Excitonic Insulator Transition Research Articles

Collective electronic excitations in charge density wave systems: The case of CuTe

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative.

Doping-controlled transition from excitonic insulator to semimetal in Ta2NiSe5

Excitonic insulator (EI) is an intriguing insulating phase of matter, where electrons and holes are bonded into pairs, so called excitons, and form a phase-coherent state via Bose-Einstein Condensation (BEC). Its theoretical concept has been proposed several decades ago, but the followed research is very limited, due to the rare occurrence of EI in natural materials and the lack of manipulating method of excitonic condensation. In this paper, we report the realization of a doping-controlled EI-to-semi-metal transition in Ta$_2$NiSe$_5$ using $in$-$situ$ potassium deposition. Combining with angle-resolved photoemission spectroscopy (ARPES), we delineate the evolution of electronic structure through the EI transition with unprecedented precision. The results not only show that Ta$ _2 $NiSe$ _5 $ (TNS) is an EI originated from a semi-metal non-interacting band structure, but also resolve two sequential transitions, which could be attributed to the phase-decoherence and pair-breaking respectively. Our results unveil the Bardeen-Cooper-Schrieffer (BCS)-BEC crossover behavior of TNS and demonstrate that its band structure and excitonic binding energy can be tuned precisely via alkali-metal deposition. This paves a way for investigations of BCS-BEC crossover phenomena, which could provide insights into the many-body physics in condensed matters and other many-body systems.

High thermoelectric performance in excitonic bilayer graphene

We consider the excitonic effects on the thermal properties in the AB-stacked bilayer graphene. The calculations are based on the bilayer generalization of the usual Hubbard model at the half-filling. The full interaction bandwidth is used without any low-energy assumption. We obtain the unusually high values for the electronic figure of merit even at the room-temperatures which is very promising for the thermoelectric applications of the AB-bilayer structure. We discuss the effects of the interlayer Coulomb interaction and temperature on different thermal parameters in the bilayer graphene and we emphasize the role of the charge neutrality point in the thermal properties and within the excitonic insulator transition scenario. The calculated values of the rate of thermoelectric conversion efficiency suggest the possibility of high-performance device applications of AB-bilayer graphene.

Strong interband interaction in the excitonic insulator phase of Ta2NiSe5

Excitonic insulator (EI) was proposed in 60's as a distinct insulating state originating from pure electronic interaction, but its material realization has been elusive with extremely few material candidates and with only limited evidence such as anomalies in transport properties, band dispersions, or optical transitions. We investigate the real-space electronic states of the low temperature phase in Ta$_2$NiSe$_5$ with an atomic resolution to clearly identify the quasiparticle energy gap together with the strong electron-hole band renormalization using scanning tunneling microscopy (STM) and spectroscopy (STS). These results are in good agreement with the EI transition scenario in Ta$_2$NiSe$_5$. Our spatially-resolved STS data and theoretical calculations reveal further the orbital inversion at band edges, which indicates the exciton condensation close to the Bardeen-Cooper-Schrieffer regime.

Layer- and substrate-dependent charge density wave criticality in 1T–TiSe2

TiSe2 exhibits an unconventional charge density wave (CDW) that has been associated with an excitonic insulator transition. Here we investigate how the CDW transition is changed for single to few layers compared to bulk TiSe2. TiSe2 grown by molecular beam epitaxy on HOPG- or MoS2-substrates is characterized by variable temperature scanning tunneling microscopy and spectroscopy. We show that the CDW state persists for the monolayer but the transition temperature TCDW is significantly increased compared to the bulk. Furthermore, TCDW is strongly dependent on the substrate material. Within the model of an excitonic insulator phase for TiSe2, the substrate dependence may be associated with variations of the excitonic binding energies by the dielectric properties of the substrate. Interestingly, for single layer TiSe2 on HOPG we also observe peaks in the tunneling spectra below 50 K, which are tentatively assigned to coherence peaks of an excitonic condensate. The peaks are observed below TCDW of ~230 K, suggesting that an excitonic insulator induced CDW can exist without a phase coherent state.

Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of <50 meV. Below TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB. Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC–EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.

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.

Excitonic insulator transition in the conjugated polymer polyacene

According to molecular orbital theory, the symmetrically positioned one-dimensional (1-D) conduction and valence bands of polyacene touch at the X point. Clearly, the exciton binding energy of this semimetal exceeds the band gap so that polyacene should be a textbook case of a semimetal undergoing a transition to an excitonic insulator. We investigate qualitatively this scenario for polyacene at zero temperature ( T=0). Collectively bound electron-hole pairs (singlet- or triplet-state excitons) become mixed into the new excitonic ground state (EGS), much like pairs of electrons become mixed into the ground state of a Bardeen–Cooper–Schrieffer (BCS) superconductor. But quite unlike the latter case, the bound electron-hole pairs in the EGS can be directly photoionized by light with energy ℏ ω≥2Δ, where 2Δ is the mean excitonic energy gap of the polymer. Thus, a new (IR) optical absorption appears in the polymer, polarized along the chain direction ( σ 1( ω)≠0) and rendering the polymer an IR photoconductor. To our knowledge, the optical absorption of the excitonic insulator for ℏ ω≥2Δ has not been previously investigated. The prospect of a polymeric quasi-1-D excitonic insulator is an interesting one.

Ghost excitonic insulator transition in layered graphite.

