arXiv subjects

Anurag Gupta

Publications and source records attributed to Anurag Gupta.

At least 55 records · Page 3Linked to original sources

arXiv2016

Ultrasensitive interplay between ferromagnetism and superconductivity in NbGd composite thin films

A model binary hybrid system composed of a randomly distributed rare-earth ferromagnetic (Gd) part embedded in an s-wave superconducting (Nb) matrix is being manufactured to study the interplay between competing superconducting and ferromagnetic order parameters. The normal metallic to superconducting phase transition appears to be very sensitive to the magnetic counterpart and the modulation of the superconducing properties follow closely to the Abrikosov-Gorkov (AG) theory of magnetic impurity induced pair breaking mechanism. A critical concentration of Gd is obtained for the studied NbGd based composite films (CFs) above which superconductivity disappears. Besides, a magnetic ordering resembling the paramagnetic Meissner effect (PME) appears in DC magnetization measurements at temperatures close to the superconducting transition temperature. The positive magnetization related to the PME emerges upon doping Nb with Gd. The temperature dependent resistance measurements evolve in a similar fashion with the concentration of Gd as that with an external magnetic field and in both the cases, the transition curves accompany several intermediate features indicating the traces of magnetism originated either from Gd or from the external field. Finally, the signatures of magnetism appear evidently in the magnetization and transport measurements for the CFs with very low (less than 1 at. %) doping of Gd.

cond-mat.supr-con↗

arXiv2015

Material homogeneity and strain compatibility in thin elastic shells

We discuss several issues regarding material homogeneity and strain compatibility for materially uniform thin elastic shells from the viewpoint of a 3-dimensional theory, with small thickness, as well as a 2-dimensional Cosserat theory. A relationship between inhomogeneity and incompatibility measures under the two descriptions is developed. More specifically, we obtain explicit forms of intrinsic dislocation density tensors characterising inhomogeneity of a dislocated Cosserat shell. We also formulate a system of governing equations for the residual stress field emerging out of strain incompatibilities which in turn are related to inhomogeneities. The equations are simplified for several cases under the Kirchhoff-Love assumption.

math-ph↗

arXiv2015

A Three-dimensional Study of Coupled Grain Boundary Motion with Junctions

A novel continuum theory of incoherent interfaces with triple junctions is applied to study three-dimensional coupled grain boundary (GB) motion in polycrystalline materials. The kinetic relations for grain dynamics, relative sliding and migration of the boundary, and junction evolution are developed. In doing so a vectorial form of the geometric coupling factor, which relates the tangential motion at the GB to the migration, is also obtained. Diffusion along the GBs and the junctions is allowed so as to prevent nucleation of voids and overlapping of material near the GBs. The coupled dynamics has been studied in detail for two bicrystalline and one tricrystalline arrangements. The first bicrystal consists of two rectangular grains separated by a GB, while the second is composed of a spherical grain embedded inside a larger grain. The tricrystal has an arbitrary shaped grain embedded inside a much larger bicrystal made of two rectangular grains. In all these cases, analytical solutions are obtained wherever possible while emphasizing the role of various kinetic coefficients during the coupled motion.

cond-mat.mtrl-sci↗

arXiv2014

Simultaneous Grain Boundary Motion, Grain Rotation, and Sliding in a Tricrystal

Grain rotation and grain boundary (GB) sliding are two important mechanisms for grain coarsening and plastic deformation in nanocrystalline materials. They are in general coupled with GB migration and the resulting dynamics, driven by capillary and external stress, is significantly affected by the presence of junctions. Our aim is to develop and apply a novel continuum theory of incoherent interfaces with junctions to derive the kinetic relations for the coupled motion in a tricrystalline arrangement. The considered tricrystal consists of a columnar grain embedded at the center of a non-planar GB of a much larger bicrystal made of two rectangular grains. We examine the shape evolution of the embedded grain numerically using a finite difference scheme while emphasizing the role of coupled motion as well as junction mobility and external stress. The shape accommodation at the GB, necessary to maintain coherency, is achieved by allowing for GB diffusion along the boundary.

cond-mat.mtrl-sci↗

arXiv2014

Enhanced spin-orbit coupling and charge carrier density suppression in LaAl1-xCrxO3/SrTiO3 heterointerfaces

