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See detailExcitonic-insulator instability and Peierls distortion in one-dimensional semimetals
Barborini, Matteo UL; Calandra, Matteo; Mauri, Francesco et al

in Physical Review. B (2022), 105(7), 075122

The charge density wave instability in one-dimensional semimetals is usually explained through a Peierls-like mechanism, where the coupling of electrons and phonons induces a periodic lattice distortion ... [more ▼]

The charge density wave instability in one-dimensional semimetals is usually explained through a Peierls-like mechanism, where the coupling of electrons and phonons induces a periodic lattice distortion along certain modes of vibration, leading to a gap opening in the electronic band structure and to a lowering of the symmetry of the lattice. In this work, we study two prototypical Peierls systems: the one-dimensional carbon chain and the monatomic hydrogen chain with accurate ab initio calculations based on quantum Monte Carlo and hybrid density functional theory. We demonstrate that in one-dimensional semimetals at T=0, a purely electronic instability can exist independently of a lattice distortion. It is induced by spontaneous formation of low energy electron-hole pairs resulting in the electronic band gap opening, i.e., the destabilization of the semimetallic phase is due to an excitonic mechanism. [less ▲]

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See detailElectronic structure of TiSe2 from a quasi-self-consistent G0W0 approach
Hellgren, Maria; Baguet, Lucas; Calandra, Matteo et al

in Physical Review. B (2021), 103

n a previous work, it was shown that the inclusion of exact exchange is essential for a first-principles description of both the electronic and the vibrational properties of TiSe2, M. Hellgren et al ... [more ▼]

n a previous work, it was shown that the inclusion of exact exchange is essential for a first-principles description of both the electronic and the vibrational properties of TiSe2, M. Hellgren et al. [Phys. Rev. Lett. 119, 176401 (2017)]. The GW approximation provides a parameter-free description of screened exchange but is usually employed perturbatively (G0W0), making results more or less dependent on the starting point. In this work, we develop a quasi-self-consistent extension of G0W0 based on the random phase approximation (RPA) and the optimized effective potential of hybrid density functional theory. This approach generates an optimal G0W0 starting point and a hybrid exchange parameter consistent with the RPA. While self-consistency plays a minor role for systems such as Ar, BN, and ScN, it is shown to be crucial for TiS2 and TiSe2. We find the high-temperature phase of TiSe2 to be a semimetal with a band structure in good agreement with experiment. Furthermore, the optimized hybrid functional agrees well with our previous estimate and therefore accurately reproduces the low-temperature charge-density-wave phase. [less ▲]

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See detailCritical Role of the Exchange Interaction for the Electronic Structure and Charge-Density-Wave Formation in TiSe2
Hellgren, Maria; Baima, Jacopo; Bianco, Raffaello et al

in Physical Review Letters (2017), 119

We show that the inclusion of screened exchange via hybrid functionals provides a unified description of the electronic and vibrational properties of TiSe2. In contrast to local approximations in density ... [more ▼]

We show that the inclusion of screened exchange via hybrid functionals provides a unified description of the electronic and vibrational properties of TiSe2. In contrast to local approximations in density functional theory, the explicit inclusion of exact, nonlocal exchange captures the effects of the electron-electron interaction needed to both separate the Ti-d states from the Se-p states and stabilize the charge-density- wave (CDW) (or low-T) phase through the formation of a p-d hybridized state. We further show that this leads to an enhanced electron-phonon coupling that can drive the transition even if a small gap opens in the high-T phase. Finally, we demonstrate that the hybrid functionals can generate a CDW phase where the electronic bands, the geometry, and the phonon frequencies are in agreement with experiments. [less ▲]

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