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COLLOQUIA & SEMINARS 2012:
 

1. (SEMINAR)"Photoinduced currents in graphene and carbon nanotubes", Prof. Dr. E. L. Ivchenko (Ioffe Physico-Technical Institute, RUSSIA), at 4:00 pm, February 15, 2012.

1. Abstract:
   In my talk I overview experimental and theoretical investigations of photogalvanic effects induced in graphene and carbon nanotubes. The following items are to be discussed.
   (i) Photon drag effect induced in single-layer graphene under oblique incidence of the laser light, in the terahertz and infrared frequency regions [1,2]. For the terahertz radiation the photon drag effect can be considered as a dynamic Hall effect. The second-harmonic generation studied in Ref. [3] is complimentary to the drag photocurrent.
   (ii) Terahertz radiation driven chiral edge currents in graphene induced in single layer graphene samples by illumination of the graphene edges with circularly polarized terahertz radiation at normal incidence [4].
   (iii) Pure valley currents induced under the homogeneous normal-incidence photoexcitation of graphene [5]. The intravalley current appears because of the reduced symmetry D_{3h} of the valleys K and K, while the net electric current is forbidden by the overall D_{6h} point-group symmetry of graphene.
   (iv) Photogalvanic properties of graphene superlattices (SLs) formed by periodic strain [6]. Asymmetric graphene SLs act as quantum ratchets and allow helicity-dependent photocurrents under normal incidence.
   (v) Effects inherent for chiral carbon nanotubes: circular photogalvanic effect (PGE), magneto- induced linear PGE, natural circular dichroism, magneto-spatial dispersion of light absorption, magneto-chiral dc electric current quadratic in the bias voltage applied to a chiral nanotube [7].
   
   Literature
   [1] J. Karch, P. Olbrich, M. Schmalzbauer, C. Zoth, C. Brinsteiner, M. Fehrenbacher, U. Wurstbauer, M.M. Glazov, S.A. Tarasenko, E.L. Ivchenko, D. Weiss, J. Eroms, R. Yakimova, S. Lara-Avila, S. Kubatkin, S.D. Ganichev, Phys. Rev. Lett. 105, 227402 (2010).
   [2] Chongyun Jiang, V.A. Shalygin, V.Yu. Panevin, S.N. Danilov, M.M. Glazov, R. Yakimova, S. Lara-Avila, S. Kubatkin, S.D. Ganichev, Phys. Rev. B 84, 125429 (2011).
   [3] M.M. Glazov, JETP Lett. 93, 408 (2011).
   [4] J. Karch, C. Drexler, P. Olbrich, M. Fehrenbacher, M. Hirmer, M. M. Glazov, S. A. Tarasenko, E.L. Ivchenko, B. Birkner, J. Eroms, D. Weiss, R. Yakimova, S. Lara-Avila, S. Kubatkin, M. Ostler, T. Seyller, S.D. Ganichev, Phys. Rev. Lett. 107, 276601 (2011).
   [5] L.E. Golub, S.A. Tarasenko, M.V. Entin, L.I. Magarill, Phys. Rev. B 84, 195408 (2011).
   [6] L.E. Golub, Yu.Yu. Kiselev, Phys. Rev. B 84, 235440 (2011).
   [7] E.L. Ivchenko, B. Spivak, Phys. Rev. B 66, 155404 (2002); Physica E 17, 366 (2003).
   

2. (SEMINAR)"Manipulating and probing Majorana fermions using superconducting circuits", Prof. Dr. Jian-Qiang You (Fudan University) at 4:00 pm, March 16, 2012.

1. Abstract:
   Majorana fermions are long-sought exotic particles that are their own antiparticles. In this talk, we show how to utilize superconducting circuits to construct two superconducting-qubit arrays where Majorana modes can occur. Moreover, we propose to use four superconducting qubits as the smallest system to demonstrate the braiding of Majorana modes and show how the states before and after braiding Majoranas can be discriminated. This provides an experimentally realizable, relatively simple system for manipulating and probing Majorana fermions.

3. (SEMINAR)"Spin damping monopole", Asso. Prof. Dr. G. Tatara (Tokyo Metropolitan University, JAPAN) at 4:00 pm, March 21, 2012.

