2,650 research outputs found

### Soft Graviton effects on Gauge theories in de Sitter Space

We extend our investigation of soft graviton effects on the microscopic
dynamics of matter fields in de Sitter space. We evaluate the quantum equation
of motion in generic gauge theories. We find that the Lorentz invariance can be
respected and the velocity of light is not renormalized at the one-loop level.
The gauge coupling constant is universally screened by soft gravitons and
diminishes with time. These features are in common with other four dimensional
field theories with dimensionless couplings. In particular the couplings scale
with time with definite scaling exponents. Although individual scaling
exponents are gauge dependent, we argue that the relative scaling exponents are
gauge independent and should be observable. We also mention soft graviton
effects on cosmic microwave background.Comment: 13 pages, 1 figure. arXiv admin note: text overlap with
arXiv:1203.0391, arXiv:1211.387

### Soft Gravitons Screen Couplings in de Sitter Space

The scale invariance of the quantum fluctuations in de Sitter space leads to
the appearance of de Sitter symmetry breaking infra-red logarithms in the
graviton propagator. We investigate physical effects of soft gravitons on the
local dynamics of matter fields well inside the cosmological horizon. We show
that the IR logarithms do not spoil Lorentz invariance in scalar and Dirac
field theory. The leading IR logarithms can be absorbed by a time dependent
wave function renormalization factor in the both cases. In the interacting
field theory with $\lambda \phi^4$ and Yukawa interaction, we find that the
couplings become time dependent with definite scaling exponents. We argue that
the relative scaling exponents of the couplings are gauge invariant and
physical as we can use the evolution of a coupling as a physical time.Comment: 32pages, 1 figur

### Theory of electron-phonon interaction in a nonequilibrium open electronic system

We study the effects of time-independent nonequilibrium drive on an open 2D
electron gas system coupled to 2D longitudinal acoustic phonons using the
Keldysh path integral method. The layer electron-phonon system is defined at
the two-dimensional interface between a pair of three-dimensional Fermi liquid
leads, which act both as a particle pump and an infinite bath. The
nonequilibrium steady state is achieved in the layer by assuming the leads to
be thermally equilibrated at two different chemical potentials. This subjects
the layer to an out-of-plane voltage $V$ and drives a steady-state charge
current perpendicular to the system. We compute the effects of small voltages
(V\ll\w_D) on the in-plane electron-phonon scattering rate and the electron
effective mass at zero temperature. We also find that the obtained
onequilibrium modification to the acoustic phonon velocity and the Thomas-Fermi
screening length reveal the possibility of tuning these quantities with the
external voltage.Comment: 14 pages, 4 figure

### "Wormhole" geometry for entrapping topologically-protected qubits in non-Abelian quantum Hall states and probing them with voltage and noise measurements

We study a tunneling geometry defined by a single point-contact constriction
that brings to close vicinity two points sitting at the same edge of a quantum
Hall liquid, shortening the trip between the otherwise spatially separated
points along the normal chiral edge path. This ``wormhole''-like geometry
allows for entrapping bulk quasiparticles between the edge path and the tunnel
junction, possibly realizing a topologically protected qubit if the
quasiparticles have non-Abelian statistics. We show how either noise or simpler
voltage measurements along the edge can probe the non-Abelian nature of the
trapped quasiparticles.Comment: 5 pages, 2 figue

### Magnetic excitations in L-edge resonant inelastic x-ray scattering from cuprate compounds

We study the magnetic excitation spectra in L-edge resonant inelastic x-ray
scattering (RIXS) from undoped cuprates. We analyze the second-order dipole
allowed process that the strong perturbation works through the intermediate
state in which the spin degree of freedom is lost at the core-hole site. Within
the approximation neglecting the perturbation on the neighboring sites, we
derive the spin-flip final state in the scattering channel with changing the
polarization, which leads to the RIXS spectra expressed as the dynamical
structure factor of the transverse spin components. We assume a spherical form
of the spin-conserving final state in the channel without changing the
polarization, which leads to the RIXS spectra expressed as the 'exchange'-type
multi-spin correlation function. Evaluating numerically the transition
amplitudes to these final states on a finite-size cluster, we obtain a sizable
amount of the transition amplitude to the spin-conserving final state in
comparison with that to the spin-flip final state. We treat the itinerant
magnetic excitations in the final state by means of the 1/S-expansion method.
Evaluating the higher-order correction with 1/S, we find that the peak arising
from the one-magnon excitation is reduced with its weight, and the continuous
spectra arising from the three-magnon excitations come out. The interaction
between two magnons is treated by summing up the ladder diagrams. On the basis
of these results, we analyze the L_3-edge RIXS spectra in Sr_2CuO_2Cl_2 in
comparison with the experiment. It is shown that the three-magnon excitations
as well as the two-magnon excitations give rise to the intensity in the high
energy side of the one-magnon peak, making the spectral shape asymmetric with
wide width, in good agreement with the experiment.Comment: 18 pages, 10 figures, Revte

