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Register a new userIntermediate scattering potential strength in electron - irradiated $text{YBa}_{2}text{Cu}_{3}text{O}_{7-delta}$ from London penetration depth measurements
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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.
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The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7 GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin film shows that the $H_{rf}$--induced flux penetration is due to the increase of the film temperature.
YBa$_{2}$Cu$_{3}$O$_{7-{delta}}$ coated conductors (CCs) have achieved high critical current densities ($textit{J}_{c}$) that can be further increased through the introduction of additional defects using particle irradiation. However, these gains are accompanied by increases in the flux creep rate, a manifestation of competition between the different types of defects. Here, we study this competition to better understand how to design pinning landscapes that simultaneously increase $textit{J}_{c}$ and reduce creep. CCs grown by metal organic deposition show non-monotonic changes in the temperature-dependent creep rate, $textit{S}(textit{T})$. Notably, in low fields, there is a conspicuous dip to low $textit{S}$ as temperature ($textit{T}$) increases from ~20 K to ~65 K. Oxygen-, proton-, and Au-irradiation substantially increase $textit{S}$ in this temperature range. Focusing on an oxygen-irradiated CC, we investigate the contribution of different types of irradiation-induced defects to the flux creep rate. Specifically, we study $textit{S}(textit{T})$ as we tune the relative density of point defects to larger defects by annealing both an as-grown and an irradiated CC in O$_{2}$ at temperatures $textit{T}_{A}$ = 250${deg}$C to 600${deg}$C. We observe a steady decrease in $textit{S}$($textit{T}$ > 20 K) with increasing $textit{T}_{A}$, unveiling the role of pre-existing nanoparticle precipitates in creating the dip in $textit{S}(textit{T})$ and point defects and clusters in increasing $textit{S}$ at intermediate temperatures.
The London penetration depth $lambda$ is the basic length scale for electromagnetic behavior in a superconductor. Precise measurements of $lambda$ as a function of temperature, field, and impurity scattering have been instrumental in revealing the nature of the order parameter and pairing interactions in a variety of superconductors discovered over the past decades. Here we recount our development of the tunnel-diode resonator technique to measure $lambda$ as a function of temperature and field in small single crystal samples. We discuss the principles and applications of this technique to study unconventional superconductivity in the copper oxides and other materials such as iron-based superconductors. The technique has now been employed by several groups worldwide as a precision measurement tool for the exploration of new superconductors.
Most measurements of critical current densities in YBa$_2$Cu$_3$O$_{7-delta}$ thin films to date have been performed on films where the textit{c}-axis is grown normal to the film surface. With such films, the analysis of the dependence of $j_c$ on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of $j_c$, such as the measurement geometry and disposition of pinning centres, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed FLL structure models on the basis of angular critical current density measurements on textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of $j_c$ in YBa$_2$Cu$_3$O$_{7-delta}$ are reviewed. Measurements on YBa$_2$Cu$_3$O$_{7-delta}$ thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the FLL in YBa$_2$Cu$_3$O$_{7-delta}$. There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation and temperature. As a consequence, a simple scaling law can not, by itself, describe the observed critical current anisotropy in YBa$_2$Cu$_3$O$_{7-delta}$. The experimentally obtained $j_c(theta)$ behaviour of YBCO is successfully described in terms of a kinked vortex structure for fields applied near parallel to the textit{a-b} planes.
We report the first direct observation of the oxygen-isotope ($^{16}$O/$^{18}$O) effect on the in-plane penetration depth $lambda_{ab}$ in a nearly optimally doped YBa$_2$Cu$_3$O$_{7-delta}$ film using the novel low-energy muon-spin rotation technique. Spin polarized low energy muons are implanted in the film at a known depth $z$ beneath the surface and precess in the local magnetic field $B(z)$. This feature allows us to measure directly the profile $B(z)$ of the magnetic field inside the superconducting film in the Meissner state and to make a model independent determination of $lambda_{ab}$. A substantial isotope shift $Deltalambda_{ab}/lambda_{ab}=2.8(7)$% at 4 K is observed, implying that the in-plane effective supercarrier mass $m_{ab}^ast$ is oxygen-isotope dependent with $Delta m_{ab}^ast/m_{ab}^ast = 5.5(1.4)%$.
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