Leon Balents (University of
California, Santa Barbara)
Quantum spin liquid theory for quantum spin ice
Recent experiments show that strong quantum fluctuations may exist
in some spin-ice-like materials. Theory shows that quantum spin
liquid states are possible in this class of Hamiltonians. We will
review the
microscopic theory for such quantum spin liquids, and then discuss
the characteristics of the quantum spin liquid and nearby states,
and report on the evolution of the ground states with increasing
temperature.
Collin Broholm (Johns Hopkins Univ)
Incommensurate correlations & mesoscopic spin resonance in
YbRh2Si2
Andrea Damascelli (Univ of British
Columbia)
From p-wave superconductors to relativistic-Mott insulators via spin-orbit
interaction in solids
Spin-orbit coupling is essential to the quantum-mechanical description
of atomic energy levels. Yet its most spectacular consequences are
found in the low-energy electronic structure of solids, where this
atomic-like interaction plays a key role in the emergence of some
of the most unconventional quantum phenomena. In this talk I will
show how spin and angle-resolved photoemission spectroscopy, in
combination with in-situ doping techniques, can be used to unveil
the role of spin-orbit interaction in the emergence of p-wave superconductivity
in Sr2RuO4 [1,2] and relativistic Mott insulating behavior in Na2IrO3
[3].
[1] M.W. Haverkort et al., Phys. Rev. Lett. 101, 026406 (2008).
[2] C.N. Veenstra et al., in preparation (2012).
[3] R. Comin et al., arXiv:1204.4471 (2012).
Marcel Franz (University of British Columbia)
Lattice model for the surface states of a topological insulator
A surface of a strong topological insulator (STI) is characterized
by an odd number of linearly dispersing gapless electronic surface
states. It is well known that such a surface cannot be described
by an effective two-dimensional lattice model (without breaking
the time-reversal symmetry), which often hampers theoretical efforts
to quantitatively understand some of the properties of such surfaces,
including the effect of strong disorder, interactions and various
symmetry-breaking instabilities. Here I describe a lattice model
that can be used to describe a pair of STI surfaces and has an odd
number of Dirac fermion states with wavefunctions localized on each
surface. The Hamiltonian consists of two planar tight-binding models
with spin-orbit coupling, representing the two surfaces, weakly
coupled to each other by terms that remove the redundant Dirac points
from the low-energy spectrum. The utility of this model is illustrated
by studying the magnetic and exciton instabilities of the STI surface
state driven by short-range repulsive interactions.
Bruce Gaulin (McMaster University)
Effective Spin 1/2 Hamiltonians in the Pyrochlore Magnets Er2Ti2O7
and Yb2Ti2O7
New neutron scattering instrumentation offers unprecedented opportunities
for mapping out the full dispersion and dynamic susceptibility of
magnetic materials. In turn, these measurements can be exploited
to determine their microscopic spin Hamiltonians in great detail.
We've used these techniques to examine two pyrochlore magnets with
unusual and exotic ground states, Er2Ti2O7 and Yb2Ti2O7. These materials
are both known to display anisotropic g-tensors with XY anisotropy,
yet their ground state properties are very different. Collaborative
work with Lucile Savary and Leon Balents has modelled our spin wave
data in terms of an anisotropic exchange Hamiltonian on the pyrochlore
lattice. As a result we can understand Er2Ti2O7's ordered ground
state on the basis of selection by an order-by-quantum-disorder
mechanism[1], while Yb2Ti2O7's ground state is shown to be in reasonably
close proximity to spin liquid and other exotic ground states[2].
[1] K.A. Ross, L. Savary, B.D. Gaulin and L. Balents, Phys. Rev
X, 1, 021022, 2011.
[2] L. Savary, K.A. Ross, B.D. Gaulin, J.P.C. Ruff, and L. Balents,
arXiv:1204.1320 and to appear, Phys. Rev. Lett.
Michael Gingras (University of Waterloo)
The Tb2Ti2O7 Pyrochlore Antiferromagnet: the Platypus of Frustrated
Magnetic Systems
The Tb2Ti2O7 pyrochlore antiferromagnet was first found to lack long
range order down to 50 mK at about the same time as spin ice behavior
was reported in the Ho2Ti2O7 compound. Thirteen years later, despite
numerous experimental studies and theoretical attempts, we still do
not have an understanding of what is the real nature of the low temperature
state of this material. I will review in this talk the salients facts,
some agreed upon by a most researchers and some not, regarding the
nature of the low-temperature state of Tb2Ti2O7.
Harold Hwang (Stanford)
Magnetism between nonmagnetic insulators
One of the aspirations of the study of complex oxide heterostructures
is the creation of interface states without a bulk analog. An interesting
example is the ferromagnetism which emerges at the LaAlO3/SrTiO3
interface. We will report our studies of the magnetism found here
in terms of spatial variations in the plane, stability, and spectroscopy.
