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Register a new userEvidence of a multiple boson emission in Sm$_{1-x}$Th$_x$OFeAs
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We studied a reproducible fine structure observed in dynamic conductance spectra of Andreev arrays in Sm$_{1-x}$Th$_x$OFeAs superconductors with various thorium concentrations ($x = 0.08 - 0.3$) and critical temperatures $T_c = 26-50$,K. This structure is unambiguously caused by a multiple boson emission (of the same energy) during the process of multiple Andreev reflections. The directly determined energy of the bosonic mode reaches $varepsilon_0 = 14.8 pm 2.2$,meV for optimal compound. Within the studied range of $T_c$, this energy as well as the large $Delta_L$ and the small $Delta_S$ superconducting gaps, nearly scales with critical temperature with the characteristic ratio $varepsilon_0/k_BT_c approx 3.2$ (and $2Delta_L/k_BT_c approx 5.3$, correspondingly) resembling the expected energy $Delta_L + Delta_S$ of spin resonance and spectral density enhancement in $s^{pm}$ and $s^{++}$ states, respectively.
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Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm$_{1-x}$Th$_x$OFeAs with various thorium doping and critical temperatures $T_C = 21-54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS-ratio for the large gap $2Delta_L/k_BT_C = 5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2Delta_L/k_BT_C^L = 4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2Delta_S/k_BT_C = 1.2-1.6$ remain nearly constant within all the $T_C$ range studied. The temperature dependences $Delta_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas and Coulomb repulsion constants $mu^{ast}$ are finite in Sm-based oxypnictides.
We studied SNS- and S-N-S-N-...-S contacts (where S - superconductor, N - normal metal) formed by break-junction technique in polycrystalline Sm$_{1-x}$Th$_x$OFeAs superconductor samples with critical temperatures $T_C = (34 div 45)$ K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on these effects, we detected two independent bulk order parameters and determined their magnitudes. Theoretical analysis of the large and the small gap temperature dependences revealed superconducting properties of Sm$_{1-x}$Th$_x$OFeAs to be driven by intraband coupling, and $sqrt{V_{11}V_{22}}/V_{12} approx 14$ (where $V_{ij}$ - electron-boson interaction matrix elements), whereas the ratio between density of states for the bands with the small and the large gap, $N_2/N_1$, correspondingly, was roughly of an order. We estimated solo BCS-ratio values in a hypothetic case of zero interband coupling ($V_{i e j} = 0$) for each condensate as $2Delta_{L,S}/k_BT_C^{L,S} le 4.5$. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.
We report conventional and time-resolved infrared spectroscopy on LaFeAsO$_{1-x}$F$_x$ superconducting thin films. The far-infrared transmission can be quantitatively explained by a two-component model including a conventional s-wave superconducting term and a Drude term, suggesting at least one carrier system has a full superconducting gap. Photo-induced studies of excess quasiparticle dynamics reveal a nanosecond effective recombination time and temperature dependence that agree with a recombination bottleneck in the presence of a full gap. The two experiments provide consistent evidence of a full, nodeless though not necessarily isotropic, gap for at least one carrier system in LaFeAsO$_{1-x}$F$_x$.
High resolution photoemission measurements have been carried out on non-superconducting SmOFeAs parent compound and superconducting Sm(O$_{1-x}$F$_x$)FeAs (x=0.12, and 0.15) compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state which suggests the Fermi surface sheet(s) around the $Gamma$ point may not be gapped in this multiband superconductors. A robust feature at 13 meV is identified in all these samples. Spectral weight suppression near E$_F$ with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.
Motivated by the possibility of observing the co-existence between magnetism and unconventional superconductivity in heavy-fermion Ce$_{1-x}$Sm$_x$CoIn$_5$ alloys, we studied how the samarium substitution on the cerium site affects the magnetic field-tuned-quantum criticality of stoicheometric CeCoIn$_5$ by performing specific heat and resistivity measurements. By applying an external magnetic field, we have observed Fermi-liquid to non-Fermi-liquid crossovers in the temperature dependence of the electronic specific heat normalized by temperature and of the resistivity. We obtained the magnetic-field-induced quantum critical point (QCP) by extrapolating to zero temperature the temperature - magnetic field dependence at which the crossovers take place. Furthermore, a scaling analysis of the electronic specific heat is used to confirm the existence of the QCP. We have found that the magnitude of the magnetic-field-induced QCP decreases with increasing samarium concentration. Our analysis of heat capacity and resistivity data reveals a zero-field QCP for $x_textrm{cr} approx 0.15$, which falls inside the region where Sm ions antiferromagnetism and superconductivity co-exist.
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