Fèlix Casanova

Controlling graphene plasmons with resonant metal antennas and spatial conductivity patterns.

A controlled launch for plasmons To create nanophotonic devices, engineers must combine large-scale optics with tiny nanoelectronics. Plasmons, the collective light-induced excitations of electrons at a metals surface, can bridge that difference in size scales. Alonso-Gonzalez et al. placed structured gold “antennas” on top of a graphene layer to launch and propagate plasmonic excitations into the graphene. By carefully designing the antennas, the researchers could engineer the wavefronts of the plasmons and control the direction of propagation. This approach illustrates a versatile approach for the development of nanophotonics. Science, this issue p. 1369 Structured gold antennas are used to launch plasmons into graphene, engineer their wavefronts, and control their propagation. Graphene plasmons promise unique possibilities for controlling light in nanoscale devices and for merging optics with electronics. We developed a versatile platform technology based on resonant optical antennas and conductivity patterns for launching and control of propagating graphene plasmons, an essential step for the development of graphene plasmonic circuits. We launched and focused infrared graphene plasmons with geometrically tailored antennas and observed how they refracted when passing through a two-dimensional conductivity pattern, here a prism-shaped bilayer. To that end, we directly mapped the graphene plasmon wavefronts by means of an imaging method that will be useful in testing future design concepts for nanoscale graphene plasmonic circuits and devices.

Real-space mapping of Fano interference in plasmonic metamolecules

An unprecedented control of the spectral response of plasmonic nanoantennas has recently been achieved by designing structures that exhibit Fano resonances. This new insight is paving the way for a variety of applications, such as biochemical sensing and surface-enhanced Raman spectroscopy. Here we use scattering-type near-field optical microscopy to map the spatial field distribution of Fano modes in infrared plasmonic systems. We observe in real space the interference of narrow (dark) and broad (bright) plasmonic resonances, yielding intensity and phase toggling between different portions of the plasmonic metamolecules when either their geometric sizes or the illumination wavelength is varied.

Experimental verification of the spectral shift between near- and far-field peak intensities of plasmonic infrared nanoantennas.

Theory predicts a distinct spectral shift between the near- and far-field optical response of plasmonic antennas. Here we combine near-field optical microscopy and far-field spectroscopy of individual infrared-resonant nanoantennas to verify experimentally this spectral shift. Numerical calculations corroborate our experimental results. We furthermore discuss the implications of this effect in surface-enhanced infrared spectroscopy.

Room-temperature spin transport in C60-based spin valves.

A IO N Spintronics, or the possibility of performing electronics with the spin of the electron, has been fundamental for the exponential growth of digital data storage which has occurred in the last decades. Indeed, hard-disk drives read-heads are the maximum exponent of what is currently being called fi rstgeneration spintronic devices. Current read-heads, although technologically very complex, are scientifi cally based simply on the tunnel magnetoresistance effect (TMR; magnetoresistance being the change in electrical resistance of a device under the application of an external magnetic fi eld). A tunnel magnetoresistive vertical spin valve is composed of two ferromagnetic layers separated by a thin (around 1 nm) insulating layer, and the resistance of the structure can be switched between two different values upon the application of a magnetic fi eld capable of rotating the magnetization vector of the ferromagnetic layers from parallel to antiparallel. [ 1 ] For the eventual success of a second-generation of spintronic devices, more complex mechanisms than the nanometre-distance spin transport in metallic or insulating materials have to be obtained. In particular, coherent spin transport at distances above a few nm and spin manipulation are unavoidable requirements for the production of sophisticated prototypes of, for example, spin transistors or spin light-emitting diodes. [ 2 , 3 ] Organic semiconductors (OS) have emerged as promising materials for advanced spintronics applications. Their spin relaxation mechanisms, mainly represented by spin orbit interaction and hyperfi ne interaction with protons, [ 4 ] are very small, and long spin lifetimes have been consistently detected. [ 5 ] Moreover, in spite of the relatively low carrier mobility of these materials, organic vertical spin valves with semiconducting channels thicker than 100 nm have been demonstrated. [ 6–11 ] In parallel, OS ultrathin layers perform successfully as spin tunnel junctions, and extremely high ( > 300%) magnetoresistance (MR) values have been obtained at low temperatures. [ 12 ] Regarding possible applications of spin transport in OS, a basic operational requirement is the room temperature (RT) operation of the devices. So far, only organic spin tunnel junctions have shown any signifi cant MR effect at RT. [ 13–17 ] By contrast, most devices employing thicker organic layer ( > 15 nm) show a clear decay of the MR well below RT. [ 6–11 , 18 ]