Takashi Imai (Department of Physics and
Astronomy, McMaster University, Hamilton, and Canadian Institute for
Advanced Research)
NMR Search for the Spin Nematic State in LaFeAsO
The mechanism of high Tc superconductivity in iron-pnictides remains
controversial. While earlier NMR measurements provide ample evidence
for the enhancement of spin fluctuations near the optimized superconducting
transition [1], the softening of spins is accompanied by that of
the lattice [2]. A theoretical analysis of the magnetic properties
of the FeAs planes based on the frustrated J1-J2 model suggests
that the SDW (spin density wave) transition may be highly unconventional
due to the strong magneto-elastic coupling between spins and the
lattice [3]. Moreover, the intermediate temperature range between
the tetragonal-orthorhombic structural phase transition at TTO and
the SDW transition at TSDW may be a realization of the spin nematic
state [4].
NMR is an ideal probe to investigate these effects, thanks to its
sensitivity to both the spin and lattice degrees of freedom. Moreover,
the angle-dependent measurements of the spin-lattice relaxation
rate 1/T1 has been proposed to be an effective probe of the spin
nematic phase [5]. In this talk, we report a 75As single crystal
NMR investigation of LaFeAsO, the parent phase of a pnictide high
Tc superconductor [6]. We demonstrate that spin dynamics develop
a strong two-fold anisotropy within each orthorhombic domain in
the orthorhombic phase below TTO~156 K, prior to the SDW transition
at TSDW ~ 142K. This intermediate state with a dynamical breaking
of the rotational symmetry freezes progressively into a SDW state
below TSDW ~ 142 K. Our findings are consistent with the presence
of a spin nematic state below TTO with an incipient magnetic order.
The work at McMaster was supported by NSERC and CIFAR.
[1] F. L. Ning, T.I. et al., Phys. Rev. Lett. 104, 037001 (2010).
[2] R. M. Fernandes et al., Phys. Rev. Lett. 105, 157003 (2010).
[3] C. Xu, M. Müller, and S. Sachdev, Phys. Rev. B 78, 020501R,
(2008).
[4] C. Fang, W. F. Tsao, J. P. Hu, and S. A. Kivelson, Phys. Rev.
B 77, 224509 (2008).
[5] A. Smerald and N. Shannon, Phys. Rev. B 84, 184437 (2011).
[6] M. Fu, D. A. Torchetti, T. Imai, F. L. Ning, J.-Q. Yan, and
A. S. Sefat, arXiv:1208.5652.
Catherine Kallin (McMaster University)
Anomalous Hall effect in chiral superconductors and density wave
states
The polar Kerr effect is a sensitive and direct probe of broken
time-reversal symmetry. A non-zero Kerr effect, which implies an
anomalous Hall effect, has been observed in the superconducting
state of Sr2RuO4 (SRO) as well as in the pseudogap phase of several
cuprate materials. Here, we discuss recent theoretical work on the
anomalous Hall effect in clean chiral superconductors, which suggests
new experiments that might identify the nature of the superconductivity,
as well as which bands are primarily active, in SRO. The anomalous
Hall conductivity of non-superconducting chiral density wave states,
e.g., chiral d-density wave order, will also be discussed and contrasted
to the superconducting case.
Sung-Sik Lee (Perimeter Institute/McMaster)
Chiral non-Fermi liquids in two dimensions
We propose a renormalization group scheme which is suitable for
theories with Fermi surface. Low energy modes near the Fermi surface
are viewed as a collection of one dimensional fermions with a
continuous flavour labelling the momentum along the Fermi surface.
Based on this approach, we provide a non-perturbative argument for
the stability of the chiral non-Fermi Liquid states where one patch
of
Fermi surface is coupled with a gapless boson in two dimensions.
We point out that the validity (or breakdown) of local patch description
in momentum space is closely related to a phenomenon known as the
UV/IR mixing where infrared singularity is controlled by a UV cut-off
scale. Finally, we will suggest a possible experimental realization
ofa chiral non-Fermi liquid state.
Young S. Lee (MIT)
Experimental signatures of spin liquid physics on the S=1/2 kagome
lattice
Materials based on the kagome lattice appear to be ideal hosts
for the possibility of a quantum spin liquid ground state in two-dimensions.
I will discuss our 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).
Recent susceptibility measurements yield valuable information on
the additional terms in the spin Hamiltonian beyond nearest neighbor
Heisenberg exchange, and anomalous x-ray diffraction yields detailed
information on the presence of a small amount of atomic impurities.
