Principles Band Calculation Research Articles

Molecular beam epitaxy growth and doping modulation of topological semimetal NiTe2

In this study, high-quality thin films of the topological semimetal phase NiTe2 were prepared using the molecular beam epitaxy technique, confirmed through x-ray diffraction with pronounced Laue oscillations. Electrical transport experiments reveal that thick films have properties similar to bulk materials. By employing co-deposition, we introduced either magnetic or non-magnetic elements during the growth of thinner films, significantly altering their electrical properties. Notably, the magnetic element Cr induces long-range ferromagnetic ordering, leading to the observation of a significant anomalous Hall effect in NiTe2 thin films. The Hall conductivity remains nearly constant well below the Curie temperature, indicating the correlation with the intrinsic topological nature of the band structure. Theoretical first principles band calculations support the generation of the Weyl semimetal state in the material through magnetic doping. These findings pave the way for exploring more magnetic Weyl semimetals and related low-dimensional quantum devices based on topological semimetals.

First Principles Study on the Thermoelectric Performance of CaAl2Si2-type Zintl Phase Compounds

We investigate the thermoelectric properties of CaAl$_2$Si$_2$-type Zintl phase compounds $AB_2X_2$ ($A$ = Mg, Ca, Sr, Ba, $B$ = Mg, Zn, Cd, and $X$ = P, As, Sb) using first principles band calculations within the Boltzmann transport theory assuming the constant relaxation time approximation. We introduce the effective degree of valley degeneracy $n_{TE}$ to focus on the relationship between the thermoelectric properties and the multivalley character of the electronic band structure around the Fermi level. We also introduce a quantity $\gamma_{TE}$, which takes into account $n_{TE}$ and anisotropy of the valley structure, and it is found that $\gamma_{TE}$ enables us to well understand the overall trend of the material dependence of the power factor. We finally suggest promising thermoelectric materials, e.g. BaMg$_2$P$_2$ for PF $\sim 20\mu$W/cmK$^2$ and $ZT > 0.2$ at 300K and SrZn$_2$As$_2$ for PF $\sim 35\mu$W/cmK$^2$ and $ZT > 0.35$ at 300K assuming a relaxation time of 10 fs and a lattice thermal conductivity value of 2 W/mK.

Odd-parity superconductivity in bilayer transition metal dichalcogenides

Spin-orbit coupling in transition metal dichalcogenides (TMDCs) causes spin-valley locking giving rise to unconventional optical, transport, and superconducting properties. In this paper, we propose exotic superconductivity in bilayer group-IV TMDCs by symmetry control. The sublattice-dependent hidden spin-orbit coupling arising from local inversion symmetry breaking in the crystal structure may stabilize the odd-parity superconductivity by purely $s$-wave local pairing interaction. The stability of the odd-parity superconducting state depends on the bilayer stacking. The 2H$_b$ stacking in MoX$_2$ and WX$_2$ (X =S, Se) favors the odd-parity superconductivity due to interlayer quantum interference. On the other hand, the odd-parity superconductivity is suppressed by the 2H$_a$ stacking of NbSe$_2$. Calculating the phase diagram of the tight-binding model derived from first principles band calculations, we conclude that the intercalated bilayer MoS$_2$ and WS$_2$ are candidates for a new class of odd-parity superconductors by spin-orbit coupling.

Enhancement of electron correlation due to the molecular dimerization in organic superconductorsβ−(BDA-TTP)2X(X=I3, SbF6)

We perform a first principles band calculation for quasi-two-dimensional organic superconductors $\beta$-(BDA-TTP)$_{2}$I$_{3}$ and $\beta$-(BDA-TTP)$_{2}$SbF$_{6}$. The first principles band structures between the I$_{3}$ and SbF$_{6}$ salts are apparently different. We construct a tight-binding model for each material which accurately reproduces the first principles band structure. The obtained transfer energies give the differences such as (i) larger dimerization in the I$_{3}$ salt than the SbF$_{6}$ salt, and (ii) different signs and directions of the inter-stacking transfer energies. To decompose the origin of the difference into the dimerization and the inter-stacking transfer energies, we adopt a simplified model by eliminating the dimerization effect and extract the difference caused by the inter-stacking transfer energies. From the analysis using the simplified model, we find that the difference of the band structure comes mainly from the strength of dimerization. To compare the strength of the electron correlation having roots in the band structure, we calculate the physical properties originated from the effect of the electron correlation such as the spin susceptibility applying two particle self-consistent (TPSC) method. We find that the maximum value of the spin susceptibility of the I$_{3}$ salt is larger than that of the SbF$_{6}$ salt. Hypothetically decreasing the dimerization within the model of the I$_{3}$ salt, the spin susceptibility takes almost the same value as that of the SbF$_6$ salt for the same magnitude of the dimerization. We expect that the different ground state between the I$_{3}$ and SbF$_{6}$ salt mainly comes from the strength of the dimerization which is apparently masked in the band calculation along a particular $k$-path.

