Recently there have been significant theoretical and experimental efforts to understand and identify the so-called topological phases of matter in interacting electron systems. These topological phases may be characterized by different kinds of topological properties such as non-trivial edge/surface states and/or unusual elementary excitations in the bulk or surface. Notable examples include quantum spin liquids, topological insulators, and other closely related phases. One of the main challenges is to come up with theoretical criteria that can be used to identify or predict correlated materials that hold promise for the emergence of such topological phases. We discuss recent theoretical and experimental developments in this direction, along with a brief introduction to some of the proposed topological phases. In particular, we focus on correlated materials with strong spin-orbit coupling and/or near a metal-insulator transition. 

Refreshments will be served in CP 179 at 3:15 PM

A revision of our system of units, the SI, is currently discussed and may be implemented as early as 2018. The new SI is a logical extension of an argument made in 1983 when the meter was redefined to be based on the exact value of the speed of light. In the new SI all units will be derived from seven fundamental reference constants, thus replacing the seven base units of the current system. For example, the unit of mass, the kilogram, is currently defined by an artifact called the International Prototype of the Kilogram (IPK). In the future we will be able to realize the unit of mass, not just at the kilogram level, from a fixed value of the Planck constant, which has units of kg m^2/s. One condition for redefinition is agreement between different measurements of the Planck constant. Currently two measurement strategies lead to values with relative uncertainties less than 100 parts per billion (ppb): (1) Avogadro’s number can be determined by estimating the number of atoms in a well characterized crystal. From Avogadro’s number h can be calculated using the Rydberg constant, which is known with much smaller uncertainty (2) A watt balance can be used to measure mechanical power in units of electrical power. Electrical power can be measured as the product of the Planck constant and two frequencies by utilizing the Josephson effect and the Quantum Hall effect. NIST has carried out measurements of h with watt balances for over 20 years. In the past 18 months a new team has performed a largely independent determination of h. I will describe this measurement and measurements from other laboratories.
 

Refreshments will be served in CP 179 at 3:15 PM

Since Tony Skyrme's discovery of skyrmions in particle physics in the 1960's, its notion has been generalized to a certain type of mathematical object (topologically stable whirls) that are realized in different areas of physics. This colloquium focuses on magnets without inversion symmetry, like MnSi, where in 2009 a skyrmion crystal was observed as a new magnetic state. In recent years, magnetic skyrmions have attracted a great deal of interest as they have been found in different materials (metals, semiconductors, and even insulators), and on different length and temperature scales. Furthermore, the peculiar twist of the magnetization in the skyrmion crystal leads to a very efficient coupling to electric currents which makes it also interesting for spintronics. We study the interplay of electric currents and skyrmions as well as the induced forces onto each other. Very characteristic for the skyrmion crystal is a finite and quantized Magnus force which can be understood in close analogy to the Magnus force acting on a spinning ball leading to famous “banana kicks” in soccer.
 

Refreshments will be served in CP 179 at 3:15 PM