arXiv subjects

Thomas Frauenheim

Publications and source records attributed to Thomas Frauenheim.

At least 55 records · Page 3Linked to original sources

arXiv2018

Electronic and Optical Properties of Functionalized GaN (10-10) Surfaces using Hybrid-Density Functionals

Adsorption of small ligands on semiconductor surfaces is a possible route to modify these surfaces so that they can be used in biosensing and optoelectronic devices. In this work we perform density-functional theory calculations of electronic and optical properties of small ligands on GaN-(10$\bar1$0) surfaces. From the investigated anchor groups we show that thiol groups introduce states into the GaN band gap. However, these state are not optically active, at least for these perfect surfaces. This means that more realistic surfaces need to be considered to suggest how surface modification can enhance the optical properties of GaN non-polar surfaces.

cond-mat.mtrl-sci↗

arXiv2018

Hybrid density-functional theory calculations of electronic and optical properties of mercaptocarboxylic acids on ZnO $(10{\overline 1}0)$ surfaces

In this work we investigate the electronic properties of mercaptocarboxylic acids with several carbon chain lengths adsorbed on ZnO-(10-10) surfaces via density functional theory calculations using semi-local and hybrid exchange-correlation functionals. Amongst the investigated structures, we identify the monodentate adsorption mode to be stable. Moreover, this mode introduces optically active states in the ZnO gap, is further confirmed by the calculation of the dielectric function at PBE0 and TD-PBE0 levels. One interesting finding is that adsorption mode and the dielectric properties of the hybrid system are both rather insensitive to the chain length, since the acceptor molecular state is very localized on the sulphur atom. This indicates that even small molecules can be used to stabilize ZnO surface and to enhance its functionality for opto-electronic applications.

cond-mat.mtrl-sci↗

arXiv2018

Vibronic Dephasing Model for Coherent-to-Incoherent Crossover in DNA

In this work we investigate the interplay between coherent and incoherent charge transport in cytosine-guanine (GC) rich DNA molecules. Our objective is to introduce physically grounded approach to dephasing in large molecules and to understand the length dependent charge transport characteristics and especially the crossover from coherent tunneling to incoherent hopping regime at different temperatures. Therefore, we apply the vibronic dephasing model and compare the results to the Büttiker probe model which is commonly used to describe decoherence effects in charge transport. Using the full ladder model and simplified 1D model of DNA, we consider molecular junctions with alternating and stacked GC sequences and compare our results to recent experimental measurements.

cond-mat.mes-hall↗

arXiv2018

Non-Markovian quantum processes: complete framework and efficient characterisation

Currently, there is no systematic way to describe a quantum process with memory solely in terms of experimentally accessible quantities. However, recent technological advances mean we have control over systems at scales where memory effects are non-negligible. The lack of such an operational description has hindered advances in understanding physical, chemical and biological processes, where often unjustified theoretical assumptions are made to render a dynamical description tractable. This has led to theories plagued with unphysical results and no consensus on what a quantum Markov (memoryless) process is. Here, we develop a universal framework to characterise arbitrary non-Markovian quantum processes. We show how a multi-time non-Markovian process can be reconstructed experimentally, and that it has a natural representation as a many body quantum state, where temporal correlations are mapped to spatial ones. Moreover, this state is expected to have an efficient matrix product operator form in many cases. Our framework constitutes a systematic tool for the effective description of memory-bearing open-system evolutions.

quant-ph↗

arXiv2018

Operational Markov condition for quantum processes

We derive a necessary and sufficient condition for a quantum process to be Markovian which coincides with the classical one in the relevant limit. Our condition unifies all previously known definitions for quantum Markov processes by accounting for all potentially detectable memory effects. We then derive a family of measures of non-Markovianity with clear operational interpretations, such as the size of the memory required to simulate a process, or the experimental falsifiability of a Markovian hypothesis.

quant-ph↗

arXiv2017

Optically and electrically controllable adatom spin-orbital dynamics in transition metal dichalcogenides

