NbSe2, a true 2-D superconductor

November 6, 2015, Elhuyar Fundazioa
Atomically resolved STM image of the NbSe2 surface showing CDW modulation. Credit: CIC nanoGUNE

An international team led by Miguel M. Ugeda and Michael F. Crommie has demonstrated the coexistence of superconductivity and charge density wave order in a single layer of NbSe2, a model transition metal dichalcogenide metal. The demonstration that a single layer of NbSe2 is a true 2-D superconductor is a breakthrough in the field of 2-D materials. This work has been recently reported in Nature Physics.

Very few 2D exist in nature, and single-layer NbSe2 is the first among them that remains a superconductor in its isolated, 2D form without the need of a special substrate. Furthermore, CDW order—spatial modulation of both the electron density and the atomic lattice (see figure below)—turns out to be a genuine 2D electronic phenomenon in NbSe2. This work has been recently reported in Nature Physics.

Many-body interactions in solids are at the core of stunning collective electronic phenomena such as CDW order and superconductivity, which are found to exist in some TMD metals in their 3D bulk form. In reduced dimensions, materials usually exhibit remarkable differences in their properties with respect to their 3D bulk counterparts due to quantum confinement effects. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below 7.2 K these electronic states coexist but their microscopic formation mechanisms in bulk remain controversial after nearly four decades of intense research. Furthermore, the fate of its CDW and superconducting phases in the ultimate single-layer limit has also remained mysterious until now. Writing in Nature Physics, the team has characterized the electronic structure of a single layer of NbSe2 directly grown on graphene, which experimentally confirms that NbSe2 undergoes a reduction in the number of electronic bands involved in its collective phases (from three bands in bulk NbSe2 to just one for single-layer NbSe2). Despite significant changes in NbSe2 electronic structure upon dimensional reduction, the authors show that in stark contrast to recent theoretical predictions, the CDW phase of single-layer NbSe2 remains unaffected when the material is thinned down to the single-layer limit. Because of the robust 2D character of the CDW in NbSe2, researchers experimentally ruled out two well-known mechanisms proposed to explain the origin of the CDW in NbSe2. On the contrary to the CDW phase, the material remains a superconductor although with a critical temperature TC = 1.9 K, a significant departure from the behavior of bulk NbSe2 (TC = 7.2 K).

In summary, these results paint a clear picture of the effects of reduced dimensionality on the CDW and superconducting phases of a model strongly correlated system. This creates exciting new opportunities for directly exploring the interplay between superconductivity and other competing collective phases in 2D.

Explore further: Electrochemical etching down to one-monolayer towards high-Tc superconductivity

More information: Miguel M. Ugeda et al. Characterization of collective ground states in single-layer NbSe2, Nature Physics (2015). DOI: 10.1038/nphys3527

Related Stories

Ocean upwelling and increasing winds

September 3, 2015

As the southern westerly winds drive the Antarctic circumpolar current around Antarctica, deep waters are forced up to the surface south of the polar front. Changes in the intensity with which this relatively warm, nutrient ...

High-temperature superconductivity in atomically thin films

June 2, 2015

A research group at Tohoku University has succeeded in fabricating an atomically thin, high-temperature superconductor film with a superconducting transition temperature (Tc) of up to 60 K (-213°C). The team, led by Prof. ...

Recommended for you

Researchers capture an image of negative capacitance in action

January 21, 2019

For the first time ever, an international team of researchers imaged the microscopic state of negative capacitance. This novel result provides researchers with fundamental, atomistic insight into the physics of negative capacitance, ...

Toward ultrafast spintronics

January 21, 2019

Electronics have advanced through continuous improvements in microprocessor technology since the 1960s. However, this process of refinement is projected to stall in the near future due to constraints imposed by the laws of ...

New thermoelectric material delivers record performance

January 17, 2019

Taking advantage of recent advances in using theoretical calculations to predict the properties of new materials, researchers reported Thursday the discovery of a new class of half-Heusler thermoelectric compounds, including ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.