We report measurements of the in-plane magnetic penetration depth $λ$ in single crystals of SmFeAsO$_{1-x}$F$_y$ ($x\simeq y \simeq 0.2$) with $T_c \simeq 45$ K. We find that at low temperature $λ$ has an exponential temperature dependence which suggests that the Fermi surface is fully gapped. The magnitude of the minimum energy gap, $Δ_1=1.1\pm 0.1 k_BT_c$ at T=0 K, is significantly smaller than the BCS weak-coupling value suggesting that the gap is either strongly anisotropic or varies significantly between the different Fermi surface sheets. Our data fits well a two gap model with an additional larger gap of magnitude $Δ_2 = 1.8\pm0.2 k_BT_c$ which is associated with $\sim 80$% of the total superfluid density.
Measurements of the in-plane magnetic field penetration depth λ_{ab} in Fe-based superconductors with the nominal composition SmFeAsO_0.85 (T_c\simeq52K) and NdFeAsO_0.85 (T_c\simeq51K) were carried out by means of muon-spin-rotation. The absolute values of λ_{ab} at T=0 were found to be 189(5)nm and 195(5)nm for Sm and Nd substituted samples, respectively. The analysis of the magnetic penetration depth data within the Uemura classification scheme, which considers the correlation between the superconducting transition temperature T_c and the effective Fermi temperature T_F, reveal that both fa…
Single crystals of the oxypnictide superconductors SmFeAsO_(0.8)F_(0.2) and NdFeAsO_(0.8)F_(0.2) with T_c in the range of 44 K to 48 K were investigated by torque magnetometry. An analysis of the data in terms of a recently proposed model for the anisotropic magnetization in the superconducting state, treating the penetration depth anisotropy differently than the upper critical field anisotropy, provides evidence that in the oxypnictide superconductors two distinct anisotropies are present. As a result the penetration depth anisotropy differs significantly in magnitude and in temperature depen…
Muon-spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO$_{1-x}$F$_{x}$ with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth $λ_{ab}(0)=190(5)$\thinspace nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between $T_{\mathrm{c}}$ and $λ_{ab}(0)^{-2}$.
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…
Superconducting poly- and single-crystalline samples of Sm1-xThxFeAsO with partial substitution of Sm3+ by Th4+ were synthesized and grown under high pressure and their structural, magnetic and transport properties are studied. The superconducting Tc reaches values higher than 50 K. Bulk superconducting samples (x = 0.08, 0.15, 0.3) do not show any signs of a phase transition from tetragonal to orthorhombic crystal structure at low temperatures. With Th substitution the unit cell parameters a and c shrink and the fractional atomic coordinate of the As site zAs remains almost unchanged, while t…
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…
We report measurements of resistivity, magnetoresistivity, Hall effect, Seebeck coefficient, infrared reflectivity of undoped SmFeAsO and lightly doped SmFeAs(O0.93F0.07) oxypnictides. All the properties measured on SmFeAsO are characterized by clear signatures of the magnetic instability. A self-consistent picture emerges in which below the magnetic transition carrier condensation occurs due to the opening of spin density wave (SDW) gap. This is accompanied by the mobility increase of not gapped carriers due to the suppression of electron-electron scattering. SmFeAs(O0.93F0.07) exhibits an in…
