64 research outputs found
Poor electronic screening in lightly doped Mott insulators observed with scanning tunneling microscopy
The effective Mott gap measured by scanning tunneling microscopy (STM) in the
lightly doped Mott insulator differs
greatly from values reported by photoemission and optical experiments. Here, we
show that this is a consequence of the poor electronic screening of the
tip-induced electric field in this material. Such effects are well known from
STM experiments on semiconductors, and go under the name of tip-induced band
bending (TIBB). We show that this phenomenon also exists in the lightly doped
Mott insulator and that, at doping
concentrations of , it causes the measured energy gap in the sample
density of states to be bigger than the one measured with other techniques. We
develop a model able to retrieve the intrinsic energy gap leading to a value
which is in rough agreement with other experiments, bridging the apparent
contradiction. At doping we further observe circular features
in the conductance layers that point to the emergence of a significant density
of free carriers in this doping range, and to the presence of a small
concentration of donor atoms. We illustrate the importance of considering the
presence of TIBB when doing STM experiments on correlated-electron systems and
discuss the similarities and differences between STM measurements on
semiconductors and lightly doped Mott insulators.Comment: 9 pages, 5 figure
A weakly correlated Fermi liquid state with a small Fermi surface in lightly doped SrIrO
We characterize the electron doping evolution of
(SrLa)IrO by means of angle-resolved photoemission.
Concomitant with the metal insulator transition around we find
the emergence of coherent quasiparticle states forming a closed small Fermi
surface of volume , where is the independently measured La
concentration. The quasiparticle weight remains large along the entire
Fermi surface, consistent with the moderate renormalization of the low-energy
dispersion. This indicates a conventional, weakly correlated Fermi liquid state
with a momentum independent residue in lightly doped
SrIrO$_7&.Comment: 5 pages, 4 figure
Anisotropic exchange and spin-wave damping in pure and electron-doped SrIrO
The collective magnetic excitations in the spin-orbit Mott insulator
(SrLa)IrO () were investigated by
means of resonant inelastic x-ray scattering. We report significant magnon
energy gaps at both the crystallographic and antiferromagnetic zone centers at
all doping levels, along with a remarkably pronounced momentum-dependent
lifetime broadening. The spin-wave gap is accounted for by a significant
anisotropy in the interactions between isospins, thus
marking the departure of SrIrO from the essentially isotropic
Heisenberg model appropriate for the superconducting cuprates.Comment: 6 pages, 4 figure
A laser-ARPES study of LaNiO3 thin films grown by sputter deposition
Thin films of the correlated transition-metal oxide LaNiO undergo a
metal-insulator transition when their thickness is reduced to a few unit cells.
Here, we use angle-resolved photoemission spectroscopy to study the evolution
of the electronic structure across this transition in a series of epitaxial
LaNiO films of thicknesses ranging from 19 to 2 u.c. grown in situ by RF
magnetron sputtering. Our data show a strong reduction of the electronic mean
free path as the thickness is reduced below 5 u.c. This prevents the system
from becoming electronically two-dimensional, as confirmed by the largely
unchanged Fermi surface seen in our experiments. In the insulating state we
observe a strong suppression of the coherent quasiparticle peak but no clear
gap. These features resemble previous observations of the insulating state of
NdNiO.Comment: Submitted to APL Material
Functional diversification of Argonautes in nematodes:an expanding universe
In the last decade, many diverse RNAi (RNA interference) pathways have been discovered that mediate gene silencing at epigenetic, transcriptional and post-transcriptional levels. The diversity of RNAi pathways is inherently linked to the evolution of Ago (Argonaute) proteins, the central protein component of RISCs (RNA-induced silencing complexes). An increasing number of diverse Agos have been identified in different species. The functions of most of these proteins are not yet known, but they are generally assumed to play roles in development, genome stability and/or protection against viruses. Recent research in the nematode Caenorhabditis elegans has expanded the breadth of RNAi functions to include transgenerational epigenetic memory and, possibly, environmental sensing. These functions are inherently linked to the production of secondary siRNAs (small interfering RNAs) that bind to members of a clade of WAGOs (worm-specific Agos). In the present article, we review briefly what is known about the evolution and function of Ago proteins in eukaryotes, including the expansion of WAGOs in nematodes. We postulate that the rapid evolution of WAGOs enables the exceptional functional plasticity of nematodes, including their capacity for parasitism
