962 research outputs found
Modified-gravity wormholes without exotic matter
A fundamental ingredient in wormhole physics is the flaring-out condition at
the throat which, in classical general relativity, entails the violation of the
null energy condition. In this work, we present the most general conditions in
the context of modified gravity, in which the matter threading the wormhole
throat satisfies all of the energy conditions, and it is the higher order
curvature terms, which may be interpreted as a gravitational fluid, that
support these nonstandard wormhole geometries. Thus, we explicitly show that
wormhole geometries can be theoretically constructed without the presence of
exotic matter, but are sustained in the context of modified gravity.Comment: 4 pages. V2: Slight change in title, discussion on the stability and
references added; version to appear in PRD. V3: reference adde
Single hole dynamics in dimerized spin liquids
The dynamics of a single hole in quantum antiferromagnets is influenced by
magnetic fluctuations. In the present work we consider two situations. The
first one corresponds to a single hole in the two leg t-J spin ladder. In this
case the wave function renormalization is relatively small and the
quasiparticle residue of the S=1/2 state remains close to unity. However at
large t/J there are higher spin (S=3/2,5/2,..) bound states of the hole with
the magnetic excitations, and therefore there is a crossover from
quasiparticles with S=1/2 to quasiparticles with higher spin.
The second situation corresponds to a single hole in two coupled
antiferromagnetic planes very close to the point of antiferromagnetic
instability. In this case the hole wave function renormalization is very strong
and the quasiparticle residue vanishes at the point of instability.Comment: 12 pages, 3 figure
Long-range dynamics of magnetic impurities coupled to a two-dimensional Heisenberg antiferromagnet
We consider a two-dimensional Heisenberg antiferromagnet on a square lattice
with weakly coupled impurities, i.e. additional spins interacting with the host
magnet by a small dimensionless coupling constant g<<1. Using linear spin-wave
theory, we find that the magnetization disturbance at distance r from a single
impurity behaves as g/r for 1>1/g. Surprisingly
the disturbance is inversely proportional to the coupling constant! The
interaction between two impurities separated by a distance r is proportional to
g^2/r for 1>1/g. Hence at large distances, the
interaction is universal and independent of the coupling constant. We also find
that the frequency of Rabi oscillations between two impurities is proportional
to g^2 ln(gr) at 1<<r<<1/g, logarithmically enhanced compared to the spin-wave
width. This leads to a new mechanism for NMR, NQR and EPR line broadening. All
these astonishing results are due to the gapless spectrum of the magnetic
excitations in the quantum antiferromagnet.Comment: 6 pages, 5 figure
Low-energy singlet and triplet excitations in the spin-liquid phase of the two-dimensional J1-J2 model
We analyze the stability of the spontaneously dimerized spin-liquid phase of
the frustrated Heisenberg antiferromagnet - the J1-J2 model. The lowest triplet
excitation, corresponding to breaking of a singlet bond, is found to be stable
in the region 0.38 < J2/J1 < 0.62. In addition we find a stable low-energy
collective singlet mode, which is closely related to the spontaneous violation
of the discrete symmetry. Both modes are gapped in the quantum disordered phase
and become gapless at the transition point to the Neel ordered phase
(J2/J1=0.38). The spontaneous dimerization vanishes at the transition and we
argue that the disappearance of dimer order is related to the vanishing of the
singlet gap. We also present exact diagonalization data on a small (4x4)
cluster which indeed show a structure of the spectrum, consistent with that of
a system with a four-fold degenerate (spontaneously dimerized) ground state.Comment: 4 pages, 4 figures, small changes, published versio
Stability of the spiral phase in the 2D extended t-J model
We analyze the t-t'-t''-J model at low doping by chiral perturbation theory
and show that the (1,0) spiral state is stabilized by the presence of t',t''
above critical values around 0.2J, assuming t/J=3.1. We find that the (magnon
mediated) hole-hole interactions have an important effect on the region of
charge stability in the space of parameters t',t'', generally increasing
stability, while the stability in the magnetic sector is guaranteed by the
presence of spin quantum fluctuations (order from disorder effect). These
conclusions are based on perturbative analysis performed up to two loops, with
very good convergence.Comment: 7 pages, 6 figure
Superconducting Spiral Phase in the two-dimensional t-J model
We analyse the t-t'-t''-J model, relevant to the superconducting cuprates. By
using chiral perturbation theory we have determined the ground state to be a
spiral for small doping \delta << 1 near half filling. In this limit the
solution does not contain any uncontrolled approximations. We evaluate the
spin-wave Green's functions and address the issue of stability of the spiral
state, leading to the phase diagram of the model. At t'=t''=0 the spiral state
is unstable towards a local enhancement of the spiral pitch, and the nature of
the true ground state remains unclear. However, for values of t' and t''
corresponding to real cuprates the (1,0) spiral state is stabilized by quantum
fluctuations (``order from disorder'' effect). We show that at \delta = 0.119
the spiral is commensurate with the lattice with a period of 8 lattice
spacings. It is also demonstrated that spin-wave mediated superconductivity
develops in the spiral state and a lower limit for the superconducting gap is
derived. Even though one cannot classify the gap symmetry according to the
lattice representations (s,p,d,...) since the symmetry of the lattice is
spontaneously broken by the spiral, the gap always has lines of nodes along the
(1,\pm 1) directions.Comment: 17 pages, 11 figure
Molecular CP-violating magnetic moment
A concept of CP-violating (T,P-odd) permanent molecular magnetic moments
is introduced. We relate the moments to the electric dipole moment
of electron (eEDM) and estimate for several diamagnetic polar
molecules. The moments exhibit a steep, Z^5, scaling with the nuclear charge Z
of the heavier molecular constituent. A measurement of the CP-violating
magnetization of a polarized sample of heavy molecules may improve the present
limit on eEDM by several orders of magnitude.Comment: 4 pages, no figures, submitted to PR
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