In the doped topological insulator SrxBi2Se3, a pronounced in-plane twofold symmetry is observed in electronic properties below the superconducting transition temperature Tc <^> 3 K, despite the threefold symmetry of the observed R3m space group. The axis of twofold symmetry is nominally pinned to one of three rotationally equivalent crystallographic directions and crystallographic strain has been proposed to be the origin of this pinning. We carried out multimodal synchrotron diffraction and resistivity measurements down to <^>0.68 K and in magnetic fields up to 45 kG on a single crystal of SrxBi2Se3 to probe the effect of superconductivity on the crystallographic distortion. Our results indicate that there is no in-plane crystallographic distortion at the level of 1 x 10-5 associated with the superconducting transition. These results further support the model that the large twofold in-plane anisotropy of superconducting properties of SrxBi2Se3 is not structural in origin, but electronic, namely, it is caused by a nematic superconducting order parameter of Eu symmetry.

In the doped topological insulator Sr_xBi_2Se_3, a pronounced in-plane two-fold symmetry is observed in electronic properties below the superconducting transition temperature T_c ∼ 3 K, despite the three-fold symmetry of the observed R3̅m space group. The axis of two-fold symmetry is nominally pinned to one of three rotational equivalent directions and crystallographic strain has been proposed to be the origin of this pinning. We carried out multimodal synchrotron diffraction and resistivity measurements down to ∼0.68 K and in magnetic fields up to 45 kG on a single crystal of Sr_0.1Bi_2Se_3 to probe the effect of superconductivity on the crystallographic distortion. Our results indicate that there is no in-plane crystallographic distortion at the level of 1x10^-5 associated with the superconducting transition. These results further support the model that the large two-fold in-plane anisotropy of superconducting properties of Sr_xBi_2Sr_3 is not structural in origin but electronic, namely it is caused by a nematic superconducting order parameter of Eu symmetry.

Van Hove singularities (vHs) located close to the Fermi level in Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have profound influence on their electronic and transport characteristics. Specifically, magneto-transport and susceptibility measurements on CsV3Sb5 reveal an anomalous temperature dependence of the upper critical field H_c2 (T), characterized by a pronounced upward curvature for both in-plane and c-axis magnetic fields, with zero-temperature H_c2 values of 6.0 T and 1.2 T, respectively. Our theoretical analysis, using a newly developed single-band model incorporating vHs and gap anisotropy, suggests that the observed upper critical field behavior is predominantly driven by the anisotropy of the Fermi velocity originating from vHs, instead of multi-band effects or gap anisotropy. Increased electron scattering introduced by proton irradiation defects smears out the vHs, reduces anisotropy, and recovers the conventional H_c2 (T) behavior, corroborating our proposed model.

Pd_3Bi_2Se_2 is a rare realization of a superconducting metal with a non-zero Z_2 topological invariant. We report the growth of high-quality single crystals of layered Pd_3Bi_2Se_2 with a superconducting transition at T_c 0.80 K and upper critical fields of 10 mT and 5 mT for the in-plane and out-of-plane directions, respectively. Our density functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level, all displaying clear three-dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd_3Bi_2Se_2 is evident in magneto-transport measurements, yielding a sign-changing Hall resistivity at low temperatures. The magnetoresistance is non-saturating and follows Kohler's scaling rule. We interpret the magneto-transport data in terms of open orbits that are revealed in the DFT-calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: F_α = (150 ± 26)T, F_β = (293 ± 10)T, F_γ = (375 ± 20)T, and F_η = (1017 ± 12)T, with the low frequency dominating the spectrum. Through the measurement of angular dependent dHvA oscillations and DFT calculations, we identify the F_α frequency with an approximately ellipsoidal electron pocket centered on the L_2 point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass: m^* = (0.11 ± 0.02) m_0 and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd_3Bi_2Se_2 as a potential candidate for exploring topological superconductivity.

Pd3Bi2Se2 is a rare realization of a superconducting metal with a nonzero Z(2) topological invariant. Here, we report the growth of high-quality single crystals of layered Pd3Bi2Se2 with a superconducting transition at T-c approximate to 0.80 K and upper critical fields of similar to 10 and similar to 5 mT for the in-plane and out-of-plane directions, respectively. Our density-functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level, all displaying clear three-dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd3Bi2Se2 is evident in magnetotransport measurements, yielding a sign-changing Hall resistivity at low temperatures. The magnetoresistance is nonsaturating and follows Kohler's scaling rule. We interpret the magnetotransport data in terms of open orbits that are revealed in the DFT-calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: F-alpha = 150 +/- 26 T, F-beta = 293 +/- 10 T, F-gamma = 375 +/- 20 T, and F-eta = 1017 +/- 12 T, with the low frequency dominating the spectrum. Through the measurement of angular-dependent dHvA oscillations and DFT calculations we identify the F-alpha frequency with an approximately ellipsoidal electron pocket centered on the L-2 point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass m* = (0.11 +/- 0.02)m(0) and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd3Bi2Se2 as a potential candidate for exploring topological superconductivity.

