Archiv 2016

In this very informal talk I will first briefly present our view on superconductivity in alkali doped fullerenes. In particular, I will discuss how it is at all possible that M3C60 has an s-wave condensate despite the sizeable intra-molecular Coulomb repulsion relatively to the narrow molecular bandwidth. In the second part of the talk I will review what it is known about the MIR optical properties of doped fullerenes and try to build a link with the remarkable Hamburg observation of a large enhancement of Tc under intense THz pumping. [mehr]

There exists a size difference between molecules and visible wavelengths. Nanofabrication techniques can be adopted to obtain designed meta-molecules or meta-materials, with interesting optical properties. [mehr]

Complex quantum many-body systems are ubiquitous in nature, but their behaviour often remains very challenging to predict with analytical or numerical calculations - especially when it comes to dynamics. However, using ultracold atoms in optical lattices it is possible to create precisely tunable, yet very accessible artificial solids, which can be probed with a large arsenal of observables. Using this experimental set-up, we demonstrate how a periodically modulated lattice can be described by an effective Floquet-Hamiltonian on longer time scales - even when driving the system far from equilibrium. This allows for implementing Haldane's model for a topological insulator, and mapping out its topological transitions, by applying an oscillating force to a honeycomb lattice. Using an oscillating magnetic field gradient, we also engineer spin-dependent bands. By adding interactions to the optical lattice system, we create a pure realisation of the Hubbard model and study how the distribution of anti-ferromagnetic correlations therein depends on the geometry of the lattice. We investigate how fast correlations can re-arrange, by deforming the lattice geometry on time-scales ranging from the sudden to the adiabatic regime. Finally, we explore how an oscillating force applied to this interacting system may be used to tune and enhance the magnetic exchange energy beyond the regimes accessible within the Hubbard model. [mehr]

The physics of the “ultra-fast” and “ultra-small” are often closely linked, motivating experiments that access these extremes. In this talk, I will discuss the application of ultrashort light pulses, from THz to X-rays, to the study of dynamics and emergent correlations in quantum materials. [mehr]

When an exciton transition and a resonant optical mode exchange energy faster than any competing dissipation process, it can lead to light-matter strong coupling. This brings about interesting properties possessed by neither the original exciton nor the optical mode and leads to new possibilities such as enhanced conductivity of organic semiconductors. In this case, the enhancement stems from the delocalized nature of the hybrid states over the spatial extent of the optical mode which is also expected to affect energy transport according to recent theoretical studies. [mehr]

TiSe₂ is one of the simplest charge density wave (CDW) materials, forming a 2×2 superlattice below a transition temperature T_C = 200 K, but the origin of this phase is controversial. Its nearly inverted band structure led early authors to identify TiSe₂ as an “excitonic insulator,” which is an electronic instability involving spontaneous proliferation of excitons. The problem is that the CDW also exhibits a sizeable lattice distortion, leading later authors to identify it as a conventional Peierls phase. That said, an excitonic phase would also create an incidental lattice distortion, since the interaction with phonons can’t be switched off. The arguments on the matter have gone in circles for decades. [mehr]

A characteristic of ferroic materials is the emergence of a temporally static finite expectation value of an order parameter. Here, we introduce a new mechanism [1] for ferroic order, in which a non-zero quasi-static magnetoelectric quadrupolar order appears, mediated by a strong coupling of spin and phonon fluctuations. We show that our proposed mechanism is consistent, to our knowledge, with many experimental observations for the onset of the pseudo-gap phase in cuprate superconductors and therefore propose the quasi-static magnetoelectric quadrupole as a possible pseudo-gap order parameter. By using first-principles calculations in combination with our recent developed formalism [2,3], to calculate multipole moments within a Berry phase approach, we calculate the magnitude of the effect for the the prototypical cuprate superconductor, HgBa2CuO4+δ. Using these results we finally show that our mechanism embraces several key findings of experimental reports and in addition also aspects of previous theoretical models. [mehr]

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