arXiv subjects

M. E. Tobar

Publications and source records attributed to M. E. Tobar.

At least 19 recordsLinked to original sources

arXiv2025

Dynamically tuneable helicity in twisted electromagnetic resonators

We report the generation of helical electromagnetic radiation in a microwave cavity resonator, achieved by introducing mirror asymmetry, i.e., chirality, through a controlled geometric twist of the conducting boundary conditions. The emergence of electromagnetic helicity is attributed to a nonzero spatial overlap between the electric and magnetic mode eigenvectors, quantified by $\text{Im}\left[\vec{\mathbf{E}}_i(\vec{r})\cdot{\vec{\mathbf{H}}}_i^*(\vec{r})\right]$, a feature not observed in conventional cavity resonators. This phenomenon originates from magnetoelectric coupling between nearly degenerate transverse electric (TE) and transverse magnetic (TM) modes, resulting in a measurable frequency shift of the resonant modes as a function of the twist angle, $\phi$. In addition to the bulk helicity induced by global geometric twist, internal helical corrugations break structural symmetry on the surface, introducing an effective surface chirality $\kappa_{\text{eff}}$, which perturbs the resonant conditions and contributes to asymmetric frequency tuning. By dynamically varying $\phi$, we demonstrate real-time, macroscopic manipulation of both electromagnetic helicity and resonant frequency. Furthermore, we investigate the underlying mode-coupling dynamics of the system, highlighting strong photon-photon interactions.

physics.optics↗

arXiv2025

Distinct Berry Phases in a Single Triangular M\"{o}bius Microwave Resonator

We report the experimental observation of two distinct Berry phases ($+\frac{2\pi}{3}$ and $-\frac{2\pi}{3}$) generated on the surface of a M\"{o}bius cavity resonator at microwave frequencies supporting the TE$_{1,0,n}$ mode family. This resonator consists of a twisted, mirror-asymmetric prism with a cross-section of the triangular $D_3$ symmetry group, bent around on itself to form a ring. This geometric class supports resonant modes with nonzero electromagnetic helicity (i.e. nonzero $\vec{E}\cdot\vec{B}$ product) at microwave frequencies. There exist modes with three-fold rotational symmetry as well as those that exhibit no rotational symmetry. The latter result in an accumulated Berry phase whilst the former do not, which is determined from the measured frequency shift of the modes when compared to a mirror-symmetric resonator of otherwise equivalent geometry.

physics.class-ph↗

arXiv2025

Ferroelectric phase transition and crystal asymmetry monitoring of $SrTiO_3$ using quasi $TE_{m,1,1}$ and quasi $TM_{m,1,1}$ modes

Dielectric spectroscopy of $SrTiO_3$ single crystal over a broad range of microwave frequency using quasi $TE_{m,1,1}$ and quasi $TM_{m,1,1}$ modes reveals crystal asymmetry from typical measurement of $Q$-factor, transmission or frequency characteristics in continuous cooling down to a few Kelvin. The properties of the modes due to the crystal asymmetry is validated by implementing a quasiharmonic phonon approximation. The observed ferroelectric phase transition temperature is around $51~K$, and quantum-mechanical stabilization of the paraelectric phase arises below $5~K$ with very high permittivity. Also, an antiferroelectric distortive transition was indicated at $105~K$. Landau's theory of correlation length supports the observation of an extra loss term so the transition may be identified near the $Q$-factor maxima or transmission maxima, depending on the other loss terms present in the cavity. Thus, the ferroelectric phase transition with respect to temperature may be identified when this extra-loss term causes a discontinuity in the derivative of the temperature characteristic near the minimum of total cavity loss (maxim Q-factor or maximum transmission temperature characteristic). This temperature is confirmed by transmission amplitude variation under 200 V dc electric field showing existence of the soft-mode. These measurements support a typical polarization model and explicit temperature dependency of the soft-mode incorporating an imaginary frequency.

