arXiv subjects

C. Schindelin

Publications and source records attributed to C. Schindelin.

6 recordsLinked to original sources

arXiv2001

Magnetic order in the quasi-two-dimensional easy-plane XXZ model

A Green's-function theory of antiferromagnetic short-range and long-range order (LRO) in the $S=1/2$ quasi-two-dimensional easy-plane XXZ model is presented. As the main new result, {\it two} phase transitions due to the combined influence of spatial and spin anisotropy are found, where below the higher and lower Néel temperature there occurs LRO in the transverse and in both the transverse and longitudinal spin correlators, respectively. Comparing the theory with neutron-scattering data for the correlation length of $\rm La_2CuO_4$, a very good agreement in the whole temperature dependence is obtained. Moreover, for $\rm La_2CuO_4$, $\rm Sr_2CuO_2Cl_2$, and $\rm Ca_{0.85}Sr_{0.15}CuO_2$ the second phase with longitudinal LRO is predicted to appear far below room temperature.

cond-mat.str-el↗

arXiv2000

Spin correlation functions and susceptibilities in the easy-plane XXZ chain

We present a Green's-function theory of magnetic short-range order in the $S=1/2$ easy-plane XXZ chain based on the projection method for the dynamic spin susceptibility and a decoupling of three-spin operator products introducing vertex parameters. The longitudinal and transverse static susceptibilities and two-point correlation functions of arbitrary range are calculated self-consistently for all wavenumbers, temperatures, and anisotropy parameters $ -1\leq Δ\leq 1$. In the easy-plane ferromagnetic region $(Δ< 0)$, the longitudinal correlators of spins at distance $n$ change sign at a finite temperature $T_0(n,Δ)$, in reasonable agreement with recent data obtained by finite-chain diagonalizations. The temperature dependence of the uniform static susceptibilities exhibits a maximum which is explained as an effect of magnetic short-range order which decreases with increasing temperature.

cond-mat.str-el↗

arXiv2000

Quantum to classical crossover in the 2D easy-plane XXZ model

Ground-state and thermodynamical properties of the spin-1/2 two-dimensional easy-plane XXZ model are investigated by both a Green's-function approach and by Lanczos diagonalizations on lattices with up to 36 sites. We calculate the spatial and temperature dependences of various spin correlation functions, as well as the wave-vector dependence of the spin susceptibility for all anisotropy parameters $Δ$. In the easy--plane ferromagnetic region $(-1< Δ< 0)$, the longitudinal correlators of spins at distance $r$ change sign at a finite temperature $T_0(Δ, {\bf r})$. This transition, observed in the 2D case for the first time, can be interpreted as a quantum to classical crossover.

cond-mat.str-el↗

arXiv1999

Spin correlation functions and Néel order in the 2D Heisenberg model: Effects of spatial anisotropy

The ground-state properties of the square-lattice spin-1/2 Heisenberg antiferromagnet with spatially anisotropic couplings are investigated by Green's-function projection approaches. The staggered magnetization and the two-spin correlators are calculated; the competition between magnetic long- and short-range order is discussed in comparison with experiments on $\rm Sr_2[Ca_2]CuO_3$.

cond-mat.str-el↗

arXiv1999

Magnetic Order-Disorder Transition in the Two-Dimensional Spatially Anisotropic Heisenberg Model at Zero Temperature

The ground-state properties of the spin-1/2 antiferromagnetic Heisenberg model with spatially anisotropic couplings on a square lattice are investigated by a spin-rotation-invariant Green's-function approach and by Lanczos diagonalizations on lattices up to 36 sites supplemented by finite-size scaling. We focus on the anisotropy-driven transition from the Néel state to a paramagnetic state with antiferromagnetic short-range order and on the spatial dependence of spin correlation functions. Our principal result is that a rather sharp crossover in the magnetic behavior occurs at the coupling ratio $R_0\simeq 0.2$ ($R=J_y/J_x$).

cond-mat.str-el↗

arXiv1996

Theory of short-range magnetic order for the t-J model

We present a self-consistent theory of magnetic short-range order based on a spin-rotation-invariant slave-boson representation of the 2D t-J model. In the functional-integral scheme, at the nearest-neighbour pair-approximation level, the bosonized t-J Lagrangian is transformed to a classical Heisenberg model with an effective (doping-dependent) exchange interaction which takes into account the interrelation of ``itinerant'' and ``localized'' magnetic behaviour. Evaluating the theory in the saddle-point approximation, we find a suppression of antiferromagnetic and incommensurate spiral long-range-ordered phases in the favour of a paramagnetic phase with pronounced antiferromagnetic short-range correlations.

cond-mat↗