Proximity Exchange Research Articles

Chirality-Dependent Valley Polarization in Magnetic van der Waals Heterostructures via Spin-Selective Charge Transfer.

Magnetic proximity interaction provides a promising route to manipulate the spin and valley degrees of freedom in van der Waals heterostructures. Here, we report a control of valley pseudospin in the WS2/MoSe2 heterostructure by utilizing the magnetic proximity effect of few-layered CrBr3 and, for the first time, observe a substantial difference in valley polarization of intra/interlayer excitons under different circularly polarized laser excitations, referred to as chirality-dependent valley polarization. Theoretical and experimental results reveal that the spin-selective charge transfer between MoSe2 and CrBr3, as well as between MoSe2 and WS2, is mostly responsible for the chiral feature of valley polarization in comparison with the proximity exchange field. This means that a long-distance manipulation of exciton behaviors in multilayer heterostructures can be achieved through spin-selective charge transfer. This work marks a significant advancement in the control of spin and valley pseudospin in multilayer structures.

Emergent Correlated Phases in Rhombohedral Trilayer Graphene Induced by Proximity Spin-Orbit and Exchange Coupling.

The impact of proximity-induced spin-orbit and exchange coupling on the correlated phase diagram of rhombohedral trilayer graphene (RTG) is investigated theoretically. By employing abinitio-fitted effective models of RTG encapsulated by transition metal dichalcogenides (spin-orbit proximity effect) and ferromagnetic Cr_{2}Ge_{2}Te_{6} (exchange proximity effect), we incorporate the Coulomb interactions within the random-phase approximation to explore potential correlated phases at different displacement fields and doping. We find a rich spectrum of spin-valley resolved Stoner and intervalley coherence instabilities induced by the spin-orbit proximity effects, such as the emergence of a spin-valley-coherent phase due to the presence of valley-Zeeman coupling. Similarly, proximity exchange removes the phase degeneracies by biasing the spin direction, enabling a magnetocorrelation effect-strong sensitivity of the correlated phases to the relative magnetization orientations (parallel or antiparallel) of the encapsulating ferromagnetic layers.

Phase-Dependent Magnetic Proximity Modulations on Valley Polarization and Splitting.

Proximate-induced magnetic interactions present a promising strategy for precise manipulation of valley degrees of freedom. Taking advantage of the splendid valleytronic platform of transition metal dichalcogenides, magnetic two-dimensional VSe2 with different phases are introduced to intervene in the spin of electrons and modulate their valleytronic properties. When constructing the heterostructures, 1T-VSe2/WX2 (X = S and Se) showcases significant improvement in the valley polarizations at room temperature, while 2H-VSe2/WX2 exhibits superior performance at low temperatures and demonstrates heightened sensitivity to the external magnetic field. Simultaneously, considerable valley splitting with a large geff factor up to -29.0 is observed in 2H-VSe2/WS2, while it is negligible in 1T-VSe2/WX2. First-principles calculations reveal a phase-dependent magnetic proximity mechanism on the valleytronic modulations, which is dominated by interfacial charge transfer in 1T-VSe2/WX2 and the proximity exchange field in 2H-VSe2/WX2 heterostructures. The effective control over valley degrees of freedom will bridge the valleytronic physics and devices, rendering enormous potential in the field of valley quantum applications.

Creativity, Auto-Ethnography, and the Reinvention of Sacred Space in New Age Spirituality and Healing during the COVID-19 Pandemic

New Age practices and their ritualized actions have been primarily based on the creation of sacred spaces through immediate interaction, proximity, affect, healing, bodily engagement, and emotional exchange. In the COVID 19 pandemic context, however, such spiritual intimacy has been challenged if not compromised. New Age practitioners have faced the necessity to become ritually and spiritually innovative and establish new forms of sacred spaces to accommodate their performances. Drawing on long-term fieldwork on the theme of New Age spirituality and healing in Lisbon, Portugal, and Athens, Greece, this article offers an account of how New Age spiritual creativity was performed in the context of the pandemic, while exploring how different yet intertwined sacred spaces are created, and the role that (auto)ethnographic embodiment and research knowledge plays in this process.

