Session F4: Condensed Matter I

Imaging Transport in Nanowires with NSOM

A novel system has been developed for the imaging of carrier transport within semiconductor nanostructures by operating a near field scanning optical microscopy (NSOM) within a scanning electron microscope. Luminescence associated with carrier recombination is collected with high spatial resolution to monitor the motion and recombination of charge generated by use of an electron beam as an independent point source. Light is collected in the near field from a scanning fiber using tuning fork feedback in an open architecture combined AFM/NSOM system allowing for independent motion of sample and tip. From a single image, it is possible to obtain a direct measure of minority carrier diffusion length. This technique has been used in the near-field collection mode to image the diffusion of holes in n-type GaN-AlGaN core-shell nanowires, grown via Ni-catalyzed MOCVD. Measurements were made on tapered nanowires ranging in diameter from 500 to 800 nm, with lengths up to $\sim 30 \mu m$. The average 1-dimensional carrier diffusion length was measured to be 1.2 +/- 0.2 $\mu$m in the low injection limit. In addition, it is possible to map the luminescence that is wave-guided to the end of the structure, imaging waveguide modes   

Little and Large:Topological Defects in Cosmology and Condensed Matter Theory

Cosmology and condensed matter theory seem to be worlds apart, and yet are ubiquitously linked. Testing our current understanding of phenomena that occur on galactic scales can now be realized in the laboratory. The coming-together of cosmology and condensed matter theory is facilitated by the phase transitions and defect formation that is common to both areas. A recurring question in cosmology has concerned whether the vacuum is empty or contains vortex-strings or other topological defects. Understanding the formation and evolution of these topological defects plays a significant role in our understanding of cosmology and the early universe. Condensed matter systems provide an important starting point to studying the phenomena of phase transitions and the formation of topological defects. In both the cosmological and condensed matter scenarios, symmetry breaking causes a change to a degenerate vacuum manifold with non-trivial topology. This occurrence will be discussed along with experimental results in superfluid Helium and superconductors.   

Search for anomalous spin-mass coupling with a rubidium magnetometer

We report on progress of our experiment using a dual-isotope rubidium magnetometer to search for a hypothetical long-range coupling between Rb nuclear spins and the mass of the Earth. The valence electron dominates magnetic interactions and serves as a precise co-magnetometer for the nuclei in a simultaneous measurement of Rb-85 and Rb-87 spin precession frequencies, enabling accurate subtraction of magnetic perturbations. The construction and optimization of the apparatus is nearly complete, and we are now addressing several technical sources of noise and studying potential sources of systematic error. The optimized dual-isotope Rb magnetometer has sufficient shot- noise-projected sensitivity to improve experimental limits on long-range spin-mass couplings by an order of magnitude in general and by two orders of magnitude for the proton spin in particular.   

Light Propagation in Liquid Crystals with a Chiral Dopant

This project will investigate the design and feasibility of a novel liquid crystal sensor that could be used to detect the presence and amount of foreign biological and/or chemical airborne agents. Such a sensor would have the advantage of being very portable. As such could have particular value in detecting biological or chemical weapons in the field of military operations. It would also be of use in a rapid response to a chemical or biological terrorist attack. The device would operate on the basic principal that when certain types of molecules bind to a liquid crystal molecule, the conformation of the liquid crystal molecule changes. This would in turn lead to a change in the overall arrangement of the liquid crystal, which could be detected using polarized light. In the absence of a contaminant the average molecular direction (optical axis, $\hat{n}$ ) is constant throughout the liquid crystal. The dopant adds a chirality or twist so that $\hat{n}$ precesses as a function of depth. We first solve for the reflected and transmitted light off of the air-liquid crystal boundary in the simplified case where there is linear chirality or a spiral configuration which repeats itself over some fixed interval (or pitch). We then generalize for cases in which this repeat distance varies with crystal depth. Finally we will obtain an expression for the contaminated crystal configuration which should depend on time and a diffusion constant and examine how the light properties change with respect to intensity and duration of exposure to the contaminant.   

Towards microwave modulation in a wavelength-tuned magneto-optical trap

In this project, I present a new method for trapping Rubidium-87 atoms. The method proposed is microwave modulation of an external cavity diode laser. The modulation is designed so as to produce frequency sidebands for hyperfine pumping in addition to the main cooling frequency. It is designed for use in magneto-optical trapping.   

