29 research outputs found
Time- and Angle-Resolved Photoemission Studies of Quantum Materials
Angle-resolved photoemission spectroscopy (ARPES) -- with its exceptional
sensitivity to both the binding energy and momentum of valence electrons in
solids -- provides unparalleled insights into the electronic structure of
quantum materials. Over the last two decades, the advent of femtosecond lasers,
which can deliver ultrashort and coherent light pulses, has ushered the ARPES
technique into the time domain. Now, time-resolved ARPES (TR-ARPES) can probe
ultrafast electron dynamics and the out-of-equilibrium electronic structure,
providing a wealth of information otherwise unattainable in conventional ARPES
experiments. This paper begins with an introduction to the theoretical
underpinnings of TR-ARPES followed by a description of recent advances in
state-of-the-art ultrafast sources and optical excitation schemes. It then
reviews paradigmatic phenomena investigated by TR-ARPES thus far, such as
out-of-equilibrium electronic states and their spin dynamics, Floquet-Volkov
states, photoinduced phase transitions, electron-phonon coupling, and surface
photovoltage effects. Each section highlights TR-ARPES data from diverse
classes of quantum materials, including semiconductors, charge-ordered systems,
topological materials, excitonic insulators, van der Waals materials, and
unconventional superconductors. These examples demonstrate how TR-ARPES has
played a critical role in unraveling the complex dynamical properties of
quantum materials. The conclusion outlines possible future directions and
opportunities for this powerful technique.Comment: To appear in Reviews of Modern Physic
A basic model for the propagation of ideologies
Ideas and ideologies move the world and are involved in almost every aspect of human life and society. This paper presents a mathematical model for the propagation of two different ideologies in a group of people that could convert or not to one of the ideologies. This model allowed us to analyze which relations between parameters influence the survival and dominance of an ideology. The basic reproductive number was computed and numerical simulations were performed to analyze different scenarios.UCR::VicerrectorÃa de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de MatemáticaUCR::VicerrectorÃa de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones en Matemáticas Puras y Aplicadas (CIMPA
Establishing non-thermal regimes in pump-probe electron-relaxation dynamics
Time- and angle-resolved photoemission spectroscopy (TR-ARPES) accesses the
electronic structure of solids under optical excitation, and is a powerful
technique for studying the coupling between electrons and collective modes. One
approach to infer electron-boson coupling is through the relaxation dynamics of
optically-excited electrons, and the characteristic timescales of energy
redistribution. A common description of electron relaxation dynamics is through
the effective electronic temperature. Such a description requires that
thermodynamic quantities are well-defined, an assumption that is generally
violated at early delays. Additionally, precise estimation of the non-thermal
window -- within which effective temperature models may not be applied -- is
challenging. We perform TR-ARPES on graphite and show that Boltzmann rate
equations can be used to calculate the time-dependent electronic occupation
function, and reproduce experimental features given by non-thermal electron
occupation. Using this model, we define a quantitative measure of non-thermal
electron occupation and use it to define distinct phases of electron relaxation
in the fluence-delay phase space. More generally, this approach can be used to
inform the non-thermal-to-thermal crossover in pump-probe experiments.Comment: 18 pages, 10 figure
A versatile laser-based apparatus for time-resolved ARPES with micro-scale spatial resolution
We present the development of a versatile apparatus for a 6.2 eV laser-based
time and angle-resolved photoemission spectroscopy with micrometer spatial
resolution (time-resolved -ARPES). With a combination of tunable spatial
resolution down to 11 m, high energy resolution (11 meV),
near-transform-limited temporal resolution (280 fs), and tunable 1.55 eV
pump fluence up to 3 mJ/cm, this time-resolved -ARPES system
enables the measurement of ultrafast electron dynamics in exfoliated and
inhomogeneous materials. We demonstrate the performance of our system by
correlating the spectral broadening of the topological surface state of
BiSe with the spatial dimension of the probe pulse, as well as
resolving the spatial inhomogeneity contribution to the observed spectral
broadening. Finally, after in-situ exfoliation, we performed time-resolved
-ARPES on a 30 m few-layer-thick flake of transition metal
dichalcogenide WTe, thus demonstrating the ability to access ultrafast
electron dynamics with momentum resolution on micro-exfoliated and twisted
materials
Direct determination of mode-projected electron-phonon coupling in the time-domain
Ultrafast spectroscopies have become an important tool for elucidating the
microscopic description and dynamical properties of quantum materials. In
particular, by tracking the dynamics of non-thermal electrons, a material's
dominant scattering processes -- and thus the many-body interactions between
electrons and collective excitations -- can be revealed. Here we present a new
method for extracting the electron-phonon coupling strength in the time domain,
by means of time and angle-resolved photoemission spectroscopy (TR-ARPES). This
method is demonstrated in graphite, where we investigate the dynamics of
photo-injected electrons at the K point, detecting quantized energy-loss
processes that correspond to the emission of strongly-coupled optical phonons.
