Revealing Hidden Spin Polarization in Centrosymmetric van der Waals Materials on Ultrafast Timescales

: One of the key challenges for spintronic and novel quantum technologies is to achieve active control of the spin angular momentum of electrons in nanoscale materials on ultrafast, femtosecond timescales. While conventional ferromagnetic materials and materials supporting spin texture suffer both from conceptional limitations in miniaturization and in efficiency of optical and electronic manipulation, non-magnetic centrosymmetric layered materials with hidden spin polarization may offer an alternative pathway to manipulate the spin degree of freedom by external stimuli. Here we demonstrate a novel approach to generate transient spin polarization on a femtosecond timescale in the otherwise spin-unpolarized band structure of the centrosymmetric 2H-stacked group VI transition metal dichalcogenide WSe 2 . Using ultrafast optical excitation of a fullerene layer grown on top of WSe 2 , we trigger an ultrafast interlayer electron transfer from the fullerene layer into the WSe 2 crystal. The resulting transient charging of the C 60 /WSe 2 interface leads to a substantial interfacial electric field that by means of spin-layer-valley locking ultimately creates ultrafast spin polarization without the need of an external magnetic field. Our findings hence open a novel pathway for optically engineering spin functionalities such as the sub-picosecond generation and manipulation of ultrafast spin currents in 2D heterostructures.

Fundamental to the advance of spintronics and the creation of novel quantum functionalities in solids is the ability to encode, manipulate and store information onto the spin angular momentum of electrons with high efficiency and low volatility [1,2].Ferromagnetic materials have long been the natural driving target for these efforts.However, their intrinsic limitations regarding miniaturization (i.e., governed by the super-paramagnetic limit) and their susceptibility to external stray fields has triggered a search for non-magnetic materials that can nevertheless support advanced spin functionalities.
One highly promising alternative to ferromagnets are non-magnetic bulk materials with broken symmetries and strong spin-orbit coupling.The combination of both properties results in the spin-splitting of bands in momentum space either through the Dresselhaus effect [3] or the bulk Rashba effect [4][5][6], and leads to intriguing spin functionalities such as the interconversion of charge and spin [7][8][9].Unfortunately, while the resulting spin-momentum locking and associated spin texture does indeed enable controlling the spin degree of freedom, it also limits the type of spin operations that can be realized in such materials: For instance, an unpolarized charge current can only be converted into a transverse spin current by the spin Hall effect [10].In addition, electric and optical gating, needed for fast operations, can only manipulate the magnitude of the momentum-dependent spin splitting, and both are limited to the picosecond timescale due to the intrinsic buildup time of the photovoltage [11,12].These fundamental limitations underline the need for new paradigms to tailor and manipulate the spin degrees of freedom, ideally by directly creating and manipulating spin polarization rather than spin texture.
In this regard, the discovery of the so-called hidden spin polarization in non-magnetic materials with centrosymmetric crystal symmetry suggests a pathway toward realizing spin manipulation in a much larger class of materials [13][14][15].Hidden spin polarizations emerge in centrosymmetric layered structures containing subunits with broken inversion symmetry.Typical examples are, for instance, 2H-stacked group VI transition metal dichalcogenides (TMDs), of which one of the most prominent example 2H-WSe2 is the focus of the present study.A cartoon of the salient features of the spin-and layer-dependent valence band structure of this material is shown in Fig. 1a: It is characterized by spin-split valence bands, localized within each individual layer of the 2H-stacked structure [14,[16][17][18], and whose spin is reversed between the valleys at the high symmetry point K and its time-reversal couple K'.Inversion symmetry of the full bulk unit cell, which contains two layers, leads to an inversion of the valence band spin polarization at each high symmetry point in successive layers, resulting as expected in an overall spindegenerate bulk band structure.If however the inversion symmetry in otherwise centrosymmetric 2H-WSe2 can be broken between two adjacent layers, e.g. by addressing individual layers differentially, then the emergence of previously hidden spin polarization may be expected, enabling manipulation of spin degrees of freedom without magnetic fields and potentially on ultrafast timescales.
