Day 2 :
University of Tennessee Oak Ridge National Laboratory, USA
Time : 09:00-09:30
Takeshi Egami has completed his PhD from University of Pennsylvania, and Postdoctoral studies from University of Sussex and Max-Planck Institute in Sttutgart. After teaching at Penn for 30 years he moved to the University of Tennessee/Oak Ridge National Laboratory as Distinguished Scientist. He was the Director of Joint Institute for Neutron Sciences. He has published more than 500 papers and one book, gave more than 300 invited presentations at conferences, and was cited more than 18000 times. He has been the Editor of Advances in Physics and Division Associate Editor of Physical Review Letters.
The science of liquid and glass is seriously under-developed compared to that of crystalline solids, because most of the theoretical tools of condensed-matter-physics assume lattice periodicity, and thus are powerless for liquid and glass in which the structure has no periodicity and many-body correlations dominate. However, owing to recent advances in computational power and experimental tools we are making significant progress. We found that the origin of viscosity in high-temperature liquid is the local topological excitation, the elementary excitation to change the local topology of atomic connectivity which we named anankeon, and gave the direct experimental proof of this mechanism for water through the inelastic x-ray scattering (IXS). The results of the IXS were double-Fourier transformed into the van Hove function, g(r, t), which describes the two-body atomic correlation in real space and time (Fig. 1). We have also determined the van Hove function by inelastic neutron scattering (INS) for liquid metals and superfluid helium. Whereas Landau explained superfluidity from the energy dissipation side, the real space mechanism has not been known. Using INS we have determined a new real space mechanism of superfluidity in terms of coherent atomic tunneling. On the theoretical side we are developing a new statistical mechanics of liquid in terms of topological identification of the state using the graph theory, which greatly simplifies the statistics and make it possible to evaluate the configurationally entropy. These advances are making the creation of the field of liquid-state-physics a real possibility.
Advanced Studies in Physics Centre of the Romanian Academy, Romania
Keynote: Unitary relativistic quantum theory
Time : 09:30-10:00
Eliade Stefanescu graduated the Faculty of Electronics, Section of Physicist Engineers, in 1970, and obtained a PhD in Theoretical Physics in 1990. As a Scientist from 1976, a Senior Scientist III from 1978, he worked in physics and technology of semiconductor devices, and from 1978, he worked in physics of optoelectronic devices. From 1987, and from 1990 as a Senior Scientist II, he worked in the field of open quantum physics. In the years 1995-2000, he held a course called Dissipative Systems for the master degree. In 1991 he discovered that the penetrability of a potential barrier can be increased by coupling to a dissipative system, and described the decay spectrum of some cold fission modes. As a Senior Scientist I, from 1997 he developed a microscopic theory of open quantum systems, and discovered a physical principle for the heat conversion into usable energy. In 2014, he produced a unitary relativistic quantum theory. He received the Prize of the Romanian Academy for Physics in 1983, Diploma as Ordinary Member of the Academy of Romanian Scientists, Diploma and Golden Plate as Founder of the Academy of Romanian Scientists, and the Prize “Serban Titeica” (2014) for the book “Open Quantum Physics”.
Generally, the theory of relativity and quantum mechanics are conceived as two different theories, the first for the classical particles, and the second referring to the smallest conceivable particles, called quantum particles, which satisfy the wave equation of Schrödinger. However, when we tried to understand a quantum particle as a packet of waves as solutions of a Schrödinger equation, we came to a serious inconsistency: the group velocities of the wave packets in the two conjugated spaces, of the coordinates and of the momenta, were not in agreement with the Hamilton equations. For this agreement, instead of the Hamiltonian in the time-dependent phase of a particle wave function, one has to consider the Lagrangian. More than that, we find that, with the relativistic Lagrangian, a particle spectrum takes the physical form of a bound one, for spectrum cut-off velocity c (Figure 1). In this framework, we defined a relativistic quantum principle, asserting that any quantum particle is described by waves with scalar time dependent phases, invariant to any change of coordinates. Based on this principle, we obtain the relativistic kinematics and dynamics of a quantum particle. Describing the particle dynamics in an electromagnetic field by terms of the time dependent phase, with a vector potential conjugated to the coordinate variations, and a scalar potential conjugated to the time variation, we reobtain the Lorentz force and the Maxwell equations. These equations describe field waves propagating with the “light velocity”, which, for the physical consistency, is considered the same as the cutoff velocity c. In this framework, a Schrödinger wave function is only a slowly varying amplitude of a particle wave function with a rapid oscillation given by the particle rest mass. We reobtain the proper rotation of a quantum particle called spin, and demonstrate the spin-statistics relation. We generalize a particle wave packet for a curvilinear system of coordinates and consider the gravitational field as a deformation of these coordinates. In this framework, a quantum particle is considered as a continuous distribution of matter according the theory of the general relativity.
