Showing papers by "Silke Paschen published in 2018"

Weyl–Kondo semimetal in heavy-fermion systems

Abstract: 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 Ce3Bi4Pd3. Our findings provide the much-needed theoretical foundation for the experimental search of topological metals with strong correlations and open up an avenue for systematic studies of such quantum phases that naturally entangle multiple degrees of freedom.

Singular charge fluctuations at a magnetic quantum critical point

Abstract: Strange metal behavior is ubiquitous to correlated materials ranging from cuprate superconductors to bilayer graphene. There is increasing recognition that it arises from physics beyond the quantum fluctuations of a Landau order parameter which, in quantum critical heavy fermion antiferromagnets, may be realized as critical Kondo entanglement of spin and charge. The dynamics of the associated electronic delocalization transition could be ideally probed by optical conductivity, but experiments in the corresponding frequency and temperature ranges have remained elusive. We present terahertz time-domain transmission spectroscopy on molecular beam epitaxy-grown thin films of YbRh$_2$Si$_2$, a model strange metal compound. We observe frequency over temperature scaling of the optical conductivity as a hallmark of beyond-Landau quantum criticality. Our discovery implicates critical charge fluctuations as playing a central role in the strange metal behavior, thereby elucidating one of the longstanding mysteries of correlated quantum matter.

Suppression of vacancies boosts thermoelectric performance in type-I clathrates

Abstract: 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,□)46, where □ 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.

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

Abstract: Kondo insulators are emerging as a simplified setting to study both magnetic and insulator-to-metal quantum phase transitions. Here, we study the half-filled Anderson lattice model defined on a magnetically frustrated Shastry-Sutherland geometry. We determine a ``global'' phase diagram that applies to both the local-moment and intermediate-valence regimes. This provides the theoretical basis for understanding how tuning a Kondo insulator by external parameters can close its hybridization gap, liberate the local-moment spins from the conduction electrons, and lead to a magnetically correlated metal. We also calculate the momentum distribution of the single-particle excitations in the Kondo insulating state, and show how Fermi-surface-like features emerge as a precursor to the actual Fermi surfaces of the Kondo-destroyed metals. The implications for an incipient Fermi surface and quantum phase transitions of Kondo insulators including ${\mathrm{SmB}}_{6}$ are discussed.

Determining the local low-energy excitations in the Kondo semimetal CeRu4Sn6 using resonant inelastic x-ray scattering

Abstract: We have investigated the local low-energy excitations in CeRu4Sn6, 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 F-2(5/2) and F-2(7/2) excitations of the Ce 4f(1) configuration in the spectra allows for the determination of the crystal-electric field (CEF) parameters that explain quantitatively the high-temperature anisotropy of the magnetic susceptibility. The absence of an azimuthal dependence in the spectra indicates that all CEF states are close to being rotational symmetric. We show further that the non-negligible impact of the (sic)(6)(0) parameter on the ground state of CeRu4Sn6 leads to a reduction of the magnetic moment mu(c) due to multiplet intermixing. This improves the agreement between CEF calculations and the experimentally determined magnetic susceptibility considerably at low temperatures. Deviations that persist at low temperatures for fields within the tetragonal plane are attributed to the Kondo interaction between 4f and conduction electrons. The RIXS results are consistent with inelastic neutron scattering data and are compared to the predictions from ab initio based electronic structure calculations.

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

Abstract: 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.

Mechanism of a strange metal state near a heavy-fermion quantum critical point

Abstract: Unconventional metallic or strange metal (SM) behavior with non-Fermi liquid (NFL) properties, generic features of heavy-fermion systems near quantum phase transitions, are yet to be understood microscopically. A paradigmatic example is the magnetic field-tuned quantum critical heavy-fermion metal ${\mathrm{YbRh}}_{2}{\mathrm{Si}}_{2}$, revealing a possible SM state over a finite range of fields at low temperatures when substituted with Ge. Above a critical field, the SM state gives way to a heavy Fermi liquid with Kondo correlation. The NFL behavior, most notably a linear-in-temperature electrical resistivity and a logarithmic-in-temperature followed by a power-law singularity in the specific heat coefficient at low temperatures, still lacks a definite understanding. We propose the following mechanism as origin of the experimentally observed behavior: a quasi-$2d$ fluctuating short-ranged resonating-valence-bond spin liquid competing with the Kondo correlation. Applying a field-theoretical renormalization group analysis on an effective field theory beyond a large-$N$ approach to an antiferromagnetic Kondo-Heisenberg model, we identify the critical point and explain remarkably well the SM behavior. Our theory goes beyond the well-established framework of quantum phase transitions and serves as a basis to address open issues in quantum critical heavy-fermion systems.

ARPES and STM view of heavy-electron quantum criticality: perspectives and challenges

Abstract: 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.

Crystal Chemistry and Thermoelectric Properties of Type-I Clathrate Ba8Ni∼3.8SixGe42.2-x (x = 0, 10, 20, 42.2)

Abstract: Thermoelectric materials are actively considered for waste heat recovery applications. To improve the heat to electricity conversion efficiency, fundamental understanding on composition, crystal structure, and interrelation with the thermoelectric properties is necessary. Here, we report the chemical and thermoelectric properties of type-I clathrates Ba 8 Ni 3.8 Si x Ge 42.2 − x (x = 0, 10, 20, 42.2), to show that the Si substitution can retain the low lattice thermal conductivity as in pure Ge-based clathrates by adding defects (cage distortion) scattering and/or alloying effect, and the charge carrier concentration can be optimized and thus the electronic properties can be improved by tailoring the vacancy content. We demonstrate the vacancies in the pure Ge-based compound by Rietveld refinement, and possible vacancies in the quaternary compound by transport property measurements. We also show that, for intrinsic property studies in these compounds with such a complex crystal structure, a heat treatment for as cast alloys is necessary for phase purity and composition homogeneity. The highest Z T value of 0.19 at 550 ° C is reached in the compound with x = 10 .

Structure and Thermoelectric Properties of Nanostructured Bi 1- x Sb x Alloys Synthesized by Mechanical Alloying

Abstract: We report on the synthesis of Bi1−xSbx alloys and the investigation of the relationship between their structural and thermoelectric properties. In order to produce a compound that will work efficiently even above room temperature, Bi1−xSbx alloys were chosen, as they are known to be the best suited n-type thermoelectric materials in the low-temperature regime (200 K). Using a top–down method, we produced nanostructured Bi1−xSbx powders by ball-milling in the whole composition range of 0 < x < 1.0. Nanostructuring of Bi1−xSbx alloys increases the band gap and thus results in an enlargement of the semiconducting composition region (0 ≤ x ≤ 0.5) compared to its bulk counterpart (0.07 ≤ x ≤ 0.22). The enhancement of the band gap strongly affects the transport properties of the alloys, i.e. the electrical conductivity and the Seebeck coefficient. Moreover, nanostructuring reduces the thermal conductivity through the implementation of grain boundaries as phonon-scattering centers, leading to a significant enhancement of the thermoelectric properties. The highest figure-of-merit observed in this study is 0.25 which was found for Bi0.87Sb0.13 at 280 K.