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4 Corners Southwest Ontario Condensed Matter Physics Symposium 2014

Conference Date: 
Jeudi, Mai 1, 2014 (All day)
Scientific Areas: 
Condensed Matter

 

This seventh annual one-day symposium aims to provide an opportunity for condensed matter researchers in southwest Ontario to gather together and discuss informally their most recent research.  The general format of the meeting consists of 2 guest speakers and 5-7 contributed talks.  The names of the contributing speakers and title of their talks will be announced later.  Registration begins at 9:30 am.  The meeting is expected to start around 9:45 am and end between 5-5:30 pm.  A lunch will be provided by the Black Hole Bistro.  

There will be two keynote speakers for the symposium.  Professor David Huse from Princeton University and Professor Young Lee from the Massachusetts Institute of Technology.  Professor Huse will discuss a topic pertaining to cold atoms and Professor Lee will talk about quantum spin liquids.

Past speakers for this event were as follows:

2013

  • Steve Kivelson, Stanford University
  • Radu Coldea, University of Oxford

2012

  • Paul Chaikin, New York University
  • Patrick Lee, Massachusetts Institute of Technology

2011

  • Seamus Davis, Cornell University
  • Eduardo Fradkin, University of Illinois at Urbana-Champaign

2010

  • Leon Balents, Kavli Institute for Theoretical Physics

2009

  • Piers Coleman, Rutgers University
  • Bill Halperin, Northwestern University

2008

  • Collin Broholm, Johns Hopkins University
  • Subir Sachdev, Harvard University

To register for this event, please click here.

 

Dima Abanin, Perimeter Institute
Gang Chen, University of Toronto
Jaime Forrest, Perimeter Institute
Mingxuan Fu, McMaster University
Bruce Gaulin, McMaster University
David Huse, Princeton University
Edwin Kermarrec, McMaster University
Young Lee, Massachusetts Institute of Technology
Adrian Lupascu, University of Waterloo
Ipsita Mandal, Perimeter Institute
Jeff Rau, University of Toronto
Evelyn Tang, Massachusetts Institute of Technology
Juven Wang, Massachusetts Institute of Technology

  • Phillip Ashby, McMaster University
  • Bill Atkinson, Trent University
  • Peter Barfuss, University of Waterloo
  • John Berlinsky, Perimeter Institute
  • Anton Burkov, University of Waterloo
  • Eamon Campbell,  Trent University
  • Jules Carbotte, McMaster University
  • Hilary Carteret, Wilfrid Laurier University
  • Gang Chen, University of Toronto
  • Yangang Chen, Perimeter Institute
  • Lukasz Cincio, Perimeter Institute
  • Lucy Clark, McMaster University
  • Denis Dalidovich, Perimeter Institute
  • Alexandre Day, University of Waterloo
  • Jaime Forrest, Perimeter Institute
  • Mingxuan Fu, McMaster University
  • Jonathan Gaudet, McMaster University
  • Bruce Gaulin, McMaster University
  • Sedigh Ghamari, McMaster University
  • Michel Gingras, University of Waterloo
  • John Greedan, McMaster University
  • Alannah Hallas, McMaster University
  • Zhihao Hao, Univeristy of Waterloo
  • Lauren Hayward, University of Waterloo
  • Robert Hill, University of Waterloo
  • Wen Huang, McMaster University
  • David Huse, Princeton University
  • Kyusung Hwang, University of Toronto
  • Behnam Javanparast, University of Waterloo
  • Catherine Kallin, McMaster University
  • Edwin Kermarrec, McMaster University
  • Isaac Kim, Perimeter Institute
  • Bohdan Kulchytskyy, University of Waterloo
  • Patrick Lafleur, Trent University
  • Eric Lee, University of Toronto
  • Keith Lee, Perimeter Institute
  • Sung-Sik Lee, Perimeter Institute
  • Young Lee, Massachusetts Institute of Technology
  • Zhou Li, McMaster University
  • Taoran Lin, University of Waterloo
  • Graeme Luke, McMaster University
  • Peter Lunts, Perimeter Institute
  • Adrian Lupascu, University of Waterloo
  • Dalini Maharaj, McMaster University
  • John Malcolm, University of Guelph
  • Ipsita Mandal, Perimeter Institute
  • Roger Melko, Perimeter Institute & University of Waterloo
  • Guoxing Miao, University of Waterloo
  • Christopher Mitchelitis, University of Waterloo
  • Lister Mulindwa, The University of Zambia
  • Rodrigo Muniz, University of Toronto
  • Rob Myers, Perimeter Institute
  • Ray Ng, McMaster University
  • Elisabeth Nicol, University of Guelph
  • Duncan O'Dell, McMaster University
  • Zlatko Papic, Perimeter Institute
  • Boris Pavlovic, University of Guelph
  • Jayanayana Perera, Trent University
  • David Pomaranski, University of Waterloo
  • Feresheteh Rajabi, University of Wesern Ontario
  • Jeff Rau, University of Toronto
  • Robert Schaffer, University of Toronto
  • Vijay Shankar, University of Toronto
  • Fil Simovic, University of Waterloo
  • Erik Sorensen, McMaster University
  • Shouvik Sur, McMaster University
  • Calvin Tabert, University of Guelph
  • Makoto Tachibana, McMaster University
  • Evelyn Tang, Massachusetts Institute of Technology
  • Edward Taylor, McMaster University
  • Corey Thompson, McMaster University
  • Guifre Vidal, Perimeter Institute
  • Yidun Wan, Perimeter Institute
  • Juven Wang, Perimeter Institute 
  • Brian Yee, University of Guelph
  • William Toews, University of Waterloo
  • Teresa Medina, McMaster University
  • Murray Wilson, McMaster University
  • Jerod Wagman, McMaster University