Giant proximity exchange and flat Chern band in 2D magnet-semiconductor heterostructures

van der Waals (vdW) heterostructures formed by two-dimensional (2D) magnets and semiconductors have provided a fertile ground for fundamental science and spintronics. We present first-principles calculations finding a proximity exchange splitting of 14 meV (equivalent to an effective Zeeman field of 120 T) in the vdW magnet-semiconductor heterostructure MoS2/CrBr3, leading to a 2D spin-polarized half-metal with carrier densities ranging up to 1013 cm-2. We consequently explore the effect of large exchange coupling on the electronic band structure when the magnetic layer hosts chiral spin textures such as skyrmions. A flat Chern band is found at a "magic" value of magnetization [Formula: see text] for Schrödinger electrons, and it generally occurs for Dirac electrons. The magnetic proximity-induced anomalous Hall effect enables transport-based detection of chiral spin textures, and flat Chern bands provide an avenue for engineering various strongly correlated states.

Scaling with MgO thickness of the proximity exchange field in a Fe/MgO/Si contact

The scaling of the proximity exchange field, which ferromagnetic Fe exerts on the spin accumulation at the MgO/Si interface of Fe/MgO/Si tunnel contacts, was investigated using the inverted Hanle effect. Data as a function of the applied bias voltage shows that the exchange field varies with bias in the same way as the square of the tunnel spin polarization (TSP). The decay of the exchange field with increasing MgO thickness ${t}_{\mathrm{MgO}}$ is rather weak and nonexponential, which we argue to be related to the improved epitaxial quality of the MgO, and the, consequently, improved spin-filtering properties at larger MgO thickness. Using the TSP data to correct for this yields an exponential decay of the exchange field with ${t}_{\mathrm{MgO}}$ with a decay length of $\ensuremath{\lambda}=\ensuremath{-}0.42$ nm. Comparing this to the decay length for the tunnel conductance ($\ensuremath{\lambda}=\ensuremath{-}0.21$ nm) suggests that the exchange interaction is dominated by two-step tunneling processes.

Strong manipulation of the valley splitting upon twisting and gating in MoSe2/CrI3 and WSe2/CrI3 van der Waals heterostructures

We investigate the twist-angle and gate dependence of the proximity-induced exchange coupling in the monolayer transition-metal dichalcogenides (TMDCs) MoSe$_2$ and WSe$_2$ due to the vdW coupling to the ferromagnetic semiconductor CrI$_3$, from first-principles calculations. A model Hamiltonian, that captures the relevant band edges at the $K/K^{\prime}$ valleys of the proximitized TMDCs, is employed to quantify the proximity-induced exchange. Upon twisting from 0{\deg} to 30{\deg}, we find a transition of the TMDC valence band (VB) edge exchange splitting from about $-2$ to $2$ meV, while the conduction band (CB) edge exchange splitting remains nearly unchanged at around $-3$ meV. For the VB of WSe$_2$ (MoSe$_2$) on CrI$_3$, the exchange coupling changes sign at around 8{\deg} (16{\deg}). We find that even at the angles with almost zero spin splittings of the VB, the real-space spin polarization profile of holes at the band edge is highly non-uniform, with alternating spin up and spin down orbitals. Furthermore, a giant tunability of the proximity-induced exchange coupling is provided by a transverse electric field of a few V/nm. We complement our \textit{ab initio} results by calculating the excitonic valley splitting to provide experimentally verifiable optical signatures of the proximity exchange. Specifically, we predict that the valley splitting increases almost linearly as a function of the twist angle. Furthermore, the proximity exchange is highly tunable by gating, allowing to tailor the valley splitting in the range of 0 to 12 meV in WSe$_2$/CrI$_3$, which is equivalent to external magnetic fields of up to about 60 Tesla. Our results highlight the important impact of the twist angle and gating when employing magnetic vdW heterostructures in experimental geometries.

