Table of contents

A detailed study of energy differences between the highest occupied and lowest unoccupied molecular orbitals (HOMO–LUMO gaps) in protein systems and water clusters is presented. Recent work questioning the applicability of Kohn–Sham density-functional theory to proteins and large water clusters (Rudberg 2012 J. Phys.: Condens. Matter 24 072202) has demonstrated vanishing HOMO–LUMO gaps for these systems, which is generally attributed to the treatment of exchange in the functional used. The present work shows that the vanishing gap is, in fact, an electrostatic artefact of the method used to prepare the system. Practical solutions for ensuring the gap is maintained when the system size is increased are demonstrated. This work has important implications for the use of large-scale density-functional theory in biomolecular systems, particularly in the simulation of photoemission, optical absorption and electronic transport, all of which depend critically on differences between energies of molecular orbitals.

The phase diagram of carbon is mapped to high pressure using a computationally-tractable potential model. The use of a relatively simple (Tersoff-II) potential model allows a large range of phase space to be explored. The coexistence (melting) curve for the diamond crystal/liquid dyad is mapped directly by modelling the solid/liquid interfaces. The melting curve is found to be re-entrant and belongs to a conformal class of diamond/liquid coexistence curves. On supercooling the liquid a phase transition to a tetrahedral amorphous form (ta-C) is observed. The liquid ⟷ amorphous coexistence curve is mapped onto the pT plane and is found to also be re-entrant. The entropy changes for both melting and the amorphous ⟶ liquid transitions are obtained from the respective coexistence curves and the associated changes in molar volume. The structural change on amorphization is analysed at different points on the coexistence curve including for transitions that are both isochoric and isocoordinate (no change in nearest-neighbour coordination number). The conformal nature of the melting curve is highlighted with respect to the known behaviour of Si. The relationship of the observed liquid/amorphous coexistence curve to the Si low- and high-density amorphous (LDA/HDA) transition is discussed.

We investigate the influence of dimensionality of a sample on the properties of magnetic dipolar soft spheres. Molecular dynamics simulations and diagram expansion are employed to analyze the pressure and microscopic structure of model monodisperse magnetic fluids in a bulk and in a monolayer. We found that, for a broad range of densities and dipolar interaction strengths, strong geometrical confinement weakens the influence of the dipole–dipole interaction on the pressure and, as a result, steric repulsion of dipolar particles provides the main contribution to the thermodynamic properties of ferrofluids in strong confinement.

A reformulation and generalization of the Zwanzig model (ZW model) for ideal homopolymer chains poly-X, where X represents any of the twenty naturally occurring proteinogenic amino acid residues is presented. This reformulation and generalization provides a direct connection between coarse-grained parameters originally proposed in the ZW model with variables from the Lifson–Roig (LR) theory, such as the helical propensity per residue ω, and new variables introduced here, such as the energy gap Δ between unfolded and folded structures, as well as the ratio f of the energy scales involved. This enables us to discover the relevance of the energy spectrum E to the onset of configurational phase transitions. From the configurational partition function , thermodynamic properties such as the configurational entropy S, specific heat C_v and average energy 〈E〉 are calculated in terms of the number of residues K, temperature T, helical propensity ω and energy barrier ΔH for different poly-X chains in vacuo. Results obtained here provide substantial evidence that configurational phase transitions for ideal poly-X chains correspond to first-order phase transitions. An anomalous behavior of the thermodynamic functions 〈E〉, Cv, S with respect to the number K of residues is also highlighted. On-going methods of solution are outlined.

The motion and annihilation of a grain boundary (GB) in graphene are investigated by tight-binding molecular dynamics (TBMD) simulation and ab initio local density approximation total energy calculation. A meandering structure of the GB is found to be energetically more favorable than other structures, in good agreement with experiment. It is observed in the TBMD simulation that evaporation of carbon dimers and sequential Stone–Wales transformations of carbon bonds lead to rapid motion and annihilation of the GB. The dimer erection and evaporation are found to proceed by formation of an adatom due to bond breaking. These results shed interesting light on the fabrication of high-quality graphene.

Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS₂) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown.

We produced graphene-based field-effect transistors by contacting mono- and bi-layer graphene by sputtering Ni or Ti as metal electrodes. We performed electrical characterization of the devices by measuring their transfer and output characteristics. We clearly observed the presence of a double-dip feature in the conductance curve for Ni-contacted transistors, and we explain it in terms of charge transfer and graphene doping under the metal contacts. We also studied the contact resistance between the graphene and the metal electrodes with larger values of ∼30 kΩμm² recorded for Ti contacts. Importantly, we prove that the contact resistance is modulated by the back-gate voltage.