We analyze the interplay of spin-valley coupling, orbital physics and magnetic anisotropy taking place at single magnetic atoms adsorbed on semiconducting transition-metal dichalcogenides, MX$_2$ (M = Mo, W; X = S, Se). Orbital selection rules turn out to govern the kinetic exchange coupling between the adatom and charge carriers in the MX$_2$ and lead to highly orbitally dependent spin-flip scattering rates, as we illustrate for the example of transition metal adatoms with $d^9$ configuration. Our ab initio calculations suggest that $d^9$ configurations are realizable by single Co, Rh, or Ir adatoms on MoS$_2$, which additionally exhibit a sizable magnetic anisotropy. We find that the interaction of the adatom with carriers in the MX$_2$ allows to tune its behavior from a quantum regime with full Kondo screening to a regime of "Ising spintronics" where its spin-orbital moment acts as classical bit, which can be erased and written electronically and optically.

cond-mat.mes-hall↗

arXiv2017

Dephasing-Controlled Particle Transport Devices

We study the role of dephasing in transport through different structures. We show that interference effects invalidate Kirchhoff's circuit laws in quantum devices. We present circuits where the particle transport and the direction of rectification can be controlled through the dephasing strength. This suggests the possibility of constructing molecular devices with new functionalities which use dephasing as a control parameter.

quant-ph↗

arXiv2016

Rectangular Tantalum Carbide Halides TaCX (X = Cl, Br, I) monolayer: Novel Large-Gap Quantum Spin Hall Insulator

Quantum spin Hall (QSH) insulators possess edge states that are topologically protected from backscattering. However, known QSH materials (e.g. HgTe/CdTe and InAs/GaSb quantum wells) exhibit very small energy gap and only work at low temperature, hindering their applications for room temperature devices. Based on the first-principles calculations, we predict a novel family of QSH insulators in monolayer tantalum carbide halide TaCX (X = Cl, Br, and I) with unique rectangular lattice and large direct energy gaps larger than 0.2 eV, accurately, 0.23$-$0.36 eV. The mechanism for 2D QSH effect in this system originates from a intrinsic d$-$d band inversion, different from conventional QSH systems with band inversion between s$-$p or p$-$p orbitals. Further, stain and intrinsic electric field can be used to tune the electronic structure and enhance the energy gap. TaCX nanoribbon, which has single-Dirac-cone edge states crossing the bulk band gap, exhibits a linear dispersion with a high Fermi velocity comparable to that of graphene. These 2D materials with considerable nontrivial gaps promise great application potential in the new generation of dissipationless electronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2016

New Quantum Spin Hall Insulator in Two-dimensional MoS$_2$ with Periodically Distributed Pores

MoS$_2$, one of transition metal dichalcogenides (TMDs), has caused a lot of attentions for its excellent semiconductor characteristics and potential applications. Here, based on the density functional theory methods, we predict a novel 2D quantum spin hall (QSH) insulator in the porous allotrope of monolayer MoS$_2$ (g-MoS$_2$), consisting of MoS$_2$ square and hexagon. The g-MoS$_2$ has a nontrivial gap as large as 109 meV, comparable with previous reported 1T'-MoS$_2$ (80 meV), so-MoS$_2$ (25 meV). We demonstrate that the origin of 2D QSH effect in g-MoS$_2$ originates from the pure d-d band interaction, different from conventional band inversion between s$-$p, p$-$p or d$-$p orbitals. Such new polymorph greatly enriches the TMDs family and its stabilities are confirmed by phonon spectrum analysis. In particular, porous structure also endows it potential application in efficient gas separation and energy storage.

cond-mat.mtrl-sci↗

arXiv2015

The Driven Liouville von Neumann Equation in Lindblad Form

The Driven Liouville von Neumann approach [J. Chem. Theory Comput. 10, 2927-2941 (2014)] is a computationally efficient simulation method for modeling electron dynamics in molecular electronics junctions. Previous numerical simulations have shown that the method can reproduce the exact single-particle dynamics while avoiding density matrix positivity violation found in earlier implementations. In this study we prove that, in the limit of infinite lead models, the underlying equation of motion can be cast in Lindblad form. This provides a formal justification for the positivity and trace preservation obtained numerically.

physics.chem-ph↗

arXiv2015

New Family of Quantum Spin Hall Insulators in Two-dimensional Transition-Metal Halide with Large Nontrivial Band Gaps

