To account for the dark-matter content in our Universe, postinflationary scenarios predict for the QCD axion a mass in the range $(10–{10}^3)\mu \mathrm{eV}$. Searches with haloscope experiments in this mass range require the monitoring of resonant cavity modes with frequency above 5 GHz, where several experimental limitations occur due to linear amplifiers, small volumes, and low quality factors of copper resonant cavities. In this paper, we deal with the last issue, presenting the result of a search for galactic axions using a haloscope based on a $36{\mathrm{cm}}^3$ NbTi superconducting cavity. The cavity worked at $T=4\mathrm{K}$ in a 2 T magnetic field and exhibited a quality factor $Q_0=4.5\times {10}^5$ for the TM010 mode at 9 GHz. With such values of $Q$, the axion signal is significantly increased with respect to copper cavity haloscopes. Operating this setup, we set the limit $g_{a\gamma \gamma }<1.03\times {10}^{-12}{\mathrm{GeV}}^{-1}$ on the axion photon coupling for a mass of about $37\mu \mathrm{eV}$. A comprehensive study of the NbTi cavity at different magnetic fields, temperatures, and frequencies is also presented.

A ferromagnetic axion haloscope searches for dark matter in the form of axions by exploiting their interaction with electronic spins. It is composed of an axion-to-electromagnetic field transducer coupled to a sensitive rf detector. The former is a photon-magnon hybrid system, and the latter is based on a quantum-limited Josephson parametric amplifier. The hybrid system consists of ten 2.1 mm diameter yttrium iron garnet spheres coupled to a single microwave cavity mode by means of a static magnetic field. Our setup is the most sensitive rf spin magnetometer ever realized. The minimum detectable field is $5.5\times {10}^{-19}\mathrm{T}$ with 9 h integration time, corresponding to a limit on the axion-electron coupling constant $g_{aee}\le 1.7\times {10}^{-11}$ at 95% C.L. The scientific run of our haloscope resulted in the best limit on dark matter axions to electron coupling constant in a frequency span of about 120 MHz, corresponding to the axion-mass range $42.4–43.1\mu \mathrm{eV}$. This is also the first apparatus to perform a wide axion-mass scanning by only changing the static magnetic field.

A haloscope of the QUAX–$a\gamma$ experiment composed of an oxygen-free high thermal conductivity-Cu cavity inside an 8.1 T magnet and cooled to $\sim 200\mathrm{mK}$ is put in operation for the search of galactic axion with mass $m_a\simeq 43\mu \mathrm{eV}$. The power emitted by the resonant cavity is amplified with a Josephson parametric amplifier whose noise fluctuations are at the standard quantum limit. With the data collected in about 1 h at the cavity frequency ${\nu }_c=10.40176\mathrm{GHz}$, the experiment reaches the sensitivity necessary for the detection of galactic QCD-axion, setting the 90% confidence level limit to the axion-photon coupling $g_{a\gamma \gamma }<0.766\times {10}^{-13}{\mathrm{GeV}}^{-1}$.

Electronic functionalities in materials from silicon to transition metal oxides are, to a large extent, controlled by defects and their relative arrangement. Outstanding examples are the oxides of copper, where defect order is correlated with their high superconducting transition temperatures. The oxygen defect order can be highly inhomogeneous, even in optimal superconducting samples, which raises the question of the nature of the sample regions where the order does not exist but which nonetheless form the “glue” binding the ordered regions together. Here we use scanning X-ray microdiffraction (with a beam 300 nm in diameter) to show that for La ₂ CuO _{4+ y} , the glue regions contain incommensurate modulated local lattice distortions, whose spatial extent is most pronounced for the best superconducting samples. For an underdoped single crystal with mobile oxygen interstitials in the spacer La ₂ O _{2+ y} layers intercalated between the CuO ₂ layers, the incommensurate modulated local lattice distortions form droplets anticorrelated with the ordered oxygen interstitials, and whose spatial extent is most pronounced for the best superconducting samples. In this simplest of high temperature superconductors, there are therefore not one, but two networks of ordered defects which can be tuned to achieve optimal superconductivity. For a given stoichiometry, the highest transition temperature is obtained when both the ordered oxygen and lattice defects form fractal patterns, as opposed to appearing in isolated spots. We speculate that the relationship between material complexity and superconducting transition temperature T _c is actually underpinned by a fundamental relation between T _c and the distribution of ordered defect networks supported by the materials.

A haloscope of the QUAX–$a\gamma$ experiment, composed of an high-$Q$ resonant cavity immersed in a 8 T magnet and cooled to $\sim 4.5\mathrm{K}$ is operated to search for galactic axions with mass $m_a\simeq 42.8\mu \mathrm{eV}$, not accessible to other running experiments. The design of the cavity with hollow dielectric cylinders concentrically inserted in a OFHC Cu cavity, allowed us to maintain a loaded quality-factor $\mathrm{Q}\sim 300000$ during the measurements in presence of magnetic field. Through the cavity tuning mechanism it was possible to modulate the resonance frequency of the haloscope in the region $10.35337-10.35345\mathrm{GHz}$ and thus acquire different datasets at different resonance frequencies. Acquiring each datasets for about 50 minutes, combining them and correcting for the axion’s signal estimation-efficiency, we set a limit on the axion-photon coupling $g_{a\gamma \gamma }<0.731\times {10}^{-13}{\mathrm{GeV}}^{-1}$ with the confidence level set at 90%.

Axions, originally proposed to solve the strong CP problem of quantum chromodynamics, emerge now as leading candidates of WISP dark matter. The rich phenomenology associated to the light and stable...