The London penetration depth, $λ(T)$, has been measured in several single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $λ(T)$ follows a power-law, $Δλ(T) \sim T^n$ with $n=2.4 \pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which indicate an anisotropi…
Ac susceptibility and static magnetization measurements were performed in the nearly-optimally doped LaFeAsO$_{0.9}$F$_{0.1}$ and CeFeAsO$_{0.92}$F$_{0.08}$ superconductors, complementing earlier results on SmFeAsO$_{0.8}$F$_{0.2}$ [Phys. Rev. {\bf B 83}, 174514 (2011)]. The magnetic field -- temperature phase diagram of the mixed superconducting state is drawn for the three materials, displaying a sizeable reduction of the liquid phase upon increasing $T_{c}$ in the range of applied fields ($H \leq 5$ T). This result indicates that SmFeAsO$_{0.8}$F$_{0.2}$ is the most interesting compound amo…
We report measurements of the in-plane London penetration depth $λ$ in single crystals of the $α$-PdBi$_{2}$ superconductor --- the $α$-phase counterpart of the putative topological superconductor $β$-PdBi$_{2}$, down to 0.35~K using a high-resolution tunnel-diode-based technique. Both $λ$ and superfluid density $ρ_{s}$ exhibit an exponential behavior for $T\leq$ 0.35$T_{c}$, with $Δ(0)/k_{B}T_{c}\sim$2.0, $ΔC/γT_{c}$$\sim$2.0 and $λ(0)$$\sim$140~nm, showing that $α$-PdBi$_{2}$ is a moderately-coupling, fully-gapped superconductor. The values of $Δ(0)$ and $ΔC/γT_{c}$ are consistent with each …
The recent discovery of the topologically protected surface states in the beta-phase of PdBi2 has reignited the research interest in this class of superconductors. Here, we show results of our muon spin relaxation and rotation (muSR) measurements carried out to investigate the superconducting and magnetic properties and the topological effect in the superconducting ground state of beta-PdBi2. Zero-field muSR data reveal that no sizeable spontaneous magnetization arises with the onset of superconductivity implying that the time reversal symmetry is preserved in the superconducting state of beta…
We report the $^{207}$Pb nuclear magnetic resonance (NMR) measurements on polycrystalline sample of PbTaSe$_2$ with noncentrosymmetric crystal structure and topological electronic band. The nuclear spin-lattice relaxation rate $1/T_1$ shows a suppressed coherence peak below the superconducting transition temperature $T_{\rm c}$ = 4.05 K and decreases as an exponential function of temperature. The penetration depth derived from the NMR spectrum is almost temperature independent below $T$ = 0.7 $T_{\rm c}$. The Knight shift $K$ decreases below $T_{\rm c}$. These results suggest spin-singlet supe…
From measurements of the magnetic penetration depth, $λ(T)$, from 1.6 K to $T_c$ in films of electron-doped cuprates La$_{2-x}$Ce$_x$CuO$_{4-y}$ and Pr$_{2-x}$Ce$_x$CuO$_{4-y}$ we obtain the normalized density of states, $N_s(E)$ at T=0 by using a simple model. In this framework, the flat behavior of $λ^{-2}(T)$ at low $T$ implies $N_s(E)$ is small, possibly gapped, at low energies. The upward curvature in $λ^{-2}(T)$ near $T_c$ seen in overdoped films implies that superfluid comes from an anomalously small energy band within about $3k_BT_c$ of the Fermi surface.
