Showing 1–18 of 18 results for author: Dogan, M

Ultrafast laser synthesis of zeolites

Authors: Sezin Galioglu, Mehdi Hagverdiyev, Meryem M. Doğan, Özgün Yavuz, Ü. Seleme Nizam, Ghaith Makey, Aladin Choura, Mesut Laçin, Burcu Akata Kurç, Parviz Elahi, F. Ömer Ilday, Serim Ilday

Abstract: Research has demonstrated that zeolite nucleation and growth can be controlled by fine-tuning chemical composition, temperature, and pressure, resulting in structures with diverse porosities and functionalities. Nevertheless, current energy delivery methods lack the finesse required to operate on the femto- and picosecond timescales of silica polymerisation and depolymerisation, limiting their abi… ▽ More Research has demonstrated that zeolite nucleation and growth can be controlled by fine-tuning chemical composition, temperature, and pressure, resulting in structures with diverse porosities and functionalities. Nevertheless, current energy delivery methods lack the finesse required to operate on the femto- and picosecond timescales of silica polymerisation and depolymerisation, limiting their ability to direct synthesis with high precision. To overcome this limitation, we introduce an ultrafast laser synthesis technique capable of delivering energy at these timescales with unprecedented spatiotemporal precision. Unlike conventional or emerging approaches, this method bypasses the need for specific temperature and pressure settings, as nucleation and growth are governed by dynamic phenomena arising from nonlinear light-matter interactions, such as convective flows, cavitation bubbles, plasma formation, and shock waves. These processes can be initiated, paused, and resumed within fractions of a second, effectively freezing structures at any stage of self-assembly. Using this approach, we traced the entire nucleation and growth pathway of laser-synthesized TPA-silicate-1 zeolites, from early oligomer formation to fully developed crystals. The unprecedented spatiotemporal control of this technique unlocks new avenues for manipulating reaction pathways and exploring the vast configurational space of zeolites. △ Less

Submitted 26 February, 2025; v1 submitted 4 February, 2025; originally announced February 2025.

Comments: main text, 6 figures, 4 video files, supplementary information

Anisotropy and Isotope Effect in Superconducting Solid Hydrogen

Authors: Mehmet Dogan, James R. Chelikowsky, Marvin L. Cohen

Abstract: Elucidating the phase diagram of solid hydrogen is a key objective in condensed matter physics. Several decades ago, it was proposed that at low temperatures and high pressures, solid hydrogen would be a metal with a high superconducting transition temperature. This transition to a metallic state can happen through the closing of the energy gap in the molecular solid or through a transition to an… ▽ More Elucidating the phase diagram of solid hydrogen is a key objective in condensed matter physics. Several decades ago, it was proposed that at low temperatures and high pressures, solid hydrogen would be a metal with a high superconducting transition temperature. This transition to a metallic state can happen through the closing of the energy gap in the molecular solid or through a transition to an atomic solid. Recent experiments have managed to reach pressures in the range of 400-500 GPa, providing valuable insights. There is strong evidence suggesting that metallization via either of these mechanisms occurs within this pressure range. Computational and experimental studies have identified multiple promising crystal phases, but the limited accuracy of calculations and the limited capabilities of experiments prevent us from determining unequivocally the observed phase or phases. Therefore, it is crucial to investigate the superconducting properties of all the candidate phases. Recently, we reported the superconducting properties of the C2/c-24, Cmca-12, Cmca-4 and I41/amd-2 phases, including anharmonic effects. Here, we report the effects of anisotropy on superconducting properties using Eliashberg theory. Then, we investigate the superconducting properties of deuterium and estimate the size of the isotope effect for each phase. We find that the isotope effect on superconductivity is diminished by anharmonicity in the C2/c-24 and Cmca-12 phases and enlarged in the Cmca-4 and I41/amd-2 phases. Our anharmonic calculations of the C2/c-24 phase of deuterium agree closely with the most recent experiment by Loubeyre et al. [Phys. Rev. Lett. 29, 035501 (2022)], indicating that the C2/c-24 phase remains the leading candidate in this pressure range, and has a strong anharmonic character. These characteristics can serve to distinguish among crystal phases in experiment. △ Less

Submitted 6 July, 2023; originally announced July 2023.

