At a surface or interface the electron spin can form specific patterns but it remains in the surface plane. Helmholtz Zentrum Berlin (HZB) researchers have now succeeded in turning the spin out of the plane, and they explain why this is a principle property. The results were published on 27. July 2015 in ("Tunable Fermi level and hedgehog spin texture in gapped graphene"). They are building on previous work published earlier in 2011 in ("Effect of sublattice asymmetry and spin-orbit interaction on out-of-plane spin polarization of photoelectrons"). 
The hedgehog-configuration of the spins and the Fermi-Level is shown. (Illustration: Thomas Splettstößer/HZB) If an electron bounces back from an obstruction it runs, as one should think, exactly back the way it came from. Quantum mechanics, however, has its own rules when it comes to electrons and particularly when it comes to electrons in graphene. When an electron in graphene runs head on against an obstruction and is scattered back, it does change it course by 180°. Its spin, however, should also turn by 180° but it rotates only be 90°. Indeed, an electron has to be rotated by 720° to get it back into its original states. High spin-orbit interaction plus bandgap = Hedgehog texture To do this experiment, several preconditions have to be met. First of all, the electron spin property has to be imparted on the graphen. Varykhalov and coworkers have much experience since they succeeded in this in a remarkable experiment in 2008. They squeezed gold atoms underneath the graphene und thereby enhanced the spin-orbit interaction in the graphene by a factor of 10,000. Precondition no. 2 is to allow for the 180° backscattering. This is challenging since graphene is first and foremost famous the absence of backscattering. To this end, Varykhalov et al. created a band gap in the graphene. This means nothing else than sending electrons back by 180°. If both is fulfilled, the spins in this band gap have to be oriented perpendicular to the graphene plane, more far away, however, in the plane. The continuous transition between the two has the appearance of the prickles of a hedgehog. Model calculations have been performed by theoreticians from Budapest and confirm the experimental results. Construction of a spin filter For symmetry reasons the hedgehog structure has to be reversed elsewhere in the graphene. This does not mean that the hedgehog had no influence on the graphene. On the contrary, the so-called valley Hall effect can be used to realize a spin filter. This effect means that the electrons in the graphene are deflected to the right or left depending on which valley they are in. Because according to the results by Varykhalov et al. the two valleys correspond to two spin orientations, the two spins assemble at opposite sides of the graphene sample.
read more "Spins in graphene with a hedgehog texture"
The hedgehog-configuration of the spins and the Fermi-Level is shown. (Illustration: Thomas Splettstößer/HZB) If an electron bounces back from an obstruction it runs, as one should think, exactly back the way it came from. Quantum mechanics, however, has its own rules when it comes to electrons and particularly when it comes to electrons in graphene. When an electron in graphene runs head on against an obstruction and is scattered back, it does change it course by 180°. Its spin, however, should also turn by 180° but it rotates only be 90°. Indeed, an electron has to be rotated by 720° to get it back into its original states. High spin-orbit interaction plus bandgap = Hedgehog texture To do this experiment, several preconditions have to be met. First of all, the electron spin property has to be imparted on the graphen. Varykhalov and coworkers have much experience since they succeeded in this in a remarkable experiment in 2008. They squeezed gold atoms underneath the graphene und thereby enhanced the spin-orbit interaction in the graphene by a factor of 10,000. Precondition no. 2 is to allow for the 180° backscattering. This is challenging since graphene is first and foremost famous the absence of backscattering. To this end, Varykhalov et al. created a band gap in the graphene. This means nothing else than sending electrons back by 180°. If both is fulfilled, the spins in this band gap have to be oriented perpendicular to the graphene plane, more far away, however, in the plane. The continuous transition between the two has the appearance of the prickles of a hedgehog. Model calculations have been performed by theoreticians from Budapest and confirm the experimental results. Construction of a spin filter For symmetry reasons the hedgehog structure has to be reversed elsewhere in the graphene. This does not mean that the hedgehog had no influence on the graphene. On the contrary, the so-called valley Hall effect can be used to realize a spin filter. This effect means that the electrons in the graphene are deflected to the right or left depending on which valley they are in. Because according to the results by Varykhalov et al. the two valleys correspond to two spin orientations, the two spins assemble at opposite sides of the graphene sample.
