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COLLOQUIA & SEMINARS 2014:
 

1. (SEMINAR)"Recent progress on spin-orbit coupled quantum gases", Dr. Ren-Yuan Liao (Fujian Normal University), at 4:00 pm, March 20, 2014.

1. Abstract:
   There have been surging developments in the area of spin-orbit coupled quantum gases. In this talk, I will review the recent experimental and theoretical advances in understanding these fascinating many-body systems. Then I will turn to my current theoretical work on spin-oribt coupled quantum gases. Finally I will conclude with some interesting directions and some challenging problems.

2. (SEMINAR)"Spin photocurrents in semiconductors", Researcher Dr. Yong-Hai Chen (Institute of Semiconductors, CAS), at 4:00 pm, March 28, 2014.

1. Abstract:
   Spin photocurrents induced by optical excitation with circularly polarized radiation in semiconductors are reviewed. The absorption of circularly polarized light results in optical spin orientation due to the transfer of the angular momentum of photons to electrons, which causes a directed motion of electrons in unbiased semiconductors. A characteristic feature of this spin photocurrent is that it reverses its direction upon changing the radiation helicity. Three kinds of spin photocurrents, i.e., circular photogalvanic effect, anomalous circular photogalvanic effect and light-induced anomalous Hall effect, and their microscopic mechanisms are discussed. The important role of spin-orbit coupling is revealed.

3. (HUMANITIES FORUM)"Einstein's Thought on Human Rights with Its Influence on Chinese Intellectuals", Mr. Guo-Yong Fu (Historian, Freelance Writer), at 7:00 pm, April 15, 2014.
   

4. (SEMINAR)"Itinerancy enhanced quantum fluctuation of magnetic moments and bimagnon excitations in iron-based superconductors", Prof. Dr. Dao-Xin Yao (Sun Yat-Sen University), at 4:00 pm, April 18, 2014.

1. Abstract:
   First, I will introduce the itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors. We demonstrate the active role of electron itinerancy in modulating strong anisotropic quantum spatial fluctuation and tuning the ordered anti-ferromagnetic moments. This is performed by first integrating out the itinerant degree of freedom of a degenerate spin-fermion model with Hund's coupling, followed by estimation of quantum fluctuation via spin-wave theory. Our results complement current emphasis on the temporal fluctuation in the literature, and highlight the essential interplay between itinerant and local degree of freedoms. Second, I will talk about the bimagnon excitations in the spatially frustrated Jx?Jy?J2 Heisenberg model on a square lattice in both the antiferromagnetic and the collinear antiferromagnetic phase. Based on an interacting spin wave theory study, we find the appearance of a robust two-peak structure over a wide range of the transferred momenta in both magnetically ordered phases. The unfrustrated model has a single-peak structure with a two-peak splitting originating due to spatial anisotropy and frustrated interactions. Our predicted two-peak structure can be realized in iron pnictides.
   
   

5. (SEMINAR)"Ultrafast Nonlinear Optical Properties of Two-Dimensional Transition Metal Dichalcogenides", Researcher Dr. Jun Wang (Shanghai Institute of Optics and Fine Mechanics, CAS), at 4:00 pm, April 22, 2014.

1. Abstract:
   Owing to the specific two-dimensional (2D) confinement of electron motion and the absence of interlayer perturbation, 2D semiconductors possess unique optoelectronic properties and has become a research hot-spot in recent years. Whereas the electronic and luminescent properties of 2D transition metal dichalcogenide (TMDC) nanosheets, say, MoS2, MoSe2, WS2, etc., have been generating much research interest, the ultrafast nonlinear optical (NLO) properties remain largely unexplored. Realized that the sizable and thickness-dependent bandgap offers TMDCs a huge potential in the development of photonic devices with high performance and unique functions, we studied extensively the ultrafast NLO property of a range of TMDC nanosheets. 2D TMDC nanosheets with high-quality layered nanosheets were prepared using liquid-phase exfoliation and CVD techniques. Ultrafast saturable absorption, two-photon absorption, ultrafast nonlinear photoluminescence were observed from the 2D nanostructures. The exciting results open up the door to 2D photonic nano-devices, such as optical switches, pulse shaping devices, mode-lockers, optical limiters, etc., capable of ultrafast response and broadband tunability.

6. (SEMINAR)"Polarized Valley Exciton Dynamics in MoS2 and WSe2 monolayers", Prof. Dr. Xavier Marie (Universite de Toulouse, FRANCE), at 4:00 pm, May 19, 2014.