Stacking and tuning effects on magneto-electronic and electric contact features for arsenene/Fe3GeTe2 van der Waals heterostructure

Recently, experimentally available Fe3GeTe2 (FGT) monolayer has attracted tremendous research interest due to its long-range ferromagnetic (FM) order. Here, we systematically study the magneto-electronic and electric contact properties of FGT-based van der Waals (vdW) heterostructures integrated by arsenene (As) with multiple stacking patterns. The low binding energy (−117.69 to −52.69 meV atom−1) proves their highly geometric stability, and the high magnetized energy (91.61–213.61 meV/unit cell) and magnetic exchange energy (64.53–84.43 meV/unit cell) provide a strong evidence for magnetism stability. Particularly, the magnetic proximity exchange effect as well as spin transfer and spin rearrangement can enhance the FM coupling in heterostructures, so that the highest Curie temperature T C = 173.93 K is achieved, being 19.73% higher than isolated FGT. Meanwhile, the arsenene is magnetized as well. In particular, metal-induced gap states appear in band gap of original As monolayer. Besides, metal-semiconductor contact performance is improved in constructed heterostructures by lowering electronic Schottky barrier height to nearly obtain Ohmic contact. Magnetic exchange energy rises further to improve magnetic stability with the interlayer spacing narrowed, and T C is increased up to 184.80 K, an increase of 27.21% compared to FGT monolayer. Being attributed to the proximity exchange effect strengthened with a shrunk interlayer spacing, and the electric contact behavior is also boosted further by this compressive strain to realize high-performance electric junction. Our findings provide a new route to extend the applications of FGT by constructing suitable vdW heterostructures.

Coastal micro-phytoplankton community changes during the toxigenic Alexandrium minutum blooms in a semi-enclosed tropical coastal lagoon (Malaysia, South China Sea)

Estuarine lagoons often experience eutrophication due to anthropogenic activities in proximity and poor water exchange. Deterioration of water quality in the lagoon has driven the shifts in micro-phytoplankton communities that often form coastal harmful algal blooms (HABs). This study investigated changes in the micro-phytoplankton community in a tropical semi-enclosed lagoon, Geting Lagoon, for two consecutive years during the study periods of 2014/2015 (September 2014–March 2015) and 2015/2016 (August 2015–March 2016). A total of 38 phytoplankton taxa were identified throughout the sampling periods, with 17 taxa of harmful algae identified microscopically. The diatoms Skeletonema costatum and Chaetoceros were predominant in the lagoon. The lagoon was significantly enriched in macronutrients, and high overall nitrogen and phosphorous concentrations likely supported substantial phytoplankton blooms in the lagoon throughout the studied periods. It was observed that with similar temperatures and nutrient availability between the 2014/2015 and 2015/2016 periods, the density of A. minutum in 2014/2015 did not reach as high as in those of the 2015/2016 period, in which higher precipitations, streamflow, and decreased in salinity stratification index had prevented the development of A. minutum bloom. The findings of this study revealed that the coastal micro-phytoplankton community was dependent on the interplay of environmental variables and unique strategic adaptations of the functional types in the assemblages. This study provides valuable insights into the phytoplankton community dynamics in a tropical eutrophic lagoon.

Strain and electric field dependent spin polarization in two-dimensional arsenene/CrI3 heterostructure

Spin polarized van der Waals (vdW) heterostructures have attracted considerable interest owing to the spin splitting manipulation. In this paper, first-principles calculations are employed to explore the two-dimensional arsenene/CrI3 vdW heterostructure as a promising spin polarized material, as well as the spin polarization under strain and electronic field are investigated. The most stable stacking configuration and ferromagnetic (FM) property of the arsenene/CrI3 vdW heterostructure have been confirmed. The detailed calculations show that the Curie temperature (Tc) of the FM coupling CrI3 layer in the heterostructure can be enhanced up to 61 K, which is attributed to both superexchange interaction and proximity exchange effect. The electronic structures suggest that this heterostructure possess an intrinsic type-II band alignment and diluted magnetic semiconductor property. Interestingly, we found a transition from diluted magnetic semiconductor to half-metal and gradually to metal induced by the biaxial strains, and the extra electronic field can modulate the “maxican hat” of the valance band maximum (VBM) of arsenene. Our work provides not only application prospects of the arsenene/CrI3 vdW heterostructure nanodevices but also theoretical effective support for the research and development of the spin electronics and flexible electronics.

