Showing 1–24 of 24 results for author: Marom, N

Combining Quasiparticle Self-Consistent $GW$ and Machine-Learned DFT+$U$ to Assess Half-Metallicity in Co- and Ni-Based Heuslers

Authors: Zefeng Cai, Malcolm J. A. Jardine, Maituo Yu, Chenbo Min, Jiatian Wu, Hantian Liu, Derek Dardzinski, Christopher J. Palmstrøm, Noa Marom

Abstract: Half-metallic Heusler compounds are of significant interest for spintronics. For device fabrication, compounds that can be epitaxially grown on III-V semiconductors are particularly attractive. We present a first-principles investigation of four Co-based and two Ni-based Heusler compounds that are lattice-matched to InAs. The results of density functional theory (DFT) using semi-local and hybrid f… ▽ More Half-metallic Heusler compounds are of significant interest for spintronics. For device fabrication, compounds that can be epitaxially grown on III-V semiconductors are particularly attractive. We present a first-principles investigation of four Co-based and two Ni-based Heusler compounds that are lattice-matched to InAs. The results of density functional theory (DFT) using semi-local and hybrid functionals are compared to quasiparticle self-consistent $GW$ (QP$GW$). We also consider DFT with machine-learned Hubbard $U$ corrections [npj Computational Materials 6, 180 (2020)] with a new Bayesian optimization (BO) objective function to determine the $U$ values that yield the closest agreement with the QP$GW$ band structure and magnetic moments. We find that DFT+$U$(BO) can adequately reproduce the key QP$GW$ features in most cases. Our results reveal a strong method dependence of the degree of spin polarization at the Fermi level and, in some cases, even the dominant spin channel (majority or minority). Of the materials studied here, Co$_2$TiSn and Co$_2$ZrAl are the most likely to be half-metals. △ Less

Submitted 11 August, 2026; v1 submitted 24 February, 2026; originally announced February 2026.

Visualization of Tunable Electronic Structure of Monolayer TaIrTe$_4$

Authors: Sandy Adhitia Ekahana, Aalok Tiwari, Souvik Sasmal, Zefeng Cai, Ravi Kumar Bandapelli, I-Hsuan Kao, Jian Tang, Chenbo Min, Tiema Qian, Kenji Watanabe, Takashi Taniguchi, Ni Ni, Qiong Ma, Chris Jozwiak, Eli Rotenberg, Aaron Bostwick, Simranjeet Singh, Noa Marom, Jyoti Katoch

Abstract: Monolayer TaIrTe$_4$ has emerged as an attractive material platform to study intriguing phenomena related to topology and strong electron correlations. Recently, strong interactions have been demonstrated to induce strain and dielectric screening tunable topological phases such as quantum spin Hall insulator (QSHI), trivial insulator, higher-order topological insulator, and metallic phase, in the… ▽ More Monolayer TaIrTe$_4$ has emerged as an attractive material platform to study intriguing phenomena related to topology and strong electron correlations. Recently, strong interactions have been demonstrated to induce strain and dielectric screening tunable topological phases such as quantum spin Hall insulator (QSHI), trivial insulator, higher-order topological insulator, and metallic phase, in the ground state of monolayer TaIrTe$_4$. Moreover, charge dosing has been demonstrated to convert the QSHI into a dual QSHI state. Although the band structure of monolayer TaIrTe$_4$ is central to interpreting its topological phases in transport experiments, direct experimental access to its intrinsic electronic structure has so far remained elusive. Here we report direct measurements of the monolayer TaIrTe$_4$ band structure using spatially resolved micro-angle-resolved photoemission spectroscopy (microARPES) with micrometre-scale resolution. The observed dispersions show quantitative agreement with density functional theory calculations using the Heyd-Scuseria-Ernzerhof hybrid functional, establishing the insulating ground state and revealing no evidence for strong electronic correlations. We further uncover a pronounced electron-hole asymmetry in the doping response. Whereas hole doping is readily induced by electrostatic gating, attempts to introduce electrons via gating or alkali metal deposition do not yield a rigid upward shift of the Fermi level. Fractional charge calculations demonstrate that added electrons instead drive band renormalization and shrink the band gap. Taken together, our experimental and theoretical results identify the microscopic mechanism by which induced charges reshape the band topology of monolayer TaIrTe$_4$, showing that doping can fundamentally alter the electronic structure beyond the rigid band behaviour that is typically assumed. △ Less

Submitted 16 January, 2026; originally announced January 2026.

