Excitonic Insulator Research Articles

Screening and gap generation in bilayer graphene

A fully self-consistent treatment for gap generation and Coulomb screening in excitonic insulators is presented. The method is based on the equations of motion for the relevant dynamical variables combined with a variational approach. Applying the theory for a model system of bilayer graphene, an excitonic ground state with a gap exceeding 10 meV is predicted.

Instability of the chiral phase and electronic ferroelectricity in the extended Falicov–Kimball model

AbstractWe consider a spinless extended Falicov–Kimball model (EFKM) at half‐filling, for the case of opposite‐parity bands. Within the Hartree–Fock approach, we calculate the excitation energies in the chiral phase, which is a possible mean‐field solution in the presence of a hybridisation. It is shown that the chiral phase is unstable. We then briefly review the accumulated results on stability and degeneracies of the excitonic insulator phase. Based on these, we conclude that the presence of both hybridisation and narrow‐band hopping is required for electronic ferroelectricity (EFE).

Time-domain evidence for an excitonic insulator

Time- and angle-resolved photoemission spectroscopy with a high–harmonic- generation source is used to classify the potential excitonic insulator 1T -TiSe2 and the reference Peierls-Mott insulator 1T -TaS2 on the basis of the melting times of “spectroscopic order parameters”.

A BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach

Motivated by recent experimental and theoretical studies of excitonic insulators, we use the variational cluster approximation to calculate the single-particle Green's function and anomalous Green's function of the Falicov-Kimball model extended by including a finite valence bandwidth. We thus determine the single-particle excitation gap due to the bound-state formation of an electron and a hole. We also evaluate the order parameter of the excitonic insulating state indicating quantum coherence of condensed excitons. We thereby discuss the excitonic insulator that typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of preformed excitons (strong-coupling regime). A BCS-BEC crossover of the model thus manifests itself as a function of the Coulombic coupling strength.

Electronic structure of Ta2NiSe5 as a candidate for excitonic insulators

We make the electronic structure calculations of Ta2NiSe5 known as a candidate for excitonic insulators where the semiconducting or semi-metallic ground state becomes unstable against the coherent formation of excitons. We use the generalized gradient approximation (GGA) in the density functional theory, where the Hubbard-type repulsive interaction U is taken into account (GGA+U). We find that the material has a very simple quasi-one-dimensional (1D) electronic structure which relates to the formation of excitons. From the calculated partial density of states, band dispersion and spatial distribution of charge densities, we find in the first approximation that the conduction band is configured by the Ta 1D chain and the valence band is configured by the Ni-Se 1D chain.

Variational cluster approach to the extended Falicov-Kimball model: A BCS-BEC crossover in the Excitonic insulators

We study the Coulomb-interaction induced spontaneous symmetry breaking of the excitonic insulator state in the two-dimensional extended Falicov-Kimball model. Using the variational cluster approximation, we evaluate the order parameter, single-particle excitation gap, momentum distribution functions, and anomalous Green's function as a function of U at zero temperature. We find that in the weak-to-intermediate couplinb regime, the Fermi surface is well-defined and the calculated results can be understood in the close correspondence with the BCS theory, whereas in the strong-coupling regime, the Fermi surface is ill-defined and the results are consistent with the picture of BEC.

Excitonic instabilities and insulating states in bilayer graphene

The competing ground states of bilayer graphene are studied by applying renormalization group techniques to a bilayer honeycomb lattice with nearest neighbor hopping. In the absence of interactions, the Fermi surface of this model at half-filling consists of two nodal points with momenta $\mathbf{K}$, ${\mathbf{K}}^{\ensuremath{'}}$, where the conduction band and valence band touch each other, yielding a semimetal. Since near these two points the energy dispersion is quadratic with perfect particle-hole symmetry, excitonic instabilities are inevitable if interband interactions are present. Using a perturbative renormalization group analysis up to the one-loop level, we find different competing ordered ground states, including ferromagnetism, superconductivity, spin and charge density wave states with ordering vector $\mathbf{Q}=\mathbf{K}\ensuremath{-}{\mathbf{K}}^{\ensuremath{'}}$, and excitonic insulator states. In addition, two states with valley symmetry breaking are found in the excitonic insulating and ferromagnetic phases. This analysis strongly suggests that the ground state of bilayer graphene should be gapped, and with the exception of superconductivity, all other possible ground states are insulating.

