arXiv subjects

V. P. S. Awana

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

At least 109 records · Page 6Linked to original sources

arXiv2015

Structural, Magnetic and Magneto-caloric studies of Ni50Mn30Sn20Shape Memory Alloy

We have synthesized a nominal composition of Ni50Mn30Sn20 alloy using arc melting technique. Rietveld refinement confirms the austenite L21 structure in Fm-3m space group. Electrical resistivity has been found to clearly exhibiting two different phenomena viz. a magnetic transition from paramagnetic to ferromagnetic and a structural transition from austenite to martensitic phase. Thermo-magnetization measurements M(T) confirms ferromagnetic transition temperature TC at 222 K and martensitic transition starting at 127 K(MS). Magnetization measurement M(H) at 10 K confirms the ferromagnetic state. Frequency dependence of ac susceptibility \c{hi}' at low temperature suggests spin glass behavior in the system. The isothermal magnetic entropy change values have been found to be 1.14 J/Kg.K, 2.69 J/Kg.K and 3.9 J/Kg.K, with refrigeration capacities of 19.6 J/kg, 37.8 J/kg and 54.6 J/kg for the field change of 1, 2 and 3 Tesla respectively at 227 K.

cond-mat.str-el↗

arXiv2015

Electron correlation effects and scattering rates in Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductor

Using angle-resolved photoemission spectroscopy we have studied the low-energy electronic structure and the Fermi surface topology of Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors. Similar to the known iron pnictides we observe hole pockets at the center and electron pockets at the corner of the Brillouin zone (BZ). However, on a finer level, the electronic structure around the $Γ$- and $Z$-points in $k$-space is substantially different from other iron pnictides, in that we observe two hole pockets at the $Γ$-point, and more interestingly only one hole pocket is seen at the $Z$-point, whereas in $1111$-, $111$-, and $122$-type compounds, three hole pockets could be readily found at the zone center. Another major difference noted in the Fe$_{1+y}$Te$_{1-x}$Se$_x$ superconductors is that the top of innermost hole-like band moves away from the Fermi level to higher binding energy on going from $Γ$ to $Z$, quite opposite to the iron pnictides. The polarization dependence of the observed features was used to aid the attribution of the orbital character of the observed bands. Photon energy dependent measurements suggest a weak $k_z$ dispersion for the outer hole pocket and a moderate $k_z$ dispersion for the inner hole pocket. By evaluating the momentum and energy dependent spectral widths, the single-particle self-energy was extracted and interestingly this shows a pronounced non-Fermi liquid behaviour for these compounds. The experimental observations are discussed in context of electronic band structure calculations and models for the self-energy such as the spin-fermion model and the marginal-Fermi liquid.

cond-mat.supr-con↗

arXiv2015

Improved superconducting properties of skutterudite La3Co4Sn13 with indium substitution

We report on two fold increase in superconducting transition temperature of La3Co4Sn13 by substituting indium at the tin site. The transition temperature of this skutterudite is observed to increase from 2.5 K to 5.1 K for 10 % indium substituted sample. The band structure and density of states calculations also indicate such a possibility. The compounds exhibit type - II superconductivity and the values of lower critical field (Hc1), upper critical field (Hc2), Ginzburg - Landau coherence length , penetration depth and GL parameter are estimated to be 0.0028 T, 0.68 T, 21.6 nm, 33.2 nm and 1.53 respectively for La3 Co4Sn11.7In1.3. Hydrostatic external pressure leads to decrease in transition temperature and the calculated pressure coefficient is -0.311 K/GPa . Flux pinning and vortex activation energies also improved with indium addition. Only positive frequencies are observed in phonon dispersion curve that relate to the absence of charge density wave or structural instability in the normal state.

cond-mat.supr-con↗

arXiv2015

Flux free growth of large FeSe1/2Te1/2 superconducting single crystals by an easy high temperature melt and slow cooling method

