Session D3: Condensed Matter Physics I

Room Temperature Antiferromagnetism in c-FeSi

We will be reporting on a recent experimental discovery [1] of a new antiferromagnetic material, having important fundamental and practical implications for nanomagnetism and nanotechnology. Using spin-polarized scanning tunneling microscopy (SP-STM) we observed room-temperature antiferromagnetic spin ordering in thin epitaxial films of c-FeSi on Si(111). Although some earlier GMR experiments [2] clearly indicated such a possibility, antiferromagnetism in this material has never been directly observed or predicted in theory. Using Fe-terminated STM tip, we found unusually high (75{\%}) spin polarization of tunneling current. We also found atomically narrow spin-domain-boundaries, indicating that c-FeSi can be used for atomic scale magnetic memory storage. Our data analysis suggests that c-FeSi represents a Mott-Hubbard antiferromagnet.\\[4pt] [1] Igor Altfeder, Wei Yi, and V. Narayanamurti, ``Spin Polarized Scanning Tunneling Microscopy of the Room Temperature Antiferromagnet c-FeSi'', Rapid Communication, Physical Review B 87, 020403(R) (2013).\\[0pt] [2] J. M. Pruneda, R. Robles, S. Bouarab, Ferrer, and A. Vega, ``Antiferromagnetic interlayer coupling in Fe/c-SiFe/Fe sandwiches and multilayers'', Phys. Rev. B 65, 024440 (2001).   

Managing thermal effects in z-scan measurements on PTCDA films

We study the two-photon absorption in micrometer thick polycrystalline PTCDA (perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride) films using the open aperture z-scan technique. The films were grown by organic molecular beam deposition on Pyrex substrate and have been excited with 150 fs high repetition rate laser pulses at a wavelength of 820 nm. The pulses are focused onto the sample with a 10 x or a 20 x long distance microscope objective lens. The excitation intensities have been kept the same in both cases. To study the influence of sample heating the laser repetition rate has been varied from 4 MHz to 50 kHz by an acousto-optic pulse selector. At laser repetition rates larger than 200 kHz and 1 MHz for the 10 x and 20 x microscope lenses, respectively, we observe a reduction of the z-scan transmission dip. This reduction is attributed to a counteracting thermal effect due to film heating in the focus area. The reduced thermal effect using a 20 x microscope lens is attributed to faster thermal diffusion from the smaller focus area into the unexcited film. At lower repetition rates the z-scan dip is independent of the repetition rate and the two-photon absorption coefficient in PTCDA films was determined to be approximately 5 cm/GW.   

Exciton Dynamics in Hexagonal InP Nanowires

We study the exciton dynamics in InP nanowire (NW) ensembles by intensity/temperature-dependent photoluminescence (PL) measurements, time-correlated-single-photon-counting (TCSPC) and heterodyne detected four-wave-mixing experiments (HFWM). The InP NW were grown on fused silica substrate by 50 nm gold catalyst metal-organic-vapor-phase-epitaxy at T$=$ 450$^{\circ}$ C resulting in nearly WZ type NWs. The PL measurements at 15 K show three emission band at 820 nm, 837 nm and 860 nm. The bands are attributed to free, trapped and indirect WZ-ZB related exciton transitions, respectively. With increasing T the free-exciton band gains importance relative to the dominating trapped exciton band while the low energy band vanishes. TCSPC results show an increasing PL decay rate of all emission bands with increasing T, most pronounced for the low energy band. The result agrees with the exciton population dynamics obtained from three-beam HFWM measurements. Photon echo experiments at 80 K reveal an ultrafast exciton dephasing time of less than 100 fs which is attributed to scattering with a high carrier background in these NWs.   

Atomicscale Investigation of Proximity Effect at a Molecular-Superconductor-Metal Boundary

Proximity effects of nanoscale molecular clusters, (BETS)$_{2}$-GaCl$_{4}$, on Ag(111)have been investigated by using low temperature tunneling spectroscopy and spectroscopic mapping. At a far distance from the superconductor-metal boundary, the dI/dV tunneling spectroscopic data show the surface state on-set of two-dimensional nearly free electrons on Ag(111) surface at -65 mV. Strikingly, the intensity of the surface state rapidly decreases when the dI/dV data are recorded closer to the boundary. At $\sim$ 2nm distance, the surface state is completely quenched. Moreover, the formation of pseudo-gap state is already observed far from the metal-superconductor boundary, which later transform into the superconducting gap. This work provides detailed electronic structure evolution across a metal-superconductor boundary at an atomic scale.   

