arXiv subjects

Sugata Chowdhury

Publications and source records attributed to Sugata Chowdhury.

34 records · Page 2Linked to original sources

arXiv2022

On the Role of Defects in the Electronic Structure of MnBi$_{2-x}$Sb$_x$Te$_4$

Elemental substitution is a proven method of Fermi level tuning in topological insulators, which is needed for device applications. Through static and time resolved photoemission, we show that in MnBi$_2$Te$_4$, elemental substitution of Bi with Sb indeed tunes the Fermi level towards the bulk band gap, making the material charge neutral at 35\% Sb concentration. For the first time, we are able to directly probe the excited state band structure at this doping level, and their dynamics, which show that the decay channels at the Fermi level are severely restricted. However, elemental substitution widens the surface state gap, which we attribute to the increase in antisite defects resulting from Sb substitution. This hypothesis is supported by DFT calculations that include defects, which show a sensitivity of the topological surface state to their inclusion. Our results emphasize the need for defect control if MnBi$_{2-x}$Sb$_x$Te$_4$ is to be used for device applications.

cond-mat.mtrl-sci↗

arXiv2022

Topological Phases of Mn$A_{2}$$X_{4}$ (A=Bi, Sb; X = Se, Te) under Magnetic Field

The concept of electronic topology and the associated topological protection brings excellent opportunities for developing next-generation devices. Ideally, magnetic topological materials (MTM) should have their Dirac/Weyl points and/or associated mass gaps at the Fermi energy (EF) or be readily tunable such that they can be placed at EF via external perturbations such as electric field gating, chemical substitutions, or doping. Three-dimensional antiferromagnetic (AFM) materials like Mn$Bi_{2}$$X_{4}$ (X=Se, Te) that have strong spin-orbit coupling (SOC) and broken time-reversal symmetry (TRS) due to magnetic ordering have been the subject of enormous interest.. In this work, using density functional theory (DFT), we have studied the electronic properties and topological phases of the first intrinsic magnetic topological insulator family Mn$Bi_{2}$$Te_{4}$ (MBT) in the presence of an external magnetic field. Our calculations reveal that the topological phase of bulk rhombohedral (R$\overline{3}$m) Mn$A_{2}$$X_{4}$ (A = Sb, Bi; X=Se, Te) depends on the spin direction and the chemistry. The antiferromagnetic (AFM) ground state of Mn$Sb_{2}$$Se_{4}$ (MSS) is a trivial insulator, whereas the AFM ground state of Mn$Bi_{2}$$Se_{4}$(MBS) is an Axion insulator. Both materials become nodal point or nodal line Weyl semimetals in the presence of a sufficiently strong external magnetic field. The AFM ground state of Mn$Sb_{2}$$Te_{4}$ (MST) is an Axion insulator. MST is a type-II Weyl semimetal with spins aligned in the $\hat{Z}$-direction, but becomes insulating with an inverted band gap for spins in-plane. Similarly, the AFM phase of MBT is an Axion insulator, but remains insulating with an inverted gap in the ferromagnetic phase. Additionally, we demonstrated the evolution of the topological phase of Mn$Bi_{2}$$Te_{4}$ (MBT) by substituting the Bi atoms with the Sb atoms.

cond-mat.mtrl-sci↗

arXiv2022

Anisotropy of the magnetic and transport properties in EuZn$_2$As$_2$

Several recent studies have shown that the anisotropy in the magnetic structure of \ECA\ plays a significant role in stabilizing the Weyl nodes. To investigate the relationship between magnetic anisotropy and Weyl physics, we present a comparative study between EuZn$_2$As$_2$ and EuCd$_2$As$_2$ that are isostructural but with different magnetic anisotropy. We performed structural analysis, electronic transport, and magnetization experiments on millimeter-sized single crystals of EuZn$_2$As$_2$, and compared the results to those of EuCd$_2$As$_2$. By combining the first principle calculations and neutron diffraction experiment, we identify the magnetic ground state of EuZn$_2$As$_2$ as A-type antiferromagnetic order with a transition temperature ($T_\mathrm{N}$ = 19.6 K) twice that of EuCd$_2$As$_2$. Like EuCd$_2$As$_2$, the negative magnetoresistance of EuZn$_2$As$_2$ is observed after suppressing the resistivity peak at $T_\mathrm{N}$ with increasing fields. However, the anisotropy in both transport and magnetization are much reduced in EuZn$_2$As$_2$. The difference could be ascribed to the weaker spin-orbit coupling, more localized $d$-orbitals, and a larger contribution from the Eu $s$-orbitals in the zinc compound, as suggested by the electronic band calculations. The same band structure effect could be also responsible for the observation of a smaller non-linear anomalous Hall effect in EuZn$_2$As$_2$ compared to EuCd$_2$As$_2$.

