Degenerate Valence Band Research Articles

Low lattice thermal conductivity induced by rattling-like vibration in Zintl phase Na2CaCdSb2 compound with high ZT from two-channel model

Zintl phase compounds, characterized by the traits of a phonon-glass and electron-crystal, stand out as promising candidates for thermoelectric applications. This study delves into the thermoelectric properties of Zintl phase Na2CaCdSb2 compound through a synergy of first-principles calculations, machine learning interatomic potential approach based on the two-channel model, and high-throughput screening calculations. With an indirect bandgap of 1.02 eV, Na2CaCdSb2 compoundpossesses an high carrier mobility. The high valence band degeneracy and anisotropic band (light-heavy band) lead to a high power factor for the p-type Na2CaCdSb2 compound. The strong fourth-order anharmonicity of the rattling-like Na atom manifests the low lattice thermal conductivity on the basis of two-channel model. The loosely bonding Na atom displays random rotations and dynamically disordered orientations, leading to the weakly disordered nature of Na2CaCdSb2 compound. The unique crystal structure of Na2CaCdSb2 compound coupled with the abundance of diffusion-like phonons play a pivotal role in the phonon transport. Considering the low lattice thermal conductivity and high power factor with the framework of two-channel model, the n-type and p-type Na2CaCdSb2 compounds exhibit optimal figure-of-merits (ZT) of 1.0 and 2.0 at 600 K, showcasing excellent thermoelectric performance. Meanwhile, an optimal ZT of 2.3 is achieved for p-type Na2CaCdSb2 compound at the same temperature along the x-direction. Our present study not only provides fundamental insights into the thermal and electronic transport properties of Na2CaCdSb2 compound under the two-channel model in combination with the four-phonon scattering and multiple carrier scattering rates, but also rationalizes the high-order phonon interactions and thermal transport properties under two-channel model.

Enhancing thermoelectric performance of AB2Sb2-type Zintl phase through band shaping and lattice distortion

AB2Sb2-type Zintl phases, especially Mg3Sb2, have garnered widespread attention in virtue of their nontoxic constituent elements and outstanding thermoelectric performance in n-type materials. However, the p-type counterpart of AB2Sb2 still struggles with a low figure of merit zT, primarily because of the limited valence band degeneracy, large band effective mass, and relatively high thermal conductivity. Here we simultaneously optimize the electrical and thermal transports of p-type AB2Sb2-based materials through shaping the band edge and distorting the crystal lattice. By alloying Zn at Mg2 site and Ca at Mg1 site, we successfully decorate the crystal lattice of Mg3Sb2 in real space and thereby modify the band structure in reciprocal space. Zn alloying promotes the band degeneracy while Ca alloying reduce the single band effective mass, which largely promotes the transport of charge carriers. Moreover, the distorted crystal lattice effectively blocks the heat-carry phonons to reduce the lattice thermal conductivity κL. A high zT of 0.81 is finally realized at 750 K in our Zn/Ca co-alloyed samples, which is three times larger than that of Mg3Sb2 matrix. Our work opens new possibilities for the synthetic optimization of both electrical and thermal transports in Zintl phase and other thermoelectric systems.

Enhanced thermoelectric properties of Mm-filled p-type (Fe, Co)Sb3 skutterudites via Co/Fe ratio regulation and extra Sb compensation

The band degeneracy thus thermoelectric performance of (Fe, Co)Sb3 based p-type skutterudites can be well modulated by adjusting Co/Fe ratio. Combining systematic experiments and density function theory (DFT) calculations, we revealed in this work that the variation of Co/Fe ratio could result in complicated and mutually coupled effects on the individual thermoelectric parameters (band degeneracy, band gap, Fermi level, filling limit of fillers, phonon spectrum, etc.) and that using Co/Fe ratio to improve the overall thermoelectric figure of merit is nontrivial. We eventually found that the optimal Co/Fe ratio for Mm (Misch metal) filled (Fe, Co)Sb3 lies at 1.3:2.7, which is somewhat off the Co/Fe = 1:3 point where the largest valence band degeneracy appears. Extra Sb was then introduced to compensate Sb volatilization for improved electrical transport performance. An overall peak figure of merit ZTmax of 1.06 was eventually achieved in the sample Mm0.8Fe2.7Co1.3Sb12.60, which is among the highest ones for all reported (Fe, Co)Sb3 based p-type skutterudites so far.

