NEWS: The CQM Distinguished Lecture series has been established in the Fall of 2015 to bring to Stony Brook University the renown experts in the physics of quantum matter.
October 2016 – March 2017
Organometallic methyl ammonium metal halide compounds, with the general composition of MA-MX3, have been first synthesized nearly 40 years ago. (MA stands for methyl ammonium, M is a metal ion (e.g. Pb2+, Sn2+) and X represents Cl-, Br-, I- or F-.) One of the simplest compound in the series, CH3NH3PbI3 , has been tested for solar energy conversion in 2009, and since then this family of materials is the subject of intense interest. CH3NH3PbI3 exhibits high light conversion efficiency, it lases in red color, and it can serve as the basis for light emitting diodes and photodetectors. I will review the properties the status of the current research in this area, and I will talk about the work that I did in collaboration with Laszlo Forro’s group in Lausanne.
Thermal vibrations (phonons) carry heat. This is true in all solids, but in good metals, electronic heat conduction can dominate thermal conductivity k, defined by jH= –kdT/dx. The electrical conductivity s of metals provides an interesting contrast. Conductivity is limited by electron-phonon and electron-electron interactions, and affected by Fermi statistics. Vibrations have instead anharmonic Bose-Einstein-type phonon-phonon interactions. Similarities in behavior suggest questions. Does vibrational thermal resistance (1/k) “saturate”, in analogy to the saturation of r=1/s in metals with large electron-phonon or impurity scattering? Will study of k aid in understanding more generally the breakdown of the quasiparticle picture? Should we expect a phonon analog of an Anderson insulator? Finally, the subject of non-locality of thermal response is now very important. An electron analog in metals was worked out many years ago: the “anomalous skin effect.” Analogous behavior can be recognized in thermal conductivity, and this recognition can be exploited. In all of these analogs, the vibrational k is messier than the electronic s, because of the diverse nature of phonons thermally equilibrated at different T, compared to the more homogeneous nature of electrons near the Fermi surface, which have a small parameter, kBT/EF.
We study the time evolution -induced by a quench- of local excitations in one dimension. We focus on interaction quenches: the considered protocol consists in creating a stable localized excitation propagating through the system, and then operating a sudden change of the interaction between the particles. To highlight the effect of the quench, we take the initial excitation to be a soliton. The quench splits the excitation into two packets moving in opposite directions, whose characteristics can be expressed in a universal way. Our treatment, which is hydrodynamic in nature, allows to describe the internal dynamics of these two packets in terms of the different velocities of their components. We confirm our analytical predictions through numerical simulations performed with the Gross-Pitaevskii equation and with the Calogero model (as an example of long range interactions and solvable with a parabolic confinement). Through the Calogero model we also discuss the effect of an external trapping on the protocol. The hydrodynamic approach shows that there is a difference between the bulk velocities of the propagating packets and the velocities of their peaks: it is possible to discriminate the two quantities, as we show through the comparison between numerical simulations and analytical estimates. In the realizations of the discussed quench protocol in a cold atom experiment, these different velocities are accessible through different measurement procedures.
- Fabio Franchini, Andrey Gromov, Manas Kulkarni, & Andrea Trombettoni, J. Phys. A: Math. Theor. 48 (2015) 28FT01
- Fabio Franchini, Manas Kulkarni, & Andrea Trombettoni, arXiv:1603.03051 (Accepted to NJP)
Graphene plasmons, which are collective oscillations of Dirac fermions in graphene, are of broad interests in both fundamental research and technological applications. In this talk, we present the first nano-infrared (IR) imaging studies of graphene plasmons using the scattering-type scanning near-field optical microscopy – a unique technique allowing efficient excitation and high-resolution imaging of graphene plasmons. With this technique, we were able to show that common graphene/SiO2/Si back-gated structures support propagating surface plasmons in the IR frequencies. The observed plasmons are highly confined surface modes with a wavelength around 200 nm that are conveniently tunable by the back gate voltages [Nature 487, 82–85 (2012)]. In addition, we were able to map and characterize grain boundaries inside CVD graphene film by examining the distinct plasmonic interference patterns triggered by these line defects. Our modeling and analysis unveiled unique electronic properties associated with grain boundaries [Nature Nanotech. 8, 821–825 (2013)]. Furthermore, we investigate the plasmonic properties of Bernal-stacking bilayer graphene (BLG) and find that BLG supports gate-tunable IR plasmons with higher confinement compared to graphene and randomly-stacked graphene layers. We also found that BLG plasmons can be turned off completely in a wide voltage range close to the charge neutrality point. Those unique plasmonic properties are attributed to both interlayer electron tunneling and bandgap opening in BLG [Nano Lett. 15, 4983-4978 (2015)]. Finally, we observed peculiar one-dimensional edge plasmons propagating strictly along the edges of patterned graphene nanostructures. Compared to commonly known two-dimensional surface plasmons, these one-dimensional edge modes have shown a slightly smaller plasmon wavelength [Nano Lett. 15, 8271-8276 (2015)].