We report successful growth of flux free large single crystals of superconducting FeSe1/2Te1/2 with typical dimensions of up to few cm. The AC and DC magnetic measurements revealed the superconducting transition temperature (Tc) value of around 11.5K and the iso-thermal MH showed typical type-II superconducting behavior. The lower critical field being estimated by measuring the low field iso-thermal magnetization in superconducting regime is found to be above 200 Oe at 0K.

cond-mat.supr-con↗

arXiv2015

Pressure enhanced superconductivity at 10 K in La doped EuBiS2F

Polycrystalline Eu0.5La0.5BiS2F was synthesized by solid state reaction which crystallizes in the tetragonal CeOBiS2 structure (P4/nmm). We report here enhancement of Tc to 2.2 K in Eu0.5La0.5BiS2F (by electron doping in EuBiS2F with Tc ~ 0.3 K). Eu0.5La0.5BiS2F is semiconducting down to 3 K and an onset of superconductivity is seen at 2.2 K at ambient pressure. Upon application of pressure the Tc could be enhanced upto 10 K. Step like features are seen in the resistivity curves at intermediate pressures (0.5 - 1 GPa) which hints towards the possible existence of two phases with different Tc. At a pressure above 1.38GPa, the Tconset remains invariant at 10 K but the Tc(\r{ho}=0) is increased to above 8.2 K. There is a possible transformation from a low Tc phase to a high Tc phase by application of pressure.

cond-mat.supr-con↗

arXiv2015

Impact of Ni doping on critical parameters of PdTe type-II BCS superconductor

We report the impact of Ni doping on superconductivity of PdTe superconductor. The superconducting parameters like critical temperature (Tc), upper critical field (Hc2) and normalized specific-heat jump are reported for Ni doped Pd1-xNixTe. The samples of series Pd1-xNixTe with nominal compositions x=0, .01, 0.05, 0.07, 0.1, 0.15, 0.2, 0.3 and 1.0 are synthesized via solid state reaction route. All the studied samples of series Pd1-xNixTe (x = 0.0 to 1.0) are crystallized in hexagonal crystal structure within the space group P63/mmc. Unit cell volume shrinks almost linearly upon Ni doping in Pd1-xNixTe. The normal state residual resistivity increases with Ni substitution on Pd site. Both the electrical resistivity and magnetic measurements revealed that Tc decreases with increase of Ni concentration in Pd1-xNixTe and is not observed down to 2K for x=0.30 i.e., 30% of Ni doping at Pd site. Interestingly, this is unusual for magnetic Ni doping in a known type-II BCS type superconductor. Magnetic Ni must suppress the superconductivity much faster. Interestingly, the isothermal magnetization measurements for NiTe revealed that Ni is non-magnetic in Pd1-xNixTe structure and hence the Tc depression is mainly due to disorder. The magneto-transport measurements revealed that flux is better pinned for 20% Ni doped PdTe as compared to other compositions of Pd1-xNixTe. The magnetic field dependence of specific heat of Pd1-xNixTe for x=0.01 was studied and the estimated value of the normalized specific-heat jump,is found to be 1.42, which is under BCS weak-coupling limit. Summarily, we report the impact of Ni doping in Pd1-xNixTe superconductor and conclude that Ni substitutes at Pd site, suppress superconductivity moderately and is of non magnetic nature in this system. To best of our knowledge this is the first study on Ni substitution in PdTe superconductor.

cond-mat.supr-con↗

arXiv2015

Anomalous impact of hydrostatic pressure on superconductivity of polycrystalline LaO0.5F0.5BiSe2

We report bulk superconductivity at 2.5K in LaO0.5F0.5BiSe2 compound through the DC magnetic susceptibility and electrical resistivity measurements. The synthesized LaO0.5F0.5BiSe2 compound is crystallized in tetragonal structure with space group P4/nmm and Reitveld refined lattice parameters are a= 4.15(1)A and c=14.02(2)A. The lower critical field of Hc1= 40Oe, at temperature 2K is estimated through the low field magnetization measurements. The LaO0.5F0.5BiSe2 compound showed metallic normal state electrical resistivity with residual resistivity value of 1.35mΩ-cm. The compound is type-II superconductor, and the estimated Hc2(0) value obtained by WHH formula is above 20kOe for 90percent Rn criteria. The superconducting transition temperature decreases with applied pressure till around 1.68GPa and with further higher pressures a high Tc phase emerges with possible onset Tc of above 5K for 2.5GPa.

