Electron Valley Research Articles

Synergistic Band and Defect Engineering in ZrNiSn Alloys via Ni-Vacancy Regulation for Realizing High Thermoelectric Performance

Of the n-type half-Heusler material systems, Zr1-xHfxNiSn1-ySby alloys have excellent thermoelectric properties. Typically, the Hf atoms replace Zr site atoms, thus reducing the lattice thermal conductivity. Nonetheless, the excess use of Hf increases the cost of half-Heusler thermoelectric materials, limiting their commercial use. In this study, we used band and defect engineering concurrently to enhance the thermoelectric properties of ZrNiSn-based alloys through the modulation of Ni-structural vacancies. The induction of Ni-structural vacancies not only enhances phonon scattering, decreasing both bipolar and lattice thermal conductivities at high temperatures, but also promotes the formation of multiple electronic valleys close to the Fermi level, enhancing the electrical transport properties. Concurrently, doping ZrNi1-xSn samples with Nb increases the stability of the crystal structure and further refines the thermoelectric parameters. Notably, a thermoelectric figure of merit (ZT) of 1.16 was achieved at 1073 K for Zr0.96Nb0.04Ni0.9Sn, as high as that of Zr1-xHfxNiSn1-ySby samples but without the addition of Hf. This is a novel example of integrating multiple electronic valleys into a ZrNiSn system by introducing Ni-structural vacancies. Crucially, this approach to band structure engineering could be applied to other thermoelectric materials.

Engineering Two-Dimensional Magnetic Heterostructures: A Theoretical Perspective.

Two-dimensional (2D) magnetic materials have attracted great attention due to their promise for applications in future high-speed, low-energy quantum computing and memory devices. By integrating 2D magnetic materials with other magnetic or nonmagnetic materials to form heterostructures, the synergistic effects of interlayer orbital hybridization, spin-orbit coupling, and symmetry breaking can surpass the performance of single-layer materials and lead to novel physical phenomena. This review provides a comprehensive theoretical analysis of engineering 2D magnetic heterostructures, emphasizing the fundamental physics of interlayer interactions and the resulting enhancements and novel properties. It reviews the mechanisms and progress in tuning the magnetic ordering, enhancing the Curie temperature (Tc) and modulating properties such as topological magnetic structures, spin polarization, electronic band topology, valley polarization, and magnetoelectric coupling through the construction of 2D magnetic heterostructures. Additionally, this review discusses the current challenges faced by 2D magnetic heterostructures, aiming to guide the future design of higher-performance magnetic heterostructures.

Predictable Gate-Field Control of Spin in Altermagnets with Spin-Layer Coupling.

Spintronics, a technology harnessing electron spin for information transmission, offers a promising avenue to surpass the limitations of conventional electronic devices. While the spin directly interacts with the magnetic field, its control through the electric field is generally more practical, and has become a focal point in the field. Here, we propose a mechanism to realize static and almost uniform effective magnetic field by gate-electric field. Our method employs two-dimensional altermagnets with valley-mediated spin-layer coupling (SLC), in which electronic states display valley-contrasted spin and layer polarization. For the low-energy valley electrons, a uniform gate field is approximately identical to a uniform magnetic field, leading to predictable control of spin. Through symmetry analysis and abinitio calculations, we predict altermagnetic monolayer Ca(CoN)_{2} and its family materials as potential candidates hosting SLC. We show that an almost uniform magnetic field (B_{z}) indeed is generated by gate field (E_{z}) in Ca(CoN)_{2} with B_{z}∝E_{z} in a wide range, and B_{z} reaches as high as about 10^{3} T when E_{z}=0.2 eV/Å. Furthermore, owing to the clean band structure and SLC, one can achieve perfect and switchable spin and valley currents and significant tunneling magnetoresistance in Ca(CoN)_{2} solely using the gate field. Our work provides new opportunities to generate predictable control of spin and design spintronic devices that can be controlled by purely electric means.

