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Titanium nitride is a material of interest for many superconducting devices such as nanowire microwave resonators and photon detectors. Thus, controlling the growth of TiN thin films with desirable properties is of high importance. This work aims to explore effects in ion beam-assisted sputtering (IBAS), were an observed increase in nominal critical temperature and upper critical fields are in tandem with previous work on Niobium nitride (NbN). We grow thin films of titanium nitride by both, the conventional method of DC reactive magnetron sputtering and the IBAS method, to compare their superconducting critical temperatures $$T_{c}$$ T c as functions of thickness, sheet resistance, and nitrogen flow rate. We perform electrical and structural characterizations by electric transport and x-ray diffraction measurements. Compared to the conventional method of reactive sputtering, the IBAS technique has demonstrated a 10% increase in nominal critical temperature without noticeable variation in the lattice structure. Additionally, we explore the behavior of superconducting $$T_c$$ T c in ultra-thin films. Trends in films grown at high nitrogen concentrations follow predictions of mean-field theory in disordered films and show suppression of superconducting $$T_c$$ T c due to geometric effects, while nitride films grown at low nitrogen concentrations strongly deviate from the theoretical models.

We report on temperature-dependent size and anisotropy of the Fermi pockets in graphite revealed by magnetotransport measurements. The magnetoresistances obtained in fields along the c-axis obey an extended Kohler's rule, with the carrier density following prediction of a temperature-dependent Fermi energy, indicating a change in the Fermi pocket size with temperature. The angle-dependent magnetoresistivities at a given temperature exhibit a scaling behavior. The scaling factor that reflects the anisotropy of the Fermi surface is also found to vary with temperature. Our results demonstrate that temperature-driven changes in Fermi surface can be ubiquitous and need to be considered in understanding the temperature-dependent carrier density and magnetoresistance anisotropy in semimetals., To appear in Physical Review B

We demonstrate microwave-mediated distant magnon-magnon coupling on a superconducting circuit platform, incorporating chip-mounted single-crystal Y$_3$Fe$_5$O$_{12}$ (YIG) spheres. Coherent level repulsion and dissipative level attraction between the magnon modes of the two YIG spheres are demonstrated. The former is mediated by cavity photons of a superconducting resonator, and the latter is mediated by propagating photons of a coplanar waveguide. Our results open new avenues towards exploring integrated hybrid magnonic networks for coherent information processing on a quantum-compatible superconducting platform., Comment: 6 pages, 4 figures, Accepted in Phys. Rev. Lett

Recently, anomalies in the temperature dependences of the carrier density and/or mobility derived from analysis of the magnetoresistivities using the conventional two-band model have been used to unveil intriguing temperature-induced Lifshitz transitions in various materials. For instance, two temperature-driven Lifshitz transitions were inferred to exist in the Dirac nodal-line semimetal ZrSiSe, based on two-band model analysis of the Hall magnetoconductivities where the second band exhibits a change in the carrier type from holes to electrons when the temperature decreases below T = 106 K and a dip is observed in the mobility versus temperature curve at T = 80 K. Here, we revisit the experiments and two-band model analysis on ZrSiSe. We show that the anomalies in the second band may be spurious, because the first band dominates the Hall magnetoconductivities at T > 80 K, making the carrier type and mobility obtained for the second band from the two-band model analysis unreliable. That is, care must be taken in interpreting these anomalies as evidences for temperature-driven Lifshitz transitions. Our skepticism on the existence of such phase transitions in ZrSiSe is further supported by the validation of the Kohler's rule for magnetoresistances at temperatures below 180 K. This work showcases potential issues in interpreting anomalies in the temperature dependence of the carrier density and mobility derived from the analysis of magnetoconductivities or magnetoresistivities using the conventional two-band model.