We report a gradual suppression of the two-dimensional electron gas (2DEG) at the LaAlO3/SrTiO3 interface on substitution of chromium at the Al sites. The sheet carrier density at the interface (ns) drops monotonically from 2.2x10(14) cm-2 to 2.5x10(13) cm-2 on replacing nearly 60 percent of Al sites by Cr and the sheet resistance (Rs) exceeds the quantum limit for localization (h/2e2) in the concentrating range 40 to 60 percent of Cr.

cond-mat.str-el↗

arXiv2014

Coupled Grain Boundary Motion with Triple Junctions

Coupled grain boundary (GB) motion has been studied in a two-dimensional tricrystal where a cylindrical grain is embedded at the center of the planar GB of a large bicrystal. Kinetic relations for GB dynamics, grain orientations, and junctions have been derived within the framework of Gibbs thermodynamics. These are solved numerically to investigate the shrinkage of the embedded grain while emphasizing the role of coupled motion as well as junction mobility in the shape evolution.

cond-mat.mtrl-sci↗

arXiv2013

Geometry of Defects in Solids

The appealing connection between non-Euclidean geometries and defects in solids is brought forth in this article. Drawing a correspondence between the nature of a defect and a specific geometric property of the material space not only illuminates the underlying structure of defects in solids but also provides an unambiguous way to represent defect densities within a physical theory. We present a rigorous background of the relevant concepts from classical differential geometry, as well as illustrations of isolated defects in a lattice, to motivate the relationship between continuous defect densities and various tensor fields in differential geometry. We identify the Riemann-Christoffel tensor (or curvature tensor), the Cartan tensor (or torsion tensor), and the nonmetricity tensor (obtained from the covariant derivative of the metric tensor), associated with the material space, with the density of disclinations, dislocations, and point-defects (vacancies, interstitials, substitutional), respectively. We end our discussion with remarks on the elastic stress field associated with defect distribution and on the analogy between the present theory and the general theory of relativity.

cond-mat.mtrl-sci↗

arXiv2013

Giant coercivity enhancement and dimensional crossover of superconductivity in Co2FeSi-NbN nanoscale bilayers

Proximity coupling and magnetic switching dynamics are shown to be correlated in bilayers (B) of fully spin polarized ferromagnet (F) Co2FeSi and superconductor (S) NbN. The upper critical field derived from resistivity measurements shows a dimensional crossover with a reduced effective thickness of the S layer. At temperatures (T) << superconducting Tc, the measured M-H loops show two step switching; one at T-independent value ~ 7 Oe and other at strongly T-dependent value becoming very large ~ 1 kOe at 2 K. These results reveal induced ferromagnetism in S-layer at the S/F interface with vortex pinning influenced dynamics.

cond-mat.supr-con↗

arXiv2011

Plastic flow in solids with interfaces

A non-equilibrium theory of isothermal and diffusionless evolution of incoherent interfaces within a plastically deforming solid is developed. The irreversible dynamics of the interface are driven by its normal motion, incoherency (slip and misorientation), and an intrinsic plastic flow; and purely by plastic deformation in the bulk away from the interface. Using the continuum theory for defect distribution (in bulk and over the interface) we formulate a general kinematical framework, derive relevant balance laws and jump conditions, and prescribe a thermodynamically consistent constitutive/kinetic structure for interface evolution.

cond-mat.mtrl-sci↗

arXiv2006

Physical property characterization of Fe-tube encapsulated and vacuum annealed bulk MgB2

We report phase formation, and detailed study of magnetization and resistivity under magnetic field of MgB2 polycrystalline bulk samples prepared by Fe-tube encapsulated and vacuum (10-5 torr) annealed (750 0C) route. Zero-field-cooled magnetic susceptibility (cZFC) measurements exhibited sharp transition to superconducting state with a sizeable diamagnetic signal at 39 K (Tc). The measured magnetization loops of the samples, despite the presence of flux jumps, exhibited a stable current density (Jc) of around 2.4 x 105 A/cm2 in up to 2 T (Tesla) field and at temperatures (T) up to 10 K. The upper critical field is estimated from resistivity measurements in various fields and shows a typical value of 8 T at 21 K. Further, cFC measurements at an applied field of 0.1 T reveal paramagnetic Meissner effect (PME) that is briefly discussed.