1. Abstract:
   We present a theoretical evidence that a monopole emerges in dynamic magnetic systems in the presence of the spin-orbit interaction. The monopole field is expressed in terms of the spin damping associated with the magnetization dynamics. We demonstrate that the observation of this spin damping monopole is accomplished electrically by use of the Ampere's law for the monopole current. Our discovery suggests the possibility of the integration of monopoles into the electronics, namely the monopolotronics.

4. (SEMINAR)"Electrical Control of the Spin Coherence in (111) Quantum Wells", Prof. Dr. Xavier Marie (Universite de Toulous, FRANCE) at 4:00 pm, March 23, 2012.

1. Abstract:
   We have measured the electron spin relaxation time in (111)-oriented GaAs quantum wells (QW) by time-resolved photoluminescence spectroscopy. By embedding the QWs in a PIN or NIP structure we demonstrate the tuning of the conduction band spin splitting and hence the spin relaxation time with an applied external electric field applied along the growth z direction.
   The application of an external electric field of 50 kV/cm yields a two-order of magnitude increase of the spin relaxation time which can reach values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one.
   Experiments performed under transverse magnetic fields demonstrate that in addition to the spin lifetime, the spin coherence time can be significantly increased.
   The role of the Dresselhaus cubic terms on both the temperature dependence of the effect and the anisotropy of the spin relaxation will be discussed. The tuning or suppression of the Dyakonov-Perel electron spin relaxation demonstrated here for GaAs/AlGaAs quantum wells is also possible in many other III-V and II-VI zinc-blende nanostructures since the principle relies only on symmetry considerations.

5. (SEMINAR)"Spin measurement and Rabi spin oscillations in driven systems", Prof. Dr. E. Ya. Sherman (University of Basque Country, SPAIN) at 4:00 pm, May 7, 2012.

1. Abstract:
   Here we build the bridge between spin measurements and spin dynamics in driven systems. First, we analyze the physical meaning of a possible experiment aimed at the simultaneous measurement of two non-commuting spin components. We demonstrate that switching of a strong spin-orbit interaction in a solid or in a cold-atom system for a short time simulates a simultaneous von Neumann measurement of non-commuting spin components. By mapping spin dynamics onto coordinate-space motion such a measurement determines time averages of spin operators which, unlike their instantaneous values, may be evaluated simultaneously to an arbitrary accuracy. As a result, some spin processes in the presence of spin-orbit coupling should demonstrate the Zeno effect: a frequent observation of the system prohibits the evolution.
   Next, we consider joint effects of tunneling and spin-orbit coupling on driven spin dynamics in a multi-level system. Tunneling plays the crucial role in the formation of the Rabi-like spin-flip transitions. In contrast to the weak electric field behavior, the spin flip rate becomes much smaller than expected for the two-level system and shows oscillating dependence on the driving field amplitude. In addition, the full spin flip becomes very difficult to achieve. These two effects can be seen as the Zeno effect of slowing down the dynamics of an observable by its measurement.

6. (SEMINAR)"Some recent progress on Geometrically frustrated spin systems and quantum spin systems", Prof. Dr. Dao-Xin Yao (Sun Yat-sen University) at 4:00 pm, May 11, 2012.

1. Abstract:
   There are many degenerate ground states in the geometrically frustrated spin systems, which can produce novel states. I will introduce an extremely frustrated lattice-Triangular Kagome Lattice (TKL). We obtain the exact solutions for the Ising model, Quantum+Ising model, and Dimer model on the TKL. The rich phase diagrams are found. Especially a topological phase - Kagome loop gas is found in the Quantum+Ising model on the TKL. The residual zero-temperature entropies for the most frustrated cases on the TKL are found to be ln72=4.2767 for the Ising model and 2.5258 for the Quantum+Ising model respectively. By using the cellular dynamical mean-field theory, we obtain the rich phase diagrams in the Hubbard model on the TKL. A short rang resonating valence-bond spin liquid phase is claimed. In the second part, I will talk about the dynamics of two initially correlated qubits coupled to their own separate spin baths modeled by a XY spin chain. We find the analytical expression of the quantum discord for the system. A sudden transition is found to exist between classical and quantum decoherence by choosing certain initial states. We find that the sudden transition happens near the critical point, which provides a new way to characterize the quantum phase transition. Furthermore, we propose a scheme to prolong the transition time of the quantum discord by applying the bang-bang pulses.

