First-principles Band Structure Calculations 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. Beside 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.

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.

ZrOsSi: a Z2 topological metal with a superconducting ground state

The silicide superconductors (Ta, Nb, Zr)OsSi are among the best candidate materials for investigating the interplay of topological order and superconductivity. Here, we investigate in detail the normal-state topological properties of (Ta, Nb, Zr)OsSi, focusing on ZrOsSi, by employing a combination of 29Si nuclear magnetic resonance (NMR) measurements and first-principles band-structure calculations. We show that, while (Ta, Nb)OsSi behave as almost ideal metals, characterized by weak electronic correlations and a relatively low density of states, the replacement of Ta (or Nb) with Zr expands the crystal lattice and shifts ZrOsSi towards an insulator. Our ab initio calculations indicate that ZrOsSi is a Z2 topological metal with clear surface Dirac cones and properties similar to a doped strong topological insulator.

Synthesis, crystal and energy structure of the Ag8SnS6 crystal

The Ag8SnS6 crystal was synthesized by directly melting a high-purity stoichiometric mixture of elementary Ag, Sn, and S in a sealed quartz ampoule. This argyrodite crystallizes in the orthorhombic structure (Pna21 space group (No. 33)) at room temperature. The theoretical first-principle calculations of the electronic band structure and density of states of a αʹʹ-Ag8SnS6 crystal are estimated by the generalized gradient approximation (GGA) and local density approximation (LDA). A Perdew–Burke–Ernzerhof functional (PBE) and (PBEsol) were utilized for GGA calculation. All calculated parameters correlate well with known experimental data. Based on the electronic band structure, the effective mass of electrons and holes was calculated. The anisotropic behavior of electronic band structure is discussed.

Photoluminescence and electronic structure of α'-Ag8GeS6 crystals

The Ag8GeS6 crystal was synthesized by the direct melting of the elementary Ag, Ge, and S high-purity grade stoichiometric mixture in a sealed quartz ampoule. This argyrodite crystallizes in orthorhombic structure [Pna21 space group (No. 33)] below ≈483 K. The theoretical first-principle calculations of the electronic band structure and density of states of an α'-Ag8GeS6 crystal are estimated by the generalized gradient approximation. The Perdew–Burke–Ernzerhof functional was utilized. All of the calculated parameters correlate well with the known experimental data. Based on the electronic band structure, the effective mass of the electrons and holes was calculated. The anisotropic behavior of electronic band structure is discussed. Temperature and carrier concentration dependence of the Seebeck coefficient is calculated. The temperature behavior of α'-Ag8GeS6 photoluminescence spectra is presented. The emission spectra are studied in the temperature range between 17 and 130 K and laser power excitation between 4 and 512 mW and in the spectral range of 750–1650 nm. Two main emission bands at ≈1.35 and ≈1.57 eV are observed and discussed.

Spin-polarized two-dimensional electron/hole gas at the interface of nonmagnetic semiconducting half-Heusler compounds: Modified Slater-Pauling rule for half-metallicity at the interface

Half-Heusler compounds with 18 valence electrons per unit cell are well-known nonmagnetic semiconductors. Employing first-principles electronic band-structure calculations, we study the interface properties of the half-Heusler heterojunctions based on FeVSb, CoTiSb, CoVSn, and NiTiSn compounds, which belong to this category of materials. Our results show that several of these heterojunction interfaces become not only metallic but also magnetic. The emergence of spin-polarization is accompanied by the formation of two-dimensional electron gas or hole gas at the interface. A qualitative analysis of the origin of the spin polarization at the interfaces suggests that strong correlations are responsible. For the cases of magnetic interfaces where half-metallicity is also present, we propose a modified Slater-Pauling rule similar to the one for bulk half-metallic half-Heusler compounds. Additionally, we calculate exchange parameters, Curie temperatures, and magnetic anisotropy energies for magnetic interfaces. Our study, along with recent experimental evidence confirming the presence of two-dimensional electron gas at CoTiSb/NiTiSn heterojunctions, may motivate future efforts to explore and realize device applications using these heterojunctions.

