621 research outputs found
Nematicity as a route to a magnetic field-induced spin density wave order; application to the high temperature cuprates
The electronic nematic order characterized by broken rotational symmetry has
been suggested to play an important role in the phase diagram of the high
temperature cuprates. We study the interplay between the electronic nematic
order and a spin density wave order in the presence of a magnetic field. We
show that a cooperation of the nematicity and the magnetic field induces a
finite coupling between the spin density wave and spin-triplet staggered flux
orders. As a consequence of such a coupling, the magnon gap decreases as the
magnetic field increases, and it eventually condenses beyond a critical
magnetic field leading to a field-induced spin density wave order. Both
commensurate and incommensurate orders are studied, and the experimental
implications of our findings are discussed.Comment: 5 pages, 3 figure
Surprising phenomena in a rich new class of inflationary models
We report on a new class of fast-roll inflationary models. In a huge part of
its parameter space, inflationary perturbations exhibit quite unusual phenomena
such as scalar and tensor modes freezing out at widely different times, as well
as scalar modes reentering the horizon during inflation. In another, narrower
range of parameters, this class of models agrees with observations. One
specific point in parameter space is characterized by extraordinary behavior of
the scalar perturbations. Freeze-out of scalar perturbations as well as
particle production at horizon crossing are absent. Also the behavior of the
perturbations around this quasi-de Sitter background is dual to a quantum field
theory in flat space-time. Finally, the form of the primordial power spectrum
is determined by the interaction between different modes of scalar
perturbations.Comment: 12 pages, 5 figures, 1 table, references + comments added, errors
corrected, conclusions unchanged, version published in JCA
Accelerating black hole in 2+1 dimensions and 3+1 black (st)ring
A C-metric type solution for general relativity with cosmological constant is
presented in 2+1 dimensions. It is interpreted as a three-dimensional black
hole accelerated by a strut. Positive values of the cosmological constant are
admissible too. Some embeddings of this metric in the 3+1 space-time are
considered: accelerating BTZ black string and a black ring where the
gravitational force is sustained by the acceleration.Comment: 12 pages, 2 figures, JHEP 1101:114,201
Chaotic dynamics in preheating after inflation
We study chaotic dynamics in preheating after inflation in which an inflaton
is coupled to another scalar field through an interaction
. We first estimate the size of the quasi-homogeneous
field at the beginning of reheating for large-field inflaton potentials
by evaluating the amplitude of the fluctuations on
scales larger than the Hubble radius at the end of inflation. Parametric
excitations of the field during preheating can give rise to chaos
between two dynamical scalar fields. For the quartic potential (,
) chaos actually occurs for in a
linear regime before which the backreaction of created particles becomes
important. This analysis is supported by several different criteria for the
existence of chaos. For the quadratic potential () the signature of chaos
is not found by the time at which the backreaction begins to work, similar to
the case of the quartic potential with .Comment: 12 pages, 10 figures, Version to appear in Classical and Quantum
Gravit
SO(4) Theory of Competition between Triplet Superconductivity and Antiferromagnetism in Bechgaard Salts
Motivated by recent experiments with Bechgaard salts, we investigate the
competition between antiferromagnetism and triplet superconductivity in quasi
one-dimensional electron systems. We unify the two orders in an SO(4) symmetric
framework, and demonstrate the existence of such symmetry in one-dimensional
Luttinger liquids. SO(4) symmetry, which strongly constrains the phase diagram,
can explain coexistence regions between antiferromagnetic, superconducting, and
normal phases, as observed in (TMTSF)PF. We predict a sharp neutron
scattering resonance in superconducting samples.Comment: 5 pages, 3 figures; Added discussion of applicability of SO(4)
symmetry for strongly anisotropic Fermi liquids; Added reference
DEFROST: A New Code for Simulating Preheating after Inflation
At the end of inflation, dynamical instability can rapidly deposit the energy
of homogeneous cold inflaton into excitations of other fields. This process,
known as preheating, is rather violent, inhomogeneous and non-linear, and has
to be studied numerically. This paper presents a new code for simulating scalar
field dynamics in expanding universe written for that purpose. Compared to
available alternatives, it significantly improves both the speed and the
accuracy of calculations, and is fully instrumented for 3D visualization. We
reproduce previously published results on preheating in simple chaotic
inflation models, and further investigate non-linear dynamics of the inflaton
decay. Surprisingly, we find that the fields do not want to thermalize quite
the way one would think. Instead of directly reaching equilibrium, the
evolution appears to be stuck in a rather simple but quite inhomogeneous state.
In particular, one-point distribution function of total energy density appears
to be universal among various two-field preheating models, and is exceedingly
well described by a lognormal distribution. It is tempting to attribute this
state to scalar field turbulence.Comment: RevTeX 4.0; 16 pages, 9 figure
The vacuum bubbles in de Sitter background and black hole pair creation
We study the possible types of the nucleation of vacuum bubbles. We classify
vacuum bubbles in de Sitter background and present some numerical solutions.
The thin-wall approximation is employed to obtain the nucleation rate and the
radius of vacuum bubbles. With careful analysis we confirm that Parke's formula
is also applicable to the large true vacuum bubbles. The nucleation of the
false vacuum bubble in de Sitter background is also evaluated. The tunneling
process in the potential with degenerate vacua is analyzed as the limiting
cases of the large true vacuum bubble and false vacuum bubble. Next, we
consider the pair creation of black holes in the background of bubble
solutions. We obtain static bubble wall solutions of junction equation with
black hole pair. The masses of created black holes are uniquely determined by
the cosmological constant and surface tension on the wall. Finally, we obtain
the rate of pair creation of black holes.Comment: 3 figures, minor including errors and typos corrected, and refs.
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Visualizing the microscopic coexistence of spin density wave and superconductivity in underdoped NaFe1-xCoxAs
Although the origin of high temperature superconductivity in the iron
pnictides is still under debate, it is widely believed that magnetic
interactions or fluctuations play an important role in triggering Cooper
pairing. Because of the relevance of magnetism to pairing, the question of
whether long range spin magnetic order can coexist with superconductivity
microscopically has attracted strong interests. The available experimental
methods used to answer this question are either bulk probes or local ones
without control of probing position, thus the answers range from mutual
exclusion to homogeneous coexistence. To definitively answer this question,
here we use scanning tunneling microscopy to investigate the local electronic
structure of an underdoped NaFe1-xCoxAs near the spin density wave (SDW) and
superconducting (SC) phase boundary. Spatially resolved spectroscopy directly
reveal both the SDW and SC gap features at the same atomic location, providing
compelling evidence for the microscopic coexistence of the two phases. The
strengths of the SDW and SC features are shown to anti correlate with each
other, indicating the competition of the two orders. The microscopic
coexistence clearly indicates that Cooper pairing occurs when portions of the
Fermi surface (FS) are already gapped by the SDW order. The regime TC < T <
TSDW thus show a strong resemblance to the pseudogap phase of the cuprates
where growing experimental evidences suggest a FS reconstruction due to certain
density wave order. In this phase of the pnictides, the residual FS has a
favorable topology for magnetically mediated pairing when the ordering moment
of the SDW is small.Comment: 18 pages, 4 figure
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