Thermoelectric performance of ternary Cu-based chalcogenide Cu2TiTe3

In this work, we report the ternary Cu-based chalcogenide, Cu2TiTe3, as a promising thermoelectric material in middle-temperature range. The bonding interaction between Te p and Ti d states is observed in the Ti–Te octahedron, which drives the side bands up converging with Γ band, yielding a high valence band degeneracy of 5. A high electronic quality factor of 2.2 μW cm−1 K−2 and a decent power factor of 7.5 μW cm−1 K−2 at 300 K are achieved for Cu2TiTe3. Likewise, Cu2TiTe3 demonstrates low lattice thermal conductivity throughout the measured temperature range, which is attributed to the low frequency vibration related to the global motion of Ti–Te–Cu clusters. Finally, a maximum figure-of-merit of 0.38 was obtained for Cu2TiTe3 at 600 K.

PtS2/WSe2 heterostructure for thermoelectric and Li-ion battery Applications: A First-Principles study

Van der Waals’ two-dimensional heterostructure engineering offers great potential in applications of electronic devices, battery applications, thermoelectric. Encouraged by the successful synthesis of PtS2/WSe2 heterostructure, we explore its potential in thermoelectric and Li-ion battery applications by first-principles calculations. Due to these multi-valley degenerate valence and conduction bands, a large power factor of 0.007 W/mK2 is attained. Due to the significant interface effect, PtS2/WSe2 heterostructure can strongly adsorb Li atoms with −2.69 eV adsorption strength on the middle of PtS2/WSe2 heterostructure layers. Also, an extremely fast Li diffusion rate was found in this heterostructure (as low as 20 meV), which is close to the recently reported MXene. Large theoretical volumetric storage capacity of 1283.98 mA h/cm3, in the meanwhile a rather low average Open-circuit voltage (0.07 V) also purports that PtS2/WSe2 heterostructure is a quite promising anode material in Li-ion batteries with high power densities and fast charge/discharge rates.

First principles study of stability and thermoelectric response of GeH monolayer

Thermoelectric characteristics of two-dimensional GeH structure have been studied using first-principles density functional technique and Boltzmann transport theory. Positive phonon modes and negative formation energy both attest to the stability of structure. A high value of elastic constant suggests strong chemical bonding and hence high mechanical strength. A direct band gap semiconducting character of GeH monolayer with 1.64 eV band gap is revealed by examination of electronic band structure. Flat conduction band in one direction and degenerate valence band near Fermi level improves density of states. As a consequence, both Seebeck coefficient and electrical conductivity shows excellent response and results in high power factor. At room temperature, the ZeT value of 5.51 (5.45) is obtained for p-(n) type doping, which shows its high practical capability of thermoelectric conversion. These findings suggest that GeH monolayer is an exciting possibility for 2D thermoelectric applications.

Multi-zone resonance spectrometry of inelastic electron scattering of light and the manifestation of strong spin-orbit interaction in nanostructures with quantum dots

Highly sensitive methods of inelastic light scattering spectrometry have been developed to detect individual photons in the visible and near-infrared spectral ranges (0.4-1.35) μm. Precision optical measurements of vibrational and electronic states of nanostructures with quantum objects are performed, using the example of quantum-dimensional nanoheterostructures (311)In si-GaAs/InAs with InAs quantum dots. A distinctive attractive feature of such structures with a degenerate valence band of symmetry 8 is the presence of a strong spin-orbit interaction. In this case, the intrinsic moment of the holes can effectively interact with the electric field of the incident light wave. It was found that such interaction causes intense luminescence and inelastic scattering of the light wave in the IR region of the 0.9-1.35 μm spectrum. Effective generation of nonequilibrium electron-hole (e-h) plasma with concentrations n=p=1.0·1017 cm-3 and T_e=T_h=25 K,was revealed under resonant excitation conditions, while the lattice temperature T_L=5.1 K. New mechanisms have been discovered for the formation of an abnormally intense integration spectrum of multiplex inelastic light scattering of a rather complex shape formed by various separable resonant contributions of the processes of quasi-elastic scattering by charge carriers, light scattering by acoustic plasmons of electron-hole plasma, as well as inelastic intra- and inter-band light scattering by heavy holes. It has been established that the anomalous intensity gain of such multi-zone selective resonant light scattering is more than 105 times higher than the intensity of Thomson light scattering on individual charge carriers. There is agreement between the estimated calculated and experimental spectra, and for the most part for the difficult-to-interpret significant width of the observed light scattering line. It is shown that new mechanisms of various contributions to the formation of scattered photon radiation are important diagnostic elements that are clearly manifested in the spectra of the resulting enhanced inelastic light scattering. Keywords: nanostructures, quantum objects, resonant scattering of light by charge carriers, exchange interaction, plasma diagnostics.

