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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 detailNetwork model and four-terminal transport in minimally twisted bilayer graphene
De Beule, Christophe UL; Dominguez, Fernando; Recher, Patrik

in Physical Review. B (2021), 104

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See detailEffects of topological band structure on thermoelectric transport of bismuthene
Gaffar, Muhammad; Wella, Sasfan Arman; Hasdeo, Eddwi Hesky UL

in Physical Review. B (2021), 104(20), 205105

Two-dimensional bismuth (Bi) layer, known as bismuthene, exhibits Z2 topological bulk states due to large spin-orbit coupling that inverts the bands. Using the tight-binding method, we calculate the band ... [more ▼]

Two-dimensional bismuth (Bi) layer, known as bismuthene, exhibits Z2 topological bulk states due to large spin-orbit coupling that inverts the bands. Using the tight-binding method, we calculate the band structure of buckled bismuthene to understand its topological and trivial phases. We determine the thermoelectric properties for some considered phases, incorporating the edge states contribution, by using the linearized Boltzmann transport equation with a constant relaxation time approximation. It is shown that the thermoelectric figure of merit, ZT, actually drops in undoped topological bismuthene due to the edge effects. Surprisingly, the topological edge states enhance ZT at large doping with the Fermi energy near the bottom of bulk bands when bismuthene is nearly metallic. [less ▲]

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See detailKerr effect in tilted nodal loop semimetals
Ekström, Carl Johan Ingvar UL; Hasdeo, Eddwi Hesky UL; Farias, Maria Belen UL et al

in Physical Review. B (2021), 104(12), 125411

We investigate the optical activity of tilted nodal loop semimetals. We calculate the full conductivity matrix for a band structure containing a nodal loop with possible tilt in the x−y plane, which ... [more ▼]

We investigate the optical activity of tilted nodal loop semimetals. We calculate the full conductivity matrix for a band structure containing a nodal loop with possible tilt in the x−y plane, which allows us to study the Kerr rotation and ellipticity both for a thin film and a bulk material. We find signatures in the Kerr signal that give direct information about the tilt velocity and direction, the radius of the nodal loop and the internal chemical potential of the system. These findings should serve as guide to understanding optical measurements of nodal loop semimetals and as an additional tool to characterize them. [less ▲]

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See detailEffective Floquet model for minimally twisted bilayer graphene
De Beule, Christophe UL; Dominguez, Fernando; Recher, Patrik

in Physical Review. B (2021), 103

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See detailElectron hydrodynamics of anomalous Hall materials
Hasdeo, Eddwi Hesky UL; Ekström, Carl Johan Ingvar UL; Idrisov, Edvin UL et al

in Physical Review. B (2021), 103(12), 125106

We study two-dimensional electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological ... [more ▼]

We study two-dimensional electron systems in the hydrodynamic regime. We show that a geometrical Berry curvature modifies the effective Navier-Stokes equation for viscous electron flow in topological materials. For small electric fields, the Hall current becomes negligible compared to the viscous longitudinal current. In this regime, we highlight an unconventional Poiseuille flow with an asymmetric profile and a deviation of the maximum of the current from the center of the system. In a two-dimensional infinite geometry, the Berry curvature leads to current whirlpools and an asymmetry of potential profile. This phenomenon can be probed by measuring the asymmetric non-local resistance profile. [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 detailCollective electronic excitations in charge density wave systems: The case of CuTe
Cudazzo, Pier Luigi UL; Wirtz, Ludger UL

in Physical Review. B (2021), 104(12), 125101

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW ... [more ▼]

The study of neutral electronic excitations directly probed by electron energy loss spectroscopy experiments allows obtaining important insight about the physical origin of the charge density wave (CDW) transition in solids. In particular it allows us to disentangle purely phononic mechanisms from the excitonic insulator scenario which is associated to a purely electronic mechanism. As a matter of fact, while the the loss function of the excitonic insulators should display negative dispersive features associated to the softening of neutral electronic excitations at the CDW wave vector above the critical temperature, no softening is expected when the driving force is purely phononic. Here we perform a microscopic analysis of the dynamical charge response of CuTe, a material that displays a low-temperature Peierls-like CDW instability. By means of first-principles time-dependent density functional calculations of the loss function, we characterize the plasmon dispersion along the different directions, highlighting the role of the intrinsic structural anisotropy and the effects of the crystal local fields that are responsible for the periodic reappearance of the spectra of the first Brillouin zone as well as the formation of an acousticlike plasmon. Finally, we demonstrate that also in this system, in analogy with other materials displaying excitonic insulator instability, the low energy region of the loss function presents negative dispersive structures at momentum transfer close to the CDW wave vector. This is a feature common to both excitonic insulator transition and Peierls distortion that further highlights how the difference between the two mechanisms is at most quantitative. [less ▲]

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See detailHeat transport in overdamped quantum systems
Kadijani, Sadeq S.; Schmidt, Thomas UL; Esposito, Massimiliano UL et al

in Physical Review. B (2020), 102

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See detailFirst-principles description of the exciton-phonon interaction: A cumulant approach
Cudazzo, Pier Luigi UL

in Physical Review. B (2020), 102

Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic ... [more ▼]

Electron-phonon coupling leads to intriguing effects in the spectra of materials. Current approximations to calculate spectra most often describe this coupling insufficiently. Starting from basic equations of many-body perturbation theory, we derived a cumulant formulation for neutral excitation spectra that contains excitonic effects and the coupling between excitons and phonons. The cumulant approach allows us to include dynamical effects arising from the electron-phonon coupling in a simple and intuitive way. It can be implemented as a postprocessing of state-of-the-art GW-plus-Bethe-Salpeter calculation of excitonic states and a density functional perturbation theory calculation of phonons and electron-phonon coupling. We demonstrate that, in order to obtain a consistent treatment of exciton-phonon coupling, diagrams have to be taken into account that can be neglected when the effect of lattice vibrations is treated in a static or quasistatic approximation. From the application of this approach to a model system, we analyzed the main features of the exciton-phonon interaction and provided a general picture of their link with the properties of materials such as exciton mass and exciton Bohr radius. [less ▲]

