Enhanced NMR relaxation of Tomonaga-Luttinger liquids and the magnitude of the carbon hyperfine coupling in single-wall carbon nanotubes

Recent transport measurements [Churchill \textit{et al.} Nat. Phys. \textbf{5}, 321 (2009)] found a surprisingly large, 2-3 orders of magnitude larger than usual $^{13}$C hyperfine coupling (HFC) in $^{13}$C enriched single-wall carbon nanotubes (SWCNTs). We formulate the theory of the nuclear relaxation time in the framework of the Tomonaga-Luttinger liquid theory to enable the determination of the HFC from recent data by Ihara \textit{et al.} [Ihara \textit{et al.} EPL \textbf{90}, 17004 (2010)]. Though we find that $1/T_1$ is orders of magnitude enhanced with respect to a Fermi-liquid behavior, the HFC has its usual, small value. Then, we reexamine the theoretical description used to extract the HFC from transport experiments and show that similar features could be obtained with HFC-independent system parameters.Comment: 5 pages plus 2 supplementary material

Effective elastic properties of two dimensional multiplanar hexagonal nanostructures

A generalized analytical approach is presented to derive closed-form formulae for the elastic moduli of hexagonal multiplanar nano-structures. Hexagonal nano-structural forms are common for various materials. Four different classes of materials (single layer) from a structural point of view are proposed to demonstrate the validity and prospective application of the developed formulae. For example, graphene, an allotrope of carbon, consists of only carbon atoms to form a honeycomb like hexagonal lattice in a single plane, while hexagonal boron nitride (hBN) consists of boron and nitrogen atoms to form the hexagonal lattice in a single plane. Unlike graphene and hBN, there are plenty of other materials with hexagonal nano-structures that have the atoms placed in multiple planes such as stanene (consists of only Sn atoms) and molybdenum disulfide (consists of two different atoms: Mo and S). The physics based high-fidelity analytical model developed in this article are capable of obtaining the elastic properties in a computationally efficient manner for wide range of such materials with hexagonal nano-structures that are broadly classified in four classes from structural viewpoint. Results are provided for materials belonging to all the four classes, wherein a good agreement between the elastic moduli obtained using the proposed formulae and available scientific literature is observed

High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe

A decade of intense research on two-dimensional (2D) atomic crystals has revealed that their properties can differ greatly from those of the parent compound. These differences are governed by changes in the band structure due to quantum confinement and are most profound if the underlying lattice symmetry changes. Here we report a high-quality 2D electron gas in few-layer InSe encapsulated in hexagonal boron nitride under an inert atmosphere. Carrier mobilities are found to exceed 103cm2V-1s-1and 104cm2V-1s-1at room and liquid-helium temperatures, respectively, allowing the observation of the fully developed quantum Hall effect. The conduction electrons occupy a single 2D subband and have a small effective mass. Photoluminescence spectroscopy reveals that the bandgap increases by more than 0.5eV with decreasing the thickness from bulk to bilayer InSe. The band-edge optical response vanishes in monolayer InSe, which is attributed to the monolayer's mirror-plane symmetry. Encapsulated 2D InSe expands the family of graphene-like semiconductors and, in terms of quality, is competitive with atomically thin dichalcogenides and black phosphorus.EU, EPSRC. The Royal Societ

trans-Tango: driving gene activity across the vertebrate synapse

Understanding brain function requires the identification of neuronal connections that mediate cognition and behavior. Conventional labeling methods and reconstruction by serial electron microscopy uncover synaptic partners but do not permit manipulation of identified circuits. trans-Tango, first developed in Drosophila, is a genetic approach for anterograde labeling across the synapse that not only identifies post-synaptic neurons but also permits their transcriptional regulation. A ligand modified from human glucagon is expressed under Gal4 control and tethered to the pre-synaptic membrane of defined neurons where it activates a signaling pathway in the corresponding post-synaptic neurons. Binding of the ligand to the human glucagon receptor coupled to the QF transcription factor through a protease cleavage site recruits an arrestin-protease fusion protein, that cleaves QF. This allows QF to translocate to the nucleus and promote the expression of any gene under the control of the upstream activating sequence (QUAS) where it binds. We adapted trans-Tango for use in the zebrafish nervous system and validated its effectiveness both in transient assays using Tol2 trans-Tango plasmids and in stable transgenic lines. We injected the trans-Tango ligand, receptor, and arrestin constructs along with Tol2 mRNA into embryos from fish-bearing different neuron-specific Gal4 driver lines and UAS:GFP mated to fish containing Tg(QUAS:mApple-CAAX). GFP labeled pre-synaptic neurons were detected in close proximity to mApple labeled putative post-synaptic neurons in Gal4-dependent patterns. Connectivity was validated by examining known synaptic partners in the retina as well as tectal neurons with known morphology. To confirm that gene expression is regulated through signaling across the synapse, we used optogenetics to selectively activate retinal ganglion cells and Tg(QUAS:GCaMP6f) to detect calcium transients in post-synaptic neurons in the optic tectum. These exciting results lay the foundation for genetic approaches to reveal synaptic partners and to access an arsenal of tools to monitor or modulate neural circuits

