Abstract
We study the zero temperature properties of the sub-Ohmic spin-boson model with quadratic spin-boson coupling. This model describes experimental setups at the optimal working point where the linear coupling between the qubit (spin) and the environmental noise (bosons) is zero and the leading coupling is quadratic. In the strong coupling regime, we find that the existence of spin induces quantum phase transitions (QPTs) between two states of environment: the normal state and a state with local distortions. The phase diagram contains both continuous and the first-order QPTs, with nontrivial critical properties obtained exactly. At the QPTs, the equilibrium state spin dynamics bears power-law dependence in the small frequency limit and a robust coherent Rabi oscillation at high frequency. We discuss the feasibility of observing such environmental QPTs in the qubit-related experiments.
8 More- Received 30 December 2016
- Revised 4 August 2018
DOI:https://doi.org/10.1103/PhysRevB.98.115131
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