# Is there any material insulating in one direction?

Is it possible for a material to be insulating in z direction but conducting in x and y direction? If so want would be the DOS diagram looks like? Will there be a band gap or not?

• en.m.wikipedia.org/wiki/Topological_insulator – physicopath Jan 9 '18 at 23:52
• Hi and welcome to the Physics SE! What are your thoughts? Please note that you are expected to have thoroughly searched for an answer before asking your question. And it's important to detail where you're stuck and why, in order to attract good answers. You can consider checking the advice on writing good questions. – stafusa Jan 10 '18 at 0:28
• @physicopath It is the surface state that is conducting in the case of topological insulator. I was wondering if strong anisotropy would lead to conduction in one direction (bulk state) but insulating in another. – etudiant Jan 10 '18 at 22:50
• what about graphene? – physicopath Jan 11 '18 at 11:14

It depends, it depends. Its difficult to answer your question compeltely because there are so many possible definitions for "materials", and what do you mean by direction? I'm not trying to be pernickety, but are you talking about individual molecules, crystals, or macroscopic electronic devices? All will have different answers. Do you mean direction on the molecular scale, or on a device?

But, what you might find interesting to consider is conjugated polymers.

These are long, thin conducting plastics. The simplest is called polyacetylene. It looks like this

We can make it as long as we want. It conducts because of the double carbon bondss. One valence electron on each double carbon atom resides in an orbital pointing upwards, which is orthogonal to the other three sigma-bonds.

All the upwards pointing orbitals combine with each other, to form a molecule wide delocalised set of orbitals. Through some oxidation reaction, some of these electrons will be lost.

Now you have a one-dimension electronic band, and the electrons within this band become mobile when it is partially emptied. If you want a mathematical rigour this can be described in the tight-binding model. The DOS looks like

Where $E_{v}$ corresponds to the top of the valence band.

These polymers will have good conductivity in one direction. Conductivity in the perpendicular direction depends on how well you can align your molecules and how close they are together. You could extend this to be two dimensional by attaching strips on the ends of the polymers.

We can adapt this molecule to be a good semiconductor by adding some more fancy molecules onto the backbone. A particular one of research interest at the moment is pentacene:

These have HOMO and LUMO levels, analogous in many ways to the valence band and the conduction band, and are organic semiconductors. According to Sigma - its HOMO and LUMO energies are HOMO $5eV$, LUMO $3eV$ (with respect to the vacuum level). - Thats a band gap of $2eV$

## tl;dr

Is it possible for a material to be insulating in z direction but conducting in x and y direction? - Yes - molecules which only have one dimension.

If so want would be the DOS diagram looks like? - Depends on the orbital geometry.

Will there be a band gap or not? - Depends on the material (but generally yes, but you have to think of the orbitals - its quite different from metals, but thats a story for another day).

Consider 3D crystalline system. in x and y-direction, there is a hopping term but there is no hopping in the z-direction. consequently, the system has a dissipation in XY plane but in the z-direction, the system has a dissipationless character.

In an effective manner, we try to describe a system with anisotropic mass which for a z-direction diverge. this system has a metallic character in XY plane but insulation in the z-direction.

• I don't see how this provides an answer to the question. – JMac Jan 15 '18 at 17:32
• There is no band gap but slop of dispersion along z-direction is vanish. – Rasoul-Ghadimi Jan 16 '18 at 13:19
• That isn't really a 3D crystalline system - it's just a stack of completely unrelated 2D systems. – Emilio Pisanty Jan 19 '18 at 15:25
• You must provide anisotropic situation which can be constructed by the stack of 2D systems. – Rasoul-Ghadimi Jan 20 '18 at 12:59