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Register a new userUnravelling the mechanism of the semiconducting-like behavior and its relation to superconductivity in (CaFe$_{1-x}$Pt$_{x}$As)$_{10}$Pt$_{3}$As$_{8}$
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The temperature-dependence of the in-plane optical properties of (CaFe$_{1-x}$Pt$_{x}$As)$_{10}$Pt$_{3}$As$_{8}$ have been investigated for the undoped ($x=$0) parent compound, and the optimally-doped ($x=$0.1) superconducting material ($T_{c}simeq$ 12 K) over a wide frequency range. The optical conductivity has been described using two free-carrier (Drude) components, in combination with oscillators to describe interband transitions. At room temperature, the parent compound may be described by a strong, broad Drude term, as well as a narrow, weaker Drude component. Below the structural and magnetic transitions at $simeq$ 96 and 83 K, respectively, strength is transferred from the free-carrier components into a bound excitation at $simeq$ 1000 cm$^{-1}$, and the material exhibits semiconducting-like behavior. In the optimally-doped sample, at room temperature the optical properties are again described by narrow and broad Drude responses comparable to the parent compound; however, below $T^ast simeq$ 100 K, strength from the narrow Drude is transferred into a newly-emergent low-energy peak at $simeq$ 120 cm$^{-1}$, which arises from a localization process, resulting in semiconducting-like behavior. Interestingly, below $T_{c}$, this peak also contributes to the superfluid weight, indicating that some localized electrons condense into Cooper pairs; this observation may provide insight into the pairing mechanism in iron-based superconductors.
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The upper critical fields, $H_{c2}$($T$), of single crystals of the superconductor Ca$_{10}$(Pt$_{4-delta}$As$_{8}$)((Fe$_{0.97}$Pt$_{0.03}$)$_{2}$As$_{2}$)$_{5}$ ($delta$ $approx$ 0.246) are determined over a wide range of temperatures down to $T$ = 1.42 K and magnetic fields of up to $mu_{0}H$ $simeq$ 92 T. The measurements of anisotropic $H_{c2}$($T$) curves are performed in pulsed magnetic fields using radio-frequency contactless penetration depth measurements for magnetic field applied both parallel and perpendicular to the textbf{ab}-plane. Whereas a clear upward curvature in $H_{c2}^{paralleltextbf{c}}$($T$) along textbf{H}$parallel$textbf{c} is observed with decreasing temperature, the $H_{c2}^{paralleltextbf{ab}}$($T$) along textbf{H}$parallel$textbf{ab} shows a flattening at low temperatures. The rapid increase of the $H_{c2}^{paralleltextbf{c}}$($T$) at low temperatures suggests that the superconductivity can be described by two dominating bands. The anisotropy parameter, $gamma_{H}$ $equiv$ $H_{c2}^{paralleltextbf{ab}}/H_{c2}^{paralleltextbf{c}}$, is $sim$7 close to $T_{c}$ and decreases considerably to $sim$1 with decreasing temperature, showing rather weak anisotropy at low temperatures.
We have measured the temperature dependence of the absolute value of the magnetic penetration depth $lambda(T)$ in a Ca$_{10}$(Pt$_{3}$As$_{8}$)[(Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$]$_{5}$ (x=0.097) single crystal using a low-temperature magnetic force microscope (MFM). We obtain $lambda_{ab}$(0)$approx$1000 nm via extrapolating the data to $T = 0$. This large $lambda$ and pronounced anisotropy in this system are responsible for large thermal fluctuations and the presence of a liquid vortex phase in this low-temperature superconductor with critical temperature of 11 K, consistent with the interpretation of the electrical transport data. The superconducting parameters obtained from $lambda$ and coherence length $xi$ place this compound in the extreme type MakeUppercase{romannumeral 2} regime. Meissner responses (via MFM) at different locations across the sample are similar to each other, indicating good homogeneity of the superconducting state on a sub-micron scale.
The platinum iron arsenides Ca$_{10}$(Fe$_{1-x}$Pt$_x$As)$_{10}$(Pt$_n$As$_8$) are the first Fe based superconductors with metallic spacer layers. Furthermore they display a large variation in their critical temperatures depending on the amount of Pt in their spacer layers: $(n=3,4)$. To gain more insight into the role of the spacer layer the electronic structures of the iron arsenic platenides are represented in the momentum space of the underlying Fe sublattice using a first principles unfolding method. We find that Ca$_{10}$(FeAs)$_{10}$(Pt$_4$As$_8$), contrary to Ca$_{10}$(FeAs)$_{10}$(Pt$_3$As$_8$), shows a net electron doping and a non-negligible interlayer coupling. Both effects could account for the difference in the critical temperatures.
Inelastic neutron scattering studies have been carried out on selected phonons and magnetic excitations of a crystal of superconducting (SC) Ca$_{10}$Pt$_4$As$_8$(Fe$_{1-x}$Pt$_x$As)$_{10}$ with the onset transition temperature $T_{rm c}^{rm onset} sim$ 33 K to investigate the role that orbital fluctuations play in the Cooper pairing. The spectral weight of the magnetic excitations, $chi ({bm Q}, omega)$ at ${bm Q} = {bm Q}_{rm M}$ (magnetic $Gamma$ points) is suppressed (enhanced) in the relatively low (high) $omega$ region. The maximum of the enhancement is located at $omega = omega_{rm p} sim$ 18 meV at temperature $T = 3$ K corresponding to the $omega_{rm p}/k_{rm B}T_{rm c}^{rm onset} sim$ 6.3. This large value is rather favorable to the orbital-fluctuation mechanism of the superconductivity with the so-called $S_{++}$ symmetry. In the phonon measurements, we observed slight softening of the in-plane transverse acoustic mode corresponding to the elastic constant $C_{66}$. This softening starts at $T$ well above the superconducting $T_{rm c}$, as $T$ decreases. An anomalously large increase in the phonon spectral weight of in-plane optical modes was observed in the range of $35 < omega < 40$ meV with decreasing $T$ from far above $T_{rm c}$. Because this $omega$ region mainly corresponds to the in-plane vibrations of Fe atoms, the finding presents information on the coupling between the orbital fluctuation of the Fe 3$d$ electrons and lattice system, useful for studying possible roles of the orbital fluctuation in the pairing mechanism and appearance of the so-called nematic phase.
The compound Y2PdGe3 was earlier reported by us to be one of the very few ternary superconducting compounds (T_c= 3 K) belonging to the same structure as that of MgB2. Here we report the results of electrical resistivity, magnetization and heat capacity measurements at low temperatures on the solid solution with a nominal starting composition, Y2Pd(1-x)Pt(x)Ge3, to understand the influence of gradual replacement of Pd by Pt on T_c. The superconducting properties of this solution is distinctly interesting in the sense that the Tc varies monotonically with increasing x in sharp contrast to the non-monotonic variation for other isostructural solid solutions reported recently.
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