arXiv subjects

Sudipta Roy Barman

Publications and source records attributed to Sudipta Roy Barman.

16 recordsLinked to original sources

arXiv2025

Escape-Induced Temporally Correlated Noise Driven Universality Crossover

Universal behavior in far-from-equilibrium systems is driven by interactions between transport processes and noise structure. The Kardar-Parisi-Zhang (KPZ) framework predicts that extensions incorporating conserved currents or temporally correlated noise give rise to distinct growth morphologies and universality classes, yet direct experimental realization has remained elusive. Here, we report atomically resolved Sn thin-film growth on Sb-doped MnBi$_2$Te$_4$, revealing a sharp dynamical crossover between two fundamentally different regimes. Early stage growth follows conserved KPZ scaling, forming two-dimensional islands and stanene layers. Beyond a critical deposition time, temporally correlated noise dominates, driving the nucleation of $α$ -Sn clusters, their evolution into faceted grains, and coexistence with faceted $β$-Sn. Molecular dynamics simulation and Auger electron spectroscopy show adatom escape as the microscopic origin of temporally correlated noise, providing a microscopic mechanism for the universality crossover. These findings establish, for the first time, that temporal noise correlations can fundamentally alter the scaling class of a growing interface, linking atomistic kinetics to emergent universal behavior.

cond-mat.mtrl-sci↗

arXiv2024

Experimental detection of topological electronic state and large linear magnetoresistance in $SrSn_{4}$ superconductor

While recent experiments confirm the existence of hundreds of topological electronic materials, only a few exhibit the coexistence of superconductivity and a topological electronic state. These compounds attract significant attention in forefront research because of the potential for the existence of topological superconductivity, paving the way for future technological advancements. $SrSn_{4}$ is known for exhibiting unusual superconductivity below the transition temperature ($T_{C}$) of 4.8 K. Recent theory predicts a topological electronic state in this compound, which is yet to be confirmed by experiments. Systematic and detailed studies of the magnetotransport properties of $SrSn_{4}$ and its Fermi surface characterizations are also absent. For the first time, a quantum oscillation study reveals a nontrivial $π$ Berry phase, very light effective mass, and high quantum mobility of charge carriers in $SrSn_{4}$. Magnetotransport experiment unveils large linear transverse magnetoresistance (TMR) of more than 1200% at 5 K and 14 T. Angle-dependent transport experiments detect anisotropic and four-fold symmetric TMR, with the maximum value ($\sim$ 2000%) occurring when the angle between the magnetic field and the crystallographic b-axis is 45 degree. Our results suggest that $SrSn_{4}$ is the first topological material with superconductivity above the boiling point of helium that displays such high magnetoresistance.

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↗

arXiv2024

Intrinsic and extrinsic plasmons in the hard x-ray photoelectron spectra of nearly free electron metals

Collective plasmon excitations in solids that result from the process of photoemission are an important area of fundamental research. In this study, we identify a significant number ($n$) of multiple bulk plasmons ($nω_p$) in the hard x-ray photoelectron spectra of the core levels and valence bands (VBs) of two well-known, nearly free electron metals, aluminum (Al) and magnesium (Mg). On the basis of earlier theoretical works, we estimate the contributions of extrinsic, intrinsic, and interference processes to the intensities of 1$s$ to 2$s$ core level plasmons. The intrinsic contribution diminishes from 22% for 1$ω_p$, to 4.4% for 2$ω_p$, and becomes negligible thereafter (0.5% for 3$ω_p$). The extrinsic and intrinsic plasmon contributions do not vary significantly across a broad range of photoelectron kinetic energies, and also between the two metals (Al and Mg). The interference contribution varies from negative to zero as $n$ increases. An asymmetric line shape is observed for the bulk plasmons, which is most pronounced for 1$ω_p$. Signature of the surface plasmon is detected in normal emission, and it exhibits a significantly increased intensity in the grazing emission. The VB spectra of Al and Mg, which are dominated by $s$-like states, exhibit excellent agreement with the calculated VB based on density functional theory. The VB exhibits four multiple bulk plasmon peaks in the loss region, which are influenced by an intrinsic process in addition to the extrinsic process. On a completely oxidized aluminum surface, the relative intensity of the Al metal bulk plasmon remains nearly unaltered, while the surface plasmon is completely attenuated.