La-139 NMR in La$_{4}$Ni$_{3}$O$_{8}$: a possible analog to the cuprate high temperature superconductors

The Ni$^{1+}$/Ni$^{2+}$ states in the nickelates have identical electronic configurations as Cu$^{2+}$/Cu$^{3+}$ in the high temperature superconducting cuprates (3d$^{9}$/3d$^{8})$, and may exhibit similar properties. However, the Ni$^{1+}$ state is rare and cannot be easily stabilized. Recently, Martha Greenblatt and collaborators at Rutgers University have succeeded in growing a family of such compounds, Ln$_{n+1}$Ni$_{n}$O$_{2n+2 }$with a layered structure similar to the cuprates. The La$_{4}$Ni$_{3}$O$_{8}$ compound is particularly interesting as it undergoes an antiferromagnetic transition at T$_{N}$ = 100 K. We have done La NMR on powder samples to investigate the nature of this phase. Our spin lattice relaxation rate measurements clearly reveal a second order electronic phase transition similar to that observed in other antiferromagnets. Although we found clear signatures of changes to the spectra below T$_{N}$, we are unable to assign these changes to the presence of an internal field from the antiferromagnetic structure, or changes to the electric field gradient at the La site.   

Quantum Phases of Atom-Molecule Mixtures of Fermionic Atoms

Cold atom experiments have observed atom-molecule mixtures by tuning the interactions between particles.\footnote{M.L. Olsen, J. D. Perreault, T. D. Cumby, and D. S. Jin, Phys. Rev. A 80, 030701(R) (2009)} We study many particle interactions by examaning a simple model that describes the destruction of fermionic atom pairs to form single bosonic molecules and vice versa. A set of functional Renomalization Group equations\footnote{R. Shankar, Rev. Mod. Phys., Vol 66 No. 1, January 1994}$^,$\footnote{S.W. Tsai, A.H. Castro Neto, R. Shankar, D.K. Campbell, Phys. Rev. B 72, 054531 (2005)} describing these processes are set up and solved numerically. The Self Energy of the fermions are attained as a function of frequency and we search for frequency dependent instabilities that could denote a transition from a disordered liquid to a BCS phase. (Financial support from NSF DMR-084781 and UC-Lab Fees Research Program.)   

Knight Shift Probe of Onset of Coherence in Heavy Electron Superconductor CeIrIn5

The CeMIn$_{5}$ compounds, where M = Co, Ir, Rh, are a novel class of superconductors discovered about 10 years ago. The 115 compounds are Kondo lattice materials: the compound's conduction electrons are coupled to an ordered lattice of local moments through the Kondo effect. These materials have a large effective electronic mass at low temperatures. We present new NMR Knight shift data in single crystals of CeIrIn5 between 2K and 120K. We find that the Knight shift of the In(1) site in this material is proportional to the bulk magnetic susceptibility above a temperature T* $\sim $ 30(?)K. Below this temperature, the Knight shift fails to track the susceptibility. We interpret these results in terms of the two-fluid model, in which the susceptibility of the heavy electron component, chi{\_}cf, grows in intensity with decreasing temperature. We find that K{\_}cf $\sim $ chi{\_}cf $\sim $ log(T/T*), in agreement with other heavy fermion compounds. Our results confirm the predictions of the dynamical mean field theory calculations of Haule et al. for the onset of coherence in this compound.   

Cyclotron Resonance Vanishing effect in Correlated 2D Electron Systems

``Cyclotron Resonance - Vanishing effect'' (CRV) arise on magnetospectra of cyclotron resonance line (CR) as a well-defined gap that reduce to zero CR effect. CRV have been discovered due to experimental study of terahertz radiation transmission and photoresistivity magnetospectra at CR conditions in two-dimensional electron system (2DES) of GaAs/AlGaAs nanostructures with higher electron mobility at low (non-quantized Hall effect) magnetic fields. Unique experimental approach based on study of 2DES with photoresistivity and transmission techniques allows to get complementary data. One of the more significant results is that CRV-line shape (and consequently CRV effect) independent from testing THz power. We will discuss experimental study of ``CR- Vanishing effect'' and theoretical analysis that indicates on appearance of new fundamental correlated states of electrons at CRV conditions. To study CRV effect in detail we are working to create new model taking into account models for quantum Hall effect, magneto-plasma waves, non-linear zero-resistance states, and others that was develop for comparable experimental conditions.   

Steady-State and Transient Photoconductivity in the Poly(2,7-Carbazole) Copolymer PCDTBT, and in Bulk Heterojunction Composites with PC$_{70}$BM

We have studied the nature of carrier generation in an alternating donor-acceptor low bandgap copolymer and in composites of that polymer with a soluble fullerene derivative, using steady-state and transient photoconductivity. The Poly(2,7-Carbazole) copolymer PCDTBT that we studied represents a class of donor-acceptor copolymers that hold promise for photovoltaic applications because of the ability to tune the electronic energy levels by changing the acceptor unit (see Blouin, N.; Michaud, A.; Leclerc, M. \textit{Adv. Mater.} \textbf{2007}, $19$, 2295 - 2300). Photovoltaic devices fabricated from PCDTBT in composites with the soluble fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC$_{70}$BM) have exhibited a higher solar cell power conversion efficiency than has been achieved in P3HT based devices. In PCDTBT, the absorption extends out to 700 nm, with two distinct but broad absorption bands that are centered at $\sim $400 nm and $\sim $600 nm. We have used steady-state and transient photoconductivity to investigate the carrier generation and collection efficiency of PCDTBT and its composite with PC$_{70}$BM after photoexcitation at each of its distinct absorption bands.   