cond-mat.mtrl-sci↗

arXiv2025

Axion Dark Matter eXperiment around 3.3 μeV with Dine-Fischler-Srednicki-Zhitnitsky Discovery Ability

We report the results of a QCD axion dark matter search with discovery ability for Dine Fischler Srednicki Zhitnitsky (DFSZ) axions using an axion haloscope. Sub-Kelvin noise temperatures are reached with an ultra low-noise Josephson parametric amplifier cooled by a dilution refrigerator. This work excludes (with a 90% confidence level) DFSZ axions with masses between 3.27 to 3.34 $μ$eV, assuming a standard halo model with a local energy density of 0.45 GeV/cm$^3$ made up 100% of axions.

hep-ex↗

arXiv2025

Search for Axion Dark Matter from 1.1 to 1.3 GHz with ADMX

Axion dark matter can satisfy the conditions needed to account for all of the dark matter and solve the strong CP problem. The Axion Dark Matter eXperiment (ADMX) is a direct dark matter search using a haloscope to convert axions to photons in an external magnetic field. Key to this conversion is the use of a microwave resonator that enhances the sensitivity at the frequency of interest. The ADMX experiment boosts its sensitivity using a dilution refrigerator and near quantum-limited amplifier to reduce the noise level in the experimental apparatus. In the most recent run, ADMX searched for axions between 1.10-1.31 GHz to extended Kim-Shifman-Vainshtein-Zakharov (KSVZ) sensitivity. This Letter reports on the results of that run, as well as unique aspects of this experimental setup.

hep-ex↗

arXiv2025

Electromagnetic Helicity in Twisted Cavity Resonators

Through left- or right-handed twisting, we investigate the impact of mirror-asymmetry (chirality) of the conducting boundary conditions of an equilaterial triangular cross-section electromagnetic resonator. We observe the generation of eigenmodes with non-zero electromagnetic helicity as a result of the coupling of near degenerate TE$_{11(p+1)}$ and TM$_{11p}$ modes. This can be interpreted as an emergence of magneto-electric coupling, which in turn produces a measurable shift in resonant mode frequency as a function of twist angle. We show that this coupling mechanism is equivalent to introducing a non-zero chirality material parameter $\kappa_\text{eff}$ or axion field $\theta_{\text{eff}}$ to the medium. Our findings demonstrate the potential for real-time, macroscopic manipulation of electromagnetic helicity.

physics.class-ph↗

arXiv2025

Improved Receiver Noise Calibration for ADMX Axion Search: 4.54 to 5.41 $μ$eV

Axions are a well-motivated candidate for dark matter. The preeminent method to search for axion dark matter is known as the axion haloscope, which makes use of the conversion of axions to photons in a large magnetic field. Due to the weak coupling of axions to photons however, the expected signal strength is exceptionally small. To increase signal strength, many haloscopes make use of resonant enhancement and high gain amplifiers, while also taking measures to keep receiver noise as low as possible such as the use of dilution refrigerators and ultra low-noise electronics. In this paper we derive the theoretical noise model based on the sources of noise found within a typical axion haloscope receiver chain, using the Axion Dark Matter eXperiment (ADMX) as a case study. We present examples of different noise calibration measurements at 1280~MHz taken during ADMX's most recent data-taking run. These new results shed light on a previously unidentified interaction between the cavity and JPA, as well as provide a better understanding of the systematic uncertainty on the system noise temperature used in the axion search analysis for this data-taking run. Finally, the consistency between the measurements and the detailed model provide suggestions for future improvements within ADMX and other axion haloscopes to reach a lower noise temperature.

hep-ex↗

arXiv2024

Upconversion of Phonon Modes into Microwave Photons in a Lithium Niobate Bulk Acoustic Wave Resonator Coupled to a Microwave Cavity

