Highlights of 2014

A pedagogical essay is given that alerts researchers to the errors inherent in assigning linear I(V) current–voltage dependences to Ohmic conduction. Such a linear I(V) is necessary but not sufficient, since other mechanisms, including Simmons' modification of the basic Schottky emission theory, also give linear I(V) at small applied voltages. Discrimination among Ohmic, Schottky, space-charge limited, and other models requires accurate thickness dependence I(d) data, where for Ohmic conduction I = a/d, whereas for interface-limited mechanisms such as Simmons/Schottky, I is nearly independent of d.

Resistivity, dc magnetization, and heat capacity measurements are reported for superconducting NaBi. T_c, the electronic contribution to the specific heat γ, the ΔC_p/γT_c ratio, and the Debye temperature are found to be 2.15 K, 3.4 mJ mol⁻¹ K⁻², 0.78, and 140 K respectively. The calculated electron–phonon coupling constant (λ_ep = 0.62) implies that NaBi is a moderately coupled superconductor. The upper critical field and coherence length are found to be 250 Oe and 115 nm, respectively. Electronic structure calculations show NaBi to be a good metal, in agreement with the experiments; the p_x and p_y orbitals of Bi dominate the electronic states at the Fermi Energy.

We use density functional theory to explore the interplay between octahedral rotations and ferroelectricity in the model compound SrTiO₃. We find that over the experimentally relevant range octahedral rotations suppress ferroelectricity as is generally assumed in the literature. Somewhat surprisingly, we observe that at larger angles the previously weakened ferroelectric instability strengthens significantly. By analyzing geometry changes, energetics, force constants and charges, we explain the mechanisms behind this transition from competition to cooperation with increasing octahedral rotation angle.

We study subsurface arsenic dopants in a hydrogen-terminated Si(001) sample at 77 K, using scanning tunnelling microscopy and spectroscopy. We observe a number of different dopant-related features that fall into two classes, which we call As1 and As2. When imaged in occupied states, the As1 features appear as anisotropic protrusions superimposed on the silicon surface topography and have maximum intensities lying along particular crystallographic orientations. In empty-state images the features all exhibit long-range circular protrusions. The images are consistent with buried dopants that are in the electrically neutral (D⁰) charge state when imaged in filled states, but become positively charged (D⁺) through electrostatic ionization when imaged under empty-state conditions, similar to previous observations of acceptors in GaAs. Density functional theory calculations predict that As dopants in the third layer of the sample induce two states lying just below the conduction-band edge, which hybridize with the surface structure creating features with the surface symmetry consistent with our STM images. The As2 features have the surprising characteristic of appearing as a protrusion in filled-state images and an isotropic depression in empty-state images, suggesting they are negatively charged at all biases. We discuss the possible origins of this feature.

We propose a nonadiabatic time-dependent spin-density functional theory (TDSDFT) approach for studying single-electron excited states and the ultrafast response of systems with strong electron correlations. The correlation part of the nonadiabatic exchange–correlation (XC) kernel is constructed by using exact results for the Hubbard model of strongly correlated electrons. We demonstrate that the corresponding nonadiabatic XC kernel reproduces the main features of the spectrum of the Hubbard dimer and the 2D, 3D and infinite-dimensional Hubbard models, some of which are impossible to obtain within the adiabatic approach. The formalism may be applied for ab initio examination of strongly correlated electron systems in- and out-of-equilibrium within the TDSDFT, extending it beyond the metallic and semiconductor structures with plasmons, excitons and other excitations.

We have investigated the growth of Pt on Ge(1 1 0) using scanning tunneling microscopy and spectroscopy. The deposition of several monolayers of Pt on Ge(1 1 0) followed by annealing at 1100 K results in the formation of 3D metallic Pt-Ge nanocrystals. The outermost layer of these crystals exhibits a honeycomb structure. The honeycomb structure is composed of two hexagonal sub-lattices that are displaced vertically by 0.2 Å with respect to each other. The nearest-neighbor distance of the atoms in the honeycomb lattice is 2.5  ±  0.1 Å, i.e. very close to the predicted nearest-neighbor distance in germanene (2.4 Å). Scanning tunneling spectroscopy reveals that the atomic layer underneath the honeycomb layer is more metallic than the honeycomb layer itself. These observations are in line with a model recently proposed for metal di-(silicides/)germanides: a hexagonal crystal with metal layers separated by semiconductor layers with a honeycomb lattice. Based on our observations we propose that the outermost layer of the Ge₂Pt nanocrystal is a germanene layer.

Research efforts addressing spin waves (magnons) in micro- and nanostructured ferromagnetic materials have increased tremendously in recent years. Corresponding experimental and theoretical work in magnonics faces significant challenges in that spin-wave dispersion relations are highly anisotropic and different magnetic states might be realized via, for example, the magnetic field history. At the same time, these features offer novel opportunities for wave control in solids going beyond photonics and plasmonics. In this topical review we address materials with a periodic modulation of magnetic parameters that give rise to artificially tailored band structures and allow unprecedented control of spin waves. In particular, we discuss recent achievements and perspectives of reconfigurable magnonic devices for which band structures can be reprogrammed during operation. Such characteristics might be useful for multifunctional microwave and logic devices operating over a broad frequency regime on either the macro- or nanoscale.

