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Institute of Advanced Studies
Workshop on
Topological Phase Transitions and
New Developments5 to 8 June 2017
Nanyang Technological University, Singapore
Sponsored by
Contents
Foreword ………………………………………………………….....................2
Organising Committee………………………..………….……..........3
Programme…………………………………………….…………………….....4
Abstracts of Speakers………………….……………………..........11
Poster Presenters & Titles…………………….…….................40
Useful Contacts………………………………………..…………………43
1
Foreword We would like to welcome all of you to the Workshop on Topological Phase Transitions and New Developments!
In recent years, ideas from topology, a branch of mathematics dealing with the properties of space that are preserved under continuous deforming such as stretching and bending, have rapidly infiltrated physics and drive both theoretical and experimental efforts into the study of new exotic materials. Topological phase transitions involving vortices that appear at critical temperatures in thin superconductors, superfluids or magnets have provided immense insight into the mechanism of some physical systems. Moreover, these topological properties remain stable even if the system is subject to perturbations. An example of the application of such robustness to perturbations is the search for encoding information in quantum computers, providing the platform for fault-tolerant quantum computation.
In the past three decades, the discovery of topological phases has not only engendered great interest among researchers in condensed matter physics but it has also attracted the interest of researchers working on quantum information, quantum materials and simulations, high energy physics and string theory. This workshop aims to bring together researchers working on various aspects of topological phases and topological phase transitions.
We hope you will actively participate in this exciting workshop!
Professor Kok Khoo Phua Institute of Advanced Studies Nanyang Technological University
Professor Lars Brink Chalmers University of Technology
Professor Mike Gunn University of Birmingham
Professor Jorge José Indiana University
Professor J. Michael Kosterlitz Nobel Laureate in Physics 2016
2
Organising Committee
Co-Chairs NAME INSTITUTION
Lars Brink Chalmers University of Technology
Mike Gunn University of Birmingham
Jorge José Indiana University
J. Michael Kosterlitz Nobel Laureate in Physics 2016
Kok Khoo Phua Institute of Advanced Studies, NTU
Local Organising Committee NAME INSTITUTION
Shafiqque Adam National University of Singapore
Yidong Chong School of Physical & Mathematical Sciences, NTU
Rainer Dumke School of Physical & Mathematical Sciences, NTU
Yuan Ping Feng National University of Singapore
Leong Chuan Kwek Institute of Advanced Studies, NTU & Centre for Quantum Technologies
Hwee Boon Low Institute of Advanced Studies, NTU
Choo Hiap Oh Institute of Advanced Studies, NTU & National University of Singapore
Pinaki Sengupta School of Physical & Mathematical Sciences, NTU
Justin Song School of Physical & Mathematical Sciences, NTU
David Wilkowski School of Physical & Mathematical Sciences, NTU
Chi Xiong Institute of Advanced Studies, NTU & School of Physical & Mathematical Sciences, NTU
3
Programme
4
Day 1 – Monday, 5 June 2017
08.00 – 09.00 Registration
Opening Ceremony
09.00 – 09.15 Welcome Address by:Kok Khoo Phua (Institute of Advanced Studies, NTU)Jorge José (Indiana University)Mike Gunn (University of Birmingham)
Session Chair: Kok Khoo Phua (Institute of Advanced Studies, NTU)
09:15 – 10:00 Thierry Giamarchi (University of Geneva)Clean and Dirty Bosons in 1D Lattices
10:00 – 10:45 Rosario Fazio (Scuola Normale Superiore)Majorana Quasi-Particles with Fermionic Cold Atoms in Optical Lattices
10:45 – 11:30 Group Photo & Coffee Break
Session Chair: Rosario Fazio (Scuola Normale Superiore)
11:30 – 12:15 J. Michael Kosterlitz (Brown University)Topological Defects and Phase Transitions
12:15 – 13:00 Jorge José (Indiana University)Theoretical Physics Developments After 1974 and Their Consequences for the Kosterlitz-Thouless Theory
13:00 – 14:00 Lunch
5
Day 1 – Monday, 5 June 2017
Session Chair: Thierry Giamarchi (University of Geneva)
14:00 – 14:45 John Reppy (Cornell University) Superfluidity, Phase Transitions, and Topology
14:45 – 15:30 Wang Yao (The University of Hong Kong) Topological Phenomena in the Moire Pattern of Van Der Waals Heterostructures
15:30 – 16:00 Coffee Break
Session Chair: Shafiqque Adam (National University of Singapore)
16:00 – 16:45 Arthur Hebard (University of Florida) Heterogeneous Interfaces for Teasing out the Physics of Embedded Surface States
16:45 – 17:30 Pieralberto Marchetti (University of Padova) Attraction Between Topological Quantum Vortices as Origin of Superconductivity in Cuprates
17:30 – 19:00 Poster Session @ Education Wing Atrium
18:00 Dinner at NTU Peach Garden (by invitation only)
** Dinner at NTU Peach Garden Two-way transport will be provided. Invited guests please be reminded to gather at the NEC Guest Wing Lobby (Level 1) by 5.50pm.
