Quantum magnets doped with holes

Magnetism is a phenomenon that we experience in everyday-life quite often. The property, which is observed in materials such as such as iron, is caused by the alignment of electron spins. Even more interesting effects are expected in case that the magnetic crystals exhibit holes, i.e., lattice sites that are not occupied with an electron. Because of the interplay between the motion of the defect and the magnetic correlations of the electron spins, the magnetic order seems to be suppressed. In general, solid state physicists are not able to separate the two processes, so they cannot answer the question, whether the magnetic order is indeed reduced, or whether it is just hidden. Now a team of scientists around Dr. Christian Groß from the Quantum Many-Body Systems Division (director Professor Immanuel Bloch) at the Max Planck Institute of Quantum Optics has demonstrated that in one-dimensional quantum magnets the magnetic order is preserved even when they are doped with holes – a direct manifestation of spin-charge (density) separation. The quantum crystals were prepared by chains of ultracold atoms in an optical lattice. The observation was made possible with a unique tool which allows tracking the motion of holes and the spin excitations separately in one measurement process (Science, 4 August 2017). In the next step the scientists plan to extend the method to two-dimensional systems. Here the interaction between holes and magnetic correlations is by far more complex. It could lead to the detection of exotic many-body phases that might be responsible for the occurrence of high-temperature superconductivity. The Garching team starts with cooling an ensemble of fermionic lithium-6 atoms down to extremely low temperatures, a millionth of a Kelvin above absolute zero. The atoms are then captured in a single plane in a two-dimensional optical lattice that is created by laser beams. The plane in turn is split into about 10 one-dimensional tubes along which the atoms can move. In the last step, the tubes are superimposed with an optical lattice which mimics the periodic potential that electrons see in a real material. In analogy to electrons lithium atoms carry a spin-1/2 (or magnetic moment) which can point either upwards or downwards. In a previous experiment with a similar system the scientists have shown that below a certain temperature the magnetic moments of neighbouring atoms align in opposite directions such that antiferromagnetic correlations emerge. In the follow up experiment they investigate the influence of holes on the degree of order of the quantum crystal. “We achieve a certain amount of hole doping by making sure that the number of atoms loaded into the optical lattice is smaller than the number of lattice sites,” says Timon Hilker, first author and doctoral candidate at the experiment. “Now the questions arise, whether the holes are fixed or whether they can move, and how they affect the magnetic order of the system.””