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Physics Conference Room, SB B326
Coffee starts at 12:00 PM and talk starts at 12:15 PM
- Monday, September 8, 2014
ABSTRACT: Quantum optics arose with the invention of the laser. Early work focussed on developing a quantum theory of the laser and on better understanding the nature of the quantized electromagnetic field. It was for this latter work that Roy Glauber won the 2005 Nobel Prize in Physics. The fields produced by nonlinear optical devices also received attention, because of their unusual correlation properties. In the 1980's two major areas of study were quantum metrology, using nonclassical states of the electromagnetic field to improve the accuracy of measurements, and micromasers and microlasers, optical devices that are pumped by a single atom at a time. In the 1990's the field split into three parts. Some researchers turned their attention to the study of Bose-Einstein condensates and related phenomena in matter-wave physics. Another group pursued the newly emerging field of quantum information, while a third continued with work on mainstream quantum optics. Today all of these efforts are alive and doing very well, and they have been joined more recently by the study of quantum opto-mechanics. A broad overview of these trends will be presented as well as more detailed discussions of some selected topics.
- Monday, September 22, 2014
ABSTRACT: Coherent backscattering, also known as weak localization, is one of the most striking examples of the subtle interplay between interference and disorder in scattering samples. Due to constructive interference between time-reversed paths inside the sample, the backscattered intensity exhibits a peak exactly at the direction opposite to the incident wave. Recently we have discovered an analogue of coherent back scattering in multimode optical fibers with strong mode mixing. I will discuss how fibers provide new opportunities to directly access and control the relative phase between the time-reversed paths, allowing us to flip the backscattered peak, and turn it into a dip. I will then show how we utilize correlations between the time-reversed paths for secure optical key distribution.
- Monday, September 29, 2014
ABSTRACT: I review such aspects of low-D physics as quantum number fractionalization, non-Fermi liquid behavior, and spontaneous mass generation.
- Monday, October 6, 2014
ABSTRACT: We will discuss the role of symmetries in three different areas of large scale structure: 1. how to test the equivalence principle using black holes in centers of galaxies; 2. how to measure gravitational redshift on cosmological scales using parity-violating signatures in correlation functions; 3. how spontaneously broken symmetries give us non-perturbative relations between different correlation functions.
- Monday, October 20, 2014
ABSTRACT: Advances in X-ray astronomy open the possibility for high precision spin and mass determination for astrophysical black holes starting the era of precision black hole physics. These observations turn astrophysical black holes into sensitive probes of ultra-light axion-like particles motivated by the strong CP problem and string theory.
When the axion Compton wavelength matches the black hole size, the axions develop "superradiant" atomic bound states around the black hole "nucleus" through the Penrose superradiance process. Their occupation number grows exponentially by extracting rotational energy from the ergosphere, culminating in a rotating Bose-Einstein axion condensate emitting gravitational waves. This transfer of angular momentum from the black hole to the axion condensate results in mass gaps in the spectrum of rapidly rotating black holes and gives rise to distinctive gravity wave signals.
Lea Ferreira dos Santos
- Monday, October 27, 2014
ABSTRACT: We consider one-dimensional isolated interacting quantum systems that are taken out of equilibrium instantaneously. Three aspects are addressed: (i) the relaxation process, (ii) the size of the temporal fluctuations after relaxation, (iii) the conditions to reach thermal equilibrium. The relaxation process and the size of the fluctuations depend on the interplay between the initial state and the Hamiltonian after the perturbation, rather than on the regime of the system. They may be very similar for both chaotic and integrable systems. The general picture associating chaos with the onset of thermalization is also further elaborated. It is argued that thermalization may not occur in the chaotic regime if the energy of the initial state is close to the edges of the spectrum, and it may occur in integrable systems provided the initial state is sufficiently delocalized.
- Monday, November 3, 2014
ABSTRACT: Proton exchange membrane fuel cells (PEMFC) have attracted great attention because of their high power density, low operation temperature ] and almost pollution-free emission . In fuel cells power is generated via the conduction of H+ ions through a polyelectrolyte membrane, commonly composed of sulfonated tetrafluoroethylene based fluoropolymer-copolymer. The function of the fuel cell constitutes a balance between hydrogen oxidation and oxygen reduction reactions where Pt nanoparticles are used to catalyze the reactions at the electrodes . Although the hydrogen oxidation process is a fast electrochemical reaction, challenges come up when impure hydrogen is used. Carbon monoxide is well known to poison the catalyst by blocking active sites on the catalyst’s surface , which prevents the hydrogen adsorption and subsequent oxidation propane, or alcohols can be an inexpensive alternative to pure hydrogen gas, but the high CO and CO2 content of reformed gas, even after purification, poses a major drawback. Hence the ability to engineer a CO resistant system would be a critical step towards enabling the commercialization of a competitively priced hydrogen fuel cell. We have developed a technique whereby Au particle nanoplatelets, 3nm in diameter, and only three atomic layers thick, could be reproducibly formed at the air water interface, and then coated onto any arbitrary surface simply by using the LB technique. Here we show that when this layer of NP is deposited directly onto the membrane of a PEM fuel cell, the efficiency of the cell running in ambient conditions is enhanced by more than 80% and an H2 stream with as much as 20% CO2 is tolerated. DFT calculations indicate that a two-step oxidation process is also present in this case, which enables the oxidation reaction to take place in a broad temperature range, extending well below ambient.
- Monday, November 17, 2014
ABSTRACT: The development of collective long-range order by means of phase transitions occurs by the spontaneous breaking of fundamental symmetries. Magnetism is a consequence of broken time-reversal symmetry, whereas superfluidity results from broken gauge invariance. The broken symmetry that develops below 17.5 kelvin in the heavy-fermion compound URu2Si2 has long eluded such identification. Here we show that the recent observations of Ising quasiparticles in URu2Si2 results from a spinor order parameter that breaks double time-reversal invariance, mixing states of integer and half-integer spin. Such "hastatic" order hybridizes uranium-atom conduction electrons with Ising 5f2 states to produce Ising quasiparticles; it accounts for the large entropy of condensation and the magnetic anomaly observed in torque magnetometry. Hastatic order predicts a tiny transverse moment in the conduction-electron sea, a collosal Ising anisotropy in the nonlinear susceptibility anomaly and a resonant, energy-depedent nematicity in the tunnelling density of states. We also discuss the microscopic origin of hastatic order, identifying it as a fractionalization of three-body body bound-states into integer spin fermions and half-integer spin bosons.
Work done with Piers Coleman and Rebecca Flint.
References: PC, P. Coleman and R. Flint Nature 493, 421 (2013)
- Monday, November 24, 2014
ABSTRACT: Metamaterials with properties that vary from point to point in space and time are suitable for new applications such as an “optical cloak”. Colloidal dispersions of metal nano-rods in dielectric fluids are appropriate to construct such spatially varying and electrically reconfigurable optical metamaterials. An applied electric field can be used to control the orientation and concentration of nano-rods, and thus modulate the optical properties of the dispersion. For example, by using gold nano-rods dispersed in toluene, we demonstrate an electrically induced change in refractive index on the order of 0.1 which was used to change the visibility of metal object. This approach is also valid to design an omnidirectional broadband optical “black-hole”.
- Monday, December 8, 2014