XYZ-polarisation analysis of diffuse magnetic neutron scattering from single crystals

Studies of diffuse magnetic scattering largely benefit from the use of a multi-detector covering wide scattering angles. Therefore, the different contributions to the diffuse scattering that originate from magnetic, nuclear coherent, and nuclear spin-incoherent scattering can be separated by the so-called XYZ-polarization analysis. In the past this method has been successfully applied to the analysis of diffuse scattering by polycrystalline samples of magnetic disordered materials. Single crystal studies that exploit the vector properties of spin correlations are of particular interest for furthering our understanding of frustration effects in magnetism. Based on the symmetry properties of polarised scattering a suitable extension of the conventional XYZ method has been derived, which allows for the complete separation and the analysis of features of diffuse magnetic scattering from single crystals.Comment: 6 pages 2 figures, revised as published, one Eq. removed, minor corrections, typos correcte

Frustrated impurity spins in ordered two-dimensional quantum antiferromagnets

Dynamical properties of an impurity spin coupled symmetrically to sublattices of ordered 2D Heisenberg quantum antiferromagnet (i.e., frustrated impurity spin) are discussed at $T\ge0$ (existence of a small interaction stabilizing the long range order at $T\ne0$ is implied). We continue our study on this subject started in Phys.Rev.B 72, 174419 (2005), where spin-1/2 defect is discussed and the host spins fluctuations are considered within the spin-wave approximation (SWA). In the present paper we i) go beyond SWA and ii) study impurities with spins $S\ge1/2$. It is demonstrated that in contrast to defects coupled to sublattices asymmetrically longitudinal host spins fluctuations play important role in the frustrated impurity dynamics. The spectral function, that is proportional to $\omega^2$ within SWA, acquires new terms proportional to $\omega^2$ and $\omega T^2$. It is observed that the spin-1/2 impurity susceptibility has the same structure as that obtained within SWA: the Lorenz peak and the non-resonant term. The difference is that the width of the peak becomes larger being proportional to $f^2(T/J)^3$ rather than $f^4(T/J)^3$, where $f$ is the dimensionless coupling parameter. We show that transverse static susceptibility acquires a new negative logarithmic contribution. In accordance with previous works we find that host spins fluctuations lead to an effective one-ion anisotropy on the impurity site. Then defects with $S>1/2$ appears to be split. We observe strong reduction of the value of the splitting due to longitudinal host spins fluctuations. We demonstrate that the dynamical impurity susceptibility contains $2S$ Lorenz peaks corresponding to transitions between the levels, and the non-resonant term.Comment: 17 pages, 7 figures, to appear in PR

RKKY interaction in Layered Superconductors with Anisotropic Pairing

The RKKY interaction between rare-earth (RE) ions in high-$T_c$ superconductors is considered at $T\ll T_c$. It is shown that this interaction consists of two terms: conventional oscillating one and the positive term, which is proportional to the gap function and decreases in the $2D$ case inversely proportional to the distance. In the antiferromagnetic state of the RE subsystem this positive interaction gives rise for frustrations which diminishes the Neel temperature. In the case of strongly anisotropic gap function this frustration produces two different values of the effective nearest neighbor exchange coupling between RE ions along the $a$ and $b$. This anisotropy has been established experimentally in Ref.\cite{6,7,8}.Comment: 10 pages, REVTEX, no figure

Magnon Bose condensation in symmetry breaking magnetic field

Magnon Bose condensation (BC)in the symmetry breaking magnetic field is a result of unusual form of the Zeeman energy, which has terms linear in the spin-wave operators and terms mixing excitations differ in the Wave-vector of the magnetic structure. The following examples are considered: simple easy-plane tetragonal antiferromagnets (AF), frustrated AF family$R_2Cu O_4$ where $R=Pr,Nd$ etc. and cubic magnets with the Dzyaloshinskii-Moriya interaction ($Mn Si$ etc.). In all cases the BC becomes important when the magnetic field becomes comparable with the spin-wave gap. The theory is illustrated by existing experimental results.Comment: Submitted to J. of Phys. Condens. Matter (Proceedings of International Conference "Highly Frustrated Magnets", Osaka (Japan), August 2006). 8 pages, 5 figure

Quantum Phase Transition in Pr2CuO4 to Collinear Spin State in Inclined Magnetic Field: A Neutron Diffraction Observation

In the external field slightly inclined to the $x$- or y-axis of the frustrated tetragonal atiferromagnet Pr2CuO4, a transition is discovered from the phase with orthogonal antiferromagnetic spin subsystems along [1,0,0] and [0,1,0] to the phase with the collinear spins. This phase is shown to be due to the pseudodipolar interaction, and transforms into the spin-flop phase S perp H asymptotically at very high field. The discovered phase transition holds at T=0 and is a quantum one, with the transition field being the critical point and the angle between two subsystems being the order parameter