We report the observation of two gaps in the superconductor SmFeAsO$_{0.9}$F$_{0.1}$ (F-SmFeAsO) with $T_c=51.5K$ as measured by point-contact spectroscopy. Both gaps decrease with temperature and vanish at $T_c$ and the temperature dependence of the gaps are described by the theoretical prediction of the Bardeen-Cooper-Schrieffer (BCS) theory. A zero-bias conductance peak (ZBCP) was observed, indicating the presence of Andreev bound states at the surface of F-SmFeAsO. Our results strongly suggest an unconventional nodal superconductivity with multiple gaps in F-SmFeAsO.
We studied the effects of isoelectronic Ru substitution at the Fe site on the energy gaps of optimally F-doped SmFeAsO by means of point-contact Andreev reflection spectroscopy. The results show that the SmFe_{1-x}Ru_{x}AsO_{0.85}F_{0.15} system keeps a multigap character at least up to x=0.50, and that the gap amplitudes Delta_1 and Delta_2 scale almost linearly with the local critical temperature TcA. The gap ratios 2Delta_i/(kB Tc) remain approximately constant only as long as Tc > 30 K, but increase dramatically when Tc decreases further. This trend seems to be common to many Fe-based supe…
Point-contact Andreev-reflection measurements were performed in SmFeAsO_{0.8}F_{0.2} polycrystals with T_c \simeq 53 K. The experimental conductance curves reproducibly exhibit peaks around \pm 6 mV and shoulders at V \sim 16-20 mV, indicating the presence of two nodeless superconducting gaps. While the single-band Blonder-Tinkham-Klapwijk fit can only reproduce a small central portion of the conductance curve, the two-gap one accounts remarkably well for the shape of the whole experimental dI/dV. The fits of the normalized curves give Delta_1(0) = 6.15 \pm 0.45 meV and Delta_2(0) = 18 \pm 3 m…
The magnetic field dependence of the in-plane magnetic penetration depth for optimally doped SmFeAsO_(1-x)F_y was investigated by combining torque magnetometry, SQUID magnetometry, and muon-spin rotation. The results obtained from these techniques show all a pronounced decrease of the superfluid density as the field is increased up to 1.4 T. This behavior is analysed within a two-band model with self-consistently derived coupled gaps, where the superfluid density related to the larger gap is field independent and the superfluid density related to the smaller gap is strongly suppressed with inc…
Low temperature specific heat (SH) was measured on the FeAs-based superconducting single crystals Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ and high pressure synthesized polycrystalline samples SmFeAsO$_{0.9}$F$_{0.1}$. It is found that the sharp SH anomaly $ΔC/T|_{T_c}$ in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ reaches an unexpected high value of 98 mJ/mol K$^2$, about one order of magnitude larger than that of SmFeAsO$_{0.9}$F$_{0.1}$ ($6\sim8$ mJ/mol K$^2$) samples, suggesting very high normal state quasiparticle density of states in FeAs-122 than in FeAs-1111. Furthermore, we found that the electronic SH c…
To probe manifestations of multiband superconductivity in oxypnictides, we measured the angular dependence of magnetic torque $τ(θ)$ in the mixed state of SmO$_{0.8}$F$_{0.2}$FeAs single crystals as functions of temperature $T$ and high magnetic field $H$ up to 30 T. We show that the effective mass anisotropy parameter $γ$ extracted from $τ(θ)$, can be greatly overestimated if the strong paramagnetism of Sm or Fe ions is not properly taken into account. The correctly extracted $γ$ depends on both $T$ and $H$, saturating at $γ\simeq 9$ at lower temperatures. Neither the London penetration depth…
Recent angle-resolved photoemission electron spectroscopy (ARPES) experiments demonstrate that the momentum dependence of the spectral gap in underdoped cuprates does not follow a pure $d$-wave form [H. Anzai et a., Nat. Comm. {\bf 4}, 1815 (2013)]. This deviation is highly controversial. It has often been interpretated as a proof of the non-superconducting origin of the antinodal gap in the underdoped regime. In this article, we show that the measured angular dependence of the spectral gap can be explained by the basic nature of pairs in high-T$_c$ cuprates. Hole pairs, or {\it pairons}, form…
