The addition of transition metals (TM) to III-V semiconductors radically changes their electronic, magnetic and structural properties. In contrast to the conventional semiconductor alloys, the lattice parameter in magnetic semiconductor alloys, including the ones with diluted concentration (the diluted magnetic semiconductors - DMS), cannot be determined uniquely from the composition. By using first-principles calculations, we find a direct correlation between the magnetic moment and the anion-TM bond lengths. We derive a simple formula that determines the lattice parameter of a particular magnetic semiconductor by considering both the composition and magnetic moment. The formula makes accurate predictions of the lattice parameter behavior of AlMnN, AlCrN, GaMnN, GaCrN, GaCrAs and GaMnAs alloys. This new dependence can explain some of the hitherto puzzling experimentally observed anomalies, as well as, stimulate other kind of theoretical and experimental investigations.
The bandgap and band bowing parameter of semiconductor alloys are calculated with a fast and realistic approach. The method is a dielectric scaling approximation that is based on a scissor approximation. It adds an energy shift to the bandgap provided by the local density approximation (LDA) of the density functional theory (DFT). The energy shift consists of a material-independent constant weighted by the inverse of the high-frequency dielectric constant. The salient feature of the approach is the fast calculation of the dielectric constant of alloys via the Green function (GF) of the TB-LMTO…
Tuning the opto-electronic properties through alloying is essential for semiconductor technology. Currently, mostly isovalent and isostructural alloys are used (e.g., group-IV and III-V), but a vast and unexplored space of novel functional materials is conceivable when considering more complex alloys by mixing aliovalent and heterostructural constituents. The real challenge lies in the quantitative property prediction for such complex alloys to guide their experimental exploration. We developed an approach to predict compositional dependence of both band-structure and electrical properties fro…
Employing $ab$ $intito$ electronic structure calculations, we have investigated electronic and magnetic properties of the Zr-based quanternary Heusler alloys: ZrCoVIn, ZrFeVGe, ZrCoFeP, ZrCoCrBe and ZrFeCrZ (Z=In and Ga). Our $ab$ $intito$ calculation results show that all the alloys are (or nearly) spin-gapless semiconductors. All the alloys have large band gaps, indicating the stability of them at room temperature. The Slater-Pauling behaviours of these alloys are discussed as well. The values of Curie temperature of all the alloys are estimated. And it is found that the values of the Curie …
The structural properties of the $Zn_xMg_{1-xS_ySe_{1-y}}$ solid solutions are determined by a combination of the computational alchemy and the cluster expansion methods with Monte Carlo simulations. We determine the phase diagram of the alloy and show that the homogeneous phase is characterized by a large amount of short-range order occurring among first-nearest neighbors. Electronic-structure calculations performed using the special quasi-random structures approach indicate that the energy gap of the alloy is rather sensitive to this short-range order.
Alloy disorder in II-VI diluted magnetic semiconductors (DMS) is typically reduced when the local magnetic spins align in an applied magnetic field. An important and untested expectation of current models of alloy disorder, however, is that alloy fluctuations in many DMS compounds should increase again in very large magnetic fields of order 100 tesla. Here we measure the disorder potential in a Zn$_{.70}$Cd$_{.22}$Mn$_{.08}$Se quantum well via the low temperature photoluminescence linewidth, using a new magnet system to 89 T. Above 70 T, the linewidth is observed to increase again, in accord w…
This paper studies the bond valence method (BVM) and its application in the non-isovalent semiconductor alloy (GaN)$_{\rm{1-x}}$(ZnO)$_{\rm{x}}$. Particular attention is paid to the role of short-range order (SRO). A physical interpretation based on atomic orbital interaction is proposed and examined by density-functional theory (DFT) calculations. Combining BVM with Monte-Carlo simulations and a DFT-based cluster expansion model, bond-length distributions and bond-angle variations are predicted. The correlation between bond valence and bond stiffness is also revealed. Finally the concept of b…
Deviations from randomicity in In(x)Ga(1-x) As semiconductor alloys induced by elastic effects are investigated within the Keating potential. Our model is based on Monte Carlo simulations on large (4096 atoms) supercells, performed with two types of boundary conditions: Fully periodic boundary conditions represent the bulk, while periodic boundary conditions along the x and y directions and a free surface in the z direction simulate the epitaxial growth environment. We show that In-In correlations identified in the bulk tend to be enhanced in the epitaxially grown samples.
