Table of contents

Dating back to the late 1980s, bulk metallic glasses (BMGs) are relatively new materials that exhibit exceptional mechanical properties (strength, hardness, fracture toughness, stored elastic energy ...), compared to those of most crystalline metallic alloys. Their apparent brittleness under uniaxial loading, however, is still a major obstacle to their industrialization. Moreover, BMGs often contain crystalline defects developed, intentionally or not, during their complex and delicate elaboration. These flaws are known to affect their fracture toughness and their plastic behavior. This paper reviews twenty years of works about this subject on Zr-based BMGs that may contain a low volume fraction of crystalline defects of different natures, e.g. dendrites or spherulites, depending on the synthesis method. Dedicated experimental set-ups, mainly bending tests on notched beams, were developed to create in the specimen a proper pre-crack by fatigue and then load it monotonically up to fracture. The measured fracture toughness and the fractographic observations allow to conclude that these crystalline defects facilitate pre-cracking, but result in an embrittlement that is more or less significant depending on their type. The loading mode of the crack — mode I, II or mixed — as well as the temperature were shown to play a key role in crack initiation and propagation, whether steadily or catastrophically, in the BMG. By means of finite element computations analyses, explanations on how the crystalline flaws presence can affect fracture toughness and perturbate crack growth, under mode I and mode II, were proposed. Finally, the relevance of these experimental techniques as well as the link between crystalline defects, fracture toughness and their consequences on the ductility of a structural component are discussed.

In this work, we incorporated an ionic liquid (IL), 1-n-butyl-3-methylimidazolium methyl sulfate ([BMIM][MeSO₄]) into two different metal organic frameworks (MOFs), UiO-66, and its amino-functionalized counterpart, NH₂-UiO-66, to investigate the effects of ligand-functionalization on the thermal stability limits of IL/MOF composites. The as-synthesized IL/MOF composites were characterized in detail by combining x-ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller analysis, x-ray fluorescence, infrared spectroscopies (FTIR), and their thermal stability limits were determined by thermogravimetric analysis (TGA). Characterization data confirmed the successful incorporation of the IL into each MOF and indicated the presence of direct interactions between them. A comparison of the interactions in [BMIM][MeSO₄]-incorporated UiO-66 and NH₂-UiO-66 with those in their 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆])-incorporated counterparts showed that the hydrophilic IL, [BMIM][MeSO₄], interacts with the 1,4-benzenedicarboxylate (BDC) ligand of the UiO-66, while the hydrophobic IL, [BMIM][PF₆], is interacting with the joints where zirconium metal cluster coordinates with BDC ligand. The TGA data demonstrated that the composite with the ligand-functionalized MOF, NH₂-UiO-66, exhibited a lower percentage decrease in the maximum tolerable temperature compared to those of IL/UiO-66 composites. Moreover, it is discovered that when the IL is hydrophilic, its hydrogen bonding ability can be utilized to designate an interaction site on MOF's ligand structure, leads to a lower reduction in thermal stability limits. These results provide insights for the rational design of IL/MOF composites and contribute towards the complete elucidation of structural factors controlling the thermal stability.

We study the effects of pseudo-magnetic fields on Weyl semimetals with over-tilted Weyl cones, or type II cones. We compare the phenomenology of the resulting pseudo-Landau levels in the type II Weyl semimetal to the known case of type I cones. We predict that due to the nature of the chiral Landau level resulting from a magnetic field, a pseudo-magnetic field, or their combination, the optical conductivity can be utilized to detect a type II phase and deduce the direction of the tilt. Finally, we discuss ways to engineer homogeneous and inhomogeneous type II semimetals via generalizations of known layered constructions in order to create controlled pseudo-magnetic fields and over-tilted cones.

