arXiv subjects

B. N. Dev

Publications and source records attributed to B. N. Dev.

At least 19 recordsLinked to original sources

arXiv2024

Two superconducting thin films systems with potential integration of different quantum functionalities

Quantum computation based on superconducting circuits utilizes superconducting qubits with Josephson tunnel junctions. Engineering high-coherence qubits requires materials optimization. In this work, we present two superconducting thin film systems, grown on silicon (Si), and one obtained from the other via annealing. Cobalt (Co) thin films grown on Si were found to be superconducting [EPL 131 (2020) 47001]. These films also happen to be a self-organised hybrid superconductor/ferromagnet/superconductor (S/F/S) structure. The S/F/S hybrids are important for superconducting $\pi$-qubits [PRL 95 (2005) 097001] and in quantum information processing. Here we present our results on the superconductivity of a hybrid Co film followed by the superconductivity of a CoSi$_2$ film, which was prepared by annealing the Co film. CoSi$_2$, with its $1/f$ noise about three orders of magnitude smaller compared to the most commonly used superconductor aluminium (Al), is a promising material for high-coherence qubits. The hybrid Co film revealed superconducting transition temperature $T_c$ = 5 K and anisotropy in the upper critical field between the in-plane and out-of-plane directions. The anisotropy was of the order of ratio of lateral dimensions to thickness of the superconducting Co grains, suggesting a quasi-2D nature of superconductivity. On the other hand, CoSi$_2$ film showed a $T_c$ of 900 mK. In the resistivity vs. temperature curve, we observe a peak near $T_c$. Magnetic field scan as a function of $T$ shows a monotonic increase in intensity of this peak with temperature. The origin of the peak has been explained in terms of parallel resistive model for the particular measurement configuration. Although our CoSi$_2$ film contains grain boundaries, we observed a perpendicular critical field of 15 mT and a critical current density of 3.8x10$^7$ A/m$^2$, comparable with epitaxial CoSi$_2$ films.

cond-mat.supr-con↗

arXiv2024

Effect of UHV annealing on morphology and roughness of sputtered $Si(111)-(7\times7)$ surfaces

$Ar^+$ ion has been used regularly for the cleaning of semiconductor, metal surfaces for epitaxial nanostructures growth. We have investigated the effect of low-energy $Ar^+$ ion sputtering and subsequent annealing on the $Si(111)-(7\times7)$ surfaces under ultrahigh vacuum (UHV) condition. Using $in-situ$ scanning tunnelling microscopy (STM) we have compared the morphological changes to the $Si(111)-(7\times7)$ surfaces before and after the sputtering process. Following $500~eV Ar^+$ ion sputtering, the atomically flat $Si(111)-(7\times7)$ surface becomes amorphous. The average root mean square (rms) surface roughness $({\sigma}_{avg})$ of the sputtered surface and that following post-annealing at different temperatures $(500^\circ-700^\circ)C$ under UHV have been measured as a function of STM scan size. While, annealing at $\sim 500^\circ C$ shows no detectable changes in the surface morphology, recrystallization process starts at $\sim 600^\circ C$. For the sputtered samples annealed at temperatures $\geq 600^\circ C, \,log~\sigma_{avg}$ varies linearly at lower length scales and approaches a saturation value of $\sim 0.6 nm$ for the higher length scales confirming the self-affine fractal nature. The correlation length increases with annealing temperature indicating gradual improvement in crystallinity. For the present experimental conditions, $650^\circ C$ is the optimal annealing temperature for recrystallization. The results offer a method to engineer the crystallinity of sputtered surface during nanofabrication process.

cond-mat.mtrl-sci↗

arXiv2023

Understanding the Role of Four-Phonon Scattering in the Lattice Thermal Transport of Monolayer MoS$_{2}$

