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Doping is one of the most relevant paths to tune the functionality of cuprates, it determines carrier density and the overall physical properties of these impressive superconducting materials. We present an oxygen doping study of YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) thin films from underdoped to overdoped state, correlating the measured charge carrier density, $n_\textrm{H}$, the hole doping, $p$, and the critical current density, $J_\textrm{c}$. Our results show a continuous increase of $J_\textrm{c}$ with charge carrier density, reaching 90 MA/cm$^2$ at 5 K for $p$-doping at the Quantum Critical Point (QCP), linked to an increase of the superconducting condensation energy. The ultra-high $J_\textrm{c}$ achived corresponds to a third of the depairing current, i.e. a value 60 % higher than ever reported in YBCO films. The overdoped regime is characterized by a sudden increase of $n_\textrm{H}$, associated to the reconstruction of the Fermi-surface at the QCP. Overdoping YBCO opens a promising route to extend the current carrying capabilities of REBCO coated conductors for applications.

Anisotropy is an intrinsic factor that dictates the magnetic properties of YBCO, thus with great impact for many applications. Artificial pinning centres are often introduced in an attempt to mitigate its effect, resulting in less anisotropic electrical and magnetic properties. However, the nanoengineering of the superconductor makes the quantification of the anisotropy itself uncertain: the intrinsic anisotropy due to the layered structure, quantified by the anisotropy factor $\gamma$, mixes up with the additional anisotropy due to pinning. As a consequence, there is no consensus on the experimental anisotropy factor $\gamma$ that can result in YBCO when directional (twin planes, nanorods) or isotropic defects are present. We present here measurements of the magnetic field and angular dependent surface impedance in very different nanostructured YBCO films, grown by chemical route and by pulsed laser deposition, with different kind of defects (nanorods, twin planes, nanoparticles). We show that the surface impedance measurements are able to disentangle the intrinsic anisotropy from the directional pinning anisotropy, thanks to the possibility to extract the true anisotropic flux--flow resistivity and by correctly exploiting the angular scaling. We find in all films that the intrinsic anisotropy $\gamma = 5.3\pm0.7$. By contrast, the pinning anisotropy determines a much complex, feature--rich and nonuniversal, sample--dependent angular landscape., Comment: 24 pages, 11 figures, submitted to IOP

Enabling temperature dependent experiments in Atomic Force Microscopy is of great interest to study materials and surface properties at the nanoscale. By studying Curie temperature of multiferroic materials, temperature based phase transition on crystalline structures or resistive switching phenomena are only a few examples of applications. We present an equipment capable of cooling samples using a thermoelectric cooling stage down to -61.4 C in a 15x15 mm sample plate. The equipment uses a four-unit thermoelectric stack to achieve maximum temperature range, with low electrical and mechanical noise. The equipment is installed into a Keysight 5500LS Atomic Force Microscopy maintaining its compatibility with all Electrical and Mechanical modes of operation. We study the contribution of the liquid cooling pump vibration into the cantilever static deflection noise and the temperature dependence of the cantilever deflection. A La0.7Sr0.3MnO3-y thin film sample is used to demonstrate the performance of the equipment and its usability by analysing the resistive switching phenomena associated with this oxide perovskite.

A nanometric needle sensor mounted in an Atomic Force Microscopy allows systematic picometer-range distance measurements. This force sensing device is used in Piezoresponse Force Microscopy (PFM) as a distance sensor, by employing the cantilever spring constant as the conversion factor opening a pathway to explore the piezoelectric effect at the nanoscale. The force-distance equivalence is achieved if the force sensor does not disturb the system to study, solely. In this manuscript we report a systematic study in which different Lead Zirconate Titanate (PZT) materials, having different d33 values, are measured following the standard theory available for PFM. Both in resonance and out of resonance measurements demonstrate that PFM cannot be considered quantitative. After performing the measurements, we propose a correction of the standard theory employed in PFM by considering the force exerted by the material as a variable. The g33 parameter, inherent to piezoelectricity, governs the amount of force available from the system. A comparison of piezoelectric stiffness for the case of a nanoscale site contact region, similar to the one it is found while performing PFM, is provided. Such stiffness is well below the cantilever stiffness, limiting and diminishing the material movement, as the piezoelectric material does not have enough stroke to induce the intended displacement. A correction factor, named Open Piezopotential Gauge, accounts for these effects, which is used to correct the measurements carried out in PZT materials towards a real quantitative PFM., Comment: 3 figures

The magnetic and electronic properties of Eu2Ru2O7 are discussed in terms of the local ruthenium and europium coordinations, electronic band structure calculations and molecular orbital energy levels. A preliminary electronic structure was calculated within the LDA and LSDA+U approximations. The molecular orbital energy level diagrams have been used to interpret the Eu-Ru ligand spectrum and the ensuing magnetic properties. The orbital hybridizations and bonds are discussed., Comment: 4 pages

The critical current (Jc) of highly twinned YBa2Cu3O7 films has been measured as a function of temperature, magnetic field and angle. For much of the parameter space we observe a strong suppression of Jc for fields in the twin boundary (TB) directions; this is quantitatively modeled as flux-cutting-mediated vortex channeling. For certain temperatures and fields a cross-over occurs to a regime in which channeling is blocked and the TBs act as planar pinning centers so that TB pinning enhances the overall Jc. In this regime, intrinsic pinning along the TBs is comparable to that between the twins., Comment: Submitted to PRL