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DESCRIPTION:Collin Broholm\, John Hopkins University\n\nMagne
tic materials are typically found in one of two qualitatively different st
ates: Thermally disordered at high temperatures or spin ordered at low tem
peratures. In this talk I describe a third distinct state of an interactin
g spin system: quantum ordered magnetism. I present neutron scattering dat
a that provide evidence for quantum order in zero\, one\, two\, and three-
dimensional spin systems. La_{4}Cu_{3}MoO_{12} con
tains spin-trimers that develop quantum order at low temperature where eac
h trimer becomes a composite spin-1/2 degree of freedom. Y_{2}BaNiO_{5} is an antiferromagnetic spin-1 chain with an ext
ensive one-dimensional Haldane ground state. I present scattering data tha
t provide clear evidence for long range coherence in the absence of conven
tional spin order^{2}. (C_{4}H_{12}N_{2})C
u_{2}C1_{6} (PHCC) is a frustrated bi-layer antiferromagne
t with interactions that span a two-dimensional plane. I show that there a
re coherent triplet excitations and argue that competing interactions favo
r quantum order over spin order^{3}. Cu_{2}(C_{5}H
_{12}N_{2})_{2}Cl_{4} (CuHpCl) has a coope
rative singlet ground state and was initially thought to be a spin ladder.
However\, neutron scattering data show that it is in fact a three dimensi
onal frustrated system with quantum order. Apart from describ
ing 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 access
ed experimentally in several of these systems. \n

\n

\n<
FONT SIZE=-1>**REFERENCES**\n

\n1. Y. Qiu\, C. Broholm\, S. Ishiwata
\, M. Azuma\, M. Takano\, R. Bewley\, and W. J. L. Buyers\, cond-mat/0205
018.

\n2. Guangyong Xu\, J. F. DiTusa\, T. Ito\, H. Takagi\, K. Oka\,
C. Broholm and G. Aeppli\, Phys. Rev. B** 54**\, R6827 (1996).

\n3.
M. B. Stone\, I. A. Zaliznyak\, Daniel H. Reich\, and C. Broholm\, Phys. R
ev. B **64**\, 144405 (2001).

\n4. M. B. Stone\, J. Rittner\, Y. Che
n\, H. Yardimci\, D. H. Reich\, C. Broholm\, D. V. Ferraris\, and T. Lectk
a\, Phys. Rev. B 65\, 064423 (2002).

\n5. 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).

\n6. Y. Chen\,
Z. Honda\, A. Zheludev\, C. Broholm\, K. Katsumata\, and S. M. Shapiro Phy
s. Rev. Lett. **86**\, 1618 (2001).

DTSTART:20031121T210000Z
LOCATION:Physics Building\, Room 204
SUMMARY:Quantum Coherence in Magnets
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