Table of contents

The spin lattice appropriate for azurite Cu₃(CO₃)₂(OH)₂ was determined by evaluating its spin exchange interactions on the basis of first principles density functional calculations. It is found that azurite is not well described as an isolated diamond chain with no spin frustration, but is better modeled as a two-dimensional spin lattice in which diamond chains with spin frustration interact through the interchain spin exchange in the ab-plane. Our analysis indicates that the magnetic properties of azurite at low temperatures can be approximated on the basis of two independent contributions, i.e., isolated dimer and effective uniform chain contributions. This prediction was verified by analyzing the magnetic susceptibility and specific heat data for azurite.

We report on the first nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) investigation of inorganic fullerene-like MoS₂ nanoparticles. Spectra of bulk 2H-MoS₂ samples have also been measured for comparison. The similarity between the measured quadrupole coupling constants and chemical shielding anisotropy parameters for bulk and fullerene-like MoS₂ reflects the nearly identical local crystalline environments of the Mo atoms in these two materials. EPR measurements show that fullerene-like MoS₂ exhibits a larger density of dangling bonds carrying unpaired electrons, indicative of them having a more defective structure than the bulk sample. The latter observation explains the increase in the spin–lattice relaxation rate observed in the NMR measurements for this sample in comparison with the bulk 2H- MoS₂ ones.

We present a theoretical approach, based on the effective mass approximation model, on the quantum-confinement Stark effects for spherical semiconducting quantum dots in the regime of strong confinement of interactive electron–hole pairs and limiting weak electric field. The respective roles of Coulomb potential and polarization energy are investigated in detail. Under reasonable physical assumptions, analytical calculations can be performed. They show that the Stark shift is a quadratic function of the electric field amplitude in this regime. The computed numerical values obtained from this approach are found to be in good agreement with experimental data over a significant domain of quantum dot sizes.

In this study, we discuss the behavior of the Fano factor in a double quantum dot (DQD) connected with Luttinger liquid (LL) electrodes. At the Toulouse point, we study the dependence of the Fano factor on the bias voltage, the energy level of the dots, the interdot coupling, and the asymmetry parameter. We show that the behavior of the Fano factor in a DQD is similar to that in a single quantum dot (SQD); however, it behaves nonmonotonically with bias voltage and three local extrema can occur. The condition for the occurrence of nonmonotonic behavior is determined, and it is shown that local extrema result from the mixing of the bare energy levels of the dots caused by the interdot coupling. The influence of the Klein factor on the conductance in a DQD and the limitation of the perturbation calculation for a DQD are discussed.

We simulate with a transfer-matrix methodology the rectification properties of geometrically asymmetric metal–vacuum–metal junctions in which one of the metals is flat while the other is extended by a tip. We consider both tungsten and silver as the material for the tip and we study the influence of the dielectric function of these materials on the rectification properties of the junction. We determine in particular the power that these junctions could provide to an external load when subject to a bias whose typical frequency is in the infrared or optical domain. We also study the rectification ratio of these junctions, which characterizes their ability to rectify the external bias by providing currents with a strong dc component. The results show that these quantities exhibit a significant enhancement for frequencies Ω that correspond to a resonant polarization of the tip. With silver and the geometry considered in this paper, this arises for values of the order of 3.1 eV in the visible range. Our results hence indicate that the frequency at which the device is the most efficient for the rectification of external signals could be controlled by the geometry or the material used for the tip.

The transferable force constant model of van de Walle et al (2002 Rev. Mod. Phys. 74 11) has been combined with the itinerant coherent potential approximation to calculate the complete phonon spectra and elastic constants in the magnetic type-II alloy Pd_xFe_1−x across the concentration range. The calculated dispersion curves and elastic constants agree very well with the experiments. We discuss the results in the light of the behavior of inter-atomic force constants between various pairs of chemical species. The results demonstrate that the combination of the transferable force constant model and the ICPA method for configuration averaging serve as an efficient and reliable first-principles-based tool to compute the phonon spectra for disordered alloys at any arbitrary concentration.

Emission and excitation spectra of Cs₂NaLnCl₆ (Ln = Y, Eu, Gd, Er, Yb), Cs₂NaYCl₆:Ce and Cs₂NaYCl₆:Tm have been recorded using synchrotron radiation. With the possible exception of the case for Ce³⁺, no transitions are observed and the emission spectra are entirely assigned to intraconfigurational transitions of Ln³⁺ in LnCl₆³⁻ and impurity species. The excitation spectra comprise intraconfigurational, charge transfer and band-to-band transitions. Trace impurities of oxy-species or of other lanthanide ions have a profound effect upon the spectra. The 4f–5d absorption spectra have been simulated by employing the suite of programs of Professor M F Reid and the results have been included together with the experimental spectra.

QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

Ab initio calculations of the elastic constants for several cubic ordered structures of zirconium carbonitride (ZrC_xN_1−x) and zirconium–titanium carbide (Zr_xTi_1−xC) alloys were carried out. The calculations of total and formation energies, bulk modulus and elastic constants as functions of composition were performed with an ab initio pseudo-potential method. The predicted equilibrium lattice parameters are slightly higher than those found experimentally (on average by 0.2–0.4%). The predicted formation energies indicate that the ZrC_xN_1−x alloys are stable even at 0 K in the whole concentration range, while the homogeneous Zr_xTi_1−xC alloys can be stabilized only at high temperatures. Spinodal decomposition of the latter alloys into cubic domains takes place over a wide range of compositions and temperatures. For the carbonitrides, the shear modulus G, the Young's modulus E and the Poisson ratio σ reach an extremum for carbon-rich alloys, and this is attributed to a maximum value of the shear modulus C₄₄ that corresponds to a valence-electron concentration in the range of 8.2–8.3. This extremal behavior finds its origin in the response of the band structure of ZrC_xN_1−x alloys for 0≤x≤1, caused by the monoclinic strain that determines this shear modulus. In contrast, the other shear modulus does not exhibit any extremum over the whole composition range. These results are in contrast with those for Zr–Ti carbides for which the elastic properties gradually increase from ZrC to TiC.

Vacuum ultraviolet excitation spectra at ∼10 K have been recorded for transitions of Cs₂NaYF₆:Ln³⁺ (Ln = Nd, Sm, Eu, Tb, Ho, Er, Tm). In these high bandgap hosts the lanthanide ions occupy octahedral symmetry sites. The spectra comprise broad, structured bands and in most cases the individual vibronic structure is not resolved. Simulations of the relative intensities and band positions in the spectra have been made by using parameter values from previous studies and/or by employing values from similar systems or estimating trends across the lanthanide series, without data fitting or parameter adjustments. The agreement with experimental results is reasonable except where the luminescent state being monitored is not efficiently populated nonradiatively from the 4f^N−15d state, or where additional bands are present. The latter are readily assigned to charge transfer transitions or the near-excitonic band. Comparison of the spectra has been made with those of other high symmetry lanthanide ion systems.

To describe the interaction of molecular vibrations with electrons at a quantum dot contacted to metallic leads, we extend an analytical approach that we previously developed for the many-polaron problem. Our scheme is based on an incomplete variational Lang–Firsov transformation, combined with a perturbative calculation of the electron–phonon self-energy in the framework of generalized Matsubara functions. This allows us to describe the system at weak-to-strong coupling and intermediate-to-large phonon frequencies. We present results for the quantum dot spectral function and for the kinetic coefficient that characterizes the electron transport through the dot. With these results we critically examine the strengths and limitations of our approach, and discuss the properties of the molecular quantum dot in the context of polaron physics. We place particular emphasis on the importance of corrections to the concept of an anti-adiabatic dot polaron suggested by the complete Lang–Firsov transformation.

We investigate the time evolution of entanglement under various models of decoherence: a general heuristic model based on local relaxation and dephasing times, and two microscopic models describing decoherence of electron spin qubits in quantum dots due to the hyperfine interaction with the nuclei. For each of the decoherence models, we investigate and compare how long the entanglement can be detected. We also introduce filtered witness operators, which extend the available detection time and investigate this detection time for various multipartite entangled states. By comparing the time required for detection with the time required for generation and manipulation of entanglement, we estimate for a range of different entangled states how many qubits can be entangled in a one-dimensional array of electron spin qubits.

The relation between the dynamical regimes (weak and strong coupling) and entanglement for a dissipative quantum dot microcavity system is studied. In the framework of a phenomenological temperature model an analysis in both temporal (population dynamics) and frequency domain (photoluminescence) is carried out in order to identify the associated dynamical behavior. The Wigner function and concurrence are employed to quantify the entanglement in each regime. We find that sudden death of entanglement is a typical characteristic of the strong coupling regime.