Sr$_2$RuO$_4$ has been under intensive scrutiny over the past years after new NMR measurements unveiled that the superconducting state might be a spin singlet. One of the best order parameter candidates in light of these new experiments is a chiral d-wave state with $d_{xz}+id_{yz}$ symmetry. This order parameter has been overlooked given the strong two-dimensional character of the normal state electronic structure. Recently, a phenomenological proposal based on local interactions with a three-dimensional electronic structure showed that a chiral d-wave state can be stable in Sr$_2$RuO$_4$ onc…
Low-temperature thermal conductivity measurements were performed on single crystal of PbTaSe$_2$, a noncentrosymmetric superconductor with topological bulk nodal lines in the electronic band structure. It is found that the residual linear term $κ_0/T$ is negligible in zero magnetic field. Furthermore, the field dependence of $κ_0/T$ exhibits a clear "$S$"-shape curve. These results suggest that PbTaSe$_2$ has multiple nodeless superconducting gaps. Therefore, the spin-triplet state with gap nodes does not play an important role in this noncentrosymmetric superconductor with strong spin-orbital…
Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to Tc = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)], however only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our a…
Low temperature specific heat has been measured in superconductor $β$-FeS with T$_c$ = 4.55 K. It is found that the low temperature electronic specific heat C$_e$/T can be fitted to a linear relation in the low temperature region, but fails to be described by an exponential relation as expected by an s-wave gap. We try fittings to the data with different gap structures and find that a model with one or two nodal gaps can fit the data. Under a magnetic field, the field induced specific heat $Δγ$=[C$_e$(H)-C$_e$(0)]/T shows the Volovik relation $Δγ_e(H)\propto \sqrt{H}$, suggesting the presence …
Although more than twenty years have passed since the discovery of high temperature cuprate superconductivity, the identification of the superconducting order parameter is still under debate. Here, we show that the nodal gap component is the best candidate for the superconducting order parameter. It scales with the critical temperature $T_c$ over a wide doping range and displays a significant temperature dependence below $T_c$ in both the underdoped and the overdoped regimes of the phase diagram. In contrast, the antinodal gap component does not scale with $T_c$ in the underdoped side and appe…
Mechanism for Nodal Topological Superconductivity on PtBi$_2$ Surface
Institute for Theoretical Physics and Astrophysics, University of Würzburg（维尔茨堡大学理论物理与天体物理研究所）；Würzburg-Dresden Cluster of Excellence ctd.qmat（维尔茨堡-德累斯顿卓越集群ctd.qmat）
Using a precursor superconductivity scenario for the cuprates we present a theory for the temperature dependent behavior of the spectral gaps associated with four distinct spectroscopies: angle resolved photoemission (ARPES), differential conductance $dI/dV$, quasi-particle interference spectroscopy, and the autocorrelated ARPES pattern. We find good agreement for a range of existing experiments and make predictions for others. Our theory, which incorporates the necessary (observed) contrast between the nodal and anti-nodal response, shows how different nodal gap shapes are associated with the…
We report an investigation of the London penetration depth $Δλ(T)$ on single crystals of the layered superconductor Ta$_4$Pd$_3$Te$_{16}$, where the crystal structure has quasi-one-dimensional characteristics. A linear temperature dependence of $Δλ(T)$ is observed for $T\ll T_c$, in contrast to the exponential decay of fully gapped superconductors. This indicates the existence of line nodes in the superconducting energy gap. A detailed analysis shows that the normalized superfluid density $ρ_s(T)$, which is converted from $Δλ(T)$, can be well described by a multigap scenario, with nodes in one…
