Low-dimensional magnetism

Electronic transport in low-dimensional copper oxides has been the focus of much activity during the past decade. In particular, unusual superconducting states have been observed in layered cuprates, and more recently also in cuprates with quasi-one-dimensional structures. One-dimensional systems are more amenable to analytical and numerical calculations and may well hold lessons for the two-dimensional systems as well. Unfortunately, quasi-one-dimensional cuprates that can be doped, such as (Sr, Ca)₁₄Cu₂₄O₄₁, are structurally very complicated, with several electronically active units. Structurally simple quasi-one-dimensional copper oxides such as CuGeO₃, SrCuO₂, and Sr₂CuO₃ have also been widely studied, but attempts to introduce mobile charge carriers into the copper oxide chains have thus far been unsuccessful.

The discovery of a new class of copper oxides with quasi-one-dimensional electronic structure in Jansen's department [1] opens up a new perspective for a detailed description of the interplay between magnetism and charge order in the cuprates. These compounds are free of chemical disorder and can be highly doped. We have established the presence of helical order due to long-range competing exchange interactions in NaCu₂O₂, the insulating parent compound of this family (Fig. 1a) [2,3]. We also characterized its magnetic and dielectric properties [3] as well as the magnetic and lattice dynamical excitations [4] of NaCu₂O₂. An investigation of the optical properties of a related copper-oxide chain compound has led to the discovery of novel excitonic modes whose spectral weight is controlled by one-dimensional spin correlations [5]. These results are an excellent basis for ongoing experiments on doped analogs. Initial experiments on Na₈Cu₅O₁₀, a compound exhibiting an intricate charge-ordering pattern (Fig. 1b), have already revealed an unusual spin-density modulation [6]. These projects are carried out in close cooperation with chemists in Jansen's department, and with theorists in Metzner's department.

In another line of research, we are using the newly developed TRISP spectrometer to probe the lifetimes of collective excitations in low-dimensional quantum magnets, following successful experiments on a three-dimensional classical magnet [7]. These experiments allow detailed tests of long-standing theoretical predictions.