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Using small-angle neutron scattering we have studied the superconducting vortex lattice (VL) phase diagram in MgB2 as the applied magnetic field is rotated away from the c axis and towards the basal plane. The field rotation gradually suppresses the intermediate VL phase which exists between end states aligned with two high symmetry directions in the hexagonal basal plane for H || c. Above a critical angle, the intermediate state disappears, and the previously continuous transition becomes discontinuous. The evolution towards the discontinuous transition can be parameterized by a vanishing twelvefold anisotropy term in the VL free energy., 6 pages, 4 figures; [v2]: minor revisions to text and figures; [v3] typos fixed, journal reference added

Chiral superconductors are novel topological materials with finite angular momentum Cooper pairs circulating around a unique chiral axis, thereby spontaneously breaking time-reversal symmetry. They are rather scarce and usually feature triplet pairing: a canonical example is the chiral p-wave state realized in the A-phase of superfluid He3. Chiral triplet superconductors are, however, topologically fragile with the corresponding gapless boundary modes only weakly protected against symmetry-preserving perturbations in contrast to their singlet counterparts. Using muon spin relaxation measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral superconductor: spontaneous magnetic fields inside the superconducting state indicating broken time-reversal symmetry and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is a chiral d-wave singlet., Nature Communications, 12 (1), ISSN:2041-1723

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing., 12 pages, 9 figures. Title changed, minor amendments in the text

The vortex lattice (VL) in MgB2 is characterized by the presence of long-lived metastable states (MSs), which arise from cooling or heating across the equilibrium phase boundaries. A return to the equilibrium configuration can be achieved by inducing vortex motion. Here we report on small-angle neutron scattering studies of MgB2, focusing on the structural properties of the VL as it is gradually driven from metastable to equilibrium states (ESs) by an AC magnetic field. Measurements were performed using initial MSs obtained either by cooling or heating across the equilibrium phase transition. In all cases, the longitudinal correlation length remains constant and comparable to the sample thickness. Correspondingly, the VL may be considered as a system of straight rods, where the formation and growth of ES domains only occurs in the two-dimensional plane perpendicular to the applied field direction. Spatially resolved raster scans of the sample were performed with apertures as small as 80 μm, corresponding to only 1.2 × 106 vortices for an applied field of 0.5 T. These revealed spatial variations in the metastable and equilibrium VL populations, but individual domains were not directly resolved. A statistical analysis of the data indicates an upper limit on the average domain size of approximately 50 μm., New Journal of Physics, 21, ISSN:1367-2630

Muon-spin rotation data collected at ambient pressure ($p$) and at $p=2.42$ GPa in MnP were analyzed to check their consistency with various low- and high-pressure magnetic structures reported in the literature. Our analysis confirms that in MnP the low-temperature and low-pressure helimagnetic phase is characterised by an increased value of the average magnetic moment compared to the high-temperature ferromagnetic phase. An elliptical double-helical structure with a propagation vector ${\bf Q}=(0,0,0.117)$, an $a-$axis moment elongated by approximately 18% and an additional tilt of the rotation plane towards $c-$direction by $\simeq 4-8^{\rm o}$ leads to a good agreement between the theory and the experiment. The analysis of the high-pressure $\mu$SR data reveals that the new magnetic order appearing for pressures exceeding $1.5$ GPa can not be described by keeping the propagation vector ${\bf Q} \parallel c$. Even the extreme case -- decoupling the double-helical structure into four individual helices -- remains inconsistent with the experiment. It is shown that the high-pressure magnetic phase which is a precursor of superconductivity is an incommensurate helical state with ${\bf Q} \parallel b$., Comment: 9 pages, 9 figures. arXiv admin note: substantial text overlap with arXiv:1603.03367

Using intrinsic multiple Andreev reflections effect spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors ${\mathrm{Sm}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}\mathrm{OFeAs}$ 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 $2{\mathrm{\ensuremath{\Delta}}}_{L}/{k}_{B}{T}_{C}=5.0--5.7$ and its eigen-BCS ratio (in a hypothetical case of zero interband coupling) $2{\mathrm{\ensuremath{\Delta}}}_{L}/{k}_{B}{T}_{C}^{L}=4.1--4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2{\mathrm{\ensuremath{\Delta}}}_{S}/{k}_{B}{T}_{C}=1.2--1.6$, remain nearly constant within all the ${T}_{C}$ range studied. The temperature dependences ${\mathrm{\ensuremath{\Delta}}}_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas Coulomb repulsion constants ${\ensuremath{\mu}}^{*}$ are finite in Sm-based oxypnictides.