Structure Prediction of Ionic Epitaxial Interfaces with Ogre Demonstrated for Colloidal Heterostructures of Lead Halide Perovskites

Authors: Stefano Toso, Derek Dardzinski, Liberato Manna, Noa Marom

Abstract: Colloidal epitaxial heterostructures are nanoparticles composed of two different materials connected at an interface, which can exhibit properties different from those of their individual components. Combining dissimilar materials offers opportunities to create several functional heterostructures. Yet, assessing structural compatibility (the main prerequisite for epitaxial growth) is challenging w… ▽ More Colloidal epitaxial heterostructures are nanoparticles composed of two different materials connected at an interface, which can exhibit properties different from those of their individual components. Combining dissimilar materials offers opportunities to create several functional heterostructures. Yet, assessing structural compatibility (the main prerequisite for epitaxial growth) is challenging when pairing complex materials with different lattice parameters/crystal structures. This complicates both the selection of target heterostructures for synthesis and the assignment of interface models when new heterostructures are obtained. Here, we demonstrate Ogre as a powerful tool to accelerate the design and characterization of colloidal heterostructures. To this end we implemented developments tailored for the efficient prediction of epitaxial interfaces between ionic/polar materials, which encompass most colloidal semiconductors. These include pre-screening candidate models based on charge balance at the interface and using a classical potential for fast energy evaluations, with parameters automatically extracted from the input bulk structures. These developments are validated for CsPbBr3/Pb4S3Br2 heterostructures, where Ogre produces interface models in agreement with density functional theory and experiments. Furthermore, we use Ogre to rationalize the templating effect of CsPbCl3 on the growth of lead sulfochlorides, where perovskite seeds induce the formation of Pb4S3Cl2 rather than Pb3S2Cl2 due to better epitaxial compatibility. Combining Ogre simulations with experimental data enables us to unravel the structure and composition of the hitherto unsolved CsPbBr3/BixPbySz interface and assign a structure to many other reported metal halide/oxide based interfaces. The Ogre package is available on GitHub or via the OgreInterface desktop application, available for Windows, Linux and Mac. △ Less

Submitted 11 December, 2025; originally announced December 2025.

Comments: 103 pages, 48 figures

Journal ref: ACS Nano 2025, 19, 5, 5326-5341

Near-half-metallic state in the half Heusler PtMnSb film on a III-V substrate

Authors: Shinichi Nishihaya, Malcolm J. A. Jardine, Hadass S. Inbar, Aranya Goswami, Jason T. Dong, Aaron N. Engel, Yu-Hao Chang, Connor P. Dempsey, Makoto Hashimoto, Donghui Lu, Noa Marom, Chris J. Palmstrøm

Abstract: The interplay between half-metallic ferromagnetism and spin-orbit coupling within the inversion symmetry-broken structure of half Heuslers provides an ideal platform for various spintronics functionalities. Taking advantage of good lattice matching, it is highly desired to epitaxially integrate promising Heuslers into III-V semiconductor-based devices. PtMnSb is one of the first half Heuslers pred… ▽ More The interplay between half-metallic ferromagnetism and spin-orbit coupling within the inversion symmetry-broken structure of half Heuslers provides an ideal platform for various spintronics functionalities. Taking advantage of good lattice matching, it is highly desired to epitaxially integrate promising Heuslers into III-V semiconductor-based devices. PtMnSb is one of the first half Heuslers predicted to be an above-room-temperature half-metal with large spin orbit coupling, however, its half-metallicity and potential as a spintronics material has remained elusive due to lack of high quality samples. Here we demonstrate epitaxial growth of single crystal PtMnSb(001) film on GaSb(001) substrates using molecular beam epitaxy. Direct observation of the band structure via angle-resolved photoemission spectroscopy and many-body perturbation theory within the quasiparticle self-consistent GW approximation (QPGW) reveal that PtMnSb hosts rather a near-halfmetallic state with both spin bands crossing the Fermi level and with high spin polarization over 90%. Temperature dependence of magnetization also shows an anomalous enhancement below 60 K, which can be associated with the development of such a near-half-metallic state at low temperatures. Epitaxial growth of high crystalline PtMnSb on a III-V paves the way for systematic clarification of its spin transport properties with fine-tuning of strain in heterostructure devices. △ Less

Submitted 16 December, 2024; originally announced December 2024.

Comments: 20 pages, 6 figures

Structure Prediction of Epitaxial Organic Interfaces with Ogre, Demonstrated for TCNQ on TTF

Authors: Saeed Moayedpour, Imaneul Bier, Wen Wen, Derek Dardzinski, Olexandr Isayev, Noa Marom

Abstract: Highly ordered epitaxial interfaces between organic semiconductors are considered as a promising avenue for enhancing the performance of organic electronic devices including solar cells, light emitting diodes, and transistors, thanks to their well-controlled, uniform electronic properties and high carrier mobilities. Although the phenomenon of organic epitaxy has been known for decades, computatio… ▽ More Highly ordered epitaxial interfaces between organic semiconductors are considered as a promising avenue for enhancing the performance of organic electronic devices including solar cells, light emitting diodes, and transistors, thanks to their well-controlled, uniform electronic properties and high carrier mobilities. Although the phenomenon of organic epitaxy has been known for decades, computational methods for structure prediction of epitaxial organic interfaces have lagged far behind the existing methods for their inorganic counterparts. We present a method for structure prediction of epitaxial organic interfaces based on lattice matching followed by surface matching, implemented in the open-source Python package, Ogre. The lattice matching step produces domain-matched interfaces, where commensurability is achieved with different integer multiples of the substrate and film unit cells. In the surface matching step, Bayesian optimization (BO) is used to find the interfacial distance and registry between the substrate and film. The BO objective function is based on dispersion corrected deep neural network interatomic potentials, shown to be in excellent agreement with density functional theory (DFT). The application of Ogre is demonstrated for an epitaxial interface of 7,7,8,8-tetracyanoquinodimethane (TCNQ) on tetrathiafulvalene (TTF), whose electronic structure has been probed by ultraviolet photoemission spectroscopy (UPS), but whose structure had been hitherto unknown [Organic Electronics 48, 371 (2017)]. We find that TCNQ(001) on top of TTF(100) is the most stable interface configuration, closely followed by TCNQ(010) on top of TTF(100). The density of states, calculated using DFT, is in excellent agreement with UPS, including the presence of an interface charge transfer state. △ Less

Submitted 18 January, 2023; originally announced January 2023.

