353 research outputs found
Transport Properties through Double Barrier Structure in Graphene
The mode-dependent transmission of relativistic ballistic massless Dirac
fermion through a graphene based double barrier structure is being investigated
for various barrier parameters. We compare our results with already published
work and point out the relevance of these findings to a systematic study of the
transport properties in double barrier structures. An interesting situation
arises when we set the potential in the leads to zero, then our 2D problem
reduces effectively to a 1D massive Dirac equation with an effective mass
proportional to the quantized wave number along the transverse direction.
Furthermore we have shown that the minimal conductivity and maximal Fano factor
remain insensitive to the ratio between the two potentials V_2/V_1=\alpha.Comment: 18 pages, 12 figures, clarifications and reference added, misprints
corrected. Version to appear in JLT
Perturbative renormalization and thermodynamics of quantum crystalline membranes
Contains fulltext :
251301.pdf (Publisher’s version ) (Open Access
Nonquasiparticle states in the half-metallic ferromagnet NiMnSb
Nonquasiparticle states above the Fermi energy are studied by first-principle
dynamical mean field calculations for a prototype half-metallic ferromagnet,
NiMnSb. We present a quantitative evaluation of the spectral weight of this
characteristic feature and discuss the possible experimental investigation
(BIS, NMR, STM and Andreev reflection) to clarify the existence of these
states.Comment: 15 pages, 4 figures, acepted in PR
Scattering by flexural phonons in suspended graphene under back gate induced strain
We have studied electron scattering by out-of-plane (flexural) phonon modes
in doped suspended graphene and its effect on charge transport. In the
free-standing case (absence of strain) the flexural branch shows a quadratic
dispersion relation, which becomes linear at long wavelength when the sample is
under tension due to the rotation symmetry breaking. In the non-strained case,
scattering by flexural phonons is the main limitation to electron mobility.
This picture changes drastically when strains above are considered. Here we study in particular the
case of back gate induced strain, and apply our theoretical findings to recent
experiments in suspended graphene.Comment: 4 pages, 3 figures, published versio
Theory of Doping: Monovalent Adsorbates
Contains fulltext :
224022.pdf (Publisher’s version ) (Open Access
Electron self-trapping at quantum and classical critical points
Using Feynman path integral technique estimations of the ground state energy
have been found for a conduction electron interacting with order parameter
fluctuations near quantum critical points. In some cases only \textit{singular}
perturbation theory in the coupling constant emerges for the electron ground
state energy. It is shown that an autolocalized state (quantum fluctuon) can be
formed and its characteristics have been calculated depending on critical
exponents for both weak and strong coupling regimes. The concept of fluctuon is
considered also for the classical critical point (at finite temperatures) and
the difference between quantum and classical cases has been investigated. It is
shown that, whereas the quantum fluctuon energy is connected with a true
boundary of the energy spectrum, for classical fluctuon it is just a
saddle-point solution for the chemical potential in the exponential density of
states fluctuation tail.Comment: 45 pages, 1 eps figure, elsart, submitted to Annals of Physic
Dual boson approach to collective excitations in correlated fermionic systems
We develop a general theory of a boson decomposition for both local and
non-local interactions in lattice fermion models which allows us to describe
fermionic degrees of freedom and collective charge and spin excitations on
equal footing. An efficient perturbation theory in the interaction of the
fermionic and the bosonic degrees of freedom is constructed in so-called dual
variables in the path-integral formalism. This theory takes into account all
local correlations of fermions and collective bosonic modes and interpolates
between itinerant and localized regimes of electrons in solids. The zero-order
approximation of this theory corresponds to extended dynamical mean-field
theory (EDMFT), a regular way to calculate nonlocal corrections to EDMFT is
provided. It is shown that dual ladder summation gives a conserving
approximation beyond EDMFT. The method is especially suitable for consideration
of collective magnetic and charge excitations and allows to calculate their
renormalization with respect to "bare" RPA-like characteristics. General
expression for the plasmonic dispersion in correlated media is obtained. As an
illustration it is shown that effective superexchange interactions in the
half-filled Hubbard model can be derived within the dual-ladder approximation.Comment: Extended version, 17 pages, 5 figure
Correlation effects and orbital magnetism of Co clusters
Recent experiments on isolated Co clusters have shown huge orbital magnetic
moments in comparison with their bulk and surface counterparts. These clusters
hence provide the unique possibility to study the evolution of the orbital
magnetic moment with respect to the cluster size and how competing interactions
contribute to the quenching of orbital magnetism. We investigate here different
theoretical methods to calculate the spin and orbital moments of Co clusters,
and assess the performances of the methods in comparison with experiments. It
is shown that density functional theory in conventional local density or
generalized gradient approximations, or even with a hybrid functional, severely
underestimates the orbital moment. As natural extensions/corrections we
considered the orbital polarization correction, the LDA+U approximation as well
as the LDA+DMFT method. Our theory shows that of the considered methods, only
the LDA+DMFT method provides orbital moments in agreement with experiment, thus
emphasizing the importance of dynamic correlations effects for determining
fundamental magnetic properties of magnets in the nano-size regime
Dual boson approach with instantaneous interaction
Contains fulltext :
208999.pdf (publisher's version ) (Open Access
Demonstration of a quantum nondemolition sum gate
The sum gate is the canonical two-mode gate for universal quantum computation
based on continuous quantum variables. It represents the natural analogue to a
qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum
nondemolition (QND) interaction between the quadrature components of two light
fields. We experimentally demonstrate a QND sum gate, employing the scheme by
R. Filip, P. Marek, and U.L. Andersen [\pra {\bf 71}, 042308 (2005)], solely
based on offline squeezed states, homodyne measurements, and feedforward. The
results are verified by simultaneously satisfying the criteria for QND
measurements in both conjugate quadratures.Comment: 4 pages, 4 figure
- …