arXiv subjects

Troels Markussen

Publications and source records attributed to Troels Markussen.

At least 19 recordsLinked to original sources

arXiv2022

Resonance-enhanced vibrational spectroscopy of molecules on a superconductor

Molecular vibrational spectroscopy with the scanning tunneling microscope is feasible but usually detects few vibrational modes. We harness sharp Yu-Shiba-Rusinov (YSR) states observed from molecules on a superconductor to significantly enhance the vibrational signal. From a lead phthalocyanin molecule 46 vibrational peaks are resolved enabling a comparison with calculated modes. The energy resolution is improved beyond the thermal broadening limit and shifts induced by neighbor molecules or the position of the microscope tip are determined. Vice versa, spectra of vibrational modes are used to measure the effect of an electrical field on the energy of YSR states. The method may help to further probe the interaction of molecules with their environment and to better understand selection rules for vibrational excitations.

cond-mat.mes-hall↗

arXiv2020

First principles evaluation of fcc ruthenium for use in advanced interconnects

As the semiconductor industry turns to alternate conductors to replace Cu for future interconnect nodes, much attention as been focused on evaluating the electrical performance of Ru. The typical hexagonal close-packed (hcp) phase has been extensively studied, but relatively little attention has been paid to the face-centered cubic (fcc) phase, which has been shown to nucleate in confined structures and may be present in tight-pitch interconnects. Using \emph{ab initio} techniques, we benchmark the performance of fcc Ru. We find that the phonon-limited bulk resistivity of the fcc Ru is less than half of that of hcp Ru, a feature we trace back to the stronger electron-phonon coupling elements that are geometrically inherited from the modified Fermi surface shape of the fcc crystal. Despite this benefit of the fcc phase, high grain boundary scattering results in increased resistivity compared to Cu-based interconnects with similar average grain size. We find, however, that the line resistance of fcc Ru is lower than that of Cu below 21 nm line width due to the conductor volume lost to adhesion and wetting liners. In addition to studying bulk transport properties, we evaluate the performance of adhesion liners for fcc Ru. We find that it is energetically more favorable for fcc Ru to bind directly to silicon dioxide than through conventional adhesion liners such as TaN and TiN. In the case that a thin liner is necessary for the Ru deposition technique, we find that the vertical resistance penalty of a liner for fcc Ru can be up to eight times lower than that calculated for conventional liners used for Cu interconnects. Our calculations, therefore, suggest that the formation of the fcc phase of Ru may be a beneficial for advanced, low-resistance interconnects.

cond-mat.mtrl-sci↗

arXiv2019

QuantumATK: An integrated platform of electronic and atomic-scale modelling tools

QuantumATK is an integrated set of atomic-scale modelling tools developed since 2003 by professional software engineers in collaboration with academic researchers. While different aspects and individual modules of the platform have been previously presented, the purpose of this paper is to give a general overview of the platform. The QuantumATK simulation engines enable electronic-structure calculations using density functional theory or tight-binding model Hamiltonians, and also offers bonded or reactive empirical force fields in many different parametrizations. Density functional theory is implemented using either a plane-wave basis or expansion of electronic states in a linear combination of atomic orbitals. The platform includes a long list of advanced modules, including Green's-function methods for electron transport simulations and surface calculations, first-principles electron-phonon and electron-photon couplings, simulation of atomic-scale heat transport, ion dynamics, spintronics, optical properties of materials, static polarization, and more. Seamless integration of the different simulation engines into a common platform allows for easy combination of different simulation methods into complex workflows. Besides giving a general overview and presenting a number of implementation details not previously published, we also present four different application examples. These are calculations of the phonon-limited mobility of Cu, Ag and Au, electron transport in a gated 2D device, multi-model simulation of lithium ion drift through a battery cathode in an external electric field, and electronic-structure calculations of the composition-dependent band gap of SiGe alloys.

cond-mat.mtrl-sci↗

arXiv2018

First-principles quantum transport modeling of spin-transfer and spin-orbit torques in magnetic multilayers

We review a unified approach for computing: (i) spin-transfer torque in magnetic trilayers like spin-valves and magnetic tunnel junction, where injected charge current flows perpendicularly to interfaces; and (ii) spin-orbit torque in magnetic bilayers of the type ferromagnet/spin-orbit-coupled-material, where injected charge current flows parallel to the interface. Our approach requires to construct the torque operator for a given Hamiltonian of the device and the steady-state nonequilibrium density matrix, where the latter is expressed in terms of the nonequilibrium Green's functions and split into three contributions. Tracing these contributions with the torque operator automatically yields field-like and damping-like components of spin-transfer torque or spin-orbit torque vector, which is particularly advantageous for spin-orbit torque where the direction of these components depends on the unknown-in-advance orientation of the current-driven nonequilibrium spin density in the presence of spin-orbit coupling. We provide illustrative examples by computing spin-transfer torque in a one-dimensional toy model of a magnetic tunnel junction and realistic Co/Cu/Co spin-valve, both of which are described by first-principles Hamiltonians obtained from noncollinear density functional theory calculations; as well as spin-orbit torque in a ferromagnetic layer described by a tight-binding Hamiltonian which includes spin-orbit proximity effect within ferromagnetic monolayers assumed to be generated by the adjacent monolayer transition metal dichalcogenide.