Enhanced Thermoelectric Performance in GeTe by Synergy of Midgap state and Band Convergence

AbstractThe good co‐existence of midgap state and valence band degeneracy is realized in Bi‐alloyed GeTe through the In‐Cd codoping to play different but complementary roles in the valence band structure modification. In doping induces midgap state and results in a considerably improved Seebeck coefficient near room temperature, while Cd doping significantly increases the Seebeck coefficient in the mid‐high temperature region by promoting the valence band convergence. The synergistic effects obviously increase the density of state effective mass from 1.39 to 2.65 m0, and the corresponding carrier mobility still reaches 34.3 cm2 V−1 s−1 at room temperature. Moreover, the Bi‐In‐Cd co‐alloying introduces various phonon scattering centers including nanoprecipitates and strain field fluctuations and suppresses the lattice thermal conductivity to a rather low value of 0.56 W m−1 K−1 at 600 K. As a result, the Ge0.89Bi0.06In0.01Cd0.04Te sample obtains excellent thermoelectric properties of zTmax ≈2.12 at 650 K and zTavg ≈1.43 between 300 and 773 K. This study illustrates that the thermoelectric performance of GeTe can be optimized in a wide temperature range through the synergy of midgap state and valence band convergence.

Full-momentum exciton landscape and macroscopic quantum phase diagram in two-dimensional gallium oxides

High-temperature exciton macroscopic quantum phases have a wide application prospect in many fields such as ultrahigh power photonics and quantum information, and are extremely difficult to realize due to the strong thermal dissociation effect. In this study, the exciton dispersions and macroscopic quantum phase diagrams of two-dimensional (2D) ultrawide bandgap semiconductors I–Ga2O3 and II-Ga2O3 are investigated for the first time. Both I–Ga2O3 and II-Ga2O3 possess the complex excitonic composition and morphology, resulting from their highly degenerate and flat valence bands. For the exciton B1 (B1') with an ultra-large binding energy of 1.14 (1.03) eV in I–Ga2O3, the phase transition temperature from free exciton to Bose-Einstein condensation (TBEC) is as high as 356 (679) K, respectively. The critical carrier densities for the formation of electron-hole liquid at room temperature (NEHL) are 1.3 × 1013 cm−2 and 1.8 × 1013 cm−2, respectively. For the free exciton B1 with an ultra-large binding energy of 1.49 eV in II-Ga2O3, TBEC reaches 365 K and NEHL is 5.9 × 1013 cm−2. Our findings extend the material basis for the research of exciton macroscopic quantum phases and will boost their applications in novel optoelectronic devices.

(Invited) Optoelectronic Quantum Information Processing: An All-Group IV Integrated Platform

