
Quantum Materials Summer School
May 79, 2007

Summer School Lecturers (click to expand abstract)
Jess Brewer (UBC):
Topics in muSR


Three part lecture series by:
Joshua Folk (UBC):
Measurement of single spins in solid state systems

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The first demonstration of electron spin detection in solid state
materials came with the advent of electron spin resonance in the
1940's. Electron spin resonance (ESR) is an ensemble
measurement, sensitive to the minute magnetic moment of an electron
spin only when integrated over 10^9 or more electrons. Over the past
5 years, a variety of experimental techniques have been developed
that push the detection limit down to the single electron level.
These efforts have been motivated in part by aspirations of quantum
information processing, and in part by a more fundamental interest in
spin decoherence mechanisms and interactions with the spin degree of
freedom in a solid state material. This series of seminars will
present an overview of these developments, their motivations, and an
outlook for the future.
Paul Haljan (SFU): Quantum Simulation with Trapped Ions

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Trapped ions are a promising experimental platform for preparing large
amounts of quantum entanglement, eventually on a scale large enough for
quantum computing applications. The laser and iontrap technologies which
have been developed so far offer the opportunity to apply them to more
specialized and potentially less demanding applications in the short term.
One of these applications involves using trapped ions and tailored
laserinduced forces between them to perform quantum simulations of
designer Hamiltonians on the scale of tens of ions. Laseraddressed arrays
of ions are naturally adept at simulating networks of interacting spins,
which can exhibit quantum magnetism analogous to familiar condensed matter
models. The ultimate goal would be to reveal quantum fluctuations,
correlations, and dynamics in a quantum magnet, all at the level of single
atoms; measurements in the short term on small numbers of ions will
provide
a specific experiment framework with which to address several questions,
including what the technical and fundamental limitations are to iontrap
simulation.
Andrew Hines (UBC):
Introduction to Quantum Computation

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A quantum computer aspires to perform computations that a "classical"
computer can only perform very poorly, requiring exponentially large
memory or processing time. Tasks like factoring (Shor), database searching
(Grover) and efficiently simulating manybody quantum systems, are
examples of problems where quantum algorithms are known and are faster
than the best (known) classical ones. However, realizing quantum
information processing in the lab is extremely difficult. It requires two
almost mutually exclusive conditions  weak coupling to the environment
for low decoherence, along with strong coupling to the user for precise
control and measurement. In this lecture I will introduce the concept of
quantum information processing, before focusing on its implementation 
the requirements, the obstacles and the (possible) solutions.
Gil Lonzarich (Cambridge):
Quantum Phase Transitions


Andrew Macfarlene (UBC):
What Can Magnetic Resonance Radiotracers Tell Us About Condensed
Matter?


Babak Seradjeh (UBC):
Anyons in a weakly interacting condensedmatter system

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Anyons are particles with exchange phase other than that of fermions (pi)
or bosons (2pi). This intriguing property is only possible intwo spatial
dimensions. A simple physical picture, due to Wilczek, of such a particle
is a composite of charge and magnetic flux. Anyons were proposed to exist
as the excitations of Laughlin's strongly interacting fractional quantum
Hall states and were recently claimed to have been observed
experimentally. The search for other condensed matter systems exhibiting
these exotic particles is ongoing. In this talk I will review the relevant
concepts and describe our recent theoretical proposal for a system whose
excitations are anyons that, remarkably, can be built by filling a set of
singleparticle states of essentially noninteracting electrons. The system
consists of an artificially structured typeII superconducting film
adjacent to a 2D electron gas (2DEG) in the integer quantum Hall regime
with unit filling fraction. A vacancy in an otherwise periodic vortex
lattice in the superconductor creates a bound state in the 2DEG with total
charge e/2. The composite of this fractionally charged hole and the
missing flux due to the vacancy is an almost literal realization of
Wilczek's picture of an anyon with exchange phase pi/4.
In collaboration with Marcel Franz, Conan Weeks, and Gili Rosenberg
Reference: condmat/0703001 and references therein.
Three part lecture series by:
Philip Stamp (UBC):
Topics in Quantum Magnetism

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 Some basic ideas and techniques in quantum magnetism
 Quantum Spin glasses and spin liquids
 Largescale quantum phenomena in spin systems
C. C. Tsuei (IBM Watson):
Determination
and consequences of dwave pairing symmetry in cuprate
superconductors:
the road to dwave and beyond

