26 research outputs found
Theory of ultrafast quasiparticle dynamics in high-temperature superconductors: Pump fluence dependence
We present a theory for the time-resolved optical spectroscopy of
high-temperature superconductors at high excitation densities with strongly
anisotropic electron-phonon coupling. A signature of the strong coupling
between the out-of-plane, out-of-phase O buckling mode () and
electronic states near the antinode is observed as a higher-energy peak in the
time-resolved optical conductivity and Raman spectra, while no evidence of the
strong coupling between the in-plane Cu-O breathing mode and nodal electronic
states is observed. More interestingly, it is observed that under appropriate
conditions of pump fluence, this signature exhibits a re-entrant behavior with
time delay, following the fate of the superconducting condensate.Comment: 5 pages, 3 embedded eps figures, to appear in PR
Induced ferroelectric phases in SrTiO3 by a nanocomposite approach
Inducing new phases in thick films via vertical lattice strain is one of the critical advantages of vertically aligned nanocomposites (VANs). In SrTiO3 (STO), the ground state is ferroelastic, and the ferroelectricity in STO is suppressed by the orthorhombic transition. Here, we explore whether vertical lattice strain in three-dimensional VANs can be used to induce new ferroelectric phases in SrTiO3:MgO (STO:MgO) VAN thin films. The STO:MgO system incorporates ordered, vertically aligned MgO nanopillars into a STO film matrix. Strong lattice coupling between STO and MgO imposes a large lattice strain in the STO film. We have investigated ferroelectricity in the STO phase, existing up to room temperature, using piezoresponse force microscopy, phase field simulation and second harmonic generation. We also serendipitously discovered the formation of metastable TiO nanocores in MgO nanopillars embedded in the STO film matrix. Our results emphasize the design of new phases via vertical epitaxial strain in VAN thin films
Light-Driven Nanoscale Vectorial Currents
Controlled charge flows are fundamental to many areas of science and
technology, serving as carriers of energy and information, as probes of
material properties and dynamics, and as a means of revealing or even inducing
broken symmetries. Emerging methods for light-based current control offer
promising routes beyond the speed and adaptability limitations of conventional
voltage-driven systems. However, optical manipulation of currents at nanometer
spatial scales remains a basic challenge and a key step toward scalable
optoelectronic systems and local probes. Here, we introduce vectorial
optoelectronic metasurfaces as a new class of metamaterial in which ultrafast
charge flows are driven by light pulses, with actively-tunable directionality
and arbitrary patterning down to sub-diffractive nanometer scales. In the
prototypical metasurfaces studied herein, asymmetric plasmonic nanoantennas
locally induce directional, linear current responses within underlying
graphene. Nanoscale unit cell symmetries are read out via polarization- and
wavelength-sensitive currents and emitted terahertz (THz) radiation. Global
vectorial current distributions are revealed by spatial mapping of the THz
field polarization, also demonstrating the direct generation of elusive
broadband THz vector beams. We show that a detailed interplay between
electrodynamic, thermodynamic, and hydrodynamic degrees of freedom gives rise
to these currents through rapidly-evolving nanoscale forces and charge flows
under extreme spatial and temporal localization. These results set the stage
for versatile patterning and optical control over nanoscale currents in
materials diagnostics, nano-magnetism, microelectronics, and ultrafast
information science
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Competing Interface and Bulk Effect-Driven Magnetoelectric Coupling in Vertically Aligned Nanocomposites.
