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Acta Physica Sinica

Dual regulation mechanism of non-Hermitian dissipation on superfluid states: Promoting BCS and suppressing finite-momentum pairing

ZHOU Lihong

Published Jan 1, 2026• 0 citations
Publisher

Abstract

非厄米量子多体物理是当前物理学的前沿热点，其中冷原子体系为探索非厄米多体效应提供了理想的实验平台。本文基于非厄米平均场理论，研究了具有单体耗散的二维自旋-轨道耦合费米气体，系统分析了质心动量为零（BCS态）与非零（有限动量态）条件下的超流特性。研究发现，单体耗散对超流态具有显著的双重调控作用：一方面，它能有效促进常规的BCS超流，表现为序参量增大及能量差降低；另一方面，它显著抑制了有限动量超流态的形成。进一步分析表明，这种抑制效应源于非厄米耗散破坏了由实塞曼场诱导的费米面不对称性，从而驱动系统趋向于对称的BCS超流态。本工作揭示了非厄米耗散在超流调控中的关键作用，为理解开放量子多体系统中的新奇物理提供了新视角。

DOI
10.7498/aps.75.20260748

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We investigate a two-dimensional non-Hermitian Fermi system with spin imbalance and dissipative interaction. We employ the mean-field approximation to derive and minimize the thermodynamic potential, and determine the zero-temperature phase diagram upon variation of imbalance and dissipation. Notably, we identify a Fulde−Ferrell−Larkin−Ovchinnikov (FFLO) state characterized by a pairing order parameter with finite center-of-mass momentum, along with the normal, Bardeen−Cooper−Schrieffer (BCS), and metastable BCS states. The FFLO state is favored with increasing spin imbalance but diminishes and eventually disappears with elevated dissipation. Additionally, we present the quasiparticle energy spectrum and momentum distribution, highlighting the spatial asymmetry features that can be used to deterministically identify the FFLO state.
The Higgs and Nambu-Goldstone modes are paradigmatic collective excitations in superconductors and superfluids. These modes are commonly analyzed within the pseudospin formulation of BCS theory, where the dynamics are obtained from the Heisenberg equations of motion for pseudospins. However, this construction becomes inconsistent when directly extended to non-Hermitian systems. In this work, we develop a consistent pseudospin framework for non-Hermitian fermionic superfluids based on the metricized formulation of non-Hermitian quantum mechanics. We apply this formalism to a driven non-Hermitian BCS-type Hamiltonian with complex pairing interaction and analyze its collective excitation spectrum. We find that, in addition to the conventional Higgs (amplitude) mode, the system hosts a novel phase mode that has no counterpart in Hermitian superfluids. Remarkably, this mode is gapped even in the absence of the Anderson-Higgs mechanism, as is the case in neutral superfluids. Furthermore, the resonance spectrum depends explicitly on the initial nongauge phase of the complex order parameter and the dynamical response remains finite at resonance, in contrast to the divergence characteristic of Hermitian systems. These resonances disappear upon the emergence of exceptional points.
We investigate superfluid pairing in a two-dimensional Rashba spin-orbit-coupled Fermi gas subject to spin-selective one-body loss. Within the non-Hermitian mean-field framework, we self- consistently solve the gap and number equations and find that moderate dissipation can significantly enhance the pairing gap, resulting in a pronounced nonmonotonic dependence on the dissipation strength. Dissipation also provides an additional control parameter for driving the system across the BCS-BEC crossover. We further analyze the quasi-particle spectrum and identify two distinct superfluid regimes characterized by one and three exceptional rings, separated by an exceptional spectral transition. Interestingly, dissipation can enhance both pairing channels while simultaneously inducing a momentum-dependent phase twist in the triplet component. These results demonstrate that spin-selective dissipation provides a versatile non-Hermitian control knob for manipulating superfluid pairing, spectral structure, and crossover physics in spin-orbit-coupled quantum gases.