Abstract
High-entropy alloys (HEAs) are characterized by a simultaneous presence of a crystal lattice and an amorphous-type chemical (substitutional) disorder. In order to unravel the effect of ...crystal-glass duality on the electronic transport properties of HEAs, we performed a comparative study of the electronic transport coefficients of a 6-component alloy Al
0.5
TiZrPdCuNi that can be prepared either as a HEA or as a metallic glass (MG) at the same chemical composition. The HEA and the MG states of the Al
0.5
TiZrPdCuNi alloy both show large, negative-temperature-coefficient resistivity, positive thermopower, positive Hall coefficient and small thermal conductivity. The transport coefficients were reproduced analytically by the spectral conductivity model, using the Kubo-Greenwood formalism. For both modifications of the material (HEA and MG), contribution of phonons to the transport coefficients was found small, so that their temperature dependence originates predominantly from the temperature dependence of the Fermi–Dirac function and the variation of the spectral conductivity and the related electronic density of states with energy within the Fermi-level region. The very similar electronic transport coefficients of the HEA and the MG states point towards essential role of the immense chemical disorder.
Detecting the metallic Dirac electronic states on the surface of Topological Insulators (TIs) is critical for the study of important surface quantum properties (SQPs), such as Majorana zero modes, ...where simultaneous probing of the bulk and edge electron states is required. However, there is a particular shortage of experimental methods, showing at atomic resolution how Dirac electrons extend and interact with the bulk interior of nanoscaled TI systems. Herein, by applying advanced broadband solid-state
Te nuclear magnetic resonance (NMR) methods on Bi
Te
nanoplatelets, we succeeded in uncovering the hitherto invisible NMR signals with magnetic shielding that is influenced by the Dirac electrons, and we subsequently showed how the Dirac electrons spread inside the nanoplatelets. In this way, the spin and orbital magnetic susceptibilities induced by the bulk and edge electron states were simultaneously measured at atomic scale resolution, providing a pertinent experimental approach in the study of SQPs.
We have determined the crystal structure and the magnetic state of the CeFe9Si4 intermetallic compound. Our revised structural model (fully ordered tetragonal unit cell, I4/mcm) agrees with the ...previous literature report, except for some minor quantitative differences. Magnetically, the CeFe9Si4 undergoes a ferromagnetic transition at the temperature TC ≈ 94 K. Ferromagnetism in the combined Ce–Fe spin system is a result of interplay between the localized magnetism of the Ce sublattice and the Fe band (itinerant) magnetism. Ferromagnetic ordering obeys the rather general rule that the exchange spin coupling between atoms possessing more than half-full d shells with atoms possessing less than half-full d shells is antiferromagnetic (where the Ce atoms are considered as light d elements). Since in rare-earth metals from the light half of the lanthanide series, the magnetic moment is directed opposite to the spin, this results in ferromagnetism. The magnetoresistance and the magnetic specific heat show an additional temperature-dependent feature (a shoulder) deep inside the ferromagnetic phase that is considered to originate from the influence of the magnetization on the electronic band structure via the magnetoelastic coupling, which alters the Fe band magnetism below TC . The ferromagnetic phase of CeFe9Si4 is magnetically soft.
In a search for unconventional heavy-Fermion compounds with the localized 4f moments distributed quasiperiodically instead of a conventional distribution on a regular, translationally periodic ...lattice, we have successfully synthesized a stable Ce3Au13Ge4 Tsai-type 1/1 quasicrystalline approximant of the off-stoichiometric composition Ce3+x Au13+y Ge4+z (x = 0.17, y = 0.49, z = 1.08) and determined its structural model. The structure is body-centered-cubic (bcc), with space group Im3̅, unit cell parameter a = 14.874(3) Å, and Pearson symbol cI174, and can be described as a bcc packing of partially interpenetrating multishell rhombic triacontahedral clusters. The cerium sublattice, corresponding to the magnetic sublattice, consists of a bcc packing of Ce icosahedra with an additional Ce atom in a partially occupied site (occupation 0.7) at the center of each icosahedron. The measurements of its magnetic properties and the specific heat have demonstrated that it is a regular intermetallic compound with no resemblance to heavy-Fermion systems. The partially occupied Ce2 site in the center of each Ce1 icosahedron, the mixed-occupied Au/Ge ligand sites between the Ce2 and Ce1 atoms, and the random compositional fluctuations due to nonstoichiometry of the investigated Ce3+x Au13+y Ge4+z alloy introduce randomness into the Ce magnetic sublattice, which causes a distribution of the indirect-exchange antiferromagnetic interactions between the spins. Together with the geometric frustration of the triangularly distributed Ce moments, this leads to a spin-glass phase below the spin freezing temperature T f ≈ 0.28 K.
We investigated molecular dynamics in two ammonium borane systems from the group of promising ion conductors. The investigation was performed by means of 1H and 11B NMR spectroscopy and spin–lattice ...relaxation techniques. We identified two reorientational processes, the rotations of NH4 units that are present already at low temperatures and rotations of large boron cages, B10H10 or B12H12, which are thermally activated and become prominent above 250 K. Activation energies for these processes were determined. In addition, solid-state ion conductivity measurements were conducted to determine poor NH4 + conductivity of both systems.
