Analyzing the electronic states of a heavily overdoped high temperature superconductor
 
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Analyzing the electronic states of a heavily
overdoped high temperature superconductor

SUMMARY

Superconductors are materials in which some of the electrons are able to lose their individual identity by pairing up and 'condensing' into a single macroscopic quantum mechanical state. This produces exotic and useful properties including the ability to conduct electrical current with zero resistance. Some common materials are superconductors at very low temperatures (e.g. lead at 7K, mercury at 4K, aluminum at 1K), and in these materials the pairing mechanism is well understood. However, in the mid-1980s, a new class of multi-layered copper-based superconductors was discovered with transition temperatures ranging above 100K (-173°C, which counts as "high temperature" in this field). These posed two of the most important unsolved questions in physics: what makes these materials superconductors, and is it possible to achieve superconductivity at room temperature?

The Bi_2Sr_2CaCu_2O_8+x unit cell, and a photograph of the heavily overdoped sample used in the Davis Group study.

Figure 1: The Bi2Sr2CaCu2O8+x unit cell, and a photograph of the heavily overdoped sample used in the Davis Group study.


One of the most widely studied high temperature superconductors is Bi2Sr2CaCu2O8+x (Bi-2212, figure 1). Like the rest of its family, the superconductor is produced by doping its insulating parent compound with a certain quantity of excess charge carriers (in this case, by adding more oxygen). To understand what makes these materials work, it is necessary to understand how the doping produces the superconductivity, and why it disappears when the materials are doped too much. One path is to study crystals that are heavily overdoped yet still superconducting, to see what states begin to evolve, and to test various competing theoretical predictions about what should happen.

Using a purpose-built scanning tunneling microscope (STM) in the ultra-low vibration research laboratory of the Davis Group at Cornell University, graduate student James Slezak produced a topograph (figure 2) of the surface of the crystal, and a spectroscopic line cut of the density of states (figure 3). The shape and features of this 3D surface provides important information about the electronic state of the material and its spatial evolution.

 

Atomic resolution topographic image of the heavily overdoped superconductor Bi-2212, with red line indicating position and direction of spectroscopic line-cut.

Figure 2 : Atomic resolution topographic image of the heavily overdoped superconductor Bi-2212, with red line indicating position and direction of spectroscopic line-cut.


3D Wire Surface Color Map graph of a line cut representing the density of electronic states in the superconductor along the line indicated in figure 2.

Figure 3: A line cut representing the density of electronic states in the superconductor along the line indicated in figure 2.

ANALYSIS IN ORIGIN

The line cut data were imported from the STM control software using Origin's ASCII data Import Wizard. There was one spectrum for each point along the line, and these were combined onto one worksheet using the copy and paste functions. Each spectrum gives the tunneling conductance of the STM tip-sample junction as a function of the bias voltage, which is conventionally interpreted as the density of states of the material as a function of energy.

The horizontal axis of the worksheet corresponded to distance along the line cut, so the top row of the worksheet contained the position, in nanometers, of each spectrum in the column below. To produce the 3D graph, the worksheet was converted to a matrix (using Origin's direct conversion method). To reduce the number of points plotted, the size of the matrix along each axis was halved using the "shrink matrix" command and smoothed. The graph was produced as a 3D color map surface plot.

ACKNOWLEDGMENTS

  Origin's Direct Conversion to Matrix dialog

This research was conducted at the J. C. Davis Group, Laboratory of Atomic and Solid State Physics, at Cornell University. The samples were prepared by Kazuhiro Fujita (U. Tokyo), Dr. Hiroshi Eisaki (AIST-Tokyo) and Prof. Shin-ichi Uchida (U. Tokyo) in Japan, and were overdoped at the Davis Group labs at Cornell by Kazuhiro Fujita and James Slezak. The STM experiment was performed by James Slezak, Kazuhiro Fujita, Dr. Jinho Lee (Cornell) and Alfred Wang (Cornell).

James Slezak is a Ph.D. student in the Department of Physics at Cornell University.

 

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