Excitonic Phase Transition Research Articles

One-dimensional Dexter-type excitonic topological phase transition

The concept of the Su-Schrieffer-Heeger model, which was successfully introduced in topological photonics, can also be applied to the study of topological excitonics. Here we study the topological properties of a one-dimensional excitonic tight-binding model formed by two-level systems by taking into account the charge transfer and local excitations. The interactions between the two types of excitons give rise to a rich spectrum of physics, including the nontrivial topological phase in the uniform chain, unlike the conventional Su-Schrieffer-Heeger model, the topologically nontrivial flat bands, and, most importantly, the excitonic topological phase transition assisted by the Dexter electron exchange process. The excitonic topological phase leads to the development of “chiral superposition.” The topological edge states are robust because they are protected by the inversion symmetry. Based on our calculations, experiments for observing the edge states optically in a molecular chain are proposed. Published by the American Physical Society 2024

Photoinduced chiral charge density wave in TiSe2

$1T\text{\ensuremath{-}}{\mathrm{TiSe}}_{2}$ has been found to host a chiral charge density wave (CDW). Some studies suggest the microscopic origin of this phase is due to electron-phonon coupling while other studies suggest it is due to an excitonic insulator phase transition based on nonthermal melting of the charge density wave. First, we propose these interpretations can be reconciled if one analyzes the available experimental and theoretical data within a formal definition of what constitutes an excitonic insulator as initially proposed by Keldysh and Kopaev. Next, we present pump-probe measurements of circularly polarized optical transitions and first-principles calculations to highlight the role of elevated electronic temperatures on structural distortions to understand the nonthermal melting of the CDW phase. We also uncover a noncentrosymmetric CDW structure that explains the finite chirality of the optical transitions observed in the CDW phase of ${\mathrm{TiSe}}_{2}$.

Towards high-temperature electron-hole condensate phases in monolayer tetrels metal halides: Ultra-long excitonic lifetimes, phase diagram and exciton dynamics

Excitons as composite bosons have been predicted to condense into Bose Einstein Condensation (BEC), Berezinskii-Kosterlitz-Thouless (BKT) superfluid or electron-hole liquid (EHL) states. However, short lifetime and small binding energy are the main obstacles that hinder the formation and observation of excitonic condensate at high temperature. Here we demonstrate excellent two-dimensional Tetrel Metal Halides (TMHs) platforms for excitonic quantum phase transition researches. We find that excitons in those monolayers possess effective masses similar to the free electron, promising large thermal de Broglie wavelengths. They can maintain stable bosonic characteristic at high temperature and concentration up to 1012 cm−2. The calculation on PbI2, PbBr2, PbCl2, SnI2, SnBr2, SnCl2 indicates BEC transition temperatures at 49 K, 103 K, 252 K, 36 K, 78 K, 152 K, comparable to the experimentally observed exciton BEC in MoSe2–WSe2 bilayer and 1 T-TiSe2 at 100 K and 190 K respectively. Besides, we confirm that the thermal equilibrium of excitonic subsystem can be established really fast, up to ∼10–100 fs. The excitons can live for ∼1–100 μs without direct radiation, beneficial for the formation and observation of condensate state. Our result paves the way not only for studies of quantum phase transition researches, but also for high-temperature excitonic devices.

Failed excitonic quantum phase transition in Ta2Ni(Se1−xSx)5

We study the electronic phase diagram of the excitonic insulator candidates Ta$_2$Ni(Se$_{1-x}$S$_x$)$_5$ [x=0, ... ,1] using Raman spectroscopy. Critical excitonic fluctuations are observed, that diminish with $x$ and ultimately shift to high energies, characteristic of a quantum phase transition. Nonetheless, a symmetry-breaking transition at finite temperatures is detected for all $x$, exposing a cooperating lattice instability that takes over for large $x$. Our study reveals a failed excitonic quantum phase transition, masked by a preemptive structural order.

Theory of thermal conductivity of excitonic insulators

We study the thermal conductivity in the excitonic insulator using a simple quasi one-dimensional two-band model consisting of electron and hole bands with the Coulomb interactions between these bands. Based on the linear response theory within a mean-field scheme, we develop a method to identify the contributions to thermal conductivity driven by excitonic insulator. It is found that there is an additional heat current operator owing to the excitonic phase transition, and that it gives contributions to thermal conductivity which are not expressed in the form of Sommerfeld-Bethe relations written in the form of an imaginary-time derivative of the electric current operator. Finally, we discuss the relationship between the newly-found additional contribution and the heat current carried by excitons.

