SwePub - search: WFRF:(Berland Kristian)

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1.	Akabli, H., et al. (author) Intersubband energies in Al1-yInyN/Ga1-xInxN heterostructures with lattice constant close to aGaN 2012 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 52:1, s. 70-77 Journal article (peer-reviewed)abstract We have studied the conduction band profile and the intersubband transition energy, E-12, of Al1-yInyN/Ga1-xInxN quantum well structures. We have considered how material parameters such as non-parabolicity and the uncertainty in the bowing parameter affect E-12 and the corresponding wavelength, lambda(12). The calculations include strain and cover the transition range from telecommunication wavelengths (1.55 mu m) to the mid-infrared (similar to 10 mu m). Our results show that the transition energies of strain-free Al1-yInyN/Ga1-xInxN quantum well structures, which are lattice-matched to GaN (y = 17.7%, x = 0), resulted in wavelengths above similar to 2 mu m. To reach shorter wavelengths, we explored structures with other indium concentrations but maintaining a small mismatch to GaN. For similar to 1% lattice mismatch the wavelength lambda(12) could be reduced to less than 1.55 mu m. The results serve as a starting point for designing and epitaxial growth of photonic intersubband structures.
2.	Andersson, Thorvald, 1946, et al. (author) Design and Fabrication of AlN/GaN Heterostructures for Intersubband Technology 2011 In: ISPlasma 2011, March 6-9, 2011 Nagoya, Japan. Conference paper (other academic/artistic)
3.	Bergvall, Anders, 1981, et al. (author) Graphene nanogap for gate-tunable quantum-coherent single-molecule electronics 2011 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:15 Journal article (peer-reviewed)abstract We present atomistic calculations of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules can be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitates control of the device conductance in a transistor effect with high on-off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.
4.	Berland, Kristian, 1983 (author) A general solution to the Schrodinger-Poisson equation for a charged hard wall: Application to potential profile of an AlN/GaN barrier structure 2011 In: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 50:4, s. 411-418 Journal article (peer-reviewed)abstract A general, system-independent, formulation of the parabolic Schrodinger-Poisson equation is presented for a charged hard wall in the limit of complete screening by the ground state. It is solved numerically using iteration and asymptotic boundary conditions. The solution gives a simple relation between the band bending and sheet charge density at an interface. Approximative analytical expressions for the potential profile and wave function are developed based on properties of the exact solution. Specific tests of the validity of the assumptions leading to the general solution are made. The assumption of complete screening by the ground state is found be a limitation; however, the general solution provides a fair approximate account of the potential profile when the bulk is doped. The general solution is further used in a simple model for the potential profile of an AlN/GaN barrier structure. The result compares well with the solution of the full Schrodinger-Poisson equation.
5.	Berland, Kristian, 1983, et al. (author) A van der Waals density functional study of adenine on graphene: Single-molecular adsorption and overlayer binding 2011 In: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 23, s. 135001- Journal article (peer-reviewed)abstract The adsorption of an adenine molecule on graphene is studied using a first-principles van der Waals functional, vdW-DF (Dion et al 2004 Phys. Rev. Lett. 92 246401). The cohesive energy of an ordered adenine overlayer is also estimated. For the adsorption of a single molecule, we determine the optimal binding configuration and adsorption energy by translating and rotating the molecule. The adsorption energy for a single molecule of adenine is found to be 711 meV, which is close to the calculated adsorption energy of the similarly sized naphthalene. On the basis of the single-molecular binding configuration, we estimate the cohesive energy of a two-dimensional ordered overlayer. We find a significantly stronger binding energy for the ordered overlayer than for single-molecule adsorption.
6.	Berland, Kristian, 1983, et al. (author) Analysis of van der Waals density functional components: Binding and corrugation of benzene and C-60 on boron nitride and graphene 2013 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 87:20 Journal article (peer-reviewed)abstract The adsorption of benzene and C60 on graphene and boron nitride is studied using density functional theory with the van der Waals density functional (vdW-DF). By comparing these systems we can systematically investigate their adsorption nature and differences between the two functional versions vdW-DF1 and vdW-DF2. The bigger size of the C60 molecule makes it bind stronger to the surface than benzene, yet the interfaces between the molecules and the sheets are similar in nature. The binding separation is more sensitive to the exchange variant used in vdW-DF than to the correlation version. This result is related to the exchange and correlation components of the potential energy curve. We show that a moderate dipole forms for C60 on graphene, unlike for the other adsorption systems. We find that the corrugation (at the atomic scale) is very sensitive to the variant or version of vdW-DF used, in particular, the exchange. Further, we show that this sensitivity arises indirectly through the shift in binding separation caused by changing the vdW-DF variant. Based on our results, we suggest a concerted theory-experiment approach to assess the exchange and correlation contributions to physisorption. Using DFT calculations, the corrugation can be linked to the optimal separation, allowing us to extract the exchange-correlation part of the adsorption energy. Molecules with the same interfaces to the surface, but different geometries, can in turn cast light on the role of van der Waals forces.
