201 research outputs found
Interplay of charge, spin and lattice degrees of freedom on the spectral properties of the one-dimensional Hubbard-Holstein model
We calculate the spectral function of the one dimensional Hubbard-Holstein
model using the time dependent Density Matrix Renormalization Group (tDMRG),
focusing on the regime of large local Coulomb repulsion, and away from
electronic half-filling. We argue that, from weak to intermediate
electron-phonon coupling, phonons interact only with the electronic charge, and
not with the spin degrees of freedom. For strong electron-phonon interaction,
spinon and holon bands are not discernible anymore and the system is well
described by a spinless polaronic liquid. In this regime, we observe multiple
peaks in the spectrum with an energy separation corresponding to the energy of
the lattice vibrations (i.e., phonons). We support the numerical results by
introducing a well controlled analytical approach based on Ogata-Shiba's
factorized wave-function, showing that the spectrum can be understood as a
convolution of three contributions, originating from charge, spin, and lattice
sectors. We recognize and interpret these signatures in the spectral properties
and discuss the experimental implications.Comment: 8 pages, 7 figure
Behavior of quantum entropies in polaronic systems
Quantum entropies and state distances are analyzed in polaronic systems with
short range (Holstein model) and long range (Frhlich model)
electron-phonon coupling. These quantities are extracted by a variational wave
function which describes very accurately polaron systems with arbitrary size in
all the relevant parameter regimes. With the use of quantum information tools,
the crossover region from weak to strong coupling regime can be characterized
with high precision. Then, the linear entropy is found to be very sensitive to
the range of the electron-phonon coupling and the adiabatic ratio. Finally, the
entanglement entropy is studied as a function of the system size pointing out
that it not bounded, but scales as the logarithm of the size either for weak
electron-phonon coupling or for short range interaction. This behavior is
ascribed to the peculiar coupling induced by the single electron itinerant
dynamics on the phonon subsystem.Comment: 4 figures, to be published in Phys. Rev.
Dynamic nuclear polarisation in biased quantum wires with spin-orbit interaction
We propose a new method for dynamic nuclear polarisation in a quasi
one-dimensional quantum wire utilising the spin-orbit interaction, the
hyperfine interaction, and a finite source-drain potential difference. In
contrast with current methods, our scheme does not rely on external magnetic or
optical sources which makes independent control of closely placed devices much
more feasible. Using this method, a significant polarisation of a few per cent
is possible in currently available InAs wires which may be detected by
conductance measurements. This may prove useful for nuclear-magnetic-resonance
studies in nanoscale systems as well as in spin-based devices where external
magnetic and optical sources will not be suitable.Comment: 6 pages, published versio
Pseudo-Automorphisms of positive entropy on the blowups of products of projective spaces
We use a concise method to construct pseudo-automorphisms f_n of the first
dynamical degree d_1(f_n) > 1 on the blowups of the projective n-space for all
n > 1 and more generally on the blowups of products of projective spaces. These
f_n, for n = 3 have positive entropy, and for n > 3 seem to be the first
examples of pseudo-automorphisms with d_1(f_n) > 1 (and of non-product type) on
rational varieties of higher dimensions.Comment: Mathematische Annalen (to appear
Rashba quantum wire: exact solution and ballistic transport
The effect of Rashba spin-orbit interaction in quantum wires with hard-wall
boundaries is discussed. The exact wave function and eigenvalue equation are
worked out pointing out the mixing between the spin and spatial parts. The
spectral properties are also studied within the perturbation theory with
respect to the strength of the spin-orbit interaction and diagonalization
procedure. A comparison is done with the results of a simple model, the
two-band model, that takes account only of the first two sub-bands of the wire.
