Electronic Origin of Tc in Bulk and Monolayer FeSe

FeSe is classed as a Hund’s metal, with a multiplicity of d bands near the Fermi level. Correlations in Hund’s metals mostly originate from the exchange parameter J, which can drive a strong orbital selectivity in the correlations. The Fe-chalcogens are the most strongly correlated of the Fe-based superconductors, with dxy the most correlated orbital. Yet little is understood whether and how such correlations directly affect the superconducting instability in Hund’s systems. By applying a recently developed ab initio theory, we show explicitly the connections between correlations in dxy and the superconducting critical temperature Tc. Starting from the ab initio results as a reference, we consider various kinds of excursions in parameter space around the reference to determine what controls Tc. We show small excursions in J can cause colossal changes in Tc. Additionally we consider changes in hopping by varying the Fe-Se bond length in bulk, in the free standing monolayer M-FeSe, and M-FeSe on a SrTiO3 substrate (M-FeSe/STO). The twin conditions of proximity of the dxy state to the Fermi energy, and the strength of J emerge as the primary criteria for incoherent spectral response and enhanced single- and two-particle scattering that in turn controls Tc. Using c-RPA, we show further that FeSe in monolayer form (M-FeSe) provides a natural mechanism to enhance J. We explain why M-FeSe/STO has a high Tc, whereas M-FeSe in isolation should not. Our study opens a paradigm for a unified understanding what controls Tc in bulk, layers, and interfaces of Hund’s metals by hole pocket and electron screening cloud engineering.

Modeling the Magnetic Properties of La0.62Er0.05Ba0.33Fe0.2Mn0.8O3: Mean-Field Study and Bean Rodbell Model

The Brillouin function, the phase transition and the related magnetic properties in La0.62Er0.05Ba0.33Fe0.2Mn0.8O3 perovskite have been studied using Bean-Rodbell model. The Brillouin function allows determining the total momentum J and the mean filed exchange parameter λ of the perovskite. The mean-filed equation draws the system to second order phase transition. These constants were used to stimulate the experimental isotherms M (H, T) by meanfield theory. The predicted results are compared to the available experimental data. It is noted that a good agreement has been found, with minor discrepancies, between theoretical and experimental data.

Twinned olivenite from Cap Garonne, Mine du Pradet – structure and magnetic behavior

Systematically twinned olivenite (Cu2(AsO4)OH) single crystals from Cap Garonne, Mine du Pradet, France, were studied by X-ray diffraction: P 21/n, a = 822.69(6) pm, b = 861.88(9) pm, c = 594.06(9) pm, β = 90.000(6)°, wR = 0.0224, 1621 F2 values, 79 variables and a domain ratio of 0.501(1)/0.499(1). The temperature dependence of the magnetic susceptibility was well reproduced with a square-spin cluster model and an antiferromagnetic spin-exchange parameter of J/kB = 157(3) K.

Stability criteria and critical runway conditions of propylene glycol manufacture in a continuous stirred tank reactor

<p>Here, a new method for the analysis of the steady state and the safety operational conditions of the hydrolysis of propylene oxide with excess of water, in a Continuous Stirred Tank Reactor (CSTR), was developed. For industrial operational typical values, at first, the generated and removed heat balances were examined. Next, the effect of coolant fluid temperature in the critical ignition and extinction temperatures (T_CI and T_CE, respectively) was analyzed. The influence of the heat exchange parameter (<em>hS</em>) on coolant and critical temperatures was also studied. Finally, the steady state operation areas were defined. The existence of multiple stable states was recognized when the heat exchange parameter was in the range 6.636 &lt; <em>hS</em> kJ/(min.K) &lt; 11.125. Unstable operation area was located between the T_CI and T_CEvalues, restricting the reactor operation area to the low stable temperatures.</p>

THE QUANTUM REFRIGERATOR IN A TWO-QUBIT XXZ HEISENBERG MODEL

The four-level entangled quantum refrigerator (QR) is studied in the XXZ Heisenberg model for the two-qubits. The Hamiltonian of the problem includes the exchange parameters Jx = Jy = J and Jz = αJ along the x-, y- and z-directions, respectively, and constant external magnetic field B in the z-direction. The parameter α is introduced into the model which controls the strength of the exchange parameter Jz in comparison to Jx and Jy, thus, our investigation of QR includes the XX (α = 0.0), XXX (α = 1.0) and XXZ (for other α's) Heisenberg models. The two-qubits are assumed to be in contact with two heat reservoirs at different temperatures. The concurrences for a two-qubit are used as a measure of entanglement and then the expressions for the amount of heat transferred, the work performed and the efficiency are derived. The contour, i.e., the isoline maps, and some two-dimensional plots of the above mentioned thermodynamic quantities are illustrated.

THE ENTANGLED QUANTUM HEAT ENGINE IN THE VARIOUS HEISENBERG MODELS FOR A TWO-QUBIT SYSTEM

The four-level entangled quantum heat engine (QHE) is analyzed in the various Heisenberg models for a two-qubit. The QHE is examined for the XX, XXX and XXZ Heisenberg models by introducing a parameter x which controls the strength of the exchange parameter Jz = xJ along the z-axis with respect to the ones along the x- and y-axes, i.e. Jx = Jy = J, respectively. It is assumed that the two-qubit is entangled and in contact with two heat reservoirs at different temperatures and under the effect of a constant magnetic field. The concurrences (C) are used as a measure of entanglement and then the expressions for the amount of heat transferred, the work performed and the efficiency of the QHE are derived. The contour, i.e. the isoline maps, and some two-dimensional plots of the above mentioned thermodynamic quantities are calculated and some interesting features are found.

Study of Magnetism of Two-Dimensional Ferromagnetic Graphene

In this paper, the magnetic properties of ferromagnetic graphene nanostructures, especially the dependence of the magnetism on finite temperature, are investigated by use of the many-body Green’s function method of quantum statistical theory. The spontaneous magnetization increases with spin quantum number, and decreases with temperature. Curie temperature increases with exchange parameter J or the strength K2 of single-ion anisotropy and spin quantum number. The Curie temperature TC is directly proportional to the exchange parameter J. The spin-wave energy drops with temperature rising, and becomes zero as temperature reaches Curie temperature. As J(p,q)=0, ω1=ω2, the spin wave energy is degenerate, and the corresponding vector k=(p, q) is called the Dirac point. This study contributes to theoretical analysis for pristine two-dimensional magnetic nanomaterials that may occur in advanced experiments.