Magnetic Resonance States In Monolayer FeSe Superconductor

The Fermi surface of single-layer FeSe is composed of two electronic-like pockets at the M-point in the first Brillioun zone.This Fermi surface is very simple,however,the superconductivity transition temperature is extremely high,?100 K.The single-layer FeSe superconductor is an ideal material for researchers to study the superconductivity mechanism of high temperature superconductors because of its unique properties and two-dimensional structure.Pairing symmetry of superconductors is a crucial clue to go further into the study of the superconductivity mechanisms in unconventional superconductors.There are many experimental and theoretical investigations on the pairing symmetry of single-layer FeSe superconductor,plain s-wave,s ±wave,d-wave,coexistence of s-wave and d-wave pairing symmetries are reported in literatures.So far,a confident conclusion on the pairing symmetry of monolayer FeSe superconductor has not been established.Electron states near the Fermi surface can be described by low-energy effective theory near the Fermi surface and group theory analysis,three possible pairing symmetries have been proposed for single-layer FeSe superconductor:s-wave pairing,d-wave pairing and p-wave pairing.In this dissertation,we study the resonance states induced by quantum magnetic impurity in single-layer FeSe superconductor for different pairing symmetries.In order to distinguish the three pairing symmetries,the effect of spin-orbit coupling on impurity resonance states is also considered.Using group theory analysis,one can find that,the pairing symmetries of single layer FeSe can be sorted into two classes:the 'even class'(s-wave pairing and d-wave pairing),and the 'odd class'(p-wave pairing)by time-reversal symmetry.The total Hamiltonian of superconductor with s-wave pairing,d-wave pairing and p-wave pairing is constructed respectively.Slave-boson mean-field approximation is used to derive the Green function and calculate the local density of states by introducing slave-boson operators and fermionic operators,and simplifying the total Hamiltonian with the multiplier.In the strong coupling regime,some suitable parameters are selected to calculate the magnetic resonance states for s-wave pairing,d-wave pairing and p-wave pairing.Firstly,we calculate the local density of states under the two types of hybridization:One type is that the impurity hybridizes to only the X-pocket,another is that hybridization only occurs between impurity and Y-pocket.The numerical results show that the distribution of local density of states versus energy is identical for the two kinds of hybridizations listed above.There are two in-gap resonance peaks located symmetrically at the two sides of the Fermi energy for all the pairing symmetries we studied.This is consistent with experimental results.In addition,for the two types of hybridizations listed above,the spatial distribution of the local density of states is mirror symmetric.This is consistent with the theoretical prediction.The intensities of the two peaks are comparable for s-wave and d-wave pairings.However,for the p-wave pairing,we find that the intensity of the positive resonance peak is much smaller than the negative resonance peak.This feature can be considered as evidence to distinguish p-wave pairing from s-wave and d-wave pairings.When the magnetic impurity hybridizes with two electronic pockets simultaneously and the effect of spin-orbit coupling is no negligible,the two resonance peaks will be split into four for d-wave pairing.Numerical calculations show that this splitting is absent for s-wave and p-wave pairings in a large parameter regime.This feature can be used to distinguish d-wave pairing from s-wave pairing or p-wave pairing.Our study provides some guidance to determine the pairing symmetry of the monolayer FeSe superconductor and may be useful for further investigation on the mechanism of high-temperature superconductivity.