arXiv subjects

C. S. Sundar

Publications and source records attributed to C. S. Sundar.

At least 19 recordsLinked to original sources

arXiv2016

Effect of Sb substitution on the Topological Surface States in Bi_{2}Se_{3} single crystals: a magneto-transport study

Magneto-transport measurements have been carried out on Bi2-xSbxSe3 (x = 0, 0.05, 0.1, 0.3, 0.5) single crystals at 4.2 K temperature in the magnetic field range of -15 T to 15 T. Shubnikov-de Haas (SdH) oscillations of 2D nature were observed in samples with Sb concentration upto x = 0.3. The analyses of SdH oscillations observed in magneto-resistance data using Lifshitz-Kosevich equation reveal a systematic decrease in the Fermi surface area with Sb substitution. The Berry phase obtained from the Landau Level fan diagram suggests the occurrence of 2D oscillations arising from a Topological Surface State (TSS) for Sb concentrations of x = 0, 0.05 and 0.1; while 2D oscillation seen at higher Sb concentration is attributed to surface 2D electron gas consequent to downward band bending.

cond-mat.mtrl-sci↗

arXiv2015

Magneto-transport behaviour of Bi2Se3-xTex: Role of disorder

Magnetoresistance and Hall resistance measurements have been carried out in fastcooled single crystals of Bi2Se3-xTex (x: 0 to 2) in 4 to 300 K temperature range, under magnetic fields up to 15 T. The variation of resistivity with temperature that points to a metallic behaviour in Bi2Se3, shows an upturn at low temperatures in the Te doped samples. Magnetoresistance measurements in Bi2Se3 show clear signatures of Shubnikov de Hass oscillations that gets suppressed in the Te doped samples. In the Bi2SeTe2 sample, the magneto-resistance shows a cusp like positive magneto-resistance at low magnetic fields and low temperatures, a feature associated with weak antilocalisation (WAL), that crosses over to negative magneto-resistance at higher fields. The qualitatively different magnetotransport behaviour seen in Bi2SeTe2 as compared to Bi2Se3 is rationalised in terms of the disorder, through an estimate of the carrier density, carrier mobility and an analysis in terms of the Ioffe Regel criterion with support from Hall Effect measurements.

cond-mat.mtrl-sci↗

arXiv2015

Magnetization and magneto-transport studies on Fe$_2$VAl$_{1-x}$Si$_x$

We report on magnetoresistance, Hall and magnetization measurements of Fe2VAl1-xSix Heusler compounds for x= 0.005, 0.015, 0.02. There is a systematic change in the temperature coefficient of resistance (TCR) from negative to positive as the Si composition is increased. The Hall co-efficient shows that the carriers are electron like and the carrier density increases with Si concentration. Resistance measurements under magnetic field indicate a decreasing behavior under the application of magnetic field at low temperature region (T< 60 K), suggesting the suppression of scattering by magnetic field. Temperature and field dependent magnetization measurements did not show any significant change apart from the fact that the presence of super paramagnetic (SPM) cluster and its ordering at low temperatures. Arrott plot analysis of magnetization versus field also indicates the magnetic ordering with applied field below 60 K.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic behavior of the metal organic framework solid: [(CH3)2NH2][Co(HCOO)3]

In this study we examine the phase transitions in single crystals of [(CH3)2NH2]Co(HCOO)3], using magnetization and specific heat measurements as a function of temperature and magnetic field. Magnetisation measurements indicate a transition at 15 K that is associated with an antiferromagnetic transition. The results of the isothermal magnetization versus magnetic field curves demonstrate the presence of a single-ion magnet phase, coexisting with antiferromagnetism. A peak in specific heat is seen at 15 K, corresponding to the magnetic transition. The enthalpy of the transition evaluated from the area under the specific heat peak decreases with the application of magnetic field of upto8 T. This is suggestive of long range antiferromagnetic magnetic order, giving way to single-ion magnetic behavior under external field. In experiments at high temperatures, corresponding to the well-known structural transition in this system, the specific heat measurements, shows a peak at ~155K, that is insensitive to applied magnetic field. The magnetisation in this temperature range, while it exhibits a paramagnetic behavior, shows a distinct jump that has been attributed to a spin-state transition of Co2+ associated with the structural transition.

