Session F07: Correlated Topological Metals and Other Quantum Fluids

Impact of nematic fluctuations on the Hall viscosity of electronic fluids

Electronic nematic order and fluctuations are of great importance to a wide class of correlated electronic materials, such as unconventional superconductors, doped topological insulators, and twisted moiré systems. Because of the intertwining between nematicity and elasticity, the manifestations of nematic fluctuations on elastic properties of the lattice, such as the shear modulus, have been widely investigated. In this talk, we explore the impact of nematic fluctuations on the elastic properties of the electronic fluid itself. In particular, we focus on the Hall viscosity coefficient, which is the electronic counterpart of the dissipationless generation of stress by a time-varying strain in the presence of time-reversal symmetry-breaking. We consider the case of Dirac materials, which is a model electronic fluid, and compute the contribution to the Hall viscosity arising from electronic nematicity, highlighting the role played by dynamic nematic fluctuations.   

How to measure the quantum geometric tensors of Bloch electrons in solids

The geometric properties of quantum states are characterized by the quantum geometric tensors whose real and imaginary parts correspond to the quantum metric and Berry curvature, respectively. Despite its fundamental importance, it is quite challenging to directly visualize their momentum space distribution. Recently, using the circular-dichroic (CD) angle resolved photoemission spectroscopy (ARPES), the momentum space distribution of the local angular momentum was measured. As the local angular momentum is closely related to the local Berry curvature, it was possible to visualize the momentum space distribution of the Berry curvature. However, the distribution of the quantum metric in momentum space has never been observed in solids yet. In this talk, I will explain how the quantum metric as well as the Berry curvature can be measured using the APRES data. Apply this idea to CoSn, I'll show the corresponding distribution of the quantum geometric tensors probed in recent CD-APRES measurement.   

Quantum Geometric Low-Frequency Optical Response of Correlated Metals

While single-particle electromagnetic responses of band insulators and semimetals are increasingly well-understood to interrogate geometrical and topological properties of Bloch states, optical signatures of quantum geometry in correlated electron systems remain a key open question. Here, we show that the low-frequency optical conductivity in correlated metals can directly probe the quantum geometry of the Fermi surface. This effect emerges from effective photon-dressed Coulomb scattering and can be equivalently understood to arise from light-induced perturbations of the band's Wannier functions. We illustrate ramifications for dilute-doped higher-angular-momentum topological band inversions in two dimensions with approximate Galilean invariance and show that the intraband optical conductivity due to electronic interactions becomes purely quantum-geometric in nature. Our results provide a new optical probe of quantum geometry in correlated electron systems.   

Topological heavy fermions in magnetic field.

The recently introduced topological heavy fermion model (THFM) provides a means for interpreting the low-energy electronic degrees of freedom of the magic angle twisted bilayer graphene as hybridization amidst highly dispersing topological conduction and weakly dispersing localized heavy fermions. In order to understand the Landau quantization of the ensuing electronic spectrum, a generalization of THFM to include the magnetic field B is desired. However the generalization of THFM to magnetic flux seemed elusive because of the failure of the conventional canonical substitution method to account for the correct number of total magnetic subbands within the narrow bands, i.e. its total Chern number. To this end we provide a systematic derivation of the THFM in B and solve the resulting model to obtain the interacting Hofstadter spectra for single particle charged excitations at three different fillings of the moiré unit cell. The presented analytical results offer an intuitive understanding of the nature of the (strongly interacting) Hofstadter bands.   

Surface states and effects of local surface chemistry in f-electron Weyl Semimetal CeCoGe

Topological quantum materials have defining surface states whose presentation in ARPES (angle resolved photoemission spectroscopy) can depend on the sometimes irregular chemistry of the cleaved surface. Local surface structure and chemistry can produce additional effects in the band structure of novel materials probed via photoemission. CeCoGe3 is a strongly correlated f-electron Weyl semimetal (WSM) with enhanced effective mass and low-temperature magnetism. I will present both XPS (x-ray photoemission spectroscopy) and ARPES measurements of CeCoGe3, showing correlations between surface environment and local electronic structure. 

