arXiv subjects

V. P. S. Awana

Publications and source records attributed to V. P. S. Awana.

At least 127 records · Page 7Linked to original sources

arXiv2014

Crossover from charge order to strain glass in phase separated manganite thin films: Impact of thermal cycling and substrate induced strain

The magnetic and magnetotransport properties of single crystalline La1-x-yPryCaxMnO3 (x=0.42, y=0.40) thin films (~140 nm) deposited on (110) oriented LaAlO3 and SrTiO3 substrates exhibit a crossover from the high temperature antiferromagnetic-charge ordered insulator (AFM-COI) phase (T>TN) to strain glass (T<Tg). At intermediate temperatures (Tg<T<TN) dynamical liquid having prominent thermal-magneto-resistive hysteresis dominates in the cooling cycle, while in the warming cycle it is preceded by ferromagnetic metal (FMM) phase. Magnetic field required to drive AFM-COI to FMM phase transition are higher than that for the strain glass. The magneto-electric nature and temperature span of the distinct magnetic regimes are sensitive to the thermal cycling and substrate induced strain.

cond-mat.mtrl-sci↗

arXiv2014

Superconductivity at 25K under hydrostatic pressure for FeTe0.5Se0.5 superconductor

We report the impact of hydrostatic pressure on the superconductivity and normal state resistivity of FeTe0.5Se0.5 superconductor. At the ambient pressure the FeTe0.5Se0.5 compound shows the superconducting transition temperature Tconset at above 13K and TcR=0 at 11.5K. We measure pressure dependent resistivity from 250K to 5K, which shows that the normal state resistivity increases initially for the applied pressures of up to 0.55GPa and then the same is decreased monotonically with increasing pressure of up to 1.97GPa. On the other hand the superconducting transition temperatures (Tconset and TcR=0) increase monotonically with increasing pressure. Namely the Tconset increases from 13K to 25K and TcR=0 from 11.5K to 20K for the pressures range of 0-1.97GPa. Our results suggest that superconductivity in this class of Fe-based compounds is very sensitive to pressure as the estimated pressure coefficient dTc(onset)/dP is 5.8K/GPa. It may be suggested that FeTe0.5Se0.5 superconductor is a strong electron correlated system. The enhancement of Tc with applying pressure is mainly attributed to an increase of charge carriers at Fermi surface.

cond-mat.supr-con↗

arXiv2014

Significant enhancement of superconductivity under hydrostatic pressure in CeO0.5F0.5BiS2 superconductor

We report the effect of hydrostatic pressure (0-1.97GPa) on the superconductivity of BiS2 based CeO0.5F0.5BiS2 compound. The CeO0.5F0.5BiS2 superconductor was synthesized by the solid state reaction route and the compound is crystallized in tetragonal P4/nmm space group. The studied compound shows superconductivity with transition temperature of 2.5K (Tconset) at ambient pressure, which has been enhanced to 8 K at applied pressure of 1.97 GPa. The observed normal resistivity exhibited semiconducting behavior. The data of normal state resistivity R(T) has been fitted by activation type equation and it is found that the energy gap is significantly reduced with pressure. Resistivity measurements under magnetic field for the highest applied pressure of 1.97GPa (Tconset = 8K) exhibits the upper critical field of above 5Tesla. The observation of fourfold increase in Tc accompanied with improved normal state conduction under hydrostatic pressure on CeO0.5F0.5BiS2 superconductor calls for the attention of solid state physics community.

cond-mat.supr-con↗

arXiv2014

Magnetotransport studies of FeSe under hydrostatic pressure

The discoveries of iron-based superconductors with relatively high transition temperature are under intense experimental and theoretical investigation. Here we present magnetotransport measurements on FeSe superconductor under hydrostatic pressure. We show that in Fe-deficient tetragonal FeSe binary compound, the onset of superconducting transition is almost doubled under 1.98GPa pressure and the estimated upper critical field of 26.7Tesla is increased to 47.5Tesla.