cond-mat.supr-con↗

arXiv2006

Phase formation and superconductivity of Fe-tube encapsulated and vacuum annealed MgB2

We report optimization of the synthesis parameters viz. heating temperature (TH), and hold time (thold) for vacuum (10-5 torr) annealed and LN2 (liquid nitrogen) quenched MgB2 compound. These are single-phase compounds crystallizing in the hexagonal structure (space group P6/mmm) at room temperature. Our XRD results indicated that for phase-pure MgB2, the TH for 10-5 torr annealed and LN2 quenched samples is 750 0C. The right stoichiometry i.e., MgB2 of the compound corresponding to 10-5 Torr and TH of 750 0C is found for the hold time (thold) of 2.30 hours. With varying thold from 1- 4 hours at fixed TH (750 0C) and vacuum (10-5 torr), the c-lattice parameter decreases first and later increases with thold (hours) before a near saturation, while the a-lattice parameter first increase and later decreases beyond thold of 2.30 hours. c/a ratio versus thold plot showed an inverted bell shape curve, touching the lowest value of 1.141 which is reported value for perfect stoichiometry of MgB2. The optimized stoichimetric MgB2 compound exhibited superconductivity at 39.2 K with transition width of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum annealed MgB2 compound are optimized and are compared with widely reported Ta tube encapsulated samples.

cond-mat.supr-con↗

arXiv2005

Impact of Zn substitution on phase formation and superconductivity of Bi1.6Pb0.4 Sr2Ca2Cu3-xZnxO10 with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12

Samples of series Bi1.6Pb0.4Sr2Ca2Cu3-xZnxO10 with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi-2223 (Bi2Sr2Ca2Cu3O10) phase (c-lattice parameter ~ 36 A0). The proportion of Bi-2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase (Bi-2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [(T)] and AC magnetic susceptibility [(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (~10K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi-2223 system are compared with Zn doped other HTSC (High temperature superconducting) systems, namely the RE-123 (REBa2Cu3O7) and La-214 ((La,Sr)2CuO4).

cond-mat.supr-con↗

arXiv2004

Effect of Ca doping for Y on structural / microstructural and superconducting properties of YBa2Cu 3O7-d

We report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O7-d bulk samples with 0.20 x 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu3O7-d leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation.

cond-mat.supr-con↗

arXiv2004

Revival of Superconductivity by Y3+/Ca2+ substitution in YBa2Cu2.7Co0.3O7 without reported phase transformation

Results of phase formation, resistivity (r), and thermo-electric power (S), are reported on Y1-xCaxBa2Cu2.7Co0.3O7 compounds with x = 0.1 and 0.2. Pristine compound i.e. without Co or Ca substitution crystallizes in orthorhombic structure with space group P/mmm. Cu-site Co substituted compound i.e. YBa2Cu2.7Co0.3O7 is tetragonal. With simultaneous doping of Ca at Y site in Co substituted compound i.e. Y1-xCaxBa2Cu2.7Co0.3O7 the tetragonal nature still remains. r(T) measurements showed superconducting transition temperature (Tc) to decrease from 90K (YBa2Cu3O7) to 33 K for YBa2Cu2.7Co0.3O7 which with further Ca substitution increases from 33K to 53K (Y0.9 Ca0.1Ba2Cu2.7Co0.3O7) and 67 K for Y0.8 Ca0.2Ba2Cu2.7Co0.3O7. Tc decreases first with Cu-site Co substitution by hole-filling and later recovers by simultaneous hole creation by Y site Ca substitution. Room temperature thermoelectric power S(300 K), which is an indirect measure of mobile carriers shows the decrease of carriers with Co doping and creation by Ca substitution. Our results demonstrate the hole filling by Co substitution is compensated by simultaneous Ca substitution.

cond-mat.supr-con↗

arXiv2004

Resistivity and Thermoelectric power of NaxCoO2 (x =1.0, 0.7 and 0.6) system

Results of thermo-electric power (S) and electrical resistivity (r) measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P63/mmc) at room temperature. Thermo-electric power values at 300K (S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is \~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300-5K temperature range and decreases monotonically to zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.