Most interestingly, inelastic neutron scattering measurements of
the spin correlations in a single crystal sample reveal a continuum
of spinon excitations in this two-dimensional insulating magnet.
Such fractionalized excitations are a long-sought hallmark of the
quantum spin liquid.
Graeme Luke (McMaster University)
Ground State and Excitations in Spin Ice and Quantum Spin Ice
Magnetic rare earth pyrochlore systems exhibit a rich variety of
phenomena arising from geometrical frustration of their magnetic
interactions. Dy2Ti2O7 and Ho2Ti2O7 have been identified as dipolar
spin ices, where the combination of ferromagnetic interactions and
crystal field scheme leads to a ground state consisting of two spins
pointing in and two spins pointing out of each tetrahedral unit
in the crystal structure, in analogy to water ice where each oxygen
has two closely bound and two weakly bound hydrogen neighbours.
Yb2Ti2O7 has been proposed to be an example of a related quantum
spin ice state, though the precise determination of the ground state
remains controversial.
I will present muon spin relaxation measurements of Dy2Ti2O7 where
we find that the ground state exhibits a form of persistent spin
dynamics to low temperature which exist in addition to any proposed
emergent magnetic monopoles. In the case of Yb2Ti2O7, we find that
the spins remain dynamic to the lowest temperature, though there
is a distinct change in the local spin susceptibility in both single
crystal and polycrystalline samples at the temperature where specific
heat measurements identify a phase transition. Our results show
that there are no static magnetic moments in Yb2Ti2O7.
Tae Won Noh (IBS Center for Functional Interfaces
of Correlated Electron System, & Dept of Physics and Astronomy,
Seoul National University)
Electronic band structures of LaNiO3 ultrathin films in-situ studied
by angle resolved photoemission spectroscopy
Bulk LaNiO3 (R: rare earth) is metal and has d7 electronic configuration
with fully occupied t2g and partially filled eg electrons, similar
to the cuprates. Recent theoretical calculations predicted the cuprate-like
eg orbital reconstruction and a gap opening in LaNiO3/LaAlO3 heterostructure.[1,2]
To confirm these theoretical predictions, the orbital characters in
LaNiO3/LaAlO3 heterostructure have been studied by using synchrotron-based
x-ray measurements.[3-5] However, fundamental understandings on the
eg orbital reconstruction are still elusive due to the absence of
direct band structure measurements.
Recently, we performed in-situ angle-resolved photoemission spectroscopy
(ARPES) studies on LaNiO3 ultrathin films. (1) By using LaAlO3, NdGaO3,
and SrTiO3 substrates, we could obtain LaNiO3 films which were under
compressive, nearly free, and tensile strains, respectively. The measured
electronic band structure of the LaNiO3 film on NdGaO3 substrate can
be explained by the DMFT calculations (not by the LDA calculations),
indicating the importance of the correlation effects. In addition,
the electronic band structure of the LaNiO3 film under compressive
strain can be explained in terms of octahedral elongation. On the
other hand, under tensile stress, we found that the octahedral elongation
is not enough to explain the electronic structure. (2) We also deposited
LaNiO3 ultrathin films on SrTiO3 substrates with thickness between
1 and 5 unit cells (u.c.) We observed that the dimensional crossover
of the band structure occurs between 4 u.c. to 3 u.c. of LaNiO3 film.
Contrary to earlier theoretical works, our LaNiO3 ultrathin film has
a Fermi surface down to 1 u.c.. Compared to 3 dimensional LaNiO3 thick
films, we found that the electronic correlation effect was getting
weaker in 2 dimensional LaNiO3 ultrathin films. Further details will
be discussed more in presentation.
1. Jir?i´ Chaloupka et al., Phys. Rev. Lett. 100, 016404 (2008).
2. P. Hansmann et al., Phys. Rev. Lett. 103, 016401 (2009).
3. Eva Benckiser et al., Nature Mater. 10, 189 (2011).
4. Jian Liu et al., Phys. Rev. B 83, 161102(R) (2011).
5. J. Chakhalian et al., Phys. Rev. Lett. 107, 116805 (2011).
Je-Geun Park (Seoul National University)
Spin dynamics of multiferroic BiFeO3 and unusual low energy features
1 IBS Center for Functional Interfaces of Correlated Electron Systems,
Seoul National University, Seoul 151-742, Korea
2 Department of Physics & Astronomy, Seoul National University,
Seoul 151-742, Korea
3 Center for Strongly Correlated Materials Research, Seoul National
University, Seoul, 151-742, Korea
Multiferroic materials having a coexistence of otherwise seemingly
incompatible phases of magnetic and ferroelectric ground states
have been the focus of intensive materials researches recently.