We present a perspective on a new critical-current-by-design paradigm to tailor and enhance the current-carrying capacity of applied superconductors. Critical current by design is based on large-scale simulations of vortex matter pinning in high-temperature superconductors and has qualitative and quantitative predictive powers to elucidate vortex dynamics under realistic conditions and to propose vortex pinning defects that could enhance the critical current, particularly at high magnetic fields. The simulations are validated with controlled experiments and demonstrate a powerful tool for designing high-performance superconductors for targeted applications., 20 pages, 11 figures

Significance Lossless transport is the Holy Grail of energy science in general and superconductivity research in particular. The main obstacle is the dissipative motion of Abrikosov vortices, which can be reduced or eliminated by pinning at nonsuperconducting defects. Pinning effectiveness nontrivially depends on various factors such as the shape, concentration, and spatial distribution of defects, rendering the optimization of the vortex pinning landscape highly difficult. Here, we use concepts from biological evolution to develop an efficient strategy for vortex pinning improvement. We replace natural selection with targeted selection, where only pinning configurations with better vortex immobilization survive. In combination with high-performance numerical algorithms, it allows us to dynamically evolve the defect landscape into the best possible pinning configuration with maximal lossless current., The ability of type II superconductors to carry large amounts of current at high magnetic fields is a key requirement for future design innovations in high-field magnets for accelerators and compact fusion reactors, and largely depends on the vortex pinning landscape comprised of material defects. The complex interaction of vortices with defects that can be grown chemically, e.g., self-assembled nanoparticles and nanorods, or introduced by postsynthesis particle irradiation precludes a priori prediction of the critical current and can result in highly nontrivial effects on the critical current. Here, we borrow concepts from biological evolution to create a vortex pinning genome based on a genetic algorithm, naturally evolving the pinning landscape to accommodate vortex pinning and determine the best possible configuration of inclusions for two different scenarios: a natural evolution process initiating from a pristine system and one starting with preexisting defects to demonstrate the potential for a postprocessing approach to enhance critical currents. Furthermore, the presented approach is even more general and can be adapted to address various other targeted material optimization problems.

We use irradiation with 50-MeV Cu-ions to create vortex pinning defects in high-temperature superconducting Y1Ba2Cu3O7-x coated conductors using a beam-rastering approach that allows for the uniform irradiation of large ample areas. Our samples contain barium zirconate nanorods as pre-existing vortex pinning defects. By irradiating the samples at angles of 0o, 15oand 30o from the crystallographic c-axis we explore the interplay between pre-existing and irradiation-induced pinning and find that irradiation at 30o leads to a moderate enhancement of Jc at 5 K at high fields (greater than 2 Tesla). In contrast, Jc was suppressed for all temperatures and fields for other angles of irradiation. Optimized particle irradiation procedures offer a way for improving the performance of high-temperature superconducting wires for use in high magnetic fields without the need for changing wire synthesis protocols.

We designed and characterized a microstrip pattern of planar patch antennas compatible with a cuprate high-T

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.

A notable phenomenon in topological semimetals is the violation of Kohler$^,$s rule, which dictates that the magnetoresistance $MR$ obeys a scaling behavior of $MR = f(H/\rho_0$), where $MR = [\rho_H-\rho_0]/\rho_0$ and $H$ is the magnetic field, with $\rho_H$ and $\rho_0$ being the resistivity at $H$ and zero field, respectively. Here we report a violation originating from thermally-induced change in the carrier density. We find that the magnetoresistance of the Weyl semimetal, TaP, follows an extended Kohler$^,$s rule $MR = f[H/(n_T\rho_0)]$, with $n_T$ describing the temperature dependence of the carrier density. We show that $n_T$ is associated with the Fermi level and the dispersion relation of the semimetal, providing a new way to reveal information on the electronic bandstructure. We offer a fundamental understanding of the violation and validity of Kohler$^,$s rule in terms of different temperature-responses of $n_T$. We apply our extended Kohler$^,$s rule to BaFe$_2$(As$_{1-x}$P$_x$)$_2$ to settle a long-standing debate on the scaling behavior of the normal-state magnetoresistance of a superconductor, namely, $MR$ ~ $tan^2\theta_H$, where $\theta_H$ is the Hall angle. We further validate the extended Kohler$^,$s rule and demonstrate its generality in a semiconductor, InSb, where the temperature-dependent carrier density can be reliably determined both theoretically and experimentally.

The physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full-field microscopy and resonant magnetic X-ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin-orbit Mott insulator Sr

Recently, the niobium (Nb) doped topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ layers, has been shown to exhibit both superconductivity with ${T}_{c}\ensuremath{\simeq}3$ K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth ${\ensuremath{\lambda}}_{\mathrm{eff}}\left(T\right)$ in the layered topological superconductor candidate ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. The exponential temperature dependence of ${\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature ${T}_{c}=2.9\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the gap to ${T}_{c}$ ratio $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}=3.95(19)$. We also reveal that the ratio ${T}_{c}/{\ensuremath{\lambda}}_{\mathrm{eff}}^{\ensuremath{-}2}$ 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 $\ensuremath{\mu}\mathrm{SR}$ 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 ${\mathrm{Nb}}_{0.25}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$.

Real-space modulated charge density waves (CDW) are a ubiquitous feature in many families of superconductors. In particular, how CDW relates to superconductivity is an active and open question that has recently gathered much interest since CDWs have been discovered in many cuprates superconductors. Here we show that disorder induced by proton irradiation is a full-fledged tuning parameter that can bring essential information to answer this question as it affects CDW and superconductivity with different and unequivocal mechanisms. Specifically, in the model CDW superconductor ${\mathrm{Lu}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ that develops a 1D CDW below 77 K and s-wave superconductivity below 4 K, we show that disorder enhances the superconducting critical temperature ${T}_{\mathrm{c}}$ and ${H}_{\mathrm{c}2}$ while it suppresses the CDW. Discussing how disorder affects both superconductivity and the CDW, we make a compelling case that superconductivity and CDW are competing for electronic density of states at the Fermi level in ${\mathrm{Lu}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$, and we reconcile the results obtained via the more common tuning parameters of pressure and doping. Owing to its prototypical, 1D, Peierls type CDW and the s-wave, weak-coupling nature of its superconductivity, this irradiation study of ${\mathrm{Lu}}_{5}{\mathrm{Ir}}_{4}{\mathrm{Si}}_{10}$ provides the basis to understand and extend such studies to the more complex cases of density waves and superconductivity coexistence in heavy fermions, Fe-based, or cuprates superconductors.

The high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 contains stacked ‘intrinsic’ Josephson junctions, with unrivaled packing density and a high superconducting gap energy. Cuboid ‘mesa’ devices constructed from this material are consequently a promising technology for coherent, continuouswave radiation in the ‘terahertz gap’ range, spanning from approximately 0.3-1.5 THz. A key issue for practical applications of such devices is their cryocooling requirements, and it is therefore highly desirable to optimize their performance at temperatures that can be achieved by nitrogen cryogenics. Here we report generation of 0.13 milliwatts of coherent emission power at 0.461 THz, at a bath temperature of 77.4 Kelvin. This was achieved by exciting the (3, 0) cavity mode of a stack containing 579 junctions, and with T c of 86.5 Kelvin. In order to minimize selfheating, the THz source was mounted on a copper substrate using PbSn solder. We will discuss the choice of mesa dimensions and cavity mode, and implications for the design of devices which are intended to operate close to the material’s superconducting critical temperature.

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

A friendly coexistence of superconductivity and magnetism is quite an unusual physical phenomenon. However, the authors find that in the new high-T${}_{c}$ magnetic superconductor RbEuFe${}_{4}$As${}_{4}$ the magnetic and superconducting subsystems show a remarkable cooperative response. In the external magnetic field, the magnetic Eu sublattice acts as an internal pump of the magnetic flux, while the superconducting component works as a valve controlling the flux entry through the pinning of flux-carrying Abrikosov vortices and establishes the self-organized critical state.

We investigate the effect of Ni doping on the Fe site in single crystals of the magnetic superconductor $\mathrm{Rb}\mathrm{Eu}{\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ 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 dopant atoms while the shape of the curve and thus the electronic structure appear largely unchanged. The observed step $\mathrm{\ensuremath{\Delta}}C/{T}_{c}$ at the superconducting transition decreases strongly for increasing Ni doping in agreement with expectations based on a model of multiband superconductivity and strong interband pairing. The upper critical field slopes are reduced upon Ni doping for in-plane 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 Berezinskii-Kosterlitz-Thouless behavior close to the transition temperature ${T}_{m}$ for all doping levels. The transition temperature is essentially unchanged upon doping. The in-plane 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.

