arXiv subjects

Silke Paschen

Publications and source records attributed to Silke Paschen.

53 records · Page 3Linked to original sources

arXiv2020

Anisotropic Physical Properties of the Kondo Semimetal CeCu$_{1.11}$As$_2$

The recently proposed novel materials class called Weyl-Kondo semimetal (WKSM) is a time reversal invariant but inversion symmetry broken Kondo semimetal in which Weyl nodes are pushed to the Fermi level by the Kondo interaction. Here we explore whether CeCu$_{1+x}$As$_2$ may be a new WKSM candidate. We report on its single-crystal growth, structure determination and physical properties investigation. Previously published studies on polycrystalline samples suggest that it is indeed a Kondo semimetal, which is confirmed by our investigations on single crystals. X-ray diffraction reveals that CeCu$_{1+x}$As$_2$ crystallizes in a tetragonal centrosymmetric structure, although the inversion symmetry could still be broken locally due to partially occupied Cu sites. Chemical analysis results in an average occupation $x$ = 0.11(1). The electrical resistivity increases logarithmically with decreasing temperature, and saturates below 10 K. A Kondo temperature $T_{\mathrm{K}}$ $\approx$ 4 K is extracted from entropy, estimated from the specific heat measurements. From Hall effect experiments, a charge carrier density of $8.8 \times 10^{20}$ cm$^{-3}$ is extracted, a value characteristic of a semimetal. The magnetization shows pronounced anisotropy, with no evidence of magnetic ordering down to 0.4 K. We thus classify CeCu$_{1.11}$As$_2$ as a tetragonal Kondo semimetal with anisotropic magnetic properties, with a possibly broken inversion symmetry, thus fulfilling the necessary conditions for a WKSM state.

cond-mat.str-el↗

arXiv2020

Heavy-electron quantum criticality and single-particle spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) have become indispensable tools in the study of correlated quantum materials. Both probe complementary aspects of the single-particle excitation spectrum. Taken together, ARPES and STM have the potential to explore properties of the electronic Green's function, a central object of many-body theory. This review explicates this potential with a focus on heavy-electron quantum criticality, especially the role of Kondo destruction. A discussion on how to probe the Kondo destruction effect across the quantum-critical point using ARPES and STM measurements is presented. Particular emphasis is placed on the question of how to distinguish between the signatures of the initial onset of hybridization-gap formation, which is the "high-energy" physics to be expected in all heavy-electron systems, and those of Kondo destruction, which characterizes the low-energy physics and, hence, the nature of quantum criticality. Recent progress and possible challenges in the experimental investigations are surveyed, the STM and ARPES spectra for several quantum-critical heavy-electron compounds are compared, and the prospects for further advances are outlined.

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arXiv2020

$^{105}$Pd NMR and NQR study of the cubic heavy fermion system Ce$_3$Pd$_{20}$Si$_6$

We report $^{105}$Pd NMR and NQR measurements on a single crystal of Ce$_3$Pd$_{20}$Si$_6$, where antiferroquadrupolar and antiferromagnetic orders develop at low temperature. From the analysis of NQR and NMR spectra, we have determined the electric field gradient (EFG) tensors and the anisotropic Knight shift ($K$) components for both inequivalent Pd sites - Pd($32f$) and Pd($48h$). The observed EFG values are in excellent agreement with our state-of-the-art DFT calculations. The principal values of the quadrupolar coupling are $(20.37 \pm 0.02)$ MHz and $(5.45 \pm 0.02)$ MHz, for the Pd($32f$) and Pd($48h$) site, respectively, which is large compared to the Larmor frequency defined by the gyromagnetic constant $γ= 1.94838$ MHz/T for $^{105}$Pd. Therefore, the complete knowledge of $K$ and the EFG tensors is crucial to establish the correspondence between NMR spectra and crystallographic sites, which is needed for a complete analysis of the magnetic structure, static spin susceptibility, and the spin-lattice relaxation rate data and a better understanding of the groundstate of Ce$_3$Pd$_{20}$Si$_6$.

