arXiv subjects

Sam Posen

Publications and source records attributed to Sam Posen.

At least 19 recordsLinked to original sources

arXiv2026

Cryogenic RF characterization of the MAGO cavity for high-frequency gravitational-wave detection

Superconducting radio-frequency (SRF) cavities are promising resonant sensors for gravitational-wave detection in the kHz-MHz frequency range. We report the cryogenic RF characterization of a prototype superconducting niobium cavity with an unconventional geometry designed for narrow electromagnetic mode separation. Following an adapted surface preparation procedure, cryogenic tests were performed at Fermilab and DESY at temperatures down to 2\,K. Mechanical tuning at room temperature achieved a mode splitting of approximately 11\,kHz at cryogenic temperature. High electromagnetic quality factors consistent with previous prototype cavities were measured. The measurements further revealed phase transfer characteristics relevant for stable low-level RF control as well as indications of mode coupling potentially caused by one-point multipacting. In addition, first cryogenic measurements of the mechanical eigenmodes yielded mechanical quality factors significantly below commonly assumed theoretical values. These results demonstrate the successful application of established SRF preparation and characterization techniques to a non-standard resonator geometry and provide important experimental input for the development of future SRF-based gravitational-wave detectors.

physics.ins-det↗

arXiv2026

Microwave Vortex Motion Characterization of Nb$_3$Sn Coatings for Applications in High Magnetic Fields

In this work, microwave measurements carried out in dielectric-loaded resonators exposed to high magnetic fields are exploited to yield the surface impedance of Nb$_3$Sn superconducting coatings deposited via two different techniques: vapor tin diffusion, and DC magnetron sputtering. The obtained data lead to qualitative interpretations on both the Nb$_3$Sn superconducting properties, and vortex-dynamics and pinning, of each coating separately, as well as simple distinctive features when comparing those. When examining the respective surface impedances at varying field, it is expected that the studied films perform at substantially diverse magnitudes of flux-flow resistivity, but also in well-differentiated pinning regimes, yet the obtained surface resistances of both samples are comparable, thus demonstrating that there is room for film optimization at the expense of certain compromise between the parameters involved.

cond-mat.supr-con↗

arXiv2026

Microstructural Characterization of Nb3Sn Thin Films Using FIB Tomography

The accelerating gradient of Nb3Sn superconducting radiofrequency (SRF) cavities is currently limited, and the underlying cause remains an open question in the field. One leading hypothesis attributes this limitation to the presence of tin-deficient regions within the Nb3Sn coating, which can suppress the superheating field. Due to the relatively large coherence length of Nb3Sn, defects near the surface may significantly interact with the RF field. However, these subsurface defects have proven difficult to characterize. This research aims to investigate the structure and distribution of subsurface Sn deficient regions to better understand their influence on cavity performance. We employ focused ion beam (FIB) tomography to analyze the subsurface microstructure of Nb3Sn thin films. This technique enables three-dimensional reconstruction of both the tin distribution and the grain structure within the film. By correlating Sn content with grain structure, we find that Sn deficient regions are more prevalent that previously thought. However, the Sn deficient regions are consistently located below the surface of the film where RF fields are strongly attenuated by supercurrent screening and are likely not a limiting factor for cavity performance.

cond-mat.mtrl-sci↗

arXiv2025

Improved Dark Photon Sensitivity from the Dark SRF Experiment

We report the refined dark-photon exclusion bound from Dark SRF's pathfinder run. Our new result is driven by improved theoretical modeling of frequency instability in high-quality resonant experiments. Our analysis leads to a constraint that is an order of magnitude stronger than previously reported (corresponding to a signal-to-noise ratio that is four orders of magnitude larger). This result represents the world-leading constraint on non-dark-matter dark photons over a wide range of masses below $6\,\rm \mu eV$ and translates to the best laboratory-based limit on the photon mass $m_\gamma<2.9\times 10^{-48}\,\rm g$.

hep-ex↗

arXiv2025

Nuclear spin coherence in superconducting Nb$_3$Sn

We have investigated the normal and superconducting states of the technologically important compound Nb$_3$Sn using $^{93}$Nb nuclear magnetic resonance. From spin-lattice relaxation we find strong suppression of the zero-temperature superconducting order parameter by magnetic field. Additionally we have identified an anomalously large electron-nuclear exchange interaction from spin-spin relaxation measurements, an order of magnitude beyond that of the nuclear dipolar coupling. This RKKY interaction evolves from normal to superconducting states, becoming essentially Lorentzian in the low temperature limit.

cond-mat.supr-con↗

arXiv2025

Analysis of Niobium Electropolishing Using a Generalized Distribution of Relaxation Times Method

