Congratulations! Yong-Bin’s work on WTe2 Josephson junction in international collaboration team is published in Nature Materials.

Higher order topology found in 2D crystal

Evidence of Higher Order Topology in Multilayer WTe2 from Josephson Coupling through Anisotropic Hinge States
(Nature Materials, published on-line)

For the last decade or so, condensed matter physics has been experiencing a golden-age with novel materials, properties and technologies being developed at a break neck pace due to the advent of topological physics. The field took flight in 2008 with the discovery of the “topological insulator” – a type of material that is electrically insulating in the bulk but is metallic on the surface. Since then, scientists have found more and more exotic topological phases including Dirac semimetals, Weyl semimetals, and Axionic insulators. But most recently, the so-called “higher-order topological insulators” – materials which are insulating in the bulk and on the surfaces/edges, but are metallic only on the hinges or at the corners, were theoretically predicted. These bizarre new materials are extremely rare and so far only elemental Bismuth has been experimentally shown to possibly belong to this category.

What is a hinge state anyway? Imagine a box – longer and wider than tall – with flaps on the top and bottom that you can open in order to put things inside. The inside of this box would be called the “bulk”. Most materials which conduct electricity do so in the bulk. However, in topological insulators, the bulk of the box is electrically insulating, but the top and bottom, where the flaps are, are metallic and have “surface states”. For some materials, the bulk, top and bottom of the box are insulating, but the sides (a.k.a. the edges) are metallic. These have “edge states” which have been shown in magnetic topological insulators. Finally, in higher order topological insulators, the bulk, top, bottom and sides of the box are all insulating. But the hinges and corners of the box are metallic and have “hinge states” and “corner states”. The hinge states in particular are expected to be promising for spintronics (because the direction of their propagation is tied to their spin) as well as for realizing Majorana fermions which are actively being investigated for their applications to fault-tolerant quantum computing.

Now an international team of scientists from the United States, Hong Kong, Germany, and South Korea have discovered a new higher-order topological insulator – the layered, 2 dimensional transition metal dichalcogenide (TMDC), WTe2. This is a famous material in condensed matter physics, showcasing a variety of exotic properties ranging from titanic magnetoresistance to the quantized Spin hall effect. It was the first example of a Type-II Weyl semimetal and is exfoliatable, like graphene, so it can be made into devices just one layer thick. WTe2 has also been showed to superconduct (where electrons form pairs and a supercurrent that travel without any resistance) under pressure.

Adding to this carnival of properties, theoretical physicists in 2019 predicted WTe2 (and sister material MoTe2) to also be higher order topological insulators with metallic hinge states. Many research groups around the world have since been searching for evidence of these exotic states in WTe2 and MoTe2 and some recent results have shown that there are extra conductive states at the “edges” of the samples; but researchers were unable to tell if these were truly edge states or the highly sought after hinge states.

In a study published in Nature Materials on July 6th 2020, the team led by Kin Chung Fong (Raytheon BBN Technologies), Mazhar N. Ali (Max Plank Institute of Microstructure Physics and also Material Mind Inc.), Kam Tuen Law (Hong Kong University of Science and Technology) and Gil-Ho Lee (Pohang University of Science and Technology and the Asia Pacific Center for Theoretical Physics) took a new approach and used Josephson Junctions to spatially resolve the supercurrent flow and show that WTe2 does indeed appear to have hinge states and be a higher order topological insulator.

Josephson Junctions are incredibly important devices and tools in physics. They are used in a variety of technological applications including MRI (magnetic resonance imaging) machines as well as Qubits (the building block of quantum computers). These junctions are formed when two superconducting electrodes (like Nb) are connected by a non- superconducting bridge (like high-quality WTe2) in a thin film device. When the temperature of the experiment is lowered enough, the supercurrent that is injected into the WTe2 from one Nb electrode can travel across the bridge, without resistance, to the other Nb electrode. So the overall device shows zero resistance and is said to be superconducting. However, you can’t send an infinite amount of supercurrent across the bridge while maintaining superconductivity; when the injected current exceeds a so- called critical current, the junction turns into a ‘normal’ state and shows a finite resistance. The Josephson effect says that as a function of the applied magnetic field, the critical current will oscillate between high and low values due to the changing phase of the superconducting wave- function across the sample, in a so-called Fraunhofer pattern.

The team realized that hidden in this oscillation is information of where in the sample the supercurrent is travelling. By taking an inverse Fourier transform of the Fraunhofer pattern, they were able to visualize the supercurrent flow in the sample and found there was indeed supercurrent travelling on the sides of their WTe2 device. However, this wasn’t enough to distinguish the edge states from the hinge states.

As shown in the figure below, due to a quirk in the symmetry-based origin of the hinge states, they are not equivalent on all of the hinges of a WTe2 sample. There are metallic hinge states, for example, on the top left and bottom right hinges of the sample, but not on the top right and bottom left. This is different from an edge state, which would simply be existing on the entirety of the left and right sides of the sample. “We used this difference to our advantage: by connecting superconducting electrodes to just the top half of the sample and not the bottom half, we realized we would see a different Fraunhofer pattern if hinge states existed and not edge states,” explains Kin Chung Fong at Raytheon BBN Technologies. “In this configuration electrodes would connect to only one of the hinge states (i.e. top left and not bottom right), which would show a distinct Fraunhofer pattern. If there were edge states, this configuration wouldn’t be any different than connecting to both the bottom and top halves of the sample and the Fraunhofer would look the same.” When they carried out this challenging experiment, they saw the hallmark of the hinge state not the edge state.

“But that’s not all. WTe2 is a fairly low-symmetry orthorhombic material with high crystalline anisotropy. The different directions in the crystal are not equivalent and we also theorized, and confirmed, that the hinge states existing in WTe2 aren’t all equivalent either. In some directions, they mix into the bulk while in other directions they don’t,” explains Kam Tuen Law at Hong Kong University of Science and Technology.

“There is a variety of exciting physics to be explored in these compounds in the near future now that hinge states have been found in WTe2. The possibility for dissipationless interconnections, true 1D superconducting nano-wires and spintronics devices, topological superconductivity, Majorana fermions and correspondingly topological quantum computers are all on the horizon,” says Gil-Ho Lee at Pohang University of Science and Technology.

“WTe2 may be the second material shown to host hinge states, but it is very different from the other candidate, Bismuth. WTe2, being 2D, is easily fabricable into nano-devices with controlled surfaces, can be layered on top of other 2D materials in heterostructures, and even on top of itself, when slightly twisted, to form a Moire superlattice. Also, the sister material, MoTe2, is expected to show the same hinge states, but is an intrinsic superconductor at low temperature. How can these hinge states be modified, controlled, and used? There are a lot of exciting research opportunities ahead.” says Mazhar N. Ali at the Max Plank Institute of Microstructure Physics.

This work was funded by Samsung Science & Technology Foundation and National Research Foundation of Korea.

[2020/07] Yong-Bin and Jinho’s work on WTe2 Josephson junction is published in Nature Materials.