arXiv subjects

Abhishek K. Singh

Publications and source records attributed to Abhishek K. Singh.

At least 19 recordsLinked to original sources

arXiv2024

Magnetocaloric effect near room temperature in chromium telluride (Cr2Te3)

Transition metal telluride compositions are explored extensively for their unique magnetic behavior. Since chromium telluride (Cr2Te3) exhibits a near-room-temperature phase transition, the material can be effectively used in applications such as magnetic refrigeration. Compared to existing magnetocaloric materials, Heusler alloys, and rare-earth-based alloys, the large-scale synthesis of Cr2Te3 involves less complexity, resulting in a stable composition. Compared to existing tellurides, Cr2Te3 exhibited a large magnetic entropy change of 2.36 J/kg-K at a very small magnetic field of 0.1 T. The refrigeration capacity (RC) of 160 J/kg was determined from entropy change versus temperature curve. The results were comparable with the existing Cr compounds. The telluride system, Cr2Te3 compared to pure gadolinium, reveals an enhanced room temperature magnetocaloric effect (MCE) with a broad working temperature range. The heating cycle of MCE was successfully visualized using a thermal imaging setup. To confirm the observed magnetic properties of Cr2Te3, first-principles calculations were conducted. Through density functional theory (DFT) studies, we were able to determine both Curie temperature (TC) and Neel temperature (TN) which validated our experimental transitions at the same temperatures. Structural transition was also observed using first principles DFT calculation which is responsible for magnetic behavior.

cond-mat.mtrl-sci↗

arXiv2023

Probing angle dependent thermal conductivity in twisted bilayer MoSe2

Twisted bilayer (t-BL) transition metal dichalcogenides (TMDCs) attracted considerable attention in recent years due to their distinctive electronic properties, which arise due to the moire superlattices that lead to the emergence of flat bands and correlated electron phenomena. Also, these materials can exhibit interesting thermal properties, including a reduction in thermal conductivity. In this article, we report the thermal conductivity of monolayer (1L) and t-BL MoSe2 at some specific twist angles around two symmetric stacking AB (0 degree) and AB' (60 degree) and one intermediate angle 31 (degree) using the optothermal Raman technique. The observed thermal conductivity values are found to be 13, 23, and 30 W m-1K-1 for twist angle = 58 (degree), 31 (degree) and, 3 (degree) respectively, which is well supported by our first-principles calculation results. The reduction in thermal conductivity in t-BL MoSe2 compared to monolayer (38 W m-1K-1) can be explained by the occurrence of phonon scattering caused by the formation of a moire super-lattice. Herein, the emergence of multiple folded phonon branches and modification in the Brillouin zone caused by in-plane rotation are also accountable for the decrease in thermal conductivity observed in t-BL MoSe2. The theoretical phonon lifetime study and electron localization function (ELF) analysis further reveals the origin of angle-dependent thermal conductivity in t-BL MoSe2. This work paves the way towards tuning the angle-dependent thermal conductivity for any bilayer TMDCs system.

cond-mat.mtrl-sci↗

arXiv2023

Interfacial Layers between Ion and Water Detected by Terahertz Spectroscopy

Dynamic fluctuations in hydrogen-bond network of water occur from femto- to nano-second timescale and provides insights into structural/dynamical aspects of water at ion-water interfaces. Employing terahertz spectroscopy assisted with molecular dynamics simulations, we study aqueous chloride solutions of five monovalent cations, namely, Li, Na, K, Rb and Cs. We show that ions modify the behavior of surrounding water molecules and form interfacial layers of water around them with physical properties distinct from that of bulk water. Small cations with high charge densities influence the kinetics of water well beyond the first solvation shell. At terahertz frequencies, we observe an emergence of fast relaxation processes of water with their magnitude following the ionic order Cs>Rb>K>Na>Li, revealing an enhanced population density of weakly coordinated water at ion-water interface. The results shed light on the structure breaking tendency of monovalent cations and provide insights into the properties of ionic solutions at the molecular level.

cond-mat.soft↗

arXiv2021

Strain-Modulated Interlayer Charge and Energy Transfers in MoS2/WS2 Heterobilayer

