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Machine learning approaches have recently been applied to the study of various problems in physics. Most of these studies are focused on interpreting the data generated by conventional numerical methods or the data on an existing experimental database. An interesting question is whether it is possible to use a machine learning approach, in particular a neural network, for solving the many-body problem. In this paper, we present a neural network solver for the single impurity Anderson model, the paradigm of an interacting quantum problem in small clusters. We demonstrate that the neural-network-based solver provides quantitative accurate results for the spectral function as compared to the exact diagonalization method. This opens the possibility of utilizing the neural network approach as an impurity solver for other many-body numerical approaches, such as the dynamical mean field theory. [ABSTRACT FROM AUTHOR]

The electronic structure of aluminum oxide α-Al2O3 (corundum) in the presence of s- (Li, Mg), p- (C, N, Si, Ga, Ge, Sn), and 4d-element (Zr, Nb, Mo) impurities and nonstoichiometry in both sublattices of α-Al2O3 is calculated using the coherent potential approximation. The changes in the electronic spectrum in the vicinity of the energy gap of α-Al2O3, which are caused by single-component and multicomponent doping of corundum, are studied. The mono C-, N-, Zr-, Nb-, Mo-, Si-, and Ge-doped and oxygen-deficient compositions are found to exhibit additional bands of 2p, 4d, and s symmetry inside the corundum energy gap, which agrees well with the available experimental data. The calculation results are used to develop theoretical principles of designing doped multicomponent aluminum oxide with a certain energy gap. Recommendations are given on the set and concentrations of impurities at the aluminum and oxygen sites to achieve the given characteristics of the electronic spectrum in the ultraviolet, visible, and infrared regions. Compositions promising for use as alternative oxide insulators and photoactive materials are proposed. [ABSTRACT FROM AUTHOR]

The electronic structure of metal‐doped α‐Al2O3 with the disordered distribution of multi‐d‐ (Zr, Nb or/and Mo) and s‐ (Mg) impurities in the aluminum sublattice under the variation of their concentrations have been calculated using the coherent potential approximation method. Herein, an energy gap reduction due to appearance of new isolated occupied or empty Zr, Nb, and Mo d bands near the conduction band bottom of undoped α‐Al2O3 is shown. New multi d‐ and s‐doped α‐Al2O3 dielectrics Al2−x−yZr(Nb,Mo)xMgyO3 with energy gap values of 7.1, 5.6, and 4.3 eV are found. Four multi‐d‐doped α‐Al2O3 semiconductors Al1.91Zr0.06Nb0.03O3, Al1.91Nb0.03Mo0.06O3, Al1.88Zr0.06Mo0.06O3, and Al1.86Zr0.10Nb0.02Mo0.02O3 with energy gap values of 0.2–0.4 eV can be considered as promising materials for optoelectronic devices. [ABSTRACT FROM AUTHOR]

Machine learning approaches have recently been applied to the study of various problems in physics. Most of these studies are focused on interpreting the data generated by conventional numerical methods or the data on an existing experimental database. An interesting question is whether it is possible to use a machine learning approach, in particular a neural network, for solving the many-body problem. In this paper, we present a neural network solver for the single impurity Anderson model, the paradigm of an interacting quantum problem in small clusters. We demonstrate that the neural-network-based solver provides quantitative accurate results for the spectral function as compared to the exact diagonalization method. This opens the possibility of utilizing the neural network approach as an impurity solver for other many-body numerical approaches, such as the dynamical mean field theory.

The coherent potential approximation (CPA) within full counting statistics (FCS) formalism is shown to be a suitable method to investigate average electric conductance, shot noise as well as higher order cumulants in disordered systems. We develop a similar FCS-CPA formalism for phonon transport through disordered systems. As a byproduct, we derive relations among coefficients of different phonon current cumulants. We apply the FCS-CPA method to investigate phonon transport properties of graphene systems in the presence of disorders. For binary disorders as well as Anderson disorders, we calculate up to the 8-th phonon transmission moments and demonstrate that the numerical results of the FCS-CPA method agree very well with that of the brute force method. The benchmark shows that the FCS-CPA method achieves 20 times more speedup ratio. Collective features of phonon current cumulants are also revealed. [ABSTRACT FROM AUTHOR]

