Proximity effect thermometer for local temperature measurements on mesoscopic samples

Using the strong temperature dependent resistance of a normal metal wire in proximity to a superconductor, we have been able to measure the local temperature of electrons heated by flowing a dc current in a metallic wire to within a few tens of millikelvin at low temperatures. By placing two such thermometers at different parts of a sample, we have been able to measure the temperature difference induced by a dc current flowing in the sample. This technique may provide a flexible means of making quantitative thermal and thermoelectric measurements on mesoscopic metallic samples

Relativistic materials from an alternative viewpoint

Electrons in materials containing heavy elements are fundamentally relativistic and should in principle be described using the Dirac equation. However, the current standard for treatment of electrons in such materials involves density functional theory methods originally formulated from the Schr\"{o}dinger equation. While some extensions of the Schr\"{o}dinger-based formulation have been explored, such as the scalar relativistic approximation with or without spin-orbit coupling, these solutions do not provide a way to fully account for all relativistic effects of electrons, and the language used to describe such solutions are still based in the language of the Schr\"{o}dinger equation. In this article, we provide a different method for translating between the Dirac and Schr\"{o}dinger viewpoints in the context of a Coulomb potential. By retaining the Dirac four-vector notation and terminology in taking the non-relativistic limit, we see a much deeper connection between the Dirac and Schr\"{o}dinger equation solutions that allow us to more directly compare the effects of relativity in the angular and radial functions. Through this viewpoint, we introduce the concepts of densitals and Dirac spherical harmonics that allow us to translate more easily between the Dirac and Schr\"{o}dinger solutions. These concepts allow us to establish a useful language for discussing relativistic effects in materials containing elements throughout the full periodic table and thereby enable a more fundamental understanding of the effects of relativity on electronic structure

Phase formation in ion‐irradiated and annealed Ni‐rich Ni‐Al thin films

Phase formation was studied in ion‐irradiated multilayer and coevaporated Ni‐20 at. % Al films supported by Cu, Mo, and Ni transmission electron microscopy (TEM) grids. Irradiation with either 700‐keV Xe or 1.7‐MeV Xe, to doses sufficient to homogenize the multilayers (≥7.5×1015 cm−2), resulted in the formation of metastable supersaturated γ and HCP phases in both film types. Post‐irradiation annealing of multilayers at 450 °C for 1 h transformed the metastable phases to a two‐phase γ+γ′ microstructure. In the absence of Cu, the formation of γ′ appeared to proceed by a traditional diffusional growth mechanism, resulting in small (<50 Å) γ′ precipitates in γ matrix grains. The presence of Cu caused the formation of a dual‐phase γ+γ′ structure (i.e., distinct, equal‐sized grains of γ and γ′) during post‐irradiation annealing. It is suggested that copper affected the nucleation of γ′ precipitates and increased the kinetics of growth resulting in the dual‐phase morphology. Strong irradiation‐induced textures were observed in the multilayers that were less pronounced in the coevaporated films. The texture in the multilayers was attributed to the presence of a slight as‐evaporated texture combined with the enhanced atomic mobility due to the heat‐of‐mixing released during irradiation. The irradiation‐induced texture appeared to be necessary for the formation of the dual‐phase structure since it likely provided high‐diffusivity paths for Cu to diffuse into the film from the TEM grid.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70874/2/JAPIAU-69-4-2021-1.pd

The heat‐of‐mixing effect on ion‐induced grain growth

Irradiation experiments were conducted on multilayer (ML) and coevaporated (CO) thin films in order to examine the role that the heat‐of‐mixing (ΔHmix) has in ion‐induced grain growth. Room‐temperature irradiations using 1.7‐MeV Xe ions were performed in the High Voltage Electron Microscope at Argonne National Laboratory. The ML films (Pt‐Ti, Pt‐V, Pt‐Ni, Au‐Co, and Ni‐Al) spanned a large range of calculated ΔHmix values. Comparison of grain growth rates between ML and CO films of a given alloy confirmed a heat‐of‐mixing effect. With the exception of the Pt‐V system, differences in grain growth rates between ML and CO films varied according to the sign of the calculated ΔHmix of the system. Substantial variations in growth rates among CO alloy films experiencing similar displacement damage demonstrated that a purely collisional approach is inadequate for describing ion‐induced grain growth. Therefore consideration must also be given to material‐specific properties, such as cohesive energy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70305/2/JAPIAU-70-3-1252-1.pd

Complete Characterization of Quantum-Optical Processes

The technologies of quantum information and quantum control are rapidly improving, but full exploitation of their capabilities requires complete characterization and assessment of processes that occur within quantum devices. We present a method for characterizing, with arbitrarily high accuracy, any quantum optical process. Our protocol recovers complete knowledge of the process by studying, via homodyne tomography, its effect on a set of coherent states, i.e. classical fields produced by common laser sources. We demonstrate the capability of our protocol by evaluating and experimentally verifying the effect of a test process on squeezed vacuum.Comment: 5 pages, 4 figure

The role of gamma rays and freely-migrating defects in reactor pressure vessel embrittlement

Gamma ray effects are often neglected when evaluating reactor pressure vessel (RPV) embrittlement. However, recent analyses indicate that in newer style light water reactors, gamma damage can be a substantial fraction of the total displacement damage experienced by the (RPV); ignoring this damage will lead to errors in embrittlement predictions. Furthermore, gamma rays may be more efficient than fast neutrons at producing freely-migrating defects and as such can impact certain embrittlement mechanisms more effectively than fast neutrons. Consideration of these gamma effects are therefore essential for a more complete understanding of radiation embrittlement

Excited States in Warm and Hot Dense Matter

Accurate modeling of warm and hot dense matter is challenging in part due to the multitude of excited states that must be considered. In thermal density functional theory, these excited states are averaged over to produce a single, averaged, thermal ground state. Here we present a variational framework and model that includes explicit excited states. In this framework an excited state is defined by a set of effective one-electron occupation factors and the corresponding energy is defined by the effective one-body energy with an exchange and correlation term. The variational framework is applied to an atom-in-plasma model (a generalization of the so-called average atom model). Comparisons with a density functional theory based average atom model generally reveal good agreement in the calculated pressure, but the new model also gives access to the excitation energies and charge state distributions