arXiv subjects

X. Blase

Publications and source records attributed to X. Blase.

16 recordsLinked to original sources

arXiv2024

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↗

arXiv2015

Exploring approximations to the GW self-energy ionic gradients

The accuracy of the many-body perturbation theory GW formalism to calculate electron-phonon coupling matrix elements has been recently demonstrated in the case of a few important systems. However, the related computational costs are high and thus represent strong limitations to its widespread application. In the present study, we explore two less demanding alternatives for the calculation of electron-phonon coupling matrix elements on the many-body perturbation theory level. Namely, we test the accuracy of the static Coulomb-hole plus screened-exchange (COHSEX) approximation and further of the constant screening approach, where variations of the screened Coulomb potential W upon small changes of the atomic positions along the vibrational eigenmodes are neglected. We find this latter approximation to be the most reliable, whereas the static COHSEX ansatz leads to substantial errors. Our conclusions are validated in a few paradigmatic cases: diamond, graphene and the C60 fullerene. These findings open the way for combining the present many-body perturbation approach with efficient linear-response theories.

physics.comp-ph↗

arXiv2012

Many-body Green's function study of coumarins for dye-sensitized solar cells

We study within the many-body Green's function $GW$ and Bethe-Salpeter formalisms the excitation energies of several coumarin dyes proposed as an efficient alternative to ruthenium complexes for dye-sensitized solar cells. Due to their internal donor-acceptor structure, these chromophores present low-lying excitations showing a strong intramolecular charge-transfer character. We show that combining $GW$ and Bethe-Salpeter calculations leads to charge-transfer excitation energies and oscillator strengths in excellent agreement with reference range-separated functional studies or coupled-cluster calculations. The present results confirm the ability of this family of approaches to describe accurately Frenkel and charge-transfer photo-excitations in both extended and finite size systems without any system-dependent adjustable parameter, paving the way to the study of dye-sensitized semiconducting surfaces.

cond-mat.mtrl-sci↗

arXiv2012

Molecular fingerprints in the electronic properties of crystalline organic semiconductors: from experiment to theory

By comparing photoemission spectroscopy with a non-perturbative dynamical mean field theory extension to many-body ab initio calculations, we show in the prominent case of pentacene crystals that an excellent agreement with experiment for the bandwidth, dispersion and lifetime of the hole carrier bands can be achieved in organic semiconductors provided that one properly accounts for the coupling to molecular vibrational modes and the presence of disorder. Our findings rationalize the growing experimental evidence that even the best band structure theories based on a many-body treatment of electronic interactions cannot reproduce the experimental photoemission data in this important class of materials.

cond-mat.mtrl-sci↗

arXiv2012

Short to long-range charge-transfer excitations in the zincbacteriochlorin-bacteriochlorin complex: a Bethe-Salpeter study

We study using the Bethe-Salpeter formalism the excitation energies of the zincbacteriochlorinbacteriochlorin dyad, a paradigmatic photosynthetic complex. In great contrast with standard timedependent density functional theory calculations with (semi)local kernels, charge transfer excitations are correctly located above the intramolecular Q-bands transitions found to be in excellent agreement with experiment. Further, the asymptotic Coulomb behavior towards the true quasiparticle gap for charge transfer excitations at long distance is correctly reproduced, showing that the present scheme allows to study with the same accuracy intramolecular and charge transfer excitations at various spatial range and screening environment without any adjustable parameter.

cond-mat.mtrl-sci↗

arXiv2011

Electron-phonon coupling in the C60 fullerene within the many-body GW approach

We study the electron-phonon coupling in the C60 fullerene within the first-principles GW approach, focusing on the lowest unoccupied t1u three-fold electronic state which is relevant for the superconducting transition in electron doped fullerides. It is shown that the strength of the coupling is significantly enhanced as compared to standard density functional theory calculations with (semi)local functionals, with a 48% increase of the electron-phonon potential Vep. The calculated GW value for the contribution from the Hg modes of 93 meV comes within 4% of the most recent experimental values. The present results call for a reinvestigation of previous density functional based calculations of electron-phonon coupling in covalent systems in general.

cond-mat.mtrl-sci↗

arXiv2011

Charge-transfer excitations in molecular donor-acceptor complexes within the many-body Bethe-Salpeter approach

We study within the perturbative many-body $GW$ and Bethe-Salpeter approach the low lying singlet charge-transfer excitations in molecular donor-acceptor complexes associating benzene, naphtalene and anthracene derivatives with the tetracyanoethylene acceptor. Our calculations demonstrate that such techniques can reproduce the experimental data with a mean average error of 0.1-0.15 eV for the present set of dimers, in excellent agreement with the best time-dependent density functional studies with optimized range-separated functionals. The present results pave the way to the study of photoinduced charge transfer processes in photovoltaic devices with a parameter-free \textit{ab initio} approach showing equivalent accuracy for finite and extended systems.

cond-mat.mtrl-sci↗

arXiv2010

Chemically Functionalized Semiconducting Carbon Nanotubes: Limits for High Conductance Performance