Influence of the Vibrational Modes from the Organic Moieties in 2D Lead Halides on Excitonic Recombination and Phase Transition

Abstract2D metal halide semiconductors have been intensively studied in the past few years due to their unique optical properties and potential for new‐generation photonic devices. Despite the large number of recent works, this class of materials is still in need of further understanding due to their complex structural and optical characteristics. In this work, a molecular‐level explanation for the dual band emission in the 2D (C4H9NH3)2PbI4 in its bulk form is presented, demonstrating that this feature is caused by a strong exciton–phonon coupling. Temperature‐dependent photoluminescence with Raman and IR spectroscopies reveals that vibrations involving the C‐NH3+ butylammonium polar head are responsible for this exciton–phonon coupling. Additionally, experimental shifts in the mean phonon frequencies coupled with the electronic excitation, combined with a theoretical model, show that these vibrational modes present a soft‐mode behavior in the phase transition of this material.

Near-Unity Light Absorption in a Monolayer WS2 Van der Waals Heterostructure Cavity.

Excitons in monolayer transition-metal-dichalcogenides (TMDs) dominate their optical response and exhibit strong light-matter interactions with lifetime-limited emission. While various approaches have been applied to enhance light-exciton interactions in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based van der Waals heterostructure cavity that provides near-unity excitonic absorption, and emission of excitonic complexes that are observed at ultralow excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, nonradiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D systems. This enhanced light-exciton interaction provides a platform for studying excitonic phase-transitions and quantum nonlinearities and enables new possibilities for 2D semiconductor-based optoelectronic devices.

Dual excitonic emissions and structural phase transition of octylammonium lead iodide 2D layered perovskite single crystal

Two dimensional (2D) layered hybrid lead halide perovskites exhibit interesting electronic and optical properties because of their repeating quantum well structure. These materials are now being explored for various optoelectronic applications, including solar cells and light emitting diodes. But still there are surprises regarding the basic band gap or excitonic absorption and emission of these layered hybrid perovskites. Recently, we showed possibility of dual band gaps or dual excitonic emissions from 2D butylammonium lead iodide (BA2PbI4). In the present paper, we extend the study to octylammonium lead iodide (OA)2PbI4 single crystals. The longer organic cations in (OA)2PbI4 separates the adjacent inorganic Pb-I layers further, reducing any possible electronic interactions between the inorganic layers. Interestingly, (OA)2PbI4 also exhibits dual excitonic emission at room temperature. A minor structural inhomogeneity between near-surface and interior regions of (OA)2PbI4 single crystals might be responsible for the two different excitonic emissions. To explore the correlation between crystal structure and excitonic emissions, we have studied single crystal XRD and photoluminescence at variable temperatures. The results show a small change in tilting of Pb-I6 octahedra can distinctly change the band gap and the excitonic emission energy.

Excitonic gap formation and condensation in the bilayer graphene structure

We have studied the excitonic gap formation in the Bernal Stacked, bilayer graphene (BLG) structures at half-filling. Considering the local Coulomb interaction between the layers, we calculate the excitonic gap parameter and we discuss the role of the interlayer and intralayer Coulomb interactions and the interlayer hopping on the excitonic pair formation in the BLG. Particularly, we predict the origin of excitonic gap formation and condensation, in relation to the furthermost interband optical transition spectrum. The general diagram of excitonic phase transition is given, explaining different interlayer correlation regimes. The temperature dependence of the excitonic gap parameter is shown and the role of the chemical potential, in the BLG, is discussed in details.

Excitonic Phase Transition in the Extended Three-Dimensional Falicov–Kimball Model

We study the excitonic phase transition in a system of the conduction band electrons and valence band holes described by the three-dimensional (3D) extended Falicov-Kimball (EFKM) model with the tunable Coulomb interaction $U$ between both species. By lowering the temperature, the electron-hole system may become unstable with respect to the formation of the excitons, i.e, electron-hole pairs at temperature $T=T_{\Delta}$, exhibiting a gap $\Delta$ in the particle excitation spectrum. To this end we implement the functional integral formulation of the EFKM, where the Coulomb interaction term is expressed in terms of U(1) phase variables conjugate to the local particle number, providing a useful representation of strongly correlated system. The effective action formalism allows us to formulate a problem in the phase-only action in the form of the quantum rotor model and to obtain analytical formula for the critical lines and other quantities of physical interest like charge gap, chemical potential and the correlation length.

Superconducting and excitonic quantum phase transitions in doped Dirac electronic systems

We investigate the effect of superconducting and excitonic interactions, as well as their competition, on Dirac electrons on a bipartite planar lattice. It is shown that, at half-filling, Cooper pairs and excitons coexist if the superconducting and excitonic coupling parameters are equal and above a threshold corresponding to a quantum critical point. In the case where only the excitonic interaction is present, we obtain a critical chemical potential, as a function of the interaction strength. Conversely, if only the superconducting interaction is considered, we show that the superconducting gap displays a characteristic dome as charge carriers are doped into the system. We also show that, as the chemical potential increases, superconductivity tends to suppress the excitonic order parameter.