Find partners: Transgenic tools for trans-synaptic tracing

Techniques for directional tracing of neural circuits will propel our understanding of the organization and function of the brain. We adapted trans-Tango, a powerful genetic approach for anterograde circuit tracing in Drosophila, to a vertebrate nervous system, that of zebrafish. trans-Tango involves a synthetic signaling pathway introduced into all neurons. Specificity comes from genetically defined neurons presenting a tethered ligand at the synapse. Binding of ligand to a G protein-coupled receptor activates the pathway in post-synaptic neurons, resulting in recruitment of an arrestin-protease fusion protein, site-specific proteolysis of the receptor, and release of the QF transcription factor allowing its translocation to the nucleus. There QF binds to upstream regulatory sequences (QUAS) to drive expression of genes such as fluorescent reporters, which label and identify post-synaptic cells. The trans-Tango components were modified for zebrafish by placing receptor and arrestin fusion protein constructs downstream of the elavl3 promoter in Tol2 transposition vectors. Ligand variants differing in their pre-synaptic targeting sequences were also cloned into Tol2 plasmids under control of the Gal4 transcription factor. To test functionality, plasmids encoding ligand, receptor and arrestin, as well as Tol2 transposase RNA, were co-injected into 1-cell stage embryos, the progeny of a Gal4 driver that promotes strong expression of Tg(UAS:GFP) in the hindbrain mated with the Tg(QUAS:mApple-CAAX) reporter. Confocal imaging revealed GFP labeled axon terminals closely apposed to mApple labeled neurons, suggesting that trans-Tango is effective in transient expression assays. With such assays, we determined the optimal ligand for activating the receptor in post-synaptic neurons without off-target labeling. We are currently characterizing trans-synaptic labeling in stable transgenic lines with verified expression of trans-Tango components. We are also using optogenetics and calcium imaging to confirm that identified connections are functional synapses and developing a related configuration for retrograde circuit tracing. This work lays the foundation for mapping neural connectivity in vivo for both invertebrate and vertebrate nervous systems

trans-Tango: Driving gene activity across the vertebrate synapse

A greater understanding of brain function requires tools to unravel the neuronal connections that underlie behavior. We adapted trans-Tango, a powerful genetic strategy for anterograde circuit tracing in Drosophila,to a vertebrate nervous system, that of zebrafish. trans-Tango is introduced into all neurons. Specificity comes from genetically defined neurons presenting a tethered ligand at the pre-synaptic membrane, which activates a receptor in post-synaptic partners. The receptor is coupled to the QF transcription factor through a protease cleavage site and once activated, recruits an Arrestin-protease 22 fusion protein. This results in site-specific proteolysis and release of QF that then translocates to the nucleus to drive expression of any gene under control of its upstream activating sequence (QUAS). To test functionality, Tol2 plasmids containing ligand, Arrestin and receptor constructs, along with Tol2 transposase RNA, were co-injected into 1-cell stage embryos. Embryos were derived from mating fish carrying a Gal4 driver line that promotes strong neural-specific expression of Tg(UAS:GFP) with fish from a Tg(QUAS;mApple-CAAX) reporter line. As larvae, mApple labeled neurons were detected adjacent to the GFP labeled axons of pre-synaptic neurons in distinct patterns for each Gal4 driver line. To confirm that gene expression is induced through signaling across the synapse, we used optogenetics to selectively activate retinal ganglion cells and the genetically encoded calcium indicator Tg(QUAS:GCaMP6f) to measure neural activity in post-synaptic targets. Increased QF-dependent calcium transients were observed in the optic tectum in neurons with known morphology as well as in a newly identified cell type. The results confirm that trans-Tango is effective both in transient assays and in stable transgenic lines for revealing and interrogating neural circuits in the vertebrate brain. This work is funded by the NIH Brain initative 3RF1MH123213

Electron spin resonance in alkali doped SWCNTs

Electron spin resonance (ESR) measurements on SWCNTs doped with alkali atoms (potassium) using a solid state reaction is reported. We find the emergence of the ESR signal of itinerant electrons upon doping with a signal intensity that is comparable to that expected from band structure calculations. The ESR line-width and microwave conductivity weakly depend on the temperature indicating taht doped SWCNTs are bad metals. It is argued to result from the lack of crystalline order in the tubes and the large impurity concentration. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

Identifying the electron spin resonance of conduction electrons in alkali doped SWCNTs

We study the electron spin resonance (ESR) signal of pristine and potassium doped SWCNTs. We identify signals of a super-paramagnetic background, a low intensity impurity, and of the conduction electron spin resonance (CESR). The latter only appears upon the alkali atom doping. To identify the CESR signal, we critically assess whether it could come from residual graphitic carbon, which we clearly exclude. We give accurate values for the signal intensities and the corresponding concentration of spins and for the g-factors. The CESR signal intensity allows to determine the density of states on the SWCNT assembly