Finally, the transport properties within the ballistic regime are analytically
calculated for the two-band model and through a tight-binding Green function
for the entire system. Single and double interfaces separating regions with
different strengths of spin-orbit interaction are analyzed injecting carriers
into the first and the second sub-band. It is shown that in the case of a
single interface the spin polarization in the Rashba region is different from
zero, and in the case of two interfaces the spin polarization shows
oscillations due to spin selective bound states
Spin-orbital polarization of Majorana edge states in oxides nanowires
We investigate a paradigmatic case of topological superconductivity in a
one-dimensional nanowire with orbitals and a strong interplay of
spin-orbital degrees of freedom due to the competition of orbital Rashba
interaction, atomic spin-orbit coupling, and structural distortions. We
demonstrate that the resulting electronic structure exhibits an orbital
dependent magnetic anisotropy which affects the topological phase diagram and
the character of the Majorana bound states (MBSs). The inspection of the
electronic component of the MBSs reveals that the spin-orbital polarization
generally occurs along the direction of the applied Zeeeman magnetic field, and
transverse to the magnetic and orbital Rashba fields. The competition of
symmetric and antisymmetric spin-orbit coupling remarkably leads to a
misalignment of the spin and orbital moments transverse to the orbital Rashba
fields, whose manifestation is essentially orbital dependent. The behavior of
the spin-orbital polarization along the applied Zeeman field reflects the
presence of multiple Fermi points with inequivalent orbital character in the
normal state. Additionally, the response to variation of the electronic
parameters related with the degree of spin-orbital entanglement leads to
distinctive evolution of the spin-orbital polarization of the MBSs. These
findings unveil novel paths to single-out hallmarks relevant for the
experimental detection of MBSs.Comment: 14 pages, 8 figure
Modelling of strain effects in manganite films
Thickness dependence and strain effects in films of
perovskites are analyzed in the colossal magnetoresistance regime. The
calculations are based on a generalization of a variational approach previously
proposed for the study of manganite bulk. It is found that a reduction in the
thickness of the film causes a decrease of critical temperature and
magnetization, and an increase of resistivity at low temperatures. The strain
is introduced through the modifications of in-plane and out-of-plane electron
hopping amplitudes due to substrate-induced distortions of the film unit cell.
The strain effects on the transition temperature and transport properties are
in good agreement with experimental data only if the dependence of the hopping
matrix elements on the bond angle is properly taken into account.
Finally variations of the electron-phonon coupling linked to the presence of
strain turn out important in influencing the balance of coexisting phases in
the filmComment: 7 figures. To be published on Physical Review
Multiple double-exchange mechanism by Mn-doping in manganite compounds
Double-exchange mechanisms in REAEMnO manganites (where
RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies
on the strong exchange interaction between two Mn and Mn ions
through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions
has ever been considered "silent" with respect to the DE conducting mechanisms.
Here we show that a new path for DE-mechanism is indeed possible by partially
replacing the RE-AE elements by Mn-ions, in La-deficient
LaMnO thin films. X-ray absorption spectroscopy demonstrated
the relevant presence of Mn ions, which is unambiguously proved to be
substituted at La-site by Resonant Inelastic X-ray Scattering. Mn is
proved to be directly correlated to the enhanced magneto-transport properties
because of an additional hopping mechanism trough interfiling Mn-ions,
theoretically confirmed by calculations within the effective single band model.
The very idea to use Mn both as a doping element and an ions
electronically involved in the conduction mechanism, has never been foreseen,
revealing a new phenomena in transport properties of manganites. More
important, such a strategy might be also pursed in other strongly correlated
materials.Comment: 6 pages, 5 figure
Interplay between charge-lattice interaction and strong electron correlations in cuprates: phonon anomaly and spectral kinks
We investigate the interplay between strong electron correlations and
charge-lattice interaction in cuprates. The coupling between half breathing
bond stretching phonons and doped holes in the t-t'-J model is studied by
limited phonon basis exact diagonalization method. Nonadiabatic electron-phonon
interaction leads to the splitting of the phonon spectral function at half-way
to the zone boundary at and
to low energy kink feature in the electron dispersion, in agreement with
experimental observations. Another kink due to strong electron correlation
effects is observed at higher energy, depending on the strength of the
charge-lattice coupling.Comment: 4 pages, 3 figure
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