53 research outputs found
Temperature-resolution anomalies in the reconstruction of time dynamics from energy-loss experiments
Inelastic scattering techniques provide a powerful approach to studying
electron and nuclear dynamics, via reconstruction of a propagator that
quantifies the time evolution of a system. There is now growing interest in
applying such methods to very low energy excitations, such as lattice
vibrations, but in this limit the cross section is no longer proportional to a
propagator. Significant deviations occur due to the finite temperature Bose
statistics of the excitations. Here we consider this issue in the context of
high-resolution electron energy loss experiments on the copper-oxide
superconductor BiSrCaCuO. We find that simple division of a
Bose factor yields an accurate propagator on energy scales greater than the
resolution width. However, at low energy scales, the effects of resolution and
finite temperature conspire to create anomalies in the dynamics at long times.
We compare two practical ways for dealing with such anomalies, and discuss the
range of validity of the technique in light of this comparison.Comment: 19 pages, 2 figures, submitted to Journal of Physics
Collective excitations in layered materials with momentum-resolved electron energy loss spectroscopy
Strong Coulomb interactions are either suspected or known to play a prominent role in material classes such as high temperature superconductors, charge density waves, and Mott insulators among many others. These interactions are quantified by the charge density response function, chi(q,w) (or the closely related inverse dielectric function). The measurement of the energy- and momentum-resolved chi(q,w) over a large phase space of q and w, however, presents a significant experimental challenge. Traditional methods to measure chi(q,w) have suffered from either one or more major drawbacks. To address this problem, the development of a spectroscopic technique, momentum-resolved electron energy loss spectroscopy (M-EELS), was undertaken. Because many of the material classes that exhibit these unusual ground states tend to be layered or quasi-two dimensional, M-EELS presents a promising approach to measuring the energy- and momentum-resolved charge density response. Since the technique is not widely used, however, the M-EELS results obtained as part of this thesis were compared to other probes in the relevant ranges of phase space to ensure consistency. Furthermore, a theoretical framework was worked out to demonstrate explicitly the relationship between the scattering cross section and c(q,w). M-EELS experiments were conducted on a high-temperature superconductor, Bi2Sr2CaCu2O8+d, a charge density wave material, TiSe2, and a topological insulator, Bi2Se3. It was determined that the bosonic origin of quasiparticle kinks often seen in angle-resolved photoemission studies can be identified using M-EELS. Lastly, the observation of a novel electronic collective mode in TiSe2 is presented as strong evidence for an excitonic insulator phase in this compound
Adsorption properties and third sound propagation in superfluid He films on carbon nanotubes
We consider the adsorption properties of superfluid He films on carbon
nanotubes. One major factor in the adsorption is the surface tension force
arising from the very small diameter of the nanotubes. Calculations show that
surface tension keeps the film thickness on the tubes very thin even when the
helium vapor is increased to the saturated pressure. The weakened Van der Waals
force due to the cylindrical geometry also contributes to this. Both of these
effects act to lower the predicted velocity of third sound propagation along
the tubes. It does not appear that superfluidity will be possible on
single-walled nanotubes of diameter about one nm, since the film thickness is
less than 3 atomic layers even at saturation. Superfluidity is possible on
larger-diameter nanotube bundles and multi-walled nanotubes, however. We have
observed third sound signals on nanotube bundles of average diameter 5 nm which
are sprayed onto a Plexiglass surface, forming a network of tubes.Comment: 4 pages, accepted for Journal of Physics: Conference Series
(Proceedings of LT25
Amplitude dynamics of charge density wave in LaTe: theoretical description of pump-probe experiments
We formulate a dynamical model to describe a photo-induced charge density
wave (CDW) quench transition and apply it to recent multi-probe experiments on
LaTe [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies on
coupled time-dependent Ginzburg-Landau equations tracking two order parameters
that represent the modulations of the electronic density and the ionic
positions. We aim at describing the amplitude of the order parameters under the
assumption that they are homogeneous in space. This description is supplemented
by a three-temperature model, which treats separately the electronic
temperature, temperature of the lattice phonons with stronger couplings to the
electronic subsystem, and temperature of all other phonons. The broad scope of
available data for LaTe and similar materials as well as the synergy
between different time-resolved spectroscopies allow us to extract model
parameters. The resulting calculations are in good agreement with ultra-fast
electron diffraction experiments, reproducing qualitative and quantitative
features of the CDW amplitude evolution during the initial few picoseconds
after photoexcitation.Comment: 21 pages, 14 figures; this version is almost identical to the
published version; comparing to the earlier arXiv submission, current version
contains a new figure (Fig.10), and a broader discussion of theoretical
results and approximation
A reexamination of the effective fine structure constant of graphene, as measured in graphite
We present a refined and improved study of the influence of screening on the
effective fine structure constant of graphene, , as measured in
graphite using inelastic x-ray scattering. This follow-up to our previous study
[J. P. Reed, et al., Science 330, 805 (2010)] was carried out with two times
better energy resolution, five times better momentum resolution, and improved
experimental setup with lower background. We compare our results to RPA
calculations and evaluate the relative importance of interlayer hopping,
excitonic corrections, and screening from high energy excitations involving the
bands. We find that the static, limiting value of falls in
the range 0.25 to 0.35, which is higher than our previous result of 0.14, but
