Valley Polarization Research Articles

Valley polarization in a two-dimensional high-temperature semiconducting TiInTe[formula omitted] honeycomb ferromagnet

Exploring valley-contrasting physics in scarce intrinsic high-temperature ferromagnetic semiconductors with feasible synthesis route is of great significance for ever-evolving moden information technology. By first-principles calculations, the TiInTe3 monolayer is predicted to demonstrate excellent structural stability and easy-plane semiconducting ferromagnetism arising from indirect superexchange mechanism. Monte Carlo simulations based on the Heisenberg model reveal a magnetic phase transition at 583 K well above room temperature. Due to the inversion and time-reversal asymmetries, tunable valley polarization in the conduction band up to 110 meV can be achieved by varying the magnetization orientation, which is further validated by a first-order perturbation theory. More excitingly, electron doping for realizing the anomalous valley Hall effect gives rise to an out-of-plane preference for magnetization in the TiInTe3 monolayer and thus generates spontaneous and larger valley polarizations. After doping, the ferromagnetism above room temperature remains robust in the experimentally attainable electron density regime. Our findings highlight that the TiInTe3 monolayer is a promising ferrovalley material for developing high-performance spintronic and valleytronic nanodevices.

Possible way to achieve valley-polarized quantum anomalous Hall insulator

The valley-polarized quantum anomalous Hall insulator (VQAHI), which combines valleytronics and topology in one material, provides a path toward integrating valleytronics, topological quantum effects, and spintronics. Here, we propose a way of realizing VQAHIs in tetragonal quantum anomalous Hall insulators (QAHIs) by introducing valley polarization, which can be achieved by breaking C4 rotational symmetry. We use a concrete example of a QAHI Fe2I2 monolayer, where there are equivalent valleys along the Γ-X and Γ-Y lines in momentum space, to illustrate our proposal through first-principle calculations. By applying uniaxial strain along the x direction, the rotational symmetry C4 is reduced into C2, which induces valley polarization. With uniaxial strain from a compressive one to a tensile one, valley polarization transition can be induced. Moreover, the nonzero Berry curvature around valleys can produce the anomalous valley Hall effect. With the considered strain range, the quantum anomalous Hall properties can be maintained. Therefore, a VQAHI can be realized in strained Fe2I2. Our works propose an experimentally feasible way to realize valley polarization and VQAHIs.

Magnetic and Ferroelectric Manipulation of Valley Physics in Janus Piezoelectric Materials.

The realization of multiferroic materials offers the possibility of multifunctional electronic device design. However, the coupling between the multiferroicity and piezoelectricity in Janus materials is rarely reported. In this study, we propose a mechanism for manipulating valley physics by magnetization reversing and ferroelectric switching in multiferroic and piezoelectric material. The ferromagnetic VSiGeP4 monolayer exhibits a large valley polarization up to 100 meV, which can be effectively operated by reversing magnetization. Interestingly, the antiferromagnetic VSiGeP4 bilayers with AB and BA stacking configurations allow the coexistence of valley polarization and ferroelectricity, supporting the proposed strategy for manipulating valley physics via ferroelectric switching and interlayer sliding. In addition, the VSiGeP4 monolayer contains remarkable tunable piezoelectricity regulated by electron correlation U. This study proposes a feasible idea for regulating valley polarization and a general design idea for multifunctional devices with multiferroic and piezoelectric properties, facilitating the miniaturization and integration of nanodevices.

Orbital multiferroicity in pentalayer rhombohedral graphene.

