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1.
S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,55(4):371-376
The energy gap of the 1D AF-Heisenberg model in the presence of both uniform (H) and staggered (h) magnetic fields is investigated
using the exact
diagonalization technique. The opening of the gap in the presence of a
staggered field is found to scale with hν, where ν=ν(H) is the
critical exponent, and depends on the uniform field. With respect to the range
of the staggered magnetic field, two regimes are identified through which the
dependence of the real critical exponent ν(H) on H can be calculated
numerically. Our numerical results are in good agreement with the results
obtained by other theoretical approaches. 相似文献
2.
We investigate an extended spin ladder with diagonal frustrated exchanges in a wide parameter regime. By representing the
model as a sum of semidefinite positive projection operators, we prove that this model has exactly a dimer ground state. Smoothly
changing parameters may lead the model cover several exactly known models. Starting from this ladder model, we proposed two
two-dimensional net models with exact ground states. The quantum phase transition of the ground state, due to the change of
exchange strengths along perpendicular rungs, is also discussed.
Received 13 October 2002 Published online 27 January 2003
RID="a"
ID="a"e-mail: schen@thphy.uni.duesseldorf.de 相似文献
3.
P. Thalmeier A.N. Yaresko 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):495-508
An investigation of the spin excitation spectrum of charge ordered (CO) NaV2O5 is presented. We discuss several different exchange models which may be relevant for this compound, namely in-line and zig-zag
chain models with weak as well as strong inter-chain coupling and also a ladder model and a CO/MV (mixed valent) model. We
put special emphasis on the importance of large additional exchange across the diagonals of V-ladders and the presence of
exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches
may both be interpreted as anisotropy effect as well as acoustic-optic mode splitting in the weakly coupled chain models.
In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter-chain
coupling, as suggested by recent LDA + U results, an additional high energy optical excitation appears and the spin gap is
determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which
agrees very well with recent results obtained by inelastic neutron scattering.
Received 30 April 1999 and Received in final form 5 October 1999 相似文献
4.
H. Niggemann J. Zittartz 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):731-738
We present a class of optimum ground states for quantum spin- models on the Cayley tree with coordination number 3. The interaction is restricted to nearest neighbours and contains 5
continuous parameters. For all values of these parameters the Hamiltonian has parity invariance, spin-flip invariance, and
rotational symmetry in the xy-plane of spin space. The global ground states are constructed in terms of a 1-parametric vertex state model, which is a direct generalization of the well-known matrix product ground state approach. By using recursion relations and the transfer matrix technique we derive exact analytical expressions for local
fluctuations and longitudinal and transversal two-point correlation functions.
Received 1 March 1999 相似文献
5.
A. Honecker M. Kaulke K.D. Schotte 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(3):423-434
CsCuCl3 is a ferromagnetically stacked triangular spin-1/2 antiferromagnet. We discuss models for its zero-temperature magnetization
process. The models range from three antiferromagnetically coupled ferromagnetic chains to the full three-dimensional situation.
The situation with spin-1/2 is treated by expansions around the Ising limit and exact diagonalization. Further, weak-coupling
perturbation theory is used mainly for three coupled chains which are also investigated numerically using the density-matrix
renormalization group technique. We find that already the three-chain model gives rise to the plateau-like feature at one
third of the saturation magnetization which is observed in magnetization experiments on CsCuCl3 for a magnetic field perpendicular to the crystal axis. For a magnetic field parallel to the crystal axis, a jump is observed
in the experimental magnetization curve in the region of again about one third of the saturation magnetization. In contrast
to earlier spinwave computations, we do not find any evidence for such a jump with the model in the appropriate parameter
region.
Received 25 October 1999 and Received in final form 30 December 1999 相似文献
6.
G. M. Rocha-Filho A. S.T. Pires M. E. Gouvêa 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,56(1):7-14
We study the one-dimensional isotropic spin-1 Heisenberg
magnet with antiferromagnetic nearest-neighbor (nn) and
next-nearest-neighbor (nnn) interactions by using the modified spin
wave theory (MSWT). The ground state energy and the singlet-triplet
energy gap are obtained for several values of j, defined as the
ratio of the nnn interaction constant to the nn one. We also compare
two different ways of implementing the MSWT currently found in the
literature, and show that, despite the remarkable differences between
the equations to be solved in each procedure, the results given by
both are equivalent, except for the predicted value of the
jmax, the maximum value of j accessible in each treatment.
Here, we suggest that jmax is related to the disorder point of
the first kind. Our results show that the ground state and the gap
energies increase with j, for j ≤jmax, in accordance to
previous numerical results. 相似文献
7.
