Scanning Probe Microscopy

The Physical Review is marking the 35th Anniversary of Scanning Tunneling Microscopy (STM) and 30 years of Atomic Force Microscopy (AFM) with free access to notable papers from the Physical Review journals in these two experimental techniques.

So check them out!

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Tunneling Experiment Proposed To Distinguish Superconducting Pairing Origin

It certainly was a nice coincidence that I mentioned about the paper on tunneling spectroscopy on superconductors earlier. It turns out that a new paper proposes an experiment using Josephson tunneling junction to distinguish between the pairing model proposed by 5 different theories.

Now, She et al. lay out in parallel the theoretical expectations for the pair susceptibility of 5 different theories of superconductivity in quantum critical metals. These scenarios include the orthodox BCS theory with a simple Einstein-oscillator pairing function, BCS with a Hertz-Millis-type criticality of the bosonic spectrum, BCS with a simple pairing function and quantum critical electrons, and two limits of the recently developed holographic superconductivity that borrow mathematical concepts from string theory [anti–de Sitter/conformal field theory (AdS/CFT) correspondence] in order to handle scaling near a quantum critical point.

It will be interesting to see which group gets to do this test first, and whether the results can actually distinguish one versus the others. Still, this is a prime example of the tunneling phenomenon being used to study other things.

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Point-Contact Tunneling Spectroscopy

I came across this and said "Whoa! A major deja vu moment!"

This is a very nice paper on the principle of tunneling, and its use in tunneling spectroscopy that gives us quite a bit of information on the properties of a conventional superconductor. It describes an experiment for an undergraduate laboratory exercise. I had a deja vu moment because my PhD research was in tunneling spectroscopy using such point-contact method, but done on high-Tc superconductors which, at that time, was a "hot" commodity. I think we were one of the first groups that got a very clear V-shaped gap structure that was consistent with the d-wave symmetry.

In any case, this is a good paper to read if you want to learn an experimental technique, and certainly a good introduction to what we can learn by using such tunneling phenomenon. Of course, if you want to learn quite a bit more, than you have to read the same text that I used when I was a struggling graduate student - Wolf's standard text "Principle of Electron Tunneling Spectroscopy", which I think is still the bible for electron tunneling studies in solids.

This is also a very good opportunity to emphasize, especially those who are outside physics, of another example where a quantum phenomenon (tunneling) that is so well-known, that we are using it to study other things, in this case, the properties of a superconductor. While many may think that these quantum phenomena are nothing more than some esoteric properties that have no bearing on reality, the truth is that we make use of many of them, often to understand the stuff that we use everyday!

BTW, these undergraduate labs are getting to be more and more advanced. We already have an undergraduate experiment that showed the particle nature of light with a which-way-type experiment.

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STM Images Molecular Orbital

Scanning tunneling microscopy has been extremely useful in condensed matter/material science. Now, it might finally be an important tool in chemistry. A new report shows a very neat experiment using a STM with a tip that has been "functionalized" with a CO molecule. So in this case the tunneling goes through the p-wave orbital of the molecule and allows for an imaging of the nodal direction.

With their new results, Gross et al. show that combining CO-functionalized tips and alkali-halide interlayers allows them to image the nodal pattern of the orbitals of flat organic molecules (Fig. 1). Here, again, the two CO 2π^* orbitals play an important role. These derive from the p orbitals of the carbon and oxygen atoms that stand perpendicular to the molecular axis. Figure 1 shows one of the 2π^* orbitals: it has four lobes (drawn in red and light blue, representing their phase or sign) separated by two nodal planes. The other 2π^* orbital, perpendicular to the one in Fig. 1, is not shown. By bringing the tip close to a sample molecule (pentacene in Fig. 1) separated by a salt layer from a metallic substrate, and tuning the voltage between tip and substrate to address a particular orbital of the pentacene molecule, the lobes of the CO 2π^* orbital and those of the pentacene orbital (shown in purple and rose) come into and out of registry, as the tip is scanned across the molecule.

Amazing work. You can get the paper for free at the link above.

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Unconventional s-Wave Superconductivity in Fe(Se,Te)

This is an amazing experiment in more ways than one.

