arXiv subjects

Brijesh Kumar

Publications and source records attributed to Brijesh Kumar.

At least 73 records · Page 4Linked to original sources

arXiv2017

Quantum to classical transition in the ground state of a spin-$S$ quantum antiferromagnet

We study a frustrated spin-$S$ staggered-dimer Heisenberg model on square lattice by using the bond-operator representation for quantum spins, and investigate the emergence of classical magnetic order from the quantum mechanical (staggered-dimer singlet) ground state for increasing $S$. Using triplon analysis, we find the critical couplings for this quantum phase transition to scale as $1/S(S+1)$. We extend the triplon analysis to include the effect of quintet dimer-states, which proves to be essential for establishing the classical order (Néel or collinear in the present study) for large $S$, both in the purely Heisenberg case and also in the model with single-ion anisotropy.

cond-mat.str-el↗

arXiv2017

Level crossings induced by a longitudinal coupling in the transverse field Ising chain

We study the effect of antiferromagnetic longitudinal coupling on the one-dimensional transverse field Ising model with nearest-neighbour couplings. In the topological phase where, in the thermodynamic limit, the ground state is twofold degenerate, we show that, for a finite system of N sites, the longitudinal coupling induces N level crossings between the two lowest lying states as a function of the field. We also provide strong arguments suggesting that these N level crossings all appear simultaneously as soon as the longitudinal coupling is switched on. This conclusion is based on perturbation theory, and a mapping of the problem onto the open Kitaev chain, for which we write down the complete solution in terms of Majorana fermions.

cond-mat.str-el↗

arXiv2016

Intertwined nematic orders in a frustrated ferromagnet

We investigate the quantum phases of the frustrated spin-$\frac{1}{2}$ $J_1$-$J_2$-$J_3$ Heisenberg model on the square lattice with ferromagnetic $J_1$ and antiferromagnetic $J_2$ and $J_3$ interactions. Using the pseudo-fermion functional renormalization group technique, we find an intermediate paramagnetic phase located between classically ordered ferromagnetic, stripy antiferromagnetic, and incommensurate spiral phases. We observe that quantum fluctuations lead to significant shifts of the spiral pitch angles compared to the classical limit. By computing the response of the system with respect to various spin rotation and lattice symmetry-breaking perturbations, we identify a complex interplay between different nematic spin states in the paramagnetic phase. While retaining time-reversal invariance, these phases either break spin-rotation symmetry, lattice-rotation symmetry, or a combination of both. We therefore propose the $J_1$-$J_2$-$J_3$ Heisenberg model on the square lattice as a paradigmatic example where different intimately connected types of nematic orders emerge in the same model.

cond-mat.str-el↗

arXiv2016

Gate Tunable In- and Out-Of-Plane Spin-Orbit Coupling and Spin Splitting Anisotropy at LaAlO3/SrTiO3 (110) Interface

Manipulating spin-orbit coupling (SOC) is important for devices such as spin-orbit torque based memory and its understanding is neccessary to answer several fundamental open questions in triplet state superconductivity, topological insulators and Majorana fermions. Here we report spin splitting of 25 meV at the LaAlO3/SrTiO3 (110) interface for in-plane spins at a current density of 1.4x104 A/cm2, which is large compared to that found in semiconductor heterostructures or the LaAlO3/SrTiO3 (100) interface, and in addition it is anisotropic. The anisotropy arises from the difference in electron effective mass along the [001] and [1-10] directions. Our study predicts a spin splitting energy > 1000 meV at a current density of 107 A/cm2, which is enormous compared to metallic systems and will be an ideal spin polarized source. In addition to the in-plane effect, there is an unexpected gate-tunable out-of-plane SOC at the LaAlO3/SrTiO3 (110) interface when the spins lie out-of-plane due to broken symmetry in the plane of the interface. We demonstrate that this can be manipulated by varying the LaAlO3 thickness showing that this interface can be engineered for spin-orbit torque devices.

cond-mat.mes-hall↗

arXiv2016

Core-Collapse supernovae and its progenitors

Massive stars unable to sustain gravitational collapse, at the end of nuclear burning stage, turns out into core-collapse supernovae, leaving behind compact objects like neutron stars or black holes. The progenitor properties like mass and metallicity primarily governs the explosion parameters and type of compact remnant. In this contribution we summarize observational study of three Core Collapse type IIP SNe 2012aw, 2013ab and 2013ej, which are rigorously observed from ARIES and other Indian observatories and discuss their progenitor and explosion properties.