Excitonic properties in 2D heterobilayers are closely governed by charge transfer (CT) and excitonic energy transfer (ET) at van der Waals interfaces. Various means have been employed to modulate the interlayer CT and ET, including electrical gating and modifying interlayer spacing, but with limited extent in their controllability. Here, we report a novel method to modulate these transfers in MoS2/WS2 heterobilayer by applying compressive strain under hydrostatic pressure. Raman and photoluminescence measurements, combined with density functional theory calculations show pressure-enhanced interlayer interaction of the heterobilayer. Photoluminescence enhancement factor η of WS2 in heterobilayer decreases by five times up to ~4 GPa, suggesting a strong ET, whereas it increases by an order of magnitude at higher pressures and reaches almost unity, indicating enhanced CT. Theoretical calculations show that orbital switching in the conduction bands is responsible for the modulation of the transfers. Our findings provide a compelling approach towards effective mechanical control of CT and ET in 2D excitonic devices.

cond-mat.mtrl-sci↗

arXiv2020

High Entropy Alloy Nanoparticles Decorated, p-type 2D-Molybdenum Disulphide (MoS2) and Gold Schottky Junction Enhanced Hydrogen Sensing

Molybdenum Disulphide (MoS2) is an interesting material which exists in atomically thin multilayers and can be exfoliated into monolayer or a few layers for multiple applications. It has emerged as a promising material for development of such efficient sensors. Here, we have exfoliated and decorated MoS2 flakes with novel, single phase High Entropy Alloy (HEA) nanoparticles using facile and scalable cryomilling technique, followed by sonochemical method. It is found that the decoration of HEA nanoparticles impart surface enhanced Raman scattering effect and reduction in the work function of the material from 4.9 to 4.75 eV as measured by UV photoelectron spectroscopy. This change in the work function results in a schottky barrier between the gold contact and HEA decorated MoS2 flakes as a result of drastic changes in the surface chemical non-stoichiometry. The response to hydrogen gas is studied at temperatures 30 to 100 oC and it shows unusual p-type nature due to surface adsorbed oxygen species. The nanoscale junction formed between HEA and MoS2 shows 10 times increase in the response towards hydrogen gas at 80 oC. The experimental observations are further explained with DFT simulation showing more favourable hydrogen adsorption on HEA decorated MoS2 resulting for enhanced response.

cond-mat.mtrl-sci↗

arXiv2020

Stacking Order Driven Optical Properties and Carrier Dynamics in ReS2

Two distinct stacking orders in ReS2 are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one-unit cell displacement along the a axis. First-principle calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm-1 for AA stacking, and 20 cm-1 for AB stacking, making a simple tool for determining the stacking orders in ReS2. Polarized photoluminescence (PL) reveals that AB stacking possesses blue-shifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular to b axis. Our findings underscore the stacking-order driven optical properties and carrier dynamics of ReS2, mediate many seemingly contradictory results in literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order.

cond-mat.mtrl-sci↗

arXiv2019

Carrier dynamics in ultrathin gold nanowires: Role of Auger processes

Carrier dynamics in metallic nanostructures is strongly influenced by their confining dimensions. Gold nanoparticles of size $\sim 2$ nm lie at the boundary separating metallic and non-metallic behavior. In this work, we report carrier dynamics in high aspect ratio ultrathin gold nanowires (Au-UNWs) of average diameter $\sim$2 nm using pump (3.1 eV) and coherent white light continuum as probe in the spectral range of 1.15 eV to 2.75 eV. The transient carrier dynamics in Au-UNWs under extreme excitation regime is slower than predicted by the often used two temperature model. We identify Auger-assisted carrier heating process which slows down the hot carrier cooling dynamics. The rate equation model fitted to the data yields an estimate of Auger coefficient for gold nanostructures.

