N. Cavadini

Bose–Einstein condensation of the triplet states in the magnetic insulator TlCuCl3

Bose–Einstein condensation denotes the formation of a collective quantum ground state of identical particles with integer spin or intrinsic angular momentum. In magnetic insulators, the magnetic properties are due to the unpaired shell electrons that have half-integer spin. However, in some such compounds (KCuCl3 and TlCuCl3), two Cu2+ ions are antiferromagnetically coupled to form a dimer in a crystalline network: the dimer ground state is a spin singlet (total spin zero), separated by an energy gap from the excited triplet state (total spin one). In these dimer compounds, Bose–Einstein condensation becomes theoretically possible. At a critical external magnetic field, the energy of one of the Zeeman split triplet components (a type of boson) intersects the ground-state singlet, resulting in long-range magnetic order; this transition represents a quantum critical point at which Bose–Einstein condensation occurs. Here we report an experimental investigation of the excitation spectrum in such a field-induced magnetically ordered state, using inelastic neutron scattering measurements of TlCuCl3 single crystals. We verify unambiguously the theoretically predicted gapless Goldstone mode characteristic of the Bose–Einstein condensation of the triplet states.

Quantum magnetic interactions in S = 1/2 KCuCl3

KCuCl3 is an S = 1/2 magnetic insulator with a singlet ground state and a finite spin excitation gap. Above the gap, dispersive triplet excitation modes propagate in the whole reciprocal space. From single-crystal inelastic neutron investigations the three-dimensional coupling scheme is rationalized in the framework of a dimer Heisenberg model, and related to the structural features of KCuCl3. The experimental and theoretical characterization presented completes earlier works on the compound under investigation, providing also higher-order expressions for the singlet-triplet dispersion relation. The latter may also be of relevance for the parent quantum systems TlCuCl3 and NH4CuCl3, albeit at different coupling ratios with respect to KCuCl3.

Dynamical structure factors for dimerized spin systems

We discuss the transition strength between the disordered ground state and the basic low-lying triplet excitation for interacting dimer materials by presenting theoretical calculations and series expansions as well as inelastic neutron scattering results for the material KCuCl3. We describe in detail the features resulting from the presence of two differently oriented dimers per unit cell and show how energies and spectral weights of the resulting two modes are related to each other. We present results from the perturbation expansion in the interdimer interaction strength and thus demonstrate that the wavevector dependence of the simple dimer approximation is modified in higher orders. Explicit results are given in tenth order for dimers coupled in 1D, and in second order for dimers coupled in 3D with application to KCuCl3 and TlCuCl3.

Magnetic excitations and spin gap in KCuCl3

Abstract Detailed knowledge of the dynamical spin–spin correlation function S(κ, ω) turns out to be a necessity in characterizing unconventional quantum spin systems — in particular if a finite spin excitation gap separates a singlet ground state from excited states. Basic predictions relying on a Heisenberg dimer model are presented in the context of neutron scattering investigations on the S= 1 2 Cu 2+ compound KCuCl3. Expressions concerning dispersion relation, intensity and field dependence of the observed singlet–triplet modes above the gap are derived and compared to the experimental results. The agreement between theory and experiment is found to be remarkably good, supporting the characterization of KCuCl3 as a quantum spin liquid near to the dimer limit.

Local spin susceptibility in KCuCl3

Abstract S= 1 2 KCuCl 3 has a singlet ground state with a finite spin energy gap Δ=31 K to triplet excited states. The bulk magnetic susceptibility vanishes for T ⪡ Δ according to the singlet nature of the ground state. Astonishingly, recent muon spin relaxation (μSR) measurements reveal fluctuating internal fields down to T=5 K , with the onset of a quasi-static regime below T=5 K . The presence of free electronic magnetic moments and their possible freezing at low T contradicts the bulk picture of KCuCl 3 . By means of polarized neutron scattering and μSR measurements on single crystals of the same batch, the nature of the local spin susceptibility is studied. The claims of a substantial muon influence in KCuCl 3 are reinforced.