Quasiparticle relaxation dynamics in underdoped Bi2Sr2CaCu2O

Yasunori Toda

¹

,

T. Mertelj

²

,

P. Kusar

³

et al.

Abstract

We investigate the relaxation dynamics of photoexcited quasiparticles (QPs) in underdoped Bi 2 Sr 2 CaCu 2 O 8+δ (T c = 78 K). By changing the excitation energy and polarization of the probe beam, two different types of relaxation dynamics, associated with superconducting (SC) and pseudogap (PG) QPs, are quantitatively analyzed independently. From the temperature dependencies, we obtained the SC gap, SC (0) = 24 meV, using BCStype temperature-dependent gap and the pseudogap, PG = 41 meV. The pump fluence (F) dependence of the SC-dominated transients shows a contribution of the PG component above the saturation condition of the SC component (F th = 16 μJ/cm 2 ), where Cooper pairs with long-range order are fully destroyed within the photoexcited volume. Assuming a temperature-independent PG decay time, we successfully isolate the native SC transient even above F th by subtracting the PG response from the original data. In the saturation regime, the exponential decay (recovery of SC) is fast (τ SC ∼ 2-3 ps), suggesting an efficient nonequilibrium phonon relaxation in this compound. We also find a flat-top response preceding the exponential decay at F > F th , which appears as a delay of SC recovery in the original data. This response is visible over the whole temperature range below T c and its duration increases with increasing F. The response is attributable to a photoinduced SC to non-SC phase transition arising from excitation by the nonthermal QPs and/or high-frequency phonons. The consistently near-constant magnitude of the PG response at the start of the SC state recovery from the non-SC phase suggests a correlation between the SC and PG QPs.

Ultrafast pump-probe spectroscopic signatures of superconducting and pseudogap phases in YBa2Cu3O7−δ films

“…For example, in the previous measurements on optimally-doped (OD) YBa 2 Cu 3 O 7-δ (YBCO) samples, the observed sign of time-resolved differential reflectivity (ΔR/R) was reported to be either positive or negative. 9,11,25,26 This fundamental discrepancy was attributed to the dependence of ΔR/R on several factors including laser power, 15,23 wavelength, 25,27 polarization, 25,28 and repetition rate 24,29 . Furthermore, in a recent effort to uncover the transient optical signatures of the pseudogap (PG) phase, [11][12][13][14]28,[30][31][32] and elucidate a possible relationship between the PG state and SC state in OD YBCO [31][32] , the PG phase was either identified as a weak transient feature 11,30 or found to be completely absent from the spectra.…”

Theory of photoexcited carrier relaxation across the energy gap of phase-ordered materials

“…Usually the anomalous T dependence of τ exp is observed in many gapped systems, for example, phaseordered systems showing an energy gap opening in the electron band below the transition temperature T c . It has been found that a wide variety of superconductors [1][2][3][4][5][6][7][8][9][10][11][12][13] and density-wave compounds [14][15][16][17][18][19] show diverging behavior of τ exp near T c , as confirmed by femtosecond time-resolved optical spectroscopy. 1,4,5,[7][8][9][10][11][12][13][14][15][16][17]19 The divergence of τ exp in these gapped systems is believed to result from a recursive energy transfer between electrons and phonons.…”

Dynamics of superconducting order parameter through ultrafast normal-to-superconducting phase transition in Bi2Sr2CaCu2O8+δ

“…While the laser ARPES can directly resolve the momentum dependent 14 quasiparticle (QP) distribution function, all-optical techniques offer better bulk sensitivity and greater flexibility. The lack of momentum resolution of an optical probe can be partially compensated by use of the optical dipole transition selection rules that depend on the probe-photon polarization 15,16 and energy 8,17 and enable selection of different parts of the Brillouin zone (BZ).…”

“…The ultrafast S→N→S transition in the cuprate superconductors has been initially studied by all-optical [5][6][7][8][9] pump-probe technique followed by laser ARPES [10][11][12][13] . While the laser ARPES can directly resolve the momentum dependent 14 quasiparticle (QP) distribution function, all-optical techniques offer better bulk sensitivity and greater flexibility.…”

“…By means of the all-optical multi-pulse technique combined with the polarization selective optical probe we were able to separate the SC state recovery dynamics from the previously studied 22 PG state recovery dynamic and enable discrimination between relaxation in the nodal and anti-nodal BZ regions. The material has been studied previously by timeresolved techniques 7,8,[10][11][12]23 , but thus far there has been no systematic study of the the non-equilibrium transitions in this material, especially by the 3-pulse technique.…”