We study spin-resolved noise in Coulomb blockaded double quantum dots coupled to ferromagnetic electrodes. The modulation of the interdot coupling and spin polarization in the electrodes gives rise to an intriguing dynamical spin ↑–↑ (↓–↓) blockade mechanism: bunching of up (down) spins due to dynamical blockade of an up (down) spin. In contrast to the conventional dynamical spin ↑–↓ bunching (bunching of up spins associated with a dynamical blockade of a down spin), this new bunching behavior is found to be intimately associated with the spin mutual-correlation, i.e. the noise fluctuation between opposite spin currents. We further demonstrate that the dynamical spin ↑–↑ and ↑–↓ bunching of tunneling events may be coexistent in the regime of weak interdot coupling and low spin polarization.

The possibilities of an enhanced thermoelectric figure of merit value, ZT, in a nanostructured junction are examined for a wide range of parameter values in a theoretical model. Our research shows that the figure of merit can take a very large maximum, which depends both on the length and the energy gap values. The maximum of ZT is achieved when the Fermi level of the electrodes is aligned to the edge of the electronic transmission function of the junction, where both the conductance and the Seebeck constant are significantly enhanced. On the basis of our results, we conclude that nanowires and molecular junctions form a special class of systems where a large ZT can be expected in some cases.

The pressure dependence of various inter- and intra-layer Raman modes has been studied on pristine matlockite compound, PbFCl, up to ∼41 GPa. The low-frequency interlayer vibrational modes, A_1g(1) and E_g(1), identified as rigid layer modes, exhibit non-monotonic behavior with increasing pressure. They exhibit points of inflexion at ∼24 GPa and ∼31 GPa respectively, indicating the onset of a subtle instability. The emergence of a new Raman mode (∼181 cm⁻¹) at ∼24 GPa and a sudden large increase in the intensity of the A_1g(1) mode signify the occurrence of a symmetry lowering structural transition of the parent tetragonal phase with enhanced interlayer coupling. Two more modes appear at higher pressures (∼33 GPa) at frequencies below the A_1g(1) mode and are ascribed to a monoclinically distorted phase (space group P2₁/m). High pressure x-ray diffraction studies performed up to ∼47 GPa confirm the occurrence of the structural transitions with decreasing crystal symmetry. These observations are consistent with a picture in which the structural distortion involves destabilization of the tetragonal unit cell following a gradual change in the bonding nature from layer-like (2D) to non-layer like (3D) involving the Cl-bilayers along the c direction.

In this work we apply a Lagrangian kernel-based estimator of continuum fields to atomic data to estimate the J-integral for the emission dislocations from a crack tip. Face-centered cubic (fcc) gold and body-centered cubic (bcc) iron modeled with embedded atom method (EAM) potentials are used as example systems. The results of a single crack with a K-loading compare well to an analytical solution from anisotropic linear elastic fracture mechanics. We also discovered that in the post-emission of dislocations from the crack tip there is a loop size-dependent contribution to the J-integral. For a system with a finite width crack loaded in simple tension, the finite size effects for the systems that were feasible to compute prevented precise agreement with theory. However, our results indicate that there is a trend towards convergence.

The electronic structures, densities of states, Fermi surfaces and elastic properties of AB₃ (A  =La, Y; B  =Pb, In, Tl) compounds are studied under pressure using the full-potential linear augmented plane wave (FP-LAPW) method within the local density approximation for the exchange–correlation functional and including spin–orbit coupling. Fermi surface topology changes are found for all the isostructural AB₃ compounds under compression (at V/V₀ = 0.90 for LaPb₃ (pressure = 8 GPa), at V/V₀ = 0.98 for AIn₃ (pressure = 1.5 GPa), at V/V₀ = 0.80 for ATl₃ (pressure in excess of 18 GPa)) apart from YPb₃, although its electronic structure at zero pressure is very similar to that of LaPb₃. For LaPb₃ a softening of the C₄₄ elastic constant under pressure (equivalent to 8 GPa) may be related to the appearance of a new hole pocket around the X point. From the calculated elastic properties and other mechanical properties, all the compounds investigated are found to be ductile in nature with elastic anisotropy. The states at the Fermi level (E_F) are dominated by B p states with significant contributions from the A d states. For the La compounds, small hybridizations of the La f states also occur around E_F.

