arXiv subjects

Silke Paschen

Publications and source records attributed to Silke Paschen.

At least 37 records · Page 2Linked to original sources

arXiv2023

Shot noise in a strange metal

Strange-metal behavior has been observed in materials ranging from high-temperature superconductors to heavy fermion metals. In conventional metals, current is carried by quasiparticles; although it has been suggested that quasiparticles are absent in strange metals, direct experimental evidence is lacking. We measured shot noise to probe the granularity of the current-carrying excitations in nanowires of the heavy fermion strange metal YbRh2Si2. When compared with conventional metals, shot noise in these nanowires is strongly suppressed. This suppression cannot be attributed to either electron-phonon or electron-electron interactions in a Fermi liquid, which suggests that the current is not carried by well-defined quasiparticles in the strange-metal regime that we probed. Our work sets the stage for similar studies of other strange metals.

cond-mat.str-el↗

arXiv2023

Topological Diagnosis of Strongly Correlated Electron Systems

The intersection of electronic topology and strong correlations offers a rich platform to discover exotic quantum phases of matter and unusual materials. An overarching challenge that impedes the discovery is how to diagnose topology in strongly correlated settings, as exemplified by Mott insulators. Here, we develop a general framework to address this outstanding question and illustrate its power in the case of Mott insulators. The concept of Green's function Berry curvature -- which is frequency dependent -- is introduced. We apply this notion in a system that contains symmetry-protected nodes in its noninteracting bandstructure; strong correlations drive the system into a Mott insulating state, creating contours in frequency-momentum space where the Green's function vanishes. The Green's function Berry flux of such zeros is found to be quantized, and is as such direct probe of the system's topology. Our framework allows for a comprehensive search of strongly correlated topological materials with Green's function topology.

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arXiv2023

How to identify and characterize strongly correlated topological semimetals

How strong correlations and topology interplay is a topic of great current interest. In this perspective paper, we focus on correlation-driven gapless phases. We take the time-reversal symmetric Weyl semimetal as an example because it is expected to have clear (albeit nonquantized) topological signatures in the Hall response and because the first strongly correlated representative, the noncentrosymmetric Weyl-Kondo semimetal Ce$_3$Bi$_4$Pd$_3$, has recently been discovered. We summarize its key characteristics and use them to construct a prototype Weyl-Kondo semimetal temperature-magnetic field phase diagram. This allows for a substantiated assessment of other Weyl-Kondo semimetal candidate materials. We also put forward scaling plots of the intrinsic Berry-curvature-induced Hall response vs the inverse Weyl velocity -- a measure of correlation strength, and vs the inverse charge carrier concentration -- a measure of the proximity of Weyl nodes to the Fermi level. They suggest that the topological Hall response is maximized by strong correlations and small carrier concentrations. We hope that our work will guide the search for new Weyl-Kondo semimetals and correlated topological semimetals in general, and also trigger new theoretical work.

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arXiv2023

Quantum Criticality Enabled by Intertwined Degrees of Freedom

Strange metals appear in a wide range of correlated materials. Electronic localization-delocalization and the expected loss of quasiparticles characterize beyond-Landau metallic quantum critical points and the associated strange metals. Typical settings involve local spins. Systems that contain entwined degrees of freedom offer new platforms to realize novel forms of quantum criticality. Here, we study the fate of an SU(4) spin-orbital Kondo state in a multipolar Bose-Fermi Kondo model, which provides an effective description of a multipolar Kondo lattice, using a renormalization-group method. We show that at zero temperature a generic trajectory in the model's parameter space contains two quantum critical points, which are associated with the destruction of Kondo entanglement in the orbital and spin channels respectively. Our asymptotically exact results reveal an overall phase diagram, provide the theoretical basis to understand puzzling recent experiments of a multipolar heavy fermion metal, and point to a means of designing new forms of quantum criticality and strange metallicity in a variety of strongly correlated systems.

cond-mat.str-el↗

arXiv2023

Dark Matter Detection with Strongly Correlated Topological Materials: Flatband Effect