Funding acknowledgement: BSF Grant 2020067   

Weyl-Kondo semimetals in chiral and magnetic settings

Weyl-Kondo semimetals represent a class of gapless topological states driven by strong correlations. Here, we consider such phases in chiral or magnetic settings. Chiral crystals belong to a class of crystalline materials that lack mirror and inversion symmetry. In such crystals, electrons with different chirality are not related by any symmetry operations. Therefore, they serve as the platform for chiral phenomena in condensed matter settings. Here, we consider the chiral crystal environment for Kondo lattice systems. We find that the Kondo effect cooperates with the symmetry constraints of the chiral lattice to produce non-degenerate Weyl fermions near the Fermi energy. The corresponding Berry charge gives rise to experimentally measurable signatures. Among the latter are the circular photogalvanic effect (CPGE). The effects are significantly renormalized by the strong electron-electron interaction in the Kondo effect. Our work connects with the recent experiments on Ce3Rh4Sn13, and sheds light on the further experimental exploration of this and related heavy fermion materials. Separately, I will discuss Weyl-Kondo semimetals with magnetic orders and their possible material realizations.   

Non-fermi liquid signatures of quadratic band touching and phonon anomalies in metallic Pr2Ir2O7

A single Fermi node in Pr2Ir2O7 protected by time reversal and inversion symmetry resulting from an interplay between strong electronic correlations and spin-orbit coupling (SOC) has attracted significant attention. Here, we report a detailed temperature-dependent Raman study of the ”quadratic band touching Luttinger semimetal” Pr2Ir2O7. In addition to phonons, we observe an underlying flat electronic continuum due to electronic Raman scattering (ERS) that is dominant below a characteristic temperature TQ. The observed ERS is understood using the Raman susceptibility of a non-Fermi liquid expected in a quadratic band touching system.The phonon frequencies and linewidths show anomalous temperature dependence below TQ ∼ 150 K attributed to strong electron-phonon coupling (EPC) in a quadratic band touching (QBT) state. The Ir-O-Ir bending Eg mode shows pronounced Fano asymmetry as well as anomalous linewidth broadening below TQ. Further, a Raman line at ∼460 cm−1 involving crystal field transition couples strongly to the T2g phonon at ∼350 cm−1. Our results establish the enhanced scale of electron-electron correlations and electron-phonon coupling in the QBT state of metallic Pr2Ir2O7.   

Dirac nodal lines in the optical conductivity of Ba(Co,Ni)S2

The solid solution between BaCoS₂ and BaNiS₂ shows anarray of quantum properties. The Co material is a semi-metal dominated by strong correlations. At 28% Ni doping it undergoes an electronic metal-insulator phase transition to a Drude metal. The metallic states persists all the way to the pure Ni compound, where bands with a linear dispersion producing dispersive Dirac nodal lines compete with the bulk Drude carriers. We combined ab-initio calculations with mesurements of the optical conductivity of these materials to reverse engineer the contribution of each band in the optical response of these materials. We explained uncommon features in their optical response such as a linear dispersion of the optical conductivity [1] and the existence of an isosbestic line separating a spectral-weight transfer across Dirac nodal states [2].   

Low temperature magneto-transport of epitaxial samarium hexaboride films

The interplay between localized magnetic magnetic moments and itinerant electrons is at the core of a wide-range of phenomena from the Kondo effect to heavy-fermion behavior. A particular instance of this occurs in samarium hexaboride (SmB₆), where the localized f-moments lie energetically close to a dispersing d-band. The resulting f-d hybridization leads to an insulating state, dubbed a Kondo insulator. Moreover, due to band-inversion between the f-d bands, SmB₆ has been proposed to be a topological Kondo insulator hosting Dirac surface states. Though measurements of bulk SmB₆ crystals have shown agreement with this picture, similar efforts on thin-films have yielded mixed results due to issues with film growth. Hence, optimization of growth recipes and an identification of quantifiable metrics that signify the quality of films is crucial for progress.