1. Abstract:
   The spectacular progress in controlling the electronic properties of graphene has triggered research in alternative atomically thin two-dimensional crystals. Monolayers (ML) of transition-metal dichalcogenides such as MoS2 have emerged as very promising nanostructures for optical and electronic applications for mainly two reasons. First, the indirect bulk semiconductor MoS2 becomes direct when thinned to 1ML, resulting in efficient optical absorption and emission. Second, inversion symmetry breaking (usually absent in graphene) together with the large spin-orbit interaction leads to a coupling of carrier spin and k-space valley physics, i.e., the circular polarization (¦Ò+ or ¦Ò?) of the absorbed or emitted photon can be directly associated with selective carrier excitation in one of the two nonequivalent k valleys (K+ or K?, respectively). In this talk we will review our recent results based on cw and time-resolved photoluminescence experiments performed on MoS2 and WSe2 MLs. The inherent chiral optical selectivity allows exciting one of these valleys, and close to 90% polarized emission at 4 K is observed with ~40% polarization remaining at 300 K. The high polarization degree of the emission remains unchanged in transverse magnetic fields up to 9 T [1,2]. We have performed the first time resolved photoluminescence polarization measurements in MoS2 [2] and WSe2 [3] MLs providing vital information on the electron and exciton valley dynamics [4]. Moreover the combination of non-linear and linear optical spectroscopy techniques allows us to uncover the excited exciton spectrum which does not follow the usual Rydberg series. An exciton binding energy as large as 600 meV has been measured for WSe2 ML [5]. We will also present micro-Raman and photoluminescence results measured at 300 K to investigate the influence of uniaxial tensile strain on the vibrational and optoelectronic properties of monolayer and bilayer MoS2 on a flexible substrate [6].
   [1] Sallen et al, Phys. Rev. B 86, 081301(R) (2012)
   [2] Lagarde et al, Phys. Rev. Lett. 112 47401 (2014)
   [3] Wang et al, ArXiv 1402.6009 (2014)
   [4] Glazov et al, ArXiv 1403.0108 (2014)
   [5] Wang et al, ArXiv?1404, 0056 (2014)
   [6] Wang et al, Phys. Rev. B 88, 121301(R) (2013)

7. (SEMINAR)"New Physics in Multi-orbital systems with strong correlation", Researcher Dr. Xi Dai (Institute of Physics, CAS), at 4:00 pm, May 29, 2014.

1. Abstract:
   In this talk, I will first review the progress which has been made in the past two decades for the Mott transition in single-band Hubbard model. Then I will introduce three new phenomenons existing only in multi-orbital systems, namely the orbital selective Mott transition, the Hund's metal phase and the interaction enhanced spin-orbital coupling. Two popular computational tools in this field, the dynamical mean field theory and Gutzwiller variational method will also be introduced briefly.

8. (SEMINAR)"Hidden Sign in Iron-Based Superconductors", Researcher Dr. J. P. Hu (Institute of Physics, CAS), at 4:00 pm, June 9, 2014.

1. Abstract:
   A superconducting state is described by a complex order parameter. In an unconventional superconductor, the order parameter may exhibit a sign change in both momentum and real spaces. Normally, a sign change order parameter, such as the d-wave pairing in cuprates, leads to gapless nodes or nodal lines in the superconducting state which can be detected by many experimental techniques.
   Iron-based superconductors , which was discovered six years ago, exhibit a nodeless full superconducting gap on Fermi surfaces. Nevertheless, there are strong indirect experimental evidence that indicates the existence of sign change on the superconducting order parameters. How to compromise the sign change and full gap becomes an essential question. Determining the sign change pattern and understanding the origin of the sign change are also critical to solve pairing mechanism.
   In this talk, I will review both experimental evidence and predictions or explanations from different theories on pairing symmetries and sign change issues on iron-based superconductors. I will discuss a new solution which can provide an unified understanding for sign changes on all iron-based superconductors.

9. (SEMINAR)"Spin-orbit coupled BEC: spin measurement and interference", Prof. Dr. E. Ya. Sherman (University of Basque Country, SPAIN), at 4:00 pm, June 24, 2014.