Effects of strain-tunable valleys on charge transport in bismuth

The manipulation of the valley degree of freedom can boost the technological development of novel functional devices based on valleytronics. The current mainstream platform for valleytronics is to produce a monolayer with inversion asymmetry, in which the strain-band engineering through the substrates can serve to improve the performance of valley-based devices. However, pinpointing the effective role of strain is inevitable for the precise design of the desired valley structure. Here, we demonstrate the charge transport under continuously controllable external strain for bulk bismuth crystals with three equivalent electron valleys and one hole valley. The strain response of resistance, namely elastoresistance, exhibits evolutions in both antisymmetric and symmetric channels with decreasing temperature. The elastoresistance behaviors mainly reflect the significant changes in valley density depending on the symmetry of induced strain, evidenced by our strain-dependent quantum oscillation measurements and first-principles band calculations under strain. These facts suggest the successful tuning and evaluation of the valley populations through strain-dependent charge valley transport. Published by the American Physical Society 2024

Optically and electrically controlled switchers and electron beam splitters based on topological edge states

Recently, topological edge states have attracted rapid attention for designing low-power consumption electronics due to their topological protection and suppression of backscattering. We study the transport properties of inner and outer edge states in a three-terminal device composed of hybrid zigzag graphene nanoribbons based on our derived formulas of local bond current and transmission coefficient with the wave-function matching technique. Particularly, by electrically modulating inner edge state in the conductor region, the valley current can be flipped as the charge current that is switched into one lead or split into two transmitted leads, leading to the charge switchers and electron beam splitters when the incident lead and transmitted leads are set as antichiral inner edge state and chiral edge states, respectively. Furthermore, when three leads are both modulated as antichiral inner edge state by the spatially varying off-resonant circularly polarized (ORCP) light, the charge switchers and electron beam splitters are accordingly converted into the valley switchers and electron beam splitters. The behavior of the switchers and splitters originates from the cooperative effects of the lager wave-vector mismatch between different valleys, and the distributions of lateral and vertical inner edge states. Moreover, the proposed devices could be cut off by the staggered electric field due to the gap effect, and are robust against the edge magnetism and the spin-orbit coupling. These findings are expected to apply for optically and electrically controlled topological devices.

Prediction of large spin-valley polarization in the Janus 2H–WSSe monolayer on VN magnetic substrate

Two-dimensional heterostructures based on transition metal dichalcogenides (TMDs) exhibit broad application prospects in valleytronics due to the space-reversal symmetry breaking and strong spin–orbit coupling. In this work, the electronic structure, magnetic anisotropy and valley polarization of 2H–WSSe/VN van der Waals heterostructure under various interlayer spacings, magnetic angle and in-plane strain are investigated in detail by first-principles calculations. The stacked configuration of Se-C2-1 with most stable structure shows the largest valley polarization of 386.5[Formula: see text]meV. By adjusting the interlayer spacing of heterostructure, the largest valley polarization of 702.7[Formula: see text]meV appears in Se-C2-1 stacked configuration with interlayer spacing of 2.24 Å. The magnetic angle [Formula: see text] exhibits significant effects on valley polarization and magnetic anisotropy of 2H–WSSe/VN heterostructures. The stability and valley polarization of 2H–WSSe/VN heterostructure decrease after the in-plane biaxial strain is applied. The large and tunable valley polarization as well as magnetic anisotropy in the 2H–WSSe/VN heterostructures make it potential applications in valleytronic devices.

Hybrid Interface Chemistry Enabling Mixed Conducting via Ultrafast Microwave Polarization Toward Dendrite-Free Zn Anodes.

Zn metal anodes in aqueous electrolytes suffer from interface issues including uncontrolled dendrite growth and undesired side reactions, resulting in their limited application in terms of short circuits and cell failure. Herein, a hybrid interface chemistry strategy is developed through ultrafast microwave polarization at the skin region of bare Zn. Owing to efficient Joule heating directed by abundant local hot spots at electron valleys, the rapid establishment of a dense interfacial layer can be realized within a minute. Stabilized Zn with suppressed side reactions or surface corrosion is therefore achieved due to the interfacial protection. Importantly, hybrid zincophilic sites involving laterally/vertically interconnected Cu-Zn intermetallic compound and Zn2+-conductive oxide species ensure mixed charge conducting (denoted as CuHL@Zn), featuring uniformly distributed electric field and boosted Zn2+ diffusion kinetics. As a consequence, CuHL@Zn in symmetric cells affords lifespans of 2800 and 3200h with ultra-low polarization voltages (≈19 and 56mV) at a plating capacity of 1.0mAhcm-2 for 1 and 5mAcm-2, respectively. The CuHL@Zn||MnO2 full cell further exhibits cycling stability with a capacity retention of over 80% for 500 cycles at 2Ag-1.

