27 research outputs found
Focal laryngeal dystonia: diagnostics, therapeutics and novelties in neurophysiologic research
Aim of study: To present and discuss diagnostic procedures, therapeutic options and novelties in
neurophysiological research of laryngeal dystonia (LD), a rare movement disorder with an unknown cause
affecting the intrinsic muscles of the larynx. Material and methods: 1) Literature overview of the on the
standard guidelines in the diagnostics and treatment of LD; 2) The use of transcranial magnetic stimulation
(TMS) in the neurophysiologic research of LD by investigation of cortical silent period (cSP) reflecting the
intracortical inhibitory process mediated by GABAA and GABAB receptors; 3) The use of navigated TMS in
mapping the laryngeal motor cortex by investigating the duration of cortical silent period in two LD cases,
abductor and adductor LD type. Results: The study presents standard diagnostic, treatment of LD disorder, and
results of investigation by groups from School of Medicine, University of Minnesota, Minneapolis, MN, USA
and School of Medicine, University of Split, Croatia on the duration of cSP in vocal and cricothyroid muscles
in LD patients and healthy control subjects. The research groups presented norms for cSP duration in vocal
and cricothyroid muscle in healthy subjects and pointed to the altered (shortened) duration of the cSP in LD
patients (adductor type). Conclusion: The cSP measure might be useful neurophysiologic biomarker for
understanding the LD disorder. In LD, the cortical activation during phonation may not be efficiently or
effectively associated with inhibitory processes, leading to muscular dysfunction. Promising techniques such
as TMS might bring new light to the diagnosis and treatment of LD disorder
Thermopower in the strongly overdoped region of single-layer Bi2Sr2CuO6+d superconductor
The evolution of the thermoelectric power S(T) with doping, p, of
single-layer Bi2Sr2CuO6+d ceramics in the strongly overdoped region is studied
in detail. Analysis in term of drag and diffusion contributions indicates a
departure of the diffusion from the T-linear metallic behavior. This effect is
increased in the strongly overdoped range (p~0.2-0.28) and should reflect the
proximity of some topological change.Comment: 4 pages, 4 figure
Vienamečių dviskilčių piktžolių naikinimas cukrinių runkelių pasėliuose herbicidų deriniais
Optical studies of gap, hopping energies and the Anderson-Hubbard parameter in the zigzag-chain compound SrCuO2
We have investigated the electronic structure of the zigzag ladder (chain)
compound SrCuO2 combining polarized optical absorption, reflection,
photoreflectance and pseudo-dielectric function measurements with the model
calculations. These measurements yield an energy gap of 1.42 eV (1.77 eV) at
300 K along (perpendicular) to the Cu-O chains. We have found that the lowest
energy gap, the correlation gap, is temperature independent. The electronic
structure of this oxide is calculated using both the
local-spin-density-approximation with gradient correction method, and the
tight-binding theory for the correlated electrons. The calculated density of
electronic states for non-correlated and correlated electrons shows
quasi-one-dimensional character. The correlation gap values of 1.42 eV
(indirect transition) and 1.88 eV (direct transition) have been calculated with
the electron hopping parameters t = 0.30 eV (along a chain), t_yz = 0.12 eV
(between chains) and the Anderson-Hubbard repulsion on copper sites U= 2.0 eV.
We concluded that SrCuO_2 belongs to the correlated-gap insulators.Comment: 24 pages, 8 figures, to be published in Phys.Rev.
Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in -NaVO studied by far infrared spectroscopy
We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3
and 200cm-1 in polarizations of incident light parallel to a, b, and c
crystallographic axes in magnetic fields up to 33T. The triplet origin of an
excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The
magnitude of the spin gap at low temperatures is found to be magnetic field
independent at least up to 33T. All other infrared-active transitions appearing
below Tc are ascribed to zone-folded phonons. Two different dynamic
Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to
the oscillator strength of the otherwise forbidden singlet to triplet
transition. 1. The strongest singlet to triplet transition is an electric
dipole transition where the polarization of the incident light's electric field
is parallel to the ladder rungs, and is allowed by the dynamic DM interaction
created by a high frequency optical a-axis phonon. 2. In the incident light
polarization perpendicular to the ladder planes an enhancement of the singlet
to triplet transition is observed when the applied magnetic field shifts the
singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon
energy. The origin of this mechanism is the dynamic DM interaction created by
the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated
for both mechanisms using the presented theory.Comment: 21 pages, 22 figures. Version 2 with replaced fig. 18 were labels had
been los
Electronic Properties of \alpha'-NaV_2O_5
We studied electronic excitations in NaV2O5 by Raman. Three main topics are
discussed. The first is related to a broad continuum of excitations found in
the 200-1500 cm-1 range and peaked around 680 cm-1. The resonant Raman profile
of this excitation, the polarization selection rules and the presence of its
overtone in resonance conditions allowed us to conclude that the origin of this
feature is magnetic. We proposed that it arises as a result of light coupling
to multi-spinon Raman excitations. Within this scenario we also argued for a
scenario explaining the puzzling temperature dependence of the magnetic
continuum in terms of an increasing role of next nearest neighbor frustration
and in the context of a strongly fluctuating low temperature phase. The second
topic is related to the observation of a folded S = 1 magnetic mode which
displayed very clear selection rules as a function of the magnetic field
orientation. We proposed that the coupling of the photon field to this
excitation takes place via the antisymmetric, Dzyaloshinskii-Moriya (DM),
interaction which, in a simple dimer model, can also explain the observed
selection rules: no splitting or shifts for magnetic fields parallel to the DM
vector and the observation of two (upward and downward) dispersing branches for
fields perpendicular to the DM vector. Thirdly, we discuss the nature of
several new resonances seen below T_c and focus on the possibilities that they
are either folded phonons or singlet bound states of two triplet excitations.
In particular we emphasized the existence of two modes at 66 and 105 cm-1, the
first one being degenerate with one of the spin gap modes.Comment: 29 pages, 19 figure
Thermodynamics of Spin S = 1/2 Antiferromagnetic Uniform and Alternating-Exchange Heisenberg Chains
The magnetic susceptibility chi and specific heat C versus temperature T of
the spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenberg
chain are studied for the entire range 0 \leq alpha \leq 1 of the alternation
parameter alpha = J2/J1. For the uniform chain (alpha = 1), detailed
comparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemper
and Johnston are made with the asymptotically exact low-T field theory
predictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T)
are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 is
extracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the above
numerical data are obtained. We examine in detail the theoretical predictions
of Bulaevskii for chi(alpha,T) and compare them with our results. Our
experimental chi(T) and C(T) data for NaV2O5 single crystals are modeled in
detail. The chi(T) data above the spin dimerization temperature Tc = 34 K are
not in agreement with the prediction for the uniform Heisenberg chain, but can
be explained if there is a moderate ferromagnetic interchain coupling and/or if
J changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2)
K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) and
average exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) are
derived from the data. A spin pseudogap with a large magnitude \approx 0.4
Delta(0) is consistently found just above Tc, which decreases with increasing
T. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropy
change due to the transition at Tc and associated order parameter fluctuations
arise from the lattice and/or charge degrees of freedom and less than 23% from
the spin degrees of freedom.Comment: 53 two-column REVTeX pages, 50 embedded figures, 7 tables. Revisions
required due to incorrect Eq. (39) in Ref. 51 which gives the low-T
approximation for the specific heat of a S = 1/2 1D system with a spin gap;
no conclusions were changed. Additional minor revisions made. Phys. Rev. B
(in press