Using the α-T₃ model, we carried out analytical and numerical calculations for the static and dynamic polarization functions in the presence of a perpendicular magnetic field. The model involves a parameter α which is the ratio of the hopping strength from an atom at the center of a honeycomb lattice to one of the atoms on the hexagon to the hopping strength around its rim. Our results were employed to determine the longitudinal dielectric function and the magnetoplasmon dispersion relation. The magnetic field splits the continuous valence, conduction and flat energy subband into discrete Landau levels which present significant effects on the polarization function and magnetoplasmon dispersion. This includes the fact that the energies of the Landau levels are valley dependent which leads to different behaviors of the polarization function as the hopping parameter α (or ϕ = tan⁻¹α) is reduced continuously toward zero. This essential critical behavior of the polarization function leads to a softening of a magnetoplasmon mode. We present results for a doped layer in the integer quantum Hall regime for fixed hopping parameter α and various magnetic fields as well as chosen magnetic field and different α in the random phase approximation.

This work presents a Green's function approach, originally implemented in graphene with well-defined edges, to the surface of a strong 3D topological insulator with a sequence of proximitized superconducting (S) and ferromagnetic (F) surfaces. This consists of the derivation of the Green's functions for each region by the asymptotic solutions method and their coupling by a tight-binding Hamiltonian with the Dyson equation to obtain the full Green's functions of the system. These functions allow the direct calculation of the momentum-resolved spectral density of states, the identification of subgap interface states and the derivation of the differential conductance for a wide variety of configurations of the junctions. We illustrate the application of this method for some simple systems with two and three regions, finding the characteristic chiral state of the quantum anomalous Hall effect at the NF interfaces, and chiral Majorana modes at the NS interfaces. Finally, we discuss some geometrical effects present in three-region junctions such as weak Fabry–Pérot resonances and Andreev bound states.

High-energy x-ray diffraction (HE-XRD) experiments combined with an analysis based on atomic-pair-distribution functions can be an effective tool for probing low-dimensional materials. Here, we show how such an analysis can be used to gain insight into structural properties of PbTe nanoparticles (NPs). We interpret our HE-XRD data using an orthorhombic Pnma phase of PbTe, which is an orthorhombic distortion of the rocksalt phase. Although local crystal geometry can vary substantially with particle size at scales below 10 nm, and for very small NPs the particle size itself influences x-ray diffraction patterns, our study shows that HE-XRD can provide a unique nano-characterization tool for unraveling structural properties of nanoscale systems.

We have used fluctuation electron microscopy (FEM) to investigate the nucleation stage of TiO₂ crystal formation in binary TiO₂–SiO₂ glasses with heat treatment. It was found that spatial fluctuations of electron scattering in the glass with 13 wt% TiO₂ increases with heat treatment above 800 °C but before any crystals precipitate, i.e. before crystals are detectable by electron diffraction. We have attributed this to TiO₂ clustering and increasing medium-range order due to gradual ordering of TiO₂ phase. Moreover, we have found that FEM is sensitive to structural changes at temperatures as low as 400 °C but the nature of the changes yet to be determined. This demonstrates that FEM can be sensitive to structural changes in oxide glasses occurring during thermal treatment but preceding detection of the first crystals.

Symmetry indicates that low energy spectra of materials could be richer than well-known Dirac, semi-Dirac, or quadratic, hosting some unusual quasiparticles. Performing the systematic study of exact forms of low energy effective Hamiltonians and dispersions in high-symmetry points with fourfold degeneracy of bands, we found new, previously unreported dispersion, which we named poppy flower (PF) after its shape. This massless fermion exists in non-magnetic two-dimensional (2D) crystals with spin–orbit coupling (SOC), which are invariant under one of the proposed ten noncentrosymmetric layer groups. We suggest real three-dimensional (3D) layered materials suitable for exfoliation, having layers that belong to these symmetry groups as candidates for realization of PF fermions. In 2D systems without spin–orbit interaction, fortune teller (FT)-like fermions were theoretically predicted, and afterward experimentally verified in the electronic structure of surface layer of silicon. Herein, we show that such fermions can also be hosted in 2D crystals with SOC, invariant under additional two noncentrosymmetric layer groups. This prediction is confirmed by density functional based calculation: layered BiIO₄, which has been synthesized already as a 3D crystal, exfoliates to stable monolayer with symmetry pb2₁a, and FT fermion is observed in the band structure. Analytically calculated density of states (DOS) of the PF shows semimetallic characteristic, in contrast to metallic nature of FT having non-zero DOS at the bands contact energy. We indicate possibilities for symmetry breaking patterns which correspond to the robustness of the proposed dispersions as well as to the transition from Dirac centrosymmetric semimetal to PF.

