Structural and chemical properties of UHV-prepared CeRu4Sn6 (001) and (010) surfaces
Recently, it has been theoretically predicted that CeRu4Sn6, a heavy fermion system, belongs to a new class of strongly correlated topological materials [1]. The calculated electronic structure exhibits Weyl fermions, opening a new direction of research in condensed matter physics. A way to demonstrate the presence of such quasiparticles is to use angle-resolved photoemission spectroscopy (ARPES) in combination with other techniques. ARPES is a surface sensitive tool hence requiring clean and atomically flat surfaces under ultra-high vacuum. A common way to experimentally proceed is to use in situ cleaved single crystals. However, cleavage might lead to different surface layers with different composition and high rugosity directly impacting band structure and measurement reproducibility. Moreover, many large samples are necessary. An alternative way is to prepare surfaces by Ar+ sputtering and annealing cycles. Evidently, the main benefit is that a single crystal with controlled surface composition and orientation can be reused at will for reliable investigations. In this work, we investigate the morphology and chemical composition of CeRu4Sn6 surfaces with either (001) or (010) orientation cleaned by sputtering and annealing cycles as a function of annealing temperature (Ta). Using low-energy electron diffraction, scanning tunneling microscopy and X-ray photoemission, we show that the surface exhibits flat terraces and 3D islands with sharp facets and that the Sn content at the surface varies with Ta. Surface energies of possible bulk truncated models have been computed based on density functional theory calculations. Preliminary ARPES results will be presented and discussed.The work in Vienna was supported by the Austrian Science Fund (I4047, P29279).
References:
[1] Y. Xu et al., Phys. Rev. X 7 (2017) 011027
Adsorption energies determined by machine learning on the low-index Al13Co4 complex surfaces
Complex phases and quasicrystal-related compounds have recently emerged as promising catalysts. Understanding the structure-catalytic properties relationships requires the determination of the adsorption energies in a first step. The most efficient methods to draw the adsorption energy landscape of surfaces are based on Density Functional Theory (DFT). These approaches are however quite time consuming when complex surfaces are considered. Here, we investigate how machine learning approaches can accurately provide adsorption energies from a few DFT estimates. Our study focus on the adsorption properties of atomic H, O and Pb on quasicristalline Al13Co4 low-index surfaces. For instance, we see on figure (1) that the root mean squeare error (RMSE), representing the precision of the prediction, increase with the mass of the atom adsorbed by the compound.Finally, the accuracy and the transferability of the different models are discussed and compared to the existing literature.
Supported (ultra)-thin films: how strong is the impact of the substrate to enforce a specific structure?
The crystal growth of a film over a substrate is a fascinating field, due to the complexity of the parameters involved. Cristalline, amorphous or polycristalline structures can emerge, induced by key factors such as the interplay between the relative strengths of adsorbate-adsorbate and adsorbate-substrate interactions, the system’s temperature, the complexity of the substrate surface, and the flux of incoming species [1]. Quasicrystal surfaces have been found as perfect template structures for the growth of single-element pseudomorphic thin films. But quasicrystalline order can also emerge on periodic metals, as recently discovered by Forster et al. [2]. In this work, we focus on two types of model structures to investigate interfaces with a quasiperiodic character. First, a quasicrystaline approximant oxide deposited over a periodic metallic substrate (BaTiO3 /Pt(111)) [3], and second, a thin alumina phase (amorph and ?) deposited over a complex intermetallic substrate (Al9Co2/Al2O3). Calculations based on Density Functional Theory are performed to determine the influence of the substrate (showing a short or larger periodicity) over different types of oxides films. As a preliminary result, the adhesive energy is stronger (-0.27 eV/Å2) in the BaTiO3 /Pt(111) system than in the Al9Co2/Al2O3 (-0.11 and -0.15 eV/Å2 for θ and amorph, respectively). Further studies will aim to investigate the impact of other factors on the adhesion, such as the atomic density of the film or the charge transfer.References:
[1] R. McGrath et al. In: Phil. Trans. R. Soc. A. 370.1969 (June 2012), pp. 2930–2948. issn: 1364-503X, 1471-2962.
[2] Stefan Főrster et al. In: Nature 502.7470 (Oct. 2013), pp. 215–218. issn: 0028-0836, 1476-4687.
[3] Thiago Trevizam Dorini et al. In: Nanoscale 13.24 (2021), pp. 10771–10779. issn: 2040-3364, 2040-3372.
Pd-In intermetallic nanoalloys stability: a DFT study of shape, size, and composition.
