Possibility of a superfluid phase in a Bose condensed excitonic state

Wächter, P.; Bucher, B.; Malar, J.

doi:10.1103/physrevb.69.094502

Cited by 81 publications

(121 citation statements)

References 17 publications

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“…45 Te 0.55 , are further candidates for exciton condensation. Fine tuning the excitonic level to the narrow 4f valence band, a rather large number of about 10 20 − 10 21 cm −3 (small-to-intermediate sized) excitons can be created, which presumably condense into an EI state, at temperatures below 20 K in the pressure range between 5 and 11 kbar [22][23][24] . Clearly, for these rather complex transition-metal/rare-earth compounds with strong electronic correlations, simple effective-massmodel based theories will be too crude.…”

Section: Introductionmentioning

confidence: 99%

Existence of excitonic insulator phase in the extended Falicov-Kimball model: SO(2)-invariant slave-boson approach

et al. 2010

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We re-examine the three-dimensional spinless Falicov-Kimball model with dispersive f electrons at half-filling, addressing the dispute about the formation of an excitonic condensate, which is closely related to the problem of electronic ferroelectricity. To this end, we work out a slave-boson functional integral representation of the suchlike extended Falicov-Kimball model that preserves the SO(2) ⊗ U (1) ⊗2 invariance of the action. We find a spontaneous pairing of c electrons with f holes, building an excitonic insulator state at low temperatures, also for the case of initially nondegenerate orbitals. This is in contrast to recent predictions of scalar slave-boson mean-field theory but corroborates previous Hartree-Fock and RPA results. Our more precise treatment of correlation effects, however, leads to a substantial reduction of the critical temperature. The different behavior of the partial densities of states in the weak and strong inter-orbital Coulomb interaction regimes supports a BCS-BEC transition scenario.

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Section: Introductionmentioning

confidence: 99%

Existence of excitonic insulator phase in the extended Falicov-Kimball model: SO(2)-invariant slave-boson approach

et al. 2010

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“…Excitonic correlations [14,27,28] are believed to play a role at similar densities in TmSe 0:45 Te 0:55 [29].…”

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Infrared Conductivity of Elemental Bismuth under Pressure: Evidence for an Avoided Lifshitz-Type Semimetal-Semiconductor Transition

et al. 2010

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The application of pressure to elemental bismuth reduces its conduction-valence band overlap, and results in a semimetal-semiconductor (SMSC) transition around 25 kbar. This transition is nominally of the topological ''Lifshitz'' Fermi surface variety, but there are open questions about the role of interactions at low charge densities. Using a novel pressure cell with optical access, we have performed an extensive study of bismuth's infrared conductivity under pressure. In contrast to the expected pure band behavior we find signatures of enhanced interaction effects, including strongly coupled charge-plasmon (plasmaron) features and a plasma frequency that remains finite up to the transition. These effect are inconsistent with a pure Lifshitz bandlike transition. We postulate that interactions play a central role in driving the transition.

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“…A phase diagram has been deduced out of the resistivity, thermal diffusity and heat conductivity data, which contains, below 20 K and in the pressure range between 5 and 11 kbar, a superfluid Bose condensed state. 4 The experimental claims for excitonic condensation in TmSe 0.45 Te 0.55 have been analysed from a theoretical point of view. 5,6 Adapting the standard effective-mass, (statically) screened Coulomb interaction model to the Tm[Se,Te] electron-hole system, the valence-band-hole conduction-band-electron mass asymmetry was found to suppress the excitonic insulator (EI) phase on the semimetallic side, as observed experimentally.…”

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confidence: 99%

Bound state formation and the nature of the excitonic insulator phase in the extended Falicov-Kimball model

et al. 2008

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Motivated by the possibility of pressure-induced exciton condensation in intermediate-valence Tm[Se,Te] compounds we study the Falicov-Kimball model extended by a finite f-hole valence bandwidth. Calculating the Frenkel-type exciton propagator we obtain excitonic bound states above a characteristic value of the local interband Coulomb attraction. Depending on the system parameters coherence between c-and f-states may be established at low temperatures, leading to an excitonic insulator phase. We find strong evidence that the excitonic insulator typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate (BEC) of preformed excitons (strong-coupling regime), which points towards a BCS-BEC transition scenario as Coulomb correlations increase.PACS numbers: 71.28.+d, 71.35.Lk, 71.30.+h, 71.28.+d. 71.27.+a That excitons in solids might condense into a macroscopic phase-coherent quantum state-the excitonic insulator-was theoretically proposed about more than four decades ago 1 , for a recent review see Ref.2. The experimental confirmation has proved challenging, because excitonic quasiparticles are not the ground state but bound electron-hole excitations that tend to decay on a very short timescale. Thus a large number of excitons has to be created, e.g. by optical pumping, with sufficiently long lifetimes as a steady-state precondition for the Bose-Einstein condensate (BEC) realizing process.The obstacles to produce a BEC out of the faroff-equilibrium situation caused by optical excitation might be circumvented by pressure-induced generation of excitons. Hints that pressure-sensitive, narrow-gap semiconducting materials, such as intermediate-valent TmSe 0.45 Te 0.55 , might host an excitonic BEC in solids came from a series of electric and thermal transport measurements. 3 Fine-tuning the excitonic level, by applying pressure, to the level of electrons in the narrow 4f-valence band, excitons can form near the semiconductor semimetal transition in thermodynamical equilibrium and might give rise to collective excitonic phases. A phase diagram has been deduced out of the resistivity, thermal diffusity and heat conductivity data, which contains, below 20 K and in the pressure range between 5 and 11 kbar, a superfluid Bose condensed state. 4 The experimental claims for excitonic condensation in TmSe 0.45 Te 0.55 have been analysed from a theoretical point of view. 5,6 Adapting the standard effective-mass, (statically) screened Coulomb interaction model to the Tm[Se,Te] electron-hole system, the valence-band-hole conduction-band-electron mass asymmetry was found to suppress the excitonic insulator (EI) phase on the semimetallic side, as observed experimentally. But also on the semiconducting side, the EI instability might be prevented-within this model-by either electron-hole liquid phases 6,7 or, at very large electron-hole mass ratios ( 100), by Coulomb crystallization. 8 The effective-mass Mott-Wannier-type exciton model neglects, however, important band structure effec...

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Possibility of a superfluid phase in a Bose condensed excitonic state

Cited by 81 publications

References 17 publications

Existence of excitonic insulator phase in the extended Falicov-Kimball model: SO(2)-invariant slave-boson approach

Existence of excitonic insulator phase in the extended Falicov-Kimball model: SO(2)-invariant slave-boson approach

Infrared Conductivity of Elemental Bismuth under Pressure: Evidence for an Avoided Lifshitz-Type Semimetal-Semiconductor Transition

Bound state formation and the nature of the excitonic insulator phase in the extended Falicov-Kimball model

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