Proximity effects in graphene on monolayers of transition-metal phosphorus trichalcogenides MPX3 (M:Mn, Fe, Ni, Co, and X: S, Se)

We investigate the electronic band structure of graphene on a series of two-dimensional magnetic transition-metal phosphorus trichalcogenide monolayers, ${M\text{P}X}_{3}$ with $M={\mathrm{Mn},\mathrm{Fe},\mathrm{Ni},\mathrm{Co}}$ and $X={\mathrm{S},\mathrm{Se}}$, with first-principles calculations. A symmetry-based model Hamiltonian is employed to extract orbital parameters and sublattice resolved proximity-induced exchange couplings (${\ensuremath{\lambda}}_{\text{ex}}^{\text{A}}$ and ${\ensuremath{\lambda}}_{\text{ex}}^{\text{B}}$) from the low-energy Dirac bands of the proximitized graphene. Depending on the magnetic phase of the ${M\text{P}X}_{3}$ layer (ferromagnetic and three antiferromagnetic ones), completely different Dirac dispersions can be realized with exchange splittings ranging from 0 to 10 meV. Remarkably, not only the magnitude of the exchange couplings depends on the magnetic phase, but also the global sign and the type. Important, one can realize uniform $({\ensuremath{\lambda}}_{\text{ex}}^{\text{A}}\ensuremath{\approx}{\ensuremath{\lambda}}_{\text{ex}}^{\text{B}})$ and staggered $({\ensuremath{\lambda}}_{\text{ex}}^{\text{A}}\ensuremath{\approx}\ensuremath{-}{\ensuremath{\lambda}}_{\text{ex}}^{\text{B}})$ exchange couplings in graphene. From selected cases we find that the interlayer distance, as well as a transverse electric field, are efficient tuning knobs for the exchange splittings of the Dirac bands. More specifically, decreasing the interlayer distance by only about 10%, a giant fivefold enhancement of proximity exchange is found, while applying few V/nm of electric field, provides tunability of proximity exchange by tens of percent. We have also studied the dependence on the Hubbard $U$ parameter and find it to be weak. Moreover, we find that the effect of SOC on the proximitized Dirac dispersion is negligible compared to the exchange coupling.

Electrical and thermal transport in a twisted heterostructure of transition metal dichalcogenide and CrI3 connected to a superconductor

The broad tunability of the proximity exchange effect between transition-metal dichalcogenides (TMDCs) and chromium iodide (CrI$_3$) heterostructures offers intriguing possibilities for the use of TMDCs in two-dimensional magnetoelectrics. In this work, the influence of the twist angle and the gate electric field on the electric and thermal transport in a TMDC/CrI$_3$ junction is investigated using the Dirac -Bogoliubov-de Gennes equation. We show that significant amounts can be controlled by spin-splitting of band structures due to spin-orbit interaction, and that the exchange-splitting of bands arises from the proximity effect. The property of the Andreev reflection (AR) process is highly dependent on the spin valley polarized states due to spin-orbit coupling. Remarkably, perfect spin valley polarized AR is possible over a wide bias range by using a gate voltage to tune the local Fermi energy and varying the type of charge doping. The proposed structure with $p$-type doping is found to have larger spin valley polarized Andreev conductance and high thermal conductance. We further show that, depending on the TMDC material and chemical potential of the TMDC/CrI$_3$ layer, twisting can lead to suppression or a significant increase in Andreev conductance as well as enhancement of thermal conductance for chemical potentials smaller than that of the superconducting regime.