7.	Berland, Kristian, 1983, et al. (author) Assessment of two hybrid van der Waals density functionals for covalent and non-covalent binding of molecules 2017 In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 146:23 Journal article (peer-reviewed)abstract Two hybrid van der Waals density functionals (vdW-DFs) are developed using 25% Fock exchange with (i) the consistent-exchange vdW-DF-cx functional [K. Berland and P. Hyldgaard, Phys. Rev. B89, 035412 (2014)] and (ii) with the vdW-DF2 functional [K. Lee et al., Phys. Rev. B 82, 081101 (2010)]. The ability to describe covalent and non-covalent binding properties of molecules is assessed. For properties related to covalent binding, atomization energies (G2-1 set), molecular reaction energies (G2RC set), and ionization energies (G21IP set) are benchmarked against experimental reference values. We find that hybrid-vdW-DF-cx yields results that are rather similar to those of the standard non-empirical hybrid PBE0 [C. Adamo and V. Barone, J. Chem. Phys. 110, 6158 (1999)], with mean average deviations (MADs) of 4.9 and 5.0 kcal/mol for the G2-1 set, respectively. In this comparison, experimental reference values are used, back corrected by wavefunction-based quantum-chemistry calculations of zero-point energies. Hybrid vdW-DF2 follows somewhat different trends, showing on average significantly larger deviations from the reference energies, with a MAD of 14.5 kcal/mol for the G2-1 set. Non-covalent binding properties of molecules are assessed using the S22 benchmark set of non-covalently bonded dimers and the X40 set of dimers of small halogenated molecules, using wavefunction-based quantum chemistry results as references. For the S22 set, hybrid-vdW-DF-cx performs better than standard vdW-DF-cx for the mostly hydrogen-bonded systems, with MAD dropping from 0.6 to 0.3 kcal/mol, but worse for purely dispersion-bonded systems, with MAD increasing from 0.2 to 0.6 kcal/mol. Hybrid-vdW-DF2 offers a slight improvement over standard vdW-DF2. Similar trends are found for the X40 set, with hybrid-vdW-DF-cx performing particularly well for binding energies involving the strongly polar hydrogen halides, but poorly for systems with tiny binding energies. Our study of the X40 set reveals the potential of mixing Fock exchange with vdW-DF, but also highlights shortcomings of the hybrids constructed here. The solid performance of hybrid-vdW-DF-cx for covalent-bonded systems, as well as the strengths and issues uncovered for non-covalently bonded systems, makes this study a good starting point for developing even more accurate hybrid vdW-DFs.
8.	Berland, Kristian, 1983 (author) Bound by long-range interactions: Molecular crystals and benzene on Cu(111) 2009 Licentiate thesis (other academic/artistic)abstract This thesis investigates molecular systems bound by long-range interactions. Westudy molecular crystals held together by van der Waals forces and the physisorption of benzene (Bz) on Cu(111). We also study the self-organization of benzeneon the Cu(111) surface which is influenced both by the (semi) long-range van derWaals forces, and the very-long-ranged indirect electronic interactions mediated bya metallic surface state on the Cu(111) surface. The van der Waals bound systems are investigated from first-principle principlesusing a van der Waals density functional (vdW-DF), [Phys. Rev. Lett. 92, 246401 (2004)]. We examine the character of these interactions, by studying the asymptoticaccount of the van der Waals forces, and compare full vdW-DF with an asymptotic atom-based pair potentials extracted from vdW-DF. The results show that the charac-ter of the binding differs in the two cases, with vdW-DF being significantly enhanced at relevant binding separations. This suggests that the use of asymptotic atom-based pair potentials as an approximation for non-local correlation is not consistent. Formolecular crystals, we study the structure and cohesive energy of hexamine and the platonic hydrocarbons cubane and dodecahedrane. The calculated lattice parame-ters and cohesion energy agree well with experiments. For adsorption of benzene on Cu(111), we obtain a binding energy in good agreement with experiment. We alsostudy diffusion barriers and find that the molecules can move almost freely along a honeycomb web of “corridors” passing between fcc and hcp hollow sites via bridgesites. To account for the surface-state mediated interactions, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this in-direct interaction. This description is based on input from DFT calculations of the metallic surface state on Cu(111), in addition to a parameterization of the benzene-induced scattering properties. We combine vdW-DF calculations of benzene dimers and diffusion barriers with the estimates of the surface-state mediated interactionsand find that the more compact (hexagonal) Bz-overlayer phase, with lattice constant ̊a = 6.74 A, is due to direct Bz-Bz vdW attraction, and the stability of the sparser hexagonal Bz phase, a = 10.24 A, is due to surface-state mediated interactions. Thus, we can account well for benzene self-organization on Cu(111). These results illustrate the current state of development of quantum-physical based approaches to systems bound by long-range interactions.
