Search

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]

Bismuth-based compounds are of great interest due to their unique and important structural, physical, and chemical properties. Recently, a new layered material MgNi2Bi4 has been successfully synthesized in experiments, which is a Pauli paramagnetic metal. The weak interlayer interactions imply that it may be a potential two-dimensional material. The elastic and electronic properties of the bulk and monolayer MgNi2Bi4 have been studied by first-principles calculation. It is found that bulk MgNi2Bi4 is a typical non-magnetic and ductile metal with complex Fermi surfaces. The small cleavage energy implies that the MgNi2Bi4 monolayer could be possibly obtained in experiments. The calculation indicates that the monolayer MgNi2Bi4 is also a metal. The calculation results and analysis have revealed the basic physical properties of MgNi2Bi4 and could be useful in future studies on this material. [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.

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]

Bismuth - based compounds are of great interest due to their unique and important structural, physical, and chemical properties. Recently, a new layered material MgNi2Bi4 has been successfully synthesized in experiments, which is a Pauli paramagnetic metal. The weak interlayer interactions imply that it may be a potential two - dimensional material. The elastic and electronic properties of the bulk and monolayer MgNi2Bi4 have been studied by first - principles calculation. It is found that bulk MgNi2Bi4 is a typical non - magnetic and ductile metal with complex Fermi surfaces. The small cleavage energy implies that the MgNi2Bi4 monolayer could be possibly obtained in experiments. The calculation indicates that the monolayer MgNi2Bi4 is also a metal. The calculation results and analysis have revealed the basic physical properties ofMgNi2Bi4 and could be useful in future studies on this material. [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 review recent progress in single crystal growth and study of electronic properties in. Czocharalski pulling, using purified uranium metal and subsequent annealing under ultra-high vacuum, is successfully applied to this compound, and it yields the highest residual resistivity ratio. These high-quality single crystals allow us to investigate Fermi surfaces using quantum oscillation and to make detailed transport measurements at low temperature.