Session Z1: Invited Session: Time- and Angle- Resolved Photoemission Spectroscopy of Complex Materials

Ultrafast Optical Excitation of a Persistent Surface-State Population in the Topological Insulator Bi$_2$Se$_3$

Bi$_2$Se$_3$ is a material which has gained great attention since it was recognized to be a topological insulator. Due to their topologically-protected spin-textured Dirac surface states, topological insulators have been recognized for their potential in device applications, particularly for spintronics. Thus, much of the experimental focus has been on ways to electronically or optically couple to the surface spin-texture. Using time- and angle- resolved photoemission spectroscopy (TR-ARPES), we optically excite p-type Bi$_2$Se$_3$ and study the dynamical response of its electronic structure on a femtosecond timescale. The strength of this technique is that its energy- and angle- resolution allows us to study these dynamics directly within the electronic band structure, so that surface and bulk contributions can be separately resolved. We find that optical excitation produces a metastable population of bulk carriers due to the presence of the bandgap. We discuss the coupling of these carriers to the Dirac surface state, which results in a long-lived nonequilibrium surface carrier distribution. This spin-textured population may present a channel in which to drive transient spin-polarized currents.   

Ultrafast momentum-dependent quasiparticle dynamics in high-$T_{c}$ superconductors

Femtosecond time- and angle-resolved photoelectron spectroscopy trARPES facilitates insight into electronic relaxation and electronic structure of non-equilibrium states of matter [1]. Hot electrons and holes relax in metals on ultrafast time scales due to the screened Coulomb interaction [2]. In superconductors the relaxation rates of quasiparticles at energies close to the superconducting gap edge are reduced because of the loss of quasiparticle states near $E_{F}$. Since in the superconducting state the relaxation of optically excited carriers proceeds partly by Cooper pair reformation, the study of the quasiparticle dynamics bears the potential to analyze the interaction responsible for Cooper pair formation. Results of trARPES will be discussed for optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in the superconducting state [2] and on EuFe$_{2}$As$_{2}$ in the antiferromagnetic state [3]. In the cuprate system we find a predominant excitation of quasiparticles at momenta near the antinode. We show furthermore, that at excitation densities of several 10 $\mu $J/cm$^{2}$ quasiparticle relaxation is dominated by Cooper pair reformation, which again proceeds near the antinode. In the Fe-pnictide material we monitor a difference in the relaxation rate for electrons and holes near the Fermi momentum, which disappears above the Neel temperature. We conclude that this anisotropic relaxation of electrons and holes is a consequence of the optical modification of the antiferromagnetic order. Analysis of energy transfer from electrons to phonons allows to determine the momentum averaged electron-phonon coupling constant $\lambda $. We find values below 0.25 for Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ [5] and below 0.15 for EuFe$_{2}$As$_{2}$ [4]. \\[4pt] [1] Bovensiepen and Kirchmann, Laser Photonics Rev. 6, 589 (2012).\\[0pt] [2] Kirchmann et al., Nature Physics 6, 782 (2010).\\[0pt] [3] Cortes et al., Phys. Rev. Lett. 107, 097002 (2011).\\[0pt] [4] Rettig et al., Phys. Rev. Lett. 108, 097002 (2012).\\[0pt] [5] Perfetti et al., Phys. Rev. Lett. 99, 197001 (2007).   

Measurement of intrinsic Dirac fermion cooling of a topological insulator with time- and angle- resolved photoemission spectroscopy

