First-principles Band Structure Research Articles

Enhancement of the thermoelectric figure of merit in the Dirac semimetal Cd3As2 by band-structure and -filling control

ABSTRACT Topological materials attract a considerable research interest because of their characteristic band structure giving rise to various new phenomena in quantum physics. Besides this, they are tempting from a functional materials point of view: Topological materials bear potential for an enhanced thermoelectric efficiency because they possess the required ingredients, such as intermediate carrier concentrations, large mobilities, heavy elements etc. Against this background, this work reports an enhanced thermoelectric performance of the topological Dirac semimetal Cd3As2 upon alloying the trivial semiconductor Zn3As2. This allows to gain fine-tuned control over both the band filling and the band topology in Cd3-x Zn x As2. As a result, the thermoelectric figure of merit exceeds 0.5 around x = 0.6 and x = 1.2 at elevated temperatures. The former is due to an enhancement of the power factor, while the latter is a consequence of a strong suppression of the thermal conductivity. In addition, in terms of first-principle band structure calculations, the thermopower in this system is theoretically evaluated, which suggests that the topological aspects of the band structure change when traversing x = 1.2 .

Unraveling the origin of Kondo-like behavior in the 3d-electron heavy-fermion compound YFe2Ge2

The heavy fermion (HF) state of [Formula: see text]-electron systems is of great current interest since it exhibits various exotic phases and phenomena that are reminiscent of the Kondo effect in [Formula: see text]-electron HF systems. Here, we present a combined infrared spectroscopy and first-principles band structure calculation study of the [Formula: see text]-electron HF compound YFe[Formula: see text]Ge[Formula: see text]. The infrared response exhibits several charge-dynamical hallmarks of HF and a corresponding scaling behavior that resemble those of the [Formula: see text]-electron HF systems. In particular, the low-temperature spectra reveal a dramatic narrowing of the Drude response along with the appearance of a hybridization gap ([Formula: see text] 50 meV) and a strongly enhanced quasiparticle effective mass. Moreover, the temperature dependence of the infrared response indicates a crossover around [Formula: see text] 100 K from a coherent state at low temperature to a quasi-incoherent one at high temperature. Despite of these striking similarities, our band structure calculations suggest that the mechanism underlying the HF behavior in YFe[Formula: see text]Ge[Formula: see text] is distinct from the Kondo scenario of the [Formula: see text]-electron HF compounds and even from that of the [Formula: see text]-electron iron-arsenide superconductor KFe[Formula: see text]As[Formula: see text]. For the latter, the HF state is driven by orbital-selective correlations due to a strong Hund's coupling. Instead, for YFe[Formula: see text]Ge[Formula: see text] the HF behavior originates from the band flatness near the Fermi level induced by the combined effects of kinetic frustration from a destructive interference between the direct Fe-Fe and indirect Fe-Ge-Fe hoppings, band hybridization involving Fe [Formula: see text] and Y [Formula: see text] electrons, and electron correlations. This highlights that rather different mechanisms can be at the heart of the HF state in [Formula: see text]-electron systems.

Understanding the Adiabatic Evolution of Surface States in Tetradymite Topological Insulators under Electrochemical Conditions.

Nontrivial surface states in topological materials have emerged as exciting targets for surface chemistry research. In particular, topological insulators have been used as electrodes in electrocatalytic reactions. Herein, we investigate the robustness of the topological surface states and band topology under electrochemical conditions, specifically in the presence of an electric double layer. First-principles band structure calculations are performed on the electrified (111) surfaces of Bi2Te3, Bi2Se3, and Sb2Te3 using an implicit electrolyte model. Our results demonstrate the adiabatic evolution of the surface states upon surface charging. Under oxidizing potentials, the surface states are shifted upward in energy, preserving the Dirac point on the surface and the band inversion in the bulk. Conversely, under reduced potentials, hybridization is observed between the surface and bulk states, suggesting a likely breakdown of topological protection. The position of the Fermi level, which dictates the working states in catalytic reactions, should ideally be confined within the bulk bandgap. This requirement defines a potential window for the effective application of topological electrocatalysis.