The University of Electro-Communications, Japan
Time : 10:00-10:30
Taro Toyoda has completed his DSc from Tokyo Metropolitan University and was a Research Associate at National Research Council of Canada (NRCC). He is now a Project Professor of The University of Electro-Communications. His research focuses on basic studies of optical properties in semiconductor quantum dots including photoexcited carrier dynamics and their applications to photovoltaic solar cells. He has published more than 200 papers in reputed journals.
One of the main factors determining the photovoltaic performance in sensitized solar cell is the morphology of the TiO2 electrode. Using a suitable morphology can lead to improvements in the photovoltaic conversion efficiency. The present study focuses on a comparison between the electronic structure of inverse opal (IO)- and nanoparticulate (NP)-TiO2 electrodes. A higher open circuit voltage, Voc, was observed with IO-TiO2 electrodes compared to conventional NP-TiO2 electrodes. It appears that fundamental studies are needed to shed light on the underlying physics and chemistry governing the enhancement of Voc. Optical absorption measurements by the photoacoustic spectroscopy showed that indirect and direct transitions can be observed in IO- and NP-TiO2. The indirect bandgaps of IO- and NP-TiO2 are similar to each other (~ 3.2eV) in good agreement with previously reported, and the direct bandgaps of them are ~ 3.6 eV and ~ 3.5 eV, respectively, indicating difference in the electronic structure. There is a possibility that the density of states in the conduction band of IOTiO2 is larger than that of NP-TiO2. Analysis of the Urbach tail shows that there is a higher exciton-phonon interaction in IOTiO2 than in NP-TiO2. Indirect photoluminescence (PL) and exciton PL can be observed. Also, PL due to oxygen vacancies was observed. The PL spectra suggest difference in the valence band structure between IO- and NP-TiO2. The position of valence band maximum for IO-TiO2 is higher than that for NP-TiO2 measured by photoelectron yield spectroscopy, indicating that the surface of IO-TiO2 is polarized with more positive dipole moment toward the vacuum level than that of NP-TiO2. Hence, the formation of a double layer in the former is different from that in the latter due to the differences in the formation of oxygen vacancies, suggesting a correlation with the increased Voc in sensitized solar cells.
Free University of Berlin, Germany
Keynote: The interplay between off-stoichiometry and intrinsic point defects in quaternary compound semiconductors
Time : 10:30-11:00
Susan Schorr has obtained her PhD in physics from the Technical University Berlin in 1995. She was Postdoc in the inelastic neutron scattering group at the Hahn-Meitner-Institute Berlin and Visiting Scientist at the Los Alamos National Laboratory, US. She started as a Research Associate at the University Leipzig where she finished her Habilitation in 2006. At this time she started to work on multinary compound semiconductors for PV applications and developed the average neutron scattering length analysis method to evaluate the materials intrinsic point defects. She went back to the Hahn-Meitner-Institute Berlin (now HZB) to join the Institute of Technology in the Solar Energy Division as a Group Leader. In 2008, she was appointed as Professor for Geo-Materials Research at the Freie Universitat Berlin and became Head of the Department Structure and Dynamics of Energy Materials at the Helmholtz-Zentrum Berlin for Materials and Energy (HZB).
Thin film photovoltaic is an emerging alternative technology because of short energy payback time and minimum use of high purity materials, addressing the urgent need for cost-competitive renewable energy technologies. Compound semiconductors, like chalcopyrite type Cu(In,Ga)(Se,S)2 (CIGSe), are the most advanced and most efficient absorber materials. Such solar cells show present record lab efficiencies of >22%. Since the availability of indium is an object of concern regarding the large scale production of solar cells, its replacement with Zn and Sn is beneficial in this sense. Compounds like Cu2ZnSn(S,Se)4 (CZTS, CZTSe) are an alternative. One of the reasons for the success of CIGSe based thin film solar cells is the remarkable flexibility of its chalcopyrite type crystal structure. This flexibility is a key also for the quaternary kesterite type compounds CZTS, Se because the thin film growth is in fact a non-equilibrium process. The absorber layers of high efficient solar cells exhibits an overall off-stoichiometric composition, thus the existence of intrinsic point defects is strongly correlated with the chemical potential and therefore dependent on the composition of the material. These structural defects influence the electronic properties of the final device sensitively. A high density of bulk defects and structural disorder (Cu/Zn disorder) will cause extreme band tailing which could account for a significant part of the Voc loss, the main limitation for the performance of CZTS, Se-based PV devices. Our research focuses on the correlation between off-stoichiometry, point defects and physical properties of kesterites. We have demonstrated that kesterite type CZTSe can self-adapt to Cu-poor and Cu-rich compositions without any structural change except the cation distribution. The ability to accept deviations from stoichiometry, which can be categorized in off-stoichiometry types (A-L), is correlated to a Cu/Zn disorder and the formation of intrinsic point defects (see fig. 1).On the other hand, Cu/Zn disorder correlates with physical properties, like a shift of the hotoluminescence (PL) peak position. Hence we were able to show quantitatively that Cu/Zn disorder in kesterites causes shifts in the energy band gap giving raise to band tailing.