arXiv subjects

Sobhit Singh

Publications and source records attributed to Sobhit Singh.

47 records · Page 3Linked to original sources

arXiv2019

Topology of Triple-Point Metals

We discuss and illustrate the appearance of topological fermions and bosons in triple-point metals, where a band crossing of three electronic bands occurs close to the Fermi level. Topological bosons appear in the phonon spectrum of certain triple-point metals, depending on the mass of atoms that form the binary triple-point metal. We first provide a classification of possible triple-point electronic topological phases possible in crystalline compounds and discuss the consequences of these topological phases, seen in Fermi arcs, topological Lifshitz transitions and transport anomalies. Then we show how the topological phase of phonon modes can be extracted and proven for relevant compounds. Finally, we show how the interplay of electronic and phononic topologies in triple-point metals puts these \textit{metallic} materials into the list of the most efficient \textit{metallic} thermoelectrics known to date.

cond-mat.mtrl-sci↗

arXiv2019

Low energy phases of bilayer Bi predicted by structure search in two dimensions

We employ an ab-initio structure search algorithm to explore the configurational space of Bi in quasi two dimensions. A confinement potential restricts the movement of atoms within a pre-defined thickness during structure search calculations within the minima hopping method to find the stable and metastable forms of bilayer Bi. In addition to recovering the two known low-energy structures (puckered monoclinic and buckled hexagonal), our calculations predict three new structures of bilayer Bi. We call these structures the $α$, $β$, and $γ$ phases of bilayer Bi, which are, respectively, 63, 72, and 83 meV/atom higher in energy than that of the monoclinic ground state, and thus potentially synthesizable using appropriate substrates. We also compare the structural, electronic, and vibrational properties of the different phases. The puckered monoclinic, buckled hexagonal, and $β$ phases exhibit a semiconducting energy gap, whereas $α$ and $γ$ phases are metallic. We notice an unusual Mexican-hat type band dispersion leading to a van Hove singularity in the buckled hexagonal bilayer Bi. Notably, we find symmetry-protected topological Dirac points in the electronic spectrum of the $γ$ phase. The new structures suggest that bilayer Bi provides a novel playground to study distortion-mediated metal-insulator phase transitions.

cond-mat.mtrl-sci↗

arXiv2018

First-principles investigation of graphene/MoS2 bilayer heterostructures using Tkatchenko-Scheffler van der Waals method

Graphene/MoS$_2$ van der Waals (vdW) heterostructures have promising technological applications due to their unique properties and functionalities. Many experimental and theoretical research groups across the globe have made outstanding contributions to benchmark the properties of graphene/MoS$_2$ heterostructures. Even though some research groups have already made an attempt to model the graphene/MoS$_2$ heterostructures using {\it first-principles} calculations, there exists several discrepancies in the results from different theoretical research groups and the experimental findings. In the present work, we revisit this problem by first principles approach and address the existing discrepancies about the interlayer spacing between graphene and MoS$_2$ monolayers in graphene/MoS$_2$ heterostructures, and the location of Dirac points near Fermi-level. We find that the Tkatchenko--Scheffler method efficiently evaluates the long-range vdW interactions and accurately predicts interlayer spacing between graphene and MoS$_2$ sheets. We further investigate the electronic, mechanical and vibrational properties of the optimized graphene/MoS$_2$ heterostructures created using 5$\times$5/4$\times$4 and 4$\times$4/3$\times$3 supercell geometries having different magnitudes of lattice mismatch. The effect of the varying interlayer spacing on the electronic properties of heterostructures is discussed. Our phonon calculations reveal that the interlayer shear and breathing phonon modes, which are very sensitive to the weak vdW interactions, play vital role in describing the thermal properties of the studied systems. The thermodynamic and elastic properties of heterostructures are further discussed. A comparison between our results and the results reported from other research groups is presented.

cond-mat.mtrl-sci↗

arXiv2018

Topological Thermoelectricity in Metals

Recently published discoveries of acoustic and optical mode inversion in the phonon spectrum of certain metals became the first realistic example of non-interacting topological bosonic excitations in existing materials. However, the observable physical and technological use of such topological phonon phases remained unclear. In this work we provide a strong theoretical and numerical evidence that for a class of metallic compounds (known as triple point topological metals), the points in the phonon spectrum, at which three (two optical and one acoustic) phonon modes (bands) cross, represent a well-defined topological material phase, in which the hosting metals have very strong thermoelectric response. The triple point bosonic collective excitations appearing due to these topological phonon band-crossing points significantly suppress the lattice thermal conductivity, making such metals phonon-glass like. At the same time, the topological triple-point and Weyl fermionic quasiparticle excitations present in these metals yield good electrical transport (electron-crystal) and cause a local enhancement in the electronic density of states near the Fermi level, which considerably improves the thermopower. This combination of "phonon-glass" and "electron-crystal" is the key for high thermoelectric performance in metals. We call these materials topological thermoelectric metals and propose several newly predicted compounds for this phase (TaSb and TaBi). We hope that this work will lead researchers in physics and materials science to the detailed study of topological phonon phases in electronic materials, and the possibility of these phases to introduce novel and more efficient use of thermoelectric materials in many everyday technological applications.

