Session P19: Invited Session: Dynamics of Strongly Correlated Systems: Control and Ultrafast X-Ray Probes

Controlling Quantum Condensed Matter With Light

In this talk I will discuss some of our recent work aimed at controlling the properties of quantum condensed matter with light, for instance in High-Tc cuprates or in Mott Insulators. The focus is on the use of high-field THz radiation rather than near-visible excitations, thus using photon energies that do not destroy the broken symmetry state of the solid, for example by breaking Cooper pairs. Rather we study the non-linear electrodynamics of the solid, for example by manipulating low-lying lattice vibrations coherently or by driving the phase excitations in superconducting condensates. A straightforward conceptual analogy can be found with experiments that that study driven dynamics of strongly correlated atomic gases in optical lattices. Due to the short lengthscales and the fast timescales involved in condensed matter, femtosecond x-ray scattering and spectroscopy experiments with the LCLS Free Electron Laser are necessary to interrogate the microscopic non-equilibrium paths explored by the stimulated solid.   

Light induced meltdown of quasiparticles in high temperature superconductors

Ultrafast \textit{time}- and angle- resolved photoemission spectroscopy (tr-ARPES) is emerging as a powerful tool to access the non-equilibrium quasiparticles dynamics and Cooper pairs formation in unconventional superconductors. In a tr-ARPES experiment a non-equilibrium transient state is created by pumping with an infrared pulse and is then measured via photoemission with an ultraviolet probe pulse. By varying the time delay between pump and probe we can directly access the recovery of the superconducting gap and the non --equilibrium quasiparticles population decay. Here we present a detailed momentum, temperature, doping and density dependent study of the response of a high temperature Bi2212 superconductor to a femtosecond pump-probe. In particular, through systematic pump fluence dependence we have induced the meltdown of quasiparticles and have driven the system normal by inducing a collapse of the superconducting gap. Interestingly we observed that only quasiparticles beyond a particular boson mode respond to the pump laser excitations, while the others remain untouched and that both quasiparticles recombination and gap dynamics are a density, momentum and doping dependent process, showing a crossover from a weakly perturbed to a strongly perturbed regime. These results point to a new dichotomy between the ultrafast gap and quasiparticles response within and beyond the Fermi arc and reveal a new window into the nature of the pairing interaction in high Tc superconductors.   

Shooting the electronic structure movie: Femtosecond time-resolved photoemission of layered charge-density-wave systems

Charge-density waves (CDWs) are broken-symmetry states of low-dimensional materials that are brought about by strong electron-phonon interaction. Yet surprisingly, a universal microscopic understanding beyond this statement has not really evolved for this classical paradigm of condensed matter physics. In quasi-two-dimensional systems, for example, the common approaches based on ARPES band structure results--looking for nested sections of the Fermi surface or for a peak in the electronic susceptibility--have almost no predictive power. Apparently, more successful explanations have to take into account the delicate balance between several factors including not only electronic and phononic structure, but also electron-electron and electron-phonon interactions. Here, we will show that femtosecond time-resolved XPS and ARPES using pulsed extreme ultraviolet radiation generated by the free-electron laser FLASH [1] as well as by a table-top high-harmonic-generation source [2] can provide novel insights into the relative roles that the various factors play in CDW formation. We will focus on three conspicuous CDWs in prominent members of the family of layered transition-metal dichalcogenides: the $(\sqrt{13}\times\sqrt{13})$ CDW in the Mott insulator $1T$-TaS$_2$, the $c(2\sqrt{3}\times4)rect.$ CDW in the Peierls insulator Rb$_x$TaS$_2$, and the $(2\times2\times2)$ CDW in the possible excitonic insulator $1T$-TiSe$_2$. The specific program is to reveal the relative importance of electronic and phononic contributions to the various CDW transitions by relating measured melting and relaxation times of CDW-induced spectral features to typical elementary time scales in layered materials. \\[4pt] [1] S. Hellmann {\it et al.}, Phys. Rev. Lett. {\bf 105}, 187401 (2010). \\[0pt] [2] T. Rohwer {\it et al.}, Nature {\bf 471}, 490 (2011).   

Theoretical description of photo-doping in Mott and charge-transfer insulators

Many aspects of photo-excited insulator-to-metal transitions in Mott and charge-transfer systems are theoretically not well understood: How is the photo-doped state related to a chemically doped state? On what timescale do we expect the formation of quasiparticles? To describe the electronic dynamics of Mott insulators, we have used nonequilibrium dynamical mean-field theory (DMFT) in combination with Quantum Monte Carlo and various weak and strong-coupling [1] techniques. In the talk, I will briefly present the current status of this approach and of related cluster approaches for nonequilibrium. I will then discuss results for the photo-doping in the Hubbard model, and in a in a p-d model for charge-transfer insulators. When the onsite Coulomb repulsion U is much larger than the hopping, rapid thermalization of the pump-excited Mott insulator is inhibited by the energetic stabilization of doublon-hole pairs [2], and various types of non-thermal states can arise. Immediately after the excitation process, the system of doublons and holes is too hot to form quasiparticle states, but coupling to a heat-bath of phonons can drive the system into a metallic state with well developed doublon and hole bands. Close to the metal-insulator transition, on the other hand, when U is of the order as the hopping, doublons and holes rapidly thermalize due to the electron-electron interaction, which makes the system a bad metal rather than a Fermi liquid. \\[4pt] [1] M. Eckstein and Ph. Werner, Phys. Rev. B {\bf 82}, 115115 (2010).\\[0pt] [2] M. Eckstein and Ph. Werner, Phys. Rev. B {\bf 84}, 035122 (2011).   

High frequency properties of individual metallic carbon nanotubes

We study the electrical and electrothermal dynamics of individual metallic single-walled carbon nanotubes (SWNT). Using Johnson noise thermometry, we characterize the dependence of the electron temperature on the dc bias current. This allows us to determine the thermal conductance associated with cooling of the nanotube electron system as a function of both temperature and nanotube length [1]. This thermal conductance can be used to predict the measured radio frequency (rf) bolometric response. At low temperatures and low bias current, an additional rf response is observed from the (non-thermal) electrical nonlinearity of the contacts [2]. Finally, we compare these rf measurements with measurements of terahertz (THz) detection. The THz measurements are used as a probe of plasmon standing wave resonances on the SWNT. \\[4pt] [1] D.F. Santavicca, J.D. Chudow, D.E. Prober, M.S. Purewal, and P. Kim, Nano Lett. 10, 4538 (2010). \\[0pt] [2] D.F. Santavicca, J.D. Chudow, D.E. Prober, M.S. Purewal, and P. Kim, Appl. Phys. Lett. 98, 223503 (2011).