In the calculations of lattice thermal conductivity ($\kappa_{\text{L}}$), vital contributions stemming from four-phonon scattering are often neglected. The significance of four-phonon scattering in the thermal transport properties of monolayer (ML) MoS$_{2}$ has been unraveled using first-principles calculations combined with the Boltzmann transport equation. If only three-phonon scattering processes are considered then the $\kappa_{\text{L}}$ is found to be significantly overestimated ($\sim$ 115.8 Wm$^{-1}$K$^{-1}$ at 300 K). With the incorporation of the four-phonon scattering processes, the $\kappa_{\text{L}}$ reduces to 24.6 Wm$^{-1}$K$^{-1}$, which is found to be closer to the experimentally measured $\kappa_{\text{L}}$ of 34.5 Wm$^{-1}$K$^{-1}$. Four-phonon scattering significantly impacts the carrier lifetime ($\tau$) of the low-energy out-of-plane acoustic mode (ZA) phonons and thereby, suppresses its contribution in $\kappa_{\text{L}}$ from 64% (for three-phonon scattering) to 16% (for both three- and four-phonon scatterings). The unusually high four-phonon scattering rate ($\tau_{4}^{-1}$) of the ZA phonons is found to result from the simultaneous effect of the acoustic-optical frequency gap, strong anharmonicity, and the reflection symmetry imposed selection rule. The strong coupling between the quadratic dispersion of the ZA mode and the $\tau_{4}^{-1}$ is discovered by the application of mechanical strain. The strain induced increase in the linearity of the ZA mode dispersion dramatically reduces the significance of the four-phonon scattering in the strained ML-MoS$_{2}$, both qualitatively and quantitatively. These conclusions will provide significant insights into the thermal transport phenomena in ML-MoS$_{2}$, as well as any other 2D material.

cond-mat.mtrl-sci↗

arXiv2023

Tensile Strain Induced Anomalous Enhancement in the Lattice Thermal Transport of Monolayer ZnO: A First Principles Study

Density functional theory based calculations have been performed for solving the phonon Boltzmann transport equation to investigate the thermal transport properties of monolayer (ML) ZnO under in-plane isotropic biaxial tensile strain. The in-plane lattice thermal conductivity ($\kappa_{\text{L}}$) of ML-ZnO increases dramatically in response to the biaxial tensile strain ranging from 0% to 10%, conflicting with the general belief. The strain-induced stiffening of the ZA phonon mode and the resulting concomitant increase in group velocity and decrease in phonon population is found to play a significant role behind the unusual enhancement of $\kappa_{\text{L}}$. The mode resolved analysis shows the tensile strain driven competitive behavior between different phonon properties, mainly the group velocity and phonon lifetimes, being responsible for the observed unusual enhancement in $\kappa_{\text{L}}$. Additionally, the phonon scattering calculations show the importance of inclusion of 4-phonon scattering in the thermal transport calculations suggesting the significance of higher-order anharmonicity in ML-ZnO. A strikingly high 4-phonon scattering strength in ML-ZnO primarily results from the strong anharmonicity, quadratic ZA mode dispersion, large frequency gap in phonon dispersion, and reflection symmetry induced selection rule. The incorporation of 4-phonon scattering significantly alters the transport characteristics of all the phonon modes, in general and ZA phonons, in particular. At large strains, a linear dispersion of the ZA mode and closure of the frequency gap is observed, which results in a significant reduction of 4-phonon scattering strength in ML-ZnO.

cond-mat.mtrl-sci↗

arXiv2023

Hydrostatic Pressure Induced Anomalous Enhancement in the Thermoelectric Performance of Monolayer MoS$_{2}$

The hydrostatic pressure induced changes in the transport properties of monolayer (ML) MoS$_2$ have been investigated using first-principles density functional theory based calculations. The application of pressure induces shift in the conduction band minimum (CBM) from K to $\Lambda$, while retaining the band extrema at K in around the same energy at a pressure of 10 GPa. This increase in valley degeneracy is found to have a significant impact on the electronic transport properties of ML-MoS$_2$ via enhancement of the thermopower (S) by up to 140\% and power factor (S$^{2}$$\sigma$/$\tau$) by up to 310\% at 300 K. Besides, the very low deformation potential (E$_\text{DP}$) associated with the CB-$\Lambda$ valley results in a remarkably high electronic mobility ($\mu$) and relaxation time ($\tau$). Additionally, the application of pressure reduces the room temperature lattice thermal conductivity ($\kappa_\text{L}$) by 20\% of its unstrained value, owing to the increased anharmonicity and resulting increase in the intrinsic phonon scattering rates. The hydrostatic pressure induced increase in power factor (S$^{2}$$\sigma$) and the decrease in $\kappa_\text{L}$ act in unison to result in a substantial improvement in the overall thermoelectric performance (zT) of ML-MoS$_2$. At 900 K with an external pressure of 25 GPa, zT values of 1.63 and 1.21 are obtained for electron and hole doping, respectively, which are significantly higher compared to the zT values at zero pressure. For the implementation in a thermoelectric module where both n-type and p-type legs should be preferably made of the same material, the concomitant increase in zT of ML-MoS$_2$ for both types of doping with hydrostatic pressure can be highly beneficial.

