Condensed-Matter Physics & Materials Science Seminar

Introducing holes into low-dimensional quantum antiferromagnets significantly affects their magnetic properties and yields intriguing
ground states. Although there is no doubt that the number of holes and their mobility are crucial issues in the 2D cuprates, the effect of
the holes on the spin degrees of freedom is still under discussion in these compounds. We have thus investigated the influence of holes on
the quasi-one-dimensional (1D) cuprate spin chain systems (Sr,Ca,La)14Cu24O41 and Li2CuO2. Here, S = 1/2 CuO2 spin chains are realised in
which the number of holes can be adjusted between 0 and 60%, thereby providing model systems for the interplay of spin and charge in low
dimensional cuprates.
We present nuclear magnetic resonance (NMR), (in)elastic neutron scattering (INS/NS) and pulsed field magnetization data in magnetic fields
up to 60T together with measurements of the specific heat and the thermal expansion. Our data show that spin order and charge mobility are
intimately connected. For 60% hole doping, a static charge order and a dimer state are well established. For 50% holes, our NMR and INS
data imply the presence of local charge ordering and dimer formation on the NMR/IMS time scales but the absence of a static spin gap. We
discuss our results in terms of dynamic dimers, in which two Cu spins are antiferromagnetically coupled via an intermediate
Zhang-Rice-Singlet and magnetically well separated from other Cu spins. At low hole doping (~10%), there is indirect evidence for a coupled
(possibly short ranged) charge stripe order and a 3D spin order. Our data suggest that the positions of the holes in the chains are
affected by the magnetic field: small magnetic fields induce the motion of the holes along the chains, thereby frustrating the magnetic
interchain coupling and destroying the charge and spin order. Thus, we present a simple model system where it is possible to drive the hole
motion by a tuneable external parameter and to study