Sommerfeld Theory Colloquium (ASC)

Sommerfeld Theory Colloquium, To pinpoint the microscopic mechanism for superconductivity has

proven to be one of the most outstanding challenges in the physics of

correlated quantum matter. Thus far, the most direct evidence for

an electronic pairing mechanism is the observation of a new symmetry

of the order-parameter, as done in the cuprate high-temperature

superconductors. Like distinctions based on the symmetry of a locally

defi�ned order-parameter, global, topological invariants allow for

a sharp discrimination between states of matter that cannot be transformed

into each other adiabatically. Here we propose an unconventional

pairing state for the electron

uid in two-dimensional oxide

interfaces and establish a direct link to the emergence of nontrivial

topological invariants. Topological superconductivity and Majorana edge states can then be used to detect the microscopic origin for

superconductivity. In addition, we show that also the density wave

states that compete with superconductivity sensitively depend on the

nature of the pairing interaction. Our conclusion is based on the special

role played by the spin-orbit coupling and the shape of the Fermi

surface in SrTiO3/LaAlO3-interfaces and closely related systems.

Sommerfeld Theory Colloquium

Sommerfeld Theory Colloquium, The role of Type Ia supernovae in observational cosmology has evolved

from being ”avant-garde” in the early 1990’s until today’s mature status of precision cosmology. Several large transient surveys have been detecting supernovae routinely, near and far, with the aim of probing what is causing the accelerated expansion of the Universe.

With time, the focus has changed towards addressing the intricacies of astrophysical effects that could bias the fits of cosmological parameters,

most notably the nature of dark energy. In this talk, aimed at high-energy physics theorists(!), I will try to convey the status of

the field, high-lighting some progress and set-backs, based on studies

of the closest Type Ia SN in modern time that exploded in the

beginning of 2014 in the near-by galaxy M82.

Sommerfeld Theory Colloquium, Various aspects of lattice gauge theory will be briefly discussed including, general principles, sources of systematic errors, dynamical fermions, QCD phenomenology, the FLAG project and, if time allows, some applications of lattice theory to other non-perturbative BSM phenomena.

Sommerfeld Theory Colloquium, Equivalence principle violation, deviations from the inverse-square

law of gravity, and long-range interactions between particle spins

o�er powerful tests of beyond-the-standard-model physics. Using a

torsion pendulum, a technique that dates to the 18th century, is still

the most sensitive way to search for these signatures of new physics.

I will talk about how the experiments are done and give an update on the current and future status of such torsion balance experiments in the Eot-Wash group.

Sommerfeld Theory Colloquium, Several new techniques are currently being employed to probe the strong gravitational fi�eld in the vicinity of black holes. Long baselineinterferometry at sub-millimeter wavelengths sets constraints on the

silhouette of the black holes in the Galactic center (SgrA*) and M87.

Stars which get tidally disrupted as they orbit too close to a single

black hole are being discovered at cosmological distances. Electromagnetic counterparts of black hole binaries in galaxy mergers are being identifi�ed, and can be used to calibrate the rate of gravitational wave sources. Most interestingly, the recoil induced by the

anisotropic emission of gravitational waves in the �nal plunge of binaries leaves unusual imprints on their host galaxies. Finally, the

lecture will describe new constraints on the contribution of primordial black holes to the dark matter.

Sommerfeld Theory Colloquium, Three-dimensional Einstein gravity has no local dynamical degree of

freedom. Yet, it is far from being trivial when the cosmological constant is negative. (i) It admits black hole solutions. (ii) It easily allows for consistently interacting and tractable higher-spin extensions.

(iii) It possesses remarkable asymptotic properties at infi�nity where

an infi�nite-dimensional symmetry algebra emerges. These features

make three-dimensional gravity a perfect "theoretical laboratory" in which to explore the conceptual issues related to (i), (ii) and (iii) in

a simpler context. The talk will not only discuss three-dimensional

gravity assuming no previous knowledge on the subject, but will also

provide access to recent work where new results on points (i), (ii)

and (iii) are developed.

Sommerfeld Theory Colloquium, In this seminar I will describe some of the hidden structures recently

discovered in the scattering amplitudes of elementary particles, such

as those measured at the Large Hadron Collider. These structures are

responsible for the mysterious simplicity of these quantities, which is

completely obscured by a calculation based on textbook techniques

such as Feynman diagrams. I will then move on to discuss form factors. These are slightly off-shell quantities and, similarly to amplitudes,

are also much simpler than what expected based on conventional

approaches. In particular I will focus on form factors of particular (half-BPS) operators in a special theory, known as N=4 super Yang-Mills, and briefly discuss some unexpected connections

to scattering amplitudes in phenomenologically relevant theories.

Sommerfeld Theory Colloquium, In the standard picture of a quantum phase transition, a single quantum critical point separates the phases at zero temperature. Here

we show that the two-dimensional case is considerably more complex. Instead of the single point separating the antiferromagnet from

the normal metal, we have discovered a broad region between these two phases where the magnetic order is destroyed but certain areas of the Fermi surface are closed by a large gap. This gap reflects the formation of a novel quantum state characterized by a superposition of d-wave superconductivity and a quadrupole density wave

(QDW), which builds a checkerboard pattern with a period incommensurate with that of the original spin density wave. At moderate temperatures both orders co-exist over comparatively large distances but thermal fluctuations destroy the long-range order. Below

a critical temperature the

fluctuations are less essential and super-

conductivity becomes stable. Applying a magnetic field destroys the superconductivity but establishes QDW. In addition to these phases

we obtain also a charge density wave (CDW) arising as a result of interaction of electrons with superconducting fluctuations. This phase

is possible when the superconductivity is destroyed by either thermal fluctuations or a magnetic field. The results of our theory can serve as explanation of recent experiments on cuprates performed with the help of STM, NMR, hard and resonant soft X-ray scattering, sound

propagation, and other techniques.

Sommerfeld Theory Colloquium, Einstein is well known for his rejection of quantum mechanics in the form it emerged from the work of Heisenberg, Born and Schrodinger in 1926. Much less appreciated are the many seminal contributions

he made to quantum theory prior to his �final scientifi�c verdict, that

the theory was at best incomplete. In this talk I present an overview

of Einsteins many conceptual breakthroughs and place them in historical context. I argue that Einstein, much more than Planck, introduced the concept of quantization of energy in atomic mechanics.

Einstein proposed the photon, the fi�rst force-carrying particle discovered for a fundamental interaction, and put forward the notion of wave-particle duality, based on sound statistical arguments 14 years before De Broglies work. He was the fi�rst to recognize the intrinsic

randomness in atomic processes, and introduced the notion of transition probabilities, embodied in the A and B coeffi�cients for atomic emission and absorption. He also preceded Born in suggesting the interpretation of wave fi�elds as probability densities for particles, photons, in the case of the electromagnetic �field. Finally, stimulated by

Bose, he introduced the notion of indistinguishable particles in the

quantum sense and derived the condensed phase of bosons, which is

one of the fundamental states of matter at low temperatures. His

work on quantum statistics in turn directly stimulated Schrodinger

towards his discovery of the wave equation of quantum mechanics. It

was only due to his rejection of the �final theory that he is not generally recognized as the most central �figure in this historic achievement

of human civilization.