Electrical control of magnetization is of crucial importance for integrated spintronics devices. Spin-orbit torques (SOT) in heavy-metal/ferromagnetic heterostructures have emerged as promising tool to achieve efficiently current-induced magnetization reversal. However, the microscopic origin of the SOT is being debated,with the spin Hall effect (SHE) due to nonlocal spin currents and the spin Rashba-Edelstein effect (SREE) due to local spin polarization at the interface being the primary candidates. We investigate the electrically induced out-of-equilibrium spin and orbital polarizations in pure Pt films and in Pt/3d-metal (Co, Ni, Cu) bilayer films using ab initio electronic structure methods and linear-response theory. We compute atom-resolved response quantities that allow us to identify the induced spin-polarization contributions that lead to fieldlike SOTs, mostly associated with the SREE, and dampinglike (DL) SOTs, mostly associated with the SHE, and compare their relative magnitude, dependence on the magnetization direction, as well as their Pt-layer thickness dependence. We find that both the FL and DL components contribute to the resulting SOT at the Pt/Co and Pt/Ni interfaces, with the former contributions being larger at the Pt interface layer and the latter larger in the Co or Ni layers. Our calculations show that the electrically-induced transverse orbital polarization is exceedingly larger than the induced spin polarization and present even without spin-orbit coupling, in contrast to the spin polarization.
Moiré structures in small-angle-twisted bilayers of two-dimensional semiconductors with a broken-symmetry interface form arrays of ferroelectric domains with periodically alternating out-of-plane polarization. Here, we propose a network theory for the tunability of such FE domain structure by applying an electric field perpendicular to the 2D crystal. Using multiscale analysis, we derive a fully parametrized string-theory-like description of the domain wall network and show that it undergoes a qualitative change, after the arcs of partial dislocation like domain walls merge (near the network n…
Twisted graphene bilayers provide a versatile platform to engineer metamaterials with novel emergent properties by exploiting the resulting geometric moiré superlattice. Such superlattices are known to host bulk valley currents at tiny angles ($α\approx 0.3 ^\circ$) and flat bands at magic angles ($α\approx 1^\circ$). We show that tuning the twist angle to $α^*\approx 0.8^\circ$ generates flat bands away from charge neutrality with a triangular superlattice periodicity. When doped with $\pm 6$ electrons per moiré cell, these bands are half-filled and electronic interactions produce a symmetry-…
Optoelectronic devices which allow rerouting, modulation and detection of the optical signals would be extremely beneficial for telecommunication technology. One of the most promising platforms for such devices are excitonic devices, as they offer very efficient coupling to light. Of especial importance are those based on indirect excitons, because of their long lifetime. Here we demonstrate excitonic transistor and router based on bilayer of WSe2. Due to their strong dipole moment, excitons in bilayer WSe2 can be controlled by transverse electric field. At the same time, unlike indirect excit…
Twisted van der Waals heterostructures with flat electronic bands have recently emerged as a platform for realizing correlated and topological states with an extraordinary degree of control and tunability. In graphene-based moiré heterostructures, the correlated phase diagram and band topology depend strongly on the number of graphene layers, their relative stacking arrangement, and details of the external environment from the encapsulating crystals. Here, we report that the system of twisted monolayer-bilayer graphene (tMBG) hosts a variety of correlated metallic and insulating states, as wel…
We present a theory of spatially indirect exciton condensate states in systems composed of a pair of electrically isolated Bernal graphene bilayers. The ground state phase diagram in a two-dimensional displacement-field/inter-bilayer-bias space includes layer-polarized semiconductors, spin-density-wave states, exciton condensates, and states with mixed excitonic and spin order. We find that two different condensate states, distinguished by a chirality index, are stable under different electrical control conditions.
The twist degree of freedom provides a powerful new tool for engineering the electrical and optical properties of van der Waals heterostructures. Here, we show that the twist angle can be used to control the spin-valley properties of transition metal dichalcogenide bilayers by changing the momentum alignment of the valleys in the two layers. Specifically, we observe that the interlayer excitons in twisted WSe$_2$/WSe$_2$ bilayers exhibit a high (>60%) degree of circular polarization (DOCP) and long valley lifetimes (>40 ns) at zero electric and magnetic fields. The valley lifetime can be tuned…
The possibility of triggering correlated phenomena by placing a singularity of the density of states near the Fermi energy remains an intriguing avenue towards engineering the properties of quantum materials. Twisted bilayer graphene is a key material in this regard because the superlattice produced by the rotated graphene layers introduces a van Hove singularity and flat bands near the Fermi energy that cause the emergence of numerous correlated phases, including superconductivity. While the twist angle-dependence of these properties has been explored, direct demonstration of electrostatic co…
Moiré superlattice-induced sub-bands in twisted van der Waals homo- and hetero-structures govern their optical and electrical properties, rendering additional degrees of freedom such as twist angle. Here, we demonstrate the moiré superlattice effects on the intralayer excitons and trions in a twisted bilayer of MoS$_2$ of H-type stacking at marginal twist angles. We identify the emission from localized and multiple delocalized sub-bands of intralayer moiré excitons and show their electrical modulation by the corresponding trion formation. The electrical control of the oscillator strength of th…
We investigate the effect of a direct current on propagating spin waves in a CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering technique, we observe that the spin wave amplitude may be attenuated or amplified depending on the direction of the current and the applied magnetic field. Our work suggests an effective approach for electrically controlling the propagation of spin waves in a magnetic waveguide and may be useful in a number of applications such as phase locked nano-oscillators and hybrid information processing devices.
