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Seminars (AGAW03)

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Event When Speaker Title Presentation Material
AGAW03 10th April 2007
10:00 to 11:00
Quantum swimming

I shall describe a theory of quantum swimming developed jointly with Boris Gutkin and David Oaknin. The theory has the remarkable feature that swimming in a one dimensional Fermi sea at T=0 is quantized. The theory is closely related to quantum pumping and to adiabatic, time dependent scattering on graphs.

AGAW03 10th April 2007
11:30 to 12:30
Quantum fields and scale invariance on star graph

We construct quantum fields and vertex algebras on star graphs. Our construction uses a specific deformation of the canonical algebra, encoding the interaction at the vertex of the graph. Special attention is devoted to the scale invariant interactions, which determine the critical properties of the system. We classify the critical points and investigate their features. The physical observables, we focus on, are the Casimir energy density and the conductance. Among the examples illustrating nontrivial interactions in the bulk of the graph, we consider the nonlinear Schrodinger and the massless Thirring models.

AGAW03 10th April 2007
14:00 to 15:00
N March Electronic states in ordered and disordered quantum networks: with applications to graphene and to C and B nanotubes

The idea behing the quantum network (QN) model is simple enough. One joins each atom to its neighbours, and then treat electron (though quantum mechanically of course) as though they flowed through one-dimensional wires as in an electrical circuit obeying Kirfhhoff's Laws at every node. Here we will begin with two periodic systems: namely an infinite graphene layer and a two-dimensional sheet of boson atoms. This will be followed by a discussion of C and especially B nanotubes. Finally a brief summary will be given on the nature of the electronic states in a disorderd network, via an approximate treatment related to Boltzmann's equation.

AGAW03 10th April 2007
15:30 to 16:30
Quantum network model of zigzag carbon nanotube

We consider the quantum network which is a model for a zigzag carbon nanotube under assumption that the confinement potential created by sigma electrons restricts the pi electrons to the network. A parallel magnetic field and a wide range of periodic scalar potentials are taken into account. The spectral properties of the corresponding Schroedinger operator are analyzed using the Lyapunov function technique. We explicitly calculate the point spectrum and all localized eigenstates. We prove that there are stable spectral gaps, where the endpoints are periodic and anti-periodic eigenvalues, and resonance spectral gaps, where the endpoints are resonances; the asymptotics of all endpoints at high energies are provided. All finite gap potentials are described. The dependence of the spectrum on magnetic field is also investigated.

AGAW03 10th April 2007
16:30 to 17:30
On the spectra of carbon nano-structures

An explicit derivation of dispersion relations and spectra for periodic Schrödinger operators on carbon nano-structures (including graphene and all types of single-wall nano-tubes) is provided.

AGAW03 11th April 2007
09:00 to 10:00
Excitonic influence on transport coefficients in low dimensional quantum systems

This is joint work with P. Duclos and B. Ricaud. Consider a many-body fermionic Hamiltonian defined with periodic boundary conditions on a two dimensional torus (which models a very long and thin nano-ring). We will study the low lying spectrum of this operator, and show that in the Hartree-Fock approximation one can obtain effective one-particle models describing these particular states. Even though it sounds more physically oriented, the main purpose of this talk is to formulate clear mathematical problems related to PDEs, unbounded linear self-adjoint operators, and integral equations.

AGAW03 11th April 2007
10:00 to 11:00
Quantum transport in networks of weakly disordered metallic wires

I will consider the quantum transport in networks of weakly disordered metallic wires. Quantum interferences of reversed trajectories are responsible for a small contribution to the conductance, known as the "weak localization correction" (WL). From the experimental point of view the study of the WL provides an efficient tool to probe phase coherence in weakly disordered metals. The WL is identified through its magnetic field dependence: for example, the conductance of a ring presents oscillations as a function of the flux with period $h/2e$, known as Al'tshuler-Aronov-Spivak (AAS) oscillations. The contributions of interfering reversed trajectories are encoded in the so-called "Cooperon". I will show how the Cooperon must be properly integrated into a multiterminal network connected to reservoirs and will emphasize the role on nonlocality of quantum transport.

In a second part I will discuss the effect of decoherence due to electron-electron interaction and more specifically how the AAS oscillations are affected by electron-electron interaction in several networks.

AGAW03 11th April 2007
11:30 to 12:30
Localization under external interactions in periodic quantum graphs

We discuss the creation of eigenvalues in the T3-shaped quantum graph using magnetic field and Rashba interaction. For some combinations of parameters the whole spectrum degenerates into a series of isolated flat bands.