Influence of metal atom substitution on the electronic and optical properties of solid-state Cd0.75X0.25Te (X= Cu, Ag and Au) solutions

The solid-state Cd0.75X0.25Te (X= Cu, Ag, and Au) solutions crystallize in the cubic structure and are studied in the framework of density functional theory. The theoretical first-principle calculations of the electronic band structure, density of states, and refractive index of solid-state Cd0.75X0.25Te (X= Cu, Ag, and Au) solutions are estimated by the generalized gradient approximation (GGA). A Perdew–Burke–Ernzerhof functional (PBE) was utilized. Formation energy is calculated based on the results of the total energy of the study samples. The effective mass of the electrons and holes was calculated based on the electronic band structure. The influence of atom substitution on electron conductivity and mobility is discussed. To study the optical properties was use a complex dielectric function ε(ħω). The spectral behaviour of the refractive index was calculated based on the dielectric function.

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.

Charge order with unusual star-of-David lattice in monolayer NbTe2

Interplay between fermiology and electron correlation is crucial for realizing exotic quantum phases. Transition-metal dichalcogenide (TMD) 1T-TaS2 has sparked a tremendous attention owing to its unique Mott-insulating phase coexisting with the charge-density wave (CDW). However, how the fermiology and electron correlation are associated with such properties has yet to be claried. Here we demonstrate that monolayer 1T-NbTe2 is a new class of two-dimensional TMD which has the star-of-David lattice similarly to bulk TaS2 and isostructural monolayer NbSe2, but exhibits a metallic ground state with an unusual lattice periodicity root19xroot19 characterized by the sparsely occupied star-of-David lattice. By using angle-resolved photoemission and scanning-tunneling spectroscopies in combination with first-principles band-structure calculations, we found that the hidden Fermi-surface nesting and associated CDW formation are a primary cause to realize this unique correlated metallic state with no signature of Mott gap. The present result points to a vital role of underlying fermiology to characterize the Mott phase of TMDs.

Prediction of ferromagnetic Weyl semimetal and antiferromagnetic topological insulator phases in MnHg2Te3

Based on first-principles band-structure calculations, we predict that FM MnHg2Te3 is a Weyl semimetal candidate. When the direction of spin polarization is toward the c-axis, there are six Weyl points in the whole Brillouin zone. With spin orientation along the a-axis, there exist eight Weyl points. For AFM MnHg2Te3, when the spin direction is along the c-axis, the band structure is fully gapped. The calculation of the Z2 number confirms that AFM-c MnHg2Te3 is a 3D AFM topological insulator. Adjusting the spin direction from the c-axis to the a-axis only changes the bandgap without affecting the topological properties of this system. The gapless surface-state on the (100) surface is also obtained, the results of which correspond with the properties of the AFM topological insulator.

Dirac semimetal phase and switching of band inversion in XMg2Bi2 (X\u2009=\u2009Ba and Sr)

Topological Dirac semimetals (TDSs) offer an excellent opportunity to realize outstanding physical properties distinct from those of topological insulators. Since TDSs verified so far have their own problems such as high reactivity in the atmosphere and difficulty in controlling topological phases via chemical substitution, it is highly desirable to find a new material platform of TDSs. By angle-resolved photoemission spectroscopy combined with first-principles band-structure calculations, we show that ternary compound BaMg2Bi2 is a TDS with a simple Dirac-band crossing around the Brillouin-zone center protected by the C3 symmetry of crystal. We also found that isostructural SrMg2Bi2 is an ordinary insulator characterized by the absence of band inversion due to the reduction of spin–orbit coupling. Thus, XMg2Bi2 (X = Sr, Ba, etc.) serves as a useful platform to study the interplay among crystal symmetry, spin–orbit coupling, and topological phase transition around the TDS phase.

