arXiv subjects

Evgeny Plekhanov

Publications and source records attributed to Evgeny Plekhanov.

At least 19 recordsLinked to original sources

arXiv2026

Correlation-driven phonon renormalisation and the equation of state of $γ$-cerium

We investigate the thermodynamic properties of elemental cerium by assessing the crucial role of phonon free energy within the framework of dynamical mean-field theory (DMFT). While conventional density functional theory (DFT) often fails to capture the intricate energy landscape of $f$-electron materials, our approach integrates many-body electronic correlations with lattice dynamics to achieve a more rigorous description of the equation of state. We calculate the total energy as a function of the lattice constant at both the DFT and DFT+DMFT levels, subsequently incorporating the vibrational free energy derived from the phonon density of states. Our findings reveal that electronic renormalisation of the force constants significantly alters the phonon spectra, particularly in the strongly correlated $γ$-phase. By applying these phonon corrections to the energy profiles, we observe a substantial refinement in the predicted equilibrium volumes. Using principal-component-based machine learning, we interpolate phonon dispersions continuously from a finite set of first-principles calculations and compare them to experiment, finding significantly closer agreement compared to conventional DFT and DFT+U calculations that neglect dynamical many-body correlations. This study underlines the necessity of accounting for both electronic and vibrational entropy when evaluating the phase stability and structural transitions of lanthanide systems under varying pressures and temperatures.

cond-mat.str-el↗

arXiv2025

Simulation of Atomic Layer Deposition with a Quantum Computer

In this work, we present the study of an atomic layer deposition (ALD) of zirconium by means of a quantum computation on an emulator representing the features of an architecture based on qubits implemented on carbon nanotubes. ALD process control is key in several technological applications such as spintronics, catalysis and renewable energy storage. We first derive a large ab-initio model of the precursor molecule approaching the infinite hydroxylated silicon (100) surface. In particular, we optimize geometry in three configurations: reactants, transition state and products. Subsequently, we derive an effective small cluster model for each state. Atomic valence active space (AVAS) transformation is then performed on these small clusters, leading to an effective qubit Hamiltonian, which is solved using the Variational Quantum Eigensolver (VQE) algorithm. We study the convergence of the reaction activation barrier with respect to the active space size and benchmark quantum calculations on a noiseless emulator and on an emulator representing a carbon nanotube qubit architecture, including an appropriate noise model and post-selection error mitigation. These calculations reveal an excellent agreement between the two emulation modes. Our VQE calculations provide the multi-configurational corrections to the single determinant DFT and HF states and pave the way for the routine quantum calculations of ALD reactions.

quant-ph↗

arXiv2024

Quantum Computed Green's Functions using a Cumulant Expansion of the Lanczos Method

In this paper, we present a quantum computational method to calculate the many-body Green's function matrix in a spin orbital basis. We apply our approach to finite-sized fermionic Hubbard models and related impurity models within Dynamical Mean Field Theory, and demonstrate the calculation of Green's functions on Quantinuum's H1-1 trapped-ion quantum computer. Our approach involves a cumulant expansion of the Lanczos method, using Hamiltonian moments as measurable expectation values. This bypasses the need for a large overhead in the number of measurements due to repeated applications of the variational quantum eigensolver (VQE), and instead measures the expectation value of the moments with one set of measurement circuits. From the measured moments, the tridiagonalised Hamiltonian matrix can be computed, which in turn yields the Green's function via continued fractions. While we use a variational algorithm to prepare the ground state in this work, we note that the modularity of our implementation allows for other (non-variational) approaches to be used for the ground state.

cond-mat.str-el↗

arXiv2024

Platinum-based Catalysts for Oxygen Reduction Reaction simulated with a Quantum Computer

