What is quantum matter?

It may depend on who you ask.
It is interesting that even twenty years ago the phrase "quantum matter" was rarely used.
Now we have

Department of Quantum Matter Science, Hiroshima University

Quantum Matter Institute, University of British Columbia

Shoenberg Laboratory for Quantum Matter, University of Cambridge

So, what is quantum matter?

To some it is any material system (solid, liquid, or gas) where the quantum statistics of the constituent particles significantly affect the properties of the system. One could argue on some level this is any state of matter! After all, the Pauli exclusion principle is key to chemistry!

The above departments are largely concerned with studying what used to be called "strongly correlated electron systems". Hence, one also often sees the phrase "correlated quantum matter". I think David Pines and Piers Coleman may be two of the people who have most promoted the phrase. Coleman and Andy Schofield use the phrase "quantum matter" repeatedly in their 2005 Nature review Quantum criticality. Pines has a nice tutorial article Emergent behavior in quantum matter.
Does anyone have a better etymology?

To me the key idea is that there are states of matter [quantum many-body systems] with emergent macroscopic properties that are intrinsically quantum mechanical. Superconductivity is the classic example, being described by a macroscopic quantum mechanical wave function. Furthermore, there may not be broken symmetries. Instead, the many-body states of quantum matter may require concepts such as topological order, the most common examples being found in fractional quantum Hall effect and topological insulators. In some sense different metallic states: bad metals, "quantum critical metals", and the "strange metal" in the cuprates are all quantum matter.

The notion of quantum matter is useful as a unifying concept for describing many of the common themes of interest in two culturally distinct research communities: those studying ultracold atomic gases and correlated electron materials.

There is also a puzzling somewhat philosophical question:
Is quantum matter itself emergent or does quantum matter have emergent properties?

Should you be concerned about Massive On-Line Courses (MOOC)?

Yes.
MOOCs are all the rage in some circles, particularly amongst politicians and university administrators. On Doug Natelson's blog he has a helpful post which links to two thoughtful, critical and challenging articles. I think the social justice issues raised by the Philosophy Department at San Jose State University are particularly pertinent.

I agree with Doug's point:

I do think it's worth thinking hard about the purpose of MOOCs.  Are they about idealistically providing access to fantastic educational opportunities at very low cost to the student for millions of potential pupils who have an internet connection?  Are they about cynically slashing the operating costs of universities by restructuring the educational experience and potentially eliminating large numbers of faculty jobs?  These are not mutually exclusive.

Ice X is quantum

Solid water (ice) is amazing having more than ten distinct phases. Ice X exists above pressures of about 70 GPa. It is of particular interest because it represents a case of strong hydrogen bonding where protons are equidistant between oxygen atoms.

There is a nice Nature paper from 1998 Tunnelling and zero-point motion in high pressure ice by Benoit, Marx, and Parrinello. The figure below is of particular interest to me. It shows the OH bond length as a function of the O-O distance (bottom scale) in the crystal. The latter can be tuned continuously with pressure (top scale).

The non-solid points are from a classical calculation at two different temperatures. The solid points are when one takes into account the full quantum dynamics of the protons, thus taking into account the effects of tunneling and zero point motion.

The proton becomes equidistant between the two oxygen atoms (solid line) for pressures larger than about 70 kbar, consistent with experiment.

The figure above is similar to a figure in my recent paper about hydrogen bonding, and is consistent with comments I made there about the importance of zero-point motion.

I thank Christiaan Bekker [who is doing an undergraduate research project with me] for bringing the paper to my attention.

Listen to experimentalists (sometimes)

Over the years I have benefited greatly from my interactions with experimentalists. These interactions have varied from informal discussions, listening to talks, and reading papers.
These interactions have led me to work on interesting and important problems and helped make my theoretical work sharper and more relevant to experiment.

But on reflection, I regret I have also wasted significant amounts of time, energy, and money because I have listened (too much) to experimentalists.

So is there a key to getting the benefits without the liabilities?

I think the key is to listen to the "broad brush strokes" and not get distracted or hung up on the details.

Experimentalists can teach us

• what measurable quantities we should aim to calculate [e.g. thermopower vs. temperature, interlayer magnetoresistance vs. magnetic field direction]
• what specific systems or materials are of particular interest
• what is actually known about specific systems or materials [e.g. the shape of the Fermi surface]
• orders of magnitude and reasonable parameter ranges for experimental observables
• we should not believe every published experimental result. Things can go wrong. If you visit a lab and see how complicated and delicate some of the apparatus are you may wonder why there aren't more spurious results!

Theory papers often contain things like a graph of ground state energy vs. a variational parameter for a trial wave function. That may be of some theoretical relevance or importance. But, it is not something an experimentalist is going to be interested in. It is not something they can measure!

