arXiv subjects

J. S. Bobowski

Publications and source records attributed to J. S. Bobowski.

At least 19 recordsLinked to original sources

arXiv2023

Transients in lossy transmission lines

Numerical inverse Laplace transforms are used to analyze the transient response of non-ideal transmission lines to a voltage step. We find that the detailed shape of the transient response is sensitive conductor losses, but insensitive to dielectric losses. Furthermore, we find that the low-loss approximations for the transmission line propagation constant and characteristic impedance in the complex-frequency domain are sufficient to accurately model the observed transient response. We also investigate the effects of: (1) a parasitic capacitance terminating the open end of the coaxial transmission line, (2) the input impedance of the oscilloscope used to make the measurements, and (3) the finite rise time of the voltage step. Finally, we cool a semi-rigid coaxial cable in liquid nitrogen so as to reduced conductor lossless and observe a transient response that is closer to that expected from an ideal lossless line.

physics.comp-ph↗

arXiv2023

Non-local electrodynamics in ultra-pure PdCoO$_{2}$

The motion of electrons in the vast majority of conductors is diffusive, obeying Ohm's law. However, the recent discovery and growth of high-purity materials with extremely long electronic mean free paths has sparked interest in non-ohmic alternatives, including viscous and ballistic flow. Although non-ohmic transport regimes have been discovered across a range of materials, including two-dimensional electron gases, graphene, topological semimetals, and the delafossite metals, determining their nature has proved to be challenging. Here, we report on a new approach to the problem, employing broadband microwave spectroscopy of the delafossite metal PdCoO$_{2}$ in three distinct sample geometries that would be identical for diffusive transport. The observed differences, which go as far as differing power laws, take advantage of the hexagonal symmetry of PdCoO$_{2}$. This permits a particularly elegant symmetry-based diagnostic for non-local electrodynamics, with the result favouring ballistic over strictly hydrodynamic flow. Furthermore, it uncovers a new effect for ballistic electron flow, owing to the highly facetted shape of the hexagonal Fermi surface. We combine our extensive dataset with an analysis of the Boltzmann equation to characterize the non-local regime in PdCoO$_{2}$. More broadly, our results highlight the potential of broadband microwave spectroscopy to play a central role in investigating exotic transport regimes in the new generation of ultra-high conductivity materials.

cond-mat.mes-hall↗

arXiv2021

Inductive Power Transfer Through Saltwater

We investigated inductive power transfer (IPT) through a rectangular slab of saltwater. Our inductively-coupled transmitters and receivers were made from loop-gap resonators (LGRs) having resonant frequencies near 100 MHz. Electric fields are confined within the narrow gaps of the LGRs making it possible to strongly suppress the power dissipation associated with electric fields in a conductive medium. Therefore, the power transfer efficiency in our system was limited by magnetic field dissipation in the conducting medium. We measured the power transfer efficiency as a function of both the conductivity of the water and the resonant frequency of the LGRs. We also present an equivalent circuit model that can be used to model IPT through a conductive medium. Finally, we show that using dividers to partition the saltwater volume provides another means of enhancing power transfer efficiency.

physics.app-ph↗

arXiv2021

SNEWS 2.0: A Next-Generation SuperNova Early Warning System for Multi-messenger Astronomy

The next core-collapse supernova in the Milky Way or its satellites will represent a once-in-a-generation opportunity to obtain detailed information about the explosion of a star and provide significant scientific insight for a variety of fields because of the extreme conditions found within. Supernovae in our galaxy are not only rare on a human timescale but also happen at unscheduled times, so it is crucial to be ready and use all available instruments to capture all possible information from the event. The first indication of a potential stellar explosion will be the arrival of a bright burst of neutrinos. Its observation by multiple detectors worldwide can provide an early warning for the subsequent electromagnetic fireworks, as well as signal to other detectors with significant backgrounds so they can store their recent data. The Supernova Early Warning System (SNEWS) has been operating as a simple coincidence between neutrino experiments in automated mode since 2005. In the current era of multi-messenger astronomy there are new opportunities for SNEWS to optimize sensitivity to science from the next Galactic supernova beyond the simple early alert. This document is the product of a workshop in June 2019 towards design of SNEWS 2.0, an upgraded SNEWS with enhanced capabilities exploiting the unique advantages of prompt neutrino detection to maximize the science gained from such a valuable event.

