Orbital structure and magnetic ordering in stoichiometric and doped crednerite CuMnO2

The exchange interactions and magnetic structure in layered system CuMnO2 (mineral crednerite) and in nonstoichiometric system Cu1.04Mn0.96O2, with triangular layers distorted due to orbital ordering of the Mn3+ ions, are studied by ab-initio band-structure calculations, which were performed within the GGA+U approximation. The exchange interaction parameters for the Heisenberg model within the Mn-planes and between the Mn-planes were estimated. We explain the observed in-plane magnetic structure by the dominant mechanism of the direct d-d exchange between neighboring Mn ions. The superexchange via O ions, with 90 degree Mn-O-Mn bonds, plays less important role for the in-plane exchange. The interlayer coupling is largely dominated by one exchange path between the half-filled 3z^2-r^2 orbitals of Mn3+. The change of interlayer coupling from antiferromagnetic in pure CuMnO2 to ferromagnetic in doped material is also explained by our calculations

Geomagnetic control of the spectrum of traveling ionospheric disturbances based on data from a global GPS network

In this paper an attempt is made to verify the hypothesis on the role of geomagnetic disturbances as a factor determining the intensity of traveling ionospheric disturbances (TIDs). To improve the statistical validity of the data, we have used the based on the new GLOBDET technology method involving a global spatial averaging of disturbance spectra of the total electron content (TEC). To characterize the TID intensity quantitatively, we suggest that a new global index of the degree of disturbance should be used, which is equal to the mean value of the rms variations in TEC within the selected range of spectral periods (of 20-60 min in the present case). It was found that power spectra of daytime TEC variations in the range of 20-60 min periods under quiet conditions have a power-law form, with the slope index k = -2.5. With an increase of the level of magnetic disturbance, there is an increase in total intensity of TIDs, with a concurrent kink of the spectrum caused by an increase in oscillation intensity in the range of 20-60 min. It was found that an increase in the level of geomagnetic activity is accompanied by an increase in total intensity of TEC; however, it correlates not with the absolute level of Dst, but with the value of the time derivative of Dst (a maximum correlation coefficient reaches -0.94). The delay of the TID response of the order of 2 hours is consistent with the view that TIDs are generated in auroral regions, and propagate equatorward with the velocity of about 300-400 m/s.Comment: LaTeX2.09, 16 pages, 5 figures, 1 table, egs.cls, egs.bst (the style files

Electronic structure of V4_4O7_7: charge ordering, metal-insulator transition and magnetism

The low and high-temperature phases of V$_4$O$_7$ have been studied by \textit{ab initio} calculations. At high temperature, all V atoms are electronically equivalent and the material is metallic. Charge and orbital ordering, associated with the distortions in the V pseudo-rutile chains, occur below the metal-insulator transition. Orbital ordering in the low-temperature phase, different in V$^{3+}$ and V$^{4+}$ chains, allows to explain the distortion pattern in the insulating phase of V$_4$O$_7$. The in-chain magnetic couplings in the low-temperature phase turn out to be antiferromagnetic, but very different in the various V$^{4+}$ and V$^{3+}$ bonds. The V$^{4+}$ dimers formed below the transition temperature form spin singlets, but V$^{3+}$ ions, despite dimerization, apparently participate in magnetic ordering.Comment: 10 pages, 6 figures, 2 table

Energy and Emission Characteristics of Superlattice Quantum-Cascade Structures

Energy levels, wave functions, and matrix elements of optical dipole transitions have been numerically calculated for superlattice quantum-cascade structures. An expression for the gain has been obtained with regard to the complete set of energy levels in different models of spectral-line broadening. A universal relation between the gain and emission spectra for unipolar laser structures has been proposed. The effect of spectral broadening on the shape of emission spectra is estimated. The electroluminescence spectra are compared with the calculated spontaneous recombination spectra and good agreement between the results is shown

Geomagnetic storms and the occurrence of phase slips in the reception of GPS signals

We have investigated a dependence of the relative density of GPS phase slips on the geomagnetic disturbance level. The study is based on using Internet-available selected data from the global GPS network, with the simultaneously handled number of receiving stations ranging from 160 to 323. The analysis used four days from the period 1999-2000, with the values of the geomagnetic field disturbance index Dst from 5 to – 300 nT. During strong magnetic storms, the relative density of phase slips on mid-latitudes exceeds that for magnetically quiet days by one-two orders of magnitude as a minimum, and reaches a few percent of the total density of observations. Furthermore, the level of phase slips for the GPS satellites located on the sunward side of the Earth was by a factor of 5-10 larger compared with the opposite side of the Earth. The level of slips of L 1 phase measurements at the fundamental GPS frequency is at least one order of magnitude lower than that in L 1 – L 2 measurements. The slips of L 1 – L 2 measurements are most likely to be caused by the high level of slips of L 2 phase measurements at the auxiliary frequency. As an alternative, we developed and tested a new method for determining TEC variations using only data on the pseudo-range and phase measurements at fundamental frequency L 1 . The standard deviation of the TEC variations which were obtained in phase measurements at two frequencies, L 1 – L 2 , and at fundamental frequency L 1 , does not exceed 0.1 TECU, which permits this method to be used in strong disturbance conditions when phase slips at auxiliary frequency L 2 are observed

Structural transition in AuAgTe4 under pressure

Gold is inert and forms very few compounds. One of the most interesting of those is calaverite AuTe2, which has incommensurate structure and which becomes superconducting when doped or under pressure. There exist a "sibling" of AuTe2 the mineral sylvanite AuAgTe4, which properties are almost unknown. In sylvanite Au and Ag ions are ordered in stripes, and Te6 octahedra around metals are distorted in such a way that Ag becomes linearly coordinated, what is typical for Ag^{1+}, whereas Au is square coordinated - it is typical for d^8 configurations, i.e. one can assign to Au the valence 3+. Our theoretical study shows that at pressure P_C ~ 5 GPa there should occur in it a structural transition such that above this critical pressure Te6 octahedra around Au and Ag become regular and practically identical. Simultaneously Te-Te dimers, existing at P = 0 GPa, disappear, and material from a bad metal becomes a usual metal with predominantly Te 5p states at the Fermi energy. We expect that, similar to AuTe2, AuAgTe4 should become superconducting above P_C.Comment: 8 pages, 4 figure