All talks are in Room
B-131, except when otherwise noted. Regular seminar time is Friday
1:30PM. Follow the links to see the schedule in past semesters.
Nikolai Nezlobin
University of Minnesota
Phase-Locking in a Non-Uniform Array
of
Tunnel Junctions
A non-uniform, one-dimensional tunnel junction array that is designed to promote phase-locking to an ac pump signal is proposed and theoretically investigated for possible information processing applications. The tunnel resistances of the junctions in one part of the array are tailored to provide phase restoration for electrons tunneling in the array, while the tunnel resistances in the other part of the array are designed to provide time delay with minimal jitter. It is shown that phase-locking of the tunneling is possible at the fundamental and at the one-half sub-harmonic of the pump frequency. While the required parameter ranges and the necessary degree of control of this design are not practical for tunneling phase logic based on conventional tunnel junction technologies, the proposed design could potentially be realized in a molecular system.
Host: Likharev
Friday, August 13, 1:30PM
Hideaki Takayanagi
NTT Basic Research Laboratories,
Japan
Superconducting Flux Qubit as a Macroscopic Artificial Atom
First I will give a very short introduction of research activities at NTT Basic Research Laboratories. They cover wide areas from Nano-bioscience to Quantum information technology. Then I focus on recent results on a superconducting flux qubit. We have been working on the superconducting flux qubit which consists of three Josephson junctions in a loop. We have achieved the single-shot readout of the quantum state by carefully optimizing the operating condition of a quantum detector, the dc-SQUID. We have also observed multi-photon transition between superposition of clockwise and counter-clockwise macroscopic persistent supercurrents in the qubit loop. Resonant peaks and dips of up to three photon processes were observed. The width of these multiphoton dips was well explained by the Bessel functions. Recently, we have observed coherent oscillation between the two lowest-energy quantum levels of a qubit. Resonant microwave pulses induce coherent quantum oscillations between these macroscopic quantum states. We used a design in which qubit and a detector SQUID are spatially separated. This design has the distinct advantage of being immune to invasion of quasiparticles which are generated on the switching of the detector SQUID to a voltage state and are thought as one of serious sources of decoherence
Host: Averin
Friday September 24, 1:30PM
Igor O. Kulik
Stony Brook
Hypothetic Quantum Computation and Novel Classical Computation
1. Can a quantum computer solve non-tractable mathematical problems in
polynomial time?
2. Spontaneous and persistent currents as quantum computational tools.
3. Double-conversion classical computational doctrine and its
implementation: the ABC compiler.
Host: Likharev
Friday October 1, 1:30PM
Kenneth S. Burch
UC San Diego
Optical Properties of Magnetic Semiconductors
Recently there has been growing interest in III-V semiconductors doped
with transitions metals (TM), due to their potential applications in
devices which exploit both the charge and spin of the carriers.
Additionally in Dilute Magnetic Semiconductors (DMS) the Fermi energy
is less than or equal to the exchange energy. However a full
experimental description of the charge dynamics and band structure of
DMS is still incomplete. In this talk I will discuss recent infrared
and ellipsometry measurements of Mn doped GaAs. We examine random
alloys in the doping regime from x=0% (LT-GaAs) to x=6.6%. Through
comparison with LT-GaAs we uncover the optical signatures of defects in
GaMnAs spectra. The critical points in the spectra are revealed through
an analysis of the second derivatives of the dielectric function. The
evolution of these critical points and their relationship to the
development of the band structure with increased Mn doping will be
reviewed. We also report on the electromagnetic response of Digital
Ferromagnetic Heterostructures (DFH): systems with delta-doped MnAs
layers separated by GaAs spacers of variable thickness. The gross
features of the infrared conductivity of DFH samples are consistent
with the notion that digital structures are GaAs/GaMnAs superlattices.
This conclusion is supported by a combination of spectral weight
analysis and effective medium theory. The optical properties of DFH
also provide insights into the evolution of their magnetic properties
with GaAs spacing. In DFH a low-lying mobility gap materializes in the
energy dependent conductivity, a feature never previously observed to
coexist with ferromagnetism in III-V semiconductors.
Host: Mihaly
Friday October 8, 1:30PM
Peter Sutter
Brookhaven National Lab
New Approaches to High-Resolution Imaging and Spectroscopy in Scanning Tunneling Microscopy
Host: Allen
Friday October 15, 1:30PM
Marshall Newton
Brookhaven National Lab
Electron transfer kinetics and conductance through oligomeric molecules: similarities and differences
Host: Allen
Friday October 22, 1:30PM
Raymond Teller
Argonne National Lab
Condensed Matter Research at the Intense Pulsed Neutron Source (IPNS)
I will discuss the opportunities for condensed matter research at
Argonne National Laboratory's IPNS facility. IPNS is a DOE funded
national user facility; 75% of instrument time is allocated by a peer
review process to outside users. In a typical year IPNS hosts 250 users
(ca. 600 participants) conducting 400 experiments. We operate 26 weeks
per year and welcome novices as well as experts to visit our facility.
A suite of 13 neutron scattering instruments are capable of diffraction
(single crystal, powder as well as amorphous materials), spectroscopy,
small angle scattering and reflectometry experiments. I will describe
some recent results that highlight how neutrons provide unique insights
into materials and processes, including magnetic structures of
materials, thin films, the structure and dynamics of hydrogenous
materials, structures of macromolecules, self assembly processes, and
in-situ structural and spectroscopic measurements. More information can
be found at www.pns.anl.gov.
