28 research outputs found
Anisotropic compression in the high pressure regime of pure and Cr-doped vanadium dioxide
We present structural studies of VCrO (pure, 0.7% and 2.5% Cr
doped) compounds at room temperature in a diamond anvil cell for pressures up
to 20 GPa using synchrotron x-ray powder diffraction. All the samples studied
show a persistence of the monoclinic symmetry between 4 and 12 GPa. Above
12 GPa, the monoclinic symmetry changes to isostructural phase
(space group ) with a significant anisotropy in lattice compression of
the - plane of the phase. This behavior can be reconciled
invoking the pressure induced charge-delocalization
Population Inversion in Monolayer and Bilayer Graphene
The recent demonstration of saturable absorption and negative optical
conductivity in the Terahertz range in graphene has opened up new opportunities
for optoelectronic applications based on this and other low dimensional
materials. Recently, population inversion across the Dirac point has been
observed directly by time- and angle-resolved photoemission spectroscopy
(tr-ARPES), revealing a relaxation time of only ~ 130 femtoseconds. This
severely limits the applicability of single layer graphene to, for example,
Terahertz light amplification. Here we use tr-ARPES to demonstrate long-lived
population inversion in bilayer graphene. The effect is attributed to the small
band gap found in this compound. We propose a microscopic model for these
observations and speculate that an enhancement of both the pump photon energy
and the pump fluence may further increase this lifetime.Comment: 18 pages, 6 figure
Witnessing Light-Driven Entanglement using Time-Resolved Resonant Inelastic X-Ray Scattering
Characterizing and controlling entanglement in quantum materials is crucial
for next-generation quantum technologies. However, defining a quantifiable
figure of merit for entanglement in a material is theoretically and
experimentally challenging. At equilibrium, the presence of entanglement can be
diagnosed by extracting entanglement witnesses from spectroscopies and
extending this approach to nonequilibrium states could lead to the discovery of
novel dynamical phenomena. Here, we propose a systematic approach to quantify
the time-dependent quantum Fisher information and entanglement depth of
transient states of quantum materials through time-resolved resonant inelastic
x-ray scattering, a recently developed solid-state pump-probe technique. Using
a quarter-filled extended Hubbard model as an example, we benchmark the
efficiency of this approach and predict a light-enhanced quantum entanglement,
due to the proximity to a phase boundary. Our work sets the stage for
experimentally witnessing and controlling entanglement in light-driven quantum
materials via solid-state accessible ultrafast spectroscopic measurements.Comment: 11 pages, 6 figure
Measurement of the dynamic charge response of materials using low-energy, momentum-resolved electron energy-loss spectroscopy (M-EELS)
One of the most fundamental properties of an interacting electron system is
its frequency- and wave-vector-dependent density response function, . The imaginary part, , defines the
fundamental bosonic charge excitations of the system, exhibiting peaks wherever
collective modes are present. quantifies the electronic compressibility
of a material, its response to external fields, its ability to screen charge,
and its tendency to form charge density waves. Unfortunately, there has never
been a fully momentum-resolved means to measure at the
meV energy scale relevant to modern elecronic materials. Here, we demonstrate a
way to measure with quantitative momentum resolution by applying
alignment techniques from x-ray and neutron scattering to surface
high-resolution electron energy-loss spectroscopy (HR-EELS). This approach,
which we refer to here as "M-EELS," allows direct measurement of with meV resolution while controlling the momentum with an accuracy
better than a percent of a typical Brillouin zone. We apply this technique to
finite-q excitations in the optimally-doped high temperature superconductor,
BiSrCaCuO (Bi2212), which exhibits several phonons
potentially relevant to dispersion anomalies observed in ARPES and STM
experiments. Our study defines a path to studying the long-sought collective
charge modes in quantum materials at the meV scale and with full momentum
control.Comment: 26 pages, 10 sections, 7 figures, and an appendi
Phonon-pump XUV-photoemission-probe in graphene: evidence for non-adiabatic heating of Dirac carriers by lattice deformation
We modulate the atomic structure of bilayer graphene by driving its lattice
at resonance with the in-plane E1u lattice vibration at 6.3um. Using time- and
angle-resolved photoemission spectroscopy (tr-ARPES) with extreme ultra-violet
(XUV) pulses, we measure the response of the Dirac electrons near the K-point.
We observe that lattice modulation causes anomalous carrier dynamics, with the
Dirac electrons reaching lower peak temperatures and relaxing at faster rate
compared to when the excitation is applied away from the phonon resonance or in
monolayer samples. Frozen phonon calculations predict dramatic band structure
changes when the E1u vibration is driven, which we use to explain the anomalous
dynamics observed in the experiment.Comment: 16 pages, 8 figure
Snapshots of non-equilibrium Dirac carrier distributions in graphene
The optical properties of graphene are made unique by the linear band
structure and the vanishing density of states at the Dirac point. It has been
proposed that even in the absence of a semiconducting bandgap, a relaxation
bottleneck at the Dirac point may allow for population inversion and lasing at
arbitrarily long wavelengths. Furthermore, efficient carrier multiplication by
impact ionization has been discussed in the context of light harvesting
applications. However, all these effects are difficult to test quantitatively
by measuring the transient optical properties alone, as these only indirectly
reflect the energy and momentum dependent carrier distributions. Here, we use
time- and angle-resolved photoemission spectroscopy with femtosecond extreme
ultra-violet (EUV) pulses at 31.5 eV photon energy to directly probe the
non-equilibrium response of Dirac electrons near the K-point of the Brillouin
zone. In lightly hole-doped epitaxial graphene samples, we explore excitation
in the mid- and near-infrared, both below and above the minimum photon energy
for direct interband transitions. While excitation in the mid-infrared results
only in heating of the equilibrium carrier distribution, interband excitations
give rise to population inversion, suggesting that terahertz lasing may be
possible. However, in neither excitation regime do we find indication for
carrier multiplication, questioning the applicability of graphene for light
harvesting. Time-resolved photoemission spectroscopy in the EUV emerges as the
technique of choice to assess the suitability of new materials for
optoelectronics, providing quantitatively accurate measurements of
non-equilibrium carriers at all energies and wavevectors.Comment: 16 pages, 7 figure