Physical Dependence of the Sensitivity and Room-Temperature Stability of AuxGe1-x Thin Film Resistive Thermometers on Annealing Conditions

The reported nearly constant temperature sensitivity of appropriately annealed polycrystalline AuxGe1-x thin films at cryogenic temperatures would appear to make them promising materials for low mass, rapid thermal response resistive thermometers, but their adoption has been limited by difficulties in fabrication and uncertainties in annealing. In this work, we present a method of fabrication and annealing which allows control of the two most important parameters for these films: the room-temperature resistivity ρRT and the temperature sensitivity η(T), where η ≡ -d In R/d In T. We find that the dependence of ρRT on total anneal duration t for x≈0.18 is given by ρRT=ρ∞[1-Aexp(-t/τ)], where the limiting room-temperature resistivity ρ∞, the annealing coefficient A, and relaxation time τ are annealing temperature dependent parameters. The dependence of ρRT and temperature calibration ρ(T) on anneal duration can be minimized by annealing above 250 °C. Like ρRT, the sensitivity η(T) also depends on annealing temperature, with higher annealing temperatures corresponding to lower cryogenic sensitivities. In all cases η(T) can be well described by a polynomial expansion in In T from room temperature down to at least 2 K

Wide Range Thin-Film Ceramic Metal-Alloy Thermometers with Low Magnetoresistance

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 Ni0.8Cr0.2 and the insulator silcon dioxide SiO2. 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

Calorimetric Measurements of Magnetic-Field-Induced Inhomogeneous Superconductivity Above The Paramagnetic Limit

We report the first magneto-caloric and calorimetric observations of a magnetic-field-induced phase transition within a superconducting state to the long-sought exotic "FFLO" superconducting state first predicted over 50 years ago. Through the combination of bulk thermodynamic calorimetric and magnetocaloric measurements in the organic superconductor $\kappa$ - (BEDT-TTF)$_2$Cu(NCS)$_2$, as a function of temperature, magnetic field strength, and magnetic field orientation, we establish for the first time that this field-induced first-order phase transition at the paramagnetic limit $H_p$ for traditional superconductivity is to a higher entropy superconducting phase uniquely characteristic of the FFLO state. We also establish that this high-field superconducting state displays the bulk paramagnetic ordering of spin domains required of the FFLO state. These results rule out the alternate possibility of spin-density wave (SDW) ordering in the high field superconducting phase. The phase diagram determined from our measurements --- including the observation of a phase transition into the FFLO phase at $H_p$ --- is in good agreement with recent NMR results and our own earlier tunnel-diode magnetic penetration depth experiments, but is in disagreement with the only previous calorimetric report.Comment: 5 pages, 5 figure

Calorimetric Measurements of Magnetic-Field-Induced Inhomogeneous Superconductivity Above The Paramagnetic Limit

We report the first magnetocaloric and calorimetric observations of a magnetic-field-induced phase transition within a superconducting state to the long-sought exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state, first predicted over 50 years ago. Through the combination of bulk thermodynamic calorimetric and magnetocaloric measurements in the organic superconductor κ−(BEDT−TTF)2Cu(NCS)2 as a function of temperature, magnetic field strength, and magnetic field orientation, we establish for the first time that this field-induced first-order phase transition at the paramagnetic limit Hp is a transition to a higher-entropy superconducting phase, uniquely characteristic of the FFLO state. We also establish that this high-field superconducting state displays the bulk paramagnetic ordering of spin domains required of the FFLO state. These results rule out the alternate possibility of spin-density wave ordering in the high-field superconducting phase. The phase diagram determined from our measurements—including the observation of a phase transition into the FFLO phase at Hp—is in good agreement with recent NMR results and our own earlier tunnel-diode magnetic penetration depth experiments but is in disagreement with the only previous calorimetric report

Specific ion channels contribute to key elements of pathology during secondary degeneration following neurotrauma

