Nearly free electrons in a 5d delafossite oxide metal

Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit–assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO2. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μΩ-cm), establishing PtCoO2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along kz. Despite being predominantly composed of d-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14me. Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below EF, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO2 as a model nearly-free–electron system in a 5d delafossite transition-metal oxide.

Pressure-tuned Transport Properties of Novel Iron-based Superconductors and Topological Insulators

Andrea Pisoni

The majority of interactions in solids strongly depend on the interatomic distances. The application of pressure changes the lattice parameters and modifies the electronic and the phononic energy spectra of a material avoiding some of the undesirable effects induced by chemical doping, like lattice disorder, impurity phases and phase separations. Layered materials are particularly affected by the application of pressure because their anisotropic structure eases the contraction of the lattice along specific crystallographic directions. In this thesis I study the effect of pressure on the transport properties of two new layered materials: L4Fe2As2Te(1-x)O4 (L = Pr, Sm, Gd) superconductors and beta-Bi4I4 topological insulator. The discovery of Fe-based superconductors (Fe-SCs) provided a new material base for studying the mechanism of high temperature superconductivity. These materials present unexpectedly high critical temperatures (Tc ) despite the presence of magnetic atoms. The parent compounds of Fe-SCs are semimetals with an antiferromagnetic ground state in which the itinerant electrons form a periodic modulation of spin density called spin density wave (SDW). By changing the structural properties or by adding/removing carriers the SDW order can be suppressed and superconductivity emerges. Our group recently succeeded in synthesizing a new family of Fe-SCs that presents Tc up to 45 K upon optimum chemical doping and substitution. I performed a systematic study of the upper critical field and the pressure-dependent electrical resistivity of single crystals of Pr4Fe2As2Te(1-x)O4 and Sm4Fe2As2Te(1-x)O(4-y)Fy . Hall coefficient and magnetoresistance of Pr4Fe2As2Te(1-x)O4 revealed electrons as the dominant type of charge carriers. The results can be successfully fitted by a two-band model that allows the estimation of the temperature dependent electron and hole densities and mobilities. In view of the exceptionally high structural anisotropy of this new class of Fe-SCs I investigated the electronic anisotropy of Sm4Fe2As2Te(1-x)O(4-y)Fy by means of microfabrication techniques. The results show a ratio between the out-of-plane and in-plane electrical resistivities that is the highest reported so far in Fe-SCs. Topological insulators (TIs) form another class of materials that is currently attracting a lot of attention both for fundamental physics and potential applications. In TIs metallic surface states coexist with an insulating bulk. Most of the TIs known so far are either three-dimensional strongly bonded bulk materials or layered van der Waals crystals. Beta-Bi4I4 is a material composed of quasi-one-dimensional molecular chains that was theoretically predicted, and only recently experimentally verified, to be a novel strong topological insulator. Due to its particular structure, beta-Bi4I4 offers the possibility to study the interplay between different ground states like topological insulating phase, charge-density-wave and superconductivity. Here I present, for the first time, a study of the thermoelectric properties of beta-Bi4I4 single crystals. Electrical resistivity, Seebeck coefficient, thermal conductivity and Hall coefficient measurements reveal a possible charge density-wave-order (CDW) at low temperature. According to resistivity and thermoelectric power measurements, pressure suppresses the CDW order. Resistivity measurements performed in a diamond anvil cell up to 20 GPa demonstrate that superconductivity is induced in beta-Bi4I4 for pressure above 10 GPa. Preliminary X-ray diffraction measurements by synchrotron radiation, performed on the sample recovered at ambient pressure after the experiment, reveal that a new Bi-I phase is formed at high pressure. The possibility that the topological insulator state could coexist with superconductivity in beta-Bi4I4 is certainly a fascinating option but it remains for now an open question.

