Field-controlled phase separation at the impurity-induced magnetic ordering in the spin-Peierls Magnet CuGeO3.

The paramagnetic fraction surviving at the impurity-induced antiferromagnetic phase transition in the spin-Peierls magnet CuGeO3 is found to increase with an external magnetic field. This effect is explained by the competition of the Zeeman interaction and of the exchange interaction of local antiferromagnetic clusters formed on the spin-gap background near impurities.

Magnetic-field-induced condensation of triplons in Han Purple pigment BaCuSi2O6.

Besides being an ancient pigment, BaCuSi2O6 is a quasi-2D magnetic insulator with a gapped spin dimer ground state. The application of strong magnetic fields closes this gap, creating a gas of bosonic spin triplet excitations. The topology of the spin lattice makes BaCuSi2O6 an ideal candidate for studying the Bose-Einstein condensation of triplet excitations as a function of the external magnetic field, which acts as a chemical potential. In agreement with quantum Monte Carlo numerical simulations, we observe a distinct lambda anomaly in the specific heat together with a maximum in the magnetic susceptibility upon cooling down to liquid helium temperatures.

Cooperative ordering of gapped and gapless spin networks in Cu2Fe2Ge4O13.

The unusual magnetic properties of a novel low-dimensional quantum ferrimagnet Cu2Fe2Ge4O13 are studied using bulk methods, neutron diffraction, and inelastic neutron scattering. It is shown that this material can be described in terms of two low-dimensional quantum spin subsystems, one gapped and the other gapless, characterized by two distinct energy scales. Long-range magnetic ordering observed at low temperatures is a cooperative phenomenon caused by weak coupling of these two spin networks.

Dynamics and scaling in a quantum spin chain material with bond randomness.

Single crystal inelastic neutron scattering is used to study dynamic spin correlations in the quasi-one-dimensional quantum antiferromagnet BaCu2(Si0.5Ge0.5)2O7, where the exchange constant fluctuates due to a random distribution of Si and Ge atoms. The measured low-energy spectrum is dominated by localized excitations and can be understood in the framework of the random singlet model. The observed scaling relations for the frequency dependencies of the correlation length and structure factor are in excellent agreement with recent theoretical predictions for the renormalization group fixed point.

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7.

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.

Dominance of the excitation continuum in the longitudinal spectrum of weakly coupled Heisenberg S=1/2 chains.

A low-field spin-flop transition in the quasi-one-dimensional antiferromagnet BaCu2Si2O7 is exploited to study the polarization dependence of low-energy magnetic excitations. The measured longitudinal spectrum is best described as a single broad continuum, with no sharp "longitudinal mode," in apparent contradiction with the commonly used chain-mean-field and random phase approximation (MF/RPA) theories. The observed behavior is also quite different than that previously seen in the related KCuF3 material, presumably due to a large difference in the relative strength of interchain interactions. The results highlight the limitations of the chain-MF/RPA approach.

Two-stage spin-flop transitions in the S = 1/2 antiferromagnetic spin chain BaCu(2)Si(2)O(7).

Two-stage spin-flop transitions are observed in the quasi-one-dimensional antiferromagnet BaCu(2)Si(2)O(7). A magnetic field applied along the easy axis induces a spin-flop transition at 2.0 T followed by a second transition at 4.9 T. The magnetic susceptibility indicates the presence of Dzyaloshinskii-Moriya (DM) antisymmetric interactions between the intrachain neighboring spins. We discuss a possible mechanism whereby the geometrical competition between DM and interchain interactions, as discussed for the two-dimensional antiferromagnet La(2)CuO(4), causes the two-stage spin-flop transitions.

Energy separation of single-particle and continuum states in an S = 1/2 weakly coupled chains antiferromagnet

Inelastic neutron scattering is used to study transverse-polarized magnetic excitations in the quasi-one-dimensional S = 1/2 antiferromagnet BaCu2Si2O7, where the saturation value for the Neel order parameter is m(0) = 0.12&mgr;(B) per spin. At low energies the spectrum is totally dominated by resolution-limited spin-wave-like excitations. An excitation continuum sets in above a well-defined threshold frequency. Experimental results are discussed in the context of current theories for weakly interacting quantum half-integer-spin chains.