1,695 research outputs found
Modeling the secular evolution of migrating planet pairs
The subject of this paper is the secular behaviour of a pair of planets
evolving under dissipative forces. In particular, we investigate the case when
dissipative forces affect the planetary semi-major axes and the planets move
inward/outward the central star, in a process known as planet migration. To
perform this investigation, we introduce fundamental concepts of conservative
and dissipative dynamics of the three-body problem. Based on these concepts, we
develop a qualitative model of the secular evolution of the migrating planetary
pair. Our approach is based on analysis of the energy and the orbital angular
momentum exchange between the two-planet system and an external medium; thus no
specific kind of dissipative forces is invoked. We show that, under assumption
that dissipation is weak and slow, the evolutionary routes of the migrating
planets are traced by the Mode I and Mode II stationary solutions of the
conservative secular problem. The ultimate convergence and the evolution of the
system along one of these secular modes of motion is determined uniquely by the
condition that the dissipation rate is sufficiently smaller than the proper
secular frequency of the system. We show that it is possible to reassemble the
starting configurations and migration history of the systems on the basis of
their final states and consequently to constrain the parameters of the physical
processes involved.Comment: 20 pages, 17 figures. Accepted for publication in MNRA
Possible Z2 phase and spin-charge separation in electron doped cuprate superconductors
The SU(2) slave-boson mean-field theory for the tt'J model is analyzed. The
role of next-nearest-neighbor hopping t' on the phase-diagram is studied. We
find a pseudogap phase in hole-doped materials (where t'<0). The pseudo-gap
phase is a U(1) spin liquid (the staggered-flux phase) with a U(1) gauge
interaction and no fractionalization. This agrees with experiments on hole
doped samples. The same calculation also indicates that a positive t' favors a
Z2 state with true spin-charge separation. The Z2 state that exists when t' >
0.5J can be a candidate for the pseudo-gap phase of electron-doped cuprates (if
such a phase exists). The experimental situation in electron-doped materials is
also addressed.Comment: 6 pages, 2 figures, RevTeX4. Homepage http://dao.mit.edu/~wen
Anomalous electronic structure and pseudogap effects in Nd_1.85Ce_0.15CuO_4
We report a high-resolution angle-resolved photoemission spectroscopic
(ARPES) study of the electron-doped (n-type) cuprate superconductor
Nd_1.85Ce_0.15CuO_4. We observe regions along the Fermi surface where the
near-E_F intensity is suppressed and the spectral features are broad in a
manner reminiscent of the high-energy ``pseudogap'' in the underdoped p-type
(hole doped) cuprates. However, instead of occurring near the (pi, 0) region,
as in the p-type materials, this pseudogap falls near the intersection of the
underlying Fermi surface with the antiferromagnetic Brillouin zone boundary.Comment: 5 pages, 3 figures, RevTex, submitted Phys. Rev. Lett. December 21,
200
The Frequency Dependent Conductivity of Electron Glasses
Results of DC and frequency dependent conductivity in the quantum limit, i.e.
