The full H-T phase diagram in the nematic superconductor FeSe is mapped out using specific-heat and thermal-expansion measurements down to 0.7 K and up to 30 T for both field directions. A clear thermodynamic signal of an underlying vortex-melting tr
ansition is found in both datasets and could be followed down to low temperatures. The existence of significant Gaussian thermal superconducting fluctuations is demonstrated by a scaling analysis, which also yields the mean-field upper critical field Hc2(T). For both field orientations, Hc2(T) shows Pauli-limiting behavior. Whereas the temperature dependence of the vortex-melting line is well described by the model of Houghton et al., Phys. Rev. B 40, 6763 (1989) down to the lowest temperatures for H $perp$ FeSe layers, the vortex-melting line exhibits an unusual behavior for fields parallel to the planes, where the Pauli limitation is much stronger. Here, the vortex-melting anomaly is only observed down to T*= 2-3 K, and then merges with the Hc2(T) line as predicted by Adachi and Ikeda, Phys. Rev. B 68 184510 (2003). Below T*, Hc2(T) also exhibits a slight upturn possibly related to the occurence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state.
We report on a detailed study of the optical response and $T_c-rho$ phase diagram ($T_c$ being the superconducting critical temperature and $rho$ the normal state resistivity of the film) of granular aluminum, combining transport measurements and a h
igh resolution optical spectroscopy technique. The $T_c-rho$ phase diagram is discussed as resulting from an interplay between the phase stiffness, the Coulomb repulsion and the superconducting gap $Delta$. We provide a direct evidence for two different types of well resolved sub-gap absorptions, at $omega_1simeqDelta$ and at $Deltalesssimomega_2lesssim2Delta$ (decreasing with increasing resistivity).
The introduction of crystalline defects or dopants can give rise to so-called dirty superconductors, characterized by reduced coherence length and quasiparticle mean free path. In particular, granular superconductors such as Granular Aluminum (GrAl),
consisting of remarkably uniform grains connected by Josephson contacts have attracted interest since the sixties thanks to their rich phase diagram and practical advantages, like increased critical temperature, critical field, and kinetic inductance. Here we report the measurement and modeling of circuit quantum electrodynamics properties of GrAl microwave resonators in a wide frequency range, up to the spectral superconducting gap. Interestingly, we observe self-Kerr coefficients ranging from $10^{-2}$ Hz to $10^5$ Hz, within an order of magnitude from analytic calculations based on GrAl microstructure. This amenable nonlinearity, combined with the relatively high quality factors in the $10^5$ range, open new avenues for applications in quantum information processing and kinetic inductance detectors.
Cho et al. [Phys. Rev. B, 84, 174502 (2011)] have reported on the temperature dependence of the London penetration depth deduced from Tunnel Diode Oscillator (TDO) measurements in optimally doped Fe(Se,Te) single crystals. According to their analysis
, these measurements chould suggest a nodeless two-gap pairing symmetry with strong pair breaking effects. However, to reach this conclusion, the authors fit the temperature dependence of the superfluid density with a two band {it clean} limit model which is incompatible with the presence of strong pair breaking effects, deduced from the $T^n$ temperature dependence of the London penetration depth below $T_c/3$. Moreover they claim that their results are also ruling out the suggestion that surface conditions can significantly affect the TDO data but this conclusion is based on one very specific damaging process, and is completely ignoring the large dispersion in the previously published TDO data.
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 $C_p$ mea
surements 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.
The upper and lower critical fields have been deduced from specific heat and Hall probe magnetization measurements in non-optimally doped NdFeAs(O,F) single crystals ($T_c sim 32-35$K). The anisoptropy of the penetration depth ($Gamma_lambda$) is tem
perature independent and on the order of $4.0 pm 1.5$. Similarly specific heat data lead an anisotropy of the coherence lenght $Gamma_xi sim 5.5 pm 1.5$ close to $T_c$. Our results suggest the presence of rather large thermal fluctuations and to the existence of a vortex liquid phase over a broad temperature range ($sim 5$K large at 2T).
