876 research outputs found
Symmetry-protected topological phases of alkaline-earth cold fermionic atoms in one dimension
We investigate the existence of symmetry-protected topological phases in
one-dimensional alkaline-earth cold fermionic atoms with general half-integer
nuclear spin I at half filling. In this respect, some orbital degrees of
freedom are required. They can be introduced by considering either the
metastable excited state of alkaline-earth atoms or the p-band of the optical
lattice. Using complementary techniques, we show that SU(2) Haldane topological
phases are stabilised from these orbital degrees of freedom. On top of these
phases, we find the emergence of topological phases with enlarged SU(2I+1)
symmetry which depend only on the nuclear spin degrees of freedom. The main
physical properties of the latter phases are further studied using a
matrix-product state approach. On the one hand, we find that these phases are
symmetry-protected topological phases, with respect to inversion symmetry, when
I=1/2,5/2,9/2,..., which is directly relevant to ytterbium and strontium cold
fermions. On the other hand, for the other values of I(=half-odd integer),
these topological phases are stabilised only in the presence of exact
SU(2I+1)-symmetry
Low-lying excitations and magnetization process of coupled tetrahedral systems
We investigate low-lying singlet and triplet excitations and the
magnetization process of quasi-1D spin systems composed of tetrahedral spin
clusters. For a class of such models, we found various exact low-lying
excitations; some of them are responsible for the first-order transition
between two different ground states formed by local singlets. Moreover, we find
that there are two different kinds of magnetization plateaus which are
separated by a first-order transition.Comment: To appear in Phys.Rev.B (Issue 01 August 2002). A short comment is
adde
Magnetization plateaus in weakly coupled dimer spin system
I study a spin system consisting of strongly coupled dimers which are in turn
weakly coupled in a plane by zigzag interactions. The model can be viewed as
the strong-coupling limit of a two-dimensional zigzag chain structure typical,
e.g., for the -planes of KCuCl_3. It is shown that the magnetization
curve in this model has plateaus at 1/3 and 2/3 of the saturation
magnetization, and an additional plateau at 1/2 can appear in a certain range
of the model parameters; the critical fields are calculated perturbatively. It
is argued that for the three-dimensional lattice structure of the KCuCl_3
family the plateaus at 1/4 and 3/4 of the saturation can be favored in a
similar way, which might be relevant to the recent experiments on NH_4CuCl_3 by
Shiramura et al., J. Phys. Soc. Jpn. {\bf 67}, 1548 (1998).Comment: serious changes in Sect. II,III, final version to appear in PR
Magnetization Curves of Antiferromagnetic Heisenberg Spin-1/2 Ladders
Magnetization processes of spin-1/2 Heisenberg ladders are studied using
strong-coupling expansions, numerical diagonalization of finite systems and a
bosonization approach. We find that the magnetization exhibits plateaux as a
function of the applied field at certain rational fractions of the saturation
value. Our main focus are ladders with 3 legs where plateaux with magnetization
one third of the saturation value are shown to exist.Comment: 5 pages REVTeX, 4 PostScript figures included using psfig.sty; this
is the final version to appear in Phys. Rev. Let
Maximized string order parameters in the valence bond solid states of quantum integer spin chains
We propose a set of maximized string order parameters to describe the hidden
topological order in the valence bond solid states of quantum integer spin-S
chains. These optimized string order parameters involve spin-twist angles
corresponding to rotations around or -axes, suggesting a
hidden symmetry. Our results also suggest that a local
triplet excitation in the valence bond solid states carries a
topological charge measured by these maximized string order parameters.Comment: 5 pages, 1 figur
Series expansion analysis of a tetrahedral cluster spin chain
Using series expansion by continuous unitary transformations we study the
magnetic properties of a frustrated tetrahedral spin-1/2 chain. Starting from
the limit of isolated tetrahedra we analyze the evolution of the ground state
energy and the elementary triplet dispersion as a function of the
inter-tetrahedral coupling. The quantum phase diagram is evaluated and is shown
to incorporate a singlet product, a dimer, and a Haldane phase. Comparison of
our results with those from several other techniques, such as density matrix
renormalization group, exact diagonalization and bond-operator theory are
provided and convincing agreement is found.Comment: 6 pages, 5 figures, 1 tabl
Magnetization plateaus in antiferromagnetic-(ferromagnetic)_{n} polymerized S=1/2 XXZ chains
The plateau-non-plateau transition in the
antiferromagnetic-(ferromagnetic) polymerized XXZ chains under
the magnetic field is investigated. The universality class of this transition
belongs to the Brezinskii-Kosterlitz-Thouless (BKT) type. The critical points
are determined by level spectroscopy analysis of the numerical diagonalization
data for where is the size of a unit cell.
It is found that the critical strength of ferromagnetic coupling decreases with
for small but increases for larger enough . It is also found that
the plateau for large is wide enough for moderate values of exchange
coupling so that it should be easily observed experimentally. This is in
contrast to the plateaus for chains which are narrow for a wide range
of exchange coupling even away from the critical point
Massive and Massless Behavior in Dimerized Spin Ladders
We investigate the conditions under which a gap vanishes in the spectrum of
dimerized coupled spin-1/2 chains by means of Abelian bosonization and Lanczos
diagonalization techniques. Although both interchain () and dimerization
() couplings favor a gapful phase, it is shown that a suitable choice
of these interactions yields massless spin excitations. We also discuss the
influence of different arrays of relative dimerization on the appearance of
non-trivial magnetization plateaus.Comment: 5 pages, RevTex, 5 Postscript figure
Magnetization plateaus as insulator-superfluid transitions in quantum spin systems
We study the magnetization process in two-dimensional S=1/2 spin systems, to
discuss the appearance of a plateau structure. The following three cases are
considered: (1) the Heisenberg antiferromagnet and multiple-spin exchange model
on the triangular lattice, (2) Shastry-Sutherland type lattice, [which is a
possible model for SrCu2(BO3)2,] (3) 1/5-depleted lattice (for CaV4O9). We find
in these systems that magnetization plateaus can appear owing to a transition
from superfluid to a Mott insulator of magnetic excitations. The plateau states
have CDW order of the excitations. The magnetizations of the plateaus depend on
components of the magnetic excitations, range of the repulsive interaction, and
the geometry of the lattice.Comment: 5 pages, RevTeX, 7 figures, note and reference adde
- âŠ