80 research outputs found
Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction
Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism
of superconductivity for underdoped cuprates. The gluing force of the
superconducting mechanism is an attraction between spin vortices on two
different N\'eel sublattices, centered around the empty sites described in
terms of fermionic holons. The spin fluctuations are described by bosonic
spinons with a gap generated by the spin vortices. Due to the no-double
occupation constraint, there is a gauge attraction between holon and spinon
binding them into a physical hole. Through gauge interaction the spin vortex
attraction induces the formation of spin-singlet (RVB) spin pairs with a
owering of the spinon gap. Lowering the temperature the approach exhibits two
crossover temperatures: at the higher crossover a finite density of incoherent
holon pairs are formed leading to a reduction of the hole spectral weight, at
the lower crossover also a finite density of incoherent spinon RVB pairs are
formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic
vortices appear in the plasma phase. Finally, at a even lower temperature the
hole pairs become coherent, the magnetic vortices become dilute and
superconductivity appears. The superconducting mechanism is not of BCS-type
since it involves a gain in kinetic energy (for spinons) coming from the spin
interactions.Comment: 4 pages, 3 figures, accepted by the proceedings of SNS2010 conferenc
Spin-charge gauge symmetry: A way to tackle HTS cuprates?
We propose an explanation of several experimental features of transport
phenomena in the normal state of high Tc cuprates in terms of a spin-charge
gauge theory of the 2D t-J model. The calculated doping-temperature dependence
for a number of physical quantities is found in qualitative agreement with
data. In particular, we recover: in the ``pseudogap phase'' the metal-insulator
crossover of the in-plane resistivity and of the NMR ``relaxation time'' and
the insulating behavior of the out-of-plane resistivity; in the ``strange metal
phase'' (at higher temperature or doping) the linear in T behavior of the above
quantities; the appearance of maxima in the in-plane far-infrared conductivity
in strongly underdoped and overdoped samples.Comment: 8 pages, 14 figures; to appear without figures in Journal of Physics
and Chemistry of Solid
Variational principle and energy-momentum tensor for relativistic Electrodynamics of point charges
We give a new representation as tempered distribution for the energy-momentum
tensor of a system of charged point-particles, which is free from divergent
self-interactions, manifestly Lorentz-invariant and symmetric, and conserved.
We present a covariant action for this system, that gives rise to the known
Lorentz-Dirac equations for the particles and entails, via Noether theorem,
this energy-momentum tensor. Our action is obtained from the standard action
for classical Electrodynamics, by means of a new Lorentz-invariant
regularization procedure, followed by a renormalization. The method introduced
here extends naturally to charged p-branes and arbitrary dimensions.Comment: 36 pages, no figures, refs. and comments adde
Dirac Branes, Characteristic Currents and Anomaly Cancellations in 5-Branes
The aim of this note is to discuss, in a somewhat informal language, the
cancellation of anomalies (in topologically trivial space-time) for 5-branes
using as "building blocks": i) a generalization to p-branes of the Dirac
strings of monopoles (Dirac branes) and a refinement of this idea involving a
geometric regularization of Dirac branes, leading to the formalism of
"characteristic currents" ii) the PST formalism . As an example of the
potentiality of the developed framework we discuss in some detail the anomaly
cancellation in the D=10 effective theory of heterotic string and 5-brane
coupled to supergravity, where the anomaly inflow is automatically generated.
Some remarks are also made on a similar approach to the problem of anomaly
cancellation in the effective theory of M5-brane coupled to D=11 supergravity,
developed in collaboration with M.Tonin, where however still as open problem
remains a Dirac anomaly.Comment: LaTeX file, espcrc2.sty, 6 pages. Based on the talk given by P.M. at
the D.V. Volkov Memorial Conference "Supersymmetry and Quantum Field Theory"
(25-29 July, 2000, Kharkov, Ukraine). To be published in Nuclear Physics B
Conference Supplement
Gauge-invariant charged, monopole and dyon fields in gauge theories
We propose explicit recipes to construct the euclidean Green functions of
gauge-invariant charged, monopole and dyon fields in four-dimensional gauge
theories whose phase diagram contains phases with deconfined electric and/or
magnetic charges. In theories with only either abelian electric or magnetic
charges, our construction is an euclidean version of Dirac's original proposal,
the magnetic dual of his proposal, respectively. Rigorous mathematical control
is achieved for a class of abelian lattice theories. In theories where electric
and magnetic charges coexist, our construction of Green functions of
electrically or magnetically charged fields involves taking an average over
Mandelstam strings or the dual magnetic flux tubes, in accordance with Dirac's
flux quantization condition. We apply our construction to 't Hooft-Polyakov
monopoles and Julia-Zee dyons. Connections between our construction and the
semiclassical approach are discussed
Duality-invariant Quantum Field Theories of Charges and Monopoles
We present a manifestly Lorentz- and SO(2)-Duality-invariant local Quantum
Field Theory of electric charges, Dirac magnetic monopoles and dyons. The
manifest invariances are achieved by means of the PST-mechanism. The dynamics
for classical point particles is described by an action functional living on a
circle, if the Dirac-Schwinger quantization condition for electric and magnetic
charges holds. The inconsistent classical field theory depends on an arbitrary,
but fixed, external vector field, a generalization of the Dirac-string.
