79 research outputs found
Correlation effects for semiconducting single wall carbon nanotube: a density matrix renormalization group study
In this paper, we report the applicability of the density matrix
renormalization group(DMRG) approach to the cylindrical single wall carbon
nanotube (SWCN) for purpose of its correlation effect. By applying the DMRG
approach to the ++ model, with and being the hopping and
Coulomb energies between the nearest neighboring sites, respectively, and
the onsite Coulomb energy, we calculate the phase diagram for the SWCN with
chiral numbers (), which reflects the competition between the
correlation energy and . Within reasonable parameter ranges, we
investigate possible correlated groundstates, the lowest excitations and the
corresponding correlation functions in which the connection with the excitonic
insulator is particularly addressed.Comment: 1 source files, 5 figure
Midgap States in Antiferromagnetic Heisenberg Chains with A Staggered Field
We study low-energy excitations in antiferromagnetic Heisenberg chains with a
staggered field which splits the spectrum into a longitudinal and a transverse
branch. Bound states are found to exist inside the field induced gap in both
branches. They originate from the edge effects and are inherent to spin-chain
materials. The sine-Gordon scaling (: the
staggered field) provides an accurate description for the gap and midgap
energies in the transverse branch for and the midgap energies in both
branches for over a wide range of magnetic field; however, it can fit
other low-energy excitations only at much lower field. Moreover, the
integer-spin S=1 chain displays scaling behavior that does not fit this scaling
law. These results reveal intriguing features of magnetic excitations in
spin-chain materials that deserve further investigation.Comment: 4 pages, 4figure
United Nations Educational Scientific and Cultural Organization and International Atomic Energy Agency THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS BOND-VERSUS-SITE DOPING MODELS FOR OFF-CHAIN-DOPED HALDANE-GAP SYSTEM Y 2 Ba Ni O 5
Abstract Using the density matrix renormalization-group technique, we calculate the impurity energy levels for two different effective models of off-chain doping for quasi-one-dimensional Heisenberg chain compound Y2 Ba Ni O5: ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping. Thresholds of the impurity strength for the appearance of localized states are found for both models. However, the ground-state and low-energy excitations for weak impurity strength are different for these two models and the difference can be detected by experiments
Field-induced gap in the spin-1/2 antiferromagnetic Heisenberg chain: A density matrix renormalization group study
We study the spin-1/2 antiferromagnetic Heisenberg chain in both uniform and
(perpendicular) staggered magnetic fields using the density-matrix
renormalization-group method. This model has been shown earlier to describe the
physics of the copper benzoate materials in magnetic field. In the present
work, we extend the study to more general case for a systematic investigation
of the field-induced gap and related properties of the spin-1/2
antiferromagnetic Heisenberg chain. In particular, we explore the high magnetic
field regime where interesting behaviors in the field-induced gap,
magnetization, and spin correlation functions are found. Careful examination of
the low energy properties and magnetization reveals interesting competing
effects of the staggered and uniform fields. The incommensurate behavior in the
spin correlation functions is demonstrated and discussed in detail. The present
work reproduces earlier results in good agreement with experimental data on
copper benzoate and predicts new interesting field-induced features at very
high magnetic field.Comment: 8 pages, 6 figure
Topological effects at short antiferromagnetic Heisenberg chains
The manifestations of topological effects in finite antiferromagnetic
Heisenberg chains is examined by density matrix renormalization group technique
in this paper. We find that difference between integer and half-integer spin
chains shows up in ground state energy per site when length of spin chain is
longer than , where is a spin-spin correlation
length, for spin magnitude S up to 5/2. For open chains with spin magnitudes
to S=5, we verify that end states with fractional spin quantum numbers
exist and are visible even when the chain length is much smaller than the
correlation length . The end states manifest themselves in the structure
of the low energy excitation spectrum.Comment: 4 pages, 6 figure
Effects of substituting rare-earth ion R by non-magnetic impurities in - theory and numerical DMRG results
In this paper we study the effect of substituting R (rare-earth ion) by
non-magnetic ions in the spin-1 chain material . Using a
strong-coupling expansion and numerical density matrix renormalization group
calculations, we show that spin-wave bound states are formed at the impurity
site. Experimental consequences of the bound states are pointed out.Comment: 5 pages, 4 postscript figure
Low-energy properties and magnetization plateaus in a 2-leg mixed spin ladder
Using the density matrix renormalization group technique we investigate the
low-energy properties and the magnetization plateau behavior in a 2-leg mixed
spin ladder consisting of a spin-1/2 chain coupled with a spin-1 chain. The
calculated results show that the system is in the same universality class as
the spin-3/2 chain when the interchain coupling is strongly ferromagnetic, but
the similarity between the two systems is less clear under other coupling
conditions. We have identified two types of magnetization plateau phases. The
calculation of the magnetization distribution on the spin-1/2 and the spin-1
chains on the ladder shows that one plateau phase is related to the partially
magnetized valence-bond-solid state, and the other plateau state contains
strongly coupled S=1 and s=1/2 spins on the rung.Comment: 6 pages with 8 eps figure
Bond-versus-site doping models for off-chain-doped Haldane-gap system Y Ba Ni O
Using the density matrix renormalization-group technique, we calculate the
impurity energy levels for two different effective models of off-chain doping
for quasi-one-dimensional Heisenberg chain compound Y Ba Ni O:
ferromagnetic bond doping and antiferromagnetic site spin-1/2 doping.
Thresholds of the impurity strength for the appearance of localized states are
found for both models. However, the ground-state and low-energy excitations for
weak impurity strength are different for these two models and the difference
can be detected by experiments.Comment: 5 pages, 5 eps figures included, to be published in Phys. Rev.
Logarithmic corrections from ferromagnetic impurity ending bonds of open antiferromagnetic host chains
We analyze the logarithmic corrections due to ferromagnetic impurity ending
bonds of open spin 1/2 antiferromagnetic chains, using the density matrix
renormalization group technique. A universal finite size scaling for impurity contributions in the quasi-degenerate ground state
energy is demonstrated for a zigzag spin 1/2 chain at the critical next nearest
neighbor coupling and the standard Heisenberg spin 1/2 chain, in the long chain
limit. Using an exact solution for the latter case it is argued that one can
extract the impurity contributions to the entropy and specific heat from the
scaling analysis. It is also shown that a pure spin 3/2 open Heisenberg chain
belongs to the same universality class.Comment: 4 pages, 7 eps figure
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