cond-mat.str-el↗

arXiv2022

Review of Theoretical and Computational Methods for 2D Materials Exhibiting Charge Density Waves

Two-dimensional (2D) materials that exhibit charge density waves (CDWs) have generated many research endeavors in the hopes of employing their exotic properties for various quantum-based technologies. Early investigations surrounding CDWs were mostly focused on bulk materials. However, applications for quantum devices have since required devices to be constructed from few-layer material to fully utilize the material's properties. This field has greatly expanded over the decades, warranting a focus on the computational efforts surrounding CDWs in 2D materials. In this review, we will cover ground in the following relevant, theory-driven subtopics for TaS2 and TaSe2: summary of general computational techniques and methods, atomic structures, Raman modes, and effects of confinement and dimensionality. Through understanding how the computational methods have enabled incredible advancements in quantum materials, one may anticipate the ever-expanding directions available for continued pursuit as the field brings us through the 21st century.

cond-mat.mtrl-sci↗

arXiv2021

Bonding characteristics of the interfacial buffer layer in epitaxial graphene via density functional theory

Monolayer epitaxial graphene is an appropriate candidate for a wide variety of electronic and optical applications. One advantage of growing graphene on the Si face of SiC is that it develops as a single crystal, as does the layer underneath, commonly referred to as the interfacial buffer layer. The properties of this supporting layer include a band gap, making it of interest to groups seeking to build devices with on-off capabilities. In this work, using density functional theory, we have calculated the bonding characteristics of the buffer layer to the SiC substrate beneath. These calculations were used to determine a periodic length between the covalent bonds acting as anchor points in this interface. Additionally, it is evident that the formation of these anchor points depends on the lattice mismatch between the graphene layer and SiC.

cond-mat.mtrl-sci↗

arXiv2021

Tunable spin polarization and electronic structure of bottom-up synthesized MoSi$_2$N$_4$ materials

Manipulation of spin-polarized electronic states of two-dimensional (2D) materials under ambient conditions is necessary for developing new quantum devices with small physical dimensions. Here, we explore spin-dependent electronic structures of ultra-thin films of recently introduced 2D synthetic materials MSi$_2$Z$_4$ (M = Mo or W and Z = N or As) using first-principles modeling. Stacking of MSi$_2$Z$_4$ monolayers is found to generate dynamically stable bilayer and bulk materials with thickness-dependent properties. When spin-orbit coupling (SOC) is included in the computations, MSi$_2$N$_4$ monolayers display indirect bandgaps and large spin-split states at the $K$ and $K'$ symmetry points at the corners of the Brillouin zone with nearly 100\% spin polarization. The spins are locked in opposite directions along an out-of-the-plane direction at $K$ and $K'$, leading to spin-valley coupling effects. As expected, spin polarization is absent in the pristine bilayers due to the presence of inversion symmetry, but it can be induced via an external out-of-plane electric field much like the case of Mo(W)S$_2$ bilayers. A transition from an indirect to a direct bandgap can be driven by replacing N by As in MSi$_2$(N, As)$_4$ monolayers. Our study indicates that the MSi$_2$Z$_4$ materials can provide a viable alternative to the MoS$_2$ class of 2D materials for valleytronics and optoelectronics applications.

cond-mat.mes-hall↗

arXiv2021

Influence of Dimensionality on the Charge Density Wave Phase of 2H-TaSe$_{2}$

Metallic transition metal dichalcogenides like tantalum diselenide (TaSe$_{2}$) exhibit exciting behaviors at low temperatures, including the emergence of charge density wave (CDW) states. In this work, density functional theory (DFT) is used to investigate how structural, electronic, and Raman spectral properties of the CDW configuration change as a function of thickness. Such findings highlight the influence of dimensionality change (from 2D to 3D) and van der Waals (vdW) interactions on the system properties. The vdW effect is most strongly present in bulk TaSe$_{2}$ in the spectral range 165 cm$^{-1}$ to 215 cm$^{-1}$. The phonons seen in the experimental Raman spectra are compared with the results calculated from the DFT models as a function of temperature and layer number. The matching of data and calculations substantiates the model's description of the CDW structural formation as a function of thickness, which is shown in depth for 1L through 6L systems. These results highlight the importance of understanding interlayer interactions, which are pervasive in many quantum phenomena involving two-dimensional confinement.

cond-mat.mtrl-sci↗

arXiv2021

Topological surface states of MnBi$_2$Te$_4$ at finite temperatures and at domain walls

