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 La$_{1.75}$Sr$_{0.25}$NiO$_{4}$, 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

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

Ultrafast Mid-Infrared Intra-Excitonic Response of Individualized Single-Walled Carbon Nanotubes

The quasi-1D confinement and reduced screening of photoexcited charges in single-walled carbon nanotubes (SWNTs) entails strongly-enhanced Coulomb interactions and exciton binding energies. Such amplified electron-hole (e-h) correlations have important implications for both fundamental physics and optoelectronic applications of nanotubes. The availability of"individualized" SWNT ensembles with bright and structured luminescence has rendered specific tube chiralities experimentally accessible. In these samples, evidence for excitonic behavior was found in absorption-luminescence maps, two-photon excited luminescence, or ultrafast carrier dynamics. Here, we report ultrafast mid-infrared (mid-IR) studies of individualized SWNTs, evidencing strong photoinduced absorption around 200 meV in semiconducting tubes of (6,5) and (7,5) chiralities. This manifests the observation of quasi-1D intra-excitonic transitions between different relative-momentum states, in agreement with the binding energy and calculated oscillator strength. Our measurements further reveal a saturation of the photoinduced absorption with increasing phase-space filling of the correlated e-h pairs. The transient mid-IR response represents a new tool, unhindered by restrictions of momentum or interband dipole moment, to investigate the density and dynamics of SWNT excitons