arXiv subjects

A. K. Sood

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

At least 145 records · Page 8Linked to original sources

arXiv2003

Electron Paramagnetic Resonance Study of Phase Segregation in Nd$_{0.5}$Sr$_{0.5}$MnO$_3$

We present results of an electron paramagnetic resonance (EPR) study of Nd$_{1-x}$Sr$_x$MnO$_3$ with x = 0.5 across the paramagnetic to ferromagnetic, insulator to metal transition at 260 K (T$_c$) and the antiferromagnetic, charge ordering transition (T$_N$ = T$_{co}$) at 150 K. The results are compared with those on Nd$_{0.45}$Sr$_{0.55}$MnO$_3$ which undergoes a transition to a homogeneous A-type antiferromagnetic phase at T$_N$ = 230 K and on La$_{0.77}$Ca$_{0.23}$MnO$_3$ which undergoes a transition to coexisting ferromagnetic metallic and ferromagnetic insulating phases. For x = 0.5, the EPR signals below T$_c$ consist of two Lorentzian components attributable to the coexistence of two phases. From the analysis of the temperature dependence of the resonant fields and intensities, we conclude that in the mixed phase ferromagnetic and A-type antiferromagnetic (AFM) phases coexist. The x = 0.55 compound shows a single Lorentzian throughout the temperature range. The signal persists for a few degrees below T_N. The behaviour of the A-type AFM phase is contrasted with that of the two ferromagnetic phases present in La$_{0.77}$Ca$_{0.23}$MnO$_3$. The comparison of behaviour of A-type AFM signal observed in both Nd$_{0.5}$Sr$_{0.5}$MnO$_3$and Nd$_{0.45}$Sr$_{0.55}$MnO$_3with the two FM phases of La$_{0.77}$Ca$_{0.23}$MnO$_3$, vis-a-vis the shift of resonances with respect to the paramagnetic phases and the behaviour of EPR intensity as a function of temperature conclusively prove that the Nd$_{0.5}$Sr$_{0.5}$MnO$_3$undergoes phase separation into A-type AFM and FM phases.

cond-mat.str-el↗

arXiv2003

Brillouin Scattering Studies of La_{0.77}Ca_{0.23}MnO_3 Across Metal-Insulator Transition

Temperature-dependent Brillouin scattering studies have been carried out on La_{0.77}Ca_{0.23}MnO_3 across the paramagnetic insulator - ferromagnetic metal (I-M) transition. The spectra show a surface Rayleigh wave (SRW) and a high velocity pseudo surface acoustic wave (HVPSAW) besides bulk acoustic waves (BAW). The Brillouin shifts associated with SRW and HVPSAW show blue-shifts, where as the frequencies of the BAW decrease below the I-M transition temperature (T_C) of 230 K. These results can be understood based on the temperature dependence of the elastic constants. We also observe a central peak whose width is maximum at T_C.

cond-mat.mtrl-sci↗

arXiv2002

An Electron Paramagnetic Resonance Study of Electron-Hole Asymmetry in Charge Ordered Pr_1-x Ca_xMnO_3 (x=0.64, 0.36)

We present and compare the results of temperature-dependent electron paramagnetic resonance (EPR) studies on Pr_1-x Ca_xMnO_3 for x = 0.64 which is electron-doped with those on the hole doped x = 0.36 composition. The temperature dependence of the various parameters obtained from the powder and single crystal spectra show significant differences between the two manganites. At room temperature the `g' parameter for the electron doped system is less than the free electron `g' value `g_e', whereas for the hole-doped system it is more than `g_e'. Further, the `g' value and the linewidth obtained from the powder spectra as well as the single crystal spectra show different functional dependences on temperature in the two systems. Quite strikingly, the peak observed at T_{co} in the temperature dependence of the asymmetry parameter, alpha, of the single crystal spectra in the hole-doped system is absent in the electron-doped system. We understand this contrasting behaviour of the EPR parameters in the two systems in terms of very different nature of microscopic interactions in them.

cond-mat.str-el↗

arXiv2002

Raman Scattering in charge ordered Pr_{0.63}Ca_{0.37}MnO_{3} : Anomalous Temperature Dependence of Linewidth

We report the evolution of the Raman active A_g phonon modes in the charge-ordered manganite Pr_{0.63}Ca_{0.37}MnO_{3} as a function of temperature from 300K to 25K. Our studies reveal that the linewidths of A_g(2) and A_g(4) phonons increase as temperature decreases. This anomalous temperature dependence of phonon lineshapes, seen for the first time in charge ordered manganites, can be quantitatively understood in terms of strong spin-phonon coupling involving t_{2g} spins and A_g phonons.