9.	Berland, Kristian, 1983 (author) Connected by voids: Interactions and screening in sparse matter 2012 Doctoral thesis (other academic/artistic)abstract Using theory and computations, we study structural and electronic properties of nanoscale systems where regions of low density are important for response and function. Materials with such properties are called sparse matter. Examples include assemblies of organic molecules and layered structures like graphite. Binding in molecular crystals and physisorption onto surfaces is studied using density-functional theory (DFT) with a van der Waals density functional (vdW-DF) account of exchange-orrelation. We examine binding properties of molecular crystals of high symmetry (hexamine, dodeacahedrane, cubane, C60, graphite) as they allow straightforward analysis of binding nature. We find that vdW-DF describes a non-local attraction that is significantly enhanced at shorter separations compared to the 1/r6 form common in pair-potential descriptions. The binding of adenine, benzene, and C60 on graphene are studied. Several properties of the non-local vdW-DF correlation is elucidated, like sensitivity to geometry, length-scales, and role of exchange companion. Adsorption of H2 and benzene on Cu(111) is studied. We find that benzene affects the surface state and that the corrugation is small enough to allow it to react to surface-state mediated (SSM) interactions. In all cases, the results of vdW-DF calculations compare well to experimental observations.Noble metal surfaces like Cu(111) harbor a metallic Shockley surface state that supports two-dimensional electron gas (2DEG). This surface state is believed to play a crucial role in the self-assembly of molecules. Using DFT, we study this surfacestate and how an external field and physisorbed molecules influence its properties. To aid our study, we developed a method to enhance slab-based calculations of surfacestates. We also investigate the self-assembly of benzene on Cu(111), where we include a simple analytical expression for the SSM interaction in addition to the direct van der Waals interactions. The resulting binding curves have two separate minima that coincide with the periodicity of the measured overlayer phases.Finally, we draw a link to AlN/GaN heterostructures. In analogy to sparse matter, they exhibit wide regions of low conduction electron density. Associated with these depletion regions are inversion layers supporting 2DEGs. This charge redistribution arise from internal fields originating in intrinsic polarization. These structures are studied using a custom-built Schrödinger-Poisson solver. A systematic design strategy to remove the internal fields in the leads is developed to aid the design of devices based on transport perpendicular to the interfaces.
10.	Berland, Kristian, 1983, et al. (author) Exchange functional that tests the robustness of the plasmon description of the van der Waals density functional 2014 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 89:3, s. art. no. 035412- Journal article (peer-reviewed)abstract Is the plasmon description within the nonlocal correlation of the van der Waals density functional by Dion and coworkers (vdW-DF) robust enough to describe all exchange-correlation components? To address this question, we design an exchange functional based on this plasmon description as well as recent analysis on exchange in the large-s regime. In the regime with reduced gradients s = \|del n\|/2nk(F)(n) smaller than approximate to 2.5, dominating the nonlocal correlation part of the binding energy, the enhancement factor F-x(s) closely resembles the Langreth-Vosko screened exchange. In the s regime beyond, dominated by exchange, F-x(s) passes smoothly over to the revised Perdew-Wang-86 form. We term the specific exchange functional LV-PW86r, wheras the full van der Waals functional version emphasizing consistent handling of exchange is termed vdW-DF-cx. Our tests indicate that vdW-DF-cx produces accurate separations and binding energies of the S22 data set of molecular dimers as well as accurate lattice constants and bulk moduli of layered materials and tightly bound solids. These results suggest that the plasmon description within vdW-DF gives a good description of both exchange and correlation effects in the low-to-moderate s regime.
11.	Berland, Kristian, 1983, et al. (author) Harris-type van der Waals density functional scheme 2013 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 88:4 Journal article (peer-reviewed)abstract Biomolecular systems that involve thousands of atoms are difficult to address with standard density functional theory (DFT) calculations. With the development of sparse-matter methods such as the van der Waals density functional (vdW-DF) method [M. Dion et al., Phys. Rev. Lett. 92, 246401 (2004)], it is now possible to include the dispersive forces in DFT which are necessary to describe the cohesion and behavior of these systems. vdW-DF implementations can be as efficient as those for traditional DFT. Yet, the computational costs of self-consistently determining the electron wave functions and hence the kinetic-energy repulsion still limit the scope of sparse-matter DFT. We propose to speed up sparse-matter calculations by using the Harris scheme [J. Harris, Phys. Rev. B 31, 1770 (1985)]; that is, we propose to perform electronic relaxations only for separated fragments (molecules) and use a superposition of fragment densities as a starting point to obtain the total energy non-self-consistently. We evaluate the feasibility of this approach for an adaption of the Harris scheme for non-self-consistent vdW-DF (sfd-vdW-DF). We study four molecular dimers with varying degrees of polarity and find that the sfd scheme accurately reproduces standard non-self-consistent vdW-DF for van der Waals dominated systems but is less accurate for those dominated by polar interactions. Results for the S22 set of typical organic molecular dimers are promising.
