Special Colloquium
Thursday, January 26, 2012
Note Special Day
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Utpal Chatterjee
[Host: Seunghun Lee]
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3:30 PM, Room 204
Note Special Time
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Argonne National Laboratory |
| Physics Building |
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“Characterizing phase diagram of High Temperature Superconductors via Angle Resolved Photoemission Spectroscopy” |
ABSTRACT:
| High Temperature Superconductors (HTSCs) were discovered more than 25 years ago.
However, a microscopic theory of them is yet to be realized. In order to identify the
mechanism behind superconductivity in these systems, we must understand the normal
state from which superconductivity emerges. From our detailed Angle Resolved
Photoemission Spectroscopy (ARPES) measurements on Bi2Sr2CaCu2O8+δ (BISCO 2212) HTSCs we have found that unlike conventional superconductors, where there is a single temperature scale Tc separating the normal from the superconducting state, HTSCs
are associated with two additional temperature scales. One is the so-called pseudogap
scale T*, below which electronic states are partially gapped, while the second one is the
coherence scale Tcoh, characterizing the onset of a significant enhancement in electronic
lifetime. We have observed that both T* and Tcoh change strongly with carrier
concentration and they cross each other near optimal doping, i.e. the carrier concentration
at which an HTSC attains its maximum Tc. Furthermore, there is an unusual phase in the
normal state where the electronic excitations are gapped as well as coherent. Quite
remarkably, this is the phase from which the superconductivity with maximum Tc
emerges. Our experimental finding that T* and Tcoh intersect is not compatible with the
theories invoking “single quantum critical” point near optimal doping, rather it is more
naturally consistent with the theories of superconductivity for doped Mott insulators. |
ABSTRACT:
The polaron is a mathematical model for a “dressed”
particle consisting of an electron together with its entourage of
local excitations of a quantized phonon field. We will give a brief
historical review of the polaron, including the analysis of its ground
state by a Brownian motion functional integral and by a related
variational expression.
For the case of two or more electrons,
the interaction of the electrons with the phonon field gives rise to
an effective attraction between electrons that causes the particles to
bind together. For N electrons, N → ∞, the
systems are unstable in the sense that the binding energy grows faster
than linearly in N. We will discuss recent work with Frank,
Lieb, and Seiringer which shows that sufficiently strong Coulomb
repulsion between electrons can compensate for this binding and
provide stability for polaron systems for large N. |
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Friday, February 3, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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Special Colloquium
Thursday, February 9, 2012
Note Special Day
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Kenjiro Gomes
[Host: Seunghun Lee]
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3:30 PM, Room 204
Note Special Time
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Stanford University |
| Physics Building |
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“Tailoring Dirac Fermions in Molecular Graphene” |
ABSTRACT:
| The dynamics of electrons in solids is tied to the band structure created by a periodic atomic potential. The design of artificial lattices, assembled through atomic manipulation, opens the door to engineer electronic band structure and to create novel quantum states. We present scanning tunneling spectroscopic measurements of a nanoassembled honeycomb lattice displaying a Dirac fermion band structure. The artificial lattice is created by atomic manipulation of single CO molecules with the scanning tunneling microscope on the surface of Cu(111). The periodic potential generated by the assembled CO molecules reshapes the band structure of the two-dimensional electron gas, present as a surface state of Cu(111), into a "molecular graphene" system. We characterize the band structure through Fourier transform analysis of impurity scattering maps. We tailor this new tunable class of graphene to reveal signature topological properties: an emergent mass and energy gap created by breaking the pseudospin symmetry with a Kekule bond distortion; gauge fields generated by applying atomically engineered strains; and the condensation of electrons into quantum Hall-like states and topologically confined phases. |
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Friday, February 10, 2012
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Robert W. Michaels
[Host: Xiaochao Zheng]
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4:00 PM, Room 204
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Thomas Jefferson National Accelerator Facility |
| Physics Building |
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“The Lead Radius Experiment PREX” |
ABSTRACT:
| The Lead Radius Experiment PREX ran in the Spring of 2010 in Hall A at the Thomas Jefferson
National Accelerator Facility (JLab). The experiment measures the parity-violating asymmetry in the elastic scattering of longitudinally polarized electrons from a 208Pb nucleus at an energy of 1.06 GeV and a scattering angle of 5◦. The Z boson that mediates the weak neutral interaction couples mainly to neutrons and provides a clean, model-independent measurement of the RMS radius Rn of the neutron distribution in the nucleus. This measurement is a fundamental test of nuclear structure theory, and our result establishes the existence of the neutron skin, i.e. that Rn > Rp. A precise measurement of Rn pins down the density-dependence of the symmetry energy of neutronrich
