2D Electronic Structure Research Articles

Structure and thermodynamics of nonaqueous solvation by integral equation theory

Electronic structure theory under the influence of apolar solvents suffers from substantial methodical difficulties since in this case the solvent-induced solute polarization originates mainly from specific directional interactions and higher electric multipoles. Continuum solvation models based on the dielectric solvent response such as the PCM approach ignore such interactions and can therefore not adequately model solvation effects in nonaqueous environments. The “embedded cluster reference interaction site model” (EC-RISM) [1] retains the granularity of the solvent and represents a microscopically more detailed and therefore improved approach towards solvation modeling. EC-RISM is based on a self-consistent solution of solvent distribution functions described by a 3D integral equation theory and solute electronic structure by mapping the solvent charge distribution onto discrete, solute-embedding point charges. In aqueous solution EC-RISM theory is capable of calculating pKa shifts [1] and tautomer ratios relatively fast and with high accuracy [2]. Here we outline the strength of the integral equation model by studying benzene and hexafluorobenzene solutions. In particular, the thermodynamics of differential solvation is quantified for organic compounds dissolved in these media. Moreover, it is shown that the respective solvent structures around particular solutes differ strongly, possibly leading to changes in the thermodynamic stability scale of various isomers which are not reproduced by the PCM model.

Electrical resistivity, Curie temperature and band structure of Ho(Fe1−xCox)2 compounds

Electrical resistivities for Ho(Fe1−xCox)2 C15 Laves phases were measured in a wide temperature region (13–1000 K) and the residual, phonon and magnetic resistivities contributing to them were separated. Moreover, the parameters characterizing resistivity dependence on temperature and composition, including the Debye temperature, were determined. The differential of the magnetic part of electrical resistivity against temperature was used to estimate Curie temperatures. The Curie temperature changes approximately parabolically, increases with x, reaches a maximum at x = 0.3 and then strongly decreases for higher x values. Some results of 3d and 4s electronic band structure, calculated using the full-potential linearized augmented plane wave (FLAPW) method, are presented. A distribution function for the densities of states is introduced, and a way of defining 3d and 4s band half-widths as well as band splitting energy is proposed. The calculated 3d and 4s band splitting energies for Fe, Co and stoichiometrically weighted transition metal M are presented. A simple statistical-type formula describing the Curie temperature of Ho(Fe1−xCox)2 intermetallics related to FLAPW results is proposed.

Optical and structural studies of a two-dimensional organic Mott insulator dimethylphenazine-tetrafluorotetracyanoquinodimethane

Electronic structure of an organic charge-transfer complex, M${}_{2}$P-TCNQF${}_{4}$ (M${}_{2}$P: 5,10-dihydro-5,10-dimethylphenazine; TCNQF${}_{4}$: 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane), was investigated by means of optical reflection spectroscopy and x-ray structural analyses. This is an ionic compound and has a unique quasi-two-dimensional (2D) crystal structure in which donor ($D$) molecules of M${}_{2}$P and acceptor ($A$) molecules of TCNQF${}_{4}$ stack respectively along the [100] direction, and $D$ and $A$ molecules also stack alternately along the [111] direction. We evaluated the transfer energy $t$($AA$) between the neighboring $A$ molecules, $t$($DD$) between the neighboring $D$ molecules along the [100] direction, and $t$($DA$) between the neighboring $D$ and $A$ molecules along the [111] direction to be 43 meV, 29 meV, and 67 meV at room temperature, respectively. This demonstrates that an anisotropic 2D electronic structure is formed. By comparing the spectra of the imaginary part of the dielectric constant ${\ensuremath{\varepsilon}}_{2}$ obtained from the polarized reflectivity spectra with the absorption spectrum of K-TCNQF${}_{4}$ and the ${\ensuremath{\varepsilon}}_{2}$ spectra of M${}_{2}$P-PF${}_{6}$, which are composed of 1D $A$ stacks and $D$ stacks, respectively, it was revealed that in M${}_{2}$P-TCNQF${}_{4}$ the optical gap corresponds to the Mott gap transition from ${A}^{\ensuremath{-}}$ to ${A}^{\ensuremath{-}}$, and the ${D}^{+}$ to ${D}^{+}$ transition and the ${A}^{\ensuremath{-}}$ to ${D}^{+}$ transition or equivalently the charge-transfer (CT) transition located at the higher energies. Below the structural and magnetic phase transition temperature ${T}_{c}$ \ensuremath{\sim} 122 K, spin-singlet states are formed via displacements of both $D$ and $A$ molecules. The pattern of molecular displacements was found to be very unique, indicating that the phase transition cannot be attributed to a spin-Peierls-like mechanism. The nature of the phase transition is discussed from the temperature dependences of molecular displacements as well as of the Mott-gap transition and the CT transition.

