Solid State Phys Research Articles

Determining the Band Alignment at the BiVO4|NiOOH Interface Using the Hybrid DFT Technique

It is important to understand the band offsets between semiconductors, which are crucial to determine the direction of electron transfer at the interfaces. Two methods are normally used to determine the direction from the first principles: alternating slabs put in contact (without empty spaces between them) and separate calculations for each material surface in the presence of vacuum spaces. The first method may introduce distortions due to insufficient epitaxial match, which may lead to bandgap changes, and the second may neglect electron transfer at the interface, which may be important in systems exhibiting very different average electronegativities. This can also imply a spill of electronic density into the vacuum spaces, which will not be present at real interfaces. Herein, both approaches were used to study the BiVO4/NiOOH interface, and the results were compared; the results are here relevant for photoelectrochemistry. The method is based on hybrid Density Functional Theory methods which give for the bulk phases Bandgap values that agree with the experimental ones (in one case, a value reflecting the theoretical value). The distances between the (hybrid DFT-derived) band positions and the corresponding profiles of the Hartree electrostatic potential were transferred to the interfaces. This helps determining the appropriate positions of the valence and conduction bands (as has been suggested by C. G. Van de Walle & R. M. Martin, Phys. Rev. B 1987, 35, 8154). It is ensured that the interfaces are nonpolar (Tasker’s criterion: P.W. Tasker, J. Phys. C: Solid State Phys. 1979, 12, 4977).

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Over 15% efficient wide-band-gap Cu(In,Ga)S2 solar cell: Suppressing bulk and interface recombination through composition engineering

Chiral Magnonics: Reprogrammable Nanoscale Spin Wave Networks Based on Chiral Domain Walls.

SummarySpin waves offer promising perspectives as information carriers for future computational architectures beyond conventional complementary metal-oxide-semiconductor (CMOS) technology, owing to their benefits for device minimizations and low-ohmic losses. Although plenty of magnonic devices have been proposed previously, scalable nanoscale networks based on spin waves are still missing. Here, we demonstrate a reprogrammable two-dimensional spin wave network by combining the chiral exchange spin waves and chiral domain walls. The spin-wave network can be extended to two dimensions and offers unprecedented control of exchange spin waves. Each cell in the network can excite, transmit, and detect spin waves independently in the chiral domain wall, and spin-wave logics are also demonstrated. Our results open up perspectives for integrating spin waves into future logic and computing circuits and networks.

Grüneisen parameters and thermal conductivity in the phase change compound GeTe

Thermal conductivity is one of the key properties for the application of phase change materials in nonvolatile memories. In this work we compute the mode Gruneisen parameters of the phase change compound GeTe by means of density-functional perturbation theory. The Gruneisen parameters are then used to estimate the bulk thermal conductivity with a phenomenological formula (Slack in Solid State Phys 34:1–71, 1979). This estimate is compared with the full solution of the Boltzmann Transport Equation we obtained in a previous work within the same theoretical framework. This comparison allowed us to validate the phenomenological formula also for the prototypical phase change compound GeTe.

Band-gap tuning and optical response of two-dimensionalSixC1−x: A first-principles real-space study of disordered two-dimensional materials

We present a real-space formulation for calculating the electronic structure and optical conductivity of such random alloys based on the Kubo-Greenwood formalism interfaced with the augmented space recursion (ASR) [A. Mookerjee, J. Phys. C: Solid State Phys. {\bf 6}, 1340 (1973)] formulated with the Tight-binding Linear Muffin-tin Orbitals (TB-LMTO) basis with van Leeuwen-Baerends corrected exchange (vLB) [Singh et al, Phys. Rev B {\bf 93}, 085204, (2016)]. This approach has been used to quantitatively analyze the effect of chemical disorder on the configuration averaged electronic properties and optical response of 2D honeycomb siliphene Si$_{x}$C$_{1-x}$ beyond the usual Dirac-cone approximation. We predicted the quantitative effect of disorder on both the electronic-structure and optical response over a wide energy range, and the results discussed in the light of the available experimental and other theoretical data. Our proposed formalism may open up a facile way for planned band gap engineering in opto-electronic applications.