nuclear matter, which has impacts on neutron star structure, heavy ion collisions, and atomic
parity violation experiments. The experiment involves all aspects of the JLab accelerator, from
the polarized source to the detector, and capitalizes on JLab’s unique strengths for carrying out high-precision parity experiments. In addition to the 2010 data, several technical challenges will be described, as well as prospects for future measurements at JLab from 208Pb and other nuclei such as 48Ca. |
Special Colloquium
Monday, February 13, 2012
Note Special Day
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Sami Amasha
[Host: Seunghun Lee]
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3:30 PM, Room 204
Note Special Time
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Stanford University |
| Physics Building |
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“Pseudo-spin Resolved Transport Spectroscopy of the Kondo Effect” |
ABSTRACT:
| In strongly-correlated materials, such as high-temperature superconductors and heavy fermion compounds, electrons form many-body states with properties different from those of non-interacting electrons. A simpler and better understood example of electron correlations is the Kondo effect, which describes how spins of conduction electrons screen the spin of a localized electron that has degenerate spin states (spin-up and spin-down in the case of a localized spin-1/2 electron). This screening generates spin correlations. Electrical transport measurements of a single quantum dot can probe Kondo physics; however, to directly access the spin correlations one needs spin-resolved measurements. We address this challenge by using the orbital states of a double quantum dot as pseudo-spin states: an electron on the left/right dot is associated with pseudo-spin up/down. When the energies of these pseudo-spin states are degenerate, Kondo screening occurs. We establish a correspondence between spin Kondo in a single dot and pseudo-spin Kondo in double dots. We use this to show that our pseudo-spin resolved spectroscopy measurements of the Kondo state in a double dot correspond to predictions for spin-resolved spectroscopy of spin Kondo. Finally, we explore the interplay between orbital and spin degeneracy in this double dot system. |
Special Colloquium
Thursday, February 16, 2012
Note Special Day
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Cheng Cen
[Host: Seunghun Lee]
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3:30 PM, Room 204
Note Special Time
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IBM |
| Physics Building |
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“Oxide Nanoelectronics on Demand” |
ABSTRACT:
| Complex oxides and their heterostructures have exhibited a great collection of novel functionalities and are considered one of the most promising candidate for next generation technological materials. At the interface formed between LaAlO3 and SrTiO3, by scanning a biased conducting atomic force microscope (AFM) tip along a programmed trajectory at room temperature, we can reversibly control in nanoscale the metal-insulator transition. With this technique, a variety of rewritable nanoscale devices and structures have been studied. These nanostructures, which are mainly assembled from basic elements including conductive wires and dots with characteristic dimensions just a few nanometers, show great performance as field effect transistors, nanodiodes and photodetectors. At low temperatures, a variety of electronic, spintronic and superconducting properties are observed, with enormous potential for exploitation in quantum devices. |
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Friday, February 17, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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Special Colloquium
Monday, February 20, 2012
Note Special Day
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Haidong Zhou
[Host: Seunghun Lee]
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3:30 PM, Room 204
Note Special Time
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National High Magnetic Field Lab |
| Physics Building |
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“Spin Ice and Quantum Spin Liquid in Geometrically Frustrated Magnets” |
ABSTRACT:
| In geometrically frustrated magnets (GFMs), the incompatibility between the interactions of the magnetic degrees of freedom in a lattice and the underling crystal geometry leads to the frustration. The massive level of degeneracy introduced by this frustration can persist to low temperatures to enhance the spin fluctuations and suppress the magnetic ordering, therefore resulting exotic spin ground states with abnormal thermo-dynamics. In this talk, two GFMs will be introduced: (i) Spin ice with pyrochlore structure, in which the ground state is a short range ordering of the “two spin in two spin out” configurations on tetrahedrons following the “ice rule”; (ii) Quantum spin liquid (QSL), in which the strong quantum fluctuations of the spins with small number (S = 1/2 and 1) destroy the magnetic ordering and lead to a spin-liquid like ground state. Following the introduction, we present our recently studies on new pyrochlore materials Pr2Sn2O7 and Dy2Ge2O7, and new QSL materials Ba3CuSb2O9 and Ba3NiSb2O9 with a triangular lattice of S = 1/2 and S = 1, respectively. |
Special Colloquium
Thursday, February 23, 2012
Note Special Day
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Xiang Cheng
[Host: Seunghun Lee]