cond-mat.supr-con↗

arXiv2014

Robust superconductivity with large upper critical field in Nb2PdS5

We report synthesis, structural details and complete superconducting characterization of very recently discovered Nb2PdS5 new superconductor. The synthesized compound is crystallized in mono-clinic structure. Bulk superconductivity is seen in both magnetic susceptibility and electrical resistivity measurements with superconducting transition temperature (Tc) at 6K. The upper critical field (Hc2), being estimated from high field magneto-transport measure-ments is above 240kOe. The estimated Hc2(0) is clearly above the Pauli paramagnetic limit. Heat capacity measurements show clear transition with well defined peak at Tc, but with lower jump than as expected for a BCS type superconductor. The Sommerfield constant and Debye temperature as determined from low temperature fitting of heat capacity data are 32mJ/moleK2 and 263K respectively. Hall coefficients and resistivity in conjugation with electronic heat capacity indicates multiple gap superconductivity signatures in Nb2PdS5. We also studied the impact of hydrostatic pressure on superconductivity of Nb2PdS5 and found nearly no change in Tc for the given pressure range.

cond-mat.supr-con↗

arXiv2014

Impact of hydrostatic pressure on superconductivity of Sr0.5La0.5FBiS2

We report the impact of hydrostatic pressure (0-1.97GPa) on superconductivity of recently discovered 2K superconductor Sr0.5La0.5FBiS2. Resistivity under pressure measurements are performed by using HPC-33 Piston type pressure cell with Quantum design DC resistivity Option. The superconducting transition temperature (Tc) is increased by 5 fold to around 10K with just above 1GPa pressure, which remains nearly unaltered for studied higher pressures of up to 1.97GPa. The fivefold increase in Tc of Sr0.5La0.5FBiS2 with just above 1GPa pressure suggests very strong electron correlations in these systems and the same calls for the attention of solid state physics community.

cond-mat.supr-con↗

arXiv2014

Superconducting performance of ex-situ SiC-doped MgB2 mono-filamentary tapes

We report on the superconducting performance of the ex-situ SiC doped MgB2 monofilamentary tapes. Polycrystalline powders of MgB2 doped with 5 and 10 wt% SiC were synthesized by conventional solid-state reaction route and characterized for their superconducting performance. It is found that superconducting parameters i.e. upper critical field (Hc2), irreversibility field (Hirr) and critical current density (Jc) are all improved significantly with SiC addition. Also it was found that relatively lower synthesis temperature (700 C) resulted in further improved superconducting parameters. As synthesized powders are used for ex-situ powder-in-tube (PIT) monofilamentary tapes and superconducting parameters are determined. Albeit the superconducting transition temperature (Tc) is decreased slightly (2K) for SiC doped tapes, the superconducting performance in terms of critical current density (Jc), being determined from both magnetization and transport measurements, is improved significantly. In particular the SiC doped and 700 °C synthesized MgB2 tapes exhibited the transport Jc of nearly 10^4 A/cm2 under applied fields of as high as 7 Tesla. Further it is found that the Jc anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB2 tapes are responsible for decreased anisotropy and improved Jc(H) performance.

cond-mat.supr-con↗

arXiv2014

Local structural distortions and their role in superconductivity in SmFeAsO$_{1-x}$F$_x$ superconductors

EXAFS studies at the As K edge as a function of temperature have been carried out in SmFeAsO$_{1-x}$F$_x$ ($x$ = 0 and 0.2) compounds to understand the role of local structural distortions superconductivity observed in F doped compound. A significant correlation between thermal variation of local structural parameters like anion height and superconducting onset is found in the fluorinated compound. Such a variation in anion height is absent in non-superconducting compound. Increase in Fe-As bond distance just below superconducting onset temperature indicates a similarity in distortions observed in high T$_C$ cuprates and these Fe based superconductors.

cond-mat.supr-con↗

arXiv2014

Effect of external pressure on the magnetic properties of $R$CoAsO ($R$ = La, Pr, Sm): a $μ$SR study