Journal ref: Journal of Physics: Condensed Matter 36 01LT01 (2024)

Solving the electronic structure problem for over 100,000 atoms in real-space

Authors: Mehmet Dogan, Kai-Hsin Liou, James R. Chelikowsky

Abstract: Using a real-space high order finite-difference approach, we investigate the electronic structure of large spherical silicon nanoclusters. Within Kohn-Sham density functional theory and using pseudopotentials, we report the self-consistent field convergence of a system with over 100,000 atoms: a Si(107,641)H(9,084) nanocluster with a diameter of 16 nm. Our approach uses Chebyshev-filtered subspace… ▽ More Using a real-space high order finite-difference approach, we investigate the electronic structure of large spherical silicon nanoclusters. Within Kohn-Sham density functional theory and using pseudopotentials, we report the self-consistent field convergence of a system with over 100,000 atoms: a Si(107,641)H(9,084) nanocluster with a diameter of 16 nm. Our approach uses Chebyshev-filtered subspace iteration to speed-up the convergence of the eigenspace, and blockwise Hilbert space filling curves to speed-up sparse matrix-vector multiplications, all of which is implemented in the PARSEC code. For the largest system, we utilized 2048 nodes (114,688 processors) on the Frontera machine in the Texas Advanced Computing Center. Our quantitative analysis of the electronic structure shows how it gradually approaches its bulk counterpart as a function of the nanocluster size. The band gap is enlarged due to quantum confinement in nanoclusters, but decreases as the system size increases, as expected. Our work serves as a proof-of-concept for the capacity of the real-space approach in efficiently parallelizing very large calculations using high performance computer platforms, which can straightforwardly be replicated in other systems with more than $10^5$ atoms. △ Less

Submitted 1 March, 2023; originally announced March 2023.

Journal ref: Phys. Rev. Materials 7, L063001 (2023)

Roadmap on Electronic Structure Codes in the Exascale Era

Authors: Vikram Gavini, Stefano Baroni, Volker Blum, David R. Bowler, Alexander Buccheri, James R. Chelikowsky, Sambit Das, William Dawson, Pietro Delugas, Mehmet Dogan, Claudia Draxl, Giulia Galli, Luigi Genovese, Paolo Giannozzi, Matteo Giantomassi, Xavier Gonze, Marco Govoni, Andris Gulans, François Gygi, John M. Herbert, Sebastian Kokott, Thomas D. Kühne, Kai-Hsin Liou, Tsuyoshi Miyazaki, Phani Motamarri , et al. (16 additional authors not shown)

Abstract: Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources d… ▽ More Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing. △ Less

Submitted 26 September, 2022; originally announced September 2022.

Comments: Submitted as a roadmap article to Modelling and Simulation in Materials Science and Engineering; Address any correspondence to Vikram Gavini (vikramg@umich.edu) and Danny Perez (danny_perez@lanl.gov)

Magnetism and Interlayer Bonding in Pores of Bernal-Stacked Hexagonal Boron Nitride