7. (SEMINAR)"Probing electronic excitation in single- and multi-layer graphenes by Raman spectroscopy", Researcher Dr. Ping-Heng Tan (Institute of Semiconductors, CAS) at 4:00 pm, May 21, 2012.

1. Abstract:
   The quest for materials capable of realizing the next generation of electronic and photonic devices continues to fuel research on the electronic, optical and vibrational properties of graphene. Single-layer graphene (SLG) and multi-layer graphene (MLG) flakes with less than ten layers each show a distinctive band structure. How to understand the electronic excitation of SLG and FLG is a crucial issue in the fundamental physics and device application. Raman spectroscopy is one of the most useful and versatile tools to access phonons in graphene samples. The interaction between phonon and electronic excitation is a direct way to probe the electronic properties of SLG and MLG.
   We show that by performing Raman measurements at different excitation energies, the variation of the 2D intensity relative to the G peak of heavily-doped SLG with excitation energy allows one to assess its Fermi energy for a given doping level, and a Fermi level shift of up to ~0.85 eV can be measured for Stage-1 intercalation graphite compounds by FeCl3, where each graphene layer in compounds behaves as a decoupled heavily doped SLG. We also show that detection of Raman modes down to ~10 cm-1 is possible using three BragGrate notch filters in combination with a single monochromator.
   We focus on the low-energy shear mode in graphene layers, whose frequency corresponds to ~42 cm-1 of the E2g mode in bulk graphite. We uncover the equivalent mode for MLGs and show that it provides a direct measurement of the interlayer coupling. The corresponding shear modes can be well-fitted with a Breit-Wagner-Fano lineshape, which arises as quantum interference between the shear mode and a continuum of Raman-active electronic transitions. This makes it a probe for the quasiparticles near the Dirac point by quantum interference.
   Graphite is not the only layered material. MoS2 is another typical layered material. The unique valley-selective circular dichroism in monolayer MoS2 by circularly polarized photoluminescence will be briefly reported.

8. (SEMINAR)"Spin coherence and transport in the persistent photoconductor AlGaAs", Prof. Dr. Peng Xiong (Florida State University, USA) at 4:00 pm, July 2, 2012.

1. Abstract:
   Spin coherence and coherent spin transport are among the central issues of semiconductor spintronics. A host of optical studies have revealed a pronounced, nonmonotonic variation of the carrier spin lifetime with the doping level in GaAs, with a peak near the insulator-metal transition (IMT). While some aspects of these intriguing observations can be understood qualitatively based on the competition between the main spin relaxation mechanisms, a full account of the nontrivial dependence of the spin lifetime with carrier density in semiconductors remains elusive. Using the persistent photoconductor Al0.3Ga0.7As as the medium, Fe/GaAs graded Schottky junctions as the spin injector, and all-electric nonlocal detection of spin accumulation/transport, we carry out a detailed examination of the carrier spin transport/relaxation as a function of its carrier concentration, especially near the IMT. Persistent photoconductivity permits in situ photodoping of the AlGaAs channel and detailed determination of the spin lifetimes in the neighborhood of the IMT, all in one and the same sample. Characterization of the photo-induced IMT in the AlGaAs epilayer and electronic properties of the Fe/AlGaAs heterostructures, as well as initial measurements of the spin accumulation/transport with varying carrier density, will be presented.

9. (SEMINAR)"Magnon mediated electric current drag across a ferromagnetic insulator layer", Prof. Dr. Shu-Feng Zhang (University of Arizona, USA) at 4:00 pm, July 5, 2012.

1. Abstract:
   In semiconductor heterostructure, the Coulomb interaction is responsible for the electric current drag between two 2-d electron gases across an electron impenetrable insulator. For two metallic layers separated by a ferromagnetic insulator (FI) layer, the electric current drag can be mediated by a nonequilibrium magnon current of the FI. In this talk, I describe how an electron spin current in metals converts into magnon currents in a FI layer. By Using the semiclassical Boltzmann approach with proper boundary conditions, we predict the electric drag current.

10. (SEMINAR)"Ultrafast dynamics of photoexcited carriers in graphene", Prof. Dr. Tian-Shu Lai (Sun Yat-sen University), at 9:00 am, August 9, 2012.

