arXiv subjects

A. K. Sood

Publications and source records attributed to A. K. Sood.

At least 109 records · Page 6Linked to original sources

arXiv2010

Probing top-gated field effect transistor of reduced graphene oxide monolayer made by dielectrophoresis

We demonstrate top-gated field effect transistor made of reduced graphene oxide (RGO) monolayer (graphene) by dielectrophoresis. Raman spectrum of RGO flakes of typical size of 5μm x 5μm show a single 2D band at 2687 cm-1, characteristic of a single layer graphene. The two probe current - voltage measurements of RGO flakes, deposited in between the patterned electrodes with a gap of 2.5 μm using a.c. dielectrophoresis show ohmic behavior with a resistance of ~ 37kΩ. The temperature dependence of the resistance (R) of RGO measured between temperatures 305 K to 393 K yields temperature coefficient of resistance [dR/dT]/R ~ -9.5x10-4 K-1, same as mechanically exfoliated single layer graphene. The field effect transistor action was obtained by electrochemical top-gating using solid polymer electrolyte (PEO + LiClO4) and Pt wire. Ambipolar nature of graphene flakes is observed upto a doping level of ~ 6x1012/cm2 and carrier mobility of ~ 50 cm2V-1sec-1. The source - drain current characteristics shows a tendency of current saturation at high source - drain voltage which is analyzed quantitatively by a diffusive transport model.

cond-mat.mes-hall↗

arXiv2010

Temperature-Dependent Infrared Reflectivity Studies of Multiferroic TbMnO_{3}: Evidence for Spin-Phonon Coupling

We have measured near normal incidence far infrared (FIR) reflectivity spectra of a single crystal of TbMnO3 from 10K to 300K in the spectral range of 50 cm$^{-1}$ to 700 cm$^{-1}$. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function $ε_2$($ω$) and energy loss function Im(-1/$ε$($ω$)), respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature T$_N$ (~ 46K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the super-exchange integral between the Mn$^{3+}$ spins. The effective charge of oxygen (Z$_O$) calculated using the measured LO-TO splitting increases below T$_N$.

cond-mat.mtrl-sci↗

arXiv2010

Anomalous Raman scattering from phonons and electrons of superconducting FeSe$_{0.82}$

We report interesting anomalies in the temperature dependent Raman spectra of FeSe$_{0.82}$ measured from 3K to 300K in the spectral range from 60 to 1800 cm$^{-1}$ and determine their origin using complementary first-principles density functional calculations. A phonon mode near 100 cm$^{-1}$ exhibits a sharp increase by $\sim$ 5% in frequency below a temperature T$_s$ ($\sim$ 100 K) attributed to strong spin-phonon coupling and onset of short-range antiferromagnetic order. In addition, two high frequency modes are observed at 1350 cm$^{-1}$ and 1600 cm$^{-1}$, attributed to electronic Raman scattering from ($x^2-y^2$)to $xz$ / $yz$ $d$-orbitals of Fe.

cond-mat.supr-con↗

arXiv2010

Raman evidence for Orbiton-Mediated Multiphonon Scattering in Multiferroic TbMnO$_3$

Temperature-dependent Raman spectra of TbMnO$_3$ from 5 K to 300 K in the spectral range of 200 to 1525 cm$^{-1}$ show five first-order Raman allowed modes and two high frequency modes. The intensity ratio of the high frequency Raman band to the corresponding first order Raman mode is nearly constant and high ($\sim$ 0.6) at all temperatures, suggesting a orbiton-phonon mixed nature of the high frequency mode. One of the first order phonon modes shows anomalous softening below T$_N$ ($\sim$ 46 K), suggesting a strong spin-phonon coupling.