Through a direct comparison of specific heat and magnetoresistance, we critically asses the nature of superconducting fluctuations in the same nanogram crystal of SmFeAs(O,F). We show that although the superconducting fluctuation contribution to conductivity scales well within the two-dimensional lowest Landau level (2D-LLL) scheme, its predictions contrast with the inherently 3D nature of SmFeAs(O,F) in the vicinity of ${T}_{c}$. Furthermore, the transition seen in specific heat cannot be satisfactory described either by the LLL or the $XY$ scaling. Additionally we have validated, through comparing ${H}_{c2}$ values obtained from the entropy conservation construction (${H}_{c2//ab}=\ensuremath{-}19.5\phantom{\rule{0.16em}{0ex}}\mathrm{T}/\mathrm{K}$ and ${H}_{c2//c}=\ensuremath{-}2.9\phantom{\rule{0.16em}{0ex}}\mathrm{T}/\mathrm{K})$, the analysis of fluctuation contribution to conductivity as a reasonable method for estimating the $H{}_{c2}$ slope.

We studied iron-based superconductors of various families with critical temperatures covering almost all range $T_C = 9 - 53$ K. In natural arrays of contacts formed in these materials we observed intrinsic multiple Andreev reflections effect (IMARE). By using IMARE spectroscopy, we detected the two-gap superconductivity, determined the value of the large and the small superconducting gaps, and the corresponding BCS-ratios. The temperature dependencies of the large and the small gaps $\Delta_{L,S}(T)$ are similar for various families of the Fe-based superconductors and could be well-fitted in the framework of the two-band model by Moskalenko and Suhl. We concluded on the extended s-wave symmetry of the $\Delta_L$ order parameter (20-30 % anisotropy in k-space) and on the absence of nodes for $\Delta_S$. The BCS-ratio $2\Delta_L/k_BT_C \approx 5.2$ is nearly constant within the whole range of $T_C$ (this means that coupling rate is unchanged), reflecting the 20 % reduction of the $T_C^{local}$ in relation to the eigen $T_C^L$, and the large gap roughly corresponds to the energy of magnetic resonance $2\Delta_L \approx E_{res}$. This result requires a special theoretical consideration. Our estimation of the relative coupling constants and eigen parameters of each condensate (in a hypothetical case of a zero interband interaction) $2\Delta_L/k_BT_C^L = 4.2 - 4.8$ and $2\Delta_S/k_BT_C^S = 3.5 - 4.5$ leads to indirect conclusion that namely a strong electron-phonon interaction in each condensate described in the framework of the Eliashberg theory plays the key role in the superconductivity of iron-based oxypnictides. With it, the two condensates interact weakly with each other. The observed scaling of $\Delta_{L,S}$ with $T_C$, as was discussed above, is caused mainly by changing of the density of states $N_{L,S}$ in the bands, whereas Ln-O spacers act as charge reservoirs.

The evolution of flux distribution in MgB2 single crystals during their remagnetization was imaged with magneto-optical technique. Meissner holes, formed as the areas where the annihilation of vortices and antivortices takes place, were found at the boundary between oppositely magnetized parts of the crystal. Gradient of magnetic induction in the vicinity of Meissner holes was found to be enhanced. Finger-like structures of convex shape, formed during the penetration of magnetic flux inside the crystal, were observed and explained as an effect of inhomogeneous pinning and demagnetizing field redistribution in the sample.

We studied SNS and S-N-S-N-...-S contacts (where S is a superconductor and N is a normal metal) formed by “break-junction” technique in polycrystalline Sm1 − x Th x OFeAs superconductor samples with critical temperatures T C = 34–45 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 Sm1 − x Th x OFeAs to be driven by intraband coupling, and $$\sqrt {V_{11} V_{22} } /V_{12} \approx 14$$ (where V ij are the 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 ≠ j = 0) for each condensate as 2ΔL, S/k B T C L,S ≤ 4.5. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.

Superconducting MgB$_2$ shows an E$_{2g}$ zone center phonon, as measured by Raman spectroscopy, that is very broad in energy and temperature dependent. The Raman shift and lifetime show large differences with the values elsewhere in the Brillouin Zone measured by Inelastic X-ray Scattering (IXS), where its dispersion can be accounted for by standard harmonic phonon theory, adding only a moderate electron-phonon coupling. Here we show that the effects rapidly disappear when electron-phonon coupling is switched off by Al substitution on the Mg sites. Moreover, using IXS with very high wave-vector resolution in MgB$_2$, we can follow the dispersion connecting the Raman and the IXS signal, in agreement with a theory using only electron-phonon coupling but without strong anharmonic terms. The observation is important in order to understand the effects of electron-phonon coupling on zone center phonons modes in MgB$_2$, but also in all metals characterized by a small Fermi velocity in a particular direction, typical for layered compounds.

We report measurements of the phonon density of states as measured with inelastic x-ray scattering in SmFeAsO$_{1-x}$F$_y$ powders. An unexpected strong renormalization of phonon branches around 23 meV is observed as fluorine is substituted for oxygen. Phonon dispersion measurements on SmFeAsO$_{1-x}$F$_y$ single crystals allow us to identify the 21 meV A$_{1g}$ in-phase (Sm,As) and the 26 meV B$_{1g}$ (Fe,O) modes to be responsible for this renormalization, and may reveal unusual electron-phonon coupling through the spin channel in iron-based superconductors., 4 pages, 3 figures, submitted for SNS2010 conference proceedings

The vortex structure in the iron pnictide single crystals Ba 1– x K x Fe 2 As 2 and Sr 1– x K x Fe 2 As 2 of the 122 type and SmFeAsO 1– xFx of the 1111 type has been investigated using the decoration method. In all of the crystals under investigation, no regular vortex lattice has been revealed in the magnetic field range up to 200 Oe. The disordered vortex structure is discussed in view of the vortex pinning in single crystals.