Large magnetoresistance of a compensated metal Cu2Sb correlated with its Fermi surface topology

We report electrical transport properties and electronic structure of a nonmagnetic metal ${\mathrm{Cu}}_{2}\mathrm{Sb}$ single crystal. ${\mathrm{Cu}}_{2}\mathrm{Sb}$ was found to be a compensated metal with high carrier density $\ensuremath{\sim}{10}^{22}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$ and high carrier mobility $\ensuremath{\ge}{10}^{3}\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ for both electron and hole carriers. The current-in-plane magnetoresistance at 2 K and 9 T was 730%, while the current-perpendicular-to-plane magnetoresistance at 2 K and 9 T was 2700% without the saturation. Angle-resolved photoemission spectroscopy throughout the three-dimensional (3D) bulk Brillouin zone signified a quasi-two-dimensional (2D) electron pocket axially centered along the M-A line and a 3D hole pocket at the \ensuremath{\Gamma} point, in accordance with the electron-hole compensated nature. The presence of quasi-2D open Fermi surface, in line with the first-principles band-structure calculations, is likely responsible for the observed nonsaturating current-in-plane magnetoresistance. The present result lays the foundation for realizing large magnetoresistance via Fermiology engineering in compensated metals.

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.

Excited states of a phosphorus pair in silicon: Combining valley-orbital interaction and electron-electron interactions

Excitations of impurity complexes in semiconductors can not only provide a route to fill the terahertz gap in optical technologies, but can also connect local quantum bits to scale up solid-state quantum-computing devices. However, taking into account both the interactions among electrons/holes, and the host band structures, is challenging. Here we combine first-principles band-structure calculations with quantum-chemistry methodology to evaluate the ground and excited states of a pair of phosphorous donors in silicon within s single framework. We use a broken-symmetry Hartree-Fock approach, followed by a time-dependent Hartree-Fock method to compute the excited states. Our Hamiltonian for each valley includes an anisotropic kinetic energy term, which splits the 2p_0 and 2p_+- transitions of isolated donors by ~4 meV, in good agreement with experiments. Our single-valley calculations show the optical response is a strong function of the optical polarisation, and suggest the use of valley polarisation to control optics and reduce oscillations in exchange interactions. When taking into account all valleys, including valley-orbital interactions, we find a gap opens between the 1s to 2p transition and low-energy charge-transfer states within 1s manifolds (which become optically allowed because of inter-donor interactions). In contrast to the single-valley case, we find charge-transfer excited states also in the triplet sector, thanks to the valley degrees of freedom. These states have a qualitatively correct energy as compared with the previous experiments; additionally, we predict new excitations below 20 meV that have not been analysed previously. A statistical average of nearest-neighbour pairs at different separations suggests that THz radiation could be used to excite pairs spin-selectively. Our approach can readily be extended to other donors and to other semiconducting hosts.

Electronic states of multilayer VTe2 : Quasi-one-dimensional Fermi surface and implications for charge density waves

We have performed angle-resolved photoemission spectroscopy on epitaxial $\mathrm{V}{\mathrm{Te}}_{2}$ films to elucidate the relationship between the fermiology and charge density waves (CDWs). We found that a two-dimensional triangular pocket in 1 monolayer (ML) $\mathrm{V}{\mathrm{Te}}_{2}$ is converted to a strongly warped quasi-one-dimensional (1D) Fermi surface in the 6 ML counterpart, likely associated with the $1T$-to-$1{T}^{\ensuremath{'}}$ structural phase transition. We also revealed a metallic Fermi edge on the entire Fermi surface in 6 ML at low temperature distinct from anisotropic pseudogap in 1 ML, signifying a contrast behavior of CDW that is also supported by first-principles band-structure calculations. The present result points to the importance of simultaneously controlling the structural phase and fermiology to manipulate the CDW properties in ultrathin transition-metal dichalcogenides.

Pressure-induced band anticrossing in two adamantine ordered-vacancy compounds: CdGa2S4 and HgGa2S4

This paper reports a joint experimental and theoretical study of the electronic band structure of two ordered-vacancy compounds with defect-chalcopyrite structure: CdGa2S4 and HgGa2S4. High-pressure optical-absorption experiments (up to around 17 GPa) combined with first-principles electronic band-structure calculations provide compelling evidence of strong nonlinear pressure dependence of the bandgap in both compounds. The nonlinear pressure dependence is well accounted for by the band anticrossing model that was previously established mostly for selenides with defect chalcopyrite structure. Therefore, our results on two sulfides with defect chalcopyrite structure under compression provide definitive evidence that the nonlinear pressure dependence of the direct bandgap is a common feature of adamantine ordered-vacancy compounds and does not depend on the type of anion.

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 $\mathrm{Zr}{\mathrm{Te}}_{5}$.