162 research outputs found
Photoemission kinks and phonons in cuprates
One of the possible mechanisms of high Tc superconductivity is Cooper pairing
with the help of bosons, which change the slope of the electronic dispersion as
observed by photoemission. Giustino et al. calculated that in the high
temperature superconductor La1.85Sr0.15CuO4 crystal lattice vibrations
(phonons) should have a negligible effect on photoemission spectra and
concluded that phonons do not play an important role. We show that the
calculations employed by Giustino et al. fail to reproduce huge influence of
electron-phonon coupling on important phonons observed in experiments. Thus one
would expect these calculations to similarly fail in explaining the role of
electron-phonon coupling for the electronic dispersion.Comment: To appear in Nature as a Brief Communiction Arisin
Interplay between electron-phonon and Coulomb interactions in cuprates
Evidence for strong electron-phonon coupling in high-Tc cuprates is reviewed,
with emphasis on the electron and phonon spectral functions. Effects due to the
interplay between the Coulomb and electron-phonon interactions are studied. For
weakly doped cuprates, the phonon self-energy is strongly reduced due to
correlation effects, while there is no corresponding strong reduction for the
electron self-energy. Polaron formation is studied, focusing on effects of
Coulomb interaction and antiferromagnetic correlations. It is argued that
experimental indications of polaron formation in undoped cuprates are due to a
strong electron-phonon interaction for these systems.Comment: 43 pages and 22 figure
Carrier-mediated magnetoelectricity in complex oxide heterostructures
While tremendous success has been achieved to date in creating both single
phase and composite magnetoelectric materials, the quintessential
electric-field control of magnetism remains elusive. In this work, we
demonstrate a linear magnetoelectric effect which arises from a novel
carrier-mediated mechanism, and is a universal feature of the interface between
a dielectric and a spin-polarized metal. Using first-principles density
functional calculations, we illustrate this effect at the SrRuO/SrTiO
interface and describe its origin. To formally quantify the magnetic response
of such an interface to an applied electric field, we introduce and define the
concept of spin capacitance. In addition to its magnetoelectric and spin
capacitive behavior, the interface displays a spatial coexistence of magnetism
and dielectric polarization suggesting a route to a new type of interfacial
multiferroic
Giant phonon anomalies and central peak due to charge density wave formation in YBaCuO
The electron-phonon interaction is a major factor influencing the competition
between collective instabilities in correlated-electron materials, but its role
in driving high-temperature superconductivity in the cuprates remains poorly
understood. We have used high-resolution inelastic x-ray scattering to monitor
low-energy phonons in YBaCuO (superconducting
K), which is close to a charge density wave (CDW) instability. Phonons in a
narrow range of momentum space around the CDW ordering vector exhibit extremely
large superconductivity-induced lineshape renormalizations. These results imply
that the electron-phonon interaction has sufficient strength to generate
various anomalies in electronic spectra, but does not contribute significantly
to Cooper pairing. In addition, a quasi-elastic "central peak" due to CDW
nanodomains is observed in a wide temperature range above and below ,
suggesting that the gradual onset of a spatially inhomogeneous CDW domain state
with decreasing temperature is a generic feature of the underdoped cuprates
An ARPES view on the high-Tc problem: phonons vs spin-fluctuations
We review the search for a mediator of high-Tc superconductivity focusing on
ARPES experiment. In case of HTSC cuprates, we summarize and discuss a
consistent view of electronic interactions that provides natural explanation of
both the origin of the pseudogap state and the mechanism for high temperature
superconductivity. Within this scenario, the spin-fluctuations play a decisive
role in formation of the fermionic excitation spectrum in the normal state and
are sufficient to explain the high transition temperatures to the
superconducting state while the pseudogap phenomenon is a consequence of a
Peierls-type intrinsic instability of electronic system to formation of an
incommensurate density wave. On the other hand, a similar analysis being
applied to the iron pnictides reveals especially strong electron-phonon
coupling that suggests important role of phonons for high-Tc superconductivity
in pnictides.Comment: A summary of the ARPES part of the Research Unit FOR538,
http://for538.wmi.badw.d
Electron-Phonon Interactions in Graphene, Bilayer Graphene, and Graphite