Symmetry and magnetic direction dependent spin/valley splitting and anomalous Hall conductivity of antiferromagnetic monolayer MnPTe3

Due to the time reversal symmetry breaking or spin-valley coupling, there are intrinsic valley splittings in two-dimensional magnetic materials, which are closely related to the symmetry and magnetic order. Based on MnPTe3 monolayers with different symmetries, we present an understanding of direction dependent spin/valley splitting and anomalous Hall conductivity from the perspective of orbital interactions. Unlike T-MnPTe3 with central inversion symmetry, in-plane magnetization leads to further non-degeneracy of valley electrons of H–MnPTe3, and the splitted valley states are spin polarized, which is perpendicular to the in-plane magnetization direction. The model and first-principle calculations consistently indicate that the splitting of the valley states is almost proportional to sinθxz, which originates from the difference in spin-up and spin-down interactions introduced by the breaking of spatial inversion symmetry. Regardless of spin degeneracy, non-zero anomalous Hall conductivity occurs with out-of-plane magnetization, and this direction dependent anomalous Hall effect is caused by mirror symmetry breaking from the neighboring Te atoms and spin-orbit coupling. Magnetization direction is an important means of regulating valley/spin splittings and anomalous Hall effect, which has certain significance for realizing spin or valley polarized Hall effect and the research of spintronic or valleytronic devices.

Tunable valley splitting in RuClF bilayer

Valleytronic devices are increasingly gaining attention for their unique capability to encode and process information by leveraging electronic valley degrees of freedom. In this study, we performed first-principle calculations to analyze the band dispersions of double-layer antiferromagnetic valley material RuClF subjected to an external electric field. Our findings show that the double-layered structure exhibits interlayer antiferromagnetic order and that the bilayer bands display opposite valley splitting. We further categorized the atomic contact surfaces of the top and bottom layers into three interface conditions: F/F, Cl/Cl, and Cl/F. Under F/F and Cl/Cl interface conditions, when the electric field changes by 0.1 V/nm, the valley splitting values change by 12.2 meV and 9.5 meV, respectively. However, under the Cl/F interface condition, the structure exhibited metallic characteristics and was as such excluded from our analysis. These findings substantiate the practicality of employing an electric field to regulate the valley splitting of the bilayer structure, offering a new control strategy for valleytronic devices.

Valley polarization and magnetic anisotropy of two-dimensional Ni2Cl3I3/MoSe2 heterostructures.

Two-dimensional (2D) Janus trihalides have attracted widespread attention due to their potential applications in spintronics. In this work, the valley polarization of MoSe2 at the K' and K points can be modulated by Ni2Cl3I3, a new 2D Janus trihalide. The Ni2Cl3I3/MoSe2 heterostructure has an in-plane magnetic anisotropy energy (IMA) and is characterized by three distinct electronic structures: metallic, semiconducting, and half-metallic. It is noted that the semiconducting state features a band gap of 0.07 eV. When spin-orbit coupling (SOC) is considered, valley polarization is exhibited in the Ni2Cl3I3/MoSe2 heterostructure, with the degree of valley polarization varying across different configurations and reaching a maximum value of 4.6 meV. The electronic properties, valley polarization and MAE of the system can be tuned by biaxial strains. The application of a biaxial strain ranging from -6% to +6% can enhance the valley polarization value from 0.9 meV to 12.9 meV. The directions of MAE of the Ni2Cl3I3/MoSe2 heterostructure can be changed at biaxial strains of -6%, +2%, +4% and +6%. The above calculation results show that the heterostructure system possesses rich electronic properties and tunability, with extensive potential applications in the fields of spintronic and valleytronic devices.

