Valley Polarization Research Articles

Ultra-wide band gap and large exciton effect in 2D ferrovalley material H-FeCl2.

Ferrovalley materials are valleytronic materials with intrinsic ferromagnetism, in which the presence of spontaneous valley polarization is more conducive to practical applications. The optical properties of ferrovalley are important for selectively exciting electrons at the valley. In this paper, the electronic and optical spectrum of the H-phase FeCl2monolayer is studied using first-principles calculations as an example. We use hybrid functional HSE06 and GW0 methods with spin-orbit coupling for our calculations, the band gap of H-FeCl2is about 3.975 and 4.072 eV at K and -K valley, which is significantly larger than that obtained by the PBE method, with a 97 meV valley splitting. It is shown that the monolayer H-FeCl2is a ferrovalley material with an ultra-wide band gap and large intrinsic valley polarization, which has strong electronic correlation and many-body effects. Calculation of the imaginary part of the dielectric function using GW-BSE method shows that the energy corresponding to the exciton peak is 2.421 and 2.491 eV, much smaller than the GW band gap. The exciton binding energy is about 1.554 and 1.581 eV at K and -K valley, indicating a large exciton effect. And the exciton binding energy of the two valleys are unequal, with a difference of 27 meV. It is found that splitting occurs at the first exciton peak in the ferrovalley material, and the splitting value is inequivalent to the bandgap splitting at the valley, which is instructive for further research as well as application of the valleytronics.

Fluctuating magnetism and Pomeranchuk effect in multilayer graphene.

Magnetism typically arises from the effect of exchange interactions on highly localized fermionic wavefunctions in f- and d-atomic orbitals. By contrast, in rhombohedral multilayer graphene (RMG), magnetism-manifesting as spontaneous polarization into one or more spin and valley flavours1-7-originates from itinerant electrons near a Van Hove singularity. Here we show experimentally that the electronic entropy in this system indicates signatures typically associated with disordered local magnetic moments, unexpected for electrons in a fully itinerant metal. Specifically, we find a contribution ΔS ≈ 1 kB per charge carrier that begins at the Curie temperature and survives more than one order of magnitude in temperature. First-order phase transitions show an isospin 'Pomeranchuk effect' in which the fluctuating moment phase is entropically favoured over the nearby symmetric Fermi liquid8,9. Our results imply that, despite the itinerant nature of the electron wavefunctions, the spin and valley polarization of individual electrons is decoupled, a phenomenon typically associated with localized moments, as happens, for example, in solid 3He (ref. 10). Transport measurements, surprisingly, show a finite-temperature resistance minimum in the fluctuating moment regime, which we attribute to the interplay of fluctuating magnetic moments and electron-phonon scattering. Our results highlight the universality of soft isospin modes to two-dimensional flat-band systems.

Spin-splitting above room-temperature in Janus Mn2ClSeH antiferromagnetic semiconductor with a large out-of-plane piezoelectricity

Two-dimensional (2D) antiferromagnets have garnered considerable research interest due to their robustness against external magnetic perturbation, ultrafast dynamics, and magneto-transport effects. However, the lack of spin-splitting in antiferromagnetic (AFM) materials severely limits their potential in spintronics applications. Inspired by inherent out-of-plane potential gradient of Janus structure, we predict three stable AFM Janus Mn2ClXH (X = O, S, and Se) monolayers with spontaneous spin-splitting based on first-principles calculations. Notably, Janus Mn2ClSeH exhibits a high Néel temperature of up to 510 K, robust perpendicular magnetocrystalline anisotropy, outstanding out-of-plane piezoelectricity of 0.454 × 10−10 C/m, and sizeable spontaneous valley polarization of 17.2 meV. Moreover, the spin-splitting can be significantly enhanced through appropriate synergistic regulation of biaxial strain and external electric field. These results demonstrate that the Janus Mn2ClSeH monolayer is a very potential candidate for designing intriguing antiferromagnet-based devices with fantastic piezoelectric and valleytronic characteristics.