First-principles study on the electronic structure of n-type magnetic semiconductor Ba(Zn 1−x Co x )2As2

We perform systematic first-principles calculations on the electronic structure of n-type magnetic semiconductor Ba(Zn 1−x Co x )2As2 with the facilitation of HSE06 hybrid functional. Supercells are used to consider the doping of Co atoms, and the first-principles band structures are unfolded for clarity. Based on the calculation results, magnetic states are preferred by individual Co atoms doped in Ba(Zn 1−x Co x )2As2 at diluted limit, and carriers are originated mainly from situations where only one Co atom exists in the nearest neighbor Zn sites out of certain doped Co atoms. The origination of carriers can be explained by the density of states and the unfolded band structure, where it is found that the scattering effects from single Co atom is small but quite large when more Co atoms are located at adjacent Zn sites. The large scattering effects of two adjacent Co atoms will alter the band structures near the Fermi-level. Carriers in Ba(Zn 1−x Co x )2As2 mainly originate from the As-4p orbitals, with partial contributions from the Co-3d orbitals. Our work provides new insights into the origin of the n-type carriers in magnetic semiconductors and will inspire the development of new magnetic semiconducting systems.

Thermoelectric properties of M[formula omitted]AgAlBr[formula omitted] (M = K, Rb, Cs) double perovskites: A first principles study

Structural, electronic, and transport properties of aluminum-based double halide perovskite M2AgAlBr6 (M = K, Rb, Cs) are here subject of investigation, using first-principles approaches based on density functional theory, as implemented in the plane-wave pseudopotential method. Thermoelectric performance was evaluated by analyzing the results of first-principles band structure calculations and semiclassical Boltzmann theory within the constant relaxation time approximation. The theoretically predicted values for the electronic contribution for the figure of merit are close to 1.0 at room temperature, suggesting that all compounds studied are promising candidates for applications in thermoelectric devices.

Band Structure Evolution during Reversible Interconversion between Dirac and Standard Fermions in Organic Charge-Transfer Salts

Materials containing Dirac fermions (DFs) have been actively researched because they often alter electrical and magnetic properties in an unprecedented manner. Although many studies have suggested the transformation between standard fermions (SFs) and DFs, the non-availability of appropriate samples has prevented the observation of the transformation process. We observed the interconversion process of DFs and SFs using organic charge-transfer (CT) salts. The samples are unique in that the constituents (the donor D and acceptor A species) are particularly close to each other in energy, leading to the temperature- and D-A-combination-sensitive CT interactions in the solid states. The three-dimensional weak D–A CT interactions in low-symmetry crystals induced the continuous reshaping of flat-bottomed bands into Dirac cones with decreasing temperature; this is a characteristic shape of bands that converts the behavior of SFs into that of DFs. Based on the first-principles band structures supported by the observed electronic properties, round-apex-Dirac-cone-like features appear and disappear with temperature variation. These band-structure snapshots are expected to add further detailed understanding to the related research fields.

Optical properties of AgxCu1–xI alloy thin films

We report on the excitonic transition energy E0 and spin–orbit split-off energy Δ0 of γ-AgxCu1–xI alloy thin films studied by using reflectivity measurements at temperatures between 20 K and 290 K. The observed bowing behavior of the E0 transition as a function of the alloy composition is explained based on first-principles band structure calculations in terms of different physical and chemical contributions within the description of ordered alloys. The spin–orbit coupling is found to increase from a value of 640 meV for CuI to approximately 790 meV for AgI. Furthermore, we show that the temperature-dependent bandgap shift between 20 K and 290 K decreases with increasing Ag-content from 25 meV for CuI to 6 meV for AgI. We attribute this behavior mostly to changes in the contribution of thermal lattice expansion to the bandgap shift.

