Te 5p States Research Articles

Small compressive strain tuning of α-TeO2 and β-TeO2 polymorphs: Electronic, linear, and nonlinear optical properties from first-principles calculations

This study examines the effects of 2 % compressive strain on the structural, electronic, and optical properties of α-TeO2 and β-TeO2 polymorphs. α-TeO2’s bandgap increases from 3.64 eV to 3.70 eV, while β-TeO2’s decreases from 3.632 to 3.53 eV. Lattice parameters uniformly reduce in α-TeO2, but vary dissimilarly in β-TeO2. PDOS analysis shows O-2p and Te-5p states dominate near the band edges. Optical behavior is anisotropic, with enhanced polarizability and refractive indices under strain. Nonlinear optical properties improve with increased strain, especially the non-linear susceptibility components, which are relevant for second-harmonic generation. Born Effective Charges show increased Te polarization and reduced oxygen polarization, impacting dielectric response.

Emergence of Weyl points and large anomalous Hall conductivity in layered Bi2TeMnI2.

In recent years, narrow band gap layered materials were reported as an interesting candidate for energy efficient devices. Here, we chose BiTeI, a layered material that has significant Rashba spin splitting, for charge modification with the purpose of exploring the electronic, magnetic and topological properties. Chemical doping with an Mn atom is done to the Te site in BiTeI. On the basis of density functional theory calculations, we found that the parent material BiTeI is a semiconductor with an indirect band gap of ∼0.46 eV within full-relativistic mode. The orbital contributions around the Fermi level are found to be mainly from the Bi-6p, I-5p and Te-5p states in the electronic structure. Upon chemical doping by Mn to Bi, Te and I separately, doping to the Te site is energetically favorable with a ferromagnetic ground state and a semimetallic behaviour. The doped material, i.e., Bi2TeMnI2, is found to be a magnetic Weyl semimetal with six Weyl points close to the Fermi level (around 100 meV in the conduction region). Our calculations suggest Bi2TeMnI2 as a probable candidate of a Weyl semimetal. The emergence of Weyl points gives rise to a large intrinsic anomalous Hall conductivity of up to ∼750 Ω-1 cm-1. The calculated negative value of formation energy (-0.233 eV) and the positive phonon frequency suggests Bi2TeMnI2 to be thermodynamically favorable and dynamically stable. This work deserves a transport experiment to confirm our claim, which might provide insights towards discovering new quantum materials suitable for high-speed electronics, spintronics and quantum computing.

Effect of Hubbard Potential (U) on Mn Doped BiTeI

In recent years, Bismuth tellurohalide has gained significant attention as a possible candidate for spintronics applications. Among them, BiTeI is one such layered material that shows Rashba spin splitting responsible for transport phenomenon. On the basis of density functional theory calculations using full-potential local-orbital code, we studied the electronic and magnetic properties of Bi2TeMnI2 compound. Our study shows an energy band gap of 1.18 eV for the parent material BiTeI. The main contribution around the Fermi level are from the Bi-6p, I-5p and Te-5p states. Upon doping Mn atom independently to Bi, Te and I site, we found that Te site is energetically most favorable. With Mn doped to Te site, the material is found to transform to ferromagnetic semiconducting state with magnetic easy-axis along [001] with magnetic moment of 3 μB per unit cell. The magnetic moment is found to alter interestingly with the implementation of Hubbard-potential (U). The value first reduces to 1 μB for U = 3 eV and starts changing with increasing value of U. Due to internal ferromagnetic ordering and strong correlation effect, this compound seems to be a promising candidate for spintronics and quantum computing.

Origins of midgap states in Te-based Ovonic threshold switch materials

The non-linear threshold-type current-voltage behavior that characterizes selector devices for three-dimensional (3D) crossbar-type nonvolatile memory devices relies upon a phenomenon known as Ovonic threshold switching (OTS). Because current practical OTS materials are based on toxic elements, such as Se and As, Te-based OTS materials are expected to offer a more environmentally friendly option. However, the electronic structure that determines the OTS behavior of Te-based OTS materials is not well understood. In this paper, the electronic structure of amorphous Si0.29Te0.71, has been explored using hard X-ray photoelectron spectroscopy (HAXPES) in conjunction with density functional theory (DFT) calculations. The HAXPES results show that the Si0.29Te0.71 amorphous network of the simulated amorphous structure is based upon Te-Te, Te-Si, and Si-Si bonding. DFT calculations revealed that Si3p and Te5p states contribute to bonding, whereas occupied non-bonding Te5p states form the top of the valence state. A projected local density of states analysis shows that the Si site forms conduction-tail states, whereas the Te site forms both conduction- and valence-tail states. Furthermore, Te-Te dimers contribute significantly to the midgap states that characterize the OTS behavior. Finally, the valence-tail state extension within the mobility gap of Si0.29Te0.71 was experimentally demonstrated.