cond-mat.mtrl-sci↗

arXiv2023

Ab-initio Study of Electronic and Lattice Dynamical Properties of monolayer ZnO under Strain

First-principles density functional theory based calculations have been performed to investigate the strain-induced modifications in the electronic and vibrational properties of monolayer (ML) ZnO. Wide range of in-plane tensile and compressive strains along different directions are applied to analyse the modifications in detail. The electronic band gap reduces under both tensile and compressive strains and a direct to indirect band gap transition occurs for high values of biaxial tensile strain. The relatively low rate of decrease of band gap and large required strain for direct to indirect band gap transition compared to other $2$D materials are analysed. Systematic decrease in the frequency of the in-plane and increase in the out-of-plane optical phonon modes with increasing tensile strain are observed. The in-plane acoustic modes show linear dispersion for unstrained as well as strained cases. However, the out-of-plane acoustic mode (ZA), which shows quadratic dispersion in the unstrained condition, turns linear with strain. The dispersion of the ZA mode is analysed using the shell elasticity theory and the possibility of ripple formation with strain is analysed. The strain-induced linearity of the ZA mode indicates the absence of rippling under strain. Finally, the stability limit of ML-ZnO is investigated and found that for $18\%$ biaxial tensile strain the structure shows instability with the emergence of imaginary phonon modes. Furthermore, the potential of ML-ZnO to be a good thermoelectric material is analyzed in an intuitive way based on the calculated electronic and phononic properties. Our results, thus, not only highlight the significance of strain-engineering in tailoring the electronic and vibrational properties but also provide a thorough understanding of the lattice dynamics and mechanical strength of ML-ZnO.

cond-mat.mtrl-sci↗

arXiv2022

Strain Driven Anomalous Anisotropic Enhancement in the Thermoelectric Performance of monolayer MoS$_{2}$

First principles density functional theory based calculations have been performed to investigate the strain and temperature induced tunability of the thermoelectric properties of monolayer (ML) MoS$_2$. Modifications in the electronic and phononic transport properties, under two anisotropic uniaxial strains along the armchair (AC) and zigzag (ZZ) directions, have been explored in detail. Considering the intrinsic carrier-phonon scattering, we found that the charge carrier mobility ($\mu$) and relaxation time ($\tau$) increase remarkably for strains along the ZZ direction. Concomitantly, strain along the ZZ direction significantly reduces the lattice thermal conductivity ($\kappa_\text{L}$) of ML-MoS$_2$. The combined effect of shortened phonon relaxation time and group velocity, and the reduced Debye temperature is found to be the driving force behind the lowering of $\kappa_\text{L}$. The large reduction in $\kappa_\text{L}$ and increase in $\tau$, associated with the strains along the ZZ direction, act in unison to result in enhanced efficiency and hence, improved thermoelectric performance. Nearly $150\%$ enhancement in the thermoelectric efficiency can be achieved with the optimal doping concentration. We, therefore, highlight the significance of in-plane tensile strains, in general, and strains along the ZZ direction, in particular, in improving the thermoelectric performance of ML-MoS$_2$.

cond-mat.mtrl-sci↗

arXiv2022

Strain induced effects on the electronic and phononic properties of 2H and 1T$^{\prime}$ monolayer MoS$_{2}$

