Brigitte Leridon

Conductivity of Underdoped YBa2Cu3O7delta: Evidence for Incoherent Pair Correlations in the Pseudogap Regime

A two-channel scenario for the conductivity of underdoped YBa 2Cu 3O (7-delta) is proposed. One is the single-particle excitations channel, which dominates in the optimally doped material, whose resistivity is linear as a function of temperature. The other one gives a contribution which merges the 3D Aslamazov-Larkin fluctuation conductivity at low temperature and obeys a power law at high temperature, depending on two superconductive parameters (T(c) and the zero temperature coherence length xi(c0)) and an energy scale Delta(*). This allows one to address the nature of the pseudogap in favor of incoherent pairing.

Spin-bottleneck due to spin-charge separation in a superconductor

An experimental device was designed to measure the effect of the injection of spin-polarized carriers on the superconductive gap and density-of-states (DOS). Quasiparticles were injected from a ferromagnet (

Transport measurements in YBa2Cu3O7−δ/PrBa2Cu3−xGaxO7−δ superlattices: experimental evidence for underdoping

Ni_{0.8}Fe_{0.2}

Paraconductivity of underdoped La2-xSrxCuO4 thin-film superconductors using high magnetic fields

) through a tunnel junction into a conventional superconductor (Nb), while charge neutrality was maintained by a supercurrent. The DOS of the superconductor was measured through a second tunnel junction with a normal paramagnetic metal. No significant decrease of the superconductive gap was observed while a noticeable heating of the quasiparticles of the superconductor was measured. A similar experiment performed with current injected from a paramagnet (Al or Ag) showed no heating of quasiparticles. These observations are consistent with spin-charge separation of Bogoliubov quasi-particles and spin-bottleneck due to the enhanced recombination time of pure spin-excitations.

PRONAOS: a balloon-borne experiment for submillimeter radioastronomy with a 380-GHz heterodyne spectrometer

Abstract The normal state resistivity of YBa2Cu3O7−δ/PrBa2Cu3−xGaxO7 (x=0.2) superlattices grown by Pulsed Laser Deposition is studied as a function of temperature from Tc to 325 K. The respective YBa2Cu3O7−δ (YBCO) and PrBa2Cu3−xGaxO7−δ (PBCGO) thickness are varied. A decrease and a broadening of Tc as well as an increase of ρ(100 K) are observed with increasing PrBa2Cu3−xGaxO7−δ content. For temperatures lower than 200 K, the PrBa2Cu3−xGaxO7−δ does not contribute to the conductivity of the superlattice, which shows an analogy with the one for underdoped compounds. The data are compared to similar measurements on oxygen-deficient YBCO thin films and YBa2Cu3−xCoxO7−δ single crystals: this allows to conclude to a underdoping effect of the superconducting CuO2 planes in the superlattices, probably due to charge transfer from the YBCO layers to the PrBa2Cu3−xGaxO7 layers.

BaCu3O4: High-temperature magnetic order in one-dimensionalS=12diamond chains

The contribution of superconducting fluctuations to the conductivity, or paraconductivity, is studied in the underdoped regime of La2-xSrxCuO4 cuprates. A perpendicular magnetic field up to 50T is applied to suppress the superconductivity and obtain the normal state resistivity, which is then used to calculate the paraconductivity. Surprisingly enough, it is consistent with a two-dimensional Aslamazov-Larkin regime of Gaussian fluctuations close to the critical temperature. At higher temperature, the paraconductivity exhibits a power-law decrease in temperature (as T-α ), as was previously shown for underdoped YBa2Cu3O7-δ and Bi2Sr2CaCu2O8+δ samples. Our observations are not consistent with the existence of Kosterlitz-Thouless fluctuations. This tends to indicate that the superconducting pair amplitude is not already defined above TC in the pseudogap state.

New elements for analysis of series arrays of superconducting junctions for submillimeter heterodyne detection

A balloon-borne observatory (PRONAOS) including a two meter telescope associated with a submillimeter heterodyne spectrometer for radioastronomy is supported by the French space agency (CNES) to prepare the future space programs in astrophysics. This instrument will be used to simultaneously detect the 368 GHz O2 and the 380 GHz H2O lines in the interstellar medium. Observations in this part of the spectrum require low atmospheric water vapor and oxygen molecule emission, so that the telescope will fly under a 1,000,000 m3 balloon at an altitude of approximately equals 37 km. The receiver, under development at Meudon Observatory, includes a SIS mixer using Nb/AlOx/Nb tunnel junctions operating at 4 K, a 6 GHz IF low-noise cooled preamplifier, a LO quasi-optically injected with a phase locked Gunn oscillator and two cascaded frequency multipliers. Noise temperature as low as 300 K has been obtained; less than 200 K is expected. An 800 MHz Acousto-Optical Spectrometer (AOS) is used for the high resolution (800 kHz) spectral analysis. The optimization of the frequency resolution of the spectrometer was obtained in the design and building of a new kind of Bragg cell in LiNbO3 centered at 2 GHz.