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The layered compound $\mathrm{Ta_2NiSe_5}$ is a quasi-one-dimensional and narrow-gap semiconductor, which is proposed to undergo a transition to an excitonic insulator at $T_\mathrm{c}=326$ K. We found a clear anomaly at $T_\mathrm{c}$ in…
The structure of the ground state exciton of Cd(Se,Te) quantum dots embedded in ZnTe matrix is studied experimentally using photoluminescence spectroscopy and theoretically using ${\bf k}\cdot{\bf p}$ and configuration interaction methods.…
Excitonic insulators host the condensates of bound electron-hole pairs, offering a platform for studying correlated bosonic quantum states. Yet, how macroscopic coherence emerges from locally collapsed pairing remains elusive. Here, using…
Electron-hole bound pairs, or excitons, are common excitations in semiconductors. They can spontaneously form and ``condense'' into a new insulating ground state -- the so-called excitonic insulator -- when the energy of electron-hole…
We report on the charge dynamics of Ta$_2$Pd$_3$Te$_5$ using temperature-dependent optical spectroscopy with polarized light. We observe a metal-insulator transition characterized by the collapse of Drude response and the emergence of sharp…
The opening of an energy gap in the electronic structure generally indicates the presence of interactions. In materials with low carrier density and short screening length, long-range Coulomb interaction favors the spontaneous formation of…
We explore the collective response in an excitonic insulator phase in Ta$_2$NiSe$_5$ using a semirealistic model including relevant lattice and electronic instabilities. We calculate order-parameter susceptibility and Raman response within…
The influence of phonons on the formation of the excitonic insulator has hardly been analyzed so far. Recent experiments on $\rm Ta_2NiSe_5$, 1$T$-$\rm TiSe_2$, and $\rm TmSe_{0.45}Te_{0.55}$, being candidates for realizing the…
Ta2Ni3Te5, a layered transition metal chalcogenide with quasi-one-dimensional electronic states, exhibits rich topological and correlated phenomena. Using first-principles calculations, we explore Ta2Ni3Te5 nanoribbons, demonstrating…
The excitonic insulator is an elusive electronic phase exhibiting a correlated excitonic ground state. Materials with such a phase are expected to have intriguing properties such as excitonic high-temperature superconductivity. However,…
We present a generic procedure for quantifying the interplay of electronic and lattice degrees of freedom in photo-doped insulators through a comparative analysis of theoretical many-body simulations and time- and angle-resolved…
TiSe$_2$ has long been considered one of the best candidate materials to host the elusive excitonic insulator (EI) phase. However, a finite coupling to the lattice can generically be expected, while a lack of "smoking-gun" signatures for…
We reveal an ultrafast purely electronic phase transition in \TNS, which is a plausible excitonic insulator, after excited by an ultrafast infrared laser pulse. Specifically, the order parameter of the strong electron-hole binding shrinks…
Excitons are bosonic quasiparticles with a variety of applications in optoelectronics, photosyn thesis, and dissipationless informatics, and their lifetime can become sufficiently long to form a quantum condensate. While exciton…
During a band-gap-tuned semimetal-to-semiconductor transition, Coulomb attraction between electrons and holes can cause spontaneously formed excitons near the zero-band-gap point, or the Lifshitz transition point. This has become an…
Utilizing the unbiased time-dependent density-matrix renormalization group technique, we examine the photoemission spectra in the extended Falicov-Kimball model at zero and finite temperatures, particularly with regard to the excitonic…
We present a combined study from angle-resolved photoemission and density-functional theory calculations of the temperature-dependent electronic structure in the excitonic insulator candidate Ta$_2$NiSe$_5$. Our experimental measurements…
The electronic structures and topological properties of the orthorhombic and monoclinic phases of the quasi-one-dimensional excitonic insulator Ta2NiSe5 are investigated based on density functional theory. In contrast to a single parity or…
The microscopic quantum interference associated with excitonic condensation in Ta$_2$NiSe$_5$ is studied in the BCS-type mean-field approximation. We show that in ultrasonic attenuation the coherence peak appears just below the transition…
The transition from insulator to conductor can be achieved in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a mechanism the…