Minority Band Gap Research Articles

Characterization of Electronic Transport Properties of Narrow-Band Gap Fe(Se1-xTex)2 Alloys via the Two-Band Model

Environmentally sustainable thermoelectric technologies can be more broadly applied in industries once the performance of thermoelectric materials is improved. Several approaches have been proposed to improve the electronic transport properties of thermoelectric materials. The effects of each approach on the electronic properties can be evaluated by changes in the band parameters. The Single Parabolic Band (SPB) model has been widely used to determine the effect of different materials engineering strategies on band parameters, such as the density-of-states effective mass and deformation potential. However, when the material has a narrow band gap, the Two-Band (TB) model better describes the changes in band parameters, as it includes the bipolar conduction from the minority carrier band. Here, the band parameters of previously reported Fe(Se1-xTex)2 (x/i> = 0, 0.2, 0.6, 0.8, 1) alloys, whose band gap significantly decreases with x/i>, have been estimated using the SPB and TB models. While the x-dependent band parameters obtained via the SPB model varied abruptly with x/i>, all the band parameters estimated by the TB model changed linearly with x/i>. The abruptness observed in the band parameters of the SPB model can be attributed to artifacts reflected in the single band, which occurs when the minority carrier band and band gap change are not included. The bipolar thermal conductivity of Fe(Se1-xTex)2 alloys was also calculated using the TB model, and is understood in terms of changes in the weighted mobility ratio, Hall carrier concentration, and band gap in terms of alloy composition, x/i>.

First-principles study to probe the effect of substitution at X and Z sites on the electronic, magnetic and transport properties of Co2X(V, Nb, Ta)Z(Al, Ga, In, Si, Ge, Sn) Heusler alloys

In the present work, we have used density functional theory based electronic structure calculations to probe the role of isovalent substitution at X@(0.5, 0.5, 0.5) and Z@(0, 0, 0) site of Co2X(= Ta, Nb, V)Z(= Al, Ga, In). Further, we probed the role of electron-rich substitution at Z site on the electronic, magnetic and transport properties of half-metallic(HM) Co2TaZ( = Al,Ga,In) full Heusler alloys(FHA). One of our main aims is to study the effect of electron-rich substitution at Z site on the total and partial moments of X and Z sites. We have used the modified Becke–Johnson(mBJ) exchange–correlation term to calculate the HM band-gap. We observe that, isovalent substitution at X and Z sites provides a way to tune the lattice constant in range of ∼5.75 Å to ∼6.16 Å with sustained HM character. Further, it is noticed that the HM band-gap decreases with isovalent substitution (with low atomic number) at X site with maximum and minimum values of 1.09 eV and 0.5 eV, respectively. Isovalent substitution does not affect the total magnetic moment, however, it affects the atomic magnetic moment values. Further, we observe that 25% of substitution of tetravalent atoms (Si, Ge, Sn) at atomic site Z (having Al, Ga, In) enhances the total magnetic moment with a small decrease in the minority band-gap. However, 75 to 100 % of substitutions destroy the half-metallicity, showing an enhancement in the magnetic moment. Temperature dependent spin Seebeck coefficients corresponding to the 25% and 75% of substitution of tetravalent element at Z site exhibit values close to 150 μV/K and 25–75 μV/K, respectively, for temperatures above 400 K. Interestingly, electron rich substitution at Z cite, changes the orientation of the magnetic moment corresponding to the X (Ta) site element. In order to understand in detail the impact of substitution on the local magnetic moment of Co and X site atoms (V, Nb, Ta), we have performed calculations of respective X-ray magnetic circular dichroism(XMCD) spectra and analyzed the results in terms of unoccupied density of states. XMCD spectra of L2,3 edges of Co atom show only small deviation (in the intensity) with the changes in the X and Z sites. Unlike the case of Co atom, we find that the XMCD spectra of M2,3 edges of Ta atom exhibit substantial changes when tetravalent atoms replace the trivalent atoms at the Z site.

Mini band gap generation in magnetic beta-borophene: effects of optical phonon interaction

In this work we report for the first time, to the best of our knowledge, the tunable electronic properties of beta-borophene (BB) on the polar substrate (). We provide an analytical prescription for the calculation of ground state energy in the presence of the electron–phonon (e-ph) interaction, within the framework of the Lee-Low-Pines theory. In the theoretical investigation of the polaron formation in BB, we describe its effective masses, polaronic band-gap, mobility, and Fermi velocities, which are different in each coordinate due to the out-of-plane buckling structure. We also analyze how the average effective mass and Fermi velocities of the charge carriers in the buckling structure are affected by the external magnetic field. It is shown that the polaronic energy becomes more effective in presence of a magnetic field, which we confirm through an analytical prescription. The characteristics of the evolution of average effective mass and average effective Fermi velocity in the presence of a magnetic field are critically discussed. We find that the average effective Fermi velocity is less sensitive, while the mass is greatly influenced by the magnetic field.

