Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userComment on Direct observation of excitonic instability in Ta$_2$NiSe$_5$ [arXiv:2007.08212 (2020)]
211
0
0.0
(
0
)
Authors
G. Blumberg
Ask ChatGPT about the research
No Arabic abstract
The origin of the second order phase transition at 328K in Ta$_2$NiSe$_5$, a prominent candidate for direct gap excitonic insulator, remains under fervent debate. The driving force for the transition can be revealed by identification of the soft modes origin that may be deducted from polarization resolved Raman scattering experiments. Such studies were recently reported in [arXiv:2007.07344 (2020)], [arXiv:2102.07912 (2021)], [arXiv:2007.01723 (2020)] and [arXiv:2007.08212 (2020)]. In this Comment, it is shown that the parameters derived in a recent arXiv by Kwangrae Kim et. al. [arXiv:2007.08212 (2020)], including the Weiss temperature for excitonic transition, are based on inconsistent data.
rate research
Read More
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 temperature $T_{rm c}$ whereas in NMR spin-lattice relaxation the rate rapidly decreases below $T_{rm c}$; these observations can offer a crucial experimental test for the validity of the excitonic condensation scenario in Ta$_2$NiSe$_5$. We also show that the excitonic condensation manifests itself in a jump of the heat capacity at $T_{rm c}$ as well as in softening of the elastic shear constant, in accordance with the second-order phase transition observed in Ta$_2$NiSe$_5$.
In the presence of electron-phonon coupling, an excitonic insulator harbors two degenerate ground states described by an Ising-type order parameter. Starting from a microscopic Hamiltonian, we derive the equations of motion for the Ising order parameter in the phonon coupled excitonic insulator Ta$_2$NiSe$_5$ and show that it can be controllably reversed on ultrashort timescales using appropriate laser pulse sequences. Using a combination of theory and time-resolved optical reflectivity measurements, we report evidence of such order parameter reversal in Ta$_2$NiSe$_5$ based on the anomalous behavior of its coherently excited order-parameter-coupled phonons. Our work expands the field of ultrafast order parameter control beyond spin and charge ordered materials.
We analyze the measured optical conductivity spectra using the density-functional-theory-based electronic structure calculation and density-matrix renormalization group calculation of an effective model. We show that, in contrast to a conventional description, the Bose-Einstein condensation of preformed excitons occurs in Ta$_2$NiSe$_5$, despite the fact that a noninteracting band structure is a band-overlap semimetal rather than a small band-gap semiconductor. The system above the transition temperature is therefore not a semimetal, but rather a state of preformed excitons with a finite band gap. A novel insulator state caused by the strong electron-hole attraction is thus established in a real material.
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.
The three-chain Hubbard model for Ta$_2$NiSe$_5$ known as a candidate material for the excitonic insulator is investigated over the wide range of energy gap $D$ between the two-fold degenerate conduction bands and the nondegenerate valence band including both semiconducting ($D>0$) and semimetallic ($D<0$) cases. In the semimetallic case, the difference of the band degeneracy inevitably causes the imbalance of each Fermi wavenumber, resulting in a remarkable excitonic state characterized by the condensation of excitons with finite center-of-mass momentum $q$, the so-called Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) excitonic state. With decreasing $D$ corresponding to increasing pressure, the obtained excitonic phase diagram shows a crossover from BEC ($Dsimg 0$) to BCS ($Dsiml 0$) regime, and then shows a distinct phase transition at a certain critical value $D_c(<0)$ from the uniform ($q=0$) to the FFLO ($q e 0$) excitonic state, as expected to be observed in Ta$_2$NiSe$_5$ under high pressure.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