Ferroic orders describe spontaneous polarization of spin, charge and lattice degrees of freedom in materials. Materials exhibiting multiple ferroic orders, known as multiferroics, have important parts in multifunctional electrical and magnetic device applications1-4. Two-dimensional materials with honeycomb lattices offer opportunities to engineer unconventional multiferroicity, in which the ferroic orders are driven purely by the orbital degrees of freedom and not by electron spin. These include ferro-valleytricity corresponding to the electron valley5 and ferro-orbital-magnetism6 supported by quantum geometric effects. These orbital multiferroics could offer strong valley-magnetic couplings and large responses to external fields-enabling device applications such as multiple-state memory elements and electric control of the valley and magnetic states. Here we report orbital multiferroicity in pentalayer rhombohedral graphene using low-temperature magneto-transport measurements. We observed anomalous Hall signals Rxy with an exceptionally large Hall angle (tanΘH > 0.6) and orbital magnetic hysteresis at hole doping. There are four such states with different valley polarizations and orbital magnetizations, forming a valley-magnetic quartet. By sweeping the gate electric field E, we observed a butterfly-shaped hysteresis of Rxy connecting the quartet. This hysteresis indicates a ferro-valleytronic order that couples to the composite fieldE·B(where Bis the magnetic field), but not to the individual fields. Tuning E would switch each ferroic order independently and achieve non-volatile switching of them together. Our observations demonstrate a previously unknown type of multiferroics and point to electrically tunable ultralow-power valleytronic and magnetic devices.

Synergistic Effect of Chiral Metasurface and Hot Carrier Injection Enabling Manipulation of Valley Polarization of WSe2 at Room Temperature

AbstractThe unique band structure of monolayer transition metal dichalcogenides (TMDC) provides an important platform for spintronic and valleytronic devices. Various approaches, such as in‐plane electric field and out‐of‐plane magnetic field, have been proposed to actively control the valley polarization. In this work, we propose a synergistic effect involving chiral near‐field interactions and hot carrier injection to actively control the valley polarization emission of WSe2 at room temperature (RT). The degree of valley polarization is enhanced from near zero (for pure WSe2) to 20% under non‐resonant optical excitation (532 nm) when monolayer WSe2 is coupled with the chiral near field of plasmonic metasurface. More importantly, the application of near‐infrared light (wavelength of 970 to 1600 nm) illumination further enhances the valley polarization from 20% to 30%, which is attributed to plasmonic‐induced hot carrier injection from the metasurface to WSe2. The synergistic effect of the chiral near field and infrared light pumping offers another strategy to manipulate the valley polarization emission in monolayer TMDs at room temperature, paving the way for future applications of opto‐valleytronic/spintronic devices based on these 2D materials.

Identification of magnetic state of transition metal dichalcogenides via photonic spin Hall effect

Monolayer transition metal dichalcogenides with magnetic exchange fields have been demonstrated to display the remarkable valley polarization and magnetooptical behaviors. However, the explorations of their photonic spin Hall effects are lacking. Here, we show that the reflected spin shift of monolayer transition metal dichalcogenides with magnetic exchange field is significantly different from that of the pristine one and it exhibits the distinctive dependence on the size of the magnetic exchange field. In addition, we can manipulate the reflected spin shift of monolayer transition metal dichalcogenides with the magnetic exchange field via its chemical potential. This work unravels the potential of the photonic spin Hall effect on identifying the magnetic state of monolayer transition metal dichalcogenides or the substrate, which might promote their potential applications in the spin photonic devices.

Valley-Selective High Harmonic Generation and Polarization Induced by an Orthogonal Two-Color Laser Field

The valley pseudospin properties of electrons in two-dimensional hexagonal materials result in many fascinating physical phenomena, which opens up the new field of valleytronics. The valley-contrasting physics aims at distinguishing the valley degree of freedom based on valley-dependent effects. Here, we theoretically demonstrate that both of the valley-selective high harmonic generation and valley-selective electronic excitation can be achieved by using an orthogonal two-color (OTC) laser field in gapped graphene. It is shown that the asymmetry degrees of harmonic yields in the plateaus, cutoff energies of generated harmonics and electron populations from two different valleys can be precisely controlled by the relative phase of the OTC laser field. Thus, the selectivity of the dominant valley for the harmonic radiation and electronic polarization can be switched by adjusting the relative phase of the OTC laser field. Our work offers an all-optical route to produce the valley-resolved high harmonic emissions and manipulate the ultrafast valley polarization on a femtosecond timescale in condensed matter.

