Temperature-dependent London penetration depth, $lambda(T)$, of a high quality optimally-doped $text{YBa}_{2}text{Cu}_{3}text{O}_{7-delta}$ single crystal was measured using tunnel-diode-resonator technique. Controlled artificial disorder was induced
by low-temperature (20~K) irradiation by 2.5 MeV electrons at two large doses of $3.8times10^{19}$and $5.3times10^{19}$ electrons per $textrm{cm}^{2}$. The irradiation caused significant suppression of the superconductors critical temperature, $T_{c}$, from 94.6 K to 90.0 K, and to 78.7 K, respectively. The low-temperature behavior of $lambdaleft(Tright)$ evolves from a $T-$ linear in pristine state to a $T^{2}-$ behavior after irradiation, expected for a line-nodal $d-$wave superconductor. However, the original theory that explained such behavior assumed a unitary limit of the scattering potential, whereas usually in normal metals and semiconductors, Born scattering is sufficient to describe the experiment. To estimate the scattering potential strength, we calculated the superfluid density, $rho_{s}=lambda^{2}left(0right)/lambda^{2}left(Tright)$, varying the amount and strength of non-magnetic scattering using a self-consistent $t-$matrix theory. Comparing experimental and theoretical coefficients $A$ and $B$ of the low-temperature power series, $rho_{s}approx1-At-Bt^{2}$, we determine the amplitude of the scattering phase shift to be around 65$^{o}$. Knowing this value is important for further theoretical analysis of the microscopic mechanisms of superconductivity in $text{YBa}_{2}text{Cu}_{3}text{O}_{7-delta}$ high$-T_{c}$ superconductor.
In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped Ba$_{1-x}$K$_x$Fe$_2$As$_2$ in the composition range $0.21 leq x leq 0.26$ where anisotropy changes sign. Low-temperature ($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 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 re-entrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $Delta rho equiv rho_a(T)-rho_b(T)$ is found in pristine samples with $x=$0.213 and $x=$0.219, without similar signatures in either $rho_a(T)$ or $rho_b(T)$. We show that this feature can be reproduced by a phenomenological model of R.~M.~Fernandes {it et al.} Phys. Rev. Lett. {bf 107},217002 (2011). We speculate that onset of fluctuations of nematic order on approaching the instability towards the re-entrant tetragonal phase contributes to this unusual dependence.
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 mea
surements 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-Gorkov 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.
Controlled point-like disorder introduced by 2.5 MeV electron irradiation was used to probe the superconducting state of single crystals of CaKx superconductor at $x = 0$ and 0.05 doping levels. Both compositions show an increase of the residual resi
stivity and a decrease of the superconducting transition temperature, $T_c$ at the rate of $dT_c/drho(T_c) approx$ 0.19 K(textmu$Omega$cm)$^{-1}$ for $x=0$ and 0.38 K(textmu$Omega$cm)$^{-1}$ for $x=:$0.05, respectively. In Ni - doped, $x = 0.05$, compound the coexisting spin-vortex crystal (SVC) magnetic phase is suppressed at the rate of $dT_N/drho(T_N)approx$ 0.16 K(textmu$Omega$cm)$^{-1}$. Low - temperature variation of London penetration depth is well approximated by the power law, $Delta lambda (T) = AT^n$ with $napprox,$2.5 for $x=0$ and $napprox,$1.9 for $x=0.05$ in the pristine state. Electron irradiation leads to the exponent $n$ increase above 2 in $x=0.05$ suggesting superconducting gap with significant anisotropy that is smeared by the disorder scattering. Detailed analysis of $lambda (T)$ and (T_{c}) evolution with disorder is consistent with two effective nodeless superconducting energy gaps due to robust s$_{pm}$ pairing. Overall the behavior of CaKx at $x = 0$ is similar to a slightly overdoped BaK at $y approx$ 0.5 and at $x= 0.05$ to an underdoped composition at $y approx$ 0.2.
The interplay between superconductivity and charge density waves (CDW) in $H$-NbSe2 is not fully understood despite decades of study. Artificially introduced disorder can tip the delicate balance between two competing forms of long-range order, and r
eveal the underlying interactions that give rise to them. Here we introduce disorders by electron irradiation and measure in-plane resistivity, Hall resistivity, X-ray scattering, and London penetration depth. With increasing disorder, $T_{textrm{c}}$ varies nonmonotonically, whereas $T_{textrm{CDW}}$ monotonically decreases and becomes unresolvable above a critical irradiation dose where $T_{textrm{c}}$ drops sharply. Our results imply that 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.
We show that the dynamical freezing of vortex structures nucleated at diluted densities in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ samples with a dense distribution of columnar defects, $B sim 10^{-2} B_{Phi}$ with $B_{Phi}=5$,kG, results in configurations
with liquid-like correlations. We propose a freezing model considering a relaxation dynamics dominated by double-kink excitations driven by the local stresses obtained directly from experimental images. With this model we estimate the relaxation barrier and the freezing temperature. We argue that the low-field frozen vortex structures nucleated in a dense distribution of columnar defects thus correspond to an out-of-equilibrium non-entangled liquid with strongly reduced mobility rather than to a snapshot of a metastable state with divergent activation barriers as for instance expected for the Bose-glass phase at equilibrium.
A single crystal of isovalently substituted Ba(Fe$_{1-x}$Ru$_{x}$)$_2$As$_2$ ($x=0.24$) was sequentially irradiated with 2.5 MeV electrons up to a maximum dose of $2.1 times 10^{19}$ electrons/cm^2. The electrical resistivity was measured textit{in -
situ} at $T=$22 K during the irradiation and textit{ex - situ} as a function of temperature between subsequent irradiation runs. Upon irradiation, the superconducting transition temperature, $T_c$, decreases and the residual resistivity, $rho_0$, increases. We find that electron irradiation leads to the fastest suppression of $T_c$ compared to other types of artificially introduced disorder, probably due to the strong short-range potential of the point-like irradiation defects. A more detailed analysis within a multiband scenario with variable scattering potential strength shows that the observed $T_c$ vs. $rho_0$ is fully compatible with $s_pm$ pairing, in contrast to earlier claims that this model leads to a too rapid a suppression of $T_c$ with scattering.
The persistence of the first-order transition line in the phase diagram of mesoscopic Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ vortex matter is detected down to a system size of less than hundred vortices. Precise and highly-sensitive to bulk currents AC ma
gnetization techniques proved to be mandatory in order to obtain this information. The location of the vortex matter first-order transition lines are not altered by decreasing the sample size down to 20 $mu$m. Nevertheless, the onset of irreversible magnetization is affected by increasing the sample surface-to-volume ratio producing a noticeable enlargement of the irreversible vortex region above the second-peak transition.
Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortices oriented along the c-axis moving parallel to the
ab-plane, and to vortices in the ab-plane moving perpendicular to, and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter, {epsilon}_{xi}. This allows extraction of {epsilon}_{xi} at magnetic fields far below the upper critical field B_c2. We find that increasing magnetic field decreases the anisotropy of the coherence length.
Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopi
c scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred nm. These variations do not go beyond 5% - we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-T fields is characterized by the presence of a peak effect, the location of which in the (B,T)-plane is consistent with an order-disorder transition of the vortex lattice.