Using first-principles techniques, we calculate the renormalization of the
electron Fermi velocity and the vibrational lifetimes arising from
electron-phonon interactions in doped bilayer graphene and in graphite and
compare the results with the corresponding quantities in graphene. For similar
levels of doping, the Fermi velocity renormalization in bilayer graphene and in
graphite is found to be approximately 30% larger than that in graphene. In the
case of bilayer graphene, this difference is shown to arise from the interlayer
interaction. We discuss our findings in the light of recent photoemission and
Raman spectroscopy experiments.Comment: 6 pages, 4 figure
Small Polarons in Transition Metal Oxides
The formation of polarons is a pervasive phenomenon in transition metal oxide
compounds, with a strong impact on the physical properties and functionalities
of the hosting materials. In its original formulation the polaron problem
considers a single charge carrier in a polar crystal interacting with its
surrounding lattice. Depending on the spatial extension of the polaron
quasiparticle, originating from the coupling between the excess charge and the
phonon field, one speaks of small or large polarons. This chapter discusses the
modeling of small polarons in real materials, with a particular focus on the
archetypal polaron material TiO2. After an introductory part, surveying the
fundamental theoretical and experimental aspects of the physics of polarons,
the chapter examines how to model small polarons using first principles schemes
in order to predict, understand and interpret a variety of polaron properties
in bulk phases and surfaces. Following the spirit of this handbook, different
types of computational procedures and prescriptions are presented with specific
instructions on the setup required to model polaron effects.Comment: 36 pages, 12 figure
Strength of the Spin-Fluctuation-Mediated Pairing Interaction in a High-Temperature Superconductor
Theories based on the coupling between spin fluctuations and fermionic
quasiparticles are among the leading contenders to explain the origin of
high-temperature superconductivity, but estimates of the strength of this
interaction differ widely. Here we analyze the charge- and spin-excitation
spectra determined by angle-resolved photoemission and inelastic neutron
scattering, respectively, on the same crystals of the high-temperature
superconductor YBa2Cu3O6.6. We show that a self-consistent description of both
spectra can be obtained by adjusting a single parameter, the spin-fermion
coupling constant. In particular, we find a quantitative link between two
spectral features that have been established as universal for the cuprates,
namely high-energy spin excitations and "kinks" in the fermionic band
dispersions along the nodal direction. The superconducting transition
temperature computed with this coupling constant exceeds 150 K, demonstrating
that spin fluctuations have sufficient strength to mediate high-temperature
superconductivity.Comment: 25 pages, 7 figures, including supplementary information, accepted
for publication in Nature Physic
STANJE I PERSPEKTIVA RAZVOJA VINOGRADARSTVA I VINARSTVA ISTRE I HRVATSKOG PRIMORJA
Strong many-body interactions in solids yield a host of fascinating and potentially useful physical properties. Here, from angle-resolved photoemission experiments and ab initio many-body calculations, we demonstrate how a strong coupling of conduction electrons with collective plasmon excitations of their own Fermi sea leads to the formation of plasmonic polarons in the doped ferromagnetic semiconductor EuO. We observe how these exhibit a significant tunability with charge carrier doping, leading to a polaronic liquid that is qualitatively distinct from its more conventional lattice-dominated analogue. Our study thus suggests powerful opportunities for tailoring quantum many-body interactions in solids via dilute charge carrier doping
Phase diagram of the two-dimensional Hubbard-Holstein model
The electron\u2013electron and electron\u2013phonon interactions play an important role in correlated materials, being key features for spin, charge and pair correlations. Thus, here we investigate their effects in strongly correlated systems by performing unbiased quantum Monte Carlo simulations in the square lattice Hubbard-Holstein model at half-filling. We study the competition and interplay between antiferromagnetism (AFM) and charge-density wave (CDW), establishing its very rich phase diagram. In the region between AFM and CDW phases, we have found an enhancement of superconducting pairing correlations, favouring (nonlocal) s-wave pairs. Our study sheds light over past inconsistencies in the literature, in particular the emergence of CDW in the pure Holstein model case
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