The electronic structure of β-TeO2 as wide bandgap p-type oxide semiconductor

Wide bandgap oxide semiconductors have gained significant attention in the fields from flat panel displays to solar cells, but their uses have been limited by the lack of high mobility p-type oxide semiconductors. Recently, β-phase TeO2 has been identified as a promising p-type oxide semiconductor with exceptional device performance. In this Letter, we report on the electronic structure of β-TeO2 studied by a combination of high-resolution x-ray spectroscopy and hybrid density functional theory calculations. The bulk bandgap of β-TeO2 is determined to be 3.7 eV. Direct comparisons between experimental and computational results demonstrate that the top of a valence band (VB) of β-TeO2 is composed of the hybridized Te 5s, Te 5p, and O 2p states, whereas a conduction band (CB) is dominated by unoccupied Te 5p states. The hybridization between spatially dispersive Te 5s2 states and O 2p orbitals helps us to alleviate the strong localization in the VB, leading to small hole effective mass and high hole mobility in β-TeO2. The Te 5p states provide stabilizing effect to the hybridized Te 5s-O 2p states, which is enabled by structural distortions of a β-TeO2 lattice. The multiple advantages of large bandgap, high hole mobility, two-dimensional structure, and excellent stability make β-TeO2 a highly competitive material for next-generation opto-electronic devices.

Dynamic Lone Pair Expression as Chemical Bonding Origin of Giant Phonon Anharmonicity in Thermoelectric InTe

AbstractLoosely bonded (“rattling”) atoms withs2lone pair electrons are usually associated with strong anharmonicity and unexpectedly low thermal conductivity, yet their detailed correlation remains largely unknown. Here we resolve this correlation in thermoelectric InTe by combining chemical bonding analysis, inelastic X‐ray and neutron scattering, and first principles phonon calculations. We successfully probe soft low‐lying transverse phonons dominated by large In1+z‐axis motions, and their giant anharmonicity. We show that the highly anharmonic phonons arise from the dynamic lone pair expression with unstable occupied antibonding states induced by the covalency between delocalized In1+5s2lone pair electrons and Te 5pstates. This work pinpoints the microscopic origin of strong anharmonicity driven by rattling atoms with stereochemical lone pair activity, important for designing efficient materials for thermoelectric energy conversion.

Dynamic Lone Pair Expression as Chemical Bonding Origin of Giant Phonon Anharmonicity in Thermoelectric InTe.

Loosely bonded ("rattling") atoms with s2 lone pair electrons are usually associated with strong anharmonicity and unexpectedly low thermal conductivity, yet their detailed correlation remains largely unknown. Here we resolve this correlation in thermoelectric InTe by combining chemical bonding analysis, inelastic X-ray and neutron scattering, and first principles phonon calculations. We successfully probe soft low-lying transverse phonons dominated by large In1+ z-axis motions, and their giant anharmonicity. We show that the highly anharmonic phonons arise from the dynamic lone pair expression with unstable occupied antibonding states induced by the covalency between delocalized In1+ 5s2 lone pair electrons and Te 5p states. This work pinpoints the microscopic origin of strong anharmonicity driven by rattling atoms with stereochemical lone pair activity, important for designing efficient materials for thermoelectric energy conversion.

Revealing trend of electron momentum densities, electronic and optical response of dichalcogenides MS2-xTex (M=Ti, Zr; x=0,1,2)

Role of replacement of S atom by Te atoms in transition metal layered dichalcogenides (MS2; M = Ti and Zr) in amending the electronic response is reported. Different exchange and correlation potentials within linear combination of atomic orbitals (LCAO) have been used to interpret the experimental momentum densities, electronic and optical properties. It is seen that Compton profiles (CPs) based on hybrid functional combining Hartree-Fock with density functional theory namely WC1LYP reconciles better with the measured momentum densities than other considered schemes of exchange and correlation potentials. More covalent character of TiSTe than that in ZrSTe as predicted by Mulliken's population is in accordance with the measured equal-valence-electron-density profiles. Moreover, electronic and optical response using the modified Becke-Johnson (mBJ) potential as facilitated in the full potential linearized augmented plane wave method are reported. From energy bands and DOS, we conclude that concentration of Te reduces band gap in ZrS2 due to lowering of Te-5p states in the unoccupied region, while the TiS2 based mixed compounds depict metallic character.

