Ab initio Simulations of Fe-based Ferric Wheels

Based on first-principles density-functional theory calculations we investigate the electronic structure of hexanuclear "ferric wheels" M Fe_6[N(CH_2 CH_2 O)_3]_6 Cl (M = Li, Na) in their antiferromagnetic ground state. The electronic structure is presented in form of spin- and site-resolved local densities of states. The latter clearly indicate that the magnetic moment is distributed over several sites. The local moment at the iron site is still the largest one with about 4 mu_B, thus indicating the valence state of iron to be closer to Fe(II) than to commonly accepted Fe(III). The local spin of S=5/2 per iron site, following from magnetization measurements, is perfectly reproduced if one takes the moments on the neighbor atoms into account. The largest magnetic polarization is found on the apical oxygen atom, followed by nitrogen bridging oxygens. These findings are confirmed by a map of spatial spin density. A further goal of the present study has been a comparative test of two different DFT implementations, Siesta and NRLMOL. They yield a very good agreement down to small details in the electronic structure.Comment: 10 pages, 3 embedded postscript figures, to be published in Molecular Physics Reports (proceedings of the Summer School on New Magnetics - Bedlewo, Poland, September 2003). Two references update

Electronic and magnetic properties of Co_n Mo_m nanoclusters with n+m = x and 2<=x<=6 atoms from DFT calculations

We present the results of the density functional theory study of Co_n Mo_m nanoclusters with n+m=x and 2<=x<=6 atoms on the all-electron level using the generalized gradient approximation. The discussion of the properties of the pure cobalt and molybdenium cluster is followed by an analysis of the respective mixed clusters of each cluster size x. We found that the magnetic moment of a given cluster is mainly due to the Co content and increases with increasing n. The magnetic anisotropy on the other hand becomes smaller for larger magnetic moments S. We observe an increase in the binding energy, electron affinity, and average bond length with increasing cluster size as well as a decrease in the ionization potential, chemical potential, molecular hardness and the HOMO-LUMO gap.Comment: 12 pages, 6 figures, 1 tabl

Masoński kod w twórczości Władimira Nabokowa. Próba interpretacji powieści „Zaproszenie na egzekucję” w kluczu symboliki masońskiej

This article is an attempt to interpet a work by Vladimir Nabokov, Invitation to a Beheading, with the most important determinants of Masonic culture. There are few studies that discuss this outstanding prose writer in terms of freemasonry and the author of this article discusses this issue with particular attention to the symbolism  of the Masonic initiation ritual.This article is an attempt to interpet a work by Vladimir Nabokov, Invitation to a Beheading, with the most important determinants of Masonic culture. There are few studies that discuss this outstanding prose writer in terms of freemasonry and the author of this article discusses this issue with particular attention to the symbolism  of the Masonic initiation ritual

Gate Controlled Molecular Switch Based on Picene/F4TCNQ Charge-Transfer Material

We show that the recently synthesized charge-transfer material picene/F4TCNQ can be used as a gate-voltage controlled molecular switch. The picene/F4TCNQ system is compared with the extensive characterized anthraquinone-based molecular system, which is known to exhibit large switching ratios due to quantum interference effects. In case of picene/F4TCNQ we find switching ratios larger by one order of magnitude. Further, our calculations reveal that the picene/F4TCNQ system resembles remarkable well the I-V characteristics of a classical diode. The reverse-bias current of this molecular diode can be increased two orders of magnitude by an external gate voltage. Based on density-functional theory calculations we show that the hybrid states formed by the picene/F4TCNQ system are playing the key role to determine the transport properties. We further conclude that the tuning of quantum transport properties through hybrid states is a general concept which opens a new route towards functional materials for molecular electronics

Wavevector-dependent optical properties from wavevector-independent proper conductivity tensor

We discuss the calculation of the refractive index by means of the ab initio scalar dielectric function and point out its inherent limitations. To overcome these, we start from the recently proposed fundamental, microscopic wave equation in materials in terms of the frequency- and wavevector-dependent dielectric tensor, and investigate under which conditions the standard treatment can be justified. Thereby, we address the question of neglecting the wavelength dependence of microscopic response functions. Furthermore, we analyze in how far the fundamental, microscopic wave equation is equivalent to the standard wave equation used in theoretical optics. In particular, we clarify the relation of the "effective" dielectric tensor used there to the microscopic dielectric tensor defined in ab initio physics.Comment: consistent with published version in Eur. Phys. J. B (2020

MgB2 thick film with TC = 40.2 K deposited on sapphire substrate

We have successfully deposited thick MgB2 film on the (0001) crystalline surface of sapphire by the method of hybrid physical-chemical vapor deposition (HPCVD). The film thickness is about 1.3 micron. It has a dense and interlaced structure. The film surface, shown by SEM, is stacked with MgB2 microcrystals. Transport measurements by the 4-probe technique have demonstrated that its critical temperature is about 40.2 K, with a sharp transition width of 0.15 K. The residual resistivity ratio (RRR) is about 11. By extrapolation, HC2(0) is determined as 13.7 T from the magneto-transport measurement. Also by hysteresis measurement and applying the Bean model, the critical current density is estimated as 5*1010 A/m2 in zero magnetic field. The present work has demonstrated that HPCVD is an effective technique to fabricate the MgB2 thick film with decent superconducting properties. Hence, it is important for the future superconducting application, in particular, as a crucial preliminary stage to fabricate superconducting tape.Comment: 7 pages with 4 figures included, Phys. Stat. Sol. (a) In pres

Three-dimensional MgB2_{2}-type superconductivity in hole-doped diamond

We substantiate by calculations that the recently discovered superconductivity below 4 K in 3% boron-doped diamond is caused by electron-phonon coupling of the same type as in MgB$_2$, albeit in 3 dimensions. Holes at the top of the zone-centered, degenerate $\sigma$-bonding valence band couple strongly to the optical bond-stretching modes. The increase from 2 to 3 dimensions reduces the mode-softening crucial for $T_{c}$ reaching 40 K in MgB$_{2}.$ Even if diamond had the same \emph{bare} coupling constant as MgB$_{2},$ which could be achieved with 10% doping, $T_{c}$ would only be 25 K. Superconductivity above 1 K in Si (Ge) requires hole-doping beyond 5% (10%).Comment: revised version, accepted by PR