Tailoring interlayer coupling has emerged as a powerful tool to tune the electronic structure of van der Waals (vdW) bilayers. One example is the usage of the moire pattern to create controllable two-dimensional electronic superlattices through the configurational dependence of interlayer electronic couplings. This approach has led to some remarkable discoveries in twisted graphene bilayers, and transition metal dichalcogenide (TMD) homo- and hetero-bilayers. However, a largely unexplored factor is the interlayer distance, d, which can impact the interlayer coupling strength exponentially. In …
A striking feature of bilayer graphene is the induction of a significant band gap in the electronic states by the application of a perpendicular electric field. Thicker graphene layers are also highly attractive materials. The ability to produce a band gap in these systems is of great fundamental and practical interest. Both experimental and theoretical investigations of graphene trilayers with the typical ABA layer stacking have, however, revealed the lack of any appreciable induced gap. Here we contrast this behavior with that exhibited by graphene trilayers with ABC crystallographic stackin…
Electrical detection of topological magnetic textures such as skyrmions is currently limited to conducting materials. While magnetic insulators offer key advantages for skyrmion technologies with high speed and low loss, they have not yet been explored electrically. Here, we report a prominent topological Hall effect in Pt/Tm$_3$Fe$_5$O$_{12}$ bilayers, where the pristine Tm$_3$Fe$_5$O$_{12}$ epitaxial films down to 1.25 unit cell thickness allow for tuning of topological Hall stability over a broad range from 200 to 465 K through atomic-scale thickness control. Although Tm$_3$Fe$_5$O$_{12}$ i…
Recently discovered relativistic spin torques induced by a lateral current at a ferromagnet/paramagnet interface are a candidate spintronic technology for a new generation of electrically-controlled magnetic memory devices. Phenomenologically, the torques have field-like and antidamping-like components with distinct symmetries. Microscopically, they are considered to have two possible origins. In one picture, a spin-current generated in the paramagnet via the relativistic spin Hall effect (SHE) is absorbed in the ferromagnet and induces the spin transfer torque (STT). In the other picture, a n…
In epitaxial (111) oriented Ni$_{80}$Fe$_{20}$/Fe$_{50}$Mn$_{50}$ bilayers, we separate two distinct behaviors: unidirectional anisotropy (exchange bias) in thick Fe$_{50}$Mn$_{50}$, and enhanced coercivity in thin Fe$_{50}$Mn$_{50}$. By measuring the magnetization response to a rotating magnetic field, we quantitatively determine the relevant anisotropies, and demonstrate that the enhanced coercivity is related to the rotatable magnetic anisotropy of Fe$_{50}$Mn$_{50}$. We also demonstrate the consequences of the anisotropy changes with temperature.
