arXiv subjects

Catherine Stampfl

Publications and source records attributed to Catherine Stampfl.

At least 19 recordsLinked to original sources

arXiv2026

Symmetry Guided Band-Gap Opening via Periodic Topological Defects in Graphene

Graphene lacks an intrinsic band-gap, which limits its use in electronic applications. Here we demonstrate that periodic arrays of topological defects can open and control a band-gap in a predictable manner governed by defect spacing and lattice symmetry. Using first-principles density functional theory calculations supported by tight-binding models, we investigate graphene superlattices containing Stone-Wales and flower-like defects over a range of $N \times N$ periodicities, where $N$ determines the defect separation. We show that band-gap opening occurs only when translation symmetry is reduced in a specific way: for supercells with $N$ a multiple of three, Brillouin-zone folding brings the Dirac cones at $K$ and $K'$ to the same momentum in the reduced Brillouin zone. In particular, flower-like defect superlattices produce larger and tunable band-gaps, whose magnitude decreases systematically with increasing defect separation and approaches zero in the dilute-defect limit. These results establish a predictive framework for band-gap engineering in defect-patterned graphene and clarify the microscopic mechanism underlying gap formation in periodically reconstructed lattices.

cond-mat.mtrl-sci↗

arXiv2026

Long-Lived Interlayer Excitons and Type-II Band Alignment in Janus MoTe2/CrSBr van der Waals Heterostructures

Identifying two-dimensional heterostructures with exceptional electronic and optical properties remains an active area of research in advanced optoelectronics. Here, we present a comprehensive first-principles investigation of the electronic, optical, and excitonic properties of a MoTe2/CrSBr van der Waals heterostructure using density functional theory combined with fully relativistic GW and Bethe-Salpeter equation calculations. The close lattice matching between the two monolayers enables the formation of stable heterobilayers with two inequivalent interfaces (Te-S and Te-Br) arising from the Janus nature of CrSBr. Both interfaces are dynamically and thermally stable and exhibit type-II band alignment with a direct quasiparticle gap, promoting efficient spatial separation of electrons and holes. The heterostructure hosts interlayer excitons with lifetimes 18-45 ps significantly longer than those of the intralayer excitons in the isolated MoTe2, 3.6 ps, and CrSBr, 8.1 ps, monolayers. Moreover, the optical gap, exciton binding energy, and exciton lifetime of the heterostructure are strongly modulated by the built-in electric field associated with the Janus layer. These results establish the MoTe2/CrSBr heterostructure as a versatile platform for engineering long-lived interlayer excitons and highlight its potential for next-generation optoelectronic and light-harvesting applications.

cond-mat.mtrl-sci↗

arXiv2025

Kolmogorov-Arnold Networks in Thermoelectric Materials Design

The discovery of high-performance thermoelectric materials requires models that are both accurate and interpretable. Traditional machine learning approaches, while effective at property prediction, often act as black boxes and provide limited physical insight. In this work, we introduce Kolmogorov--Arnold Networks (KANs) for the prediction of thermoelectric properties, focusing on the Seebeck coefficient and band gap. Compared to multilayer perceptrons (MLPs), KANs achieve comparable predictive accuracy while offering explicit symbolic representations of structure--property relationships. This dual capability enables both reliable predictions and the extraction of physically meaningful functional forms. Benchmarking against literature models further highlights the robustness and generalisability of the approach. Our findings demonstrate that KANs provide a powerful framework for reverse engineering materials with targeted thermoelectric properties, bridging the gap between predictive performance and scientific interpretability.

cond-mat.mtrl-sci↗

arXiv2019

Prediction of fundamental properties of Be-B-Ta based novel ternary compounds from first-principles calculations

