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...

We present electron-spin-resonance data of Ti 3ϩ (3d 1 ) ions in single crystals of the novel layered quantum spin magnet TiOCl. The analysis of the g tensor yields direct evidence that the d xy orbital from the t 2g set is predominantly occupied and owing to the occurrence of orbital order a linear spin chain forms along the crystallographic b axis. This result corroborates recent theoretical local-density approximation ϩ U calculations of the band structure. The temperature dependence of the parameters of the resonance signal suggests a strong coupling between spin and lattice degrees of freedom and gives evidence for a transition to a nonmagnetic ground state at 67 K.Transition-metal ͑TM͒ oxides with low-dimensional structural elements provide a fascinating ''playground'' to study novel phenomena such as high-temperature superconductivity, spin-charge separation, spin-gap states, and quantum disorder. 1-3 Until recently, the emphasis has been put on Cu-based oxides, where a Cu 2ϩ (3d 9 ) ion has a single hole in the e g orbitals with spin Sϭ1/2, and its orbital momentum is almost completely quenched by the crystal field. The ions at the beginning of the TM elements row, such as Ti 3ϩ and V 4ϩ , have, in contrast, a single d electron which occupies one of the t 2g orbitals. Because these orbitals are much less Jahn-Teller active, their near degeneracy may yield more complicated physics, involving not only the spin and charge, but also the orbital sector. 4 As an example, the threedimensional cubic perovskite LaTiO 3 has been proposed to realize a quantum orbital liquid. 5,6 . However, recent x-ray and neutron structural data suggest the ordering of the orbitals. 7 The structural dimensionality is reduced in TiOCl, where ͓TiO 4 Cl 2 ͔ octahedra are arranged in bilayers separated from each other along the c axis ͓Fig. 1͑a͔͒. In fact, for quite a while this compound has been considered as a twodimensional ͑2D͒ antiferromagnet, an electron analog to the high-T c cuprates, 9 owing to an almost T-independent magnetic susceptibility reported in Ref. 8. However, very recently TiOCl has emerged in an entirely new light as a 1D antiferromagnet 10 and is proposed as the second example of an inorganic spin-Peierls compound after CuGeO 3 . 11 LDAϩU ͑where LDAϩU means local-density approximation including Hubbard U͒ band-structure calculations 10 suggest ordering of the t 2g orbitals in TiOCl which produces quasi-1D antiferromagnetic ͑AF͒ Sϭ1/2 chains. This calculation favors the occupancy of the d xy orbitals ͓Fig. 1͑b͔͒ which form a uniform chain along the b axis. A transition to a nonmagnetic state at T c ϭ67 K has been observed in the static magnetization. 10 Remarkably, nuclear-magnetic resonance ͑NMR͒ data reveal the preexisting pseudo-spin-gap already above T c which is ascribed to strong orbital fluctuations. 12 In this paper we present electron-spin-resonance ͑ESR͒ data of Ti 3ϩ (3d 1 ) in single crystals of TiOCl. By measuring the anisotropy of the g factor and comparing it with our theoretical estimates in the fra...

Collective orbital excitations in orbitally ordered YVO3 and HoVO3 E Benckiser, R Rückamp, T Möller et al. Abstract. The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO 3 (R = La, Sm and Y), LaMnO 3 , Y 2 BaNiO 5 , CaCu 2 O 3 and K 4 Cu 4 OCl 10 , ranging from early to late transition-metal ions, from t 2g to e g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO 3 , we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yields good agreement, demonstrating that the coupling to the lattice is important for a 7 Author to whom any correspondence should be addressed. quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g. the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular, in case of the orbital excitations at ≈0.25 eV in RTiO 3 . Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing. Contents

The structural and thermal behavior of all members of the homologous series of neodymium(III) alkanoates, ranging from neodymium(III) butyrate to neodymium(III) eicosanoate are described. Neodymium(III) butyrate monohydrate, Nd(C 3 H 7 COO) 3 ‚H 2 O crystallizes in space group P1 h (No. 2), Z ) 2. The lattice parameters are a ) 9.824(2) Å, b ) 11.974(2) Å, c ) 14.633(2) Å, R ) 86.21(2)°, ) 75.92(2)°, γ ) 77.97(2)°. The crystal structure consists of ionic layers of neodymium ions, separated by bilayers of butyrate anions. In the ionic layers, the neodymium ions are connected by bridging tridentate carboxylate groups to zigzag chains, whereas the chains are connected among themselves by bridging bidentate carboxylate groups. The two crystallographically different neodymium ions are both having coordination number 9, with a geometry close to a monocapped square antiprism. The structure of the higher homologues can be derived from the structure of neodymium butyrate by extending the alkyl chains. These compounds have a lamellar bilayer structure with planes of neodymium(III) ions coordinated to the carboxylate groups and with the alkyl chains in an all-trans conformation. All homologous compounds from neodymium(III) pentanoate to neodymium(III) pentadecanoate display a thermotropic mesophase, which was identified by high-temperature X-ray diffraction as a smectic A phase. For the series from neodymium(III) pentanoate to neodymium(III) undecanoate an additional high viscosity mesophase is present between the crystalline state and the smectic A mesophase.