arXiv subjects

Marios Zacharias

Publications and source records attributed to Marios Zacharias.

31 records · Page 2Linked to original sources

arXiv2022

Direct view of phonon dynamics in atomically thin MoS$_{2}$

Transition metal dichalcogenide monolayers and heterostructures are highly tunable material systems that provide excellent models for physical phenomena at the two-dimensional (2D) limit. While most studies to date have focused on electrons and electron-hole pairs, phonons also play essential roles. Here, we apply ultrafast electron diffraction and diffuse scattering to directly quantify, with time and momentum resolution, electron-phonon coupling (EPC) in monolayer molybdenum disulfide (MoS$_{2}$) and phonon transport from the monolayer to a silicon nitride (Si$_3$N$_4$) substrate. Optically generated hot carriers result in a profoundly anisotropic distribution of phonons in the monolayer on the $\sim$ 5 ps time scale. A quantitative comparison with ab-initio ultrafast dynamics simulations reveals the essential role of dielectric screening in weakening EPC. Thermal transport from the monolayer to the substrate occurs with the phonon system far from equilibrium. While screening in 2D is known to strongly affect equilibrium properties, our findings extend this understanding to the dynamic regime.

cond-mat.mtrl-sci↗

arXiv2021

Designing a Trusted Data Brokerage Framework in the Aviation Domain

In recent years, there is growing interest in the ways the European aviation industry can leverage the multi-source data fusion towards augmented domain intelligence. However, privacy, legal and organisational policies together with technical limitations, hinder data sharing and, thus, its benefits. The current paper presents the ICARUS data policy and assets brokerage framework, which aims to (a) formalise the data attributes and qualities that affect how aviation data assets can be shared and handled subsequently to their acquisition, including licenses, IPR, characterisation of sensitivity and privacy risks, and (b) enable the creation of machine-processable data contracts for the aviation industry. This involves expressing contractual terms pertaining to data trading agreements into a machine-processable language and supporting the diverse interactions among stakeholders in aviation data sharing scenarios through a trusted and robust system based on the Ethereum platform.

cs.AI↗

arXiv2021

Quantum Confinement of Electron-Phonon Coupling in Graphene Quantum Dots

On the basis of first-principles calculations and the special displacement method, we demonstrate the quantum confinement scaling law of the phonon-induced gap renormalization of graphene quantum dots (GQDs). We employ zigzag-edged GQDs with hydrogen passivation and embedded in hexagonal boron nitride. Our calculations for GQDs in the sub-10 nm region reveal strong quantum confinement of the zero-point renormalization ranging from 20 to 250 meV. To obtain these values we introduce a correction to the Allen-Heine theory of temperature-dependent energy levels that arises from the phonon-induced splitting of 2-fold degenerate edge states. This correction amounts to more than 50% of the gap renormalization. We also present momentum-resolved spectral functions of GQDs, which are not reported in previous contributions. Our results lay the foundation to systematically engineer temperature-dependent electronic structures of GQDs for applications in solar cells, electronic transport, and quantum computing devices.

cond-mat.mtrl-sci↗

arXiv2021

Multi-phonon diffuse scattering in solids from first-principles: Application to layered crystals and 2D materials

Time-resolved diffuse scattering experiments have gained increasing attention due to their potential to reveal non-equilibrium dynamics of crystal lattice vibrations with full momentum resolution. Although progress has been made in interpreting experimental data on the basis of one-phonon scattering, understanding the role of individual phonons can be sometimes hindered by multi-phonon excitations. In Ref. [{\it arXiv:2103.10108}] we have introduced a rigorous approach for the calculation of the all-phonon inelastic scattering intensity of solids from first-principles. In the present work, we describe our implementation in detail and show that multi-phonon interactions are captured efficiently by exploiting translational and time-reversal symmetries of the crystal. We demonstrate its predictive power by calculating the scattering patterns of monolayer molybdenum disulfide (MoS$_2$), bulk MoS$_2$, and black phosphorus (bP), and we obtain excellent agreement with our measurements of thermal electron diffuse scattering. Remarkably, our results show that multi-phonon excitations dominate in bP across multiple Brillouin zones, while in MoS$_2$ they play a less pronounced role. We expand our analysis for each system and examine the effect of individual atomic and interatomic vibrational motion on the diffuse scattering signals. We further demonstrate the high-throughput capability of our approach by reporting all-phonon scattering maps of 2D MoSe2, WSe2, WS2, graphene, and CdI2, rationalizing in each case the effect of multi-phonon processes. As a side point, we show that the special displacement method reproduces the thermally distorted configuration that generates precisely the all-phonon diffuse pattern.