Dirac materials have been proposed as a new class of electron-based detectors for light dark-matter (DM) scattering or absorption, with predicted sensitivities far exceeding superconductors and superfluid helium. The superiority of Dirac materials originates from a significantly reduced in-medium dielectric response winning over the suppression of DM scattering owing to the limited phase space at the point-like Fermi surface. Here we propose a new route to enhance significantly the DM detection efficiency via strongly correlated topological semimetals. Specifically, by considering a strongly correlated Weyl semimetal model system, we demonstrate that the strong correlation-induced flatband effects can amplify the coupling and detection sensitivity to light DM particles by expanding the scattering phase space, while maintaining a weak dielectric in-medium response.

cond-mat.str-el↗

arXiv2023

Size effect on the thermal conductivity of a type-I clathrate

Clathrates are a materials class with an extremely low phonon thermal conductivity, which is a key ingredient for a high thermoelectric conversion efficiency. Here, we present a study on the type-I clathrate La$_{1.2}$Ba$_{6.8}$Au$_{5.8}$Si$_{38.8}\square_{1.4}$ directed at lowering the phonon thermal conductivity even further by forming mesoscopic wires out of it. Our hypothesis is that the interaction of the low-energy rattling modes of the guest atoms (La and Ba) with the acoustic modes, which originate mainly from the type-I clathrate framework (formed by Au and Si atoms, with some vacancies $\square$), cuts off their dispersion and thereby tilts the balance of phonons relevant for thermal transport to long-wavelength ones. Thus, size effects are expected to set in at relatively long length scales. The structuring was carried out using a top-down approach, where the wires, ranging from 1260 nm to 630 nm in diameter, were cut from a piece of single crystal using a focused ion beam technique. Measurements of the thermal conductivity were performed with a self-heating $3ω$ technique down to 80 K. Indeed, they reveal a reduction of the room-temperature phonon thermal conductivity by a sizable fraction of $\sim$40 % for our thinnest wire, thereby confirming our hypothesis.

cond-mat.mtrl-sci↗

arXiv2023

Is the optical conductivity of heavy fermion strange metals Planckian?

Strange metal behavior appears across a variety of condensed matter settings and beyond, and achieving a universal understanding is an exciting prospect. The beyond-Landau quantum criticality of Kondo destruction has had considerable success in describing the behavior of strange metal heavy fermion compounds, and there is some evidence that the associated partial localization-delocalization nature can be generalized to diverse materials classes. Other potential overarching principles at play are also being explored. An intriguing proposal is that Planckian scattering, with a rate of $k_{\rm B}T/\hbar$, leads to the linear temperature dependence of the (dc) electrical resistivity, which is a hallmark of strange metal behavior. Here we extend a previously introduced analysis scheme based on the Drude description of the dc resistivity to optical conductivity data. When they are well described by a simple (ac) Drude model, the scattering rate can be directly extracted. This avoids the need to determine the ratio of charge carrier concentration to effective mass, which has complicated previous analyses based on the dc resistivity. However, we point out that strange metals typically exhibit strong deviations from Drude behavior, as exemplified by the ``extreme'' strange metal YbRh$_2$Si$_2$. This calls for alternative approaches, and we point to the power of strange metal dynamical (energy-over-temperature) scaling analyses for the inelastic part of the optical conductivity. If such scaling extends to the low-frequency limit, a strange metal relaxation rate can be estimated, and may ultimately be used to test whether strange metals relax in a Planckian manner.

cond-mat.str-el↗

arXiv2022

Topological semimetals without quasiparticles

The interplay between interactions and topology in quantum materials is of extensive current interest. Strong correlations are known to be important for insulating topological states, as exemplified by the fractional quantum Hall effect. For the metallic case, whether and how they can drive topological states that have no free-electron counterparts is an open and pressing question. We introduce a general framework for lattice symmetries to constrain single-particle excitations even when they are not quasiparticles, and substantiate it in a periodic Anderson model with two channels of conduction electrons. We demonstrate that symmetry constrains correlation-induced emergent excitations to produce non-Fermi liquid topological phases. The loss of quasiparticles in these phases is manifested in a non-Fermi liquid form of spectral and transport properties, whereas its topological nature is characterized by surface states and valley and spin Hall conductivities. We also identify candidate materials to realize the proposed phases. Our work opens a door to a variety of non-Fermi liquid topological phases in a broad range of strongly correlated materials.