Versatile device structures in MoS2: magnetic switching of perfect spin and valley polarization states as well as tunneling magnetoresistance

Abstract Valley polarization has been achieved in transition metal dichalcogenides (TMDs), particularly
molybdenum disulfide (MoS2), using polarized light. Two valley polarized states have also been
reported in ferromagnetic MoS2 junctions with circularly polarized light. The polarized states are
accessible by switching the polarization direction. Despite these advances in valley polarization, more
amenable options for electronic devices, such as electrostatic and/or magnetic gating, are desirable.
Here, we explore the possibility of having two well-defined valley polarization states in ferromagnetic
MoS2 junctions accessible by switching the magnetization direction. In addition, spin polarization
and tunneling magnetoresistance (TMR), opening the door for versatile devices in TMDs. In the
case of Ferromagnetic/Ferromagnetic Insulator/Normal Metal (FM/FI/NM) junctions, we find spin
valley polarization and negative TMR. However, two well-defined valley polarization states accessible
by reversing the magnetization direction are not possible. As an alternative, we study FM/NM/FI
MoS2 junctions, finding two well-defined polarization states for both the spin and valley degrees of
freedom. The states are accessible by changing the magnetization direction in the FI region. Here,
it is fundamental to apply electrostatic gating in the NM region to ensure the participation of holes
in the electronic transport or n-p-n transport. FM/NM/FI MoS2 junctions also show high positive
TMR values at the edges of the valence and conduction band.

Coexisting Triferroic and Multiple Types of Valley Polarization by Structural Phase Transition in 2D Materials

AbstractThe multiferroic materials, which coexist magnetism, ferroelectric, and ferrovalley, have broad practical application prospects in promoting the miniaturization and integration of spintronic and valleytronic devices. However, it is rare that there are triferroic orders and multiple types of valley polarization in a real material. Here, a mechanism is proposed to realize triferroic order coexistence and multiple types of valley polarization by structural phase transition in 2D materials. The 1T and 2H phase OsBr2 monolayers exhibit non‐magnetic semiconductor and ferromagnetic semiconductor with valley polarization up to 175.49 meV, respectively. Interestingly, the 1T phase OsBr2 bilayer shows the tri‐state valley polarization due to lattice symmetry breaking, while the valley polarization of 2H phase bilayer originates from the combined effect of time‐reversal symmetry breaking and spin‐orbit coupling. Furthermore, the valley polarization and ferroelectric polarization of 1T phase AB stackings and 2H phase AA stackings can be manipulated via interlayer sliding. Importantly, it is verified that the 2H phase can be transformed to 1T phase by Li+ ion intercalation, while the 2H phase can occur the structural phase transition into the 1T phase by infrared laser induction. This work provides a feasible strategy for manipulating valley polarization and a design idea for nano‐devices with nonvolatile multiferroic properties.

Valley polarization dynamics of photoinjected carriers at the band edge in room-temperature silicon studied by terahertz polarimetry

Valley polarization dynamics of photoinjected carriers at the band edge in room-temperature silicon studied by terahertz polarimetry

Combining THz and Infrared Light to Control Valley Charge and Current in Gapless Graphene.

The absence of a selection rule coupling oppositely circularly polarized light to conjugate valleys, the basis of "lightwave valleytronics", would appear to curtail the possibilities of graphene as a valleytronic platform. Here we show that combining linearly polarized THz light and circularly polarized infrared light allows not only near perfect valley charge polarization but also complete light control over the associated valley current. The underlying mechanism involves a THz induced momentum space shift of valley uncontrasted charge generated by the circularly polarized component, yielding charge at the K and Γ points. Ab initio simulation reveals an ultrafast de-excitation of this latter charge, resulting in perfect valley polarization. We argue that this pulse design will allow control of the valley current and charge in a broad range of gapless systems, including the Xene family and various few layer graphene systems, opening the way to potentially utilize these materials in valleytronics applications.

