Tuning the adhesive geometry of neurons: length and polarity control.

Neurons acquire their functional and morphological axo-dendritic polarity by extending, from competing minor processes (neurites), one long axon among numerous dendrites. We employed complementary sets of micropatterns built from 2 and 6 µm wide stripes of various lengths to constrain hippocampal neuron shapes. Using these geometries, we have (i) limited the number of neuronal extensions to obtain a minimal in vitro system of bipolar neurons and (ii) controlled the neurite width during growth by the generation of a progressive cell shape asymmetry on either side of the cellular body. From this geometrical approach, we gained a high level of control of each neurite length and of the localization of axonal specification. To analyze these results, we developed a model based on a width and polarization dependent neurite elongation rate and on the existence of a critical neurite length that sets the axonal fate. Our data on the four series of micro-patterns developed for this study are described by a single set of growth parameters, well supported by experiments. The control of neuronal shapes by adhesive micro-patterns thereby offers a novel paradigm to follow the dynamical process of neurite lengthening and competition through the process of axonal polarization.

Large magnetoresistance in the iron-free pnictide superconductor LaRu2P2.

The magnetoresistance (MR) of iron pnictide superconductors is often dominated by electron-electron correlations and deviates from theH2or saturating behaviors expected for uncorrelated metals. Contrary to similar Fe-based pnictide systems, the superconductor LaRu2P2(Tc= 4 K) shows no enhancement of electron-electron correlations. Here we report a non-saturating MR deviating from theH2or saturating behaviors in LaRu2P2. We present results in single crystals of LaRu2P2, where we observe a MR followingH1.3up to 22 T. We discuss our result by comparing the bandstructure of LaRu2P2with that of Fe based pnictide superconductors. The different orbital structures of Fe and Ru leads to a 3D Fermi surface with negligible bandwidth renormalization in LaRu2P2, that contains a large open sheet over the whole Brillouin zone. We show that the large MR in LaRu2P2is unrelated to the one obtained in materials with strong electron-electron correlations and that it is compatible instead with conduction due to open orbits on the rather complex Fermi surface structure of LaRu2P2.

Millikelvin scanning tunneling microscope at 20/22 T with a graphite enabled stick-slip approach and an energy resolution below 8 µeV: Application to conductance quantization at 20 T in single atom point contacts of Al and Au and to the charge density wave of 2H-NbSe2.

We describe a scanning tunneling microscope (STM) that operates at magnetic fields up to 22 T and temperatures down to 80 mK. We discuss the design of the STM head, with an improved coarse approach, the vibration isolation system, and efforts to improve the energy resolution using compact filters for multiple lines. We measure the superconducting gap and Josephson effect in aluminum and show that we can resolve features in the density of states as small as 8 µeV. We measure the quantization of conductance in atomic size contacts and make atomic resolution and density of states images in the layered material 2H-NbSe2. The latter experiments are performed by continuously operating the STM at magnetic fields of 20 T in periods of several days without interruption.

Simplified feedback control system for scanning tunneling microscopy.

A Scanning Tunneling Microscope (STM) is one of the most important scanning probe tools available to study and manipulate matter at the nanoscale. In a STM, a tip is scanned on top of a surface with a separation of a few Å. Often, the tunneling current between the tip and the sample is maintained constant by modifying the distance between the tip apex and the surface through a feedback mechanism acting on a piezoelectric transducer. This produces very detailed images of the electronic properties of the surface. The feedback mechanism is nearly always made using a digital processing circuit separate from the user computer. Here, we discuss another approach using a computer and data acquisition through the universal serial bus port. We find that it allows successful ultralow noise studies of surfaces at cryogenic temperatures. We show results on different compounds including a type II Weyl semimetal (WTe2), a quasi-two-dimensional dichalcogenide superconductor (2H-NbSe2), a magnetic Weyl semimetal (Co3Sn2S2), and an iron pnictide superconductor (FeSe).

Pairing mechanism in the ferromagnetic superconductor UCoGe.

Superconductivity is a unique manifestation of quantum mechanics on a macroscopic scale, and one of the rare examples of many-body phenomena that can be explained by predictive, quantitative theories. The superconducting ground state is described as a condensate of Cooper pairs, and a major challenge has been to understand which mechanisms could lead to a bound state between two electrons, despite the large Coulomb repulsion. An even bigger challenge is to identify experimentally this pairing mechanism, notably in unconventional superconductors dominated by strong electronic correlations, like in high-Tc cuprates, iron pnictides or heavy-fermion compounds. Here we show that in the ferromagnetic superconductor UCoGe, the field dependence of the pairing strength influences dramatically its macroscopic properties like the superconducting upper critical field, in a way that can be quantitatively understood. This provides a simple demonstration of the dominant role of ferromagnetic spin fluctuations in the pairing mechanism.