150 research outputs found
Monte Carlo Study of the Separation of Energy Scales in Quantum Spin 1/2 Chains with Bond Disorder
One-dimensional Heisenberg spin 1/2 chains with random ferro- and
antiferromagnetic bonds are realized in systems such as . We have investigated numerically the thermodynamic properties of a
generic random bond model and of a realistic model of by the quantum Monte Carlo loop algorithm. For the first time we
demonstrate the separation into three different temperature regimes for the
original Hamiltonian based on an exact treatment, especially we show that the
intermediate temperature regime is well-defined and observable in both the
specific heat and the magnetic susceptibility. The crossover between the
regimes is indicated by peaks in the specific heat. The uniform magnetic
susceptibility shows Curie-like behavior in the high-, intermediate- and
low-temperature regime, with different values of the Curie constant in each
regime. We show that these regimes are overlapping in the realistic model and
give numerical data for the analysis of experimental tests.Comment: 7 pages, 5 eps-figures included, typeset using JPSJ.sty, accepted for
publication in J. Phys. Soc. Jpn. 68, Vol. 3. (1999
Lightly Doped t-J Three-Leg Ladders - an Analog for the Underdoped Cuprates
The three-leg ladder has one odd-parity and two even-parity channels. At low
doping these behave quite differently. Numerical calculations for a t-J model
show that the initial phase upon hole doping has two components - a conducting
Luttinger liquid in the odd-parity channel, coexisting with an insulating (i.e.
undoped) spin liquid phase in the even-parity channels. This phase has a
partially truncated Fermi surface and violates the Luttinger theorem. This
coexistence of conducting fermionic and insulating paired bosonic degrees of
freedom is similar to the recent proposal of Geshkenbein, Ioffe, and Larkin for
the underdoped spin-gap normal phase of the cuprates. A mean field
approximation is derived which has many similarities to the numerical results.
One difference however is an induced hole pairing in the odd-parity channel at
arbitrary small dopings, similar to that proposed by Geshkenbein, Ioffe, and
Larkin for the two-dimensional case. At higher dopings, we propose that a
quantum phase transition will occur as holes enter the even-parity channels,
resulting in a Luther-Emery liquid with hole pairing with essentially d-wave
character. In the mean field approximation a crossover occurs which we
interpret as a reflection of this quantum phase transition deduced from the
numerical results.Comment: RevTex, 36 pages with 16 figure
Numerical renormalization-group study of spin correlations in one-dimensional random spin chains
We calculate the ground-state two-spin correlation functions of spin-1/2
quantum Heisenberg chains with random exchange couplings using the real-space
renormalization group scheme. We extend the conventional scheme to take account
of the contribution of local higher multiplet excitations in each decimation
step. This extended scheme can provide highly accurate numerical data for large
systems. The random average of staggered spin correlations of the chains with
random antiferromagnetic (AF) couplings shows algebraic decay like ,
which verifies the Fisher's analytic results. For chains with random
ferromagnetic (FM) and AF couplings, the random average of generalized
staggered correlations is found to decay more slowly than a power-law, in the
form close to . The difference between the distribution functions of
the spin correlations of the random AF chains and of the random FM-AF chains is
also discussed.Comment: 14 pages including 8 figures, REVTeX, submitted to Physical Review
Thermodynamics of Random Ferromagnetic Antiferromagnetic Spin-1/2 Chains
Using the quantum Monte Carlo Loop algorithm, we calculate the temperature
dependence of the uniform susceptibility, the specific heat, the correlation
length, the generalized staggered susceptibility and magnetization of a
spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings, down
to very low temperatures. Our data show a consistent scaling behavior in all
the quantities and support strongly the conjecture drawn from the approximate
real-space renormalization group treatment.A statistical analysis scheme is
developed which will be useful for the search of scaling behavior in numerical
and experimental data of random spin chains.Comment: 13 pages, 13 figures, RevTe
Quantum magnetism in the stripe phase: bond- versus site order
It is argued that the spin dynamics in the charge-ordered stripe phase might
be revealing with regards to the nature of the anomalous spin dynamics in
cuprate superconductors. Specifically, if the stripes are bond ordered much of
the spin fluctuation will originate in the spin sector itself, while site
ordered stripes require the charge sector as the driving force for the strong
quantum spin fluctuations.Comment: 4 pages, 3 figures, LaTe
The Non Linear Sigma Model and Spin Ladders
The well known Haldane map from spin chains into the non linear sigma
model is generalized to the case of spin ladders. This map allows us to explain
the different qualitative behaviour between even and odd ladders, exactly in
the same way it explains the difference between integer and half-integer spin
chains. Namely, for even ladders the topological term in the sigma model action
is absent, while for odd ladders the parameter, which multiplies the
topological term, is equal to , where is the spin of the ladder.