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO0.8F0.2 state-of-the-art polycrystals, with critical temperature Tc≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB2 and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to Tc, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ1=6.15±0.45 meV and Δ2=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same Tc. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.

To elucidate the effect of the mass renormalization on the superconducting state in A Os 2 O 6 , we have performed thermodynamic and transport measurements on K x Rb 1− x Os 2 O 6 with x = 0–1. We show that KOs 2 O 6 (RbOs 2 O 6 ) is an intermediate to strong-coupling superconductor with a coupling parameter λ ep ≈ 1.2 ( λ ep ≈ 1). Quantifying λ ep allows us to determine the mass enhancement over the calculated band electronic density of states in addition to the electron–phonon term. We correlate the additional enhancement with the upper critical field slope which is determined by orbital magnetism. In the normal state, the charge carriers scatter at a low-energetic phonon. This rattling motion appears as an Einstein contribution with three modes per Rb in the RbOs 2 O 6 specific heat data, suggesting the rattler being the alkali-ion.

Layered oxypnictide Sm$_{1-x}$Th$_x$OFeAs (Sm-1111) is an ideal candidate to be probed by intrinsic multiple Andreev reflections effect (IMARE) spectroscopy. Using the classical "break-junction" technique, we formed ballistic Andreev arrays of identical S--n--S-contacts, where $S$ is superconductor, and $n$ is a layer of normal metal. For $T < T_C$, the I(V) curve shows an excess-current and a subharmonic gap structure (SGS): a set of sharp dI(V)/dV-dips at positions which depend on the superconducting gap value, the number of junctions in the array, and the natural subharmonic order, thus manifesting the effect of intrinsic multiple Andreev reflections. Here we present the I(V) and dI(V)/dV with up to 4 SGS dips for Andreev arrays formed in optimally doped Sm-1111 with critical temperatures $T_C \approx 49$\,K, as well as in underdoped samples with $T_C \approx 37$\,K. We show that a number of Andreev subharmonics facilitates the determination of the superconducting gap with a better accuracy., 4 pages, 4 figures, accepted to J. Supercond. Novel Magn

In the family of the iron-based superconductors, the $RE$FeAsO-type compounds (with $RE$ being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures ($T_{\mathrm{c}}$) up to $55\ \textrm{K}$ and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe$_{0.92}$Co$_{0.08}$AsO ($T_{\mathrm{c}}=18\ \textrm{K}$) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to the $RE$FeAsO-type materials and if so, whether their exact topology is responsible for the aforementioned record $T_{\mathrm{c}}$. In this work, we use angle-resolved photoemission spectroscopy (ARPES) to investigate the inherent electronic structure of the NdFeAsO$_{0.6}$F$_{0.4}$ compound with a twice higher $T_{\mathrm{c}}=38\ \textrm{K}$. We find a similarly singular Fermi surface and further demonstrate that the dramatic enhancement of superconductivity in this compound correlates closely with the fine-tuning of one of the band-edge singularities to within a fraction of the superconducting energy gap $\Delta$ below the Fermi level. Our results provide compelling evidence that the band-structure singularities near the Fermi level in the iron-based superconductors must be explicitly accounted for in any attempt to understand the mechanism of superconducting pairing in these materials., Comment: Open access article available online at http://www.nature.com/articles/srep18273

It has recently been established that the high temperature (high-Tc) superconducting state coexists with short-range charge-density-wave order and quenched disorder arising from dopants and strain. This complex, multiscale phase separation invites the development of theories of high temperature superconductivity that include complexity. The nature of the spatial interplay between charge and dopant order that provides a basis for nanoscale phase separation remains a key open question, because experiments have yet to probe the unknown spatial distribution at both the nanoscale and mescoscale (between atomic and macroscopic scale). Here we report micro X-ray diffraction imaging of the spatial distribution of both the charge-density-wave puddles (domains with only a few wavelengths) and quenched disorder in HgBa2CuO4+y, the single layer cuprate with the highest Tc, 95 kelvin. We found that the charge-density-wave puddles, like the steam bubbles in boiling water, have a fat-tailed size distribution that is typical of self-organization near a critical point. However, the quenched disorder, which arises from oxygen interstitials, has a distribution that is contrary to the usual assumed random, uncorrelated distribution. The interstitials-oxygen-rich domains are spatially anti-correlated with the charge-density-wave domains, leading to a complex emergent geometry of the spatial landscape for superconductivity., 11 pages, 3 figures

