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While recent experiments confirm the existence of hundreds of topological electronic materials, only a few exhibit the coexistence of superconductivity (SC) and a topological electronic state. These compounds attract significant attention in forefront research because of the potential for the existence of topological SC, paving the way for future technological advancements. SrSn4 is known for exhibiting unusual SC below the transition temperature (TC) of 4.8 K. Recent theory predicts a topological electronic state in this compound, which is yet to be confirmed by experiments. Systematic and detailed studies of the magnetotransport properties of SrSn4 and its Fermi surface characterizations are also absent. For the first time, a quantum oscillation study reveals a nontrivial 흅‐Berry phase, very light effective mass, and high quantum mobility of charge carriers in SrSn4. Magnetotransport experiment unveils large linear transverse magnetoresistance (TMR) of more than 1200% at 5 K and 14 T. Angle‐dependent transport experiments detect anisotropic and fourfold symmetric TMR, with the maximum value (≈2000%) occurring when the angle between the magnetic field and the crystallographic b‐axis is 45°. The results suggest that SrSn4 is the first topological material with SC above the boiling point of helium that displays such high magnetoresistance. [ABSTRACT FROM AUTHOR]

In the present work, we numerically and experimentally study the Co 3 - x Ni x O 4 (spinel-like oxides) system. Using the perturbative density functional theory (p-DFT) method, we start the study from the homogeneous sample ( x = 0 ), obtaining the main electronic properties (band structure (BS), density of states (DOS), and Fermi surface (FS)). Subsequently, we doped (x) with Ni atoms in different proportions (0–7% respectively, taking 56 atoms as 100% and the percentage of doping, on this percentage). As we increase the doping (x ≠ 0) , we have found that the forbidden gap decreases and the Fermi energy (FE) decreases, causing the material to exhibit a transition phase for a particular doping value. In addition, we find that more bands are generated when the system is doped, which would be responsible for the phase transition. The data from the theoretical analysis carried out in this paper was compared with the experimental data of various widely accepted works. Some of the results, when compared with the information available from the experimental ones, show good agreement. [ABSTRACT FROM AUTHOR]

The electronic properties of calcium-intercalated graphite (CaC6) as a function of pressure are revisited using density functional theory (DFT). The electronic band structures of CaC6, like many other layered superconducting materials, display cosine-shaped bands at or near the Fermi level (FL). Such bands encompass bonding/antibonding information with a strong connection to superconducting properties. Using a hexagonal cell representation for CaC6, the construction of a double supercell in the c-direction effects six-folding in the reciprocal space of the full cosine function, explicitly revealing the bonding/antibonding relationship divide at the cosine midpoint. Similarly, folding of the Fermi surface (FS) reveals physical phenomena relevant to electronic topological transitions (ETTs) with the application of pressure. The ETT is characterised by a transition of open FS loops to closed loops as a function of pressure. As the highest transition temperature is reached with pressure, the dominant continuous, open FS loops shift to a different region of the FS. For CaC6, the peak value for the superconducting transition temperature, Tc, occurs at about 7.5 GPa, near the observed pressure of the calculated ETT. At this pressure, the radius of the nearly spherical Ca 4s-orbital FS coincides with three times the distance from the Γ centre point to the Brillouin zone (BZ) boundary of the 2c supercell. In addition, the ETT coincides with the alignment of the nonbonding (inflection) point of the cosine band with the FL. At other calculated pressure conditions, the Ca 4s-orbital FS undergoes topological changes that correspond and can be correlated with experimentally determined changes in Tc. The ETT is a key mechanism that circumscribes the known significant drop in Tc for CaC6 as a function of increasing pressure. Consistent calculated responses of the ETT to pressure match experimental measurements and validate the examination of superlattices as important criteria for understanding mechanisms driving superconductivity. [ABSTRACT FROM AUTHOR]

The superconductivity of CaC6 as a function of pressure and Ca isotopic composition was revisited using DFT calculations on a 2c–double hexagonal superlattice. The introduction of superlattices was motivated by previous synchrotron absorption and Raman spectroscopy results on other superconductors that showed evidence of superlattice vibrations at low (THz) frequencies. For CaC6, superlattices have previously been invoked to explain the ARPES data. A superlattice along the hexagonal c-axis direction is also illustrative of atomic orbital symmetry and periodicity, including bonding and antibonding s-orbital character implied by cosine-modulated electronic bands. Inspection of the cosine band revealed that the cosine function has a small (meV) energy difference between the bonding and antibonding regions, relative to a midpoint non-bonding energy. Fermi surface nesting was apparent in an appropriately folded Fermi surface using a superlattice construct. Nesting relationships identified phonon vectors for the conservation of energy and for phase coherency between coupled electrons at opposite sides of the Fermi surface. A detailed analysis of this Fermi surface nesting provided accurate estimates of the superconducting gaps for CaC6 with the change in applied pressure. The recognition of superlattices within a rhombohedral or hexagonal representation provides consistent mechanistic insight on superconductivity and electron−phonon coupling in CaC6. [ABSTRACT FROM AUTHOR]

In this paper, we have explored the electronic and magnetic properties of MS2(M = Co , Ni) using first-principles calculations. Our data show rather high tunability of the electronic and magnetic properties of the alloy Co 1 − x NixS2 (0. 0 ≤ x ≤ 1. 0) with the emergence of half-metallicity that persisted up to the intermediate doping concentration. The half-metallic ground state is characterized by large spin polarization at the Fermi level ( E F ). Beyond the critical doping concentration x ∼ 0. 6 , we obtained a metallic solution followed by an antiferromagnetic ground state at a larger doping concentration. This study provides the underlying physics to understand the low-energy Hilbert space and reports the role of the Fermi surface in controlling the electron transport and thus elucidating the anomalous electronic and magnetic behavior of Co 1 − x NixS2. [ABSTRACT FROM AUTHOR]