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The recent development of phasesensitive techniques has finally settled
the decade  long dwave versus swave debate in favor of an order
parameter with d_{ x2 y2 } symmetry
in various cuprate superconductors. We will briefly review the experiments
using the halffluxquantum effect as a definitive signature for dwave
pairing symmetry. In particular, a series of recent tricrystal experiments
will be presented to demonstrate that the dwave pair state in this class
of superconductors is robust against time reversal symmetry breaking, and
a widerange of temperature and doping variations. We will discuss the
consequences of establishing the dwave pair state for the nature of the
superconducting ground state and its low energy excitations in high
temperature superconductors. Also will be addressed is the question of
whether unconventional (dwave) superconductivity (pairing mechanism,
isotope effect, nmscale charge inhomogeneity  ) can be understood in
terms of conventional wisdom, the Fermiliquid based BCS
theory.
Carl Wieman (UBC, 2001 Nobel Laureate):
Introduction to BEC in dilute gases and Feshbach resonances


Student talks
(some with accompanying posters) in order of presentation
Mangala Singh (Université de Sherbrooke):
Magnetic and magnetodielectric properties of PLDgrown
La_{2}CoMnO_{6} films

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Presence of
multifunctional coupling in a material offers new set of physics and
enables to design novel devices. This has prompted us to investigate
the growth and properties of wellordered La_{2}CoMnO_{6}
films on SrTiO_{3}. The Bsite ordering is achieved by growing
the films at relatively high temperatures (~ 800 ^{o}C) and
O_{2} pressures (~ 600 mTorr). We present the structural and
magnetic properties of these films. Despite their single ferromagnetic
transition around 240 K, we demonstrate that they possess a bidomain
structure with distinct magnetic characteristics. Films exhibit a maximum
5.8 µ_{B}/f.u. saturation magnetization and magnetic easy axis
parallel to SrTiO_{3} (110). Further films display a nonlinear
magnetodielectric effect at low temperature. The
origin of the bidomain structure and magnetodielectric effect is briefly
discussed.
Supported by NSERC, FQRNT, CIAR and
CFI.
Stéphane Savard (Université de Sherbrooke):
Terahertz Superconducting Antennas

[]

Radiation properties
of superconducting terahertz antennas are related to relaxation mechanisms
of electronic excitations. Past studies focused mainly on modifying
their configurations to improve performance. Looking for the influence
of the structural properties of these films on radiation, we compare
the characteristics of YBa_{2}Cu_{3}O_{7d}
antennas made on LaAlO_{3} and MgO substrates. In order to get
only the intrinsic YBCO properties with no geometrical dependence, we
have explored several geometries. Our results clearly show that their
emission spectrum is plagued with resonant signatures arising from the
geometry. This study allows us to find the key parameters that govern
terahertz radiation processes in high temperature superconductors.
Rahul Roy (UIUC): Unconventional insulators with time reversal
symmetry

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Traditionally, band insulators which obey time reversal symmetry have
only received limited attention. Recently it has been realized that
due to spinorbit coupling, novel spin transport phenomena such as the
spin Hall effect can occur in these systems. Certain insulators can
have localized gapless states at the edge, analogous to quantum Hall
systems. An analog of the Hall conductance for these systems is the
"Z_{2} invariant." I describe a new approach to the Z_{2}
characterization
of two dimensional band insulators which clarifies the connection with
robust edge states. I also present a new characterization of three
dimensional band insulators and other systems with time reversal
symmetry. Systems which might realize the exotic phases of this
characterization and the connection with chiral lattice gauge theory
will also be discussed.
Andriy Nevidomskyy (Université de Sherbrooke):
Frustration in the Hubbard model: a quantum cluster study

[]

