Picosecond dynamics of the superconducting state in MgB 2 Y. Xu a, * , M. Khafizov a,1 , L. Satrapinsky a,b ,P.K u  s c , A. Plecenik c , J. Karpinski d , J. Jun d , S.M. Kazakov d , Roman Sobolewski a, * ,2 a Department of Electrical and Computer Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627-0231, USA b Institute of Electrical Engineering, Slovak Academy of Science, SK-84239 Bratislava, Slovak Republic c Department of Solid State Physics, FMFI Comenius University, SK-84248 Bratislava, Slovak Republic d Solid State Physics Laboratory, ETH, CH-8093 Zurich, Switzerland Abstract We present time-resolved pump–probe studies of carrier dynamics in superconducting MgB 2 thin films and single crystals. The samples were excited by 800-nm-wavelength, 100-fs-wide optical pulses and their photoinduced, transient reflectivity change was measured by the second, time-delayed, 100-fs-wide optical probe. In the superconducting state, the photoresponse of MgB 2 was governed by three relaxation processes: the initial sub-picosecond electron–phonon relaxation, followed by few-picosecond-long electron–phonon component, and the nanosecond-long, phonon cooling/ escape time limited, recombination of quasiparticles into Cooper pairs. In thin films, the temperature dependence of the amplitude of the superconducting component followed the two-gap anomalous temperature dependence with the gaps closing at two different T c ’s. The picosecond electron–phonon relaxation time constant experienced a double-divergence behavior at the same T c values. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Magnesium diboride; Femtosecond spectroscopy; Optical photoresponse; Carrier dynamics; Two-gap structure 1. Introduction The discovery of superconductivity at 39 K in mag- nesium diborides [1] has stimulated very intensive investigations of both the basic physical properties and possible practical applications of this new supercon- ductor.RecentworksdemonstratedthatMgB 2 isaBCS, phonon-mediated superconductor with the two distinct superconducting energy gaps Ds [2–6]. It was stressed by Liu et al. [3] that the two Ds could be observed only in the clean limit, when the interband scattering is weak. In the dirty limit, one can measure only one average D.The temperature dependence of this D deviates considerably from the BCS DðT Þ curve and exhibits a hump at tem- peratures below T c [7].However,aswasshownin[7],the existence of the hump can be explained using the two- gap model with two different gap closing T c values. The dirty limit is applicable for post-annealed MgB 2 thin films, while the clean limit describes single crystals. 2. Experimental Two types of MgB 2 superconducting samples were prepared and investigated, namely, thin films and single crystals. The MgB 2 films were prepared by co-evapora- tion of Mg and B components on unheated sapphire * Corresponding authors. Tel.: +1-585-275-1551; fax: +1- 585-275-2073. E-mail addresses: yixu@ece.rochester.edu (Y. Xu), roman.sobolewski@rochester.edu (R. Sobolewski). 1 Also with the Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA. 2 Also with the Institute of Physics, Polish Academy of Sciences, PL-02668 Warszawa, Poland. 0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.02.043 www.elsevier.com/locate/physc Physica C 408–410 (2004) 90–92