We investigate temperature and concentration driven phase transitions (structural and reentrant phase transitions included) in magnetic and superconducting systems with the use of a wide class of model Hamiltonians applied to rare earth (Re) based compounds and alloys (integer and fluctuating valence systems). Studying the temperature or concentration dependence of the chemical potential we observe small but distinct and well localized kinks at all critical points as evidence for phase transitions. For systems with, at least, two kinds of interacting electrons the kinks at critical temperatures or concentrations occur also in the electronic average occupation numbers (critical electron redistribution). These observations suggest a direct and universal experimental application of the chemical potential as a detector of phase transitions for temperature and concentration driven phase transitions, as well as, for pressure-or external field-induced transitions in solids. The agreement between the cal...

On the new universal possibility to detect phase transitions in correlated electron systems

Generalized Multiple Gap Model For The Superconductivity In Fe-As Based Superconductors Anuj Nuwal1, Shubhra Kakani2, Shyam Lal Kakani3* 1 Department of Physics Sangam University N.H. No. 79, Bhilwara By-Pass, Chittor Road, BHILWARA -311001 (RAJ.), INDIA 2 Department of Physics, M. L. V. Govt. P. G. College, BHILWARA -311001 (RAJ.), INDIA 3 4-G-45, Shastri Nagar, New Housing Board, BHILWARA -311001 (RAJ.), INDIA *Corresponding author: slkakani28@gmail.com

http://www.scipublish.com/journals/TPHY/papers/download/2204-100.pdf

Generalized Multiple Gap Model For The Superconductivity In Fe-As Based Superconductors

We consider an array of magnetically ordered 4 f ions forming s.c. lattice. In the center of each cube, we place one `effective' nonmagnetic ion and we consider a model taking into account all necessary interactions of electrons belonging to both sorts of ions leading to a competition between ferromagnetism and superconductivity. Two kinds of solutions of the model can be found. One of them describes the reentrant superconductivity with three critical temperatures T S 1 , T S 2 (lower and upper superconducting transition temperature) and T C (Curie temperature) which fulfil the relation T S 1 T C T S 2 . The magnetizations due to 4 f electrons, and net magnetization originating from two sorts of electrons (two different sorts of atoms) are all parallel in this case. Here, we observe a great similarity of the properties of the model to the observed properties of the real HoMo 6 S 8 compound. The second kind of solutions of the model corresponds to the situation where the net magnetization originating from two sorts of electrons vanishes (antiparallel aligning of magnetic moments in different conduction subbands) and we observe the superconductivity in the temperature range T∈[0,T S 2 ] (T S 1 =0) where T C T S 2 or T C > T S 2 (coexistence of ferromagnetism and superconductivity of nonreentrant type). The latter situation seems to be similar to that observed in the real Y 9 Co 7 compound. In all cases, we observe that the relative occupation numbers connected with two sorts of interacting electrons exhibit at critical temperatures small but distinctive kinks that give the evidence about the electron redistribution at critical temperatures (critical electron redistribution). Hence, we suggest that the secondary information about the critical temperatures of the real solids can be predicted from the experimental data providing that the temperature dependence of the density of states is available.

Reentrant and nonreentrant properties of magnetic superconductors. Critical electron redistribution

A study of photoinduced high- T c superconductivity is presented by canonical two-band BCS model containing Fermi surfaces of p and d holes. We have obtained two superconducting gaps from this model. Studies of chemical potential and hole concentration dependences on critical temperature ( T c ) are made. The enhancement of T c is found due to doping. The study of specific heat and density of states based on this model is also presented. The dependence T c ( n h ) for the system YBa 2 Cu 3 O 7− x (1 2 3) obtained theoretically agrees with the available experimental data.

Theoretical study of photoinduced superconductors in a two band model

A polaronic model of superconductivity in doped fulleride systems is presented. The normal and anomalous one-particle Green's functions are derived for a system with strong electron phonon coupling. The study of collapse of the electron band and the phonon vacuum is presented within the mean-field approximation. Self-consistent equation for the superconductivity order parameter (Δ) is derived using Green's fucntion technique and following Lang and Firsov transformations. Expressions for specific heat, density of states, free energy and critical field based on this model have been derived. The theory is applied to explain the experimental results in the systems K3C60 and Rb3C60. These results are in good agreement with the available experimental data.

Polaronic mechanism of superconductivity in doped fulleride systems

Density of states and specific heat of itinerant ferromagnetic superconductors are studied by using the mean field approximation. The results are applied to the system Y9Co7. There is satisfactory agreement between theory and experimental results.



Die Zustandsdichte und spezifische Warme von itineranten ferromagnetischen Supraleitern werden mit der Mittelfeldnaherung untersucht. Diese Ergebnisse werden auf das System Y9Co7 angewendet. Es besteht befriedigende Ubereinstimmung zwischen Theorie und experimentellen Ergebnissen.

R. K. Paliwal

Papers

Theoretical Study of Density of States and Specific Heat of Itinerant Ferromagnetic Superconductors