12.	Berland, Kristian, 1983, et al. (author) Molecular systems, growth and overlayers: van der Waals density functional and first-principles thermodynamics calculations 2010 In: SNIC progress report 08/09. Swedish national infrastructure for computing.. Book chapter (other academic/artistic)
13.	Berland, Kristian, 1983, et al. (author) Polarization-balanced design of heterostructures: Application to AlN/GaN double-barrier structures 2011 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:24 Journal article (peer-reviewed)abstract Inversion and depletion regions generally form at the interfaces between doped leads ( cladding layers) and the active region of polar heterostructures like AlN/GaN and other nitride compounds. The band bending in the depletion region sets up a barrier that may seriously impede perpendicular electronic transport. This may ruin the performance of devices such as quantum-cascade lasers and resonant-tunneling diodes. Here we introduce the concepts of polarization balance and polarization-balanced designs: A structure is polarization balanced when the applied bias match the voltage drop arising from spontaneous and piezeolectric fields. Devices designed to operate at this bias have polarization-balanced designs. These concepts offer a systematic approach to avoid the formation of depletion regions. As a test case, we consider the design of AlN/GaN double-barrier structures with Al((x) over tilde)Ga(1-(x) over tilde)N leads. To guide our efforts, we derive a simple relation between the intrinsic voltage drop arising from polar effects, average alloy composition of the active region, and the alloy concentration of the leads. Polarization-balanced designs secure good filling of the ground state for unbiased structures, while for biased structures with efficient emptying of the active region they remove the depletion barriers.
14.	Berland, Kristian, 1983, et al. (author) Response of the Shockley surface state to an external electrical field: A density-functional theory study of Cu(111) 2012 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 85:3 Journal article (peer-reviewed)abstract The response of the Cu(111) Shockley surface state to an external electrical field is characterized by combining a density-functional theory calculation for a slab geometry with an analysis of the Kohn-Sham wave functions. Our analysis is facilitated by a decoupling of the Kohn-Sham states via a rotation in Hilbert space. We find that the surface state displays isotropic dispersion, quadratic until the Fermi wave vector but with a significant quartic contribution beyond. We calculate the shift in energetic position and effective mass of the surface state for an electrical field perpendicular to the Cu(111) surface; the response is linear over a broad range of field strengths. We find that charge transfer occurs beyond the outermost copper atoms and that accumulation of electrons is responsible for a quarter of the screening of the electrical field. This allows us to provide well converged determinations of the field-induced changes in the surface state for a moderate number of layers in the slab geometry.
15.	Berland, Kristian, 1983, et al. (author) Rings sliding on a honeycomb network: Adsorption contours, interactions, and assembly of benzene on Cu(111) 2009 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 80:15, s. 155431- Journal article (peer-reviewed)abstract Using a van der Waals density functional (vdW-DF) [Phys. Rev. Lett. 92, 246401 (2004)], we perform ab initio calculations for the adsorption energy of benzene (Bz) on Cu(111) as a function of lateral position and height. We find that the vdW-DF inclusion of nonlocal correlations (responsible for dispersive interactions) changes the relative stability of eight binding-position options and increases the binding energy by over an order of magnitude, achieving good agreement with experiment. The admolecules can move almost freely along a honeycomb web of “corridors” passing between fcc and hcp hollow sites via bridge sites. Our diffusion barriers (for dilute and two condensed adsorbate phases) are consistent with experimental observations. Further vdW-DF calculations suggest that the more compact (hexagonal) Bz-overlayer phase, with lattice constant a=6.74 Å, is due to direct Bz-Bz vdW attraction, which extends to ~8 Å. We attribute the second, sparser hexagonal Bz phase, with a=10.24 Å, to indirect electronic interactions mediated by the metallic surface state on Cu(111). To support this claim, we use a formal Harris-functional approach to evaluate nonperturbationally the asymptotic form of this indirect interaction. Thus, we can account well for benzene self-organization on Cu(111).