cond-mat.str-el↗

arXiv2010

Banded spatiotemporal chaos in sheared nematogenic fluids

We present the results of a numerical study of a model of the hydrodynamics of a sheared nematogenic fluid, taking into account the effects of order parameter stresses on the velocity profile, but allowing spatial variations only in the gradient direction. When parameter values are such that the stress from orientational distortions is comparable to the bare viscous stress, the system exhibits steady states with the characteristics of shear banding. In addition, nonlinearity in the coupling of extensional flow to orientation leads to the appearance of a new steady state in which the features of both spatiotemporal chaos and shear banding are present.

cond-mat.soft↗

arXiv2010

Femtosecond carrier dynamics and saturable absorption in graphene suspensions

Nonlinear optical properties and carrier relaxation dynamics in graphene, suspended in three different solvents, are investigated using femtosecond (80 fs pulses) Z-scan and degenerate pumpprobe spectroscopy at 790 nm. The results demonstrate saturable absorption property of graphene with a nonlinear absorption coefficient, $beta$, of ~2 to 9x10^-8 cm/W. Two distinct time scales associated with the relaxation of photoexcited carriers, a fast one in the range of 130-330 fs (related to carrier-carrier scattering) followed by a slower one in 3.5-4.9 ps range (associated with carrier-phonon scattering) are observed.

cond-mat.mtrl-sci↗

arXiv2009

Giant low frequency velocity fluctuations in a driven vortex lattice

The driven state of a well-ordered flux line lattice in a single crystal of 2H-NbSe2 in the time domain has revealed the presence of substantial fluctuations in velocity, with large and distinct time periods (~ seconds). A superposition of a periodic drive in the driven vortex lattice causes distinct changes in these fluctuations. We propose that prior to onset of the peak effect there exist hithertofore unexplored regime of coherent dynamics, with unexpected behaviour in velocity fluctuations.

cond-mat.supr-con↗

arXiv2009

Formation of p-n junction in polymer electrolyte-top gated bilayer graphene transistor

We show simultaneous p and n type carrier injection in bilayer graphene channel by varying the longitudinal bias across the channel and the top gate voltage. The top gate is applied electrochemically using solid polymer electrolyte and the gate capacitance is measured to be 1.5 $μF/cm^2$, a value about 125 times higher than the conventional SiO$_2$ back gate capacitance. Unlike the single layer graphene, the drain-source current does not saturate on varying the drain-source bias voltage. The energy gap opened between the valence and conduction bands using top and back gate geometry is estimated.

cond-mat.mes-hall↗

arXiv2009

Low temperature and high pressure Raman and x-ray studies of pyrochlore Tb$_2$Ti$_2$O$_7$ : phonon anomalies and possible phase transition

We have carried out temperature and pressure-dependent Raman and x-ray measurements on single crystals of Tb$_2$Ti$_2$O$_7$. We attribute the observed anomalous temperature dependence of phonons to phonon-phonon anharmonic interactions. The quasiharmonic and anharmonic contributions to the temperature-dependent changes in phonon frequencies are estimated quantitatively using mode Grüneisen parameters derived from pressure-dependent Raman experiments and bulk modulus from high pressure x-ray measurements. Further, our Raman and x-ray data suggest a subtle structural deformation of the pyrochlore lattice at $\sim$ 9 GPa. We discuss possible implications of our results on the spin-liquid behaviour of Tb$_2$Ti$_2$O$_7$.

cond-mat.mtrl-sci↗

arXiv2008

Temperature dependent Raman and x-ray studies of spin-ice pyrochlore $Dy_2Ti_2O_7$ and non-magnetic pyrochlore $Lu_2Ti_2O_7$