In the family of iron-based superconductors, LaFeAsO-type materials possess the simplest electronic structure due to their pronounced two-dimensionality. And yet they host superconductivity with the highest transition temperature Tc ≈ 55K. Early theoretical predictions of their electronic structure revealed multiple large circular portions of the Fermi surface with a very good geometrical overlap (nesting), believed to enhance the pairing interaction and thus superconductivity. The prevalence of such large circular features in the Fermi surface has since been associated with many other iron-based compounds and has grown to be generally accepted in the field. In this work we show that a prototypical compound of the 1111-type, SmFe0.92Co0.08AsO , is at odds with this description and possesses a distinctly different Fermi surface, which consists of two singular constructs formed by the edges of several bands, pulled to the Fermi level from the depths of the theoretically predicted band structure by strong electronic interactions. Such singularities dramatically affect the low-energy electronic properties of the material, including superconductivity. We further argue that occurrence of these singularities correlates with the maximum superconducting transition temperature attainable in each material class over the entire family of iron-based superconductors., Scientific Reports, 5, ISSN:2045-2322

We demonstrate that images of flux vortices in a superconductor taken with a transmission electron microscope can be used to measure the penetration depth and coherence length in all directions at the same temperature and magnetic field. This is particularly useful for MgB2, where these quantities vary with the applied magnetic field and values are difficult to obtain at low field or in the c direction. We obtained images of flux vortices from a MgB2 single crystal cut in the ac plane by focused ion beam milling and tilted to 45 degrees. with respect to the electron beam about the crystallographic a axis. A new method was developed to simulate these images that accounted for vortices with a nonzero core in a thin, anisotropic superconductor and a simplex algorithm was used to make a quantitative comparison between the images and simulations to measure the penetration depths and coherence lengths. This gave penetration depths Lambda(ab) = 100 +/- 35 nm and Lambda(c) = 120 +/- 15 nm at 10.8 K in a field of 4.8 mT. The large error in Lambda(ab) is a consequence of tilting the sample about a and had it been tilted about c, the errors on Lambda(ab) and Lambda(c) would be reversed. Thus obtaining the most precise values requires taking images of the flux lattice with the sample tilted in more than one direction. In a previous paper [J. C. Loudon et al., Phys. Rev. B 87, 144515 (2013)], we obtained a more precise value for Lambda(ab) using a sample cut in the ab plane. Using this value gives Lambda(ab) = 107 +/- 8 nm, Lambda(c) = 120 +/- 15 nm, xi(ab) = 39 +/- 11 nm, and xi(c) = 35 +/- 10 nm, which agree well with measurements made using other techniques. The experiment required two days to conduct and does not require large-scale facilities. It was performed on a very small sample, 30 x 15 mu m and 200-nm thick, so this method could prove useful for superconductors where only small single crystals are available, as is the case for some iron-based superconductors.

The recent discovery of pressure (p) induced superconductivity in the binary helimagnet CrAs has raised questions on how superconductivity emerges from the magnetic state and on the mechanism of the superconducting pairing. In the present work the suppression of magnetism and the occurrence of superconductivity in CrAs were studied by means of muon spin rotation. The magnetism remains bulk up to p 3.5 kbar while its volume fraction gradually decreases with increasing pressure until it vanishes at p 7 kbar. At 3.5 kbar superconductivity abruptly appears with its maximum Tc 1.2 K which decreases upon increasing the pressure. In the intermediate pressure region (3.5 p 7 kbar) the superconducting and the magnetic volume fractions are spatially phase separated and compete for phase volume. Our results indicate that the less conductive magnetic phase provides additional carriers (doping) to the superconducting parts of the CrAs sample thus leading to an increase of the transition temperature (Tc) and of the superfluid density (ρs). A scaling of ρs with as well as the phase separation between magnetism and superconductivity point to a conventional mechanism of the Cooper-pairing in CrAs., Scientific Reports, 5, ISSN:2045-2322

One of the pivotal questions in the physics of high-temperature superconductors is whether the low-energy dynamics of the charge carriers is mediated by bosons with a characteristic timescale. This issue has remained elusive since electronic correlations are expected to dramatically speed up the electron-boson scattering processes, confining them to the very femtosecond timescale that is hard to access even with state-of-the-art ultrafast techniques. Here we simultaneously push the time resolution and the frequency range of transient reflectivity measurements up to an unprecedented level that enables us to directly observe the 16 fs build-up of the effective electron-boson interaction in hole-doped copper oxides. This extremely fast timescale is in agreement with numerical calculations based on the t-J model and the repulsive Hubbard model, in which the relaxation of the photo-excited charges is achieved via inelastic scattering with short-range antiferromagnetic excitations., Comment: to appear in Nature Physics

In the first part of the present paper we discuss the fabrication and the characterization of an MgB 2 -based SQUID magnetometer with a directly coupled large-area pick-up loop, made on an MgB 2 film deposited by an all in situ technique. The coarse structure of the SQUID was defined by optical lithography and Ar-ion milling, while the two nanobridges acting as weak links in the superconducting loop were made by focused ion beam (FIB) milling. The device was characterized at different temperatures and showed Josephson quantum interference up to 20 K as well as a noise level already compatible with the recording of an adult magnetocardiogram. In the second part, concerning the fundamental physics of MgB 2 , we present the results of very recent point-contact measurements on Mg 1− x Mn x B 2 single crystals with 34.1 ⩾ T c ⩾ 13.3 K (i.e. 0.37% ⩽ x ⩽ 1.5%). The experimental conductance curves were fitted with the generalized two-band BTK model and their behaviour in magnetic fields was studied to check if both the order parameters (OPs) of the σ and π bands were present in the whole doping range. The dependence of the OPs (evaluated through the fit) on the Andreev critical temperature of the junctions is analyzed in the framework of the two-band Eliashberg theory by including the effects of magnetic impurities. The results give an evidence of a dominant effect of the magnetic impurities on the σ-band channel.

