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We show that finite temperature variational cluster approximation (VCA) calculations on an ex-

Using the band structure calculation and mean-field analysis of the derived three-chain Hubbard model with phonon degrees of freedom, we discuss the origin of the orthorhombic-to-monoclinic phase transition of the layered chalcogenide Ta2NiSe5. We show that the Bose-Einstein condensation of excitonic electron-hole pairs cooperatively induces the instability of the phonon mode at momentum q → 0 in the quasi-one-dimensional Ta-NiSe-Ta chain, resulting in the structural phase transition of the system. The calculated single-particle spectra reproduce the deformation of the band structure observed in the angle-resolved photoemission spectroscopy experiment. PACS numbers: 71.10.Fd, 71.30.+h, The electron-hole pair condensation in thermal equilibrium into the excitonic insulator (EI) state has attracted renewed interest in recent years because of the discoveries of a number of new materials. The idea of EI was proposed about half a century ago [1,2], where the EI state was predicted to be realized either in a semiconductor with a small band gap or in a semimetal with a small band overlap. The formation of excitons is driven by poorly screened Coulomb interaction between conduction-band electrons and valence-band holes under condition of a low carrier concentration. If the binding energy of excitons is larger than the band gap, they may spontaneously condense at low temperatures and drive the system into a new ground state with exotic properties. This new state, which is a condensed state of a macroscopic number of excitons acquiring quantum phase coherence, is called EI [3,4]. It has been pointed out that the semimetal-EI transition may be described in analogy with the BCS theory of superconductivity and the semiconductor-EI transition is discussed in terms of a Bose-Einstein condensation (BEC) of preformed excitons [5][6][7][8][9].An example of materials for possible realization of EI is Tm(Se,Te), where it has been claimed that a transition into EI occurs by applying pressure [10][11][12] and that the superfluidity of condensed excitons is responsible for the observed anomalous properties [13]. Another promising candidate for EI is CaB 6 [14], where the observed weak ferromagnetism with an unexpectedly high Curie temperature was interpreted in terms of a doped EI [15,16]. Also known as a candidate for EI is TiSe 2 , where the observed charge-density-wave state was interpreted to be of an excitonic type [17][18][19]. The spin-density-wave state of the iron-pnictide superconductors has also been argued to be of the excitonic type [20][21][22]. Semiconductor bilayer systems have attracted attention as well in relation to the BEC of excitons [23].Recently, a transition-metal chalcogenide Ta 2 NiSe 5 has been studied in this respect [24,25]. This mate-rial has a layered structure stacked loosely by a weak van der Waals interaction, and in each layer, Ni single chains and Ta double chains are running along the aaxis of the lattice to form a quasi-one-dimensional (1D) chain structure [26]. The observed resistivity shows a se...

The spin and orbital moments of Au/Co/Au trilayers grown on a W(110) single crystal substrate have been investigated by means of x-ray magnetic circular dichroism. Our findings suggest that the orbital moment of Co does not obtain a maximum value along the easy axis, in contrast with previous experience. This is attributed to the large spin-orbit interaction within the Au caps. Both second order perturbation theory and first principles calculations show how the magnetocrystalline anisotropy (MCA) is dramatically influenced by this effect, and how this leads to the fact that the orbital moment anisotropy is not proportional to the MCA.

A surface-functionalized liposome with a high C70 concentration (LMIC70) was synthesized by exchange reaction from an unstable fullerene complex, C70·γ-CDx. The DNA photocleavage ability of LMIC70 is extremely higher than that of LMIC60 or C60·γ-CDx.