arXiv subjects

V. Olevano

Publications and source records attributed to V. Olevano.

12 recordsLinked to original sources

arXiv2025

Pressure and doping effects on the electronic structure and magnetism of the single-layer nickelate La$_2$NiO$_4$

La$_2$NiO$_4$ is a prototypical member of the Ruddlesden-Popper nickelate series that offers a valuable reference point for elucidating the key ingredients behind the intriguing properties of these systems. However, the structural and electronic properties of La$_2$NiO$_4$ under pressure and doping remain surprisingly underexplored. Here, we investigate these properties using density-functional-theory calculations. We find that its tetragonal $I4/mmm$ structure can be stabilized, not only under pressure, but also at ambient pressure via the partial substitution of La with Ba. In both cases, we find a pronounced magnetostructural interplay that manifests, in particular, as anomalies in the lattice-parameter evolution with composition, deviating from Vegard's law. Moreover, we show that the combined effects of Ba substitution and pressure leads to qualitative changes in the electronic structure towards the formal $d^{7.5}$ configuration of the superconducting bilayer nickelates. Further, while La$_2$NiO$_4$ can undergo a insulator-metal transition with pressure retaining G-type antiferromagnetic order, La$_{1.5}$Ba$_{0.5}$NiO$_4$ exhibits metallic behavior with an enhanced competition between different magnetic states. Our results thus offer new insights into the interplay of structure, doping, and magnetism across the Ruddlesden-Popper nickelate series.

cond-mat.supr-con↗

arXiv2023

Preempted phonon-mediated superconductivity in the infinite-layer nickelates

Nickelate superconductors are outstanding materials with intriguing analogies with the cuprates. These analogies suggest that their superconducting mechanism may be unconventional, although this fundamental question is currently under debate. Here, we scrutinize the role played by electronic correlations in enhancing the electron-phonon coupling in the infinite-layer nickelates and the extent to which this may promote superconductivity. Specifically, we use $ab$ $initio$ many-body perturbation theory to perform state-of-the-art $GW$ and Eliashberg-theory calculations. We find that the electron-phonon coupling is effectively enhanced compared to density-functional-theory calculations. This enhancement may lead to low-$T_c$ superconductivity in the parent compounds already. However, it remains marginal in the sense that it cannot explain the record $T_c$s obtained with doping. This circumstance implies that conventional superconductivity is preempted by another pairing mechanism in the infinite-layer nickelates.

cond-mat.supr-con↗

arXiv2019

Infinite-layer fluoro-nickelates as $d^9$ model materials

We study theoretically the fluoro-nickelate series $A$NiF$_2$ ($A=$ Li, Na, K, Rb, Cs) in the tetragonal $P4/mmm$ infinite-layer structure. We use density functional theory to determine the structural parameters and the electronic band structure of these unprecedented compounds. Thus, we predict these materials as model $d^9$ systems where the Ni$^{1+}$ oxidation is realized and the low-energy physics is completely determined by the Ni-3$d$ bands only. Fluoro-nickelates of this class thus offer an ideal platform for the study of intriguing physics that emerges out of the special $d^9$ electronic configuration, notably high-temperature unconventional superconductivity.

cond-mat.supr-con↗

arXiv2019

Magnetic penetration depth and $T_c$ in superconducting nickelates

We compute the nominal magnetic penetration depth of $R$NiO$_2$ ($R =$ La, Nd) from first principles calculations and discuss the results in relation to the superconducting $T_c$. We find a marked discrepancy with the well established phenomenology that correlates these two quantities in cuprates (Uemura plot). We also consider the 2D ultrathin limit and estimate the maximum attainable $T_c$ to be $\sim 180$ K according to the Nelson-Kosterliz universal relation between the superfluid density and the transition temperature.

cond-mat.supr-con↗

arXiv2016

Emerging giant resonant exciton induced by Ta-substitution in anatase TiO$_{2}$: a tunable correlation effect

Titanium dioxide (TiO$_2$) has rich physical properties with potential implications in both fundamental physics and new applications. Up-to-date, the main focus of applied research is to tune its optical properties, which is usually done via doping and/or nano-engineering. However, understanding the role of $d$-electrons in materials and possible functionalization of $d$-electron properties are still major challenges. Herewith, within a combination of an innovative experimental technique, high energy optical conductivity, and of the state-of-the-art {\it ab initio} electronic structure calculations, we report an emerging, novel resonant exciton in the deep ultraviolet region of the optical response. The resonant exciton evolves upon low concentration Ta-substitution in anatase TiO$_{2}$ films. It is surprisingly robust and related to strong electron-electron and electron-hole interactions. The $d$- and $f$- orbitals localization, due to Ta-substitution, plays an unexpected role, activating strong electronic correlations and dominating the optical response under photoexcitation. Our results shed light on a new optical phenomenon in anatase TiO$_{2}$ films and on the possibility of tuning electronic properties by Ta substitution.

