1,720,975 research outputs found
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 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
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
TRANSIENT OPTICAL NONLINEARITIES ENGENDERED BY FEMTOSECOND LASER FILAMENTATION IN GASES
No full textThe laser filamentation process in gases and its consequences have been at the center of interest over the three recent decades. The filament wake channel is formed by the laser pulse as a highly nonequilibrium and optically underdense plasma column. The contents of the plasma evolve towards equilibrium, giving rise to various transient optical effects. When filamentation occurs in a dense gas, it leads to the production of the excessively high density of excited atoms as compared to the density of ions. We used a kinetic model of the competing electron-collisional processes in the case of high-pressure argon gas and explored the sensitivity of the resulting excited-to-ionized atoms number density ratio to the envelope shape of the driving laser pulse. Considering three different families of the pulse shapes, we have shown that the ratio of excited atoms to ions in the dense gas can be manipulated and further increased. To further investigate the structure of the plasma column, we studied the filamentation process at the crossing of two laser beams. We have shown that in this case the process is significantly affected by the transient intensity grating caused by the beam interference in the crossing area, which leads to the formation of a microscopically structured filament wake channel. In particular, the grating of excited atom density is formed in the channel. We obtained characteristics of such excitation gratings that are controlled by the spatial and temporal characteristics of the crossing pulses. A nonlinear optical effect that is crucial in the context of excess excited atoms is the Rabi sideband generation. The Rabi sideband patterns from a one-dimensional plasma channel have already been studied. We considered theoretically the probing of the above-mentioned excitation gratings by a picosecond laser beam of 800 nm carrier wavelength and the formation of the characteristic spatial-spectral patterns of the Rabi sidebands. We demonstrated the sensitivity of these Rabi sideband patterns towards the grating characteristics, probe beam shape and wavelength and to the position of the observation screen and the observation slit on the screen. As our capstone work, we explored filamentation of long-wavelength laser pulses in atmospheric-pressure gases, as this situation effectively meets the dense gas criteria. We worked at transforming the theoretical and computational techniques that we developed for high-pressure gases at typical laser wavelengths (~800 nm) to be applicable to atmospheric-pressure gases at longer laser wavelengths (~3900 nm). Intense, ultrashort laser pulses of these latter carrier wavelength values just recently have become available for experiments and carry a great promise for applications in atmospheric optics, atmospheric chemistry, and related disciplines.Physic
DEGENERATE SECOND ORDER NONLINEAR OPTICAL SPECTROSCOPY OF CHIRAL WEYL SEMIMETALS
No full textThis thesis focuses on the development of nonlinear optical techniques and the measurement of topological properties of the Weyl semimetals. The first portion of this thesis describes technical developments of the nonlinear optical spectroscopic probes rotational anisotropy second harmonic generation (RA-SHG) and transient grating. In our work on SHG, we describe a fast-reflective optic-based rotational anisotropy nonlinear harmonic generation spectrometer built upon synchronization of stepper motors and a voice-coil fast turning motor with data recorded by a data acquisition card. This device enables fast accumulation of significantly more data points than traditional SHG spectrometers and further allows spectral measurement over a broad wavelength range to be performed without optical realignment. We then describe the Fourier domain RA-SHG, allows direct measurements of the RA-SHG signal components of Cn symmetry. This method is based on the fast scanning RA-SHG device described above and operates by recording the nth harmonics of the fast scanning signal using a lock-in amplifier. Finally, we describe a novel method of performing transient grating measurements based on low power laser diodes, a laser diode pulser, a digital delay generator, and a data acquisition card. The RA-SHG technique was applied to the chiral Weyl semimetal RhSi, where a spectrum of the sole SHG tensor element χ(2) i jk was measured over the unprecedented 0.275-1.5 eV incoming photon energy range. Our data shows evidence of a strong surface state response and are detailed enough to reveal the second order corrections to the linear band structure as well as the Pauli blocking condition which was observed to occur at ∼630 meV. We also describe measurements of the linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE) in RhSi deriving from topological Fermi arc states. While the magnitude of the CPGE response broadly matched theoretical predictions, the data also exhibit an inexplicably high degree of symmetry in the response as a function of incoming polarization in both CPGE and LPGE channels.
