arXiv subjects
Publications and source records attributed to K. -H. Mütter.
We investigate the critical exponents $η_3(α,M), η_1(α,M)$ associated with the singularities in the longitudinal and transverse structure factors of the one dimensional antiferromagnetic Heisenberg model with nearest (J_1) and next to nearest (J_2) neighbour coupling of relative strength $α= {J_2}/{J_1}$ and an external field $B$ with magnetization $M(B)$.
We investigate groundstate energies and magnetization curves in the one dimensional XXZ-model with next to nearest neighbour coupling $α>0$ and anisotropy $Δ$ ($-1 \le Δ\le 1$) at T=0. In between the familiar ferro- and antiferromagnetic phase we find a transition region -- called metamagnetic phase -- where the magnetization curve is discontinuous at a critical field $B_c(α,Δ)$.
We study the one-dimensional spin-1/2 model with nearest and next-to-nearest-neighbor couplings exposed to a homogeneous magnetic field $h_{3}$ and a dimer field with period $q$ and strength $δ$. The latter generates a magnetization plateau at $M=(1-q/π)/2$, which evolves with strength $δ$ of the perturbation as $δ^ε$, where $ε=ε(h_{3},α)$ is related to the $η$-exponent which describes the critical behavior of the dimer structure factor, if the perturbation is switched of ($δ=0$). We also discuss the appearance of magnetization plateaus in ladder systems with $l$ legs.
We study the one-dimensional spin-1/2 antiferromagnetic Heisenberg model exposed to an external field, which is a superposition of a homogeneous field $h_{3}$ and a small periodic field of strength $h_{1}$. For the case of a transverse staggered field a gap opens, which scales with $h_{1}^{ε_{1}}$, where $ε_{1}=ε_{1}(h_{3})$ is given by the critical exponent $η_{1}(M(h_{3}))$ defined through the transverse structure factor of the model at $h_{1}=0$. For the case of a longitudinal periodic field with wave vector $q=π/2$ and strength $h_{q}$ a plateau is found in the magnetization curve at $M=1/4$. The difference of the upper- and lower magnetic field scales with $h_{3}^{u}-h_{3}^{l}\sim h_{q}^{ε_{3}}$, where $ε_{3}=ε_{3}(h_{3})$ is given by the critical exponent $η_{3}(M(h_{3}))$ defined through the longitudinal structure factor of the model at $h_{q}=0$.
We investigate the scaling properties of the excitation energies and transition amplitudes of the one-dimensional spin-$1\over 2$ antiferromagnetic Heisenberg model exposed to an external perturbation. Two types of perturbations are discussed in detail: a staggered field and a dimerized field.
We study the first derivative of the staggered magnetization squared $dm^†(θ)^2/dθ$ and the second derivative $d^2e_0(θ)/dθ^2$ of the ground state energy per site. The parameter $θ$ controls the anisotropy between horizontal and vertical couplings in a two-dimensional (2D) spin-1/2 antiferromagnetic Heisenberg model. It is shown, that both derivatives diverge at $θ=1$, where the anisotropic 2D model reduces to the 1D model.
The finite size effects of the dynamical structure factors in the XXZ-model are studied in the euclidean time $(τ)$-representation. Away from the critical momentum $p=π$ finite size effects turn out to be small except for the large $τ$ limit. The large finite size effects at the critical momentum $p=π$ signal the emergence of infrared singularities in the spectral $(ω)$-representation of the dynamical structure factors.
The static structure factors of the XXZ model in the presence of uniform field are determined from an exact computation of the groundstates at given total spin on rings with $N=4,6,\ldots,28$ sites. In contrast to the naive expectation a weak uniform field strengthens the antiferromagnetic order in the transverse structure factor for the isotropic case.
Based on the Lieb-Schultz-Mattis construction we present a five parameter family of Spin-1 Hamiltonians with degenerate groundstate. Starting from the critical $SU(3)$ symmetric Hamiltonian, we look for those perturbations of the $SU(3)$ symmetry, which leave the groundstate degenerate. We also discuss the spin-3/2 $SU(4)$-case.
We perform a finite size analysis of the longitudinal and transverse structure factors $S_j(p,γ,N),j=1,3$ in the groundstate of the spin-$\frac{1}{2}$ XXZ model. Comparison with the exact results of Tonegawa for the XX model yields excellent agreement. Comparison with the conjecture of Müller, Thomas, Puga and Beck reveals discrepancies in the momentum dependence of the longitudinal structure factors.
An evolution equation for the expectation values of the Boltzmann factor between monomer valence bond states is derived. It contains the whole information on the thermodynamical and magnetic properties of the spin $\frac{1}{2}$ quantum Heisenberg model. A new approximation scheme is proposed and studied in the thermodynamical limit.
The evolution equation for the expectation values of the Boltzmann factor between valence bond states is evaluated in lowest order of the dimer cluster expansion. Explicit formulas are given for the internal energy and the specific heat of the d-dimensional antiferromagnetic Heisenberg model.