Mobility spectrum analytical approach for intrinsic band picture of Ba(FeAs)2

Unconventional high temperature superconductivity as well as three-dimensional bulk Dirac cone quantum states arising from the unique d-orbital topology have comprised an intriguing research area in physics. Here we apply a special analytical approach using a mobility spectrum, in which the carrier number is conveniently described as a function of mobility without any hypothesis, both on the types and the numbers of carriers, for the interpretations of longitudinal and transverse electric transport of high quality single crystal Ba(FeAs)2 in a wide range of magnetic fields. We show that the majority carriers are accommodated in large parabolic hole and electron pockets with very different topology as well as remarkably different mobility spectra, whereas the minority carriers reside in Dirac quantum states with the largest mobility as high as 70,000 cm2(Vs). The deduced mobility spectra are discussed and compared to the reported sophisticated first principle band calculations.

Theoretical Analysis on the Band Structure Variance of the Electron Doped 1111 Iron-based Superconductors

Abstract We perform first principles band calculation of electron doped iron-based superconductors adopting the virtual crystal approximation. We find that when electrons are doped by element substitution in the blocking layer, the band structure near the Fermi level is affected due to the increase of the positive charge in the layer. On the other hand, when Fe in the conducting layer is substituted by Co, the band structure is barely affected. This difference should be a key factor in understanding the phase diagram of the heavily doped electron doped systems Ln FeAsO 1-x H x .

Superconductivity in Ca10(Ir4As8)(Fe2As2)5 with Square-Planar Coordination of Iridium

We report the unprecedented square-planar coordination of iridium in the iron iridium arsenide Ca10(Ir4As8)(Fe2As2)5. This material experiences superconductivity at 16 K. X-ray photoemission spectroscopy and first-principles band calculation suggest Ir(II) oxidation state, which yields electrically conductive Ir4As8 layers. Such metallic spacer layers are thought to enhance the interlayer coupling of Fe2As2, in which superconductivity emerges, thus offering a way to control the superconducting transition temperature.

Ideal band shape in the potential thermoelectric material CuAlO2: Comparison to NaxCoO2

A potential thermoelectric material CuAlO${}_{2}$ is theoretically studied. We first construct a model Hamiltonian of CuAlO${}_{2}$ based on the first principles band calculation, and calculate the Seebeck coefficient. Then, we compare the model with that of a well-known thermoelectric material Na${}_{x}$CoO${}_{2}$, and discuss the similarities and the differences. It is found that the two materials are similar from an electronic structure viewpoint in that they have a peculiar pudding-mold type band shape, which is advantageous for thermoelectric materials. There are, however, some differences, and we analyze the origin of the difference from a microscopic viewpoint. The band shape (a very flat band top but with an overall wide bandwidth) of CuAlO${}_{2}$ is found to be even more ideal than that of Na${}_{x}$CoO${}_{2}$, and we predict that once a significant amount of holes is doped in CuAlO${}_{2}$, thermoelectric properties (especially the power factor) even better than those of Na${}_{x}$CoO${}_{2}$ can be expected.

Understanding the reentrant superconducting phase diagram of the iron pnictide Ca4Al2O6Fe2(As1−xPx)2: First-principles calculations

Recently, a very rich phase diagram has been obtained for an iron-based superconductor Ca4Al2O6Fe2(As1-xPx)2. It has been revealed that nodeless (x=0) and nodal (x = 1) superconductivity are separated by an antiferromagnetic phase. Here we study the origin of this peculiar phase diagram using a five orbital model constructed from first principles band calculation, and applying the fluctuation exchange approximation assuming spin fluctuation mediated pairing. Based on the calculation results, we propose a scenario where the frustration in momentum space degrades superconductivity in the intermediate x regime, while antiferromangetism takes place due to a very good nesting. In order to see whether the present theoretical scenario is consistent with the actual nature of the competition between superconductivity and antiferromagnetism, we also perform hydrostatic pressure experiment for Ca4Al2O6Fe2(As1-xPx)2. In the intermediate x regime where antiferromagnetism occurs at ambient pressure, applying hydrostatic pressure smears out the antiferromagnetic transition, but superconductivity does not take place. This supports our scenario that superconductivity is suppressed by the momentum space frustration in the intermediate x regime, apart from the presence of the antiferromangnetism.

