Condensed Matter Seminars
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Condensed Matter Seminars
are in Room NPB 2205
on Mondays @ 4:05 pm t0 4:55 pm |
| August 26 | ||
| Speaker | Zuhair Khandker
(Yale University) |
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| Title | Viscosity Sum Rules for Fermions Near Unitarity |
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| Abstract | In 2004, a new type of fermionic superfluid was
experimentally realized using ultracold atomic gases in
optical traps. These systems are modeled as spin-1/2
particles interacting with S-wave scattering length a.
The novel and fascinating feature of these experiments is
that manipulation of external magnetic fields allows in
principle for the scattering length a to be tuned to
any arbitrary value. This provides an ideal arena for
studying the properties of interacting fermions through a
variety of phases: from BCS (1/a ~ - infinity) to BEC
(1/a ~ + infinity) superfluidity and an intermediate
region (1/a ~ 0) called unitarity. The
unitarity regime is particularly important for understanding
BCS-BEC cross-over physics as well as strong-interaction
dynamics and is the subject of much current interest in many subfields, including condensed matter, nuclear, and particle physics. I will review the unitarity limit and then describe our work studying hydrodynamic properties of these systems using the operator product expansion. |
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| Host | Pradeep Kumar |
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| September 2 (No seminar - Labor day) | ||
| Speaker | ||
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| September 9 | ||
| Speaker | Mark Meisel (UF) |
|
| Title | A snapshot of the history of Bell
Labs (an informal discussion in the tradition of the Fulbright Program*) |
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| Abstract |
This discussion is motivated by a recent book, “The Idea Factory: Bell Labs and the Great Age of American Innovation” by Jon Gertner (Penguin Press, 2012), and by my recent Fulbright Scholar (six-month) visit in The Center for Low Temperature Physics, Pavol Jozef Šafárik University in Košice, Slovakia. In the world of condensed matter science, “Bell Labs” is often recognized as a past leading laboratory of scientific research. How did it start? How was it sustained? What were the management philosophies underlying the lab? What happened? Why did it close? Let’s have some fun discussing these questions by asking “What can be learned from this history?” and “Where are we going in the future?” Can any of these lessons be applied to the new initiatives at UF to hire new faculty? Prizes will be awarded! * “Man’s struggle to be rational about himself, about his relationship to his own society and to other peoples and nations, involves a constant search for understanding among all peoples and all cultures -- a search that can only be effective when learning is pursued on a worldwide basis.” Senator J. William Fulbright, from the Forward of “The Fulbright Program: A History” by W. Johnson and F. J. Colligan (University of Chicago Press, 1965). |
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| Host | Mark Meisel |
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| September 16 | ||
| Speaker | Saurabh Maiti (NHMFL Dirac Postdoctoral Fellow at UF) | |
| Title | Peculiarities under the Superconducting dome in
Iron based Superconductors |
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| Abstract |
It is well known that the Iron based superconductors fall
under the high Tc category. They, however, display various
features that are unique to them and not seen in other
high Tc materials like the heavy fermions and the
cuprates. This is one of the reasons why these materials
continue to draw researchers’ interest. In this talk, I
hope to discuss some of these interesting features which
are connected to the superconducting phase of these
materials. The peculiarities are spread out all across the
phase diagram - the underdoped region (where there is
coexistence of magnetism and superconductivity), the
optimally doped region (where the Tc is the highest) and
the overdoped regions (where there are only one kind of
Fermi-pockets). We will see how the anisotropy and the
multi-band nature of these materials can result in
features like Z2 symmetry breaking, peculiar gap structure
evolution with doping, and a special type of spontaneous
time-reversal symmetry breaking(s+is). A very interesting
result about the number of possible quantum phase
transitions under the superconducting dome will also be
discussed. |
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| Host | Dmitrii Maslov |
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| September 23 | ||
| Speaker | Maitri Warusawithana (Florida State
Univ.) |
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| Title | Film stoichiometry, "polar catastrophe" and the 2-D
electron liquid at LaAlO3/SrTiO3
interfaces |
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| Abstract |
The strongly correlated electrons that form a
two-dimensional electron liquid (2-DEL) at the interface
between the insulators, LaAlO3 and SrTiO3,
has fueled interesting research on emergent collective
phenomena, including superconductivity and magnetism.