### Boltzmann Collision Term

We derive the Boltzmann equation for scalar fields using the
Schwinger-Keldysh formalism. The focus lies on the derivation of the collision
term. We show that the relevant self-energy diagrams have a factorization
property. The collision term assumes the Boltzmann-like form of scattering
probability times statistical factors for those self-energy diagrams which
correspond to tree level scattering processes. Our proof covers scattering
processes with any number of external particles, which come from self-energy
diagrams with any number of loops.Comment: 17 pages, 4 figure

### Real-time effective-action approach to the Anderson quantum dot

The non-equilibrium time evolution of an Anderson quantum dot is
investigated. The quantum dot is coupled between two leads forming a
chemical-potential gradient. We use Kadanoff-Baym dynamic equations within a
non-perturbative resummation of the s-channel bubble chains. The effect of the
resummation leads to the introduction of a frequency-dependent 4-point vertex.
The tunneling to the leads is taken into account exactly. The method allows the
determination of the transient as well as stationary transport through the
quantum dot, and results are compared with different schemes discussed in the
literature (fRG, ISPI, tDMRG and QMC).Comment: 12 pages, 13 figure

### Separation of Equilibration Time-Scales in the Gradient Expansion

We study thermalization by applying gradient expansion to the Kadanoff-Baym
equations of the 2PI effective action to two-loop in a theory with Dirac
fermions coupled to scalars. In addition to those chemical potentials which
equilibrate in the on-shell limit, we identify modes which are conserved in
this approximation, but which relax when off-shell effects are taken into
account. This implies that chemical equilibration does not require higher loop
contributions to the effective action and is compatible with the gradient
expansion. We explicitly calculate the damping time-scales of both, on- and
off-shell, chemical equilibration rates. It is shown that off-shell
equilibration is suppressed by the thermal width of the particles in the
plasma, which explains the separation of on- and off-shell chemical
equilibration time-scales.Comment: 20 pages, 3 figures, published versio

### Current driven quantum criticality in itinerant electron ferromagnets

We determine the effect of an in-plane current flow on the critical
properties of a 2d itinerant electron system near a ferromagnetic-paramagnetic
quantum critical point. We study a model in which a nonequilibrium steady state
is established as a result of exchange of particles and energy with an
underlying substrate. the current $\vec{j}$ gives rise not only to an effective
temperature equal to the voltage drop over a distance of order the mean free
path, but also to symmetry breaking terms of the form $\vec{j}\cdot
\vec{nabla}$ in the effective action. The effect of the symmetry breaking on
the fluctuational and critical properties is found to be small although (in
agreement with previous results) if rotational degrees of freedom are
important, the current can make the classically ordered state dynamically
unstable.Comment: 4 pages, published versio

### Current-induced domain wall motion in Rashba spin-orbit system

Current-induced magnetic domain wall motion, induced by transfer of spin
transfer effect due to exchange interaction, is expected to be useful for next
generation high-density storages. We here show that efficient domain wall
manipulation can be achieved by introduction of Rashba spin-orbit interaction,
which induces spin precession of conduction electron and acts as an effective
magnetic field. Its effect on domain wall motion depends on the wall
configuration. We found that the effect is significant for Bloch wall with the
hard axis along the current, since the effective field works as $\beta$ or
field-like term and removes the threshold current if in extrinsic pinning is
absent. For N\'eel wall and Bloch wall with easy axis perpendicular to Rashba
plane, the effective field induces a step motion of wall corresponding to a
rotation of wall plane by the angle of approximately $\pi$ at current lower
than intrinsic threshold. Rashba interaction would therefore be useful to
assist efficient motion of domain walls at low current

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