Research
Moiré magnetism
2D magnet chromium triiodide (CrI3) exhibits unique stacking-dependent interlayer exchange interactions. Changing the layer stacking from monoclinic to rhombohedral can switch the interlayer magnetic coupling, and thus the magnetic ground state, from antiferromagnetic (AFM) to ferromagnetic (FM). In twisted CrI3, using magneto-optics and single-spin quantum magnetometry, we observe the coexistence of AFM and FM domains with periodic patterns, revealing moiré magnetism in magnetic moiré superlattices.
T. Song et al., Science (2021).
T. Song et al., Nature Materials (2019).
Q.-C. Sun*, T. Song* et al., Nat. Commun. (2021).
Van der Waals magnetic tunnel junctions
Atomically thin CrI3 exhibits unique layer-dependent magnetism enabled by the antiferromagnetic interlayer coupling. The resulting layered-antiferromagnetism gives rise to the spin-filtering effect because the electron tunneling process is spin-conserving. In the vdW magnetic tunnel junctions, atomically thin CrI3 acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We observe giant tunneling magnetoresistance that is drastically enhanced with increasing CrI3 layer thickness, reaching a record high for magnetic multilayer structures at low temperatures.
T. Song et al., Science (2018).
X. Cai, T. Song et al., Nano Letters (2019).
2D superconductivity and topology
Monolayer tungsten ditelluride (WTe2) is so exotic that it hosts superconductivity and nontrivial topology in a single crystalline material. 2D superconductivity occurs at an unusually low carrier density that can readily be induced through electrostatic doping. It undergoes superconductor-insulator transition, and becomes quantum spin Hall insulator at charge neutrality, which survives up to 100 K. With the increasingly advanced nanofabrication techniques, quantum devices based on WTe2 and 2D magnets provide an exciting platform for exploring the interplay between topology, correlations, and magnetism, as well as engineering new quantum phases in 2D systems.
T. Song et al., Nature Physics (2024).
W. Zhao, Z. Fei, T. Song et al., Nature Materials (2020).
2D magnetism
Theoretically, magnetic order is prohibited in the two-dimensional isotropic Heisenberg model at finite temperatures by the Mermin–Wagner theorem. However, this restriction can be removed by magnetic anisotropy, which can enable, for instance, the occurrence of two-dimensional Ising ferromagnetism. We develop and employ several experimental techniques to probe 2D magnetism, for example, electron tunneling, magneto-optical Kerr effect, reflective magnetic circular dichroism, and single-spin quantum magnetometry.
B. Huang et al., Nature (2017).
T. Song et al., Science (2018).
Z. Sun*, Y. Yi*, T. Song et al., Nature (2019).
Q.-C. Sun*, T. Song* et al., Nat. Commun. (2021).
C.-C. Tseng*, T. Song* et al., Nano Letters (2022).
Spectroscopy and microscopy
We employ versatile spectroscopy and microscopy measurements (home-built optical setups) to investigate 2D materials and their vdW heterostructures. For example, polarization-resolved Raman and magneto-optical Raman spectroscopy, circular polarization-resolved reflection and absorption spectroscopy, polarization-resolved second-harmonic generation, magneto-photoluminescence spectroscopy, scanning magnetic circular dichroism and scanning photocurrent microscopy.
T. Song et al., Nature Materials (2019).
T. Song et al., Science Advances (2021).
T. Song et al., Nano Letters (2019).
Pressure tunning of interlayer magnetism
The physical properties of two-dimensional van der Waals crystals can be sensitive to interlayer coupling. We demonstrate pressure tuning of interlayer magnetism in 2D magnet CrI3. The magnetic states are probed by using electron tunneling and scanning magnetic circular dichroism microscopy measurements. The interlayer magnetic coupling can be more than doubled by applying hydrostatic pressure. In bilayer CrI3, pressure induces a transition from layered antiferromagnetic to ferromagnetic phase. In trilayer CrI3, pressure can create coexisting domains of three phases, one ferromagnetic and two antiferromagnetic. The observed changes in magnetic order can be explained by changes in the stacking arrangement.
T. Song et al., Nature Materials (2019).
Spin photovoltaic effect
With the recent discovery of 2D magnets, the control of the spin degree of freedom can be integrated to realize 2D spin-optoelectronics. We report spin photovoltaic effects in vdW heterostructures of 2D magnet CrI3 sandwiched by graphene contacts. The photocurrent displays a distinct dependence on light helicity, which can be tuned by varying the magnetic states and photon energy. Circular polarization-resolved absorption measurements reveal that these observations originate from magnetic order–coupled and, thus, helicity-dependent charge-transfer excitons. The photocurrent displays multiple plateaus as the magnetic field is swept, associated with different CrI3 spin configurations
T. Song et al., Science Advances (2021).
Voltage control of magnetic states
Voltage-controlled switching between bistable magnetic states can be employed in energy efficient magnetic memory and logic technologies. We demonstrate electrical control of tunneling magnetoresistance formed by four-layer CrI3 sandwiched by monolayer graphene contacts in a dual-gated structure. By varying the gate voltages at fixed magnetic field, the device can be switched reversibly between bistable magnetic states with the same net magnetization but drastically different resistance (by a factor of 10 or more). This work demonstrates new kinds of magnetically moderated transistor action and opens up possibilities for voltage-controlled vdW spintronic devices.
T. Song et al., Nano Letters (2019).