arXiv subjects

Michael E Tobar

Publications and source records attributed to Michael E Tobar.

6 recordsLinked to original sources

arXiv2025

Dielectric Properties of Single Crystal Calcium Tungstate

This investigation employed microwave whispering gallery mode (WGM) analysis to characterize the dielectric properties of a cylindrical, single-crystal sample of calcium tungstate (CaWO$_4$). Through investigation of quasi-transverse\hyp{}magnetic and quasi-transverse\hyp{}electric mode families, we can assess loss mechanisms and relative permittivity from room temperature down to cryogenic conditions. We report the biaxial permittivity values of $\epsilon_{||} = 9.029 \pm 0.009$ and $\epsilon_{\perp} = 10.761 \pm 0.01$ at $295$ K, and $\epsilon_{||} = 8.794 \pm 0.009$ and $\epsilon_{\perp} = 10.440 \pm 0.01$ at $4$ K. Components are denoted with respect to the c\hyp{}axis of the crystal unit cell. The parallel component agrees well with the published literature at MHz frequencies; however, the perpendicular component is $4.8$\% lower. The WGM technique offers greater precision, with accuracy limited primarily by the uncertainty in the crystal's dimensions. WGMs also serve as sensitive probes of lattice dynamics, enabling monitoring of temperature-dependent loss mechanisms. At room temperature, the measured loss tangents were $\tan\delta_{||}^{295,\mathrm{K}} = (4.1 \pm 1.4) \times 10^{-5}$ and $\tan\delta_{\perp}^{295,\mathrm{K}} = (3.64 \pm 0.92) \times 10^{-5}$. Upon cooling to 4 K, the loss tangents improved by approximately two orders of magnitude, reaching $\tan\delta_{||}^{4,\mathrm{K}} = (1.56 \pm 0.52) \times 10^{-7}$ and $\tan\delta_{\perp}^{4,\mathrm{K}} = (2.05 \pm 0.79) \times 10^{-7}$. These cryogenic values are higher than those reported in prior studies, likely due to a magnetic loss channel associated with an unidentified paramagnetic spin ensemble. These findings have implications for the use of CaWO$_4$ in applications such as spin-based quantum systems and cryogenic bolometry, highlighting the potential of WGMs for novel sensing applications.

cond-mat.mtrl-sci↗

arXiv2025

The Study of Thermal Fluctuations in Microwave and Mechanical Resonators

We report high-resolution measurements of thermal fluctuations in microwave and mechanical resonators using a dual-channel readout system. The latter comprises a low-noise amplifier, an I/Q-mixer, and a cross-correlator. We discovered that, under certain conditions, the intrinsic fluctuations of the low-noise amplifier, which are common to both channels of the readout system, are averaged out when computing the voltage noise cross-spectrum between the mixer's outputs. The suppression of the amplifier's technical fluctuations significantly improves the contrast of the thermal noise peaks exhibited by the resonators. Thus, for the room-temperature-stabilized 9 GHz sapphire-loaded cavity resonator, we observed more than 16 dB improvement in the thermal noise peak contrast relative to the single-channel measurements. The ability of the dual-channel readout system to discriminate between the broad- and narrow-band fluctuations may benefit the search for dark matter, which relies on the use of cryogenic microwave resonators.

physics.ins-det↗

arXiv2024

Precision Multi-Mode Microwave Spectroscopy of Paramagnetic and Rare-Earth Ion Spin Defects in Single Crystal Calcium Tungstate

We present experimental observations of dilute ion spin ensemble defects in a low-loss single crystal cylindrical sample of CaWO$_4$ cooled to $30$ mK in temperature. Crystal field perturbations were elucidated by constructing a dielectrically loaded microwave cavity resonator from the crystal. The resonator exhibited numerous whispering gallery modes with high $Q$-factors of up to $3\times 10^7$, equivalent to a loss tangent of $\sim 3\times 10^{-8}$. The low loss allowed precision multi-mode spectroscopy of numerous high $Q$-factor photon-spin interactions. Measurements between 7 to 22 GHz revealed the presence of Gd$^{3+}$, Fe$^{3+}$, and another trace species, inferred to be rare-earth, at concentrations on the order of parts per billion. These findings motivate further exploration of prospective uses of this low-loss dielectric material for applications regarding precision and quantum metrology, as well as tests for beyond standard model physics.

