Leopold Infeld Colloquium
2006/2007 | 2007/2008 | 2008/2009 | 2009/2010 | 2010/2011 | 2011/2012 | 2012/2013 | 2013/2014 | 2014/2015 | 2015/2016 | 2016/2017 | 2017/2018
Understanding the origin of ferromagnetism in semiconductors
To memory of Jan Gaj (1943-2011)
In course of the years, the origin of spontaneous magnetisation that has been observed in numerous semiconductors and oxides has arguably become one of the most controversial topics in the contemporary physics of condensed matter. After a general introduction to spintronics and magnetically doped semiconductors, I will argue [1] that surprising properties of these systems have two distinct roots (i) an intricate interplay between hole-mediated ferromagnetism and AndersonMott localisation and (ii) a highly non-random distribution of magnetic cations driven by a significant contribution of open d shells to the cohesive energy.
[1] see, T. Dietl, Nature Mat. 9, 965 (2010), and references therein.
Magnetic and color superfluid order in multiflavor Fermi gases
i) We investigate antiferromagnetic ordering of trapped spin-1/2 fermions using large-scale dynamical mean-field theory simulations. We find a clear experimental signature - enhanced double occupancy - for the onset of magnetic order at low temperatures in current experiments.
ii) We study the properties of three-flavor fermions in an optical lattice, where new exotic quantum states such as color superfluids arise in partial analogy to Quantum Chromodynamics. Low-temperature properties of this system are addressed using DMRG and dynamical mean-field theory. Wefind a strong interplay between magnetization and color superfluidity.
The size of the proton
The charge radius Rp of the proton has so far been known only with a surprisingly low precision of about 1% from both electron scattering and precision spectroscopy of hydrogen.
We have recently determined Rp by means of laser spectroscopy of the exotic "muonic hydrogen" atom. Here, the muon, which is the 200 times heavier cousin of the electron, orbits the proton with a 200 times smaller Bohr radius. This enhances the sensitivity to the proton's finite size tremendously.
Our new value Rp = 0.84184 (67) fm is ten times more precise than the generally accepted CODATA value, but it differs by 5 standard deviations from it. A lively discussion about possible solutions to the "proton size puzzle" has started.