Excitonic Insulator Research Articles

The Thermoelectric Power of the Excitonic Phase

Impurities in excitonic insulators are classified into two categories corresponding to non-magnetic and magnetic impurities in the case of superconductivity. Electrical conductivity, thermoelectric power and thermal conductivity are calculated for the cases of respective impurities. The heat current in existence of the vector potential A ( x , t ) is discussed. In order to understand the thermoelectric power of Cr and its alloys it seems necessary to assume the different sizes of the fermi spheres for electrons and holes, respectively.

Note on superconductivity of the excitonic insulator

A discussion is given of the superconductive properties of the excitonic insulator. The conclusion of Kohn and Sherrington that the excitonic insulator cannot superconduct is shown to be invalid.

Magnetic-field-induced semiconductor-semimetal transition in Bi-Sb alloys

Accurate and detailed measurements of the temperature dependence of the longitudinal magnetoresistance of single-crystal Bi-Sb alloys have been made, with static magnetic fields in the range 0–100 kG oriented parallel to the trigonal axis. Alloy concentrations were in the range 8–12 at.% Sb, and temperatures in the range 1–35 K. At very high fields the resistance increases with increasing temperature in a metallic manner with “ideal” and “residual” components, in contrast to the semiconductor behavior observed at zero field or low fields. For the high-field semimetal regime the electrical resistance behaves in a simple manner similar to a metal in zero field, in contrast to the complicated magnetoresistance phenomena for metals in low fields. This behavior can be understood in terms of a simple quasi-one-dimensional extreme-quantum-limit regime. The magnetic-field-induced semiconductor-semimetal transition is associated with an energy gap and changes of the energy-band structure which are of order 1 meV. Thermal activation energies for electrical conduction manifest this gap only at temperatures below approximately 20 K. Activation energies an order of magnitude larger which have been measured at considerably higher temperatures are apparently the direct gap at theL-point in the Brillouin zone and are not directly connected with the semiconductor-semimetal transition. Our results indicate that the zero-field indirectL-T energy gap increases from zero somewhere near 7–8 at. % Sb to values only as large as approximately 1.5 meV at 12 at. % Sb. At the magnetic-field induced transition there occurs evidence of an intermediate “excitonic insulator” phase, a resistance minimum below 10 K reminiscent of the Kondo alloy behavior. This anomalous regime is a property of the semiconductor-to-semimetal transition and cannot be associated with the well-known temperature and magnetic-field “freeze-out” of charge carriers in extrinsic semiconductors, or with magnetic ordering of the Kondo type.

Superconductive Properties of the Excitonic Insulator

Analysis of a simple model of the excitonic insulator shows that the ordered phase exhibits electrical superconductivity whenever the conduction-band mass differs from the valence-band mass. Interband scattering of electrons by the magnetic vector potential plays an essential role. States of finite electric persistent current are demonstrated explicitly. The excitonic insulator is a system where diagonal versus off-diagonal long-range order is a function of one's bookkeeping.

Excitonic Insulator in One Dimension

Excitonic Insulator in One Dimension

Hall Effect in Excitonic Insulator

Hall Effect in Excitonic Insulator

Dielectric Screening of the Electron-Hole Interaction in Small-Gap Semiconductors

The dielectric function that screens the electron-hole interaction in semiconductors has been derived from first principles, and is found to differ in an essential way from the usual random-phase-approximation (RPA) dielectric function. This screening function determines the binding energy of the exciton, and hence it is important for the theory of the excitonic insulator. We have studied the binding energy of the exciton for a simple band structure, and find that the case considered does not become unstable toward exciton formation, whereas the usual dielectric function in RPA predicts the excitonic instability.

The transition metal dichalcogenides discussion and interpretation of the observed optical, electrical and structural properties

Abstract The transition metal dichalcogenides are about 60 in number. Two-thirds of these assume layer structures. Crystals of such materials can be cleaved down to less than 1000 Å and are then transparent in the region of direct band-to-band transitions. The transmission spectra of the family have been correlated group by group with the wide range of electrical and structural data available to yield useful working band models that are in accord with a molecular orbital approach. Several special topics have arisen; these include exciton screening, d-band formation, and the metal/insulator transition; also magnetism and superconductivity in such compounds. High pressure work seems to offer the possibility for testing the recent theory of excitonic insulators.