We employed a state-of-the-art first-principles many-body approach, namely the density functional theory in combination with the single-site dynamical mean-field theory, to study the 4f electronic structures in cerium monopnictides (CeX, where X = N, P, As, Sb, and Bi). We find that the 4f electrons in CeN are highly itinerant and mixed-valence, showing a prominent quasiparticle peak near the Fermi level. On the contrary, they become well localized and display weak valence fluctuation in CeBi. It means that a 4f itinerant-localized crossover could emerge upon changing the X atom from N to Bi. Moreover, according to the low-energy behaviors of 4f self-energy functions, we could conclude that the 4f electrons in CeX also demonstrate interesting orbital-selective electronic correlations, which are similar to the other cerium-based heavy fermion compounds.

Motivated by recent experiments in the Kitaev honeycomb lattice, Kondo insulators, and the 'Luttinger's theorem-violating' Fermi liquid phase of the underdoped cuprates, we extend the theoretical machinery of Landau–Fermi liquid theory to a system of itinerant, interacting Majorana-like particles. Building upon a previously introduced model of 'nearly self-conjugate' fermionic polarons, a Landau–Majorana kinetic equation is introduced to describe the collective modes and Fermi surface instabilities in a fluid of particles whose fermionic degrees of freedom obey the Majorana reality condition. At large screening, we show that the Landau–Majorana liquid harbors a Lifshitz transition for specific values of the driving frequency. Moreover, we find the dispersion of the zero sound collective mode in such a system, showing that there exists a specific limit where the Landau–Majorana liquid harbors a stability against Pomeranchuk deformations unseen in the conventional Landau–Fermi liquid. With these results, our work paves the way for possible extensions of the Landau quasiparticle paradigm to nontrivial metallic phases of matter.

Ferroelectric systems with multi-phase co-existence are found to exhibit anomalous photovoltaic response. In this work, detailed photovoltaic studies are carried out under 405 nm light illumination on Ba_1−x(Bi_0.5Li_0.5)_xTiO₃ ferroelectric oxides having the co-existence of tetragonal and orthorhombic phases. The linear and sinusoidal photocurrent-dependence as a function of light intensity and polarization-direction, respectively elucidate the experimental evidence for linear bulk-photovoltaic effect. Importantly, the temperature-dependent photovoltaic studies display 2-fold enhancement in photovoltage near the ferroelectric transition temperature (T_C). The observed features in photovoltage follow inverse temperature-dependence of the photoconductivity. The linear relationship between the calculated bulk-photovoltaic tensor component and the photocurrent established from the proposed phenomenological model is verified through their composition-dependent studies. These studies provide the desired design parameters to engineer the ferroelectric system for better photovoltaic characteristics suitable for device applications.

Index matching of guided modes in birefringent multilayered organic waveguides opens new prospects for the design of mode coupling and mode switching devices. We demonstrate index matching of guided modes in two multilayered structures, in (a) a PTCDA-Alq₃-PTCDA three-layer and (b) a PTCDA-Alq₃ effective medium multilayer waveguide. The optical waveguides were grown on a Pyrex substrate by organic molecular beam deposition. The occurrence of index matching was investigated both experimentally by measuring the effective refractive index dispersion of transverse electric and magnetic modes using the m-line technique and theoretically by modelling the index dispersion with a transfer matrix algorithm.