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arXiv2019

Comment on "Tuning low-energy scales in YbRh$_2$Si$_2$ by non-isoelectronic substitution and pressure"

In Ref. 1, Schubert et al. [Phys. Rev. Research 1, 032004 (2019)] reported measurements of the isothermal magnetoresistance of Fe- and Ni-substituted YbRh$_2$Si$_2$, based on which they raised questions about the Kondo destruction description for the magnetic field-induced quantum critical point (QCP) of pristine YbRh$_2$Si$_2$. Here we make three points. Firstly, as shown by studies on pristine YbRh$_2$Si$_2$ in Paschen et al. and Friedemann et al., isothermal crossed-field and single-field Hall effect measurements are necessary to ascertain the evolution of the Fermi surface across this QCP. Because Schubert et al. did not carry out such measurements, their results on Fe- and Ni-substituted YbRh$_2$Si$_2$ cannot be used to assess the validity of the Kondo destruction picture neither for substituted nor for pristine YbRh$_2$Si$_2$. Secondly, when referring to the data of Friedemann et al. on the isothermal crossover of YbRh$_2$Si$_2$, they did not recognize the implications of the crossover width, quantified by the full width at half maximum (FWHM), being linear in temperature, with zero offset, over about $1.5$ decades in temperature, from 30 mK to 1 K. Finally, in claiming deviations of Hall crossover FWHM data of Friedemann et al. from the above linear-in-$T$ dependence they neglected the error bars of these measurements and discarded some of the data points. The claims of Schubert et al. are thus not supported by data, neither previously published nor new (Ref. 1). As such they cannot invalidate the evidence that has been reported for Kondo destruction quantum criticality in YbRh$_2$Si$_2$.

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arXiv2019

Dynamical Kondo effect and Kondo destruction in effective models for quantum-critical heavy fermion metals

Quantum criticality in certain heavy-fermion metals is believed to go beyond the Landau framework of order-parameter fluctuations. In particular, there is considerable evidence for Kondo destruction: a disappearance of the static Kondo singlet amplitude that results in a sudden reconstruction of Fermi surface across the quantum critical point and an extra critical energy scale. This effect can be analyzed in terms of a dynamical interplay between the Kondo and RKKY interactions. In the Kondo-destroyed phase, a well-defined Kondo resonance is lost, but Kondo singlet correlations remain at nonzero frequencies. This dynamical effect allows for mass enhancement in the Kondo-destroyed phase. Here, we elucidate the dynamical Kondo effect in Bose-Fermi Kondo/Anderson models, which unambiguously exhibit Kondo-destruction quantum critical points. We show that a simple physical quantity---the expectation value $\langle {\bf S}_{f} \cdot {\bf s}_{c} \rangle$ for the dot product of the local ($f$) and conduction-electron ($c$) spins---varies continuously across such quantum critical points. A nonzero $\langle {\bf S}_{f} \cdot {\bf s}_{c} \rangle$ manifests the dynamical Kondo effect that operates in the Kondo-destroyed phase. Implications are discussed for the stability of Kondo-destruction quantum criticality as well as the understanding of experimental results in quantum critical heavy-fermion metals.

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arXiv2018

Low-carrier density and fragile magnetism in a Kondo lattice system

Kondo-based semimetals and semiconductors are of extensive current interest as a viable platform for strongly correlated states. It is thus important to understand the routes towards such dilute-carrier correlated states. One established pathway is through Kondo effect in metallic non-magnetic analogues. Here we advance a new mechanism, through which Kondo-based semimetals develop out of conduction electrons with a low carrier-density in the presence of an even number of rare-earth sites. We demonstrate this effect by studying the Kondo material Yb3Ir4Ge13 along with its closed-f-shell counterpart, Lu3Ir4Ge13. Through magnetotransport, optical conductivity and thermodynamic measurements, we establish that the correlated semimetallic state of Yb3Ir4Ge13 below its Kondo temperature originates from the Kondo effect of a low carrier conduction-electron background. In addition, it displays fragile magnetism at very low temperatures, which, in turn, can be tuned to a non Fermi liquid regime through Lu-for-Yb substitution. These findings are connected with recent theoretical studies in simplified models. Our results open an entirely new venue to explore the strong correlation physics in a semimetallic environment.