The role of frustration in the Hubbard model is studied on the square
lattice with nearest and nextnearest neighbour hoppings t and t' using
the Variational cluster approximation[1]. We find two phases with
longrange magnetic order: the usual antiferromagnet (AF1) phase, stable
at small values of t'/t, and the AF2 phase with the ordering wavevector
(0,pi), stable at large frustrations. These are separated by a phase
with no magnetic order. We also find the dwave superconductivity for
small values of U<5t and a broad range of frustrations. The MottHubbard
transition is discussed in this context.
Our findings may be applied to a variety of experimental systems with
inplane magnetic frustrations, including the cobaltates. The results
are compared with the classical phase diagram obtained from the largeU
expansion and from the J1J2 frustrated Heisenberg model.
[1] M. Potthoff, M. Aichhorn, C. Dahnken, Phys. Rev. Lett. 91, 206402
(2003).
Suman
Hossain (UBC): Impuritycontrolled valence, spin, and orbital
state in Sr_{3}Ru_{2}O_{7}

[]

Impurity doping is one of the most effective means of tuning the
properties of materials. We have discovered a possible way for orbital
hierarchy inversion, valence control, and magnetocrystalline anisotropy
rotation in complex oxides, by embedding ‘localized’ impurity orbitals in
a ‘delocalized’ host matrix. Here we present a comprehensive analysis of
experimental and theoretical results on Sr3(Ru1xMnx)2O7 by a combination
of xray absorption spectroscopy (XAS), density functional theory (LSDA)
and cluster multiplet calculations. We find that Mn impurities do not
exhibit the same 4+ valence of Ru, but behave as Mn3+ acceptors;
furthermore, the extra electron occupies inplane eg orbitals instead of
the outofplane 3z2  r2 orbital predicted by crystal field theory, a
counterintuitive result which might be termed as “ inverse JahnTeller
effect”. This behavior has profound implications for the spin and orbital
ordering of the system as a whole, with a magnetocrystalline anisotropy
that gradually goes from outofplane to inplane as the Mn doping level
is increased. Overall, the behavior is intriguingly similar to that of Mn
doped GaAs and may be relevant to the physics of dilute magnetic
semiconductors. This may also indicate new pathways toward the design of
novel orbital and magnetic materials, e.g. dilute 3d impurities in 4d or
even 5d hosts.
David Le Boeuf (Université de
Sherbrooke):
Quantum oscillations in underdoped YBCO

[]

After 20 years
of research, the hightemperature copper oxide superconductors still
remain of fundamental interest. One question arising as one goes from
the overdoped regime, where a large well defined Fermi surface is
exhibited,
to the underdoped regime, characterized by the pseudogap phase, concerns
the manner nature uses to bridge those two regimes.
One way to
deal with this issue, is to explore the ground state of the pseudogap
phase. We hence performed resistivity measurement on ultrahigh quality
YBa_{2}Cu_{3}O_{6.5} Ortho II ordered samples,
under high magnetic field as a method to reveal the ground state of
underdoped cuprates. We observed quantum oscillations, the most direct
and robust bulk probe of a Fermi surface. The low frequency of those
oscillations indicates that the electronic ground state is made of small
pocket(s), as opposed to the large Fermi surface observed in the overdoped
regime. We shall interpret this result in the context of band structure
calculations, and also in the light of different theories predicting
small Fermi pockets in the underdoped regime.
Student posters
JeanSebastien Bernier (Toronto): Mott phases and superfluidinsulator
transition of dipolar spinthree bosons in an optical lattice:
implications for Cr atoms

Maxime Boissonneault (Sherbrooke): Quantum Optics in Superconducting
Circuits

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Several recent experiments have demonstrated that superconducting
circuits are ideal systems for the study of quantum mechanical effects
on large scale and promising candidates for quantum computation. It
was recently proposed [1] and experimentally demonstrated [2,3] that
superconducting circuits fabricated inside a high quality onchip
transmission line resonator can be used to study solidstate analogs of
quantum optics and, in particular, to reach the strongcoupling regime
of cavity quantum electrodynamics (CQED). Moreover, in the dispersive
regime, were the qubit transition frequency is well detuned from the
resonator frequency, the resonator can be used as a tool to readout the
quantum state of the superconducting qubit in a quantum nondemolition
way. In this poster, we study important nonlinear effects and how they
affect the measurement process. In particular, we find that in the
presence of pure dephasing of the qubit, the resonator can act as a heat
bath on the qubit, thereby possibly ruining its quantum nondemolition
properties.
[1] A. Blais, R.S. Huang, A. Walraff, S. M. Girvin and R. J.
Schoelkopf, Phys. Rev. A 69, 062320 (2004).
[2] A. Wallraff, D. Schuster, A. Blais, L. Frunzio, R.S. Huang, J.
Majer, S. Kumar, S. M. Girvin and R. J. Schoelkopf, Nature 431, 162
(2004).
[3] D. I. Schuster, A. A. Houck, J. A. Schreier, A. Wallraff, J. M.
Gambetta, A. Blais, L. Frunzio, B. Johnson, M. H. Devoret, S. M. Girvin
and R. J. Schoelkopf. Nature, 445, 515 (2007).
Patrick Clancy (McMaster): Suppression of commensurate spinPeierls
order in Scdoped TiOCl

