The idea of confinement states that in certain systems constituent particles can be discerned only indirectly being bound by an interaction whose strength increases with increasing particle separation. Though the most famous example is the confinement of quarks to form baryons and mesons in (3+1)-dimensional Quantum Chromodynamics, confinement can also be realized in condensed matter physics systems such as spin-ladders which consist of two spin-1/2 antiferromagnetic chains coupled together by spin exchange interactions. Excitations of individual chains (spinons) carrying spin S=1/2, are confined even by an infinitesimal interchain coupling. The realizations studied so far cannot illustrate this process due to the large strength of their interchain coupling which leaves no energy window for the spinon excitations of individual chains. Here we present neutron scattering experiments for a weakly-coupled ladder material. At high energies the spectral function approaches that of individual chains; at low energies it is dominated by spin 0,1 excitations of strongly-coupled chains.The experiments presented in this paper illustrate the condensed matter realization of the confinement idea. The original and most popularized form of this idea comes from particle theory, more specifically from the theory of strong interactions. It is suggested that heavy particles (baryons and mesons) are made of quarks. The latter particles possess properties (more precisely, quantum numbers) which cannot be directly observed, such as fractional electric charge (±2e/3,±e/3). In a similar fashion in spin ladders excitations of individual spin-1/2 chains (spinons) carry quantum numbers which which are forbidden as soon as the chains are coupled. Quarks are held together by the Yang-Mills (or colour) gauge field which quanta are called gluons. As for any gauge field at smallest distances this interaction obeys the Coulomb law, but at larger distances instead of decreasing it progressively increases due to the gluon-gluon interaction. The popular image is of gluon field lines sticking together and creating some kind of "string" between the quarks. This picture is very appealing since quarks being just end points of a string can under no circumstances appear as individual particles provided the string has a finite tension. Even if one allows the string to snap, none of its pieces can have just one end and hence single quarks still cannot appear. A finite string tension generates an effective potential between the quarks which grows with distance leading to their confinement. Since such a potential well apparently contains infinite number of energy levels, corresponding to different internal energies and hence by the E = mc 2 relation to different particles masses, one would imagine that there is an infinite number of particles with the same quantum numbers, but different masses. This picture is oversimplified, however, failing to take into account the quantum nature of the gluon

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We present experimental results for the thermal conductivity kappa of the pseudo-two-leg ladder material CaCu2O3. The strong buckling of the ladder rungs renders this material a good approximation to a S=1/2 Heisenberg chain. Despite a strong suppression of the thermal conductivity of this material in all crystal directions due to inherent disorder, we find a dominant magnetic contribution kappa mag along the chain direction. kappa mag is linear in temperature, resembling the low-temperature limit of the thermal Drude weight D th of the S=1/2 Heisenberg chain. The comparison of kappamag and Dth yields a magnetic mean-free path of l mag approximately 22+/-5 A, in good agreement with magnetic measurements.

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