Quantum oscillation studies in the unconventional superconductor YFe₂Ge₂ and in the Dirac semimetal candidates NbXSb (X = Ge/Si)

We have performed experiments to probe the ground state dynamics of YFe₂Ge₂, NbGeSb and NbSiSb. An unusually large Sommerfeld coefficient (γ ~ 100 mJmol⁻¹K⁻²) has been observed in the d-electron system YFe₂Ge₂. It also shows an anomalous power law temperature dependence of the electrical resistivity (ρ = ρ0 + AT³⁄²) which indicates Fermi liquid breakdown, possibly connected to its vicinity to a quantum critical point. The materials NbXSb (X = Ge/Si) have been theoretically predicted via initial density functional theory (DFT) calculations to harbour Dirac/Weyl-like features in their electronic band structure. Quantum oscillations were measured in the range 5-18 T using the de Haas-van Alphen effect. The observed quantum oscillation frequencies have been compared to DFT calculations of the electronic structure. Several Fermi surface sheets predicted by DFT have been observed and their quasiparticle masses have been measured. The context of these measurements with regards to the electronic structure in YFe₂Ge₂ has been discussed. It was found that the quasiparticle effective mass enhancement of the detected Fermi surface sheets can account for only ~40% of the mass enhancement previously observed from specific heat measurements. Quantum oscillations were measured over the range 3-18 T using the Shubnikov-de Haas effect in NbXSb. Many frequencies have been observed in NbSiSb which also shows an extremely large magnetoresistance. We have considered the origin of parts of the quantum oscillation spectrum at high fields and briefly discussed this in relation to magnetic interactions and magnetic breakdown. One clear frequency has been seen in NbGeSb, which also shows an unusual negative magnetoresistance at low fields. Quantum oscillation frequencies in both materials have been compared to DFT calculations. The Berry phase has been estimated for the observed Fermi surface sheets in both materials, suggesting a topological nature in these materials

Modular thermal Hall effect measurement setup for fast-turnaround screening of materials over wide temperature range using capacitive thermometry

We demonstrate a simple and easy-to-build probe designed to be loaded into a widely available Quantum Design Physical Properties Measurement System (PPMS) cryostat, with a detachable shielded sample puck section and robust heat sinking of three pairs of coaxial cables. It can be in principle used with any low-temperature cryostat. Our modular puck design has a radiation shield for thermal isolation and protection of the delicate sample space while handling and allows any variety of experimental setup benefiting from shielded coaxial wiring to be constructed on a selection of sample pucks. Pucks can be quickly and easily switched, and the system makes use of the simple yet extremely stable temperature and magnetic field control of the easy-to-use PPMS system. We focus on a setup designed for measurements of the thermal Hall effect and show that this system can yield unprecedented resolution over a wide temperature range and with rapid sample mounting or changing—allowing a large collection of potential samples to be screened for this novel physics. Our design aims to make these sensitive but challenging measurements quick, reliable, cheap, and accessible, through the use of a standard, widespread base cryostat and a system of modular removable sample stage pucks to allow quick turnaround and screening of a large number of candidate samples for potential new thermal Hall physics

Supporting data for 'Fermi surface and mass renormalization in the iron-based superconductor YFe2Ge2'

This deposit holds all the data underlying the figures shown in the linked publication. Please see readme file for detail

Research data supporting "Unconventional bulk superconductivity in YFe2Ge2 single crystals"

Low temperature heat capacity, electrical transport and magnetisation data obtained on high quality crystals of YFe2Ge2, showing the dependence of crystalline quality on the growth method and demonstrating the influence of disorder level on the superconducting transition

Unconventional Bulk Superconductivity in YFe_{2}Ge_{2} Single Crystals.

Sharp superconducting transition anomalies observed in a new generation of single crystals establish that bulk superconductivity is intrinsic to high purity YFe_{2}Ge_{2}. Low temperature heat capacity measurements suggest a disorder and field dependent residual Sommerfeld coefficient, consistent with disorder-induced in-gap states as expected for a sign-changing order parameter. The sevenfold reduction in disorder scattering in these new crystals to residual resistivities ≃0.45  μΩ cm was achieved using a new liquid transport growth technique, paving the way for multiprobe experiments investigating the normal and superconducting states of YFe_{2}Ge_{2}

Fermi Surface and Mass Renormalization in the Iron-Based Superconductor YFe_{2}Ge_{2}.

Interaction-enhanced carrier masses are central to the phenomenology of iron-based superconductors. Quantum oscillation measurements in the new unconventional superconductor YFe_{2}Ge_{2} resolve all four Fermi surface pockets expected from band structure calculations, which predict an electron pocket in the Brillouin zone corner and three hole pockets enveloping the centers of the top and bottom of the Brillouin zone. Carrier masses reach up to 20 times the bare electron mass and are among the highest ever observed in any iron-based material, accounting for the enhanced heat capacity Sommerfeld coefficient ≃100  mJ/mol K^{2}. Mass renormalization is uniform across reciprocal space, suggesting predominantly local correlations, as in the Hund's metal scenario

The influence of depth, site exposure and season on the intensity of iceberg scouring in nearshore Antarctic waters

Ice scour disturbance has a significant effect on the physical and biological characteristics of polar benthos. A series of grids, each consisting of 25 markers, were deployed along depth transects and replicated at two contrasting study sites at Adelaide Island, West Antarctic Peninsula. Markers were surveyed and replaced every 3 months for 2 years in order to assess the frequency and intensity of iceberg impacts. Depth, site, season and year were all highly significant factors influencing ice scouring frequency. We observed a high variation in the duration of winter fast ice between sites and years, which had a marked effect on ice scouring frequency. The ecological effects of the disturbance regime are likely to include depth zonation of benthic assemblages, patchiness of communities at varying stages of recovery and the near denudation of sessile fauna in the shallow subtidal