Session Y29: Strongly Correlated Systems, Including Quantum Fluids and Solids XXI

Long Exact Sequence in the Symmetry Broken Phase: defect anomaly matching and Berry phase protection.

We investigate 't Hooft anomaly matching in the symmetry broken phase by studying the Jackwi-Rebbi gapless modes living on defects such as domain wall and vortices. We show that the anomaly of the effective lower dimensional system living on the core of the defect matches the bulk anomaly under the Smith homomorphism map, extending the results of Callan and Harvey. We extend this map to a long exact sequence and relate it to physical phenomona such as Berry phase protecting the diabolical point and higher dimensional version of bulk-boundary correspondence. Additionally, we give a crystalline-symmetric interpretation for this anomaly matching condition and the long exact sequence. The twisting of the symmetry on the defect is explained by the crystalline equivalence principle (CEP), and the anomaly matching is related to Lieb-Schultz-Mattis (LSM) theorems. Lastly, we explicitly apply these anomaly matching conditions to free fermion symmetry breaking and abelian Higgs models.   

Exact emergent higher-form symmetries and their physical consequences

Generalized global symmetries provide a unifying perspective of quantum-many body systems. However, since condensed matter systems rarely have these symmetries, it is natural to wonder if the phases of a lattice model without generalized symmetries can be meaningfully characterized by emergent generalized symmetries. Here we explicitly demonstrate this possibility in two bosonic Hamiltonian models. While these UV models have no higher-form symmetries, there is a region of parameter space where a higher-form symmetry emerges in the mid-IR. This emergent symmetry is robust against local UV perturbations and is an exact symmetry of the full mid-IR effective theory in the thermodynamic limit. Therefore, it is an exact emergent symmetry: it is not a UV symmetry but constrains the IR as if it were. We discuss in detail the physical consequences of this. For instance, there is a spontaneous symmetry breaking (SSB) phase with an exactly gapless Goldstone mode or exact ground state degeneracy. Additionally, there can be an SPT phase protected by the exact emergent symmetry. Furthermore, there are topological defects that are gapped in the SSB phase but condensed in the symmetric phase, and at energies below their gap there is a new exact emergent higher-form symmetry and an emergent mixed 't Hooft anomaly. We conclude by discussing these properties of exact emergent generalized symmetries in a more general, model-independent way.   

Ab initio calculations of the dielectric function of Eu5In2Sb6 for light dark matter detection

Quantum materials with narrow meV-scale band gaps have recently emerged as promising targets for the detection of light dark matter (DM) in the largely unexplored mass range of keV-MeV DM. Eu₅In₂Sb₆, a narrow band gap material with complex antiferromagnetic ordering and potentially non-trivial band topology, is currently under investigation for DM detection, but many questions about its electronic structure remain open. Here, we present first-principles density functional theory calculations of the electronic structure and complex dielectric function of Eu₅In₂Sb₆, addressing the effects of magnetic ordering, spin-orbit coupling, and GW self-energy corrections. We will discuss the challenges of accurately calculating the electronic structure and dielectric function of narrow band gap quantum materials, such as Eu₅In₂Sb₆, and the implications this has for accurate prediction of the projected reach of light DM scattering and absorption.

    

Using Magnetic Tunnel Junctions (MTJs) Instead of Induction Coils in Nuclear Magnetic Resonance (NMR) to Detect Dark Matter

While dark matter remains elusive, a successful detection seems to require unconventional approaches with high sensitivity for small signals. One of the proposed methods to directly probe dark matter has been the utilization of pulsed nuclear magnetic resonance (NMR) to measure an oscillating torque on nuclear spins exerted by the axion-like darkmatter's oscillating magnetic field. However, all such NMR based experiments have relied on inductive coil detection, which innately has low sensitivity. Instead, we propose the application of magnetic tunnel junctions (MTJs), which uses the tunnel magnetoresistance (TMR) between two ferromagnets separated by a thin insulator to detect changes in the magnetic field. We explore how the sensitivity of MTJ based sensors compares to traditional inductive detection for axionlike dark matter detection.   

String-like Theory of Quantum Hall Interfaces

We derive the effective theories for quantum hall droplets with attractive interaction among the constituent particles. In the absence of confining potentials such droplets are defined by their freely moving interfaces (or boundaries) with the vacuum. We demonstrate the effective theories take forms similar to string theories. Generalization to interfaces between different quantum Hall liquids is discussed.   

Symmetric Mass Generation in the 1+1 Dimensional Chiral Fermion 3-4-5-0 Model

Lattice regularization of chiral fermions has been a long-standing problem in physics. In this Letter, we present the density matrix renormalization group simulation of the 3-4-5-0 model of (1+1)D chiral fermions with an anomaly-free chiral U(1) symmetry, which contains two left-moving and two rightmoving fermions carrying U(1) charges 3,4 and 5,0, respectively. Following the Wang-Wen chiral fermion model, we realize the chiral fermions and their mirror partners on the opposite boundaries of a thin strip of ð2 þ 1ÞD lattice model of multilayer Chern insulator, whose finite width implies the quantum system is effectively (1+1)D. By introducing two sets of carefully designed six-fermion local interactions to the mirror sector only, we demonstrate that the mirror fermions can be gapped out by the interaction beyond a critical strength without breaking the chiral U(1) symmetry, via the symmetric mass generation mechanism. We show that the interaction-driven gapping transition is in the Berezinskii-Kosterlitz-Thouless universality class. We determine the evolution of Luttinger parameters before the transition, which confirms that the transition happens exactly at the point when the interaction term becomes marginal. As the mirror sector is gapped after the transition, we check that the fermions in the light chiral fermion sector remain gapless, which provides the desired lattice regularization of chiral fermions.   

