D. Jaccard

Superconducting interfaces between insulating oxides.

At interfaces between complex oxides, electronic systems with unusual electronic properties can be generated. We report on superconductivity in the electron gas formed at the interface between two insulating dielectric perovskite oxides, LaAlO3 and SrTiO3. The behavior of the electron gas is that of a two-dimensional superconductor, confined to a thin sheet at the interface. The superconducting transition temperature of ≅ 200 millikelvin provides a strict upper limit to the thickness of the superconducting layer of ≅ 10 nanometers.

Pressure induced heavy fermion superconductivity of CeCu2Ge2

Systematic trends observed in the thermopower of heavy fermion compounds have led us to study CeCu2Ge2 under high pressure. As expected, the transport properties of this compound above 70 kbar were found to be quite similar to the normal pressure ones of CeCu2Si2. Around this pressure, magnetic ordering vanishes and superconductivity appears. At 101 kbar, the transition temperature Tc≈0.64 K and the critical field Bc2(0)≈2 T.

Inelastic neutron scattering study of cerium heavy fermion compounds

We will review inelastic neutron scattering experiments performed on single crystals of the heavy fermion compounds CeRu2Si2 and CeCu6. At high temperatures, the magnetic scattering can be described by a single quasi-elastic Lorentzian peak. At low temperatures antiferro and incommensurate magnetic correlations develop below 70 and 10 K in CeRu2Si2 and CeCu6, respectively; the associated wave vectors are k1 = (0.3, 0, 0) and k2 = (0.3, 0.3, 0)for CeRu2Si2 and k1 = (0, 0, 1) and k1 = (0, 0, 1) andk2 = (0.85, 0, 0) for CeCu6. These magnetic correlations are destroyed by a magnetic field applied along the easy axis (Hc ≈ 25 kOe for CeCu6, Hc ≈ 83 kOe for CeRu2Si2). These high field experiments allow us to establish that in both compounds, at low T and H = 0, the magnetic scattering is the superposition of two contributions: i) a q-independent (single site) quasi-elastic contribution of Lorentzian type, slowly decreasing at high field, ii) a strongly peaked inelastic contribution associated with magnetic correlations, centered at a finite energy ħω0 with characteristic energy width Γ ≈ ħω0 ≈ 1.2 meV and 0.2 meV for CeRu2Si2 and CeCu6, respectively.

Thermodynamic and transport properties of CeCu6

Measurements of the specific heat, magnetization, and transport properties (resistivity, thermoelectric power, and thermal conductivity) of single-crystal CeCu6 are reported with special emphasis on the effect of magnetic field. The relative field variations of the differential susceptibility, the coefficients of the linear temperature term of the specific heat, and theT2 resistivity term are presented. They are directly related to the field curvature of the magnetization, indicating the itinerant nature of thef electrons. The Wilson ratio reaches a maximum atH=H*, which may correspond to a crossover from a low-field phase in which antiferromagnetic correlations dominate, to a highly polarized phase. Intersite coupling seems to play an important role in heavy-fermion compounds. Another interesting result is the occurrence of residual positive magnetoresistivity, which also appears to be a general feature of heavy-fermion compounds. The properties of CeCu6 are compared to those of otherf instability compounds and to present theories.

Signatures of valence fluctuations in CeCu 2 Si 2 under high pressure

Simultaneous resistivity and ac specific heat measurements have been performed under pressure on singlecrystalline CeCu2Si2 to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies was observed around the pressure coinciding with a maximum in the superconducting critical temperature, Tc max . These anomalies can be linked with an abrupt change of the Ce valence and suggest a second quantum critical point at a pressure Pv.4.5 GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability—and associated superconductivity—is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f electrons. We explain the T-linear resistivity found at Pv in this picture, while other anomalies found around Pv can be qualitatively understood using the same model

On the thermoelectricity of correlated electrons in the zero-temperature limit

The Seebeck coefficient of a metal is expected to display a linear temperature dependence in the zero-temperature limit. To attain this regime, it is often necessary to cool the system well below 1 K. We put under scrutiny the magnitude of this term in different families of strongly interacting electronic systems. For a wide range of compounds (including heavy-fermion, organic and various oxide families) a remarkable correlation between this term and the electronic specific heat is found. We argue that a dimensionless ratio relating these two signatures of mass renormalization contains interesting information about the ground state of each system. The absolute value of this ratio remains close to unity in a wide range of strongly correlated electron systems.

Local switching of two-dimensional superconductivity using the ferroelectric field effect

Correlated oxides display a variety of extraordinary physical properties including high-temperature superconductivity and colossal magnetoresistance. In these materials, strong electronic correlations often lead to competing ground states that are sensitive to many parameters—in particular the doping level—so that complex phase diagrams are observed. A flexible way to explore the role of doping is to tune the electron or hole concentration with electric fields, as is done in standard semiconductor field effect transistors. Here we demonstrate a model oxide system based on high-quality heterostructures in which the ferroelectric field effect approach can be studied. We use a single-crystal film of the perovskite superconductor Nb-doped SrTiO3 as the superconducting channel and ferroelectric Pb(Zr,Ti)O3 as the gate oxide. Atomic force microscopy is used to locally reverse the ferroelectric polarization, thus inducing large resistivity and carrier modulations, resulting in a clear shift in the superconducting critical temperature. Field-induced switching from the normal state to the (zero resistance) superconducting state was achieved at a well-defined temperature. This unique system could lead to a field of research in which devices are realized by locally defining in the same material superconducting and normal regions with ‘perfect’ interfaces, the interface being purely electronic. Using this approach, one could potentially design one-dimensional superconducting wires, superconducting rings and junctions, superconducting quantum interference devices (SQUIDs) or arrays of pinning centres.

Anisotropy of the superconducting transport properties of the LaAlO3/SrTiO3 interface

The superconducting transport properties of the conducting LaAlO3/SrTiO3 interface have been investigated in perpendicular and parallel magnetic fields. A large anisotropy in the transport properties is measured and the two-dimensional nature of the superconducting gas is confirmed. Analyses of the resistance versus temperature and magnetic field, as well as of the correlation length as a function of the magnetic field close to the superconducting critical temperature (about 200 mK), yield an estimate of ∼10 nm for the superconducting layer thickness.

Magnetism and spin fluctuations in a weak itinerant ferromagnet : MnSi

Abstract Magnetoresistivity experiments under hydrostatic pressure ( p ) are described on MnSi, which is considered to be a weak ferromagnet at T c = 29.1 K for p = 0 and which becomes a Pauli paramagnet above 15 kbar. Attention is given to the pressure dependencies of T c , to the power law observed in the temperature dependence of the resistivity at very low temperature and to the restoration of the T 2 Fermi liquid power law with the magnetic polarization. No track of superconductivity was detected from p = 41 kbar down to 30 mK

Transport properties of CeCu6 single crystals

The electrical resistivity ϱ and the thermoelectric power of CeCu6 single crystals are strongly anisotropic. The inverse of the temperature of the Kondo resistivity maximum (Tmax) roughly scales the linear temperature coefficient B of ϱ as well as the residual value (ϱ0 ÷ B ÷ 1/Tmax). Along the [1 0 0] direction ϱ follows a T2 Fermi-liquid law between 30 and 90 mK. The thermoelectric power is positive over the investigated temperature range (1–300 K) and shows two contributions.