The superconducting gap is a pivotal character for a superconductor. While the cuprates and conventional phonon-mediated superconductors are characterized by distinct d-wave and s-wave pairing symmetry with nodal and nodeless gap distribution respectively, the superconducting gap distributions in iron-based superconductors are rather diversified. While nodeless gap distributions have been directly observed in Ba1-xKxFe2As2, BaFe2-xCoxAs2, KxFe2-ySe2, and FeTe1-xSex, the signatures of nodal superconducting gap have been reported in LaOFeP, LiFeP, KFe2As2, BaFe2(As1-xPx)2, BaFe2-xRuxAs2 and FeSe…
Both electronic Raman scattering (ERS) and angle-resolved photoemission spectra (ARPES) revealed two energy scales for the gap in different momentum spaces in the cuprates. However, the interpretations were different, and the gap values were also different in two experiments. In order to clarify the origin of these discrepancies, we directly compared ERS and ARPES by calculating ERS from the experimental data of ARPES through the Kubo formula. The calculated ERS spectra were in good agreement with the experimental results except for the B$_{1g}$ peak energies. The doping-dependent B$_{2g}$ pea…
A deep understanding of the character of superconductivity in the recently discovered Fe-based oxypnictides ReFeAsO1-xFx (Re = rare-earth) necessarily requires the determination of the number of the gaps and their symmetry in k space, which are fundamental ingredients of any model for the pairing mechanism in these new superconductors. In the present paper, we show that point-contact Andreev-reflection experiments performed on LaFeAsO1-xFx (La-1111) polycrystals with Tc ~ 27 K and SmFeAsO0.8F0.2 (Sm-1111) ones with Tc ~ 53 K gave differential conductance curves exhibiting two peaks at low bias…
Recently the kagome compounds have inspired enormous interest and made some great progress such as in the field of superconductivity and topology. Here we predict a different kagome magnesium triboride (MgB3) superconductor with a calculated Tc ~12.2 K and Tc ~15.4 K by external stress, the potentially highest among the reported diverse kagome-type superconductors. We reveal its various exotic physical properties including the van Hove singularity, flat-band, multiple Dirac points, and nontrivial topology. The system can be described by a two-band model with highly anisotropic superconducting …
The symmetry of the order parameter in iron-based superconductors, especially the presence or absence of nodes, is still a question of debate. While contradictory experiments can be explained by appropriately tuned theories of nodeless superconductivity in the iron-arsenide compounds, for LaOFeP all experiments clearly point to a nodal order parameter. We put forward a scenario that naturally explains the difference between the order-parameter character in these two sets of compounds, and use functional renormalization group (fRG) techniques to analyze it in detail. Our results show that, due …
We present differential conductance measurements of Cobalt / Cobalt-Oxide / Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range of barriers, from very low to a tunnel barrier. The characteristic conductance of all the junctions show a V-shape structure at low bias instead of the U-shape characteristic of a s-wave order parameter. The bias of the conductance peaks is, for all junctions, larger than the gap of indium. Both properties exclude pure s-wave pairing. The data is well fitted by a model that assumes the coexistence of s-wave singlet and equal spin p-wave tri…
Identifying the uniqueness of FeP-based superconductors may shed new lights on the mechanism of superconductivity in iron-pnictides. Here, we report nuclear magnetic resonance(NMR) studies on LiFeP and LiFeAs which have the same crystal structure but different pnictogen atoms. The NMR spectrum is sensitive to inhomogeneous magnetic fields in the vortex state and can provide the information on the superconducting pairing symmetry through the temperature dependence of London penetration depth $λ_L$. We find that $λ_L$ saturates below $T \sim 0.2$ $T_c$ in LiFeAs, where $T_c$ is the superconducti…