Electrical and thermal transport properties of quasicrystals are reviewed on the examples of i-Ag-In-Yb and i-Al-Cu-Fe icosahedral phases and d-Al-Co-Ni decagonal phase. Using samples of single-grain ...morphology and high structural quality, and performing the measurements along well-defined crystallographic directions, the following basic questions in the context of physical properties of quasicrystals are addressed, both experimentally and theoretically: (1) are the unusual transport properties of quasicrystals introduced by the quasiperiodicity of the structure or are they a consequence of complex local atomic order with no direct relationship to the quasiperiodicity; (2) what is the role of the electronic structure of quasicrystals in their electronic transport properties, especially the pseudogap in the electronic density of states in the vicinity of the Fermi energy; (3) what is the anisotropy of the transport coefficients along different crystallographic directions for icosahedral and decagonal quasicrystals and (4) what are the true intrinsic properties of quasicrystalline phases?
Abstract
We report on the dynamics of a magnetic-field-driven antiferromagnetic-to-paramagnetic quantum phase transition in monocrystalline Ce
3
Al via transverse-field muon spin rotation (TF-
µ
SR) ...experiments down to low temperature of
$$\sim$$
∼
80 mK. The quantum phase transition is of a spin-flip type and takes place on the Ce–Al magnetic chains as a result of competition between the indirect exchange and the Zeeman interaction of the Ce moments with the external field, applied along the chain direction (also the direction of the antiferromagnetic axis). The Ce moments are not static at
$$T \to$$
T
→
0, but fluctuate in their direction due to the Heisenberg uncertainty principle. Upon applying the magnetic field sweep, the fluctuations exhibit the largest amplitude at the quantum critical point, manifested in a maximum of the muon transverse relaxation rate at the critical field. The quantum nature of fluctuations observed in the TF-
µ
SR experiments is reflected in the temperature independence of the average local magnetic field component along the external magnetic field at the muon stopping site(s) and the muon transverse relaxation rate within the investigated temperature range 1.5 K–80 mK. Quantum fluctuations are fast on the muon Larmor frequency scale,
$$\tau_{0} <$$
τ
0
<
10
–10
s.
In this study, we investigate the scandium-containing Sc-Hf-Nb-Ta-Ti-Zr system of refractory high-entropy alloys (HEAs). Using the arc-melting method, we synthesized nine equimolar alloys (five 4-, ...three 5- and one 6-component), with all of them containing Sc. The alloys were characterized by XRD, electron microscopy and EDS, while superconductivity was investigated via electrical resistivity, specific heat and the Meissner effect. The results were compared to the parent Hf-Nb-Ta-Ti-Zr refractory HEAs, forming a single-phase body-centered cubic (bcc) structure and quite homogeneous microstructure. The addition of Sc produces a two-phase structure in the Sc-Hf-Nb-Ta-Ti-Zr alloys, with one phase being bcc and the other hexagonal close-packed (hcp). The hcp phase absorbs practically all Sc, whereas the Sc-poor bcc phase is identical to the bcc phase in the Hf-Nb-Ta-Ti-Zr parent system. Upon the Sc addition, the microstructure becomes very inhomogeneous. Large bcc dendrites (10-100 µm) are homogeneous in the central parts, but become a fine dispersion of sub-micron precipitates of the bcc and hcp phases close to the edges. The interdendritic regions are also a fine dispersion of the two phases. Superconductivity of the Sc-Hf-Nb-Ta-Ti-Zr alloys originates from the bcc phase fraction, which demonstrates identical superconducting parameters as the bcc Hf-Nb-Ta-Ti-Zr parent alloys, while the Sc-containing hcp phase fraction is non-superconducting.
In the search for electronic phenomena in high-entropy alloys (HEAs) that go beyond the independent-electron description, we have synthesized a series of hexagonal rare earth (RE)-based HEAs: ...CexLaLuScY (x = 0.05–1.0). The measurements of electrical resistivity, magnetic susceptibility and specific heat have shown that the CexLaLuScY HEAs exhibit the Kondo effect, which is of a single impurity type in the entire range of employed Ce concentrations despite the alloys being classified as dense (concentrated) Kondo systems. A comparison to other known dense Kondo systems has revealed that the Kondo effect in the CexLaLuScY HEAs behaves quite differently from the chemically ordered Kondo lattices but quite similar to the RE-containing magnetic metallic glasses and randomly chemically disordered Kondo lattices of the chemical formula RE1xRE21−xM (with RE1 being magnetic and RE2 being nonmagnetic). The main reason for the similarity between HEAs and the metallic glasses and chemically disordered Kondo lattices appears to be the absence of a periodic 4f sublattice in these systems, which prevents the formation of a coherent state between the 4f-scattering sites in the T→ 0 limit. The crystal-glass duality of HEAs does not bring conceptually new features to the Kondo effect that would not be already present in other disordered dense Kondo systems. This study broadens the classification of HEAs to correlated electron systems.
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