27,556 research outputs found
Ground state correlations and structure of odd spherical nuclei
It is well known that the Pauli principle plays a substantial role at low
energies because the phonon operators are not ideal boson operators.
Calculating the exact commutators between the quasiparticle and phonon
operators one can take into account the Pauli principle corrections. Besides
the ground state correlations due to the quasiparticle interaction in the
ground state influence the single particle fragmentation as well. In this
paper, we generalize the basic QPM equations to account for both mentioned
effects. As an illustration of our approach, calculations on the structure of
the low-lying states in Ba have been performed.Comment: 12 pages, 1 figur
Fermi surface nesting in several transition metal dichalcogenides
By means of high-resolution angle resolved photoelectron spectroscopy (ARPES)
we have studied the fermiology of 2H transition metal dichalcogenide polytypes
TaSe2, NbSe2, and Cu0.2NbS2. The tight-binding model of the electronic
structure, extracted from ARPES spectra for all three compounds, was used to
calculate the Lindhard function (bare spin susceptibility), which reflects the
propensity to charge density wave (CDW) instabilities observed in TaSe2 and
NbSe2. We show that though the Fermi surfaces of all three compounds possess an
incommensurate nesting vector in the close vicinity of the CDW wave vector, the
nesting and ordering wave vectors do not exactly coincide, and there is no
direct relationship between the magnitude of the susceptibility at the nesting
vector and the CDW transition temperature. The nesting vector persists across
the incommensurate CDW transition in TaSe2 as a function of temperature despite
the observable variations of the Fermi surface geometry in this temperature
range. In Cu0.2NbS2 the nesting vector is present despite different doping
level, which lets us expect a possible enhancement of the CDW instability with
Cu-intercalation in the CuxNbS2 family of materials.Comment: Accepted to New J. Phy
Ocean acidification reduces hardness and stiffness of the Portuguese oyster shell with impaired microstructure: a hierarchical analysis
The rapidly intensifying process of ocean acidification (OA) due to anthropogenic CO2 is not only depleting carbonate ions necessary for calcification but also causing acidosis and disrupting internal pH homeostasis in several marine organisms. These negative consequences of OA on marine calcifiers, i.e. oyster species, have been very well documented in recent studies; however, the consequences of reduced or impaired calcification on the end-product, shells or skeletons, still remain one of the major research gaps. Shells produced by marine organisms under OA are expected to show signs of dissolution, disorganized microstructure and reduced mechanical properties. To bridge this knowledge gap and to test the above hypothesis, we investigated the effect of OA on juvenile shells of the commercially important oyster species, Magallana angulata, at ecologically and climatically relevant OA levels (using pH 8.1, 7.8, 7.5, 7.2). In lower pH conditions, a drop of shell hardness and stiffness was revealed by nanoindentation tests, while an evident porous internal microstructure was detected by scanning electron microscopy. Crystallographic orientation, on the other hand, showed no significant difference with decreasing pH using electron back-scattered diffraction (EBSD). These results indicate the porous internal microstructure may be the cause of the reduction in shell hardness and stiffness. The overall decrease of shell density observed from micro-computed tomography analysis indicates the porous internal microstructure may run through the shell, thus inevitably limiting the effectiveness of the shell's defensive function. This study shows the potential deterioration of oyster shells induced by OA, especially in their early life stage. This knowledge is critical to estimate the survival and production of edible oysters in the future ocean
Geometrically Reduced Number of Protein Ground State Candidates
Geometrical properties of protein ground states are studied using an
algebraic approach. It is shown that independent from inter-monomer
interactions, the collection of ground state candidates for any folded protein
is unexpectedly small: For the case of a two-parameter Hydrophobic-Polar
lattice model for -mers, the number of these candidates grows only as .
Moreover, the space of the interaction parameters of the model breaks up into
well-defined domains, each corresponding to one ground state candidate, which
are separated by sharp boundaries. In addition, by exact enumeration, we show
there are some sequences which have one absolute unique native state. These
absolute ground states have perfect stability against change of inter-monomer
interaction potential.Comment: 9 page, 4 ps figures are include
Thermodynamical aspects of the Casimir force between real metals at nonzero temperature
We investigate the thermodynamical aspects of the Casimir effect in the case
of plane parallel plates made of real metals. The thermal corrections to the
Casimir force between real metals were recently computed by several authors
using different approaches based on the Lifshitz formula with diverse results.