We investigate the effect of the ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ mesa sidewall on the spontaneous synchronization of intrinsic Josephson junctions and concomitant terahertz radiation. We develop a lithography technique to control the sidewall angles. Using a Fourier interferometer, we characterize the radiation emitted from a series of mesas with different sidewall angles. We determine that a uniform distribution of the oscillation frequencies is essential to produce coherent radiation outside the mesa. It is also noted that in the resonant state, the input excess energy is transferred to the excitation of the coherent state.

Intrinsic Josephson junctions in high-temperature superconductor Bi2Sr2CaCu2O8 are known for their capability to emit high-power terahertz photons with widely tunable frequencies. Hotspots, as inhomogeneous temperature distributions across the junctions, are believed to play a critical role in synchronizing the gauge-invariant phase difference among the junctions, so as to achieve coherent strong emission. Previous optical imaging techniques have indirectly suggested that the hotspot temperature can go higher than the superconductor critical temperature. However, such optical approaches often disturb the local temperature profile and are too slow for device applications. In this paper, we demonstrate an on-chip in situ sensing technique that can precisely quantify the local temperature profile. This is achieved by fabricating a series of micro "sensor" junctions on top of an "emitter" junction and measuring the critical current on the sensors versus the bias current applied to the emitter. This fully electronic on-chip design could enable efficient close-loop control of hotspots in BSCCO junctions and significantly enhance the functionality of superconducting terahertz emitters., 6 pages, 5 figures

Fascination with glassy states has persisted since Fisher introduced the vortex-glass as a new thermodynamic phase that is a true superconductor that lacks conventional long-range order. Though Fisher’s original model considered point disorder, it was later predicted that columnar defects (CDs) could also induce glassiness — specifically, a Bose-glass phase. In YBa2Cu3O7−x (YBCO), glassy states can cause distinct behavior in the temperature (T ) dependent rate of thermally activated vortex motion (S). The vortex-glass state produces a plateau in S(T ) whereas a Bose-glass can transition into a state hosting vortex excitations called double-kinks that can expand, creating a large peak in S(T ). Although glass phases have been well-studied in YBCO, few studies exist of other materials containing CDs that could contribute to distinguishing universal behavior. Here, we report on the effectiveness of CDs tilted ~30° from the c-axis in reducing S in a NbSe2 crystal. The magnetization is 5 times higher and S is minimized when the field is parallel to the defects versus aligned with the c-axis. We see signatures of glassiness in both field orientations, but do not observe a peak in S(T ) nor a plateau at values observed in YBCO. Finally, we discuss the possibility that competing disorder induces a field-orientation-driven transition from a Bose-glass to an anisotropic glass involving both point and columnar disorder.

RbEuFe4As4 exhibits both superconducting order in the FeAs layers and ferromagnetic order in the Eu layers, providing a good platform to study the interaction and microscopic coexistence of these two traditionally incompatible orders. Growing high-quality RbEuFe4As4 single crystals is essential to investigate these phenomena at a deeper level. Here we report the successful growth of the RbEuFe4As4 single crystals with millimeter-size dimensions using a RbAs flux. The high-quality crystals were characterized via resistivity, magnetization, and heat capacity measurements. Single crystal X-ray diffraction data refinements reveal almost regular FeAs4 tetrahedra (As1–Fe–As1 = 108.60(7)°, As2–Fe–As2 = 109.01(8)°, and As1–Fe–As2 = 109.81(2)°) in RbEuFe4As4, providing structural support for the highest superconducting transition temperature (Tc = 36.8 K) among all known AAeFe4As4 (A = K, Rb, Cs; Ae = Ca, Sr, Eu) compounds. Our flux method using RbAs can also be employed to grow other desired transition metal comp...

The use of second-generation high-temperature superconducting-coated conductors enables an enhancement of the performance of undulator magnets. However, preventing the motion of the wire and providing sufficient conduction cooling to the winding stacks have remained challenges. In this study, we have evaluated epoxy impregnation techniques to address these issues. Epoxy resin was prepared with different nanopowders and the effect on the performance of the undulator coil pack was investigated. All epoxy impregnated coils showed smaller n values and some degree of deterioration of the critical current I c . The I c degradation was most pronounced for epoxy mixed with high aspect ratio multiwalled carbon nanotubes (MWCNTs). It has been found that the crack formation in the epoxy results in plastic deformation of the copper stabilizer layer, which causes the underlying ceramic REBCO superconducting layer to crack resulting in degradation of the superconducting tape performance. Careful adjustment of epoxy thickness surrounding the superconductor and the powder ratio in the epoxy eliminate the performance degradation.

