*ITAMP Lunch Seminar* *Speaker: * Florentin Reiter (Harvard) *Date:* Thursday, September 27th *Time:* 12:00-1:00 pm Includes Pizza. *Title:* *Cooperative Breakdown of the Oscillator Blockade in the Dicke Model* *Abstract: *The Dicke model [1] describes the coupling of an ensemble of spins to a harmonic oscillator. Its superradiant phase transition [2] constitutes a prime example of a non-equilibrium phase transition, which can be observed experimentally [3]. We demonstrate that, in addition, a dissipative Dicke model can undergo a second phase transition to a non-stationary phase, characterized by unlimited heating of the harmonic oscillator. Identifying the mechanism of the phase transition, we conclude that the novel phase transition can be understood as a cooperative breakdown of the oscillator blockade [4] which otherwise prevents higher excitation of the system. We discuss the relationship to previously observed breakdown phenomena [5] and derive the scaling of the critical coupling with the system size. The model can be realized using standard experimental platforms such as trapped ions. [1] R. H. Dicke, Phys. Rev. 93, 99 (1954). [2] K. Hepp and E. H. Lieb, Ann. Phys. 76, 360 (1973); Y. K. Wang and F. T. Hioe, Phys. Rev. A 7, 831 (1973). [3] K. Baumann et al., Nature 464, 1301 (2010); Z. Zhiqiang et al., Optica 4, 424 (2017). [4] F. Reiter, T. L. Nguyen, J. P. Home, and S. F. Yelin, arxiv:1807.06026 (2018). [5] H. J. Carmichael, Phys. Rev. X 5, 031028 (2015); J. Fink et al., Phys. Rev. X 7, 011012 (2017). *Location: *B-106 @ Center for Astrophysics (60 Garden Street <https://maps.google.com/?q=60+Garden+Street&entry=gmail&source=g>) *Directions: *After entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, B-106 will be at the end of the hallway on the left side.