16.	Berland, Kristian, 1983, et al. (author) Structure and binding in crystals of cagelike molecules: Hexamine and platonic hydrocarbons 2010 In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 132:13, s. (Published: April 7 2010) (Article Number: 134705)- Journal article (peer-reviewed)abstract In this paper, we show that first-principle calculations using a van der Waals density functional (vdW-DF) [M. Dion, H. Rydberg, E. Schroumlder, D. C. Langreth, and B. I. Lundqvist, Phys. Rev. Lett. 92, 246401 (2004)] permit the determination of molecular crystal structure within density functional theory (DFT). We study the crystal structures of hexamine and the platonic hydrocarbons (cubane and dodecahedrane). The calculated lattice parameters and cohesion energy agree well with experiments. Further, we examine the asymptotic accounts of the van der Waals forces by comparing full vdW-DF with asymptotic atom-based pair potentials extracted from vdW-DF. The character of the binding differs in the two cases, with vdW-DF giving a significant enhancement at intermediate and relevant binding separations. We analyze consequences of this result for methods such as DFT-D and question DFT-D's transferability over the full range of separations.
17.	Berland, Kristian, 1983, et al. (author) Temperature stability of intersubband transitions in AlN/GaN quantum wells 2010 In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 97:4, s. 043507- Journal article (peer-reviewed)abstract Temperature dependence of intersubband transitions in AlN/GaN multiple quantum wells grown with molecular beam epitaxy is investigated both by absorption studies at different temperatures and modeling of conduction-band electrons. For the absorption study, the sample is heated in increments up to 400 degrees C. The self-consistent Schroumldinger-Poisson modeling includes temperature effects of the band gap and the influence of thermal expansion on the piezoelectric field. We find that the intersubband absorption energy decreases only by similar to 6 meV at 400 degrees C relative to its room temperature value.
18.	Berland, Kristian, 1983, et al. (author) Title: van der Waals density functional calculations of binding in molecular crystals 2010 Journal article (other academic/artistic)abstract A recent paper [J. Chem. Phys. 132, 134705 (2010)] illustrated the potential of the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92, 246401 (2004)] for efficient first-principle accounts of structure and cohesion in molecular crystals. Since then, modifications of the original vdW-DF version (identified as vdW-DF1) has been proposed, and there is also a new version called vdW-DF2 [ArXiv 1003.5255], within the vdW-DF framework. Here we investigate the performance and nature of the modifications and the new version for the binding of a set of simple molecular crystals: hexamine, dodecahedrane, C60, and graphite. These extended systems provide benchmarks for computational methods dealing with sparse matter. We show that a previously documented enhancement of non-local correlations of vdW-DF1 over an asymptotic atom-based account close to and a few A, beyond binding separation persists in vdW-DF2. The calculation and analysis of the binding in molecular crystals requires appropriate computational tools. In this paper, we also present details on our real-space parallel implementation of the vdW-DF correlation and on the method used to generate asymptotic atom-based pair potentials based on vdW-DF.
19.	Berland, Kristian, 1983, et al. (author) Van der Waals density functional calculations of binding in molecular crystals 2011 In: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 182:9, s. 1800-1804 Journal article (peer-reviewed)abstract A recent paper [J. Chem. Phys. 132 (2010) 134705] illustrated the potential of the van der Waals density functional (vdW-DF) method [Phys. Rev. Lett. 92 (2004) 246401] for efficient first-principle accounts of structure and cohesion in molecular crystals. Since then, modifications of the original vdW-DF version (identified as vdW-DF1) have been proposed, and there is also a new version called vdW-DF2 [Phys. Rev. B 82 (2010) 081101(R)], within the vdW-DF framework. Here we investigate the performance and nature of the modifications and the new version for the binding of a set of simple molecular crystals: hexamine, dodecahedrane, C60, and graphite. These extended systems provide benchmarks for computational methods dealing with sparse matter. We show that a previously documented enhancement of non-local correlations of vdW-DF1 over an asymptotic atom-based account close to and a few A beyond binding separation persists in vdW-DF2. The calculation and analysis of the binding in molecular crystals require appropriate computational tools. In this paper, we also present details on our real-space parallel implementation of the vdW-DF correlation and on the method used to generate asymptotic atom-based pair potentials based on vdW-DF.
