Sommerfeld Lecture Series (ASC)

Eleventh ASC Lecture Series, Recent developments in cosmology suggest that the big bang was not a unique event in the cosmic history. Other big bangs constantly erupt in remote parts of the uni- verse, producing new worlds with great variety of physical properties. Some of these worlds are similar to ours, while others are strikingly different and even obey different laws of physics. I will discuss the origin of this new worldview, its possible observational tests, and some of its bizarre implications.

Sommerfeld Lecture Series, According to the Landau description of Fermi liquids, low- energy excitations in metals are constructed out of quasiparticles – long-lived excitations which have the same quantum numbers as those of an electron in vacuum. In metals with strong correlations however, quasiparticles become fragile: they are destroyed above a characteristic energy or temperature scale, the quasiparticle coherence scale. This energy scale can be remarkably low, even in materials which are not close to a Mott metal-insulator transition, for example as a result of the Hund's rule coupling. I will provide evidence that this is relevant for many materials, especially oxides of the 4d transition metals. In other materials, such as cuprates, quasiparticles are destroyed selectively in specific regions of momentum-space. The understanding of charge and thermal transport in such ``bad metals'' is a key issue, with both fundamental and practical implications.

Sommerfeld Lecture Series, From copper-oxide superconductors to rare-earth compounds, materials with strong electronic correlations have focused enormous attention over the last two decades. Solid-state chemistry, new elaboration techniques and improved experimental probes are constantly providing us with examples of novel materials with surprising electronic properties, the latest example being the recent discovery of iron-based high-temperature superconductors.

In this colloquium, I will emphasize that the classic paradigm of solid-state physics, in which electrons form a gas of wave-like quasiparticles, must be seriously revised for strongly correlated materials. Instead, a description accounting for both atomic-like excitations in real-space and quasiparticle excitations in momentum space is requested. I will review how Dynamical Mean-Field Theory -an approach that has led to significant advances in our understanding of strongly correlated materials- fulfills this goal.

New frontiers are also opening up, which bring together condensed-matter physics and quantum optics. `Artificial materials' made of ultra-cold atoms trapped by laser beams can be engineered with a remarkable level of controllability, and allow for the study of strong- correlation physics in previously unexplored regimes.

Sommerfeld Lecture Series, Which property of a material is more familiar to us than its color? And yet, the strange laws of quantum mechanics, which rule atoms, electrons and photons, are key to the understanding of this most beautiful feature! The invention and engineering of novel materials has shaped human civilization, from the Bronze age to the Silicon age. This lecture is an invitation to explore materials down to the scale of their intimate constituents – atoms and electrons. We'll address questions such as: do we master quantum mechanics well enough today to explain how materials behave from the only knowledge of the atoms which build them? Have we reached the stage where the principles of quantum mechanics allow for the design of a novel material with specific functionalities?

Advances in light sources and time resolved spectroscopy have made it

possible to excite specific atomic vibrations in solids and to observe the

resulting changes in electronic properties. I argue that in narrow-band

systems the dominant symmetry-allowed coupling between electron density

and dipole active modes implies an electron density-dependent squeezing of

the phonon state which provides an attractive contribution to the electron-electron

interaction, independent of the sign of the bare electron-phonon

coupling and with a magnitude proportional to the degree of laser-induced

phonon excitation. Reasonable excitation amplitudes lead to non-negligible

attractive interactions that may cause significant transient changes in

electronic properties including superconductivity. The mechanism is

generically applicable to a wide range of systems, offering a promising route

to manipulating and controlling electronic phase behavior in novel materials.

This talk will present an overview of recent progress towards a solution of one

of the grand-challenges of modern science: understanding the properties of

interacting electrons in molecules and solids. After an introduction to the

physics I will argue our theoretical understanding of a basic model system,

the two dimensional Hubbard model, has reached the level that we can say

with confidence that its superconducting properties capture key aspect of the

high-Tc superconductivity in copper-oxide materials. I will then summarize

the current status of our extension of the methods to fully physically realistic

systems, emphasizing the areas of theoretical uncertainty and the prospects

for resolution.

Superconductivity, the ability of certain materials to conduct electricity with

no resistance whatsoever, has fascinated scientists since its discovery by

Kammerlingh-Onnes in 1911. While much has been understood, the question

of predicting which materials will become superconducting, and at what

temperatures, remains one of the grand challenges of modern materials

theory. This talk will outline the evolution of our understanding as the subject

has progressed from its primitive beginnings through the ''bronze age''

marked by the 1986 discovery of high temperature superconductivity in

copper-oxide compounds to the present-day ''iron age'' of the Fe-As based

superconducting materials. The current status of the theory of the origin of

superconductivity will be described.

A combination of ideas originating from Condensed Matter physics, Supersymmetric Field Theory, and AdS/CFT has led to a detailed web of conjectured dualities. These relate the long distance behavior of different short distance theories. These dualities clarify a large number of confusing and controversial issues in Condensed Matter physics and in the study of 2+1 dimensional quantum field theory.

Global symmetries and gauge symmetries have played a crucial role in physics. The

idea of duality demonstrates that gauge symmetries can be emergent and might not

be fundamental. During the past decades it became clear that the circle of ideas

about emergent gauge symmetries and duality is central in different branches of

physics including Condensed Matter Physics, Quantum Field Theory, and Quantum

Gravity. We will review these developments, which highlight the unity of physics.

In recent decades, physicists and astronomers have discovered two beautiful

Standard Models, one for the quantum world of extremely short distances, and one

for the universe as a whole. Both models have had spectacular success, but there are

also strong arguments for new physics beyond these models. In this lecture, we will

review these models, their successes and their shortfalls. We will describe how

experiments in the near future could point to new physics suggesting a profound

conceptual revolution, which could change our view of the world.