1,721,355 research outputs found
Nonmagnetic molecular Jahn-Teller Mott insulators
No full textNarrow-band conductors may turn insulating and magnetic as a consequence of strong electron-electron correlation. In molecular conductors, the concomitance of a strong Jahn-Teller coupling may give rise to the alternative possibility of a nonmagnetic insulator, with or without a static cooperative Jahn-Teller distortion. In the latter case the insulator has Mott-like properties, with an interesting interplay between electron-electron repulsion and the Jahn-Teller effect, which is dynamical. We study this kind of nonmagnetic insulator in a very simple E circle times e Jahn-Teller model and we discuss its general properties in a more general context, also in connection with the insulating state of K4C60 and Rb4C60
The electronic structure of 4d transition-metal monatomic wires
No full textMonatomic nanowires of the nonmagnetic transition metals Ru, Rh, and Pd have been studied theoretically, using first-principles computational techniques, in order to investigate the possible onset of magnetism in these nanosystems. Our fully relativistic spin-polarized all-electron density functional calculations reveal the onset of Hund's rule magnetism in nanowires of all three metals, with mean-field moments of 1.1, 0.3, and 0.7 mu(B), respectively, at the equilibrium bond length. An analysis of the band structures indicates that the nanocontact superparamagnetic state suggested by our calculations should affect the ballistic conductance between tips made of Ru, Rh or Pd, leading to possible temperature and magnetic field dependent conductance
Jahn-Teller effect in the magnesium phthalocyanine anion
No full textWe present an ab initio study of the static Jahn–Teller (JT) effect in the magnesium phthalocyanine anion (MgPc−). The neutral MgPc has D4h symmetry with a doubly degenerate eg affinity level and 14 B1g plus 14 B2g JT active modes. MgPc− is thus a physical realization of a eg ⊗ (B1g ⊕ B2g) JT problem. We carried out a first principles DFT/LDA calculation of the electronic structure, full vibrational spectrum, and linear electron–vibration coupling constants for this system. From that we calculate a static JT energy gain of 50 meV – essentially due to a B1g distortion – characterizing this as a relatively weak-coupling system. It is anticipated that quantum mechanical dynamic JT tunneling between the two equivalent B1g distorted JT minima should be quite important. Two B2g distorted saddle points are identified as the relevant transition states, with a barrier of 27 meV above the minima. Previous work on this JT problem is also addressed
Connection between magnetism and structure in Fe double chains on the Ir(100) surface
No full textThe magnetic ground state of nanosized systems such as Fe double chains, chains recently shown to form in the early stages of Fe deposition on Ir(100), is generally nontrivial. Using ab initio density functional theory we find that the straight ferromagnetic (FM) state typical of bulk Fe as well as of isolated Fe chains and double chains is disfavored after deposition on Ir(100) for all the experimentally relevant double chain structures considered. So long as spin-orbit coupling (SOC) is neglected, the double chain lowest energy state is generally antiferromagnetic (AFM), a state which appears to prevail over the FM state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further elements, namely, a magnetocrystalline anisotropy and a Dzyaloshinskii-Moriya (DM) spin-spin interaction; the former stabilizing the collinear AFM state and the latter favoring a long-period spin modulation. We find that anisotropy is most important when the double chain is adsorbed on the partially deconstructed Ir(100)-a state which we find to be substantially lower in energy than any reconstructed structure-so that in this case the Fe double chain should remain collinear AFM. Alternatively, when the same Fe double chain is adsorbed in a metastable state onto the (5 x 1) fully reconstructed Ir(100) surface, the FM-AFM energy difference is very much reduced and the DM interaction is expected to prevail, probably yielding a helical spin structure
Microsurgery of lymphatic vessels [MICROCHIRURGIA DEI VASI LINFATICI. DALLE LEGATURE ANTIGRAVITAZIONALI AGLI INNESTI DI LINFATICI E DI VENE]
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Magnetic phenomena in 5d transition metal nanowires
No full textWe have carried out fully relativistic full-potential, spin-polarized, all-electron density-functional calculations for straight, monatomic nanowires of the 5d transition and noble metals Os, Ir, Pt, and Au. We find that, of these metal nanowires, Os and Pt have mean-field magnetic moments for values of the bond length at equilibrium. In the case of Ir, the wire needs to be slightly stretched in order to spin polarize. An analysis of the band structures of the wires indicate that the superparamagnetic state, which our calculations suggest will affect the conductance through the wires—though not by a large amount—at least in the absence of magnetic domain walls. It should thus lead to a characteristic temperature-and field-dependent conductance, and may also cause a significant spin polarization of the transmitted current
Surface theory of melting
No full textWe demonstrate that melting is a surface initiated process. The surface becomes unstable before the bulk and the process of melting consists in the unstable surface that proceeds into the otherwise stable bul
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