Manipulation of valley-pseudospin in 2D WTe2/CrI3 van der Waals heterostructure by magnetic proximity effect

Lifting the valley degeneracy is an essential condition for exploiting the valley degrees of freedom to process and store information in valleytronics. The magnetic proximity effect (MPE) has been proven to be an effective method for introducing a magnetic field via interlayer exchange interaction, which can be conveniently achieved by two-dimensional (2D) van der Waals (vdW) heterostructure engineering. We have investigated the electronic properties and valley physics of 2D WTe2/CrI3 heterostructure using first-principles calculations. The results show that a significant valley splitting of 34.01 meV can be obtained in the WTe2/CrI3 heterostructure due to the simultaneous breaking of inversion and time-reversal symmetry, which sensitively depends on the atomic overlap of the projected position in the specific stacking model. In addition, we also show that the valley splitting can be enhanced by applying a vertical external electric field (E-field) or reducing the interlayer distance. More than that, it can be boosted by an order of magnitude (∼115.25 meV) through Cr ion intercalation. This is attributed to that the self-intercalated Cr ion has ferromagnetic (FM) ordering and perpendicular magnetic anisotropy due to the exchange coupling of the CrI3 layer. This result provides fundamental insights and valuable guidance for the application of 2D WTe2/CrI3 heterostructure in novel spintronic and valleytronic devices.

Tunable valley polarization in two-dimensional H-HfI2/T-VBrCl van der Waals heterostructure

The electronic band structure and valley splitting of monolayer H-HfI2 and H-HfI2/T-VBrCl heterostructure have been investigated by first principles calculations. The monolayer H-HfI2 exhibits valleytronic feature at K and K’ points of the valence bands. The valley polarization in monolayer H-HfI2 arises from the magnetic proximity effect induced by T-VBrCl monolayer. The stacking configurations have a significant influence on the valley polarization of the H-HfI2/T-VBrCl heterostructure. The maximum valley polarization can be as high as −20 meV. It is possible to manipulate the magnitude of valley polarization by adjusting the layer spacing and applying biaxial strain. The Berry curvature between two valleys indicates that the H-HfI2/T-VBrCl heterostructure should be a potential candidate for valleytronic devices.

Phonon-Assisted Auger-Meitner Recombination in Silicon from First Principles.

We present a consistent first-principles methodology to study both direct and phonon-assisted Auger-Meitner recombination (AMR) in indirect-gap semiconductors that we apply to investigate the microscopic origin of AMR processes in silicon. Our results are in excellent agreement with experimental measurements and show that phonon-assisted contributions dominate the recombination rate in both n-type and p-type silicon, demonstrating the critical role of phonons in enabling AMR. We also decompose the overall rates into contributions from specific phonons and electronic valleys to further elucidate the microscopic origins of AMR. Our results highlight potential pathways to modify the AMR rate in silicon via strain engineering.

Tunable valley polarization and magneto-crystal anisotropy in two-dimensional VI3/WSe2 Van der Waals heterostructure

In this work, the effects of magnetic proximity coupling on electronic structures, valley polarization and magneto-crystal anisotropy of VI3/WSe2 heterostructure are studied in detail based on density functional theory. The results show that the spin-valley polarization characters of WSe2 monolayer can be induced by the proximity effect of ferromagnetic VI3 monolayer. Based on the effective Hamiltonian [Formula: see text] model, a large valley polarization of 8.7[Formula: see text]meV is observed, which is mainly contributed by the SOC effect and not magnetic exchange field. Meanwhile, the magneto-crystal anisotropy of VI3 layer is also obviously altered from the [100] to [001] magnetic axis compared to the isolated VI3 monolayer. The reduction in the interlayer space strengths the W–V atomic coupling, resulting in a significant increase in the spin and valley polarization. Compression strain shows weakening effect on the valley polarization, while the tensile strain largely enhances the valley polarization of VI3/WSe2 heterostructure, which can reach the maximum value of 29.1[Formula: see text]meV at the tensile strain of 8%. The V spin direction ([Formula: see text]) has significant effect on valley polarization of WSe2 layer, which is positively correlated with the magnetization strength of V atom in the vertical direction. Furthermore, constructing the VI3/WSe2/VI3 sandwich heterostructure can induce a larger valley polarization, which can reach a maximum value of 32.4[Formula: see text]meV. These results indicate that the VI3/WSe2 heterostructure is a potential candidate for valleytronics.

