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P.H.M. and M.D.B. received PhD studentship support from the UK Engineering and Physical Science Research Council via Grant No. EP/L015110/1. C.P. and P.J.W.M. are supported by the European Research Council under the European Union's Horizon 2020 research and innovation programme (Microstructured Topological Materials Grant No. 715730). E. Z. acknowledges support from the International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM). Irradiation experiments performed on the SIRIUS platform were supported by the French National Network of Accelerators for Irradiation and Analysis of Molecules and Materials (EMIR&A) under Project No. EMIR 2019 18-7099. The delafossite metal PtCoO2 is among the highest-purity materials known, with low-temperature mean free path up to 5 μm in the best as-grown single crystals. It exhibits a strongly faceted, nearly hexagonal Fermi surface. This property has profound consequences for nonlocal transport within this material, such as in the classic ballistic-regime measurement of bend resistance in mesoscopic squares. Here, we report the results of experiments in which high-energy electron irradiation was used to introduce pointlike disorder into such squares, reducing the mean free path and therefore the strength of the ballistic-regime transport phenomena. We demonstrate that high-energy electron irradiation is a well-controlled technique to cross from nonlocal to local transport behavior and therefore determine the nature and extent of unconventional transport regimes. Using this technique, we confirm the origins of the directional ballistic effects observed in delafossite metals and demonstrate how the strongly faceted Fermi surface both leads to unconventional transport behavior and enhances the length scale over which such effects are important. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the "https://creativecommons.org/licenses/by/4.0/"Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Open access publication funded by the Max Planck Society. Publisher PDF

Temperature variation of the lower critical field in the noncentrosymmetric superconductor $\alpha$-BiPd was probed by local magnetization measurements using Hall micromagnetometry, performed down to 0.3 K in a magnetic field applied along the crystallographic $b$ axis. Below a critical temperature $T_c \simeq$ 3.8 K, a conventional $H_{c1}(T)$ dependence was found, typical for a single-band $s$-wave BCS superconductor. The obtained data imply an absence of spin-triplet component in the superconducting wavefunction and marginal multiband effects in this material, which contradicts some literature reports., Comment: 14 pages, 5 figures

The quantum anomalous Hall effect1,2 is a fundamental transport response of a topological insulator in zero magnetic field. Its physical origin is a result of an intrinsically inverted electronic band structure and ferromagnetism3, and its most important manifestation is the dissipationless flow of chiral charge currents at the edges of the system4, a property that has the potential to transform future quantum electronics5,6. Here, we report a Berry-curvature-driven4,7 anomalous Hall regime at temperatures of several Kelvin in the magnetic topological bulk crystals in which Mn ions self-organize into a period-ordered MnBi2Te4/Bi2Te3 superlattice. Robust ferromagnetism of the MnBi2Te4 monolayers opens a surface gap8–10, and when the Fermi level is tuned to be within this gap, the anomalous Hall conductance reaches an e2/h quantization plateau, which is a clear indication of chiral transport through the edge states. The quantization in this regime is not obstructed by the bulk conduction channels and therefore should be present in a broad family of topological magnets. A three-dimensional topological magnetic superlattice structure exhibits the quantum anomalous Hall effect when the Fermi energy is tuned into the correct energy window.

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Order-disorder transitions between glassy phases are common in nature and yet a comprehensive survey on the entailed structural changes is challenging since the constituents are in the micro-scale. Vortex matter in type-II superconductors is a model system where some of these experimental challenges can be tackled. Samples with point disorder present a glassy transition on increasing the density of vortices. A glassy yet quasi-crystalline phase, the Bragg glass, nucleates at low densities. The vortex glass stable at high densities is expected to be disordered, however its detailed structural properties remained experimentally elusive. Here we show that the vortex glass has large crystallites with in-plane positional displacements growing algebraically and short-range orientational order. Furthermore, the vortex glass has a finite and almost constant correlation length along the direction of vortices, in sharp contrast with strong entanglement. These results are important for the understanding of disorder-driven phase transitions in glassy condensed matter. Fil: Aragón Sánchez, Jazmín. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina Fil: Cortés Maldonado, Raúl. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina Fil: Cejas Bolecek, Néstor René. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina Fil: Rumi, Gonzalo Agustín. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina Fil: Pedrazzini, Pablo. Comisión Nacional de Energía Atómica. Gerencia del Area de Investigación y Aplicaciones No Nucleares. Gerencia de Física (Centro Atómico Bariloche). División Bajas Temperaturas; Argentina Fil: Dolz, Moira Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; Argentina Fil: Nieva, Gladys Leonor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina Fil: Van Der Beek, Cornelis Jacominus. Université Paris Sud; Francia. Centre National de la Recherche Scientifique; Francia Fil: Konczykowski, Marcin. Ecole Polytechnique. Departement Physique. Laboratoire Des Solides Irradies; Francia Fil: Dewhurst, Charles D.. Institut Laue Langevin; Francia Fil: Cubitt, Robert. Institut Laue Langevin; Francia Fil: Kolton, Alejandro Benedykt. Comisión Nacional de Energía Atómica. Gerencia del Área Investigaciones y Aplicaciones No Nucleares. Gerencia de Física (CAB). Grupo de Teoría de Sólidos; Argentina Fil: Pautrat, Alain. École Nationale Supérieure d’Ingénieurs de Caen. Centre de Recherche; Francia Fil: Fasano, Yanina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche | Comisión Nacional de Energía Atómica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología. Unidad Ejecutora Instituto de Nanociencia y Nanotecnología - Nodo Bariloche; Argentina