We develop a real space cluster extension of the typical medium theory (cluster-TMT) to study Anderson localization. By construction, the cluster-TMT approach is formally equivalent to the real space cluster extension of the dynamical mean field theory. Applying the developed method to the 3D Anderson model with a box disorder distribution, we demonstrate that cluster-TMT successfully captures the localization phenomena in all disorder regimes. As a function of the cluster size, our method obtains the correct critical disorder strength for the Anderson localization in 3D, and systematically recovers the re-entrance behavior of the mobility edge. From a general perspective, our developed methodology offers the potential to study Anderson localization at surfaces within quantum embedding theory. This opens the door to studying the interplay between topology and Anderson localization from first principles.

Impurity doping is a necessary technology for the application of semiconductor materials in microelectronic devices. The quantification of doping effects is crucial for controlling the transport properties of semiconductors. Here, taking two-dimensional (2D) hexagonal boron phosphide semiconductor as an example, we employ coherent potential approximation method to investigate the electronic properties of 2D semiconductor materials at low doping concentrations, which cannot be exploited with conventional density function theory. The results demonstrate that the positive or negative impurity potential in 2D semiconductors determines whether it is p-type or n-type doping, while the impurity potential strength decides whether it is shallow-level or deep-level doping. Impurity concentration has important impacts on not only the intensity but also the broadening of impurity peak in band gap. Importantly, we provide the operating temperature range of hexagonal boron phosphide as a semiconductor device under different impurity concentrations and impurity potentials. The methodology of this study can be applied to other 2D semiconductors, which is of great significance for quantitative research on the application of 2D semiconductors for electronic devices., (© 2024 IOP Publishing Ltd.)

Thermal conductivity of nanocrystalline and ultra-nanocrystalline films is analyzed with effective medium theory (EMT) models. The existing EMT models use the spherical inclusion approximation. Although this approximation works quite well it is inconsistent, mostly with respect to the maximal packing of 74%, which may be unrealistic for polycrystalline films. To check the consistency of these models we devise an EMT model with arbitrarily shaped inclusions. We pick the EMT model with cubic inclusions and we compare its results with the results of the EMT model with spherical inclusions. It is found a very good agreement between both calculations. This agreement is explained by general geometrical arguments. We further employ these models to analyze thermal conductivity of nanocrystalline and ultra-nanocrystalline diamond films. It is noticed that the effective conductivity is strongly affected not only by the boundary Kapitza resistance but also by intra-grain scattering for grain sizes below 100 nm. Generally, both intra-grain conductivity and Kapitza resistance increase with grain size. However, the effect of Kapitza resistance increase is negligible due to the geometrical factor accompanying Kapitza resistance contribution to the effective conductivity. Unlabelled Image • An effective medium model with arbitrarily shaped inclusions is given for thermal conductivity of polycrystalline films • A more appropriate model with cubic crystallite gives similar results like the existing models with spherical crystallites • The theory is applied to model thermal conductivity of nanocrystalline and ultra-nanocrystalline diamond films • Thermal conductivity is strongly affected by Kapitza resistance and intra-grain scattering for grain sizes below 100 nm • Intra-grain conductivity and Kapitza resistance increase with grain size, but the effect of Kapitza resistance is small [ABSTRACT FROM AUTHOR]

Electronic structures and magnetic properties of dilute aluminium pnictides (Al1-xTx)M are calculated on the basis of density functional theory, where T denotes 3d transition metal (TM) elements and M denotes As and P elements. Among the TM elements, V, Cr, Fe and Co are used as dopants, which are used sequentially at the cation sites of the pnictide semiconductors. Ferromagnetic (FM) critical temperature (TC) is calculated using the mean-field approximation combined with the Korringa-Kohn- Rostoker Green's function method. Self-consistent energy minimisation scheme is used to select the stable magnetic state as well as the calculation of TC between the FM state and disordered local moment (DLM) state. Furthermore, electronic structures are calculated for the stable magnetic states of these compounds. A significant change in TC is obtained for varying doping concentrations. Saturated magnetisations of those stable compounds are consistent with the variation of doping concentrations, whereas spin magnetisations are found to be nearly independent of doping concentrations. [ABSTRACT FROM AUTHOR]