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We have performed xray scattering measurements on single crystals of the
doped spinPeierls compound Ti{1x}Sc{x}OCl (x = 0, 0.01, 0.03). These
measurements reveal that the introduction of nonmagnetic impurities, even
at the level of 1%, has a profound effect on the unconventional
spinPeierls behavior of the system. Scdoping results in the suppression
of commensurate fluctuations in both the pseudogap and incommensurate
spinPeierls phases of TiOCl, and completely prevents the formation of a
longrange ordered spinPeierls state. We have observed broad
incommensurate scattering in the doped compound, which arises at Tc2 ~ 93
K and persists down to ~ 7 K with no evidence of a lockin transition. The
width of this incommensurate scattering is largely independent of
temperature, and suggests correlation lengths on the order of ~ 12 A below
Tc2. The wavevector of the incommensurate modulation, (delta, 0.5, 0),
decreases between Tc2 and Tc1 ~ 63 K, but remains constant at ~ (0.055,
0.5, 0) below Tc1. While the concentration of nonmagnetic impurities does
not appear to affect the width or the wavevector of the incommensurate
scattering, increased Scdoping does result in a significant decrease in
the observed scattering intensity.
Denis Dalidovic (SFU): Superfluid Density near T_{c} in the
Presence of Random Planar Defects

Ramzy Daou (Sherbrooke): Probing LBCO with heat transport: stripes and
nodes

[]

At the magic 1/8 doping, superconductivity in
La_{2x}Ba_{x}CuO_{4} is strongly suppressed.
However, ARPES measurements above T_c show that there is still a gap in
the single particle excitation spectrum with dwave symmetry and
pointlike nodes. Here we present low temperature thermal conductivity
measurements in the superconducting and the field induced normal state
which are compatible with this scenario. We also discuss the recent
observations of 2D superconducting fluctuations in this compound.
John Hopkinson (Toronto): Classical antiferromagnet on a hyperkagome
lattice

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Motivated by recent experiments on
Na_{4}Ir_{3}O_{8} [Takagi,unpublished], we study
the classical antiferromagnet on a frustrated threedimensional lattice
obtained by selectively removing one of four sites in each tetrahedron of
the pyrochlore lattice. This "hyperkagome" lattice consists of
cornersharing triangles. We present the results of largeN mean field
theory and Monte Carlo computations on O(N) classical spin models. We find
the classical ground states to be highly degenerate. Nonetheless, at low
temperatures, nematic order emerges via "order by disorder" in the
Heisenberg model ($N$=3), representing the dominance of coplanar spin
configurations. Above this transition, the spinspin correlations show a
dipolar form which can be understood to arise from a generalized "Gauss'
law" constraint. Implications for ongoing experiments are discussed.
Jungseek Hwang (McMaster)

Shunichiro Kittaka (Kyoto): Two distinct superconducting
transitions in the Sr_{3}Ru_{2}O_{7} region
of Sr_{2}RuO_{4}Sr_{3}Ru_{2}O_{7}
eutectic crystals

[]

We report superconducting
properties of
Sr_{2}RuO_{4}Sr_{3}Ru_{2}O_{7}
eutectic crystals, consisting of the spintriplet superconductor
Sr_{2}RuO_{4}
with the monolayer stacking of RuO_{2} planes and the metamagnetic
normal metal Sr_{3}Ru_{2}O_{7} with the bilayer
stacking. Sr_{3}Ru_{2}O_{7}
has not been reported to become superconducting until now, but our AC
susceptibility measurements reveal two superconducting transitions
reproducibly
occurring in the Sr_{3}Ru_{2}O_{7} region of
the eutectic crystals. The shielding fraction of the superconductivity
reaches essentially 100% at low AC fields. However, it is easily
suppressed
by AC fields larger than 0.1 mT. Moreover, no anomaly is observed in
the specific heat. These facts suggest that the superconductivity observed
in the Sr_{3}Ru_{2}O_{7} region does not come
from bulk superconductors. Among other possibilities, we discuss a model
with monolayers of RuO_{2} planes contained as stacking faults
to explain these experimental results.
Jonathan Laverdière (Sherbrooke)

