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Dhar, S. K.

Publications and source records attributed to Dhar, S. K..

9 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Magnetostructural behavior in the non-centrosymmetric compound Nd 7 Pd 3

A systematic study of the physical properties and microscopic magnetism of Nd 7 Pd 3 compound, which in the paramagnetic state crystallizes in the non-centrosymmetric hexagonal Th 7 Fe 3 -type structure [hP20-P6 3 mc; with a = 10.1367(1) Å and c = 6.3847(1) Å at 300 K], confirms multiple magnetic ordering transitions that occur upon cooling. Antiferromagnetic transition is observed at T N = 37 K, which is followed by ferromagnetic transformation at T C = 33 K. The first-order magnetic transition at TC is magnetoelastic: it involves a change of crystal symmetry from P6 3 mc to Cmc2 1 and leads to anisotropic changes of the unit cell parameters. While the antiferromagnetic structure is symmetry allowed in P6 3 mc, the ferromagnetic structure with magnetic moments along the a-direction of the original hexagonal unit cell induces the first order transition to Cmc21. Density functional theory calculations confirm the experimentally observed ground state with the a-axis as the easy magnetization direction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Spin correlation in trigonal EuMn 2 As 2

The trigonal structure of EuMn 2 As 2 is an anomaly in the tetragonal 122-type pnictide family. Here, we report a detailed investigation of the underlying magnetic correlations in single crystal EuMn 2 As 2 using high-resolution elastic neutron-scattering measurements. The system undergoes two successive antiferromagnetic transitions at T = 135 K and 14.4 K, respectively. Numerical modeling of the experimental data reveals the long-range antiferromagnetic correlation of Mn-ions in the a - b plane below T N 1 = 135 K . Mn spins are aligned closer to the diagonal axis of the unit cell. The lower temperature transition, below T N 2 = 14.4 K , is discovered to arise due to the long-range antiferromagnetic correlation of Eu spins that are rotated by θ = 55 ° from the c axis of the unit cell.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Gapped excitation in dense Kondo lattice CePtZn

In this work, we report on neutron scattering and muon spin relaxation measurements of dense Kondo lattice CePtZn. The system develops long-range incommensurate magnetic order as the temperature is reduced below TN=1.75 K. Interestingly, a Q-independent gap at E=0.65 meV in the energy spectrum is found to co-exist with the long-range magnetic order. The gap persists to a very high temperature of T≃100 K. The Q-independent characteristic and its persistence to high temperature hint that the gapped excitation may be manifesting the excited state of the ground-state doublet of the crystal-field energy levels. However, the observed broadness in the linewidth with distinct temperature and field dependencies makes it a nontrivial phenomenon. Qualitative analysis of the experimental data suggests the possible co-existence of a local critical behavior, which is onset near the critical field of H≃3 T, with the crystal-field excitation in the dynamic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Ti{sub 3}CrCu{sub 4}: A possible 2-D ferromagnetic spin fluctuating system

Ti{sub 3}CrCu{sub 4} is a new ternary compound which crystallizes in the tetragonal Ti{sub 3}Pd{sub 5} structure type. The Cr atoms form square nets in the a-b plane (a = 3.124 Å) which are separated by an unusually large distance c = 11.228 Å along the tetragonal axis, thus forming a -2-D Cr-sublattice. The paramagnetic susceptibility is characterized by a low effective moment, μ{sub eff} = 1.1 μ{sub B}, a low paramagnetic Curie temperature θ{sub P} (below 7 K) and a temperature independent χ{sub 0} = 6.7 x 10{sup −4} emu/mol. The magnetization at 1.8 K increases rapidly with field nearly saturating to 0.2 μ{sub B}/f.u. The zero field heat capacity C/T shows an upturn below 7 K (∼190 mJ/mol K{sup 2} at ∼0.1K) which is suppressed in applied magnetic fields and interpreted as suggesting the presence of spin fluctuations. The resistivity at low temperatures shows non-Fermi liquid behavior. Overall, the experimental data thus reveal an unusual magnetic state in Ti{sub 3}CrCu{sub 4}, which likely has its origin in the layered nature of the Cr sub-lattice and ferromagnetic spin fluctuations. Density functional theoretical calculations reveal a sharp Cr density of states peak just above the Fermi level, indicating the propensity of Ti{sub 3}CrCu{sub 4} to become magnetic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Magnetic structures of R 5 Ni 2 In 4 and R 11 Ni 4 In 9 ( R = Tb and Ho): Strong hierarchy in the temperature dependence of the magnetic ordering in the multiple rare-earth sublattices