Collaborative work on the SHG spectrum from TaAs is also described, from which we attribute the origin of the SHG response peak to the third cumulant of the Bloch wavefunction. Further collaborative studies of the CPGE in RhSi (111) revealed a response that was likely due to the topological band structure, but that also shows that the theoretically predicted quantized CPGE was not observed due to impurities and from contributions from sources other than the Weyl nodes. Finally, we briefly summarized how the crystal structure of PrAlGe1-xSix was revealed to be non-centrosymmetric using the RA-SHG technique. Transition from intrinsic to extrinsic anomalous Hall effect by tuning the dopant concentration x was studied in this ferromagnetic Weyl semimetal.Physic
Development of Broadband Time-Resolved Spectroscopy and Investigation of the Dynamics of Photoexcited Carriers and Lattice Excitations in Chiral Weyl Semimetals
Get PDFThis thesis presents both technological developments of broadband time-resolved ultrafast spectroscopy and investigation of the dynamics of photoexcited carriers and topological phonons in chiral Weyl semimetals. In the first portion of this thesis, we describe a novel, optical pump supercontinuum probe technique based upon a nonlinear photonic crystal fiber and a digital micro-mirror device enabling the use of single element detector and lock-in demodulation of the signal, along with rapid, repeated averaging over the spectrum. Using a bismuth test sample, we demonstrate that the apparatus is capable of measuring time-resolved changes in reflectivity ΔR of a sample over the 1.5 - 3.0 eV energy range with 25 fs temporal resolution, while also being sensitive to relative changes in reflectivity as low as ΔR/R ~ 10^(-4).
A more conventional broadband pump-probe technique was applied to the chiral Weyl semimetal RhSi (S.G. 198) with the aim of investigating the dynamics of chiral single particle excitations and collective modes of topological, Weyl quasiparticles. In S.G. 198 materials, lack of crystallographic mirror symmetries allows for Weyl nodes to exist with a relative displacement of ~ 330 meV in energy, permitting optical investigation of the dynamics of a single Weyl node without interference from other bands or the opposite chirality Weyl node. The probe wavelength was independently scanned over the 0.4 -1.0 eV energy range to monitor the photoinduced changes in reflectivity fixing the pump wavelength at energies 0.57, 0.73 & 1.03 eV in order to excite one node, two nodes, and non-topological portions of the band structure, respectively. A single fast decay process (relaxation time τ ~ 300 - 500 fs) was observed over the entire energy regime studied, consistent with previous measurements of photoexcited charge dynamics in other Dirac and Weyl semimetals. Significantly, measurements of time-resolved Kerr effect spectroscopy yielded evidence of a T representation chiral lattice excitation whose observed frequency matched the calculated frequency using density functional theory.Physic
NONLINEAR AND ULTRAFAST OPTICAL STUDIES OF INTERFACIAL PROCESSES IN PHOTOVOLTAIC NANOMATERIALS
Get PDFThe development of efficient solar energy conversion devices has attracted much attention. Despite the fact that progress have been achieved, a fundamental understanding examining why efficiency can be improved remains elusive. For example, dye-sensitized solar cells (DSSC) exhibit high conversion efficiency when acetonitrile is used to prepare both the working electrode and the electrolyte. However, the mechanism explaining exactly how solvent influences device performance has not yet been systematically investigated. Another prominent example is the metal/semiconductor heterojunction systems. While it has been demonstrated that such mixed systems can significantly improve solar conversion efficiency, the mechanism of the electron dynamics driving these systems remains controversial. This stems in part from the fact that the experimentally deduced time constants, which are characteristic of such systems, are only ever extracted from phenomenological models and therefore cannot be assigned to specific physical processes. Ultimately, the development of a physical model is necessary to obtain an unambiguous physical picture of the solar conversion process.
In this dissertation, the ultrafast nonlinear spectroscopic methods, second harmonic light scattering (SHS) and transient absorption (TA) spectroscopy, have been employed to study dye molecular adsorption and charge transfer dynamics in several solar energy conversion systems, including 1) DSSC, where solvent effects are investigated to understand why acetonitrile is the most effective solvent; 2) Ag/TiO2 heterostructure system, where a physical model is proposed to quantitively analyze the electron dynamics; 3) porphyrin/Ag/TiO2 nanocomposite, where we found there is no electron injection from porphyrin to TiO2 and plasmonic metal can enhance the porphyrin dye adsorption to improve the device efficiency.
