We present measurements of the magnetic susceptibility and of the thermal expansion of a LaCoO3 single crystal. Both quantities show a strongly anomalous temperature dependence. Our data are consistently described in terms of a spin-state transition of the Co 3+ ions with increasing temperature from a low-spin ground state (t ) without (100 K -500 K) and with (> 500 K) orbital degeneracy. We attribute the lack of orbital degeneracy up to 500 K to (probably local) Jahn-Teller distortions of the CoO6 octahedra. A strong reduction or disappearance of the Jahn-Teller distortions seems to arise from the insulator-to-metal transition around 500 K.Transition-metal oxides have fascinating physical properties as e.g. high-temperature superconductivity in the cuprates or colossal magnetoresistance in the manganites. Their properties are often governed by a complex interplay of charge, magnetic, structural, and orbital degrees of freedom. Moreover, for a given oxidation state some transition metals display different spin states as it is the case in various cobalt oxides. Quite recently a class of layered cobalt compounds with the chemical composition REBaCo 2 O 5+δ (RE = rare earth) has attracted considerable interest. These compounds show a broad variety of ordering phenomena and other transitions, e.g. (antiferro-and/or ferro-) magnetic order, charge and/or orbital order, metal-insulator transitions or spin-state transitions [1,2,3,4,5,6,7,8,9]. For TlSr 2 CoO 5 it has been proposed that a metal-insulator transition is driven by a spin disproportionation, which consists of an alternating ordering of Co The occurrence of Co 3+ in different spin states is known since the 1950s from LaCoO 3 [12, 13], which transforms with increasing temperature from a non-magnetic insulator to a paramagnetic insulator around 100 K and shows an insulator-to-metal transition around 500 K. But even for this rather simple pseudo-cubic perovskite the nature of these transitions is still unclear. The ground state is usually attributed to the low-spin configuration (LS: t 6 2g e 0 g ; S = 0) and the paramagnetic behavior above 100 K to the thermal population of an excited state. However, the question whether the excited state has to be identified with the HS or the IS state is subject of controversial discussions. Early publications often assume a LS/HS scenario [14,15,16]. In order to explain the insulating nature up to 500 K an ordering of LS and HS Co 3+ ions has been proposed which vanishes at the insulatorto-metal transition [17,18]. Yet the presence of a HS configuration below 400 K has been questioned on the basis of X-ray absorption and photoemission experiments [19]. Alternative descriptions of LaCoO 3 favoring a LS/IS scenario [20,21,22,23,24] are mainly based on the results of LDA+U calculations [25], which propose that due to a strong hybridization between Co-e g levels and O-2p levels the IS state is lower in energy than the HS state. Within this scenario the occurrence of orbital order and its melting have been proposed in order to e...

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.

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We report on the magnetic, thermodynamic and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g. the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.PACS numbers: 75.10. Jm, 75.40.Cx, 71.70.Ch Low-dimensional quantum spin systems exhibit a multitude of interesting phenomena. For instance a onedimensional (1D) S=1/2 chain coupled to the lattice may show a spin-Peierls transition to a non-magnetic, dimerized ground state. In recent years, detailed studies of the first inorganic spin-Peierls compound CuGeO 3 have deepened the understanding of spin-Peierls systems substantially [1]. Even richer physics is expected if the spins are coupled additionally to orbital or charge degrees of freedom. A prominent example is the complex behavior of NaV 2 O 5 , which arises from the interplay of spin dimerization, orbital order and charge order [1]. Recently, TiOCl and TiOBr have been discussed as spinPeierls compounds with strong orbital fluctuations [2-9], assuming a near degeneracy of the t 2g orbitals in these 3d 1 systems. Different quantities such as the magnetic susceptibility [2], the specific heat [9], ESR data [3] and NMR spectra [4] point towards the existence of two successive phase transitions, which clearly goes beyond a canonical spin-Peierls scenario in which a single secondorder phase transition is expected. The high transition temperatures of T c1 =67 K and T c2 =91 K found in TiOCl are fascinating in a spin-Peierls context.The structure of TiOX consists of 2D Ti-O bilayers within the ab plane which are well separated by X=Cl/Br ions [10]. Quasi-1D S=1/2 chains are formed due to the occupation of the d y 2 −z 2 orbital in the ground state (see below), giving rise to strong direct exchange between neighboring Ti sites along the b axis (y direction) and negligible coupling in the other directions. Accordingly, the magnetic susceptibility of TiOCl is well described at high temperatures by the 1D S=1/2 Heisenberg model with an exchange constant of J ≈ 676 K [2,3]. In the non-magnetic low-temperature phase, a doubling of the unit cell along the chain direction has been observed by x-ray measurements for both TiOCl [10] and TiOBr [11], supporting a spin-Peierls scenario. However, the following experimental facts are not expected in a canonical spin-Peierls system: (i) the existence of two successive phase transitions [2-4,9], (ii) the first-order character of the low-temperature phase transition [9][10][11], (iii) the observation of inequivalen...