Nanoalloys combine finite size effects and alloy effects, significantly extending the range of nanomaterial properties. A crucial step for the control of these properties is to characterize structural and chemical arrangements.Experiments have made impressive progress to synthesize and characterize nanoalloys of controlled size, morphology and composition. However, the precise chemical arrangement at the surface is still challenging to observe, meaning experiments often need simulations to refine their interpretations. It is particular significant in catalysis since the catalytic properties depend on the atomic arrangments at the surface. Pd-In nanoalloys exhibit higher activity and selectivity than the monometallic Pd catalyst for selective acetylene hydrogenation but the performance of this catalyst depend on the composition1, PdIn surfaces present good catalytic properties for CO2 hydrogenation to methanol2. Intermetallic systems such as Pd-In are well known to present a high enthalpy formation in the bulk and no surface segration. By reducing the system size, from alloy to nanoalloy, a lot of questions arise: Is chemical ordering conserved? What is the morphology of nanoalloys? Is there surface segreation?
We propose a DFT study of the stability of Pd-In nanoalloys as a function of size, shape and chemical arrangements to answer these questions. This is a crucial step before any investigation of catalytic properties.
References:
[1] Y. Cao, Z. Sui, Y. Zhu, X. Zhou, and D. Chen, ACS Catal. 2017, 7, 7835-7846
[2] P. Wu, B. Yang, Catal. Sci. Technol. 2019, 9, 6102-6113
Ammonia synthesis on the LaRuSi Electride
Recently, transition metal electrides have attracted the attention of scientists as promising catalysts for ammonia synthesis. But very little is known about the atomic processes involved. In this work, extensive DFT calculations are performed to explore the detailed mechanism of ammonia synthesis catalyzed by LaRuSi, a typical electride catalyst. In sharp contrast to the recent proposal involving the high surface energy Ru-termination of LaRuSi(001), we show that an alternative reaction path on the most stable La-termination is more likely. Our study addresses the contrasted catalytic properties of the isostructural LaRuSi electride and CaRuSi non-electride compounds. It points the role of surface La atoms in the catalytic performances of LaRuSi, and shows that active sites are not necessarily transition metal atoms. Our findings open up future explorations of transition metal free catalysts for ammonia synthesis, active under mild conditions [1].References:
[1] F. Brix, G. Frapper and E. Gaudry, Ammonia synthesis on the LaRuSi Electride Catalyst: New DFT Mechanistic Insights Revealing the Active La-terminated (001) Surface, submitted
De Haas-van Alphen oscillations in ZrxHf1-xSiS
Among three-dimensional (3D) systems with non-trivial topological states, topological nodal-line (TNL) semimetals attract much attention due to their particularly interesting physics. In topological nodal-line semimetal Dirac band crossings occur along a line or form a loop in momentum space. Representatives of this type of material are ZrSiS and HfSiS. Since Zr and Hf are isovalent and of similar size, ZrxHf1-xSiS compound is stable for x ∈ [0,1].In this poster we present synthesis of ZrxHf1-xSiS monocrystals for various x and show De Haas-van Alphen quantum oscillations in magnetization for several directions of external field up to 7 T.
Investigation of chemical order in Gd14AuxAl86-x quasicrystal 1/1 approximants
Quasicrystals (QCs) exhibit crystallographically forbidden symmetries and aperiodic long range atomic order. whereas approximants of quasicrystals (ACs) possess conventional periodic crystal structures with similar chemical composition and local atom arrangments as their related QCs. For the past 40 years, there has been considerable interest in finding new QCs and ACs, and investigating their physical properties. The effect of chemical composition on magnetic behavior has been recently reported for ACs Gd14AuxAl86-x. [1]. In particular it was found that with increasing Au concentration magnetism changes from spin glass behavior to FM to AFM In order to better understand the underlying reasons for the observed magnetic properties in it was deemed necessary to determine the crystal structures of Gd14AuxAl86-x. 1/1 ACs for a broader range of x. Therefore, high quality single crystals with different Au/Al ratio have been synthesized using the self-flux method. The phase purity of the samples have been confirmed by EDX and powder XRD techniques. The crystal structure was refined from single crystal XRD data. As a result, some atomic sites show strong chemical preference while others are resilient. The specifics of the chemical ordering phenomenon in Gd14AuxAl86-x 1/1 ACs will be discussed.References:
[1] A. Ishikawa et al., Composition-driven spin glass to ferromagnetic transition in the quasicrystal approximant Au-Al-Gd., Phys. Rev. B 93, 024416 (2016).
