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L. C. Gupta

Publications and source records attributed to L. C. Gupta.

12 recordsLinked to original sources
arXiv2017

Magnetic structures of rare earth intermetallic compounds RCuAs$_2$ (R = Pr, Nd, Tb, Dy, Ho, and Yb)

Neutron scattering studies have been carried out on polycrystalline samples of a series of rare earth intermetallic compounds RCuAs$_2$ (R = Pr, Nd, Dy, Tb, Ho and Yb) as a function of temperature to determine the magnetic structures and the order parameters. These compounds crystallize in the ZrCuSi$_2$ type structure, which is similar to that of the RFeAsO (space group P4/nmm) class of iron-based superconductors. PrCuAs$_2$ develops commensurate magnetic order with K = (0, 0, 0.5) below T$_N$ = 6.4 (2) K, with the ordered moments pointing along the c-axis. The irreducible representation analysis shows either a $Γ$$^1_2$ or $Γ$$^1_3$ representation. NdCuAs$_2$ and DyCuAs$_2$ order below T$_N$ = 3.54(5) K and T$_N$ = 10.1(2) K, respectively, with the same ordering wave vector but the moments lying in the a-b plane (with a $Γ$$^2_9$ or $Γ$$^2_{10}$ representation). TbCuAs$_2$ and HoCuAs$_2$ exhibit incommensurate magnetic structures below T$_N$ = 9.44(7) and 4.41(2) K, respectively. For TbCuAs$_2$, two separate magnetic ordering wave vectors are established as K$_{1(Tb)}$ = (0.240,0.155,0.48) and K$_{2(Tb)}$ = (0.205, 0.115, 0.28), whereas HoCuAs$_2$ forms a single K$_{(Ho)}$ = (0.121, 0.041, 0.376) magnetic structure with 3$^{rd}$ order harmonic magnetic peaks. YbCuAs$_2$ does not exhibit any magnetic Bragg peaks at 1.5 K, while susceptibility measurements indicate an antiferromagnetic-like transition at 4 K, suggesting that either the ordering is not long range in nature or the ordered moment is below the sensitivity limit of $\approx$ 0.2 $μ_B$.

cond-mat.mtrl-sci↗
arXiv2016

High spin polarization and the origin of unique ferromagnetic ground state in CuFeSb

CuFeSb is isostructural to the ferro-pnictide and chalcogenide superconductors and it is one of the few materials in the family that are known to stabilize in a ferromagnetic ground state. Majority of the members of this family are either superconductors or antiferromagnets. Therefore, CuFeSb may be used as an ideal source of spin polarized current in spin-transport devices involving pnictide and the chalcogenide superconductors. However, for that the Fermi surface of CuFeSb needs to be sufficiently spin polarized. In this paper we report direct measurement of transport spin polarization in CuFeSb by spin-resolved Andreev reflection spectroscopy. From a number of measurements using multiple superconducting tips we found that the intrinsic transport spin polarization in CuFeSb is high ($\sim$ 47\%). In order to understand the unique ground state of CuFeSb and the origin of large spin polarization at the Fermi level, we have evaluated the spin-polarized band structure of CuFeSb through first principles calculations. Apart from supporting the observed 47\% transport spin polarization, such calculations also indicate that the Sb-Fe-Sb angles and the height of Sb from the Fe plane is strikingly different for CuFeSb than the equivalent parameters in other members of the same family thereby explaining the origin of the unique ground state of CuFeSb.

cond-mat.supr-con↗
arXiv2016

Superconductivity in La1-xSmxO0.5F0.5BiS2 (x = 0.2, 0.8) under hydrostatic pressure