1. Abstract:
   Cold atoms recently became the testbed for studies of spin-orbit coupling phenomena which cannot be seen in solids. It is well-known that spin-orbit coupling leads to a spin-coordinate entanglement producing mixed states in reduced spin subspace. We study two consequences of this entanglement for spin dynamics in Bose-Einstein condensates. First, spin-orbit coupling simulates a von Neumann measurement of a spin ? since spatial separation of spin-up and spin-down states decoheres the spin density matrix and allows one reading the spin with the particle coordinate. We consider measurement of spin of a particle in a magnetic field as can be simulated by dynamics of a spin-orbit coupled BEC and study in detail the effects of decoherence which appear in this measurement [1]. Second, we consider interference of two spin-orbit coupled BECs as determined by mutual orientation of their spins. In addition, we study the effect of a quantum backflow in the spin-dependent interference [2]. These two effects: spin measurement and interference, can be useful for quantum computation with the qubits based on the spin-orbit coupled BECs.
   [1] E. Ya. Sherman and D. Sokolovski?, New J. Phys. 16 015013 (2014)
   [2] S. Mardonov, M. Palmero, J.G. Muga, M. Modugno, and E.Ya. Sherman, Europhysics Letters, in print

10. (SEMINAR)"New insights of spin pumping from linear response approach", Prof. Dr. S. F. Zhang (University of Arizona, USA), at 4:00 pm, July 10, 2014.

1. Abstract:
   Spin pumping has become one of the most important phenomena in spintronics. Yet, the theoretical work on the spin pumping remains limited to the original framework of the scattering approach which is more suitable for mesoscopic transport. For a disordered system, a natural approach is to utilize the linear response theory in which we reformulate the spin pump current in terms of spectral density functions. We believe that this work will immediately impact the field of spintronics since many experimental results on spin pumping should be re-evaluated within our new formalism. In addition, our theory predicts a spin pumping current inside the processing magnetic layers in which the scattering theory fails to predict. Finally, we present a coherent explanation to recent experimental data.

11. (SEMINAR)"3D topological states from layer construction", Prof. Dr. Xiao-Liang Qi (Stanford University, USA), at 9:00 am, July 11, 2014.

1. Abstract:
   While the topological order in two dimensions has been studied extensively since the discover of the integer and fractional quantum Hall systems, topological states in 3 spatial dimensions are much less understood. In this work, we propose a general formalism for constructing a large class of three-dimensional topological states by stacking layers of 2D topological states and introducing coupling between them. Using this construction, different types of topological states can be obtained, including those with only surface topological order and no bulk topological quasiparticles, and those with topological order both in the bulk and at the surface. For both classes of states we study its generic properties and present several explicit examples. As an interesting consequence of this construction, we obtain example systems with nontrivial braiding statistics between string excitations. In addition to studying the string-string braiding in the example system, we propose a generic topological field theory description which can capture both string-particle and string-string braiding statistics. Lastly, we provide a proof of a general identity for Abelian string statistics, and discuss an example system with non-Abelian strings.

12. (COLLOQUIUM)"Quantum Engineered Interband Cascade Structures for Optoelectronic Devices", Prof. Dr. Rui Q. Yang (University of Oklahoma, USA), at 4:00 pm, July 15, 2014.

1. Abstract:
   Interband cascade (IC) structures were originally introduced for achieving efficient mid-infrared (IR) lasers [1]. By taking advantage of the broken band-gap alignment in type-II InAs/Ga(In)Sb quantum wells to form interband cascade stages, IC lasers reuse injected electrons for photon generation with high quantum efficiency. Unlike intraband quantum cascade lasers, IC lasers use interband (conduction to valence bands) transitions for photon emission without involving fast phonon scattering, making it possible to significantly lower the threshold current density. Significant progress has been achieved in the development of high-performance IC lasers [2-3]. Some outstanding performance features such as low threshold current densities (e.g. <2 A/cm2 at 80 K, <200 A/cm2 at 300 K) and low power consumption (e.g. <0.1 W in cw operation at room temperature) partially verified the advantages of IC lasers. Also, efficient IC lasers have been demonstrated in the long wavelength IR region (>10 ?m) based on plasmon waveguides. Recently, quantum engineered IC structures have been explored for other optoelectronic devices such as infrared photodetectors and photovoltaic cells with certain advantages [4-6]. The unique features and status of these IC structures and relevant optoelectronic devices will be reviewed and discussed with recent experimental results.
   1. R. Q. Yang, at 7th Inter. Conf. on Superlattices, Microstructures and Microdevices, Banff, Canada, August, 1994; Superlattices and Microstructures 17, 77 (1995); ¡°Novel concepts and structures for infrared lasers¡±, chapter 2 in Long Wavelength Infrared Emitters Based on Quantum Wells and Superlattices, M. Helm, editor, Gordon and Breach, Singapore, 2000.
   2. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, ¡°Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption¡±, Nature Communications, 2, 585 (2011).
   3. R. Q. Yang, et al. ¡°Recent progress in development of InAs-based Interband Cascade Lasers¡± Proc. SPIE, 8640, 86400Q (2013); and references therein.
   4. R. Q. Yang, Z. Tian, Z. Cai, J. F. Klem, M. B. Johnson, and H. C. Liu, ¡°Interband cascade infrared photodetectors with superlattice absorbers¡±, J. Appl. Phys. 107, No. 5, 054514 (2010).
   5. H. Lotfi, R. T. Hinkey, L. Li, R. Q. Yang, J. F. Klem, M. B. Johnson, ¡°Narrow-Bandgap photovoltaic devices operating at room temperature and above with high open-circuit voltage¡±, Appl. Phys. Lett, 102, 211103 (2013).
   6. R. T. Hinkey and R. Q. Yang, ¡°Theory of Multiple-Stage Interband Photovoltaic Devices and Ultimate Performance Limit Comparison of Multiple-Stage and Single-Stage Interband Infrared Detectors¡±, J. Appl. Phys. 114, 104506 (2013).

