arXiv subjects

Matteo Calandra

Publications and source records attributed to Matteo Calandra.

At least 55 records · Page 3Linked to original sources

arXiv2022

Highly tunable optics across a topological transition in organic polymers

Controllable topological phase transitions are appealing as they allow for tunable single particle electronic properties. Here, by using state-of-the-art manybody perturbation theory techniques, we show that the topological $Z_2$ phase transition occurring in the single particle spectrum of the recently synthetized ethynylene bridged polyacene polymers is accompanied by a topological excitonic phase transition: the band inversion in the non-trivial phase yields real-space exciton wave functions in which electrons and holes exchange orbital characters with respect to the trivial phase. The topological excitonic phase transition results in a broad tunability of the singlet-triplet splittings, opening appealing perspectives for the occurrence of singlet fission. Finally, the flatness of the single-particle electronic structure in the topological non trivial phase leads to negatively dispersing triplet excitons in a large portion of the Brillouin zone, opening a route for spontaneously coherent energy transport at room temperature.

cond-mat.mes-hall↗

arXiv2021

Why Mercury is a superconductor

Despite being the oldest known superconductor, solid mercury is mysteriously absent from all current computational databases of superconductors. In this work, we present a critical study of its superconducting properties based on state-of-the-art superconducting density-functional theory. Our calculations reveal numerous anomalies in electronic and lattice properties, which can mostly be handled, with due care, by modern ab-initio techniques. In particular, we highlight an anomalous role of (i) electron-electron correlations on structural properties (ii) spin-orbit coupling on the dynamical stability, and (iii) semicore $d$ levels on the effective Coulomb interaction and, ultimately, the critical temperature.

cond-mat.supr-con↗

arXiv2021

Light-tunable charge density wave orders in MoTe2 and WTe2 single layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure and bandgap. Moreover, optical pumping of the semimetallic 1T' phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

cond-mat.mtrl-sci↗

arXiv2021

Charge density wave in single-layer Pb/Ge(111) driven by Pb-substrate exchange interaction

Single layer Pb on top of (111) surfaces of group IV semiconductors hosts charge density wave and superconductivity depending on the coverage and on the substrate. These systems are normally considered to be experimental realizations of single band Hubbard models and their properties are mostly investigated using lattice models with frozen structural degrees of freedom, although the reliability of this approximation is unclear. Here, we consider the case of Pb/Ge(111) at 1/3 coverage, for which surface X-ray diffraction and ARPES data are available. By performing first principles calculations, we demonstrate that the non-local exchange between Pb and the substrate drives the system into a $3\times 3$ charge density wave. The electronic structure of this charge ordered phase is mainly determined by two effects: the magnitude of the Pb distortion and the large spin-orbit coupling. Finally, we show that the effect applies also to the $3\times 3$ phase of Pb/Si(111) where the Pb-substrate exchange interaction increases the bandwidth by more than a factor 1.5 with respect to DFT+U, in better agreement with STS data. The delicate interplay between substrate, structural and electronic degrees of freedom invalidates the widespread interpretation available in literature considering these compounds as physical realizations of single band Hubbard models.

cond-mat.str-el↗

arXiv2021

Lattice dynamics of photoexcited insulators from constrained density-functional perturbation theory

We present a constrained density functional perturbation theory scheme for the calculation of structural and harmonic vibrational properties of insulators in the presence of an excited and thermalized electron-hole plasma. The method is ideal to tame ultrafast light induced structural transitions in the regime where the photocarriers thermalize faster than the lattice, the electron-hole recombination time is longer than the phonon period and the photocarrier concentration is large enough to be approximated by an electron-hole plasma. The complete derivation presented here includes total energy, forces and stress tensor, variable cell structural optimization, harmonic vibrational properties and the electron-phonon interaction. We discuss in detail the case of zone center optical phonons not conserving the number of electrons and inducing a Fermi shift in the photo-electron and hole distributions. We validate our implementation by comparing with finite differences in Te and VSe2. By calculating the evolution of the phonon spectrum of Te, Si and GaAs as a function of the fluence of the incoming laser light, we demonstrate that even at low fluences, corresponding to approximately 0.1 photocarriers per cell, the phonon spectrum is substantially modified with respect to the ground state one with new Kohn anomalies appearing and a substantial softening of zone center optical phonons. Our implementation can be efficiently used to detect reversible transient phases and irreversible structural transition induced by ultrafast light absorption.

cond-mat.mtrl-sci↗

arXiv2021

Intrinsic vibrational angular momentum from non-adiabatic effects in non-collinear magnetic molecules