Superconductivity in the topological non-trivial Dirac semimetal PdTe$_2$ was recently shown to be type-I. We here report measurements of the relative magnetic penetration depth, $ Δλ$, on several single crystals using a high precision tunnel diode oscillator technique. The temperature variation $Δλ(T)$ follows an exponential function for $T/T_c < 0.4$, consistent with a fully-gapped superconducting state and weak or moderately coupling superconductivity. By fitting the data we extract a $λ(0)$-value of $\sim 500$~nm. The normalized superfluid density is in good agreement with the computed cur…
We use magnetic force microscopy (MFM) and scanning SQUID susceptometry to measure the local superfluid density $ρ_{s}$ in Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$ single crystals from 0.4 K to the critical temperature $T_c=18.5$ K. We observe that the penetration depth $λ$ varies about ten times more slowly with temperature than previously published, with a dependence that can be well described by a clean two-band fully gapped model. We demonstrate that MFM can measure the important and hard-to-determine absolute value of $λ$, as well as obtain its temperature dependence and spatial homogeneity. …
The noncentrosymmetric superconductor Re$_6$Zr has attracted much interest due to the observation of broken time-reversal symmetry in the superconducting state. Here we report an investigation of the superconducting gap structure of Re$_6$Zr single crystals by measuring the magnetic penetration depth shift $Δλ(T)$ and electronic specific heat $C_e(T)$. $Δλ(T)$ exhibits an exponential temperature dependence behavior for $T~\ll~T_c$, which indicates a fully-open superconducting gap. Our analysis shows that a single gap $s$-wave model is sufficient to describe both the superfluid density $ρ_s(T)$…
The nature of the pairing symmetry of the first heavy fermion superconductor CeCu$_2$Si$_2$ has recently become the subject of controversy. While CeCu$_2$Si$_2$ was generally believed to be a $d$-wave superconductor, recent low temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped $d+d$ band-mixing pairing…
The recent observation of fully-gapped superconductivity in Yb doped CeCoIn5 poses a paradox, for the disappearance of nodes suggests that they are accidental, yet d-wave symmetry with protected nodes is we ll established by experiment. Here, we show that composite pairing provides a natural resolution: in this scenario, Yb doping drives a Lifshitz transition of the nodal Fermi surface, forming a fully-gapped d-wave molecular superfluid of composite pairs. The T4 dependence of the penetration depth associated with the sound mode of this condensate is in accord with observation.
The in-plane magnetic penetration depth, $λ_m(T)$, was measured in single crystals of SrPd$_2$Ge$_2$ superconductor in a dilution refrigerator down to T=60 mK and in magnetic fields up to $H_{dc} = 1$ T by using a tunnel diode resonator. The London penetration depth, $λ$, saturates exponentially approaching $T\rightarrow 0$ indicating fully gapped superconductivity. The thermodynamic Rutgers formula was used to estimate $λ(0) = 426$ nm which was used to calculate the superfluid density, $ρ_s(T)=λ^2(0)/λ^2(T)$. Analysis of $ρ_s(T)$ in the full temperature range shows that it is best described b…
We report on the superconducting gap and pairing symmetry in layered superconductor Bi4O4S3. The measurement of temperature dependence of magnetic penetration depth was carried out using tunnel diode oscillator technique. It is observed that Bi4O4S3 is a conventional s-wave type superconductor with fully developed gap. The zero-temperature value of the superconducting energy gap Δ0 was found to be 1.54 meV, corresponding to the ratio 2Δ0/kBTc=7.2 which is much higher than the BCS value of 3.53. In the superconducting range, superfluid density is very well described by single gap s- wave model.
Measurements of London penetration depth are a sensitive tool to study multi-band superconductivity and it has provided several important insights to the behavior of Fe-based superconductors. We first briefly review the "experimentalist - friendly" self-consistent Eilenberger model that relates the measurable superfluid density and structure of superconducting gaps. Then we focus on the \BaFe -derived materials, for which the results are consistent with 1) two distinct superconducting gaps; 2) development of strong in-plane gap anisotropy with the departure from the optimal doping; 3) appearan…