Hydrogen has emerged as a promising energy source, holding the key to achieve low-carbon and sustainable mobility. However, its applications are still limited by modest conversion efficiency in the electrocatalytic oxygen reduction reaction (ORR) within fuel cells. Consequently, the development of novel catalysts and a profound understanding of the underlying reactions have become of paramount importance. The complex nature of the ORR potential energy landscape and the presence of strong electronic correlations present challenges to atomistic modelling using classical computers. This scenario opens new avenues for the implementation of novel quantum computing workflows to address these molecular systems. Here, we present a pioneering study that combines classical and quantum computational approaches to investigate the ORR on pure platinum and platinum/cobalt surfaces. Our research demonstrates, for the first time, the feasibility of implementing this workflow on the H1-series trapped-ion quantum computer and identify the challenges of the quantum chemistry modelling of this reaction. The results highlight the involvement of strongly correlated species in the cobalt-containing catalyst, suggesting their potential as ideal candidates for showcasing quantum advantage in future applications.

quant-ph↗

arXiv2023

Computational prediction of samarium hydride at megabar pressure

Samarium hydrides, belonging to the broad class of lanthanide polyhydrides, have yet to be experimentally tested at high pressure. In this study, we use random structure searches to explore multiple possible stoichiometries and propose SmH$_2$ with a layered hexagonal structure in the $P6/mmm$ space group and SmH$_6$ with hydrogen clathrate structures in the Im-3m space group as theoretically stable phases of samarium hydrides at a wide range of pressures centered around $200$ GPa. We further combine the first-principles methods of density functional theory and dynamical mean-field theory to explore many-body correlations in samarium hydrides, reporting electron and phonon dispersions and densities of states, and also evaluate the electron-phonon driven superconductivity to investigate low critical temperatures at $200$ GPa.

cond-mat.str-el↗

arXiv2022

Engineering superconducting properties of multiferroic copper oxide heterostructures

Oxide heterostructures have repeatedly been shown to display apical properties at the interfaces, some of which favorable to the formation of two-dimensional electron systems, as well as high transition temperature superconductivity. In this study, we propose a novel heterostructure to potentially achieve near room-temperature superconductivity, via the carrier injection in cuprate interfaces with ferro-electrics. Using a digital design approach guided by density-functional theory, the systems of XTiO3/XCuO3/XTiO3 are thoroughly examined, confirming the formation of a two-dimensional electron gas at the cuprous oxide interface. Via the manipulation of lattice parameters, the key ingredients for two-dimensional electron gas formation is explored. We apply cluster dynamical mean-field theory on the cuprous oxide plane and probe the superconducting properties of the system XTiO3/XCuO3/XTiO3 . As a result, we see a marked increase of superconducting ordering parameter near the fully occupied regime, a known marker for the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2022

Exploring the Effect of the Number of Hydrogen Atoms on the Properties of Lanthanide Hydrides by DMFT

Lanthanide hydrogen-rich materials have long been considered as one of the candidates with high-temperature superconducting properties in condensed matter physics, and have attracted great interest. Attempts to investigate the effects of different compositions of lanthanide hydrogen-rich materials are ongoing, with predictions and experimental studies in recent years having shown that substances such as LaH 10 , CeH 9 , and LaH 16 exhibit extremely high superconducting temperatures between 150-250 GPa. In particular, researchers have noted that in those materials an increase in the f character at the Fermi level leads to an increase in the superconducting temperature. Here, we further elaborate on the effect of the ratios of lanthanide to hydrogen in these substances with the aim to bring more clarity to the study of superhydrides in these extreme cases by comparing a variety of lanthanide hydrogen-rich materials with different ratios using the DMFT method, and provide ideas for later structural predictions and material property studies.

cond-mat.supr-con↗

arXiv2022

High temperature superconductivity in the lanthanide hydrides at extreme pressures