We discuss various aspects of nonlocal electrical transport in anisotropic metals. For a metal with circular Fermi surface, the scattering rates entering the local conductivity and viscosity tensors are well-defined, corresponding to eigenfrequencies of the linearized collision operator. For anisotropic metals, we provide generalized formulas for these scattering rates and use a variational approximation to show how they relate to microscopic transition probabilities. We develop a simple model of a collision operator for a metal of arbitrary Fermi surface with finite number of quasi-conserved quantities, and derive expressions for the wavevector-dependent conductivity σ (q) and the spatially-varying conductivity σ (x) for a long, narrow channel. We apply this to the case of different rates for momentum-conserving and momentum-relaxing scattering, deriving closed-form expressions for σ (q) and σ (x) — beyond generalizing from circular to arbitrary Fermi surface geometry, this represents an improvement over existing methods which solve the relevant differential equation numerically rather than in closed form. For the specific case of a diamond Fermi surface, we show that, if transport signatures were interpreted via a model for a circular Fermi surface, the diagnosis of the underlying transport regime would differ based on experimental orientation and based on whether σ (q) or σ (x) was considered. Finally, we discuss the bulk conductivity. While the common lore is that "momentum"-conserving scattering does not affect bulk resistivity, we show that crystal momentum-conserving scattering — such as normal electron-electron scattering — can affect the bulk resistivity for an anisotropic Fermi surface. We derive a simple formula for this contribution. [ABSTRACT FROM AUTHOR]

The electronic properties of calcium-intercalated graphite (CaC6) as a function of pressure are revisited using density functional theory (DFT). The electronic band structures of CaC6, like many other layered superconducting materials, display cosine-shaped bands at or near the Fermi level (FL). Such bands encompass bonding/antibonding information with a strong connection to superconducting properties. Using a hexagonal cell representation for CaC6, the construction of a double supercell in the c-direction effects six-folding in the reciprocal space of the full cosine function, explicitly revealing the bonding/antibonding relationship divide at the cosine midpoint. Similarly, folding of the Fermi surface (FS) reveals physical phenomena relevant to electronic topological transitions (ETTs) with the application of pressure. The ETT is characterised by a transition of open FS loops to closed loops as a function of pressure. As the highest transition temperature is reached with pressure, the dominant continuous, open FS loops shift to a different region of the FS. For CaC6, the peak value for the superconducting transition temperature, Tc, occurs at about 7.5 GPa, near the observed pressure of the calculated ETT. At this pressure, the radius of the nearly spherical Ca 4s-orbital FS coincides with three times the distance from the Γ centre point to the Brillouin zone (BZ) boundary of the 2c supercell. In addition, the ETT coincides with the alignment of the nonbonding (inflection) point of the cosine band with the FL. At other calculated pressure conditions, the Ca 4s-orbital FS undergoes topological changes that correspond and can be correlated with experimentally determined changes in Tc. The ETT is a key mechanism that circumscribes the known significant drop in Tc for CaC6 as a function of increasing pressure. Consistent calculated responses of the ETT to pressure match experimental measurements and validate the examination of superlattices as important criteria for understanding mechanisms driving superconductivity.

Raman and synchrotron THz absorption spectral measurements on MgB2 provide experimental evidence for electron orbital superlattices. In earlier work, we have detected THz spectra that show superlattice absorption peaks with low wavenumbers, for which spectral density evolves and intensifies after cooling below the superconducting transition temperature for MgB2. In this work, we show how these observations indicate a direct connection to superconducting properties and mechanisms. Bonding–antibonding orbital character is identified in calculated electronic band structures and Fermi surfaces consistent with superlattice structures along the c-axis. DFT calculations show that superlattice folding of reciprocal space generates Brillouin zone boundary reflections, Umklapp processes, and substantially enhances nesting relationships. Tight binding equations are compared with expected charge density waves from nesting relationships and adjusted to explicitly accommodate these linked processes. Systematic analysis of electronic band structures and Fermi surfaces allows for direct identification of Cooper pairing and the superconducting gap, particularly when the k-grid resolution of a calculation is suitably calibrated to structural parameters. Thus, we detail a robust and accurate DFT re-interpretation of BCS superconductivity for MgB2. [ABSTRACT FROM AUTHOR]

The superconductivity of CaC6 as a function of pressure and Ca isotopic composition was revisited using DFT calculations on a 2c–double hexagonal superlattice. The introduction of superlattices was motivated by previous synchrotron absorption and Raman spectroscopy results on other superconductors that showed evidence of superlattice vibrations at low (THz) frequencies. For CaC6, superlattices have previously been invoked to explain the ARPES data. A superlattice along the hexagonal c-axis direction is also illustrative of atomic orbital symmetry and periodicity, including bonding and antibonding s-orbital character implied by cosine-modulated electronic bands. Inspection of the cosine band revealed that the cosine function has a small (meV) energy difference between the bonding and antibonding regions, relative to a midpoint non-bonding energy. Fermi surface nesting was apparent in an appropriately folded Fermi surface using a superlattice construct. Nesting relationships identified phonon vectors for the conservation of energy and for phase coherency between coupled electrons at opposite sides of the Fermi surface. A detailed analysis of this Fermi surface nesting provided accurate estimates of the superconducting gaps for CaC6 with the change in applied pressure. The recognition of superlattices within a rhombohedral or hexagonal representation provides consistent mechanistic insight on superconductivity and electron−phonon coupling in CaC6.

