امواج چگالي بار و ابر رسانايي در تركيبات ديكالكوژن- فلزات واسطه

چكيده انگليسي :

Charge Density Waves and Supercoductivity in Transition Metal Dichalcogenides Sedighe Yaghoobi s yaghoobi@ph iut ac ir 3 5 2012 Department of Physics Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor Dr Peyman Sahebsara Sahebsara@cc iut ac irAbstractQuasi two dimensional structures are among the most interesting structures in the study of high Tc super conductors In strongly correlated electron systems superconductivity is usually accompanied by mag netic or charge ordering Transition metal dichalcogenide compounds are consisted of layered structurewhich undergo to charge density waves by decreasing temperature NbSe is one of these compoundswhich shows charge density waves and superconductivity below 33 5K and 7 2K respectively Thesetwo phases coexist at lower temperatures Unlike the other charge density wave conductors NbSe be comes a better conductor within the transition to charge density waves There are many attempts to find themechanism of charge density waves in these materials Two main mechanisms have proposed for it a nesting in Fermi surfaces b saddle point In this thesis at first the interplay between antiferromagnetismand d wave superconductivity is studied Besides the condition of appearance of the spin triplet amplitudeand its robustness in square lattice are evaluated The dynamical generation of the new order parameter isnot restricted to a system with antiferromagnetism and d wave superconductor but is a generic feature forfermionic systems The antiferromagnetism and superconductivity occur in strongly correlated electronsystems Therefore we use the extended Hubbard model for these systems In spite of the simplicity ofthis Hamiltonian its exact study is complicated and even impossible Therefore for its solution severalapproximations are used One of these approximations is the mean field theory To study the orderedphases in finite temperatures it is necessary to calculate free energy in the mean field approximation andto use optimization procedure Therefore a self consistent equation is obtained for each order parameter By numerical solution of these equations the order parameters are obtained in various temperatures anddoppings Then one can draw the phase diagrams of the system In the following section to study thecharge density waves for the family of NbSe materials for simplicity we consider the lattice as a triangu lar lattice with three atom basis But the calculation of its Hamiltonian is complicated too Therefore wereduce our calculations to the triangular lattice with one atom basis Then we calculate the ground stateenergy and optimize it to find out the order parameters and draw the ground state energy versus orderparameters for which we can determine the ground state phase and the behavior of the system at zerotemperature Finally we study the antiferromagnetism and charge density waves in the honeycomb latticeand the interplay between them In this case we will draw the order parameter versus on site and off siteamplitudes of coulomb interaction We will calculate the critical amplitude Therefore we will be able toexplain the behavior of the honeycomb lattice in zero temperature Keywords Extended Hubbard model Mean field theory Transition metal dichalcogenide NbSe Honeycomb lat tice Charge density waves Antiferromagnetism Superconductivity triplet

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امواج چگالي بار و ابر رسانايي در تركيبات ديكالكوژن- فلزات واسطه

هاميلتوني هابارد گسترش يافته , نظريه ميدان ميانگين , پادفرومغناطيس