In this work and by using spin-and time-resolved angle-resolved photoemission spectroscopy (ARPES), we overcome this challenge for the first time and demonstrate a new approach to generate transient spin polarization by lifting the spin degeneracy of the bulk band structure at the interface of a C60/2H-WSe2 heterostructure.Using ultrafast optical excitation, we are able to generate large interfacial electric fields that ultimately result in ultrafast spin polarization.Conceptionally, this scheme is based on the coupled spin, spin-like valley, and layer pseudospin degrees of freedom that characterize the hidden spin polarization of 2H-WSe2 in the valence band and near the K-points, as expressed by the Hamiltonian [16]: Here, the first term describes the coupling between the spin (  ), valley-pseudospin (  ), and layer-pseudospin (   ) degrees of freedom mediated by spin-orbit coupling   (SOC), and the second term describes the coupling of the weak interlayer hopping ( ⊥ ) in WSe2 to the layer pseudospin.Importantly, carrier population in a specific layer represents an interlayer electronic polarization, and hence the layer pseudospin can be considered as an electrical polarizability that can mediate interactions between this spin-like quantity and an external, transient electric field via the Hamiltonian (1) [17][18][19].
In order to generate the layer-dependent ultrafast electric field, we take advantage of the unique properties of our hybrid organic/inorganic heterostructure by driving interfacial charge-transfer from C60 to WSe2 (Fig. 1b).The resulting transient band structure engineering by interfacial electric fields presents the first key step towards ultrafast generation of hole-like spin currents at the interface of TMD bulk materials by fs light excitation, without the need for large external magnetic fields, time-reversal or structural inversion symmetry breaking.Our conclusions are enabled by multi-dimensional photoemission spectroscopy of a C60/WSe2 heterostructure with an ultrathin C60 layer that allows us to directly access and uncover transient changes of the hidden spin polarization after optical excitation.In this way, we demonstrate that we are able for the first time to trace both the excited state and spin-dependent band structure dynamics at this hybrid heterointerface on the fs timescale.
Our sample consists of an in situ prepared surface of a 2H-WSe2 bulk crystal covered with approx.0.8 ML of C60 (see method section for more details).The energy level alignment of the valence band structure of the C60/WSe2 heterostructure prior to ultrafast excitation can be deduced from the momentum-resolved photoemission map in Fig. 1c, recorded along the Г-Σ-K high symmetry direction and shown in the vicinity of the K-point.The spin-split valence bands of WSe2 appear as hole-like parabolic features at the K-point with an energy splitting of 450 meV, similar to the bare WSe2 surface (see SI and Ref. [14]), indicative of physisorptive interactions at the C60/WSe2 interface.The non-dispersive feature at E-EVB = 1.3 eV is attributed to the C60 valence state, i.e. the highest occupied molecular orbital (HOMO), and reflects the large ionization energy of C60 [20,21].The spin-resolved photoemission yield of the valence band structure is shown on the right for a selected electron momentum (indicated by a white vertical line in Fig. 1c).The red curve corresponds to the yield of spin-up electrons (out-of-plane spin direction), and the blue curve to the yield of spin-down electrons.The C60 HOMO (H) is not spin-polarized, as expected for molecular films on non-magnetic surfaces.In contrast, we find strong spin polarization for both SOC-split WSe2 valence bands (VB1 and VB2).Though bulk WSe2 does not support a spin-split density of states [13], the layer-dependent hidden spin polarization of inversion-symmetric bulk WSe2 is made apparent by the extreme surface sensitivity of the photoemission process [14].In ARPES, we primarily probe the top WSe2 layer, and hence the photoemission yield at VB1 carries mostly spin-up electrons (green curve/area) near K, while the smaller signal in the spin-down channel (blue curve/area) stems from the inverted spin-polarization of VB1 of the second layer.The opposite is true for the lower valence band VB2.Key to these observations is the fact that our spin-and momentum-resolved photoemission experiment also provides layer-sensitivity.This allows us to disentangle ultrafast momentum-, spin-and layer-dependent band structure changes in the C60/WSe2 heterostructure following optical excitation.
