Towfiq Ahmed - Academia.edu (original) (raw)

Papers by Towfiq Ahmed

Research paper thumbnail of Area-selective deposition of germanium on patterned graphene/monolayer molybdenum disulfide stacks via dipole engineering

APL materials, Mar 1, 2024

Effect of fluorine ion irradiation on the properties of monolayer molybdenum disulfide

Research paper thumbnail of Theoretical prediction of giant Hall high harmonic generation in monolayer NbSe$_2$

arXiv (Cornell University), Oct 3, 2023

et al., "A review on mechanics and mechanical properties of 2d materials-graphene and beyond," Ex... more et al., "A review on mechanics and mechanical properties of 2d materials-graphene and beyond," Extreme Mechanics Letters 13, 42-77 (2017).

Research paper thumbnail of Magnetic, electronic, and optical properties of double perovskite Bi<sub>2</sub>FeMnO<sub>6</sub>

APL Materials, Mar 1, 2017

Research paper thumbnail of Electronic structure and lattice dynamics at the interface of single layer FeSe and SrTiO$_{\mathrm{3}}$

Bulletin of the American Physical Society, Mar 16, 2016

Research paper thumbnail of Edge states and local electronic structure around an adsorbed impurity in a topological superconductor

Physical Review B, Nov 16, 2015

Recently topological superconducting states has attracted a lot of interest. In this work, we con... more Recently topological superconducting states has attracted a lot of interest. In this work, we consider a topological superconductor with Z2 topological mirror order [1] and s±-wave superconducting pairing symmetry, within a two-orbital model originally designed for iron-based superconductivity [2]. We predict the existence of gapless edge states. We also study the local electronic structure around an adsorbed interstitial magnetic impurity in the system, and find the existence of low-energy in-gap bound states even with a weak spin polarization on the impurity. We also discuss the relevance of our results to the recent STM experiment on Fe(Te,Se) compound with adsorbed Fe impurity [3], for which our density functional calculations show the Fe impurity is spin polarized.

Research paper thumbnail of Experimental and theoretical comparison of ion properties from nanosecond laser-produced plasmas of metal targets

Journal of Applied Physics, May 22, 2023

The ion emission properties of laser-produced plasmas as a function of laser intensities between ... more The ion emission properties of laser-produced plasmas as a function of laser intensities between 4-50 GW cm À2 and varying angles with respect to the target normal were investigated. The plasmas were produced by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser on various metal targets. The targets used for this study include Ti, Mo, and Gd (Z ¼ 22; 42; 64). It is noted that all ion profiles are composed of multiple peaks-a prompt emission peak trailed by three ion peaks (ultrafast, fast, and thermal). Experimentally, it is shown that each of these ion peaks follows a unique trend as a function of laser intensity, angle, and distance away from the target. Theoretically, it is shown that simple analytical models can be used to explain the properties of the ions. The variations in the ion velocity and density as a function of laser intensity are found to be in good agreement with theoretical models of sheath acceleration, isothermal self-similar expansion, and ablative plasma flow for various ion peaks.

Research paper thumbnail of f -electron structure database (FESD): A high-throughput data analysis and materials discovery tool for strongly correlated materials

Bulletin of the American Physical Society, Mar 6, 2018

Research paper thumbnail of First-principles study of magnetic, electronic and optical properties of double perovskite Bi$_2$FeMnO$_6$

