232 research outputs found
Dynamical Dzyaloshinsky-Moriya interaction in KCuF3: Raman evidence for an antiferrodistortive lattice instability
In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet
KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25%
and 13%) between room temperature and the characteristic temperature T_S = 50
K. In this temperature range a freezing-in of F ion dynamic displacements is
proposed to occur. In addition, the Eg mode at about 260 cm-1 clearly splits
below T_S. The width of the phonon lines above T_S follows an activated
behavior with an activation energy of about 50 K. Our observations clearly
evidence a reduction of the structural symmetry below T_S and indicate a strong
coupling of lattice and spin fluctuations for T>T_S.Comment: 7 pages, 9 figure
Topologically protected charge transfer along the edge of a chiral -wave superconductor
The Majorana fermions propagating along the edge of a topological
superconductor with pairing deliver a shot noise power of
per eV of voltage bias. We calculate the full
counting statistics of the transferred charge and find that it becomes
trinomial in the low-temperature limit, distinct from the binomial statistics
of charge- transfer in a single-mode nanowire or charge- transfer
through a normal-superconductor interface. All even-order correlators of
current fluctuations have a universal quantized value, insensitive to disorder
and decoherence. These electrical signatures are experimentally accessible,
because they persist for temperatures and voltages large compared to the
Thouless energy.Comment: 5 pages, 4 figures. v3 [post-publication]: added an appendix on the
effect of a tunnel barrier at the normal-superconductor contac
Towards a Better Understanding of the Local Attractor in Particle Swarm Optimization: Speed and Solution Quality
Particle Swarm Optimization (PSO) is a popular nature-inspired meta-heuristic
for solving continuous optimization problems. Although this technique is widely
used, the understanding of the mechanisms that make swarms so successful is
still limited. We present the first substantial experimental investigation of
the influence of the local attractor on the quality of exploration and
exploitation. We compare in detail classical PSO with the social-only variant
where local attractors are ignored. To measure the exploration capabilities, we
determine how frequently both variants return results in the neighborhood of
the global optimum. We measure the quality of exploitation by considering only
function values from runs that reached a search point sufficiently close to the
global optimum and then comparing in how many digits such values still deviate
from the global minimum value. It turns out that the local attractor
significantly improves the exploration, but sometimes reduces the quality of
the exploitation. As a compromise, we propose and evaluate a hybrid PSO which
switches off its local attractors at a certain point in time. The effects
mentioned can also be observed by measuring the potential of the swarm
Magnetic excitations and phonons in the spin-chain compound NaCu2O2
We report an inelastic light scattering study of single-crystalline
NaCuO, a spin-chain compound known to exhibit a phase with helical
magnetic order at low temperatures. Phonon excitations were studied as a
function of temperature and light polarization, and the phonon frequencies are
compared to the results of ab-initio lattice dynamical calculations, which are
also reported here. The good agreement between the observed and calculated
modes allows an assignment of the phonon eigenvectors. Two distinct high-energy
two-magnon features as well as a sharp low-energy one-magnon peak were also
observed. These features are discussed in terms of the magnon modes expected in
a helically ordered state. Their polarization dependence provides evidence of
substantial exchange interactions between two closely spaced spin chains within
a unit cell. At high temperatures, the spectral features attributable to
magnetic excitations are replaced by a broad, quasielastic mode due to
overdamped spin excitations
Dynamical lattice instability versus spin liquid state in a frustrated spin chain system
The low-dimensional s=1/2 compound (NO)[Cu(NO3)3] has recently been suggested
to follow the Nersesyan-Tsvelik model of coupled spin chains. Such a system
shows unbound spinon excitations and a resonating valence bond ground state due
spin frustration. Our Raman scattering study demonstrates phonon anomalies as
well as the suppression of a broad magnetic scattering continuum for
temperatures below a characteristic temperature, T<T*=100K. We interpret these
effects as evidence for a dynamical interplay of spin and lattice degrees of
freedom that might lead to a further transition into a dimerized or
structurally distorted phase at lower temperatures.Comment: 5 pages, 6 figure
Enhanced quasiparticle dynamics of quantum well states: the giant Rashba system BiTeI and topological insulators
In the giant Rashba semiconductor BiTeI electronic surface scattering with
Lorentzian linewidth is observed that shows a strong dependence on surface
termination and surface potential shifts. A comparison with the topological
insulator Bi2Se3 evidences that surface confined quantum well states are the
origin of these processes. We notice an enhanced quasiparticle dynamics of
these states with scattering rates that are comparable to polaronic systems in
the collision dominated regime. The Eg symmetry of the Lorentzian scattering
contribution is different from the chiral (RL) symmetry of the corresponding
signal in the topological insulator although both systems have spin-split
surface states.Comment: 6 pages, 5 figure
Low temperature mixed spin state of Co3+ in LaCoO3 evidenced from Jahn-Teller lattice distortions
One- and multi-phonon excitations of the single crystalline LaCoO3 were
studied using Raman spectroscopy in the temperature region of 5 K - 300 K.
First-order Raman spectra show a larger number of phonon modes than allowed for
the rhombohedral structure. Additional phonon modes are interpreted in terms of
activated modes due to lattice distortions, arising from the Jahn-Teller (JT)
activity of the intermediate-spin (IS) state of Co3+ ions. In particular, the
608-cm-1 stretching-type mode shows anomalous behavior in peak energy and
scattering intensity as a function of temperature. The anomalous temperature
dependence of the second-order phonon excitations spectra is in accordance with
the Franck-Condon mechanism that is characteristic for a JT orbital order.Comment: 11 pages, 9 figures, to be published in J. Low. Temp. Physic
Optical phonons, spin correlations, and spin-phonon coupling in the frustrated pyrochlore magnets CdCr2O4 and ZnCr2O4
We report on infrared, Raman, magnetic susceptibility, and specific heat
measurements on CdCr2O4 and ZnCr2O4 single crystals. We estimate the
nearest-neighbor and next-nearest neighbor exchange constants from the magnetic
susceptibility and extract the spin-spin correlation functions obtained from
the magnetic susceptibility and the magnetic contribution to the specific heat.
By comparing with the frequency shift of the infrared optical phonons above TN
, we derive estimates for the spin-phonon coupling constants in these systems.
The observation of phonon modes which are both Raman and infrared active
suggest the loss of inversion symmetry below the Neel temperature in CdCr2O4 in
agreement with theoretical predictions by Chern and coworkers [Phys. Rev. B 74,
060405 (2006)]. In ZnCr2O4 several new modes appear below TN, but no phonon
modes could be detected which are both Raman and infrared active indicating the
conservation of inversion symmetry in the low temperature phase.Comment: 11 pages, 13 figure
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