Hydrogen-rich superhydrides are promising high-Tc superconductors, with superconductivity experimentally observed near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g. LaH10 at 170 GPa and CeH9 at 150 GPa. Superconductivity is believed to be closely related with the high vibrational modes of the bound hydrogen ions. Here we study the limit of extreme pressures (above 200 GPa) where lanthanide hydrides with large hydrogen content have been reported. We focus on LaH16 and CeH16, two prototype candidates for achieving a large electronic contribution from hydrogen in the electron-phonon coupling. In this work, we propose a first principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce-H and La-H systems and to understand the structure, stability and superconductivity of these systems at ultra-high pressure. We provide a practical approach to further investigate conventional superconductivity in hydrogen rich superhydrides. We report that density functional theory provides accurate structure and phonon frequencies, but many-body corrections lead to an increase of the critical temperature, that is associated with spectral weight transfer of the f-states.

cond-mat.supr-con↗

arXiv2021

Calculating DMFT forces in ab-initio ultrasoft pseudopotential formalism

In this paper, we show how to calculate analytical atomic forces within self-consistent density functional theory + dynamical mean-field theory (DFT+DMFT) approach in the case when ultra-soft or norm-conserving pseudopotentials are used. We show how to treat the non-local projection terms arising within the pseudopotential formalism and circumvent the problem of non-orthogonality of the Kohn-Sham eigenvectors. Our approach is, in principle, independent of the DMFT solver employed, and here was tested with the Hubbard I solver. We benchmark our formalism by comparing against the forces calculated in Ce$_{2}$O$_{3}$ and PrO$_2$ by numerical differentiation of the total free energy, as well as by comparing the energy profiles against the numerically integrated analytical forces.

cond-mat.str-el↗

arXiv2021

Computational Materials Discovery for Lanthanide Hydrides at high pressure: predicting High Temperature superconductivity

Hydrogen-rich superhydrides are believed to be very promising high-T$_c$ superconductors, with experimentally observed critical temperatures near room temperature, as shown in recently discovered lanthanide superhydrides at very high pressures, e.g. LaH$_{10}$ at 170 GPa and CeH$_9$ at 150 GPa. With the motivation of discovering new hydrogen-rich high-T$_c$ superconductors at lowest possible pressure, quantitative theoretical predictions are needed. In these promising compounds, superconductivity is mediated by the highly energetic lattice vibrations associated with hydrogen and their interplay with the electronic structure, requiring fine descriptions of the electronic properties, notoriously challenging for correlated $f$ systems. In this work, we propose a first-principles calculation platform with the inclusion of many-body corrections to evaluate the detailed physical properties of the Ce-H system and to understand the structure, stability and superconductivity of CeH$_9$ at high pressure. We report how the prediction of T$_c$ is affected by the hierarchy of many-body corrections, and obtain a compelling increase of T$_c$ at the highest level of theory, which goes in the direction of experimental observations. Our findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. We provide a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides.

cond-mat.supr-con↗

arXiv2021

Pressure induced electronic transitions in Samarium monochalcogenides

Pressure induced isostructural insulator to metal transition for SmS is characterised by the presence of an intermediate valence state at higher pressure which cannot be captured by the density functional theory. As a direct outcome of including the charge and spin fluctuations incorporated in dynamical mean field theory, we see the emergence of insulating and metallic phases with increasing pressure as a function of changing valence. This is accompanied by significantly improved predictions of the equilibrium lattice constants and bulk moduli for all Sm-monochalcogenides verifying experiments. Nudged Elastic Band analysis reveals the insulating states to have a finite quasiparticle weight, decreasing as the gap closes rendering the transition to be not Mott-like, and classifies these materials as correlated band insulators. The difference between the discontinuous and continuous natures of these transitions can be attributed to the closeness of the sharply resonant Sm-4f peaks to the fermi level in the predicted metallic states in SmS as compared to SmSe and SmTe.

cond-mat.str-el↗

arXiv2019

Tuning topological surface states by cleavage angle in topological crystalline insulators

