|speaker||Pau Gomez (Prof. Mitchell's group, ICFO)|
|title||A Spinor BEC Co-Magnetometer for Studies of Magnetic Symmetry Breaking|
The "co-magnetometer" is a technology developed for rotation sensing and searches for physics beyond the standard model, consisting of two different magnetically-sensitive systems operating in the same volume and thus experiencing the same magnetic field. A differential measurement can then reject true magnetic influences (which typically are strong and noisy), while sensitively detecting small signals that differently affect the two components. Here we report a spinor BEC (SBEC) co-magnetometer, with the two components being the F=1 and F=2 ground state populations, independently detected using Faraday rotation probing , an extension of our recently-reported single-domain magnetic SBEC . We study spin oscillation and spin amplification in F=2, using the F=1 component as a reference. This novel scheme gives accurate information on both the amplitude and phase of the F=2 SBEC as it rotates in a magnetic field, allowing tomographic study of spontaneous symmetry breaking, spin squeezing, and quantum entropy generation in a magnetically-polarized system. |
 M. Koschorreck et al., "Sub-projection-noise sensitivity in broadband atomic magnetometry", Phys. Rev. Lett. 104, 093602 (2010).
 S. Palacios et al., "Multi-second magnetic coherence in a single domain spinor Bose-Einstein condensate", preprint arXiv:1707.09607 (2017).
|speaker||Prof. Minoru Yamashita (ISSP, Univ. of Tokyo)|
|title||Universal thermal Hall conductivity of a kagome antiferromagnet|
|speaker||安藤 陽一 氏 （ケルン大学物理学科教授）|
|speaker||Prof. Ryuichi Shindou (Peking Univ.)|
|title||Theories of topological spin-nematic excitonic insulators in graphite under high magnetic field and quantum multicriticality in disordered Weyl semimetal|
In the first part of my talk, I will discuss our phenomenological
theory for metal-insulator transitions in graphite under high
magnetic field [1,2]. Graphite under high magnetic field exhibits
consecutive metal-insulator (MI) transitions as well as re-entrant
insulator-metal (IM) transition at low temperature. We explain
these enigmatic insulator phases as manifestation of topological
excitonic insulator phases with spin nematic orderings.
We first argue that graphite under high magnetic field (> 20 Tesla)
is in the charge neutrality region. Based on this observation,
we employ models with electron and hole pocket(s), to construct
a bosonized Hamiltonian that comprises of displacement field
along the field direction and its conjugate fields. Using a renormalization
group argument, we show that there exists a critical interaction
strength above which a umklapp term becomes relevant and the
system enters excitonic insulator phase with a long-range ordering
of spin superfluid phase field, i.e. "spin nematic excitonic insulator
(SNEI)". We argue that, when a pair of electron and hole pockets get
smaller in size, a quantum fluctuation of the spin superfluid phase
becomes larger and destabilizes the excitonic insulator phases, which
results in the re-entrant IM transition. We explain field- and
temperature-dependences of in-plane resistivity in graphite experiment
by surface transports via novel surface states in topological
SNEI phases [1,2].|
In the second part of my talk, I will discuss our recent theory on multicriticality in disordered Weyl semimetal [3,4]. In electronic band structure of solid state material, two band touching points with linear dispersion (called as `Weyl node') appear in pair in the momentum space (`Nielsen-Ninomiya' theorem). When they annihilate with each other, the system undergoes a quantum phase transition from three-dimensional Weyl semimetal (WSM) phase to a band insulator (BI) phase. The phase transition is described by a new critical theory with a `magnetic dipole' like object in the momentum space. We reveal that the critical theory hosts a novel disorder-driven quantum multicritical point, which is encompassed by three quantum phases, WSM phase, BI phase, and diffusive metal (DM) phase. Based on the renormalization group argument, we first clarify scaling properties around the band touching points at the quantum multicritical point as well as all phase boundaries among these three phases . We argue that a localization-delocalization transition between the BI phase and a WSM phase is controlled by a clean-limit fixed point with spatially anisotropic scale invariance. We show that the anisotropic scale invariance is reflected on unconventional scaling function forms in the quantum phase transition between BI and WSM phases. We verify the proposed scaling function forms in terms of transfer-matrix calculations of conductance and localization length in the tight-binding model .
 Z. Pan, X.-T. Zhang, and R. Shindou, arXiv:1802.10253.
 in preparation
 X. Luo, T. Ohtsuki, and R. Shindou, arXiv:1803.09051.
 X. Luo, B. Xu, T. Ohtsuki, and R. Shindou, Phys. Rev. B 97, 045129 (2018).
|speaker||Prof. Nir Navon (Yale Univ.)|
|title||Turbulence in a quantum gas|
Many turbulent flows form so-called cascades, where excitations injected at
large length scales, are transported to gradually smaller scales until they
reach a dissipation scale. We initiate a turbulent cascade in a dilute Bose
fluid by pumping energy at the container scale of an optical box trap using
an oscillating magnetic force [1,2]. In contrast to classical fluids where
the dissipation scale is set by the viscosity of the fluid, the turbulent
cascade of our quantum gas finishes when the particles kinetic energy
exceeds the laser-trap depth. This mechanism allows us to effectively
tune the dissipation scale where particles (and energy) are lost, and
measure the particle flux in the cascade at the dissipation scale. Our
measurements are in very good agreement with simulations of the Gross-
Pitaevskii equation including dissipation.|
 A.L. Gaunt, T.F. Schmidutz, I. Gotlibovych, R.P. Smith, Z. Hadzibabic, Phys. Rev. Lett. 110, 200406 (2013).
 N. Navon, A.L. Gaunt, R.P. Smith, Z. Hadzibabic, Nature 539, 72 (2016) .