Spin-dependent band-gap formation for the L-gap surface state on the reconstructed Au(111) surface

We elucidate how the free-electron-like energy dispersion of the L-gap surface state on a Au(111)-(1 × 1) surface is modified by the experimentally observed uniaxial reconstruction of the topmost atomic layer. For this purpose, we perform a first-principles embedded Green’s function calculation for the reconstructed semi-infinite Au(111) surface. The obtained band structure unfolded into the surface Brillouin zone of the (1 × 1) surface can be understood in terms of two spin-split parabolic bands centered at the point, their umklapp-induced replicas centered at reciprocal lattice vectors of the superlattice (SL) with much weaker intensities, and mini band gaps at the crossing of two of them. More importantly, it is revealed that the band-gap size depends not only on the amplitude of the SL potential but also on mutual spin orientations of two crossing bands. Furthermore, we demonstrate that the band-gap size and the charge density distribution of the surface states are closely correlated with spatial profile of the SL potential.

Spin wave propagation and nonreciprocity in metallic magnonic quasi-crystals

The characteristics of spin waves (SWs) propagating in Fibonacci magnonic quasi-crystals (MQCs) were investigated in micromagnetic simulations. The SWs feel 1/3 of the characteristic Fibonacci sequence length as a period, and mini band gaps reflected by MQCs are formed. The effect of the MQC on the SW’s propagation becomes prominent above the first band gap frequency. The properties of SWs in MQCs generally depend on the propagation direction, because SWs feel different structures depending on the direction. Therefore, the nonreciprocity (NR) characteristics becomes complex. The NR characteristics change at every band gap frequency and hence across the frequency regions defined by them. In particular, some frequency regions have almost no NR, while others have enhanced NR and some have even negative NR. These characteristics seem to provide a new way to control NR.

Miniband engineering and topological phase transitions in topological-insulator–normal-insulator superlattices

Periodic stacking of topologically trivial and non-trivial layers with opposite symmetry of the valence and conduction bands induces topological interface states that, in the strong coupling limit, hybridize both across the topological and normal insulator layers. Using band structure engineering, such superlattices can be effectively realized using the IV-VI lead tin chalcogenides. This leads to emergent minibands with a tunable topology as demonstrated both by theory and experiments. The topological minibands are proven by magneto-optical spectroscopy, revealing Landau level transitions both at the center and edges of the artificial superlattice mini Brillouin zone. Their topological character is identified by the topological phase transitions within the minibands observed as a function of temperature. The critical temperature of this transition as well as the miniband gap and miniband width can be precisely controlled by the layer thicknesses and compositions. This witnesses the generation of a new fully tunable quasi-3D topological state that provides a template for realization of magnetic Weyl semimetals and other strongly interacting topological phases.

Band structure tuning of Heusler compounds: Spin- and momentum-resolved electronic structure analysis of compounds with different band filling

Half-metallic ferromagnetic Heusler compounds represent an important class of materials for spintronic applications. We experimentally observe the dispersion of electronic bands with spin resolution in three representative Heusler compounds (${\mathrm{Co}}_{2}\mathrm{MnGa}, {\mathrm{Co}}_{2}\mathrm{MnSi}$, and ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$). Bulk-sensitive measurements at very low photon energy are complemented by spin-integrated hard x-ray angular resolved photoemission spectroscopy. The dispersion of majority and minority electrons allows us to link exchange splitting and band filling to the number of valence electrons ${N}_{v}$. Photoexcitation at $h\ensuremath{\nu}=6.05\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ gives access to the spin-polarization texture $\mathbit{P}({E}_{B},{k}_{x},{k}_{y})$ of the bulk bands covering a $({k}_{x},{k}_{y})$ range of 60% of the Brillouin zone. We find that ${\mathrm{Co}}_{2}\mathrm{MnSi}$ and ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ exhibit minority band gaps of 0.5 and 0.35 eV, i.e., decreasing band gap with increasing ${N}_{\mathrm{V}}$ in contrast to the rigid-band expectation. For ${\mathrm{Co}}_{2}\mathrm{MnGa}$, the minority valence state maximum lies at approx. 0.2 eV above ${E}_{\mathrm{F}}$.