Superconducting gap structure was probed in type-II Dirac semimetal PdTe$_2$ by measuring the London penetration depth using tunnel diode resonator technique. At low temperatures, the data for two samples are well described by weak coupling exponential fit yielding $λ(T=0)=230$~nm as the only fit parameter at a fixed $Δ(0)/T_c\approx 1.76$, and the calculated superfluid density is consistent with a fully gapped superconducting state characterized by a single gap scale. Electrical resistivity measurements for in-plane and inter-plane current directions find very low and nearly temperature-indep…
The London penetration depth for the unconventional superconductor Sr$_2$RuO$_4$ is analyzed assuming an order parameter which breaks time reversal symmetry and parity simultaneously. Such a superconducting state possesses chiral quasiparticle states with subgap energies at the surface. We show that these subgap states can give a significant contribution to the low-temperature behavior of the London penetration depth yielding a $ T^2 $ power-law even though bulk quasiparticle spectrum is gapped. The presence of several electron bands gives rise to interband transition among the subgap surface …
A true critical current density, $j_{c}$, as opposite to commonly measured relaxed persistent (Bean) current, $j_{B}$, was extracted from the Campbell penetration depth, $λ_{C}(T,H)$ measured in single crystals of LiFeAs. The effective pinning potential is non-parabolic, which follows from the magnetic field - dependent Labusch parameter $α$. At the equilibrium (upon field - cooling), $α(H)$ is non-monotonic, but it is monotonic at a finite gradient of the vortex density. This behavior leads to a faster magnetic relaxation at the lower fields and provides a natural \emph{dynamic} explanation f…
The London penetration depth is evaluated for isotropic materials for any transport and pair-breaking Born scattering rates. Besides known results, a number of new features are found. The slope $|dρ/dθ|$ of the normalized superfluid density $ρ=λ^2(0)/λ^2(θ)$ at the transition $θ=T/T_c=1$ has a minimum near the value of the pair-breaking parameter separating gapped and gapless states. The low-$T$ exponentially flat part of $ρ$ for the s-wave materials is suppressed by increasing pair breaking. For strong $T_c $ suppression by magnetic impurities the "Homes scaling" $λ^{-2}(0) \propto σT_c$ with…
We report a $μ$SR study on the antiperovskite oxide superconductor Sr$_{3-x}$SnO. With transverse-field $μ$SR, we observed the increase of the muon relaxation rate upon cooling below the superconducting transition temperature $T_{\mathrm{c}}=5.4$ K, evidencing bulk superconductivity. The exponential temperature dependence of the relaxation rate $σ$ at low temperatures suggests a fully gapped superconducting state. We evaluated the zero-temperature penetration depth $λ(0)\propto1/\sqrt{σ(0)}$ to be around 320-1020 nm. Such a large value is consistent with the picture of a doped Dirac semimetal.…
Transverse-field muon spin relaxation rates in single crystals of the heavy-fermion superconductor PrOs4Sb12 (Tc = 1.85 K) are nearly constant in the vortex state for temperatures below ~0.5Tc. This suggests that the superconducting penetration depth lambda(T) is temperature-independent at low temperatures, consistent with a gapped quasiparticle excitation spectrum. In contrast, radiofrequency measurements yield a stronger temperature dependence of lambda(T), indicative of point nodes in the gap. A similar discrepancy exists in superconducting Sr2RuO4 which, like PrOs4Sb12, breaks time-reversa…
We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a c-axis oriented superconductor CaC6 in the mixed state. Using both a simple second moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field independent in-plane magnetic penetration depth λab (0) = 72(3) nm. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical, and both are well represented by the clean limit BCS model with 2Δ/kB Tc = 3.6(1), suggesting that CaC6 is a fully gapped BCS …
We study the temperature dependence of the magnetic penetration depth in a 3D topological superconductor (TSC), incorporating the paramagnetic current due to the surface states. A TSC is predicted to host a gapless 2D surface Majorana fluid. In addition to the bulk-dominated London response, we identify a $T^3$ power-law-in-temperature contribution from the surface, valid in the low-temperature limit. Our system is fully gapped in the bulk, and should be compared to bulk nodal superconductivity, which also exhibits power-law behavior. Power-law temperature dependence of the penetration depth c…