These collective phenomena distinguish this rich system
from conventional two-dimensional electron gases at
compound semiconductor interfaces. Averting a polar
catastrophe – a diverging potential due to the polar
discontinuity at the LaAlO3 and TiO2-terminated
(100) SrTiO3 interface – proposed as the origin
of this 2-DEL, however, has been highly debated with focus
on the role of defects in the SrTiO3 while the
LaAlO3 has been assumed perfect. In this talk I
will discuss our experiments and first principles
calculations, which show that the cation stoichiometry of
the nominal LaAlO3 layer is key to 2-DEL
formation [1]: only Al-rich LaAlO3 results in a
2-DEL. While extrinsic defects including oxygen deficiency
are known to render LaAlO3/SrTiO3
samples conducting, our results show that in the absence
of such extrinsic defects, an interface 2-DEL can form.
Its origin is consistent with an intrinsic electronic
reconstruction occurring to counteract a polarization
catastrophe. While the requirement of a critical thickness
for interface conductivity has been established [2], our
experiments find a key missing piece of the polar
catastrophe puzzle – a critical La/Al ratio (determined to
be ≤ 0.97 ± 0.03), which is also required for the
formation of a 2-DEL. |
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| Host | Amlan Biswas |
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| September 30
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| Speaker | Eduardo M. Calleja (U. Colorado at Boulder and UF Physics BS degree) | |
| Title | Imaging Magnetic Impurities in High Tc Materials Using
Low Noise Scanning Tunneling Microscopy |
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| Abstract | Doping into strongly correlated electronic systems can lead to physically interesting effects such as colossal magneto resistance, anomalous thermopower, and high temperature superconductivity. Variations in doping and temperature often reveal rich phase diagrams, as the one found in the copper oxide superconductors, where hole doping the parent anti-ferromagnetic insulator reveals high temperature superconductivity. The interplay between the antiferromagnetism and the superconductivity remains a mystery to this day. The real space nature of the strong interactions makes them inherently susceptible to disorder on a nanometer length scale. This makes local probes such as Spectroscopic-Imagining Scanning Tunneling Microscopy (SI-STM) extremely valuable for studying the electronic structure of these materials. While many of the interesting phases that occur at low doping in the copper oxide superconductors have been studied in the charge degree of freedom using local probes, accessing the spin degree of freedom has been experimentally challenging. By inserting magnetic single atom impurities into a copper oxide superconductor and using SI-STM we observe a Kondo like effect which leads to a new way to probe the spin degree of freedom in the material. Using our low noise spectroscopy technique our data shows strong evidence of a magnetic phase competing with superconductivity, allowing us to gain a handle on the interplay of magnetism and superconductivity. | |
| Host | Yoon Lee | |
| October 7 | ||
| Speaker | Jean-Louis Pichard (CEA-Saclay, France) | |
| Title |
Scanning Gate
Microscopy and Quantum Point Contacts |
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| Abstract |
The conductance of the electron interferometer formed in a
two dimensional electron gas between a quantum contact and
the depletion region induced by the charged tip of a
scanning gate microscope exhibits very unusual interference
behaviors. This can be seen when one studies the
interferometer conductance as a function of the position of
the tip, of the opening of the contact and of the
temperature. Using a solvable model where the contact
exhibits a simple spin-degenerate Breit-Wigner resonance, I
shall explain the enhancement of the interference fringes
induced by increasing the temperature [1] and the
interference rings occurring in addition to fringes space by
half the Fermi wavelength when the resonance is split by a
Zeeman Field [2]. The
observation of these two phenomena using either a quantum
point contact or a Kondo dot above its Kondo temperature for
the contact region will be eventually discussed. [1] “Thermal Enhancement of Interference Effects in Quantum point Contacts”, Adel Abbout, Gabriel Lemarié and Jean-Louis Pichard, Phys. Rev. Lett. 106, 156810 (2011). [2] “Scanning Gate Microscopy of Kondo Dots: Fabry-Pérot Interferences and Thermally Induced Rings”, Andrii Kleshchonok, Geneviève Fleury and Jean-Louis Pichard, arXiv:1305.0106 (2013). |
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| Host | K. Muttalib |
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| October 14 |
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| Speaker | Christian D. Batista (LANL) |
