99 research outputs found
Lifetime of d-holes at Cu surfaces: Theory and experiment
We have investigated the hole dynamics at copper surfaces by high-resolution
angle-resolved photoemission experiments and many-body quasiparticle GW
calculations. Large deviations from a free-electron-like picture are observed
both in the magnitude and the energy dependence of the lifetimes, with a clear
indication that holes exhibit longer lifetimes than electrons with the same
excitation energy. Our calculations show that the small overlap of d- and
sp-states below the Fermi level is responsible for the observed enhancement.
Although there is qualitative good agreement of our theoretical predictions and
the measured lifetimes, there still exist some discrepancies pointing to the
need of a better description of the actual band structure of the solid.Comment: 15 pages, 7 figures, 1 table, to appear in Phys. Rev.
Orbital and spin correlations in CaSrRuO: A mean field study
The alloy CaSrRuO exhibits a complex phase diagram with
peculiar magnetic metallic phases. In this paper some aspects of this alloy are
discussed based on a mean field theory for an effective Kugel-Khomskii model of
localized orbital and spin degrees of freedom. This model results from an
orbital selective Mott transition which in the three-band system localized two
orbitals while leaving the third one itinerant. Special attention is given to
the region around a structure quantum phase transition at
where the crystal lattice changes from tetragonal to orthorhombic symmetry
while leaving the system metallic. This transition yields, a change from
ferromagnetic to antiferromagnetic spin correlations. The complete mean field
phase diagram for this transition is given including orbital and spin order.
The anisotropy of spin susceptibility, a consequence of spin-orbit coupling and
orbital correlation, is a tell-tale sign of one of these phases. In the
predominantly antiferromagnetic phase we describe a metamagnetic transition in
a magnetic field and show that coupling of the itinerant band to the localized
degrees of freedom yields an anomalous longitudinal magnetoresistance
transition. Both phenomena are connected with the evolution of the
ferromagnetic and antiferromagnetic domains in the external magnetic field and
agree qualitatively with the experimental findings.Comment: 12 pages, accepted for publication in EPJ
Hole dynamics in noble metals
We present a detailed analysis of hole dynamics in noble metals (Cu and Au),
by means of first-principles many-body calculations. While holes in a
free-electron gas are known to live shorter than electrons with the same
excitation energy, our results indicate that d-holes in noble metals exhibit
longer inelastic lifetimes than excited sp-electrons, in agreement with
experiment. The density of states available for d-hole decay is larger than
that for the decay of excited electrons; however, the small overlap between d-
and sp-states below the Fermi level increases the d-hole lifetime. The impact
of d-hole dynamics on electron-hole correlation effects, which are of relevance
in the analysis of time-resolved two-photon photoemission experiments, is also
addressed.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
Ethical considerations in on-ground applications of the ecosystem services concept
The ecosystem services (ES) concept is one of the main avenues for conveying society's dependence on natural ecosystems. On-ground applications of the concept are now widespread and diverse and include its use as a communication tool, for policy guidance and priority setting, and for designing economic instruments for conservation. Each application raises ethical considerations beyond traditional controversies related to the monetary valuation of nature. We review ethical considerations across major on-ground applications and group them into the following categories: anthropocentric framing, economic metaphor, monetary valuation, commodification, sociocultural impact, changes in motivations, and equity implications. Different applications of the ES concept raise different suites of ethical issues, and we propose methods to address the issues most relevant to each application. We conclude that the ES concept should be considered as only one among various alternative approaches to valuing nature and that reliance on economic metaphors can exclude other motivations for protecting ecosystems
Symmetry-Breaking Phase Transition without Peierls Mechanism in Conducting Monoatomic Chains
The one-dimensional (1D) model system Au/Ge(001), consisting of linear chains
of single atoms on a surface, is scrutinized for lattice instabilities
predicted in the Peierls paradigm. By scanning tunneling microscopy and
electron diffraction we reveal a second-order phase transition at 585 K. It
leads to charge ordering with transversal and vertical displacements and
complex interchain correlations. However, the structural phase transition is
not accompanied by the electronic signatures of a charge density wave, thus
precluding a Peierls instability as origin. Instead, this symmetry-breaking
transition exhibits three-dimensional critical behavior. This reflects a
dichotomy between the decoupled 1D electron system and the structural elements
that interact via the substrate. Such substrate-mediated coupling between the
wires thus appears to have been underestimated also in related chain systems.Comment: 5 pages, 4 figures, accepted at Physical Review Letters 09/201
Phenomenological theory of the 3 Kelvin phase in Sr2RuO4
We model the 3K-phase of Sr2RuO4 with Ru-metal inclusion as interface state
with locally enhanced transition temperatures. The resulting 3K-phase must have