cond-mat.str-el↗

arXiv2001

Temperature-dependent electron paramagnetic resonance studies of charge ordered Nd_0.5Ca_0.5MnO_3

We report Electron Paramagnetic Resonance measurements on single crystalline and powder samples of Nd_0.5Ca_0.5MnO_3 across the charge-ordering transition at T_co=240 K down to the antiferromagnetic ordering transition at T_N = 140 K. The changes in the linewidth, g-factor and intensity as functions of temperature are studied to understand the nature of spin-dynamics in the system. We explain the observed large decrease in the linewidth from T_N to T_co in terms of motional narrowing caused by the hopping of the Jahn-Teller polarons yielding an activation energy of E_a = 0.13 eV. Similar analysis of data on Pr_0.6Ca_0.4MnO_3 published earlier gives E_a=0.2 eV. Below T_co, the g-value increases continuously suggesting a gradual strengthening of the orbital ordering. We give a qualitative explanation of the temperature dependence and of the maximum in the asymmetry ratio A/B observed at T_co in single crystal spectra which also supports the themodel of motional narrowing.

cond-mat.str-el↗

arXiv2001

Origin of the unusual dependence of Raman D band on excitation wavelength in graphite-like materials

We have revisited the still unresolved puzzle of the dispersion of the Raman disordered-induced D band as a function of laser excitation photon energy E$_L$ in graphite-like materials. We propose that the D-mode is a combination of an optic phonon at the K-point in the Brillioun zone and an acoustic phonon whose momentum is determined uniquely by the double resonance condition. The fit of the experimental data with the double-resonance model yields the reduced effective mass of 0.025m$_{e}$ for the electron-hole pairs corresponding to the A$_{2}$ transition, in agreement with other experiments. The model can also explain the difference between $ω_S$ and $ω_{AS}$ for D and D$^{\star}$ modes, and predicts its dependence on the Raman excitation frequency.

cond-mat.dis-nn↗

arXiv2001

Chaotic Dynamics in Shear-thickening Surfactant Solutions

We report the observation of dynamical behaviour in dilute, aqueous solutions of a surfactant CTAT (cetyl trimethylammonium p-toluenesulphonate), below the overlap concentration c$^{\star}$. At these concentrations, CTAT forms cylindrical micelles and shows a pronounced shear thickening transition above a concentration-dependent critical shear rate $\dotγ_{c}$. An analysis of the time-series of the stress relaxations at controlled shear rates in the shear-thickening regime shows the existence of correlation dimensions greater than two and positive Lyapunov exponents. This indicates the existence of deterministic chaos in the dynamics of stress relaxation at these concentrations and shear rates. The observed chaotic behaviour may be attributed to the stick-slip between the shear - induced structure (SIS) formed in the sheared surfactant solution and the coexisting dilute phase. At still higher shear rates, when the SIS spans the Couette, there is a transition to higher-dimensional dynamics arising out of the breakage and recombination of the SIS.

cond-mat.soft↗

arXiv2001

Photon transport in thin disordered slabs

We examine using Monte Carlo simulations, photon transport in optically `thin' slabs whose thickness L is only a few times the transport mean free path $l^{*}$, with particles of different scattering anisotropies. The confined geometry causes an auto-selection of only photons with looping paths to remain within the slab. The results of the Monte Carlo simulations are borne out by our analytical treatment which incorporates the directional persistence by the use of the Ornstein-Uhlenbeck process, which interpolates between the short time ballistic and long time diffusive regimes.

cond-mat.dis-nn↗

arXiv2001

Non-resonant microwave absorption studies of superconducting MgB_2

Non-resonant microwave absorption(NRMA) studies of superconducting MgB_2 at a frequency of 9.43 GHz in the field range -50 Gauss to 5000 Gauss are reported. The NRMA results indicate near absence of intergranular weak links. A linear temperature dependence of the lower critical field H_c1 is observed indicating a non s-wave superconductivity. However, the phase reversal of the NRMA signal which could suggest d-wave symmetry is also not observed.

cond-mat.supr-con↗

arXiv2000

Effect of Screening of Intermicellar Interactions on the Linear and Nonlinear Rheology of a Viscoelastic Gel

