1,164 research outputs found
Ab initio calculation of the shift photocurrent by Wannier interpolation
We describe and implement a first-principles algorithm based on
maximally-localized Wannier functions for calculating the shift-current
response of piezoelectric crystals in the independent-particle approximation.
The proposed algorithm presents several advantages over existing ones,
including full gauge invariance, low computational cost, and a correct
treatment of the optical matrix elements with nonlocal pseudopotentials.
Band-truncation errors are avoided by a careful formulation of
perturbation theory within the subspace of wannierized bands. The needed
ingredients are the matrix elements of the Hamiltonian and of the position
operator in the Wannier basis, which are readily available at the end of the
wannierization step. If the off-diagonal matrix elements of the position
operator are discarded, our expressions reduce to the ones that have been used
in recent tight-binding calculations of the shift current. We find that this
`diagonal' approximation can introduce sizeable errors, highlighting the
importance of carefully embedding the tight-binding model in real space for an
accurate description of the charge transfer that gives rise to the shift
current.Comment: 13 pages, 7 figure
Tuning paramagnetic spin-excitations of single adatoms
Around 50 years ago, Doniach [Proc. Phys. Soc. 91, 86 (1967)] predicted the
existence of paramagnons in nearly ferromagnetic materials, recently measured
in bulk Pd [Phys. Rev. Lett. 105, 027207 (2010)]. Here we predict the analogous
effect for single adatoms, namely paramagnetic spin-excitations (PSE). Based on
time-dependent density functional theory, we demonstrate that these overdamped
excitations acquire a well-defined peak structure in the meV energy region when
the adatom's Stoner criterion for magnetism is close to the critical point. In
addition, our calculations reveal a subtle tunability and enhancement of PSE by
external magnetic fields, exceeding by far the response of bulk paramagnons and
even featuring the atomic version of a quantum phase transition. We further
demonstrate how PSE can be detected as moving steps in the
signal of state-of-the-art inelastic scanning tunneling spectroscopy, opening a
potential route for experimentally accessing fundamental electronic properties
of non-magnetic adatoms, such as the Stoner parameter.Comment: 6 pages, 3 figure
Zero-point quantum swing of magnetic couples
Quantum fluctuations are ubiquitous in physics. Ranging from conventional
examples like the harmonic oscillator to intricate theories on the origin of
the universe, they alter virtually all aspects of matter -- including
superconductivity, phase transitions and nanoscale processes. As a rule of
thumb, the smaller the object, the larger their impact. This poses a serious
challenge to modern nanotechnology, which aims total control via atom-by-atom
engineered devices. In magnetic nanostructures, high stability of the magnetic
signal is crucial when targeting realistic applications in information
technology, e.g. miniaturized bits. Here, we demonstrate that zero-point
spin-fluctuations are paramount in determining the fundamental magnetic
exchange interactions that dictate the nature and stability of the magnetic
state. Hinging on the fluctuation-dissipation theorem, we establish that
quantum fluctuations correctly account for the large overestimation of the
interactions as obtained from conventional static first-principles frameworks,
filling in a crucial gap between theory and experiment [1,2]. Our analysis
further reveals that zero-point spin-fluctuations tend to promote the
non-collinearity and stability of chiral magnetic textures such as skyrmions --
a counter-intuitive quantum effect that inspires practical guidelines for
designing disruptive nanodevices
A Geometry of the Generations
We propose a geometric theory of flavor based on the discrete group
, in the context of the minimal supersymmetric standard model. The
group treats three objects symmetrically, while making fundamental distinctions
between the generations. The top quark is the only heavy quark in the symmetry
limit, and the first and second generation squarks are degenerate. The
hierarchical nature of Yukawa matrices is a consequence of a sequential
breaking of .Comment: 10 pages, 1 EPS figure as uuencoded tar-compressed file, uses
psfig.st
Radiative Electroweak Symmetry Breaking and the Infrared Fixed Point of the Top Quark Mass
The infrared quasi fixed point solution for the top quark mass in the Minimal
Supersymmetric Standard Model explains in a natural way large values of the top
quark mass and appears as a prediction in many interesting theoretical schemes.
Moreover, as has been recently pointed out, for moderate values of ,
in order to achieve gauge and bottom-tau Yukawa coupling unification, the top
quark mass must be within of its fixed point value. In this work we
show that the convergence of the top quark mass to its fixed point value has
relevant consequences for the (assumed) universal soft supersymmetry breaking
parameters at the grand unification scale. In particular, we show that the low
energy parameters do not depend on and but on the combination
. Hence, there is a reduction in the number of
independent parameters. Most interesting, the radiative
breaking condition implies strong correlations between the supersymmetric mass
parameter and the supersymmetry breaking parameters and
or . These correlations, which become stronger for , may have some fundamental origin, which would imply the need of a
reformulation of the naive fine tuning criteria.Comment: 17 pages, 5 figures, CERN-TH.7060/9
Self-similar static solutions admitting a two-space of constant curvature
A recent result by Haggag and Hajj-Boutros is reviewed within the framework
of self-similar space-times, extending, in some sense, their results and
presenting a family of metrics consisting of all the static spherically
symmetric perfect fluid solutions admitting a homothety.Comment: 6 page
Inverting the Supersymmetric Standard Model Spectrum: from Physical to Lagrangian Ino Parameters
We examine the possibility of recovering the supersymmetric (and soft
supersymmetry breaking) Lagrangian parameters as direct {\em analytical}
expressions of appropriate physical masses, for the unconstrained (but CP and
R-parity conserving) minimal supersymmetric standard model. We concentrate
mainly on the algebraically non-trivial "inversion" for the ino parameters, and
obtain, for given values of , simple analytical expressions for the
, and parameters in terms of three arbitrary input physical
masses, namely either two chargino and one neutralino masses, or alternatively
one chargino and two neutralino masses. We illustrate and discuss in detail the
possible occurrence of ambiguities in this reconstruction. The dependence of
the resulting ino Lagrangian parameters upon physical masses is illustrated,
and some simple generic behaviour uncovered in this way. We finally briefly
sketch generalizing such an inversion to the full set of MSSM Lagrangian
parameters.Comment: Latex, 28 pages, 6 figures, 1 table, some typos corrected, one
paragraph extended in section 4.2. Version to appear in Phys. Rev.
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