2020 Abstracts 2D

Kenneth Burch

 

Precise control of electronic charge at the nanoscale has been crucial in creating new phases of matter and devices. Here I will present results on the 2D magnet RuCl3 that demonstrate it is able to induce large charge on short length scales in other materials. I will discuss its ability to work with various systems, and potential for control via relative twist angle. I will also review the limitations of this technique in terms of ultimate charge doping and homogeneity. Time permitting I will briefly discuss the unique magnetic excitations in this system useful for topological computing, and implications for heterostructures of RuCl3 with other 2D magnets.

Bart van Wees

In recent years it was demonstrated that magnons can be efficient transporters of spins, making new devices and functionalities possible with (insulating) magnonic systems. I will give an introduction into magnon spin transport in ferro/ferri/and anti-ferrro magnetic systems. I will discuss how charge current information can be transformed into (electronic) spin information by the spin Hall effect, which can then generate a magnon spin current in the ferrimagnetic insulators yttrium iron garnet (YIG) [1]. Magnon spins can then be detected via the inverse spin Hall effect, and converted back into a charge signal. These experiments have led to a better understanding of electrically and  thermally induced magnon currents (spin Seebeck effect) and emphasize the role of the nonequilibrium magnon chemical potential as the driving force for magnon currents [2] Based on these concepts a magnon transistor geometry was fabricated in which the magnon density was controlled by a magnon injecting gate electrode [3]. It was also shown that magnons in antiferromagnets can effectively transport spins, and experiments demonstrated this in multilayer 2D Van der Waals antiferromagnets[4]  I will discuss our recent results on magnon spin caloritronics, including magnon spin Seebeck effect and anomalous Nernst effects, in CrBr3 based ferromagnetic van der Waals systems.

• J. Cornelissen et al., Nat. Phys. 11, 1022 (2015)
• J. Cornelissen et al., Phys. Rev. B94, 014412 (2016)
• J. Cornelissen et al., Phys. Rev. Lett. 120, 097702 (2018)
• Xing et al., Phys. Rev. X9, 011026 (2019)
• Liu et al., Phys. Rev. B 101, 205407 (2020)

Cheng Gong

Magnetism, one of the most fundamental physical properties, has revolutionized significant technologies such as data storage and biomedical imaging, and continues to bring forth new phenomena in emerging materials and reduced dimensions. The recently discovered magnetic 2D van der Waals materials (hereafter abbreviated as “2D magnets”) provide ideal platforms to enable the atomic-thin, flexible, lightweight magneto-optic and magnetoelectric devices. The seamless integration of 2D magnets with dissimilar electronic and photonic materials further opens up exciting possibilities for unprecedented properties and functionalities. In this tutorial, I will start with the fundamentals on 2D magnetism, and continue to speak on our experimental observation of 2D ferromagnet, analyze the current progress and the existing challenges in this emerging field, and show how we push the boundary by exploring the potential of 2D antiferromagnets for spintronics.

Jianting Ye

Many recent discoveries on novel electronic states were made on 2D materials. Especially, by making artificial bilayer systems, new electronic states such as superconductivity and ferromagnetism have been reported. This talk will discuss quantum phase transitions and Ising superconductivity induced in 2D transition metal dichalcogenides. Using ionic gating, quantum phases such as superconductivity can be induced by field-effect on many 2D materials. In transition metal dichalcogenides, Ising-like paring states can form at K and K’ point of the hexagonal Brillouin zone. Also, we will discuss how to couple two Ising superconducting states through Josephson coupling by inducing superconductivity symmetrically in a suspended bilayer. This method can access electronic states with broken local inversion symmetry while maintaining the global inversion symmetry [3]. Controlling the Josephson coupling and spin-orbit coupling is an essential step for realizing many exotic electronics states predicted for the coupled bilayer superconducting system with strong spin-orbit interactions.

[1] Lu, J. M. Zheliuk O, et al., Science 350 1353 (2015).
[2] Lu, J. M. Zheliuk O, et al., Proceedings of the National Academy of Sciences 115 3551 (2018).
[3] Zheliuk O, Lu, J. M., et al., Nature Nanotechnology 14 1123 (2019).