Abstract: The superconducting state is characterized by a pairing of electrons with a superconducting gap on the Fermi surface. In iron-based superconductors, an unconventional pairing state has been argued for theoretically. We used scanning tunneling microscopy on Fe(Se,Te) single crystals to image the quasi-particle scattering interference patterns in the superconducting state. By applying a magnetic field to break the time-reversal symmetry, the relative sign of the superconducting gap can be determined from the magnetic-field dependence of quasi-particle scattering amplitudes. Our results indicate that the sign is reversed between the hole and the electron Fermi-surface pockets (s±-wave), favoring the unconventional pairing mechanism associated with spin fluctuations.

T. Hanaguri et al., Science v.328, p.474 (2010).

What is amazing is that they not only managed to determine the pairing symmetry for this family of superconductors, but also were able, using STM no less, to detect the s±-wave pairing symmetry, which is not easy! See J.E. Hoffman article reviewing this work in the same issue of Science to understand why.

Is this the first ever experiment to actually make an observation of this pairing symmetry? I'm sure that it is the first ever using an STM.

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Quantum Tunneling Composite

Chalk this up for another direct application of QM. This new technology that makes use of quantum tunneling could introduce a "third dimension" to your 2D touchscreen that is so prevalent in many smart phones (i.e. iPhone lookalike) and upcoming tablets like iPad.

The composite works by using spiky conducting nanoparticles, similar to tiny medieval maces, dispersed evenly in a polymer.

None of these spiky balls actually touch, but the closer they get to each other, the more likely they are to undergo a quantum physics phenomenon known as tunnelling.

Tunnelling is one of several effects in quantum mechanics that defies explanation in terms of the "classical" physics that preceded it.

Simply put, quantum mechanics says that there is a tiny probability that a particle shot at a wall will pass through it in an effect known as tunnelling.
QTC-enabled handset

Similarly, the material that surrounds the spiky balls acts like a wall to electric current. But as the balls draw closer together, when squashed or deformed by a finger's pressure, the probability of a charge tunnelling through increases.

When I first read this, my first reaction was "oh, they finally found an application for that!" Let me explain. I did my Ph.D research in tunneling spectroscopy, or more precisely, in point-contact tunneling spectroscopy. What I had was this sharp, pointy tip, and this tip was pushed onto the sample that we want to study. The native oxide barrier on the sample acts as an insulator, and thus, the potential barrier. So one sets up a tunnel junction consisting of the tip-oxide insulator-sample. One can measure the I-V curve if one so desires due to the tunneling current. The more one pushes the tip into the insulating layer, the larger the junction conductance, and consequently, the larger the tunnel current.

{Of course, there is a limit to how hard one can push the tip into the material. At some point, you'll either get an Ohmic contact, i.e. a short, or you break the sample.}

The same can be said about STM system, but here, the vacuum between the tip and the sample acts as the insulating barrier. How much one varies the distance between the tip and the sample surface dictates the conductance and the amount of tunneling current.

So in essence, this new device, the QTC, is the application of what many of us in condensed matter have already known and made use. I guess with the new nanoparticles, they finally managed to find an application for it.

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Is the Pseudogap Competing With The Superconducting Gap?

The issue on the nature of the pseudogap in the spectra of high-Tc superconductors is one of the most puzzling aspect of this family of material. Briefly, the pseudogap is the gap seen in the single-particle spectrum ABOVE Tc, i.e. before it condenses into the superconducting state where the superconducting gap develops. The pseudogap occurs above Tc without any kind of condensation, and it is more pronounced in the underdoped regime. Not only that, as one go further into the underdoped regime, the upper temperature that the pseudogap exists also increases, meaning that you start seeing this gap in the spectrum at an even higher temperature as the doping decreases.

Since it was first discovered, the main question that has been circulating is whether this pseudogap is simply a precursor to the superconducting gap? Is this the signature of electron pairing, forming Cooper pairs, but without the long-range coherence needed to form the superconducting fluid? This is the pre-formed pair scenario. The other school of thought is that the pseudogap is simply a pairing that competes with superconductivity. The charge carriers that are forming the pairing are taken out of the "pool" of carriers that later on will form the Cooper pair and condenses into the supercurrent below Tc.

This question has continued till today, with evidence being presented for one camp or the other. One of the latest by H.B. Yang et al. using ARPES[1]. When they reconstructed the particle-hole symmetry, they arrive at the conclusion that the result supports the preformed pair scenario. However, another report J.H. Ma et al.[2] using STM and ARPES results reported two distinct gaps that behave differently from each other, supporting the idea that the pseudogap competes with superconductivity. This work even had a press release. So is this a done deal?

