| Physics at the University of Virginia | ||||||
| Academics | People | Research | Announcements | Facilities | Administration | Classes |
| Thursday, August 27, 2009 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 3, 2009 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 10, 2009 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, September 17, 2009 | Alioscia Hamma [Host: Israel Klich] | |
| 4:00 PM, Room 204 | Perimeter Institute | |
| Physics Building | “Toric-boson model: Toward a topological quantum memory at finite temperature” |
| We discuss the existence of stable topological quantum memory at finite temperature. At stake here is the fundamental question of whether it is, in principle, possible to store quantum information for macroscopic times without the intervention from the external world, that is, without error correction. We study the toric code in two dimensions with an additional bosonic field that couples to the defects, in the presence of a generic environment at finite temperature: the toric-boson model. Although the coupling constants for the bare model are not finite in the thermodynamic limit, the model has a finite spectrum. We show that in the topological phase, there is a finite temperature below which open strings are confined and therefore the lifetime of the memory can be made arbitrarily (polynomially) long in system size. The interaction with the bosonic field yields a long-range attractive force between the end points of open strings but leaves closed strings and topological order intact. |
| Thursday, September 24, 2009 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, October 1, 2009 | Available | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, October 15, 2009 | Stephen Powell [Host: Andrew James] | |
| 4:00 PM, Room 204 | Joint Quantum Institute | |
| Physics Building | “Classical-quantum mappings for geometrically frustrated systems” |
| Several systems have recently been demonstrated to show "non-LGW" quantum phase transitions, with different orderings on two sides of a continuous transition. We present two examples of classical statistical systems --- spin ice in a [100] magnetic field and an ordering transition of close-packed dimers on a cubic lattice --- that appear to show continuous (second-order) transitions that lie outside the Landau paradigm. In both cases, strong local constraints mean that neither of the neighboring phases can be understood as thermally disordered, excluding the standard route to a continuum critical theory. Instead, we derive critical theories for both transitions by mapping from three-dimensional classical problems to two-dimensional quantum problems. For the dimer model, this mapping provides a direct connection to previous work on non-LGW transitions of lattice bosons at fractional filling factors. |
| Thursday, October 22, 2009 | Sukumar Rajauria [Host: Bellave Shivaram] | |
| 4:00 PM, Room 204 | NIST, Maryland | |
| Physics Building | “Refrigeration using superconducting tunnel junctions” |
| In the recent years, nano-refrigeration using electron tunneling in hybrid Normal metal - Insulator - Superconductor (N-I-S) junctions has gained increasing attention. Its basic principle is the energy selective tunneling due to the presence of an energy gap in the superconductor density of states. With a sub-gap voltage bias, only the most energetic electrons can tunnel out of the normal metal, leaving behind the electrons with less energy. We have measured with a high resolution the differential conductance of S-I-N-I-S junctions, whose analysis gives us an access to the normal metal electronic temperature as a function of the voltage. A quantitative model is proposed, that includes the electron-phonon coupling and the Kapitza resistance at the interface with the substrate. With this model, we have achieved a thorough description of the charge and heat currents. We have also shown that the normal metal phonon temperature drops significantly below the substrate temperature. At very low temperature (T < 200mK) and low bias, the coherent Andreev current dominates the quasi-particle current. By analyzing quantitatively the heat balance in the S-I-N-I-S junction, we demonstrate that the Andreev current does carry heat. This thermal contribution heats the normal metal electrons, overriding over a large voltage range the tunneling-based cooling. References [1] S. Rajauria, P. S. Luo, T. Fournier, F. W. J. Hekking, H. Courtois, and B. Pannetier, Phy. Rev. Lett. 99, 047004 (2007). [2] S. Rajauria, Ph. Gandit, T. Fournier, F. W. J. Hekking, B. Pannetier, and H. Courtois, Phy. Rev. Lett. 100, 047004 (2008). [3] S. Rajauria, H. Courtois and B. Pannetier, submitted (2009). |
| Thursday, October 29, 2009 | Brian Leroy [Host: Keith Williams] | |
| 4:00 PM, Room 204 | Univ. of Arizona | |
| Physics Building | “Local electronic properties of graphene” |
| Combining scanning probe microscopy with electrical transport measurements is a powerful approach to probe low-dimensional systems. The local information provided by scanning probe microscopy is invaluable for studying effects such as electron-electron interactions and scattering. Using this approach, we have probed the local electronic properties of mono- and bilayer graphene with atomic resolution. We studied the effect of ripples, charged impurities and defects on the local density of states. We find that long-range scattering from ripples and impurities shifts the Dirac point leading to electron and hole puddles. Short-range scattering from lattice defects mixes the two sublattices of graphene and tends to be strongly suppressed away from the Fermi energy. In addition, in bilayer graphene we observe an opening of a band gap due to the application of a transverse electric field. |
