Sobhit Singh

Giant tunable Rashba spin splitting in a two-dimensional BiSb monolayer and in 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 eV\AA, 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.

Investigation of novel crystal structures of Bi–Sb binaries predicted using the minima hopping method

Semi-conducting alloys BixSb1-x have emerged as a potential candidate for topological insulators and are well known for their novel thermoelectric properties. In this work, we present a systematic study of the low-energy phases of 35 different compositions of BixSb1-x (0 < x < 1) at zero temperature and zero pressure. We explore the potential energy surface of BixSb1-x as a function of Sb concentration by using the ab initio minima hopping structural search method. Even though Bi and Sb crystallize in the same R3[combining macron]m space group, our calculations indicate that BixSb1-x alloys can have several other thermodynamically stable crystal structures. In addition to the configurations on the convex hull, we find a large number of metastable structures which are dynamically stable. The electronic band structure calculations of several stable phases reveal the presence of strong spin-orbit interaction leading to the Rashba-Dresselhaus spin-splitting of bands which is of great interest for spintronics applications. We also find an orthorhombic structure of BiSb in the Imm2 space group which exhibits signatures of type-II Weyl semimetal. Additionally, we have studied the thermoelectric properties of the selected structures. Regarding thermoelectric properties, we find that the compositions which crystallize in the rhombohedral structure exhibit values of the Seebeck coefficient and the power factor similar to that of Bi2Te3 at room temperature, while the theoretical maximum figure of merit (ZeT) is smaller than that of Bi2Te3. We observe enhancement in the thermopower with the increase in the strength of the Rashba-Dresselhaus spin-splitting effect.

Prediction and control of spin polarization in a Weyl semimetallic phase of BiSb

Sobhit Singh, A. C. Garcia-Castro, 3 Irais Valencia-Jaime, 2 Francisco Muñoz, 5 and Aldo H. Romero Department of Physics and Astronomy, West Virginia University, Morgantown, WV-26505-6315, USA Centro de Investigación y Estudios Avanzados del IPN, MX-76230, Querétaro, México Physique Théorique des Matériaux, Université de Liège, B-4000 Sart-Tilman, Belgium Departamento de F́ısica, Facultad de Ciencias, Universidad de Chile, Casilla 653, Santiago, Chile Centro para el Desarrollo de la Nanosciencia y la Nanotechnoloǵıa CEDENNA, Santiago, Chile

Synthesis, structural characterization and magnetic properties of Fe/Pt core-shell nanoparticles

Structural and magnetic properties of Fe/Pt core-shell nanostructure prepared by a sequential reduction process are reported. Transmission electron microscopy shows nearly spherical particles fitting a lognormal size distribution with Do = 3.0 nm and distribution width λD = 0.31. In x-ray diffraction, Bragg lines only from the Pt shell are clearly identified with line-widths yielding crystallite size = 3.1 nm. Measurements of magnetization M vs. T (2 K–350 K) in magnetic fields up to 90 kOe show a blocking temperature TB = 13 K below which hysteresis loops are observed with coercivity HC increasing with decreasing T reaching HC = 750 Oe at 2 K. Temperature dependence of the ac susceptibilities at frequencies fm = 10 Hz–5 kHz is measured to determine the change in TB with fm using the Vogel-Fulcher law. This analysis shows the presence of significant interparticle interaction, the Neel-Brown relaxation frequency fo = 5.3 × 1010 Hz and anisotropy constant Ka = 3.6 × 106 ergs/cm3. A fit of the M vs. H data u...

Size-dependent structural, magnetic, and optical properties of MnCo2O4 nanocrystallites

Finite-size (d = 5.4–112 nm) and surface effects on the structural, optical, and magnetic properties of ferrimagnetic inverse-spinel MnCo2O4 are reported. For d ≥ 87 nm, partial tetragonal distortion of the inverse spinel-lattice was observed. The Curie temperature TC of MnCo2O4 nanostructures, as determined by dc-magnetic susceptibility (χ) measurements, follows a finite-size scaling relation TC(d) = TC(∞)[1−(ξ0/d)λ] with a shift exponent λ = 0.75 ± 0.15 and microscopic correlation length ξ0 = 1.4 ± 0.3 nm, which is consistent with the mean field theory. For T > TC, χ(T) fits Neels expression for the two-sublattice model with antiferromagnetic molecular field (exchange) constants NBB ∼ 85.16 (JBB ∼ 2.94 × 10−22 J), NAB ∼ 110.96 (JAB ∼ 1.91 × 10−22 J), and NAA ∼ 43.8 (JAA ∼ 1.13 × 10−22 J) and asymptotic Curie temperature Ta ∼ 717.63 K. The optical energy bandgap Eg, evaluated from the Kubelka-Munk function ( [ F ( R ∞ ) ℏ ω ] 2 = C2( ℏ ω - Eg)) is blueshifted to 2.4 eV (d ∼ 5.4 nm) from 1.73 eV (d ∼ 112...

