Slivers of FlowerpotsPreparing the Surface of Single Crystal High Temperature Superconductors Using the UltracutI. Whats a high Tc?Properties which define superconductivitySpecific kinds of superconductors: high Tc cuprate familyII. Why try to cut them?Theyre as hard as flowerpots (ceramic), but the physicsis compelling!III. Crystal Preparation and CuttingUsing the Ultracut @ the Microscopy Suite to Prepare Surfaces.IV. Prospective MeasurementsK.P.Singh, Research Scholar, Department of PhysicsM.Shahabuddin, Jamia Millia Islamia,Reader of PhysicsThanks to Dept. of Physics.

What is a superconductor?Superconductors are characterized by a few central properties:

ZERO RESISTANCE below a certain temperature, Tc the critical temperature

Most wires possess some amount of resistance tothe flow of electricity. This leads to energy loss.

Superconductors allow electricity to flow freely withoutloss of energy. Theyre perfect conductors.

DIAMAGNETISM that is, a superconductor cannot be magnetized, It expels all magnetic flux when a field is applied.

Kinds of superconductorsSuperconductors are classified based on their Tc , among other properties.

LOW Tc SUPERCONDUCTORS: familiar materials (metallic elements)

HIGH Tc SUPERCONDUCTORS: not so familiar materials (ceramics)Rare earths + copper + oxygen = CUPRATES Discovered in 1986, Bednorz and MuellerTc(K)LeadIndiumMercury...

Alloys of these

Cuprate of Choice: Bi2Sr2Ca1Cu2O8 (BSCCO)(pronounced Bis-co)

Well-Studied High Temperature Superconductor Tc~ 92K Grown as single crystals (Argonne National Labs., MSD, D.G. Hinks)

Whats so special about these superconductors?

1. We dont understand fully how they superconduct.How electrons pair to form a superconductor is a complex issue.

Bardeen, Cooper and Schreiffer (all from UIUC)figured out how low Tcs superconduct (late 1950s), but thehigh Tcs are a mystery.

2. Unlike the low Tc materials, these high Tcs are anisotropic, that is properties are different indifferent directions.

Can the anisotropy of certain properties tell us anything about the mechanism for high Tc superconductivity?

Why would we want to cut these?In order to measure the anisotropy of different properties, we have tocut the crystals somehow in order to expose different crystallographic orientations.

Our measurements are surface-sensitive, so we need a techniquethat allows us to prepare a very smooth (to within a few 10s of nm)surface.Use the Ultracut!We were wary at first, since high temperaturesuperconductors are ceramics.

Could we cut a flowerpot-like material with a delicate diamond knife?

BSCCOAttach Au wires to the base ofthe crystal using silver paint.

Use a Teflon block to holdcrystal + leads in place insidea cylindrical mold and fillwith epoxy.

Result is a structure that is partially wired and readyto be ultramicrotomed.

Grown of Single Crystal

The Ultracut is no more than a refined paper cutter for our system!

Our material is ordered like a stack of papers.Each sheet of paper is a Cu-O plane. We can choose to cut along the stack height or parallelto the stack surface. The direction of the cut influences the surface roughness. ORAgainst the grain Causes some buckling at the surface, thus rough featuresAlong the grain Smoother features

The physics were interested in is for samples that are cut against the grain. It is possible to cleave the crystal perpendicular to the planes, but it is not possible to cleave parallel to the planes.

This is why ultramicrotomy is so important here:

Wed like to measure properties of a material in a direction that does not cleave naturally.

In order to make those measurements, we need a smooth surface (so that we dont average over othercrystal directions as much).

OK!NO!

AFM of a BSCCO Surface Cut Against the GrainOther techniques to prepare the surface, such as polishingwith diamond paper, dont come close!

Prospective Measurements1. Measurements of the Superconducting Density of StatesThis tells us the energy spectrum of electronsin a superconductor and how they pair with one another.

2. Preparation of other Unconventional Superconductors for Density of States Measurements

Organic Superconductors

There are many superconductive systems that wedont understand fully. Using ultramicrotomy as a wayto prepare surfaces will help!

SummaryThe Ultracut is a great tool for surface preparation of non-biological samples.

It has been used to cut hard samples, with only nominal damage to the diamond knife.

It is possible prepare smooth surfaces that do not naturally cleave, allowing measurements of anisotropic properties of superconductors.

Surface-sensitive measurements will benefit from the Ultracut,since the surfaces created by the knife are only a few nanometers rough. This is a dramatic improvement upon current surface prep. techniques such as polishing.

K.P. Singh, University of Jamia Millia Islamia, Jan 2003