AGAW03 11th April 2007
15:00 to 16:00
Graphs which sound the same

After a short review of the conditions for unique spectral inversion for quantum graphs, I shall describe a method for constructing families of isospectral yet not isometric garphs: "graphs which sound the same". I shall then discuss the conjecture that graphs which sound the same can be resolved by the difference between their sequences of counts of nodal domains, and will present a proof that this is indeed the case for a simple yet non trivial example.

AGAW03 11th April 2007
16:00 to 17:00
V Kostrykin Inverse scattering problems for quantum graphs

The talk is devoted to inverse scattering problems for Laplace operators on metric graphs. Some possible applications to network design will also be discussed. The talk is based on a joint work with R. Schrader.

AGAW03 11th April 2007
17:00 to 18:00
On the skeleton method

In the spectral analysis of few one dimensional quantum particles interacting through delta potentials it is well known that one can recast the problem into the spectral analysis of an integral operator (the skeleton) living on the submanifold which supports the delta interactions. We shall present several tools which allow direct insights into the spectral structure of this skeleton. Application to effective models of excitons in nanotubes as well as some nets of quantum wires will be given. This is a work in progress with H. Cornean and B. Ricaud, see e.g.

Three quantum charged particles interacting through delta potentials}, Few-Body Systems 38(2-4), 125-131, 2006, ArXiv math-ph/0604003

AGAW03 12th April 2007
09:00 to 10:00
Perfect transfer of quantum information across graphs

Quantum information is encoded in quantum mechanical states of physical systems. Hence, reliable transmission of quantum information from one location to another entails the perfect transfer of quantum mechanical states between these locations. We consider the situation in which the system used for this information transmission consists of N interacting spins and we address the problem of arranging the spins in a network in a manner which would allow perfect state transfer over the largest possible distance. The network is described by a graph G, with the vertices representing the locations of the spins and the edges connecting spins which interact with each other. State transfer is achieved by the time evolution of the spin system under a suitable Hamiltonian. This can be equivalently viewed as a continuous time quantum walk on the graph G. We find the maximal distance of perfect state transfer and prove that the corresponding quantum walk exhibits an exponential speed-up over its classical counterpart.

AGAW03 12th April 2007
10:00 to 11:00
Optimal computation with noisy quantum walks

Quantum versions of random walks on the line and cycle show a quadratic improvement in their spreading rate and mixing times respectively. The addition of decoherence to the quantum walk produces a more uniform distribution on the line, and even faster mixing on the cycle by removing the need for time-averaging to obtain a uniform distribution. By calculating the entanglement between the coin and the position of the quantum walker, the optimal decoherence rates are found to be such that all the entanglement is just removed by the time the final measurement is made.

This requires only O(log T) random bits for a quantum walk of T steps.

AGAW03 12th April 2007
11:30 to 12:30
Finding eigenvalues and resonances of the Laplacian on domains with regular ends

In this joint work with Marco Marletta (Cardiff), we present a simple uniform algorithm for finding eigenvalues (if they exist) lying below or embedded into the continuous spectrum, as well as complex resonances, of the Laplace operator on infinite domains with regular ends - e.g. cylindrical.

AGAW03 12th April 2007
14:00 to 15:00
D Grieser Spectral asymptotics of the Dirichlet Laplacian on fat graphs

We investigate the behavior of the eigenvalues of the Laplacian, or a similar operator, on a family of Riemannian manifolds with boundary, called fat graphs, obtained by associating to the edges of a given finite graph cross-sectional Riemannian manifolds with boundary, and also to the vertices certain Riemannian manifolds with boundary, glueing them according to the graph structure, and scaling them by a factor of $\varepsilon$ while keeping the lengths of the edges fixed. The simplest model of this is the $\varepsilon$-neighborhood of a the graph embedded with straight edges in $R^n$.

We determine the asymptotics of the eigenvalues with various boundary conditions as $\varepsilon\to 0$ in terms of combinatorial and scattering data.