Electronic properties of III-V semiconductor alloys are examined using first principles with the focus on the spatial localization of electronic states. We compare localization at the band edges due to various isovalent impurities in a host GaAs including its impact on the photoluminescence line widths and carrier mobilities. The extremity of localization at the band edges is correlated with the ability of individual elements to change the band gap and the relative band alignment. Additionally, the formation energies of substitutional defects are calculated and linked to challenges associated …
Applying the approximate DFT-1/2 quasiparticle scheme, band structure unfolding, and generalized quasichemical approximation to describe chemical and structural disorder, we investigate the electronic structure of Bi$_{1-x}$Sb$_x$ alloys from first principles. We calculate the important energy levels near the Fermi energy versus the Sb concentration $x$ where the trivial-topological (TT) and semimetal-semiconductor (SMSC) transitions occur. We demonstrate that the energy variation of the relevant states deviates significantly from linear behavior and that the bowings are important to correctly…
We have investigated the anomalous Ettingshausen effect (AEE) in iron-carbon alloys, i.e., cast irons and steel, using the lock-in thermography. All the alloys exhibit the clear AEE-induced temperature modulation, and their anomalous Ettingshausen coefficient is an order of magnitude greater than that of the pure iron at room temperature. The dimensionless figure of merit for AEE in the ductile cast iron is 55 times greater than that in the pure iron owing to the significant increase of the anomalous Ettingshausen coefficient. Our result reveals a potential of iron-carbon alloys as transverse …
We have studied growth and properties of quaternary alloy magnetic semiconductor (InGaMn)As grown both on GaAs substrates and on InP substrates by low-temperature molecular-beam epitaxy (LT-MBE). (InGaMn)As thin films were ferromagnetic below ~30 K, exhibiting strong magneto-optical effect. The lattice constant of [(InyGa1-y)1-xMnx]As, whose Mn concentration x is below 4%, is slightly smaller than that of InyGa1-yAs with the same In/Ga content ratio. We have also obtained very smooth surface morphology of nearly lattice matched (InGaMn)As thin films grown on InP substrates, which is important …
The standard theoretical understanding of the lattice thermal conductivity, $κ_{\ell}$, of semiconductor alloys assumes that mass disorder is the most important source of phonon scattering. In contrast, we show that the hitherto neglected contribution of force-constant (IFC) disorder is essential to accurately predict the $κ_{\ell}$ of those polar compounds characterized by a complex atomic-scale structure. We have developed an \emph{ab initio} method based on special quasirandom structures and Green's functions, and including the role of IFC disorder, and applied it in order to calculate the …
This paper studies short-range order (SRO) in the semiconductor alloy (GaN)$_{1-x}$(ZnO)$_x$. Monte Carlo simulations performed on a density functional theory (DFT)-based cluster expansion model show that the heterovalent alloys exhibit strong SRO because of the energetic preference for the valence-matched nearest-neighbor Ga-N and Zn-O pairs. To represent the SRO-related structural correlations, we introduce the concept of Special Quasi-ordered Structure (SQoS). Subsequent DFT calculations reveal dramatic influence of SRO on the atomic, electronic and vibrational properties of the (GaN)$_{1-x…
We propose a simple and effective approach to construct the empirical tight-binding parameters of ternary alloys in the virtual crystal approximation. This combines a new, compact formulation of the strain parameters and a linear interpolation of the hamiltonians of binary materials strained to the alloy equilibrium lattice parameter. We show that it is possible to obtain a perfect description of the bandgap bowing of ternary alloys in the InGaAsSb family of materials. Furthermore, this approach is in a good agreement with supercell calculations using the same set of parameters. This scheme op…
