Magnetic excitation spectrum of dimerized antiferromagnetic chains

Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladder

The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

Two-dimensional zeolite-like network in the new caesium copper aluminate Cs2CuAl4O8

Monoclinic dicaesium copper tetraaluminate, Cs2CuAl4O8, space group P2(1)/c, a = 8.4551 (7), b = 10.012 (1), c = 17.073 (2) Å, β = 101.643 (9)°, Z = 6, was obtained by high-temperature crystallization from a phosphate flux. Its microporous crystal structure presents the first example of double layers built from [AlO4] tetrahedra combined in 4-, 6- and 8-rings, topologically similar to those found in the ATT-type zeolites and isostructural minerals armstrongite, davanite and dalyite. These layers show a rare arrangement of three [AlO4] tetrahedra sharing one oxygen vertex. The aluminate slabs are further linked by chains of edge-sharing [CuO4] square planes to form a mixed anionic three-dimensional framework with Cs(+) cations in channels and cavities. An unusually short Cu···Cs distance of 3.166 Å is ascribed to the strong Jahn-Teller effect of Cu(2+). The magnetic subsystem demonstrates properties of an alternating antiferromagnetic chain with a gap in the spectrum of magnetic excitations.

Generalized Moment Method for Gap Estimation and Quantum Monte Carlo Level Spectroscopy

We formulate a convergent sequence for the energy gap estimation in the worldline quantum Monte Carlo method. The ambiguity left in the conventional gap calculation for quantum systems is eliminated. Our estimation will be unbiased in the low-temperature limit, and also the error bar is reliably estimated. The level spectroscopy from quantum Monte Carlo data is developed as an application of the unbiased gap estimation. From the spectral analysis, we precisely determine the Kosterlitz-Thouless quantum phase-transition point of the spin-Peierls model. It is established that the quantum phonon with a finite frequency is essential to the critical theory governed by the antiadiabatic limit, i.e., the k=1 SU(2) Wess-Zumino-Witten model.

Evidence for dimer crystal melting in the frustrated spin-ladder system BiCu2PO6

In the spin ladder compound BiCu(2)PO(6), there exists a decisive dynamics of spin excitations that we classify and characterize using inelastic light scattering. We observe an interladder singlet bound mode at 24  cm(-1) and two intraladder bound states at 62 and 108  cm(-1) in the leg (bb) and the rung (cc) polarization as well as a broad triplon continuum extending from 36  cm(-1) to 700  cm(-1). Though isolated spin ladder physics can roughly account for the observed excitations at high energies, frustration and interladder interactions need to be considered to fully describe the spectral distribution and scattering selection rules at low and intermediate energies. In addition, we attribute the rich spectrum of singlet bound modes to a melting of a dimer crystal. Our study provides evidence for a Z(2) quantum phase transition from a dimer to a resonating valence bond state driven by singlet fluctuations.

Magnetic properties of the spin S = 1/2 Heisenberg chain with hexamer modulation of exchange

We consider the spin-1/2 Heisenberg chain with alternating spin exchange in the presence of additional modulation of exchange on odd bonds with period 3. We study the ground state magnetic phase diagram of this hexamer spin chain in the limit of very strong antiferromagnetic (AF) exchange on odd bonds using the numerical Lanczos method and bosonization approach. In the limit of strong magnetic field commensurate with the dominating AF exchange, the model is mapped onto an effective XXZ Heisenberg chain in the presence of uniform and spatially modulated fields, which is studied using the standard continuum-limit bosonization approach. In the absence of additional hexamer modulation, the model undergoes a quantum phase transition from a gapped phase into the only one gapless Lüttinger liquid (LL) phase by increasing the magnetic field. In the presence of hexamer modulation, two new gapped phases are identified in the ground state at magnetization equal to [Formula: see text] and [Formula: see text] of the saturation value. These phases reveal themselves also in the magnetization curve as plateaus at corresponding values of magnetization. As a result, the magnetic phase diagram of the hexamer chain shows seven different quantum phases, four gapped and three gapless, and the system is characterized by six critical fields which mark quantum phase transitions between the ordered gapped and the LL gapless phases.

