arXiv subjects

Silke Paschen

Publications and source records attributed to Silke Paschen.

At least 19 recordsLinked to original sources

arXiv2026

Nonperturbative Nonlinear Hall Effect in Nonequilibrium Steady States

The nonlinear Hall effect in quantum materials has attracted broad interest, yet most existing studies focus on the weak-field, perturbative regime. Here we develop a nonperturbative approach based on nonequilibrium steady-state Green's functions for dc-field-driven lattice systems, with dissipation and interactions incorporated through self-energies beyond the constant relaxation-time approximation and interband transitions treated alongside their intraband counterparts. Applied to a two-band semimetal model, our approach provides direct access to the strong-field Hall response beyond the nonperturbative crossover where the edge of the nonequilibrium distribution reaches Berry-curvature hot spots, a regime in which constant relaxation-time estimates and Berry curvature dipole calculations become unreliable. We further demonstrate that interaction and electron-phonon self-energies within dynamical mean-field theory can substantially change the Hall signal. Our framework enables quantitative simulations of nonequilibrium nonlinear Hall phenomena and provides guidance for strong-field transport experiments.

cond-mat.str-el↗

arXiv2026

Chiral Weyl-Kondo semimetals and hexagonal heavy fermion systems

Strong correlation, in concert with symmetry and topology, engenders novel gapless phases of matter, though only a tip of the iceberg has been seen. An exemplary framework is provided by Weyl-Kondo semimetals, in which Weyl fermions develop through crystalline symmetry constraints on the emergent low-energy heavy-fermion excitations. This paradigm has opened up new opportunities to explore correlated topologies without a noninteracting counterpart, but fully realizing this potential requires a large base of candidate materials. Here we confront the challenge on both fronts by studying heavy fermion systems with hexagonal space groups. This family contains a large number of chiral nonsymmorphic crystal structures that promote Weyl degeneracies and, in addition, feature geometric frustration in the $f$-electron magnetism. Our calculations for the heavy fermion states identify Weyl-Kondo semimetals with chiral or achiral Weyl nodes in the respective structural classes. We also develop the first search strategy of any kind for the difficult case of strongly correlated materials, which is also suitable for automation, using a combination of materials database, symmetry classification and search for desired experimental properties, and propose as candidate topological heavy fermion systems the chiral CePt$_2$B and achiral Ce$_2$NiGe$_3$ and Ce$_6$Co$_{2-\delta}$Si$_3$. Our findings raise the prospect for strongly correlated metallic topology in the unusual setting of exotic quantum magnetism and, moreover, point a way to go beyond serendipity in the search for novel strongly correlated quantum materials.

cond-mat.str-el↗

arXiv2025

Amplified response of cavity-coupled quantum-critical systems

A quantum critical point develops when matter undergoes a continuous transformation between distinct ground states at absolute zero. It hosts pronounced quantum fluctuations, which render the system highly susceptible to external perturbations. While light-matter coupling has rapidly moved forward as a means to probe and control quantum materials, the capacity of quantum critical fluctuations in the photon-mediated responses has been largely unexplored. Here we advance the notion that directly coupling a quantum critical mode to a quantized cavity field dramatically facilitates the realization of the elusive superradiant phase transition in equilibrium, circumventing at once the key obstacles that have prevented its attainment in spite of decades of pursuit. The superradiant phase transition develops far below the ultrastrong regime of light-matter couplings, and the transition is accompanied by the hybrid system showing strongly enhanced intrinsic squeezing and amplified quantum Fisher information. We also identify candidate cavity quantum materials platforms for validating the proposed effect. Our findings suggest a general principle by which quantum criticality amplifies the response to cavity photons. They also demonstrate that cavity coupling accesses the elevated quantum entanglement of the underlying matter at quantum criticality, thereby pointing to a pathway towards realizing the potential of highly collective quantum materials to expand the capacities of quantum information science.