6
Day 2 – Tuesday, 6 June 2017
Session Chair: Rainer Helmet Dumke (School of Physical & Mathematical Sciences, NTU)
09:00 – 09:45 F. Duncan M. Haldane (Princeton University) Geometry of Flux Attachment in the Fractional Quantum Hall Effect
09:45 – 10:30 Nicolas Regnault (Ecole Normale Supérieure de Paris) Emergent Particle-Hole Symmetry in Spinful Bosonic Quantum Hall Systems
10:30 – 11:00 Coffee Break
11:00 – 11:45 Frank Pollmann (Technical University of Munich) Dynamical Signatures of Quantum Spin Liquids
11:45 – 12:30 Gerardo Ortiz (Indiana University) Topological Superfluidity with Repulsive Fermionic Atoms
12:30 – 13:45 Lunch
Session Chair: Jorge José (Indiana University)
13:45 – 14:30 Christopher Lobb (University of Maryland) – Via SKYPE Getting the Jump in the Kosterlitz-Thouless Transition
14:30 – 15:15 Valerii Vinokour (Argonne National Laboratory, Lemont) Topological BKT Phases in Disordered Materials
15:15 – 15:45 Coffee Break
7
Day 2 – Tuesday, 6 June 2017
Session Chair: Lu Yu (Institute of Physics, Chinese Academy of Sciences)
15:45 – 17:15 Panel Discussion on Looking into the Future of Topological Phase Transitions
Rosario Fazio (Scuola Normale Superiore)
F. Duncan M. Haldane (Princeton University)
J. Michael Kosterlitz (Brown University)
Nicolas Regnault (Ecole Normale Supérieure de Paris)
Valerii M. Vinokour (Argonne National Laboratory, Lemont)
18:00 Depart for Workshop Banquet at Orchard Hotel, Ballroom 3 (Level 3)
19:00 – 21:30 Workshop Banquet (by invitation only)
** Workshop Banquet at Orchard Hotel Two-way transport will be provided. Invited guests please be reminded to gather at the NEC Guest Wing Lobby (Level 1) by 5.50pm.
8
Day 3 – Wednesday, 7 June 2017
Session Chair: Mike Gunn (University of Birmingham)
09:30 – 10:15 Stephen Teitel (University of Rochester) Phase Transitions: From Josephson Junction Arrays to Flowing Granular Matter
10:15 – 11:00 Yidong Chong (Nanyang Technological University) Topological Phase Transitions in Photonic Lattices
11:00– 11:30 Coffee Break
Session Chair: Justin Song (Nanyang Technological University)
11:30 – 12:15 Mike Gunn (University of Birmingham) Instabilities and Solitary Waves of Light and Atoms in Photonic Crystal Fibres
12:15 – 13:00 Zohar Nussinov (Washington University in St. Louis) A Universal Collapse of the Viscosity of All Supercooled Liquids as a Consequence of Equilibrium Melting Transitions
13:00 – 14.15 Lunch
Session Chair: Pinaki Sengupta (School of Physical & Mathematical Sciences, NTU)
14:15 – 15:00 Baile Zhang (Nanyang Technological University) Some Topological Phases for Sound
15:00 – 15:45 Christos Panagopoulos (Nanyang Technological University) Spin Topology Architectures in Low Dimensional Magnets
15:45 – 16:15 Coffee Break
16:15 – 17:00 Philip W Phillips (University of Illinois at Urbana–Champaign) Origin of Bose Metal in 2D Superconducting Films
17:00– 17:45 Martin Freer (University of Birmingham) The Influence of Topology in Nuclear Structure
17:45 – 20:00 Dinner at Function Hall 1, Level 3 (above NEC Auditorium)
9
Day 4 – Thursday, 8 June 2017
Session Chair: Leong Chuan Kwek (Institute of Advanced Studies, NTU)
09:00 – 09:45 Zidan Wang (The University of Hong Kong) Realizing and Manipulating Topological Metals and Their Exotic Properties
09:45 – 10:30 Herbert Fertig (Indiana University) Magnetic Ordering on the Surface of a Topological Crystalline Insulator
10:30 – 11:15 Coffee Break
Session Chair: Xiong Chi (Institute of Advanced Studies, NTU)
11:15 – 12:00 John Saunders (Royal Holloway University of London) Helium Films at Ultralow Temperatures: From Strongly Correlated Atomically Layered Films to Topological Mesoscopic Superfluidity
12:00 – 12:45 Yayu Wang (Tsinghua University) Tuning Magnetism and Topology in Topological Insulators with Broken Time Reversal Symmetry
12:45 – 13:30 Justin Song (Nanyang Technological University) Anomalous Collective Modes in Topological Matter
13:30 Lunch
End of Workshop Note: All talks include 5 minutes of Question and Answers.
10
Abstracts | Invited Talks
11
Author
: Yidong Chong
Affiliation
: Nanyang Technological University
Title
: Topological Phase Transitions in Photonic Lattices
Abstract
Topologically non-trivial bandstructures are not limited to condensed-matter systems, but they
can also be realized in a variety of specially-designed photonic systems, such as photonic
crystals and arrays of coupled optical waveguides. In some of these photonic systems, it is
exceptionally easy to tune system parameters to drive the bandstructure through a topological
phase transition, which can produce various interesting effects. In this talk, I will discuss an
optical waveguide array that exhibits a particularly accessible transition between a 2D
conventional insulator and a 2D topological insulator phase. The transition point corresponds
in the full 3D bandstructure to a Type-II Weyl point, the first to be found experimentally in
photonics. Moreover, in the regime of nonlinear optics, I will show that the topological phase
transition gives rise to a novel family of “self-induced topological solitons”, which inherit the
properties of gap solitons as well as topological edge states.
