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## Cambridge - Leiden: easyMeeting on Quantum Matter |

It is well
established by now that s-wave superconductivity can be induced in ferromagnets over typical distances varying from 20 nm (weak
magnets) to 1 nm (strong magnets). A more intringuing
possibility is that an 'odd-frequency triplet' order parameter can be induced,
in particular in halfmetallic ferromagnets.
This would lead to a much longer range of the superconducting correlations, of
the order of a micrometer. Experiments indicating the existence of the effect
have been reported, but are still controversial. I shall give a brief overview
of the field, and the current
We calculate
the critical temperature for Bose-Einstein condensation in a gas of bosonic atoms across a Feshbach
resonance, and show how medium effects at negative scattering lengths give rise
to pairs reminiscent of the ones responsible for fermionic
superfluidity. We find that the formation of pairs
leads to a large suppression of the critical temperature. Within the formalism
developed by Nozieres and Schmitt-Rink the gas
appears mechanically stable throughout the entire crossover region, but when
interactions between pairs are taken into account we show that the gas becomes
unstable close to the critical temperature. We discuss prospects of observing
these effects in a gas of ultracold Cs133 atoms where
recent measurements indicate that the gas may be sufficiently long-lived to
explore the many-body physics around a Feshbach
resonance.
Having just
celebrated their first birthday, the iron-pnictide
high-T In this talk,
I'll present our latest data dealing with the electronic structure of these new
members of the unconventional superconductor family. Starting with simple yet
important questions regarding the degree to which these systems should be
considered as strongly correlated, I will then discuss recent data mapping the
electronic states of the MFe This research
was carried out in collaboration: Y. Huang, S. de Jong,
F. Massee, E. Huisman, E.
van Heumen, A. de Visser,
J.B. Goedkoop, J. Fink, S. Thurupathaih,
R. Ovsyannikov, R. Follath,
M. Gorgoi, F. Schäfers,
H.A. Dürr, C. Felser, S. Dastjani Farahani, M. Rotter, D. Johrendt, A. GLoskovskii, Y.Z. Zhang, H.O. Jeschke,
R. Valenti, H.S. Jeevan, P.
Gegenwwart, A. Erb. Support
from FOM and EU (I
I will present
an overview of the notion of quantum criticality and current theoretical
activity on the subject in the i.
Attempts to understand the anomalous phase that pre-empts the metamagnetic quantum critical point in Sr ii.
Inconsistency in the Hertz-Millis theory of itinerant magnetic criticality and
a recent reformulation. iii. The magnetothermoelectric response near to a superconductor
insulator transition. I will use
these to reveal some broader aspects of current theoretical (and to an extent experimental ) activity in the
The occurrence
of a 2-dimensional conducting layer between insulating oxides, most notably in
SrTiO However,
analysis of the experimental data shows that also oxygen vacancies play an
important role for various of the reported
experimental results. In this talk I
will provide an update on the recent developments in this field.
In this talk,
I will review our group's efforts to elucidate the temperature and momentum
dependence of the scattering rate in high temperature superconductors, the link
between anisotropic scattering and superconductivity, and the possible role of
intense scattering on the development of the pseudogap
in underdoped cuprates.
Microcavity polaritons - superpositions
of confined photons and excitons in quantum wells -
have been the subject of much experimental and theoretical work recently,
following the experimental realisation of
condensation of polaritons. One important difference between polariton condensates and previous examples such as Helium
or cold atoms is that polaritons have relatively
short lifetimes. On the other hand, compared to lasers, polaritons
are more strongly interacting, and therefore much better able to thermalise than are photons. This combination leads to a picture
of non-equilibrium condensation, in which there is a continual flux of
particles through the system. I will discuss
our microscopic model[1] for non-equilibrium polariton condensation, consisting of a coupled exciton-photon system, in which both excitons
and photons are also coupled to external baths, driving a flux of particles
through the system. I will
discuss how the steady states in the presence of pumping and decay can be
described, considering both the nature of the steady states,
and fluctuations about them[2]. Motivated
by these microscopic results, I will then discuss more macroscopic features,
such as the non-equilibrium spectrum, and how it affects the possibility of superfluidity away from equilibrium, and conclude by
talking about the way that pumping and decay change the large-scale structure
expected in a non-equilibrium condensate[3]. [1] J.
Keeling, F. M. Marchetti, M. H. Szymanska,
and P. B. Littlewood, Semicond.