Variational approach to the excitonic phase transition in graphene

We analyze the Coulomb interacting problem in undoped graphene layers by using an excitonic variational ansatz. By minimizing the energy, we derive a gap equation which reproduces and extends known results. We show that a full treatment of the exchange term, which includes the renormalization of the Fermi velocity, tends to suppress the phase transition by increasing the critical coupling at which the excitonic instability takes place.

Quantum Hall Exciton Condensation at Full Spin Polarization

Using Coulomb drag as a probe, we explore the excitonic phase transition in quantum Hall bilayers at nu(T) = 1 as a function of Zeeman energy E(Z). The critical layer separation (d/l)(c) for exciton condensation initially increases rapidly with E(Z), but then reaches a maximum and begins a gentle decline. At high E(Z), where both the excitonic phase at small d/l and the compressible phase at large d/l are fully spin polarized, we find that the width of the transition, as a function of d/l, is much larger than at small E(Z) and persists in the limit of zero temperature. We discuss these results in the context of two models in which the system contains a mixture of the two fluids.

Excitonic gap, phase transition, and quantum Hall effect in graphene

We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of an excitonic gap. The experimental form of the Hall conductivity is reproduced and the main characteristics of the dynamics are described. Predictions of the behavior of the gap as a function of temperature and a gate voltage are made.

Prediction of an Excitonic Ground State inInAs/InSbQuantum Dots

Using atomistic pseudopotential and configuration-interaction many-body calculations, we predict an excitonic ground state in the InAs/InSb quantum-dot system. For large dots, the conduction band minimum of the InAs dot lies below the valence band maximum of the InSb matrix. Due to quantum confinement, at a critical size calculated here for various shapes, the gap E(g) between InAs conduction states and InSb valence states vanishes. Strong electron-hole correlation effects are induced by the spatial proximity of the electron and hole wave functions, and by the lack of strong (exciton unbinding) screening, afforded by the existence of discrete 0D confined energy levels. These correlation effects overcome E(g), leading to the formation of a biexcitonic ground state (two electrons in InAs and two holes in InSb) being energetically more favorable (by approximately 15 meV) than the dot without excitons.

Excitonic absorption spectra and phase transitions in thin films of the ferroelastics Cs2CdI4 and Rb2CdI4

The absorption spectrum of thin films of the ferroelastics M2CdI4 (M=Cs, Rb) with a structure of the β-K2SO4 type is investigated in the energy range 3–6 eV and temperature range 90–420 K. It is found that both compounds are direct-gap insulators, and the low-frequency electronic and excitonic excitations are localized in the CdI42− structural elements of their crystal lattice. From the temperature dependence of the spectral absorption and the half-widths of the low-frequency excitonic bands in Rb2CdI4 it is found that there is a phase transition at 380 K (paraphase→incommensurate), a first-order phase transition at 320 K (incommensurate→first ferroelastic phase), and a second-order phase transition at 210 K (first→second ferroelastic phase). Similar, but less pronounced phase transitions are found in Cs2CdI4 at lower temperatures. In the ferroelastic phases of these compounds an additional broadening of the bands appears, apparently due to the scattering of excitons on strain fluctuations in the domain-wall regions.

Excitonic spin instabilities and continuous phase transitions in quantum Hall magnets

Inelastic light scattering from collective excitations has emerged as a powerful tool to explore novel ground states of quantum Hall liquids. We present here a short review of light scattering experiments in coupled electron bilayers in GaAs double quantum wells. At even integer filling factors the bilayers display soft collective excitation modes that are linked to phase transformations in the spin degree of freedom. In this work light scattering emerges as a method that offers novel venues for the study of fascinating quantum phase transitions of electrons in artificially layered semiconductors. The experiments reveal that excitonic terms of electron interactions play key roles and offer remarkable insights on finite temperature physics. We focus on studies of collective mode behaviour and on states of spin created by the interplay between Zeeman energies, tunnelling gaps and electron exchange interactions.

Imaging of excitonic transport in semiconductors

Excitonic matter produced by photoexcitation of a semiconductor crystal at low temperature is able to diffuse or drift macroscopic distances from the point of its creation. This motion can be observed by time-resolved imaging of recombination luminescence or optical absorption. At low densities, the simplest neutral particles — excitons — generally exhibit the diffusive motion of classical gases. At high densities, however, the transport behavior can deviate wildly from classical diffusion, as excitonic phase transitions and quantum statistics come into play. In this paper, I will survey a few of the current observations.

On the critical behaviour of systems exhibiting an excitonic phase transition

The Landau expansion for the grand thermodynamic potential is obtained for a system which possesses a multicritical point of a new type, a hybrid of a tricritical point and a Lifshits point. The role of fluctuations in such a system is investigated.

On the order of the excitonic phase transition

The correct phase diagram in the density-temperature plane is constructed for some mean-field models describing the excitonic transition. The phase transition is of first order at low temperature, in disagreement with the accepted picture. The source of error is pointed out.