12
Author : Rosario Fazio
Affiliation : Scuola Normale Superiore
Title : Majorana Quasi-Particles with Fermionic Cold Atoms in Optical Lattices
Email : [email protected]
Abstract The combination of interactions and static gauge fields plays a pivotal role in our
understanding of strongly-correlated quantum matter. Cold atomic gases are emerging as an
ideal platform to experimentally address this interplay in quasi-one-dimensional systems. I will
discuss how angular momentum conservation can stabilise a symmetry-protected quasi-
topological phase of matter supporting Majorana quasi-particles as edge modes in one-
dimensional cold atom gases. In particular I will analyse a number-conserving four-species
Hubbard model in the presence of spin-orbit coupling. These results pave the way toward the
observation of Majorana edge modes with alkaline-earth-like fermions in optical lattices,
where all basic ingredients for our recipe - spin-orbit coupling and strong inter-orbital
interactions - have been experimentally realized over the last two years.
13
Author
: Herbert Fertig
Affiliation
: Indiana University
Title
: Magnetic Ordering on the Surface of a Topological Crystalline Insulator
Abstract
Topological Crystalline Insulators (TCI's) are a class of materials which can support non-trivial
band topology protected by crystalline symmetry. Using analytic and numerical methods, we
will study the effect of bulk magnetic impurities on a model of (Sn,Pb)Te alloys which are
believed to be TCI’s in their topological state. When the Fermi energy is in a bulk gap, gapless
surface states are occupied. Their energies are affected by the magnetic ordering on the
surface, particularly when it breaks the mirror symmetry. Focusing on the (111) surface, which
supports four independent surface Dirac cones (each associated with one of the four L points of
the bulk spectrum), we will derive the surface states and their coupling to the magnetic
moments. Ordering of the moments on the surface opens gaps in the surface spectra, with gap
sizes depending on the orientations of the magnetic moments relative to the different Γ-L
directions in the bulk. This leads to ferromagnetic ordering with magnetization direction
sensitive to the doping of the system. In particular we find a transition from a state with a two-
fold easy axis to one with six degenerate minima as a function of the surface state filling.
Computations of the spin stiffness confirm the linear stability of these states. Possible
experimental signatures of these magnetic orderings and transitions among them are
discussed.
14
Author
: Martin Freer
Affiliation
: University of Birmingham
Title
: The Influence of Topology in Nuclear Structure
Abstract
The simple textbook image of nuclei as a homogenous distribution of protons and neutrons in
light nuclear systems is far from the truth. Here the nucleus precipitates out into clusters,
particularly alpha-particles, and these clusters strongly influence the structural modes. As a
simple example, the nucleus 8Be is composed of 2 alpha-particles; 9Be 2 alpha-particles and a
covalently exchanged neutron, 11Be a 10Be core and 1 weakly coupled neutron in a neutron-
halo. The neighbouring 11Li is a Borromean system of 9Li+2n, where no pair of constituents is
bound. 10C is a nucleus which is formed from 4 constituents in which no combination form a
bound system. Understanding the shapes and structures of these nuclei is a central
experimental challenge as is their influence on formation of elements in stars. In this latter
instance the famous Hoyle-state in 12C is of central importance. This is the nuclear state
through which carbon-12 is synthesised in the triple-alpha process. Understanding the
geometric arrangement of the alpha-particles in this state has been a holy grail, which 50 years
after its discovery, is now being resolved.
15
Author
: Thierry Giamarchi
Affiliation
: University of Geneva
Title
: Clean and Dirty Bosons in 1D lattices
Abstract
I will discuss the phase transitions occurring for one-dimensional quantum bosons, both for
clean systems and systems where disorder or quasi-periodic potentials are present. Using a
technique of bosonisation of bosons, originally due to Haldane, this class of problems is
directly related to the Berezinskii–Kosterlitz–Thouless transition for the clean case or related
transitions for the disordered case. I will discuss the recent results that were obtained, in
particular for the quasi-periodic potentials and discuss the various possible experimental
realization of such questions both in the field of cold atomic gases and in one of quantum
antiferromagnets.
16
Author
: Mike Gunn
Affiliation
: University of Birmingham
Title
: Instabilities and Solitary Waves of Light and Atoms in Photonic Crystal
Fibres
Abstract
The unexpected experimental discovery of the topologically-ordered Fractional Quantum Hall
(FQH) states showed that the powerful diagrammatic perturbation theoretic methods of the time
were only useful for a subclass of problems adiabatically related to free-particle problems, and
instead, Laughlin’s discovery of a model state that describes “flux attachment” to form composite
particles has been the source of most subsequent understanding of the effect. In recent years, it
has become apparent that “flux attachment” has important sort-distance geometrical properties
as well as long-distance topological entanglement properties. I will describe geometric analogies
between the unit cell of a solid and the “composite boson” which is the elementary unit of
incompressible FQH liquids, and the place for “composite fermions” in their description.
*Work performed in collaboration with Jack Gartlan and Nicola Wilkin.
17
Author
: F. Duncan M. Haldane
Affiliation
: Princeton University
Title
: Geometry of Flux Attachment in the Fractional Quantum Hall Effect States
Abstract
The unexpected experimental discovery of the topologically-ordered Fractional Quantum Hall
(FQH) states showed that the powerful diagrammatic perturbation theoretic methods of the time
were only useful for a subclass of problems adiabatically related to free-particle problems, and
instead, Laughlin’s discovery of a model state that describes “flux attachment” to form composite
particles has been the source of most subsequent understanding of the effect. In recent years, it
has become apparent that “flux attachment” has important sort-distance geometrical properties
as well as long-distance topological entanglement properties. I will describe geometric analogies
between the unit cell of a solid and the “composite boson” which is the elementary unit of
incompressible FQH liquids, and the place for “composite fermions” in their description.
18
Author
: Arthur Hebard
Affiliation
: University of Florida
Title
: Heterogeneous Interfaces for Teasing out the Physics of Embedded
Surface States
Abstract
The interface between two dissimilar materials in intimate contact can usually be probed by
either current-voltage or capacitance-voltage measurements to reveal unique surface state
manifestations of bulk physical phenomena occurring in either (or both) contacting materials.