Magnetic topological semimetals, the latest member of topological quantum materials, are attracting extensive attention as they may lead to topologically driven spintronics. Currently, magnetotransport investigations on these materials are focused on the anomalous Hall effect. Here, we report on the magnetoresistance anisotropy of topological semimetal CeBi, which has tunable magnetic structures arising from localized Ce $4f$ electrons and exhibits both negative and positive magnetoresistances, depending on the temperature. We found that the angle dependence of the negative magnetoresistance, regardless of its large variation with the magnitude of the magnetic field and with temperature, is solely dictated by the field-induced magnetization that is orientated along a primary crystalline axis and flops under the influence of a rotating magnetic field. The results reveal the strong interaction between conduction electrons and magnetization in CeBi. They also indicate that magnetoresistance anisotropy can be used to uncover the magnetic behavior and the correlation between transport phenomena and magnetism in magnetic topological semimetals.

Thermal fluctuations always melt the vortex lattice in type-II superconductors at temperatures below the mean-field transition. The extent of the intermediate vortex-liquid state varies widely among different materials. Exploring the vortex-lattice melting in the magnetic iron arsenide RbEuFe${}_{4}$As${}_{4}$ via calorimetry and transport, the authors demonstrate here that the vortex liquid occupies a significant portion of the phase diagram and that the location of the melting line agrees with theoretical predictions without fitting parameters. Comparison with other materials reveals the universality of melting behavior.

We present a study of the upper critical field ${H}_{c2}$ of pristine and proton-irradiated ${\mathrm{RbEuFe}}_{4}{\mathrm{As}}_{4}$ crystals in pulsed magnetic fields of up to 65 T. The data for ${H}_{c2}$ reveal pronounced downwards curvature, particularly for the in-plane field orientation, and a superconducting anisotropy that decreases with decreasing temperature. These features are indicative of Pauli paramagnetic limiting. For the interpretation of these data, we use a model of a clean single-band superconductor with an open Fermi surface in the shape of a warped cylinder, which takes into account Zeeman spin splitting. Fits to the data reveal that the in-plane upper critical field is Pauli paramagnetic limited, while the out-of-plane upper critical field is orbitally limited and that the orbital and paramagnetic fields have opposite anisotropies. A consequence of this particular combination is the unusual inversion of the anisotropy ${H}_{c2}^{ab}l{H}_{c2}^{c}$ of the irradiated sample at temperatures below 10 K. The fits also yield an in-plane Maki parameter ${\ensuremath{\alpha}}_{M}^{110}\ensuremath{\approx}2.6$ exceeding the critical value for the formation of the Fulde-Ferrell-Larkin-Ovchinnikov state. Nevertheless, the current measurements did not reveal direct evidence for the occurrence of this state.

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).

We report an unusual enhancement of the magnetic induction in single crystals of the magnetic superconductor ${\mathrm{RbEuFe}}_{4}{\mathrm{As}}_{4}$, highlighting the interplay between superconducting and magnetic subsystems in this material. Contrary to the conventional Meissner expulsion of magnetic flux below the superconducting transition temperature, we observe a substantial boost of the magnetic flux density upon approaching the magnetic transition temperature, ${T}_{m}$. Direct imaging of the flux evolution with a magneto-optical technique, shows that the magnetic subsystem serves as an internal magnetic flux pump, drawing Abrikosov vortices from the surface, while the superconducting subsystem controls their conveyance into the bulk of the magnetic superconductor via a peculiar self-organized critical state.

