Prof. John Wei bio

John Y.T. Wei Associate Professor Condensed Matter Physics
	60 St. George Street, Toronto, ON M5S1A7 Canada Phone: (416) 946-5943 Office: MP 081 Fax: (416) 978-2537 E-mail: wei@physics.utoronto.ca
Scholar, Canadian Institute for Advanced Research (since 1999) Member, Brockhouse Institute for Materials Research (since 2003) Member, Center for Quantum Information and Quantum Control (since 2004) Postdoctoral Scholar & Senior Research Fellow, California Institute of Technology (1996-99) Joint Research Student, IBM T. J. Watson Research Center (1992-95) M.S. & Ph.D., Columbia University (1991 & 1996) B.A., University of California Berkeley (1987) *Research Interests: Nanoscale studies of exotic superconductors & magnets, high-temperature & heavy-fermion superconductivity, colossal magnetoresistance, spin-injection & phase-slip phenomena. Materials: Cuprates, intermetallics, manganites, superconducting & ferromagnetic nanostructures. Techniques: Cryomagnetic scanning tunneling microscopy (STM), quasiparticle tunneling & Andreev reflection spectroscopy, pulsed laser deposition (PLD), electron beam lithography. My research is devoted to nanoscale studies of novel electronic materials, using both top-down and bottom-up experimental approaches. These materials are so-called quantum materials where quantum mechanical effects of electrons are manifest. They incldue exotic superconducting cuprates, ferromagnetic manganites and thin-film heterostructures comprising these novel materials. These materials are remarkable in that they are characterized by strong electron-electron correlations, share a similar perovskite crystal structure, but exhibit extremely different physical behaviors depending on the chemical composition. These experiments are intended to study the fundamental problems of high-temperature superconductivity, half-metallic ferromagnetism and their dynamic interplay. For the cuprates, I am primarily interested in the symmetry of the Cooper pairs, which can be determined by measuring the anisotropy of quasiparticle tunneling and Andreev reflection spectroscopy, as a means to elucidate the microscopic mechanism underlying high temperature superconductivity. Because of the short superconducting coherence length (<10nm) in the cuprates, it is important to probe these pairs locally. A complex pairing symmetry, such as d+is* or d+id', could provide a locally phase-coherent and naturally doubly-degenerate system for use as a quantum computational bit, i.e. a superconducting QUBIT. And because the cuprates are known to have spatial inhomegeneities involving spin/charge separation, it would be very useful to be able to discern between spin and charge excitations, e.g. by using a tunable spin-polarized scanning probe. For the manganites, my focus is on the ferromagnetic double-exchange interaction which spin-polarizes the itinerant band electrons, and on how this half-metallic character affects the electrical conduction in a phenomenon known as colossal magnetoresistivity. It is important to probe the spin-polarized electronic spectrum locally and under quasiparticle injection, in order to understand the role of tunneling magnetoresistance between microdomains, and to determine the spin-diffusion length and spin-relaxation time scales which are vital to spintronic applications. By combining these lattice-compatible materials into superconductor/ferromagnet epitaxial heterostructures, with the ferromagnet serving as a source for spin-polarized quasiparticle injection, the dynamics of both Cooper pair-breaking in the cuprates and itinerant magnetism in the manganites can also be examined. Such studies could reveal important phenomenological links between the superconductivity and the magnetism which occur in the perovskite system. My experimental expertise involves the technique of scanning tunneling microscopy (STM), which provides a highly sensitive probe of the electronic density of states in conjunction with atomic-scale spatial resolution. Our current plan is to extend these STM spectroscopy measurements to ultra-low temperatures (below 100mK) and high magnetic fields (up to 18Tesla), using apparatus my students and I are expressly building for these purposes. We are studying both single-crystal and epitaxial-film samples, the latter to be grown by a multiple-target pulsed laser-ablated deposition (PLD) system which we have installed. The multilayer epitaxial films can also be patterned into nanostructures using electron-beam lithography facilities which we have also installed in-house . Courses Taught Physics 1850F: Foundations of Condensed Matter Physics Physics 225H: Undergraduate Laboratory Physics 281S: Introduction to Quantum Physics Select Papers: Conductance characteristics between a normal metal and a 2D Fulde-Ferrell-Larkin-Ovchinnikov superconductor Qinghong Cui, C.-R. Hu , J. Y. T. Wei , and Kun Yang, Physical Review B 73, 214514 (2006). Point Contact Spectroscopy Study of the Ferromagnetic Superconductor ZrZn_2 C. Turel, M. Tanatar, R. Hill, E. Yelland, S. Hayden, J. Y. T. Wei, preprint, to appear in AIP Conf. Proc. for LT24 (2006). Evidence for Current-Driven Phase Slips in YBa_2Cu_3O_7-d Microstrips P. Morales, M. DiCiano, and J. Y. T. Wei, preprint, to appear in AIP Conf. Proc. for LT24 (2006). Nanoscale Studies of Cuprate Superconductors in the Current-Carrying State J. Y. T. Wei, J. Ngai, and P. Morales, Proceedings of the SPIE Int. Soc. Opt. Eng. 5932, 593210 (2005). Nuclear Spin Relaxation of ⁸Li in a Thin Film of La_0.67Ca_0.33MnO_3 R. I. Miller et al., preprint, to appear in Physica B (2005). Suppression of low-energy Andreev states by a supercurrent in YBa_2Cu_3O_7-d J. Ngai, P. Morales and J. Y. T. Wei, Physical Review B 72, 054513 (2005). Spectroscopic Evidence for Multiple Order Parameter Components in the Heavy Fermion Superconductor CeCoIn_5 P. M. C. Rourke et al., Physical Review Letters 94, 107005 (2005). Selective epitaxial growth of sub-micron complex oxide structures by amorphous SrTiO_3 P. Morales, M. DiCiano, and J. Y. T. Wei, Applied Physics Letters 86, 192509 (2005). Scanning tunneling spectroscopy under pulsed spin injection J. Ngai, et al., Applied Physics Letters 84, 1907 (2004). Nanoscale high-temperature superconductivity P. Mohanty, J. Y.T. Wei, V. Ananth, P. Morales, W. Skocpol, Physica C 408–410, 666 (2004). Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures, J. Y. T. Wei et al., Journal of Superconductivity 15-1, 67 (2002). Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in cuprate/manganite heterostructures, J. Y. T. Wei et al., Journal of Applied Physics 85, 5350 (1999). Directional tunneling and Andreev reflection on YBa_2Cu_3O_{7-delta} single crystals: Predominance of d-wave pairing symmetry verified with the generalized BTK theory, J. Y. T. Wei et al., Physical Review Letters 81, 2542 (1998).
Physics Homepage	University of Toronto