The propensity for surface adsorption of two related dyes, ortho-ethyl red (o-ER) and para-ethyl red (p-ER), onto TiO2 particles is studied with SHS. While p-ER readily adsorbs onto TiO2, o-ER does not. It is suggested that this difference is linked to the effects of the steric hindrance of the adsorbate. The influence of the solvent on the adsorption of p-ER onto TiO2 is also investigated. Of significance, p-ER can only chemically bond to the TiO2 surface in aprotic solvents, where adsorption free energy scales with solvent polarity. For protic solvents, preferential adsorption of the solvent shell ultimately prevents direct adsorption of p-ER onto the surface of TiO2. Likewise, solvent effects on charge transfer from p-ER to TiO2 are studied by TA. The electron injection rate is shown to be positively related to solvent polarity. Overall, highly polar aprotic solvents are shown to facilitate dye adsorption and electron injection, which helps improve the efficiency of DSSC devices.
Ultrafast dynamics of plasmon-induced hot electrons from Ag to TiO2 nanorods are probed by TA. The observed transient signal, which corresponds to the lifetime of the optically generated electrons, is analyzed using a physical model including electron injection, relaxation, band edge annihilation, the surface to bulk diffusion, and back diffusion from the bulk to the surface. A ca. 13 fs electron injection time is deduced for Ag to TiO2, which is faster than that generated in Au and dyes. Additionally, the excited state exciton dynamics of a porphyrin J-aggregate are investigated and subsequently modeled. More rapid dynamics are found following aggregation of the porphyrin, which can be attributed to the inclusion of more efficient relaxation channels. However, no electron injection from the J-aggregate to TiO2 is observed. This likely stems from the negatively charged repulsion between the two components. Further, when the J-aggregate is introduced into an Ag/TiO2 system, optical excitation occurs predominantly in the J-aggregate. This stems either from direct excitation of the J-aggregate or indirect excitation through plasmon-induced resonant energy transfer from Ag. Our results indicate that plasmon can enhance the dye adsorption, which has great potential for designing more efficient plasmonic DSSC devices.Chemistr
HIDDEN QUANTUM INTERFERENCE AND ACHIRAL SYMMETRY BREAKING REVEALED BY NONLINEAR OPTICAL HARMONIC GENERATION SPECTROSCOPY
No full textRotational anisotropy second harmonic generation measurements conducted with incoming photon energies range of 1.1 - 1.8 eV revealed the presence of strong magnetic dipole (MD) transitions that we assign to the trivalent lanthanide ions. Extracting the spectra of the MD transition susceptibility tensor, we observe an asymmetric resonance at 1.5 eV, consistent with a Fano lineshape. Comparison of our data with a band structure obtained from density function theory revealed the hybridization of an unoccupied band deriving from unassigned orbitals in LaAlSi and CeAlSi, the unoccupied state of PrAlSi, and occupied state in NdAlSi. Moreover,polarimetry measurement was conducted at normal incidence in the paramagnetic phase of LnAlSi revealed a novel, nonlinear form of electromagnetically induced chirality (EIC) that derives
from resonant MD transitions. This form of nonlinear magnetoelectricity is expressed as , which causes the emission of elliptically polarized SHG.
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In a separate study, we used the second and third harmonic nonlinear harmonic generation spectroscopy to study \ce{1T-TiSe2} below its charge density wave phase transition temperature at 200 K. The lack of a second harmonic signal below and above the transition temperature indicated that the low-temperature symmetry-breaking phase is achiral. Further study using rotational anisotropy nonlinear third harmonic generation revealed that \ce{1T-TiSe2} experiences a non-trivial phase transition at 180 K within the charge density phase that comprises two separate domain types that randomly rearrange upon thermal cycling. Taken together, our data support the onset of orbital ordering at 180 K without the emergence of an electronically chiral state.\\\par
Finally, we describe THz emission spectroscopy to study the circular photogalvanic effect (CPGE ) and linear photogalvanic effect (LPGE) spectra of the structurally chiral Weyl semimetal PdGa. We reveal that CPGE and LPGE spectra produce signals of opposite signs for the two PdGa enantiomers, as predicted by theory. Measurement of the spectra deriving from the material's bulk reveals a peak in the photogalvanic spectra at 0.68 eV, which is due to optical transitions between the parallel bands near the Weyl nodes dispersing from to R and similar transitions between M and R. Surface sensitive measurement are consistent with helicoidally dispersing states of opposite helicity from the two different chirality samples.Physic
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
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