1,720,982 research outputs found
Observation of a metal-to-insulator transition with both Mott-Hubbard and Slater characteristics in Sr2IrO4 from time-resolved photocarrier dynamics
No full textWe perform a time-resolved optical study of Sr[subscript 2]IrO[subscript 4] to understand the influence of magnetic ordering on the low energy electronic structure of a strongly spin-orbit coupled J[subscript eff] = 1/2 Mott insulator. By studying the recovery dynamics of photoexcited carriers, we find that upon cooling through the Néel temperature T[subscript N] the system evolves continuously from a metal-like phase with fast (∼50 fs) and excitation density independent relaxation dynamics to a gapped phase characterized by slower (∼500 fs) excitation density-dependent bimolecular recombination dynamics, which is a hallmark of a Slater-type metal-to-insulator transition. However our data indicate that the high energy reflectivity associated with optical transitions into the unoccupied J[subscript eff] = 1/2 band undergoes the sharpest upturn at T[subscript N], which is consistent with a Mott-Hubbard type metal-to-insulator transition involving spectral weight transfer into an upper Hubbard band. These findings show Sr[subscript 2]IrO[subscript 4] to be a unique system in which Slater- and Mott-Hubbard-type behaviors coexist and naturally explain the absence of anomalies at T[subscript N] in transport and thermodynamic measurements.United States. Army Research Office (Grant W911NF-11-1-0331
Fluctuating charge-density waves in a cuprate superconductor
Get PDFCuprate materials hosting high-temperature superconductivity (HTS) also exhibit various forms of charge and spin ordering1, 2, 3, 4, 5, 6 whose significance is not fully understood7. So far, static charge-density waves8 (CDWs) have been detected by diffraction probes only at particular doping levels9, 10, 11 or in an applied external field12 . However, dynamic CDWs may also be present more broadly and their detection, characterization and relationship with HTS remain open problems. Here we present a method based on ultrafast spectroscopy to detect the presence and measure the lifetimes of CDW fluctuations in cuprates. In an underdoped La[subscript 1.9]Sr[subscript 0.1]CuO4 film (T[subscript c] = 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 that decreases to 0.5 ps at T = 100 K. In contrast, in an optimally doped La[subscript 1.84]Sr[subscript 0.16]CuO[subscript ]4 film (T[subscript c] = 38.5 K), we detect no signatures of fluctuating CDWs at any temperature, favouring the competition scenario. This work forges a path for studying fluctuating order parameters in various superconductors and other materials
High frequency longitudinal and shear acoustic waves in glass-forming liquids
No full textThree different picosecond ultrasonic techniques for longitudinal and transverse acoustic pulse generation have been combined with Impulsive Stimulated Thermal Scattering (ISTS) to probe structural relaxation dynamics in glycerol and DC704 (tetramethyl tetraphenyl trisiloxane) at megahertz and gigahertz frequencies (∼ 50 MHz - 100 GHz) from below their respective glass transition temperatures up to 370 K.United States. Department of Energy (Grant DE-FG02-00ER15087)National Science Foundation (U.S.) (Grant CHE-0616939)National Science Foundation (U.S.) (Grant DMR-0414895
Band-dependent Quasiparticle Dynamics in Single Crystals of the Ba[subscript 0.6]K[subscript 0.4]Fe[subscript 2]As[subscript 2] Superconductor Revealed by Pump-Probe Spectroscopy
No full textWe report on band-dependent quasiparticle dynamics in Ba[subscript 0.6]K[subscript 0.4]Fe[subscript 2]As[subscript 2](Tc=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.United States. Dept. of Energy (Grant No. DE-FG02-08ER46521)National Natural Science Foundation of China (NSFC)China. Ministry of Science and Technology (project 973
Band-dependent quasiparticle dynamics in the hole-doped Ba-122 iron pnictides
Get PDFWe 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
Toward broadband mechanical spectroscopy
No full textDiverse material classes exhibit qualitatively similar behavior when made viscous upon cooling toward the glass transition, suggesting a common theoretical basis. We used seven different measurement methods to determine the mechanical relaxation kinetics of a prototype molecular glass former over a temporal range of 13 decades and over a temperature range spanning liquid to glassy states. The data conform to time–temperature superposition for the main (alpha) process and to a scaling relation of schematic mode-coupling theory. The broadband mechanical measurements demonstrated have fundamental and practical applications in polymer science, geophysics, multifunctional materials, and other areas. Keywords: viscous liquids; broadband mechanical spectroscopy; mode-coupling theoryNational Science Foundation (U.S.) (Grant CHE-1111557)United States. Department of Energy (Grant DE-FG02-00ER15087
