97 research outputs found
Buried double CuO chains in YBaCuO uncovered by nano-ARPES
The electron dynamics in the CuO chains has been elusive in Y-Ba-Cu-O cuprate
systems by means of standard angle-resolved photoemission spectroscopy (ARPES);
cleaved sample exhibits areas terminated by both CuO-chain or BaO layers, and
the size of a typical beam results in ARPES signals that are superposed from
both terminations. Here, we employ spatially-resolved ARPES with submicrometric
beam (nano-ARPES) to reveal the surface-termination-dependent electronic
structures of the double CuO chains in YBaCuO. We present the first
observation of sharp metallic dispersions and Fermi surfaces of the double CuO
chains buried underneath the CuO-plane block on the BaO terminated surface.
While the observed Fermi surfaces of the CuO chains are highly one-dimensional,
the electrons in the CuO-chains do not undergo significant electron
correlations and no signature of a Tomonaga-Luttinger liquid nor a marginal
Fermi liquid is found. Our works represent an important experimental step
toward understanding of the charge dynamics and provides a starting basis for
modelling the high- superconductivity in YBCO cuprate systems.Comment: 10 pages, 5 figures including supplementary material (4 pages, 2
figures
Observation of band crossings protected by nonsymmorphic symmetry in the layered ternary telluride Ta3SiTe6
We have performed angle-resolved photoemission spectroscopy of layered
ternary telluride Ta3SiTe6 which is predicted to host nodal lines associated
with nonsymmorphic crystal symmetry. We found that the energy bands in the
valence-band region show Dirac-like dispersions which present a band degeneracy
at the R point of the bulk orthorhombic Brillouin zone. This band degeneracy
extends one-dimensionally along the whole SR high-symmetry line, forming the
nodal lines protected by the glide mirror symmetry of the crystal. We also
observed a small band splitting near EF which supports the existence of
hourglass-type dispersions predicted by the calculation. The present results
provide an excellent opportunity to investigate the interplay between exotic
nodal fermions and nonsymmorphic crystal symmetry.Comment: 6 pages, 4 figure
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
Spectral signatures of a unique charge density wave in TaNiSe
Charge Density Waves (CDW) are commonly associated with the presence of
near-Fermi level states which are separated from others, or "nested", by a
wavector of . Here we use Angle-Resolved Photo Emission
Spectroscopy (ARPES) on the CDW material TaNiSe and identify a total
absence of any plausible nesting of states at the primary CDW wavevector
. Nevertheless we observe spectral intensity on replicas of the
hole-like valence bands, shifted by a wavevector of , which appears
with the CDW transition. In contrast, we find that there is a possible nesting
at , and associate the characters of these bands with the reported
atomic modulations at . Our comprehensive electronic structure
perspective shows that the CDW-like transition of TaNiSe is unique,
with the primary wavevector being unrelated to any low-energy
states, but suggests that the reported modulation at , which would
plausibly connect low-energy states, might be more important for the overall
energetics of the problem
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
One-dimensional electronic states in a natural misfit structure
Misfit compounds are thermodynamically stable stacks of two-dimensional
materials, forming a three-dimensional structure that remains incommensurate in
one direction parallel to the layers. As a consequence, no true bonding is
expected between the layers, with their interaction being dominated by charge
transfer. In contrast to this well-established picture, we show that interlayer
coupling can strongly influence the electronic properties of one type of layer
in a misfit structure, in a similar way to the creation of modified band
structures in an artificial moir\'e structure between two-dimensional
materials. Using angle-resolved photoemission spectroscopy with a micron-scale
light focus, we selectively probe the electronic properties of hexagonal
NbSe and square BiSe layers that terminate the surface of the
(BiSe)NbSe misfit compound. We show that the band structure in
the BiSe layers is strongly affected by the presence of the hexagonal NbSe
layers, leading to quasi one-dimensional electronic features. The electronic
structure of the NbSe layers, on the other hand, is hardly influenced by
the presence of the BiSe. Using density functional theory calculations of the
unfolded band structures, we argue that the preferred modification of one type
of bands is mainly due to the atomic and orbital character of the states
involved, opening a promising way to design novel electronic states that
exploit the partially incommensurate character of the misfit compounds
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
Probing the structure and dynamics of molecular clusters using rotational wavepackets
The chemical and physical properties of molecular clusters can heavily depend
on their size, which makes them very attractive for the design of new materials
with tailored properties. Deriving the structure and dynamics of clusters is
therefore of major interest in science. Weakly bound clusters can be studied
using conventional spectroscopic techniques, but the number of lines observed
is often too small for a comprehensive structural analysis. Impulsive alignment
generates rotational wavepackets, which provides simultaneous information on
structure and dynamics, as has been demonstrated successfully for isolated
molecules. Here, we apply this technique for the firsttime to clusters
comprising of a molecule and a single helium atom. By forcing the population of
high rotational levels in intense laser fields we demonstrate the generation of
rich rotational line spectra for this system, establishing the highly
delocalised structure and the coherence of rotational wavepacket propagation.
Our findings enable studies of clusters of different sizes and complexity as
well as incipient superfluidity effects using wavepacket methods.Comment: 5 pages, 6 figure
Ramifications of Optical Pumping on the Interpretation of Time-Resolved Photoemission Experiments on Graphene
In pump-probe time and angle-resolved photoemission spectroscopy (TR-ARPES)
experiments the presence of the pump pulse adds a new level of complexity to
the photoemission process in comparison to conventional ARPES. This is
evidenced by pump-induced vacuum space-charge effects and surface
photovoltages, as well as multiple pump excitations due to internal reflections
in the sample-substrate system. These processes can severely affect a correct
interpretation of the data by masking the out-of-equilibrium electron dynamics
intrinsic to the sample. In this study, we show that such effects indeed
influence TR-ARPES data of graphene on a silicon carbide (SiC) substrate. In
particular, we find a time- and laser fluence-dependent spectral shift and
broadening of the acquired spectra, and unambiguously show the presence of a
double pump excitation. The dynamics of these effects is slower than the
electron dynamics in the graphene sample, thereby permitting us to deconvolve
the signals in the time domain. Our results demonstrate that complex
pump-related processes should always be considered in the experimental setup
and data analysis.Comment: 9 pages, 4 figure
- …