We investigate bias-driven non-equilibrium quantum phase transitions in a paradigmatic quantum-transport setup: an interacting quantum dot coupled to non-interacting metallic leads. Using the Random Phase Approximation, which is exact in the limit of a large number of dot levels, we map out the zero-temperature non-equilibrium phase diagram as a function of interaction strength and applied bias. We focus our analysis on the behavior of the charge susceptibility and the current noise in the vicinity of the transition. Remarkably, despite the intrinsically non-equilibrium nature of the steady state, critical charge fluctuations admit an effective-temperature description, $T_{\text{eff}}(T,V)$ , that collapses the steady-state behavior onto its equilibrium form. In sharp contrast, current fluctuations exhibit genuinely non-equilibrium features: the fluctuation-dissipation ratio becomes negative in the ordered phase, corresponding to a negative effective temperature for the current degrees of freedom. These results establish current noise as a sensitive probe of critical fluctuations at non-equilibrium quantum phase transitions and open new directions for exploring voltage-driven critical phenomena in quantum transport systems.We investigate bias-driven non-equilibrium quantum phase transitions in a paradigmatic quantum-transport setup: an interacting quantum dot coupled to non-interacting metallic leads. Using the Random Phase Approximation, which is exact in the limit of a large number of dot levels, we map out the zero-temperature non-equilibrium phase diagram as a function of interaction strength and applied bias. We focus our analysis on the behavior of the charge susceptibility and the current noise in the vicinity of the transition. Remarkably, despite the intrinsically non-equilibrium nature of the steady state, critical charge fluctuations admit an effective-temperature description, $T_{\text{eff}}(T,V)$, that collapses the steady-state behavior onto its equilibrium form. In sharp contrast, current fluctuations exhibit genuinely non-equilibrium features: the fluctuation-dissipation ratio becomes negative in the ordered phase, corresponding to a negative effective temperature for the current degrees of freedom. These results establish current noise as a sensitive probe of critical fluctuations at non-equilibrium quantum phase transitions and open new directions for exploring voltage-driven critical phenomena in quantum transport systems.

cond-mat.str-elCross-listedarXiv:2601.20474Jan 29, 2026

Quantum phase transitions in the presence of disorder and dissipation

Chetan Kotabage

Jan 1, 20111 citation

Universality in equilibrium and away from it: A personal perspective

Miguel A. Muñoz, Pedro L. Garrido, Joaquín Marro +1 more

In this talk/paper I discuss the concept of universality in phase transitions and the question of whether universality classes are more robust in equilibrium than away from it. In both of these situations, the main ingredients determining universality are symmetries, conservation laws, the dimension of the space and of the order‐parameter and the presence of long‐range interactions or quenched disorder. The existence of detailed‐balance and fluctuation‐dissipation theorems imposes severe constraints on equilibrium systems, allowing to define universality classes in a very robust way; instead, non‐equilibrium allows for more variability. Still, quite robust non‐equilibrium universality classes have been identified in the last decades. Here, I discuss some examples in which (i) non‐equilibrium phase transitions are simply controlled by equilibrium critical points, i.e. non‐equilibrium ingredients turn out to be irrelevant in the renormalization group sense and (ii) non‐equilibrium situations in which equilibrium seems to come out of the blue, generating an adequate effective description of intrinsically non‐equilibrium problems. Afterwards, I shall describe different genuinely non‐equilibrium phase transitions in which introducing small, apparently innocuous changes (namely: presence or absence of an underlying lattice, parity conservation in the overall number of particles, existence of an un‐accessible vacuum state, deterministic versus stochastic microscopic rules, presence or absence of a Fermionic constraint), the critical behavior is altered, making the case for lack of robustness. However, it will be argued that in all these examples, there is an underlying good reason (in terms of general principles) for universality to be altered. The final conclusions are that: (i) robust universality classes exist both in equilibrium and non‐equilibrium; (ii) symmetry and conservation principles are crucial in both, (iii) non‐equilibrium allows for more variability (i.e. it is less constrained).In this talk/paper I discuss the concept of universality in phase transitions and the question of whether universality classes are more robust in equilibrium than away from it. In both of these situations, the main ingredients determining universality are symmetries, conservation laws, the dimension of the space and of the order‐parameter and the presence of long‐range interactions or quenched disorder. The existence of detailed‐balance and fluctuation‐dissipation theorems imposes severe constraints on equilibrium systems, allowing to define universality classes in a very robust way; instead, non‐equilibrium allows for more variability. Still, quite robust non‐equilibrium universality classes have been identified in the last decades. Here, I discuss some examples in which (i) non‐equilibrium phase transitions are simply controlled by equilibrium critical points, i.e. non‐equilibrium ingredients turn out to be irrelevant in the renormalization group sense and (ii) non‐equilibrium situations in which equilibrium seems to come out of the blue, generating an adequate effective description of intrinsically non‐equilibrium problems. Afterwards, I shall describe different genuinely non‐equilibrium phase transitions in which introducing small, apparently innocuous changes (namely: presence or absence of an underlying lattice, parity conservation in the overall number of particles, existence of an un‐accessible vacuum state, deterministic versus stochastic microscopic rules, presence or absence of a Fermionic constraint), the critical behavior is altered, making the case for lack of robustness. However, it will be argued that in all these examples, there is an underlying good reason (in terms of general principles) for universality to be altered. The final conclusions are that: (i) robust universality classes exist both in equilibrium and non‐equilibrium; (ii) symmetry and conservation principles are crucial in both, (iii) non‐equilibrium allows for more variability (i.e. it is less constrained).

AIP conference proceedingsJan 1, 20110 citations

Universality in equilibrium and away from it: A personal perspective

Miguel A. Muñoz

AIP conference proceedingsJan 1, 20111 citation

Equilibrium and Nonequilibrium Aspects of Phase Transitions in Quantum Physics

Ricardo Puebla

Springer thesesJan 1, 20184 citations

T_{\text{eff}}(T,V)

, that collapses the steady-state behavior onto its equilibrium form. In sharp contrast, current fluctuations exhibit genuinely non-equilibrium features: the fluctuation-dissipation ratio becomes negative in the ordered phase, corresponding to a negative effective temperature for the current degrees of freedom. These results establish current noise as a sensitive probe of critical fluctuations at non-equilibrium quantum phase transitions and open new directions for exploring voltage-driven critical phenomena in quantum transport systems.