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arXiv2025

Weyl nodes in CeRu$_4$Sn$_6$ studied by dynamical mean-field theory

The heavy fermion compound CeRu$_4$Sn$_6$ was recently shown to exhibit a spontaneous nonlinear Hall effect, indicating its topological nature. This is consistent with the lack of inversion symmetry that allows for the existence of Weyl nodes. Here, we employ density functional theory combined with dynamical mean-field theory, which is state-of-the-art for strongly correlated materials, and study the topology of CeRu$_4$Sn$_6$. We find five inequivalent Weyl nodes of either type I or II, each having either eight or sixteen symmetry-related replicas. These Weyl nodes bridge the Kondo insulating gap, which is a direct but not an indirect gap. The Weyl points closest to the Fermi level are situated only 0.5 meV below it, and have a very flat dispersion. Our ab initio results establish CeRu$_4$Sn$_6$ as a model system for investigating the interplay between strong electronic correlations and nontrivial topology. These findings provide a theoretical foundation for future studies of quantum transport and interaction-driven topological phases in heavy-fermion systems.

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arXiv2025

Tracking flat bands via phonon-mediated interband scattering

Flat-band (FB) materials have emerged as promising platforms for exploring exotic quantum phases. While numerous candidates have recently been identified through spectroscopic techniques such as angle-resolved photoemission spectroscopy, central challenges remain on how to tune FBs towards the Fermi level $E_F$ and to understand their impact on low-energy excitations probed in electronic transport experiments. Here, we show that, by attributing the temperature dependence of the electrical resistivity at elevated temperatures to electron-phonon interband scattering, one can infer the position of FBs near $E_F$ across diverse material classes. As charge carriers scatter off phonons, interband transitions into FB states lead to distinctive sub- or superlinear resistivity at elevated temperatures, governed by the proximity of the FB to $E_F$. Our phenomenological model captures these universal transport behaviors observed across several recently studied FB compounds and offers a simple, broadly applicable method for detecting flat bands.

cond-mat.str-el↗

arXiv2025

Suppression of shot noise at a Kondo destruction quantum critical point

Strange metal behavior has been observed in an expanding list of quantum materials, with heavy fermion metals serving as a prototype setting. Among the intriguing questions is the nature of charge carriers; there is an increasing recognition that the quasiparticles are lost, as captured by Kondo destruction quantum criticality. Among the recent experimental advances is the measurement of shot noise in a heavy-fermion strange metal. We are thus motivated to study current fluctuations by advancing a minimal Bose-Fermi Kondo lattice model, which admits a well-defined large-$N$ limit. Showing that the model in equilibrium captures the essential physics of Kondo destruction, we proceed to derive quantum kinetic equations and compute shot noise to the leading nontrivial order in $1/N$. Our results reveal a strong suppression of the shot noise at the Kondo destruction quantum critical point, thereby providing the understanding of the striking experiment. Broader implications of our results are discussed.

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arXiv2025

Fully nonequilibrium Hall response from Berry curvature

In topological materials, Berry curvature leads to intrinsic Hall responses. Focusing on time-reversal symmetric systems with broken inversion symmetry, a spontaneoous (zero magnetic field) Hall effect is expected to develop under an applied electric field. Motivated by recent developments in Weyl-Kondo semimetals, here we advance a fully nonequilibrium (FNE) Hall response due to the Berry curvature. In particular, we show that, while the spontaneous Hall current is quadratic in the previously described regime of weak electric field, due to the contribution from the dipole moment of the Berry curvature, the FNE Hall response for non-perturbative electric fields is not controlled by the Berry curvature dipole. Remarkably, the FNE Hall response resembles what happens in systems that break the microscopic time-reversal symmetry. We illustrate the universality of these results by comparing them with their counterparts in systems with any higher-multipole of the Berry curvature. The implications of our results for the understanding of strongly correlated topological semimetals are discussed.

