We present first results and future plans for the Oscillating Resonant Group AxioN (ORGAN) experiment, a microwave cavity axion haloscope situated in Perth, Western Australia designed to probe for high mass axions motivated by several theoretical models. The first stage focuses around 26.6 GHz in order to directly test a claimed result, which suggests axions exist at the corresponding mass of 110 µeV. Later stages will move to a wider scan range of 15-50 GHz (60 − 210 µeV). We present the results of the pathfinding run, which sets a limit on g aγγ of 2.02 × 10 −12 eV −1 at 26.531 GHz, or 110 µeV, in a span of 2.5 neV (shaped by the Lorentzian resonance) with 90% confidence. Furthermore, we outline the current design and future strategies to eventually attain the sensitivity to search for well known axion models over the wider mass range.

With the axion being a prime candidate for dark matter, there has been some recent interest in direct detection through a so called 'Ferromagnetic haloscope.' Such devices exploit the coupling between axions and electrons in the form of collective spin excitations of magnetic materials with the readout through a microwave cavity. Here, we present a new, general, theoretical treatment of such experiments in a Hamiltonian formulation for strongly coupled magnons and photons, which hybridise as cavity-magnon polaritons. Such strongly coupled systems have an extended measurable dispersive regime. Thus, we extend the analysis and operation of such experiments into the dispersive regime, which allows any ferromagnetic haloscope to achieve improved bandwidth with respect to the axion mass parameter space. This experiment was implemented in a cryogenic setup, and initial search results are presented setting laboratory limits on the axion-electron coupling strength of g aee > 3.7 × 10 −9 in the range 33.79 µeV< m a < 33.94 µeV with 95% confidence. The potential bandwidth of the Ferromagnetic haloscope was calculated to be in two bands, the first of about 1 GHz around 8.24 GHz (or 4.1 µeV mass range around 34.1 µeV) and the second of about 1.6 GHz around 10 GHz (6.6 µeV mass range around 41.4 µeV). Frequency tuning may also be easily achieved via an external magnetic field which changes the ferromagnetic resonant frequency with respect to the cavity frequency. The requirements necessary for future improvements to reach the DFSZ axion model band are discussed in the paper.

Several experimental implementations of cavity-magnon systems are presented. First an Yttrium Iron Garnet (YIG) block is placed inside a re-entrant cavity where the resulting hybrid mode is measured to be in the ultra strong coupling (USC) regime. When fully hybridised the ratio between the coupling rate and uncoupled mode frequencies is determined to be g/ω=0.46. Next a thin YIG cylinder is placed inside a loop gap cavity. The bright mode of this cavity couples to the YIG sample and is similarly measured to be in the USC regime with ratio of coupling rate to uncoupled mode frequencies as g/ω=0.34. A larger spin density medium such as lithium ferrite (LiFe) is expected to improve couplings by a factor of 1.46 in both systems as coupling strength is shown to be proportional to the square root of spin density and magnetic moment. Such strongly coupled systems are potentially useful for cavity QED, hybrid quantum systems and precision dark matter detection experiments. The YIG disc in the loop gap cavity, is, in particular, shown to be a strong candidate for dark matter detection. Finally, a LiFe sphere inside a two post re-entrant cavity is considered. In past work it was shown that the magnon mode in the sample has a turnover point in frequency (Goryachev et al 2018 Phys. Rev. B 97 155129). Additionally, it was predicted that if the system was engineered such that it fully hybridised at this turnover point the cavity-magnon polariton transition frequency would become insensitive to both first and second order magnetic bias field fluctuations, a result useful for precision frequency applications. This work implements such a system by engineering the cavity mode frequency to near this turnover point, with suppression in sensitivity to second order bias magnetic field fluctuations shown.

We present frequency tuning mechanisms for dielectric resonators, which undergo "super-mode" interactions as they tune. The tunable schemes are based on dielectric materials strategically placed inside traditional cylindrical resonant cavities, necessarily operating in Transverse Magnetic modes for use in axion haloscopes. The first technique is based on multiple dielectric disks with radii smaller than that of the cavity. The second scheme relies on hollow dielectric cylinders similar to a Bragg resonator, but of different location and dimension. In particular we engineer a significant increase in form factor for the TM030 mode utilising a variation of a Distributed Bragg Reflector Resonator. Additionally, we demonstrate application of traditional Distributed Bragg Reflectors in TM modes, which may be applied to a haloscope. This is the first demonstration of Bragg resonators applied to TM modes, as well as the first application of super-modes to tune Bragg resonators, or haloscope resonators. Theory and experimental results are presented showing an increase in Q-factor and tunability due to the super-mode effect. The TM030 ring resonator mode offers between 1 and 2orders-of-magnitude improvement in axion sensitivity over current conventional cavity systems and will be employed in the forthcoming ORGAN experiment.

Reentrant cavities are microwave resonant devices employed in a number of different areas of physics. They are appealing due to their simple frequency tuning mechanism, which offers large tuning ranges. Reentrant cavities are, in essence, 3D lumped LC circuits consisting of a conducting central post embedded in a resonant cavity. The lowest order reentrant mode (which transforms from the T M 010 mode) has been extensively studied in past publications. In this work we show the existence of higher order reentrant post modes (which transform from the T M 01n mode family). We characterize these new modes in terms of their frequency tuning, filling factors and quality factors, as well as discuss some possible applications of these modes in fundamental physics tests.

A method for determining the internal DC magnetic field inside a superconducting cavity is presented. The method relies on the relationship between the magnetic field and frequency of the Kittel mode of a ferrimagnetic sphere, hybridized in the dispersive regime of the superconducting cavity. Results were used to experimentally determine the level of screening that a superconducting Nb cavity provides as it changes from perfect diamagnetism to no screening. Two cavity geometries were tested, a cylinder and single post re-entrant cavity. Both demonstrated a consistent value of field that enters the cavity, expected to be the superheating critical field. Hysteresis in the screened field during ramp up and ramp down of the external magnetic field due to trapped vortices was also observed. Some abnormal behavior was observed in the cylindrical cavity in the form of plateaus in the internal field above the first critical field, and we discuss the potential origin of this behavior. The measurement approach would be a useful diagnosis for axion dark matter searches, which plans on using superconducting materials but needs to know precisely the internal magnetic field.

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