# Tag Archives: Technology

## What Happens in 2D Stays in 2D.

There was a recent paper published in Nature Nanotechnology demonstrating that single-layer NbSe$_2$ exhibits a charge density wave transition at 145K and superconductivity at 2K. Bulk NbSe$_2$ has a CDW transition at ~34K and a superconducting transition at ~7.5K. The authors speculate (plausibly) that the enhanced CDW transition temperature occurs because of an increase in electron-phonon coupling due to the reduction in screening. An important detail is that the authors used a sapphire substrate for the experiments.

This paper is among a general trend of papers that examine the physics of solids in the 2D limit in single-layer form or at the interface between two solids. This frontier was opened up by the discovery of graphene and also by the discovery of superconductivity and ferromagnetism in the 2D electron gas at the LAO/STO interface. The nature of these transitions at the LAO/STO interface is a prominent area of research in condensed matter physics. Part of the reason for this interest stems from researchers having been ingrained with the Mermin-Wagner theorem. I have written before about the limitations of such theorems.

Nevertheless, it has now been found that the transition temperatures of materials can be significantly enhanced in single layer form. Besides the NbSe$_2$ case, it was found that the CDW transition temperature in single-layer TiSe$_2$ was also enhanced by about 40K in monolayer form. Probably most spectacularly, it was reported that single-layer FeSe on an STO substrate exhibited superconductivity at temperatures higher than 100K  (bulk FeSe only exhibits superconductivity at 8K). It should be mentioned that in bulk form the aforementioned materials are all quasi-2D and layered.

The phase transitions in these compounds obviously raise some fundamental questions about the nature of solids in 2D. One would expect, naively, for the transition temperature to be suppressed in reduced dimensions due to enhanced fluctuations. Obviously, this is not experimentally observed, and there must therefore be a boost from another parameter, such as the electron-phonon coupling in the NbSe$_2$ case, that must be taken into account.

I find this trend towards studying 2D compounds a particularly interesting avenue in the current condensed matter physics climate for a few reasons: (1) whether or not these phase transitions make sense within the Kosterlitz-Thouless paradigm (which works well to explain transitions in 2D superfluid and superconducting films) still needs to be investigated, (2) the need for adequate probes to study interfacial and monolayer compounds will necessarily lead to new experimental techniques and (3) qualitatively different phenomena can occur in the 2D limit that do not necessarily occur in their 3D counterparts (the quantum hall effect being a prime example).

Sometimes trends in condensed matter physics can lead to intellectual atrophy — I think that this one may lead to some fundamental and major discoveries in the years to come on the theoretical, experimental and perhaps even on the technological fronts.

Update: The day after I wrote this post, I also came upon an article demonstrating evidence for a ferroelectric phase transition in thin Strontium Titanate (STO), a material known to exhibit no ferroelectric phase transition in bulk form at all.

## Social and Moral Responsibility of a Scientist

To what extent do scientists and engineers have a responsibility to try to solve the world’s great problems at large? Let me state right at the beginning of the post that I do not have an answer to this question, but I just wanted to raise a few points to start a discussion.

During the WWII period in the United States, many of the nation’s top physicists were corralled to Los Alamos, New Mexico to work at the Manhattan Project in effort to build a nuclear weapon. Moreover, the scientists at Bell Labs, a private laboratory under the auspices of AT&T, aided in the war effort most notably by working on radar technology and also by enabling secure communication among the Allies by developing SIGSALY.

During the Cold War Space Race, US scientists and engineers were again called upon, this time at NASA, to make sure the the United States was able to land an astronaut on the moon before the Soviets. With a lot of money and effort, scientists were able to deliver on the promise by John F. Kennedy to do so before 1970.

While these were under different circumstances, i.e. wartime, scientists responded when called upon by the government. There are other numerous examples outside the US, where scientists have worked in close quarters with the government, such as in the former USSR.

Today, however, the issues are a little different. The looming potential problems caused by greenhouse gas emissions due to rapid industrial development are a “peacetime” concern. This time, also, it isn’t a single government that has to corral the scientists, it is all of them.

The question now is, even in the absence of large-scale government action on these matters, to what extent are physicists and other scientists responsible for addressing these problems? Many engineering departments and national labs are currently engaged in developing battery technologies, more efficient solar cells, transparent solar cells, etc. (funded by the government). Many physicists continue to work on superconductivity with the hope that it may solve the energy transportation and storage problem. But the urgency is clearly not the same as in wartime.

It is largely public money that educates most of us, funds most of our research, yet much of the research we undertake does not have foreseeable implications for the grander problems at large. The fact that scientists and engineers are some of the best placed in terms of education and technical ability to solve these problems does put some burden on us.

Left alone, I would love to spend all my time doing basic science without looking up to see that the world is facing some pretty grand challenges. Unfortunately, I don’t have that luxury, and I do think it would be fair for governments to require us to address these problems by requesting PIs to spend a certain percentage of their research time devoted to these kinds of pressing problems. Perhaps a wartime mindset is needed to solve this problem.

Lastly, I would like to stress that in the two cases mentioned above, WWII and the Space Race, the US economy came out faring better after the heavy investment in science and technology. Industrialized nations can do the same in the present time by investing more in the world’s greener energy technologies, which undoubtedly must be the future of humans.