Spin-induced valley polarization in heterobilayer Janus transition-metal dichalcogenides

Inspired by potential application prospects of spintronics and valleytronics, a novel heterobilayer Janus structure is designed by replacing the chalcogenide atomic layers in the original bilayer MoS2. Based on first-principles calculations, it is found that the SMoS/SeMoS structure exhibits a direct band-gap semiconductor and a typical type-II band alignment with longer carrier lifetime. The transition metal (TM) atom represented by V/Cr/Mn can be stably adsorbed on the heterobilayer Janus SMoS/SeMoS sheet and effectively introduce magnetic moments (m). The calculation results demonstrate that the most stable adsorption site of the TM atom is CX(A), and the TM (V/Cr/Mn) adatom modified SMoS/SeMoS system is converted into metal (V-) or half-metal (Cr/Mn-), respectively. Under the coupling of different indirect exchange interactions, the structure exhibits stable intrinsic anti-ferromagnetic interactions for V-SMoS/SeMoS and ferromagnetic ground state for Cr/Mn-SMoS/SeMoS, respectively, and the magnetic transition temperature (T c) reaches a high temperature or even room temperature. Moreover, the robust out-of-plane magnetocrystalline anisotropy energy ensures stable long-range magnetic order. Specifically, the combination of spin injection and strong spin–orbit coupling interaction effectively breaks the time-reversal symmetry, which leads to valley polarization of the system. Based on this, the biaxial strain can effectively regulate the electronic structure, magnetic properties and valley polarization of TM-SMoS/SeMoS nanosheets with double breaking of spatial-inversion and time-reversal symmetry.

Valley-Selective Polarization in Twisted Bilayer Graphene Controlled by a Counter-Rotating Bicircular Laser Field

The electron valley pseudospin in two-dimensional hexagonal materials is a crucial degree of freedom for achieving their potential application in valleytronic devices. Here, bringing valleytronics to layered van der Waals materials, we theoretically investigate lightwave-controlled valley-selective excitation in twisted bilayer graphene (tBLG) with a large twist angle. It is demonstrated that the counter-rotating bicircular light field, consisting of a fundamental circularly-polarized pulse and its counter-rotating second harmonic, can manipulate the sub-cycle valley transport dynamics by controlling the relative phase between two colors. In comparison with monolayer graphene, the unique interlayer coupling of tBLG renders its valley selectivity highly sensitive to duration, leading to a noticeable valley asymmetry that is excited by single-cycle pulses. We also describe the distinct signatures of the valley pseudospin change in terms of observing the valley-selective circularly-polarized high-harmonic generation. The results show that the valley pseudospin dynamics can still leave visible fingerprints in the modulation of harmonic signals with a two-color relative phase. This work could assist experimental researchers in selecting the appropriate protocols and parameters to obtain ideal control and characterization of valley polarization in tBLG.

Investigating valley-dependent current generation due to asymmetric energy dispersion for charge-transfer from a quantum dot to single-walled carbon nanotube

Single-wall carbon nanotubes (SWCNT), which consist of a two-dimensional hexagonal lattice of carbon atoms, possess unique mechanical, electrical, optical and thermal properties. SWCNT can be synthesized in diverse chiral indexes to determine certain attributes. This work theoretically investigates electron transport in different directions along SWCNT. The electron studied in this research transfers from the quantum dot that can possibly move to the right or left direction in SWCNT with different valley-dependent probability. These results show that valley polarized current is present. The valley current in the right and left directions has a composition of valley degrees of freedom where its components (K and K′) are not identical. Such a result can be traced theoretically by certain effects. That firstly is the curvature effect on SWCNT in which the hopping integral between pi electrons from the flat graphene is altered, and another is curvature-inducing sigma -pi mixture. Due to these effects, the band structure of SWCNT is asymmetric in certain chiral indexes leading to the asymmetry of valley electron transport. Our results exhibit that the zigzag chiral indexes is the only type making electron transport symmetrical that is different to the result from the other chiral index types which are the armchair and chiral. This work also illustrates the characteristic of the electron wave function propagating from the initial point to the tip of the tube over time, and the current density of the probability in specific times. Additionally, our research simulates the result from the dipole interaction between the electron in QD and the tube that impacts the lifetime of the electron being in QD. The simulation portrays that more dipole interaction encourages the electron transfer to the tube, thereby shortening the lifetime. We as well suggest the reversed electron transfer from the tube to QD that the time duration of such transfer is much less than the opposite transfer owing to the different orbital of the electron’s states. Valley polarized current in SWCNTs may also be used in the development of energy storage devices such as batteries and supercapacitors. The performance and effectiveness of nanoscale devices, including transistors, solar cells, artificial antennas, quantum computers, and nano electronic circuits, must be improved in order to achieve a variety of benefits.