Some unusual properties of layered graphite, including a linear energy dependence of the quasiparticle damping and weak ferromagnetism at low doping, are explained as a result of the proximity of a single graphene sheet to the excitonic insulator phase which can be further stabilized in a doped system of many layers stacked in the staggered ( ABAB...) configuration.

Ghost Excitonic Insulator Transition in Layered Graphite

Ghost Excitonic Insulator Transition in Layered Graphite

The dipolar Frenkel excitonic insulator phase of an impurity in a liquid solvent: theory

A theory of the dipolar Frenkel excitonic insulator (EI) phase developed in an earlier paper is extended to spatially disordered systems. Using the Hartree approximation studied previously, the authors derive, for a given atomic centre-of-mass configuration, a self-consistency equation for the atomic dipole moments, a non-zero solution to which indicates an EI phase. They obtain as a special case the microscopic Yvon-Kirkwood equations of classical dielectric theory. For the experimentally relevant case of an impurity at infinite dilution in a solvent or disordered matrix, they derive an explicit expression for the impurity dipole moment. To take into account the ensemble of atomic configurations a mean field approximation is developed, numerical results for which, within the class of linear approximations of classical liquid state theory, will be given in a subsequent paper. The authors also examine the dynamic response of the impurity system to an oscillating electric field. They locate the lowest excited state of the system in both the normal insulating and dipolar EI phases, and show that it is degenerate with the ground state at the EI transition, thus making contact with exciton theories of the EI phase.

The dipolar Frenkel excitonic insulator phase of an impurity in a liquid solvent: results

In a previous paper the authors have developed a mean field theory for the dipolar Frenkel excitonic insulator (EI) phase of an impurity at infinite dilution in a liquid solvent or disordered matrix, a situation of experimental relevance. Based on this, they here present numerical results for the location of the EI transition, and the impurity dipole moment in the dipolar EI phase, using linear classical liquid state theories. For a non-polar polarizable solvent, the authors consider impurity and solvent atoms of (i) identical hard-sphere diameter, and (ii) differing hard-sphere diameter. They also generalize the previously studied model to allow for dipolar polarizable solvent molecules, and present example results. The authors consider in particular the case of alkali metal atoms dissolved in methylamine, and conclude that a dipolar EI phase is possible for Li and Cs.

The electronic structure of a liquid of interacting hydrogenic atoms: A prototype for expanded liquid metals

The electronic structure of a metal heated above its critical temperature (an ‘‘expanded liquid metal’’) shows dramatic changes as the density is increased. There is some experimental evidence that as one proceeds from insulating to metallic behavior the substance can even go through two separate phase transitions: from an ordinary insulator to a so-called excitonic insulator, and then from an excitonic insulator to a metal. In an effort to study how the metallic phase is approached, we have used discretized-path-integral methods to look at the statistical mechanics and the electronic structure of a model liquid. In the gas phase, the atoms in the model have a single valence electron constrained to occupy one of the s or p orbitals of the valence shell, but at higher densities, the orbitals hybridize, leading to instantaneous dipole–dipole interaction. We show that formulating this electronic structure in terms of occupation numbers allows us to monitor the hybridization evolution via an imaginary-time correlation function—which we calculate from an analytical solution to the mean-spherical approximation for the model. The numerical results strongly suggest that the model has a sudden hybridization transition, suggesting, in turn, that it might be profitable to think of the excitonic insulator transition in this language.

Mean field theory of a dipolar excitonic insulator transition in matrix-bound systems

A mean field theory is developed for a dipolar excitonic insulator transition occurring in matrix-bound systems, paradigms of which are low temperature alkali-doped rare gas solids at low impurity concentration. The excitonic insulator transition is driven primarily by changes in the host matrix density, and is a transition from the normal insulating domain in which the impurity electronic ground state is spherically symmetric, to an excitonic phase in which the impurity atoms possess electric dipole moments. A description is given of the electrical and optical characteristics of the system in both the normal insulating phase and the dipolar excitonic state, and it is shown that a variety of properties characteristic of the excitonic state can be deduced from experiment. Comparison is made with experimental results, with which the theory is shown to be compatible, and it is suggested that the dipolar excitonic state may have been observed widely over many years.

Frenkel Excitonic Insulator Transitions in Expanded Metals

AbstractWe present a discussion of the dipolar excitonic insulator transition believed to occur in certain expanded fluid metals, notably Hg. A mean field theory is developed which, employing ideas from continuum dielectric theory, leads to the construction of a free energy function representing the change in free energy of the system consequent upon producing a dipolar atom, and relative to the normal non‐polar insulating ground state. Analysis of this function predicts the existence of, and leads to a simple criterion for, the excitonic insulator instability. The transition to a dipolar atomic state is predicted to occur at a density where the optical absorption edge is at a finite energy. Particular attention is given to expanded fluid Hg, although fluid Li is also discussed and is concluded to be a viable candidate for an excitonic insulator transition. Consideration is also given as to whether the excitonic state of Hg is paraelectric or ferroelectric in nature.

Excitonic insulator transition in thin films

A gap Δ 2 in the elementary excitation spectrum of a thin semimetal film is calculated. The gap associated with electron-hole pairing is shown to be much larger in a two-dimensional system than in a three-dimensional one. It is noted that the gap decreases significantly if the film is placed on a metallic substrate. The possibility of anisotropic semimetal-insulator (or, at the triplet pairing, antiferromagnetic) transition when the sample thickness decreases is considered.

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.