We report our studies of the linear and nonlinear rheology of aqueous solutions of the surfactant cetyl trimethylammonium tosylate (CTAT) with varying amounts of sodium chloride (NaCl). The CTAT concentration is fixed at 42mM and the salt concentration is varied between 0mM to 120mM. On increasing the salt (NaCl) concentration, we see three distinct regimes in the zero-shear viscosity and the high frequency plateau modulus data. In regime I, the zero-shear viscosity shows a weak increase with salt concentration due to enhanced micellar growth. The decrease in the zero-shear viscosities with salt concentration in regimes I I and III can be explained in terms of inter-micellar branching. The most intriguing feature of our data however, is the anomalous behavior of the high frequency plateau modulus in regime II (0.12 $\le \frac{[NaCl]}{[CTAT]} \le$ 1. 42). In this regime, the plateau modulus {\it increases} with an increase in NaCl concentration. This is highly counter-intuitive, since the correlation length of concentration fluctuations and hence the plateau modulus $G_{\circ}$ are not expected to change appreciably in the semi-dilute regime. We propose to explain the changes in regime II in terms of the unbinding of the organic counterions (tosylate) from the CTA$^{+}$ surfaces on the addition of NaCl. In the nonlinear flow curves of the samples with high salt content, significant deviations from the predictions of the Giesekus model for entangled micelles are observed.

cond-mat.soft↗

arXiv2000

Rheology of Semi-dilute Solutions of Calf-thymus DNA

We study the rheology of semi-dilute solutions of the sodium salt of calf-thymus DNA in the linear and nonlinear regimes. The frequency response data can be fitted very well to the hybrid model with two dominant relaxation times $τ_{\circ}$ and $τ_{1}$. The ratio $\frac{τ_{\circ}}{τ_{1}} \sim$ 5 is seen to be fairly constant on changing the temperature from 20$^{\circ}$C to 30$^{\circ}$C. The shear rate dependence of viscosity can be fitted to the Carreau model.

cond-mat.soft↗

arXiv2000

Dynamical Behaviour in the Nonlinear Rheology of Surfactant Solutions

Several surfactant molecules self-assemble in solution to form long, flexible wormlike micelles which get entangled with each other, leading to viscoelastic gel phases. We discuss our recent work on the rheology of such a gel formed in the dilute aqueous solutions of a surfactant CTAT. In the linear rheology regime, the storage modulus $G^{\prime}(ω)$ and loss modulus $G^{\prime\prime}(ω)$ have been measured over a wide frequency range. In the nonlinear regime, the shear stress $σ$ shows a plateau as a function of the shear rate $\dotγ$ above a certain cutoff shear rate $\dotγ_c$. Under controlled shear rate conditions in the plateau regime, the shear stress and the first normal stress difference show oscillatory time-dependence. The analysis of the measured time series of shear stress and normal stress has been done using several methods incorporating state space reconstruction by embedding of time delay vectors.The analysis shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics of the observed mechanical instability can be described by that of a dynamical system with a strange attractor of dimension varying from 2.4 to 2.9.

cond-mat.soft↗

arXiv2000

An electron paramagnetic resonance study of Pr_{0.6}Ca_{0.4}MnO_{3} across the charge ordering transition

We report the first electron paramagnetic resonance studies of single crystals and powders of Pr_{0.6}Ca_{0.4}MnO_{3} in the 300-4.2 K range, covering the charge ordering transition at ~ 240 K and antiferromagnetic transition (T_N) at ~ 170 K. The asymmetry parameter for the Dysonian single crystal spectra shows anomalous increase at T_{co}. Below T_{co} the g-value increases continuously, suggesting a gradual strengthening of orbital ordering. The linewidth undergoes a sudden increase at T_{co} and continues to increase down to T_N. The intensity increases as the temperature is decreased till T_{co} due to the renormalization of magnetic susceptibility arising from the build up of ferromagnetic correlations. The value of the exchange constant, J, is estimated to be 154 K.

cond-mat.str-el↗

arXiv2000

Optical Limiting in Single-walled Carbon Nanotube Suspensions

Optical limiting behaviour of suspensions of single-walled carbon nanotubes in water, ethanol and ethylene glycol is reported. Experiments with 532 nm, 15 nsec duration laser pulses show that optical limiting occurs mainly due to nonlinear scattering. The observed host liquid dependence of optical limiting in different suspensions suggests that the scattering originates from microbubbles formed due to absorption-induced heating.