Both the Drude and plasma models were used to describe a real metal. We
calculate the entropy density of photons between metallic plates as a function
of the surface separation and temperature. Some of these approaches are
demonstrated to lead to negative values of entropy and to nonzero entropy at
zero temperature depending on the parameters of the system. The conclusion is
that these approaches are in contradiction with the third law of thermodynamics
and must be rejected. It is shown that the plasma dielectric function in
combination with the unmodified Lifshitz formula is in perfect agreement with
the general principles of thermodynamics. As to the Drude dielectric function,
the modification of the zero-frequency term of the Lifshitz formula is outlined
that not to violate the laws of thermodynamics.Comment: 8pages, 4 figures; Phys. Rev. A, to appea
On classical super-radiance in Kerr-Newman-anti-de Sitter black holes
We study in detail the modes of a classical scalar field on a
Kerr-Newman-anti-de Sitter (KN-AdS) black hole. We construct sets of basis
modes appropriate to the two possible boundary conditions (``reflective'' and
``transparent'') at time-like infinity, and consider whether super-radiance is
possible. If we employ ``reflective'' boundary conditions, all modes are
non-super-radiant. On the other hand, for ``transparent'' boundary conditions,
the presence of super-radiance depends on our definition of positive frequency.
For those KN-AdS black holes having a globally time-like Killing vector, the
natural choice of positive frequency leads to no super-radiance. For other
KN-AdS black holes, there is a choice of positive frequency which gives no
super-radiance, but for other choices there will, in general, be
super-radiance.Comment: 23 pages, 3 figures, v2: minor changes, references adde
Entanglement transfer between bipartite systems
The problem of a controlled transfer of an entanglement initially encoded
into two two-level atoms that are successively sent through two single-mode
cavities is investigated. The atoms and the cavity modes form a four qubit
system and we demonstrate under which conditions the initial entanglement
encoded into the atoms can be completely transferred to other pairs of qubits.
We find that in the case of a nonzero detuning between the atomic transition
frequencies and the cavity mode frequencies, no complete transfer of the
initial entanglement is possible to any of the other pairs of qubits. In the
case of exact resonance and equal coupling strengths of the atoms to the cavity
modes, an initial maximally entangled state of the atoms can be completely
transferred to the cavity modes. The complete transfer of the entanglement is
restricted to the cavity modes only with the transfer to the other pairs being
limited to up to 50%. We have found that the complete transfer of an initial
entanglement to other pairs of qubits may take place if the initial state is
not the maximally entangled state and the atoms couple to the cavity modes with
unequal strengths. Depending on the ratio between the coupling strengths, the
optimal entanglement can be created between the atoms and one of the cavity
modes.Comment: 3 figures. Oral talk presented in CEWQO 18, Madrid 201
Energetic Components of Cooperative Protein Folding
A new lattice protein model with a four-helix bundle ground state is analyzed
by a parameter-space Monte Carlo histogram technique to evaluate the effects of
an extensive variety of model potentials on folding thermodynamics. Cooperative
helical formation and contact energies based on a 5-letter alphabet are found
to be insufficient to satisfy calorimetric and other experimental criteria for
two-state folding. Such proteinlike behaviors are predicted, however, by models
with polypeptide-like local conformational restrictions and
environment-dependent hydrogen bonding-like interactions.Comment: 11 pages, 4 postscripts figures, Phys. Rev. Lett. (in press
Bulk Tunneling at Integer Quantum Hall Transitions
The tunneling into the {\em bulk} of a 2D electron system (2DES) in strong
magnetic field is studied near the integer quantum Hall transitions. We present
a nonperturbative calculation of the tunneling density of states (TDOS) for
both Coulomb and short-ranged electron-electron interactions. In the case of
Coulomb interaction, the TDOS exhibits a 2D quantum Coulomb gap behavior,
\nu(\ve)=C_Q\ave/e^4, with a nonuniversal coefficient of quantum
mechanical origin. For short-ranged interactions, we find that the TDOS at low
bias follows \nu(\ve)/\nu (0)=1+(\ave/\ve_0)^\gamma, where is a
universal exponent determined by the scaling dimension of short-ranged
interactions.Comment: 4 pages, revtex, final version to appear in Phys. Rev. Let
Generic master equations for quasi-normal frequencies
Generic master equations governing the highly-damped quasi-normal frequencies
[QNFs] of one-horizon, two-horizon, and even three-horizon spacetimes can be
obtained through either semi-analytic or monodromy techniques. While many
technical details differ, both between the semi-analytic and monodromy
approaches, and quite often among various authors seeking to apply the
monodromy technique, there is nevertheless widespread agreement regarding the
the general form of the QNF master equations. Within this class of generic
master equations we can establish some rather general results, relating the
existence of "families" of QNFs of the form omega_{a,n} = (offset)_a + i n
(gap) to the question of whether or not certain ratios of parameters are
rational or irrational.Comment: 23 pages; V2: Minor additions, typos fixed. Matches published versio
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