L. Pisani

Electronic structure and magnetic properties of graphitic ribbons

First-principles calculations are used to establish that the electronic structure of graphene ribbons with zigzag edges is unstable with respect to magnetic polarization of the edge states. The magnetic interaction between edge states is found to be remarkably long ranged and intimately connected to the electronic structure of the ribbon. Various treatments of electronic exchange and correlation are used to examine the sensitivity of this result to details of the electron-electron interactions, and the qualitative features are found to be independent of the details of the approximation. The possibility of other stablization mechanisms, such as charge ordering and a Peierls distortion, are explicitly considered and found to be unfavorable for ribbons of reasonable width. These results have direct implications for the control of the spin-dependent conductance in graphitic nanoribbons using suitably modulated magnetic fields.

A defective graphene phase predicted to be a room temperature ferromagnetic semiconductor

Theoretical calculations, based on the hybrid exchange density functional theory, are used to show that in graphene, a periodic array of defects generates a ferromagnetic ground state at room temperature for unexpectedly large defect separations. This is demonstrated for defects that consist of a carbon vacancy in which two of the dangling bonds are saturated with H atoms. The magnetic coupling mechanism is analysed and found to be due to an instability in the ?-electron system with respect to a long-range spin polarization characterized by alternation in the spin direction between adjacent carbon atoms. The disruption of the ?-bonding opens a semiconducting gap at the Fermi edge. The size of the energy gap and the magnetic coupling strength are strong functions of the defect separation and can thus be controlled by varying the defect concentration. The position of the semiconducting energy gap and the electron effective mass are strongly spin-dependent and this is expected to result in a spin asymmetry in the transport properties of the system. A defective graphene sheet is, therefore, a very promising material with an in-built mechanism for tailoring the properties of future spintronic devices.

Effects of fe substitution on the electronic, transport, and magnetic properties of ZnGa2O4 : A systematic ab initio study

We present a density functional study of Fe doped into the tetrahedral and octahedral cation sites of the wide-band-gap spinel

Density functional study of the electronic and vibrational properties of TiOCl

\mathrm{Zn}{\mathrm{Ga}}_{2}{\mathrm{O}}_{4}

Electronic structure of the spin- 1 2 quantum magnet TiOCl

. We calculate the electronic structure for different substitutions and discuss the magnetic and transport properties for each case considering different approximations for the exchange-correlation potential. We show that for certain doped cases, significant differences in the predicted behavior are obtained depending on the exchange-correlation potential adopted. Possible applications of the doped systems as magnetic semiconductors are outlined.

One-dimensional versus two-dimensional correlation effects in the oxyhalides TiOCl and TiOBr

We present the phonon spectrum of TiOCl computed using hybrid density functional theory DFT .A complete analysis of the spectrum is performed for the space group Pmmn high-symmetry phase and the space group P21 /m low-symmetry phase, which is the symmetry of the spin-Peierls phase. We show that the nonlocal correlations present in the hybrid DFT approach are important for understanding the electron-lattice interactions in TiOCl. The computed frequencies compare well with those observed in Raman and infrared spectroscopy experiments, and we identify the origin of an anomalous phonon observed in Raman spectroscopy. The relationship between relevant zone boundary phonons in the high-symmetry phase and the zone center counterparts in the P21 /m symmetry allow us to speculate about the origin of the spin-Peierls phonon.

Unusual quasi-one-dimensional electron dispersions in the spin-1/2 quantum magnet TiOCl

We have studied the electronic structure of the spin-1/2 quantum magnet TiOCl by polarization-dependent momentum-resolved photoelectron spectroscopy. From that, we confirm the quasi-one-dimensional nature of the electronic structure along the crystallographic b-axis and find no evidence for sizable phonon-induced orbital fluctuations as origin for the non-canonical phenomenology of the spin-Peierls transition in this compound. A comparison of the experimental data to our own LDA+U and Hubbard model calculations reveals a striking lack of understanding regarding the quasi-one-dimensional electron dispersions in the normal state of this compound.

Ab initio phonon calculations for the layered compound TiOCi

We have performed a comparative study of the electronic structures of the spin-Peierls systems TiOCl and TiOBr by means of photoemission spectroscopy and density-functional calculations. While the overall electronic structure of these isostructural compounds is qualitatively similar, the bromide appears to be less one-dimensional. We present a quantitative analysis of the experimental dispersions in terms of exchange constant

Electronic structure of the spin-12quantum magnet TiOCl

J

Density functional study of the magnetic coupling in V(TCNE)-2

and hopping integral