T−T-linear resistivity from magneto-elastic scattering: application to PdCrO2_2

An electronic solid with itinerant carriers and localized magnetic moments represents a paradigmatic strongly correlated system. The electrical transport properties associated with the itinerant carriers, as they scatter off these local moments, has been scrutinized across a number of materials. Here we analyze the transport characteristics associated with ultra-clean PdCrO$_2$ -- a quasi two-dimensional material consisting of alternating layers of itinerant Pd-electrons and Mott-insulating CrO$_2$ layers -- which shows a pronounced regime of $T-$linear resistivity over a wide-range of intermediate temperatures. By contrasting these observations to the transport properties in a closely related material PdCoO$_2$, where the CoO$_2$ layers are band-insulators, we can rule out the traditional electron-phonon interactions as being responsible for this interesting regime. We propose a previously ignored electron-magnetoelastic interaction between the Pd-electrons, the Cr local-moments and an out-of-plane phonon as the main scattering mechanism that leads to the significant enhancement of resistivity and a $T-$linear regime in PdCrO$_2$ at temperatures far in excess of the magnetic ordering temperature. We suggest a number of future experiments to confirm this picture in PdCrO$_2$, as well as other layered metallic/Mott-insulating materials.Comment: 7 pages, 3 figures. Supplementary material: 10 pages, 3 figure

Investigation of Planckian behavior in a high-conductivity oxide : PdCrO2

Funding: JFMV and DC are supported by faculty startup grants at Cornell University. ET and EB were supported by the European Research Council (ERC) under grant HQMAT (Grant Agreement No. 817799), the Israel-US Binational Science Foundation (BSF), and the Minerva Foundation.The layered delafossite metal PdCrO2 is a natural heterostructure ofhighly conductive Pd layers Kondo coupled to localized spins in the adjacentMott insulating CrO2 layers. At high temperatures T it has a T-linearresistivity which is not seen in the isostructural but non-magnetic PdCoO2.The strength of the Kondo coupling is known, as-grown crystals are extremelyhigh purity and the Fermi surface is both very simple and experimentally known.It is therefore an ideal material platform in which to investigate 'Planckianmetal' physics. We do this by means of controlled introduction of pointdisorder, measurement of the thermal conductivity and Lorenz ratio and studyingthe sources of its high temperature entropy. The T-linear resistivity is seento be due mainly to elastic scattering and to arise from a sum of severalscattering mechanisms. Remarkably, this sum leads to a scattering rate within10% of the Planckian value of kBT/ℏ.Publisher PDFPeer reviewe

Directional ballistic transport in the two-dimensional metal PdCoO2

This project was supported by the Max Planck Society and the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (MiTopMat, grant agreement no. 715730). M.D.B. and P.H.M. acknowledge EPSRC for PhD studentship support through grant number EP/L015110/1. Research in Dresden benefits from the environment of the Excellence Cluster ct.qmat. A.S. acknowledges support from an ARCS Foundation Fellowship, a Ford Foundation Predoctoral Fellowship and a National Science Foundation Graduate Research Fellowship. A.S. would thanks Z. Gomez and E. Huang for helpful discussions and T. Devereaux for letting us use his group cluster. Computational work was performed on the Sherlock cluster at Stanford University and on resources of the National Energy Research Scientific Computing Center, supported by the DOE under contract DE_AC02-05CH11231. T.S. acknowledges support from the Emergent Phenomena in Quantum Systems initiative of the Gordon and Betty Moore Foundation, and from the Natural Sciences and Engineering Research Council of Canada (NSERC), in particular the Discovery Grant (RGPIN-2020-05842), Accelerator Supplement (RGPAS-2020-00060) and Discovery Launch Supplement (DGECR-2020-00222). T.S. contributed to this work prior to joining AWS. D.G.-G.’s and A.W.B.’s involvement in calculations was supported by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC02-76SF00515. E.Z. and M.M. thank the International Max Planck Research School for Chemistry and Physics of Quantum Materials (IMPRS-CPQM) for financial support. G.B. and D.A.B. acknowledge support from the Natural Sciences and Engineering Research Council of Canada (NSERC Discovery Grant RGPIN-2018-04280) and from the Canada First Research Excellence Fund.In an idealized infinite crystal, the material properties are constrained by the symmetries of the unit cell. The point-group symmetry is broken by the sample shape of any finite crystal, but this is commonly unobservable in macroscopic metals. To sense the shape-induced symmetry lowering in such metals, long-lived bulk states originating from an anisotropic Fermi surface are needed. Here we show how a strongly facetted Fermi surface and the long quasiparticle mean free path present in microstructures of PdCoO2 yield an in-plane resistivity anisotropy that is forbidden by symmetry on an infinite hexagonal lattice. We fabricate bar-shaped transport devices narrower than the mean free path from single crystals using focused ion beam milling, such that the ballistic charge carriers at low temperatures frequently collide with both of the side walls that define the channel. Two symmetry-forbidden transport signatures appear: the in-plane resistivity anisotropy exceeds a factor of 2, and a transverse voltage appears in zero magnetic field. Using ballistic Monte Carlo simulations and a numerical solution of the Boltzmann equation, we identify the orientation of the narrow channel as the source of symmetry breaking.Publisher PDFPeer reviewe

