Weyl and Dirac semimetals, characterized by their unique band structures with linear energy dispersion (E vs k) near the Fermi level (EF), have emerged as promising candidates for next-generation technology based on thermoelectric materials. Their exceptional electronic properties, notably high carrier mobility and substantial Berry curvature, offer the potential to surmount the limitations inherent in conventional thermoelectric materials. A comprehensive understanding of the fundamental physics underlying these materials is essential. This chapter mainly focused into the topological properties and distinctive electronic band structures of Weyl and Dirac semimetals, providing a theoretical framework for comprehending their thermoelectric transport properties such as Seebeck coefficients, electrical and thermal conductivity. The pivotal role of Berry curvature in enhancing Seebeck coefficients while reducing thermal conductivity is a key focus. Experimental advancements in synthesizing single crystals and characterizing these materials have been significant. Recent development in material growth and characterization techniques have propelled research forward. The intricate relationship between material properties, such as carrier concentration, electronic bandgap, and crystal structure, and thermoelectric performance is explored. Realizing the potential of Weyl and Dirac semimetals for practical thermoelectric applications necessitates overcoming specific challenges. This chapter outlines strategies to optimize thermoelectric figures of merit (ZT) through band engineering, carrier doping, and nanostructuring. Moreover, the exploration of hybrid materials and heterostructures offers promising avenues for enhancing thermoelectric performance for renewable energy applications.

Using X-ray photoelectron spectroscopy (XPS), we show that Au forms an Ohmic contact with the surface of the narrow-bandgap Mott insulator Sr3Ir2O7 thin films. XPS reveals no significant binding energy shift with increasing Au overlayer thickness on epitaxial Sr3Ir2O7(001) thin films, consistent with the linear current-voltage (I-V) behavior and indicative of negligible band bending. The absence of Schottky barrier formation at the Au/Sr3Ir2O7 interface corroborates the metallic surface character of the Sr3Ir2O7 thin film, as observed in angle-resolved photoemission spectroscopy (ARPES) studies, suggesting that the interface behaves effectively as a metal-on-metal contact. Furthermore, XPS points to the presence of distinct surface/interface and bulk electronic states for Sr3Ir2O7. This study provides critical material information for designing iridate-based electronic devices.

Cadmium sulfide (CdS) quantum dots (QDs) exhibit strong potential in biomedical applications; however, their antimicrobial properties remain insufficiently studied, particularly concerning key synthesis parameters. This work investigates the impact of synthesis temperature, particle size, Cd:S atomic ratio, CdS concentration, and surfactant type on the physicochemical and antimicrobial properties of cadmium sulfide (CdS) quantum dots (QDs). Using a cost-effective aqueous chemical method, both bulk and quantum dot cadmium sulfide (CdS) were synthesised. Characterization via XRD, FTIR, UV-Vis, and TEM confirmed the formation of CdS, quantum confinement, and spherical morphology. Photoluminescence analysis showed stable fluorescence across variations, indicating surfactants (PEG and PVPP) reduce toxicity without altering optical properties. Notably, CdS QDs synthesized at 50 degrees C with a 1:0.5 Cd:S ratio, and PEG/PVPP surfactants exhibited excellent antibacterial and antifungal activity at just 5-10 mg/mL-the lowest effective concentration reported to date. These findings support the design of CdS quantum dots (QDs) with enhanced luminescence and antimicrobial activity for bioimaging and drug delivery applications.

