Spontaneous exciton condensation in 1T-TiSe2: a BCS-like approach

Recently strong evidence has been found in favor of a BCS-like condensation of excitons in 1\textit{T}-TiSe$_2$. Theoretical photoemission intensity maps have been generated by the spectral function calculated within the excitonic condensate phase model and set against experimental angle-resolved photoemission spectroscopy data. Here, the calculations in the framework of this model are presented in detail. They represent an extension of the original excitonic insulator phase model of J\'erome \textit{et al.} [Phys. Rev. {\bf 158}, 462 (1967)] to three dimensional and anisotropic band dispersions. A detailed analysis of its properties and further comparison with experiment are also discussedComment: Submitted to PRB, 11 pages, 7 figure

Activity of raltitrexed and gemcitabine in advanced pancreatic cancer

Background: Gemcitabine has evolved as standard therapy in advanced pancreatic cancer since the demonstration of a significant clinical benefit. Phase II trials have shown that gemcitabine can be successfully combined with thymidylate synthase (TS) inhibitors such as continuous-infusion 5-fluorouracil (5-FU). However, continuous-infusion 5-FU is inconvenient because of the need for a central venous access. The aim of this study was to assess the efficacy and safety of gemcitabine in combination with raltitrexed (Tomudex), a novel and selective TS inhibitor that has the advantage of a 3-weekly treatment interval and manageable toxicity. Patients and methods: Chemotherapy-naïve patients with measurable advanced pancreatic cancer were treated with raltitrexed 3 mg/m2 as a 15-min infusion on day 1 and gemcitabine 1000 mg/m2 on days 1 and 8, every 21 days. Results: Twenty-five eligible patients (17 male, eight female) with metastatic (21 patients) or locally advanced (four patients) disease entered the study. The median number of courses per patient was four (range 1-14). One patient was not evaluable for response. There were three partial remissions [12%; 95% confidence interval (CI) 2.6% to 31.2%] and nine stable disease situations (36%; 95% CI 18.0% to 57.5%), while the tumours of 12 patients (48%; 95% CI 27.8% to 68.7%) showed progressive disease after three treatment cycles. WHO grade 3/4 toxicity was rare and symptomatic in only one patient, who experienced grade 4 diarrhoea and grade 3 nausea and vomiting. Symptomatic benefit was seen in 12 patients. Median survival was 185 days (95% CI 129-241) with six patients still alive. Conclusions: The efficacy of raltitrexed plus gemcitabine is limited, but compares well with other chemotherapy treatment options in advanced pancreatic cancer. However, this combination is convenient and symptomatic toxicity is rare. Thus, raltitrexed and gemcitabine should be investigated further in combination with drugs interfering with specific molecular target

Quasiparticle spectrum in a nearly antiferromagnetic Fermi liquid: shadow and flat bands

We consider a two-dimensional Fermi liquid in the vicinity of a spin-density-wave transition to a phase with commensurate antiferromagnetic long-range order. We assume that near the transition, the Fermi surface is large and crosses the magnetic Brillouin zone boundary. We show that under these conditions, the self-energy corrections to the dynamical spin susceptibility, $\chi (q, \omega)$, and to the quasiparticle spectral function function, $A(k, \omega)$, are divergent near the transition. We identify and sum the series of most singular diagrams, and obtain a solution for $\chi(q, \omega)$ and an approximate solution for $A(k, \omega)$. We show that (i) $A(k)$ at a given, small $\omega$ has an extra peak at $k = k_F + \pi$ (`shadow band'), and (ii) the dispersion near the crossing points is much flatter than for free electrons. The relevance of these results to recent photoemission experiments in $YBCO$ and $Bi2212$ systems is discussed.Comment: a sign and amplitude of the vertex renormalization and few typos are correcte

Theory for the excitation spectrum of High-T$_c superconductors : quasiparticle dispersion and shadows of the Fermi surface

Using a new method for the solution of the FLEX-equations, which allows the determination of the self energy $\Sigma_{\bf k}(\omega)$ of the $2D$ Hubbard model on the real frequency axis, we calculate the doping dependence of the quasi-particle excitations of High-T$_c$ superconductors. We obtain new results for the shadows of the Fermi surface, their dependence on the deformation of the quasi particle dispersion, an anomalous $\omega$-dependence of ${\rm Im}\Sigma_{\bf k}(\omega)$ and a related violation of the Luttinger theorem. This sheds new light on the influence of short range magnetic order on the low energy excitations and its significance for photoemission experiments.Comment: 4 pages (REVTeX) with 3 figure

Temperature dependent photoemission on 1T-TiSe2: Interpretation within the exciton condensate phase model

The charge density wave phase transition of 1T-TiSe2 is studied by angle-resolved photoemission over a wide temperature range. An important chemical potential shift which strongly evolves with temperature is evidenced. In the framework of the exciton condensate phase, the detailed temperature dependence of the associated order parameter is extracted. Having a mean-field-like behaviour at low temperature, it exhibits a non-zero value above the transition, interpreted as the signature of strong excitonic fluctuations, reminiscent of the pseudo-gap phase of high temperature superconductors. Integrated intensity around the Fermi level is found to display a trend similar to the measured resistivity and is discussed within the model.Comment: 8 pages, 6 figure

Inverse photoemission in strongly correlated electron systems

Based on exact results for small clusters of 2D t-J model we demonstrate the existence of several distinct `channels' in its inverse photoemission (IPES) spectrum. Hole-like quasiparticles can either be annihilated completely, or leave behind a variable number of spin excitations, which formed the `dressing cloud' of the annihilated hole. In the physical parameter regime the latter processes carry the bulk of IPES weight and although the Fermi surface takes the form of hole pockets, the distribution of spectal weight including these `magnon-bands' in the IPES spectrum is reminiscent of free electrons. The emerging scenario for Fermiology and spectral weight distribution is shown to be consistent with photoemission, inverse photemission and de Haas--van Alphen experiments on cuprate superconductors.Comment: Revtex file, 4 PRB pages + three figures appended as uu-encoded postscript. Hardcopies of figures (or the entire manuscript) can also be obtained by e-mail request to: [email protected]

Spectral weight function for the half-filled Hubbard model: a singular value decomposition approach

The singular value decomposition technique is used to reconstruct the electronic spectral weight function for a half-filled Hubbard model with on-site repulsion $U=4t$ from Quantum Monte Carlo data. A two-band structure for the single-particle excitation spectrum is found to persist as the lattice size exceeds the spin-spin correlation length. The observed bands are flat in the vicinity of the $(0,\pi),(\pi,0)$ points in the Brillouin zone, in accordance with experimental data for high-temperature superconducting compounds.Comment: 4 pages, Revtex

Measuring the gap in ARPES experiments

Angle-resolved photoemission spectroscopy (ARPES) is considered as the only experimental tool from which the momentum distribution of both the superconducting and pseudo-gap can be quantitatively derived. The binding energy of the leading edge of the photoemission spectrum, usually called the leading edge gap (LEG), is the model-independent quantity which can be measured in the modern ARPES experiments with the very high accuracy--better than 1 meV. This, however, may be useless as long as the relation between the LEG and the real gap is unknown. We present a systematic study of the LEG as a function of a number of physical and experimental parameters. The absolute gap values which have been derived from the numerical simulation prove, for example that the nodal direction in the underdoped Bi-2212 in superconducting state is really the node--the gap is zero. The other consequences of the simulations are discussed.Comment: revtex4, 9 pages, 6 figure