Peter B. Littlewood

Angular distribution of photoluminescence as a probe of bose condensation of trapped excitons.

Recent experiments on two-dimensional exciton systems have shown that excitons collect in shallow in-plane traps. We find that Bose condensation in a trap results in a dramatic change of the exciton photoluminescence (PL) angular distribution. The long-range coherence of the condensed state gives rise to a sharply focused peak of radiation in the direction normal to the plane. By comparing the PL profile with and without Bose condensation, we provide a simple diagnostic for the existence of a Bose condensate. The PL peak has strong temperature dependence due to the thermal order parameter phase fluctuations across the system. The angular PL distribution can also be used for imaging vortices in the trapped condensate. Vortex phase spatial variation leads to destructive interference of PL radiation in certain directions, creating nodes in the PL distribution that imprint the vortex configuration.

Polariton condensation with saturable molecules dressed by vibrational modes

Polaritons, mixed light-matter quasiparticles, undergo a transition to a condensed, macroscopically coherent state at low temperatures or high densities. Recent experiments show that coupling light to organic molecules inside a microcavity allows condensation at room temperature. The molecules act as saturable absorbers with transitions dressed by molecular vibrational modes. Motivated by this, we calculate the phase diagram and spectrum of a modified Tavis-Cummings model, describing vibrationally dressed two-level systems, coupled to a cavity mode. Coupling to vibrational modes can induce re-entrance, i.e. a normal-condensed-normal sequence with decreasing temperature and can drive the transition first-order.

Large isotropic negative thermal expansion above a structural quantum phase transition

Perovskite structured materials contain myriad tunable ordered phases of electronic and magnetic origin with proven technological importance and strong promise for a variety of energy solutions. An always-contributing influence beneath these cooperative and competing interactions is the lattice, whose physics may be obscured in complex perovskites by the many coupled degrees of freedom, which makes these systems interesting. Here, we report signatures of an approach to a quantum phase transition very near the ground state of the nonmagnetic, ionic insulating, simple cubic perovskite material

Why is the electrocaloric effect so small in ferroelectrics

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Research Update: Plentiful magnetic moments in oxygen deficient SrTiO3

, and show that its physical properties are strongly effected as much as 100 K above the putative transition. Spatial and temporal correlations in the high-symmetry cubic phase determined using energy- and momentum-resolved inelastic x-ray scattering as well as x-ray diffraction reveal that soft mode, central peak, and thermal expansion phenomena are all strongly influenced by the transition.

Optical recombination of biexcitons in semiconductors

Ferroelectrics are attractive candidate materials for environmentally friendly solid state refrigeration free of greenhouse gases. Their thermal response upon variations of external electric fields is largest in the vicinity of their phase transitions, which may occur near room temperature. The magnitude of the effect, however, is too small for useful cooling applications even when they are driven close to dielectric breakdown. Insight from microscopic theory is therefore needed to characterize materials and provide guiding principles to search for new ones with enhanced electrocaloric performance. Here, we derive from well-known microscopic models of ferroelectricity meaningful figures of merit for a wide class of ferroelectric materials. Such figures of merit provide insight into the relation between the strength of the effect and the characteristic interactions of ferroelectrics such as dipolar forces. We find that the long range nature of these interactions results in a small effect. A strategy is pro...

Non-equilibrium Properties of a Pumped-Decaying Bose-Condensed Electron–Hole Gas in the BCS–BEC Crossover Region

Correlated band theory is employed to investigate the magnetic and electronic properties of different arrangements of oxygen di- and tri-vacancy clusters in SrTiO3. Hole and electron doping of oxygen deficient SrTiO3 yields various degrees of magnetization as a result of the interaction between localized magnetic moments at the defect sites. Different kinds of Ti atomic orbital hybridization are described as a function of the doping level and defect geometry. We find that magnetism in SrTiO3−δ is sensitive to the arrangement of neighbouring vacancy sites, charge carrier density, and vacancy-vacancy interaction. Permanent magnetic moments in the absence of vacancy doping electrons are observed. Our description of the charged clusters of oxygen vacancies widens the previous descriptions of mono- and multi-vacancies and points out the importance of the controlled formation at the atomic level of defects for the realization of transition metal oxide based devices with a desirable magnetic performance.

Photoluminescence and gain/absorption spectra of a driven-dissipative electron-hole-photon condensate

We calculate the photoluminescence spectrum and lifetime of a biexciton in a semiconductor using Fermis golden rule. Our biexciton wavefunction is obtained using a Quantum Monte Carlo calculation. We consider a recombination process where one of the excitons within the biexciton annihilates. For hole masses greater than or equal to the electron mass, we find that the surviving exciton is most likely to populate the ground state. We also investigate how the confinement of excitons in a quantum dot would modify the lifetime in the limit of a large quantum dot where confinement principally affects the centre of mass wavefunction. The lifetimes we obtain are in reasonable agreement with experimental values. Our calculation can be used as a benchmark for comparison with approximate methods.

Non-Equilibrium Bose-Einstein Condensation in a Dissipative Environment

We theoretically investigate a Bose-condensed exciton gas out of equilibrium. Within the framework of the combined BCS-Leggett strong-coupling theory with the non-equilibrium Keldysh formalism, we show how the Bose–Einstein condensation (BEC) of excitons is suppressed to eventually disappear, when the system is in the non-equilibrium steady state. The supply of electrons and holes from the bath is shown to induce quasi-particle excitations, leading to the partial occupation of the upper branch of Bogoliubov single-particle excitation spectrum. We also discuss how this quasi-particle induction is related to the suppression of exciton BEC, as well as the stability of the steady state.

Novel Quantum Condensates in Excitonic Matter

We investigate theoretically nonequilibrium effects on photoluminescence and gain/absorption spectra of a driven-dissipative exciton-polariton condensate, by employing the combined Hartree-Fock-Bogoliubov theory with the generalized random phase approximation extended to the Keldysh formalism. Our calculated photoluminescence spectra is in semiquantitative agreement with experiments, where features such as a blue shift of the emission from the condensate, the appearance of the dispersionless feature of a diffusive Goldstone mode, and the suppression of the dispersive profile of the mode are obtained. We show that the nonequilibrium nature of the exciton-polariton condensate strongly suppresses the visibility of the Bogoliubov dispersion in the negative energy branch (ghost branch) in photoluminescence spectra. We also show that the trace of this branch can be captured as a hole burning effect in gain/absorption spectra. Our results indicate that the nonequilibrium nature of the exciton-polariton condensate strongly reduces quantum depletion, while a scattering channel to the ghost branch is still present.