Recent developments in the emerging field of plasmonics in graphene and other Dirac systems are reviewed and a comprehensive introduction to the standard models and techniques is given. In particular, we discuss intrinsic plasmon excitation of single- and bilayer graphene via hydrodynamic equations and the random phase approximation, but also comment on double and multilayer structures. Additionally, we address Dirac systems in the retardation limit and also with large spin–orbit coupling including topological insulators. Finally, we summarize basic properties of the charge, current and photon linear response functions in an appendix.

The response of oxide thin films to polar discontinuities at interfaces and surfaces has generated enormous activity due to the variety of interesting effects that it gives rise to. A case in point is the discovery of the electron gas at the interface between LaAlO₃ and SrTiO₃, which has since been shown to be quasi-two-dimensional, switchable, magnetic and/or superconducting. Despite these findings, the origin of the two-dimensional electron gas is highly debated and several possible mechanisms remain. Here we review the main proposed mechanisms and attempt to model expected effects in a quantitative way with the ambition of better constraining what effects can/cannot explain the observed phenomenology. We do it in the framework of a phenomenological model constructed to provide an understanding of the electronic and/or redox screening of the chemical charge in oxide heterostructures. We also discuss the effect of intermixing, both conserving and not conserving the total stoichiometry.

Atomistic modelling of magnetic materials provides unprecedented detail about the underlying physical processes that govern their macroscopic properties, and allows the simulation of complex effects such as surface anisotropy, ultrafast laser-induced spin dynamics, exchange bias, and microstructural effects. Here we present the key methods used in atomistic spin models which are then applied to a range of magnetic problems. We detail the parallelization strategies used which enable the routine simulation of extended systems with full atomistic resolution.

We review the state of the art of surface magnetic property control with non-magnetic means, concentrating on metallic surfaces and techniques such as charge-doping or external electric field (EEF) application.

Magneto-electric coupling via EEF-based charge manipulation is discussed as a way to tailor single adatom spins, exchange interaction between adsorbates or anisotropies of layered systems. The mechanisms of paramagnetic and spin-dependent electric field screening and the effect thereof on surface magnetism are discussed in the framework of theoretical and experimental studies.

The possibility to enhance the effect of EEF by immersing the target system into an electrolyte or ionic liquid is discussed by the example of substitutional impurities and metallic alloy multilayers.

A similar physics is pointed out for the case of charge traps, metallic systems decoupled from a bulk electron bath. In that case the charging provides the charge carrier density changes necessary to affect the magnetic moments and anisotropies in the system.

Finally, the option of using quasi-free electrons rather than localized atomic spins for surface magnetism control is discussed with the example of Shockley-type metallic surface states confined to magnetic nanoislands.

The field of graphene research has developed rapidly since its first isolation by mechanical exfoliation in 2004. Due to the relativistic Dirac nature of its charge carriers, graphene is both a promising material for next-generation electronic devices and a convenient low-energy testbed for intrinsically high-energy physical phenomena. Both of these research branches require the facile fabrication of clean graphene devices so as not to obscure its intrinsic physical properties. Hexagonal boron nitride has emerged as a promising substrate for graphene devices as it is insulating, atomically flat and provides a clean charge environment for the graphene. Additionally, the interaction between graphene and boron nitride provides a path for the study of new physical phenomena not present in bare graphene devices. This review focuses on recent advancements in the study of graphene on hexagonal boron nitride devices from the perspective of scanning tunneling microscopy with highlights of some important results from electrical transport measurements.

Over the past ten years, the all-atom molecular dynamics method has grown in the scale of both systems and processes amenable to it and in its ability to make quantitative predictions about the behavior of experimental systems. The field of computational DNA research is no exception, witnessing a dramatic increase in the size of systems simulated with atomic resolution, the duration of individual simulations and the realism of the simulation outcomes. In this topical review, we describe the hallmark physical properties of DNA from the perspective of all-atom simulations. We demonstrate the amazing ability of such simulations to reveal the microscopic physical origins of experimentally observed phenomena. We also discuss the frustrating limitations associated with imperfections of present atomic force fields and inadequate sampling. The review is focused on the following four physical properties of DNA: effective electric charge, response to an external mechanical force, interaction with other DNA molecules and behavior in an external electric field.

Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological attention in view of the exploitation of its exotic electronic properties. This novel material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. By employing low-energy electron diffraction and microscopy, we have studied the growth of Si on Ag(1 1 1) and observed a rich variety of rotationally non-equivalent silicene structures. Our results highlight a very complex formation diagram, reflecting the coexistence of different and nearly degenerate silicene phases, whose relative abundance can be controlled by varying the Si coverage and growth temperature. At variance with other studies, we find that the formation of single-phase silicene monolayers cannot be achieved on Ag(1 1 1).

By means of first-principles calculations we predict the stability of silicene layers as buckled honeycomb lattices on Cl-passivated Si(1 1 1) and clean CaF₂(1 1 1) surfaces. The van der Waals interaction between silicene and the inert substrate stabilizes the adsorbate system while not destroying the Si p_z-derived linear bands forming Dirac cones at the Brillouin zone corners. Only small gaps of about 3 and 52 meV are opened.