We show that in non-collinear magnetic molecules, non-adiabatic (dynamical) effects due to the electron-vibron coupling are time-reversal symmetry breaking interactions for the vibrational field. As in these systems the electronic wavefunction can not be chosen as real, a nonzero geometric vector potential (Berry connection) arises. As a result, an intrinsic nonzero vibrational angular momentum occurs even for non-degenerate modes and in the absence of external probes. The vibronic modes can then be seen as elementary quantum particles carrying a sizeable angular momentum. As a proof of concept, we demonstrate the magnitude of this topological effect by performing non-adiabatic first principles calculations on platinum clusters and by showing that these molecules host sizeable intrinsic phonon angular momenta comparable to the orbital electronic ones in itinerant ferromagnets.

physics.chem-ph↗

arXiv2021

The Stochastic Self-Consistent Harmonic Approximation: Calculating Vibrational Properties of Materials with Full Quantum and Anharmonic Effects

The efficient and accurate calculation of how ionic quantum and thermal fluctuations impact the free energy of a crystal, its atomic structure, and phonon spectrum is one of the main challenges of solid state physics, especially when strong anharmonicy invalidates any perturbative approach. To tackle this problem, we present the implementation on a modular Python code of the stochastic self-consistent harmonic approximation method. This technique rigorously describes the full thermodyamics of crystals accounting for nuclear quantum and thermal anharmonic fluctuations. The approach requires the evaluation of the Born-Oppenheimer energy, as well as its derivatives with respect to ionic positions (forces) and cell parameters (stress tensor) in supercells, which can be provided, for instance, by first principles density-functional-theory codes. The method performs crystal geometry relaxation on the quantum free energy landscape, optimizing the free energy with respect to all degrees of freedom of the crystal structure. It can be used to determine the phase diagram of any crystal at finite temperature. It enables the calculation of phase boundaries for both first-order and second-order phase transitions from the Hessian of the free energy. Finally, the code can also compute the anharmonic phonon spectra, including the phonon linewidths, as well as phonon spectral functions. We review the theoretical framework of the stochastic self-consistent harmonic approximation and its dynamical extension, making particular emphasis on the physical interpretation of the variables present in the theory that can enlighten the comparison with any other anharmonic theory. A modular and flexible Python environment is used for the implementation, which allows for a clean interaction with other packages. We briefly present a toy-model calculation to illustrate the potential of the code.

cond-mat.mtrl-sci↗

arXiv2021

Ab-initio energetics of graphite and multilayer graphene: stability of Bernal versus rhombohedral stacking

There has been a lot of excitement around the observation of superconductivity in twisted bilayer graphene, associated to flat bands close to the Fermi level. Such correlated electronic states also occur in multilayer rhombohedral stacked graphene (RG), which has been receiving increasing attention in the last years. In both natural and artificial samples however, multilayer stacked Bernal graphene (BG) occurs more frequently, making it desirable to determine what is their relative stability and under which conditions RG might be favored. Here, we study the energetics of BG and RG in bulk and also multilayer stacked graphene using first-principles calculations. It is shown that the electronic temperature, not accounted for in previous studies, plays a crucial role in determining which phase is preferred. We also show that the low energy states at room temperature consist of BG, RG and mixed BG-RG systems with a particular type of interface. Energies of all stacking sequences (SSs) are calculated for N = 12 layers, and an Ising model is used to fit them, which can be used for larger N as well. In this way, the ordering of low energy SSs can be determined and analyzed in terms of a few parameters. Our work clarifies inconsistent results in the literature, and sets the basis to studying the effect of external factors on the stability of multilayer graphene systems in first principles calculations.

cond-mat.mtrl-sci↗

arXiv2021

Electronic structure of TiSe$_2$ from a quasi-self-consistent G$_0$W$_0$ approach

In 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 TiSe$_2$, 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 ($G_0W_0$) making results more or less dependent on the starting point. In this work, we develop a quasi-self-consistent extension of $G_0W_0$ based on the random phase approximation (RPA) and the optimized effective potential of hybrid density functional theory. This approach generates an optimal $G_0W_0$ 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 TiS$_2$ and TiSe$_2$. We find the high-temperature phase of TiSe$_2$ to be a semi-metal 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.

cond-mat.mtrl-sci↗

arXiv2020

Misfit layer compounds: a platform for heavily-doped two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) display a rich variety of instabilities such as spin and charge orders, Ising superconductivity and topological properties. Their physical properties can be controlled by doping in electric double-layer field-effect transistors (FET). However, for the case of single layer NbSe$_2$, FET doping is limited to $\approx 1\times 10^{14}$ cm$^{-2}$, while a somewhat larger charge injection can be obtained via deposition of K atoms. Here, by performing ARPES, STM, quasiparticle interference measurements, and first principles calculations we show that a misfit compound formed by sandwiching NbSe$_2$ and LaSe layers behaves as a NbSe$_2$ single layer with a rigid doping of $0.55-0.6$ electrons per Nb atom or $\approx 6\times 10^{14}$ cm$^{-2}$. Due to this huge doping, the $3\times3$ charge density wave is replaced by a $2\times2$ order with very short coherence length. As a tremendous number of different misfit compounds can be obtained by sandwiching TMDs layers with rock salt or other layers, our work paves the way to the exploration of heavily doped 2D TMDs over an unprecedented wide range of doping.

