46 research outputs found
Density and current response functions in strongly disordered electron systems: Diffusion, electrical conductivity and Einstein relation
We study consequences of gauge invariance and charge conservation of an
electron gas in a strong random potential perturbed by a weak electromagnetic
field. We use quantum equations of motion and Ward identities for one- and
two-particle averaged Green functions to establish exact relations between
density and current response functions. In particular we find precise
conditions under which we can extract the current-current correlation function
from the density-density correlation function and vice versa. We use these
results in two different ways to extend validity of a formula associating the
density response function with the electrical conductivity from semiclassical
equilibrium to quantum nonequilibrium systems. Finally we introduce quantum
diffusion via a response relating the current with the negative gradient of the
charge density. With the aid of this response function we derive a quantum
version of the Einstein relation and prove the existence of the diffusion pole
in the zero-temperature electron-hole correlation function with the the
long-range spatial fluctuations controlled by the static diffusion constant.Comment: 16 pages, REVTeX4, 6 EPS figure
Unified character of correlation effects in unconventional Pu-based superconductors and \delta-Pu
Electronic structure calculations combining the local-density approximation
with an exact diagonalization of the Anderson impurity model show an
intermediate 5f^5-5f^6-valence ground state and delocalization of the 5f^5
multiplet of the Pu atom 5f-shell in PuCoIn_5, PuCoGa_5, and \delta-Pu. The
5f-local magnetic moment is compensated by a moment formed in the surrounding
cloud of conduction electrons. For PuCoGa_5 and \delta-Pu the compensation is
complete and the Anderson impurity ground state is a singlet. For PuCoIn_5 the
compensation is partial and the Pu ground state is magnetic. We suggest that
the unconventional d-wave superconductivity is likely mediated by the 5f-states
antiferromagnetic fluctuations in PuCoIn_5, and by valence fluctuations in
PuCoGa_5.Comment: 5 pages, 3 figure
Many-body nodal hypersurface and domain averages for correlated wave functions
We outline the basic notions of nodal hypersurface and domain averages for
antisymmetric wave functions. We illustrate their properties and analyze the
results for a few electron explicitly solvable cases and discuss possible
further developments
The magnetic exciton of EuS revealed by resonant inelastic x-ray scattering
We report the valence-to-core resonant inelastic x-ray scattering (RIXS) of
EuS measured at the L3 edge of Eu. The obtained data reveal two sets of
excitations: one set is composed of a hole in the S 3p bands and an electron
excited to extended Eu 5d band states, the other is made up from a hole in the
Eu 4f states and an electron in localized Eu 5d states bound to the 4f hole by
its Coulomb potential. The delocalized excitations arise from the
dipole-allowed 5d to 2p emissions, whereas the localized excitations result
from the dipole-forbidden (quadrupole-allowed) 4f to 2p emissions. Both these
emission channels have a comparable intensity thanks to a small number of
occupied 5d states (approximately 0.6) combined with a large number of occupied
4f states (seven). We identify the localized electron-hole pairs with the
"magnetic excitons" suggested in the past as an interpretation of the sharp
features seen in the optical absorption spectra. Our observations provide a
direct experimental evidence of these excitons which has been missing up to
now.Comment: 7 pages, 4 figures, supplemental material (PDF, 8 pages) added as an
ancillary fil
Transition metal oxides using quantum Monte Carlo
The transition metal-oxygen bond appears prominently throughout chemistry and
solid-state physics. Many materials, from biomolecules to ferroelectrics to the
components of supernova remnants contain this bond in some form. Many of these
materials' properties strongly depend on fine details of the TM-O bond and
intricate correlation effects, which make accurate calculations of their
properties very challenging. We present quantum Monte Carlo, an explicitly
correlated class of methods, to improve the accuracy of electronic structure
calculations over more traditional methods like density functional theory. We
find that unlike s-p type bonding, the amount of hybridization of the d-p bond
in TM-O materials is strongly dependant on electronic correlation.Comment: 20 pages, 4 figures, to appear as a topical review in J. Physics:
Condensed Matte
Strong-coupling d-wave superconductivity in PuCoGa5 probed by point-contact spectroscopy
Superconductivity is due to an attractive interaction between electrons that, below a critical temperature, drives them to form Cooper pairs and to condense into a ground state separated by an energy gap from the unpaired states. In the simplest cases, the pairing is mediated by lattice vibrations and the wavefunction of the pairs is isotropic. Less conventional pairing mechanisms can favour more exotic symmetries of the Cooper pairs. Here, we report on point-contact spectroscopy measurements in PuCoGa5, a moderate heavy-fermion superconductor with a record high critical temperature Tc=18.5 K. The results prove that the wavefunction of the paired electrons has a d-wave symmetry, with four lobes and nodes, and show that the pairing is likely to be mediated by spin fluctuations. Electronic structure calculations, which take into account the full structure of the f-orbital multiplets of Pu, provide a hint of the possible origin of these fluctuations