The deposition of Mn on to reconstructed InSb and GaAs surfaces, without coincident As or Sb flux, has been studied by reflection high energy electron diffraction, atomic force microscopy and scanning tunnelling microscopy. On both Ga- and As-terminated GaAs(0 0 1), (2 × n) Mn-induced reconstruction domains arise with n = 2 for the most well ordered reconstructions. On the Ga-terminated (4 × 6), the Mn-induced (2 × 2) persists up to around 0.5 ML Mn followed by Mn nano-cluster formation. For deposition on initially β2(2 × 4)-reconstructed GaAs(0 0 1), the characteristic trench structure of the reconstruction is partially preserved even beyond 1 monolayer Mn coverage. On both the β2(2 × 4) and c(4 × 4) surfaces, MnAs-like nano-clusters form alongside the reconstruction changes. In contrast, there are no new Mn-induced surface reconstructions on InSb. Instead, the Sb-terminated surfaces of InSb (0 0 1), (1 1 1)A and (1 1 1)B revert to reconstructions characteristic of clean In-rich surfaces after well defined coverages of Mn proportional to the Sb content of the starting reconstruction. These surfaces are decorated with self-assembled MnSb nanoclusters. These results are discussed in terms of basic thermodynamic quantities and the generalized electron counting rule.

The presence of plasmonic material influences the optical properties of nearby molecules in untrivial ways due to the dynamical plasmon-molecule coupling. We combine quantum and classical calculation schemes to study this phenomenon in a hybrid system that consists of a Na₂ molecule located in the gap between two Au/Ag nanoparticles. The molecule is treated quantum-mechanically with time-dependent density-functional theory, and the nanoparticles with quasistatic classical electrodynamics. The nanoparticle dimer has a plasmon resonance in the visible part of the electromagnetic spectrum, and the Na₂ molecule has an electron-hole excitation in the same energy range. Due to the dynamical interaction of the two subsystems the plasmon and the molecular excitations couple, creating a hybridized molecular-plasmon excited state. This state has unique properties that yield e.g. enhanced photoabsorption compared to the freestanding Na₂ molecule. The computational approach used enables decoupling of the mutual plasmon-molecule interaction, and our analysis verifies that it is not legitimate to neglect the backcoupling effect when describing the dynamical interaction between plasmonic material and nearby molecules. Time-resolved analysis shows nearly instantaneous formation of the coupled state, and provides an intuitive picture of the underlying physics.

The application of low temperature spin-polarized scanning tunneling microscopy and spectroscopy in magnetic fields for the quantitative characterization of spin polarization, magnetization reversal and magnetic anisotropy of individual nano structures is reviewed. We find that structural relaxation, spin polarization and magnetic anisotropy vary on the nm scale near the border of a bilayer Co island on Cu(1 1 1). This relaxation is lifted by perimetric decoration with Fe. We discuss the role of spatial variations of the spin-dependent electronic properties within and at the edge of a single nano structure for its magnetic properties.

The structure of the electrolyte/electrode interface plays a significant role in electrochemical processes. To date, most studies are focusing on understanding the interfacial structure in pure ionic liquids. In this paper in situ scanning tunnelling microscopy (STM) has been employed to elucidate the structure of the charged Au(111)–ionic liquid (1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, [Py_1,4]FAP) interface in the presence of 0.1 M LiCl. The addition of the Li salt to the ionic liquid has a strong influence on the interfacial structure. In the first STM scan in situ measurements reveal that Au(111) undergoes the ($22 \times \surd 3$) 'herringbone' reconstruction in a certain potential regime, and there is strong evidence that the gold surface dissolves at negative electrode potentials in [Py_1,4]FAP containing LiCl. Bulk deposition of Li is obtained at −2.9 V in the second STM scan.

The structural behavior of GeO₂ glass has been investigated up to 64 GPa using results from x-ray absorption spectroscopy in a diamond anvil cell combined with previously reported density measurements. The difference between the nearest Ge–O distances of glassy and rutile-type GeO₂ disappears at the Ge–O distance maximum at 20 GPa, indicating completion of the tetrahedral–octahedral transition in GeO₂ glass. The mean-square displacement σ² of the Ge–O distance in the first Ge–O shell increases progressively to a maximum at 10 GPa, followed by a substantial reduction at higher pressures. The octahedral glass is, as expected, less dense and has a higher compressibility than the corresponding crystalline phase, but the differences in Ge–O distance and density between the glass and the crystals are gradually eliminated over the 20–40 GPa pressure range. Above 40 GPa, GeO₂ forms a dense octahedral glass with a compressibility similar to that of the corresponding crystalline phase (α-PbO₂ type). The EXAFS and XANES spectra show evidence for subtle changes in the dense glass continuing to occur at these high pressures. The Ge–O bond distance shows little change between 45–64 GPa, and this may reflect a balance between bond shortening and a gradual coordination number increase with compression. The density of the glass is similar to that of the α–PbO₂-type phase, but the Ge–O distance is longer and is close to that in the higher-coordination pyrite-type phase which is stable above ∼60 GPa. The density data provide evidence for a possible discontinuity and change in compressibility at 40–45 GPa, but there are no major changes in the corresponding EXAFS spectra. A pyrite-type local structural model for the glass can provide a reasonable fitting to the XAFS spectra at 64 GPa.

