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Asymmetric magnetic proximity interactions in MoSe2/CrBr3 van der Waals heterostructures

Nature Materials volume 22, pages 305–310 (2023)Cite this article

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Abstract

Magnetic proximity interactions between atomically thin semiconductors and two-dimensional magnets provide a means to manipulate spin and valley degrees of freedom in non-magnetic monolayers, without using applied magnetic fields1,2,3. In such van der Waals heterostructures, magnetic proximity interactions originate in the nanometre-scale coupling between spin-dependent electronic wavefunctions in the two materials, and typically their overall effect is regarded as an effective magnetic field acting on the semiconductor monolayer4,5,6,7,8. Here we demonstrate that magnetic proximity interactions in van der Waals heterostructures can in fact be markedly asymmetric. Valley-resolved reflection spectroscopy of MoSe2/CrBr3 van der Waals structures reveals strikingly different energy shifts in the K and K′ valleys of the MoSe2 due to ferromagnetism in the CrBr3 layer. Density functional calculations indicate that valley-asymmetric magnetic proximity interactions depend sensitively on the spin-dependent hybridization of overlapping bands and as such are likely a general feature of hybrid van der Waals structures. These studies suggest routes to control specific spin and valley states in monolayer semiconductors9,10.

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Fig. 1: Strong MPIs in a MoSe2/CrBr3 vdW heterostructure.
Fig. 2: Spatially imaging MPIs in a MoSe2/CrBr3 heterostructure.
Fig. 3: Asymmetric MPIs in MoSe2/CrBr3 heterostructures.
Fig. 4: Calculated electronic band dispersion of MoSe2/CrBr3 and asymmetric valley shifts.

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All data are available from the corresponding author upon reasonable request.

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Acknowledgements

We gratefully acknowledge I. Žutić and B. Urbaszek for helpful discussions. Experimental studies at the National High Magnetic Field Laboratory were supported by the Los Alamos Laboratory Directed Research and Development programme (J.C. and S.A.C.). The National High Magnetic Field Laboratory is supported by National Science Foundation DMR-1644779, the State of Florida and the US Department of Energy. Computational studies were supported in part by the Center for Integrated Nanotechnologies, a US Department of Energy Basic Energy Sciences user facility, in partnership with the Los Alamos National Laboratory Institutional Computing Program for computational resources (C.L. and J.-X.Z). Additional computations were performed at the National Energy Research Scientific Computing Center, a US Department of Energy Office of Science user facility located at Lawrence Berkeley National Laboratory, operated under contract no. DE-AC02-05CH11231 using National Energy Research Scientific Computing Center award ERCAP0020494.

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Authors and Affiliations

  1. National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM, USA

    Junho Choi & Scott A. Crooker

  2. Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM, USA

    Christopher Lane & Jian-Xin Zhu

  3. Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, NM, USA

    Christopher Lane & Jian-Xin Zhu

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  1. Junho Choi
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Contributions

J.C. and S.A.C. conceived the project. J.C. prepared the samples and performed the optical experiments. C.L. and J.-X.Z. performed the DFT calculations. All authors contributed to writing the manuscript.

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Correspondence to Scott A. Crooker.

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Extended data

Extended Data Fig. 1 Additional MCD data and MCD spectra.

a, Hysteresis loops of the MCD signal from the MoSe2/CrBr3 heterostructure, acquired at the B-exciton transition of the MoSe2 monolayer (photon energy = 1.83 eV; cf. Fig. 1e of the main text). b, MCD spectra of an isolated ‘control’ MoSe2 monolayer, compared with the MCD spectra of the MoSe2/CrBr3 heterostructure, at 4 K. Each spectrum was obtained after the CrBr3 was magnetized by applied magnetic fields (cf. Fig. 1d of the main text).

Extended Data Fig. 2 MCD images of magnetic proximity effects on a different MoSe2/CrBr3 heterostructure.

The CrBr3 thickness is 7 monolayers. a, Optical microscope image of the vdW structure. b, Magnetic hysteresis of the MCD signal, using light tuned to the A-exciton transition of the MoSe2 monolayer. c, MCD images acquired at 4 K, using probe light tuned to the peak of the A-exciton MCD resonance (photon energy = 1.64 eV), at selected applied magnetic fields Bz. The MoSe2/CrBr3 heterostructure region is indicated by the black dashed lines, and red and blue colors reveal magnetic domains oriented along \(+\hat{z}\) and \(-\hat{z}\), respectively. Magnetic domains in CrBr3 have been studied using a variety of techniques over the past several decades; see for example44,45.

