Ultrafast Loss of Lattice Coherence in the Light-Induced Structural Phase Transition of V_{2}O_{3}.

In solids, the response of the lattice to photoexcitation is often described by the inertial evolution on an impulsively modified potential energy surface which leads to coherent motion. However, it remains unknown if vibrational coherence is sustained through a phase transition, during which coupling between modes can be strong and may lead to rapid loss of coherence. Here we use coherent phonon spectroscopy to track lattice coherence in the structural phase transition of V_{2}O_{3}. In both the low and high symmetry phases unique coherent phonon modes are generated at low fluence. However, coherence is lost when driving between the low and high symmetry phases. Our results suggest strongly damped noninertial dynamics dominate during the phase transition due to disorder and multimode coupling.

Relaxor ferroelectricity and magnetoelectric coupling in ZnO-Co nanocomposite thin films: beyond multiferroic composites.

ZnO-Co nanocomposite thin films are synthesized by combination of pulsed laser deposition of ZnO and Co ion implantation. Both superparamagnetism and relaxor ferroelectricity as well as magnetoelectric coupling in the nanocomposites have been demonstrated. The unexpected relaxor ferroelectricity is believed to be the result of the local lattice distortion induced by the incorporation of the Co nanoparticles. Magnetoelectric coupling can be attributed to the interaction between the electric dipole moments and the magnetic moments, which are both induced by the incorporation of Co. The introduced ZnO-Co nanocomposite thin films are different from conventional strain-mediated multiferroic composites.

Tuning quantum corrections and magnetoresistance in ZnO nanowires by ion implantation.

Using ion implantation, the electrical as well as the magnetotransport properties of individual ZnO nanowires (NWs) can be tuned. The virgin NWs are configured as field-effect transistors which are in the enhancement mode. Al-implanted NWs reveal a three-dimensional metallic-like behavior, for which the magnetoresistance is well described by a semiempirical model that takes into account the presence of doping induced local magnetic moments and of two conduction bands. On the other hand, one-dimensional electron transport is observed in Co-implanted NWs. At low magnetic fields, the anisotropic magnetoresistance can be described in the framework of weak electron localization in the presence of strong spin-orbit scattering. From the weak localization, a large phase coherence length is inferred that reaches up to 800 nm at 2.5 K. The temperature-dependent dephasing is shown to result from a one-dimensional Nyquist noise-related mechanism. At the lowest temperatures, the phase coherence length becomes limited by magnetic scattering.

Unexpected optical response of single ZnO nanowires probed using controllable electrical contacts.

Relying on combined electron-beam lithography and lift-off methods Au/Ti bilayer electrical contacts were attached to individual ZnO nanowires (NWs) that were grown by a vapor phase deposition method. Reliable Schottky-type as well as ohmic contacts were obtained depending on whether or not an ion milling process was used. The response of the ZnO NWs to ultraviolet light was found to be sensitive to the type of contacts. The intrinsic electronic properties of the ZnO NWs were studied in a field-effect transistor configuration. The transfer characteristics, including gate threshold voltage, hysteresis and operational mode, were demonstrated to unexpectedly respond to visible light. The origin of this effect could be accounted for by the presence of point defects in the ZnO NWs.

Antiferromagnetic LaFeO(3) thin films and their effect on exchange bias.

Antiferromagnetic (AFM) orthoferrites are interesting model systems for exploring the correlation between their crystalline and AFM domains and the resulting exchange bias when coupled to a ferromagnetic layer. In particular, LaFeO(3) (LFO) has a Néel temperature, T(N) = 740 K, which is the highest in the orthoferrite family. The recent developments of synchrotron radiation-based photoelectron emission microscopy (PEEM) have provided the possibility of studying AFM domain structures as well as the magnetic coupling between the AFM and the adjacent ferromagnetic (FM) layer, domain by domain. Thin films of LFO have proved excellent candidates for such studies because their AFM domains are well defined and large enough to be readily imaged by PEEM. This paper reviews the growth, structural and magnetic properties of LFO thin films as well as exchange coupling to a FM layer. The strong correlation between structural and AFM domains in this material allows us to investigate the exchange coupling as a function of the domain configuration, which can be changed by using different substrate material and substrate orientation. A significant increase of the exchange bias field by a factor of about 10 was obtained when LFO was diluted with Ni atoms in the volume part. In this sample, the structural domain boundary became corrugated due to substitutional defects. Our results indicate that the details of the precise domain boundary configuration strongly affect the exchange coupling.

Electrostatic modulation of the superfluid density in an ultrathin film.

By capacitively charging an underdoped ultrathin La2-xSrxCuO4 film with an electric field applied across a gate insulator with a high dielectric constant, relative changes of the areal superfluid density ns of unprecedented strength were observed in measurements of the film kinetic inductance. Although ns appears to be substantially reduced by disorder, the data provide, for the first time on the same sample, direct compelling evidence for the Uemura relation Tc proportional to ns(T=0) in the underdoped regime of copper-oxide superconductors.

Studies of the magnetic structure at the ferromagnet-antiferromagnet interface.

Antiferromagnetic layers are a scientifically challenging component in magnetoelectronic devices, such as magnetic sensors in hard-disk heads, or magnetic random-access memory (RAM) elements. In this paper, it is shown that photoelectron emission microscopy (PEEM) is capable of determining the magnetic structure at the interface of ferromagnets and antiferromagnets with high spatial resolution (down to 20 nm). Dichroism effects at the L edges of the magnetic 3d transition metals, using circularly or linearly polarized soft X-rays from a synchrotron source, give rise to a magnetic image contrast. Images, acquired with the PEEM2 experiment at the Advanced Light Source, show magnetic contrast for antiferromagnetic LaFeO3, microscopically resolving the magnetic domain structure in an antiferromagnetically ordered thin film for the first time. Magnetic coupling between LaFeO3 and an adjacent Co layer results in a complete correlation of their magnetic domain structures. From field-dependent measurements, a unidirectional anisotropy resulting in a local exchange bias of up to 30 Oe in single domains could be deduced. The elemental specificity and the quantitative magnetic sensitivity render PEEM a perfect tool to study magnetic coupling effects in multilayered thin-film samples.