50 research outputs found
NMR evidence for a strong modulation of the Bose-Einstein Condensate in BaCuSiO
We present a Cu and Si NMR study of the quasi-2D coupled
spin 1/2 dimer compound BaCuSiO in the magnetic field range 13-26 T and
at temperatures as low as 50 mK. NMR data in the gapped phase reveal that below
90 K different intra-dimer exchange couplings and different gaps
( = 1.16) exist in every second plane along
the c-axis, in addition to a planar incommensurate (IC) modulation. Si
spectra in the field induced magnetic ordered phase reveal that close to the
quantum critical point at = 23.35 T the average boson density
of the Bose-Einstein condensate is strongly modulated along the
c-axis with a density ratio for every second plane
. An IC modulation of the local
density is also present in each plane. This adds new constraints for the
understanding of the 2D value = 1 of the critical exponent describing
the phase boundary
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
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
Caesium 6P fine-structure mixing and quenching induced by collisions with ground-state caesium atoms and molecules
Correlation between acoustic divergence and phylogenetic distance in soniferous European gobiids (Gobiidae; Gobius lineage)
In fish, species identity can be encoded by sounds, which have been thoroughly investigated
in European gobiids (Gobiidae, Gobius lineage). Recent evolutionary studies suggest
that deterministic and/or stochastic forces could generate acoustic differences among
related animal species, though this has not been investigated in any teleost group to date. In
the present comparative study, we analysed the sounds from nine soniferous gobiids and
quantitatively assessed their acoustic variability. Our interspecific acoustic study, incorporating
for the first time the representative acoustic signals from the majority of soniferous
gobiids, suggested that their sounds are truly species-specific (92% of sounds correctly
classified into exact species) and each taxon possesses a unique set of spectro-temporal
variables. In addition, we reconstructed phylogenetic relationships from a concatenated
molecular dataset consisting of multiple molecular markers to track the evolution of acoustic
signals in soniferous gobiids. The results of this study indicated that the genus Padogobius
is polyphyletic, since P. nigricans was nested within the Ponto-Caspian clade, while the congeneric
P. bonelli turned out to be a sister taxon to the remaining investigated soniferous
species. Lastly, by extracting the acoustic and genetic distance matrices, sound variability
and genetic distance were correlated for the first time to assess whether sound evolution follows
a similar phylogenetic pattern. The positive correlation between the sound variability
and genetic distance obtained here emphasizes that certain acoustic features from representative
sounds could carry the phylogenetic signal in soniferous gobiids. Our study was
the first attempt to evaluate the mutual relationship between acoustic variation and genetic
divergence in any teleost fish
Fine structure in the off-resonance conductance of small Coulomb blockade systems
We show how a fine, multiple-peak structure can arise in the off-resonance,
zero-bias conductance of Coulomb blockade systems. In order to understand how
this effect comes about one must abandon the orthodox, mean-field understanding
of the Coulomb blockade phenomenon and consider quantum fluctuations in the
occupation of the single-particle electronic levels. We illustrate such an
effect with a spinless Anderson-like model for multi-level systems and an
equation-of-motion method for calculating Green's functions that combines two
simple decoupling schemes.Comment: 5 pages, 3 figures, postscript file also available at
http://www.pa.uky.edu/~palacios/papers/eom.ps One figure added. Discussion of
results extende
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.
Multiband model of high Tc superconductors
We propose an extension to other high T_{c } compounds of a model introduced
earlier for YBCO. In the ''self-doped'' compounds we assume that the doping
part (namely the BiO, HgO, TlO planes in BSCCO, HBCCO, TBCCO respectively) is
metallic, which leads to a multiband model. This assumption is supported by
band structure calculations. Taking a repulsive pairing interaction between
these doping bands and the CuO_{2} bands leads to opposite signs for the order
parameter on these bands and to nodes whenever the Fermi surfaces of these
bands cross. We show that in BSCCO the low temperature dependence of the
penetration depth is reasonably accounted for. In this case the nodes are not
located near the 45^{o} direction, which makes the experimental determination
of the node locations an important test for our model. The situation in HBCCO
and TBCCO is rather analogous to BSCCO. We consider the indications given by
NMR and find that they rather favor a metallic character for the doping bands.
Finally we discuss the cases of NCCO and LSCO which are not ''self-doped'' and
where our model does not give nodes.Comment: 11 pages, revtex, 1 figure
Renormalized perturbation calculations for the single-impurity Anderson model
We illustrate the renormalized perturbation expansion method by applying it
to a single impurity Anderson model. Previously, we have shown that this
approach gives the {\it exact} leading order results for the specific heat,
spin and charge susceptibilities and leading order temperature dependence of
the resistivity for this model in the Fermi-liquid regime, when carried out to
second order in the renormalized interaction . Here we consider the
effects of higher order quasi-particle scattering and calculate the third order
contributions to the term in the impurity magnetization for the symmetric
model in a weak magnetic field . The result is asymptotically exact in the
weak coupling regime, and is very close to the exact Bethe ansatz result in the
Kondo regime. We also calculate the quasi-particle density of states in a
magnetic field, which is of interest in relation to recent experimental work on
quantum dots