1,478 research outputs found
Correlations between axial stiffness and microstructure of a species of bamboo
Bamboo is a ubiquitous monocotyledonous flowering plant and is a member of the true grass family Poaceae . In many parts of the world, it is widely used as a structural material especially in scaffolding and buildings. In spite of its wide use, there is no accepted methodology for standardizing a species of bamboo for a particular structural purpose. The task of developing structure–property correlations is complicated by the fact that bamboo is a hierarchical material whose structure at the nanoscopic level is not very well explored. However, we show that as far as stiffness is concerned, it is possible to obtain reliable estimates of important structural properties like the axial modulus from the knowledge of certain key elements of the microstructure. Stiffness of bamboo depends most sensitively on the size and arrangement of the fibre sheaths surrounding the vascular bundles and the arrangement of crystalline cellulose microfibrils in their secondary cell walls. For the species of bamboo studied in this work, we have quantitatively determined the radial gradation that the arrangement of fibres renders to the structure. The arrangement of the fibres gives bamboo a radially graded property variation across its cross section
Low temperature properties of holographic condensates
In the current work we study various models of holographic superconductors at
low temperature. Generically the zero temperature limit of those models are
solitonic solution with a zero sized horizon. Here we generalized simple
version of those zero temperature solutions to small but non-zero temperature
T. We confine ourselves to cases where near horizon geometry is AdS^4. At a
non-zero temperature a small horizon would form deep inside this AdS^4 which
does not disturb the UV physics. The resulting geometry may be matched with the
zero temperature solution at an intermediate length scale. We understand this
matching from separation of scales by setting up a perturbative expansion in
gauge potential. We have a better analytic control in abelian case and
quantities may be expressed in terms of hypergeometric function. From this we
calculate low temperature behavior of various quatities like entropy, charge
density and specific heat etc. We also calculate various energy gaps associated
with p-wave holographic superconductor to understand the underlying pairing
mechanism. The result deviates significantly from the corresponding weak
coupling BCS counterpart.Comment: 17 Page
Quantum Critical Superfluid Flows and Anisotropic Domain Walls
We construct charged anisotropic AdS domain walls as solutions of a
consistent truncation of type IIB string theory. These are a one-parameter
family of solutions that flow to an AdS fixed point in the IR, exhibiting
emergent conformal invariance and quantum criticality. They represent the
zero-temperature limit of the holographic superfluids at finite superfluid
velocity constructed in arXiv:1010.5777. We show that these domain walls exist
only for velocities less than a critical value, agreeing in detail with a
conjecture made there. We also comment about the IR limits of flows with
velocities higher than this critical value, and point out an intriguing
similarity between the phase diagrams of holographic superfluid flows and those
of ordinary superconductors with imbalanced chemical potential.Comment: 11 pages, 3 figures. V2: Very minor corrections. JHEP versio
Small Hairy Black Holes in Global AdS Spacetime
We study small charged black holes in global AdS spacetime in the presence of
a charged massless minimally coupled scalar field. In a certain parameter range
these black holes suffer from well known superradiant instabilities. We
demonstrate that the end point of the resultant tachyon condensation process is
a hairy black hole which we construct analytically in a perturbative expansion
in the black hole radius. At leading order our solution is a small undeformed
RNAdS black hole immersed into a charged scalar condensate that fills the AdS
`box'. These hairy black hole solutions appear in a two parameter family
labelled by their mass and charge. Their mass is bounded from below by a
function of their charge; at the lower bound a hairy black hole reduces to a
regular horizon free soliton which can also be thought of as a nonlinear Bose
condensate. We compute the microcanonical phase diagram of our system at small
mass, and demonstrate that it exhibits a second order `phase transition'
between the RNAdS black hole and the hairy black hole phases.Comment: 68+1 pages, 18 figures, JHEP format. v2 : small typos corrected and a
reference adde
Type IIB Holographic Superfluid Flows
We construct fully backreacted holographic superfluid flow solutions in a
five-dimensional theory that arises as a consistent truncation of low energy
type IIB string theory. We construct a black hole with scalar and vector hair
