Superconductivity was recently observed in the iron-arsenic-based compounds with a superconducting transition temperature (Tc) as high as 56K [1-7], naturally raising comparisons with the high Tc copper oxides. The copper oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculations that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K [8,9]. Early work on the iron-arsenic compounds seemed to support this view [7,10]. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously-known layered superconductors [9,11], and indicates that reduced dimensionality in these compounds is not a prerequisite for high-temperature superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenide compounds, which appears to be much more three dimensional than that of the copper oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.
Unconventional superconductivity is commonly linked to electronic pairing mechanisms, since it is believed that the conventional electron-phonon interaction (EPI) cannot cause sign-changing superconducting gap symmetries. Here, we show that this common understanding needs to be revised when one considers a more elaborate theory of electron-phonon superconductivity beyond standard approximations. We selfconsistently solve the full-bandwidth, anisotropic Eliashberg equations including vertex corrections beyond Migdal's approximation assuming the usual isotropic EPI for cuprate, Fe-based and heav…
Fermi liquid and isotropic superconductivity of Hund scenario for bilayer nickelates
Hangzhou Normal University（杭州师范大学）；Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Songshan Lake Materials Laboratory（松山湖材料实验室）
Understanding the superconductivity at the interface of FeSe/SrTiO3 is a problem of great contemporary interest due to the significant increase in critical temperature (Tc) compared to that of bulk FeSe, as well as the possibility of an unconventional pairing mechanism and topological superconductivity. We report a study of the influence of a capping layer on superconductivity in thin films of FeSe grown on SrTiO3 using molecular beam epitaxy. We used in vacuo four-probe electrical resistance measurements and ex situ magneto-transport measurements to examine the effect of three capping layers …
The unconventional multi-gap superconductivity in elemental Pb were reported previously by surface sensitive tunneling experiments, as well as predicted by several theory works. To obtain bulk evidence for such multiple gap behavior, the thermodynamic critical field $B_{\rm c}$ was measured along three different crystallographic directions ([100], [110], and [111]) in a high-quality Pb single crystal by means of muon spin rotation/relaxation. No difference in temperature evolution of $B_{\rm c}$ for all three directions was detected. The average reduced gap $α=Δ/k_{\rm B}T_{\rm c}=2.312(3)$ ($…
High-entropy alloys (HEA) form through the random arrangement of five or more chemical elements on a crystalline lattice. Despite the significant amount of resulting compositional disorder, a subset of HEAs enters a superconducting state below critical temperatures $T_c<10$ K. The superconducting properties of the known HEAs seem to suffice a Bardeen-Cooper-Schrieffer (BCS) description, but little is known about their superconducting order parameter and the microscopic role of disorder. We report on magnetic susceptibility measurements on films of the superconducting HEA (TaNb)$_{1-x}$(ZrHfTi)…
We study temperature dependent upper critical field $H_{\rm c2}$ of a SrFe$_{1.85}$Co$_{0.15}$As$_{2}$ single crystal (\textit{T$_c$}=20.2 K) along \textit{ab}-plane and \textit{c}-axis through resistivity measurements up to 50 T. For the both crystalline directions, $H_{\rm c2}$ becomes nearly isotropic at zero temperature limit, reaching $\sim$ 48 T. The temperature dependence of the $H_{\rm c2}$ curves is explained by interplay between orbital and Pauli limiting behaviors combined with the two band effects.
Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that glues electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local or Kondo-breakdown quantum-critical point that is revealed in isotropic scatte…
The recent observation of superconductivity in thin film infinite-layer nickelates$^{1-3}$ offers a different angle to investigate superconductivity in layered oxides$^{4}$. A wide range of candidate models have been proposed$^{5-10}$, emphasizing single- or multi-orbital electronic structure, Kondo or Hund's coupling, and analogies to cuprates. Clearly, further experimental characterization of the superconducting state is needed to develop a full understanding of the nickelates. Here we use magnetotransport measurements to probe the superconducting anisotropy in Nd$_{0.775}$Sr$_{0.225}$NiO$_{…
While the layered 122 iron arsenide superconductors are highly anisotropic, unconventional, and exhibit several forms of electronic orders that coexist or compete with superconductivity in different regions of their phase diagrams, we find in the absence of iron in the structure that the superconducting characteristics of the end member BaPd2As2 are surprisingly conventional. Here we report on complementary measurements of specific heat, magnetic susceptibility, resistivity measurements, Andreev spectroscopy, and synchrotron high pressure x-ray diffraction measurements supplemented with theore…
We reported a controllable way of removing excess Fe in Fe1+yTe0.6Se0.4 by annealing in a fixed amount of O2. Compared with the weak superconductivity induced by dilute acids, O2 annealing can successfully induce bulk superconductivity. Proper O2 annealing changes the temperature dependence of resistivity at low temperatures from semiconducting to metallic, which comes from the deintercalation of excess Fe. Critical current densities with field along the c-axis and ab-plane are found almost isotropic with large values of 3*10^5 A/cm2 and 2.5*10^5 A/cm2 at 2 K. Furthermore, magneto-optical imag…
In an experiment in a diamond anvil cell utilizing helium pressure medium, yttrium metal displays a superconducting transition temperature which increases monotonically from Tc ? 3.5 K at 30 GPa to 17 K at 89.3 GPa, one of the highest transition temperatures for any elemental superconductor. The pressure dependence of Tc differs substantially from that observed in previous studies under quasihydrostatic pressure to 30 GPa. Remarkably, the dependence of Tc on relative volume V/Vo is linear over the entire pressure range above 33 GPa, implying that higher values of Tc are likely at higher pressu…
Thermal conductivity, point contact spectroscopy, angle-resolved photoemission and Raman spectroscopy measurements were performed on BaFe1.9Pt0.1As2 single crystals obtained from the same synthesis batch in order to investigate the superconducting energy gap structure using multiple techniques. Low temperature thermal conductivity was measured in the superconducting state as a function of temperature and magnetic field, revealing an absence of quasiparticle excitations in the T=0 limit up to 15 T applied magnetic fields. Point-contact Andreev reflection spectroscopy measurements were performed…
The classical criterion for classification of superconductors as type-I or type-II based on the isotropic Ginzburg-Landau theory is generalized to arbitrary temperatures for materials with anisotropic Fermi surfaces and order parameters. We argue that the relevant quantity for this classification is the ratio of the upper and thermodynamic critical fields, $H_{c2}/H_c$, rather than the traditional ratio of the penetration depth and the coherence length, $λ/ξ$. Even in the isotropic case, $H_{c2}/H_c$ coincides with $\sqrt{2}λ/ξ$ only at the critical temperature $T_c$ and they differ as $T$ dec…
Local superconducting gap structure is studied as a function of nanoscale depth in electric-field-induced surface superconductivity such as in SrTiO3. We examine solutions of Bogoliubov-de Gennes equation in two limiting confinement potential cases of electric field with and without screening effects. As unique properties different from bulk superconductivity, there appear in-gap states even for isotropic s-wave pairing, due to multiple gap structure of sub-band dependent surface superconductivity. These determine the depth-dependence of local superconductivity.
Nearly Isotropic Quantum-Critical Transport in Single-Crystal CeNiC2
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；School of Physical Sciences, University of Chinese Academy of Sciences（中国科学院大学）；Institute for Solid State Physics, The University of Tokyo（东京大学固体科学研究所）；Graduate School of Science and Technology, Yamaguchi University（山口大学理工学研究科）；Faculty of Science and Engineering, Tokyo City University（东京都立大学理工学部）
We address an important issue as to whether bulk-sensitive data of Raman scattering, optical conductivity, magnetic penetration depth, directional point-contact tunneling spectra, and nonmagnetic pair-breaking effect in optimally electron-doped Nd_{1.85}Ce_{0.15}CuO_{4-y} support a nodeless s-wave or d-wave superconducting gap. We numerically calculate Raman intensities, directional point-contact tunneling spectra, and nonmagnetic pair-breaking effect in terms of both s-wave and d-wave gap symmetries. We find that all these bulk-sensitive data are in quantitative agreement with a nearly isotro…
It was suggested by Rasolt and Tešanović that the Landau level quantization in isotropic superconductors could enhance superconductivity in a very strong magnetic field, above the upper critical field ($H_{c2}$). We derive a generalized Harper equation for superconducting systems, and show that a similar reentrant behavior appears in a lattice model, even though the Landau--level--structure is destroyed by the periodic potential in that case. Both the orbital and the Zeeman field--induced effects are taken into account.