Parameters of the energy gap and, consequently, electronic, magnetic and galvanomagnetic properties in different X$_2$YZ Heusler alloys can vary quite strongly. In particular, half-metallic ferromagnets (HMFs) and spin gapless semiconductors (SGSs) with almost 100% spin polarization of charge carriers are promising materials for spintronics. The changes in the electrical, magnetic and galvanomagnetic properties of the Co$_2$YSi (Y = Ti, V, Cr, Mn, Fe) and Co$_2$MnZ Heusler alloys (Z = Al, Si, Ga, Ge) in possible HMF and/or SGS states were followed and their interconnection was established. Sig…
A comprehensive magnetotransport study including resistivity ($ρ_{xx}$) at various fields, isothermal magnetoresistance and Hall resistivity ($ρ_{xy}$) has been carried out at different temperatures on the Co$_{2}$TiAl Heusler alloy. Co$_{2}$TiAl alloy shows a paramagnetic (PM) to ferromagnetic (FM) transition below the curie temperature (T$_{C}$) $\sim$ 125 K. In the FM region, resistivity and magnetoresistance reveals a spin flip electron-magnon scattering and the Hall resistivity unveils the anomalous Hall resistivity ($ρ_{xy}^{AH}$). Scaling of anomalous Hall resistivity with resistivity e…
From electronic-structure-based thermodynamic linear-response, we establish chemical ordering behavior in complex solid solutions versus how Gibbs' space is traversed -- applying it on prototype refractory A2 Ta-Nb-Mo-W high-entropy alloys. Near ideal stoichiometry, this alloy has anomalous, intricate chemical ordering tendencies, with long-ranged chemical interactions that produce competing short-range order (SRO) with a crossover to spinodal segregation. This atypical SRO arises from canonical band behavior that, with alloying, create features near the Fermi-surface (well-defined even with d…
The evolution of the optical phonons in layered semiconductor alloys SnSe1-xSx is studied as a function of the composition by using polarized Raman spectroscopy with six different excitation wavelengths (784.8, 632.8, 532, 514.5, 488, and 441.6 nm). The polarization dependences of the phonon modes are compared with transmission electron diffraction measurements to determine the crystallographic orientation of the samples. Some of the Raman modes show significant variation in their polarization behavior depending on the excitation wavelengths. It is established that the maximum intensity direct…
Shining light on short-range atomic ordering in semiconductors alloys
Stanford University（斯坦福大学）；SLAC National Accelerator Laboratory（斯坦福直线加速器中心）；George Washington University（乔治华盛顿大学）；National Institute of Standards and Technology（美国国家标准与技术研究院）；University of California Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Laboratory（劳伦斯伯克利国家实验室）；Max Planck Institute for Sustainable Materials（马克斯·普朗克可持续材料研究所）
An amorphous Ti50Ni25Cu25 alloy was produced by 19 h of mechanical alloying. Anomalous wide angle x-ray scattering data were collected at six energies and six total scattering factors were obtained. By considering the data collected at two energies close to the Ni and Cu K edges, two differential anomalous scattering factors around the Ni and Cu atoms were obtained, showing the chemical environments around these atoms are different. The eight factors were used as input data to the reverse Monte Carlo method used to compute the partial structure factors STi-Ti(K), STi-Cu(K), STi-Ni(K), SCu-Cu(K…
Ultra-wide direct band gap semiconductors hold great promise for deep ultraviolet opto-electronic applications. Here we evaluate the potential of MgSiN$_2$-GaN alloys for this purpose. Although MgSiN$_2$ itself has an indirect gap $\sim$0.4 eV below its direct gap of $\sim$6.5 eV, its different sign lattice mismatch from GaN in two different basal plane directions could avoid the tensile strain which limits Al$_x$Ga$_{1-x}$N on GaN for high $x$. Two octet-rule preserving structures (with space groups $Pmn2_1$ and $P1n1$) of a 50% alloy of MgSiN$_2$ and GaN are investigated and are both found t…
First-principles simulations have been performed for [001]-ordered Mn/Ge and Mn/GaAs "digital alloys", focusing on the effects of i) a larger band-gap and ii) a different semiconducting host on the electronic structure of the magnetic semiconductors of interest. Our results for the exchange constants in Mn/Ge, evaluated using a frozen-magnon scheme, show that a larger band-gap tends to give a stronger nearest-neighbor ferromagnetic coupling and an overall enhanced in-plane ferromagnetic coupling even for longer-ranged coupling constants. As for the chemical effects on the exchange constants, w…