On the transition temperature of an excitonic insulator

The dependence of the excitonic insulator transition temperature on the shift in chemical potential due to the presence of various types of impurities (donors, acceptors or normal impurities) and on the change of band masses due to small crystal deformations is investigated in terms of a mathematical parameter, ηp, which involves all these physical factors. It is shown that both in the semiconductor region and in the semi-metal region, the transition temperature is independent of the sign of this parameter but is a maximum when this parameter is zero.

Possible Cooperative Phase for Electrons in Double-Stranded DNA

The possibility of a superfluid phase occurring in macromolecules such as double-stranded DNA is discussed. The restricted geometry for electron states in macromolecules is probably more propitious for the excitonic insulator than for superconductivity. The excitonic insulator in restricted geometry is discussed, and some qualitative comments are made on the band structure of double-stranded DNA.

Excitonic Insulator in GaAs Junctions

In this paper a case will be given for the existence of an excitonic insulating region in semiconductor junctions (either metal-semiconductor or p−n semiconductor).Received 11 April 1968DOI:https://doi.org/10.1103/PhysRevLett.21.284©1968 American Physical Society

Excitonic Insulator in a Magnetic Field

The effects of large magnetic fields on the excitonic insulator are considered. Since the excitonic insulator does not display a Meissner effect (according to a recent paper by Jerome, Rice, and Kohn), a Hartree-Fock ground state can be expressed in terms of Landau-level basis functions. A magnetic-field-dependent phase diagram and the magnetoconductivity are discussed. The excitonic insulator, not yet observed experimentally, may be observable in high magnetic fields at normal pressure.

Nonexistence of excitonic insulators in one and two dimensions

The Bogoliubov inequality is used to prove the nonexistence of the excitonic insulating phase in one and two dimensions for systems of electrons described by three different many-band models with interactions. The proof is carried through (1) for arbitrary two-particle interactions with an interaction potential which goes to zero faster than [Formula: see text] (n being the number of dimensions) at large [Formula: see text], (2) for two-particle interactions of arbitrary range, provided only the so-called Coulomb and exchange parts of the interaction are retained and the remainder is neglected, and (3) for the simplified isotropic two-band model suggested for the excitonic insulator by Des Cloizeaux.

Transport Properties of the "Excitonic Insulator": Thermal Conductivity

The thermal conductivity of the excitonic insulator is calculated in the semimetallic limit. At low temperatures the main scattering mechanism is assumed to be due to impurities. Despite a recent claim of "super-thermal conductivity" of the system, which can be described as a condensate of electron-hole pairs, we find that the thermal conductivity is well behaved. It is shown that the thermal conductivity in the semimetallic limit of the underlying two-band model is almost identical to the thermal conductivity of a superconductor containing magnetic impurities as scatterers. The analogy stems from the fact that the thermodynamic properties in the two cases are similar, as was discussed recently.

Transport Properties of the "Excitonic Insulator": Electrical Conductivity

The dc conductivity of the insulator recently discussed in the literature is calculated in the semimetallic region. The calculations are based on recent work on the description of the excitonic phase in the presence of impurities. It is shown that the conductivity decreases below the transition temperature to the excitonic state. For low impurity concentrations the system acquires insulating properties. For higher impurity concentrations the conductivity is still nonzero at $T=0$. Thus, metallic properties prevail in the excitonic phase. It is pointed out that this behavior depends essentially on the form of the excitation spectrum of the system, i.e., the presence or absence of a gap. At the transition temperature the conductivity-versus-temperature curve has a finite slope.

Theory of the Excitonic Insulator in the Presence of Normal Impurities

Recently several papers have discussed the possibility of a new kind of insulating phase in semimetals or semiconductors with small band gap. This phase can be described as a condensate of electron-hole pairs (excitons) due to an effective interaction between valence and conduction electrons. This paper extends the analysis of the excitonic phase in the semimetallic region in the presence of randomly distributed normal (i.e., nonmagnetic) impurities. It is shown that the impurities have a pair-breaking effect similar to the case of magnetic impurities in superconductors. The Abrikosov-Gorkov theory developed for the latter case is applied with minor modifications to the excitonic phase. It is shown that beyond a critical impurity concentration the excitonic phase cannot exist. Changes in the transition temperature and in the order parameter are calculated, as well as the density of states. It is found that in a region close to the critical concentration the excitation spectrum of the system has no energy gap.