可调量子临界性与伪临界性在各向异性自旋-玻色模型中固定点湮灭中的表现
Tunable quantum criticality and pseudocriticality across the fixed-point annihilation in the anisotropic spin-boson model
AI总结：
该研究利用精确量子蒙特卡罗方法，探讨了各向异性自旋-玻色模型中固定点湮灭对相图和相变的影响，揭示了连续相变、对称增强的一阶相变及伪临界性的可调性。
摘要：
自旋-玻色模型是量子耗散系统的简单例子，但也是量子磁性中的有效模型，并表现出非平凡的临界性。最近，它们已被确立为研究非平凡的重整化群（RG）场景——固定点湮灭的平台，在此场景中，两个中间耦合RG固定点碰撞并产生极慢的RG流。对于玻色Kondo模型，一个单个S=1/2自旋，其中每个自旋分量通过耗散强度α_i（i∈{x,y,z}）耦合到独立的玻色子浴，其谱比例为ω^s的幂律谱，这种现象在U(1)-对称模型中依次出现于α_z=0，以及SU(2)-对称情况中α_z=α_xy，当浴指数s<1被调节时。这里我们利用精确的虫洞量子蒙特卡罗方法来研究这种非平凡的固定点结构如何影响各向异性模型的相图和相变。特别是，我们展示了在对称增强的临界流形内固定点湮灭如何导致（i）超出Landau范式的连续序到序相变，（ii）对称增强的一阶相变，以及（iii）伪临界性，后者可通过浴指数s进行调节。我们提取了连续相变处的临界指数，但也在对称增强的一阶相变处发现了标度行为，其中相关长度倒数指数由浴指数s给出。此外，我们提供了固定点碰撞两侧的伪临界标度的直接数值证据，这表现为相关长度指数的极慢漂移。我们还研究了远离SU(2)-对称情况的交叉，并确定了扩展U(1)-对称临界相的相边界。
英文摘要：
Spin-boson models are simple examples of quantum dissipative systems, but also serve as effective models in quantum magnetism and exhibit nontrivial 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 $S=1/2$ spin where each spin component couples to an independent bosonic bath with power-law spectrum $\propto ω^s$ via dissipation strengths $α_i$, $i\in\{x,y,z\}$, such phenomena occur sequentially for the U(1)-symmetric model at $α_z=0$ and the SU(2)-symmetric case at $α_z = α_{xy}$, 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 model. In particular, we show how fixed-point annihilation within a symmetry-enhanced critical manifold leads to (i) a continuous order-to-order transition beyond the Landau paradigm, (ii) a symmetry-enhanced first-order transition, and (iii) pseudocriticality, which can be tuned into each other via the bath exponent $s$. 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. We also study the crossover away from the SU(2)-symmetric case and determine the phase boundary of an extended U(1)-symmetric critical phase.
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