Superconductivity of Co-Doped CaKFeAs Investigated via Point-Contact Spectroscopy and London Penetration Depth Measurements.

The iron-based superconductors (IBSs) of the recently discovered 1144 class, unlike many other IBSs, display superconductivity in their stoichiometric form and are intrinsically hole doped. The effects of chemical substitutions with electron donors are thus particularly interesting to investigate. Here, we study the effect of Co substitution in the Fe site of CaKFe4As4 single crystals on the critical temperature, on the energy gaps, and on the superfluid density by using transport, point-contact Andreev-reflection spectroscopy (PCARS), and London penetration depth measurements.

High-Frequency ac Susceptibility of Iron-Based Superconductors.

A microwave technique suitable for investigating the AC magnetic susceptibility of small samples in the GHz frequency range is presented. The method-which is based on the use of a coplanar waveguide resonator, within the resonator perturbation approach-allows one to obtain the absolute value of the complex susceptibility, from which the penetration depth and the superfluid density can be determined. We report on the characterization of several iron-based superconducting systems, belonging to the 11, 122, 1144, and 12442 families.

Critical Current Density and Vortex Dynamics in Pristine and Irradiated KCaFeAsF.

We report the critical current density () and vortex pinning properties in single crystals of a novel iron-based superconductor (IBS) KCaFeAsF with large in the pristine state, before and after introduction of artificial defects by swift-particle irradiation. The effects of 2.6 GeV U and 3 MeV proton irradiations in KCaFeAsF single crystals on transition temperature and , including its dose dependence, are systematically studied.

Effect of Controlled Artificial Disorder on the Magnetic Properties of EuFe(AsP) Ferromagnetic Superconductor.

Static (DC) and dynamic (AC, at 14 MHz and 8 GHz) magnetic susceptibilities of single crystals of a ferromagnetic superconductor, EuFe2(As1-xPx)2 ( = 0.23), were measured in pristine state and after different doses of 2.5 MeV electron or 3.

Scaling laws for ion irradiation effects in iron-based superconductors.

We report on ion irradiation experiments performed on compounds belonging to the [Formula: see text] family, each one involving the partial substitution of an atom of the parent compound (K for Ba, Co for Fe, and P for As), with an optimal composition to maximize the superconducting critical temperature [Formula: see text]. Employed ion beams were 3.5-MeV protons, 250-MeV Au ions, and 1.

Quasiparticle Evidence for the Nematic State above T_{c} in Sr_{x}Bi_{2}Se_{3}.

In the electronic nematic state, an electronic system has a lower symmetry than the crystal structure of the same system. Electronic nematic states have been observed in various unconventional superconductors such as cuprate, iron-based, heavy-fermion, and topological superconductors. The relation between nematicity and superconductivity is a major unsolved problem in condensed matter physics.

Zero-energy vortex bound state in the superconducting topological surface state of Fe(Se,Te).

Majorana quasiparticles in condensed matter are important for topological quantum computing, but remain elusive. Vortex cores of topological superconductors may accommodate Majorana quasiparticles that appear as the Majorana bound state (MBS) at zero energy. The iron-based superconductor Fe(Se,Te) possesses a superconducting topological surface state that was investigated by scanning tunnelling microscopy (STM) studies, which suggest such a zero-energy vortex bound state (ZVBS).

Domain Meissner state and spontaneous vortex-antivortex generation in the ferromagnetic superconductor EuFe(AsP).

The interplay between superconductivity and magnetism is one of the oldest enigmas in physics. Usually, the strong exchange field of ferromagnet suppresses singlet superconductivity via the paramagnetic effect. In EuFe(AsP), a material that becomes not only superconducting at 24.

Tuning the structure of the Josephson vortex lattice in BiSrCaCuO single crystals with pancake vortices.