     My research is devoted to nanoscale studies of novel electronic materials, using both top-down and bottom-up experimental approaches. These materials are so-called quantum materials where quantum mechanical effects of electrons are manifest. They incldue exotic superconducting cuprates, ferromagnetic manganites and thin-film heterostructures comprising these novel materials. These materials are remarkable in that they are characterized by strong electron-electron correlations, share a similar perovskite crystal structure, but exhibit extremely different physical behaviors depending on the chemical composition. These experiments are intended to study the fundamental problems of high-temperature superconductivity, half-metallic ferromagnetism and their dynamic interplay.
     For the cuprates, I am primarily interested in the symmetry of the Cooper pairs, which can be determined by measuring the anisotropy of quasiparticle tunneling and Andreev reflection spectroscopy, as a means to elucidate the microscopic mechanism underlying high temperature superconductivity. Because of the short superconducting coherence length (<10nm) in the cuprates, it is important to probe these pairs locally. A complex pairing symmetry, such as d+is or d+id', could provide a locally phase-coherent and naturally doubly-degenerate system for use as a quantum computational bit, i.e. a superconducting QUBIT. And because the cuprates are known to have spatial inhomegeneities involving spin/charge separation, it would be very useful to be able to discern between spin and charge excitations, e.g. by using a tunable spin-polarized scanning probe.
     For the manganites, my focus is on the ferromagnetic double-exchange interaction which spin-polarizes the itinerant band electrons, and on how this half-metallic character affects the electrical conduction in a phenomenon known as colossal magnetoresistivity. It is important to probe the spin-polarized electronic spectrum locally and under quasiparticle injection, in order to understand the role of tunneling magnetoresistance between microdomains, and to determine the spin-diffusion length and spin-relaxation time scales which are vital to spintronic applications.
     By combining these lattice-compatible materials into superconductor/ferromagnet epitaxial heterostructures, with the ferromagnet serving as a source for spin-polarized quasiparticle injection, the dynamics of both Cooper pair-breaking in the cuprates and itinerant magnetism in the manganites can also be examined. Such studies could reveal important phenomenological links between the superconductivity and the magnetism which occur in the perovskite system.
     My experimental expertise involves the technique of scanning tunneling microscopy (STM), which provides a highly sensitive probe of the electronic density of states in conjunction with atomic-scale spatial resolution. Our current plan is to extend these STM spectroscopy measurements to ultra-low temperatures (below 100mK) and high magnetic fields (up to 18Tesla), using apparatus my students and I are expressly building for these purposes. We are studying both single-crystal and epitaxial-film samples, the latter to be grown by a multiple-target pulsed laser-ablated deposition (PLD) system which we have installed. The multilayer epitaxial films can also be patterned into nanostructures using electron-beam lithography facilities which we have also installed in-house .