astro-ph.HE↗

arXiv2015

On the nature of wettability of van der Waals heterostructures

Wetting behaviour of surfaces is believed to be affected by van der Waals (vdW) forces, however, there is no clear demonstration of this. With the isolation of two-dimensional vdW layered materials it is possible to test this hypothesis. In this paper, we report the wetting behaviour of vdW heterostructures which include, chemical vapor deposition (CVD) grown graphene, molybdenum disulfide (MoS2) and tungsten disulfide (WS2) on few layers of hexagon boron nitride (h-BN) and SiO2/Si. Our study clearly shows that while this class of two-dimensional materials are not wetting transparent, there seems to be a significant amount of influence on their wetting properties by the underlying substrate due to dominant vdW forces. Contact angle measurements indicate that graphene and graphene-like layered transitional metal dichalcogenides invariably have intrinsically dispersive surfaces with a dominating London-vdW force-mediated wettability. Electric field controlled wetting studies of MoS2/WS2/SiO2/Si heterostructures were performed and no notable changes to the water contact angle was seen with applied voltage although two orders of magnitude change in resistance was observed. We postulate that the highly dispersive nature of these surfaces arising from the predominant London-vdW forces could be the reason for such observation.

cond-mat.mes-hall↗

arXiv2015

Photometric and polarimetric observations of fast declining Type II supernovae 2013hj and 2014G

We present broadband photometric and polarimetric observations of two type II supernovae (SNe) 2013hj and 2014G. SN 2014G is a spectroscopically classified type IIL event, which we also confirm photometrically as its light curve show characteristic features (plateau slope of 2.55 mag (100 d)$ ^{-1} $ in V-band and duration of $ \sim77 $d) of a generic IIL SN. On the other hand SN 2013hj also shows high plateau decline rate of $ 1.5 $ mag (100 d)$ ^{-1} $ in V-band, similar to SNe IIL, but marginally lower than SNe IIL template light curves. Our high cadence photometric observations of SNe 2013hj and 2014G enables us to cover all characteristic phases up to radioactive tail of optical light curves. Broadband polarimetric observations reveal some polarization in SN 2013hj with subtle enhancement as SN evolves towards plateau end, however the polarization angle remains constant throughout the evolution. This characteristic is consistent with the idea that the evolving SN with recombining hydrogen envelope is slowly revealing more asymmetric central region of explosion. Modelling of bolometric light curve yields a progenitor mass of $ \sim11 $M$_{\odot}$ with a radius of $ \sim700 $R$_{\odot}$ for SN 2013hj, while for SN 2014G model estimated progenitor mass is $ \sim9 $M$_{\odot}$ with a radius of $ \sim630 $R$_{\odot}$, both having a typical energy budget of $ \sim2\times10^{51} $ erg.

astro-ph.HE↗

arXiv2015

SN 2013ej - A type IIL supernova with weak signs of interaction

We present optical photometric and spectroscopic observations of supernova 2013ej. It is one of the brightest type II supernovae exploded in a nearby ($\sim 10$ Mpc) galaxy NGC 628. The light curve characteristics are similar to type II SNe, but with a relatively shorter ($ \sim85 $ day) and steeper ($ \sim1.7 $ mag (100 d)$^{-1} $ in V) plateau phase. The SN shows a large drop of 2.4 mag in V band brightness during plateau to nebular transition. The absolute ultraviolet (UV) light curves are identical to SN 2012aw, showing a similar UV plateau trend extending up to 85 days. The radioactive $^{56}$Ni mass estimated from the tail luminosity is $ 0.02 $M$_{\odot}$ which is significantly lower than typical type IIP SNe. The characteristics of spectral features and evolution of line velocities indicate that SN 2013ej is a type II event. However, light curve characteristics and some spectroscopic features provide strong support in classifying it as a type IIL event. A detailed SYNOW modelling of spectra indicates the presence of some high velocity components in H$α$ and H$β$ profiles, implying possible ejecta-CSM interaction. The nebular phase spectrum shows an unusual notch in the H$α$ emission which may indicate bipolar distribution of $^{56}$Ni. Modelling of the bolometric light curve yields a progenitor mass of $ \sim14 $M$_{\odot}$ and a radius of $ \sim450 $R$_{\odot}$, with a total explosion energy of $ \sim2.3\times10^{51} $ erg.

