Spin-density wave state and superconductivity in quasi-twodimensional systems
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PALISTRANT, Maria; URSU, Vitalie. Spin-density wave state and superconductivity in quasi-twodimensional systems. In: Materials Science and Condensed Matter Physics. Editia 7, 16-19 septembrie 2014, Chișinău. Chișinău, Republica Moldova: Institutul de Fizică Aplicată, 2014, p. 60.
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Materials Science and Condensed Matter Physics
Editia 7, 2014
Conferința "Materials Science and Condensed Matter Physics"
Chișinău, Moldova, 16-19 septembrie 2014

Spin-density wave state and superconductivity in quasi-twodimensional systems


Pag. 60-60

Palistrant Maria, Ursu Vitalie
 
Institute of Applied Physics, Academy of Sciences of Moldova
 
Disponibil în IBN: 25 februarie 2019


Rezumat

Beginning from 2008 year, it is received a large number of iron - based compounds (iron - oxipnictides) with relatively high superconducting transition temperature Tc ~ 55 K (see [1]). New compounds have a number of features that contributed to the existence of antifeITomagnetic state and superconductivity (SC). At present there is a need into account a number of features associated with the prope1iies oflayers of FeAs. In this paper we consider a single-band superconductor and the main attention is devoted to the high-temperature superconductivity (in a commensurate and noncommensurate phases) and also to the existence of deviation from half filling of the energy band. In this situation it is necessaiy to consider sepai·ately the cases of the given chemical potential (µ) and vai·iable impurity concentration (x). In paiiiculai·, we consider the case of the introduction of impurities into the system (case of given µ see in [2]). Hamiltonian of the system contains a tenn responsible for superconductivity, as well as the Coulomb interaction of electrons with opposite spins. On the basis of this Hamiltonian it is obtained the system of basic equations for the order paraineters of spindensiti wave (SDW)(M), supercondactivity (Ll) and chemical potential µ for n01mal processes. A series of transfo1mations ai·e perfo1med, contributing to the accounting of unclapp - processes and leading to noncommensurate SC state, when the wave vector of the spin density wave I Q I * 2KF shifts on the Fe1mi surface and it is fonned a gapless state. Behavior of the1modynainic quantities strongly depends on the impurity concentration x. Let us first consider SDW state in the absence of superconductivity (Ll * 0). The system of equations defining the magnetic transition temperature TM as a function of x has a branch point of solutions for x = Xe, if we consider only the n01mal processes (Q = 2KF) and becomes unstable for x >Xe.Accounting ofunclapp- processes (Q * 2KF) leads to a new order parameter with components 1Ja and 7Jb- The existence of this pai·ameter stabilizes the SDW state, transfonning it into noncommensurable phase, which is the basis for the superconducting state. In the mixed phase (SC + SDW) it is necessaiy to solve a system of equations with Ll * 0 HM* 0. Moreover, it is necessary that free energy difference 8 F = F (Ll, M) - F(0, M) will be negative, which coITesponds to the profitability of origin of SC against the background of the magnetic state. In some limiting cases it was possible to obtain analytical solutions. hnpo1iant role in the behavior of the system plays the relation between the temperature seed values of magnetic and SC transitions (TMo and Teo). Two limiting cases are examined: Tc ~ TM H Tc << TM. We obtain that in the case 0 < µ < µc SC does not exist, but for µ >µc - it exists. With the decrease of the order pai·ameter M value Tc increases. In the second case SC arises provided M is less than at least one of the parameters µ'p (a; {J = ±1), µ! = µ + arya + fJTJb. In the case of µ'p < M origin of SC against the background of SC is impossible. It also follows that in the SC system (Tc > TM ) magnetism does not ai·ise. It is also proved that the conditions for the existence of SC in magnetic systems ai·e more stringent than those in the case of a Paierls insulator.