The coupling between acoustic vibrations in a lithium niobate bulk acoustic wave resonator and microwave photons of a re-entrant microwave cavity was investigated at a temperature close to 4 K. Coupling was achieved by placing the acoustic resonator in the location of the re-entrant cavity electric field maxima, in a symmetric "split-post" configuration, with a large overlap between the microwave field and the acoustic mode, allowing acoustic modulations of the microwave frequency. We show that the acoustic modes in this setup retain large inherent quality factors of greater than $10^6$. A maximum optomechanical coupling rate was determined to be $g_0$ = 0.014 mHz, four orders of magnitude larger than previous results obtained using a quartz BAW at 4 K in a similar experimental setup, but using a single post-re-entrant cavity resonator.

quant-ph↗

arXiv2024

Search for non-virialized axions with 3.3-4.2 $μ$eV mass at selected resolving powers

The Axion Dark Matter eXperiment is sensitive to narrow axion flows, given axions compose a fraction of the dark matter with a non-negligible local density. Detecting these low-velocity dispersion flows requires a high spectral resolution and careful attention to the expected signal modulation due to Earth's motion. We report an exclusion on the local axion dark matter density in narrow flows of $ρ_a \gtrsim 0.03\,\mathrm{GeV/cm^3}$ and $ρ_a \gtrsim 0.004\,\mathrm{GeV/cm^3}$ for Dine-Fischler-Srednicki-Zhitnitski and Kim-Shifman-Vainshtein-Zakharov axion-photon couplings, respectively, over the mass range $3.3-4.2\,μ\text{eV}$. Measurements were made at selected resolving powers to allow for a range of possible velocity dispersions.

astro-ph.CO↗

arXiv2024

Aggregate Frequency Width, Nuclear Hyperfine Coupling and Jahn-Teller Effect of $Cu^{2+}$ Impurity Ion ESR in $SrLaAlO_4$ Dielectric Resonator at $20$ Millikelvin

The impurity paramagnetic ion, $Cu^{2+}$ substitutes $Al$ in the $SrLaAlO_4$ single crystal lattice, this results in a $CuO_6$ elongated octahedron, the resulting measured g-factors shows four-fold axes variation condition. The aggregate frequency width of the electron spin resonance with the required minimum level of impurity concentration has been evaluated in single crystal $SrLaAlO_4$ at $20$ millikelvin. Measured parallel hyperfine constants, $A_{\scriptscriptstyle\parallel Cu}$, were determined to be $-155.7\times10^{-4}~cm^{-1},~ -163.0\times10^{-4}~cm^{-1},~ -178.3\times10^{-4}~cm^{-1} $ and$~-211.1\times10^{-4}~cm^{-1}$ at $9.072~GHz~(WGH_{4,1,1})$ for the nuclear magnetic quantum number $M_I=+\frac{3}{2},+\frac{1}{2},-\frac{1}{2}$,~and$-\frac{3}{2}$ respectively. The anisotropy of the hyperfine structure reveals a characteristics of static Jahn-Teller effect. The second-order-anisotropy-term, $\sim (\frac{spin-orbit~coupling}{10D_q})^2$, is significant and can not be disregarded, with the local strain dominating over the observed Zeeman-anisotropy-energy difference. The Bohr electron magneton, $β=9.23\times 10^{-24} JT^{-1}$, (within $-0.43\%$ so-called experimental error) has been found using the measured spin-Hamiltonian parameters. Measured nuclear dipolar hyperfine structure parameter $P_{\scriptscriptstyle\parallel}=12.3\times10^{-4}~cm^{-1}$ shows that the mean inverse third power of the electron distance from the nucleus is $\langle r^{-3}_q\rangle\simeq 5.23$ a.u. for $Cu^{2+}$ ion in the substituted $Al^{3+}$ ion site assuming nuclear electric quadruple moment $Q=-0.211$ barn.

quant-ph↗

arXiv2024

Rigorous ESR spectroscopy of $Fe^{3+}$ impurity ion with oxygen vacancy in ferroelectric $SrTiO_3$ crystal at 20 mK

