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Orbital Glasses



Our results in FeCr2S4 provide evidence that this material belongs to a new class of glass-like materials: A novel state of matter, an "orbital glass", arises in this spinel compound at low temperatures.

In an "orbital glass" the electron clouds, forming the outer shells of atoms in solid matter ("orbitals"), freeze into a disordered state at low temperature, in a manner very similar to the continuous arrest of molecular motion occurring when glass blowers perform their craft. In contrast to canonical glasses as glassforming liquids or silica glasses, this exotic type of glassy material has a regular crystalline lattice but the electronic orbitals are disordered with respect to the orientational degrees of freedom (see figure below). In this respect these materials resemble the so-called spin-glasses, where the magnetic moments are disordered, and the plastic crystals and orientational glasses, which exhibit disorder of the orientations of asymmetric molecules.

 

orbital glass

Schematic view of an orbital liquid (above; fast transitions between different orbital configurations), orbital order (left; regular arrangement of the orbitals), and an orbital glass (right; statistical disorder of orbital degrees of freedom). The rhombuses symbolize the surrounding ligands. [from Physik in unserer Zeit 36, 112 (2005)]

 

In FeCr2S4, via the Jahn-Teller effect, the orientational rearrangement of the non-spherical d-orbitals of the Fe2+-ions is accompanied by a distortion of the tetrahedron formed by the surrounding sulfur ions. By measuring the specific heat and the dielectric response of the material in the audio- to radio-frequency range (see Figs. below), we found that at low temperatures the reorientational motion of the electronic orbitals dramatically slows down to form a glass-like state. This "glassy freezing" of orbital motion exhibits typical characteristics known from canonical glasses. However, amazingly here electronic degrees of freedom slow down, becoming as slow as 0.1 s, while usually electrons are known to exhibit much faster dynamics (in the order of 10-15s). In contrast to the conventional glass transition, a complete arrest at the lowest temperatures is suppressed by quantum-mechanical tunneling.

 

specific heat

Specific heat of polycrystalline (PC) and single-crystalline (SC) FeCr2S4. While the poly-crystal shows a pronounced anomaly due to orbital order, the single crystal exhibits the signature of glassy freezing.

dielectric constant

Temperature-dependent dielectric constant of FeCr2S4 for various frequencies. The shoulders indicated by the arrows show the signature of glassy freezing, attributed to the formation of an orbital glass state. The inset demonstrates the absence of this feature in the polycrystal. [from Phys. Rev. Lett. 94, 027601 (2005)]

 

To learn more, see:

R. Fichtl, V. Tsurkan, P.Lunkenheimer, J.Hemberger, V. Fritsch, H.-A. Krug von Nidda, E.-W. Scheidt, and A. Loidl, Phys. Rev. Lett. 94, 027601 (2005),
Physics News Update, Number 716 #3 and
A. Loidl and P. Lunkenheimer, Physik in unserer Zeit 36, 112 (2005).

For further information please contact: peter.lunkenheimer@physik.uni-augsburg.de