astro-ph.HE↗

arXiv2015

SN 2013ab : A normal type IIP supernova in NGC 5669

We present densely-sampled ultraviolet/optical photometric and low-resolution optical spectroscopic observations of the type IIP supernova 2013ab in the nearby ($\sim$24 Mpc) galaxy NGC 5669, from 2 to 190d after explosion. Continuous photometric observations, with the cadence of typically a day to one week, were acquired with the 1-2m class telescopes in the LCOGT network, ARIES telescopes in India and various other telescopes around the globe. The light curve and spectra suggest that the SN is a normal type IIP event with a plateau duration of $ \sim80 $ days with mid plateau absolute visual magnitude of -16.7, although with a steeper decline during the plateau (0.92 mag 100 d$ ^{-1} $ in $ V $ band) relative to other archetypal SNe of similar brightness. The velocity profile of SN 2013ab shows striking resemblance with those of SNe 1999em and 2012aw. Following the Rabinak & Waxman (2011) prescription, the initial temperature evolution of the SN emission allows us to estimate the progenitor radius to be $ \sim $ 800 R$_{\odot}$, indicating that the SN originated from a red supergiant star. The distance to the SN host galaxy is estimated to be 24.3 Mpc from expanding photosphere method (EPM). From our observations, we estimate that 0.064 M$_{\odot}$ of $^{56}$Ni was synthesized in the explosion. General relativistic, radiation hydrodynamical modeling of the SN infers an explosion energy of $ 0.35\times10^{51} $ erg, a progenitor mass (at the time of explosion) of $ \sim9 $ M$_{\odot}$ and an initial radius of $ \sim600 $ R$_{\odot}$.

astro-ph.HE↗

arXiv2014

Effective models for the magnetization behavior of Shastry-Sutherland model

The minimal effective model for the magnetisation plateaus below 1/2 in the Shastry-Sutherland model is derived to be an Ising model of certain unit-cell hard-core bosons with anisotropic repulsive interactions on isosceles triangular lattice. It unambiguously gives the prominent plateaus at 1/8, 1/6, 1/4, 1/3 and an additional one at 3/8, related through a particle-hole (p-h) transformation. The Dzyaloshinsky-Moriya (DM) interaction dresses it up with an inhomogeneous transverse field that also causes p-h asymmetry due to an in-plane component of the inter-dimer DM vector. This explains the asymmetry between the magnetisation below and above 1/4 in SrCu$_2$(BO$_3$)$_2$. The effective model above 1/2 plateau is an XXZ model with stronger XY parts. It gives no magnetisation plateaus, but exhibits chiral order.

cond-mat.str-el↗

arXiv2014

Edge modes in a frustrated quantum Ising chain

The ground state properties of an Ising chain with nearest ($J_{1}$) and next-nearest neighbor ($J_{2}$) interactions in a transverse field are investigated using the density matrix renormalization group and cluster mean-field theory methods. Its quantum phase diagram has two ordered regions (separated by a quantum disordered phase), one of which has simple ferromagnetic or Néel order depending upon the sign of $J_{1}$, and the other is the double-staggered antiferromagnetic phase. The presence of the Majorana-like edge-modes in these ordered phases is inferred by calculating the end-to-end spin-spin correlations on open chains. It is found that there occur four edge modes in the double-staggered phase, while the ferromagnetic and Néel phases support two edge modes, except very near $J_{1}=0$ where it seems they have four edge modes like the exact case at $J_{1}=0$ itself.