First-principles calculations, within the framework of density functional theory, have been performed on the well-studied 2H and the less explored 1T$^{\prime}$ phase of single-layer MoS$_{2}$. We have addressed the strain-induced tunability of the electronic and phononic properties of both phases, and compared their stability against the applied strain. By considering a large number of strain profiles for both tensile and compressive stress, we have found that the electronic properties of both 2H and 1T$^{\prime}$ phases are sensitive to the direction of the applied strain and can be tuned in a controlled way. For the 2H phase, in most cases, a direct to indirect band gap transition at lower strain and a semiconductor to metal transition at higher strain is observed. The applied strain destroys the semimetallic nature of the 1T$^{\prime}$ phase via the overlapping of the bulk states with the topologically protected edge states. Significant strain-induced changes in the phononic properties, in the frequency of the phonon branches, as well as in the nature of the dispersion curves, are observed. A systematic change in the frequency of the optical phonon modes at the zone centre is seen for both phases. With increasing strain, the out-of-plane acoustic mode (ZA) turns imaginary, indicating a possibility of phase transition or instability of the crystal structure. The 2H phase appears to withstand a larger amount of strain and therefore possesses better stability compared to the 1T$^{\prime}$ phase since the imaginary branch starts to appear at much lower values of strain in the latter case. We highlight the significance of strain engineering in tuning the electronic and phononic properties and the safe limit of the strain application in different polymorphs of single-layer MoS$_{2}$.

cond-mat.mtrl-sci↗

arXiv2017

Superconductivity of Cobalt in Thin Films

Due to competing long range ferromagnetic order, the transition metals Fe, Co and Ni are not superconductors at ambient pressure. While superconductivity was observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to be discovered in Co and Ni under any experimental conditions. Here, we report emergence of superconductivity in the recently discovered high-density nonmagnetic face centered cubic phase in Co thin films below a transition temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact spectroscopy and resistance, and four-probe measurements of resistance at ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of Co within first-principles density functional theory, and show that its superconductivity below 5 K originates from anomalous softening of zone-boundary phonons and their enhanced coupling with electrons upon biaxial strain.

cond-mat.supr-con↗

arXiv2017

High density nonmagnetic cobalt in thin films

Recently high density (HD) nonmagnetic (NM) cobalt has been discovered in a cobalt thin film, grown on Si(111). This cobalt film had a natural cobalt oxide at the top. The oxide layer forms when the film is taken out of the electron-beam deposition chamber and exposed to air. Thin HD NM cobalt layers were found near the cobalt/silicon and the cobalt-oxide/cobalt interfaces, while the thicker mid-depth region of the film was hcp cobalt with normal density and normal magnetic moment. If an ultrathin film of gold is grown on the cobalt layer, before exposing it to air, the oxidation of the cobalt surface layer is prevented. It is important to investigate whether the growth of HD NM cobalt layers in the thin film depends on (i) capping of the film by the gold layer, (ii) the film thickness and (iii) the nature of the substrate. The results of such investigations, presented here, indicates that for cobalt films capped with a thin gold layer, and for various film thicknesses, HD NM cobalt layers are still observed. However, instead of a Si substrate, when the cobalt films are grown on oxide substrates, such as silicon oxide or cobalt oxide, HD NM cobalt layers are not observed.

cond-mat.mtrl-sci↗

arXiv2016

Nanoscale η-NiSi formation via ion irradiation of Si/Ni/Si

Nickel monosilicide (NiSi) has emerged as an excellent material of choice for source-drain contact applications below 45 nm node complementary metal-oxide-semiconductor (CMOS) technology. We have investigated the formation of nanoscale NiSi by ion irradiation of Si (~5 nm)/Ni(~15 nm)/Si, grown under ultrahigh vacuum environment. Irradiation was carried out at room temperature with 1 MeV Si+ ions. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed for analysis. With increasing ion fluence ion beam mixing occurs and more and more Si is incorporated into the Ni layer and this layer gets amorphized. At an even higher fluence a recrystallized uniform nickel monosilicide (η-NiSi) layer is formed. Several planar spacings of different Miller indices of η-NiSi have been observed in XRD and TEM. Additionally, an oscillatory amorphization and recrystallization has been observed in the substrate Si with increasing ion fluence. To our knowledge, this has never been observed in ion irradiation of bare Si in decades of work in this area. The oscillatory amorphization/recrystallization in Si is apparently Ni-induced. Irradiation displaces Ni and produces a distribution of Ni in amorphized Si. Irradiation at a higher fluence produces two recrystallized Si bands in amorphous Si with concomitant accumulation of Ni at the amorphous/crystalline interfaces. On further increase of irradiation fluence the recrystallized Si bands again pass through amorphization and recrystallization. The total thickness of recrystallized as well as amorphous Si shows an oscillatory behavior as a function of ion fluence.

