2,822 research outputs found
Adventures in Holographic Dimer Models
We abstract the essential features of holographic dimer models, and develop
several new applications of these models. First, semi-holographically coupling
free band fermions to holographic dimers, we uncover novel phase transitions
between conventional Fermi liquids and non-Fermi liquids, accompanied by a
change in the structure of the Fermi surface. Second, we make dimer vibrations
propagate through the whole crystal by way of double trace deformations,
obtaining nontrivial band structure. In a simple toy model, the topology of the
band structure experiences an interesting reorganization as we vary the
strength of the double trace deformations. Finally, we develop tools that would
allow one to build, in a bottom-up fashion, a holographic avatar of the Hubbard
model.Comment: 22 pages, 8 figures; v2: brief description of case of pure D5 lattice
added in sec.3; v3: minor typo fixed; v4: minor change
Notes on Properties of Holographic Matter
Probe branes with finite worldvolume electric flux in the background created
by a stack of Dp branes describe holographically strongly interacting
fundamental matter at finite density. We identify two quantities whose leading
low temperature behavior is independent of the dimensionality of the probe
branes: specific heat and DC conductivity. This behavior can be inferred from
the dynamics of the fundamental strings which provide a good description of the
probe branes in the regime of low temperatures and finite densities. We also
comment on the speed of sound on the branes and the temperature dependence of
DC conductivity at vanishing charge density.Comment: 18 pages, 2 figures; v2: corrected error in Section 6, conclusions
unchanged; v3: improved figures and added clarifying comment
The holographic quantum effective potential at finite temperature and density
We develop a formalism that allows the computation of the quantum effective
potential of a scalar order parameter in a class of holographic theories at
finite temperature and charge density. The effective potential is a valuable
tool for studying the ground state of the theory, symmetry breaking patterns
and phase transitions. We derive general formulae for the effective potential
and apply them to determine the phase transition temperature and density in the
scaling region.Comment: 27 page
Pan-Africanism: a contorted delirium or a pseudonationalist paradigm? Revivalist critique
This essaic-article goes against established conventions that there is anything ethno-cultural (and hence national) about the so-called African tribes. Drawing largely from the culture history of precolonial/prepolitical Africans—that is, the Bantu/Cushitic-Ethiopians (Azanians)—the author has demonstrated vividly that far from being distinct ethno-culture national communities, the so-called tribes of African states are better considered subculture groups, whose regional culture practices erstwhile paid tribute to their nation’s main culture center in Karnak. For example, using the culture symbols and practices of some local groups and linking them to the predynastic and dynastic Pharaonic periods, I argued that there is compelling evidence against qualifying Africa’s tribes as distinct ethno-culture national entities. In genuine culture context, I stressed that the Ritual of Resurrection and its twin culture process of the mummification of deceased indigenous Pharaohs tend to suggest that the object of the Bantu/Cushitic-Ethiopians national culture was life (in its eternal manifestation) and then resurrection later, and that there are recurring (culturally sanctioned) ethical examples among the culture custodians of these subculture groups that generally pay tribute to the overarching culture norm. Furthermore, the fact that the Ritual of Resurrection began in the Delta region and ended at the Sources of the Nile, where the spirit of the deceased indigenous Pharaohs was introduced into the spiritual world of their ancestors, contradicts conventional perceptions that ancient Egypt was a distinct national community isolated from precolonial/prepolitical Africa/Azania
Strange metals and the AdS/CFT correspondence
I begin with a review of quantum impurity models in condensed matter physics,
in which a localized spin degree of freedom is coupled to an interacting
conformal field theory in d = 2 spatial dimensions. Their properties are
similar to those of supersymmetric generalizations which can be solved by the
AdS/CFT correspondence; the low energy limit of the latter models is described
by a AdS2 geometry. Then I turn to Kondo lattice models, which can be described
by a mean- field theory obtained by a mapping to a quantum impurity coupled to
a self-consistent environment. Such a theory yields a 'fractionalized Fermi
liquid' phase of conduction electrons coupled to a critical spin liquid state,
and is an attractive mean-field theory of strange metals. The recent
holographic description of strange metals with a AdS2 x R2 geometry is argued
to be related to such mean-field solutions of Kondo lattice models.Comment: 19 pages, 4 figures; Plenary talk at Statphys24, Cairns, Australia,
July 2010; (v2) added refs; (v3) more ref
Theory of band gap bowing of disordered substitutional II-VI and III-V semiconductor alloys
For a wide class of technologically relevant compound III-V and II-VI
semiconductor materials AC and BC mixed crystals (alloys) of the type
A(x)B(1-x)C can be realized. As the electronic properties like the bulk band
gap vary continuously with x, any band gap in between that of the pure AC and
BC systems can be obtained by choosing the appropriate concentration x, granted
that the respective ratio is miscible and thermodynamically stable. In most