We report a type-II intermetallic superconductor ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$ with a transition temperature $({T}_{\mathrm{c}})$ of \ensuremath{\sim}5.8 K, which is confirmed by zero resistivity, diamagnetic magnetic susceptibility, and specific-heat jump. Single crystals of ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$ were grown from a reactive indium flux. ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$ crystallizes in the tetragonal space group $P4/m$ and features a $[\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}]$ polyanionic network with Y atoms located in three different channels. The upper critical fields of ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$ at 0 K are determined to be \ensuremath{\sim}5.3 and 2.4 T along the $c$ axis and the $ab$ plane, respectively. The estimated coherence length along the $c$ axis (\ensuremath{\sim}174 \AA{}) is much larger than the estimated mean free path along the $c$ axis (\ensuremath{\sim}29 \AA{}) in ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$, placing its superconductivity in the so-called dirty regime. The compound exhibits a large superconducting specific-heat jump $\phantom{\rule{4pt}{0ex}}\frac{\mathrm{\ensuremath{\Delta}}C}{{\ensuremath{\gamma}}_{e}{T}_{\mathrm{c}}}\ensuremath{\approx}2.4$, significantly well above the weak-coupling Barden-Cooper-Schrieffer theoretical value of 1.43 and pointing to a strong-coupling scenario in ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$. Density-functional-theory calculations show that the density of states in ${\mathrm{Y}}_{7}\mathrm{R}{\mathrm{u}}_{4}\mathrm{InG}{\mathrm{e}}_{12}$ exhibits a broad peak near the Fermi level which mainly derives from $\mathrm{Y}\text{\ensuremath{-}}4d,\mathrm{Ru}\text{\ensuremath{-}}4d$, and $\mathrm{Ge}\text{\ensuremath{-}}4p$ states.

We designed and characterized a microstrip pattern of planar patch antennas compatible with a cuprate high-T c superconducting terahertz emitter. Antenna parameters were optimized using an electromagnetic simulator. We observed repeatable sub-terahertz emissions from each mesa fabricated on identical Bi2Sr2CaCu2O8+δ base crystals in a continuous frequency range of 0.35–0.85 THz. Although there was no significant output power enhancement, a plateau behavior at a fixed frequency was observed below 40 K, indicating moderate impedance matching attributable to the ambient microstrip pattern. A remarkably anisotropic polarization at an axial ratio of up to 16.9 indicates a mode-locking effect. Our results enable constructing compactly assembled, monolithic, and broadly tunable superconducting terahertz sources.

We demonstrate a twofold increase in the in-field critical current of AMSC's standard 2G coil wire by irradiation with 18-MeV Au ions. The optimum pinning enhancement is achieved with a dose of $6\times 10^{11}\ \text{Au}\ \text{ions/cm}^{2}$ . Although the 77 K, self-field critical current is reduced by about 35%, the in-field critical current (H//c) shows a significant enhancement between 4 and 50 K in fields > 1 T. The process was used for the roll-to-roll irradiation of AMSC's standard 46-mm-wide production coated conductor strips, which were further processed into standard copper laminated coil wire. The long-length wires show the same enhancement as attained with short static irradiated samples. The roll-to-roll irradiation process can be incorporated in the standard 2G wire manufacturing, with no modifications to the current process. The enhanced performance of the wire will benefit rotating machine and magnet applications.

YBa2Cu3O7-based coated conductors (CCs) achieve the highest critical current densities (J c) of any known superconductor and are a key technology for applications such as rotatory machines, high-field magnets and power transmission. Incorporation of nano-sized non-superconducting second phases as additional vortex pinning centers has been considered the most amenable route to further enhance J c at an industrial scale, and has been successfully used in commercial CCs. The resulting pinning landscape is quite complex, with both synergistic and competing interactions among the various types of defects. Particle irradiation, on the other hand, allows for a controlled post-processing incorporation of a well-defined defect morphology. We have previously shown that irradiation with protons and other light ions can further enhance the in-field J c in commercial state-of-the-art CCs. Here we develop a combined irradiation process that increases J c above values previously achieved by irradiating with only one species. Our new approach involves sequentially irradiating with 250 MeV Au ions and 4 MeV protons. For example, at T∼ 27 K (liquid neon) and µ 0 H∼ 4 T, a region of interest for rotatory machines applications, we obtain J c ∼ 5 MA cm−2, which is about 40% higher than the values produced by the individual irradiations. Finally, we conclude that this is due to the synergistic pinning effects of the introduced splayed, non-uniform columnar defects and small clusters.