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe2(As1−xPx)2 (x = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.5 MeV proton irradiation. The superconducting transition temperature, Tc(H), shows an extraordinarily large decrease. It starts at Tc(H=0)≈24K in the pristine sample for both AC and DC measurements, but moves to almost half of that value after moderate irradiation dose. Remarkably, after the irradiation not only Tc moves significantly below the FM transition, its values differ drastically for measurements at different frequencies, ≈16 K in AC measurements and ≈12 K in a DC regime. We attribute such a large difference in Tc to the appearance of the spontaneous internal magnetic field below the FM transition, so that the superconductivity develops directly into the mixed spontaneous vortex-antivortex state where the onset of diamagnetism is known to be frequency-dependent. We also examined the response to the applied DC magnetic fields and studied the annealing of irradiated samples, which almost completely restores the superconducting transition. Overall, our results suggest that in EuFe2(As1−xPx)2 superconductivity is affected by local-moment ferromagnetism mostly via the spontaneous internal magnetic fields induced by the FM subsystem. Another mechanism is revealed upon irradiation where magnetic defects created in ordered Eu2+ lattice act as efficient pairbreakers leading to a significant Tc reduction upon irradiation compared to other 122 compounds. On the other hand, the exchange interactions seem to be weakly screened by the superconducting phase leading to a modest increase of Tm (less than 1 K) after the irradiation drives Tc to below Tm. Our results suggest that FM and SC phases coexist microscopically in the same volume.

Summary Topological insulators (TIs) are bulk insulators with metallic surface states that can be described by a single Dirac cone. However, low-dimensional solids such as nanowires (NWs) are a challenge, due to the difficulty of separating surface contributions from bulk carriers. Fabrication of NWs with high surface-to-volume ratio can be realized by different methods such as chemical vapor transport, molecular beam epitaxy, and electrodeposition. The last method is used in the present work allowing the growth of structures such as p-n junctions, intercalation of magnetic or superconducting dots. We report the synthesis of high-quality TI NW: Bi2Te3, Sb2Te3 and p-n junction via electrodeposition. Structural, morphological, and nanostructure properties of NWs have been investigated by various characterization techniques. Interface structures and lateral heterojunctions (LHJ) in p-n junction NWs has also been made.

Topological insulators (TI) belong to category of phases which go beyond the theory of spontaneous symmetry breaking, well describing classical phases. TI are materials of strong spin-orbit interaction that leads to the inversed band structure. Thus, they belong to different topological class than surrounding “normal” world. Consequently, these materials behave as insulators in their volume while their surface hosts metallic states, that appear as a result of the need to meet boundary conditions. The metallic states have the unusual spin structure described by the Dirac-type Hamiltonian, with the electron spin locked to its momentum. They are protected by the time reversal symmetry, thus are resistant to non-magnetic disturbances. Introducing magnetic impurities breaks the time reversal symmetry, opening the energy gap at the Dirac point and eventually modifying spin texture. In research of magnetically doped TI there are still many challenges and open questions. Here, I will present results of our recent studies of three-dimensional TI from the Bi2-xSbxTe3-ySey family, doped with Mn ions. I will discuss possible locations of Mn impurity in the crystal host lattice, the influence of doping on the crystal structure and magnetic properties. Ferromagnetism was successfully obtained in Bi2Te3 and BiSbTe3 doped with 1.5-2 at. % of Mn, with the Curie temperature of the order of ~ 15 K. The role of free carriers in ferromagnetic interactions is not clear. Ferromagnetism is observed at diluted Mn concentrations suggesting a need for a medium mediating the long-range ferromagnetic order, but the Tc does not scale with the concentration of free carriers. We would like to acknowledge National Science Center, Poland, grant no 2016/21/B/ST3/02565.

MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$ materials system has recently generated strong interest as a natural platform for realization of the quantum anomalous Hall (QAH) state. The system is magnetically much better ordered than substitutionally doped materials, however, the detrimental effects of certain disorders are becoming increasingly acknowledged. Here, from compiling structural, compositional, and magnetic metrics of disorder in ferromagnetic MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$ it is found that migration of Mn between MnBi$_{2}$T$e_{4}$ septuple layers (SLs) and otherwise non-magnetic Bi$_{2}$Te$_{3}$ quintuple layers (QLs) has systemic consequences - it induces ferromagnetic coupling of Mn-depleted SLs with Mn-doped QLs, seen in ferromagnetic resonance as an acoustic and optical resonance mode of the two coupled spin subsystems. Even for a large SL separation (n $\gtrsim$ 4 QLs) the structure cannot be considered as a stack of uncoupled two-dimensional layers. Angle-resolved photoemission spectroscopy and density functional theory studies show that Mn disorder within an SL causes delocalization of electron wavefunctions and a change of the surface bandstructure as compared to the ideal MnBi$_{2}$Te$_{4}/$(Bi$_{2}$Te$_{3}$)$_{n}$. These findings highlight the critical importance of inter- and intra-SL disorder towards achieving new QAH platforms as well as exploring novel axion physics in intrinsic topological magnets., Comment: 16 pages, 5 figures, corrected typos, corrected references to figures for section 5

The authors investigate the impact of atomic defects on cage structure in filled skutterudite LaRu${}_{4}$As${}_{12}$ through high-resolution heat capacity measurements, and show the suppression of the anharmonic phonons and $s$-wave superconductivity with increasing atomic defects.

Robustness to disorder - the defining property of any topological state - has been mostly tested in low-disorder translationally-invariant materials systems where the protecting underlying symmetry, such as time reversal, is preserved. The ultimate disorder limits to topological protection are still unknown, however, a number of theories predict that even in the amorphous state a quantized conductance might yet reemerge. Here we report a directly detected robust spin response in structurally disordered thin films of the topological material Sb2Te3 free of extrinsic magnetic dopants, which we controllably tune from a strong (amorphous) to a weak crystalline) disorder state. The magnetic signal onsets at a surprisingly high temperature (~ 200 K) and eventually ceases within the crystalline state. We demonstrate that in a strongly disordered state disorder-induced spin correlations dominate the transport of charge - they engender a spin memory phenomenon, generated by the nonequilibrium charge currents controlled by localized spins. The negative magnetoresistance (MR) in the extensive spin-memory phase space is isotropic. Within the crystalline state, it transitions into a positive MR corresponding to the weak antilocalization (WAL) quantum interference effect, with a 2D scaling characteristic of the topological state. Our findings demonstrate that these nonequilibrium currents set a disorder threshold to the topological state; they lay out a path to tunable spin-dependent charge transport and point to new possibilities of spin control by disorder engineering of topological materials

In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ in the composition range $0.21\ensuremath{\le}x\ensuremath{\le}0.26$ where anisotropy changes sign. Low-temperature ($\ensuremath{\sim}20$ K) irradiation with relativistic 2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to the conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic ${C}_{2}$ phase above the reentrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $\mathrm{\ensuremath{\Delta}}\ensuremath{\rho}\ensuremath{\equiv}{\ensuremath{\rho}}_{a}(T)\ensuremath{-}{\ensuremath{\rho}}_{b}(T)$ is found in pristine samples with $x=0.213$ and $x=0.219$, without similar signatures in either ${\ensuremath{\rho}}_{a}(T)$ or ${\ensuremath{\rho}}_{b}(T)$. We show that this feature can be reproduced by a phenomenological model of R. M. Fernandes et al. [Phys. Rev. Lett. 107, 217002 (2011)]. We speculate that onset of fluctuations of nematic order on approaching the instability towards the reentrant tetragonal phase contributes to this unusual dependence.

Funding: This project was funded by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant no. 715730, MiTopMat) and also was supported by the Max Planck Society. A.P.M. and R.M. acknowledge support from the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter (EXC 2147). M.D.B., P.M., and V.S. acknowledge studentship funding from the EPSRC under grant no. EP/L015110/1. A.S. was supported by the Israel Science Foundation, the European Research Council (Project LEGOTOP), and the DFG through projectno.CRC-183. M.K. acknowledges support from the SIRIUS irradiation facility through project no. EMIR 2019 18-7099. Microstructures can be carefully designed to reveal the quantum phase of the wave-like nature of electrons in a metal. Here, we report phase-coherent oscillations of out-of-plane magnetoresistance in the layered delafossites PdCoO2 and PtCoO2 The oscillation period is equivalent to that determined by the magnetic flux quantum, h/e, threading an area defined by the atomic interlayer separation and the sample width, where h is Planck's constant and e is the charge of an electron. The phase of the electron wave function appears robust over length scales exceeding 10 micrometers and persisting up to temperatures of T > 50 kelvin. We show that the experimental signal stems from a periodic field modulation of the out-of-plane hopping. These results demonstrate extraordinary single-particle quantum coherence lengths in delafossites. Postprint

