Table of contents

We investigate the structure of gluten polymer-like gels in a binary mixture of water/ethanol, 50/50 v/v, a good solvent for gluten proteins. Gluten comprises two main families of proteins, monomeric gliadins and polymer glutenins. In the semi-dilute regime, scattering experiments highlight two classes of behavior, akin to standard polymer solution and polymer gel, depending on the protein composition. We demonstrate that these two classes are encoded in the structural features of the proteins in very dilute solution, and are correlated with the presence of proteins assemblies of typical size tens of nanometers. The assemblies only exist when the protein mixture is sufficiently enriched in glutenins. They are found directly associated to the presence in the gel of domains enriched in non-exchangeable H-bonds and of size comparable to that of the protein assemblies. The domains are probed in neutron scattering experiments thanks to their unique contrast. We show that the sample visco-elasticity is also directly correlated to the quantity of domains enriched in H-bonds, showing the key role of H-bonds in ruling the visco-elasticity of polymer gluten gels.

Two-dimensional (2D) materials with both auxetic effect and ferroelasticity are rare, however, have great application potential in next generation microelectromechanical and nanoelectronic devices. Here, we report the findings of an extraordinary combination half-auxetic effect and ferroelasticity in a single p2mm-type TiSe monolayer by performing first-principles calculations. The unique half-auxetic effect, namely the material expand laterally under both uniaxial tensile strain, and compressive strain, is reported and explained by considering both the nearest and the next-nearest interactions. The ferroelasticity is stemming from the degeneracy breaking of the 3d-orbitals of Ti atoms in a distorted tetrahedron crystal field, or the so-called Jahn–Teller effect. The results provide a guideline for the future design of novel 2D multiple functional materials at the nanoscale.

Contacts between black phosphorene (BP) and metal electrodes are critical components of BP-based devices and can dramatically affect device performance. In this paper, we adopted first-principles calculations to explore binding energies, electronic structures, spatial potential distribution of monolayer BP–Ni interfaces in surface contact and edge contact types, and used density functional theoretical coupled with nonequilibrium Green's function method to investigate the electrical transport properties for transport systems of monolayer BP with Ni electrodes. Our calculated results indicate that contact type between monolayer BP and metal Ni electrodes may much affect the transport properties of monolayer BP–Ni devices. Interfacial interaction between Ni and monolayer BP in edge contact type is stronger than that in surface contact type. The potential distributions indicate that edge contact type is more beneficial for reducing contact resistance of monolayer BP–Ni contacts and conducive to improve the performance of BP–Ni electrode device.

Motivated by the spin–momentum locking of electrons at the boundaries of certain topological insulators, we study a one-dimensional system of spin–orbit coupled massless Dirac electrons with s-wave superconducting pairing. As a result of the spin–orbit coupling, our model has only two kinds of linearly dispersing modes, and we take these to be right-moving spin-up and left-moving spin-down. Both lattice and continuum models are studied. In the lattice model, we find that a single Majorana zero energy mode appears at each end of a finite system provided that the s-wave pairing has an extended form, with the nearest-neighbor pairing being larger than the on-site pairing. We confirm this both numerically and analytically by calculating the winding number. We find that the continuum model also has zero energy end modes. Next we study a lattice version of a model with both Schrödinger and Dirac-like terms and find that the model hosts a topological transition between topologically trivial and non-trivial phases depending on the relative strength of the Schrödinger and Dirac terms. We then study a continuum system consisting of two s-wave superconductors with different phases of the pairing, with a δ-function potential barrier lying at the junction of the two superconductors. Remarkably, we find that the system has a single Andreev bound state (ABS) which is localized at the junction. When the pairing phase difference crosses a multiple of 2π, an ABS touches the top of the superconducting gap and disappears, and a different state appears from the bottom of the gap. We also study the AC Josephson effect in such a junction with a voltage bias that has both a constant V₀ and a term which oscillates with a frequency ω. We find that, in contrast to standard Josephson junctions, Shapiro plateaus appear when the Josephson frequency ω_J = 2eV₀/ℏ is a rational fraction of ω. We discuss experiments which can realize such junctions.

Since the concept of ferroelectric metal predicted in the 1960s has been experimentally realized in the bulk Weyl semimetal WTe₂ [Sharma et al 2019 Sci. Adv.5, eaax5080], it is significant to find the ultrathin polar metal or ferroelectric metal due to the demand of miniature of electronic nanodevices. Here, 2D buckled monolayers composed of group-IV elements such as SiGe, SiSn, and GeSn are selected as prototype. Then, the stability of 2D ferroelectricity in the above monolayers are confirmed based on the results of first-principles calculations. Most interesting, a robustly metallic polar state has been found in the above 2D ferrolectrics under both the electron doping and hole doping, and the polar distortion becomes even more remarkable when the electrons are doped as compared with the undoped system. Thus, the coexistence of polar state and conduction is theoretically verified in the doped group-IV monolayers. We hope the 2D ferroelectric materials can be used as a starting point to look for the polar metals with atomic thickness, and further broaden their applications in 2D electronics or spintronics in the future.