Theorists will sometimes be not that concerned with the actual magnitude of energy, temperature, or field scales. Instead they just work with some model parameter, e.g. t and U in a Hubbard model. But, experimentalists really want to know how this translates to temperature or magnetic field. Earlier I have posted about this issue for the case of magnetic fields in Hubbard model calculations.

So when should theorists ignore experimentalists? When do I wish I had ignored them?

I think my mistake has been to sometimes get caught up in the puzzle of specific results of individual experimentalists.

Suppose an experimentalist comes to you and says "We have these interesting/weird results on this exotic material using our fancy new fangled measuring technique. Maybe you can help us explain them." I recommend listening politely and not getting involved. Wait until another group has independently observed similar results on a different sample with a different technique.

I also think that sometimes experimentalists get hung up on small discrepancies between theory and experiment and try to get us to explain them. There are important historical cases where this has been important. But, I think often the benefits to theorists can be marginal.

I have just focussed on the scientific benefits from the fruitful interaction of theorists with experimentalists. I think there can also be some significant career benefits (and liabilities) because generally experimentalists have more money and clout than theorists and so being liked by them can be a significant career booster. But, that is another story....

I welcome comments and peoples own experiences.
I would also be interested to hear an experimentalists view on listening (not too much) to theorists!

Shameless spam from ACS publications

Occasionally I scan my Junk Mail folder because sometimes useful email does turn up. In amongst all the spam from Linked In, ResearchGate, administrators, Nigerian widows, conferences and journals I have never heard of... there was an email from American Chemical Society (ACS) Publications.

In the world of networked science, it isn't enough to be published — you have to be found.

Optimizing your papers for search isn't a skill taught in grad school. Ensure your research gets found by the right people. Download your free guide: Writing Scientific Manuscripts for the Digital Age. Your research is important. Help it get the impact it deserves.

---

Guide Highlights:

• Optimizing your keywords and visuals for search
• The abstract: importance of your mini-manuscript
• Selecting the optimal journal to publish your research
• Broadening your reach with social media
• Measuring the influence of your article

This sounded great. I also thought it might be good blog fodder. 

First the catch: you can't directly access the article. When you click on the link there is a request for your email address and name.

They then send you an email giving you a link to the actual article.

I must say I was very disappointed. 

The "article" is really just a shameless marketing blurb for ACS journals and social media.

I found no real useful tips beyond the obvious: include keywords that may show up in searches in your title and abstract.

Enumerating classical and quantum Hall effects

The helpful figure below is taken from The Complete Quantum Hall Trio, a recent Perspective in Science by Seongshik Oh.

Most decisions are binary: yes or no?

I find reminding myself of this fact. It makes decisions a lot easier.
It may also help in influencing decision makers.

These days I have to make many decisions:
Should this paper be published in PRL?
Should this person get a grant?
Should this person get tenure?
Should I interview this person for a postdoc?
Should this student be allowed to continue their Ph.D?

Making these decisions can consume large amounts of time and energy.
However, I have found it is important and somewhat liberating to sharpen the decision down to a simple yes/no question. It is easy to get distracted from this.

For example when reviewing a grant application it is easy to get distracted by secondary questions:
Does this young applicant deserve to get a grant?
Is the applicants last paper valid, important and significant?
How much should I let citations influence my decision?
Is the budget reasonable and realistic?

But the real question is more like:
Given the competition, the funds available, and the goals and criteria of the funding agency should I recommend the person get a grant?
The answer to that can often be decided very quickly. I don't have to research all the scientific subtleties behind the proposed project or wade through the budget details or fluff about impact factors and committee service....
Furthermore, once I have the yes/no answer I don't see the need to write a long and detailed report justifying it. The decision makers (at the next stage) reading my report are mostly interested in the yes/no not the subtleties behind it.

I don't deny that the answers to secondary questions influence the yes/no answer to the primary question. But, I have found it is easy to get to distracted by them.

I also find this focus on the binary character of decisions helps in trying to influence the decisions that affect me. It particularly affects how much time I spend on "preparing my case." 

Suppose I want to get a travel grant which has a 70 per cent success rate. All I care about is whether I get the grant. Yes/no. Whether my proposal is highly ranked or is appreciated because it contains a beautiful literature review is really irrelevant.

Suppose I want an editor at PRL to accept my paper after some critical referee reports. I don't really care if the editor thinks I have given a particularly cogent response to all of the criticisms (e.g. three different counter arguments for each criticism). I just want the editor to be convinced that it is o.k. to publish the paper, even if she/he has misgivings.
Yes/no.

I welcome your thoughts. Am I ruthless and superficial? Is this a helpful idea?