quant-ph↗

arXiv2024

Scalar Gravitational Aharonov-Bohm Effect: Generalization of the Gravitational Redshift

The Aharonov-Bohm effect is a quantum mechanical phenomenon that demonstrates how potentials can have observable effects even when the classical fields associated with those potentials are absent. Initially proposed for electromagnetic interactions, this effect has been experimentally confirmed and extensively studied over the years. More recently, the effect has been observed in the context of gravitational interactions using atom interferometry. Additionally, recent predictions suggest that temporal variations in the phase of an electron wave function will induce modulation sidebands in the energy levels of an atomic clock, solely driven by a time-varying scalar gravitational potential [1]. In this study, we consider the atomic clock as a two-level system undergoing continuous Rabi oscillations between the electron's ground and excited state. We assume the photons driving the transition are precisely frequency-stabilized to match the transition, enabling accurate clock comparisons. Our analysis takes into account, that when an atom transitions from its ground state to an excited state, it absorbs energy, increasing its mass according to the mass-energy equivalence principle. Due to the mass difference between the two energy levels, we predict that an atomic clock in an eccentric orbit will exhibit a constant frequency shift relative to a ground clock corresponding to the orbit's average gravitational redshift, with additional modulation sidebands due to the time-varying gravitational potential.

quant-ph↗

arXiv2022

Poynting vector controversy in axion modified electrodynamics

The most sensitive haloscopes that search for axion dark matter through the two photon electromagnetic anomaly, convert axions into photons through the mixing of axions with a large DC magnetic field. In this work we apply Poynting theorem to the resulting axion modified electrodynamics and identify two possible Poynting vectors, one similar to the Abraham Poynting vector and the other to the Minkowski Poynting vector in electrodynamics. The latter picks up the extra non-conservative terms while the former does not. To understand the source of energy conversion and power flow in the detection systems, we apply the two Poynting theorems to axion modified electrodynamics, for both the resonant cavity and broadband low-mass axion detectors. We show that both Poynting theorems give the same sensitivity for a resonant cavity axion haloscope, but predict markedly different sensitivity for a low-mass broadband capacitive haloscope. Hence we ask the question, can understanding which one is the correct one for axion dark matter detection, be considered under the framework of the Abraham-Minkowski controversy? In reality, this should be confirmed by experiment when the axion is detected. However, many electrodynamic experiments have ruled in favour of the Minkowski Poynting vector when considering the canonical momentum in dielectric media. In light of this, we show that the axion modified Minkowski Poynting vector should indeed be taken seriously for sensitivity calculation for low-mass axion haloscope detectors in the quasi static limit, and predict orders of magnitude better sensitivity than the Abraham Poynting vector equivalent.

hep-ph↗

arXiv2015

Acoustic tests of Lorentz symmetry using quartz oscillators

We propose and demonstrate a test of Lorentz symmetry based on new, compact, and reliable quartz oscillator technology. Violations of Lorentz invariance in the matter and photon-sector of the standard model extension (SME) generate anisotropies in particles' inertial masses and the elastic constants, giving rise to measurable anisotopies in the resonance frequencies of acoustic modes in solids. A first realization of such a "phonon-sector" test of Lorentz symmetry using room-temperature SC-cut crystals provides a limit of $\tilde c_Q^{\rm n}=(-1.8 \pm 2.2)\times 10^{-14}$\,GeV on the most weakly constrained neutron-sector $c-$coefficient of the SME. Future experiments with cryogenic oscillators promise significant improvements in accuracy, opening up the potential for improved limits on Lorentz violation in the neutron, proton, electron and photon sector.

gr-qc↗