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 three-dimensional spin systems. La4Cu3MoO12 contains spin-trimers that develop quantum order at low temperature where each trimer becomes a composite spin-1/2 degree of freedom. Y2BaNiO5 is an antiferromagnetic spin-1 chain with an extensive one-dimensional Haldane ground state. I present scattering data that provide clear evidence for long range coherence in the absence of conventional spin order2. (C4H12N2)Cu2C16 (PHCC) is a frustrated bi-layer antiferromagnet with interactions that span a two-dimensional plane. I show that there are coherent triplet excitations and argue that competing interactions favor quantum order over spin order3. Cu2(C5H12N2)2Cl4 (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 impurities5 and the field driven quantum phase transitions that can be accessed experimentally in several of these systems.
1. Y. Qiu, C. Broholm, S. Ishiwata, M. Azuma, M. Takano, R. Bewley, and W. J. L. Buyers, cond-mat/0205018.
2. Guangyong Xu, J. F. DiTusa, T. Ito, H. Takagi, K. Oka, C. Broholm and G. Aeppli, Phys. Rev. B 54, R6827 (1996).
3. M. B. Stone, I. A. Zaliznyak, Daniel H. Reich, and C. Broholm, Phys. Rev. B 64, 144405 (2001).
4. M. B. Stone, J. Rittner, Y. Chen, H. Yardimci, D. H. Reich, C. Broholm, D. V. Ferraris, and T. Lectka, Phys. Rev. B 65, 064423 (2002).
5. M. Kenzelmann, G. Xu, I. A. Zaliznyak, C. Broholm, J. F. DiTusa, G. Aeppli, T. Ito, K. Oka, and H. Takagi. Phys. Rev. Lett. 90, 087202 (2003).
6. Y. Chen, Z. Honda, A. Zheludev, C. Broholm, K. Katsumata, and S. M. Shapiro Phys. Rev. Lett. 86, 1618 (2001).