Anisotropic Band Research Articles

Band anisotropy and effective mass renormalization in strained metallic VO2 (101) thin films

We explore how strain impacts the band structure of metallic-phase VO2 thin films deposited on TiO2(101) substrates. Employing a combination of X-ray absorption linear dichroism and valence band measurements, we demonstrate that strain can alter the intrinsic band structure anisotropy of metallic VO2. Our findings reveal that reducing the thickness of VO2 films leads to a more isotropic band structure. This observation is further supported by an analysis of the electronic population redistribution in the d||−π*\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${d}_{{||}}{-}{\\pi }^{* }$$\\end{document} bands, which affects the screening length and induces effective mass renormalization. Overall, our results underscore the potential of strain manipulation in tailoring the electronic structure uniformity of thin films, thereby expanding the scope for engineering VO2 functionalities.

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.

Electronic structure investigation and anisotropic phonon anharmonicity in ternary ZrGeTe4 single crystals

Ternary two-dimensional (2D) transition metal chalcogenides have gained immense attention because of their ability to overcome the intrinsic limitations of their binary counterparts. Layered 2D materials are important for future electronic and photonic devices owing to their low structural symmetry and in-plane anisotropy with tunable bandgap. Herein, the electronic structure and detailed vibrational properties of bulk ZrGeTe4 layered single crystals were investigated using angle-resolved photoemission spectroscopy (ARPES) and Raman scattering (RS). The ARPES results revealed an anisotropic Fermi surface of different momentum along kx and ky from the zone center and an anisotropic band structure with varying band curvatures along the high-symmetry directions. Furthermore, the RS of ZrGeTe4 was investigated under different polarizations and varying temperatures. The polarized RS exhibited twofold and fourfold symmetry orientations in different configurations, revealing the anisotropic phonon dispersions for bulk ZrGeTe4. The observed softening of Raman modes was corroborated with the anharmonic phonon dispersion, which was further supported by our third-order force constant calculations of thermal transport using density functional theory. Low lattice thermal conductivity with increasing temperature is linked with enhanced phonon–phonon scattering, which is evident from the decreased phonon lifetime and peak linewidth. In addition to these fundamental aspects, the anisotropic nature and unique layered structure of such materials reveal their bright future for next-generation nanoelectronic applications.

Effects of thickness, temperature and substrate on phonon anisotropy in layered Ta2NiSe5

Anisotropy in crystal structure has provided a new degree of freedom to tune the physical and photoelectric properties of low-symmetrical materials, enabling the anisotropic band structure and polarization-resolved applications. Thus, the identification and control on the anisotropy is of great importance for the development of polarization-integrated devices. In this work, we introduce a promising two-dimensional (2D) dimetal chalcogenide Ta2NiSe5 with strong in-plane anisotropic structure and explore the effects of thickness, temperature and substrate on its in-plane anisotropy via an angle-resolved polarized Raman spectra. It is shown that the Raman mode softens with increasing temperature, revealing the anharmonic phonon properties of the Ta2NiSe5 flakes. We found that the anisotropy of phonon vibration is strongly related with the thickness, temperature and used substrate, which is more obvious at thinner samples, higher temperature and opaque silicon substrate compared to sapphire and suspended conditions. This work first reveals the influence factor on the anisotropy of phonon mode, which will guide the fundamental research of anisotropic physics and experimental design for the angle-resolved electronics.

Anisotropic flat band and charge density wave in quasi-one-dimensional indium telluride

Anisotropic flat band and charge density wave in quasi-one-dimensional indium telluride

Antiferromagnetic superlattices: Anisotropic band and spin-valley valve in buckled two-dimensional materials

Antiferromagnetic superlattices: Anisotropic band and spin-valley valve in buckled two-dimensional materials

Strong Vertical Piezoelectricity and Broad-pH-Value Photocatalyst in Ferroelastic Y2Se2BrF Monolayer.

With the development of miniaturized devices, there is an increasing demand for 2D multifunctional materials. Six ferroelastic semiconductors, Y2Se2XX' (X, X' = I, Br, Cl, or F; X ≠ X') monolayers, are theoretically predicted here. Their in-plane anisotropic band structure, elastic and piezoelectric properties can be switched by ferroelastic strain. Moderate energy barriers can prevent the undesired ferroelastic switching that minor interferences produce. These monolayers exhibit high carrier mobilities (up to 104 cm2 V-1 s-1) with strong in-plane anisotropy. Furthermore, their wide bandgaps and high potential differences make them broad-pH-value and high-performance photocatalysts at pH value of 0-14. Strikingly, Y2Se2BrF possesses outstanding d33 (d33 = -405.97 pm/V), greatly outperforming CuInP2S6 by 4.26 times. Overall, the nano Y2Se2BrF is a hopeful candidate for multifunctional devices to generate a direct current and achieve solar-free photocatalysis. This work provides a new paradigm for the design of multifunctional energy materials.

