铁磷属元素化物超导体Ba $_{0.6}$ K $_{0.4}$ Fe $_2$ As $_2$的混合态超流密度（$n_s$）是由最佳掺杂样品（$x=0.4$）的μ子自旋旋转确定的。$n_s$的温度依赖性由用于s波配对的常规BCS模型完美地再现，其中序参数可以是具有$Δ=8.35(6)$ meV [ $2Δ/k_BT_c=5.09(4)$ ]的单隙结构，或者具有$Δ_1=12$ meV（固定）[ $2Δ_1/k_BT_c=7.3$ ]和$Δ_2=6.8(3)$ meV [ $2Δ_2/k_BT_c=4.1(2)$ ]的双隙结构。后者与角分辨光发射光谱的最新结果一致。大的能隙参数（$2Δ/k_BT_c$）表明载流子与玻色子的极强耦合，介导了库珀配对。
英文摘要：
Superfluid density ($n_s$) in the mixed state of an iron pnictide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ is determined by muon spin rotation for a sample with optimal doping ($x=0.4$). The temperature dependence of $n_s$ is perfectly reproduced by the conventional BCS model for s-wave paring, where the order parameter can be either a single-gap with $Δ=8.35(6)$ meV [$2Δ/k_BT_c=5.09(4)$], or double-gap structure with $Δ_1=12$ meV (fixed) [$2Δ_1/k_BT_c=7.3$] and $Δ_2=6.8(3)$ meV [$2Δ_2/k_BT_c=4.1(2)$]. The latter is consistent with the recent result of angle-resolved photo-emssion spectroscopy. The large gap parameters ($2Δ/k_BT_c$) indicate extremely strong coupling of carriers to bosons that mediate the Cooper pairing.
A muon-spin relaxation (muSR) investigation is presented for the molecular superconductor kappa-(BEDT-TTF)2Cu[N(CN)2Br]. Evidence is found for low-temperature phase-separation, with only a fraction of the sample showing a superconducting signal, even for slow cooling. Rapid cooling reduces the superconducting fraction still further. For the superconducting phase, the in-plane penetration depth is measured to be lambda_{parallel} = 0.47(1) mu m and evidence is seen for a vortex decoupling transition in applied fields above 40 mT. The magnetic fluctuations in the normal state produce Korringa be…
The anisotropy and temperature dependence of the magnetic field penetration depth in the iron-based superconductor KFe$_2$As$_2$ have been measured down to 20 mK by means of muon spin rotation. We have observed that the flux-line lattice forms triangular symmetry when we applied the field parallel to c-axis, suggesting the interaction between vortices should be isotropic. The temperature dependence of the penetration depth observd with an applied field parallel and perpendicular to c-axis are different. These results can be accounted for by a superconducting gap function with a horizontal line…
Zero field and longitudinal field muon spin relaxation measurements have been performed in optimally doped and overdoped superconductors LaFeAsO$_{1-x}$F$_x$ in order to investigate the magnetic fluctuation over a wide range of temperature and longitudinal field. We have observed no sign of magnetic fluctuation against temperature in the muons' time window (10$^{-5}\sim$ 10$^{-9}$s). Considering the current results and other results, i.e., spin fluctuation observed by neutron scattering, pseudogap-like behaviors by NMR and photoemission spectroscopy, it is suggested that not only the spin fluc…
Systems with strong spin-orbit coupling have been a topic of fundamental interest in condensed matter physics due to the exotic topological phases and the unconventional phenomenon they exhibit. In this particular study, we have investigated the superconductivity in the transition-metal ternary noncentrosymmetric compounds LaMSi (M = Ni, Pt) with different spin-orbit coupling strength, using muon-spin rotation and relaxation measurements. Transverse-field measurements made in the vortex state indicate that the superconductivity in both materials is fully gapped, with a conventional s-wave pair…
The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T'-structured cuprate, T'-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_c\simeq21$ K) in its {\sl undoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $\sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the "parent" compound of the $n$-type cuprates is not a Mott insulat…
We report on a pressure-induced evolution of exotic superconductivity and spin correlations in CeIr(In$_{1-x}$Cd$_{x}$)$_5$ by means of In-Nuclear-Quadrupole-Resonance (NQR) studies. Measurements of an NQR spectrum and nuclear-spin-lattice-relaxation rate $1/T_1$ have revealed that antiferromagnetism induced by the Cd-doping emerges locally around Cd dopants, but superconductivity is suddenly induced at $T_c$ = 0.7 and 0.9 K at 2.34 and 2.75 GPa, respectively. The unique superconducting characteristics with a large fraction of the residual density of state at the Fermi level that increases wit…
The superconducting properties of Lu2Fe3Si5 with Tc = 6.1 K have been investigated using low-temperature transverse-field muon spin rotation ($μ$SR) and specific heat measurements. The magnetic penetration depth at zero temperature is 353(1) nm. However, the temperature dependence of the magnetic penetration depth is consistent with a two gap s+s-wave model. Low-temperature specific heat measurements on the same sample also show evidence of two distinct superconducting gaps.
