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B. D. Simons

Publications and source records attributed to B. D. Simons.

At least 37 records · Page 2Linked to original sources
arXiv2009

Polaritons and Pairing Phenomena in Bose--Hubbard Mixtures

Motivated by recent experiments on cold atomic gases in ultra high finesse optical cavities, we consider the problem of a two-band Bose--Hubbard model coupled to quantum light. Photoexcitation promotes carriers between the bands and we study the non-trivial interplay between Mott insulating behavior and superfluidity. The model displays a global U(1) X U(1) symmetry which supports the coexistence of Mott insulating and superfluid phases, and yields a rich phase diagram with multicritical points. This symmetry property is shared by several other problems of current experimental interest, including two-component Bose gases in optical lattices, and the bosonic BEC-BCS crossover problem for atom-molecule mixtures induced by a Feshbach resonance. We corroborate our findings by numerical simulations.

cond-mat.str-el↗
arXiv2008

Superfluidity at the BEC-BCS crossover in two-dimensional Fermi gases with population and mass imbalance

We explore the zero temperature phase behavior of a two-dimensional two-component atomic Fermi gas with population and mass imbalance in the regime of the BEC-BCS crossover. Working in the mean-field approximation, we show that the normal and homogeneous balanced superfluid phases are separated by an inhomogeneous superfluid phase of Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type. We obtain an analytical expression for the line of continuous transitions separating the normal and inhomogeneous FFLO phases. We further show that the transition from the FFLO phase to the homogeneous balanced superfluid is discontinuous leading to phase separation. If the species have different masses, the superfluid phase is favored when the lighter species is in excess. We explore the implications of these findings for the properties of the two-component Fermi gas in the atomic trap geometry. Finally, we compare and contrast our findings with the predicted phase behavior of the electron-hole bilayer system.

cond-mat.str-el↗
arXiv2008

Coexistence of SDW, d-wave singlet and staggered $π$-triplet superconductivity

We have studied the competition and coexistence of staggered triplet SC with d-wave singlet SC and SDW in the mean-field approximation. Detailed numerical studies demonstrate that particle-hole asymmetry mixes these states and therefore they are simultaneously present. Even more interesting were the results of our study of the influence of a uniform magnetic field. We observe novel transitions that show the characteristics of Fulde-Ferrel phases, yet they concern transitions to different combinations of the above orders. For example, above a given field, in a particle-hole symmetric system we observe a transition from d-wave singlet SC to a state in which d-wave singlet SC coexists with staggered triplet SC and SDW. We believe our results may provide, among others, a direct explanation to recent puzzles about the Fulde Ferrel like states that are apparently observed in CeCoIn5.

cond-mat.supr-con↗
arXiv2007

Finite temperature phase diagram of a polarised Fermi condensate

The two-component Fermi gas is the simplest fermion system displaying superfluidity, and as such finds applications ranging from the theory of superconductivity to QCD. Ultracold atomic gases provide an exceptionally clean realization of this system, where the interatomic interaction and the atom species population are both independent, tuneable parameters. This allows one to investigate the Fermi gas with imbalanced spin populations, which had previously been experimentally elusive, and this prospect has stimulated much theoretical activity. Here we show that the finite temperature phase diagram contains a region of phase separation between the superfluid and normal states that touches the boundary of second-order superfluid transitions at a tricritical point, reminiscent of the phase diagram of $^3$He-$^4$He mixtures. A variation of interaction strength then results in a line of tricritical points that terminates at zero temperature on the molecular Bose-Einstein condensate (BEC) side. On this basis, we argue that tricritical points will play an important role in the recent experiments on polarised atomic Fermi gases.

cond-mat.other↗
arXiv2007

Polarized Fermi condensates with unequal masses: Tuning the tricritical point

We consider a two-component atomic Fermi gas within a mean-field, single-channel model, where both the mass and population of each component are unequal. We show that the tricritical point at zero temperature evolves smoothly from the BEC- to BCS-side of the resonance as a function of mass ratio r. We find that the interior gap state proposed by Liu and Wilczek is always unstable to phase separation, while the breached pair state with one Fermi surface for the excess fermions exhibits differences in its DoSs and pair correlation functions depending on which side of the resonance it lies. Finally, we show that, when r > 3.95, the finite temperature phase diagram of trapped gases at unitarity becomes topologically distinct from the equal mass system.