Exploring spin photovoltaics in defective armchair phosphorene nanoribbons

This study explores the spin photovoltaic potential within armchair phosphorene nanoribbons (APNRs) that feature a periodic distribution of monovacancies (MVs) under the influence of light radiation. We investigate spin-semiconducting behavior induced by MV defects by utilizing both the mean-field Hubbard approximation and the self-consistent non-equilibrium Green's function model. This behavior is characterized by localized and anisotropic band structures around the Fermi energy, particularly within the antiferromagnetic phase. The existence of spin-splitting band gaps in defective APNRs not only enables the crafting of spin-optoelectronic nanodevices but also allows for the manipulation of electronic structure behavior with applied electric fields in both the vertical and transverse directions. Notably, the implementation of electric fields, offering tunability in electronic structure, results in varied spin photovoltaic responses encompassing a broad spectrum of photon energies from visible to ultraviolet. This research reveals promising avenues for advancing the field of spin-optoelectronic devices by MVs in APNRs.

Band Degeneracy and Anisotropy Enhances Thermoelectric Performance from Sb2Si2Te6 to Sc2Si2Te6.

The complex interrelationships among thermoelectric parameters mean that a priori design of high-performing materials is difficult. However, band engineering can allow the power factor to be optimized through enhancement of the Seebeck coefficient. Herein, using layered Sb2Si2Te6 and Sc2Si2Te6 as model systems, we comprehensively investigate and compare their thermoelectric properties by employing density functional theory combined with semiclassical Boltzmann transport theory. Our simulations reveal that Sb2Si2Te6 exhibits superior electrical conductivity compared to Sc2Si2Te6 due to lower scattering rates and more pronounced band dispersion. Remarkably, despite Sb2Si2Te6 exhibiting a lower lattice thermal conductivity and superior electrical conductivity, Sc2Si2Te6 is predicted to achieve an extraordinary dimensionless figure of merit (ZT) of 3.51 at 1000 K, which significantly surpasses the predicted maximum ZT of 2.76 for Sb2Si2Te6 at 900 K. We find the origin of this behavior to be a combined increase in band (valley) degeneracy and anisotropy upon switching the conduction band orbital character from Sb p to Sc d, yielding a significantly improved Seebeck coefficient. This work suggests that enhancing band degeneracy and anisotropy (complexity) through compositional variation is an effective strategy for improving the thermoelectric performance of layered materials.

Optical conductivity and linear dichroism in monolayer C[formula omitted]N

We theoretically study the optical properties of monolayer C3N within the framework of linear response theory by using a two band effective k⋅p Hamiltonian. We find that both the effective masses and optical conductivities are anisotropic arising from the anisotropic band structure at the M point. Although the inter-band coupling only exists in the armchair (x-)direction, the effective mass along the armchair direction is larger than that along the zigzag (y-)direction. Interestingly, the absorption part of the optical conductivity Re σxx excited by linearly polarized light along the armchair direction is an order of magnitude larger than Re σyy excited by linearly polarized light along the zigzag direction, resulting in a strong linear dichroism, because the inter-band optical conductivity is dominated by inter-band coupling which only exists in the armchair direction. Finite doping can adjust the optical conductivities i.e., Re σxx and Re σyy and modulate the linear dichroism, but the effects of n-type doping and p -type doping are different because of the electron–hole asymmetry. Our results provide further understanding of the electronic state of monolayer C3N and maybe useful to design polarized optical devices based on it.