cond-mat.mes-hall↗

arXiv2025

Crystal field excitations in rattling clathrate CeBa$_7$Au$_6$Si$_{40}$

We investigate the local crystal-field environment of cerium in the clathrate compound CeBa$_7$Au$_6$Si$_{40}$ (Ce-BAS) using resonant inelastic x-ray scattering (RIXS) and magnetic susceptibility measurements. Ce-BAS is a rare example of a system where heavy-fermion physics coexists with low-energy rattling phonon modes, making it a candidate for enhanced thermoelectric performance. Magnetic susceptibility measurements reveal a temperature-dependent local moment that cannot be explained within a static crystal-field model. A fit to the susceptibility data requires strong mixing between the j = 5/2 and j = 7/2 crystal-field states, which implies large internal splittings inconsistent with RIXS spectra. In contrast, RIXS data are well described by a model with negligible j = 5/2 - j = 7/2 mixing and an energy separation of 12 meV between the ground and first excited crystal-field states. The inability to reconcile these two datasets within a static framework points to a dynamical modification of the crystal-field potential. We attribute this to coupling between the Ce 4f electrons and low-energy phonons associated with the Ce rattling motion. This interpretation is consistent with theoretical predictions of phonon-enhanced Kondo effects and dynamical Jahn-Teller distortions. Our results highlight the need for multi-orbital impurity models that include phonon coupling to fully describe the low-energy physics of Ce-BAS.

cond-mat.str-el↗

arXiv2025

Correlated flat-band physics in a bilayer kagome metal based on compact molecular orbitals

Flat bands, when located close to the Fermi energy, can considerably enhance the influence of electron correlations on the low energy physics in kagome and other frustrated-lattice metals. A major challenge in describing the interaction effects in such bulk materials is that the flat band is often intermixed with a large number of other bands. Here we show that the recently introduced notion of compact molecular orbitals (CMOs) enable a path forward in describing the dominant effect of the Coulomb interactions in spite of the complexity of the bandstructure. Our materials-based analysis allows for the understanding of the scanning-tunneling-microscopy experiment [J. C. Souza et al., preprint (2024)] of the bilayer kagome metal Ni$_3$In in terms of the CMO notion. From the resulting CMO, an effective Anderson lattice model can be set up. This CMO-based approach enables the calculation of correlation effects that is difficult to do based on the atomic orbitals. Furthermore, it suggests an enriched phase diagram for the strange metal physics of the kagome metal, which can be tested by future experiments. We discuss the implications of our results for the general correlation physics of flat band systems and beyond.

cond-mat.str-el↗

arXiv2025

Metallic quantum criticality enabled by flat bands in a kagome lattice

Strange metals arise in a variety of platforms for strongly correlated electrons, ranging from the cuprates, heavy fermions to flat band systems. Motivated by recent experiments in kagome metals, we study a Hubbard model on a kagome lattice whose noninteracting limit contains flat bands. A Kondo lattice description is constructed, in which the correlation effects are captured by symmetry preserving and exponentially localized molecular orbitals. These compact molecular orbitals represent the local degrees of freedom that emerge from topological flat bands. We identify a quantum critical point at which quasiparticles are lost and strange metallicity emerges. Our theoretical work opens up a new route for realizing beyond-Landau quantum criticality, as well as the associated strange metallicity and emergent quantum phases.

cond-mat.str-el↗

arXiv2025

Amplified multipartite entanglement witnessed in a quantum critical metal

Strong correlations in matter promote a landscape of quantum phases and associated quantum critical points. For metallic systems, there is increasing recognition that the quantum criticality goes beyond the Landau framework and, thus, novel means are needed to characterize the quantum critical fluid. Here we do so by studying an entanglement quantity, the quantum Fisher information, in a strange metal system, focusing on the exemplary case of an Anderson/Kondo lattice model near its Kondo destruction quantum critical point. The spin quantum Fisher information peaks at the quantum critical point and indicates a strongly entangled ground state. Our results are supported by the quantum Fisher information extracted from inelastic neutron scattering measurements in heavy fermion metals. Our work elucidates the loss of quasiparticles in strange metals, opens a quantum information avenue to advance the understanding of metallic quantum criticality in a broad range of strongly correlated systems, and points to a novel regime of quantum matter to realize amplified entanglement.

