26 research outputs found
The Role of Lower Leg Muscle Activity in Blood Pressure Maintenance of Older Adults
Purpose. Age-associated muscle weakness, postural instability, and orthostatic hypotension have been identified as contributing factors to falls , but the relationships among them are not clear. Therefore, the purpose of this study, a two-way factorial design, was to investigate the differences in lower extremity (LE) muscle activity, blood pressure (BP), and heart rate (HR) between young and older individuals in an upright position. Methods. Ten young males (20-24 yrs.) and 10 older males (65-82 yrs.) stood for 15 minutes while BP, HR, and LE electromyography (EMG) were recorded at one minute intervals . A two-way ANOVA was used for data analysis , p=.05. Results. Mean arterial pressure of both groups significantly increased from supine values within one minute of standing (young = 86.5±1.68 to 96.9±3.16 mmHg, old = 100.3±4.42 to 114.0±5.40 mmHg). BP variables remained elevated during the 15 minutes of standing despite a significantly attenuated HR response in the older group (young = 85±4.51 bpm, old = 73±3.98 bpm). Standing EMG activity of the older group was significantly greater than the young group. Conclusion. This study suggests that increased LE muscle activity may play a role in the ability of older individuals to maintain BP in the standing position
Negative flat band magnetism in a spin-orbit coupled correlated kagome magnet
It has long been speculated that electronic flat band systems can be a
fertile ground for hosting novel emergent phenomena including unconventional
magnetism and superconductivity. Although flat bands are known to exist in a
few systems such as heavy fermion materials and twisted bilayer graphene, their
microscopic roles and underlying mechanisms in generating emergent behavior
remain elusive. Here we use scanning tunneling microscopy to elucidate the
atomically resolved electronic states and their magnetic response in the kagome
magnet Co3Sn2S2. We observe a pronounced peak at the Fermi level, which is
identified to arise from the kinetically frustrated kagome flat band.
Increasing magnetic field up to +-8T, this state exhibits an anomalous
magnetization-polarized Zeeman shift, dominated by an orbital moment in
opposite to the field direction. Such negative magnetism can be understood as
spin-orbit coupling induced quantum phase effects tied to non-trivial flat band
systems. We image the flat band peak, resolve the associated negative
magnetism, and provide its connection to the Berry curvature field, showing
that Co3Sn2S2 is a rare example of kagome magnet where the low energy physics
can be dominated by the spin-orbit coupled flat band. Our methodology of
probing band-resolved ordering phenomena such as spin-orbit magnetism can also
be applied in future experiments to elucidate other exotic phenomena including
flat band superconductivity and anomalous quantum transport.Comment: Nature Physics onlin
Imaging real-space flat band localization in kagome magnet FeSn
Kagome lattices host flat bands due to their frustrated lattice geometry,
which leads to destructive quantum interference of electron wave functions.
Here, we report imaging of the kagome flat band localization in real-space
using scanning tunneling microscopy. We identify both the Fe3Sn kagome lattice
layer and the Sn2 honeycomb layer with atomic resolution in kagome
antiferromagnet FeSn. On the Fe3Sn lattice, at the flat band energy determined
by the angle resolved photoemission spectroscopy, tunneling spectroscopy
detects an unusual state localized uniquely at the Fe kagome lattice network.
We further show that the vectorial in-plane magnetic field manipulates the
spatial anisotropy of the localization state within each kagome unit cell. Our
results are consistent with the real-space flat band localization in the
magnetic kagome lattice. We further discuss the magnetic tuning of flat band
localization under the spin-orbit coupled magnetic kagome lattice model.Comment: To appear in Communications Material
Observation of sixfold degenerate fermions in PdSb
Three types of fermions have been extensively studied in topological quantum
materials: Dirac, Weyl, and Majorana fermions. Beyond the fundamental fermions
in high energy physics, exotic fermions are allowed in condensed matter systems
residing in three-, six- or eightfold degenerate band crossings. Here, we use
angle-resolved photoemission spectroscopy to directly visualize
three-doubly-degenerate bands in PdSb. The ultrahigh energy resolution we
are able to achieve allows for the confirmation of all the sixfold degenerate
bands at the R point, in remarkable consistency with first-principles
calculations. Moreover, we find that this sixfold degenerate crossing has
quadratic dispersion as predicted by theory. Finally, we compare sixfold
degenerate fermions with previously confirmed fermions to demonstrate the
importance of this work: our study indicates a topological fermion beyond the
constraints of high energy physics
Fermion-boson many-body interplay in a frustrated kagome paramagnet
Kagome-net, appearing in areas of fundamental physics, materials, photonic
and cold-atom systems, hosts frustrated fermionic and bosonic excitations.
However, it is extremely rare to find a system to study both fermionic and
bosonic modes to gain insights into their many-body interplay. Here we use
state-of-the-art scanning tunneling microscopy and spectroscopy to discover
unusual electronic coupling to flat-band phonons in a layered kagome
paramagnet. Our results reveal the kagome structure with unprecedented atomic
resolution and observe the striking bosonic mode interacting with dispersive
kagome electrons near the Fermi surface. At this mode energy, the fermionic
quasi-particle dispersion exhibits a pronounced renormalization, signaling a
giant coupling to bosons. Through a combination of self-energy analysis,
first-principles calculation, and a lattice vibration model, we present
evidence that this mode arises from the geometrically frustrated phonon
flat-band, which is the lattice analog of kagome electron flat-band. Our
findings provide the first example of kagome bosonic mode (flat-band phonon) in
electronic excitations and its strong interaction with fermionic degrees of
freedom in kagome-net materials.Comment: To appear in Nature Communications (2020
Discovery of unconventional chiral charge order in kagome superconductor KV3Sb5
Intertwining quantum order and nontrivial topology is at the frontier of
condensed matter physics. A charge density wave (CDW) like order with orbital
currents has been proposed as a powerful resource for achieving the quantum
anomalous Hall effect in topological materials and for the hidden phase in
cuprate high-temperature superconductors. However, the experimental realization
of such an order is challenging. Here we use high-resolution scanning
tunnelling microscopy (STM) to discover an unconventional charge order in a
kagome material KV3Sb5, with both a topological band structure and a
superconducting ground state. Through both topography and spectroscopic
imaging, we observe a robust 2x2 superlattice. Spectroscopically, an energy gap
opens at the Fermi level, across which the 2x2 charge modulation exhibits an
intensity reversal in real-space, signaling charge ordering. At
impurity-pinning free region, the strength of intrinsic charge modulations
further exhibits chiral anisotropy with unusual magnetic field response.
Theoretical analysis of our experiments suggests a tantalizing unconventional
chiral CDW in the frustrated kagome lattice, which can not only lead to large
anomalous Hall effect with orbital magnetism, but also be a precursor of
unconventional superconductivity.Comment: Orbital magnetism calculation adde