MgAl2O4 spinel matrix nanocomposite with TiC, AlTi3, and Al2O3 was directly fabricated using the mechanical alloying (MA) method with Mg, TiO2, Al, and graphite as the starting powder mixture. This methodology was used to synthesize magnesium aluminate spinel (MSA) matrix nanocomposite with having the advantage of no subsequent heat treatment. The mixture of the powders was milled at room temperature using a high-energy planetary ball mill with a vial rotation speed of 400 rpm in the air. The phases and morphological analysis after the MA process were characterized by X-ray diffraction and scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectroscopy (EDS). Followed by 20 h of ball-milling, all the desired phases of MgAl2O4, TiC, AlTi3, and Al2O3 appeared in XRD results, and also a small fraction of raw materials remained. The major reactions to synthesize MgAl2O4/TiC-AlTi3-Al2O3 spinel matrix nanocomposite were the reduction of TiO2 by Al and Mg. [ABSTRACT FROM AUTHOR]

This study examines the physical properties of germanium-based halide perovskite through Density Functional Theory (DFT) computations. The physical, optical, mechanical, and magnetic properties of NaGeX _3 (X = Cl, Br, and I) were examined with the effects of hydrostatic pressure applied externally. The compounds were subjected to pressure variations ranging from 0 to 5 GPa. The results indicate a decrease in the band gap from the infrared to the visible spectrum. For NaGeCl _3 , NaGeBr _3 , and NaGeI _3 the band gap decreased from 0.766 eV, 0.497 eV, and 0.400 eV to 0 eV, respectively, indicating the metallic behavior. The mechanical properties of NaGeX _3 (X = Cl, Br, and I) demonstrate that for all three compounds, Bulk Modulus (B), Shear Modulus (G), Young’s Modulus (E), Poisson’s ratio (ν), and Pugh’s ratio $\left(B/G\right)$ all increase with increasing pressure. It demonstrates that all these NaGeX _3 (X = Cl, Br, I) compounds are ductile in nature. The compounds are determined to be diamagnetic based on their magnetic property investigation, which reveals no notable changes in behavior up to 5 GPa of rising pressure. To gain a better understanding of the properties of the material when incident light strikes its surface, researchers also looked in at optical absorption, reflectivity, dielectric constants, refractive index, conductivity, and loss functions. Pressure-induced NaGeX _3 perovskite compounds, where X = Cl, Br, and I, show an increase in dielectric constant as pressure rises, suggesting a decrease in charge carrier recombination rates and a possibility for higher optoelectronic device efficiency. For all NaGeX _3 compounds (where X = Cl, Br, and I), the maximum absorption coefficient peaks are located around 3 eV, indicating that increasing pressure increases optical conductivity. Additionally, they have significantly low reflectance throughout the visible spectrum and very narrow band gap, which indicates significant absorption and the possibility of effective Near-Infrared (NIR) Sensors, photodetector etc applications.

The topological properties of the systems lead to the emergence of surface states that can be observed experimentally within the angle-resolved photoemission spectroscopy measurements. Recently, the topological properties of Pb2Pd were reported. In this paper, we discuss the role of surface termination on the realized surface states. We discuss the termination dependence of the surface state for surfaces (001) and (110). We demonstrate that the Pd terminated (001) surface allows the realization of the Dirac cone-like surface state. In the case of (110), we observe clearly visible surface states with a parabolic-like dispersion relation in close vicinity of the Fermi level. [ABSTRACT FROM AUTHOR]

AV _3 Sb _5 (A = Cs, K, Rb) is a recently discovered superconducting system ( $T_\mathrm{c}\sim0.9$ –2.5 K) in which the vanadium atoms adopt the kagome structure. Intriguingly, these systems enter a charge-density-wave (CDW) phase ( $T_\mathrm{CDW}\sim80$ –100 K), and further evidence shows that the time-reversal symmetry is broken in the CDW phase. Concurrently, the anomalous Hall effect (AHE) has been observed in KV _3 Sb _5 and CsV _3 Sb _5 inside the novel CDW phase. Here, we report a comprehensive study of a high-quality RbV _3 Sb _5 single crystal with magnetotransport measurements. Our data demonstrate the emergence of the AHE in RbV _3 Sb _5 when the CDW state develops. The magnitude of the anomalous Hall resistivity in the low temperature limit is comparable to the reported values in KV _3 Sb _5 and CsV _3 Sb _5 . The magnetoresistance channel further reveals a rich spectrum of quantum oscillation frequencies, many of which have not been reported before. In particular, a large quantum oscillation frequency (2235 T), which occupies ∼56% of the Brillouin zone area, was recorded. For the quantum oscillation frequencies with sufficient signal-to-noise ratio, we further perform field angle-dependent measurements, and our data indicate two-dimensional Fermi surfaces in RbV _3 Sb _5 . Our results provide indispensable information for understanding the AHE and band structure in kagome metal AV _3 Sb _5 .

