Anisotropic Physical Properties of the Kondo Semimetal CeCu1.11_{1.11}As2_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.Comment: 6 pages, 4 figures, Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2019

CeCuAs tief 2

Abweichender Titel nach Übersetzung der Verfasserin/des VerfassersKondo insulators and heavy fermion metals are strongly correlated materials that have been investigated for more than 40 years. In these materials, the hybridization between conduction bands and localized states, typically arising from a rare earth element with open 4f shell, leads to the formation of a nonmagnetic ground state with heavy quasiparticles. If the rare earth atoms form a regular lattice, the hybridization with the conduction electrons leads to the opening of a gap. Then, depending on whether the Fermi energy is situated in the gap or within one of the hybridized bands, a Kondo insulator or a heavy fermion metal results. Questions of current interest include: Can one transform these two states into each other? How to understand Kondo materials that are neither metals nor insulators, but have semimetallic properties? Is topology playing an important role in such semimetals? To address these questions we explored the tetragonal intermetallic compound CeCuAs2. Published electrical resistivity data on polycrystalline samples suggest that it may be a Kondo semimetal. Very recently, the group of Arumugam Thamizhavel at the Tata Institute of Fundamental Research in India succeeded to grow single crystals of this compound and provided them to us for the initial physical property characterization. In this master thesis, electrical transport, magnetization, and specific heat measurements were performed on single crystalline samples from several growth batches. The primary goal was to distinguish between intrinsic properties of the compound and effects related to defects, for instance in the form of a deficiency in As. Indeed, the examined samples display either an "insulator-like" increase of the electrical resistivity with decreasing temperature or a "metal-like" decrease; this goes along with pronounced differences in the magnetorestistance and Hall effect. The "insulator-like" behavior is seen in more stoichiometric samples and is thus identified as the intrinsic behavior. For all samples a pronounced anisotropy is observed in the magnetization, with no evidence of magnetic ordering down to 0.4 K. From an entropy estimation of the specific heat data a Kondo temperature of about 4 K is extracted. Thus, the most stoichiometric CeCuAs2 samples available to date classify as Kondo semimetal. To address these questions we explored the tetragonal intermetallic compound CeCuAs2. Published electrical resistivity data on polycrystalline samples suggest that it may be a Kondo semimetal. Very recently, the group of Arumugam Thamizhavel at the Tata Institute of Fundamental Research in India succeeded to grow single crystals of this compound and provided them to us for the initial physical property characterization. In this master thesis, electrical transport, magnetization, and specific heat measurements were performed on single crystalline samples from several growth batches. The primary goal was to distinguish between intrinsic properties of the compound and effects related to defects, for instance in the form of a deficiency in As. Indeed, the examined samples display either an "insulator-like'' increase of the electrical resistivity with decreasing temperature or a "metal-like'' decrease; this goes along with pronounced differences in the magnetoresistance and the Hall effect. The "insulator-like'' behavior is seen in more stoichiometric samples and is thus identified as the intrinsic behavior. For all samples a pronounced anisotropy is observed in the magnetization, with no evidence of magnetic ordering down to 2 K. A Kondo temperature of about 4 K is extracted from entropy, estimated from the specific heat measurements. Thus, the most stoichiometric CeCuAs2 samples available to date can be classified as Kondo semimetals.7

Oxygen vacancy and hydrogen in amorphous HfO2_2

International audienceAmorphous hafnium dioxide (a-HfO$_2$) is widely used in electronic devices, such as ultra-scaled field-effect transistors and resistive memory cells. The density of oxygen vacancy (OV) defects in a-HfO 2 strongly influences the conductivity of the amorphous material. Ultimately, OV defects are responsible for the formation and rupture of conductive filament paths which are exploited in novel resistive switching devices. In this work, we studied neutral OV in a-HfO$_2$ using ab initio methods. We investigated the formation energy of OV, the binding energy of di-OVs, the OV migration, unperturbed and in the close presence of a hydrogen atom, as well as the migration of hydrogen atom towards OV. A shallow and short-range OV migration barrier (0.6 eV) exists in a-HfO$_2$ in contrast to the barrier (2.4 eV) in crystalline HfO 2 . Nearby hydrogen has a limited impact on the OV migration; however, hydrogen can diffuse easily by hopping among OVs

Modeling the Initial Stages of Si(100) Thermal Oxidation: An Ab-initio Approach

Silicon together with its native oxide SiO$_2$ was recognized as an outstanding material system for the semiconductor industry in the 1950s. In state-of-the-art device technology, SiO$_2$ is widely used as an insulator in combination with high-$k$ dielectrics such as HfO$_2$, demanding fabrication of ultra-thin interfacial layers. The classical standard model derived by Deal and Grove accurately describes the oxidation of Si in a progressed stage, however, strongly underestimates growth rates for thin oxide layers. Recent studies report a variety of oxidation mechanisms during the growth of oxide films in the range of \SI{10}{\angstrom} with various details still under debate. This paper presents a first-principles based approach to theoretically assess the thermal oxidation process of the technologically relevant Si(100) surfaceduring this initial stage. Our investigations range from the chemisorption of single O$_2$ molecules onto the $p(2\times2)$ reconstructed Si surface to oxidized Si surface layers with a thickness of up to \SI{20}{\angstrom}. The initially observed enhanced growth rate is assigned to barrierless O$_2$ chemisorption events upon which the oxygen molecule dissociate. We present strong evidence for an immediate amorphization of the oxide layer from the onset of oxidation. Surface reactions dominate until the surface is saturated with oxygen and separated from the Si substrate by a \SI{5}{\angstrom} transition region. The saturated surface becomes inert to dissociative reactions and enables the diffusion of molecular oxygen to the \interface interface as assumed within the Deal-Grove model. Further oxidation of the Si substrate is then provided by O$_2$ dissociations at the interface due to the same charge transfer process responsible for the chemisorption at the surface.Comment: 12 pages, 8 figure