We report an investigation of the superconducting properties of the hexagonal noncentrosymmetric compound LaPdIn. Electrical resistivity, specific heat and ac susceptibility measurements demonstrate the presence of bulk superconductivity below $T_c$ = 1.6 K. The specific heat, together with the penetration depth measured using transverse-field muon spin rotation and the tunnel diode oscillator based method, are well described by single gap $s$-wave superconductivity, with a gap magnitude of 1.8$k_BT_c$. From zero-field muon spin relaxation results no evidence is found for the spontaneous emerg…
The effective superconducting penetration depth measured in the vortex state of PrOs4Sb12 using transverse-field muon spin rotation (TF-muSR) exhibits an activated temperature dependence at low temperatures, consistent with a nonzero gap for quasiparticle excitations. In contrast, Meissner-state radiofrequency (rf) inductive measurements of the penetration depth yield a T^2 temperature dependence, suggestive of point nodes in the gap. A scenario based on the recent discovery of extreme two-band superconductivity in PrOs4Sb12 is proposed to resolve this difference. In this picture a large diffe…
Transverse-field muon spin rotation ($μ$SR) experiments in the heavy-fermion superconductor PrOs$_{4}$Sb$_{12}$ ($T_{c}=1.85$ K) suggest that the superconducting penetration depth $λ(T)$ is temperature-independent at low temperatures, consistent with a gapped quasiparticle excitation spectrum. In contrast, radiofrequency (rf) inductive measurements yield a stronger temperature dependence of $λ(T)$, indicative of point nodes in the gap. This discrepancy appears to be related to the multiband structure of PrOs$_{4}$Sb$_{12}$. Muon Knight shift measurements in PrOs$_{4}$Sb$_{12}$ suggest that the…
We have studied the in- and out-of-plane magnetic penetration depths in the hole- doped iron based superconductor Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($T_{c}\approx$ 30K). The study was performed on single crystals grown from different fluxes and we find that the results are nearly the same. The in-plane London penetration depth $λ_{ab}$ does not show exponential saturation at low temperature, as would be expected from a fully gapped superconductor. Instead, $λ_{ab}(T)$ shows a power-law behavior, $λ\propto T^{n}$ ($n\approx 2$), down to $T\approx 0.02 T_{c}$, similar to the electron doped Ba(Fe…
We report on the superconducting properties of In doped SnTe which has recently been explored as a topological crystalline superconductor. Single crystals of Sn0.5In0.5Te have been synthesized by modified Bridgman method. Resistivity measurement performed in the range 1.6K to 300K shows metallic normal state with onset of superconducting transition at Tc = 4.5K. Bulk superconductivity has also been confirmed by DC magnetization, AC susceptibility and rf penetration depth measurements. The zero temperature upper critical field, lower critical field, coherence length, penetration depth are estim…
Recent high-precision measurements employing different experimental techniques have unveiled an anomalous peak in the doping dependence of the London penetration depth which is accompanied by anomalies in the heat capacity in iron-pnictide superconductors at the optimal composition associated with the hidden antiferromagnetic quantum critical point. We argue that finite temperature effects can be a cause of observed features. Specifically we show that quantum critical magnetic fluctuations under superconducting dome can give rise to a nodal-like temperature dependence of both specific heat and…
The recent discovery of superconductivity in NaSn$_2$As$_2$ with a van der Waals layered structure raises immediate questions on its pairing mechanism and underlying electronic structure. Here, we present measurements of the temperature-dependent magnetic penetration depth $λ(T)$ in single crystals of NaSn$_2$As$_2$ down to $\sim40$ mK. We find a very long penetration depth $λ(0) = 960$ nm, which is strongly enhanced from the estimate of first-principles calculations. This enhancement comes from a short mean free path $\ell \approx 1.7$ nm, indicating atomic scale disorder possibly associated …