cond-mat.str-el↗

arXiv2024

Shot noise and universal Fano factor as characterization of strongly correlated metals

Shot noise measures out-of-equilibrium current fluctuations and is a powerful tool to probe the nature of current-carrying excitations in quantum systems. Recent shot noise measurements in the heavy fermion strange metal YbRh$_2$Si$_2$ exhibit a strong suppression of the Fano factor ($F$) -- the ratio of the current noise to the average current in the DC limit. This system is representative of metals in which electron correlations are extremely strong. Here we carry out the first theoretical study on the shot noise of diffusive metals in the regime of strong correlations. A Boltzmann-Langevin equation formulation is constructed in a quasiparticle description in the presence of strong correlations. We find that $F = \sqrt{ 3}/{4}$ in such a correlation regime. Thus, we establish the aforementioned Fano factor as universal to Fermi liquids, and show that the Fano factor suppression observed in experiments on YbRh$_2$Si$_2$ necessitates a loss of the quasiparticles. Our work opens the door to systematic theoretical studies of shot noise as a means of characterizing strongly correlated metallic phases and materials.

cond-mat.str-el↗

arXiv2024

Magnetic Weyl-Kondo semimetals induced by quantum fluctuations

Weyl-Kondo semimetals are strongly correlated topological semimetals that develop through the cooperation of the Kondo effect with space group symmetries. The Kondo effect, capturing quantum fluctuations associated with strong correlations, is usually suppressed by magnetic order. Here we develop the theory of magnetic Weyl-Kondo semimetal. The key of the proposed mechanism is that the magnetic order comes from conduction $d$ electrons, such that the local $f$ moments can still fluctuate. We illustrate the extreme case where the magnetic space group symmetries prevent any spontaneous magnetization on the sites with the $f$-orbitals. In this case, topological degeneracies, including hourglass Weyl-Kondo nodal lines, appear when the magnetic space group symmetry constrains the Kondo-driven low-energy excitations; they lead to a third-order nonlinear anomalous Hall response. Based on the proposed mechanism, we explore the interplay between strong correlations and symmetries with database search leading to several candidate materials. The most prominent candidates are antiferromagnetic $\rm UNiGa$ and $\rm UNiAl$, with a third-order anomalous Hall response, as well as ferromagnetic $\rm USbTe$ and $\rm CeCoPO$, with a first-order one. Our findings pave the way for future experimental and theoretical investigations that promise to further advance the overarching theme of strongly correlated topology.

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arXiv2024

Anomalous Hall effect and quantum criticality in geometrically frustrated heavy fermion metals

Studies on the heavy-fermion pyrochlore iridate (Pr$_2$Ir$_2$O$_7$) point to the role of time-reversal-symmetry breaking in geometrically frustrated Kondo lattices. With this motivation, here we study the effect of Kondo coupling and chiral spin liquids in a frustrated $J_1-J_2$ model on a square lattice. We treat the Kondo effect within a slave-fermion approach, and discuss our results in the context of a proposed global phase diagram for heavy fermion metals. We calculate the anomalous Hall response for the chiral states of both the Kondo destroyed and Kondo screened phases. Across the quantum critical point, a reconstruction of the Fermi surface leads to a sudden change of the Berry curvature distribution and, consequently, a jump of the anomalous Hall conductance. We discuss the implications of our results for the heavy-fermion pyrochlore iridate and propose an interface structure based on Kondo insulators to further explore such effects.

cond-mat.str-el↗

arXiv2024

Symmetry constraints and spectral crossing in a Mott insulator with Green's function zeros

