Oxford Symposium on Quantum Materials 2013
Somerville College, 10 May 2013
Abstracts
Oxford Symposium on Quantum Materials 2013
Oxford Symposium on Quantum Materials 2013 – Welcome
Paolo Radaelli
Oxford Physics
Talk: 9.00-09.15 am
QM@oxford
Oxford Symposium on Quantum Materials 2013
Forging a Theory of Iron-Based Superconductors
Peter Hirschfeld
Florida State University, USA
In contrast to cuprate superconductors, which manifest low-temperature properties
which correspond to a gap with a common (dx2-y2) form, the gap structure in Febased superconductors appears to be far from universal [1]. I argue that, while
aesthetically unappealing from a traditional theoretical perspective, such a situation
affords a novel opportunity to develop a quantitative theory of unconventional
superconductivity. I will review approaches to this problem that focus on pairing
from spin and orbital fluctuations in the Fe d states, and discuss how well they allow
us to understand current experiments on the superconducting state of various Febased systems. I will review experimental evidence for the sign-changing s-wave
pair state, and discuss recent electron irradiation experiments which appear capable
of resolving the debate on the role of disorder in these materials.
[1] "Gap symmetry and structure of Fe-based superconductors", P.J. Hirschfeld, M.M.
Korshunov, and I.I. Mazin, Rep. Prog. Phys. 74, 124508 (2011).
Talk: 09.15-10.00 am
QM@oxford
Oxford Symposium on Quantum Materials 2013
Chemistry of Layered Chalcogenides and Oxide Chalcogenides
Simon Clarke
Oxford Chemistry
The chemistry of layered chalcogenides and oxide chalcogenides in which
intercalation or deintercalation reactions are used to tune the formal oxidation
states of mid transition metals will be described. The systems will include mixedvalent manganites and high temperature superconductors derived from iron
selenide.
Talk: 10.00-10.20 am
QM@oxford
Oxford Symposium on Quantum Materials 2013
Field-Induced Magnetic Transition in an Iron Pnictide
Superconductor, Ca10(Pt3As8)((Fe/Pt)2As2)5
Matthew Watson
Oxford Physics
I will present a high magnetic field study (up to 55 T) of a10(Pt3As8)(Fe1−xPtxAs2)5
(x = 0.097) with a TC ˜ 10 K using magnetic torque, tunnel diode oscillator
technique and transport measurements. The superconducting phase diagram is
determined, revealing an anisotropy of the irreversibility field up to a factor of 10
near TC. Unexpectedly, beyond the superconducting state we find a field-induced
magnetic transition at ˜22 T in the magnetic torque data when the magnetic field is
applied perpendicular to the conducting layers. This transition becomes significantly
sharper by suppressing the thermal fluctuations while lowering the temperature to
0.3 K. The origin of the anomaly is likely to be the stabilization of a collinear
magnetic ordering of the Fe or Pt sublattices by the magnetic field.
Talk:10.50-11.05 am
QM@oxford
Oxford Symposium on Quantum Materials 2013
High Resolution Microstructural Analysis of Phase Separation
Phenomena in Fe-Based Superconductors
Susie Speller
Oxford Materials
The co-existence of magnetic order and superconductivity is a common feature of
the iron-based superconductors, raising the question of whether these phases are
spatially distinct or whether the same electrons are responsible for both
phenomena. To investigate the nature of the phase separation in Fe-based single
crystals, we have employed a combination of microanalytic techniques to assess the
chemical and structural uniformity of single crystalline samples used for
fundamental property measurements. In particular we have used the high resolution
Electron Backscatter Diffraction (EBSD) technique, pioneered for strain mapping in
aerospace alloys [1], to map local variations in lattice parameter with exceptional
precision and sub-micron spatial resolution for a range of different iron-based
compounds including Fe(Se,Te) [2] and AFe2Se2 (A=Cs,Rb) [3]. In AFe2Se2, the
intrinsic phase separation is very extreme, with significant chemical and structural
differences associated with the spatially distinct electronic phases. Here we will
present recent results on the microstructural development in AFe2Se2 and
CaFe2As2 single crystals.
Talk:11.05 -11.25 am
QM@oxford
Oxford Symposium on Quantum Materials 2013
The Upper Critical Field of NaFe1-x CoxAs Superconductors
Saman Ghannadzadeh
Oxford Physics
The recent discovery of iron-pnictide superconductors has generated great interest,
leading to an increased worldwide effort towards understanding the
superconducting mechanism. A crucial part of this effort is the measurement of the
upper critical field, which is a fundamental property of type-II superconductors. The
critical fields give the coherence and penetration depths, and provide information
on the pair-breaking mechanism. Furthermore, the upper critical field anisotropy
sheds light on the multi-band nature of the superconductivity, and is sensitive to
the dimensionality and the underlying electronic structure of the system.
