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Register a new userTemperature dependence of correlated electronic states in archetypal kagome metal CoSn
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Hexagonal CoSn is a newly-discovered frustrated kagome metal. It shows close-to-textbook flat bands and orbital-selective Dirac fermions, which are largely associated with its strongly correlated Co-3$d$ orbitals. Because correlated electronic states are easily regulated by external conditions (such as chemical doping, pressure, and temperature), the fate of these kagome-derived electronic bands upon temperature becomes an interesting and unsolved question. In this work, we try to study the temperature-dependent electronic structures of hexagonal CoSn by means of the density functional theory in conjunction with the embedded dynamical mean-field theory. We find that hexagonal CoSn is in close proximity to a Mott insulating state at ambient condition. Special attention is devoted to the evolution of its Co-3$d$ electronic states with respect to temperature. At least six different temperatures (or energy scales), namely $T^{*}$, $T_{text{FL}}$, $T_{text{S1}}$ (and $T_{text{S2}}$), $T_{text{SF}}$, and $bar{T}$, are figured out. They are related to stabilization of the pseudogap state, emergence of the non-Fermi-liquid phase, onset (and completeness) of the intermediate spin state, occurrence of the spin-frozen phase, beginning of the orbital freezing transition, respectively.
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The field of topological electronic materials has seen rapid growth in recent years, in particular with the increasing number of weakly interacting systems predicted and observed to host topologically non-trivial bands. Given the broad appearance of topology in such systems, it is expected that correlated electronic systems should also be capable of hosting topologically non-trivial states. Interest in correlated platforms is heightened by the prospect that collective behavior therein may give rise to new types of topological states and phenomena not possible in non-interacting systems. However, to date only a limited number of correlated topological materials have been definitively reported due to both the challenge in calculation of their electronic properties and the experimental complexity of correlation effects imposed on the topological aspects of their electronic structure. Here, we report a de Haas-van Alphen (dHvA) study of the recently discovered kagome metal Fe$_3$Sn$_2$ mapping the massive Dirac states strongly coupled to the intrinsic ferromagnetic order. We observe a pair of quasi-two-dimensional Fermi surfaces arising from the massive Dirac states previously detected by spectroscopic probes and show that these band areas and effective masses are systematically modulated by the rotation of the ferromagnetic moment. Combined with measurements of Berry curvature induced Hall conductivity, we find that along with the Dirac fermion mass, velocity, and energy are suppressed with rotation of the moment towards the kagome plane. These observations demonstrate that strong coupling of magnetic order to electronic structure similar to that observed in elemental ferromagnets can be extended to topologically non-trivial electronic systems, suggesting pathways for connecting topological states to robust spintronic technologies.
CoSn is a Pauli paramagnet with relatively flat d-bands centered about 100 meV below the Fermi energy Ef. Single crystals of CoSn lightly doped with Fe, In, or Ni are investigated using x-ray and neutron scattering, magnetic susceptibility and magnetization, ac susceptibility, specific heat and resistivity measurements. Within the rigid band approximation, hole doping with a few percent of Fe or In should move the flat bands closer to Ef, whereas electron doping with Ni should move the flat bands further away from Ef. We provide evidence that this indeed occurs. Fe and In doping drive CoSn toward magnetism, while Ni doping suppresses CoSns already weak magnetic response. The resulting ground state is different for Fe versus In doping. For Fe-doped crystals, Co1-xFexSn, with 0.02 < x < 0.27, the magnetic and specific heat data are consistent with the formation of a spin glass, with a glass transition temperature, Tg, ranging from 1 K for x=0.02 to 10 K for x= 0.27. Powder and single crystal neutron diffraction found no evidence of long-range magnetic order below Tg with x = 0.17. For In-doped crystals, CoSn1-yIny, both the magnetic susceptibility and the Sommerfeld coefficient, gamma, increase substantially relative to pure CoSn, but with no clear indication of a magnetic transition for 0.05 < y < 0.2. CoSn crystals doped with Ni (Co0.93Ni0.07Sn) have a significantly smaller magnetic susceptibility and gamma than pure CoSn, consistent with the flat bands further from Ef.
A typical f-electron Kondo lattice system Ce exhibits the well-known isostructural transition, the so-called gamma-alpha transition, accompanied by an enormous volume collapse. Most interestingly, we have discovered that a topological-phase transition also takes place in elemental Ce, concurrently with the gamma-alpha transition. Based on the dynamical mean-field theory approach combined with density functional theory, we have unravelled that the non-trivial topology in alpha-Ce is driven by the f-d band inversion, which arises from the formation of coherent 4f band around the Fermi level. We captured the formation of the 4f quasi-particle band that is responsible for the Lifshitz transition and the non-trivial Z2 topology establishment across the phase boundary. This discovery provides a concept of topology switch for topological Kondo systems. The on and off switching knob in Ce is versatile in a sense that it is controlled by available pressure (around 1 GPa) at room temperature.
The search for materials with novel and unusual electronic properties is at the heart of condensed matter physics as well as the basis to develop conceptual new technologies. In this context, the correlated honeycomb transition metal oxides attract large attention for both, being a possible experimental realization of the theoretically predicted magnetic Kitaev exchange and the theoretical prospect of topological nontriviality. The Mott insulating sodium iridate is prototypical among these materials with the promising prospect to bridge the field of strongly correlated systems with topology, finally opening a path to a wide band gap material with exotic surface properties. Here, we report a profound study of the electronic properties of ultra-high-vacuum cleaved surfaces combining transport measurements with scanning tunneling techniques, showing that multiple conductive channels with differing nature are simultaneously apparent in this material. Most importantly, a V-shaped density of states and a low sheet resistance, in spite of a large defect concentration, point towards a topologically protected surface conductivity contribution. By incorporating the issue of the addressability of electronic states in the tunneling process, we develop a framework connecting previous experimental results as well as theoretical considerations.
Negative compressibility is a sign of thermodynamic instability of open or non-equilibrium systems. In quantum materials consisting of multiple mutually coupled subsystems, the compressibility of one subsystem can be negative if it is countered by positive compressibility of the others. Manifestations of this effect have so far been limited to low-dimensional dilute electron systems. Here we present evidence from angle-resolved photoemission spectroscopy (ARPES) for negative electronic compressibility (NEC) in the quasi-three-dimensional (3D) spin-orbit correlated metal (Sr1-xLax)3Ir2O7. Increased electron filling accompanies an anomalous decrease of the chemical potential, as indicated by the overall movement of the deep valence bands. Such anomaly, suggestive of NEC, is shown to be primarily driven by the lowering in energy of the conduction band as the correlated bandgap reduces. Our finding points to a distinct pathway towards an uncharted territory of NEC featuring bulk correlated metals with unique potential for applications in low-power nanoelectronics and novel metamaterials.
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