MnBi$_2$Te$_4$ has recently been the subject of intensive study, due to the prediction of axion insulator, Weyl semimetal, and quantum anomalous Hall insulator phases, depending on the structure and magnetic ordering. Experimental results have confirmed some aspects of this picture, but several experiments have seen zero-gap surfaces states at low temperature, in conflict with expectations. In this work, we develop a first-principles-based tight-binding model that allows for arbitrary control of the local spin direction and spin-orbit coupling, enabling us to accurately treat large unit-cells. Using this model, we examine the behavior of the topological surface state as a function of temperature, finding a gap closure only above the Néel temperature. In addition, we examine the effect of magnetic domains on the electronic structure, and we find that the domain wall zero-gap states extend over many unit-cells. These domain wall states can appear similar to the high temperature topological surface state when many domain sizes are averaged, potentially reconciling theoretical results with experiments.

cond-mat.mtrl-sci↗

arXiv2020

Examining Experimental Raman Mode Behavior in Mono- and Bi-layer 2H-TaSe$_{2}$ via Density Functional Theory

Tantalum diselenide (TaSe$_{2}$) is a metallic transition metal dichalcogenide whose equilibrium structure and vibrational behavior strongly depends on temperature and thickness, including the emergence of charge density wave (CDW) states at very low T. In this work, observed modes for mono- and bi-layer are described across several spectral regions and com-pared to the bulk ones. Such modes, including an experimentally observed forbidden Raman mode and low frequency CDW modes, are then matched to corresponding density functional theory (DFT) predicted vibrations, to unveil their inner working. The excellent match between experimental and computational results justifies the presented vibrational visualizations of these modes. Additional support is provided by experimental phonons seen in Raman spectra as a function of temperature and thickness. These results highlight the importance of understanding interlayer interactions and their effects on mode behaviors.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic Skyrmions in Atomic Thin CrI$_3$ Monolayer

In this letter, we report the visualization of topologically protected spin textures, in the form of magnetic skyrmions, in recently discovered monoatomic-thin two-dimensional CrI$_3$. By combining density functional theory and atomistic spin dynamic simulation, we demonstrate that an application of out-of-plane electric field to CrI$_3$ lattice favors the formation of sub-10 nm skyrmions at 0 K temperature. The spin texture arises due to a strong correlation between magneto-crystalline anisotropy, Dzyaloshinskii-Moriya interaction and the vertical electric field, whose shape and size could be tuned with the magnetic field. Such finding will open new avenues for atomic-scale quantum engineering and precision sensing.

cond-mat.mtrl-sci↗

arXiv2018

Strain Controlled Magnetic and Optical Properties of Monolayer 2H-TaSe$_2$

First-principles calculations are used to probe the effects of mechanical strain on the magnetic and optical properties of monolayer (ML) 2H-TaSe$_2$. A complex dependence of these physical properties on strain results in unexpected behavior, such as ferromagnetism under uniaxial, in-plane, tensile strain and a lifting of the Raman-active E' phonon degeneracy. While ferromagnetism is observed under compression along x-axis and expansion along y-axis, no magnetic order occurs when interchanging the strain direction. The calculations show that the magnetic behavior of the system depends on the exchange within the 5d orbitals of the Ta atoms. The magnetic moment per Ta atom persists even when an additional compressive strain along the z-axis is added to a biaxially-strained ML, which suggests stability of the magnetic order. Exploring the effects of this mechanical strain on the Raman-active phonon modes, we find that the E$^"$ and E' modes are red-shifted due to Ta-Se bond elongation, and that strain lifts the E' mode degeneracy, except for the symmetrical biaxial tensile case. Our results demonstrate the possibility of tuning the properties of 2D-materials for nanoelectronic applications through strain.

cond-mat.mtrl-sci↗

arXiv2018

Rutile TiO2 Bulk Structural and Vibrational Properties: A DFT Study on the Importance of Pseudopotentials

Using first-principal density functional theory (DFT) we explained the importance of pseudopotential for decribing the electronic and vibrational properties of rutile TiO2 (R-TiO2). Calculations were performed using the generalized gradient approximation (GGA), local density approximation (LDA) and hybrid functional (HSE), with normconserving, ultrasoft and projector augmented wave (PAW) pseudopotential. The choice of an appropriate exchange-correlation functionals and the pseudopotential within DFT is critical to explain the different properties of R-TiO2. We found that the lattice parameters predicted by the norm-conserving pseudopotential and GGA are in excellent agreement with experimental data. Our studies reveal that norm-conserving pseudopotentials with GGA exchange-correlations provide much improvement for the prediction of the electronic properties of TiO2. We observed that, using LDA and ultrasoft pseudopotential is an excellent combination to describe the vibrational properties of R-TiO2. Furthermore, we observed the temperature dependent asymmetric nature of vibrational modes due to changes of bond lengths and different lattice vibrations.