We propose the concept of 'topological Hamiltonian' for topological insulators and superconductors in interacting systems. The eigenvalues of the topological Hamiltonian are significantly different from the physical energy spectra, but we show that the topological Hamiltonian contains the information of gapless surface states, therefore it is an exact tool for topological invariants.

The multi-order Raman scattering is studied up to fourth order for a detwinned LaMnO₃ crystal. Based on a comprehensive data analysis of the polarization-dependent Raman spectra, we show that the anomalous features in the multi-order scattering could be the sidebands on the low-energy mode at about 25 cm⁻¹. We suggest that this low-energy mode stems from the tunneling transition between the potential energy minima arising near the Jahn–Teller Mn³⁺ ion due to the lattice anharmonicity and that the multi-order scattering is activated by this low-energy electronic motion. The sidebands are dominated by the oxygen contribution to the phonon density-of-states, however, there is an admixture of an additional component, which may arise from coupling between the low-energy electronic motion and the vibrational modes.

The antiferromagnetic correlation plays an important role in high-T_c superconductors. Considering this effect, the magnetic excitations in n-type cuprates near the optimal doping are studied within the spin-density-wave description. The magnetic excitations are commensurate in the low-energy regime and further develop into spin-wave-like dispersion at higher energy, consistent with the inelastic neutron scattering measurements. We clearly demonstrate that the commensurability originates from the band splitting and Fermi surface topology. The commensurability is a normal state property and has nothing to do with d-wave superconductivity. Our results strongly suggest the essential role of antiferromagnetic correlations in the cuprates.

We study the formation and characteristics of 'spin droplets', i.e. compact spin-polarized configurations in the highest occupied Landau level, in an etched quantum Hall device at filling factors 2 ≤ ν ≤ 3. The confining potential for electrons is obtained with self-consistent electrostatic calculations on a GaAs/AlGaAs heterostructure with experimental system parameters. Real-space spin-density-functional calculations for electrons confined in the obtained potential show the appearance of stable spin droplets at ν ∼ 5/2. The qualitative features of the spin droplet are similar to those in idealized (parabolic) quantum-dot systems. The universal stability of the state against geometric deformations underlines the applicability of spin droplets in, for example, spin-transport through quantum point contacts.

We report an electric field driven destabilization of the insulating state in nominally pure LaMnO₃ single crystal with a moderate field which leads to a resistive state transition below 300 K. The transition is between the insulating state in LaMnO₃ and a high resistance bad metallic state that has a temperature independent resistivity. The transition occurs at a threshold field (E_th) that shows a steep enhancement on cooling. While at lower temperatures the transition is sharp and involves a large change in resistance, it softens on heating and is eventually absent above 280 K. When the Mn⁴⁺ content is increased by Sr substitution up to x = 0.1, the observed transition, although observable in a certain temperature range, softens considerably. This observation has been explained as a bias driven percolation type transition between two co-existing phases, where the majority phase is a charge and orbitally ordered polaronic insulating phase and the minority phase is a bad metallic phase. The mobile fraction f of the bad metallic phase deduced from the experimental data follows an activated kinetics as f = f_o(E)exp(−Δ/k_BT) with the activation energy Δ ≈ 200 meV, and the pre-factor f_o(E) is a strong function of the field that leads to a rapid enhancement of f on application of field, leading to the resistive state transition. We suggest likely scenarios for such co-existing phases in nominally pure LaMnO₃ that can lead to the bias driven percolation type transition.

Low-temperature dc-magnetization, ac electrical resistivity and specific heat measurements were performed on single crystals of the intermetallic compound β-IrSn₄. The compound crystallizes in the tetragonal MoSn₄-type structure (space group I4₁/acd) and exhibits superconductivity below T_c = 0.9 ± 0.05 K. Further, the magnitude of the ratios ΔC_p/(γ_nk_BT_c) = 1.29, 2Δ/(k_BT_c) = 3.55 and of the electron–phonon coupling ${\overline{\lambda }}_{e-p h}=0.5$ imply that superconductivity in β-IrSn₄ can be ascribed to a s-wave weak coupling regime. We determined crucial thermodynamic characteristics of the superconducting state. It turned out that depending on the assumption of either a spherical or non-spherical Fermi surface, the superconductivity can be ascribed to either a type-I and type-II/1 or type-II in clean limit, respectively. However, the behavior of the upper critical field and the anisotropic crystalline structure of the studied compound provide strong support to the type-II superconductivity. In the normal state the resistivity exhibits a prominent quadratic temperature dependence, which together with a large Kadowaki–Woods ratio and with the enhanced effective mass indicate that the electrons in β-IrSn₄ are strongly correlated.

