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Contact Information: Office:152F Davey Lab,
Phone:1-814-863-0090 Last Updated:2-12-2009 |
Most
superconducting materials studied thus far, including high-Tc
cuprates, are even-parity, spin-singlet (s- or d-wave)
superconductors. Theoretical studies of systems where spin-triplet
pairing may be realized, ranging from neutron stars to superconductor-ferromagnet
nanostructures, have revealed exotic properties of these unconventional
superconductors, raising the possibility of using them for novel
applications. Until recently, superfluid 3He
had
long been considered the only established example of odd-parity pairing.
However,
work in our laboratory [1] has shown that odd-parity, spin-triplet
pairing also occurs in Sr2RuO4 (strontium ruthenate), the
only layered perovskite that becomes superconducting without the
presence of Cu. Our work on Sr2RuO4 has focused on tunneling and phase-sensitive measurements using high-quality single crystals grown by our collaborators. Our work on Pb-Sr2RuO4-Pb junctions revealed that superconductivity in Sr2RuO4 actually suppressed the Josephson coupling between two Pb electrodes, providing an early piece of evidence for unconventional, non-s-wave pairing in Sr2RuO4 [2]. We showed that the Josephson coupling between Sr2RuO4 and Indium was possible only along the in-plane direction [3], and demonstrated that the Andreev surface bound states exist in both the bulk and the 3K phases of Sr2RuO4 [4], further revealing the unconventional aspects of superconductivity in this material. We also imaged vortices in Sr2RuO4 in collaboration with other groups. A
phase-sensitive experiment provides the most stringent test on the
pairing symmetry of a superconductor, as shown in the high-Tc
work. In
[1] we prepared Au0.5In0.5-Sr2RuO4
superconducting quantum interference devices (SQUIDs) and showed that
the phase of the order parameter in Sr2RuO4
changes by pi under inversion. Prior to this experiment, the constant spin susceptibility observed in the
nuclear magnetic resonance Knight shift measurements was considered the strongest
evidence for spin-triplet superconductivity in Sr2RuO4.
However, reasons unrelated to pairing symmetry are also known to lead to
constant spin susceptibility. For example, the Knight shift of vanadium,
an s-wave superconductor, is unchanged across Tc.
Our
phase-sensitive work provided the first definitive
experimental proof that Sr2RuO4 is an odd-parity
superconductor, and will likely be the last push for Sr2RuO4
to find its way into textbooks. We
are currently doing experiments to clarify the exact form of the order parameter using
phase-sensitive techniques, the possibility of d-vector
rotation, and the existence and manipulation of domains and domain walls.
[1] K.D. Nelson, Z.Q. Mao, Y. Maeno,
and Y. Liu, “Odd-parity superconductivity in Sr2RuO4,”
Science 12, 1151-1154 (2004). [2] R. Jin, Yu. Zadorozhny, Y. Liu,
D.G. Schlom, Y. Mori, Y. Maeno, “Observation of anomalous temperature
dependence of the critical current in Pb/ Sr2RuO4/Pb
junctions,” Phys. Rev. B 59, 4433-4438 (1999) [3] R. Jin, Y. Liu, Z.Q.
Mao, and Y. Maeno, “Experimental observation of the selection rule in
Josephson coupling between In and Sr2RuO4,” Europhys. Lett. 51,
341-347 (2000) [4] Z.Q. Mao, K.D. Nelson, R.Jin,
Y.Liu, and Y.Maeno, “Observation of Andreev surface bound states in
the 3-K phase region of Sr2RuO4.,” Phys. Rev.
Lett. 87, 037003 (2001) Contacts: Ronald Myers, rjm31 @ psu.edu Yiqun Alex Ying, yzy116 @ psu.edu Neal Staley, nes151 @ psu.edu
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