20 research outputs found
Electron-hole asymmetry of quantum collective excitations in high- copper oxides
We carry out a systematic study of collective spin- and charge excitations
for the canonical single-band Hubbard, --, and - models of
high-temperature copper-oxide superconductors, both on electron- and hole-doped
side of the phase diagram. Recently developed variational wave function
approach, combined with the expansion in inverse number of fermionic flavors,
is employed. All three models exhibit a substantial electron-hole asymmetry of
magnetic excitations, with a robust paramagnon emerging for hole-doping, in
agreement with available resonant inelastic -ray scattering data for the
cuprates. The - model yields additional high-energy peak in the magnetic
spectrum that is not unambiguously identified in spectroscopy. For all
considered Hamiltonians, the dynamical charge susceptibility contains a
coherent mode for both hole- and electron doping, with overall bandwidth
renormalization controlled by the on-site Coulomb repulsion. Away from the
strong-coupling limit, the antiferromagnetic ordering tendency is more
pronounced on electron-doped side of the phase diagram
Origin of longitudinal spin excitations in iron-pnictide parent compounds
We investigate longitudinal spin excitations (LSEs) as a probe of microscopic origin of magnetic ordering in parent pnictides BaFe2As2 and NaFeAs. Currently adopted interpretation of LSEs as bottom of particle-hole continuum points unambiguously toward itinerant-electron magnetism, but is difficult to reconcile with available optical measurements. We study the possibility that the LSEs originate from multi-magnon processes which are not energetically constrained by optical spectroscopy and do not sharply distinguish between local-moment and itinerant scenarios. Two mechanisms, capable of enhancing multi-magnon continuum to the level indicated by neutron scattering experiments, are proposed. The first emphasizes itinerant electrons and is based on electronic transitions between magnetically split bands, while the other relies on purely spin fluctuations close to a magnetic quantum phase transition. Electronic excitations enhance multi-magnon contribution to LSEs for small Fermi surface taking part in the SDW instability, but are insufficient to account for measured intensities. The correct order of LSEs, on the other hand, can be reproduced by the spin fluctuation mechanism for a reasonable set of parameters
Toward complementary characterization of the chemical bond
A precise discussion of a single bond requires consideration of two-particle wave function for the particles involved. Here we define and determine rigorously the intrinsic covalency and connected characteristics of the canonical example of the H2 molecule. This is achieved by starting from an analytic form for the two-particle wave function for electrons forming the bond, in which we single out the atomic contribution (atomicity) in an unequivocal manner. The presence of the atomicity and ionicity factors complements the existing attributes of the bond. In this way, a gradual evolution of the molecular state to its two-atom correspondent is traced systematically with increasing interatomic distance. In effect, a direct relation to the onset of incipient Mott-Hubbard atomicity (Mottness) to the intrinsic covalency and ionicity is established. This goal is achieved formally by combining the single-particle wave function readjustment in the entangled state with a simultaneous determination of two-particle states in the particle (second quantization) representation
Many-particle covalency, ionicity, and atomicity revisited for a few simple example molecules
We analyze two-particle binding factors of , , and molecules/ions with the help of our original exact diagonalization ab initio approach. The interelectronic correlations are taken into account rigorously within the second quantization scheme for restricted basis of renormalized single-particle wave functions, i.e., with their size readjusted in the correlated state. This allows us to determine the many-particle covalency and ionicity factors in a natural and intuitive manner in terms of the microscopic single-particle and interaction parameters, also determined within our method. We discuss the limitations of those basic characteristics and introduce the concept of atomicity, corresponding to the Mott and Hubbard criterion concerning localization threshold in many-particle systems. This addition introduces an atomic ingredient into the electron states and thus removes a spurious behavior of covalency with the increasing interatomic distance, as well as provides a more complete physical interpretation of bonding
Universal collective modes from strong electronic correlations: Modified theory with application to high- cuprates
A nonzero-temperature technique for strongly correlated electron lattice
systems, combining elements of both variational wave function (VWF) approach
and expansion in the inverse number of fermionic flavors (),
is developed. The departure point, VWF method, goes beyond the renormalized
mean-field theory and provides semi-quantitative description of principal
equilibrium properties of high- superconducting cuprates. The developed
here scheme of VWF+, in the leading order provides dynamical
spin and charge responses around the VWF solution, generalizing the
weak-coupling spin-fluctuation theory to the regime of strong correlations.