Achieving coherent optical photon-to-spin conversion is a long-sought-after strategy for surmounting current fundamental limits in optical schemes that hinder the long-distance distribution of entanglement. Moreover, photon-to-spin interfaces are also essential for a direct mapping of the quantum information encoded in photon flying qubits to stationary spin processors. However, the lack of scalable materials offering an efficient interaction with optical photons along with optimal spin properties remains a formidable obstacle hindering the development of these quantum technological components. With this perspective, this presentation will discuss strategies to address these challenges by leveraging the degrees of freedom offered by group IV (Si)GeSn semiconductors, namely strain and composition, to tailor the electronic structure and eventually fulfill these prerequisites. These innovative systems do not only have the potential to enable coherent photon-to-spin interfaces, but because of their compatibility with the semiconductor industry they will also offer scalability, manufacturability, and cost-effectiveness.We will show that this family of semiconductors provide an additional flexibility to control the charge carrier states and achieve a selective confinement of holes. The latter benefit from a quiet quantum environment that has been at the core of increasingly reliable quantum processors and memories. However, most if not all available experimental studies of two-dimensional gas systems have been thus far focused on heavy-hole (HH) states. This is attributed to the nature of the heterostructures currently available (e.g, Ge/SiGe, InGaAs/GaAs), where compressive strain lifts the valence band degeneracy and leaves HH states energetically well above the light-hole (LH) states. We will demonstrate that tensile strained Ge/GeSn quantum wells alleviate these limitations and allow to selectively confine LH provided the strain is higher than 1%. This requires strain relaxed, high Sn content GeSn buffer layers to be used to grow Ge quantum wells with LH ground state, high g-factor anisotropy, and a tunable splitting of the hole subbands. The optical and electronic properties of these low-dimensional systems will be described and discussed. Spin injection and coherent control will also be addressed. Additionally, qubit designs exploiting the ability to engineer LH states and the Ge large spin-orbit coupling allowing fast all-electrical spin-manipulation schemes will also be presented and discussed.

Band engineering and improved thermoelectric performance in p-type SmMg2Sb2: A first-principles study

The thermoelectric properties of p-type SmMg2Sb2 are investigated by using the density functional theory and the Boltzmann transport equation method. It is found that SmMg2Sb2 shows a narrow band gap characteristic, and the crystal field splitting and spin-orbital coupling effect result in a singly degenerate valence band maximum. Meanwhile, SmMg2Sb2 exhibits an intrinsic low lattice thermal conductivity owing to the obvious phonon softening in the low-frequency acoustic branches. Based on a careful analysis of the band structure, we propose a scheme of metallic dopant at Sm site to promote the valence band convergence and improve the Seebeck coefficient. The improved thermoelectric properties are numerically confirmed in the Ba-doped p-type SmMg2Sb2, where a significantly improved ZT of 1.6 at 750 K is predicted with the synergistical effects of band engineering and enhanced phonon scattering.

Machine-learned model Hamiltonian and strength of spin–orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb)

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds Mg2Si, Mg2Ge, Mg2Sn, and Mg2Pb subject to strain. The MMTB models incorporate spin–orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding (TB) model reproduces the key aspects of the valence band structures in the entire Brillouin zone. The five-band model reveals that compressive strain localizes the contribution of the 3s orbital of Mg to the conduction bands and the outer shell p orbitals of X (X = Si, Ge, Sn, Pb) to the valence bands. In contrast, tensile strain has a reversed effect as it weakens the contribution of the 3s orbital of Mg and the outer shell p orbitals of X to the conduction bands and valence bands, respectively. The π bonding in the Mg2X compounds is negligible compared to the σ bonding components, which follow the hierarchy , and the largest variation against strain belongs to σ pp . The five-band model allows for estimating the strength of spin–orbit coupling (SOC) in Mg2X and obtaining its dependence on the atomic number of X and strain. Further, the band structure calculations demonstrate a significant band gap tuning and band splitting due to strain. A compressive strain of can open a band gap at the Γ point in metallic Mg2Pb, whereas a tensile strain of closes the semiconducting band gap of Mg2Si. A tensile strain of removes the three-fold degeneracy of valence bands at the Γ point in semiconducting Mg2Ge. The presented MMTB models can be extended for various materials and simulations (band structure, transport, classical molecular dynamics), and the obtained results can help in designing devices made of Mg2X.