Authors: Mehmet Dogan, Marvin L. Cohen

Abstract: When single-layer h-BN is subjected to a high-energy electron beam, triangular pores with nitrogen edges are formed. Because of the broken sp2 bonds, these pores are known to possess magnetic states. We report on the magnetism and electronic structure of triangular pores as a function of their size. Moreover, in the Bernal-stacked h-BN (AB-h-BN), multilayer pores with parallel edges can be created… ▽ More When single-layer h-BN is subjected to a high-energy electron beam, triangular pores with nitrogen edges are formed. Because of the broken sp2 bonds, these pores are known to possess magnetic states. We report on the magnetism and electronic structure of triangular pores as a function of their size. Moreover, in the Bernal-stacked h-BN (AB-h-BN), multilayer pores with parallel edges can be created, which is not possible in the commonly fabricated multilayer AA'-h-BN. Given that these pores can be manufactured in a well-controlled fashion using an electron beam, it is important to understand the interactions of pores in neighboring layers. We find that in certain configurations, the edges of the neighboring pores remain open and retain their magnetism, and in others, they form interlayer bonds. We present a comprehensive report on these configurations for small nanopores. We find that at low temperatures, these pores have near degenerate magnetic configurations, and may be utilized in magnetoresistance and spintronics applications. In the process of forming larger multilayer nanopores, interlayer bonds can form, reducing the magnetization. Yet, unbonded parallel multilayer edges remain available at all sizes. Understanding these pores is also helpful in a multitude of applications such as DNA sequencing and quantum emission. △ Less

Submitted 11 May, 2022; v1 submitted 27 February, 2022; originally announced February 2022.

Journal ref: Phys. Chem. Chem. Phys., 2022, 24, 20882-20890

High Temperature Superconductivity in the Candidate Phases of Solid Hydrogen

Authors: Mehmet Dogan, Sehoon Oh, Marvin L. Cohen

Abstract: As the simplest element in nature, unraveling the phase diagram of hydrogen is a primary task for condensed matter physics. As conjectured many decades ago, in the low-temperature and high-pressure part of the phase diagram, solid hydrogen is expected to become metallic with a high superconducting transition temperature. The metallization may occur via band gap closure in the molecular solid or vi… ▽ More As the simplest element in nature, unraveling the phase diagram of hydrogen is a primary task for condensed matter physics. As conjectured many decades ago, in the low-temperature and high-pressure part of the phase diagram, solid hydrogen is expected to become metallic with a high superconducting transition temperature. The metallization may occur via band gap closure in the molecular solid or via a transition to the atomic solid. Recently, a few experimental studies pushed the achievable pressures into the 400 - 500 GPa range. There are strong indications that at some pressure in this range metallization via either of these mechanisms occurs, although there are disagreements between experimental reports. Furthermore, there are multiple good candidate crystal phases that have emerged from recent computational and experimental studies which may be realized in upcoming experiments. Therefore, it is crucial to determine the superconducting properties of these candidate phases. In a recent study, we reported the superconducting properties of the C2/c-24 phase, which we believe to be a strong candidate for metallization via band gap closure [M. Dogan et al., arXiv:2107.03889 (2021)]. Here, we report the superconducting properties of the Cmca-12, Cmca-4 and I41/amd-2 phases including the anharmonic effects using a Wannier function-based dense k-point and q-point sampling. We find that the Cmca-12 phase has a superconducting transition temperature that rises from 86 K at 400 GPa to 212 K at 500 GPa, whereas the Cmca-4 and I41/amd-2 phases show a less pressure-dependent behavior with their Tc in the 74 - 94 K and 307 - 343 K ranges, respectively. These properties can be used to distinguish between crystal phases in future experiments. Understanding superconductivity in pure hydrogen is also important in the study of high-Tc hydrides. △ Less

Submitted 4 October, 2021; originally announced October 2021.

Journal ref: Journal of Physics: Condensed Matter 34, 15LT01 (2022)

Prediction of High Temperature Superconductivity in C2/c-24 Solid Hydrogen

Authors: Mehmet Dogan, Sehoon Oh, Marvin L. Cohen

Abstract: Recent experimental developments in hydrogen-rich materials in high pressures have put this class of materials above others in the race toward room temperature superconductivity. As it is the basis of all the materials in this class, the efforts to determine the properties of pure solid hydrogen at high pressures remain intense. Most notably, a recent experimental study of the metallization of hyd… ▽ More Recent experimental developments in hydrogen-rich materials in high pressures have put this class of materials above others in the race toward room temperature superconductivity. As it is the basis of all the materials in this class, the efforts to determine the properties of pure solid hydrogen at high pressures remain intense. Most notably, a recent experimental study of the metallization of hydrogen identified the crystal phase of the solid as the C2/c-24 molecular phase up to ~425 GPa. It is possible that the observed metallization is caused by band structure effects and not a structural phase transition, and the material remains in this crystal phase up to higher pressures. Therefore it is of crucial importance to determine the superconducting properties of the C2/c-24 phase. Here, we employ a Wannier function-based dense k-point and q-point sampling to compute the electron-phonon coupling and superconducting properties of molecular hydrogen in the C2/c-24 phase. We find that the material has a high superconducting transition temperature of 242 K at 500 GPa. We also find that the transition temperature rapidly increases with pressure in the 400 - 500 GPa range. △ Less

