RESEARCH OVERVIEW

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Research Areas
Our studies span a wide range of quantum matter systems, including superconductors, superfluids, supersolids, electronic liquid crystals, topological insulators superconductors & superfluids, heavy fermions, and spin liquids. Throughout, the focus is on development of innovative techniques and approaches to each problem.

Superfluid Josephson Effects
Superfluid 3He Josephson junctions use nano-aperture arrays fabricated at Cornell Nanofabrication Center. Using these devices, we discovered Josephson oscillations in superfluid 3He (Nature 388, 449 (1997)), the current-phase relationship of a superfluid Josephson junction (Science 278, 1435-1438 (1997)), p-states within Josephson junction of a topological p-wave superfluid weak link (Nature 392, 687-690 (1998), Nature 396, 554-557 (1998)) and the first superfluid DC-SQUID (Nature 412, 55 (2001)).

These projects were in collaboration with Prof. R. Packard of U.C. Berkeley.

Solid and ‘Supersolid’ 4He
A possible supersolid phase has been reported at high pressure in solid 4He. We have developed the first SQUID-based torsional oscillator system for supersolid studies. Using this new approach, we found evidence for a ‘superglass’ state in solid 4He (Science 324, 632(2009)) and were able to identify a unified relationship between rotational, relaxational, and shear dynamics of this quantum solid (Science 332, 821, (2011)).

These projects are in collaboration with Dr. A.V. Balatsky of LANL, New Mexico, USA, and Prof. M. Yamashita of Kyoto University, Japan.

Copper-based High Temperature Superconductivity
In 1999 we introduced spectroscopic imaging STM for visualization of electronic structure in complex electronic matter. We have used this approach extensively for studies of copper-based high temperature superconductors. We imaged electronic bound-states at individual impurity atoms (Science 285, 88 (1999)) including non-magnetic Zn impurity atoms on in Bi2Sr2CaCu2O8+δ, Nature 403, 746 (2000) and magnetic Ni impurity atoms in Bi2Sr2CaCu2O8+δ (Nature 411, 920 (2001)). We discovered the granular electronic structure of cuprates in underdoped Bi2Sr2CaCu2O8+δ (Nature 413,282 (2001), Nature 415, 412 (2002)). We also discovered the famous electronic density waves in underdoped cuprates (Science 266, 455 (2002), Nature 430, 1001 (2004), Science 314, 1914 (2006)), and intra unit cell symmetry breaking that generates and electronic nematic state (Science 315, 1380 (2007), Nature 466, 374 (2010), Science 333, 426 (2011)).

We introduced the quasiparticle interference (QPI) imaging for determination of momentum-space electronic structure in complex electronic materials (Science 297, 1148 (2002), Nature 422, 520 (2003)) and used these techniques to determine effects of dopant atom and the approach of Mott insulator state (Science 309, 1048 (2005), Nature 454, 1072 (2008)). We studied the microscopic pairing mechanism via the interplay of electron-lattice interactions and superconductivity in (QPI) (Nature 442, 546 (2006)), we found the spectroscopic fingerprint of phase incoherent d-wave superconductivity (Science 325,1099 (2009)), and we identified the hidden critical point underpinning high-Tc superconductivity phase diagram (Science 344, 612 (2014)).

The Bi2Sr2CaCu2O8+δ project is in collaboration with Prof. S. Uchida of Tokyo University and Dr. H Eisaki of AIST Tsukuba, Japan, the Ca2-xNaxCuO2Cl2 project is in collaboration with Prof. H. Takagi of Tokyo University and RIKEN, Japan. Theoretical collaborations are with Prof. D. -H. Lee of UC Berkeley, and Profs. E. -A. Kim and M. J. Lawler of Cornell University.

Iron-Based High Temperature Superconductivity
In 2009 we introduced spectroscopic imaging STM and quasiparticle interference imaging for visualization of electronic structure in iron-based superconductors. We used this approach to discover the nematic electronic phase in iron-based superconductors (Science 327, 181 (2010)), the impact of individual dopant atoms and of high energy radiation damage (Nature Physics 9, 220 (2013), Science Advances 1, 1500033, (2015)), and the superconducting electronic structure (Science 336, 563 (2012)) and magnetically mediated pairing mechanism of iron-based superconductivity (Nature Physics 11, 117 (2015)).

The iron-based high-Tc superconductivity project is in collaboration with Prof. P. Canfield of Ames National Lab., Dr. H. Eisaki of AIST, Tsukuba, Japan, and Prof. A. Mackenzie of St. Andrews University, Scotland. Theoretical collaborations are with Profs. E. -A. Kim and M. J. Lawler of Cornell University.

Topological Quantum Matter
Topological insulators and topological superfluids (3He A, 3He B) are well known states of matter. Topological superconductors are proposed to exist but no definitive proof has so far emerged – although SrRuO-214 is widely believed to be a time-reversal violating odd-parity superconductor. We pursue a program of studies of topological insulator surface states and searches for topological superconductivity. In the field of ferromagnetic topological insulators (FM TI) where a formally equivalent topological order should exist, by introducing the Dirac-mass 'gapmap' technique, i.e. we discovered intense nanoscale disorder in the Dirac-mass and demonstrated that this is directly related to fluctuations in the magnetic-dopant atom density n(r) (Proc. Nat. Acad. Sci. 112, 1316 (2015)).

The SrRuO-214 project is in collaboration with Prof. A. Mackenzie of University of St. Andrews & Max Planck Inst. Dresden. The CrBiSbTe FM TI project is in collaboration with Dr. Genda Gu of Brookhaven Nat. Lab.

Heavy Fermion Superconductivity & Quantum Criticality
Heavy fermions are composite quantum objects made by quantum superposition of free electrons and fixed magnetic-spins. The result is an exotic fluid of electronic particles that are free to move through a material but have effective mass thousands of times that of a free electron. In 2010 we achieved the first visualization of heavy fermions (Nature 465, 570 (2010)), followed by the discovery the electronic structure of heavy fermion superconductivity (Nature Physics 9, 468(2013)), and the magnetically mediated pairing mechanism of heavy fermion superconductivity (Proc. Nat. Acad. Sci. 111, 11663 (2014)).

The URu2Si2 project is in collaboration with Prof. G. Luke, McMaster University, Canada. The CeCoIn5 project is in collaboration with Dr. C. Petrovic of Brookhaven National Lab., and Prof. A.P. Mackenzie of St. Andrews University.

Spin and Monopole Liquids
In 2015, by introducing novel techniques for magnetic fluid flow studies, we discovered that the magnetic fluid in the canonical pyrochlore material Dy2Ti2O7 is a supercooled spin liquid – an unprecedented quantum magnetic state (Proc. Nat. Acad. Sci. 112, 8549 (2015)).

The pyrochlore spin liquid project is in collaboration with Prof. G. Luke, McMaster University, Canada.