T. Fournier

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.

Measurement of the thermal conductance of silicon nanowires at low temperature

We have performed thermal conductance measurements on individual single-crystalline silicon suspended nanowires. The nanowires (130nm thick and 200nm wide) are fabricated by e-beam lithography and suspended between two separated pads on silicon on insulator substrate. We measure the thermal conductance of the phonon waveguide by the 3ω method. The cross section of the nanowire approaches the dominant phonon wavelength in silicon which is of the order of 100nm at 1K. Above 1.3K the conductance behaves as T3, but a deviation is measured at the lowest temperature which can be attributed to the reduced geometry.

Phase diagram of the system Bi1.6Pb0.4Sr2CuO6-CaCuO2 between 825°C and 1100°C

Abstract The phase diagram of the lead-substituted Bi 2 O 3 -SrO-CaO-CuO system has been investigated in the temperature range 825°C–1100°C, and in static air along the line Bi 1.6 Pb 0.4 Sr 2 Ca n −1 Cu n O 2 n +4+ x . It involves nine distinct solid phases and two liquid ones. The transformation and melting lines of the superconducting n = 1, 2, 3 phases are located below 900°C. Below the solidus (∼855°C), the sequence of stable { n } phases is {1}, {1}+{2}, {2}, {2}+{3}, and {3}. The {3} phase (110K) exists as the dominant phase between ∼835°C and ∼875°C for 3 n t >∼900°C) mainly contains the transformation and melting lines of CuO, (Ca, Sr) 2 CuO 3 , and (Ca, Sr) 14 Cu 24 O y . The liquids involves a eutectic point at (∼930°C, n =2) and a critical end point of miscibility of the two liquids L 1 (bismuth rich) and L 2 . The presence of Pb shifts the transformation lines of the superconducting phases downwards by 10–15°C. The practical implications of this diagram are discussed.

Mesoscopic Size Effects on the Thermal Conductance of Silicon Nanowire

We report the measurement of thermal conductance of silicon nanowires at low temperature. It is demonstrated that the roughness at the nanometer scale plays a crucial role for the phonon transport in low-dimensional samples. To this end, using e-beam lithography, nanowires of size 200 nm by 100 nm and 10 microm long have been nanofabricated. Their thermal properties have been measured using the 3 omega method between 0.3 and 6 K. The change in the temperature behavior of the thermal conductance (quadratic temperature dependence of K(T)) is a signature of an intermediate regime lying between the classical Casimir regime and the quantum regime. The Casimir-Ziman model is used to show that this specific behavior originates in mesoscopic samples where the dominant phonon wavelength becomes commensurate to the characteristic length of the roughness of the nanowire surfaces.

Josephson junctions and superconducting quantum interference devices made by local oxidation of niobium ultrathin films

We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3–6.5 nm thick under the voltage-biased tip of an atomic force microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated: lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4 K. The modulation shape and depth of SQUIDs based on variable thickness bridges indicate that the weak link size becomes comparable to the superconducting film coherence length ξ which is of the order of 10 nm.

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.

The superconducting “copper/carbonate cuprates” An electron microscopy study

Abstract By means of electron microscopy and diffraction together with EDX and EELS, we have confirmed the main characteristics of the structure of the recently discovered HTSC family of “copper/carbonate cuprates”. These can be obtained at high pressure and have critical temperatures of the order of 117 K. It consists of a copper/carbonate ordered variant of the tetragonal “single X-O layered” cuprate structure. The unit cell is given by a =2 a t , b = b t and c =2 c t , where subindex t corresponds to the tetragonal single layer cell.

Phonon Heat Conduction in Corrugated Silicon Nanowires Below the Casimir Limit

The thermal conductance of straight and corrugated monocrystalline silicon nanowires has been measured between 0.3 K and 5 K. It is demonstrated that the corrugation strongly reduces the thermal transport by reducing the mean free path of the phonons. The experimental averaged mean free path is remarkably smaller than the smaller diameter of the nanowire, evidencing a phonon thermal transport reduced below the Casimir limit. Monte Carlo simulations highlight that this effect can be attributed to significant multiple scattering of ballistic phonons occurring on the corrugated surfaces. This result suggests an original approach to transforming a monocrystalline material into a phonon glass.

Batch processing of nanometer-scale electrical circuitry based on in-situ grown single-walled carbon nanotubes

We present a fabrication method for a nanometer-scale conducting network made of self-assembled single-walled carbon nanotubes. The electrical connection of the suspended nanotubes to the metallic contacts is obtained during the nanotube synthesis itself, which involves the hot-filament CVD technique. We directly characterize, without any further processing, the electronic transport properties of samples with different pad geometries. At room temperature, all tested samples show ohmic behavior in the kΩ range, for both two-probe and four-probe geometries. At low temperature, non-linear transport is observed and a large discrepancy of resistance arises between two-probe and four-probe geometries, suggesting the dominant influence of the contact resistance.

Pb3Sr3Cu3O8+δCl: A new layered copper oxychloride

Abstract The crystal structure and basic properties of a new layered copper oxychloride, Pb 3 Sr 3 Cu 3 O 8+δ Cl, are described. The new structure type is related to that of Pb 2 Sr 2 RECu 3 O 8+δ , with the RE (rare-earth) layer replaced by an (Sr, Pb)-Cl-(Sr, Pb) layer. A wide range of oxygen stoichiometry is observed. The new compound has the planes of CuO 5 pyramids required for high- T c superconductivity, but we have been unable to introduce enough (positive or negative) charge to drive it superconducting.