Out-of-plane transport in ZrSiS and ZrSiSe microstructures

A recent class of topological nodal-line semimetals with the general formula MSiX (M = Zr, Hf and X = S, Se, Te) has attracted much experimental and theoretical interest due to their properties, particularly their large magnetoresistances and high carrier mobilities. The plateletlike nature of the MSiX crystals and their extremely low residual resistivities make measurements of the resistivity along the [001] direction extremely challenging. To accomplish such measurements, microstructures of single crystals were prepared using focused ion beam techniques. Microstructures prepared in this manner have very well-defined geometries and maintain their high crystal quality, verified by the observations of quantum oscillations. We present magnetoresistance and quantum oscillation data for currents applied along both [001] and [100] in ZrSiS and ZrSiSe, which are consistent with the nontrivial topology of the Dirac line-node, as determined by a measured pi Berry phase. Surprisingly, we find that, despite the three dimensional nature of both the Fermi surfaces of ZrSiS and ZrSiSe, both the resistivity anisotropy under applied magnetic fields and the in-plane angular dependent magnetoresistance differ considerably between the two compounds. Finally, we discuss the role microstructuring can play in the study of these materials and our ability to make these microstructures free-standing. (C) 2019 Author(s).

Scaling between magnetic field and temperature in the high-temperature superconductor BaFe2(As1−xP x )2

Philip Johannes Walter Moll

Many exotic metallic systems have a resistivity that varies linearly with temperature, and the physics behind this is thought to be connected to high-temperature superconductivity in the cuprates and iron pnictides1,2,3,4,5,6,7,8,9. Although this phenomenon has attracted considerable attention, it is unclear how the relevant physics manifests in other transport properties, for example their response to an applied magnetic field. We report measurements of the high-field magnetoresistance of the iron pnictide superconductor BaFe2(As1−xPx)2 and find that it obeys an unusual scaling relationship between applied magnetic field and temperature, with a conversion factor given simply by the ratio of the Bohr magneton and the Boltzmann constant. This suggests that magnetic fields probe the same physics that gives rise to the T-linear resistivity, providing a new experimental clue to this long-standing puzzle.

2016

Sr2Pt8−x As: a layered incommensurately modulated metal with saturated resistivity

Alla Arakcheeva, Gabriel Albert Autes, Maja Deborah Bachmann, László Forró, Sergiy Katrych, Edoardo Martino, Philip Johannes Walter Moll, Andrea Pisoni, Oleg Yazyev

The high-pressure synthesis and incommensurately modulated structure are reported for the new compound Sr2Pt8-xAs, with x = 0.715 (5). The structure consists of Sr2Pt3 As layers alternating with Pt-only corrugated grids. Ab initio calculations predict a metallic character with a dominant role of the Pt d electrons. The electrical resistivity (rho) and Seebeck coefficient confirm the metallic character, but surprisingly, rho showed a near-flat temperature dependence. This observation fits the description of the Mooij correlation for electrical resistivity in disordered metals, originally developed for statistically distributed point defects. The discussed material has a long-range crystallographic order, but the high concentration of Pt vacancies, incommensurately ordered, strongly influences the electronic conduction properties. This result extends the range of validity of the Mooij correlation to long-range ordered incommensurately modulated vacancies. Motivated by the layered structure, the resistivity anisotropy was measured in a focused-ion-beam micro-fabricated well oriented single crystal. A low resistivity anisotropy indicates that the layers are electrically coupled and conduction channels along different directions are intermixed.

2018

Super-geometric electron focusing on the hexagonal Fermi surface of PdCoO2

Maja Deborah Bachmann, Philip Johannes Walter Moll, Matthias Carsten Putzke

Geometric electron optics may be implemented in solids when electron transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a single crystal of PdCoO2 by use of focused ion beam (FIB) micro-machining, defining crystalline ballistic circuits with features as small as 250 nm. The peculiar hexagonal Fermi surface naturally leads to enhanced electron self-focusing effects in a magnetic field compared to circular Fermi surfaces. This super-geometric focusing can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material. These results suggest a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermi surfaces.

2019