cases the band gap does not vary linearly with x, but a pronounced bowing
behavior as a function of the concentration is observed. In this paper we show
that the electronic properties of such A(x)B(1-x)C semiconductors and, in
particular, the band gap bowing can well be described and understood starting
from empirical tight binding models for the pure AC and BC systems. The
electronic properties of the A(x)B(1-x)C system can be described by choosing
the tight-binding parameters of the AC or BC system with probabilities x and
1-x, respectively. We demonstrate this by exact diagonalization of finite but
large supercells and by means of calculations within the established coherent
potential approximation (CPA). We apply this treatment to the II-VI system
Cd(x)Zn(1-x)Se, to the III-V system In(x)Ga(1-x)As and to the III-nitride
system Ga(x)Al(1-x)N.Comment: 14 pages, 10 figure
Three-Dimensional FDTD Simulation of Biomaterial Exposure to Electromagnetic Nanopulses
Ultra-wideband (UWB) electromagnetic pulses of nanosecond duration, or
nanopulses, have been recently approved by the Federal Communications
Commission for a number of various applications. They are also being explored
for applications in biotechnology and medicine. The simulation of the
propagation of a nanopulse through biological matter, previously performed
using a two-dimensional finite difference-time domain method (FDTD), has been
extended here into a full three-dimensional computation. To account for the UWB
frequency range, a geometrical resolution of the exposed sample was ,
and the dielectric properties of biological matter were accurately described in
terms of the Debye model. The results obtained from three-dimensional
computation support the previously obtained results: the electromagnetic field
inside a biological tissue depends on the incident pulse rise time and width,
with increased importance of the rise time as the conductivity increases; no
thermal effects are possible for the low pulse repetition rates, supported by
recent experiments. New results show that the dielectric sample exposed to
nanopulses behaves as a dielectric resonator. For a sample in a cuvette, we
obtained the dominant resonant frequency and the -factor of the resonator.Comment: 15 pages, 8 figure
A conical deficit in the AdS4/CFT3 correspondence
Inspired by the AdS/CFT correspondence we propose a new duality that allow
the study of strongly coupled field theories living in a 2+1 conical
space-time. Solving the 4-d Einstein equations in the presence of an infinite
static string and negative cosmological constant we obtain a conical AdS4
space-time whose boundary is identified with the 2+1 cone found by Deser,
Jackiw and 't Hooft. Using the AdS4/CFT3 correspondence we calculate retarded
Green's functions of scalar operators living in the cone.Comment: v3, 14 pages. We reinterpret our results for the Green's functions in
the con
Large-density field theory, viscosity, and "" singularities from string duals
We analyze systems where an effective large-N expansion arises naturally in
gauge theories without a large number of colors: a sufficiently large charge
density alone can produce a perturbative string ('tHooft) expansion. One
example is simply the well-known NS5/F1 system dual to , here viewed as a 5+1 dimensional theory at finite density. This model is
completely stable, and we find that the existing string-theoretic solution of
this model yields two interesting results. First, it indicates that the shear
viscosity is not corrected by effects in this system. For flow
perpendicular to the F1 strings the viscosity to entropy ratio take the usual
value , but for flow parallel to the F1's it vanishes as at low
temperature. Secondly, it encodes singularities in correlation functions coming
from low-frequency modes at a finite value of the momentum along the
directions. This may provide a strong coupling analogue of finite density
condensed matter systems for which fermionic constituents of larger operators
contribute so-called "" singularities. In the NS5/F1 example, stretched
strings on the gravity side play the role of these composite operators. We
explore the analogue for our system of the Luttinger relation between charge
density and the volume bounded by these singular surfaces. This model provides
a clean example where the string-theoretic UV completion of the gravity dual to
a finite density field theory plays a significant and calculable role.Comment: 28 pages. v2: added reference
Deep Memory Networks for Attitude Identification
We consider the task of identifying attitudes towards a given set of entities
from text. Conventionally, this task is decomposed into two separate subtasks:
target detection that identifies whether each entity is mentioned in the text,
either explicitly or implicitly, and polarity classification that classifies
the exact sentiment towards an identified entity (the target) into positive,
negative, or neutral.
Instead, we show that attitude identification can be solved with an
end-to-end machine learning architecture, in which the two subtasks are
interleaved by a deep memory network. In this way, signals produced in target
detection provide clues for polarity classification, and reversely, the
predicted polarity provides feedback to the identification of targets.
Moreover, the treatments for the set of targets also influence each other --
the learned representations may share the same semantics for some targets but
vary for others. The proposed deep memory network, the AttNet, outperforms
methods that do not consider the interactions between the subtasks or those
among the targets, including conventional machine learning methods and the
state-of-the-art deep learning models.Comment: Accepted to WSDM'1
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