astro-ph.HE↗

arXiv2020

The Hydraulic Analogues of Basic Circuits: Labs for Online Learning Environments

In this paper we describe a couple of exercises that we developed for our online first-year EM course. The labs that we describe are focused on basic concepts from circuit analysis, namely Ohm's law and charging a capacitor with a constant voltage source and a series resistor. To give students a visual demonstration of these concepts and, simultaneously, provide a means for students to collect their own unique dataset, hydraulic analogues of these electric circuits were built and a series of videos were recorded. Incidentally, the visual demonstrations may also help to alleviate some of the commonly-held student misconceptions about basic circuits. For this reason, even once in-person labs resume, we expect that the hydraulic analogue demonstrations will continue to be useful.

physics.ed-ph↗

arXiv2020

Modeling and measuring the non-ideal characteristics of transmission lines

We describe a simple method to experimentally determine the frequency dependencies of the per-unit-length resistance and conductance of transmission lines. The experiment is intended as a supplement to the classic measurement of the transient response of a transmission line to a voltage step or pulse. In the transient experiment, an ideal (lossless) model of the transmission line is used to determine the characteristic impedance and signal propagation speed. In our experiment, the insertion losses of various coaxial cables are measured as a function of frequency from 1 to 2000 MHz. A full distributed circuit model of the transmission line that includes both conductor and dielectric losses is needed to fit the frequency dependence of the measured insertion losses. Our model assumes physically-sensible frequency dependencies for the per-unit-length resistance and conductance that are determined by the geometry of the coaxial transmission lines used in the measurements.

physics.ed-ph↗

arXiv2020

Permittivity of waste-activated sludge by an open-ended coaxial line

The complex permittivity of thickened waste activated sludge (WAS) was measured from 3 MHz to 40 GHz. The solid content of the thickened WAS sample was varied from 4.5% to 18% by weight. The permittivity spectra exhibit features typical of biological tissues that have a high water content. At high frequencies, a Debye-type dispersion is observed with a relaxation rate of 19 GHz characteristic of the bulk water in the sample ($γ$-dispersion). At lower frequencies, the solid content of the samples determines the properties of the permittivity. The onset of the so-called $β$-dispersion, attributed to the charging of cell membranes, occurs between 10-100 MHz. For samples with higher solid concentrations, a weak dispersion of the real part of the permittivity, characteristic of bound water, was observed at intermediate frequencies ($δ$-dispersion).

physics.app-ph↗

arXiv2020

Permittivity measurements of biological samples by an open-ended coaxial line

We previously reported on the complex permittivity and dc conductivity of waste-activated sludge. The measurements, spanning a frequency range of 3 MHz to 40 GHz, were made using an open-ended coaxial transmission line. Although this technique is well established in the literature, we found that it was necessary to combine methods from several papers to use the open-ended coaxial probe to reliably characterize biological samples having a high dc conductivity. Here, we provide a set of detailed and practical guidelines that can be used to determine the permittivity and conductivity of biological samples over a broad frequency range. Due to the electrode polarization effect, low frequency measurements of conducting samples require corrections to extract the intrinsic electrical properties. We describe one practical correction scheme and verify its reliability using a control sample.

physics.app-ph↗

arXiv2018

Automation of the Cavendish Experiment to 'Weigh the Earth'

We describe a simple and inexpensive method for automating the data collection in the well-known Cavendish torsion balance experiment to determine the gravitational constant $G$. The method uses a linear array of phototransistors and requires no moving parts. Multiplexers and a data-acquisition device are used to sample the state of each phototransistor sequentially. If the sampled phototransistor is illuminated by the laser spot, the position and time are recorded to a data file. The recorded data does an excellent job of capturing the damped harmonic oscillations. The resulting data were analysed to extract an experimental value of $G$ that was within 5% of the accepted value.

physics.ed-ph↗

arXiv2018

Design and characterization of a novel toroidal split-ring resonator

The design and characterization of a novel toroidal split-ring resonator (SRR) is described in detail. In conventional cylindrical SRRs, there is a large magnetic flux within the bore of the resonator. However, there also exists a non-negligible magnetic flux in the free space surrounding the resonator. The energy losses associated with this radiated power diminish the resonator's quality factor. In the toroidal SRR, on the other hand, the magnetic field lines are strongly confined within the bore of the resonator resulting in high intrinsic quality factors and stable resonance frequencies without requiring additional electromagnetic shielding. This paper describes the design and construction of a toroidal SRR as well as an experimental investigation of its cw response in the frequency-domain and its time-domain response to an rf pulse. Additionally, the dependence of the toroidal SRR's resonant frequency and quality factor on the strength of inductive coupling to external circuits is investigated both theoretically and experimentally.