Be-B/B-Ta based compounds are very attractive to researchers because of their high density and ultra-hardness. But ternary Be-B-Ta compounds are neither synthesized nor predicted. In this paper, variable composition evolutionary crystal structure prediction calculations based on first-principles method have been performed to find the stable crystal structure containing Be-B-Ta at ambient condition. The predicted five compounds BeB$_2$Ta, BeB$_3$Ta$_2$ (high-pressure phase), BeBTa, BeBTa$_2$, and Be$_2$B$_2$Ta have been found to be highly dense and very hard materials. All these compounds are metallic and spin-orbit coupling (SOC) effect is significant in them. Only two of them (BeB$_2$Ta and Be$_2$B$_2$Ta) have been found to be superconductors within Migdal-Eliashberg theory. The calculated critical temperature including SOC effect is 8 and 9 K for BeB$_2$Ta and Be$_2$B$_2$Ta, respectively. Because of their energetic and dynamic stability, these compounds might be favorable to synthesize in the laboratory.

cond-mat.supr-con↗

arXiv2018

First-principles prediction of phonon-mediated superconductivity in XBC (X= Mg, Ca, Sr, Ba)

From first-principles calculations, we predict four new intercalated hexagonal $X$BC ($X$=Mg, Ca, Sr, Ba) compounds to be dynamically stable and phonon-mediated superconductors. These compounds form a LiBC like structure but are metallic. The calculated superconducting critical temperature, $T{_c}$, of MgBC is 51 K. The strong attractive interaction between $σ$-bonding electrons and the B${_{1g}}$ phonon mode gives rise to a larger electron-phonon coupling constant (1.135) and hence high $T_c$; notably, higher than that of MgB$_2$. The other compounds have a low superconducting critical temperature (4-17 K) due to the interaction between $σ$-bonding electrons and low energy phonons (E${_{2u}}$ modes). Due to their energetic and dynamic stability, we envisage that these compounds can be synthesized experimentally.

cond-mat.supr-con↗

arXiv2018

van der Waals forces control the internal chemical structure of monolayers within ABP2X6 lamellar materials

Following the recent demonstration that van der Waals forces control the ferroelectric ordering of layers within nanoflakes and bulk samples of CuBiP2Se6 and CuInP2S6, it is demonstrated that they also control the internal geometrical structure of isolated monolayers of these materials. This internal structure involves large displacements of the copper atoms, either normal to the layer plane or else within the plane, that change its ligation environment. In both cases, the van der Waals dispersion force out-competes traditional bonding effects to control structure. However, we find that the aspects of the dispersion force giving rise to each effect are uncorrelated: long range effects control inter-layer ferroelectric ordering whereas short-range effects control internal layer structure. These conclusions are drawn considering predicted properties of monolayers, bilayers, and bulk materials obtained using 14 density-functional-theory based methods. While the different methods used often predict starkly different quantitative results, they concur as to the basic nature of ABP2X6 materials. Of the methods used, only the PBE-D3 and optPBEvdW methods were found to predict a wide range of observed properties without serious disparity. Finding optimal computational methods remains a significant challenge for which the unusual multi-scale nature of the van der Waals interactions in ABP2X6 materials provides demanding criteria.

cond-mat.mtrl-sci↗

arXiv2018

Superconductivity in intercalated buckled two-dimensional materials: KGe$_2$

Germanene has emerged as a novel two-dimensional material with various interesting properties and applications. Here we report the possibility of superconductivity in a stable potassium intercalated germanene compound, KGe$_2$, with a transition temperature $T_c \sim 11$ K, and an electron-phonon coupling of 1.9. Applying a 5\% tensile strain, which reduces the buckling height by 4.5\%, leads to the reduction of the electron-phonon coupling by 11\% and a slight increase in $T_c \sim 12$ K. That is, strong electron-phonon coupling results from the buckled structure of the germanene layers. Despite being an intercalated van der Waals material similar to intercalated graphite superconductors, it does not possess an occupied interlayer state.

