Abstract
The phonon-mediated attractive interaction between carriers leads to the Cooper pair formation in conventional superconductors. Despite decades of research, the glue holding Cooper pairs in high-temperature superconducting cuprates is still controversial, and the same is true for the relative involvement of structural and electronic degrees of freedom. Ultrafast electron crystallography (UEC) offers, through observation of spatiotemporally resolved diffraction, the means for determining structural dynamics and the possible role of electron-lattice interaction. A polarized femtosecond (fs) laser pulse excites the charge carriers, which relax through electron-electron and electron-phonon couplings, and the consequential structural distortion is followed diffracting fs electron pulses. In this paper, the recent findings obtained on cuprates are summarized. In particular, we discuss the strength and symmetry of the directional electron-phonon coupling inBi2Sr2CaCu2O8+δ(BSCCO), as well as thec-axis structural instability induced by near-infrared pulses inLa2CuO4(LCO). The theoretical implications of these results are discussed with focus on the possibility of charge stripes being significant in accounting for the polarization anisotropy of BSCCO, and cohesion energy (Madelung) calculations being descriptive of thec-axis instability in LCO.
Highlights
Despite two decades of intense research, the mechanism of high temperature superconductivity in cuprates is still unclear [1]
The theoretical implications of these results are discussed with focus on the possibility of charge stripes being significant in accounting for the polarization anisotropy of BSCCO, and cohesion energy (Madelung) calculations being descriptive of the c-axis instability in LCO
Information on structural dynamics has been obtained through Raman spectroscopy [4], X-ray absorption spectroscopy (XAS) [5], Angle resolved photoemission spectroscopy (ARPES) [6,7,8], and oxygen isotope substitution studies [9,10,11,12]
Summary
Despite two decades of intense research, the mechanism of high temperature superconductivity in cuprates is still unclear [1]. By changing the wavelength of the probe pulse from terahertz [31] to midinfrared [27], and to optical frequencies [25, 28], dynamics of the superconducting condensate or quasiparticle subsystem could be studied as a function of time These experiments yielded both the elastic and inelastic quasiparticle scattering rate [32], the quasiparticle diffusion rate [33] as a function of temperature, and the excitation density (and doping) in cuprates [28]. Different compositions were investigated, by varying the doping level and number of Cu–O planes per unit cell in the BSCCO family, and by probing optimally doped LCO (La, Cu, O) nanoislands In these experiments, the initial fs excitation drives the system from the superconducting into the metallic phase [29], breaking Cooper pairs [28]. We will show that simple cohesion energy calculations can account for the observed c-axis instability in LCO, based on the assumption that infrared pulses photo-dope the Cu– O planes through a charge transfer process
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