1,101 research outputs found
Joint superexchange--Jahn-Teller mechanism for A-type antiferromagnetism in
We propose a mechanism for A-type antiferromagnetism in orthorombic LaMnO_3,
compatible with the large Jahn-Teller splitting inferred from structural data.
Orbital ordering resulting from Jahn-Teller distortions effectively leads to
A-type ordering (antiferromagnetic in the c axis and ferromagnetic in the ab
plane) provided the in-plane distorsion Q_2 is large enough, a condition
generally fulfilled in existing data.Comment: 4 pages Late
Phase separation and long wave-length charge instabilities in spin-orbit coupled systems
We investigate a two-dimensional electron model with Rashba spin-orbit
interaction where the coupling constant depends on the electronic
density. It is shown that this dependence may drive the system unstable towards
a long-wave length charge density wave (CDW) where the associated second order
instability occurs in close vicinity to global phase separation. For very low
electron densities the CDW instability is nesting-induced and the modulation
follows the Fermi momentum . At higher density the instability criterion
becomes independent of and the system may become unstable in a broad
momentum range. Finally, upon filling the upper spin-orbit split band, finite
momentum instabilities disappear in favor of phase separation alone. We discuss
our results with regard to the inhomogeneous phases observed at the
LaAlO/SrTiO or LaTiO/SrTiO interfaces.Comment: 6 pages, 6 figure
Phase diagrams of voltage-gated oxide interfaces with strong Rashba coupling
We propose a model for the two-dimensional electron gas formed at the
interface of oxide heterostructures that includes a Rashba spin-orbit coupling
proportional to an electric field oriented perpendicularly to the interface.
Taking into account the electron density dependence of this electric field
confining the electron gas at the interface, we report the occurrence of a
phase separation instability (signaled by a negative compressibility) for
realistic values of the spin-orbit coupling and of the electronic
band-structure parameters at zero temperature. We extend the analysis to finite
temperatures and in the presence of an in-plane magnetic field, thereby
obtaining two phase diagrams which exhibit a phase separation dome. By varying
the gating potential the phase separation dome may shrink and vanish at zero
temperature into a quantum critical point where the charge fluctuates
dynamically. Similarly the phase separation may be spoiled by a planar magnetic
field even at zero temperature leading to a line of quantum critical points.Comment: 17 pages, 17 figure
Spectroscopic evidences of quantum critical charge fluctuations in cuprates
We calculate the optical conductivity in a clean system of quasiparticles
coupled to charge-ordering collective modes. The absorption induced by these
modes may produce an anomalous frequency and temperature dependence of
low-energy optical absorption in some cuprates. However, the coupling with
lattice degrees of freedom introduces a non-universal energy scale leading to
scaling violation in low-temperature optical conductivity.Comment: Proceedings of M2S 2006. To appear in Physica
Application of In Situ Fiberization for fabrication of improved strain isolation pads and graphite epoxy composites
The feasibility of applying the in situ fiberization process to the fabrication of strain isolation pads (SIP) for the Space Shuttle and to the fabrication of graphite-epoxy composites was evaluated. The ISF process involves the formation of interconnected polymer fiber networks by agitation of dilute polymer solutions under controlled conditions. High temperature polymers suitable for SIP use were fiberized and a successful fiberization of polychloro trifluoroethylene, a relatively high melting polymer, was achieved. Attempts to fiberize polymers with greater thermal stability were unsuccessful, apparently due to characteristics caused by the presence of aromaticity in the backbone of such materials. Graphite-epoxy composites were fabricated by interconnecting two dimensional arrays of graphite fiber with polypropylene IS fibers with subsequent epoxy resin impregnation. Mechanical property tests were performed on laminated panels of this material to evaluate intralaminar and interlaminar shear strength, and thus fracture toughness. Test results were generally unpromising
Optical excitation of phase modes in strongly disordered superconductors
According to the Goldstone theorem the breaking of a continuous U(1) symmetry
comes along with the existence of low-energy collective modes. In the context
of superconductivity these excitations are related to the phase of the
superconducting (SC) order parameter and for clean systems are optically
inactive. Here we show that for strongly disordered superconductors phase modes
acquire a dipole moment and appear as a subgap spectral feature in the optical
conductivity. This finding is obtained with both a gauge-invariant random-phase
approximation scheme based on a fermionic Bogoliubov-de Gennes state as well as
with a prototypical bosonic model for disordered superconductors. In the