First Principles Assessment of CdTe as a Tunnel Barrier at the $\mathbfα$-Sn/InSb Interface

Authors: Malcolm J. A. Jardine, Derek Dardzinski, Maituo Yu, Amrita Purkayastha, A. -H. Chen, Yu-Hao Chang, Aaron Engel, Vladimir N. Strocov, Moïra Hocevar, Chris J. Palmstrøm, Sergey M. Frolov, Noa Marom

Abstract: Majorana zero modes, with prospective applications in topological quantum computing, are expected to arise in superconductor/semiconductor interfaces, such as $β$-Sn and InSb. However, proximity to the superconductor may also adversely affect the semiconductor's local properties. A tunnel barrier inserted at the interface could resolve this issue. We assess the wide band gap semiconductor, CdTe, a… ▽ More Majorana zero modes, with prospective applications in topological quantum computing, are expected to arise in superconductor/semiconductor interfaces, such as $β$-Sn and InSb. However, proximity to the superconductor may also adversely affect the semiconductor's local properties. A tunnel barrier inserted at the interface could resolve this issue. We assess the wide band gap semiconductor, CdTe, as a candidate material to mediate the coupling at the lattice-matched interface between $α$-Sn and InSb. To this end, we use density functional theory (DFT) with Hubbard U corrections, whose values are machine-learned via Bayesian optimization (BO) [npj Computational Materials 6, 180 (2020)]. The results of DFT+U(BO) are validated against angle resolved photoemission spectroscopy (ARPES) experiments for $α$-Sn and CdTe. For CdTe, the z-unfolding method [Advanced Quantum Technologies, 5, 2100033 (2022)] is used to resolve the contributions of different $k_z$ values to the ARPES. We then study the band offsets and the penetration depth of metal-induced gap states (MIGS) in bilayer interfaces of InSb/$α$-Sn, InSb/CdTe, and CdTe/$α$-Sn, as well as in tri-layer interfaces of InSb/CdTe/$α$-Sn with increasing thickness of CdTe. We find that 16 atomic layers (3.5 nm) of CdTe can serve as a tunnel barrier, effectively shielding the InSb from MIGS from the $α$-Sn. This may guide the choice of dimensions of the CdTe barrier to mediate the coupling in semiconductor-superconductor devices in future Majorana zero modes experiments. △ Less

Submitted 7 January, 2023; originally announced January 2023.

Journal ref: ACS Appl. Mater. Interfaces 2023

First Principles Study of the Electronic Structure of the Ni$_2$MnIn/InAs and Ti$_2$MnIn/InSb interfaces

Authors: Brett Heischmidt, Maituo Yu, Derek Dardzinski, James Etheridge, Saeed Moayedpour, Vlad S. Pribiag, Paul A. Crowell, Noa Marom

Abstract: We present a first-principles study of the electronic and magnetic properties of epitaxial interfaces between the Heusler compounds Ti$_2$MnIn and Ni$_2$MnIn and the III-V semiconductors, InSb and InAs, respectively. We use density functional theory (DFT) with a machine-learned Hubbard $U$ correction determined by Bayesian optimization. We evaluate these interfaces for prospective applications in… ▽ More We present a first-principles study of the electronic and magnetic properties of epitaxial interfaces between the Heusler compounds Ti$_2$MnIn and Ni$_2$MnIn and the III-V semiconductors, InSb and InAs, respectively. We use density functional theory (DFT) with a machine-learned Hubbard $U$ correction determined by Bayesian optimization. We evaluate these interfaces for prospective applications in Majorana-based quantum computing and spintronics. In both interfaces, states from the Heusler penetrate into the gap of the semiconductor, decaying within a few atomic layers. The magnetic interactions at the interface are weak and local in space and energy. Magnetic moments of less than 0.1 $μ_B$ are induced in the two atomic layers closest to the interface. The induced spin polarization around the Fermi level of the semiconductor also decays within a few atomic layers. The decisive factor for the induced spin polarization around the Fermi level of the semiconductor is the spin polarization around the Fermi level in the Heusler, rather than the overall magnetic moment. As a result, the ferrimagnetic narrow-gap semiconductor Ti$_2$MnIn induces a more significant spin polarization in the InSb than the ferromagnetic metal Ni$_2$MnIn induces in the InAs. This is explained by the position of the transition metal $d$ states in the Heusler with respect to the Fermi level. Based on our results, these interfaces are unlikely to be useful for Majorana devices but could be of interest for spintronics. △ Less

Submitted 7 January, 2023; v1 submitted 28 September, 2022; originally announced September 2022.