The isovalent-substituted iron pnictide compound SrFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ exhibits multiple evidence for nodal superconductivity via various experimental probes, such as the penetration depth, nuclear magnetic resonance and specific heat measurements. The direct identification of the nodal superconducting (SC) gap structure is challenging, partly because the presence of nodes is not protected by symmetry but instead caused by an accidental sign change of the order parameter, and also because of the three-dimensionality of the electronic structure. We have studied the SC gaps of SrFe…
Identifying the superconducting (SC) gap structure of the iron-based high-temperature superconductors (Fe-HTS's) remains a key issue for the understanding of superconductivity in these materials. In contrast to other unconventional superconductors, in the Fe-HTS's both $d$-wave and extended s-wave pairing symmetries are close in energy, with the latter believed to be generally favored over the former. Probing the proximity between these very different SC states and identifying experimental parameters that can tune them, are of central interest. Here we report high-pressure muon spin rotation e…
The 12442 compounds are a recently discovered family of iron-based superconductors, that share several features with the cuprates due to their strongly anisotropic structure, but are so far poorly understood. Here, we report on the gap structure and anisotropy of RbCa2(Fe1-xNix)4As4F2 single crystals, investigated by a combination of directional point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements. Two gaps were identified, with clear signatures of d-wave-like nodal structures which persist upon Ni doping, well described by a two-band d-d state with symme…
We introduce and study the nodal liquid, a novel zero-temperature quantum phase obtained by quantum-disordering a d-wave superconductor. It has numerous remarkable properties which lead us to suggest it as an explanation of the pseudo-gap state in underdoped high-temperature superconductors. In the absence of impurities, these include power-law magnetic order, a T-linear spin susceptibility, non-trivial thermal conductivity, and two- and one-particle charge gaps, the latter evidenced, e.g. in transport and electron photoemission (which exhibits pronounced fourfold anisotropy inherited from the…
The in-plane resistivity $ρ$ and thermal conductivity $κ$ of FeAs-based superconductor KFe$_2$As$_2$ single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior $ρ(T) \sim T^{1.5}$ at $H_{c_2}$ = 5 T, and the development of a Fermi liquid state with $ρ(T) \sim T^2$ when further increasing field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field $H_{c_2}$. In zero field there is a large residual linear term $κ_0/T$, and the field dependence of $κ_0/T$ mimics that in d-wave cuprate superconductors. This indicates that…
To understand the origin of the nodal gap structure realized in BaFe$_2$(As,P)$_2$, we study the three-dimensional gap structure based on the three-dimensional ten-orbital Hubbard model with quadrupole interaction. In this model, strong spin and orbital fluctuations develop by using the random-phase-approximation. By solving the Eliashberg gap equation, we obtain the fully-gapped s-wave state with (without) sign reversal between hole-like and electron-like Fermi surfaces due to strong spin (orbital) fluctuations, so called the $s_\pm$-wave ($s_{++}$-wave) state. When both spin and orbital fluc…
The magnetic field dependence of low temperature specific heat in YNi2B2C was measured and analyzed using various pairing order parameters. At zero magnetic field, the two-gap model which has been successfully applied to MgB2 and the point-node model, appear to describe the superconducting gap function of YNi2B2C better than other models based on the isotropic s-wave, the d-wave line nodes, or the s+g wave. The two energy gaps, delta_L=2.67 meV and delta_S=1.19 meV are obtained. The observed nonlinear field dependence of electronic specific heat coefficient, gamma(H)~H0.47, is quantitatively c…
