Fermi surface induced lattice distortion in NbTe2_2

The origin of the monoclinic distortion and domain formation in the quasi two-dimensional layer compound NbTe$_2$ is investigated. Angle-resolved photoemission shows that the Fermi surface is pseudogapped over large portions of the Brillouin zone. Ab initio calculation of the electron and phonon bandstructure as well as the static RPA susceptibility lead us to conclude that Fermi surface nesting and electron-phonon coupling play a key role in the lowering of the crystal symmetry and in the formation of the charge density wave phase

Hole Pockets in the Doped 2D Hubbard Model

The electronic momentum distribution ${\rm n({\bf k})}$ of the two dimensional Hubbard model is studied for different values of the coupling ${\rm U/t}$, electronic density ${\rm \langle n \rangle}$, and temperature, using quantum Monte Carlo techniques. A detailed analysis of the data on $8\times 8$ clusters shows that features consistent with hole pockets at momenta ${\rm {\bf k}=(\pm {\pi\over{2}},\pm {\pi\over{2}})}$ appear as the system is doped away from half-filling. Our results are consistent with recent experimental data for the cuprates discussed by Aebi et al. (Phys. Rev. Lett. {\bf 72}, 2757 (1994)). In the range of couplings studied, the depth of the pockets is maximum at ${\rm \langle n \rangle \approx 0.9}$, and it increases with decreasing temperature. The apparent absence of hole pockets in previous numerical studies of this model is explained.Comment: 11 pages, 4 postscript figures appended, RevTeX (version 3.0

Hot Spots on the Fermi Surface of Bi2212: Stripes versus Superstructure

In a recent paper Saini et al. have reported evidence for a pseudogap around (pi,0) at room temperature in the optimally doped superconductor Bi2212. This result is in contradiction with previous ARPES measurements. Furthermore they observed at certain points on the Fermi surface hot spots of high spectral intensity which they relate to the existence of stripes in the CuO planes. They also claim to have identified a new electronic band along Gamma-M1 whose one dimensional character provides further evidence for stripes. We demonstrate in this Comment that all the measured features can be simply understood by correctly considering the superstructure (umklapp) and shadow bands which occur in Bi2212.Comment: 1 page, revtex, 1 encapsulated postscript figure (color

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

Doping nature of native defects in 1T-TiSe2

The transition metal dichalcogenide 1T-TiSe2 is a quasi two-dimensional layered material with a charge density wave (CDW) transition temperature of TCDW 200 K. Self-doping effects for crystals grown at different temperatures introduce structural defects, modify the temperature dependent resistivity and strongly perturbate the CDW phase. Here we study the structural and doping nature of such native defects combining scanning tunneling microscopy/spectroscopy and ab initio calculations. The dominant native single atom dopants we identify in our single crystals are intercalated Ti atoms, Se vacancies and Se substitutions by residual iodine and oxygen.Comment: 5 pages, 3 figure

Hypermethylation of the DPYD promoter region is not a major predictor of severe toxicity in 5-fluorouracil based chemotherapy

<p>Abstract</p> <p>Background </p> <p>The activity of dihydropyrimidine dehydrogenase (DPD), the key enzyme of pyrimidine catabolism, is thought to be an important determinant for the occurrence of severe toxic reactions to 5-fluorouracil (5-FU), which is one of the most commonly prescribed chemotherapeutic agents for the treatment of solid cancers. Genetic variation in the DPD gene (DPYD) has been proposed as a main factor for variation in DPD activity in the population. However, only a small proportion of severe toxicities in 5-FU based chemotherapy can be explained with such rare deleterious DPYD mutations resulting in severe enzyme deficiencies. Recently, hypermethylation of the DPYD promoter region has been proposed as an alternative mechanism for DPD deficiency and thus as a major cause of severe 5-FU toxicity.</p> <p>Methods </p> <p>Here, the prognostic significance of this epigenetic marker with respect to severe 5-FU toxicity was assessed in 27 cancer patients receiving 5-FU based chemotherapy, including 17 patients experiencing severe toxic side effects following drug administration, none of which were carriers of a known deleterious DPYD mutation, and ten control patients. The methylation status of the DPYD promoter region in peripheral blood mononuclear cells was evaluated by analysing for each patient between 19 and 30 different clones of a PCR-amplified 209 base pair fragment of the bisulfite-modified DPYD promoter region. The fragments were sequenced to detect bisulfite-induced, methylation-dependent sequence differences.</p> <p>Results </p> <p>No evidence of DPYD promoter methylation was observed in any of the investigated patient samples, whereas in a control experiment, as little as 10% methylated genomic DNA could be detected.</p> <p>Conclusion </p> <p>Our results indicate that DYPD promoter hypermethylation is not of major importance as a prognostic factor for severe toxicity in 5-FU based chemotherapy.</p