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تسجيل مستخدم جديدCharge and nematic orders in AFe$_2$As$_2$ (A= Rb,Cs) superconductors
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We discuss the results of $^{75}$As Nuclear Quadrupole Resonance (NQR) and muon spin relaxation measurements in AFe$_2$As$_2$ (A= Cs, Rb) iron-based superconductors. We demonstrate that the crossover detected in the nuclear spin-lattice relaxation rate $1/T_1$ (around 150 K in RbFe$_2$As$_2$ and around 75 K in CsFe$_2$As$_2$), from a high temperature nearly localized to a low temperature delocalized behaviour, is associated with the onset of an inhomogeneous local charge distribution causing the broadening or even the splitting of the NQR spectra as well as an increase in the muon spin relaxation rate. We argue that this crossover, occurring at temperatures well above the phase transition to the nematic long-range order, is associated with a charge disproportionation at the Fe sites induced by competing Hunds and Coulomb couplings. In RbFe$_2$As$_2$ around 35 K, far below that crossover temperature, we observe a peak in the NQR $1/T_1$ which is possibly associated with the critical slowing down of electronic nematic fluctuations on approaching the transition to the nematic long-range order.
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The origin of diverse nematicity and their order parameters in Fe-based superconductors have been attracting increasing attention. Recently, a new type of nematic order has been discovered in heavily hole-doped ($n_d=5.5$) compound AFe$_2$As$_2$ (A=C
s, Rb). The discovered nematicity has $B_{2g}$ (=$d_{xy}$) symmetry, rotated by $45^circ$ from the $B_{1g}$ (=$d_{x^2-y^2}$) nematicity in usual compounds with $n_dapprox6$. We predict that the nematic bond order, which is the symmetry-breaking of the correlated hopping, is responsible for the $B_{2g}$ nematic order in AFe$_2$As$_2$. The Dirac pockets in AFe$_2$As$_2$ is essential to stabilize the $B_{2g}$ bond order. Both $B_{1g}$ and $B_{2g}$ nematicity in A$_{1-x}$Ba$_x$Fe$_2$As$_2$ are naturally induced by the Aslamazov-Larkin many-body process, which describes the spin-fluctuation-driven charge instability. The present study gives a great hint to control the nature of charge nematicity by modifying the orbital character and the topology of the Fermi surface.
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