Based on a two-dimensional model of coupled two-leg spin ladders, we derive a unified picture of recent neutron scattering data of stripe-ordered La 15/8 Ba 1/8 CuO4, namely of the low-energy magnons around the superstructure satellites and of the triplon excitations at higher energies. The resonance peak at the antiferromagnetic wave vector QAF in the stripe-ordered phase corresponds to a saddle point in the dispersion of the magnetic excitations. Quantitative agreement with the neutron data is obtained for J = 130 − 160 meV and Jcyc/J = 0.2 − 0.25.PACS numbers: 74.25. Ha, 75.40.Gb, 75.10.Jm, 75.50.Ee Quantum magnetism in the cuprate superconductors is an intriguing issue. A detailed understanding of the dynamic spin susceptibility as measured by inelastic neutron scattering (INS) experiments should allow to clarify the role of magnetism in the mechanism of high-T c superconductivity. In particular two features have been in the focus of interest: the appearance of the socalled resonance peak [1,2] in the superconducting phase at the antiferromagnetic wave vector Q AF = (1/2, 1/2) (see Fig. 1b) at finite energies (e.g. 41 meV in optimally doped YBa 2 Cu 3 O 7−δ (YBCO)) and the existence of stripe order which manifests itself in superstructure satellites around Q AF (Fig. 1b) [2][3][4]. In general, these superstructure satellites are incommensurate, but they may become commensurate for certain doping concentrations. For many years, these two features have been regarded as separate issues, each of them apparent in only one of the two families of cuprates on which most neutron studies have focused: La 2−x Sr x CuO 4 (LSCO) and YBCO. But recent experimental results show that the resonance peak in YBCO is accompanied at lower energies by incommensurate reflections [5,6], and that stripe order appears also in YBCO [7][8][9]. Very recently, Tranquada et al.[10] observed a resonance peak at Q AF also in stripe-ordered La 15/8 Ba 1/8 CuO 4 (for T > T c ).An S=1 collective mode (the resonance peak) in the superconducting phase is a prominent feature of many different theoretical scenarios. Its interpretation ranges from a particle-hole bound state (see Refs. in [1,2]) to a particle-particle bound state in SO(5) theory [11]. In the stripe-ordered phase the choice of the microscopic model is straightforward. Static stripe order corresponds to a segregation into hole-rich charge stripes and hole-poor spin ladders. Tranquada et al.[10] analyzed their INS data at high energies (including the resonance) in terms of the elementary triplet excitations (triplons [12]) of isolated two-leg ladders (Fig. 1a) which are realized in case of bond-centered stripes [13]. But the incommensurate low-energy excitations were described in a separate model as spin waves (magnons), motivated by the existence of weak long-range order. Based on a model of coupled two-leg S = 1/2 ladders, we derive a unified description of the low-energy superstructure modes, of the resonance peak and of the highenergy excitations observed in Ref. [10]. The super...