455 research outputs found
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
Non-Fermi-liquid scattering rates and anomalous band dispersion in ferropnictides
Angle-resolved photoemission spectroscopy (ARPES) is used to study the band
dispersion and the quasiparticle scattering rates in two ferropnictides
systems. Our ARPES results show linear-in-energy dependent scattering rates
which are constant in a wide range of control parameter and which depend on the
orbital character of the bands. We demonstrate that the linear energy
dependence gives rise to weakly dispersing band with a strong mass enhancement
when the band maximum crosses the chemical potential. In the superconducting
phase the related small effective Fermi energy favors a
Bardeen-Cooper-Schrieffer (BCS)\,\cite{Bardeen1957}-Bose-Einstein
(BE)\,\cite{Bose1924} crossover state.Comment: 5 pages, 4 figures Supplement 4 pages, 6 figure
Infinitesimal and local convexity of a hypersurface in a semi-Riemannian manifold
Given a Riemannian manifold M and a hypersurface H in M, it is well known
that infinitesimal convexity on a neighborhood of a point in H implies local
convexity. We show in this note that the same result holds in a semi-Riemannian
manifold. We make some remarks for the case when only timelike, null or
spacelike geodesics are involved. The notion of geometric convexity is also
reviewed and some applications to geodesic connectedness of an open subset of a
Lorentzian manifold are given.Comment: 14 pages, AMSLaTex, 2 figures. v2: typos fixed, added one reference
and several comments, statement of last proposition correcte
Superconducting properties of Nb thin films deposited on porous silicon templates
Porous silicon, obtained by electrochemical etching, has been used as a
substrate for the growth of nanoperforated Nb thin films. The films, deposited
by UHV magnetron sputtering on the porous Si substrates, inherited their
structure made of holes of 5 or 10 nm diameter and of 10 to 40 nm spacing,
which provide an artificial pinning structure. The superconducting properties
were investigated by transport measurements performed in the presence of
magnetic field for different film thickness and substrates with different
interpore spacing. Perpendicular upper critical fields measurements present
peculiar features such as a change in the H_c2(T) curvature and oscillations in
the field dependence of the superconducting resistive transition width at H=1
Tesla. This field value is much higher than typical matching fields in
perforated superconductors, as a consequence of the small interpore distance.Comment: accepted for publication on Journal of Applied Physic
Direct observation of the spin texture in strongly correlated SmB6 as evidence of the topological Kondo insulator
The concept of a topological Kondo insulator (TKI) has been brought forward
as a new class of topological insulators in which non-trivial surface states
reside in the bulk Kondo band gap at low temperature due to the strong
spin-orbit coupling [1-3]. In contrast to other three-dimensional (3D)
topological insulators (e.g. Bi2Se3), a TKI is truly insulating in the bulk
[4]. Furthermore, strong electron correlations are present in the system, which
may interact with the novel topological phase. Applying spin- and
angle-resolved photoemission spectroscopy (SARPES) to the Kondo insulator SmB6,
a promising TKI candidate, we reveal that the surface states of SmB6 are spin
polarized, and the spin is locked to the crystal momentum. Counter-propagating
states (i.e. at k and -k) have opposite spin polarizations protected by
time-reversal symmetry. Together with the odd number of Fermi surfaces of
surface states between the 4 time-reversal invariant momenta in the surface
Brillouin zone [5], these findings prove, for the first time, that SmB6 can
host non-trivial topological surface states in a full insulating gap in the
bulk stemming from the Kondo effect. Hence our experimental results establish
that SmB6 is the first realization of a 3D TKI. It can also serve as an ideal
platform for the systematic study of the interplay between novel topological
quantum states with emergent effects and competing order induced by strongly
correlated electrons.Comment: 4 figure
Control-volume representation of molecular dynamics
A Molecular Dynamics (MD) parallel to the Control Volume (CV) formulation of
fluid mechanics is developed by integrating the formulas of Irving and
Kirkwood, J. Chem. Phys. 18, 817 (1950) over a finite cubic volume of molecular
dimensions. The Lagrangian molecular system is expressed in terms of an
Eulerian CV, which yields an equivalent to Reynolds' Transport Theorem for the
discrete system. This approach casts the dynamics of the molecular system into
a form that can be readily compared to the continuum equations. The MD
equations of motion are reinterpreted in terms of a
Lagrangian-to-Control-Volume (\CV) conversion function , for
each molecule . The \CV function and its spatial derivatives are used to
express fluxes and relevant forces across the control surfaces. The
relationship between the local pressures computed using the Volume Average (VA,
Lutsko, J. Appl. Phys 64, 1152 (1988)) techniques and the Method of Planes
(MOP, Todd et al, Phys. Rev. E 52, 1627 (1995)) emerges naturally from the
treatment. Numerical experiments using the MD CV method are reported for
equilibrium and non-equilibrium (start-up Couette flow) model liquids, which
demonstrate the advantages of the formulation. The CV formulation of the MD is
shown to be exactly conservative, and is therefore ideally suited to obtain
macroscopic properties from a discrete system.Comment: 19 pages, 15 figure
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