We have investigated the pressure effect on the newly discovered samarium doped La1-xSmxO0.5F0.5BiS2 superconductors. More than threefold increase in Tc (10.3 K) is observed with external pressure (at ~1.74 GPa at a rate of 4.08 K/GPa)) for x = 0.2 composition. There is a concomitant large improvement in the quality of the superconducting transition. Beyond this pressure Tc decreases monotonously at the rate of -2.09 K/GPa. In the x = 0.8 sample, we do not observe any enhancement in Tc with application of pressure (up to 1.76 GPa). The semiconducting behavior observed in the normal state resistivity of both of the samples is significantly subdued with the application of pressure which, if interpreted invoking thermal activation process, implies that the activation energy gap of the carriers is significantly reduced with pressure. We believe these observations should generate further interest in the La1-xSmxO0.5F0.5BiS2 superconductors.

cond-mat.supr-con↗
arXiv2015

Investigation of Arrott plot and magnetocaloric effect in the complex CaMn7O12 perovskite

Detailed magnetic studies including magneto-caloric measurements on magnetic multiferroic quadruple perovskite CaMn7O12 are presented. Based on the collective response of Arrott plots and ΔSM (T), a magnetic phase diagram of CaMn7O12 is suggested. A new magnetic transition at TN3 ~20 K where the system changes from noncollinear AFM to collinear AFM is reported. An anomaly observed in both ΔSM (T) and Arrott plots around 64 K has been attributed to high external field induced spin canting leading to change in magnetic order inducing phase transition. Magneto-caloric effect in this material is presented for the first time. The peak value of change in isothermal magnetic entropy is 1.3 J/K-Kg and the value of refrigeration capacity is reported to be 34.5 J/Kg for the field of 7 T.

cond-mat.mtrl-sci↗
arXiv2015

Coexistence of superconductivity and itinerant ferromagnetism in Sr0.5Ce0.5FBiS2-xSex (x = 0.5 and 1.0), the first non-U material with Tc < TFM

We have carried out detailed magnetic and transport studies of the new Sr0.5Ce0.5FBiS2-xSex (x = 0.5, 1) superconductors derived by doping Se in Sr0.5Ce0.5FBiS2. Se-doping produces several effects: it suppresses semiconducting like behavior observed in the undoped Sr0.5Ce0.5FBiS2, ferromagnetic ordering temperature, TFM, decreases considerably from 7.5 K (in Sr0.5Ce0.5FBiS2) to 3.5 K and superconducting transition temperature, Tc, gets enhanced slightly to 2.9 - 3.3 K. Thus in these Se-doped materials, TFM is just marginally higher than Tc. Magnetization studies provide an evidence of bulk superconductivity in Sr0.5Ce0.5FBiS2-xSex. Quite remarkably, as compared with the effective paramagnetic Ce-moment (~ 2.2 muB), the ferromagnetically ordered Ce-moment in the superconducting state is rather small (~ 0.1 muB). To the best of our knowledge, the title compounds are the first Ce-based superconducting itinerant ferromagnetic materials (Tc < TFM). We stress that Ce-4f electrons are responsible for both superconductivity and ferromagnetism just as U-5f electrons are in UCoGe. Furthermore, a novel feature of these materials is a dual hysteresis loop corresponding to both the ferromagnetism and the coexisting superconductivity. Such features of Sr0.5Ce0.5FBiS2-xSex put these materials apart from the well known U-containing superconducting ferromagnets reported so far.

cond-mat.supr-con↗
arXiv2015

Structural and magnetic properties of a new and ordered quaternary alloy MnNiCuSb (SG: F-43m)

We have synthesized a new crystallographically ordered quaternary Heusler alloy, MnNiCuSb. The crystal structure of the alloy has been determined by Rietveld refinement of the powder x-ray diffraction data. This alloy crystallizes in the LiMgPdSb type structure with F-43m space group. MnNiCuSb is a ferromagnet with a high TC ~ 690K and magnetic moment of 3.85MuB/f.u. Besides this we have also studied two other off-stoichiometric compositions; one Cu rich and the other Ni rich (MnNi0.9Cu1.1Sb and MnNi1.1Cu0.9Sb) which are also ferromagnets. It must be stressed that MnNiCuSb is one of the very few known, non-Fe containing quaternary Heusler alloys with 1: 1: 1: 1 composition.