cond-mat.mtrl-sci↗

arXiv2017

First principles investigation of quantum emission from hBN defects

Hexagonal boron nitride (hBN) has recently emerged as a fascinating platform for room-temperature quantum photonics due to the discovery of robust visible light single-photon emitters. In order to utilize these emitters, it is necessary to have a clear understanding of their atomic structure and the associated excitation processes that give rise to this single photon emission. Here we perform density-functional theory (DFT) and constrained DFT calculations for a range of hBN point defects in order to identify potential emission candidates. By applying a number of criteria on the electronic structure of the ground state and the atomic structure of the excited states of the considered defects, and then calculating the Huang-Rhys (HR) factor, we find that the CBVN defect, in which a carbon atom substitutes a boron atom and the opposite nitrogen atom is removed, is a potential emission source with a HR factor of 1.66, in good agreement with the experimental HR factor. We calculate the photoluminescence (PL) line shape for this defect and find that it reproduces a number of key features in the the experimental PL lineshape.

cond-mat.mtrl-sci↗

arXiv2017

Near-perfect spin filtering and negative differential resistance in an Fe(II)S complex

Density functional theory and nonequilibrium Green's function calculations have been used to explore spin-resolved transport through the high-spin state of an iron(II)sulfur single molecular magnet. Our results show that this molecule exhibits near-perfect spin filtering, where the spin-filtering efficiency is above 99%, as well as significant negative differential resistance centered at a low bias voltage. The rise in the spin-up conductivity up to the bias voltage of 0.4 V is dominated by a conductive lowest unoccupied molecular orbital, and this is accompanied by a slight increase in the magnetic moment of the Fe atom. The subsequent drop in the spin-up conductivity is because the conductive channel moves to the highest occupied molecular orbital which has a lower conductance contribution. This is accompanied by a drop in the magnetic moment of the Fe atom. These two exceptional properties, and the fact that the onset of negative differential resistance occurs at low bias voltage, suggests the potential of the molecule in nanoelectronic and nanospintronic applications.

cond-mat.mtrl-sci↗

arXiv2016

Multiferroic crossover in perovskite oxides

Recently, the perovskite BiCoO$_3$ has been shown experimentally to be isostructural with PbTiO$_3$, while simultaneously the $d^6$ Co$^{3+}$ ion has a high spin ground state with $C$-type antiferromagnetic ordering. Using hybrid density functional calculations, we investigate the atomic, electronic and magnetic structure of BiCoO$_3$ to elucidate the origin of the multiferroic state. To begin with, we perform a qualitative trend sudy of the role of $d$ electrons in affecting the tendency for perovskite materials to exhibit a ferroelectric distortion; this work initially explores a qualitative trend study in artificial cubic and tetragonal LaBO$_3$ perovskites. We choose La as the A-cation so as to remove the effects of Bi $6s$ hybridization. Through first-principles calculations of the LaBO$_3$ series, where B is a $d^0 - d^8$ cation from the $3d$-block, the trend study reveals that increasing the $d$ orbital occupation initially removes the tendency for a polar distortion, as expected. However, for high spin $d^5-d^7$ and $d^8$ cations a strong ferroelectric instability is recovered. We explained this effect in terms of the pseudo Jahn-Teller theory for ferroelectricity. It is shown that, in some cases, unpaired electron spins actually drive ferroelectricity, rather than inhibit it, which represents a shift in the understanding of how ferroelectricity and magnetism interact in perovskite oxides. It follows, that for the case of BiCoO$_3$, the Co$^{3+}$ ion plays a major role in the ferroelectric lattice instability. Importantly, the ferroelectric polarization is greatly enhanced when the Co$^{3+}$ ion is in the high spin state, when compared to the nonmagnetic, low spin state, and a large coupling of the electrical and magnetic polarization is present. Importantly, it is demonstrated that the ground spin state is switched by reducing the internal ferroelectric polarization.

cond-mat.mtrl-sci↗

arXiv2015

Unraveling the origins of conduction band valley degeneracies in Mg2Si-1xSnx thermoelectrics

