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Due to the COVID-19 outbreak, academic institutions across the globe have been forced to move to online learning environments. It has been particularly challenging for the experimental sciences to develop and deliver authentic lab-based experiences. Some of the strategies that have been adopted for first-year physics labs include: providing a video demonstration and supplying data for students to analyze, using online simulations, having students construct simple apparatuses using commonly available materials, collecting data with the aid of a smartphone, and, when dealing with smaller class sizes, providing a custom lab kit to each student. Many institutions have chosen to adopt a blend of the various strategies.

We describe efficient four-coil inductive power transfer (IPT) systems that operate at 100 MHz. The magnetically-coupled transmitter and receiver were made from electrically-small and high-Q loop-gap resonators (LGRs). In contrast to the commonly-used helical and spiral resonators, the LGR design has the distinct advantage that electric fields are strongly confined to the capacitive gap of the resonator. With negligible fringing electric fields in the surrounding space, the IPT system is immune to interference from nearby dielectric objects, even when they are in close proximity to the transmitter and/or receiver. We experimented with both cylindrical and split-toroidal LGR geometries. Although both systems performed well under laboratory conditions, the toroidal geometry has the additional advantage that the magnetic flux is weak everywhere except within the bore of the LGR and in the space directly between the transmitter and receiver. Furthermore, we show that the toroidal LGR system can be operated efficiently at a fixed frequency for a wide range of transmitter-receiver distances. The experimental results are complimented by 3-D finite-element simulations which were used to investigate the electromagnetic field profiles and surface current density distributions. Finally, we demonstrate the use of our IPT system at powers up to 32 W and discuss possible applications., Comment: 18 pages, 12 figures

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., Comment: 10 pages, 6 figures

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., 5 pages, 5 figures

This work has been financially supported by the Deutsche Forschungsgemeinschaft (GR 4667/1, GRK 1621 and SFB 1143 projects C02 and C09) and the Max Planck Society. Y.M., T.M. and J.S.B. acknowledge the financial support of JSPS Kakenhi (JP15H5852, JP15K21717 and JP17H06136) and the JSPS Core-to-Core Program. N.K. acknowledges the financial support from JSPS Kakenhi (no. JP18K04715) and JST-Mirai Program (no. JPMJMI18A3). A.N. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement no. 701647. Strontium ruthenate (Sr2RuO4) continues to present an important test of our understanding of unconventional superconductivity, because while its normal-state electronic structure is known with precision, its superconductivity remains unexplained. There is evidence that its order parameter is chiral, but reconciling this with recent observations of the spin part of the pairing requires an order parameter that is either finely tuned or implies a new form of pairing. Therefore, a definitive resolution of whether the superconductivity of Sr2RuO4 is chiral is important for the study of superconductivity. Here we report the measurement of zero-field muon spin relaxation—a probe sensitive to weak magnetism—on samples under uniaxial stresses. We observe stress-induced splitting between the onset temperatures of superconductivity and time-reversal symmetry breaking—consistent with the qualitative expectations for a chiral order parameter—and argue that this observation cannot be explained by conventional magnetism. In addition, we report the appearance of bulk magnetic order under higher uniaxial stress, above the critical pressure at which a Lifshitz transition occurs in Sr2RuO4. Postprint

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., 28 pages, 10 figures

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., Comment: 11 pages, 8 figures

We describe a simple AC susceptometer built in-house that can be used to make high-resolution measurements of the magnetic susceptibility of high-temperature superconductors in an undergraduate physics lab. Our system, cooled using liquid nitrogen, can reach a base temperature of 77 K. Our apparatus does not require gas handling systems or PID temperature controllers. Instead, it makes use of a thermal circuit that is designed to allow the sample to cool on a time scale that is suitable for an undergraduate lab. Furthermore, the temperature drift rate at the superconducting transition temperature $T_\mathrm{c}$ is low enough to allow for precise measurements of the complex magnetic susceptibility through $T_\mathrm{c}$, even for single-crystal samples with exceedingly sharp superconducting transitions. Using an electromagnet, we were able to apply static magnetic fields up to 63 mT at the sample site. By measuring the change in susceptibility as a function of the strength of an applied of static magnetic field, we were able to estimate the lower critical field $H_\mathrm{c1}$ of a single-crystal sample of optimally-doped YBa$_2$Cu$_3$O$_{6.95}$ at 77 K. We also investigated the mixed state of a sintered polycrystalline sample of YBa$_2$Cu$_3$O$_{6+y}$., Comment: 19 pages, 11 figures

