arXiv subjects

B. Satpati

Publications and source records attributed to B. Satpati.

17 recordsLinked to original sources

arXiv2019

Evidence of topological gap opening in the surface state of Bi$_2$Se$_3$ by proximity to a magnetic insulator

Topological insulators are bulk insulators with exotic surface states, protected under time-reversal symmetry, that hold promise in observing many exciting condensed-matter phenomena. In this report, we show that by having a topological insulator (Bi$_2$Se$_3$) in proximity to a magnetic insulator (EuS), a metal-to-insulator transition in the surface state, attributed to opening of an exchange gap, can be observed whose properties are tunable using bottom gate voltage and external magnetic field. Our study provides evidence of gate-controlled enhanced interface magnetism with the signature of half-integer quantum Hall effect when the Fermi level is tuned into the exchange gap. These results pave the way for using magnetic proximity effect in developing topological electronic devices.

cond-mat.mes-hall↗

arXiv2019

Superconductivity-driven magnetization modulation in YBa2Cu3O7-δ /SrTiO3/La0.67Sr0.33MnO3 heterostructures

Using spin polarized neutron reflectivity experiments, we demonstrate an unusual proximity behaviour when the superconductor (SC) and the ferromagnet (FM) are coupled through an insulator (I) in YBa2Cu3O7-δ (SC)/SrTiO3 (I)/La0.67Sr0.33MnO3 (FM) heterostructures. We have observed an unexpected magnetic modulation at the interface region of the FM below the superconducting transition temperature. The magnetization of the FM layer at the I/FM interface was drastically reduced as compared to the magnetization in the rest of the FM layer. This result indicates that the Cooper pairs tunnel across the insulator and interact with the local magnetization at the interface region (extending ~ 30 Å) of the FM causing modification of the magnetization at the interface. This unexpected magnetic behavior cannot be explained on the basis of the existing theoretical models. However, the length scale associated here clearly suggests the long range proximity effect as a result of tunneling of Cooper pairs.

cond-mat.supr-con↗

arXiv2018

Modification of microstructure and micromagnetic properties in Gd-Fe thin films by rapid thermal processing

Impact of rapid thermal processing (RTP) on microstructure and magnetic properties of Gd-Fe thin films have been investigated with a special emphasis to magnetic microstructure. 100 nm thick amorphous Gd-Fe film shows elongated stripe domains with characteristic feature size of 122 nm, which signifies the development of perpendicular magnetic anisotropy (PMA) in this system. RTP at 550^oC for different time intervals viz. 5, 10, 15, 20 minutes induces the crystallization of Fe over the amorphous Gd-Fe matrix. Cross-sectional imaging with transmission electron microscope reveals the presence of Fe nanocrystal and the reacted film-substrate interface, which has also been verified by Rutherford backscattering spectrometry. With increasing RTP time, the characteristic size and the threshold height of the topographic feature increases. While the magnetic contrast drops down for the processed films, the increasing threshold height of the nanocrystalline grains results a topography dominated mixed phase in the magnetic force microscopy images. The magnetization measurements clearly indicate a re-orientation of the magnetization direction from perpendicular to the plane of the surface. To unravel the effect of anisotropy modification on domain structure, 3D micromagnetic simulations have been performed, which showed that with 50% reduction of original anisotropy, the magnetization tries to orient in the plane of the surface, whereas an in-plane (along xz plane) tilt angle of 30^o has been estimated to be the threshold for the decrease in perpendicular component of magnetization in the investigated Gd-Fe thin film system.

cond-mat.mes-hall↗

arXiv2017

Large nonsaturating magnetoresistance and signature of non-degenerate Dirac nodes in ZrSiS