Nearly all experimental observations of quasi-ballistic heat flow are interpreted using Fourier theory with modified thermal conductivity. Detailed Boltzmann transport equation (BTE) analysis, however, reveals that the quasi-ballistic motion of thermal energy in semiconductor alloys is no longer Brownian but instead exhibits Lévy dynamics with fractal dimension $α< 2$. Here, we present a framework that enables full 3D experimental analysis by retaining all essential physics of the quasi-ballistic BTE dynamics phenomenologically. A stochastic process with just two fitting parameters describes t…
Semiconductor alloys exhibit a strong dependence of effective thermal conductivity on measurement frequency. So far this quasi-ballistic behaviour has only been interpreted phenomenologically, providing limited insight into the underlying thermal transport dynamics. Here, we show that quasi-ballistic heat conduction in semiconductor alloys is governed by Lévy superdiffusion. By solving the Boltzmann transport equation (BTE) with ab initio phonon dispersions and scattering rates, we reveal a transport regime with fractal space dimension $1 < α< 2$ and superlinear time evolution of mean square e…
A review of theoretical and experimental studies of the electronic structure, electronic and magnetic properties of various systems of Heusler alloys in the states of a half-metallic ferromagnet, a spin gapless semiconductor, and a topological semimetal is presented. These substances have unusual, highly sensitive to external influences, magnetic and electronic characteristics, which is associated with the presence of energy gaps and exotic excitations in them. The features of the behavior and evolution of the electronic structure and properties in each of these states, as well as during the t…
We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity $σ_{xy}$. This method is based on the different microscopic conductive processes of each origin of $σ_{xy}$ in the Kubo--Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value…
The control of spin without magnetic field is one of challenges in developing spintronic devices. In an attempt to solve this problem, we proposed a novel hypothetic LaMn0.5Zn0.5AsO alloy from two experimentally synthesized rare earth element transition metal arsenide oxides, i.e. LaMnAsO and LaZnAsO. On the basis of the first-principles calculations with strong-correlated correction, we found that the LaMn0.5Zn0.5AsO alloy is an antiferromagnetic semiconductor at ground state, while bipolar magnetic semiconductor at ferromagnetic state. Both electron and hole doping in the LaMn0.5Zn0.5AsO all…
Edge Dislocation Mediated Anomalous Charge Transfer in Face Centered Cubic High Entropy Alloys
School of Materials Science and Technology, IIT-BHU（印度理工联合学院瓦拉纳西分校材料科学与技术学院）；National Centre for Computational Sciences, Oak Ridge National Laboratory（橡树岭国家实验室国家计算科学中心）；Department of Metallurgy and Materials Engineering, IIEST Shibpur（IIEST Shibpur冶金与材料工程系）
We observe that pressure-induced amorphization of Ga2SeTe2 (a III-VI semiconductor) is directly influenced by the periodicity of its intrinsic defect structures. Specimens with periodic and semi-periodic two-dimensional vacancy structures become amorphous around 10-11 GPa in contrast to those with aperiodic structures, which amorphize around 7-8 GPa. The result is a notable instance of altering material phase-change properties via rearrangement of stoichiometric vacancies as opposed to adjusting their concentrations. Based on our experimental findings, we posit that periodic two-dimensional va…
We report a new model for highly mismatched semiconductor (HMS) alloys. Based on the Anderson impurity Hamiltonian, the model generalizes the recent band anti-crossing (BAC) model, which successfully explains the band bowing in highly mismatched semiconductors. Our model is formulated in empirical tight-binding (ETB) theory and uses the so called sp$^{3}$s* parameterization. It does not need extra parameters other than bulk ones. The model has been applied to BeSe$_{x}$Te$_{1 - x}$ alloy. BeTe and BeSe are wide-band gap and highly mismatched semiconductors. Calculations show large band bowing,…
