We present a theory of phonon-assisted optical twomagnon absorption in two-leg spin-ladders. Based on the strong intra-rung-coupling limit we show that collective excitations of total spin S = 0, 1 and 2 exist outside of the two-magnon continuum. It is demonstrated that the singlet collective state has a clear signature in the optical spectrum. 71.27.+a, 75.10.Jm, 75.50.Ee, 75.30.Et, Recent design of materials with localized spin-1/2 moments arranged in ladder, as well as frustrated and dimerized chain geometries has enormously intensified research on low-dimensional quantum magnetism [1]. Many of the novel materials are likely candidates for a spin-liquid ground state and display a gapped spinexcitation spectrum. On the two-leg ladder the spin-gap is a phenomenon well established by various theoretical approaches, i.e. exact diagonalization [2,3], strong coupling expansion [4,5], density-matrix renormalization group [6], and bosonization [7]. Optical spectroscopy, and in particular Raman scattering, has proven to be a valuable tool to investigate the spin excitations of the new quantum magnets [8]. Apart from Raman scattering, optical absorption has has been studied, however, only on the single-chain cuprate Sr 2 CuO 3 [9,10] where it has provided direct evidence for the gapless twospinon spectrum of the spin-1/2 Heisenberg chain [11]. In Sr 2 CuO 3 absorption is due to phonon assisted twomagnon emission (PME) [12,13]. While several of the new spin-ladder materials, eg. SrCu 2 O 3 and LaCuO 2.5 , may allow for PME as well, corresponding studies are lacking. In this work we propose a simple theory of PME for spin-ladders. We show that bound states in the spingap which have no analogy in the Heisenberg chain have a substantial effect on the optical absorption spectrum.We start our discussion with the two-leg spin-ladder Hamiltonianwhere S α µl with α = x, y, z is a spin-1/2 operator on site l of leg µ and H is measured in units of J ⊥ with λ = J /J ⊥ . In this paper we focus on the limit of large intrarung coupling λ → 0 in which a reformulation of (1) in terms of the rung-spin S α l = S α 1l + S α 2l and the operatorwhere S l = 0(1) for a rung singlet(triplet). The rung-spin eigenbasis is given by |s2 where the first(second) entry in the kets refers to a site on leg '1(2)' of the ladder. As usual S α |s = 0 and S α |t β = iε αβγ |t γ with the LeviCivita symbol ε αβγ . The action of T α on the rung basis is given by T α |s = |t α and T α |t β = δ αβ |s .For vanishing inter-rung coupling the ground state of (2) is a pure rung-singlet product-state | = ⊗ l |s l . The excited states are products of singlets and triplets |{t mα } = ⊗ {l} |s l ⊗ {mα} |t mα with an excitation energy given by the number of triplets N [{t mα }]. At finite λ the action of the ladder-Hamiltonian on these product states can be read off easily from (2): (i) H 2 creates(destroys) pairs of nearest-neighbor (NN) triplets of equal α-index, (ii) given a pair of NN sites in a relative state of one singlet and one triplet H 2 generates NN hopping o...

We present a bond-operator mean-field theory for the Kondo lattice model at half filling in two and three dimensions ͑3D͒. A continuous quantum phase transition from an antiferromagnetic to a spin-gapped singlet ground state is found at J/tϭ1.505 (1.833) in 2D (3D). Additionally we evaluate the quasiparticle dispersions as well as the staggered magnetic moment and provide a comparison with complementary numerical approaches. The Kondo lattice model ͑KLM͒ describes the exchangescattering of a band of itinerant conduction electrons at a lattice of localized magnetic moments. It serves as a basic model for heavy fermion materials in the integral-valent limit. 1 At half filling of the conduction band it is believed to give a description of Kondo insulators, which have been of considerable interest in recent years. 2 In one dimension the ground state at half filling is a spin singlet for all values of exchange-scattering and conduction-band widths. 3 In higher dimensions it has been suggested early on, that the competition between Kondo screening and the Ruderman-Kittel-Kasuya-Yoshida ͑RKKY͒ interaction leads to a quantum phase transition between a global spin-singlet and an antiferromagnetically ordered phase. 4,5 This scenario has been corroborated in two dimensions by variational 6 calculations, series expansion 7 and mean-field 8 approaches. Numerically exact results in two dimensions ͑2D͒ have been obtained recently by QMC. 9,10 In 3D only series expansion is available. 7 The purpose of this work is to introduce a mean-field theory for the KLM at half filling in two and three dimensions. In contrast to other mean-field calculations 5,8 our approach is based on a bond-operator representation of the KLM which is suitable for strong exchange scattering and has proven to be useful in dimerized spin systems. 12 Moreover, our treatment goes beyond recent mean-field work focusing on the Kondo-necklace problem which neglects conduction-electron charge fluctuations. 11 The KLM reads

We present a unified account for the coupled single-holeand spin-dynamics in the spin-gap phase of dimerized and frustrated spin-chains and two-leg spin ladders. Based on the strong dimer-limit of a one-dimensional t1,2,3-J1,2,3-model a diagrammatic approach is presented which employs a mapping of the spin-Hamiltonian onto a pseudo-fermion bondboson model. Results for the single-hole spectrum are detailed. A finite quasi-particle weight is observed and studied for a variety of system parameters. A comparison with existing exact diagonalization data is performed and good agreement is found. 71.27.+a, 71.10.Fd, 75.10.Jm

We demonstrate that a two-triplet resonance strongly renormalizes the Raman spectrum of two-leg spin ladders and moreover suggest this to be the origin of the asymmetry of the magnetic Raman continuum observed in CaV2O5.Comment: 2 pages, 1 figur

We consider the Kondo lattice model in two dimensions at half filling. In addition to the fermionic hopping integral t and the superexchange coupling J the role of a Coulomb repulsion U in the conduction band is investigated. We find the model to display a magnetic order-disorder transition in the U -J plane with a critical value of Jc which is decreasing as a function of U . The single particle spectral function A( k, ω) is computed across this transition. For all values of J > 0, and apart from shadow features present in the ordered state, A( k, ω) remains insensitive to the magnetic phase transition with the first low-energy hole states residing at momenta k = (±π, ±π). As J → 0 the model maps onto the Hubbard Hamiltonian. Only in this limit, the low-energy spectral weight at k = (±π, ±π) vanishes with first electron removalstates emerging at wave vectors on the magnetic Brillouin zone boundary. Thus, we conclude that (i) the local screening of impurity spins determines the low energy behavior of the spectral function and (ii) one cannot deform continuously the spectral function of the Mott-Hubbard insulator at J = 0 to that of the Kondo insulator at J > Jc. Our results are based on both, T = 0 Quantum Monte-Carlo simulations and a bond-operator mean-field theory.