We report the discovery of a strong and tunable spin lifetime anisotropy with excellent spin lifetimes up to 7.8 ns in dual-gated bilayer graphene. Remarkably, this realizes the manipulation of spins in graphene by electrically-controlled spin-orbit fields, which is unexpected due to graphene's weak intrinsic spin-orbit coupling. We utilize both the in-plane magnetic field Hanle precession and oblique Hanle precession measurements to directly compare the lifetimes of out-of-plane vs. in-plane spins. We find that near the charge neutrality point, the application of a perpendicular electric fiel…
Continuous wave optically and electrically detected magnetic resonance spectroscopy (cwODMR/cwEDMR) allow the investigation of paramagnetic states involved in spin-dependent transitions, like recombination and transport. Although experimentally similar to conventional electron spin resonance (ESR), there exist limitations when applying models originally developed for ESR to observables (luminescence and electric current) of cwODMR and cwEDMR. Here we present closed-form solutions for the modulation frequency dependence of cwODMR and cwEDMR based on an intermediate pair recombination model and …
We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe$_2$O$_{\textrm{4-x}}$ ($4\geq x \geq 0$) bilayers. We separate the voltage generated by the spin Seebeck effect from the anomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect (ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to its proximity to the FM. Further, we probe the dependence of these effects on the electrical conductivity and the band gap energy of the FM film varying from nearly insulating NiFe$_2$O$_4$ to metalli…
Assessing quantitative phase reliability in electron ptychography using a twisted trilayer WS2 thickness staircase
University of Science and Technology of China（中国科学技术大学）；Yanshan University（燕山大学）；Zhejiang University of Technology（浙江工业大学）；University of Chinese Academy of Sciences（中国科学院大学）
A modified effective field model was developed to quantitatively interpret the angular dependent magnetization reversal processes in exchange biased Fe/IrMn bilayers. Several kinds of multi-step loops with distinct magnetization reversal routes were observed for the samples measured at various field orientations. Two types of angular dependent switching fields are observed and their transitions are investigated, which are found to be driven by both Fe and IrMn layer thicknesses. Our modified effective field model can nicely describe all the switching field behaviors including the critical effe…
Prediction of a layer nonlinear Hall effect in bilayer nonmagnetic or antiferromagnetic systems
School of Science, Hunan Institute of Technology（湖南工学院理学院）；Key Laboratory of Low Dimensional Materials and Application Technology of Ministry of Education, School of Materials Science and Engineering, Xiangtan University（教育部低维材料与应用技术重点实验室、湘潭大学材料科学与工程学院）；Hunan Provincial Key laboratory of Thin Film Materials and Devices, School of Materials Science and Engineering, Xiangtan University（湖南省薄膜材料与器件重点实验室、湘潭大学材料科学与工程学院）
HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the 'single-valley' analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe …
Electric-field-driven magnetic switching and tightly bound interlayer excitons in bilayer CrSBr
Wuhan National High Magnetic Field Center and School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院）；School of Physics and Technology, Wuhan University（武汉大学物理科学与技术学院）；National Institute for Material Science（日本国立材料研究所）
Coupling degrees of freedom of distinct nature plays a critical role in numerous physical phenomena. The recent emergence of layered materials provides a laboratory for studying the interplay between internal quantum degrees of freedom of electrons. Here, we report experimental signatures of new coupling phenomena connecting real spin with layer pseudospins in bilayer WSe2. In polarization-resolved photoluminescence measurements, we observe large spin orientation of neutral and charged excitons generated by both circularly and linearly polarized light, with a splitting of the trion spectrum in…
We demonstrate pronounced electric-field-induced second-harmonic generation in naturally inversion symmetric 2H stacked bilayer MoS$_{2}$ embedded into microcapacitor devices. By applying strong external electric field perturbations ($|F| = \pm 2.6 MVcm^{-1}$) perpendicular to the basal plane of the crystal we control the inversion symmetry breaking and, hereby, tune the nonlinear conversion efficiency. Strong tunability of the nonlinear response is observed throughout the energy range ($E_ω \sim 1.25 eV - 1.47 eV$) probed by measuring the second-harmonic response at $E_{2ω}$, spectrally detun…
Van der Waals (vdW) materials have greatly expanded our design space of heterostructures by allowing individual layers to be stacked at non-equilibrium configurations, for example via control of the twist angle. Such heterostructures not only combine characteristics of the individual building blocks, but can also exhibit emergent physical properties absent in the parent compounds through interlayer interactions. Here we report on a new family of emergent, nanometer-thick, semiconductor 2D ferroelectrics, where the individual constituents are well-studied non-ferroelectric monolayer transition …
Theory of Electrically Detected Magnetic Resonance of Silicon Vacancy-Related Spin Pairs in Silicon Carbide
University of Iowa（爱荷华大学）；Jet Propulsion Laboratory, California Institute of Technology（加州理工学院喷气推进实验室）；Donostia International Physics Center（圣塞巴斯蒂安国际物理中心）；Coastal Carolina University（海岸卡罗来纳大学）；The Pennsylvania State University（宾夕法尼亚州立大学）；Eindhoven University of Technology（埃因霍温理工大学）
Phosphorene, a two-dimensional (2D) monolayer of black phosphorus, has attracted considerable theoretical interest, although the experimental realization of monolayer, bilayer, and few-layer flakes has been a significant challenge. Here we systematically survey conditions for liquid exfoliation to achieve the first large-scale production of monolayer, bilayer, and few-layer phosphorus, with exfoliation demonstrated at the 10-gram scale. We describe a rapid approach for quantifying the thickness of 2D phosphorus and show that monolayer and few-layer flakes produced by our approach are crystalli…
We investigated a possible correlation between spin Hall angles and exchange bias in Ni$_{81}$Fe$_{19}$/IrMn samples by performing spin torque ferromagnetic resonance measurements. This correlation is probed by patterning of Ni$_{81}$Fe$_{19}$/IrMn bilayers in different relative orientations with respect to the exchange bias direction. The measured voltage spectra allow a quantitative determination of spin Hall angles, which are independent of the orientation around 2.8\pm0.3%.