Design and fabrication of a micro-sample holder for thermopower measurement using photolithography
The application of the temperature gradient along the sample, which is at a uniform chemical potential, creates an energy imbalance per charge that eventually leads to charge diffusion from one side of the sample to the other. This leads to the generation of the electric potential difference. To a linear approximation, it is proportional to the applied temperature gradient, the Seebeck coefficient being the coefficient of proportionality. In general, it gives information about the topology of the Fermi surface, but is generally difficult to model.The measurement of the Seebeck coefficient is also not an easy task. The main difficulty is to measure accurately the temperature difference along the sample. Normally, we measure it with a differential thermocouple consisting of three thin wires whose connections are electrically linked, for example by soldering or spot welding. To overcome these difficulties, we have developed a sample holder on a chip with low thermal conductivity. The chromel constantan thermocouple is deposited on it by a combination of a two-step process involving magnetron sputtering and micropatterning by photolithography. In this work we describe the fabrication process in detail.
Investigation of CeRu4Sn6 under hydrostatic pressure
The landscape of novel quantum phases in strongly correlated systems is expected to be further enriched in the presence of nontrivial electronic topology. A candidate material to explore this regime is the tetragonal, non-centrosymmetric Kondo semimetal CeRu4Sn6. Recent theoretical work predicted that it hosts Weyl points in its renormalized bandstructure near the Fermi level [1]. In addition, inelastic neutron scattering experiments revealed that CeRu4Sn6 is quantum critical without tuning [2]. These results raise the interesting question of whether the Weyl-Kondo semimetal may be an emergent phase stabilized by quantum fluctuations. In this work we address this question by investigating the specific heat and magnetotransport properties of CeRu4Sn6 single crystals under hydrostatic pressure. Our specific heat results suggest that applying pressure tunes CeRu4Sn6 away from quantum criticality. At the same time, electrical transport indicates that a “background” Kondo-insulator gap is enlarged by pressure. We also study putative signatures of Weyl-Kondo physics.Acknowledgment:
The work in Vienna was supported by the Austrian Science Fund (FWF grants I4047 and P29279) and the European Microkelvin Platform (H2020 project 824109).
References:
[1] Y. Xu et al., Phys. Rev. X 7, 011027 (2017)
[2] W. T. Fuhrman et al., Sci. Adv. 7, eabf9134 (2021)
Investigation of bulk ZnPd catalysts using high resolution electron microscopy
Methanol Steam Reforming (MSR) is an important reaction converting methanol into hydrogen (CH3OH + H2O 🠢 3 H2 + CO2) holding promise for easy fuel handling in hydrogen based applications like fuel cells. ZnPd/ZnO nanoparticles prove to be promising MSR catalysts due to their high CO2 selectivity, stability and activity.1 Their activity and selectivity depends very sensitively on their Zn/Pd ratio, even though the relevant intermetallic PdZn phase has a wide compositonal existance range.2 In order to elucidate the relationship between composition, microstructure, and catalytic performance, ZnPd samples with different composition were characterized using high resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDX). Furthermore, first identical location and in-situ experiments under MSR conditions are conducted to elucidate the microstructural changes of the ZnPd system under realistic operation conditions.References:
[1] Főttinger, K., Van Bokhoven, et al J. Phys. Chem. Lett. 2, 428–433 (2011).
[2] Friedrich, M., Teschner et al J. Catal. 285, 41–47 (2012).
Ba6Li8Ga8Nx with isolated [Ga4] tetrahedra: synthesis, modifications and twinning
In the context of our recent work in the M-Li-Ga systems (M = Sr, Ba, Eu) with the aim of synthesizing compounds with isolated [Gan] clusters [1,2], we have obtained Ba6Li8Ga8Nx, which is stabilized only by the presence of nitrogen. The synthesis was performed by using stochiometric amounts of elemental Ba and Ga, excess of Li as flux for crystal growth and different amounts of Li3N as nitrogen source. The flux was subsequently removed by high-temperature centrifugation-aided filtration.Two modifications are obtained. Whereas the hexagonal h-Ba6Li8Ga8Nx (space group P63mc, a = 9.8495(5) Å, c = 6.8092(7) Å) is formed by nitrogen deficit, the orthorhombic o-Ba6Li8Ga8Nx (space group Pmnn, a = 9.82388(6) Å, b = 17.0474(1) Å, c = 6.84496(4) Å, (b/a)2 = 3.011) is obtained if larger amounts of nitrogen are present. A series of experiments revealed that the critical amount of nitrogen needed for the transition from P63mc to Pmnn modifications is approximately 10 % of the stochiometric amount in Ba6Li8Ga8N. The single-crystal investigation of o-Ba6Li8Ga8Nx is hampered by twinning (three-domains).