We study the local vortex-vortex interaction force fi of the structure frozen during a field-cooling process in an electron-irradiated Bi2Sr2CaCu2O8+δ sample. We compute this magnitude from snapshots of the vortex structure obtained via magnetic decoration experiments at various fields H in the same sample. Since the observed structures correspond to the equilibrium ones frozen at T∼Tirr(H)[1], at this temperature the local modulus of fi roughly equals the local pinning force at the decorated surface of the sample. We estimate the most probable local pinning force from the mode value of the fi(r)distribution, fpm. We found that fpm grows algebraically with H and in electron-irradiated samples is 50-20% smaller than for samples with columnar defects.

We study the entropy jump associated with the first-order vortex melting transition (FOT) in Bi2Sr2CaCu2O8+δ crystals by means of Hall probe magnetometry. The samples present a diluted distribution of columnar defects (CD) introduced by irradiation with Xe ions. The FOT is detected in ac transmittivity measurements as a paramagnetic peak, the height of which is proportional to the enthalpy difference entailed by the transition. By applying the Clausius-Clapeyron relation, we quantify the evolution of the entropy jump Δs as a function of the FOT temperature, TFOT, in both pristine crystals and crystals with CD. On increasing the density of CD, Δs decreases monotonically with respect to values found in pristine samples. The Δs versus TFOT dependence in the case of pristine samples follows reasonably well the theoretical prediction of dominant electromagnetic coupling for a model neglecting the effect of disorder. The data for samples with a diluted distribution of CD are not properly described by such a theoretical model.

The longitudinal magnetoresistance of the array of parallel-oriented bismuth nanowires each 100 nm in diameter grown by electrochemical deposition in nanopores of an Al2O3 membrane has been studied in magnetic fields up to 14 T and at temperatures down to 0.3 K. The resistance increases with the field and reaches a broad maximum in fields about 10 T. An anomalous increase in the resistance in weak fields is qualitatively consistent with the suppression of the antilocalization correction to the resistance, and the maximum is qualitatively associated with the classical size effect. Near the maximum at temperatures below 0.8 K, manifestations of reproducible magneto-oscillations of the resistance, which are periodic in field, have been detected. The period of these oscillations is close to a value corresponding to the passage of the flux quantum hc/e through the section of a nanowire. The Fourier analysis also confirms that the oscillations are periodic. This result is similar to the manifestation the Aharonov–Bohm effect caused by conducting surface states of Dirac fermions occupying L-valleys of bismuth.

Using time-resolved multi-dimensional angle-resolved photoelectron spectroscopy (ARPES) we explore the angular momentum transfer of low energy polarized photons to two prototype topological insulators, Bi 2 Te 2 Se and Bi 2 Se 3 . Our comparative study is based on the analysis of circular dichroism in the photoemission yield of photoexcited Dirac states, and reveals that the spin vector of in-gap Dirac electrons in Bi 2 Te 2 Se presents a more pronounced out-of-plane component compared to that of Bi 2 Se 3 . We show that the multi-dimensional ARPES approach can be effectively used to observe the spin texture of photoexcited topological insulators, and to unambiguously disentangle experimental geometry and matrix element effects.

We report on direct imaging of vortex matter nucleated in micron-sized Bi 2 Sr 2 CaCu 2 O 8 + y superconducting samples that incidentally present a bridge structure. We find that when nucleating vortices in a field-cooling condition the deck of the bridge acts as a converging lens for vortices. By means of Bitter decoration images allowing us to quantify the enhancement of vortex-vortex interaction energy per unit length in the deck of the bridge, we are able to estimate that the deck is thinner than ∼ 0.6 μ m. We show that the structural properties of vortex matter nucleated in micron-sized thin samples are not significantly affected by sample-thickness variations of the order of half a micron, an important information for type-II superconductors-based mesoscopic technological devices.

This paper reports the effect of randomness on a quantum spin liquid state of 1T-TaS${}_{2}$ with a two-dimensional perfect triangular lattice. Systematic measurements of heat capacity and thermal conductivity in pure, Se-substituted, and electron-irradiated crystals reveal the microscopic coexistence of localized orphan spins that form random valence bonds and itinerant spinons that appear to form a Fermi surface.