Electronic heat generation in semiconductors: Non-equilibrium excitation and evolution of zone-edge phonons via electron–phonon scattering in photo-excited germanium

We investigate experimentally and using first-principles theory the generation of phonons and the relaxation of carriers on picosecond timescales across the Brillouin zone of photo-excited Ge by inter-valley electron–phonon scattering. The phonons generated are typical of those generated in semiconductor devices, contributing to the accumulation of heat within the material. We simulate the time-evolution of phonon populations, based on first-principles band structure and electron–phonon and phonon–phonon matrix elements, and compare them to data from time-resolved x-ray diffuse scattering experiments, performed at the Linac Coherent Light Source x-ray free-electron laser facility, following photo-excitation by a 50 fs near-infrared optical pulse. We show that the intensity of the non-thermal x-ray diffuse scattering signal, which is observed to grow substantially near the L-point of the Brillouin zone over 3–5 ps, is due to phonons generated by scattering of carriers between the Δ and L valleys. These phonons have low group velocities, resulting in a heat bottleneck. With the inclusion of phonon decay through 3-phonon processes, the simulations also account for other non-thermal features observed in the x-ray diffuse scattering intensity, which are due to anharmonic phonon–phonon scattering of the phonons initially generated by electron–phonon scattering.

Topological pz-wave nodal-line superconductivity with flat surface bands in the AHxCr3As3 (A = Na, K, Rb, Cs) superconductors

We study the pairing symmetry and the topological properties of the hydrogen-doped ACr3As3 superconductors. Based on our first-principle band structure with spin-orbit coupling (SOC), we construct a tight-binding model including the on-site SOC terms, equipped with the multi-orbital Hubbard interactions. Then using the random-phase-approximation (RPA) approach, we calculate the pairing phase diagram of this model. Our RPA results yield the triplet p z -wave pairing in the spin channel to be the leading pairing symmetry all over the experiment relevant hydrogen-doping regimes. This pairing state belongs to the spin-U(1)-symmetry–protected time-reversal–invariant topological nodal-line superconductivity. Determined by the momentum-dependent topological invariant Z(k x , k y ), the whole (001) surface Brillouin zone is covered with topological flat bands with different regimes covered with different numbers of flat surface bands, which can be detected by the scanning tunneling microscope experiments.

Single-Component Molecular Conductors — Multi-Orbital Correlated π-d Electron Systems

Abstract Traditional molecular conductors are composed of more than two chemical species. Two prerequisites for the design of molecular metals have long been considered to be 1) forming of the electronic band and 2) existence of charge carriers created by the intermolecular charge transfer between the molecules constructing the band and other chemical species. On the other hand, a single-component molecular metal, [Ni(tmdt)2] (tmdt = trimethylenetetrathiafulvalenedithiolate), was developed in 2001; it is a planar nickel complex coordinated by the extended-TTF dithiolate ligands, tmdt from both sides. Since then, various types of single-component molecular conductors with a variety of extended-TTF dithiolate ligands have been developed. In this account, we briefly describe the recent progress in research on single-component molecular conductors. First, single-component molecular conductors in isostructural systems, [M(tmdt)2] (M = Ni, Pd, Pt, Au, and Cu) are described. Recent orbital-selective 13C and 1H NMR experiments have genealogically elucidated the differences in the electronic states and physical properties of these systems, that is, their various unusual phenomena are produced from their multi-orbital correlated π or π-d electron systems. Next, we describe [Ni(hfdt)2] (hfdt = bis(trifluoromethyl)tetrathiafulvalenedithiolate), the first single-component molecular superconductor, which was revealed by high-pressure resistivity measurements with a diamond anvil cell (DAC). The superconducting transition occurred around 7.5–8.7 GPa with a maximum Tc (onset temperature) of 5.5 K. Recent theoretical calculation has revealed that [Ni(hfdt)2] will be a new molecular Dirac electron system. In the final section, we briefly introduce molecular Dirac electron systems. Recently, a new series of semimetals, [M(dmdt)2] (M = Pt and Ni; dmdt = dimethyltetrathiafulvalenedithiolate) was synthesized. They belong to a three-dimensional ambient-pressure molecular massless Dirac electron system. The first-principles band structure calculations of [M(dmdt)2] (M = Pt and Ni) revealed that Dirac cones emerge along the a* direction and form Dirac nodal lines.

