Density Functional Band Structure Calculations Research Articles

Observation of infrared interband luminescence in magnesium by femtosecond spectroscopy

Ultrafast luminescence in Mg was investigated in the infrared region, between 0.35 and 1.05 eV, and compared with the results for Al, using a luminescence upconversion technique. The luminescence intensity of these metals at 0.9 eV was higher than that of platinum with a similar surface roughness under the same excitation density. Although the Mg and Al are adjacent to each other in the periodic table and belong to “light metals,” having similar band structures, their luminescence spectra differ significantly. Pronounced peak structures were found for Mg and these were attributed to interband transitions within the conduction bands consisting of 3s and 3p orbitals overlapped on the intraband continuum, based on density functional theory band structure calculation. This result is in contrast to the interband luminescence in noble metals (Au, Ag, and Cu) under continuous-wave blue laser excitation, where the final states have been assigned to the d bands. A comparison of the spectra of rough and specular surfaces suggested that the surface roughness is not essential for mitigating wavenumber mismatch for intraband transitions. The luminescence from light metals, which are harmless to humans, will be attractive for biomedical applications.

(CN3H6)[FeIIFeIII(SO4)3(H2O)3]: A Framework Iron Sulfate with a Mixed S = 2 and S = 5/2 Honeycomb Lattice.

A new guanidinium-templated hydrated iron sulfate, [CN3H6][FeIIFeIII(SO4)3(H2O)3] (1), was prepared from strongly acidic aqueous solutions. Its crystal structure is comprised from FeIIIO6 and FeIIO3(H2O)3 octahedra linked by sulfate bridges forming a [FeIIFeII(SO4)3(H2O)3]- 3D framework with a layer-by-layer ordering of ferric and ferrous cations. The structural topology of the framework is related to the anhydrous rhombohedral mikasaite Fe2(SO4)3. The removal of part of the sulfate tetrahedra and the partial replacement of the Fe3+ cations in the [Fe3+2(SO4)3]0 framework by Fe2+ provide a negative charge and allow the incorporation of the protonated organic species in the voids. The compound 1 has been characterized by single-crystal X-ray diffraction, TG and DSC analyses, UV-vis-NIR spectroscopy, magnetic susceptibility, Mössbauer spectroscopy, IR and Raman spectroscopy, and density functional band-structure calculations. The magnetic behavior of 1 shows an interplay of FeII (S = 2) and FeIII (S = 5/2) sublattices that exhibit different types of antiferromagnetic couplings, one FeIII-FeIII (J1 ∼ 6.1 K) and two FeII-FeIII couplings (J2 ∼ 1 K, J3 ∼ 5.9 K) within corrugated honeycomb layers. These ferrimagnetic layers are coupled antiparallel to each other, resulting in an overall antiferromagnetic order below TN = 31 K.

Optical signatures of the Jeff = 12 state in Ir4+ halides

We study the vibrational and electronic excitations in cubic iridium halides (${\mathrm{NH}}_{4}{)}_{2}{\mathrm{IrCl}}_{6}, {\mathrm{K}}_{2}{\mathrm{IrCl}}_{6}$, and ${\mathrm{K}}_{2}{\mathrm{IrBr}}_{6}$ by infrared reflectivity and transmission measurements of single crystals. All three compounds show optical excitations in the energy range of 0.5--0.8 eV, which can be ascribed to ${j}_{\mathrm{eff}}=\frac{3}{2}$ to ${j}_{\mathrm{eff}}=\frac{1}{2}$ transitions (spin-orbit excitons), and ${t}_{2g}$-to-${e}_{g}$ excitations above $\ensuremath{\sim}1$ eV. We observe at least two peaks due to spin-orbit excitons in the bromide and four peaks in both chlorides, suggesting local deviations from the cubic symmetry already at room temperature. We further show that the ${e}_{g}$ states lie at lower energies in the bromide compared to the chlorides, in agreement with density-functional band-structure calculations.

Formation and Polymorphism of Semiconducting K2SiH6 and Strategy for Metallization.