We describe and demonstrate the use of a novel toroidal split-ring resonator to make accurate and precise measurements of the permittivity of liquids and gases. We first analytically show how the resonance frequency and quality factor of the resonator are related to the complex permittivity of the material filling its gap. We then use the resonator to experimentally determine the permittivity of a number of different materials. First, the compact and high-Q resonator is used to measure both the real and imaginary parts of the complex permittivity of methyl alcohol at 185 MHz. Second, the resonator was placed inside a vacuum-tight Dewar. We measured the resonance frequency with the resonator suspended in vacuum and then immersed in an atmosphere of air. From these data, the dielectric constant of air was accurately determined. Next, the resonator was submerged in liquid nitrogen and the boiling temperature of the nitrogen bath was manipulated by regulating its vapor pressure. This system allowed for a precise measurement of the dielectric constant of liquid nitrogen over a temperature range of 64 to 77 K. Finally, we monitored the quality factor of the copper resonator as its temperature drifted from 80 K to room temperature. From these data, we extracted the linear temperature dependence of copper's resistivity., 7 pages, 8 figures

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 ($\gamma$-dispersion). At lower frequencies, the solid content of the samples determines the properties of the permittivity. The onset of the so-called $\beta$-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 ($\delta$-dispersion)., Comment: 11 pages, 3 figures

We have measured the relative permeability of split-ring resonator (SRR) arrays used in metamaterials designed to have $��^\prime< 0$ over a narrow range of microwave frequencies. The SRR arrays were loaded into the bore of a loop-gap resonator (LGR) and reflection coefficient measurements were used to determine both the real and imaginary parts of the array's effective permeability. Data were collected as a function of array size and SRR spacing. The results were compared to those obtained from continuous extended split-ring resonators (ESRRs). The arrays of planar SRRs exhibited enhanced damping and a narrower range of frequencies with $��^\prime, 11 pages, 5 figure

A single-contact voltage sensor designed for accurate measurements of ac voltages across a pair of conductors is described. The sensor design is motivated by remote monitoring applications where accurate voltage measurement of high-voltage transmission lines is required. The body of the sensor is electrically and mechanically attached to a single conductor: either the neutral or high-voltage conductor. A capacitive sensing plate attached to the sensor creates a capacitive voltage divider using the stray capacitance to the non-contacted line. A very high-impedance buffer is used to measure the voltage across the divider output and estimate the line voltage. An important part of the work includes a method of calibrating the sensor such that blind voltage measurements can be made without knowing the exact geometry of the conductors. Other important aspects of the design include a two-stage voltage divider for retaining accuracy and increasing the voltage range of the sensor. The work is supported by extensive numerical simulation models which were used to determine the optimum design for the sensing plate and to evaluate the sensitivity to different configurations including conductor spacing and the height above ground. For calibration values which are accurate to 1%, the line voltage can be measured with an accuracy of 10%. The paper describes the theory, design, and experimental verification of the sensor up to a line voltage of 7.5 kVrms., Comment: 12 pages, 13 figures

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., 9 pages, 6 figures

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., 7 pages, 9 figures

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, Comment: 10 pages, 7 figures

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., Comment: 11 pages, 9 figures

A method is proposed to experimentally determine the effective complex permeability of split-ring resonator (SRR) arrays used in the design of metamaterials at microwave frequencies. We analyze the microwave response of a loop-gap resonator (LGR) whose bore has been partially loaded with one or more SRRs. Our analysis reveals that the resonance frequency, magnetic plasma frequency, and damping constant of the effective permeability of the SRR array can be extracted from fits to the reflection coefficient (S11) of an inductively-coupled LGR. We propose LGR designs that would allow both a one-dimensional array of SRRs and small three-dimensional arrays of SRRs to be characterized. Finally, we demonstrate the method using a toroidal LGR loaded with a single extended SRR of length z., 12 pages, 10 figures

The complex permeability of an iron-oxide nanoparticle suspension has been measured as a function of magnetic field strength at radio and microwave frequencies using a loop-gap resonator. The system studied was a water-based ferrofluid containing Dextran-coated Fe3O4 particles with an average diameter of 8 nm. 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 ferrofluid was found to be 1.86 ± 0.07.