Noncentrosymmetric materials are promising paradigm to explore unconventional superconductivity. In particular, several Re containing noncentrosymmetric materials have attracted considerable attention due to a superconducting state with a broken time reversal symmetry. A comprehensive study on the superconducting ground state of NbReSi was investigated using magnetization, resistivity, and muon spin rotation/relaxation measurements. Zero field muon spectroscopy results showed the absence of any spontaneous magnetic field below the superconducting transition temperature, T$ _{c} $ = 6.29 K, ind…
We present our results of a local probe study on EuFe$_{2}$(As$_{1-x}$P$_{x}$)$_{2}$ single crystals with $x$=0.13, 0.19 and 0.28 by means of muon spin rotation and ${}^{57}$Fe Mössbauer spectroscopy. We focus our discussion on the sample with $x$=0.19 viz. at the optimal substitution level, where bulk superconductivity ($T_{\text{SC}}=28$ K) sets in above static europium order ($T^{\text{Eu}}=20$K) but well below the onset of the iron antiferromagnetic (AFM) transition ($\sim$100 K). We find enhanced spin dynamics in the Fe sublattice closely above $T_{\text{SC}}$ and propose that these are r…
Superconductors usually display either type-I or type-II superconductivity and the coexistence of these two types in the same material, for example at different temperatures is rare in nature. We the employed muon spin rotation (muSR) technique to unveil the superconducting phase diagram of the dodecaboride ZrB12 and obtained clear evidence of both type-I and type-II characteristics. Most importantly, we found a region showing unusual behavior where the usually mutually exclusive muSR signatures of type-I and type-II superconductivity coexist. We reproduced that behavior in theoretical modelin…
We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7/PrBa2Cu3O7 trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures we show that a finite superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7 layers when juxtaposed to YBa2Cu3O7 "electrodes" while the intrinsic antiferromagnetic order is unaffected.
The superconducting state of the caged type compound Lu$_5$Rh$_6$Sn$_{18}$ has been investigated by using magnetization, heat capacity, and muon-spin relaxation or rotation ($μ$SR) measurements and the results interpreted on the basis of the group theoretical classifications of the possible pairing symmetries and a simple model of the resulting quasiparticle spectra. Our zero-field $μ$SR measurements clearly reveal the spontaneous appearance of an internal magnetic field below the transition temperature, which indicates that the superconducting state in this material is characterized by the br…
Single-layer FeSe film on SrTiO3(001) was recently found to be the champion of interfacial superconducting systems, with a much enhanced superconductivity than the bulk iron-based superconductors. Its superconducting mechanism is of great interest. Although the film has a simple Fermi surface topology, its pairing symmetry is unsettled. Here by using low-temperature scanning tunneling microscopy (STM), we systematically investigated the superconductivity of single-layer FeSe/SrTiO3(001) films. We observed fully gapped tunneling spectrum and magnetic vortex lattice in the film. Quasi-particle i…
We report bulk superconductivity (SC) in Eu$_{0.2}$Sr$_{0.8}$(Fe$_{0.86}$Co$_{0.14}$)$_{2}$As$_{2}$ single crystals by means of electrical resistivity, magnetic susceptibility, and specific heat measurements with $T$$_{\mathrm{c}}$ $\simeq$ 20 K with an antiferromagnetic (AFM) ordering of Eu$^{2+}$ moments at $T$$_{\mathrm{N}}$ $\simeq$ 2.0 K in zero field. $^{75}$As NMR experiments have been performed in the two external field directions (H$\|ab$) and (H$\|c$). $^{75}$As-NMR spectra are analyzed in terms of first order quadrupolar interaction. Spin-lattice relaxation rates (1/$T_{1}$) follow …
We present a comprehensive study on superconducting properties of Re$_7$B$_3$ and Re$_3$B through specific heat, magnetic susceptibility, resistivity, and transverse and zero-field muon spin rotation/relaxation ($μ$SR) experiments on polycrystalline samples. Re$_7$B$_3$ (T$_C$ = 3.2~K) is a non-centrosymmetric type-II ($κ$ $\approx$ 9.27) superconductor in the weak coupling ($λ_{e-ph}$ = 0.54) regime. On the other hand, Re$_3$B (T$_C$ = 5.19~K) is a centrosymmetric type-II ($κ$ $\approx$ 34.55) superconductor in the moderate coupling ($λ_{e-ph}$ = 0.64) regime. Our transverse-field $μ$SR measu…