Local density of states and scanning tunneling currents in graphene

Graphene consists of an atom-thick layer of carbon atoms arranged in a honeycomb lattice and has been intensively studied due to its fascinating properties. We calculate the local density of states in graphene with different chemical substitution impurities, such as boron and nitrogen atoms, as well as for vacancies. We give exact analytical expressions for the local density of states for the whole energy range including energies beyond the Dirac cone approximation. The momentum maps of the local density of states for different impurities and discussion of their interpretation are given. We also present exact analytical calculations of scanning tunneling currents in locally disorded graphene using a multimode description of the microscope. [N.~M.~R. Peres, L. Yang, and S. - W. Tsai, New J. Phys. {\bf 11}, 095007(2009)]   

Arsenic nuclear magnetic resonance in CaFe2As2

We present $^{75}$As nuclear magnetic resonance measurements in the paramagnetic and antiferromagnetic states of CaFe$_{2}$As$_{2}$. Single crystals were produced using a Sn flux method and characterized via powder X-Ray diffraction, susceptibility, and specific heat measurements. The NMR data show that the internal hyperfine field and the electric field gradient change discontinuously at T$_{0}$ = 169 K. The observed hyperfine field is consistent with stripe antiferromagnetic ordering of the Fe spins in the a-b plane. Spin lattice relaxation data show metallic T$_{1}$$^{-1}$ $\sim$ T for T $\leq$ T$_{0}$/3. However, T$_{1}$$^{-1}$ shows a small peak at 10 K attributed to slow spin fluctuations that could indicate the emergence of antiferromagnetic domain wall motion.   

Growth of Iridium on Ge(111) Studied by STM and LEEM

Iridium on germanium is a system which is useful for understanding the interaction of 5d metals with semiconductors, with potential applications to electronic contacts. We have used both scanning tunneling microscopy (STM) and low energy electron microscopy (LEEM) to characterize the submonolayer growth of iridium onto Ge(111) as a function of coverage, deposition temperature, and annealing temperature. Ir deposited onto the Ge(111) c(2x8) surface forms a ($\surd $3x$\surd $3)R30$^{\circ}$ phase with the island size dependent upon substrate temperature during deposition. Deposition at a sample temperature of 670 C yields large micron-sized regions of continuous ($\surd $3x$\surd $3)R30$^{\circ}$ coverage, as seen by LEEM. Deposition at 400 C produces Ir islands of three different sizes, all of which are too small to be resolved in LEEM but can be easily observed in room temperature STM images: large islands of roughly 10 to 20 nm diameter, consisting of multiple layers; medium-sized islands of roughly 4 nm in diameter, and small islands about 1 nm in diameter. Heating the sample over 640 C yields islands of large enough size to be resolved with LEEM, with the island size dependent upon annealing temperature. Ostwald ripening was observed in LEEM movies.   

A model of electron spin relaxation momentum time in GaAs cyclindrical quantum dots: including the Dresshaus effect

A model of GaAs quantum dots embedded in a quantum wire is studied. We show that how the electron spin relaxation momentum time (SRT) is varying with some physical parameters. Under this model, a general conclusion is given : SRT decreases while the four parameters (external magnetic field, surrounding temperatures, quantum wire width and thickness) are increasing. The phenonmena is understood by more and more phonon modes resulted in a higher electron-phonon scattering probability when the system is under high magnetic field and high temperature. Thus the SRT is reduced. The most important reason for us to study such topics is that it is related with quantum information processing ability. In the present model, we deal with rectangular acoustic strain with deformation potential theory including the Dresshaus effect. Next step, we want to deal with how the SRT varies of quantum dots under very low temperature. A model of piezoelectric scattering with cyclindrical acoustic strain is considered in which the ionic displacement field (IDF) based on Born-Huang equation is shown. We are working on this line.   

Scanning Conductive Force Microscopy for Characterization of Model Molecular Devices

We have used scanning conductive atomic force microscopy as a tool to characterize molecular devices. Using self-assembled monolayers such as n-decanethiolate or n-octadecanethiolate as a matrix, we inlaid device components such as dendrimers or gold nanoparticles. All model systems were studied under constant force mode in air, while topography, lateral force, and current images were acquired. This configuration enables high resolution atomic force microscopy imaging, such as revealing of etch pits among the self-assembled monolayers, which is otherwise difficult to attain. In addition, the local conductivity can be correlated with the topographic features such as thiolate domains or surface defects. Both the technical development associated with this method and the detailed results will be discussed.