August 22, 2014 Fermion space charge in narrowband gap semiconductors, Weyl semimetals and around highly charged nuclei Prof. Eugene B. Kolomeisky
University of Virginia
TBA 
August 28, 2014
TBA
Prof. Predrag Nikolic
George Mason University
TBA 
September 2, 2014
Quantum Engineering Robust 2D Topological Insulator in InAs/GaSb Bilayers
Prof. RuiRui Du
Rice University

September 8, 2014
Rare fluctuation effects in disordered Dirac fermion systems
Dr. Rahul M. Nandkishore
Princeton Center for Theoretical Science Abstract:
The traditional theory of disordered systems is based on concepts such
as nonlinear sigma models and Born approximations. However, the usual
implementations of these ideas ignore the contributions of rare regions
(`quantum Griffiths effects’), which can dominate the behavior of
disordered systems of Dirac fermions. I illustrate this physics with a
discussion of two sets of phenomena that are dominated by rare regions:
superconductivity on the surface of a topological insulator, and
transport near charge neutrality in a three dimensional Dirac
semimetal. In both cases, I explain how traditional analyses yield
qualitatively incorrect answers, and how the physics is dominated by
rare region effects. I also explain how sigma model based analyses may
be modified to incorporate the effect of rare regions. References: Phys. Rev. B 87, 174511 (2013), Phys. Rev. B 89, 245110 (2014), arXiv: 1407.4830 
Novermber 17, 2014
NONABELIAN EXCITATIONS IN A 2 DIMENSIONAL ELECTRON SYSTEM Dr. Robert Willett
Bell Labs
Abstract:
Correlation of charges in twodimensional electron systems can induce
numerous novel physical properties, including fractional charge,
fractional statistics, and transitions between statistical classes.
Perhaps the most unique of the properties within this system is the
predicted demonstration of nonAbelian statistics by a specific set of
excitations. In this talk we will review these concepts from an
experimental perspective, and present charge interference measurements
that demonstrate distinct nonAbelian effects. 
Novermber 18, 2014
SPINORBIT TUNED GROUND STATES IN SINGLECRYSTAL IRIDATESProf. Gang Cao University of Kentucky Abstract:
The iridates have become a fertile ground for studies of new physics
driven by spinorbit coupling (SOC) that is comparable to the onsite
Coulomb and crystalline electric field interactions. This unique
circumstance creates a delicate balance between interactions that
drives complex magnetic and dielectric behaviors and exotic states
seldom or never seen in other materials. A profound manifestation of
this competition is the novel “Jeff = 1/2 Mott state” that was observed
in the layered iridates with tetravalent Ir4+(5d5) ions. On the other
hand, very little attention has been drawn to iridates having
pentavalent Ir5+(5d4) ions, primarily because the strong SOC limit is
expected to impose a nonmagnetic singlet ground state (Jeff = 0). In
this talk, we review the underlying physical properties of the iridates
including perovskites, honeycomb lattices and double perovskites with
pentavalent Ir5+ ions, and report results of our recent studies that
emphasize spinorbittuned ground states stabilized by chemical doping,
application of pressure and magnetic field. In addition, we address the
urgent question that the Jeff states may not survive in the presence of
strong noncubic crystal fields and/or exchange interactions. 
November 19, 2014
Valley Zeeman effect in a twodimensional semiconductor
Dr. Ajit Srivastava
ETH Zürich Abstract:
A monolayer of transition metal dichalcogenide (TMD) such as WSe2 is an
atomically thin direct bandgap semiconductor with a honeycomb lattice
which breaks inversion symmetry. The existence of two inequivalent
minima or valleys in the Brillouin zone represents a pseudospin degree
of freedom which can be optically addressed using circularly polarized
light. Furthermore, due to timereversal symmetry but broken inversion
symmetry, there is equal but opposite Berry curvature and a closely
related quantity called intercellular orbital magnetic moment, in the
two valleys. This unique contribution to the total magnetic
moment can be thought to arise from the selfrotation of a wavepacket
constructed from the Bloch states of a band and points outofplane for
a twodimensional material. In this talk, I will present our
recent results providing evidence for the observation of such an
orbital magnetic moment in monolayer WSe2. We perform magnetooptical
spectroscopy and observe a splitting of the exciton peak corresponding
to a magnetic moment of ~ 4.3 Bohr magneton only in a magnetic field
perpendicular to the sample. The intercellular orbital magnetic moment
calculated from a particlehole asymmetric tightbinding Hamiltonian
agrees well with the experimental value when strong
excitonic interactions are taken into account. On the other hand,
the magnetic moment of the trion is found to be anomalously large in
comparison with exciton which we attribute to the large
exchangeinduced Berry curvature of trions. As an outlook, I will
discuss the possibility of using stronglight matter interactions in
TMDs to tune the trion Berry curvature by forming cavity
trionpolaritons. 