Lan Luan (Stanford): Dragging individual vortices to probe the
dimensionality of pinning in
YBa_{2}Cu_{3}O_{7\delta}

Patrick Morales (Toronto)

Jesse Petersen (SFU): Visible PumpTHz Probe Spectroscopy of the
Undoped Cuprate Sr_{2}CuCl_{2}O_{2}

Christoph Puetter (Toronto)

Jean Philippe Reid (Sherbrooke)

Patrick Rourke (Toronto): New de Haasvan Alphen effect measurement
electronics

Junliang Song (UBC): Quantum FluctuationInduced Uniaxial and Biaxial
Spin Nematics of F=2 Cold Atoms

Hiroshi Takatsu (Kyoto): Roles of high frequency optical phonons in the
properties of the conductive delafossite PdCoO_{2}

[]

Layered oxides continue to provide interesting superconducting states. In
addition to the square lattice in the layered cuprates and ruthenates, a
triangular lattice of cobalt and oxygen in
Na_{x}(H_{3}O)_{z}CoO_{2}?yH_{2}O
yields unconventional superconductivity. We report here physical
properties of another layered oxide having a triangular lattice,
PdCoO_{2}. Its delafossite structure is closely related to that of
NaCoO_{2}, a mother compound of
Na_{x}(H_{3}O)_{z}CoO_{2}?yH_{2}O,
but with a different layer stacking. It also contains a triangular lattice
of Pd. Using single crystals, we identified two prominent phonon modes at
520 cm^{1} and 712 cm^{1} in the Raman spectra, and
absorption from 600 to 1200 cm^{1} in the infrared spectra. We
clarified that these high frequency phonon modes account for the unusual
temperature dependence of the specific heat, which reaches only 65% of the
classical Dulong and Petit’s value even at 250 K. These optic phonons also
account for the deviation from the Tlinear behavior of the
resistivity expected for typical electronacoustic phonon scattering.
Despite the influence of these highfrequency phonon modes, no sign of
superconductivity was detected down to 15 mK.
So Takei (Toronto): Theory of electronphonon interaction in a
nonequilibrium open electronic system

[]

We study the effects of timeindependent nonequilibrium drive on an open
2D electron gas system coupled to 2D longitudinal acoustic phonons using
the Keldysh path integral method. The layer electronphonon system is
defined at the twodimensional interface between a pair of
threedimensional Fermi liquid leads, which act both as a particle pump
and an infinite bath. The nonequilibrium steady state is achieved in the
layer by assuming the leads to be thermally equilibrated at two different
chemical potentials. This subjects the layer to an outofplane voltage
and drives a steadystate charge current perpendicular to the system. We
compute the effects of small voltages on the inplane electronphonon
scattering rate and the electron effective mass at zero temperature. We
also find that the obtained nonequilibrium modification to the acoustic
phonon velocity and the ThomasFermi screening length reveal the
possibility of tuning these quantities with the external voltage.
Hiroki Wadati (UBC): Insitu
photoemission study of Pr_{1x}Ca_{x}MnO_{3}
epitaxial thin films

[]

Pr_{1x}Ca_{x}MnO_{3}
(PCMO) has been extensively investigated due to its wide composition
range of charge ordering (CO). In order to see the effect of epitaxial
strain on the electronic structure, we have performed an insitu
photoemission study of PCMO thin films grown on LaAlO_{3} (001)
substrates. From the corelevel photoemission study, we found that the
chemical potential shift was not suppressed unlike bulk PCMO. In the
valenceband spectra, we found no spectral weight at the Fermi level
(E_{F}) or dopinginduced spectral weight transfer
toward E_{F}
also unlike bulk PCMO. These results are a spectroscopic evidence for
the absence of the same type of CO in thin films.
Fan Wang (Toronto): Reentrant spin glass transition in
LuFe_{2}O_{4}

Jing Yang (McMaster): Magnetic resonance in the midinfrared region