In this work, we work with SmB₆ films epitaxially grown on Si by molecular-beam epitaxy and perform magneto-transport with film thickness from 10 to 80 nm. Measurements are performed on unpatterned films via the van der Pauw method and through micro-fabricated Hall bars etched via Ar ion-milling. Hall measurements at 1.5 K suggest a sheet carrier density on the order of 1E16 cm^-2, with parallel conduction, presumably from bulk carriers, freezing out below 10 K. A critical analysis of sheet resistance as a function of  film thickness indicates some challenges with growth on Si and points towards strategies that can lead to further optimization of such films.   

Charge dynamics in elemental bismuth measured with momentum-resolved EELS

The topology of the quantum mechanical wavefunction has been a core interest of the condensed matter physics and material science community over the past decade. Bismuth-based compounds have played a significant role in this regard owing to the large spin-orbit coupling, even though elemental bismuth was believed to be topologically trivial. Recent studies have proposed that bismuth is likely a higher order topological insulator [1] and could also be a potential candidate for a first order topological crystalline insulator [2]. Here we have studied the charge dynamics in bismuth single crystals using momentum resolved electron energy loss spectroscopy (m-EELS). Signatures of a strongly temperature dependent plasmon are observed. While the plasmon frequency at q = 0 is consistent with infrared spectroscopy [3], the lifetime appears to be significantly shorter. We measure the dispersion of the plasmon excitation and compare it to the calculated dynamical charge susceptibility using Lindhard theory in the RPA. The results shed light on the collective excitations in the proposed topological surface states in bismuth.

[1] Schindler, Frank, et al. Nature physics 14.9: 918-924 (2018).

[2] Hsu, Chuang-Han, et al. Proceedings of the National Academy of Sciences 116.27: 13255-13259 (2019).

[3] Tediosi, Riccardo, et al. Physical review letters 99.1 : 016406 (2007).   

Nonlinear bosonization and the anomalous Hall effect

We study various aspects of anomalous Hall effect using nonlinear bosonization. Nonlinear bosonization offers both perturbative and non-perturbative techniques to study Fermi and non-Fermi liquids. This approach naturally leads to a multidimensional bosonized description with nonlinear corrections fixed by the geometry of the Fermi surface. We find how Berry curvature can be captured in the nonlinear bosonization framework, establishing the connections with the anomalous Hall effect.   

Spontaneous electric potential generation on the surface of the topological Kondo insulator SmB6

In recent years, Samarium Hexaboride (SmB₆) has received renewed attention due to its identification as a three-dimensional topological Kondo insulator (3D TKI). Understanding the precise mechanisms behind the development of the topological surface states in SmB₆ and how the relativistic quasiparticles achieve thermal equilibrium remains a scientific challenge. In this study, we delve into the intricate behaviors of SmB₆, with a focus on the intermediate temperature region (5-15 K) where topological surface states begin to develop whereas the bulk is already a gapped insulator. Most notably, we observed stable electrical voltage and current between two electrodes placed on different surface regions of SmB₆, which persist over macroscopic timescales. Moreover, our investigation reveals a strong connection between the spontaneous surface potential and the presence of topological surface states. Thus, we infer that the formation of localized inhomogeneity of topological surface states at intermediate temperature range underlies this intriguing behavior. Our observations not only reveal a novel electrical transport behavior in SmB₆ but also demonstrate the potential of SmB₆ as a platform for converting thermal energy from the environment into electrical energy.   

Magnetization of superfluid He-3 in periodic structures

Triplet p-wave superfluid He-3 has multiple superfluid phases with different spin structures and different magnetic properties, that are used to identify these phases experimentally. Similar techniques are important for identification of other unconventional superconductors.  These properties are modified in confined geometries and near impurities, such as aerogel, due to presence of surface Andreev states. Andreev quasiparticle states near fully reflective surfaces are a manifestation of the topological properties of the superfluid, and they create special magnetic environment different from the bulk.  Using the scattering matrix approach and quasiclassical technique, we investigate the spectrum of Andreev states near semi-transparent interfaces, including periodic structures made out of such interfaces, and focus on their influence on the magnetization of the superfluid states.

This research is supported by NSF grant DMR-2023928.