In this paper we give a general theoretical framework that describes the sedimentation of multicomponent mixtures of particles with sizes ranging from molecules to macroscopic bodies. Both equilibrium sedimentation profiles and the dynamic process of settling, or its converse, creaming, are modeled. Equilibrium profiles are found to be in perfect agreement with experiments. Our model reconciles two apparently contradicting points of view about buoyancy, thereby resolving a long-lived paradox about the correct choice of the buoyant density. On the one hand, the buoyancy force follows necessarily from the suspension density, as it relates to the hydrostatic pressure gradient. On the other hand, sedimentation profiles of colloidal suspensions can be calculated directly using the fluid density as apparent buoyant density in colloidal systems in sedimentation–diffusion equilibrium (SDE) as a result of balancing gravitational and thermodynamic forces. Surprisingly, this balance also holds in multicomponent mixtures. This analysis resolves the ongoing debate of the correct choice of buoyant density (fluid or suspension): both approaches can be used in their own domain. We present calculations of equilibrium sedimentation profiles and dynamic sedimentation that show the consequences of these insights. In bidisperse mixtures of colloids, particles with a lower mass density than the homogeneous suspension will first cream and then settle, whereas particles with a suspension-matched mass density form transient, bimodal particle distributions during sedimentation, which disappear when equilibrium is reached. In all these cases, the centers of the distributions of the particles with the lowest mass density of the two, regardless of their actual mass, will be located in equilibrium above the so-called isopycnic point, a natural consequence of their hard-sphere interactions. We include these interactions using the Boublik–Mansoori–Carnahan–Starling–Leland (BMCSL) equation of state. Finally, we demonstrate that our model is not limited to hard spheres, by extending it to charged spherical particles, and to dumbbells, trimers and short chains of connected beads.

Junctions comprised of ferromagnets and nonmagnetic materials are one of the key building blocks in spintronics. With the recent breakthroughs of spin injection in ferromagnet/graphene junctions it is possible to consider spin-based applications that are not limited to magnetoresistive effects. However, for critical studies of such structures it is crucial to establish accurate predictive methods that would yield atomically resolved information on interfacial properties. By focusing on Co(0001)/graphene junctions and their electronic structure, we illustrate the inequivalence of different spin polarizations. We show atomically resolved spin polarization maps as a useful approach to assess the relevance of Co(0001)/graphene for different spintronics applications.

Recent experiments on current-induced domain-wall motion in chiral domain walls reveal important contributions both from spin–orbit torques (SOTs) and from the Dzyaloshinskii–Moriya interaction (DMI). We derive a Berry phase expression for the DMI and show that within this Berry phase theory DMI and SOTs are intimately related, in a way formally analogous to the relation between orbital magnetization (OM) and anomalous Hall effect (AHE). We introduce the concept of the twist torque moment, which probes the internal twist of wavepackets in chiral magnets in a similar way as the orbital moment probes the wavepacket's internal self-rotation. We propose to interpret the Berry phase theory of DMI as a theory of spiralization in analogy to the modern theory of OM. We show that the twist torque moment and the spiralization together give rise to a Berry phase governing the response of the SOT to thermal gradients, in analogy to the intrinsic anomalous Nernst effect. The Berry phase theory of DMI is computationally very efficient because it only needs the electronic structure of the collinear magnetic system as input. As an application of the formalism we compute the DMI in Co/Pt(111), O/Co/Pt(111) and Al/Co/Pt(111) magnetic bi- and trilayers and show that the DMI is highly anisotropic in these systems.

We investigate the electrical and magneto-transport properties of Pt–C granular metals prepared by focused electron beam induced deposition. In particular, we consider samples close to the metal–insulator transition obtained from as-grown deposits by means of a low-energy electron irradiation treatment. The temperature dependence of the conductivity shows a σ ∼lnT behavior, with a transition to $\sigma \sim \sqrt {T}$ at low temperature, as expected for systems in the strong coupling tunneling regime. The magnetoresistance is positive and is described within the wavefunction shrinkage model, normally used for disordered systems in the weak coupling regime. In order to fit the experimental data, spin-dependent tunneling has to be taken into account. In the discussion we attribute the origin of the spin-dependency to the confinement effects of Pt nano-grains embedded in the carbon matrix.

The modification of the electronic structure of bilayer MoS₂ by an external electric field can have potential applications in optoelectronics and valleytronics. Nevertheless, the underlying physical mechanism is not clearly understood, especially the effects of the van der Waals interaction. In this study, the spin orbit-coupled electronic structure of bilayer MoS₂ has been investigated using the first-principle density functional theory. We find that the van der Waals interaction as well as the interlayer distance has significant effects on the band structure. When the interlayer distance of bilayer MoS₂ increases from 0.614 nm to 0.71 nm, the indirect gap between the Γ and Λ points increases from 1.25 eV to 1.70 eV. Meanwhile, the energy gap of bilayer MoS₂ transforms from an indirect one to a direct one. An external electric field can shift down (up) the energy bands of the bottom (top) MoS₂ layer and also breaks the inversion symmetry of bilayer MoS₂. As a result, the electric field can affect the band gaps, the spin-orbit interaction and splits the valance bands into two groups. The present study can help us understand more about the electronic structures of MoS₂ materials for potential applications in electronics and optoelectronics.