We present here temperature-dependent Raman, x-ray diffraction and specific heat studies between room temperature and 12 K on single crystals of spin-ice pyrochlore compound $Dy_2Ti_2O_7$ and its non-magnetic analogue $Lu_2Ti_2O_7$. Raman data show a "new" band not predicted by factor group analysis of Raman-active modes for the pyrochlore structure in $Dy_2Ti_2O_7$, appearing below a temperature of $T_c=$110 K with a concomitant contraction of the cubic unit cell volume as determined from the powder x-ray diffraction analysis. Low temperature Raman experiments on O$^{18}$-isotope substituted $Dy_2Ti_2O_7$ confirm the phonon origin of the "new" mode. These findings, absent in $Lu_2Ti_2O_7$, suggest that the room temperature cubic lattice of the pyrochlore $Dy_2Ti_2O_7$ undergoes a "subtle" structural transformation near $T_c$. We find anomalous \textit{red-shift} of some of the phonon modes in both the $Dy_2Ti_2O_7$ and the $Lu_2Ti_2O_7$ as the temperature decreases, which is attributed to strong phonon-phonon anharmonic interactions.

cond-mat.mtrl-sci↗

arXiv2008

Nonequilibrium Fluctuation Relation for Sheared Micellar Gel in a Jammed State

We show that the shear rate at a fixed shear stress in a micellar gel in a jammed state exhibits large fluctuations, showing positive and negative values, with the mean shear rate being positive. The resulting probability distribution functions (PDF's) of the global power flux to the system vary from Gaussian to non-Gaussian, depending on the driving stress and in all cases show similar symmetry properties as predicted by Gallavotti-Cohen steady state fluctuation relation. The fluctuation relation allows us to determine an effective temperature related to the structural constraints of the jammed state. We have measured the stress dependence of the effective temperature. Further, experiments reveal that the effective temperature and the standard deviation of the shear rate fluctuations increase with the decrease of the system size.

cond-mat.soft↗

arXiv2008

Phonon renormalisation in doped bilayer graphene

We report phonon renormalisation in bilayer graphene as a function of doping. The Raman G peak stiffens and sharpens for both electron and hole doping, as a result of the non-adiabatic Kohn anomaly at the $Γ$ point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This results in a change of slope in the variation of G peak position with doping, which allows a direct measurement of the interlayer coupling strength.

cond-mat.mes-hall↗

arXiv2008

Ultrafast switching time and third order nonlinear coefficients of microwave treated single walled carbon nanotube suspensions

Microwave treated water soluble and amide functionalized single walled carbon nanotubes have been investigated using femtosecond degenerate pump-probe and nonlinear transmission experiments. The time resolved differential transmission using 75 femtosecond pulse with the central wavelength of 790 nm shows a bi-exponential ultrafast photo-bleaching with time constants of of 160 fs (130 fs) and 920 fs (300 fs) for water soluble (amide functionalized) nanotubes. Open and closed aperture z-scans show saturation absorption and positive (negative) nonlinear refraction for water soluble (amide functionalized) nanotubes. Two photon absorption coefficient,beata ~250 cm/GW (650 cm/GW) and nonlinear index, gamma ~ 15 cm^2/pW (-30 cm^2/pW) are obtained from the theoretical fit in the saturation limit to the data for two types of nanotubes.

cond-mat.mtrl-sci↗

arXiv2008

Ultrafast electron dynamics and cubic optical nonlinearity of free standing thin film of double walled carbon nanotubes

Ultrafast degenerate pump-probe experiments performed on a free standing film of double walled carbon nanotubes near the first metallic transition energy of outer tube show ultrafast ($97 fs$) photobleaching followed by a photo-induced absorption with a slow relaxation of $1.8 ps$. Femtosecond closed and open aperture z-scan experiments carried out at the same excitation energy show saturation absorption and negative cubic nonlinearity. From these measurements, real and imaginary part of the third order nonlinear susceptibility are estimated to be Re($χ^{(3)}) \sim -2.2 \times 10 ^{-9} esu$ and Im($χ^{(3)}) \sim 1.1 \times 10 ^{-11} esu$.