cond-mat.mes-hall↗

arXiv2018

Efficient first-principles calculation of phonon assisted photocurrent in large-scale solar cell devices

We present a straightforward and computationally cheap method to obtain the phonon-assisted photocurrent in large-scale devices from first-principles transport calculations. The photocurrent is calculated using nonequilibrium Green's function with light-matter interaction from the first-order Born approximation while electron-phonon coupling (EPC) is included through special thermal displacements (STD). We apply the method to a silicon solar cell device and demonstrate the impact of including EPC in order to properly describe the current due to the indirect band-to-band transitions. The first-principles results are successfully compared to experimental measurements of the temperature and light intensity dependence of the open-circuit voltage of a silicon photovoltaic module. Our calculations illustrate the pivotal role played by EPC in photocurrent modelling to avoid underestimation of the open-circuit voltage, short-circuit current and maximum power. This work represents a recipe for computational characterization of future photovoltaic devices including the combined effects of light-matter interaction, phonon-assisted tunneling and the device potential at finite bias from the level of first-principles simulations.

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arXiv2017

First-Principles Electron Transport with Phonon Coupling: Large-Scale at Low Cost

Phonon-assisted tunneling plays a crucial role for electronic device performance and even more so with future size down-scaling. We show how one can include this effect in large-scale first-principles calculations using a single "special thermal displacement" (STD) of the atomic coordinates at almost the same cost as elastic transport calculations. We apply the method to ultra-scaled silicon devices and demonstrate the importance of phonon-assisted band-to-band and source-to-drain tunneling. In a diode the phonons lead to a rectification ratio suppression in good agreement with experiments, while in an ultra-thin body transistor the phonons increase off-currents by four orders of magnitude, and the subthreshold swing by a factor of four, in agreement with perturbation theory.

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arXiv2017

Interface band gap narrowing behind open circuit voltage losses in Cu$_2$ZnSnS$_4$ solar cells

We present evidence that band gap narrowing at the heterointerface may be a major cause of the large open circuit voltage deficit of Cu$_2$ZnSnS$_4$/CdS solar cells. Band gap narrowing is caused by surface states that extend the Cu$_2$ZnSnS$_4$ valence band into the forbidden gap. Those surface states are consistently found in Cu$_2$ZnSnS$_4$, but not in Cu$_2$ZnSnSe$_4$, by first-principles calculations. They do not simply arise from defects at surfaces but are an intrinsic feature of Cu$_2$ZnSnS$_4$ surfaces. By including those states in a device model, the outcome of previously published temperature-dependent open circuit voltage measurements on Cu$_2$ZnSnS$_4$ solar cells can be reproduced quantitatively without necessarily assuming a cliff-like conduction band offset with the CdS buffer layer. Our first-principles calculations indicate that Zn-based alternative buffer layers are advantageous due to the ability of Zn to passivate those surface states. Focusing future research on Zn-based buffers is expected to significantly improve the open circuit voltage and efficiency of pure-sulfide Cu$_2$ZnSnS$_4$ solar cells.

cond-mat.mes-hall↗

arXiv2017

Electron-phonon scattering from Green's function transport combined with Molecular Dynamics: Applications to mobility predictions

We present a conceptually simple method for treating electron-phonon scattering and phonon limited mobilities. By combining Green's function based transport calculations and molecular dynamics (MD), we obtain a temperature dependent transmission from which we evaluate the mobility. We validate our approach by comparing to mobilities and conductivies obtained by the Boltzmann transport equation (BTE) for different bulk and one-dimensional systems. For bulk silicon and gold we successfully compare against experimental values. We discuss limitations and advantages of each of the computational approaches.

cond-mat.mes-hall↗

arXiv2016

Inelastic vibrational signals in electron transport across graphene nanoconstrictions