cond-mat.stat-mech↗

arXiv2014

The globular cluster system of NGC1316. III. Kinematic complexity

The merger remnant NGC 1316 (Fornax A) is one of the most important objects regarding the investigation of merger-related processes. We use kinematical data of globular clusters (GCs) and the diffuse stellar light to investigate the global structure of NGC 1316 and to constrain the dark matter content. We perform multi-object-spectroscopy with VLT/FORS2 and MXU. Out of 562 slits, we extract radial velocities for 177 GCs. Moreover, we measure radial velocities of the integrated galaxy light, using slits with a sufficiently bright "sky". To these data, we add 20 cluster velocities from Goudfrooij et al. (2001). In an appendix, we identify new morphological features of NGC 1316 and its companion galaxy NGC 1317. The GC sample based on radial velocities confirms the colour peaks already found in our photometric study. The bright clusters, which probably have their origin in a 2 Gyr-old starburst and younger star formation events, avoid the systemic velocity. A Gaussian velocity distribution is found only for clusters fainter than about m_R=22 mag. The velocity distribution of clusters shows a pronounced peak at 1600 km/s. These clusters populate a wide area in the south-western region which we suspect to be a disk population. Globular clusters or subsamples of them do not show a clear rotation signal. This is different from the galaxy light, where rotation along the major axis is discernable out to 3 arcmin radius. A simple spherical model like that suggested by dynamical analyses of planetary nebulae reproduces also the velocity dispersions of the faint GCs. The central dark matter density of the present model resembles a giant elliptical galaxy. This contradicts population properties which indicate spiral galaxies as pre-merger components. MOND would provide a solution, but the kinematical complexity of NGC 1316 does not allow a really firm conclusion. (abridged)

astro-ph.GA↗

arXiv2014

Apex determination and detection of stellar clumps in the open cluster M 67

We determined the cluster apex coordinates, studied the substructures and performed membership analysis in the central part (34'X33') of the open cluster M 67. We used the individual stellar apexes method developed earlier and classical technique of proper motion diagrams in coordinate system connected with apex. The neighbour-to-neighbour distance technique was applied to detect space details. The membership list was corrected and some stars were excluded from the most probable members list. The apex coordinates have been determined as: A0=132.97deg+/-0.81deg and D0=11.85deg+/-0.90deg. The 2D-space star density field was analysed and high degree of inhomogeneity was found.

astro-ph.GA↗

arXiv2014

Distance Determination To Eight Galaxies Using Expanding Photosphere Method

Type IIP supernovae are recognized as independent extragalactic distance indicators, however, keeping in view of the diverse nature of their observed properties as well as the availability of good quality data, more and newer events need to be tested for their applicability as a reliable distance indicators. We use early photometric and spectroscopic data of eight type-IIP SNe to derive distances to their host galaxies using the expanding photosphere method (EPM). For five of these, EPM is applied for the first time. In this work, we improved EPM application by using SYNOW estimated velocities and by semi-deconvolving the broadband filter responses while deriving color temperatures and black-body angular radii. We find that the derived EPM distances are consistent with that derived using other redshift independent methods.

astro-ph.CO↗

arXiv2013

X-ray flares observed from six young stars located in the region of star clusters NGC 869 and IC 2602

We present an analysis of seven intense X-ray flares observed from six stars (LAV 796, LAV 1174, SHM2002 3734, 2MASS 02191082+5707324, V553 Car, V557 Car) for the first time. These stars are located in the region of young open star clusters NGC 869 and IC 2602. These flares detected in the XMM-Newton data show a rapid rise (10-40 minutes) and a slow decay (20-90 minutes). The X-ray luminosities during the flares in the energy band 0.3-7.5 keV are in the range of $10^{29.9}$ to $10^{31.7}$ erg s$^{-1}$. The strongest flare was observed with the ratio $\sim 13$ for count rates at peak of the flare to the quiescent intensity. The maximum temperature during the flares has been found to be $\sim$100 MK. The semi loop lengths for the flaring loops are estimated to be of the order of $\rm{10^{10}}$ cm. The physical parameters of the flaring structure, the peak density, pressure, and minimum magnetic field required to confine the plasma have been derived and found to be consistent with flares from pre-main sequence stars in the Orion and the Taurus-Auriga-Perseus region.

astro-ph.SR↗

arXiv2013

X-ray observations of eight young open star clusters : I. Membership and X-ray Luminosity