cond-mat.mtrl-sci↗

arXiv2021

Efficient first-principles methodology for the calculation of the all-phonon inelastic scattering in solids

Inelastic scattering experiments are key methods for mapping the full dispersion of fundamental excitations of solids in the ground as well as non-equilibrium states. A quantitative analysis of inelastic scattering in terms of phonon excitations requires identifying the role of multi-phonon processes. Here, we develop an efficient first-principles methodology for calculating the all-phonon quantum mechanical structure factor of solids. We demonstrate our method by obtaining excellent agreement between measurements and calculations of the diffuse scattering patterns of black phosphorus, showing that multi-phonon processes play a substantial role. The present approach constitutes a step towards the interpretation of static and time-resolved electron, X-ray, and neutron inelastic scattering data.

cond-mat.mtrl-sci↗

arXiv2021

Two-dimensional plasmonic polarons in n-doped monolayer MoS2

We report experimental and theoretical evidence of strong electron-plasmon interaction in n-doped single-layer MoS2. Angle-resolved photoemission spectroscopy (ARPES) measurements reveal the emergence of distinctive signatures of polaronic coupling in the electron spectral function. Calculations based on many-body perturbation theory illustrate that electronic coupling to two-dimensional (2D) carrier plasmons provides an exhaustive explanation of the experimental spectral features and their energies. These results constitute compelling evidence of the formation of plasmon-induced polaronic quasiparticles, suggesting that highly-doped transition-metal dichalcogenides may provide a new platform to explore strong-coupling phenomena between electrons and plasmons in 2D.

cond-mat.str-el↗

arXiv2021

Accessing the anisotropic non-thermal phonon populations in black phosphorus

We combine femtosecond electron diffuse scattering experiments and first-principles calculations of the coupled electron-phonon dynamics to provide a detailed momentum-resolved picture of the ultrafast lattice thermalization in a thin film of black phosphorus. The measurements reveal the emergence of highly anisotropic non-thermal phonon populations which persist for several picoseconds following excitation of the electrons with a light pulse. Combining ultrafast dynamics simulations based on the time-dependent Boltzmann formalism and calculations of the structure factor, we reproduce the experimental data and identify the vibrational modes primarily responsible for the carrier relaxation via electron-phonon coupling and the subsequent lattice thermalization via phonon-phonon scattering. In particular, we attribute the non-equilibrium lattice dynamics of black phosphorus to highly-anisotropic phonon-assisted scattering processes, which are primarily mediated by high-energy optical phonons. Our approach paves the way towards unravelling and controlling microscopic energy-flow pathways in two-dimensional materials and van der Waals heterostructures, and may also be extended to other non-equilibrium phenomena involving coupled electron-phonon dynamics such as superconductivity, phase transitions or polaron physics.

cond-mat.mes-hall↗

arXiv2021

Temperature-dependent electronic ground state charge transfer in van der Waals heterostructures

Electronic charge rearrangement between components of a heterostructure is the fundamental principle to reach the electronic ground state. It is acknowledged that the density of states distribution of the components governs the amount of charge transfer, but a notable dependence on temperature has not yet been considered, particularly for weakly interacting systems. Here, we experimentally observe that the amount of ground state charge transfer in a van der Waals heterostructure formed by monolayer MoS2 sandwiched between graphite and a molecular electron acceptor layer increases by a factor of three when going from 7 K to room temperature. State-of-the-art electronic structure calculations of the full heterostructure that account for nuclear thermal fluctuations reveal intra-component electron-phonon coupling and inter-component electronic coupling as the key factors determining the amount of charge transfer. This conclusion is rationalized by a model applicable to multi-component van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2020

Fully Anharmonic, Non-Perturbative Theory of Vibronically Renormalized Electronic Band Structures

We develop a first-principles approach for the treatment of vibronic interactions in solids that overcomes the main limitations of state-of-the-art electron-phonon coupling formalisms. In particular, anharmonic effects in the nuclear dynamics are accounted to all orders via ab initio molecular dynamics simulations. This non-perturbative, self-consistent approach evaluates the response of the wave functions along the computed anharmonic trajectory; thus it fully considers the coupling between nuclear and electronic degrees of freedom. We validate and demonstrate the merits of the concept by calculating temperature-dependent, momentum-resolved spectral functions for silicon and the cubic perovskite SrTiO3, a strongly anharmonic material featuring soft modes. In the latter case, our approach reveals that anharmonicity and higher-order vibronic couplings contribute substantially to the electronic-structure at finite-temperatures, noticeably affecting band gaps and effective masses, and hence macroscopic properties such as transport coefficients.

