Department of Physics & Astronomy
Where: HZ 116
When: Mondays at 4:00 p.m.
| September 19,
Periodic Systems: New Insight into their Properties
Dr. Emil Prodan
Quantum systems with periodic potentials play an important role when it comes about our understanding of the condensed matter. In a seminal paper, Bloch showed that the solutions of the Schrodinger equation for electrons in periodic potentials are still waves and, ever since, these solutions were labeled Bloch waves. In 1959, Walter Kohn published a 1D study on these functions, in which he revealed the analytic structure of Bloch waves as a function of the k vector. Understanding this structure, for real as well as for complex k vectors, turned out to be the key for understanding, besides many other things, the exponential localization of different correlation functions in periodic insulators. In this talk I will discuss recent and first extensions of these results to higher dimensions. I will present the analytic structure of the Bloch functions for linear molecular chains and cubic crystals and discuss several applications which include: estimating the changes of the particle density and local density of states in periodic crystals due to impurities, surfaces and interfaces
| September 27, 2005
Generalized Bose-Einstein Condensation in Many-Fermion Systems
Prof. Manuel de Llano
University of Houston
By recognizing the vital importance of two-hole Cooper pairs in addition to the standard two-fermion ones in a many-fermion system such as a superconductor or neutral-fermion superfluid, the concept of pairing has been re-examined with striking conclusions. Based on this, Bose-Einstein condensation (BEC) theory is generalized to include not boson-boson interactions (also neglected in BCS theory) but rather boson-fermion interactions. The new formalism reduces to all the old known statistical theories as special cases---including the so-called BCS-Bose crossover picture which in turn generalizes BCS theory. With no adjustable parameters, it yields substantially higher superconducting transition temperatures without invoking non-phonon dynamics. The implications of all this in neutral-fermion superfluids like trapped ultracold Fermi atomic clouds, is still to be explored.
| October 3, 2005
Quantum Criticality in Ferromagnetic Single-Electron Transistors
Quantum Dots and Single-Electron Transistors have been used over the past decade to model Fermi liquid or Kondo states in and near
equilibrium. The universal features of the Kondo effect and its significance in describing strongly correlated electron materials have led to a strong interest both from a technological as well as scientific point of view.
Here, we introduce what we believe to be the first realistic system - a quantum dot attached to ferromagnetic leads - that models non-Fermi liquid states near a quantum phase transition. We theoretically demonstrate a gate-voltage induced quantum phase transition. At the transition the Kondo effect becomes quantum critical, leading to distinct, universal properties. We find a fractional-power-law dependence of the conductance on temperature (T). The AC conductance and thermal noise spectrum have related power-law dependences on frequency (omega) and, in addition, show an (omega/T) scaling. Our results imply that the ferromagnetic nanostructure constitutes a robust and realistic model system to elucidate magnetic quantum criticality that is central to the heavy fermions and other strongly correlated electron systems with non-Fermi liquid behavior.
Non-Fermi-Liquid Physics in a Magnetically Frustrated Nanostructure
Prof. Kevin Ingersent
University of Florida
Interest in the Kondo effect has been revived by experimental manifestation of this many-body phenomenon in tunneling through a quantum dot and through a magnetic adatom on a metallic surface. Studies have begun of multiple-"impurity" configurations in which Kondo screening of local spins competes with magnetic ordering of the same spins. In this context, a cluster of three antiferromagnetically coupled spins is of fundamental importance as the simplest example of frustration, a feature of many magnetic systems. Scanning tunneling microscopy points to the existence of a novel Kondo state in Cr trimers on a gold surface, but the nature of this state is somewhat controversial. Interest is also growing in the possible interplay between Kondo physics and interdot quantum entanglement in triangular quantum-dot devices.
After reviewing this background, the talk will describe a non-Fermi liquid phase of the Kondo model for three half-integer spins with frustrating antiferromagnetic interactions. The phase, which is stabilized by the triangular symmetry of the model, arises without fine-tuning of couplings, and (unlike the non-Fermi liquid regimes found in other impurity models) is stable against magnetic fields and breaking of particle-hole symmetry. Various exact, universal low-energy properties will be presented. Signatures predicted in electrical transport may be testable in scanning tunneling microscopy on adatom trimers or in transport through quantum dots.
Microwave and rf spectroscopy of two-dimensional electron solids in high magnetic field
Prof. Lloyd Engel
Florida State University
Two-dimensional electron systems (2DES), hosted in GaAs, are best known for exhibiting the integer and fractional quantum Hall effects. The electrons in such systems are predicted to form electronic crystals in a number of different ranges of magnetic field, B. The archetypical electron solid, expected to be the ground state of disorder-free 2DES in the high B or low density limit, is the Wigner crystal, a triangular lattice of electrons stabilized by their mutual repulsion.
We have found that a striking rf or microwave resonance in the spectrum is a generic feature of the electron solids. The observed resonance frequencies range from 70 MHz to 10 GHz. The electron solids are insulators due to pinning by disorder, and the resonances are interpreted as "pinning modes" of the solids, in which crystalline domains oscillate within the disorder potential. After introducing the phenomenology of the resonances, I will present results for three distinct types of electron solid: 1) the phases terminating the fractional quantum Hall series at high magnetic field, 2) electron crystals concomitant with integer quantum Hall effects, and 3) bubble phases, which are crystals with clusters of electron guiding centers at each site, and which are observed when multiple Landau levels are occupied.