Spin\u2013orbit torque engineering in \u03b2-W/CoFeB heterostructures with W\u2013Ta or W\u2013V alloy layers between \u03b2-W and CoFeB

The spin–orbit torque (SOT) resulting from a spin current generated in a nonmagnetic transition metal layer offers a promising magnetization switching mechanism for spintronic devices. To fully exploit this mechanism, in practice, materials with high SOT efficiencies are indispensable. Moreover, new materials need to be compatible with semiconductor processing. This study introduces W–Ta and W–V alloy layers between nonmagnetic β-W and ferromagnetic CoFeB layers in β-W/CoFeB/MgO/Ta heterostructures. We carry out first-principles band structure calculations for W–Ta and W–V alloy structures to estimate the spin Hall conductivity. While the predicted spin Hall conductivity values of W–Ta alloys decrease monotonically from −0.82 × 103 S/cm for W100 at% as the Ta concentration increases, those of W–V alloys increase to −1.98 × 103 S/cm for W75V25 at% and then gradually decrease. Subsequently, we measure the spin Hall conductivities of both alloys. Experimentally, when β-W is alloyed with 20 at% V, the absolute value of the spin Hall conductivity considerably increases by 36% compared to that of the pristine β-W. We confirm that the W–V alloy also improves the SOT switching efficiency by approximately 40% compared to that of pristine β-W. This study demonstrates a new material that can act as a spin current-generating layer, leading to energy-efficient spintronic devices.

First-principles study of electron transport in ScN

We investigate the conduction-band structure and electron mobility in rocksalt ScN based on density functional theory. The first-principles band structure allows us to obtain band velocities and effective masses as a function of energy. Electron-phonon scattering is assessed by explicitly computing the $q$-dependent electron-phonon matrix elements, with the inclusion of the long-range electrostatic interaction. The influence of free-carrier screening on the electron transport is assessed using the random-phase approximation. We find a notable enhancement of electron mobility when the carrier concentration exceeds ${10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We calculate the room-temperature electron mobility in ScN to be 587 ${\mathrm{cm}}^{2}/\mathrm{Vs}$ at low carrier concentrations. When the carrier concentration is increased, the electron mobility starts to decrease significantly around $n={10}^{19}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and drops to 240 ${\mathrm{cm}}^{2}/\mathrm{Vs}$ at $n={10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. We also explore the influence of strain in (111)- and (100)-oriented ScN films. For (111) films, we find that a 1.0% compressive epitaxial strain increases the in-plane mobility by 72 ${\mathrm{cm}}^{2}/\mathrm{Vs}$ and the out-of-plane mobility by 50 ${\mathrm{cm}}^{2}/\mathrm{Vs}$. For (100) films, a 1.0% compressive epitaxial strain increases the out-of-plane mobility by as much as 172 ${\mathrm{cm}}^{2}/\mathrm{Vs}$, but has a weak impact on the in-plane mobility. Our study sheds light on electron transport in ScN at different electron concentrations and shows how strain engineering could increase the electron mobility.

First-principles calculations for topological quantum materials

Discoveries of topological states and topological materials reshape our understanding of physics and materials over the last 15 years. First-principles calculations have been playing a significant role in bridging the theory of topology and experiments by predicting realistic topological materials. In this article, we overview the first-principles methodology on topological quantum materials. First, we unify different concepts of topological states in the same band inversion scenario. Then, we discuss the topology using first-principles band structures and newly-established topological materials databases. We stress challenges in characterizing symmetry-independent Weyl semimetals and calculating topological surface states, closing with an outlook on the exciting transport and optical phenomena induced by the topology.