Kang L Wang

 

Layered materials have recently been investigated for exploring magnetic properties.  This talk will discuss the magnetism of layered materials including those of magnetic doped materials as well the interface proximity-induced ferromagnetism by proximity with antiferromagnetic materials.  We will begin by describing layered magnetic doped topological insulators (TI), SbBiTe, for achieving quantum anomalous Hall.  Then we will discuss the proximity-induced magnetism in doped and undoped TIs when interfaced with different kinds of antiferromagnets, such as CrSb and MnTe, with a perpendicular and an in-plane Nel orders, respectively.  Antiferromagnets interfaced with a magnet is shown being to yield skyrmions, whose topological charge can be controlled by cooling under applied magnetic fields.   Atomically thin 2-D van der Waals magnetic materials (FeGeTe and alike) also have drawn significant interests. We observed interface Nel-type skyrmions in FeGeTe/WeTe₂ heterostructures from the topological Hall effect below the temperature of 150 K, with the varying sizes for different temperatures, and the skyrmions were also confirmed by Lorentz transmission electron microscopy. A Dzyalosinskii-Moriya interaction with an energy of 1.0 mJ/m², obtained from the aligned and stripe-like domain structure, is shown to be sufficiently large to support and stabilize the skyrmions.

 

 

 

Wei Han

 

The two-dimensional (2D) van der Waals magnets have provided new platforms for exploring quantum magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices.

 

In this talk, I will discuss the spin transport in magnetic 2D materials and their heterostructures. Firstly, I will discuss magnon-mediated spin transport in an insulating 2D van der Waals antiferromagnetic MnPS₃. Long distance magnon transport over several micrometers is observed in quasi-2D MnPS₃. The transport of magnons could be described using magnon-dependent chemical potential, and long magnon relaxation length of several micrometers are obtained. Then, I will discuss the spin transport in a metallic 2D van der Waals ferromagnetic Fe_0.29TaS₂ and its heterostructures. Via systematically measuring Fe_0.29TaS₂ devices with different thickness, it is found that the dominant AHE mechanism is skew scattering in bulk single crystal, and the contribution from intrinsic mechanism emerges and become more relevant as the Fe_0.29TaS₂ thickness decrease. The spin-dependent scattering at the Fe_0.29TaS₂/superconductor interface will be also discussed, which reveals a large magnetoresistance that can be explained by the anisotropic Andreev reflection.

José H. Garciá A. 

The manifestations of spin-orbit coupling in two-dimensional materials with reduced symmetries, such as MoTe2 or WTe2 in their 1T' or 1Td phases, can lead to hitherto unexplored ways to control the electronic spins. In this talk, I will present numerical simulations that demonstrate that due to a combination of a persistent canted spin texture and hotspot of the berry curvature, transition metal dichalcogenides show a tunable canted spin Hall effect. The canting angle depends on the microscopic spin-orbit coupling parameters and can be tuned through the electronic environment. Moreover, the persistent spin texture spam over a broad energy range allowing for long spin relaxations even in the metallic regime.  These findings vividly emphasize how crystal symmetry governs the intrinsic spin phenomenology and how it can be exploited to broaden the range and efficiency of spintronic functionalities. We also propose specific experimental guidelines for the confirmation of the effect.

Jing Shi

2D van der Waals (vdW) magnetic materials offer exciting new opportunities to study interfacial phenomena arising from or enhanced by the atomically flat interfaces. I will present our recent studies on three types of bilayer systems composed of vdW magnet and Pt: Cr₂Ge₂Te₆/Pt, Fe₃GeTe₂/Pt, and Pt/CI₃.  In each bilayer, the exfoliated vdW magnet consists of 10’s atomic layer units and the sputtered 5 nm Pt layer is either below or above the vdW magnet. In Cr₂Ge₂Te₆/Pt and Pt/CI₃, both Cr₂Ge₂Te₆ and CI₃ are insulating, we use induced magneto-transport properties in Pt to probe the spin states and magnetic domains in the insulating magnets [1-3]. Unlike these two insulating magnets, Fe₃GeTe₂ is a metallic ferromagnet which has the highest Curie temperature among all 2D vdW magnets, strong perpendicular magnetic anisotropy, and more resistive than Pt; therefore, it is an excellent 2D magnet for investigating the spin-orbit torque effects. We demonstrate that Fe₃GeTe₂/Pt has a spin-orbit torque efficiency comparable with that in the best bilayers made with 3D magnets and the Fe₃GeTe₂ magnetization can be switched with a relatively low critical current density [4]. These excellent properties show great potential of 2D materials for spintronic applications.

1. B. Niu, T. Su, et al., Nano Lett. 20, 553 (2020). DOI: 10.1021/acs.nanolett.9b04282.
2. M. Lohmann, et al., Nano Lett. 19, 2397 (2019). DOI: 10.1021/acs.nanolett.8b05121.
3. T. Su, et al., 2D Materials 7, 045006 (2020). DOI:10.1088/2053-1583/ab9dd5.
4. M. Alghamdi, M. Lohmann, et al., Nano Lett. 19, 4400 (2019). DOI: 10.1021/acs.nanolett.9b01043.