In this work, we study the spin polarization in the MoS(Se)₂–WS(Se)₂ transition metal dichalcogenide heterostructures by using the non-equilibrium Green's function method and a three-band tight-binding model near the edges of the first Brillouin zone. Although it has been shown that the structures have no significant spin polarization in a specific range of energy of electrons, by applying a transverse electric field in the sheet of the metal atoms, shedding light on the sample, and under a small bias voltage, a significant spin polarization in the structure could be created. Besides, by applying a suitable bias voltage between leads and applying the electric field, a noticeable spin polarization can be found even without shedding the light on the heterostructures.

Electronic excitation energy transfer is a ubiquitous process that has generated prime research interest since its discovery. Recently developed variational polaron transformation-based second-order master equation is capable of interpolating between Förster and Redfield limits with exceptional accuracy. Forms of spectral density functions studied so far through the variational approach provide theoretical support for various experiments. Recently introduced ohmic like spectral density function that can account for logarithmic perturbations provides generality and exposition to a unique and practical set of environments. In this paper, we exploit the energy transfer dynamics of a two-level system attached to an ohmic like spectral density function with logarithmic perturbations using a variational polaron transformed master equation. Our results demonstrate that even for a relatively large bath coupling strength, quantum coherence effects can be increased by introducing logarithmic perturbations of the order of one and two in super-ohmic environments. Moreover, for particular values of the ohmicity parameter, the effect of logarithmic perturbations is observed to be insignificant for the overall dynamics. In regard to ohmic environments, as logarithmic perturbations increase, damping characteristics of the coherent transient dynamics also increase in general. It is also shown that, having logarithmic perturbations of the order of one in an ohmic environment can result in a less efficient energy transfer for relatively larger system bath coupling strengths.

The present work discusses the possibility to achieve a high degree of spin polarization in a three-terminal quantum system. Irradiating the system, subjected to Rashba spin–orbit (SO) interaction, we find high degree of spin polarization under a suitable input condition along with different magnitudes and phases at the two output leads. The system is described within a tight-binding (TB) framework and the effect of irradiation is incorporated following the Floquet–Bloch (FB) ansatz. All the spin-dependent transmission probabilities are evaluated through Green's function technique using Landauer–Büttiker formalism. Several possible aspects are included to make the system more realistic and examined rigorously in the present work. To name a few, the effects of irradiation, SO interaction, interface sensitivity, system size, system temperature are investigated, and finally, the role of correlated impurities are studied. Despite having numerous proposals available to generate and manipulate spin-selective transmissions, such a prescription exploiting the irradiation effect is relatively new to the best of our concern.

The magnetoresistance (MR) of iron pnictide superconductors is often dominated by electron–electron correlations and deviates from the H² or saturating behaviors expected for uncorrelated metals. Contrary to similar Fe-based pnictide systems, the superconductor LaRu₂P₂ (T_c = 4 K) shows no enhancement of electron–electron correlations. Here we report a non-saturating MR deviating from the H² or saturating behaviors in LaRu₂P₂. We present results in single crystals of LaRu₂P₂, where we observe a MR following H^1.3 up to 22 T. We discuss our result by comparing the bandstructure of LaRu₂P₂ with that of Fe based pnictide superconductors. The different orbital structures of Fe and Ru leads to a 3D Fermi surface with negligible bandwidth renormalization in LaRu₂P₂, that contains a large open sheet over the whole Brillouin zone. We show that the large MR in LaRu₂P₂ is unrelated to the one obtained in materials with strong electron–electron correlations and that it is compatible instead with conduction due to open orbits on the rather complex Fermi surface structure of LaRu₂P₂.

Density functional theory is used to calculate the energy of electron–hole liquid and the equilibrium density of electron-hole pairs in quantum wells. Nonlinear Kohn–Sham equations for electrons and holes are solved numerically. The influence of the depth and width of the quantum well, the ratio of the hole and electron masses, and the spin splitting of the hole band on the properties of electron–hole liquid is studied. The critical temperature of electron–hole liquid in quantum wells is estimated. Good agreement between the calculations and experimental results is obtained.

