Scanning Tunneling Spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of superconducting MgB2 with a critical temperature of 39K. In zero magnetic field the conductance spectra can be analyzed in terms of the standard BCS theory with a smearing parameter Gamma. The value of the superconducting gap is 5.2 meV at 4.2 K, with no experimentally significant variation across the surface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this novel superconductor. The application of a magnetic field induces strong pair-breaking as seen in the conductance spectra in fields up to 6 T.Comment: 4 pages, 4 figure

Predominantly Superconducting Origin of Large Energy Gaps in Underdoped Bi2Sr2CaCu2O8-d from Tunneling Spectroscopy

New tunneling data are reported in underdoped Bi2Sr2CaCu2O8-d using superconductor-insulator-superconductor break junctions. Energy gaps, Delta, of 51+2, 54+2 and 57+3 meV are observed for three crystals with Tc=77, 74, and 70 K respectively. These energy gaps are nearly three times larger than for overdoped crystals with similar Tc. Detailed examination of tunneling spectra over a wide doping range from underdoped to overdoped, including the Josephson IcRn product, indicate that these energy gaps are predominantly of superconducting origin.Comment: 10 pages, 4 figures, 1 tabl

Coherent quasiparticle weight and its connection to high-T_c superconductivity from angle-resolved photoemission

In conventional superconductors, the pairing energy gap (\Delta) and superconducting phase coherence go hand-in-hand. As the temperature is lowered, both the energy gap and phase coherence appear at the transition temperature T_c. In contrast, in underdoped high-T_c superconductors (HTSCs), a pseudogap appears at a much higher temperature T^*, smoothly evolving into the superconducting gap at T_c. Phase coherence on the other hand is only established at T_c, signaled by the appearance of a sharp quasiparticle (QP) peak in the excitation spectrum. Another important difference between the two types of superconductors is in the ratio of 2\Delta / T_c=R. In BCS theory, R~3.5, is constant. In the HTSCs this ratio varies widely, continuing to increase in the underdoped region, where the gap increases while T_c decreases. Here we report that in HTSCs it is the ratio z_A\Delta_m/T_c which is approximately constant, where \Delta_m is the maximum value of the d-wave gap, and z_A is the weight of the coherent excitations in the spectral function. This is highly unusual, since in nearly all phase transitions, T_c is determined by an energy scale alone. We further show that in the low-temperature limit, z_{\it A} increases monotonically with increasing doping x. The growth is linear, i.e. z_A(x)\propto x, in the underdoped to optimally doped regimes, and slows down in overdoped samples. The reduction of z_A with increasing temperature resembles that of the c-axis superfluid density.Comment: 11 pages, 5 figures, revised versio

Scanning tunneling spectroscopy of high-temperature superconductors

Tunneling spectroscopy played a central role in the experimental verification of the microscopic theory of superconductivity in the classical superconductors. Initial attempts to apply the same approach to high-temperature superconductors were hampered by various problems related to the complexity of these materials. The use of scanning tunneling microscopy/spectroscopy (STM/STS) on these compounds allowed to overcome the main difficulties. This success motivated a rapidly growing scientific community to apply this technique to high-temperature superconductors. This paper reviews the experimental highlights obtained over the last decade. We first recall the crucial efforts to gain control over the technique and to obtain reproducible results. We then discuss how the STM/STS technique has contributed to the study of some of the most unusual and remarkable properties of high-temperature superconductors: the unusual large gap values and the absence of scaling with the critical temperature; the pseudogap and its relation to superconductivity; the unprecedented small size of the vortex cores and its influence on vortex matter; the unexpected electronic properties of the vortex cores; the combination of atomic resolution and spectroscopy leading to the observation of periodic local density of states modulations in the superconducting and pseudogap states, and in the vortex cores.Comment: To appear in RMP; 65 pages, 62 figure

Theory of tunneling conductance for normal metal/insulator/triplet superconductor junctions

Tunneling conductance spectra of normal metal/insulator/triplet superconductor junctions are investigated theoretically. As triplet paring states we select several types of symmetries that are promising candidates for the superconducting states in UPt$_{3}$ and in Sr$_{2}$RuO$_{4}$. The calculated conductance spectra are sensitive to the orientation of the junction which reflects the anisotropy of the pairing states. They show either zero-bias conductance peaks or gap-like structures depending on the orientation of the junctions. The existence of a residual density of states, peculiar to nonunitary states, is shown to have a significant influence on the properties of the conductance spectra. Present results serve as a guidefor the experimental determination of the symmetry of the pair potentials in UPt$_{3}$ and Sr$_{2}$RuO$_{4}$.Comment: 10 pages, 11 eps figures, J.Phys.Soc.Jpn.67,No.9(1998

Cost Effectiveness of a CYP2C19 Genotype-Guided Strategy in Patients with Acute Myocardial Infarction:Results from the POPular Genetics Trial

