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INFLUENCE OF STRONG DISORDER ON SUPERCONDUCTIVITY OF MgB2 THIN FILMS Th. Koch1, 4, V.I. Zdravkov1, 3, A. Surdu1, E. Condrea1, A. Socrovisciuc1, A.N. Rossolenko2, V.V. Ryazanov2, B. Straumal2, R. Tidecks3, A. Wixforth3, and A.S. Sidorenko1, 4
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Institute of Electronic Engineering and Industrial Technologies, Academy of Sciences of Moldova, 3/3, Academiei str., MD-2028, Chisinau, Republic of Moldova 2 Institute of Solid State Physics RAS, 142432, Chernogolovka, Russia 3 Institute of Physics, University of Augsburg, D-86135, Augsburg, Germany 4 University of Erlangen-Nuremberg, 4, Cauer str., D-91058, Erlangen, Germany (Received 12 May 2008) Abstract

We investigated the influence of disorder on superconductivity of MgB2 films, prepared in a single deposition run onto two different substrates: (100)-MgO and on (128° rot)-LiNbO3 with trigonal crystal structure which does not match to the MgB2 structure. As expected, the microstructure of both films crucially differs. The REM study shows a very homogeneous, smooth morphology of the MgB2 film on MgO, but a very rough inhomogeneous film structure on LiNbO3. Although the MgB2 film on LiNbO3 is strongly disordered, its critical temperature Tc=33.5 K is practically the same as for the film deposited on MgO. Possible reasons of such unusual Tc behavior are discussed. Introduction The recent discovery of superconductivity in MgB2 raised questions about the origin and peculiarities of the superconducting state in this compound. MgB2 exhibits a hexagonal crystal structure with boron planes separated by magnesium layers [1], strongly influencing their superconducting properties [2]. Thin films with a high quality crystal structure and a smooth surface are the basis of microelectronic applications. Unfortunately, Mg is highly volatile and easily reacts with ambient materials, so that the deposition of high-quality MgB2 thin films has proven to be very difficult. Two-step ex-situ techniques, which require Mg diffusion into precursor films at high temperatures, have been quite successful in growing films with high Tc and preferentially oriented grains. However, the surface quality for microelectronic-device applications is still far from being achieved. The dc-magnetron sputtering is a well developed method to obtain films with smooth surface and suitable microstructure. In the present paper we report about the growth and properties of high-quality films prepared by this technique. Sample preparation Samples were grown by a two-step process MgB2 composite target. Before sputtering a target phase during the film deposition and to provide sputtering, a composite target was used containing using dc-magnetron sputtering from a Mghas to be prepared. To generate the MgB2 sufficient target conductance for the dcMgB2 and pure Mg in approximately equal


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amounts. The Mg-MgB2 target was sputtered in a 99.999% purity argon atmosphere at a pressure of 3 Pa. The deposition rate was about 1.3 nm/s. The substrate temperature was held at 200 °C during 15 min and then increased up to 600 °C. Next, the films were annealed in a saturated Mg vapor atmosphere during 1 hour ex-situ. The annealing was performed at 850 °C in Ta envelope, placed inside an evacuated quartz tube. The procedure of MgB2 film preparation is described in detail in [3, 4]. The films were deposited on two different substrates: (100)-MgO and (128° rot)LiNbO3 with trigonal crystal structure, the latter not matching to the MgB2 structure. The thickness of the films was about 3 m. The resistivity was measured by a standard four-probe technique using Cu wires soldered with In.

Fig. 1. REM image of MgB2 film deposited on MgO substrate.

Fig. 2. REM image of 3 µm thick MgB2 film deposited on LiNbO3 substrate. 165


Moldavian Journal of the Physical Sciences, Vol.7, N2, 2008

Results and discussion The elementary analysis (EDX) was made for all prepared films. The EDX spectra for one of the samples, prepared on LiNbO3 substrate, are shown in Fig. 3 and demonstrate the absence of ambient contaminations in prepared film.

Fig. 3. EDX spectrum for 3 µm thick MgB2 film deposited on LiNbO3 substrate.

Resistive measurements, performed in the temperature range 300-10K in "Quantum Design" cryosystem, demonstrate a sharp transitions R(T), as shown in Fig. 4. Specific resistance of the MgB2 film on LiNbO3, n = 4.6.10-4 Ohm.cm, is higher by a factor of ~10 than n for MgB2 film of better quality on MgO substrate. The ratio R300/RTc was 1.5 for film on MgO and 1.13 for film on LiNbO3. As one expected, the microstructure of both films, crucially differs. REM study detected a very homogeneous, smooth morphology of the MgB2 film, deposited onto MgO (Fig. 1), but very rough inhomogeneous film structure on LiNbO3 (Fig. 2).

Fig. 4. Resistive curves for MgB2 films on MgO and LiNbO3 substrates. 166


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These experimental data evidence strong disorder of the film on LiNbO3. Apparently, the films were formed by two different mechanisms of growth. In spite of strong disorder of the MgB2 film on LiNbO3, its critical temperature Tc= 33.5K is the same as for the film deposited on MgO (Fig. 4) although with some broadening of superconducting transition. This phenomenon seems to be unexpected, because the critical temperature for most superconductors crucially depends on quality of the films; disorder and strains in superconducting films usually suppress their Tc. The similar independence of Tc on disorder and presence of impurities was observed only for Pb. This phenomenon may be one of peculiarities of the superconducting state of the multiband-superconductor, MgB2, and needs further investigation. Acknowledgements The work was supported by RFBR-Moldova "Fundamentals of development of metallomatrix composites with superconducting layers of magnesium diboride with the aid of grain boundary wetting" (Project Nr. 43/R), and BMBF grant MDA01/007. References [1] J. Namagatsu, N. Nagakawa, T. Muranaka, Y. Zenitani, and J. Akimitsu, Nature, 410, 63, (2001). [2] A.S. Sidorenko, L.R. Tagirov, A.N. Rossolenko, N.S. Sidorov, V.I. Zdravkov, V.V. Ryazanov, M. Klemm, S. Horn, and R. Tidecks, JETP Letters, 76, 20, (2002). [3] A. Sidorenko, V. Zdravkov, V. Ryazanov, S. Horn, S. Klimm, R. Tidecks, A. Wixforth, Th. Koch, and Th. Schimmel, Philosophical Magazine, 85, 16, 1783, (2005). [4] V. Zdravkov, A. Sidorenko, A. Rossolenko, V. Ryazanov, I. Bdikin, O. KrЖmer, E. Nold, Th. Koch, and Th. Schimmel, Journ. of Physics: Conference Series, 61, 606, (2007).

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