The magnetic susceptibility of the 1/1 approximants to icosahedral quasicrystals in a series of Cd_85−xMg_xTb₁₅ (x = 5, 10, 15, 20) alloys was investigated in detail. The occurrence of antiferromagnetic (AFM) to spin-glass (SG)-like transition was noticed by increasing Mg. Transmission electron microscopy analysis evidenced a correlation between the magnetic transition and suppression of the monoclinic superlattice ordering with respect to the orientation of the Cd₄ tetrahedron at T > 100 K. The possible origins of this phenomenon were discussed in detail. The occurrence of the AFM to SG-like magnetic transition is associated with the combination of chemical disorder due to a randomized substitution of Cd with Mg and the orientational disorder of the Cd₄ tetrahedra.

The structural and magnetic behavior of Mn-site doped intermetallic manganese silicide alloys of nominal compositions Mn_5−xA_xSi₃ (x = 0.05, 0.1, 0.2 and A = Ni, Cr) have been investigated with a focus to the inverted hysteresis behavior and thermomagnetic irreversibility. Room temperature x-ray powder diffraction data confirm that all the doped alloys crystallize in hexagonal D8₈ type structure with space group P6₃/mcm. The doped alloys are found to show paramagnetic–collinear antiferromagnetic (AFM2)–noncollinear antiferromagnetic (AFM1) transitions during cooling from room temperature. A significant decrease in the critical values of both AFM1–AFM2 transition temperatures and fields have been observed with the increasing Ni/Cr concentration. Inverted hysteresis loop, field-induced arrest, and thermomagnetic arrest, the key features of the undoped Mn₅Si₃ alloy, are found to be significantly affected by the Mn-site doping and eventually vanishes with 4% doping.

As an exotic material in spintronics, Gd-doped GaN is known as a room temperature ferromagnetic material that possesses a large magnetic moment (4000 μ_B per Gd ion). This paper theoretically proposes that the large magnetic moment and room temperature ferromagnetism observed in Gd-doped GaN is caused by N 2p holes based on the assumption that Ga-vacancies result from the introduction of Gd ions. This causes that the too large magnetic moment is estimated for Gd ions if only Gd ions contributed the magnetic moment.

Magnetization measurements have been performed to understand the role of the magnetic structure on the superconducting properties of epitaxial thin films of Ba_1−xLa_xFe₂As₂ (x = 0.08, 0.13, and 0.18) deposited on single crystal (001)-oriented MgO substrates by pulsed laser deposition. All samples exhibit a reentrant-spinglass like behavior at normal state. At lower temperatures, we observe the same magnetic state coexisting with superconductivity and it is also observed a prominent non-linear giant diamagnetism in an intermediate temperature range just above the superconducting phase transition temperature. Furthermore, no significant change in the magnetic domain structure was detected by the onset of superconductivity. Based on their magnetic states, we claim that each domain (as a disconnected superconducting island) has its own bulk superconducting properties. Finally, we discussed the dual character played by the La atoms in the superconducting properties. That duality character has been also confirmed by analyzing resistivity data.

Including the orbital off-diagonal spin and charge condensates in the self consistent determination of magnetic order within a realistic three-orbital model for the 4d⁴ compound Ca₂RuO₄, reveals a host of novel features including strong and anisotropic spin–orbit coupling (SOC) renormalization, coupling of strong orbital magnetic moments to orbital fields, and a magnetic reorientation transition. Highlighting the rich interplay between orbital geometry and overlap, SOC, Coulomb interactions, tetragonal distortion, and staggered octahedral tilting and rotation, our investigation yields a planar antiferromagnetic (AFM) order for moderate tetragonal distortion, with easy a–b plane and easy b axis anisotropies, along with small canting of the dominantly yz, xz orbital moments. With decreasing tetragonal distortion, we find a magnetic reorientation transition from the dominantly planar AFM order to a dominantly c axis ferromagnetic order with significant xy orbital moment.

