Spin Alignment Studies on the Muon-Site Determination in La2CuO4

Here we report spin-alignment contributions to muon coordinate calculated utilizing density functional theory (DFT) calculation. We estimated four different antiferromagnetic (AF) spin alignments in La2CuO4. We observed small changes by adjusting spin configurations in DFT calculations. Cu-spin value of 0.61 µB is constant in all calculations. The insulating gap of 1.9 eV is unchanged in all configurations. Muon coordinate was defined as the most minimum energy in atomic potential distribution. By assuming that Cu-spin is a point dipole for each atom, internal fields for muon were calculated and compared to known experimental results.

Magnetic field-tuned Fermi liquid in a Kondo insulator

Kondo insulators are expected to transform into metals under a sufficiently strong magnetic field. The closure of the insulating gap stems from the coupling of a magnetic field to the electron spin, yet the required strength of the magnetic field–typically of order 100 T–means that very little is known about this insulator-metal transition. Here we show that Ce$${}_{3}$$3Bi$${}_{4}$$4Pd$${}_{3}$$3, owing to its fortuitously small gap, provides an ideal Kondo insulator for this investigation. A metallic Fermi liquid state is established above a critical magnetic field of only $${B}_{{\rm{c}}}\approx$$Bc≈ 11 T. A peak in the strength of electronic correlations near $${B}_{{\rm{c}}}$$Bc, which is evident in transport and susceptibility measurements, suggests that Ce$${}_{3}$$3Bi$${}_{4}$$4Pd$${}_{3}$$3 may exhibit quantum criticality analogous to that reported in Kondo insulators under pressure. Metamagnetism and the breakdown of the Kondo coupling are also discussed.

Blocking Characteristics of Photoconductive Switches Based on Semi-Insulating GaP and GaN

This article presents results of research work aimed at manufacturing photoconductive semiconductor switches (PCSSs) based on semi-insulating (SI) gallium phosphide (GaP) and gallium nitride (GaN). Currently, the work is in progress to determine the optimal values of PCSS parameters. In this article, the parameters of the selected semiconductor materials used for making PCSSs, the device operation principle, and possible areas of use are presented. The paper demonstrates the construction of test PCSSs based on SI GaP and SI GaN and results of blocking characteristics measurements without the illumination, as well as with illumination with a small photon flux. Further research directions are presented also.

Numerical study of an electrode-based resistivity tool for fracture diagnostics in steel-cased wellbores

The efficiency of a hydraulic fracture treatment depends primarily on the dimensions and orientation of propped fractures. We have developed a novel electrode-based resistivity tool concept for mapping proppant distribution in hydraulic fractures in steel-cased wellbores. The proposed tool configuration is shown to overcome the severe limitations of induction tools for the detection and resolution of propped fracture geometries in such wellbores. The concept makes use of an array of electrically insulating gap subsections, which are installed and cemented as permanent parts of the casing string, separating the casing sections. By imposing voltages on the insulating gaps, the conductive casing is excited directly, thus avoiding through-casing signal degradation caused by its high electrical conductivity. This allows us to detect subsurface fractures propped with conductive proppant. The envisioned measurements are performed by running a bottom-hole assembly into the fractured zone on a coiled tubing to impose a voltage across each insulating gap at a time, before and after hydraulic fracture operations. For each excited insulating gap, the voltages across all other insulating gaps are recorded by the electronics embedded in the insulating gaps. To interpret the envisioned measurements, a forward model of the tool, based on a finite volume method, is developed, and the design’s sensitivity to the fracture parameters is demonstrated via case studies. The results indicate that measurements made based on the proposed concept will be highly sensitive to a fracture’s location, size, and angle, and less sensitive to a fracture’s shape. Simulations also indicate that direct contact of the fracture with an excited casing section enables the differentiation of fractures of up to a 100 m radius. Fractures with angles greater than 30° or aspect ratios greater than two can also be distinguished from the ones orthogonal to the well or with an aspect ratio of one.

Extracting the Single-Particle Gap in Carbon Nanotubes with Lattice Quantum Monte Carlo

We show how lattice Quantum Monte Carlo simulations can be used to calculate electronic properties of carbon nanotubes in the presence of strong electron-electron correlations. We employ the path integral formalism and use methods developed within the lattice QCD community for our numerical work and compare our results to empirical data of the Anti-Ferromagnetic Mott Insulating gap in large diameter tubes.