The temperature dependencies of the lower critical field ${H}_{c1}(T)$ of several filled-skutterudite superconductors were investigated by local magnetization measurements. While ${\mathrm{LaOs}}_{4}{\mathrm{As}}_{12}$ and ${\mathrm{PrRu}}_{4}{\mathrm{As}}_{12}$ exhibit the ${H}_{c1}(T)$ dependencies consistent with the single-band BCS prediction, for ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$ (the superconducting temperature ${T}_{c}=10.4\text{ }\text{ }\mathrm{K}$) with a similar three-dimensional Fermi surface, we observe a sudden increase in ${H}_{c1}(T)$ deep in a superconducting state below about $0.32{T}_{c}$. Remarkably, a rapid rise of ${H}_{c1}(T)$ at approximately the same reduced temperature $0.27{T}_{c}$ is also found for the heavy-fermion compound ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ (${T}_{c}\ensuremath{\simeq}1.78\text{ }\text{ }\mathrm{K}$), in fair accord with the earlier macroscopic study. We attribute the unusual ${H}_{c1}(T)$ dependencies of ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$ and ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$ to a kink structure in their superfluid densities due to different contributions from two nearly decoupled bands. Whereas ${\mathrm{LaRu}}_{4}{\mathrm{As}}_{12}$ is established as a two-band isotropic $s$-wave superconductor, nonsaturating behavior of ${H}_{c1}(T)$ is observed for ${\mathrm{PrOs}}_{4}{\mathrm{Sb}}_{12}$, indicative of an anisotropic structure of a smaller gap. For this superconductor with broken time-reversal symmetry, our findings suggest a superconducting state with multiple symmetries of the order parameters.

Low temperature ($\sim$ 20~K) electron irradiation with 2.5 MeV relativistic electrons was used to study the effect of controlled non-magnetic disorder on the normal and superconducting properties of the type-II Dirac semimetal PdTe$_2$. We report measurements of longitudinal and Hall resistivity, thermal conductivity and London penetration depth using tunnel-diode resonator technique for various irradiation doses. The normal state electrical resistivity follows Matthiessen rule with an increase of the residual resistivity at a rate of $\sim$0.77$ \mu \Omega$cm/$(\textrm{C}/\textrm{cm}^2)$. London penetration depth and thermal conductivity results show that the superconducting state remains fully gapped. The superconducting transition temperature is suppressed at a non-zero rate that is about sixteen times slower than described by the Abrikosov-Gor'kov dependence, applicable to magnetic impurity scattering in isotropic, single-band $s$-wave superconductors. To gain information about the gap structure and symmetry of the pairing state, we perform a detailed analysis of these experimental results based on insight from a generalized Anderson theorem for multi-band superconductors. This imposes quantitative constraints on the gap anisotropies for each of the possible pairing candidate states. We conclude that the most likely pairing candidate is an unconventional $A_{1g}^{+-}$ state. While we cannot exclude the conventional $A_{1g}^{++}$ and the triplet $A_{1u}$, we demonstrate that these states require additional assumptions about the orbital structure of the disorder potential to be consistent with our experimental results, e.g., a ratio of inter- to intra-band scattering for the singlet state significantly larger than one. Due to the generality of our theoretical framework, we think that it will also be useful for irradiation studies in other spin-orbit-coupled multi-orbital systems., Comment: 22 pages, 12 figures

The delafossite metals PdCoO2, PtCoO2, and PdCrO2 are among the highest conductivity materials known, with low-temperature mean free paths of tens of microns in the best as-grown single crystals. A key question is whether these very low resistive scattering rates result from strongly suppressed backscattering due to special features of the electronic structure or are a consequence of highly unusual levels of crystalline perfection. We report the results of experiments in which high-energy electron irradiation was used to introduce point disorder to the Pd and Pt layers in which the conduction occurs. We obtain the cross section for formation of Frenkel pairs in absolute units, and cross-check our analysis with first-principles calculations of the relevant atomic displacement energies. We observe an increase of resistivity that is linear in defect density with a slope consistent with scattering in the unitary limit. Our results enable us to deduce that the as-grown crystals contain extremely low levels of in-plane defects of approximately 0.001%. This confirms that crystalline perfection is the most important factor in realizing the long mean free paths and highlights how unusual these delafossite metals are in comparison with the vast majority of other multicomponent oxides and alloys. We discuss the implications of our findings for future materials research.

Inferring the nature of disorder in the media where elastic objects are nucleated is of crucial importance for many applications but remains a challenging basic-science problem. Here we propose a method to discern whether weak-point or strong-correlated disorder dominates based on characterizing the distribution of the interaction forces between objects mapped in large fields-of-view. We illustrate our proposal with the case-study system of vortex structures nucleated in type-II superconductors with different pinning landscapes. Interaction force distributions are computed from individual vortex positions imaged in thousands-vortices fields-of-view in a two-orders-of-magnitude-wide vortex-density range. Vortex structures nucleated in point-disordered media present Gaussian distributions of the interaction force components. In contrast, if the media have dilute and randomly-distributed correlated disorder, these distributions present non-Gaussian algebraically-decaying tails for large force magnitudes. We propose that detecting this deviation from the Gaussian behavior is a fingerprint of strong disorder, in our case originated from a dilute distribution of correlated pinning centers., Comment: 12 pages, 6 figures

LaRu4As12 with the critical temperature T c = 10.4 K displays several features which point at a non-singlet superconducting order parameter, although the bcc crystal structure of the filled skutterudites does not favour the emergence of multiple energy gaps. LaRu4As12 displays an unexpected enhancement of the lower critical field deep in superconducting state which can be attributed to the existence of two superconducting gaps. At T = 0.4 K, the local magnetization measurements were performed utilizing miniaturized Hall sensors.