physics.ins-det↗

arXiv2018

Non-radiative mid-range wireless power transfer: An experiment for senior physics undergraduates

A wireless power transfer experiment suitable for senior physics undergraduates that operates between 3 and 4 MHz is described and demonstrated in detail. The apparatus consists of a pair of identical resonant coils that can be moved relative to one another. A signal generator circuit is inductively coupled to the transmitting coil using a single loop of wire. Likewise, a single loop of wire couples the receiving coil to a load impedance. A matching circuit was used to tune the impedance of the system of coupled resonators to match the 50-$\ohm$ output impedance of the signal generator. A low-cost vector network analyzer was used to characterize the system performance as the distance between the pair of coils was changed. When the distance between the transmitting and receiving coils was small, a double resonance emerged. The frequency difference between the pair resonances was inversely proportional to the distance between the coils. After the system was fully characterized, it was operated with up to 20 W of incident power. Our measurements revealed that, at weak coupling, the fraction of the incident power lost to radiation increases linearly with the distance between the coils. Finally, we were able to transmit enough power to dimly light a 60-W incandescent light bulb.

physics.ed-ph↗

arXiv2018

Improved Single-Crystal Growth of Sr$_2$RuO$_4$

High-quality single crystals are essentially needed for the investigation of the novel bulk properties of unconventional superconductors. The availability of such crystals grown by the floating-zone method has helped to unveil the unconventional superconductivity of the layered perovskite Sr$_2$RuO$_4$, which is considered as a strong candidate of a topological spin-triplet superconductor. Yet, recent progress of investigations urges further efforts to obtain ultimately high-quality crystalline samples. In this paper, we focus on the method of preparation of feed rods for the floating-zone melting and report on the improvements of the crystal growth. We present details of the improved methods used to obtain crystals with superconducting transition temperatures $T_\mathrm{c}$ that are consistently as high as 1.4 K, as well as the properties of these crystals.

cond-mat.supr-con↗

arXiv2018

Using split-ring resonators to measure the electromagnetic properties of materials: An experiment for senior physics undergraduates

A spilt-ring resonator experiment suitable for senior physics undergraduates is described and demonstrated in detail. The apparatus consists of a conducting hollow cylinder with a narrow slit along its length and can be accurately modelled as a series $LRC$ circuit. The resonance frequency and quality factor of the split-ring resonator are measured when the apparatus is suspended in air, submerged in water, and submerged in an aqueous solution of various concentrations of NaCl. The experimental results are used to extract the dielectric constant of water and to investigate the dependence of the resonator quality factor on the conductivity of the NaCl solution. The apparatus provides opportunities to experimentally examine radiative losses, complex permittivity, the electromagnetic skin depth, and cutoff frequencies of rf propagation in cylindrical waveguides, which are all concepts introduced in an undergraduate course in electrodynamics. To connect with current research, the use of split-ring resonators as a tool to precisely measure the electromagnetic properties of materials is emphasized

physics.ed-ph↗

arXiv2018

Quantitative Stirling Cycle Measurements: P-V Diagram and Refrigeration

This paper describes simple modifications to a demonstration Stirling engine that allows one to make quantitative measurements of the Stirling cycle. First, we describe measurements of the $P$-$V$ diagram using an inexpensive pressure sensor and a common photogate. Then, as a supplement, the engine was run as a refrigerator by using a rotary tool to turn the flywheel of the engine. A small, but measurable, temperature difference developed across the body of the engine.

physics.ed-ph↗

arXiv2018

Ferromagnetic resonance in the complex permeability of an Fe$_3$O$_4$ nanosuspension at radio and microwave frequencies

