Low-lying excitations and magnetization process of coupled tetrahedral systems

We investigate low-lying singlet and triplet excitations and the magnetization process of quasi-1D spin systems composed of tetrahedral spin clusters. For a class of such models, we found various exact low-lying excitations; some of them are responsible for the first-order transition between two different ground states formed by local singlets. Moreover, we find that there are two different kinds of magnetization plateaus which are separated by a first-order transition.Comment: To appear in Phys.Rev.B (Issue 01 August 2002). A short comment is adde

Spin 1/2 Magnetic Impurity in a 2D Magnetic System Close to Quantum Critical Point

We consider a magnetic impurity in a spin liquid state of a magnetic system which is close to the quantum phase transition to the magnetically ordered state. There is similarity between this problem and the Kondo problem. We derive the impurity Green's function, consider renormalizations of the magnetic moments of the impurity, calculate critical indexes for the magnetic susceptibilities and finally consider specific heat and magnetic interaction of two impurities.Comment: 9 pages, 9 figure

RT-2 Detection of Quasi-Periodic Pulsations in the 2009 July 5 Solar Hard X-ray Flare

We present the results of an analysis of hard X-ray observations of the C2.7 solar flare detected by the RT-2 Experiment onboard the Coronas - Photon satellite. We detect hard X-ray pulsations at periods of ~12 s and ~15 s. We find a marginal evidence for a decrease in period with time. We have augmented these results using the publicly available data from the RHESSI satellite. We present a spectral analysis and measure the spectral parameters.Comment: 12 pages, 8 figures and 3 tables, accepted for publication in The Astrophysical Journa

Critical Dynamics of Singlet Excitations in a Frustrated Spin System

We construct and analyze a two-dimensional frustrated quantum spin model with plaquette order, in which the low-energy dynamics is controlled by spin singlets. At a critical value of frustration the singlet spectrum becomes gapless, indicating a quantum transition to a phase with dimer order. This T=0 transition belongs to the 3D Ising universality class, while at finite temperature a 2D Ising critical line separates the plaquette and dimerized phases. The magnetic susceptibility has an activated form throughout the phase diagram, whereas the specific heat exhibits a rich structure and a power law dependence on temperature at the quantum critical point. We argue that the novel quantum critical behavior associated with singlet criticality discussed in this work can be relevant to a wide class of quantum spin systems, such as antiferromagnets on Kagome and pyrochlore lattices, where the low-energy excitations are known to be spin singlets, as well as to the CAVO lattice and several recently discovered strongly frustrated square-lattice antiferromagnets.Comment: 5 pages, 5 figures, additional discussion and figure added, to appear in Phys. Rev.

Magnetic Impurity in the two-dimensional Heisenberg Antiferromagnet

We analyze the ground state properties of the two-dimensional quantum antiferromagnet with a S=1/2 Kondo impurity. Perturbation theory around the strong Kondo coupling limit is developed and the results compared with studies, based on exact diagonalization of small clusters. We find that at intermediate coupling the impurity is partially screened and the magnetization locally suppressed. A local singlet between the impurity and the host spin is formed asymptotically.Comment: 12 REVTex pages, 4 Postscript figure

Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration

We have studied the low-energy excitation spectrum of a dimerized and frustrated antiferromagnetic Heisenberg chain. We use an analytic approach, based on a description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. Additional singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of the elementary triplets. We have also calculated the contributions of the elementary and collective excitations into the spin structure factor. Our results are in excellent agreement with exact diagonalizations and dimer series expansions data as long as the dimerization parameter $\delta$ is not too small ($\delta>0.1$), i.e. while the elementary triplets can be treated as localized objects.Comment: 18 pages, 13 figure

Excitation spectrum of the S=1/2 quantum spin ladder with frustration: elementary quasiparticles and many-particle bound states

We study the excitation spectrum of the two-chain S=1/2 Heisenberg spin ladder with additional inter-chain second-neighbor frustrating interactions. The one and two-particle excitations are analyzed by using a mapping of the model onto a Bose gas of hard-core triplets. We find that low-lying singlet and triplet two-particle bound states are present and their binding energy increases with increasing frustration. In addition, many-particle bound states are found by a combination of variational and exact diagonalization techniques. We prove that the larger the number of bound quasiparticles the larger the binding energy. Thus the excitation spectrum has a complex structure and consists of elementary triplets and collective many-particle singlet and triplet excitations which generally mix with the elementary ones. The model exhibits a quantum phase transition from an antiferromagnetic ladder phase (small frustration) into Haldane phase (effectively ferromagnetic ladder for large frustration). We argue that near the transition point the spectrum in both triplet and singlet channels becomes gapless. The excitation wave function is dominated by large-size bound states which leads to the vanishing of the quasiparticle residue.Comment: RevTeX, 23 pages, 12 figure

Collective Singlet Excitations and Evolution of Raman Spectral Weights in the 2D Spin Dimer Compound SrCu2(BO3)2

We present a Raman light scattering study of the two-dimensional quantum spin system SrCu2(BO3)2 and show that the magnetic excitation spectrum has a rich structure, including several well-defined bound state modes at low temperature, and a scattering continuum and quasielastic light scattering contributions at high temperature. The key to the understanding of the unique features of SrCu2(BO3)2 is the presence of strong interactions between well-localized triplet excitations in the network of orthogonal spin dimers realized in this compound. Based on our analysis of the Heisenberg model relevant for this material, we argue that the collective excitations involving two and three-particle singlet bound states have large binding energies and are observed as well-defined peaks in the Raman spectrum.Comment: 5 pages, 2 figures. Revised version, to appear in Phys. Rev. Lett. (2000

Incommensurate magnetism in the coupled spin tetrahedra system Cu2Te2O5Cl2

Neutron scattering studies on powder and single crystals have provided new evidences for unconventional magnetism in Cu2Te2O5Cl2. The compound is built from tetrahedral clusters of S=1/2 Cu2+ spins located on a tetragonal lattice. Magnetic ordering, emerging at TN=18.2 K, leads to a very complex multi-domain, most likely degenerate, ground state, which is characterized by an incommensurate (ICM) wave vector k ~ [0.15, 0.42,1/2]. The Cu2+ ions carry a magnetic moment of 0.67(1) mB/ Cu2+ at 1.5 K and form a four helices spin arrangement with two canted pairs within the tetrahedra. A domain redistribution is observed when a magnetic field is applied in the tetragonal plane (Hc≈0.5 T), but not for H||c up to 4 T. The excitation spectrum is characterized by two well-defined modes, one completely dispersionless at 6.0 meV, the other strongly dispersing to a gap of 2 meV. The reason for such complex ground state and spin excitations may be geometrical frustration of the Cu2+ spins within the tetrahedra, intra- and inter-tetrahedral couplings having similar strengths and strong Dzyaloshinski-Moriya anisotropy. Candidates for the dominant intra- and inter-tetrahedral interactions are proposed