3,314 research outputs found
Frequency-dependent spin susceptibility in the two-dimensional Hubbard model
A Quantum Monte Carlo calculation of dynamical spin susceptibility in the
half-filled 2D Hubbard model is presented for temperature and an
intermediate on-site repulsion . Using the singular value decomposition
technique we succeed in analytically continuing the Matsubara Green's function
into the real frequency domain and in deriving the spectral representation for
the longitudinal and transverse spin susceptibility. The simulation results,
while contradicting the random-phase approximation prediction of
antiferromagnetic long-range order at this temperature, are in agreement with
an extension of a self-consistent renormalization approach of Moriya. The
static susceptibility calculated using this technique is qualitatively
consistent with the simulation results.Comment: 4 pages, Revtex, encoded figs.uu file with 3 figures enclose
A factorization of a super-conformal map
A super-conformal map and a minimal surface are factored into a product of
two maps by modeling the Euclidean four-space and the complex Euclidean plane
on the set of all quaternions. One of these two maps is a holomorphic map or a
meromorphic map. These conformal maps adopt properties of a holomorphic
function or a meromorphic function. Analogs of the Liouville theorem, the
Schwarz lemma, the Schwarz-Pick theorem, the Weierstrass factorization theorem,
the Abel-Jacobi theorem, and a relation between zeros of a minimal surface and
branch points of a super-conformal map are obtained.Comment: 21 page
Superconductivity and Pseudogap in Quasi-Two-Dimensional Metals around the Antiferromagnetic Quantum Critical Point
Spin fluctuations (SF) and SF-mediated superconductivity (SC) in
quasi-two-dimensional metals around the antiferrromagnetic (AF) quantum
critical point (QCP) are investigated by using the self-consistent
renormalization theory for SF and the strong coupling theory for SC. We
introduce a parameter y0 as a measure for the distance from the AFQCP which is
approximately proportional to (x-xc), x being the electron (e) or hole (h)
doping concentration to the half-filled band and xc being the value at the
AFQCP. We present phase diagrams in the T-y0 plane including contour maps of
the AF correlation length and AF and SC transition temperatures TN and Tc,
respectively. The Tc curve is dome-shaped with a maximum at around the AFQCP.
The calculated one-electron spectral density shows a pseudogap in the
high-density-of-states region near (pi,0) below around a certain temperature T*
and gives a contour map at the Fermi energy reminiscent of the Fermi arc. These
results are discussed in comparison with e- and h-doped high-Tc cuprates.Comment: 5 pages, 3 figure
Diagrammatic perturbation theory and the pseudogap
We study a model of quasiparticles on a two-dimensional square lattice
coupled to Gaussian distributed dynamical fields. The model describes
quasiparticles coupled to spin or charge fluctuations and is solved by a Monte
Carlo sampling of the molecular field distributions. The non-perturbative
solution is compared to various approximations based on diagrammatic
perturbation theory. When the molecular field correlations are sufficiently
weak, the diagrammatic calculations capture the qualitative aspects of the
quasiparticle spectrum. For a range of model parameters near the magnetic
boundary, we find that the quasiparticle spectrum is qualitatively different
from that of a Fermi liquid in that it shows a double peak structure, and that
the diagrammatic approximations we consider fail to reproduce, even
qualitatively, the results of the Monte Carlo calculations. This suggests that
the pseudogap induced by a coupling to antiferromagnetic fluctuations and the
spin-splitting of the quasiparticle peak induced by a coupling to ferromagnetic
spin-fluctuations lie beyond diagrammatic perturbation theory
Can Frustration Preserve a Quasi-Two-Dimensional Spin Fluid?
Using spin-wave theory, we show that geometric frustration fails to preserve
a two-dimensional spin fluid. Even though frustration can remove the interlayer
coupling in the ground-state of a classical anti-ferromagnet, spin layers
innevitably develop a quantum-mechanical coupling via the mechanism of ``order
from disorder''. We show how the order from disorder coupling mechanism can be
viewed as a result of magnon pair tunneling, a process closely analogous to
pair tunneling in the Josephson effect. In the spin system, the Josephson
coupling manifests itself as a a biquadratic spin coupling between layers, and
for quantum spins, these coupling terms are as large as the inplane coupling.
