Remnant Fermi surface in the presence of an underlying instability in layered 1T-TaS_2

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS_2 in the quasi commensurate (QC) and the commensurate (C) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS even when passing from the QC-phase to the C-phase. Possibly, this Mott localization induced transition represents the underlying instability responsible for the pseudogapped FS

Geometric approach to Fletcher's ideal penalty function

Original article can be found at: www.springerlink.com Copyright Springer. [Originally produced as UH Technical Report 280, 1993]In this note, we derive a geometric formulation of an ideal penalty function for equality constrained problems. This differentiable penalty function requires no parameter estimation or adjustment, has numerical conditioning similar to that of the target function from which it is constructed, and also has the desirable property that the strict second-order constrained minima of the target function are precisely those strict second-order unconstrained minima of the penalty function which satisfy the constraints. Such a penalty function can be used to establish termination properties for algorithms which avoid ill-conditioned steps. Numerical values for the penalty function and its derivatives can be calculated efficiently using automatic differentiation techniques.Peer reviewe

Le donne nella magistratura italiana: 1960-1990

In un contesto caratterizzato da una limitata presenza di donne nell’attività retribuita in Italia, che ci pone ai livelli più bassi riscontrati in Europa rispetto ai tassi di occupazione femminile, questo paper si interroga sui fattori determinanti la bassa partecipazione delle donne al mercato del lavoro e fra questi individua anche i limiti nell’accesso alle professioni. Il presente lavoro analizza una particolare professione che, sino al 1963, era preclusa alle donne in Italia: la Magistratura. Esso ricostruisce il contesto storico che ha portato all’approvazione della Legge n. 66 del 9 febbraio 1963 ed estende l’analisi sino agli anni novanta, rilevando che la persistenza di evidenti criticità di tipo retributivo e preclusivo alle carriere direttive riscontrate dalle donne in Italia affonda nel passato le sue radici. Ricostruite le linee generali degli aspetti storici e giuridici ed evidenziate le principali difficoltà che le donne hanno dovuto affrontare, il presente lavoro ha l’obiettivo di far emergere il lungo e tortuoso percorso che ha consentito l’entrata delle donne in Magistratura, ponendo in evidenza i limiti che tuttora impediscono una piena ed effettiva integrazione delle donne in Italia a tutti i livelli di questo settore

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

Angle-scanned photoemission: Fermi surface mapping and structural determination

A brief survey of the angle-scanned photoemission technique is given. It incorporates two complementary methods in one:http://www.sciencedirect.com/science/article/B6TVX-3X82696-4F/1/e2ffd4efc660238ad8d4ad8f685991b

Orbital textures and charge density waves in transition metal dichalcogenides

Low-dimensional electron systems, as realized naturally in graphene or created artificially at the interfaces of heterostructures, exhibit a variety of fascinating quantum phenomena with great prospects for future applications. Once electrons are confined to low dimensions, they also tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves; a state of matter that currently attracts enormous attention because of its relation to various unconventional electronic properties. In this study we reveal a remarkable and surprising feature of charge density waves (CDWs), namely their intimate relation to orbital order. For the prototypical material 1T-TaS2 we not only show that the CDW within the two-dimensional TaS2-layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow to manipulate salient features of the electronic structure. Indeed, these orbital effects enable to switch the properties of 1T-TaS2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultra-fast devices based on layered transition metal dichalcogenides

Angle-scanned photoemission: Fermi surface mapping and structural determination

A brief survey of the angle-scanned photoemission technique is given. It incorporates two complementary methods in one:http://www.sciencedirect.com/science/article/B6TVX-3X82696-4F/1/e2ffd4efc660238ad8d4ad8f685991b

Formation of a stable deacagonal quasicrystalline Al-Pd-Mn surface layer

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the 5-fold symmetric surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, low-energy electron diffraction and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied and compared to its icosahedral counterpart.Comment: Submitted to Phys. Ref. Lett. (18 July 2001