第5回 物理工学科教室談話会(講師:Dr. Cristian D. Batista)

日時: 2015年3月26日(木) 15:00 – 16:30
場所: 工学部6号館1階大会議室
講師: Dr. Cristian D. Batista (Theoretical Division of Los Alamos National Laboratory)
題目: Vortex Crystals in Frustrated Mott Insulators

The Bose-Einstein condensate is the simplest example of an ordered quatum state of matter. Since its discovery in 1924, this phenomenon has captivated several generations of physicists, including the most recent ones. The Nobel Prize in Physics 2001 that was awarded jointly to Eric A. Cornell, Wolfgang Ketterle and Carl E. Wieman for the achievement of Bose-Einstein condensation in dilute gases of alkali atoms is a testimony of this long standing interest.
An attractive aspect of this state of matter resides in its multiple incarnations, which include superconductivity (condensation of Cooper pairs) and superfluidity (condensation of neutral atoms). Quantum magnets provide an alternative realization of bosonic gases because magnetic excitations obey Bose statistics. An attractive aspect of these particular incarnations is that the single-particle kinetic energy can exhibit minima at multiple wave-vectors whenever the magnet exhibits a property known as frustration. In this situation, the bosons can in principle condense in a state that is a linear combination of Bloch waves with the different wave-vectors that minimize the single-particle dispersion. The particular linear combination is dictated by the interactions between bosons.
By considering different realizations of highly frustrated quantum magnets (Mott insulators) that appear in nature, I will demonstrate that Bose-Einstein condensation of magnetic excitations in n-fold symmetric lattices (with n≥3) can lead to spontaneous magnetic vortex crystals analogous to the Abrikosov vortex lattices that appear in type II superconductors under the effect of an external magnetic field. We will see that electric and spin currents circulate around the cores of these magnetic vortices and that they also induce an electronic charge redistribution or charge density wave.