Magneto-optic effects of monolayer transition metal dichalcogenides induced by ferrimagnetic proximity effect

Monolayer transition metal dichalcogenides upon the application of proximity exchange interactions have recently attracted great attention due to their abilities of producing the anomalous Hall effect and tunable spin polarized edge currents. However, the magnetic distribution of the magnetic exchange field mainly focuses on the ferromagnetic order. Here, we argue that the nonzero magneto-optical responses can be excited in monolayer transition metal dichalcogenides subject to magnetic exchange field with ferrimagnetic order, which are the hallmarks of magneto-optical materials. This unusual effect comes from the joint contribution of spin orbit coupling and the ferrimagnetic order of magnetic exchange field, whose broken time reversal symmetry is confirmed by the topology of energy spectra. Our results suggest an alternative pathway to control the magnetooptical effects of monolayer transition metal dichalcogenides, which might stimulate further theoretical and experimental interest.

Low-temperature quantum correction to anisotropic magnetoresistance in Tm3Fe5O12/Pt heterostructures

Spin transport in ferromagnetic insulator/heavy metal heterojunctions has attracted much attention because of its rich physics and potential applications. Here, we report an anomalous anisotropic magnetoresistance (AMR) at low temperature in a ${\mathrm{Tm}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}\text{/Pt}$ bilayer, which has often been neglected for its weak signal compared with spin Hall magnetoresistance. In our study, the magnetic proximity effect is weak above 50 K so that the AMR signal is mainly caused by ordinary magnetoresistance and spin-related scattering. However, at low temperatures, the AMR changes dramatically due to the weak localization (WL) and weak anti-localization (WAL) related to the quantum diffusion mechanism. Moreover, the dominated mechanism changes from WL to WAL in a Cu-inserted structure of TmIG/Cu/Pt without the magnetic proximity exchange coupling at the interfaces, accompanied by an AMR variation from negative to positive. Our study reveals the quantum correction effect on the AMR, which provides a possible explanation of the complex resistive behavior at low temperatures.

It Is All About Money! What Drives Interest Groups’ Relations with Political Parties in Central and Eastern Europe?

This article studies contacts between interest groups and political parties. Existing research suggests that times of close and formal cooperation between parties and groups in Western Europe are over as the contacts have become more pragmatic and sometimes spontaneous. Studies usually point to ideological proximity and resource exchange as the main factors behind contacts, however, focusing mainly on parties’ power and ideological position. Here, by drawing on data from Poland, Slovenia, and Lithuania, we focus on young democracies from Eastern Europe by taking into account interest groups’ resources, their typology, and the system of interest groups’ representation. The article shows that financial resources are the main factor behind seeking contacts with the large number of political parties.

Proximity spin-orbit and exchange coupling in ABA and ABC trilayer graphene van der Waals heterostructures

We investigate the proximity spin-orbit and exchange couplings in ABA and ABC trilayer graphene encapsulated within monolayers of semiconducting transition-metal dichalcogenides and the ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$. Employing first-principles calculations we obtain the electronic structures of the multilayer stacks and extract the relevant proximity-induced orbital and spin interaction parameters by fitting the low-energy bands to model Hamiltonians. We also demonstrate the tunability of the proximity effects by a transverse electric field. Using the model Hamiltonians we also study mixed spin-orbit/exchange coupling encapsulation, which allows to tailor the spin interactions very efficiently by the applied field. We also summarize the spin-orbit physics of bare ABA, ABC, and ABB trilayers, and provide, along with the first-principles results of the electronic band structures, density of states, spin splittings, and electric-field tunabilities of the bands, qualitative understanding of the observed behavior and realistic model parameters as a resource for model simulations of transport and correlation physics in trilayer graphene.

Engineering Proximity Exchange by Twisting: Reversal of Ferromagnetic and Emergence of Antiferromagnetic Dirac Bands in Graphene/Cr_{2}Ge_{2}Te_{6}.

