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...

We investigate the influence of a cyclic spin exchange Jcyc on the one-and two-triplet excitations of an undoped two-leg S=1/2 ladder, using the density matrix renormalization group (DMRG). The dispersion of the S=0 two-triplet bound state is dramatically reduced by Jcyc due to a repulsion between triplets on neighboring rungs. In (La,Ca)14Cu24O41 a consistent description of both the spin gap and the S=0 bound state requires Jcyc/J ⊥ ≈ 0.20-0.27 and J/J ⊥ ≈ 1.25-1.35. With these coupling ratios the recently developed dynamical DMRG yields an excellent description of the entire S=0 excitation spectrum observed in the optical conductivity, including the continuum contribution.PACS numbers: 75.10. Jm, 75.40.Gb, 75.40.Mg, 74.72.Jt, 75.30.Et The antiferromagnetic parent compounds of the high-T c cuprates are thought to be the best representatives of the two-dimensional S=1/2 square-lattice Heisenberg model. Understanding their magnetic properties is of utmost importance due to the intimate relationship of magnetic correlations and high-T c superconductivity. Recently, the question how to set up a minimal model which accounts for these magnetic properties has been readdressed. High-resolution inelastic neutron scattering experiments performed on the two-dimensional S=1/2 antiferromagnet La 2 CuO 4 [1] exhibit a magnon dispersion at the zone boundary which cannot be obtained within a nearest-neighbor Heisenberg model. It has been argued that the inclusion of a cyclic spin exchange term of about 20% would reproduce this dispersion [2]. This cyclic spin exchange emerges as a correction to the nearestneighbor Heisenberg Hamiltonian in order t 4 /U 3 from a t/U -expansion of the one-band Hubbard model [3]. It is expected to be the dominant correction within a more realistic three-band description of the CuO 2 -planes because there the cyclic permutation of 4 spins on a plaquette can take place without double occupancy [4,5]. Similar cyclic spin exchange processes have proven to be significant in other systems, e.g. in cuprate spin chains a ferromagnetic 2-spin cyclic exchange is responsible for the unusually strong exchange anisotropy [6].Cuprate spin ladders offer an alternative approach to decide about the existence and potential implications of a cyclic spin exchange term. They are composed of the same corner-sharing Cu-O plaquettes as the 2D cuprates, thus similar exchange couplings are expected for all spin products. In fact the inclusion of a cyclic spin exchange has been suggested in order to explain the smallness of the ladder spin gap observed in (La,Ca) 14 Cu 24 O 41 [7][8][9]. However, it is impossible to extract a unique set of coupling parameters or even to settle the existence of a cyclic spin exchange term from an analysis of the spin gap only. Here, the optical conductivity σ(ω) [10,11] can provide the missing information. Magnetic excitations can be observed in σ(ω) via the simultaneous excitation of a phonon [12,13]. In (La,Ca) 14 Cu 24 O 41 , two peaks in σ(ω) were identified as S=0 bound states of tw...

Combining infrared reflectivity, transport, susceptibility and several diffraction techniques, we find compelling evidence that CaCrO3 is a rare case of a metallic and antiferromagnetic transitionmetal oxide with a three-dimensional electronic structure. LSDA calculations correctly describe the metallic behavior as well as the anisotropic magnetic ordering pattern of C type: The high Cr valence state induces via sizeable pd hybridization remarkably strong next-nearest neighbor interactions stabilizing this ordering. The subtle balance of magnetic interactions gives rise to magneto-elastic coupling, explaining pronounced structural anomalies observed at the magnetic ordering transition.Strongly correlated electron systems including the wide class of transition-metal oxides exhibit a quite general relation between magnetic order and electrical conductivity [1]: ferromagnetism typically coexists with metallic conductivity, whereas insulators usually exhibit antiferromagnetism. It is always a challenge to understand exceptions from this rule. The rare observations of ferromagnetism in insulating transition-metal oxides most often are due to a particular type of orbital ordering [2]. The few examples of antiferromagnetic (AFM) metals, e.g., (La/Sr) 3 Mn 2 O 7 [3] or Ca 3 Ru 2 O 7 [4], are characterized by reduced electronic and structural dimensionality, and the antiferromagnetic order corresponds to a stacking of ferromagnetic (FM) layers. Here we report the discovery of a three-dimensional transition-metal oxide with metallic conductivity, antiferromagnetic exchange interactions, and C-type antiferromagnetic order: the perovskite CaCrO 3 .Perovskites containing Cr 4+ (CaCrO 3 , SrCrO 3 , and PbCrO 3 ) were already studied previously [5,6,7,8,9,10], but neither the details of the crystal structure nor the nature of the magnetic ordering are known. Only very recently evidence for C-type AFM order was reported in multi-phase samples of SrCrO 3 [10]. Regarding the conductivity, the existing data are controversial. In Refs. [7,9] CaCrO 3 was claimed to be metallic, but more recently insulating behavior has been reported [5]. A similar controversy persists also for SrCrO 3 , which should definitely be more metallic than CaCrO 3 due to the less distorted crystal structure, but metallic behavior was observed in Ref.[5] only under pressure. These controversies most likely are connected with the difficulty to prepare high-quality stoichiometric materials and with the lack of large single crystals.CaCrO 3 exhibits an orthorhombic GdFeO 3 -type perovskite structure and early magnetization measurements indicate a magnetic transition at 90 K [8], which is confirmed in our samples. Two electrons occupy the Cr 3d shell (S=1), rendering the material electronically similar to insulating RVO 3 [11] (also 3d 2 ) and to metallic (Ca/Sr)RuO 3 (4d 2 ) [12]. CaCrO 3 shows an unusually high transition-metal valence, Cr 4+ , which may lead to a small or even negative charge-transfer gap [13,14], i.e., holes in the O band. In CrO 2 with ruti...

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