The ²²⁹thorium isotope presents an extremely low-energy isomer state of the nucleus which is expected around 7.8 eV, in the vacuum ultraviolet (VUV) regime. This unique system may bridge between atomic and nuclear physics, enabling coherent manipulation and precision spectroscopy of nuclear quantum states using laser light. It has been proposed to implant ²²⁹thorium into VUV transparent crystal matrices to facilitate laser spectroscopy and possibly realize a solid-state nuclear clock. In this work, we validate the feasibility of this approach by computer modelling of thorium doping into calcium fluoride single crystals. Using atomistic modelling and full electronic structure calculations, we find a persistent large band gap and no additional electronic levels emerging in the middle of the gap due to the presence of the dopant, which should allow direct optical interrogation of the nuclear transition.

Based on the electronic structure, we estimate the thorium nuclear quantum levels within the solid-state environment. Precision laser spectroscopy of these levels will allow the study of a broad range of crystal field effects, transferring Mössbauer spectroscopy into the optical regime.

A many-body potential model for the description of actinide oxide systems, which is robust at high temperatures, is reported for the first time. The embedded atom method is used to describe many-body interactions ensuring good reproduction of a range of thermophysical properties (lattice parameter, bulk modulus, enthalpy and specific heat) between 300 and 3000 K for AmO₂, CeO₂, CmO₂, NpO₂, ThO₂, PuO₂ and UO₂. Additionally, the model predicts a melting point for UO₂ between 3000 and 3100 K, in close agreement with experiment. Oxygen–oxygen interactions are fixed across the actinide oxide series because it facilitates the modelling of oxide solid solutions. The new potential is also used to predict the energies of Schottky and Frenkel pair disorder processes.

By means of first-principles calculations, various properties of SrRuO₃ are investigated, focusing on its lattice dynamical properties. Despite having a Goldschmidt tolerance factor very close to 1, the phonon dispersion curves of the high-temperature cubic phase of SrRuO₃ show strong antiferrodistortive instabilities. The energetics of metastable phases with different tilt patterns are discussed, concluding that the coupling of oxygen rotation modes with anti-polar Sr motion plays a key role in stabilizing the Pnma phase with respect to alternative rotation patterns. Our systematic analysis confirms previous expectations and contributes to rationalizing better why many ABO₃ perovskites, including metallic compounds, exhibit an orthorhombic ground state. The zone-center phonon modes of the Pnma phase have been computed, from which we propose partial reassignment of available experimental data. The full dispersion curves have also been obtained, constituting benchmark results for the interpretation of future measurements and providing access to thermodynamical properties.

We present a genetic algorithm (GA) for structural search that combines the speed of structure exploration by classical potentials with the accuracy of density functional theory (DFT) calculations in an adaptive and iterative way. This strategy increases the efficiency of the DFT-based GA by several orders of magnitude. This gain allows a considerable increase in the size and complexity of systems that can be studied by first principles. The performance of the method is illustrated by successful structure identifications of complex binary and ternary intermetallic compounds with 36 and 54 atoms per cell, respectively. The discovery of a multi-TPa Mg-silicate phase with unit cell containing up to 56 atoms is also reported. Such a phase is likely to be an essential component of terrestrial exoplanetary mantles.

High-sensitivity ²⁷Al nuclear magnetic resonance (NMR) measurements of aluminum metal under hydrostatic pressure of up to 10.1 GPa reveal an unexpected negative curvature in the pressure dependence of the electronic density of states measured through shift and relaxation, which violates free electron behavior. A careful analysis of the Fermiology of aluminum shows that pressure induces an electronic topological transition (Lifshitz transition) that is responsible for the measured change in the density of states. The experiments also reveal a sudden increase in the NMR linewidth above 4.2 GPa from quadrupole interaction, which is not in agreement with the metal's cubic symmetry.

Van der Waals interactions play an important role in layered materials such as MoS₂ and MoO₃. Within density functional theory, several methods have been developed to explicitly include van der Waals interactions. We compare the performance of several of these functionals in describing the structural and electronic properties of MoS₂ and MoO₃. We include functionals based on the local density or generalized gradient approximations, but also based on hybrid functionals. The coupling of the semiempirical Grimme D2 method with the hybrid functional HSE06 is shown to lead to a very good description of both structural and electronic properties.

Motivated by an experimental report of iridate superlattices, we performed first-principle electronic structure calculations for SrIrO₃/SrTiO₃. Heterostructuring causes SrIrO₃ to become Sr₂IrO₄-like, and the system has the well-defined j_eff = 1/2 states near the Fermi level as well as a canted antiferromagnetic order within the quasi-two-dimensional IrO₂ plane. In response to a larger tensile strain, the band gap is increased due to the resulting increase in bond length and the bandwidth reduction. The ground state magnetic properties are discussed in comparison to the metastable collinear antiferromagnetic state. Our work sheds new light on understanding the recent experimental results on the iridate heterostructures.