cond-mat.mtrl-sci↗

arXiv2016

Evidence of Formation of Superdense Nonmagnetic Cobalt

Magnetism of 3d transition metals at high density has always received wide interest due to existence of magnetism at the core of the Earth. For ferromagnetic cobalt, although there is a theoretical prediction that its magnetic moment would vanish when the density increases to about 1.4 times the normal value, so far there is no experimental evidence. We have discovered the existence of ultrathin superdense nonmagnetic cobalt layers in a polycrystalline cobalt thin film. The densities of these layers are about 1.3-1.4 times the normal density of Co. This has been revealed by X-ray reflectometry experiments, which provide a depth profile of the electron scattering length density, and corroborated by polarized neutron reflectometry (PNR) experiments. The superdense Co layer has an fcc structure, unlike hcp structure for bulk Co, as revealed by transmission electron microscopy. The magnetic depth profile, obtained by PNR, shows that the superdense Co layers at the film-substrate interface and near the top of the film are nonmagnetic. The major part of the Co film has the usual density and magnetic moment.

cond-mat.mtrl-sci↗

arXiv2015

Ion irradiation effects on a magnetic Si/Ni/Si trilayer and lateral magnetic-nonmagnetic multistrip patterning by focused ion beam

Fabrication of a multistrip magnetic/nonmagnetic structure in a thin sandwiched Ni layer [Si(5 nm)/Ni(10 nm)/Si] by a focused ion beam (FIB) irradiation has been attempted. A control experiment was initially performed by irradiation with a standard 30 keV Ga ion beam at various fluences. Analyses were carried out by Rutherford backscattering spectrometry, X-ray reflectivity, magnetooptical Kerr effect (MOKE) measurements and MOKE microscopy. With increasing ion fluence, the coercivity as well as Kerr rotation decreases. A threshold ion fluence has been identified, where ferromagnetism of the Ni layer is lost at room temperature and due to Si incorporation into the Ni layer, a Ni0.68Si0.32 alloy layer is formed. This fluence was used in FIB irradiation of parallel 50 nm wide stripes, leaving 1 micrometer wide unirradiated stripes in between. MOKE microscopy on this FIB-patterned sample has revealed interacting magnetic domains across several stripes. Considering shape anisotropy effects, which would favor an alignment of magnetization parallel to the stripe axis, the opposite behavior is observed. Magneto-elastic effects introducing a stress-induced anisotropy component oriented perpendicular to the stripe axis are the most plausible explanation for the observed behavior.

cond-mat.mtrl-sci↗

arXiv2014

Evolution of Fermi Level State Density in Ultrathin Films Near the Two Dimensional Limit: Experiment and Theory

Electronic density of states (DOS) at Fermi level has been investigated in ultrathin Ag films grown on Si(111)-(7x7) down to the two dimensional limit of a single atomic layer. Measurement of DOS at Fermi level by scanning tunneling spectroscopy shows an approximate (1 - γ/d) dependence, where γ is a constant and d is the film thickness. The results are explained in the light of an analytical theory as well as our density functional theory (DFT) calculations. DFT results also show that in the proximity of the interface the DOS values of the film and the substrate are mutually affected by each other.

cond-mat.mes-hall↗

arXiv2013

Crossover regimes in lower dimensional structures

Modern growth and fabrication techniques can produce lower dimensional structures in the crossover regime. Such structures in the crossover regime can provide tunability of various properties. For example, a zero-dimensional (0-D) structure evolving towards a 2-D structure shows electronic structure which is neither 0-D-like, nor 2-D-like. Within the crossover regime the electronic density of states (DOS) at Fermi level (Ef) keeps on changing as the size of the system changes. DOS at Ef determines many properties of materials, such as electronic specific heat, spin susceptibility etc. Keeping the importance of DOS at Ef in mind, we determine their values and other details of electronic structure of lower dimensional structures of metals, in the 0-D to 1-D, 1-D to 2-D, 2-D to 3-D, 0-D to 2-D, 0-D to 3-Dand 1-D to 3-Dcrossover regimes, in a simple free electron model. We compare our results with analytical theory and experimental results, wherever available. We also present some results obtained by scanning tunneling spectroscopy measurements on Ag islands on Si(111) evolving from a 0-D to a 2-D structure. This simple model is quite useful in understanding lower dimensional structures in the crossover regimes.