Excitonic Phases from Weyl Semimetals

Systems with strong spin-orbit coupling, which competes with other interactions and energy scales, offer a fertile playground to explore new correlated phases of matter. Weyl semimetals are an example where the phenomenon leads to a low-energy effective theory in terms of massless linearly dispersing fermions in three dimensions. In the absence of interactions chirality is a conserved quantum number, protecting the semimetallic physics against perturbations that are translationally invariant. In this Letter we show that the interplay between interaction and topology yields a novel chiral excitonic insulator. The state is characterized by a complex vectorial order parameter leading to a gapping out of the Weyl nodes. A striking feature is that it is ferromagnetic, with the phase of the order parameter determining the direction of the induced magnetic moment.

Collective excitations and stability of the excitonic phase in the extended Falicov-Kimball model

We consider the excitonic insulator state (often associated with electronic ferroelectricity), which arises on the phase diagram of an extended spinless Falicov-Kimball model (FKM) at half-filling. Within the Hartree--Fock approach, we calculate the spectrum of low-energy collective excitations in this state up to second order in the narrow-band hopping and/or hybridization. This allows to probe the mean-field stability of the excitonic insulator. The latter is found to be unstable when the case of the pure FKM (no hybridization with a fully localized band) is approached. The instability is due to the presence of another, lower-lying ground state and not to the degeneracy of the excitonic phase in the pure FKM. The excitonic phase, however, may be stabilized further away from the pure FKM limit. In this case, the low-energy excitation spectrum contains important information about the properties of the excitonic condensate (including the strongly suppressed critical temperature).

Phases of the excitonic condensate in two-layer graphene

Two graphene monolayers that are oppositely charged and placed close to each other are considered. Taking into account valley and spin degeneracy of electrons, we analyze the symmetry of the excitonic insulator states in such a system and build a phase diagram that takes into account the effect of the symmetry breaking due to the external in-plane magnetic field and the carrier density imbalance between the layers.

Two-dimensional semimetal-insulator transition in HgTe-based quantum wells induced by a longitudinal magnetic field

A metal-insulator transition in a two-dimensional semimetal based on HgTe quantum wells is discovered. The transition is induced by a magnetic field applied parallel to the plane of the quantum well. The threshold behavior of the activation energy as a function of the magnetic-field strength and an abrupt reduction of the Hall resistance at the onset of the transition suggest that the observed effect originates from the formation of an excitonic insulator.

Ab initiomany-body effects in TiSe2: A possible excitonic insulator scenario from GW band-shape renormalization

We present both theoretical ab-initio results within the Hedin's GW approximation and experimental angle-resolved photoemission and scanning tunneling spectroscopy measurements on TiSe${}_{2}$. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a $\ensuremath{\approx}$0.2-eV band-gap insulator consistent with our STS spectra at 5 K. The highest valence and the lowest conduction bands are strongly renormalized, with a loss of ${k}^{2}$ parabolic dispersion toward a ${k}^{4}$ shape. In particular, GW moves the top of valence moved toward a circle of points away from $\ensuremath{\Gamma}$, arising in a Mexican hat shape commonly associated with an excitonic insulator. Our calculations are in good agreement with experiment.

A new type of low temperature conductivity in InSb doped with Mn

We investigated unusual low temperature transport properties of InSb single crystals doped with manganese to concentrations in the range of 1017<NMn<2*1017cm-3 which includes the critical concentration for metal-insulator transition Ncr= 2*1017cm-3. It was found that in the temperature range T=10÷1.5K the resisitivity had an unusual behavior described by an exponential quadratic function ρ ∼ exp(Δ1/kT)2 where Δ1 varied from Δ1=0.3meV to Δ1=0. Coexistence of electrons and holes at temperatures around the metal-insulator transition in p-InSb(Mn) crystals gives the ground to explain the unusual temperature - resistivity dependence in the framework of excitonic insulator phase model (L. V. Keldysh and Yu. V. Kopaev, Sov. Phys. Solid State 6, 2219 (1965)).

Excitonic insulator state in the two-orbital Hubbard model: Variational cluster approach

The variational cluster approximation is used to study the spontaneous symmetry breaking of the excitonic insulator (EI) state in the two-orbital Hubbard model defined on the two-dimensional square lattice. Evaluating the order parameter of the EI state, staggered magnetization, and number of particles on each orbital, we obtain the ground-state phase diagram of the model in a wide parameter space of the intra and interorbital Coulomb interactions. We also calculate the single-particle and anomalous Green's functions. We show that the normal metallic phase is unstable to the formation of the EI state and the EI phase appears in a wide parameter region between the band-insulator and Mott-insulator phases.