We investigate the twist-angle and gate dependence of the proximity exchange coupling in twisted graphene on monolayer Cr_{2}Ge_{2}Te_{6} from first principles. The proximitized Dirac band dispersions of graphene are fitted to a model Hamiltonian, yielding effective sublattice-resolved proximity-induced exchange parameters (λ_{ex}^{A} and λ_{ex}^{B}) for a series of twist angles between 0° and 30°. For aligned layers (0° twist angle), the exchange coupling of graphene is the same on both sublattices, λ_{ex}^{A}≈λ_{ex}^{B}≈4 meV, while the coupling is reversed at 30° (with λ_{ex}^{A}≈λ_{ex}^{B}≈-4 meV). Remarkably, at 19.1° the induced exchange coupling becomes antiferromagnetic: λ_{ex}^{A}<0, λ_{ex}^{B}>0. Further tuning is provided by a transverse electric field and the interlayer distance. The predicted proximity magnetization reversal and emergence of an antiferromagnetic Dirac dispersion make twisted graphene/Cr_{2}Ge_{2}Te_{6} bilayers a versatile platform for realizing topological phases and for spintronics applications.

Majorana Zero Modes in Ferromagnetic Wires without Spin-Orbit Coupling

We present a novel controllable platform for engineering Majorana zero modes. The platform consists of a ferromagnetic metallic wire placed among conventional superconductors, which are in proximity to ferromagnetic insulators. We demonstrate that Majorana zero modes emerge localised at the edges of the ferromagnetic wire, due to the interplay of the applied supercurrents and the induced by proximity exchange fields with conventional superconductivity. Our mechanism does not rely on the pairing of helical fermions by combining conventional superconductivity with spin-orbit coupling, but rather exploits the misalignment between the magnetization of the ferromagnetic insulators and that of the ferromagnetic wire.

Giant spin-valve effect in Al/Y3Fe5O12 heterostructures

Superconducting phase transition in the aluminium stripes fabricated on yttrium-iron garnet and on oxidized silicon substrates is studied. Reduction of critical temperature, widening of superconducting transition depending upon the mutual current and magnetization orientations as well as on current strength are observed in aluminium on garnet comparing with aluminium on silicon. The proximity and triplet proximity effects, Andreev reflection, exchange interaction, spin-orbit coupling and self YIG magnetic field impacts on the observed effects are discussed. We show that only accounting for the spin-orbit coupling and self-magnetic field of YIG is allowed to explain the observed phenomenon of asymmetric change in the transition under the variation of the current and magnetization alignment. Based on the obtained results, we suggest a new geometry of spin-valve with hundreds percent variation of the resistance controlled by the current.

Bilayer graphene encapsulated within monolayers of WS2 or Cr2Ge2Te6 : Tunable proximity spin-orbit or exchange coupling

Van der Waals (vdW) heterostructures consisting of bilayer graphene (BLG) encapsulated within monolayers of strong spin-orbit semiconductor WS$_2$ or ferromagnetic semiconductor Cr$_2$Ge$_2$Te$_6$ (CGT), are investigated. By performing realistic first-principles calculations we capture the essential BLG band structure features, including layer- and sublattice-resolved proximity spin-orbit or exchange couplings. For different relative twist angles (0 or 60$^{\circ}$) of the WS$_2$ layers, and the magnetizations (parallel or antiparallel) of the CGT layers, with respect to BLG, the low energy bands are found and characterized by a series of fit parameters of model Hamiltonians. These effective models are then employed to investigate the tunability of the relevant energy dispersions by a gate field. For WS$_2$/BLG/WS$_2$ encapsulation we find that twisting allows to turn off the spin splittings away from the $K$ points, due to opposite proximity-induced valley-Zeeman couplings in the two sheets of BLG. Close to the $K$ points the electron spins are polarized out of the plane. This polarization can be flipped by applying a gate field. As for magnetic CGT/BLG/CGT structures, we realize the recently proposed spin-valve effect, whereby a gap opens for antiparallel magnetizations of the CGT layers. Furthermore, we find that for the antiferromagnetic orientation the electron states away from $K$ have vanishingly weak proximity exchange, while the states close to $K$ remain spin polarized in the presence of an electric field. The induced magnetization can be flipped by changing the gate field. These findings should be useful for spin transport, spin filtering, and spin relaxation anisotropy studies of BLG-based vdW heterostructures.