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arXiv2018

Kondo Destruction and Multipolar Order-- Implications for Heavy Fermion Quantum Criticality

Quantum criticality beyond the Landau paradigm represents a fundamental problem in condensed matter and statistical physics. Heavy fermion systems with multipolar degrees of freedom can play an important role in the search for its universal description. We consider a Kondo lattice model with both spin and quadrupole degrees of freedom, which we show to exhibit an antiferroquadrupolar phase. Using a field theoretical representation of the model, we find that Kondo couplings are exactly marginal in the renormalization group sense in this phase. This contrasts with the relevant nature of the Kondo couplings in the paramagnetic phase and, as such, it implies that a Kondo destruction and a concomitant small to large Fermi surface jump must occur as the system is tuned from the antiferroquadrupolar ordered to the paramagnetic phase. Implications of our results for multipolar heavy fermion physics in particular and metallic quantum criticality in general are discussed.

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arXiv2018

Determining the local low-energy excitations in the Kondo semimetal CeRu$_4$Sn$_6$ using resonant inelastic x-ray scattering

We have investigated the local low-energy excitations in CeRu$_4$Sn$_6$, a material discussed recently in the framework of strongly correlated Weyl semimetals, by means of Ce $M_5$ resonant inelastic x-ray scattering (RIXS). The availability of both $^2$F$_\frac{5}{2}$ and $^2$F$_\frac{7}{2}$ excitations of the Ce $4f^1$ configuration in the spectra allows for the determination of the crystal-electric field parameters that explain quantitatively the temperature dependence and anisotropy of the magnetic susceptibility. The absence of an azimuthal dependence in the spectra indicates that all crystal-electric field states are close to being rotational symmetric. We show further that the non-negligible impact of the $\check A_6^0$ parameter on the ground state of CeRu$_4$Sn$_6$ leads to a reduction of the magnetic moment due to multiplet intermixing. The RIXS results are consistent with inelastic neutron scattering (INS) data and are compared to the predictions from \textsl{ab-initio} based electronic structure calculations.

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arXiv2018

Weyl-Kondo Semimetal in Heavy Fermion Systems

Insulating states can be topologically nontrivial, a well-established notion that is exemplified by the quantum Hall effect and topological insulators. By contrast, topological metals have not been experimentally evidenced until recently. In systems with strong correlations, they have yet to be identified. Heavy fermion semimetals are a prototype of strongly correlated systems and, given their strong spin-orbit coupling, present a natural setting to make progress. Here we advance a Weyl-Kondo semimetal phase in a periodic Anderson model on a noncentrosymmetric lattice. The quasiparticles near the Weyl nodes develop out of the Kondo effect, as do the surface states that feature Fermi arcs. We determine the key signatures of this phase, which are realized in the heavy fermion semimetal Ce$_3$Bi$_4$Pd$_3$. Our findings provide the much-needed theoretical foundation for the experimental search of topological metals with strong correlations, and open up a new avenue for systematic studies of such quantum phases that naturally entangle multiple degrees of freedom.