Topological nodal surface semimetal states in Sr5X3 compounds (X=As,Sb,Bi)

Topological nodal surface semimetals are a class of materials characterized by degenerate conduction and valence bands on closed nodal surfaces in the Brillouin zone that are protected by crystalline symmetries. These materials attract considerable attention for exhibiting intriguing, topologically driven quantum phenomena that are fundamentally interesting and practically useful. Here we identify by systematic ab initio calculations and topological quantum chemistry analysis prominent topological nodal surface semimetal states in ${\mathrm{Sr}}_{5}{X}_{3}(X$ = As, Sb, Bi) compounds in hexagonal $P{6}_{3}/mcm$ (${D}_{6h}^{3}$) symmetry. These nodal surface states comprise degenerate the highest occupied band and the lowest unoccupied band that overlap in a single band at the ${k}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane and are protected by time-reversal and nonsymmorphic twofold screw rotation symmetry. Topological quantum chemistry theory analysis reveals a twofold band degeneracy without spin-orbit coupling at the high-symmetry and generic $\mathbit{k}$ points located at the ${k}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane, which is formed by the bands of (${\mathrm{A}}_{1}\ensuremath{\bigoplus}{\mathrm{B}}_{2})@6g$ band representations. Since the compounds studied here have been already experimentally synthesized, the present results have an immediate impact on the experimental detection and characterization of the theoretically predicted topological physics.

Band-edge absorption characteristics of semi-insulating indium phosphide under unified Franz-Keldysh and Einstein models

The foundational Franz-Keldysh effect and Einstein model are applied in this work to characterize semiconductor band-edge absorption---and its departures from ideality. We unify the Franz-Keldysh and Einstein models to fully characterize the field-induced tunneling of photoexcited electrons from degenerate valence bands into the conduction band, with encroachment into the band gap arising as an Urbach tail. Our unified model is implemented for semi-insulating indium phosphide (SI-InP) with strong agreement seen between the theoretical and experimental results for varied photon energies and electric fields.

Low thermal conductivity and high thermoelectric performance via Cd underbonding in half-Heusler PCdNa

Half-Heusler compounds are a very large class of materials that have received attention as promising high-temperature thermoelectric materials due to their favorable electrical transport behavior and other properties beneficial for device fabrication. A particular challenge is that while half-Heusler compounds show an exceptionally large range of thermal conductivities, these are still generally higher than other state-of-the-art thermoelectric materials. Here, we investigate PCdNa, which was reported to be a material that may form in the half-Heusler structure and have an unusually low thermal conductivity. This is in order to understand its very low thermal conductivity and elucidate its electronic properties in relation to thermoelectricity. We find that the low thermal conductivity of 2.6 W/mK at 300 K is a consequence of underbonding of the Cd ions within the network formed by the P. This underbonding leads to anharmonicity and relatively low frequency phonons and may be regarded as a generalized effective rattling phenomenon. The electronic structure shows a high valence band degeneracy, which we find leads to favorable good electrical transport properties. The combination of the low lattice thermal conductivity and favorable electrical transport properties results in high $p$-type thermoelectric performance. The figure of merit is predicted to reach $ZT=3.3$ at 900 K for optimum $p$-type doping. This suggests that PCdNa is a promising $p$-type high-temperature half-Heusler thermoelectric material.

First-principles study of the structural, electronic, optical, and thermoelectric properties of the RhVZ (Z= Si, Ge, Sn)

In this work, we have explored half-Heusler RhVZ (Z ​= ​Si, Ge, Sn) with 18 valence electrons per unit cell within the Density Functional Theory (DFT). The structural, electronic, optical, thermoelectric, and phonon properties have been investigated. The structural properties such as lattice constant, bulk modulus, and pressure derivative of bulk modulus have been reported. All materials are semiconductors with a narrow bandgap as it is concluded from the electronic band structure, and the phonon dispersion approve the dynamical stability, meanwhile, the absorption coefficient is found to be around 104 ​cm−1 in the visible region indicating that these materials can be used in Photovoltaic applications. The semi-classical Boltzmann transport equation used to calculate the thermoelectric characteristics and all materials under investigation show high performance in the p-type region due to the valence band degeneracy. The thermoelectric analysis provided show that the power factor of the materials lies on 3.12 ​× ​10−3, 3.26 ​× ​10−3, and 6.52 ​× ​10−3 ​W/msK2 for RhVSi, RhVGe, and RhVSn respectively, the figure of merit ZT reaches the maximum value of 0.8 ​at room temperature indicating that those materials are suitable for thermo-generation application at near room temperature. However, we found that RhVSn is more suitable for thermoelectric applications with its high ZT values more than RhVSi, and RhVGe compounds.