The conducting states, recently discovered at the surface of two special class of insulators -- topological insulators and topological crystalline insulators - are distinguished by their insensitivity to local and non-magnetic surface defects at a level of disorder, sufficiently small to be described within the perturbation theory. However, the behavior of the surface states in case of non local macroscopic imperfections is not clear. Here, we propose a systematic study of the topological surface states on vicinal planes (deviations from perfect surface cleavage) in a topological crystalline insulator of the tin telluride family, by using realistic first-principles-derived tight-binding models. The theoretical framework proposed is quite general and easily permits the extensions to other topological insulator families.

cond-mat.mtrl-sci↗

arXiv2019

Calculating Bardeen-Cooper-Schrieffer and Magnetic Superstructure Electronic States with $ΘΦ$

We propose the $ΘΦ$ (Theta-Phi) package which addresses two of the most important extensions of the essentially single-particle mean-field paradigm of the computational solid state physics: the admission of the Bardeen-Cooper-Schrieffer electronic ground state and allowance of the magnetically ordered states with an arbitrary superstructure (pitch) wave vector. Both features are implemented in the context of multi-band systems which paves the way to an interplay with the solid state quantum physics packages eventually providing access to the first-principles estimates of the relevant matrix elements of the model Hamiltonians derived from the standard DFT calculations. Several examples showing the workability of the proposed code are given.

cond-mat.str-el↗

arXiv2018

A novel continuous time quantum Monte Carlo solver for dynamical mean field theory in the compact Legendre representation

Dynamical mean-field theory (DMFT) is one of the most widely-used methods to treat accurately electron correlation effects in ab-initio real material calculations. Many modern large-scale implementations of DMFT in electronic structure codes involve solving a quantum impurity model with a Continuous-Time Quantum Monte Carlo (CT-QMC) solver. The main advantage of CT-QMC is that, unlike standard quantum Monte Carlo approaches, it is able to generate the local Green's functions of the correlated system on an arbitrarily fine imaginary time grid, and is free of any systematic errors. In this work, we extend a hybrid QMC solver proposed by Khatami et al. and Rost et al. to a multi-orbital context. This has the advantage of enabling impurity solver QMC calculations to scale linearly with inverse temperature and permit its application to d and f band materials. In addition, we present a novel Green's function processing scheme which generates accurate quasi-continuous imaginary time solutions of the impurity problem which overcome errors inherent to standard QMC approaches. This solver and processing scheme are incorporated into a full DFT+DMFT calculation using the CASTEP DFT code. Benchmark calculations for strontium vanadate properties are presented.

cond-mat.str-el↗

arXiv2018

Self-assembly of correlated Kondo lattices: The Mott to Kondo transition in diluted superlattices

In the field of condensed matter, the quest to obtain an experimental realization of a Kondo lattice has generated a tremendous effort of the community, from both standpoints of experiments and theory. The pursuit of obtaining independent magnetic moments, via charge localization through Coulomb interactions, is paramount for applications in nanotechnology. In particular, systems with simultaneous charge and spin degrees of freedom can manifest both Kondo spin quenching and Mott-Hubbard charge localization. A unified experimental framework illuminating the pathway between the two phenomena is of physical and technological interest, and is (as of yet) hardly observed in real condensed matter systems. Recent developments in the ability to control densities and temperatures of strongly correlated Fermionic impurities on surfaces and substrates has opened up a new paradigm of possibilities for this pathway. In particular, a milestone was recently surpassed through the observation of self-assembled superlattices of f band adatoms on metallic surfaces, such as the deposition of Ce on Ag(111). Such lattices have introduced a mechanism of diluted correlated lattices where the interaction between Kondo and Mott physics can be methodically studied. However, it remains difficult to control the adatom distances and substrate densities in these systems, and the interplay between Kondo physics and charge localization remains elusive. In this work, we systematically investigate the phase diagram of superlattice structures of heavy f elements deposited on metallic substrates, and assess the required conditions to obtain Kondo lattices in superlattices. We unveil a unique pathway between Kondo quenching and Mott localization, and identify a non-trivial charge density wave phase emerging from the competition of charge localization and Kondo physics.