Valley-dependent topological phase transition in monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br)

Due to the breaking of the time reversal symmetry and spatial inversion symmetry, hexagonal ferrovalley materials have intrinsic large valley polarization. Model analysis shows that tuning the two different band gaps of valleys can realize phase transitions between ferrovalley semiconductors, half valley metals, and valley-polarized quantum anomalous Hall semiconductors. Through first-principle calculations, monolayer ferrovalley materials RuXY (X, Y = F, Cl, Br), which exhibit valley splitting at the top valence band and the bottom conduction band, are predicted to achieve this valley-dependent topological phase transition. Due to the different orbital proportions of d orbitals, the valley splitting at the top valence band is much greater than that at the bottom conduction band. Strain can regulate the interaction between orbitals, thus producing valley-dependent band inversion, leading to the quantum spin or valley Hall effect. The chiral edge states are demonstrated under appropriate biaxial strain. The topological phase transition is related to the inversion of the band structure and Berry curvatures at K and K′ valleys. These results have certain significance for the design of two-dimensional valley-dependent quantum materials and the application of valleytronic devices.

Strain-modulated band structure and high harmonic generations in two-dimensional MoS2

Two-dimensional (2D) condensed matter is a material that is restricted in one direction while being periodic in the other. Since the restricted size of 2D materials is comparable to the wavelength of electrons, a quantum confinement effect may occur. Moreover, the absence of periodicity provides weak screening in 2D materials, which brings novel physical properties such as the quantum well, which is widely applied in quantum information, and the fine absorption structures in graphene. Among the 2D materials, the monolayer transition metal chalcogenides represented by MoS2 have attracted wide attention due to the direct band gap in the visible light region (1.8 eV) and valley polarizations, which are prospective for solar cells as well as photoelectric devices. High-harmonic generation (HHG) is a strong non-linear process during which a high-energy laser impulse is applied to materials and high-harmonic radiations are yielded. As a typical ultrafast dynamic, HHG has important applications in laser generation, such as EUV lithographic metrology and high-resolution coherent imaging. According to the Bloch oscillation model, HHG is highly dependent on band structure. Here, we report the strain-dependent HHG dynamics in MoS2. Further investigation reveals that the strain-dependence of HHG is caused by band modulation under different strains, which is dominant during HHG. Our research sheds light on ways to achieve effective modulations in ultrafast dynamics, implying an all-optical measurement band structure in strained materials.

Composite Fermi Liquid at Zero Magnetic Field in Twisted MoTe_{2}.

The pursuit of exotic phases of matter outside of the extreme conditions of a quantizing magnetic field is a long-standing quest of solid state physics. Recent experiments have observed spontaneous valley polarization and fractional Chern insulators in zero magnetic field in twisted bilayers of MoTe_{2}, at partial filling of the topological valence band (ν=-2/3 and -3/5). We study the topological valence band at half filling, using exact diagonalization and density matrix renormalization group calculations. We discover a composite Fermi liquid (CFL) phase even at zero magnetic field that covers a large portion of the phase diagram near twist angle ∼3.6°. The CFL is a non-Fermi liquid phase with metallic behavior despite the absence of Landau quasiparticles. We discuss experimental implications including the competition between the CFL and a Fermi liquid, which can be tuned with a displacement field. The topological valence band has excellent quantum geometry over a wide range of twist angles and a small bandwidth that is, remarkably, reduced by interactions. These key properties stabilize the exotic zero field quantum Hall phases. Finally, we present an optical signature involving "extinguished" optical responses that detects Chern bands with ideal quantum geometry.

Valley polarization dependence of nonreciprocal transport in a chiral semiconductor

Valley polarization dependence of nonreciprocal transport in a chiral semiconductor

Stacking order, charge doping, and strain-induced switching between AFM and FM in bilayer GdI2

GdI2 monolayer is a promising material for spintronics applications due to its robust room-temperature ferromagnetism and sizable valley polarization. In two-dimensional van der Waals magnets, interlayer magnetic coupling plays a crucial role in device applications. The performance of these devices can be effectively tuned by adjusting the stacking order, charge doping, and strain. By performing first-principles calculations, we have demonstrated that the interlayer magnetic coupling in bilayer GdI2 is highly dependent on the stacking order, which can be tuned between ferromagnetic (FM) and antiferromagnetic orders through lateral shifting. Furthermore, the interlayer magnetic coupling can also be tuned by charge doping and strain, where both electron and hole doping can enhance the FM coupling interaction between layers, and the interlayer FM coupling can be strengthened with increasing biaxial tensile strain. These results show that bilayer GdI2 has rich tunable interlayer magnetic interactions, which can be used in designing interesting spin tunnel field-effect transistor devices.

