282 research outputs found
Investigation of unconventional reconstruction and electronic properties on the Na2IrO3 surface
Na2IrO3 is an intriguing material for which spin-orbit coupling plays a key
role. Theoretical predictions, so far unverified, have been made that the
surface of Na2IrO3 should exhibit a clear signature of the quantum spin Hall
effect. We studied the surface of Na2IrO3 using scanning tunneling microscopy
and density-functional theory calculations. We observed atomic level resolution
of the surface and two types of terminations with different surface periodicity
and Na content. By comparing bias-dependent experimental topographic images to
simulated images, we determined the detailed atomistic structure of both
observed surfaces. One of these reveals a strong relaxation to the surface of
Na atoms from the subsurface region two atomic layers below. Such dramatic
structural changes at the surface cast doubt on any prediction of surface
properties based on bulk electronic structure. Indeed, using spatially resolved
tunneling spectroscopy we found no indication of the predicted quantum spin
Hall behavior
Surface resonance of the (2Ă1) reconstructed lanthanum hexaboride (001)-cleavage plane : a combined STM and DFT study
We performed a combined study of the (001)-cleavage plane of lanthanum hexaboride (LaB6) using scanning tunneling microscopy and density-functional theory (DFT). Experimentally, we found a (2Ă1) reconstructed surface on a local scale. The reconstruction is only short-range ordered and tends to order perpendicularly to step edges. At larger distances from surface steps, the reconstruction evolves to a labyrinthlike pattern. These findings are supported by low-energy electron diffraction experiments. Slab calculations within the framework of DFT show that the atomic structure consists of parallel lanthanum chains on top of boron octahedra. Scanning tunneling spectroscopy shows a prominent spectral feature at â0.6eV. Using DFT, we identify this structure as a surface resonance of the (2Ă1) reconstructed LaB6 (100) surface which is dominated by boron dangling bond states and lanthanum d states
Finger somatotopy is preserved after tetraplegia but deteriorates over time
Previous studies showed reorganised and/or altered activity in the primary sensorimotor cortex after a spinal cord injury (SCI), suggested to reflect abnormal processing. However, little is known about whether somatotopically specific representations can be activated despite reduced or absent afferent hand inputs. In this observational study, we used functional MRI and a (attempted) finger movement task in tetraplegic patients to characterise the somatotopic hand layout in primary somatosensory cortex. We further used structural MRI to assess spared spinal tissue bridges. We found that somatotopic hand representations can be activated through attempted finger movements in the absence of sensory and motor hand functioning, and no spared spinal tissue bridges. Such preserved hand somatotopy could be exploited by rehabilitation approaches that aim to establish new hand-brain functional connections after SCI (e.g. neuroprosthetics). However, over years since SCI the hand representation somatotopy deteriorated, suggesting that somatotopic hand representations are more easily targeted within the first years after SCI
On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs
The electronic properties of shallow acceptors in p-doped GaAs{110} are
investigated with scanning tunneling microscopy at low temperature. Shallow
acceptors are known to exhibit distinct triangular contrasts in STM images for
certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to
identify their energetic origin and behavior. A crucial parameter - the STM
tip's work function - is determined experimentally. The voltage dependent
potential configuration and band bending situation is derived. Ways to validate
the calculations with the experiment are discussed. Differential conductivity
maps reveal that the triangular contrasts are only observed with a depletion
layer present under the STM tip. The tunnel process leading to the anisotropic
contrasts calls for electrons to tunnel through vacuum gap and a finite region
in the semiconductor.Comment: 11 pages, 8 figure
Concurrent adaptation to opposing visual displacements during an alternating movement.
