Optical study of shear and longitudinal acoustic waves and complex relaxation dynamics of glass forming liquids

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 259-277).The spectroscopic technique Impulsive Stimulated Scattering (ISS) was refined and used to study the complex structural relaxation dynamics of glass forming liquids, allowing both empirical modeling and testing of the predictions of the mode-coupling theory (MCT). Longitudinal and shear acoustic waves throughout much of the MHz frequency range, time-dependent thermal expansion on nanosecond and microsecond scales, and slower thermal diffusion were all monitored in real time. The data were used to construct complex longitudinal modulus spectra spanning from, 30 kHZ to 3 GHz, and complex shear modulus spectra from - 10 MHz to 1 GHz. In the liquid tetramethyl tetraphenyl trisiloxane, experiments which verified timetemperature superposition of its relaxation dynamics permitted construction of a master plot of scaled relaxation spectra in the entire temperature range studied. MCT predictions of power-law frequency dependencies of the high and low frequency wings of the loss modulus yielded a high-frequency exponent parameter in good agreement with the width of the non-exponential relaxation kinetics. The low-frequency exponent did not agree with the predicted value. In triphenyl phosphite, measurements of the measured shear relaxation spectrum over two decades in frequency revealed that it does not match the previously measured longitudinal spectrum, suggesting that different underlying degrees of freedom contribute to shear and compressional relaxation. Measurement of shear wave propagation as a function of temperature lent credence to the dominance of the temperature dependence of the transport by the instantaneous shear modulus. These measurements also call into question other relationships drawn between glass mechanical behavior and the supercooled liquid fragility. In work conducted collaboratively, the ISS technique was employed in singles hot measurements of liquid benzene under conditions of shock loading. The results indicate that benzene remains in a liquid state for at least 200 ns after the shock's arrival. ISS was also used to characterize both the thermal transport and mechanical properties of nanofluids.(cont.) Finally, results of ISS acoustic measurements of thin films and their relationship with the study of glass forming liquids are briefly discussed.by Darius H. Torchinsky.Ph.D

Fluctuating charge density waves in a cuprate superconductor

Cuprate materials hosting high-temperature superconductivity (HTS) also exhibit various forms of charge and/or spin ordering whose significance is not fully understood. To date, static charge-density waves (CDWs) have been detected by diffraction probes only at special doping or in an applied external field. However, dynamic CDWs may also be present more broadly and their detection, characterization and relationship with HTS remain open problems. Here, we present a new method, based on ultrafast spectroscopy, to detect the presence and measure the lifetimes of CDW fluctuations in cuprates. In an underdoped La1.9Sr0.1CuO4 film (Tc = 26 K), we observe collective excitations of CDW that persist up to 100 K. This dynamic CDW fluctuates with a characteristic lifetime of 2 ps at T = 5 K which decreases to 0.5 ps at T = 100 K. In contrast, in an optimally doped La1.84Sr0.16CuO4 film (Tc = 38.5 K), we detect no signatures of fluctuating CDWs at any temperature, favoring the competition scenario. This work forges a path for studying fluctuating order parameters in various superconductors and other materials.Comment: 16 pages, 4 figures, accepted to Nature Material

Band-dependent Quasiparticle Dynamics in Single Crystals of the Ba0.6_{0.6}K0.4_{0.4}Fe2_2As2_2 Superconductor Revealed by Pump-Probe Spectroscopy

We report on band-dependent quasiparticle dynamics in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ ($T_c = 37 K$) measured using ultrafast pump-probe spectroscopy. In the superconducting state, we observe two distinct relaxation processes: a fast component whose decay rate increases linearly with excitation density and a slow component with an excitation density independent decay rate. We argue that these two components reflect the recombination of quasiparticles in the two hole bands through intraband and interband processes. We also find that the thermal recombination rate of quasiparticles increases quadratically with temperature. The temperature and excitation density dependence of the decays indicates fully gapped hole bands and nodal or very anisotropic electron bands.Comment: 4 pages, 4 figure

Band-dependent quasiparticle dynamics in the hole-doped Ba-122 iron pnictides

We report on band-dependent quasiparticle dynamics in the hole-doped Ba-122 pnictides measured by ultrafast pump-probe spectroscopy. In the superconducting state of the optimal and over hole-doped samples, we observe two distinct relaxation processes: a fast component whose decay rate increases linearly with excitation density and a slow component whose relaxation is independent of excitation strength. We argue that these two components reflect the recombination of quasiparticles in the two hole bands through intraband and interband processes. We also find that the thermal recombination rate of quasiparticles increases quadratically with temperature in all samples. The temperature and excitation density dependence of the decays indicates fully gapped hole bands and nodal or very anisotropic electron bands.United States. Department of Energy (Grant No. DE-FG02-08ER46521)National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (award number DMR - 0819762

α\alpha-Scale Decoupling of the Mechanical Relaxation and Diverging Shear Wave Propagation Lengthscale in Triphenylphosphite

We have performed depolarized Impulsive Stimulated Scattering experiments to observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from $\sim$10 - 500 MHz. These measurements, in tandem with previously performed longitudinal and shear measurements, permit further analyses of the relaxation dynamics of TPP within the framework of the mode coupling theory (MCT). Our results provide evidence of $\alpha$ coupling between the shear and longitudinal degrees of freedom up to a decoupling temperature $T_c$ = 231 K. A lower bound length scale of shear wave propagation in liquids verified the exponent predicted by theory in the vicinity of the decoupling temperature

A Low Temperature Nonlinear Optical Rotational Anisotropy Spectrometer for the Determination of Crystallographic and Electronic Symmetries

Nonlinear optical generation from a crystalline material can reveal the symmetries of both its lattice structure and underlying ordered electronic phases and can therefore be exploited as a complementary technique to diffraction based scattering probes. Although this technique has been successfully used to study the lattice and magnetic structures of systems such as semiconductor surfaces, multiferroic crystals, magnetic thin films and multilayers, challenging technical requirements have prevented its application to the plethora of complex electronic phases found in strongly correlated electron systems. These requirements include an ability to probe small bulk single crystals at the micron length scale, a need for sensitivity to the entire nonlinear optical susceptibility tensor, oblique light incidence reflection geometry and incident light frequency tunability among others. These measurements are further complicated by the need for extreme sample environments such as ultra low temperatures, high magnetic fields or high pressures. In this review we present a novel experimental construction using a rotating light scattering plane that meets all the aforementioned requirements. We demonstrate the efficacy of our scheme by making symmetry measurements on a micron scale facet of a small bulk single crystal of Sr$_2$IrO$_4$ using optical second and third harmonic generation.Comment: 8 pages, 5 figure