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Register a new userLarge magnetic penetration depth and thermal fluctuations in a Ca$_{10}$(Pt$_{3}$As$_{8}$)[(Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$]$_{5}$ (x=0.097) single crystal
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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.
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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$1 with decreasing temperature, showing rather weak anisotropy at low temperatures.
Inelastic neutron scattering studies have been carried out on selected phonons and magnetic excitations of a crystal of superconducting (SC) Ca$_{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 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.
We measured the pressure dependence of in-plane resistivity $rho_{ab}$ in the recently-discovered iron-based superconductor Ca$_{10}$(Ir$_{4}$As$_{8}$)(Fe$_{2-x}$Ir$_{x}$As$_{2}$)$_{5}$, which shows a unique structural phase transition in the absence of magnetic ordering, with a superconducting transition temperature $T_{rm c}$ = 16 K and structural phase transition temperature $T_{rm s}$ $simeq$ 100 K at ambient pressure. $T_{rm c}$ and $T_{rm s}$ are suppressed on applying pressure and disappear at approximately 0.5 GPa, suggesting a relationship between superconductivity and structure. Ca$_{10}$(Ir$_{4}$As$_{8}$)(Fe$_{2-x}$Ir$_{x}$As$_{2}$)$_{5}$ is a rather rare example in which the superconductivity appears only in a low-temperature ordered phase. The fact that the change in the crystal structure is directly linked with superconductivity suggests that the crystal structure as well as magnetism are important factors governing superconductivity in iron pnictides.
We report the magnetic field -- temperature ($H-T$) phase diagram of Ca$_{10}$(Pt$_4$As$_8$)[(Fe$_{1-x}$Pt$_x$)$_2$As$_2$]$_5$ ($xapprox 0.05$) single crystals, which consists of normal, vortex liquid, plastic creep and elastic creep phases. The upper critical field anisotropy is determined by a radio frequency technique via the measurements of magnetic penetration depth, $lambda$. Both, irreversibility line, $H_{irr}(T)$, and flux creep line, $H^{SPM}(T)$, are obtained by measuring the magnetization. We find that $H_{irr}(T)$ is well described by the Lindemann criterion with parameters similar to those for cuprates, while small $H^{SPM}(T)$ results in a wide plastic creep regime. The flux creep rates in the elastic creep regime are in qualitative agreement with the collective creep theory for random point defects. A gradual crossover from a single vortex to a bundles regime is observed. Moreover, we obtain $lambda(4~ text K) = 260(26)$ nm through the direct measurement of the London penetration depth by magnetic force microscopy.
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