23 Search Results

Here this manuscript reports on the structural and magnetic properties of NdCuGa₃ using powder and single crystal X-ray diffraction (XRD), zero-field single crystal neutron diffraction, magnetization, and specific heat measurements. Our XRD on a single crystal specimen of NdCuGa3 confirmed that it crystallizes in the tetragonal BaNiSi₃-type structure. A magnetic phase transition at T_N = 3.3 K is assessed using specific heat and ac magnetic susceptibility measurements. No additional anomaly below T_N down to 50 mK was detected by performing specific heat measurements. Neutron single crystal diffraction data collected at T = 300 mK confirm the antiferromagnetic phase below T_N=more » 3.3 K with the propagation vector $$\vec{\tau}$$ = (0.2, 0, 0). Possible magnetic structure solutions of NdCuGa₃ are discussed« less

Abstract The addition of artificial pinning centers has led to an impressive increase in the critical current density ( J _c ) of superconductors, enabling record-breaking all-superconducting magnets and other applications. The J _c of superconductors has reached ~0.2–0.3 J _d , where J _d is the depairing current density, and the numerical factor depends on the pinning optimization. By modifying λ and/or ξ, the penetration depth and coherence length, respectively, we can increase J _d . For (Y _0.77 Gd _0.23 )Ba ₂ Cu ₃ O _y ((Y,Gd)123), we can achieve this by controlling the carrier density,more » which is related to λ and ξ. We can also tune λ and ξ by controlling the chemical pressure in Fe-based superconductors, i.e., BaFe ₂ (As _{1− x} P _x ) ₂ films. The variation in λ and ξ leads to an intrinsic improvement in J _c via J _d , allowing extremely high values of J _c of 130 MA/cm ² and 8.0 MA/cm ² at 4.2 K, consistent with an enhancement in J _d of a factor of 2 for both incoherent nanoparticle-doped (Y,Gd)123 coated conductors (CCs) and BaFe ₂ (As _{1− x} P _x ) ₂ films, showing that this new material design is useful for achieving high critical current densities in a wide array of superconductors. The remarkably high vortex-pinning force in combination with this thermodynamic and pinning optimization route for the (Y,Gd)123 CCs reached ~3.17 TN/m ³ at 4.2 K and 18 T ( H || c ), the highest values ever reported for any superconductor.« less

The goal of this project was to study the applicability of specific innovative instrumentation techniques for assessing parameters needed for safe small modular reactor operation and safeguarding of nuclear material. Small modular reactor core designs are currently being developed to provide energy more effectively and efficiently than in the past because they can be built as modules at fabrication sites and then transported to a power-producing facility. However, these modules and/or the final core will often be sealed and not accessible again until disposal. Instead of instruments that access the core directly, during and after operation, to monitor flux/dose andmore » structural integrity, as current power plants use, new sensors need to be designed that can be built into the reactor initially to determine operating history, structural integrity through operation of the system, and nuclear material accountancy after shutdown and before disposition of the core. Embedded sensors already exist that can provide neutron flux, gamma dose, and temperatures, but techniques to expand upon these for assessing structural health and material inventory in the system over time need to be developed. Structural health assessments include the detection and imaging of cracks in the components that could eventually cause radioactive fission products to be released.« less

Abstract Polycrystalline materials can have complex anisotropic properties depending on their crystallographic texture and crystal structure. In this study, we use resonant ultrasound spectroscopy (RUS) to nondestructively quantify the elastic anisotropy in extruded aluminum alloy 1100-O, an inherently low-anisotropy material. Further, we show that RUS can be used to indirectly provide a description of the material’s texture, which in the present case is found to be transversely isotropic. By determining the entire elastic tensor, we can identify the level and orientation of the anisotropy originated during extrusion. The relative anisotropy of the compressive (c ₁₁ /c ₃₃ ) and shearmore » (c ₄₄ /c ₆₆ ) elastic constants is 1.5% ± 0.5% and 5.7% ± 0.5%, respectively, where the elastic constants (five independent elastic constants for transversely isotropic) are those associated with the extrusion axis that defines the symmetry of the texture. These results indicate that the texture is expected to have transversely isotropic symmetry. This finding is confirmed by two additional approaches. First, we confirm elastic constants and the degree of elastic anisotropy by direct sound velocity measurements using ultrasonic pulse echo. Second, neutron diffraction (ND) data confirm the symmetry of the bulk texture consistent with extrusion-induced anisotropy, and polycrystal elasticity simulations using the elastic self-consistent model with input from ND textures and aluminum single-crystal elastic constants render similar levels of polycrystal elastic anisotropy to those measured by RUS. We demonstrate the ability of RUS to detect texture-induced anisotropy in inherently low-anisotropy materials. Therefore, as many other common materials have intrinsically higher elastic anisotropy, this technique should be applicable for similar levels of texture, providing an efficient general diagnostic and characterization tool.« less