strongly disordered regime, where the system displays an effective granularity
of the SC properties, the optically active dipoles are linked to the isolated
SC islands, offering a new perspective for realizing microwave optical devices
Possible mechanisms of electronic phase separation in oxide interfaces
LaAlO3/SrTiO3 ad LaTiO3/SrTiO3 interfaces are known to host a strongly
inhomogeneous (nearly) two-dimensional electron gas (2DEG). In this work we
present three unconventional electronic mechanisms of electronic phase
separation (EPS) in a 2DEG as a possible source of inhomogeneity in oxide
interfaces. Common to all three mechanisms is the dependence of some
(interaction) potential on the 2DEG's density. We first consider a mechanism
resulting from a sizable density-dependent Rashba spin-orbit coupling. Next, we
point out that an EPS may also occur in the case of a density-dependent
superconducting pairing interaction. Finally, we show that the confinement of
the 2DEG to the interface by a density-dependent, self-consistent electrostatic
potential can by itself cause an EPS.Comment: 4 pages and 4 figures, Proceedings of the International Conference
"Superstripes 2014", 25-31 July 2015, Erice, Ital
Preparation and decay of a single quantum of vibration at ambient conditions
A single quantum of excitation of a mechanical oscillator is a textbook
example of the principles of quantum physics. Mechanical oscillators, despite
their pervasive presence in nature and modern technology, do not generically
exist in an excited Fock state. In the past few years, careful isolation of
GHz-frequency nano-scale oscillators has allowed experimenters to prepare such
states at milli-Kelvin temperatures. These developments illustrate the tension
between the basic predictions of quantum mechanics that should apply to all
mechanical oscillators existing even at ambient conditions, and the complex
experiments in extreme conditions required to observe those predictions. We
resolve the tension by creating a single Fock state of a vibration mode of a
crystal at room temperature using a technique that can be applied to any
Raman-active system. After exciting a bulk diamond with a femtosecond laser
pulse and detecting a Stokes-shifted photon, the 40~THz Raman-active internal
vibrational mode is prepared in the Fock state with probability.
The vibrational state is read out by a subsequent pulse, which when subjected
to a Hanbury-Brown-Twiss intensity correlation measurement reveals the
sub-Poisson number statistics of the vibrational mode. By controlling the delay
between the two pulses we are able to witness the decay of the vibrational Fock
state over its ps lifetime at room temperature. Our technique is agnostic
to specific selection rules, and should thus be applicable to any Raman-active
medium, opening a new generic approach to the experimental study of quantum
effects related to vibrational degrees of freedom in molecules and solid-state
systems
Time-dependent Gutzwiller theory of magnetic excitations in the Hubbard model
We use a spin-rotational invariant Gutzwiller energy functional to compute
random-phase-approximation-like (RPA) fluctuations on top of the Gutzwiller
approximation (GA). The method can be viewed as an extension of the previously
developed GA+RPA approach for the charge sector [G. Seibold and J. Lorenzana,
Phys. Rev. Lett. {\bf 86}, 2605 (2001)] with respect to the inclusion of the
magnetic excitations. Unlike the charge case, no assumptions about the time
evolution of the double occupancy are needed in this case. Interestingly, in a
spin-rotational invariant system, we find the correct degeneracy between
triplet excitations, showing the consistency of both computations. Since no
restrictions are imposed on the symmetry of the underlying saddle-point
solution, our approach is suitable for the evaluation of the magnetic
susceptibility and dynamical structure factor in strongly correlated
inhomogeneous systems. We present a detailed study of the quality of our
approach by comparing with exact diagonalization results and show its much
higher accuracy compared to the conventional Hartree-Fock+RPA theory. In
infinite dimensions, where the GA becomes exact for the Gutzwiller variational
energy, we evaluate ferromagnetic and antiferromagnetic instabilities from the
transverse magnetic susceptibility. The resulting phase diagram is in complete
agreement with previous variational computations.Comment: 12 pages, 8 figure
Temperature dependence of the collective mode and its influence on the band splitting in bilayer cuprates
The recently observed bilayer splitting in high-T cuprates is analyzed
within a model where the charge carriers are coupled to a phenomenological
bosonic spectrum which interpolates between the marginal Fermi liquid structure
and collective mode type behavior as a function of temperature. We argue that
the origin of the collective mode is probably associated with dynamic
incommensurate charge density waves. Moreover it is shown that the resulting
temperature dependence of the self-energy is in good agreement with
as extracted from angle-resolved photoemission data.Comment: 6 pages, 4 figures, accepted for PR
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