[vcg3rd]

Anyone feel like explaining it to me like I'm 5? High School physics was over 40 years ago, and while I like reading "layman's" science this one only made me think about the discussion of diminsions in 3 Body Problem, which also confused me. Frankly I've never figured out why people write of dimensions as if Time isn't one (e.g. isn't the 2D material discussed actually 3D?) Is it just understood that Time is a given one doesn't have to name?

TIA

[GlibMonkeyDeath]

There's quite a few layers to this onion, I'll try to go from simplest to what is described in the article:

Coordinate definition: Current travels along x, magnetic field applied along z, and voltage is measured across y:

Hall Effect: A transverse voltage (y) that appears along a conductor carrying current (along x) in the presence of a transverse (z) magnetic field. Basically, the magnetic field pushes the traveling charge to one side or the other (+/- y) as it moves, causing a charge separation along y, and hence generating an external "Hall" voltage. (BTW, this is a handy way to detect a magnetic field of a small permanent magnet, hence all the "Hall effect sensors" you can buy to measure if e.g. your door is closed, without need for a physical electrical contact...)

Quantum Hall Effect: The observed voltage has quantized "Steps" because electrons must form orbits that follow the rules of quantum mechanics (so the quantum phase of the carriers must be an integer along the orbit) It's usually only observed at very low temperatures (so the carriers aren't getting their phases bashed around) and in "2D" materials (i.e., carriers restricted to the say X-Y plane, as in a MOSFET just under the oxide gate.) That way all the orbits have to lie in the same plane.

Fractional Quantum Hall Effect: Wait, the orbits can sometimes be some other integer fraction (like 4/7, 2/3 etc.)? How can that be? Well, basically the carriers interact with the outside edges of the material (the orbits "bounce against" the edges, making it "topological" i.e. dependent on the physical shape of the sample.)

Anomalous (Quantum or not, Fractional or not) Hall Effect: Rather than supplying the external magnetic field, the sample itself (via some spin-orbit mechanism, i.e. the little orbits of the electrons around the atoms) effectively supply the magnetic field.

It isn't obvious that this material should exhibit fractional, anomalous quantum Hall effect behavior, so theorists are super excited to work on something new. And who knows? Lots of times things that seem boring and unimportant turn out to be useful...

[tsimionescu]

Time is not a dimension in the same sense as the spatial dimensions. It's a coordinate for events, of course, but you can't for example cross from one side of a line on a 2D plane to the other without ever intersecting the line by moving in the time dimension like you can if you have a 3rd space dimension (e.g. by flying over the line).

Even in relativity, the fourth time-related dimension is not of the same nature as the other two (the distance between two points counts the time dimension differently than the space ones).

[PlunderBunny]

By coincidence (?), Futility Closet has just posted an interesting argument by philosopher Ned Markosianto that 'time is not the fourth dimension':

https://www.futilitycloset.com/2024/03/27/sideways-music/

[finally2049]

> you can't for example cross from one side of a line on a 2D plane to the other without ever intersecting the line by moving in the time dimension

Can't you though? Time travel to before the line existed, move over, time travel back.

[emporas]

Well, absolute zero temperatures (or close to it) are the usual way to create quantum computation. Absolute zero temperatures however are totally impractical economically also size of the machine (see MRI machines) and other reasons i cannot recall right now.

In absolute zero temperatures, the electrons behave in a discreet way, like digits, they behave less unpredictably. Temperature is one way to control their movement, but i think magnetism is another one.

What these two discoveries mean (MoTe2 and the graphene one), is that there is another way to control their movement, less unpredictable movement once again, in normal temperatures and without magnetism. They call that "fractional quantum anomalous Hall effect (FQAHE)".

That's my take on it. Still not GPT-4 level but getting there.

EDIT: Also 2d material means a material one atom thick. Graphene is exactly that, that's the definition of graphene: graphite one atom thick. The two materials they describe, are not 2D exactly, but they are thin enough that they consider them 2D. They essentially mean sheets of atoms, instead of them being just an atom thick, a little bit more, like 5 or 10 atoms thick.

[behringer]

Time is not a spatial dimension. It's not relavent in most spatial discussions.

[luxuryballs]

I’m not sure if you can measure time without employing something that changes spatially though can you?

[bee_rider]

I think when nanodevice engineers talk about 2D problems, they are talking about devices in which the interesting physics happen in 2D.

The actual devices actually exist in the 3 spatial dimensions of course, and evolve in the time dimension. They just have physics that involve very flat sheets of things, or all of the interesting stuff happens on the surface of something.

The big deal stuff about time and space both being dimensions is more of a relativity thing I think. IIRC in quantum physics time is really annoying and everybody hates it.

[luxuryballs]

yeah I think time is just an abstraction for measuring things happening in space