arXiv subjects

Manas Sardar

Publications and source records attributed to Manas Sardar.

12 recordsLinked to original sources

arXiv2017

VO2 microcrystals as advanced smart window material at semiconductor to metal transition

Textured VO2(011) microcrystals are grown in the monoclinic, M1 phase which undergo a reversible first order semiconductor to metal transition (SMT) accompanied by a structural phase transition to rutile tetragonal, R phase. Around the phase transition, VO2 also experiences noticeable change in its optical and electrical properties. A change in color of the VO2 micro crystals from white to cyan around the transition temperature is observed, which is further understood by absorption of red light using temperature dependent ultraviolet-visible spectroscopic analysis and photoluminescence studies. The absorption of light in the red region is explained by the optical transition between Hubbard states, confirming the electronic correlation as the driving force for SMT in VO2. The thermochromism in VO2 has been studied for smart window applications so far in the IR region, which supports the opening of the band gap in semiconducting phase; whereas there is hardly any report in the management of visible light. The filtering of blue light along with reflection of infrared above the semiconductor to metal transition temperature make VO2 applicable as advanced smart windows for overall heat management of a closure.

cond-mat.mtrl-sci↗

arXiv2017

Role of 1-D finite size Heisenberg chain in increasing metal to insulator transition temperature in hole rich VO2

VO2 samples are grown with different oxygen concentrations leading to different monoclinic, M1 and triclinic, T insulating phases which undergo a first order metal to insulator transition (MIT) followed by a structural phase transition (SPT) to rutile tetragonal phase. The metal insulator transition temperature (Tc) was found to be increased with increasing native defects. Vanadium vacancy (VV) is envisaged to create local strains in the lattice which prevents twisting of the V-V dimers promoting metastable monoclinic, M2 and T phases at intermediate temperatures. It is argued that MIT is driven by strong electronic correlation. The low temperature insulating phase can be considered as a collection of one-dimensional (1-D) half-filled band, which undergoes Mott transition to 1-D infinitely long Heisenberg spin 1/2 chains leading to structural distortion due to spin-phonon coupling. Presence of VV creates localized holes (d0) in the nearest neighbor, thereby fragmenting the spin 1/2 chains at nanoscale, which in turn increase the Tc value more than that of an infinitely long one. The Tc value scales inversely with the average size of fragmented Heisenberg spin 1/2 chains following a critical exponent of 2/3, which is exactly the same predicted theoretically for Heisenberg spin 1/2 chain at nanoscale undergoing SPT (spin-Peierls transition). Thus, the observation of MIT and SPT at the same time in VO2 can be explained from our phenomenological model of reduced 1-D Heisenberg spin 1/2 chains. The reported increase (decrease) in Tc value of VO2 by doping with metal having valency less (more) than four, can also be understood easily with our unified model, for the first time, considering finite size scaling of Heisenberg chains.

cond-mat.mtrl-sci↗

arXiv2016

Far field photoluminescence imaging of single AlGaN nanowire in the sub-diffraction length scale using optical confinement of polarized light

Till now the nanoscale focussing and imaging in the sub-diffraction limit is achieved mainly with the help of plasmonic field enhancement assisted with noble metal nanoparticles. Using far field imaging technique, we have recorded polarized spectroscopic photoluminescence (PL) imaging of a single AlGaN nanowire (NW) of diameter ~ 100 nm using confinement of polarized light. The nanowires on the substrate have a nematic ordering. It is found that the PL from a single NW is influenced by the proximity to other NWs with the PL intensity scaling as 1/(lxd), where l and d are the NW length and the separation from the neighbouring NW, respectively. We show that this proximity induced PL intensity enhancement can be understood, if we assume the existence of reasonably long lived photons in the intervening space between the NWs. A nonzero non-equilibrium population of such photons causes stimulated emission leading to the enhanced PL emission with the intensity scaling as 1/(lxd). The effect is analogous to the Purcell enhancement of polarized optical emissions induced by confined photons in micro-cavities. The enhancement of PL emission facilitated the far field spectroscopic imaging of a single semiconducting nanowire in the sub-diffraction regime.

cond-mat.mtrl-sci↗

arXiv2015

Surface functionalization enhanced magnetism in SnO_2 nanoparticles and its correlation to photoluminescence properties

High value of magnetic moment 0.08 emu/g at room temperature for SnO_2 nanoparticles (NPs) was observed. Surface functionalization with octadecyltrichlorosilane (OTS) enhanced the saturation magnetic moment of NPs to an anomalously high value of 0.187 emu/g by altering the electronic configuration on NPs surface. Surface functionalization also suppressed photoluminescence (PL) peaks arising from oxygen defects around 2 eV and caused an increase in the intensities of two peaks near violet region (2.6 - 3 eV). PL studies under uniform external magnetic field enriched understanding of the role of OTS. Both OTS and external magnetic field significantly modulated the luminescence spectra, by altering the surface electronic structure of NPs. Extra spins on the surface of SnO2 NPs created by the surface functionalization process and their influence on resultant magnetic moment and luminescence properties are discussed in details.

cond-mat.mtrl-sci↗

arXiv2007

Bulk ferromagnetism and large changes in photoluminescence intensity by magnetic field in $β$-Ga$_2$O$_3$

In this letter we report observation of room temperature ferromagnetism in bulk Ga$_2$O$_3$ . We also observe large (10-60%) increase/decrease in photoluminescence. In the red(700 nm wavelength)/blue(500 nm), with the application of small magnetic field(0.4 Tesla). We argue, that ferromagnetism occurs entirely due to chains of oxygen(O(3) sites, Fig. 5) vacancies. We propose a simple model to explain, origin and location of moment, formation of ferromagnetic dislocation needles, and strong increase/decrease ofred/blue photoluminescence intensity with magnetic field.

cond-mat.mtrl-sci↗

arXiv2004

Is Li$_2$Pd$_3$B a self-doped hole superconductor ?

