Large phonon drag thermopower boosted by massive electrons and phonon leaking in LaAlO3 / LaNiO3 / LaAlO3 heterostructure
M. Kimura, X. He, T. Katase, T. Tadano, J. M. Tomczak, M. Minohara, R. Aso, H. Yoshida, K. Ide, S. Ueda, H. Hiramatsu, H. Kumigashira, H. Hosono, T. Kamiya
Nano Lett. 21, 9240 (2021)

An unusually large thermopower (S) enhancement is induced by heterostructuring thin films of the strongly correlated electron oxide LaNiO₃. The phonon-drag effect, which is not observed in bulk LaNiO₃, enhances S for thin films compressively strained by LaAlO₃ substrates. By a reduction in the layer thickness down to three unit cells and subsequent LaAlO₃ surface termination, a 10 times S enhancement over the bulk value is observed due to large phonon drag. The enhancement originates from the coupling of lattice vibration to d electrons with large effective mass, and the electron–phonon interaction is largely enhanced by the phonon leakage from the LaAlO₃ substrate and the capping layer. Transition-metal oxide heterostructures emerge as a new playground to manipulate electronic and phononic properties in the quest for high-performance thermoelectrics.

Breaking of thermopower-conductivity trade-off in LaTiO3 film near Mott insulator to metal transition
T. Katase, X. He, T. Tadano, J. M. Tomczak, T. Onozato, K. Ide, B. Feng, T. Tohei, H. Hiramatsu, H. Ohta, Y. Ikuhara, H. Hosono, T. Kamiya
Advanced Science 8, 2102097 (2021)

Introducing artificial strain in epitaxial thin films is an effective strategy to alter electronic structures of transition metal oxides (TMOs) and to induce novel phenomena and functionalities not realized in bulk crystals. This study reports a breaking of the conventional trade-off relation in thermopower (S)–conductivity (σ) and demonstrates a 2 orders of magnitude enhancement of power factor (PF) in compressively strained LaTiO₃ (LTO) films.

Strain-engineering Mott-insulating La2CuO4
O. Ivashko, M. Horio, W. Wan, N. B. Christensen, D. E. McNally, E. Paris, Y. Tseng, N. E. Shaik, H. M. Rønnow, H. I. Wei, C. Adamo, C. Lichtensteiger, M. Gibert, M. R. Beasley, K. M. Shen, J. M. Tomczak, T. Schmitt, J. Chang
Nat. Commun. 10, 786 (2019)
arXiv:1805.07173

The transition temperature T_c of unconventional superconductivity is often tunable: For a monolayer of FeSe the sweet spot is uniquely bound to titanium-oxide substrates, while for La_2−xSr_xCuO₄ thin films, such substrates are sub-optimal and the highest T_c is instead obtained using LaSrAlO₄. An outstanding challenge is thus to understand the optimal conditions for superconductivity in thin films: which microscopic parameters drive the change in T_c and how can we tune them? Here we demonstrate, by a combination of x-ray absorption, resonant inelastic x-ray scattering spectroscopy, and theoretical simulations, how the Coulomb and magnetic-exchange interaction of La₂CuO₄ thin films can be enhanced by compressive strain. We establish that the substrate producing the largest T_c under doping also generates the largest nearest neighbour hopping integral, Coulomb and magnetic-exchange interaction. We hence suggest optimising the parent Mott state as a strategy for enhancing the superconducting transition temperature in cuprates.

Strain-induced tuning of the electronic Coulomb interaction in 3d transition metal oxide perovskites
B. Kim, P. Liu, J. M. Tomczak, C. Franchini
Phys. Rev. B 98, 075130 (2018) arXiv:1806.05503

Epitaxial strain offers an effective route to tune the physical parameters in transition metal oxides. So far, most studies have focused on the effects of strain on the bandwidths and crystal field splitting, but recent experimental and theoretical works have shown that also the effective Coulomb interaction changes upon structural modifications. This effect is expected to be of paramount importance in current material engineering studies based on epitaxy-based material synthetization. Here, we perform constrained random phase approximation calculations for prototypical oxides with a different occupation of the d shell, LaTiO3 (d1), LaVO3 (d2), and LaCrO3 (d3), and systematically study the evolution of the effective Coulomb interactions (Hubbard U and Hund's J) when applying epitaxial strain.

Effective Coulomb interactions in solids under pressure
J. M. Tomczak, T. Miyake, R. Sakuma, and F. Aryasetiawan
Phys. Rev. B 79 (23), 235133 (2009)
arXiv:0906.4398

While the effect of pressure on structures and bands has been investigated extensively, the pressure dependence of electron-electron interactions had received little attention. In this pioneering work, we examine the paradigmatic pressure dependence of Coulomb interactions in a Wannier-function setting.