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Register a new userCoexistence of ferromagnetism and superconductivity in CeO$_{0.3}$F$_{0.7}$BiS$_{2}$
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Bulk magnetization, transport and neutron scattering measurements were performed to investigate the electronic and magnetic properties of a polycrystalline sample of the newly discovered ferromagnetic superconductor, CeO$_{0.3}$F$_{0.7}$BiS$_{2}$. Ferromagnetism develops below T$_{FM}$ = 6.54(8) K and superconductivity is found to coexist with the ferromagnetic state below T$_{SC}$ ~ 4.5 K. Inelastic neutron scattering measurements reveal a very weakly dispersive magnetic excitation at 1.8 meV that can be explained by an Ising-like spin Hamiltonian. Under application of an external magnetic field, the direction of the magnetic moment changes from the c-axis to the ab-plane and the 1.8 meV excitation splits into two modes. A possible mechanism for the unusual magnetism and its relation to superconductivity is discussed.
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We have studied EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ by the measurements of x-ray diffraction, electrical resistivity, thermopower, magnetic susceptibility, magnetoresistance and specific heat. Partial substitution of As with P results in the shrinkage of lattice, which generates chemical pressure to the system. It is found that EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ undergoes a superconducting transition at 26 K, followed by ferromagnetic ordering of Eu$^{2+}$ moments at 20 K. This finding is the first observation of superconductivity stabilized by internal chemical pressure, and supplies a rare example showing coexistence of superconductivity and ferromagnetism in the ferro-arsenide family.
We measure the magnetic penetration depth $Deltalambda(T)$ for NdO$_{1-x}$F$_{x}$BiS$_{2}$ ($x$ = 0.3 and 0.5) using the tunnel diode oscillator technique. The $Deltalambda(T)$ shows an upturn in the low-temperature limit which is attributed to the paramagnetism of Nd ions. After subtracting the paramagnetic contributions, the penetration depth $Deltalambda(T)$ follows exponential-type temperature dependence at $Tll T_c$. Both $Deltalambda(T)$ and the corresponding superfluid density $rho_s(T)$ can be described by the BCS model with an energy gap of $Delta(0)$ $approx$ 2.0 $k_BT_c$ for both $x$ = 0.3 and 0.5, suggesting strong-coupling BCS superconductivity in the presence of localized moments for NdO$_{1-x}$F$_{x}$BiS$_{2}$.
We present scanning tunneling microscopy measurements on a cleaved surface of the recently discovered superconductor NdO$_{0.7}$F$_{0.3}$BiS$_{2}$ with a transition temperature ($T_{mathrm{c}}$) of 5.1 K.Tunneling spectra at 4.2 K (below $T_{mathrm{c}}$) and 22 K (above $T_{mathrm{c}}$) show a large spectroscopic gap ($sim$40 mV), which is inconsistent with the metallic nature demonstrated in bulk measurements. Moreover, we find two interesting real-space electronic features. The first feature is a `checkerboard stripe electronic state characterized by an alternating arrangement of two types of nanocluster. In one cluster, one-dimensional electronic stripes run along one Bi-Bi direction, whereas, in the other cluster, the stripes run along the other Bi-Bi direction. The second feature is a nanoscale electronic inhomogeneity whose microscopic source seems to be atomic defects on the cleaved surface or dopant F atoms.
We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) measurements on CeO$_{1-x}$F$_x$BiS$_2$, in which the superconductivity of the BiS$_2$ layer and the ferromagnetism of the CeO$_{1-x}$F$_x$ layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce $L_3$-edge XAS spectrum of CeOBiS$_2$ exhibits coexistence of $4f^1$ (Ce$^{3+}$) and $4f^0$ (Ce$^{4+}$) state transitions revealing Ce mixed valency in this system. The spectral weight of the $4f^0$ state decreases with the F-doping and completely disappears for $x>0.4$ where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS$_2$ layer and the ferromagnetic CeO$_{1-x}$F$_x$ layer.
We have investigated the electronic structure of BiS$_2$-based CeO$_{0.5}$F$_{0.5}$BiS$_2$ superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around X point, the straight portion parallel to the $k_y$ direction is dominated by Bi $6p_x$ character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the BiS$_2$ system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.
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