cond-mat.mtrl-sci↗

arXiv2014

Role of Se vacancies on Shubnikov de Haas oscillations in Bi2Se3: a combined magneto-resistance and positron annihilation study

Magneto resistance measurements coupled with positron lifetime measurements, to characterize the vacancy type defects, have been carried out on the topological insulator (TI) system Bi2Se3, of varying Se/Bi ratio. Pronounced Shubnikov de Haas (SdH) oscillations are seen in nominal Bi2Se3.1 crystals for measurements performed in magnetic fields up to 15 T in the 4 K to 10 K temperature range, with field applied perpendicular to the (001) plane of the crystal. The quantum oscillations, characteristic of 2D electronic structure, are seen only in the crystals that have a lower concentration of Se vacancies, as inferred from positron annihilation spectroscopy.

cond-mat.mtrl-sci↗

arXiv2014

Structural investigations in BaFe2-xRuxAs2 as a function of Ru and temperature

We present Synchrotron XRD measurements on powdered single crystal samples of BaFe2-xRuxAs2 samples, as a function of Ru content at room temperature. The Rietveld refinements reveal that the a-axis increases with Ru substitution, while the c-axis decreases. The variation of positional co-ordinates of As (zAs), the Fe-As bond length and the As-Fe-As bond angles have been determined from the Rietveld refinements. In the sample with x=0.1, temperature dependent XRD measurements were carried out. The results indicate that while the orthorhombicity shows the characteristic increase with decrease in temperature, the As-Fe-As bond angles, Fe-As bond length and positional co-ordinate of the As show definite anomalies close to the structural transition. First principle ab-initio simulations are performed in order to understand the experimentally observed anomalies in structural parameters. The experimental observations are discussed in the context of the simulation results.

cond-mat.supr-con↗

arXiv2014

Unification of the Pressure and Composition Dependence of Superconductivity in Ru substituted BaFe2As2

Temperature dependent high pressure electrical resistivity studies has been carried out on Ba(Fe_{1-x}Ru_{x})_{2}As_{2} single crystals with x = 0.12, 0.26 and 0.35, which correspond to under doped, optimally doped and over doped composition regimes respectively. The evolution of structural/magnetic (T_{S-M}) and superconducting transition (T_(c)) temperatures, with pressure for various compositions have been obtained. The normal state resistivity has been analyzed in terms of a model that incorporates both spin fluctuations and the opening of the gap in the spin density wave (SDW)phase. It is shown that Tc scales with the strength of the spin fluctuation, B, and T_{S-M} scales with the SDW gap parameter, Δ. This provides a prescription for the unification of the composition and pressure induced superconductivity in BaFe2As2.

cond-mat.supr-con↗

arXiv2012

Critical current density and magnetic phase diagram of BaFe1.29Ru0.71As2 Single Crystals

The critical current density has been measured on single crystals of Ru substituted BaFe2As2 superconductor at several temperatures and in fields up to 16 T. The magnetisation versus field isotherms reveal the occurrence of a clear second magnetisation peak (SMP) also known as fish-tail effect for both H parallel and perpendicular to c-axis of the crystal. The in-field resistance and magnetisation data are used to put forth a vortex phase diagram. The nature of the vortices have been determined from scaling behaviour of the pinning force density extracted from the Jc-H isotherms. The scaled JC versus reduced temperature behaviour seems to fit to a power law that indicates unambiguously that pinning in this system arises due to the spatial variation in the mean free path, viz. delta-l pinning.