Multifunctional physical properties are usually a consequence of a rich electronic-structural interplay. To advance our understanding in this direction, we reinvestigate the structural properties of the LaPdSb and CePdSb intermetallic compounds using single-crystal neutron and X-ray diffraction. We establish that both compounds can be described by the non-centrosymmetric space group P63mc, where the Pd/Sb planes are puckered and show ionic order rather than ionic disorder as was previously proposed. In particular, at 300 K, the (h, k, 10)-layer contains diffuse scattering features consistent with the Pd/Sb puckered layers. The experimental results are further rationalized within the framework of DFT and DFT+ embedded DMFT methods, which confirm that a puckered structure is energetically more favorable. We also find strong correspondence between puckering strength and band topology. Namely, strong puckering removes the bands and, consequently, the Fermi surface pockets at the M point. In addition, the Pd-d band character is reduced with puckering strength. Thus, these calculations provide further insights into the microscopic origin of the puckering, especially the correspondence between the band's character, Fermi surfaces, and the strength of the puckering. [ABSTRACT FROM AUTHOR]

The metal–insulator transition in the organic quasi-two-dimensional metal κ-(BEDT-TTF)2Hg(SCN)2Br at TMI ≈ 90 K has been investigated. The crystal structure changes during this transition from monoclinic above TMI to triclinic below TMI. A theoretical study suggested that this phase transition should be of the metal-to-metal type and brings about a substantial change of the Fermi surface. Apparently, the electronic system in the triclinic phase is unstable toward a Mott insulating state, leading to the growth of the resistance when the temperature drops below TMI ≈ 90 K. The application of external pressure suppresses the Mott transition and restores the metallic electronic structure of the triclinic phase. The observed quantum oscillations of the magnetoresistance are in good agreement with the calculated Fermi surface for the triclinic phase, providing a plausible explanation for the puzzling behavior of κ-(BEDT-TTF)2Hg(SCN)2Br as a function of temperature and pressure around 100 K. The present study points out interesting differences in the structural and physical behaviors of the two room temperature isostructural salts of κ-(BEDT-TTF)2Hg(SCN)2X with X = Br, Cl. [ABSTRACT FROM AUTHOR]

The magnetotransport behavior inside the nematic phase of bulk FeSe reveals unusual multiband effects that cannot be reconciled with a simple two-band approximation proposed by surface-sensitive spectroscopic probes. In order to understand the role played by the multiband electronic structure and the degree of two-dimensionality, we have investigated the electronic properties of exfoliated flakes of FeSe by reducing their thickness. Based on magnetotransport and Hall resistivity measurements, we assess the mobility spectrum that suggests an unusual asymmetry between the mobilities of the electrons and holes, with the electron carriers becoming localized inside the nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the presence of a hole-like quasiparticle with a lighter effective mass and a quantum scattering time three times shorter, as compared with bulk FeSe. The observed localization of negative charge carriers by reducing dimensionality can be driven by orbitally dependent correlation effects, enhanced interband spin fluctuations, or a Lifshitz-like transition, which affect mainly the electron bands. The electronic localization leads to a fragile two-dimensional superconductivity in thin flakes of FeSe, in contrast to the two-dimensional high-Tc induced with electron doping via dosing or using a suitable interface. [ABSTRACT FROM AUTHOR]

A large number of hitherto unexplored elastic, thermophysical, acoustic, and optoelectronic properties of a double Weyl semimetal SrSi2 have been investigated in this study. Density functional theory (DFT) based methodology has been employed. Analyses of computed elastic parameters reveal that SrSi2 is a mechanically stable, ductile, moderately machinable, and relatively soft material. The compound is predicted to be dynamically stable and possesses significant metallic bonding. Study of thermophysical properties, namely, Debye temperature, Grüneisen parameter, acoustic parameters, melting temperature, heat capacity, thermal expansion coefficient, and dominant phonon mode is also indicative of soft nature of SrSi2. The electronic band structure calculations without and with spin–orbit coupling disclose semimetallic character with clear Weyl nodes close to the Fermi level. The electronic dispersion is anisotropic as evidenced by nearly flat and linear regions within the Brillouin zone. Optical parameters at different photon energies are investigated. SrSi2 shows excellent nonselective reflection spectrum across an extended range of energy encompassing the visible range implying that the compound under study has significant potential to be used as an efficient solar energy reflector. SrSi2 absorbs ultraviolet light quite efficiently. The compound also possesses high refractive index in the low energy. All these optical features can be useful in optoelectronic device applications.

Calculation results regarding the concentration dependence of the electrical transfer parameters of resistivity, mean free path and Fermi energy for high-entropy alloys HEA (number of components from 4 to 6) in the form of film or bulk alloys are presented. Calculations were made assuming the additivity of parameters: ρ = ∑n i=1 ciρi, λ-1 = ∑n i=1 ciλ-1i and εF = ∑n i=1 ciεFi is the resistivity; λ is the mean free path of electrons; εF is the Fermi energy), which in the case of resistivity is confirmed experimentally. Based on the obtained results and analysis of the Fermi surface form for HEA, a qualitative conclusion is made regarding the fact that studied in the work HEA should not be classified as new substances, since they are one of the types of metallic solid solutions with electronic parameters close in magnitude to metals. [ABSTRACT FROM AUTHOR]

In the present paper, we report the structural optimization of intercalated CrTiS 2 compound by using Density Functional Theory (DFT) with Generalized Gradient Approximation (GGA) through Quantum ESPRESSO code. All the computations are carried out by using an ultra-soft pseudopotential. The effect of charge transfer from guest 3d transition metal Cr-atom to self-intercalated compound TiS 2 has been studied. In electronic properties, the energy band structure, total density of states (TDOS), partial density of states (PDOS) and Fermi surface have carried out. From the energy band structure, we conclude that the TiS 2 -intercalated compound has a small bandgap while the doped compound with guest Cr-atom has metallic behavior as shown form its overlapped band structure.

