Volume 84, Number 3, November 2008
Article Number 36005
Number of page(s) 6
Section Condensed Matter: Structural, Mechanical and Thermal Properties
Published online 30 October 2008
EPL, 84 (2008) 36005
DOI: 10.1209/0295-5075/84/36005

Orbital order driven quantum criticality

Z. Nussinov1 and G. Ortiz2

1   Department of Physics, Washington University - St. Louis, MO 63160, USA
2   Department of Physics, Indiana University - Bloomington, IN 47405, USA

received 6 March 2008; accepted in final form 29 September 2008; published November 2008
published online 30 October 2008

Charge, spin, and orbital degrees of freedom underlie the physics of transition metal compounds. Much work has revealed quantum critical points associated with spin and charge degrees of freedom in many of these systems. Here we illustrate that the simplest models that embody the orbital degrees of freedom —the two- and three-dimensional quantum orbital compass models— exhibit an exact quantum critical behavior on diluted square and cubic lattices (with doping $\delta$ = 1/4 and $\delta$ = 1/2, respectively). This raises the possibility of quantum critical points triggered by the degradation of orbital order upon doping (or applying pressure to) such transition metal systems. We prove the existence of an orbital spin glass in several related systems in which the orbital couplings are made non-uniform. Moreover, a new orbital Larmor precession (i.e., a periodic change in the orbital state) is predicted when uniaxial pressure is applied.

64.70.Tg - Quantum phase transitions.
71.20.Be - Transition metals and alloys.
71.10.-w - Theories and models of many-electron systems.

© EPLA 2008