A general rule of negative effective U(U_eff) system caused by (i) exchange correlation and (ii) charge excitation mechanisms is proposed. Based on the general rule, we perform ab initio electronic structure calculations by generalized gradient approximation (GGA) + U method for hole-doped chalcopyrite CuFeS₂ [Cu⁺(d¹⁰)Fe³⁺(d⁵)S²⁻(s²p⁶)₂]. It is found from our calculations that the hole-doped CuFeS₂ has the negative U_eff = −0.44 eV, where U_eff ≡ E(N + 1) + E(N − 1) − 2E(N) < 0 and E(N) is the total energy of the hole-doped CuFeS₂. The negative U_eff is caused by the charge-excitation in the hole-doped Cu²⁺(d⁹) and S⁻(s²p⁵), and also caused by the exchange-correlation in the hole-doped Fe⁴⁺(d⁴). The strong attractive electron–electron interaction (U_eff = −0.44 eV ∼ −5000 K) originates from the purely electronic mechanism. The closed shell of the d¹⁰ electronic configuration is more stable than the d⁹ electronic configuration, since the first excited state with the d⁹s¹ electronic configuration and the ground state with the d¹⁰ electronic configuration are very close, then these two states repel very strongly through the second order perturbation. Therefore, the spin-polarized total energy curve for the hole-doped CuFeS₂ shows the strong upward convexity with N − 1, N and N + 1 electronic configurations leading to the negative U_eff. The hole-doped paramagnetic and metallic CuFeS₂ with the negative U_eff may cause a possible high-T_c superconductor (T_c ∼ 1000 K, if 2Δ/k_BT_c ≈ 10 by assuming a strong coupling regime) because of the strong attractive electron–electron interactions (superconducting gap Δ ≈ |U_eff| ∼ 5000 K). Finally, we propose a new computational materials design methodology to design ultra high-T_c superconductors by using three steps starting from the atomic number only.

In this paper, we study the two-dimensional magnetic topological insulators from the correlated Chern insulator and the correlated Z₂ topological insulator at finite temperature. For the 2D correlated Chern insulator, we find that the thermal-fluctuation-induced magnetic topological insulator (MTI) appears in the intermediate interaction region of the correlated Chern insulator. On the contrary, for the correlated Z₂ topological insulator, thermal-fluctuation-induced MTI does not exist. Finally, we offer an explanation on the difference between the two cases.

Electrical transport measurements have been made on a series of trivalent-ion-doped tungsten bronzes M_xWO₃, with M = Y (0.05 ≤ x ≤ 0.12) or La (0.05 ≤ x ≤ 0.19), over the temperature range 2–300 K. The results are consistent with a metal–insulator transition (MIT) at a critical concentration x_C ≃ 0.06, which corresponds to an electron concentration n_C ≃ 3.3 × 10²¹ cm⁻³. The appearance of small concentrations of non-cubic phases for x ∼ x_C does not have a significant impact on the evolution of the electronic properties of the trivalent bronzes in the low x range. Analysis of the transport results, and a comparison of the findings with those obtained by other workers for the sodium tungsten bronzes, suggest that electron–electron interaction effects play a significant role in inducing the MIT in this type of disordered system.

We report inelastic neutron scattering experiments on a single crystal of the intermediate valence compound CePd₃. At 300 K the magnetic scattering is quasielastic, with half-width Γ = 23 meV, and is independent of momentum transfer Q. At low temperature, the Q-averaged magnetic spectrum is inelastic, exhibiting a broad peak centered near E_max = 55 meV. These results, together with the temperature dependence of the susceptibility, 4f occupation number, and specific heat, can be fit by the Kondo/Anderson impurity model. The low temperature scattering near E_max, however, shows significant variations with Q, reflecting the coherence of the 4f lattice. The intensity is maximal at (1/2, 1/2, 0), intermediate at (1/2, 0, 0) and (0, 0, 0), and weak at (1/2, 1/2, 1/2). We discuss this Q-dependence in terms of current ideas about coherence in heavy fermion systems.

We have synthesized polycrystalline samples of superconducting LaC₂ and investigated them by x-ray and neutron powder diffraction, magnetic susceptibility and heat capacity measurements. Depending on the preparation conditions we find superconductivity below ∼1.8 K. A comparison of the superconducting anomaly in the heat capacity with theoretical predictions indicates LaC₂ to be a weak-coupling BCS-type superconductor. Evidence for a structural phase transition has not been found from the neutron powder diffraction experiments carried out down to 4 K. A negative thermal expansion of the c lattice parameter was observed below ∼50 K. The electronic structure of LaC₂ has been calculated ab initio and it is compared with that of YC₂. The carbon–carbon distance of LaC₂ has been determined from the neutron powder diffraction experiments and it is compared and discussed with respect to those observed in other superconducting binary and ternary La and Y carbides and carbide halides.

We report superconductivity at T_c ≈ 2.6 K in a new layered bismuth oxyselenide LaO_0.5F_0.5BiSe₂ with the ZrCuSiAs-type structure composed of alternating superconducting BiSe₂ and blocking LaO layers. The superconducting properties of LaO_0.5F_0.5BiSe₂ were investigated by means of dc magnetization, resistivity and muon-spin rotation experiments, revealing the appearance of bulk superconductivity with a rather large superconducting volume fraction of ≈ 70% at 1.8 K.