AGAW03 12th April 2007
15:30 to 16:30
Dirichlet eigenvalues in a narrow strip
We study the Dirichlet Laplacian. We also show that convergence of eigenvalues here is a consequence of some ‘generalized’ version of the convergence in norm of the resolvents. A modificaton of the standard resolvent convergence is necessary, since the operators ?e for different ?, as well as the operator H, act in different spaces.
AGAW03 12th April 2007
16:30 to 17:30
M Harmer Spin filtering and the Rashba effect

We discuss quantum graphs with the Rashba Hamiltonian with application to spin filtering

AGAW03 13th April 2007
09:00 to 10:00
Phase transitions on lattice graphs

We explain the random-cluster (or FK) representation of the classical Potts/Ising models, when the underlying graph is a lattice. There is a continuum version of the random-cluster model that may be used to study the quantum Ising model with transverse field. A selection of open problems will be included.

AGAW03 13th April 2007
10:00 to 11:00
Slow light in photonic crystals

The problem of slowing down light by orders of magnitude has been extensively discussed in the literature. Such a possibility can be useful in a variety of optical and microwave applications. Many qualitatively different approaches have been explored. Here we discuss how this goal can be achieved in linear dispersive media, such as photonic crystals. The existence of slowly propagating electromagnetic waves in photonic crystals is quite obvious and well known. The main problem, though, has been how to convert the input radiation into the slow mode without losing a significant portion of the incident light energy to absorption, reflection, etc. We show that the so-called frozen mode regime offers a unique solution to the above problem. Under the frozen mode regime, the incident light enters the photonic crystal with little reflection and, subsequently, is completely converted into the frozen mode with huge amplitude and almost zero group velocity. The linearity of the above effect allows the slowing of light regardless of its intensity. An additional advantage of photonic crystals over other methods of slowing down light is that photonic crystals can preserve both time and space coherence of the input electromagnetic wave.

AGAW03 13th April 2007
11:30 to 12:30
Simulation of quantum graphs by microwave networks

Experimental and theoretical study of irregular microwave graphs (networks) consisting of coaxial cables connected by the joints are presented. The spectra of the microwave networksare measured for bidirectional and directional microwave networks consisting of coaxial cables and Faraday isolators for which the time reversal symmetry is broken. In this way the statistical properties of the graphs such as the integrated nearest neighbor spacing distribution and the spectral rigidity are obtained.We show that microwave irregular microwave networks with absorption can be used to experimental studies of the distributions of Wigner's reaction K matrix. The distributions of the imaginary and real parts of K matrix, P(v) and P(u), respectively, are obtained from the measurements and numerical calculations of the scattering matrix S of the networks. We demonstrate that the experimental and numerical results are in good agreement with the theoretical predictions. Furthermore, we present the results of studies of parameter dependent velocity correlation functions in the graphs.

AGAW03 13th April 2007
14:00 to 15:00
Localized shelf waves on a curved coast - existence of eigenvalues of a linear operator pencil in a curved waveguide

The study of the possibility of the non-propagating, trapped continental shelf waves along curved coasts reduces mathematically to a spectral problem for a self-adjoint operator pencil in a curved strip. Using the methods developed in the setting of the waveguide trapped mode problem, we show that such continental shelf waves exist for a wide class of coast curvature and depth profiles. This is joint work with Ted Johnson (UCL) and Michael Levitin (Heriot-Watt)

AGAW03 13th April 2007
15:30 to 16:30
O Turek Approximations of strongly singular couplings at quantum graph vertices

Approximation of a general strongly singular coupling in the center of a star graph by means of a delta-coupling and delta-interactions will be presented. We will (i) formulate necessary conditions under which the approximation is possible; (ii) show a way how to realize such approximation; (iii) explain its character: we will demonstrate that the coupling is approximated in the norm-resolvent sense.

AGAW03 13th April 2007
16:30 to 17:30
Twisting versus bending in quantum waveguides

The Dirichlet Laplacian in tubular domains is a simple but remarkably successful model for the quantum Hamiltonian in mesoscopic waveguide systems. We make an overview of mathematical results established so far about the spectrum of the Dirichlet Laplacian in infinite curved three-dimensional tubes with arbitrary cross-section and mention consequences for the electronic transport. We focus on the interplay between bending and twisting as regards the existence of quantum bound states, associated with the discrete spectrum of the Laplacian. As the most recent result, we show that twisting of an infinite straight three-dimensional tube with non-circular cross-section gives rise to a Hardy-type inequality, with important consequences for the stability of the spectrum. We also discuss similar effects induced by curvature of the ambient space or switch of boundary conditions.

Related Links

University of Cambridge Research Councils UK
    Clay Mathematics Institute London Mathematical Society NM Rothschild and Sons