The superconductivity mosaic

A research team from the Donostia International Physics Center (DIPC) has made a key breakthrough in understanding superconductivity in a family of materials known as Transition Metal Dichalcogenides (TMDs). Their work on how superconductivity develops in the material on a microscopic scale in the presence of a charge density wave has been published in the latest edition of the prestigious journal NanoLetters, and features on the cover.

TMDs are extremely versatile two-dimensional materials. They can be exfoliated like graphene, and two-dimensional layers can be obtained; that makes them ideal candidates for designing materials with specific properties. What characterizes TMDs is the fact that, by simply changing the transition metal, they exhibit radically different electronic properties: from semiconductors (MoS₂), metals (TiSe₂) and even to superconductors (NbSe₂).

Some of these TMD materials are superconductors and, in fact, they usually coexist with other electronic phases; the phase known as the charge density wave (CDW), a kind of “wave” in the electronic distribution on the material, features among them. In one of the TMD materials explored in this work (TaS₂), the presence of this density wave turns the material into an insulator, thus rendering it incompatible with superconductivity. However, in this family of materials, when sulfur (S) is gradually replaced by selenium (Se), superconductivity arises spontaneously at a certain point. This is the case of the material studied, TaSSe, a TMD with half the number of sulfur and selenium atoms.

The study by the group of DIPC researchers aims to find out why superconductivity arises in these alloys when the CDW phase, in principle, gives them an insulating character. Curiously, superconductivity arises in TaSSe because the CDW of the material undergoes a very radical change, in which “it breaks up”, displaying a mosaic appearance (as this new CDW phase is known) with domains of a few nanometers acting as “tesserae”, each of the small pieces that form a mosaic. This new mosaic phase (see image) generates a large number of edges between the tesserae that have been related to the metallic state of the material and, therefore, to the origin of the superconductivity.

The DIPC researchers explored the complex mutual dependence between the CDW phase and superconductivity in these materials. Using high-resolution scanning tunneling microscopy (STM) measurements, the team were able to demonstrate that, although it is necessary for the CDW to display a “mosaic” appearance for superconductivity to emerge, the latter does not emerge on the edges between the “tesserae”. On the contrary, “what we can see using STM is that, even above the critical superconducting temperature, the entire material is metallic, and not just on the edges”, explained Miguel Moreno-Ugeda, an Ikerbasque Research Professor at the DIPC and one of the authors of the work. “When the temperature is lowered even further and the material is turned into a superconductor, we can see that, indeed, superconductivity is not restricted to specific areas, but extends uniformly throughout the material,” added Ugeda.

They are also proposing a new explanation: the emergence of superconductivity is not due to these metal edges, but to the vertical disorder of the CDW which induces the mosaic phase by emerging independently in each layer of the material —one can imagine a TMD as a kind of puff pastry or stack of misaligned sheets—. This transforms the system into a metal and, subsequently, allows superconductivity to emerge.

The study was carried out almost entirely by DIPC research staff. “It’s been great to be able to carry out such a comprehensive piece of work practically at home; personally, I am really excited about it,” said Miguel Moreno-Ugeda, who stressed the value of internal collaboration in achieving high-impact results.

Although this is basic research, this type of superconductivity, which emerges from the disorder in two-dimensional systems, could be important in the design of new quantum devices. For the moment, the work provides a key to understanding the interaction between different electronic phases that coexist in these highly important materials.