A DMI Guide to Magnets Micro-World

Journal of Experimental and Theoretical Physics - Dzyaloshinskii–Moriya interaction, DMI in short, represents an antisymmetric type of magnetic interactions that favor orthogonal orientation...     

Phase behaviour of the discotic mesogen 5,10,15,20-tetrakis (4-n-dodecylphenyl)porphyrin under pressure

The phase behaviour of the discotic mesogen 5,10,15,20-tetrakis(4-n -dodecylphenyl)porphyrin (C12TPP) was investigated under hydrostatic pressures up to 300MPa by high pressure DTA and wide angle X-ray diffraction methods. The typical enantiotropic phase transitions of C12TPP, low- to high-temperature crystal (Cr2-Cr1), Cr1-discotic lamellar phase (DL), and DL-isotropic liquid (I) are observed at pressures up to 10MPa. Application of hydrostatic pressure to the sample generates a pressure-induced crystal polymorph (Cr3) between the Cr2 and Cr1 phases, and the phase transitions Cr2-Cr3-Cr1-DL-I occur reversibly in the pressure region between 10 and 180MPa. On heating at higher pressures above 180MPa, the fourth crystal polymorph (Cr4) is formed between the Cr2 and Cr3 phases at lower temperatures, and at the same time the fifth crystal polymorph (Cr5) appears abruptly between the Cr1 and DL phases at high temperatures. The Cr2-Cr4-Cr3-C1-(Cr5)-DL-I transition processes were observed at 180 200MPa. Further increasing the pressure above 270MPa induces entirely different thermal behaviour: only two peaks for the pressure-induced transition between the sixth and fifth polymorphs (Cr6-Cr5) and the Cr5-I transitions are detected at low and high temperatures on heating, while both the DTA and WAXD experiments on cooling show the formation of the DL phase as a monotropic phase between the I and Cr5 phases, indicating the I DL Cr5 Cr6 process. The thermal behaviour was ambiguous and complex in the pressure region between 200 and 260MPa because the peaks for the intermediate crystal transitions were too small to detect with confidence. The two different sequences of the Cr2-Cr4-Cr3-Cr1-DL-I and Cr6-Cr5-(DL)-I processes seems to occur competitively. The T vs. P phase diagram of a sample cooled at 300MPa was studied to determine the triple point of the DL phase and to investigate the phase stability of the pressure-induced crystal polymorphs. The Cr6-Cr5-I transition process was observed on heating at 200 and 300MPa, while the Cr6-Cr5-DL-I process was detected at lower pressures below 100MPa. Since the Cr5-DL transition temperature changes linearly with a slope dT/dP 40 degrees C/100 MPa, while the DL-I transition temperature changes slightly (dT/dP 5.5 degrees C/100MPa), the DL phase forms a triangle in the T vs. P diagram. The triple point of the DL phase was found to be 240.8MPa and 168.8 C. The Cr6 polymorph reorganized to the stable Cr2 form under atmospheric pressure on annealing at room temperature overnight.     

New allotropic forms of carbon based on С<Subscript>60</Subscript> and С<Subscript>20</Subscript> fullerenes with specific mechanical characteristics

New allotropic forms of carbon based on С₆₀ and С₂₀ fullerenes are considered. The most stable carbon compounds are found using an evolution algorithm, and their crystal structure (X-ray diffraction spectra) and electron (band structure) and mechanical (moduli of elasticity, hardness) characteristics are studied. The carbon phase with the tetragonal symmetry with mechanical properties close to those of a diamond crystal and having a narrow band gap is found.     

Wave-Packet Dynamics Study of the Transport Characteristics of Perforated Bilayer Graphene Nanoribbons

JETP Letters - The electrical conduction characteristics of perforated bilayer graphene nanoribbons are studied by the wave-packet dynamics method. The transport characteristics for typical...     

Theoretical Study of the Electronic and Transport Properties of Lateral 2D–1D–2D Graphene–CNT–Graphene Structures

JETP Letters - The electronic and transport properties of new hybrid 2D–1D–2D structures of carbon atoms, which are graphene sheets continuously connected through a fragment of a...     

Solar modulation of cosmic deuteron fluxes in the PAMELA experiment

The preliminary results from measurements of deuteron fluxes in galactic cosmic rays (GCR) in the vicinity of the Earth in 2006–2009 are presented. The results are obtained by analyzing data from the PAMELA experiment aboard the Resurs DK-1 satellite. High-precision detection instruments provided an opportunity to identify GCR deuterons and measure their spectrum in the energy interval of 90–650MeV/nucleon. Spectra averaged over six-month intervals from the summer of 2006 to the summer of 2009 (the solar activity minimum) are presented. The influence of solar modulation on the observed spectrum is clearly seen in the results.     

Trapped Positrons and Electrons in the Inner Radiation Belt According to Data of the PAMELA Experiment

Measurements of secondary-electron and secondary-positron fluxes below the geomagnetic cutoff in near-Earth space were performed by means of the PAMELA magnetic spectrometer installed on board the Resurs-DK1 satellite launched on June 15, 2006, in an elliptical orbit of inclination 70° and altitude 350 to 600 km. This spectrometer permits measuring the fluxes of electrons and positrons over a wide energy range, as well as determining their spatial distributions to a precision of about 2°. A calculation of particle trajectories in the geomagnetic field makes it possible to separate electrons and positrons originating from cosmic-ray interactions in the Earth’s magnetosphere. The spatial distributions of quasitrapped, trapped, and short-lived albedo positrons and electrons of energy above 70 MeV in the radiation belt were analyzed. The ratio of the electron-to-positron fluxes and the energy spectra of the electrons and positrons in question are indicative of different productionmechanisms for stably trapped and quasitrapped secondary particles.     

Formation of graphene quantum dots by “Planting” hydrogen atoms at a graphene nanoribbon

Different technological approaches for creating graphene quantum dots by the adsorption of hydrogen atoms are considered. The adsorption can occur both at convex portions of a distorted graphene nanoribbon and in the structure formed by two distorted graphene nanoribbon rows superimposed on each other at the places free from the ribbon crossings. It is shown that settlement of hydrogen atoms at convex portions of the nanoribbons is energetically favorable. This gives rise to the creation of insulating graphane (CH) nanodomains separating the conducting regions. As a result, a graphene quantum dot appears. The variation of the electron spectra of graphene quantum dots with the length of these graphane regions is discussed.     

Modernization and Methods of Maintaining the Operating Mode of the OGRAN (Optoacoustic Gravity Antenna) Setup

Physics of Atomic Nuclei - An upgraded version of the combined optoacoustic gravitational radiation detector OGRAN is considered. This underground setup located at the Baksan Neutrino Observatory...     

Isothermal slip of a Fermi gas with specular-diffuse reflection from the boundary

A solution of the Kramers problem on isothermal slip of a quantum Fermi gas along a plane solid surface has been obtained. The reflection of molecules from the wall was supposed to be specular-diffuse. A model kinetic equation with the τ-model collision integral and with the collision frequency proportional to the molecular velocity was used. The slip velocity has been analyzed in relation to a parameter representing the ratio of the chemical potential to the product of Boltzmann’s constant by the absolute temperature.