In the first part of the talk, I will discuss the effects of short- range interactions in generalized Weyl semimetals with a monopole charge (n) greater than one. I will show that a strong enough short range interaction may lead to the onset of a translational symmetry breaking axion insulator or a rotational symmetry breaking gapless nematic state . To address this problem, I will use a new renormalization group scheme in which the monopole charge is an expansion parameter. The computed correlation length exponent ν=n/2, and therefore generalized interacting Weyl semimetals with n>1 provide rare examples of a non-Gaussian itinerant quantum criticality in three dimensions. I will discuss experimental signatures of the symmetry breaking phases in transport and thermodynamic responses. The second part of the talk is devoted to the role of the long-range Coulomb interaction in a simple (n=1) Weyl semimetal. In particular, I will show that this interaction leads to a universal enhancement of the zero-temperature optical conductivity that depends solely on the number of Weyl points at the Fermi level . This scaling is a remarkable consequence of an interplay between the quantum-critical nature of an interacting Weyl liquid, marginal irrelevance of the long-range Coulomb interaction and the violation of hyperscaling in three dimensions, and is directly measurable in recently discovered Weyl and Dirac materials.  B. Roy, P. Goswami, and V. Juricic, Phys. Rev. B 95, 201102 (R) (2017).  B. Roy and V. Juricic, Phys. Rev. B 96, 155117 (2017).
Classical fluorescence microscopy is limited in resolution by the wavelength of light (diffraction limit) restricting lateral resolution to ca. 200 nm, and axial resolution to ca. 500 nm (at typical excitation and emission wavelengths around 500 nm). However, recent years have seen a tremendous development in high- and super-resolution techniques of fluorescence microscopy, pushing spatial resolution to its diffraction-dictated limits and much beyond. One of these techniques is Image Scanning Microscopy (ISM). In ISM, the focus of a conventional laser-scanning confocal microscope (LCSM) is scanned over the sample, but instead of recording only the total fluorescence intensity for each scan position, as done in conventional operation of an LCSM, one records a small image of the illuminated region. The result is a four-dimensional stack of data: two dimensions refer to the lateral scan position, and two dimensions to the pixel position on the chip of the image-recording camera. This set of data can then be used to obtain a super-resolved image with doubled resolution, completely analogously to what is achieved with Structured Illumination Microscopy. However, ISM is conceptually and technically much simpler, suffers less from sample imperfections like refractive index variations, and can easily be implemented into any existing LSCM. I will also present recent results of combining ISM with two-photon excitation, which is important for deep-tissue imaging of e.g. neuronal tissue, and for performing non-linear coherent microscopy such as second-harmonic generation. A second method which I will present is concerned with achieving nanometer resolution along the optical axis. It is called Metal Induced Energy Transfer or MIET and is based on the fact that, when placing a fluorescent molecule close to a metal, its fluorescence properties change dramatically. In particular, one observes a strongly modified lifetime of its excited state (Purcell effect). This coupling between an excited emitter and a metal film is strongly dependent on the emitter’s distance from the metal. We have used this effect for mapping the basal membrane of live cells with an axial accuracy of ~3 nm. The method is easy to implement and does not require any change to a conventional fluorescence lifetime microscope; it can be applied to any biological system of interest, and is compatible with most other super-resolution microscopy techniques which enhance the lateral resolution of imaging. Moreover, it is even applicable to localizing individual molecules, thus offering the prospect of three-dimensional single-molecule localization microscopy with nanometer isotropic resolution for structural biology.
One hundred and fifty years ago, Maxwell first posed the thought experiment that become known as “Maxwell’s demon.” Designed to understand more deeply the nature of the newly formulated second law of thermodynamics, the demon was to play a long, controversial role in the development of statistical physics. Just two months later, Maxwell’s paper “On governors” gave the first analysis of a feedback system. These two foundational works reflect the fundamental and practical aspects of control. I will present an experiment that unites the two: using feedback to create “impossible” dynamics, we make a Maxwell demon that can reach the fundamental limits to control set by thermodynamics. We test—and then extend—Rolf Landauer’s 1961 prediction that information erasure requires at least as much work as can be extracted from a system by virtue of information. These fundamental thermodynamic limits are benchmarks for evaluating the performance of practical information engines, such as those active within cells and other complex systems.