Using electrochemical impedance spectroscopy, we have devised a method of sensing the microscopic surface conditions on the surface of niobium as it is undergoing an electrochemical polishing (EP) treatment. The method uses electrochemical impedance spectroscopy (EIS) to gather information on the surface state of the electrode without disrupting the polishing reaction. The EIS data is analyzed using a so-called distribution of relaxation times (DRT) method. Using DRT, the EIS data can be deconvolved into discrete relaxation time peaks without any a priori knowledge of the electrode dynamics. By analyzing the relaxation time peaks, we are able to distinguish two distinct modes of the EP reaction. As the polishing voltage is increased, the electrode transitions from the low voltage EP mode, characterized by a single relaxation time peaks, to the high voltage EP mode, characterized by two relaxation time peaks. We theorize that this second peak is caused by the formation of an oxide layer on the electrode. We also find that this oxide induced peak transitions from to a negative relaxation time, which is indicative of a blocking electrode process. By analyzing EPed samples, we show that samples polished in the low voltage mode have significantly higher surface roughness due to grain etching and faceting. We find that the surface roughness of the samples only improves when the oxide film peak is present and in the negative relaxation time region. This shows that EIS combined with DRT analysis can be used to predict etching on EPed Nb. This method can also be performed before or during the EP, which could allow for adjustment of polishing parameters to guarantee a smooth cavity surface finish.

cond-mat.mtrl-sci↗

arXiv2024

First characterisation of the MAGO cavity, a superconducting RF detector for kHz-MHz gravitational waves

Heterodyne detection using microwave cavities is a promising method for detecting high-frequency gravitational waves or ultralight axion dark matter. In this work, we report on studies conducted on a spherical 2-cell cavity developed by the MAGO collaboration for high-frequency gravitational waves detection. Although fabricated around 20 years ago, the cavity had not been used since. Due to deviations from the nominal geometry, we conducted a mechanical survey and performed room-temperature plastic tuning. Measurements and simulations of the mechanical resonances and electromagnetic properties were carried out, as these are critical for estimating the cavity's gravitational wave coupling potential. Based on these results, we plan further studies in a cryogenic environment. The cavity characterisation does not only provide valuable experience for a planned physics run but also informs the future development of improved cavity designs.

gr-qc↗

arXiv2024

An Improved Bound on Nonlinear Quantum Mechanics using a Cryogenic Radio Frequency Experiment

There are strong arguments that quantum mechanics may be nonlinear in its dynamics. A discovery of nonlinearity would hint at a novel understanding of the interplay between gravity and quantum field theory, for example. As such, experiments searching for potential nonlinear effects in the electromagnetic sector are important. Here we outline such an experiment, consisting of a stream of random bits (which were generated using Rigetti's Aspen-M-3 chip) as input to an RF signal generator coupled to a cryogenic detector. Projective measurements of the qubit state, which is originally prepared in an equal superposition, serve as the random binary output of a signal generator. Thereafter, spectral analysis of the RF detector would yield a detectable excess signal predicted to arise from such a nonlinear effect. A comparison between the projective measurements of the quantum bits vs the classical baseline showed no power excess. This sets a new limit on the electromagnetic nonlinearity parameter $|\epsilon| \lessapprox 1.15 \times 10^{-12}$, at a 90.0% confidence level. This is the most stringent limit on nonlinear quantum mechanics thus far and an improvement by nearly a factor of 50 over the previous experimental limit.

quant-ph↗

arXiv2024

A Precision Gyroscope from the Helicity of Light

We describe a gyroscope that measures rotation based on the effects of the rotation on the polarization of light. Rotation induces a differential phase shift in the propagation of left- and right-circularly polarized light and this phase shift can be measured in suitably designed interferometric setups. The signal in this setup is independent of the frequency of light, unlike various sources of noise such as vibrations, which cause phase shifts that depend on the frequency. Such vibrations are the practical limit on the sensitivity of conventional Sagnac-style optical interferometers that are typically used as gyroscopes. In the proposed setup, one can potentially mitigate this source of noise by simultaneously using two (or more) sources of light that have different frequencies. The signal in this setup scales with the total storage time of the light. Due to its frequency independence, it is thus most optimal to measure the signal using superconducting radio-frequency systems where the high finesse of the available cavities enables considerably longer storage times than is possible in an optical setup.

physics.optics↗

arXiv2024

Healing Gradient Degradation in Nb3Sn SRF Cavities Using a Recoating Method

Despite having advantageous superconducting properties, Nb3Sn superconducting radiofrequency (SRF) cavities still have practical challenges compared to Nb SRF cavities due to the brittle nature of Nb3Sn. Performance degradation can occur when a Nb3Sn SRF cavity experiences mechanical stresses such as during handling and tuning of the cavity. In this study, we present a potential treatment for SRF cavities that have experienced stress induced performance degradation that involves a recoating procedure. The degraded cavity is coated with a small amount of Sn using a single step vapor-diffusion methodology. Using this approach we can recover a significant portion of the lost performance of a Nb3Sn SRF cavity.