A many-particle system must posses long-range interactions in order to be hyperuniform at thermal equilibrium. Hydrodynamic arguments and numerical simulations show, nevertheless, that a three-dimensional elastic-line array with short-ranged repulsive interactions, such as vortex matter in a type-II superconductor, forms at equilibrium a class-II hyperuniform two-dimensional point pattern for any constant-$z$ cross section. In this case, density fluctuations vanish isotropically as $\sim q^{\alpha}$ at small wave-vectors $q$, with $\alpha=1$. This prediction includes the solid and liquid vortex phases in the ideal clean case, and the liquid in presence of weak uncorrelated disorder. We also show that the three-dimensional Bragg glass phase is marginally hyperuniform, while the Bose glass and the liquid phase with correlated disorder are expected to be non-hyperuniform at equilibrium. Furthermore, we compare these predictions with experimental results on the large-wavelength vortex density fluctuations of magnetically decorated vortex structures nucleated in pristine, electron-irradiated and heavy-ion irradiated superconducting BiSCCO samples in the mixed state. For most cases we find hyperuniform two-dimensional point patterns at the superconductor surface with an effective exponent $\alpha_{\text{eff}} \approx 1$. We interpret these results in terms of a large-scale memory of the high-temperature line-liquid phase retained in the glassy dynamics when field-cooling the vortex structures into the solid phase. We also discuss the crossovers expected from the dispersivity of the elastic constants at intermediate length-scales, and the lack of hyperuniformity in the $x\,-y$ plane for lengths $q^{-1}$ larger than the sample thickness due to finite-size effects in the $z$-direction., Comment: 17 pages, 8 figures

We study the field for the penetration of a first vortex, HP, for vortex matter nucleated in micron-sized samples with edges aligned along the nodal and antinodal directions of the d-wave superconducting order parameter of Bi2Sr2CaCu2O8-δ. Here we present evidence that the HP for vortex matter nucleated in mesoscopic samples with edges parallel to the nodal direction is larger than for the antinodal case, ∼72% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce an anomalous Meissner current that increases the Bean-Livingston barrier for vortex penetration. Fil: Dolz, Moira Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; Argentina Fil: Cejas Bolecek, Néstor René. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Puig, Joaquin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Pastoriza, Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Nieva, Gladys Leonor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Guimpel, Julio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Van Der Beek, Cornelis Jacominus. Ecole Polytechnique. Departement Physique. Laboratoire Des Solides Irradies; Francia. Centre National de la Recherche Scientifique; Francia Fil: Konczykowski, M.. Centre National de la Recherche Scientifique; Francia. Ecole Polytechnique. Departement Physique. Laboratoire Des Solides Irradies; Francia Fil: Fasano, Yanina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina

International audience; Uniaxial stress is used to detwin samples of orthorhombic iron based superconductors to study their intrinsic electronic anisotropy. Here we describe the development of a new detwinning setup enabling variable-load stress-detwinning with easy sample mounting/dismounting without the need to re-solder the contacts. It enables the systematic study of the anisotropy evolution as a function of an external parameter when the sample is modified between the measurements. In our case, the external parameter is the dose of 2.5 MeV electron irradiation at low temperature. We illustrate the approach by studying resistivity anisotropy in single crystals of Ba$_{1−x}$K$_x$Fe$_2$As$_2$ at $x$ =0.25 where the much discussed unusual re-entrance of tetragonal C 4 phase, C 4 → C 2 → C 4 , is observed on cooling. With the described technique we found a significant anisotropy increase in $C_2$ phase after electron irradiation with dose of 2.35 C/cm$^2$

Low-temperature (22~K) irradiation with 2.5~MeV electrons was used to study the competition between stripe ${\rm C_2}$ and tetragonal ${\rm C_4}$ antiferromagnetic phases which exist in a narrow doping range around $x=$0.25 in hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$. In nearby compositions outside of this range, at $x=$0.22 and $x=$0.19, the temperatures of both the concomitant orthorhombic/stripe antiferromagnetic transition $T_{\rm C2}$ and the superconducting transition $T_{\rm c}$ are monotonically suppressed by added disorder at similar rates of about 0.1~K/$\mu \Omega$cm, as revealed through using resistivity variation as an intrinsic measure of scattering rate. In a stark contrast, a rapid suppression of the ${\rm C_4}$ phase at the rate of 0.24 K/$\mu \Omega \cdot$cm is found at $x=$0.25. Moreover, this suppression of the ${\rm C_4}$ phase is accompanied by unusual disorder-induced stabilization of the ${\rm C_2}$ phase, determined by resistivity and specific heat measurements. The rate of the ${\rm C_4}$ phase suppression is notably higher than the suppression rate of the spin-vortex phase in the Ni-doped CaKFe$_4$As$_4$ (0.16 K/$\mu \Omega$cm).

