Superfluid Density in Ultrathin YBCO Films
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Superfluid Density in Ultrathin YBCO Films
Lyuba A. Delimova, Igor V. Grekhov, Ivan A. Liniichuk, Ivan A. Veselovsky, Konstantin B. Traito, Erkki Lahderanta1, Ioffe Institute RAS, 26 Politekhnicheskaya, St.Petersburg 194021, Russia 1 Wihuri Physical Laboratory University of Turku, FIN – 20014 Turku, Finland
ABSTRACT
We studied the transition of 8 nm-thick YBCO films deposited with and without YBaCuNbO buffer from the superconducting to resistive state due to the temperature or transport current increase. For both film types, the current –voltage characteristics and resistive transition followed the Kosterlitz-Thouless transition model. The superfluid densities were found from the Kosterlitz-Thouless transition temperature TKT to be about 3% and 1.3% of the total carrier density in YBCO with and without buffer, respectively. We believe that the low superfluid densities indicate the presence of weak links in the film.
INTRODUCTION
A study of superconducting transition in ultrathin (less than 10 nm in thickness) HTSC films is of great interest due to their fundamental HTSC properties and possible application as the basic element of such a promising device as SuFET. It can be especially important for YBa2Cu3O7 (YBCO) material which is known to show a dramatic suppress of the critical temperature Tc for ultrasmall thickness [1,2]. Much effort has recently been made to understand the reasons for the Tc drop, and the problem of how ultrathin YBCO achieves the zero-resistance state has been a subject of much interest [3]. YBCO has been investigated extensively in terms of the phase transition in a quasi-two-dimensional (2D) system. In the extreme 2D case, the superconducting transition can be described by the Kosterlitz-Thouless (KT) transition model [4,5 ]. Evidence for the vortex-antivortex unbinding transition has lately been observed by many research groups in the YBCO system [ 6,7 ]. Recently, we suggested a new YBaCuNbO buffer which was shown in our previous research to improve the superconducting properties of ultrathin YBCO films [8]. This paper considers the transition of 8 nm YBCO films deposited on a SrTiO3 substrate with and without an YBaCuNbO buffer from the superconducting to resistive state due to the temperature or transport current increase.
EXPERIMENTAL DETAILS We studied laser ablated 8 nm-thick YBCO films deposited on a SrTiO3 substrate with and without YBaCuNbO buffer. The buffer is a Nb-doped YBaCuO (YBaCuNbO) composed of the II6.5.1
elements (Y:Ba:Cu:Nb:O) in the following proportion: (1:2:2.7:0.3:7). The buffer is a dielectric with the room temperature resistivity of 6⋅103 ⋅cm and was found from X-ray diffraction analysis to be a mixture of two phases: YBCO and YBa2NbO6 insulator[ 8]. As for the buffer superconductivity, let us consider the results presented in Figs. 1-2. Fig.1a shows the magnetic moment of 8 nm-thick YBCO deposited on a buffer (YBCO/buf/STO) as a function of a normally applied magnetic field B, the transition width was found from the film resistance measurements to be -8
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