The complex permeability of an iron-oxide nanosuspension has been measured as a function of magnetic field strength at RF and microwave frequencies using a loop-gap resonator. The particles were suspended in water and had an 8-nm diameter Fe$_3$O$_4$ core that was coated by Dextran. The real part of the permeability increased sharply beyond a frequency-dependent threshold value of the static magnetic field before saturating. Just beyond this threshold field, there was a peak in the imaginary part of the permeability. The permeability measurements, which exhibited features associated with ferromagnetic resonance, were used to determine the dependence of the microwave absorption on static magnetic field strength. Using the absorption data, the $g$-factor of the nanosuspension was found to be $1.86\pm 0.07$.

cond-mat.mes-hall↗

arXiv2010

Precision Microwave Electrodynamic Measurements of K- and Co-doped BaFe$_2$As$_2$

We have studied the microwave electrodynamics of single crystal iron-based superconductors Ba$_{0.72}$K$_{0.28}$Fe$_2$As$_2$ (hole-doped, $T_\mathrm{c}$ $\approx $30 K) and Ba(Fe$_{0.95}$Co$_{0.05}$)$_2$As$_2$ (electron-doped, $T_\mathrm{c}$ $\approx$20 K), by cavity perturbation and broadband spectroscopy. SQUID magnetometry was used to confirm the quality and homogeneity of the samples under study. Through cavity perturbation techniques, the temperature dependence of the in-plane London penetration depth $Δλ(T)$, and therefore the superfluid phase stiffness $λ^2(0)/λ^2(T)$ was measured. Down to 0.4 K, the data do not show the exponential saturation at low temperatures expected from a singly-, fully-gapped superconductor. Rather, both the electron- and the hole-doped systems seem to be best described by a power law behavior, with $λ^2(0)/λ^2(T)$ $\sim$ $T^n$ and \emph{n} $\approx$ 2.5. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near $T/T_\mathrm{c}$ =~0.04, hinting at a corresponding low energy feature in the superconducting density of states. The data can also be relatively well-described by a simple two-gap s-wave model of the order parameter, but this yields parameters which seem unrealistic and dependent on the fit range. Broadband surface resistance measurements reveal a sample dependent residual loss whose origin is unclear. The data from the \FeAs samples can be made to scale as $ω^2$ if the extrinsic loss is treated as an additive component, indicating large scattering rates. Finally, the temperature dependence of the surface resistance at 13 GHz obeys a power law very similar to those observed for $Δλ(T)$.

cond-mat.supr-con↗

arXiv2008

Stability of nodal quasiparticles in underdoped YBa2Cu3O6+y probed by penetration depth and microwave spectroscopy

High resolution measurements of superfluid density and broadband quasiparticle conductivity have been used to probe the low energy excitation spectrum of nodal quasiparticles in underdoped YBCO. Penetration depth is measured to temperatures as low as 0.05 K. Microwave conductivity is measured from 0.1 to 20 GHz and is a direct probe of zero-energy quasiparticles. The data are compared with predictions for a number of theoretical scenarios that compete with or otherwise modify pure d-wave superconductivity, in particular commensurate and incommensurate spin and charge density waves; d + i s and d + i d superconductivity; circulating current phases; and the BCS--BEC crossover. We conclude that the data are consistent with a pure d-wave state in the presence of a small amount of strong scattering disorder, and are able to rule out most candidate competing states either completely, or to a level set by the energy scale of the disorder, ~ 4 K. Commensurate spin and charge density orders, however, are not expected to alter the nodal spectrum and therefore cannot be excluded.

cond-mat.supr-con↗

arXiv2006

Oxygen Chain Disorder as the Weak Scattering Source in YBa$_2$Cu$_3$O$_{6.50}$

The microwave conductivity of an ultra-pure single crystal of YBa$_2$Cu$_3$O$_{6.50}$ has been measured deep in the superconducting state as a continuous function of frequency from $0.5\to20$ GHz. Conductivity spectra were first measured at four temperatures below 10 K after having prepared the crystal in the so called ortho-II phase in which the CuO chain oxygen are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities (small scattering phaseshift) in an otherwise clean d-wave superconductor. The measurements were repeated on the same crystal after heating and then rapidly quenching the sample to reduce the degree of oxygen order in the CuO chains. With the increased disorder, the conductivity spectra retain the distinctive weak-limit scattering features, but have increased widths reflecting an increase in quasiparticle scattering. These measurements unambiguously establish that CuO chain oxygen disorder is the dominant source of in-plane quasiparticle scattering in high purity YBCO.

cond-mat.supr-con↗