ABSTRACT:
Magnetic materials are typically found in one of two qualitatively different states: Thermally disordered at high temperatures or spin ordered at low temperatures. In this talk I describe a third distinct state of an interacting spin system: quantum ordered magnetism. I present neutron scattering data that provide evidence for quantum order in zero, one, two, and threedimensional spin systems. La_{4}Cu_{3}MoO_{12} contains spintrimers that develop quantum order at low temperature where each trimer becomes a composite spin1/2 degree of freedom. Y_{2}BaNiO_{5} is an antiferromagnetic spin1 chain with an extensive onedimensional Haldane ground state. I present scattering data that provide clear evidence for long range coherence in the absence of conventional spin order^{2}. (C_{4}H_{12}N_{2})Cu_{2}C1_{6} (PHCC) is a frustrated bilayer antiferromagnet with interactions that span a twodimensional plane. I show that there are coherent triplet excitations and argue that competing interactions favor quantum order over spin order^{3}. Cu_{2}(C_{5}H_{12}N_{2})_{2}Cl_{4} (CuHpCl) has a cooperative singlet ground state and was initially thought to be a spin ladder. However, neutron scattering data show that it is in fact a three dimensional frustrated system with quantum order. Apart from describing and comparing the low temperature quantum ordered states in these pure systems, I shall also touch on the fascinating effects of impurities^{5} and the field driven quantum phase transitions that can be accessed experimentally in several of these systems.
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