Bulletin of the American Physical Society, Mar 5, 2015

Submitted for the MAR15 Meeting of The American Physical Society First-principles study of magnet... more Submitted for the MAR15 Meeting of The American Physical Society First-principles study of magnetic, electronic and optical properties of double perovskite Bi 2 FeMnO 6 1 TOWFIQ AHMED, DZMITRY YAROTSKI, QUANXI JIA, JIAN-XIN ZHU, Los Alamos Natl Lab-We study magnetic, electronic and optical properties of double perovskite Bi2FeMnO6 (BFMO) using density functional theory. In these systems, the exchange interaction between Fe and Mn sites gives rise to a ferrimagnetic ordering, which is captured in our ab initio calculations. Thin film Bi2FeMnO6 (BFMO) are generally grown on substrates such as SrTiO3 and Si. Significant strain has been experimentally observed in BFMO unit cells due to slight lattice mismatch between the thin film and substrate unit cells. In this work, we find that the net magnetic moment in BFMO depends on the "c/a" ratio of the unit cell, suggesting the strain dependence of magnetization in such system. We further calculate x-ray magnetic dichroism (XMCD) signals of Fe and Mn ions in BFMO for L2 and L3 edges. By applying the XMCD sum rules, we adopted an alternative approach to estimate the spin and orbital magnetic moment from our DFT calculations. We find qualitative agreement between our calculated values and the experimental measurements based on different techniques.Moreover, we study spin resolved optical conductivity and density of states in BFMO. These calculations give insight into electronic structure near Fermi energy, and dominant electronic excitations in the valence-conduction region of BFMO.

Research paper thumbnail of Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Scientific Reports, Mar 13, 2015

Research paper thumbnail of Combining electronic structure and many-body theory with large databases: A method for predicting the nature of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>4</mml:mn><mml:mi>f</mml:mi></mml:mrow></mml:math> states in Ce compounds

Physical Review Materials, Aug 28, 2017

Recent progress in materials informatics has opened up the possibility of a new approach to acces... more Recent progress in materials informatics has opened up the possibility of a new approach to accessing properties of materials in which one assays the aggregate properties of a large set of materials within the same class in addition to a detailed investigation of each compound in that class. Here we present the first large scale investigation of electronic properties and correlated magnetism in Ce-based compounds accompanied by a systematic study of the electronic structure and 4f-hybridization function of a large body of Ce compounds. We systematically study the electronic structure and 4f-hybridization function of a large body of Ce compounds with the goal of elucidating the nature of the 4f states and their interrelation with the measured Kondo energy in these compounds. The hybridization function has been analyzed for more than 350 data sets (being part of the IMS data base) of cubic Ce compounds using electronic structure theory that relies on a full-potential approach. We demonstrate that the strength of the hybridization function, evaluated in this way, allows us to draw precise conclusions about the degree of localization of the 4f states in these compounds. The theoretical results are entirely consistent with all experimental information, relevant to the degree of 4f localization for all investigated materials. Furthermore, a more detailed analysis of the electronic structure and the hybridization function allows us to make precise statements about Kondo correlations in these systems. The calculated hybridization functions, together with the corresponding density of states, reproduce the expected exponential behavior of the observed Kondo temperatures and prove a consistent trend in real materials. This trend allows us to predict which systems may be correctly identified as Kondo systems. A strong anti-correlation between the size of the hybridization function and the volume of the systems has been observed. The information entropy for this set of systems is about 0.42. Our approach demonstrates the predictive power of materials informatics when a large number of materials is used to establish significant trends. This predictive power can be used to design new materials with desired properties. The applicability of this approach for other correlated electron systems is discussed.

Research paper thumbnail of A real-time TDDFT study of CDW phase under femtosecond optical pulse in monolayer 2H-NbSe2

Bulletin of the American Physical Society, Mar 5, 2020

Research paper thumbnail of Multiscale Modeling of DNA Translocation through Multiple Nanopores

APS March Meeting Abstracts, 2019

Research paper thumbnail of Temperature dependent vibrational modes of Ammonium Nitrate in terahertz regime

Bulletin of the American Physical Society, Mar 5, 2019

Research paper thumbnail of Temperature dependent vibrational modes of TNT and CL-20 cocrystal in terahertz regime

Bulletin of the American Physical Society, 2020

Research paper thumbnail of Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Scientific reports, 2015

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), compo... more Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution to realize magnetoelectric coupling between ferromagnetic and ferroelectric order parameters. Despite having antiferromagnetic order, BiFeO3 (BFO) has nevertheless been a key material due to excellent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La0.7Sr0.3MnO3 (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO, an insulating superlattice, is demonstrated using polarized neutron reflectometry. Remarkably, the magnetization enables magnetic field to change the dielectric properties of the superlattice, which we cite as an example of synthetic magnetoelectric coupling. Importantly, controlled creation of magnetic moment in BFO is a much needed path toward design and implementation of integrated oxide devices for next generation magnetoelectric...