We present calculations of the inelastic vibrational signals in the electrical current through a graphene nanoconstriction. We find that the inelastic signals are only present when the Fermi-level position is tuned to electron transmission resonances, thus, providing a fingerprint which can link an electron transmission resonance to originate from the nanoconstriction. The calculations are based on a novel first-principles method which includes the phonon broadening due to coupling with phonons in the electrodes. We find that the signals are modified due to the strong coupling to the electrodes, however, still remain as robust fingerprints of the vibrations in the nanoconstriction. We investigate the effect of including the full self-consistent potential drop due to finite bias and gate doping on the calculations and find this to be of minor importance.

cond-mat.mes-hall↗

arXiv2015

First-principles method for electron-phonon coupling and electron mobility: Applications to 2D materials

We present density functional theory calculations of the phonon-limited mobility in n-type monolayer graphene, silicene and MoS$_2$. The material properties, including the electron-phonon interaction, are calculated from first-principles. We provide a detailed description of the normalized full-band relaxation time approximation for the linearized Boltzmann transport equation (BTE) that includes inelastic scattering processes. The bulk electron-phonon coupling is evaluated by a supercell method. The method employed is fully numerical and does therefore not require a semi-analytic treatment of part of the problem and, importantly, it keeps the anisotropy information stored in the coupling as well as the band structure. In addition, we perform calculations of the low-field mobility and its dependence on carrier density and temperature to obtain a better understanding of transport in graphene, silicene and monolayer MoS$_2$. Unlike graphene, the carriers in silicene show strong interaction with the out-of-plane modes. We find that graphene has more than an order of magnitude higher mobility compared to silicene. For MoS$_2$, we obtain several orders of magnitude lower mobilities in agreement with other recent theoretical results. The simulations illustrate the predictive capabilities of the newly implemented BTE solver applied in simulation tools based on first-principles and localized basis sets.

cond-mat.mes-hall↗

arXiv2015

Nonequilibrium spin texture within a thin layer below the surface of current-carrying topological insulator Bi$_2$Se$_3$: A first-principles quantum transport study

We predict that unpolarized charge current injected into a ballistic thin film of prototypical topological insulator (TI) Bi$_2$Se$_3$ will generate a {\it noncollinear spin texture} $\mathbf{S}(\mathbf{r})$ on its surface. Furthermore, the nonequilibrium spin texture will extend into $\simeq 2$ nm thick layer below the TI surfaces due to penetration of evanescent wavefunctions from the metallic surfaces into the bulk of TI. Averaging $\mathbf{S}(\mathbf{r})$ over few Å along the longitudinal direction defined by the current flow reveals large component pointing in the transverse direction. In addition, we find an order of magnitude smaller out-of-plane component when the direction of injected current with respect to Bi and Se atoms probes the largest hexagonal warping of the Dirac-cone dispersion on TI surface. Our analysis is based on an extension of the nonequilibrium Green functions combined with density functional theory (NEGF+DFT) to situations involving noncollinear spins and spin-orbit coupling. We also demonstrate how DFT calculations with properly optimized local orbital basis set can precisely match putatively more accurate calculations with plane-wave basis set for the supercell of Bi$_2$Se$_3$.

cond-mat.mes-hall↗

arXiv2013

Electron transport across a metal/MoS$_2$ interface: dependence on contact area and binding distance

We investigate the nature of electron transport through monolayer molybdenum dichalcogenides (MoX$_2$, X=S, Se) suspended between Au and Ti metallic contacts. The monolayer is placed ontop of the close-packed surfaces of the metal electrodes and we focus on the role of the metal-MoX$_2$ binding distance and the contact area. Based on \emph{ab initio} transport calculations we identify two different scattering mechanisms which depend differently on the metal-MoX$_2$ binding distance: (i) An interface resistance between the metal and the supported part of MoX$_2$ which decreases with decreasing binding distance and increasing contact area. (ii) An edge resistance across the 1D interface between metal-supported and free-standing MoX$_2$ which increases with decreasing binding distance and is independent on contact area. The origin of the edge resistance is a metal-induced potential shift within the MoX$_2$ layer. The optimal metal thus depends on the junction geometry. In the case of MoS$_2$, we find that for short contacts, L$<$6 nm, Ti electrodes (with short binding distance) gives the lowest resistance, while for longer contacts, Au (large binding distance) is a better electrode metal.

cond-mat.mes-hall↗

arXiv2013

Carbon nanotubes as heat dissipaters in microelectronics

We review our recent modelling work of carbon nanotubes as potential candidates for heat dissipation in microelectronics cooling. In the first part, we analyze the impact of nanotube defects on their thermal transport properties. In the second part, we investigate the loss of thermal properties of nanotubes in presence of an interface with various substances, including air and water. Comparison with previous works is established whenever is possible.

