Ruff, J. P. C.Clancy, J. P.Bourque, A.White, M. A.Ramazanoglu, M.Gardner, J. S.Qiu, Y.Copley, J. R. D.Johnson, M. B.Dabkowska, H. A.Gaulin, B. D.2013-08-232013-08-232008-10Ruff, J. P. C., J. P. Clancy, A. Bourque, M. A. White, et al. 2008. "Spin waves and quantum criticality in the frustrated XY pyrochlore antiferromagnet Er2Ti2O7." Physical Review Letters 101(14): 147205-147205. Copyright © 2008 American Physical Society.0031-9007http://dx.doi.org/10.1103/PhysRevLett.101.147205http://hdl.handle.net/10222/36209We report detailed measurements of the low temperature magnetic phase diagram of Er2Ti2O7. Heat capacity and time- of- flight neutron scattering studies of single crystals reveal unconventional low- energy states. Er3+ magnetic ions reside on a pyrochlore lattice in Er2Ti2O7, where local XY anisotropy and antiferromagnetic interactions give rise to a unique frustrated system. In zero field, the ground state exhibits coexisting short and long- range order, accompanied by soft collective spin excitations previously believed to be absent. The application of finite magnetic fields tunes the ground state continuously through a landscape of noncollinear phases, divided by a zero temperature phase transition at mu(0)Hc similar to 1.5 T. The characteristic energy scale for spin fluctuations is seen to vanish at the critical point, as expected for a second order quantum phase transition driven by quantum fluctuations.article10114147205