Optical excitation of the heterostructure with 3.2 eV sub-50 fs pulses creates a transient electric field across the C60/WSe2 interface: At this energy, the CT2 state of C60 is excited (see energy level alignment diagram in Fig. 2a), supported by previous studies of the two materials [19,22,23].This state is associated with the formation of intermolecular charge transfer excitons.Using time-and momentum-resolved photoemission, we follow the ultrafast chargecarrier dynamics subsequent to excitation at 3.2 eV.Example energy vs. momentum cuts from recently identified as a spectroscopic signature of charge-transfer excitons in molecular films [22,23].Crucially, we only observe a marginal depletion of the WSe2 valence states (see SI).This proves that the formation of charge-transfer excitons in the C60 layer is indeed the dominant optical excitation path and that direct excitation of WSe2 [24] does not play a dominant role here.
This broad electron distribution (t = 0 fs) evolves to populating the K-and Σ-valley of the WSe2 conduction band (t = 340 fs), clearly indicating ultrafast electron-transfer from C60 to WSe2.
To gain a more quantitative understanding of the interlayer and intervalley scattering processes at the C60/WSe2 interface, we model the changes in electron and hole populations using a rateequation model (see SI).We extract the transient electron population at the K-and Σ-valley of the WSe2 conduction band and the hole population of the C60 HOMO by analysis of the ARPES times larger than previously reported for bare WSe2 [19].We believe that this can be attributed to a sample temperature of approx.40 K, much lower than in the previous report and causing significantly reduced electron-phonon scattering [27].A detailed analysis of the hole population dynamics in Fig. 2c reveals a clear persistence of holes in the C60 layer.Detailed analysis using exponential fit functions (discussed in Ref. [22]) shows instantaneous depletion of the C60 HOMO within our experimental resolution, as expected from resonant excitation, followed by decay of the hole population in a two-step process with a fast recovery time constant of 730±50 fs and a significantly slower second time constant of approx.7.5 ps.
Summarized in Figs.3a and b, the key processes involve ultrafast interfacial charge-transfer from C60 to WSe2, resulting in a hole located on C60 and an electron in the K-valley of the top layer of WSe2.This is followed by intervalley scattering to Σ, whose electron density spans both the first and second layer of WSe2 [19].This charge separation between C60 and WSe2 establishes a strong and transient interfacial electric field along the surface normal.As we show below, this field is ultimately responsible for revealing the hidden broken inversion-symmetry in WSe2.Eventually, the photoexcited electrons delocalize into the bulk WSe2 crystal and the interfacial electric field decays.
We next discuss the influence of the transient electric field on the interfacial energy level alignment.As can be seen in Fig. 3c  Initially, upon optical excitation (t = 0 fs), no changes in the energy level alignment and the spin polarization are observed for any of the WSe2 valence bands, and all spectral changes can be attributed to an instantaneous inhomogeneous linewidth broadening caused by the formation of the C60-based CT2 exciton.However, once interlayer charge transfer takes place and the chargeseparated state is created (t = 950 fs), the WSe2 valence bands shift.In both spin channels, the valence bands of the first layer (green Gaussian curves) transiently shift rigidly towards larger binding energies by 50±20 meV, while the valence bands of the second layer reveal only a minor shift of 20±20 meV.Thus, interlayer charge transfer at the C60/WSe2 heterointerface modifies the band structure in a layer-dependent fashion.Crucially, the electric field gradient within the first two WSe2 layers is strong enough to lead to a sizeable relative shift of the spin-polarized bands of the first vs. the second WSe2 layer, thus creating a transient ferromagnetic-like spin polarization in the WSe2 valence bands by revealing the hidden spin polarization in the surface region of the bulk crystal on ultrafast timescales.
