Sommerfeld Lecture Series (ASC)

Seventeenth Arnold Sommerfeld Lecture Series, Our understanding of simple solids, is firmly grounded on the Fermi

liquid concept and powerful computational techniques built around the

density functional theory. These ideas form the basis of our “standard

model” of solid state physics and have provided us with an accurate

description of many materials of great technological significance.

Correlated electron systems are materials for which the the standard

model of solid state physics fails dramatically. The best known example

being the copper oxide high temperature superconductors. Correlated

electron materials continue to be discovered accidentally and surprise us

with their exceptional physical properties and their potential for new

applications. The most recent example is provided by the iron arsenide

based high temperature superconductors.

From a theoretical perspective describing strongly correlated electron

systems pose one of the most difficult non-perturbative challenges in

physics. In this colloquium I will give an elementary introduction to the

field of strongly correlated electron materials and Dynamical Mean Field

Theory (DMFT) a non perturbative method which provides a zeroth order

picture of the strong correlation phenomena in close analogy with the

Weiss mean field theory in statistical mechanics. Applications materials

containing f and d electrons will be presented to show how the

anomalous properties of correlated materials emerge from their atomic

constituents.

I will conclude with an outlook of the challenges ahead and the

perspectives for a rational material design.

Seventeenth Arnold Sommerfeld Lecture Series, Strongly correlated metals exhibit anomalous transport properties

which have puzzled condensed matter physicists for many years.

They are characterized by large resistivities which exceed the Mott

Ioffe Reggel limit and large thermoelectric responses, which cannot

be explained in terms of standard Fermi liquid quasiparticles.

Dynamical Mean Field Theory (DMFT) calculations [1,2] carried out

on a doped one band Hubbard model suggest that this behavior

originate in the strong temperature dependence of thee parameters

of the underlying resilient (non-Landau) quasiparticles.

We will test these ideas by analyzing low energy optical spectroscopy

measurements in several prototypical compounds starting with

the archetypal correlated material Sesquioxide V2O3. We will also

show first principles, material specific, LDA+DMFT calculations

which are in very good agreement with the experiments [3].

Seventeenth Arnold Sommerfeld Lecture Series, Superconductivity is a state of matter where electrons can flow without

resistance and where magnetic fields are expelled. It was discovered

serendipitously more than a hundred years ago. Today, superconductors

are essential components of medical imaging devices as well as high

energy particles accelerators.

Understanding this phenomena was one of the greatest intellectual

challenges of the twentieth century. A dramatic advance was provided by

the BCS (Bardeen Cooper Schrieffer) theory 45 years after. It posits that

superconductivity is the result of macroscopic condensation of electron

pairs, which are held together by the vibrations of the lattice. The condensate

is a macroscopic quantum objects and its rigidity accounts for its

striking macroscopic properties.

The BCS theory was so successful that by the early 70’s superconductivity

was considered a completely understood subject with the maximum

achievable critical temperature having been reached experimentally

around 30K. In the late 80’s this field of research took a dramatically turn

with the discovery of new ceramic compounds which superconduct at

temperatures as high as 160 K. These materials, cannot be described by

straightforward extensions of the BCS theory. Scientists are still working

on finding new explanations for these materials and we will describe the

challenge they pose. The quest for room temperature superconductivity

thus continues. A breakthrough in this field would have unimaginable

consequences, changing the way we transmit electricity from its

generation to its consumption to the way we design computers.

Fourteenth ASC Lecture Series, Does the world embody beautiful ideas? Pythagoras and Plato intuited that it should, Newton and Maxwell showed, in impressive examples, how it could. Modern physics demonstrates, in depth and detail, that it does. I will narrate, through notable examples, how the concept of beauty in physical law has evolved – and how it continues to guide our quest for ultimate understanding.

