EDUCATION:

D.Sc. 1982, University of Tokyo

RESEARCH:

Since I joined Columbia Physics Faculty in 1988, I have been working on experimental studies of superconductivity and magnetism, using the muon spin relaxation (MuSR) technique.  At the TRIUMF meson facility in Vancouver, our team from Columbia has performed extensive MuSR measurements of the magnetic field penetration depth and magnetic ordering in high-Tc cuprate (HTSC), organic, heavy-fermions, and other superconductors and their family materials. These studies have revealed remarkable correlations between Tc and the superconducting carrier density n_{s} in these exotic superconductors.   We established a nearly linear relationship between T_{c} and n_{s}/m^{*} in various HTSC systems.  Combined with subsequent findings of the “pseudo-gap” behavior, our results suggest that the condensation mechanism in HTSC systems is fundamentally different from that described in the BCS theory, while being rather similar to that in systems which undergo Bose Einstein condensation, such as superfluid ^{4}He. 

We have also studied magnetism in frustrated and/or low dimensional spin systems and in itinerant ferromagnets, such as MnSi, which exhibit quantum phase transitions in crossover to correlated paramagnets.  In these studies, we found: (1) dynamic spin fluctuations persisting to T = 0 in Kagome lattice and some other frustrated spin systems; (2) appearance of local magnetic field below T_{c} which breaks time reversal symmetry in odd-parity superconductors Sr2RuO4 and UPt3; (3) phase separation and suppression of dynamic critical behavior in quantum magnetic phase transitions in MnSi and SrRuO3; (4) spatial phase separation of regions with superconductivity and static magnetism in various HTSC systems; and (5) external-field induced staggered spin polarization in HTSC systems starting well above T_{c}.   

 
During 1983-88, I performed a number of neutron-scattering experiments using the high flux beam reactor at Brookhaven National Laboratory to study spin fluctuations and excitations in random magnetic systems, such as spin glasses and fractal spin networks.  Neutron scattering and MuSR are representative applications of "Big Science" facilities (accelerator and reactor) to solid-state physics. MuSR is a probe in coordinate space and time, while neutron scattering probes in momentum and energy. They are also complementary in their time windows for dynamic phenomena. We would like to study current topics of solid-state physics by combining experimental results from MuSR and neutrons, and by developing phenomenological pictures of underlying physics mechanisms.

SELECTED PUBLICATIONS:

Click for a complete listing of publications.

"Muon Spin Relaxation in AuFe and CuMn Spin Glasses", Phys. Rev. Lett. 45, 583 (1980); Phys. Rev. B31, 546 (1985) [234 citations].  

"Magnons and Fractons in the Diluted Antiferromagnet MnxZn1-x F2", Phys. Rev. Lett. 57, 1947 (1986); Phys. Rev. B36, 7026 (1987).

"Universal Correlations Between Tc and ns/m* (Carrier Density/Effective Mass) in High-Tc Cuprate Superconductors", Phys. Rev. Lett. 62, 2317 (1989) [595 citations]; Phys. Rev. Lett. 66, 2665  (1991) [408 citations].

                   
“Spin Fluctuations in Frustrated Kagome Lattice System SrCr8Ga4O19 Studied by Muon Spin Relaxation”, Phys. Rev. Lett. 73 (1994) 3306.

“Time-Reversal Symmetry Breaking Superconductivity in Sr2RuO4”, Nature 394 (1998) 558 [314 citations].

“Muon Spin Relaxation Studies of Incommensurate Magnetism and Superconductivity in stage-4 La2CuO(4.11) and La(1.88)Sr(0.12)CuO4”, Phys. Rev. B66 (2002) 014524.

“Muon Spin Relaxation Studies of Magnetic-Field-Induced Effects in High-Tc Superconductors”, Phys. Rev. Lett. 95 (2005) 157001.

“Twin spin/charge roton mode and superfluid density: Primary determining factors of Tc in high-Tc superconductors observed by neutron, ARPES, and MuSR (Yamazaki Prize Lecture)”, Presented at the International Conference on Muon Spin Rotation MuSR2005, in Oxford, UK, August 8-12, 2005, Physica B374-375 (2006) 1.

“Phase separation and suppression of critical dynamics at quantum transitions of itinerant magnets: MnSi and (Sr,Ca)RuO3”, to appear in Nature Physics (2006).

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