Asim Kumar Ghosh 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,82(1):19-27
We consider a one-dimensional Ising-like S =
\frac12\frac{1}{2} Heisenberg antiferromagnetic Hamiltonian and study the dynamics of domain wall excitations in presence of both uniform and
staggered Dzyaloshinskii-Moriya interactions. We obtain dispersion relations and dynamical spin
correlation functions, S
x
x
(q, ω) using the basis of domain wall pair states. It is shown that the line shapes of S
x
x
(q, ω) are highly asymmetric over the whole Brillouin zone such that the spectral weights mainly concentrate in the low energy
side. It is observed that presence of staggered Dzyaloshinskii-Moriya interaction explains the experimental results on the
Ising-like
antiferromagnetic compounds CsCoCl3 and CsCoBr3 very well. 相似文献
8.
G. Misguich P. Sindzingre 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(3):305-309
We compute the magnetic susceptibility and specific heat of the
spin-
Heisenberg model on the kagome lattice with
high-temperature expansions and exact diagonalizations. We compare
the results with the experimental data on ZnCu3(OH)6Cl2 obtained by Helton
et al. [Phys. Rev. Lett. 98, 107204 (2007)]. Down to
kBT/J≃0.2, our calculations reproduce accurately the
experimental susceptibility, with an exchange interaction
J≃190 K and a contribution of 3.7% of weakly interacting
impurity spins. The comparison between our calculations of the
specific heat and the experiments indicate that the low-temperature
entropy (below ~20 K) is smaller in ZnCu3(OH)6Cl2 than in the kagome
Heisenberg model, a likely signature of other interactions in the
system. 相似文献
9.
R. Zinke J. Schulenburg J. Richter 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(2):147-152
We investigate the ground-state magnetic long-range order of
quasi-one-dimensional quantum Heisenberg antiferromagnets for spin
quantum numbers s = 1/2 and s = 1. We use the coupled
cluster method to calculate the sublattice magnetization and
its dependence on the inter-chain coupling J⊥.
We find that for the unfrustrated spin-1/2 system,
an infinitesimal inter-chain coupling is sufficient to stabilize
magnetic long-range order, in agreement with results obtained
by other methods.
For s = 1, we find that a finite inter-chain coupling
is necessary to stabilize magnetic long-range order.
Furthermore, we consider a quasi one-dimensional spin-1/2 system,
where a frustrating next-nearest neighbor in-chain coupling is included.
We find that for stronger frustration as well, a finite inter-chain coupling
is necessary to have magnetic long-range order in the ground state, and that the strength of the inter-chain coupling necessary
to establish magnetic long-range order is related to the size of the spin gap of the isolated chain. 相似文献
10.
M.A. Garcia-Bach 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):439-448
For w-legged antiferromagnetic spin-1/2 Heisenberg ladders, a long-range spin pairing order can be identified which enables the
separation of the space spanned by finite-range (covalent) valence-bond configurations into w
+1 subspaces. Since every subspace has an equivalent counter subspace connected by translational symmetry, twofold degeneracy,
breaking translational symmetry is found except for the subspace where the ground state of w = even belongs to. In terms of energy ordering, (non)degeneracy and the discontinuities introduced in the long-range spin
pairing order by topological spin defects, the differences between even and odd ladders are explained in a general and systematic
way.
Received 19 July 1999 and Received in final form 8 October 1999 相似文献
11.
S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,59(3):285-289
We have studied the behavior of the energy gap of the 1D AF
spin-
XXZ model in a transverse magnetic field (h)
using the exact diagonalization technique. The ground state phase
diagram consists of two spin-flop and paramagnetic phases. Using a
modified finite-size scaling approach, we have computed the
critical exponent of the energy gap in the vicinity of the
critical transverse field hc(Δ). Our numerical results
confirm that the continuous phase transition from the spin-flop
phase to the paramagnetic one is in the universality class of the
Ising model in the transverse field (ITF). By applying conformal
estimates of a small perturbation (h≪1), we have also
justified our numerical results. 相似文献
12.
M. Dudzinski J. Sznajd J. Zittartz 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(4):575-579
The zero temperature phase diagram of a one-dimensional ferromagnet with cubic single ion anisotropy in an external magnetic
field is studied. The mean-field approximation and the density-matrix renormalization group method are applied. Two phases
at finite magnetic fields are identified: a canted phase with spontaneously broken symmetry and a phase with magnetization
along the magnetic field. Both methods predict that the canted phase exists even for the single-ion anisotropy strong enough
to destroy the magnetic order at zero magnetic field. In contrast to the mean-field theory, the density-matrix renormalization
group predicts a reentrant behavior for the model. The character of the phase transition at finite magnetic field has also
been considered and the critical index has been found.
Received 9 May 2000 and Received in final form 5 July 2000 相似文献
13.