Single molecule motor with dipolar arms

One of the goals of nanotechnology is to have billions of nano-molecular machines packed in a tiny area that can operate under control. Simultaneous operation of such nano-machines requires developing a system in which the nanomachines can communicate each other. Here we investigate a double-decker class molecular motor adsorbed on Au(111) and Cu(111) surfaces using low temperature scanning tunneling microscopy in an ultrahigh vacuum environment. Our molecular motor is formed by a porphyrin based stator, and a dipole active rotor. An Eu atom is used to link the rotation and stationary parts of the motor, and therefore it acts as a single atom ball bearing. On Cu(111) surface, however, the molecules form self-assembled structures with a hexagonal pattern. The stable positions are maintained by balancing two interactions; the internal interactions between the upper and lower decks, and the dipolar interactions with the neighboring rotors. On Au(111) surface, however, we observe thermal induced rotations of the individual molecular motors at 80K. This work is a step forward in the development of molecular machines for nanoscale information transport.   

Directionally controlled switching of a molecular rotor

Directional control in the operation of molecular devices is a key hurdle in the path toward functional synthetic devices at the nanoscale. We demonstrate the operation of a molecular rotor whose rotator is decoupled from substrate interactions via a threefold symmetric stator with a single ruthenium atom at its apex, acting as a bearing around which the rotator is controlled. Five ``arms'' extend from the apex of the stator, one of which is truncated to facilitate observation of device rotations via molecular asymmetry. Molecular rotations are driven via inelastic tunneling electrons from the tip of a scanning tunneling microscope, and the direction of rotation is determined by the choice of electron injection site with respect to the internal molecular structure.   

Insight into Topological Insulators from Infrared Spectroscopy

Topological insulators are certain band-insulator compounds that exhibit a new phase of quantum matter with inverted insolating energy gap. As a result, these bulk insulators have conducting states on the surface. This new state of matter is a consequence of strong spin-orbit coupling in these materials. We use infrared reflectance spectroscopy to probe electrodynamics of charge carriers at different temperatures and we conduct magneto-optical spectroscopy experiments to probe magneto-electric coupling. We present findings on several second-generation Bi-based topological insulators at temperatures ranging from 10 to 300 K and in magnetic fields up to 18 T. In addition, we discuss the charge inhomogeneities in these novel materials.   

Molecular Kondo Chain

Functional molecular systems, for spin-electronic devices, require careful characterizations of magnetic interactions and intermolecular couplings. We present a scanning tunneling microscopic (STM) study of a surface supported molecular system of coupled cobalt ion centers, formed by a surface catalyzed ring coupling reaction. Binding between individual molecular units is confirmed by vertical and lateral manipulations. Spectroscopy (STS) is used to probe Kondo magnetic interaction. Curve fitting of the Kondo spectroscopic feature is used to extract Kondo temperatures, a measure of the interaction energy, for individual magnetic ion sites. Local point spectroscopy and spectroscopic mapping reveal that the magnetic interactions are localized to the cobalt ion sites, and that chain length influences the interaction strength. Analysis of data as a function of chain length and magnetic ion position is combined with density functional theory (DFT) results to argue for antiferromagnetic (AFM) coupling between magnetic centers.   

Relating Thermal Properties of Polymeric Materials to Structural and Filler Characteristics

Thermal properties of polymeric systems and the relationship of these properties to a polymer's structure and reinforcing fillers are studied. Differential Scanning Calorimetry (DSC) and Thermal Gravimetric Analysis (TGA) are used to examine basic thermal properties (specific heat capacity, glass transition temperature, melting temperature, melting enthalpy, and decomposition temperature) of selected polymeric materials. The experimental results are in turn related to structure and reinforcing fillers for the following materials: polyethylene, nitrile rubber, and EPDM (ethylene propylene diene rubber.)