When an exciton acts like a hole

Aug 27, 2014 by Mark Wolverton
Selective mapping of the two-exciton modes and their comparison to SCBA calculations. (a) Image plot of RIXS spectra measured along high-symmetry lines in the normal and grazing incidence geometry. (b) SCBA calculations.

(Phys.org) —When is an electron hole like a quasiparticle (QP)? More specifically, what happens when a single electron hole is doped into a two-dimensional quantum antiferromagnet? Quasiparticle phenomena in such a system are predicted by theory, but have eluded observation, complicating the understanding of electron behavior in high-temperature superconducting cuprates. A team of experimenters working at the U.S. Department of Energy's (DOE's) Advanced Photon Source at Argonne National Laboratory have taken a different approach to the problem with their recent observation of an excitonic quasiparticle in strontium iridate (Sr2IrO4), a quasi-two-dimensional, spin-1/2, antiferromagnetic Mott insulator. Their work was published in Nature Communications.

Strontium iridate is a newly-discovered pseudospin-1/2 Heisenberg antiferromagnet in which superconductivity has also been predicted but not yet observed. The experimenters from Argonne National Laboratory and the Institute for Theoretical Solid State Physics and Max Planck Institute for Solid State Research (Germany) used resonant inelastic x-ray scattering (RIXS) at the X-ray Science Division 30-ID-B,C beamline of the Advanced Photon Source, a DOE Office of Science user facility. They carried out their studies utilizing the MERIX spectrometer, a medium-energy-resolution diffractometer for non-resonant and resonant inelastic x-ray scattering

(RIXS) measurements of samples that had been prepared at the Argonne Materials Science Division. Earlier RIXS studies of Sr2IrO4 showed that the dispersions of an electron hole excited across spin-orbit coupling levels could be related to the QP problem. The investigators attempted to reflect the behavior of an electron hole in Sr2IrO4 by using an exciton analog.

Using a theoretical model structured around the observation that the propagation of an orbital excitation through a Mott insulator such as Sr2IrO4 can be mapped on the motion of a single hole, the researchers employed a self-consistent Born approximation (SCBA) for comparison and interpretation of the RIXS data. They found an excellent agreement between the effective t-J model (used to calculate high-Tc states) calculated within the SCBA and the experimental spectra. Energy distribution curves showed a sharp exciton peak, which is both narrower than the sharpest peak measured in Sr2IrO4 by angle-resolved photoemission spectroscopy (ARPES) and smaller than the total bandwidth of about 112 meV, which reveals the exciton to be a QP.

While this demonstrates that a coherent particle can propagate through a quantum antiferromagnet, it does not yet explain the lack of a QP with a single-hole excitation. The researchers attribute this to the charge-neutral nature of an exciton compared to the charged-particle character of a hole. Because a charged hole interacts much more strongly with the lattice structure and the inevitable impurities present in an insulator, it also tends to dampen or completely wash out QP phenomena, particularly in ARPES measurements. On the other hand, a charge-neutral exciton is not subject to these effects and thus can more readily reveal subtle QP dynamics that can elude experimenters.

The work confirms that QPs can definitely be observed in Mott insulators and that these materials can display striking parallels with high-temperature cuprate superconductors. By showing that an exciton in a spin-1/2 antiferromagnet can display fundamentally similar dynamics to a charged hole doped into the same kind of system, the researchers have cast new light on a classic problem in condensed matter physics and opened a new pathway for the study of high-temperature superconductors.

Explore further: Physicists discover 'quantum droplet' in semiconductor

More information: Jungho Kim1, M. Daghofer2, A.H. Said1, T. Gog1, J. van den Brink2, G. Khaliullin3, and B.J. Kim3,1*, "Excitonic quasiparticles in a spin–orbit Mott insulator ," Nat. Commun. 5, article No. 4453 (published on line 17 July 2014). DOI: 10.1038/ncomms5453

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not rated yet Aug 28, 2014
This article has to be the most dense usage of purely technical terms seen yet...
not rated yet Aug 28, 2014
From high-level layman perspective the whole stuff is working in the following way: the magnetic field is known to be expelled out of superconductors. But what will happen, if the superconductor is composed of paramagnetic atoms by itself? Under such a situation the superconductivity has a problem even in the bulk of material and it tends to become separated there into many individual zones, which travel back and forth rather freely. What does it mean for electrons here?

The superconductivity is usually constrained to so-called hole stripes: the holes are places of positively charged atoms within lattice, to which the electrons are attracted like the hungry hens to the feeders. The jam of electrons around holes is the effect responsible for free conduction of charge in these areas. Now, when the areas rich of electrons can move so freely, as above explained, they will create a ripples in similar way like the electrons at the surface of metals, which form a plasmon quasiparticles.
not rated yet Aug 28, 2014
So try to imagine a poultry farm, equipped with feeders hanging from the ceiling in long rows. The hungry hens are therefore concentrated around feeders in a rows and they fight for their place. When we sway the feeders, then the hens will follow them desperately and they will form a large ripples across whole farm.

When we do the same experiments with electrons, then the bulk ripples of electrons around electron holes can be detected easily with spectra - they will resonate and absorb microwaves at narrow range of frequencies. This experiment demonstrates the volatility and complexity of superconductivity effect inside of paramagnetic superconductors. Theoretically it could be utilized for special types of generators of terahertz waves or similar purposes - but given the cost of ruthenium and necessity of low temperature I'd say, this option is rather hypothetical in this moment.
Whydening Gyre
not rated yet Aug 28, 2014
Love the new chicken analogies. (Much more fun then just water...)
not rated yet Aug 28, 2014
You can imagine the ducklings at the place of chicken. The ducks are already compatible with AWT in full extent...

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