Submitted 1 February, 2022; v1 submitted 8 July, 2021; originally announced July 2021.

Journal ref: Physical Review B 105, L020509 (2022)

Anomalous behavior in high-pressure carbonaceous sulfur hydride

Authors: Mehmet Dogan, Marvin L. Cohen

Abstract: A new experimental study by Snider et al. [Nature 586, 373-377 (2020)] reported behavior in a high-pressure carbon-sulfur-hydrogen system that has been interpreted by the authors as superconductivity at room temperature. The sudden drop of electrical resistance at a critical temperature and the change of the R vs. T behavior with an applied magnetic field point to superconductivity. This is a very… ▽ More A new experimental study by Snider et al. [Nature 586, 373-377 (2020)] reported behavior in a high-pressure carbon-sulfur-hydrogen system that has been interpreted by the authors as superconductivity at room temperature. The sudden drop of electrical resistance at a critical temperature and the change of the R vs. T behavior with an applied magnetic field point to superconductivity. This is a very exciting study in one of the most important areas of science, hence, it is crucial for the community to investigate these findings and hopefully reproduce these results. In this comment, we present calculations that expand upon the arguments put forth by Hirsch and Marsiglio [arXiv:2010.10307], and offer some speculations about physical mechanisms that might explain the observed data. In agreement with Hirsch and Marsiglio, we show that there are errors in the analysis presented in the experimental paper, and with the correct analysis, the reported R vs. T data significantly deviate from the expected behavior. In particular, the extremely sharp change in resistance at the superconducting transition is not consistent with a strongly type II superconductor. △ Less

Submitted 9 March, 2021; v1 submitted 19 December, 2020; originally announced December 2020.

Journal ref: Physica C: Superconductivity and its Applications, 2021, 583, 1353851

Ultra-Narrow TaS2 Nanoribbons

Authors: Jeffrey D. Cain, Sehoon Oh, Amin Azizi, Scott Stonemeyer, Mehmet Dogan, Markus Thiel, Peter Ercius, Marvin L. Cohen, Alex Zettl

Abstract: Imposing additional confinement in two-dimensional (2D) materials can yield further control over the associated electronic, optical, and topological properties. However, synthesis of ultra-narrow nanoribbons (NRs) remains a challenge, particularly for the transition metal dichalcogenides (TMDs), and synthesizing TMD NRs narrower than 50 nm has remained elusive. Here, we report the vapor-phase synt… ▽ More Imposing additional confinement in two-dimensional (2D) materials can yield further control over the associated electronic, optical, and topological properties. However, synthesis of ultra-narrow nanoribbons (NRs) remains a challenge, particularly for the transition metal dichalcogenides (TMDs), and synthesizing TMD NRs narrower than 50 nm has remained elusive. Here, we report the vapor-phase synthesis of ultra-narrow TaS2 NRs. The NRs are grown within the hollow cavity of carbon nanotubes, thereby limiting their lateral dimensions and layer number, while simultaneously stabilizing them against the environment. The NRs reach the monolayer (ML) limit and exhibit widths as low as 2.5 nm. Atomic-resolution scanning transmission electron microscopy (STEM) reveals the detailed atomic structure of the ultra-narrow NRs and we observe a hitherto unseen atomic structure supermodulation phenomenon of ordered defect arrays within the NRs. First-principles calculations based on density functional theory (DFT) show the presence of flat bands, as well as edge- and boundary-localized states, and help identify the atomic configuration of the supermodulation. Nanotube-templated synthesis represents a unique, transferable, and broadly deployable route toward ultra-narrow TMD NR growth. △ Less

Submitted 9 December, 2020; originally announced December 2020.