physics.acc-ph↗

arXiv2023

Nuclear Magnetic Resonance Investigation of Superconducting and Normal State Nb$_3$Sn

The superconductor Nb$_3$Sn has a high critical temperature and high critical field, widely used for high-field superconducting magnets. In this work we investigate its microscopic electronic structure with $^{93}$Nb nuclear magnetic resonance (NMR). The high-quality Nb$_3$Sn powder sample was studied in both 3.2T and 7T magnetic fields in the temperature range from 1.5K to 300K. From measurement of the spectrum and its theoretical analysis, we find evidence for anisotropy despite its cubic crystal structure. This anisotropy is manifest in alignment of powder grains under certain temperature and field cycling conditions. The Knight shift and spin-lattice relaxation rate, $T_1^{-1}$, were measured in the normal state. Additionally, $T_1^{-1}$ was measured in the superconducting state and compared with BCS theory revealing a weak field dependence, with an energy gap $\Delta(0)=2.0\pm0.08k_B T_c$ at 3.2T and $\Delta(0)=1.73\pm0.08k_B T_c$ at 7T, indicating suppression of the order parameter by magnetic field.

cond-mat.supr-con↗

arXiv2023

Phase-controlled improvement of photon lifetime in coupled superconducting cavities

High-quality cavities are crucial for various fundamental physical studies and applications. Here we find that by coupling two cavities directly or via a phase-tunable coupling channel, the photon lifetime of the local field can exceed that of the bare cavities. The cavity photon lifetime is modified by the phases of the initial states and the phase accumulation on the coupling channel which affect the interference between cavities. In experiments, by coupling superconducting radio-frequency cavities via phase-tunable cables, we realize a factor of two improvement in the cavity photon lifetime. The results can bring rich revenue to quantum information science, sensing, and high-energy physics.

quant-ph↗

arXiv2023

Improving Nb3Sn Cavity Performance Using Centrifugal Barrel Polishing

In this study we will show a new method of polishing for Nb3Sn cavities known as centrifugal barrel polishing (CBP). Using this method, Nb3Sn coated samples are polished to a surface roughness comparable to a traditional Nb cavity after electropolishing (EP). We also investigate different methods of cleaning the Nb3Sn surface after CBP to remove residual abrasive particles. The polished Nb3Sn surface is analyzed using confocal laser microscopy, and scanning electron microscopy (SEM) is used to image the surface and measure the surface roughness after polishing. Transmission electron microscopy (TEM) is also used for high resolution analysis of the surface after polishing. Finally, we show that centrifugal barrel polishing can improve the performance of a Nb3Sn SRF cavity.

physics.acc-ph↗

arXiv2023

Measuring axion gradients with photon interferometry (MAGPI)

We propose a novel search technique for axions with a $CP$-violating monopole coupling $\tilde{g}_Q$ to bulk Standard Model charges $Q \in \{B,L,B-L\}$. Gradients in the static axion field configurations sourced by matter induce achromatic circular photon birefringence via the axion-photon coupling $g_{\phi\gamma}$. Circularly polarized light fed into an optical or (open) radio-frequency (RF) Fabry-P\'erot (FP) cavity develops a phase shift that accumulates up to the cavity finesse: the fixed axion spatial gradient prevents a cancellation known to occur for an axion dark-matter search. The relative phase shift between two FP cavities fed with opposite circular polarizations can be detected interferometrically. This time-independent signal can be modulated up to non-zero frequency by altering the cavity orientations with respect to the field gradient. Multi-wavelength co-metrology techniques can be used to address chromatic measurement systematics and noise sources. With Earth as the axion source, we project reach beyond current constraints on the product of couplings $\tilde{g}_Q g_{\phi\gamma}$ for axion masses $m_{\phi} \lesssim 10^{-5} \mathrm{eV}$. If shot-noise-limited sensitivity can be achieved, an experiment using high-finesse RF FP cavities could reach a factor of $\sim 10^{5}$ into new parameter space for $\tilde{g}_Q g_{\phi\gamma}$ for masses $m_\phi \lesssim 4\times 10^{-11} \mathrm{eV}$.

hep-ph↗

arXiv2023

Selective thermal evolution of native oxide layer in Nb and Nb3Sn-coated SRF grade Nb: An in-situ angular XPS study