Commentary on ‘Ordering, metastability and phase transitions in two-dimensional systems’ J M Kosterlitz and D J Thouless (1973 J. Phys. C: Solid State Phys. 6 1181–203)—the early basis of the successful Kosterlitz–Thouless theory

Commentary on ‘Ordering, metastability and phase transitions in two-dimensional systems’ J M Kosterlitz and D J Thouless (1973 J. Phys. C: Solid State Phys. 6 1181–203)—the early basis of the successful Kosterlitz–Thouless theory

Spin decoherence in n-type GaAs: The effectiveness of the third-body rejection method for electron-electron scattering

We study the spin decoherence in n-type bulk GaAs for moderate electronic densities at room temperature using the Ensemble Monte Carlo method. We demonstrate that a technique called “third-body rejection method” devised by B. K. Ridley, J. Phys. C: Solid State Phys. 10, 1589 (1977) can be successfully adapted to Ensemble Monte Carlo method and used to tackle the problem of the electron-electron contribution to spin decoherence in the parameter region under study, where the electron-electron interaction can be reasonably described by a Yukawa potential. This scattering technique is employed in a doping region where one can expect that multiple collisions may play a role in carrier dynamics. By this technique, we are able to calculate spin relaxation times which are in very good agreement with the experimental results found by Oertel et al., Appl. Phys. Lett. 93, 13 (2008). Through this method, we show that the electron-electron scattering is overestimated in Born approximation, in agreement with previous results obtained by C. A. Kukkonen and H. Smith, Phys. Rev. B 8, 4601 (1973).

Analysis of simulation methodology for calculation of the heat of transport for vacancy thermodiffusion

Computation of the heat of transport Qa* in monatomic crystalline solids is investigated using the methodology first developed by Gillan [J. Phys. C: Solid State Phys. 11, 4469 (1978)] and further developed by Grout and coworkers [Philos. Mag. Lett. 74, 217 (1996)], referred to as the Grout-Gillan method. In the case of pair potentials, the hopping of a vacancy results in a heat wave that persists for up to 10 ps, consistent with previous studies. This leads to generally positive values for Qa* which can be quite large and are strongly dependent on the specific details of the pair potential. By contrast, when the interactions are described using the embedded atom model, there is no evidence of a heat wave, and Qa* is found to be negative. This demonstrates that the dynamics of vacancy hopping depends strongly on the details of the empirical potential. However, the results obtained here are in strong disagreement with experiment. Arguments are presented which demonstrate that there is a fundamental error made in the Grout-Gillan method due to the fact that the ensemble of states only includes successful atom hops and hence does not represent an equilibrium ensemble. This places the interpretation of the quantity computed in the Grout-Gillan method as the heat of transport in doubt. It is demonstrated that trajectories which do not yield hopping events are nevertheless relevant to computation of the heat of transport Qa*.

Nuclear magnetic resonance relaxometry of water in two and quasi-two dimensions

Molecular dynamics (MD) and Monte Carlo (MC) methods are used to determine the spin-pair correlation function G(*)(t) for the diffusion of bulk water in three dimensions (3D) and pore water in two dimensions (2D) and quasi-two dimensions (Q2D). The correlation function is required for the determination of the nuclear magnetic resonance spin-lattice and spin-spin relaxation times T(1) and T(2). It is shown that the analytic form of the powder-average correlation function, introduced by Sholl [Sholl, J. Phys. C: Solid State Phys. 7, 3378 (1974)] for the diffusion of spins on a 3D lattice, is of general validity. An analytic expression for G(*)(t) for a uniform spin fluid is derived in 2D. An analytic expression for the long-time behavior of G(*)(t) is derived for spins diffusing on 3D, 2D, and Q2D lattices. An analytic correction term, which accounts for spin pairs outside the scope of the numerical simulations, is derived for 3D and 2D and shown to improve the accuracy of the simulations. The contributions to T(1) due to translational and rotational motion obtained from the MD simulation of bulk water at 300 K are 7.4 s and 10±1 s, respectively, at 150 MHz, leading to an overall time of 4.3±0.4 s compared to the experimental value of 3.8 s. In Q2D systems, in which water is confined by alpha-quartz surfaces to thicknesses of 1-5 nm, T(1) for both translational and rotational relaxation is reduced due to the orientation and adsorption of spins at the surfaces. A method of parametrizing the MC lattice-diffusion simulations in 3D, 2D, and Q2D systems is presented. MC results for G(*)(t) for 3D and 2D systems are found to be consistent with an analytic uniform fluid model for t~/>40 ps. The value of TT(1) for translational diffusion obtained from the MC simulation of bulk water is found to be 4.8 s at 15 MHz. G(*)(t) obtained from MC simulations of Q2D systems, where water is confined by hard walls, is found to execute a distinct transition from 3D to 2D behavior. The T(1) is found to be similar to the 3D bulk water result at all pore thicknesses.

