Reversible, Opto-Mechanically Induced Spin-Switching in a Nanoribbon-Spiropyran Hybrid Material

It has recently been shown that electronic transport in zigzag graphene nanoribbons becomes spin-polarized upon application of an electric field across the nanoribbon width. However, the electric fields required to experimentally induce this magnetic state are typically large and difficult to apply in practice. Here, using both first-principles density functional theory (DFT) and time-dependent DFT, we show that a new spiropyran-based, mechanochromic polymer noncovalently deposited on a nanoribbon can collectively function as a dual opto-mechanical switch for modulating its own spin-polarization. These calculations demonstrate that upon mechanical stress or photoabsorption, the spiropyran chromophore isomerizes from a closed-configuration ground-state to a zwitterionic excited-state, resulting in a large change in dipole moment that alters the electrostatic environment of the nanoribbon. We show that the electronic spin-distribution in the nanoribbon-spiropyran hybrid material can be reversibly modulated via noninvasive optical and mechanical stimuli without the need for large external electric fields. Our results suggest that the reversible spintronic properties inherent to the nanoribbon-spiropyran material allow the possibility of using this hybrid structure as a resettable, molecular-logic quantum sensor where opto-mechanical stimuli are used as inputs and the spin-polarized current induced in the nanoribbon substrate is the measured output.Comment: Accepted by Nanoscal

Generation of photovoltage in graphene on a femtosecond time scale through efficient carrier heating

Graphene is a promising material for ultrafast and broadband photodetection. Earlier studies addressed the general operation of graphene-based photo-thermoelectric devices, and the switching speed, which is limited by the charge carrier cooling time, on the order of picoseconds. However, the generation of the photovoltage could occur at a much faster time scale, as it is associated with the carrier heating time. Here, we measure the photovoltage generation time and find it to be faster than 50 femtoseconds. As a proof-of-principle application of this ultrafast photodetector, we use graphene to directly measure, electrically, the pulse duration of a sub-50 femtosecond laser pulse. The observation that carrier heating is ultrafast suggests that energy from absorbed photons can be efficiently transferred to carrier heat. To study this, we examine the spectral response and find a constant spectral responsivity between 500 and 1500 nm. This is consistent with efficient electron heating. These results are promising for ultrafast femtosecond and broadband photodetector applications.Comment: 6 pages, 4 figure

Critical solutions in topologically gauged N=8 CFTs in three dimensions

In this paper we discuss some special (critical) background solutions that arise in topological gauged ${\mathcal N}=8$ three-dimensional CFTs with SO(N) gauge group. These solutions solve the TMG equations (containing the parameters $\mu$ and $l$) for a certain set of values of $\mu l$ obtained by varying the number of scalar fields with a VEV. Apart from Minkowski, chiral round $AdS_3$ and null-warped $AdS_3$ (or Schr\"odinger(z=2)) we identify also a more exotic solution recently found in $TMG$ by Ertl, Grumiller and Johansson. We also discuss the spectrum, symmetry breaking pattern and the supermultiplet structure in the various backgrounds and argue that some properties are due to their common origin in a conformal phase. Some of the scalar fields, including all higgsed ones, turn out to satisfy three-dimensional singleton field equations. Finally, we note that topologically gauged ${\mathcal N}=6$ ABJ(M) theories have a similar, but more restricted, set of background solutions.Comment: 34 pages, v2: minor corrections, note about a new solution added in final section, v3: two footnotes adde

Energy Level Modulation of HOMO, LUMO, and Band‐Gap in Conjugated Polymers for Organic Photovoltaic Applications

To devise a reliable strategy for achieving specific HOMO and LUMO energy level modulation via alternating donor‐acceptor monomer units, we investigate a series of conjugated polymers (CPs) in which the electron withdrawing power of the acceptor group is varied, while maintaining the same donor group and the same conjugated chain conformation. Through experiment and DFT calculations, good correlation is identified between the withdrawing strength of the acceptor group, the HOMO and LUMO levels, and the degree of orbital localization, which allows reliable design principles for CPs. Increasing the acceptor strength results in an enhanced charge transfer upon combination with a donor monomer and a more pronounced decrease of the LUMO level. Moreover, while HOMO states remain delocalized along the polymer chain, LUMO states are strongly localized at specific bonds within the acceptor group. The degree of LUMO localization increases with increasing polymer length, which results in a further drop of the LUMO level and converges to its final value when the number of repeat units reaches the characteristic conjugation length. Based on these insights we designed PBT8PT, which exhibits 6.78% power conversion efficiency after device optimization via the additive assisted annealing, demonstrating the effectiveness of our predictive design approach. A strong acceptor lowers both the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of conjugated polymers (CPs), ultimately producing a narrowed band‐gap. The energy level difference between the CP and the constituent monomers converge to a constant value, providing an energy level prediction tool. Organic photovoltaic performance is correlated with the CP's energy levels, and a 6.78% power conversion efficiency is achieved.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96311/1/adfm_201201385_sm_suppl.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/96311/2/439_ftp.pd

