Universal collisionless transport of graphene

The impact of the electron-electron Coulomb interaction on the optical conductivity of graphene has led to a controversy that calls into question the universality of collisionless transport in this and other Dirac materials. Using a lattice calculation that avoids divergences present in previous nodal Dirac approaches, our work settles this controversy and obtains results in quantitative agreement with experiment over a wide frequency range. We also demonstrate that dimensional regularization methods agree, as long as the scaling properties of the conductivity and the regularization of the theory in modified dimension are correctly implemented. Tight-binding lattice and nodal Dirac theory calculations are shown to coincide at low energies even when the non-zero size of the atomic orbital wave function is included, conclusively demonstrating the universality of the optical conductivity of graphene.Comment: 4+ pages,4 figures; includes Supplemental Material (18 pages, 2 figures

Elastic response of the electron fluid in intrinsic graphene: The collisionless regime

The elastic response of an electron fluid at finite frequencies is defined by the electron viscosity $\eta(\omega)$. We determine $\eta(\omega)$ for graphene at the charge neutrality point in the collisionless regime, including the leading corrections due to the electron-electron Coulomb interaction. We find interaction corrections to $\eta(\omega)$ that are significantly larger if compared to the corresponding corrections to the optical conductivity. In addition, we find comparable contributions to the dynamic momentum flux due to single-particle and many-particle effects. We also demonstrate that $\eta(\omega)$ is directly related to the nonlocal energy-flow response of graphene at the Dirac point. The viscosity in the collisionless regime is determined with the help of the strain generators in the Kubo formalism. Here, the pseudo-spin of graphene describing its two sublattices plays an important role in obtaining a viscosity tensor that fulfills the symmetry properties of a rotationally symmetric system.Comment: 18 pages, 5 figure

Complete zero-energy flat bands of surface states in fully gapped chiral noncentrosymmetric superconductors

Noncentrosymmetric superconductors can support flat bands of zero-energy surface states in part of their surface Brillouin zone. This requires that they obey time-reversal symmetry and have a sufficiently strong triplet-to-singlet-pairing ratio to exhibit nodal lines in the bulk. These bands are protected by a winding number that relies on chiral symmetry, which is realized as the product of time-reversal and particle-hole symmetry. We reveal a way to stabilize a flat band in the entire surface Brillouin zone, while the bulk dispersion is fully gapped. This idea could lead to a robust platform for quantum computation and represents an alternative route to strongly correlated flat bands in two dimensions, besides twisted bilayer graphene. The necessary ingredient is an additional spin-rotation symmetry that forces the direction of the spin-orbit-coupling vector not to depend on the momentum component normal to the surface. We define a winding number which leads to flat zero-energy surface bands due to bulk-boundary correspondence. We discuss under which conditions this winding number is nonzero in the entire surface Brillouin zone and verify the occurrence of zero-energy surface states by exact numerical diagonalization of the Bogoliubov-de Gennes Hamiltonian for a slab. In addition, we consider how a weak breaking of the additional symmetry affects the surface band, employing first-order perturbation theory and a quasiclassical approximation. We find that the surface states and the bulk gap persist for weak breaking of the additional symmetry but that the band does not remain perfectly flat. The broadening of the band strongly depends on the deviation of the spin-orbit-coupling vector from its unperturbed direction as well as on the spin-orbit-coupling strength and the triplet-pairing amplitude.Comment: 18 pages, 6 figure

Dielectric monitoring of the PAN fiber stabilization process

[EN] The production of carbon fibers based on Polyacrylonitrile (PAN) precursor fibers is a very energy intensive process. In order to increase the energy efficiency, microwave plasma processes and, alternatively, dielectric heating with microwaves are in the focus. In this latter case, the knowledge of the dielectric properties over temperature is the key in the design of an appropriate microwave heating system. A measurement system was developed that is capable to monitor in-situ the change of the dielectric properties during the stabilization process of the PAN fiber. First results show that the loss tangent of the PAN fiber is strongly varying with temperature. Additionally, the dielectric properties are changing during the stabilization process due to the chemical transformation.The authors acknowledge the financial support by the Federal Ministry for Economic Affairs and Energy of Germany in the project REINFORCE (project number ZF4204603SY7) and the DSC measurements done by the project partner KCTECH.Hofele, J.; Jung, M.; Link, G.; Jelonnek, J. (2019). Dielectric monitoring of the PAN fiber stabilization process. En AMPERE 2019. 17th International Conference on Microwave and High Frequency Heating. Editorial Universitat Politècnica de València. 365-372. https://doi.org/10.4995/AMPERE2019.2019.9788OCS36537

Are Elevated Levels of IGF-1 Caused by Coronary Arteriesoclerosis?: Molecular and Clinical Analysis

The importance of insulin-like growth factor-1 (IGF-1) in coronary artery disease (CAD) due to wide range of its biological effects and its therapeutic potential, has already been described. Our aim was to evaluate possible influence of IGF-1 serum level changes on coronary atherosclerosis. In case of existence of such association our further aim was to verify and explain this phenomenon by examination of promoter P1 of IGF-1gene and receptor gene for IGF-1. The study was performed in 101 consecutive patients undergo for routine coronary angiography. Quantitative and qualitative assessment of coronary atherosclerosis was performed respectively by estimation of the number of culprit lesions in coronary arteries and by Gensini score calculation. IGF-1, IGFBP3 and plasma lipoproteins were measured in all patients. In addition, we evaluated DNA from 101 patients, isolated from blood cells, which was amplified by using PCR with sophisticated primers for P1 promoter of IGF-1 gene and IGF-1 receptor gene, then analyzed utilizing SSCP technique and automatically sequenced. We observed significant increase of serum IGF-1 levels in patients with “3 vessel disease” and with high score in Gensini scale when compared to those without any narrowing lesions in coronary arteries and 0 Gensini score (in group with 3 vessel disease 215.0 ± 71.3 versuss 176.7 ± 34.2 ng/ml p = 0.04 and with high Gensini score 231.4 ± 59.3 versus 181.0 ± 37.8 ng/ml p = 0.01).We found different genotypes for five P1 promoter polymorphisms of IGF-1 gene (RS35767, RS5742612, RS228837, RS11829693, RS17879774). There were no significant associations between the observed single nucleotide polymorphism (SNP) and coronary atherosclerosis nor with levels of circulating IGF-1. We found no structural polymorphism in receptor gene for IGF-1 nor in its extracellular domain(exon 2–4) nor in internal domain (exon 16–21). The effect of increased IGF-1 serum level in our study was probably independent from structural polymorphism in promoter P1 for IGF-1 or in receptor gene for IGF-1