cond-mat.str-el↗

arXiv2004

Induction of superconductivity in Y0.4Pr0.6Ba2-xSrxCu3O7 system with increasing Sr substitution

Samples of Y0.4Pr0.6Ba2-xSrxCu3O7 have been synthesized by a solid- state reaction route. The samples with x = 0.60 crystallize in an orthorhombic structure (with orthorhombic distortion decreasing with increasing x), while the samples with x=0.80 and 1.0 crystallize in a pseudo tetragonal structure. Resistance (R) measurements as a function of temperature (T) show that x = 0.0 sample is highly semiconducting with R2K/R300K ratio of ~ 65. This ratio decreases to only ~23 and 5 for x=0.20 and x=0.4 samples, respectively. Further, the x=0.60 sample shows onset of a broad superconducting transition temperature (Tconset) at around 20 K without achieving the TcR=0 state down to 2 K. The TcR=0 state is observed in x = 0.80 and 1.00 samples at around 5 and 14 K, respectively. Thermo-electric power (S), also exhibits TcS~0 state at around 11 K and 16 K, respectively for x = 0.8 and 1.00 samples. Thermo electric power at room temperature is positive and decreases with increasing x, indicating enhanced number of mobile holes. These results demonstrate that substitution of Sr at Ba-site systematically induces metallicity and eventually superconductivity in Y0.4Pr0.6Ba2-xSrxCu3O7 system. The results are explained on the basis of reduced Pr(4f) orbital hybridization with O(2p) in supercondu cting Cu-O2 planes, resulting in delocalization of the mobile carriers. The role of decreased Pr/Ba intermixing disorder is also considered.

cond-mat.supr-con↗

arXiv2004

Possible Competition between superconductivity and magnetism in RuSr2Gd1.5Ce0.5Cu2O10-d (Ru-1222) Rutheno-cuprate compounds

The RuSr2Gd1.5Ce0.5Cu2O10-d (Ru-1222) compounds, with varying oxygen content, crystallize in a tetragonal crystal structure (space group I4/mmm). Resistance (R) versus temperature (T) measurements show that the air-annealed samples exhibit superconductivity with superconducting transition temperature (Tc) onset at around 32 K and R=0 at 3.5 K. On the other hand, the N2-annealed sample is semiconducting down to 2 K. Magneto-transport measurements on air-annealed sample in applied magnetic fields of 3 and 6 Tesla show a decrease in both Tc onset and TR=0. Magnetoresistance of up to 20% is observed in N2-annealed sample at 2 K and 3 T applied field. The DC magnetization data (M vs. T) reveal magnetic transitions (Tmag.) at 100 K and 106 K, respectively, for both air- and N2-annealed samples. Ferromagnetic components in the magnetization are observed for both samples at 5K and 20 K. The superconducting transition temperature (Tc) seems to compete with the magnetic transition temperature (Tmag.). Our results suggest that the magnetic ordering temperature (Tmag.) of Ru moments in RuO6 octahedra may have direct influence/connection with the appearance of superconductivity in Cu-O2 planes of Ru-1222 compounds. Key words: Ruthenocuprates, Superconductivity and Magnetism, Magneto transport and Magneto resistance. PACS: 74.25. Ha, 74.72. Jt, 75.25. +z, 75.30. Cr.

cond-mat.supr-con↗

arXiv2004

Micro-structure and Magnetization of the 80-K Superconductor, TbSr2Cu2.7Mo0.3O7+d

The results of micro-structural and detailed magnetization studies are reported here for our recently published TbSr2Cu2.7Mo0.3O7+d superconductor with a transition temperature (Tc) at as high as 80 K [1]. From XRD and EDX the sample was confirmed to be of single-phase and possess the nominal cation stoichiometry. The SEM images showed well-developed grains for this ceramic high-Tc superconductor. Magnetization measurements at various temperatures and fields revealed bulk superconductivity below 80 K. Magnetization versus applied field (M vs. H) loops at various temperatures below the Tc exhibited clear full penetration field HP. In normal state (> 100 K) the susceptibility follows the Curie-Weiss behavior with an effective paramagnetic moment of 9.83 mB.

cond-mat.supr-con↗