286 research outputs found
Valence transition in the periodic Anderson model
A very rich phase diagram has recently been found in CeCuSi from
high pressure experiments where, in particular, a transition between an
intermediate valence configuration and an integral valent heavy fermion state
has been observed. We show that such a valence transition can be understood in
the framework of the periodic Anderson model. In particular, our results show a
breakdown of a mixed-valence state which is accompanied by a drastic change in
the \textit{f} occupation in agreement with experiment. This valence transition
can possibly be interpreted as a collapse of the large Fermi surface of the
heavy fermion state which incorporates not only the conduction electrons but
also the localized \textit{f} electrons. The theoretical approach used in this
paper is based on the novel projector-based renormalization method (PRM). With
respect to the periodic Anderson model, the method was before only employed in
combination with the basic approximations of the well-known slave-boson
mean-field theory. In this paper, the PRM treatment is performed in a more
sophisticated manner where both mixed as well as integral valent solutions have
been obtained. Furthermore, we argue that the presented PRM approach might be a
promising starting point to study the competing interactions in
CeCuSi and related compounds.Comment: 9 pages, 3 figures included; v2: completely revised and extended
versio
Analytical approach to the quantum-phase transition in the one-dimensional spinless Holstein model
We study the one-dimensional Holstein model of spinless fermions interacting
with dispersion-less phonons by using a recently developed projector-based
renormalization method (PRM). At half-filling the system shows a
metal-insulator transition to a Peierls distorted state at a critical
electron-phonon coupling where both phases are described within the same
theoretical framework. The transition is accompanied by a phonon softening at
the Brillouin zone boundary and a gap in the electronic spectrum. For different
filling, the phonon softening appears away from the Brillouin zone boundary and
thus reflects a different type of broken symmetry state.Comment: 8 pages, 4 figures included; v2: completely revised and extended; v3:
minor changes, final version, to be published in Eur. Phys. J.
Renormalization of the periodic Anderson model: an alternative analytical approach to heavy Fermion behavior
In this paper a recently developed projector-based renormalization method
(PRM) for many-particle Hamiltonians is applied to the periodic Anderson model
(PAM) with the aim to describe heavy Fermion behavior. In this method
high-energetic excitation operators instead of high energetic states are
eliminated. We arrive at an effective Hamiltonian for a quasi-free system which
consists of two non-interacting heavy-quasiparticle bands. The resulting
renormalization equations for the parameters of the Hamiltonian are valid for
large as well as small degeneracy of the angular momentum. An expansion
in is avoided. Within an additional approximation which adapts the
idea of a fixed renormalized \textit{f} level , we obtain
coupled equations for and the averaged \textit{f}
occupation . These equations resemble to a certain extent those of the
usual slave boson mean-field (SB) treatment. In particular, for large
the results for the PRM and the SB approach agree perfectly whereas
considerable differences are found for small .Comment: 26 pages, 5 figures included, discussion of the DOS added in v2,
accepted for publication in Phys. Rev.
Charge order induced by electron-lattice interaction in NaV2O5
We present Density Matrix Renormalization Group calculations of the
ground-state properties of quarter-filled ladders including static
electron-lattice coupling. Isolated ladders and two coupled ladders are
considered, with model parameters obtained from band-structure calculations for
-NaVO. The relevant Holstein coupling to the lattice
causes static out-of-plane lattice distortions, which appear concurrently with
a charge-ordered state and which exhibit the same zigzag pattern observed in
experiments. The inclusion of electron-lattice coupling drastically reduces the
critical nearest-neighbor Coulomb repulsion needed to obtain the
charge-ordered state. No spin gap is present in the ordered phase. The charge
ordering is driven by the Coulomb repulsion and the electron-lattice
interaction. With electron-lattice interaction, coupling two ladders has
virtually no effect on or on the characteristics of the charge-ordered
phase. At V=0.46\eV, a value consistent with previous estimates, the lattice
distortion, charge gap, charge order parameter, and the effective spin coupling
are in good agreement with experimental data for NaVO_5$.Comment: 7 pages, 9 figure
On the Geometry and Homology of Certain Simple Stratified Varieties
We study certain mild degenerations of algebraic varieties which appear in
the analysis of a large class of supersymmetric theories, including superstring
theory. We analyze Witten's sigma-model and find that the non-transversality of
the superpotential induces a singularization and stratification of the ground
state variety. This stratified variety (the union of the singular ground state
variety and its exo-curve strata) admit homology groups which, excepting the
middle dimension, satisfy the "Kahler package" of requirements, extend the
"flopped" pair of small resolutions to an "(exo)flopped" triple, and is
compatible with mirror symmetry and string theory. Finally, we revisit the
conifold transition as it applies to our formalism.Comment: LaTeX 2e, 18 pages, 4 figure
Optical conductivity of wet DNA
Motivated by recent experiments we have studied the optical conductivity of
DNA in its natural environment containing water molecules and counter ions. Our
density functional theory calculations (using SIESTA) for four base pair B-DNA
with order 250 surrounding water molecules suggest a thermally activated doping
of the DNA by water states which generically leads to an electronic
contribution to low-frequency absorption. The main contributions to the doping
result from water near DNA ends, breaks, or nicks and are thus potentially
associated with temporal or structural defects in the DNA.Comment: 4 pages, 4 figures included, final version, accepted for publication
in Phys. Rev. Let
Dominant particle-hole contributions to the phonon dynamics in the spinless one-dimensional Holstein model
In the spinless Holstein model at half-filling the coupling of electrons to
phonons is responsible for a phase transition from a metallic state at small
coupling to a Peierls distorted insulated state when the electron-phonon
coupling exceeds a critical value. For the adiabatic case of small phonon
frequencies, the transition is accompanied by a phonon softening at the
Brillouin zone boundary whereas a hardening of the phonon mode occurs in the
anti-adiabatic case. The phonon dynamics studied in this letter do not only
reveal the expected renormalization of the phonon modes but also show
remarkable additional contributions due to electronic particle-hole
excitations.Comment: 7 pages, 4 figures and 1 table included; v2: discussion of Luttinger
liquid parameters adde
PaaSword: A Holistic Data Privacy and Security by Design Framework for Cloud Services
Enterprises increasingly recognize the compelling economic and operational benefits from virtualizing and pooling IT resources in the cloud. Nevertheless, the significant and valuable transformation of organizations that adopt cloud computing is accompanied by a number of security threats that should be considered. In this position paper, we outline significant security challenges presented when migrating to a cloud environment and propose PaaSword - a novel holistic framework that aspires to alleviate these challenges. Specifically, this proposed framework involves a context-aware security model, the necessary policies enforcement mechanism along with a physical distribution, encryption and query middleware
On the Construction and the Structure of Off-Shell Supermultiplet Quotients
Recent efforts to classify representations of supersymmetry with no central
charge have focused on supermultiplets that are aptly depicted by Adinkras,
wherein every supersymmetry generator transforms each component field into
precisely one other component field or its derivative. Herein, we study
gauge-quotients of direct sums of Adinkras by a supersymmetric image of another
Adinkra and thus solve a puzzle from Ref.[2]: The so-defined supermultiplets do
not produce Adinkras but more general types of supermultiplets, each depicted
as a connected network of Adinkras. Iterating this gauge-quotient construction
then yields an indefinite sequence of ever larger supermultiplets, reminiscent
of Weyl's construction that is known to produce all finite-dimensional unitary
representations in Lie algebras.Comment: 20 pages, revised to clarify the problem addressed and solve
- …