Among the different methods of vortex study in superconductors, Scanning Tunneling Microscopy (STM) and Spectroscopy (STS), occupies a particular place. Due to its extraordinary spatial and energy resolution STS allows the study of vortices in real space on a nanometeric scale. In practice however, the success of the observation of vortices by STS depends strongly on the surface quality. Usually highly uniform surfaces are needed to distinguish vortices. As a result of this limitation, the vortex lattice has been successfully studied only in a limited number of superconductors. Here we suggest a simple method extending the vortex analysis to 'difficult cases' of 'bad' superconductors, thin films and even granular materials. In this method the STM tip is positioned in a chosen location of the sample and the local tunneling spectroscopy is continuously performed while the magnetic field is slowly swept. The vortices entering the sample move under the tip and are detected due to the change in the shape of tunneling spectra. We show that the method allows not only to follow the vortex penetration but also to estimate essential superconducting parameters such as the penetration and the coherence lengths. Moreover, it authorizes the study of the vortex cores in the cases where scanning is difficult or impossible. (c) 2005 Elsevier Ltd. All rights reserved.

We report the results of directional point-contact measurements in Mg(B\(_{1-x}\)C\(_{x})_{2}\) single crystals. The amplitudes of the gaps, \(\Delta_{\pi}\) and \(\Delta_{\sigma}\), were determined for each C content by fitting the experimental low-temperature normalized conductance curves of our “soft” point contacts with the BTK model generalized to the two-band case. We found that, on increasing the carbon content, \(\Delta_{\sigma}\) decreases almost linearly with \(T_{c}\) and \(\Delta_{\pi}\) slightly increases until, at \(x=0.132\) (where \(T_{c}=19\) K), they assume the same value \(\Delta =3.2 \pm 0.9\) meV. This result is confirmed by the temperature and magnetic-field dependence of the conductance curves at this C content, which do not show any evidence of two distinct gap values. In particular, the Δ versus T curve follows very well a standard BCS curve, with a gap ratio \(2\Delta /k_{B} T_{c}=3.9\). These experimental findings are compared to the theoretical predictions of the two-band model in the Eliashberg formulation.

In layered superconductors the order parameter may be modulated within the unit cell, leading to non-trivial modifications of the vortex core if the interlayer coherence length $\xi_c(T)$ is comparable to the interlayer distance. In the iron-pnictide SmFeAs(O,F) ($T_c \approx 50$K) this occurs below a cross-over temperature $T^\star \approx 41$K, which separates two regimes of vortices: anisotropic Abrikosov-like at high and Josephson-like at low temperatures. Yet in the transition region around $T^\star$, hybrid vortices between these two characteristics appear. Only in this region around $T^\star$ and for magnetic fields well aligned with the FeAs layers, we observe oscillations of the c-axis critical current $j_c(H)$ periodic in $\frac{1}{\sqrt{H}}$ due to a delicate balance of intervortex forces and interaction with the layered potential. $j_c(H)$ shows pronounced maxima when a hexagonal vortex lattice is commensurate with the crystal structure. The narrow temperature window in which oscillations are observed suggests a significant suppression of the order parameter between the superconducting layers in SmFeAs(O,F), despite its low coherence length anisotropy ($\gamma_\xi \approx 3-5$)., Comment: accepted for PRL

The magnetic penetration depth ($\lambda$) as a function of applied magnetic field and temperature in SrPt$_3$P($T_c\simeq8.4$ K) was studied by means of muon-spin rotation ($\mu$SR). The dependence of $\lambda^{-2}$ on temperature suggests the existence of a single $s-$wave energy gap with the zero-temperature value $\Delta=1.58(2)$ meV. At the same time $\lambda$ was found to be strongly field dependent which is the characteristic feature of the nodal gap and/or multi-gap systems. The multi-gap nature of the superconduicting state is further confirmed by observation of an upward curvature of the upper critical field. This apparent contradiction would be resolved with SrPt$_3$P being a two-band superconductor with equal gaps but different coherence lengths within the two Fermi surface sheets., Comment: 6 pages, 4 figures