Here, we will present and discuss the results of our recent study on the upper
critical fields of the NaFe1-xCoxAs superconductor family, which was carried out at
the Nicholas Kurti High Magnetic Field facility in fields of up to 45 T.
Talk: 11.25 – 11.40 am
QM@oxford
Oxford Symposium on Quantum Materials 2013
Next Generation of High Field Superconducting Magnets
Ziad Melhem
Oxford Instruments
Users need for superconducting magnet greater than 23T for physical sciences, NMR
or ICR applications will require new innovation in superconducting magnet
engineering. The talk presents an overview of high fields for research and industry
and the dependence of the future systems on the use of advanced superconducting
materials. The production of high magnetic fields using low temperature
superconductors (LTS) has become common place. However, the large magnet sizes
and their associated high cooling costs have often precluded the full utilization of
these research capabilities. Recent advances in internal Sn superconductors and
Cryofree® technology together with advances in high temperature superconductors
(HTS) have opened up a new era in superconducting magnet technology. A new
generation of superconducting magnets is undergoing development and exploits
the current performance of HTS and LTS materials together with innovative solutions
in engineering the integration of coils from different materials.
Talk: 11.40 am -12.00 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Probing Frustration: Muons and Spin Ice
Stephen Blundell
Oxford Physics
The detection of a current of monopoles in spin ice has proved controversial. An
analysis is presented of the behavior of muons in the low-temperature state in spin
ice [1]. It is shown in detail how the behavior observed in some previous muon
experiments on spin ice in a weak transverse field may result from the macroscopic
stray field of magnetized spin ice. A model is presented which allows these
macroscopic field effects to be simulated and the results agree with experiment.
The persistent spin dynamics at low temperature originate from the sample and
could be a muon-induced implantation effect that is operative in out-of-equilibrium
systems with long relaxation times. New data on various other pyrochlore systems
will be presented to help assess the role of muon diffusion (which has been
proposed [2] to account for some of the behaviour observed) and also quantum spin
ice behaviour (which can be realised in some Pr-containing pyrochlores).
[1] S. J. Blundell, Phys. Rev. Lett. 108, 147601 (2012).
[2] P. Quémerais, P. McClarty, and R. Moessner, Phys. Rev. Lett. 109, 127601
Talk: 14.00-14.20 pm
M@oxford
Oxford Symposium on Quantum Materials 2013
Crystal Field States in Candidate Quantum Spin Ice Pr2Sn2O7
Andrew Princep
Oxford Physics
Magnetic pyrochlores A2B2O7 have very strong geometric frustration, leading to
exotic phenomena such as classical spin liquids and magnetic monopole excitations
[1]. Recently the idea of a “quantum” spin ice, where the quantum fluctuations
preventing spin freezing, has attracted considerable interest. This state should
realise an analogue of quantum electromagnetism with linearly dispersing magnetic
excitations equivalent to photons [2]. It has been proposed that Pr2Sn2O7 is one of
the most promising candidates for quantum spin ice as Neutron diffraction
measurements have shown a strong quasielastic peak, but no evidence for longrange magnetic order down to 200 mK [3]. Specific heat measurements also indicate
a greater residual entropy than other spin ice compounds. All these results point
towards a ground state which is dynamic at T = 0.
The specific admixture of angular momentum states in the ground doublet, together
with the strengths of the exchange and dipolar interactions, control whether or not
a material can exhibit quantum ice behaviour. When the ground state is not of the
Kramers type, the mapping to a quantum ice model is non-trivial and the details of
the wavefunction are important for the quantitative calculations needed to confirm
whether or not the system falls within the realm of spin-ice models [4]. By
performing inelastic neutron scattering on the MERLIN instrument at ISIS, we have
determined the single-ion ground state wavefunction of the Pr3+ ions, and lowest
lying level is indeed a non-kramers doublet. We also observe and reproduce
intermultiplet transitions at approximately 280 meV, and show that an excitation
around 38 meV, previously assigned to a crystal field transition, is actually a
phonon.