α-Scale decoupling of the mechanical relaxation and diverging shear wave propagation length scale in triphenylphosphite
No full textWe have performed depolarized impulsive stimulated scattering experiments to observe shear acoustic phonons in supercooled triphenylphosphite (TPP) from ∼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. Our results provide evidence of α coupling between the shear and longitudinal degrees of freedom up to a decoupling temperature T [subscript 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.National Science Foundation (U.S.) (Grant CHE-0616939)National Science Foundation (U.S.) (Grant IMR-0414895
Theoretical and experimental study of second harmonic generation from the surface of the topological insulator Bi[subscript 2]Se[subscript 3]
No full textWe develop a theoretical model that describes the second harmonic generation of light from the surface of the topological insulator Bi[subscript 2]Se[subscript 3] and experimentally demonstrate that the technique is sensitive to the surface electrons. By performing a crystal symmetry analysis of Bi[subscript 2]Se[subscript 3](111) we determine the nonlinear electric susceptibility tensor elements that give rise to second harmonic generation. Using these results, we present a phenomenological model that shows that the relative magnitudes of these tensor elements can be determined by measuring the polarization and intensity of the radiated second harmonic light as a function of the in-plane crystal orientation and incident laser polarization. We describe optical techniques capable of isolating second harmonic light and, using these techniques, we measure the first-order linear optical and second-order nonlinear optical responses as a function of crystal orientation and laser polarization on bulk single crystals of Bi[subscript 2]Se[subscript 3](111). The experimental results are consistent with our theoretical description. By comparing the data to our theoretical model we determine that a portion of the measured second harmonic light originates from the accumulation region of Bi[subscript 2]Se[subscript 3](111), which we confirm by performing surface doping-dependent studies. Our results show that second harmonic generation is a promising tool for spectroscopic studies of topological surfaces and buried interfaces.United States. Dept. of Energy (DE-FG02- 08ER46521
Nonequilibrium quasiparticle relaxation dynamics in single crystals of hole- and electron-doped BaFe2As2
Get PDFWe report on the nonequilibrium quasiparticle dynamics in BaFe[subscript 2]As[subscript 2] on both the hole-doped (Ba[subscript 1−x]K[subscript x]Fe[subscript 2]As[subscript 2]) and electron-doped (BaFe[subscript 2−y]Co[subscript y]As[subscript 2]) sides of the phase diagram using ultrafast pump-probe spectroscopy. Below T[subscript c], measurements conducted at low photoinjected quasiparticle densities in the optimally and overdoped Ba[subscript 1−x]K[subscript x]Fe[subscript 2]As[subscript 2] samples reveal 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 processes 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 these samples. The temperature and excitation density dependence of the decays indicates fully gapped hole bands and nodal or very anisotropic electron bands. At higher excitation densities and lower hole dopings, the dependence of the dynamics on quasiparticle density disappears as the data are more readily understood in terms of a model which accounts for the quasiequilibrium temperature attained by the sample. In the BaFe[subscript 2−y]Co[subscript y]As[subscript 2] samples, dependence of the recombination rate on quasiparticle density at low dopings (i.e., y=0.12) is suppressed upon submergence of the inner hole band and quasiparticle relaxation occurs in a slow, density-independent manner.United States. Dept. of Energy (Grant No. DE-FG02-08ER46521)MRSEC Program of the National Science Foundation (Award No. DMR-0819762)National Natural Science Foundation (China)Chinese Academy of SciencesNational Basic Research Program of China (973 Program
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