arXiv subjects

Joydipto Bhattacharya

Publications and source records attributed to Joydipto Bhattacharya.

8 recordsLinked to original sources

arXiv2025

Strain induced stabilization of high symmetry phase in MAPbBr3 perovskite

Structural phase transitions in the organic inorganic metal halide perovskites are driven via rearrangement of methylammonium cation and distortion in the PbX6 octahedra. Compositional tuning is usually incorporated for suppression of the structural phase transition in these systems with cation or anion tuning. Along with the compositional tuning, behaviour of strain present in the system can also lead to stabilization of single phase in these systems. In the present investigation, two different samples of CH3NH3PbBr3 perovskite were studied and it is observed that structural phase transition is absent for one of the sample while it is present in the other sample. The non-observance of structural phase transition and stabilization of single phase has been attributed to the perceived tensile strain in the system contrary to the compressive strain observed in the system showing structural phase transitions. This observation is further supported by theoretical calculations. Extended X-ray absorption fine structure measurements revealed distorted octahedra with varying bond lengths along planar and axial directions in both samples, along with observed increase in bond lengths in one of the sample. This stabilization of the cubic phase can enhance device performance and increase overall environmental stability, making these systems more effective for practical applications.

cond-mat.mtrl-sci↗

arXiv2024

Growth of bilayer stanene on a magnetic topological insulator aided by a buffer layer

Stanene, a two-dimensional counterpart to graphene, has the potential to exhibit novel quantum phenomena when grown on a magnetic topological insulator (MTI). This work demonstrates the formation of up to bilayer stanene on 30\% Sb-doped MnBi$_{2}$Te$_4$ (MBST), a well known MTI, albeit with a buffer layer (BL) in between. Angle-resolved photoemission spectroscopy (ARPES), when combined with density functional theory (DFT), reveals stanene related bands such as two hole-like bands and an inverted parabolic band around the $\overlineΓ$ point. An outer hole-like band traverses the Fermi level (\ef) and gives rise to a hexagonal Fermi surface, showing that stanene on MBST is metallic. In contrast, a bandgap of 0.8 eV is observed at the $\overline{K}$ point. We find that DFT shows good agreement with ARPES only when the BL and hydrogen passivation of the top Sn layer are considered in the calculation. Scanning tunneling microscopy (STM) establishes the honeycomb buckled structure of stanene. A stanene-related component is also detected in the Sn $d$ core level spectra, in addition to a BL-related component. The BL, which forms because of the chemical bonding between Sn and the top two layers of MBST, has an ordered crystal lattice with random anti-site defects. The composition of the BL is estimated to be Sn:Te:Bi/Sb $\approx$ 2:1:1 from x-ray photoelectron spectroscopy. Low energy electron diffraction shows that the lattice constant of stanene is marginally larger than that of MBST, and the STM result aligns with this. The BL bridges this disparity and provides a platform for stanene growth.

cond-mat.mtrl-sci↗

arXiv2024

Kramers nodal line in the charge density wave state of YTe$_3$ and the influence of twin domains

Recent studies have focused on the relationship between charge density wave (CDW) collective electronic ground states and nontrivial topological states. Using angle-resolved photoemission and density functional theory, we establish that YTe$_3$ is a CDW-induced Kramers nodal line (KNL) metal, a newly proposed topological state of matter. YTe$_3$ is a non-magnetic quasi-2D chalcogenide with a CDW wave vector ($q_{\rm cdw}$) of 0.2907c$^*$. Scanning tunneling microscopy and low energy electron diffraction revealed two orthogonal CDW domains, each with a unidirectional CDW and similar YTe$_3$. The effective band structure (EBS) computations, using DFT-calculated folded bands, show excellent agreement with ARPES because a realistic x-ray crystal structure and twin domains are considered in the calculations. The Fermi surface and ARPES intensity plots show weak shadow bands displaced by $q_{\rm cdw}$ from the main bands. These are linked to CDW modulation, as the EBS calculation confirms. Bilayer split main and shadow bands suggest the existence of crossings, according to theory and experiment. DFT bands, including spin-orbit coupling, indicate a nodal line along the $Σ$ line from multiple band crossings perpendicular to the KNL. Additionally, doubly degenerate bands are only found along the KNL at all energies, with some bands dispersing through the Fermi level.

cond-mat.mtrl-sci↗

arXiv2023

Coherent Tunneling and Strain Sensitivity of an All Heusler Alloy Magnetic Tunneling Junction: A First-Principles Study

Half-metallic Co-based full Heusler alloys have captured considerable attention of the researchers in the realm of spintronic applications, owing to their remarkable characteristics such as exceptionally high spin polarization at Fermi level, ultra-low Gilbert damping, and high Curie temperature. In this comprehensive study, employing density functional theory, we delve into the stability and electron transport properties of a magnetic tunneling junction (MTJ) comprising a Co$_2$MnSb/HfIrSb interface. Utilizing a standard model given by Julliere, we estimate the tunnel magnetoresistance (TMR) ratio of this heterojunction under external electric field, revealing a significantly high TMR ratio (500%) that remains almost unaltered for electric field magnitudes up to 0.5 V/A. In-depth investigation of K-dependent majority spin transmissions uncovers the occurrence of coherent tunneling for the Mn-Mn/Ir interface, particularly when a spacer layer beyond a certain thickness is employed. Additionally, we explore the impact of bi-axial strain on the MTJ by varying the in-plane lattice constants between -4% and +4%. Our spin-dependent transmission calculations demonstrate that the Mn-Mn/Ir interface manifests strain-sensitive transmission properties under both compressive and tensile strain, and yields a remarkable three-fold increase in majority spin transmission under tensile strain conditions. These compelling outcomes place the Co2MnSb/HfIrSb junction among the highly promising candidates for nanoscale spintronic devices, emphasizing the potential significance of the system in the advancement of the field.