It has been suggested that, during tasks in which subjects are exposed to a visual rotation of cursor feedback, alternating bimanual adaptation to opposing rotations is as rapid as unimanual adaptation to a single rotation (Bock et al. in Exp Brain Res 162:513â519, 2005). However, that experiment did not test strict alternation of the limbs but short alternate blocks of trials. We have therefore tested adaptation under alternate left/right hand movement with opposing rotations. It was clear that the left and right hand, within the alternating conditions, learnt to adapt to the opposing displacements at a similar rate suggesting that two adaptive states were formed concurrently. We suggest that the separate limbs are used as contextual cues to switch between the relevant adaptive states. However, we found that during online correction the alternating conditions had a significantly slower rate of adaptation in comparison to the unimanual conditions. Control conditions indicate that the results are not directly due the alternation between limbs or to the constant switching of vision between the two eyes. The negative interference may originate from the requirement to dissociate the visual information of these two alternating displacements to allow online control of the two arms
Surface resonance of the (21) reconstructed lanthanum hexaboride (001)-cleavage plane: a combined STM and DFT study
We performed a combined study of the (001)-cleavage plane of lanthanum
hexaboride (LaB) using scanning tunneling microscopy (STM) and
density functional theory (DFT). Experimentally, we found a (21)
reconstructed surface on a local scale. The reconstruction is only short-range
ordered and tends to order perpendicularly to step edges. At larger distances
from surface steps, the reconstruction evolves to a labyrinth-like pattern.
These findings are supported by low-energy electron diffraction (LEED)
experiments. Slab calculations within the framework of DFT shows that the
atomic structure consists of parallel lanthanum chains on top of boron
octahedra. Scanning tunneling spectroscopy (STS) shows a prominent spectral
feature at -0.6 eV. Using DFT, we identify this structure as a surface
resonance of the (21) reconstructed LaB (100)-surface which
is dominated by boron dangling bond-states and lanthanum d-states.Comment: 10 pages, 16 figure
Surface analysis of the PrB6 (001) cleavage plane by scanning tunneling microscopy and spectroscopy
Scanning tunneling microscopy and spectroscopy were performed on the (001) cleavage plane of praseodymium hexaboride (PrB6). We found three different ordered morphologies, namely, a chainlike (2 Ă 1) reconstruction and two uniform terminations. The chainlike (2 Ă 1) reconstruction is rationalized as parallel Pr rows on top of a complete B6 network. The two uniform terminations are identified as complete Pr or B6 layers. Although the uniform terminations could be expected to be simply (1 Ă 1) reconstructed, one of them shows a rather stripelike atomic corrugation for close tip-sample distances. All morphologies share two spectral features at â0.2 and +0.2 eV around EF. In addition, one uniform termination shows an additional peak in the differential conductance at â0.7 eV. Similarly, the chainlike (2 Ă 1) reconstruction reveals a feature in the differential conductance at â1.1 eV when moving the tip closer to the surface. The distance dependency points towards rather localized electronic states, which we tentatively attribute to a 4 f -related feature
Investigation of Single Boron Acceptors at the Cleaved Si:B (111) Surface
The cleaved and (2 x 1) reconstructed (111) surface of p-type Si is
investigated by scanning tunneling microscopy (STM). Single B acceptors are
identified due to their characteristic voltage-dependent contrast which is
explained by a local energetic shift of the electronic density of states caused
by the Coulomb potential of the negatively charged acceptor. In addition,
detailed analysis of the STM images shows that apparently one orbital is
missing at the B site at sample voltages of 0.4 - 0.6 V, corresponding to the
absence of a localized dangling-bond state. Scanning tunneling spectroscopy
confirms a strongly altered density of states at the B atom due to the
different electronic structure of B compared to Si.Comment: 6 pages, 7 figure
Local biases drive, but do not determine, the perception of illusory trajectories
When a dot moves horizontally across a set of tilted lines of alternating orientations, the dot appears to be moving up and down along its trajectory. This perceptual phenomenon, known as the slalom illusion, reveals a mismatch between the veridical motion signals and the subjective percept of the motion trajectory, which has not been comprehensively explained. In the present study, we investigated the empirical boundaries of the slalom illusion using psychophysical methods. The phenomenon was found to occur both under conditions of smooth pursuit eye movements and constant fixation, and to be consistently amplified by intermittently occluding the dot trajectory. When the motion direction of the dot was not constant, however, the stimulus display did not elicit the expected illusory percept. These findings confirm that a local bias towards perpendicularity at the intersection points between the dot trajectory and the tilted lines cause the illusion, but also highlight that higher-level cortical processes are involved in interpreting and amplifying the biased local motion signals into a global illusion of trajectory perception
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