Calculations of the thermodynamic quantities, and the spectra of spinons, triplet excitations, and two-particle spin-singlet (Δ S z = 0,±1) excitations for a weakly coupled antiferromagnetic S = 1/2 spin chain are made using a mean-field approximation for the interchain couplings ( J 2 ) by the quantum Monte Carlo method. The mass gaps in these excitation spectra are estimated as functions of the interchain coupling. The critical field H c , and the temperatures at which the spinon and singlet gaps close ( T 1 and T 2 , respectively) are obtained. Some peculiarities of the staggered magnetization, the specific heat, and the spin-spin correlation function are found at T 1 = 0.9 J 2 , T 2 = 1.5 J 2 , H c = 3.8 J 2 , as determined by linear function fitting for J 2 / J 1 <0.15. The intrachain and interchain couplings, and the staggered magnetization for KCuF 3 , Sr 2 CuO 3 , and Ca 2 CuO 3 are estimated, and mass gaps are predicted to exist in the spectra of the spinons and the singlet two-particle excitations.
We calculate the effects of a single impurity in antiferromagnetic quantum spin-1/2 chains with the help of one-dimensional quantum field theory and renormalization group tech niques in the low temperature limit. We are able to present numerical evidence from ex act diagonalization, numerical Bethe ansatz, and quantum Monte Carlo methods, which support our findings. Special emphasis has been put on impurity effects on the local susceptibility in the chain, because of the experimental relevance of this quantity. We propose a muon spin resonance experiment on quasi one-dimensional spin compounds, which may show some of the impurity effects.
The thermodynamics of finite open antiferromagnetic XXZ chains is studied using field theory, Bethe ansatz and quantum Monte Carlo methods. For the susceptibility a parameter-free result as a function of the number of sites L and temperature T beyond the scaling limit is derived. The limiting cases ( J being the exchange constant), where the boundary correction shows a logarithmically suppressed Curie behaviour, and , where a crossover to the ground state behaviour takes place, are discussed in detail. On the basis of this analysis we present a simple formula for the averaged susceptibility of a spin-1/2 chain doped with non-magnetic impurities. We show that the effective Curie constant has a highly non-trivial temperature dependence and shows scaling in the low temperature limit. Finally, corrections due to intrachain and interchain couplings and implications for experiments on Sr 2 Cu 1− x Pd x O 3+δ and related compounds are discussed.
We study the effect of anisotropy of spin-
S impurities on an antiferromagnetic SU(2) Heisenberg chain. The magnetic impurities are assumed to have an XXZ-type anisotropic exchange with their neighboring sites. First, using density-matrix renormalization group technique, we examine spin-spin correlation function, instability of N\'eel order, and local spin susceptibility in the presence of a single spin-
S impurity. Based on the results, we find that the types of spin-
S impurities are classified into two groups: (i) nonmagnetic and
S=1 impurities enhance only short-range antiferromagnetic correlation and (ii)
S=21 and
S>1 impurities can, in contrast, stabilize a long-range N\'eel order in the disordered SU(2) Heisenberg chain. Then, we focus on the case of
S=21 impurity as a representative of (ii) and investigate the evolution of some experimentally observable quantities such as magnetization, specific heat, and magnetic susceptibility, as a function of concentration and XXZ anisotropy strength of the impurity. We confirm that the N\'eel order is induced in the bulk spin chain in the presence of finite amount of easy-axis XXZ
S=21 impurities. Furthermore, we recover some of the aforementioned features using cluster mean-field theory, which allows us to present results on experimentally accessible quantities at finite temperatures. Interestingly, in the presence of uniform magnetic field, the total magnetization exhibits a pseudogap behavior for low values of applied field. We also discuss the dependence of NMR spectrum on various XXZ impurities and identify that the spin state of Co impurity in
SrCu0.99Co0.01O2 is
S=23.
We consider a magnetic impurity in the antiferromagnetic spin-
1/2 chain which is equivalent to the two-channel Kondo problem in terms of the field theoretical description. Using a modification of the transfer-matrix density matrix renormalization group we are able to determine the crossover function for the impurity susceptibility over a large temperature range, which exhibits universal data collapse. We also calculate the local susceptibilities near the impurity, which show an interesting competition of boundary effects. This results in quantitative predictions for experiments on doped spin-
1/2 chains, which could observe two-channel Kondo physics directly.
By extending White's density-matrix renormalization group (DMRG) technique to finite temperatures, we calculate the impurity susceptibility for the following systems: (a) a spin-1/2 impurity coupled to a single site of an antiferromagnetic Heisenberg spin-1/2 chain and (b) an impurity coupled symmetrically to the two spins at the open ends of the chain. We also perform a Monte Carlo simulation and show that the DMRG adaptation gives reliable results. The calculated impurity susceptibility shows Pauli-like behavior at low temperatures in the former case while a ln T divergence is found in the latter case, in agreement with predictions based on conformal field theory.