cond-mat.mtrl-sci↗

arXiv2018

Proximity-induced topological transition and strain-induced charge transfer in graphene/MoS2 bilayer heterostructures

Graphene/MoS2 heterostructures are formed by combining the nanosheets of graphene and monolayer MoS2. The electronic features of both constituent monolayers are rather well-preserved in the resultant heterostructure due to the weak van der Waals interaction between the layers. However, the proximity of MoS2 induces strong spin orbit coupling effect of strength ~1 meV in graphene, which is nearly three orders of magnitude larger than the intrinsic spin orbit coupling of pristine graphene. This opens a bandgap in graphene and further causes anticrossings of the spin-nondegenerate bands near the Dirac point. Lattice incommensurate graphene/MoS2 heterostructure exhibits interesting moire' patterns which have been observed in experiments. The electronic bandstructure of heterostructure is very sensitive to biaxial strain and interlayer twist. Although the Dirac cone of graphene remains intact and no charge-transfer between graphene and MoS2 layers occurs at ambient conditions, a strain-induced charge-transfer can be realized in graphene/MoS2 heterostructure. Application of a gate voltage reveals the occurrence of a topological phase transition in graphene/MoS2 heterostructure. In this chapter, we discuss the crystal structure, interlayer effects, electronic structure, spin states, and effects due to strain and substrate proximity on the electronic properties of graphene/MoS2 heterostructure. We further present an overview of the distinct topological quantum phases of graphene/MoS2 heterostructure and review the recent advancements in this field.

cond-mat.mtrl-sci↗

arXiv2018

Effect of spin-orbit coupling on the elastic, mechanical, and thermodynamic properties of Bi-Sb binaries

Using first principles calculations, we systematically study the elastic stiffness constants, mechanical properties, elastic wave velocities, Debye temperature, melting temperature, and specific heat of several thermodynamically stable crystal structures of Bi$_{x}$Sb$_{1-x}$ ($0 < x < 1$) binaries, which are of great interest due to their numerous inherent rich properties, such as thermoelectricity, thermomagnetic cooling, strong spin-orbit coupling (SOC) effects, and topological features in the electronic bandstructure. We analyze the bulk modulus ($B$), Young's modulus ($E$), shear modulus ($G$), $B/G$ ratio, and Poisson's ratio ($ν$) as a function of the Bi concentration in Bi$_{x}$Sb$_{1-x}$. The effect of SOC on above mentioned properties is further investigated. In general, we observe that the SOC effects cause elastic softening in most of the studied structures. Three monoclinic structures of Bi-Sb binaries are found to exhibit significantly large auxeticity. The Debye temperature and the magnitude of the elastic wave velocities monotonically decrease with increasing Bi-concentration. We also discuss the specific heat capacity versus temperature data for all studied binaries. Our theoretical results are in excellent agreement with the existing experimental and theoretical data. The comprehensive understanding of the material properties such as hardness, mechanical strength, melting temperature, propagation of the elastic waves, auxeticity, and heat capacity is vital for practical applications of the studied binaries.

cond-mat.mtrl-sci↗

arXiv2017

Size dependence of the effective magnetic anisotropy in Co, Ni, Fe, and magnetite nanoparticles: Testing the core-shell-surface-layer (CSSL) model

The stability of the stored information in recording media depends on the anisotropy energy Ea = KeffV of nanoparticles (NPs) of volume V or diameter D. Therefore, the knowledge of how the effective magnetic anisotropy Keff varies with D for a given system is important for technological applications. In a recent paper [Appl. Phys. Lett. 110 (22), 222409 (2017)], the variation of Keff versus D in NPs of maghemite (γ-Fe2O3) was best described by the Eq.: Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]-3 -1}, where Kb, KS, and Ksh are the anisotropy constants of spins in the core, surface layer, and a shell of thickness d, respectively. This core-shell-surface layer (CSSL) model is an extension of the often used core-surface layer (CSL) model described by Keff = Kb + (6KS/D) [Phys. Rev. Lett. 72, 282 (1994)]. The additional term involving Ksh was found to be necessary to fit the data for smaller NPs of γ-Fe2O3 with D < 5 nm. Here we check the validity of the CSSL model for metallic magnetic NPs of Co, Ni, Fe and magnetite using Keff vs. D data from published literature. Care was taken in selecting data only for those NPs for which the effects of interparticle interactions has been taken into account in determining Keff. The importance of the new CSSL model is that it describes well the Keff vs. D variation for all particles sizes whereas the core-surface layer model often fails for smaller particles with the notable exception of Fe NPs. The verification of the CSSL model for metallic NPs of Co, Ni, and magnetite along with NPs of NiO and γ-Fe2O3 validates its general applicability.

cond-mat.mtrl-sci↗

arXiv2017

A core-shell-surface layer model to explain the size dependence of effective magnetic anisotropy in magnetic nanoparticles