Magnetic Multilayer Edges in Bernal-Stacked Hexagonal Boron Nitride

Authors: Mehmet Dogan, Marvin L. Cohen

Abstract: Single-layer $\it{h}$-BN is known to have edges with unique magnetism, however, in the commonly fabricated multilayer $\text{AA}^{\prime}$-$\it{h}$-BN, edge relaxations occur that create interlayer bonds and eliminate the unpaired electrons at the edge. Recently, a robust method of growing the unconventional Bernal-stacked $\it{h}$-BN (AB-$\it{h}$-BN) has been reported. Here, we use theoretical ap… ▽ More Single-layer $\it{h}$-BN is known to have edges with unique magnetism, however, in the commonly fabricated multilayer $\text{AA}^{\prime}$-$\it{h}$-BN, edge relaxations occur that create interlayer bonds and eliminate the unpaired electrons at the edge. Recently, a robust method of growing the unconventional Bernal-stacked $\it{h}$-BN (AB-$\it{h}$-BN) has been reported. Here, we use theoretical approaches to investigate the nitrogen-terminated zigzag edges in AB-$\it{h}$-BN that can be formed in a controlled fashion using a high-energy electron beam. We find that these "open" edges remain intact in bilayer and multilayer AB-$\it{h}$-BN, enabling researchers potentially to investigate these edge states experimentally. We also investigate the thermodynamics of the spin configurations at the edge by constructing a lattice model that is based on parameters extracted from a set of first-principles calculations. We find that the edge spins in neighboring layers interact very weakly, resulting in a sequence of independent spin chains in multilayer samples. By solving this model using Monte Carlo simulations, we can determine nm-scale correlation lengths at liquid-N$_{2}$ temperatures and lower. At low temperatures, these edges may be utilized in magnetoresistance and spintronics applications. △ Less

Submitted 30 July, 2020; originally announced July 2020.

Journal ref: Phys. Rev. B 102, 155419 (2020)

Observed Metallization of Hydrogen Interpreted as a Band Structure Effect

Authors: Mehmet Dogan, Sehoon Oh, Marvin L. Cohen

Abstract: A recent experimental study of the metallization of hydrogen tracked the direct band gap and vibron frequency via infrared measurements up to ~425 GPa [P. Loubeyre et al., Nature 577, 631 (2020)]. Above this pressure, the direct gap has a discontinuous drop to below the minimum experimentally accessible energy (~0.1 eV). The authors suggested that this observation is caused by a structural phase t… ▽ More A recent experimental study of the metallization of hydrogen tracked the direct band gap and vibron frequency via infrared measurements up to ~425 GPa [P. Loubeyre et al., Nature 577, 631 (2020)]. Above this pressure, the direct gap has a discontinuous drop to below the minimum experimentally accessible energy (~0.1 eV). The authors suggested that this observation is caused by a structural phase transition between the C2/c-24 molecular phase to another molecular phase such as Cmca-12. Here, through ab initio calculations of pressure dependent vibron frequency and direct band gap, we find that the experimental data is consistent with the C2/c-24 phase up to 425 GPa, and suggest that this consistency extends beyond that pressure. Specifically, we find that qualitative changes in the band structure of the C2/c-24 phase lead to a discontinuous drop of the direct band gap, which can explain the observed drop without a structural transition. This alternative scenario naturally explains the absence of hysteresis in the measurements. △ Less

Submitted 28 July, 2020; v1 submitted 29 June, 2020; originally announced June 2020.

