Session F43: Strong Electronic Correlations in Topological Materials I

Live

SmB₆ is a mysterious compound that is electrically insulating but yet it exhibits quantum oscillations, which are a telltale signature of the metallic state. Adding to the enigma is the possibility that SmB₆ is a topological Kondo insulator. Here, we report first-principles, parameter-free all-electron electronic-structure calculations on SmB₆, which yield the band structure and crystal-field splittings within the f -electron complex in accord with experiments. Predicted energies of several magnetic phases where charge, spin and lattice degrees of freedom are treated on an equal footing are found to be extremely close, indicating the key role of spin fluctuations in SmB₆. Our analysis shows that the topological Kondo state of SmB₆ is robust regardless of its magnetic configuration. However, it lies near the metal-insulator transition where the Fermi surface derived from the predicted ground state explains the experimentally observed bulk quantum oscillations. The calculated effective electron mass at the Fermi surface and specific heat further explain how the material is essentially insulating in agreement with the experimental results.

References:
R. Zhang, et al., arXiv:2003.11052
M. Hartstein, et al., Nat. Phys. 14, 166–172 (2018)   

Live

The interplay of non-trivial topology and Kondo physics has stimulated a flourish of experimental reports in the mixed valence SmB₆ compound, some of which appear to be in contradiction. The origin of such discrepancies may lie on the fragility of the Kondo insulating phase. We locally explore Al-flux grown Sm_1-xGd_xB₆ single crystals (0 ≤ x ≤ 0.02) with electron spin resonance (ESR) and bulk measurements [1]. Although the Gd³⁺ ESR spectra in highly dilute systems (x ≈ 0.0004) show a cubic insulating environment, the increase of Gd³⁺ concentration changes the ESR response to a metallic ESR lineshape. Remarkably, all the samples still show insulating dc electrical resistivity. Our results hint that the Kondo insulating state is destroyed locally around impurities before a global percolation occurs. These results not only explain the discrepancy between dc and ac conductivity [2,3], but also point to a scenario to explain the presence of quantum oscillations in magnetization in absence of oscillations in electrical resistivity [4].

[1] J. C. Souza et al., Phys. Rev. Research 2(4):043181 (2020).
[2] N. J. Laurita et al. Phys. Rev. B 94(16):165154 (2016).
[3] Y. S. Eo et al. Proc. of Nat. Acad. Soc. 116(26): 12638-12641 (2019).
[4] Y. S. Eo et al. Phys. Rev. B, 101(15):155109 (2020).   

Live

Hydrodynamic electron flow is a unique signature of strong electron interactions in a material. This effect has been observed in 2D materials, but observations in bulk materials are intriguing as high-carrier density should screen the interactions. In this work, we study hydrodynamic flow in the semimetal WTe2 to gain insight into the microscopic origin of its electron interactions.

We image the spatial profile of the electric current by using a nitrogen-vacancy scanning tip. Using coherent quantum sensing, we obtain magnetic field resolution of ~10nT and spatial resolution of ~100nm. The current pattern we observe differs substantially from the flat profile of a normal metal, and indicates correlated flow through the semimetal. The pattern also shows non-monotonic temperature dependence, with hydrodynamic effects peaking at ~20 K.

We compare our results to a model which combines ab initio electron scattering rates and the electronic Boltzmann transport equation.
The model shows quantitative agreement with our measurement, allowing us to extract the strength of electron-electron interactions in our material. Furthermore, we conclude that electron interactions are phonon-mediated. This result opens a path for hydrodynamic flow and strong interactions in a variety of new materials.   

Live

Hydrodynamic flow of electrons has recently been the focus of numerous studies, indicating electrons in condensed matter can exhibit fluid phenomena. Depending on the length scales of momentum conserving (l_mc) and relaxing (l_mr) scattering and the conductor size (d), current flow may shift from ohmic to ballistic or hydrodynamic. Distinguishing these regimes, however, requires an accurate experimental method to estimate the mean-free-paths. We present Sondheimer oscillations (SO), as a method to obtain l_mr even when l_mr»d. SO manifest as periodic-in-B oscillations of the resistivity due to helical motion of carriers along a magnetic field. The field sets the cyclotron radius and thus determines at which point the helical motion is cut off by scattering from the device surface, making a positive or negative contribution to the conduction. As this effect requires there to be no scattering in the bulk, it is sensitive to the mean-free-path and enables its extraction.
Using WP₂ as a model system, we demonstrate the presence of SO and extract l_mr over a wide range of temperatures. We find excellent agreement with theoretical predictions, demonstrating the effectiveness of SO as a tool to determine l_mr and study non-Ohmic electron flow.   

