471 research outputs found
Local Compressibility Measurements of Correlated States in Suspended Bilayer Graphene
Bilayer graphene has attracted considerable interest due to the important
role played by many-body effects, particularly at low energies. Here we report
local compressibility measurements of a suspended graphene bilayer. We find
that the energy gaps at filling factors v = 4 do not vanish at low fields, but
instead merge into an incompressible region near the charge neutrality point at
zero electric and magnetic field. These results indicate the existence of a
zero-field ordered state and are consistent with the formation of either an
anomalous quantum Hall state or a nematic phase with broken rotational
symmetry. At higher fields, we measure the intrinsic energy gaps of
broken-symmetry states at v = 0, 1 and 2, and find that they scale linearly
with magnetic field, yet another manifestation of the strong Coulomb
interactions in bilayer graphene.Comment: 9 pages, including 4 figures and supplementary material
Notch Signaling Regulates Motor Neuron Differentiation of Human Embryonic Stem Cells
In the pMN domain of the spinal cord, Notch signaling regulates the balance between motor neuron differentiation and maintenance of the progenitor state for later oligodendrocyte differentiation. Here, we sought to study the role of Notch signaling in regulation of the switch from the pMN progenitor state to differentiated motor neurons in a human model system. Human embryonic stem cells (hESCs) were directed to differentiate to pMNâlike progenitor cells by the inductive action of retinoic acid and a Shh agonist, purmorphamine. We found that the expression of the Notch signaling effector Hes5 was induced in hESCâderived pMNâlike progenitors and remained highly expressed when they were cultured under conditions favoring motor neuron differentiation. Inhibition of Notch signaling by a Îłâsecretase inhibitor in the differentiating pMNâlike progenitor cells decreased Hes5 expression and enhanced the differentiation toward motor neurons. Conversely, overâexpression of Hes5 in pMNâlike progenitor cells during the differentiation interfered with retinoic acidâ and purmorphamineâinduced motor neuron differentiation and inhibited the emergence of motor neurons. Inhibition of Notch signaling had a permissive rather than an inductive effect on motor neuron differentiation. Our results indicate that Notch signaling has a regulatory role in the switch from the pMN progenitor to the differentiated motor neuron state. Inhibition of Notch signaling can be harnessed to enhance the differentiation of hESCs toward motor neurons. Stem Cells 2015;33:403â415Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110586/1/stem1873.pd
1/4 is the new 1/2: Interaction-induced Unification of Quantum Anomalous and Spin Hall Effects
We introduce interactions into two general models for quantum spin Hall
physics. Although the traditional picture is that such physics appears when the
two lower spinful bands are occupied, that is, half-filling, we show using
determinantal quantum Monte Carlo as well as from an exactly solvable model
that in the presence of strong interactions, the quarter-filled state instead
exhibits the quantum spin Hall effect at high temperature. A topological Mott
insulator is the underlying cause. The peak in the spin susceptibility is
consistent with a possible ferromagnetic state at . The onset of such
magnetism would convert the quantum spin Hall to a quantum anomalous Hall
effect. We argue that it is the consistency with the Lieb-Schultz-Mattis
theorem\cite{lsm1,lsm2} for interacting systems with an odd number of charges
per unit cell that underlies the emergence of the quantum anomalous Hall effect
as a low-temperature symmetry-broken phase of the quantum spin Hall effect.
While such a symmetry-broken phase typically is accompanied by a gap, we find
that the interaction strength must exceed a critical value for the gap to form
using quantum Monte Carlo dynamical cluster approximation simulations. Hence,
we predict that topology can obtain in a gapless phase but only in the presence
of interactions in dispersive bands. These results are applied to recent
experiments on moir\'e systems and shown to be consistent with valley-coherent
quantum anomalous Hall physics.Comment: Figure 4e,f added as well as a referenc
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Permeabilization of the Blood-Brain Barrier via Mucosal Engrafting: Implications for Drug Delivery to the Brain
Utilization of neuropharmaceuticals for central nervous system(CNS) disease is highly limited due to the blood-brain barrier(BBB) which restricts molecules larger than 500Da from reaching the CNS. The development of a reliable method to bypass the BBB would represent an enormous advance in neuropharmacology enabling the use of many potential disease modifying therapies. Previous attempts such as transcranial catheter implantation have proven to be temporary and associated with multiple complications. Here we describe a novel method of creating a semipermeable window in the BBB using purely autologous tissues to allow for high molecular weight(HMW) drug delivery to the CNS. This approach is inspired by recent advances in human endoscopic transnasal skull base surgical techniques and involves engrafting semipermeable nasal mucosa within a surgical defect in the BBB. The mucosal graft thereby creates a permanent transmucosal conduit for drugs to access the CNS. The main objective of this study was to develop a murine model of this technique and use it to evaluate transmucosal permeability for the purpose of direct drug delivery to the brain. Using this model we demonstrate that mucosal grafts allow for the transport of molecules up to 500 kDa directly to the brain in both a time and molecular weight dependent fashion. Markers up to 40 kDa were found within the striatum suggesting a potential role for this technique in the treatment of Parkinsonâs disease. This proof of principle study demonstrates that mucosal engrafting represents the first permanent and stable method of bypassing the BBB thereby providing a pathway for HMW therapeutics directly into the CNS
Mapping twist-tuned multi-band topology in bilayer WSe
Semiconductor moir\'e superlattices have been shown to host a wide array of
interaction-driven ground states. However, twisted homobilayers have been
difficult to study in the limit of large moir\'e wavelength, where interactions
are most dominant, and despite numerous predictions of nontrivial topology in
these homobilayers, experimental evidence has remained elusive. Here, we
conduct local electronic compressibility measurements of twisted bilayer
WSe at small twist angles. We demonstrate multiple topological bands which
host a series of Chern insulators at zero magnetic field near a 'magic angle'
around . Using a locally applied electric field, we induce a
topological quantum phase transition at one hole per moir\'e unit cell.
