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Register a new userUnusual Relationship between Magnetism and Superconductivity in FeTe$_{0.5}$Se$_{0.5}$
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We use neutron scattering, to study magnetic excitations in crystals near the ideal superconducting composition of FeTe$_{0.5}$Se$_{0.5}$. Two types of excitations are found, a resonance at (0.5, 0.5, 0) and incommensurate fluctuations on either side of this position. We show that the two sets of magnetic excitations behave differently with doping, with the resonance being fixed in position while the incommensurate excitations move as the doping is changed. These unusual results show that a common behavior of the low energy magnetic excitations is not necessary for pairing in these materials.
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Inelastic neutron scattering measurements have been performed on a superconducting single crystal FeTe$_{0.5}$Se$_{0.5}$ to examine the ${bf Q}$-dependent enhancement of the dynamical structure factor, $S({bf Q},E)$, from ${bf Q}$ = (0, 0) to ($pi$, $pi$), including ($pi$, 0) in the superconducting state. In most of iron-based superconductors, $S({bf Q},E)$ is enhanced at ${bf Q}$ = ($pi$, 0), where the magnetic resonance mode is commonly observed in the unfolded Brillouin zone. Constant-$E$ cuts of $S({bf Q},E)$ suggest that the enhancement is not uniform in the magnetic excitation, and limited around ${bf Q}$ = ($pi$, 0). This result is consistent with the theoretical simulation of the magnetic resonance mode due to the Bardeen$-$Cooper$-$Schrieffer coherence factor with the sign-reversing order parameter of s$_{pm}$ wave.
We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron--phonon coupling for newly discovered superconductor LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$. Significant similarity between LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is found, i.e. there is a strong Fermi surface nesting at ($pi $,$pi $,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long--range order. The calculation shows that LaO$_{0.5}$F$_{0.5}$BiSe$_{2}$ is highly anisotropic, and same as LaO$_{0.5}$F$_{0.5}$BiS$_{2}$, this compound is also a conventional electron-phonon coupling induced superconductor.
We report on specific heat ($C_p$), transport, Hall probe and penetration depth measurements performed on Fe(Se$_{0.5}$Te$_{0.5}$) single crystals ($T_c sim 14$ K). The thermodynamic upper critical field $H_{c2}$ lines has been deduced from $C_p$ measurements up to 28 T for both $H|c$ and $H|ab$, and compared to the lines deduced from transport measurements (up to 55 T in pulsed magnetic fields). We show that this {it thermodynamic} $H_{c2}$ line presents a very strong downward curvature for $T rightarrow T_c$ which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirm that $H_{c2}$ is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to $T/T_c sim 0.99$ (for $H|ab$) which is the consequence of a very small value of the coherence length $xi_c(0) sim 4 AA$ (and $xi_{ab}(0) sim 15 AA$), confirming the strong renormalisation of the effective mass (as compared to DMFT calculations) previously observed in ARPES measurements [Phys. Rev. Lett. 104, 097002 (2010)]. $H_{c1}$ measurements lead to $lambda_{ab}(0) = 430 pm 50$ nm and $lambda_c(0) = 1600 pm 200$ nm and the corresponding anisotropy is approximatively temperature independent ($sim 4$), being close to the anisotropy of $H_{c2}$ for $Trightarrow T_c$. The temperature dependence of both $lambda$ ($propto T^2$) and the electronic contribution to the specific heat confirm the non conventional coupling mechanism in this system.
Indium substitution turns the topological crystalline insulator (TCI) Pb$_{0.5}$Sn$_{0.5}$Te into a possible topological superconductor. To investigate the effect of the indium concentration on the crystal structure and superconducting properties of (Pb$_{0.5}$Sn$_{0.5}$)$_{1-x}$In$_{x}$Te, we have grown high-quality single crystals using a modified floating-zone method, and have performed systematic studies for indium content in the range $0leq xleq 0.35$. We find that the single crystals retain the rock salt structure up to the solubility limit of indium ($xsim0.30$). Experimental dependences of the superconducting transition temperature ($T_c$) and the upper critical magnetic field ($H_{c2}$) on the indium content $x$ have been measured. The maximum $T_c$ is determined to be 4.7 K at $x=0.30$, with $mu_0H_{c2}(T=0)approx 5$ T.
The temperature ($T$) and magnetic field ($H$) dependence of the magnetic penetration depth, $lambda(T,H)$, in Ca(Al$_{0.5}$Si$_{0.5}$)$_2$ exhibits significant deviation from that expected for conventional BCS superconductors. In particular, it is inferred from a field dependence of $lambda(H)$ ($propto H$) at 2.0 K that the quasiparticle excitation is strongly enhanced by the Doppler shift. This suggests that the superconducting order parameter in Ca(Al$_{0.5}$Si$_{0.5}$)$_2$ is characterized by a small energy scale $Delta_S/k_Ble 2$ K originating either from anisotropy or multi-gap structure.
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