We analytically calculate the intrinsic spin-Hall conductivities (ISHCs) (${\sigma }_{x y}^{z}$ and ${\sigma }_{y x}^{z}$) in a clean, two-dimensional system with generic k-linear spin–orbit interaction. The coefficients of the product of the momentum and spin components form a spin–orbit matrix $\tilde {\beta }$. We find that the determinant of the spin–orbit matrix $\mathrm{det}\tilde {\beta }$ describes the effective coupling of the spin s_z and orbital motion L_z. The decoupling of spin and orbital motion results in a sign change of the ISHC and the band-overlapping phenomenon. Furthermore, we show that the ISHC is in general unsymmetrical (${\sigma }_{x y}^{z}\not = -{\sigma }_{y x}^{z}$), and it is governed by the asymmetric response function $\Delta \tilde {\beta }$, which is the difference in band-splitting along two directions: those of the applied electric field and the spin-Hall current. The obtained non-vanishing asymmetric response function also implies that the ISHC can be larger than e/8π, but has an upper bound value of e/4π. We will show that the unsymmetrical properties of the ISHC can also be deduced from the manifestation of the Berry curvature in the nearly degenerate area. On the other hand, by investigating the equilibrium spin current, we find that $\mathrm{det}\tilde {\beta }$ determines the field strength of the SU(2) non-Abelian gauge field.

We study electron transfer across a two-terminal quantum ring using a time-dependent description of the scattering process. For the considered scattering event the quantum ring is initially charged with one or two electrons, with another electron incident to the ring from the input channel. We study the electron transfer probability (T) as a function of the external magnetic field. We determine the periodicity of T for a varied number of electrons confined within the ring. For that purpose we develop a method to describe the wave packet dynamics for a few electrons participating in the scattering process, taking into full account the electron–electron correlations. We find that electron transfer across the quantum ring initially charged by a single electron acquires a distinct periodicity of half of the magnetic flux quantum (Φ₀/2), corresponding to the formation of a transient two-electron state inside the ring. In the case of a three-electron scattering problem with two electrons initially occupying the ring, a period of Φ₀/3 for T is formed in the limit of thin channels. The effect of disorder present in the confinement potential of the ring is also discussed.

Cubic–tetragonal phase transitions of [100]-oriented BaTiO₃ crystals have been studied by means of an acoustic emission (AE) method, under an external electric field. The Curie temperature (T_c) reveals a nontrivial behavior at the small field, namely, it initially decreases with enhancement of the field, attains a minimum at the threshold field E_th = 0.256 kV cm⁻¹, accompanied by the AE count rate $\dot {N}\approx 9~{\mathrm{s}}^{-1}$, and then starts increasing with a rate of 1.5 K m kV⁻¹ as the field is enhanced. Such nontrivial behavior of T_c is explained as due to the presence of polar nanoregions, recently revealed in BaTiO₃ crystals (Ko et al 2011 Phys. Rev. B 84 094123).

The effect of temperature on the pressure-induced structural changes in perovskite-type (ABO₃) relaxor ferroelectrics is studied by in situ high-temperature high-pressure Raman spectroscopy on single crystals of PbSc_1/2Ta_1/2O₃ (PST) and PbSc_1/2Nb_1/2O₃ (PSN), which allowed us to elucidate the interplay between the polar and antiferrodistortive order coexisting on the mesoscopic scale at ambient conditions. High-pressure experiments were carried out at elevated temperatures below and above the characteristic intermediate temperature T*. The results were compared with those obtained at room temperature, which for PST is just above the paraelectric–ferroelectric phase transition T_C, whereas for PSN is below T_C. It is shown that the first critical pressure p_c1, at which a transition from a relaxor to a non-polar rhombohedral state with antiphase octahedral tilt ordering occurs, decreases at elevated temperatures due to the weakening of the polar coupling, which in turn facilitates the evolution of the preexisting medium-range antiferrodistortive order into a long-range order. The critical pressure p_c2 of the second phase transition, involving a change in the type of the antiferrodistortive order, is not affected by temperature, i.e. it is independent of the state of polar coupling and is mainly related to the initial correlation length of antiferrodistortive order. The strong influence of temperature on p_c1, which occurs only when the mesoscopic polar order is suppressed, emphasizes the importance of coexisting ferroelectric and antiferrodistortive coupling for the occurrence of the relaxor states.