Tatiana G. Rappoport

The field of spintronics blossomed in the last decade, driven by the use of spin-orbit coupling to generate and manipulate spin currents in non-magnetic materials. In these systems, the efficient conversion between charge and spin currents is mediated by spin-orbit. Great progress in the manipulation of the orbital angular momentum of light has also been achieved in the last decades, leading to a large number of relevant applications. Still, electron orbitals in solids were less exploited, even though they are known to be essential in several underlying physical processes in material science. The orbital-Hall effect (OHE), similarly to the spin-Hall effect (SHE), refers to the creation of a transverse flow of orbital angular momentum that is induced by a longitudinally applied electric field. The OHE has been explored mostly in three dimensional metallic systems, where it can be quite strong. However, several of its features remain unexplored in two-dimensional (2D) materials.

We then investigate the OHE in multi-orbital 2D insulators, such as transition metal dichalcogenides. We show that the OHE in these systems is associated with exotic momentum-space orbital textures. This intrinsic property emerges from the interplay between orbital attributes and crystalline symmetries and does not rely on the spin-orbit coupling. Our results indicate that multi-orbital 2D materials can display robust OHE that may be used to generate orbital angular momentum accumulation, and produce strong orbital torques that are of great interest for developing novel spin-orbitronic devices.

Angela R. Hight Walker

Raman spectroscopy, imaging, and mapping are powerful non-contact, non-destructive optical probes of fundamental physics in graphene and other related two-dimensional (2D) materials, including layered, quantum materials that are candidates for use in the next quantum revolution. An amazing amount of information can be quantified from the Raman spectra, including layer thickness, disorder, edge and grain boundaries, doping, strain, thermal conductivity, magnetic ordering, and unique excitations such as charge density waves. Most interestingly for quantum materials is that Raman efficiently probes the evolution of the electronic structure and the electron-phonon, spin-phonon, and magnon-phonon interactions as a function of temperature, laser energy, and polarization. Our unique magneto-Raman spectroscopic capabilities will be detailed, enabling diffraction-limited, spatially-resolved Raman measurements while simultaneously varying the temperature (1.6 K to 400 K), laser wavelength (tunability from visible to near infrared), and magnetic field (up to 9 T) to study the photo-physics of nanomaterials. Additionally, coupling to a triple grating spectrometer provides access to low-frequency (down to 6 cm-1, or 0.75 meV) phonon and magnon modes, which are sensitive to coupling. By utilizing electrical feedthroughs, studying the strain-dependent effects on magnetic materials utilizing MEMs devices is also a novel opportunity. Current results on intriguing quantum materials will be presented to highlight our capabilities and research directions. One example leverages the Raman spectra from α-RuCl3 to probe this Kitaev magnet and possible quantum spin liquid1. Within a single layer, the honeycomb lattice exhibits a small distortion, reducing the symmetry from hexagonal to orthorhombic. We utilize polarization-dependent Raman spectroscopy to study this distortion, including polarizations both parallel and perpendicular to the c-axis. Coupling of the phonons to a continuum is also investigated. Using Raman spectroscopy to probe magnetic phenomena in the antiferromagnetic metal phosphorus trichalcogenide family2, we highlight FePS3 and MnPSe3. Using magneto-Raman spectroscopy as an optical probe of magnetic structure, we show that in FePS3 one of the Raman-active modes in the magnetically ordered state is actually a magnon with a frequency of ≈3.7 THz (122 cm−1). In addition, the surprising symmetry behavior of the magnon is studied by polarization-dependent Raman spectroscopy and explained using the magnetic point group of FePS3. Using resonant Raman scattering, we studied the Neel-type antiferromagnet MnPSe3 through its ordering temperature and also as a function of applied external magnetic field. Surprisingly, the previously assigned one-magnon scattering peak showed no change in frequency with an increasing in-plane magnetic field. Instead, its temperature dependence revealed a more surprising story. Combined with first-principle calculations, the potential origin of this Raman scattering will be discussed.

Finally, the magnetic field- and temperature-dependence of an exciting ferromagnetic 2D material, CrI3, will be presented3. We report a magneto-Raman spectroscopy study on multilayered CrI3, focusing on two new features in the spectra which appear below the magnetic ordering temperature and were previously assigned to high frequency magnons. Instead, we conclude these modes are actually zone-folded phonons. We observe a striking evolution of the Raman spectra with increasing magnetic field applied perpendicular to the atomic layers in which clear, sudden changes in intensities of the modes are attributed to the interlayer ordering changing from antiferromagnetic to ferromagnetic at a critical magnetic field. Our work highlights the sensitivity of the Raman modes to weak interlayer spin ordering in CrI3.

[1] PHYSICAL REVIEW B 100, 134419 (2019)
[2] PHYSICAL REVIEW B 101, 064416 (2020)
[3] https://arxiv.org/abs/1910.01237 (in press @Nature Comm)