We report on a detailed investigation of the itinerant ferromagnets LaCoAsO, PrCoAsO and SmCoAsO performed by means of muon spin spectroscopy upon the application of external hydrostatic pressures $p$ up to $2.4$ GPa. These materials are shown to be magnetically hard in view of the weak dependence of both critical temperatures $T_{C}$ and internal fields at the muon site on $p$. In the cases $R$ = La and Sm, the behaviour of the internal field is substantially unaltered up to $p = 2.4$ GPa. A much richer phenomenology is detected in PrCoAsO instead, possibly associated with a strong $p$ dependence of the statistical population of the two different crystallographic sites for the muon. Surprisingly, results are notably different from what is observed in the case of the isostructural compounds $R$CoPO, where the full As/P substitution is already inducing a strong chemical pressure within the lattice but $p$ is still very effective in further affecting the magnetic properties.

cond-mat.str-el↗

arXiv2014

Superconducting properties of BiS2 based superconductor NdO1-xFxBiS2 (x=0 to 0.9)

We report the phase diagram of NdO1-xFxBiS2 (x=0.1, 0.3, 0.5, 0.7, 0.9) compound. The samples synthesized by the vacuum encapsulation technique are crystallized in a tetragonal P4/nmm space group. The superconductivity of all samples is confirmed by DC magnetic susceptibility and electrical transport measurements. The maximum Tc of 5.25K is seen for x = 0.7 sample. The upper critical field is calculated from magneto transport measurements. The Thermal activated flux flow - TAFF studies are also carried out. Hall measurements show that electrons are the dominating charge carriers in these systems.

cond-mat.supr-con↗

arXiv2014

Evolution of superconductivity in PrFe1-xCoxAsO with x = 0.0 to 1.0

We report the synthesis and physical property characterization of PrFe1-xCoxAsO with x = 0.0 to 1.0. The studied samples are synthesized by solid state reaction route via vacuum encapsulation method. The pristine compound PrFeAsO does not show superconductivity, but rather exhibits a metallic step like transition due to spin density wave ordering of Fe moments below 150 K, followed by another upward step due to anomalous ordering of Pr moments at 12 K. Both the Fe-SDW and Pr-TN temperatures decrease monotonically with Co substitution at Fe site. Superconductivity appears in a narrow range of x from 0.07 to 0.25 with maximum Tc at 11.12 K for x = 0.15. Samples, with x = 0.25 exhibit metallic behavior right from 300 K down to 2 K, without any Fe-SDW or Pr-TN steps in resistivity. In fact, though Fe-SDW decreases monotonically, the Pr-TN is disappeared even with x = 0.02. The magneto transport measurements below 14 Tesla on superconducting polycrystalline Co doped PrFeAsO lead to extrapolated values of the upper critical fields [Hc2(0)] of up to 60 Tesla.

cond-mat.supr-con↗

arXiv2014

Electrical and Magnetic behaviour of PrFeAsO0.8F0.2 superconductor

The superconducting and ground state samples of PrFeAsO0.8F0.2 and PrFeAsO have been synthesised via easy and versatile single step solid state reaction route. X-ray & Reitveld refine parameters of the synthesised samples are in good agreement to the earlier reported value of the structure. The ground state of the pristine compound (PrFeAsO) exhibited a metallic like step in resistivity below 150K followed by another step at 12K. The former is associated with the spin density wave (SDW) like ordering of Fe spins and later to the anomalous magnetic ordering for Pr moments. Both the resistivity anomalies are absent in case of superconducting PrFeAsO0.8F0.2 sample. Detailed high field (up to 12Tesla) electrical and magnetization measurements are carried out for superconducting PrFeAsO0.8F0.2 sample. The PrFeAsO0.8F0.2 exhibited superconducting onset (Tconset) at around 47K with Tc(ρ =0) at 38K. Though the Tconset remains nearly invariant, the Tc(ρ =0) is decreased with applied field, and the same is around 23K under applied field of 12Tesla. The upper critical field (Hc2) is estimated from the Ginzburg Landau equation (GL) fitting, which is found to be ~ 182Tesla. Critical current density (Jc) being calculated from high field isothermal magnetization (MH) loops with the help of Beans critical state model, is found to be of the order of 103 A/cm2. Summarily, the superconductivity characterization of single step synthesised PrFeAsO0.8F0.2 superconductor is presented.