G. I. Japaridze S. Mahdavifar 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,68(1):59-66
The ground-state magnetic phase diagram of a spin S=1/2 two-leg ladder with alternating rung exchange J⊥(n)=J⊥[1 + (-1)n δ] is studied using the analytical and numerical approaches. In the limit where the rung exchange is dominant, we have mapped
the model onto the effective quantum sine-Gordon model with topological term and identified two quantum phase transitions
at magnetization equal to the half of saturation value from a gapped to the gapless regime. These quantum transitions belong
to the universality class of the commensurate-incommensurate phase transition. We have also shown that the magnetization curve
of the system exhibits a plateau at magnetization equal to the half of the saturation value. We also present a detailed numerical
analysis of the low energy excitation spectrum and the ground state magnetic phase diagram of the ladder with rung-exchange
alternation using Lanczos method of numerical diagonalizations for ladders with number of sites up to N = 28. We have calculated
numerically the magnetic field dependence of the low-energy excitation spectrum, magnetization and the on-rung spin-spin correlation
function. We have also calculated the width of the magnetization plateau and show that it scales as δν, where critical exponent varies from ν = 0.87±0.01 in the case of a ladder with isotropic antiferromagnetic legs to ν = 1.82±0.01
in the case of ladder with ferromagnetic legs. Obtained numerical results are in an complete agreement with estimations made
within the continuum-limit approach. 相似文献
14.
S. Chen L. Wang Y. P. Wang 《The European Physical Journal B - Condensed Matter and Complex Systems》2007,57(3):265-270
We study the ground-state properties of frustrated Heisenberg ferrimagnetic
ladders with antiferromagnetic exchange interactions and two types of
alternating sublattice spins. In the limit of strong rung couplings, we show
that the mixed spin-1/2 and spin-1 ladders can be systematically mapped onto
a spin-1/2 Heisenberg model with additional next-nearest-neighbor exchanges.
The system is either in a ferrimagnetic state or in a critical spin-liquid
state depending on the competition between the spin exchanges along the legs
and the diagonal exchanges. 相似文献
15.
E. Rastelli A. Tassi 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):341-347
Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J1 develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J2 is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find
that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive
for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit.
Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997 相似文献
16.
Large spin systems as given by magnetic macromolecules or two-dimensional spin arrays rule out an exact diagonalization of
the Hamiltonian. Nevertheless, it is possible to derive upper and lower bounds of the minimal energies, i.e. the smallest energies for a given total spin S. The energy bounds are derived under additional assumptions on the topology of the coupling between the spins. The upper
bound follows from “n-cyclicity", which roughly means that the graph of interactions can be wrapped round a ring with n vertices. The lower bound improves earlier results and follows from “n-homogeneity", i.e. from the assumption that the set of spins can be decomposed into n subsets where the interactions inside and between spins of different subsets fulfill certain homogeneity conditions. Many
Heisenberg spin systems comply with both concepts such that both bounds are available. By investigating small systems which
can be numerically diagonalized we find that the upper bounds are considerably closer to the true minimal energies than the
lower ones.
Received 22 October 2002 / Received in final form 4 April 2003 Published online 20 June 2003
RID="a"
ID="a"e-mail: jschnack@uos.de 相似文献
17.
Y. Zhou G.-F. Zhang 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2008,47(2):227-231
We study quantum teleportation via a two-qubit Heisenberg
XXZ chain under an inhomogeneous magnetic field. We first consider entanglement
teleportation, and then focus on the teleportation fidelity under different
conditions. The effects of anisotropy and the magnetic field, both uniform
and inhomogeneous, are discussed. We also find that, though entanglement
teleportation does require an entangled quantum channel, a nonzero critical
value of minimum entanglement is not always necessary. 相似文献
18.
C. Waldtmann H.-U. Everts B. Bernu C. Lhuillier P. Sindzingre P. Lecheminant L. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):501-507
We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis
of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather
unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless
continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these
states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg
antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably
they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.
Received: 27 November 1997 / Accepted: 29 January 1998 相似文献
19.
In this paper, we investigate the four-qubit spin-\(\frac {1}{2}\) XXZ Heisenberg chain with Dzyaloshinskii-Moriya interaction by topological basis method, and research the relationship between the topological basis states and the ground states. In order to study the Hamiltonian system beyond XXZ model, we introduce two Temperley-Lieb algebra generators and two other generalized generators. Then we investigate the relationship between topological basis and Heisenberg XXZ model with Dzyaloshinskii-Moriya interaction. The results show that the ground state of this model falls on the topological basis state for anti-ferromagnetic case and gapless phase case. 相似文献
20.
A. Avella F. Mancini E. Plekhanov 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,66(3):295-299
By means of the Density Matrix Renormalization Group technique, we have studied the region where XXZ-like behavior is most
likely to emerge within the phase diagram of the F-AF anisotropic extended (J-J’) Heisenberg chain. We have analyzed, in great
detail, the equal-time two-spin correlation functions, both in- and out-of- plane, as functions of the distance (and momentum).
Then, we have extracted, through an accurate fitting procedure, the exponents of the asymptotic power-law decay of the spatial
correlations. We have used the exact solution of XXZ model (J’ = 0) to benchmark our results, which clearly show the expected
agreement. A critical value of J’ has been found where the relevant power-law decay exponent is independent of the in-plane
nearest-neighbor coupling. 相似文献