in this theory, and study the phase diagram. As expected, the superfluid phase
ceases to exist for high enough superfluid velocity, but we show that the phase
transition between normal and superfluid phases is always second order. We also
analyze the zero temperature limit of these solutions. Interestingly, we find
evidence that the emergent IR conformal symmetry of the zero-temperature domain
wall is broken at high enough velocity.Comment: v3: Published version. Figures 5 and 6 corrected. 24 pages, 7 figure
Supersymmetric Electromagnetic Waves on Giants and Dual-Giants
We set up the BPS equations for a D3-brane moving in AdS_5 \times S^5 which
preserves two supercharges and with all bosonic fields turned on in the
world-volume theory. By solving these, we find generalizations of Mikhailov
giants and wobbling dual-giants that include electromagnetic waves propagating
on their world-volume. For these giants (dual-giants) we show that the BPS
field strength is the real part of the pull-back of a holomorphic 2-form in the
ambient space C^3 (C^{1,2}) onto the world-volume.Comment: 18 page
N=8 Superspace Constraints for Three-dimensional Gauge Theories
We present a systematic analysis of the N=8 superspace constraints in three
space-time dimensions. The general coupling between vector and scalar
supermultiplets is encoded in an SO(8) tensor W_{AB} which is a function of the
matter fields and subject to a set of algebraic and super-differential
relations. We show how the conformal BLG model as well as three-dimensional
super Yang-Mills theory provide solutions to these constraints and can both be
formulated in this universal framework.Comment: 34 + 10 pages; added references, minor correction
p-wave Holographic Superconductors and five-dimensional gauged Supergravity
We explore five-dimensional and
SO(6) gauged supergravities as frameworks for condensed matter applications.
These theories contain charged (dilatonic) black holes and 2-forms which have
non-trivial quantum numbers with respect to U(1) subgroups of SO(6). A question
of interest is whether they also contain black holes with two-form hair with
the required asymptotic to give rise to holographic superconductivity. We first
consider the case, which contains a complex two-form potential
which has U(1) charge . We find that a slight
generalization, where the two-form potential has an arbitrary charge , leads
to a five-dimensional model that exhibits second-order superconducting
transitions of p-wave type where the role of order parameter is played by
, provided . We identify the operator that condenses
in the dual CFT, which is closely related to Super Yang-Mills
theory with chemical potentials. Similar phase transitions between R-charged
black holes and black holes with 2-form hair are found in a generalized version
of the gauged supergravity Lagrangian where the two-forms have
charge .Comment: 35 pages, 14 figure
Pointlike probes of superstring-theoretic superfluids
In analogy with an experimental setup used in liquid helium, we use a
pointlike probe to study superfluids which have a gravity dual. In the gravity
description, the probe is represented by a hanging string. We demonstrate that
there is a critical velocity below which the probe particle feels neither drag
nor stochastic forces. Above this critical velocity, there is power-law scaling
for the drag force, and the stochastic forces are characterized by a finite,
velocity-dependent temperature. This temperature participates in two simple and
general relations between the drag force and stochastic forces. The formula we
derive for the critical velocity indicates that the low-energy excitations are
massless, and they demonstrate the power of stringy methods in describing
strongly coupled superfluids.Comment: 17 pages, 2 figures, added a figure, a reference, and moved material
to an appendi
Holographic Superconductors from Einstein-Maxwell-Dilaton Gravity
We construct holographic superconductors from Einstein-Maxwell-dilaton
gravity in 3+1 dimensions with two adjustable couplings and the charge
carried by the scalar field. For the values of and we
consider, there is always a critical temperature at which a second order phase
transition occurs between a hairy black hole and the AdS RN black hole in the
canonical ensemble, which can be identified with the superconducting phase
transition of the dual field theory. We calculate the electric conductivity of
the dual superconductor and find that for the values of and where
is small the dual superconductor has similar properties to the
minimal model, while for the values of and where is
large enough, the electric conductivity of the dual superconductor exhibits
novel properties at low frequencies where it shows a "Drude Peak" in the real
part of the conductivity.Comment: 25 pages, 13 figures; v2, typos corrected; v3, refs added, to appear
in JHE
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