Scale-dependent optimized homoepitaxy of InAs(111)A

Authors: Steffen Zelzer, Rajib Batabyal, Derek Dardzinski, Noa Marom, Kasper Grove-Rasmussen, Peter Krogstrup

Abstract: We combined in-situ scanning tunneling microscopy (STM) with the conventional growth characterization methods of atomic force microscopy (AFM) and reflection high energy electron diffraction (RHEED) to simultaneously assess atomic-scale impurities and the larger-scale surface morphology of molecular beam epitaxy (MBE) grown homoepitaxial InAs(111)A. By keeping a constant substrate temperature and… ▽ More We combined in-situ scanning tunneling microscopy (STM) with the conventional growth characterization methods of atomic force microscopy (AFM) and reflection high energy electron diffraction (RHEED) to simultaneously assess atomic-scale impurities and the larger-scale surface morphology of molecular beam epitaxy (MBE) grown homoepitaxial InAs(111)A. By keeping a constant substrate temperature and indium flux while increasing the As$_2$ flux, we find two differing MBE growth parameter regions for optimized surface roughness on the macro and atomic scale. In particular, we show that a pure step-flow regime with strong suppression of hillock formation can be achieved, even on substrates without intentional offcut. On the other hand, an indium adatom deficient, low atomic defect surface can be observed for a high hillock density. We identify the main remaining point defect on the latter surface by comparison to STM simulations. Furthermore, we provide a method for extracting root-mean-square surface roughness values and discuss their use for surface quality optimization by comparison to scale-dependent, technologically relevant surface metrics. Finally, mapping the separately optimized regions of the growth parameter space should provide a guide for future device engineering involving epitaxial InAs(111)A growth. △ Less

Submitted 23 May, 2022; originally announced May 2022.

Report number: NBI QDEV 2022

Dependence of the electronic structure of the EuS/InAs interface on the bonding configuration

Authors: Maituo Yu, Saeed Moayedpour, Shuyang Yang, Derek Dardzinski, Chunzhi Wu, Vlad S. Pribiag, Noa Marom

Abstract: Recently, the EuS/InAs interface has attracted attention for the possibility of inducing magnetic exchange correlations in a strong spin-orbit semiconductor, which could be useful for topological quantum devices. We use density functional theory (DFT) with a machine-learned Hubbard $U$ correction [npj Comput. Mater. 6, 180 (2020)] to elucidate the effect of the bonding configuration at the interfa… ▽ More Recently, the EuS/InAs interface has attracted attention for the possibility of inducing magnetic exchange correlations in a strong spin-orbit semiconductor, which could be useful for topological quantum devices. We use density functional theory (DFT) with a machine-learned Hubbard $U$ correction [npj Comput. Mater. 6, 180 (2020)] to elucidate the effect of the bonding configuration at the interface on the electronic structure. For all interface configurations considered here, we find that the EuS valence band maximum (VBM) lies below the InAs VBM. In addition, dispersed states emerge at the top of the InAs VBM at the interface, which do not exist in either material separately. These states are contributed mainly by the InAs layer adjacent to the interface. They are localized at the interface and may be attributed to charge transfer from the EuS to the InAs. The interface configuration affects the position of the EuS VBM with respect to the InAs VBM, as well as the dispersion of the interface state. For all interface configurations studied here, the induced magnetic moment in the InAs is small. This suggests that this interface, in its coherent form studied here, is not promising for inducing equilibrium magnetic properties in InAs. △ Less

Submitted 4 April, 2021; originally announced April 2021.

Journal ref: Phys. Rev. Materials 5, 064606 (2021)

Structure Prediction of Epitaxial Inorganic Interfaces by Lattice and Surface Matching with Ogre

Authors: Saeed Moayedpour, Derek Dardzinski, Shuyang Yang, Andrea Hwang, Noa Marom

Abstract: We present a new version of the Ogre open source Python package with the capability to perform structure prediction of epitaxial inorganic interfaces by lattice and surface matching. In the lattice matching step a scan over combinations of substrate and film Miller indices is performed to identify the domain-matched interfaces with the lowest mismatch. Subsequently, surface matching is conducted b… ▽ More We present a new version of the Ogre open source Python package with the capability to perform structure prediction of epitaxial inorganic interfaces by lattice and surface matching. In the lattice matching step a scan over combinations of substrate and film Miller indices is performed to identify the domain-matched interfaces with the lowest mismatch. Subsequently, surface matching is conducted by Bayesian optimization to find the optimal interfacial distance and in-plane registry between the substrate and film. For the objective function, a geometric score function is proposed, based on the overlap and empty space between atomic spheres at the interface. The score function reproduces the results of density functional theory (DFT) at a fraction of the computational cost. The optimized interfaces are pre-ranked using a score function based on the similarity of the atomic environment at the interface to the bulk environment. Final ranking of the top candidate structures is performed with DFT. Ogre streamlines DFT calculations of interface energies and electronic properties by automating the construction of interface models. The application of Ogre is demonstrated for two interfaces of interest for quantum computing and spintronics, Al/InAs and Fe/InSb. △ Less

Submitted 25 March, 2021; originally announced March 2021.