Relation between the interband dipole and momentum matrix elements in semiconductors

It is shown that a frequently used relation between the interband momentum and dipole matrix elements (shortened to the ``p-r relation'') in semiconductors acquires an additional correction term if applied to finite-volume crystals treated with periodic boundary conditions. The correction term, which is a generalization of the one obtained by Yafet [Phys. Rev. 106, 679 (1957)] for infinite crystals, does not vanish in the limit of infinite volume. We illustrate this with numerical examples for bulk GaAs and GaAs superlattices. The persistence of the correction term is traced to the subtle nature of the dipole matrix element with spatially extended wave functions. In contrast, a straightforward application of the findings by Blount [Solid State Phys. 13, 305 (1962)] and Haug [Theoretical Solid State Physics (Pergamon, Oxford, 1972)] yields the usual p-r relation in the distribution sense, without any corrections, when Bloch wave functions normalized to delta functions in crystal momentum space are used. Our findings therefore show that, for the interband dipole matrix element, using Bloch wave functions under periodic boundary conditions is not the proper way to approach the infinite-volume limit. From our numerical evaluations, we find that the correction term is large in the case of interband transitions in bulk GaAs, and that it can be chosen to be small in the case of intersubband transitions in superlattices, which are important in the context of terahertz (THz) radiation. We also show that one can interpret the infinite-volume p-r relation in terms of a limiting procedure using progressively broadened wave packet states that approach delta-normalized Bloch wave functions. Finally, we discuss the p-r relation for nanostructures in the envelope function approximation and show that the cell-envelope factorization of the nanostructure dipole matrix element into a cell-matrix element and an envelope overlap integral involves the cell gradient-$k$ rather than the cell dipole matrix element.

On the use of modification functions when Fourier transforming total scattering data

When Fourier transforming radiation total scattering data to the pair distribution function it is common to use a `modification' function to help reduce the termination ripples that would otherwise occur as a result of the finite range and counting statistics of the scattering data. One of the most common functions employed was proposed by Lorch [J. Phys. C Solid State Phys.(1969),2, 229–237]. In a recent article [Soper & Barney (2011).J. Appl. Cryst.44, 714–726] a revised version of this function was proposed. Here the effectiveness of these two functions at removing spurious structure from Fourier transformed data is compared. It is found that the two functions produce equivalent results, unless the broadening is allowed to increase withr, in which case the revised Lorch function is better at suppressing spurious oscillations. The presence of counting statistics produces a marked increase in the amplitude of the truncation oscillations.

Anisotropic generalization of Stinchcombe's solution for the conductivity of random resistor networks on a Bethe lattice

Our study is based on the work of Stinchcombe (1974 J. Phys. C: Solid State Phys. 7 179) and is devoted to the calculations of average conductivity of random resistor networks placed on an anisotropic Bethe lattice. The structure of the Bethe lattice is assumed to represent the normal directions of the regular lattice. We calculate the anisotropic conductivity as an expansion in powers of the inverse coordination number of the Bethe lattice. The expansion terms retained deliver an accurate approximation of the conductivity at resistor concentrations above the percolation threshold. We make a comparison of our analytical results with those of Bernasconi (1974 Phys. Rev. B 9 4575) for the regular lattice.

Photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal at room temperature

Excitation photon energy and carrier density dependence of spin dynamics in bulk CdTe crystal was studied by time resolved pump-probe reflectivity technique at room temperature. The results show that spin relaxation time decreases monotonously. While with increasing excitation carrier density, the time constants increases initially then decreases after reaching a maximum value. Our experimental results reveal that both D’yakonov–Perel’ [M. I. D’yakonov and V. I. Perel’, Sov. Phys. JETP 38, 177 (1974)] and Elliot–Yafet [R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, Solid State Phys. 14, 1 (1963)] mechanisms dominate the spin relaxation process in CdTe crystal.

On the operator equivalents and the crystal-field and spin Hamiltonian parameters

Operator equivalents used in magnetic resonance (electron paramagnetic resonance [EPR], electron-nuclear double resonance, nuclear magnetic resonance, etc.) are considered. The known non-Hermitian tensor operators O ±q (k) (0 ≤ q ≤ k) defined by D. Smith and J. H. M. Thornley (Proc. Phys. Soc. 89, 779–781, 1966) after H. A. Buckmaster (Can. J. Phys. 40, 1670–1677, 1962) are expressed in terms of Hermitian cosine and sine operators O kq c and O kq s . These new normalized operators are related to the nonnormalized extended Stevens operators O k q and O k −q ≡ Ω k q defined by C. Rudowicz (J. Phys. C.: Solid State Phys. 18, 1415–1430, 1985), which are commonly used in EPR of transition ions. The relations between O kq c,s and some other normalized operators occurring in the literature are also given. New crystal-field or spin Hamiltonian parameters B kq c and B kq s associated with the corresponding operators O kq c and O kq s are discussed.

