During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science—promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008

Ab-initio simulations of materials using VASP: Density-functional theory and beyond.

Materials for hydrogen storage

Transparent conductors as solar energy materials: A panoramic review

https://uhra.herts.ac.uk/bitstream/handle/2299/11214/904970.pdf;sequence=2

A review of solar collectors and thermal energy storage in solar thermal applications

We give a general introduction to quantum phase transitions in strongly-correlated electron systems. These transitions which occur at zero temperature when a non-thermal parameter $g$ like pressure, chemical composition or magnetic field is tuned to a critical value are characterized by a dynamic exponent $z$ related to the energy and length scales $\Delta$ and $\xi$. Simple arguments based on an expansion to first order in the effective interaction allow to define an upper-critical dimension $D_{C}=4$ (where $D=d+z$ and $d$ is the spatial dimension) below which mean-field description is no longer valid. We emphasize the role of pertubative renormalization group (RG) approaches and self-consistent renormalized spin fluctuation (SCR-SF) theories to understand the quantum-classical crossover in the vicinity of the quantum critical point with generalization to the Kondo effect in heavy-fermion systems. Finally we quote some recent inelastic neutron scattering experiments performed on heavy-fermions which lead to unusual scaling law in $\omega /T$ for the dynamical spin susceptibility revealing critical local modes beyond the itinerant magnetism scheme and mention new attempts to describe this local quantum critical point.

https://arxiv.org/pdf/cond-mat/0102119.pdf

Quantum Phase Transitions

Apart from a reflecting and a transparent state, rare-earth-Mg alloys (RE-Mg) exhibit also a highly absorbing, black state during loading with hydrogen The occurrence of such a black state is due to the disproportionation into subwavelength size ${\mathrm{REH}}_{2+\ensuremath{\epsilon}}$ and Mg grains during the first hydrogen loading While the optical properties of ${\mathrm{REH}}_{x}$ change continuously with a further increase in hydrogen concentration x, Mg changes abruptly from a good reflector to a transparent insulator $({\mathrm{MgH}}_{2})$ Thin pure Mg films also show this black state when (un)loaded carefully at elevated temperatures By using the Bruggeman effective medium approximation in combination with the transfer matrix method it is shown that the coexistence of Mg and ${\mathrm{MgH}}_{2}$ grains is the cause of this high absorption Furthermore, we compare this phenomenon to the high absorption of light observed in metal-dielectric composites

/pdf/highly-absorbing-black-mg-and-rare-earth-mg-switchable-407n609aia.pdf

Highly absorbing black Mg and rare-earth-Mg switchable mirrors

At low temperatures dislocations are the dominant flux-pinning centers in thin films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ deposited on (100) ${\mathrm{SrTiO}}_{3}$ substrates [B. Dam et al., Nature (London) 399, 439 (1999)]. Using a wet-chemical etching technique in combination with atomic force microscopy, both the length and the lateral dislocation distribution are determined in laser ablated ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films. We find that (i) dislocations are induced in the first stages of film growth, i.e., close to the substrate-film interface, and persist all the way up to the film surface parallel to the c axis, resulting in a uniform length distribution, and (ii) the radial dislocation distribution function exhibits a universal behavior: it approaches zero at small distances, indicating short-range ordering of the defects. This self-organization of the growth-induced correlated disorder makes epitaxial films completely different from single crystals with randomly distributed columnar defects created by means of heavy-ion irradiation. Since the substrate temperature can be used to tune the dislocation density ${n}_{\mathrm{disl}}$ over almost two orders of magnitude (\ensuremath{\sim}1--100/\ensuremath{\mu}${\mathrm{m}}^{2}$), the mechanism by which dislocations are induced is closely related to the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.