We present a detailed investigation of X-ray source contents of eight young open clusters with ages between 4 to 46 Myr using archival X-ray data from XMM-Newton. The probable cluster memberships of the X-ray sources have been established on the basis of multi-wavelength archival data, and samples of 152 pre-main sequence (PMS) low mass (< 2 M_\odot), 36 intermediate mass (2 - 10 M_\odot) and 16 massive (> 10 M_\odot) stars have been generated. X-ray spectral analyses of high mass stars reveal the presence of high temperature plasma with temperature <2 keV, and mean L_X/L_{bol} of 10^{-6.9}. In the case of PMS low mass stars, the plasma temperatures have been found to be in the range of 0.2 keV to 3 keV with a median value of ~1.3 keV, with no significant difference in plasma temperatures during their evolution from 4 to 46 Myr. The X-ray luminosity distributions of the PMS low mass stars have been found to be similar in the young star clusters under study. This may suggest a nearly uniform X-ray activity in the PMS low mass stars of ages ~4--14 Myr. These observed values of L_X/L_{bol} are found to have a mean value of 10^{-3.6\pm0.4}, which is below the X-ray saturation level.The L_X/L_{bol} values for the PMS low mass stars are well correlated with their bolometric luminosities, that implies its dependence on the internal structure of the low mass stars. The difference between the X-ray luminosity distributions of the intermediate mass stars and the PMS low mass stars has not been found to be statistically significant. Their L_X/L_{bol} values, however have been found to be significantly different from each other with a confidence level greater than 99.999%. The possibility of X-ray emission from the intermediate mass stars due to a low mass star in close proximity of the intermediate mass star can not be ruled out due to the limited spatial resolution of XMM-Newton.

astro-ph.SR↗

arXiv2013

A photometric study of the Open Cluster II: Stellar population and dynamical evolution in NGC 559

We present UBVRI photometry of stars in the field of the intermediate-age open cluster NGC 559. By determining the stellar membership probabilities derived through a photometric and kinematic study of the cluster, we identify the 22 most probable cluster members. These are used to obtain robust cluster parameters. The mean proper motion of the cluster is μ_x = -3.29+/-0.35, μ_y = -1.24+/-0.28 mas/yr. The radial distribution of the stellar surface density gives a cluster radius of 4'.5+/-0'.2 (3.2+/-0.2 pc). By fitting solar metallicity stellar isochrones to the colour-colour and colour-magnitude diagrams, we find a uniform cluster reddening of E(B-V) = 0.82+/-0.02. The cluster has an age of 224+/-25 Myr and is at a distance of 2.43+/-0.23 kpc. From the optical and near-infrared two-colour diagrams, we obtain colour excesses in the direction of the cluster E(V-K) = 2.14+/-0.02, E(J-K) = 0.37+/-0.01, and E(B-V)= 0.76+/-0.04. A total-to-selective extinction of Rv=3.5+/-0.1 is found in the direction of the cluster which is marginally higher than the normal value. We derive the luminosity function and the mass function for the cluster main sequence. The mass function slope is found to be -2.1+/-0.3. We find evidence of mass segregation in this dynamically relaxed cluster.

astro-ph.GA↗

arXiv2013

SN 2007uy - metamorphosis of an aspheric Type Ib explosion

The supernovae of Type Ibc are rare and the detailed characteristics of these explosions have been studied only for a few events. Unlike Type II SNe, the progenitors of Type Ibc have never been detected in pre-explosion images. So, to understand the nature of their progenitors and the characteristics of the explosions, investigation of proximate events are necessary. Here we present the results of multi-wavelength observations of Type Ib SN 2007uy in the nearby ($\sim$ 29.5 Mpc) galaxy NGC 2770. Analysis of the photometric observations revealed this explosion as an energetic event with peak absolute R band magnitude $-18.5\pm0.16$, which is about one mag brighter than the mean value ($-17.6\pm0.6$) derived for well observed Type Ibc events. The SN is highly extinguished, E(B-V) = 0.63$\pm$0.15 mag, mainly due to foreground material present in the host galaxy. From optical light curve modeling we determine that about 0.3 M$_{\odot}$ radioactive $^{56}$Ni is produced and roughly 4.4 M$_{\odot}$ material is ejected during this explosion with liberated energy $\sim 15\times10^{51}$ erg, indicating the event to be an energetic one. Through optical spectroscopy, we have noticed a clear aspheric evolution of several line forming regions, but no dependency of asymmetry is seen on the distribution of $^{56}$Ni inside the ejecta. The SN shock interaction with the circumburst material is clearly noticeable in radio follow-up, presenting a Synchrotron Self Absorption (SSA) dominated light curve with a contribution of Free Free Absorption (FFA) during the early phases. Assuming a WR star, with wind velocity $\ga 10^3 {\rm km s}^{-1}$, as a progenitor, we derive a lower limit to the mass loss rate inferred from the radio data as $\dot{M} \ga 2.4\times10^{-5}$ M$_{\odot}$, yr$^{-1}$, which is consistent with the results obtained for other Type Ibc SNe bright at radio frequencies.

astro-ph.HE↗