Ideal Spin Filters: Theoretical Study of Electron Transmission Through Ordered and Disordered Interfaces Between Ferromagnetic Metals and Semiconductors

It is predicted that certain atomically ordered interfaces between some ferromagnetic metals (F) and semiconductors (S) should act as ideal spin filters that transmit electrons only from the majority spin bands or only from the minority spin bands of the F to the S at the Fermi energy, even for F with both majority and minority bands at the Fermi level. Criteria for determining which combinations of F, S and interface should be ideal spin filters are formulated. The criteria depend only on the bulk band structures of the S and F and on the translational symmetries of the S, F and interface. Several examples of systems that meet these criteria to a high degree of precision are identified. Disordered interfaces between F and S are also studied and it is found that intermixing between the S and F can result in interfaces with spin anti-filtering properties, the transmitted electrons being much less spin polarized than those in the ferromagnetic metal at the Fermi energy. A patent application based on this work has been commenced by Simon Fraser University.Comment: RevTeX, 12 pages, 5 figure

Human fetal brain self-organizes into long-term expanding organoids

Human brain development involves an orchestrated, massive neural progenitor expansion while a multi-cellular tissue architecture is established. Continuously expanding organoids can be grown directly from multiple somatic tissues, yet to date, brain organoids can solely be established from pluripotent stem cells. Here, we show that healthy human fetal brain in vitro self-organizes into organoids (FeBOs), phenocopying aspects of in vivo cellular heterogeneity and complex organization. FeBOs can be expanded over long time periods. FeBO growth requires maintenance of tissue integrity, which ensures production of a tissue-like extracellular matrix (ECM) niche, ultimately endowing FeBO expansion. FeBO lines derived from different areas of the central nervous system (CNS), including dorsal and ventral forebrain, preserve their regional identity and allow to probe aspects of positional identity. Using CRISPR-Cas9, we showcase the generation of syngeneic mutant FeBO lines for the study of brain cancer. Taken together, FeBOs constitute a complementary CNS organoid platform.</p

Dissipative superfluid dynamics from gravity

Charged asymptotically AdS black branes in five dimensions are sometimes unstable to the condensation of charged scalar fields. For fields of infinite charge and squared mass -4 Herzog was able to analytically determine the phase transition temperature and compute the endpoint of this instability in the neighborhood of the phase transition. We generalize Herzog's construction by perturbing away from infinite charge in an expansion in inverse charge and use the solutions so obtained as input for the fluid gravity map. Our tube wise construction of patched up locally hairy black brane solutions yields a one to one map from the space of solutions of superfluid dynamics to the long wavelength solutions of the Einstein Maxwell system. We obtain explicit expressions for the metric, gauge field and scalar field dual to an arbitrary superfluid flow at first order in the derivative expansion. Our construction allows us to read off the the leading dissipative corrections to the perfect superfluid stress tensor, current and Josephson equations. A general framework for dissipative superfluid dynamics was worked out by Landau and Lifshitz for zero superfluid velocity and generalized to nonzero fluid velocity by Clark and Putterman. Our gravitational results do not fit into the 13 parameter Clark-Putterman framework. Purely within fluid dynamics we present a consistent new generalization of Clark and Putterman's equations to a set of superfluid equations parameterized by 14 dissipative parameters. The results of our gravitational calculation fit perfectly into this enlarged framework. In particular we compute all the dissipative constants for the gravitational superfluid.Comment: v1: 58 + 1 pages; v2: 83 + 1 page

Early motion and directed exercise (EMADE) versus usual care post ankle fracture fixation: study protocol for a pragmatic randomised controlled trial