The presence of magnetic impurities in topological insulators can disrupt their time reversal symmetry and lead to the emergence of an energy gap. This study delves into the energy band structure and the Kondo effect through the introduction of Gadolinium (Gd) magnetic perturbations (at levels of x=0.1,0.16) into a pure Bi2Se3 single crystal. In the case of the Bi1.9Gd0.1Se3 (5%) single crystal, the Kondo effect becomes observable at temperatures below 50 K. However, the unaltered parent and Bi1.84Gd0.16Se3 (8%) exhibit typical metallic behavior. The pure sample displays the highest magnetoresistance (MR) of around 225% and demonstrates quantum oscillations driven by a nontrivial berry phase. The sample doped with 5% Gd undergoes a transition from negative MR to positive MR due to a presence of mixed magnetic state resulting from the opening of a gap at the Dirac point. This gap opening is confirmed through angle-resolved photoemission spectroscopy (ARPES) measurements. The comparison of the parameters obtained from the SdH and ARPES measurements, the reduction in the kF values in the magnetotransport measurements is likely due to the band bending induced by the Schottky barrier. Thermoelectric properties are assessed across all prepared samples. The undoped sample displays the highest Seebeck coefficient and power factor values of -398.02 mu V K-1 and 6.83mWmK-2, respectively, at room temperature. These values are notably high for thermoelectric applications at room temperature.

Using angle-resolved photoemission spectroscopy (ARPES) with spin resolution, scanning tunneling microscopy/spectroscopy (STM/STS) and density functional theory (DFT) methods, we study the electronic structure of graphene-covered and bare Au/Co(0001) systems and reveal intriguing features, arising from the ferrimagnetic order in graphene and the underlying gold monolayer. In particular, a spin-polarized Dirac-cone-like state, intrinsically related to the induced magnetization of Au, was discovered at point. We have obtained a good agreement between experiment and theory for bare and graphene-covered Au/Co(0001) and have proven that both Au ferrimagnetism and the Dirac-cone-like band are intimately linked to the triangular loop dislocations present at the Au/Co interface. STM measurements and simulation of the local density of states reveal a magnetic band gap in the electronic structure of graphene for out-of-plane magnetization. This gap is promising for achieving a quantum anomalous Hall state in graphene.

Electronic and crystal structures of Ce(Ru1-xRhx)2Al10 have been studied using x-ray emission spectroscopy, photoelectron spectroscopy (PES), and x-ray diffraction. No structural phase transition was observed up to x = 0.1, while the x-ray absorption spectra showed a Ce valence transition between x = 0.03 and 0.05, accompanied by an increase in c-f hybridization strength, suggesting a Lifshitz transition. Angle-resolved PES conducted on the x = 0.1 sample revealed stronger hybridization along the b axis compared to the other axes. Direction-dependent hybridization introduces complexity in applying conventional phase diagrams. For the first time, we propose a direction-dependent phase diagram for Ce(Ru1-xRhx)2Al10, advancing beyond the traditional Doniach phase diagram.

Occupied and unoccupied electronic states of altermagnetic MnTe(0001) single crystals were studied by photoemission and inverse-photoemission spectroscopies after establishing a reproducible surface cleaning procedure involving repeated sputtering and annealing cycles. The angle-resolved photoemission spectroscopy (ARPES) exhibited a hole-like band dispersion centered at the Γ¯ point, which was consistent with the reported ARPES results and our density functional theory (DFT) calculations with the on-site Coulomb interaction U. The observed Mn 3d↑-derived peak at −3.5 eV, however, significantly deviated from the DFT + U calculations. Meanwhile, the Mn 3d↓-derived peak at +3.0 eV observed by inverse-photoemission spectroscopy agreed well with the DFT + U results. Based on simulations of the spectral function employing an w-dependent model self-energy, we found significant relaxation effects in the electron-removal process, while such effects were negligible in the electron-addition process. Our study provides a comprehensive picture of electronic states, forming a solid foundation for understanding the magnetic and transport properties of MnTe.