Extended Data Fig. 3 Reflection spectra from control sample.

Reflection spectra from an isolated ‘control’ MoSe2 monolayer (raw data), showing that the energy of the exciton resonance shifts negligibly at the low temperatures (T≤30 K) considered in this work. Black and red traces were acquired at 6 K and 30 K, respectively.

Extended Data Fig. 4 Total (not valley-resolved) MPI-induced valley splitting.

a, The energy difference between the K and \(K^{\prime}\) A-exciton resonance energies shown in Fig. 3c of the main text. b, The energy difference between the K and \(K^{\prime}\) B-exciton resonance energies shown in Fig. 3f of the main text. These plots are shown for completeness – as the main text emphasizes, plotting energy differences obscure the marked valley asymmetry that arises due to MPIs. Error bars represent 95% confidence intervals on the fitting of the resonance energy.

Extended Data Fig. 5 Calculated electronic band structures.

Theoretical electronic band structures for various CrBr3 magnetic configurations. The unfolded electronic band structure of the MoSe2/CrBr3 heterostructure for CrBr3 ferromagnetically polarized along \(+\hat{z}\) (a-b), \(-\hat{z}\) (c-d), and in the planar-antiferromagnetic phase (e-f). Top and bottom panels show the MoSe2 and CrBr3 layer projections, respectively. The sizes of the red (blue) dots are proportional to the fractional weights of the spin-up (-down) MoSe2 and CrBr3 layer projections, respectively. In the first two columns, the CrBr3 conduction states clearly cut through the MoSe2 unoccupied bands, thereby generating substantial level mixing and repulsion. The hybridization between these sets of bands is marked by the pronounced ‘shadow’ of CrBr3 bands in the MoSe2 projected states (a,c). In the right-most column (panels e,f), minimal band mixing between the layers is observed. Since the chromium magnetic moments lie in the plane, the electron overlap integrals connecting the CrBr3 layer and the spin-polarized bands in MoSe2 are substantially reduced. Furthermore, since there is no net magnetic moment in the planar-antiferromagnetic phase, the valley degeneracy is preserved.

Extended Data Fig. 6 Crystal structure of MoSe2/CrBr3 for various stacking arrangements.

The MoSe2/CrBr3 bilayer structure viewed from the top for AA, AB and AC stacking configurations. The green, violet, orange and blue spheres denote the selenium, molybdenum, bromine and chromium, respectively. The black line indicates the unit cell boundary.

Extended Data Fig. 7 Calculated valley shifts vs. U for AA stacking.

Calculated shifts of the optical transition energies in the K and \({K}^{{\prime} }\) valleys as a function of U for AA stacked MoSe2/CrBr3. Dashed horizontal lines show experimentally measured values. As the effective Hubbard U on the chromium atomic sites is increased from 0 to 2 eV the optical transition energies in the K and \({K}^{{\prime} }\) valleys (solid lines with dots) display a non-monotonic evolution for both \(+\hat{z}\) and \(-\hat{z}\) chromium spin polarizations. Since the relative band alignment between CrBr3 and MoSe2 states changes with increased U, the resulting resonant avoided crossing phenomena produces a blue or red shift in the optical transition energies. A U = 0.9 eV (denoted by vertical dotted line) is found to simultaneously reproduce the experimental band gap of CrBr3 and qualitatively capture the 1−3 meV magnitude of the asymmetric exciton shifts in the two valleys.

Extended Data Fig. 8 Calculated valley shifts vs. U for AB stacking.

Same as Extended Data Fig. 7, except for AB stacked MoSe2/CrBr3.

Extended Data Fig. 9 Calculated valley shifts vs. U for AC stacking.

Same as Extended Data Fig. 7, except for AC stacked MoSe2/CrBr3.

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Choi, J., Lane, C., Zhu, JX. et al. Asymmetric magnetic proximity interactions in MoSe2/CrBr3 van der Waals heterostructures. Nat. Mater. 22, 305–310 (2023). https://doi.org/10.1038/s41563-022-01424-w

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