K2SiH6, crystallizing in the cubic K2PtCl6 structure type (Fm3̅m), features unusual hypervalent SiH62- complexes. Here, the formation of K2SiH6 at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH3 as a precursor. At the investigated pressures, 8 and 13 GPa, K2SiH6 adopts the trigonal (NH4)2SiF6 structure type (P3̅m1) upon formation. The trigonal polymorph is stable up to 725 °C at 13 GPa. At room temperature, the transition into an ambient pressure recoverable cubic form occurs below 6.7 GPa. Theory suggests the existence of an additional, hexagonal, variant in the pressure interval 3-5 GPa. According to density functional theory band structure calculations, K2SiH6 is a semiconductor with a band gap around 2 eV. Nonbonding H-dominated states are situated below and Si-H anti-bonding states are located above the Fermi level. Enthalpically feasible and dynamically stable metallic variants of K2SiH6 may be obtained when substituting Si partially by Al or P, thus inducing p- and n-type metallicity, respectively. Yet, electron-phonon coupling appears weak, and calculated superconducting transition temperatures are <1 K.

Bulk and surface electronic structure of NiBi3

We present a high-resolution, angle-resolved photoemission spectroscopy study of the normal electronic state of the superconducting ${\mathrm{NiBi}}_{3}$. Our experimental results show a complex Fermi surface structure with many sheets along the $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}X$ and $\mathrm{\ensuremath{\Gamma}}\text{\ensuremath{-}}Y$ directions of the Brillouin zone. The band structure presents a topological surface state (TSS) at the high symmetry $\mathrm{\ensuremath{\Gamma}}$ point with a surface Dirac point at the energy $\ensuremath{-}0.185$ eV. The Dirac-like cone presents a linear dispersion along ${\mathrm{k}}_{x}$ while it presents saddle-like states along ${\mathrm{k}}_{y}$ located in the vicinity of the surface Dirac point. Our results are in good agreement with results of density functional theory band structure calculations. We also discuss the topological band structure of the ${\mathrm{NiBi}}_{3}$ compound.

Deformed spin- 12 square lattice in antiferromagnetic NaZnVOPO4(HPO4)

We report the structural and magnetic properties of a new spin-$\frac12$ antiferromagnet NaZnVOPO$_4$(HPO$_4$) studied via x-ray diffraction, magnetic susceptibility, high-field magnetization, specific heat, and $^{31}$P nuclear magnetic resonance (NMR) measurements, as well as density-functional band-structure calculations. While thermodynamic properties of this compound are well described by the $J_1-J_2$ square-lattice model, \textit{ab initio} calculations suggest a significant deformation of the spin lattice. From fits to the magnetic susceptibility we determine the averaged nearest-neighbor and second-neighbor exchange couplings of $\bar J_1\simeq -1.3$ K and $\bar J_2\simeq 5.6$ K, respectively. Experimental saturation field of 15.3\,T is consistent with these estimates if 20\% spatial anisotropy in $J_1$ is taken into account. Specific heat data signal the onset of a magnetic long-range order at $T_{\rm N} \simeq 2.1$ K, which is further supported by a sharp peak in the NMR spin-lattice relaxation rate. The NMR spectra mark the superposition of two P lines due to two noneqivalent P sites where the broad line with the strong hyperfine coupling and short $T_1$ is identified as the P(1) site located within the magnetic planes, while the narrow line with the weak hyperfine coupling and long $T_1$ is designated as the P(2) site located between the planes.

Quantum magnetism of ferromagnetic spin dimers in α−KVOPO4

Magnetism of the spin-$\frac12$ $\alpha$-KVOPO$_4$ is studied by thermodynamic measurements, $^{31}$P nuclear magnetic resonance (NMR), neutron diffraction, and density-functional band-structure calculations. Ferromagnetic Curie-Weiss temperature of $\theta_{\rm CW}\simeq 15.9$ K and the saturation field of $\mu_0H_s\simeq 11.3$ T suggest the predominant ferromagnetic coupling augmented by a weaker antiferromagnetic exchange that leads to a short-range order below 5 K and the long-range antiferromagnetic order below $T_{\rm N}\simeq 2.7$ K in zero field. Magnetic structure with the propagation vector $\mathbf k=(0,\frac12,0)$ and the ordered magnetic moment of 0.58 $\mu_B$ at 1.5 K exposes a non-trivial spin lattice where strong ferromagnetic dimers are coupled antiferromagnetically. The reduction in the ordered magnetic moment with respect to the classical value (1 $\mu_{\rm B}$) indicates sizable quantum fluctuations in this setting, despite the predominance of ferromagnetic exchange. We interpret this tendency toward ferromagnetism as arising from the effective orbital order in the folded chains of the VO$_6$ octahedra.