The phonon dynamics of the low-temperature superconductor Sr₂RuO₄ is calculated quantitatively in linear response theory and compared with that of the structurally isomorphic high-temperature superconductor La₂CuO₄. Our calculation corrects for a typical deficiency of local density approximation-based calculations, which always predict too large an electronic k_z-dispersion insufficient for describing the c-axis response of real materials. With a more realistic computation of the electronic band structure, the frequency and wavevector dependent irreducible polarization part of the density response function is determined and used for adiabatic and nonadiabatic phonon calculations. Our analysis for Sr₂RuO₄ reveals important differences from the lattice dynamics of p- and n-doped cuprates. Consistently with experimental evidence from inelastic neutron scattering, the anomalous doping related softening of the strongly coupling high-frequency oxygen bond-stretching modes which is generic for the cuprate superconductors is largely suppressed or completely absent, respectively, depending on the actual value of the on-site Coulomb repulsion of the Ru 4d orbitals. Also the presence of a characteristic Λ₁ mode in La₂CuO₄ with a very steep dispersion coupled strongly to the electrons is not found for Sr₂RuO₄. Moreover, we evaluate the possibility of a phonon–plasmon scenario for Sr₂RuO₄, which has been shown recently to be realistic for La₂CuO₄. In contrast to the case for La₂CuO₄, in Sr₂RuO₄ the plasmons that are very low lying are overdamped along the c-axis.

(As_0.4S_0.6)_100−xAg_x glasses (x = 0, 4, 8, 12 at.%) have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at As and Ag K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. Analysis of the partial pair correlation functions and coordination numbers extracted from the model atomic configurations revealed that silver preferentially bonds to sulfur in the As₂S₃–Ag ternary glasses, which results in the formation of homoatomic As–As bonds. Upon the addition of Ag, a small proportion of Ag–As bonds (N_AgAs≈0.3) are formed in all three ternary compositions, while the direct Ag–Ag bonds (N_AgAg≈ 0.4) appear only in the glass with the highest Ag content (12 at.%). Similar to the g- As₂S₃ binary, the mean coordination number of arsenic is close to three, and that of sulfur is close to two, in the As₂S₃–Ag ternary glasses. The first sharp diffraction peak on the total structure factors of As₂S₃ binary and (As_0.4S_0.6)_100−xAg_x ternary glasses is related to the As–As and As–S correlations.

The influence of substrate effects on the ferroelectric and magnetic properties in multiferroic thin films is studied based on the Heisenberg and transverse Ising model. Green's function technique allows the calculation of static and dynamic properties in the dependence on temperature, film thickness and different substrates. It is demonstrated that the polarization, the magnetization, the critical temperatures and the spin-wave energies are very sensitive to the exchange interaction constants between the surface and the substrate and could be increased or decreased by using different kinds of substrates. The dependence on the film thickness is also discussed. The results are in qualitative accordance with the experimental data.

Here we report a systematic theoretical study of the equilibrium structures, electronic and magnetic properties of FePd_n−1 clusters with n = 1–13, within the framework of density functional theory. The results show that the doping of a single Fe impurity enhances the binding energies as well as the magnetic moment of the Pd_n clusters. Interestingly, in the mid-size region (n = 5–7), Fe substitution in Pd_n clusters results in a three fold enhancement in the magnetic moment. We find that the geometries of the host clusters do not change significantly after the addition of an Fe atom, except for n = 6, 7, 11, 12. In the lowest energy configurations, the Fe atom tries to increase its coordination number by moving from the convex to the interior site as the number of Pd atoms varies from 2 to 12.

We present a comprehensive study of the magnetization switching of a uniaxial nanoparticle driven by a circularly polarized magnetic field rotated in the plane perpendicular to the easy axis. The conditions for the existence of the uniform and non-uniform precessions of the nanoparticle magnetic moment are derived. In addition, the differences between switchings via uniform and non-uniform precession are determined, and the essential role of field polarization is demonstrated. The dependence of the switching time on the field amplitude and frequency are calculated numerically. We show that a permanent magnetic field can reduce the amplitude and frequency of the switching rotating field, and that the combined action of these fields is characterized by an extremely strong dependence of the switching time on the field parameters. We also demonstrate that the transition process caused by an external magnetic field pulse can decrease the switching amplitude in comparison with the value predicted from analysis of the stability criterion. We discuss the advantages of switching the magnetization by means of the action of a rotating field over the magnetization switching using a steady field applied perpendicular to the easy axis.