cond-mat.str-el↗

arXiv2022

Topological Semimetal driven by Strong Correlations and Crystalline Symmetry

Electron correlations amplify quantum fluctuations and, as such, they have been recognized as the origin of a rich landscape of quantum phases. Whether and how they lead to gapless topological states is an outstanding question, and a framework that allows for determining novel phases and identifying new materials is in pressing need. Here we advance a general approach, in which strong correlations cooperate with crystalline symmetry to drive gapless topological states. We test this materials design principle by exploring Kondo lattice models and materials whose space group symmetries may promote different kinds of electronic degeneracies, with a particular focus on square-net systems. Weyl-Kondo nodal-line semimetals -- with nodes pinned to the Fermi energy -- are identified. We describe how this approach can be applied to discover strongly correlated topological semimetals, identify three heavy fermion compounds as new candidates, provide first direct experimental evidence for our prediction in Ce$_2$Au$_3$In$_5$, and discuss how our approach may lead to many more. Our findings illustrate the potential of the proposed materials design principle to guide the search for new topological metals in a broad range of strongly correlated systems.

cond-mat.str-el↗

arXiv2022

Are Heavy Fermion Strange Metals Planckian?

Strange metal behavior refers to a linear temperature dependence of the electrical resistivity at temperatures below the Mott-Ioffe-Regel limit. It is seen in numerous strongly correlated electron systems, from the heavy fermion compounds, via transition metal oxides and iron pnictides, to magic angle twisted bi-layer graphene, frequently in connection with unconventional or "high temperature" superconductivity. To achieve a unified understanding of these phenomena across the different materials classes is a central open problem in condensed matter physics. Tests whether the linear-in-temperature law might be dictated by Planckian dissipation - scattering with the rate $\sim k_{\rm B}T/\hbar$, are receiving considerable attention. Here we assess the situation for strange metal heavy fermion compounds. They allow to probe the regime of extreme correlation strength, with effective mass or Fermi velocity renormalizations in excess of three orders of magnitude. Adopting the same procedure as done in previous studies, i.e., assuming a simple Drude conductivity with the above scattering rate, we find that for these strongly renormalized quasiparticles, scattering is much weaker than Planckian, implying that the linear temperature dependence should be due to other effects. We discuss implications of this finding and point to directions for further work.

cond-mat.str-el↗

arXiv2021

The many faces (phases) of strong correlations

There has been considerable recent progress in discovering and understanding quantum phases and fluctuations produced by strong correlations. Heavy fermion systems are an ideal platform for systematic studies because low and competing energy scales make them highly tunable. As such the phases (faces) of strong correlations transform continuously into one another.

cond-mat.str-el↗

arXiv2021

Kondomania

Originally the Kondo effect describes a scattering mechanism of electrons in metals that have defects with internal quantum mechanical degrees of freedom. The characteristic dynamic interplay between localized and itinerant states occurs in many different forms. It is now clear that the Kondo effect shapes the properties of various classes of quantum materials and is a key to understanding their unusual properties. In the following we outline a selection of the most important developments in this Kondomania.

cond-mat.str-el↗

arXiv2021

Extreme topological tunability of Weyl-Kondo semimetal to Zeeman coupling

There is considerable interest in the intersection of correlations and topology, especially in metallic systems. Among the outstanding questions are how strong correlations drive novel topological states and whether such states can be readily controlled. Here we study the effect of a Zeeman coupling on a Weyl-Kondo semimetal in a nonsymmorphic and noncentrosymmetric Kondo-lattice model. A sequence of distinct and topologically nontrivial semimetal regimes are found, each containing Kondo-driven and Fermi-energy-bound Weyl nodes. The nodes annihilate at a magnetic field that is smaller than what it takes to suppress the Kondo effect. As such, we demonstrate an extreme topological tunability that is isolated from the tuning of the strong correlations per se. Our results are important for experiments in strongly correlated systems, and set the stage for mapping out a global phase diagram for strongly correlated topology.