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| Title | Vortex Crystals in Frustrated Mott Insulators |
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| Abstract | Chiral spin textures with different length scales emerge
in some itinerant magnets and are generating an increasing
interest in the study of magneto-transport and possible
applications to magnetic data storage and spin-electronic
devices [1-5]. It is natural to ask if similar topological
textures can emerge in Mott insulators and also lead to
magneto-electric effects. In this talk I will show that this
is indeed possible when the exchange interactions are
geometrically frustrated. For this purpose, I will consider
a frustrated S=1/2 XXZ Hamiltonian that is a low-energy
effective model for Ba3Mn2O8,
a layered spin-dimer compound, comprising magnetic dimers of
Mn5+ ions arranged on triangular planes
[6-8]. Successive layers are stacked following an
‘ABC’ sequence, such that the dimer units on adjacent planes
are positioned in the center of the triangular plaquettes of
the layers above and below. The effective exchange
anisotropy of the low-energy model results from frustration
between exchange interactions connecting the same pair of
dimmers. The competition between intra and inter-layer
exchange interactions leads to a triplon dispersion with
six-fold degenerate minima at incommensurate wave vectors ±Qn
(1 ≤ n ≤ 3). This degeneracy leads to a very rich
quantum phase diagram near the magnetic field induced
quantum critical point, that is constructed by adding ladder
diagrams and minimizing the resulting energy functional [9].
The phase diagram includes different multi-Q magnetic
orderings, which combine up to the six degenerate
incommensurate lowest-energy modes ±Qn (1
≤ n ≤ 3). In particular, it includes six-Q states
that are crystals of magnetic vortices and other complex
spin textures associated with different multi-Q ordered
states. [1] Chappert, C.; Fert, A.; Van Dau, F. N. Nat. Mater. 2007, 6, 813− 823. [2] Shinjo, T. Nanomagnetism and spintronics; Elsevier B. V: Amsterdam, 2009. [3] Tokura, Y.; Nagaosa, N. Science 2000, 288, 462−468. [4] Ivar Martin and C. D. Batista, Phys. Rev. Lett. 101, 156402 (2008). [5] Tonumura et. al., Nano Lett. 12, 1673 (2012). [6] M. Uchida et al., PRB 66, 054429 (2002). [7] E. C. Samulon et al., PRB 7 7, 214441 (2008). [8] E. C. Samulon et al., PRB 81, 104421 (2010). [9] S. T. Beliaev, Sov. Phys. JETP 7, 299 (1958). |
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| Host | Dmitrii Maslov |
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| October 21 | ||
| Speaker | Guillaume Gervais (McGill
University) |
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| Title |
Quantum Matter "On-a-chip"! |
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| Abstract |
Quantum physical phenomena are inherently different in dimensions lower than 3D. For instance, one can observe bizarre electronic quantum states in 2D with the properties of charge fractionalization, and in even lower dimension (1D) one can observe the conductance of a wire to be given by a single quantum corresponding to G = 2e2/h. In this talk, I will describe a few low temperature experiments, all performed on some sort of "chip", with the hope to elucidate bizarre quantum phenomena. These include: [2] the 1D-1D Coulomb drag of electrons in two closed-pack quantum wires, separated by only ~15 nm; and [3] a nano-engineered quantum faucet (for real fluids!). It's a lot of (quantum) fun! |
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| Host | Mark Meisel |
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| October 28 | ||
| Speaker | James A. Sauls (Northwestern) |
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| Title |
TBA |
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| Abstract | TBA |
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| Host | Mark Meisel |
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| November 4 | ||
| Speaker | Jennifer Andrew (UF MSE) |
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| Title | TBA |
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| Abstract |
TBA |
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| Host | Amlan Biswas |
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| Nobember 11 (No seminar - Veteran's Day) | ||
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| November 18 | ||
| Speaker | Stephen M. Holmes (Univ. Missouri-St. Louis) | |
| Title | TBA |
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| Abstract |
TBA |
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| Host | Mark Meisel |
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| November 25 (Monday of Thanksgiving Week) | ||
| Speaker | Junichiro Kono (Rice University) | |
| Title | Superfluorescence from a Quantum-Degenerate
Electron-Hole Gas |
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| Abstract | Quantum particles sometimes cooperate to develop an