 

Time

Event

Location

9:00 – 9:40

Registration and Coffee

Atrium

9:40 – 9:45

Michel Gingras, University of Waterloo
Welcome Address

Lazaridis Theatre

 

Morning Session
 Chair: Rob Hill, University of Waterloo

Lazaridis Theatre

9:45 – 10:45

David Huse, Princeton University
"Localization-Protected Quantum Order"

Lazaridis Theatre

10:45 – 11:15

Dima Abanin, Perimeter Institute
“Entanglement, Ergodicity, and Many-Body Localization”

Lazaridis Theatre

11:15 – 11:35

Edwin Kermarrec, McMaster University
"Exotic Magnetism on the FCC Lattice of 5dn Double Perovskites”

Lazaridis Theatre

11:35 – 11:55

Ipsita Mandal, Perimeter Institute
“Renormalization Group Analysis of a Non-Fermi Liquid System”

Lazaridis Theatre

11:55 – 12:15

Juven Wang, MIT
Non-Abelian String and Particle Braiding in 3+1D Topological Order

Lazaridis Theatre

12:15 – 1:20

Lunch Break

Black Hole Bistro

1:20 – 1:30

Conference Photo

Atrium

 

 

Afternoon Session #1
Chair: Bruce Gaulin, McMaster University

Lazaridis Theatre

1:30 – 2:30

Young Lee, Massachusetts Institue of Technology
“Experimental Sightings of the Quantum Spin Liquid”

Lazaridis Theatre

2:30 – 2:50

Mingxuan Fu, McMaster University
"Revealing the Local Magnetism of S=1/2 Kagomé Lattice
ZnCu3(OH)6Cl2 using Single Crystal NMR”

Lazaridis Theatre

2:50 – 3:10

Gang Chen, University of Toronto
“Fractionalized Charge Excitations in a Spin Liquid on Partially-Filled
Pyrochlore Lattice”

Lazaridis Theatre

3:10 – 3:30

Jeffrey Rau, University of Toronto
“Generic Spin Model for Honeycomb Iridates Beyond the Kitaev Limit”

Lazaridis Theatre

3:30 – 4:00

Afternoon Coffee Break

Black Hole Bistro

 

Afternoon Session #2
Chair: Roger Melko, University of Waterloo

Lazaridis Theatre

4:00 – 4:30

Jamie Forrest, University of Waterloo
“Using β-NMR to Solve Hard Problems in Soft Condensed Matter”

