5,288 research outputs found
Calorons, instantons and constituent monopoles in SU(3) lattice gauge theory
We analyze the zero-modes of the Dirac operator in quenched SU(3) gauge
configurations at non-zero temperature and compare periodic and anti-periodic
temporal boundary conditions for the fermions. It is demonstrated that for the
different boundary conditions often the modes are localized at different
space-time points and have different sizes. Our observations are consistent
with patterns expected for Kraan - van Baal solutions of the classical
Yang-Mills equations. These solutions consist of constituent monopoles and the
zero-modes are localized on different constituents for different boundary
conditions. Our findings indicate that the excitations of the QCD vacuum are
more structured than simple instanton-like lumps.Comment: Remarks added. To appear in Phys. Rev.
Testing the self-duality of topological lumps in SU(3) lattice gauge theory
We discuss a simple formula which connects the field-strength tensor to a
spectral sum over certain quadratic forms of the eigenvectors of the lattice
Dirac operator. We analyze these terms for the near zero-modes and find that
they give rise to contributions which are essentially either self-dual or anti
self-dual. Modes with larger eigenvalues in the bulk of the spectrum are more
dominated by quantum fluctuations and are less (anti) self-dual. In the high
temperature phase of QCD we find considerably reduced (anti) self-duality for
the modes near the edge of the spectral gap.Comment: Remarks added, to appear in Phys. Rev. Let
Electronic Structure of Atoms in Magnetic Quadrupole Traps
We investigate the electronic structure and properties of atoms exposed to a
magnetic quadrupole field. The spin-spatial as well as generalized time
reversal symmetries are established and shown to lead to a two-fold degeneracy
of the electronic states in the presence of the field. Low-lying as well as
highly excited Rydberg states are computed and analyzed for a broad regime of
field gradients. The delicate interplay between the Coulomb and various
magnetic interactions leads to complex patterns of the spatial spin
polarization of individual excited states. Electromagnetic transitions in the
quadrupole field are studied in detail thereby providing the selection rules
and in particular the transition wavelengths and corresponding dipole
strengths. The peculiar property that the quadrupole magnetic field induces
permanent electric dipole moments of the atoms is derived and discussed.Comment: 17 pages, 13 figures, accepted for publication in PR
Locality and topology with fat link overlap actions
We study the locality and topological properties of fat link clover overlap
(FCO) actions. We find that a small amount of fattening (2-4 steps of APE or 1
step of HYP) already results in greatly improved properties compared to the
Wilson overlap (WO). We present a detailed study of the localisation of the FCO
and its connection to the density of low modes of . In contrast to
the Wilson overlap, on quenched gauge backgrounds we do not find any dependence
of the localization of the FCO on the gauge coupling. This suggests that the
FCO remains local in the continuum limit. The FCO also faithfully reproduces
the zero mode wave functions of typical lattice instantons, not like the Wilson
overlap. After a general discussion of different lattice definitions of the
topological charge we also show that the FCO together with the Boulder charge
are likely to satisfy the index theorem in the continuum limit. Finally, we
present a high statistics computation of the quenched topological
susceptibility with the FCO action.Comment: 19 pages, LaTe
Numerical Study of the Antiferrromagnetic Spin Chain with Bond Alternation
We study the quantum spin chain with bond alternation {\cal H}=\sum _i
(1-(-1)^i\delta)\vect{S}_i\cdot \vect{S}_{i+1} by the density matrix
renormalization group method recently proposed by Steven R. White
(\PRL{69}{3844}{1993}). We find a massless point at .
We also find the edge states in the region under the open
boundary condition, which disappear in the region . At the
massless point, the spin wave velocity is and the central
charge is . Our results indicate that a continuous phase
transition occurs at the massless point accompanying
breaking of the hidden symmetry.Comment: 9 pages and 1 PostScript figure, Revtex 3.0 (Minor corrections in
TEX-file format to remove possible compilatory troubles.
Instantons and Chiral Symmetry on the Lattice
I address the question of how much of QCD in the chiral limit is reproduced
by instantons. After reconstructing the instanton content of smoothed Monte
Carlo lattice configurations, I compare hadron spectroscopy on this instanton
ensemble to the spectroscopy on the original ``physical'' smoothed
configurations using a chirally optimised clover fermion action. By studying
the zero mode zone in simple instances I find that the optimised action gives a
satisfactory description of it. Through the Banks-Casher formula, instantons by
themselves are shown to break chiral symmetry but hadron correlators on the
instanton backgrounds are strongly influenced by free quark propagation. This
results in unnaturally light hadrons and a small splitting between the vector
and the pseudoscalar meson channels. Superimposing a perturbative ensemble of
zero momentum gauge field fluctuations (torons) on the instantons is found to
be enough to eliminate the free quarks and restore the physical hadron
correlators. I argue that the torons that are present only in finite volumes,
are probably needed to compensate the unnaturally large finite size effects due
to the lack of confinement in the instanton ensemble.Comment: 32 pages, LaTeX with 14 eps figure
Thermodynamics of quantum Heisenberg spin chains
Thermodynamic properties of the quantum Heisenberg spin chains with S = 1/2,
1, and 3/2 are investigated using the transfer-matrix renormalization-group
method. The temperature dependence of the magnetization, susceptibility,
specific heat, spin-spin correlation length, and several other physical
quantities in a zero or finite applied field are calculated and compared. Our
data agree well with the Bethe ansatz, exact diagonalization, and quantum Monte
Carlo results and provide further insight into the quantum effects in the
antiferromagnetic Heisenberg spin chains.Comment: 9 pages, 10 figure
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