An alternative mechanism for decoupling spin layers occurs in classical XY
models in which decoupled "sliding phases" of spin fluid can form in certain
finely tuned conditions. Unfortunately, these finely tuned situations appear
equally susceptible to the strong-coupling effects of quantum tunneling,
forcing us to conclude that in general, geometric frustration cannot preserve a
two-dimensional spin fluid.Comment: 12 pages, 3 figure
Anisotropy, Itineracy, and Magnetic Frustration in High-Tc Iron Pnictides
Using first-principle density functional theory calculations combined with
insight from a tight-binding representation, dynamical mean field theory, and
linear response theory, we have extensively investigated the electronic
structures and magnetic interactions of nine ferropnictides representing three
different structural classes. The calculated magnetic interactions are found to
be short-range, and the nearest () and next-nearest () exchange
constants follow the universal trend of J_{1a}/2J_{2}\sim 1, despite their
itinerant origin and extreme sensitivity to the z-position of As. These results
bear on the discussion of itineracy versus magnetic frustration as the key
factor in stabilizing the superconducting ground state. The calculated spin
wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast
to cuprates.Comment: Fig.4 updated: Phys. Rev. Lett (in press
iPTF14hls as a variable hyper-wind from a very massive star
The origin of iPTF14hls, which had Type IIP supernova-like spectra but kept bright for almost two years with little spectral evolution, is still unclear. We here propose that iPTF14hls was not a sudden outburst like supernovae but rather a long-term outflow similar to stellar winds. The properties of iPTF14hls, which are at odds with a supernova scenario, become natural when interpreted as a stellar wind with variable mass-loss rate. Based on the wind hypothesis, we estimate the mass-loss rates of iPTF14hls in the bright phase. We find that the instantaneous mass-loss rate of iPTF14hls during the 2-yr bright phase was more than a few M⊙ yr−1 (‘hyper-wind’) and it reached as much as 10 M⊙ yr−1 . The total mass lost over two years was about 10 M⊙. Interestingly, we find that the light curve of iPTF14hls has a very similar shape to that of η Carinae during the Great Eruption, which also experienced a similar but less extreme brightening accompanied by extraordinary mass-loss, shedding more than 10 M⊙ in 10 yr. The progenitor of iPTF14hls is less than 150 M⊙ if it still exists, which is similar to η Carinae. The two phenomena may be related to a continuum-driven extreme wind from very massive stars
Itinerant ferromagnetism in a two-dimensional atomic gas
Motivated by the first experimental evidence of ferromagnetic behavior in a
three-dimensional ultracold atomic gas, we explore the possibility of itinerant
ferromagnetism in a trapped two-dimensional atomic gas. Firstly, we develop a
formalism that demonstrates how quantum fluctuations drive the ferromagnetic
reconstruction first order, and consider the consequences of an imposed
population imbalance. Secondly, we adapt this formalism to elucidate the key
experimental signatures of ferromagnetism in a realistic trapped geometry.Comment: Accepted for publication in Phys. Rev.
Superconductivity in CeCoIn5-xSnx: Veil Over an Ordered State or Novel Quantum Critical Point?
Measurements of specific heat and electrical resistivity in magnetic fields
up to 9 T along [001] and temperatures down to 50 mK of Sn-substituted CeCoIn5
are reported. The maximal -ln(T) divergence of the specific heat at the upper
critical field H_{c2} down to the lowest temperature characteristic of
non-Fermi liquid systems at the quantum critical point (QCP), the universal
scaling of the Sommerfeld coefficient, and agreement of the data with
spin-fluctuation theory, provide strong evidence for quantum criticality at
H_{c2} for all x < 0.12 in CeCoIn5-xSnx. These results indicate the
"accidental" coincidence of the QCP located near H_{c2} in pure CeCoIn5, in
actuality, constitute a novel quantum critical point associated with
unconventional superconductivity.Comment: 12 pages, 4 figure
Field-tuned quantum critical point of antiferromagnetic metals
A magnetic field applied to a three-dimensional antiferromagnetic metal can
destroy the long-range order and thereby induce a quantum critical point. Such
field-induced quantum critical behavior is the focus of many recent
experiments. We investigate theoretically the quantum critical behavior of
clean antiferromagnetic metals subject to a static, spatially uniform external
magnetic field. The external field does not only suppress (or induce in some
systems) antiferromagnetism but also influences the dynamics of the order
parameter by inducing spin precession. This leads to an exactly marginal
correction to spin-fluctuation theory. We investigate how the interplay of
precession and damping determines the specific heat, magnetization,
magnetocaloric effect, susceptibility and scattering rates. We point out that
the precession can change the sign of the leading \sqrt{T} correction to the
specific heat coefficient c(T)/T and can induce a characteristic maximum in
c(T)/T for certain parameters. We argue that the susceptibility \chi =\partial
M/\partial B is the thermodynamic quantity which shows the most significant
change upon approaching the quantum critical point and which gives experimental
access to the (dangerously irrelevant) spin-spin interactions.Comment: 12 pages, 8 figure
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