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COLLOQUIA & SEMINARS 2011:

1. (SEMINAR)"STM Study of Topological Insulators Grown by MBE", Prof. Dr. Xi Chen (Tsinghua University), at 4:00 pm, January 5, 2011.

1. Abstract:
   I will summarize our recent activity in the molecular beam epitaxy (MBE) growth and characterization of the nontrivial surface states of topological insulator films of Bi_2Te_3 and Bi_2Se_3. We demonstrate the atomically flat morphology and intrinsic topological property of the resulted films by angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS). By direct imaging standing waves associated with nonmagnetic impurities and steps on Bi_2Te_3 and Bi_2Se_3 (111) surfaces, we show that the topological states have a surface nature and are protected by the time reversal symmetry. The Dirac cone structure is also indicated by the Landau quantization of the topological states in high magnetic field.

2. (SEMINAR)"Semiconductor Quantum Nanophotonic Devices", Prof. Dr. H. C. Liu (National Research Council of Canada, CANADA), at 4:00 pm, January 11, 2011.

1. Abstract:
   Semiconductor quantum structures have not only enabled discoveries of new phenomena but also realizations of novel photonic devices. In this talk I will introduce those devices that I have been working on and discuss my future plans. The first one is the, by now, well-established quantum-well infrared photodetector (QWIP). Here I will discuss its potential for applications in laboratories and environmental sensing in the infrared (IR) spectrum. I will also discuss its integration with light emitting diode for pixelless upconversion imaging. Quantum cascade laser (QCL) is another emerging and promising device. I will review the current OCL status and our record-holding three-well resonant-phonon design for the terahertz (THz) region. Together with QWIPs, QCLs form a very promising technology base for sensing.

3. (SEMINAR)"Spin Injection in Organic Semiconductors", Prof. Dr. Y. L. Gao (University of Rochester, USA) at 4:00 pm, March 11, 2011.

1. Abstract:
   It is observed that there can be a strong spin valve effect in organic materials, showing that organic materials can be promising for spintronics applications. The mechanism and efficiency of spin injection into organic materials, however, is largely unknown. Using spin- and time-resolved two-photon photoemission (STR-2PPE) spectroscopy, we have investigated spin injection from Co and GaAs to organic semiconductor copper phthalocyanine (CuPc). With STR-2PPE, the dynamics of both electron and spin relaxation can be studied with femtosecond time resolution and high surface sensitivity. We observed that the spin injection efficiency for hot electrons in is high, ~90% from Co to CuPc. We also observed that the spin polarization increases with time at low energies, revealing scattering processes that can be strongly energy-dissipating but spin-preserving.

4. (SEMINAR)"Oscillatory Magnetic Anisotropy Originating From Quantum Well States In Fe Films", Prof. Dr. Y. Z. Wu (Fudan University) at 4:00 pm, April 20, 2011.

1. Abstract:
   The electrons in the nanometer scale thin films could be confined to form quantum well states (QWS), and many novel thickness-dependent oscillation of physical properties induced by QWS in non-magnetic films were discovered. Modern magnetoeletronics devices are based on metallic films with thicknesses on the nanometer scale, so it would be very important to experimentally explore how the QWS in ferromagnetic films affect the intrinsic magnetic properties in magnetic nanostructure research. In this talk, we will report the study on thickness- and temperature-dependent step-induced in-plane magnetic anisotropy of Fe films grown on vicinal Ag(1,1,10) surface utilizing magneto-optic Kerr effect. At low temperatures, below 200 K, the magnetic in-plane uniaxial anisotropy strongly oscillates as a function of Fe thickness with the period of 5.7 monolayers(ML). For the sample covered with Au, also the easy magnetization axis oscillates between perpendicular and parallel to the steps with the same period. Our results also indicate the out-of-plane magnetic anisotropy of Fe films oscillates with the Fe thickness. Such oscillation of magnetic anisotropy is attributed to the quantum well states (QWS) of the d-band electrons in Ferromagnetic Fe films, and this QWS was directly observed in Fe/Ag(001) system by the angular-resolved photoemission spectroscopy(ARPES). The lack of oscillation at RT may result from the thermal reduction of the electron mean free path.