We report 31P-NMR and specific heat measurements on an iron (Fe)-based superconductor SrFe2(As0.65P0.35)2 with Tc=26 K, which have revealed the development of antiferromagnetic correlations in the normal state and the unconventional superconductivity(SC) with nodal gap dominated by the gapless low-lying quasiparticle excitations. The results are consistently argued with an unconventional multiband SC state with the gap-size ratio of different bands being significantly large; the large full gaps in s\pm-wave state keep Tc high, whereas a small gap with a nodal-structure causes gapless feature u…
Simultaneous transport and tunneling spectroscopy of moiré graphene: Distinct observation of the superconducting gap and signatures of nodal superconductivity
By an exact formulation of tunnelling into a multi-band superconductor in terms of Green's functions, we demonstrate that the multi-band feature of the iron-based superconductors can lead to novel interference between Andreev reflections and decoherence effect of quasi-particles to Andreev reflections in a tunnelling junction. These effects depend on the relative sign of the gaps for s-wave pairing, and the sign of the Fermi velocities also matters for tunnelling along the nodal direction of d-wave gaps. Experimental probe of such effects could determine both the pairing symmetry and the relat…
We use angle-resolved photoemission spectroscopy applied to deeply underdoped cuprate superconductors Bi2Sr2(Ca,Y)Cu2O8 (Bi2212) to reveal the presence of two distinct energy gaps exhibiting different doping dependence. One gap, associated with the antinodal region where no coherent peak is observed, increases with underdoping - a behavior known for more than a decade and considered as the general gap behavior in the underdoped regime. The other gap, associated with the near nodal regime where a coherent peak in the spectrum can be observed, does not increase with less doping - a behavior not …
We find evidence that the newly discovered Fe-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ ($T_c~=~33.36(7)$~K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation ($μ$SR) measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully-gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband $s$-wave model, yet are well described by two-gap models with line nodes on either one or both of the gap…
By employing a series of experimental techniques, we provide clear evidence that CaPtAs represents a rare example of a noncentrosymmetric superconductor which simultaneously exhibits nodes in the superconducting gap and broken time-reversal symmetry (TRS) in its superconducting state (below $T_c$ $\approx$ 1.5 K). Unlike in fully-gapped superconductors, the magnetic penetration depth $λ(T)$ does not saturate at low temperatures, but instead it shows a $T^2$-dependence, characteristic of gap nodes. Both the superfluid density and the electronic specific heat are best described by a two-gap mode…
The evidence of edge-gaps around the M-points in the metallic state of underdoped cuprates has triggered a very active debate on their origin. We first consider the possibility that this spectroscopic feature results from a quasi-static charge ordering taking place in the underdoped regime. It comes out that to explain the coexistence of gaps and arcs on the Fermi surface the charge modulation should be in an eggbox form. In the lack of evidences for that, we then investigate the local pairing induced by charge-stripe fluctuations. A proper description of the strong anisotropy of both the inte…
We report a nontrivial transition in the core structure of vortices in two-band superconductors as a function of interband impurity scattering. We demonstrate that, in addition to singular zeros of the order parameter, the vortices there can acquire a circular nodal line around the singular point in one of the superconducting components. It results in the formation of the peculiar "moat"-like profile in one of the superconducting gaps. The moat-core vortices occur generically in the vicinity of the impurity-induced crossover between $s_{\pm}$ and $s_{++}$ states.