While the discovery of Dirac and Weyl type excitations in electronic systems is a major breakthrough in recent condensed matter physics, finding appropriate materials for fundamental physics and technological applications, is an experimental challenge. In all the reported materials, linear dispersion survives only up to a few hundred meV from the Dirac or Weyl nodes. On the other hand, real materials are subject to uncontrolled doping during preparation and thermal effect near room temperature can hinder the rich physics. In ZrSiS, ARPES measurements have shown an unusually robust linear dispersion (up to $\sim$2 eV) with multiple non-degenerate Dirac nodes. In this context, we present the magnetotransport study on ZrSiS crystal, which represents a large family of materials (\textit{WHM} with \textit{W} = Zr, Hf; \textit{H} = Si, Ge, Sn; \textit{M} = O, S, Se, Te) with identical band topology. Along with extremely large and non-saturating magnetoresistance (MR), $\sim$ 1.4 $\times$ 10$^{5}$ \% at 2 K and 9 T, it shows strong anisotropy depending on the direction of the magnetic field. Quantum oscillation and Hall effect measurements have revealed large hole and small electron Fermi pockets. Non-trivial $π$ Berry phase confirms the Dirac fermionic nature for both types of charge carriers. The long-sought relativistic phenomenon of massless Dirac fermions, known as Adler-Bell-Jackiw chiral anomaly, has also been observed.

cond-mat.mtrl-sci↗

arXiv2016

Tuning the scattering mechanism in three-dimensional Dirac semimetal Cd$_{3}$As$_{2}$

To probe the charge scattering mechanism in Cd$_{3}$As$_{2}$ single crystal, we have analyzed the temperature and magnetic field dependence of the Seebeck coefficient ($S$). The large saturation value of $S$ at high field clearly demonstrates the linear energy dispersion of three-dimensional Dirac fermion. A wide tunability of the charge scattering mechanism has been realized by varying the strength of the magnetic field and carrier density via In doping. With the increase in magnetic field, the scattering time crosses over from being nearly energy independent to a regime of linear dependence. On the other hand, the scattering time enters into the inverse energy-dependent regime and the Fermi surface strongly modifies with 2\% In doping at Cd site. With further increase in In content from 2 to 4\%, we did not observe any Shubnikov-de Haas oscillation up to 9 T field, but the magnetoresistance is found to be quite large as in the case of undoped sample.

cond-mat.str-el↗

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↗

arXiv2013

Formation of isolated islands by size-selected copper nanocluster deposition

Deposition of size-selected metal nanoclusters on a substrate with very low kinetic energy helps to keep the clusters intact with respect to their shape and size as compared to clusters in ight condition. Here we report formation of isolated monodispersed islands of copper of desired size on carbon film by deposition of size selected copper clusters (~ 3 nm) in soft-landing method. Copper clusters were produced by a magnetron based gas aggregation type source equipped with a quardrupole mass filter (QMF) to select size of clusters before landing. Transmission Electron Microscopy (TEM) study shows that diffusion of islands is very low.

cond-mat.mtrl-sci↗

arXiv2009

The mechanism of ion induced amorphization in Si

Some results on damage build up in, and amorphization of, Si, induced by 25-30 keV Al$_5^-$, Si$_5^-$ and Cs$^-$ ions, at room temperature, are reported. We show that at low energy, amorphization is a nucleation and growth process, based on the direct impact mechanism. With an Avrami exponent $\sim 1.6$, the growth towards amorphization seems to be diffusion limited. A transition to a completely amorphized state is indicated at a dose exceeding 17 eV/atom, which is higher than 6-12 eV/atom as predicted by simulations. The observed higher threshold could be due to temperature effects although an underestimation of keV-energy recoils, in simulation, may not be ruled out.

cond-mat.mtrl-sci↗

arXiv2005

Magnetism in C60 Films Induced by Proton Irradiation

It is shown that polycrystalline fullerene thin films on hydrogen passivated Si(111) substrates irradiated by 2 MeV protons displays ferromagnetic-like behavior at 5 K. At 300 K both the pristine and the irradiated film show diamagnetic behavior. Magnetization data in the temperature range 2 - 300 K, in 1 Tesla applied field, for the irradiated film shows much stronger temperature dependence compared to the pristine film. Possible origin of ferromagnetic-like signals in the irradiated films are discussed.

cond-mat.mtrl-sci↗

arXiv2005

Size distribution of sputtered particles from Au nanoislands due to MeV self-ion bombardment