Using ultrasonic techniques, the phonon diffusion coefficient and density of states have been measured at intermediate frequencies in a porous solid network of sintered glass beads. A transition to a very strong scattering regime is observed, whereupon the diffusion coefficient becomes independent of frequency. Near crossover, the density of states is enhanced and exhibits a broad maximum. These data are in good overall agreement with a lattice-based percolation model, and give additional insight into the effects of strong scattering on phonon transport in disorderd media. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A novel low temperature bolometric method has been devised and implemented for high-precision measurements of the microwave surface resistance of small single-crystal platelet samples having very low absorption, as a continuous function of frequency. The key to the success of this non-resonant method is the in-situ use of a normal metal reference sample that calibrates the absolute rf field strength. The sample temperature can be controlled independently of the 1.2 K liquid helium bath, allowing for measurements of the temperature evolution of the absorption. However, the instrument's sensitivity decreases at higher temperatures, placing a limit on the useful temperature range. Using this method, the minimum detectable power at 1.3 K is 1.5 pW, corresponding to a surface resistance sensitivity of $\approx$1 $\mu\Omega$ for a typical 1 mm$\times$1 mm platelet sample., Comment: 13 pages, 12 figures, submitted to Review of Scientific Instruments

Microwave (2.45 GHz, 1200 W) and conventional heating (custom pressure vessel) pretreatments were applied to dewatered municipal waste sludge (18% total solids) using identical heating profiles that span a wide range of temperatures (80-160 °C). Fourteen lab-scale semi-continuous digesters were set up to optimize the energy (methane) output and sludge retention time (SRT) requirements of untreated (control) and thermally pretreated anaerobic digesters operated under mesophilic and thermophilic temperatures. Both pretreatment methods indicated that in the pretreatment range of 80-160 °C, temperature was a statistically significant factor (p-value0.05) for increasing solubilization of chemical oxygen demand and biopolymers (proteins, sugars, humic acids) of the waste sludge. However, the type of pretreatment method, i.e. microwave versus conventional heating, had no statistically significant effect (p-value0.05) on sludge solubilization. With the exception of the control digesters at a 5-d SRT, all control and pretreated digesters achieved steady state at all three SRTs, corresponding to volumetric organic loading rates of 1.74-6.96 g chemical oxygen demand/L/d. At an SRT of 5 d, both mesophilic and thermophilic controls stopped producing biogas after 20 d of operation with total volatile fatty acids concentrations exceeding 1818 mg/L at pH5.64 for mesophilic and 2853 mg/L at pH7.02 for thermophilic controls, while the pretreated digesters continued producing biogas. Furthermore, relative (to control) organic removal efficiencies dramatically increased as SRT was shortened from 20 to 10 and then 5 d, indicating that the control digesters were challenged as the organic loading rate was increased. Energy analysis showed that, at an elevated temperature of 160 °C, the amount of methane recovered was not enough to compensate for the energy input. Among the digesters with positive net energy productions, control and pretreated digesters at 80 °C were more favorable at an SRT of 10 d.

The microwave conductivity of ultrapure single crystals of ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.5}$ has been measured deep in the superconducting state as a continuous function of frequency using a broadband spectrometer. 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 Cu-O chain oxygens are ordered into alternating full and empty chains. These spectra exhibit features expected for quasiparticle scattering from dilute weak impurities (small scattering phase shift) in a $d$-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 Cu-O chains. The disordered conductivity spectra retained the weak-limit scattering features. However, the spectra were broadened due to increased quasiparticle scattering which we have identified as increased scattering from disorder in the Cu-O chains. The quasiparticle conductivity spectral widths of a new generation of ultrahigh-purity ortho-II ordered ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.5}$ samples, with an order of magnitude lower cation impurity concentration, were also measured and found to be unchanged from those in the older generation samples. It is concluded for both generations of ortho-II ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6.5}$ crystals that the widths of the quasiparticle conductivity spectra are largely determined by residual disorder in the Cu-O chains.

