62 research outputs found
Ultrafast Angle-Resolved Photoemission Spectroscopy of Quantum Materials
Techniques in time- and angle-resolved photoemission spectroscopy have
facilitated a number of recent advances in the study of quantum materials. We
review developments in this field related to the study of incoherent
nonequilibrium electron dynamics, the analysis of interactions between
electrons and collective excitations, the exploration of dressed-state physics,
and the illumination of unoccupied band structure. Future prospects are also
discussed.Comment: 7 pages, 6 figure
Broadband THz study of excitonic resonances in the high-density regime
We report the first terahertz study of the intra-excitonic 1s-2p transition
at high excitation densities in GaAs/AlGaAs quantum wells. A strong shift,
broadening, and ultimately the disappearance of this resonance occurs with
increasing density, after ultrafast photoexcitation at the near-infrared
exciton line. Densities of excitons and unbound electron-hole pairs are
followed quantitatively using a model of the composite terahertz dielectric
response. Comparison with near-infrared absorption changes reveals a
significantly enhanced energy shift and broadening of the intra-excitonic
resonance.Comment: 4 pages, 4 figure
Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate
The ability to probe symmetry breaking transitions on their natural time
scales is one of the key challenges in nonequilibrium physics. Stripe ordering
represents an intriguing type of broken symmetry, where complex interactions
result in atomic-scale lines of charge and spin density. Although phonon
anomalies and periodic distortions attest the importance of electron-phonon
coupling in the formation of stripe phases, a direct time-domain view of
vibrational symmetry breaking is lacking. We report experiments that track the
transient multi-THz response of the model stripe compound
LaSrNiO, yielding novel insight into its electronic and
structural dynamics following an ultrafast optical quench. We find that
although electronic carriers are immediately delocalized, the crystal symmetry
remains initially frozen - as witnessed by time-delayed suppression of
zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry.
Longitudinal and transverse vibrations react with different speeds, indicating
a strong directionality and an important role of polar interactions. The hidden
complexity of electronic and structural coupling during stripe melting and
formation, captured here within a single terahertz spectrum, opens new paths to
understanding symmetry breaking dynamics in solids.Comment: 21 pages, 4 figures; updated version with journal re
Denoising Scanning Tunneling Microscopy Images of Graphene with Supervised Machine Learning
Machine learning (ML) methods are extraordinarily successful at denoising
photographic images. The application of such denoising methods to scientific
images is, however, often complicated by the difficulty in experimentally
obtaining a suitable expected result as an input to training the ML network.
Here, we propose and demonstrate a simulation-based approach to address this
challenge for denoising atomic-scale scanning tunneling microscopy (STM)
images, which consists of training a convolutional neural network on STM images
simulated based on a tight-binding electronic structure model. As model
materials, we consider graphite and its mono- and few-layer counterpart,
graphene. With the goal of applying it to any experimental STM image obtained
on graphitic systems, the network was trained on a set of simulated images with
varying characteristics such as tip height, sample bias, atomic-scale defects,
and non-linear background. Denoising of both simulated and experimental images
with this approach is compared to that of commonly-used filters, revealing a
superior outcome of the ML method in the removal of noise as well as scanning
artifacts - including on features not simulated in the training set. An
extension to larger STM images is further discussed, along with intrinsic
limitations arising from training set biases that discourage application to
fundamentally unknown surface features. The approach demonstrated here provides
an effective way to remove noise and artifacts from typical STM images,
yielding the basis for further feature discernment and automated processing.Comment: Includes S
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Ultrabroadband 50-130 THz pulses generated via phase-matcheddifference frequency mixing in LiIO3
We report the generation of ultrabroadband pulses spanningthe 50-130 THz frequency range via phase-matched difference frequencymixing within the broad spectrum of sub-10 fs pulses in LiIO_3. Modelcalculations reproduce the octave-spanning spectra and predict few-cycleTHz pulse durations less than 20~;fs. The applicability of this scheme isdemonstrated with 9-fs pulses from a Ti:sapphire oscillator and with 7-fsamplified pulses from a hollow fiber compressor as pumpsources
Coherent two-dimensional Fourier transform spectroscopy using a 25 Tesla resistive magnet.
We performed nonlinear optical two-dimensional Fourier transform spectroscopy measurements using an optical resistive high-field magnet on GaAs quantum wells. Magnetic fields up to 25 T can be achieved using the split helix resistive magnet. Two-dimensional spectroscopy measurements based on the coherent four-wave mixing signal require phase stability. Therefore, these measurements are difficult to perform in environments prone to mechanical vibrations. Large resistive magnets use extensive quantities of cooling water, which causes mechanical vibrations, making two-dimensional Fourier transform spectroscopy very challenging. Here, we report on the strategies we used to overcome these challenges and maintain the required phase-stability throughout the measurement. A self-contained portable platform was used to set up the experiments within the time frame provided by a user facility. Furthermore, this platform was floated above the optical table in order to isolate it from vibrations originating from the resistive magnet. Finally, we present two-dimensional Fourier transform spectra obtained from GaAs quantum wells at magnetic fields up to 25 T and demonstrate the utility of this technique in providing important details, which are obscured in one dimensional spectroscopy
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