Due to the complexity of the materials used as heterogeneous catalysts, and the difficulty of interrogating them under reaction conditions, catalysts are generally characterized only before and after chemical reactions. Thus, the dynamic active phases formed in catalysts under reaction conditions, which generate nanometer sized multifunctional active centers at metal/oxide/modifiers interfaces, and the associated reaction mechanisms are unknown. Since the structure of catalysts change as reacting molecules interact with it in route to forming products, only in-situ techniques allow following the dynamic processes involved on the surface of a catalyst. But interrogating these special interfacial sites experimentally is challenging. There has been considerable progress in the development and use of surface science techniques to follow catalytic reactions in-situ. Photon-in/photon-out techniques, such as IR spectroscopy, were used early on but photo-electron spectroscopic techniques, where electrons interact strongly with the reactants in the gas phase, took much longer to become widely available. Electron based structural techniques, such as scanning tunneling microscopy (STM), are still only available on a limited number of laboratories. I will present case studies showing how complementary in situ techniques including ambient pressure (AP) x-ray photoelectron spectroscopy (AP-XPS), infrared reflection absorption spectroscopy (AP-IRRAS) and AP-STM can be applied to study heterogeneous interfaces in model catalysts.
“Tuning the Properties of Cu-Based Catalysts via Molecular in Situ Studies of Model Systems”
Acc. Chem. Res. 48, 2151-2158, (2015).
“Highly Active Copper-Ceria and Copper-Ceria-Titania Catalysts for Methanol Synthesis from CO2” Science 345, 546-550, (2014).
Dr. Stacchiola is the Interface Science and Catalysis Group Leader at the Center for Functional Nanomaterials-Brookhaven National Laboratory (CFN-BNL) and Adjunct Professor at Michigan Technological University. He obtained his B.S. degree (1997) at UNSL (Argentina) and his Ph.D. (2002) at the University of Wisconsin Milwaukee and was a Humboldt Research Fellow at the Fritz-Haber-Institute in Berlin (2005−2007). His research focuses in surface chemistry studied by in-situ tools, and in particular on structure–reactivity relationships in catalysis . The use of well-defined model catalysts allows the characterization of systems at the molecular and atomic level. Studies of these model catalysts by complimentary in-situ ambient pressure spectroscopy and microscopy gives direct evidence of dynamically generated active phases and adsorbed surface species which are produced or stabilized only under reaction conditions.
We present a time-domain analysis of the response of a BCS
superconductor (in the low temperature limit) to a few cycle THz pulse
having spectral content limited to just below the absorption threshold
for breaking pairs. The analysis is based on the finite-difference
time-domain (FDTD) approach, in combination with a model susceptibility
for a superconductor that includes an explicit dependence on the energy
gap. The FDTD approach allows us to calculate the THz induced
current density, from which we determine the modified energy gap at each
instant of time during the THz wave’s passage. The resulting non-linear
susceptibility causes up-conversion of the incident THz wave into odd
harmonics. The FDTD results are compared with experiment for thin NbN
films in both linear and non-linear regimes. Additionally, some
experimental results for the response of a NbN film to a very strong,
single-cycle, sub-THz pulse will be shown, indicating that the
superconducting state can be disrupted on a ~100fs time scale.