Van Hove Singularity-Induced Non-Equilibrium Anomalous Valley Hall Effect in a Two-Dimensional Lattice.

van Hove singularity and valley represent two fundamental phenomena in materials science and condensed matter physics, which have recently attracted considerable interest. Here, we propose that the interplay between van Hove singularity and valley can generate a previously unreported anomalous valley Hall effect (AVHE) in a two-dimensional (2D) lattice, termed the non-equilibrium AVHE, which is characterized by the non-equilibrium transverse accumulation of valley carriers from different valleys. The physics relates to van Hove singularity-induced breaking of time-reversal symmetry and the resulting valley polarization under carrier doping, which guarantees the unique properties of non-equilibrium valley carriers. Remarkably, in contrast to typical AVHE relying on intrinsic magnetic materials, the non-equilibrium AVHE is rooted in 2D nonmagnetic systems. Using first-principles calculations, we validate the proposed mechanism and the predicted phenomena in monolayer In2Se3, known to exhibit nonmagnetic and ferroelectric natures. These findings open an avenue for exploring unconventional AVHE in 2D nonmagnetic systems.

Valley polarization in two-dimensional zero-net-magnetization magnets

Valleytronics in two-dimensional (2D) zero-net-magnetization magnets exhibits ultradense and ultrafast potential due to their intrinsic advantages of zero stray field and terahertz dynamics. The zero-net-magnetization magnets mainly include PT-antiferromagnet [the joint symmetry (PT) of space inversion symmetry (P) and time-reversal symmetry (T)], altermagnet, and fully compensated ferrimagnet. In these magnets, achieving controllable valley polarization is extremely important to the application of valleytronics. In this perspective article, we provide some possible design strategies to achieve valley polarization and spin-splitting in 2D zero-net-magnetization magnets. Furthermore, the anomalous valley Hall effect can be achieved in these zero-net-magnetization magnets. These proposed design strategies can encourage more theoretical and experimental works to explore valley polarization in these eminent magnets.

Altermagnetism, piezovalley, and ferroelectricity in two-dimensional Cr2SeO altermagnet

The study on altermagnet is receiving extensive research efforts due to its directional-dependent spin-polarized band structure. Here, we investigate the multifunctional properties of the Janus Cr2SeO monolayer system. The Cr2SeO monolayer exhibits an in-plane magnetic anisotropy energy of -0.28 meV/cell and a Neel temperature of around 280 K, which is further enhanced to 315 K under 1% uniaxial compressive strain. The Janus Cr2SeO exhibits spin band inversion with a large spin splitting of 0.38 eV at the Y and X high-symmetry points. Nonetheless, the valleys are degenerated due to the diagonal mirror symmetry. The uniaxial strain effectively breaks the diagonal mirror symmetry in the Janus Cr2SeO monolayer, and the strain-induced valley polarization of up to 90 meV is achieved. Furthermore, the uniaxial strain tunes the band gap between 0.30 eV to 0.76 eV. Moreover, Cr2SeO demonstrates ferroelectricity, with a total electric polarization of 5.34 pC/m, which increases to 5.52 pC/m under small uniaxial strain. Besides, we found a large vertical piezoelectric coefficient d31 of 1.18 pm/V. Our study suggests that the Janus Cr2SeO is a promising platform for Valleytronics and ferroelectric memory applications at room temperature.

Acoustic Valley Filter, Valve, and Diverter.

The discovery of valley degrees of freedom in electronic and classical waves opened the field of valleytronics and offered the prospect for new devices based on valleys. However, the implementation of valley-based devices remains challenging in practice. Here, by taking advantage of the flexibility of phononic crystals in design and fabrication, the realizations of valley devices, or filters, valves, and diverters for acoustic waves are reported. All the devices are configured as the structures of input and output ports bridged by channels. The phononic crystals serving as ports allow the propagation of both valley polarizations, whereas the phononic crystals serving as channels, as they are narrow, only allow the propagation of single polarizations. For valley filters that achieve valley-polarized currents, the bridge channel is simply a straight single phononic crystal, but for valley valves that can turn off the valley-polarized currents, the channel consists of two parts, allowing the propagation of opposite valley polarizations. The valley diverters have one input port, and two output ports, and thus a branched channel, and the three parts in the channel allow the propagation of the same valley polarizations, so that the energy flow can be partitioned. The results may serve as a basis for developing advanced acoustic valley devices.