For example, we have shown that the Van der Waals interface between freshly prepared
graphene and a variety of doped semiconductors manifests high quality Schottky barriers with
pronounced barrier height (Fermi energy) modulations achieved by the application of electric
fields or chemical coatings. Similar rectifying structures made with flakes of transition metal
dichalcogenides (TaS2, TiSe2, NbSe2) harbouring charge density waves (CDWs), Bi2Se3 with
topologically protected surface states or high Tc superconducting cuprates (Bi-2212) pressed
against appropriately doped semiconductors reveal surprising phenomenology including shifts
in Fermi energies and the redistribution of interfacial charges. By replacing the semiconductor
with freshly cleaved Bi-2212 flakes we have also studied proximity-induced high-Tc
superconductivity in Bi-2212/1T-TaS2 junctions driven by coupling to the metastable metallic
phase coexisting within the Mott commensurate CDW phase of this material. Encouraged by
these observations, we will discuss the insights gained by applying these techniques to a
variety of layered materials with putative topological states residing at heterogeneous Van der
Waals interfaces.
19
Author
: Jorge José
Affiliation
: Indiana University
Title
: Theoretical Physics Developments After 1974 and Their Consequences
for the Kosterlitz-Thouless Theory
Abstract
Kosterlitz and Thouless (KT) published their seminal papers in 1972-1973. Berezinskii had
similar ideas about vortex pairs unbinding in his 1971 paper (which was in Russian). In 1974
Kosterlitz derived Renormalization-Group equations, within the 2-D Coulomb gas
approximation, inspired by a paper from Anderson and Yuval that dealt with a 1-D Ising model
with quadratic decaying interactions. After their publications, there were some questions raised
by some people about their heuristic leading approximations. It was after their papers were
published that new developments occurred in Theoretical Physics, to wit: Lattice Gauge
theories, Topological excitations in quantum field theories, Duality Transformations and new
RG approaches, designed for this type of problem. They all played an important role in
providing a solid mathematical foundation for the KT theory. I will also discuss extensions of
the KT theory for systems that have more general types of gauge symmetries leading to very
rich phase diagrams.
20
Author
: J. Michael Kosterlitz
Affiliation
: Brown University
Title
: Topological Defects and Phase Transitions
Abstract
This talk reviews some of the applications of topology and topological defects in phase
transitions in two-dimensional systems for which Kosterlitz and Thouless split half the 2016
Physics Nobel Prize. The theoretical predictions and experimental verification in two
dimensional superfluids, superconductors and crystals will be reviewed as they provide very
convincing quantitative agreement with topological defect theories.
21
Author
: Christopher Lobb
Affiliation
: University of Maryland
Title
: Getting the Jump in the Kosterlitz-Thouless Transition
Abstract
Kosterlitz and Thouless (KT) predicted that their eponymous phase transition should occur in a
variety of two-dimensional systems. They correctly excluded two-dimensional superconductors
from their list because the vortex-vortex interaction does not have the necessary logarithmic
form [1]. Fortunately for experimenters, later theory [2-5] showed that in the right samples
vortex-vortex interactions are close enough to logarithmic to cause the KT transition to occur.
Experimental results from two different systems, one with [6] and one without [7] the transition,
illustrate when the transition occurs, and when it doesn’t.
References [1] J M Kosterlitz and D J Thouless, J. Phys. C: Solid State Phys. 6, 1973.
[2] M. R. Beasley, J. E. Mooij, and T. P. Orlando, Phys. Rev. Lett. 42, 1165 (1979).
[3] S. Doniach and B. A. Huberman, Phys. Rev. Lett. 42, 1169 (1979).
[4] B. I. Halperin and David R. Nelson, J. Low Temp. Phys. 36, 1165 {1979).
[5] C. J. Lobb et al., Phys. Rev. B 27, 150 (1983).
[6] David W. Abraham, et al., Phys. Rev. B 26, 5268(R) (1982).
[7] J. M. Repaci, et al., Phys. Rev. B 54, R9674 (1996).
22
Author
: Pieralberto Marchetti
Affiliation
: University of Padova
Title
: Attraction Between Topological Quantum Vortices as Origin of
Superconductivity in Cuprates
Abstract
We propose a “topological” mechanism for superconductivity in cuprates based on a
description of Zhang-Rice singlets as holes of the ‘t-J model’, a standard model for
cuprates. Holes are composites of spin (spinon) and charge (holon) excitations, bound
together by the gauge interaction originated from the no-double occupancy constraint.
An attempt to optimize the energy in both t and J terms using the Chern-Simons
theory generates quantum spin vortices centred on holons, with opposite vorticity for centers in
different Néel sublattices. They interact like the Kosterlitz-Thouless (KT), generating charge
pairing below a KT crossover. The gauge interaction induces, at a lower
temperature, Resonance Valence Bound (RVB) pairing of spinons, gapped by scattering
against vortices, thus giving rise to incoherent hole pairs. The superconducting transition
occurs, as a 3D XY-like transition, by lowering the spinon gap (since RVB pairs do not
contribute anymore to it) at an even lower temperature.
Short-range antiferromagnetism and charge pairing originate from the same term in the
rewriting of the t- J Hamiltonian, incorporating a strong interplay between them.
We can reproduce the general structure of the experimental phase diagram and many features
of superfluid density in underdoped cuprates (universality, critical exponent, Uemura relation).