5. (SEMINAR)"Electron fractionalization and the phase diagram of doped antiferromagnets", Researcher Dr. Chu-Shun Tian (Tsinghua University) at 4:00 pm, May 12, 2011.

1. Abstract:
   "Elementary particles are the architect of the nature." It is this simple philosophy that shaped natural sciences. Interestingly enough, in some electronic materials such as doped Mott insulators, at short scales, electrons can be effectively fractionalized into more "elementary" particles-holons and spinons-carrying the charge and spin degrees of freedom. What kind of quantum statistics do they obey? How do they form quantum matters at large scales? It has been recognized very recently that these questions may provide a key to many hard-core issues in strongly correlated systems such as high Tc superconductivity. In this talk, I will present a case study of these questions in the context of lightly doped antiferromagnets (spin 1/2). I will first explain how canonical theories of phase transitions are challenged by "electron fractionalization". Then, I will discuss how a peculiar statistics, the so-called mutual statistics, emerges due to electron fractionalization. Finally, I will explain how the zero-temperature phase diagram deeply roots in such a peculiar statistics and is characterized by a pair of Wilson loops. I will show the existence of a novel phase-the Bose insulator phase-that separates the antiferromagnetic and superconducting phases, and the quantum transitions between them are far beyond the Landau-Ginzburg-Wilson paradigm.

6. (SEMINAR)"Spintronics at multiferroic oxide interfaces", Prof. Dr. Jamal Berakdar (Martin-Luther University Halle-Wittenberg, GERMANY) at 4:00 pm, May 22, 2011.

1. Abstract:
   Recent experiments [1] evidence the emergence of a metallic phase with a high carrier mobility at the interface of some insulating oxides such as LaTiO3 /SrTiO3 or LaAlO3 /SrTiO3. As demon- strated experimentally [2], this new type of two-dimensional electron gas (2DEG) can be laterally confined and patterned to achieve nanometer-size tunnel junctions and to exploit these structure as field-effect transistors. Exciting applications in the field of spintronic are expected when spiral multiferroic oxides are utilized, such as RMnO3 (R=Tb, Dy, Gd) [3]. In this presentation we will discuss how the magnetoelectric coupling [4] can be exploited to functionalize the 2DEG at the interface of these compounds for spintronic applications. In particular, an electrically controllable pure spin current can be generated and the principle of a spin-field-effect transistor and flash- memory device are demonstrated [5]. We also discuss the magneto and thermal transport through in these systems [6] and in tunnel junctions involving multiferroic spacers and demonstrate the existence of anisotropic tunnel magnetoresistance [7] that can be controlled by an applied electric field with a strength in the range of several keV/cm.
   [1] A. Ohtomo, and H. Y. Hwang Nature 427, 423 (2004); 441, 120 (2006); A. Ohtomo, et al. ibid. 419, 378 (2002); S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, and J. Mannhart Science 313, 1942 (2006).
   [2] C. Cen, S. Thiel, J. Mannhart, and J. Levy Science 323, 1026 (2009); E. Singh-Bhalla et al. Nature Phys. 7, 80 (2011).
   [3] Mannhart, et al. MRS Bull. 33, 1027 (2008).
   [4] H. Katsura, N. Nagaosa, and A. V. Balatsky Phys. Rev. Lett. 95, 057205 (2005); C.L. Jia, J. Berakdar Euro.Phys.Lett. 85, 57004 (2009).
   [5] C.L. Jia, J. Berakdar Appl. Phys. Lett. 95, 012105 (2009); Phys. Rev. B 80, 014432 (2009).
   [6] C.L. Jia, J. Berakdar Appl. Phys. Lett. 98, 042110 (2011); Phys. Rev. B 83, 045309 (2011).
   [7] C.L. Jia, J. Berakdar Phys. Rev. B 81, 052406 (2010).

