22 research outputs found
Thermodynamic Properties of XXZ model in a Transverse Field
We have numerically studied the thermodynamic properties of the spin 1/2 XXZ
chain in the presence of a transverse (non commuting) magnetic field. The
thermal, field dependence of specific heat and correlation functions for chains
up to 20 sites have been calculated. The area where the specific heat decays
exponentially is considered as a measure of the energy gap. We have also
obtained the exchange interaction between chains in a bulk material using the
random phase approximation and derived the phase diagram of the three
dimensional material with this approximation. The behavior of the structure
factor at different momenta verifies the antiferromagnetic long range order in
y-direction for the three dimensional case. Moreover, we have concluded that
the Low Temperature Lanczos results [M. Aichhorn et al., Phys. Rev. B 67,
161103(R) (2003)] are more accurate for low temperatures and closer to the full
diagonalization ones than the results of Finite Temperature Lanczos Method [J.
Jaklic and P. Prelovsek, Phys. Rev. B 49, 5065 (1994)].Comment: 7 pages, 10 eps figure
Ground-state fidelity of the spin-1 Heisenberg chain with single ion anisotropy: quantum renormalization group and exact diagonalization approaches
We study the phase diagram of the anisotropic spin-1 Heisenberg chain with
single ion anisotropy (D) using a ground-state fidelity approach. The
ground-state fidelity and its corresponding susceptibility are calculated
within the quantum renormalization group scheme where we obtained the
renormalization of fidelity preventing to calculate the ground state. Using
this approach, the phase boundaries between the antiferromagnetic N\'{e}el,
Haldane and large-D phases are obtained for the whole phase diagram, which
justifies the application of quantum renormalization group to trace the
symmetery protected topological phases. In addition, we present numerical exact
diagonalization (Lanczos) results in, which we employ a recently introduced
non-local order parameter to locate the transition from Haldane to large-D
phase accurately.Comment: 9 pages, 11 figures, 1 table, to appear in JPC
Moment screening in the correlated Kondo lattice model
The magnetic correlations, local moments and the susceptibility in the
correlated 2D Kondo lattice model at half filling are investigated. We
calculate their systematic dependence on the control parameters J_K/t and U/t.
An unbiased and reliable exact diagonalization (ED) approach for ground state
properties as well as the finite temperature Lanczos method (FTLM) for specific
heat and the uniform susceptibility are employed for small tiles on the square
lattice. They lead to two major results: Firstly we show that the screened
local moment exhibits non-monotonic behavior as a function of U for weak Kondo
coupling J_K. Secondly the temperature dependence of the susceptibility
obtained from FTLM allows to extract the dependence of the characteristic Kondo
temperature scale T* on the correlation strength U. A monotonic increase of T*
for small U is found resolving the ambiguity from earlier investigations. In
the large U limit the model is equivalent to the 2D Kondo necklace model with
two types of localized spins. In this limit the numerical results can be
compared to those of the analytical bond operator method in mean field
treatment and excellent agreement for the total paramagnetic moment is found,
supporting the reliability of both methods.Comment: 19 pages, 9 figure
Thermodynamic behavior of the XXZ Heisenberg s=1/2 chain around the factorizing magnetic field
We have investigated the zero and finite temperature behaviors of the
anisotropic antiferromagnetic Heisenberg XXZ spin-1/2 chain in the presence of
a transverse magnetic field (h). The attention is concentrated on an interval
of magnetic field between the factorizing field (h_f) and the critical one
(h_c). The model presents a spin-flop phase for 0<h<h_f with an energy scale
which is defined by the long range antiferromagnetic order while it undergoes
an entanglement phase transition at h=h_f. The entanglement estimators clearly
show that the entanglement is lost exactly at h=h_f which justifies different
quantum correlations on both sides of the factorizing field. As a consequence
of zero entanglement (at h=h_f) the ground state is known exactly as a product
of single particle states which is the starting point for initiating a spin
wave theory. The linear spin wave theory is implemented to obtain the specific
heat and thermal entanglement of the model in the interested region. A double
peak structure is found in the specific heat around h=h_f which manifests the
existence of two energy scales in the system as a result of two competing
orders before the critical point. These results are confirmed by the low
temperature Lanczos data which we have computed.Comment: Will be published in JPCM (2010), 7 figure
The two-dimensional frustrated Heisenberg model on the orthorhombic lattice
We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.Comment: 4 pages, 3 figures, accepted for publication in Journal of Physics:
Conference Serie
Phase Diagram and Entanglement of Ising Model With Dzyaloshinskii-Moriya Interaction
We have studied the phase diagram and entanglement of the one dimensional
Ising model with Dzyaloshinskii-Moriya (DM) interaction. We have applied the
quantum renormalization group (QRG) approach to get the stable fixed points,
critical point and the scaling of coupling constants. This model has two
phases, antiferromagnetic and saturated chiral ones. We have shown that the
staggered magnetization is the order parameter of the system and DM interaction
produces the chiral order in both phases. We have also implemented the exact
diagonalization (Lanczos) method to calculate the static structure factors. The
divergence of structure factor at the ordering momentum as the size of systems
goes to infinity defines the critical point of the model. Moreover, we have
analyzed the relevance of the entanglement in the model which allows us to shed
insight on how the critical point is touched as the size of the system becomes
large. Nonanalytic behavior of entanglement and finite size scaling have been
analyzed which is tightly connected to the critical properties of the model. It
is also suggested that a spin-fluid phase has a chiral order in terms of new
spin operators which are defined by a nonlocal transformation.Comment: 11page, 15 figures, Accepted in Physical Review
Frustrated local-moment models for iron pnictide magnetism
The low energy spin excitations of the Fe pnictide parent compounds have been
determined by inelastic neutron scattering and interpreted within the local
moment J_1a,b-J_2 Heisenberg model with orthorhombic symmetry. This has led to
alternative exchange models that strongly differ in the size of anisotropy.
Although the compounds are itinerant the localised spin model can explain basic
features of the excitations. The inherent frustration of this model leads to
quantum fluctuations and possible moment reduction. We investigate this
question in detail using spin wave approximation and partly exact
diagonalisation Lanczos calculations for finite clusters. We find that the
orthorhombic anisotropy stabilizes the columnar AF phase and its moment. For
the exchange models proposed from inelastic neutron scattering we can exclude a
strong influcence of frustration on the moment size. We also investigate
dependence of magnetisation and susceptibility on field and temperature.Comment: 16 pages, 9 figures, minor changes to the text, references update
Field-induced staggered moment stabilization in frustrated quantum magnets
For low-dimensional frustrated quantum magnets, the dependence of the staggered moment on a magnetic field is nonmonotonic: For small and intermediate fields, quantum fluctuations are gradually suppressed, leading to an increase of the staggered moment as a function of the field strength. For large applied magnetic fields, the classically expected field dependence is recovered, namely a monotonous decrease with increasing field strength. The staggered moment is eventually suppressed when reaching the fully polarized state at the saturation field. The quantitative analysis of this behavior is an excellent tool to determine the frustration parameter of a magnetic compound. We have developed a general finite-size scaling scheme for numerical exact-diagonalization data of low-dimensional frustrated magnets, which we apply to the recently measured field dependence of the magnetic neutron scattering intensity of Cu(pz)(2)(ClO4)(2) in the framework of the S = 1/2 two-dimensional (2D) J (1)-J (2) Heisenberg model. We also apply linear spin-wave theory to complement our numerical findings. Our results show that Cu(pz)(2)(ClO4)(2) is a quasi-2D antiferromagnet with intermediate frustration J (2)/J (1) = 0.2