In conclusion, our work has demonstrated a novel approach to transiently engineer the spinpolarized valence band structure in the otherwise spin-degenerate layered bulk material 2H-WSe2.Specifically, the ultrafast electron transfer from an optically excited C60 layer grown on top of WSe2 leads to a layer-dependent shift of the spin-valley-layer locked WSe2 valence band structure that ultimately reveals the hidden spin polarization of the system on a femtosecond timescale.Our optical manipulation scheme for generating a ferromagnetic-like spin polarization in the valence band without an external magnetic field constitutes not only an avenue for optically engineering new spin functionalities, such as the generation of spin-polarized hole currents in WSe2, on ultrafast, sub-picosecond timescales, but also opens the intriguing possibility for exploiting and manipulating the orbital degree of freedom of layered TMDs thus paving the way for pushing the emergent field of orbitronics [33] towards ultrafast timescales.

Methods:
Sample preparation: All sample preparation and measurement steps were performed under ultrahigh vacuum (UHV) conditions.The WSe2 single crystals were obtained from HQ graphene and cleaved prior to the experiments resulting in a clean and flat surface.C60 molecules were evaporated onto the surface at a pressure <10 -8 mbar using a Knudsen-type evaporation source (Kentax GmbH).The molecular flux was calibrated using a quartz crystal oscillator gauge and the molecular coverage was estimated using the integrated intensity signal of the highest occupied molecular orbital of C60 as a reference.

Spin-and time-resolved angle-resolved photoemission spectroscopy (ARPES):
The multidimensional photoemission experiments were conducted with a hemispherical analyzer (SPECS Phoibos 150) that is equipped with both a CCD detector system and the commercial spin detector (Focus FERRUM [34]) that is mounted in a 90° geometry after the hemispherical analyzer's exit slit plane.All spin-resolved photoemission data were recorded for the out-ofplane spin component, i.e., the spin component parallel to the optical axis of the analyzer lens optics.The spin sensitivity or Sherman function (S) of this very-low-energy electron diffraction (VLEED) detector was determined to be 0.29 for the out-of-plane spin component.
As excitation sources, we used the monochromatic He Iα radiation (21.2 eV, Scienta VUV5k) of a high-flux He discharge source as well as a pulsed femtosecond extreme ultraviolet (fs-XUV) light source.The fs-XUV radiation (22.2 eV, horizontal (p) polarization) was obtained by high harmonic generation (HHG) using the second harmonic (390 nm) of a titanium sapphire laser amplifier system (repetition rate 10 kHz, pulse duration < 40 fs) to drive the HHG process [35].
The optical excitation of the organic material was also performed with the second harmonic of the amplifier system (3.17 ± 0.04 eV, bandwidth 80 meV, horizontal (p) polarization).Prior to each time-resolved experiment, the spatial overlap between the pump and the probe pulse was optimized directly on the sample plate, which was placed at the focus position of the analyzer.
The spatial overlap was actively stabilized during the experiment to correct for spatial drift of the pump and probe beams.This is achieved by constantly monitoring the beam position of the fundamental laser beam at two well-defined positions in the laser beamline using two CCD cameras.Any lateral draft of the laser beam is compensated by two motorized mirrors installed in the beamline.All time-resolved photoemission experiments were conducted in normal incidence geometry and an emission angle of approx.45°.A detailed description of the data analysis procedure can be found in the Supplementary Information.

B. Electrostatic Model Simulations
We used a simple electrostatic model to determine the magnitude and direction of the transient electric fields due to charge transfer at the interface of C60 and WSe2.Beyond a minor static interfacial dipole, an additional electrostatic potential step emerges as a result of interfacial charge transfer from C60 to WSe2.We model the magnitude of this transient electrostatic potential by an array of physical dipole moments  per area, , by the Helmholtz equation: where  0 is the vacuum permittivity, and  e the charge separated by distance, .By considering a finite array of dipole moments made up of point charges, we model the electrostatic potential of this interface.
