Excitonic Insulator State Research Articles

Excitonic Insulator State inTa2NiSe5Probed by Photoemission Spectroscopy

We report on a photoemission study of Ta2NiSe5 that has a quasi-one-dimensional structure and an insulating ground state. Ni 2p core-level spectra show that the Ni 3d subshell is partially occupied and the Ni 3d states are heavily hybridized with the Se 4p states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni 3d-Se 4p hole and the Ta 5d electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Slave-boson theory of the extended Falicov-Kimball model

The extended Falicov-Kimball model, with both an on-site hybridization potential and dispersive narrow band, is examined within the saddle-point approximation to the Kotliar-Ruckenstein [Phys. Rev. Lett. 57, 1362 (1986)] slave-boson theory. We first set the hybridization potential to zero and find that the phase diagram depends strongly on the orbital structure: for degenerate orbitals, a correlated-insulating state is found at sufficiently strong interaction strengths, whereas a finite orbital energy difference can lead to discontinuous valence transitions. The obtained phase diagram is very sensitive to the presence of a finite hybridization potential. As in Hartree-Fock theory, we find an enhancement of the hybridization by the interorbital Coulomb repulsion. The more precise treatment of correlation effects, however, leads to large deviations from the Hartree-Fock results. In the limit of vanishing hybridization, an excitonic insulator state is only found when the orbitals are degenerate, which restricts this phase to a much smaller parameter space than in other available mean-field theories.

Graphite as a bose metal

Although a considerable amount of the research work has been done on graphite, its physical properties are still not well understood, and novel phenomena such as the magnetic-field-driven metal-insulator transition (MIT), the quantum Hall effect, ferromagnetic and superconducting correlations have recently been revealed. Theoretical analysis suggests that the MIT in graphite is the condensed-matter realization of the magnetic catalysis phenomenon known in relativistic theories of (2 + 1) - dimensional Dirac fermions (DF), i. e. that the applied field opens an insulating gap in the spectrum of DF, associated with the electron-hole pairing. On the other hand, we demonstrate in this paper that a two parameter scaling analysis proposed by Das and Doniach [D. Das and S. Doniach, Phys. Rev. B 64, 134511 (2001)] to characterize the magnetic-field-tuned Bose metal - insulator transition can be well applied to the MIT observed in graphite. We discuss the possibility that the MIT in graphite is associated with the transition between Bose metal and excitonic insulator states.

Possibilities for exciton condensation in semiconductor quantum-well structures

Recent progress in understanding the behavior of condensed excitonic systems is reviewed. The Bose-Einstein condensed excitonic insulator is studied numerically within the mean-field pairing approximation. In particular, we examine in some detail a system of spatially separated, two-dimensional (2D) electrons and holes, as a candidate system for study in semiconductor heterostructures. Based on these calculations, we suggest that it may be possible to create a droplet of high density exciton condensate fluid in semiconductor quantum-well structures. In heterostructures, the recombination of electrons and holes is dipole-allowed, which presents both difficulties and opportunities. The excitonic insulator state should be superradiant, which provides a direct measure of the coherence of the ground state. Additionally there is a gap between the absorption and emission spectrum, and collective modes lying within this quasiparticle gap. Coupling of the condensate to radiative modes of an optical cavity may offer an opportunity to manipulate the condensate and its optical response.

Excitonic instability and origin of the mid-gap states

In the framework of the two-band model of a doped semiconductor the self-consistent equations describing the transition into the excitonic insulator state are obtained for the 2D case. It is found that due to the exciton-electron interactions the excitonic phase may arise with doping in a semiconductor stable initially with respect to excitonic transition in the absence of doping. The effects of the strong interactions between electron (hole) Fermi-liquid (FL) and excitonic subsystems can lead to the appearance of the states lying in the middle of the insulating gap.

Non-stationary excitonic-insulator states in photoexcited semiconductors

Time dependent solutions of the semiconductor Bloch equations (SBE) are presented for zero external field which can be identified with a possible dynamics of the so called excitonic-insulator state (EIS) of a semiconductor after the laser pulse has switched off. The collective oscillation of the macroscopic polarization locks into a definite frequency depending on the energy absorbed during the interaction time with the ultra short laser pulse. This dynamical state is different from a stationary EIS proposed previously to be the final state for detunings above the exciton instability and will be demonstrated to appear already for off-resonant excitation. Numerical results are presented for a quasi 1d-model with long-range Coulomb interaction.

Precursors of the excitonic insulator in excited quantum wells

The theory of the magnetooptical properties of highly excited quantum wells is reviewed with emphasis on recent experimental work in a collaboration led by the Chernogolovka group. Realistic numerical calculations strongly support the interpretation of the experimental magnetoluminescence spectra in terms of bound electron-hole pairs, but at the experimental plasma temperatures of ~40-50 K phase coherence and thus observation of the excitonic insulator state is not yet achieved. Important screening effects beyond the Hartree-Fock approximation exist even at magnetic fields of ~10T. At lower densities, evidence of dynamical screening of the electron-hole interaction is found.

Electron-hole magnetoplasmas in quantum wells.

The coupling of a photon probe to electron-hole magnetoplasmas in quantum wells is calculated for the first time including electron-hole pairing and excitonic effects. Optical experiments on modulation-doped quantum wells support the validity of the mean-field approximation involved. The condensation of quasi-two-dimensional, neutral, electron-hole plasmas into an excitonic-insulator state is accompanied by strong modifications of magneto-optical spectra. Significant magnetic field and well-width dependences on dynamical Stark effect and exciton saturation are predicted.