cond-mat.str-el↗

arXiv2012

The bandstructure of gold from many-body perturbation theory

The bandstructure of gold is calculated using many-body perturbation theory (MBPT). Different approximations within the GW approach are considered. Standard single shot G0W0 corrections shift the unoccupied bands up by ~0.2 eV and the first sp-like occupied band down by ~0.4 eV, while leaving unchanged the 5d occupied bands. Beyond G0W0, quasiparticle self-consistency on the wavefunctions lowers the occupied 5d bands by 0.35 eV. Globally, many-body effects achieve an opening of the interband gap (5d-6sp gap) of 0.35 to 0.75 eV approaching the experimental results. Finally, the quasiparticle bandstructure is compared to the one obtained by the widely used HSE (Heyd, Scuseria, and Ernzerhof) hybrid functional.

cond-mat.mtrl-sci↗

arXiv2011

Many-body effects in TiSe2: Can GW describe an Excitonic Insulator?

We present both theoretical ab initio GW and experimental angle-resolved photoemission (ARPES) and scanning tunneling (STS) spectroscopy results on TiSe2. With respect to the density-functional Kohn-Sham metallic picture, the many-body GW self-energy leads to a ~ 0.2 eV band gap insulator consistent with our STS spectra at 5 K. The band shape is strongly renormalized, with the top-of-valence moved towards a circle of points away from Γ, arising in a mexican hat feature typical of an excitonic insulator. Our calculations are in good agreement with experiment.

cond-mat.str-el↗

arXiv2011

Transport properties of molecular junctions from many-body perturbation theory

The conductance of single molecule junctions is calculated using a Landauer approach combined to many-body perturbation theory MBPT) to account for electron correlation. The mere correction of the density-functional theory eigenvalues, which is the standard procedure for quasiparticle calculations within MBPT, is found not to affect noticeably the zero-bias conductance. To reduce it and so improve the agreement with the experiments, the wavefunctions also need to be updated by including the non-diagonal elements of the self-energy operator.

cond-mat.mes-hall↗

arXiv2008

Ab initio GW many-body effects in graphene

We present an {\it ab initio} many-body GW calculation of the self-energy, the quasiparticle band plot and the spectral functions in free-standing undoped graphene. With respect to other approaches, we numerically take into account the full ionic and electronic structure of real graphene and we introduce electron-electron interaction and correlation effects from first principles. Both non-hermitian and also dynamical components of the self-energy are fully taken into account. With respect to DFT-LDA, the Fermi velocity is substantially renormalized and raised by a 17%, in better agreement with magnetotransport experiments. Furthermore, close to the Dirac point the linear dispersion is modified by the presence of a kink, as observed in ARPES experiments. Our calculations show that the kink is due to low-energy $π\to π^*$ single-particle excitations and to the $π$ plasmon. Finally, the GW self-energy does not open the band gap.

cond-mat.mtrl-sci↗

arXiv2008

Specification of an extensible and portable file format for electronic structure and crystallographic data

In order to allow different software applications, in constant evolution, to interact and exchange data, flexible file formats are needed. A file format specification for different types of content has been elaborated to allow communication of data for the software developed within the European Network of Excellence "NANOQUANTA", focusing on first-principles calculations of materials and nanosystems. It might be used by other software as well, and is described here in detail. The format relies on the NetCDF binary input/output library, already used in many different scientific communities, that provides flexibility as well as portability accross languages and platforms. Thanks to NetCDF, the content can be accessed by keywords, ensuring the file format is extensible and backward compatible.

cs.DL↗

arXiv2008

Linear plasmon dispersion in single-wall carbon nanotubes and the collective excitation spectrum of graphene

We have measured a strictly linear pi-plasmon dispersion along the axis of individualized single wall carbon nanotubes, which is completely different from plasmon dispersions of graphite or bundled single wall carbon nanotubes. Comparative ab initio studies on graphene based systems allow us to reproduce the different dispersions. This suggests that individualized nanotubes provide viable experimental access to collective electronic excitations of graphene, and it validates the use of graphene to understand electronic excitations of carbon nanotubes. In particular, the calculations reveal that local field effects (LFE) cause a mixing of electronic transitions, including the 'Dirac cone', resulting in the observed linear dispersion.

cond-mat.str-el↗

arXiv2003

Optical absorption in small BN and C nanotubes

We present a theoretical study of the optical absorption spectrum of small boron-nitride and carbon nanotubes using time-dependent density-functional theory and the random phase approximation. Both for C and BN tubes, the absorption of light polarized perpendicular to the tube-axis is strongly suppressed due to local field effects. Since BN-tubes are wide band-gap insulators, they only absorb in the ultra-violet energy regime, independently of chirality and diameter. In comparison with the spectra of the single C and BN-sheets, the tubes display additional fine-structure which stems from the (quasi-) one-dimensionality of the tubes and sensitively depends on the chirality and tube diameter. This fine structure can provide additional information for the assignment of tube indices in high resolution optical absorption spectroscopy.

cond-mat.mtrl-sci↗