Nanoisland gold films, deposited by vacuum evaporation of gold onto Si(100) substrates, were irradiated with 1.5 MeV Au$^{2+}$ ions up to a fluence of $5\times 10^{14}$ ions cm$^{-2}$ and at incidence angles up to $60^{\circ}$ with respect to the surface normal. The sputtered particles were collected on carbon coated grids (catcher grid) during ion irradiation and were analyzed with transmission electron microscopy and Rutherford backscattering spectrometry. The average sputtered particle size and the areal coverage are determined from transmission electron microscopy measurements, whereas the amount of gold on the substrate is found by Rutherford backscattering spectrometry. The size distributions of larger particles (number of atoms/particle, $n$ $\ge$ 1,000) show an inverse power-law with an exponent of $\sim$ -1 in broad agreement with a molecular dynamics simulation of ion impact on cluster targets.

cond-mat.mtrl-sci↗

arXiv2005

MeV ion-induced strain at nanoisland-semiconductor surface and interfaces

Strain at surfaces and interfaces play an important role in the optical and electronic properties of materials. MeV ion-induced strain determination in single crystal silicon substrates and in Ag (nanoisland)/Si(111) at surface and interfaces has been carried out using transmission electron microscopy (TEM) and surface-sensitive X-ray diffraction. Ag nanoislands are grown under various surface treatments using thermal evaporation in high vacuum conditions. Irradiation has been carried out with 1.5 MeV Au^{2+} ions at various fluences and impact angles. Selected area electron diffraction (SAED) and lattice imaging (using TEM) has been used to determine the strain at surface and interfaces. Preliminary results on the use of surface-sensitive asymmetric x-ray Bragg reflection method have been discussed. The TEM results directly indicate a contraction in the silicon lattice due to ion-induced effects. The nanoislands have shadowed the ion beam resulting in lesser strain beneath the island structures in silicon substrates. High-resolution lattice imaging has also been used to determine the strain in around amorphization zones caused by the ion irradiation.

cond-mat.mtrl-sci↗

arXiv2005

Ion Irradiation Induced Effects in Metal Nanostructures

High resolution transmission electron microscopy (HRTEM) and Rutherford backscattering spectrometry (RBS) are used to study the ion induced effects in Au, Ag nanostructures grown on Si and thermally grown SiO2 substrates. Au and Ag films (~2 nm) are prepared by thermal evaporation under high vacuum condition at room temperature (RT). These films were irradiated with MeV Au ions also at RT. Very thin films of Au and Ag deposited on silicon substrates (with native oxide) form isolated nano-island structures due to the non-wetting nature of Au and Ag. Ion irradiation causes embedding of these nanoislands into the substrate. For Ag nanoislands with diameter 15 - 45 nm, the depth of the embedding increases with ion fluence and the nano particles are fully submerged into Si and SiO$_2$ substrate at a fluence of 5*10^14 ions /cm^2 without any mixing. Au nanoparticles (diameter 6 - 20 nm), upon ion irradiation, forms embedded gold-silicide in the case of Si substrate and show lack of mixing and silicide formation in the case of SiO2 substrate system.

cond-mat.mtrl-sci↗

arXiv2005

Energy dependent sputtering of nano-clusters from a nanodisperse target and embedding of nanoparticles into a substrate

Au nanoparticles, prepared by thermal evaporation under high vacuum condition on Si substrate, are irradiated with Au ions at different ion energies. During ion irradiation, embedding of nanoparticles as well as ejection of nano-clusters is observed. Ejected particles (usually smaller than those on the Si substrate) due to sputtering are collected on carbon-coated transmission electron microscopy (TEM) grids. Both the TEM grids and the ion-irradiated samples are analyzed with TEM. Unirradiated as well as irradiated samples are also analyzed by Rutherford backscattering spectrometry (RBS). In the case of low energy (32 keV) ions, where the nuclear energy loss is dominant, both sputtering and embedding are less compared to medium energy (1.5 MeV). In the high energy regime (100 MeV), where the electronic energy loss is dominant, sputtering is maximum but practically there is no embedding. Ion bombardment of surfaces at an angle with respect to the surface-normal produces enhanced embedding compared to normal-incidence bombardment. The depth of embedding increases with larger angle of incidence. Au nanoparticles after ion irradiation form embedded gold-silicide. Size distribution of the sputtered Au clusters on the TEM grids for different ion energy regimes are presented.