We study the transition temperature Tc(H) of a quasi-one-dimensional (Q1D) superconductivity which is derived from the quantum effect of an electron motion in a strong magnetic field. We calculate Tc(H) of both isotropic and anisotropic superconductivity by taking account of the optimal momentum of the Cooper pairs and the effect of higher harmonic terms along second conducting axis in the tight-binding model. We find that, although Tc(H) of the spin-singlet superconductivity is suppressed strongly by the Zeeman effect, the suppression of Tc(H) is not very severe if we take the optimal pair-mo…
By interpreting various experimental data for the new high temperature FeAs type superconductors in terms of lattice mediated multi gap superconductivity, it is shown that these systems strongly resemble MgB2, however, with the distinction that local polaronic lattice effects exist. This fact establishes a connection to cuprate high temperature superconductors where polaron formation is essential for the pseudogap phase and the unconventional isotope effects observed there. However, similar to MgB2 and in contrast to cuprates the two superconducting gaps in the Fe-As based materials are isotro…
It has long remained elusive whether CuCo$_{2}$S$_{4}$ thiospinel shows bulk superconductivity. Here we clarify the issue by studying on the samples of sulfur-deficient CuCo$_{2}$S$_{3.7}$ and sulfurized CuCo$_{2}$S$_{4}$. The sample CuCo$_{2}$S$_{3.7}$ has a smaller lattice constant of $a=9.454$ Å, and it is not superconducting down to 1.8 K. After a full sulfurization, the $a$ axis of the thiospinel phase increases to 9.475 Å, and the thiospinel becomes nearly stoichiometric CuCo$_{2}$S$_{4}$, although a secondary phase of slightly Cu-doped CoS$_2$ forms. Bulk superconductivity at 4.2 K and …
The dependence of the superconducting transition temperature T_{c} on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, T_{c} rising rapidly to ~ 14 K at 30 GPa. The T_{c}(P)-dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and…
In magnetic fields applied within the angular range of the surface superconductivity state a magnetically anisotropic layered medium is created in structurally isotropic, sufficiently thick niobium films. Surface (Kulik) vortices residing in the superconducting sheaths on both main film surfaces in tilted fields are shown to undergo a decoupling transition from a coherent to an independent behavior, similar to the behavior observed for Giaever transformer. At the transition a feature in pinning properties is measured, which implies different pinning for the lattice of surface vortices coherent…
LiFeAs is unique among the arsenic based Fe-pnictide superconductors because it is the only nearly stoichiometric compound which does not exhibit magnetic order. This is at odds with electronic structure calculations which and a very stable magnetic state and predict cylindrical hole- and electron-like Fermi surface sheets whose geometry suggests spin uctuations and a possible instability toward long-range ordering at the nesting vector. In fact, a complex magnetic phase-diagram is indeed observed in the isostructural NaFeAs compound. Previous angle resolved photoemission (ARPES) experiments r…
Superconductivity with $T_{\rm c}$ $\simeq$ 2 K was discovered in the intermetallic binary compounds Rh$_2$Ga$_9$ and Ir$_2$Ga$_9$. This is the first observation of superconductivity in the Rh-Ga and Ir-Ga binary systems. Both compounds crystallize in a distorted Co$_2$Al$_9$-type structure (monoclinic, space group: $Pc$), which lacks spatial inversion symmetry. Specific heat measurements revealed that both compounds are weak-coupling BCS superconductors having an isotropic superconducting gap. Measurements in magnetic fields indicated type-I superconductivity with a critical field $H_{\rm c}(…