Live

Nonsymmorphic structures are promising to host Dirac and Weyl fermions due to the presence of protected band crossings. The Kondo effect, present in Ce-based compounds, may pin these crossings to the Fermi level, favoring non-trivial topological effects. Here, we report the properties of stoichiometric CeAuBi₂. It crystallizes in the ZrSiS tetragonal P4/nmm nonsymmorphic structure and orders antiferromagnetically below 19 K with a propagation vector (0 0 1/2). At low temperatures, magnetic fields parallel to the c-axis induce several metamagnetic transitions, similar to CeAuSb₂, and a linear positive magnetoresistance is observed, which suggests the presence of band crossings near the Fermi level. The application of external pressure favors the antiferromagnetism. Finally, neutron magnetic diffraction as a function of magnetic field will be discussed.   

Not Participating

Dirac materials, which can be well described at low energy by Dirac fermions theories, constitute an interesting class of systems that can undergo a quantum phase transition at zero temperature, when a non-thermal parameter reaches the quantum critical point (QCP). In the vicinity of the QCP, physical observables are affected by thermal and quantum fluctuations. We study the longitudinal and Hall conductivity in a deformed Dirac fermions CFT and in the Gross-Neveu model in large N in 2+1 dimensions. We consider the effect of a detuning from the QCP and a nonzero temperature. In the large frequency regime, we show that the structure of the conductivities can be understood by using operator product expansion and conformal perturbation theory.   

Live

We study the effect of long-range correlated quenched disorder on quantum phase transitions
in two-dimensional Dirac semimetals described by the chiral Ising, XY, and Heisenberg Gross-Neveu-Yukawa
models, using a one-loop renormalization group analysis in a triple epsilon expansion. In all three classes of
models we discover new finite-randomness multicritical points, some of which are of stable-focus type and
exhibit oscillatory corrections to scaling. For the XY and Heisenberg models, we find a supercritical Hopf
bifurcation accompanied by unusual critical behavior controlled by a stable limit cycle.   

Live

Clean Dirac liquids are expected to show hydrodynamic transport properties dominated by inelastic scattering. Nowadays, this hydrodynamic limit can be accessed in high quality graphene samples. We investigate whether the customary two fluid Boltzmann treatment of electrons and holes and their inelastic scattering correctly captures the heat conductivity. We include a third dynamical degree of freedom, particle-hole pairs, and find that is modifies the heat transport characteristics under realistic conditions. We conclude that a quantitative modelling of thermo-electric transport properties in clean graphene should include collective excitations.   

Live

YbB₁₂, a representative Kondo insulator, has attracted attention in recent years as it exhibits Shubnikov-de Haas effect in intense magnetic fields [1]. With the magnetic field further increasing, YbB₁₂ goes through an insulator-to-metal transition. We systematically studied the high field metallic state in YbB₁₂ in pulsed magnetic fields. The temperature dependence of the resistivity in this state reveals a crossover from non-Fermi-liquid behavior above 4 K to Fermi-liquid-like behavior below 2 K. This Fermi-liquid-like state also shows unusually large Kadowaki-Woods ratio. Results of the quantum oscillation measurements provide evidence that the electronic band structure is depicted by a two-fluid picture where unusual quasiparticles, contributing little or nothing to charge transport, coexist with the conventional fermions.

[1] Z. Xiang, et al. Science 362, 65-69 (2018).   

Live

The crystal-field (CF) splitting of the ⁶H_5/2 Hund's rule ground state of Sm³⁺ in the strongly correlated topological insulator^1,2 SmB₆ has been determined with high resolution resonant inelastic x-ray scattering (RIXS) at the Sm M₅ edge³. The valence selectivity of RIXS allows to isolate the crystal-field-split excited multiplets of the Sm³⁺ (4f⁵) configuration from those of Sm²⁺ (4f⁶) in intermediate valent SmB₆. We find that the quartet Γ₈ ground state⁴ and the doublet Γ₇ excited state are split by Δ=20meV±10meV. Considering this as an upper limit for the 4f bandwidth points to an extremely large mass renormalization from the band structure value, which can be linked to the small coefficient of fractional parentage for the hopping of the 4f electrons⁵. The tiny band width complies with the small value of the gap and may be used to put constraints to the energies of the topological surface states.
[1] M. Dzero et al, PRL 104, 106408 (2010)
[2] T. Takimoto, J. Phys. Soc. Jpn. 80, 123710 (2011)
[3] A. Amorese et al, PRB 100, 241107(R) (2019)
[4] M. Sundermann et al, PRL 120, 016402 (2018)
[5] G. Sawatzky & R. Green, edt. E. Pavarini & E. Koch (FZ Julich), 1.1 (2016), in Quantum Materials: Experiments and Theory   

Not Participating

The layered semimetal WTe₂ behaves in the monolayer limit as a topological insulator but the nature of the insulating state in the interior bulk is unclear. We study its conductivity as a function of gate doping, temperature, and current direction. Care is needed to eliminate helical edge conduction from the measurements, including along cracks, which we locate by microwave impedance microscopy. The conductivity is found to be highly anisotropic for hole doping and much less so for electron doping. Surprisingly, for hole doping the conductivity is about three times lower along the a-axis, the direction of the tungsten chains, than along the b-axis, perpendicular to the chains. We consider the implications of this observation for the possibility that the state is a kind of excitonic insulator.   