Co 2 Cr(Ga,Si) shape memory alloys have a wide application temperature range due to the unique re-entrant martensite phase transformation (RMT) behavior. Nevertheless, the microscopic mechanism of RMT remains elusive and unsystematic. The heart of this investigation lies the comprehensive exploration of phase stability, phase transformation path, and martensite slip direction during RMT from martensite (D0 22 -PM) to austenite (L2 1 -FM). In this work, we systematically investigate the re-entrant martensitic transformation of Co 2 Cr(Ga,Si) SMAs using first-principles methods for the first time. Firstly, the density of states (DOS) calculation indicates that the stability of L2 1 -FM is higher than D0 22 -PM. The charge density calculation further indicates that the bonding strength between Co atoms and Cr (Si) atoms in the L2 1 -FM phase is the highest. In addition, Fermi surface calculation further reveals that phase instability is due to the Fermi surface nesting caused by electron-phonon coupling. Moreover, the minimum energy path was calculated by the G-SSNEB method for the first time, which indicates the RMT can occur. Finally, the calculation of the elastic constants indicates that RMT is caused by the crystal plane slip of the D0 22 -PM phase along the <110> direction. Our calculations provide the electronic-level and thermodynamic mechanism for RMT of Co 2 Cr(Ga,Si) alloy and shed some light on the revelation of the phase transformation mechanism. • The re-entrant martensitic transformation mechanism of Co 2 Cr(Ga,Si) alloy has been studied systematically for the first time. • The Fermi surface of Co 2 Cr(Ga,Si) SMAs was calculated for the first time. • The minimum energy path of Co 2 Cr(Ga,Si) alloy is studied by G-SSNEB method for the first time, [ABSTRACT FROM AUTHOR]

In collinear antiferromagnetic (AFM) order, the structural evolution and electronic structure of AFe2As2 (A = Ba, Sr) under hydrostatic and uniaxial pressures have been studied. The Fe‐Fe distance is closely related to the AFM suppression as well as the Fe‐As bond. Through analysis of Fermi surface and band diagram, the uniaxial pressure along c‐axis can make the band near the Γ point to produce Lifshitz transition under the condition of lower pressure, and it is easier to suppress Fe magnetic moment. If the pressure component along c‐axis is larger in the experiment, it is easier to cause the superconducting. The superconductivity of BaFe2As2 and SrFe2As2 under pressure may be due to the increasing of Fe 3d‐ and As 4p‐hybridized bands width near the Fermi level. [ABSTRACT FROM AUTHOR]

This study reported a novel method to obtain rutile TiO2 with excellent photocatalytic activity for degradation of organic dyes. In this study, the concentrated HCl was selected as the inhibitor to make TiO2 precursor hardly hydrolyzed at room temperature. And a certain amount of urea was added, which results in TiO2 precursor hydrolyzed to produce rutile TiO2 due to urea thermally decomposed into alkaline substances to neutralize the concentrated HCl. To further explore the mechanism of excellent photocatalytic performance of rutile TiO2, a series of experiments, characterizations, and DFT computations were carried out. Based on DFT computations and experimental results, it could be concluded that the introduction of surface oxygen vacancies was the main reason for the excellent photocatalytic performance of the samples, and the concentration of surface oxygen vacancies would affect the physical and chemical properties of rutile TiO2. Meaningfully, this unique and innovative work broke the traditional preconception of rutile TiO2 and provided a theoretical possibility for rutile TiO2 to be applied in other research fields. [ABSTRACT FROM AUTHOR]

The article analyses the existing set of data on the temperature dependences of the main kinetic coefficients and the IR-reflection spectra R(ν) of the PbSb2Te4 crystals. The form of the Fermi surface is discussed. The anisotropy of holes’ effective mass and relaxation time is estimated. The complex structure of the valence zone is confirmed, the values of these parameters are refined. Complex X-ray studies of the structure and composition of PbSb2Te4 samples gave the possibilities to clarify the real crystal structure and explain the number of features in the IR-reflection spectra.

The article analyses the existing set of data on the temperature dependences of the main kinetic coefficients and the IR-reflection spectra R(ν) of the PbSb2Te4 crystals. The form of the Fermi surface is discussed. The anisotropy of holes’ effective mass and relaxation time is estimated. The complex structure of the valence zone is confirmed, the values of these parameters are refined. Complex X-ray studies of the structure and composition of PbSb2Te4 samples gave the possibilities to clarify the real crystal structure and explain the number of features in the IR-reflection spectra.

In this short note, we prove a sector counting lemma for a class of Fermi surface on the plane which are C 2 -differentiable and strictly convex. This result generalizes the one proved in Feldman et al. (Rev Math Phys 15(9):1121–1169, 2003) for the class of C 2 + r -differentiable, r ≥ 3 , strictly convex and strongly asymmetric Fermi surfaces, and the one proved in Benfatto et al. (Ann Henri Poincaré 7:809–898, 2006) and Feldman et al. (Helv Phys Acta 65:679–721, 1992), for the class of C 2 -differentiable, strictly convex and central symmetric Fermi surfaces. This new sector counting lemma can be used to construct interacting many-fermion models for the doped graphene, in which the Fermi surface is extended and quasi-symmetric. [ABSTRACT FROM AUTHOR]

