Spin-boson models are simple examples of quantum dissipative systems, but also serve as effective models in quantum magnetism and exhibit nontrivial quantum criticality. Recently, they have been established as a platform to study the nontrivial renormalization-group (RG) scenario of fixed-point annihilation, in which two intermediate-coupling RG fixed points collide and generate an extremely slow RG flow near the collision. For the Bose Kondo model, a single $S=\frac{1}{2}$ spin where each spin component couples to an independent bosonic bath with power-law spectrum $\propto {\omega }^s$ via dissipation strengths ${\alpha }_i, i\in \{x,y,z\}$, such phenomena occur sequentially for the U(1)-symmetric model at ${\alpha }_z=0$ and the SU(2)-symmetric case at ${\alpha }_z={\alpha }_{xy}$, as the bath exponent $s<1$ is tuned. Here we use an exact wormhole quantum Monte Carlo method for retarded interactions to explore how this nontrivial fixed-point structure affects the phase diagram and phase transitions of the anisotropic spin-boson model. In particular, we show how fixed-point annihilation within a symmetry-enhanced critical manifold leads to a variety of anisotropy-driven critical phenomena: (i) a continuous order-to-order transition beyond the Landau paradigm, (ii) a symmetry-enhanced first-order transition, and (iii) pseudocriticality. Depending on whether the attractive fixed point within the critical manifold corresponds to a critical or a localized phase, the transition between the two long-range ordered localized phases can be tuned from continuous to strongly first order and even becomes weakly first order in an extended regime close to the fixed-point collision. We extract critical exponents at the continuous transition, but also find scaling behavior at the symmetry-enhanced first-order transition, for which the inverse correlation-length exponent is given by the bath exponent $s$. Moreover, we provide direct numerical evidence for pseudocritical scaling on both sides of the fixed-point collision, which manifests in an extremely slow drift of the correlation-length exponent, even at the continuous transition. In addition, we also study the crossover behavior away from the SU(2)-symmetric case and determine the phase boundary of an extended U(1)-symmetric critical phase for ${\alpha }_z<{\alpha }_{xy}$. Our work establishes the spin-boson model as a paradigmatic example to access tunable criticality and pseudocriticality across the fixed-point collision in large-scale simulations, which is reminiscent of a scenario discussed in the context of deconfined criticality.

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• Phase diagrams
• Quantum criticality
• Strongly correlated systems
• Finite-size scaling
• Quantum Monte Carlo
• Spin-boson model