still below the value expected from RPA. We show the reduced value is not a
consequence of interlayer hopping effects, which were ignored in our previous
analysis, but of a combination of excitonic effects in the particle-hole continuum, and background screening from the
-bonded electrons. We find that -band screening is extremely
strong at distances of the order of a few nm, and should be highly effective at
screening out short-distance, Hubbard-like interactions in graphene, as well as
other carbon allotropes.Comment: 23 pages, 5 figure
Recommended from our members
Amplitude dynamics of the charge density wave in LaTe3: Theoretical description of pump-probe experiments
We formulate a dynamical model to describe a photo-induced charge density
wave (CDW) quench transition and apply it to recent multi-probe experiments on
LaTe [A. Zong et al., Nat. Phys. 15, 27 (2019)]. Our approach relies on
coupled time-dependent Ginzburg-Landau equations tracking two order parameters
that represent the modulations of the electronic density and the ionic
positions. We aim at describing the amplitude of the order parameters under the
assumption that they are homogeneous in space. This description is supplemented
by a three-temperature model, which treats separately the electronic
temperature, temperature of the lattice phonons with stronger couplings to the
electronic subsystem, and temperature of all other phonons. The broad scope of
available data for LaTe and similar materials as well as the synergy
between different time-resolved spectroscopies allow us to extract model
parameters. The resulting calculations are in good agreement with ultra-fast
electron diffraction experiments, reproducing qualitative and quantitative
features of the CDW amplitude evolution during the initial few picoseconds
after photoexcitation
Second harmonic generation as a probe of broken mirror symmetry
The notion of spontaneous symmetry breaking has been used to describe phase
transitions in a variety of physical systems. In crystalline solids, the
breaking of certain symmetries, such as mirror symmetry, is difficult to detect
unambiguously. Using 1-TaS, we demonstrate here that
rotational-anisotropy second harmonic generation (RA-SHG) is not only a
sensitive technique for the detection of broken mirror symmetry, but also that
it can differentiate between mirror symmetry-broken structures of opposite
planar chirality. We also show that our analysis is applicable to a wide class
of different materials with mirror symmetry-breaking transitions. Lastly, we
find evidence for bulk mirror symmetry-breaking in the incommensurate charge
density wave phase of 1-TaS. Our results pave the way for RA-SHG to
probe candidate materials where broken mirror symmetry may play a pivotal role.Comment: 13 pages, 10 figures. Edited (v2) to include Bilal G\"okce in the
authors list who was mistakenly excluded. Edited again (v3) to incorporate
modifications recommended by referees. Replaced (v4) with version published
in Physical Review
Second harmonic generation as a probe of broken mirror symmetry
The notion of spontaneous symmetry breaking has been used to describe phase
transitions in a variety of physical systems. In crystalline solids, the
breaking of certain symmetries, such as mirror symmetry, is difficult to detect
unambiguously. Using 1-TaS, we demonstrate here that
rotational-anisotropy second harmonic generation (RA-SHG) is not only a
sensitive technique for the detection of broken mirror symmetry, but also that
it can differentiate between mirror symmetry-broken structures of opposite
planar chirality. We also show that our analysis is applicable to a wide class
of different materials with mirror symmetry-breaking transitions. Lastly, we
find evidence for bulk mirror symmetry-breaking in the incommensurate charge
density wave phase of 1-TaS. Our results pave the way for RA-SHG to
probe candidate materials where broken mirror symmetry may play a pivotal role
Crystallographic refinement of collective excitations using standing wave inelastic x-ray scattering
We propose a method for realizing true, real-space imaging of charge dynamics
in a periodic system, with angstrom spatial resolution and attosecond time
resolution. In this method, inelastic x-ray scattering (IXS) is carried out
with a coherent, standing wave source, which provides the off-diagonal elements
of the generalized dynamic structure factor, S(q_1,q_2,\omega), allowing
complete reconstruction of the inhomogeneous response function of the system,
\chi(x_1,x_2,t). The quantity \chi has the physical meaning of a propagator for
charge, so allows one to observe - in real time - the disturbance in the
electron density created by a point source placed at a specified location, x_1
(on an atom vs. between atoms, for example). This method may be thought of as a
generalization of x-ray crystallography that allows refinement of the excited
states of a periodic system, rather than just its ground state.Comment: 28 pages, 8 figures, submitted to Chemical Physic
Manipulating the electronic polarization in a magnetoelectric antiferromagnet via the two-photon Stark effect
When intense light is shone through a transparent medium, the strong,
time-periodic potential from the radiation field reshapes the many-body
Hamiltonian. Crucially, this interaction does not involve light absorption and,
in principle, does not generate any heat. The incident radiation can
nonetheless be used to optically tailor various degrees of freedom, leading to
the possibility of photo-controlling macroscopic properties of matter. Here, we
show that when inversion symmetry is broken by the antiferromganetic spin
arrangement in CrO, transmitting linearly polarized light through the
crystal gives rise to a purely electronic dipole moment by way of a two-photon
Stark effect. Using interferometric time-resolved second harmonic generation,
we show that the threefold rotational symmetry of the crystal is broken only
while the pump pulse is present; the timescale indicates that an electronic
response is generated without affecting the magnetic or crystal structures. The
orientation of the induced moment depends on the incident light polarization,
which allows for contact-free control of the dipole moment vector. Our results
establish a dissipationless optical protocol by which to selectively polarize
the electronic subsystem and provides a method to manipulate electronic
symmetries in noncentrosymmetric insulators
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