Theory of Enhanced Interlayer Tunneling in Optically Driven High-TcSuperconductors

Abstract

Motivated by recent pump-probe experiments indicating enhanced coherent c-axis transport in underdoped YBCO, we study Josephson junctions periodically driven by optical pulses. We propose a mechanism for this observation by demonstrating that a parametrically driven Josephson junction shows an enhanced imaginary part of the low-frequency conductivity when the driving frequency is above the plasma frequency, implying an effectively enhanced Josephson coupling. We generalize this analysis to a bilayer system of Josephson junctions modeling YBCO. Again, the Josephson coupling is enhanced when the pump frequency is blue detuned to either of the two plasma frequencies of the material. We show that the emergent driven state is a genuine, nonequilibrium superconducting state, in which equilibrium relations between the Josephson coupling, current fluctuations, and the critical current no longer hold. DOI: 10.1103/PhysRevLett.117.227001 Recent pump-probe experiments on high-temperature superconductors such as YBCO revealed transiently enhanced superconducting-like states both below and above their critical temperatures T c [1][2][3][4]. The origin of these transient superconducting states has not been identified yet. Several ideas have been proposed: nonlinear phononic effects [5], parametric cooling [6,7], competing orders [8,9], and redistribution of phase fluctuations [10]. In this Letter, we model the layered structure of YBCO as a Josephson junction chain, and demonstrate that below the critical temperature the parametric driving of these junctions enhances the Josephson coupling in the steady state [11]. We extract this quantity from the 1=ω divergence of the imaginary part of the conductivity σðωÞ as the frequency ω approaches zero.So far, most of the theoretical studies have considered quantities such as the power spectrum of currents [10], or the dc current response [6]. While these quantities give important insight into the system, here we discuss the optical conductivity itself by including the probing field in our calculation to obtain the actual nonequilibrium response. The Kubo formula or other equilibrium methods are not used. This achievement is crucially important because the conclusions of Refs. [1][2][3][4] are based precisely on this quantity. We note that while experiments indicate stiffening of the superfluid density both below and above T c , here we focus on cases below T c , in which condensed Cooper pairs are safely assumed. We will discuss cases with fluctuating Cooper pairs just above T c (below the pseudogap temperature) elsewhere. Complete understanding how the interlayer coherence is dynamically enhanced may provide new theoretical and experimental means to investigate fluctuating orders above T c .In this Letter, we first study a single Josephson junction. We show that a driving frequency just above the plasma frequency leads to strong enhancement of the Josephson coupling, derived from the low-frequency conductivity. This arises because the nonlinear driving term couples the probe puls...

Transiently enhanced interlayer tunneling in optically driven high- Tc superconductors

“…These results expand on the response function that was derived in Ref. 26 for a single driving frequency. We assume that the driving pulse has an admixture of the second harmonic frequency, A(t) = A 0 cos(ω e t) + A 1 exp(i2ω e t) + A * 1 exp(−i2ω e t) = A 0 cos(ω e t) + |A 1 | cos(2ω e t + φ 1 ),…”

“…As we have shown in Ref. 26, the interlayer tunneling is enhanced (suppressed) at the blue-(red-) detuned side, and the driven steady state value is approximately…”

“…As we have shown in Ref. 26, this is due to the fact that the fluctuations of the current j(t) ≡ sin ϕ(t) are increased when integrated over all frequencies dω |j(ω)| 2 . This increased noise induces phase slips and destroys the phase coherence, even though the low-frequency part of the fluctuations, which determines J eff , is reduced.…”

“…These intriguing experimental results were studied theoretically in Refs. [22][23][24][25][26][27]. However, these studies primarily focused on the steady state of this driven system, while the experimental operation uses a pump pulse, with a pulse length that is typically around five times of the inverse optical frequency.…”

Nonequilibrium steady states and transient dynamics of conventional superconductors under phonon driving

“…In particular, the resonant excitation of mid-infrared phonon modes has opened a new pathway for manipulating properties such as metallicity [2], magnetism [3], and superconductivity (SC) [4][5][6][7][8][9] and for modifying lattice structures in ways which may not be achievable in equilibrium. Especially, the light-induced SC-like behavior in the optical conductivity for high T c cuprates and doped fullerides [4][5][6][7][8][9] has stimulated several theoretical investigations [10][11][12][13][14][15][16][17][18][19]. So far, two possible scenarios for the enhancement of SC have been discussed in the literature, based on the enhanced pairing interaction out of equilibrium: (i) Under time-periodic driving, the time-averaged Hamiltonian can be modified in such a way that SC is favored [10,14,18], and (ii) the interaction can be effectively enhanced in the driven state by dynamical effects [11][12][13]16].…”