The superfluid density or superconducting (SC) carrier concentration n_sc of cuprates has been the subject of intense investigations but there is not any single theory capable to explain all the available data. Here we show that the behavior of n_sc in under and overdoped cuprates are a consequence of an SC interaction based on charge fluctuations in the incommensurate charge-density-waves (CDW) domains. We have shown that this interaction scales with the CDW amplitude or the pseudogap (PG) energy, yielding local SC amplitudes and Josephson currents. The average Josephson energy $\left\langle {E}_{\mathrm{J}}\right\rangle$ is proportional to the phase stiffness or superfluid density ρ_sc ∝ n_sc. We find that n_sc(p) increases almost linearly with doping p in the underdoped region and in the charge abundant overdoped only a few fractions of the holes condense leading to two kinds of carriers, a recently confirmed feature. The calculations and the ρ_sc data uncover how the PG–CDW–SC intertwined orders operate to yield cuprates properties.

We report the electronic structure and magnetic properties of Co₂Ti_1−xGe_xO₄ (0 ⩽ x ⩽ 1) spinel by means of the first-principle methods of density functional theory involving generalized gradient approximation along with the on-site Coulomb interaction (U_eff) in the exchange-correlation energy functional. Special emphasis has been given to explore the site occupancy of Ge atoms in the spinel lattice by introducing the cationic disorder parameter (y) which is done in such a way that one can tailor the pyrochlore geometry and determine the electronic/magnetic structure quantitatively. For all the compositions (x), the system exhibits weak tetragonal distortion (c/a ≠ 1) due to the non-degenerate ${d}_{{z}^{2}}$ and ${d}_{{x}^{2}-{y}^{2}}$ states (e_g orbitals) of the B-site Co. We observe large exchange splitting (Δ_EX ∼ 9 eV) between the up and down spin bands of t_2g and e_g states, respectively, of tetrahedral and octahedral Co²⁺ (⁴A_2(g)(F)) and moderate crystal-field splitting (Δ_CF ∼ 4 eV) and the Jahn–Teller distortion (Δ_JT ∼ 0.9 eV). These features indicate the strong intra-atomic interaction which is also responsible for the alteration of energy band-gap (1.7 eV ⩽ E_g ⩽ 3.3 eV). The exchange interaction (J_BB ∼ −4.8 meV, for (x, y) = (0.25, 0)) between the Co²⁺ dominates the overall antiferromagnetic behaviour of the system for all 'x' as compared to J_AA (∼−2.2 meV, for (x, y) = (0.25, 0)) and J_AB (∼−1.8 meV, for (x, y) = (0.25, 0)). For all the compositions without any disorderness in the system, the net ferrimagnetic moment (Δμ) remains constant, however, increases progressively with increasing x due to the imbalance of Co spins between the A- and B-sites.

We studied the applicability of Heusler alloys Mn₂RuZ (Z = Al, Ga, Ge, Si) to the electrode materials of MgO-based magnetic tunnel junctions. All these alloys possess Hg₂CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized Δ₁ band, which is robust against atomic disorder. Next we studied the electronic structure of Mn₂RuAl/MgO and Mn₂RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn₂RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn₂RuAl/MgO junction, high spin polarization of bulk Mn₂RuAl is preserved independent of its termination.

We study the sodium-iridates model on the honeycomb lattice with both BCS pairing potential and Hubbard interaction term. It is shown that this model can be exactly solved with appropriate choices of amplitude of pairing gaps, where the interacting terms are transformed to external field terms. The band structures of these exact solutions on both torus and cylinder geometry are discussed in great details. It is found that the ground state assumes an anti-ferromagnetic configuration, which breaks the time reversal symmetry spontaneously and renders the superconductor topologically trivial. On the other hand, the nontrivial topology is preserved with ferromagnetic configuration and can be characterized by the isospin Chern number.

Wannier functions have been widely applied in the study of topological properties and Floquet–Bloch bands of materials. Usually, the real-space Wannier functions are linked to the k-space Hamiltonian by two types of Fourier transform (FT), namely lattice-gauge FT (LGFT) and atomic-gauge FT (AGFT), but the differences between these two FTs on Floquet–Bloch bands have rarely been addressed. Taking monolayer graphene as an example, we demonstrate that LGFT gives different topological descriptions on the Floquet–Bloch bands for the structurally equivalent directions which are obviously unphysical, while AGFT is immune to this dilemma. We introduce the atomic-laser periodic effect to explain the different Floquet–Bloch bands between the LGFT and AGFT. Using AGFT, we showed that linearly polarized laser could effectively manipulate the properties of the Dirac fermions in graphene, such as the location, generation and annihilation of Dirac points. This proposal offers not only deeper understanding on the role of Wannier functions in solving the Floquet systems, but also a promising platform to study the interaction between the time-periodic laser field and materials.