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

In magnetic nanoparticles (NPs), the observed increase in the effective magnetic anisotropy Keff with the decrease in particle size D is often interpreted, sometimes unsuccessfully, using the equation Keff = Kb + (6KS/D), where Kb is the bulk-like anisotropy of the core spins and KS is the anisotropy of spins in the surface layer. Here, we test the validity of this relation in γ-Fe2O3 NPs for sizes D from 15 nm to 2.5 nm. The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3, and 7.0 nm investigated here, with results on 14 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. For the γ-Fe2O3 NPs with D < 5 nm, an unusual enhancement of Keff with decreasing D, well above the magnitudes predicted by the above equation, is observed. Instead the variation of Keff vs. D is best described by an extension of the above equation by including Ksh term from spins in a shell of thickness d. Based on this core-shell-surface layer model, the data are fit to the equation Keff = Kb + (6KS/D) + Ksh{[1−(2d/D)]−3−1} with Kb = 1.9 × 105 ergs/cm3, KS = 0.035 ergs/cm2, and Ksh = 1.057 × 104 ergs/cm3 as the contribution of spins in the shell of thickness d = 1.1 nm. Significance of this result is discussed.In magnetic nanoparticles (NPs), the observed increase in the effective magnetic anisotropy Keff with the decrease in particle size D is often interpreted, sometimes unsuccessfully, using the equation Keff = Kb + (6KS/D), where Kb is the bulk-like anisotropy of the core spins and KS is the anisotropy of spins in the surface layer. Here, we test the validity of this relation in γ-Fe2O3 NPs for sizes D from 15 nm to 2.5 nm. The samples include oleic acid-coated NPs with D = 2.5, 3.4, 6.3, and 7.0 nm investigated here, with results on 14 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. For the γ-Fe2O3 NPs with D < 5 nm, an unusual enhancement of Keff with decreasing D, well above the magnitudes predicted by the above equation, is observed. Instead the variation of Keff vs. D is best described by an extension of the above equation by including Ksh term from spins...

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 BixSb1−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 BixSb1−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.

Nature of Magnetic Ordering in Cobalt‐Based Spinels

In this chapter, the nature of magnetic ordering in cobalt‐based spinels Co3O4, Co2SnO4, Co2TiO4, and Co2MnO4 is reviewed, and some new results that have not been reported before are presented. A systematic comparative analysis of various results available in the literature is presented with a focus on how occupation of the different cations on the A‐ and B‐sites and their electronic states affect the magnetic properties. This chapter specifically focuses on the issues related to (i) surface and finite‐size effects in pure Co3O4, (ii) magnetic‐compensation effect, (iii) co‐existence of ferrimagnetism and spin‐glass‐like ordering, (iv) giant coercivity (HC) and exchange bias (HEB) below the glassy state, and (v) sign‐reversal behavior of HEB across the ferri/ antiferromagnetic Néel temperature (TN) in Co2TiO4 and Co2SnO4. Finally, some results on the low‐temperature anomalous magnetic behavior of Co2MnO4 spinels are presented.

The role of surface effects on the optical behavior of nanocrystalline NiO

A detailed analysis of the optical properties of nickel oxide under reduced dimensions has been reported. The role of crystallite size on the optical process of NiO has been demonstrated using the diffuse-reflectance-spectroscopy. A significant increase (5.24 eV) in the optical energy band-gap (Eg) has been observed with decreasing the average crystallite size (4.1 nm) due to the quantum size effect and Burstein-Moss shift reported for the degenerated oxide semiconductors. Moreover, the series of absorption bands observed in the visible and near infrared regime are associated with the inter-valence metal-to-metal (d-d) charge transfer type transitions from the ground state of 3A2g to 1A1g, 3T1g, 1T2g, 3T1g, 1Eg, 1T1g, and 3T2g centered at 3.25 eV, 2.91 eV, 2.7 eV, 2.31 eV, 1.71 eV, 1.17 eV, and 0.88 eV respectively. The position of these inter-valence transitions remains invariant even after reducing the crystallite sizes below the critical size of 20 nm.

Electrostatic potential and valence modulation in La0.7Sr0.3MnO3 thin films

The Mn valence in thin film La0.7Sr0.3MnO3 was studied as a function of film thickness in the range of 1–16 unit cells with a combination of non-destructive bulk and surface sensitive X-ray absorption spectroscopy techniques. Using a layer-by-layer valence model, it was found that while the bulk averaged valence hovers around its expected value of 3.3, a significant deviation occurs within several unit cells of the surface and interface. These results were supported by first principles calculations. The surface valence increases to up to Mn3.7+, whereas the interface valence reduces down to Mn2.5+. The change in valence from the expected bulk value is consistent with charge redistribution due to the polar discontinuity at the film-substrate interface. The comparison with theory employed here illustrates how this layer-by-layer valence evolves with film thickness and allows for a deeper understanding of the microscopic mechanisms at play in this effect. These results offer insight on how the two-dimensional electron gas is created in thin film oxide alloys and how the magnetic ordering is reduced with dimensionality.