P. Gandit

Nanocalorimeter for high resolution measurements of low temperature heat capacities of thin films and single crystals

An innovative nanocalorimeter has been developed for measuring specific heats of thin films, multilayers (typical thickness: 1000 A) and single crystals (mass: 1 μg) in the temperature range of 1.5–20 K. The addenda of the device are as small as 3 nJ/K at 4 K (0.5 nJ/K at 1.5 K), thus samples with a heat capacity of the order of nJ/K at 4 K can be measured. Heat capacity differences as a function of temperature or an external magnetic field (5 T) were determined with a resolution of ΔC/C≃10−4. This way we have seen heat capacity variations of less than a pJ/K. We present as an example measurements on very small Mn12O12 acetate single crystals and a measurement of a thin superconducting Pb layer. In the latter measurement we could evidence via specific heat a finite size effect.

Thermodynamic and transport properties of UPt3

In the normal phase of UPt3, the magnetoresistivity is large and positive with the striking feature of a quasi-independence of the temperature and magnetic field terms; the thermoelectric power has aT2 dependence and the susceptibility is almost constant up to 4 K. The superconducting transition is broadened and the specific heat jump is weak due to the strong anisotropy of its normal phase. FromTc to 146 mK (the lowest experimental temperature), a largeT2 contribution is observed in the specific heat, but the thermal conductivity has the same dependence only below 150 mK. These results are compared with the predictions given for polar odd-pairing superconductivity.

A very sensitive microcalorimetry technique for measuring specific heat of μg single crystals

A microcalorimetry technique allowing one to measure the specific heat of bulk single crystals of a mass typically 10 μg, with a relative resolution Δc/c of 10−4 is presented. The well known ac steady state technique is employed. The sample holder is a polymer membrane with lithographically patterned thermometer and heaters made out of metallic thin films. The diameter of the useful area is 0.6 mm and the specific heat of the addendum is 1.5 μJ/K at 100 K. The sample is thermally coupled to the membrane through a gold thin film sputtered on its surface. The thermal coupling time constant of the sample is ∼5 ms, allowing to work at frequencies of 10 Hz. The method was applied to small single crystals of YBCO, providing the specific heat jump at the superconducting transition with a relative resolution of 10−2 and a precision of 10−1. The method is optimized at the temperature range 40–160 K, but can easily be modified for lower temperatures.

Andreev Current-Induced Dissipation in a Hybrid Superconducting Tunnel Junction

We have studied hybrid superconducting microcoolers made of a double superconductor-insulator-normal metal tunnel junction. Under subgap conditions, the Andreev current is found to dominate the single-particle tunnel current. We show that the Andreev current introduces additional dissipation in the normal metal equivalent to Joule heating. By analyzing quantitatively the heat balance in the system, we provide a full description of the evolution of the electronic temperature with the voltage. The dissipation induced by the Andreev current is found to dominate the quasiparticle tunneling-based cooling over a large bias range.

Room temperature magnetic detection of spin switching in nanosized spin-crossover materials.

Recently, nanoscale spin-crossover (SCO) particles have been the subject of great interest. The change in the 3d electronic configuration of the metal ion results in significant changes in the metal-ligand bond length and geometry, as well as in the molecular volume. Hence the spin switching process is accompanied by a remarkable change in the color, mechanical properties, dielectric properties, and magnetic susceptibility. The synthesis and investigation of these materials at reduced length scales is central not only to the exploration of fundamental effects of size reduction in these systems, but also for the development of new functional materials with applications, including guest molecule sensing, memory devices, and molecular switches

Anisotropy and angular variation of the giant magnetoresistance in magnetic multilayers (invited)

The giant magnetoresistance (GMR) of magnetic multilayers is usually considered as isotropic, i.e., independent of the direction of the sensing current with respect to the applied field. In spin‐valve samples of the form NiFe/Cu/NiFe/FeMn it is possible to accurately determine the amplitude of the GMR (without any contribution from the usual anisotropic magnetoresistance) for various direction of the current with respect to the direction of the magnetization of the two ferromagnetic layers, both in the parallel and antiparallel magnetic configurations. In three series of spin‐valve samples of the composition F tF/Cu tCu/NiFe/FeMn, we have observed that the GMR amplitude is larger when the current is perpendicular to the magnetizations than when it is parallel to it. This intrinsic anisotropy in the GMR shows a pronounced maximum (relative amplitude of the anisotropy of the order of 10% at the maximum) for a thickness of the ferromagnetic layer of the order of 150 A. In contrast, this anisotropy depends ve...

Measurement of the Derivatives of the Magnetization of a Superconducting Network

Measurements are reported of the derivatives dM/dH and dM/dT of the magnetization of a superconducting network near the superconducting transition temperature in a weak transverse magnetic field. The a.c. susceptibility dM/dH exhibits sharp peaks at integer and fractional number of flux quanta per cell as expected from the phase diagram (H, T) of the network. In contrast, the dM/dT measurements reveal distinct features, characteristic of the equilibrium state below the superconducting transition. These results are compared to recent theoretical predictions for the mixed state of a superconducting network.

New methods to measure the current perpendicular to the plane magnetoresistance of multilayers

The experimental devices presented here enable us to measure variations with magnetic field of electrical resistances in the range (0.1 nΩ, 1 μΩ), for applied magnetic fields up to 3 T, and for temperatures less than 9 K. The setups work with three measurement methods. Two provide direct access to R(H), one in constant current, the other in alternating current. Their absolute sensitivity is about 1 pΩ, and their relative resolution ΔR/R is limited to 0.3%. The third method is a sinusoidal field modulation technique giving the field derivative of the resistance: dR/dH. It is sensitive to variations of the resistance as small as 1 pΩ independent of the absolute resistance of the sample, thus the resulting resolution ΔR/R is, for example, 10−6 for a 1 μΩ sample. With these devices, the magnetoresistance of multilayers can be studied with the current perpendicular to the plane.

Magnetic properties of a superconducting network

Abstract We present direct measurements of the derivatives dM/dH and dM/dT of a superconducting square network near the superconducting transition temperature. The a.c. susceptibility exhibits sharp peaks at integer and fractional number quanta of flux per cell in relation to the critical line of the superconducting transition. In contrast, the dM/dT has distinct features and presents symmetric and asymmetric jumps. These types of behaviour are compared to recent theoretical predictions for the mixed state of a superconducting network in terms of the Berry phase.

Inherent Thermometry in a Hybrid Superconducting Tunnel Junction

We discuss inherent thermometry in a Superconductor-Normal metal-Superconductor tunnel junction. In this configuration, the energy selectivity of single-particle tunneling can provide a significant electron cooling, depending on the bias voltage. The usual approach for measuring the electron temperature consists in using an additional pair of superconducting tunnel junctions as probes. In this paper, we discuss our experiment performed on a different design with no such thermometer. The quasi-equilibrium in the central metallic island is discussed in terms of a kinetic equation including injection and relaxation terms. We determine the electron temperature by comparing the micro-cooler experimental current-voltage characteristic with isothermal theoretical predictions. The limits of validity of this approach, due to the junctions asymmetry, the Andreev reflection or the presence of sub-gap states are discussed.