INTRODUCTION: The POPular Genetics trial demonstrated that a CYP2C19 genotype-guided P2Y12 inhibitor strategy reduced bleeding rates compared with standard treatment with ticagrelor or prasugrel without increasing thrombotic event rates after primary percutaneous coronary intervention (PCI). OBJECTIVE: In this analysis, we aimed to evaluate the cost effectiveness of a genotype-guided strategy compared with standard treatment with ticagrelor or prasugrel. METHODS: A 1-year decision tree based on the POPular Genetics trial in combination with a lifelong Markov model was developed to compare costs and quality-adjusted life-years (QALYs) between a genotype-guided and a standard P2Y12 inhibitor strategy in patients with myocardial infarction undergoing primary PCI. The cost-effectiveness analysis was conducted from a Dutch healthcare system perspective. Within-trial survival and utility data were combined with lifetime projections to evaluate lifetime cost effectiveness for a cohort of 1000 patients. Costs and utilities were discounted at 4 and 1.5%, respectively, according to Dutch guidelines for health economic studies. Besides deterministic and probabilistic sensitivity analyses, several scenario analyses were also conducted (different time horizons, different discount rates, equal prices for P2Y12 inhibitors, and equal distribution of thrombotic events between the two strategies). RESULTS: Base-case analysis with a hypothetical cohort of 1000 subjects demonstrated 8.98 QALYs gained and €725,550.69 in cost savings for the genotype-guided strategy (dominant). The deterministic and probabilistic sensitivity analysis confirmed the robustness of the model and the cost-effectiveness results. In scenario analyses, the genotype-guided strategy remained dominant. CONCLUSION: In patients undergoing primary PCI, a CYP2C19 genotype-guided strategy compared with standard treatment with ticagrelor or prasugrel resulted in QALYs gained and cost savings. TRIAL REGISTRATION: Clinicaltrials.gov number: NCT01761786, Netherlands trial register number: NL2872

C-axis electronic Raman scattering in Bi_2Sr_2CaCu_2O_{8+\delta}

We report a c-axis-polarized electronic Raman scattering study of Bi_2Sr_2CaCu_2O_{8+\delta} single crystals. In the normal state, a resonant electronic continuum extends to 1.5 eV and gains significant intensity as the incoming photon energy increases. In the superconducting state, a coherence 2\Delta peak appears around 50 meV, with a suppression of the scattering intensity at frequencies below the peak position. The peak energy, which is higher than that seen with in-plane polarizations, signifies distinctly different dynamics of quasiparticle excitations created with out-of-plane polarization.Comment: 12 pages, REVTEX, 3 postscript figure

Selective inhibitors of the PSEN1-gamma-secretase complex

Clinical development of Y-secretases, a family of intramembrane cleaving proteases, as therapeutic targets for a variety of disorders including cancer and Alzheimer’s disease was aborted because of serious mechanism-based side effects in the phase III trials of unselective inhibitors. Selective inhibition of specific Y-secretase complexes, containing either PSEN1 or PSEN2 as the catalytic subunit and APH1A or APH1B as supporting subunits, does provide a feasible therapeutic window in preclinical models of these disorders. We explore here the pharmacophoric features required for PSEN1 versus PSEN2 selective inhibition. We synthesized a series of brain penetrant 2-azabicyclo[2,2,2]octane sulfonamides and identified a compound with low nanomolar potency and high selectivity (>250-fold) toward the PSEN1–APH1B subcomplex versus PSEN2 subcomplexes. We used modeling and site-directed mutagenesis to identify critical amino acids along the entry part of this inhibitor into the catalytic site of PSEN1. Specific targeting one of the different Y-secretase complexes might provide safer drugs in the future.The work was supported by an AIO-project (no. HBC.2016.0884). This project received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme (grant agreement no. ERC-834682 CELLPHASE_AD). This work was supported by the Flanders Institute for Biotechnology (VIB vzw), a Methusalem grant from KU Leuven and the Flemish Government, the Fonds voor Wetenschappelijk Onderzoek, KU Leuven, The Queen Elisabeth Medical Foundation for Neurosciences, the Opening the Future campaign of the Leuven Universitair Fonds, the Belgian Alzheimer Research Foundation (SAO-FRA), and the Alzheimer’s Association USA.Peer ReviewedPostprint (published version

Nesting properties and anisotropy of the Fermi surface of LuNi2_{2}B2_{2}C

The rare earth nickel borocarbides, with the generic formula $R$Ni$_{2}$B$_{2}$C, have recently been shown to display a rich variety of phenomena. Most striking has been the competition between, and even coexistence of, antiferromagnetism and superconductivity. We have measured the Fermi surface (FS) of LuNi$_{2}$B$_{2}$C, and shown that it possesses nesting features capable of explaining some of the phenomena experimentally observed. In particular, it had previously been conjectured that a particular sheet of FS is responsible for the modulated magnetic structures manifest in some of the series. We report the first direct experimental observation of this sheet.Comment: 4 pages, 4 PS figure