20.	Berland, Kristian, 1983, et al. (author) van der Waals density functionals built upon the electron-gas tradition: Facing the challenge of competing interactions 2014 In: Journal of Chemical Physics. - : AIP Publishing. - 1089-7690 .- 0021-9606. ; 140:18, s. 18A539 - Journal article (peer-reviewed)abstract The theoretical description of sparse matter attracts much interest, in particular for those ground-state properties that can be described by density functional theory. One proposed approach, the van der Waals density functional (vdW-DF) method, rests on strong physical foundations and offers simple yet accurate and robust functionals. A very recent functional within this method called vdW-DF-cx [K. Berland and P. Hyldgaard, Phys. Rev. B89, 035412 (2014)] stands out in its attempt to use an exchange energy derived from the same plasmon-based theory from which the nonlocal correlation energy was derived. Encouraged by its good performance for solids, layered materials, and aromatic molecules, we apply it to several systems that are characterized by competing interactions. These include the ferroelectric response in PbTiO3, the adsorption of small molecules within metal-organic frameworks, the graphite/diamond phase transition, and the adsorption of an aromatic-molecule on the Ag(111) surface. Our results indicate that vdW-DF-cx is overall well suited to tackle these challenging systems. In addition to being a competitive density functional for sparse matter, the vdW-DF-cx construction presents a more robust general-purpose functional that could be applied to a range of materials problems with a variety of competing interactions.
21.	Berland, Kristian, 1983, et al. (author) van der Waals forces in density functional theory: a review of the vdW-DF method 2015 In: Reports on Progress in Physics. - : IOP Publishing. - 0034-4885 .- 1361-6633. ; 78:6, s. 066501- Journal article (peer-reviewed)abstract A density functional theory (DFT) that accounts for van der Waals (vdW) interactions incondensed matter, materials physics, chemistry, and biology is reviewed. The insights that ledto the construction of the Rutgers–Chalmers van der Waals density functional (vdW-DF) arepresented with the aim of giving a historical perspective, while also emphasizing more recentefforts which have sought to improve its accuracy. In addition to technical details, we discussa range of recent applications that illustrate the necessity of including dispersion interactionsin DFT. This review highlights the value of the vdW-DF method as a general-purpose method,not only for dispersion bound systems, but also in densely packed systems where these typesof interactions are traditionally thought to be negligible.
22.	Hamed, Tareq Abu, et al. (author) Multiscale in modelling and validation for solar photovoltaics 2018 In: EPJ Photovoltaics. - : EDP Sciences. - 2105-0716. ; 9 Journal article (peer-reviewed)abstract Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges.
23.	Hyldgaard, Per, 1964, et al. (author) Interpretation of van der Waals density functionals 2014 In: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 90:7 Journal article (peer-reviewed)abstract The nonlocal correlation energy in the van der Waals density functional (vdW-DF) method can be interpreted in terms of a coupling of zero-point energies of characteristic modes of semilocal exchange-correlation (xc) holes. These xc holes reflect the internal functional in the framework of the vdW-DF method. We explore the internal xc hole components, showing that they share properties with those of the generalized-gradient approximation. We use these results to illustrate the nonlocality in the vdW-DF description and analyze the vdW-DF formulation of nonlocal correlation.
24.	Ive, Tommy, 1968, et al. (author) Design and Fabrication of AlN/GaN Heterostructures for Intersubband Technology 2012 In: Japanese Journal of Applied Physics. - 1347-4065 .- 0021-4922. ; 51:1, s. Article Number: 01AG07 - Journal article (peer-reviewed)abstract We have used models based on the effective-mass approximation and Schrodinger-Poisson to design AlN/GaN multiple quantum well structures for intersubband transitions between two or three energy levels. The structures were realized by molecular beam epitaxy and the surface morphology and structural quality were investigated. We also investigated GaN waveguides that were fabricated using standard cleanroom techniques. Our work is focused on the various challenges associated to the fabrication of quantum cascade lasers based on group III-nitrides. These challenges are discussed in the light of our results.
25.	Lee, Kyuho, et al. (author) Benchmarking van der Waals density functionals with experimental data: potential-energy curves for H2 molecules on Cu(111), (100) and (110) surfaces 2012 In: Journal of Physics: Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 24:42 Journal article (peer-reviewed)abstract Detailed physisorption data from experiment for the H2 molecule on low-index Cu surfaces challenge theory. Recently, density functional theory (DFT) has been developed to account for nonlocal correlation effects, including van der Waals (dispersion) forces. We show that the functional vdW-DF2 gives a potential-energy curve, potential-well energy levels and difference in lateral corrugation promisingly close to the results obtained by resonant elastic backscattering–diffraction experiments. The backscattering barrier is sensitive to the choice of exchange functional approximation. Further, the DFT-D3 and TS-vdW corrections to traditional DFT formulations are also benchmarked, and deviations are analyzed.
26.	Lee, Kuyho, et al. (author) Evaluation of a density functional with account of van der Waals forces using experimental data of H2 physisorption on Cu(111) 2011 In: Physical Review B. - 1098-0121. ; 84, s. 193408- Journal article (peer-reviewed)abstract Detailed experimental data for physisorption potential-energy curves of H2 on low-indexed faces of Cu challenge theory. Recently, density-functional theory has been developed to also account for nonlocal correlation effects, including van der Waals forces. We show that one functional, denoted vdW-DF2, gives a potential-energy curve promisingly close to the experiment-derived physisorption-energy curve. The comparison also gives indications for further improvements of the functionals.
