2,068 research outputs found
String Theory
This is a rendering of a review talk on the state of String Theory, given at
the EPS-2003 Conference, intended for a wide audience of experimental and
theoretical physicists. It emphasizes general ideas rather than technical
aspects.Comment: 11 pages. Contribution to the EPS2003 conferenc
Identification of slow relaxing spin components by pulse EPR techniques in graphene-related materials
Electron Paramagnetic Resonance (EPR) is a powerful technique that is suitable to study graphene-related materials. The challenging ability requested to the spectroscopy is its capability to resolve the variety of structures, relatively similar, that are obtained in materials produced through
different methods, but that also coexist inside a single sample. In general, because of the intrinsic inhomogeneity of the samples, the EPR spectra are therefore a superposition of spectra coming from different structures. We show that by pulse EPR techniques (echo-detected EPR, ESEEM and Mims
ENDOR) we can identify and characterize species with slow spin relaxing properties. These species are generally called molecular states, and are likely small pieces of graphenic structures of limited dimensions, thus conveniently described by a molecular approach. We have studied commercial reduced graphene oxide and chemically exfoliated graphite, which are characterized by different EPR spectra. Hyperfine spectroscopies enabled us to characterize the molecular components of the different materials, especially in terms of the interaction of the unpaired electrons with protons (number of protons and hyperfine coupling constants). We also obtained useful precious information about extent of delocalization of the molecular states
On the Nature of the Hagedorn Transition in NCOS Systems
We extend the study of the nature of the Hagedorn transition in NCOS systems
in various dimensions. The canonical analysis results in a microscopic
ionization picture of a bound state system in which the Hagedorn transition is
postponed till irrelevancy. A microcanonical analysis leads to a limiting
Hagedorn behaviour dominated by highly excited, long open strings. The study of
the full phase diagram of the NCOS system using the AdS/CFT correspondence
suggests that the microscopic ionization picture is the correct one. We discuss
some refinements of the ionization mechanism for NCOS systems, including
the formation of a temperature-dependent barrier for the process. Some possible
consequences of this behaviour, including a potential puzzle for , are
discussed. Phase diagrams of a regularized form of NCOS systems are introduced
and do accomodate a phase of long open strings which disappears in the strict
NCOS limit.Comment: 37 pages, 3 Postscript figure
Long time scales and eternal black holes
We discuss the various scales determining the temporal behaviour of
correlation functions in the presence of eternal black holes. We point out the
origins of the failure of the semiclassical gravity approximation to respect a
unitarity-based bound suggested by Maldacena. We find that the presence of a
subleading (in the large-N approximation involved) master field does restore
the compliance with one bound but additional configurations are needed to
explain the more detailed expected time dependence of the Poincare recurrences
and their magnitude.Comment: 10 pages, 6 figures. Presented at Johns Hopkins 2003 and Ahrenshoop
2003 workshop
Fast Scramblers Of Small Size
We investigate various geometrical aspects of the notion of `optical depth'
in the thermal atmosphere of black hole horizons. Optical depth has been
proposed as a measure of fast-crambling times in such black hole systems, and
the associated optical metric suggests that classical chaos plays a leading
role in the actual scrambling mechanism. We study the behavior of the optical
depth with the size of the system and find that AdS/CFT phase transitions with
topology change occur naturally as the scrambler becomes smaller than its
thermal length. In the context of detailed AdS/CFT models based on D-branes,
T-duality implies that small scramblers are described in terms of matrix
quantum mechanics.Comment: 14 pages, 3 figures. Added reference
Remarks on Black Hole Instabilities and Closed String Tachyons
Physical arguments stemming from the theory of black-hole thermodynamics are
used to put constraints on the dynamics of closed-string tachyon condensation
in Scherk--Schwarz compactifications. A geometrical interpretation of the
tachyon condensation involves an effective capping of a noncontractible cycle,
thus removing the very topology that supports the tachyons. A semiclassical
regime is identified in which the matching between the tachyon condensation and
the black-hole instability flow is possible. We formulate a generalized
correspondence principle and illustrate it in several different circumstances:
an Euclidean interpretation of the transition from strings to black holes
across the Hagedorn temperature and instabilities in the brane-antibrane
system.Comment: harvmac, 20 pp, 4 eps figures. Contribution to Jacob Bekenstein's
Festschrif
Noncommutative Field Theory and the Dynamics of Quantum Hall Fluids
We study the spectrum of density fluctuations of Fractional Hall Fluids in
the context of the noncommutative hidrodynamical model of Susskind. We show
that, within the weak-field expansion, the leading correction to the
noncommutative Chern--Simons Lagrangian (a Maxwell term in the effective
action,) destroys the incompressibility of the Hall fluid due to strong UV/IR
effects at one loop. We speculate on possible relations of this instability
with the transition to the Wigner crystal, and conclude that calculations
within the weak-field expansion must be carried out with an explicit
ultraviolet cutoff at the noncommutativity scale. We point out that the
noncommutative dipoles exactly match the spatial structure of the
Halperin--Kallin quasiexcitons. Therefore, we propose that the noncommutative
formalism must describe accurately the spectrum at very large momenta, provided
no weak-field approximations are made. We further conjecture that the
noncommutative open Wilson lines are `vertex operators' for the quasiexcitons.Comment: 20 pages, harvma
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