9,665 research outputs found
Superconductivity and incommensurate spin fluctuations in a generalized t-J model for the cuprates
We consider the slave-fermion Schwinger-boson decomposition of an effective
model obtained through a systematic low-energy reduction of the three-band
Hubbard Hamiltonian. The model includes a three-site term t'' similar to that
obtained in the large-U limit of the Hubbard model but of opposite sign for
realistic or large O-O hopping. For parameters close to the most realistic ones
for the cuprates, the mean-field solution exhibits d+s superconductivity
(predominantly d_{x^2-y^2}) with a dependence on doping x very similar to the
experimentally observed. We also obtained incommensurate peaks at wave vectors
near in the spin structure factor, which also agree with
experiment.Comment: 9 pages, latex, 2 figures, to appear in Europhys. Let
Tipping time of a quantum rod
The behaviour of a quantum rod, pivoted at its lower end on an impenetrable
floor and restricted to moving in the vertical plane under the gravitational
potential is studied analytically under the approximation that the rod is
initially localised to a small-enough neighbourhood around the point of
classical unstable equilibrium. It is shown that the rod evolves out of this
neighbourhood. The time required for this to happen, i.e., the tipping time is
calculated using the semi-classical path integral. It is shown that equilibrium
is recovered in the classical limit, and that our calculations are consistent
with the uncertainty principle.Comment: 10 pages, 1 figure, To appear in Euro. J. Phy
Hamiltonian Reduction and the Construction of q-Deformed Extensions of the Virasoro Algebra
In this paper we employ the construction of Dirac bracket for the remaining
current of deformed Kac-Moody algebra when constraints similar to
those connecting the -WZW model and the Liouville theory are imposed and
show that it satisfy the q-Virasoro algebra proposed by Frenkel and
Reshetikhin. The crucial assumption considered in our calculation is the
existence of a classical Poisson bracket algebra induced, in a consistent
manner by the correspondence principle, mapping the quantum generators into
commuting objects of classical nature preserving their algebra.Comment: 6 pages, late
Non Abelian Sugawara Construction and the q-deformed N=2 Superconformal Algebra
The construction of a q-deformed N=2 superconformal algebra is proposed in
terms of level 1 currents of quantum affine
Lie algebra and a single real Fermi field. In particular, it suggests the
expression for the q-deformed Energy-Momentum tensor in the Sugawara form. Its
constituents generate two isomorphic quadratic algebraic structures. The
generalization to is also proposed.Comment: AMSLATEX, 21page
Statistics of finite-time Lyapunov exponents in the Ulam map
The statistical properties of finite-time Lyapunov exponents at the Ulam
point of the logistic map are investigated. The exact analytical expression for
the autocorrelation function of one-step Lyapunov exponents is obtained,
allowing the calculation of the variance of exponents computed over time
intervals of length . The variance anomalously decays as . The
probability density of finite-time exponents noticeably deviates from the
Gaussian shape, decaying with exponential tails and presenting spikes
that narrow and accumulate close to the mean value with increasing . The
asymptotic expression for this probability distribution function is derived. It
provides an adequate smooth approximation to describe numerical histograms
built for not too small , where the finiteness of bin size trimmes the sharp
peaks.Comment: 6 pages, 4 figures, to appear in Phys. Rev.
Iron oxide doped boron nitride nanotubes: structural and magnetic properties
A first-principles formalism is employed to investigate the interaction of
iron oxide (FeO) with a boron nitride (BN) nanotube. The stable structure of
the FeO-nanotube has Fe atoms binding N atoms, with bond length of roughly
2.1 \AA, and binding between O and B atoms, with bond length of 1.55 \AA.
In case of small FeO concentrations, the total magnetic moment is
(4) times the number of Fe atoms in the unit cell and it is
energetically favorable to FeO units to aggregate rather than randomly bind to
the tube. As a larger FeO concentration case, we study a BN nanotube fully
covered by a single layer of FeO. We found that such a structure has square FeO
lattice with Fe-O bond length of 2.11 \AA, similar to that of FeO bulk, and
total magnetic moment of 3.94 per Fe atom. Consistently with
experimental results, the FeO covered nanotube is a semi-half-metal which can
become a half-metal if a small change in the Fermi level is induced. Such a
structure may be important in the spintronics context.Comment: 10 pages, 3 figure
Quantum critical 5f-electrons avoid singularities in U(Ru,Rh)2Si2
We present specific heat measurements of 4% Rh-doped U(Ru,Rh)2Si2 at magnetic
fields above the proposed metamagnetic transition field Hm~34 T, revealing
striking similarities to the isotructural Ce analog CeRu2Si2, suggesting that
strongly renormalized hybridized band models apply equally well to both
systems. The vanishing bandwidths as H --> Hm are consistent with a putative
quantum critical point close to Hm. The existence of a phase transition into an
ordered phase in the vicinity of Hm for 4% Rh-doped U(Ru,Rh)2Si2, but not for
CeRu2Si2, is consistent with a stronger super-exchange in the case of the U 5-f
system, with irreversible processes at the transition revealing a strong
coupling of the 5f orbitals to the lattice, most suggestive of orbital or
electric quadrupolar order.Comment: 4 pages, 4 figure
Detecting relevant tweets in very large tweet collections: the London Riots case study
In this paper we propose to approach the subject of detecting relevant tweets when in the presence of very large tweet collections containing a large number of different trending topics. We use a large database of tweets collected during the 2011 London Riots as a case study to demonstrate the application of the proposed techniques. In order to extract relevant content, we extend, formalize and apply a recent technique, called Twitter Topic Fuzzy Fingerprints, which, in the scope of social media, outperforms other well known text based classification methods, while being less computationally demanding, an essential feature when processing large volumes of streaming data. Using this technique we were able to detect 45% additional relevant tweets within the database
Stick-Slip Motion and Phase Transition in a Block-Spring System
We study numerically stick slip motions in a model of blocks and springs
being pulled slowly. The sliding friction is assumed to change dynamically with
a state variable. The transition from steady sliding to stick-slip is
subcritical in a single block and spring system. However, we find that the
transition is continuous in a long chain of blocks and springs. The size
distribution of stick-slip motions exhibits a power law at the critical point.Comment: 8 figure
Quasiparticle photoemission intensity in doped two-dimensional quantum antiferromagnets
Using the self-consistent Born approximation, and the corresponding wave
function of the magnetic polaron, we calculate the quasiparticle weight
corresponding to destruction of a real electron (in contrast to creation of a
spinless holon), as a funtion of wave vector for one hole in a generalized
model and the strong coupling limit of a generalized Hubbard model. The
results are in excellent agreement with those obtained by exact diagonalization
of a sufficiently large cluster. Only the Hubbard weigth compares very well
with photoemission measurements in Sr_2CuO_2Cl_2.Comment: 11 pages, latex, 3 figure
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