The synthesis and structural and physical properties of iron lanthanide oxypnictide superconductors, ${L}_{4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}{\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{O}}_{4}$ $(L\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}\mathrm{Pr},\phantom{\rule{0.16em}{0ex}}\mathrm{Sm},\phantom{\rule{0.16em}{0ex}}\mathrm{Gd})$, with transition temperature at \ensuremath{\sim}25 K are reported. Single crystals have been grown at high pressure using the cubic anvil technique. The crystal structure consists of layers of ${L}_{2}{\mathrm{O}}_{2}$ tetrahedra separated by alternating layers of chains of Te and of ${\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ tetrahedra: $\ensuremath{-}{L}_{2}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{Te}\ensuremath{-}{L}_{2}{\mathrm{O}}_{2}\ensuremath{-}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}\ensuremath{-}{L}_{2}{\mathrm{O}}_{2}\ensuremath{-}\mathrm{Te}\ensuremath{-}{L}_{2}{\mathrm{O}}_{2}\ensuremath{-}$ (space group: $I\mathrm{4/}\mathit{mmm}$, $a\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}4.0$, $c\phantom{\rule{0.16em}{0ex}}\ensuremath{\sim}\phantom{\rule{0.16em}{0ex}}29.6\phantom{\rule{0.16em}{0ex}}\AA{})$. Substitution of oxygen by fluorine increases the critical temperature, e.g., in ${\mathrm{Gd}}_{4}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}{\mathrm{Te}}_{1\ensuremath{-}x}{\mathrm{O}}_{4\ensuremath{-}y}{\mathrm{F}}_{y}$ up to 45 K. Magnetic torque measurements reveal an anisotropy of the penetration depths of \ensuremath{\sim}31.

Spin-orbit coupling (SOC) is a fundamental interaction in solids which can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. Using angle-resolved photoemission spectroscopy we directly observe a sizeable spin-orbit splitting in all main families of IBS. We demonstrate that its impact on the low-energy electronic structure and details of the Fermi surface topology is much stronger than that of possible nematic ordering. Intriguingly, the largest pairing gap is always supported exactly by SOC-induced Fermi surfaces., Comment: Significant spin-orbit coupling is observed in all families of IBS. Figures 3 and 4 are new. Supplementary information is extended

Imaging of flux vortices in high quality MgB$_2$ single crystals has been successfully performed in a commercial Field Emission Gun-based Transmission Electron Microscope. In Cryo-Lorentz Microscopy, the sample quality and the vortex lattice can be monitored simultaneously, allowing one to relate microscopically the surface quality and the vortex dynamics. Such a vortex motion ultimately determines the flow resistivity, $\rho_{f}$, the knowledge of which is indispensable for practical applications such as superconducting magnets or wires for Magnetic Resonance Imaging. The observed patterns have been analyzed and compared with other studies by Cryo-Lorentz Microscopy or Bitter decoration. We find that the vortex lattice arrangement depends strongly on the surface quality obtained during the specimen preparation, and tends to form an hexagonal Abrikosov lattice at a relatively low magnetic field. Stripes or gossamer-like patterns, recently suggested as potential signatures of an unconventional behavior of MgB$_2$, were not observed.

Vortex patterns at very low vortex densities in high quality MgB 2 single crystals are studied by scanning SQUID microscopy. In observations, characteristically vortices tend to line up locally at very low vortex densities and then form vortex cluster structure with increasing applied field. The evolution of vortex clusters is elucidated by molecular dynamics (MD) simulations based on a two-gap Ginzburg–Landau (GL) theory with type-1.5 conditions. In both observations and simulations, the average intervortex distance in clusters is independent of the applied field. The agreement confirms that high quality MgB 2 single crystals lie in the type-1.5 regime.

Recently, extensive vortex lattice metastability was reported in MgB2 in connection with a second-order rotational phase transition. However, the mechanism responsible for these well-ordered metastable vortex lattice phases is not well understood. Using small-angle neutron scattering, we studied the vortex lattice in MgB2 as it was driven from a metastable to the ground state through a series of small changes in the applied magnetic field. Our results show that metastable vortex lattice domains persist in the presence of substantial vortex motion and directly demonstrate that the metastability is not due to vortex pinning. Instead, we propose that it is due to the jamming of counterrotated vortex lattice domains which prevents a rotation to the ground state orientation., 5 pages, 4 fgures

We report crystal growth at high pressure, structure determination, and magnetic and transport studies of an oxypnictide superconductor Pr4Fe2As2Te1-xO4. Its structure resembles the known 1111 phase except for the considerably larger c lattice constant and intercalated tellurium atoms and it crystallizes in a tetragonal lattice [a = 4.0165(2) angstrom, c = 29.8572(16) angstrom, and space group I4/mmm (no. 139)]. The electrical resistivity rho(T) and magnetization measurement show a transition at T-c approximate to 25 K. The lower (H-c1) and upper (H-c2) critical fields are 2 mT and 6.5 T, respectively. The Ginzburg- Landau parameter of kappa approximate to 80 places this compound in the family of strong type-II superconductors.

We investigate temperature and fluence dependent dynamics of the photoexcited quasi-particle relaxation and low-energy electronic structure in electron-doped 1111-structure Sm(Fe_{0.93}Co_{0.07})AsO single crystal. We find that the behavior is qualitatively identical to the 122-structure Ba(Fe,Co)_{2}As_{2} including the presence of a normal state pseudogap and a marked 2-fold symmetry breaking in the tetragonal phase that we relate to the electronic nematicity. The 2-fold symmetry breaking appears to be a general feature of the electron doped iron pnictides.