Live

We combine STM/STS with bulk measurements to investigate the Zintl phase Eu₅In₂Sb₆, a non-symmorphic antiferromagnetic insulator. This research was motivated by the theoretical predictions of non-trivial Fermi surface topology stabilized by non-symmorphic symmetry.

Eu₅In₂Sb₆ crystallizes in an orthorhombic structure (Pbam), characterized by infinite [In₂Sb₆]¹⁰⁻ double chains along the c-axis. At low temperatures, two magnetic transitions (T_N1 ≈ 14 K and T_M2 ≈ 7 K), revealed in our magnetic and transport measurements, point to an intricate magnetic structure. Interestingly, the reported emergence of colossal magnetoresistance (CMR) suggests the formation of magnetic polarons¹.

Performing STM/STS measurements, we obtained local insight into the electronic structure and surface morphology. The samples were cleaved in situ in UHV and at low temperatures. In the (010) plane, we obtained striped patterns that can be correlated to the stacking of double-chains. The attempted cleavage along the a-axis revealed a more complex pattern, which could be indicative of cleavage along the (1-10) plane. So far, direct experimental evidence of topological surface states remains elusive.

[1] P. Rosa et al., npj Quantum Materials (2020) 5:52.   

Live

Recently, transition-metal based kagome magnets, consisting of stacked kagome lattices, have shown to provide a natural platform to study the effect of the interplay between magnetism and electronic topology. For example, the ferromagnets Fe₃Sn₂ and Co₃Sn₂S₂ and the antiferromagnets Mn₃Ge and Mn₃Sn have revealed a large observed intrinsic anomalous Hall effect (AHE) and topological band degeneracies. Of particular interest, the ternary kagome magnet TbMn₆Sn₆ has very recently revealed a large intrinsic Chern gapped phase associated with Landau quantization upon application of a magnetic field at low temperature. Here we present our recent findings on the role of the rare-earth metal terbium in the complex interplay of magnetism and topology in the electronic structure of TbMn₆Sn₆. The magnetic structure consists of a collinear ferrimagnet below the Curie temperature of 423K and a spin reorientation process at 310K. The intrinsic AHE is negligible at low temperatures but scales non-monotonically with magnetization at higher temperatures leading to the manifestation of an unconventional intrinsic contribution.   

Live

Certain frustrated 2D quantum magnets may be described at low energy by a Dirac spin liquid (DSL) which is a version of quantum electrodynamics with 2N flavors of two-component gapless Dirac spinons. A DSL also hosts monopole excitations that drive the confinement of the spinons. We revisit the hierarchy among monopole operators with different quantum numbers at the quantum critical point (QCP) between a DSL and an antiferromagnet. We describe the organization of monopoles in multiplets of the QCP flavor symmetry group SU(2) x SU(N). The scaling dimension in each magnetic spin sector is obtained at leading order in N using the state-operator correspondence. We also discuss 1/N corrections to the scaling dimension of monopoles at a critical point between a DSL and a chiral spin liquid.   

Live

Kondo insulators have been intensively re-investigated recently due to non-trivial topology [1], unconventional quantum oscillations (QOs) [2], and evidence for existence of itinerant charge-neutral fermions (CNFs) [3]. Although it has been proposed the CNFs may produce the QO signals, the origin of the CNFs is highly elusive and under debate.
YbIr₃Si₇ is a newly discovered Kondo lattice compound, where c-f interaction leads to insulating ground state with antiferromagnetic (AF) ordering [4]. The AF ordering can be easily tuned by an external magnetic field. Here we present specific heat C and thermal conductivity κ measurements of YbIr₃Si₇. Between 2 and 3 T, where the AF order is suppressed, C is strongly enhanced and a finite residual linear term in the thermal conductivity κ/T (T → 0) is observed. A likely possible explanation for the present results is that CNFs are coupled directly to the spin degrees of freedom of the underlying Kondo lattice. YbIr₃Si₇ thus provides an intriguing platform to study the CNFs and their relationship with magnetism in strongly correlated insulators.

[1] M. Dzero et al., Annu. Rev. Con. Mat. Phys. 7, 249 (2016).
[2] Z. Xiang et al., Science 362, 65 (2018).
[3] Y. Sato et al., Nature Physics 15, 954 (2019).
[4] M. Stavinoha et al., arXiv:1908.11336v2.