Absmore » tract Magnetic skyrmions are well-suited for encoding information because they are nano-sized, topologically stable, and only require ultra-low critical current densities j _c to depin from the underlying atomic lattice. Above j _c skyrmions exhibit well-controlled motion, making them prime candidates for race-track memories. In thin films thermally-activated creep motion of isolated skyrmions was observed below j _c as predicted by theory. Uncontrolled skyrmion motion is detrimental for race-track memories and is not fully understood. Notably, the creep of skyrmion lattices in bulk materials remains to be explored. Here we show using resonant ultrasound spectroscopy—a probe highly sensitive to the coupling between skyrmion and atomic lattices—that in the prototypical skyrmion lattice material MnSi depinning occurs at $$$${j}_{c}^{* }$$$$ $j_c^{*}$ that is only 4 percent of j _c . Our experiments are in excellent agreement with Anderson-Kim theory for creep and allow us to reveal a new dynamic regime at ultra-low current densities characterized by thermally-activated skyrmion-lattice-creep with important consequences for applications.« less

The mechanical interaction between the fuel and cladding that occurs during operation of a nuclear reactor is important to understand as it can lead to cladding failures and release of radioactive material into the coolant. In order to develop better models of pellet-cladding mechanical interactions when accident tolerant fuel candidates are used, the mechanical properties of the fuel at relevant operating temperatures, like the elastic moduli, are needed for current and advanced accident tolerant fuels (ATFs). In this work, elevated temperature nanoindentation and resonant ultrasound spectroscopy were used to measure the moduli and hardness of several fluorite materials (CeO2, ThO2,more » UO2) and several accident tolerant fuel candidates (ATF) (U3Si2, UN, UB2). In addition, a comparison of the two techniques was performed in this study to independently validate the mechanical properties.« less

Here, we describe construction of a new multiphase equation of state (EOS) for beryllium calibrated almost entirely to density functional theory calculations based on the AM05 exchange-correlation functional. The EOS is in tabular form (as SESAME 92025) and includes hcp, bcc, and liquid/plasma phases. We find hcp→bcc transitions at ~325 GPa at zero and room temperatures, and at ~150 GPa on the Hugoniot. Shock melting from the bcc phase proceeds at 205-230 GPa. We also present experimental results for the isentropic bulk modulus as a function of temperature, based on resonant ultrasound spectroscopy measurements.

The past years have witnessed major advancements in all-electrical doping control on cuprates. In the vast majority of cases, the tuning of charge carrier density has been achieved via electric field effect by means of either a ferroelectric polarization or using a dielectric or electrolyte gating. Unfortunately, these approaches are constrained to rather thin superconducting layers and require large electric fields in order to ensure sizable carrier modulations. In this work, we focus on the investigation of oxygen doping in an extended region through current-stimulated oxygen migration in YBa₂Cu₃O_7–δ superconducting bridges. The underlying methodology is rather simple and avoids sophisticatedmore » nanofabrication process steps and complex electronics. Here, a patterned multiterminal transport bridge configuration allows us to electrically assess the directional counterflow of oxygen atoms and vacancies. Importantly, the emerging propagating front of current-dependent doping δ is probed in situ by optical microscopy and scanning electron microscopy. The resulting imaging techniques, together with photoinduced conductivity and Raman scattering investigations, reveal an inhomogeneous oxygen vacancy distribution with a controllable propagation speed permitting us to estimate the oxygen diffusivity. These findings provide direct evidence that the microscopic mechanism at play in electrical doping of cuprates involves diffusion of oxygen atoms with the applied current. The resulting fine control of the oxygen content would permit a systematic study of complex phase diagrams and the design of electrically addressable devices.« less

Resonant Ultrasound Spectroscopy (RUS) is an ultrasound-based minimal-effort high-accuracy elastic modulus measurement technique. RUS as described here uses the mechanical resonances (normal modes of vibration or just modes) of rectangular parallelepiped or cylindrical specimens with a dimension of from a fraction of a millimeter to as large as will fit into the apparatus. Provided here is all that is needed so that the reader can construct and use a state-of-the-art RUS system. Included are links to open-source circuit diagrams, links to download Los Alamos National Laboratory open-source data acquisition software, links to request free analysis software, procedures for acquiring measurements,more » considerations on building transducers, 3-D printed stage designs, and a full mathematical explanation of how the analysis software extracts elastic moduli from resonances.« less

Nonlinear electrical transport studies at high pulsed magnetic fields, above the range accessible by dc magnets, are of direct fundamental relevance to the physics of superconductors, domain-wall, charge-density waves, and topological semimetal. All-superconducting very high field magnets also make it technologically relevant to study vortex matter in this regime. However, pulsed magnetic fields reaching 100 T in milliseconds impose technical and fundamental challenges that have prevented the realization of these studies. Here, we present a technique for sub-microsecond smart current-voltage measurements, which enables the determination of the superconducting critical current in pulsed magnetic fields, beyond the reach of any dcmore » magnet. We demonstrate the excellent agreement of this technique with low dc field measurements on Y_0.77Gd_0.23Ba₂Cu₃O₇ coated conductors with and without BaHfO₃ nanoparticles. Exploring the uncharted high-magnetic-field region, we discover a characteristic influence of the magnetic field rate of change (dH/dt) on the current-voltage curves in a superconductor. As a result, we fully capture this unexplored vortex physics through a theoretical model based on the asymmetry of the vortex-velocity profile produced by the applied current.« less