cond-mat.mtrl-sci↗

arXiv2020

Weak Dimensionality Dependence and Dominant Role of Ionic Fluctuations in the Charge-Density-Wave Transition of NbSe$_2$

Contradictory experiments have been reported about the dimensionality effect on the charge-density-wave transition in 2H NbSe$_2$. While scanning tunnelling experiments on single layers grown by molecular beam epitaxy measure a charge-density-wave transition temperature in the monolayer similar to the bulk, around 33 K, Raman experiments on exfoliated samples observe a large enhancement of the transition temperature up to 145 K. By employing a non-perturbative approach to deal with anharmonicity, we calculate from first principles the temperature dependence of the phonon spectra both for bulk and monolayer. In both cases, the charge-density-wave transition temperature is estimated as the temperature at which the phonon energy of the mode driving the structural instability vanishes. The obtained transition temperature in the bulk is around 59 K, in rather good agreement with experiments, and it is just slightly increased in the single-layer limit to 73 K, showing the weak dependence of the transition on dimensionality. Environmental factors could motivate the disagreement between the transition temperatures reported by experiments. Our analysis also demonstrates the predominance of ionic fluctuations over electronic ones in the melting of the charge-density-wave order.

cond-mat.mes-hall↗

arXiv2020

Electronic structure of pristine and Ni-substituted LaFeO$_3$ from near edge x-ray absorption fine structure experiments and first-principles simulations

We present a joint theoretical and experimental study of the oxygen $K$-edge spectra for LaFeO$_3$ and homovalent Ni-substituted LaFeO$_3$ (LaFe$_{0.75}$Ni$_{0.25}$O$_3$), using first-principles simulations based on density-functional theory with extended Hubbard functionals and x-ray absorption near edge structure (XANES) measurements. Ground-state and excited-state XANES calculations employ Hubbard on-site $U$ and inter-site $V$ parameters determined from first principles and the Lanczos recursive method to obtain absorption cross sections, which allows for a reliable description of XANES spectra in transition-metal compounds in a very broad energy range, with an accuracy comparable to that of hybrid functionals but at a substantially lower cost. We show that standard gradient-corrected exchange-correlation functionals fail in capturing accurately the electronic properties of both materials. In particular, for LaFe$_{0.75}$Ni$_{0.25}$O$_3$ they do not reproduce its semiconducting behaviour and provide a poor description of the pre-edge features at the O $K$ edge. The inclusion of Hubbard interactions leads to a drastic improvement, accounting for the semiconducting ground state of LaFe$_{0.75}$Ni$_{0.25}$O$_3$ and for a good agreement between calculated and measured XANES spectra. We show that the partial substitution of Fe for Ni affects the conduction-band bottom by generating a strongly hybridized O($2p$)-Ni($3d$) minority-spin empty electronic state. The present work, based on a consistent correction of self-interaction errors, outlines the crucial role of extended Hubbard functionals to describe the electronic structure of complex transition-metal oxides such as LaFeO$_3$ and LaFe$_{0.75}$Ni$_{0.25}$O$_3$ and paves the way to future studies on similar systems.

cond-mat.str-el↗

arXiv2020

Phonon collapse and van der Waals melting of the 3D charge density wave of VSe$_2$

Among transition metal dichalcogenides (TMDs), VSe$_2$ is considered to develop a purely 3-dimensional (3D) charge-density wave (CDW) at T$_{CDW}$=110 K. Here, by means of high resolution inelastic x-ray scattering (IXS), we show that the CDW transition is driven by the collapse of an acoustic mode at the critical wavevector \textit{q}$_{CDW}$= (2.25 0 0.7) r.l.u. and critical temperature T$_{CDW}$=110 K. The softening of this mode starts to be pronounced for temperatures below 2$\times$ T$_{CDW}$ and expands over a rather wide region of the Brillouin zone, suggesting a large contribution of the electron-phonon interaction to the CDW formation. This interpretation is supported by our first principles calculations that determine a large momentum-dependence of the electron-phonon interaction, peaking at the CDW wavevector, in the presence of nesting. Fully anharmonic {\it ab initio} calculations confirm the softening of one acoustic branch at \textit{q}$_{CDW}$ as responsible for the CDW formation and show that van der Waals interactions are crucial to melt the CDW. Our work also highlights the important role of out-of-plane interactions to describe 3D CDWs in TMDs.

cond-mat.mtrl-sci↗

arXiv2020

Theory of the thickness dependence of the charge density wave transition in 1T-TiTe$_2$