We present electronic structure calculations of rare earth doped Ca1−xRxFeAs2, 112 Fe-based high temperature superconducting materials with special emphasis on Sb doping at the chain As-site. The electronic structures obtained within Density functional Theory based first principles calculations on 112 compounds with Sb doping for different rare earth (Pr, Nd, Sm) co-doping reveal a significant increase in partial density of states of As at the Fermi level. An increase in density of states at the Fermi level indicates increased availability of electrons for Cooper pairing and hence possible enhancement of superconductivity. Our results indicate possible preference of Sm doping over other rare earth elements as far as enhancement of superconductivity via Sb doping at As-chain is concerned. [ABSTRACT FROM AUTHOR]

The metal–insulator transition in the organic quasi-two-dimensional metal κ-(BEDT-TTF)2Hg(SCN)2Br at TMI ≈ 90 K has been investigated. The crystal structure changes during this transition from monoclinic above TMI to triclinic below TMI. A theoretical study suggested that this phase transition should be of the metal-to-metal type and brings about a substantial change of the Fermi surface. Apparently, the electronic system in the triclinic phase is unstable toward a Mott insulating state, leading to the growth of the resistance when the temperature drops below TMI ≈ 90 K. The application of external pressure suppresses the Mott transition and restores the metallic electronic structure of the triclinic phase. The observed quantum oscillations of the magnetoresistance are in good agreement with the calculated Fermi surface for the triclinic phase, providing a plausible explanation for the puzzling behavior of κ-(BEDT-TTF)2Hg(SCN)2Br as a function of temperature and pressure around 100 K. The present study points out interesting differences in the structural and physical behaviors of the two room temperature isostructural salts of κ-(BEDT-TTF)2Hg(SCN)2X with X = Br, Cl.

Andreev reflection (AR) refers to the electron-hole conversion at the normal metal-superconductor interface. In a three-dimensional metal with a spherical Fermi surface, retro (specular) AR can occur with the sign reversal of all three (a single) components of particle velocity. Here, we predict a novel type of AR with the inversion of two velocity components, dubbed "anomalous Andreev reflection" (AAR), which can be realized in a class of materials with a torus-shaped Fermi surface, such as doped nodal line semimetals. For its toroidal circle perpendicular to the interface, the Fermi torus doubles the AR channels and generates multiple AR processes. In particular, the AAR and retro AR are found to dominate electron transport in the light and heavy doping regimes, respectively. We show that the AAR visibly manifests itself as a ridge structure in the spatially resolved nonlocal conductance, in contrast to the peak structure for the retro AR. Our work opens a new avenue for the AR spectroscopy and offers a clear transport signature of the torus-shaped Fermi surface. [ABSTRACT FROM AUTHOR]

We present a review of our investigations on the increasing specularity of surface scattering of conduction electrons on the W(011), Mo(011), and W(001) surfaces after covering the surfaces with well-ordered (sub)monolayers of adsorbed hydrogen or deuterium. The degree of the specularity of surface scattering was estimated on the base of measurements of magnetoresistance of single-crystalline plates, cooled with liquid He, under ultra-high vacuum conditions, and in a strong magnetic field. The structures of adsorbed hydrogen and deuterium layers were determined from LEED patterns. It is found that the magnetoresistance non-monotonically depends on submonolayer hydrogen coverage and degree of the ordering of adsorbed layers, showing a drastic increase of the specularity (which is higher than for the atomically-clean surfaces!) after the formation of the well-ordered (1 × 1) -H structures and a pronounced increase of the specularity after the formation of the well-ordered (2 × 2) -2H structure. Changes in the magnetoresistance induced by the ordering of structures of adsorbed H(D) layers are found to corroborate with the adsorption-induced transformation of Fermi contours of surface electronic states obtained in performed DFT calculations. The increase of the specularity of the scattering caused by the ordering of the surface structures is attributed to the decrease of the probability of electronic transitions between the surface and bulk electronic states induced by surface scattering. [ABSTRACT FROM AUTHOR]

We use the composite operator method (COM) to analyze the strongly correlated repulsive Hubbard model, investigating the effect of nearest-neighbor hoppings up to fourth order on a square lattice. We consider two sets of self-consistent equations, one enforcing the Pauli principle and the other imposing charge-charge, spin-spin, and pair-pair correlations using a decoupling scheme developed by Roth (1969 Phys. Rev. 184 451-9). We extract three distinct solutions from these equations: COM1 and COM2 by imposing the Pauli principle and one from Roth decoupling. An overview of the method studying the validity of particle-hole symmetry and the Luttinger theorem for each solution is presented. Additionally, we extend the initial basis to study superconductivity, concluding that it is induced by the Van Hove singularity. Finally, we include higher-order hoppings using realistic estimates for tight binding parameters and compare our results with ARPES measurements on cuprates., (© 2024 IOP Publishing Ltd.)

We study the main intra-band and inter-band transport properties at zero temperature of free electron-like system undergoing a topological reconstruction of the Fermi surface for the two-dimensional and three-dimensional case. The calculated intra-band properties include the single-particle density of states, the total and the effective concentrations of electrons and the thermopower. As for the inter-band case, the real part of the conductivity has been calculated within the vanishing inter-band relaxation approximation as a function of the incident photon energy. Within this approach, it is shown that the optical conductivity has a nonvanishing component parallel to the reconstruction wave vector and the shape which depends on the value of the Fermi energy. Each dimensionality has its particular features in the transport quantities presented in the paper, which are discussed and compared with those in the free electron scenario. Finally, we identify the signature of the topological reconstruction of the Fermi surface in the intra-band and inter-band transport functions.