Reliability and Simulation of Lead-Free Solder Joint Behavior in 3D Packaging Structure

This paper deals with behavior of lead-free solder joint applied in 3D electronic packaging structure. There are plenty of factors which influences reliability and life-time of solder compound connection. Type of substrates, selection of materials and process arrangement are the main areas to investigate. The content consists from three main parts. The first deals with introduction of designed test pattern, material, and process arrangement and applied testing methods. Second part continues with ANSYS software simulation of solder interconnections during thermo-mechanical stress tests and third part is aimed to experimental evaluation of results in the comparison to simulated results.

Observation of a ubiquitous three-dimensional superconducting gap function in optimally doped Ba0.6K0.4Fe2As2

The iron-pnictide superconductors have a layered structureformed by stacks of FeAs planes from which the superconductivity originates. Given the multiband and quasi three-dimensional \cite{3D_SC} (3D) electronic structure of these high-temperature superconductors, knowledge of the quasi-3D superconducting (SC) gap is essential for understanding the superconducting mechanism. By using the \KZ-capability of angle-resolved photoemission, we completely determined the SC gap on all five Fermi surfaces (FSs) in three dimensions on \BKFAOP samples. We found a marked \KZ dispersion of the SC gap, which can derive only from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intralayer $\Delta_1$ and interlayer $\Delta_2$ pairing. The anisotropy ratio $\Delta_2/\Delta_1$, determined from the gap function, is close to the c-axis anisotropy ratio of the magnetic exchange coupling $J_c/J_{ab}$ in the parent compound \cite{NeutronParent}. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. A suitable candidate could arise from short-range antiferromagnetic fluctuations.

Altered Reactivity and the Emergence of Ionic Metal Ordered Structures in Li-Cs at High Pressures

We show how pressure fundamentally alters the repulsive nature of the nonreactive Li-Cs mixture, converting it from strongly phase separating at ambient pressure to strongly long-range ordering at high pressures. The ordered phases found via a global space group optimization within the density-functional theory are Li7Cs in the Cmmm structure, LiCs in the B2 structure, and Li7Cs in the C2/m structure. These structures are remarkably stabilized by a pressure-induced increase in charge transfer from Cs to Li unit, an unusual effect concerning two elements from the same group (isovalent). These high-pressure phases exhibit interesting behaviors: (i)LiCs (B2) has its Cs(5p) core states nearly merged with the valence Cs(6s) states, indicating that core states can become valence states at high pressures; (ii)Li7Cs (Cmmm) structure exhibits an interesting 1D electronic structure within a 3D crystal structure.

A Fast Parallel Algorithm for Selected Inversion of Structured Sparse Matrices with Application to 2D Electronic Structure Calculations

An efficient parallel algorithm is presented for computing selected components of $A^{-1}$ where $A$ is a structured symmetric sparse matrix. Calculations of this type are useful for several applications, including electronic structure analysis of materials in which the diagonal elements of the Green's functions are needed. The algorithm proposed here is a direct method based on a block $LDL^T$ factorization. The selected elements of $A^{-1}$ we compute lie in the nonzero positions of $L+L^T$. We use the elimination tree associated with the block $LDL^T$ factorization to organize the parallel algorithm, and reduce the synchronization overhead by passing the data level by level along this tree using the technique of local buffers and relative indices. We demonstrate the efficiency of our parallel implementation by applying it to a discretized two dimensional Hamiltonian matrix. We analyze the performance of the parallel algorithm by examining its load balance and communication overhead, and show that our parallel implementation exhibits an excellent weak scaling on a large-scale high performance distributed-memory parallel machine.