Determination of Silicon Electrical Properties Using First Principles Approach

Silicon nanowires have attracted attention as basis for reconfigurable electronics. However, as the size decreases, the electronic properties of the nanowires vary as a result of confinement, strain and crystal topology effects. Thus, at the thin diameter regime the band gap of Silicon nanowires can no longer be derived from a simple extrapolation of the isotropic bulk behaviour. This study compares band gap parameters in sub 10nm nanowires obtained from first-principles density-functional band structure calculations with extrapolations using continuum theory in order to rationalize the changes of the overall conductance, resistance and band gap. The device consists of silicon nanowire of size between 1 nm to 6nm. The results indicate an increase of, both the energy gap and the resistance along with reduced conductivity for the thinnest wires and a dependence on the crystal orientation with gaps reaching up to 4.3 eV along <111>, 4.0 eV along <110>, and 3.7 along <100>.

Ultrathin epitaxial Bi film growth on 2D HfTe2 template

Among ultrathin monoelemental two-dimensional (2D) materials, bismuthene, the single layer of heavier group-VΑ element bismuth (Bi), has been predicted to have large non trivial gap. Here, we demonstrate the growth of Bi films by molecular beam epitaxy on 2D-HfTe2 template. At the initial stage of Bi deposition (1–2 bilayers, BL), both the pseudocubic Bi(110) and the hexagonal Bi(111) phases are formed. When reaching 3 BL Bi, a transformation to pure hexagonal Bi(111) occurs. The electronic band structure of 3 BL Bi(111) films was measured by angle-resolved photoemission spectroscopy showing very good matching with the density functional theory band structure calculations of 3 BL free standing Bi(111). The grown Bi(111) thin film was capped with a protective Al2O3 layer and its stability under ambient conditions, necessary for practical applications and device fabrication, was confirmed by x-ray photoelectron spectroscopy and Raman spectroscopy.

Universal fluctuating regime in triangular chromate antiferromagnets

We report x-ray diffraction, magnetic susceptibility, heat capacity, $^{1}\mathrm{H}$ nuclear magnetic resonance (NMR), and muon spin relaxation $(\ensuremath{\mu}\mathrm{SR})$ measurements, as well as density-functional band-structure calculations for the frustrated $S=3/2$ triangular lattice Heisenberg antiferromagnet (TLHAF) $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{HCrO}}_{2}$ (trigonal, space group: $R\overline{3}m$). This compound undergoes a clear magnetic transition at ${T}_{\mathrm{N}}\ensuremath{\simeq}22.5$ K, as seen from the drop in the muon paramagnetic fraction and concurrent anomalies in the magnetic susceptibility and specific heat capacity. Local probes (NMR and $\ensuremath{\mu}\mathrm{SR}$) reveal a broad regime with slow fluctuations down to $0.7\phantom{\rule{0.16em}{0ex}}{T}_{\mathrm{N}}$, this temperature corresponding to the maximum in the $\ensuremath{\mu}\mathrm{SR}$ relaxation rate and in the NMR wipe-out. From the comparison with ${\mathrm{NaCrO}}_{2}$ and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{KCrO}}_{2}$, the fluctuating regime and slow dynamics below ${T}_{\mathrm{N}}$ appear to be hallmarks of the TLHAF with $ABC$ stacking. We discuss the role of interlayer frustration, which may have impacted recent spin-liquid candidates with triangular geometry.

Low-dimensional magnetism of BaCuTe2O6

X-ray diffraction, thermodynamic measurements, and density-functional band-structure calculations are used to study the magnetic behavior of BaCuTe$_{2}$O$_{6}$, a member of the $A$CuTe$_2$O$_6$ structural family that hosts complex three-dimensional frustrated spin networks with possible spin-liquid physics. Temperature-dependent magnetic susceptibility and heat capacity of the Ba compound are well described by the one-dimensional spin-$\frac12$ Heisenberg chain model reminiscent of the Sr analog SrCuTe$_2$O$_6$. While the intrachain coupling $J/k_{\rm B}\simeq 37$ K is reduced compared to 49 K in the Sr compound, the N\'eel temperature increases from 5.5 K (Sr) to 6.1 K (Ba). Unlike the Sr compound, BaCuTe$_2$O$_6$ undergoes only one magnetic transition as a function of temperature and shows signatures of weak spin canting. We elucidate the microscopic difference between the Sr and Ba compounds and suggest that one of the interchain couplings changes sign as a result of negative pressure caused by the Sr/Ba substitution. The N\'eel temperature of BaCuTe$_2$O$_6$ is remarkably insensitive to the magnetic dilution with Zn$^{2+}$ up to the highest reachable level of about 20%.

