Femtosecond Pump-probe studies of Carrier Dynamics in the Normal State of the Unconventional Superconductor Sr 2 RuO 4
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Femtosecond Pump-probe studies of Carrier Dynamics in the Normal State of the Unconventional Superconductor Sr2RuO 4 P. Guptasarma1 , S.L. Sendelbach3+, M.L. Schneider3,*, M.S. Williamsen1 , G. Taft2 , A. Souslov1,4 and M. Onellion3 1.
Physics Department, University of Wisconsin, Milwaukee, WI Physics Department, University of Wisconsin, Stevens Point, WI 53706 3. Physics Department, University of Wisconsin, Madison, WI 4. National High Magnetic Field Laboratory, Tallahassee, FL 32310 2.
1. Introduction: The microscopic origin of unconventional superconductivity1 is one of the most fascinating questions 2-3 in condensed matter physics. In recent years, important questions have been raised about the nature of the normal (metallic, non-superconducting) state above the superconducting critical transition temperature (Tc). In fact, it is now generally agreed4,5 that a better understanding of the nature of the normal state is crucial to our understanding of the microscopic origin of the superconducting state. Strontium Ruthenate (Sr2 RuO 4 ) – perhaps the only known perovskite superconductor without copper – has shown striking evidence in support of unconventional superconductivity6,7. Of particular interest is the exciting possibility of a spin-triplet paired8 , p-wave superconducting ground state which breaks time reversal symmetry9 . Also interesting is its similarity to yet another layered perovskite: La 2-x Bax CuO 4 , with which it is iso-structural. Early suggestions 10 of p-wave triplet pairing in Sr2 RuO 4 mediated through ferromagnetic fluctuations, perhaps inspired by ferromagnetic order in SrRuO 3, continue to be investigated in recent experiments11 . Also under investigation are studies of the properties of the gap in this system12 . Substitution of Lanthanum and/or Calcium at the Sr site in the Sr2 RuO 4 structure lead to phase diagrams showing a wide range of competing ground states, which include unconventional magnetic ground states, a metal-to- insulator Mott transition and possible meta- magnetic behavior13-15 . In this proceedings article, we report our ongoing investigations of the normal state of highly pure crystals of Sr2 RuO 4 using a femtosecond time-resolved optical pump-probe technique.
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2. Experimental Details 2.1 Single Crystals: Strontium Ruthenate has been shown to have two phases: the better-known phase shows up in the vicinity of 1 Kelvin, also called the “1–K phase”. Yet another, the so-called “3-K phase”, a possible interface-superconducting state16 , shows up under certain crystal growing conditions (not the subject of this report). Superconductivity in this “1K phase” of Sr2 RuO 4 has been shown to be very sensitive to impurities17 – minor levels of impurity drive Tc down to non-superconducting. In fact, residual resistivity versus Tc of bulk Sr2 RuO 4 , when fitted with the Abrikosov-Gorkov pair breaking function and extrapolated18 , predicts a theoretical optimum Tc = 1.5 K for Sr2 RuO 4 in the clean limit. For residual resistivity greater than 1 µΩcm, the mean
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