## Humboldt Research Award for Professor Andrzej M. Oleś

**Professor Andrzej M. Oleś** received Humboldt Research Award (Humboldt-Forschungspreis) in recognition of his entire achievements in the theoretical physics of condensed matter.

The works of A. M. Oleś advanced the field of electron correlations in transition metals, Mott insulators, and high-T_{c} superconductors. He introduced important concepts in the theory of quantum materials with several interacting degrees of freedom, in particular discovered the quantum aspects of spin-orbital physics, including such phenomena as fluctuations and entanglement. The most important discoveries were completed in the collaboration with his collegues from Quantum Many-Body Theory Department in Max Planck Institute for Solid State Research.

_{x},I)-plane for fixed AF interaction J

_{z }= 1. Long-range order is stabilized by any finite Heisenberg interaction I. Square and diamond at the compass line indicate multi-critical points, where four ordered phases meet. The spin order of the different phases is depicted in a corresponding inset, and the subscript indicates the type of symmetry breaking in spin space. Straight lines separating the phases follow from symmetry; in two cases they are modified by quantum corrections.

*F. Trousselet, A. M. Ole***, Magnetic Properties of Nanoscale Compass-Heisenberg Planar Clusters, Physical Review B**

*ś, P. Horsch***86**, 134412 (2012).

_{x},I)-plane for fixed AF interaction J

_{z }= 1. Long-range order is stabilized by any finite Heisenberg interaction I. Square and diamond at the compass line indicate multi-critical points, where four ordered phases meet. The spin order of the different phases is depicted in a corresponding inset, and the subscript indicates the type of symmetry breaking in spin space. Straight lines separating the phases follow from symmetry; in two cases they are modified by quantum corrections.

*F. Trousselet, A. M. Ole***, Magnetic Properties of Nanoscale Compass-Heisenberg Planar Clusters, Physical Review B**

*ś, P. Horsch***86**, 134412 (2012).

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**The selected papers listed here introduce new fundamental concepts:**

The importance of novel quantum orbital fluctuations in spin-orbital systems was discovered by Feiner, Oleś, and Zaanen in [1]. The interplay between spin and orbital degrees of freedom in transition metal oxides is described in detail in [2], including some cases where the interpretation requires spin-orbital *entanglement* [3]. For a hole doped Mott insulator, as e. g. LaVO_{3}, he introduced *spin-orbital polarons*, explaining that the orbital degrees of freedom confine the spin dynamics and generate the string potential of the joint spin-orbital character [4]. Recent research concentrates on the impact of various defect states in spin-orbital systems, including the *orbital dilution* in doped ruthenates [5].

**[1]** **L. F. Feiner, A. M. Oleś, J. Zaanen**

Quantum Melting of Magnetic Order due to Orbital Fluctuations

Physical Review Letters **78**, 2799-2802 (1997)

**[2]** **A. M. Oleś, G. Khaliullin, P. Horsch, L. F. Feiner**

Fingerprints of Spin-Orbital Physics in Cubic Mott Insulators:

Magnetic Exchange Interactions and Optical Spectral Weights

Physical Review B **72**, 214431/1-32 (2005)

**[3] A. M. Oleś, P. Horsch, L. F. Feiner, G. Khaliullin**

Spin-Orbital Entanglement and Violation of the Goodenough-Kanamori Rules

Physical Review Letters

**96**, 147205/1-4 (2006)

**[4] K. Wohlfeld, A. M. Oleś, P. Horsch**

Orbitally Induced String Formation in the Spin-Orbital Polarons

Physical Review B

**79**, 224433/1-17 (2009)

[Editor's choice and Viewpoint in Physics]

**[5]** **W. Brzezicki, A. M. Oleś, M. Cuoco**

Spin-Orbital Order Modified by Orbital Dilution in Transition Metal Oxides:

From Spin Defects to Frustrated Spins Polarizing Host Orbitals

Physical Review X **5**, 011037/1-28 (2015)