cond-mat.other↗

arXiv2007

Strong Correlations and Fickian Water Diffusion in Narrow Carbon Nanotubes

We have used atomistic molecular dynamics (MD) simulations to study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules. The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube show solid-like ordering at room temperature, which we quantify by calculating the pair correlation function. It is shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube is Fickian and not sub-diffusive. We also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. We find good agreement between the mean-square displacements calculated from the random walk model and from MD simulations, thereby confirming that the water molecules undergo normal-mode diffusion inside the nanotube. We attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube is shown to scale quadratically with the nanotube length.

cond-mat.stat-mech↗

arXiv2007

Manifestation of geometric frustration on magnetic and thermodynamic properties of pyrochlores $Sm_2X_2O_7$ (X=Ti, Zr)

We present here magnetization, specific heat and Raman studies on single-crystalline specimens of the first pyrochlore member $Sm_2Ti_2O_7$ of the rare-earth titanate series. Its analogous compound $Sm_2Zr_2O_7$ in the rare-earth zirconate series is also investigated in the polycrystalline form. The Sm spins in $Sm_2Ti_2O_7$ remain unordered down to at least T = 0.5 K. The absence of magnetic ordering is attributed to very small values of exchange ($θ_{cw} ~ -0.26 K$) and dipolar interaction ($μ_{eff} ~ 0.15 μ_B$) between the $Sm^{3+}$ spins in this pyrochlore. In contrast, the pyrochlore $Sm_2Zr_2O_7$ is characterized by a relatively large value of Sm-Sm spin exchange ($θ_{cw} ~ - 10 K$); however, long-range ordering of the $Sm^{3+}$ spins is not established at least down to T = 0.67 K, due to frustration of the $Sm^{3+}$ spins on the pyrochlore lattice. The ground state of $Sm^{3+}$ ions in both pyrochlores is a well-isolated Kramer's doublet. The higher-lying crystal field excitations are observed in the low-frequency region of the Raman spectra of the two compounds recorded at T = 10 K. At higher temperatures, the magnetic susceptibility of $Sm_2Ti_2O_7$ shows a broad maximum at T = 140 K while that of $Sm_2Zr_2O_7$ changes monotonically. Whereas $Sm_2Ti_2O_7$ is a promising candidate for investigating spin-fluctuations on a frustrated lattice as indicated by our data, the properties of $Sm_2Zr_2O_7$ seem to conform to a conventional scenario where geometrical frustration of the spin exclude their long-range ordering.

cond-mat.mtrl-sci↗

arXiv2007

Large nonlinear absorption and refraction coefficients of carbon nanotubes estimated from femtosecond Z-scan measurements

Nonlinear transmission of 80 and 140 femtosecond pulsed light with $0.79 μm$ wavelength through single walled carbon nanotubes suspended in water containing sodium dodecyl sulphate is studied. Pulse-width independent saturation absorption and negative cubic nonlinearity are observed, respectively, in open and closed aperture Z-scan experiments. The theoretical expressions derived to analyze the z-dependent transmission in the saturable limit require two photon absorption coefficient $β_0\sim$ $1.4 cm/MW$ and a nonlinear index $γ\sim -5.5 \times10^{-11} cm^2/W$ to fit the data.

cond-mat.mtrl-sci↗

arXiv2007

Granger Causality and Cross Recurrence Plots in Rheochaos

Our stress relaxation measurements on wormlike micelles using a Rheo-SALS (rheology + small angle light scattering) apparatus allow simultaneous measurements of the stress and the scattered depolarised intensity. The latter is sensitive to orientational ordering of the micelles. To determine the presence of causal influences between the stress and the depolarised intensity time series, we have used the technique of linear and nonlinear Granger causality. We find there exists a feedback mechanism between the two time series and that the orientational order has a stronger causal effect on the stress than vice versa. We have also studied the phase space dynamics of the stress and the depolarised intensity time series using the recently developed technique of cross recurrence plots (CRPs). The presence of diagonal line structures in the CRPs unambiguously proves that the two time series share similar phase space dynamics.

nlin.CD↗