We present the first study of thermal conductivity in superconducting SrTi$_{1-x}$Nb$_{x}$O$_{3}$, sufficiently doped to be near its maximum critical temperature. The bulk critical temperature, determined by the jump in specific heat, occurs at a significantly lower temperature than the resistive T$_{c}$. Thermal conductivity, dominated by the electron contribution, deviates from its normal-state magnitude at bulk T$_{c}$, following a Bardeen-Rickayzen-Tewordt (BRT) behavior, expected for thermal transport by Bogoliubov excitations. Absence of a T-linear term at very low temperatures rules out…
Determination of the symmetry property of superconducting gaps has been a central issue in studies to understand the mechanisms of unconventional superconductivity. Although it is often difficult to completely achieve the aforementioned goal, the existence of superconducting nodes, one of the few important experimental signatures of unconventional superconductivity, plays a vital role in exploring the possibility of unconventional superconductivity. The interplay between superconducting nodes and topology has been actively investigated, and intensive research in the past decade has revealed va…
Topological materials and topological phases have recently become a hot topic in condensed matter physics. In this work, we report a topological nodal-line semimetal InxTaSe2, in the presence of both charge density wave (CDW) and superconductivity. In the x = 0.58 samples, the 2 * /3 commensurate CDW (CCDW) and the 2 * 2 CCDW are observed below 116 K and 77 K, respectively. Consistent with theoretical calculations, the spin-orbital coupling gives rise to two two-fold-degenerate nodal rings (Weyl rings) connected by drumhead surface states, confirmed by angle-resolved photoemission spectroscopy…
Point-contact Andreev reflection spectroscopy was performed on single crystals of the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$, down to 90mK and up to 3Tesla. The conductance spectra showed multiple structures, including zero-bias peaks and spectral dips, which are interpreted as signatures of multiple pairing gaps, one clearly having nodes. Samples with 2% Ru replacing Os were also measured, showing pronounced spectral humps consistent with the emergence of a non-nodal gap. Detailed analysis of the spectral evolution revealed a magnetic field-\emph{vs.}-temperature phase diagram for P…
The two-dimensional d-p model (or extended Hubbard model) on a square lattice is investigated for fermion pairing by a slave boson method. The inter-site d-fermion interaction is introduced additionally. The momentum space counterpart of this interaction is taken to be separable and expanded in terms of basis functions corresponding to mixed symmetry states. The investigation leads to anomalous pairing of fermion fields only for pure d-wave symmetry. The charge and spin ordering gaps appear in the single- particle spectrum when d(x,y) state is taken into account.The non-Fermi liquid behavior i…
We discuss how to analyze the optical conductivity and Raman spectra of multi-orbital systems using the velocity and the Raman vertices in a similar way Raman vertices were used to disentangle nodal and antinodal regions in cuprates. We apply this method to iron superconductors in the magnetic and non-magnetic states, studied at the mean field level. We find that the anisotropy in the optical conductivity at low frequencies reflects the difference between the magnetic gaps at the X and Y electron pockets. Both gaps are sampled by Raman spectroscopy. We also show that the Drude weight anisotrop…
The magnetic penetration depth ($λ$) 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 ($μ$SR). The dependence of $λ^{-2}$ on temperature suggests the existence of a single $s-$wave energy gap with the zero-temperature value $Δ=1.58(2)$ meV. At the same time $λ$ 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…
The Fermi-liquid-like (FL) resistivity recently observed in clean HgBa2CuO4 below the pseudogap temperature was related to carriers at the nodal points on the Fermi surface [4]. We show that this necessitates important implications for the electronic spectrum of underdoped (UD) cuprates in whole. Photoemission experiments picture the spectrum as of metallic arcs separated from each other by regions with large energy gaps. We solved the kinetic equation in such model rigorously. The Fermi arcs carriers contribute to FL resistivity, if scattering between the opposite nodal points admits the Umkl…
Generic chiral superconductors with three-dimensional electronic structure have nodal gaps and are not strictly topological. Nevertheless, they exhibit a spontaneous thermal Hall effect (THE), i.e. a transverse temperature gradient in response to a heat current even in the absence of an external magnetic field. While in some cases this THE can be quantized analogous to the Quantum Hall effect, this is not the case for nodal superconductors in general. In this study we determine the spontaneous THE for tight binding models with tetragonal and hexagonal crystal symmetry with chiral $p$- and d-wa…
