Quasiparticle relaxation dynamics in superconductors with different gap structures: Theory and experiments onYBa2Cu3

Abstract

Photoexcited quasiparticle relaxation dynamics are investigated in a YBa2Cu3O 7−δ superconductor as a function of doping δ and temperature T using ultrafast time-resolved optical spectroscopy. A model calculation is presented which describes the temperature dependence of the photoinduced quasiparticle population npe, photoinduced transmission ∆T /T and relaxation time τ for three different superconducting gaps: (i) a temperature-dependent collective gap such that ∆(T ) → 0 as T → Tc, (ii) a temperature-independent gap, which might arise for the case of a superconductor with pre-formed pairs and (iii) an anisotropic (e.g. d-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission |∆T /T |, reflection |∆R/R| and quasiparticle recombination time τ in YBa2Cu3O 7−δ over a very wide range of doping (0.1 < δ < 0.48) is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping (δ < 0.1) and a temperature-independent isotropic gap in underdoped YBa2Cu3O 7−δ (0.15 < δ < 0.48). A pure d-wave gap was found to be inconsistent with the data.

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.…”

“…It is based on the assumption that photoexcited carrier relaxation in gapped systems is regulated by the anharmonic slow decay of phonons. 1,23 By the absorption of a pump laser photon, electrons in the valence band are excited far above the initial states, and they rapidly accumulate in the upper end of the gap through carrier-carrier and carrier-phonon interactions. Then, the carriers emit high-energy phonons (HEPs) whose energies are higher than the gap width 2 to relax into the lower end of the gap.…”

“…The magnitude of the change is proportional to the number of the nonequilibrium carriers. 1 Due to the phonon bottleneck effect, the nonequilibrium carriers relax down to states near the gap via electron-electron and electronphonon scatterings, resulting in quasi-equilibrium populations of carriers and HEPs. They then reach equilibrium by HEP decay.…”

Spin Polaron-Bipolaron Scenario of Three Gap-Like Energy Scales in Superconducting Cuprates

“…The term pseudogap has been coined to describe this kind of physics. The presence of pseudogap has been established through the measurements of the nuclear magnetic resonance (NMR) [5], the scanning tunneling spectroscopy [6][7][8][9], the analysis of the electronic Raman scattering [10][11][12], through time-resolved optical spectroscopy [13][14][15][16][17][18], and through the ARPES [19][20][21][22][23][24][25]. The observed strong Nernst effect in cuprates has been also attributed to it [26][27][28].…”

Theory of ultrafast quasiparticle dynamics in high-temperature superconductors: The dependence on pump fluence

“…Understanding the non-equilibrium dynamics of quantum manybody systems has, in fact, posed a theoretical challenge. Historically, theoretical attempts to model the time evolution of properties have either used quasi-equilibrium models such as T * and µ * models 20 to describe non-equilibrium excitations created by a pump pulse 21 or rate equation approaches based on the phenomenological Rothwarf-Taylor model 22 to describe the recovery dynamics of the superconducting state. Recently, the time evolution of the optical conductivity has been studied within a microscopic model that treats the excitation and relaxation dynamics on the same footing.…”