/pdf/natural-strong-pinning-in-laser-ablated-yba2cu3o7-d-thin-1bezhgs9ts.pdf

Natural strong pinning in laser ablated YBa2Cu3O7-d thin films

By means of high-sensitivity capacitance torque magnetometers we have measured the superconducting current j s and the dynamic magnetic-moment relaxation of YBa 2 Cu 3 O 7 and YBa 2 Cu 4 O 8 films of typically 100 nm thickness at temperatures between 2 K and T c in magnetic fields up to 6 T. For the measurements of the dynamic relaxation rate Q ≡d ln j s /d ln (d B e /d t ) magnetic-field sweep rates were varied between 0.5 and 40 mT/s. At low fields (typically 0.5 T) the dynamical relaxation rate exhibits a plateau at Q ≈0.06 in YBa 2 Cu 3 O 7 and 0.04 in YBa 2 Cu 4 O 8 . At high fields ( B e = μ 0 H e ≈ 6 T) the plateaus have completely disappeared and Q increases almost linearly with increasing temperature. At all fields a sharp increase up to Q≊1 is observed when the irreversibility line is approached. By means of the generalized inversion scheme (GIS), the j s ( T , B e ) and Q ( T , B e ) data are used to determined the current dependent activation energy U ( j , T , B e ) for thermally activted flux creep. Although the GIS does not make any a priori assumptions about the explicit functional dependences on T and j , the U ( j , T =0, B e ) function derived from the experimental data by means of the GIS can remarkably well be described with the collective-creep interpolation formula U ( j )=( U c / μ )[( j c / j ) μ −1] with μ ≈0.6 for currents j >0.15 j c ( T =0, B e ) where J c ( T =0, B e ) is the critical current at T =0, and wi U c depending on B e . At lower current densities U ( j , T =0, B e ) does not diverge as j −0.6 but shifts gradually to a weaker ln( j c / j ) dependence. At low temperatures the current and relaxation data cannot be explained in terms of a thermally activated flux-motion model. Quantum creep has an influence up to ∼13 K.

Critical current, magnetization relaxation and activation energies for YBa2Cu3O7 and YBa2Cu4O8 films

An efficient inversion scheme is given to derive the local currents in a superconductor from the z component of the magnetic field measured above its surface, as is done using magneto-optical indicators. The method works for samples of arbitrary thickness provided that the current vector has only x and y components. Data storage is much lower and convergence much faster than in previously reported schemes. The influence of the distance of observation and of the sample aspect ratio on the measured field Hz is investigated. The current calculated from the magneto-optical observation of a real sample ~ aB i 2Sr 2CaCu 2O 8 single crystal! is in good agreement with the value measured directly by means of torque magnetometry. @S0163-1829~96!07534-0#

/pdf/determination-of-two-dimensional-current-patterns-in-flat-1i3wl7scoh.pdf

Determination of two-dimensional current patterns in flat superconductors from magneto-optical measurements: An efficient inversion scheme

In contrast to the binary switchable mirror films (YHx, LaHx, REHx with RE:rare earth) which have a weak red transparency window in their metallic dihydride phase, rare-earth alloys containing magnesium are remarkable for the large contrast between their metallic dihydride and transparent trihydride phase. By means of structural, optical transmittance, and electrical resistivity measurements on a series of Y1−yMgyHx, films we show that this is due to a disproportionation of the alloy. While the yttrium dihydride phase is formed, Mg separates out, remaining in its metallic state. Upon further loading, insulating MgH2 is formed together with cubic YH3−δ. In this way Mg acts essentially as a microscopic optical shutter, enhancing the reflectivity of these switchable mirrors in the metallic state and increasing the optical gap in the transparent state.

R.P. Griessen

Papers

Highly absorbing black Mg and rare-earth-Mg switchable mirrors

Natural strong pinning in laser ablated YBa2Cu3O7-d thin films

Critical current, magnetization relaxation and activation energies for YBa2Cu3O7 and YBa2Cu4O8 films

Determination of two-dimensional current patterns in flat superconductors from magneto-optical measurements: An efficient inversion scheme

Contrast enhancement of rare-earth switchable mirrors through microscopic shutter effect