cond-mat.supr-con↗

arXiv2014

Effect of Se doping in recently discovered layered Bi4O4S3 Superconductor

We report suppression of superconductivity in Bi4O4S3 compound by Se doping at S site. Bulk polycrystalline samples are synthesized by solid state reaction route. The Rietveld refined of XRD data of all the studied samples show that the same are crystallized in tetragonal I4/mmm space group with slightly increased c lattice constant with the Se doping. Superconductivity is observed at below 4.5 K in both dc magnetization and resistivity with temperature measurements for all Bi4O4S3-xSex (x = 0, 0.03, 0.09 and 0.15) samples, though the same is decreased slightly with increase in Se content. The upper critical field of the Bi4O4S3 is estimated from resistivity under magnetic field [R(T)H] measurements of up to 2T (Tesla), and the same decreases for Se doped samples. The flux flow activation energies being obtained by fitting to the the Arrhenius equation are 18.60 meV for Bi4O4S3, 13.69 meV for Bi4O4S2.97Se0.03 and 13.16 meV for Bi4O4S2.85Se0.15 samples in applied magnetic field of 200 Oe. In conclusion the S site Se substitution showed detrimental effect on superconductivity of Bi4O4S3.

cond-mat.supr-con↗

arXiv2014

Superconductivity in layered CeO0.5F0.5BiS2

We report appearance of superconductivity in CeO0.5F0.5BiS2. The bulk polycrystalline samples CeOBiS2 and CeO0.5F0.5BiS2 are synthesized by conventional solid state reaction route via vacuum encapsulation technique. Detailed structural analysis showed that the studied CeO0.5F0.5BiS2 compound is crystallized in tetragonal P4/nmm space group with lattice parameters a = 4.016(3) A, c = 13.604(2) A. DC magnetization measurement (MT-curve) shows the ferromagnetic signal at the low temperature region. The superconductivity is established in CeO0.5F0.5BiS2 at Tconset = 2.5K by electrical transport measurement. Under applied magnetic field both Tc onset and Tc (ρ =0) decrease to lower temperatures and an upper critical field [Hc2(0)] above 1.2Tesla is estimated. The results suggest coexistence of ferromagnetism and superconductivity for the CeO0.5F0.5BiS2 sample.

cond-mat.supr-con↗

arXiv2014

Anomalous magnetism of Pr in PrCoAsO

Detailed magnetization and magneto-transport measurements studies are carried out to unearth the anomalous magnetism of Pr in PrCoAsO compound. The studied PrCoAsO sample is single phase and crystallized in the tetragonal structure with space group P4/nmm in analogy of ZrCuSiAs type compounds. Detailed magnetization measurements showed that Co moments in PrCoAsO exhibit weakly itinerant ferromagnetic Co spins ordering at below 80 K with a small magnetic moments of 0.12 muB/f.u. High temperatures Curie-Weiss fit, resulted in effective paramagnetic moment mueff (exp) of 5.91 muB/f.u., which can be theoretically assigned to 3d Co (3.88 muB) and 4f Pr (3.58 muB). Further, a positive Curie- Weiss temperature of 136 K is seen, indicating predominant ferromagnetic interactions in PrCoAsO. Detailed transport measurements showed that PrCoAsO exhibit metallic behavior and negative magneto-resistance below ferro-magnetically (FM) ordered state. Surprisingly, the situation of PrCoAsO is similar to non magnetic La containing LaCoAsO and strikingly different than that as reported for magnetic Nd, Sm and Gd i.e., (Nd/Sm/Gd)CoAsO. The magnetic behavior of PrCoAsO being closed to LaCoAsO and strikingly different to that of (Nd/Sm/Gd)CoAsO is unusual.

cond-mat.supr-con↗

arXiv2014

Superconducting and magneto-transport properties of BiS2 based superconductor PrO1-xFxBiS2 (x = 0 to 0.9)

