84 research outputs found
Recovering the chiral critical end-point via delocalization of quark interactions
We show that for the lower branch of the quark condensate and values higher
than approximately the chiral critical end-point in
the Nambu--Jona-Lasinio model does not occur in the phase diagram. By using
lattice motivated non-local quark interactions, we demonstrate that the
critical end-point can be recovered. We study this behavior for a range of
condensate values and find that the variation in the position of the critical
end-point is more pronounced as the condensate is increased.Comment: title changed, minor changes in text, version to match the one
published in PR
Spin dynamics of the spin-Peierls compound CuGeO_3 under magnetic field
The magnetic field--driven transition in the spin-Peierls system CuGeO_3
associated with the closing of the spin gap is investigated numerically. The
field dependence of the spin dynamical structure factor (seen by inelastic
neutron scattering) and of the momentum dependent static susceptibility are
calculated. In the dimerized phase (H<H_c), we suggest that the strong field
dependence of the transverse susceptibility could be experimentally seen from
the low temperature spin-echo relaxation rate 1/T_{2G} or the second moment of
the NMR spectrum. Above H_c low energy spin excitations appear at
incommensurate wave vectors where the longitudinal susceptibility chi_{zz}(q)
peaks.Comment: 4 pages, LaTeX, postscript figures include
Magnetic Field Effects on Quasiparticles in Strongly Correlated Local Systems
We show that quasiparticles in a magnetic field of arbitrary strength can
be described by field dependent parameters. We illustrate this approach in the
case of an Anderson impurity model and use the numerical renormalization group
(NRG) to calculate the renormalized parameters for the levels with spin
, , resonance width
and the effective local quasiparticle interaction . In the Kondo or strong correlation limit of the model the progressive
de-renormalization of the quasiparticles can be followed as the magnetic field
is increased. The low temperature behaviour, including the conductivity, in
arbitrary magnetic field can be calculated in terms of the field dependent
parameters using the renormalized perturbation expansion. Using the NRG the
field dependence of the spectral density on higher scales is also calculated.Comment: 15 pages, 17 figure
Endemic fish calling: Acoustics and reproductive behaviour of the Neretva dwarf goby Orsinigobius croaticus
The Neretva dwarf goby Orsinigobius croaticus (Gobiiformes, Gobionellidae) is an endemic
fish native to the freshwaters of the Adriatic Basin in Croatia and Bosnia and
Herzegovina, a Mediterranean Biodiversity Hotspot. Due to its limited distribution
range, specific karst habitat and endangered status, laboratory studies on reproductive
biology are scarce but crucial. Herein, we investigated the sound production and
acoustic behaviour of the endangered O. croaticus during reproductive intersexual
laboratory encounters, utilising an interdisciplinary approach. We also performed dissections
and micro-computed tomography (μCT) scanning of the pectoral girdle to
explore its potential involvement in sound production. Finally, comparative acoustic
analysis was conducted on sounds produced by previously recorded soniferous sand
gobies to investigate whether acoustic features are species-specific. The endemic
O. croaticus is a soniferous species. Males of this species emit pulsatile sounds composed
of a variable number of short (~15 ms) consecutive pulses when interacting with
females, usually during the pre-spawning phase in the nest, but also during courtship
outside the nest. Pulsatile sounds were low-frequency and short pulse trains (~140 Hz,
<1000 ms). Male visual behaviour rate was higher when co-occurring with sounds
and females entered the male's nest significantly more frequently when sounds were
present. Characteristic body movements accompanied male sound production, such
as head thrust and fin spreading. Furthermore, μCT scans and dissections suggest that
O. croaticus shares certain anatomical similarities of the pectoral girdle (i.e. osseous
elements and arrangement of levator pectoralis muscles) to previously studied sand
gobies that could be involved in sound production. Multivariate comparisons, using
sounds produced by eight soniferous European sand gobies, effectively distinguished
soniferous (and sympatric) species based on their acoustic properties. However, the
discrimination success decreased when temperature-dependent features (sound
duration and pulse repetition rate) were excluded from the analysis. Therefore, we
suggest both spectral and temporal features are important for the acoustic differentiation
of sand gobies
Multiple Magnon Modes and Consequences for the Bose-Einstein Condensed Phase in BaCuSi2O6
The compound BaCuSi2O6 is a quantum magnet with antiferromagnetic dimers of S
= 1/2 moments on a quasi-2D square lattice. We have investigated its spin
dynamics by inelastic neutron scattering experiments on single crystals with an
energy resolution considerably higher than in an earlier study. We observe