This contribution discusses the results of an in-situ angular XPS study on the thermal evolution of the native oxide layer on Nb3Sn and pure Nb. XPS data were recorded with conventional spectrometers using an AlK(alpha) X-ray source for spectra collected up to 600 C, and an MgK(Alpha) X-rays source for temperatures above 600 C. The effect of the thickness, composition, and thermal stability of that oxide layer is relevant to understanding the functional properties of superconducting radiofrequency (SRF) cavities used in particle accelerators. There is a consensus that oxide plays a role in surface resistance (Rs). The focus of this study is Nb3Sn, which is a promising material that is used in the manufacturing of superconducting radiofrequency (SRF) cavities as well as in quantum sensing, and pure Nb, which was included in the study for comparison. The thermal evolution of the oxide layer in these two materials is found to be quite different, which is ascribed to the influence of the Sn atom on the reactivity of the Nb atom in Nb3Sn films. Nb and Sn atoms in this intermetallic solid have different electronegativity, and the Sn atom can reduce electron density around neighbouring Nb atoms in the solid, thus reducing their reactivity for oxygen. This is shown in the thickness, composition, and thermal stability of the oxide layer formed on Nb3Sn. The XPS spectra were complemented by grazing incident XRD patterns collected using the ESRF synchrotron radiation facility. The results discussed herein shed light on oxide evolution in the Nb3Sn compound and guide its processing for potential applications of the Nb3Sn-based SRF cavities in accelerators and other superconducting devices.

cond-mat.mtrl-sci↗

arXiv2023

Evaluation of predictive correlation between flux expulsion and grain growth for superconducting radio frequency cavities

A series of experiments were carried out in an effort to develop a simple method for predicting magnetic flux expulsion behavior of high purity niobium used to fabricate superconducting radio frequency (SRF) cavities. Using conventional metallographic characterizations in conjunction with high spatial resolution electron backscattered diffraction-orientation imaging microscopy (EBSD-OIM), we found that the flux expulsion behavior of 1.3 GHz single cell SRF Nb cavities is significantly associated with the grain growth of the Nb material during heat treatment. Most of Nb grains rapidly grew during 900C heat treatment, and likely full-recrystallized with 1000C HT. With comparison of the magnetic flux expulsion ratio (Bsc/Bnc) at dT = 5 K, the flux expulsion efficiency of the cavities increases along with increasing of grain size. Most interestingly, 900C HT shows a roughly linear trend that suggests this criterion could be used to predict appropriate heat treatment temperature for sufficient flux expulsion behavior in SRF-grade Nb. This result would be used to see if flux expulsion can be predicted by examining the materials coming from the Nb vendor, prior to cavity fabrication.

physics.acc-ph↗

arXiv2022

RF Accelerator Technology R&D: Report of AF7-rf Topical Group to Snowmass 2021

Accelerator radio frequency (RF) technology has been and remains critical for modern high energy physics (HEP) experiments based on particle accelerators. Tremendous progress in advancing this technology has been achieved over the past decade in several areas highlighted in this report. These achievements and new results expected from continued R&D efforts could pave the way for upgrades of existing facilities, improvements to accelerators already under construction (e.g., PIP-II), well-developed proposals (e.g., ILC, CLIC), and/or enable concepts under development, such as FCC-ee, CEPC, C3, HELEN, multi-MW Fermilab Proton Intensity Upgrade, future Muon Colloder, etc. Advances in RF technology have impact beyond HEP on accelerators built for nuclear physics, basic energy sciences, and other areas. Recent examples of such accelerators are European XFEL, LCLS-II and LCLS-II-HE, SHINE, SNS, ESS, FRIB, and EIC. To support and enable new accelerator-based applications and even make some of them feasible, we must continue addressing their challenges via a comprehensive RF R&D program that would advance the existing RF technologies and explore the nascent ones.

physics.acc-ph↗

arXiv2022

Deepest sensitivity to wavelike dark photon dark matter with superconducting radio frequency cavities

Wavelike, bosonic dark matter candidates like axions and dark photons can be detected using microwave cavities known as haloscopes. Traditionally, haloscopes consist of tunable copper cavities operating in the TM$_{010}$ mode, but ohmic losses have limited their performance. In contrast, superconducting radio frequency (SRF) cavities can achieve quality factors of $\sim 10^{10}$, perhaps five orders of magnitude better than copper cavities, leading to more sensitive dark matter detectors. In this paper, we first derive that the scan rate of a haloscope experiment is proportional to the loaded quality factor $Q_L$, even if the cavity bandwidth is much narrower than the dark matter halo line shape. We then present a proof-of-concept search for dark photon dark matter using a nontunable ultrahigh quality SRF cavity. We exclude dark photon dark matter with kinetic mixing strengths of $\chi > 1.5\times 10^{-16}$ for a dark photon mass of $m_{A^{\prime}} = 5.35\mu$eV, achieving the deepest exclusion to wavelike dark photons by almost an order of magnitude.

hep-ex↗