Power law Kohn anomalies and the excitonic transition in graphene

Dirac electrons in graphene in the presence of Coulomb interactions of strength β have been shown to display power law behavior with β dependent exponents in certain correlation functions, which we call the mass susceptibilities of the system. In this work, we first discuss how this phenomenon is intimately related to the excitonic insulator transition, showing the explicit relation between the gap equation and response function approaches to this problem. We then provide a general computation of these mass susceptibilities in the ladder approximation, and present an analytical computation of the static exponent within a simplified kernel model, obtaining η0=1−β/βc. Finally we emphasize that the behavior of these susceptibilities provides new experimental signatures of interactions, such as power law Kohn anomalies in the dispersion of several phonons, which could potentially be used as a measurement of β.

Photoemission Spectroscopy of Ta2NiSe5

We report temperature-dependent angle-resolved photoemission spectroscopy measurement of Ta2NiSe5 which shows a semiconductor-semiconductor structural phase transition at around 330 K. Characteristically, flat band at the top of the valence band is observed, which is ascribed to the excitonic insulator effect. The top valence band shifts to higher binding energy and its bandwidth increases as the temperature decreases.

Electron-hole pair condensation at the semimetal-semiconductor transition: A BCS-BEC crossover scenario

We act on the suggestion that an excitonic insulator state might separate---at very low temperatures---a semimetal from a semiconductor and ask for the nature of these transitions. Based on the analysis of electron-hole pairing in the extended Falicov-Kimball model, we show that tuning the Coulomb attraction between both species, a continuous crossover between a BCS-like transition of Cooper-type pairs and a Bose-Einstein condensation of preformed tightly-bound excitons might be achieved in a solid-state system. The precursor of this crossover in the normal state might cause the transport anomalies observed in several strongly correlated mixed-valence compounds.

Time-domain classification of charge-density-wave insulators

Distinguishing insulators by the dominant type of interaction is a central problem in condensed matter physics. Basic models include the Bloch-Wilson and the Peierls insulator due to electron-lattice interactions, the Mott and the excitonic insulator caused by electron-electron interactions, and the Anderson insulator arising from electron-impurity interactions. In real materials, however, all the interactions are simultaneously present so that classification is often not straightforward. Here, we show that time- and angle-resolved photoemission spectroscopy can directly measure the melting times of electronic order parameters and thus identify-via systematic temporal discrimination of elementary electronic and structural processes-the dominant interaction. Specifically, we resolve the debates about the nature of two peculiar charge-density-wave states in the family of transition-metal dichalcogenides, and show that Rb intercalated 1T-TaS(2) is a Peierls insulator and that the ultrafast response of 1T-TiSe(2) is highly suggestive of an excitonic insulator.

Properties of an excitonic insulator in the superconducting state

The properties of an excitonic insulator with embedded (nondissipative) current are studied using the self-consistent field approximation, in which the wavefunction of the system has the form of the known Bardeen-Cooper-Schrieffer trial function with time-dependent coefficients; the equations for these coefficients are derived. Such a formulation holds for the homogeneous case (in the absence of a coordinate dependence). We consider two problems: (i) time evolution of the system in the case when an embedded current exists at the initial instant; and (ii) the response of the system to an abrupt perturbation (the vector potential changes jumpwise from zero to a certain finite value). In both cases, the state of the system depends on time, but some characteristics (e.g., undamped current) tend to a constant value. For a weak perturbation, the system behaves as an insulator. If the perturbation is not small (on the order of the gap in the spectrum), nonlinear effects lead to substantial differences: a certain part of the embedded current is preserved in the former case, while the initial current in the latter case acquires a certain addition.

Transport and magnetotransport effects in indium antimonide doped with manganese within quasimetal‐insulator transition

AbstractInSb crystals doped with manganese within the critical concentration of metal‐insulator transition (Ncr = 2×1017 cm‐3) demonstrate colossal negative magnetoresitance and Hall constant sign inversion at low temperatures in magnetic field – unusual for nonmagnetic materials. Studies of the magnetotransport and conductivity dependency on temperature and manganese concentration revealed that both holes and electrons influence the effects. An excitonic insulator phase conception is proposed as a new approach for the explanation of conductivity and magnetotransport anomalies in p‐InSb(Mn). (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)