International audience; One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi 2 Te 3 , we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time-and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states.

The interplay between superconductivity and charge-density wave (CDW) in 2H-NbSe2 is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing long-range orders, and reveal the underlying interactions that give rise to them. Here we introduce disorder by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, the superconducting transition temperature, Tc, varies non-monotonically, whereas the CDW transition temperature, TCDW, monotonically decreases and becomes unresolvable above a critical irradiation dose where Tc drops sharply. Our results imply that the CDW order initially competes with superconductivity, but eventually assists it. We argue that at the transition where the long-range CDW order disappears, the cooperation with superconductivity is dramatically suppressed. X-ray scattering and Hall resistivity measurements reveal that the short-range CDW survives above the transition. Superconductivity persists to much higher dose levels, consistent with fully gapped superconductivity and moderate interband pairing., The interplay between superconductivity and charge density wave (CDW) in 2H-NbSe2 is still not fully understood. Here, Cho et al. use controlled disorder to probe the interplay between these two phases in 2H-NbSe2 and find that superconductivity initially competes with CDW but eventually long-range CDW order assists superconductivity.

Using femtosecond time- and angle-resolved photoemission spectroscopy, we explore the out-of-equilibrium dynamics of surface fermions in the topological system ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{2}\mathrm{Se}$. We show that the presence of localized states from defects at the surface is one of the key material parameters undergirding the long relaxation time of photoexcited Dirac electrons lying in the projected band gap of the bulk-insulating pristine compound. Doping this ternary compound with Sn substituting on Bi sites, while desirably increasing the resistivity at low temperatures decreases decay times of the excited homologous Dirac electrons. On the basis of these observations, we argue that long relaxation times can be ultimately controlled by a charge transfer to the surface.

The recent discovery of superconductivity in ${\mathrm{NaSn}}_{2}{\mathrm{As}}_{2}$ 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 $\ensuremath{\lambda}(T)$ in single crystals of ${\mathrm{NaSn}}_{2}{\mathrm{As}}_{2}$ down to $\ensuremath{\sim}40$ mK. We find a very long penetration depth $\ensuremath{\lambda}(0)=960\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, which is strongly enhanced from the estimate of first-principles calculations. This enhancement comes from a short mean free path $\ensuremath{\ell}\ensuremath{\approx}1.7\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, indicating atomic scale disorder possibly associated with the valence-skipping states of Sn. The temperature dependence of superfluid density is fully consistent with the conventional fully gapped $s$-wave state in the dirty limit. These results suggest that ${\mathrm{NaSn}}_{2}{\mathrm{As}}_{2}$ is an ideal material to study quantum phase fluctuations in strongly disordered superconductors with its controllable dimensionality.

Controlled pointlike disorder introduced by 2.5-MeV electron irradiation was used to probe the superconducting state of single crystals of $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$ superconductor at $x=0$ and 0.05 doping levels. Both compositions show an increase of the residual resistivity and a decrease of the superconducting transition temperature, ${T}_{c}$, at the rate of $d{T}_{c}/d\ensuremath{\rho}({T}_{c})\ensuremath{\approx}0.19$ K/($\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$) for $x=0$ and 0.38 K/($\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$) for $x=0.05$, respectively. In the Ni-doped compound ($x=0.05$), the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of $d{T}_{N}/d\ensuremath{\rho}({T}_{N})\ensuremath{\approx}$ 0.16 K/($\ensuremath{\mu}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$). The low-temperature variation of London penetration depth is well approximated by the power-law function, $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}(T)=A{T}^{n}$, with $n\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}2.5$ for $x=0$ and $n\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}1.9$ for $x=0.05$ in the pristine state. Detailed analysis of $\ensuremath{\lambda}(T)$ and ${T}_{c}$ evolution with disorder is consistent with two effective nodeless energy gaps in the density of states due to robust ${s}_{\ifmmode\pm\else\textpm\fi{}}$ pairing. Overall the behavior of $\mathrm{CaK}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$ at $x=0$ is similar to a slightly overdoped ${\mathrm{Ba}}_{1\ensuremath{-}y}{\mathrm{K}}_{y}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ at $y\ensuremath{\approx}$ 0.5, and at $x=0.05$ to an underdoped composition at $y\ensuremath{\approx}$ 0.2.