27.	Li, Yang, et al. (author) Origin of anomalously stabilizing ice layers on methane gas hydrates near rock surface 2023 In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 25:9, s. 6636-6652 Journal article (peer-reviewed)abstract Gas hydrates (GHs) in water close to freezing temperatures can be stabilised via the formation of ice layers. In a recent work [Bostrom et al., Astron. Astrophys., A54, 650, 2021], it was found that a surface region with partial gas dilution could be essential for obtaining nano- to micron-sized anomalously stabilizing ice layers. In this paper, it is demonstrated that the Casimir-Lifshitz free energy in multi-layer systems could induce thinner, but more stable, ice layers in cavities than those found for gas hydrates in a large reservoir of cold water. The thickness and stability of such ice layers in a pore filled with cold water could influence the leakage of gas molecules. Additional contributions, e.g. from salt-induced stresses, can also be of importance, and are briefly discussed.
28.	Mahan, G. D., et al. (author) Theory of polar corrections to donor binding 2011 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 84:23 Journal article (peer-reviewed)abstract We calculate the optical-phonon correction to the binding energy of electrons to donors in cubic materials. Previous theories calculated the Rydberg energy reduced by the effective mass and the static dielectric function. They omitted an important energy term from the long-range polarization of the ionized donor, which vanishes for the neutral donor. They also omitted the donor-phonon interaction. The inclusion of these terms yields a new formula for the donor binding energy.
29.	Rangel, Tonatiuh, et al. (author) Structural and excited-state properties of oligoacene crystals from first principles 2016 In: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 93 Journal article (peer-reviewed)abstract Molecular crystals are a prototypical class of van der Waals (vdW) bound organic materials with excited state properties relevant for optoelectronic applications. Predicting the structure and excited state properties of molecular crystals presents a challenge for electronic structure theory, as standard approximations to density functional theory (DFT) does not cover the long ranged vdW dispersion interactions and do not yield excited state properties. In this work we use a a combination of DFT including vdW forces -- using both nonlocal functionals and pair-wise correction methods -- together with many-body perturbation theory (MBPT) to study the geometry and excited states, respectively, of the entire set of oligoacene crystals, from benzene to hexazene. We find that vdW methods can predict lattice constants within 1 percent of the experimental measurements, on par with the previously reported accuracy of the pair-wise approximations for the same systems. We further find that the excitations energies are sensitive to the geometry, but if optimized geometries are used MBPT can yield excited state properties within a few tenths of an eV from experiments. We elucidate trends in MBPT-charge and neutral excitation energies across the acene series and discuss the role of common approximations used in MBPT.
30.	Schröder, Elsebeth, 1966, et al. (author) The vdW-DF Family of Non-Local Exchange-Correlation Functionals 2017 In: Non-covalent Interactions in Quantum Chemistry and Physics: Theory and Applications. ; , s. Chapter 8- Book chapter (other academic/artistic)abstract van der Waals interactions are a phenomenon where charge fluctuations in one part of a system correlate with fluctuations in another, resulting in an attractive force. Such interactions are thus a truly non-local correlation effect. While the full—albeit unknown—density functional does include these interactions, standard local and semi-local density functionals cannot account for these non-local effects by construction and yield qualitatively erroneous predictions. The simplest expression of a non-local functional of the density rho(r) takes the formIntegral[d3r d3r' rho(r) phi(r,r') rho(r')], but it was not until the end of the last century that the means to find a physically motivated, general, and transferable kernel rho emerged. The present chapter discusses the work on this kernel rho leading to the development of the successful van der Waals density functional (vdW-DF) and its variants.
31.	Stattin, Martin, 1983, et al. (author) Waveguides for nitride based quantum cascade lasers 2011 In: Physica Status Solidi (C) Current Topics in Solid State Physics. - : Wiley. - 1610-1642 .- 1862-6351. ; 8:7-8, s. 2357-2359 Journal article (peer-reviewed)abstract Waveguide designs for AlGaN-based near-infrared quantum cascade lasers are investigated using a finite-difference method mode solver. Because of the negligibly small refractive index difference between the AlN/GaN/AlGaN gain region and the surrounding AlGaN current injection and extraction layers, a low refractive index substrate (sapphire) and a low refractive index dielectric (SiO2) are used for vertical confinement of the optical field. A ridge waveguide with an off-center contact metallization is used for lateral confinement. The off-center contact allows for the propagation of a TM mode with low metal induced loss and sufficient optical confinement in the gain region. A viable waveguide design with a metal-induced loss of 6.1 cm-1 and a confinement factor of 0.52 is demonstrated.