13. (SEMINAR)"Studying graphene carrier dynamics with obliquely polarized incident beams", Dr. Xiao-Qing Yan (Nankai University), at 4:00 pm, September 12, 2014.

1. Abstract:
   Graphene carrier dynamics and optical absorption property have attracted lots of interest due to its potential application in photonics and optoelectronic devices. Before, normally incident beam was wieldy used to study the carrier relaxation process and optical absorption of graphene. In this talk, I will present our pump-probe experimental work on graphene carrier dynamics process with obliquely polarized beams. Pump polarization dependence of differential reflectivity signal from degenerate and nondegenerate measurements reveals the evolution of photoexcited carriers from anisotropic at excitation state to fully isotropic in the energy band. The experiment also suggests graphene optical absorbance for in-plane and out-of-plane optical fields is identical. Then, I will present the experimental results on sign reversing of differential signal and give some discussion. The exact nature of such a sign reversing is unclear at present. The observed storng dependence of the sign-reversal on the probe-light polarization may provide the pathway to find the origin of sign reversing.

14. (SEMINAR)"Transport properties of magnetically doped topological insulator thin films", Prof. Dr. Ya-Yu Wang (Tsinghua University), at 4:00 pm, October 10, 2014.

1. Abstract:
   Topological insulators (TIs) are novel quantum materials with topologically nontrivial band structure induced by strong spin-orbit coupling. Breaking the time reversal symmetry (TRS) in TIs has been predicted to create a variety of exotic topological magnetoelectric effects such as image magnetic monopole and quantized anomalous Hall effect. In this talk we report transport studies of magnetically doped TI ultrathin films grown by molecular beam epitaxy (MBE), aiming to reveal the unique properties of the topological surface states and realize the quantum anomalous Hall effect.
   In the Bi2Se3 films doped with Cr, we found a systematic crossover from weak antilocalization to weak localization induced by magnetic doping. We show that the evolution of the localization behavior indicates the transformation of the system from a topologically nontrivial TI to a topologically trivial dilute magnetic semiconductor. In a new ternary TI system (Bi1-xSbx)2Te3 with depleted bulk carriers, Cr dopants induce a long-range ferromagnetic ordering. More interestingly, the ferromagnetism exists both in the presence of hole- and electron-type Dirac fermions with widely varied carrier concentrations. The carrier-independent ferromagnetism in TIs is consistent with the Van Vleck mechanism mediated by the band electrons. This picture is further supported by recent observations of a topology-driven magnetic quantum phase transition, in which ferromagnetic ordering is strongly favored by the nontrivial bulk band topology. In the end, we will present experimental observation of the quantum anomalous Hall effect, i.e., the quantum Hall effect without a magnetic field, in magnetic TIs.

15. (SEMINAR)"Quasiparticle collapsing and charge modulation as manifestations of Mott physics", Prof. Dr. Zheng-Yu Weng (Tsinghua University), at 10:00 am, October 20, 2014.

1. Abstract:
   Quasiparticle collapsing is a central issue in the study of strongly correlated electron systems. In the one-dimensional case, the quasiparticle collapsing in a form of spin-charge separation has been well established, but the problem remains elusive in dimensions higher than one. By using density matrix renormalization group (DMRG) algorithm, we show that in an anisotropic two-leg t-J ladder, an injected single hole behaves like a well-defined quasiparticle in the strong rung limit, but undergoes a ``phase transition'' with the effective mass diverging at a critical point towards the isotropic limit. After the transition, the quasiparticle collapses into a composite object of a self-localized charge (holon) and a deconfined spin-1/2 (spinon), accompanied by a substantially enhanced binding energy between two holes. We discuss the underlying novel mechanism, which may be generic for a doped Mott insulator of any dimensions.