multiple magnon modes, indicating clearly the presence of magnetically
inequivalent dimer sites. This more complex spin Hamiltonian leads to a
distinct form of magnon Bose-Einstein condensate (BEC) phase with a spatially
modulated condensate amplitude.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let
Dynamic susceptibilities of the single impurity Anderson model within an enhanced non-crossing approximation
The single impurity Anderson model (SIAM) is studied within an enhanced
non-crossing approximation (ENCA). This method is extended to the calculation
of susceptibilities and thoroughly tested, also in order to prepare
applications as a building block for the calculation of susceptibilities and
phase transitions in correlated lattice systems. A wide range of model
parameters, such as impurity occupancy, temperature, local Coulomb repulsion
and hybridization strength, are studied. Results for the spin and charge
susceptibilities are presented. By comparing the static quantities to exact
Bethe ansatz results, it is shown that the description of the magnetic
excitations of the impurity within the ENCA is excellent, even in situations
with large valence fluctuations or vanishing Coulomb repulsion. The description
of the charge susceptibility is quite accurate in situations where the singly
occupied ionic configuration is the unperturbed ground state; however, it seems
to overestimate charge fluctuations in the asymmetric model at too low
temperatures. The dynamic spin excitation spectra is dominated by the
Kondo-screening of the impurity spin through the conduction band, i.e. the
formation of the local Kondo-singlet. A finite local Coulomb interaction U
leads to a drastic reduction of the charge response via processes involving the
doubly occupied impurity state. In the asymmetric model, the charge
susceptibility is enhanced for excitation energies smaller than the Kondo scale
T_K due to the influence of valence fluctuations.Comment: 16 pages, 13 figure
Caesium 6P fine-structure mixing and quenching induced by collisions with ground-state caesium atoms and molecules
Cu(2) nuclear resonance evidence for an original magnetic phase in aged 60K-superconductors RBa2Cu3O6+x (R=Tm,Y)
It is widely believed that the long-range antiferromagnetic order in the
RBa2Cu3O6+x compounds (R=Y and rare earths except of Ce, Pr, Tb) is totally
suppressed for the oxygen index x>0.4 (AFM insulator-metal transition). We
present the results of the copper NQR/NMR studies of aged RBa2Cu3O6+x (R=Tm,Y)
samples showing that a magnetic order can still be present at oxygen contents x
up to at least 0.7 and at temperatures as high as 77K.Comment: 7 pages, 6 figures. Submitted to Phys.Rev.
Size-dependent standard deviation for growth rates: empirical results and theoretical modeling
We study annual logarithmic growth rates R of various economic variables such as exports, imports, and foreign debt. For each of these variables we find that the distributions of R can be approximated by double exponential (Laplace) distributions in the central parts and power-law distributions in the tails. For each of these variables we further find a power-law dependence of the standard deviation σ(R) on the average size of the economic variable with a scaling exponent surprisingly close to that found for the gross domestic product (GDP) [Phys. Rev. Lett. 81, 3275 (1998)]. By analyzing annual logarithmic growth rates R of wages of 161 different occupations, we find a power-law dependence of the standard deviation σ(R) on the average value of the wages with a scaling exponent β≈0.14 close to those found for the growth of exports, imports, debt, and the growth of the GDP. In contrast to these findings, we observe for payroll data collected from 50 states of the USA that the standard deviation σ(R) of the annual logarithmic growth rate R increases monotonically with the average value of payroll. However, also in this case we observe a power-law dependence of σ(R) on the average payroll with a scaling exponent β≈−0.08. Based on these observations we propose a stochastic process for multiple cross-correlated variables where for each variable (i) the distribution of logarithmic growth rates decays exponentially in the central part, (ii) the distribution of the logarithmic growth rate decays algebraically in the far tails, and (iii) the standard deviation of the logarithmic growth rate depends algebraically on the average size of the stochastic variable
Possible Localized Modes in the Uniform Quantum Heisenberg Chains of Sr2CuO3
A model of mobile-bond defects is tentatively proposed to analyze the
"anomalies" observed on the NMR spectrum of the quantum Heisenberg chains of
Sr2CuO3. A bond-defect is a local change in the exchange coupling. It results
in a local alternating magnetization (LAM), which when the defect moves,
creates a flipping process of the local field seen by each nuclear spin. At low
temperature, when the overlap of the LAM becomes large, the defects form a
periodic structure, which extends over almost all the chains. In that regime,
the density of bond-defects decreases linearly with T.Comment: 4 pages + 3 figures. To appear in Physical Review
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