# Presentation: 293981

## Spectroscopy with a Twist

- Infrared magneto-polarization measurements
**John Cerne, University at Buffalo, SUNY, DMR 0449899**

- In addition to providing new challenges to our basic understanding of materials, strange metals (such as ruthenate oxides and high temperature superconductors) and magnetic semiconductors hold great technological promise. By exploring how polarization (the direction the electric field in a light wave oscillates) of transmitted and reflected infrared light changes in a magnetic field, we find new structure that is hidden from more conventional measurements, such as infrared spectroscopy, which only measures the intensity (the amplitude of the electric field in a light wave) of transmitted and reflected light.
- We have used these techniques (Kim et al., Phys. Rev B 75, 214416 (2007)) in a collaboration with the Drew (U. Maryland) and Millis (Columbia U.) groups to explore the superconductor Pr2-xCexCuO4. One of the key questions is whether there are any remnants of the anti-ferromagnetic insulator (AFI) phase once the superconducting (SC) phase is reached by adding electrons to the compound by doping. Different experimental techniques produce different answers. Since our measurements are sensitive to small differences in the chirality (left or right handedness) of absorption of infrared light, dramatic gap-like features associated with the AFI state (circled zero crossings for x=0.12 and 0.15 samples in figure) are clearly seen even for optimally doped Pr2-xCexCuO4, which has the highest (optimum) superconducting temperature. This is the first direct optical evidence that the AFI state is still present even for samples that have the highest superconducting temperature. The overdoped (x=0.18) sample also shows unusual behavior, which is currently being investigated.

**Notes:**
We have developed new measurement and analysis techniques that allow us to extract both the infrared (1-11 micron wavelength light) longitudinal conductivity σxx, which describes how charges move along an applied electric field, as well as the Hall conductivity σxy, which describes how charges move perpendicular to an applied electric field in the presence of a magnetic field, by measuring the polarization of light (Kim et al., Phys. Rev B 75, 214416 (2007)). We have used these techniques in a collaboration with the Drew (U. Maryland) and Millis (Columbia U.) groups to explore σxy in the superconductor Pr2-xCexCuO4 (see figure and Zimmers et al., Phys. Rev. B 76, 064515 (2007)). Since σxy is sensitive to small differences in the chirality (left or right handedness) of absorption of infrared light, dramatic gap-like features evidenced by Im σxy crossing zero (x=0.12 and 0.15 samples in figure) are clearly seen even for optimally doped Pr2-xCexCuO4. This is the first direct optical evidence that the AFI state is still present even for samples that have the highest superconducting temperature. The overdoped (x=0.18) sample also shows unusual behavior, which is currently being investigated.

## CLAW: Conceptual Learning Approach to Waves

- Getting a physical grasp on waves and polarized light
**John Cerne, University at Buffalo, SUNY, DMR 0449899**

- Waves represent one of the most important concepts in physics, playing a crucial role in topics ranging from acoustical phenomena, electricity and magnetism, optics, Fourier analysis, and even quantum mechanics. However, since waves have both a temporal and spatial dependence (often in more than one dimension) that may be difficult to visualize, many undergraduate and graduate students have a poor understanding of even basic wave concepts. We are creating a web site (_[http://electron.physics.buffalo.edu/claw](http://electron.physics.buffalo.edu/claw/)__[/](http://electron.physics.buffalo.edu/claw/)_) that explains many basic wave concepts using dynamic and interactive graphical simulations. There are many excellent web sites using similar graphical interactive tools, but they tend to focus on mechanics, electrostatics, and magnetism. I am actively using this site for my introductory physics courses, as well as a magneto-polarimetry teaching lab that I have created (_[http://www.physics.buffalo.edu/cerne/education/moke_manual.pdf](http://www.physics.buffalo.edu/cerne/education/moke_manual.pdf)_).

**Phase of a sine wave **