cond-mat.supr-con↗

arXiv2011

Pressure Induced Metallization of BaMn2As2

The temperature and pressure dependent electrical resistivity rho(T,P) studies have been performed on BaMn2As2 single crystal in the 4.2 to 300 K range upto of 8.2 GPa to investigate the evolution of its ground state properties. The rho(T) shows a negative co-efficient of resistivity under pressure upto 3.2 GPa. The occurrence of an insulator to metal transition (MIT) in an external P ~4.5 GPa is indicated by a change in the temperature co-efficient in the rho(T) data at ~36 K . However complete metallization in entire temperature range is seen at a P~5.8 GPa. High pressure XRD studies carried out at room temperature also shows an anomaly in the pressure versus volume curve around P ~ 5 GPa, without a change in crystal structure, indicative of an electronic transition. Further, a clear precipitous drop in rho(T) at ~17 K is seen for P ~5.8 GPa which suggests the possibility of the system going over to a superconducting ground state.

cond-mat.str-el↗

arXiv2011

Efficacy of surface error corrections to density functional theory calculations of vacancy formation energy in transition metals

We calculate properties like equilibrium lattice parameter, bulk modulus and monovacancy formation energy for nickel (Ni), iron (Fe) and chromium (Cr) using Kohn-Sham density functional theory (DFT). We compare relative performance of local density approximation (LDA) and generalized gradient approximation (GGA) for predicting such physical properties for these metals. We also make a relative study between two different flavors of GGA exchange correlation functional, namely, PW91 and PBE. These calculations show that there is a discrepancy between DFT calculations and experimental data. In order to understand this discrepancy in the calculation of vacancy formation energy, we introduce a correction for the surface intrinsic error corresponding to an exchange correlation functional using the scheme implemented by Mattsson et al. [Phys. Rev. B 73, 195123 (2006)] and compare the effectiveness of the correction scheme for Al and the 3d-transition metals.

cond-mat.mtrl-sci↗

arXiv2011

Dislocation Driven Chromium Precipitation in Fe-9Cr Binary Alloy: A Positron Lifetime Study

The influence of initial heat treatment on anomalous Cr precipitation within high temperature solubility region in Fe-9Cr alloy has been investigated using positron lifetime studies. Air-quenched samples with pre-existing dislocations exhibited a distinct annealing stage in positron lifetime between 800 and 1100 K corresponding to Cr-precipitation. During this stage, Transmission Electron Microscopy showed fine precipitates of average size 4 nm, dispersed throughout the sample and from EDS analysis they are found to be Cr-enriched. The existence of dislocations is found to be responsible for Cr precipitation.

cond-mat.mtrl-sci↗

arXiv2011

Polaronic transport in the ferromagnetic phase of Gd1-xCaxBaCo2O5.53

Temperature dependent electrical resistivity and thermopower measurements were carried out on Gd1-xCaxBaCo2O5.53 with x varying between 0 and 0.25. Ca subsitution leads to the incorporation of holes (Co4+) into the system that leads to a reduction in resistivity and a stabilisation of the ferromagnetic phase at low temperatures. The temperature dependence of resistivity and thermopower are markedly different in the Ca doped sample, with a dramatic reduction in the resistivity, as compared to that in the pristine sample. The variation in both the resistivity and thermopower with temperature is explained in terms of the transport of polarons in the ferromagnetic phase of Ca doped system.

cond-mat.str-el↗

arXiv2011

High intrinsic flux pinning strength of BaFe2-xCoxAs2 superconductor

The field dependence of flux pinning potential, U0, for under-doped, optimally doped and over-doped Ba(Fe,Co)2As2 single crystals has been investigated. U0 is determined using the Thermally Activated Flux Flow (TAFF) model, for both H || ab and H || c configurations. The observed power law dependencies of U0 versus H suggest that for fields upto 12 T for H || ab and 7 T for H || c, individual flux pinning, and for fields beyond 7 T for H || c, collective pinning becomes operative. A comparison of U0 for the Ba(Fe,Co)2As2 system with some other superconductors is presented

cond-mat.supr-con↗

arXiv2011

Upper critical field anisotropy in BaFe2-xCoxAs2 single crystals synthesized without flux