We have studied the microwave electrodynamics of single-crystal iron-based superconductors ${\text{Ba}}_{0.72}{\text{K}}_{0.28}{\text{Fe}}_{2}{\text{As}}_{2}$ (hole doped, ${T}_{\text{c}}\ensuremath{\approx}30\text{ }\text{K}$) and $\text{Ba}{({\text{Fe}}_{0.95}{\text{Co}}_{0.05})}_{2}{\text{As}}_{2}$ (electron doped, ${T}_{\text{c}}\ensuremath{\approx}20\text{ }\text{K}$), by cavity perturbation and broadband spectroscopy. Meissner curves were 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 $\ensuremath{\Delta}\ensuremath{\lambda}(T)$, and therefore the superfluid phase stiffness ${\ensuremath{\lambda}}^{2}(0)/{\ensuremath{\lambda}}^{2}(T)$ 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 ${\ensuremath{\lambda}}^{2}(0)/{\ensuremath{\lambda}}^{2}(T)\ensuremath{\sim}{T}^{n}$ and $n\ensuremath{\approx}2.5$. In the three samples we studied, a weak feature near the sensitivity limit of our measurements appears near $T/{T}_{\text{c}}=0.04$, 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 ${\text{Ba}}_{0.72}{\text{K}}_{0.28}{\text{Fe}}_{2}{\text{As}}_{2}$ samples can be made to scale as ${\ensuremath{\omega}}^{2}$ 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 $\ensuremath{\Delta}\ensuremath{\lambda}(T)$.

High-resolution measurements of superfluid density ${\ensuremath{\rho}}_{s}(T)$ and broadband quasiparticle conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\Omega})$ have been used to probe the low-energy excitation spectrum of nodal quasiparticles in underdoped ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{6+y}$. Penetration depth $\ensuremath{\lambda}(T)$ is measured to temperatures as low as 0.05 K. ${\ensuremath{\sigma}}_{1}(\ensuremath{\Omega})$ 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 ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ superconductivity, in particular, commensurate and incommensurate spin and charge-density waves; ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+is$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}+i{d}_{xy}$ superconductivity; circulating current phases; and the BCS-BEC crossover. We conclude that the data are consistent with a pure ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ 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, ${T}_{d}\ensuremath{\sim}4\text{ }\text{K}$. Commensurate spin and charge-density orders, however, are not expected to alter the nodal spectrum and therefore cannot be excluded.

Extensive measurements of the microwave conductivity of highly pure and oxygen-ordered $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ single crystals have been performed as a means of exploring the intrinsic charge dynamics of a $d$-wave superconductor. Broadband and fixed-frequency microwave apparatus together provide a very clear picture of the electrodynamics of the superconducting condensate and its thermally excited nodal quasiparticles. The measurements reveal the existence of very long-lived excitations deep in the superconducting state, as evidenced by sharp cusplike conductivity spectra with widths that fall well within our experimental bandwidth. We present a phenomenological model of the microwave conductivity that captures the physics of energy-dependent quasiparticle dynamics in a $d$-wave superconductor which, in turn, allows us to examine the scattering rate and oscillator strength of the thermally excited quasiparticles as functions of temperature. Our results are in close agreement with the Ferrell-Glover-Tinkham sum rule, giving confidence in both our experiments and the phenomenological model. Separate experiments for currents along the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{a}$ and $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{b}$ directions of detwinned crystals allow us to isolate the role of the CuO chain layers in $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$, and a model is presented that incorporates both one-dimensional conduction from the chain electrons and two-dimensional transport associated with the $\mathrm{Cu}{\mathrm{O}}_{2}$ plane layers.