We present a study of the superconducting properties of the candidate topological superconductor Ti_(3)Sb. Electrical transport measurements show zero resistance with a T_(c,onset) of 5.9 K with a transition width ΔT_c 0.6 K. The superconducting phase boundaries as derived from magneto-transport and magnetic susceptibility measurements agree well. We estimate an upper critical field Bc2(0) 4.5 T. A Ginzburg-Landau (GL) analysis yields values of the coherence length and penetration depth of ζ = 6.2 nm and λ = 340 nm, respectively, and a GL parameter 55, indicating extreme type-II behavior. Furthermore, we observed a step height in the specific heat (ΔC_e)/(γT_c ) 1.61, a value larger than the Bardeen-Cooper-Schrieffer (BCS) value of 1.43, suggesting modest coupling. Measurements of the temperature dependence of the London penetration depth via the tunnel-diode oscillator (TDO) technique down to 450 mK show a full superconducting gap, consistent with a conventional s-wave gap structure.

The recently discovered layered kagome metals of composition AV_{3}Sb_{5} (A=K, Rb, Cs) exhibit a complex interplay among superconductivity, charge density wave order, topologically nontrivial electronic band structure and geometrical frustration. Here, we probe the electronic band structure underlying these exotic correlated electronic states in CsV_{3}Sb_{5} with quantum oscillation measurements in pulsed fields up to 86 T. The high-field data reveal a sequence of magnetic breakdown orbits that allows the construction of a model for the folded Fermi surface of CsV_{3}Sb_{5}. The dominant features are large triangular Fermi surface sheets that cover almost half the folded Brillouin zone. These sheets have not yet been detected in angle resolved photoemission spectroscopy and display pronounced nesting. The Berry phases of the electron orbits have been deduced from Landau level fan diagrams near the quantum limit without the need for extrapolations, thereby unambiguously establishing the nontrivial topological character of several electron bands in this kagome lattice superconductor.

M. P. Smylie, 2 Kaya Kobayashi, J. Z. Dans, H. Hebbeker, R. Chapai, W.-K. Kwok, and U. Welp Department of Physics and Astronomy, Hofstra University, Hempstead, New York 11549, USA Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Lemont, Illinois 60439, USA Research Institute for Interdisciplinary Science, Okayama University, Okayama 700-8530, Japan Department of Physics, Okayama University, Okayama 700-8530, Japan

We report the synthesis and characterization of phase pure Ta 3 Sb, a material predicted to be topological with eightfold degenerate fermionic states [Science 353, aaf5037 (2016)] and to exhibit a large spin Hall effect [Sci. Adv. 5, eaav8575 (2019]. We observe superconductivity in Ta 3 Sb with 𝑇 𝑐 ~ 0.67 K in both electrical resistivity ρ( T ) and specific heat C ( T ) measurements. Field dependent measurements yield the superconducting phase diagram with an upper critical field of 𝐻 𝑐2 (0)~ 0.95 T, corresponding to a superconducting coherence length of ξ  18.6 nm. The gap ratio deduced from specific heat anomaly, 2∆ 0 /𝑘 𝐵 𝑇 𝑐 is 3.46, a value close to the Bardeen-Cooper-Schrieffer (BCS) value of 3.53. From a detailed analysis of both the transport and thermodynamic data within the Ginsburg-Landau (GL) framework, a GL parameter of κ  90 is obtained identifying Ta 3 Sb as an extreme type-II superconductor. The observation of superconductivity in an eightfold degenerate fermionic compound with topological surface states and predicted large spin Hall conductance positions Ta 3 Sb as an appealing platform to further explore exotic quantum states in multifold

The discovery of a rotational symmetry breaking, or nematic, state in the superconducting doped topological insulator M x Bi 2 Se 3 (M = Cu, Sr, Nb) indicates a multicomponent superconducting order parameter which is topological. Many questions remain in this family of materials. In particular, the nematic axis of twofold symmetry is always pinned along one of three rotationally equivalent directions in the threefold symmetric basal plane in the rhombohedral crystal. Crystallographic strain has been proposed as the origin of the choice of pinning axis. Here, we discuss nematic pinning of the superconducting state, summarizing our magnetotransport, magnetization, penetration depth, and calorimetry measurements, which support the theoretical picture of topological superconductivity. We discuss the impact of controlled disorder via proton irradiation on the nematic superconducting state and discuss results of our multimodal technique for searching for crystallographic strain below 1 K via simultaneous diffraction and magnetotransport measurements.

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 paper we present measurements of the temperature dependence of the London penetration depth Delta lambda (T ) in the compound In0.8Pb0.2Te, which shows a bulk T-c,T-onset of similar to 4.75 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 measured below 1 K and other superconducting parameters are also presented.