Images of flux vortices in superconductors acquired by transmission electron microscopy should allow a quantitative determination of their magnetic structure but so far, only visual comparisons have been made between experimental images and simulations. Here, we make a quantitative comparison between Fresnel images and simulations based on the modified London equation to investigate the magnetic structure of flux vortices in MgB2. This technique gives an absolute, low-field (~30 Oe) measurement of the penetration depth from images of single vortices. We found that these simulations gave a good fit to the experimental images and that if all the other parameters in the fit were known, the penetration depth for individual vortices could be measured with an accuracy of +/- 5 nm. Averaging over 17 vortices gave a penetration depth in the ab plane of 113 +/- 2 at 10.8 K assuming that the entire thickness of the sample was superconducting. The main uncertainty in this measurement was the proportion of the specimen which was superconducting. Allowing for a non-superconducting layer of up to 50 nm thickness on the specimen surfaces gave a penetration depth in the range 100-115 nm, close to values of 90 +/- 2 nm obtained by small-angle neutron scattering and 118-138 nm obtained by radio-frequency measurements. We also discuss the use of the transport of intensity equation which should, in principle, give a model-independent measure of the magnetic structure of flux vortices., 9 pages, 6 figures. Equation (2) has been corrected. Published in Physical Review B (http://prb.aps.org/abstract/PRB/v87/i14/e144515)

We report an angle-dependent study of the magnetic torque $\tau(\theta)$ within the vortex state of single-crystalline LaO$_{0.9}$F$_{0.1}$FeAs and SmO$_{0.9}$F$_{0.1}$FeAs as a function of both temperature $T$ and magnetic field $H$. Sharp peaks are observed at a critical angle $\theta_c$ at either side of $\theta=90^{\circ}$, where $\theta$ is the angle between $H$ and the inter-planar \emph{c}-axis. $\theta_c$ is interpreted as the critical depinning angle where the vortex lattice, pinned and locked by the intrinsic planar structure, unlocks and acquires a component perpendicular to the planes. We observe a series of smaller replica peaks as a function of $\theta$ and as $\theta$ is swept away from the planar direction. These suggest commensurability effects between the period of the vortex lattice and the inter-planar distance leading to additional kinked vortex configurations., Comment: 5 pages, 5 figures

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds., 7 pages, 4 figures. Journal of Superconductivity and Novel Magnetism, in press

Iron pnictides are layered high Tc superconductors with moderate material anisotropy and thus Abrikosov vortices are expected in the mixed state. Yet, we have discovered a distinct change in the nature of the vortices from Abrikosov-like to Josephson-like in the pnictide superconductor SmFeAs(O,F) with Tc~48–50 K on cooling below a temperature T*~41–42 K, despite its moderate electronic anisotropy γ~4–6. This transition is hallmarked by a sharp drop in the critical current and accordingly a jump in the flux-flow voltage in a magnetic field precisely aligned along the FeAs layers, indicative of highly mobile vortices. T* coincides well with the temperature where the coherence length ξc perpendicular to the layers matches half of the FeAs-layer spacing. For fields slightly out-of-plane (> 0.1°– 0.15°) the vortices are completely immobilized as well-pinned Abrikosov segments are introduced when the vortex crosses the FeAs layers. We interpret these findings as a transition from well-pinned, slow moving Abrikosov vortices at high temperatures to weakly pinned, fast flowing Josephson vortices at low temperatures. This vortex dynamics could become technologically relevant as superconducting applications will always operate deep in the Josephson regime.

Spin–orbit coupling (SOC) is a fundamental interaction in solids that can induce a broad spectrum of unusual physical properties from topologically non-trivial insulating states to unconventional pairing in superconductors. In iron-based superconductors (IBS) its role has so far been considered insignificant with the models based on spin- or orbital fluctuations pairing being the most advanced in the field. In this review we discuss the recent direct observation of SOC by angle-resolved photoemission spectroscopy (ARPES). Electron-like Fermi surface of LiFeAs split by SOC.

Single crystals of the $Ln$FeAsO ($Ln$1111, $Ln$ $=$ Pr, Nd, and Sm) family with lateral dimensions up to 1 mm were grown from NaAs and KAs flux at high pressure. The crystals are of good structural quality and become superconducting when O is partially substituted by F (PrFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ and NdFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$) or when Fe is substituted by Co (SmFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO). From magnetization measurements, we estimate the temperature dependence and anisotropy of the upper critical field and the critical current density of underdoped PrFeAsO${}_{0.7}$F${}_{0.3}$ crystal with ${T}_{c}$ \ensuremath{\approx} 25 K. Single crystals of SmFe${}_{1\ensuremath{-}x}$Co${}_{x}$AsO with maximal ${T}_{c}$ up to 16.3 K for $x$ \ensuremath{\approx} 0.08 were grown. From transport and magnetic measurements, we estimate the critical fields and their anisotropy and find these superconducting properties to be quite comparable to the ones in SmFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ with a much higher ${T}_{c}$ \ensuremath{\approx} 50 K. The magnetically measured critical current densities are as high as 10${}^{9}$ A/m${}^{2}$ at 2 K up to 7 T, with indication of the usual fishtail effect. The upper critical field estimated from resistivity measurements is anisotropic with slopes of \ensuremath{\sim}\ensuremath{-}8.7 T/K ($H$||$ab$ plane) and \ensuremath{\sim}\ensuremath{-}1.7 T/K ($H$||$c$ axis). This anisotropy (\ensuremath{\sim}5) is similar to that in other $Ln$1111 crystals with various higher ${T}_{c}$'s.