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See detailCasimir force between Weyl semimetals in a chiral medium
Farias, Maria Belen UL; Zyuzin, Alexander A.; Schmidt, Thomas UL

in Physical Review. B (2020), 101

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See detailPhonon coupling and shallow defects in CuInS2
Lomuscio, Alberto UL; Sood, Mohit UL; Melchiorre, Michele UL et al

in Physical Review. B (2020), 101(8), 085119-

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See detailImpact of crystalline anisotropy on the extrinsic spin Hall effect in ultrathin films
Herschbach, Christian; Fedorov, Dmitry UL; Gradhand, Martin et al

in Physical Review. B (2020), 102

An efficient conversion of a charge current into a spin current is a crucial point for application of the spin Hall effect in practical spintronic devices. Recently, we revealed that this goal can be ... [more ▼]

An efficient conversion of a charge current into a spin current is a crucial point for application of the spin Hall effect in practical spintronic devices. Recently, we revealed that this goal can be achieved by using ultrathin fcc (111) and (001) noble metal films doped with Bi impurities, which possess spin Hall angles up to 80%. Here, we show that the effect can be further amplified in monolayer films with a strong crystalline anisotropy. This is demonstrated by considering noble metal films with fcc (110) geometry. Our theoretical study predicts related spin Hall angles exceeding 100% especially when the crystalline anisotropy is increased, which tunes the Fermi surface topology. [less ▲]

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See detailIon-induced interactions in a Tomonaga-Luttinger liquid
Michelsen, Andreas Nicolai Bock UL; Valiente, Manuel; Zinner, Nikolaj Thomas et al

in Physical Review. B (2019), 100(20),

We investigate the physics of a Tomonaga-Luttinger liquid of spin-polarized fermions superimposed on an ion chain. This compound system features (attractive) long-range interspecies interactions. By means ... [more ▼]

We investigate the physics of a Tomonaga-Luttinger liquid of spin-polarized fermions superimposed on an ion chain. This compound system features (attractive) long-range interspecies interactions. By means of density matrix renormalization group techniques we compute the Tomonaga-Luttinger-liquid parameter and speed of sound as a function of the relative atom/ion density and the two quantum defect parameters, namely, the even and odd short-range phases which characterize the short-range part of the atom-ion polarization potential. The presence of ions is found to allow critical tuning of the atom-atom interaction, and the properties of the system are found to depend significantly on the short-range phases due to the atom-ion interaction. These latter dependencies can be controlled, for instance, by manipulating the ions' internal state. This allows modification of the static properties of the quantum liquid via external driving of the ionic impurities. [less ▲]

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See detailFinite frequency noise in a chiral Luttinger liquid coupled to phonons
Idrisov, Edvin UL

in Physical Review. B (2019)

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See detailEntropy production in one-dimensional quantum fluids
Idrisov, Edvin UL; Schmidt, Thomas UL

in Physical Review. B (2019)

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See detailLattice dynamics and Raman spectrum of BaZrO3 single crystals
Toulouse, Constance UL; Amoroso, Danila; Xin, Cong et al

in Physical Review. B (2019), 100

BaZrO3 is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with ... [more ▼]

BaZrO3 is a perovskite that remains in the simple cubic phase at all temperatures, hence with no first-order Raman-active phonon mode allowed by symmetry. Yet, it exhibits an intense Raman spectrum with sharp and well-defined features. Here, we report the evolution of the Raman spectrum of BaZrO3 single crystals in a broad temperature range (4-1200 K) and discuss its origin with the support of detailed first-principle calculations of the lattice dynamics. Phonon calculations are performed not only for the cubic phase of BaZrO3, but also for the low-symmetry phases with octahedra tilts that have been suspected to exist at the nanoscale. We show that the Raman spectrum shows no direct evidence for these nanodomains, but can instead be explained by classical second-order Raman scattering. We provide an assignment of the dominant features to phonon mode combinations. In particular, we show that the high frequency range of the spectrum is dominated by overtones and shows an image of the phonon density of states corresponding to the stretching modes of the oxygen octahedra. [less ▲]

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See detailHigh-pressure structural change in the ferroelectric layered perovskite Sr2Nb2O7
Alanis, Javier; Joazet Ojeda-Galvan, Hiram; Rodriguez-Aranda, M. C. et al

in Physical Review. B (2019), 100(5),

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See detailTheory of resonant Raman scattering: Towards a comprehensive ab initio description
Reichardt, Sven UL; Wirtz, Ludger UL

in Physical Review. B (2019), 99(17), 174312-15

We develop a general, fully quantum mechanical theory of Raman scattering from first principles in terms of many-body correlation functions. In order to arrive at expressions that are practically useful ... [more ▼]

We develop a general, fully quantum mechanical theory of Raman scattering from first principles in terms of many-body correlation functions. In order to arrive at expressions that are practically useful in the context of condensed matter physics, we adopt the Lehmann-Symanzik-Zimmermann reduction formula from high-energy physics and formulate it in the language of many-body perturbation theory. This enables us to derive a general and practically useful expression for the Raman scattering rate in terms of quantities that can be computed ab initio. Our work paves the way toward a comprehensive computational approach to the calculation of Raman spectra that goes beyond the current state of the art by capturing both excitonic and nonadiabatic effects. [less ▲]

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