In extremely anisotropic cuprate superconductors a lattice of stacks of pancake vortices nucleates when a magnetic field is applied perpendicular to the copper oxide layers, while an orthogonal lattice of highly elliptical Josephson vortices forms when the applied field is parallel to the layers. Under tilted magnetic fields these sublattices can interact in complex ways to form systems of vortex chains and composite vortex lattices. Here we have used high-resolution scanning Hall microscopy (SHM) to map the rich tilted-field vortex phase diagram in an underdoped BiSrCaCuO single crystal.

Effects of disorder induced by heavy-ion irradiation on (Ba K )FeAs single crystals, within the three-band Eliashberg s± wave model.

One of the open issues concerning iron-based superconductors is whether the s± wave model is able to account for the overall effects of impurity scattering, including the low rate of decrease of the critical temperature with the impurity concentration. Here we investigate Ba K FeAs crystals where disorder is introduced by Au-ion irradiation. Critical temperature, T , and London penetration depth, λ , were measured by a microwave resonator technique, for different values of the irradiation fluence.

Influence of interstitial Fe to the phase diagram of Fe1+yTe1-xSex single crystals.

Superconductivity (SC) with the suppression of long-range antiferromagnetic (AFM) order is observed in the parent compounds of both iron-based and cuprate superconductors. The AFM wave vectors are bicollinear (π, 0) in the parent compound FeTe different from the collinear AFM order (π, π) in most iron pnictides. Study of the phase diagram of Fe1+yTe1-xSex is the most direct way to investigate the competition between bicollinear AFM and SC.

Bipartite electronic superstructures in the vortex core of Bi2Sr2CaCu2O8+δ.

The central issue in the physics of cuprate superconductivity is the mutual relationship among superconductivity, pseudogap and broken-spatial-symmetry states. A magnetic field B suppresses superconductivity, providing an opportunity to investigate the competition among these states. Although various B-induced electronic superstructures have been reported, their energy, spatial and momentum-space structures are unclear.

Dynamics and mechanism of oxygen annealing in Fe1+yTe0.6Se0.4 single crystal.

Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K.

High sensitivity differential magneto-optical imaging with a compact Faraday-modulator.

We present here the design of a sensitive compact Faraday-modulator (CFM) based optical magnetometer for imaging the distribution of weak local magnetic fields inside hysteretic magnetic materials. The system developed has a root-mean-square noise level of 50 mG Hz(-1/2) at a full frame rate of 1 fps (frame per second) with each frame being of size 512 × 512 pixels. By measuring the local magnetic field distribution in different superconducting samples we show that our magnetometer provides an order of magnitude improvement in the signal-to-noise ratio at low fields as compared to ordinary magneto-optical imaging technique.

Lamellar solid-liquid mesophase nucleated by Josephson vortices at the melting of the vortex lattice in Bi2Sr2CaCu2O(8+δ) superconductor.

The local effect of the Josephson vortices on the vortex lattice melting process in Bi2Sr2CaCu2O(8+δ) crystals in the presence of an in-plane field H(ab) is studied by differential magneto-optical imaging. The melting process is found to commence along the Josephson vortex stacks, forming a mesomorphic phase of periodic liquid and solid lamellas, the direction and spacing of which are controlled by H(ab). The reduction of the local melting field H(m) along the Josephson vortex stacks is more than an order of magnitude larger than the reduction of the average bulk H(m) by HH(ab).

Two-gap superconductivity in FeSi (=Lu, Sc) and ScIrSi.

FeSi (= Sc, Y, Lu) contains nonmagnetic iron and has a relatively high superconducting transition temperature among iron-containing superconductors. An anomalous temperature dependence of specific heat () has been reported for polycrystalline samples down to 1 K. We have grown FeSi single crystals, confirmed the anomalous () dependence, and found a second drop in specific heat below 1 K.

Multiple changes of order of the vortex melting transition in Bi2Sr2CaCu2O8 with dilute columnar defects.