We study the interplay of fluctuations and superconductivity in BaFe$_2$As$_2$ (Ba-122) compounds with Ba and Fe substituted by K ($p$ doping) and Co ($n$ doping), respectively. To this end we measured electronic Raman spectra as a function of polarisation and temperature. We observe gap excitations and fluctuations for all doping levels studied. The response from fluctuations is much stronger for Co substitution and, according to the selection rules and the temperature dependence, originates from the exchange of two critical spin fluctuations with characteristic wave vectors $(\pmπ, 0)$ and $…
When certain elemental metals like Cu, Sr and Nb are intercalated between the layers of Bi$_2$Se$_3$, a topological insulator, the intercalated systems superconduct with critical temperatures around 3 K. Naturally, in all these cases, the possibility of topological superconductivity was suggested and explored. However, in cases of Cu and Sr intercalated systems, the low-temperature scanning tunneling microscopy (STM) experiments revealed fully formed gaps where no signature of low-energy states, a requisite for topological superconductivity, was found. Here, through STM spectroscopy down to 40…
Recently, a new family of iron-based superconductors called 12442 was discovered and the muon spin relaxation ($μ$SR) measurements on KCa$_2$Fe$_4$As$_4$F$_2$ and CsCa$_2$Fe$_4$As$_4$F$_2$ polycrystals, two members of the family, indicated that both have a nodal superconducting gap structure with $s+d$ pairing symmetry. Here we report the ultralow-temperature thermal conductivity measurements on CsCa$_2$Fe$_4$As$_4$F$_2$ single crystals ($T_c$ = 29.3 K). A negligible residual linear term $κ_0/T$ in zero field and the field dependence of $κ_0/T$ suggest multiple nodeless superconducting gaps in…
We have investigated the superconducting ground state of the newly discovered superconductor ThFeAsN with a tetragonal layered crystal structure using resistivity, magnetization, heat capacity and transverse-field (TF) muon-spin rotation ($μ$SR) measurements. Our resistivity and magnetization measurements reveal an onset of bulk superconductivity with $T_{\bf c}\sim$ 30 K. The heat capacity results show a very small anomaly, $Δ$C$_{ele}$$\sim$0.214 (J/mol-K) at $T_{\bf c}\sim$ 30 K and exhibits exponential behavior below $T_{\bf c}$, which fits better to two superconducting gaps rather than a …
Angle-dependent studies of the gap function provide evidence for the coexistence of two distinct gaps in hole doped cuprates, where the gap near the nodal direction scales with the superconducting transition temperature $T_c$, while that in the antinodal direction scales with the pseudogap temperature. We present model calculations which show that most of the characteristic features observed in the recent angle-resolved photoemission spectroscopy (ARPES) as well as scanning tunneling microscopy (STM) two-gap studies are consistent with a scenario in which the pseudogap has a non-superconductin…
Here, we report an overview of the phase diagram of single layered and double layered Fe arsenide superconductors at high magnetic fields. Our systematic magnetotransport measurements of polycrystalline SmFeAsO$_{1-x}$F$_x$ at different doping levels confirm the upward curvature of the upper critical magnetic field $H_{c2}(T)$ as a function of temperature $T$ defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single crystals that this feature, w…
A model of charged hole-pair bosons, with long range Coulomb interactions and very weak interlayer coupling, is used to calculate the order parameter -Phi- of underdoped cuprates. Model parameters are extracted from experimental superfluid densities and plasma frequencies. The temperature dependence -Phi(T)- is characterized by a 'trapezoidal' shape. At low temperatures, it declines slowly due to harmonic phase fluctuations which are suppressed by anisotropic plasma gaps. Above the single layer Berezinski-Kosterlitz-Thouless (BKT) temperature, Phi(T) falls rapidly toward the three dimensional …
Low-temperature thermal conductivity measurements were performed on single crystals of TlNi$_2$Se$_2$, a nickel-chalcogenide heavy-electron superconductor with $T_c$ $\simeq$ 3.7 K. In zero field, the residual electronic contribution at $T$ $\rightarrow$ 0 K ($κ_0/T$) was well separated from the total thermal conductivity, which is less than 0.45\% of its normal-state value. Such a tiny residual $κ_0/T$ is unlikely contributed by the nodal quasiparticles. Nodeless gap structure is supported by the very weak field dependence of $κ_0(H)/T$ in low magnetic fields. In the whole field range, $κ_0(H…