Lazaridis Theatre

4:30 – 4:50

Evelyn Tang, MIT
“Strain-Induced Helical Flat Band & Interface Superconductivity in
Topological Crystalline Insulators”

Lazaridis Theatre

4:50 – 5:20

Adrian Lupascu, University of Waterloo
“Measurements of Noise in Condensed Matter Systems Using
Superconducting Qubits and Resonators”

Lazaridis Theatre

5:20 – 5:50

Bruce Gaulin, McMaster University
“Frozen Spin Ice Ground States in the Pyrochlore Magnet Tb2Ti2O7

Lazaridis Theatre

5:50

Michel Gingras – Closing

Lazaridis Theatre

 

KEYNOTE SPEAKERS

David Huse, Princeton University

Localization-protected quantum order

Systems that are many-body Anderson localized can exhibit symmetry-breaking long-range order or topological order in regimes where such order would be destroyed at equilibrium by thermal fluctuations.  The ordering is dynamical: an ordered initial state stays ordered, being "protected" by the localization of all fluctuations.  The simplest examples are quantum Ising models with static randomness.  For the random quantum Ising chain this feature has been "known" but apparently not appreciated for close to half a century. 

Ref.: Huse, et al., Phys. Rev. B 88, 014206 (2013).

Young Lee,  Massachusetts Institute of Technology

Experimental sightings of the quantum spin liquid

New states of matter may be produced if quantum effects and frustration conspire to prevent the ground state from achieving classical order. An example of a new quantum phase is the quantum spin liquid. Such spin liquids cannot be characterized by local order parameters; rather, they are distinctive by their possession of long range quantum entanglement. I will describe recent experimental progress in the quest to study quantum spin liquids in frustrated magnets. The kagome lattice, composed of corner-sharing triangles, is highly frustrated for antiferromagnetic spins. Materials based on the kagome lattice with spin-1/2 are ideal hosts for quantum spin liquid ground states. I will discuss our group’s work which includes single crystal growth, bulk characterization, and neutron scattering measurements of the S=1/2 kagome lattice material ZnCu3(OH)6Cl2 (also known as herbertsmithite). Our inelastic neutron scattering measurements of the spin correlations in a single crystal sample reveal that the excitations are fractionalized, a hallmark signature of spin liquid physics.

SPEAKERS

Dima Abanin, Perimeter Institute

Entanglement, Ergodicity, and Many-Body Localization

We are used to describing systems of many particles by statistical mechanics. However, the basic postulate of statistical mechanics – ergodicity – breaks down in so-called many-body localized systems, where disorder prevents particle transport and thermalization. In this talk, I will present a theory of the many-body localized (MBL) phase, based on new insights from quantum entanglement. I will argue that, in contrast to ergodic systems, MBL eigenstates are not highly entangled. I will use this fact to show that MBL phase is characterized by an infinite number of emergent local conservation laws, in terms of which the Hamiltonian acquires a universal form. Turning to the experimental implications, I will describe the response of MBL systems to quenches: surprisingly, entanglement shows logarithmic in time growth, reminiscent of glasses, while local observables exhibit power-law approach to “equilibrium” values. I will support the presented theory with results of numerical experiments. I will close by discussing other directions in exploring ergodicity and its breaking in quantum many-body systems.