The fractional quantum Hall (FQH) effect was discovered in two-dimensional electron systems subject to a large perpendicular magnetic field nearly four decades ago. It helped launch the field of topological phases, and in addition, because of the quenching of the kinetic energy, gave new meaning to the phrase "correlated matter." Most FQH phases are gapped like insulators and superconductors; however, a small subset with even denominator fractional fillings ν of the Landau level, typified by ν = 1 / 2 , is found to be gapless, with a Fermi surface akin to metals. We discuss our results, obtained numerically using the infinite density matrix renormalization group scheme, on the effect of non-isotropic distortions with discrete N-fold rotational symmetry of the Fermi surface at zero magnetic field on the Fermi surface of the correlated ν = 1 / 2 state. We find that while the response for N = 2 (elliptical) distortions is significant (and in agreement with experimental observations with no adjustable parameters), it decreases very rapidly as N is increased. Other anomalies, like resilience to breaking the Fermi surface into disjoint pieces, are also found. This highlights the difference between Fermi surfaces formed from the kinetic energy, and those formed of purely potential energy terms in the Hamiltonian. [ABSTRACT FROM AUTHOR]

We prove that the Fermi surface of a connected doubly periodic self-adjoint discrete graph operator is irreducible at all but finitely many energies provided that the graph (1) can be drawn in the plane without crossing edges, (2) has positive coupling coefficients, (3) has two vertices per period. If "positive" is relaxed to "complex," the only cases of reducible Fermi surface occur for the graph of the tetrakis square tiling, and these can be explicitly parameterized when the coupling coefficients are real. The irreducibility result applies to weighted graph Laplacians with positive weights [ABSTRACT FROM AUTHOR]

KFe2As2 is one of the representatives of iron-based superconductors. Many interesting features distinguish this compound from other iron-based superconductors, e.g., a realization of the Pauli limit or an occurrence of the superconducting gap with nodal lines. Moreover, with increasing pressure, the isostructural phase transition from the tetragonal to collapsed tetragonal phase is experimentally observed. We discuss the structural, electronic, and superconducting properties of the KFe2As2 under pressure using the ab initio density functional theory (DFT) methods. We analyze the untypical properties of this superconductor considering, among others, the Fermi surfaces or the dependence of the anion height from the iron layer on the superconducting critical temperature. [ABSTRACT FROM AUTHOR]

A suite of density functional theory (DFT) approaches that provide accurate and reliable superconductivity parameters without post-calculation corrections is presented. The sensitivity of phonon dispersion (PD) outputs to slight variations in calculation and external parameters enables identification of the most electron-coupled phonon mode(s). These mode(s) are directly linked, in a range of superconductors, to specific band structure characteristics and to the response of electrons to deformation potentials. These approaches are applied successfully to MgB2 under a wide range of variable external conditions, including isotopic state, metal substitutions, pressure, and temperature. This approach has also been extended to other chemical systems with significantly different composition and crystal symmetry, such as YB6, H3S under pressure, B-doped diamond, B-doped silicon, and A15 compounds, to name a few. The effectiveness of these approaches that link both PDs and electronic band structures (EBSs) is largely dependent on reliable and accurate DFT results at the meV range that resolve Fermi surface (FS) detail in close reciprocal space proximity. Such a level of meV resolution is possible only by using extremely fine k-grids and large cutoff radii, much smaller and larger, respectively, to what has typically been used in DFT calculations reported in the literature. [ABSTRACT FROM AUTHOR]

The topology of the Fermi surface of an extended d–p Hubbard model is investigated using Green's function technique in a n-pole approximation. The effects of the d–p hybridisation on the Fermi surface are the main focus in the present work. Nevertheless, the effects of doping, Coulomb interaction and hopping to second-nearest-neighbours on the Fermi surface, are also studied. Particularly, it is shown that the crossover from hole-like to electron-like Fermi surface (Lifshitz transition) is deeply affected by the d–p hybridisation. Moreover, the pseudogap present in the low doping regime is also affected by the hybridisation. The results show that both the doping and the hybridisation act in the sense of suppresses the pseudogap. Therefore, the systematic investigation of the Fermi surface topology, shows that not only the doping but also the hybridisation can be considered as a control parameter for both the pseudogap and the Lifshitz transition. Assuming that the hybridisation is sensitive to external pressure, the present results agree qualitatively with recent experimental data for the cuprate Nd-LSCO. [ABSTRACT FROM AUTHOR]

The interaction of hydrogen with terbium was correlated and the structural and electronic structure properties for the cubic rare earth terbium dihydride (TbH 2) using density functional theory (DFT) have been studied. The electronic band structure and density of states (DOS) for spin-up, spin-down and non-spin cases were studied and furthermore influence of higher pressure was also investigated. The charge contribution shows that TbH 2 has metallic characteristic with a mixed covalent and ionic bonding. The Fermi surfaces show hole type structure and provide a space where hydrogen can be absorbed and filled easily. The Löwdin population analysis show that the decrease in charge density of Tb atom which explain absorption of H atom by Tb. The charge density of TbH 2 also show decrease in differential charge density which verify the Löwdin population analysis. Furthermore, we have also analysed phonon dispersion curve at ambient and higher pressure and changes were studied. The TbH 2 can be a good alternative to develop a hydrogen storage device as mass ratio percentage is around 1.27%. • We have studied the structural, electronic and vibrational properties of terbium dihydride (TbH 2) at ambient and higher pressure. • Hydrogen storage capacity depend on pressure is explain by electronic band structure and density of state. • The soften mode in phonon dispersion curve at higher pressure show structural phase transition. • Löwdin population is also analyzing to determine the distribution of electron among the constituent atoms. [ABSTRACT FROM AUTHOR]