GRAPHENE NANOSTRUCTURES: Physics and Device
Prof. Li Lu
Chinese Academy of Sciences
Considerable efforts have been made in recent years to explore using carbon nanotubes as the building blocks for future nanoelectronic circuits. In this approach, however, there are unsolved difficulties such as how to grow carbon nanotubes with the desired diameter and chirality and in particular with intramolecular junctions, and how to select nanotubes of a desired type, then to assemble them in the right positions in circuits. In this talk I will report our proposal and investigation on a completely new approach, in which a top-down technique is incorporated to tailor graphene sheets directly to nano-devices and nanoelectronic circuits. We demonstrate the feasibility of this idea by fabricating specially designed multi-terminal graphene patterns down to a minimum strip width of 50 nm. Electron tunneling measurement confirms the formation of quasi-one-dimensional subbands due to the quantum size confinement of the electrons in the fabricated strips. This new approach would in the future provide an efficient way of producing numerous layers of identical graphene nanoelectronic circuits.
Magnetism, Superconductivity and Quantum Criticality in the Heavy-Fermion Compound CeRhIn5
Prof. Joe D. Thompson
Though a long-standing problem, the relationship between magnetism and superconductivity has become a particularly lively topic, especially in the context of strongly correlated heavy-fermion materials, such as CeRhIn5. In these systems, unconventional superconductivity emerges as a magnetic phase transition is tuned by applied pressure toward zero-temperature. Long-range magnetic order, however, terminates abruptly when the magnetic and superconducting transition temperatures become equal, and there is no evidence that the magnetic transition actually reaches T=0. Recent measurements reveal the emergence of field-induced antiferromagnetism in the superconducting state of CeRhIn5. This magnetism, hidden by superconductivity in zero field, reappears in an applied field and terminates at the expected quantum-critical point where the effective mass of quasiparticles diverges and the Fermi-surface volume increases without a change in topology. The relationship between magnetism, superconductivity and quantum criticality found in CeRhIn5 may be applicable to other strongly correlated systems, such as those based on copper-oxide and plutonium.
The Anomalous Hall Effect in a Paramagnetic 2DEG
Prof. John Cumings
University of Maryland
In ferromagnetic conductors, the Hall Effect generally acquires an additional contribution, even in the absence of a magnetic-field, due to spin-orbit coupling of the carriers and their inherent spin polarization. This is known as the Anomalous Hall Effect (AHE). Despite being discovered more than a century ago, the basic physics behind the AHE is still a subject of intense debate. In recent measurements, we have shown that this phenomenon can also be exhibited in a paramagnetic semiconductor. The two-dimensional nature of the system allows us to independently characterize the material through quantum transport and also to tune the effect with an applied electric field. Our results provide clues to the origin of the AHE and even track the effect into the little-explored realm of carrier localization.
ATOMIC FERMI GASES WITH UNEQUAL SPIN POPULATIONSDr. Meera Parish
I will investigate the properties of a gas of fermionic atoms where the two spin populations are unequal. By considering how the ground state evolves as a function of inter-atomic interaction and population imbalance, I will determine what the measurable differences are between the various theoretical models of the BCS-BEC crossover in atomic gases. In addition, I will present the phase diagram at finite temperature.
POSITRON BINDING ON MOLECULES IN POSITRON EMISSION TOMOGRAPHY
Prof. Lukas Pichl
International Christian University, Tokyo
Positron binding on molecules have been long believed to be rather of theoretical than practical interest, not only because of the ultimate annihilation (i.e., quasi-bound states of positron), but especially because of the positronium formation
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States in Exotic PairingFermionic Superfluids with Unbalanced Pairing Species
Prof. Kung Yang
National High Magnetic Field Lab, Florida
Superfluidity in fermionic systems originates from pairing of fermions,and Bose condensation of these Cooper pairs. The Cooper pairs are usually made of fermions of different species; for example in superconductors they are pairs of electrons with opposite spins. Thus the most favorable situation for pairing and superfluidity is when the two species of fermions that form pairs have the same density, a situation successfully described by the Bardeen-Cooper-Schrieffer theory. It has become clear recently that pairing and superfluidity can also occur when the fermion species have different densities, in systems ranging from superconductors in a strong magnetic field to trapped cold atom systems, and quark matter in the core of neutron stars. Such a situation will necessarily lead to unpaired fermions in the ground state, and possibly non-trivial spatial structure in the superfluid order parameter. In this talk I will discuss the physics of such exotic pairing states, and their possible realization and detection in superconductors and cold atom systems.
Observation of the Two-Channel Kondo Effect in a Semiconductor Nanostructure
Dr. Ilena Rau
two-channel Kondo Hamiltonian is a prototype for studying non-Fermi
liquid behavior in strongly correlated electron systems. Quantum dots
have proved to be excellent systems for studying the single-channel
Kondo effect, that is, the many-body ground state resulting from an
excess spin _ electron in the quantum dot interacting with a
reservoir of conduction electrons. The two-channel Kondo effect is
achieved by coupling the dot with equal strength to two independent
reservoirs. In this configuration the screening of the excess spin is
not successful anymore as the reservoirs compete in an attempt to
form the Kondo state with the dot.
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