Valley-dependent transport in a mescoscopic twisted bilayer graphene device

We study the valley-dependent electron transport in a four-terminal mesoscopic device of the two monolayer graphene nanoribbons vertically stacked together, where the intersection forms a bilayer graphene lattice with a controllable twist angle. Using a tight-binding lattice model, we show that the longitudinal and transverse conductances exhibit significant valley polarization in the low energy regime for small twist angles. As the twist angle increases, the valley polarization shifts to the high energy regime. This arises from the regrouping effect of the electron band in the twisted bilayer graphene region. But for relatively large twist angles, no significant valley polarization is observed. These results are consistent with the spectral densities of the twisted bilayer graphene.

Optical spectrum of ferrovalley materials: A case study of Janus H-VSSe

As opposed to the prototypical MoS2 with centroasymmetry, Janus ferrovalley materials such as H-VSSe are less symmetric with the mirror symmetry and time reversal symmetry broken, and hence possess spontaneous valley polarization and strong ferroelasticity. The optical transition is an important means to excite the valley carriers. We investigate the optical spectrum of H-VSSe by using the many-body perturbation-based GW approach and solving the Bethe–Salpeter equation (BSE) to include the electron–hole interactions. It is found that after the GW correction, the band gaps of the quasiparticle bands are much larger than those obtained by the normal density functional theory. The system is ferromagnetic and the valley gaps become non-degenerate due to spin–orbit coupling (SOC). The position of the lowest BSE peak is much lower than the quasiparticle band gap, indicating that the excitonic effect is large. The peak is split into two peaks by the SOC. The binding energy difference between these two BSE peaks is about the same as the difference between the inequivalent valley gaps. Our results show that in Janus H-VSSe the two lowest exciton peaks are from the two inequivalent valleys with different gaps, in contrast to the A and B exciton peaks of MoS2 which are from the same valley.

Magnetic exchange field signature in ultrafast electron dynamics in monolayer WSe2 by ultrashort optical pulse

We theoretically investigate the ultrafast electron dynamics in monolayer tungsten diselenide interacting with an irradiated ultrashort strong optical pulse in the presence of a magnetic exchange field. The dynamics of Dirac fermions in monolayer WSe2 is affected by spin and valley splitting, of course, enhanced by applied magnetic exchange field. There exists remarkable interaction between the exchange field and valley index because of huge spin–orbit coupling at the valence band. Regarding the optical pulse duration is smaller than the electron scattering time, the electron dynamics remains coherent and is described by the time-dependent Schrodinger equation. The strong electric field of the pulse, interacted with monolayer WSe2 causes appearance of a dipole moment connecting two valence and conduction bands. Because of the K and K′ valleys behave differently by the magnetic exchange field, the perfect valley-spin polarization effect is achieved. We explore the irreversible conduction band electron transition affected by a magnetic exchange field. The signature of the exchange field on such transitions is quite obvious. The conduction band electron redistribution around the Dirac points is represented, when the pulse ends. Asymmetric hot spots pattern in two different K and K′ valleys depends more or less on the magnitude of the magnetic exchange field. By reversing the direction of the exchange field, the process of spin polarization is completely reversed and it results in the electron distribution pattern. The high spin-valley resolved splitting in monolayer WSe2 provides a unique platform to investigate the magnetic exchange field driven phenomena, leading to valley polarization effect.

Room temperature valley polarization via spin selective charge transfer

The two degenerate valleys in transition metal dichalcogenides can be used to store and process information for quantum information science and technology. A major challenge is maintaining valley polarization at room temperature where phonon-induced intervalley scattering is prominent. Here we demonstrate room temperature valley polarization in heterostructures of monolayer MoS2 and naphthylethylammine based one-dimensional chiral lead halide perovskite. By optically exciting the heterostructures with linearly polarized light close to resonance and measuring the helicity resolved photoluminescence, we obtain a degree of polarization of up to −7% and 8% in MoS2/right-handed (R-(+)-) and left-handed (S-(-)-) 1-(1-naphthyl)ethylammonium lead iodide perovskite, respectively. We attribute this to spin selective charge transfer from MoS2 to the chiral perovskites, where the perovskites act as a spin filter due to their chiral nature. Our study provides a simple, yet robust route to obtain room temperature valley polarization, paving the way for practical valleytronics devices.

