Separation device

A newly proposed separation devices has come out (http://physics.aps.org/articles/v5/6).

Rubí and Peter Hänggi of the University of Augsburg, Germany, led a team that has developed a new approach to these ratchet sorters. They start with a mathematical framework in which the entropy of the system is treated like potential energy, with entropy “barriers” that repel particles. These are regions where particles are restricted to a small space, which reduces the number of states (locations and velocities) that a particle can occupy. Fewer states means lower entropy. Like balls rolling down a hill, particles tend to move away from these low entropy spots.

The team applies this formalism to a tube with walls that periodically ramp from a narrow diameter to a wide diameter and back, with an asymmetric or “sawtooth” profile. This shape forms distinct but still connected chambers, or segments, each of which is a few microns long. Entropic barriers inhibit travel between segments; however, the barriers are steeper going to the left, so the net motion of the particles is to the right.

In order to clearly see the entropic effect in their computer simulation and analytical calculations, the researchers apply an oscillating force that essentially shakes the particles back and forth inside the tube. In a real experiment, this force could be an oscillating electric field.

New design of transistors

These are of course seminal to future in electronics industry.

The team has created a two-layer GaAs/AlGaAs quantum well heterostructure, in which the wave function of one layer extends into the second to modulate the tunneling current between the layers. In this design, a voltage on the first quantum well causes that layer to be depleted of carriers, which changes the subband energy level in the well. As the subband energy approaches the top of the quantum well potential, the wave function extends further and further out toward the second layer. When the wave function overlaps the second layer, the tunneling current can increase as much as two orders of magnitude, a substantial degree of gating leverage.

Although the reported design only works at cryogenic temperatures, a different choice of materials, for example, graphene, may allow operation at more technologically relevant temperatures. – David Voss

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.

Giant Electroresistance

There are many devices that are of fundamental interest. The most latest and famous may be the one invented by Fert et al., which displays giant magnetoresistance. In the past few years, there have appeared a flurry of work exploiting ferroelectrics as the barrier layer. I have mentioned quite a number of them in this blog. Here comes a new one. It is made up by sandwiching a BTO layer with two LSMO at different dopings. All these efforts are clearly directed to manipulate the interweaving properties of spin, charge and orbital degrees.

A giant tunneling electroresistance effect may be achieved in a ferroelectric tunnel junction by exploiting the magnetoelectric effect at the interface between the ferroelectric barrier and a magnetic La1 xSrxMnO3 electrode. Using first-principles density-functional theory we demonstrate that a few magnetic monolayers of La1 xSrxMnO3 near the interface act, in response to ferroelectric polarization
reversal, as an atomic-scale spin valve by filtering spin-dependent current. This produces more than an order of magnitude change in conductance, and thus constitutes a giant resistive switching effect. [PRL 106, 157203 (2011)]

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)].

The breakdown of Born-Oppenheimer approximation

In dealing with electrons attached to nuclei, the Born-Oppenheimer approximation is often invoked, in which the motions of the nuclei are treated adiabatically relative to that of electrons. However, it will breakdown if the nuclei is light and the electron-nuclei coupling is very strong. In this case, the evolution of such electron-nuclei systems become coherent and entangled. A direct experimental demonstration of this breakdown was recently attained [Nature Physics, 1802(2010)].

Molecules filtering spins

Using STM with a magnetic tip can be used to probe the magnetic feature of a surface. The tunneling current shall be sensitive to the alignment (collimation) between the spin orientation of the surface and that of the tip. If the tunneling, as is usually the case, is non-magnetic, then parallel alignment yield a bigger current. A very valuable aspect of STM is that this device probes the local properties of a material. This makes it especially useful in investigating defects or impurities of a surface. Now these authors [Phys. Rev. Lett. 105, 066601 (2010)] came to examine what will happen to the signal if the electrons tunnel from the Fe surface into the tip through a single organic molecule with Beneze rings. The result is this: this molecule allows more spin-up electrons to pass. So, it works as a selective valve, which may be tailored to specific applications that needs manipulate spin current. This phenomenon was predicted 3 years ago in Ref.[3], where the computation was implemented in the aid of DFT. However, it may prove more elucidating if a simple model description is prescribed.
For convenience, some references are attested on this subject:

1. Atodiresei, N. et al. Phys. Rev. Lett. 105, 066601 (2010).
2. Brede, J. et al. Phys. Rev. Lett. 105, 047204 (2010).
3. Rocha, A. R. & Sanvito, S. J. Appl. Phys. 101, 09B102 (2007).
4. Barraud, C. et al. Nature Phys. 6, 615–620 (2010).
5. Sanvito, S. Nature Phys. 6, 562–564 (2010).
6. Cinchetti, M. et al. Nature Mater. 8, 115–119 (2009).
7. Drew, A. J. et al. Nature Mater. 8, 109–114 (2009).
8. Szulczewski, G., Sanvito, S. & Coey, J. M. D. Nature Mater. 8, 693–695 (2009)

crossover from tunneling to hopping

It was reported in Science [1] of observing directly the crossover in energy transfer from single step tunneling to multi-step hopping in a donor-bridge-acceptor molecular structure (DBA). The electrons carry energy from D to A by passing through the B. This passage can be via tunneling or hopping, depending on the bridge length.

Triplet energy transfer (TT), a key process in molecular and organic electronics, generally occurs by either strongly distance-dependent single-step tunneling or weakly distance-dependent multistep hopping. We have synthesized a series of p-stacked molecules consisting of a benzophenone donor, one to three fluorene bridges, and a naphthalene acceptor, and studied the rate of TT from benzophenone to naphthalene across the fluorene bridge using femtosecond transient absorption
spectroscopy. We show that the dominant TT mechanism switches from tunneling to wire-like hopping between bridge lengths 1 and 2. The crossover observed for TT can be determined by direct observation of the bridge-occupied state.

[1] SCIENCE VOL 328 18 JUNE 2010