The particle size (D) dependence of the effective magnetic anisotropy Keff of magnetic nanoparticles (NPs) usually shows Keff increasing with decreasing D. This dependence is often interpreted using the Eq.: Keff = Kb + (6Ks/D) where Kb and Ks are the anisotropy constants of the spins in the bulk-like core and surface layer, respectively. Here, we show that this model is inadequate to explain the observed size-dependency of Keff for smaller nanoparticles with D < 5 nm. Instead the results in NPs of maghemite (γ-Fe2O3), NiO and Ni are best described by an extension of the above model leading to the variation given by Keff = Kb + (6Ks/D) +Ksh{[1-(2d/D)]^(-3) -1}, where the last term is due to the spins in a shell of thickness d with anisotropy Ksh. The validation of this core-shell-surface layer (CSSL) model for three different magnetic NPs systems viz. ferrimagnetic γ-Fe2O3, ferromagnetic Ni and antiferromagnetic NiO suggests its possible applicability for all magnetic nanoparticles.

cond-mat.mtrl-sci↗

arXiv2017

Unusual enhancement of effective magnetic anisotropy with decreasing particle size in maghemite nanoparticles

Experimental results and a model are presented to explain the observed unusual enhancement of the effective magnetic anisotropy Keff with decreasing particle size D from 15 nm to 2.5 nm in γ-Fe2O3 nanoparticles (NPs). The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3 and 7.0 nm investigated here, with the results on other sizes taken from literature. Keff is determined from the analysis of the frequency dependence of the blocking temperature TB after considering the effects of interparticle interactions on TB. The data of Keff vs. D is fit to the derived core-shell-based relation: Keff = Kb + (6KS/D) +Ksh{[1-(2d/D)]^(-3) -1}, with Kb = 1.9 x10^5 ergs/cm^3 as the bulk-like contribution of the core, KS = 0.035 ergs/cm^2 as the contribution of the surface layer, and Ksh = 1.057 x10^4 ergs/cm^3 as the contributions of spins in the shell of thickness d = 1.1 nm. This equation represents an extension of the often-used Eq.: Keff = Kb + (6KS/D). Significance of this new result is discussed.

cond-mat.mtrl-sci↗

arXiv2017

Giant tunable Rashba spin splitting in two-dimensional BiSb monolayer and BiSb/AlN heterostructures

Search of novel two-dimensional giant Rashba semiconductors is a crucial step in the development of the forthcoming nano-spintronics technology. Using first-principle calculations, we study a stable two-dimensional crystal phase of BiSb having buckled honeycomb lattice geometry, which is yet unexplored. The phonon, room temperature molecular dynamics and elastic constant calculations verify the dynamical and mechanical stability of the monolayer at 0~K and at room temperature. The calculated electronic bandstructure reveals the direct bandgap semiconducting nature of BiSb monolayer with presence of highly mobile two-dimensional electron gas (2DEG) near Fermi-level. Inclusion of spin-orbit coupling (SOC) yields the giant Rashba spin-splitting of 2DEG near Fermi-level. The calculated Rashba energy and Rashba splitting constant are 13 meV and 2.3 eVÅ, respectively. The strength of the Rashba splitting is amongst the largest yet known 2D Rashba semiconductors. We demonstrate that the strength of the Rashba spin-splitting can be significantly tuned by applying in-plane bi-axial strain on the BiSb monolayer. Presence of the giant Rashba spin-splitting together with the large electronic bandgap (1.6 eV) makes this system of peculiar interest for optoelectronics applications. Furthermore, we study the electronic properties of BiSb/AlN heterostructures having a lattice mismatch of 1.3\% at the interface. Our results suggest that BiSb monolayer and heterostructure systems could be potentially used to develop highly efficient spin field-effect transistors, optoelectronics and nano-spintronics devices. Thus, this comprehensive study of two-dimensional BiSb systems can expand the range of possible applications in the future spintronics technology.

cond-mat.mes-hall↗

arXiv2016

Prediction of a controllable Weyl semi-metallic phase in inversion-asymmetric BiSb

Recent experimental realization of long sought Weyl fermions in non-magnetic crystals has greatly motivated condensed matter physicists to search for materials supporting Weyl fermions. Weyl fermions appear to be very promising for future electronics, often referred as Weyltronics. Here, by means of first-principle calculations, we report a stoichiometric crystal structure of BiSb with broken space-inversion symmetry. This structure is insulating in bulk and has non-trivial band topology. We observe a pressure driven Weyl semi-metallic phase transition in this crystal structure. The obtained Weyl semi-metallic phase exists in the 4.0 - 6.0 GPa pressure range. We find that a total 6 pairs of Weyl points, 6 monopoles and 6 antimonopoles, exist in the Brillouin zone. The Weyl points with opposite chirality are located at different energy values yielding separate electron and hole Fermi-surfaces. Additionally, the spin-texture of the bulk BiSb compound appears to be electrically controllable when the interlink between pressure and an electric field is exploited. This produces novel manipulable topological transport properties in this system which are very promising for implementation of this kind of materials in next-generation Weyltronics and spintronic devices.

cond-mat.mtrl-sci↗