physics.app-ph↗

arXiv2023

Bulk Electronic Structure of Ni2MnGa studied by Density Functional Theory and Hard X-ray Photoelectron Spectroscopy

A combined study employing density functional theory (DFT) using the experimentally determined modulated structures and bulk-sensitive hard x-ray photoelectron spectroscopy on single-crystalline Ni$_2$MnGa is presented in this work. For the aforementioned modulated structures, all of the characteristic features in the experimental valence band (VB) are in excellent agreement with the theoretical VB calculated from DFT, evincing that it is the true representation of Ni$_2$MnGa in the martensite phase. We establish the existence of a charge density wave (CDW) state in the martensite phase from the shape of the VB near $E_F$ that shows a transfer of spectral weight in excellent agreement with DFT. Furthermore, presence of a pseudogap is established by fitting the near $E_F$ region with a power law function predicted theoretically for the CDW phase. Thus, the present work emphasizes that the atomic modulation plays an important role in hosting the CDW phase in bulk stoichiometric Ni$_2$MnGa.

cond-mat.mtrl-sci↗

arXiv2023

Ab initio Prediction of Mechanical, Electronic, Magnetic and Transport Properties of Bulk and Heterostructure of a Novel Fe-Cr based Full Heusler Chalcogenide

Using electronic structure calculations based on density functional theory, we predict and study the structural, mechanical, electronic, magnetic and transport properties of a new full Heusler chalcogenide, namely, Fe$_2$CrTe, both in bulk and heterostructure form. The system shows a ferromagnetic and half-metallic(HM) like behavior, with a very high (about 95%) spin polarization at the Fermi level, in its cubic phase. Interestingly, under tetragonal distortion, a clear minimum (with almost the same energy as the cubic phase) has also been found, at a c/a value of 1.26, which, however, shows a ferrimagnetic and fully metallic nature. The compound has been found to be dynamically stable in both the phases against the lattice vibration. The elastic properties indicate that the compound is mechanically stable in both the phases, following the stability criteria of the cubic and tetragonal phases. The elastic parameters unveil the mechanically anisotropic and ductile nature of the alloy system. Due to the HM-like behavior of the cubic phase and keeping in mind the practical aspects, we probe the effect of strain as well as substrate on various physical properties of this alloy. Transmission profile of the Fe$_2$CrTe/MgO/Fe$_2$CrTe heterojunction has been calculated to probe it as a magnetic tunneling junction (MTJ) material in both the cubic and tetragonal phases. Considerably large tunneling magnetoresistance ratio (TMR) of 1000% is observed for the tetragonal phase, which is found to be one order of magnitude larger than that of the cubic phase.

cond-mat.mtrl-sci↗

arXiv2022

Charge density wave induced nodal lines in LaTe$_3$

LaTe$_3$ is a noncentrosymmetric (NC) material with time reversal (TR) symmetry in which the charge density wave (CDW) is hosted by the Te bilayers. Here, we show that LaTe$_3$ hosts a Kramers nodal line (KNL), a twofold degenerate nodal line that connects the TR invariant momenta in NC achiral systems, using angle resolved photoemission spectroscopy (ARPES), density functional theory (DFT), effective band structure (EBS) calculated by band unfolding, and symmetry arguments. DFT incorporating spin-orbit coupling (SOC) reveals that the KNL -- protected by the TR and lattice symmetries -- imposes gapless crossings between the bilayer-split CDW-induced shadow bands and the main bands. In excellent agreement with the EBS, ARPES data corroborate the presence of the KNL and show that the crossings traverse the Fermi level. Furthermore, spinless nodal lines - entirely gapped out by the SOC - are formed by the linear crossings of the shadow and main bands with a high Fermi velocity.

cond-mat.mtrl-sci↗

arXiv2021

Electron spin relaxation in X-valley of indirect bandgap AlxGa1-xAs: A new horizon for the realization of next generation spin-photonic devices

GaAs/AlGaAs quantum well (QW) system is utilized to investigate the electron spin relaxation in the satellite X-valley of indirect band gap Al0.63Ga0.37As epitaxial layers through polarization resolved photo-luminescence excitation spectroscopy. Solving the rate equations, steady state electronic distribution in various valleys of AlxGa1-xAs is estimated against continues photo carrier generation and an expression for the degree of circular polarization (DCP) of photoluminescence coming from the adjacent quantum well (QW) is derived. Amalgamating the experimental results with analytical expressions, the X-valley electron spin relaxation time (τ_S^X) is determined to be 2.7 +/- 0.1 ps at 10 K. To crosscheck its validity, theoretical calculations are performed based on Density Functional Theory within the framework of the projector augmented wave method, which support the experimental result quite well. Further, temperature dependence of τ_S^X is studied over 10-80 K range, which is explained by considering the intra-valley scattering of carriers in the X-valley of indirect band gap AlGaAs barrier layer. It is learnt that the strain induced modification of band structure lifts the degeneracy in X-valley, which dominates the electron spin relaxation beyond 50 K. Furthermore, the DCP spectra of hot electrons in indirect band gap AlGaAs layers is found to be significantly different compared to that of direct bandgap AlGaAs. It is understood as a consequence of linear k dependent Dresselhaus spin splitting and faster energy relaxation procedure in the X-valley. Findings of this work could provide a new horizon for the realization of next generation spin-photonic devices which are less sensitive to Joule heating.

cond-mat.mtrl-sci↗