Physics of Earth’s core is crucial to give insight into the origin and behavior of the Earth and other terrestrial planets. In order to interpret the behavior of tetrataenite L10 FeNi at Earth’s inner core boundary and Earth’s center conditions, we report electronic properties namely electronic charge density and Fermi surface of tetrataenite L10 FeNi using the first-principles plane wave self-consistent method under the framework of density functional theory. For structural and electronic properties of tetrataenite L10 FeNi at the Earth’s core conditions, we used spin polarization and ultrasoft pseudopotential with the exchange correlation of Perdew-Burke-Ernzerhof (PBE). Variable cell optimization (VC-relax) calculation using Wentzcovitch dynamics as implemented in Quantum ESPRESSO code is used for estimating equilibrium lattice constant of tetrataenite phase of L10 FeNi at 0 K. Bonding character between Fe-Fe, Ni-Ni and Fe- Ni metal atoms is discussed at 0 K and extreme Earth’s core conditions. The complicated shape of comprehensive Fermi surface is observed which occurred from the merging of all individual Fermi surfaces due to corresponding band crossing at Fermi level EF in the electronic band structure. Relation between crossing of each band in the electronic band structure with high symmetrical points of the Brillouin zone and the corresponding shape of Fermi surfaces are discussed. Conclusions based on the electronic charge density plot and Fermi surface topology of tetrataenite L10 FeNi at Earth’s core conditions are summarized. [ABSTRACT FROM AUTHOR]

Structural and electronic properties, Fermi surface topology, and chemical bonding features in layered 1111-oxyarsenides LaRhAsO and LaIrAsO are studied and compared using ab initio simulations. The nonmagnetic metal LaIrAsO is found to be weakly sensitive to both electronic and hole doping. However, the Rh-containing phase is predicted to exhibit weak band magnetism and can turn into a nonmagnetic state by hole doping. Therefore, the LaRhAsO oxyarsenide can be considered as a possible "electronic" analogue of the LaFeAsO compound, the basic phase of layered FeAs superconductors. [ABSTRACT FROM AUTHOR]

The ternary MAX phase Cr 2 AlC exhibits a noticeable variation of in-plane Hall coefficient with temperature, T. In previous work, this was assumed to be due to temperature dependent electron and hole concentrations, an unlikely conjecture for a highly metallic-like conducting ceramics like Cr 2 AlC. Herein we show that a realistic analytic approximation of the Fermi surface as measured by Angle Resolved Photo-Emission Spectroscopy or as computed by Density Functional Theory allows us to recover the main features of the in-plane magneto-transport (temperature variations of Hall coefficient, resistivity and magnetoresistance). The temperature variation is explained by the combined contribution of electron and hole bands, taking into account the local curvature of the Fermi surface and an appropriate and physically sound temperature variation of the relaxation time in each band. The model also allows us to retrieve the apparent (and erroneous) carrier densities as extracted when using conventional methods. We suggest that our explanation could be generalized to other MAX phases, for which more "involved" explanations have been advanced so far. [ABSTRACT FROM AUTHOR]

• Hall resistivity of Lu(Co 0.91 Al 0.09) 2 was measured at various temperatures at 0 GPa and 0.5 GPa. • The normal Hall coefficient was estimated for ferromagnetic and paramagnetic states. • The results were compared with the theoretical values calculated for ferromagnetic and paramagnetic LuCo 2. • The sign of the normal Hall coefficient was discussed by considering the effect of electron doping. We have studied the Hall effect of Lu(Co 0.91 Al 0.09) 2 at a pressure of 0.5 GPa as well as at ambient pressure. The compound shows itinerant electron metamagnetism (IEM) above T C at ambient pressure and at low temperatures at 0.5 GPa. The Hall resistivity ρ xy was measured as a function of magnetic field B at various temperatures. In the ferromagnetic state, the ρ xy – B curves show rapid falls at low fields due to the anomalous Hall effect (AHE). Above the saturation field, ρ xy increases nearly linearly with increasing B. In the paramagnetic state, ρ xy initially decreases with increasing field. When IEM takes place, ρ xy rapidly drops due to the AHE. At high fields where the ferromagnetic state is induced, the ρ xy – B curves have positive slopes. We estimated the normal (ordinary) Hall coefficient in the ferromagnetic state R N f and that in the paramagnetic state R N p. It is found that R N f is positive, while R N p is negative in this compound. The estimated normal Hall coefficients are compared with the theoretical values calculated from the energy bands for ferromagnetic and nonmagnetic LuCo 2. [ABSTRACT FROM AUTHOR]

From the first-principles DFT calculations, we explore the optoelectronic and thermoelectric properties of Yb 3 Q 5 (Q= Ge, Si). The ground state properties of crystalline materials can precisely be estimated using the FP-LAPW technique implemented in the WIEN2K code. It can be determined that the compounds under consideration are metallic based on their band structures. Fermi surfaces of Yb 3 Q 5 (Q= Ge, Si) are also calculated to investigate the electronic conductivity of these materials at the Fermi level. The phenomenon of absorption and dispersion of incident photons can be explained using obtained dielectric function ε(ω) for Yb 3 Q 5 (Q= Ge, Si) compounds. The infrared (IR) region is where Yb 3 Q 5 (Q= Ge, Si) compounds absorb most light packets, however, their absorption in the visible region is also not negligible. These materials are also potential candidates for antireflecting coatings in the upper UV region. Yb 3 Q 5 (Q= Ge, Si) are brittle compounds as their B/G values are less than 1.75. Thermodynamic parameters show that these compounds are thermodynamically stable. [ABSTRACT FROM AUTHOR]