Crossing point phenomena (T* = 2.7 K) in specific heat curves of superconducting ferromagnets RuSr2Gd1.4Ce0.6Cu2O10-{\delta}

Crossing point phenomena are one of the interesting and still puzzling effects in strongly correlated electron systems. We have synthesized RuSr2Gd1.4Ce0.6Cu2O10-{\delta} (GdRu-1222) magneto-superconductor through standard solid state reaction route and measured its magnetic, transport and thermal properties. We also synthesized RuSr2Eu1.4Ce0.6Cu2O10-{\delta} (EuRu-1222) then measured its heat capacity in zero magnetic fields for reference. The studied compounds crystallized in tetragonal structure with space group I4/mmm. GdRu-1222 is a reported magneto-superconductor with Ru spins magnetic ordering at temperature around 110 K and superconductivity in Cu-O2 planes below around 40 K. To explore the crossing point phenomena, the specific heat [Cp (T)] was investigated in temperature range 1.9-250 K, under magnetic field of up to 70 kOe. Unfortunately though no magnetic and superconducting transitions are observed in specific heat, a Schottky type anomaly is observed at low temperatures below 20 K. This low temperature Schottky type anomaly can be attributed to splitting of the ground state spectroscopic term 8S7/2 of paramagnetic Gd3+ ions by both internal and external magnetic fields. It was also observed that Cp (T) being measured for different values of magnetic field, possesses the same crossing point (T* = 2.7 K), up to the applied magnetic field 70 kOe. A quantitative explanation of this phenomenon, based on its shape and temperature dependence of the associated generalized heat capacity (Cp), is presented. This effect supports the crossing point phenomena, which is supposed to be inherent for strongly correlated systems.Comment: 12 pages Text+Figs ([email protected]

High-Order Coupled Cluster Method Calculations for the Ground- and Excited-State Properties of the Spin-Half XXZ Model

In this article, we present new results of high-order coupled cluster method (CCM) calculations, based on a N\'eel model state with spins aligned in the $z$-direction, for both the ground- and excited-state properties of the spin-half {\it XXZ} model on the linear chain, the square lattice, and the simple cubic lattice. In particular, the high-order CCM formalism is extended to treat the excited states of lattice quantum spin systems for the first time. Completely new results for the excitation energy gap of the spin-half {\it XXZ} model for these lattices are thus determined. These high-order calculations are based on a localised approximation scheme called the LSUB$m$ scheme in which we retain all $k$-body correlations defined on all possible locales of $m$ adjacent lattice sites ($k \le m$). The ``raw'' CCM LSUB$m$ results are seen to provide very good results for the ground-state energy, sublattice magnetisation, and the value of the lowest-lying excitation energy for each of these systems. However, in order to obtain even better results, two types of extrapolation scheme of the LSUB$m$ results to the limit $m \to \infty$ (i.e., the exact solution in the thermodynamic limit) are presented. The extrapolated results provide extremely accurate results for the ground- and excited-state properties of these systems across a wide range of values of the anisotropy parameter.Comment: 31 Pages, 5 Figure