Charge density waves are a common occurrence in all families of high critical temperature superconducting cuprates. Although consistently observed in the underdoped region of the phase diagram and at relatively low temperatures, it is still unclear to what extent they influence the unusual properties of these systems. Using resonant x-ray scattering we carefully determined the temperature dependence of charge density modulations in (Y,Nd)Ba2Cu3O7-δ for three doping levels. We discovered short-range dynamical charge density fluctuations besides the previously known quasi-critical charge density waves. They persist up to well above the pseudogap temperature T*, are characterized by energies of few meV and pervade a large area of the phase diagram, so that they can play a key role in shaping the peculiar normal-state properties of cuprates.Main text: High-Tc superconductors (HTS) are doped Mott insulators, where the quasi-twodimensionality of the layered structure and the large electron-electron repulsion (responsible, e.g., for the robust short-range antiferromagnetic correlations) make them deviating from the Landau Fermi liquid paradigm. The doping-temperature (p-T) phase diagram encompasses, at low T, the antiferromagnetic and the superconducting orders and, at higher T, the pseudogap region, which marks, below the cross-over temperature T*, a reduction of the quasiparticle density of states in some sections of the Fermi surface. In the pseudogap state and up to optimal doping p0.17, short/medium range incommensurate charge density waves (CDW) emerge as an order weakly competing with superconductivity.CDW were proposed theoretically since the early times of the high temperature superconductivity age (1,2,3); experimental evidence by surface and bulk sensitive techniques came initially in selected materials (4,5,6,7), and later in all cuprate families (8,9,10,11,12). Moreover long-range tridimensional CDW (3D-CDW) order has been observed inside the superconductive dome (for p0.08-0.17) in special circumstances, e.g. in high magnetic fields that weaken superconductivity or in epitaxially grown samples (13,14,15). Finally, it has come as a surprise the recent observation of CDW modulations in overdoped (Bi,Pb)2.12Sr1.88CuO6+δ outside the pseudogap regime too (16), hinting at a wider than expected occurrence of this phenomenon.

We have grown and characterized 30 nm thick YBa2Cu3O 7−δ (YBCO) films, deposited by pulsed laser deposition on both MgO (110) and SrTiO3 (001) substrates, which induce opposite strain to the superconducting layer. By carefully tuning the in-situ post-annealing oxygen pressure, we achieved, in a reproducible way, films at different oxygen doping, spanning from the slightly overdoped down to the strongly underdoped region of the phase diagram. The transport properties of the films, investigated through resistance versus temperature measurements, are in perfect qualitative agreement with single crystals. Starting from these films, we have also successfully fabricated nanowires with widths down to 65 nm, at different oxygen doping. The nanostructures exhibit characteristic temperatures (as the critical temperature Tc and the pseudogap temperature T * ) similar to those of the as-grown films and carry critical current densities Jc close to the critical depairing value, limited by vortex entry. This implies that the superconducting and the normal state properties of underdoped YBCO are preserved in our films, and they can be studied as a function of the dimensionality of the system, down to the nanoscale. arXiv:1712.02008v1 [cond-mat.supr-con]

We present noise measurements performed on a YBa 2 Cu 3 O 7−δ nanoscale weak-linkbased magnetometer consisting of a Superconducting QUantum Interference Device (SQUID) galvanically coupled to a 3.5 × 3.5 mm 2 pick-up loop, reaching white flux noise levels and magnetic noise levels as low as 6 µΦ 0 / √ Hz and 100 fT/ √ Hz at T = 77 K, respectively. The low noise is achieved by introducing Grooved Dayem Bridges (GDBs), a new concept of weak-link. A fabrication technique has been developed for the realization of nanoscale grooved bridges, which substitutes standard Dayem bridge weak links. The introduction of these novel key blocks reduces the parasitic inductance of the weak links and increases the differential resistance of the SQUIDs. This greatly improves the device performance, thus resulting in a reduction of the white noise.

The development of quantum limited magnetic flux sensors has recently gained a lot of attention for the possibility of detecting the magnetic moment of nanoscaled systems. Here, the ultimate goal is the observation of a single spin. Such sensors are of fundamental importance for applications, ranging from spintronics and spin-based quantum information processing, to fundamental studies of nano-magnetism in molecules and magnetic nanoclusters. A nano-scale superconducting quantum interference device (nanoSQUID) is indeed a promising candidate to reach this ambitious goal. Nanowires, fabricated of high critical temperature superconductors (HTS), have been shown to be a valid candidate for the realization of nanoSQUIDs. A crucial requirement to achieve the necessary flux sensitivity and spatial resolution, is a SQUID loop on the nanometer scale. Moreover, HTS nanowire-based SQUIDs in combination with large area pickup loops or flux transformers might become instrumental in magnetometer applications, such as magneto encephalography and low field magnetic resonance imaging, where low intrinsic magnetic field noise is required. In this review we will give a survey on the state of the art of YBa2Cu3O7−δ thin film nanowires and their implementation in low noise nanoSQUIDs and magnetometers.

We have grown untwinned YBa2Cu3O 7−δ (YBCO) films on (110) MgO substrates that were preannealed at high temperature in oxygen atmosphere. The annealing results in surface reconstruction with shallow facets, which induce the suppression of the YBCO twinning domains, and the preferential alignment of the CuO chains along one of the in-plane directions of the substrate. Because of the large mismatch between the in-plane lattice parameters of film and substrate, the strain induced by the MgO into the YBCO layer is strong and very peculiar. The YBCO film is compressed, with respect to the bulk, and presents a unidirectional buckling of the atomic planes, along the chains' direction, due to a deformation of the copper-oxygen octahedra. The YBCO films, which can be grown with thicknesses down to few unit cells and oxygen doping levels spanning most of the superconducting dome, are patterned into nanowires with dimensions down to 50 nm. The anisotropies due to the untwinning state are preserved in these structures; moreover, additional anisotropies appear, in ultrathin structures where strain effects become more pronounced. Such untwinned and compressively strained films can therefore be used as a platform to study the interplay between strain and the various local orders in the normal state of YBCO. * riccardo.arpaia@chalmers.se † floriana.lombardi@chalmers.se duced by the substrate. In the case of YBa 2 Cu 3 O 7−δ (YBCO), this requires the growth of films which must be untwinned, to preserve the anisotropies related to the orthorhombicity of the unit cell, and at the same time compressively strained. The growth of untwinned films, where aligned CuO chains are present throughout the sample, is not an easy task since c-axis oriented YBCO films usually grow twinned on the substrates commonly used for the deposition of cuprates. This twinning, caused by a random exchange of the in-plane a and b axis, influences the electric and magnetic properties of the films [15][16][17][18]. The best untwinned YBCO films, reported so far, have been achieved on SrTiO 3 (STO) substrates with a vicinal miscut angle, i.e. with a small misorientation, with respect to the (001) plane direction [19,20]. However, the STO substrate induces a tensile stress into the YBCO films.Here we report on the growth of untwinned YBCO films under compressive strain. We succeeded in this task, by an in-situ thermal treatment of (110) oriented MgO substrates, prior to the deposition of the YBCO thin films. The thermal treatment results in a reconstruction of the substrate surface, favoring the growth of untwinned YBCO. Moreover, the film is subject to a very peculiar compressive strain, as a result of the large mismatch between the in-plane lattice parameters of the film and the substrate: the b axis shrinks, and tilts around the normal to the MgO surface.The manuscript is organized as follows: In Section II, we start with an overview of the past results on un-arXiv:1908.02637v2 [cond-mat.supr-con]