cond-mat.other↗
arXiv2006

Graphite intercalation compounds under pressure

Motivated by recent experimental work, we use first-principles density functional theory methods to conduct an extensive search for low enthalpy structures of C$_6$Ca under pressure. As well as a range of buckled structures, which are energetically competitive over an intermediate range of pressures, we show that the high pressure system ($\gtrsim 18$ GPa) is unstable towards the formation of a novel class of layered structures, with the most stable compound involving carbon sheets containing five- and eight-membered rings. As well as discussing the energetics of the different classes of low enthalpy structures, we comment on the electronic structure of the high pressure compound and its implications for superconductivity.

cond-mat.supr-con↗
arXiv2005

Electronic structure of superconducting graphite intercalate compounds: The role of the interlayer state

Although not an intrinsic superconductor, it has been long--known that, when intercalated with certain dopants, graphite is capable of exhibiting superconductivity. Of the family of graphite--based materials which are known to superconduct, perhaps the most well--studied are the alkali metal--graphite intercalation compounds (GIC) and, of these, the most easily fabricated is the C${}_8$K system which exhibits a transition temperature $\bm{T_c\simeq 0.14} $K. By increasing the alkali metal concentration (through high pressure fabrication techniques), the transition temperature has been shown to increase to as much as $\bm 5 $K in C${}_2$Na. Lately, in an important recent development, Weller \emph{et al.} have shown that, at ambient conditions, the intercalated compounds \cyb and \cca exhibit superconductivity with transition temperatures $\bm{T_c\simeq 6.5} $K and $\bm{11.5} $K respectively, in excess of that presently reported for other graphite--based compounds. We explore the architecture of the states near the Fermi level and identify characteristics of the electronic band structure generic to GICs. As expected, we find that charge transfer from the intercalant atoms to the graphene sheets results in the occupation of the $\bmπ$--bands. Yet, remarkably, in all those -- and only those -- compounds that superconduct, we find that an interlayer state, which is well separated from the carbon sheets, also becomes occupied. We show that the energy of the interlayer band is controlled by a combination of its occupancy and the separation between the carbon layers.

cond-mat.supr-con↗
arXiv2005

Fermion-mediated BCS-BEC Crossover in Ultracold Potassium-40 Gases

Studies of Feshbach resonance phenomena in fermionic alkali gases have drawn heavily on the intuition afforded by a Fermi-Bose theory which presents the Feshbach molecule as a featureless Bose particle. While this model may provide a suitable platform to explore the lithium-6 system, we argue that its application to potassium-40, where the hyperfine structure is inverted, is inappropriate. Introducing a three-state Fermi model, where a spin state is shared by the open and closed channel states, we show that effects of ``Pauli blocking'' are recorded in the internal structure of the condensate wave function.

cond-mat.other↗
arXiv2005

Condensation and Lasing of Microcavity Polaritons: Comparison between two Models

Condensation of microcavity polaritons and the substantial influence of pair-breaking disorder and decoherence leading to a laser regime has been recently considered using two different models: a model for direct two band excitons in a disordered quantum well coupled to light and a model where the cavity mode couples instead to a medium of localised excitons, represented by two-level oscillators in the presence of dephasing processes. Even if complementary from the point of view of assumptions, the models share most of the main conclusions and show similar phase diagrams. The issue whether excitons are propagating or localised seems secondary for the polariton condensation and the way in which pair-breaking disorder and decoherence processes influence the condensation and drive the microcavity into a lasing regime is, within the approximations used in each model, generic. The reasons for the similarities between the two physical situations are analysed and explained.

cond-mat.mes-hall↗
arXiv2005

Phase Bifurcation and Quantum Fluctuations in Sr3Ru2O7

The bilayer ruthenate Sr_3Ru_2O_7 has been cited as a textbook example of itinerant metamagnetic quantum criticality. However, recent studies of the ultra-pure system have revealed striking anomalies in magnetism and transport in the vicinity of the quantum critical point. Drawing on new experimental data, we propose that the anomalous phase behavior can be accommodated by a mechanism which relies on the effects of strong magnetoelastic coupling. We argue that the phenomenological theory identified here is more generic and raises the possibility of quantum tricritical behaviour.