Host: Stephens
Wednesday October 27, 4:00PM
Nilay Pradhan
University of California, Irvine
Vibronic Spectroscopy of Oxide Supported C60 Molecules and Atomic Scale Transistor Action Observed with a Scanning Tunneling Microscope
The single molecular sensitivity of the STM to detect the vibrational motions of molecules has
now been extended to surfaces that are poorly conducting near zero bias. This was achieved by
first adsorbing the system (either isolated C60 molecules or C60 monolayers) on an ultrathin
alumina film formed by the partial oxidation of the NiAl(110) substrate. This makes it possible
to discern the successive vibronic excitations in the differential conductance (dI/dV) spectra
which occur as the sample bias is gradually raised. The spectra of isolated adsorbed molecules
were compared to those of molecules adsorbed as parts of monolayers. In both cases, the spectra
show variety, based on differences in adsorption geometry and the local electronic structure of
the oxide film. The applicability of the STM was further expanded by enabling it to function
as a three-terminal device. The role of the gate bias was simulated by the ionization of a single
impurity alkali metal atom adsorbed on a C60 monolayer prepared in the manner described above.
Due to the presence of the oxide film, the bias value at which the ionization occurs depends on
the lateral position of the tip with respect to the impurity as well as the tip-sample separation.
As a result, it becomes possible to regulate the current flowing through the junction by controlling
the position of the tip. Additional imaging and spectroscopic capability of the STM reveals the
role of the C60 bands and the symmetry of the tip in this process.
Host: Lukens
Friday October 29, 1:00PM
M. Isabel Alonso
Institut de Ciència de Materials de Barcelona,
Consejo Superior de Investigaciones Científicas,
Campus de la Universitat Autònoma de Barcelona, Bellaterra
Linear optical properties of anisotropic crystals studied by
spectroscopic ellipsometry
Technologically relevant materials are very often structurally anisotropic, with accordingly
anisotropic physical properties. Determination of the linear optical properties given by the
dielectric tensor in a general case is not well established yet. In my talk I will review the
analysis of ellipsometric spectra of general anisotropic materials from an experimental point
of view, illustrating the practical use of ellipsometry to measure the complex dielectric
tensor. Among the different examples I will present results in systems of reduced symmetry
where principal axes of the dielectric tensor are a priori unknown, as monoclinic PTCDA
(perylene tetracarboxylic dianhidride) and anthracene.
Host: Mendez
Friday November 12, 1:30PM Cancelled
J. W. Davenport
Brookhaven National Lab
Analytic Tight Binding Model: Application to NanoScale Metallic Clusters
It seems not to be widely recognized that the simple cubic s band tight
binding model can be exactly solved for finite clusters [1]. Even more
surprising is the generalization to fcc and bcc clusters [2]. I derive
these results in a novel way use them to calculate the density of states
(DOS) for clusters ranging up to one million atoms. For clusters larger
than 100,000 atoms, the DOS is essentially the same as the bulk. However,
in the 1000 - 100,000 atom range there are substantial differences between
the cluster and the bulk. The formulas are easily generalized to include 1
or 2 dimensions and mixed boundary conditions (periodic or finite) so that
flat films, tubes, and surfaces can also be studied.
[1] R. P. Messmer, Phys. Rev. B 15, 1811 (1977).
[2] G-H. Ryu and H. Kim, Bull. Korean Chem. Soc, 12, 544 (1991).
Host: Allen
Friday December 3, 1:30PM
Jessica Thomas
Department of Physics, Brookhaven National Lab
Resonant x-ray scattering in manganites
Like many transition metal oxides, manganites show a complex evolution of the magnetic and electronic properties with temperature, pressure, magnetic field and change in carrier concentration. The reason is that the macroscopic properties are highly sensitive to the "local" degrees of freedom associated with the manganese ion, such as the average valence, spin and crystal symmetry. Interactions between these various degrees of freedom often lead to correlated phases, such as long range magnetic, charge or orbital order.
In this talk, I will describe the use of resonant x-ray scattering to look at correlated phases in manganite single crystals and manganite-ferroelectric heterostructures. Diffraction measurements performed in the vicinity of a Mn absorption edge, particularly the 2p -> 3d absorption, enhance the sensitivity to the chemical and magnetic environment of the Mn valence electrons and provide a direct probe of orbital and spin order in manganites. Resonant magnetic scattering with circularly polarized light provides sensitivity to the ferromagnetic density near the interface of a manganite-ferroelectric heterostructure. This technique can therefore be used to study the modification of the magnetic properties in device structures.
Host: Stephens
Friday December 10, 1:30PM
Vasili Perebeinos
IBM - Yorktown Heights
Excitons and polarons in carbon nanotubes
Due to their low dimensionality, carbon nanotubes (CNTs) have striking
and novel properties, quite different from these of traditional
semiconductors, with important implications for technology. Performance
of electronic devices rely on mobility, which is extraordinary high at
low fields. However, at high fields mobility is dramatically reduced due
to the inelastic optical phonon scattering. Optical properties of CNTs,
essential for electro-optical devices, are dominated by excitons with
binding energies and oscillator strengths orders of magnitude larger
than those in traditional semiconductors. We calculate electronic
transport and optical properties of CNTs taking into account lattice
dynamics and electronic structure of CNTs.
* This work is a collaboration with Jerry Tersoff and Phaedon Avouris.
Host: Allen