Motivated by the recent discovery of superconductivity in infinite-layer nickelate thin films, we report on a synthesis and magnetization study on bulk samples of the parent compounds RNiO₂ (R = La, Pr, Nd). The frequency-dependent peaks of the alternating current magnetic susceptibility, along with remarkable memory effects, characterize spin-glass states. Furthermore, various phenomenological parameters via different spin glass models show strong similarity within these three compounds as well as with other rare-earth metal nickelates. The universal spin-glass behaviour distinguishes the nickelates from the parent compound CaCuO₂ of cuprate superconductors, which has the same crystal structure and d ⁹ electronic configuration but undergoes a long-range antiferromagnetic order. Our investigations may indicate a distinctly different nature of magnetism and superconductivity in the bulk nickelates than in the cuprates.

We report on the inelastic-scattering rate of electrons on phonons and relaxation of electron energy studied by means of magnetoconductance, and photoresponse, respectively, in a series of strongly disordered superconducting NbN films. The studied films with thicknesses in the range from 3 to 33 nm are characterized by different Ioffe-Regel parameters but an almost constant product $q_{T}l$ ($q_T$ is the wave vector of thermal phonons and $l$ is the elastic mean free path of electrons). In the temperature range 14–30 K, the electron-phonon scattering rates obey temperature dependencies close to the power law $1/{\tau }_{\mathrm{e}\text{−}\mathrm{ph}}\sim T^{\mathrm{n}}$ with the exponents $n\approx 3.2–3.8$. We found that in this temperature range ${\tau }_{\mathrm{e}\text{−}\mathrm{ph}}$ and $n$ of studied films vary weakly with the thickness and square resistance. At 10 K electron-phonon scattering times are in the range 11.9–17.5 ps. The data extracted from magnetoconductance measurements were used to describe the experimental photoresponse with the two-temperature model. For thick films, the photoresponse is reasonably well described without fitting parameters, however, for thinner films, the fit requires a smaller heat capacity of phonons. We attribute this finding to the reduced density of phonon states in thin films at low temperatures. We also show that the estimated Debye temperature in the studied NbN films is noticeably smaller than in bulk material.

As a member of the Ruddlesden-Popper $L{n}_{\mathrm{n}+1}\mathrm{N}{\mathrm{i}}_{\mathrm{n}}{\mathrm{O}}_{3\mathrm{n}+1}$ series rare-earth-nickelates, $\mathrm{P}{\mathrm{r}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ consists of infinite quasi-two-dimensional perovskite-like Ni-O based layers. Although a metal-to-metal phase transition at $T_{pt}\approx 157\mathrm{K}$ has been revealed by previous studies, a comprehensive study of physical properties associated with this transition has not yet been performed. We have grown single crystals of $\mathrm{P}{\mathrm{r}}_4\mathrm{N}{\mathrm{i}}_3{\mathrm{O}}_{10}$ at high oxygen pressure, and report on the physical properties around that phase transition, such as heat-capacity, electric-transport, and magnetization. We observe a distinctly anisotropic behavior between in-plane and out-of-plane properties: a metal-to-metal transition at $T_{pt}$ within the a-b plane, and a metal-to-insulator-like transition along the $c$ axis with decreasing temperature. Moreover, an anisotropic and anomalous negative magnetoresistance is observed at $T_{pt}$ that we attribute to a slight suppression of the first-order transition with magnetic field. The magnetic susceptibility can be well described by a Curie-Weiss law, with different Curie constants and Pauli-spin susceptibilities between the high-temperature and the low-temperature phases. The single crystal x-ray diffraction measurements show a shape variation of the different $\mathrm{Ni}{\mathrm{O}}_6$ octahedra from the high-temperature phase to the low-temperature phase. This subtle change of the environment of the Ni sites is likely responsible for the different physical properties at high and low temperatures.

We describe the Bose-Einstein condensate of magnetic bosonic quasiparticles in insulating spin systems using a phenomenological standard functional method for $T=0$. We show that results that are already known from advanced computational techniques immediately follow. The inclusion of a perturbative anisotropy term that violates the axial symmetry allows us to remarkably well explain a number of experimental features of the dimerized spin-1/2 system ${\text{TlCuCl}}_3$. Based on an energetic argument we predict a general intrinsic instability of an axially symmetric magnetic condensate toward a violation of this symmetry, which leads to the spontaneous formation of an anisotropy gap in the energy spectrum above the critical field. We, therefore, expect that a true Goldstone mode in insulating spin systems, i.e., a strictly linear energy-dispersion relation down to arbitrarily small excitations energies, cannot be observed in any real material.

We report on the preparation and the physical properties of superconducting ${\left(\mathrm{TaNb}\right)}_{1-x}{\left(\mathrm{ZrHfTi}\right)}_x$ high-entropy alloy films. The films were prepared by means of magnetron sputtering at room temperature, with $x$ ranging from 0 to 1 with an average thickness of 600–950 nm. All films crystallize in a pseudo body-centered cubic (BCC) structure. For samples with $x<0.65$, the normal-state properties are metallic, while for $x\ge 0.65$ the films are weakly insulating. The transition from metallic to weakly insulating occurs right at the near-equimolar stoichiometry. We find all films, except for $x=0$ or 1, to become superconducting at low temperatures, and we interpret their superconducting properties within the Bardeen-Cooper-Schrieffer (BCS) framework. The highest transition temperature $T_c=6.9\mathrm{K}$ of the solid solution is observed for $x\sim 0.43$. The highest upper-critical field $B_{c2}\left(0\right)=11.05$ T is found for the near-equimolar ratio $x\sim 0.65$, where the mixing entropy is the largest. The superconducting parameters derived for all the films from transport measurements are found to be close to those that are reported for amorphous superconductors. Our results indicate that these films of high-entropy alloys are promising candidates for superconducting device fabrication.

We have synthesized a series of the Ruddlesden-Popper nickelate solid solution $A_{4–x}{B}_x{\mathrm{Ni}}_3{\mathrm{O}}_{10}$ ($A$ and $B=\mathrm{La},\mathrm{Pr},\text{and} \mathrm{Nd};x=0,1,2,\text{and} 3$) via the citrate precursor method in different reacting atmospheres. Both the electronic-transport and magnetization measurements on these samples show well-defined phase transitions at temperatures between 135 and 165 K. These transition temperatures as well as the room-temperature resistivities strongly correlate with the Goldschmidt tolerance factor $t$, irrespective of the combination of the magnetic rare-earth ions with nonmagnetic $\mathrm{L}{\mathrm{a}}^{3+}$. We conclude that these changes of the electronic properties are mostly a consequence of the distortion of the $\mathrm{Ni}{\mathrm{O}}_6$ octahedra at the phase transition which is intimately related to the tolerance factor $t$, but appear to be rather insensitive to the average magnetic moment of the rare-earth ions upon exchanging La by Pr or Nd.