1. Abstract:
   Ultrafast dynamics of photoinjected carriers is studied in graphene grown by CVD. A optical conductivity model including intra- and inter-band transitions is used to fit the untrafast dynamics. Multiple physical parameters are extracted. It is surprised to find that the scattering time of carriers is in sub-femtosecond time scale which is much less than currently adopted value between several and a few hundred of femtoseconds. Carrier density and energy dependence of the ultrafast dynamics is also studied. Results are briefly discussed.

11. (SEMINAR)"Active electromagnetic metamaterial based on spin torque oscillators", Asso. Prof. Dr. Gen Tatara (Tokyo Metropolitan University, JAPAN) at 4:00 pm, August 18, 2012.

1. Abstract:
   We propose an active material for electromagnetic radiation with frequency of GHz-THz by use of spin-torque oscillators. The origin of the amplication is the energy supplied to the magnetization by the injected current. We show theoretically that close to a resonance with current-driven magnetization , the imaginary part of magnetic permeability becomes indeed negative for either of the two circular polarizations, resulting in negative imaginary part of refractive index. Besides, the real part of the refractive index is also manipulated by the current. Our system thus realizes an active filter to obtain circular polarized radiation and/or an electromagnetic metamaterial having negative refractive index, both controlled electrically.

12. (COLLOQUIUM)"BEC transition of paraexcitons in a Cu2O crystal", Prof. Dr. Makoto Kuwata-Gonokami (Tokyo University, JAPAN) at 9:00 am, August 20, 2012.

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13. (COLLOQUIUM)"Towards first-principles method for open systems: from holographic electron density theorem to quantum mechanical simulation of transient current through a metallic wire", Prof. Dr. Guan-Hua Chen (Hong Kong University) at 3:00 pm, August 20, 2012.

1. Abstract:
   Conventional quantum dissipation theory is often limited to model systems which contain a few levels, while quantum chemistry methods are routinely applied to real complex systems containing hundreds or thousands of orbitals. Basing on the holographic electron density theorem, an exact first-principles framework is proposed for open systems, in particular, a time-dependent density-fucntional theory is developed for quantum transport, and has been applied to simulate the transient currents through various nanoscopic electronic devices. In particular, transient currents through a set of metallic wires of different sizes are calculated, and a surprising linear relationship between the current and time is observed.

14. (Ph. D. Dissertation Defense)"Hole Spin Relaxation in Semiconductors and Landau-Lifshitz-Gilbert Equation in Ferromagnetic Semiconductors", Mr. Ka Shen, [Committee Members: Prof. Bang-Fen Zhu (Chair, Tsinghua Univ.); Prof. Dr. Guan-Hua Chen (Hongkong University); Prof. Dr. Makoto Kuwata-Gonokami (Tokyo University); Asso. Prof. Dr. Gen Tatara (Tokyo Metropolitan University); Prof. Dr. Zheng-Yu Weng (Tsinghua University); Prof. Dr. Xin-Cheng Xie (Peking University); Prof. Dr. Qi-Kun Xue (Tsinghua University); Prof. Dr. Hang Zheng (Shanghai Jiaotong University)], at 6:30 pm, August 20, 2012.

1. Please browse  SUPERVISED PH. D DISSERTATIONS.
   

15. (SEMINAR)"Kondo Effect - a well-known but still under active study strongly correlated phenomenon", Prof. Dr. Hong-Gang Luo (Lanzhou University) at 4:00 pm, October 17, 2012.

1. Abstract:
   The Kondo effect, a phenomenon found in 1930's and interpreted by Kondo in 1964, still attracts much attention in recent years due to the great advance of the nanotechnology. In this talk, we firstly give a brief review on the Kondo physics and its experimental studies. Then, we focus on several explicit systems to show the nature of the Kondo effect like the single magnetic atom on the surface of the normal metal or graphene, quantum dot systems, and so on. Finally, a brief summary and outlook are given.

16. (COLLOQUIUM)"THz interaction with Bio-Molecules", Prof. Dr. H. L. Cui (New York University Polytechnic Institute, USA) at 4:00 pm, October 26, 2012.