Room-temperature magnetoelectric (ME) coupling is developed in artificial multilayers and nanocomposites composed of magnetostrictive and electrostrictive materials. While the coupling mechanisms and strengths in multilayers are widely studied, they are largely unexplored in vertically aligned nanocomposites (VANs), even though theory has predicted that VANs exhibit much larger ME coupling coefficients than multilayer structures. Here, strong transverse and longitudinal ME coupling in epitaxial BaTiO3:CoFe2O4 VANs measured by both optical second harmonic generation and piezoresponse force microscopy under magnetic fields is reported. Phase field simulations have shown that the ME coupling strength strongly depends on the vertical interfacial area which is ultimately controlled by pillar size. The ME coupling in VANs is determined by the competition between the vertical interface coupling effect and the bulk volume conservation effect. The revealed mechanisms shed light on the physical insights of vertical interface coupling in VANs in general, which can be applied to a variety of nanocomposites with different functionalities beyond the studied ME coupling effect.The work at Los Alamos National Laboratory was supported by the NNSA's Laboratory Directed Research and Development Program and was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is managed by Triad National Security, LLC for the U.S. Department of Energy's NNSA, under contract 89233218CNA000001. Angular‐dependent magnetization studies (L.C.) were partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Materials Sciences, and Engineering Division
Correlation between quantum charge fluctuations and magnetic ordering in multiferroic LuFe
We examine the interplay between quantum charge fluctuations and magnetic ordering in
multiferroic LuFe2O4 and show that this can couple spin and charge degrees of
freedom in a LuFe2O4 bilayer below the Neel temperature TN. Our analysis supports
the idea that the double exchange mechanism normally used in metallic systems can be
applied to charge-ordered insulators. This causes ferrimagnetic spin order to reduce the
transfer integrals between Fe2+ and Fe3+ in LuFe2O4, decreasing charge fluctuations and increasing the
polarization in this system below TN. This work thus
provides a more detailed understanding of the mechanism for spin-charge coupling in
LuFe2O4
Optical techniques for solid-state materials characterization
This book has comprehensively covered the essential optical approaches needed for solid-state materials characterization. Written by experts in the field, this will be a great reference for students, engineers, and scientists.-Professor Yoke Khin Yap, Michigan Technical University
Dynamic light-matter coupling across multiple spatial dimensions in a quantum dots-in-a-well heterostructure
Semiconductor heterostructures incorporating multiple degrees of spatial confinement have recently attracted substantial interest for photonic applications. One example is the quantum dots-in-a-well (DWELL) heterostructure, consisting of zero-dimensional quantum dots embedded in a two-dimensional quantum well and surrounded by three-dimensional bulk material. This structure offers several advantages over conventional photonic devices while providing a model system for the study of light-matter interactions across multiple spatial dimensions. Here, we use ultrafast differential transmission spectroscopy2 to temporally and spectrally resolve density-dependent carrier dynamics in a DWELL heterostructure. We observe excitation-dependent shifts of the quantum dot energy levels at low densities, while at high densities we observe an anomalous induced absorption at the quantum dot excited state that is correlated to quantum well population dynamics. These studies of density-dependent light-matter interactions across multiple coupled spatial dimensions provide clues to the underlying physics governing quantum dot properties, with important implications for DWELL-based photonic devices
Ultrafast terahertz emission from emerging symmetry-broken materials
Abstract Nonlinear optical spectroscopies are powerful tools for investigating both static material properties and light-induced dynamics. Terahertz (THz) emission spectroscopy has emerged in the past several decades as a versatile method for directly tracking the ultrafast evolution of physical properties, quasiparticle distributions, and order parameters within bulk materials and nanoscale interfaces. Ultrafast optically-induced THz radiation is often analyzed mechanistically in terms of relative contributions from nonlinear polarization, magnetization, and various transient free charge currents. While this offers material-specific insights, more fundamental symmetry considerations enable the generalization of measured nonlinear tensors to much broader classes of systems. We thus frame the present discussion in terms of underlying broken symmetries, which enable THz emission by defining a system directionality in space and/or time, as well as more detailed point group symmetries that determine the nonlinear response tensors. Within this framework, we survey a selection of recent studies that utilize THz emission spectroscopy to uncover basic properties and complex behaviors of emerging materials, including strongly correlated, magnetic, multiferroic, and topological systems. We then turn to low-dimensional systems to explore the role of designer nanoscale structuring and corresponding symmetries that enable or enhance THz emission. This serves as a promising route for probing nanoscale physics and ultrafast light-matter interactions, as well as facilitating advances in integrated THz systems. Furthermore, the interplay between intrinsic and extrinsic material symmetries, in addition to hybrid structuring, may stimulate the discovery of exotic properties and phenomena beyond existing material paradigms
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