Transient superconductivity without superconductivity

Abstract

Recent experiments on K3C60 and layered copper-oxide materials have reported substantial changes in the optical response following application of an intense THz pulse. These data have been interpreted as the stimulation of a transient superconducting state even at temperatures well above the equilibrium transition temperature. We propose an alternative phenomenology based on the assumption that the pulse creates a non-superconducting, though non-equilibrium situation in which the linear response conductivity is negative. The negative conductivity implies that the spatially uniform pre-pulse state is unstable and evolves to a new state with a spontaneous electric polarization. This state exhibits coupled oscillations of entropy and electric charge whose coupling to incident probe radiation modifies the reflectivity, leading to an apparently superconducting-like response that resembles the data. Dependencies of the reflectivity on polarization and angle of incidence of the probe are predicted and other experimental consequences are discussed.There has been substantial interest in the use of intense radiation fields to drive materials into non-equilibrium states [1]. Particular excitement has been generated by reports [2-4] of dramatic changes in the electromagnetic response of K 3 C 60 and layered copper-oxide materials after their exposure to intense THz radiation. The key features of the data are: i) before the application of the pump pulse, the material is in the normal (unbroken symmetry) state; ii) after photo-excitation of the material by the pump, the reflectivity R(ω) is measured as a function of the frequency ω of a probe field; iii) for some time after the pump excitation, R(ω) is found to be substantially enhanced at low frequency, see the insets in Fig. 1. This enhancement has been interpreted in terms of the creation, by the pulse, of a superconducting (SC) state.Theories proposed to date [5][6][7][8][9][10][11] are all based on the premise that the pump pulse changes the interactions and/or structure in a way that enables a transition to a broken symmetry SC state at a temperature much higher than that of the equilibrium transition. In this work we point out that the data do not require this interpretation; instead the observations can be understood within a general phenomenology that does not involve SC.The essence of our model is: i) we argue on general grounds that a non-equilibrium system can exhibit a negative linear response conductivity; ii) in this case the spatially homogeneous state is unstable and evolves to a new state characterized by domains of constant electric field bounded by sheets of charge, Fig. 2a; iii) in the experimentally relevant situation where the non-equilibrium state is produced by a pulse and thereafter evolves with a conserved energy, we show that the system sustains collective modes strongly coupled to incident radiation, leading to the reflectivity curves shown in Fig. 1. i) Consider the system out of equilibrium. The sample occupies the half space z > 0. Pump radiatio...

Non-equilibrium Phenomena in Superconductors Probed by Femtosecond Time-Domain Spectroscopy

“…The resulting temperature of the excited volume, assuming the system is quasithermalized on the 1 ns timescale, can be estimated from the temperature dependence of the total specific heat of K 3 C 60 [141], and amounts to about 500 K! Here we should mention a recently proposed alternative scenario, where the experimental data are accounted for by considering that the pulse, resonant with a phonon mode, creates a (nonsuperconducting) nonequilibrium state in which the linear response conductivity becomes negative [133,134].…”

Multi-component Time-dependent Ginzburg–Landau Study of Photo-excitation Effects on Superconducting Fluctuation

“…To understand the behaviors, the theoretical studies examine the change in the electronphonon coupling for the cause of the enhancement of the superconductivity [6][7][8][9][10][11][12][13][14][15][16]. On the other hand, the induced lights also excite the electron system directly, and such possibility is also theoretically studied [17,18].…”

Negative absolute conductivity in photoexcited metals

“…The results in Fig. 3a neglect the instability associated to a negative conductivity: for σ < 0 any charge fluctuation grows exponentially with a characteristic time τ −1 M = 4π|σ| [7,8]; for a typical metal τ M 1 fs. This time is much smaller than the typical values of τ pulse , so we can assume that the system instantaneously tunes itself to a state with a spontaneous polarization E such that σ(E ) = 0.…”

“…This paper is concerned with the less commonly realized situation of negative absolute conductivity (NAC), σ ≡ j/E < 0. A negative absolute conductivity state is possible away from thermal equilibrium because the entropy decrease implied by the σE 2 term can be compensated by other sources of entropy production, and would lead to remarkable phenomenological consequences including novel response properties [4][5][6] spontaneously generated internal electric fields [7], and new collective modes [8] that might be relevant to recent experimental studies of the transient optical properties in photoexcited K 3 C 60 [9]. It is therefore important to understand the circumstances under which a negative absolute conductivity can occur.…”

“…Finally to make a connection to the phenomenological analysis of Ref. [8], we estimate the parameter ω E , i.e. the sensitivity of entropy production to perturbations of the total energy.…”