ordered state, where macroscopic coherence appears
spontaneously. Here, we demonstrate that such
spontaneous appearance of coherence occurs in an optically
excited semiconductor quantum well in a high magnetic field
[1-3]. When we create a dense electron-hole (e-h)
plasma with an intense laser pulse, after a certain delay,
an ultrashort burst of coherent radiation emerges. We
interpret this striking phenomenon as a manifestation of
superfluorescence (SF), in which a macroscopic polarization
spontaneously builds up from an initially incoherent
ensemble of excited quantum oscillators and then decays
abruptly, producing giant pulses of coherent
radiation. SF has been observed in atomic gases, but
the present work represents the first observation of SF in a
semiconductor, where not only real-photon exchange but also
virtual-photon exchange (Coulomb interactions) is
responsible for the formation of macroscopic
coherence. We found that Coulomb interactions
dramatically enhance and modify the collective superradiant
decay of the e-h plasma. Unlike typical spontaneous
emission from semiconductors, which occurs at the band edge,
the observed SF occurs at the quasi-Fermi energy of the
highly degenerate carrier distribution, up to 150 meV above
the band edge. As the carriers are consumed by
ultrafast radiative recombination, the quasi-Fermi energy
goes down, and we observe a continuously red-shifting streak
of SF at zero magnetic field and a series of sequential SF
bursts from higher to lower Landau levels in a magnetic
field. This Coulomb enhancement allows the magnitude
of the giant dipole to exceed even the maximum possible
value for ordinary SF (i.e., the total sum of in-phase
oscillations of individual dipoles), making e-h SF even more
“super” than atomic SF. 1. G. T. Noe et al., Nature Physics 8, 219 (2012). 2. G. T. Noe et al., Fortschritte der Physik 61, 393 (2013). 3. J.-H. Kim et al., Physical Review B 87, 045304 (2013). |
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| Host | Mark Meisel and Chris Stanton |
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| December 2 (Monday after Thanksgiving Holiday) | ||
| Speaker | Various CM Faculty Speakers |
|
| Title | Meet the Faculty Shindig |
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| Abstract |
Various Condensed Matter (CM) Faculty speakers
make a pitch about what is hot in their research groups
and why |
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| Host | Meisel and Muttalib |
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Condensed Matter Seminars
|
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Condensed Matter Seminars
are in Room NPB 2205
on Mondays @ 4:05 pm t0 4:55 pm |
| January 13 | ||
| Speaker | Peter Kopietz
(Goethe-Universität, Frankfurt) |
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| Title | TBA |
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| Abstract | TBA |
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| Host | Muttalib |
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| January 20 (No seminar - Martin Luther King Jr. Day) | ||
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| January 27 | ||
| Speaker | Wei Pan (Sandia National Lab) | |
| Title | Exotic fractional quantum Hall states: New Physics | |
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TBA |
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| Host | Neil Sullivan |
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| February 3 | ||
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| February 10 | ||
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| February 17
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| February 24 | ||
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| March 3 (No Seminar -
UF Spring Break Week AND APS March Meeting
in Denver) |
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| March 10 | ||
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| March 17 |
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| March 24 | ||
| Speaker | Jamie Manson (Eastern Washington
Univ.) |
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| Title | TBA |
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TBA |
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| Host | Mark Meisel |
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| March 31 | ||
| Speaker | Don McCarty (UF Horticultural Sciences) |
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| Title | TBA |
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| Abstract | TBA |
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| Host | Mark Meisel |
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| April 7 | ||
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| April 14 | ||
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| April 21 | ||
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