Investigation of transport regimes in restricted geometries of ultra-pure natural heterostructures

This thesis describes investigations into the origins of the unconventional electrical transport of the non-magnetic delafossite metals PtCoO₂ and PdCoO₂ and the magnetic delafossite metal PdCrO₂ using focused ion beam microstructuring techniques. These compounds are among the highest conductivity materials known, with an extreme purity of up to 1 defect in 120,000 atoms. This remarkable purity, together with the hexagonal Fermi surface, opens the possibility of studying novel regimes of mesoscopic physics. This work is split into two parts. In the first part, I will review the key properties of non-magnetic delafossite metals and the application of focused ion beam microstructuring to transport measurements within low resistivity materials. The related experimental chapter describes an investigation which uses the high energy electron irradiation investigation to probe the effects of a non-circular Fermi surface on the transport within bars and four-terminal, square-shaped junctions inside the ballistic regime. The other studies were concentrated on the magnetic delafossite metal PdCrO₂. I will describe a new method of microstructure preparation which was created for PdCrO₂ transport studies but is widely applicable to other materials. This material obeys the Planckian bound at a wide range of temperatures between 200 K and 500 K. The accompanying experimental chapter details an investigation by high energy electron irradiation of the origin of this behaviour. The new method of mounting microstructures also allows, for the first time, the study of studying unconventional transport regimes in PdCrO₂

Data underpinning Elina Zhakina's thesis

The data files are embargoed until 07/02/202

Low-symmetry nonlocal transport in microstructured squares of delafossite metals

Funding: Max Planck Society. Engineering and Physical Science Research Council PhD studentship support via grant EP/L015110/1 (PHM,MDB). Deutsche Forschungsgemeinschaft Cluster of Excellence ct.qmat EXC 2147, project-id 390858490. European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (MiTopMat - grant agreement No 715730) (PJWM,CP).Intense work studying the ballistic regime of electron transport in two-dimensional systems based on semiconductors and graphene had been thought to have established most of the key experimental facts of the field. In recent years, however, additional forms of ballistic transport have become accessible in the quasi–two-dimensional delafossite metals, whose Fermi wavelength is a factor of 100 shorter than those typically studied in the previous work and whose Fermi surfaces are nearly hexagonal in shape and therefore strongly faceted. This has some profound consequences for results obtained from the classic ballistic transport experiment of studying bend and Hall resistances in mesoscopic squares fabricated from delafossite single crystals. We observe pronounced anisotropies in bend resistances and even a Hall voltage that is strongly asymmetric in magnetic field. Although some of our observations are nonintuitive at first sight, we show that they can be understood within a nonlocal Landauer-Büttiker analysis tailored to the symmetries of the square/hexagonal geometries of our combined device/Fermi surface system. Signatures of nonlocal transport can be resolved for squares of linear dimension of nearly 100 µm, approximately a factor of 15 larger than the bulk mean free path of the crystal from which the device was fabricated.Publisher PDFPeer reviewe