cond-mat.mes-hall↗

arXiv2019

Recent advances in MXenes: from fundamentals to applications

The family of MAX phases and their derivative MXenes are continuously growing in terms of both crystalline and composition varieties. In the last couple of years, several breakthroughs have been achieved that boosted the synthesis of novel MAX phases with ordered double transition metals and, consequently, the synthesis of novel MXenes with a higher chemical diversity and structural complexity, rarely seen in other families of two-dimensional (2D) materials. Considering the various elemental composition possibilities, surface functional tunability, various magnetic orders, and large spin$-$orbit coupling, MXenes can truly be considered as multifunctional materials that can be used to realize highly correlated phenomena. In addition, owing to their large surface area, hydrophilicity, adsorption ability, and high surface reactivity, MXenes have attracted attention for many applications, e.g., catalysts, ion batteries, gas storage media, and sensors. Given the fast progress of MXene-based science and technology, it is timely to update our current knowledge on various properties and possible applications. Since many theoretical predictions remain to be experimentally proven, here we mainly emphasize the physics and chemistry that can be observed in MXenes and discuss how these properties can be tuned or used for different applications.

cond-mat.mtrl-sci↗

arXiv2018

Topological phases in hydrogenated gallenene and in its group elements

Nontrivial topology of Dirac (DSMs), Weyl (WSMs) and nodal line semimetals (NLSMs) are delineated by the novel band crossings near the Fermi level in the bulk and the appearance of exotic surface states. Among them, nodal line semimetals have gained immense interest due to the formation of one-dimensional nodal ring near the Fermi level. Using density funtional theory (DFT) calculations, we report that two dimensional (2D) NLSM phase can be hosted on hydrogen passivated (010) surface of gallium (gallenene) and on other group 13 elements, without inclusion of spin-orbit coupling (SOC). NLSM in these 2D systems is protected by the presence of crystalline (CS) along with inversion (IS) and the time reversal symmetry (TRS). In the presence of SOC, aluminane preserved its topological NLSM phase while in other single layered group 13 elements, a gap opened at the nodal point due to relatively stronger SOC effect. On applying tensile strain along with the inclusion of SOC, hydrogen passivated gallenene (gallenane) evolves into quantum spin Hall insulator with an indirect bandgap of 28 meV. The appearance of long range dissipationless linearly dispersive helical edge states and a large gap in gallenane make it promising for room temperature spintronics applications.

cond-mat.mtrl-sci↗

arXiv2018

Pressure-Induced Topological Phase Transitions in CdGeSb$_2$ and CdSnSb$_2$

Topological quantum phase transitions (TQPTs) in a material induced by external perturbations are often characterized by band touching points in the Brillouin zone. The low-energy excitations near the degenerate band touching points host different types of fermions while preserving the topological protection of surface states. An interplay of different tunable topological phases offers an insight into the evolution of the topological character. In this paper, we study the occurrence of TQPTs as a function of hydrostatic pressure in CdGeSb$_2$ and CdSnSb$_2$ chalcopyrites, using the first-principles calculations. At ambient pressure, both materials are topological insulators having a finite band gap with inverted order of Sb-$s$ and Sb-$p_x$,$p_y$ orbitals of valence bands at the $Γ$ point. On the application of hydrostatic pressure the band gap reduces, and at the critical point of the phase transition, these materials turn into Dirac semimetals. On further increasing the pressure beyond the critical point, the band inversion is reverted making them trivial insulators. The pressure-induced change in band topology from non-trivial to trivial phase is also captured by Lüttinger model Hamiltonian calculations. Our model demonstrates the critical role played by a pressure-induced anisotropy in frontier bands in driving the phase transitions. These theoretical findings of peculiar coexistence of multiple topological phases in the same material provide a realistic and promising platform for the experimental realization of the TQPT.

cond-mat.mtrl-sci↗

arXiv2018

Multiple Triple-Point Fermions in Heusler Compounds

Using the density functional theoretical calculations, we report a new set of topological semimetals X$_{2}$YZ (X = \{Cu, Rh, Pd, Ag, Au, Hg\}, Y = \{Li, Na, Sc, Zn, Y, Zr, Hf, La, Pr, Pm, Sm, Tb, Dy, Ho, Tm\} and Z =\{Mg, Al, Zn, Ga, Y, Ag, Cd, In, Sn, Ta, Sm\}), which show the existence of multiple topological triple point fermions along four independent $C_{3}$ axes. These fermionic quasiparticles have no analogues elementary particle in the standard model. The angle-resolved photoemission spectroscopy is simulated to obtain the exotic topological surface states and the characteristic Fermi arcs. The inclusion of spin-orbit coupling splits the triple-point into two Dirac points. The triple-point fermions are exhibited on the easily cleavable (111) surface and are well separated from the surface $\barΓ$ point, allowing them to be resolved in the surface spectroscopic techniques. This intermediate linearly dispersive degeneracy between Weyl and Dirac points may offer prospective candidates for quantum transport applications.