cond-mat.mtrl-sci↗

arXiv2005

Ion-beam induced 1D to 3D periodic transformation in nanostructured multilayers

Ion-irradiation-induced modifications of a periodic Pt/C multilayer system containing Fe impurity have been analyzed by transmission electron microscopy (TEM). The multilayer stack with 16 Pt/C layer pairs (period 4.23 nm) was fabricated on a glass substrate. A 2 MeV Au$^{2+}$ ion beam was rastered on the sample to obtain uniformly irradiated strips with fluences from 1$\times10^{14}$ to 1$\times10^{15}$ $ions/cm^2$. Ion-irradiation has been found to cause preferential migration of Fe towards Pt layers [Nucl. Instr. Methods Phys. Res. B212 (2003) 530]. Cross-sectional transmission electron microscopy (XTEM) shows considerable atomic redistribution for irradiation at the highest ion fluence (1$\times10^{15}$ $ions/cm^2$). Individual entities in this structure is like a cluster. Periodic multilayers have periodicity only in the direction normal to the multilayer surface. However, Fourier transform of the XTEM images of the sample irradiated at the highest-fluence shows new off-normal Fourier components of superlattice periodicities arising due to ion irradiation. With a proper understanding of this phenomenon it may be possible to fabricate three dimensional periodic structures of nanoclusters.

cond-mat.mtrl-sci↗

arXiv2005

Replicating Nanostructures on Silicon by Low Energy Ion Beams

We report on a nanoscale patterning method on Si substrates using self-assembled metal islands and low-energy ion-beam irradiation. The Si nanostructures produced on the Si substrate have a one-to-one correspondence with the self-assembled metal (Ag, Au, Pt) nanoislands initially grown on the substrate. The surface morphology and the structure of the irradiated surface were studied by high-resolution transmission electron microscopy (HRTEM). TEM images of ion-beam irradiated samples show the formation of sawtooth-like structures on Si. Removing metal islands and the ion-beam induced amorphous Si by etching, we obtain a crystalline nanostructure of Si. The smallest structures emit red light when exposed to a UV light. The size of the nanostructures on Si is governed by the size of the self-assembled metal nanoparticles grown on the substrate for this replica nanopatterning. The method can easily be extended for tuning the size of the Si nanostructures by the proper choice of the metal nanoparticles and the ion energy in ion-irradiation. It is suggested that off-normal irradiation can also be used for tuning the size of the nanostructures.

cond-mat.mtrl-sci↗

arXiv2004

Electron and Phonon Confinement and New Surface Phonon Modes in CdSe-CdS Core-Shell Nanocrystals

Optical and vibrational properties of bare and CdS shelled CdSe nanocrystalline particles are investigated. To confirm the formation of such nanocrystals in our samples we estimate their average particle sizes and size distributions using TEM measurements. From the line profile analysis of the images the core-shell structure in the particles has been confirmed. The blue shift in optical absorption spectra, analyzed using theoretical estimates based on the effective bond order model, establishes the electron confinement in the nanoparticles. Unique characteristics of the nanocrystals (which are absent in the corresponding bulk material), such as confinement of optical phonons and the appearance of surface phonons, are then discussed. Making use of the dielectric response function model we are able to match the experimental and theoretical values of the frequencies of the surface phonons. We believe that our studies using optical probes provide further evidence on the existence of core-shell structures in CdSe-CdS type materials.

cond-mat.mtrl-sci↗

arXiv2003

Height preference and strain in Ag islands on Si(111)-(7x7)

Growth and strain behavior of thin Ag films on Si substrate have been investigated by scanning tunneling microscopy, cross-sectional transmission electron microscopy and high resolution x-ray diffraction studies. Ag islands formed on Si at room temperature growth show strongly preferred heights and flat top. At low coverage, islands containing two atomic layers of Ag are overwhelmingly formed. At higher coverages island height distribution shows strong peaks at relative heights corresponding to an even number of Ag atomic layers. This appears to be a quantum size effect. Hexagonal disc-like islands with flat top are formed upon annealing. The annealed film shows two closely-spaced Ag(111) diffraction peaks - one weak and broad and the other narrow and more intense. The intense peak corresponds to a shorter Ag(111) planar spacing compared to the bulk value. This can be explained in terms of changes in the Ag lattice during the heating-cooling cycle due to thermal expansion coefficient mismatch between Ag and Si.

cond-mat.mtrl-sci↗