Research paper thumbnail of Improved Performance of Si Nanowire Lithium Ion Battery Anodes by Au Catalyst Nanodot Formation on TiN

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Dec 2, 2019

Research paper thumbnail of A controlled nucleation and growth of Si nanowires by using a TiN diffusion barrier layer for lithium-ion batteries

Nanoscale advances, 2022

Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Research paper thumbnail of Machine learning and Design of Experiment for High Throughput Synthesis

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Aug 13, 2019

Research paper thumbnail of Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide

ACS Nano, Feb 9, 2022

Advances in epitaxy have enabled the preparation of high-quality material architectures consistin... more Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Research paper thumbnail of Evidence of Hexagonal Germanium Grains on Annealed Monolayer Mos2

Research paper thumbnail of Area-selective deposition of germanium on patterned graphene/monolayer molybdenum disulfide stacks via dipole engineering

APL materials, Mar 1, 2024

Effect of fluorine ion irradiation on the properties of monolayer molybdenum disulfide

Research paper thumbnail of Theoretical prediction of giant Hall high harmonic generation in monolayer NbSe$_2$

arXiv (Cornell University), Oct 3, 2023

et al., "A review on mechanics and mechanical properties of 2d materials-graphene and beyond," Ex... more et al., "A review on mechanics and mechanical properties of 2d materials-graphene and beyond," Extreme Mechanics Letters 13, 42-77 (2017).

Research paper thumbnail of Magnetic, electronic, and optical properties of double perovskite Bi<sub>2</sub>FeMnO<sub>6</sub>

APL Materials, Mar 1, 2017

Research paper thumbnail of Electronic structure and lattice dynamics at the interface of single layer FeSe and SrTiO$_{\mathrm{3}}$

Bulletin of the American Physical Society, Mar 16, 2016

Research paper thumbnail of Edge states and local electronic structure around an adsorbed impurity in a topological superconductor

Physical Review B, Nov 16, 2015

Recently topological superconducting states has attracted a lot of interest. In this work, we con... more Recently topological superconducting states has attracted a lot of interest. In this work, we consider a topological superconductor with Z2 topological mirror order [1] and s±-wave superconducting pairing symmetry, within a two-orbital model originally designed for iron-based superconductivity [2]. We predict the existence of gapless edge states. We also study the local electronic structure around an adsorbed interstitial magnetic impurity in the system, and find the existence of low-energy in-gap bound states even with a weak spin polarization on the impurity. We also discuss the relevance of our results to the recent STM experiment on Fe(Te,Se) compound with adsorbed Fe impurity [3], for which our density functional calculations show the Fe impurity is spin polarized.

Research paper thumbnail of Experimental and theoretical comparison of ion properties from nanosecond laser-produced plasmas of metal targets

Journal of Applied Physics, May 22, 2023

The ion emission properties of laser-produced plasmas as a function of laser intensities between ... more The ion emission properties of laser-produced plasmas as a function of laser intensities between 4-50 GW cm À2 and varying angles with respect to the target normal were investigated. The plasmas were produced by focusing 1064 nm, 6 ns pulses from an Nd:YAG laser on various metal targets. The targets used for this study include Ti, Mo, and Gd (Z ¼ 22; 42; 64). It is noted that all ion profiles are composed of multiple peaks-a prompt emission peak trailed by three ion peaks (ultrafast, fast, and thermal). Experimentally, it is shown that each of these ion peaks follows a unique trend as a function of laser intensity, angle, and distance away from the target. Theoretically, it is shown that simple analytical models can be used to explain the properties of the ions. The variations in the ion velocity and density as a function of laser intensity are found to be in good agreement with theoretical models of sheath acceleration, isothermal self-similar expansion, and ablative plasma flow for various ion peaks.