cond-mat.mtrl-sci↗

arXiv2023

Quasiperiodic gallium adlayer on i-Al-Pd-Mn

Using scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and density functional theory (DFT), we demonstrate the formation of quasicrystalline gallium adlayer on icosahedral ($i$)-Al-Pd-Mn. Quasiperiodic motifs are evident in the STM topography images, including the Ga white flower (GaWF) and $τ$ inflated GaWF ($τ$-GaWF), where $τ$ is the golden mean. A larger and more complicated ring motif is also identified, comprised of a bright center and an outer ring of pentagons. The fast Fourier transform of the STM images exhibits distinct quasiperiodic spots, thereby establishing quasiperiodicity on a length scale of $\sim$350 nm. Based on our DFT calculations, the preferred adsorption sites of Ga on i-Al-Pd-Mn are determined to be either the Mn atoms at the center of the Penrose P1 tile or the vertices of the P1 tile containing Pd atoms at the center of a cluster of 5 Al atoms (5-Al). The GaWF motif is modeled by an inner 6 atom Ga cluster (6-Ga) around the central Mn atom and an outer ring of 5 Ga atoms adsorbed at the centers of the 5-Al clusters, both having pentagonal symmetry. The $τ$-GaWF motif is modeled by the 6-Ga arranged on the $τ$-P1 tiling, while the ring motif is modeled by Ga atoms adsorbed at the center of 5-Al clusters above a Pd atom. The side lengths and diameters of the GaWF, $τ$-GaWF, and the ring motifs are $τ$ scaled and show excellent agreement with the DFT-based models. An additional indication of the quasiperiodic characteristics of the Ga monolayer is the 5-fold LEED patterns that were detected throughout the entire range of beam energy that was measured.

cond-mat.mtrl-sci↗

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↗

arXiv2022

Decagonal Sn clathrate on $d$-Al-Ni-Co

Decagonal quasiperiodic ordering of Sn thin film on $d$-Al-Ni-Co, is shown based on scanning tunneling microscopy (STM), low-energy electron diffraction and density functional theory (DFT). Interestingly, the decagonal structural correlations are partially retained up to a large film thickness of 10 nm grown at a 165$\pm$10 K. The nucleation centers called 'Sn white flowers' identified by STM at submonolayer thickness are recognized as valid patches of the decagonal clathrate structure with low adsorption energies. Due to the excellent lattice matching (to within 1%) between columns of Sn dodecahedra in the clathrate structure and pentagonal motifs at the $d$-Al-Ni-Co surface, the interfacial energy favors clathrate over the competing Sn crystalline forms. DFT study of the Sn/Al-Ni-Co composite model shows good mechanical stability, as shown by the work of separation of Sn from Al-Ni-Co slab that is comparable to clathrate self-separation energy. The relaxed surface terminations of the R$_2$T$_4$ clathrate approximant are in self-similarity correspondence with the motifs observed in the STM images from monolayer to thickest Sn film.

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↗

arXiv2020

Unique Dirac and Triple point fermiology in simple transition metals and their binary alloys

Noble metal surfaces (Au, Ag and Cu etc.) have been extensively studied for the Shockley type surface states (SSs). Very recently, some of these Shockley SSs have been understood from the topological consideration, with the knowledge of global properties of electronic structure. In this letter, we show the existence of Dirac like excitations in the elemental noble metal Ru, Re and Os based on symmetry analysis and first principle calculations. The unique SSs driven Fermi arcs have been investigated in details for these metals. Our calculated SSs and Fermi arcs are consistent with the previous transport and photo-emission results. We attribute these Dirac excitation mediated Fermi arc topology to be the possible reasons behind several existing transport anomalies, such as large non-saturating magneto resistance, anomalous Nernst electromotive force and its giant oscillations, magnetic breakdown etc. We further show that the Dirac like excitations in these elemental metal can further be tuned to three component Fermionic excitations, using symmetry allowed alloy mechanism.