Three-Dimensional Electronic Structure of Superconducting Iron Pnictides Observed by Angle-Resolved Photoemission Spectroscopy

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe 2- x Co x As 2 (Ba122) ( x = 0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating a three-dimensional (3D) electronic structure. This observation suggests that antiferromagnetism and superconductivity in the pnictides have to be described in terms of an orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.

Transient electronic structure of the photoinduced phase ofPr0.7Ca0.3MnO3probed with soft x-ray pulses

We use time-resolved x-ray absorption near-edge structure spectroscopy to investigate the electronic dynamics associated with the photoinduced insulator-to-metal phase transition in the colossal magnetoresistive manganite Pr{sub 0.7}Ca{sub 0.3}MnO{sub 3}. Absorption changes at the O K and Mn L edges directly monitor the evolution of the density of unoccupied states in the transient photoinduced phase. We show that the electronic structure of the photoinduced phase is remarkably similar to that of the ferromagnetic metallic phase reached in related manganites upon cooling below the Curie temperature.

Four-dimensional STEM-EELS: Enabling nano-scale chemical tomography

Advances in electron-based instrumentation have enabled the acquisition of multidimensional data sets for exploring the unique structure–property relationship of nanomaterials. In this manuscript, we report a technique for directly probing and analyzing the three-dimensional (3D) electronic structure of a material at the nano-scale. This technique, referred to here as 4D STEM-EELS, utilizes a rotation holder and pillar-shaped samples to allow STEM mode high-angle annular dark-field (HAADF) and EELS spectrum images to be recorded over a complete 180° rotation to minimize artifacts. The end result is a four-dimensional data set, containing two spatial dimensions, rotation angle and energy-loss information I( x, y, θ, Δ E), which can then be processed to extract any EELS signal as a rotation or “tilt-series” map. If the extracted properties satisfy the linear projection criteria, these maps can then be used for tomographic reconstruction to yield volumetric maps of the corresponding properties. Hence by combining STEM HAADF and energy-loss information from such a series of spectrum images, it is possible to map not only the microstructure, but also the elemental, physical and chemical state information of a material in three dimensions. Two examples are reported here to demonstrate the potential of this technique. To illustrate chemical tomography, 4D STEM-EELS was used to directly probe the 3D electronic structure of a W-to-Si contact from a semiconductor device. Core-loss data were used to reconstruct and render the composition of the W-to-Si contact in three dimensions. The fine structure of the 99 eV Si edge was analyzed with MLLS fitting to map the variations in Si bonding in 3D. To illustrate the direct probing of intrinsic material anisotropy, 4D STEM-EELS was used to probe a ZnO thin film. Subtle but systematic changes in low-loss structure were observed as a function of electron-beam orientation with respect to the ZnO crystallographic axes. Together these examples illustrate how the 4D STEM-EELS technique reported here can be used to probe the elemental, physical and chemical state information of a material in three dimensions and extend our knowledge of nano-scale structures.

Multigrid Accelerated Tensor Approximation of Function Related Multidimensional Arrays

In this paper, we describe and analyze a novel tensor approximation method for discretized multidimensional functions and operators in $\mathbb{R}^d$, based on the idea of multigrid acceleration. The approach stands on successive reiterations of the orthogonal Tucker tensor approximation on a sequence of nested refined grids. On the one hand, it provides a good initial guess for the nonlinear iterations to find the approximating subspaces on finer grids; on the other hand, it allows us to transfer from the coarse-to-fine grids the important data structure information on the location of the so-called most important fibers in directional unfolding matrices. The method indicates linear complexity with respect to the size of data representing the input tensor. In particular, if the target tensor is given by using the rank-$R$ canonical model, then our approximation method is proved to have linear scaling in the univariate grid size $n$ and in the input rank $R$. The method is tested by three-dimensional (3D) electronic structure calculations. For the multigrid accelerated low Tucker-rank approximation of the all electron densities having strong nuclear cusps, we obtain high resolution of their 3D convolution product with the Newton potential. The accuracy of order $10^{-6}$ in max-norm is achieved on large $n\times n\times n$ grids up to $n=1.6\cdot10^4$, with the time scale in several minutes.