23
Author
: Zohar Nussinov
Affiliation
: Washington University in St. Louis
Title
: A Universal Collapse of the Viscosity of All Supercooled Liquids as a
Consequence of Equilibrium Melting Transitions
Abstract
Crystals readily melt via the proliferation of topological defects. While such equilibrium
freezing/melting transitions are relatively well understood by now, how supercooled liquids
form an amorphous rigid state - the "glass"- still remains an outstanding problem. In principle,
all liquids may be supercooled to form such a glassy state. We ask whether the knowledge of
equilibrium melting transitions and basic principles of statistical physics may be used to gain
new insight into this age old problem. After introducing the problem, we will review the
underpinning of the micro-canonical ensemble and the more refined (and explicitly quantum)
"Eigenstate Thermalization Hypothesis". We will then find and apply a simple corollary of these
to analyse the evolution of a liquid upon supercooling to form a structural glass. Simple
theoretical considerations lead to a prediction for general properties of supercooled liquids. A
collapse of the viscosity and other relaxation data of glass formers is predicted from this
theory. This collapse is found to be realized over 16 decades of relaxation times for every
known glass former type (all 66 supercooled liquids for which published experimental data
exist have been analysed).
24
Author
: Gerardo Ortiz
Affiliation
: Indiana University
Title
: Topological Superfluidity with Repulsive Fermionic Atoms *
Abstract
I will discuss a novel route to topological superfluidity in repulsive fermionic systems. The
physical mechanism leading to pairing, and thus superfluid behaviour, is driven by local
kinetic-energy fluctuations and can be realized, for instance, in multiband systems with
dissimilar band localization properties. Specifically, we propose to observe this phenomenon in
an optical superlattice with alkaline-earth fermionic atoms such as Yb or Sr. The lattice is
carefully engineered to host itinerant and spatially localized atoms with flat bands whose inter-
band quantum fluctuations mediate an attractive interaction between the itinerant degrees of
freedom. This mechanism gives rise to a topological p-wave superfluid state in quasi-one-
dimensional lattices, and to a chiral px+ipy superfluid in two space dimensions. Most
importantly, we have developed several experimental probes to characterize the superfluid
state, including an ARPES-like momentum-resolved radiofrequency spectroscopic technique
and an analogue of the Edelstein magneto-electric effect.
* Work done in collaboration with Leonid Isaev and Ana Maria Rey
25
Author
: Christos Panagopoulos
Affiliation
: Nanyang Technological University
Title
: Spin Topology Architectures in Low Dimensional Magnets
Abstract
Using particle-like spin structures as a paradigm, I will demonstrate that the states induced by
spin orbit coupling and inversion symmetry breaking in magnetic multilayers open a broad
perspective with significant impact in the practical technology of spin topology. In particular, I
will discuss our effort to modulate interfacial properties for functional skyrmions at room
temperature. First, I will introduce a materials recipe to tune the skyrmion’s size and density,
thermodynamic stability parameter, as well as the crossover between isolated and disordered-
lattice configurations. Second, I will present results on the electrical signature of skyrmions
using transport and imaging experiments, and their spin dynamics and collective spin
excitation modes from high frequency measurements. Finally, I will address their nucleation,
stability, current-induced formation and dynamics in design-nanostructures.
26
Author : Philip W Phillips
Affiliation : University of Illinois at Urbana-Champaign
Title : Origin of Bose Metal in 2D Superconducting Films
Email : [email protected]
Abstract Much to the surprise of the condensed matter community, Jaeger and Goldman [1] observed in
1989 that in addition to exhibiting the usual superconducting and insulating phases, thin films of
Bi displayed a levelling of the resistivity over a wide range of system parameters, thereby
indicating a possible metallic state for bosons. While the existence of the insulating and
superconducting phases are well described by the Kosterlitz-Thouless theory, a metallic state
for bosons is problematic because bosons lack the fermionic rule that partially filled bands
conduct. Nonetheless, the observation of metallic states for bosons has persisted [2-4] and as
recent as 2015 experiments on ultra-pure samples of NbSe2 [5] reveal a metallic state that
appears well below Hc2, thus raising the possibility that bosons are the charge carriers.
Highlighted in this experiment [5] is the corroboration of the characteristics of the Bose metallic
state with a prediction [6] we made 16 years ago that the resistivity turns on in the metallic state
continuously from the superconducting state as a power of the coupling constant. I will review
the general features of the Bose metal, the theory behind it, and future experiments that can
definitively resolve the nature of the excitations in the metallic state that disrupts the KT
transition in 2D thin films.
References [1] H. M. Jaeger, D. B. Haviland, B. G. Orr, and A. M. Goldman, Phys. Rev. B 40, 182-196
(1989).
[2]D. Ephron, A. Yazdani, A. Kapitulnik, and M. R. Beasley, Phys. Rev. Lett. 76, 1529-1531
(996).
[3] A. Yazdani and A. Kapitulnik, Phys. Rev. Lett. 74, 3037-3040 (1995).
[4]N. Mason and A. Kapitulnik, Phys. Rev. Lett. 82, 5341-5344 (1999).
[5]A. W. Tsen, et al. Nat. Physics. 12, 208-212 (2016).
[6]P. Phillips and D. Dalidovich, Science 302, 243-247 (2003).
27
Author
: Frank Pollmann
Affiliation
: Technical University of Munich
Title
: Dynamical Signatures of Quantum Spin Liquids
Abstract
Condensed matter is found in a variety of phases, the vast majority of which are characterized
in terms of symmetry breaking. However, the last few decades have yielded a plethora of
theoretically proposed quantum phases of matter which fall outside this paradigm. Recent
focus lies on the search for concrete realizations of quantum spin liquids. These are
notoriously difficult to identify experimentally because of the lack of local order parameters. In
my talk, I will discuss universal properties found in dynamical response functions that are
useful to characterize these exotic states of matter.