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3:30 PM, Room 204
Note Special Time
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Cornell University |
| Physics Building |
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“Imaging the microscopic structure of shear thinning and thickening colloidal suspensions” |
ABSTRACT:
| While a simple Newtonian fluid such as water flows with a constant viscosity, many structured fluids ranging from polymer melts to surfactant solutions exhibit fascinating non-Newtonian flow behaviors including shear thinning and shear thickening. One typical example is a colloidal suspension, where its viscosity can vary by orders of magnitude depending on how quickly it is sheared. Although these non-Newtonian behaviors are believed to arise from the arrangement of suspended particles and their mutual interactions, microscopic particle dynamics in such suspensions are difficult to measure directly. Here, by combining fast confocal microscopy with simultaneous force measurements, we systematically investigate a suspension's structure as it transitions through regimes of different flow signatures. Our measurements of the microscopic single-particle dynamics unambiguously show that shear thinning results from the decreased relative contribution of entropic forces and that shear thickening arises from particle clustering induced by inter-particle hydrodynamic lubrication forces. Furthermore, we explore out-of-equilibrium structures of sheared colloidal suspensions and report a novel string phase, where particles link into log-rolling strings normal to the plane of shear. Our techniques illustrate an approach that complements current methods for determining the microscopic origins of non-Newtonian flow behavior in complex fluids. |
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Friday, February 24, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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Friday, March 2, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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Institute of Nuclear and Particle Physics Semi-annual Special Colloquium
Thursday, March 15, 2012
Note Special Day
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Bill Marciano
[Host: Brad Cox]
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4:00 PM, Room 203
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Brookhaven National Laboratory |
| Physics Building |
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“Tales from the "Darkside" of Particle Physics” |
ABSTRACT:
| The "Minimal" Standard Model of particle physics is almost complete. Interesting hints of a standard Higgs Boson are starting to appear at CERN and one can ask, Is it all that remains to be discovered? Dark matter observations suggest that an invisible universe of massive particles may exist all around us, but coupled to normal matter primarily by gravity. Can we detect dark particles and study their properties at accelerators? In this talk, I will discuss the implications of a Higgs Boson discovery and speculate on its possible connection to "dark" matter physics. In particular, properties of the "dark" photon, a hypothetical "dark" force carrier,
along with ongoing and proposed experimental efforts to discover, it will be described. |
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Friday, March 23, 2012
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Sir David King
[Host: Ian Harrison]
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4:00 PM, Room 203
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Smith School at Oxford |
| Physics Building |
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“TBA” |
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Friday, March 30, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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Friday, April 6, 2012
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Bernd Surrow
[Host: Nilanga Liyanage]
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4:00 PM, Room 204
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MIT |
| Physics Building |
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“TBA” |
Special Colloquium: Hoxton Lecture
Thursday, April 12, 2012
Note Special Day
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Edward Moses
[Host: Brad Cox]
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7:00 PM, Room 402
Note Special Time
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National Ignition Facility |
| Chemistry Building |
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“TBA” |
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Friday, April 13, 2012
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Bellave Shivaram
[Host: Brad Cox]
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4:00 PM, Room 204
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University of Virginia |
| Physics Building |
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“101 Years of Superconductivity” |
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Friday, April 20, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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Friday, April 27, 2012
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Available
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4:00 PM, Room 204
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| Physics Building |
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To add a speaker, send an email to
bbc2x@Virginia.EDU. Include the seminar type (e.g. Colloquia), date, name of the speaker, title of talk, and an abstract (if available). [Please send a copy of the email to phys-seminars@Virginia.EDU.]
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