Electron diffusion induced valley Hall effect and nonlinear galvanodiffusive transport in hexagonal two-dimensional Dirac monolayer materials

Diffusion currents are theoretically examined in two-dimensional Dirac materials, such as those of the transition metal dichalcogenides (TMD) family. The transversal effects are analogues of the valley Hall (VHE) and photogalvanic (PGE) transport phenomena in cases when the electron driving force is not an electric field but a gradient of electron density distribution in the sample. The latter can be created by a finite-sized laser spot or by the injection of electrons from other materials. We develop the theory of the diffusive VHE effect assuming the anisotropic electron-short-range-impurity skew scattering. The electron PGE-like transport caused by higher electron-density derivatives is analyzed assuming the trigonal warping anisotropy of electron valleys in a TMD monolayer. The nonlinear responses on an electron-density gradient are studied as well. The isotropic processes of electron scattering off the short-range and Coulomb centers are taken into account in the PGE-like transport theory.

Detection of hot electrons originating from an upper valley at ∼1.7eV above the Γ valley in wurtzite GaN using electron emission spectroscopy

Using electron emission spectroscopy, measurement and analysis were conducted on the energy distribution of vacuum emitted electrons from electrically driven InGaN/GaN green (peak wavelengths $\ensuremath{\lambda}\ensuremath{\approx}515\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$) light-emitting diodes (LEDs) with and without a prewell superlattice (SL). We report on the detection of a high-energy upper valley at $\ensuremath{\sim}1.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ above the $\mathrm{\ensuremath{\Gamma}}$ valley from samples with no prewell SL. We propose that these upper valley electrons originate predominantly from trap-assisted Auger recombination (TAAR) in green LEDs, as the intensity of these peaks is found to have quadratic dependence on the carrier density $n$ [see Espenlaub et al., J. Appl. Phys. 126, 184502 (2019)]. The high-energy upper valley peak was not observed in the sample with a prewell SL which is attributed to gettering by the prewell SL of still unidentified impurities that act as TAAR centers.

Valley-polarized and supercollimated electronic transport in an 8-Pmmn borophene superlattice

Analogous to real spins, valleys as carriers of information can play significant roles in physical properties of two-dimensional Dirac materials. On the other hand, utilizing external periodic potential is an efficient method to manipulate their band structures and transport properties. In this work, we investigate the valley dependent optics-like behaviors based on an 8-Pmmn borophene superlattice with the transfer matrix method and effective band approach. Firstly, it is found that the band structure is renormalized, more tilted Dirac cones are generated, and the group velocities are modified by the periodic potentials. Secondly, due to the exotic tilted Dirac cones in 8-Pmmn borophene, a perfect valley selected angle filter can be realized. The electrons with a specific incident angle can transmit completely in an energy window, which is flexibly tunable by changing the periodic potential. Thirdly, by using the Green’s function to simulate the time evolution of wave packets, electrons can be shown to propagate without any diffraction, valley electron beam supercollimation happens by modulating the potential parameters. Different from the graphene superlattice, the electron supercollimation here is valley dependent and can be used as a valley electron beam collimator. Fourthly, we can tune the polarization and supercollimation angles by changing the superlattice direction. These intriguing results in an 8-Pmmn borophene-based superlattice offer more opportunities in diverse electronic transport phenomena and may facilitate the devices applications in valleytronics and electron-optics.