Nevertheless, the Quantum Field Theory, obtained from this classical action via
a functional integral approach, turns out to be independent of the particular
vector field chosen, and thus consistent, if the Dirac-Schwinger quantization
condition holds. We provide explicit expressions for the generating functionals
of observables, proving that they are Dirac-string independent. Since
Lorentz-invariance is manifest at each step, the quantum theory admits also a
manifestly diffeomorphism invariant coupling to external gravity. Relations
with previous formulations, and with SO(2)--non invariant theories are
clarified.Comment: 49 pages, LaTeX, no figure
Chern-kernels and anomaly cancellation in M-theory
This paper deals with magnetic equations of the type dH=J where the current J
is a delta-function on a brane worldvolume and H a p-form field strength. In
many situations in M-theory this equation needs to be solved for H in terms of
a potential. A standard universality class of solutions, involving
Dirac-branes, gives rise to strong intermediate singularities in H which in
many physically relevant cases lead to inconsistencies. In this paper we
present an alternative universality class of solutions for magnetic equations
in terms of Chern-kernels, and provide relevant applications, among which the
anomaly-free effective action for open M2-branes ending on M5-branes. The
unobservability of the Dirac-brane requires a Dirac quantization condition; we
show that the requirement of ``unobservability'' of the Chern-kernel leads in
M-theory to classical gravitational anomalies which cancel precisely their
quantum counterparts.Comment: LaTex, 39 pages, references and comments adde
Nonperturbative 2D Gravity, Punctured Spheres and -Vacua in String Theories
We consider a model of 2D gravity with the coefficient of the
Einstein-Hilbert action having an imaginary part . This is equivalent to
introduce a -vacuum structure in the genus expansion whose effect is to
convert the expansion into a series of alternating signs, presumably Borel
summable. We show that the specific heat of the model has a physical behaviour.
It can be represented nonperturbatively as a series in terms of integrals over
moduli spaces of punctured spheres and the sum of the series can be rewritten
as a unique integral over a suitable moduli space of infinitely punctured
spheres. This is an explicit realization \`a la Friedan-Shenker of 2D quantum
gravity. We conjecture that the expansion in terms of punctures and the genus
expansion can be derived using the Duistermaat-Heckman theorem. We briefly
analyze expansions in terms of punctured spheres also for multicritical models.Comment: 12 pages + 2 figures available upon request. LaTeX Fil
Proposed Beam Test of a Transverse Gradient Undulator at the SINBAD Facility at DESY
While Laser Plasma Accelerators produce beams with the high output energy required for FELs, up to now the relatively high energy spread has prohibited FEL lasing. Therefore it was proposed to replace the normal FEL undulators by Transverse Gradient Undulators (TGUs). For a first, small scale test of the TGU concept, a 40 period prototype high gradient superconductive TGU was built at KIT and will be tested with beam at the ARES-linac in the new accelerator test facility SINBAD (Short Innovative Bunches and Accelerators at Desy) at DESY. The proposed tests are summarized in this paper
Mode-Locking in Driven Disordered Systems as a Boundary-Value Problem
We study mode-locking in disordered media as a boundary-value problem.
Focusing on the simplest class of mode-locking models which consists of a
single driven overdamped degree-of-freedom, we develop an analytical method to
obtain the shape of the Arnol'd tongues in the regime of low ac-driving
amplitude or high ac-driving frequency. The method is exact for a scalloped
pinning potential and easily adapted to other pinning potentials. It is
complementary to the analysis based on the well-known Shapiro's argument that
holds in the perturbative regime of large driving amplitudes or low driving
frequency, where the effect of pinning is weak.Comment: 6 pages, 7 figures, RevTeX, Submitte
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