We report a detailed low-temperature thermodynamic investigation (heat capacity and magnetization) of the superconducting state of KFe2As2 for H || c axis. Our measurements reveal that the properties of KFe2As2 are dominated by a relatively large nodeless energy gap (Delta?0 = 1.9 kBTc) which excludes dx2-y2 symmetry. We prove the existence of several additional extremely small gaps (?Delta0 < 1.0 kBTc) that have a profound impact on the low-temperature and low-field behavior, similar to MgB2, CeCoIn5 and PrOs4Sb12. The zero-field heat capacity is analyzed in a realistic self-consistent 4-band…
Co NQR studies were performed in recently discovered superconductor Na$_x$CoO$_2$$\cdot y$H$_2$O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na$_x$CoO$_2$ were examined, SC bilayer-hydrate sample with $T_c \sim 4.7$ K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate $1/T_1$ in the SC sample, it was found that the coherence peak is absent just below $T_c$ and that $1/T_1$ is proportional to temperature far below $T_c$. These results, which are in qualitatively agreement with the prev…
Nearly Isotropic Vortex Solid in $\mathbf{(La,Pr)_{3}Ni_{2}O_{7}}$ Thin Films
Hefei Institutes of Physical Science, Chinese Academy of Sciences（中国科学院合肥物质科学研究院）；Graduate School of USTC（中国科学技术大学研究生院）；Southern University of Science and Technology（南方科技大学）；Shenzhen Pinghu Laboratory（深圳平湖实验室）；Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area（粤港澳大湾区量子科学中心）
Using a diamond anvil cell with nearly hydrostatic helium pressure medium we have significantly extended the superconducting phase diagram Tc(P) of Sc, the lightest of all transition metals. We find that superconductivity is induced in Sc under pressure, Tc increasing monotonically to 8.2 K at 74.2 GPa. The Tc(P) dependences of the trivalent d-electron metals Sc, Y, La, and Lu are compared and discussed within a simple s-d charge transfer framework.
We report detailed studies of the upper critical field and low-temperature specific heat in the two-gap superconductor Lu$_{2}$Fe$_{3}$Si$_{5}$. The anisotropy of the upper critical field suggests that the active band is quasi-one-dimensional. Low-temperature specific heat in magnetic fields reveals that the virtual $H_{c2}$ in the passive band is almost isotropic. These results strongly indicate that the two bands have two different anisotropies, similar to the typical two-gap superconductor MgB$_{2}$, and their interplay may be essential to the two-gap superconductivity in Lu$_{2}$Fe$_{3}$Si…
We measured the temperature dependence of the magnetic penetration depth of La3Pd4Si4 down to 0.02 Tc. We observe a temperature-independent behaviour below 0.25 Tc, which is firm evidence for a nodeless superconducting gap in this material. The data display a very small anomaly around 1 K which we attribute to the possible presence of a superconducting impurity phase. The superfluid density is well described by a two-phase model, considering La3Pd4Si4 and the impurity phase. The present analysis suggests that the superconducting energy gap of La3Pd4Si4 is isotropic, as expected for conventiona…
Layered transition metal dichalcogenide WTe$_2$ has recently attracted significant attention due to the discovery of an extremely large magnetoresistance, a predicted type-II Weyl semimetallic state, and the pressure-induced superconducting state. By a careful measurement of the superconducting upper critical fields as a function of the magnetic field angle at a pressure as high as 98.5 kbar, we provide the first detailed examination of the dimensionality of the superconducting condensate in WTe$_2$. Despite the layered crystal structure, the upper critical field exhibits a negligible field an…
The temperature dependence of the in-plane magnetic penetration depth, $λ_{ab}(T)$, has been measured in a c-axis oriented polycrystalline CaC$_{6}$ bulk sample using a high-resolution mutual inductance technique. A clear exponential behavior of $λ_{ab}(T)$ has been observed at low temperatures, strongly suggesting isotropic s-wave pairing. Data fit using the standard BCS theory yields $λ_{ab}(0)=(720\pm 80)$ Angstroem and $Δ(0)=(1.79\pm 0.08)$ meV. The ratio $2Δ(0)/k_{_B}T_{c}=(3.6\pm 0.2)$ gives indication for a conventional weakly coupled superconductor.