Most metallic transition metal dichalcogenides undergo charge density wave (CDW) instabilities with similar or identical ordering vectors in bulk and in single layer, albeit with different critical temperatures. Metallic 1T-TiTe$_2$ is a remarkable exception as it shows no evidence of charge density wave formation in bulk, but it displays a stable $2\times2$ reconstruction in single-layer form. The mechanism for this 3D-2D crossover of the transition is still unclear, although strain from the substrate and the exchange interaction have been pointed out as possible formation mechanisms. Here, by performing non-perturbative anharmonic calculations with gradient corrected and hybrid functionals, we explain the thickness behaviour of the transition in 1T-TiTe$_2$. We demonstrate that the occurrence of the CDW in single-layer TiTe$_2$ occurs from the interplay of non-perturbative anharmonicity and an exchange enhancement of the electron-phonon interaction, larger in the single layer than in the bulk. Finally, we study the electronic and structural properties of the single-layer CDW phase and provide a complete description of its electronic structure, phonon dispersion as well as infrared and Raman active phonon modes.

cond-mat.mtrl-sci↗

arXiv2020

Anomalous high-temperature superconductivity in YH$_6$

Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.

cond-mat.supr-con↗

arXiv2020

Thermal conductivity of Bi$_2$Se$_3$ from bulk to thin films: theory and experiment

We calculate the lattice-driven in-plane $(κ_{\parallel})$ and out-of-plane $(κ_{\perp})$ thermal conductivities of Bi$_2$Se$_3$ bulk, and of films of different thicknesses, using the Boltzmann equation with phonon scattering times obtained from anharmonic third order density functional perturbation theory. We compare our results for the lattice component of the thermal conductivity with published data for $κ_{\parallel}$ on bulk samples and with our room-temperature thermoreflectance measurements of $κ_{\perp}$ on films of thickness (L) ranging from 18~nm to 191~nm, where the lattice component has been extracted via the Wiedemann-Franz law. Ab-initio theoretical calculations on bulk samples, including an effective model to account for finite sample thickness and defect scattering, compare favorably both for the bulk case (from literature) and thin films (new measurements). In the low-T limit the theoretical in-plane lattice thermal conductivity of bulk Bi$_2$Se$_3$ agrees with previous measurements by assuming the occurrence of intercalated Bi$_2$ layer defects. The measured thermal conductivity monotonically decreases by reducing $L$, its value is $κ_{\perp}\approx 0.39\pm 0.08$~W/m$\cdot$K for $L=18$ nm and $κ_{\perp}=0.68\pm0.14$~W/m$\cdot$K for $L=191$ nm. We show that the decrease of room-temperature $κ_{\perp}$ in Bi$_2$Se$_3$ thin films as a function of sample thickness can be explained by the incoherent scattering of out-of-plane momentum phonons with the film surface. Our work outlines the crucial role of sample thinning in reducing the out-of-plane thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2020

Long-range rhombohedral-stacked graphene through shear

The discovery of superconductivity and correlated electronic states in the flat bands of twisted bilayer graphene has raised a lot of excitement. Flat bands also occur in multilayer graphene flakes that present rhombohedral (ABC) stacking order on many consecutive layers. Although Bernal-stacked (AB) graphene is more stable, long-range ABC-ordered flakes involving up to 50 layers have been surprisingly observed in natural samples. Here we present a microscopic atomistic model, based on first-principles density functional theory calculations, that demonstrates how shear stress can produce long-range ABC order. A stress-angle phase diagram shows under which conditions ABC-stacked graphene can be obtained, providing an experimental guide for its synthesis.

cond-mat.mes-hall↗

arXiv2020

Black metal hydrogen above 360 GPa driven by proton quantum fluctuations

Production of metallic hydrogen is one of the top three open quests of physics. Recent low-temperature experiments report different metallization pressures, varying from 360GPa to 490GPa. In this work, we simulate structural properties, vibrational Raman, IR and optical spectra of hydrogen phase III accounting for proton quantum effects. We demonstrate that nuclear quantum fluctuations downshift the vibron frequencies by 25%, introduce a broad line-shape in the Raman spectra, and reduce the optical gap by 3eV. We show that hydrogen metallization occurs at 380GPa in phase III due to band overlap, in good agreement with transport data. By simulating the optical properties, we predict this state to be a peculiar black metal, transparent in the IR. The transparent window closes at 450GPa, but the reflectivity remains low, discarding phase III as the shiny metal observed at 490GPa. We predict the conductivity onset to increase by 70GPa and the transparent window to increase by 1.3eV when replacing hydrogen by deuterium at 0K, underlining that metallization is driven by quantum fluctuations and is thus isotope dependent. We show how hydrogen acquires metallic features (conductivity and brightness) at different pressures, explaining the apparent contradictions in existing experimental scenarios.

cond-mat.mtrl-sci↗