Conductance characteristics between a normal metal and a 2D Fulde-Ferrell-Larkin-Ovchinnikov superconductor
Qinghong Cui, C.-R. Hu , J. Y. T. Wei , and Kun Yang, Physical Review B 73, 214514 (2006).

Point Contact Spectroscopy Study of the Ferromagnetic Superconductor ZrZn_2
C. Turel, M. Tanatar, R. Hill, E. Yelland, S. Hayden, J. Y. T. Wei, preprint, to appear in AIP Conf. Proc. for LT24 (2006).

Evidence for Current-Driven Phase Slips in YBa_2Cu_3O_7-d Microstrips
P. Morales, M. DiCiano, and J. Y. T. Wei, preprint, to appear in AIP Conf. Proc. for LT24 (2006).

Nanoscale Studies of Cuprate Superconductors in the Current-Carrying State
J. Y. T. Wei, J. Ngai, and P. Morales, Proceedings of the SPIE Int. Soc. Opt. Eng. 5932, 593210 (2005).

Nuclear Spin Relaxation of ⁸Li in a Thin Film of La_0.67Ca_0.33MnO_3
R. I. Miller et al., preprint, to appear in Physica B (2005).

Suppression of low-energy Andreev states by a supercurrent in YBa_2Cu_3O_7-d
J. Ngai, P. Morales and J. Y. T. Wei, Physical Review B 72, 054513 (2005).

Spectroscopic Evidence for Multiple Order Parameter Components in the Heavy Fermion Superconductor CeCoIn_5
P. M. C. Rourke et al., Physical Review Letters 94, 107005 (2005).

Selective epitaxial growth of sub-micron complex oxide structures by amorphous SrTiO_3
P. Morales, M. DiCiano, and J. Y. T. Wei, Applied Physics Letters 86, 192509 (2005).

Scanning tunneling spectroscopy under pulsed spin injection
J. Ngai, et al., Applied Physics Letters 84, 1907 (2004).

Nanoscale high-temperature superconductivity
P. Mohanty, J. Y.T. Wei, V. Ananth, P. Morales, W. Skocpol, Physica C 408–410, 666 (2004).

Spin-Injection Quasiparticle Nonequilibrium in Cuprate/Manganite Heterostructures,
J. Y. T. Wei et al., Journal of Superconductivity 15-1, 67 (2002).

Tunneling spectroscopy study of spin-polarized quasiparticle injection effects in
cuprate/manganite heterostructures,
J. Y. T. Wei et al., Journal of Applied Physics 85, 5350 (1999).