12 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 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
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
Comparative Electronic Structures of the Chiral Helimagnets Cr1/3NbS2 and Cr1/3TaS2
Magnetic materials with noncollinear spin textures are promising for
spintronic applications. To realize practical devices, control over the length
and energy scales of such spin textures is imperative. The chiral helimagnets
Cr1/3NbS2 and Cr1/3TaS2 exhibit analogous magnetic phase diagrams with
different real-space periodicities and field dependence, positioning them as
model systems for studying the relative strengths of the microscopic mechanisms
giving rise to exotic spin textures. Here, we carry out a comparative study of
the electronic structures of Cr1/3NbS2 and Cr1/3TaS2 using angle-resolved
photoemission spectroscopy and density functional theory. We show that bands in
Cr1/3TaS2 are more dispersive than their counterparts in Cr1/3NbS2 and connect
this result to bonding and orbital overlap in these materials. We also
unambiguously distinguish exchange splitting from surface termination effects
by studying the dependence of their photoemission spectra on polarization,
temperature, and beam size. We find strong evidence that hybridization between
intercalant and host lattice electronic states mediates the magnetic exchange
interactions in these materials, suggesting that band engineering is a route
toward tuning their spin textures. Overall, these results underscore how the
modular nature of intercalated transition metal dichalcogenides translates
variation in composition and electronic structure to complex magnetism.Comment: 46 pages, 18 figures, 5 table
Nature of the current-induced insulator-to-metal transition in CaRuO as revealed by transport-ARPES
The Mott insulator CaRuO exhibits a rare insulator-to-metal
transition (IMT) induced by DC current. While structural changes associated
with this transition have been tracked by neutron diffraction, Raman
scattering, and x-ray spectroscopy, work on elucidating the response of the
electronic degrees of freedom is still in progress. Here we unveil the
current-induced modifications of the electronic states of CaRuO by
employing angle-resolved photoemission spectroscopy (ARPES) in conjunction with
four-probe transport. Two main effects emerge: a clear reduction of the Mott
gap and a modification in the dispersion of the Ru-bands. The changes in
dispersion occur exclusively along the high-symmetry direction, parallel
to the -axis where the greatest in-plane lattice change occurs. These
experimental observations are reflected in dynamical mean-field theory (DMFT)
calculations simulated exclusively from the current-induced lattice constants,
indicating a current driven structural transition as the primary mechanism of
the IMT. Furthermore, we demonstrate this phase is distinct from the
high-temperature zero-current metallic phase. Our results provide insight into
the elusive nature of the current-induced IMT of CaRuO and advance the
challenging, yet powerful, technique of transport-ARPES.Comment: 8 pages, 4 figure
Fano interference of the Higgs mode in cuprate high-Tc superconductors
Despite decades of search for the pairing boson in cuprate high-Tc
superconductors, its identity still remains debated to date. For this reason,
spectroscopic signatures of electron-boson interactions in cuprates have always
been a center of attention. For example, the kinks in the quasiparticle
dispersion observed by angle-resolved photoemission spectroscopy (ARPES)
studies have motivated a decade-long investigation of electron-phonon as well
as electron-paramagnon interactions in cuprates. On the other hand, the overlap
between the charge-order correlations and the pseudogap in the cuprate phase
diagram has also generated discussions about the potential link between them.
In the present study, we provide a fresh perspective on these intertwined
interactions using the novel approach of Higgs spectroscopy, i.e. an
investigation of the amplitude oscillations of the superconducting order
parameter driven by a terahertz radiation. Uniquely for cuprates, we observe a
Fano interference of its dynamically driven Higgs mode with another collective
mode, which we reveal to be charge density wave fluctuations from an extensive
doping- and magnetic field-dependent study. This finding is further
corroborated by a mean field model in which we describe the microscopic
mechanism underlying the interaction between the two orders. Our work
demonstrates Higgs spectroscopy as a novel and powerful technique for
investigating intertwined orders and microscopic processes in unconventional
superconductors
Correlated phenomena studied by ARPES : from 3d to 4f systems
The physics of strongly correlated materials is at the heart of current condensed matter research. The inclusion of interactions in these materials between electron themselves or with other excitations intertwines various degrees of freedom (orbital, spin, charge and lattice), leading to a number of novel phenomena like Mott-Hubbard and charge-transfer insulators, high-temperature superconductivity and mixed-valence and Kondo physics. This thesis focuses on the study of two classes of correlated materials: copper-oxide high-temperature superconductors, whose correlated physics is driven by the localized nature of the half-filled Cu 3d-orbitals, and the rare-earth hexaborides, which are characterized by the strongly correlated 4f-shell.
Recently, it has been shown that the interplay between different mechanisms underlying the formation of the superconducting condensate in the hole-doped bi-layer Bi₂Sr₂CaCu₂O₈₊δ
can be addressed in the time domain by means of time- and angle-resolved photoemission spectroscopy (TR-ARPES). Using this technique, the primary role of phase coherence has been established. By exploiting the same dynamical experimental approach, we show that such scenario also describes the ultrafast collapse of superconductivity in the single-layer compound Bi₂Sr₂CuO₆₊δ. Moreover, by performing a comprehensive study on different doping levels of both single- and bi-layer compounds, we provide new insights on the temperature evolution of the nodal quasiparticle spectral weight.
The second part of the thesis focuses on electron-doped cuprates, addressing the putative relation between the spectroscopically observed pseudogap and the robust antiferromagnetic order. Employing TR-ARPES as a tool to perform a detailed temperature dependent investigation allows us to explicitly link the momentum-resolved pseudogap spectral features to the evolution of the short-range spin-fluctuations in the optimally-doped Nd₂-xCe₂CuO₄.
Lastly, we make use of chemical substitution to investigate the mixed-valent character of the rare-earth hexaboride SmxLa₁-xB₆ series. Our combined ARPES and x-ray absorption measurements reveal a departure from a monotonic evolution of the Sm valence as a function of x and the possible emergence of a mixed-valent impurity regime.Science, Faculty ofPhysics and Astronomy, Department ofGraduat
Kramers nodal lines and Weyl fermions in SmAlSi
Abstract Kramers nodal lines (KNLs) have recently been proposed theoretically as a special type of Weyl line degeneracy connecting time-reversal invariant momenta. KNLs are robust to spin orbit coupling and are inherent to all non-centrosymmetric achiral crystal structures, leading to unusual spin, magneto-electric, and optical properties. However, their existence in in real quantum materials has not been experimentally established. Here we gather the experimental evidence pointing at the presence of KNLs in SmAlSi, a non-centrosymmetric metal that develops incommensurate spin density wave order at low temperature. Using angle-resolved photoemission spectroscopy, density functional theory calculations, and magneto-transport methods, we provide evidence suggesting the presence of KNLs, together with observing Weyl fermions under the broken inversion symmetry in the paramagnetic phase of SmAlSi. We discuss the nesting possibilities regarding the emergent magnetic orders in SmAlSi. Our results provide a solid basis of experimental observations for exploring correlated topology in SmAlS