cond-mat.mtrl-sci↗

arXiv2020

Quasiperiodic ordering in thick Sn layer on $i$-Al-Pd-Mn: A possible quasicrystalline clathrate

Realization of an elemental solid-state quasicrystal has remained a distant dream so far in spite of extensive work in this direction for almost two decades. Here, we report the discovery of quasiperiodic ordering in a thick layer of elemental Sn grown on icosahedral ($i$)-Al-Pd-Mn. The STM images and the LEED patterns of the Sn layer show specific structural signatures that portray quasiperiodicity but are distinct from the substrate. Photoemission spectroscopy reveals the existence of the pseudogap around the Fermi energy up to the maximal Sn thickness. The structure of the Sn layer is modeled as a novel form of quasicrystalline clathrate on the basis of the following: Firstly, from ab-initio theory, the energy of bulk Sn clathrate quasicrystal is lower than the high temperature metallic $β$-Sn phase, but higher than the low temperature $α$-Sn phase. A comparative study of the free slab energetics shows that surface energy favors clathrate over $α$-Sn up to about 4 nm layer thickness, and matches $β$-Sn for narrow window of slab thickness of 2-3 nm. Secondly, the bulk clathrate exhibits gap opening near Fermi energy, while the free slab form exhibits a pronouced pseudogap, which explains the pseudogap observed in photoemission. Thirdly, the STM images exhibit good agreement with clathrate model. We establish the adlayer-substrate compatibility based on very similar (within 1%) the cage-cage separation in the Sn clathrate and the pseudo-Mackay cluster-cluster separation on the $i$-Al-Pd-Mn surface. Furthermore, the nucleation centers of the Sn adlayer on the substrate are identified and these are shown to be a valid part of the Sn clathrate structure. Thus, based on both experiment and theory, we propose that 4 nm thick Sn adlayer deposited on 5-fold surface of $i$-Al-Pd-Mn substrate is in fact a metastable realization of elemental, clathrate family quasicrystal.

cond-mat.other↗

arXiv2019

Electronic structure of Au-Sn compounds grown on Au(111)

The electronic structure of Au-Sn intermetallic layers of different compositions grown on Au(111) to the thickness of several nanometers has been studied in this work. The layer, interface and the substrate related components in the Au 4$f$ and Sn 4$d$ core-level spectra obtained using x-ray photoelectron spectroscopy (XPS) vary with deposition parameters to reveal the details of the Au-Sn formation. While AuSn is grown by deposition at room temperature, Au rich compounds form as a result of heat treatment through inter diffusion of Au and Sn. Deposition at high temperature forms more Au rich compositions compared to post annealing at the same temperature due to the kinetic energy of the impinging Sn atoms in the former case. Post annealing, on the other hand, stabilizes the bulk phases such as AuSn and Au$_5$Sn and exhibits an activated behavior for transition from the former to the latter with increasing temperature. The XPS valence band spectra of AuSn and Au$_{5}$Sn layers show good agreement with the density functional theory calculation, indicating that these have the bulk structure reported in literature. However, the influence of anti-site defects is observed in Au$_5$Sn. Low energy electron diffraction study reveals that although the AuSn layer is ordered, its top surface is disordered at room temperature. Surface order is obtained by annealing or deposition at elevated temperatures and dispersing bands are observed by angle resolved photoemission spectroscopy. Both electron-like and hole-like bands are evident for the ($\sqrt{3}$$\times$$\sqrt{3}$)R30$^{\circ}$ phase, while a nearly free electron-like parabolic surface state is observed for the p(3$\times$3)R15$^{\circ}$ phase.

cond-mat.mtrl-sci↗

arXiv2019

Enhancement of the density of states at the Fermi level due to oxygen atoms in noble metals