Gang Chen, University of Toronto

Fractionalized Charge Excitations in a Spin Liquid on Partially-Filled

Electron charge may fractionalize in a quantum spin liquid Mott insulator. We study the Mott transition from a metal to a cluster Mott insulator in the 1/4- and 1/8-filled pyrochlore lattice systems. Such Mott transitions can arise due to charge localization in clusters or in tetrahedron units, driven by the nearest-neighbor repulsion. The resulting cluster Mott insulator is a quantum spin liquid with spinon Fermi surface, but at the same time a novel fractionalized charge liquid with charge excitations carrying half the electron charge. There exist two emergent U(1) gauge fields or "photons" that mediate interactions between spinons and charge excitations, and between fractionalized charge excitations themselves, respectively. In particular, it is suggested that the emergent photons associated with the fractionalized charge excitations can be measured in X-ray scattering experiments. This and other experimental signatures of the quantum spin and fractionalized charge liquid state are discussed in light of candidate materials with partially-filled bands on pyrochlore lattices. “Fractionalized Charge Excitations in a Spin Liquid on Partially-Filled Pyrochlore Lattice” Electron charge may fractionalize in a quantum spin liquid Mott insulator. We study the Mott transition from a metal to a cluster Mott insulator in the 1/4- and 1/8-filled pyrochlore lattice systems. Such Mott transitions can arise due to charge localization in clusters or in tetrahedron units, driven by the nearest-neighbor repulsion. The resulting cluster Mott insulator is a quantum spin liquid with spinon Fermi surface, but at the same time a novel fractionalized charge liquid with charge excitations carrying half the electron charge. There exist two emergent U(1) gauge fields or "photons" that mediate interactions between spinons and charge excitations, and between fractionalized charge excitations themselves, respectively. In particular, it is suggested that the emergent photons associated with the fractionalized charge excitations can be measured in X-ray scattering experiments. This and other experimental signatures of the quantum spin and fractionalized charge liquid state are discussed in light of candidate materials with partially-filled bands on pyrochlore lattices.
 
Jaime Forrest, University of Waterloo
 
Using β-NMR to Solve Hard Problems in Soft Condensed Matter
 
Beta-detected nuclear spin relaxation of 8Li+ has been used to study important problems in polymer physics. In the first case we probe the depth dependence of molecular dynamics in high- and low-molecular-weight deuterated polystyrene (PS-d8). The average nuclear spin-lattice relaxation rate, 1/T1 avg, is a measure of the spectral density of the polymer dynamics at the Larmor frequency (41MHz at 6.55Tesla). The mean fluctuation rate decreases approximately exponentially with distance from the free surface, returning to bulk behavior for depths greater than ~10nm and the effective thickness of the surface region increases with increasing temperature. These results present challenges for the current understanding of dynamics near the free surface of polymer glasses. In the second case, we use the technique to make the first quantitative measurements of surface segregation in samples that are blends of two chemically identical polymers with different degrees of polymerization.
 
Mingxuan Fu, McMaster University
 
Revealing the Local Magnetism of S=1/2 Kagomé Lattice 
 
Herbersmithite ZnCu3(OH)6Cl2 is known to be a promising candidate material hosting a quantum spin liquid ground state. The recent success in single crystal growth of ZnCu3(OH)6Cl2 as well as the discovery of a continuum of spinon excitations using inelastic neutron scattering [1] have opened a new chapter in the study of highly frustrated magnetism. However, the mechanism behind the realization of the non-magnetic ground state in ZnCu3(OH)6Cl2 remains controversial, mainly due to the difficulty in understanding the role of defects in its physical properties. To distinguish the intrinsic magnetism of the kagomé lattice from the defect contribution, we used 17O, 63Cu, 2H, and 35Cl NMR to probe the local behavior of spin susceptibility and spin dynamics, which provided invaluable insights into the nature of defects and their potential influence on the kagomé spin lattice [2].
 
*Present address: Material Sciences Division, Argonne National Laboratory, Argonne, IL 60439
 
[1] T. Han et al., Nature 492, 406 (2012).
[2] M. Fu, T. Imai et al., in preparation. Also see T. Imai et al., PRL 100, 077203 (2008); and PRB 84, 020411(R) (2012).
 
Bruce Gaulin, McMaster University
 
Frozen Spin Ice Ground States in the Pyrochlore Magnet Tb2Ti2O7
 
Tb2Ti2O7 was one of the first pyrochlore magnets to be studied as a candidate for a spin liquid or cooperative paramagnet, and its ground state has remained enigmatic for fifteen years. Recent time-of-flight neutron scattering studies have shown that it enters a glassy Spin Ice ground state, characterized by frozen short range order over about 8 conventional unit cells, and the formation of a ~ 0.08 meV gap in its spin excitation spectrum at the appropriate quasi-Bragg wave vectors. I will introduce the relevant Spin Ice physics background, and describe how the experiments are performed. The new H-T phase diagram for Tb2Ti2O7 in a [110] magnetic field will be presented. This shows that its frozen (i.e. glassy) Spin Ice ground state (at low temperature and zero field) and its conventional field-induced ordered phase (at low temperature and high fields) bracket the cooperative paramagnetic phase which generated the original interest in this fascinating magnet.
 