Fractal stripe structures throughout the superconducting dome in a cuprate superconductor

Scanning surface probes have recently revealed rich electronic textures at the nanoscale and mesoscale in many quantum materials.  We have defined new conceptual frameworks for interpreting and understanding these multiscale electronic textures by employing theoretical tools from fractal mathematics and disordered statistical mechanics. This allows us to use the rich spatial information available from scanning probes in order to diagnose criticality from the spatial structure alone, without the need of a sweep of temperature or external field.  These new methods have enabled the discovery of universal, fractal electronic textures across a variety of quantum materials.  [Nat. Commun. 10, 4568 (2019);  PRL 116, 036401 (2016); Nat. Commun. 3, 915 (2012)]  Applying these cluster techniques to scanning tunneling microscopy on Bi2−zPbzSr2−yLayCuO6+x, we show that fractal nematic clusters pervade the bulk of the material, throughout the superconducting doping range. 

    

Elastoresistivity of Three-state Nematicity in Fe1/3NbS2

When a material undergoes a transition into an electronic nematic phase, cells of the lattice distort along a certain axis, breaking rotational symmetry but preserving translational symmetry,  and the Fermi surface of charge carrying electrons distorts in a manner which can be described by an order parameter. In materials with tetragonal crystalline symmetry, the order parameter describing this phase is a scalar, called ‘Ising nematicity’, but materials with triangular or hexagonal symmetry present more options for the direction of in-plane distortions, requiring a two component order parameter to fully describe the state. This type of nematicity is called Potts or three-state nematicity. In this talk, I will report the study of elastoresistivity of three-state nematicity in the hexagonal antiferromagnet Fe_1/3NbS₂. By subjecting single crystals to strain and measuring their elastoresistivity, we are able to observe the resistivity anisotropy induced at different lattice distortions under the phase transition temperature, giving us new insight into the unique characteristics of three-state nematic systems.   

Nematicity in an Iron-based Superconductor

The emergence of unconventional superconductivity in proximity to intertwined electronic orders is especially relevant in the case of iron-based superconductors. Such order consists of an electronic nematic order and a spin density wave in these systems. BaNi₂As₂, like its well-known iron-based analogue BaFe₂As₂, also hosts a symmetry-breaking structural transition that is coupled to a unidirectional charge density wave (CDW), providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning B_1g uniaxial strain, we identify distinct spectral evidence of bulk band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in BaFe₂As₂, the structural and CDW order parameters in BaNi₂As₂ are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Our measurements point to a likely lattice origin of the CDW in BaNi₂As₂.

    

Author not Attending

Nematicity is a phenomenon seen in many strongly correlated systems, often associated with superconductivity and magnetism.  However, understanding the relationship between nematicity and the other underlying physics is often a challenge due to the complicated phase diagrams of many of these systems.  We take a model system, the series Tm_xY_1-xVO4, for which x = 1.0 has a ferroquadrupolar (nematic) transition at 2.15 K and study the underlying ferroquadrupolar interactions using frequency-dependent sound velocity and ultrasonic attenuation measurements.  We examine the evolution of the strength of the quadrupole-quadrupole coupling parameter for x = 0.01, 0.03, 0.1, and 1.0 members of this series.  We fit our data to a well-established mean field model and find that the quadrupolar interaction parameter saturates to its maximum x = 1.0 value for substitutions even as low as x = 0.1.  From this saturation value we deduce the effective quadrupolar interaction distance which is of order a few unit cells.  This short interaction length suggests that optical phonons play an important role in coupling quadrupoles in this material.   

Strain Tuning Three-state Potts Nematicity in a Correlated Antiferromagnet

Electron nematicity has become a familiar characteristic of many strongly correlated materials. A widely studied example is the discovered Ising-nematicity and its interplay with superconductivity in tetragonal iron pnictides. Since nematic directors in crystalline solids are restricted by the underlying crystal symmetry, recently emerged quantum matters with three-fold rotational symmetry offer a new platform to investigate nematic order with three-state Potts character. In this talk, we report reversible strain tuning of the three-state Potts nematicity in a zigzag antiferromagnet insulator, FePSe­₃. Probing the nematicity via optical linear dichroism, we demonstrate either 2π/3 or π/2 rotation of nematic director by uniaxial strain. The nature of the nematic phase transition can also be controlled such that it undergoes a smooth, crossover transition or a spontaneous Ising-like nematic flop transition. The ability to tune the nematic order with in-situ strain further enables the extraction of nematic susceptibility, which exhibits a divergent behavior near the magnetic ordering temperature.   