32.	Stattin, Martin, 1983, et al. (author) Waveguides for Nitride Based Quantum Cascade Lasers 2010 In: International Workshop on Nitride Semiconductors (IWN2010), Tampa, Florida, U.S.A., September 19 – 24, 2010. Conference paper (other academic/artistic)
33.	Sun, D. Z., et al. (author) Effective elastic properties of a van der Waals molecular monolayer at a metal surface 2010 In: Physical Review B - Condensed Matter and Materials Physics. - 2469-9950 .- 2469-9969. ; 82:20 Journal article (peer-reviewed)abstract Adsorbing anthracene on a Cu(111) surface results in a wide range of complex and intriguing superstructures spanning a coverage range from 1 per 17 to 1 per 15 substrate atoms. In accompanying first-principles density-functional theory calculations we show the essential role of van der Waals interactions in estimating the variation in anthracene adsorption energy and height across the sample. We can thereby evaluate the compression of the anthracene film in terms of continuum elastic properties, which results in an effective Young's modulus of 1.5 GPa and a Poisson ratio approximate to 0.1. These values suggest interpretation of the molecular monolayer as a porous material-in marked congruence with our microscopic observations.
34.	Thonhauser, T, et al. (author) Spin Signature of Nonlocal Correlation Binding in Metal-Organic Frameworks 2015 In: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 115:13 Journal article (peer-reviewed)abstract We develop a proper nonempirical spin-density formalism for the van der Waals density functional (vdW-DF) method. We show that this generalization, termed svdW-DF, is firmly rooted in the single-particle nature of exchange and we test it on a range of spin systems. We investigate in detail the role of spin in the nonlocal correlation driven adsorption of H-2 and CO2 in the linear magnets Mn-MOF74, Fe-MOF74, Co-MOF74, and Ni-MOF74. In all cases, we find that spin plays a significant role during the adsorption process despite the general weakness of the molecular-magnetic responses. The case of CO2 adsorption in Ni-MOF74 is particularly interesting, as the inclusion of spin effects results in an increased attraction, opposite to what the diamagnetic nature of CO2 would suggest. We explain this counterintuitive result, tracking the behavior to a coincidental hybridization of the O p states with the Ni d states in the down-spin channel. More generally, by providing insight on nonlocal correlation in concert with spin effects, our nonempirical svdW-DF method opens the door for a deeper understanding of weak nonlocal magnetic interactions.
35.	Wyrick, J., et al. (author) Do Two-Dimensional "Noble Gas Atoms" Produce Molecular Honeycombs at a Metal Surface? 2011 In: Nano Letters. - : American Chemical Society (ACS). - 1530-6992 .- 1530-6984. ; 11:7, s. 2944-2948 Journal article (peer-reviewed)abstract Anthraquinone self-assembles on Cu(111) into a giant honeycomb network with exactly three molecules on each side. Here we propose that the exceptional degree of order achieved in this system can be explained as a consequence of the confinement of substrate electrons in the pores, with the pore size tailored so that the confined electrons can adopt a noble-gas-like two-dimensional quasi-atom configuration with two filled shells. Formation of identical pores in a related adsorption system (at different overall periodicity due to the different molecule size) corroborates this concept. A combination of photoemission spectroscopy with density functional theory computations (including van der Waals interactions) of adsorbate-substrate interactions allows quantum mechanical modeling of the spectra of the resultant quasi-atoms and their energetics.
36.	Zamulko, Sergiy, et al. (author) Optical Properties of Cu2ZnSn(SxSe1-x)(4) by First-Principles Calculations 2018 In: Physica Status Solidi (a) applications and materials science. - : Wiley-VCH Verlagsgesellschaft. - 1862-6300 .- 1862-6319. ; 215:17 Journal article (peer-reviewed)abstract Structural, electronic, and optical properties of Cu2ZnSn(SxSe1-x)(4) semiconductors are studied theoretically for different concentration of S and Se anions. The optical properties are calculated at three levels of theory, in the generalized gradient approximation (GGA), meta-GGA, and with a hybrid functional. The GGA and meta-GGA calculations are corrected with an on-site Coulomb U-d term. Lattice constants, dielectric constants, and band-gaps are found to vary almost linearly with the concentration of S. The authors also show that a dense sampling of the Brillouin zone is required to accurately account for the shape of the dielectric function, which is hard to attain with hybrid functionals. This issue is resolved with a recently developed kp based interpolation scheme, which allows us to compare results of the hybrid functional calculations on an equal footing with the GGA and meta-GGA results. We find that the hybrid functionals provide the overall best agreement with the experimental dielectric function.

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