1. Abstract:
   This talk serves as an introduction of current status of THz science and technology, especially THz interactions with bio-molecules, and their applications in material science, biology, and optoelectronics. The talk will start with a brief introduction of the fundamentals and will use examples to highlight current research, successful applications, and potential developments.

17. (SEMINAR)"Wonderful combination of magnetic perpendicular magnet MnxGa films with semiconductor---displaying fascinating room-temperature ferromagnetism", Researcher Dr. Jian-Hua Zhao (Institute of Semiconductors, CAS) at 10:00 am, November 9, 2012.

1. Abstract:
   It is well known that magnetic materials with high coercivity, perpendicular magnetic anisotropy and magnetic energy product have great application potential in ultrahigh-density perpendicular magnetic recording, permanent magnets and spintronics. On the other hand, magnetic materials epitaxied on semiconductors allow for a direct integration of magnetic, optical and electronic devices with high performances. For spin-dependent surface-light-emitting devices, their optical recombination is sensitive to the out-of-plane spin polarization, the heterostructure consisting of magnetic perpendicular magnet and semiconductor may exclude a burden to apply a large magnetic field of the order of 20 kOe to rotate the magnetization out of plane. Noble-metal-free perpendicular magnetic L10-MnGa homogeneous film was theoretically predicted to have perpendicular magnetic anisotropy of 26 Merg/cc, magnetization of 845 emu/cc, magnetic energy product of 28 MGOe, Gilbert damping constant of 0.0003 and spin polarization up to 71% at Fermi level. So far, however, only a few L10-MnGa films on semiconductors GaN and GaAs demonstrated perpendicular easy axis with small magnetization below 230 emu/cc and coercivity below 5 kOe. Also, no results about the detailed perpendicular anisotropy of these L10-MnGa films were presented. The quest for semiconductor-compatible L10-MnxGa films with the fascinating theory-predicted properties remains a major challenge.
   
   In my talk, I will present our recent result on L10-MnGa single crystalline films grown on GaAs by molecular-beam epitaxy with pronounced magnetic properties at room temperature, including ultrahigh coercivity up to 42.8 kOe£¬giant perpendicular anisotropy of 21.7 Merg/cc and large energy products of 2.6 MGOe, respectively. Detailed studies will be also given on the tailoring of magnetism for different functional applications by controlling composition and post-growth annealing.

18. (SEMINAR)"Micro-Grid Based on Integrated Renewable Energy", Researcher Dr. Ning Dai (Shanghai Institute of Technical Physics) at 2:30 pm, November 9, 2012.

1. Abstract:
   Current research interests for renewable energy focuses on solar cells, wind power, hydrogen, as well as bio mass. None of them are, however, technologically mature or rich enough to replace the traditional energy. Solar cell and wind power are, for instance, extremey unstable while hydrogen, as a very clean and abundant energy source, still suffers from the technology barricade associated with cost and safety. It is highy expected that a micro grid that connects those renewablle energy resources are likely to overcome the existing problems and gradually take a position in application. The fundermental properties of the renewable energies and general feature of the micro grid are addressed.

19. (SEMINAR)"Optical Mass Spectrometry Based on Hybrid Nanomechanical Resonator Systems", Prof. Dr. Ka-Di Zhu (Shanghai JiaoTong University) at 4:00 pm, November 9, 2012.

1. Abstract:
   With the small mass, large quality-factor, high frequency, nanomechanical resonators (NRs) will ultimately find usage in a broad range of applications, such as electrometry, optomechanical/ electromechanical signal processing, and mass detection. In this talk, we shall focus on a particular NR application: ultrasensitive mass spectrometry in all-optical domain. The convectional electrical mass detection is usually based on the electric environment, where nanomechanical resonator should be suspended between two electrodes above a conducting plate, while a voltage applies on them. However, the heating effect and the energy loss induced by the current through it during the measurement will lead to the imprecision of mass detection. Addressed to this problem, in this talk, we propose a more sensitive optical protocol to weigh the external particles deposited onto the surface of mechanical resonator, which is so far the first method to deal with the NR based mass spectrometry in all-optical domain. This optical mass spectrometry has the potential to break through the limitation of frequency restriction and to enhance the sensitivity of mass spectrometry.