Quantum spin correlations in an organometallic alternating-sign chain

High resolution neutron scattering is used to study excitations in the organometallic magnet (CH3)2NH2CuCl3 (DMACuCl3). Combined with bulk magnetization and susceptibility studies, the new results imply that DMACuCl3 is a realization of the S=1/2 alternating antiferromagnetic-ferromagnetic (AFM-FM) chain. Coupled-cluster calculations indicate that the AFM and FM interactions have nearly the same strength, while analysis of scattering intensities shows evidence for interdimer spin correlations. Results are discussed in the context of recent ideas concerning quantum entanglement.

Spectral weight contributions of many-particle bound States and continuum

Cluster expansion methods are developed for calculating the spectral weight contributions of multiparticle excitations--continuum and bound states--to high orders. A complete 13th order calculation is carried out for the alternating Heisenberg chain. For lambda=0.27, relevant to the material Cu(NO3)(2).2.5D(2)O, we present detailed spectral weights for the two-triplet continuum and all bound states. We also examine the variation of the relative weights of one- and two-particle states with bond alternation from the dimerized to the uniform chain limit.

Excitations in one-dimensional S = 1/2 quantum antiferromagnets

The transition from dimerized to uniform phases is studied in terms of spectral weights for spin chains using continuous unitary transformations. The spectral weights in the S=1 channel are computed perturbatively around the limit of strong dimerization. We find that the spectral weight is concentrated mainly in the subspaces with a small number of elementary triplets (triplons), even for vanishing dimerization. So, besides spinons, triplons may be used as elementary excitations in spin chains. We conclude that there is no necessity to use fractional excitations in low-dimensional, undoped, or doped quantum antiferromagnets.

Coherent magnetic oscillation in the spin ladder system alpha(')- NaV2O5

We report the first observation of coherent magnetic excitations in a spin ladder system NaV2O5 by using femtosecond time-domain spectroscopy. A pronounced coherent oscillation is observed at 127 cm(-1) (nearly twice the spin gap energy) and assigned to a two-magnon bound state, based on the temperature dependence of the intensity below the charge ordering phase transition at T(C) = 34 K. This mode can be observable only when circularly polarized light is used as a pump or a probe beam, suggesting that it corresponds to a spin-flip excitation from the singlet ground state. A phonon mode strongly coupled to the spin state is also found at 303 cm(-1).

Fractional and integer excitations in quantum antiferromagnetic spin 1/2 ladders

Spectral densities are computed in unprecedented detail for quantum antiferromagnetic spin 1/2 two-leg ladders. These results were obtained due to a major methodical advance achieved by optimally chosen unitary transformations. The approach is based on dressed integer excitations. Considerable weight is found at high energies in the two-particle sector. Precursors of fractional spinon physics occur supporting the conclusion that there is no necessity to resort to fractional excitations in order to describe features at higher energies.

Observation of two-magnon bound states in the two-leg ladders of (Ca,La)(14)Cu(24)O(41)

Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and E parallel a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximately equal to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximately equal to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J parallel approximately equal to 1020-1100 cm(-1), J parallel/J perpendicular approximately equal to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T(c) cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.

Phase diagram of an asymmetric spin ladder

We investigate an asymmetric zigzag spin ladder with different exchange integrals on both legs using bosonization and renormalization group approaches. When the leg exchange integrals and frustration both are sufficiently small, renormalization group analysis shows that the Heisenberg critical point flows to an intermediate-coupling fixed point with gapless excitations and a vanishing spin velocity. When they are large, a spin gap opens and a dimer liquid is realized. Here, we find a continuous manifold of Hamiltonians with dimer product ground states, interpolating between the Majumdar-Ghosh and sawtooth spin-chain model.

Strong-coupling expansions for multiparticle excitations: continuum and bound states

We present a new linked cluster expansion for calculating properties of multiparticle excitation spectra to high orders. We use it to obtain the two-particle spectra for systems of coupled spin-half dimers. We find that even for weakly coupled dimers the spectrum is very rich, consisting of many bound states. The number of bound states depends on both geometry of coupling and frustration. Many of the bound states can only be seen by going to sufficiently high orders in the perturbation theory, showing the extended character of the pair attraction.

Collective singlet excitations and evolution of raman spectral weights in the 2D spin dimer compound SrCu2(BO3)(2)

Raman light scattering of the two-dimensional quantum spin system SrCu2(BO3)(2) shows a rich structure in the magnetic excitation spectrum, 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.