Single magnetic adsorbates on s-wave superconductors

In superconductors, magnetic impurities induce a pair-breaking potential for Cooper pairs, which locally affects the Bogoliubov quasiparticles and gives rise to Yu-Shiba-Rusinov (YSR or Shiba, in short) bound states in the density of states (DoS). These states carry information on the magnetic coupling strength of the impurity with the superconductor, which determines the many-body ground state properties of the system. Recently, the interest in Shiba physics was boosted by the prediction of topological superconductivity and Majorana modes in magnetically coupled chains and arrays of Shiba impurities. Here, we review the physical insights obtained by scanning tunneling microscopy into single magnetic adsorbates on the $s$-wave superconductor lead (Pb). We explore the tunneling processes into Shiba states, show how magnetic anisotropy affects many-body excitations, and determine the crossing of the many-body groundstate through a quantum phase transition. Finally, we discuss the coupling of impurities into dimers and chains and their relation to Majorana physics.Comment: 18 pages, 17 figures, revie

Tuning the magnetic anisotropy of single molecules

The magnetism of single atoms and molecules is governed by the atomic scale environment. In general, the reduced symmetry of the surrounding splits the $d$ states and aligns the magnetic moment along certain favorable directions. Here, we show that we can reversibly modify the magnetocrystalline anisotropy by manipulating the environment of single iron(II) porphyrin molecules adsorbed on Pb(111) with the tip of a scanning tunneling microscope. When we decrease the tip--molecule distance, we first observe a small increase followed by an exponential decrease of the axial anisotropy on the molecules. This is in contrast to the monotonous increase observed earlier for the same molecule with an additional axial Cl ligand. We ascribe the changes in the anisotropy of both species to a deformation of the molecules in the presence of the attractive force of the tip, which leads to a change in the $d$ level alignment. These experiments demonstrate the feasibility of a precise tuning of the magnetic anisotropy of an individual molecule by mechanical control.Comment: 16 pages, 5 figures; online at Nano Letters (2015

Geometrical Considerations for the Design of Liquid-phase Biochemical Sensors Using a Cantilever\u27s Fundamental In-plane Mode

The influence of the beam geometry on the quality factor and resonance frequency of resonant silicon cantilever beams vibrating in their fundamental in-plane flexural mode in water has been investigated. Compared to cantilevers vibrating in their first out-of-plane flexural mode, utilizing the in-plane mode results in reduced damping and reduced mass loading by the surrounding fluid. Quality factors as high as 86 have been measured in water for cantilevers with a 20 μm thick silicon layer. Based on the experimental data, design guidelines are established for beam dimensions that ensure maximal Q-factors and minimal mass loading by the surrounding fluid, thus improving the limit-of-detection of mass-sensitive biochemical sensors. Elementary theory is also presented to help explain the observed trends. Additional discussion focuses on the tradeoffs that exist in designing liquid-phase biochemical sensors using in-plane cantilevers

Magnetic anisotropy in Shiba bound states across a quantum phase transition

The exchange coupling between magnetic adsorbates and a superconducting substrate leads to Shiba states inside the superconducting energy gap and a Kondo resonance outside the gap. The exchange coupling strength determines whether the quantum many-body ground state is a Kondo singlet or a singlet of the paired superconducting quasiparticles. Here, we use scanning tunneling spectroscopy to identify the different quantum ground states of Manganese phthalocyanine on Pb(111). We observe Shiba states, which are split into triplets by magnetocrystalline anisotropy. Their characteristic spectral weight yields an unambiguous proof of the nature of the quantum ground state.Comment: 6 pages, 4 figure

Ethnopharmacology – From Mexican Hallucinogens to a Global Transdisciplinary Science

Psychoactive natural substances have been reported from practically all regions of the world, but Mexican indigenous cultures have played a crucial role having influenced medical, toxicological, biological, chemical, pharmaceutical, and, of course, anthropological research. Especially in the 1950’s and 1960’s peyotl, teonanacatl and other psychoactives came to the attention of researchers and revelers alike. In this overview we highlight the developments of ethnopharmacology from the initial development of the term until today using one psychoactive species as an example - Salvia divinorum. In 1962 “ethnopharmacologists”, Albert Hofmann and R. Gordon Wasson, documented and collected a flowering specimen of Ska María Pastora allowing the species botanical description as Salvia divinorum Epling & Játiva. Five years later Efron et al. (1967) organised a symposium “Ethnopharmacologic search for psychoactive drugs” which over the next decades would give its name to a discipline which today is much more broadly defined, dealing with local and traditional medicines, their biological activities and chemistry. Globalisation has resulted in a world-wide commodification of many traditional medicines and psychoactives, as exemplified by S. divinorum. This fascinating Lamiaceae has become globally recognized for its best known active constituent salvinorin A, a kappa-opioid antagonist which has a unique effect on human physiology. While today ethnopharmacology is a thriving discipline, the interest in psychoactive substances is no longer central to the discipline. The search for anti-cancer agents (which also started in earnest in the 1960’s) had been of particular relevance and today includes among its many foci: • The scientific study of local and traditional knowledge not only in remote regions, but for example, also in urban immigrant communities • Research linking ethnopharmacology to biodiversity research both in terms of a sustainable use of natural resources (ecosystems) • Pharmacological studies with the aim of understanding the effects of complex mixtures on specific diseases or disease targets • The safety of herbal medicines • Anthropological and historical approaches on the use of medicinal and food plants and the link between food and medical uses of plants and fungi. / 50 years on ethnopharmacology is very different from what D. Efron and colleagues had envisioned

Low relaxation rate in a low-Z alloy of iron

The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined by the intrinsic (Gilbert) relaxation rate \emph{$G$}. For many years, pure iron (Fe) has had the lowest known value of $G=\textrm{57 Mhz}$ for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy with vanadium (V) possesses values of $G$ which are significantly reduced, to 35$\pm$5 Mhz at 27% V. The result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the atomic number $Z$.Comment: 14 pages, 4 figure

Unconventional Uses of Microcantilevers as Chemical Sensors in Gas and Liquid Media

The use of microcantilevers as (bio)chemical sensors usually involves the application of a chemically sensitive layer. The coated device operates either in a static bending regime or in a dynamic flexural mode. While some of these coated devices may be operated successfully in both the static and the dynamic modes, others may suffer from certain shortcomings depending on the type of coating, the medium of operation and the sensing application. Such shortcomings include lack of selectivity and reversibility of the sensitive coating and a reduced quality factor due to the surrounding medium. In particular, the performance of microcantilevers excited in their standard out-of-plane dynamic mode drastically decreases in viscous liquid media. Moreover, the responses of coated cantilevers operating in the static bending mode are often difficult to interpret. To resolve these performance issues, the following emerging unconventional uses of microcantilevers are reviewed in this paper: (1) dynamic-mode operation without using a sensitive coating, (2) the use of in-plane vibration modes (both flexural and longitudinal) in liquid media, and (3) incorporation of viscoelastic effects in the coatings in the static mode of operation. The advantages and drawbacks of these atypical uses of microcantilevers for chemical sensing in gas and liquid environments are discussed