Thirteenth ASC Lecture Series, This talk is devoted to quantum propagation of dipole excitations in two dimensions in the presence of disorder. This problem differs from the conventional Anderson localization due to existence of long range hops. We found that the critical wave functions of the dipoles always exist which manifest themselves by a scale independent diffusion constant. If the system is T-invariant the states are critical for all values of the parameters. Otherwise, there can be a “normal metal - perfect metal" transition between this “ordinary" diffusion and the Levy-flights (the diffusion constant logarithmically increasing with the scale). These results follow from the two-loop analysis of the modified non-linear supermatrix

Thirteenth ASC Lecture Series, During more than 100 years of its history Quantum Mechanics passed all of the experimental

checks and transformed itself from a counterintuitive concept to the undisputable foundation of the modern physics. Along with this it did not lose its ability to surprise and still allows for new astonishing discoveries such as Bose-Einstein condensation of ultracold gases. Manifestations of the quantum mechanics on the macroscopic scales are especially impressive. In recent years the interest in condensed matter physics evolved from studying bulk properties of naturally occurring materials to constructing complex materials and systems not found in nature, and controlling rather than observing quantum mechanics. Within this tendency the concept of quantum condensation remains the central one.

Controllable quantum behavior can be achieved in systems of weakly coupled locally coherent elements. An array of Josephson junctions between superconducting islands is a representative but not the exclusive example. Other examples of such systems are ultracold gases in optical lattices, excitons and photons in semiconductor cavities, etc. Global phase coherence exists in these systems can be destroyed by reducing the coupling. In Josephson arrays this destruction is manifested by the phase transition from superconducting to insulating state.

This talk is about the relation between the classical and the quantum worlds. Some of the quantum effects, e.g. interference, can be realized in classical systems, others like Einstein- Podolsky-Rosen paradox are “truly quantum”. It turns out that the quantum condensation has a classical analog: synchronization (mode-locking) in nonlinear dynamics. Discovered by Huygens almost 350 years ago the synchronization is the most fundamental nonlinear phenomenon. However the synchronization happens when the system is driven by outside forces, while one can think about BEC in thermodynamic equilibrium. On the other hand quantum systems can be also driven. One of the familiar examples is coherent state of photons

generated by a laser: this generation happens only in the presence of a pumping and does not exist in the equilibrium. The interest to the quantum systems out of equilibrium is rapidly growing due to the desire to control and manipulate quantum states. I will discuss the similarities between macroscopic quantum and classical behaviors. It looks like new interesting physics emerges on the crossroads of the quantum mechanics, condensed matter physics, and nonlinear dynamics.

Twelfth Arnold Sommerfeld Lecture Series, The last three decades have witnessed the discovery of many new superconductors, with properties dramatically different from the conventional low temperature superconductors described by the Bardeen-Cooper- Schrieffer theory. These new superconductors can have much higher critical temperature, and all display antiferromagnetism in their phase diagrams. I will introduce the theory of quantum phase transitions, and use it to interpret recent experiments on these materials.

Twelfth Arnold Sommerfeld Lecture Series, String theory was originally constructed as a unification of the quantum field theory of elementary particles with Einstein's theory of gravitation. Unexpectedly, it has led to the discovery of new "dualities" which have given us a new perspective on quantum field theories not coupled to gravity. Some of the latter theories are relevant to the strongly-interacting quantum many body problems of condensed matter physics. I will survey some of the challenging open problems associated with condensed matter experiments, and discuss the insights gained from string theory.

Twelfth Arnold Sommerfeld Lecture Series, In many modern materials, electrons quantum‐entangle with each other across long distances, and produce new phases of matter, such as high temperature super‐conductors. We face the challenge of describing the entanglement of 10^{23} electrons, which is being met by many ideas, including some drawn from string theory.

Eleventh ASC Lecture Series, Cosmic strings are linear defects that could be formed at a phase transition in the early universe. Strings are predicted in a wide class of particle physics models. In particular, fundamental strings of superstring theory can have astronomical dimensions and play the role of cosmic strings. I will discuss recent progress in understanding the evolution of cosmic strings and possible ways of detecting them.