cond-mat.mtrl-sci↗

arXiv2017

Topological semimetals with nodal-rings and nexus fermions

Based on the first-principles study, we report a new set of topological semimetals (TiS, TiSe, TiTe, HfS, HfSe, HfTe and ZrS) which show the co-existence of a nodal-ring and triply-degenerate points. The two-fold degenerate one-dimensional nodal ring structure in the bulk Brillouin zone exhibits the characteristic drumhead surface states. In addition to this, a peculiar band crossing along the $k_z$ direction takes place consisting of a point-crossing with three-fold band degeneracy. These triply-degenerate points give rise to "nexus" fermions as quasiparticles having no analogous elementary particle of the standard model. In this article, we simulate angle-resolved photoemission spectroscopy to obtain the exotic topological surface states and the characteristic Fermi arcs, and explain the evolution and separation of triple-points with the magnitude of spin-orbit coupling. This intermediate linearly dispersive degeneracy between Weyl and Dirac points may offer prospective candidates for quantum transport applications.

cond-mat.mtrl-sci↗

arXiv2017

Giant Thermal Conductivity Enhancement in Multilayer MoS2 under Highly Compressive Strain

Multilayer MoS2 possesses highly anisotropic thermal conductivities along in-plane and cross-plane directions that could hamper heat dissipation in electronics. With about 9% cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed about 12 times increase in the cross-plane thermal conductivity of multilayer MoS2. Our experimental and theoretical studies reveal that this drastic change arises from the greatly strengthened interlayer interaction and heavily modified phonon dispersions along cross-plane direction, with negligible contribution from electronic thermal conductivity, despite its enhancement of 4 orders of magnitude. The anisotropic thermal conductivity in the multilayer MoS2 at ambient environment becomes almost isotropic under highly compressive strain, effectively transitioning from 2D to 3D heat dissipation. This strain tuning approach also makes possible parallel tuning of structural, thermal and electrical properties, and can be extended to the whole family of 2D Van der Waals solids, down to two layer systems.

cond-mat.mtrl-sci↗

arXiv2017

Topologically nontrivial electronic states in CaSn$_{3}$

Based on the first-principles calculations, we theoretically propose topologically non-trivial states in a recently experimentally discovered superconducting material CaSn$_3$. When the spin-orbit coupling (SOC) is ignored, the material is a host to three-dimensional topological nodal-line semimetal states. Drumhead like surface states protected by the coexistence of time-reversal and mirror symmetry emerge within the two-dimensional regions of the surface Brillouin zone connecting the nodal lines. When SOC is included, unexpectedly, each nodal line evolves into two Weyl nodes (W1, W2) in this centrosymmetric material. Berry curvature calculations show that these nodes occur in a pair and act as either a source or sink of Berry flux. The material also has unique surface states in the form of Fermi arcs, which unlike other known Weyl semimetal, form closed loops of surface states on the Fermi surface. Our theoretical realization of topologically non-trivial states in a superconducting material paves the way towards unraveling the interconnection between topological physics and superconductivity.

cond-mat.mtrl-sci↗

arXiv2017

Half-Heusler alloy LiBaBi: A new topological semimetal with five-fold band degeneracy