cond-mat.supr-con↗

arXiv2015

Common effect of chemical and external pressures on the magnetic properties of $R$CoPO ($R$ = La, Pr, Nd, Sm). II

The direct correspondence between Co band ferromagnetism and structural parameters is investigated in the pnictide oxides $R$CoPO for different rare-earth ions ($R$ = La, Pr, Nd, Sm) by means of muon-spin spectroscopy and {\it ab-initio} calculations, complementing our results published previously [G. Prando {\it et al.}, {\it Phys. Rev. B} {\bf 87}, 064401 (2013)]. Both the transition temperature to the ferromagnetic phase $T_{_{\textrm{C}}}$ and the volume of the crystallographic unit cell $V$ are found to be conveniently tuned by the $R$ ionic radius and/or external pressure. A linear correlation between $T_{_{\textrm{C}}}$ and $V$ is reported and {\it ab-initio} calculations unambiguously demonstrate a full equivalence of chemical and external pressures. As such, $R$ ions are shown to be influencing the ferromagnetic phase only via the induced structural shrinkage without involving any active role from the electronic $f$ degrees of freedom, which are only giving a sizeable magnetic contribution at much lower temperatures.

cond-mat.str-el↗

arXiv2015

Specific heat of robust Nb2PdS5 superconductor

We report specific heat under different magnetic fields for recently discovered quasi-one dimensional Nb2PdS5 superconductor. The studied compound is superconducting below 6 K. Nb2PdS5 is quite robust against magnetic field with dHc/dT of -42 kOe/K. The estimated upper critical field [Hc2(0)] is 190 kOe, clearly surpassing the Pauli-paramagnetic limit of 1.84Tc. Low temperature heat capacity in superconducting state of Nb2PdS5 under different magnetic fields showed s-wave superconductivity with two different gaps. Two quasi-linear slopes in Somerfield-coefficient as a function of applied magnetic field and two band behavior of the electronic heat capacity demonstrate that Nb2PdS5 is a multiband su-perconductor in weak coupling limit with deltagamma/deltaTc=0.9.

cond-mat.supr-con↗

arXiv2015

Effect of pressure on superconductivity in doped topological crystalline insulator Sn0.5In0.5Te

We report on the impact of hydrostatic pressure on the superconductivity of optimally (Indium) doped SnTe which is established to be derived from a topological crystalline insulating phase. Single crystals of Sn1-xInxTe were synthesized by a modified Bridgman method that exhibited maximum superconducting Tc of 4.4 K for x= 0.5. Hydrostatic pressure upto 2.5 GPa was applied on the crystals of Sn0.5In0.5Te and electrical resistivity as a function of temperature and pressure was measured. We observed decrease in onset superconducting transition temperature from 4.4 K to 2.8 K on increasing pressure from ambient to 2.5 GPa. The normal state resistivity also decreased abruptly by an order of magnitude at 0.5 GPa but for higher pressures, the same decreased marginally. From onset, offset and zero resistivity values, dTc/dP of -0.6K/GPa was confirmed. The low temperature normal state resistivity followed T^2 dependence suggesting Fermi liquid behaviour both for ambient and high pressure data. This increase in metallic characteristics accompanied by normal state Fermi liquid behaviour is in accordance with a dome structure for Tc variation with varying carrier concentration.

cond-mat.supr-con↗

arXiv2015

Effect of hydrostatic pressures on the superconductivity of new BiS2 based REO0.5F0.5BiS2 (RE-La, Pr and Nd) superconductors

