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Since the discovery of superconductivity 1 , there has been a drive to understand the mechanisms by which it occurs. The BCS (Bardeen–Cooper–Schrieffer) model successfully treats the electrons in conventional superconductors as pairs coupled by phonons (vibrational modes of oscillation) moving through the material 2 , but there is as yet no accepted model for high-transition-temperature, organic or ‘heavy fermion’ superconductivity. Experiments that reveal unusual properties of those superconductors could therefore point the way to a deeper understanding of the underlying physics. In particular, the response of a material to a magnetic field can be revealing, because this usually reduces or quenches superconductivity. Here we report measurements of the heat capacity and magnetization that show that, for particular orientations of an external magnetic field, superconductivity in the heavy-fermion material CeCoIn 5 is enhanced through the magnetic moments (spins) of individual electrons. This enhancement occurs by fundamentally altering how the superconducting state forms, resulting in regions of superconductivity alternating with walls of spin-polarized unpaired electrons; this configuration lowers the free energy and allows superconductivity to remain stable. The large magnetic susceptibility of this material leads to an unusually strong coupling of the field to the electron spins, which dominates over the coupling to the electron orbits.

In quasi-low-dimensional systems, the existence of a particular physical state and the temperature and magnetic-field-dependence of its phase boundary often strongly depends on magnetic field orientation. To investigate magnetic field orientation dependent phase transitions in these materials, we have developed rotatable miniature and sub-miniature sample-in-vacuum calorimeters that operate in dc magnetic fields up to 18 and 45 tesla. The calorimeters cover the temperature range from below 0.1 K to above 10 K; they are able rotate a full 360 degrees relative to the applied magnetic field while remaining at base temperature. Samples are typically on the order of 1 mg in mass and up to 2 mm2 × 0.5 mm in volume.

Cs sub(2)CuBr sub(4) and Ba sub(3)NiSb sub(2)O sub(9 ) are magnetically described as quasi-two-dimensional triangular-lattice antiferromagnets with spin-1/2 and 1, respectively. We show that both systems exhibit a magnetization plateau at one-third of the saturation magnetization M sub(s) due to the interplay of spin frustration and quantum fluctuation. In Cs sub(2)CuBr sub(4) that has a spatially anisotropic triangular lattice, successive magnetic-field induced quantum phase transitions including a magnetization plateau 2/3 M sub(s) were observed. For Ba sub(3)NiSb sub(2)O sub(9 ), we performed exact diagonalization for rhombic spin clusters with up to 21-sites to analyze the magnetization process. The calculated results are in agreement with experimental observations.

We have recently reported the first direct calorimetric observation of a magnetic-field-induced first-order phase transition into a high-field FFLO superconducting state at the Clogston-Chandrasekar 'Pauli' paramagnetic limitHp in a 2D superconductor κ − (BEDT-TTF)2Cu(NCS)2. The high-field state is both higher entropy and strongly paramagnetic, as thermodynamically required for the FFLO state. Here we compare our results with theoretical predictions for the field dependence of the high-field FFLO state in the 2D limit, revealing tentative evidence for transitions between FFLO states of differing order parameter. We also present calorimetric evidence for a 1st order phase transition into the FFLO state for a second 2D organic superconductor: β″ − (BEDT-TTF)2SF5(CH)2(CF)2(SO)3.

The magnetic-field-dependent ordering temperature of the quasi-2D quantum Heisenberg antiferromagnet (QHAF) Cu(pz)2(ClO4)2 was determined by calorimetric measurement in applied dc fields up to 33 tesla. The magnetic phase diagram shows a round maximum at 5.95 K and 17.5 T (at ≈ 1/3 of its saturation field), a 40 percent enhancement of the ordering temperature above the zero field value of 4.25 K. The enhancement and reentrance are consistent with predictions of a field-induced Heisenberg to XY crossover behavior for an ideal 2D QHAF system.

An antiferromagnetic system on a 2-D triangular lattice leads to geometric topological frustration. This ideal system has been the subject of theoretical investigations. One experimental realization of this system is the compound Cs2CuCl4. Various magnetization, heat capacity, neutron scattering and NMR studies have identified several magnetic transitions when the magnetic field is applied along one of the three principal axes. The current work investigates the evolution of these phases at intermediate angles as the crystal is rotated relative to the magnetic field. These phases were investigated using a novel rotating calorimeter allowing complete coverage of the experimental parameter space. New magnetic phases only existing at intermediate angles have been found.

Many thermal measurements in high magnetic fields—including heat capacity, thermal conductivity, thermopower, magnetocaloric and thermal Hall effect measurements—require thermometers that are sensitive over a wide temperature range, are low mass, have a rapid thermal response and have a minimal, easily correctable magnetoresistance. We recently reported the development of a new granular-metal oxide ceramic composite (cermet) for this purpose formed by co-sputtering of the metallic alloy nichrome (Ni 0.8 Cr 0.2 ) and the insulator silicon dioxide (SiO 2 ). In this earlier work, we found that co-sputtering of NiCr alloy and SiO 2 in a reactive oxygen and inert argon gas mixture can produce resistive thin-film thermometers sensitive enough to be used in calorimetry and related measurements from room temperature down to below 100 mK in magnetic fields up to at least 35 T. In this work, we present results for thin cermet films grown with Cu 0.55 Ni 0.45 and Ti 0.05 Cr 0.95 . Growth of CuNi-based thin-film cermets generally requires more oxygen in the working gas compared to NiCr and TiCr and yields thermometers that are much less sensitive than comparable NiCr-based thermometers. TiCr-based cermet thin-film thermometers have somewhat higher resistivity for similar sensitivities compared to NiCr-based cermet thin-film thermometers.

In classical magnetic spin systems, geometric frustration leads to a large number of states of identical energy. We report here evidence from magnetocaloric and related measurements that in Cs2CuBr4 — a geometrically frustrated Heisenberg S 1/2 triangular antiferromagnet — quantum fluctuations stabilize a series of gapped collinear spin states bounded by first-order transitions at simple increasing fractions of the saturation magnetization for fields directed along the c axis. Only the first of these quantum phase transitions has been theoretically predicted, suggesting that quantum effects continue to dominate at fields much higher than previously considered.

Many thermal measurements in high magnetic fields require thermometers that are sensitive over a wide temperature range, are low mass, have a rapid thermal response, and have a minimal, easily correctable magnetoresistance. Here we report the development of a new granular-metal oxide ceramic composite (cermet) for this purpose formed by co-sputtering of the metallic alloy nichrome Ni\\(_{0.8}\\)Cr\\(_{0.2}\\) and the insulator silcon dioxide SiO\\(_2\\). The resulting thin films are sensitive enough to be used from room temperature down to below 100 mK in magnetic fields up to at least 35 tesla.