2,485 research outputs found
Determining the Electron-Phonon Coupling Strength in Correlated Electron Systems from Resonant Inelastic X-ray Scattering
We show that high resolution Resonant Inelastic X-ray Scattering (RIXS)
provides direct, element-specific and momentum-resolved information on the
electron-phonon (e-p) coupling strength. Our theoretical analysis demonstrates
that the e-p coupling can be extracted from RIXS spectra by determining the
differential phonon scattering cross section. An alternative, very direct
manner to extract the coupling is to use the one and two-phonon loss ratio,
which is governed by the e-p coupling strength and the core-hole life-time.
This allows measurement of the e-p coupling on an absolute energy scale.Comment: 4 pages, 3 figure
Ultrashort Lifetime Expansion for Indirect Resonant Inelastic X-ray Scattering
In indirect resonant inelastic X-ray scattering (RIXS) an intermediate state
is created with a core-hole that has a ultrashort lifetime. The core-hole
potential therefore acts as a femtosecond pulse on the valence electrons. We
show that this fact can be exploited to integrate out the intermediate states
from the expressions for the scattering cross section. By this we obtain an
effective scattering cross section that only contains the initial and final
scattering states. We derive in detail the effective cross section which turns
out to be a resonant scattering factor times a linear combination of the charge
response function and the dynamic longitudinal spin density
correlation function. This result is asymptotically exact for both strong and
weak local core-hole potentials and ultrashort lifetimes. The resonant
scattering pre-factor is shown to be weakly temperature dependent. We also
derive a sum-rule for the total scattering intensity and generalize the results
to multi-band systems. One of the remarkable outcomes is that one can change
the relative charge and spin contribution to the inelastic spectral weight by
varying the incident photon energy.Comment: 9 pages, 3 figures embedde
Theory for Magnetism and Triplet Superconductivity in LiFeAs
Superconducting pnictides are widely found to feature spin-singlet pairing in
the vicinity of an antiferromagnetic phase, for which nesting between electron
and hole Fermi surfaces is crucial. LiFeAs differs from the other pnictides by
(i) poor nesting properties and (ii) unusually shallow hole pockets.
Investigating magnetic and pairing instabilities in an electronic model that
incorporates these differences, we find antiferromagnetic order to be absent.
Instead we observe almost ferromagnetic fluctuations which drive an instability
toward spin-triplet p-wave superconductivity.Comment: Published versio
First-principles study of the interaction and charge transfer between graphene and metals
Measuring the transport of electrons through a graphene sheet necessarily
involves contacting it with metal electrodes. We study the adsorption of
graphene on metal substrates using first-principles calculations at the level
of density functional theory. The bonding of graphene to Al, Ag, Cu, Au and
Pt(111) surfaces is so weak that its unique "ultrarelativistic" electronic
structure is preserved. The interaction does, however, lead to a charge
transfer that shifts the Fermi level by up to 0.5 eV with respect to the
conical points. The crossover from p-type to n-type doping occurs for a metal
with a work function ~5.4 eV, a value much larger than the work function of
free-standing graphene, 4.5 eV. We develop a simple analytical model that
describes the Fermi level shift in graphene in terms of the metal substrate
work function. Graphene interacts with and binds more strongly to Co, Ni, Pd
and Ti. This chemisorption involves hybridization between graphene -states
and metal d-states that opens a band gap in graphene. The graphene work
function is as a result reduced considerably. In a current-in-plane device
geometry this should lead to n-type doping of graphene.Comment: 12 pages, 9 figure
Resonant Inelastic X-ray Scattering on Spin-Orbit Coupled Insulating Iridates
We determine how the elementary excitations of iridium-oxide materials, which
are dominated by a strong relativistic spin-orbit coupling, appear in Resonant
Inelastic X-ray Scattering (RIXS). Whereas the RIXS spectral weight at the L2
x-ray edge vanishes, we find it to be strong at the L3-edge. Applying this to
Sr2IrO4, we observe that RIXS, besides being sensitive to local doublet to
quartet transitions, meticulously maps out the strongly dispersive delocalized
excitations of the low-lying spin-orbit doublets.Comment: 4 pages, 3 figure
New light on magnetic excitations: indirect resonant inelastic X-ray scattering on magnons
Recent experiments show that indirect resonant inelastic X-ray scattering
(RIXS) is a new probe of spin dynamics. Here I derive the cross-section for
magnetic RIXS and determine the momentum dependent four-spin correlation
function that it measures. These results show that this technique offers
information on spin dynamics that is complementary to e.g. neutron scattering.
The RIXS spectrum of Heisenberg antiferromagnets is calculated. It turns out
that only scattering processes that involve at least two magnons are allowed.
Other selection rules imply that the scattering intensity vanishes for specific
transferred momenta , in particular for . The calculated
spectra agree very well with the experimental data.Comment: 4 pages, 3 figure
Resonant Inelastic X-ray Scattering Studies of Elementary Excitations
In the past decade, Resonant Inelastic X-ray Scattering (RIXS) has made
remarkable progress as a spectroscopic technique. This is a direct result of
the availability of high-brilliance synchrotron X-ray radiation sources and of
advanced photon detection instrumentation. The technique's unique capability to
probe elementary excitations in complex materials by measuring their energy-,
momentum-, and polarization-dependence has brought RIXS to the forefront of
experimental photon science. We review both the experimental and theoretical
RIXS investigations of the past decade, focusing on those determining the
low-energy charge, spin, orbital and lattice excitations of solids. We present
the fundamentals of RIXS as an experimental method and then review the
theoretical state of affairs, its recent developments and discuss the different
(approximate) methods to compute the dynamical RIXS response. The last decade's
body of experimental RIXS data and its interpretation is surveyed, with an
emphasis on RIXS studies of correlated electron systems, especially transition
metal compounds. Finally, we discuss the promise that RIXS holds for the near
future, particularly in view of the advent of x-ray laser photon sources.Comment: Review, 67 pages, 44 figure
Finite temperature spin-dynamics and phase transitions in spin-orbital models
We study finite temperature properties of a generic spin-orbital model
relevant to transition metal compounds, having coupled quantum Heisenberg-spin
and Ising-orbital degrees of freedom. The model system undergoes a phase
transition, consistent with that of a 2D Ising model, to an orbitally ordered
state at a temperature set by short-range magnetic order. At low temperatures
the orbital degrees of freedom freeze-out and the model maps on to a quantum
Heisenberg model. The onset of orbital excitations causes a rapid scrambling of
the spin spectral weight away from coherent spin-waves, which leads to a sharp
increase in uniform magnetic susceptibility just below the phase transition,
reminiscent of the observed behavior in the Fe-pnictide materials.Comment: 4 page
Orbital excitations in LaMnO
We study the recently observed orbital excitations, orbitons, and treat
electron-electron correlations and lattice dynamics on equal footing. It is
shown that the orbiton energy and dispersion are determined by both
correlations and lattice-vibrations. The electron-phonon coupling causes
satellite structures in the orbiton spectral function and the elementary
excitations of the system are mixed modes with both orbital and phonon
character. It is proposed that the satellite structures observed in recent
Raman-scattering experiments on LaMnO are actually orbiton derived
satellites in the phonon spectral function, caused by the phonon-orbiton
interaction.Comment: 4 pages, 3 figures embedde
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