Edwin Kermarrec, McMaster University
 
Exotic Magnetism on the FCC Lattice of 5dn Double Perovskites
 
In the search for new exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the Jeff = ½ spin orbital Mott state in the 5d5 layered perovskites iridates [1]. The double perovskite structure, where the magnetic ions form a face-centered-cubic (fcc) sublattice, can accommodate a large variety of 5d transition metal elements, and therefore offers an ideal playground for systematic studies of the exotic magnetic and non-magnetic ground states stabilized by strong spin-orbit coupling [2]. Here, we report time-of-flight neutron scattering measurements on the antiferromagnetic, frustrated, cubic double perovskite system Ba2YOsO6. Its non-distorted fcc lattice is decorated with magnetic Os5+ (5d3) ions which undergo a magnetic transition to a long range ordered antiferromagnetic state below TN = 70 K, as revealed by magnetic Bragg peaks occuring at the [100] and [110] positions. Our inelastic data reveals a large spin gap to the spin-wave excitations Δ = 19(2) meV, unexpected for an orbitally quenched, d3 electronic configuration. We compare this result to the recent observation of a Δ=5 meV spin gap in the related cubic double perovskite Ba2YRuO6 (Ru5+, 4d3) [3], and conclude to a stronger spin-orbit coupling present in the heavier, 5d, osmate system.
 
[1] B. J. Kim et al., Science 323, 1329 (2009).
[2] G. Chen, R. Pereira and L. Balents, Phys. Rev. B 82, 174440 (2010);
G. Chen and L. Balents, Phys. Rev. B 84, 094420 (2011).
[3] J. P. Carlo et al., Phys. Rev. B 88, 024418 (2013).
 
Adrian Lupascu, University of Waterloo
 
Measurements of Noise in Condensed Matter Systems Using Superconducting Qubits and Resonators
 
Superconducting qubits based on Josephson junctions and resonators are presently leading candidates for the implementation of quantum computing. These systems couple strongly to their environment, which often makes preservation of coherence challenging. This strong coupling can be turned into an advantage: it enables the investigation of noise and loss at low temperatures. I will discuss two topics. The first topic is the use of superconducting flux qubits to measure magnetic flux noise. The second topic is the measurement of microwave loss in amorphous dielectric materials. Experiments with superconducting coherent systems can be used to extract new information on flux noise and dielectric loss, not accessible using other methods used in the past, providing useful input to theoretical developments.
 
Ipsita Mandal, Perimeter Institute
 
Renormalization Group Analysis of a Non-Fermi Liquid System
 
We devise a renormalization group analysis for quantum field theories with Fermi surface to study scaling behaviour of non- Fermi liquid states in a controlled approximation. The non-Fermi liquid fixed points are identified from a Fermi surface in (m+1) spatial dimensions, while the co-dimension of Fermi surface is also extended to a generic value. We also study superconducting instability in such systems as a function of dimension and co-dimension of the Fermi surface. The key point in this whole analysis is that unlike in relativistic QFT, the Fermi momentum kF enters as a dimensionful parameter, thus modifying the naive scaling arguments. The effective coupling constant is found to be a combination of the original coupling constant and kF.
 
Jeff Rau, University of Toronto
 
Generic Spin Model for Honeycomb Iridates Beyond the Kitaev Limit
 
Recently, realizations of Kitaev physics have been sought in the A2IrO3 family of honeycomb iridates, originating from oxygen-mediated exchange through edge-shared octahedra. However, for the J=1/2 Mott insulator in these materials exchange from direct d-orbital overlap is relevant, and it was proposed that a Heisenberg term should be added to the Kitaev model. Here we provide the generic nearest-neighbour spin Hamiltonian when both oxygen-mediated and direct overlap are present, containing a bond dependent off-diagonal exchange in addition to Heisenberg and Kitaev terms. We analyze this complete model using a combination of classical techniques and exact diagonalization. Near the Kitaev limit, we find new magnetic phases, 120 degree and incommensurate spiral order, as well as extended regions of zigzag and stripy order. Possible applications to Na2IrO3 and Li2IrO3 are discussed.
 
