Extreme structure and spontaneous lift of spin degeneracy in doped perforated bilayer graphenes

Extreme structure and spin states of doped and undoped perforated bigraphenes was studied using DFT simulations. It was found that folded nanopores possess extremely high curvature of 0.34 Å−1. Dramatic structural deformation causes severe changes of the chemical properties of carbon atoms localized at the nanopores converting the folded edges to local oxidative fragments. It was found that asymmetrical coordination of either Li, Ca, or Al to the nanopores is coupled with electron transfer from metal to edge carbon atoms and breakdown of local inversion symmetry. Li-, Ca-, and Al-doped perforated AA bigraphene revealed ferromagnetic spin ordering with magnetic moments of 0.38, 0.14, and 0.32μB/unit cell, respectively, and spin polarization energy gain of 0.037eV for Ca-doped superlattice. It was shown that ferromagnetic spin ordering of bigraphene nanopores contradicts to the Nagaoka's theorem, which excludes strong electron correlations as a reason of spin polarization. Spontaneous lift of spin degeneracy was interpreted in terms of perturbing intense local electrostatic fields from extra electron charges localized at the nanopore edges, coupled with breakdown of space inversion and local translation invariances. It was shown that spin energy splitting is proportional to the matrix elements calculated on Bloch states with opposite wavevectors and perturbing electrostatic fields

MAGNETIC PARAMETERS FOR DETERMINING THE DEPTH OF HARDENING IN THE EXTENDED RANGE OF ITS VARIATION

The possibility of measuring the depth of surface hardening by the value of the magnetic field measured on the surface of the object in the interpolar space of a U-shaped electromagnet is theoretically and experimentally shown.Работа выполнена по программе фундаментальных исследований УрО РАН 2015-2017 гг. проект 15-17-2-5

Numerical and Experimental Evaluation of Short Folded Recieve-Only Dipoles for 9.4T Human Head Arrays

Improvement of signal-to-noise ratio (SNR) is a critical step in designing any MRI radio frequency (RF) coil. Increasing the number of surface loops in a human head receive (Rx) array improves the peripheral SNR, while the central SNR doesnt substantially change. Recent studies demonstrated that an optimal central SNR at UHF frequencies (298 MHz and higher) requires contribution of two current patterns associated with loops and dipoles. To incorporate multiple dipoles into a human head loop Rx-array, the dipole length has to be substantially reduced, which compromises its performance. Another issue of using short Rx-dipoles is a sensitivity of their resonance frequency to loading due to a large electrical field near the dipole. To reduce the sensitivity, we propose to fold dipoles towards the RF-shield. A novel array consisting of 8 transceiver surface loops and 8 folded Rx-dipoles was developed and tested. Addition of Rx-dipoles doesnt substantially change the $B_{1}^{+}$ field and the maximum local SAR of the array. At the same time, the new design improves both the central and peripheral SNR as compared 16-element array with Rx-only vertical loops and 8-element transceiver surface loop array

A practical realization of an artificial magnetic shield for preclinical birdcage RF coils

In the most of magnetic resonance imaging (MRI) systems, a conventional radiofrequency (RF) electric shield is typically placed around an RF volume coil to avoid the interaction with the other components of the system. Disadvantageously metal shields reduce the transmit efficiency of the RF coil as well as its receive sensitivity due to out-ofphase reflection of electromagnetic waves. In contrast, an ideal magnetic shield having high surface impedance provides in-phase reflection, which can be promising for improvement of RF coil's performance. In this work, we propose an artificial magnetic shield based on a cylindrical miniaturized corrugated structure to improve characteristics of a small-animal birdcage coil at 7T. The coil was simulated in the presence of the metal and ideal magnetic shield as well as the proposed structure. The results demonstrate enhancement of the coil's performance in the presence of the proposed shield, which is comparable with an ideal one.Peer reviewe

A metamaterial-inspired MR antenna independently tunable at two frequencies

International audienc

Structure and properties of exotic nano- and mesodiamonds with pentagonal symmetry

A comprehensive critical survey of structures of exotic nano-, meso- and microdiamonds with dodecahedral and icosahedral symmetry (N/MDPS) is presented. Due to their high dodecahedral or icosahedral symmetry, the unique complex atomic and electronic structure of N/MDPS leads to transport and mechanical properties very promising for photonic, quantum, and nanomechanical applications. To explain the nature of diamonds, theoretical models have been proposed based on the formation of twinned structures consisting of either 5 or 20 symmetrically equivalent tetrahedral and prismatic fragments of the face-centered cubic lattice with the formation of star-shaped or icosahedral clusters, respectively. It has been shown that these twinned nano- and mesodiamonds have limited dimensions due to accumulation of uncompensated structural stresses arising from the deviation of the angles between diamond facets from perfect 72 degrees in tetrahedral fragments of the face-centered cubic lattice to 70.5 degrees between five symmetrically equivalent twinned fragments

External electric field effect on electronic properties and charge transfer in CoI2/NiI2 spinterface

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Applications of dielectric pads, novel materials and resonators in 1.5T and 3T MRI

\u3cp\u3eIn order to boost the performance of magnetic resonance imaging without increasing the static magnetic field, it is necessary to increase its intrinsic sensitivity. This allows a reduction in the scanning time, increased spatial resolution, and can enable low-field strength systems (which are much cheaper and can be used to scan patients with metallic implants) to have a higher signal-to-noise ratio (SNR) so that they are comparable to more expensive higher field strength systems. In this contribution, we demonstrate radiofrequency field enhancing and shaping devices based on novel materials, such as high permittivity dielectric structures and metamaterials. These materials can substantially enhance SNR, thus potentially increasing image resolution or allowing faster examinations.\u3c/p\u3

Hybridized eigenmodes of periodic wire arrays and their application in radiofrequency coils for preclinical MRI

International audienc