855 research outputs found
Theory of magnetism and metal-insulator transition in layered perovskite iridates
We investigate the metal-insulator transition in the layered Ruddelsden
Popper series of strontium iridates Srn+1IrnO3n+1. Tight-binding models of t2g
orbitals for n = 1, 2, and infinity are constructed, and changes in band
dispersion due to dimensionality and spin-orbit coupling are presented.
Identifying the states near the Fermi level to be predominantly Jeff = 1/2, we
use an effective Hubbard model to study the effect of correlations. Transitions
from a metallic state to various magnetically ordered states at different
critical interactions are obtained. A canted antiferromagnetic insulator is
found for Sr2IrO4, a c-axis collinear antiferromagnetic insulator for Sr3Ir2O7,
and non-coplanar canted antiferromagnetic insulator via magnetic metal for
SrIrO3. We derive the strong-coupling spin-model and compare the magnetic
ordering patterns obtained in the weak and strong coupling limits. We find that
they are identical, indicating that magnetic ordering is not sufficient to
justify Mott physics in this series of iridates.Comment: 11 pages, 18 figure
Canonical representation for electrons and its application to the Hubbard model
A new representation for electrons is introduced, in which the electron
operators are written in terms of a spinless fermion and the Pauli operators.
This representation is canonical, invertible and constraint-free. Importantly,
it simplifies the Hubbard interaction. On a bipartite lattice, the Hubbard
model is reduced to a form in which the exchange interaction emerges simply by
decoupling the Pauli subsystem from the spinless fermion bath. This exchange
correctly reproduces the large superexchange. Also derived, for
, is the Hamiltonian to study Nagaoka ferromagnetism. In this
representation, the infinite- Hubbard problem becomes elegant and easier to
handle. Interestingly, the ferromagnetism in Hubbard model is found to be
related to the gauge invariance of the spinless fermions. Generalization of
this representation for the multicomponent fermions, a new representation for
bosons, the notion of a `soft-core' fermion, and some interesting unitary
transformations are introduced and discussed in the appendices.Comment: 10+ pages, 3 Figure
Pairing Fluctuations Determine Low Energy Electronic Spectra in Cuprate Superconductors
We describe here a minimal theory of tight binding electrons moving on the
square planar Cu lattice of the hole-doped cuprates and mixed quantum
mechanically with pairs of them (Cooper pairs). Superconductivity occurring at
the transition temperature T_c is the long-range, d-wave symmetry phase
coherence of these Cooper pairs. Fluctuations necessarily associated with
incipient long-range superconducting order have a generic large distance
behaviour near T_c. We calculate the spectral density of electrons coupled to
such Cooper pair fluctuations and show that features observed in Angle Resolved
Photo Emission Spectroscopy (ARPES) experiments on different cuprates above T_c
as a function of doping and temperature emerge naturally in this description.
These include `Fermi arcs' with temperature-dependent length and an antinodal
pseudogap which fills up linearly as the temperature increases towards the
pseudogap temperature. Our results agree quantitatively with experiment. Below
T_c, the effects of nonzero superfluid density and thermal fluctuations are
calculated and compared successfully with some recent ARPES experiments,
especially the observed `bending' or deviation of the superconducting gap from
the canonical d-wave form.Comment: 14 pages, 8 figures (to appear in Phys. Rev. B
Electrically tunable g-factors in quantum dot molecular spin states
We present a magneto-photoluminescence study of individual vertically stacked
InAs/GaAs quantum dot pairs separated by thin tunnel barriers. As an applied
electric field tunes the relative energies of the two dots, we observe a strong
resonant increase or decrease in the g-factors of different spin states that
have molecular wavefunctions distributed over both quantum dots. We propose a
phenomenological model for the change in g-factor based on resonant changes in
the amplitude of the wavefunction in the barrier due to the formation of
bonding and antibonding orbitals.Comment: 5 pages, 5 figures, Accepted by Phys. Rev. Lett. New version reflects
response to referee comment
Planar spin exchange in LiNiO_2
We study the planar spin exchange couplings in LiNiO2 using a perturbative
approach. We show that the inclusion of the trigonal crystal field splitting at
the Oxygen sites leads to the appearance of antiferromagnetic exchange
integrals in deviation from the Goodenough-Kanamori-Anderson rules for this 90
degree bond. That gives a microscopic foundation for the recently observed
coexistence of ferromagnetic and antiferromagnetic couplings in the
orbitally-frustrated state of LiNiO2. (F. Reynaud et al, Phys. Rev. Lett. 86,
3638 (2001))Comment: latex, revtex4, 6 pages, 3 figure
Spin Fine Structure in Optically Excited Quantum Dot Molecules
The interaction between spins in coupled quantum dots is revealed in distinct
fine structure patterns in the measured optical spectra of InAs/GaAs double
quantum dot molecules containing zero, one, or two excess holes. The fine
structure is explained well in terms of a uniquely molecular interplay of spin
exchange interactions, Pauli exclusion and orbital tunneling. This knowledge is
critical for converting quantum dot molecule tunneling into a means of
optically coupling not just orbitals, but spins.Comment: 10 pages, 7 figures, added material, (published
Reentrant behavior of the phase stiffness in Josephson junction arrays
The phase diagram of a 2D Josephson junction array with large substrate
resistance, described by a quantum XY model, is studied by means of Fourier
path-integral Monte Carlo. A genuine Berezinskii-Kosterlitz-Thouless transition
is found up to a threshold value g* of the quantum coupling, beyond which no
phase coherence is established. Slightly below g* the phase stiffness shows a
reentrant behavior with temperature, in connection with a low-temperature
disappearance of the superconducting phase, driven by strong nonlinear quantum
fluctuations.Comment: 4 pages, 7 figures, to appear in Phys.Rev.Let
Critical packing in granular shear bands
In a realistic three-dimensional setup, we simulate the slow deformation of
idealized granular media composed of spheres undergoing an axisymmetric
triaxial shear test. We follow the self-organization of the spontaneous strain
localization process leading to a shear band and demonstrate the existence of a
critical packing density inside this failure zone. The asymptotic criticality
arising from the dynamic equilibrium of dilation and compaction is found to be
restricted to the shear band, while the density outside of it keeps the memory
of the initial packing. The critical density of the shear band depends on
friction (and grain geometry) and in the limit of infinite friction it defines
a specific packing state, namely the \emph{dynamic random loose packing}
Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft x-ray angle-resolved photoemission spectroscopy
We present a soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES)
study of the stoichiometric pnictide superconductor LaRu2P2. The observed
electronic structure is in good agreement with density functional theory (DFT)
calculations. However, it is significantly different from its counterpart in
high-temperature superconducting Fe-pnictides. In particular the bandwidth
renormalization present in the Fe-pnictides (~2 - 3) is negligible in LaRu2P2
even though the mass enhancement is similar in both systems. Our results
suggest that the superconductivity in LaRu2P2 has a different origin with
respect to the iron pnictides. Finally we demonstrate that the increased
probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is
essential in determining the bulk electronic structure in the experiment.Comment: 4 pages, 4 figures, 1 supplemental material. Accepted for publication
in Physical Review Letter
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