The magnetic penetration depth $λ$ has been measured in MgCNi$_{3}$ single crystals using both a high precision Tunnel Diode Oscillator technique (TDO) and Hall probe magnetization (HPM). In striking contrast to previous measurements in powders, $δλ$(T) deduced from TDO measurements increases exponentially at low temperature, clearly showing that the superconducting gap is fully open over the whole Fermi surface. An absolute value at zero temperature $λ(0)=230 $nm is found from the lower critical field measured by HPM. We also discuss the observed difference of the superfluid density deduced f…
Muon-spin rotation has been used to probe vortex state in Sr$_2$RuO$_4$. At moderate fields and temperatures a lattice of triangular symmetry is observed, crossing over to a lattice of square symmetry with increasing field and temperature. At lower fields it is found that there are large regions of the sample that are completely free from vortices which grow in volume as the temperature falls. Importantly this is accompanied by {\it increasing} vortex density and increasing disorder within the vortex-cluster containing regions. Both effects are expected to result from the strongly temperature-…
Suppressed Superconductivity of the Surface Conduction Layer in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ Single Crystals Probed by {\it c}-Axis Tunneling Measurements
A number of recent experiments indicate that the iron-chalcogenide FeSe provides the long-sought possibility to study bulk superconductivity in the cross-over regime between the weakly coupled Bardeen--Cooper--Schrieffer (BCS) pairing and the strongly coupled Bose--Einstein condensation (BEC). We report on $^{77}$Se nuclear magnetic resonance experiments of FeSe, focused on the superconducting phase for strong magnetic fields applied along the $c$ axis, where a distinct state with large spin polarization was reported. We determine this high-field state as bulk superconducting with high spatial…
Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental Bismuth (transition temperature $T_{\rm c}\simeq7.05$ K) was investigated. The Ginzburg-Landau parameter $κ=λ/ξ=30(6)$ ($λ$ is the magnetic penetration depth, $ξ$ is the coherence length) was estimated which is the highest among single element superconductors. The temperature dependence of the superconducting energy gap [$Δ(T)$] reconstructed from $λ^{-2}(T)$ deviates from the weak-coupled BCS prediction. The coupling strength $2Δ/k_{\rm B}T_{\rm c}\simeq 4.34$ was estimated thus implying that Bi-I…
We report muon spin relaxation and rotation ($μ$SR) measurements on hydrothermally-grown single crystals of the tetragonal superconductor~FeS, which help to clarify the controversial magnetic state and superconducting gap symmetry of this compound. $μ$SR time spectra were obtained from 280~K down to 0.025~K in zero field (ZF) and applied fields up to 20 mT. In ZF the observed loss of initial asymmetry (signal amplitude) and increase of depolarization rate~$Λ_\mathrm{ZF}$ below 10~K indicate the onset of static magnetism, which coexists with superconductivity below $T_c$. Transverse-field $μ$SR…
Possible Anisotropic Order Parameter in Pyrochlore Superconductor KOs$_2$O$_6$ Probed by Muon Spin Rotation
用μ子自旋转动（\msr）方法研究了烧绿石超导体KOs $_2$ O $_6$的磁穿透深度（$λ$）.横向自旋弛豫速率的温度依赖性，这是成比例的$λ^{-2}$，表现出相当大的偏离预期的经验双流体模型（$\propto T^4$），而服从$T^2$的趋势。此外，$λ$明显增加，随着外加电场的增加，这表明存在非线性和非局域效应。这些结果强烈地表明，超导序参量在KOs $_2$ O $_6$是各向异性的。
The de Gennes extrapolation length is a direction dependent measure of the spatial evolution of the pairing gap near the boundary of a superconductor, and thus provides a viable means to probe its symmetry. It is expected to be infinite and isotropic for plain s-wave pairing, and finite and anisotropic for d-wave. Here, we synthesize single layer FeSe films on SrTiO3(001) (STO) substrates by molecular beam epitaxy, and measure the de Gennes extrapolation length by scanning tunneling microscopy/spectroscopy. We find a 40% reduction of the superconducting gap near specular [110]Fe edges, yieldin…
The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate. On quite general grounds, one expects that due to their low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties in this part of the phase diagram. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding …
We present $^{57}$Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ ($T_c$ = 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate $(1/T_1)$ demonstrate the development of wave-number ($q$)-dependent spin fluctuations, except at $q$ = 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the $^{57}$Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-s…
Coexistence of magnetic fluctuations and superconductivity in the pnictide high temperature superconductor SmFeAsO$_{1-x}$F$_{x}$ measured by muon spin rotation