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arXiv2017

Suppression of vacancies boosts thermoelectric performance in type-I clathrates

Intermetallic type-I clathrates continue to attract attention as promising thermoelectric materials. Here we present structural and thermoelectric properties of single crystalline Ba8(Cu,Ga,Ge,v)46, where v denotes a vacancy. By single crystal X-ray diffraction on crystals without Ga we find clear evidence for the presence of vacancies at the 6c site in the structure. With increasing Ga content, vacancies are successively filled. This increases the charge carrier mobility strongly, even within a small range of Ga substitution, leading to reduced electrical resistivity and enhanced thermoelectric performance. The largest figure of merit ZT =0.9 at 900 K is found for a single crystal of approximate composition Ba8Cu4.6Ga1.0Ge40.4. This value, that may further increase at higher temperatures, is one of the largest to date found in transition metal element-based clathrates.

cond-mat.mtrl-sci↗

arXiv2017

Strange metal state near a heavy-fermion quantum critical point

Recent experiments on quantum criticality in the Ge-substituted heavy-electron material YbRh2Si2 under magnetic field have revealed a possible non-Fermi liquid (NFL) strange metal (SM) state over a finite range of fields at low temperatures, which still remains a puzzle. In the SM region, the zero-field antiferromagnetism is suppressed. Above a critical field, it gives way to a heavy Fermi liquid with Kondo correlation. The T (temperature)-linear resistivity and the T-logarithmic followed by a power-law singularity in the specific heat coefficient at low T, salient NFL behaviours in the SM region, are un-explained. We offer a mechanism to address these open issues theoretically based on the competition between a quasi-2d fluctuating short-ranged resonant- valence-bonds (RVB) spin-liquid and the Kondo correlation near criticality. Via a field-theoretical renormalization group analysis on an effective field theory beyond a large-N approach to an anti- ferromagnetic Kondo-Heisenberg model, we identify the critical point, and explain remarkably well both the crossovers and the SM behaviour.

cond-mat.str-el↗

arXiv2016

Giant isotropic Nernst effect in an anisotropic Kondo semimetal

The "failed Kondo insulator" CeNiSn has long been suspected to be a nodal metal, with a node in the hybridization matrix elements. Here we carry out a series of Nernst effect experiments to delineate whether the severely anisotropic magnetotransport coefficients do indeed derive from a nodal metal or can simply be explained by a highly anisotropic Fermi surface. Our experiments reveal that despite an almost 20-fold anisotropy in the Hall conductivity, the large Nernst signal is isotropic. Taken in conjunction with the magnetotransport anisotropy, these results provide strong support for an isotropic Fermi surface with a large anisotropy in quasiparticle mass derived from a nodal hybridization.

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arXiv2015

Global Phase Diagram and Momentum Distribution of Single-Particle Excitations in Kondo insulators

Kondo insulators are emerging as a simplified setting to study both magnetic and metal-to-insulator quantum phase transitions. Here, we study a half-filled Kondo lattice model defined on a magnetically frustrated Shastry-Sutherland geometry. We determine a "global" phase diagram that features a variety of zero-temperature phases; these include Kondo-destroyed antiferromagnetic and paramagnetic metallic phases in addition to the Kondo-insulator phase. Our result provides the theoretical basis for understanding how applying pressure to a Kondo insulator can close its hybridization gap, liberate the local-moment spins from the conduction electrons, and lead to a magnetically correlated metal. We also study the momentum distribution of the single-particle excitations in the Kondo insulating state, and illustrate how Fermi-surface-like features emerge as a precursor to the actual Fermi surfaces of the Kondo-destroyed metals. We discuss the implications of our results for Kondo insulators including SmB$_6$.

cond-mat.str-el↗

arXiv2013

Quantum phase transitions in heavy fermion metals and Kondo insulators

Strongly correlated electron systems at the border of magnetism are of active current interest, particularly because the accompanying quantum criticality provides a route towards both strange-metal non-Fermi liquid behavior and unconventional superconductivity. Among the many important questions is whether the magnetism acts simply as a source of fluctuations in the textbook Landau framework, or instead serves as a proxy for some unexpected new physics. We put into this general context the recent developments on quantum phase transitions in antiferromagnetic heavy fermion metals. Among these are the extensive recent theoretical and experimental studies on the physics of Kondo destruction in a class of beyond-Landau quantum critical points. Also discussed are the theoretical basis for a global phase diagram of antiferromagnetic heavy fermion metals, and the recent surge of materials suitable for studying this phase diagram. Furthermore, we address the generalization of this global phase diagram to the case of Kondo insulators, and consider the future prospect to study the interplay among Kondo coherence, magnetism and topological states. Finally, we touch upon related issues beyond the antiferromagnetic settings, arising in mixed valent, ferromagnetic, quadrupolar, or spin glass f-electron systems, as well as some general issues on emergent phases near quantum critical points.