Background: Following surgical fixation of ankle fractures, the traditional management has included immobilisation for 6 weeks in a below-knee cast. However, this can lead to disuse atrophy of the affected leg and joint stiffness. While early rehabilitation from 2 weeks post surgery is viewed as safe, controversy remains regarding its benefits. We will compare the effectiveness of early motion and directed exercise (EMADE) ankle rehabilitation, against usual care, i.e. 6 weeks’ immobilisation in a below-knee cast. Method/design: We have designed a pragmatic randomised controlled trial (p-RCT) to compare the EMADE intervention against usual care. We will recruit 144 independently living adult participants, absent of tissue-healing comorbidities, who have undergone surgical stabilisation of isolated Weber B ankle fractures. The EMADE intervention consists of a non-weight-bearing progressive home exercise programme, complemented with manual therapy and education. Usual care consists of immobilisation in a non-weight-bearing below-knee cast. The intervention period is between week 2 and week 6 post surgery. The primary outcome is the Olerud and Molander Ankle Score (OMAS) patient-reported outcome measure (PROM) at 12 weeks post surgery. Secondary PROMs include the EQ-5D-5 L questionnaire, return to work and return to driving, with objective outcomes including ankle range of motion. Analysis will be on an intention-to-treat basis. An economic evaluation will be included. Discussion: The EMADE intervention is a package of care designed to address the detrimental effects of disuse atrophy and joint stiffness. An advantage of the OMAS is the potential of meta-analysis with other designs. Within the economic evaluation, the cost-utility analysis, may be used by commissioners, while the use of patient-relevant outcomes, such as return to work and driving, will ensure that the study remains pertinent to patients and their families. As it is being conducted in the clinical environment, this p-RCT has high external validity. Accordingly, if significant clinical benefits and cost-effectiveness are demonstrated, EMADE should become a worthwhile treatment option. A larger-scale, multicentre trial may be required to influence national guidelines. Trial registration: ISRCTN, ID: ISRCTN11212729. Registered retrospectively on 20 March 2017

Holographic c-theorems in arbitrary dimensions

We re-examine holographic versions of the c-theorem and entanglement entropy in the context of higher curvature gravity and the AdS/CFT correspondence. We select the gravity theories by tuning the gravitational couplings to eliminate non-unitary operators in the boundary theory and demonstrate that all of these theories obey a holographic c-theorem. In cases where the dual CFT is even-dimensional, we show that the quantity that flows is the central charge associated with the A-type trace anomaly. Here, unlike in conventional holographic constructions with Einstein gravity, we are able to distinguish this quantity from other central charges or the leading coefficient in the entropy density of a thermal bath. In general, we are also able to identify this quantity with the coefficient of a universal contribution to the entanglement entropy in a particular construction. Our results suggest that these coefficients appearing in entanglement entropy play the role of central charges in odd-dimensional CFT's. We conjecture a new c-theorem on the space of odd-dimensional field theories, which extends Cardy's proposal for even dimensions. Beyond holography, we were able to show that for any even-dimensional CFT, the universal coefficient appearing the entanglement entropy which we calculate is precisely the A-type central charge.Comment: 62 pages, 4 figures, few typo's correcte

High-resolution numerical modeling of wave-supported gravity-driven mudflows

Author Posting. © American Geophysical Union, 2009. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 114 (2009): C05014, doi:10.1029/2008JC005006.Wave-supported gravity-driven mudflow has been identified as a major offshore fine sediment transport mechanism of terrestrial sediment into the coastal ocean. This transport process essentially occurs within the wave boundary layer. In this study, wave-supported gravity-driven mudflow is investigated via a wave-phase-resolving high-resolution numerical model for fluid mud transport. The model results are verified with field observation of sediment concentration and near-bed flow velocities at Po prodelta. The characteristics of wave-supported gravity-driven mudflows are diagnosed by varying the bed erodibility, floc properties (fractal dimension), and rheological stresses in the numerical simulations. Model results for moderate concentration suggest that using an appropriately specified fractal dimension, the dynamics of wave-supported gravity-driven mudflow can be predicted without explicitly incorporating rheological stress. However, incorporating rheological stress makes the results less sensitive to prescribed fractal dimension. For high-concentration conditions, it is necessary to incorporate rheological stress in order to match observed intensity of downslope gravity-driven current. Model results are further analyzed to evaluate and calibrate simple parameterizations. Analysis suggests that when neglecting rheological stress, the drag coefficient decreases with increasing wave intensity and seems to follow a power law. However, when rheological stress is incorporated, the resulting drag coefficient is more or less constant (around 0.0013) for different wave intensities. Model results further suggest the bulk Richardson number has a magnitude smaller than 0.25 and is essentially determined by the amount of available soft mud (i.e., the erodibility), suggesting a supply limited condition for unconsolidated mud.This study is supported by the Office of Naval Research grant N00014-09-1-0134 and grant N00014-06-1-0945 as part of the Community Sediment Transport Modeling System (CSTMS) through the National Oceanographic Partnership Program (NOPP). This study is also partially supported by National Science Foundation (OCE- 0644497)