Quantum phases and transitions with a macroscopic magnetic moment
Exploring the vicinity of a quantum critical point requires availability of a tuning
parameter (other than the temperature) which can modify parameters in the Hamiltonian.
One of the most convenient is an applied magnetic field. It couples via the Zeeman term
to the total spin, and can thus drive quantum phase transitions between phases with
and without a macroscopic magnetic moment. Often, the total spin operator commutes
with the Hamiltonian, and in these cases the conservation law has
strong consequences for the theory of the quantum critical point. The papers below explore
a number of experimentally relevant quantum critical points of this type, and show how
the conservation of total spin leads has clear experimental signatures in the quantum critical
fluctuations.
Experimental examples of such transitions are in B. C. Watson et al., Physical Review Letters 86, 5168 (2001), S. A. Vitkalov et al., Physical Review Letters 87, 086401 (2001), and G. Chaboussant et al., Physical Review Letters 80, 2713 (1998).
PAPERS

Quantum phase transitions and conserved charges,
S. Sachdev, Zeitschrift fur Physik B 94, 469 (1994); condmat/9312018.

Finite temperature properties of quantum antiferromagnets
in a uniform magnetic field in one and two dimensions, S. Sachdev,
T. Senthil, and R. Shankar, Physical Review B 50, 258 (1994); condmat/9401040.

Continuum quantum ferromagnets at finite temperature
and the quantum Hall effect, N. Read and S. Sachdev, Physical Review
Letters 75, 3509 (1995); condmat/9507103.

Zero temperature phase transitions in quantum Heisenberg
ferromagnets, S. Sachdev and T. Senthil, Annals of Physics 251,
76 (1996); condmat/9602028.

Universal low temperature properties of quantum and classical
ferromagnetic chains, M. Takahashi, H. Nakamura, and S. Sachdev, Physical
Review B 54, R744 (1996); condmat/9602114.

Multicritical crossovers near the dilute Bose gas quantum
critical point, K. Damle and S. Sachdev, Physical Review Letters 76,
4412 (1996); condmat/9602073.

Universal critical temperature for KosterlitzThouless
transitions in bilayer quantum magnets, M. Troyer and S. Sachdev, Physical
Review Letters 81, 5418 (1998); condmat/9807393.

Quantum spin glass transition in the two dimensional electron gas, S. Sachdev, Pramana 58, 285 (2002); condmat/0109309.

Quantum phases and phase transitions of Mott insulators , S. Sachdev in Quantum magnetism, U. Schollwock, J. Richter, D. J. J. Farnell and R. A. Bishop eds,
Lecture Notes in Physics, Springer, Berlin (2004), condmat/0401041.
 Quantum critical dynamics of the twodimensional Bose gas , S. Sachdev and E. R. Dunkel, Physical Review B 73, 085116 (2006); condmat/0510365.
 Magnetization of the ShastrySutherland antiferromagnet
near the Ising limit, F. Liu and S. Sachdev, arXiv:0904.3018.
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