Based on first-principles study, we report the finding of a new topological semimetal LiBaBi in half-Heusler phase. The remarkable feature of this nonmagnetic, inversion-symmetry-breaking material is that it consists of only simple $s$- and $p$-block elements. Interestingly, the material is ordinary insulator in the absence of spin-orbit coupling (SOC) and becomes nodal-surface topological semimetal showing drumhead states when SOC is included. This is in stark contrast to other nodal-line and nodal-surface semimetals, where the extended nodal structure is destroyed once SOC is included. Importantly, the linear band crossings host three-, four-, five- and six-fold degeneracies near the Fermi level, making this compound very attractive for the study of `unconventional' fermions. The band crossing points form a three-dimensional nodal structure around the zone center at the Fermi level. We identify the surface states responsible for the appearance of the drumhead states. The alloy also shows a phase transition from topological semimetal to a trivial insulator on application of pressure. In addition to revealing an intriguing effect of SOC on the nodal structure, our findings introduce a new half-Heusler alloy in the family of topological semimetals, thus creating more avenues for experimental exploration.

cond-mat.mtrl-sci↗

arXiv2016

Diffusive nature of thermal transport in stanene

Using the phonon Boltzmann transport formalism and density functional theory based calculations, we show that stanene has a low thermal conductivity. For a sample size of 1$\times$1 $μ$m$^{2}$ ($L\times W$), the lattice thermal conductivities along the zigzag and armchair directions are 10.83 W/m-K and 9.2 W/m-K respectively, at room temperature, indicating anisotropy in the thermal transport. The low values of thermal conductivity are due to large anharmonicity in the crystal resulting in high Grüneisen parameters, and low group velocities. The room temperature effective phonon mean free path is found to be around 17 nm indicating that the thermal transport in stanene is completely diffusive in nature. Furthermore, our study brings out the relative importance of the contributing phonon branches and reveals that, at very low temperatures, the contribution to lattice thermal conductivity comes from the flexural acoustic (ZA) branch and at higher temperatures it is dominated by the longitudinal acoustic (LA) branch. We also show that lattice thermal conductivity of stanene can further be reduced by tuning the sample size and creating rough surfaces at the edges. Such tunability in the lattice thermal conductivity in stanene suggests its applications in thermoelectric devices.

cond-mat.mes-hall↗

arXiv2015

From semiconductor to metal: A reversible tuning of electronic properties of mono to multilayered SnS$_\mathrm{2}$ under applied strain

Controlled variation of the electronic properties of 2D materials by applying strain has emerged as a promising way to design materials for customized applications. Using first principles density functional theory calculations, we show that while the electronic structure and indirect band gap of SnS$_\mathrm{2}$ do not change significantly with the number of layers, they can be reversibly tuned by applying biaxial tensile (BT), biaxial compressive (BC), and normal compressive (NC) strains. Mono to multilayered SnS$_\mathrm{2}$ exhibit a reversible semiconductor to metal transition (S-M) at strain values of 0.17, $-$0.26, and $-$0.24 under BT, BC, and NC strains, respectively. Due to weaker interlayer coupling, the critical strain value required to achieve S-M transition in SnS$_\text{2}$ under NC strain is much higher than for MoS$_\mathrm{2}$. The S-M transition for BT, BC, and NC strains is caused by the interaction between the S-$p_z$ and Sn-$s$, S-$p_x$/$p_y$ and Sn-$s$, and S-$p_z$ and Sn-$s$ orbitals, respectively.

cond-mat.mtrl-sci↗

arXiv2015

Non-perturbative quantum effects in stringy degenerate geometries: Vacuum created pair of $(D{\bar D})_3$-brane by a two form

We obtain axionic charged primordial black holes on a vacuum created gravitational pair of $(3{\bar 3})$-brane by the Kalb-Ramond field on a $D_4$-brane in presence of a background open string metric. The new geometries on an emergent pair of $(3{\bar 3})$-brane universe is shown to be influenced by the non-perturbative quantum effects underlying a geometric torsion in a second order formalism. The presence of small extra dimensions transverse to the pair in the formalism ensures dynamical scalar fields hidden to a $3$-brane or an anti 3-brane universe. We investigate the non-perturbative quantum vacua for their characteristic properties to explain the accelerated expansion of our universe. Interestingly the emergent brane universe is shown to possess its origin in a degenerate stringy de Sitter vacua at an early epoch. A non-perturbative $D_p$-brane world volume correction for $p < 3$ is worked out to explain some of the quantum effects underlying a quintessence axion in the string-brane setup. Our analysis reveals that a $D$-instanton can be a potential candidate to source the dark energy in our brane universe.

hep-th↗