Usual properties of lithium vanadate explained
Today’s high-tech devices would not exist without a good theory to predict how electrons move through semiconductor crystals. But gaps remain in the theory—some insulating materials are erroneously predicted to conduct electricity, for example. Resolving these problems could lead to a more robust theory that enables new breakthroughs in electronics.
As part of this effort, a RIKEN researcher and his colleagues have developed a new theoretical method for predicting how electrons will behave at very low temperatures, called the projective quantum Monte Carlo (PQMC) method, and used it to solve a puzzle about the unusual electrical conductivity, heat capacity and magnetic properties of a material called lithium vanadate (LiV2O4) (Fig. 1- Click on link below).
In a single atom, electrons can only occupy certain energy levels, known as orbitals. But in a crystal made up of trillions of atoms these orbitals smear into bands, each representing a range of energies available to the electrons. In semiconductors, electrons must acquire enough energy to hop into the lowest unoccupied band before they start to flow.
In certain materials, there is a magnetic attraction between the electrons flowing through the conduction band and those which remain trapped on the stationary atoms. This can slow the conduction electrons down, and make the trapped electrons, known as ‘heavy fermions’, behave as if they were much heavier than normal. Lithium vanadate is the first material where electrons residing in a lower orbital, called 3d, are responsible for heavy fermion behavior that appears below about -240 ˚C.
Despite the chilly conditions, there has been hot debate about exactly how this works. Now Ryotaro Arita, of RIKEN’s Discovery Research Institute in Wako, and his colleagues believe they have the answer. The team has combined two established ways of calculating how electrons spread around atoms, and used PQMC to extrapolate the case to very low temperatures.
Their calculations show that the orbitals around the vanadium atoms are subdivided into a lower energy band (a1g) where electrons remain tethered to their parent atoms, while those in an upper level (eg) effectively form a conduction band. The lower band acts as a Mott insulator: such materials do not conduct electricity because the mutual repulsion between neighboring electrons forces them to stick by their parent atoms. This makes the a1g band primarily responsible for the heavy fermion effects seen in LiV2O4.
The team now hopes to refine their theory further to explain other anomalous behavior in metals, insulators and semiconductors, says Arita.
Reference
1. Arita, R., Held, K., Lukoyanov, A. V. & Anisimov V. I. Doped Mott insulator as the origin of heavy-fermion behavior in LiV2O4. Physical Review Letters 98, 166402 (2007).
