PILLAR: Electron dynamics in laterally confined quasi-two-dimensional metals

• Applicant: Professor Dr. Philip Moll
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 501654252

Strongly layered materials have offered a wealth of exciting physical phenomena andnovel applications. These quasi-2D crystals (q2D) are defined by strong in-plane bondsand weak out-of-plane bonding. Exfoliation leads to often atomically thin flakes withelectronic, magnetic, optical and chemical properties that are strikingly different fromthe bulk. The key to their novel behavior is the reduction of the effective dimensionalityto the two in-plane dimensions. The most prominent examples of the concept ofexfoliating are undoubtedly graphite/graphene and the transition-metaldichalcogenides (TMD).If a crystal in three dimensions can be mechanically shaped to behave akin to a 2Dsystem, it must also be possible to generate quasi-1D states from it (3D = q2D + q1D).We call this limit "pillar", an infinite stack of mesoscopic (<1 micron) q2D flakes. Justlike their more familiar q2D cousins, dimensional reduction is expected to lead toelectronic behavior that is strikingly different to the bulk. Each plane of the pillar canbe viewed as an individual island that is weakly coupled to its upper and lowerneighbors, hence forming a new kind of q1D island chain. Here we propose to realizedand explore this state, by breaking the strong bonds while preserving the weak ones.While q2D sheets are easily prepared by exfoliation, pillars oppose the natural bondanisotropy and hence are difficult to obtain by conventional methods. We propose touse Focused Ion Beam machining as a gentle kinetic technique to carve such pillarsfrom bulk crystals.We plan to explore three main scientific topics based on the unique properties of q1Dpillars:1) A new regime of quantum transport in mesoscopic pillars in whichin-plane standing waves are coherently transported between the stacked sheets. Itshallmark is a novel magnetoconductance oscillation akin to the Aharonov-Bohm-effect,which we recently demonstrated in FIB-carved pillars of PdCoO2. Is this a unique caseof this delafossite metal, or a general property? Does it survive strong correlations and interact with superconductivity, e.g.in Sr2RuO4?2) Bloch-oscillations have never been observed (yet) in bulk crystalline metals. Wepropose that Landau-quantization in pillars induces shallow (flat) Landau-bands,which are ideally suited to realize Bloch oscillations. This will both be tackled viadc-transport and by building optical micro-resonators. If successful, it woulddemonstrate a field-tunable, solid-state frequency source in the THz range.3) Extreme conductivity anisotropy in layered metals provides a new approach to thetechnologically relevant field of transparent conductors. We plan to fabricate uniquepillar and slab structures of Sr2RuO4 with their atomic-interlayer direction in theplane of the substrate. Such material should be an excellent in-plane conductor whiletransmitting visible light when polarized perpendicular to the planes.

• DFG Programme: Research Grants