2,232 research outputs found
Axial anomaly and magnetism of nuclear and quark matter
We consider the response of the QCD ground state at finite baryon density to
a strong magnetic field B. We point out the dominant role played by the
coupling of neutral Goldstone bosons, such as pi^0, to the magnetic field via
the axial triangle anomaly. We show that, in vacuum, above a value of B ~
m_pi^2/e, a metastable object appears - the pi^0 domain wall. Because of the
axial anomaly, the wall carries a baryon number surface density proportional to
B. As a result, for B ~ 10^{19} G a stack of parallel pi^0 domain walls is
energetically more favorable than nuclear matter at the same density.
Similarly, at higher densities, somewhat weaker magnetic fields of order B ~
10^{17}-10^{18} G transform the color-superconducting ground state of QCD into
new phases containing stacks of axial isoscalar (eta or eta') domain walls. We
also show that a quark-matter state known as ``Goldstone current state,'' in
which a gradient of a Goldstone field is spontaneously generated, is
ferromagnetic due to the axial anomaly. We estimate the size of the fields
created by such a state in a typical neutron star to be of order
10^{14}-10^{15} G.Comment: 18 pages, v2: added a discussion of the energy cost of neutralizing
the domain wall charg
ZnSe/GaAs(001) heterostructures with defected interfaces: structural, thermodynamic and electronic properties
We have performed accurate \emph{ab--initio} pseudopotential calculations for
the structural and electronic properties of ZnSe/GaAs(001) heterostructures
with interface configurations accounting for charge neutrality prescriptions.
Beside the simplest configurations with atomic interdiffusion we consider also
some configurations characterized by As depletion and cation vacancies,
motivated by the recent successfull growth of ZnSe/GaAs pseudomorphic
structures with minimum stacking fault density characterized by the presence of
a defected (Zn,Ga)Se alloy in the interface region. We find that--under
particular thermodynamic conditions--some defected configurations are favoured
with respect to undefected ones with simple anion or cation mixing, and that
the calculated band offsets for some defected structures are compatible with
those measured. Although it is not possible to extract indications about the
precise interface composition and vacancy concentration, our results support
the experimental indication of (Zn,Ga)Se defected compounds in high-quality
ZnSe/GaAs(001) heterojunctions with low native stacking fault density. The
range of measured band offset suggests that different atoms at interfaces
rearrange, with possible presence of vacancies, in such a way that not only
local charges but also ionic dipoles are vanishing.Comment: 26 pages. 5 figures, revised version, in press (Physical Review B
Mott-Superfluid transition in bosonic ladders
We show that in a commensurate bosonic ladder, a quantum phase transition
occurs between a Mott insulator and a superfluid when interchain hopping
increases. We analyse the properties of such a transition as well as the
physical properties of the two phases. We discuss the physical consequences for
experimental systems such as Josephson Junction arrays.Comment: 4 pages, 2 figures, revtex
New Pseudo-Phase Structure for -Pu
In this paper we propose a new pseudo-phase crystal structure, based on an
orthorhombic distortion of the diamond structure, for the ground-state
-phase of plutonium. Electronic-structure calculations in the
generalized-gradient approximation give approximately the same total energy for
the two structures. Interestingly, our new pseudo-phase structure is the same
as the Pu -phase structure except with very different b/a and c/a
ratios. We show how the contraction relative to the phase, principally
in the direction, leads to an -like structure in the [0,1,1] plane.