Carrier dynamics of straininduced quantum dots and THz radiation‐induced exciton cooling

AbstractWe have modeled strain‐induced quantum dots and analyzed previous anomalous experimental observations of a cooling of radiative quantum dot (QD) excitons by THz radiation. We have made three‐dimensional (3D) modeling of the elastic strain and piezoelectric fields. The 3D electronic structure was simulated using multiband envelope function techniques and used as a basis for Monte‐Carlo simulations of the carrier dynamics.We have found that the THz radiation‐induced QD exciton cooling is due to an electron‐hole separation and a related THz radiation excitation of holes within the quantum dot. This phenomenon is maintained by an effective relaxation of holes into states which are well separated from the confined electrons. The relaxation of holes is attributed to acoustic phonon emission and, at high carrier densities, to Auger processes. The electron and hole populations of the quantum dot tend towards a highly polarized state that cannot be broken without external excitations of the holes (by e.g. THz radiation or a thermal bath). (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stereo-PEEM for three-dimensional atomic and electronic structures of microscopic materials

The principle and design of a new photoemission electron microscope (PEEM), which is called Stereo-PEEM, is described here. Stereo-PEEM can display not only the image of microscopic materials but also the angular distribution of high-energy photoelectrons up to about ±60°, which is about 100-fold the acceptance angle of usual PEEM. This wide angle acceptance for high-energy photoelectrons enables the three-dimensional (3D) display of atomic structure as well as the 3D electronic structure of individual micromaterials. The 3D atomic structure of a sample can be observed directly by taking stereophotographs using photons with angular momentum.

Dual Character of the Electronic Structure ofYBa2Cu4O8: The Conduction Bands ofCuO2Planes and CuO Chains

We use microprobe angle-resolved photoemission spectroscopy (microARPES) to separately investigate the electronic properties of CuO2 planes and CuO chains in the high temperature superconductor, YBa2Cu4O8. For the CuO2 planes, a two-dimensional (2D) electronic structure is observed and, in contrast to Bi2Sr2CaCu2O8+delta, the bilayer splitting is almost isotropic and 50% larger, which strongly suggests that bilayer splitting has no direct effect on the superconducting properties. In addition, the scattering rate for the bonding band is about 1.5 times stronger than the antibonding band and is independent of momentum. For the CuO chains, the electronic structure is quasi-one-dimensional and consists of a conduction and insulating band. Finally, we find that the conduction electrons are well confined within the planes and chains with a nontrivial hybridization.

Ti 2p soft X-ray emission spectroscopy of Ti 2O 3

Ti 3d electronic structure of Ti 2O 3 has been investigated by means of soft X-ray photoemission spectroscopy (SXES) in the Ti 2p–3d absorption region. In the SXES spectra, the Raman peaks due to the a 1 g – e g π and a 1 g – e g σ excitations are observed at 1.3 and 3.3 eV below the incident photon energy, respectively. The intensity of the a 1 g – e g σ peak in the SXES spectrum measured between the 2p 3/2–3d and 2p 1/2–3d absorption peaks depends on the polarization of the incident photon E; the peak observed clearly in case of E parallel to the c-axis, almost disappears in case of E perpendicular to the c-axis.

