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

We study the spontaneous symmetry breaking of the excitonic insulator state induced by the Coulomb interaction U in the two-dimensional extended Falicov-Kimball model. Using the variational cluster approximation (VCA) and Hartree-Fock approximation (HFA), we evaluate the order parameter, single-particle excitation gap, momentum distribution functions, coherence length of excitons, and single-particle and anomalous excitation spectra, as a function of U at zero temperature. We find that in the weak-to-intermediate coupling regime, the Fermi surface plays an essential role and calculated results can be understood in close correspondence with the BCS theory, whereas in the strong-coupling regime, the Fermi surface plays no role and results are consistent with the picture of BEC. Moreover, we find that HFA works well both in the weak-and strong-coupling regime, and that the difference between the results of VCA and HFA mostly appears in the intermediate-coupling regime. The reason for this is discussed from a viewpoint of the self-energy. We thereby clarify the excitonic insulator state that typifies either a BCS condensate of electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of preformed excitons (strong-coupling regime).

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...

Cerebral blood flow (CBF) and rate of oxygen metabolism (CMRO 2 ) may be quantified using positron emission tomography (PET) with 15 O-tracers, but the conventional three-step technique requires a relatively long study period, attributed to the need for separate acquisition for each of 15 O 2 , H 2 15 O, and C 15 O tracers, which makes the multiple measurements at different physiologic conditions difficult. In this study, we present a novel, faster technique that provides a pixel-by-pixel calculation of CBF and CMRO 2 from a single PET acquisition with a sequential administration of 15 O 2 and H 2 15 O. Experiments were performed on six anesthetized monkeys to validate this technique. The global CBF, oxygen extraction fraction (OEF), and CMRO 2 obtained by the present technique at rest were not significantly different from those obtained with three-step method. The global OEF (gOEF) also agreed with that determined by simultaneous arterio-sinus blood sampling (gOEF AÀV ) for a physiologically wide range when changing the arterial PaCO 2 (gOEF ¼ 1.03gOEF AÀV þ 0.01, Po0.001). The regional values, as well as the image quality were identical between the present technique and three-step method for CBF, OEF, and CMRO 2 . In addition, a simulation study showed that error sensitivity of the present technique to delay or dispersion of the input function, and the error in the partition coefficient was equivalent to that observed for three-step method. Error sensitivity to cerebral blood volume (CBV) was also identical to that in the three-step and reasonably small, suggesting that a single CBV assessment is sufficient for repeated measures of CBF/CMRO 2 . These results show that this fast technique has an ability for accurate assessment of CBF/CMRO 2 and also allows multiple assessment at different physiologic conditions.

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