Highlights • Fe/Cu-doped YCo5 studied within density-functional theory. • System studied as prototype for cell boundary phase of commercial Sm-Co magnets. • We show magnetocrystalline anisotropy sensitive to dopant location. • No large difference found between Cu-rich and equal Cu-Fe doping. Abstract We present a computational study of the compound Y Co 1 - x - y Fe x Cu y 5 for 0 ⩽ x , y ⩽ 0.2. This compound was chosen as a prototype for investigating the cell boundary phase believed to play a key role in establishing the high coercivity of commercial Sm-Co 2:17 magnets. Using density-functional theory, we have calculated the magnetization and magnetocrystalline anisotropy at zero temperature for a range of compositions, modeling the doped compounds within the coherent potential approximation. We have also performed finite temperature calculations for YCo 5 , Y Co 0.838 Cu 0.162 5 and Y Co 0.838 Fe 0.081 Cu 0.081 5 within the disordered local moment picture. Our calculations find that substituting Co with small amounts of either Fe or Cu boosts the magnetocrystalline anisotropy K , but the change in K depends strongly on the location of the dopants. Furthermore, the calculations do not show a particularly large difference between the magnetic properties of Cu-rich Y Co 0.838 Cu 0.162 5 and equal Fe-Cu Y Co 0.838 Fe 0.081 Cu 0.081 5 , despite these two compositions showing different coercivity behavior when found in the cell boundary phase of 2:17 magnets. Our study lays the groundwork for studying the rare earth contribution to the anisotropy of Sm Co 1 - x - y Fe x Cu y 5 , and also shows how a small amount of transition metal substitution can boost the anisotropy field of YCo 5 . [ABSTRACT FROM AUTHOR]

The coherent potential method has been applied for the investigation of Co substitutional doping influence on the spectral and magnetic properties of TiO2 rutile (100) surface. Both stoichiometric and nonstoichiometric oxygen compositions have been considered. For the stoichiometric case, the energy spectrum corresponds to half-metallic ferromagnet. The Co ions are in 2 + formal oxidation state with a spin magnetic moment of 0.8 μB. The presence of oxygen vacancies makes the surface insulating and increases the Co spin magnetic moment value. [ABSTRACT FROM AUTHOR]

A new methodology is suggested for determining and using the Coulomb interaction parameter U(t 2g), taking into account the different energy localizations of t 2g and e g states in disordered transition metal perovskite-like oxides, based on the local spin density approximation with Coulomb interaction correction method and the coherent potential approximation. The adjustment of U(t 2g)'s relies on the best coincidence of the experimental and calculated maxima in X-ray absorption/reflection spectra at simultaneous demand for the calculated magnetic moments to closely correspond to their experimental values in cubic oxides SrFeO 3 , SrMnO 3 , and SrCoO 3. Then, the obtained U's are utilized in order to construct the Green functions for calculating the electronic properties of strongly correlated perovskite-like solid solutions within the coherent potential method. The high efficiency of the proposed methodology has been demonstrated by a close coincidence of the experimentally observed and calculated X-ray emission/absorption spectra, magnetic moments, density of states at the Fermi level, and electronic conductivity in the disordered non-stoichiometric SrFeO 3-δ , SrFe 0.75 Mn 0.25 O 3-δ , and SrFe 0.85 Co 0.15 O 3-δ phases. The composition SrFe 0.85 Co 0.15 O 3-δ is proposed as a promising electrode material with enhanced characteristics of electric transport. [Display omitted] • New methodology for selecting the Coulomb parameter U for TM t 2g shell. • Electronic spectra of SrFe 1-x (Mn,Co) x O 3-δ calculated within CPA agree with experiments. • SrFe 0.85 Co 0.15 O 3-δ with the largest n (E F) is promising for electrode applications. [ABSTRACT FROM AUTHOR]

FeSe in bulk, film or layer form as well as its alloys have shown many intriguing magnetic and superconducting properties. In this paper, we examine the role of interstitial disorder in changing the various electronic, magnetic and superconducting properties of FeSe using ab-initio method. We have studied the electronic properties of FeSe doped interstitially with the transition metal (TM) impurities, Fe(TM) _x Se (where TM ≡ V, Mn, Co, Ni, and x = 0.0, 0.01, 0.04 and 0.10). Our results, analyzed in terms of density of states (DOS), band structure, Fermi surface (FS), and local and total magnetic moments, show that the excess of the transition-metal atoms substantially modify the Fermi surface of FeSe. We have also estimated the value of the electron-phonon mass enhancement factor, the Hopfield parameter and the superconducting transition temperature for these alloys.