cond-mat.mtrl-sci↗

arXiv2000

Linear and nonlinear rheology of wormlike micelles

Several surfactant molecules self-assemble in solution to form long, cylindrical, flexible wormlike micelles. These micelles can be entangled with each other leading to viscoelastic phases. The rheological properties of such phases are very interesting and have been the subject of a large number of experimental and theoretical studies in recent years. We shall report on our recent work on the macrorheology, microrheology and nonlinear flow behaviour of dilute aqueous solutions of a surfactant CTAT (Cetyltrimethylammonium Tosilate). This system forms elongated micelles and exhibits strong viscoelasticity at low concentrations ($\sim$ 0.9 wt%) without the addition of electrolytes. Microrheology measurements of $G(ω)$ have been done using diffusing wave spectroscopy which will be compared with the conventional frequency sweep measurements done using a cone and plate rheometer. The second part of the paper deals with the nonlinear rheology where the measured shear stress $σ$ is a nonmonotonic function of the shear rate $\dotγ$. In stress-controlled experiments, the shear stress shows a plateau for $\dotγ$ larger than some critical strain rate, similar to the earlier reports on CPyCl/NaSal system. Cates et al have proposed that the plateau is a signature of mechanical instability in the form of shear bands. We have carried out extensive experiments under controlled strain rate conditions, to study the time-dependence of shear stress. The measured time series of shear stress has been analysed in terms of correlation integrals and Lyapunov exponents to show unambiguously that the behaviour is typical of low dimensional dynamical systems.

cond-mat.soft↗

arXiv1999

Observation of Chaotic Dynamics in Dilute Sheared Aqueous Solutions of CTAT

The nonlinear flow behaviour of a viscoelastic gel formed due to entangled, cylindrical micelles in aqueous solutions of the surfactant CTAT has been studied. On subjecting the system to a step shear rate lying above a certain value, the shear and normal stresses show interesting time dependent behaviour. The analysis of the measured time series shows the existence of a finite correlation dimension and a positive Lyapunov exponent, unambiguously implying that the dynamics can be described by that of a dynamical system with a strange attractor whose dimension increases with the increase in shear rate.

cond-mat.soft↗

arXiv1999

The backscattering of polarised light from turbid media - An analysis of the azimuthal intensity variations and its implications for the position of the source of diffusing radiation

We study the azimuthal variation in the backscattered intensity that is seen when polarised light is scattered by a turbid medium. We present experimental observations of these intensity variations in colloidal suspensions over a wide range of optical densities. For a medium composed of spherical scatterers, we have developed using Mie scattering theory and Monte Carlo simulations of photon transport, a model which calculates the constant intensity contours of the backscattered intensity. Comparisons of calculated and experimentally obtained contours show very good agreement. To our knowledge, this is the first model to provide a quantitative comparison with experimental data. Close to the exact backscattering direction, where we have made our intensity measuements, we show that the patterns are formed by what we have called `reflected snake photons'. These are photons that have been backscattered once and have maintained their direction of propagation thereafter until they exit the medium. We also find that the reflected snake photons originate within a depth of about $4l^{*}$ from the point of entry of the incident beam, where $l^{*}$ is the photon transport mean free path. Further, in a novel approach, we have used these patterns as a probe of the assumptions underlying the diffusion approximation and present new results on the position of the apparent source of diffusing radiation within the medium. Possible applications are also discussed.

cond-mat.dis-nn↗

arXiv1999

Imaging in turbid media using quasi-ballistic photons

We study by means of experiments and Monte Carlo simulations, the scattering of light in random media, to determine the distance upto which photons travel along almost undeviated paths within a scattering medium, and are therefore capable of casting a shadow of an opaque inclusion embedded within the medium. Such photons are isolated by polarisation discrimination wherein the plane of linear polarisation of the input light is continuously rotated and the polarisation preserving component of the emerging light is extracted by means of a Fourier transform. This technique is a software implementation of lock-in detection. We find that images may be recovered to a depth far in excess of what is predicted by the diffusion theory of photon propagation. To understand our experimental results, we perform Monte Carlo simulations to model the random walk behaviour of the multiply scattered photons. We present a new definition of a diffusing photon in terms of the memory of its initial direction of propagation, which we then quantify in terms of an angular correlation function. This redefinition yields the penetration depth of the polarisation preserving photons. Based on these results, we have formulated a model to understand shadow formation in a turbid medium, the predictions of which are in good agreement with our experimental results.

cond-mat.dis-nn↗