Computational screening of magnetocaloric alloys

An exciting development over the past few decades has been the use of high-throughput computational screening as a means of identifying promising candidate materials for a variety of structural or functional properties. Experimentally, it is often found that the highest-performing materials contain substantial atomic site disorder. These are frequently overlooked in high-throughput computational searches however, due to difficulties in dealing with materials that do not possess simple, well-defined crystallographic unit cells. Here we demonstrate that the screening of magnetocaloric materials with the help of the density functional theory-based magnetic deformation proxy can be extended to systems with atomic site disorder. This is accomplished by thermodynamic averaging of the magnetic deformation for ordered supercells across a solid solution. We show that the highly non-monotonic magnetocaloric properties of the disordered solid solutions Mn(Co$_{1-x}$Fe$_x$)Ge and (Mn$_{1-x}$Ni$_x$)CoGe are successfully captured using this method.Comment: Main text: 8 pages, 6 figures. Supplemental Material: 2 pages, 2 figure

PROTON STRUCTURE FUNCTION CALCULATION BY THERMODYNAMICAL BAG MODEL

This paper focuses on finding proton structure functions in deep inelastic scattering of leptons on nucleons by MIT Bag model. This model proposed by V. Devanathan and S. Karthiyayini assumes that nucleon is a hot bag containing quarks, which interact with bosons. The nucleon structure function is then expressed in terms of Parton distribution functions where both are functions of Bjorken variable ð‘¥ only. The structure functions calculated by this model are found to be in good agreement with the data obtained from CERN for Bjorken variable ð‘¥ greater than 0.2 only

Spin-induced symmetry breaking in orbitally ordered NiCr_2O_4 and CuCr_2O_4

At room temperature, the normal oxide spinels NiCr_2O_4 and CuCr_2O_4 are tetragonally distorted and crystallize in the I4_1/amd space group due to cooperative Jahn-Teller ordering driven by the orbital degeneracy of tetrahedral Ni$^{2+}$ ($t_2^4$) and Cu$^{2+}$ ($t_2^5$). Upon cooling, these compounds undergo magnetic ordering transitions; interactions being somewhat frustrated for NiCr_2O_4 but not for CuCr_2O_4. We employ variable-temperature high-resolution synchrotron X-ray powder diffraction to establish that at the magnetic ordering temperatures there are further structural changes, which result in both compounds distorting to an orthorhombic structure consistent with the Fddd space group. NiCr_2O_4 exhibits additional distortion, likely within the same space group, at a yet-lower transition temperature of $T$ = 30 K. The tetragonal to orthorhombic structural transition in these compounds appears to primarily involve changes in NiO_4 and CuO_4 tetrahedra

Moduli of parahoric G-torsors on a compact Riemann surface

Let χ be an irreducible smooth projective algebraic curve of genus g ≥ 2 over the ground field C and let G be a semisimple simply connected algebraic group. The aim of this paper is to introduce the notion of a semistable and stable parahoric torsor under a certain Bruhat-Tits group scheme G, construct the moduli space of semistable parahoric G-torsors and identify the underlying topological space of this moduli space with certain spaces of homomorphisms of Fuchsian groups into a maximal compact subgroup of G. The results give a complete generalization of the earlier results of Mehta and Seshadri on parabolic vector bundles

Dielectric anomalies and spiral magnetic order in CoCr2O4

We have investigated the structural, magnetic, thermodynamic, and dielectric properties of polycrystalline CoCr$_2$O$_4$, an insulating spinel exhibiting both ferrimagnetic and spiral magnetic structures. Below $T_c$ = 94 K the sample develops long-range ferrimagnetic order, and we attribute a sharp phase transition at $T_N$ $\approx$ 25 K with the onset of long-range spiral magnetic order. Neutron measurements confirm that while the structure remains cubic at 80 K and at 11 K; there is complex magnetic ordering by 11 K. Density functional theory supports the view of a ferrimagnetic semiconductor with magnetic interactions consistent with non-collinear ordering. Capacitance measurements on CoCr$_2$O$_4$, show a sharp decrease in the dielectric constant at $T_N$, but also an anomaly showing thermal hysteresis falling between approximately $T$ = 50 K and $T$ = 57 K. We tentatively attribute the appearance of this higher temperature dielectric anomaly to the development of \textit{short-range} spiral magnetic order, and discuss these results in the context of utilizing dielectric spectroscopy to investigate non-collinear short-range magnetic structures.Comment: & Figure