Expansive open Fermi arcs and connectivity changes induced by infrared phonons in ZrTe5

Expansive open Fermi arcs covering most of the surface Brillouin zone (SBZ) are desirable for detection and control of many topological phenomena, but they have generally been reported for Kramers-Weyl points, or unconventional chiral fermions, pinned at time-reversal invariant momentum in chiral materials. Here using first-principles band structure calculations, we show that for conventional Weyl points in $\mathrm{Zr}{\mathrm{Te}}_{5}$ with the chirality of +1/\ensuremath{-}1 near the BZ center at general momentum induced by one of the infrared phonons---the second lowest ${B}_{1u}$ mode for breaking inversion symmetry---they can also form expansive open Fermi arcs across the SBZ boundary to occupy most of the SBZ when projected on the (001) surface. We reveal that such expansive open Fermi arcs are evolved from the topological surface states that connect multiple surface Dirac points on the (001) surface of the topological insulator phases without lattice distortion in $\mathrm{Zr}{\mathrm{Te}}_{5}$. Furthermore, we find that the connectivity of the induced open Fermi arcs can be changed by the magnitude of the lattice distortion of this infrared phonon mode. Thus, we propose that using coherent optical phonons to modulate lattice parameters can offer ways to induce unique topological features including expansive open Fermi arcs and to dynamically control Fermi arc connectivity in $\mathrm{Zr}{\mathrm{Te}}_{5}$.

Thermoelectric and Piezoelectric Properties in Half-Heusler Compounds TaXSn (X = Co, Rh and Ir) Based on Ab Initio Calculations

In this paper, we report a theoretical study of the structural, electronic, thermoelectric and piezoelectric properties of TaXSn (X = Co, Rh and Ir) half-Heusler compounds crystallizing with cubic MgAgAs-type structure. We have made a quantitative evaluation of thermoelectric figure of merit (ZT) and the electromechanical coupling coefficient (k14) of these compounds. Accordingly, we intend to combine the first-principles band structure calculations using (DFT)-based FP-LAPW approach and the semi-classical Boltzmann transport theory within constant scattering time approximation (CSTA) to interpret and predict the thermoelectric performance (ZTe) without the lattice thermal conductivity as a function of the chemical potential at various temperatures. Further, to obtain a reasonable estimate for (ZT) with the intrinsic lattice thermal conductivity, we have calculated the relaxation time (τ) at various temperatures using Bardeen–Shockley theory. Finally, for predicting piezoelectric coefficients, we have employed the modern theory of polarization as provided by density-functional perturbation theory (DFPT) based on plane waves and pseudo-potentials (PP-PW). Our key result is that these half-Heusler semiconductors are attractive for practical applications in energy-harvesting technology, which has a high (ZT) and (k14) of 0.89 and 0.25, respectively, at room temperature.

Aberrant electronic and structural alterations in pressure tuned perovskite NaOsO3

The perovskite NaOsO3 has a metal–insulator transition at temperature 410 K, which is delicate, intriguing, and provokes a lot of debate on its nature. Our combined electrical resistance, Raman, and synchrotron x-ray diffraction experiments show that the insulating ground state in this osmate endures under high pressure up to at least 35 GPa. In this pressure range, compression reveals hidden hysteretic resistance properties with a transient metallic state near 200 K, manifested three electronic character anomalies (at 1.7, 9.0, and 25.5 GPa), and a structural transition to the singular polar phase (at ~18 GPa). We distinguish NaOsO3 from the regular crystallographic behavior of perovskites, though the electrical specificities resemble iridates and nickelates. The theoretical first-principle band structure and lattice dynamics calculations demonstrate that the magnetically itinerant Lifshitz-type mechanism with spin–orbit and spin–phonon interactions is responsible for these pressure-induced changes. Our findings provide another new playground for the emergence of new states in 5d materials by using high-pressure methods.

Evaluating the Thermoelectric Properties of BaTiS3 by Density Functional Theory.