The interaction of oxygen with noble metals such as silver has been an important topic of research for many decades. Here, we show occurrence of a peak in the density of states (DOS) at the Fermi level ($E_F$) when oxygen atoms occupy disordered substitutional positions in noble metals such as Ag, Au or Ag-Au alloy. This results in large enhancement of DOS at $E_F$ with respect to Ag or Au metal. Its origin is attributed to O 2$p$ related disorder broadened flat band that straddles almost all the high symmetry directions of the Brillouin zone. Our work suggests that if a large concentration of disordered oxygen can be realized in nano-structures of noble metals, it may lead to interesting phenomenon.

cond-mat.mtrl-sci↗

arXiv2018

Dirac cone in a non-honeycomb surface alloy

We demonstrate unexpected occurrence of linear bands resembling Dirac cone at the zone-center of Au$_2$Sn surface alloy with $\left( \begin{smallmatrix} 2&1\\ 1&3 \end{smallmatrix} \right)$ surface structure formed by deposition of about 0.9 ML Sn on Au(111) at elevated temperature. The surface exhibits an oblique symmetry with unequal lattice constants making it the first two dimensional surface alloy to exhibit Dirac cone with a non-honeycomb lattice.

cond-mat.mtrl-sci↗

arXiv2018

Influence of anti-site disorder and electron-electron correlations on the electronic structure of CeMnNi$_4$

CeMnNi$_4$ exhibits an unusually large spin polarization, but its origin has baffled researchers for more than a decade. We use bulk sensitive hard x-ray photoelectron spectroscopy (HAXPES) and density functional theory based on the Green's function technique to demonstrate the importance of electron-electron correlations of both the Ni 3$d$ ($U_{Ni}$) and Mn 3$d$ ($U_{Mn}$) electrons in explaining the valence band of this multiply correlated material. We show that Mn-Ni anti-site disorder as well as $U_{Ni}$ play crucial role in enhancing its spin polarization: anti-site disorder broadens a Ni 3$d$ minority-spin peak close to the Fermi level ($E_F$), while an increase in $U_{Ni}$ shifts it towards $E_F$, both leading to a significant increase of minority-spin states at $E_F$. Furthermore, rare occurrence of a valence state transition between the bulk and the surface is demonstrated highlighting the importance of HAXPES in resolving the electronic structure of materials unhindered by surface effects.

cond-mat.mtrl-sci↗

arXiv2014

Magnetic properties and electronic structure of Mn-Ni-Ga magnetic shape memory alloys

Influence of disorder, antisite defects, martensite transition and compositional variation on the magnetic properties and electronic structure of Mn$_2$NiGa and Mn$_{1+x}$Ni$_{2-x}$Ga magnetic shape memory alloys have been studied by using full potential spin-polarized scalar relativistic Korringa-Kohn-Rostocker (FP-SPRKKR) method. Mn$_2$NiGa is ferrimagnetic and its total spin moment increases when disorder in the occupancy of Mn$_{\rm Ni}$ (Mn atom in Ni position) is considered. The moment further increases when Mn-Ga antisite defect[1] is included in the calculation. A reasonable estimate of $T_C$ for Mn$_2$NiGa is obtained from the exchange parameters for the disordered structure. Disorder influences the electronic structure of Mn$_2$NiGa through overall broadening of the density of states and a decrease in the exchange splitting. Inclusion of antisite defects marginally broaden the minority spin partial DOS (PDOS), while the majority spin PDOS is hardly affected. For Mn$_{1+x}$Ni$_{2-x}$Ga where 1$\geq$$x$$\geq$0, as $x$ decreases, Mn$_{\rm Mn}$ moment increases while Mn$_{\rm Ni}$ moment decreases in both austenite and martensite phases. For $x$$\geq$ 0.25, the total moment of the martensite phase is smaller compared to the austenite phase, which indicates possible occurrence of inverse magnetocaloric effect. We find that the redistribution of Ni 3$d$- Mn$_{\rm Ni}$ 3$d$ minority spin electron states close to the Fermi level is primarily responsible for the stability of the martensite phase in Mn-Ni-Ga.

cond-mat.mtrl-sci↗