Lorentz Center - Cambridge - Leiden: easyMeeting on Quantum Matter from 6 May 2009 through 7 May 2009
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    Cambridge - Leiden: easyMeeting on Quantum Matter
    from 6 May 2009 through 7 May 2009

 
Abstracts

Abstracts workshop Cambridge – Leiden, easyMeeting on Quantum Matter

 

 

Jan Aarts (Leiden)

Supercurrents in Ferromagnets

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 Leiden efforts.

 

Rembert Duine (Utrecht)

A strongly interacting Bose gas: Nozieres and Schmitt-Rink theory and beyond

 

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.

 

 

Mark Golden (Van der Waals-Zeeman Institute, University of Amsterdam)

Real and k-space cartography of iron pnictide high-Tc superconductors

 

Having just celebrated their first birthday, the iron-pnictide high-Tc superconductors currently dominate the arXiv preprint server ad seem to offer the perspective of truly useful high-Tc superconductivity, if only we could boost their critical temperatures into the super-77K regime.

 

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 MFe2As2 systems both in real space via variable temperature STM and STS, and in k-space via ARPES.

 

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 (I3) is acknowledged.

 

 

Andrew Green (St. Andrews)

New Horizons in Quantum Criticality

 

I will present an overview of the notion of quantum criticality and current theoretical activity on the subject in the UK. After a brief overview of the central ideas and a quick survey of some of the directions that are currently being pursued, I will review three problems in which I have recently been involved:

i. Attempts to understand the anomalous phase that pre-empts the metamagnetic quantum critical point in Sr3Ru2O7.

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 UK. 

 

 

Hans Hilgenkamp (Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente)

Conducting interfaces between insulating oxides

 

The occurrence of a 2-dimensional conducting layer between insulating oxides, most notably in SrTiO3-LaAlO3 combinations, has drawn a lot of attraction in recent years. It has created an awareness for the intriguing possibilities arising from ‘electronic reconstructions’ in these transition metal oxides.

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.

 

 

Nigel Hussey (Bristol)

The pseudogap, quantum criticality and superconductivity in cuprates: a transport perspective

 

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.

 

 

Jonathan Keeling (Cambridge)

Non-equilibrium polariton condensation: from microscopic theory to macroscopic phenomenology

 

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).

 

 

Achilleas Lazarides (Utrecht)

Quantum Hall bilayer with in-plane field at finite temperatures: Renormalization group approach

 

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.

 

 

Stephen Lynch (London Centre for Nanotechnology, University College London)

Silicon Doped with Group V Donors: A Model Hydrogenic Quantum 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.

 

 

Darius Sadri (Leiden)

Gravity meets condensed matter, an unlikely encounter

 

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.

 

 

Kareljan Schoutens (Amsterdam)

Paired and clustered quantum Hall states

 

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.

 

 

Luis Seabra (Bristol)

2H-AgNiO2 : Adventures in magnetic field 

 

AgNiO2 is a very unusual material. Built of stacked, two-dimensional nickel-oxygen planes, glued together by silver ions, it offers a unique combination of metallicity and magnetism, with the magnetic ions in each plane forming a perfect triangular lattice, nested within a honeycomb network of conducting sites.

 

Motivated by recent torque magnetometry experiments, we explore the high field properties of AgNiO2 using a simple empirical spin model which captures the essential features of magnetic excitations in zero field [Wheeler et al., PRB 79, 104421 (2009)].  Using a combination of mean field analysis, spin wave theory, and classical Monte Carlo simulation, we predict a cascade of phase transitions in magnetic field.  Our analysis suggests the possibility of finding a novel supersolid phase in AgNiO2 at accessible magnetic field.

 

 

Suchitra Sebastian (Cambridge)

Heavy holes as a precursor to superconductivity in antiferromagnetic CeIn3

 

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.

 

 

Steve Simon (Oxford)

Topological Phases of Matter and Why You Should Be Interested

 

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.

 

 

Dmitry Sokolov (Edinburgh)

Beating the resolution limit of scattering techniques: lattice constants of ferromagnetic UGe2 under pressure probed by Larmor diffraction

 

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.

 

 

 



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