P. Comaron, I. Carusotto, M. H. Szymanska, N. P. Proukakis
We study the 2d phase transition of a driven-dissipative system of exciton-polaritons under non-resonant pumping. Stochastic calculations are used to investigate the Berezinskii-Kosterlitz-Thouless-like phase diagram for experimentally realistic parameters, with a special attention to the non-equilibrium features.
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We observe the proliferation of vortices in the Berezinskii-Kosterlitz-Thouless regime on a two-dimensional array of Josephson-coupled Bose-Einstein condensates. As long as the Josephson (tunneling) energy J exceeds the thermal energy T, the array is vortex-free. With decreasing J/T, vortices appear in the system in ever greater numbers. We confirm thermal activation as the vortex formation mechanism and obtain information on the size of bound vortex pairs as J/T is varied.
Berezinskii-Kosterlitz-Thouless Renormalization Group Flow at a Quantum Phase Transition
本文通过密度矩阵重整化群和量子蒙特卡罗模拟，研究一维 Bose-Hubbard 模型中的 BKT 相变，解决了边界诱导的复杂性，并利用非参数贝叶斯分析高精度确定了临界相互作用强度，建立了 BKT 物理基准。
While the Berezinskii-Kosterlitz-Thouless transition (BKT) has been under intense scrutiny for decades, unambiguous experimental signatures in magnetic systems remain elusive. Here, we investigate the interplay between electronic and magnetic degrees of freedom near the BKT transition. Focusing on a metal with easy-plane ferromagnetic order, we establish a framework that accounts both for the coupling between the charge current and the flow of topological magnetic defects and for electron scattering on their inhomogeneous spin texture. We show that electron scattering is responsible for a temp…
Motivated by recent experimental progress in 2D magnetism, we theoretically study spin transport in 2D easy-plane magnets at finite temperatures across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition, by developing a duality mapping to the 2+1D electromagnetism with the full account of spin's finite lifetime. In particular, we find that the non-conservation of spin gives rise to a distinct signature across the BKT transition, with the spin current decaying with distance power-law (exponentially) below (above) the transition; this is detectable in the proposed experiment with NiPS$_3…
We study superconductivity of twisted bilayer graphene with local and non-local attractive interactions. We obtain the superfluid weight and Berezinskii-Kosterlitz-Thouless (BKT) transition temperature for microscopic tight-binding and low-energy continuum models. We predict qualitative differences between local and non-local interaction schemes which could be distinguished experimentally. In the flat band limit where the pair potential exceeds the band width we show that the superfluid weight and BKT temperature are determined by multiband processes and quantum geometry of the band.
We study a four-component polariton system in the optical parametric oscillator regime consisting of exciton/photon and signal/idler modes across the Berezinskii-Kosterlitz-Thouless (BKT) transition. We show that all four components share the same BKT critical point, and algebraic decay of spatial coherence with the same critical exponent. However, while the collective excitations in different components are strongly locked, both close to and far from criticality, the spontaneous creation of topological defects in the vicinity of the phase transition is found to be largely independent of the i…
Spin-qubit Noise Spectroscopy of Magnetic Berezinskii-Kosterlitz-Thouless Physics
利用自旋量子比特噪声磁度计探测磁Berezinskii-Kosterlitz-Thouless物理的动力学特征，揭示准长程有序相中温度依赖的幂律特性及临界以上自由涡旋的涌现。
Berezinskii-Kosterlitz-Thouless transitions in a topological superconductor-ferromagnet-superconductor junction
研究拓扑约瑟夫森结中嵌入铁磁层引发的量子相变，预测BKT相变并给出含多临界点与超对称点的完整相图猜想。
