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Tuning of the Metal–Insulator Transition via Alkali Adsorption Print

Turning a material from an insulator to a metal, or vice versa, by light irradiation, exposure to electric or magnetic fields, or applying small changes in temperature, pressure, or doping—such intriguing control of a material's electronic properties is possible by exploiting strongly interacting or "correlated" electrons. Now a team of researchers from the University of Kiel in Germany and the ALS has found a novel, surprising way to continuously transform a layered metallic transition-metal compound, TaS2, into an insulator. Using angle-resolved photoemission spectroscopy (ARPES), they have demonstrated that adsorption of alkali atoms onto this material's surface gradually makes it more insulating, although in general, alkali adsorption should lead to more metallic behavior, as alkali atoms easily give away their loosely bound outermost electron.

"Correlated Electron" Technology

Illumination at the flick of a switch. Instant communication across vast distances. A universe of information accessible from your desktop. We now enjoy a practically endless list of technological advances that owe their existence to an understanding of how electrons behave in the solid state. To put it simply, in certain materials (i.e., metals), electrons in the atoms' outer orbitals are free to wander throughout the material and thus conduct electricity. This highly simplified picture, however, does not take into account the effect that electrons have on each other, an effect known as "electron correlation." Materials with electrons that are strongly correlated are expected to provide the basis of major new technologies as we begin to exploit, not just electron charge (electricity), but the interplay of electron charge, spin, and orbital properties. Such interactions often result in dramatic effects: superconductivity, for example, or extreme sensitivity to slight changes in magnetic field, as in the phenomenon of colossal magnetoresistance. In this work, Rossnagel et al. have found a way to observe, "live," the changes in the electronic structure of a strongly correlated electron material as it transforms from a metal into an insulator.

How can a correlated material undergo a metal-to-insulator transition? Suppose we have a crystal with a simple band structure consisting of a single half-filled valence band, and we start to increase the lattice spacing between the atoms. Band theory then predicts metallic conduction, no matter how narrow the valence band becomes (as measured by the band width W).

Top: Schematic representation of the lattice-constant dependence of a simple band structure. Band theory predicts metallic conduction, no matter how narrow the band becomes. Bottom: At a critical lattice constant, itinerant electrons become mostly localized at individual sites, but can still hop from site to site with a nonnegligible probability.

What happens instead, at some critical band width, is that the itinerant (delocalized) valence electrons become mostly localized at individual atomic sites, such that they only hop from site to site with a small, but nonnegligible probability. This breakdown of metallic conduction is reflected in the formation of the so-called Hubbard subbands, which are separated by U, the energy cost an electron has to pay when it hops on an already occupied atom. Quite generally, these Hubbard subbands form gradually as the ratio U/W is increased.

Left: Hopping of a spin-up electron and a spin-down hole on the background of spin-down electrons gives rise to the so-called Hubbard subbands, separated by the Hubbard interaction term U. Right: The Hubbard subbands form gradually as the ratio U/W is increased.

To reveal such drastic changes in the electronic structure of a material, ARPES is the ideal tool, since it directly probes the valence-electron distribution in momentum space. The Kiel–Berkeley collaboration performed their ARPES measurements at the Electronic Structure Factory on ALS Beamline 7.0.1. Clean surfaces of layered TaS2 were prepared by cleavage in ultrahigh vacuum, and an alkali, rubidium in this case, was evaporated live during the photoemission measurements.

A "photoemission movie" recorded during the deposition process shows the intriguing evolution of the valence spectra as a function of deposition time. At the beginning of the movie, the high spectral intensity near the highest occupied electron energy, the Fermi energy EF, identifies TaS2 as a metal. Then rubidium deposition is started and spectral weight is continuously removed away from EF, gradually transforming the material into an insulator. The similarity of these results with the simple sketch of Hubbard subbands strongly supports the interpretation as a correlation-driven metal-to-insulator transition (note that ARPES can only probe the lower Hubbard subband). But what is the reason for this transition, as alkali adsorption onto metallic systems usually causes an increased band filling and thus more metallic behavior?

Left: ARPES movie recorded during the deposition of rubidium atoms on the surface of the layered transition-metal compound TaS2. Right: An artist's impression of the Rb intercalation process in TaS2.

The possible solution lies in the characteristic layer structure of TaS2. Remarkably, alkali deposition on such a material can result in the intercalation of some alkali atoms in the gaps between the layers, which increases the layer separation and thereby reduces the electronic band width perpendicular to the layers. Thus, the ratio U/W can be driven through the critical value for the metal-to-insulator transition.

Besides electronic correlations, there are more facets of this particular metal-to-insulator transition, namely, disorder introduced by the rubidium atoms, charge transfer from rubidium to TaS2, and electron–lattice coupling, as TaS2 also undergoes a charge-density-wave reconfiguration. But electron–electron interaction appears as the dominant cause in this case, making the rubidium/TaS2 system an ideal test object to study in detail the continuous evolution of the valence electron structure of a correlated material across the passage from the metallic to the insulating regime.

Research conducted by K. Rossnagel and L. Kipp (University of Kiel, Germany) and H. Koh, E. Rotenberg, and N. Smith (ALS).

Research funding: Deutsche Forschungsgemeinschaft, Alexander von Humboldt Foundation, and U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: K. Rossnagel, H. Koh, E. Rotenberg, N.V. Smith, and L. Kipp, "Continuous tuning of electronic correlations by alkali adsorption on layered 1T-TaS2," Phys. Rev. Lett. 95, 126403 (2005).