Thermodynamic corrections to the correlated saddle-point state arise
systematically at consecutive orders. Explicitly, VWF+ is
applied to evaluate dynamical response functions for the hole-doped Hubbard
model and compared with available determinant quantum-Monte-Carlo data,
yielding a good overall agreement in the regime of coherent collective-mode
dynamics. The emergence of well-defined spin and charge excitations from the
incoherent continua is explicitly demonstrated and a non-monotonic dependence
of the charge-excitation energy on the interaction magnitude is found. The
charge-mode energy saturates slowly when approaching the strong-coupling limit,
which calls for a reevaluation of the --model approach to the charge
dynamics in favor of more general -- and --- models. The
results are also related to recent inelastic resonant -ray and neutron
scattering experiments for the high- cuprates
Tuning topological superconductivity within the -- model of twisted bilayer cuprates
We carry out a theoretical study of unconventional superconductivity in
twisted bilayer cuprates as a function of electron density and layer twist
angle. The bilayer -- model is employed and analyzed within the
framework of a generalized variational wave function approach in the
statistically-consistent Gutzwiller formulation. The constructed phase diagram
encompasses both gapless -wave state (reflecting the pairing symmetry of
untwisted copper-oxides) and gapped phase that
breaks spontaneously time-reversal-symmetry (TRS) and is characterized by
nontrivial Chern number. We find that state occupies
a non-convex butterfly-shaped region in the doping vs. twist-angle plane, and
demonstrate the presence of previously unreported reentrant TRS-breaking phase
on the underdoped side of the phase diagram. This circumstance supports the
emergence of topological superconductivity for fine-tuned twist angles away
from . Our analysis of the microscopically derived Landau free energy
functional points toward sensitivity of the superconducting order parameter to
small perturbations close to the topological state boundary
Strain-induced Aharonov-Bohm effect at nanoscale and ground state of a carbon nanotube with zigzag edges
Magnetic flux piercing a carbon nanotube induce periodic gap oscillations which represent the Aharonov-Bohm effect at nanoscale. Here we point out, by analyzing numerically the anisotropic Hubbard model on a honeycomb lattice, that similar oscillations should be observable when uniaxial strain is applied to a nanotube. In both cases, a vector potential (magnetic- or strain-induced) may affect the measurable quantities at zero field. The analysis, carried out within the Gutzwiller Approximation, shows that for small semiconducting nanotube with zigzag edges and realistic value of the Hubbard repulsion (U/t = 1.6, with t = 2.5 eV being the equilibrium hopping integral) energy gap can be reduced by a factor of more than 100 due to the strain
High Temperature Superconductivity with Strong Correlations and Disorder: Possible Relevance to Cu-doped Apatite
We examine the properties of topological strongly correlated superconductor
with bond disorder on triangular lattice and demonstrate that our theoretical
(--) model exhibits some unique features of the Cu-doped apatite
. Namely, the paired state
appears only for carrier concentration and is followed by
a close-by phase separation into the superconducting and Mott insulating parts.
Furthermore, a moderate amount of the bond disorder () does not alter essentially the topology with robust Chern number
which diminishes beyond that limit. A room-temperature superconductivity is
attainable only for the exchange to hopping ratio if one takes
the bare bandwidth suggested by current DFT calculations. The admixture of
-wave pairing component is induced by the disorder. The results have been
obtained within statistically consistent variational approximation (SGA)