Abstract: The results of electronic structure calculations of imperfect crystals—nonstoichiometric metal oxides (TiO2 - x, TiO1 - x, Al2O3 - x) are reviewed. All calculations were performed within the density functional theory in the coherent potential approximation with vacancies randomly distributed in metal and oxygen sublattices. It is found that the deviations from stoichiometric composition are accompanied by appearance of vacancy induced electronic states inside the band gap of initial insulating oxides. [ABSTRACT FROM AUTHOR]

Abstract A density functional study of substitutional carbon impurities in ZnO-host matrix with 6.25 at. % was performed applying both the supercell method and the coherent potential approximation. The first of these methods assumes the spatially ordered arrangement of carbon impurities in oxygen sites, whereas the second one simulates the spatially disordered impurity substitution of oxygen sublattice. In conjunction with a theoretical part, the X-ray photoelectron spectra of pure and C-doped zinc oxide were measured. The mapping of the valence band of the carbon-loaded ZnO allows establishing the appearance of impurity C 2p electronic states on the valence band top. The experimental energy position of these impurity C 2p states is theoretically reproducing essentially better by the assumption about random distribution of carbon impurities in oxygen sublattice. Highlights • Density functional theory calculation for C:ZnO are performed. • Substitutional carbon impurities were treated by supercell method and coherent potential approximation. • X-ray photoelectron spectra of pure and C-doped ZnO were measured. • The experimental energy position of C 2p-states is reproduced better by assumption of random distribution of C-impurities. [ABSTRACT FROM AUTHOR]

The electron structure features in the high‐temperature cubic perovskite‐like ferrite La1−xSrxFeO3−δ are studied depending on concentration of disordered cation and anion defects by use of the Coherent Potential Approximatiom (CPA) taking into account the strong electronic correlations (U) in Fe d‐shell. The variations of electronic‐energy and magnetic characteristics are described at 0 ≤ δ ≤ 0.25 and 0 ≤ x ≤ 0.50. The spin state of iron atoms in stoichiometric δ = 0 compositions is shown to change near x = 0.4, which gives an explanation to the observed experimental data. [ABSTRACT FROM AUTHOR]

The photon propagator describing the interaction of light with a monolayer of metal particles (island film) is calculated in the coherent potential approximation. It is shown that the shift in the frequency peak of a dipole surface plasmon for a monolayer particle relative to the analogous frequency of the plasma resonance for an isolated particle is not the only manifestation of the influence of neighboring particles. Neighboring particles also produce a bimodal structure in the spectrum line of the plasmon resonance. The possibility of fine structure in the plasmon resonance spectrum lines is predicted. [ABSTRACT FROM AUTHOR]

We have performed ab-initio electronic structure calculations to investigate the ground state properties of Pd _1− _x Ni _x Te ( x = 0.0–0.20) alloys. The PdTe and all of its alloys are paramagnetic metals. For low concentrations, the band structure remains almost unchanged and at higher concentrations, a strong redistribution of spectral weights is observed. The most striking feature of the band structure is that the bands around the Fermi energy remain almost unchanged. The calculated Fermi surfaces are remarkably robust against disorder, strongly three-dimensional and have no or negligible nesting. The density of states at Fermi energy increases monotonically with concentration ( x ). Although the contribution of Ni to the density of states at Fermi energy is increasing continuously yet, Pd and Te dominate the density of states at Fermi energy. The density of states at Fermi energy and superconducting transition temperature T _c show opposite trends with respect to Ni concentration. So, density of states at Fermi level alone is not sufficient to discern the trends in T _c . We need to know the phonons and electron-phonon interactions as well, which at the moment are not available.