Impurity $Fe^{3+}$ ion electron spin resonance (ESR) spectroscopy using multiple dielectric modes in a $SrTiO_3$ dielectric resonator has been performed with a tuneable DC magnetic field of up to $1.6~T$. The $Ti^{4+}(d^0)$ ion is substituted by $Fe^{3+}$ ion forming $FeO_6$ octahedral complex with an iron-oxygen-vacancy $(Fe^{3+}-V_O)$. In such a metal-ligand complex, a giant g-factor of $g_{\scriptscriptstyle\parallel F} = 5.51$ was observed in the ferroelectric phase at $20~mK$. The the change of $Fe^{3+}$ ion center-symmetry in the $FeO_6$ complex as a soft-mode characteristics of ferroelectric phase transition and the influences of iron-oxygen-vacancy $(Fe^{3+}-V_O)$, interactively sensitive to asymmetry in the octahedral rotational parameter $Φ$ in $SrTi0_3$.

quant-ph↗

arXiv2023

Non-Virialized Axion Search Sensitive to Doppler Effects in the Milky Way Halo

The Axion Dark Matter eXperiment (ADMX) has previously excluded Dine-Fischler-Srednicki-Zhitnisky (DFSZ) axions between 680-790 MHz under the assumption that the dark matter is described by the isothermal halo model. However, the precise nature of the velocity distribution of dark matter is still unknown, and alternative models have been proposed. We report the results of a non-virialized axion search over the mass range 2.81-3.31 μeV, corresponding to the frequency range 680-800 MHz. This analysis marks the most sensitive search for non-virialized axions sensitive to Doppler effects in the Milky Way Halo to date. Accounting for frequency shifts due to the detector's motion through the Galaxy, we exclude cold flow relic axions with a velocity dispersion of order 10^-7 c with 95% confidence.

astro-ph.CO↗

arXiv2023

Search for a dark-matter induced Cosmic Axion Background with ADMX

We report the first result of a direct search for a Cosmic ${\it axion}$ Background (C$a$B) - a relativistic background of axions that is not dark matter - performed with the axion haloscope, the Axion Dark Matter eXperiment (ADMX). Conventional haloscope analyses search for a signal with a narrow bandwidth, as predicted for dark matter, whereas the C$a$B will be broad. We introduce a novel analysis strategy, which searches for a C$a$B induced daily modulation in the power measured by the haloscope. Using this, we repurpose data collected to search for dark matter to set a limit on the axion photon coupling of a C$a$B originating from dark matter cascade decay via a mediator in the 800-995 MHz frequency range. We find that the present sensitivity is limited by fluctuations in the cavity readout as the instrument scans across dark matter masses. Nevertheless, we suggest that these challenges can be surmounted using superconducting qubits as single photon counters, and allow ADMX to operate as a telescope searching for axions emerging from the decay of dark matter. The daily modulation analysis technique we introduce can be deployed for various broadband RF signals, such as other forms of a C$a$B or even high-frequency gravitational waves.

hep-ex↗

arXiv2023

Searching for Ultra-Light Axions with Twisted Cavity Resonators of Anyon Rotational Symmetry with Bulk Modes of Non-Zero Helicity

Möbius-ring resonators stem from a well-studied and fascinating geometrical structure that features a one-sided topology; the Möbius strip, and have been shown to exhibit fermion rotational symmetry with respect to a ring resonator with no twist (which exhibits boson rotational symmetry) (see PhysRevLett.101.247701). Here, we present a new type of resonator through the formation of twisted hollow structures using equilateral triangular cross-sections, which leads to the realization of a cavity with anyon rotational symmetry. Unlike all previous cavity resonators, the anyon resonator permits the existence of bulk resonant modes that exhibit non-zero electromagnetic helicity in vacuo, with a non-zero overlap of the electric and magnetic mode eigenvectors, $\int \mathbf{E}_p\cdot\mathbf{B}_p~dτ$, integrated over the cavity volume. In the upconversion limit, we show that these non-zero helical modes couple naturally to ultra-light dark matter axions within the bandwidth of the resonator by adding amplitude-modulated sidebands through the axion-photon chiral anomaly. Thus, we show a sensitive ultra-light dark matter experiment may be realized by implementing such a resonator in an ultra-stable oscillator configuration and searching for signals in the Fourier spectrum of amplitude fluctuations. This removes the typical requirement for an external magnetic field and therefore permits the use of superconducting materials to reduce surface losses and enhance sensitivity to axions.