We present a first-principles study of the electronic transport properties of micrometer long semiconducting CNTs randomly covered with carbene functional groups. Whereas prior studies suggested that metallic tubes are hardly affected by such addends, we show here that the conductance of semiconducting tubes with standard diameter is on the contrary severely damaged. The configurational averaged conductance as a function of tube diameter and with a coverage of up to one hundred functional groups is extracted. Our results indicate that the search for a conductance-preserving covalent functionalization route remains a challenging issue.

cond-mat.mes-hall↗

arXiv2009

Chemically-induced Mobility Gaps in Graphene Nanoribbons: A Route for Upscaling Device Performances

We report a first-principles based study of mesoscopic quantum transport in chemically doped graphene nanoribbons with a width up to 10 nm. The occurrence of quasibound states related to boron impurities results in mobility gaps as large as 1 eV, driven by strong electron-hole asymmetrical backscattering phenomena. This phenomenon opens new ways to overcome current limitations of graphene-based devices through the fabrication of chemically-doped graphene nanoribbons with sizes within the reach of conventional lithography.

cond-mat.mes-hall↗

arXiv2009

Anomalous Doping Effects on Charge Transport in Graphene Nanoribbons

We present first-principles calculations of quantum transport in chemically doped graphene nanoribbons with a width of up to 4 nm. The presence of boron and nitrogen impurities is shown to yield resonant backscattering, whose features are strongly dependent on the symmetry and the width of the ribbon, as well as the position of the dopants. Full suppression of backscattering is obtained on the pi-pi* plateau when the impurity preserves the mirror symmetry of armchair ribbons. Further, an unusual acceptor-donor transition is observed in zig-zag ribbons. These unconventional doping effects could be used to design novel types of switching devices.

cond-mat.mtrl-sci↗

arXiv2009

Model for the on-site matrix elements of the tight-binding hamiltonian of a strained crystal: Application to silicon, germanium and their alloys

We discuss a model for the on-site matrix elements of the sp3d5s* tight-binding hamiltonian of a strained diamond or zinc-blende crystal or nanostructure. This model features on-site, off-diagonal couplings between the s, p and d orbitals, and is able to reproduce the effects of arbitrary strains on the band energies and effective masses in the full Brillouin zone. It introduces only a few additional parameters and is free from any ambiguities that might arise from the definition of the macroscopic strains as a function of the atomic positions. We apply this model to silicon, germanium and their alloys as an illustration. In particular, we make a detailed comparison of tight-binding and ab initio data on strained Si, Ge and SiGe.

cond-mat.mtrl-sci↗

arXiv2007

Superconductivity in doped cubic silicon: an ab initio study

We study within a first-principle approach the band structure, vibrational modes and electron-phonon coupling in boron, aluminum and phosphorus doped silicon in the diamond phase. Our results provide evidences that the recently discovered superconducting transition in boron doped cubic silicon can be explained within a standard phonon-mediated mechanism. The importance of lattice compression and dopant related stretching modes are emphasized. We find that T$_C$ can be increased by one order of magnitude by adopting aluminum doping instead of boron.

cond-mat.supr-con↗

arXiv2002

Carbon cage-like materials as potential low work function metallic compounds: Case of clathrates

We present an ab-initio calculation of the electronic affinity of the hypothetical C-46 clathrate by studying its bare and hydrogenated (100) surfaces. We show that such a system shares with the diamond phase a small electronic affinity. Further, contrary to the diamond phase, the possibility of doping endohedrally these cage-like systems allows to significantly raise the position of the Fermi level, resulting in a true metal with a small work function. This is illustrated in the case of the Li8@C-46 doped compound. Such a class of materials might be of much interest for the design of electron-emitting devices.

cond-mat.mtrl-sci↗

arXiv2000

Identification of Electron Donor States in N-doped Carbon Nanotubes

Nitrogen doped carbon nanotubes have been synthesized using pyrolysis and characterized by Scanning Tunneling Spectroscopy and transmission electron microscopy. The doped nanotubes are all metallic and exhibit strong electron donor states near the Fermi level. Using tight-binding and ab initio calculations, we observe that pyridine-like N structures are responsible for the metallic behavior and the prominent features near the Fermi level. These electron rich structures are the first example of n-type nanotubes, which could pave the way to real molecular hetero-junction devices.

cond-mat.mtrl-sci↗

arXiv1995

Theoretical Study of One-dimensional Chains of Metal Atoms in Nanotubes

Using first-principles total-energy pseudopotential calculations, we have studied the properties of chains of potassium and aluminum in nanotubes. For BN tubes, there is little interaction between the metal chains and the tubes, and the conductivity of these tubes is through carriers located at the inner part of the tube. In contrast, for small radius carbon nanotubes, there are two types of interactions: charge-transfer (dominant for alkali atoms) leading to strong ionic cohesion, and hybridization (for multivalent metal atoms) resulting in a smaller cohesion. For Al-atomic chains in carbon tubes, we show that both effects contribute. New electronic properties related to these confined atomic chains of metal are analyzed.

mtrl-th↗