Sp Sander Kersten

Ultrahigh magnetoresistance at room temperature in molecular wires.

More Magnetoresistance When data is read off your computers hard drive, chances are that the read head is using the phenomenon of magnetoresistance (MR)—the dependence of electrical resistance on applied magnetic field—to interpret the magnetic signature of the data on the disk. Devices that have the large MR necessary for such tasks are usually made of layers of magnetic materials. Mahato et al. (p. 257, published online 4 July) observed a large MR effect in a nonmagnetic material—organic molecules squeezed into a zeolite crystal. Importantly for potential future applications, the effect was observed at room temperature and at low magnetic fields. The conduction of molecular wires embedded in a zeolite host crystal is almost entirely blocked in small magnetic fields. Systems featuring large magnetoresistance (MR) at room temperature and in small magnetic fields are attractive owing to their potential for applications in magnetic field sensing and data storage. Usually, the magnetic properties of materials are exploited to achieve large MR effects. Here, we report on an exceptionally large (>2000%), room-temperature, small-field (a few millitesla) MR effect in one-dimensional, nonmagnetic systems formed by molecular wires embedded in a zeolite host crystal. This ultrahigh MR effect is ascribed to spin blockade in one-dimensional electron transport. Its generic nature offers very good perspectives to exploit the effect in a wide range of low-dimensional systems.

Photoluminescence Spectra of Self-Assembling Helical Supramolecular Assemblies: A Theoretical Study

The reversible assembly of helical supramolecular polymers of chiral molecular building blocks is known to be governed by the interplay between mass action and the competition between weakly and strongly bound states of these building blocks. The highly co-operative transition from free monomers at high temperatures to long helical aggregates at low temperatures can be monitored by photoluminescence spectroscopy that probes the energetically lowest-lying optical excitations in the assemblies. In order to provide the interpretation of obtained spectroscopic data with a firm theoretical basis, we present a comprehensive model that combines a statistical theory of the equilibrium polymerization with a quantum-mechanical theory that not only accounts for the conformational properties of the assemblies but also describes the impact of correlated energetic disorder stemming from deformations within the chromophores and their interaction with solvent molecules. The theoretical predictions are compared to fluorescence spectra of chiral oligo(p-phenylene-vinylene) molecules in the solvent dodecane and we find them to qualitatively describe the red-shift of the main fluorescence peak and its decreasing intensity upon aggregation.

Theory of hyperfine field-induced organic magnetic field effects

In this chapter, we discuss the essence of hyperfine-field induced magnetic field effects in organic semiconductors originating from spindependent reactions between electron and hole polarons. The basic picture we employ is that of two spin-1/2 polarons, each localized on a site in the semiconductor, where one of the polarons incoherently hops to the other site, forming a spin-0 bipolaron or singlet exciton, or a spin-1 triplet exciton. We describe this process within the framework of the stochastic Liouville equation. We introduce the useful simplifications where the hyperfine coupling of the electronic spin with the surrounding nuclear spins is replaced by a classical random hyperfine field (semiclassical approximation) and where the hopping rate is assumed to be much smaller than the hyperfine precession rate (slow-hopping approximation). Within the resulting theoretical framework we discuss the magnetic field effects measured in the conduction and electroluminescence of unipolar and bipolar devices of organic semiconductors at low magnetic field, focusing in particular on the line shapes of the effects. We conclude with a theoretical discussion of a huge organic magnetoresistance that could be realized in donor-acceptor copolymers.

Using laser-induced spin manipulation to build magnetic nanologic elements

We present an ab initio theory of ultrafast nanologic elements and show that controlled spin manipulation is feasible with the inclusion of spin-orbit coupling in ?-processes. We show that in branched metallic chains with three magnetic centers both spin flips and spin transfers are possible within a hundred femtoseconds. A static external magnetic field and the magnetic state of one magnetic center serve as input poles (input bits), while the magnetic state of the cluster after a controlled laser pulse can be mapped to the output. Combining laser-induced spin-manipulation scenarios we are able to construct the NAND logic. Thus multicenter magnetic clusters can extend spin dynamics to optically triggered and functionalized magnetic transport on a subpicosecond timescale and nanometer spatial scale.

Tuning of narrow-bandwidth photonic crystal devices etched in InGaAsP planar waveguides by Liquid Crystal infiltration

Photonic crystal (PC) devices in the InP/InGaAsP/InP planar waveguide system exhibiting narrow bandwidth features were investigated for use as ultrasmall and tunable building blocks for photonic integrated circuits at the telecom wavelength of 1.55 μm. The H1 cavity, consisting of a single PC-hole left unetched, represents the smallest possible cavity in a dielectric material. The tuning of this cavity by temperature was investigated under the conditions as etched and after the holes were infiltrated with liquid crystal (LC), thus separating the contributions of host semiconductor and LC-infill. The shift and tuning by temperature of the MiniStopBand (MSB) in a W3 waveguide, consisting of three rows of holes left unetched, was observed after infiltrating the PC with LC. The samples finally underwent a third processing step of local wet underetching the PC to leave an InGaAsP membrane structure, which was optically assessed through the ridge waveguides that remained after the under etch and by SNOM-probing.