Chinese researchers at the Institute of Solid State Physics, Hefei Institutes of Physical Science have investigated the optical properties of lithium titanate superconducting thin films with  and  lattice orientations using the simply and standard infrared reflection measurement.
Based on this work, the temperature-induced metal-superconductor transition and the electronic localization effect in metallic phase have been observed optically.
Generally, Lithium titanate is a kind of traditional superconductor material and has been applied in many fields such as power energy, superconductor magnet, telecommunication, microelectronics, etc.
In this study, the lithium titanate superconducting thin films were fabricated on magnesium alumina by pulsed laser deposition. The samples are with  and  lattice orientations.
Actually, the results obtained from previous electrical transport measurements have shown that the metal-superconductor transition temperature of lithium titanate superconductor thin films is about 10-12 K, and there exists the anisotropic features of the transport properties for samples with  and  lattice orientations.
However, very little optical study on this superconductor material has been conducted. Given this, we take a simple and standard experimental setup to measure the near infrared (NIR) reflection spectrum of the lithium titanate superconductor thin films.
Moreover, in the NIR bandwidth (the radiation wavelength about 1-2 ¦Ìm) the condition: ¦Ø¦Ó~1can be satisfied for lithium titanate superconductor thin films, here ¦Ø is the radiation frequency and ¦Ó is the electronic relaxation time of the sample.
As a result, the optical response of the electrons in the sample can be achieved and the NIR reflection experiments can measure the dependence of the optical conductivity of the sample on temperature and radiation wavelength (see Figure 1).
Consequently, as a contactless measurement method the NIR reflectance spectroscopy can be directly applied to the study of basic physical features of lithium titanate superconductor films.
We found that the metal-superconductor transition temperatures measured optically agree with those obtained from electrical transport measurements for samples with different lattice orientations (Figure 2).
In addition to the temperature dependence of the NIR reflectivity looks very similar to that of the DC resistance (Figure 3).
More importantly, through examining the dependence of the NIR reflectivity on radiation wavelength, the electronic localization effect can be observed for lithium titanate superconductor thin films in metallic phase, such an effect cannot be directly measured via conventional electrical transport experiments.
This work demonstrates that the simple and contactless optical measurements can be applied to study and characterize the superconductor thin films. We would like to note that one of the Referee¡¯s reports to this article was with only one sentence: ¡°The paper is very good¡±.
The results have been published in the Optics Letters, entitled ¡°Study of spinel LiTi2O4 superconductors via near-infrared reflection experiments¡±.
This work was in collaboration with Shandong University and Institute of Physics£¬Chinese Academy of Sciences, supported by the National Natural Science Foundation of China (No.11574319) and the Ministry of Science and Technology of China. (No.2011YQ130018).
Figure1. NIR reflectance spectra of lithium titanate thin films at different temperatures. The left and right panels are the results for samples with  and  orientations, respectively. (Image by MEI Hongying)
Figure2. Temperature dependence of the reflectivity of lithium titanate thin films with different radiation wavelengths for  (left) and  (right) oriented samples respectively. (Image by MEI Hongying)
Fig. 3 Temperature-dependence of reflectivity (black) at a fixed NIR wavelength and DC resistivity (red) for samples with  (solid) and  (dashed) lattice directions. (Image by MEI Hongying)
Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences Hefei, Anhui 230000, China