Spontaneous rotational-symmetry breaking in the superconducting state of doped ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ has attracted significant attention as an indicator for topological superconductivity. In this paper, high-resolution calorimetry of the single-crystal ${\mathrm{Sr}}_{0.1}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ provides unequivocal evidence of a twofold rotational symmetry in the superconducting gap by a bulk thermodynamic probe, a fingerprint of nematic superconductivity. The extremely small specific heat anomaly resolved with our high-sensitivity technique is consistent with the material's low carrier concentration proving bulk superconductivity. The large basal-plane anisotropy of ${H}_{c2}$ (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{exp}}=3.5$) is attributed to a nematic phase of a two-component topological gap structure $\mathbf{\ensuremath{\eta}}=({\ensuremath{\eta}}_{1},{\ensuremath{\eta}}_{2})$ and caused by a symmetry-breaking energy term $\ensuremath{\delta}(|{\ensuremath{\eta}}_{1}{|}^{2}\ensuremath{-}|{\ensuremath{\eta}}_{2}{|}^{2}){T}_{c}$. A quantitative analysis of our data excludes more conventional sources of this twofold anisotropy and provides an estimate for the symmetry-breaking strength $\ensuremath{\delta}\ensuremath{\approx}0.1$, a value that points to an onset transition of the second order parameter component below 2 K.

We report detailed specific heat measurements on the recently discovered magnetic superconductor RbEuFe$_4$As$_4$. We investigated the superconducting transition at 37K and extract the phase boundary for in and out-of plane fields resulting in an anisotropy ratio of 1.8. An unusual cusp-like feature in the calorimetric data near 14.9K marks the onset of a magnetic phase. Studying the effect of small fields along the crystallographic $c$ axis, we resolve a shift in the cusp position moving to lower temperatures. For in-plane fields the cusp rapidly disappears and a broad shoulder that shifts to higher temperatures. We are able to reproduce our measured calorimetry data quantitatively by Monte-Carlo simulations of an anisotropic easy-plane 2D Heisenberg model. We can thus show that (i) the spins are preferably in plane, (ii) the cusp in specific heat is due to a Berezinskii-Kosterlitz-Thouless (BKT) transition, and (iii) the high-temperature hump in higher fields marks a crossover from a paramagnetically disordered to an ordered state. The extracted phase and crossover boundaries from experiment and simulations agree very well., 5 pages, 4figures, 7 pages supplementary material

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Ag-sheathed CaKFe4As4 superconducting tapes have been fabricated via the ex-situ powder-in-tube method. Thermal and X-ray diffraction analyses suggest that the CaKFe4As4 phase is unstable at high temperatures. It decomposes into the CaAgAs phase which reacts strongly with the silver sheath. We therefore sintered the tape at 500C and obtain a transport critical current density Jc(4.2 K, 0 T)~ 2.7x10^4 A/cm2. The pinning potential derived from magnetoresistance measurements is one order of magnitude lower than that of the (Ba/Sr)1-xKxFe2As2 tapes. Combining with the scanning electron microscopy and magneto-optical imaging results, we suggest that bad connectivity between superconducting grains caused by the low sintering temperature is the main factor responsible for the low Jc. However, this system is still a promising candidate for superconducting wires and tapes if we further optimize the post-annealing process to achieve better grain connectivity., 17 pages, 6 figures

With the discovery of charge density waves (CDW) in most members of the cuprate high temperature superconductors, the interplay between superconductivity and CDW has become a key point in the debate on the origin of high temperature superconductivity. Some experiments in cuprates point toward a CDW state competing with superconductivity, but others raise the possibility of a CDW-superconductivity intertwined order, or more elusive pair-density wave (PDW). Here we have used proton irradiation to induce disorder in crystals of La$_{1.875}$Ba$_{0.125}$CuO$_4$ and observed a striking 50% increase of $T_\mathrm{c}$ accompanied by a suppression of the CDW. This is in clear contradiction with the behaviour expected of a d-wave superconductor for which both magnetic and non-magnetic defects should suppress $T_\mathrm{c}$. Our results thus make an unambiguous case for the strong detrimental effect of the CDW on bulk superconductivity in La$_{1.875}$Ba$_{0.125}$CuO$_4$. Using tunnel diode oscillator (TDO) measurements, we find evidence for dynamic layer decoupling in PDW phase. Our results establish irradiation-induced disorder as a particularly relevant tuning parameter for the many families of superconductors with coexisting density waves, which we demonstrate on superconductors such as the dichalcogenides and Lu$_5$Ir$_4$Si$_{10}$., 10 pages, 7 figures

