Melissa Charalambous

Single-crystal growth and characterization of the superconductor

Single crystals of of typical size were grown inside a sealed silica tube with and without excess of precursor . The process is reproducible for high growth temperatures at which the use of an alumina crucible is necessary. For lower growth temperatures the as-prepared crystals are underdoped with . An optimal is achieved by post-annealing the crystals in flowing oxygen at . Crystals grown at higher temperatures show an increase of the c-axis length after oxygen annealing, indicating they are overdoped.

FIRST DIRECT MEASUREMENT OF THE C-AXIS RESISTIVITY IN SUPERCONDUCTING SINGLE CRYSTALS Y1BA2CU3O7-X

Abstract We have developed a new technique to directly measure ϱ c and ϱ ab of very small single crystals of Y 1 Ba 2 Cu 3 O 7-x . We used this technique to measure ϱ c in two different crystals having different T c s. In contrast with results obtained by the standard Montgomery method we find smaller values of ϱ c at 300 K, although the increase of ϱ c at T≥T c is higher. Our data indicate the presence of localized states between the CuO 2 planes due to intrinsic or extrinsic “dirty” interlayers. The c-axis resistivity follows a thermally activated law Inϱ c ∼(T 0 /T) with T 0 =410 K . A correlation is observed between the lowering of T c and the increase of ϱ c at T≥T c .

New six face lithography technique for realization of contacts on μm size Y1Ba2Cu3O7−x single crystals

We have developed a new contact technique for use on very small Y1Ba2Cu3O7−x single crystals (typical dimensions: 100×100×20 μm). A precise contact pattern (resolution ≊2 μm) is realized on the different faces of the sample. This allows the injection of a current parallel to a single crystallographic axis, without mixing of the anisotropic conduction channels. Contact resistivities are satisfactory: 1×10−5 Ω cm2 for the ab contacts, 1×10−6 Ω cm2 for the c‐axis contacts. To date the technique has been used to directly measure the c‐axis resistivity of the Y1Ba2Cu3O7−x compound, but other applications such as anisotropic thermal conductivity and thermoelectric power are possible.

Thermodynamic study of the melting transition in the vortex phase diagram of high purity YBa2Cu3O7-δ single crystals

Abstract We present a thermodynamic study of high purity YBa 2 Cu 3 O 7−δ single crystals, using a state of the art AC calorimetric set-up. Our sample exhibits a sharp vortex melting transition for a wide range of fields. This first order transition is confined in the H-T vortex phasediagram between two critical points. Close to these critical points we observe a new specific heat anomaly.

Dissymetry of critical exponants in YBCO

We report on measurements of the specific heat of μg-size YBCO single crystals very close to the superconducting transition in zero field. Detailed data points are obtained down to reduced temperatures of 10−5. Analysis of the derivative of the specific heat yields the critical exponents in an unambiguous way. α+ is positive ∼0.5; α- is negative ∼ −0.3. The value of α+ is consistent with gaussian fluctuations above Tc, however the value of α- is inconsistent with either gaussian fluctuations or the 3DXY model.

Hysteretic effects in calorimetric measurements of the vortex melting transition in high-purity YBa2Cu3O7−δ single crystals

Abstract We present specific heat measurements in high-purity, detwinned YBa 2 Cu 3 O 7− δ single crystals, with a state-of-the-art, high-resolution, AC calorimetric setup. Using a unique feature of the AC calorimetry, we demonstrate the existence of hysteretic effects at the vortex melting transition in specific heat measurements, a much-needed element for establishing the first-order character of melting.

Magnetic microscopy with small scale Hall probes (invited) (abstract)

The use of small scale Hall probes in the study of nanometer scale magnetic structures and high temperature superconductors is discussed. Micron sized Hall probes are a powerful means of studying local magnetic properties and are complementary to other local magnetometry methods such as those based on magnetic force microscopy and superconducting quantum interference device techniques. In addition to presenting an overview of our recent studies on nanofabricated magnetic samples1 and ring shaped superconductors,2 we discuss the factors that determine the spatial and field resolution of this technique. Among the important issues considered are the Hall response to inhomogeneous magnetic fields and the noise characteristics of sensor materials. A variety of materials have been studied including two-dimensional GaAs/GaAlAs heterostructures, three-dimensional InGaAs, and Bi semimetal thin films. Finally, our progress toward the realization of a variable temperatures scanning Hall probe microscope will be repo...

Irreversibility and melting in YBCO through AC susceptibility measurements

We report on the response of an untwinned YBCO single crystal to an AC magnetic field over a wide region of the (B,T) diagram (up to 11T) by means of high resolution micro-Hall probe measurements. Our results suggest the existence of two distinct behaviors of the crystal, depending on the amplitude of the DC field B. For B less than a critical value B0 (≈8T), shielding currents appear in a discontinuous way in the sample. At higher fields shielding currents appear in a continuous way. Our experimental method allows to detect a magnetic irreversibility in the sample too weak to be resolved by recent dc SQUID magnetometry experiments. This leads to suggest that a recently observed jump in dc magnetization [1,2] at the melting transition occurs simultaneously with irreversibility. AC susceptibility becomes so for the first time a very precise mean to track the melting (or irreversibility) line.