EPFL2016

Properties of La2-xSrxCuO4 under epitaxial strain

Dominique Cloëtta

The subject of this thesis is the growth and analysis of high temperature superconductor (HTSC) films and the study of their electronic structure and properties. In particular, the effect of epitaxial strain is investigated, predominantly by means of in-situ angle resolved photoemission spectroscopy (ARPES), as well as X-ray diffraction, resistivity and susceptibility measurements. In order to achieve that goal we have developed a unique experimental set-up at the Synchrotron Radiation Center (University of Wisconsin), consisting in a dedicated pulsed laser deposition system, which enables us to grow thin films with excellent surface quality. Our sample transfer procedures assure that this surface quality is not affected on the way to the ARPES analyzer, which is connected to a beamline. We have built a similar film growth system at the EPFL with the aim to connect it to the SCIENTA analyzer at the IPN and to an analyzer on a beamline at the SLS in Villigen. For the first time we were able to perform ARPES measurements on in-situ grown films of HTSC. Previously to our work all the ARPES measurements were carried out on cleaved or scraped samples, predominantly single crystals of Bi2Sr2CaCu2O8 compounds. Thin films offer the possibility to study the effect of epitaxial strain induced through lattice mismatch between the film and its substrate. Compressive strain in the CuO2 plane has been known to enhance the critical temperature (TC) up to 50%, therefore we expected to see a signature of strain in the electronic dispersion. The Fermi surface of unstrained La2-xSrxCuO4 evolves with doping as reported for scraped single crystals, but also changes strongly with strain. Our studies show, that the in-plane compressive strain changes the Fermi surface topology from hole-like to electron-like. It enhances band dispersion and the Fermi level is crossed before the Brillouin zone boundary, in sharp contrast to the "usual" saddle point remaining ~30 meV below the Fermi level measured along the direction of the Cu-O bonds on unstrained samples. The associated reduction of the density of states near the Fermi level does not diminish the superconductivity; TC is enhanced in all our compressively strained samples. This result is rather surprising since such a reduction of the density of states, according to many mean field models, does not favor the increase of TC measured in our films. By comparing the ARPES measurements on our films with measurements on bulk crystals, we could also show that the results from our relaxed films are equivalent to those on bulk crystals, therefore excluding an explanation through finite size effects other than strain. Our latest results on films under huge tensile strain (1% change in c-axis) are significantly different: ARPES shows evidence for a 3-dimensional dispersion, in contrast with the strictly 2-dimensional dispersion observed on compressively strained films. Already the conduction band of relaxed La2-xSrxCuO4 is atypical: It has considerable apical-oxygen pz and Cu3dz2-r2 out-of-plane character, while for the rest of the cuprate HTSC, those orbitals hybridize far less with the conduction band. We relate the observed z-axis dispersion with the significant displacement of the apical-oxygen towards the CuO2 plane, induced by the epitaxial strain. Resistivity measurements show an insulating behavior of films under extreme tensile strain and no TC. Films with weaker tensile strain still exhibit superconductivity, but diminished as compared to the relaxed films. In summary, while the in-plane compressive strain tends to push the apical oxygen far away from the CuO2 plane, enhances the 2-dimensional character of the dispersion and increases TC, the tensile strain seems to act exactly in the opposite direction and the resulting dispersion is 3-dimensional. We have established the shape of the Fermi surface for both cases, yet further experiments are required to clarify fine details.

EPFL2005

Disorder-induced electronic, magnetic, and optoelectronic properties of two-dimensional materials

Cheol Yeon Cheon

Technological advancement has been in cadence with material development by improving the purity of single crystals and, at the same time, controlling their imperfections. These capabilities have been especially vital for developing new technolo-gies based on two-dimensional (2D) van der Waals (vdW) materials for future electronic and optoelectronic applications. This is because the inherent properties of 2D vdW materials is highly susceptible to the presence of intrinsic structural defects and ex-trinsic disorders due to large surface-area-to-volume ratio. The successful reduction of these disorders has significantly im-proved material properties and led to the discovery of novel physical phenomena in vdW materials. On the other hand, structural defects â for instance, 0-dimensional point defects â can induce completely new properties that are otherwise absent in the pre-fect lattice. To harness the full potential of vdW materials, it is thus essential to produce high-quality crystals and understand how the disorder affects their material properties, which is the central idea of this dissertation.In this dissertation, we first present the work of high-quality epitaxial growth of NbS2. Based on atmospheric-pressure chemical vapor deposition, we have successfully synthesized the two polymorphs (2H and 3R) of NbS2 with the largest lateral size grown to date. Their distinct superconducting and metallic properties were examined under low-temperature charge transport, respectively. Our finding demonstrates the practical synthesis method for phase-controllable growth of 2D transition metal dichalcogenides and can benefit future studies in mesoscopic devices and large-area applications of 2D superconductors. Secondly, we present the work of discovering defect-induced novel properties in ultrathin layers of PtSe2. Although bulk PtSe2 is non-magnetic, we observe the appearance of magnetism in monolayer and bilayer PtSe2. We were able to measure the magnetoresistance (MR) of mono- and bilayer PtSe2 under perpendicular magnetic fields using proximitized graphene, and found antiferromagnetic and ferromagnetic MR responses for mono- and bilayer, respectively. The appearance of such different magnetic states is theoretically explained by the first-principle density functional theory calculation, suggesting the origin of induced-magnetic moments from intrinsic Pt vacancies for both layers. Moreover, we also found that structural disorder in PtSe2 can induce bulk photovoltaic effect (BPVE). The second-order optical nonlinear effects, such as BPVE, require broken structural inversion symmetry and crystal symmetry can be reduced by the presence of structural defects. The broken local inversion symmetry from structural disorder in centrosymmetric PtSe2 is manifested by the generation of zero-biased photo-current under homogenous illumination. We observe linear and circular polarization-dependent photocurrents in defective PtSe2, which is largely absent in the pristine crystal. Our findings in PtSe2 emphasize the importance of the structural disorder for generating completely new properties and stress the need for defect-engineering for realizing the practical use of PtSe2 in spintronic and photovoltaic applications.

EPFL2022