Research paper thumbnail of f -electron structure database (FESD): A high-throughput data analysis and materials discovery tool for strongly correlated materials

Bulletin of the American Physical Society, Mar 6, 2018

Research paper thumbnail of First-principles study of magnetic, electronic and optical properties of double perovskite Bi$_2$FeMnO$_6$

Bulletin of the American Physical Society, Mar 5, 2015

Submitted for the MAR15 Meeting of The American Physical Society First-principles study of magnet... more Submitted for the MAR15 Meeting of The American Physical Society First-principles study of magnetic, electronic and optical properties of double perovskite Bi 2 FeMnO 6 1 TOWFIQ AHMED, DZMITRY YAROTSKI, QUANXI JIA, JIAN-XIN ZHU, Los Alamos Natl Lab-We study magnetic, electronic and optical properties of double perovskite Bi2FeMnO6 (BFMO) using density functional theory. In these systems, the exchange interaction between Fe and Mn sites gives rise to a ferrimagnetic ordering, which is captured in our ab initio calculations. Thin film Bi2FeMnO6 (BFMO) are generally grown on substrates such as SrTiO3 and Si. Significant strain has been experimentally observed in BFMO unit cells due to slight lattice mismatch between the thin film and substrate unit cells. In this work, we find that the net magnetic moment in BFMO depends on the "c/a" ratio of the unit cell, suggesting the strain dependence of magnetization in such system. We further calculate x-ray magnetic dichroism (XMCD) signals of Fe and Mn ions in BFMO for L2 and L3 edges. By applying the XMCD sum rules, we adopted an alternative approach to estimate the spin and orbital magnetic moment from our DFT calculations. We find qualitative agreement between our calculated values and the experimental measurements based on different techniques.Moreover, we study spin resolved optical conductivity and density of states in BFMO. These calculations give insight into electronic structure near Fermi energy, and dominant electronic excitations in the valence-conduction region of BFMO.

Research paper thumbnail of Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Scientific Reports, Mar 13, 2015

Research paper thumbnail of Combining electronic structure and many-body theory with large databases: A method for predicting the nature of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mn>4</mml:mn><mml:mi>f</mml:mi></mml:mrow></mml:math> states in Ce compounds

Physical Review Materials, Aug 28, 2017

Recent progress in materials informatics has opened up the possibility of a new approach to acces... more Recent progress in materials informatics has opened up the possibility of a new approach to accessing properties of materials in which one assays the aggregate properties of a large set of materials within the same class in addition to a detailed investigation of each compound in that class. Here we present the first large scale investigation of electronic properties and correlated magnetism in Ce-based compounds accompanied by a systematic study of the electronic structure and 4f-hybridization function of a large body of Ce compounds. We systematically study the electronic structure and 4f-hybridization function of a large body of Ce compounds with the goal of elucidating the nature of the 4f states and their interrelation with the measured Kondo energy in these compounds. The hybridization function has been analyzed for more than 350 data sets (being part of the IMS data base) of cubic Ce compounds using electronic structure theory that relies on a full-potential approach. We demonstrate that the strength of the hybridization function, evaluated in this way, allows us to draw precise conclusions about the degree of localization of the 4f states in these compounds. The theoretical results are entirely consistent with all experimental information, relevant to the degree of 4f localization for all investigated materials. Furthermore, a more detailed analysis of the electronic structure and the hybridization function allows us to make precise statements about Kondo correlations in these systems. The calculated hybridization functions, together with the corresponding density of states, reproduce the expected exponential behavior of the observed Kondo temperatures and prove a consistent trend in real materials. This trend allows us to predict which systems may be correctly identified as Kondo systems. A strong anti-correlation between the size of the hybridization function and the volume of the systems has been observed. The information entropy for this set of systems is about 0.42. Our approach demonstrates the predictive power of materials informatics when a large number of materials is used to establish significant trends. This predictive power can be used to design new materials with desired properties. The applicability of this approach for other correlated electron systems is discussed.