Journal ref: Journal of Physics: Condensed Matter, 2021, 33, 03LT01

High-Performance Atomically-Thin Room-Temperature NO2 Sensor

Authors: Amin Azizi, Mehmet Dogan, Hu Long, Jeffrey D. Cain, Kyunghoon Lee, Rahmatollah Eskandari, Alessandro Varieschi, Emily C. Glazer, Marvin L Cohen, Alex Zettl

Abstract: The development of room-temperature sensing devices for detecting small concentrations of molecular species is imperative for a wide range of low-power sensor applications. We demonstrate a room-temperature, highly sensitive, selective, and reversible chemical sensor based on a monolayer of the transition metal dichalcogenide Re0.5Nb0.5S2. The sensing device exhibits thickness dependent carrier ty… ▽ More The development of room-temperature sensing devices for detecting small concentrations of molecular species is imperative for a wide range of low-power sensor applications. We demonstrate a room-temperature, highly sensitive, selective, and reversible chemical sensor based on a monolayer of the transition metal dichalcogenide Re0.5Nb0.5S2. The sensing device exhibits thickness dependent carrier type, and upon exposure to NO2 molecules, its electrical resistance considerably increases or decreases depending on the layer number. The sensor is selective to NO2 with only minimal response to other gases such as NH3, CH2O, and CO2. In the presence of humidity, not only are the sensing properties not deteriorated, but also the monolayer sensor shows complete reversibility with fast recovery at room temperature. We present a theoretical analysis of the sensing platform and identify the atomically-sensitive transduction mechanism. △ Less

Submitted 16 July, 2020; v1 submitted 4 March, 2020; originally announced March 2020.

Journal ref: Nano Lett. 2020, 20, 8, 6120-6127

Electron Beam-Induced Nanopores in Bernal-Stacked Hexagonal Boron Nitride

Authors: Mehmet Dogan, S. Matt Gilbert, Thang Pham, Brian Shevitski, Peter Ercius, Shaul Aloni, Alex Zettl, Marvin L. Cohen

Abstract: Controlling the size and shape of nanopores in two-dimensional materials is a key challenge in applications such as DNA sequencing, sieving, and quantum emission in artificial atoms. We here investigate experimentally and theoretically triangular vacancies in (unconventional) Bernal-stacked AB-h-BN formed using a high-energy electron beam. Due to the geometric configuration of AB-h-BN, triangular… ▽ More Controlling the size and shape of nanopores in two-dimensional materials is a key challenge in applications such as DNA sequencing, sieving, and quantum emission in artificial atoms. We here investigate experimentally and theoretically triangular vacancies in (unconventional) Bernal-stacked AB-h-BN formed using a high-energy electron beam. Due to the geometric configuration of AB-h-BN, triangular pores in different layers are aligned, and their sizes are controlled by the duration of the electron irradiation. Interlayer covalent bonding at the vacancy edge is not favored, as opposed to what occurs in the more common AA'-stacked BN. A variety of monolayer, concentric and bilayer pores in bilayer AB-h-BN are observed in high-resolution transmission electron microscopy and characterized using ab initio simulations. Bilayer pores in AB-h-BN are commonly formed, and grow without breaking the bilayer character. Nanopores in AB-h-BN exhibit a wide range of electronic properties, ranging from half-metallic to non-magnetic and magnetic semiconducting. Therefore, because of the controllability of the pore size, the electronic structure is also highly controllable in these systems, and can potentially be tuned for particular applications. △ Less

Submitted 26 June, 2020; v1 submitted 13 February, 2020; originally announced February 2020.

Journal ref: Applied Physics Letters, 2020, 117, 023102

Causes of ferroelectricity in HfO$_{2}$-based thin films: An $\textit{ab initio}$ perspective

