421 research outputs found
Entanglement of hard-core bose gas in degenerate levels under local noise
Quantum entanglement properties of the pseudo-spin representation of the BCS model is investigated. In case of degenerate energy levels, where wave functions take a particularly simple form, spontaneous breaking of exchange symmetry under local noise is studied. Even if the Hamiltonian has the same symmetry, it is shown that there is a non-zero probability to end up with a non-symmetric final state. For small systems, total probability for symmetry breaking is found to be inversely proportional to the system size
Decoherence induced spontaneous symmetry breaking
We study time dependence of exchange symmetry properties of Bell states when
two qubits interact with local baths having identical parameters. In case of
classical noise, we consider a decoherence Hamiltonian which is invariant under
swapping the first and second qubits. We find that as the system evolves in
time, two of the three symmetric Bell states preserve their qubit exchange
symmetry with unit probability, whereas the symmetry of the remaining state
survives with a maximum probability of 0.5 at the asymptotic limit. Next, we
examine the exchange symmetry properties of the same states under local,
quantum mechanical noise which is modeled by two identical spin baths. Results
turn out to be very similar to the classical case. We identify decoherence as
the main mechanism leading to breaking of qubit exchange symmetry.Comment: 12 page
Photoinduced Changes of Reflectivity in Single Crystals of YBa2Cu3O6.5 (Ortho II)
We report measurements of the photoinduced change in reflectivity of an
untwinned single crystal of YBa2Cu3O6.5 in the ortho II structure. The decay
rate of the transient change in reflectivity is found to decrease rapidly with
decreasing temperature and, below Tc, with decreasing laser intensity. We
interpret the decay as a process of thermalization of antinodal quasiparticles,
whose rate is determined by an inelastic scattering rate of quasiparticle
pairs.Comment: 4 pages, 4 figure
Invariant quantum discord in qubit-qutrit systems under local dephasing
We investigate the dynamics of quantum discord and entanglement for a
class of mixed qubit-qutrit states assuming that only the qutrit is under the action of a dephasing channel. We demonstrate that even though the entanglement in the qubit-qutrit state disappears in a finite time interval, partial coherence left in the system enables quantum discord to remain invariant throughout the whole time evolution
Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr_2IrO_4 from time-resolved photocarrier dynamics
We perform a time-resolved optical study of Sr_2IrO_4 to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled J_(eff) = 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the Néel temperature T_N the system evolves continuously from a metal-like phase with fast (∼50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower (∼500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied J_(eff) = 1/2 band undergoes the sharpest upturn at TN, which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr_2IrO_4 to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at T_N in transport and thermodynamic measurements
Quantum correlations in a few-atom spin-1 Bose-Hubbard model
We study the thermal quantum correlations and entanglement in spin-1 Bose-Hubbard model with two and three particles. While we use negativity to calculate entanglement, more general non-classical correlations are quantified using a new measure based on a necessary and sufficient condition for zero-discord state. We demonstrate that the energy level crossings in the ground state of the system are signalled by both the behavior of thermal quantum correlations and entanglement
Band-dependent Quasiparticle Dynamics in Single Crystals of the BaKFeAs Superconductor Revealed by Pump-Probe Spectroscopy
We report on band-dependent quasiparticle dynamics in
BaKFeAs () measured using ultrafast
pump-probe spectroscopy. In the superconducting state, we observe two distinct
relaxation processes: a fast component whose decay rate increases linearly with
excitation density and a slow component with an excitation density independent
decay rate. We argue that these two components reflect the recombination of
quasiparticles in the two hole bands through intraband and interband processes.
We also find that the thermal recombination rate of quasiparticles increases
quadratically with temperature. The temperature and excitation density
dependence of the decays indicates fully gapped hole bands and nodal or very
anisotropic electron bands.Comment: 4 pages, 4 figure
Second harmonic generation as a probe of broken mirror symmetry
The notion of spontaneous symmetry breaking has been used to describe phase
transitions in a variety of physical systems. In crystalline solids, the
breaking of certain symmetries, such as mirror symmetry, is difficult to detect
unambiguously. Using 1-TaS, we demonstrate here that
rotational-anisotropy second harmonic generation (RA-SHG) is not only a
sensitive technique for the detection of broken mirror symmetry, but also that
it can differentiate between mirror symmetry-broken structures of opposite
planar chirality. We also show that our analysis is applicable to a wide class
of different materials with mirror symmetry-breaking transitions. Lastly, we
find evidence for bulk mirror symmetry-breaking in the incommensurate charge
density wave phase of 1-TaS. Our results pave the way for RA-SHG to
probe candidate materials where broken mirror symmetry may play a pivotal role
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