a different pairing symmetry than the bulk phase of Sr2RuO4, because the
symmetry at the interface is lower than in the bulk. It is invariant under time
reversal and a second transition, in general, above the onset of bulk
superconductivity is expected where time reversal symmetry is broken. The
nucleation of the 3K-phase exhibits a ``capillary effect'' which can lead to
frustration phenomena for the superconducting states on different
Ru-inclusions. Furthermore, the phase structure of the pair wave function gives
rise to zero-energy quasiparticle states which would be visible in
quasiparticle tunneling spectra. Additional characteristic properties are
associated with the upper critical field Hc2. The 3K-phase has a weaker
anisotropy of Hc2 between the inplane and z-axis orientation than the bulk
superconducting phase. This is connected with the more isotropic nature
Ru-metal which yields a stronger orbital depairing effect for the inplane
magnetic field than in the strongly layered Sr$_2RuO4. An anomalous temperature
dependence for the z-axis critical field is found due to the coupling of the
magnetic field to the order parameter texture at the interface. Various other
experiments are discussed and new measurements are suggested.Comment: 10 pages, 5 figure
Magnetic Phase Diagram of Ca2-xSrxRuO4 Governed by Structural Distortions
We constructed, by the first-principles calculations, a magnetic phase
diagram of SrRuO in the space spanned by structural distortions. Our
phase diagram can qualitatively explain the experimental one for
CaSrRuO. We found that the rotation and the tilting of RuO
octahedron are responsible for the ferro- and antiferro-magnetism,
respectively, while the flattening of RuO is the key factor to stabilize
those magnetic ground states. Our results imply that the magnetic and the
structural instabilities in SrRuO are closely correlated cooperatively
rather than competitively.Comment: 3 figures; accepted by PRB as rapid communicatio
Dynamics of Excited Electrons in Copper: Role of Auger Electrons
Within a theoretical model based on the Boltzmann equation, we analyze in
detail the structure of the unusual peak recently observed in the relaxation
time in Cu. In particular, we discuss the role of Auger electrons in the
electron dynamics and its dependence on the d-hole lifetime, the optical
transition matrix elements and the laser pulse duration. We find that the Auger
contribution to the distribution is very sensitive to both the d-hole lifetime
tau_h and the laser pulse duration tau_l and can be expressed as a monotonic
function of tau_l/tau_h. We have found that for a given tau_h, the Auger
contribution is significantly smaller for a short pulse duration than for a
longer one. We show that the relaxation time at the peak depends linearly on
the d-hole lifetime, but interestingly not on the amount of Auger electrons
generated. We provide a simple expression for the relaxation time of excited
electrons which shows that its shape can be understood by a phase space
argument and its amplitude is governed by the d-hole lifetime. We also find
that the height of the peak depends on both the ratio of the optical transition
matrix elements R=|M_{d \to sp}|^2/|M_{sp \to sp}|^2 and the laser pulse
duration. Assuming a reasonable value for the ratio, namely R = 2, and a d-hole
lifetime of tau_h=35 fs, we obtain for the calculated height of the peak Delta
tau_{th}=14 fs, in fair agreement with Delta tau_{exp} \approx 17 fs measured
for polycrystalline Cu.Comment: 6 pages, 6 figure
Atomic-scale images of charge ordering in a mixed-valence manganite
Transition-metal perovskite oxides exhibit a wide range of extraordinary but
imperfectly understood phenomena. Charge, spin, orbital, and lattice degrees of
freedom all undergo order-disorder transitions in regimes not far from where
the best-known of these phenomena, namely high-temperature superconductivity of
the copper oxides, and the 'colossal' magnetoresistance of the manganese
oxides, occur. Mostly diffraction techniques, sensitive either to the spin or
the ionic core, have been used to measure the order. Unfortunately, because
they are only weakly sensitive to valence electrons and yield superposition of
signals from distinct mesoscopic phases, they cannot directly image mesoscopic
phase coexistence and charge ordering, two key features of the manganites. Here
we describe the first experiment to image charge ordering and phase separation
in real space with atomic-scale resolution in a transition metal oxide. Our
scanning tunneling microscopy (STM) data show that charge order is correlated
with structural order, as well as with whether the material is locally metallic
or insulating, thus giving an atomic-scale basis for descriptions of the
manganites as mixtures of electronically and structurally distinct phases.Comment: 8 pages, 4 figures, 19 reference
Photoemission and x-ray absorption study of MgC_(1-x)Ni_3
We investigated electronic structure of MgC_(1-x)Ni_3 with photoemission and
x-ray absorption spectroscopy. Both results show that overall band structure is
in reasonable agreement with band structure calculations including the
existence of von Hove singularity (vHs)near E_F. However, we find that the
sharp vHs peak theoretically predicted near the E_F is substantially
suppressed. As for the Ni core level and absorption spectrum, there exist the
satellites of Ni 2p which have a little larger energy separation and reduced
intensity compared to the case of Ni-metal. These facts indicate that
correlation effects among Ni 3d electrons may be important to understand
various physical properties.Comment: 12 pages, 4 figure
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