Highlights

User Research Highlights 2017

The strongly spin-momentum coupled electronic states in topological insulators (TI) have been extensively pursued to realize efficient magnetic switching. However, previous studies show a large discrepancy of the charge-spin conversion efficiency. Moreover, current-induced magnetic switching with TI can only be observed at cryogenic temperatures. We report spin-orbit torque switching in a TI-ferrimagnet heterostructure with perpendicular magnetic anisotropy at room temperature. The obtained effective spin Hall angle of TI is substantially larger than the previously studied heavy metals. Our results demonstrate robust charge-spin conversion in TI and provide a direct avenue towards applicable TI-based spintronic devices.

Published in Phys. Rev. Lett. 119, 077702 (2017). Copyright from APS.

Also supported by: NSF DMR-1419807,  NSF Grant 1639921, SRC-NRI 2700.001.

In-House Research Highlights

Light and lattice vibrations in a crystal do not interact strongly in general, but both interact strongly with electrons. This three-way interaction allows a characterization method known as double resonance Raman spectroscopy to map out the low-energy electronic structure of materials and assign vibrational signatures to different microscopic processes. Double resonance in two-dimensional MoS2 reveals the dynamics of excitons – robust elementary excitations of a 2D crystal – between two sets of low-energy states known as valleys. The accurate assignment of vibrational signatures elucidates the essential physics limiting the performance of a novel class of “valleytronic” devices exploiting the selectivity of valleys to incident light carrying different polarizations.

Identifying crystal defects in a 2D crystal usually requires an electron microscope to directly resolve atomic details, a complex and time-consuming process on expensive equipment that can damage the sample under electron irradiation. By establishing a correlation between the modified optical response and certain defects, the MIP team and collaborators have demonstrated a quick and non-destructive method of identifying defects in 2D crystals. The reason for this correlation is identified through first-principles calculations: electrons trapped by sulfur vacancies – the absence of a sulfur atom – have energies that are forbidden for electrons in defect-free regions, and therefore emit light at wavelengths different from that of the latter.

Doping modulates the electronic, chemical, and mechanical properties of materials. For a two-dimensional tungsten disulfide, although an isolated molybdenum substitution only perturbs the host lattice negligibly, it couples strongly to common lattice defects such as sulfur vacancies, as verified by state-of-the-art electron microscopy and atomistic modeling techniques. This coupling can be potentially exploited to controllably segregate undesirable defects away from the active areas of 2D crystalline devices.

Grain boundaries are borders that separate crystals of distinct orientations and are generally considered as inevitable by-products of crystalline regions nucleating at different locations during growth. The MIP team has predicted how grain boundaries in two-dimensional crystals can form within a single grain by introducing bumps onto the substrate – the “floor” on which a 2D crystal grows. A 2D crystal warps itself as growth advances past a bump, so much that it eventually runs into itself at an angle determined by steepness of the bump. Although aesthetically jarring, grain boundaries can endow 2D crystals with desirable mechanical, electronic, and magnetic properties. The predicted topographic control of grain boundaries offers the opportunity to engineer grain boundaries into 2D crystals with atomic-scale control.

Ferromagnetic topological insulators (TIs) have promise for applications in spintronics, metrology, and quantum computing.

However, TI materials are fragile and often incompatible with nanofabrication techniques. We have developed a technique that enables persistent, micron-scale optical control of both magnetization and chemical potential in Cr-(Bi,Sb)2Te3 grown by MBE on SrTiO3. This system is uniquely positioned to enable arbitrary routing of the quantized edge states recently discovered in magnetic TIs. We also use Kerr and photocurrent microscopies to image magnetic inversion dynamics, p-n junctions, and magnetic recordings that we make in these materials. This work may enable dynamic, reconfigurable control of 1D quantum channels.

Atomically thin two-dimensional layers such as molybdenum disulfide, MoS2, are promising materials for nanoelectronics due to their exceptional electronic and optical properties. An inter-atomic potential has been developed that can accurately describe the thermodynamic and structural properties of MoS2 sheets, including defects and transitions between different structural phases. A new type of “ripple” defect has been identified as a favorable host for sulfur vacancy defects. A train of moving ripplocation defects may be able to “sweep out” sulfur vacancy defects from key regions within 2D devices.

Broader Impacts Highlights 2017

The Graphene and Beyond Workshop, in its 5th year (2nd year of joint sponsorship with 2DCC), is a collaboration between the Center for 2D and Layered Materials (2DLM) and the 2DCC-MIP at Penn State. The workshop aims to enhance synergy in the community and build toward a strong future in 2D crystal science and technology. The 2017 workshop consisted of morning tutorials and afternoon talks focused on 2D material synthesis, properties, theory and optic/electronics including talks on recent results from the 2DCC and other institutions and discussion on commercialization opportunities for 2D materials.

The poster session consisted of 35 student and 18 non-student entries. The 2DCC supported travel for 3 non-R1 institution attendees (1 PUI, 2 MSIs). A 2DCC user meeting was held prior to the workshop to evaluate facility progress, access and future directions from the user point of view.

Attendees Total: 178 (79 Students)
Unique Institutions: 56
Academic (30; including 8 non-R1)
Government (8)
Industry (17)
International (1)

2DCC Research Highlights 2016

The chemical similarity of molybdenum & tungsten suggests they should randomly distribute in WxMo1-xS2, a material of great interest for next-generation elec-tronics. The 2DCC discovered that these atoms actually form thin chains, whose very different masses make properties like heat conduction anisotropic. Stripes form due to fluctuations in the availability of sulfur, the third element in WxMo1-xS2. Paradoxically, the disorder of random sulfur variations produces ordered stripes of metal. Such ‘order from disorder’ could be a general way to create atomic order in 2D materials, extending to the 2D domain the types of structures known from 3D semiconductor as “quantum wells.”