Static and dynamic magnetic properties of normal spinel Co₂RuO₄ = (Co²⁺)${}_{A}{\left[{\mathrm{C}\mathrm{o}}^{3+}{\text{Ru}}^{3+}\right]}_{B}{\mathrm{O}}_{4}$ are reported based on our investigations of the temperature (T), magnetic field (H) and frequency (f) dependence of the ac-magnetic susceptibilities and dc-magnetization (M) covering the temperature range T = 2 K–400 K and H up to 90 kOe. These investigations show that Co₂RuO₄ exhibits an antiferromagnetic (AFM) transition at T_N ∼ 15.2 K, along with a spin-glass state at slightly lower temperature (T_SG) near 14.2 K. It is argued that T_N is mainly governed by the ordering of the spins of Co²⁺ ions occupying the A-site, whereas the exchange interaction between the Co²⁺ ions on the A-site and randomly distributed Ru³⁺ on the B-site triggers the spin-glass phase, Co³⁺ ions on the B-site being in the low-spin non-magnetic state. Analysis of measurements of M (H, T) for T < T_N are used to construct the H–T phase diagram showing that T_SG shifts to lower T varying as H^2/3.2 expected for spin-glass state whereas T_N is nearly H-independent. For T > T_N, analysis of the paramagnetic susceptibility (χ) vs. T data are fit to the modified Curie–Weiss law, χ = χ₀ + C/(T + θ), with χ₀ = 0.0015 emu mol⁻¹Oe⁻¹ yielding θ = 53 K and C = 2.16 emu-K mol⁻¹Oe⁻¹, the later yielding an effective magnetic moment μ_eff = 4.16 μ_B comparable to the expected value of μ_eff = 4.24 μ_B per Co₂RuO₄. Using T_N, θ and high temperature series for χ, dominant exchange constant J₁/k_B ∼ 6 K between the Co²⁺ on the A-sites is estimated. Analysis of the ac magnetic susceptibilities near T_SG yields the dynamical critical exponent zν = 5.2 and microscopic spin relaxation time τ₀ ∼ 1.16 × 10⁻¹⁰ sec characteristic of cluster spin-glasses and the observed time-dependence of M(t) is supportive of the spin-glass state. Large M–H loop asymmetry at low temperatures with giant exchange bias effect (H_EB ∼ 1.8 kOe) and coercivity (H_C ∼ 7 kOe) for a field cooled sample further support the mixed magnetic phase nature of this interesting spinel. The negative magnetocaloric effect observed below T_N is interpreted to be due to the AFM and SG ordering. It is argued that the observed change from positive MCE (magnetocaloric effect) for T > T_N to inverse MCE for T < T_N observed in Co₂RuO₄ (and reported previously in other systems also) is related to the change in sign of (∂M/∂T) vs. T data.

Two-dimensional transition metal dichalcogenides (TMD) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMD monolayers and heterostructures in consideration of the material's structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS₂ flakes grown on a SiO₂/ Si(001) substrate are presented.

Using density functional theory and ab initio molecular dynamics, we have investigated the elastic properties of Bi, Te and Cu as a function of temperature. We compare calculated quantities which can be used to determine the effectiveness of our proposed method, such as the bulk (K), shear (G), and Young's (E) moduli. We also computed Poisson's ratio (ν) and the Pugh ratio (γ) for each of these materials at different temperatures to investigate changes in ductility. We have used the elastic moduli to calculate the Debye temperature θ_D and minimum thermal conductivity k_min of these materials as a function of temperature. We found that the elastic properties calculated in this work are in good agreement with experimental work. The inclusion of temperature effects has allowed for the proper prediction of ductility for each of these materials, a feat that standard density functional theory calculations has previously been unable to accomplish for Bi and Te.