Hence even ladders should have a dynamically generated spin gap, while odd
ladders with half-integer spin should stay gapless, and physically equivalent
to a perturbed Wess-Zumino -Witten model in the infrared regime. We
also derive some consequences from the dependence of the sigma model coupling
constant on the ladder Heisenberg couplings constants.Comment: Latex file, 14 pages, no figure
Magnetic properties of an SU(4) spin-orbital chain
In this paper, we study the magnetic properties of the one-dimensional SU(4)
spin-orbital model by solving its Bethe ansatz solution numerically. It is
found that the magnetic properties of the system for the case of
differs from that for the case of . The magnetization curve and
susceptibility are obtained for a system of 200 sites. For , the
phase diagram depending on the magnetic field and the ratio of Land\'e factors,
, is obtained. Four phases with distinct magnetic properties are
found.Comment: 4 pages, 2 figure
Numerical Study of the One-Dimensional Spin-Orbit Coupled System with SU(2)SU(2) Symmetry
We numerically study the SU(2)SU(2) symmetric spin-orbit coupled
model as a lower symmetric generalization of the SU(4) exchange model. On the
symmetric line with respect to the spin and orbit, our result shows the
essentially singular gap formation in consistent with the analytic approach,
which is different from the previous numerical calculation. Furthermore, we
find new critical phases around the SU(4) point, surrounding the previously
known gapless symmetric line. In these novel phases either spin or orbital
excitations around momentum form massless continua which split from the
excitations belonging to the irreducible representation at the SU(4)
point. On the critical symmetric line, the additional coupled spin-orbit
excitations around originating from [] become critical, too.Comment: 6 pages with 5 figures, submitted to J. Phys. Soc. Jp
Single hole dynamics in the t-J model on two- and three-leg ladders
The dynamics of a single hole in the t-J model on two- (2LL) and three- (3LL)
leg ladders is studied using a recently developed quantum Monte Carlo
algorithm. For the 2LL it is shown that in addition to the most pronounced
features of the spectral function, well described by the limit of strong
coupling along the rungs, a clear shadow band appears in the antibonding
channel. Moreover, both the bonding band and its shadow have a finite
quasiparticle (QP) weight in the thermodynamic limit. For strong coupling along
the rungs of the 3LL, the low-energy spectrum in the antisymmetric channel is
similar to a one-dimensional chain, whereas in the two symmetric channels it
resembles the 2LL. The QP weight vanishes in the antisymmetric channel, but is
finite in the symmetric one
Low-Temperature Scaling Regime of Random Ferromagnetic-Antiferromagnetic Spin Chains
Using the Continuous Time Quantum Monte Carlo Loop algorithm, we calculate
the temperature dependence of the uniform susceptibility, and the specific heat
of a spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings,
down to very low temperatures. Our data show a consistent scaling behavior in
both quantities and support strongly the conjecture drawn from the
approximative real-space renormalization group treatment. A statistical
analysis scheme is developed which will be useful for the search scaling
behavior in numerical and experimental data of random spin chains.Comment: 4 pages and 3 figure
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