Investigating structure, magneto-electronic, and thermoelectric properties of the new d0 quaternary Heusler compounds RbCaCZ (Z = P, As, Sb) from first principle calculations

The ab initio calculations based on the density functional theory (DFT) using the self-consistent full potential linearized augmented plane wave (FPLAPW) method were performed to explore the electronic structures, magnetic and thermoelectric properties of quaternary alloys RbCaCZ (Z = P, As, Sb) with quaternary Heusler structure. Results showed that FM-Y3 is the most favorable atomic arrangement. All the compounds are found to be half-metallic ferromagnetic materials with an integer magnetic moment of 2.00 μB, which predominantly derives from the strong spin polarization of p channels of C hybridized with Z elements. The predicted minority (half-metallic) band gaps were found to be 1.86 (0.87), 1.72 (0.78), and 1.78 (0.71) eV for Z = P, As, and Sb, respectively. Thermoelectric properties of the RbCaCZ (Z = P, As, Sb) materials are additionally computed over an extensive variety of temperature and it is discovered that all compounds demonstrates higher figure of merit. The half-metallic structures of these compounds with large band gaps and adequate Seebeck coefficients mean that they are suitable for use in spintronic and thermoelectric device applications.

Miniband formation engineering in GaN/AlN superlattices with constant total effective length

In this work, we study the miniband formation procedure in GaN/AlN constant total effective length superlattices by means of the subband energy calculations. We calculate the transmission coefficients and miniband structures of the systems by means of the transfer matrix and finite difference methods, respectively. The miniband structures obtained by using these methods confirm each other. Here, we observe a nonlinear miniband gap behavior as the number of wells changes. Now, we can tune the width of each miniband and minigap by means of the number of wells and the total effective length of the system. By using the strategy of fixing the total length of the system and optimizing the number of layers inside it, we can find the same miniband widths as the usual superlattice (fixing the well width, not the total system length). However, in the earlier case (our proposal), fabrication of the device may need fewer amounts of the material and may lead to smaller device sizes, because we fixed the total system length.

Structural disorder and short-range order in full Heusler alloys Fe2VAl and Co2CrAl from first principles calculations

Heusler alloys are intensively studied and applied in various spintronic devices. First principles calculations of their electronic structure allow to predict promising compounds with high spin polarization. However the theoretical predictions usually substantially deviate from experimental data. One of the reason is incorrect structural models used in the calculations. Heusler alloys are apt to form various disordered modifications with high concentration of antisite defects. In this paper we considered the effects of structural order and disorder on the electronic structure, stability and magnetic properties of full Heusler alloys Fe2VAl and Co2CrAl. The ordered Fe2VAl is non-magnetic narrow-gap semiconductor, the ordered Co2CrAl is a ferromagnet with metal behavior for majority spin band and narrow-gap semiconductor for minority one. When disordering Fe2VAl becomes a ferromagnetic metal. Co2CrAl loses minority band gap and its spontaneous magnetization reduces by 32%. Introduction of short range order in the disordered structure by increasing the portion of clusters specific for bcc structure further raised the magnetization in Fe2VAl and did not change it in Co2CrAl.

Magnons in a Quasicrystal: Propagation, Extinction, and Localization of Spin Waves in Fibonacci Structures

Magnonic quasicrystals exceed the possibilities of spin wave (SW) manipulation offered by regular magnonic crystals, because of their more complex SW spectra with fractal characteristics. Here, we report the direct x-ray microscopic observation of propagating SWs in a magnonic quasicrystal, consisting of dipolarly coupled permalloy nanowires arranged in a one-dimensional Fibonacci sequence. SWs from the first and second band as well as evanescent waves from the band gap between them are imaged. Moreover, additional mini-band gaps in the spectrum are demonstrated, directly indicating an influence of the quasiperiodicity of the system. The experimental results are interpreted using numerical calculations and we deduce a simple model to estimate the frequency position of the magnonic gaps in quasiperiodic structures. The demonstrated features of SW spectra in one-dimensional magnonic quasicrystals allows utilizing this class of metamaterials for magnonics and makes them an ideal basis for future applications.