Mn 3d state of [formula omitted] studied by Mn 2p–3d resonant photoemission spectroscopy

Pr 0.5 Ca 0.5 MnO 3− δ ( δ ∼ 0.05 ) exhibits a charge-order transition around 240 K. We have investigated the Mn 3d states of Pr 0.5 Ca 0.5 MnO 3− δ above and below the transition temperature by using valence-band photoemission spectroscopy in the Mn 2 p → 3 d photoabsorption region. We have examined the Mn 3d electronic structure of Pr 0.5 Ca 0.5 MnO 3− δ by comparing the photoemission spectra taken with photon energies well above the Mn 2p 1/2 absorption peak and below the Mn 2p absorption edge in order that the spectrum may not be influenced by particular photo-excited states at different temperatures. After the charge ordering is developed, the obtained Mn 3d spectrum clearly shows increase in intensity around 0.8 eV below the Fermi level, whereas the off-resonance photoemission spectrum shows decrease there. These indicate that the hybridization between the Mn 3d e g orbitals with oxygen 2p decreases and the 3d e g electron is localized on the Mn 3+ site when the charge ordering of Mn ions takes place and that there exists the polaron above the transition temperature.

Sr2TMO3 (TM = Ni, Co) Compounds with 1D TM–O Chains

Sr2TMO3 (TM = Ni,Co) compounds with 1D TM-O chains have been successfully synthesized in the form of single-crystalline thin films using a combinatorial approach. The crystal structure is examined using X-ray diffraction and high-resolution TEM (see figure). Polarized absorption spectroscopy and third-harmonic generation spectroscopy reveal a 1D electronic structure and sizable nonlinear optical susceptibility, respectively, for Sr2NiO3.

DNA Vibrational Coupling Revealed with Two-Dimensional Infrared Spectroscopy: Insight into Why Vibrational Spectroscopy Is Sensitive to DNA Structure

Two-dimensional infrared (2D IR) spectroscopy was used to study the carbonyl vibrational modes of guanine and cytosine bases in A- and B-form DNA. Located between 1600 and 1700 cm(-1), these modes are often used to monitor DNA secondary structure with traditional infrared spectroscopies such as FTIR, but traditional spectroscopies lack the necessary observables to unravel the coupling mechanisms that make these modes sensitive to secondary structure. By using 2D IR spectroscopy and electronic structure calculations on d(G(5)C(5)) and d(GC)(8) model nucleic acids, we find that hydrogen-bonded guanine/cytosine base pairs are primarily electrostatically coupled and that the coupling between these modes can be modeled with a transition dipole density approach. In comparison, electrostatics is insufficient to model stacked bases because of cooperative charge-sharing effects, but the coupling can be accurately calculated using a finite difference method. We find that the coupling is very strong for both hydrogen-bonded and stacked base geometries, creating vibrational modes that extend both across the base pairs and along the lengths of the helices. Our results provide a physical basis for understanding how strong coupling gives rise to the empirically established relationship between infrared spectroscopy and DNA/RNA secondary structure.

Mn 3p–3d resonant photoemission spectroscopy of ferromagnetic semiconductor Ge 1− xMn xTe

In order to investigate the Mn 3d electronic structure in IV–VI ferromagnetic semiconductor Ge 1− x Mn x Te, we have carried out the Mn 3p–3d resonant photoemission spectroscopy (RPES) and Mn 2p–3d soft X-ray emission spectroscopy (SXES). The Mn 3d partial density of states derived from the RPES spectra and the excitation-energy-dependent SXES spectra indicate that the Mn 3d states of Ge 1− x Mn x Te are nearly localized with the divalent character.

Mn 3d states in ferromagnetic semiconductor Ge 1− xMn xTe investigated by Mn 2p-3d soft X-ray magnetic circular dichroism spectroscopy

Mn 3d electronic structure in IV–VI ferromagnetic semiconductor Ge 1− x Mn x Te ( x = 0.32) has been investigated by means of the soft X-ray magnetic circular dichroism (XMCD) measurements of the Mn 2p-3d absorption spectra (XAS spectra). The XAS and XMCD spectra show a characteristic feature of the localized Mn 2+ 3d states. From the analyses of the spectra based on the configuration interaction theory, the parameters such as the p-d hybridization and p-d exchange constant N 0β are estimated.