First principles feasibility assessment of a topological insulator at the InAs/GaSb interface

Authors: Shuyang Yang, Derek Dardzinski, Andrea Hwang, Dmitry I. Pikulin, Georg W. Winkler, Noa Marom

Abstract: First principles simulations are conducted to shed light on the question of whether a two-dimensional topological insulator (2DTI) phase may be obtained at the interface between InAs and GaSb. To this end, the InAs/GaSb interface is compared and contrasted with the HgTe/CdTe interface. Density functional theory (DFT) simulations of these interfaces are performed using a machine-learned Hubbard U c… ▽ More First principles simulations are conducted to shed light on the question of whether a two-dimensional topological insulator (2DTI) phase may be obtained at the interface between InAs and GaSb. To this end, the InAs/GaSb interface is compared and contrasted with the HgTe/CdTe interface. Density functional theory (DFT) simulations of these interfaces are performed using a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. For the HgTe/CdTe interface our simulations show that band crossing is achieved and an inverted gap is obtained at a critical thickness of 5.1 nm of HgTe, in agreement with experiment and previous DFT calculations. In contrast, for InAs/GaSb the gap narrows with increasing thickness of InAs; however the gap does not close for interfaces with up to 50 layers (about 15 nm) of each material. When an external electric field is applied across the InAs/GaSb interface, the GaSb-derived valence band maximum is shifted up in energy with respect to the InAs-derived conduction band minimum until eventually the bands cross and an inverted gap opens. Our results show that it may be possible to reach the topological regime at the InAs/GaSb interface under the right conditions. However, it may be challenging to realize these conditions experimentally, which explains the difficulty of experimentally demonstrating an inverted gap in InAs/GaSb. △ Less

Submitted 19 January, 2021; originally announced January 2021.

Journal ref: Phys. Rev. Materials 5, 084204 (2021)

Electronic structure of InAs and InSb surfaces: density functional theory and angle-resolved photoemission spectroscopy

Authors: Shuyang Yang, Niels B. M. Schröter, Sergej Schuwalow, Mohana Rajpalk, Keita Ohtani, Peter KrogstrupGeorg, W. Winkler, Jan Gukelberger, Dominik Gresch, Gabriel Aeppli, Roman M. Lutchyn, Vladimir N. Strocov, Noa Marom

Abstract: The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, we study the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces using a combination of density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES). We were able to perform large-scale… ▽ More The electronic structure of surfaces plays a key role in the properties of quantum devices. However, surfaces are also the most challenging to simulate and engineer. Here, we study the electronic structure of InAs(001), InAs(111), and InSb(110) surfaces using a combination of density functional theory (DFT) and angle-resolved photoemission spectroscopy (ARPES). We were able to perform large-scale first principles simulations and capture effects of different surface reconstructions by using DFT calculations with a machine-learned Hubbard U correction [npj Comput. Mater. 6, 180 (2020)]. To facilitate direct comparison with ARPES results, we implemented a "bulk unfolding" scheme by projecting the calculated band structure of a supercell surface slab model onto the bulk primitive cell. For all three surfaces, we find a good agreement between DFT calculations and ARPES. For InAs(001), the simulations clarify the effect of the surface reconstruction. Different reconstructions are found to produce distinctive surface states. For InAs(111) and InSb(110), the simulations help elucidate the effect of oxidation. Owing to larger charge transfer from As to O than from Sb to O, oxidation of InAs(111) leads to significant band bending and produces an electron pocket, whereas oxidation of InSb(110) does not. Our combined theoretical and experimental results may inform the design of quantum devices based on InAs and InSb semiconductors, e.g., topological qubits utilizing the Majorana zero modes. △ Less

Submitted 29 December, 2020; originally announced December 2020.

Topological Properties of SnSe/EuS and SnTe/CaTe Interfaces

Authors: Shuyang Yang, Chunzhi Wu, Noa Marom

Abstract: We use density functional theory calculations to study the electronic structure of epitaxial (111) interfaces of the topological crystalline insulators SnSe and SnTe with the magnetic insulator EuS and the non-magnetic insulator CaTe, respectively. We consider both interface slab models with a vacuum region and periodic heterostructures without vacuum. We find that gaps of 21 meV at the $Γ$ point… ▽ More We use density functional theory calculations to study the electronic structure of epitaxial (111) interfaces of the topological crystalline insulators SnSe and SnTe with the magnetic insulator EuS and the non-magnetic insulator CaTe, respectively. We consider both interface slab models with a vacuum region and periodic heterostructures without vacuum. We find that gaps of 21 meV at the $Γ$ point and 9 meV at the M point arise in the topological state at the SnSe/EuS interface, due to the magnetic proximity effect, which breaks the time reversal symmetry. The surface state at $Γ$ is shifted below the Fermi level by 88 meV and the surface state at M is shifted above the Fermi level by 47 meV, owing to band bending at the interface. By comparison, the topological state at the interface of SnTe/CaTe is unperturbed by the presence of non-magnetic CaTe. △ Less

Submitted 4 March, 2020; v1 submitted 26 February, 2020; originally announced February 2020.

