Molecular Wires With 2000% Change In Magnetoresistance!

Holy magnetic wires, Batman! This thing blows away those colossal magnetoresistance material! What's bigger than "colossal" for us to give a name to this thing?

A new paper appearing in Science today is reporting the properties of a molecular wire that has the capability of having a 2000% increase in its magnetoresistance, and all at room temperature!

Ironically, the new molecular wires aren't made with magnetic materials at all. Rather, their MR effect relies on the conductivity of nonmagnetic organic dye molecules called DXP, which the Italian automaker Ferrari once used to give their roadsters their trademark red color. Unlike conventional inorganic metals in which electrons zip through a crystalline lattice, in organics electrons must hop from one molecule to another, like pails of water being passed by a bucket brigade. To create a MR, material researchers need to switch off that bucket brigade in the presence of a magnetic field.

In organic materials researchers do this with a little help from quantum mechanics. A tenet of quantum mechanics called the Pauli Exclusion Principle states that no two fermions (particles in a family that includes electrons) can occupy the same quantum state. If two electrons with the same quantum state try to hop onto the same DXP, they can't. The bucket brigade turns off and resistance skyrockets.

Of course, we all know (don't we?) that this is the field that has been responsible for invention of computer magnetic storage disks, etc. So kids, this is another example of real-world, practical application of physics, and quantum mechanics in particular.

Zz.

Photocathode for Photoinjectors

I've been attending a workshop this week on photocathodes for photoinjectors. With more stringent requirements and more demanding environment that these photocathodes are subjected to (example: higher field gradients, extremely low emittance, etc.), there is a deliberate effort to understand even more the physics of photocathodes and photoemission processes using various materials and processing techniques.

Historically, most of the emphasis has been on getting a stable photocathode, or something with sufficient quantum efficiency (QE) with long lifetimes. Metal photocathdoes have been the workhorse for many photoinjectors (such as synchrotron light sources) because they are relatively easier to fabricate, long life times, and not very fussy. But metals such as copper or niobium have very low QE, and with new demands on producing high brightness electron beam, new materials, or new processing/treatment are being investigated.

What is very exciting now in this area of study is that, there is a new influx of experts from the condensed matter/material science field studying photocathodes specifically for accelerator photoinjectors. This is important because, while there have been such experts scattered around studying these photocathodes, there hasn't been a coordinated effort to get more of these experts in, both with theorists or experimentalists. CM theorists are needed because there are many aspects of the photoemission process that resulted in high QE and low emittance beam that needed to be modeled or explained. Experimentalists are needed because they have a wealth of material characterization knowledge that are needed to study the nature of the surface and the nature of the material, and they provide feedback to theorists to make accurate models. At this workshop, there is a major presence CM theorists and experimentalists, and I think people in both accelerator physics and condensed matter/material science/physical chemistry realize that there's A LOT of work that can be done in the study of photocathodes, even though a lot has already been known.

I'm very excited with this development. As someone who came from condensed matter physics and now working in accelerator physics, I've always realized the importance of these two fields getting together and combining their expertise to solve the various problems in photocathodes. In fact, this issue doesn't just affect the application of photocathodes to accelerator photoinjector. It has a direct consequence to many photocathode applications, such as photodetectors, and this includes things such as high energy physics detector (neutrino detectors) and even night-vision goggles. So the impact of the understanding of a better understanding of the physics can be very wide.

Zz.

Much Ado About Topological Insulators

Topological Insulators are HOT. They are the hottest thing in condensed matter physics right now. This news summary from Nature describes what they are, and why they are the 'star' material at this moment (link open only for a limited time).

Those effects go beyond the way electrons move on the surface. For example, all electrons are spinning in a quantum mechanical way. Usually, the spins are constantly knocked about by random collisions and stray magnetic fields. But spinning electrons on the surface of a topological insulator are protected from disruption by quantum effects. This could make the materials beneficial for spin-related electronics, which would use the orientation of the electron spin to encode information, thereby opening up a whole new realm of computer technology.
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Researchers also believe that the collective motions of electrons inside topological insulators will mimic several of the never-before-seen particles predicted by high-energy physicists. Among them are axions, hypothetical particles predicted in the 1970s; magnetic monopoles, single points of north and south magnetism; and Majorana particles — massless, chargeless entities that can serve as their own antiparticles.

This mimicry is not entirely surprising. Almost by definition, collective electron motions can be described by just a handful of variables obeying simple equations, says Frank Wilczek, a Nobel-prizewinning particle physicist at the Massachusetts Institute of Technology in Cambridge. "There are only a few kinds of equations that you can write down that are really simple," he says. So topological-insulator theorists and particle physicists have almost inevitably ended up in the same place.

In other words, once again, the physics that governs things in condensed matter now have implications into other areas that may be fundamental in nature! How many times have I indicated this already?

And since we're talking about topological insulators, don't miss the latest STM study on something similar that has produced quite an interesting result.

Zz.

Chemical Vapour Deposition Synthetic Diamond: Materials, Technology and Applications

This is a humongous review article on CVD diamond.

Abstract: Substantial developments have been achieved in the synthesis of chemical vapour deposition (CVD) diamond in recent years, providing engineers and designers with access to a large range of new diamond materials. CVD diamond has a number of outstanding material properties that can enable exceptional performance in applications as diverse as medical diagnostics, water treatment, radiation detection, high power electronics, consumer audio, magnetometry and novel lasers. Often the material is synthesized in planar form, however non-planar geometries are also possible and enable a number of key applications. This article reviews the material properties and characteristics of single crystal and polycrystalline CVD diamond, and how these can be utilized, focusing particularly on optics, electronics and electrochemistry. It also summarizes how CVD diamond can be tailored for specific applications, based on the ability to synthesize a consistent and engineered high performance product.

Diamond, and in particular these CVD diamonds, are not only strong and hard, but also can have properties that can be tailored to meet various needs simply by different doping. It can be a good field emitter with the proper doping. Not only is it a strong insulator electrically, but it also has a relatively good thermal conductivity for an insulator, which makes it a good heat conductor.

Zz.