Minimum Model and its Theoretical Analysis for Superconducting Materials with BiS2 Layers

We perform first principles band calculation of the newly discovered superconductor LaO1- x F x BiS2, and study the lattice structure and the fluorine doping dependence of the gap between the valence and conduction bands. We find that the distance between La and S as well as the fluorine doping significantly affects the band gap. On the other hand, the four orbital model of the BiS2 layer shows that the lattice structure does not affect this portion of the band. Still, the band gap can affect the carrier concentration in the case of light electron doping, which in turn should affect the transport properties.

Minimal electronic models for superconducting BiS2layers

We construct minimal electronic models for a newly discovered superconductor LaO$_{1-x}$F$_x$BiS$_2$ ($T_c=$ 10.6K) possessing BiS$_2$ layers based on first principles band calculation. First, we obtain a model consisting of two Bi $6p$ and two S $3p$ orbitals, which give nearly electron-hole symmetric bands. Further focusing on the bands that intersect the Fermi level, we obtain a model with two $p$ orbitals. The two bands (per BiS$_2$ layer) have quasi-one-dimensional character with a double minimum dispersion, which gives good nesting of the Fermi surface. At around $x\sim 0.5$ the topology of the Fermi surface changes, so that the density of states at the Fermi level becomes large. Possible pairing states are discussed.

Corrugated flat band as an origin of large thermopower in hole doped PtSb2

The origin of the recently discovered large thermopower in hole-doped PtSb2 is theoretically analyzed based on a model constructed from first principles band calculation. It is found that the valence band dispersion has an overall flatness combined with some local ups and downs, which gives small Fermi surfaces scattered over the entire Brillouin zone. The Seebeck coefficient is calculated using this model, which gives good agreement with the experiment. We conclude that the good thermoelectric property originates from this “corrugated flat band”, where the coexistence of large Seebeck coefficient and large electric conductivity is generally expected.

RKKY Ferromagnetism with Ising-Like Spin States in IntercalatedFe1/4TaS2

We investigated the magnetic nature of Fe(1/4)TaS2 using x-ray absorption spectroscopy, photoemission spectroscopy, and first principles band calculations. The results show a large unquenched orbital magnetic moment (∼1.0 μ(B)/Fe) at intercalated Fe sites, resulting in a gigantic magnetic anisotropy (H(A)≃60 T). The magnetic coupling is well understood in terms of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, suggesting a novel RKKY ferromagnet with Ising-type spin states. We also found that this indirect exchange coupling between the neighboring Fe spins is ferromagnetic and maximized at the Fe-Fe distance of 2×2 superstructure.

Electronic Structure and Spin-Injection of Co-Based Heusler Alloy/ Semiconductor Junctions

The electronic structures of Co-based Heusler alloys with nonstoichiometric atomic compositions as well as those at the interface of semiconductor junctions are investigated using first principles band calculations. It is shown that the electronic structure of a Co-based Heusler alloy is half-metallic, even for nonstoichiometric but Co-rich compositions, whereas the half-metallicity is lost for Co-poor compositions. It is also shown that magnetic moments at the interface of Co2MnSi/ Si junctions are sensitive to the growth direction and interface structure of the junctions. Efficient spin-injection into Si can be achieved by using a (111)-oriented Co-rich Heusler alloy and controlling the layer-by-layer stacking sequence at the interface.

Possible Three-Dimensional Nodes in thes± Superconducting Gap of BaFe2(As1-xPx)2

We examine theoretically the superconducting state of BaFe$_2$(As$_{1-x}$P$_x$)$_2$, an isovalent doping 122 iron pnictide superconductor. We construct a three dimensional ten orbital model from first principles band calculation, and investigate the superconducting gap within the spin fluctuation mediated pairing mechanism. The gap is basically $s\pm$, where the gap changes its sign between electron and hole Fermi surfaces, but three dimensional nodal structures appear in the largely warped hole Fermi surface having strong $Z^2/XZ/YZ$ orbital character. The present result, together with our previous study on 1111 systems, explains the strong material dependence of the superconducting gap in the iron pnictides.