To better understand and enhance the thermoelectric efficiency of a new class of Mg-based thermoelectrics, using hybrid density-functional theory, we study the microscopic origins of valley degeneracies in the conduction band of the solid solution Mg2Si1-xSnx and its constituent components - namely, Mg2Si and Mg2Sn. In the solid solution of Mg2Si1-xSnx, the Mg sublattice and Si/Sn sublattice are expected to undergo either tensile or compressive strain. Interestingly, both tensile strain of Mg2Si and compressive strain of Mg2Sn enhance the conduction band valley degeneracy or band convergence, which has been strongly speculated as the electronic origin of the enhanced Seebeck coefficient in the Mg2Si1-xSnx system. We also consider finite-temperature electronic band structures of these systems to account for high temperature effects. Our results clearly highlight and demonstrate the role of sublattice strain in the band valley degeneracy observed in Mg2Si1-xSnx .

cond-mat.mtrl-sci↗

arXiv2014

Re-visiting the O/Cu(111) system -- When metastable surface oxides could become an issue!

Surface oxidation processes are crucial for the functionality of Cu-based catalytic systems used for methanol synthesis, partial oxidation of methanol or the water-gas shift reaction. We assess the stability and population of the "8"-structure, a $|\begin{smallmatrix} 3&2\\ -1&2 \end{smallmatrix}|$ oxide phase, on the Cu(111) surface. This structure has been observed in x-ray photoelectron spectroscopy and low-energy electron diffraction experiments as a Cu(111) surface reconstruction that can be induced by a hyperthermal oxygen molecular beam. Using density-functional theory calculations in combination with \textit{ab initio} atomistic thermodynamics and Boltzmann statistical mechanics, we find that the proposed oxide superstructure is indeed metastable and that the population of the "8"-structure is competitive with the known "29" and "44" oxide film structures on Cu(111). We show that the configuration of O and Cu atoms in the first and second layers of the "8"-structure closely resembles the arrangement of atoms in the first two layers of Cu$_2$O(110), where the atoms in the "8"-structure are more constricted. Cu$_2$O(110) has been suggested in the literature as the most active low index facet for reactions such as water splitting under light illumination. If the "8"-structure were to form during a catalytic process, it is therefore likely to be one of the reactive phases.

cond-mat.mtrl-sci↗

arXiv2014

Stacking-dependent energetics and electronic structure of ultrathin polymorphic V$_2$VI$_3$ topological insulator nanofilms

Topological insulators represent a paradigm shift in surface physics. The most extensively studied Bi$_2$Se$_3$-type topological insulators exhibit layered structures, wherein neighboring layers are weakly bonded by van der Waals interactions. Using first principles density-functional theory calculations, we investigate the impact of the stacking sequence on the energetics and band structure properties of three polymorphs of Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$. Considering their ultrathin films up to 6 nm as a function of its layer thickness, the overall dispersion of the band structure is found to be insensitive to the stacking sequence, while the band gap is highly sensitive, which may also affect the critical thickness for the onset of the topologically nontrivial phase. Our calculations are consistent with both experimental and theoretical results, where available. We further investigate tribological layer slippage, where we find a relatively low energy barrier between two of the considered structures. Both the stacking-dependent band gap and low slippage energy barriers, suggest that polymorphic stacking modification may offer an alternative route for controlling the properties of this new state of matter.

cond-mat.mtrl-sci↗

arXiv2013

CHx adsorption (x=1-4) and thermodynamic stability on CeO2(111) surface: A first-principles investigation

We present an ab initio investigation of the interaction between methane, its dehydrogenated forms and the cerium oxide surface. In particular, the stoichiometric CeO2(111) surface and the one with oxygen vacancies are considered. We study the geometries, energetics and electronic structures of various configurations of these molecules adsorbed on the surface in vacuum, and we extend the analysis to realistic environmental conditions. A phase diagram of the adsorbate-surface system is constructed and relevant transition phases are analyzed in detail, showing the conditions where partial oxidation of methane can occur.

