Archiv 2019

The normal state of unconventional superconductors often exhibits anomalous transport properties and it is commonly referred to as a “bad” or “strange” metal. Understanding its collective charge dynamics, which defies the standard quasiparticle description of a Fermi liquid, is an outstanding challenge of modern condensed matter physics.In this talk, I will present a direct measurement of the collective charge dynamics of the strange metal using inelastic electron scattering. First, I will discuss how normal-state Bi2Sr2CaCu2O8+d is defined by a featureless, localized continuum, undergoing a low-temperature massive spectral weight redistribution. I will then describe how such a phase is found to coexist with a low-energy Fermi liquid in Sr2RuO4.These results indicate that strange metals are highly localized in space and dissipate on ultrafast timescales, seemingly bound only by quantum limits. Implications for the occurrence of high-temperature superconductivity will be discussed. [mehr]

To understand the function of condensed matter, it would be desirable to directly watch its atomistic building blocks dynamically interact on their intrinsic length and time scales. Recently, lightwave electronics has made this long-standing dream come true. The idea is to exploit the carrier wave of light as an ultrafast, contact-free bias to interrogate and control the nanocosm. I will first review how lightwaves can drive electrons in solids into surprising sub-cycle quantum motion. By combining this idea with the sub-angstrom spatial resolution of scanning tunnelling microscopy we can set an ultrashort time window for single-electron tunnelling into a single orbital and record first atom-scale slow-motion movies of individual vibrating molecules. Finally, I will show how to directly exert femtosecond atomic forces, which can selectively choreograph a coherent structural motion of a single-molecule switch in its electronic ground state. This stunningly direct access to the atomistic world may tailor key elementary dynamics in nature and steer (bio)chemical reactions or ultrafast phase transitions, on their intrinsic spatio-temporal scales. [mehr]

“Quantum materials” loosely defines a broad collection of materials whose ground states are defined by unusual quantum properties. This research largely focuses on macroscopic single crystals, yet naturally interesting quantum phenomena lie beyond their equilibrium state. My group works towards reducing the sample size onto the sub-mm length scale, following the general idea that small samples can be driven more strongly and react faster than on the macro scale. Our main tool is Focused Ion Beam machining capable of cutting single crystals into high quality quantum devices. I will present two concrete research projects showcasing how new quantum states out of equilibrium can be accessed and investigated in FIB-prepared microcrystal structures. The first concerns the heavy fermion superconductor, CeIrIn5 (T_c~400mK). When a mm-sized structure is firmly coupled to a mm-sized substrate of different thermal expansion, the microstructure is under significant strain at low temperatures. By precisely controlling its shape, the emergent strain field can be controlled. The key difference to other approaches, such as uniaxial strain, is that complex, yet well-controlled, spatially varying strain fields can be achieved. In collaboration with Katja Nowack (Cornell), we have experimentally mapped out the resulting superconducting landscape in the devices using scanning-SQUID microscopy, and show that this spatial modulation can be well captured by finite element simulations. [1] Second, I will present our ongoing efforts to experimentally identify pseudo-magnetic fields in 3D Dirac semi-metals [2,3]. Owing to their Dirac dispersion, deformation of the crystal structure does not open a gap at the nodes, but shifts the location of the nodes in k-space and hence playing the role of a “pseudo-magnetic field”, B₅. I will show how microstructuring gives us unprecedented control of such a process, and discuss how future. [mehr]

Historically charge density waves have been associated with the notions of Fermi surface nesting and, at the transition temperature, a soft phonon mode. In this talk, I will present two cases that defy this common theme. First, I will show that TiSe2 undergoes a transition due to exciton condensation, which exhibits a soft mode of a different, electronic variety. Second, when driving the system away from equilibrium, the phase transition is mediated by topological defects. These defects allow for the formation of a charge density wave that does not occur in equilibrium. This light-induced charge density wave shows some unique properties that suggest that it is not just a trivial extension of an equilibrium one. [mehr]

Nonlinear optics has recently emerged as an attractive approach for both probing topological properties and driving Dirac materials into new states. Here, I will describe our use of ultrafast nonlinear optics to study three representative Dirac materials: graphene micro-ribbons, topological insulators, and Weyl semimetals. [mehr]

Although the principles of quantum optics have yielded multiple ideas to surpass the classical limitations in optical microscopy, their application in life science imaging has remained extremely challenging. In this talk, I will present two works that apply measurements of photon correlations for the benefit of localization microscopy and image scanning microscopy (ISM). The first uses photon antibunching measurement to estimate the number of emitters in a fluctuating scene and can potentially speed-up super-resolution techniques based on localization microscopy [1]. In the second work, we employ photon antibunching as the imaging contrast itself. Measuring the spatial distribution of ‘missing’ photon pairs in an ISM architecture may enhance lateral resolution four time beyond the diffraction limit [2]. The robustness of the antibunching signal enabled super-resolved imaging of fixed cells, relying solely on a quantum contrast. [mehr]