Topological insulators (TIs) are promising for achieving dissipationless transport devices due to the robust gapless states inside the insulating bulk gap. However, currently realized 2D TIs, quantum spin Hall (QSH) insulators, suffer from ultra-high vacuum and extremely low temperature. Thus, seeking for desirable QSH insulators with high feasibility of experimental preparation and large nontrivial gap is of great importance for wide applications in spintronics. Based on the first-principles calculations, we predict a novel family of two-dimensional (2D) QSH insulators in transition-metal halide MX (M = Zr, Hf; X = Cl, Br, and I) monolayers with large nontrivial gaps of 0.12$-$0.4 eV, comparable with bismuth (111) bilayer (0.2 eV), stanene (0.3 eV) and larger than ZrTe$_5$ (0.1 eV) monolayers and graphene-based sandwiched heterstructures (30$-$70 meV). Their corresponding 3D bulk materials are weak topological insulators from stacking QSH layers, and some of bulk compounds have already been synthesized in experiment. The mechanism for 2D QSH effect in this system originates from a novel d$-$d band inversion, which is different from conventional band inversion between s$-$s orbitals, or p$-$p orbitals. The realization of pure layered MX monolayers may be prepared by exfoliation from their 3D bulk phases, thus holding great promise for nanoscale device applications and stimulating further efforts on transition metal-based QSH materials.

cond-mat.mtrl-sci↗

arXiv2015

Phosphorene and Doped Monolayers Interfaced TiO$_2$ with Type-II Band Alignments: Novel Excitonic Solar Cells

Phosphorene, a new elemental two dimensional (2D) material recently isolated by mechanical exfoliation, holds the feature of a direct band gap of around 2.0 eV, overcoming graphene's weaknesses (zero band gap) to realize the potential application in optoelectronic devices. Constructing van der Waals heterostructures is an efficient approach to modulate the band structure, to advance the charge separation efficiency, and thus to optimize the optoelectronic properties. Here, we theoretically investigated three type-II heterostructures based on perfect phosphorene and its doped monolayers interfaced with TiO$_2$(110) surface. Doping in phosphorene has a tunability on built-in potential, charge transfer, light absorbance, as well as electron dynamics, which helps to optimize the light absorption efficiency. Three excitonic solar cells (XSCs) based on the phosphorene$-$TiO$_2$ heterojunctions have been proposed, which exhibit high power conversion efficiencies dozens of times higher than conventional solar cells, comparable to MoS$_2$/WS$_2$ XSC. The nonadiabatic molecular dynamics within the time-dependent density functional theory framework shows ultrafast electron transfer time of 6.1$-$10.8 fs, and slow electron$-$hole recombination of 0.58$-$1.08 ps, yielding $>98\%$ quantum efficiency for charge separation, further guaranteeing the practical power conversion efficiencies in XSC.

cond-mat.mtrl-sci↗

arXiv2015

How the aggregation of oxygen vacancies in rutile based TiO$_{2-δ}$ phases causes memristive behavior

The results of a comprehensive and systematic ab-initio based ground-state search for the structural arrangement of oxygen vacancies in rutile phase TiO$_2$ provide new insights into their memristive properties. We find that O vacancies tend to form planar arrangements which relax into structures exhibiting metallic behavior. These meta-stable arrangements are structurally akin to, yet distinguishable from the Magnéli phase. They exhibit a more pronounced metallic nature, but are energetically less favorable. Our results confirm a clear structure-property relationship between segregated oxygen vacancy arrangement and metallic behavior in reduced oxides.

cond-mat.mtrl-sci↗

arXiv2015

Towards rational design of catalysts supported on a topological insulator substrate

Exotic and robust metallic surface states of topological insulators (TIs) have been expected to provide a promising platform for novel surface chemistry and catalysis. However, it is still an unprecedented field how TIs affect the activity of catalysts. In this work, we study the effects of topological surface states (TSSs) on the activity of transition metal clusters (Au, Ag, Cu, Pt, and Pd), which are supported on a TI Bi2Se3 substrate. It was found the adsorption energy of oxygen on the supported catalysts can be always enhanced due to the TSSs. However, it does not necessarily mean an increase of the activity in catalytic oxidation reaction. Rather, the enhanced adsorption behavior in the presence of TSSs exhibits dual effects, determined by the intrinsic reactivity of these catalysts with oxygen. For the Au case, the activity of catalytic oxidation can be improved because the intrinsic binding between Au and O is relatively weak. In contrast, a negative effect is found for the Pt and Pd clusters since the intrinsic binding of Pt and Pd with oxygen is too strong. We also found that the effect of TSSs on the activity of hydrogen evolution reaction (HER) is quite similar, i.e. the metals with original weak reactivity can gain a positive effect from TSSs. The present work can pave a way for more rational design and selection of catalysts when using TIs as substrates.