Three-dimensional topological insulator (TI) is a new phase of matter with exotic surface electronic properties.~Even though the bulk states have a bandgap, the surface electrons possess a linear energy-momentum dispersion that is protected by the nontrivial topology of TI to cross the Fermi level. These properties provide a promising platform for new physics and applications in future electronics and computers including high-speed quantum information processing, whose performance depends critically on the dynamics of hot carriers. Unlike the case in graphene, helical Dirac fermions in a TI interact not only with phonons but also with an underlying bulk reservoir of electrons. In this talk, we will present our recent results of time- and angle-resolved photoemission spectroscopy (TrARPES) study of a prototypical TI Bi2Se3. We show that TrARPES is a powerful tool to distinguish the coupled dynamics between these different degrees of freedom. With the combined sub-picosecond time resolution and energy-momentum resolution, we have directly visualized the coupling between surface and bulk electrons through phonons. At low temperature, such coupling is suppressed and the unique cooling of surface Dirac fermions by acoustic phonons is revealed through the power law cooling rate dependence on doping level. The effect on the TrARPES spectra from varying excitation photon energy will also be discussed.   

Time-resolved ARPES and f-electron coherence

The coherence temperature, T*, sets an important energy scale in correlated f-electron systems. In this scale the hybridization gap opens at or in the vicinity of the Fermi level and the gap magnitude scales with effective quasiparticle mass. The new quasiparticle bands are heavy, as demonstrated by their small dispersion, and the quasiparticle lifetime is long, as seen by the narrow width of the peaks. Unless magnetic ordering suppresses the gap or mass enhancement is observed due to, e.g., magnetic excitations, the gap scales with effective mass in a universal manner across the heavy fermion systems. Possible deviations from this pattern, e.g. a small finite gap persisting at high temperatures above T* require models beyond a mean-field approach, and may be understood within e.g. the model of periodic array of Anderson impurities with correlations described by coupling to specific boson modes. \\ Self-energy approach is commonly used in ARPES of correlated systems. The coherent part of the self-energy corresponding to the gap formation is reduced at high temperatures, and the incoherent part corresponds to quasiparticle scattering. The coherent term in the self-energy expresses the mixing of f and d bands and is directly responsible for repulsion, producing the hybridization gap. This theoretical framework provides a direction towards understanding quasiparticle dynamics in correlated electron systems through ultrafast self-energy measurements and modeling. Here we show examples of time-resolved ARPES measurements of f-electron systems, providing valuable information about the evolution of coherence and the dynamics of the related quasiparticle states. \\ References \\ 1) Phys. Rev. B 84, 161103 (Rapid Comm.) (2011). \\ 2) Phys. Rev. B 84, 161101 (Rapid Comm) (2011).\\ 3) Phys. Rev. Lett. 106, 207402 (2011).\\ 4) J.Phys.C. 23, 094211 (2011).\\ 5) Rev. Sci. Instr. 81, 073108 (2010). \\ 6) Europhys. Lett. 84, 37003 (2008). \\ 7) Phys. Rev. Lett. 101, 016403 (2008).   

Ultrafast quasiparticle dynamics and pair recombination in cuprate high-temperature superconductors

Understanding how superconductivity emerges from other competing phases and how this balance evolves through the phase diagram is one of the biggest challenges in the field of high-$T_\textrm{c}$ superconductors. By using high resolution time- and angle-resolved photoemission spectroscopy (tr-ARPES) we are able to directly probe the effects of optical excitation on the electronic structure of cuprate superconductors, and study the resulting quasiparticle, superconducting gap, and Cooper pair formation dynamics near their natural time scales. In particular, we observed a pump-induced meltdown of quasiparticles, which occurs only within the energy scale defined by a particular boson mode. This meltdown was observed only below $T_\textrm{c}$, suggesting a link between superconductivity and quasiparticles in momentum space where the superconducting gap is zero. We observed that the excited quasiparticle decay dynamics were strongly pump-fluence dependent and consistent with the picture that the observed dynamics reflect actual Cooper pair formation. Further, these quasiparticle recombination dynamics were strongly momentum dependent, increasing away from the superconducting nodes. Direct measurements of momentum dependent superconducting gap dynamics and the evolution of other non-equilibrium spectral phenomena through the phase diagram further illustrate the power of this unique time- and momentum-resolved spectroscopy. These results reveal new windows into the nature of the pairing interaction in high-$T_\textrm{c}$ superconductors.