Faculty Profile

Research Overview:

URu2Si2 is a very unique material in which the complex electronic matter self-organizes at 17.5K in yet unknown order. A large amount of entropy is lost through the second order phase transition, a proof that there is an order, but the order parameter has newer been directly detected in experiment, hence it is called hidden order. This "hidden order" has been the subject of nearly a thousand scientific papers since it was first reported in 1985 at Leiden University in the Netherlands.

The electronic structure software developed in our group, which is a combination of Dynamical Mean Field Theory and Density Functional Theory (DFT+DMFT), has become much more predictive with invention of the exact "double counting" (see Phys. Rev. Lett. 115, 196403). An important problem, which limited the predictive power of this computational method, was so called double-counting problem. This arrises because both DFT and DMFT account for some part of correlations, but it was not clear unit now what is the common part of correlations accounted by both methods. By the invention of the exact double-counting this ambiguity has been finally set to rest, which makes the predictions of the method much more accurate (for example, the prediction of the total energy of H2 molecule shows only 0.2% error: Phys. Rev. B 91, 155144 (2015).)

Soon after iron superconductors were discovered, we realized that these new superconductors are correlated materials, hosting unconventional superconductivity (PRL 100, 226402 (2008)), and we termed them Hund's metals (arXiv 6 May 2008,published in NJP, see also). In Hund's metals the Coulomb interaction among the electrons is not strong enough to fully localize them, but it significantly slows them down, such that low-energy emerging quasiparticles have a substantially enhanced mass. This enhanced mass emerges not because of the Hubbard interaction U , but because of the Hund’s rule interactions that tend to align electrons with the same spin but different orbital quantum numbers when they find themselves on the same iron atom.