Superconductivity at temperatures up to 190 K at high pressures has recently been observed in $H_2S$ and interpreted as conventional BCS-electron-phonon-driven superconductivity.\cite{h2s} Instead we propose that it is another example of the mechanism of hole superconductivity at work. Within this mechanism high temperature superconductivity arises when holes conduct through negatively charged anions in close proximity. We propose that electron transfer from $H$ to $S$ leads to conduction by holes in a nearly full band arising from direct overlap of $S^=$ $p$ orbitals in a planar structure. Th…
We measured the resistivity, diamagnetization, and low-temperature specific heat of the newly discovered noncentrosymmetric superconductor Mg_(12-delta)Ir_19B_16. The temperature dependence of specific heat is consistent with the model of an isotropic s-wave gap with value Delta_0 = 0.94 meV for the sample Tc = 5.7 K, and the ratio Delta_0/k_BTc = 1.91 indicates a slightly moderate coupling for the superconductivity. The correlations among the normal state Sommerfeld constant gamma_n, the slope -dHc2(T)/dT near Tc, and the condensation energy Ec are all consistent with the slightly moderate co…
We report the 77Se NMR Knight shift and spin-lattice relaxation studies on the superconducting state of the ternary iron selenide K0.8Fe2-xSe2 with Tc~30 K. Just below Tc, the Knight shift 77Kn shows an immediate drop, indicating a singlet pairing. The spin-lattice relaxation rate 1/77T1 decreases rapidly in the temperature range from TC to Tc/2, which can be fit with an isotropic gap of Δ~3.8\pm 0.5 k_BTc. The Hebel-Slichter coherence peak is not observed. These data give bulk evidence for a strongly coupled superconductivity with isotropic gaps in K0.8Fe2-xSe2, which is similar to other iron…
Nearly perfect Fermi surface nesting in hole-doped La$_3$Ni$_2$O$_7$ enables bulk superconductivity without pressure or strain
NYU-ECNU Institute of Physics, NYU Shanghai（纽约大学-华东师范大学物理研究所，上海纽约大学）；Key Laboratory of Polar Materials and Devices, Ministry of Education, East China Normal University（华东师范大学极化材料与器件教育部重点实验室）；Department of Physics, Brown University（布朗大学物理系）；Department of Physics, New York University（纽约大学物理系）
The recently discovered superconductivity in MgB2 has captured world attention due to its simple crystal structure and relatively high superconducting transition temperature Tc=39K. It appears to be generally accepted that it is phonon-mediated s-wave BCS-like superconductivity. Surprisingly, the strongly temperature dependent anisotropy of the upper critical field, observed experimentally in magnesium diboride single crystals, is still lacking a consistent theoretical explanation. We propose a simple single-gap anisotropic s-wave order parameter in order to compare its implications with the p…
We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Ferm…
The temperature dependence of upper critical field B_c2 was determined from the shift of resistive transition ΔT(B) in nearly optimally doped Nd_{2-x}Ce_xCuO_{4-y} single crystals. Within the experimental accuracy, the weak-field data are described by power function B_c2 ~ (ΔT)^{3/2}. This result is compared with the data on heat capacity and analyzed in the context of possible manifestations of boson effects in superconductivity. The temperature dependence of B_c2 persists down to the lowest temperatures, but the numerical values of B_c2 below 1 K are different for different samples.