Background: Following partial injury to the central nervous system, cells beyond the initial injury site undergo secondary degeneration, exacerbating loss of neurons, compact myelin and function. Changes in Ca 2+ flux are associated with metabolic and structural changes, but it is not yet clear how flux through specific ion channels contributes to the various pathologies. Here, partial optic nerve transection in adult female rats was used to model secondary degeneration. Treatment with combinations of three ion channel inhibitors was used as a tool to investigate which elements of oxidative and structural damage related to long term functional outcomes. The inhibitors employed were the voltage gated Ca 2+ channel inhibitor Lomerizine (Lom), the Ca 2+ permeable AMPA receptor inhibitor YM872 and the P2X 7 receptor inhibitor oxATP. Results: Following partial optic nerve transection, hyper-phosphorylation of Tau and acetylated tubulin immunoreactivity were increased, and Nogo-A immunoreactivity was decreased, indicating that axonal changes occurred acutely. All combinations of ion channel inhibitors reduced hyper-phosphorylation of Tau and increased Nogo-A immunoreactivity at day 3 after injury. However, only Lom/oxATP or all three inhibitors in combination significantly reduced acetylated tubulin immunoreactivity. Most combinations of ion channel inhibitors were effective in restoring the lengths of the paranode and the paranodal gap, indicative of the length of the node of Ranvier, following injury. However, only all three inhibitors in combination restored to normal Ankyrin G length at the node of Ranvier. Similarly, HNE immunoreactivity and loss of oligodendrocyte precursor cells were only limited by treatment with all three ion channel inhibitors in combination. Conclusions: Data indicate that inhibiting any of a range of ion channels preserves certain elements of axon and node structure and limits some oxidative damage following injury, whereas ionic flux through all three channels must be inhibited to prevent lipid peroxidation and preserve Ankyrin G distribution and OPCs

Competition Between Superconductivity and a New 20 K Phase in -(BEDT-TTF)2I3: Specific Heat Measurements

We present here direct calorimetric evidence for a new transition at T*=22.25 K in the metallic state of quasi-2D organic superconductor -(BEDT-TTF)2I3 at ambient pressure, as suggested by our previous measurement of the temperature-dependent Hall coefficient RH(T). The apparent reduction in the effective electronic density of states at T* quantitatively explains for the first time the suppression of Tc from the pressure-stabilized eehigh-Tcee value of 8 K to the ambient-pressure cooled eelow-Tcee value of 1.5 K

Systematic Variation of Transport and Thermodynamic Properties with Degree of Reduction in Nd1.85Ce0.15CuO4-

Superconductivity only occurs in Nd1.85Ce0.15CuO4- after heat treatment in a reducing at- mosphere; yet we find the oxygen loss during reduction to be remarkably small. To better understand the physical role of reduction, we have systematically varied the degree of reduction in a series of ceramic samples by varying the partial pressures of oxygen and argon in the reducing atmosphere, keeping the total pressure constant. We find that small increases in the degree of reduction strongly increase the apparent carrier density and the superconducting Meissner fraction. We also find that there is an optimal degree of reduction to produce a single-phase sample with a zero-resistance superconducting state. These results call into question many previous experiments performed on samples of Nd2-xCexCuO4- in which the effect of varying the degree of reduction was not systematically studied

Magnetic-field-induced 1st order transition to FFLO state at paramagnetic limit in 2D superconductors

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

Observation of Quantum Oscillations in The Low Temperature Specific Heat of SmB\u3csub\u3e6\u3c/sub\u3e

We report measurements of the low-temperature specific heat of Al-flux-grown samples of SmB6 in magnetic fields up to 32 T. Quantum oscillations periodic in \emph{1/H} are observed between 8 and 32 T at selected angles between [001] and [111]. The observed frequencies and their angular dependence are consistent with previous magnetic torque measurements of SmB6 but the effective masses inferred from Lifshitz-Kosevich theory are significantly larger and closer to those inferred from zero-field specific heat. Our results are thus consistent with a bulk density of states origin for the previously observed quantum oscillations

Specific Heat of Pure and Thoriated UBe13 at Low Temperatures in High Magnetic Fields

We have measured the specific heat of pure and thoriated (3.3% thorium) UBe13 in the temperature interval 0.3 1.8 K in applied magnetic fields up to 20 T, and find striking qualitative differences between the two systems. For pure UBe13 the electronic coefficient of specific heat el(=Cel(T)/T) is noticeably suppressed by a field of 20 T at 1.8 K; below 1 K, however, this reduction from the low-field, normal-state value tends to zero. For the 3.3% thoriated sample, the high-temperature value of el is relatively insensitive to strong magnetic fields, but at lower temperatures the suppression of el becomes much stronger, reaching nearly 40% at 0.35 K in a field of 20 T. This behavior is similar to that observed in most cerium-based heavy-fermion systems