Straininduced partially flat band, helical snake states and interface superconductivity in topological crystalline insulators
Dr. Evelyn Tang
MIT 
Correlation effects on doubleWeyl semimetals
Dr. HsinHua Lai
NHMFL 
Surface and vortex states in topological nodal superconductors
Dr. PoYao Chang
U. Illinois UrbanaChampaign 
Unconventional quantum phase transition in a quantum dimer model on the kagome lattice
Dr. Zhihao Hao
U. Waterloo 
Nanotube photonics
Dr. Baratunde Cola
Georgia Tech 
March 9, 2015
Quantum spin ice
Prof. Nic Shannon Okinawa Institute of Science and Technology Abstract:
Spin ice, with its magnetic monopole excitations, is perhaps the
outstanding example a classical, topological spin liquid. Nonetheless,
the role of quantum effects in spinice materials remains poorly
understood. This question gain fresh urgency from studies of "quantum
spinice" materials such as Yb2Ti2O7 [1,2] and Pr2Zr2O7 [3], and recent
experiments which suggest that the spin ice Dy2Ti2O7 may undergo a
phase transition at very low temperature [4]. In this talk, we
explore some of the new phenomena which can arise as a result of
quantum fluctuations in a spinice material. We show how quantum
tunnelling between different spinice configurations can convert
spinice into a quantum spin liquid with photonlike excitations [5],
review the numerical evidence that such a state exists [69], and
discuss how it might be identified in experiment [8,9]. We also
consider the nature of the quantum ground state in a realistic model of
spin ice, directly motivated by Dy2Ti2O7. We identify the principles
which govern magnetic order in the presence of longrange dipolar
interactions, and use quantum Monte Carlo simulation to show that only
a very small amount of quantum tunnelling is needed to convert these
ordered states into a quantum spin liquid [10]. [1] K. Ross et
al., Phys. Rev. X 1, 021002 (2012). [2] L.J. Chang et al., Nature
Commun. 3, 992 (2012) [3] K. Kimura et al., Nature Commun. 4, 1934
(2013) [4] D. Pomaranski et al., Nature Phys. 9, 353 (2013). [5] M.
Hermele et al., Phys. Rev. B 69, 064404 (2004). [6] A. Banerjee et al.,
Phys. Rev. Lett. 100, 047208 (2008) [7] N. Shannon et al., Phys. Rev.
Lett. 108, 067204 (2012). [8] O. Benton et al., Phys. Rev. B 86, 075154
(2012). [9] Y. Kato et al., arXiv:1411.1918 [10] P. McClarty et al.,
arXiv:1410.0451 
March 16, 2015
ENHANCEMENT OF SUPERCONDUCTIVITY NEAR A NEMATIC QUANTUM CRITICAL POINTProf. Steven Kivelson Stanford University Abstract: Two
topics that have attracted intense theoretical study over the past
decade are the nature of quantum critical phenomena in metallic systems
and what, if anything, such critical points have to do with an
unconventional mechanism of superconducting pairing. The still
unmastered subtleties of the first problem have precluded convincing
resolution of the second. For the model problem of a weakly interacting
metal in proximity to a nematic quantum critical point (NQCP), we
identify a broad regime of parameters in which the nature of the
induced superconductivity can be understood in a theoretically well
controlled manner without needing to resolve the deep, unsolved issues
of metallic criticality. We show that: 1) a BCSEliashberg treatment
remains valid outside of a parametrically narrow interval about the
NQCP; 2) the symmetry of the superconducting state (dwave, swave,
pwave) is typically determined by the noncritical interactions, but
Tc is enhanced by the nematic fluctuations in all channels; 3) in 2D,
this enhancement grows upon approach to criticality up to the point at
which the weak coupling approach breaksdown, but in 3D the enhancement
is much weaker. 
