arXiv subjects

Abhishek Pandey

Publications and source records attributed to Abhishek Pandey.

61 records · Page 4Linked to original sources

arXiv2013

Crystallographic, Electronic, Thermal and Magnetic Properties of Single-Crystal SrCo2As2

In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here, structural and thermal expansion, electrical resistivity rho, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility chi, 75As NMR and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient alpha_c is negative from 7 to 300 K, whereas alpha_a is positive over this T range. The rho(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy E_F. The band calculations exhibit an extremely sharp peak in the density of states D(E_F) arising from a flat d_{x^2 - y^2} band. A comparison of the Sommerfeld coefficient of the electronic specific heat with chi(T = 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The chi(T) does have a large magnitude, but also exhibits a broad maximum at 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we propose that metallic SrCo2As2 has a gapless quantum spin-liquid ground state.

cond-mat.supr-con↗

arXiv2013

Coexistence of Half-Metallic Itinerant Ferromagnetism with Local-Moment Antiferromagnetism in Ba{0.60}K{0.40}Mn2As2

Magnetization, nuclear magnetic resonance, high-resolution x-ray diffraction and magnetic field-dependent neutron diffraction measurements reveal a novel magnetic ground state of Ba{0.60}K{0.40}Mn2As2 in which itinerant ferromagnetism (FM) below a Curie temperature TC = 100 K arising from the doped conduction holes coexists with collinear antiferromagnetism (AFM) of the Mn local moments that order below a Neel temperature TN = 480 K. The FM ordered moments are aligned in the tetragonal ab-plane and are orthogonal to the AFM-ordered Mn moments that are aligned along the c-axis. The magnitude and nature of the low-T FM ordered moment correspond to complete polarization of the doped-hole spins (half-metallic itinerant FM) as deduced from magnetization and ab-plane electrical resistivity measurements.

cond-mat.supr-con↗

arXiv2013

Persistence of local-moment antiferromagnetic order in Ba(1-x)KxMn2As2

BaMn2As2 is a local-moment antiferromagnetic insulator with a Néel temperature of 625 K and a large ordered moment of 3.9 Bohr magneton per Mn. Remarkably, this compound can be driven metallic by the substitution of as little as 1.6% K for Ba while retaining essentially the same ordered magnetic moment and Néel temperature, as previously reported. Here, using both powder and single crystal neutron diffraction we show that the local moment antiferromagnetic order in Ba(1-x)KxMn2As2 remains robust up to x = 0.4. The ordered moment is nearly independent of x for 0 < x < 0.4 and the Néel transition temperature decreases to 480 K at x = 0.4.

cond-mat.supr-con↗

arXiv2012

Crystal Growth and Physical Properties of SrCu2As2, SrCu2Sb2 and BaCu2Sb2

We report the growth of single crystals of SrCu2As2, SrCu2Sb2, SrCu2(As{0.84}Sb{0.16})2 and BaCu2Sb2 using the self-flux technique and their structural, magnetic, thermal and transport properties that were investigated by powder x-ray diffraction (XRD), magnetic susceptibility chi, specific heat Cp and electrical resistivity rho measurements versus temperature T from 1.8 to 350 K. Rietveld refinements of XRD patterns for crushed crystals confirm that SrCu2As2 crystallizes in the ThCr2Si2-type structure and SrCu2Sb2 crystallizes in the CaBe2Ge2-type structure. However, BaCu2Sb2 is found to have a large unit cell consisting of three blocks. Here a ThCr2Si2-type block is sandwiched between two CaBe2Ge2-type blocks along the c-axis, as reported, but likely with a monoclinic distortion. The chi data of all these compounds are diamagnetic and reveal nearly T-independent anisotropic behavior. The finite values of the Sommerfeld linear specific heat coefficients gamma and the T dependences of rho reveal metallic character of all four compounds. The electronic and magnetic properties indicate that these compounds are sp metals with Cu in the 3d^{10} electronic configuration corresponding to the oxidation state Cu^{+1}, as previously predicted theoretically for (Sr,Ba)Cu2As2 by D. J. Singh. We find that SrCu2As2 has a collapsed-tetragonal structure. The electronic character of the Cu and the strength of the As-As interlayer bonding are both expected to drastically change between weakly Cu-substituted BaFe2As and pure BaCu2As2.