cond-mat.mes-hall↗

arXiv2013

Phonon Interference Effects in Molecular Junctions

We study coherent phonon transport through organic, π-conjugated molecules. Using first principles calculations and Green's function methods, we find that the phonon transmission function in cross-conjugated molecules, like meta-connected benzene, exhibits destructive quantum interference features very analogous to those observed theoretically and experimentally for electron transport in similar molecules. The destructive interference features observed in four different cross-conjugated molecules significantly reduce the thermal conductance with respect to linear conjugated analogues. Such control of the thermal conductance by chemical modifications could be important for thermoelectric applications of molecular junctions.

cond-mat.mes-hall↗

arXiv2013

Quantitatively Accurate Calculations of Conductance and Thermopower of Molecular Junctions

Thermopower measurements of molecular junctions have recently gained interest as a characterization technique that supplements the more traditional conductance measurements. Here we investigate the electronic conductance and thermopower of benzenediamine (BDA) and benzenedicarbonitrile (BDCN) connected to gold electrodes using first-principles calculations. We find excellent agreement with experiments for both molecules when exchange-correlation effects are described by the many-body GW approximation. In contrast, results from standard density functional theory (DFT) deviate from experiments by up to two orders of magnitude. The failure of DFT is particularly pronounced for the n-type BDCN junction due to the severe underestimation of the lowest unoccupied molecular orbital (LUMO). The quality of the DFT results can be improved by correcting the molecular energy levels for self-interaction errors and image charge effects. Finally, we show that the conductance and thermopower of the considered junctions are relatively insensitive to the metal-molecule bonding geometry. Our results demonstrate that electronic and thermoelectric properties of molecular junctions can be predicted from first-principles calculations when exchange-correlation effects are taken properly into account.

cond-mat.mes-hall↗

arXiv2012

Graphene antidot lattice waveguides

We introduce graphene antidot lattice waveguides: nanostructured graphene where a region of pristine graphene is sandwiched between regions of graphene antidot lattices. The band gap in the surrounding antidot lattices enable localized states to emerge in the central waveguide region. We model the waveguides via a position-dependent mass term in the Dirac approximation of graphene, and arrive at analytical results for the dispersion relation and spinor eigenstates of the localized waveguide modes. To include atomistic details we also use a tight-binding model, which is in excellent agreement with the analytical results. The waveguides resemble graphene nanoribbons, but without the particular properties of ribbons that emerge due to the details of the edge. We show that electrons can be guided through kinks without additional resistance and that transport through the waveguides is robust against structural disorder.

cond-mat.mes-hall↗

arXiv2012

Thermoelectric properties of disordered graphene antidot devices

We calculate the electronic and thermal transport properties of devices based on finite graphene antidot lattices (GALs) connected to perfect graphene leads. We use an atomistic approach based on the $π$-tight-binding model, the Brenner potential, and employing recursive Green's functions. We consider the effect of random disorder on the electronic and thermal transport properties, and examine the potential gain of thermoelectric merit by tailoring of the disorder. We propose several routes to optimize the transport properties of the GAL systems. Finally, we illustrate how quantum thermal transport can be addressed by molecular dynamics simulations, and compare to the Green's function results for the GAL systems in the ballistic limit.

cond-mat.mes-hall↗

arXiv2011

First-principles quantum transport modeling of thermoelectricity in single-molecule nanojunctions with graphene nanoribbon electrodes

We overview nonequilibrium Green function combined with density functional theory (NEGF-DFT) modeling of independent electron and phonon transport in nanojunctions with applications focused on a new class of thermoelectric devices where a single molecule is attached to two metallic zigzag graphene nanoribbons (ZGNRs) via highly transparent contacts. Such contacts make possible injection of evanescent wavefunctions from ZGNRs, so that their overlap within the molecular region generates a peak in the electronic transmission. Additionally, the spatial symmetry properties of the transverse propagating states in the ZGNR electrodes suppress hole-like contributions to the thermopower. Thus optimized thermopower, together with diminished phonon conductance through a ZGNR/molecule/ZGNR inhomogeneous structure, yields the thermoelectric figure of merit ZT~0.5 at room temperature and 0.5<ZT<2.5 below liquid nitrogen temperature. The reliance on evanescent mode transport and symmetry of propagating states in the electrodes makes the electronic-transport-determined power factor in this class of devices largely insensitive to the type of sufficiently short conjugated organic molecule, which we demonstrate by showing that both 18-annulene and C10 molecule sandwiched by the two ZGNR electrodes yield similar thermopower. Thus, one can search for molecules that will further reduce the phonon thermal conductance (in the denominator of ZT) while keeping the electronic power factor (in the nominator of ZT) optimized. We also show how often employed Brenner empirical interatomic potential for hydrocarbon systems fails to describe phonon transport in our single-molecule nanojunctions when contrasted with first-principles results obtained via NEGF-DFT methodology.

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