We have studied magnetic properties of heavily Mn-doped [(In0.44Ga0.56)0.79Mn0.21]As thin films grown by low-temperature molecular-beam epitaxy (LT-MBE) on InP substrates. The (InGaMn)As with high Mn content (21%) was obtained by decreasing the growth temperature to 190 degC. When the thickness of the [(In0.44Ga0.56)0.79Mn0.21]As layer is equal or thinner than 10 nm, the reflection high-energy electron diffraction (RHEED) pattern and transmission electron microscopy (TEM) show no MnAs clustering, indicating that a homogeneous single crystal with good quality was grown. In the magnetic circular…
The anomalous Hall effect (AHE) in L10 FePd1-xPtx alloy films is studied both experimentally and theoretically. We find that the intrinsic contribution (?sigma^int_AH) to the AHE can be significantly increased whereas the extrinsic side-jump contribution (sigma^?sj_AH) can be continuously reduced from being slightly larger than sigma^?int_AH in L10 FePd to being much smaller than sigma^?int_AH in L10 FePt, by increasing the Pt composition x. We show that this chemical composition tuning of the intrinsic contribution is afforded by the stronger spin-orbit coupling strength on the Pd/Pt site whe…
Electronic structures of Ge$_{1-x}$Sn$_{x}$ alloys (0 $\leq$ $x$ $\leq$ 1) are theoretically studied by nonlocal empirical pseudopotential method. For relaxed Ge$_{1-x}$Sn$_{x}$, a topological semimetal is found for $\textit x$ $>$ 41$\%$ with gapless and band inversion at $Γ$ point, while there is an indirect-direct bandgap transition at $x$ = 8.5$\%$. For strained Ge$_{1-x}$Sn$_{x}$ on a Ge substrate, semimetals with a negative indirect bandgap appear for $x$ $>$ 43$\%$, and the strained Ge$_{1-x}$Sn$_{x}$ on Ge is always an indirect bandgap semiconductor for $x$ $<$ 43$\%$. With appropriate…
Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and \emph{p}-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap differe…
We theoretically analyse band-to-band tunneling (BTBT) in highly-mismatched, narrow-gap dilute nitride and bismide alloys, and quantify the impact of the N- or Bi-induced perturbation of the band structure -- due to band-anticrossing (BAC) with localised impurity states -- on the electric field-dependent BTBT generation rate. For this class of semiconductors the assumptions underpinning the widely-employed Kane model of direct BTBT break down, due to the strong band edge nonparabolicity resulting from BAC interactions. Via numerical calculations based on the Wentzel-Kramers-Brillouin approxima…
We demonstrate how to overcome serious problems in understanding and classification of vibration spectra in semiconductor alloys, following from traditional use of the virtual crystal approximation (VCA). We show that such different systems as InGaAs (1-bond->1-mode behavior), InGaP (modified 2-mode) and ZnTeSe (2-bond->1-mode) obey in fact the same phonon mode behavior - hence probably a universal one - of a percolation-type (1-bond->2-mode). The change of paradigm from the `VCA insight' (an averaged microscopic one) to the `percolation insight' (a mesoscopic one) offers a promising link towa…
III-nitride alloys are wide band gap semiconductors with a broad range of applications in optoelectronic devices such as light emitting diodes and laser diodes. Indium gallium nitride light emitting diodes have been successfully produced over the past decade. But the progress of green emission light emitting devices has been limited by the incorporation of indium in the alloy, mainly due to phase separation. This difficulty could be addressed by studying the growth and thermodynamics of these alloys. Knowledge of thermodynamic phase stabilities and of pressure - temperature - composition phase…
The band gap engineering of group IV semiconductors has not been well explored theoretically and experimentally, except for SiGe. Recently, GeSn has attracted much attention due to the possibility of obtaining a direct band gap in this alloy, thereby making it suitable for light emitters. Other group IV alloys may also potentially exhibit material properties useful for device applications, expanding the space for band gap engineering in group IV. In this work the electronic band structure of all group IV semiconductor alloys is investigated. Twelve possible A:B alloys, where A is a semiconduct…
This paper reports high resolution X-ray photoelectron spectroscopy (XPS) studies on Fe$_{1-x}$Ni$_x$ (x=0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) alloys down to 10 K temperature. Core levels and Auger transitions of the alloys except the invar alloy (x=0.4) exhibit no observable temperature induced changes. The invar alloy exhibits changes in the core levels below 20 K temperature that strongly depend on the core level. Such core level dependent changes with temperature were attributed to the precipitation of spin glass like phase below 20 K only in the invar alloy. Ni L$_3$M$_{45}$M$_{45}$ Auger tr…
