Light induced Berezinskii-Kosterlitz-Thouless transition in Superconducting Films

TL;DR

The paper addresses a light-driven non-equilibrium Berezinskii-Kosterlitz-Thouless (BKT) transition in a two-dimensional superconductor at $T=0$, manifested as vortex–antivortex deconfinement under optical driving. Using a generalized time-dependent Ginzburg–Landau equation (gTDGL) with circularly polarized light and a DC bias, the authors simulate a Nb-like superconducting slab and construct an $I$-$E$ phase diagram with three dynamical regimes: confined (C), premelted (P), and deconfined (D); they analyze the phase field $\theta_s$ and observe VP$_{E+}$ and VP$_{E-}$, identifying a topological threshold where $\delta_y \theta_s$ crosses $2\pi$ signaling deconfinement. The premelted phase enables vortex unbinding within the illuminated region, and optical depairing lowers the current threshold for deconfinement (e.g., $I_{tr} \approx 0.35 I_c$ electrically, increasing to about $0.48 I_c$ with light). Deconfinement can occur without a phase slip, producing long-lived metastable vortices after the optical drive is removed, signaling a non-thermal, non-magnetic BKT-like transition with a QCD-like 2D phase diagram for quantum phases.

Abstract

We report a light-driven non-equilibrium vortex Berezinskii-Kosterlitz-Thouless (BKT) transition in a superconductor. We use a time-dependent Ginzburg-Landau model to demonstrate vortex-antivortex deconfinement via light induced fields. The transformation occurs independently of thermal fluctuations and is viewed as a quantum phase transition. The resulting phase map mirrors QCD phase diagram, delineating confined, premelted, and fully deconfined vortex phases. The nature of these phases is discussed. Transitions between phases are governed by light induced depairing and phase fluctuations, establishing a new class of light-induced topological transitions.

Figures (6)

• Figure 1: Schematic diagrams illustrating (a) the BKT phase transition of vortices in the XY model, (b) confinement–deconfinement of vortices in a superconducting (SC) state induced by electric current, and (c) the one induced by light. These three types of phase transitions occur at distinct critical values of control parameters: the transition temperature $T_{BKT}$, the transition current $I_{tr}$, and the transition optical field $E_{tr}$, respectively. Yellow regions represent the unperturbed SC order (see the color bar for suppression of SC order. Red and blue arrows regions denote vortices and antivortices, respectively. (d) Simulation configuration: the superconducting slab is placed in the $xy$-plane, with a circularly polarized light beam incident normally at its center. To introduce the DC current, the upper and lower edges of the slab are connected to a metal source and metal drain, respectively.
• Figure 2: (a) $I$–$E$ phase diagram of confinement-deconfinement transitions in vortices. The gray, orange, and blue data points, marked "$\bf{C}$", "$\bf{D}$", "$\bf{P}$" represent the confinement, deconfinement, and premelted phases, respectively. Complete results for the dynamics in respective phase are provided in the Supplementary Materials supp_PD. (b) Snapshots of $\abs{\psi}$ profiles at $t = 500 \tau_{GL}$ for different applied currents and amplitudes of light field. For the current-dependent series (fixed $E = 0.5E_0$), the applied currents are $I_1 = 0.59I_c$, $I_2 = 0.66I_c$, $I_3 = 0.73I_c$, $I_4 = 0.80I_c$, and $I_5 = 0.86I_c$. For the light-dependent series (no applied current), the amplitudes are $E_1 = 0.0E_0$, $E_2 = 0.5E_0$, $E_3 = 1.0E_0$, $E_4 = 1.5E_0$, and $E_5 = 2.0E_0$.
• Figure 3: Typical profiles of the order parameter and phase field for different dynamical phases: (a) confined (C), (b) premelted (P), (c) deconfined (D) with VP, and (d) deconfined (D) with phase slip. These snapshots are taken from simulations with parameters $t=260\tau_{GL}$$[I/I_c, E/E_0] =$ [0.53,0.0], [0.00,1.3], [0.60,1.1], [0.73,1.0], respectively. The labels $V_+$, $V_-$ indicate vortices, antivortices, respectively.
• Figure 4: (a) Results of $\delta_y \theta_s = \theta_s(x, y_2) - \theta_s(x, y_1)$ (where $y_1$ and $y_2$ locate at the bottom and top edges of the sample, respectively), scanned along the $x$-axis. The corresponding phase fields are shown in the right insets. The kinetic energies of VP$_{E+}$ and VP$_{E-}$ are 28.4 and 23.4 times greater than that of a single VP, respectively, based on Eq. \ref{['eq:Ek_approx']}. The dashed lines indicate the vortex positions at $x_1$ and $x_2$, as marked in the insets. Insets: Idealized phase fields for a single VP, VP$_{E+}$, and VP$_{E-}$. The solid arrows indicate the direction of the applied DC current, which induces phase stripes outside the VP region corresponding to $\delta_y \theta_s = 4\pi$. (b) Time evolution of $\delta_y \theta_s$ from simulation datas for three distinct phases: confinement (top), premelted (middle), and deconfinement (bottom), corresponding to parameter sets [$I/I_c$, $E/E_0$] = [0.00, 0.5], [0.34, 0.5], and [0.66, 1.0], respectively. In the premelted phase, the system exhibits alternating appearances of VP$_{E+}$ and VP$_{E-}$ states over time, manifested as the red and blue bumps as labeled, respectively. (c) Effective $\delta_y \theta_s$ as a function of applied current, comparing the electric effect (blue line) and the combined electric + optical effect (orange line). The dashed line at $\delta_y \theta_s = 2\pi$ indicates the topological restriction for vortex deconfinement. Accordingly, the estimated transition current $I_{tr}$ is approximately $0.35 I_c$ for the electric effect alone, and increases to $0.48 I_c$ when the optical effect is included ( \ref{['app:Itr']}).
• Figure 5: $I$–$V$ curve of the SC slab studied in this work. The inset illustrates the configuration of the voltage probes on the superconducting slab (gray area), including the source and drain terminals.