对磷属元素高温超导体SmFeAsO $_{1-x}$ F $_{x}$（x=0.18和0.3）进行了μ子自旋转动实验。我们观察到在超导附近的慢自旋涨落的异常增强，这表明自旋涨落有助于非常规超导态的形成。获得了面内穿透深度$λ_{ab}(0)=190(5)$ \thinspace nm的估计，这证实了磷属元素化物超导体在$T_{\mathrm{c}}$和$λ_{ab}(0)^{-2}$之间服从Uemura式关系。
Temperature dependent $^{57}$Fe Mössbauer spectroscopy and specific heat measurements for CaK(Fe$_{1-x}$Ni$_x$)$_4$As$_4$ with $x$ = 0, 0.017, 0.033, and 0.049 are presented. No magnetic hyperfine field (e.g. no static magnetic order) down to 5.5 K was detected for $x$ = 0 and 0.017 in agreement with the absence of any additional feature below superconducting transition temperature, $T_c$, in the specific heat data. The evolution of magnetic hyperfine field with temperature was studied for $x$ = 0.033 and 0.049. The long-range magnetic order in these two compounds coexists with superconductivi…
The interdependence of antiferromagnetism and superconductivity in the Bechgaard salts series of organic conductors is examined in the light of the anomalous temperature dependence of the nuclear spin-lattice relaxation rate. We apply the renormalization group approach to the electron gas model to show that the crossover from antiferromagnetism to superconductivity along with the anomalous nuclear relaxation rate of the Bechgaard salts can be well described within a unified microscopic framework. For sizable nesting deviations of the Fermi surface, scaling theory reveals how pairing correlatio…
We report a detailed $μ$SR study of the pressure evolution of the magnetic order in the manganese based pnictide MnP, which has been recently found to undergo a superconducting transition under pressure once the magnetic ground state is suppressed. Using the muon as a volume sensitive local magnetic probe, we identify a ferromagnetic state as well as two incommensurate helical states (with propagation vectors ${\bf Q}$ aligned along the crystallographic $c-$ and $b-$directions, respectively) which transform into each other through first order phase transitions as a function of pressure and tem…
We present a novel phase diagram for the antiferromagnetism and superconductivity in Ce$_2$RhIn$_8$ probed by In-NQR studies under pressure ($P$). The quasi-2D character of antiferromagnetic spin fluctuations in the paramagnetic state at $P$ = 0 evolves into a 3D character because of the suppression of antiferromagnetic order for $P > P_{\rm QCP}\sim$ 1.36 GPa (QCP: antiferromagnetic quantum critical point). Nuclear-spin-lattice-relaxation rate $1/T_1$ measurements revealed that the superconducting order occurs in the $P$ range 1.36 -- 1.84 GPa, with maximum $T_c\sim$ 0.9 K around $P_{\rm QCP}…
The superconducting systems emerging from topological insulators upon metal ion intercalation or application of high pressure are ideal for investigation of possible topological superconductivity. In this context, Sr-intercalated Bi$_2$Se$_3$ is specially interesting because it displays pressure induced re-entrant superconductivity where the high pressure phase shows almost two times higher $T_c$ than the ambient superconducting phase ( $T_C\sim$ 2.9 K). Interestingly, unlike the ambient phase, the pressure-induced superconducting phase shows strong indication of unconventional superconductivi…
Based on the possible superconducting (SC) pairing symmetries recently proposed, the quasiparticle interference (QPI) patterns in electron- and hole-doped Sr$_{2}$IrO$_{4}$ are theoretically investigated. In the electron-doped case, the QPI spectra can be explained based on a model similar to the octet model of the cuprates while in the hole-doped case, both the Fermi surface topology and the sign of the SC order parameter resemble those of the iron pnictides and there exists a QPI vector resulting from the interpocket scattering between the electron and hole pockets. In both cases, the evolut…
Superconductivity in the pseudo-binary pnictides Ru0.55Rh0.45P and Ru0.75Rh0.25As is probed by muon spin relaxation and rotation (muSR) measurements in conjuction with magnetic susceptibility, heat capacity and electrical resistivity measurements. Powder x-ray diffraction confirmed the MnP-type orthorhombic structure (space group Pnma) and showed a nearly single phase nature with small impurity phase(s) of about 5% for both the samples. The occurence of bulk superconductivity is confirmed with Tc = 3.7 K for Ru0.55Rh0.45P and T = 1.6 K for Ru0.75Rh0.25As. The superconducting state electronic h…