Electronic structure of the metallic oxide ReO3

ReO${}_{3}$ is a remarkable transition metal oxide in that it has the highest conductivity of all oxides, comparable even with that of silver. Using state of the art bulk-sensitive angle-resolved photoelectron spectroscopy, the authors are able to observe clear dispersions of the Re 5$d$ and O 2$p$ derived bands as well as the momentum splitting of the Fermi surface due to the Re 5$d$ spin-orbit interaction. The experimental results are compared quantitatively to density functional theory band structure calculations, thereby providing a deeper understanding of the material class of the 5$d$ oxides.

Syntheses, Structures, and Physical Properties of Neutral Gold Dithiolate Complex, [Au(etdt)2]·THF

In order to develop new types of single-component molecular conductors with novel electronic structures and physical properties, the neutral gold dithiolate complex with an etdt (= ethylenedithiotetrathiafulvalenedithiolate) ligand, [Au(etdt)2] was prepared. However, unlike the reported single-component molecular metals, the neutral gold complex [Au(etdt)2]·THF (2) contains a solvent molecule of tetrahydrofuran (THF). The crystals of 2 form a two-dimensional conducting layer structure, which are separated by the terminal ethylene groups and THF molecules. The fairly high room-temperature conductivity of 0.2 S/cm and semiconducting behavior with a low activation energy of 0.1 eV of 2, is consistent with the result of the density functional theory band structure calculations. The observed non-magnetic behavior of 2 is caused from the dimeric structure of [Au(etdt)2] molecules.

Giant momentum-dependent spin splitting in centrosymmetric low- Z antiferromagnets

The energy vs. crystal momentum E(k) diagram for a solid (band structure) constitutes the road map for navigating its optical, magnetic, and transport properties. By selecting crystals with specific atom types, composition and symmetries, one could design a target band structure and thus desired properties. A particularly attractive outcome would be to design energy bands that are split into spin components with a momentum-dependent splitting, as envisioned by Pekar and Rashba [Zh. Eksperim. i Teor. Fiz. 47 (1964)], enabling spintronic application. The current paper provides "design principles" for wavevector dependent spin splitting (SS) of energy bands that parallels the traditional Dresselhaus and Rashba spin-orbit coupling (SOC) - induce splitting, but originates from a fundamentally different source -- antiferromagnetism. We identify a few generic AFM prototypes with distinct SS patterns using magnetic symmetry design principles. These tools allow also the identification of specific AFM compounds with SS belonging to different prototypes. A specific compound -- centrosymmetric tetragonal MnF2 -- is used via density functional band structure calculations to quantitatively illustrate one type of AFM SS. Unlike the traditional SOC-induced effects restricted to non-centrosymmetric crystals, we show that antiferromagnetic-induced spin splitting broadens the playing field to include even centrosymmetric compounds, and gives SS comparable in magnitude to the best known ('giant') SOC effects, even without SOC, and consequently does not rely on the often-unstable high atomic number elements required for high SOC. We envision that use of the current design principles to identify an optimal antiferromagnet with spin-split energy bands would be beneficial for efficient spin-charge conversion and spin orbit torque applications without the burden of requiring compounds containing heavy elements.

Two types of alternating spin- 12 chains and their field-induced transitions in ɛ−LiVOPO4

Thermodynamic properties, $^{31}$P nuclear magnetic resonance (NMR) measurements, and density-functional band-structure calculations for $\varepsilon$-LiVOPO$_4$ are reported. This quantum magnet features a singlet ground state and comprises two types of alternating spin-$\frac12$ chains that manifest themselves by the double maxima in the susceptibility and magnetic specific heat, and by the two-step magnetization process with an intermediate $\frac12$-plateau. From thermodynamic data and band-structure calculations, we estimate the leading couplings of $J_1\simeq 20$ K and $J_2\simeq 60$ K and the alternation ratios of $\alpha_1=J_1'/J_1\simeq 0.6$ and $\alpha_2=J_2'/J_2\simeq 0.3$ within the two chains, respectively. The zero-field spin gap $\Delta_0/k_{\rm B}\simeq 7.3$ K probed by thermodynamic and NMR measurements is caused by the $J_1$-$J_1'$ spin chains and can be closed in the applied field of $\mu_{0}H_{\rm c1}\simeq 5.6$ T, giving rise to a field-induced long-range order. The NMR data reveal predominant three-dimensional spin-spin correlations at low temperatures. Field-induced magnetic ordering transition observed above $H_{c1}$ is attributed to the Bose-Einstein condensation of triplons in the sublattice formed by the $J_1$-$J_1'$ chains with weaker exchange couplings.

