399 research outputs found
Electrostatic effects and band-bending in doped topological insulators
We investigate the electrostatic effects in doped topological insulators by
developing a self consistent scheme for an interacting tight binding model. The
presence of bulk carriers, in addition to surface electrons, generates an
intrinsic inhomogeneous charge density in the vicinity of the surface and, as a
result, band bending effects are present. We find that electron doping and hole
doping produce band bending effects of similar magnitude and opposite signs.
The presence of additional surface dopants breaks this approximate
electron-hole symmetry and dramatically affects the magnitude of the band
bending. Applying a gate potential can generate a depletion zone characterized
by a vanishing carrier density. We find that the density profile in the
transition zone between the depleted region and the bulk is independent of the
applied potential. In thin films the electrostatic effects are strongly
dependent on the carrier charge density. In addition, we find that substrate
induced potentials can generate a Rashba type spin-orbit coupling in ultra thin
topological insulator films. We calculate the profiles of bulk and surface
states in topological insulator films and identify the conditions corresponding
to both types of states being localized within the same region in space.Comment: 9 pages, 10 figure
ROCK SLOPE STABILITY PROBLEMS IN NATURAL SIGHTSEEING AREAS - AN EXAMPLE FROM ARVANITIA, NAFPLIO, GREECE
The morphological and geological setting of Greece, the active tectonics and the irrational human activities results to the fact that several natural sightseeing areas or even more, archaeological sites and monuments are located in areas with unfavourable geotechnical conditions. The selection of the proper support and protection measures in most of the cases appear to be very difficult because the applied measures must reassure the minimum aesthetic destruction of the sites. The natural sightseeing area of the Arvanitia walkway, in Nafplio city, is a typical example of site, with extensive human activities, manifesting serious rockfall stability problems. The applied stability analysis pointed out the geotechnical problems and allowed the suggestion of measures for the improvement of the geotechnical behaviour of the rock mass. The measures were planned with respect to the natural beauty and the historical character of the site. Further more, the stability problems located at the slopes of the Kastoria lake walkway are briefly presented. The differences between the two sites revealed the geotechnical problems arising when the landplaning engineers do not take under consideration the engineering geological conditions during the construction of infrastructures
Quantum Criticality and Incipient Phase Separation in the Thermodynamic Properties of the Hubbard Model
Transport measurements on the cuprates suggest the presence of a quantum
critical point hiding underneath the superconducting dome near optimal hole
doping. We provide numerical evidence in support of this scenario via a
dynamical cluster quantum Monte Carlo study of the extended two-dimensional
Hubbard model. Single particle quantities, such as the spectral function, the
quasiparticle weight and the entropy, display a crossover between two distinct
ground states: a Fermi liquid at low filling and a non-Fermi liquid with a
pseudogap at high filling. Both states are found to cross over to a marginal
Fermi-liquid state at higher temperatures. For finite next-nearest-neighbor
hopping t' we find a classical critical point at temperature T_c. This
classical critical point is found to be associated with a phase separation
transition between a compressible Mott gas and an incompressible Mott liquid
corresponding to the Fermi liquid and the pseudogap state, respectively. Since
the critical temperature T_c extrapolates to zero as t' vanishes, we conclude
that a quantum critical point connects the Fermi-liquid to the pseudogap
region, and that the marginal-Fermi-liquid behavior in its vicinity is the
analogous of the supercritical region in the liquid-gas transition.Comment: 18 pages, 9 figure
Search for spin gapless semiconductors: The case of inverse Heusler compounds
We employ ab-initio electronic structure calculations to search for spin
gapless semiconductors, a recently identified new class of materials, among the
inverse Heusler compounds. The occurrence of this property is not accompanied
by a general rule and results are materials specific. The six compounds
identified show semiconducting behavior concerning the spin-down band structure
and in the spin-up band structure the valence and conduction bands touch each
other leading to 100% spin-polarized carriers. Moreover these six compounds
should exhibit also high Curie temperatures and thus are suitable for
spintronics applications.Comment: Submitted to Applied Physics Letter
Orbital magnetism in the half-metallic Heusler alloys
Using the fully-relativistic screened Korringa-Kohn-Rostoker method I study
the orbital magnetism in the half-metallic Heusler alloys. Orbital moments are
almost completely quenched and they are negligible with respect to the spin
moments. The change in the atomic-resolved orbital moments can be easily
explained in terms of the spin-orbit strength and hybridization effects.
Finally I discuss the orbital and spin moments derived from X-ray magnetic
circular dichroism experiments
Surface Half-Metallicity of CrAs in the Zinc-Blende Structure
The development of new techniques such as the molecular beam epitaxy have
enabled the growth of thin films of materials presenting novel properties.
Recently it was made possible to grow a CrAs thin-film in the zinc-blende
structure. In this contribution, the full-potential screened KKR method is used
to study the electronic and magnetic properties of bulk CrAs in this novel
phase as well as the Cr and As terminated (001) surfaces. Bulk CrAs is found to
be half-ferromagnetic for all three GaAs, AlAs and InAs experimental lattice
constants with a total spin magnetic moment of 3 . The Cr-terminated
surface retains the half-ferromagnetic character of the bulk, while in the case
of the As-termination the surface states destroy the gap in the minority-spin
band.Comment: 4 pages, 2 figures, new text, new titl
Thermodynamics of the Quantum Critical Point at Finite Doping in the 2D Hubbard Model: A Dynamical Cluster Approximation Study
We study the thermodynamics of the two-dimensional Hubbard model within the
dynamical cluster approximation. We use continuous time quantum Monte Carlo as
a cluster solver to avoid the systematic error which complicates the
calculation of the entropy and potential energy (double occupancy). We find
that at a critical filling, there is a pronounced peak in the entropy divided
by temperature, S/T, and in the normalized double occupancy as a function of
doping. At this filling, we find that specific heat divided by temperature,
C/T, increases strongly with decreasing temperature and kinetic and potential
energies vary like T^2 ln(T). These are all characteristics of quantum critical
behavior.Comment: 4 pages, 4 figures. Submitted to Phys. Rev. B Rapid Communications on
June 27, 200
Mottness on a triangular lattice
We study the physics on the paramagnetic side of the phase diagram of the
cobaltates, , with an implementation of cellular dynamical mean
field theory (CDMFT) with the non-crossing approximation (NCA) for the one-band
Hubbard model on a triangular lattice. At low doping we find that the low
energy physics is dominated by a quasi-dispersionless band. At half-filling, we
find a metal-insulator transition at which depends weakly
on the cluster size. The onset of the metallic state occurs through the growth
of a coherence peak at the chemical potential. Away from half filling, in the
electron-doped regime, the system is metallic with a large, continuous Fermi
surface as seen experimentally. Upon hole doping, a quasi non-dispersing band
emerges at the top of the lower Hubbard band and controls the low-energy
physics. This band is a clear signature of non-Fermi liquid behavior and cannot
be captured by any weakly coupled approach. This quasi non-dispersive band,
which persists in a certain range of dopings, has been observed experimentally.
We also investigate the pseudogap phenomenon in the context of a triangular
lattice and we propose a new framework for discussing the pseudogap phenomena
in general. This framework involves a momentum-dependent characterization of
the low-energy physics and links the appearance of the pseudogap to a
reconstruction of the Fermi surface without invoking any long range order or
symmetry breaking. Within this framework we predict the existence of a
pseudogap for the two dimensional Hubbard model on a triangular lattice in the
weakly hole-doped regime.Comment: 14 pages, 21 figure
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