16. (HUMANITIES FORUM)"Statistical Characteristics of Scientific and Technological Achievements in Human History", Prof. Dr. Jie-Lin Dong (Soochow University), at 4:00 pm, October 24, 2014.

17. (SEMINAR)"Few-body problems in ultracold Bose-Fermi mixture and dipolar gas", Prof. Dr. Peng Zhang (Renmin University), at 4:00 pm, November 14, 2014.

1. Abstract:
   (I) Calibration of Interaction Energy between Bose and Fermi Superfluids. (arXiv: 1409.0282). In this job we study the interaction energy in a mixture of Bose and Fermi superfluids realized in recent cold atom experiment. On the Bose-Einstein-condensate (BEC) side of a Feshbach resonance between fermionic atoms, this interaction energy can be directly related to the scattering length between a bosonic atom and a dimer composed of fermions. We calculate the atom-dimer scattering length from a three-body analysis with both a zero-range model and a separable model including the van der Waals length scale, and we find significant deviation from the result given by a mean-field approach. We also find that the multiple scattering between atom and dimer can account for such a deviation. Our results provide a calibration to the mean-field interaction energy, which can be verified by measuring the shift of collective oscillation frequency.
   (II) Effect of short-range interaction for collision of ultracold dipoles (arXiv: 1410.0545). In this job we consider the scattering of two ultracold polarized dipoles with both a short-range interaction (SRI) and a weak dipole-dipole interaction (DDI) which is far away from a shape-resonance. In previous works the scattering amplitude is usually calculated via 1st-order Born approximation. Our results show that significant derivation from this approximation can arise in some cases. In these cases the SRI can significantly modify the dipole-dipole scattering amplitude, even if the scattering amplitude for the SRI alone is negligibly smaller than the dipolar length of the DDI. We further obtain approximate analytical expressions for the inter-dipole scattering amplitude.

18. (HUMANITIES FORUM)"China in the Pre-Qin Period versus Ancient Greece", Prof. Shi-Chao Zhao (Shaanxi Normal University), at 4:00 pm, November 20, 2014.

19. (HUMANITIES FORUM)"Correlation between Cancer and the Huai River Water Environment", Prof. Gong-Huan Yang (Peking Union Medical College), at 7:00 pm, November 27, 2014.

20. (SEMINAR)"Cavity optomechanics: Controlling mechanical motion with light", Prof. Dr. Hai-Lin Wang (University of Oregon, USA), at 4:00 pm, December 5, 2014.

1. Abstract:
   It is well known that radiation pressure forces of light can be used for the manipulation of mechanical motion in microscopic systems. Notable examples include laser cooling and laser tweezers. In this talk, I will discuss recent experimental advances on the use of radiation pressure force to control mechanical motion in macroscopic systems, with a focus on potential applications in quantum optics and quantum networks. Experimental studies on storing light as a mechanical excitation and on converting coherent optical fields between vastly different optical wavelengths via optomechanical processes will be highlighted.

21. (HUMANITIES FORUM)"The New Norm and Prospects of China's Economy--The Contribution of Finance", Prof. Dr. Fu-Zhong Wang (Central University of Finance and Economics), at 7:00 pm, December 12, 2014.

22. (COLLOQUIUM)"Case study as a way to understand physics", Prof. Dr. Xiao-Feng Jin (Fudan University), at 4:00 pm, December 19, 2014.

23. (COLLOQUIUM)"50 Years of Magnetic Semiconductors", Researcher Dr. Yong-Qing Li (Institute of Physics, CAS), at 4:00 pm, December 26, 2014.

1. Abstract:
   Tailoring the properties of semiconductor and magnetic structures has been a driving force of information technology. The goal of combining ferromagnetism with semiconductors into a single material has been pursued by physicists and materials scientists for about five decades. A brief review of the history of the magnetic semiconductors will be given before presenting our recent results on HgCr2Se4, a magnetic semiconductor belonging to the spinel group. The revisit to this ¡°old¡± material allowed us to gain new insight into its remarkable electron transport properties such as colossal magnetoresistance. The prospect of a new topological phase emerging from this material will also be discussed.