Unique signatures of electronic nematic liquids via nonlinear Raman spectroscopy

It was recently proposed that the observed splitting of optical phonon lines in the tetragonal phase of BaNi₂As₂ is related to strong fluctuations of an underlying orbital-nematic Ising order parameter, leading to a liquid nematic phase [1]. Characterizing the nematic liquid phase can be challenging since linear Raman responses give similar signatures as a solid nematic phase with broken Z₂ symmetry but coexisting Ising domains. To further corroborate this view, we propose using non-linear Raman spectroscopy to unambiguously differentiate the nematic liquid from a static phase with different domains, where the Z₂ symmetry is effectively restored in the response upon domain averaging.  By computing the higher-order response functions, we show that the dynamical nematic Ising degree of freedom leads to features in the multidimensional spectrum that are absent in the solid phase, reflecting the additional quantum coherence that is only present in the liquid phase.  We also discuss experimental ways of measuring such higher-order responses.

 [1] Y. Yao, R. Willa, T. Lacmann, S.-M. Souliou, M. Frachet, K. Willa, M. Merz, F. Weber, C. Meingast, R. Heid, A.-A. Haghighirad, J. Schmalian, and M. Le Tacon, Nature Communications 13, 4535 (2022).

    

Polaron mediated density wave like fluctuations revealed by Raman spectroscopy in a strongly correlated metal, Ca3Ru2O7

Competing interactions in quantum materials can tip over the ground state on the verge of phase transitions. We report such effects in the strongly correlated metal, Ca₃Ru₂O₇, where symmetry-breaking optical phonons are shown to modulate the pseudogap as well as mediate density wave like fluctuations.  Combining temperature dependent Raman spectroscopy with density functional theory, we crucially rectify prior symmetry assignments for phonons.  This reveals two dominant and competing B₂ phonons that exhibit a cross-over change of Raman scattering across the metal-pseudogap transition. This is shown to be a consequence of a strong electron-phonon coupling of ~10 eV/Å  (comparable with graphene), as one of these phonons modulates the ground state structure closer to the pseudogap phase, while the other away from it and towards the metallic phase. A microscopic model reveals that this arises from the phonons modulating the Ru 4d orbital splitting and the bandwidth of the in-plane electron hopping. Moreover, the B₂ phonons mediate incoherent dynamical charge and spin density wave fluctuations, which are evidenced by a symmetry-dependent change in the background electronic Raman scattering. This study broadly reveals the crucial role of phonons with strong electron-phonon coupling (polarons) in enabling the electronic and magnetic phase transitions in quantum materials.

    

Nematic fluctuation driven quantum criticality and superconductivity: Fe-based and cuprate superconductors

Near the nematic critical point, remarkable quantum critical behaviors and the enhancement of superconductivity have been observed in Fe(Se,Te) [1]. Similar quantum criticalities have been reported in cuprate superconductors near the CDW phase. These charge-channel fluctuations originate from the higher-order vertex corrections [2,3]. However, it is highly nontrivial why these charge-channel fluctuations give ruse to prominent quantum critical phenomena because the electron-nematicity bare coupling constant g⁰_e-nem is very small in realistic Hubbard models. To understand this fundamental problem, we develop the “Bethe-Salpeter equation method” to calculate the nematic-fluctuation-mediated interaction accurately. We reveal that the effective coupling constant g^eff_e-nem strongly increases near the critical point due to the beyond-Migdal many-body correlations. In FeSe families, s-wave high-T_c superconductivity is realized due to nematic fluctuations, based on Hubbard models with on-site U. Critical mass enhancement and T-linear resistivity are also obtained. We also find that the CDW and magnetic fluctuations cooperatively enhance d-wave T_c in cuprate superconductors.

[1] K. Ishida, et al., PNAS 119, e2110501119 (2022).

[2] H. Kontani et al., “Unconventional density waves and superconductivities in Fe-based superconductors and other strongly correlated electron systems” arXiv:2209.00539; to be published in Adv, Phys.

[3] R. Tazai et al., Sci. Adv. 8, eabl4108 (2022).   

Orbital fluctuation induced unconventional charge density waves in Kagome metal, AV3Sb5.

Recently discovered Kagome metal, AV3Sb5 (A=K, Rb, Cs) offers intriguing opportunities to explore correlated phenomena emerging from van Hove singularities (vHs). Particularly, a variety of charge density wave (CDW) have been suggested as candidates of experimental observations. Here, we consider multiple vHs closely located in the electronic structures, so that the particle-hole condensations between them are significant to settle the kinds of CDW. Compared to the single vHs, the orbital fluctuation inside CDW orders diversifies the symmetry breaking patterns, such as nematic charge order. Different kinds of CDW orders can be interpolated as the microscopic parameters vary. Moreover, the influence to the instability of orbital current order is investigated in the presence of conventional CDW. It turns out that the orbital current is closely related to both the non-trivial band topology and unconventional superconducting orders. We discuss the experimental implications to identify these complex CDW orders.