Superfluidity of indirect momentum space dark dipolar excitons in a double layer with anisotropic tilted semi-Dirac bands

Superfluidity of indirect momentum space dark dipolar excitons in a double layer with anisotropic tilted semi-Dirac bands

Anisotropic magnetism and band evolution induced by ferromagnetic phase transition in titanium-based kagome ferromagnet SmTi3Bi4

Anisotropic magnetism and band evolution induced by ferromagnetic phase transition in titanium-based kagome ferromagnet SmTi3Bi4

Vacancy modulation dramatically enhances the thermoelectric performance of InTe single crystal

InTe single crystals have demonstrated great promise in the field of thermoelectric materials, particularly when oriented along the [110] direction. This specific crystal orientation exhibits higher electronic conductivity and lower thermal conductivity compared to other orientations of InTe. Through first-principles calculations, we identified the anisotropic valence band and phonon dispersion as the underlying factors. Moreover, reducing the density of In+ vacancies in InTe was found to lower the band effective mass and modulate carrier scattering, enhancing the material quality factor (B). To explore these findings, we systematically grew InTe single crystals, achieving exceptional thermoelectric performance. A record-breaking power factor of 12.0 μW·cm−1·K−2 and a dimensionless figure of merit (zT) of 0.5 at room temperature were obtained. Notably, InTe crystals oriented along [110] with low In+ vacancy density exhibited the highest average zT of 0.63 among InTe-based thermoelectric materials within the 300–473 K temperature range. Furthermore, we introduced an effective method of reducing In+ vacancies through Indium vapor annealing, resulting in the highest reported carrier mobility of 182 cm2·V−1·s−1 for InTe. Our study highlights the potential for improving InTe's thermoelectric performance near room temperature through vacancy modulation and crystal orientation.

Highly optical anisotropy, electronic and thermodynamic properties of the topological flat bands Kagome Nb3Cl8

This study showcases the intriguing electronic, optical, and thermodynamic properties of Nb3Cl8, an exceptional class of two-dimensional (2D) crystalline materials renowned for their Kagome structure and an exceptional band arrangement featuring remarkably flat energy bands. Nb3Cl8 was found to be a semiconductor with a narrow bandgap energy of 1.23 eV. Our study reveals pronounced anisotropic behavior in various optical aspects, extending within and perpendicular to the ab-plane. We show a distinctive anisotropy in parameters like the refractive index (4.5 in-plane compared to 2.75 out-of-plane), the optical conductivity (8000 (Ω cm)−1 in-plane compared to 100 (Ω cm)−1 in out-of-plane), and the absorption coefficient (105 cm−1 within the plane as opposed to 104 cm−1 in the out-of-plane direction). In addition, the thermodynamic calculations unveil a transition, distinguished by a Schottky anomaly occurring in the specific heat capacity at approximately 100K which is closely associated with the Néel transition of the Nb3Cl8 compound. The presence of robust interactions among quantum spins within Nb3Cl8 was supported by its low thermal conductivity, ranging from 6 to 0.7 W m−1. K−1. This study not only deepens our comprehension of the optical, electronic, and thermodynamic properties of Nb3Cl8 but also establishes a basis for future innovative technological applications.

Theory of Burstein–Moss effect in semiconductors with anisotropic energy bands

Abstract We study the peculiarities of the Burstein–Moss shift employing two-band model with an anisotropic valence band. There is a long wave tail which has a convex or concave shape depending on the ratio between the longitudinal and transverse hole masses. The width of this anisotropy-induced tail is temperature-independent and increases with increasing electron concentration and difference between the hole masses. This width also does not depend upon the value of the energy gap. Having experimentally evaluated the tail width and the position of the break in the optical absorption curve, one can deduce the values of the reduced hole masses.

Anisotropic Band Evolution of Bulk Black Phosphorus Induced by Uniaxial Tensile Strain

We investigate the anisotropic band structure and its evolution under tensile strains along different crystallographic directions in bulk black phosphorus (BP) using angle-resolved photoemission spectroscopy and density functional theory. The results show that there are band crossings in the Z–L (armchair) direction, but not in the Z–A (zigzag) direction. The corresponding dispersion-k distributions near the valence band maximum (VBM) exhibit quasi-linear or quadratic relationships, respectively. Along the armchair direction, the tensile strain expands the interlayer spacing and shifts the VBM to deeper levels with a slope of −16.2 meV/% strain. Conversely, the tensile strain along the zigzag direction compresses the interlayer spacing and causes the VBM to shift towards shallower levels with a slope of 13.1 meV/% strain. This work demonstrates an effective method for band engineering of bulk BP by uniaxial tensile strain, elucidates the mechanism behind it, and paves the way for strain-regulated optoelectronic devices based on bulk BP.

Gate tunable linear dichroism in monolayer 1T'-MoS2.