Journal ref: Phys. Rev. Materials 4, 034203 (2020)

Genarris 2.0: A Random Structure Generator for Molecular Crystals

Authors: Rithwik Tom, Timothy Rose, Imanuel Bier, Harriet O'Brien, Alvaro Vazquez-Mayagoitia, Noa Marom

Abstract: Genarris is an open-source Python package for generating random molecular crystal structures with physical constraints for seeding crystal structure prediction algorithms and training machine learning models. Here we present a new version of the code, containing several major improvements. An MPI-based parallelization scheme has been implemented, which facilitates the seamless sequential execution… ▽ More Genarris is an open-source Python package for generating random molecular crystal structures with physical constraints for seeding crystal structure prediction algorithms and training machine learning models. Here we present a new version of the code, containing several major improvements. An MPI-based parallelization scheme has been implemented, which facilitates the seamless sequential execution of user-defined workflows. A new method for estimating the unit cell volume based on the single molecule structure has been developed using a machine-learned model trained on experimental structures. A new algorithm has been implemented for generating crystal structures with molecules occupying special Wyckoff positions. A new hierarchical structure check procedure has been developed to detect unphysical close contacts efficiently and accurately. New intermolecular distance settings have been implemented for strong hydrogen bonds. To demonstrate these new features, we study two specific cases: benzene and glycine. For all polymorphs, the final pool either contained the experimental structure, or structures with similar lattice parameters, symmetry, and packing motifs. △ Less

Submitted 23 September, 2019; originally announced September 2019.

Genarris: Random Generation of Molecular Crystal Structures and Fast Screening with a Harris Approximation

Authors: Xiayue Li, Farren S. Curtis, Timothy Rose, Christoph Schober, Alvaro Vazquez-Mayagoitia, Karsten Reuter, Harald Oberhofer, Noa Marom

Abstract: We present Genarris, a Python package that performs configuration space screening for molecular crystals of rigid molecules by random sampling with physical constraints. For fast energy evaluations Genarris employs a Harris approximation, whereby the total density of a molecular crystal is constructed via superposition of single molecule densities. Dispersion-inclusive density functional theory (D… ▽ More We present Genarris, a Python package that performs configuration space screening for molecular crystals of rigid molecules by random sampling with physical constraints. For fast energy evaluations Genarris employs a Harris approximation, whereby the total density of a molecular crystal is constructed via superposition of single molecule densities. Dispersion-inclusive density functional theory (DFT) is then used for the Harris density without performing a self-consistency cycle. Genarris uses machine learning for clustering, based on a relative coordinate descriptor (RCD) developed specifically for molecular crystals, which is shown to be robust in identifying packing motif similarity. In addition to random structure generation, Genarris offers three workflows based on different sequences of successive clustering and selection steps: the "Rigorous" workflow is an exhaustive exploration of the potential energy landscape, the "Energy" workflow produces a set of low energy structures, and the "Diverse" workflow produces a maximally diverse set of structures. The latter is recommended for generating initial populations for genetic algorithms. Here, the implementation of Genarris is reported and its application is demonstrated for three test cases. △ Less

Submitted 6 March, 2018; originally announced March 2018.

GAtor: A First Principles Genetic Algorithm for Molecular Crystal Structure Prediction

Authors: Farren Curtis, Xiayue Li, Timothy Rose, Álvaro Vázquez-Mayagoitia, Saswata Bhattacharya, Luca M. Ghiringhelli, Noa Marom

Abstract: We present the implementation of GAtor, a massively parallel, first principles genetic algorithm (GA) for molecular crystal structure prediction. GAtor is written in Python and currently interfaces with the FHI-aims code to perform local optimizations and energy evaluations using dispersion-inclusive density functional theory (DFT). GAtor offers a variety of fitness evaluation, selection, crossove… ▽ More We present the implementation of GAtor, a massively parallel, first principles genetic algorithm (GA) for molecular crystal structure prediction. GAtor is written in Python and currently interfaces with the FHI-aims code to perform local optimizations and energy evaluations using dispersion-inclusive density functional theory (DFT). GAtor offers a variety of fitness evaluation, selection, crossover, and mutation schemes. Breeding operators designed specifically for molecular crystals provide a balance between exploration and exploitation. Evolutionary niching is implemented in GAtor by using machine learning to cluster the dynamically updated population by structural similarity and then employing a cluster-based fitness function. Evolutionary niching promotes uniform sampling of the potential energy surface by evolving several sub-populations, which helps overcome initial pool biases and selection biases (genetic drift). The various settings offered by GAtor increase the likelihood of locating numerous low-energy minima, including those located in disconnected, hard to reach regions of the potential energy landscape. The best structures generated are re-relaxed and re-ranked using a hierarchy of increasingly accurate DFT functionals and dispersion methods. GAtor is applied to a chemically diverse set of four past blind test targets, characterized by different types of intermolecular interactions. The experimentally observed structures and other low-energy structures are found for all four targets. In particular, for Target II, 5-cyano-3-hydroxythiophene, the top ranked putative crystal structure is a $Z^\prime$=2 structure with P$\bar{1}$ symmetry and a scaffold packing motif, which has not been reported previously. △ Less

Submitted 23 February, 2018; originally announced February 2018.