From STM data of C60 on WSe2, the reported height of C60 on WSe2 is approximately 1 nm, and the radius of C60 is 7.1 Å. [5] We assume that the holes, or array of positive charges, are localized at the bottom of the C60 cage, while the electrons, or negative charges, are confined to the topmost layer of WSe2, specifically the top-most Se-atom.Due to the van der Waals nature of interlayer binding in WSe2, we do not expect lower layers in WSe2 to participate significantly in the formation of the electrostatic field.The resulting charge separation distance of 2.9 Å establishes electrostatic potentials that qualitatively reproduce the observed maximum transient energy shifts.In what follows, we discuss all aspects of our model and built-in assumptions.
We estimate the excitation densities from the transient depopulation of the HOMO level of C60 at key time-steps.Optical excitation of C60 with 3.2 eV sub-50 fs pulses resonantly excites electrons from the HOMO level into the LUMO+1* level, leading to the formation of a charge transfer exciton in the C60 layer, the CT2 state.[6] The HOMO feature shows an instantaneous intensity reduction of approximately 35%.At the high fluences used in our experiments and based on the photoemission cross section of the C60 HOMO bands for s-polarized light [7], we estimate an excitation efficiency of about 80% of all C60 in the thin film.
Based on the high density of electron-hole pairs upon interfacial charge transfer, additional effects need to be taken into account for estimating the resulting potentials.Blumenfeld et al.
showed previously for thin films of molecules supporting a permanent dipole moment that at high enough dipole densities depolarization effects strongly impact the interfacial electrostatics [8].Depolarization results from the fact that each dipole moment in the array induces a dipole moment in the surrounding molecules that is aligned in the opposite direction, thus reducing the transient dipole moment.Following their model, the effective transient dipole moment,  �  including depolarization effects can thus be expressed as: where  dip is the density of transient dipoles,  is a geometric factor capturing the geometric arrangement of dipole moments on the surface and known as the Topping constant [9], and  �  is the zz-component of the effective polarizability tensor.We estimate the latter based on the known static polarizability of C60   = 8 • 10 −29 m 3 [10], and consider screening effects by both sides of the interface by including their respective dielectric components into the simulation.The static dielectric constant of C60 is  C60 = 4.5 and the bulk out-of-plane component of the static dielectric constant for WSe2 is  WSe2 = 7.8 [10,11].This yields the zzcomponent of the effective polarizability tensor  �  : �  =   4 0  avg where   is the average dielectric constant of the two materials.By way of comparison, we estimate an induced dipole moment of approximately 13.9 D for a charge separation of 2.9 Å.
The electrostatic potential as a function of height above the surface is shown in Figure S8 for different fractions of excited C60 molecules.From the electrostatic simulations, we identify two regimes of the electrostatic surface potential (near-and far-field, highlighted in blue and grey, respectively).At approximately 3.3 Å, we observe an isosbestic-like point where the far-field effects begin to dominate.In the far-field regime, the electrostatic potential above the dipole array decays rapidly, converging to a constant potential step, representing a transient change of the global work function of the system as a result of charge-separation.As expected and given the sign of the dipole moment, an increase in excitation density (charge density) on the surface decreases the electrostatic potential and therefore the work function.
The near-field potential, or the potential "inside" the physical dipole, gives rise to an electrostatic potential whose behavior contrasts with the far-field potential: Due to depolarization, the electrostatic potential decreases with increasing charge densities.The electrostatic potential in the near-field regime explains therefore both signs and magnitude of the Stark shifts observed in the C60 HOMO and WSe2 valence band: At lower charge densities, the magnitude of the electrostatic potential close to the dipole array increases.