X-ray spectroscopy for the magnetic study of the van der Waals ferromagnet CrSiTe3 in the few- and monolayer limit

The study of magnetic order in few- and monolayer van der Waals materials poses a challenge to the most commonly employed magnetic characterization techniques as they normally lack magnetic sensitivity and/or lateral resolution enabling their thickness-dependent probing. Here we demonstrate the usefulness of x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) measurements, carried out at the Cr and Te M 5 edges, for the study of the ferromagnetic semiconductor CrSiTe3 (CST) in the form of single- and few-layer flakes. By scanning the sample under the incident x-ray beam, a map of the exfoliated system was obtained, which reproduced the optical micrographs showing the detailed distribution and thicknesses of the flakes. In this way, XAS/XMCD was performed at selected sample areas, revealing the thickness-resolved spectroscopic and magnetic properties of the flakes, such as the spin and orbital magnetic moments. The spin moment, in line with the saturation field, is decreasing with film thickness, revealing a single-domain and out-of-plane magnetization for the thinnest films. For CST, the electronic properties are governed by the strong covalent bond between the Cr 3d(e g ) and Te 5p states, giving rise to a superexchange scenario. We observed a gradually increasing ratio of orbital to spin moment for thinner flakes, which could be due to a further increase of the covalent mixing. Hysteresis loops were recorded at the Cr L 3 edge, showing an open loop for 10 down to ∼3 layers, while the bulk shows a wasp-waist shaped loop. With the transition temperature from the soft to the hard ferromagnetic state decreasing with thickness, the monolayer shows a narrowed, closed loop at 10 K, suggesting its transition temperature 10 K. Our study demonstrates the unique capabilities of XAS/XMCD for the study of few-layer van der Waals magnets, highlighting the interplay between electron correlation and ferromagnetism in CST.

Effect of Fe on Bi2Te3: Structure, magnetic properties, and XPS valence band

The crystal structure and magnetic properties of Fe doped Bi2Te3 are presented. The samples were studied by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and magnetization measurements. Rietveld refinement results show a decrease in volume due to the decrease in c lattice parameter as iron is introduced. By XPS measurements, the Te 3d, Bi 4f, Bi 5d, Te 4d, and Fe 3d core levels were identified, and the XPS valence band (VB) was measured. The deconvolution process of high resolution XPS spectra in Fe 3d orbital shows that the Fe3+ ion is present in the samples. Theoretical calculations by DFT reveal that the Bi2Te3 compound is a semiconductor with a narrow energy band gap in agreement with experimental results. DOS analysis shows that the main contributions to the valence band are the Te 5p, Bi 6p, and Fe 3d states. XPS VB shows a decrease in the intensity at zero eV due to increased Fe 3d states and good correspondence with DOS calculated for Bi2Te3. Finally, magnetic susceptibility measurements on Bi2Te3 do not show a magnetic behavior; however, for the doped sample with x = 0.2, a weak ferromagnetic behavior (WFM) below 14 K is observed.

Spin-split valence bands of the ferromagnetic insulator Cr2Ge2Te6 studied by angle-resolved photoemission spectroscopy

The low-energy electronic structure of a ferromagnetic insulator Cr2Ge2Te6 was studied by angle-resolved photoemission spectroscopy (ARPES). A broadening of the ARPES spectra, which we ascribe to a splitting of the valence band, has been observed at low temperature. Through temperature dependent measurements, we found that the band splitting presented a transition at ∼60–70 K, which is consistent with the ferromagnetic transition temperature of Cr2Ge2Te6. Therefore, we suggested that the splitting of the valence band is due to the spin polarization effect of Te 5p states by Cr 3d states, which is compatible with the superexchange coupling through Cr-Te-Cr to form long-range ferromagnetic order in Cr2Ge2Te6.

Electronic Structure of UTe2 Studied by Photoelectron Spectroscopy

The electronic structure of the unconventional superconductor UTe2 was studied by resonant photoelectron spectroscopy (RPES) and angle-resolved photoelectron spectroscopy (ARPES) with soft X-ray synchrotron radiation. The partial U 5f density of states of UTe2 were imaged by the U 4d–5f RPES and it was found that the U 5f state has an itinerant character, but there exists an incoherent peak due to the strong electron correlation effects. Furthermore, an anomalous admixture of the U 5f states into the Te 5p bands was observed at a higher binding energy, which cannot be explained by band structure calculations. On the other hand, the band structure of UTe2 was obtained by ARPES and its overall band structure were mostly explained by band structure calculations. These results suggest that the U 5f states of UTe2 have itinerant but strongly-correlated nature with enhanced hybridization with the Te 5p states.

