搜档网

搜档网

当前位置:搜档网 > Time-Reversal Symmetry Breaking Superconductivity in Sr2RuO4

Time-Reversal Symmetry Breaking Superconductivity in Sr2RuO4

a r X i v :c o n d -m a t /9808159v 1 [c o n d -m a t . s u p r -c o n ] 14 A u g 1998

Submitted to Nature 1

Time-Reversal Symmetry Breaking Superconductivity in Sr 2RuO 4

G. M. Luke ∗ , Y. Fudamoto ∗ , K. M. Kojima ∗ , M. I. Larkin ∗ , J. Merrin ∗ , B. Nachumi ∗ , Y. J. Uemura ∗ , Y. Maeno † , Z. Q. Mao † , Y. Mori † , H. Nakamura ‡ , M. Sigrist §

Dept. of Physics, Columbia University, New York, NY 10027, U.S.A. †

Dept. of Physics, Kyoto University, Kyoto 606-8502, Japan. ‡

Dept. of Material Science and Engineering, Kyoto University, Kyoto 606-8501, Japan. §

Yukawa Instutute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan.

In addition to its importance for existing and potential applications, superconductivity[1]is one of the most interesting phenomena in condensed matter physics. Although most superconducting materials are well-described in the context of the Bardeen Cooper and Schrieffer (BCS)theory[2],considerable effort has been de-voted to the search for exotic systems whose novel properties cannot be described by the BCS theory. Conventional superconductors break only gauge symmetry by selecting a def-inite phase for the Cooper pair wavefunction; a signature of an unconventional supercon-ducting state is the breaking of additional symmetries[3].Evidence for such broken symmetries include anisotropic pairing (suchas d-wave in the high-T c cuprates) and the presence of multiple superconducting phases (UPt3and superfluid 3We have per-formed muon spin relaxation measurements of Sr 2RuO 4and observe a spontaneous internal magnetic field appearing below T c . Our measurements indicate that the supercon-ducting state in Sr 2RuO 4is characterized by broken time reversal symmetry which, when combined with symmetry considerations in-dicate that its superconductivity is of p-wave (odd-parity)type, analagous to superfluid 3

He. Despite the structural similarity with the high T c cuprates, the origin of the un-conventional superconductivity in Sr 2RuO 4is fundamentally different in nature.

Sr 2RuO 4, which is isostructural to the high-T c cuprate La 1. 85Sr 0. 15CuO 4, is to date the only known layered perovskite superconductor which does not contain copper. Although first synthesized in the 50’s,[5]its superconductivity was only found in

1994[6];T c ’s of early samples were roughly 0.7K but have increased to T c =1. 5K in recent high quality single crystals[7].Despite its low transition temperature, Sr 2RuO 4is of great interest as there is growing evidence for an unconventional supercon-ducting state. In this system, strong correlation ef-fects enhance the effective mass seen in quantum oscillation[8]and Pauli spin susceptibility measure-ments, in the same way as in 3Combining this feature with Sr 2RuO 4’s expected tendency to display ferromagnetic spin fluctuations, Rice and Sigrist[10],and later Baskaran[11]argued that the pairing in Sr 2RuO 4could be of odd parity (spintriplet) type. The strong suppression of the superconducting T c by even non-magnetic impurities suggests non-s-wave Specific heat[12]and NMR 1/T1[13]mea-surements indicate the presence of a large residual density of states (RDOS)at low temperatures (wellwithin the superconducting state); in high quality samples, this RDOS as T → 0seems to approach half of the normal state value. Several authors[14,15]have proposed so-called non-unitary p-wave super-conducting states for Sr 2RuO 4to account for this RDOS as well as the absence of a Hebel-Slichter peak in NMR measurements[13].A finite RDOS is not a unique signature of unconventional superconductiv-ity; for example it is observed in so-called gapless superconductors with isotropic s-wave pairing as a re-sult of impurity scattering (althoughthis is unlikely in the specific case of Sr 2RuO 4where the finite RDOS apparently remains in the cleanest samples). It could also be explained with a multi-band hypothesis[16]where different gaps are associated with two types of bands. Therefore, further studies are required for a definitive determination of the pairing symmetry in Sr 2RuO 4.

One aspect of the pairing symmetry, the breaking of time reversal symmetry (TRS)can be probed di-rectly. If the superconducting state has a degenerate representation (asis possible for some triplet super-conducting states) then TRS can be broken, whereas it cannot be broken for non-degenerate representa-tions (thecase for all singlet states). When spin or-bit coupling is small, the pair wave function can be written as a product of the orbital part and the spin part. Non-zero angular momentum of either the or-bital or the spin part could result in TRS breaking, although there are many such cases with conserved TRS, such as d-wave states in the high-T c cuprates. TRS breaking is also possible in the case of strong spin orbit coupling. In general, pairing states can be further classified in terms of gap functions attached to irreducible representations of a point group for a given crystal lattice symmetry of the system[3].TRS

TOP相关主题