7,082 research outputs found
Multi-level interference resonances in strongly-driven three-level systems
We study multi-photon resonances in a strongly-driven three-level quantum
system, where one level is periodically swept through a pair of levels with
constant energy separation . Near the multi-photon resonance condition
, where is an integer, we find qualitatively different
behavior for even or odd. We explain this phenomenon in terms of families
of interfering trajectories of the multi-level system. Remarkably, the behavior
is insensitive to fluctuations of the energy of the driven level, and survives
deep into the strong dephasing regime. The setup can be relevant for a variety
of solid state and atomic or molecular systems. In particular, it provides a
clear mechanism to explain recent puzzling experimental observations in
strongly-driven double quantum dots.Comment: 4 pages, 3 figure
Absence of magnetic monopoles in Maxwellian magnetoelectrics
The electromagnetic response of topological insulators is governed by axion
electrodynamics, which features a topological magnetoelectric term in the
Maxwell equations. As a consequence magnetic fields become the source of
electric fields and vice-versa, a phenomenon that is general for any material
exhibiting a linear magnetoelectric effect. Axion electrodynamics has been
associated with the possibility to create magnetic monopoles, in particular by
a electrical charge that is screened above the surface of a magnetoelectric
material. Here we present the exact solution for the electromagnetic fields in
this geometry and show that while vortex-like magnetic screening fields are
generated by the electrical charge their divergence is identically zero at
every point in space which implies a strict absence of magnetic monopoles.
Although magnetic image charges can be made explicit in the problem, no bound
state with electric charges yielding a dyon arises. A dyon-like angular
momentum follows from our analysis, but is quantized in a universal way,
because of its dependence on the dielectric constant. This is consistent with a
general argument that precludes magnetic monopoles to be generated in Maxwell
magnetoelectrics.Comment: v2: 9 pages, 3 figures; improved presentation and more detailed
appendices; added calculation of angular momentum; appendix is made more
pedagogical and now includes the detailed solution for a point charge in the
presence of a topological dielectric sphere; several references are adde
Adhesive and mechanical regulation of mesenchymal stem cell differentiation in human bone marrow and periosteum-derived progenitor cells
It has previously been demonstrated that cell shape can influence commitment of human bone marrow-derived mesenchymal stem cells (hBMCs) to adipogenic, osteogenic, chondrogenic, and other lineages. Human periosteum-derived cells (hPDCs) exhibit multipotency similar to hBMCs, but hPDCs may offer enhanced potential for osteogenesis and chondrogenesis given their apparent endogenous role in bone and cartilage repair in vivo. Here, we examined whether hPDC differentiation is regulated by adhesive and mechanical cues comparable to that reported for hBMC differentiation. When cultured in the appropriate induction media, hPDCs at high cell seeding density demonstrated enhanced levels of adipogenic or chondrogenic markers as compared with hPDCs at low cell seeding density. Cell seeding density correlated inversely with projected area of cell spreading, and directly limiting cell spreading with micropatterned substrates promoted adipogenesis or chondrogenesis while substrates promoting cell spreading supported osteogenesis. Interestingly, cell seeding density influenced differentiation through both changes in cell shape and non-shape-mediated effects: density-dependent adipogenesis and chondrogenesis were regulated primarily by cell shape whereas non-shape effects strongly influenced osteogenic potential. Inhibition of cytoskeletal contractility by adding the Rho kinase inhibitor Y27632 further enhanced adipogenic differentiation and discouraged osteogenic differentiation of hPDCs. Together, our results suggest that multipotent lineage decisions of hPDCs are impacted by cell adhesive and mechanical cues, though to different extents than hBMCs. Thus, future studies of hPDCs and other primary stem cell populations with clinical potential should consider varying biophysical metrics for more thorough optimization of stem cell differentiation.R01 EB000262 - NIBIB NIH HHS; R01 GM060692 - NIGMS NIH HHSPublished versio
A clinically relevant model of osteoinduction: a process requiring calcium phosphate and BMP/Wnt signalling
In this study, we investigated a clinically relevant model of in vivo ectopic bone formation utilizing human periosteum derived cells (HPDCs) seeded in a Collagraft carrier and explored the mechanisms by which this process is driven. Bone formation occurred after eight weeks when a minimum of one million HPDCs was loaded on Collagraft carriers and implanted subcutaneously in NMRI nu/nu mice. De novo bone matrix, mainly secreted by the HPDCs, was found juxta-proximal of the calcium phosphate (CaP) granules suggesting that CaP may have triggered the 'osteoinductive program'. Indeed, removal of the CaP granules by ethylenediaminetetraacetic acid decalcification prior to cell seeding and implantation resulted in loss of bone formation. In addition, inhibition of endogenous bone morphogenetic protein and Wnt signalling by overexpression of the secreted antagonists Noggin and Frzb, respectively, also abrogated osteoinduction. Proliferation of the engrafted HPDCs was strongly reduced in the decalcified scaffolds or when seeded with adenovirus-Noggin/Frzb transduced HPDCs indicating that cell division of the engrafted HPDCs is required for the direct bone formation cascade. These data suggest that this model of bone formation is similar to that observed during physiological intramembranous bone development and may be of importance when investigating tissue engineering strategies.Published versio
- …