In this Letter, we describe quantitative magnetic imaging of superconducting vortices in RbEuFe_{4}As_{4} in order to investigate the unique interplay between the magnetic and superconducting sublattices. Our scanning Hall microscopy data reveal a pronounced suppression of the superfluid density near the magnetic ordering temperature in good qualitative agreement with a recently developed model describing the suppression of superconductivity by correlated magnetic fluctuations. These results indicate a pronounced exchange interaction between the superconducting and magnetic subsystems in RbEuFe_{4}As_{4}, with important implications for future investigations of physical phenomena arising from the interplay between them.

High-pressure synthesis techniques have allowed for the growth of Sn1-xInxTe samples beyond the ambient In-saturation limit of x = 0.5 (T-c similar to 4.5 K). In this study we present measurements of the temperature dependence of the London penetration depth Delta lambda(T) in this superconducting doped topological insulator for x = 0.7, where T-c(,onset) approximate to 5 K. The results indicate fully gapped BCS-like behavior, ruling out odd-parity A(2u), pairing; however, odd-parity A(1u) in pairing is still possible. Critical field values measured below 1 K and other superconducting parameters are also presented.

We investigate the effect of Ni doping on the Fe-site in single crystals of the magnetic superconductor RbEuFe$_4$As$_4$ for doping concentrations of up to 4%. A clear suppression in the superconducting transition temperature is observed in specific heat, resistivity and magnetization measurements. Upon Ni-doping, the resistivity curves shift up in a parallel fashion indicating a strong increase of the residual resistivity due to scattering by charged dopand atoms while the shape of the curve and thus the electronic structure appears largely unchanged. The observed step $\Delta C/T_c$ at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multi-band superconductivity and strong inter-band pairing. The upper critical field slopes are reduced upon Ni doping for in- as well as out-of-plane fields leading to a small reduction in the superconducting anisotropy. The specific heat measurements of the magnetic transition reveal the same BKT behavior close to the transition temperature $T_m$ for all doping levels. The transition temperature is essentially unchanged upon doping. The in to out-of-plane anisotropy of Eu-magnetism observed at small magnetic fields is unaltered as compared to the undoped compound. All of these observations indicate a decoupling of the Eu magnetism from superconductivity and essentially no influence of Ni doping on the Eu magnetism in this compound.

Recently, the niobium (Nb) doped topological insulator Bi2Se3, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi2Se3 layers, has been shown to exhibit both superconductivity with T-c similar or equal to 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth lambda(eff)(T) in the layered topological superconductor candidate Nb0.25Bi2Se3. The exponential temperature dependence of lambda(-2)(eff) (T) at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature T-c = 2.9 K and the gap to T-c ratio 2 Delta/k(B)T(c) = 3.95(19). We also reveal that the ratio T-c/lambda(-2)(eff) is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field mu SR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of Nb0.25Bi2Se3.

We present a study of the magnetic-flux evolution in the magnetic superconductor RbEuFe4As4 performed using magneto-optical imaging and magnetization measurements during field cooling and warming and magnetic field cycling at temperatures above and below the magnetic transition point, T-m. The vortex patterns emerging at T less than or similar to T-m reveal that the Eu-spin subsystem serves as an internal pump of the magnetic flux while the superconducting critical current controls the delivery of magnetic-flux quanta into the bulk. The interplay of magnetic susceptibility amplifying the magnetic induction and vortex pinning attenuating the magnetic-flux entry results in a field- and temperature-dependent critical state that emulates a paramagnetic Meissner effect. The observed vortex dynamics corresponds to a nontrivial spatial current distribution and yields a self-consistent inhomogeneous enhancement of the sample magnetization.

M. P. Smylie, 2, ∗ A. E. Koshelev, K. Willa, 3 R. Willa, 4 W.-K. Kwok, J.-K. Bao, D. Y. Chung, M. G. Kanatzidis, 5 J. Singleton, F. F. Balakirev, H. Hebbeker, P. Niraula, E. Bokari, A. Kayani, and U. Welp Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Lemont, Illinois 60439 Department of Physics and Astronomy, Hofstra University, Hempstead, New York 11549 Institute for Solid-State Physics, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany Institute for Theory of Condensed Matter, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, USA National High Magnetic Field Laboratory, Los Alamos National Laboratory, MS-E536, Los Alamos, New Mexico 87545 Department of Physics, Western Michigan University, Kalamazoo, Michigan 49008 (Dated: April 17, 2019)

The iron-based superconductors are characterized by strong fluctuations due to high transition temperatures and small coherence lengths. We investigate fluctuation behavior in the magnetic iron-pnictide superconductor RbEuFe4As4 by calorimetry and transport. We find that the broadening of the specific-heat transition in magnetic fields is very well described by the lowest-Landau-level scaling. We report calorimetric and transport observations for vortex-lattice melting, which is seen as a sharp drop of the resistivity and a step of the specific heat at the magnetic-field-dependent temperature. The melting line in the temperature-magnetic field plane lies noticeably below the upper-critical-field line and its location is in quantitative agreement with theoretical predictions without fitting parameters. Finally, we compare the melting behavior of RbEuFe(4)As(4 )with other superconducting materials showing that thermal fluctuations of vortices are not as prevalent as in the high-temperature superconducting cuprates, yet they still noticeably influence the properties of the vortex matter.