7. (COLLOQUIUM)"Carrier Spin Dynamics in Semiconductors probed by Luminescence Experiments", Prof. Dr. Xavier Marie (Universite de Toulous, FRANCE) at 10:00 am, May 23, 2011.

1. Abstract:
   Numerous proposals for future spintronic and quantum information devices are based on manipulating or storing information in the form of electronic or nuclear spin polarization in semiconductor quantum dots (QD). This approach is very attractive since a long coherence time is expected, as a result of the inhibition of the classical spin relaxation mechanisms: the discrete energy levels in semiconductor quantum dots and the corresponding lack of energy dispersion lead to a slowdown of the spin relaxation processes compared to bulk or two-dimensional structures.
   We have studied the spin dynamics of electrons, holes, neutral and charged excitons in undoped and doped QD by time-resolved photoluminescence. We will give a review of recent experimental results on optical spin injection and manipulation in InAs/GaAs, GaAs/AlGaAs, GaN/AlN quantum dots and ZnO nanoparticles.
   We will also present recent results on the electrical spin injection in quantum wells or quantum dots through Co/Al2O3/GaAs or CoFeB/MgO/AlGaAs tunnel barriers.

8. (SEMINAR)"Probing Dirac Fermions in Graphene", Prof. Dr. Yuan-Bo Zhang (Fudan University) at 4:00 pm, May 27, 2011.

1. Abstract:
   Graphene, a single atomic layer of carbon, is a unique two-dimensional quantum material where electrons behave as massless particles (Dirac fermions) with an effective speed of light equal to c/300. This provides an interesting analogy to the high energy relativistic quantum mechanics in a condensed matter system. In this talk I will discuss our experiments probing the novel quantum phenomena arising from the ¡°relativistic¡± nature of the quasiparticles in graphene. I will also show that, with one more layer added, the graphene bilayer is another intriguing system whose electronic structure can be controlled by electrical gating.
   

9. (SEMINAR)"Novel Quantum Phases of Interacting Bosons on Frustrated Lattices", Prof. Dr. Yan Chen (Fudan University) at 4:00 pm, June 7, 2011.

1. Abstract:
   A duality transformation in quantum field theory is usually established first through partition functions. It is always important to explore the dual relations between various correlation functions in the transformation. Here, we explore such a dual relation to study quantum phases and phase transitions in an extended boson Hubbard model at 1/3 (2/3) filling on a triangular lattice. We develop systematically a simple and effective way to use the vortex degree of freedoms on dual lattices to characterize both the density wave and valence bond symmetry breaking patterns of the boson insulating states in the direct lattices. In addition to a checkerboard charge density wave (X-CDW) and a stripe CDW, we find a novel CDW-VBS phase which has both local CDW and local valence bond solid (VBS) orders. Implications on QMC simulations are addressed. The possible experimental realizations of cold atoms loaded on optical lattices are discussed.

10. (SEMINAR)"The Phase Problem of Macromolecular X-ray Crystallography", Prof. Dr. Wu-Pei Su (University of Houston, USA) at 4:00 pm, June 18, 2011.

1. Abstract:
   The X-ray phase problem is the problem of extracting structural information of macromolecules such as proteins directly from the diffraction intensity data of the macromolecular crystals. We have developed a simple mathematical algorithm to solve this problem at medium resolution (around 5 to 8 Angstroms). Structural details of proteins such as the envelope, alpha helices and beta sheets can be determined from the intensity data.

11. (SEMINAR)"Preparation and characterization of some one dimensional nanostructures", Prof. Dr. S. G. Yang (Nanjing University) at 4:00 pm, June 20, 2011.

1. Abstract:
   In this talk, I will introduce some works on one dimensional nanostructured materials studied in my group. Magnetic nanowires have been synthesized using AAO as the template. Enhanced coercivity and anisotropic magnetic property have been observed in these nanowire arrays. ZnO, ZnS and CdS nanowires/nanobelts have been synthesized by using thermal evaporation method. Based on the ZnO nanowires, comb-like nanostructures and hexagonal symmetry ZnO nanostructures can be formed by two step epitaxial growth. ZnO nanowires and nanorods have been epitaxially grown on sides of the ZnS nanobelts. AlN and SnO2 zigzag nanobelts have been formed in the reaction at high temperature.