Valley Polarization of Landau Levels in the ZrSiS Surface Band Driven by Residual Strain

Valley Polarization of Landau Levels in the ZrSiS Surface Band Driven by Residual Strain

First-principles prediction of intrinsic ferrovalley properties in Janus rare-earth PrXY (X ≠ Y = Cl, Br, I) monolayers.

In this work, using first-principles calculations, we predict a promising class of two-dimensional ferromagnetic semiconductors, namely Janus PrXY (X ≠ Y = Cl, Br, I) monolayers. Through first-principles calculations, we found that PrXY monolayers have excellent dynamic and thermal stability, and their band structures, influenced by magnetic exchange and spin-orbital coupling, exhibit significant valley polarization. Between K and -K valleys, the Berry curvature values are opposite to each other, resulting in the anomalous valley Hall effect. In addition, applying moderate biaxial strain can further enhance their magnetic anisotropy energy and valley polarization. These findings do not only emphasize the importance of strain in regulating spin and valley characteristics, but also provide new possibilities for the application of ferromagnetic semiconducting materials in spintronic and valleytronic devices.

Synthesis and Light-Matter Interaction of Low-Dimension Ordered-Disordered Layered Semiconductors.

Well-structured and ordered 2D layered semiconducting materials have excellent optical properties but limited advanced optoelectronic applications in their natural state. Altering their natural arrangement, through artificial heterostructures, strain and pressure engineering, chemical doping, intercalation, and alloying, can impart them with unusual optical properties and potentially enhance their performance in various applications. Among these approaches, alloying is generally difficult to control and disrupts the well-ordered homophilic crystal phase of these 2D crystals, albeit with the capability to control materials as thin as a single atomic layer. In this work, the synthesis of a low-dimension ordered-disordered layered 2D alloy of Mo(1-x)WxSe2 with clearly ordered in-plane segregations of nano-sized islands of individual MoSe2 and WSe2 across the material surface is reported. The optical analysis of this ordered-disordered layered Mo(1-x)WxSe2 reveals unique interfacial and interlayer coupling physics, such as the co-existence of intralayer (interfacial) and interlayer excitons and enhanced valley polarization of up to 50%, which is traditionally absent in ordered MoSe2, WSe2, or their heterostructures. Furthermore, the structure exhibits implies open circuit voltage (up to 1130mV), signifying its excellent open-circuit voltage potential if employed in photovoltaic devices. Overall, the reported low-dimension ordered-disordered semiconductor alloys can be useful in various optoelectronic applications.

Valley-Contrasting Linear Dichroism and Excitonic Condensation in LaOBiS2.

In conjunction with the first-principles calculations, we confirm the fascinating valley-selective linear dichroism and the possibility of excitonic condensation in multiatomic-layer LaOBiS2 of a tetragonal lattice by the effective tight-binding model and many-body perturbation theory. In LaOBiS2, which indicates a spontaneous valley polarization due to crystalline symmetry reduction rather than conventional time-reversal symmetry breaking, we unravel that the excitons in X and X' valleys exclusively coupled to x- and y-linearly polarized lights, respectively. In sharp contrast to coupled quantum wells and van der Waals architectures, a new type of spatially indirect exciton is identified in single-crystal LaOBiS2, manifesting itself as an engaging platform for investigating the excitonic Bose-Einstein condensation (BEC) and superfluidity. In light of large binding energy, large radius, and small effective mass of the exciton, the transition temperature for BEC and superfluidity achieves record-high values of 325.1 and 81.3 K, respectively.

Intrinsic Localized Excitons in MoSe2/CrSBr Heterostructure.