Eu(Fe_0.79Ru_0.21)₂As₂ is suggested to be a nodeless superconductor based on the empirical correlation between pnictogen height (h_Pn) and superconducting gap behavior, in contrast to BaFe₂(As_0.7P_0.3)₂ and Ba(Fe_0.65Ru_0.35)₂As₂. We studied the low-lying electronic structure of Eu(Fe_0.79Ru_0.21)₂As₂ with angle-resolved photoemission spectroscopy (ARPES). By photon energy dependence and polarization dependence measurements, we resolved the band structure in the three-dimensional momentum space and determined the orbital character of each band. In particular, we found that the ${{\text{d}}_{{{z}^{2}}}}$ -originated ζ band does not contribute spectral weight to the Fermi surface around Z, unlike BaFe₂(As_0.7P_0.3)₂ and Ba(Fe_0.65Ru_0.35)₂As₂. Since BaFe₂(As_0.7P_0.3)₂ and Ba(Fe_0.65Ru_0.35)₂As₂ are nodal superconductors and their h_Pn's are less than 1.33 Å, while the h_Pn of Eu(Fe_0.79Ru_0.21)₂As₂ is larger than 1.33 Å, our results provide more evidence for a direct relationship between nodes, ${{\text{d}}_{{{z}^{2}}}}$ orbital character and h_Pn. Our results help to provide an understanding of the nodal superconductivity in iron-based superconductors.

We investigate the Majorana fermions in a T-shaped semiconductor nanostructure with the Rashba spin–orbit coupling and a magnetic field in the proximity of an s-wave superconductor. It is found that the properties of the low-energy modes (including the Majorana and near-zero-energy modes) at the ends of this system are similar to those in the Majorana nanowire. However, very distinct from the nanowire, one Majorana mode emerges at the intersection of the T-shaped structure when the number of the low-energy modes at each end N is odd, whereas neither Majorana nor near-zero-energy mode appears at the intersection for even N. We also discover that the intersection Majorana mode plays an important role in the transport through the above T-shaped nanostructure with each end connected with a normal lead. Due to the presence of the intersection mode, the deviation of the zero-bias conductance from the ideal value in the long-arm limit Ne²/h is more pronounced in the regime of odd N compared to the one of even N. Furthermore, when the magnetic field increases from the regime of odd N to the one of even N + 1, the deviation from the ideal value tends to decrease. This behavior is also very distinct from that in a nanowire, where the deviation always tends to increase with the increase of magnetic field.

We have studied the effect of spin–orbital coupling (SOC) on the electronic transport properties of the thermoelectric material $\beta$ -K₂Bi₈Se₁₃ via magnetoresistance measurements. We found that the strong SOC in this material results in the weak antilocalization (WAL) effect, which can be described well by a three-dimensional weak localization model. The phase coherence length extracted from theoretical fitting exhibits a power-law temperature dependence, with an exponent around 2.1, indicating that the electron dephasing is governed by electron–transverse phonon interactions. As in topological insulators, the WAL effect in $\beta$ -K₂Bi₈Se₁₃ can be quenched by magnetic impurities (Mn) but is robust against non-magnetic impurities (Te). Although our magnetotransport studies provide no evidence for topological surface states, our analyses suggest that SOC plays an important role in determining the thermoelectric properties of $\beta$ -K₂Bi₈Se₁₃.

Quantum corrections to the conductivity due to the weak antilocalization (WAL) and electron–electron interaction (EEI) effects are investigated in Sb–Te layers to evaluate the number of independent conduction channels in the topological insulator system. We separate the two contributions in the logarithmic temperature dependence of conductivity relying on their distinct response to a magnetic field. For the WAL effect, the amplitude parameter α being − 1 observed in magnetoconductivity is confirmed. The magnitude of the EEI contribution is too large to be produced by one transport channel. The mixing between the surface and bulk states is thus indicated to be weak in the Sb–Te system. In addition, the disorder scattering appears to be less influential for the EEI effect than for the WAL effect.

Resonant ultrasound spectroscopy has been used to measure the elastic and anelastic behaviour through known structural and magnetic phase transitions in single crystal hexagonal YMnO₃. Anomalous elastic behaviour is observed at the high temperature structural transition at ∼1260 K, with a discontinuity in the elastic constants and nonlinear recovery below T_c, consistent with $\lambda e{Q}_{\mathrm{s}}^{2}$ coupling. There is no change in dissipation associated with this high temperature transition, and no evidence in the elastic or anelastic behaviour for any secondary transition at ∼920 K, thus supporting the thesis of a single high temperature transformation. Elastic stiffening is observed on cooling through T_N, in accordance with previous studies, and the excess elastic constant appears to scale with the square of the magnetic order parameter. The strains incurred at T_N are a factor of ∼20 smaller than those at the structural transition, implying very weak $\lambda e{Q}_{\mathrm{m}}^{2}$ coupling and a dominant contribution to the variation in the elastic constants from $\lambda {e}^{2}{Q}_{\mathrm{m}}^{2}$. The increased acoustic dissipation above T_N is consistent with an order–disorder process.