Lattice symmetries are central to the characterization of electronic topology. Recently, it was shown that Green's function eigenvectors form a representation of the space group. This formulation has allowed the identification of gapless topological states even when quasiparticles are absent. Here we demonstrate the profundity of the framework in the extreme case, when interactions lead to a Mott insulator, through a solvable model with long-range interactions. We find that both Mott poles and zeros are subject to the symmetry constraints, and relate the symmetry-enforced spectral crossings to degeneracies of the original non-interacting eigenstates. Our results lead to new understandings of topological quantum materials and highlight the utility of interacting Green's functions toward their symmetry-based design.

cond-mat.str-el↗

arXiv2024

Emergent flat band and topological Kondo semimetal driven by orbital-selective correlations

Flat electronic bands are expected to show proportionally enhanced electron correlations, which may generate a plethora of novel quantum phases and unusual low-energy excitations. They are increasingly being pursued in $d$-electron-based systems with crystalline lattices that feature destructive electronic interference, where they are often topological. Such flat bands, though, are generically located far away from the Fermi energy, which limits their capacity to partake in the low-energy physics. Here we show that electron correlations produce emergent flat bands that are pinned to the Fermi energy. We demonstrate this effect within a Hubbard model, in the regime described by Wannier orbitals where an effective Kondo description arises through orbital-selective Mott correlations. Moreover, the correlation effect cooperates with symmetry constraints to produce a topological Kondo semimetal. Our results motivate a novel design principle for Weyl Kondo semimetals in a new setting, viz. $d$-electron-based materials on suitable crystal lattices, and uncover interconnections among seemingly disparate systems that may inspire fresh understandings and realizations of correlated topological effects in quantum materials and beyond.

cond-mat.str-el↗

arXiv2024

Quantum Fisher information in a strange metal

A strange metal is an exotic state of correlated quantum matter; intensive efforts are ongoing to decipher its nature. Here we explore whether the quantum Fisher information (QFI), a concept from quantum metrology, can provide new insight. We use inelastic neutron scattering and quantum Monte Carlo simulations to study a Kondo destruction quantum critical point, where strange metallicity is associated with fluctuations beyond a Landau order parameter. We find that the QFI probed away from magnetic Bragg peaks, where the effect of magnetic ordering is minimized, increases strongly and without a characteristic scale as the strange metal forms with decreasing temperature, evidencing its unusual entanglement properties. Our work opens a new direction for studies across strange metal platforms.

cond-mat.str-el↗

arXiv2023

Flat bands, strange metals, and the Kondo effect

Flat band materials such as the kagome metals or moiré superlattice systems are of intense current interest. Flat bands can result from the electron motion on numerous (special) lattices and usually exhibit topological properties. Their reduced bandwidth proportionally enhances the effect of Coulomb interaction, even when the absolute magnitude of the latter is relatively small. Seemingly unrelated to these cases is the large family of strongly correlated electron systems, which includes the heavy fermion compounds, cuprate and pnictide superconductors. In addition to itinerant electrons from large, strongly overlapping orbitals, they frequently contain electrons from more localized orbitals, which are subject to a large Coulomb interaction. The question then arises as to what commonality in the physical properties and microscopic physics, if any, exists between the two broad categories of materials? A rapidly increasing body of strikingly similar phenomena across the different platforms -- from electronic localization-delocalization transitions to strange metal behavior and unconventional superconductivity -- suggests that similar underlying principles could be at play. Indeed, it has recently been suggested that flat band physics can be understood in terms of Kondo physics. Inversely, the concept of electronic topology from lattice symmetry, which is fundamental in flat band systems, is enriching the field of strongly correlated electron systems where correlation-driven topological phases are increasingly being investigated. Here we elucidate this connection, survey the new opportunities for cross-fertilization in understanding across the platforms, and assess the prospect for new insights that may be gained into both the correlation physics and its intersection with electronic topology.

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