First, we will show that the anyonic statistics of fractionalized excitations display characteristic
signatures in threshold spectroscopic measurements. The low energy onset of associated
correlation functions near the threshold shows universal behaviour depending on the statistics
of the anyons. This explains some recent theoretical results in spin systems and also provides
a route towards detecting statistics in experiments such as neutron scattering and tunneling
spectroscopy.
Second, we introduce a matrix-product state based method to efficiently obtain dynamical
response functions for two-dimensional microscopic Hamiltonians, which we apply to different
phases of the Kitaev-Heisenberg model. We find significant broad high energy features
beyond spin-wave theory even in the ordered phases proximate to spin liquids. This includes
the phase with zig-zag order of the type observed in α-RuCl3, where we find high energy
features like those seen in inelastic neutron scattering experiments.
28
Author
: Nicolas Regnault
Affiliation
: Ecole Normale Supérieure de Paris
Title
: Emergent Particle-Hole Symmetry in Spinful Bosonic Quantum Hall Systems
Abstract
When a fermionic quantum Hall system is projected into the lowest Landau level, there is an exact
particle-hole symmetry between filling fractions ν and 1−ν. We investigate whether a similar
symmetry can emerge in bosonic quantum Hall states, where it would connect states at filling
fractions ν and 2−ν. We will begin by showing that the particle-hole conjugate to a composite
fermion 'Jain state' is another Jain state, obtained by reverse flux attachment. We will show how
information such as the shift and the edge theory can be obtained for states which are particle-hole
conjugates. Using the techniques of exact diagonalization and infinite density matrix
renormalization group, we will study a system of two-component (i.e., spinful) bosons, interacting
via a δ-function potential. We first obtain real-space entanglement spectra for the bosonic integer
quantum Hall effect at ν=2, which plays the role of a filled Landau level for the bosonic system. We
then show that at ν=4/3 the system is described by a Jain state which is the particle-hole conjugate
of the Halperin (221) state at ν=2/3. We will show a similar relationship between non-singlet states
at ν=1/2 and ν=3/2. We will also study the case of ν=1, providing unambiguous evidence that the
ground state is a composite Fermi liquid. Taken together our results demonstrate that there is
indeed an emergent particle-hole symmetry in bosonic quantum Hall systems.
29
Author
: John Reppy
Affiliation
: Cornell University
Title
: Superfluidity, Phase Transitions, and Topology
Abstract
The critical point and the superfluid transition of bulk liquid 4He provide a unique opportunity
for the study of influence of geometry on phase transitions. The variation of physical properties
such as the superfluid density and heat capacity of the 4He system near a phase transition
can be characterized by exponential functions. The high purity and almost strain free condition
(particularly under microgravity conditions) of bulk liquid helium samples allows an
unparalleled determination of these exponents. Another approach has been the study of
helium adsorbed as a thin film on various porous media where the topology of the substrate
controls the character of the transition. Examples are the 3-D superfluid transition observed for
a medium such a porous Vycor glass which exhibits a random 3-D structure and opposed the
fractal structure of the aerogels. Another important case is that of the Kosterlitz-Thouless
transition which is realized for 4He adsorbed on a 2-D surface. Over the years, a range of
experimental techniques have been applied to the problem of 4He critical phenomena. These
include the observation of persistent currents in 4He, high resolution heat capacity, and
superfluid density measurements with torsional oscillators.
30
Author
: John Saunders
Affiliation
: Royal Holloway, University of London
Title
: Helium Films at Ultralow Temperatures: From Strongly Correlated
Atomically Layered Films to Topological Mesoscopic Superfluidity
Abstract Helium films provide model systems for strongly correlated quantum matter and topological
superfluidity. The top-down approach we have pursued involves the confinement of topological
superfluid 3He in engineered nanoscale geometries, with in situ tuneability of the surface
scattering of 3He quasiparticles. We can approach the quasi-two-dimensional limit in slab-like
cavities of sub-coherence length height, and fingerprint the order parameter by sensitive
SQUID NMR techniques. The study of topological superfluid 3He in height modulated
structures, in which surfaces, edges and interfaces and their excitations, arising from bulk-
edge correspondence, play a central role, opens up the field of topological mesoscopic
superfluidity. We thus have a unique model system for topological superconductivity: the
subtle interplay between symmetry and topology in that important class of topological quantum
matter is an open question experimentally, because of the lack of appropriate materials. The
bottom-up approach is to investigate helium films on the surface of graphite. We have recently
identified a novel quasi-condensate in a 4He monolayer with intertwined superfluid and density
wave order, whose symmetry precludes a Kosterlitz-Thouless transition. A further objective is
to realize superfluidity in a 3He monolayer or bilayer, grown on atomically layered superfluid 4He films. Recent progress in characterizing these systems will be discussed. The key features
here are the atomic layering of the films, and the ability to cool into the microkelvin regime. In
this coupled fermion-boson system, interfacial excitations potentially play an important role.
31
Author
: Shun-Qing Shen
Affiliation
: The University of Hong Kong
Title
: Quantum Magnetotransport in Topological Weyl/Dirac Semimetals
Abstract Weyl semimetals are three-dimensional topological states of matter, in a sense that they host
electrons that could mimic massless relativistic particles with linear dispersion in three-
dimensional momentum space, i.e., the long-sought Weyl fermions in high energy physics. It
inherits many of the properties of Weyl fermions, including the violation of chiral charge
conservation or chiral anomaly. Searching for the signature of the violation of chiral charge
conservation in solids has inspired a growing passion on the magneto-transport in topological
semimetals. One of the open questions is how the conductivity depends on magnetic fields in
a semimetal phase. Several recent experiments on both Weyl and Dirac topological
semimetals show unconventional behaviours in transport measurement. Here I will introduce
theories of magnetoconductivity of Weyl and Dirac semimetals in several different regimes,
including the weak antiloclization in a weak field, negative magnetoresistance induced by
chiral anomaly, a purely quantum mechanical phenomenon, and magnetoresistance in a
strong field.