High-resolution x-ray diffraction (XRD), Raman spectroscopy and total scattering XRD coupled to atomic pair distribution function (PDF) analysis studies of the atomic-scale structure of archetypal BaZr_xTi_1−xO₃ (x = 0.10, 0.20, 0.40) ceramics are presented over a wide temperature range (100–450 K). For x = 0.1 and 0.2 the results reveal, well above the Curie temperature, the presence of Ti-rich polar clusters which are precursors of a long-range ferroelectric order observed below T_C. Polar nanoregions (PNRs) and relaxor behaviour are observed over the whole temperature range for x = 0.4. Irrespective of ceramic composition, the polar clusters are due to locally correlated off-centre displacement of Zr/Ti cations compatible with local rhombohedral symmetry. Formation of Zr-rich clusters is indicated by Raman spectroscopy for all compositions. Considering the isovalent substitution of Ti with Zr in BaZr_xTi_1−xO₃, the mechanism of formation and growth of the PNRs is not due to charge ordering and random fields, but rather to a reduction of the local strain promoted by the large difference in ion size between Zr⁴⁺ and Ti⁴⁺. As a result, non-polar or weakly polar Zr-rich clusters and polar Ti-rich clusters are randomly distributed in a paraelectric lattice and the long-range ferroelectric order is disrupted with increasing Zr concentration.

Sodium niobate (NaNbO₃, or NNO) is known to be antiferroelectric at temperatures between 45 and 753 K. Here we show experimentally the presence of the ferroelectric phase at temperatures between 100 and 830 K in the NNO crystals obtained by top-seeded solution growth. The ferroelectric phase and new phase transitions are evidenced using a combination of thermo-optical studies by variable angle spectroscopic ellipsometry, Raman spectroscopy analysis, and photoelectron emission microscopy. The possibility for strain-induced ferroelectricity in NNO is suggested.

The exponential growth of maximum energy product that prevailed in the 20th century has stalled, leaving a market dominated by two permanent magnet materials, Nd₂Fe₁₄B and Ba(Sr)Fe₁₂O₁₉, for which the maximum theoretical energy products differ by an order of magnitude (515 kJ m⁻³ and 45 kJ m⁻³, respectively). Rather than seeking to improve on optimized Nd–Fe–B, it is suggested that some research efforts should be devoted to developing appropriately priced alternatives with energy products in the range 100–300 kJ m⁻³. The prospects for Mn-based hard magnetic materials are discussed, based on known Mn-based compounds with the tetragonal L1₀ or D0₂₂ structure or the hexagonal B8₁ structure.

The structural, magnetic and electron-transport properties of cubic Mn₃Ga have been investigated. The alloys prepared by arc melting and melt-spinning show an antiferromagnetic spin order at room temperature but undergo coupled structural and magnetic phase transitions at 600 and 800 K. First-principles calculations show that the observed magnetic properties are consistent with that of a cubic Mn₃Ga crystallizing in the disordered Cu₃Au-type structure. The samples exhibit metallic electron transport with a resistance minimum near 30 K, followed by a logarithmic upturn below the minimum. The observed anomaly in the low-temperature resistivity has been discussed as a consequence of electron scattering at the low-lying excitations of the structurally disordered Mn₃Ga lattice.

We report on our relativistic density-functional investigations of the properties of transition-metal dimers adsorbed on a graphene monolayer supported by a Cu(1 1 1) substrate, which extends our studies of dimers in the gas-phase and adsorbed on a freestanding graphene layer (Błoński and Hafner 2014 J. Phys.: Condens. Matter 26 146002). The presence of the Cu(1 1 1) substrate enhances the interaction between the dimer and the support. For homoatomic dimers such as Ir₂ and Pt₂ a flat adsorption geometry is now preferred over an upright geometry, which is stable on a graphene monolayer. The magnetic moment of the dimer is strongly reduced, the magnetic anisotropy is very low—in contrast to the strong anisotropy of free and graphene-supported Ir₂ and Pt₂ dimers. For heteroatomic IrCo and PtCo dimers the upright geometry with the Co atom located in a sixfold hollow of the graphene layer is preserved, but the stronger interaction with the support leads to a further enhancement of the large magnetic anisotropy energy of IrCo to 0.2 eV/dimer, while that of PtCo is reduced. The mechanism determining the magnetic anisotropy is discussed in relation to the electronic structure of the dimers.

In this work we investigate interfacial effects in bilayer systems integrated by La_2/3Sr_1/3MnO₃ (LSMO) thin films and different capping layers by means of surface-sensitive synchrotron radiation techniques and transport measurements. Our data reveal a complex scenario with a capping-dependent variation of the Mn oxidation state by the interface. However, irrespective of the capping material, an antiferromagnetic/insulating phase is also detected at the interface, which is likely to originate from a preferential occupancy of Mn 3d 3z²−r² e_g orbitals. This phase, which extends approximately to two unit cells, is also observed in uncapped LSMO reference samples, thus pointing to an intrinsic interfacial phase separation phenomenon, probably promoted by the structural disruption and inversion symmetry breaking at the LSMO free surface/interface. These experimental observations strongly suggest that the structural disruption, with its intrinsic inversion symmetry breaking at the LSMO interfaces, plays a major role in the observed depressed magnetotransport properties in manganite-based magnetic tunneling junctions and explains the origin of the so-called dead layer.