cond-mat.mtrl-sci↗
arXiv2015

Upper critical field, critical current density and activation energy of the new La1-xSmxOF0.5BiS2 (x = 0.2, 0.8) superconductor

Critical current density (Jc), thermal activation energy (U0), and upper critical field (Hc2) of La1-xSmxO0.5F0.5BiS2 (x = 0.2, 0.8) superconductors are investigated from magnetic field dependent \r{ho}(T) studies. The estimated upper critical field (Hc2) has low values of 1.04 T for x = 0.2 and 1.41 T for x = 0.8. These values are lower than Sm free LaO0.5F0.5BiS2 superconductor (1.9 T). The critical current density (Jc) is estimated to be 1.35*105 A/cm2 and 5.07 *105 A/cm2 (2 K) for x = 0.2 and 0.8 respectively, using the Bean's model. The thermal activation energy (U0/kB) is 61 K for x = 0.2 and 140 K for x =0.8 as calculated from Arrhenius plots at low magnetic field (1 T) and indicates a strong flux pinning potential which might be co-existing with applied magnetic field.

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

Synthesis and properties of SmO0.5F0.5BiS2 and enhancement in Tc in La1-ySmyO0.5F0.5BiS2

Crystal structure and properties of a new member of oxy-bismuth-sulfide SmO1-xFxBiS2 are reported here. The compounds SmO1-xFxBiS2 (x = 0.0 and 0.5) are found to be isostructural with LaOBiS2 and crystallize in the CeOBiS2 type structure (P4/nmm). Sm substitution in LaO0.5F0.5BiS2, (La1-ySmyO0.5F0.5BiS2), leads to a gradual decrease in a-lattice constant however the c-lattice constant does not show such a gradual trend. Enhancement in Tc is achieved upon partially substituting La by smaller Sm ion. Maximum Tc ~ 4.6 K was observed for composition with y = 0.8. Disobeying this trend Tc disappears unexpectedly in composition SmO0.5F0.5BiS2 (y = 1.0). Both the undoped and F-doped (x = 0.0 and 0.5) compounds are paramagnetic exhibiting semiconducting behavior down to 2 K.

cond-mat.supr-con↗
arXiv2009

Si-induced superconductivity and structural transformations in DyRh4B4

DyRh4B4 has been known to crystallize in the primitive tetragonal (pt)-structure and to exhibit a ferromagnetic transition at 12 K, the highest magnetic transition temperature in the entire series of the RRh4B4 materials [1]. We show here that our silicon-added samples of the nominal composition DyRh4B4Si0.2 exhibit superconductivity below Tc ~ 4.5 K and an antiferromagnetic transition below TN ~ 2.7 K. The 12 K transition observed in the pt-DyRh4B4 is completely suppressed. Our annealed samples mainly consist of domains of the chemical composition DyRh3.9B4.2Si0.08. These domains contain two crystallographic phases belonging to the body-centred tetragonal (bct)-structure and the orthorhombic (o)-structure. We have reasons to suggest that superconductivity and antiferromagnetic ordering arise from bct- DyRh4B4 phase and, therefore, coexist below TN ~ 2.7 K.

cond-mat.supr-con↗
arXiv1995

Microwave Properties of Borocarbide Superconductors LnNi2B2C (Ln = Y, Er, Tm, Ho)

We report measurements of the microwave surface impedance of the borocarbide family of superconductors LnNi2B2C (Ln=Y, Er, Tm, Ho). The experiments enable direct measurements of the superfluid density, and are particularly sensitive to the influence of magnetic pairbreaking. In HoNi2B2C the antiferromagnetic transition is clearly observed at zero field, and leads to a drastic reduction of the superfluid density, which recovers at lower temperatures. In ErNi2B2C the antiferromagnetic transition is not seen in zero field data. Magnetic effects are responsible for anomalies in the low temperature surface impedance below approximately 4K in HoNi2B2C and TmNi2B2C. The temperature dependence of the microwave impedance disagrees with simple BCS calculations.

supr-con↗