Quantum phase transitions and frustration in a three-spin-boson model with independent baths

Ron Belyansky, Seth Whitsitt, Rex Lundgren +2 more

Serge Florens, D. Venturelli, R. Narayanan

Shu He, Li-Wei Duan, Yanzhi Wang

We investigate a frustrated four-spin plaquette spin-boson model with competing nearest-neighbor and diagonal Ising couplings, where each spin is coupled to an independent bosonic bath. Combining a path-integral strong-coupling analysis with variational matrix product state simulations, we obtain the ground-state phase diagram. In the strong-dissipation limit we map the model onto a classical plaquette and derive analytic phase boundaries between ferromagnetic, Néel, and stripe phases. At intermediate dissipation we find a delocalized phase and two localized ordered phases with Néel and stripe character. We show that the localized phases are separated by a first-order line, while each is connected to the delocalized regime via a continuous (second-order) localization transition, and that these three boundaries meet at a quantum triple point. Analysis of spin correlations and reduced density matrices further reveals that entanglement concentrates on nearest-neighbor (diagonal) bonds in the Néel (stripe) phase, whereas in the delocalized regime intra-plaquette two-spin entanglement is strongly suppressed in favor of enhanced spin-bath correlations.

André Winter, Heiko Rieger

We consider multiple noninteracting quantum mechanical two-level systems coupled to a common bosonic bath and study its quantum phase transition with Monte Carlo simulations using a continuous imaginary time cluster algorithm. The common bath induces an effective ferromagnetic interaction between the otherwise independent two-level systems, which can be quantified by an effective interaction strength. For degenerate energy levels above a critical value of the bath coupling strength $\ensuremath{\alpha}$ all two-level systems freeze into the same state and the critical value ${\ensuremath{\alpha}}_{c}$ decreases asymptotically as $1/N$ with increasing $N$. For a finite number $N$ of two-level systems the quantum phase transition (at zero temperature) is in the same universality class as the single spin-boson model; in the limit $N\ensuremath{\rightarrow}\ensuremath{\infty}$ the system shows mean-field critical behavior independent of the power of the spectral function of the bosonic bath. We also study the influence of a spatial separation of the spins in a bath of bosonic modes with linear dispersion relation on the location and characteristics of the phase transition as well as on correlations between the two-level systems.

Pavel Cejnar, Jan Jolie

Junzhe Tan, Nengji Zhou, Yang Zhao

The ground state properties and quantum phase transitions of sub-Ohmic spin-boson models are investigated using the multiple Davydov D2Ansatz in conjunction with the variational principle. Three variants of the model are studied: (i) a single bath with diagonal coupling, (ii) two independent baths with diagonal and off-diagonal couplings, and (iii) a single bath with simultaneous diagonal and off-diagonal couplings. For the purely diagonal model, the multiple Davydov D2Ansatz yields critical coupling strengths that are consistent with other methodologies, validating its accuracy and efficiency. In the two-bath model, the competition between diagonal and off-diagonal couplings drives a first-order transition for both symmetric and asymmetric spectral exponents, with the von Neumann exhibiting a continuous peak only under exact symmetry. Finally, for a single bath with simultaneous diagonal and off-diagonal couplings, we demonstrate that a rotational transformation maps the system to an equivalent purely diagonal model, enabling simpler and intuitive physical interpretation and reduced computational complexity.