The linear dichroism demonstrates promising applications in the fields of polarization-resolved photodetectors and polarization optical imaging. Herein, we study the optical properties of monolayer 1T'-MoS2 based on a four-band effective k · p Hamiltonian within the framework of linear response theory. Owing to the anisotropic band structure, the k-resolved optical transition matrix elements associated with armchair(x) and zigzag(y) direction polarized light exhibit a staggered pattern. The anisotropy of the optical absorption spectrum is shown to sensitively depend on the photon energy, the light polarization and the gate voltage. A gate voltage can continuously modulate the anisotropy of the optical absorption spectra, rendering it isotropic or even reversing the initial anisotropy. This modulation leads to linear dichroism conversions across multiple wavelengths. Our findings are useful to design polarized photodetectors and sensors based on monolayer 1T'-MoS2. Our results are also applicable to other monolayer transition metal dichalcogenides with 1T' structure.

Polarization‐Sensitive Photodetectors Based on Highly In‐Plane Anisotropic Violet Phosphorus with Large Dichroic Ratio

AbstractAnisotropic 2D layered materials with distinctive anisotropic electronic band structures and optical response are promising for practical applications in polarization imaging and sensors. Nonetheless, many of the currently reported polarized photodetectors based on anisotropic 2D materials are constrained by the low polarization responsivity and low linear dichroism ratio. Here a high‐performance polarization‐sensitive photodetector based on the novel in‐plane anisotropy 2D violet phosphorus (VP) crystal is reported. The angle‐resolved polarized Raman spectroscopy (ARPRS) study and the anisotropic electrical transport measurements revealed the highly in‐plane anisotropy phonon vibration and electrical properties of VP crystal. The anisotropic ratio of electron mobility is calculated to be 2.7 at room temperature. Moreover, a polarization‐sensitive photodetector based on the VP nanosheet shows high photoresponsivity of up to 341 A W−1 and a linearly dichroic ratio of up to 3.9, which is much larger than most other anisotropic 2D materials. These results indicate that the anisotropic VP crystal would be suitable for polarization‐sensitive photodetectors, which leads to a promising perspective for future novel electronic and optoelectronic devices.

Strain-induced topological transitions and tilted Dirac cones in kagome lattices

We study effects of strain on the electronic properties of the kagome lattice in a tight-binding formalism with spin–orbit coupling (SOC). The degeneracy at the Γ point evolves into a pair of emergent tilted Dirac cones under uniaxial strain, where the anisotropy and tilting of the bands depend on the magnitude and direction of the strain field. SOC opens gaps at the emergent Dirac points, making the flatband topological, characterized by a nontrivial index. Strains of a few percent drive the system into trivial or topological phases. This confirms that moderate strain can be used to engineer anisotropic Dirac bands with tunable properties to study new phases in kagome lattices.

Linear and nonlinear spin current response of anisotropic spin-orbit coupled systems

We calculate the linear and the second harmonic (SH) spin current response of two anisotropic systems with spin–orbit (SO) interaction. General expressions of wide applicability for the these response functions are first derived for a generic two-band Hamiltonian. The first system is a two-dimensional (2D) electron gas in the presence of Rashba and k-linear Dresselhaus SO couplings. The calculations show how narrow or wide the response spectra can be, what is their overall shape and size, and frequency shiftings, depending on which crystal orientation is selected. The quantitative knowing of this makes possible a comparative study for several orientations, which would allow to select a spectrum with particular characteristic. We find that vanishing linear and second order response tensors are achievable under SU(2) symmetry conditions, characterized by a collinear SO vector field. Additional conditions under which specific tensor components vanish are possible, without having such collinearity. Thus, a proper choice of the growth direction and SO strengths allows to select the polarization of the linear and SH spin currents according to the direction of flowing. The second system is an anisotropic 2D free electron gas with anisotropic Rashba interaction, which has been employed to study the optical conductivity of 2D puckered structures with anisotropic energy bands. The presence of mass anisotropy and an energy gap open several distinct scenarios for the allowed optical interband transitions, which manifest in the linear and SH response contrastingly. The linear response displays only out-of-plane spin polarized currents, while the SH spin currents flow with spin orientation lying parallel to the plane of the system strictly. The models illustrate the possibility of the nonlinear spin Hall effect in systems with SO interaction, under the presence or absence of time-reversal symmetry. The results suggest different ways to manipulate the linear and nonlinear optical generation of spin currents which could find spintronic applications.