We report superconducting properties of PrO1-xFxBiS2 compounds, synthesized by the vacuum encapsulation technique. The synthesized PrO1-xFxBiS2 (x=0.1, 0.3, 0.5, 0.7 and 0.9) samples are crystallized in a tetragonal P4/nmm space group. Both transport and DC magnetic susceptibility measurements showed bulk superconductivity below 4 K. The maximum Tc is obtained for x = 0.7 sample. Under applied magnetic field both Tc onset and Tc (R =0) decrease to lower temperatures. We estimated highest upper critical field [Hc2(0)] for PrO0.3F0.7BiS2 sample to be above 4 T (Tesla). The thermally activated flux flow (TAFF) activation energy (U0) is estimated 54.63 meV in 0.05 Tesla field for PrO0.3F0.7BiS2 sample. Hall measurement results showed that electron charge carriers are the dominating ones in these compounds. Thermoelectric effects (Thermal conductivity and Seebeck coefficient) data suggest strong electron-electron correlations in this material.

cond-mat.supr-con↗

arXiv2013

Spin dynamics, short-range order and superparamagnetism in superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10-δ

We report structural, detailed DC and linear/non-linear AC, isothermal and thermoremanent magnetization study of the rutheno-cuprate superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10-δ (GdRu-1222). Structural analysis, by employing Rietveld refinement of X-ray diffraction pattern, reveals that GdRu-1222 crystallizes in tetragonal phase with I4/mmm space group. GdRu-1222 is a reported superconducting ferromagnet with Ru spins magnetic ordering at around 110 K and superconductivity below 40 K in Cu-O2 planes. Detailed linear/non-linear first and higher order harmonic of AC susceptibility studies unveiled the complex magnetism of GdRu-1222. A frequency dependent cusp is observed in AC susceptibility (χac) vs. T measurements. The change in cusp position with applied frequency followed the well known Vogel-Fulcher law, which is a feature to describe a spin-glass (SG) system with possibility of embedded homogeneous/non-homogeneous magnetically interacting/non-interacting ferromagnetic clusters. Such an interpretation is also supported by thermoremanent magnetization (TRM) study at T = 60 K. Detailed interpretation of AC magnetization results revealed the formation of magnetic (ferromagnetic) homogenous/non-homogenous clusters of different sizes embedded in spin-glass (SG) matrix. The magnetization vs. applied field loops do not saturate, even at high applied fields (50 kOe), resulting in the short-range magnetic ordering in the system, which causes the formation of clusters that freeze at low temperatures. Temperature variation of first- and third-order susceptibility harmonics show good agreement with Wohlfarth's model (WM), leading to the superparamagnetism (SPM) state. Detailed magnetization (DC and AC both) results and their analysis helped in explaining the temperature dependent magnetism of the GdRu-1222 system.

cond-mat.supr-con↗

arXiv2013

Impact of Gd doping on morphology and superconductivity of NbN sputtered thin films

We report effect of Gd inclusion in the NbN superconductor thin films. The films are deposited on single crystalline Silicon (100) by DC reactive sputtering technique i.e., deposition of Nb and Gd in presence of reactive N2 gas. The fabricated relatively thick films (400 nm) are crystallized in cubic structure. These films are characterized for their morphology, elemental analysis and roughness by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray spectroscopy (EDAX) and Atomic Force Microscopy (AFM) respectively. The optimized film (maximum Tc) is achieved with gas ratio of Ar:N2 (80:20) for both pristine and Gd doped films. The optimized NbN film possesses Tc (R=0) in zero and 140kOe fields are at 14.8K and 8.8K respectively. The Gd doped NbN film showed Tc (R=0) in zero and 130kOe fields at 11.2K and 6.8 K respectively. The upper critical field Hc2(0) of the studied superconducting films is calculated from the magneto-transport [R(T)H] measurements using GL equations. It is found that Gd doping deteriorated the superconducting performance of NbN.

cond-mat.supr-con↗