Quantum defects are essential to understand the non-radiative recombination processes in metal halide perovskites-based photovoltaic devices, in which Huang–Rhys factor, reflecting the coupling strength between the charge carrier and optical phonons, plays a key role in determining the non-radiative recombination via multiphonon processes. Herein, we theoretically present multiphonon Raman scattering intermediated by defects arising from the charge carrier of defect coupled with the longitudinal optical (LO) phonon in the deformation potential and Fr$\ddot {o}$hlich mechanisms, respectively. We find that the Raman scattering shows multiple LO phonon overtones at equal interval LO phonons, where Huang–Rhys factor could be evaluated by the order of the strongest overtone. Meanwhile, we give the combinational multiphonon scattering between two mechanisms. Different types of the combinational modes with the weak scattering intensities provide a possible explanation for the long non-radiative charges-carrier lifetimes in metal halide perovskites.

Polycrystalline La_1-xPb_xMnO_3±y (x = 0.3, 0.35, 0.4) solid solutions were prepared by solid state reaction method and their magnetic properties have been investigated. Rietveld refinement of x-ray powder diffraction patterns showed that all samples are single phase and crystallized with the rhombohedral structure in the R-3c space group. A second order paramagnetic to ferromagnetic (FM) phase transition was observed for all materials. The Griffiths phase (GP), identified from the temperature dependence of the inverse susceptibility, was suppressed by increasing magnetic field and showed a significant dependence on A-site chemical substitution. The critical behaviour of the compounds was investigated near to their Curie temperatures, using intrinsic magnetic field data. The critical exponents (β, γ and δ) are close to the mean-field approximation values for all three compounds. The observed mean-field like behaviour is a consequence of the GP and the formation of FM clusters. Long-range FM order is established as the result of long-range interactions between FM clusters. The magnetocaloric effect was studied in terms of the isothermal entropy change. Our study shows that the material with the lowest chemical substitution (x = 0.3) has the highest potential (among the three compounds) as magnetic refrigerant, owing to its higher relative cooling power (258 J kg⁻¹ at 5 T field) and a magnetic phase transition near room temperature.

We have studied the nearest neighbor Heisenberg model with added Dzyaloshinskii–Moriya interaction using Schwinger boson mean-field theory considering the in-plane component as well as out-of-plane component. Motivated by the experimental result of vesignieite that the ground state is in a Q = 0 long-range order state, we first looked at the classical ground state of the model and considered the mean-field ansatz which mimics the classical ground state in the large S limit. We have obtained the ground-state phase diagram of this model and calculated properties of different phases. We have also studied the above model numerically using exact diagonalization up to a system size N = 30. We have compared the obtained results from these two approaches. Our results are in agreement with the experimental result of the vesignieite.

Using relativistic density-functional calculations, we examine the magneto-crystalline anisotropy and exchange properties of transition-metal atoms adsorbed on 2D-GaAs. We show that single Mn and Mo atom (Co and Os) strongly bind on 2D-GaAs, and induce local out-of-plane (in-plane) magnetic anisotropy. When a pair of TM atoms is adsorbed on 2D-GaAs in a close range from each other, magnetisation properties change (become tunable) with respect to concentrations and ordering of the adatoms. In all cases, we reveal presence of strong Dzyaloshinskii–Moriya interaction. These results indicate novel pathways towards two-dimensional chiral magnetic materials by design, tailored for desired applications in magneto-electronics.

Molecular dynamic simulations based on a recently constructed potential reveal that quasi-repeating patterns could appear in both Fe(110)/W(110) and W(110)/Fe(110) interfaces, and that three kinds of atomic displacements of Fe atoms because of the Fe–W interaction intrinsically bring about the interesting quasi-repeating patterns of the Fe–W interfaces. It is also found that the Fe–W interface becomes more brittle with less critical strains under tensile loading than pure Fe or W, which is fundamentally attributed to the movement of the interface dislocations as a result of the lattice mismatch between Fe and W. Interestingly, the dislocation loops could be formed in the Fe–W interface under tensile loading due to the pinning of the $\left\langle 100\right\rangle$ edge dislocations by the edge dislocations of $1/2\left\langle 111\right\rangle$, whereas no dislocation loop would be generated in pure Fe or W.

By means of the Monte Carlo method, a numerical study of the vortex system in a high-temperature superconductor under the impact of pulses of magnetic field has been conducted. Various shapes and amplitudes of pulses have been considered. Samples with random and regular distributions of three different numbers of defects have been compared from the viewpoint of efficiency of flux trapping. The low-temperature behavior of vortices and their penetration into samples have been shown to be independent of the pulse shape but strongly dependent of the type of pinning distribution. Saturating dependences of density of trapped magnetic flux on the pulse amplitude have been obtained. The samples with random pinning demonstrated higher efficiency of flux trapping at lower pulse amplitudes, and the samples with a triangular lattice of defects—at higher amplitudes. If the amplitude exceeded the saturation field of both samples, the trapped field was almost equal. The increasing number of defects has lead to an increase in trapped field within the considered range of concentrations.