cond-mat.supr-con↗

arXiv2012

Structural, Thermal, Magnetic and Electronic Transport Properties of the LaNi2(Ge{1-x}P{x})2 System

Polycrystalline samples of LaNi2(Ge{1-x}P{x})2 (x = 0, 0.25, 0.50, 0.75, 1) were synthesized and their properties investigated by x-ray diffraction (XRD), heat capacity Cp, magnetic susceptibility chi, and electrical resistivity rho measurements versus temperature T. These compounds all crystallize in the body-centered-tetragonal ThCr2Si2-type structure. The rho(T) measurements indicate that all compositions in this system are metallic. The low-T Cp measurements yield a rather large Sommerfeld coefficient gamma = 12.4(2) mJ/mol K^2 for x = 0 reflecting a large density of states at the Fermi energy that is comparable with the largest values found for the AFe2As2 class of materials with the same crystal structure. The gamma decreases approximately linearly with x to 7.4(1) mJ/mol K^2 for x = 1. The chi measurements show nearly temperature-independent paramagnetic behavior across the entire range of compositions except for LaNi2Ge2, where a broad peak is observed at 300 K from chi(T) measurements up to 1000 K that may arise from short-range antiferromagnetic correlations in a quasi-two-dimensional magnetic system. High-accuracy Pade approximants representing the Debye lattice heat capacity and Bloch-Gruneisen electron-phonon resistivity functions versus T are presented and are used to analyze our experimental Cp(T) and rho(T) data, respectively. The T-dependences of rho for all samples are well-described by the Bloch-Gruneisen model, although the observed rho(300 K) values are larger than calculated from this model. A significant T-dependence of the Debye temperature determined from the Cp(T) data was observed for each composition. No clear evidence for bulk superconductivity or any other long-range phase transition was found for any of the compositions studied.

cond-mat.supr-con↗

arXiv2012

Ba{1-x}KxMn2As2: An Antiferromagnetic Local-Moment Metal

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Neel temperature TN = 625 K and a large ordered moment mu = 3.9 mu_B/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K, while retaining the same crystal and antiferromagnetic structures together with nearly the same high TN and large mu. Ba_{1-x}K_xMn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba_{1-x}K_xMn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high Tc superconductivity.

cond-mat.supr-con↗

arXiv2011

Large Miscibility Gap in the Ba(Mn_xFe_{1-x})2As2 System

The compounds BaMn2As2 and BaFe2As2 both crystallize in the body-centered-tetragonal ThCr2Si2-type (122-type) structure at room temperature but exhibit quite different unit cell volumes and very different magnetic and electronic transport properties. Evidently reflecting these disparities, we have discovered a large miscibility gap in the system Ba(Mn_xFe_{1-x})2As2. Rietveld refinements of powder x-ray diffraction (XRD) measurements on samples slow-cooled from 1000 C to room temperature (RT) reveal a two-phase mixture of BaMn2As2 and Ba(Mn_{0.12}Fe_{0.88})2As2 phases together with impurity phases for x = 0.2, 0.4, 0.5, 0.6 and 0.8. We infer that there exists a miscibility gap in this system at 300 K with composition limits 0.12 < x < 1. For samples quenched from 1000 C to 77 K, the refinements of RT XRD data indicate that the miscibility gap at RT narrows at 1000 C to 0.2 < x < 0.8. Samples with x=0.4, 0.5 and 0.6 quenched from 1100-1400 C to 77 K contain a single 122-type phase together with significant amounts of Fe_{1-x}Mn_xAs and FeAs2 impurity phases. These results indicate that the system is not a pseudo-binary system over the whole composition range and that the 122-type phase has a significant homogeneity range at these temperatures. Magnetic susceptibility, electrical resistivity and heat capacity measurements versus temperature of the single-phase quenched polycrystalline samples with x = 0.2 and 0.8 and for lightly doped BaMn2As2 crystals are reported.

cond-mat.supr-con↗