cond-mat.mtrl-sci↗

arXiv2020

Temperature dependence of the optical properties of silicon nanocrystals

Silicon nanocrystals (SiNCs) have been under active investigation in the last decades and have been considered as a promising candidate for many optoelectronic applications including highly-efficient solar cells. Some of the fundamental properties of interest in these nanostructures is the temperature dependence of their optical absorption onset, and how this is controlled by different passivation regimes. In the present work we employ first-principles calculations in conjunction with the special displacement method to study the temperature dependence of the band gap renormalization of free-standing hydrogen-terminated, and oxidized SiNCs, as well as matrix-embedded SiNCs in amorphous silica, and we obtain good agreement with experimental photoluminescence data. We also provide strong evidence that the electron-phonon interplay at the surface of the nanocrystal is suppressed by oxidation and the surrounding amorphous matrix. For the matrix-embedded SiNCs, we show a high correlation between the temperature dependence of the band gap and the Si-Si strained bonds. This result emphasizes the immanent relationship of electron-phonon coupling and thermal structural distortions. We also demonstrate that, apart from quantum confinement, Si- Si strained bonds are the major cause of zero-phonon quasidirect transitions in matrix-embedded SiNCs. As a final point, we clarify that, unlike optical absorption in bulk Si, phonon-assisted electronic transitions play a secondary role in SiNCs.

cond-mat.mtrl-sci↗

arXiv2019

Theory of the special displacement method for electronic structure calculations at finite temperature

Calculations of electronic and optical properties of solids at finite temperature including electron-phonon interactions and quantum zero-point renormalization have enjoyed considerable progress during the past few years. Among the emerging methodologies in this area, we recently proposed a new approach to compute optical spectra at finite temperature including phonon-assisted quantum processes via a single supercell calculation [Zacharias and Giustino, Phys. Rev. B 94, 075125 (2016)]. In the present work we considerably expand the scope of our previous theory starting from a compact reciprocal space formulation, and we demonstrate that this improved approach provides accurate temperature-dependent band structures in three-dimensional and two-dimensional materials, using a special set of atomic displacements in a single supercell calculation. We also demonstrate that our special displacement reproduces the thermal ellipsoids obtained from X-ray crystallography, and yields accurate thermal averages of the mean-square atomic displacements. At a more fundamental level, we show that the special displacement represents an exact single-point approximant of an imaginary-time Feynman's path integral for the lattice dynamics. This enhanced version of the special displacement method enables non-perturbative, robust, and straightforward ab initio calculations of the electronic and optical properties of solids at finite temperature, and can easily be used as a post-processing step to any electronic structure code. Given its simplicity and numerical stability, the present development is suited for high-throughput calculations of band structures, quasiparticle corrections, optical spectra, and transport coefficients at finite temperature.

cond-mat.mtrl-sci↗

arXiv2016

One-shot calculation of temperature-dependent optical spectra and phonon-induced band-gap renormalization

Recently, Zacharias et al [Phys. Rev. Lett. 115, 177401 (2015)] developed a new ab initio theory of temperature-dependent optical absorption spectra and band gaps in semiconductors and insulators. In that work the zero-point renormalization and the temperature dependence were obtained by sampling the nuclear wavefunctions using a stochastic approach. In the present work, we show that the stochastic sampling can be replaced by fully deterministic supercell calculations based on a single optimal configuration of the atomic positions. We demonstrate that a single calculation is able to capture the temperature-dependent band gap renormalization including quantum nuclear effects in direct and indirect-gap semiconductors, as well as phonon-assisted optical absorption in indirect-gap semiconductors. In order to demonstrate this methodology we calculate from first principles the temperature-dependent optical absorption spectra and the renormalization of direct and indirect band gaps in Si, C, and GaAs, and we obtain good agreement with experiment and with previous calculations. In this work we also establish the formal connection between the Williams-Lax theory of optical transitions and the related theories of indirect absorption by Hall, Bardeen, and Blatt, and of temperature-dependent band structures by Allen and Heine. Furthermore, we identify an additional band gap renormalization that arises in the case of degenerate band extrema, and which has not been considered in previous work. The present methodology enables systematic ab initio calculations of optical absorption spectra at finite temperature, including both direct and indirect transitions. This feature will be useful for high-throughput calculations of optical properties at finite temperature, and for calculating temperature-dependent optical properties using high-level theories such as GW and Bethe-Salpeter approaches.

cond-mat.mtrl-sci↗

arXiv2015

Stochastic approach to phonon-assisted optical absorption

We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling Monte Carlo scheme, and we obtain temperature-dependent lineshapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature.

cond-mat.mtrl-sci↗