Evelyn Tang, Massachusetts Institute of Technology
 
Strain-Induced Helical Flat Band & Interface Superconductivity in Topological Crystalline Insulators
 
Topological crystalline insulators in IV-VI compounds host novel topological surface states, that at low energy, consist of multi-valley massless Dirac fermions. We show that strain generically acts as an effective gauge field on these Dirac fermion surface states and creates pseudo-Landau orbitals without breaking time-reversal symmetry. We predict this is naturally realized in IV-VI semiconductor heterostructures due to the spontaneous formation of a misfit dislocation array at the interface, where the zero-energy Landau orbitals form a nearly flat band. We propose that the high density of states of this topological flat band gives rise to the experimentally observed interface superconductivity in IV-VI semiconductor multilayers at temperatures that are unusually high for semiconductors, and explains its non-BCS dependence on dislocation array period. 
 
Juven Wang, Massachusetts Institute of Technology
 

Non-Abelian String and Particle Braiding in 3+1D Topological Order

 

String and particle excitations are examined in a class of 3+1D topological order described by a discrete gauge theory with a gauge group G and a 4-cocycle twist ω4∈H4(G,R/Z) of G's cohomology group. We demonstrate the topological spin and the spin-statistics relation for the closed strings, and their multi-string braiding. The 3+1D twisted gauge theory can be characterized by a representation of SL(3,Z) modular transformation, which we find its generators Sxyz and Txy in terms of the gauge group G and the 4-cocycle ω4. As we compactify one of the 3D's direction z into a compact circle inserted with a gauge flux b, we can use the generators of SL(2,Z) subgroup of SL(3,Z), Sxy and Txy, to study the dimension reduction of the 3D topological order C3D to a direct sum of degenerate states of 2D topological orders C2Db in different fluxb sectors: C3D=⊕bC2Db. The 2D topological orders C2Db are described by 2D gauge theories of the group G twisted by the 3-cocycles ω3(b)dimensionally reduced from the 4-cocycle ω4. We show that the SL(2,Z) generators, Sxy and Txy, fully encodes a particular type of three-string braiding statistics for the connected sum of two Hopf links 221#221 configuration. With certain 4-cocycle twist, we find that, by threading a third string through two-string unlink 021 into three-string Hopf links 221#221 configuration, Abelian two-string statistics is promoted to non-Abelian three-string statistics. (Work done in arxiv.org/abs/1404.7854)

 

 

Jeudi mai 01, 2014
Speaker(s): 

Tb2Ti2O7 was one of the first pyrochlore magnets to be studied as a candidate for a spin liquid or cooperative paramagnet, and its ground state has remained enigmatic for fifteen years. Recent time-of-flight neutron scattering studies have shown that it enters a glassy Spin Ice ground state, characterized by frozen short range order over about 8 conventional unit cells, and the formation of a ~ 0.08 meV gap in its spin excitation spectrum at the appropriate quasi-Bragg wave vectors.

 

Jeudi mai 01, 2014
Speaker(s): 

Superconducting qubits based on Josephson junctions and resonators are presently leading candidates for the implementation of quantum computing. These systems couple strongly to their environment, which often makes preservation of coherence challenging. This strong coupling can be turned into an advantage: it enables the investigation of noise and loss at low temperatures. I will discuss two topics. The first topic is the use of superconducting flux qubits to measure magnetic flux noise. The second topic is the measurement of microwave loss in amorphous dielectric materials.