We carry out first-principles calculations which demonstrate the importance of the non-spin-conserving part of the spin-orbit interaction for the intrinsic anomalous Hall conductivity of ordered FePt alloys. The impact of this interaction is strongly reduced if Pt is replaced by the lighter isoelectronic element Pd. An analysis of the interband transitions responsible for the anomalous velocity reveals that spin-flip transitions occur not only at avoided band crossings near the Fermi level, but also between well-separated pairs of bands with similar dispersions. We also predict a strong anisot…
The variation of TO zone-center vibration spectra with concentration in mixed zincblende-type semiconductors can be understood within a paradigm of unified "one bond - two modes" approach, which has been recently outlined as a rather general concept, and emerges from a number of previous experimental and theoretical studies. The crucial issue is that the vibration frequency, associated with a certain cation-anion bond, depends on the length of the latter, and the bond length, in its turn, depends not only on the average alloy concentration, but on local variations of it. In an (A,B)C substitut…
The technological concept of "digital alloying" offered by molecular-beam epitaxy is demonstrated to be a very effective tool for tailoring static and dynamic magnetic properties of diluted magnetic semiconductors. Compared to common "disordered alloys" with the same Mn concentration, the spin-lattice relaxation dynamics of magnetic Mn ions has been accelerated by an order of magnitude in (Cd,Mn)Te digital alloys, without any noticeable change in the giant Zeeman spin splitting of excitonic states, i.e. without effect on the static magnetization. The strong sensitivity of the magnetization dyn…
A coherent description of the anomalous Hall effect (AHE) is presented that is applicable to pure as well as disordered alloy systems by treating all sources of the AHE on equal footing. This is achieved by an implementation of the Kubo-Středa equation using the fully relativistic Korringa-Kohn-Rostoker (KKR) Green's function method in combination with the Coherent Potential Approximation (CPA) alloy theory. Applications to the pure elemental ferromagnets bcc-Fe and fcc-Ni led to results in full accordance with previous work. For the alloy systems fcc-Fe$_x$Pd$_{1-x}$ and fcc-Ni$_x$Pd$_{1-x}$ …
We present total energy and force calculations on the (GaN)$_{1-x}$(ZnO)$_{x}$ alloy. Site-occupancy configurations are generated by Monte Carlo (MC) simulations, based on a cluster expansion (CE) model proposed in a previous study. Surprisingly large local atomic coordinate relaxations are found by density-functional calculations using a 432-atom periodic supercell, for three representative configurations at $x=0.5$. These are used to generate bond length distributions. The configurationally averaged composition- and temperature-dependent short-range order (SRO) parameters of the alloys are d…
Au-Fe alloys are of immense interest due to their biocompatibility, anomalous hall conductivity, and applications in various medical treatment. However, irrespective of the method of preparation, they often exhibit a high-level of disorder, with properties sensitive to the thermal or magnetic annealing temperatures. We calculate lattice dynamical properties of Au$_{1-x}$Fe$_x$ alloys using density functional theory methods, where, being a multisite property, reliable interatomic force constant (IFC) calculations in disordered alloys remain a challenge. We follow a two fold approach: (1) an acc…
We report a detailed investigation of the Ni$_{2}$MnGa shape memory alloy through magnetic, electronic, and thermal measurements. Our measurements of the anomalous Nernst effect (ANE) reveal that this technique is very sensitive to the onset of the pre-martensitic transition in sharp contrast to other transport measurements. With the ANE being sensitive to changes at the Fermi surface, we infer on the link between the structural modulations and the modulation of the Fermi surface via its nesting features, with the magnetic field induced strain being the mediating mechanism.
