Herve Aubin

Angular Position of Nodes in the Superconducting Gap of YBCO

The thermal conductivity of a YBa{sub 2}Cu{sub 3}O{sub 6.9} detwinned single crystal has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu-O planes. Measurements were carried out at several different temperatures below T{sub c} for a field of 30kOe. A fourfold symmetry characteristic of a superconducting gap with nodes at odd multiples of 45{degree} in k space was resolved. Experiments were performed to exclude a possible macroscopic origin for such a fourfold symmetry and our results impose an upper limit of 10% on the weight of the s-wave component of the essentially d-wave superconducting order parameter of YBCO. {copyright} {ital 1997} {ital The American Physical Society}

Electric-field-driven phase transition in vanadium dioxide

We report on local probe measurements of current-voltage and electrostatic force-voltage characteristics of electric-field-induced insulator to metal transition in VO2 thin film. In conducting AFM mode, switching from the insulating to metallic state occurs for electric-field threshold E~6.5\times10^7 Vm-1 at 300K. Upon lifting the tip above the sample surface, we find that the transition can also be observed through a change in electrostatic force and in tunneling current. In this noncontact regime, the transition is characterized by random telegraphic noise. These results show that electric field alone is sufficient to induce the transition; however, the electronic current provides a positive feedback effect that amplifies the phenomena.

Universal Heat Conduction in YBa{sub 2}Cu{sub 3}O{sub 6.9}

The thermal conductivity of YBa{sub 2}Cu {sub 3}O{sub 6.9} was measured at low temperatures in untwinned single crystals with concentrations of Zn impurities from 0{percent} to 3{percent} of Cu. A linear term {kappa}{sub 0}/T=0.19mW K{sup {minus}2}cm{sup {minus}1} is clearly resolved as T{r_arrow}0, and found to be virtually independent of Zn concentration. The existence of this residual normal fluid strongly validates the basic theory of transport in unconventional superconductors. Moreover, the observed universal behavior is in quantitative agreement with calculations for a gap function of d-wave symmetry. {copyright} {ital 1997} {ital The American Physical Society}

Infrared Photodetection Based on Colloidal Quantum-Dot Films with High Mobility and Optical Absorption up to THz

Infrared thermal imaging devices rely on narrow band gap semiconductors grown by physical methods such as molecular beam epitaxy and chemical vapor deposition. These technologies are expensive, and infrared detectors remain limited to defense and scientific applications. Colloidal quantum dots (QDs) offer a low cost alternative to infrared detector by combining inexpensive synthesis and an ease of processing, but their performances are so far limited, in terms of both wavelength and sensitivity. Herein we propose a new generation of colloidal QD-based photodetectors, which demonstrate detectivity improved by 2 orders of magnitude, and optical absorption that can be continuously tuned between 3 and 20 μm. These photodetectors are based on the novel synthesis of n-doped HgSe colloidal QDs whose size can be tuned continuously between 5 and 40 nm, and on their assembly into solid nanocrystal films with mobilities that can reach up to 100 cm(2) V(-1) s(-1). These devices can be operated at room temperature with the same level of performance as the previous generation of devices when operated at liquid nitrogen temperature. HgSe QDs can be synthesized in large scale (>10 g per batch), and we show that HgSe films can be processed to form a large scale array of pixels. Taken together, these results pave the way for the development of the next generation mid- and far-infrared low-cost detectors and camera.

Electrolyte-Gated Colloidal Nanoplatelets-Based Phototransistor and Its Use for Bicolor Detection

Colloidal nanocrystals are appealing candidates for low cost optoelectronic applications because they can combine the advantages of both organic materials, such as their easy processing, and the excellent performance of inorganic systems. Here, we report the use of two-dimensional colloidal nanoplatelets for photodetection. We show that the nanoplatelets photoresponse can be enhanced by two to three orders of magnitude when they are incorporated in an all solid electrolyte-gated phototransistor. We extend this technique to build the first colloidal quantum dot-based bicolor detector with a response switchable between the visible and near-IR.

Thermal transport in the hidden-order state of URu2Si2.

We present a study of thermal conductivity in the normal state of the heavy-fermion superconductor URu2Si2. Ordering at 18 K leads to a steep increase in thermal conductivity and (in contrast with all other cases of magnetic ordering in heavy-fermion compounds) to an enhancement of the Lorenz number. By linking this observation to several other previously reported features, we conclude that most of the carriers disappear in the ordered state and this leads to a drastic increase in both the phononic and electronic mean free path.

Observation of the Nernst signal generated by fluctuating Cooper pairs

Long-range order is destroyed in a superconductor warmed above its critical temperature (Tc). However, amplitude fluctuations of the superconducting order parameter survive1 and lead to a number of well-established phenomena, such as paraconductivity2: an excess of charge conductivity due to the presence of short-lived Cooper pairs in the normal state. According to theory3, these pairs generate a transverse thermoelectric (Nernst) signal. In two dimensions, the magnitude of the expected signal depends only on universal constants and the superconducting coherence length, so the theory can be rigorously tested. Here, we present measurements of amorphous superconducting films of Nb0.15Si0.85. In this dirty superconductor, the lifetime of Cooper pairs exceeds the elastic scattering lifetime of quasiparticles in a wide temperature range above Tc and, consequently, their Nernst response dominates that generated by the normal electrons. We resolved a Nernst signal, which persists deep inside the normal state. Its amplitude is in excellent agreement with the theoretical prediction. This result provides an unambiguous case for a Nernst effect produced by short-lived Cooper pairs.

EVIDENCE FOR FIELD-INDUCED EXCITATIONS IN LOW-TEMPERATURE THERMAL CONDUCTIVITY OF BI2SR2CACU2O8

The thermal conductivity ,

Magnetic-field-induced quantum superconductor-insulator transition in Nb0.15Si0.85

\kappa

Nernst effect in metals and superconductors: a review of concepts and experiments

, of Bi_2Sr_2CaCu_2O_8 was studied as a function of magnetic field. Above 5 K, after an initial decrease,