In this study, the magnetic properties and magnetic structures of the R 5 Ni 2 In 4 and the microfibrous R 11 Ni 4 In 9 compounds with R = Tb and Ho have been examined using magnetization, heat capacity, and neutron diffraction data. Rare earth atoms occupy three and five symmetrically inequivalent rare earth sites in R 5 Ni 2 In 4 and R 11 Ni 4 In 9 compounds, respectively. As a result of the intra- and inter-magnetic sublattice interactions, the magnetic exchange interactions are different for various rare earth sites; this leads to a cascade of magnetic transitions with a strong hierarchy in the temperature dependence of the magnetic orderings.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Complex temperature evolution of the electronic structure of CaFe{sub 2}As{sub 2}

Employing high resolution photoemission spectroscopy, we investigate the temperature evolution of the electronic structure of CaFe{sub 2}As{sub 2}, which is a parent compound of high temperature superconductors—CaFe{sub 2}As{sub 2} exhibits superconductivity under pressure as well as doping of charge carriers. Photoemission results of CaFe{sub 2}As{sub 2} in this study reveal a gradual shift of an energy band, α away from the chemical potential with decreasing temperature in addition to the spin density wave (SDW) transition induced Fermi surface reconstruction across SDW transition temperature. The corresponding hole pocket eventually disappears at lower temperatures, while the hole Fermi surface of the β band possessing finite p orbital character survives till the lowest temperature studied. These results, thus, reveal signature of complex charge redistribution among various energy bands as a function of temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1995

Quaternary borocarbides: New class of intermetallic superconductors

Our recent discovery of superconductivity (SC) in the four-element multiphase Y-Ni-B-C system at an elevated temperature (TC approximately 12 K) has opened up great possibilities of identifying new superconducting materials and generating new physics. Superconductivity with Tc (greater than 20 K) higher than that known so far in bulk intermetallics has been observed in multiphase Y-Pd-B-C and Th-Pd-B-C systems and a family of single phase materials RENi2B2C (RE= Y, rare earth) have been found. Our investigations show YNi2B2C to be a strong coupling hard type-II SC. HC2(T) exhibits an unconventional temperature dependence. Specific heat and magnetization studies reveal coexistence of SC and magnetism in RNi2B2C (R = Ho, Er, Tm) with magnetic ordering temperatures (Tc approximately 8 K, 10.5 K, 11 K and Tm approximately 5 K, approximately 7K, approximately 4 K respectively) that are remarkably higher than those in known magnetic superconductors . Mu-SR studies suggest the possibility of Ni atoms carrying a moment in TmNi2B2C. Resistivity results suggests a double re-entrant transition (SC-normal-SC) in HoNi2B2C. RENi2B2C (RE = Ce, Nd, Gd) do not show SC down to 4.2 K. The Nd- and Gd-compounds order magnetically at approximately 4.5 K and approximately 19.5 K, respectively. Two SC transitions are observed in Y-Pd-B-C (Tc approximately 22 K, approximately 10 K) and in Th-Pd-B-C (Tc approximately 20 K, approximately 14 K) systems, which indicate that there are at least two structures which support SC in these borocarbides. In our multiphase ThNi2B2C we observe SC at approximately 6 K. No SC was seen in multiphase UNi2B2C, UPd2B2C, UOs2Ge2C and UPd5B3C(0.35) down to 4.2 K. Tc in YNi2B2C is depressed by substitutions (Gd, Th and U at Y-sites and Fe, Co at Ni-sites).

Nagarajan, R.↗
NASA NTRS1993

Ovonic nickel metal hydride batteries for space applications

Ovonic nickel-metal hydride (NiMH) rechargeable batteries are easily adaptable to a variety of applications. Small consumer NiMH cells were developed and are now being manufactured by licensees throughout the world. This technology was successfully scaled up in larger prismatic cells aimed at electric vehicle applications. Sealed cells aimed at satellite power applications were also built and cycle tested by OBC and other outside agencies. Prototype batteries with high specific energy (over 80 Wh/kg), high energy density (245 Wh/L), and excellent power capability (400 W/kg) were produced. Ovonic NiMH batteries demonstrated an excellent cycle life of over 10,000 cycles at 30 percent DOD. Presently, Ovonic Battery Company is working on an advanced version of this battery for space applications as part of an SBIR contract from NASA.

Venkatesan, S.↗