Electronic structure of few-layer black phosphorus from -ARPES
Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors
because of its high mobility and thickness dependent direct band gap. However,
the quasiparticle band structure of ultrathin BP has remained inaccessible to
experiment thus far. Here we use a recently developed laser-based micro-focus
angle resolved photoemission (-ARPES) system to establish the electronic
structure of 2-9 layer BP from experiment. Our measurements unveil ladders of
anisotropic, quantized subbands at energies that deviate from the scaling
observed in conventional semiconductor quantum wells. We quantify the
anisotropy of the effective masses and determine universal tight-binding
parameters which provide an accurate description of the electronic structure
for all thicknesses.Comment: Supporting Information available upon reques
Heavy d-electron quasiparticle interference and real-space electronic structure of Sr₃Ru₂O₇
As well as providing subatomic-scale real-space images of metals, the scanning tunnelling microscope also reveals momentum–space information. Now it is possible to use this technique to image a heavy-electron liquid and obtain information on orbital structures. The intriguing idea that strongly interacting electrons can generate spatially inhomogeneous electronic liquid-crystalline phases is over a decade old, but these systems still represent an unexplored frontier of condensed-matter physics. One reason is that visualization of the many-body quantum states generated by the strong interactions, and of the resulting electronic phases, has not been achieved. Soft condensed-matter physics was transformed by microscopies that enabled imaging of real-space structures and patterns. A candidate technique for obtaining equivalent data in the purely electronic systems is spectroscopic imaging scanning tunnelling microscopy (SI-STM). The core challenge is to detect the tenuous but ‘heavy’ momentum (k)-space components of the many-body electronic state simultaneously with its real-space constituents. Sr₃Ru₂O₇ provides a particularly exciting opportunity to address these issues. It possesses a very strongly renormalized ‘heavy’ d-electron Fermi liquid and exhibits a field-induced transition to an electronic liquid-crystalline phase. Finally, as a layered compound, it can be cleaved to present an excellent surface for SI-STM.Instituto de Física de Líquidos y Sistemas Biológico
Provoking topology by octahedral tilting in strained SrNbO
Transition metal oxides with a wide variety of electronic and magnetic
properties offer an extraordinary possibility to be a platform for developing
future electronics based on unconventional quantum phenomena, for instance, the
topology. The formation of topologically non-trivial states is related to
crystalline symmetry, spin-orbit coupling, and magnetic ordering. Here, we
demonstrate how lattice distortions and octahedral rotation in SrNbO films
induce the band topology. By employing angle-resolved photoemission
spectroscopy (ARPES) and density functional theory (DFT) calculations, we
verify the presence of in-phase octahedral rotation in ultra-thin
SrNbO films, which causes the formation of topologically-protected Dirac
band crossings. Our study illustrates that octahedral engineering can be
effectively exploited for implanting and controlling quantum topological phases
in transition metal oxides.Comment: 6 pages, 4 figure
Observation of flat moir\'e bands in twisted bilayer WSe
The recent observation of correlated phases in transition metal
dichalcogenide moir\'e systems at integer and fractional filling promises new
insight into metal-insulator transitions and the unusual states of matter that
can emerge near such transitions. Here, we combine real- and momentum-space
mapping techniques to study moir\'e superlattice effects in 57.4
twisted WSe (tWSe). Our data reveal a split-off flat band that derives
from the monolayer states. Using advanced data analysis, we directly
quantify the moir\'e potential from our data. We further demonstrate that the
global valence band maximum in tWSe is close in energy to this flat band
but derives from the monolayer K-states which show weaker superlattice effects.
These results constrain theoretical models and open the perspective that
-valley flat bands might be involved in the correlated physics of
twisted WSe
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