1. Xioaxiang Xi and G.L. Carr, Supercon. Sci. & Technol. 26, 114001 (2013).
2. T. Hong et al, J. Appl. Phys. 114, 243905 (2013).
3. R. Matsunaga et al, Science 345, 1145 (2014).
Department of Materials Science and Chemical Engineering, Stony Brook University
As recently as 10-15 years ago, nanoscale metal catalysts were described in qualitative terms: oblate and hemispherical, discs and rafts. Today we admire their shapes that can be as beautiful as Platonic or Archimedian solids. We also know how to discriminate between them with unprecedented accuracy. Some of these advances are obtained using X-ray absorption fine-structure (XAFS) spectroscopy, which is a premier tool for studying structural, electronic and dynamic properties of nanoscale clusters. Negative thermal expansion, mono-metallic amorphization, metal-nonmetal transitions, increased (or decreased) Debye temperature are but a few examples of non-bulk behaviors. As an illustration, I will describe a prototypical catalytic system, a platinum particle in equilibrium with oxide support and adsorbate gas. By combining X-ray absorption and emission spectroscopies with DFT/MD simulations, I will show that many “anomalies” have their explanation in the heterogeneous structure, fluctuating over broad time-scale . By tracking the flow of charge to and from the nanoparticle in operando conditions, several competing interactions can be disentangled: metal-support, metal-adsorbate, and support-adsorbate . These results are of interest to energy sciences: by learning how to navigate these complex interactions and employ dynamics to tune up reactivity, one can learn how to rationally design a catalyst with the desired activity and selectivity.
 A. I. Frenkel, M. Cason, A. Elsen, U. Jung, M. W. Small, R. G. Nuzzo, F. D. Vila, J. J. Rehr, E. A. Stach, J. C. Yang. “Critical review: Effects of complex interactions on structure and dynamics of supported metal catalysts”, J. Vac. Sci. Technol. A 32, 020801 (2014)
 A. Elsen, U. Jung, F. D. Vila, Y. Li, O. V. Safonova, R. Thomas, M. Tromp, J. J. Rehr, R. G. Nuzzo, A. I. Frenkel. “Intracluster atomic and electronic structural heterogeneities in supported nanoscale metal catalysts “, J. Phys. Chem. C 119, 25615-25627 (2015)
In strongly correlated electron materials, the delicate interplay between spin, charge, and lattice degrees of freedom often leads to extremely rich phase diagrams exhibiting intrinsic phase inhomogeneities. The key to understand such complexities usually lies in the characterization and control of these materials at fundamental energy, time and length scales. I will use this opportunity to report the recent advances in the IR and THz spectroscopy and explain how they can be used to probe electronic/structural phase transitions with unprecedented spatial and temporal resolutions. Specifically, with scanning near-field infrared microscopy we resolved the insulator to metal phase transitions in 3d, 4d and 4f materials with ~10 nm resolution over a broad spectral range. Using ultrafast terahertz pump terahertz probe spectroscopy we can unambiguously demonstrated the insulator to metal transition at picosecond time scales via electric field-induced electron liberation. These results set the stage for future spectroscopic investigations to access the fundamental properties of complex materials.
We provide a unified description of aging in terms of record dynamics. “Aging” refers to the increasingly sluggish dynamics widely observed in the jammed state of disordered materials. Structural evolution in aging materials requires ever larger, record-sized rearrangements in an uncorrelated sequence of intermittent events (avalanches or quakes). According to record statistics, these (irreversible!) rearrangements occur at a rate ~1/t. Hence, in this log-Poisson statistics, the number of events between a waiting time t_w and any later time t integrates to ~ln(t/t_w), such that any observable inherits the t/t_w-dependence that is the hallmark of pure aging. Based on this description, we can explain the relaxation dynamics observed in a broad range of materials, such as in simulations of low-temperature spin glasses and in experiments on high-density colloids and granular piles. We have proposed a phenomenological model of record dynamics that reproduces salient aspects of the experiments, for example, persistence, intermittency, and dynamic heterogeneity. Here, we compare the predictions of the model with the data available from experiments by Yunker, et. al. [PRL103(2009)115701].