cond-mat.str-el↗
arXiv2004

Dynamics of the BCS-BEC crossover in a degenerate Fermi gas

We study the short-time dynamics of a degenerate Fermi gas positioned near a Feshbach resonance following an abrupt jump in the atomic interaction resulting from a change of external magnetic field. We investigate the dynamics of the condensate order parameter and pair wavefunction for a range of field strengths. When the abrupt jump is sufficient to span the BCS to BEC crossover, we show that the rigidity of the momentum distribution precludes any atom-molecule oscillations in the entrance channel dominated resonances observed in the 40K and 6Li. Focusing on material parameters tailored to the 40K Feshbach resonance system at 202.1 gauss, we comment on the integrity of the fast sweet projection technique as a vehicle to explore the condensed phase in the crossover region

cond-mat.other↗
arXiv2004

Condensation of Cavity Polaritons in a Disordered Environment

A model for direct two band excitons in a disordered quantum well coupled to light in a cavity is investigated. In the limit in which the exciton density is high, we assess the impact of weak `pair-breaking' disorder on the feasibility of condensation of cavity polaritons. The mean-field phase diagram shows a `lower density' region, where the condensate is dominated by electronic excitations and where disorder tends to close the condensate and quench coherence. Increasing the density of excitations in the system, partially due to the screening of Coulomb interaction, the excitations contributing to the condensate become mainly photon-like and coherence is reestablished for any value of disorder. In contrast, in the photon dominated region of the phase diagram, the energy gap of the quasi-particle spectrum still closes when the disorder strength is increased. Above mean-field, thermal, quantum and fluctuations induced by disorder are considered and the spectrum of the collective excitations is evaluated. In particular, it is shown that the angle resolved photon intensity exhibits an abrupt change in its behaviour, going from the condensed to the non-condensed region.

cond-mat.mes-hall↗
arXiv2004

Localization from sigma-model geodesics

We use a novel method based on the semi-classical analysis of sigma-models to describe the phenomenon of strong localization in quasi one-dimensional conductors, obtaining the full distribution of transmission eigenvalues. For several symmetry classes, describing random superconducting and chiral Hamiltonians, the target space of the appropriate sigma-model is a (super)group manifold. In these cases our approach turns out to be exact. The results offer a novel perspective on localization.

cond-mat.mes-hall↗
arXiv2004

Models of coherent exciton condensation

That excitons in solids might condense into a phase-coherent ground state was proposed about 40 years ago, and has been attracting experimental and theoretical attention ever since. Although experimental confirmation has been hard to come by, the concepts released by this phenomenon have been widely influential. This tutorial review discusses general aspects of the theory of exciton and polariton condensates, focussing on the reasons for coherence in the ground state wavefunction, the BCS to Bose crossover(s) for excitons and for polaritons, and the relationship of the coherent condensates to standard lasers.

cond-mat.str-el↗
arXiv2004

Critical States in Disordered Superconducting Films

When subject to a pair-breaking perturbation, the pairing susceptibility of a disordered superconductor exhibits substantial long-ranged mesoscopic fluctuations. Focusing on a thin film subject to a parallel magnetic field, it is proposed that the quantum phase transition to the bulk superconducting condensate may be preempted by the formation of a glass-like phase with multi-fractal correlations of a complex order parameter. Although not universal, we argue that such behavior may be a common feature of quantum critical phenomena in disordered environments.

cond-mat.supr-con↗
arXiv2004

Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System

The development of a Turing instability to a spatially modulated state in a photoexcited electron-hole system is proposed as a novel signature of exciton Bose statistics. We show that such an instability, which is driven by kinetics of exciton formation, can result from stimulated processes that build up near quantum degeneracy. In the spatially uniform 2d electron-hole system, the instability leads to a triangular lattice pattern while, at an electron-hole interface, a periodic 1d pattern develops. We analyze the mechanism of wavelength selection, and show that the transition is abrupt (type I) for the uniform 2d system, and continuous (type II) for the electron-hole interface.

cond-mat.mes-hall↗
arXiv2003

Universality of Parametric Spectral Correlations: Local versus Extended Perturbing Potentials

We explore the influence of an arbitrary external potential perturbation V on the spectral properties of a weakly disordered conductor. In the framework of a statistical field theory of a nonlinear sigma-model type we find, depending on the range and the profile of the external perturbation, two qualitatively different universal regimes of parametric spectral statistics (i.e. cross-correlations between the spectra of Hamiltonians H and H+V). We identify the translational invariance of the correlations in the space of Hamiltonians as the key indicator of universality, and find the connection between the coordinate system in this space which makes the translational invariance manifest, and the physically measurable properties of the system. In particular, in the case of localized perturbations, the latter turn out to be the eigenphases of the scattering matrix for scattering off the perturbing potential V. They also have a purely statistical interpretation in terms of the moments of the level velocity distribution. Finally, on the basis of this analysis, a set of results obtained recently by the authors using random matrix theory methods is shown to be applicable to a much wider class of disordered and chaotic structures.

cond-mat.mes-hall↗