Despite extensive studies on magnetic proximity effects, the fundamental excitonic properties of the 2D semiconductor-magnet heterostructures remain elusive. Here, the presence of localized excitons in MoSe2/CrSBr heterostructures is unveiled, represented by a new photoluminescence emission feature, X*. Our findings reveal that X* originates from excitons confined by intrinsic defects in the CrSBr layer. Additionally, the degrees of valley polarization of the X* and trion peaks exhibit opposite polarities under a magnetic field and closely correlate with the magnetic order of CrSBr. This is attributed to spin-dependent charge transfer across the heterointerface, supported by density functional theory calculations which reveal a type-II band alignment. Furthermore, the strong in-plane anisotropy of CrSBr induces unique polarization-dependent responses in MoSe2 emissions. This study highlights the crucial role of defects in shaping excitonic properties and offers valuable insights into spectrally resolved proximity effects in semiconductor-magnet van der Waalsheterostructures.

Electrically controlled valley polarization and anomalous valley Hall effect in GdCl2 bilayer

Electrically controlled valley polarization and anomalous valley Hall effect in GdCl2 bilayer

Giant high-order nonlinear and nonreciprocal electrical transports induced by valley flipping in Bernal bilayer graphene

We investigate the electrical transport properties of the minivalley polarized state proposed recently in slightly doped Bernal bilayer graphene (BLG) in large electric displacement fields. By minimizing the Hartree-Fock (HF) energy functional, we first confirm the appearance of minivalley polarized phase. At the low carrier doping regime, the one-pocket state will be stabilized where only one of the trigonal-wrapping-induced Fermi pockets near the atomic-valley center is filled. Then we study the electrical transport of the one-pocket state by solving the Boltzmann equation. We find that the valley polarization could be easily flopped by an in-plane electrical field, which will lead to hysteresis loop in the direct current (DC) I−V curves. Such irreversible current responses in the DC limit will directly induce strong nonlinear and nonreciprocal alternating current (AC) responses, which has been already observed in the recent experiments on BLG. Published by the American Physical Society 2025

Valley-Related Multipiezo Effect in Altermagnet Monolayer V2STeO.

The multipiezo effect realizes the coupling of strain with magnetism and electricity, which provides a new way of designing multifunctional devices. In this study, monolayer V2STeO is demonstrated to be an altermagnet semiconductor with a direct band gap of 0.41 eV. The spin splittings of monolayer V2STeO are as high as 1114 and 1257 meV at the valence and conduction bands, respectively. Moreover, a pair of energy degeneracy valleys appears at X and Y points in the first Brillouin zone. The valley polarization and reversion can be achieved by applying uniaxial strains along different directions, indicating a piezovalley effect. In addition, a net magnetization coupled with uniaxial strain and hole doping can be induced in monolayer V2STeO, presenting the piezomagnetic feature. Furthermore, due to the Janus structure, the inversion symmetry of monolayer V2STeO is naturally broken, resulting in the piezoelectric property. The integration of the altermagnet, piezovalley, piezomagnetic, and piezoelectric properties make monolayer V2STeO a promising candidate for multifunctional spintronic and valleytronic devices.

Valley-Mediated Singlet- and Triplet-Polaron Interactions and Quantum Dynamics in a Doped WSe_{2} Monolayer.

In doped transition metal dichalcogenides, optically created excitons (bound electron-hole pairs) can strongly interact with a Fermi sea of electrons to form Fermi polaron quasiparticles. When there are two distinct Fermi seas, as is the case in WSe_{2}, there are two flavors of lowest-energy (attractive) polarons-singlet and triplet-where the exciton is coupled to the Fermi sea in the same or opposite valley, respectively. Using two-dimensional coherent electronic spectroscopy, we analyze how their quantum decoherence evolves with doping density and determine the condition under which stable Fermi polarons form. Because of the large oscillator strength associated with these resonances, intrinsic quantum dynamics of polarons as well as valley coherence between coupled singlet- and triplet polarons occur on subpicosecond timescales. Surprisingly, we find that a dark-to-bright state conversion process leads to a particularly long-lived singlet polaron valley polarization, persisting up to 200-800ps. Valley coherence between the singlet- and triplet polaron is correlated with their energy fluctuations. Our finding provides valuable guidance for the electrical and optical control of spin and valley indexes in atomically thin semiconductors.