The upper critical field was measured upto 12 T for three BaFe2-xCoxAs2 single crystals with estimated Co concentrations of x = 0.082, x = 0.117 and x = 0.143. HC2 versus temperature was measured from temperature dependent resistivity, for various applied magnetic fields, H || ab and H || c. The [dHC2/dT]T=Tc, normalized with the corresponding TC, decreases with increasing Co content, for both directions. The anisotropy γ defined as HC2 || ab / HC2 || c shows a distinct increase with Co content, and its temperature dependence shows a peak close to the TC. Magneto transport measurements, in the spin density wave regions, showed significant negative MR for H || ab and positive MR of H || c in the x = 0.082 sample. The implications of these results are discussed.

cond-mat.supr-con↗

arXiv2010

Superconductivity in Ru substituted BaFe2-xRuxAs2

The occurrence of bulk superconductivity at ~22 K is reported in polycrystalline samples of BaFe2-xRuxAs2 for nominal Ru content in the range of x=0.75 to 1.125. A systematic suppression of the spin density wave transition temperature (TSDW) precedes the appearance of superconductivity in the system. A phase diagram is proposed based on the measured TSDW and superconducting transition temperature (TC) variations as a function of Ru composition. Band structure calculations, indicate introduction of electron carriers in the system upon Ru substitutiom. The calculated magnetic moment on Fe shows a minimum at x=1.0, suggesting that the suppression of the magnetic moment is associated with the emergence of superconductivity. Results of low temperature and high field Mossbauer measurements are presented. These indicate weakening of magnetic interaction with Ru substitution

cond-mat.supr-con↗

arXiv2009

Pressure induced superconductivity in BaFe2As2 single crystal

The evolution of pressure induced superconductivity in single crystal as well as polycrystalline samples of BaFe2As2 has been investigated through temperature dependent electrical resistivity studies in 0-7 GPa pressure range. While the superconducting transition remains incomplete in polycrystalline sample, a clear pressure induced superconductivity with zero resistivity at the expense of magnetic transition, associated with spin density wave (SDW), is observed in the single crystal sample. The superconducting transition temperature (TC) is seen to increase upto a moderate pressure of about ~1.5 GPa and decreases monotonically beyond this pressure. The SDW transition temperature TSDW decreases rapidly with increasing pressure and vanishes above ~1.5 GPa.

cond-mat.supr-con↗

arXiv2009

Critical properties of superconducting Ba1-xKxFe2As2

Magnetisation and magnetoresistance measurements have been carried out on superconducting Ba1-xKxFe2As2 samples with x=0.40 and 0.50. From low field magnetization data carried out at different temperatures below TC, HC1 has been extracted. The plot of HC1 versus temperature shows an anomalous increase at low temperatures. From high field magnetization hysterisis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The JC determined from the high field data is >104A/cm2 at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 Tesla. From the variation of the TC onset with applied field, dHC2/dT at TC was obtained to be -7.708 T/K and -5.57 T/K in the samples with x=0.40 and 0.50.

cond-mat.supr-con↗

arXiv2005

Phonons as a Probe of the Low Temperature Metal Insulator Transition in Na.75Co.95Ni.05O2

Nickel substitution at the cobalt site in NaxCoO2 induces a Metal Insulator Transition (MIT) and the temperature at which this occurs (TMIT) increases with Ni content. Low temperature far infrared measurements on polycrystalline samples of Na0.75CoO2 and Na0.75Co0.95Ni0.05O2 (TMIT = ~ 150K) were carried out to look for signatures of this transition. While both the samples show an evident splitting of the high frequency Co-O mode on lowering the temperature, dramatic changes are observed in the low frequency sodium mode in Na0.75Co0.95Ni0.05O2 clearly pointing out to an ordering of the sodium ions in the low temperature insulating state.

cond-mat.str-el↗