BaTiS3 is a semiconductor with a small bandgap of ∼0.5 eV and strong transport anisotropy caused primarily by structural anisotropy; it contains well-separated octahedral columns along the [0001] direction and low lattice thermal conductivity, appealing for thermoelectric applications. Here, we evaluate the prospect of BaTiS3 as a thermoelectric material by using the linearized electron and phonon Boltzmann transport theory based on the first-principles density functional band structure calculations. We find sizable values of the key thermoelectric parameters, such as the maximum power factor PF = 928 μW K–2 and the maximum figure of merit ZT = 0.48 for an electron-doped sample and PF = 74 μW K–2 and ZT = 0.17 for a hole-doped sample at room temperature, and a small doping level of ±0.25e per unit cell. The increase in temperature yields an increase in both the power factor and the figure of merit, reaching large values of PF = 3078 μW K–2 and ZT = 0.77 for the electron-doped sample and PF = 650 μW K–2 and ZT = 0.62 for the hole-doped sample at 800 K. Our results elucidate the promise of BaTiS3 as a material for the thermoelectric power generator.

Study of C K-Edge High Energy Resolution Energy-Loss Near-Edge Structures of Copper Phthalocyanine and Its Chlorinated Molecular Crystals by First-Principles Band Structure Calculations.

High energy resolution energy-loss near-edge structures (ELNES) at the carbon K-edge of copper phthalocyanine (CuPc) and its chlorinated molecular crystals were observed using electron energy-loss spectroscopy combined with a scanning transmission electron microscope equipped with a monochromator. The ELNES spectra were investigated using first-principles band structure calculations with a core-hole introduced into the 1s orbitals of the nonequivalent C atoms. The calculated spectra including half a core-hole were consistent with the experimental spectra. The spectral features could be interpreted in terms of the different contributions of the partial density of states (PDOS) of nonequivalent C atoms with different transition energies depending on the site. The core-hole effects were also discussed using the spatial distribution of unoccupied states and PDOSs, which revealed site-dependent core-hole effects, where a C atom with a strong spatial distribution intensity of the unoccupied states in the ground state (GS) are susceptible to the core-hole effects. The spectral changes due to chlorination of the CuPc molecule were mainly attributed to an increase of the threshold energy of the C atoms bonded to chlorine, and the influence of the change in the PDOS by chlorination was not significantly large.

Role of Velocity Field and Principal Axis of Tilted Dirac Cones in Effective Hamiltonian of Non-Coplanar Nodal Loop

Nodal line in single-component molecular conductor [Pd(dddt)_2] has been examined to understand the tilted Dirac cone on the non-coplanar loop. In the previous work [J. Phys. Soc. Jpn. 87, 113701 (2018)], the velocity of the cone was calculated at respective Dirac points on the nodal loop based on our first-principles band structure calculations, which was a new method to derive an effective Hamiltonian with a 2 x 2 matrix. However, the Dirac cones on the nodal line are fully reproduced only at symmetric points. In the present paper, we show that our improved method well reproduces reasonable behaviors of all the Dirac cones and a very small energy dispersion of 6~meV among the Dirac points. The variation of velocities along the nodal line are shown by using principal axes of the gap function between the conduction and valence bands. Further, the density of states close to the chemical potential and orbital magnetic susceptibility are calculated using such an effective Hamiltonian.

Orbital Selectivity and magnetic ordering in Fe intercalated dirac semimetal Bi2Se3

In this paper we investigate the intercalation effects of Iron (Fe) in the van der Waals gap of Bi2Se3 on the magnetic and transport properties using first-principles band structure estimations combined with dynamical mean-field theory. The Dirac cone in the band structure of parent Bismuth Selenide is modified via Fe intercalation at moderate densities. Further inclusion of electronic correlations found to result in the emergence of novel and exotic properties in an intercalated Bi2Se3. Accompanied by unconventional structural effects, the onset of an orbital selective metal insulator transition in the Fe 3d orbitals brings about a magnetic phase transition in the Fe intercalated Bi2Se3. Additionally we have explored the dependency of the electron-electron correlations on the magnetic ordering and the effects of intercalation in establishing new physical properties.