Jonathan B. Betts

Violation of Ohm’s law in a Weyl metal

Ohms law is a fundamental paradigm in the electrical transport of metals. Any transport signatures violating Ohms law would give an indisputable fingerprint for a novel metallic state. Here, we uncover the breakdown of Ohms law owing to a topological structure of the chiral anomaly in the Weyl metal phase. We observe nonlinear I-V characteristics in Bi0.96Sb0.04 single crystals in the diffusive limit, which occurs only for a magnetic-field-aligned electric field (E∥B). The Boltzmann transport theory with the charge pumping effect reveals the topological-in-origin nonlinear conductivity, and it leads to a universal scaling function of the longitudinal magnetoconductivity, which completely describes our experimental results. As a hallmark of Weyl metals, the nonlinear conductivity provides a venue for nonlinear electronics, optical applications, and the development of a topological Fermi-liquid theory beyond the Landau Fermi-liquid theory.

High-field properties of carbon-doped MgB2 thin films by hybrid physical–chemical vapor deposition using different carbon sources

We have studied the high-field properties of carbon-doped MgB2 thin films prepared by hybrid physical–chemical vapor deposition (HPCVD). Carbon doping was accomplished by adding carbon-containing gas, such as bis(methylcyclopentadienyl)magnesium and trimethylboron, into the hydrogen carrier gas during the deposition. In both cases, Tc drops slowly and residual resistivity increases considerably with carbon doping. Both the a and c lattice constants increase with carbon content in the films, a behavior different from that of bulk carbon-doped MgB2 samples. The films heavily doped with trimethylboron show very high parallel Hc2 over 70xa0T at low temperatures and a large temperature derivative near Tc. These behaviors are found to depend on the unique microstructure of the films, which consists of MgB2 layers a few-nanometers thick separated by non-superconducting MgB2C2 layers. This leads to an increase in the parallel Hc2 by the geometrical effect, which is in addition to the significant enhancement of Hc2 due to changes in the scattering rates within and between the two bands present in films doped using both carbon sources. The high Hc2 and high-field Jc(H) values observed in this work are very promising for the application of MgB2 in high magnetic fields.

Rhenium diboride’s monocrystal elastic constants, 308 to 5 K

The five independent moduli required to construct the complete monocrystal elastic modulus tensor of the hexagonal-symmetry superhard compound ReB(2) were measured from 308 to 5 K using resonant ultrasound spectroscopy on a special-texture polycrystal. This is possible because, confirmed by X-ray diffraction, the specimen measured was composed of grains with hexagonal axes parallel so that its polycrystal elastic response is identical to a monocrystal and because hexagonal-symmetry solids are elastically isotropic in the plane perpendicular to the hexagonal axis. Along the hexagonal (c) axis, C(33) (0) = 1021 GPa, nearly equal to C(11) of diamond, and consistent with the superhard properties. However, in the (softer) isotropic plane, C(11) (0) = 671 GPa, much lower than diamond. The changes of C(ij) with temperature are very small and smooth. The Debye temperature was computed to be 738 K, and using a high-temperature approximation, the Grüneisen parameter is γ = 1.7.

The high field ordered phase and upper critical field of the filled skutterudite system Pr(Os1-xRux)4Sb12

To study the possible competition between unconventional and Bardeen–Cooper–Schrieffer superconductivity in the filled skutterudites Pr(Os1−xRux)4Sb12, the evolution of superconductivity and the high field ordered phase in single-crystal specimens has been investigated by means of electrical resistivity measurements in magnetic fields up to 18 T. Whereas the upper critical field Hc2(T) curves have conventional shapes for x<0.4, the Hc2(T) curves are nearly linear for . For all x, Hc2(0) matches the calculated value of the orbital critical field. Features in the electrical resistivity associated with the high field ordered phase, observed clearly for PrOs4Sb12, weaken with increasing x and vanish for .

Fiber Bragg Grating Dilatometry in Extreme Magnetic Field and Cryogenic Conditions

In this work, we review single mode SiO2 fiber Bragg grating techniques for dilatometry studies of small single-crystalline samples in the extreme environments of very high, continuous, and pulsed magnetic fields of up to 150 T and at cryogenic temperatures down to <1 K. Distinct millimeter-long materials are measured as part of the technique development, including metallic, insulating, and radioactive compounds. Experimental strategies are discussed for the observation and analysis of the related thermal expansion and magnetostriction of materials, which can achieve a strain sensitivity (ΔL/L) as low as a few parts in one hundred million (≈10−8). The impact of experimental artifacts, such as those originating in the temperature dependence of the fiber’s index of diffraction, light polarization rotation in magnetic fields, and reduced strain transfer from millimeter-long specimens, is analyzed quantitatively using analytic models available in the literature. We compare the experimental results with model predictions in the small-sample limit, and discuss the uncovered discrepancies.

Elastic moduli of

We study the time evolution (aging) of the elastic moduli of an eight-year-old polycrystalline δ-Pu 2.0 at. % Ga alloy (δ-Pu:Ga) at different fixed temperatures from 295u2009K to nearly 500u2009K in real time using Resonant Ultrasound Spectroscopy. After 8 years of aging at 295u2009K, the bulk and shear moduli increase at a normalized rate of 0.2%/year and 0.6%/year, respectively. As the temperature is raised, two time dependences are observed, an exponential one of about a week, followed by a linear one (constant rate). The linear rate is thermally activated with an activation energy of 0.33u2009+u20090.06u2009eV. Above 420u2009K a qualitative change in the time evolution is observed; the bulk modulus decreases with time while the shear modulus continues to stiffen. No change in the behavior of the time evolution is observed as the α−β transition temperature is crossed as would be expected if a decomposition of δ-Pu:Ga to α-Pu and Pu3Ga occurred over the temperature range studied. Our results indicate that the main mechanism of agi...

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Resonant ultrasound spectroscopy (RUS) is a well-established method for determination of the full tensor of elastic moduli of a solid sample in a single frequency sweep. The elastic moduli, together with density, can provide information related to materials fabrication processes, providing a unique signature, or fingerprint, of a material. The goal of this study was to provide forensics for nuclear materials in solid ceramic or metallic form, including composition. The premise of this study was that it is really difficult to find two materials whose density and shear and /or bulk modulus match. We used RUS to determine the bulk and shear modulus for a total of 27 samples. The samples consisted of depleted uranium oxide (MOX) with different doping of Ce, Pu, and Nd oxides, and different methods of fabrication. They were in form of cylinders with flat and parallel faces. Two-dimensional and three-dimensional spaces were investigated, using shear modulus, bulk modulus, and density as variables. The densities...

-Pu239 reveal aging in real time

Measuring elastic moduli using resonant ultrasound spectroscopy (RUS) over a very wide temperature range requires very specialized hardware. The authors have developed both this hardware and some interesting software to help make measurements in these very challenging conditions. Solutions to these problems will be presented, along with results from some recent RUS experiments carried out at the National High Magnetic Field Laboratory in Los Alamos. [Work was supported by the National Science Foundation, State of Florida, and the U.S. Dept. of Energy.]

Nuclear material identification using resonant ultrasound spectroscopy

The parameters that measure the resistance to compression and resistance to shear of a solid are cal…