Conductance and polarization in an 8-Pmmn borophene-based superlattice

We theoretically investigate the effect of the Rashba interaction on the conductance as well as the valley and spin polarization in an 8-Pmmn borophene superlattice by solving the Dirac–like equation and using the transfer matrix approach. The valley and spin-dependent conductance and spin polarization are effectively controlled by changing the direction of superlattice. The number of the quantum electrostatic potential plays an essential role in quantum transmission properties. Furthermore, in such a superlattice, the conductance of the electrons with spin rotation can be easily adjusted by the Rashba strength, direction of superlattice and electrostatic potential. Thus, borophene superlattice with Rashba interaction can act as a spin and valley splitter. Our work provides a new structure for spin-flip and can be useful for future spintronics and valleytronics applications.

Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe2/MoSe2 heterobilayers

Transition metal dichalcogenides (TMDs) have attracted much attention in the fields of valley- and spintronics due to their property of forming valley-polarized excitons when illuminated by circularly polarized light. In TMD-heterostructures it was shown that these electron-hole pairs can scatter into valley-polarized interlayer exciton states, which exhibit long lifetimes and a twist-angle dependence. However, the question how to create a valley polarization of free charge carriers in these heterostructures after a valley selective optical excitation is unexplored, despite its relevance for opto-electronic devices. Here, we identify an interlayer transfer mechanism in twisted WSe2/MoSe2 heterobilayers that transfers the valley polarization from excitons in WSe2 to free charge carriers in MoSe2 with valley lifetimes of up to 12 ns. This mechanism is most efficient at large twist angles, whereas the valley lifetimes of free charge carriers are surprisingly short for small twist angles, despite the occurrence of interlayer excitons.

Vibrational Dichroism of Chiral Valley Phonons.

Valley degrees of freedom in transition metal dichalcogenides thoroughly influence electron-phonon coupling and its nonequilibrium dynamics. We conducted a first-principles study of the quantum kinetics of chiral phonons following valley-selective carrier excitation with circularly polarized light. Our numerical investigations treat the ultrafast dynamics of electrons and phonons on equal footing within a parameter-free ab initio framework. We report the emergence of valley-polarized phonon populations in monolayer MoS2 that can be selectively excited at either the K or K' valleys depending on the light helicity. The resulting vibrational state is characterized by a distinctive chirality, which lifts time-reversal symmetry of the lattice on transient time scales. We show that chiral valley phonons can further lead to fingerprints of vibrational dichroism detectable by ultrafast diffuse scattering and persist beyond 10 ps. The valley polarization of nonequilibrium phonon populations could be exploited as an information carrier, thereby extending the paradigm of valleytronics to the domain of vibrational excitations.

Strain‐Valley Coupling in 2D Antiferromagnetic Lattice

AbstractIn recent years, 2D valley physics has attracted tremendous attention in both fundamental research and device applications. The search for a coupling valley with exotic physical properties can enable the exploration of novel quantum devices and is therefore of significant interest. Here, using model analysis, a novel mechanism is proposed that realizes strong coupling between strain and valley physics in a 2D antiferromagnetic honeycomb lattice in a controllable fashion. This idea is to modulate the exchange interactions through strain, thereby mediating the inversion and time‐reversal symmetries. As valley physics are connected with the symmetries, this enables a strong strain‐valley coupling. On the basis of first‐principles calculations, the validity of this mechanism is further demonstrated in a series of single‐layer systems, i.e., AuCl3, CrI3, and VPTe3, which are predicted to host valley polarization and anomalous valley Hall effect. The change in bonding angle is found to play a crucial role in mediating the exchange interaction and thus the strain‐valley coupling process. The microscopic origin is also discussed. These explored phenomena and insights greatly enrich the valley physics and 2D materials research.