Anomalous local magnetic field distribution and strong pinning in CaFe1.94Co0.06As2 single crystals

Magneto-optical imaging of a single crystal of CaFe1.94Co0.06As2, shows anomalous remnant magnetization within Meissner like regions of the superconductor. The unconventional shape of the local magnetization hysteresis loop suggests admixture of superconducting and magnetic fractions governing the response. Near the superconducting transition temperature, local magnetic field exceeds the applied field resulting in a diamagnetic to positive magnetization transformation. The observed anomalies in the local magnetic field distribution are accompanied with enhanced bulk pinning in the CaFe1.94Co0.06As2 single crystals. We propose our results suggest a coexistence of superconductivity and magnetic correlations.Comment: 6 pages, 4 figures. arXiv admin note: substantial text overlap with arXiv:1201.369

Critical behavior at de-pinning of a driven disordered vortex matter in 2H-NbS2

We report unusual jamming in driven ordered vortex flow in 2H-NbS2. Reinitiating movement in these jammed vortices with a higher driving force, and halting it thereafter once again with a reduction in drive, unfolds a critical behavior centered around the de-pinning threshold via divergences in the lifetimes of transient states, validating the predictions of a recent simulation study, which also pointed out a correspondence between plastic de-pinning in vortex matter and the notion of random organization proposed in the context of sheared colloids undergoing diffusive motion.Comment: Phys. Rev. B (in press, 2012). The paper has 14 pages of Text+ Refs. with 4 figures. (Note as some of the figure files are large in size, to enable faster download, the file size has been kept small and the figure resolution are low. The online version of the paper to appear in PRB will contain the higher resolution figures

Structure and superconductivity of two different phases of Re3W

Two superconducting phases of Re(3)W have been found with different physical properties. One phase crystallizes in a noncentrosymmetric cubic (alpha-Mn) structure and has a superconducting transition temperature T(c) of 7.8 K. The other phase has a hexagonal centrosymmetric structure and is superconducting with a T(c) of 9.4 K. Switching between the two phases is possible by annealing the sample or remelting it. The properties of both phases of Re(3)W have been characterized by powder neutron diffraction, magnetization, and resistivity measurements. The temperature dependences of the lower and upper critical fields have been measured for both phases. These are used to determine the penetration depths and the coherence lengths for these systems

Visualizing a dilute vortex liquid to solid phase transition in a Bi2Sr2CaCu2O8 single crystal

Using high sensitivity magneto-optical imaging we find evidence for a jump in local vortex density associated with a vortex liquid to solid phase transition just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We find the regions of the sample where the jump in vortex density occurs are associated with low screening currents. In the field - temperature vortex phase diagram we identify phase boundaries demarcating a dilute vortex liquid phase and the vortex solid phase. The phase diagram also identifies a coexistence regime of the dilute vortex liquid and solid phases and shows the effect of pinning on the vortex liquid to solid phase transition line. We find the phase boundary lines can be fitted to the theoretically predicted expression for the low-field portion of the phase boundary delineating a dilute vortex solid from a vortex liquid phase. We show that the same theoretical fit can be used to describe the pinning dependence of the low-field phase boundary lines provided a dependence of the Lindemann number on pinning strength is considered.Comment: 16 pages and 6 figures (Published

Mode-selective vibrational control of charge transport in ππ-conjugated molecular materials

The soft character of organic materials leads to strong coupling between molecular nuclear and electronic dynamics. This coupling opens the way to control charge transport in organic electronic devices by inducing molecular vibrational motions. However, despite encouraging theoretical predictions, experimental realization of such control has remained elusive. Here we demonstrate experimentally that photoconductivity in a model organic optoelectronic device can be controlled by the selective excitation of molecular vibrations. Using an ultrafast infrared laser source to create a coherent superposition of vibrational motions in a pentacene/C60 photoresistor, we observe that excitation of certain modes in the 1500-1700 cm$^{-1}$ region leads to photocurrent enhancement. Excited vibrations affect predominantly trapped carriers. The effect depends on the nature of the vibration and its mode-specific character can be well described by the vibrational modulation of intermolecular electronic couplings. Vibrational control thus presents a new tool for studying electron-phonon coupling and charge dynamics in (bio)molecular materials.This work was supported by the Netherlands Organization for Scientific Research (NWO) through the ‘Stichting voor Fundamenteel Onderzoek der Materie’ (FOM) research programme. A.A.B. also acknowledges a VENI grant from the NWO. A.A.B. is currently a Royal Society University Research Fellow. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 639750). R.L. acknowledges a Marie Curie IE Fellowship from the EU, held at the Weizmann Institute (FP7-PEOPLE-2011-IEF no. 29866). X.Y. thanks the Council for Higher Education (Israel) for a PBC programme postdoctoral research fellowship. V.C. thanks support from the Office of Naval Research and MURI Center on Advanced Molecular Photovoltaics, award No. N00014-14-1-0580. J.L.B. acknowledges support by competitive research funding from King Abdullah University of Science and Technology (KAUST) and by ONR Global, Grant N62909-15-1-2003. D.C. thanks the Israel Science Foundation Centre of Excellence program, the Grand Centre for Sensors and Security and the Schmidt Minerva Centre for Supramolecular Architecture for partial support. D.C. holds the Sylvia and Rowland Schaefer Chair in Energy Research.This is the final published version. It first appeared at http://dx.doi.org/10.1038/ncomms888

Suppression of magnetic excitations near the surface of the topological Kondo insulator SmB6

We present a detailed investigation of the temperature and depth dependence of the magnetic properties of 3D topological Kondo insulator SmB6 , in particular near its surface. We find that local magnetic field fluctuations detected in the bulk are suppressed rapidly with decreasing depths, disappearing almost completely at the surface. We attribute the magnetic excitations to spin excitons in bulk SmB6 , which produce local magnetic fields of about ~1.8 mT fluctuating on a time scale of ~60 ns. We find that the excitonic fluctuations are suppressed when approaching the surface on a length scale of 40-90 nm, accompanied by a small enhancement in static magnetic fields. We associate this length scale to the size of the excitonic state.Comment: 5 pages, 5 figures, accepted for publication as a Rapid Communication in Phys. Rev.

The Importance of Research on the Origin of SARS-CoV-2

The origin of the severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) virus causing the COVID-19 pandemic has not yet been fully determined. Despite the consensus about the SARS-CoV-2 origin from bat CoV RaTG13, discrepancy to host tropism to other human Coronaviruses exist. SARS-CoV-2 also possesses some differences in its S protein receptor-binding domain, glycan-binding N-terminal domain and the surface of the sialic acid-binding domain. Despite similarities based on cryo-EM and biochemical studies, the SARS-CoV-2 shows higher stability and binding affinity to the ACE2 receptor. The SARS-CoV-2 does not appear to present a mutational “hot spot” as only the D614G mutation has been identified from clinical isolates. As laboratory manipulation is highly unlikely for the origin of SARS-CoV-2, the current possibilities comprise either natural selection in animal host before zoonotic transfer or natural selection in humans following zoonotic transfer. In the former case, despite SARS-CoV-2 and bat RaTG13 showing 96% identity some pangolin Coronaviruses exhibit very high similarity to particularly the receptor-binding domain of SARS-CoV-2. In the latter case, it can be hypothesized that the SARS-CoV-2 genome has adapted during human-to-human transmission and based on available data, the isolated SARS-CoV-2 genomes derive from a common origin. Before the origin of SARS-CoV-2 can be confirmed additional research is required