cond-mat.mtrl-sci↗

arXiv2008

Magneto-resistivity model and ionization energy approximation for ferromagnets

The evolution of resistivity versus temperature ($ρ(T)$) curve for different doping elements, and in the presence of various defects and clustering are explained for both diluted magnetic semiconductors (DMS) and manganites. Here, we provide unambiguous evidence that the concept of ionization energy ($E_I$), which is explicitly associated with the atomic energy levels, can be related quantitatively to transport measurements. The proposed ionization energy model is used to understand how the valence states of ions affect the evolution of $ρ(T)$ curves for different doping elements. We also explain how the $ρ(T)$ curves evolve in the presence of, and in the absence of defects and clustering. The model also complements the results obtained from first-principles calculations.

cond-mat.stat-mech↗

arXiv2008

Ag-Cu alloy surfaces in an oxidizing environment: a first-principles study

Recent experiments on model catalysts have shown that Ag-Cu alloys have improved selectivity with respect to pure silver for ethylene epoxidation. In this paper we review our first-principles investigations on the (111) surface of this alloy and present new findings on other low index surfaces. We find that, for every surface orientation, the presence of oxygen leads to copper segregation to the surface. Considering the alloy to be in equilibrium with an oxygen atmosphere and accounting for the effect of temperature and pressure, we compute the surface free energy and study the stability of several surface structures. Investigating the dependence of the surface free energy on the surface composition, we construct the phase diagram of the alloy for every surface orientation. Around the temperature, pressure and composition of interest for practical applications, we find that a limited number of structures can be present, including a thin layer of copper oxide on top of the silver surface and copper-free structures. Different surface orientations show a very similar behavior and in particular a single layer with CuO stoichiometry, significantly distorted when compared to a layer of bulk CuO, has a wide range of stability for all orientations. Our results are consistent with, and help explain, recent experimental measurements.

cond-mat.mtrl-sci↗

arXiv2007

First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment

In this paper we investigate by means of first-principles density functional theory calculations the (111) surface of the Ag-Cu alloy under varying conditions of pressure of the surrounding oxygen atmosphere and temperature. This alloy has been recently proposed as a catalyst with improved selectivity for ethylene epoxidation with respect to pure silver, the catalyst commonly used in industrial applications. Here we show that the presence of oxygen leads to copper segregation to the surface. Considering the surface free energy as a function of the surface composition, we construct the convex hull to investigate the stability of various surface structures. By including the dependence of the free surface energy on the oxygen chemical potential, we are able compute the phase diagram of the alloy as a function of temperature, pressure and surface composition. We find that, at temperature and pressure typically used in ethylene epoxidation, a number of structures can be present on the surface of the alloy, including clean Ag(111), thin layers of copper oxide and thick oxide-like structures. These results are consistent with, and help explain, recent experimental results.

cond-mat.mtrl-sci↗

arXiv2004

Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functions

Previous and present "academic" research aiming at atomic scale understanding is mainly concerned with the study of individual molecular processes possibly underlying materials science applications. Appealing properties of an individual process are then frequently discussed in terms of their direct importance for the envisioned material function, or reciprocally, the function of materials is somehow believed to be understandable by essentially one prominent elementary process only. What is often overlooked in this approach is that in macroscopic systems of technological relevance typically a large number of distinct atomic scale processes take place. Which of them are decisive for observable system properties and functions is then not only determined by the detailed individual properties of each process alone, but in many, if not most cases also the interplay of all processes, i.e. how they act together, plays a crucial role. For a "predictive materials science modeling with microscopic understanding", a description that treats the statistical interplay of a large number of microscopically well-described elementary processes must therefore be applied. Modern electronic structure theory methods such as DFT have become a standard tool for the accurate description of individual molecular processes. Here, we discuss the present status of emerging methodologies which attempt to achieve a (hopefully seamless) match of DFT with concepts from statistical mechanics or thermodynamics, in order to also address the interplay of the various molecular processes. The new quality of, and the novel insights that can be gained by, such techniques is illustrated by how they allow the description of crystal surfaces in contact with realistic gas-phase environments.

cond-mat.mtrl-sci↗