An attempt is made to find different emergent quantum phases for interacting topological state of quantum matter. Our study is based on the quantum field theoretical renormalization group (RG) calculations. The behaviour of the RG flow lines gives the emergence of different quantum phases for non-interacting and interacting topological state of quantum matter. We show explicitly electron-electron interaction can turn a topologically trivial phase into a non-trivial one and also topological non-trivial phase to topological trivial phase. We show that physics of emergent is go beyond the quantum…
Full and fractional defects across the Berezinskii-Kosterlitz-Thouless transition in a driven-dissipative spinor quantum fluid
研究线偏振泵浦驱动的二维自旋量微腔极化激元系统，发现半涡旋主导 BKT 相变临界点附近的涡旋-反涡旋配对/解离，而全涡旋在远离临界点的深度准有序态晚期成为主要激发，且半涡旋结合成全涡旋的速度远快于涡旋-反涡旋湮灭。
Influence of topological degeneracy on the boundary Berezinskii-Kosterlitz-Thouless quantum phase transition of a dissipative resonant level
本研究探讨拓扑简并与量子临界剩余简并的相互作用，发现Majorana零模对的拓扑简并可放宽对称性要求，将耗散共振能级的边界BKT量子相变从点拓展为有限区域，降低实验观测难度，也为证实Majorana零模存在提供新证据。
We develop a theory for the vortex unbinding transition in homogeneously disordered superconducting films. This theory incorporates the effects of quantum, mesoscopic and thermal fluctuations stemming from length scales ranging from the superconducting coherence length down to the Fermi wavelength. In particular, we extend the renormalization group treatment of the diffusive nonlinear sigma model to the superconducting side of the transition. Furthermore, we explore the mesoscopic fluctuations of parameters in the Ginzburg-Landau functional. Using the developed theory, we determine the depende…
We study the phase transition between conducting and insulating states taking place in disordered multi-channel Luttinger liquids with inter-channel interactions. We derive renormalisation group equations which are perturbative in disorder but nonperturbative in interaction. In the vicinity of the simultaneous phase transition in all channels, these equations become a set of coupled Berezinskii--Kosterlitz--Thouless equations, which we analyze within two models: an array of identical wires and a two-channel model with distinct channels. We show that a competition between disorder and interacti…
Berezinskii-Kosterlitz-Thouless to BCS-like superconducting transition crossover driven by weak magnetic fields in ultra-thin NbN films
研究超薄NbN薄膜中弱磁场诱导BKT转变向类BCS超导转变的交叉，发现场致自由涡旋屏蔽涡旋相互作用并抹平BKT特征。
The disorder-driven metal-insulator transition in the quantum spin Hall systems is studied by scaling analysis of the Thouless conductance $g$. Below a critical disorder strength, the conductance is independent of the sample size $M$, an indication of critically delocalized electron states. The calculated beta function $β=d\ln g/d\ln M$ indicates that the metal-insulator transition is Kosterlitz-Thouless (KT) type, which is characterized by bounding and unbounding of vortex-antivortex pairs of the local currents. The KT like metal-insulator transition is a basic characteristic of the quantum s…
The effects of microwave radiation on the transport properties of atomically thin $La_{2-x}Sr_xCuO_4$ films were studied in the 0.1-13 GHz frequency range. Resistance changes induced by microwaves were investigated at different temperatures near the superconducting transition. The nonlinear response decreases by several orders of magnitude within a few GHz of a cutoff frequency $ν_{cut} \approx$ 2 GHz. Numerical simulations that assume an ac response to follow the dc V-I characteristics of the films reproduce well the low frequency behavior, but fail above $ν_{cut}$. The results indicate that …
The quantum XY model shows a Berezinskii-Kosterlitz-Thouless (BKT) transition between a phase with quasi long-range order and a disordered one, like the corresponding classical model. The effect of the quantum fluctuations is to weaken the transition and eventually to destroy it. However, in this respect the mechanism of disappearance of the transition is not yet clear. In this work we address the problem of the quenching of the BKT in the quantum XY model in the region of small temperature and high quantum coupling. In particular, we study the phase diagram of a 2D Josephson junction array, t…
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