arXiv subjects

M. Waas

Publications and source records attributed to M. Waas.

4 recordsLinked to original sources

arXiv2023

Ce$_3$Bi$_4$Ni$_3$ $-$ A large hybridization-gap variant of Ce$_3$Bi$_4$Pt$_3$

The family of cubic noncentrosymmetric 3-4-3 compounds has become a fertile ground for the discovery of novel correlated metallic and insulating phases. Here, we report the synthesis of a new heavy fermion compound, Ce$_3$Bi$_4$Ni$_3$. It is an isoelectronic analog of the prototypical Kondo insulator Ce$_3$Bi$_4$Pt$_3$ and of the recently discovered Weyl-Kondo semimetal Ce$_3$Bi$_4$Pd$_3$. In contrast to the volume-preserving Pt-Pd substitution, structural and chemical analyses reveal a positive chemical pressure effect in Ce$_3$Bi$_4$Ni$_3$ relative to its heavier counterparts. Based on the results of electrical resistivity, Hall effect, magnetic susceptibility, and specific heat measurements, we identify an energy gap of 65-70 meV, about eight times larger than that in Ce$_3$Bi$_4$Pt$_3$ and about 45 times larger than that of the Kondo-insulating background hosting the Weyl nodes in Ce$_3$Bi$_4$Pd$_3$. We show that this gap as well as other physical properties do not evolve monotonically with increasing atomic number, i.e., in the sequence Ce$_3$Bi$_4$Ni$_3$-Ce$_3$Bi$_4$Pd$_3$-Ce$_3$Bi$_4$Pt$_3$, but instead with increasing partial electronic density of states of the $d$ orbitals at the Fermi energy. To understand under which condition topological states form in these materials is a topic for future studies.

cond-mat.str-el↗

arXiv2018

Single crystal study of the charge density wave metal LuNiC2

We report on single crystal growth, single crystal x-ray diffraction, physical properties and density functional theory (DFT) electronic structure as well as Fermi surface calculations for two ternary carbides, LuCoC2 and LuNiC2. Electrical resistivity measurements reveal for LuNiC2 a charge density wave (CDW) transition at T_{CDW}~ 450 K and, for T > T_{CDW}, a significant anisotropy of the electrical resistivity, which is lowest along the orthorhombic a-axis. The analysis of x-ray superstructure reflections suggest a commensurate CDW state with a Peierls-type distortion of the Ni atom periodicity along the orthorhombic a-axis. DFT calculations based on the CDW modulated monoclinic structure model of LuNiC2 as compared to results of the orthorhombic parent-type reveal the formation of a partial CDW gap at the Fermi level which reduces the electronic density of states from N(E_{F})= 1.03 states/eV f.u. without CDW to N(E_{F})= 0.46 states/eV f.u. in the CDW state. The corresponding bare DFT Sommerfeld value of the latter, gamma_{DFT}^{CDW}= 0.90 mJ/molK^2, reaches reasonable agreement with the experimental value gamma= 0.83(5) mJ/mol\,K^2 of LuNiC2. LuCoC2 displays a simple metallic behavior with neither CDW ordering nor superconductivity above 0.4 K. Its experimental Sommerfeld coefficient, gamma= 5.9 (1) mJ/molK^2, is in realistic correspondence with the calculated, bare Sommerfeld coefficient, gamma_{DFT}= 3.82 mJ/molK^2, of orthorhombic LuCoC2.

cond-mat.str-el↗

arXiv2017

Kondo Insulator to Semimetal Transformation Tuned by Spin-Orbit Coupling

Recent theoretical studies of topologically nontrivial electronic states in Kondo insulators have pointed to the importance of spin-orbit coupling (SOC) for stabilizing these states. However, systematic experimental studies that tune the SOC parameter $λ_{\rm{SOC}}$ in Kondo insulators remain elusive. The main reason is that variations of (chemical) pressure or doping strongly influence the Kondo coupling $J_{\text{K}}$ and the chemical potential $μ$ -- both essential parameters determining the ground state of the material -- and thus possible $λ_{\rm{SOC}}$ tuning effects have remained unnoticed. Here we present the successful growth of the substitution series Ce$_3$Bi$_4$(Pt$_{1-x}$Pd$_x$)$_3$ ($0 \le x \le 1$) of the archetypal (noncentrosymmetric) Kondo insulator Ce$_3$Bi$_4$Pt$_3$. The Pt-Pd substitution is isostructural, isoelectronic, and isosize, and therefore likely to leave $J_{\text{K}}$ and $μ$ essentially unchanged. By contrast, the large mass difference between the $5d$ element Pt and the $4d$ element Pd leads to a large difference in $λ_{\rm{SOC}}$, which thus is the dominating tuning parameter in the series. Surprisingly, with increasing $x$ (decreasing $λ_{\rm{SOC}}$), we observe a Kondo insulator to semimetal transition, demonstrating an unprecedented drastic influence of the SOC. The fully substituted end compound Ce$_3$Bi$_4$Pd$_3$ shows thermodynamic signatures of a recently predicted Weyl-Kondo semimetal.

cond-mat.str-el↗

arXiv2014

Thermopower enhancement by encapsulating cerium in clathrate cages

The increasing worldwide energy consumption calls for the design of more efficient energy systems. Thermoelectrics could be used to convert waste heat back to useful electric energy if only more efficient materials were available. The ideal thermoelectric material combines high electrical conductivity and thermopower with low thermal conductivity. In this regard, the intermetallic type-I clathrates show promise with their exceedingly low lattice thermal conductivities [1]. Here we report the successful incorporation of cerium as guest atom into the clathrate crystal structure. In many simpler intermetallic compounds, this rare earth element is known to lead, via the Kondo interaction, to strong correlation phenomena including the ocurrence of giant thermopowers at low temperatures [2]. Indeed, we observe a 50% enhancement of the thermopower compared to a rare earth-free reference material. Importantly, this enhancement occurs at high temperatures and we suggest that a `rattling' enhanced Kondo interaction [3] underlies this effect.

cond-mat.str-el↗