We study the impact of hydrostatic pressure on superconductivity of new BiS2 based layered REO0.5F0.5BiS2 (RE-La, Pr, and Nd) compounds through the measurements of dc electrical resistivity. The REO0.5F0.5BiS2 (RE-La, Pr and Nd) compounds synthesized by solid state reaction route via vacuum encapsulation are crystallized in the tetragonal P4/nmm space group. At ambient pressure the superconducting transition onset temperatures are 2.7K, 3.5K and 4.5K which are enhanced substantially under external hydrostatic pressure to 10.5K, 7.8K and 7.5K for LaO0.5F0.5BiS2, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 respectively at 1.68GPa. The normal state electrical resistivity decreases with applied pressure for REO0.5F0.5BiS2 (RE-La, Pr and Nd). The electrical resistivity under magnetic field and applied pressure has been measured to estimate upper critical field, the values of which are 15.9Tesla, 8.8Tesla and 8.2Tesla for LaO0.5F0.5BiS2, PrO0.5F0.5BiS2 and NdO0.5F0.5BiS2 compounds. Substantial enhancement of superconductivity under moderate pressures in studied new BiS2 based superconductors call for the attention of condensed matter physics community.

cond-mat.supr-con↗

arXiv2015

Appearance of bulk Superconductivity under Hydrostatic Pressure in Sr0.5RE0.5FBiS2 (RE = Ce, Nd, Pr and Sm) compounds

We report the appearance of superconductivity under hydrostatic pressure (0.35 to 2.5GPa) in Sr0.5RE0.5FBiS2 with RE = Ce, Nd, Pr and Sm. The studied compounds, synthesized by solid state reaction route, are crystallized in tetragonal P4/nmm space group. At ambient pressure though the RE = Ce exhibit the onset of superconductivity below 2.5K, the Nd, Pr and Sm samples are not superconducting down to 2K. With application of hydrostatic pressure (up to 2.5GPa), superconducting transition temperature is increased to around 10K for all the studied samples. The magneto-transport measurements are carried out on all the samples with maximum Tc i.e., at under 2.5GPa pressure and their upper critical fields are determined. The new superconducting compounds appear to be quite robust against magnetic field but within Pauli paramagnetic limit. The new superconducting compounds with various RE (Ce, Nd, Pr and Sm) belonging to Sr0.5La0.5FBiS2 family are successfully synthesized for the first time and superconductivity is induced in them under hydrostatic pressure.

cond-mat.supr-con↗

arXiv2014

Importance of structural distortions in enhancement of transition temperature in FeSe$_{1-x}$Te$_{x}$ superconductors

Temperature (12K $\le$ T $\le$ 300K) dependent extended X-ray absorption fine structure (EXAFS) studies at the Fe K edge in FeSe$_{1-x}$Te$_x$ (x = 0, 0.5 and 1.0) compounds have been carried out to understand the reasons for increase in T$_C$ upon Te doping in FeSe. While local distortions are present near superconducting onset in FeSe and FeSe$_{0.5}$Te$_{0.5}$, they seem to be absent in non superconducting FeTe. Of crucial importance is the variation of anion height. In FeSe$_{0.5}$Te$_{0.5}$, near superconducting onset, the two heights, $h_{Fe-Se}$ and $h_{Fe-Te}$ show a nearly opposite behaviour. These changes indicate a possible correlation between Fe-chalcogen hybridization and the superconducting transition temperature in these Fe-chalcogenides.

cond-mat.supr-con↗

arXiv2014

Role of MgO impurity on the superconducting properties of MgB2

We address the effect of MgO impurity on the superconducting properties of MgB2. The synthesis of MgB2 is very crucial because of sensitivity of Mg to oxidation which may lead to MgO as a secondary phase. Rietveld refinement was performed to determine the quantitative volume fraction of MgO in the samples synthesized by two different techniques. Both the samples were subjected to magnetization measurements under dc and ac applied magnetic fields and the observed results were compared as a function of temperature. Paramagnetic Meissner effect has been observed in a sample of MgB2 having more amount of MgO (with Tc = 37.1K) whereas the pure sample MgB2 having minor quantity of MgO shows diamagnetic Meissner effect with Tc = 38.8K. M-H measurements at 10K reveal a slight difference in irreversibility field which is due to MgO impurity along with wide transition observed from ac magnetic susceptibility measurements. The magnetotransport measurements R(T)H using RN = 90%, 50% and 10% criterion on pure sample of MgB2 has been used to determine the upper critical field whereas the sample having large quantity of MgO does not allow these measurements due to its high resistance.