In superconductors with unconventional pairing mechanisms, the energy gap in the excitation spectrum often has nodes, which allow quasiparticle excitations at low energies. In many cases, e.g. $d$-wave cuprate superconductors, the position and topology of nodes are imposed by the symmetry, and thus the presence of gapless excitations is protected against disorder. Here we report on the observation of distinct changes in the gap structure of iron-pnictide superconductors with increasing impurity scattering. By the successive introduction of nonmagnetic point defects into BaFe$_2$(As$_{1-x}$P$_x…
Anisotropic fully-gapped superconductivity in quasi-one-dimensional Li$_{0.9}$Mo$_6$O$_{17}$
H. H. Wills Physics Laboratory, University of Bristol（布里斯托大学 H.H. 威尔斯物理实验室）；Department of Chemistry and Chemical Biology, Rutgers University（罗格斯大学化学与化学生物学系）；School of Mathematics and Physics, Queen’s University Belfast（贝尔法斯特女王大学数学与物理学院）；Institute of Fundamental Technological Research, Polish Academy of Sciences（波兰科学院基础技术研究所学）；High Field Magnet Laboratory (HFML-FELIX) and Institute for Molecules and Materials, Radboud University（拉得堡大学分子与材料研究所及高场磁实验室（HFML-FELIX））
We have determined the superconducting and magnetic properties of four samples of Lu$_x$Zr$_{1-x}$B$_{12}$ ($x=0.04$, $0.07$, $0.17$, and $0.8$) using muon spin rotation ($μ$SR) and magnetometry measurements. We observed a strong magnetic signal in both the $μ$SR and magnetometry data in one sample ($x=0.07$), likely caused by the formation of static moments of size $\approx 1\,μ_{\rm B}$ due to a clustering effect of the Lu$^{3+}$ ions. In all other samples, we find only a small magnetic signal in the $μ$SR data thought to originate from boron nuclei in the B$_{12}$ cages. The superconductivi…
Recently, the niobium (Nb)-doped topological insulator Bi_2Se_3, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi_2Se_3 layers, has been shown to exhibit both superconductivity with T_c = 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature-dependent of effective magnetic penetration depth Lambda_eff(T) in the layered topological superconductor candidate Nb_0.25Bi_2Se_3. The exponential temperature dependence of lambda_(eff)^(-2)(T) at low temperatures suggests a fully gapped su…
High-pressure synthesis techniques have allowed for the growth of Sn$_{1-x}$In$_x$Te samples beyond the ambient In-saturation limit of $x$ = 0.5 (T$_c \sim$ 4.5 K). In this study, we present measurements of the temperature dependence of the London penetration depth $Δλ(T)$ in this superconducting doped topological insulator for $x$ = 0.7, where T$_{c,onset}\approx 5$ K. The results indicate fully gapped BCS-like behavior, ruling out odd-parity $A_{2u}$ pairing; however, odd-parity $A_{1u}$ pairing is still possible. Critical field values measured below 1 K and other superconducting parameters …
We present magnetometry and muon spin rotation ($μ$SR) measurements of the superconducting dichalcogenide Ir$_{0.95}$Pt$_{0.05}$Te$_2$. From both sets of measurements we calculate the penetration depth and thence superfluid density as a function of temperature. The temperature dependence of the superfluid densities from both sets of data indicate fully gapped superconductivity that can be fit to a conventional s-wave model and yiel fitting parameters consistent with a BCS weak coupling superconductor. We therefore see no evidence for exotic superconductivity in Ir$_{0.95}$Pt$_{0.05}$Te$_2$.
In exotic superconductors including high-$T_c$ copper-oxides, the interactions mediating electron Cooper-pairing are widely considered to have a magnetic rather than the conventional electron-phonon origin. Interest in such exotic pairing was initiated by the 1979 discovery of heavy-fermion superconductivity in CeCu$_2$Si$_2$, which exhibits strong antiferromagnetic fluctuations. A hallmark of unconventional pairing by anisotropic repulsive interactions is that the superconducting energy gap changes sign as a function of the electron momentum, often leading to nodes where the gap goes to zero.…
High-pressure synthesis techniques have allowed for the growth of samples on the indium-rich side of (Pb,In)Te, which have increased superconducting transition temperatures compared to lead-rich compounds. In this study we present measurements of the temperature dependence of the London penetration depth $Δλ(T)$ in the compound In$_{0.8}$Pb$_{0.2}$Te, which shows a bulk $T_{c,onset}$ of $\sim4.75$ K. The results indicate fully gapped BCS-like behavior, ruling out odd-parity, topologically nontrivial $A_{2u}$ pairing; however, odd-parity $A_{1u}$ pairing is still possible. Critical field values…