Not quite yet! Interestingly enough, just a few days ago, a theoretical preprint out of the Univerisity of Chicago discussed this issue[3]. They discussed how, within their preformed concept, that one can have different temperature dependence for the preformed gap that forms at the nodal and antinodal directions of the Cu-O plane of the cuprates. So such observation does not rule out the preformed pair scenario.

In other words, there is still no smoking gun to pick one over the other as far as the origin of the pseudogap. So the story continues.

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[1] H.B. Yang et al. Nature v.456, p.77 (2008).
[2] J.H. Ma et al. Phys. Rev. Lett. v.101, p.207002 (2008).
[3] C.C. Chien et al. http://arxiv.org/abs/0901.3151

How Cooper Pairs Vanish Approaching the Mott Insulator in Bi2Sr2CaCu2O8+d

This paper appeared in Nature this week, but the authors have uploaded it to ArXiv and it appeared today. Using STM/STS technique, they were able to obtained the spectroscopy in both real and momentum space as a function of doping along the nodal direction of the Fermi surface of a high-Tc superconductor. The results show that as the compound approaches the Mott insulating phase, the mobile Bogoliubov quasiparticles from the Cooper pairs are slowly being replaced by the localized electrons.

A first-rate experiment!

Edit: A BNL press release on this work can be found here.

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No "Glue" For Cuprate Superconductors?

There are finally some startling evidence that points to the very unconventional nature of the cuprate superconductors. A paper just published in Science[1], heading by Ali Yazdani, has found that while there are coupling between the electrons and a bosonic mode, this coupling may not be responsible at all for superconductivity.

You can read a review of this at the Science Daily website. This discovery certainly is consistent with what Phil Anderson has been trying to push. It also could mean that the "kink" in the ARPES spectrum that I've mentioned may be another red herring that isn't directly connected to the superconducting mechanism.

But as always, this isn't the first time something of this nature has occurred in the study of high-Tc superconductors, to later turn out to be insufficient in formulating a theory of these material. So to say that this is still a controversial (in terms of interpreting what it means theoretically) result is to put it mildly.

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[1] A.N. Pasupathy et al., Science v.320, p.196 (2008).

Single-Electron Tunneling Refrigerator

This has always been something that fascinates me, and I suppose, until now, there hasn't been anything that would allow this to be of practical use.

This report presents the latest advancement in tunneling refrigerator, which might actually be practical for cooling large particles. I like how they use the coulomb blockade junction to ensure that the electrons tunnel through one at a time, and so the "hottest" electrons would have the highest probability to tunnel first.

Another cool report [pun intended]!

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Imaging Charge Carrier Motion

This is so cool!

Using a combination of a scanning tunneling microscopy (STM) and laser illumination, a group in Japan has managed to image the motion of charge carrier density across a pn semiconductor junction. They apply different biasses and you can literally see what you have so far seen only schematically in textbooks of the depletion layer and charge diffusion across this layer.

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Superluminal Tunneling?

There have been claims made now and then of an apparent superluminal signal occuring in quantum tunneling process. Of course, quacks like to jump all over something like this and going off into their own laa-laa land to come up with their outlandish theories.

However, the issue isn't as simple, and in fact, could be explained via re-examining on what actually is being timed during tunneling. Several publications have dealt with such a thing. See the list below:

H. Winful, PRL v.90, p.023901 (2003)
M. Buttiker and S. Washburn, Nature v.422, p.271 (2003)

The most recent comprehensive treatment of this issue was published by H. Winful, where he again expanded upon his PRL paper and explained away the apparent superluminal paradox in various tunneling phenomena.

H. Winful, Phys. Rep. v.436, p.1 (2006).

Of course, this may not sit well with some people, and I'm sure there will be a lot more being discussed about this. However, the point here is that claims of superluminal tunneling is far from convincing.

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Tunneling Spectroscopy of High-Tc Superconductors

This is a very comprehensive review of the theory and experimental results of tunneling spectroscopy of high-Tc superconductors. This paper is to appear in an upcoming Review of Modern Physics journal.

This particular topic was my Ph.D research area, so it is something very dear to me. The paper cited several of my papers. It is really a double-edge sword with review papers like this. On one hand, you want to make sure your work is cited since they are doing an overall review. To be excluded would imply your work isn't important enough to be included with the whole body of knowledge of that particular subject matter. On the other hand, once a review paper like this comes out, future authors, if they're lazy enough, would tend to just cite the review paper that contains all the relevant results, and not cite the original papers. So if it is your work that they're citing from that review paper, you will miss a citation count of your paper.

Oh well, we can't have everything.