On the role of Zn doping on tuning the electronic and optical properties of MnCr2O4 spinel via Mn0.5Zn0.5Cr2O4 doping scheme: A first-principles quantum computational analysis

Herein, we report on the impact of Zn doping on electronic and optical properties of manganese di-chromium oxide spinel MnCr2O4 via Mn0.5Zn0.5Cr2O4 doping scheme. For the quantum computational analysis on the targeted properties, we employed first-principles density functional theory approach within full-potential linearized augmented plane wave method by Modified Becke–Johnson potential. The calculated partial and total density of states revealed a strong hybridization among electronic orbitals of the constituent species while the asymmetric spin-polarized profiles of density of states showed magnetic nature of the spinels. The major interspecies spin interaction in the degenerate valence band is via (Mn + Cr + O ) from −7 eV to Fermi level for spin up polarization. Interestingly, we predict a reduction in band gap of MnCr2O4 to order 1eV subjected to the Zn doping, which considerably modified the electronic and optical properties of the doped spinels Mn0.5Zn0.5Cr2O4 in contrast to its undoped counterpart. In particular, we predict an enhancement in absorption coefficient, energy loss function, reflectivity, dielectric constant and refractive index for Mn0.5Zn0.5Cr2O4 spinels at some regions of ultraviolet energy spectrum. The calculated static refractive index of MnCr2O4 is 1.59 and for Mn0.5Zn0.5Cr2O4 it is 1.4. The optoelectronic properties indicate the promising prospects of the Zn-doped spinels for potential application in optoelectronic devices.

Finding the order in complexity: The electronic structure of 14-1-11 zintl compounds

Yb14MnSb11 and Yb14MgSb11 have rapidly risen to prominence as high-performing p-type thermoelectric materials. However, the fairly complex crystal structure of A14MX11 Zintl compounds renders the interpretation of the electronic band structure obscure, making it difficult to chemically guide band engineering and optimization efforts. In this work, we delineate the valence-balanced Zintl chemistry of A14MX11 compounds using the molecular orbital theory. By analyzing the electronic band structures of Yb14MgSb11 and Yb14AlSb11, we show that the conduction band minimum is composed of either an antibonding molecular orbital originating from the (Sb3)7– trimer or a mix of atomic orbitals of A, M, and X. The singly degenerate valence band is comprised of non-bonding Sb pz orbitals primarily from the Sb atoms in the (MSb4)m– tetrahedra and of isolated Sb atoms distributed throughout the unit cell. Such a chemical understanding of the electronic structure enables strategies to engineer electronic properties (e.g., the bandgap) of A14MX11 compounds.

Performance Optimization for PbTe-Based Thermoelectric Materials

Thermoelectric material is a kind of functional material that uses the movement of carriers inside a solid to realize the direct mutual conversion of heat and electric energy. It provides a safe, reliable, pollution-free, noise-free, all-solid-state power generation and cooling method and has a wide range of application prospects. Among them, the characteristics of low valence band degeneracy, low effective quality of conduction band, strong phonon non-harmonicity, simple crystal structure and adjustable microstructure have made PbTe-based materials the focus of research in the thermoelectric field. In this review, two latest strategies to improve the thermoelectric properties of PbTe-based materials are discussed, and the challenges for the further development of PbTe-based thermoelectric materials and the prospects for the future are also outlined.

Promising Thermoelectric Performance in Two-Dimensional Semiconducting Boron Monolayer.

A heavy element is a special character for high thermoelectric performance since it generally guarantees a low lattice thermal conductivity. Here, we unexpectedly found a promising thermoelectric performance in a two-dimensional semiconducting monolayer consisting of a light boron element. Using first-principles combined with the Boltzmann transport theory, we have shown that in contrast to graphene or black phosphorus, the boron monolayer has a low lattice thermal conductivity arising from its complex crystal of hexagonal vacancies. The conduction band with an intrinsic camelback shape leads to the high DOS and a high n-type Seebeck coefficient, while the highly degenerate valence band along with the small hole effective mass contributes to the high p-type power factor. As a result, we obtained the p-type thermoelectric figure of merit up to 0.96 at 300 K, indicating that the boron monolayer is a promising p-type thermoelectric material.