We report evidence of a finite density of states at the Fermi level at the surface of epitaxial thin films of the narrow bandgap Mott insulator Sr3Ir2O7(001). The Brillouin zone critical points for Sr3Ir2O7(001) thin films have been determined by a comparison of the band mapping from angle-resolved photoemission spectroscopy and low energy electron diffraction. Angle-resolved x-ray photoemission studies reveal the surface termination of Sr3Ir2O7(001) is Sr-O. The absence of dispersion with photon energy, or changing wave vector along the surface normal, indicates the two-dimensional character of the bands contributing to the density of states close to the Fermi level for Sr3Ir2O7(001) thin films. Thus, the finite density of states at the Fermi level is attributed to surface states or surface resonances. The appearance of a finite density of states at the Fermi level is consistent with the increased conductivity with decreasing film thickness for ultrathin Sr3Ir2O7(001) films.

We have investigated the surface and bulk electronic structures of the superconducting type-II Dirac semimetal 1T-PdSeTe. The superconducting transition temperature Tc = 3.2 K was almost twice as high as Tc = 1.6 K in 1T-PdTe2. Scanning transmission electron microscopy measurements showed homogeneously mixed Se and Te atoms in the chalcogen layers, consistent with the CdI2-type crystal structure. Angle-resolved photoemission spectroscopy measurements and density functional theory calculations indicated the existence of the topological surface states, and the overall band structures were similar to those of 1T-PdTe2. These results suggest that the CdI2-type lattice symmetry dictates the band dispersion, regardless of atomic disorder in the chalcogen layers. As the electronic band dispersion and the local structures were persistent upon substitution, the enhancement of Tc is likely associated with the chemical pressure. Our results provide insight into the effects of the solid solution on the surface and bulk electronic states as well as the superconducting transition temperature.

We employ spin- and angle-resolved photoemission spectroscopy and circular-dichroism angle-resolved photoemission spectroscopy to systematically investigate the spin texture of Sb-doped MnBi_{2}Te_{4}. Our results display a hedgehog-like spin texture in this system which is signified by reversed-orienting out-of-plane spins at the Dirac gap. Our finding reveals the presence of time-reversal symmetry breaking, implying the possibility for realization of the high-temperature quantum anomalous Hall effect.

Electronic and crystal structures of Ce(Ru_{1-x}Rh_{x})_{2}Al_{10} have been studied using x-ray emission spectroscopy, photoelectron spectroscopy (PES), and x-ray diffraction. No structural phase transition was observed up to x=0.1, while the x-ray absorption spectra showed a Ce valence transition between x=0.03 and 0.05, accompanied by an increase in c-f hybridization strength, suggesting a Lifshitz transition. Angle-resolved PES conducted on the x=0.1 sample revealed stronger hybridization along the b axis compared to the other axes. Direction-dependent hybridization introduces complexity in applying conventional phase diagrams. For the first time, we propose a direction-dependent phase diagram for Ce(Ru_{1-x}Rh_{x})_{2}Al_{10}, advancing beyond the traditional Doniach phase diagram.

We report the electronic structure of the half-metal ferromagnet CrO2 by means of high-resolution angle-resolved photoemission spectroscopy (ARPES). The observed clear Fermi surface (FS) and band dispersion are in good agreement with the previous reports. Moreover, the ARPES band dispersion reveals a distinct kink structure around 68 meV, providing the first evidence from the electronic structure for the elementary excitations in CrO2. The energy scale of this feature is comparable to the Debye temperature and the A1g phonon mode, suggesting the electron-phonon interaction. From the detailed analysis, we have extracted the self-energy and found two characteristic structures in the real part of the self-energy. Assuming the existence of the electron-magnon interaction as well as the electron-phonon interaction, we have simulated the line shape for the real and imaginary parts of the self-energy and reproduced the ARPES intensity. Our spectral findings demonstrate the renormalized quasiparticle (QP) dynamics in CrO2 and provide valuable insights into the fundamental many-body interactions governing half-metallic ferromagnets.