cond-mat.str-el↗

arXiv2021

Controlling electronic topology in a strongly correlated electron system

Combining strong electron correlations [1-4] and nontrivial electronic topology [5] holds great promise for discovery. So far, this regime has been rarely accessed and systematic studies are much needed to advance the field. Here we demonstrate the control of topology in a heavy fermion system. We use magnetic field to manipulate Weyl nodes in a Weyl-Kondo semimetal [6-8], up to the point where they annihilate in a topological quantum phase transition. The suppression of the topological characteristics occurs in an intact and only weakly varying strongly correlated "background". Thus, topology is changing per se and not as a consequence of a change of the correlation state, for instance across a magnetic, electronic or structural phase transition. Our demonstration of genuine topology tuning in a strongly correlated electron system sets the stage for establishing global phase diagrams of topology, an approach that has proven highly valuable to explore and understand topologically trivial strongly correlated electron systems [1-4]. Our work also lays the ground for technological exploitations of controlled electronic topology.

cond-mat.str-el↗

arXiv2021

Quantum phases driven by strong correlations

It has long been thought that strongly correlated systems are adiabatically connected to their noninteracting counterpart. Recent developments have highlighted the fallacy of this traditional notion in a variety of settings. Here we use a class of strongly correlated electron systems as a platform to illustrate the kind of quantum phases and fluctuations that are created by strong correlations. Examples are quantum critical states that violate the Fermi liquid paradigm, unconventional superconductivity that goes beyond the BCS framework, and topological semimetals induced by the Kondo interaction. We assess the prospect of designing other exotic phases of matter, by utilizing alternative degrees of freedom or alternative interactions, and point to the potential of these correlated states for quantum technology.

cond-mat.str-el↗

arXiv2021

Giant spontaneous Hall effect in a nonmagnetic Weyl-Kondo semimetal

Nontrivial topology in condensed matter systems enriches quantum states of matter, to go beyond either the classification into metals and insulators in terms of conventional band theory or that of symmetry broken phases by Landau's order parameter framework. So far, focus has been on weakly interacting systems, and little is known about the limit of strong electron correlations. Heavy fermion systems are a highly versatile platform to explore this regime. Here we report the discovery of a giant spontaneous Hall effect in the Kondo semimetal Ce3Bi4Pd3 that is noncentrosymmetric but preserves time reversal symmetry. We attribute this finding to Weyl nodes - singularities of the Berry curvature - that emerge in the immediate vicinity of the Fermi level due to the Kondo interaction. We stress that this phenomenon is distinct from the previously detected anomalous Hall effect in materials with broken time reversal symmetry; instead, it manifests an extreme topological response that requires a beyond-perturbation-theory description of the previously proposed nonlinear Hall effect. The large magnitude of the effect in even tiny electric and zero magnetic fields, as well as its robust bulk nature may aid the exploitation in topological quantum devices.

cond-mat.str-el↗

arXiv2020

Weyl-Kondo Semimetal: Towards Control of Weyl Nodes

Heavy fermion semimetals represent a promising setting to explore topological metals driven by strong correlations. In this paper, we i) summarize the theoretical results in a Weyl-Kondo semimetal phase for a strongly correlated model with inversion-symmetry-breaking and time-reversal invariance, and the concurrent work that has experimentally discovered this phase in the non-magnetic non-centrosymmetric heavy fermion system Ce$_3$Bi$_4$Pd$_3$; and ii) describe what is expected theoretically when the time-reversal symmetry is also broken.

cond-mat.str-el↗

arXiv2020

Weyl-Kondo semimetals in nonsymmorphic systems

There is considerable current interest to explore electronic topology in strongly correlated metals, with heavy fermion systems providing a promising setting. Recently, a Weyl-Kondo semimetal phase has been concurrently discovered in theoretical and experimental studies. The theoretical work was carried out in a Kondo lattice model that is time-reversal invariant but inversion-symmetry breaking. In this paper, we show in some detail how nonsymmorphic space-group symmetry and strong correlations cooperate to form Weyl nodal excitations with highly reduced velocity and pin the resulting Weyl nodes to the Fermi energy. A tilted variation of the Weyl-Kondo solution is further analyzed here, following the recent consideration of such effect in the context of understanding a large spontaneous Hall effect in Ce$_3$Bi$_4$Pd$_3$ (Dzsaber et al., arXiv:1811.02819). We discuss the implications of our results for the enrichment of the global phase diagram of heavy fermion metals, and for the space-group symmetry enforcement of topological semimetals in other strongly correlated settings.

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