All organic molecule spin valve

I remember in my previous entry I mentioned a spin filtering effect of DNA molecules. Here comes another molecule with similar effect and can operate as a spin valve at low temperatures[http://www.nature.com/nmat/journal/v10/n7/full/nmat3061.html?WT.ec_id=NMAT-201107].

Now, writing in Nature Materials, Urdampilleta and co-workers¹ report that a single-walled carbon nanotube decorated with magnetic molecules can act in just the same way as a conventional spin valve, albeit only at low temperature.
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How is it possible for a single molecule to perform as efficiently as 10 nm of iron? The key is the ability of a chemical bond to modify the magnetic properties of a surface, which has been studied under the suggestive name of 'spinterface science'⁵. It has already been shown that an attached molecule can alter the spin-polarization of the electrons emerging from a magnetic surface^{6, 7}; the experiments of Urdampilleta and co-workers now prove the opposite effect — namely that a magnetic molecule can alter the spin polarization of the current flowing in a non-magnetic material. Two particular features make this possible. First, the magnetic centre must be sufficiently close to the conduction channel. In this respect, the case of bis-phthalocyaninato-terbium(III) is rather peculiar, because the Tb³⁺ ion (Tb³⁺ carries a total angular momentum, J = 6) is sandwiched between two phthalocyanine ligands, and it is at least 1 nm away from the nanotube — too far to transfer any magnetic information. However, there is a second source of spin in this molecule, namely a S = 1/2 radical delocalized over the two phthalocyanine ligands. These are likely to participate in the bond and help to spin-polarize the electron current. Second, the conduction channel must be sufficiently sensitive to the local magnetic moment. All the atoms in a single-walled carbon nanotube reside on the surface, so that a surface modification results in an alteration of the entire electronic structure. It is an extreme surface sensitivity that makes this spin valve work.

Spin diffuses through interacting fermi gas

This is definitely a typical non-equilibrium problem. In their experiment, "A spin current is induced by spatially separating two spin components and observing their evolution in an external trapping potential." [Nature, 472:401(2011)] They found that, "interactions can be strong enough to reverse spin currents, with components of opposite spin reflecting off each other. Near equilibrium, we obtain the spin drag coefficient, the spin diffusivity and the spin susceptibility as a function of temperature on resonance and show that they obey universal laws at high temperatures. In the degenerate regime, the spin diffusivity approaches a value set by /m, the quantum limit of diffusion, where /m is Planck’s constant divided by 2π and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state^{9, 10, 11}." For a review, click here.

Spins coupled to a mechanical resonator

Employing the spin-phonon coupling, they argued that, a tiny magnet welded with a torsional mechanical oscillator can be described by a spin-boson model [PRL, 106:147203(2011)]. What interests me is actually this, is it possible to filter spins using magnetoptical coupling schemes ? Especially, what is the implication for the DNA filtering effects that were reported earlier in this blog. See also the review [Phyics, 4:28(2011)].

Helical DNA filters spins

Spin filters might be salient devices in spintronics. One example of such filters is a ferromagnetic layer, which allows a spin in one direction to pass while blocks it in the other. The selectivity has been shown up to about 25%. Now a study using DNA makes a big difference, where the selectivity can be achieved of as high as 60%. In this case, periodic helice plays an essential role, but how this is so has not been understood yet [Science 18 February 2011: 894-897]. In general, it raises an interesting question on how an electron interacts with chiral supramolecules.

Göhler et al. (3) describe a surprisingly efficient method for electronic spin filtering. They have studied how electrons emitted by a gold substrate, upon absorption of light, pass through a self-assembled DNA monolayer on the gold surface. In particular, they have studied the spin of the electrons after their passage through the DNA layer and have found that one spin type passes through much more easily, meaning that this layer acts as a spin filter, strongly hindering the passage of the other spin type. This filter effect is observed only if the DNA is assembled on the gold surface as a closely packed ordered array of helices, and is stronger if the helices are longer, reaching selectivities of 60%. For chaotic assemblies of floppy DNA chains on the gold surface, the spin filter effect was not observed. [http://www.sciencemag.org/content/331/6019/864.full]

Visualizing the edge states in graphene

(1) Band bending and the associated spatially inhomogeneous population of Landau levels play a central role in the physics of the quantum Hall effect (QHE) by constraining the pathways for charge-carrier transport and scattering¹. Recent progress in understanding such effects in low-dimensional carrier gases in conventional semiconductors has been achieved by real-space mapping using local probes^{2, 3}. Here, we use spatially resolved photocurrent measurements in the QHE regime to study the correlation between the distribution of Landau levels and the macroscopic transport characteristics in graphene. Spatial maps show that the net photocurrent is determined by hot carriers transported to the periphery of the graphene channel, where QHE edge states provide efficient pathways for their extraction to the contacts. The photocurrent is sensitive to the local filling factor, which allows us to reconstruct the local charge density in the entire conducting channel of a graphene device. [doi:10.1038/nphys1745]

(2) Spintronics, where the spin of electrons is used to carry information, is a rapidly growing area of research^{1, 2}. There are several techniques for generating pure spin currents^{3, 4, 5, 6, 7, 8, 9, 10}; however, there is no method that can directly detect them, largely because they carry no net charge current and no net magnetization. At present, studies of pure spin currents rely on measuring the induced spin accumulation with either optical techniques^{5, 11, 12, 13} or spin-valve configurations^{14, 15, 16, 17}. However, spin accumulation does not directly reflect the spatial distribution or temporal dynamics of the pure spin current, and therefore does not give a real-time or real-space measurement. Here we demonstrate a second-order nonlinear optical effect of the pure spin current that has never been explored before, and show that it can be used for the non-invasive, non-destructive and real-time imaging of pure spin currents. The detection scheme can be applied in a wide range of materials with different electronic band structures because it does not rely on optical resonances. Furthermore, the control of nonlinear optical properties of materials with pure spin currents may have potential applications in photonics integrated with spintronics. [doi:10.1038/nphys1742]