Thermoelectric Properties of Si<SUB>2</SUB>Ti-Type Al-Mn-Si Alloys

By using the first principle band calculations, we identified that Si 2 Ti-type Al-Mn-Si C54-phase possesses an electronic structure suitable for the thermoelectric materials. The formation range of the Al-Mn-Si C54-phase was determined by preparing samples at a number of different compositions, and the samples solely consisting of the C54-phase were successfully obtained around A1: Mn: Si = I: 1: 1. The carrier concentration of the Al-Mn-Si C54-phase was controlled by the partial substitution of the constituent elements, and behaviors in electrical properties characteristic to n-type and p-type thermoelectric materials were alternatively observed for the samples of a larger electron concentration and a smaller electron concentration, respectively. The absolute value of the Seebeck coefficient |S| was found to exceed 300 μV/K for both n-type and p-type samples.

Supercell Band Calculations and Correlation for High- Copper Oxide Superconductors

First principle band calculations based on local versions of density functional theory (DFT), together with results from nearly free-electron models, can describe many typical but unusual properties of the high- copper oxides. The methods and a few of the most important results are reviewed. Some additional calculations are presented, and the problems with the commonly used approximate versions of DFT for oxides are discussed with a few ideas for corrections. It is concluded that rather modest corrections to the approximate DFT, without particular assumptions about strong correlation, can push the ground state towards antiferro magnetic (AFM) order. Spin fluctuations interacting with phonons are crucial for the mechanism of superconductivity in this scenario.

Interstitial binary nitride ReN x phases prepared by pulsed laser deposition: Structure and superconductivity dependence on nitrogen stoichiometry

Pulsed laser deposition under nitrogen radical irradiation was applied to the synthesis of ReN x crystalline phases, and the effect of the varying nitrogen stoichiometry on the crystal structure and superconductivity was investigated. X-ray diffraction revealed that, at low nitrogen content ( x ∼0.06, deposition temperature T d = 873 K), ReN x maintained the hexagonal structure of the Re metal (having a hexagonal close-packing (hcp) structure), where the nitride ions occupy interstitial sites among Re atoms. It was found that the crystal structure is cubic when x = 0.17 ( T d = 603 K) and 0.22 ( T d = room temperature). X-ray photoelectron spectroscopy detected N 1s 1/2 signals close to those for Si 3N 4 and BN, indicating the strong covalent character of Re–N bonds. First principle band calculations indicated that the density of states at Fermi energy of ReN consists of mainly Re 5d bands. The superconducting transition temperature T c increased steeply from 1.6 K (Re metal) to 4.8 K (ReN 0.22).

Kadowaki-Woods ratio for strongly coupled Fermi liquids

On the basis of the Fermi liquid theory, the Kadowaki-Woods ratio $A∕{\ensuremath{\gamma}}^{2}$ is evaluated by using a first principles band calculation for typical itinerant $d$- and $f$-electron systems. It is found as observed that the ratio for the $d$-electron systems is significantly smaller than that for the normal $f$ systems, even without considering their relatively weak correlation. The difference in the ratio value comes from different characters of the Fermi surfaces. By comparing Pd and $\mathrm{U}{\mathrm{Sn}}_{3}$ as typical cases, we discuss the importance of the Fermi-surface dependence of the quasiparticle transport relaxation.

Analysis of nano-scale MOSFET including uniaxial and biaxial strain

In this paper, we focus on uniaxial and biaxial strain technologies, and we also investigate an optimum combination of strain method and channel direction. We linked the first principles band calculation program to the FUJITSU ensemble full band Monte Carlo simulator FALCON directly, which enables to incorporate arbitrary Si band structures such as uniaxial and biaxial strained-Si into device characteristics analysis. We show that the combination of biaxial tensile strain and current for NMOS, and compressive uniaxial strain and channel for PMOS are optimum methods for current enhancement. However, considering technological difficulties and process cost, it is one of the candidate methods to use the combination of uniaxial tensile strain and channel direction for NMOS and that of uniaxial compressive strain and channel direction for PMOS.