March 17, 2015
SUPERCONDUCTORINSULATOR TRANSITION IN (Li,Fe)OHFeSe AND ENHANCED SUPERCONDUCTIVITY BETWEEN BaFe2As2 and CsFe2As2
Prof. Xianhui Chen University of Science and Technology of China Abstract:
In this talk, we present a firstorder transition from superconductor
to insulator with a strong charge doping induced by ionic gating in the
thin flakes of single crystal (Li,Fe)OHFeSe, and a novel phase diagram
of temperaturegating voltage with the superconductorinsulator
transition is mapped out. The most intriguing feature is the appearance
of an insulating phase immediately adjacent to the optimal
superconductivity, which is reported for the first time in ironbased
superconductors. Similar phase diagrams have been observed in the
twodimension organic superconductors and Cs3C60. This phase diagram of
FeSederived superconductors also bears resemblance with that of
highTccuprate superconductors. The similarity of these phase diagrams
transcends the diversity of various unconventional superconducting
materials, suggesting that all of them share a universal mechanism in
superconductivity. Moreover, our work suggests that the gatecontrolled
strong charge doping is a very powerful practice for the exploration of
novel states of matter that cannot be realized using traditional
methods. Interface superconductivity offers an alternative avenue,
different from bulk doping or external pressure, to convert a
nonsuperconducting material into a superconductor and also to explore
the physics of highTc superconductivity. The reduced dimensionality at
the interface can amplify quantum fluctuations and induce or enhance
underlying interactions, which may result in unexpected physical
properties, though the exact nature could be elusive. Here, we report
on a superconductivity with transition temperature (Tc) up to 30 K in
BaFe2As2/CsFe2As2 intergrowth crystals, where BaFe2As2 is an
antiferromagnetic metal and CsFe2As2 possesses a very low Tc of 1.8 K.
Combining structural, transport and magnetic measurements, we found
that the unexpected high transition temperature arises from the
interface between BaFe2As2 and CsFe2As2. Our finding provides a new
platform to understand the mechanism of interface superconductivity,
and also indicate that the interface superconductivity could be an
important way to achieve highTc superconductivity in FeAsbased
superconductor. 
March 24, 2015
Quantum Criticality and Unconventional Properties of Heavy Fermion Superconductor Ce1xYbxCoIn5
Dr. Yogesh Singh Kent State University Abstract:
Ce1xYbxCoIn5 is a unique member of the heavy fermion family
Ce1xRxCoIn5 where ‘R’ is a rare earth. Many of the properties observed
with Yb substitution on Ce site are highly unusual. Due the
intermediate valence nature of Yb, many of the properties of this
system appear to be influenced by Yb. In this presentation, I will talk
about some of these properties and also will discuss my experimental
findings for this material. A relatively small Yb substitution
drastically decreases quantum critical point (HQCP) of CeCoIn5 and
brings it to 0 T for a very small impurity concentration. On the other
hand superconducting transition temperature (Tc) and Kondo coherence
temperature (Tcoh) are robust and survive over the whole Yb doping
range. These results imply that superconductivity and quantum
criticality are decoupled in this system, i.e., unconventional
superconductivity is not triggered by spin fluctuations. We also find a
scaling of the normalized Tcoh and normalized Tc which suggests that
the onset of manybody coherence and emergence of superconductivity
have same physical origin: hybridization between conduction and
localized felectron states. The robustness of Tcoh and Tc with respect
to disorder for Yb compared with the other rareearth substituents is
consistent with Yb atoms forming a cooperative mixedvalence state that
significantly reduces the pairbreaking effects.

April 27, 2015
Anomalous Hall effect in graphene Prof. Bruno Uchoa University of Oklahoma When
graphene is supported on hexagonal boron nitride, the local potentials
of boron and nitrogen atoms locally break sublattice symmetry, giving
rise to a local mass term in the free Dirac Hamiltonian of graphene.
Since the two crystal lattices have a small mismatch, the mass term
modulates in real space and forms a Moire pattern with a unit cell as
large as a hundred lattice sites. The zero mass lines where the mass
term changes sign form topological domain walls containing low energy modes, similarly to edge states in the quantum Hall effect. In
this talk, I will examine the role of Coulomb interactions in the
emergence of macroscopically ordered states in this system. In
the presence of these modes, I will show that Coulomb interactions
lead to spontaneous formation of chiral loop currents in bulk and
to macroscopic spinvalley order
at zero temperature. This exotic order describes an anomalous Hall
state, which can be detected with interferometry and polar Kerr
measurements. 
July 13, 2015
Nematic quantum criticality in a 2+1dimensional metalDr. Samuel Lederer
Stanford University Evidence
continues to mount that quantum critical points lie beneath the
superconducting “dome” of both the ironbased and cuprate high
temperature superconductors. Further, these putative critical points
appear intimately related to anomalous properties of both the
superconducting and normal (metallic) states of these materials. We
consider a model for a quantum critical point in a 2+1dimensional
metal with an Ising nematic order parameter, which captures the
spontaneous breaking of fourfold rotational symmetry known to occur in
the above materials. We simulate this model using numerically exact,
signproblemfree Quantum Monte Carlo techniques, and find evidence of
a new strongly coupled fixed point with nontrivial scaling exponents.
At this fixed point, the fermions are strongly, perhaps singularly
renormalized, consistent with the expected breakdown of Fermi liquid
theory. 