| Thursday, November 5, 2009 | RESERVED | |
| 4:00 PM, Room 204 | ||
| Physics Building |
| Thursday, November 12, 2009 | Ryan Barnett [Host: Austen Lamacraft] | |
| 4:00 PM, Room 204 | Joint Quantum Institute | |
| Physics Building | “New Physics in Multicomponent Ultracold Atomic Gases” |
| Bose-Einstein Condensation in multicomponent systems often exhibits physics which goes beyond traditional condensed matter paradigms due to the extra degrees of freedom. Examples of multicomponent condensates include condensed mixtures of atoms or atoms with internal spin degrees of freedom. In this talk I will discuss examples of such multicomponent condensates focused on during my recent research. The first example I will consider is a rotating condensate composed of two types of atoms with different masses. I will discuss the structure of the vortex configurations for such mixtures, and argue the existence of a counterintuitive phase where the two superfluids and external drive all rotate at different rates. During the remainder of the talk I will focus on spinor condensates. In optical traps the macroscopic spin configuration is determined by the spin-exchange interaction. The resulting mean-field ground states have a variety of point-group symmetries. I will discuss a geometrical representation of the mean-field states of these systems, and will also describe how this method is useful for understanding the coherent spinor dynamics, collective excitations, and topological defects. I will next explain how the phenomenon of order-by-disorder from quantum magnetism is naturally exhibited in spin-two condensates. Finally, I will discuss the rich structure of the vortex lattices which can occur when spinor condensates are rotated. |
| Thursday, November 19, 2009 | Christian Flindt [Host: Israel Klich] | |
| 4:00 PM, Room 204 | Harvard | |
| Physics Building | “Counting statistics of electron transport in nanostructures” |
| Fluctuations of the electrical current running through a nano-scale conductor reveal information beyond what is contained in the conductance alone. In this talk I will give an overview of my recent works on counting statistics, the stochastic theory of charge transport in nanostructures. As an example, I will show how current fluctuations can be a useful tool to detect mechanical bistabilities and frequency shifts in nanoelectromechanical systems. While most works have focused on systems whose dynamics is Markovian, I will discuss the influence of memory effects on the counting statistics, illustrated with a model of transport through a double quantum dot in a dissipative environment. Finally, I show that high-order current-current correlation functions (cumulants) in general oscillate as functions of basically any system parameter. A prediction that has been confirmed by recent experiments, as I will describe. Recent papers: C. Flindt et al., PRL 100, 150601 (2008), PNAS 106, 10116 (2009) |
|
Monday, November 23, 2009 Note Special Day |
Andrew James [Host: Austen Lamacraft] | |
|
3:30 PM, Room 204 Note Special Time |
University of Virginia | |
| Physics Building | “Non-Fermi liquid fixed point for an imbalanced gas of fermions in 1+ ε dimensions” |
| We consider a two species gas of fermions with population imbalance. Using the renormalisation group in d=1+ ε dimensions, we show that for spinless fermions and ε > 0 a fixed point appears at finite attractive coupling where the quasiparticle residue Ζ vanishes, and identify this fixed point with the transition to Larkin--Ovchinnikov--Fulde--Ferrell order (inhomogeneous superconductivity). When the two species of fermions also carry spin degrees of freedom we find a repulsive fixed point, indicating a transition to spin density wave order. |
| Thursday, December 3, 2009 | Mike Whangbo [Host: Seunghun Lee] | |
| 4:00 PM, Room 204 | NC State University | |
| Physics Building | “Spin exchange interactions and magnetic properties” |
| This talk will briefly review how the spin exchange interactions of magnetic solids are described quantitatively and qualitatively. When the choice of a spin lattice is made by inspecting the geometrical pattern of the magnetic ion arrangement or by seeking the novelty of the physics the chosen model generates, interesting but erroneous interpretations often result. The importance of choosing a spin lattice on the basis of electronic structure considerations is emphasized. The magnetic solids to be discussed in this talk include: Cs 2 CuCl 4 , Na 3 Cu 2 SbO 6 , Bi 4 Cu 3 V 2 O 14 , Cu 3 (CO 3 ) 2 (OH) 2 , AgCrO 2 , Ca 3 CoMnO 6 , MnWO 4 , [Cu(HF 2 )(pyz) 2 ]BF 4 . |
To add a speaker, send an email to bss2d@Virginia.EDU. Include the seminar type (e.g. Condensed Matter Seminars), 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.]
|
|
|
Maintained by the Webmaster This page was generated dynamically using content derived from our departmental database. |
Support UVa’s Physics Department |