BiFeO$_3$ is arguably one of the most interesting multiferroic materials
with both magnetic and ferroelectric transitions occurring above
room temperature: T$\rm{_N}$=650 K and T$\rm{_C}$=1050 K. Moreover,
it has an unusual incommensurate magnetic transition with an extremely
long period of 650 $\stackrel{\circ}{\text{A}}$.
By using 10 single crystals co-aligned within $3^{\circ}$ of one
another, we have recently measured the spin waves of the antiferromagnetic
phase at two state-of-the-art inelastic neutron scattering instruments:
one is AMATERA of J-PARC and another MERLIN of ISIS. These two experiments
allowed us to map out the full dispersion curve, for the first time,
over the entire Brillouin zones and succeeded in determining the
essential magnetic exchange interactions: two antiferromagnetic
interactions and one Dzyaloshinskii-Moriya term.
We have further investigated the low energy spin waves using a cold
TAS of LLB to find that there exists unusual spin dynamics near
the zone center. We will also discuss our latest experimental studies
using high-resolution neutron and synchrotron diffraction studies
as well as high field studies to find that there are clear anomalies
in the temperature dependence of lattice constants. We will put
our findings in a broader context of this interesting material.
George A. Sawatzky (Physics Dept and
Max Planck-UBC Centre for Quantum Materials, University of British
Columbia)
New Magnetic Materials Based on Defects, Interfaces and Doping
This is a joint work with
Ilya Elfimov, Bayo Lau, Mirko Moeller
and Mona Berciu
Ideas based on theory and some experiments will be presented regarding
possible new magnetic materials based on extended and point defects
(1), interface engineering (2), anion substitution in oxides and hole
and electron doping of oxides. The concentration will be on rather
ionic oxides mostly not involving conventional magnetic elements.
Special attention will also be placed on surface and interface effects
involving polar surfaces as well as on the role of doped holes in
O 2p in charge transfer gap oxides. O 2p holes play an extremely important
role in the magnetism and superconductivity of oxides and new results
will be presented regarding the ferromagnetic exchange coupling they
introduce in transition metal oxides(3). They are also important in
describing the interplay between transport properties, magnetic order
and the general phase diagrams of materials involving O2p holes either
in the so called self doped case of stochiometric oxides like CrO2,
in chemically substituted systems, and for cation or anion vacancies.
We also present exact results on the spin polaron formation(4,5) and
charge propagation of doped Fermions in Ferromagnetic lattices and
the pairing interaction due to the magnetic background.
Some relevant publications:
1. I. S. Elfimov, S. Yunoki, and G. A. Sawatzky PRL 89, 216403, (2002)
2. N. Pavlenko, T. Kopp, E.Y. Tsymbal, G.A. Sawatzky, and J. Mannhart
PRB 85, 020407, (2012)
3. Bayo Lau, Mona Berciu and George A. Sawatzky, PRL 106, 036401 (2011)
4. Mona Berciu and George A Sawatzky, PRB 79, 195116 (2009)
5. Mirko Moeller, George A. Sawatzky and Mona Berciu PRL 108,216403
(2012) and PRB 86, 075128 (2012)
Senthill Todadri (McMaster University)
Quantum melting of stripes
Abstract: We describe a theory of continuous stripe melting quantum
phase transitions in two-dimensional metals and the associated Fermi
surface reconstruction. Such phase transitions are strongly coupled
but yet theoretically tractable in situations where the stripe ordering
is destroyed by proliferating doubled dislocations of the charge
stripe order. The resulting non-Landau quantum critical point
has strong stripe fluctuations which we show decouple dynamically
from the Fermi surface even though static stripe ordering reconstructs
the Fermi surface. We discuss connections to various stripe phenomena
in the cuprates. We point out several puzzling aspects of old experimental
results [G. Aeppli et al., Science 278 1432 (1997)] on singular
stripe fluctuations in the cuprates, and provide a possible
explanation within our theory. These results may thus have been
the first observation of non-Landau quantum criticality in an experiment.
Xiao-Gang Wen (Perimeter/MIT)
Symmetry protected topological/trivial (SPT) phases
SPT phases are a new kind of phases at zero-temperature that have
a symmetry and a finite energy gap. The SPT phases have the following
defining properties: (a) distinct SPT phases with a given symmetry
cannot smoothly deform into each other without phase transition,
if the deformation preserve the symmetry. (b) however, they all
can smoothly deform into the same trivial product state without
phase transition, if we break the symmetry during deformation.
Using the notion of quantum entanglement, we can say that SPT states
are short-range entangled states with a symmetry. Haldane phase
of spin-1 chain and topological insulators are examples of SPT phases.
We will discuss a classification, as well as some examples of the
SPT phases in frustrated magnets.
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