hep-ph↗

arXiv2023

Low Frequency (100-600 MHz) Searches with Axion Cavity Haloscopes

We investigate reentrant and dielectric loaded cavities for the purpose of extending the range of axion cavity haloscopes to lower masses, below the range where the Axion Dark Matter eXperiment (ADMX) has already searched. Reentrant and dielectric loaded cavities were simulated numerically to calculate and optimize their form factors and quality factors. A prototype reentrant cavity was built and its measured properties were compared with the simulations. We estimate the sensitivity of axion dark matter searches using reentrant and dielectric loaded cavities inserted in the existing ADMX magnet at the University of Washington and a large magnet being installed at Fermilab.

hep-ph↗

arXiv2022

Searching for Scalar Field Dark Matter using Cavity Resonators and Capacitors

We establish new experiments to search for dark matter based on a model of a light scalar field with a dilaton-like coupling to the electromagnetic field, which is strongly motivated by superstring theory. We estimate the power of the photon signal in the process of a non-resonant scalar-photon transition and in a cavity resonator permeated by electric and magnetic fields. We show that existing cavity resonators employed in the experiments like ADMX have a low but non-vanishing sensitivity to the scalar-photon coupling. As a result, by re-purposing the results of the ADMX experiment, we find new limits on the scalar-photon coupling in the range of the scalar field masses from 2.7 to 4.2 $μ$eV. We discuss possible modifications of this experiment, which enhance the sensitivity to the scalar field dark matter. We also propose a broadband experiment for scalar field dark matter searches based on a high-voltage capacitor. The estimated sensitivity of this experiment exceeds by nearly two orders in magnitude the sensitivity of the experiment based on molecular spectroscopy.

hep-ph↗

arXiv2021

Search for "Invisible" Axion Dark Matter in the $3.3\text{-}4.2~μ$eV Mass Range

We report the results from a haloscope search for axion dark matter in the $3.3\text{-}4.2~μ$eV mass range. This search excludes the axion-photon coupling predicted by one of the benchmark models of "invisible" axion dark matter, the KSVZ model. This sensitivity is achieved using a large-volume cavity, a superconducting magnet, an ultra low noise Josephson parametric amplifier, and sub-Kelvin temperatures. The validity of our detection procedure is ensured by injecting and detecting blind synthetic axion signals.

hep-ex↗

arXiv2021

Challenges and Opportunities of Gravitational Wave Searches at MHz to GHz Frequencies

The first direct measurement of gravitational waves by the LIGO and Virgo collaborations has opened up new avenues to explore our Universe. This white paper outlines the challenges and gains expected in gravitational wave searches at frequencies above the LIGO/Virgo band, with a particular focus on Ultra High-Frequency Gravitational Waves (UHF-GWs), covering the MHz to GHz range. The absence of known astrophysical sources in this frequency range provides a unique opportunity to discover physics beyond the Standard Model operating both in the early and late Universe, and we highlight some of the most promising gravitational sources. We review several detector concepts which have been proposed to take up this challenge, and compare their expected sensitivity with the signal strength predicted in various models. This report is the summary of the workshop "Challenges and opportunities of high-frequency gravitational wave detection" held at ICTP Trieste, Italy in October 2019, that set up the stage for the recently launched Ultra-High-Frequency Gravitational Wave (UHF-GW) initiative.

gr-qc↗