physics.comp-ph↗

arXiv2015

Controllable magnetic correlation between two impurities by spin-orbit coupling in graphene

Two magnetic impurities on the edge of a zigzag graphene nanoribbon strongly interact with each other via indirect coupling, which can be mediated by conducting carriers. By means of Quantum Monte Carlo (QMC) simulations, we find that the spin-orbit coupling $λ$ and the chemical potential $μ$ in system can be used to drive the transition of local-spin exchange from ferromagnetism to anti-ferromagnetism. Since the tunable ranges for $λ$ and $μ$ in graphene are experimentally reachable, we thus open the possibilities for its device application. The symmetry in spatial distribution is broken by the vertical and the transversal spin-spin correlations due to the effect of spin-orbit coupling, leading to the spatial anisotropy of spin exchange, which distinguish our findings from the case in normal Fermi liquid.

cond-mat.str-el↗

arXiv2015

Proximity Enhanced Quantum Spin Hall State in Graphene

Graphene is the first model system of two-dimensional topological insulator (TI), also known as quantum spin Hall (QSH) insulator. The QSH effect in graphene, however, has eluded direct experimental detection because of its extremely small energy gap due to the weak spin-orbit coupling. Here we predict by ab initio calculations a giant (three orders of magnitude) proximity induced enhancement of the TI energy gap in the graphene layer that is sandwiched between thin slabs of Sb2Te3 (or MoTe2). This gap (1.5 meV) is accessible by existing experimental techniques, and it can be further enhanced by tuning the interlayer distance via compression. We reveal by a tight-binding study that the QSH state in graphene is driven by the Kane-Mele interaction in competition with Kekulé deformation and symmetry breaking. The present work identifies a new family of graphene-based TIs with an observable and controllable bulk energy gap in the graphene layer, thus opening a new avenue for direct verification and exploration of the long-sought QSH effect in graphene.

cond-mat.mes-hall↗

arXiv2014

New Family of Robust 2D Topological Insulators in van der Waals Heterostructures

We predict a new family of robust two-dimensional (2D) topological insulators in van der Waals heterostructures comprising graphene and chalcogenides BiTeX (X=Cl, Br and I). The layered structures of both constituent materials produce a naturally smooth interface that is conducive to proximity induced new topological states. First principles calculations reveal intrinsic topologically nontrivial bulk energy gaps as large as 70-80 meV, which can be further enhanced up to 120 meV by compression. The strong spin-orbit coupling in BiTeX has a significant influence on the graphene Dirac states, resulting in the topologically nontrivial band structure, which is confirmed by calculated nontrivial Z2 index and an explicit demonstration of metallic edge states. Such heterostructures offer an unique Dirac transport system that combines the 2D Dirac states from graphene and 1D Dirac edge states from the topological insulator, and it offers new ideas for innovative device designs.

cond-mat.mtrl-sci↗

arXiv2014

Phosphorene as a superior gas sensor: Selective adsorption and distinct I-V response

Recent reports on the fabrication of phosphorene, i.e., mono- or few-layer black phosphorus, have raised exciting prospects of an outstanding two-dimensional (2D) material that exhibits excellent properties for nanodevice applications. Here we study by first-principles calculations the adsorption of CO, CO2, NH3, NO and NO2 gas molecules on a mono-layer phosphorene. Our results predict superior sensing performance of phosphorene that rivals or even surpasses other 2D materials such as graphene and MoS2. We determine the optimal adsorption positions of these molecules on the phosphorene and identify molecular doping, i.e., charge transfer between the molecules and phosphorene, as the driving mechanism for the high adsorption strength. We further calculated the current-voltage (I-V) relation using a non-equilibrium Greens function (NEGF) formalism. The transport features show large (one to two orders of magnitude) anisotropy along different (armchair or zigzag) directions, which is consistent with the anisotropic electronic band structure of phosphorene. Remarkably, the I-V relation exhibits distinct responses with a marked change of the I-V relation along either the armchair or the zigzag directions depending on the type of molecules. Such selectivity and sensitivity to adsorption makes phosphorene a superior gas sensor that promises wide-ranging applications.

cond-mat.mtrl-sci↗