cond-mat.str-el↗

arXiv2012

Phononic filter effect of rattling phonons in the thermoelectric clathrate Ba$_8$Ge$_{40+x}$Ni$_{6-x}$

One of the key requirements for good thermoelectric materials is a low lattice thermal conductivity. Here we present a combined neutron scattering and theoretical investigation of the lattice dynamics in the type I clathrate system Ba-Ge-Ni, which fulfills this requirement. We observe a strong hybridization between phonons of the Ba guest atoms and acoustic phonons of the Ge-Ni host structure over a wide region of the Brillouin zone which is in contrast with the frequently adopted picture of isolated Ba atoms in Ge-Ni host cages. It occurs without a strong decrease of the acoustic phonon lifetime which contradicts the usual assumption of strong anharmonic phonon--phonon scattering processes. Within the framework of ab-intio density functional theory calculations we interpret these hybridizations as a series of an ti-crossings which act as a low pass filter, preventing the propagation of acoustic phonons. To highlight the effect of such a phononic low pass filter on the thermal transport, we compute the contribution of acoustic phonons to the thermal conductivity of Ba$_8$Ge$_{40}$Ni$_{6}$ and compare it to those of pure Ge and a Ge$_{46}$ empty-cage model system.

cond-mat.mtrl-sci↗

arXiv2010

Discontinuous Hall coefficient at the quantum critical point in YbRh2Si2

YbRh2Si2 is a model system for quantum criticality. Particularly, Hall effect measurements helped identify the unconventional nature of its quantum critical point. Here, we present a high-resolution study of the Hall effect and magnetoresistivity on samples of different quality. We find a robust crossover on top of a sample dependent linear background contribution. Our detailed analysis provides a complete characterization of the crossover in terms of its position, width, and height. Importantly, we find in the extrapolation to zero temperature a discontinuity of the Hall coefficient occurring at the quantum critical point for all samples. Particularly, the height of the jump in the Hall coefficient remains finite in the limit of zero temperature. Hence, our data solidify the conclusion of a collapsing Fermi surface. Finally, we contrast our results to the smooth Hall-effect evolution seen in Chromium, the prototype system for a spin-density-wave quantum critical point.

cond-mat.str-el↗

arXiv2010

Fermi-surface collapse and dynamical scaling near a quantum critical point

Quantum criticality arises when a macroscopic phase of matter undergoes a continuous transformation at zero temperature. While the collective fluctuations at quantum-critical points are being increasingly recognized as playing an important role in a wide range of quantum materials, the nature of the underlying quantum-critical excitations remains poorly understood. Here we report in-depth measurements of the Hall effect in the heavy-fermion metal YbRh2Si2, a prototypical system for quantum criticality. We isolate a rapid crossover of the isothermal Hall coefficient clearly connected to the quantum-critical point from a smooth background contribution; the latter exists away from the quantum-critical point and is detectable through our studies only over a wide range of magnetic field. Importantly, the width of the critical crossover is proportional to temperature, which violates the predictions of conventional theory and is instead consistent with an energy over temperature, E/T, scaling of the quantum-critical single-electron fluctuation spectrum. Our results provide evidence that the quantum-dynamical scaling and a critical Kondo breakdown simultaneously operate in the same material. Correspondingly, we infer that macroscopic scale-invariant fluctuations emerge from the microscopic many-body excitations associated with a collapsing Fermi-surface. This insight is expected to be relevant to the unconventional finite-temperature behavior in a broad range of strongly correlated quantum systems.

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