We show that the observed characteristic timescale for spectral-weight-transfer
mediated by phonon-scattering processes allows for the direct quantitative
extraction of electron-phonon matrix elements, for specific modes, and with
unprecedented sensitivity.Comment: 19 pages, 4 figure
Unveiling the underlying interactions in Ta2NiSe5 from photo-induced lifetime change
We present a generic procedure for quantifying the interplay of electronic
and lattice degrees of freedom in photo-doped insulators through a comparative
analysis of theoretical many-body simulations and time- and angle-resolved
photoemission spectroscopy (TR-ARPES) of the transient response of the
candidate excitonic insulator Ta2NiSe5. Our analysis demonstrates that the
electron-electron interactions dominate the electron-phonon ones. In
particular, a detailed analysis of the TRARPES spectrum enables a clear
separation of the dominant broadening (electronic lifetime) effects from the
much smaller bandgap renormalization. Theoretical calculations show that the
observed strong spectral broadening arises from the electronic scattering of
the photo-excited particle-hole pairs and cannot be accounted for in a model in
which electron-phonon interactions are dominant. We demonstrate that the
magnitude of the weaker subdominant bandgap renormalization sensitively depends
on the distance from the semiconductor/semimetal transition in the
high-temperature state, which could explain apparent contradictions between
various TR-ARPES experiments. The analysis presented here indicates that
electron-electron interactions play a vital role (although not necessarily the
sole one) in stabilizing the insulating state
Detection of a two-phonon mode in a cuprate superconductor via polarimetric RIXS
Recent improvements in the energy resolution of resonant inelastic x-ray
scattering experiments (RIXS) at the Cu-L edge have enabled the study of
lattice, spin, and charge excitations. Here, we report on the detection of a
low intensity signal at 140meV, twice the energy of the bond-stretching (BS)
phonon mode, in the cuprate superconductor
(Bi-2212).
Ultra-high resolution polarimetric RIXS measurements allow us to resolve the
outgoing polarization of the signal and identify this feature as a two-phonon
excitation. Further, we study the connection between the two-phonon mode and
the BS one-phonon mode by constructing a joint density of states toy model that
reproduces the key features of the data
Doping-dependent charge order correlations in electron-doped cuprates
Understanding the interplay between charge order (CO) and other phenomena (for example, pseudogap, antiferromagnetism, and superconductivity) is one of the central questions in the cuprate high-temperature superconductors. The discovery that similar forms of CO exist in both hole- and electron-doped cuprates opened a path to determine what subset of the CO phenomenology is universal to all the cuprates. We use resonant x-ray scattering to measure the CO correlations in electron-doped cuprates (La2−xCexCuO4 and Nd2−xCexCuO4) and their relationship to antiferromagnetism, pseudogap, and superconductivity. Detailed measurements of Nd2−xCexCuO4 show that CO is present in the x = 0.059 to 0.166 range and that its doping-dependent wave vector is consistent with the separation between straight segments of the Fermi surface. The CO onset temperature is highest between x = 0.106 and 0.166 but decreases at lower doping levels, indicating that it is not tied to the appearance of antiferromagnetic correlations or the pseudogap. Near optimal doping, where the CO wave vector is also consistent with a previously observed phonon anomaly, measurements of the CO below and above the superconducting transition temperature, or in a magnetic field, show that the CO is insensitive to superconductivity. Overall, these findings indicate that, although verified in the electron-doped cuprates, material-dependent details determine whether the CO correlations acquire sufficient strength to compete for the ground state of the cuprates