Hole Concentration Dependences of Superconducting Gap and Pseudo Gap in Bi2Sr2Ca1-xYxCu2O8-y

Abstract

The superconducting mechanism in conventional superconductors have been successfully explained by the BCS theory. In many conventional superconductors, the superconducting gap and the critical temperature Tc increase with increasing charge-carrier concentration. On the other hand, in a high Tc cuprate, the decreases monotonously with increasing P, but Tc increases at first, it reaches to a maximum at optimal PðP0Þ and then decreases. Here P is a formal valence charge P in Cuþð2þPÞ and has been confirmed to increase with increasing hole concentration p. This is essential difference between conventional superconductors and high Tc cuprates. If the behavior described above is universal law in high-Tc cuprates, it intimately relates to the superconducting mechanism and is a key point in making clear the superconducting mechanism of high Tc cuprate. So far the P dependence of Tc and has been mainly examined for high Tc cuprates by controlling P with an oxygen concentration (see the references in ref. 2). To confirm that both Tc and depend on only P and are independent of oxygen concentration, it is desirable to control P by another method instead of oxygen concentration. In the present experiment, the P dependences of Tc, T and are investigated by controlling P with substituting Y for Ca in Bi2Sr2Ca1 xYxCu2O8 y (BSCYCO). Here T is a temperature at which a pseudo-gap opens. The BSCYCO single crystal is grown by slowly cooling the starting material in melting state. Here a value of x (Y concentration) is defined as the composition of Y in starting material. To keep the oxygen concentration constant, all asgrown single crystals are post-annealed under the same condition. Although the x ray diffraction pattern is not shown here, it was found from the x-ray diffraction pattern that in 0 5 x 5 0:03 the crystals undergoes no structural change and the lattice constants, a, b and c monotonously increase with increasing x. As seen in the inset of Fig. 1, the p determined by the Hall effect measurement decreases with increasing Y concentration. In the present study the Hall effect measurement was carried out for two different BSCYCO samples with the same Y concentration and the p value was found to be highly reproducible. The p is used to confirm the substitution of Ca by Y. The BSCYCO single crystal is suitable for the fabrication of planar tunnel junction, because it is easily cleaved with a high quality surface. A tunnel conductance, GðVÞ was measured by the standard voltage modulation method for the Ag–SiO– BSCYCO planar junction fabricated on the cleaved face by successively depositing SiO and Ag in a vacuum of 5 10 7 Pa. We begin with the discussion on the P dependence of Tc and the superconducting gap value 4:2K at 4.2 K. The GðVÞ characteristics at 4.2K for three different Y concentrations (1% Y, 2% Y and 3% Y) are shown in Fig. 2. The gap value of 4:2K is defined as the energy difference between two Fig. 1. The –T characteristics for BSCYCO with 1% Y (x 1⁄4 0:01), 2% Y (x 1⁄4 0:02) and 3% Y (x 1⁄4 0:03). A dotted straight line is a guide in determining T . The inset is a hole concentration characteristic as a function of Y concentration x. Hall effect measurement is performed for two different BSCYCO in each Y concentration.