On the Possibility of Singlet Fission in Crystalline Quaterrylene

Authors: Xiaopeng Wang, Xingyu Liu, Cameron Cook, Bohdan Schatschneider, Noa Marom

Abstract: Singlet fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improving organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit intermolecular SF in the solid state, most of which are acene deriva… ▽ More Singlet fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improving organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit intermolecular SF in the solid state, most of which are acene derivatives. Recently, there has been a growing interest in rylenes as potential SF materials. We use many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation (BSE) to investigate the possibility of intermolecular SF in crystalline perylene and quaterrylene. A new method is presented for determining the percent charge transfer character (%CT) of an exciton wave-function from double-Bader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (ES-2ET) and %CT, crystalline quaterrylene is a promising candidate for intermolecular SF. Furthermore, quaterrylene is attractive for OPV applications thanks to its high stability and narrow optical gap. Perylene is not expected to exhibit SF, however it is a promising candidate for harvesting sub-gap photons by triplet-triplet annihilation. △ Less

Submitted 23 February, 2018; v1 submitted 13 September, 2017; originally announced September 2017.

Computational Design of Nanoclusters by Property-Based Genetic Algorithms: Tuning the Electronic Properties of (TiO$_2$)$_n$ Clusters

Authors: Saswata Bhattacharya, Benjamin H. Sonin, Christopher J. Jumonville, Luca M. Ghiringhelli, Noa Marom

Abstract: In order to design clusters with desired properties, we have implemented a suite of genetic algorithms tailored to optimize for low total energy, high vertical electron affinity (VEA), and low vertical ionization potential (VIP). Applied to (TiO$_2$)$_n$ clusters, the property-based optimization reveals the underlying structure-property relations and the structural features that may serve as activ… ▽ More In order to design clusters with desired properties, we have implemented a suite of genetic algorithms tailored to optimize for low total energy, high vertical electron affinity (VEA), and low vertical ionization potential (VIP). Applied to (TiO$_2$)$_n$ clusters, the property-based optimization reveals the underlying structure-property relations and the structural features that may serve as active sites for catalysis. High VEA and low VIP are correlated with the presence of several dangling-O atoms and their proximity, respectively. We show that the electronic properties of (TiO$_2$)$_n$ up to n=20 correlate more strongly with the presence of these structural features than with size. △ Less

Submitted 23 January, 2015; originally announced January 2015.

Comments: 4 figs, 5 pages

Journal ref: Phys. Rev. B 91, 241115(R) (2015)

Electrodynamic Response and Stability of Molecular Crystals

Authors: Bohdan Schatschneider, Jian-Jie Liang, Anthony M. Reilly, Noa Marom, Guo-Xu Zhang, Alexandre Tkatchenko

Abstract: We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal "atom-in-a-molecule" model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystal… ▽ More We show that electrodynamic dipolar interactions, responsible for long-range fluctuations in matter, play a significant role in the stability of molecular crystals. Density functional theory calculations with van der Waals interactions determined from a semilocal "atom-in-a-molecule" model result in a large overestimation of the dielectric constants and sublimation enthalpies for polyacene crystals from naphthalene to pentacene, whereas an accurate treatment of non-local electrodynamic response leads to an agreement with the measured values for both quantities. Our findings suggest that collective response effects play a substantial role not only for optical excitations, but also for cohesive properties of non-covalently bound molecular crystals. △ Less

Submitted 7 November, 2012; originally announced November 2012.

A Benchmark of GW Methods for Azabenzenes: Is the GW Approximation Good Enough?

Authors: Noa Marom, Fabio Caruso, Xinguo Ren, Oliver Hofmann, Thomas Körzdörfer, James R. Chelikowsky, Angel Rubio, Matthias Scheffler, Patrick Rinke

Abstract: Many-body perturbation theory in the GW approximation is a useful method for describing electronic properties associated with charged excitations. A hierarchy of GW methods exists, starting from non-self-consistent G0W0, through partial self-consistency in the eigenvalues (ev-scGW) and in the Green function (scGW0), to fully self-consistent GW (scGW). Here, we assess the performance of these metho… ▽ More Many-body perturbation theory in the GW approximation is a useful method for describing electronic properties associated with charged excitations. A hierarchy of GW methods exists, starting from non-self-consistent G0W0, through partial self-consistency in the eigenvalues (ev-scGW) and in the Green function (scGW0), to fully self-consistent GW (scGW). Here, we assess the performance of these methods for benzene, pyridine, and the diazines. The quasiparticle spectra are compared to photoemission spectroscopy (PES) experiments with respect to all measured particle removal energies and the ordering of the frontier orbitals. We find that the accuracy of the calculated spectra does not match the expectations based on their level of self-consistency. In particular, for certain starting points G0W0 and scGW0 provide spectra in better agreement with the PES than scGW. △ Less

Submitted 2 November, 2012; originally announced November 2012.

