Abstract-Electromagnon excitation in cupric oxide measured by Fabry-Pérot enhanced terahertz Mueller matrix ellipsometry

Sean Knight, Dharmalingam Prabhakaran, Christian Binek,  Mathias Schubert

https://www.nature.com/articles/s41598-018-37639-8

Here we present the use of Fabry-Pérot enhanced terahertz (THz) Mueller matrix ellipsometry to measure an electromagnon excitation in monoclinic cupric oxide (CuO). As a magnetically induced ferroelectric multiferroic, CuO exhibits coupling between electric and magnetic order. This gives rise to special quasiparticle excitations at THz frequencies called electromagnons. In order to measure the electromagnons in CuO, we exploit single-crystal CuO as a THz Fabry-Pérot cavity to resonantly enhance the excitation’s signature. This enhancement technique enables the complex index of refraction to be extracted. We observe a peak in the absorption coefficient near 0.705 THz and 215 K, which corresponds to the electromagnon excitation. This absorption peak is observed along only one major polarizability axis in the monoclinic a–c plane. We show the excitation can be represented using the Lorentz oscillator model, and discuss how these Lorentz parameters evolve with temperature. Our findings are in excellent agreement with previous characterizations by THz time-domain spectroscopy (THz-TDS), which demonstrates the validity of this enhancement technique.

Abstract-Terahertz field control of in-plane orbital order in La0.5Sr1.5MnO4

• Timothy A Miller,
• Ravindra W Chhajlany,
• Luca Tagliacozzo,
• Bertram Green,
• Sergey Kovalev,
• Dharmalingam Prabhakaran,
• Maciej Lewenstein,
• Michael Gensch
• & Simon Wall

http://www.nature.com/ncomms/2015/150918/ncomms9175/full/ncomms9175.html

Nature Communications

 

6,

 

Article number:

 

8175

 

doi:10.1038/ncomms9175

Received

 

02 February 2015 

Accepted

 

27 July 2015 

Published

 

18 September 2015

In-plane anisotropic ground states are ubiquitous in correlated solids such as pnictides, cuprates and manganites. They can arise from doping Mott insulators and compete with phases such as superconductivity; however, their origins are debated. Strong coupling between lattice, charge, orbital and spin degrees of freedom results in simultaneous ordering of multiple parameters, masking the mechanism that drives the transition. Here we demonstrate that the orbital domains in a manganite can be oriented by the polarization of a pulsed THz light field. Through the application of a Hubbard model, we show that domain control can be achieved by enhancing the local Coulomb interactions, which drive domain reorientation. Our results highlight the key role played by the Coulomb interaction in the control and manipulation of orbital order in the manganites and demonstrate a new way to use THz to understand and manipulate anisotropic phases in a potentially broad range of correlated materials.