Figure 1. Path of incident light around invisibility camouflage device. (a) Existing camouflage device with optical path independent of light direction. No light can enter into device, and therefore hiding person cannot see outside. (b) Asymmetric camouflage device. Rightward-propagating light avoids hiding person, whereas leftward-propagating light travels straight to enter hiding person's eyes. Hiding person cannot be seen from onlookers on right side but can see them. Optical camouflage devices designed using transformation optics have a closed region that incident light from every direction avoids. A person hiding in this region therefore seems invisible to external onlookers (Fig. 1a). However, no light can enter the cloaked region, and consequently the person hiding therein cannot be able to see outside. This is quite inconvenient for practical use. A practical camouflage device must have unidirectional transparency such that a person inside cannot be seen from the outside but can see the outside. To overcome this problem, the research team has formulated a theory of asymmetric (or nonreciprocal) camouflage that can achieve unidirectional transparency in which "they cannot see us, but we can see them." This theory is unrelated to transformation optics but instead based on the concept of 'Lorentz/Coulomb-like forces for photons.' Unidirectional transparency needs a high-level nonreciprocity in the propagation of light. For example, as shown in Fig. 1b), rightward-propagating light have to avoid and circumvent the hiding person, whereas leftward-propagating light have to travel straight to enter the eyes of the hiding person. Such nonreciprocity can be achieved by controlling the movement of photons with two forces that are analogous to Lorentz force
Photographs of upconversion in a cuvette containing cadmium selenide/rubrene mixture. The yellow spot is emission from the rubrene originating from (a) an unfocused continuous wave 800 nm laser with an intensity of 300 W/cm2. (b) a focused continuous wave 980 nm laser with an intensity of 2000 W/cm2. The photographs, taken with an iPhone 5, were not modified in any way. (Image: Zhiyuan Huang, UC Riverside) "The infrared region of the solar spectrum passes right through the photovoltaic materials that make up today's solar cells," explained Christopher Bardeen, a professor of chemistry. The research was a collaborative effort between him and Ming Lee Tang, an assistant professor of chemistry. "This is energy lost, no matter how good your solar cell. The hybrid material we have come up with first captures two infrared photons that would normally pass right through a solar cell without being converted to electricity, then adds their energies together to make one higher energy photon. This upconverted photon is readily absorbed by photovoltaic cells, generating electricity from light that normally would be wasted." Bardeen added that these materials are essentially "reshaping the solar spectrum" so that it better matches the photovoltaic materials used today in solar cells. The ability to utilize the infrared portion of the solar spectrum could boost solar photovoltaic efficiencies by 30 percent or more. In their experiments, Bardeen and Tang worked with cadmium selenide and lead selenide semiconductor nanocrystals. The organic compounds they used to prepare the hybrids were diphenylanthracene and rubrene. The cadmium selenide nanocrystals could convert visible wavelengths to ultraviolet photons, while the lead selenide nanocrystals could convert near-infrared photons to visible photons. In lab experiments, the researchers directed 980-nanometer infrared light at the hybrid material, which then generated upconverted orange/yellow fluorescent 550-nanometer light, almost doubling the energy of the incoming photons. The researchers were able to boost the upconversion process by up to three orders of magnitude by coating the cadmium selenide nanocrystals with organic ligands, providing a route to higher efficiencies. "This 550 -- nanometer light can be absorbed by any solar cell material," Bardeen said. "The key to this research is the hybrid composite material -- combining inorganic semiconductor nanoparticles with organic compounds. Organic compounds cannot absorb in the infrared but are good at combining two lower energy photons to a higher energy photon. By using a hybrid material, the inorganic component absorbs two photons and passes their energy on to the organic component for combination. The organic compounds then produce one high-energy photon. Put simply, the inorganics in the composite material take light in; the organics get light out." Besides solar energy, the ability to upconvert two low energy photons into one high energy photon has potential applications in biological imaging, data storage and organic light-emitting diodes. Bardeen emphasized that the research could have wide-ranging implications. "The ability to move light energy from one wavelength to another, more useful region, for example, from red to blue, can impact any technology that involves photons as inputs or outputs," he said.