Journal ref: Phys. Rev. B 86 245127 (2012)

Many-Body Dispersion Interactions in Molecular Crystal Polymorphism

Authors: Noa Marom, Robert A. DiStasio Jr., Viktor Atalla, Sergey Levchenko, James R. Chelikowsky, Leslie Leiserowitz, Alexandre Tkatchenko

Abstract: Polymorphs in molecular crystals are often very close in energy, yet they may possess markedly different physical and chemical properties. The understanding and prediction of polymorphism is of paramount importance for a variety of applications, including pharmaceuticals, non-linear optics, and hydrogen storage. Here, we show that the non-additive many-body dispersion (MBD) energy beyond the stand… ▽ More Polymorphs in molecular crystals are often very close in energy, yet they may possess markedly different physical and chemical properties. The understanding and prediction of polymorphism is of paramount importance for a variety of applications, including pharmaceuticals, non-linear optics, and hydrogen storage. Here, we show that the non-additive many-body dispersion (MBD) energy beyond the standard pairwise approximation is crucial for the correct qualitative and quantitative description of polymorphism in molecular crystals. This is rationalized by the sensitive dependence of the MBD energy on the polymorph geometry and the ensuing dynamic electric fields inside molecular crystals. We use the glycine crystal as a fundamental and stringent benchmark case to demonstrate the accuracy of the DFT+MBD method. △ Less

Submitted 20 October, 2012; originally announced October 2012.

Theoretical Design of a Shallow Donor in Diamond by Lithium-Nitrogen Codoping

Authors: Jonathan E. Moussa, Noa Marom, Na Sai, James R. Chelikowsky

Abstract: We propose a new substitutional impurity complex in diamond composed of a lithium atom that is tetrahedrally coordinated by four nitrogen atoms (LiN_4). Density functional calculations are consistent with the hydrogenic impurity model, both supporting the prediction that this complex is a shallow donor with an activation energy of 0.27 +/- 0.06 eV. Three paths to the experimental realization of th… ▽ More We propose a new substitutional impurity complex in diamond composed of a lithium atom that is tetrahedrally coordinated by four nitrogen atoms (LiN_4). Density functional calculations are consistent with the hydrogenic impurity model, both supporting the prediction that this complex is a shallow donor with an activation energy of 0.27 +/- 0.06 eV. Three paths to the experimental realization of the LiN_4 complex in diamond are proposed and theoretically analyzed. △ Less

Submitted 14 May, 2012; v1 submitted 10 May, 2012; originally announced May 2012.

Comments: 6 pages, 3 figures, accepted to Physical Review Letters

Journal ref: Phys. Rev. Lett. 108, 226404 (2012)

Stacking and Registry Effects in Layered Materials: The Case of Hexagonal Boron Nitride

Authors: Noa Marom, Jonathan Bernstein, Jonathan Garel, Alexandre Tkatchenko, Ernesto Joselevich, Leeor Kronik, Oded Hod

Abstract: The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to "anchor" the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is iden… ▽ More The interlayer sliding energy landscape of hexagonal boron nitride (h-BN) is investigated via a van der Waals corrected density functional theory approach. It is found that the main role of the van der Waals forces is to "anchor" the layers at a fixed distance, whereas the electrostatic forces dictate the optimal stacking mode and the interlayer sliding energy. A nearly free-sliding path is identified, along which bandgap modulations of ~0.6 eV are obtained. We propose a simple geometrical model that quantifies the registry matching between the layers and captures the essence of the corrugated h-BN interlayer energy landscape. The simplicity of this phenomenological model opens the way to the modeling of complex layered structures, such as carbon and boron nitride nanotubes. △ Less

Submitted 8 February, 2010; originally announced February 2010.

Comments: 4 Pages, 3 Figures

Journal ref: Phys. Rev. Lett. 105, 046801 (2010)

Electronic Structure of Copper Phthalocyanine: a Comparative Density Functional Theory Study

Authors: Noa Marom, Oded Hod, Gustavo E. Scuseria, Leeor Kronik

Abstract: We present a systematic density functional theory study of the electronic structure of copper phthalocyanine (CuPc), using several different (semi)-local and hybrid functionals, and compare the results to experimental photoemission data. We show that semi-local functionals fail qualitatively for CuPc, primarily because of under-binding of localized orbitals due to self-interaction errors. We dis… ▽ More We present a systematic density functional theory study of the electronic structure of copper phthalocyanine (CuPc), using several different (semi)-local and hybrid functionals, and compare the results to experimental photoemission data. We show that semi-local functionals fail qualitatively for CuPc, primarily because of under-binding of localized orbitals due to self-interaction errors. We discuss an appropriate choice of functional for studies of CuPc/metal interfaces and suggest the Heyd-Scuseria-Ernzerhof screened hybrid functional as a suitable compromise functional. △ Less

Submitted 4 January, 2008; originally announced January 2008.

Comments: 18 pages, 4 Figure, 1 Table