Authors: Mehmet Dogan, Nanbo Gong, Tso-Ping Ma, Sohrab Ismail-Beigi

Abstract: We present a comprehensive first principles study of doped hafnia in order to understand the formation of the ferroelectric orthorhombic [001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal [100] grains can transform into orthorhombic [001] during thermal annealing, when they are laterally confined by other grain… ▽ More We present a comprehensive first principles study of doped hafnia in order to understand the formation of the ferroelectric orthorhombic [001] grains. Assuming that tetragonal grains are present during the early stages of growth, matching plane analysis shows that tetragonal [100] grains can transform into orthorhombic [001] during thermal annealing, when they are laterally confined by other grains. We show that among 0%, 2% and %4 Si doping, 4% doping provides the best conditions for the tetragonal [100] to orthorhombic [001] transformation. This also holds for Al doping. We also show that for Hf$_{x}$Zr$_{1-x}$O$_{2}$, where we have studied ${x}=1.00,0.75,0.50,0.25,0.00$, the value ${x}=0.50$ provides the most favorable conditions for the desired transformation. In order for this transformation to be preferred over the tetragonal [100] to monoclinic [100] transformation, out-of-plane confinement also needs to be present, as supplied by a top electrode. Our findings illuminate the mechanism that causes ferroelectricity in hafnia-based films and provide an explanation for common experimental observations for the optimal ranges of doping in Si:HfO$_{2}$, Al:HfO$_{2}$ and Hf$_{x}$Zr$_{1-x}$O$_{2}$. We also present model thin film heterostructure computations of Ir/HfO$_{2}$/Ir stacks in order to isolate the interface effects, which we show to be significant. △ Less

Submitted 2 April, 2019; originally announced April 2019.

Journal ref: Physical Chemistry Chemical Physics, 2019, 21, 12150-12162

Ferroelectric ZrO$_{2}$ monolayers as buffer layers between SrTiO$_{3}$ and Si

Authors: Mehmet Dogan, Sohrab Ismail-Beigi

Abstract: A monolayer of ZrO$_{2}$ has recently been grown on the Si(001) surface and shown to have ferroelectric properties, which signifies the realization of the lowest possible thickness in ferroelectric oxides [M. Dogan et al., Nano Lett., 18 (1) (2018)]. In our previous computational study, we reported on the multiple (meta)stable configurations of ZrO$_{2}$ monolayers on Si, and how switching between… ▽ More A monolayer of ZrO$_{2}$ has recently been grown on the Si(001) surface and shown to have ferroelectric properties, which signifies the realization of the lowest possible thickness in ferroelectric oxides [M. Dogan et al., Nano Lett., 18 (1) (2018)]. In our previous computational study, we reported on the multiple (meta)stable configurations of ZrO$_{2}$ monolayers on Si, and how switching between a pair of differently polarized configurations may explain the observed ferroelectric behavior of these films [M. Dogan and S. Ismail-Beigi, arXiv:1902.01022 (2019)]. In the current study, we conduct a DFT-based investigation of (i) the effect of oxygen content on the ionic polarization of the oxide, and (ii) the role of zirconia monolayers as buffer layers between silicon and a thicker oxide film that is normally paraelectric on silicon, e.g. SrTiO$_{3}$. We find that (i) total energy-vs-polarization behavior of the monolayers, as well as interface chemistry, is highly dependent on the oxygen content; and (ii) SrTiO$_{3}$/ZrO$_{2}$/Si stacks exhibit multiple (meta)stable configurations and polarization profiles, i.e. zirconia monolayers can induce ferroelectricity in oxides such as SrTiO$_{3}$ when used as a buffer layer. This may enable a robust non-volatile device architecture where the thickness of the gate oxide (here strontium titanate) can be chosen according to the desired properties. △ Less

Submitted 27 March, 2019; originally announced March 2019.