12. (SEMINAR)"Instability Mechanisms of Quantum-Cascade Lasers with Various Optical Nonlinearities", Assis. Prof. Dr. Jing Bai (University of Minnesota Duluth, USA) at 4:00 pm, July 5, 2011.

1. Abstract:
   Quantum cascade lasers (QCL¡¯s) are semiconductor lasers composed of nanometers thick layers of semiconductor materials, and emit light through intersubband transitions in the quantum confined states of the conduction band. The mid-infrared (MIR) QCLs emit at wavelengths ranging from 3 to 15 ?m, which cover the ¡°fingerprint¡± region for most of the gas molecules. The dynamic analysis of QCLs is important for ultra-short pulse generations in the MIR region for trace-gas detection. The unique combination of giant nonlinearities and ultrafast carrier dynamics of QCLs makes the dynamic behavior of QCLs very different from conventional semiconductor lasers. This talk will present the analysis procedure on phase instability and amplitude instability of QCLs. The phase instability exhibits the single-mode nature and the amplitude instability is the multi-mode Risken-Nummedal-Graham-Haken (RNGH) kind of instability. Effects from various nonlinear processes, such as the saturable absorber effect, self-phase modulation, and the spatial hole burning effect, on the instability mechanisms in QCLs will also be discussed.

13. (SEMINAR)"Superconducting ground state for a doped Mott insulator", Prof. Dr. Z. Y. Weng (Tsinghua University) at 4:00 pm, October 24, 2011.

1. Abstract:
   In this talk, I will present a d-wave superconducting ground state for a doped Mott insulator, which is distinguished from a Gutzwiller-projected BCS superconductor by an explicit separation of Cooper pairing and resonating valence bond (RVB) pairing. Such a state satisfies the precise sign structure of the t-J model, just like that a BCS state satisfies the Fermi-Dirac statistics. I will show that this new class of wavefunctions can be understood by intrinsic electron fractionalization with neutral spinons and backflow spinons forming a two-component RVB structure. While the former spinon is bosonic, originated from the superexchange correlation, the latter spinon is found to be fermionic, accompanying the hopping of bosonic holons. The low-lying emergent gauge fields associated with such a specific fractionalization are of mutual Chern-Simons type. Corresponding to this superconducting ground state, three types of elementary excitations are identified. Among them a Bogoliubov nodal quasiparticle is conventional, while the other two are neutral excitations of non-BCS type that play crucial roles in higher energy/temperature regimes. Their unique experimental implications for the cuprates will be also discussed.

14. (SEMINAR)"Non-linear transport and dissipation in carbon-based nanodevices", Prof. Dr. Jean-Pierre Leburton (University of Illinois at Urbana-Champaign, USA) at 4:00 pm, October 27, 2011.

1. Abstract:
   Carbon-based nanostructures are promising candidates for nano-electronics due to their natural small scales as well as their mechanical, thermal, and electrical properties. In this talk, I will discuss some issues related to dissipation in carbon nanotubes for use as interconnects under electric stress, as well as non-linear transport and modeling in graphene field-effect transistors. Our approach is based on self-consistent solution of the Boltzmann transport equation in the high field regime within the device geometries, and boundary conditions. We obtain the output characteristics of graphene field effect transistors by using the charge-control model for the current. Closed expressions for the conductance, transconductance and saturation voltage are derived in good agreement with existing experimental data.
   
   

15. (SEMINAR)"Spin-Orbit Coupled Quantum Gases", Researcher Dr. Hui Zhai (Tsinghua University) at 4:00 pm, November 10, 2011.

