Imaging the coherent propagation of collective modes in the excitonic insulator Ta2NiSe5 at room temperature.

Published on Jul 1, 2021in Science Advances14.143
· DOI :10.1126/SCIADV.ABD6147
Hope M. Bretscher5
Estimated H-index: 5
(University of Cambridge),
Paolo Andrich8
Estimated H-index: 8
(University of Cambridge)
+ 10 AuthorsAndrew J. Millis93
Estimated H-index: 93
(Columbia University)
Excitonic insulators host a condensate of electron-hole pairs at equilibrium, giving rise to collective many-body effects. Although several materials have emerged as excitonic insulator candidates, evidence of long-range coherence is lacking and the origin of the ordered phase in these systems remains controversial. Here, using ultrafast pump-probe microscopy, we investigate the possible excitonic insulator Ta2NiSe5. Below 328 K, we observe the anomalous micrometer-scale propagation of coherent modes at velocities of ~105 m/s, which we attribute to the hybridization between phonon modes and the phase mode of the condensate. We develop a theoretical framework to support this explanation and propose that electronic interactions provide a substantial contribution to the ordered phase in Ta2NiSe5. These results allow us to understand how the condensate’s collective modes transport energy and interact with other degrees of freedom. Our study provides a unique paradigm for the investigation and manipulation of these properties in strongly correlated materials.
#1Pavel A. Volkov (RU: Rutgers University)H-Index: 5
Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs - excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta_2iSe_5disentangling it from the lattice phonons. The e...
#1Tanusree SahaH-Index: 1
#2Denis Golež (Jožef Stefan Institute)H-Index: 18
Last. Primož Rebernik Ribič (UNG: University of Nova Gorica)H-Index: 16
view all 8 authors...
We investigate the non-equilibrium electronic structure and characteristic time scales in a candidate excitonic insulator, Ta_2iSe_5 using time- and angle-resolved photoemission spectroscopy with a temporal resolution of 50 fs. Following a strong photoexcitation, the band gap closes transiently within 100 fs, i.e., on a time scale faster than the typical lattice vibrational period. Furthermore, we find that the characteristic time associated with the rise of the photoemission intensity abo...
#6Bumjoon Kim (POSTECH: Pohang University of Science and Technology)H-Index: 25
Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scatteri...
#1Hope M. Bretscher (University of Cambridge)H-Index: 5
#2Paolo Andrich (University of Cambridge)H-Index: 8
Last. Akshay Rao (University of Cambridge)H-Index: 40
view all 7 authors...
The layered chalcogenide Ta2NiSe5 has been proposed to host an excitonic condensate in its ground state, a phase that could offer a unique platform to study and manipulate many-body states at room temperature. However, identifying the dominant microscopic contribution to the observed spontaneous symmetry breaking remains challenging, perpetuating the debate over the ground state properties. Here, using broadband ultrafast spectroscopy we investigate the out-of-equilibrium dynamics of Ta2NiSe5 an...
#1Takeshi SuzukiH-Index: 9
#2Yasushi ShinoharaH-Index: 17
Last. Kozo OkazakiH-Index: 23
view all 16 authors...
Photo-induced phase transitions have been intensively studied owing to the ability to control a material of interest in a flexible manner, which can induce exotic phases unable to be attained at equilibrium. The key mechanisms are still under debate, and how the couplings between the electron, lattice, and spin degrees of freedom are evolving during photo-induced phase transitions has currently been a central issue. However, the measurements for each degree in the ultrafast timescale require dis...
#1Honglie Ning (California Institute of Technology)H-Index: 4
#2O. Mehio (California Institute of Technology)H-Index: 1
Last. David Hsieh (California Institute of Technology)H-Index: 32
view all 7 authors...
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 {\mathrm{Ta}}_{2}{\mathrm{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 optic...
#1Min Jae Kim (University of Stuttgart)H-Index: 6
#2Armin Schulz (MPG: Max Planck Society)H-Index: 9
Last. Stefan Kaiser (MPG: Max Planck Society)H-Index: 88
view all 6 authors...
The authors study the dynamics driving the semiconductor -excitonic insulator phase transition in Ta{}_{2}iSe{}_{5}using Raman spectroscopy.
#1Sukanya Pal (IISc: Indian Institute of Science)H-Index: 2
#2Shivani Grover (Jawaharlal Nehru Centre for Advanced Scientific Research)H-Index: 2
Last. A. K. Sood (IISc: Indian Institute of Science)H-Index: 65
view all 8 authors...
The authors use high-pressure Raman spectroscopy along with a first-principles model analysis to study the role of exciton-phonon coupling in stabilizing the excitonic insulator phase of Ta{}_{2}iSe{}_{5}at room temperature.
#7Y. Zhang (PKU: Peking University)H-Index: 58
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 methods of excitonic condensation. In this Rapid Communication, we report the realization of a doping-...
#1Paolo AndrichH-Index: 8
#2Hope M. BretscherH-Index: 5
Last. Akshay RaoH-Index: 40
view all 7 authors...
We use broadband ultrafast spectroscopy to investigate the out-of-equilibrium dynamics of Ta_{2}iSe_{5} This layered chalcogenide material has recently been shown to host an excitonic insulating phase up to a critical temperature T_{c}= 328 K, however the nature of the spontaneous symmetry breaking leading to the excitonic condensation, whether electronic or structural, is unknown. Here, we first demonstrate that the transient reflectivity ({\Delta}/R) in the near infrared range is c...
Cited By3
#1Tatsuya KanekoH-Index: 14
#2Zhiyuan SunH-Index: 14
Last. Andrew J. Millis (Columbia University)H-Index: 93
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We investigate the bulk photovoltaic effect, which rectifies light into electric current, in a collective quantum state with correlation driven electronic ferroelectricity. We show via explicit real-time dynamical calculations that the effect of the applied electric field on the electronic order parameter leads to a strong enhancement of the bulk photovoltaic effect relative to the values obtained in a conventional insulator. The enhancements include both resonant enhancements at sub-bandgap fre...
#1Zhiyuan SunH-Index: 14
#2Tatsuya KanekoH-Index: 14
Last. Andrew J. Millis (Columbia University)H-Index: 93
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We show that in electron-hole bilayers with excitonic order arising from conduction and valence bands formed by atomic orbitals that transform differently under inversion, nonzero interlayer tunneling leads to a second order Josephson effect. This means the interlayer electrical current is related to the phase of the excitonic order parameter as J = J_c \sin2\thetainstead of J = J_c \sin \theta and that the system has two degenerate ground states that can be switched by an interlayer volta...
#1Arjun AshokaH-Index: 2
#2Nicolas GauriotH-Index: 3
Last. Akshay RaoH-Index: 40
view all 6 authors...
We present a novel microscopic technique to access local transient optical constants and carrier motion in thin-film materials in three dimensions, with sub-10 nm spatial precision and sub-15 fs temporal resolution. Our experimental scheme is based on imaging a traditionally defocussed plane in an interferometrically enhanced femtosecond pump-probe microscope, combining the amplitude and phase contrast between the perturbed and unperturbed probe due to the local photoinduced complex refractive i...
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