First principles study on new half-metallic ferromagnetic ternary zinc-based sulfide and telluride (Zn3VS4 and Zn3VTe4)

In this resear, we have investigated electronic and magnetic behavior and also some mechanical properties of ternary zinc-based chalcogenides Zn3VCh4 (Ch = S and Te) conform to space group with 215 space number, which have simple cubic (SC) crystal structure, by spin-polarized Generalized Gradient Approximation (GGA) within Density Functional Theory (DFT). To make extensive study within ab initio methods, after the optimization process to get optimal structural parameters in most suitable magnetic phase, the electronic band structures with the total density of electronic states and second order elastic constants to predict some thermal and elastic features have been calculated. The calculated negative formation enthalpy values indicate that, our materials are thermodynamically stable and structurally synthesizable. The observed minority (down) band gaps in the calculated electronic band structures, Eg = 2.62 eV for Zn3VS4 and Eg = 1.75 eV for Zn3VTe4, show that our crystal systems have half-metallic character. Also, the calculated elastic constants prove that our compounds are mechanically stable due to satisfy the Born-Huang criteria. Moreover, the anisotropic elastic properties have been visualized in two-dimensional (2D) and three-dimensional (3D) surfaces for Young’s modulus, linear compressibility, shear modulus and Poisson’s ratio as well as with the calculation of the anisotropic factors.

Europium Cyclooctatetraene Nanowire Carpets: A Low-Dimensional, Organometallic, and Ferromagnetic Insulator.

We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M5,4 edge XMCD, orbital and spin magnetic moments of (-0.1 ± 0.3)μB and (+7.0 ± 0.6)μB, respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

Exploring half-metallic Co-based full Heusler alloys using a DFT+U method combined with linear response approach.

A density functional theory (DFT)+U method based on linear response (LR) theory was applied to investigate the electronic structures of a Co-based ternary full Heusler alloy Co2YSi to explore half-metallic (HM) ferromagnets with a wide HM gap. The LR-based DFT+U calculations tend to obtain a reasonable correlation parameter for the Y site, while the correlation of the Co site misdirects to the unphysical ground state due to the overestimated parameter value that arises from the delocalized electronic structure of Co. Furthermore, we found that the HM gap of Co2MnSi originates from the Co orbital in the conduction state and the Co–Mn hybridizing t2g orbital in the valence state around the Fermi energy. This means that the HM gap is a tunable property by selecting the Y element and/or mixing several elements into the Y site through t2g atomic-orbital coupling. Our LR-based DFT+U method was extended to other ternary Co2YSi and quaternary Co2(Y,Mn)Si. We found that Co2(Ti0.25,Mn0.75)Si and Co2(Fe0.25,Mn0.75)Si show HM nature, with the Fermi energy being at almost the center of the minority band gap, which leads to high thermal stability.

First-Principles Study in New Hf2CoZ (Z = B, C, Al, Si, Ga, Ge, In, Sn, Tl, and Pb) Heusler Alloys

The full-potential linearized augmented plane wave method within density functional theory was used to investigate the electronic structures, magnetic properties, and half-metallicity in Hf2CoZ (Z = B,C, Al, Si, Ga, Ge, In, Sn, Tl, and Pb) alloys with AlCu2Mn- and CuHg2Ti-type structures. The AlCu2Mn-type structure is energetically favorable than the CuHg2Ti-type structure for all compounds. Hf2CoGa, Hf2CoAl, Hf2CoIn, Hf2CoTl, and Hf2CoSn compounds in the CuHg2Ti-type structure were half-metallic ferromagnets. The d-d and covalent hybridizations between transition metals Hf and Co were effective in the formation of minority band gap. The total magnetic moments of half-metallic Hf2CoZ (Z = Al, Ga, In, Sn, and Tl) compounds in the CuHg2Ti-type structure obeyed the Slater–Pauling rule (M tot = Z tot − 18). Hf2CoIn and Hf2CoTl compounds were half-metals in relatively wide ranges of lattice constants which indicates that they can be promising and interesting candidates for spintronic devices.

Impact of strain on the electronic properties of InAs/GaSb quantum well systems

Electron-hole hybridization in InAs/GaSb double quantum well structures leads to the formation of a mini band gap. We experimentally and theoretically studied the impact of strain on the transport properties of this material system. Thinned samples were mounted to piezo electric elements to exert strain along the [011] and [001] crystal directions. When the Fermi energy is tuned through the mini gap, a dramatic impact on the resistivity at the charge neutrality point is found which depends on the amount of applied external strain. In the electron and hole regimes, strain influences the Landau level structure. By analyzing the intrinsic strain from the epitaxial growth, the external strain from the piezo elements and combining our experimental results with numerical simulations of strained and unstrained quantum wells, we compellingly illustrate why the InAs/GaSb material system is regularly found to be semimetallic.

