0.7 Solid state and superconductors  (Page 6/9)

Experimental procedure

Every part of the experimental procedure has correspondent questions on the Report Form. Do not proceed until ALL questions accompanying each section have been answered and recorded.

1. Demonstration of the 1,2,3-superconductor ${\text{YBa}}_2{\text{Cu}}_3{O}_{8-}$

A pellet of the 1,2,3-superconductor ${\text{YBa}}_2{\text{Cu}}_3{O}_{8-}$ is placed on the top of an inverted paper cup. The pellet is cooled down by carefully pouring liquid nitrogen over it until the bottom of the cup is filled up. After approximately 10 seconds (when the bubbling stops) the pellet should reach the liquid nitrogen temperature. Your TA will then place a very strong magnet over the pellet.

What happens to the magnet? What happens as the superconductor warms up? What is the Meissner effect? (Write observations and answer these questions on your report form)

Warning- LIQUID NITROGEN CAN CAUSE FROST BITE! Do not directly touch anything that has come into contact with the liquid nitrogen until it is warmed up to room temperature.

NOTE TO TA: to remove a levitating magnet, simply wait until the liquid nitrogen fully evaporates or use another magnet to "grab" the floating magnet. Be careful not to lose or break these very tiny, yet expensive, magnets!!!!

2. cubic cells

There are many types of fundamental unit cells, one of which is the cubic cell. In turn, there are three subclasses of the cubic cell:

a. simple or primitive cubic (P)

b. body-centered cubic (bcc, I*)

c. face-centered cubic (fcc, F)

*The I designation for body-centered cubic comes from the German word innenzentriert.

We do not have time to build models of all of the unit cells possible, so we will focus on the cubic structure and its variations. Our investigation will include several aspects of each cell type:

• the number of atoms per unit cell
• the efficiency of the packing of atoms in the volume of each unit cell
• the number of nearest neighbors (coordination number) for each type of atom
• the stoichiometry (atom-to-atom ratio) of the compound

A.    Simple Cubic Unit Cells or Primitive Cubic Unit Cells (P)

Team a

Group 1. Single Unit Cell

·Construct a simple cubic cell using template A and its matching base.

·Insert rods in the 4 circled holes in the shaded region of the template.

·Build the first layer (z = 0) by placing a colorless sphere on each rod in the shaded region.

·Draw a picture of this layer as previously described.

·Complete the unit cell by placing 4 colorless spheres on top of the first layer.

This is the z=1 layer.

 Group 2. extended structure

• Construct an extended cubic cell using template A.
• Insert rods in the circled holes of template A in the area enclosed by the dotted lines.
• Construct a set of unit cells as described for making a single unit cell.

Look closely at the structures generated by both groups. They are called simple (or primitive) cubic.

Considering all of the cells around it, answer the corresponding questions on the report form.

B.     Body-Centered Cubic Structure (BCC)

Team B

Group 1. Single Unit Cell

·Construct a body-centered cubic (bcc) structure using template F.·Insert the rods in all 5 of the holes in the shaded region.