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.

Persistent spin dynamics in the pressurized spin-liquid candidate YbMgGaO4

Single-crystal x-ray diffraction, density-functional band-structure calculations, and muon spin relaxation ($\mu$SR) are used to probe pressure evolution of the triangular spin-liquid candidate YbMgGaO$_4$. The rhombohedral crystal structure is retained up to at least 10 GPa and shows a nearly uniform compression along both in-plane and out-of-plane directions, whereas local distortions caused by the random distribution of Mg$^{2+}$ and Ga$^{3+}$ remain mostly unchanged. The $\mu$SR data confirm persistent spin dynamics up to 2.6 GPa and down to 250 mK with no change in the muon relaxation rate. Longitudinal-field $\mu$SR reveals power-law behavior of the spin-spin autocorrelation function, both at ambient pressure and upon compression.

Optical signatures of phase transitions and structural modulation in elemental tellurium under pressure

A room-temperature infrared spectroscopy study of elemental tellurium at pressures up to 8.44 GPa in the energy range 0.015-2 eV is reported. Optical signatures of the high-pressure polymorphs are investigated and compared to the results of density-functional band-structure calculations. A Drude peak is first seen in the optical conductivity around 3.5 GPa indicating a semiconductor-to-metal transition within trigonal Te-I. A sharp increase in the Drude spectral weight and dc-conductivity around 4.3 GPa signals the transformation toward the triclinic Te-II polymorph. An absorption peak around 0.15 eV appears above 5 GPa concomitant with the gradual transformation of Te-II into the structurally similar but incommensurately modulated Te-III. Microscopically, this peak can only be reproduced within a sufficiently large commensurate approximant, suggesting the low-energy optical response as a fingerprint of the structural modulation.

Origin of up-up-down-down magnetic order in Cu2GeO4

We use density-functional band-structure calculations to explore the origin of the up-up-down-down (UUDD) magnetic order in Cu$_2$GeO$_4$ with the frustrated $J_1-J_2$ spin chains coupled into layers within the spinel-like crystal structure. In contrast to earlier studies, we find that the nearest-neighbor coupling $J_1$ may be negligibly small, owing to a nearly perfect compensation of the ferromagnetic direct exchange and antiferromagnetic superexchange. Under this condition, weak symmetric anisotropy of the exchange couplings gives rise to the UUDD order observed experimentally and also elucidates the non-trivial ordering pattern between the layers, whereas a small Dzyaloshinsky-Moriya interaction causes a spin canting that may generate local electric polarization. We argue that the buckling of the copper chains plays a crucial role in the suppression of $J_1$ in Cu$_2$GeO$_4$ and sets this compound apart from other $J_1-J_2$ chain magnets.

Freestanding Borophene and Its Hybrids.

Borophene, an elemental metallic Dirac material is predicted to have unprecedented mechanical and electronic character. Need of substrate and ultrahigh vacuum conditions for deposition of borophene restricts its large-scale applications and significantly hampers the advancement of research on borophene. Herein, a facile and large-scale synthesis of freestanding atomic sheets of borophene through a novel liquid-phase exfoliation and the reduction of borophene oxide is demonstrated. Electron microscopy confirms the presence of β12 , X3 , and their intermediate phases of borophene; X-ray photoelectron spectroscopy, and scanning tunneling microscopy, corroborated with density functional theory band structure calculations, validate the phase purity and the metallic nature. Borophene with excellent anchoring capabilities is used for sensing of light, gas, molecules, and strain. Hybrids of borophene as well as that of reduced borophene oxide with other 2D materials are synthesized, and the predicted superior performance in energy storage is explored. The specific capacity of borophene oxide is observed to be ≈4941 mAh g-1 , which significantly exceeds that of existing 2D materials and their hybrids. These freestanding borophene materials and their hybrids will create a huge breakthrough in the field of 2D materials and could help to develop future generations of devices and emerging applications.