Half-metallicity and magnetism of the full-Heusler compounds KYX 2 (Y=Ti, V, and Cr; X=C, N, and O)

Abstract The electronic structure and half-metallic (HM) properties of new alloys KYX2 (Y=Ti, V, and Cr; X=C, N, and O) containing transition metals and sp elements were investigated within the density functional theory (DFT) using the self-consistent full-potential linearized augmented plane wave (FPLAPW) method. It was found that these new compounds can be experimentally synthesized because of their negative formation energies. The total energy calculations showed that in all compounds, the stable state structure was a ferromagnetic AlCu2Mn-type structure except for KTiC2 and KTiN2 which were stable in a nonmagnetic (NM) AlCu2Mn-type structure. The KTiO2 in both structures, KCrO2 in AlCu2Mn-type structure, and KVO2 in CuHg2Ti-type structure were half-metallic ferromagnets. KVO2 in AlCu2Mn-type structure was a special case with a ferromagnetic semiconducting behavior. The origin of minority band gaps for KTiO2 in both structures was also studied using the band structure calculations. The total magnetic moments of HM compounds were integer values which were in agreement with Slater-Pauling rule (Mtot=Ztot−12). Furthermore, the regions of half-metallictiy in HM compounds were considerably wider than those of Heusler compounds including transition metals, indicating the high robustness of half-metallicity with variation of lattice constants.

The effect of pressure and alloying on half-metallicity of quaternary Heusler compounds CoMnYZ (Z = Al, Ga, and In)

AbstractIn this work, first-principles calculations of the structural, electronic and magnetic properties of Heusler alloys CoMnYAl, CoMnYGa and CoMnYIn are presented. The full potential linearized augmented plane waves (FP-LAPW) method based on the density functional theory (DFT) has been applied. The structural results showed that CoMnYZ (Z = Al, Ga, In) compounds in the stable structure of type 1+FM were true half-metallic (HM) ferromagnets. The minority (half-metallic) band gaps were found to be 0.51 (0.158), 0.59 (0.294), and 0.54 (0.195) eV for Z = Al, Ga, and In, respectively. The characteristics of energy bands and origin of minority band gaps were also studied. In addition, the effect of volumetric and tetragonal strain on HM character was studied. We also investigated the structural, electronic and magnetic properties of the doped Heusler alloys CoMnYGa1−xAlx, CoMnYAl1−xInxand CoMnYGa1−xInx(x = 0, 0.25, 0.5, 0.75, 1). The composition dependence of the lattice parameters obeys Vegard’s law. All alloy compositions exhibit HM ferromagnetic behavior with a high Curie temperature (TC).

The electronic structure and half-metallic properties of zincblende TiBi (001) surfaces and TiBi(001)/InSb(001) interface

Abstract First principles calculations were performed using full potential linearized augmented plane wave (FPLAPW) method based on density functional theory (DFT) to study bulk TiBi in rock salt (RS), hexagonal NiAs, and zincblende (ZB) structures, free (001) surfaces of ZB TiBi, and interface of ZB TiBi with InSb(001). The nonmagnetic NiAs structure was ground state structure of bulk TiBi and nonmagnetic RS and ferromagnetic ZB structures were introduced as metastable structures. It was found that ZB TiBi is a half-metallic (HM) ferromagnet with a minority band gap of about 1.43 eV. The origin of half-metallicity was also discussed. The obtained phase diagram showed more stability of the Bi (001) terminated surface compared with the Ti (001) termination. The Ti (001) termination keeps HM property, while half-metallicity was destroyed at Bi (001) termination. The ZB TiBi/InSb (001) interface revealed HM property showing that InSb semiconductor is a suitable substrate for growing ZB TiBi in spintronics.

Structural, electronic and magnetic properties of CoFeTiGa1−xSbx compounds

Structural, electronic and magnetic properties of Heusler compounds CoFeTiGa1−xSbx have been studied. X-ray diffraction patterns show that CoFeTiGa1−xSbx (x=0.25, 0.5, 0.75, 1) crystallize into LiMgPdSn-type structure. First-principles calculations indicate that CoFeTiGa1−xSbx (x=0.25, 0.5, 0.75, 1) are half-metal with considerable minority band gaps. The substitution of Sb for Ga gives rise to the spin moments of the compounds, and makes them to be half-metallic ferrimagnets.