Electronic structure and dielectric function of Mn-Bi-Te layered compounds

A comparative study of the electronic and optical properties of Mn-Bi-Te layered compounds was carried out using spectroscopic ellipsometry (SE) over a photon energy range of 0.7–6.5 eV at room temperature and density functional theory (DFT)-based first-principle calculations within the general gradient approximation with Hubbard like correction (GGA+U) and allowance for a spin-orbital coupling. The total energies of the above compounds in ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations are obtained by taking the long-range van der Waals interaction into account. The stability of the AFM state of MnBi2Te4 and MnBi4Te7 over the corresponding FM counterpart is disclosed. The SE-based and calculated dielectric functions are compared. It is shown that interband optical transitions in the accessed photon energy range mainly occur between Mn 3d + Te 5p states of the valence band and Bi 6p + Te 5p with a small admixture of Mn 3d states of the conduction band.

Structural transition in AuAgTe4 under pressure

Gold is inert and forms very few compounds. One of the most interesting of those is calaverite AuTe2, which has incommensurate structure and which becomes superconducting when doped or under pressure. There exist a ‘sibling’ of AuTe2, the mineral sylvanite AuAgTe4, which properties are almost unknown. In sylvanite Au and Ag ions are ordered in stripes, and Te6 octahedra around metals are distorted in such a way that Ag becomes linearly coordinated, what is typical for Ag1+ , whereas Au is square coordinated—it is typical for d8 configurations, i.e. one can assign to Au the valence 3+. Our theoretical study shows that at pressure GPa there should occur in it a structural transition such that above this critical pressure Te6 octahedra around Au and Ag become regular and practically identical. Simultaneously Te–Te dimers, existing at P = 0 GPa, disappear, and material from a bad metal becomes a usual metal with predominantly Te 5p states at the Fermi energy. We expect that, similar to AuTe2, AuAgTe4 should become superconducting above .

Giant contribution of the ligand states to the magnetic properties of the Cr2Ge2Te6 monolayer.

The magnetic properties of the Cr2Ge2Te6 monolayer - an important two-dimensional (2D) ferromagnetic (FM) material - are systematically investigated on the basis of ab initio electronic structure calculations within density functional theory (DFT). For these purposes we construct a minimal tight-binding model in the basis of maximally localized Wannier functions, which describes the behavior of the magnetic Cr 3d, Ge 4p, and Te 5p electrons. This model allows us to rationalize the results of conventional DFT calculations at the microscopic level. We explore the abilities of different techniques, including the Green's function perturbation theory and constraint calculations with an external magnetic field, for the analysis of magnetic interactions, that allow us to decompose these interactions in terms of partial contributions coming from different atomic sites. We argue that, although the magnetism of Cr2Ge2Te6 originates from the Hund's rule effects in the partially filled Cr 3d shell, the contributions of the ligand - and particularly Te 5p - states are crucially important and have a significant effect on the behavior of isotropic exchange interactions, magnetocrystalline anisotropy energy (MAE), and antisymmetric Dzyaloshinskii-Moriya (DM) interactions induced by an electric field. In particular, the Te 5p states increase dramatically the FM coupling and DM interactions between the Cr spins. They are also largely responsible for the behavior of MAE, manifesting themselves in non-Heisenberg type effects, which go beyond the scopes of the conventional Mermin-Wagner theorem for the analysis of 2D magnetism of Cr2Ge2Te6.

Photoemission and soft X-ray absorption spectroscopy study of Gd-substituted thermoelectric and topological Bi2Te3

The electronic structures of Gd-substituted Bi2−xGdxTe3 single crystals have been investigated by employing synchrotron-radiation spectroscopies, such as soft X-ray absorption spectroscopy (XAS), soft X-ray magnetic circular dichroism (XMCD), photoemission spectroscopy (PES), and scanning photoelectron microscopy (SPEM). SPEM measurements confirm that Gd ions are substituted homogeneously in Bi2−xGdxTe3 crystals. The measured Gd 3d XAS and XMCD spectra provide evidence that the substituted Gd ions are trivalent and magnetic. The hν-dependence of the valence-band (VB) PES spectra are described well by the hν-dependent contributions of the Te 5p and Bi 6p states. A good agreement is found between the measured VB PES spectra of Bi2−xGdxTe3 and the calculated partial densities of states, revealing that the Te 5p states will play an important role in determining the thermoelectric and topological properties of Bi2−xGdxTe3.

Electronic and Thermoelectric Properties of Layered Oxychalcogenides (BiO)Cu Ch ( Ch = S, Se, Te).