Zero-field electron spin resonance (ESR) of dilute Gd ions substituted for Y in the cuprate superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ is used as a novel technique for measuring the absolute value of the low-temperature magnetic penetration depth $\ensuremath{\lambda}(\stackrel{\ensuremath{\rightarrow}}{T}0).$ The Gd ESR spectrum of samples with $\ensuremath{\approx}1%$ substitution was obtained with a broadband microwave technique that measures power absorption bolometrically from 0.5 to 21 GHz. This ESR spectrum is determined by the crystal field that lifts the level degeneracy of the spin $7/2{\mathrm{Gd}}^{3+}$ ion and details of this spectrum provide information concerning oxygen ordering in the samples. The magnetic penetration depth is obtained by relating the number of Gd ions exposed to the microwave magnetic field to the frequency-integrated intensity of the observed ESR transitions. This technique has allowed us to determine precise values of $\ensuremath{\lambda}$ for screening currents flowing in the three crystallographic orientations $(\^a,$ $\mathrm{b\ifmmode \hat{}\else \^{}\fi{}},$ and $\ifmmode \hat{c}\else \^{c}\fi{})$ in samples of ${\mathrm{Gd}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ of three different oxygen contents $y=0.993{(T}_{c}=89\mathrm{K}),$ $y=0.77{(T}_{c}=75\mathrm{K}),$ and $y=0.52{(T}_{c}=56\mathrm{K}).$ The in-plane values are found to depart substantially from the widely reported relation ${T}_{c}\ensuremath{\propto}1/{\ensuremath{\lambda}}^{2}.$

When a magnetic field is applied perpendicular to the CuO 2 planes of cuprate high temperature superconductors, the normal fluid response is expected to include contributions from quasiparticles with relevant mean free paths greater than the penetration depth. As a result, a non-local electrodynamical response is expected. Optimally doped single crystals of YBCO have been prepared such that a microwave field can be applied parallel to the c -axis with negligible demagnetization effects. We report on efforts to identify non-local quasiparticle effects in these crystals using cavity perturbation measurements of the microwave surface resistance.

The real part of the microwave conductivity of an ultra-pure sample of YBa 2 Cu 3 O 6.50 deep in the superconducting state has been measured as a continuous function of frequency. Conductivity spectra were first measured with the Cu–O chain oxygens atoms highly ordered in the ortho-II phase (alternating full and empty chains). The measurements were then repeated after heating and quenching the sample to reduce the degree of oxygen order in the Cu–O chains which lowered T c from 55 K to 49 K. Disordering the Cu–O chains produced conductivity spectra that were more than twice as broad as those of the ortho-II ordered phase. In both cases the width of the conductivity spectra increases approximately linearly with temperature. Moreover, within our experimental resolution we find that the low-temperature quasiparticle spectral weight does not depend on the degree of oxygen orderer in the Cu–O chains. Each of these features indicates that oxygen disorder in the Cu–O chains acts as a source of weak-limit scattering for quasiparticles propagating in the CuO 2 planes.

Sintered networks of glass beads form an interesting example of a porous medium, analogous to a very porous rock, in which very strong multiple scattering of elastic waves is observed when the ultrasonic wavelength is comparable with the size of the pores. To investigate the diffusive transport of energy by multiply scattered waves, the transmission of the diffuse energy flux through finite slabs of this material has been measured. From these data, the diffusion coefficient D, as well as the absorption time, was determined by fitting the predictions of the diffusion approximation to the experimental time‐of‐flight profiles. To accurately measure D, the internal reflection of diffuse waves at the sample boundaries was taken into account by extending the method used previously for acoustic waves [J. H. Page et al., Phys. Rev. E 52, 3106 (1995)]. The frequency dependence of the diffusion coefficient was measured over an extended range of frequencies, and compared with estimates of D from ballistic measurements of the scattering mean free path and group velocity. Because of its relatively simple structure, this material may be an ideal system for probing the diffusion of elastic waves, where diffuse waves have mixed character consisting of both longitudinal and transverse polarizations.

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., Comment: 66 pages, 11 figures