Flat electronic bands can accommodate a plethora of interaction driven quantum phases, since kinetic energy is quenched therein and electronic interactions therefore prevail. Twisted bilayer graphene, near so-called the "magic angles", features \emph{slow} Dirac fermions close to the charge-neutrality point that persist up to high-energies. Starting from a continuum model of slow, but strongly interacting Dirac fermions, we show that with increasing chemical doping away from the charge-neutrality point, a time-reversal symmetry breaking, valley pseudo-spin-triplet, topological $p+ip$ supercond…
The recent observation of superconductivity at the interfaces between KTaO3 and EuO (or LaAlO3) offers a new example of emergent phenomena at oxide interfaces. This superconductivity exhibits an unusual strong dependence on the crystalline orientation of KTaO3 and its superconducting transition temperature Tc is nearly one order of magnitude higher than that of the seminal LaAlO3/SrTiO3 interface. To understand its mechanism, it is crucial to address if the formation of oxide interfaces is indispensable for the presence of superconductivity. Here, by exploiting ionic liquid (IL) gating, we obt…
The superconducting gap structure of a charge density wave (CDW) superconductor LaPt$_2$Si$_2$ ($T_c$ = 1.6~K) having a quasi two dimensional crystal structure has been investigated using muon spin rotation/relaxation ($μ$SR) measurements carried out in transverse field (TF), zero field (ZF) and longitudinal field (LF) geometries. Rigorous analysis of TF-$μ$SR spectra in the superconducting state corroborates that the temperature dependence of the effective penetration depth, $λ_L$, derived from muon spin depolarization, fits to an isotropic $s+s-$wave model suggesting that the Fermi surface c…
We report a systematic study on the electronic structure and superconducting (SC) gaps in electron doped NaFe$_{0.95}$Co$_{0.05}$As superconductor using angle-resolved photoemission spectroscopy. Hole-like Fermi sheets are at the zone center and electron-like Fermi sheets are at the zone corner, and are mainly contributed by $xz$ and $yz$ orbital characters. Our results reveal a $\fracΔ{K_B T_c}$ in the range of 1.8-2.1, suggesting a weak-coupling superconductivity in these compounds. Gap closing above the transition temperature ($T_c$) shows the absence of pseudogaps. Gap evolution with tempe…
The noncentrosymmetric superconductor AuBe have been investigated using the magnetization, resistivity, specific heat, and muon-spin relaxation/rotation measurements. AuBe crystallizes in the cubic FeSi-type B20 structure with superconducting transition temperature observed at $T_{c}$ = 3.2 $\pm$ 0.1 K. The low-temperature specific heat data, $C_{el}$(T), indicate a weakly-coupled fully gapped BCS superconductivity with an isotropic energy gap 2$Δ(0)/k_{B}T_{c}$ = 3.76, which is close to the BCS value of 3.52. Interestingly, type-I superconductivity is inferred from the $μ$SR measurements, whi…
We report the discovery of superconductivity in the ternary aluminide Nb$_{5}$Sn$_{2}$Al, which crystallizes in the W$_{5}$Si$_{3}$-type structure with one-dimensional Nb chains along the $c$-axis. It is found that the compound has a multiband nature and becomes a weakly coupled, type-II superconductor below 2.0 K. The bulk nature of superconductivity is confirmed by the specific heat jump, whose temperature dependence shows apparent deviation from a single isotropic gap behavior. The lower and upper critical fields are estimated to be 2.0 mT and 0.3 T, respectively. From these values, we deri…
We demonstrate that the differential conductance, $dI/dV$, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide FeSe$_{0.45}$Te$_{0.55}$, possesses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two hole-like Fermi surfaces, and a strongly anisotropic gap on the electron-like Fermi surface. Moreover, we show that the pinning of nematic fluctuations by defects can give rise to a dumbbell-like spatial structure of the induced impuri…
We studied the temperature-pressure phase diagram of EuFe2As2 by measurements of the electrical resistivity. The antiferromagnetic spin-density-wave transition at T_0 associated with the FeAs-layers is continuously suppressed with increasing pressure, while the antiferromagnetic ordering temperature of the Eu 2+ moments seems to be nearly pressure independent up to 2.6 GPa. Above 2 GPa a sharp drop of the resistivity, ρ(T), indicates the onset of superconductivity at T_c \approx 29.5 K. Surprisingly, on further reducing the temperature ρ(T) is increasing again and exhibiting a maximum caused b…
The Th$_{7}$Fe$_{3}$ family of superconductors provides a rich playground for unconventional superconductivity. La$_7$Ni$_3$ is the latest member of this family, which we here investigate by means of thermodynamic and muon spin rotation and relaxation measurements. Our specific heat data provides evidence for two distinct and approximately isotropic superconducting gaps. The larger gap has a value slightly higher than that of weak-coupling BCS theory, indicating the presence of significant correlations. These observations are confirmed by transverse-field muon-rotation measurements. Furthermor…
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 …
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…