Journal ref: The Journal of Physical Chemistry C, 2019, 123, 24, 15053-15061

Theory of Ferroelectric ZrO$_{2}$ Monolayers on Si

Authors: Mehmet Dogan, Sohrab Ismail-Beigi

Abstract: We use density functional theory and Monte Carlo lattice simulations to investigate the structure of ZrO$_{2}$ monolayers on Si(001). Recently, we have reported on the experimental growth of amorphous ZrO$_{2}$ monolayers on silicon and their ferroelectric properties, marking the achievement of the thinnest possible ferroelectric oxide [M. Dogan et al. Nano Lett., 18 (1) (2018)]. Here, we first de… ▽ More We use density functional theory and Monte Carlo lattice simulations to investigate the structure of ZrO$_{2}$ monolayers on Si(001). Recently, we have reported on the experimental growth of amorphous ZrO$_{2}$ monolayers on silicon and their ferroelectric properties, marking the achievement of the thinnest possible ferroelectric oxide [M. Dogan et al. Nano Lett., 18 (1) (2018)]. Here, we first describe the rich landscape of atomic configurations of monocrystalline ZrO$_{2}$ monolayers on Si and determine the local energy minima. Because of the multitude of low-energy configurations we find, we consider the coexistence of finite-sized regions of different configurations. We create a simple nearest-neighbor lattice model with parameters extracted from DFT calculations, and solve it numerically using a cluster Monte Carlo algorithm. Our results suggest that up to room temperature, the ZrO$_{2}$ monolayer consists of small domains of two low-energy configurations with opposite ferroelectric polarization. This explains the observed ferroelectric behavior in the experimental films as a collection of crystalline regions, which are a few nanometers in size, being switched with the application of an external electric field. △ Less

Submitted 3 February, 2019; originally announced February 2019.

Journal ref: The Journal of Physical Chemistry C, 2019, 123, 23, 14350-14361

Alternative Stacking Sequences in Hexagonal Boron Nitride

Authors: S. Matt Gilbert, Thang Pham, Mehmet Dogan, Sehoon Oh, Brian Shevitski, Gabe Schumm, Stanley Liu, Peter Ercius, Shaul Aloni, Marvin L. Cohen, Alex Zettl

Abstract: The relative orientation of successive sheets, i.e. the stacking sequence, in layered two-dimensional materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van… ▽ More The relative orientation of successive sheets, i.e. the stacking sequence, in layered two-dimensional materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine the total energy, electronic bandgap, and dielectric response tensor for five distinct high symmetry stacking sequences for both bulk and bilayer forms of h-BN. Two sequences, the generally assumed AA' sequence and the relatively unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low energy. We present a scalable modified chemical vapor deposition method that produces large flakes of virtually pure AB stacked h-BN; this new material complements the generally available AA' stacked h-BN. △ Less

Submitted 10 October, 2018; originally announced October 2018.

Heisenberg Uncertainty Principle as Probe of Entanglement Entropy: Application to Superradiant Quantum Phase Transitions

Authors: Pierre Nataf, Mehmet Dogan, Karyn Le Hur

Abstract: Quantum phase transitions are often embodied by the critical behavior of purely quantum quantities such as entanglement or quantum fluctuations. In critical regions, we underline a general scaling relation between the entanglement entropy and one of the most fundamental and simplest measure of the quantum fluctuations, the Heisenberg uncertainty principle. Then, we show that the latter represents… ▽ More Quantum phase transitions are often embodied by the critical behavior of purely quantum quantities such as entanglement or quantum fluctuations. In critical regions, we underline a general scaling relation between the entanglement entropy and one of the most fundamental and simplest measure of the quantum fluctuations, the Heisenberg uncertainty principle. Then, we show that the latter represents a sensitive probe of superradiant quantum phase transitions in standard models of photons such as the Dicke Hamiltonian, which embodies an ensemble of two-level systems interacting with one quadrature of a single and uniform bosonic field. We derive exact results in the thermodynamic limit and for a finite number N of two-level systems: as a reminiscence of the entanglement properties between light and the two-level systems, the product $ΔxΔp$ diverges at the quantum critical point as $N^{1/6}$. We generalize our results to the double quadrature Dicke model where the two quadratures of the bosonic field are now coupled to two independent sets of two level systems. Our findings, which show that the entanglement properties between light and matter can be accessed through the Heisenberg uncertainty principle, can be tested using Bose-Einstein condensates in optical cavities and circuit quantum electrodynamics △ Less

Submitted 5 October, 2012; v1 submitted 13 April, 2012; originally announced April 2012.

Comments: 7 pages, 3 figures. Published Version

Journal ref: Physical Review A 86, 043807 (2012)