This is an important link between two complex structures of plutonium and opens
new possibilities for exploring the very rich phase diagram of Pu through
theoretical calculations
Equatorial Pacific Subsurface Countercurrents: A Model–Data Comparison in Stream Coordinates
An isopycnal stream-coordinate analysis of velocity, transport, and potential vorticity (PV), recently applied to observations of the subsurface countercurrents (SCCs) in the equatorial Pacific Ocean, is applied here to the SCCs in a numerical general ocean circulation model, run by the Japan Marine Science and Technology Center (JAMSTEC). Each observed SCC core separates regions of nearly uniform potential vorticity: low on the equatorward side, high on the poleward side. Similar low-PV pools are found in the model, but the high-PV region poleward of the southern SCC is missing. The potential vorticity gradient in each core is weaker in the model than in observations, and relative vorticity plays only a minor role in the model. Its unusually high vertical resolution, with 55 levels, together with its weak lateral dissipation may be key factors in the JAMSTEC model\u27s ability to simulate SCCs
Zero-temperature generalized phase diagram of the 4d transition metals under pressure
We use an accurate implementation of density functional theory (DFT) to
calculate the zero-temperature generalized phase diagram of the 4 series of
transition metals from Y to Pd as a function of pressure and atomic number
. The implementation used is full-potential linearized augmented plane waves
(FP-LAPW), and we employ the exchange-correlation functional recently developed
by Wu and Cohen. For each element, we obtain the ground-state energy for
several crystal structures over a range of volumes, the energy being converged
with respect to all technical parameters to within meV/atom. The
calculated transition pressures for all the elements and all transitions we
have found are compared with experiment wherever possible, and we discuss the
origin of the significant discrepancies. Agreement with experiment for the
zero-temperature equation of state is generally excellent. The generalized
phase diagram of the 4 series shows that the major boundaries slope towards
lower with increasing for the early elements, as expected from the
pressure induced transfer of electrons from states to states, but are
almost independent of for the later elements. Our results for Mo indicate a
transition from bcc to fcc, rather than the bcc-hcp transition expected from
- transfer.Comment: 28 pages and 10 figures. Submitted to Phys. Rev.
Tight-binding study of high-pressure phase transitions in titanium: alpha to omega and beyond
We use a tight-binding total energy method, with parameters determined from a
fit to first-principles calculations, to examine the newly discovered gamma
phase of titanium. Our parameters were adjusted to accurately describe the
alpha Ti-omega Ti phase transition, which is misplaced by density functional
calculations. We find a transition from omega Ti to gamma Ti at 102 GPa, in
good agreement with the experimental value of 116 GPa. Our results suggest that
current density functional calculations will not reproduce the omega Ti-gamma
Ti phase transition, but will instead predict a transition from omega Ti to the
bcc beta Ti phase.Comment: 3 encapsulated Postscript figures, submitted to Phyical Review
Letter
Structural, elastic and thermal properties of cementite (FeC) calculated using Modified Embedded Atom Method
Structural, elastic and thermal properties of cementite (FeC) were
studied using a Modified Embedded Atom Method (MEAM) potential for iron-carbon
(Fe-C) alloys. Previously developed Fe and C single element potentials were
used to develop an Fe-C alloy MEAM potential, using a statistically-based
optimization scheme to reproduce structural and elastic properties of
cementite, the interstitial energies of C in bcc Fe as well as heat of
formation of Fe-C alloys in L and B structures. The stability of
cementite was investigated by molecular dynamics simulations at high
temperatures. The nine single crystal elastic constants for cementite were
obtained by computing total energies for strained cells. Polycrystalline
elastic moduli for cementite were calculated from the single crystal elastic
constants of cementite. The formation energies of (001), (010), and (100)
surfaces of cementite were also calculated. The melting temperature and the
variation of specific heat and volume with respect to temperature were
investigated by performing a two-phase (solid/liquid) molecular dynamics
simulation of cementite. The predictions of the potential are in good agreement
with first-principles calculations and experiments.Comment: 12 pages, 9 figure
Decays of Baryons --- Quark Model versus Large-
We study nonleptonic decays of the orbitally excited, \su6 \rep{70}-plet
baryons in order to test the hypothesis that the successes of the
nonrelativistic quark model have a natural explanation in the large- limit
of QCD. By working in a Hartree approximation, we isolate a specific set of
operators that contribute to the observed s- and d-wave decays in leading order
in . We fit our results to the current experimental decay data, and make
predictions for a number of allowed but unobserved modes. Our tentative
conclusion is that there is more to the nonrelativistic quark model of baryons
than large-.Comment: LaTeX 49pp. (38 pp. landscape), PicTex, PrePicTex, PostPicTex
required for 3 figures, Harvard Preprint HUTP-94/A008. (Two additional
operators are included, but conclusions are unchanged.
Toward a structural understanding of turbulent drag reduction: nonlinear coherent states in viscoelastic shear flows
Nontrivial steady flows have recently been found that capture the main
structures of the turbulent buffer layer. We study the effects of polymer
addition on these "exact coherent states" (ECS) in plane Couette flow. Despite
the simplicity of the ECS flows, these effects closely mirror those observed
experimentally: Structures shift to larger length scales, wall-normal
fluctuations are suppressed while streamwise ones are enhanced, and drag is
reduced. The mechanism underlying these effects is elucidated. These results
suggest that the ECS are closely related to buffer layer turbulence.Comment: 5 pages, 3 figures, published version, Phys. Rev. Lett. 89, 208301
(2002
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