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Register a new userThe spontaneous symmetry breaking in Ta$_2$NiSe$_5$ is structural in nature
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Added by
Edoardo Baldini Dr.
Publication date
2020
fields
Physics
and research's language is
English
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The excitonic insulator is an electronically-driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta$_2$NiSe$_5$ being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the materials electronic and crystal structure after light excitation reveals surprising spectroscopic fingerprints that are only compatible with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the electronic gap opening. Not only do our results uncover the long-sought mechanism driving the phase transition of Ta$_2$NiSe$_5$, but they also conclusively rule out any substantial excitonic character in this instability.
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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.
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.
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.
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 unambiguously establish the normal state as a semimetal with a significant band overlap of $>$100~meV. Our temperature-dependent measurements indicate how these low-energy states hybridise when cooling through the well-known 327~K phase transition in this system. From our calculations and polarisation-dependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridisation, driven by a shear-like structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our results thus point to the key role of the lattice distortion in enabling the phase transition of Ta$_2$NiSe$_5$.
The microscopic quantum interference associated with excitonic condensation in Ta$_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$_5$.
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