We propose that the electrons responsible for superconductivity in Li2Pd3B come from the palladium 4d-electrons. So, its electronic properties are likely to be dominated by strong electronic correlations. The basic unit in this material are Pd$_6$B octahedra which share vertices to form a 3-dimensional network. Due to the highly distorted nature of the Pd$_6$B octahedron, one far stretched Pd atom per octahedra becomes almost inactive for electronic conduction. Thus, the material escapes the fate of becoming a half- filled insulating Mott antiferromagnet by hiding extra charges at these inactive Pd sites and becomes a self-doped correlated metal. We propose a 3-dimensional single band t-J model which could be the correct minimal model for this material.

cond-mat.supr-con↗

arXiv2003

Superconductivity in Na$_x$CoO$_2$$\cdot$yH$_2$O : Is Spin-Charge Separation Protecting a d$_1$+id$_2$ State ?

Superconductivity in Na$_x$CoO$_2$$\cdot$yH$_2$O is likely to be a p or d-wave; however, experiments are unable to pinpoint the symmetry. A simple estimate of pair breaking effects from an unavoidable `Na$^+$ vacancy disorder' in an ordered Na$^+$ lattice, at an optimal $x_{\rm opt} \approx 0.30$ is shown to destroy a Fermi liquid based p or d-wave superconductivity. However, a robustness of superconducting and normal states, seen in experiments is pointed out and argued to imply presence of a `quantum protectorate', possibly a `spin-charge decoupling' that protects a d$_1$+id$_2$ and not a p-state. A calculation of Knight shift and ${1\over T_1}$ in the framework of RVB mean field theory and a fit to the data of Kobayashi ${\it et al.}$ [9] is made.

cond-mat.supr-con↗

arXiv2000

Spin polarised carrier injection into high T_c superconductor: A test for the superconductivity mechanism?

We point out in this short communique, that be it a s-wave or d-wave superconductor, in a non equalibrium situation(i.e. in presence of excess unpolarised/polarised quasi-particles maintained by an injection current) the superconducting gap suppresion by the presence of same amount excess quasi-particles, can at best differ by a factor of 2, for a conventional BCS superconductor. For the high $T_c$ superconductors on the other hand, there is a huge difference in gap suppresion between upolarised/polarised quasiparticle injection, as observed in the experiments\cite{gold}. We argue that this this large anomaly has a natural explanation within the {\it Interleyer tunneling mechanism} of Wheatley, Hsu and Anderson\cite{and}, and is due to the excess polarised quasiparticles blocking the interlayer pair tunneling process. We also point out that spin polarised quasi-particle injection in a superconductor is a very easy way to distinguish between a s-wave or an anisotropic gap superconductor.

cond-mat.supr-con↗

arXiv2000

Scaling of transition temerature and CuO_2 plane buckling in the cuprate superconductors

Recently it is seen\cite{Nature},that both $\rm{T}_c$ and the buckling of the $CuO_2$ planes goes through a maximum at the same doping level. We show that only for optimal doping concentration the Fermi surface touches the M($0,π$) point in the BZ, where the matrix element for interlayer pair tunneling amplitude is largest, so that the gain in delocalization energy by tunneling(in pairs) along the $c$ axis is largest. Buckling of the planes on the other hand modulates the separation between the planes and thereby modulates the interlayer pair tunneling amplitude. That is why both $\rm{T}_c$ and buckling angle(Oxygen atom displacement out of the plane) scales the same way with doping concentration. We have calculated $\rm{T}_c$ and buckling angle for various doping concentration. The agreement with experiment is remarkably good. We also point out the possible reason for large(about 1 percent) change of the buckling mode phonon frequency, accross the transition temperature. scattering

cond-mat↗

arXiv1996

Raman continuum in high-$T_c$ superconductors

Experimentally, Raman continuum for $A_g$ and $B_g$ geometry exhibits peaks far apart from each other (about 80 to 150 $\rm cm^{-1}$) in frequency. The former is insensitive to doping over a small range where $T_c$ does not vary much whereas the latter shifts towards higher frequencies. We calculate the electronic Raman scattering intensities using the `modified spin bag model'. We show that the calculated results have natural explanation to the observed anamolous peak separation and peculiar doping dependence.

cond-mat↗

arXiv1994

Essential Differences between $ab$ and $c$ Axis Tunneling and Zero Bias Conductance in the Cuprates

The peculiarities in tunneling characteristics have been studied in the light of the controversy between s-wave and d-wave character of High $T_c$ superconductivity. We show that anisotropic s-wave gap has the same low voltage power law conductance and two peak structure in the density of states as d-wave superconductors. The asymmetric tunneling conductance and zero bias conductance for the c-axis tunneling is shown to occur because of finite band splitting coming from the interlayer hopping parameter.

cond-mat↗

arXiv1994

Effect of Single Particle Hopping and out of Plane Magnetic Impurity on Coupled Planar Superconductors

It is shown that the single particle band motion along the $c$ axis is harmful for superconductivity. Variation of $T_c$ with $c$ axis hopping parameter is shown for both the conventional planar superconductors and for interlayer pair tunneling mechanism of Wheatley Hsu and Anderson(WHA).Effect of out of plane magnetic impurity substitution is shown to suppres $T_c$ more for conventional superconductors whereas there is very sharp decrease of $T_c$ in the WHA mechanism at larger concentrations.

cond-mat↗