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Register a new userMagnetic and lattice coupling in single-crystal SrFe$_2$As$_2$: A neutron scattering study
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A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_texttt{O}$ = $T_texttt{N}$ = (201.5 $pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than emph{T}$_texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.
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A detailed elastic neutron scattering study of the structural and magnetic phase transitions in single-crystal SrFe$_2$As$_2$ reveals that the orthorhombic (O)-tetragonal (T) and the antiferromagnetic transitions coincide at $T_texttt{O}$ = $T_texttt{N}$ = (201.5 $pm$ 0.25) K. The observation of coexisting O-T phases over a finite temperature range at the transition and the sudden onset of the O distortion provide strong evidences that the structural transition is first order. The simultaneous appearance and disappearance within 0.5 K upon cooling and within 0.25 K upon warming, respectively, indicate that the magnetic and structural transitions are intimately coupled. We find that the hysteresis in the transition temperature extends over a 1-2 K range. Based on the observation of a remnant orthorhombic phase at temperatures higher than emph{T}$_texttt{O}$, we suggest that the T-O transition may be an order-disorder transition.
We measured resistivity and specific heat of high-quality CsFe$_2$As$_2$ single crystals, which were grown by using a self-flux method. The CsFe$_2$As$_2$ crystal shows sharp superconducting transition at 1.8 K with the transition width of 0.1 K. The sharp superconducting transition and pronounced jump in specific heat indicate high quality of the crystals. Analysis on the superconducting-state specific heat supports unconventional pairing symmetry in CsFe$_2$As$_2$.
We have successfully grown high quality single crystals of SrFe$_2$As$_2$ and A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr, Ba) using flux method. The resistivity, specific heat and Hall coefficient have been measured. For parent compound SrFe$_2$As$_2$, an anisotropic resistivity with $rho_c$ / $rho_{ab}$ as large as 130 is obtained at low temperatures. A sharp drop in both in-plane and out-plane resistivity due to the SDW instability is observed below 200 K. The angular dependence of in-plane magnetoresistance shows 2-fold symmetry with field rotating within ab plane below SDW transition temperature. This is consistent with a stripe-type spin ordering in SDW state. In K doped A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr. Ba), the SDW instability is suppressed and the superconductivity appears with T$_c$ above 35 K. The rather low anisotropy in upper critical field between H$parallel$ab and H$parallel$c indicates inter-plane coupling play an important role in hole doped Fe-based superconductors.
Temperature and field-dependent magnetization $M(H,T)$ measurements and neutron scattering study of a single crystal CeSb$_2$ are presented. Several anomalies in the magnetization curves have been confirmed at low magnetic field, i.e., 15.6 K, 12 K, and 9.8 K. These three transitions are all metamagnetic transitions (MMT), which shift to lower temperatures as the magnetic field increases. The anomaly at 15.6 K has been suggested as paramagnetic (PM) to ferromagnetic (FM) phase transition. The anomaly located at around 12 K is antiferromagnetic-like transition, and this turning point will clearly split into two when the magnetic field $Hgeq0.2$ T. Neutron scattering study reveals that the low temperature ground state of CeSb$_2$ orders antiferromagnetically with commensurate propagation wave vectors $textbf{k}=(-1,pm1/6,0)$ and $textbf{k}=(pm1/6,-1,0)$, with Neel temperature $T_Nsim9.8$ K. This transition is of first-order, as shown in the hysteresis loop observed by the field cooled cooling (FCC) and field cooled warming (FCW) processes.
Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, indicating that multiple d-orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition, but find no detectable change within the experimental uncertainties.
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