[en] A brief review on unconventional oxygen-isotope (¹⁶O/¹⁸O) effects (OIE) in cuprate high-temperature superconductors (HTSC) is presented. First the doping dependence of the OIE on the superconducting transition T_c in various HTSC and investigations of the site-selective OIE in Y_1-xPr_xBa₂Cu₃O_7-δ are discussed. These experiments clearly demonstrate that the planar oxygen ions mainly (≥80%) contribute to the total OIE on T_c at all doping levels. Magnetization and magnetic torque OIE studies of the in-plane penetration depth λ_ab(0) in La_2-xSr_xCuO₄ indicate a substantial oxygen-mass dependence of λ_ab^-2(0) which increases with reduced doping. These results are in agreement with recent μSR OIE studies of λ_ab^-2(0) in polycrystalline samples of Y_1-xPr_xBa₂Cu₃O_7-δ and La_2-xSr_xCuO₄. The OIE exponents of T_c and λ_ab^-2(0) exhibit a relation that appears to be generic for various families of HTSC. The unusual isotope effects observed in HTSC strongly suggest that lattice effects play an essential role

[en] Superconductivity was discovered in 1911 by Kamerlingh Onnes and Holst in mercury at the temperature of liquid helium (4.2 K). It took almost 50 years until in 1957 a microscopic theory of superconductivity, the so-called BCS theory, was developed. Since the discovery a number of superconducting materials were found with transition temperatures up to 23 K. A breakthrough in the field happened in 1986 when Bednorz and Müller discovered a new class of superconductors, the so-called cuprate high-temperature superconductors with transition temperatures as high as 135 K. This surprising discovery initiated new efforts with respect to fundamental physics, material science, and technological applications. In this brief review the basic physics of the conventional low-temperature superconductors as well as of the high-temperature superconductors are presented with a brief introduction to applications exemplified from high-power to low-power electronic devices. Finally, a short outlook and future challenges are presented, finished with possible imaginations for applications of room-temperature superconductivity.

[en] To clarify the order parameter symmetry of cuprates, the magnetic penetration depth was measured along the crystallographic directions a,b, and c in single crystals of YBa₂Cu₄O₈ via muon spin rotation. This method is direct, bulk sensitive, and unambiguous. The temperature dependences of the penetration depth along the crystallographic a- and b-directions were found to exhibit an inflection point at low temperatures as is typical and provides evidence for two-gap superconductivity (TGS) with s+d wave character in the planes. In contrast, the penetration depth along the c-axis shows almost pure s wave character which never met the awareness of the community thereby highlighting the important role of c-axis effects. We conclude that TGS is a generic and universal feature in the bulk of cuprates

[en] In this review we consider three classes of superconductors, namely cuprate superconductors, MgB₂ and the new Fe based superconductors. All of these three systems are layered materials and multiband compounds. Their pairing mechanisms are under discussion with the exception of MgB₂, which is widely accepted to be a ‘conventional’ electron-phonon interaction mediated superconductor, but extending the Bardeen-Cooper-Schrieffer (BCS) theory to account for multiband effects. Cuprates and Fe based superconductors have higher superconducting transition temperatures and more complex structures. Superconductivity is doping dependent in these material classes unlike in MgB₂ which, as a pure compound, has the highest values of T_c and a rapid suppression of superconductivity with doping takes place. In all three material classes isotope effects have been observed, including exotic ones in the cuprates, and controversial ones in the Fe based materials. Before the area of high-temperature superconductivity, isotope effects on T_c were the signature for phonon mediated superconductivity - even when deviations from the BCS value to smaller values were observed. Since the discovery of high T_c materials this is no longer evident since competing mechanisms might exist and other mediating pairing interactions are discussed which are of purely electronic origin. In this work we will compare the three different material classes and especially discuss the experimentally observed isotope effects of all three systems and present a rather general analysis of them. Furthermore, we will concentrate on multiband signatures which are not generally accepted in cuprates even though they are manifest in various experiments, the evidence for those in MgB₂, and indications for them in the Fe based compounds. Mostly we will consider experimental data, but when possible also discuss theoretical models which are suited to explain the data. (topical review)

[en] Highlights: • Unusual hidden magnetism far above the magnetic ordering temperature • Novel phase diagram as discovered by magnetization and birefringence data • Prediction of multiferroic properties under bending conditions - Abstract: Perovskite oxides ABO₃ offer a vast playground for applications and basic research. Especially multifunctionality is a frequently targeted aim, since the replacement of the A and/or B sublattice ions easily changes the properties of the system. Here we concentrate on the perovskite EuTiO₃ (ETO) which was ignored by the scientific community for a long time. We review recent results on it and show that this compound provides many surprises which may offer novel device techniques and applications. In particular we demonstrate that far above its Néel temperature T_N = 5.7 K magnetism exists and new magneto-dielectric-elastic coupling is at work. From capacitance and muon spin rotation (μSR) data we conclude that the phase diagram of ETO is much more complex than believed and a variety of new phases are described.

[en] Full text: In the tetragonal heavy fermion system CeCoIn₅ the unconventional superconducting state is probed by means of muon spin rotation. The pressure dependence (0–1 GPa) of the basal-plane magnetic penetration depth (λ_a), the penetration depth anisotropy (γ=λ_c/λ_a) and the temperature dependence of 1/λ_i² (i=a,c) were studied in single crystals. A strong decrease of λ_a with pressure was observed, while γ and λ_i²(0)/λ_i²(T) are pressure independent. A linear relationship between 1/λ_i² (140 mK) and T_c was also found. The large decrease of λ_a with pressure is the signature of an increase of the number of superconducting quasiparticles by a factor of about 2. (author)

[en] Heterostructures consisting of magnetic/superconducting layers juxtaposed to each other are ideal systems to investigate the interplay of the two order parameters and to study possible interlayer coupling. Recently, so called giant proximity effects have been reported in Josephson devices consisting of HTS electrodes and a barrier of a cuprate in the pseudogap or AF state. Low energy μSR offers the unique possibility to measure on a nanometer scale local field distributions and identify superconducting and magnetic fractions. We used polarized low energy muons to investigate the local properties of single, bi- and tri-layers composed of superconducting YBa₂Cu₃O_7-δ and semiconducting PrBa₂Cu₃O_7-δ. Zero field measurements show that the PBCO layers (thickness 50-75 nm) display in all structures the known AF ordering. However, measurements of the field profile B(z) in the Meissner state show that below the critical temperature of YBCO, supercurrents flow without dissipation over the 50 nm thick AF barrier. The measurements indicate that a finite superfluid density may be induced in the AF layer adjacent to the superconducting layer.

[en] For classical superconductors the interaction with light results in a reduction of the energy gap due to excess of quasiparticles. Surprisingly, this is very different for cuprate superconductors where it was found that illumination with light increases the charge carrier density. Systematic investigations in YBa₂Cu₃O_7-δ showed that T_c is increased by this effect which is most pronounced at the heavily underdoped side of the phase diagram. Up to now studies of photo-induced processes have been restricted to investigations of transport properties and critical temperatures. Using low energy μSR changes in the Meissner screening profile caused by illumination were directly investigated. We will show that the photo-persistent effect in underdoped YBa₂Cu₃O_7-δ is related to an increase of the superfluid density close to the surface. We are also able to show that the oxygen ordering is a crucial ingredient of the photo-induced effect.