The temperature dependence of the London penetration depth $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}(T)$ in the superconducting doped topological crystalline insulator ${\mathrm{Sn}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te}$ was measured down to 450 mK for two different doping levels, $x\ensuremath{\approx}0.45$ (optimally doped) and $x\ensuremath{\approx}0.10$ (underdoped), bookending the range of cubic phase in the compound. The results indicate no deviation from fully gapped BCS-like behavior, eliminating several candidate unconventional gap structures. Critical field values below 1 K and other superconducting parameters are also presented. The introduction of disorder by repeated particle irradiation with 5 MeV protons does not enhance ${T}_{c}$, indicating that ferroelectric interactions do not compete with superconductivity.

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe$_4$As$_4$ using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of $T_m$ = 15 K and a superconducting transition temperature of $T_c$ = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of $\Gamma$ = 1.7. Ginzburg-Landau parameters of $\kappa_c \sim 67$ and $\kappa_{ab} \sim 108$ indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and $, Comment: 9 pages, 9 figures

Sr$_x$Bi$_2$Se$_3$ and the related compounds Cu$_x$Bi$_2$Se$_3$ and Nb$_x$Bi$_2$Se$_3$ have attracted considerable interest, as these materials may be realizations of unconventional topological superconductors. Superconductivity with T$_c$ ~ 3 K in Sr$_x$Bi$_2$Se$_3$ arises upon intercalation of Sr into the layered topological insulator Bi$_2$Se$_3$. Here we elucidate the anisotropy of the normal and superconducting state of Sr$_{0.1}$Bi$_2$Se$_3$ with angular dependent magnetotransport and thermodynamic measurements. High resolution x-ray diffraction studies underline the high crystalline quality of the samples. We demonstrate that the normal state electronic and magnetic properties of Sr$_{0.1}$Bi$_2$Se$_3$ are isotropic in the basal plane while we observe a large two-fold in-plane anisotropy of the upper critical field in the superconducting state. Our results support the recently proposed odd-parity nematic state characterized by a nodal gap of $E_u$ symmetry in Sr$_x$Bi$_2$Se$_3$., 10 pages, 7 figures, 3 supplemental pages

We present resistivity and magnetization measurements on proton-irradiated crystals demonstrating that the superconducting state in the doped topological insulator ${\mathrm{Nb}}_{x}{\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ ($x=0.25$) is surprisingly robust against disorder-induced electron scattering. The superconducting transition temperature ${T}_{c}$ decreases without indication of saturation with increasing defect concentration, and the corresponding scattering rates far surpass expectations based on conventional theory. The low-temperature variation of the London penetration depth $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}(T)$ follows a power law $[\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}(T)\ensuremath{\sim}{T}^{2}]$ indicating the presence of symmetry-protected point nodes. Our results are consistent with the proposed robust nematic ${E}_{u}$ pairing state in this material.

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 impurity bound states, and explains the related $C_2$-symmetry in the Fourier transformed differential conductance.

Micro-electronic devices often undergo significant self-heating when biased to their typical operating conditions. This paper describes a convenient optical micro-imaging technique which can be used to map and quantify such behavior. Europium thenoyltrifluoroacetonate (EuTFC) has a 612 nm luminescence line whose activation efficiency drops strongly with increasing temperature, due to T-dependent interactions between the Eu(3+) ion and the organic chelating compound. This material may be readily coated on to a sample surface by thermal sublimation in vacuum. When the coating is excited with ultraviolet light (337 nm) an optical micro-image of the 612 nm luminescent response can be converted directly into a map of the sample surface temperature. This technique offers spatial resolution limited only by the microscope optics (about 1 micron) and time resolution limited by the speed of the camera employed. It offers the additional advantages of only requiring comparatively simple and non-specialized equipment, and giving a quantitative probe of sample temperature.