hw > kT, for a broad range of dopant concentrations in nominally uncompensated,
crystalline phosphorous doped silicon and amorphous niobium-silicon alloys are
reported. These materials fall under the general category of disordered
insulating systems, which are referred to as electron glasses. Using microwave
resonant cavities and quasi-optical millimeter wave spectroscopy we are able to
study the frequency dependent response on the insulating side of the
metal-insulator transition. We identify a quantum critical regime, a Fermi
glass regime and a Coulomb glass regime. Our phenomenological results lead to a
phase diagram description, or taxonomy, of the electrodynamic response of
electron glass systems
Hot Spots and Transition from d-Wave to Another Pairing Symmetry in the Electron-Doped Cuprate Superconductors
We present a simple theoretical explanation for a transition from d-wave to
another superconducting pairing observed in the electron-doped cuprates. The
d_{x^2-y^2} pairing potential Delta, which has the maximal magnitude and
opposite signs at the hot spots on the Fermi surface, becomes suppressed with
the increase of electron doping, because the hot spots approach the Brillouin
zone diagonals, where Delta vanishes. Then, the d_{x^2-y^2} pairing is replaced
by either singlet s-wave or triplet p-wave pairing. We argue in favor of the
latter and discuss experiments to uncover it.Comment: 6 pages, 4 figures, RevTeX 4. V.2: Extra figure and many references
added. V.3: Minor update of references for the proof
Doping evolution of the phonon density of states and electron-lattice interaction in NdCeCuO
We use inelastic neutron scattering to study the evolution of the generalized
phonon density of states (GDOS) of the -type high- superconductor
NdCeCuO (NCCO), from the half-filled Mott-insulator
() to the K superconductor (). Upon doping the CuO
planes in NdCuO (NCO) with electrons by Ce substitution, the
most significant change in the GDOS is the softening of the highest phonon
branches associated with the Cu-O bond stretching and out-of-plane oxygen
vibration modes. However, the softening occurs within the first few percent of
Ce-doping and is not related to the electron doping induced
nonsuperconducting-superconducting transition (NST) at . These
results suggest that the electron-lattice coupling in the -type high-
superconductors is different from that in the -type materials.Comment: 4 pages and 3 figure
Fully Gapped Single-Particle Excitations in the Lightly Doped Cuprates
The low-energy excitations of the lightly doped cuprates were studied by
angle-resolved photoemission spectroscopy. A finite gap was measured over the
entire Brillouin zone, including along the d_{x^2 - y^2} nodal line. This
effect was observed to be generic to the normal states of numerous cuprates,
including hole-doped La_{2-x}Sr_{x}CuO_{4} and Ca_{2-x}Na_{x}CuO_{2}Cl_{2} and
electron-doped Nd_{2-x}Ce_{x}CuO_{4}. In all compounds, the gap appears to
close with increasing carrier doping. We consider various scenarios to explain
our results, including the possible effects of chemical disorder, electronic
inhomogeneity, and a competing phase.Comment: To appear in Phys. Rev.
Pulsar timing arrays and the challenge of massive black hole binary astrophysics
Pulsar timing arrays (PTAs) are designed to detect gravitational waves (GWs)
at nHz frequencies. The expected dominant signal is given by the superposition
of all waves emitted by the cosmological population of supermassive black hole
(SMBH) binaries. Such superposition creates an incoherent stochastic
background, on top of which particularly bright or nearby sources might be
individually resolved. In this contribution I describe the properties of the
expected GW signal, highlighting its dependence on the overall binary
population, the relation between SMBHs and their hosts, and their coupling with
the stellar and gaseous environment. I describe the status of current PTA
efforts, and prospect of future detection and SMBH binary astrophysics.Comment: 18 pages, 4 figures. To appear in the Proceedings of the 2014 Sant
Cugat Forum on Astrophysics. Astrophysics and Space Science Proceedings, ed.
C.Sopuerta (Berlin: Springer-Verlag
Evidence for a Nodeless Gap from the Superfluid Density of Optimally Doped Pr_{1.855}Ce_{0.145}CuO_{4-y} Films
We present measurements of the ab-plane magnetic penetration depth,
\lambda(T), in five optimally doped Pr_{1.855}Ce_{0.145}CuO_{4-y} films for 1.6
K \leq T \leq T_c \sim 24 K. Low resistivities, high superfluid densities
n_s(T)\propto \lambda^{-2}(T), high T_c's, and small transition widths are
reproducible and indicative of excellent film quality. For all five films,
\lambda^{-2}(T)/\lambda^{-2}(0) at low T is well fitted by an exponential
temperature dependence with a gap, \Delta_{min}, of 0.85 k_B T_c. This behavior
is consistent with a nodeless gap and is incompatible with d-wave
superconductivity.Comment: 5 pages, 4 figures, reorganized for clarit
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