[1] J. S. Gardner et. al., Rev. Mod. Phys., 82, (2010), 53
[2] O. Benton, O. Sikora, N, Shannon, Phys. Rev. B, 86, (2012), 075154
[3] H. D. Zhou. et. al., Phys Rev. Lett., 101, (2008). 227204
[4] S. Lee, S. Onoda, and L. Balents, Phys. Rev. B, 86, (2012), 104412
Talk: 14.20-14.35 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Dynamical Spin Waves in the Pyrochlore Magnet
Eliot Kapit
Oxford Physics
We demonstrate a numerical method for studying the dynamical behavior of the spin
excitations of a pyrochlore antiferromagnet in the large-S limit. The classical
pyrochlore magnet is a highly frustrated compound, with a massive ground state
degeneracy of disordered states, and therefore provides an ideal system to study
the effects of frustration on 3D quantum magnetism. While many previous studies
have treated the static and finite temperature spin structure of these compounds,
the dynamical behavior of the system's finite energy excitations is much less well
understood. We show that by repeatedly generating ground states using classical
Monte Carlo methods and diagonalizing the full linear spin wave Hamiltonian which
results from each state, we can calculate the time-dependent spin structure factor
S[Q,w] for systems of up to 8000 spins. In addition to the familiar massive
degeneracy of zero energy excitations, we find a coherent, linearly dispersing spin
wave mode even in the disordered ground states. Preliminary comparison between
our numerical simulations and energy-resolved neutron scattering data in MgCr2O4
shows strong qualitative agreement.
Talk: 14.35-14.50 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Dispersion Relations near Quantum Criticality in the Quasi 1D
Ising Chain CoNb2O6 in Transverse Magnetic Field
Ivelisse Cabrera
Oxford Physics
The Ising chain in a transverse magnetic field is one of the canonical examples of a
quantum phase transition. We have recently realized this model experimentally in
the quasi-one-dimensional (1D) Ising-like ferromagnet CoNb2O6 [1]. Here, we
present single-crystal inelastic neutron scattering measurements of the magnetic
dispersion relations in the full three-dimensional (3D) Brillouin zone for magnetic
fields near the critical point and in the high field paramagnetic phase. We explore
the gap dependence as a function of field and quantify the cross-over to 3D physics
at the lowest energies due to the finite interchain couplings. We parametrize the
dispersion relations in the high-field paramagnetic phase to a spin wave model to
quantify the sub-leading terms in the spin Hamiltonian beyond the dominant 1D
Ising exchange.
1. [1] R. Coldea, D.A. Tennant, E.M. Wheeler et al, Science 327 177-180 (2010).
Talk: 14.50-15.05 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Magneto-Orbital Helices: A Novel Route to Coupling Magnetism
and Ferroelectricity in Multiferroic CaMn7O12
Natasha Perks
Oxford Physics
Multiferroic materials, specifically those possessing a magnetically induced
improper ferroelectric polarisations attract considerable interest, due to both their
novel physics, and their potential technological application, exploiting cross
coupling of electrical properties. Work presented here forms part of a wider effort to
develop strongly coupled, high temperature multiferroics with enhanced
functionality.
Orbital physics drives a rich phenomenology in transition metals, providing
microscopic underpinning for effects such as colossal magnetoresistance. Magnetic
and lattice degrees of freedom are coupled through orbital ordering, and it has long
been hoped that this coupling could be exploited to create new multiferroic
materials. Here, we report an unprecedented magneto-orbital texture in multiferroic
CaMn7O12, giving rise to the largest magnetically induced polarisation measured to
date. Neutron and x-ray diffraction has characterised magnetic and structural
modulations, which are intertwined to form an ‘incommensurate magneto-orbital
helix’. Analysis of magnetic exchange shows that orbital order is crucial in
stabilising the chiral magnetic structure, subsequently allowing for electric
polarisation. The presence of a global structural rotation promotes the coupling
between this polarisation and magnetic helicity required for multiferroicity. These
observations open up the possibility of finding a new class of strongly coupled
multiferroic materials, underpinning their technological development.
Talk: 15.05-15.20 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Terahertz Frequency Electromagnon and Magnon Modes in
Multiferroic Cupric Oxide
Sam Jones
Oxford Physics
We examined the terahertz magnetoelectric response
of cupric oxide, a high-temperature multiferroic with a cycloidal
spin structure that induces an electrical polarisation. Terahertz
time-domain spectroscopy at temperatures from 10-300K and magnetic
fields 0-7T uncovered electromagnon and magnon modes, providing
insights into the spin Hamiltonian.
Talk: 15.20 – 15.35 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Antiferromagnetic Ordering in Quasi-Low Dimensional Polymeric
Magnets: a CW-ESR Study
Danielle Kaminski
Oxford Physics
In both 1D and 2D systems, the formation of long-range magnetic order is
impossible at temperature T>0K. However, attempts experimentally to realise such
systems are imperfect, resulting in 'quasi' 1D (Q-1D) and 2D (Q-2D) systems. These
have a small but finite coupling across the remaining dimensions, meaning
magnetic ordering at T>0K is a possibility.
Polymeric magnets are quasi-low dimensional and consist of magnetic ions linked
by organic groups, forming chains and layers. With the ability we have to fine-tune
their magnetic parameters, they provide a fertile test ground for aspects of the
quantum theory of magnetism [1,2].