cond-mat.supr-con↗

arXiv2014

Hydrostatic Pressure Studies on Parent Phase SrFBiS2 of BiS2-based Superconducting Family

In the present work, we measure the temperature dependent electrical resistivity from 300K down to 2K under applied hydrostatic pressure of upto 2.5GPa for SrFBiS2, which is the parent compound for the BiS2 based superconductors. Though the normal state resistivity of the compound decreases with pressure, the same is not superconducting down to 2K under applied pressure of up to 2.5Gpa.

cond-mat.supr-con↗

arXiv2014

Photoemission studies of the near Fermi level spectral weight shifts in FeSe1-xTex superconductor

Our valence band photoelectron spectroscopic studies show a temperature dependent spectral weight transfer near the Fermi level in the Fe-based superconductor FeSe1-xTex. Using theoretical band structure calculations we have shown that the weight transfer is due to the temperature induced changes in the Fe(Se,Te)4 tetrahedra. These structural changes lead to shifts in the electron occupancy from the xz/yz and x2-y2 orbitals to the 3z2-r2 orbitals indicating a temperature induced crossover from a metallic state to an Orbital Selective Mott (OSM) Phase. Our study presents the observation of a temperature induced crossover to a low temperature OSM phase in the family of Fe chalcogenides.

cond-mat.supr-con↗

arXiv2014

Superconductivity at 4K in Pd deficient layered Ta_2Pd_xS_6

Here in this short note, we report on the low dimensional 4d and 5d transition metals-chalcogenide based compounds i.e., Ta2PdxS6, showing semiconducting to superconducting transition around 4K with upper critical field outside Pauli paramagnetic limit. It seems couple of different superconducting phases do exist in these new set of compounds. Our short note in this regards is thought provoking, asking to explore various unearthed possible superconducting phases in (Nb/Ta)2Pdx(S/Se/Te)y systems.

cond-mat.supr-con↗

arXiv2014

Ferromagnetism, insulator-metal transition and magnetotransport in Pr0.58Ca0.42MnO3 films: role of microstructural perturbations

Magnetic and magnetotransport properties of oriented polycrystalline Pr0.58Ca0.42MnO3 thin films prepared in flowing oxygen and air ambient has been investigated. The magnetic ground state of both the films is a frozen cluster glass. In the air annealed film charge order (CO) is quenched and ferromagnetic (FM) transition, which appears at TC=148 K is followed by antiferromagnetic (AFM) transition at TN=104 K. This film shows self-field hysteretic insulator-metal transition (IMT) at TIMC=89 K and TIMW=148 K in the cooling and warming cycle, respectively. Application of magnetic field (H) gradually enhances TIMC and TIMW, reduces the thermoresistive hysteresis and TIM diminishes. In contrast, the film annealed in flowing oxygen shows a CO transition, which is followed by FM and AFM transitions. This film shows appreciably smaller magnetic moment and does not show IMT upto H=20 kOe. As H is increased to H=30 kOe, IMT having strong thermoresistive hysteresis and sharp resistivity jumps appears in the cooling and warming cycles. As H increases to higher values the thermoresistive hysteresis is reduced, resistivity jumps are observed to disappear and TIM decreases. In the lower temperature regime the resistivity first decreases slowly with H and then shows sharp drop. The virgin cycle is not recoverable in subsequent cycles. The decrement far more pronounced in the oxygen annealed film and occurs at much higher H suggesting that the frozen cluster glass state is more robust in this film. The microstructural analysis of the two set of films shows CO quenching, FM transition and self-field IMT in air annealed film is caused by higher density of microstructural disorder and lattice defects. The difference in growth ambience of the two films could give rise to such microstructural perturbations.

cond-mat.mtrl-sci↗