Quantum impurities in a two-dimensional Heisenberg antiferromagnet.

摘要

The study of quantum phase transitions in the presence of disorder is at the forefront of research in the field of strongly correlated electron systems, yet there have been relatively few experimental model systems. One important class of model systems for studying the effects of quenched disorder is magnetic materials with random site dilution. Percolation in classical high-spin magnetic systems has been studied extensively, and magnetic order persists in these materials up to the percolation threshold (the point at which dilution breaks the magnetic lattice into finite-sized disconnected clusters). The spin-1/2 square-lattice Heisenberg antiferromagnet (SLHAF) is of particular interest because of its connection to high-temperature superconductivity. However, previous results for magnetic dilution in the spin-1/2 SLHAF have been confined to dilution levels well below the percolation threshold, leaving many questions about this complex quantum-impurity problem unanswered.; Single crystals of La₂Cu_1−p (Zn,Mg)_pO₄ at concentrations up to and beyond the site percolation threshold provide the first experimental realization of quantum percolation in a spin-1/2 SLHAF. Complementary magnetometry, neutron scattering, and numerical experiments demonstrate that La₂Cu_1−p(Zn,Mg) _pO₄ is an excellent model material for studying this problem in the low-spin limit. Measurements of the ordered moment and spin correlations provide important quantitative information for tests of theories for this complex quantum-impurity problem. Quantum Monte Carlo results for the bilayer Heisenberg anti-ferromagnet allow the determination of the quantum-fluctuation versus geometric-disorder phase diagram, and indicate that the properties of La₂Cu_1−p(Zn,Mg) _pO₄ near the percolation threshold are controlled by the effective proximity to a new quantum critical point.