1. Abstract:
   In this talk I will introduce recent progresses in studying physics of spin-orbit coupling in degenerate atomic gases. Realization of spin-orbit coupled quantum gases opens a new avenue in cold atom physics, and also brings out a lot of new physical problems. In particular, the interplay between spin-orbit coupling and inter-atomic interaction leads to many intriguing phenomena. For instance, in a bosonic system, it gives rise to interesting stripe superfluid phase, and in a fermionic system with attractive interactions, it significantly enhances BCS transition temperature. From the discussion of these examples, the key message is that spin-orbit coupling can help enhance the interaction effects, and make the interaction effects much more dramatic even in the regime where the interaction strength itself is small.
   

16. (SEMINAR)"Transport phenomena in nanostructured graphene", Prof. Dr. Antti-Pekka Jauho (Technical University of Denmark, DENMARK) at 4:00 pm, December 5, 2011.

1. Abstract:
   In order to realize the full potential of graphene - an atomic layer of carbon atoms arranged in a hexagonal lattice - the pristine graphene sheet must be modfied so as to display a band gap, at least in certain regions in space. This can be achieved, e.g., by a selective adsorption of adatoms, by cutting the graphene sheet into a nanoribbon, or by applying a transverse electric field to a bilayer graphene. Here we describe yet another route: a regular perforation of the graphene sheet, dubbed as graphene antidot lattice (GAL). Our group developed this idea in 2008, and since then many groups have followed route, either independently, or by using the concepts we introduced. We describe the basic theoretical ideas, review the experimental situation, and report on our recent simulations of charge and thermal transport in finite GALs. We also address how very large systems of nanostructured graphne may be amenable to simulations using first-principles input.

17. (SEMINAR)"Spin dynamics in one dimension: any surprises?", Prof. Dr. E. Ya. Sherman (The University of the Basque Country, SPAIN) at 4:00 pm, December 7, 2011.    

1. Abstract:
   We consider optically injected charge and spin currents in semiconductor quantum wells. The injection can be done by quantum optics techniques with the interference of the one- and two-photon transition processes, by intersubband light absorption, and by a stimulated Raman process. The final state with the injected currents strongly depends on the injection process, producing the system very strongly out of the equilibrium for the interband transitions and relatively weakly out of the equilibrium for the Raman process. In addition to the directly injected, the resulting spin currents can arise due to the spin-dependent scattering of electrons by excited holes. The resulting patterns of charge and spin densities will be analyzed. We show that the charge current evolution can have a universal character, independent on the system details.

18. (SEMINAR)"Coexistence of Cooper Pairs and Topological Order in Bi2Se3 Thin Films", Prof. Dr. Dong Qian (Shanghai Jiaotong University) at 4:00 pm, December 20, 2011.

1. Abstract:
   Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states in which electrons have their spin locked at a right angle to their momentum under the protection of time reversal symmetry. Topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking such as superconductivity can lead to new quantum phenomena and devices. However, the existence of the superconducting states (Cooper pairs) in TI's surface has not been obtained to date experimentally. Here, we report for the first time the realization of a superconducting TI/Superconductor (SC) heterostructure characterized with Cooper pairs tunneling into TI through superconducting proximity effect, by successful growing Bi2Se3 thin films on superconductor NbSe2 substrate with atomically sharp and electronic transparent interface. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we unambiguously observed the Cooper pairs at Bi2Se3 surface (Bi2Se3/NbSe2 interface) where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana Fermions in condensed matter physics.

19. (SEMINAR)"Electronic States in Disordered Graphene", Asso. Researcher Dr. Yan-Yang Zhang (Institute of Semiconductors) at 4:00 pm, December 22, 2011.

1. Abstract:
   This talk will be divided into two parts. In the first part, I will show the effect of topological defects on the transport properties of a narrow graphene ribbon. The results show that the conductance vanishes at several discrete Fermi energies where the system develops loop orbital electric currents with certain chirality. The chirality depends on the direction of the applied bias voltage and the sign of the local curvature created by the topological defects. In the second part, electronic localization properties in disordered graphene with strong long-range impurities will be demonstrated. We find that states near the Dirac points are localized for sufficiently strong disorder (therefore inevitable intervalley scattering) and the transition between the localized and delocalized states is of Kosterlitz-Thouless type. Our results show that the transition originates from bounding and unbounding of local current vortices.