A Light-Controlled Resistive Switching Memory

Sketch of the configuration of a light-controlled resistive switching memory. Light enters through the Al(2) O(3) uncovered surface and reaches the optically active p-Si substrate, where carriers are photogenerated and subsequently injected in the Al(2) O(3) layer when a suitable voltage pulse is applied. The resistance of the Al(2) O(3) can be switched between different non-volatile states, depending on the applied voltage pulse and on the illumination conditions.

A high-sensitivity differential scanning calorimeter with magnetic field for magnetostructural transitions

We have developed a differential scanning calorimeter capable of working under applied magnetic fields of up to 5 T. The calorimeter is highly sensitive and operates over the temperature range 10–300 K. It is shown that, after a proper calibration, the system enables determination of the latent heat and entropy changes in first-order solid–solid phase transitions. The system is particularly useful for investigating materials that exhibit the giant magnetocaloric effect arising from a magnetostructural phase transition. Data for Gd5(Si0.1Ge0.9)4 are presented.

Spin Hall magnetoresistance at Pt/CoFe2O4 interfaces and texture effects

We report magnetoresistance measurements on thin Pt bars grown on epitaxial (001) and (111) CoFe2O4 (CFO) ferrimagnetic insulating films. The results can be described in terms of the recently discovered spin Hall magnetoresistance (SMR). The magnitude of the SMR depends on the interface preparation conditions, being optimal when the Pt/CFO samples are prepared in situ, in a single process. The spin-mixing interface conductance, the key parameter governing SMR and other relevant spin-dependent phenomena, such as spin pumping or spin Seebeck effect, is found to be different depending on the crystallographic orientation of CFO, highlighting the role of the composition and density of magnetic ions at the interface on spin mixing.

Temperature dependence of spin diffusion length and spin Hall angle in Au and Pt

We have studied the spin transport and the spin Hall effect as a function of temperature for platinum (Pt) and gold (Au) in lateral spin valve structures. First, by using the spin absorption technique, we extract the spin diffusion length of Pt and Au. Secondly, using the same devices, we have measured the spin Hall conductivity and analyzed its evolution with temperature to identify the dominant scattering mechanisms behind the spin Hall effect. This analysis confirms that the intrinsic mechanism dominates in Pt whereas extrinsic effects are more relevant in Au. Moreover, we identify and quantify the phonon-induced skew scattering. We show that this contribution to skew scattering becomes relevant in metals such as Au, with a low residual resistivity.

Hanle Magnetoresistance in Thin Metal Films with Strong Spin-Orbit Coupling.

We report measurements of a new type of magnetoresistance in Pt and Ta thin films. The spin accumulation created at the surfaces of the film by the spin Hall effect decreases in a magnetic field because of the Hanle effect, resulting in an increase of the electrical resistance as predicted by Dyakonov [Phys. Rev. Lett. 99, 126601 (2007)]. The angular dependence of this magnetoresistance resembles the recently discovered spin Hall magnetoresistance in Pt/Y(3)Fe(5)O(12) bilayers, although the presence of a ferromagnetic insulator is not required. We show that this Hanle magnetoresistance is an alternative simple way to quantitatively study the coupling between charge and spin currents in metals with strong spin-orbit coupling.

Tuning the spin Hall effect of Pt from the moderately dirty to the superclean regime

We systematically measure and analyze the spin diffusion length and the spin Hall effect in Pt with a wide range of conductivities using the spin absorption method in lateral spin valve devices. We observe a linear relation between the spin diffusion length and the conductivity, evidencing that the spin relaxation in Pt is governed by the Elliott-Yafet mechanism. We find a single intrinsic spin Hall conductivity (