20. (HUMANITIES FORUM)"Havel: Living in Truth", Asso. Prof. Dr. Yong Hu (Peking University) at 7:00 pm, November 9, 2012.

    
    

21. (SEMINAR)"Experimental study of electron spin dynamics in (Ga,Mn)As across the insulator-to-metal transition", Researcher Dr. Xin-Hui Zhang (Institute of Semiconductors, CAS) at 10:00 am, November 26, 2012.

1. Abstract:
   Electron spin dephasing dynamics in ferromagnetic (Ga,Mn)As with Mn concentration x spanning the wide range from 0.5% to 15% across the insulator-to-metal transition, is systematically studied by varying temperature and optical pumping density with time-resolved magneto-optical Kerr effect measurements (TR-MOKE). For (Ga,Mn)As with different Mn doping concentration, the electron spin dephasing time shows quite different temperature and photo-excitation density dependent behavior, implying different spin relaxation mechanisms are involved. In the metallic and near insulating (Ga,Mn)As, the electron spin relaxation is suggested to be dominated by Dyakonov-Perel (DP) mechanism considering screening effect together with p-d exchange coupling. In the insulating (Ga,Mn)As, BAP mechanism is suggested to be important. However, deeply into the insulating and heavily doped regimes, where abundant Mn ions don¡¯t participate in the long-ranged magnetic order, the s-d exchange scattering is discussed to play an important role for electron spin relaxation process.
   I will also present our experimental study of the nonlinear second harmonic spin wave excitation in the 10-nm epitaxially grown Fe thin film on GaAs by double femtosecond laser pulses pumping with TR-MOKE. Under single beam pumping, we observed the uniform spin-wave oscillation, however, when we applied the double laser beams with their polarization axial symmetrical to the [100] axis, a clear secondary oscillation with double frequency of the uniform mode was observed. The origin of this second harmonic spin wave mode is unknown and needs theoretical collaboration.

22. (SEMINAR)"Magnetic and Pairing Instability in Organic Superconductors", Researcher Dr. Hai-Qing Lin (Beijing Computational Science Research Center) at 4:00 pm, December 7, 2012.

1. Abstract:
   In this talk, I report our recent study on the magnetic and pair binding properties in newly discovered polycyclic aromatic hydrocarbon (PAH) superconductors including alkali-metal-doped picene, coronene, phenanthrene, and dibenzopentacene. We have performed a systematic numerical investigation on the possible structure of PAH as functions of pressure and doing. Correlation effects were studied in details. To gain a better understanding on magnetism and electron correlation in PAH, we model the ¦Ð-electrons on the carbon atoms of a single molecule by a one-orbital Hubbard model, in which the energy difference ? between carbon atoms with and without hydrogen bonds is taking into account. We demonstrate that the spin polarized ground state is realized for charged molecules in the physical parameter regions, which provides a reasonable explanation of local spins observed in PAHs. In alkali-metal-doped dibenzopentacene, our results show that electron correlation may produce an effective attraction between electrons for the charged molecule with one or three added electrons. We also propose a different doping pattern which may lead to higher transition temperature.
   
   

23. (SEMINAR)"Magnetization and polarization dynamics in a Composite Multiferroic Structure", Prof. Dr. Cheng-Long Jia (Lanzhou University) at 4:00 pm, December 21, 2012.

1. Abstract:
   Simultaneously controlling magnetism and ferroelectricity is fundamentally important and bears the potential for a wide range of applications as for instance in sensorics and multiferroic memory devices with ultra low heat dissipation. A promising route to circumvent this problem is the appropriately synthesized composite ferroelectric(FE)/ferromagnetic(FM) nano and multilayer structures that may serve as elements in quantitatively new multiferroic devices at room temperature. Several mechanisms may underlay the magnetoelectric effect in composite multiferroic junctions, for example interfacial charge rearrangements, strain effects and exchange-bias are studied as the key ingredients for the magnetoelectric coupling [2]. We further demonstrate a mutual multiferroic response and a picosecond demagnetization/depolarization of a thin FE/FM chain in the presence of a fast oscillating electric/magnetic field or an ultrafast X-ray pulse.
   

24. (HUMANITIES FORUM)"Eastern European Critical literature in the 20th century", Prof. Dr. Kai-Xuan Jing, at 7:00 pm, December 27, 2012.