The structures of both compounds are strikingly similar, only differing in the respective orientation of the isolated [Ga4] tetrahedra. The [Ba6] octahedra in both structures are interconnected into columns through face-sharing, and act as host for the N atoms resembling the Mn5Si3 structure type arrangement. The formation of different [Gan] clusters in binary and ternary gallides is discussed in dependence of valence electron concentration per Ga atoms and electronegativity of the constituent elements.
References:
[1] M. Kotsch, Yu. Prots, et al, Eur. J. Inorg. Chem., 29 (2020) 2842–2849.
[2] M. Kotsch, Yu. Prots, et al, Z. Anorg. Allg. Chem., 647 (2021) 1–8.
Al2Cu(001) studied by photo electron spectroscopy 2.0
Intermetallic compounds have received interest because of their performance as catalysts in, e.g., hydrogenation reactions. However, compared to the enormous number of surface studies on pure metals and on alloys, there are considerably fewer studies on the surface properties of intermetallic compounds.Recently, Al2Cu(001) showing a (2√2 x √2)R45° reconstruction was studied using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) in combination with ab initio calculations [1]. The aim of our work is to further characterize the Al2Cu(001) surface by additional techniques, in particular X-ray photoelectron spectroscopy (XPS) and angle-resolved photoelectron spectroscopy (ARPES). The poster will present and discuss first results of our work, focusing on surface composition and structure as well as electronic structure. We investigate the effects of surface preparation by sputtering and annealing on the stoichiometry obtained from XPS measurements and the formation of different surface reconstructions. Measurements of the electronic band structure are presented along the high-symmetry directions of the surface Brillouin zone and compared to LMTO calculations [2].
References:
[1] L. N. Serkovic Loli, et al., Phys. Rev. Lett. 108 (2019) 146101
[2] M. Armbrüster, PhD Thesis, TU Dresden (2004)
Low temperature charge transport in WNx
We report low temperature electrical properties (resistivity, thermopower and magnetoresistance) of a transitional metal nitride nitride (TMN) pelet. TMNs are interesting both because of their good mechanical properties, which make them suitable for design of new cutting tools, and because of their electrical properties, which predestine them for new electrode materials [1,2].In our samples, nitride to metal ratio is suggested to be >1 because of clear Mott-like resistance above 20K [3]. In <20K regime we observe change in resistivity.
The magnetoresistance is measured in a magnetic field directed both parallel and perpendicular to the current flow. In both cases, the magnetoresistance curves have a similar shape, typical for ferromagnets, while at the same time we observe no significant change in shape below and above 20 K.
References:
[1] F. Kawamura, H. Yusa, T. Taniguchi, J Am Ceram Soc. 101 (2018) 949.
[2] Y. Zhou, W. Guo, T. Li, Ceram. Int. 45 (2019) 21062.
[3] A. Salamat, A. L. Hector, P. Knoll, P. F. McMillan, Coord. Chem. Rev. 257 (2013) 2063.
Physical and structural properties of Cu2-xSe across
Cu2Se, as a competitive thermoelectric material, has attracted much attention recently [1], mainly due to its large figure of merit (ZT), which is among highest for single crystalline compounds (1.5 at 1000K). This is predominantly associated with its superionic behavior in the high-temperature ß-phase which also holds potential for applications in solid state batteries [2].Although high temperature β-phase was extensively investigated, comprehensive insight of low temperature α phase is still lacking. It reflects the complexity of the structural and phase relations. The transition temperature from α phase to β phase considerably depend on x and mixed phase is between them [4]. We observe large temperature hysteresis probably related to the mechanism of this phase transition. The shape of hysteresis is strongly time dependent. Here we present detailed magnetotransport and thermoelectric properties of Cu2-xSe at low temperatures complemented with structural and magnetic studies. They will allow us to understand mechanism of order-disorder phase transition in Cu2-xSe subsystem which most probably lies behind wide temperature hysteresis observed in our results.
References:
[1] Liu, H. et al., Nature Materials 11. 422–425 (2012).
[2] Hu L. et al., Nanomaterials 10. 302 (2020).
[3] Eikeland, E. et al., Polymorph Materials, UCrJ4, 476 (2017).
[4] Ogorelec, Z. et al., Journal of Materials Science 7. 967-969 (1972).