Research paper thumbnail of A real-time TDDFT study of CDW phase under femtosecond optical pulse in monolayer 2H-NbSe2

Bulletin of the American Physical Society, Mar 5, 2020

Research paper thumbnail of Multiscale Modeling of DNA Translocation through Multiple Nanopores

APS March Meeting Abstracts, 2019

Research paper thumbnail of Temperature dependent vibrational modes of Ammonium Nitrate in terahertz regime

Bulletin of the American Physical Society, Mar 5, 2019

Research paper thumbnail of Temperature dependent vibrational modes of TNT and CL-20 cocrystal in terahertz regime

Bulletin of the American Physical Society, 2020

Research paper thumbnail of Synthetic magnetoelectric coupling in a nanocomposite multiferroic

Scientific reports, 2015

Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), compo... more Given the paucity of single phase multiferroic materials (with large ferromagnetic moment), composite systems seem an attractive solution to realize magnetoelectric coupling between ferromagnetic and ferroelectric order parameters. Despite having antiferromagnetic order, BiFeO3 (BFO) has nevertheless been a key material due to excellent ferroelectric properties at room temperature. We studied a superlattice composed of 8 repetitions of 6 unit cells of La0.7Sr0.3MnO3 (LSMO) grown on 5 unit cells of BFO. Significant net uncompensated magnetization in BFO, an insulating superlattice, is demonstrated using polarized neutron reflectometry. Remarkably, the magnetization enables magnetic field to change the dielectric properties of the superlattice, which we cite as an example of synthetic magnetoelectric coupling. Importantly, controlled creation of magnetic moment in BFO is a much needed path toward design and implementation of integrated oxide devices for next generation magnetoelectric...

Research paper thumbnail of Improved Performance of Si Nanowire Lithium Ion Battery Anodes by Au Catalyst Nanodot Formation on TiN

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Dec 2, 2019

Research paper thumbnail of A controlled nucleation and growth of Si nanowires by using a TiN diffusion barrier layer for lithium-ion batteries

Nanoscale advances, 2022

Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.

Research paper thumbnail of Machine learning and Design of Experiment for High Throughput Synthesis

OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information), Aug 13, 2019

Research paper thumbnail of Fabrication of a Microcavity Prepared by Remote Epitaxy over Monolayer Molybdenum Disulfide

ACS Nano, Feb 9, 2022

Advances in epitaxy have enabled the preparation of high-quality material architectures consistin... more Advances in epitaxy have enabled the preparation of high-quality material architectures consisting of incommensurate components. Remote epitaxy based on lattice transparency of atomically thin graphene has been intensively studied for cost-effective advanced device manufacturing and heterostructure formation. However, remote epitaxy on nongraphene two-dimensional (2D) materials has rarely been studied even though it has a broad and immediate impact on various disciplines, such as many-body physics and the design of advanced devices. Herein, we report remote epitaxy of ZnO on monolayer MoS2 and the realization of a whispering-gallery-mode (WGM) cavity composed of a single crystalline ZnO nanorod and monolayer MoS2. Cross-sectional transmission electron microscopy and first-principles calculations revealed that the nongraphene 2D material interacted with overgrown and substrate layers and also exhibited lattice transparency. The WGM cavity embedding monolayer MoS2 showed enhanced luminescence of MoS2 and multimodal emission.

Research paper thumbnail of Evidence of Hexagonal Germanium Grains on Annealed Monolayer Mos2