A low concentration of columnar defects is reported to transform a first-order vortex lattice melting line in Bi2Sr2CaCu2O8 crystals into alternating segments of first- and second-order transitions separated by two critical points. As the density of columnar defects is increased, the critical points shift apart and the range of the intermediate second-order transition expands. The measurement of equilibrium magnetization and the mapping of the melting line down to 27 K was made possible by employment of the shaking technique.

Specific-heat evidence for two-gap superconductivity in the ternary-iron silicide Lu2Fe3Si5.

We report low-temperature specific-heat studies on the single-crystalline ternary-iron silicide superconductor Lu(2)Fe(3)Si(5) with T(c)=6.1 K down to approximately T(c)/20. We confirm a reduced normalized jump in specific heat at T(c), and find that the specific heat divided by temperature C/T shows a sudden drop at approximately T(c)/5 and goes to zero with further decreasing temperature.

Dynamic and thermodynamic properties of porous vortex matter in Bi(2)Sr(2)CaCu(2)O(8) in an oblique magnetic field.

Vortex matter in Bi(2)Sr(2)CaCu(2)O(8) with a low concentration of tilted columnar defects (CDs) was studied using magneto-optical measurements and molecular dynamics simulations. It is found that while the dynamic properties are significantly affected by tilting the magnetic field away from the CDs, the thermodynamic transitions are angle independent. The simulations indicate that vortex pancakes remain localized on the CDs even at large tilting angles.

Interplay of anisotropy and disorder in the doping-dependent melting and glass transitions of vortices in Bi2Sr2CaCu2O 8+delta.

We study the oxygen doping dependence of the equilibrium first-order melting and second-order glass transitions of vortices in Bi2Sr2CaCu2O 8+delta. Doping affects both anisotropy and disorder. Anisotropy scaling is shown to collapse the melting lines only where thermal fluctuations are dominant.

Dynamic order-to-metastable-disorder vortex matter transition in Bi2Sr2CaCu2O8+delta.

Magneto-optical measurements of transient vortex states in Bi2Sr2CaCu2O8+delta show enhanced effects of metastability in prism-shaped as compared to platelet crystals including a significant shift of the second magnetization peak and qualitatively different dynamics. In contrast to platelets, where dislocations are generated only at the sample edges, we propose that in prism samples the dislocations are generated dynamically in the entire sample due to distributed surface barriers. As a result, a dynamic phase transition from a Bragg glass to a metastable disordered phase may occur well below the thermodynamic transition field.

Spatiotemporal vortex matter oscillations in Bi2Sr2CaCu2O8+delta crystals.

We observed an oscillatory behavior, both in space and time, of the induction in Bi2Sr2CaCu2O8+delta crystals exposed to a steady magnetic field. This new "flux waves" phenomenon appears near the order-disorder vortex phase transition, under specific conditions of temperature and induction gradient. A theoretical description of this effect is based on two coupled equations: the Landau-Khalatnikov dynamic equation for the order parameter of the vortex phase transition and the diffusion equation for the time evolution of the magnetic induction.

Ratchet without spatial asymmetry for controlling the motion of magnetic flux quanta using time-asymmetric drives.

Initially inspired by biological motors, new types of nanodevice have been proposed for controlling the motion of nanoparticles. Structures incorporating spatially asymmetric potential profiles (ratchet substrates) have been realized experimentally to manipulate vortices in superconductors, particles in asymmetric silicon pores, as well as charged particles through artificial pores and arrays of optical tweezers. Using theoretical ideas, we demonstrate experimentally how to guide flux quanta in layered superconductors using a drive that is asymmetric in time instead of being asymmetric in space.

Equilibrium first-order melting and second-order glass transitions of the vortex matter in Bi2Sr2CaCu2O8.

The thermodynamic phase diagram of Bi2Sr2CaCu2O8 was mapped by measuring local equilibrium magnetization M(H,T) in the presence of vortex shaking. Two equally sharp first-order magnetization steps are revealed in a single temperature sweep, manifesting a liquid-solid-liquid sequence. In addition, a second-order glass transition line is revealed by a sharp break in the equilibrium M(T) slope.

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites.

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.