32
Author
: Justin Song
Affiliation
: Nanyang Technological University
Title
: Anomalous Collective Modes in Topological Matter
Abstract In the presence of electron interactions, a rich array of topological phases and behaviours are
expected to manifest. I will describe in detail how the combined action of Berry curvature and
electron interactions dramatically alters the collective behaviour of interacting electron liquids,
yielding a new class of collective excitations – Berry plasmons. Berry plasmons manifest as
chiral propagating plasmonic modes, which are confined to system boundaries, and appear
even in the absence of a magnetic field. They exhibit a rich phenomenology including split
energy dispersions for oppositely directed plasmon modes, with splitting that depends directly
on interaction strength. Berry plasmons arise generically in anomalous Hall metals, and
provide a window into the role of interactions in topological matter.
A second example are collective modes of Fermi-arc carriers in time reversal broken Weyl
semimetals. These chiral fermi arc plasmons possess open dispersions, featuring hyperbolic
constant frequency contours and group velocity vectors directed along a few specific
collimated directions. As a result, a large range of surface plasmon wave vectors can be
supported at a given frequency. Both Berry plasmons and Fermi-arc plasmons can be probed
via nanophotonic methods, and are parts of an increasingly rich new tool box to manipulate
light in a topological matter.
33
Author : Stephen Teitel
Affiliation : University of Rochester
Title : Phase Transitions: From Josephson Junction Arrays to Flowing Granular
Matter
Email : [email protected]
Abstract I will review the connection between Josephson junction arrays in a uniform applied magnetic
field and the two-dimensional uniformly frustrated XY model. I will then focus on the particular
case of the fully frustrated XY model which possesses both a continuous U(1) and a discrete
Z(2) symmetry. The interaction between excitations of these two different symmetries leads to
the sequence of phase transitions in which first the U(1) order is lost and then the Z(2) order is
lost. Topological excitations will be shown to play a key role in understanding the sequence of
the transitions. I will then turn to more recent work looking at transitions in the rheological
behaviour of sheared granular fluids.
34
Author
: Valerii M. Vinokour
Affiliation
: Argonne National Laboratory, Lemont
Title
: Topological BKT Phases in Disordered Materials
Abstract The Superconductor-Insulator Transition (SIT) is a quantum phase transition found in
disordered superconducting films and lateral Josephson Junction Arrays (JJA) that occurs at
the point where two inherently two-dimensional topological phase transitions – charge and
vortex Berezinskii-Kosterlitz-Thouless (BKT) transitions terminate each other. These dual
binding-unbinding vortex- and charge BKT transitions lead to the mirror zero-resistance and
zero-conductance low-temperature states. We will report the magnetic field driven SIT in
NbTiN films and will demonstrate that the highly resistive state superinsulating state is an
ordered charge BKT state. We will reveal the divergence of the dielectric constant of the
superinsulator as a function of the magnetic field in approaching the SIT. Our findings
establish BKT physics as a universal platform for the dual superconducting and
superinsulating states. We will show that the BKT transition of strongly disordered
logarithmically interacting topological excitations is accompanied by the Vogel-Fulcher-
Timman (VFT) critical behavior which is much more singular than the usual BKT criticality. We
find that the corresponding charge- and vortex-confined phases of superconductor and
superinsulator are nonergodic.
We propose that recent BKT and VFT criticalities observed in disordered superconducting
films are manifestations of ergodic and nonergodic superinsulators.
Our findings indicate an intimate connection between the BKT physics and many-body
localization.
35
Author
: Yayu Wang
Affiliation
: Tsinghua University
Title
: Tuning Magnetism and Topology in Topological Insulators with Broken
Time Reversal Symmetry
Abstract The interplay between nontrivial topology and broken time reversal symmetry in Topological
Insulator (TI) can lead to exotic quantum phenomenon such as the quantum anomalous Hall
effect. However, there are still many open questions regarding the mechanism of magnetic
order and magneto transport in TI. In this talk, we will present transport studies on
magnetically doped TI thin films grown by molecular beam epitaxy. In Cr doped BiSeTe, we
observe a magnetic quantum phase transition accompanied by the sign reversal of the
anomalous Hall effect induced by Se substitution of Te. ARPES band mapping reveals that the
ferromagnetic order is favoured by the nontrivial bulk band topology, revealing a close
correlation between the magnetism and topology. More recently, we found a gate-tuned
ferromagnetic to paramagnetic phase transition near the topological quantum critical point. We
propose that the most likely mechanism is the Stark effect induced electronic energy level
shift, which causes a topological quantum phase transition followed by magnetic phase
transition. In Mn doped Bi2Te3, we observed that the topological Hall effect characteristic of
magnetic skyrmions, indicating the coexistence of both real space and momentum space
topological structure in magnetic topological insulators.
36
Author
: Zidan Wang
Affiliation
: The University of Hong Kong
Title
: Realizing and Manipulating Topological Metals and Their Exotic Properties
Abstract Symmetry and topology, as the two fundamentally important concepts in physics and
mathematics, have not only manifested themselves in science, but also provided us with
profound understanding of various natural phenomena. Recently, topological gapless systems,
including Z2 topological metals/semimetals [1-3], have attracted significant research interests
both theoretically and experimentally. In this talk, we will report experimental realization and
manipulation of several topological semimetal bands in superconducting quantum circuits as
well as the detection of exotic topological characters in these systems [4][5].