The band structure of ultrathin Pd(111) thin films grown on the Cr2O3(0001) surface was studied by angule-resolved photoemission spectroscopy (ARPES) combined with first-principles calculations. The Cr2O3(0001) interface and the expanded Pd lattice constant appears to significantly affect the occupied band structure of an ultrathin palladium film. A characteristic band splitting is seen in the experimental occupied electronic structure, forming a hexagonal pattern approximately half-way from theΓ¯point to the surface Brillouin zone boundary. The ARPES spectrum near the Fermi level is reproduced by the first-principles simulation for a Pd monolayer (ML) placed on the Cr-terminated Cr2O3surface with the energetically favorable lateral stacking. In this configuration, the Pd bands inside the Cr2O3band gap have a significant, momentum-dependent exchange splitting, which matches with the experimentally observed band splitting. The experimental band structure shows a band with an electron effective mass close to the free electron mass, neglecting mass enhancement effects close to the Fermi level. From the hybridization of the Pd ML with the Cr2O3(0001) substrate, a hole band with a mass of -0.48 ± 0.02 is identified.

Angle-resolved photoelectron spectroscopy studies were performed on antiferromagnetic Kondo semiconductor CeRu2Al10 together with related (Ce0.9La0.1)Ru2Al10, and CeFe2Al10 compounds with nonmagnetic ground states. Direct evidence of anisotropy of the hybridization strength was observed in CeRu2Al10. The results suggested the itinerant character of the antiferromagnetic state was derived from anisotropic c-f hybridization. La doping in CeRu2Al10 weakened the anisotropic hybridization strength and induced comparable hybridization strength along all axis directions. In CeFe2Al10 very strong hybridization was observed in all directions.

We synthesized Nb0.5Ta0.5P, exhibiting XMR at low temperatures with charge carrier compensation up to 50 K. SdH oscillations reveal multiple Fermi pockets and non-zero Berry phase. SOC effects were analyzed via band structure calculations.

Spintronics has emerged as a viable alternative to traditional electronics-based technologies in the past few decades. While the discovery of topological phases of matter with protected spin-polarized states has opened up exciting prospects, recent revelation of intriguing nonrelativistic spin-splitting in antiferromagnetic (AFM) materials with unique symmetries facilitate a wide possibility of realizing both these features simultaneously. In this work, we report the coexistence of these two intriguing properties within a single material: GdAlSi. Single crystal of GdAlSi stabilizes in a body-centered tetragonal structure with a noncentrosymmetric space group I 4 1 md (109), which is confirmed using detailed structural analysis through x-ray diffraction (XRD) and optical second harmonic generation (SHG) measurements. The magnetization data indicates AFM ordering with an ordering temperature (TN) similar to 32 K. Ab initio calculations reveal GdAlSi to be a collinear AFM Weyl semimetal with an unconventional, momentum-dependent spin-splitting, also referred to as altermagnet. Angle-resolved photoemission spectroscopy measurements on GdAlSi single crystals subsequently hints the possible presence of Fermi arcs. Electric and magnetic multipole analysis provides a deeper understanding of the symmetry- mediated, momentum-dependent spin-splitting, which has strictly nonrelativistic origin. GdAlSi is possibly the first candidate material with noncentrosymmetric collinear AFM structure showing such momentum-dependent spin-splitting, as also confirmed by our detailed symmetry analysis, rendering GdAlSi a special and promising candidate material. We further propose a device harnessing these features, poised to enable practical and efficient topotronic applications.

In a Dirac semimetal, the massless Dirac fermion has zero chirality, leading to surface states connected adiabatically to a topologically trivial surface state as well as vanishing anomalous Hall effect. Recently, it is predicted that in the nonrelativistic limit of certain collinear antiferromagnets, there exists a type of chiral“Dirac-like” fermion, whose dispersion manifests four-fold degenerate crossing points formed by spin-degenerate linear bands, with topologically protected Fermi arcs. Such an unconventional chiral fermion, protected by a hidden SU（2） symmetry in the hierarchy of an enhanced crystallographic group, namely spin space group, is not experimentally verified yet. Here, by angle-resolved photoemission spectroscopy measurements, we reveal the surface origin of the electron pocket at the Fermi surface in collinear antiferromagnet CoNb3S6. Combining with neutron diffraction and first-principles calculations, we suggest a multidomain collinear antiferromagnetic configuration, rendering the the existence of the Fermi-arc surface states induced by chiral Dirac-like fermions.Our work provides spectral evidence of the chiral Dirac-like fermion caused by particular spin symmetry in CoNb3S6, paving an avenue for exploring new emergent phenomena in antiferromagnets with unconventional quasiparticle excitations.