physics.chem-ph↗

arXiv2018

Prediction of Weyl semimetal, AFM topological insulator, nodal line semimetal, and Chern insulator phases in Bi2MnSe4

Three dimensional materials with strong spin-orbit coupling and magnetic interactions represent an opportunity to realize a variety of rare and potentially useful topological phases. In this work, we use first principles calculations to show that the recently synthesized material Bi2MnSe4 displays a combination of band inversion and magnetic interactions, leading to several topological phases. Bi2PbSe4, also studied, also displays band inversion and is a topological insulator. In bulk form, the ferromagnetic phase of Bi2MnSe4 is either a nodal line or Weyl semimetal, depending on the direction of the spins. When the spins are arranged in a layered antiferromagnetic configuration, the combination of time reversal plus a partial translation is a new symmetry, and the material instead becomes an antiferromagnetic topological insulator. However, the intrinsic TRS breaking at the surface of Bi2MnSe4 removes the typical Dirac cone feature, allowing the observation of the half-integer quantum anomalous Hall effect (AHC). Furthermore, we show that in thin film form, for some thicknesses, Bi2MnSe4 becomes a Chern insulator with a band gap of up to 58 meV. This combination of properties in a stoichiometric magnetic material makes Bi2MnSe4 an excellent candidate for displaying robust topological behavior.

cond-mat.mtrl-sci↗

arXiv2018

Computational screening of high-performance optoelectronic materials using OptB88vdW and TB-mBJ formalisms

We perform high-throughput density functional theory (DFT) calculations for optoelectronic properties (electronic bandgap and frequency dependent dielectric function) using the OptB88vdW functional (OPT) and the Tran-Blaha modified Becke Johnson potential (MBJ). This data is distributed publicly through JARVIS-DFT database. We used this data to evaluate the differences between these two formalisms and quantify their accuracy, comparing to experimental data whenever applicable. At present, we have 17,805 OPT and 7,358 MBJ bandgaps and dielectric functions. MBJ is found to predict better bandgaps and dielectric functions than OPT, so it can be used to improve the well-known bandgap problem of DFT in a relatively inexpensive way. The peak positions in dielectric functions obtained with OPT and MBJ are in comparable agreement with experiments. The data is available on our websites http://www.ctcms.nist.gov/~knc6/JVASP.html and https://jarvis.nist.gov .

cond-mat.mtrl-sci↗

arXiv2011

Theoretical investigation of the evolution of the topological phase of Bi$_{2}$Se$_{3}$ under mechanical strain

The topological insulating phase results from inversion of the band gap due to spin-orbit coupling at an odd number of time-reversal symmetric points. In Bi$_2$Se$_3$, this inversion occurs at the $Γ$ point. For bulk Bi$_2$Se$_3$, we have analyzed the effect of arbitrary strain on the $Γ$ point band gap using Density Functional Theory. By computing the band structure both with and without spin-orbit interactions, we consider the effects of strain on the gap via Coulombic interaction and spin-orbit interaction separately. While compressive strain acts to decrease the Coulombic gap, it also increases the strength of the spin-orbit interaction, increasing the inverted gap. Comparison with Bi$_2$Te$_3$ supports the conclusion that effects on both Coulombic and spin-orbit interactions are critical to understanding the behavior of topological insulators under strain, and we propose that the topological insulating phase can be effectively manipulated by inducing strain through chemical substitution.

cond-mat.mtrl-sci↗

arXiv2009

Density functional theory study of the structure and energetics of negatively charged pyrrole oligomers

First-principles calculations are used to investigate the electronic properties of neutral and negatively charged n-pyrrole oligomers with n= 2-18. Chains of neutral oligomers are bent while the negatively charged oligomers become almost flat due to accumulation of negative charge at the end monomers. Several isomers of the short oligomers (n < 5) display negative electron affinity, although they are energetically stable. For longer oligomers with n > 5, the electron affinity turns positive, increasing with oligomer length. The doping of 12-pyrrole with lithium atoms is studied, showing that negative oxidation states are possible due to charge transfer from dopant to oligomer at locations close to dopant. These molecular regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid in the C-C backbone conjugation. Additional calculations of neutral and doped polypyrrole are conducted showing that the doped infinite polymer chain displays a substantial reduction of the energy band gap and the appearance of dopant-based bands in the gap.

cond-mat.mes-hall↗