cond-mat.str-el↗

arXiv2018

Many-body renormalisation of forces in f-electron materials

We present the implementation of Dynamical Mean-Field Theory (DMFT) in the CASTEP \emph{ab-initio} code. We explain in detail the theoretical framework for DFT+DMFT and we demonstrate our implementation for three strongly-correlated systems with $f$-shell electrons: $γ$-cerium, cerium sesquioxide Ce$_{2}$O$_{3}$ and samarium telluride SmTe by using a Hubbard I solver. We find very good agreement with previous benchmark DFT+DMFT calculations of cerium compounds, while for SmTe, which was never studied within DFT+DMFT before to the best of our knowledge, we show the improved agreement with the experimental structural parameters as compared with LDA. Our implementation works equally well for both norm-conserving and ultra-soft pseudopotentials, and we apply it to the calculation of total energy, bulk modulus, equilibrium volumes and internal forces in the two cerium compounds. In Ce$_{2}$O$_{3}$ we report a dramatic reduction of the internal forces acting on coordinates not constrained by unit cell symmetries. This reduction is induced by the many-body effects, which can only be captured at the DMFT level. In addition, we derive an alternative form for treating the high-frequency tails of the Green function in Matsubara frequency summations. Our treatment allows a reduction in the bias when calculating the correlation energies and occupation matrices to high precision.

cond-mat.str-el↗

arXiv2017

Metal-insulator transition in copper oxides induced by apex displacements

High temperature superconductivity has been found in many kinds of compounds built from planes of Cu and O, separated by spacer layers. Understanding why critical temperatures are so high has been the subject of numerous investigations and extensive controversy. To realize high temperature superconductivity, parent compounds are either hole-doped, such as {La$_{2}$CuO$_4$} (LCO) with Sr (LSCO), or electron doped, such as {Nd$_{2}$CuO$_4$} (NCO) with Ce (NCCO). In the electron doped cuprates, the antiferromagnetic phase is much more robust than the superconducting phase. However, it was recently found that the reduction of residual out-of-plane apical oxygens dramatically affects the phase diagram, driving those compounds to a superconducting phase. Here we use a recently developed first principles method to explore how displacement of the apical oxygen (A-O) in LCO affects the optical gap, spin and charge susceptibilities, and superconducting order parameter. By combining quasiparticle self-consistent GW (QS\emph{GW}) and dynamical mean field theory (DMFT), that LCO is a Mott insulator; but small displacements of the apical oxygens drive the compound to a metallic state through a localization/delocalization transition, with a concomitant maximum $d$-wave order parameter at the transition. We address the question whether NCO can be seen as the limit of LCO with large apical displacements, and elucidate the deep physical reasons why the behaviour of NCO is so different than the hole doped materials. We shed new light on the recent correlation observed between T$_c$ and the charge transfer gap, while also providing a guide towards the design of optimized high-Tc superconductors. Further our results suggest that strong correlation, enough to induce Mott gap, may not be a prerequisite for high-Tc superconductivity.

cond-mat.str-el↗

arXiv2015

Analysis of the magnetic response of the edge-sharing chain cuprate Li$_2$CuO$_2$ within TMRG

It is widely accepted that the low-energy physics in edge-sharing cuprate materials has one-dimensional (1D) character. The relevant model to study such systems is believed to be the 1D extended Heisenberg model with ferromagnetic nearest-neighbor (NN) interaction and antiferromagnetic next-nearest-neighbor one. Thus far, however, theoretical studies of such materials have been confined to the case of isotropic interactions. In the present work, we compare the spin susceptibility of the 1D extended Heisenberg model with anisotropy in the NN channel, obtained by means of the Transfer Matrix Renormalization Group method, with that of the edge-sharing chain cuprate Li$_2$CuO$_2$.

cond-mat.str-el↗