We here explore, via continuous wave electron spin resonance (CW-ESR) at variable
temperature, a group of polymeric magnets which form an antiferromagnetic state
at T<TN. Through careful extraction of linewidth and principle g-tensor
components, we are successful in probing the temperature region above TN. We
therefore propose use of this method across a wider range of compounds to gain
insight into the ordering process itself.
[1] P.A. Goddard et al., New J. Phys. 2008 (10) 083025.
[2] P.A. Goddard et al., Phys. Rev. Lett. 2012 (108) 0077208.
Talk: 15.35 – 15.50 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
New Suprises from a Famous Organic Superconductor
Francis Pratt
Rutherford Appleton Laboratory
The low field phase of the first organic superconductor (TMTSF)2ClO4 has been
studied by muon spin rotation to get information about the vortex state and an
accurate value for the penetration depth. The muon spin relaxation shows no
indication of gap nodes on the Fermi surface nor of any spontaneous fields due to
time-reversal-symmetry breaking. The overall evidence does however suggest that
the symmetry of this low field phase is odd-frequency p-wave singlet; a novel
example of odd-frequency pairing in a bulk superconductor.
16.20 – 16.40 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Unconventional Superconductivity in Strontium Ruthenate
Jesper Romers
Oxford Physics
In 1994 superconductivity was discovered in the material Sr2RuO4, strontium
ruthenate (SRO). Closely related materials show ferromagnetism in the metallic state
and by analogy it was argued that SRO might exhibit spin triplet pairing in the
superconducting state.
The spin triplet pairing is well-established experimentally by now, but the form of
the orbital part of the pairing wave function is still the subject of debate. There is
hope SRO is an electronic analogue of the A phase of He-3, a chiral p-wave
superconductor. This state of matter carries topologically protected edge modes and
supports half quantum vortices with Majorana zero modes.
However, many more types of p-wave pairing are in principle possible and to date
experiments are inconclusive and sometimes at odds with one another. From a
theoretical point of view, the problem is very rich given the fact that SRO is a layered
two-dimensional system with three bands. Most theories assume the dominant
contribution to the superconductivity comes from the 2D band - recent proposals
however involve the other two quasi-1D bands.
I will give an overview of the experimental status of the field and pinpoint where the
difficulties are in reconciling the experimental data with the theoretical models.
Furthermore I will present work we are doing right now which combines taking into
account all three energy bands as well as strong spin-orbit coupling, which is
important in SRO.
16.40 – 17.00 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Anharmonicity due to Electron-Phonon Coupling in Magnetite
Moritz Hoesch
Diamond Light Source
We present the results of inelastic x-ray scattering for magnetite and analyze the
energies and widths of the phonon modes with di
fferent symmetries in a broad range of temperature 125 < T < 293 K. The phonon
modes with X4 and
5 symmetries broaden in a nonlinear way with decreasing T
when the Verwey transition is approached. It is found that the maxima of phonon
widths occur away from high-symmetry points which points at the incommensurate
character of critical fluctuations.
Strong phonon anharmonicity induced by electron-phonon coupling is discovered
by a combination of these experimental results with ab initio calculations which take
into account local Coulomb interactions at Fe ions. It (i) explains observed
anomalous phonon broadening, and (ii) demonstrates that the Verwey transition is a
cooperative phenomenon which involves a wide spectrum of phonons coupled to the
electron charge fluctuations condensing in the low-symmetry phase.
17.00 – 17.20 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Quantum Mechanics in Superconducting Circuits
Peter Leek
Oxford Chemistry
Microwave frequency electrical circuits built from superconducting components and
cooled to millikelvin temperatures can display quantum mechanical behaviour. For
example, an electrical resonator cooled to its ground state, and coupled to one of a
variety of solid state quantum devices, realises an electrical circuit analogue of
cavity quantum electrodynamics. In this talk I will give a basic introduction to this
field, and an overview of the research within it that we are pursuing at Oxford.
17.20 – 17.35 pm
QM@oxford
Oxford Symposium on Quantum Materials 2013
Do Birds Use Entangled Spins in Their Magnetic Compass Sensor?
Peter Hore
Oxford Chemistry
Most physical scientists would probably treat with scepticism the suggestion that a
chemical reaction could respond to a magnetic field as weak as the Earth’s. After all,
than a million times smaller than the thermal energy per molecule at room
temperature. Nevertheless, the kinetics of certain chemical reactions are
magnetically sensitive. The key molecular species are pairs of transient free radicals
whose electron-nuclear spin systems evolve coherently under the influence of
internal and external magnetic interactions.
In this short talk, I will discuss aspects of the proposal that the coherent quantum
spin-dynamics of photo-induced radical pairs in cryptochromes (photo-active
proteins) could be the mechanism of the light-dependent magnetic compass sense
of migratory birds and other animals.
17.35 – 17.55 pm
QM@oxford