The vortex lattice (VL) symmetry and orientation in clean type-II superconductors depends sensitively on the host material anisotropy, vortex density and temperature, frequently leading to rich phase diagrams. Typically, a well-ordered VL is taken to imply a ground-state configuration for the vortex-vortex interaction. Using neutron scattering we studied the VL in MgB(2) for a number of field-temperature histories, discovering an unprecedented degree of metastability in connection with a known, second-order rotation transition. This allows, for the first time, structural studies of a well-ordered, nonequilibrium VL. While the mechanism responsible for the longevity of the metastable states is not resolved, we speculate it is due to a jamming of VL domains, preventing a rotation to the ground-state orientation.

The low magnetic field vortex patterns nucleation and evolution in a high-quality two-gap superconductor MgB${}_{2}$ single crystal have been investigated by low-temperature scanning Hall probe microscopy. Large areas have been imaged with single-vortex resolution while changing systematically the thermodynamic parameters for field and temperature. The obtained patterns have been studied and compared with those of a reference 2H-NbSe${}_{2}$ single crystal. We found that the observed vortex patterns in MgB${}_{2}$ (e.g., stripes, clusters) appear due to competing vortex-vortex interactions as suggested by the theory of type-1.5 superconductivity.

We use femtosecond optical spectroscopy to systematically measure the primary energy relaxation rate k1 of photoexcited carriers in cuprate and pnictide superconductors. We find that k1 increases monotonically with increased negative strain in the crystallographic a-axis. Generally, the Bardeen-Shockley deformation potential theorem and, specifically, pressure-induced Raman shifts reported in the literature suggest that increased negative strain enhances electron-phonon coupling, which implies that the observed direct correspondence between a and k1 is consistent with the canonical assignment of k1 to the electron-phonon interaction. The well-known non-monotonic dependence of the superconducting critical temperature Tc on the a-axis strain is also reflected in a systematic dependence Tc on k1, with a distinct maximum at intermediate values (~16 ps-1 at room temperature). The empirical non-monotonic systematic variation of Tc with the strength of the electron-phonon interaction provides us with unique insight into the role of electron-phonon interaction in relation to the mechanism of high-Tc superconductivity as a crossover phenomenon., Comment: manuscript as accepted in PRX, main paper (20 pages, 3 figures) plus supplementary material (25 pages, 19 figures)

To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of the magnetic torque $\ensuremath{\tau}(\ensuremath{\theta})$ in the mixed state of LaO${}_{0.9}$F${}_{0.1}$FeAs single crystals as a function of temperature $T$ and magnetic fields $H$ up to 18 T. The paramagnetic contribution of the Fe ions is properly treated in order to extract the effective mass anisotropy parameter $\ensuremath{\gamma}=({m}_{c}/{m}_{\mathit{ab}}){}^{1/2}$ from $\ensuremath{\tau}(\ensuremath{\theta})$. We show that $\ensuremath{\gamma}$ depends strongly on both $T$ and $H$, reaching a maximum value of $~$10 followed by a decrease toward values close to 1 as $T$ is lowered. The observed field dependencies of the London penetration depth ${\ensuremath{\lambda}}_{\mathit{ab}}$ and $\ensuremath{\gamma}$ suggest the onset of suppression of a superconducing gap at $H\ensuremath{\approx}{H}_{c2}/3$.

We report on the specific-heat determination of the anisotropic phase diagram of single crystals of optimally doped SmFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$. In zero field, we find a clear cusplike anomaly in $C$/$T$ with $\ensuremath{\Delta}C/{T}_{c}=24$ mJ/mol ${\mathrm{K}}^{2}$ at ${T}_{c}=49.5$ K. In magnetic fields along the $c$ axis, pronounced superconducting fluctuations induce broadening and suppression of the specific-heat anomaly which can be described using three-dimensional lowest-Landau-level scaling with an upper critical field slope of --3.5 T/K and an anisotropy of \ensuremath{\Gamma} $=$8. The small value of $\ensuremath{\Delta}C/{T}_{c}$ yields a Sommerfeld coefficient $\ensuremath{\gamma}$ \ensuremath{\sim} 8 mJ/mol ${\mathrm{K}}^{2}$, indicating that SmFeAsO${}_{1\ensuremath{-}x}$F${}_{x}$ is characterized by a modest density of states and strong coupling.