Resistivity r(T), Hall coefficient RH(T), superconducting temperature Tc, and the slope of the upper critical field -dHc2/dT were studied in poly- and single-crystalline samples of the Fe-based superconductor Lu2Fe3Si5 irradiated by fast neutrons. Atomic disordering induced by the neutron irradiation leads to a fast suppression of Tc similarly to the case of doping of Lu2Fe3Si5 with magnetic (Dy) and non-magnetic (Sc, Y) impurities. The same effect was observed in a novel FeAs-based superconductor La(O-F)FeAs after irradiation. Such behavior is accounted for by strong pair breaking that is tra…
Elemental rhenium exhibiting type-II superconductivity has been previously reported to break time-reversal symmetry in the superconducting state. We have investigated an arc-melted sample of rhenium exhibiting type-I superconductivity. Low temperature zero-field muon-spin relaxation measurements indicate that time-reversal symmetry is preserved in the superconducting state. Muon diffusion is observed, which is due to quantum mechanical tunneling between interstitial sites. The normal state behavior is characterized by the conduction electrons screening the muons and thermal broadening, and is …
Using muon spin rotation and infrared spectroscopy we study the relation between magnetism and superconductivity in Ba$ _{1-x} $K$ _{x} $Fe$ _{2} $As$ _{2} $ single crystals from the underdoped to the slightly overdoped regime. We find that the Fe magnetic moment is only moderately suppressed in most of the underdoped region where it decreases more slowly than the Néel-temperature, $ T^{\mathrm{N}} $. This applies for both the total Fe moment obtained from muon spin rotation and for the itinerant component that is deduced from the spectral weight of the spin-density-wave pair breaking peak in …
Dynamics of depletion and recovery of superconducting state in La2-xSrxCuO_4 thin films is investigated utilizing optical pump-probe and optical pump - THz probe techniques as a function of temperature and excitation fluence. The absorbed energy density required to suppress superconductivity is found to be about 8 times higher than the thermodynamically determined condensation energy density and nearly temperature independent between 4 and 25 K. These findings indicate that during the time when superconducting state suppression takes place (~0.7 ps), a large part (nearly 90%) of the energy is …
Using muon spin rotation (muSR) we investigated the magnetic and superconducting properties of a series of Ba(Fe1-xCox)2As2 single crystals with 0 =< x =< 0.15. Our study details how the antiferromagnetic order is suppressed upon Co substitution and how it coexists with superconductivity. In the non-superconducting samples at 0 < x < 0.04 the antiferromagnetic order parameter is only moderately suppressed. With the onset of superconductivity this suppression becomes faster and it is most rapid between x = 0.045 and 0.05. As was previously demonstrated by muSR at x = 0.055 [P. Marsik et al., Ph…
Among iron based superconductors, the layered iron chalcogenide Fe(Te1-xSex) is structurally the simplest and has attracted considerable attentions. It has been speculated from bulk studies that nanoscale inhomogeneous superconductivity may inherently exist in this system. However, this has not been directly observed from nanoscale transport measurements. In this work, through simple micromechanical exfoliation and high precision low-energy ion milling thinning, we prepared Fe(Te0.5Se0.5) nano-flake with various thickness and systematically studied the correlation between the thickness and sup…
Topological superconductivity is an exotic phenomenon due to the symmetry-protected topological surface state, in which a quantum system has an energy gap in the bulk but supports gapless excitations confined to its boundary. Symmetries including central and time-reversal (TRS), along with their relations with topology, are crucial for topological superconductivity. We report muon spin relaxation/rotation ($μ$SR) experiments on a topological noncentrosymmetric superconductor PbTaSe$_2$ to study its TRS and gap symmetry. Zero-field $μ$SR experiments indicate the absence of internal magnetic fie…
The kagome lattice is host to flat bands, topological electronic structures, Van Hove singularities and diverse electronic instabilities, providing an ideal platform for realizing highly tunable electronic states. Here, we report soft- and mechanical- point-contact spectroscopy (SPCS and MPCS) studies of the kagome superconductors KV$_3$Sb$_5$ and CsV$_3$Sb$_5$. Compared to the superconducting transition temperature $T_{\rm c}$ from specific heat measurements (2.8~K for CsV$_3$Sb$_5$ and 1.0~K for KV$_3$Sb$_5$), significantly enhanced values of $T_{\rm c}$ are observed via the zero-bias conduc…