Space groups describing the symmetry of lattice structure allow the emergence of fermionic quasiparticles with various degeneracy in the band structure. Theoretical efforts have predicted many materials hosting fermions with the highest degeneracy, i.e., eightfold fermions, yet lacking experimental realization. Here, we explore the band degeneracies in TaCo 2 Te 2 crystals. Through systematic experimental and theoretical analyses, we establish TaCo 2 Te 2 as a nonsymmorphic crystal with negligible spin–orbit coupling (SOC) and long-range magnetic order. These critical properties guarantee the realization of practical eightfold fermions and fourfold van Hove singularity, as directly observed by photoemission spectroscopy. TaCo 2 Te 2 serves as a topological quantum critical platform, which can be tuned into various magnetic, topologically trivial, and nontrivial phases by adding strain, magnetic field, or SOC. The latter is demonstrated by our first-principles calculations, which show that enhancing SOC in TaCo 2 Te 2 will promote the experimental observation of bulk hourglass fermions. Our results establish TaCo 2 Te 2 as a platform to explore the interplay between symmetry and band topology.

Herein, the electrical and magnetic properties of polycrystalline Eu 2 CoMnO 6 are explored. More specifically, based on transport, dielectric, and impedance measurements, a thermally activated dielectric polarization near room temperature is reported. The valence states of transition metals in the Eu 2 CoMnO 6 sample are studied by measuring the X‐ray absorption spectra at both Co‐L 2,3 and Mn‐L 2,3 edges. Moreover, based on magnetization, X‐ray absorption spectroscopy, and X‐ray magnetic circular dichroism measurements, the coexistence of competing ferromagnetic and antiferromagnetic phases is probed. Dielectric measurement shows a large dielectric constant near room temperature along with a large frequency dispersion. The temperature‐dependent resistivity, Cole–Cole plot, and imaginary part of impedance confirm the semiconducting nature of Eu 2 CoMnO 6 . Temperature‐dependent magnetization shows a second‐order magnetic transition below 125 K. Isothermal magnetization versus magnetic field loop shows first‐order metamagnetic transition, driven by mixed magnetic state owing to the presence of antisite disorder and antiphase boundary.

One of the approaches to manipulate MnBi2Te4 properties is the magnetic dilution, which inevitably affects the interplay of magnetism and band topology in the system. In this work, we carried out angle-resolved photoemission spectroscopy (ARPES) measurements and density functional theory (DFT) calculations for analysing changes in the electronic structure of Mn1−xGexBi2Te4 that occur under parameter x variation. We consider two ways of Mn/Ge substitution: (i) bulk doping of the whole system; (ii) surface doping of the first septuple layer. For the case (i), the experimental results reveal a decrease in the value of the bulk band gap, which should be reversed by an increase when the Ge concentration reaches a certain value. Ab-initio calculations show that at Ge concentrations above 50%, there is an absence of the bulk band inversion of the Te pz and Bi pz contributions at the Γ-point with significant spatial redistribution of the states at the band gap edges into the bulk, suggesting topological phase transition in the system. For case (ii) of the vertical heterostructure Mn1−xGexBi2Te4/MnBi2Te4, it was shown that an increase of Ge concentration in the first septuple layer leads to effective modulation of the Dirac gap in the absence of significant topological surface states of spatial redistribution. The results obtained indicate that surface doping compares favorably compared to bulk doping as a method for the Dirac gap value modulation.