Search for Majorana fermions renewed interest in semiconductor–superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor–superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

We report anomalous enhancement of the critical current at low temperatures in gate-tunable Josephson junctions made from topological insulator BiSbTeSe2 nanoribbons with superconducting Nb electrodes. In contrast to conventional junctions, as a function of the decreasing temperature T , the increasing critical current Ic exhibits a sharp upturn at a temperature T * around 20% of the junction critical temperatures for several different samples and various gate voltages. The Ic vs. T demonstrates a short junction behavior for T > T * , but crosses over to a long junction behavior for T < T * with an exponential T -dependence Ic ∝ exp − kBT /δ , where kB is the Boltzmann constant. The extracted characteristic energy-scale δ is found to be an order of magnitude smaller than the induced superconducting gap of the junction. We attribute the long-junction behavior with such a small δ to low-energy Andreev bound states (ABS) arising from winding of the electronic wavefunction around the circumference of the topological insulator nanoribbon (TINR). Our TINRbased Josephson junctions with low-energy ABS are promising for future topologically protected devices that may host exotic phenomena such as Majorana fermions.

S100 proteins are multifunctional calcium-binding proteins of vertebrates that act intracellularly, extracellularly, or both, and are engaged in the progression of many socially significant diseases. Their extracellular action is typically mediated by the recognition of specific receptor proteins. Recent studies indicate the ability of some S100 proteins to affect cytokine signaling through direct interaction with cytokines. S100P was shown to be the S100 protein most actively involved in interactions with some four-helical cytokines. To assess the selectivity of the S100P protein binding to four-helical cytokines, we have probed the interaction of Ca2+-bound recombinant human S100P with a panel of 32 four-helical human cytokines covering all structural families of this fold, using surface plasmon resonance spectroscopy. A total of 22 cytokines from all families of four-helical cytokines are S100P binders with the equilibrium dissociation constants, Kd, ranging from 1 nM to 3 µM (below the Kd value for the S100P complex with the V domain of its conventional receptor, receptor for advanced glycation end products, RAGE). Molecular docking and mutagenesis studies revealed the presence in the S100P molecule of a cytokine-binding site, which overlaps with the RAGE-binding site. Since S100 binding to four-helical cytokines inhibits their signaling in some cases, the revealed ability of the S100P protein to interact with ca. 71% of the four-helical cytokines indicates that S100P may serve as a poorly selective inhibitor of their action.

We report on the observation of gate-tunable proximity-induced superconductivity and multiple Andreev reflections (MAR) in a bulk-insulating BiSbTeSe 2 topological insulator nanoribbon (TINR) Josephson junction (JJ) with superconducting Nb contacts. We observe a gate-tunable critical current (I C ) for gate voltages (V g ) above the charge neutrality point (V CN P ), with I C as large as 430 nA. We also observe MAR peaks in the differential conductance (dI/dV ) versus DC voltage (V dc ) across the junction corresponding to subharmonic peaks (at V dc = V n = 2∆ N b /en, where ∆ N b is the superconducting gap of the Nb contacts and n is the sub-harmonic order). The sub-harmonic order, n, exhibits a V g -dependence and reaches n = 13 for V g = 40 V, indicating the high transparency of the Nb contacts to TINR. Our observations pave the way toward exploring the possibilities of using TINR in topologically protected devices that may host exotic physics such as Majorana fermions.Three-dimensional topological insulators (TI's) are a new class of quantum matter with an insulating bulk and conducting surface states, topologically protected against time-reversalinvariant perturbations (scattering by non-magnetic impurities such as crystalline defects and surface roughness) 1,2 . Topological superconductors (TSC's) are another important class of quantum matter and are analogous to TI's, where the superconducting gap and Majorana fermions of TSC's replace the bulk bandgap and Dirac fermion surface states of the TI, respectively 2 . Controlling the Majorana modes is considered one of the important approaches for developing topologically protected quantum computers. Three-dimensional (3D) TIs in proximity to s-wave superconductors have been proposed as one of the promising platforms to realize topological superconductivity and Majorana fermions 3 . In this context, it has been pointed out that TI nanowires (TINWs) possess various appealing features for such studies 4-8 . However, the first important step is to understand how TI nanowires, including nanoribbons (TINR's), behave in contact with superconducting leads.Superconductor normal superconductor (SNS) Josephson junctions(JJs), with topological insulators as the normal material have been experimentally realized on 3D-TI's 9-22 . However, TI materials used in many of the previous experiments have notable bulk conduction, making it challenging to distinguish from the contribution of the topological surface states. In this letter, we study S-TINR-S Josephson junctions, where S = Niobium (Nb) and the TINR's are mechanically exfoliated from bulk BiSbTeSe 2 (BSTS) TI crystals. Our BSTS is among the most bulk-insulating TI's with surface states dominated conduction, and chemical potential located close to the surface state Dirac point in the bulk bandgap 23,24 . Therefore, our study enables us to investigate the proximity effects and induced superconductivity in such "intrinsic" (bulk-insulating) and gate-tunable TINR's with both electron (n) and hole (p) dominated surface transpo...

Erythropoietin (EPO) is a clinically significant four-helical cytokine, exhibiting erythropoietic, cytoprotective, immunomodulatory, and cancer-promoting activities. Despite vast knowledge on its signaling pathways and physiological effects, extracellular factors regulating EPO activity remain underexplored. Here we show by surface plasmon resonance spectroscopy, that among eighteen members of Ca2+-binding proteins of the S100 protein family studied, only S100A2, S100A6 and S100P proteins specifically recognize EPO with equilibrium dissociation constants ranging from 81 nM to 0.5 µM. The interactions occur exclusively under calcium excess. Bioinformatics analysis showed that the EPO-S100 interactions could be relevant to progression of neoplastic diseases, including cancer, and other diseases. The detailed knowledge of distinct physiological effects of the EPO-S100 interactions could favor development of more efficient clinical implications of EPO. Summing up our data with previous findings, we conclude that S100 proteins are potentially able to directly affect functional activities of specific members of all families of four-helical cytokines, and cytokines of other structural superfamilies.