cond-mat.mes-hall↗

arXiv2012

The effect of exclusion on nonlinear reaction diffusion system in inhomogeneous media

We study a minimal model to understand the formation of clusters on surfaces in the presence of surface defects. We consider reaction diffusion model in which atoms undergoes reactions at the defect centers to form clusters. Volume exclusion between particles is introduced through a drift term in the reaction diffusion equation that arises due the repulsive force field produced by the clustering atoms. The volume exclusion terms can be derived from master equation with a concentration dependent hopping rate. Perturbative analysis is performed for both cross-exclusion and self-exclusion one dimensional system. For two dimension numerical analysis is performed. We have found that the clusterization process slows down due to exclusion. As a result the size of the clusters reduces. In this model reaction scheme has algebraic nonlinearity and plausible mechanism is also given.

cond-mat.mtrl-sci↗

arXiv2012

Nanodot to Nanowire: A strain-driven shape transition in self-organized endotaxial CoSi2 on Si (100)

We report a phenomenon of strain-driven shape transition in the growth of nanoscale self-organized endotaxial CoSi2 islands on Si (100) substrates. Small square shaped islands as small as 15\times15 nm2 have been observed. Islands grow in the square shape following the four fold symmetry of the Si (100) substrate, up to a critical size of 67 \times 67 nm2. A shape transition takes place at this critical size. Larger islands adopt a rectangular shape with ever increasing length and the width decreasing to an asymptotic value of ~25 nm. This produces long wires of nearly constant width.We have observed nanowire islands with aspect ratios as large as ~ 20:1. The long nanowire heterostructures grow partly above (~ 3 nm) the surface, but mostly into (~17 nm) the Si substrate. These self-organized nanostructures behave as nanoscale Schottky diodes. They may be useful in Si-nanofabrication and find potential application in constructing nano devices.

cond-mat.mes-hall↗

arXiv2011

Growth of narrow-neck, epitaxial and nearly spherical Ge nanoislands on air-exposed Si(111)-(7$\times$7) surfaces

Growth of narrow-neck, epitaxial as well as non-epitaxial and nearly spherical Ge islands on air-exposed Si(111)-(7\times7) surfaces has been investigated by in-situ scanning tunnelling microscopy (STM) and ex-situ high resolution cross-sectional transmission electron microscopy (HRXTEM). A thin oxide is formed on Si(111)-(7\times7) surfaces via air exposure. Ge islands are grown on this oxide. STM measurements reveal the growth of very small (~2 nm diameter) Ge islands with a high number density of about 1.8\times10^12 cm-2. The island size has been found to depend on the amount of deposited Ge as well as the substrate temperature during Ge deposition. HRXTEM micrographs reveal that the islands are nearly spherical in shape, making narrow-neck contact with the substrate surface. At 520°C growth temperature both epitaxial and non-epitaxial islands grow. However, at 550°C, Ge islands predominantly grow epitaxially by a narrow-contact with Si via voids in the oxide layer. Growth of vertically elongated Ge islands is also observed in HRXTEM measurements with a very small diameter-to-height aspect ratio (~0.5-1), a hitherto unreported feature of epitaxial Ge growth on Si surfaces. In addition, stacking fault and faceting are observed in islands as small as 5 nm diameter. Ge islands, not even in contact with the Si substrate, appear to be in epitaxial alignment with the Si substrate. The island size distribution is essentially monomodal. As the contact area of Ge islands with Si through the voids in the oxide layer can be controlled via growth temperature, the results indicate that tunability of the potential barrier at the interface and consequently the tunability of electronic levels and optical properties can be achieved by the control of growth temperature.

cond-mat.mes-hall↗