We study the new details of electronic and thermoelectric properties of polycrystalline layered oxychalcogenide systems of (BiO)Cu Ch ( Ch = Se, Te) prepared by using a solid-state reaction. The systems were characterized by using photoemission (PE) spectroscopy and four-probe temperature-dependent electrical resistivity ρ( T). PE spectra are explained by calculating the electronic properties using the generalized-gradient approximation method. PE spectra and ρ( T) show that (BiO)CuSe system is a semiconductor, while (BiO)CuTe system exhibits the metallic behavior that induces the high thermoelectric performance. The calculation of electronic properties of (BiO)Cu Ch ( Ch = S, Se, Te) confirms that the metallic behavior of (BiO)CuTe system is mainly induced by Te 5p states at Fermi energy level, while the indirect bandgaps of 0.68 and 0.40 eV are obtained for (BiO)CuS and (BiO)CuSe systems, respectively. It is also shown that the local symmetry distortion at Cu site strongly stimulates Cu 3d-t2g to be partially hybridized with Ch p orbitals. This study presents the essential properties of the inorganic systems for novel functional device applications.

First-Principles Investigation of Electronic, Half-Metallic, and Optical Properties of Ti-Doped MgTe Semiconductors with Various Concentrations of Dopant

The electronic, magnetic, and optical properties of Ti-doped semiconductors Mg1−xTi x Te at concentrations of x = 0.25, 0.50, and 0.75 have been investigated in the framework of density functional theory. Spin-polarized calculations of the electronic structure of Mg1−xTi x Te revealed that these compounds are half-metallic ferromagnetic materials with 100% spin polarization at the Fermi level. Large half-metallic gaps of 1.69 eV, 1.18 eV, and 0.98 eV were obtained for these compounds with Ti concentration of 0.25, 0.50, and 0.75, respectively. In addition, the origin of the half-metallic gap in Mg1−xTi x Te is discussed based on the partial density of states. It is found that hybridization between Ti-3d and Te-5p states and the large exchange splitting of the Ti-3d states are responsible for the half-metallic property of Mg1−xTi x Te compounds. The Curie temperatures of Mg0.5Ti0.5Te and Mg0.25Ti0.75Te were predicted to be 572 K and 959 K, respectively, within the mean field approximation. The appropriate half-metallic properties of Mg1−xTi x Te (x = 0.50 and 0.75) make them appropriate electronic materials for use in spintronics applications. The optical properties of pure and Ti-doped MgTe semiconductors, such as the dielectric function, extinction coefficient, absorption coefficient, reflectivity, and optical conductivity, were also considered. It was found that Ti doping considerably changed the optical properties of the MgTe semiconductors, especially at lower frequencies, such that these materials can be used in optical devices such as photodetectors and solar cells over wider ranges of frequency than corresponding undoped material.

Density of State Calculations for Tl3SbS3 and SbTeI

Based on the full-potential linearized augmented plane waves method (FL-LAPW) with local density approximation (LDA), the partials and totals densities of state of Tl3SbS3 and SbT eI are calculated in order to find the semiconductor character via direct or indirect gap. Tl3SbS3 and SbTeI present the most important candidates of the antimony chalcogenides family. Their densities of states curves bring out characteristic features in the valence band a core like peak, at environ 13.00 eV below the valence band maximum, originating mainly from S 3s and I 5s states respectively, and a three-peak structure at the top of the valence band from S 3p and I 5p states hybridized with Sb 5p and Te 5p states. Our results give a good agreement with other theoretical calculations and experimental data.

The 7 × 1 Fermi Surface Reconstruction in a Two-dimensional f -electron Charge Density Wave System: PrTe3.

The electronic structure of a charge density wave (CDW) system PrTe3 and its modulated structure in the CDW phase have been investigated by employing ARPES, XAS, Pr 4 f RPES, and first-principles band structure calculation. Pr ions are found to be nearly trivalent, supporting the CDW instability in the metallic Te sheets through partial filling. Finite Pr 4 f spectral weight is observed near the Fermi level, suggesting the non-negligible Pr 4 f contribution to the CDW formation through the Pr 4 f -Te 5p hybridization. The two-fold symmetric features in the measured Fermi surface (FS) of PrTe3 are explained by the calculated FS for the assumed 7 × 1 CDW supercell formation in Te sheets. The shadow bands and the corresponding very weak FSs are observed, which originate from both the band folding due to the 3D interaction of Te sheets with neighboring Pr-Te layers and that due to the CDW-induced FS reconstruction. The straight vertical FSs are observed along kz, demonstrating the nearly 2D character for the near-EF states. The observed linear dichroism reveals the in-plane orbital character of the near-EF Te 5p states.