Magnetic penetration depth measurements show that the superconducting gap in this iconic heavy-fermion superconductor is robust against the introduction of disorder, ruling out some superconductivity mechanisms as explanations.

We study the effect of quenched disorder in the thermodynamic magnitudes entailed in the first-order vortex phase transition of the extremely layered Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 + \delta}$ compound. We track the temperature-evolution of the enthalpy and the entropy-jump at the vortex solidification transition by means of AC local magnetic measurements. Quenched disorder is introduced to the pristine samples by means of heavy-ion irradiation with Pb and Xe producing a random columnar-track pins distribution with different densities (matching field $B_{\Phi}$). In contrast with previous magneto-optical reports, we find that the first-order phase transition persists for samples with $B_{\Phi}$ up to 100\,Gauss. For very low densities of quenched disorder (pristine samples), the evolution of the thermodynamic properties can be satisfactorily explained considering a negligible effect of pinning and only electromagnetic coupling between pancake vortices lying in adjacent CuO planes. This description is not satisfactory on increasing magnitude of quenched disorder.

In many classes of unconventional superconductors, the question of whether the superconductivity is enhanced by the quantum-critical fluctuations on the verge of an ordered phase remains elusive. One of the most direct ways of addressing this issue is to investigate how the superconducting dome traces a shift of the ordered phase. Here, we study how the phase diagram of the iron-based superconductor BaFe$_2$(As$_{1-x}$P$_x$)$_2$ changes with disorder via electron irradiation, which keeps the carrier concentrations intact. With increasing disorder, we find that the magneto-structural transition is suppressed, indicating that the critical concentration is shifted to the lower side. Although the superconducting transition temperature $T_c$ is depressed at high concentrations ($x\gtrsim$0.28), it shows an initial increase at lower $x$. This implies that the superconducting dome tracks the shift of the antiferromagnetic phase, supporting the view of the crucial role played by quantum-critical fluctuations in enhancing superconductivity in this iron-based high-$T_c$ family., Comment: 5 pages, 4 figures

We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the first-order transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this "healing-length" distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Bragg-glass phase. This suggests that the healing length may be a key quantity to model confinement effects in the first-order transition of extremely layered vortex nanocrystals. Fil: Cejas Bolecek, Néstor René. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Dolz, Moira Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina Fil: Pastoriza, Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Konczykowski, M.. Centre National de la Recherche Scientifique; Francia Fil: Van Der Beek, Cornelis Jacominus. Centre National de la Recherche Scientifique; Francia Fil: Kolton, Alejandro Benedykt. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Fasano, Yanina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents., Surface currents in topological insulators can be controlled by light, but the underlying mechanisms are not well understood. Here, Braun et al. report an ultrafast shift photocurrent at the surface of Ca-doped Bi2Se3, whereas injection currents are much smaller than expected from asymmetric depopulation of the Dirac cone.

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba1-x K x )Fe2As2 has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping (x ≈ 0.4) to becoming nodal at x > 0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, λ(T), across the full phase diagram for different concentrations of point-like defects introduced by 2.5-MeV electron irradiation. Fitting the low-temperature variation with the power law, Δλ ~ Tn , we find that the exponent n is the highest and the Tc suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around x ≃ 0.8, indicating the onset of nodal behavior. Our analysis using a self-consistent t-matrix approach suggests the ubiquitous and robust nature of s± pairing in IBSs and argues against a previously suggested transition to a d-wave state near x = 1 in this system.

Bulk-related conduction electron spin resonance and conduction hole spin resonance were investigated in $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$, a three-dimensional topological insulator. Electrons in the conduction band and holes in the valence band both have spin \textonehalf{}. The effective $g$-factors for conduction electrons are equal to 27.3 \ifmmode\pm\else\textpm\fi{} 0.15 for magnetic field parallel to the c axis and 19.48 \ifmmode\pm\else\textpm\fi{} 0.07 for magnetic field perpendicular to the c axis, whereas for conduction holes 29.90 \ifmmode\pm\else\textpm\fi{} 0.09 for magnetic field parallel and 18.96 \ifmmode\pm\else\textpm\fi{} 0.04 for magnetic field perpendicular to the c axis, respectively. Nonparabolicity effects were not observed in the investigated low carrier concentration range, below $8\ifmmode\times\else\texttimes\fi{}{10}^{17}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. Large $g$-factors, higher by an order of magnitude than the free electron value, are due to strong spin-orbit interactions in $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$. The striking similarity of the spin resonances due to conduction electrons and holes confirms the peculiar symmetry between the conduction and valence bands of $\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$, both having similar effective masses and spin character.