Furthermore, by measuring at a variety of local twist angles, we characterize
how the interacting ground states of the underlying honeycomb superlattice
depend on the size of the moir\'e unit cell. Our work establishes the
topological phase diagram of a generalized Kane-Mele-Hubbard model in tWSe,
demonstrating a tunable platform for strongly correlated topological phases
Spin skyrmion gaps as signatures of strong-coupling insulators in magic-angle twisted bilayer graphene
The flat electronic bands in magic-angle twisted bilayer graphene (MATBG)
host a variety of correlated insulating ground states, many of which are
predicted to support charged excitations with topologically non-trivial spin
and/or valley skyrmion textures. However, it has remained challenging to
experimentally address their ground state order and excitations, both because
some of the proposed states do not couple directly to experimental probes, and
because they are highly sensitive to spatial inhomogeneities in real samples.
Here, using a scanning single-electron transistor, we observe thermodynamic
gaps at even integer moir\'e filling factors at low magnetic fields. We find
evidence of a field-tuned crossover from charged spin skyrmions to bare
particle-like excitations, suggesting that the underlying ground state belongs
to the manifold of strong-coupling insulators. From the spatial dependence of
these states and the chemical potential variation within the flat bands, we
infer a link between the stability of the correlated ground states and local
twist angle and strain. Our work advances the microscopic understanding of the
correlated insulators in MATBG and their unconventional excitations.Comment: Supplementary information available at
https://www.nature.com/articles/s41467-023-42275-
Tunable spin and valley excitations of correlated insulators in -valley moir\'e bands
Moir\'e superlattices formed from transition metal dichalcogenides (TMDs)
have been shown to support a variety of quantum electronic phases that are
highly tunable using applied electromagnetic fields. While the valley character
of the low-energy states dramatically affects optoelectronic properties in the
constituent TMDs, this degree of freedom has yet to be fully explored in
moir\'e systems. Here, we establish twisted double bilayer WSe as an
experimental platform to study electronic correlations within -valley
moir\'e bands. Through a combination of local and global electronic
compressibility measurements, we identify charge-ordered phases at multiple
integer and fractional moir\'e band fillings . By measuring the magnetic
field dependence of their energy gaps and the chemical potential upon doping,
we reveal spin-polarized ground states with novel spin polaron quasiparticle
excitations. In addition, an applied displacement field allows us to realize a
new mechanism of metal-insulator transition at driven by tuning
between - and -valley moir\'e bands. Together, our results
demonstrate control over both the spin and valley character of the correlated
ground and excited states in this system
Hofstadter states and reentrant charge order in a semiconductor moir\'e lattice
The emergence of moir\'e materials with flat bands provides a platform to
systematically investigate and precisely control correlated electronic phases.
Here, we report local electronic compressibility measurements of a twisted
WSe/MoSe heterobilayer which reveal a rich phase diagram of
interpenetrating Hofstadter states and electron solids. We show that this
reflects the presence of both flat and dispersive moir\'e bands whose relative
energies, and therefore occupations, are tuned by density and magnetic field.
At low densities, competition between moir\'e bands leads to a transition from
commensurate arrangements of singlets at doubly occupied sites to triplet
configurations at high fields. Hofstadter states (i.e., Chern insulators) are
generally favored at high densities as dispersive bands are populated, but are
suppressed by an intervening region of reentrant charge-ordered states in which
holes originating from multiple bands cooperatively crystallize. Our results
reveal the key microscopic ingredients that favor distinct correlated ground
states in semiconductor moir\'e systems, and they demonstrate an emergent
lattice model system in which both interactions and band dispersion can be
experimentally controlled
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