A procedure is presented that combines density functional theory computations of bulk semiconductor alloys with the semiconductor Bloch equations, in order to achieve an ab initio based prediction of the optical properties of semiconductor alloy heterostructures. The parameters of an eight-band kp-Hamiltonian are fitted to the effective band structure of an appropriate alloy. The envelope function approach is applied to model the quantum well using the kp-wave functions and eigenvalues as starting point for calculating the optical properties of the heterostructure. It is shown that Luttinger p…
Inverse Heusler alloy Mn2CoAl thin films, known as a spin-gapless semiconductor (SGS), grown by three different methods: ultra-high vacuum magnetron spattering, Ar-ion beam assisted sputtering, and molecular beam epitaxy, are investigated by comparing their electric transport properties, microstructures and atomic-level structures. Of the samples, the Mn2CoAl thin film grown by MBE consists of Mn- and Co-rich phases, the structures of which are determined to be the L21B-type and disordered L21-type, respectively, according to anomalous XRD analysis. None of them forms the XA-type structure exp…
By performing first principles calculations we investigate the intrinsic anomalous Hall conductivity (AHC) and its anisotropy in ordered L1o FePt, CoPt and NiPt ferromagnets, and their intermediate alloys. We demonstrate that the AHC in this family of compounds depends strongly on the direction of the magnetization in the crystal. We predict that such pronounced orientational dependence in combination with the general decreasing trend of the AHC when going from FePt to NiPt leads to a sign change of the AHC upon rotating the magnetization direction in the crystal of CoPt alloy. We also suggest…
The origin of the anomalous, 400% increase of the piezoelectric coefficient in Sc$_x$Al$_{1-x}$N alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a strong change in the response of the internal atomic coordinates to strain and pronounced softening of C$_{33}$ elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric …
Whereas exceptional mechanical and radiation performances have been found in the emergent medium- and high-entropy alloys (MEAs and HEAs), the importance of their complex atomic environment, reflecting diversity in atomic size and chemistry, to defect transport has been largely unexplored at the atomic level. Here we adopt a local structure approach based on the atomic pair distribution function measurements in combination with density functional theory calculations to investigate a series of body-centered cubic (BCC) MEAs and HEAs. Our results demonstrate that all alloys exhibit local lattice…
Si$_{y}$Ge$_{1-x-y}$(C,Sn,Pb)$_{x}$ alloys have attracted significant attention as a route to achieve a direct-gap group-IV semiconductor. Using density functional theory (DFT) - employing local density approximation and hybrid Heyd-Scuzeria-Ernzerhof exchange-correlation functionals - we compute the lattice parameters, relaxed and inner elastic constants, and internal strain (Kleinman) parameters for elemental (diamond) group-IV materials and zinc blende IV-IV compounds. Our DFT calculations support a little-known experimental re-evaluation of the $α$-Sn elastic constants, and contradict a re…