 

Jeudi mai 01, 2014
Speaker(s): 

Topological crystalline insulators in IV-VI compounds host novel topological surface states, that at low energy, consist of multi-valley massless Dirac fermions. We show that strain generically acts as an effective gauge field on these Dirac fermion surface states and creates pseudo-Landau orbitals without breaking time-reversal symmetry. We predict this is naturally realized in IV-VI semiconductor heterostructures due to the spontaneous formation of a misfit dislocation array at the interface, where the zero-energy Landau orbitals form a nearly flat band.

 

Jeudi mai 01, 2014
Speaker(s): 

Beta-detected nuclear spin relaxation of 8Li+ has been used to study important problems in polymer physics. In the first case we probe the depth dependence of molecular dynamics in high- and low-molecular-weight deuterated polystyrene (PS-d8). The average nuclear spin-lattice relaxation rate, 1/T1 avg, is a measure of the spectral density of the polymer dynamics at the Larmor frequency (41MHz at 6.55Tesla).

 

Jeudi mai 01, 2014
Speaker(s): 

Recently, realizations of Kitaev physics have been sought in the A2IrO3 family of honeycomb iridates, originating from oxygen-mediated exchange through edge-shared octahedra. However, for the J=1/2 Mott insulator in these materials exchange from direct d-orbital overlap is relevant, and it was proposed that a Heisenberg term should be added to the Kitaev model. Here we provide the generic nearest-neighbour spin Hamiltonian when both oxygen-mediated and direct overlap are present, containing a bond dependent off-diagonal exchange in addition to Heisenberg and Kitaev terms.

 

Jeudi mai 01, 2014
Speaker(s): 

Electron charge may fractionalize in a quantum spin liquid Mott insulator. We study the Mott transition from a metal to a cluster Mott insulator in the 1/4- and 1/8-filled pyrochlore lattice systems. Such Mott transitions can arise due to charge localization in clusters or in tetrahedron units, driven by the nearest-neighbor repulsion. The resulting cluster Mott insulator is a quantum spin liquid with spinon Fermi surface, but at the same time a novel fractionalized charge liquid with charge excitations carrying half the electron charge.

 

Jeudi mai 01, 2014
Speaker(s): 

New states of matter may be produced if quantum effects and frustration conspire to prevent the ground state from achieving classical order. An example of a new quantum phase is the quantum spin liquid. Such spin liquids cannot be characterized by local order parameters; rather, they are distinctive by their possession of long range quantum entanglement. I will describe recent experimental progress in the quest to study quantum spin liquids in frustrated magnets. The kagome lattice, composed of corner-sharing triangles, is highly frustrated for antiferromagnetic spins.

 

Jeudi mai 01, 2014
Speaker(s): 

String and particle excitations are examined in a class of 3+1D topological order described by a discrete gauge theory with a gauge group G and a 4-cocycle twist ω4∈H4(G,R/Z) of G's cohomology group. We demonstrate the topological spin and the spin-statistics relation for the closed strings, and their multi-string braiding. The 3+1D twisted gauge theory can be characterized by a representation of SL(3,Z) modular transformation, which we find its generators Sxyz and Txy in terms of the gauge group G and the 4-cocycle ω4.

 

Jeudi mai 01, 2014
Speaker(s): 

We devise a renormalization group analysis for quantum field theories with Fermi surface to study scaling behaviour of non- Fermi liquid states in a controlled approximation. The non-Fermi liquid fixed points are identified from a Fermi surface in (m+1) spatial dimensions, while the co-dimension of Fermi surface is also extended to a generic value. We also study superconducting instability in such systems as a function of dimension and co-dimension of the Fermi surface.

 

Jeudi mai 01, 2014
Speaker(s): 

In the search for new exotic quantum states, the impact of strong spin-orbit interaction has been recently underlined with the discovery of the Jeff = ½ spin orbital Mott state in the 5d5 layered perovskites iridates [1]. The double perovskite structure, where the magnetic ions form a face-centered-cubic (fcc) sublattice, can accommodate a large variety of 5d transition metal elements, and therefore offers an ideal playground for systematic studies of the exotic magnetic and non-magnetic ground states stabilized by strong spin-orbit coupling [2].

Pages

Scientific Organizer:

Michel Gingras, University of Waterloo