Study of phase stability in a class of binary alloys using augmented space recursion based orbital peeling technique

In this communication we have developed a recursion based approach to the study of phase stability and transition of binary alloys. We have combined the recursion method introduced by Haydock et al. [J. Phys. C: Solid State Phys. 5 (1972) 2845] and the our augmented space approach [A. Mookerjee, J. Phys. C: Solid State Phys. 6 (1973) 1340] with the orbital peeling technique proposed by Burke [Surf. Sci. 58 (1976) 349] to determine the small energy differences involved in the discussion of phase stability. We have studied three alloy systems: one which segregates and the other two orders at lower temperatures.

Hopping conduction behavior with the Coulomb effects in colossal magnetoresistance materials

AbstractWe report on the temperature dependence of resistivity in La0.7–xPrxPb0.3MnO3 (x =0, 0.5, 0.7) bulk samples in order to explore the hopping transport behavior with the Coulomb effects for the colossal magnetoresistance materials. The experimental observations in La0.7–xPrxPb0.3MnO3 (x =0.5, 0.7) bulk samples reveal the exponential temperature dependence of resistivity ϱ (T) ∞ exp [(T1/T)n ] with n = 1/2, when system was the paramagnetic (PM) insulator at the high temperature range (T>Tp). Evidence for the hopping conduction with Coulomb effects is presented By using the approximate method of the VRH conduction in the localization regime base on Efros and Shklovskii model [A. L. Efros and B. I. Shklovskii, J. Phys. C, Solid State Phys. 8, L49 (1975)], the localization length of eg electrons can be estimated with the relationship, ξ=2.8e2/(4πε0κkBT1). The reasonable results of localization length can be evaluated to explain the possibility of the presence of the Coulomb interactions in these CMR materials. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Simulation of dislocation density: Global modeling of bulk crystal growth by a quasi-steady approach of the Alexander–Haasen concept

A global model for the simulation of dislocation density in the bulk growth of crystals is presented. The model is two-dimensional and quasi-steady; it is based on the original concept of Alexander and Haasen [Solid State Phys. 22 (1968) 27]. The model is validated by experimental results obtained from various growth techniques (LEC, VCz and VGF) with GaAs and InP crystals ranging up to diameters of 6″.

Structure and microstructure of combustion synthesized MgO nanoparticles and nanocrystalline MgO thin films synthesized by solution growth route

In this work we describe the synthesis, micro structure (XRD, SEM, AFM) of magnesium oxide nanoparticles and magnesium oxide thin films synthesized by urea-based combustion method and solution growth route using magnesium nitrate as the source of Mg. We used fuel-to-oxidizer ratio (Ψ) as a control parameter to investigate how lattice parameter, particle size, and micro strain vary with Ψ = 0.25–2 in the steps of 0.25. Earlier we have studied NiO as a substitutional solute in MgO (Rao KV, Sunandana (2005) Solid State Phys 50:235). The average crystalline size of MgO was estimated from the full width half maximum (Gaussian and lorentzian fits) of the X-ray diffraction peaks using Sherrer’s formula and Williamson–Hall plot. The particle size varies from 15(±0.3) nm to 60(±1.2) nm as Ψ is varied systematically. Surface areas of the MgO powders measured using BET method were used to calculate the particle size, which is comparable with the crystalline size calculated from XRD. We also calculated porosity and microstrain in the MgO nanoparticles with varying Ψ. Thin films of MgO are well characterized from XRD and AFM. The size of the particles and RMS roughness of the thin films were calculated using AFM.

The monoclinic polymorph ofcatena-poly[[diaquamanganese(II)]-μ-oxalato-κ4O1,O2:O1′,O2′

The title polymer, [Mn(C2O4)(H2O)2]n, was synthesized by the reaction of manganese(II) nitrate tetra­hydrate with a proton-transfer compound, (pipzH2)(ox) (where oxH2 is oxalic acid and pipz is piperazine), in aqueous solution. The structure features a six-coordinate Mn2+ ion located on a twofold rotation axis, with a distorted octa­hedral geometry [Mn—O = 2.1728 (8)–2.1914 (9) Å]. The bond angles show that the two coordinated water mol­ecules are arranged trans. One-dimensional linear chains are formed through bis-bidentate oxalate ligands bridging the MnII ions. These structural units are held together by O—H⋯O hydrogen bonds. A neutron diffraction study of [Mn(μ-ox)(D2O)2]n, using powdered samples, has been published recently in the same space group [Sledzinska, Murasik & Fischer (1987). J. Phys. Solid State Phys. 20, 2247–2259]. However, it should be noted that an ortho­rhom­bic polymorph for the title polymer is also known [Huizing, van Hal, Kwestroo, Langereis & van Loosdregt (1977). Mater. Res. Bull. 12, 605–611; Lethbridge, Congreve, Esslemont, Slawin & Lightfoot (2003). J. Solid State Chem. 172, 212–218]. The present single-crystal X-ray study deals with the monoclinic polymorph. This phase is isostructural with the CoII analogue.