About me – Designing topological materials

Phases of matter

Solid

Example: ice

Liquid

Example: water

Gas

Example: steam

The solid state

Example: diamond

Example: copper wire

Circuit board showing semiconductor electronic components

Transistor, invented in 1947.

Materials that can switch between both behaviors are called semiconductors. This property makes them the basis of modern electronics.

In reality there are plenty of other phases

Superfluids
Liquid crystals
Magnets
Superconductors
Topological materials

Liquid helium in a glass — Superfluid helium

A magnet attracting metal filings — Magnetism

;

Temperature, magnetic fields, and electric fields are knobs that help us change from one phase to another.

My two favorite phases

Example: aluminum at very low temperature

Discovered in Leiden, The Netherlands in 1908

Example: MnBi2Te4

Discovered in Grenoble, France in 1980

Both conduct perfectly!

Both need extremely low temperatures!

My research: can we design a topological superconductor?

Can we design a topological superconductor?

Why can’t we just find one?

Too many requirements:

Superconductivity needs low magnetic fields
Topological materials usually need high magnetic fields
We want clear experimental signatures
We want to be able to control/change it

Approach 1: discover or synthesize the right material

TODO list:

Choose elements
Grow crystals
Tune composition, pressure, strain, or density
Look for the desired phase

Molecular beam epitaxy system for growing quantum materials

The discovery of 2D materials

Semiconductor stacks

Technology from the 70s

Exfoliation

Method invented in 2004

We now know how to create “sheets” of different materials!

Plot twist: the proximity effect ✨

A semiconductor that is atomically close to a superconductor starts superconducting!

A quantum effect that allows materials to share properties over some distance!

Emotion: Revelation

Question: What do you think happens when a semiconductor touches a superconductor?

Clearly when ice meets water, it melts, but solids do not do that! A metal conducts. An insulator blocks current. A semiconductor can be tuned. A superconductor carries current without resistance. But quantum materials have a surprise for us: when two materials touch cleanly enough, their identities are not completely separate.

A boundary can be a place where quantum behavior is shared, and this is called the proximity effect. If a semiconductor is placed in very good contact with a superconductor, superconducting correlations can leak into the semiconductor. The semiconductor can inherit part of its behavior.

That is the plot twist: we do not need to find a semiconductor that is naturally superconducting. We can place it next to a superconductor and make it behave superconductingly, while keeping the things that make semiconductors useful — especially the ability to control them with gates.

Connector: Once materials can borrow properties from their neighbors, the design strategy changes completely.

Approach 2: build a hybrid device

Hybrid superconducting-semiconducting device

Emotion: Inventiveness

This gives us a second strategy.

Instead of searching for one material that naturally has every property we need, we build a device where each part contributes something.

The semiconductor gives us control. We can tune the number of electrons using electric gates.

The superconductor gives us superconductivity IN the semiconductor through the proximity effect. The interface is the place where the transfer happens and the geometry of the device determines where electrons can move.

This is why hybrid devices are so powerful. The phase is not only discovered. It is assembled.

If the interface is clean, it lets the quantum behavior transfer. If it is messy, it can weaken the effect or create confusing signals.

Connector: The most famous example of this assemble-the-ingredients strategy is the one used for topological superconductivity

Example: planar topological superconductor

Designing new phases

could be possible thanks to the proximity effect

Designing new phases

But stacking materials is not magic

Interfaces must be extremely clean
Each layer can disturb the others
The useful signal can be tiny
Geometry and disorder matter

Thank you!