We have developed a theory that describes the spin-wave spectra of ferromagnetic films with Dzyaloshinskii–Moriya interactions. In agreement with recent experiments (Zakeri et al 2010 Phys. Rev. Lett.104 137203), we demonstrate that the spin-wave dispersion relation is asymmetric with respect to wave vector inversion for a variety of ferromagnetic films with Dzyaloshinskii–Moriya interactions and different crystallographic classes. It is also predicted that, for non-zero wave vectors, the resonance frequency and resonance field can increase or decrease depending on the spin-wave vector orientation. We provide explicit formulas for the spin-wave dispersion relation and its asymmetry, as well as for the dynamic susceptibility for a film under microwave excitation, that can be used to understand ferromagnetic resonance as well as Brillouin light scattering experiments in these classes of magnetic thin films.

We have investigated the low energy excitations in metallic Ho by high resolution neutron spectroscopy. We found at T = 3 K clear inelastic peaks in the energy loss and energy gain sides, along with the central elastic peak. The energy of this low energy excitation, which is 26.59 ± 0.02 μeV at T = 3 K, decreased continuously and became zero at T_N ≈ 130 K. By fitting the data in the temperature range 100–127.5 K with a power law we obtained the power-law exponent β = 0.37 ± 0.02, which agrees with the expected value β = 0.367 for a three-dimensional Heisenberg model. Thus the energy of the low energy excitations can be associated with the order parameter.

Using the first-principles exact muffin-tin orbitals method in combination with the coherent potential approximation, we investigated the magnetic properties, exchange interactions, and temperature-dependent half-metallicity of the Co₂Mn(Ga_1−xZ_x) (Z=Si, Ge, Sn) alloys. The total magnetic moment follows perfectly a previously proposed Slater–Pauling relation, i.e., μ₀ = N_t − 24, with N_t being the number of valence electrons. The Co–Mn and Co1–Co2 (inter-sublattice) interactions are dominated by direct exchange, whereas the Co1–Co1 (intra-sublattice) interaction is characterized by superexchange. The Mn–Mn exchange interaction in Co₂MnGa is of long-ranged RKKY-type. However, the Mn–Mn exchange interactions in Co₂MnZ are relatively localized and can be attributed to superexchange. The Co–Mn, Co1–Co2 and Co1–Co1 total exchange interactions increase with x, whereas the Mn–Mn total exchange interactions show convex behavior. The calculated Curie temperature (T_C) increases with x. The ability of Z to enhance T_C follows the sequence of Si > Ge > Sn, in agreement with the experimental findings. The temperature dependence of the spin polarization at the Fermi level [P(T)] is investigated based on the disordered local moment model. P(T) drops abruptly at temperatures much lower than T_C. At temperatures higher than 200 K, the composition with higher T_C generally corresponds to larger P(T).

The evolution of magnetic order in Fe_1+ySe_xTe_1−x crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent density functional theory calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase becomes progressively disordered with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment 'freezes' at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.

Li co-doped ZnO:Co (Zn_0.96−yCo_0.04Li_yO , y ≤ 0.1) nanoparticles were synthesized by the sol–gel technique and the correlation between the structural, electronic and magnetic properties was investigated. All the samples show a single phase hexagonal (wurtzite) ZnO structure and no secondary phases were detected. Variational trends in lattice parameters suggest the incorporation of Li in the ZnO:Co system in both substitutional and interstitial sites. Detailed electronic studies have been performed by high-resolution x-ray photoelectron spectroscopy (XPS) to determine the states of Zn, O, Co and Li. It was determined that Co substitutes at Zn sites (Co_Zn) while the O vacancy and Zn defects did not show much variation with increasing Li concentration. Deconvolution of the Li XPS peak showed a clear non-monotonic trend in the variation of the substitutional Li (Li_Zn) and interstitial Li (Li_i) defects with increasing Li concentration in the particles. The magnetization study of the samples showed that the variation of the moment closely followed the trend of variation of the Li_Zn defects. The data are interpreted in terms of substitutional Li acting as a hole dopant and optimizing the conditions for ferromagnetism in Co-doped ZnO. Interstitial Li is not seen to be playing this role.