Sci. Technol. 22, R1 (2007) [2] M. H. Szymanska, J. Keeling, and P. B. Littlewood,
Phys. Rev. Lett. 96, 230602 (2006). [3] J. Keeling
and N. G. Berloff, Phys. Rev. Lett.
100, 250401 (2008).
Starting from
the microscopic Hamiltonian of a $\nu_T=1$ bilayer Quantum Hall system under strong magnetic fields we
formulate the problem as one of bosonic excitations
above the phase-coherent ground state. We then use this description to derive
the ground state energy as a functional of the slowly-varying phase field and
determine the ground state for the case of a tilted magnetic field, when system
displays a commensurate-incommensurate transition. We use renormalization group
arguments to find the finite temperature behaviour of
the system.
Shallow
impurities in silicon first received serious attention in the 1960s. At that
time they were the key to understanding how to engineer the electrical
characteristics of semiconductor devices in a controlled and reproducible
manner. Throughout the next 30 years a program of careful material characterisation lead to a good appreciation of both their
optical and electrical properties. Recently there have been a number of
exciting potential applications for what is now one of the oldest semiconductor
materials. These include, using shallow donors embedded in a silicon matrix as
active elements in terahertz emitters and qubits in
quantum computing. It now is clear from much of the recent work, however, that
the subject of shallow impurities in silicon now needs to be revisited and that
we need to understand the underlying physics of this material system at a much
deeper level. In particular, we need to pay attention to some of the less
explored properties of shallow donors in silicon crystals such as the
fundamental limits of lifetimes of excited states, spin relaxation, electron-phonon
interactions, and interactions between different classes of dopants.
I will address some of these issues in this presentation.
Recently,
string theory has started to make contact with condensed matter in surprising
ways, both absorbing lessons, and opening the possibility for insights into
physically relevant systems. Hints
of connections between gauge theories and gravitational theories were first
seen in the 1970's, and have made their appearance in many guises. A quick introduction to the powerful AdS/CFT duality, which gives the first concrete realization
of such a connection, will be given, before explicit examples are detailed. I will focus on recent examples such
as the viscosity to entropy ratio in condensed states of QCD as seen at the
Relativistic Heavy Ion Collider experiment, and
relevant bounds from AdS/CFT, recent work on building
gravitational duals of superconductors, and finally signatures of Fermi liquids
from AdS space.
Perhaps unusual states of matter are waiting to be discovered, and such
dualities might provide new tools for the search.
I review
fractional quantum Hall (fqH) states with pairing or
clustering correlations. Excitations over such states are non-Abelian anyons, allowing a
scenario for topological quantum computation in fqH
effect devices. I will explain how the clustering properties are analyzed with
the help of parafermionic quantum fields, discuss
experimental signatures and assess the case of paired and clustered quantum Hall states in the second Landau level.
AgNiO Motivated by
recent torque magnetometry experiments, we explore
the high field properties of AgNiO
Numerous
phenomenological parallels have been drawn between f- and d- electron systems
in an attempt to understand their display of unconventional superconductivity.
The microscopics of how electrons evolve from
participation in large moment antiferromagnetism to
superconductivity in these systems, however, remains a mystery. Here, we
present ambient pressure quantum oscillation measurements on CeIn3 that
crucially identify the electronic structure - heavy hole pockets of f-character
are revealed to undergo an unexpected effective mass divergence well before the
antiferromagnetic critical field. We thus uncover the
softening of a branch of quasiparticle excitations
located away from the traditional spin fluctuation-dominated antiferromagnetic quantum critical point. The observed
Fermi surface of dispersive f-electrons in CeIn3 could potentially explain the
emergence of Cooper pairs from within a strong moment antiferromagnet.
In two
dimensional topological phases of matter, processes depend on gross topology
rather than detailed geometry. Thinking in 2+1 dimensions, particle world lines
can be interpreted as knots or links, and the amplitude for certain processes
becomes a topological invariant of that link. While sounding rather exotic, we
believe that such phases of matter not only exist, but have actually been
observed in quantum Hall experiments, and could provide a uniquely practical
route to building a quantum computer. Possibilities have also been proposed for
creating similar physics in systems ranging from superfluid
helium to strontium ruthenate to spin systems to cold
atoms.
Resolution of
a conventional three-axis neutron spectrometer is limited by a degree of monochromaticity and by a beam divergence. I present high
resolution measurements of the lattice constants of ferromagnetic
superconductor UGe2 under pressure probed by a novel technique, which utilizes Larmor precession of polarized neutrons and surpasses the
resolution of conventional scattering methods by an order of magnitude. Growth
of large single crystals of various U ferromagnets is
briefly discussed. [Back] |