates the existence of an intrinsic length scale to the 'jamming' caused by a locally applied stress.

We provide evidence that a single mechanism-flux flow along channels-can explain the functional form of the critical current density (J c) in the low temperature superconductor Nb 3 Sn and in the high temperature superconductors (HTS) YBa 2 Cu 3 O 7-δ (YBCO) and (Bi,Pb) 2 Sr 2 Ca n-1 Cu n O x (BiSCCO) in low and high magnetic fields. In this paper, we show that standard flux pinning theories, used for the last four decades to describe J c in low temperature superconductors (LTS), cannot explain the strain dependence of J c in YBCO because J c is a function of strain but the average superconducting properties are not. We conclude that in the polycrystalline samples presented here, the channels are grain boundaries that are narrow and metallic in Nb 3 Sn and YBCO but wide and semiconducting in BiSCCO. Strain alters J c by changing the superconducting properties of the grains in Nb 3 Sn but by changing the grain boundaries in YBCO.

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Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S ¼ ½ ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

Demountable superconducting magnet coils would offer significant benefits to commercial nuclear fusion power plants. Whether large pressed joints or large soldered joints provide the solution for demountable fusion magnets, a critical component or building block for both will be the many, smaller-scale joints that enable the supercurrent to leave the superconducting layer, cross the superconducting tape and pass into the solder that lies between the tape and the conductor that eventually provides one of the demountable surfaces. This paper considers the electrical and thermal properties of this essential component part of demountable high temperature superconducting (HTS) joints by considering the fabrication and properties of jointed HTSs consisting of a thin layer of solder (In52Sn48 or Pb38Sn62) sandwiched between two rare-earth-Ba2Cu3O7 (REBCO) second generation HTS coated conductors (CCs). The HTS joints are analysed using numerical modelling, critical current and resistivity measurements on the joints from 300 to 4.2 K in applied magnetic fields up to 12 T, as well as scanning electron microscopy studies. Our results show that the copper/silver layers significantly reduce the heating in the joints to less than a few hundred mK. When the REBCO alone is superconducting, the joint resistivity (RJ) predominantly has two sources, the solder layer and an interfacial resistivity at the REBCO/silver interface (∼25 nΩ cm2) in the as-supplied CCs which together have a very weak magnetoresistance in fields up to 12 T. We achieved excellent reproducibility in the RJ of the In52Sn48 soldered joints of better than 10% at temperatures below Tc of the REBCO layer which can be compared to variations of more than two orders of magnitude in the literature. We also show that demountable joints in fusion energy magnets are viable and need only add a few percent to the total cryogenic cost for a fusion tokamak.