We report an investigation of the superconducting order parameter of the noncentrosymmetric compound PbTaSe$_2$, which is believed to have a topologically nontrivial band structure. Precise measurements of the London penetration depth $Δλ(T)$ obtained using a tunnel diode oscillator (TDO) based method show an exponential temperature dependence at $T\ll T_c$, suggesting a nodeless superconducting gap structure. A single band s-wave model well describes the corresponding normalized superfluid density, with a gap magnitude of $Δ(0)=1.85T_c$. This is very close to the value of $1.76T_c$ for weak-c…
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…
We have studied the superconducting gap structure of LaPt$_2$Si$_2$ by measuring the temperature dependence of the London penetration depth shift $Δλ(T)$ and point contact spectroscopy of single crystals. $Δλ(T)$ shows an exponential temperature dependence at low temperatures, and the derived normalized superfluid density $ρ_{s}(T)$ can be well described by a single-gap s-wave model. The point-contact conductance spectra can also be well fitted by an s-wave Blonder-Tinkham-Klapwijk model, where the gap value shows a typical BCS temperature and magnetic field dependence consistent with type-II …
We measured the in-plane optical conductivity of a nearly optimally doped (Ba,K)Fe2As2 single crystal with Tc = 39.1 K. Upon entering the superconducting state the optical conductivity below ~20 meV vanishes, strongly suggesting a fully gapped system. A BCS-like fit requires two different isotropic gaps to describe the optical response of this material. The temperature dependence of the gaps and the penetration depth suggest a strong interband coupling, but no impurity scattering induced pair breaking is present. This contrasts to the large residual conductivity observed in optimally doped Ba(…
CaSb2 is a topological nodal-line semimetal that becomes superconducting below 1.6 K, providing an ideal platform to investigate the interplay between topologically nontrivial electronic bands and superconductivity. In this work, we investigated the superconducting order parameter of CaSb2 by measuring its magnetic penetration depth change Δλ(T) down to 0.07 K, using a tunneling diode oscillator (TDO) based technique. Well inside the superconducting state, Δλ(T) shows an exponential activated behavior, and provides direct evidence for a nodeless superconducting gap. By analyzing the temperatur…
The London penetration depth was measured in optimally doped Ba0.6K0.4Fe2As2 crystals, with and without columnar defects produced by 1.4 GeV 208Pb irradiation. The low temperature behavior of unirradiated samples was consistent with a fully gapped superconducting state with a minimum energy gap delta_min/(k_B T_C) = 1. Similar gap values were observed for irradiation levels corresponding to mean column-column separations of 32 nm and 22 nm. At very high irradiation levels (column-column separation of 10 nm) a T^2 power law was observed below Tc/3, most likely due to elevated scattering. Neithe…
We report a comprehensive study of the Zr$_5$Pt$_3$C$_x$ superconductors, with interstitial carbon comprised between 0 and 0.3. At a macroscopic level, their superconductivity, with $T_c$ ranging from 4.5 to 6.3 K, was investigated via electrical-resistivity-, magnetic-susceptibility-, and specific-heat measurements. The upper critical fields $μ_0H_\mathrm{c2}$ $\sim$ 7 T were determined mostly from measurements of the electrical resistivity in applied magnetic fields. The microscopic electronic properties were investigated by means of muon-spin rotation and relaxation ($μ$SR) and nuclear magn…
While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the $T=0$ penetration depth in the electron-doped iron-based superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is remi…
We measure the penetration depth $λ_{ab}(T)$ in Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ using local techniques that do not average over the sample. The superfluid density $ρ_s(T)\equiv1/λ_{ab}(T)^2$ has three main features. First, $ρ_s(T=0)$ falls sharply on the underdoped side of the dome. Second, $λ_{ab}(T)$ is flat at low $T$ at optimal doping, indicating fully gapped superconductivity, but varies more strongly in underdoped and overdoped samples, consistent with either a power law or a small second gap. Third, $ρ_s(T)$ varies steeply near $T_c$ for optimal and underdoping. These observations are …