References
[1]. Y. X. Zhao and Z. D. Wang, Phys. Rev. Lett. 110, 240404 (2013).
[2]. Y. X. Zhao and Z. D. Wang, Phys. Rev. Lett. 116, 016401 (2016).
[3]. Y. X. Zhao, A. P. Schnyder, and Z. D. Wang, Phys. Rev. Lett. 116, 156402 (2016).
[4]. X. Tan et al., submitted (2017).
[5]. X. Tan et al., submitted (2017).
37
Author
: Wang Yao
Affiliation
: The University of Hong Kong
Title
: Topological Phenomena in the Moire Pattern of Van Der Waals
Heterostructures
Abstract In monolayer Transition Metal Dichalcogenides (TMDs), a newly emerged class of 2D
semiconductors, the low energy carriers are described by massive Dirac cones located at K
and -K corners of the hexagonal Brillouin zone. These massive Dirac Fermions at K and -K
valleys, being time reversal of each other, have interesting phenomena associated with their
valley index, including the valley optical transition selection rules, valley Hall effects, and valley
magnetic moment, which enable the use of both valley and spin (via spin-valley coupling) as
information carriers in electronics. Van der Waals stacking of 2D semiconductors into
heterostructures further provides a powerful approach towards designer quantum materials
that extend the exotic properties of the building blocks. As a generic aspect of these vdW
heterostructures, the inevitable lattice mismatch always leads to the formation of Moiré pattern
(i.e. periodic variation of local atomic registries). I will show that the vdW Moire can endow
heterostructures unprecedented properties including: (i) electrically switchable lateral
superstructures of topological insulators; (ii) nano-patterned spin optics, and spin-orbit coupled
excitonic superlattices.
38
Author
: Baile Zhang
Affiliation
: Nanyang Technological University
Title
: Some Topological Phases for Sound
Abstract Sound waves travelling through air are essentially longitudinal waves, which carry no intrinsic
spin and do not respond to magnetic fields. However, by constructing gauge fields with
different approaches, it is possible to explore magnetic-like effects for sound. We first design a
phononic crystal which maps theoretically onto the integer quantum Hall effect. Time-reversal
symmetry is broken by a circulating fluid flow in each unit cell, which corresponds to nonzero
periodic effective magnetic flux density. Numerical simulations reveal the existence of
unidirectional acoustic modes at the boundaries of the phononic crystal, which are
topologically protected against backscattering from disorder. We will then introduce a new
approach of constructing acoustic Weyl nodes by stacking one-dimensional dimerised chains
of acoustic resonators. It can be identified that its topological phase corresponds to the
recently predicted type-II Weyl semimetal phase. We will trace out the trajectories of
topological surface states in the k space at the same frequency of a type-II Weyl node, forming
a “Fermi Arc” similar to those observed in condensed matter systems. This approach also
provides the possibility of constructing acoustic topological phases at different dimensions with
the same building blocks.
39
Poster Presenters & Titles
40
Poster Presenters & Titles 1 Elvira Romera (Universidad de Granada)
Identifying Topological-Band Insulator Transitions in Silicene and Other 2D Dirac Materials by Means of RéNyi Entropies
2 Masahiro Sato (Ibaraki University) Ultrafast Generation of Topological Magnetic Defects with Vortex Beams
3 Lewis Williamson (University of Otago) Coarsening Dynamics and Z2 Vortices in an Isotropic Ferromagnetic Superfluid
4 Yunseok Seo (Hanyang University) Holography of Dirac Materials
5 Tian Feng See (National University of Singapore) Diagrammatic Approach to Multiphoton Scattering
6 Jirawat Tangpanitanon (National University of Singapore) Topological Pumping of Photons in Nonlinear Resonator Arrays
7 Mehedi Hasan (Nanyang Technological University) Pushing Topologicity with Light: A Route Towards Topotronics
8 Yongzheng Luo (National University of Singapore) Computational Design of Quantum Spin Hall Insulators through Symmetry Selection
9 Victor Manuel Bastidas Valencia (National University of Singapore) Driven Open Quantum Systems and Floquet Stroboscopic Quantum Simulation
10 Li-kun Shi (Agency for Science, Technology and Research) Large Optical Conductivity of Dirac Semimetal Fermi Arc Surfaces States
11 Eddwi Hasdeo (Agency for Science, Technology and Research) Domain Wall Plasmon in Gapped Bilayer Graphene
12 Shampy Mansha (Nanyang Technological University) Robust Edge States in Amorphous Gyromagnetic Photonic Lattices
41
Poster Presenters & Titles
13 Sanjib Ghosh (National University of Singapore) Signatures of Anderson Transitions in Momentum Space
14 Xiaoming Zhang (Singapore University of Technology and Design) Coexistence of Four-Band Nodal Rings and Triply-Degenerate Nodal Points in Centrosymmetric Metal Diborides
15 Si Li (Singapore University of Technology and Design) Type-II Nodal Loops: Theory and Material Realization
16 Ulrike Bornheimer (National University of Singapore) A SU(3) Topological Insulator in the 2D Honeycomb Lattice
17 Ying Liu (Singapore University of Technology and Design) Artificial Gravity Field, Astrophysical Analogues, and Topological Phase Transitions in Strained Topological Semimetals
18 Sujit Sarkar (Poornaprajna Institute of Scientific Research) Physics of Majorana Modes in Interacting Helical Liquid
42
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