Fig. 1 :
Fig. 1: Electronic valence band structure of the C60/WSe2 heterostructure.(a) Sketch of the local layer-and spin-dependent band structure of the two non-interacting WSe2 layers of the bulk unit cell in which the spin polarization vanishes at every point in the Brillouin zone.(b) Illustration of the optical manipulation scheme for uncovering the hidden spin polarization of WSe2.An ultrashort 3.2 eV laser pulse resonantly excites the ultrathin C60 layer grown on top of WSe2 leading to an ultrafast electron transfer into the first WSe2 layer and to a transient E-field across the C60/WSe2 interface.(c) Energy vs. momentum photoemission map of the C60/WSe2 heterostructure along the Σ-K-direction (He Iα radiation).It shows the spin split WSe2 valence bands with their hole-like dispersion (VB1, VB2) and the dispersion-less HOMO (H) of C60.The right side of (c) shows the spin-resolved photoemission yield (out-of-plane spin component) of the valence band structure obtained at a selected electron momentum (see white dashed line).The red and blue curves represent the fit to the spin-up and spin-down spectrum, respectively.The contributions of the first and second layer valence bands to the spectral yield are fitted and illustrated as green and blue Gaussian curves underneath the spectra.

Fig. 2 :
Fig. 2: Ultrafast electron and hole dynamics (a) Energy level diagram of the electronic band structures of the C60/WSe2 heterostructure.The blue arrow indicates the dominant optical transition of the 3.2 eV excitation.(b) Energy vs. momentum photoemission maps at selected pump-probe delays obtained with linearly polarized pump pulses (fluence F = 0.5 mJ/cm 2 ).The excited state region (E-EVB > 0 eV) is shown as a difference map, the valence band region as an electron intensity map (see colormaps).The energy and momentum positions of the molecular CT2 state and the WSe2 valence band are superimposed onto the experimental data (1300 fs) as gray and black curve.(c) Temporal evolution of the WSe2 excited state and C60 HOMO intensity evolution.The solid lines superimposed onto the population dynamics at the K-and Σ-point (PopK and PopΣ) were obtained by a rate equation model.The key scattering processes of this model are illustrated in (d) together with the scattering times of the best fit to the data.The temporal evolution of the HOMO is modelled with a double exponential fit function.

Fig. 3 :
Fig. 3: Charge separation, interfacial E-field, and transient changes in the energy level alignment.(a) Sketch of the real space electron densities (red shaded areas) of the wave functions at the Kto the Σ-valley of the WSe2 conduction band (adapted from Bertoni et al. [19]).(b) Illustration of the charge separation process at the C60/WSe2 interface.After the ultrafast electron transfer from the C60 CT2 state into the WSe2 K-valley, the electrons are confined to the first WSe2 layer.Only the intervalley scattering form the K-into the Σ-valley leads to a delocalization of the electrons in WSe2.(c) Temporal evolution of the valence band shifts of the WSe2 (VB) and the C60 (HOMO) valence states.The dynamics of the energy shifts was analyzed with exponential functions.(d) Electrostatic model estimating the transient valence band shifts.

Fig. 4 :
Fig. 4: Ultrafast changes of the hidden spin polarization of the WSe2 bulk band structure.(a) Time-and spin-resolved photoemission yield (out-of-plane spin component) of the valence band structure (see white dashed line) at three characteristic time delays.The data were recorded at the same electron momentum as the static data in Fig. 1c.The red and blue curve represents the fit to the spin-up and spin-down spectrum, respectively.The contributions of the first and second layer valence bands to the spectral yield are fitted and illustrated as green and blue Gaussian curves underneath the spectra.The vertical solid lines indicate the significantly larger shift of the valence band of the first WSe2 layer compared to the second layer.(b) energy level diagram illustrating the ultrafast changes of the layer-and spin-dependent WSe2 valence band structure after optical excitation with 3.2 eV photons.