Magnetic vortex crystals in frustrated Mott insulator

Quantum fluctuations become particularly relevant in highly frustrated quantum magnets and can lead to new states of matter. We provide a simple and robust scenario for inducing magnetic vortex crystals in frustrated Mott insulators. By considering a quantum paramagnet that has a gapped spectrum with six-fold degenerate low energy modes, we study the magnetic field induced condensation of these modes. We use a dilute gas approximation to demonstrate that a plethora of multi-$\mathbf{Q}$ condensates are stabilized for different combinations of exchange interactions. This rich quantum phase diagram includes magnetic vortex crystals, which are further stabilized by symmetric exchange anisotropies. Because magnetic skyrmion and domain wall crystals have already been predicted and experimentally observed, this novel vortex phase completes the picture of emergent crystals of topologically nontrivial spin configurations.

In this talk, I will show how atom-cavity arrays can be used to build quantum simulators and quantum metamaterials. As case studies, I will show how atom-cavity arrays can simulate semiconductor properties, and be used to build a cloaking device and quantum superlens.  Such a situation may happen for spin-orbital models and for ultracold alkaline-earth atoms in optical lattices.
Using a combination of analytical and numerical methods, we studied a number of such cases.
In the seminar, I will present three interesting cases:   [1][2][3]. Competition between Dzyaloshinskii-Moriya and ferromagnetic exchange interactions leads to skyrmion lattices in a class of materials that share a common crystal structure.This includes insulators, like Cu2OSeO3, that allow for energetically efficient manipulation of the magnetic textures with electric field gradients [2]. Here we propose a novel mechanism for the stabilization of magnetic vortex crystals in frustrated Mott insulators that enables tunable spin superstructures [4]. By modeling the frustrated quantum magnet Ba3Mn2O8 [5] near its magnetic field-induced quantum critical point, we show that the quantum phase diagram includes novel magnetic vortex crystals, whose lattice parameter is controlled by the ratio between inter and intra-layer exchange.
This property opens the attractive possibility of tuning the vortex density by applying pressure.
Here, we address this issue. We clarify that the winding number which characterizes the bulk Z non-triviality of these systems can appear in electromagnetic and thermal responses in a certain class of heterostructure systems.
Furthermore, we also elucidate that the winding number can be detected as a bulk response function for a novel magnetoelectric effect, i.e. "chiral charge polarization" induced by an applied magnetic field. This talk will be concerned with how emergent electronic order influences spin dynamics in complex antiferromagnetic oxides. Advances in neutron spectroscopy have made it possible to measure the complete spectrum of cooperative spin excitations in magnetically ordered systems in great detail. I will illustrate how studies of the spin dynamics can provide key insights into the nature of complex ground states. I will present recent results on a half-doped layered manganite which conclusively distinguish between different models proposed for its ground state [1], and I will show how a striking hour-glass magnetic spectrum found in layered cobalt oxides sheds light on the existence of charge stripe correlations in the copper oxide superconductors [2].   In this talk, I will describe a low-energy effective theory for this magnet in terms of a lattice gauge theory with the simplest possible mathematical structure (a group of two elements, namely Z2). I will show that the theory reproduces many characteristic features observed numerically, thereby providing a missing link between the numerics and the analytics. Furthermore, I will present theoretical predictions which could be tested in future numerical studies.   biotechnology. I will discuss how the interplay between ionic liquid structure, transport properties and dynamics affects the outcomes of chemical reactions in these novel solvents. I will present some of our recent results on photoinduced electron-transfer reactions in ionic liquids, as well as structural studies of the bulk fluids using a combination of high energy X-ray scattering and molecular dynamics simulations. To complement these structural studies, we are using 2D nuclear Overhauser effect NMR to study the interactions between ions. We are introducing these 2D NMR methods to study specific interactions with a molecular solute and the surrounding cations and anions. Current measurement is a basic technique to investigate the nature of a system. In particular, the variance of the current, i.e. noise, has been extensively studied because it includes useful information about intrinsic temperature in the equilibrium noise, and about the low-energy excitations in the shot noise. It is, however, often observed that the measured noise is larger than the intrinsic one expected from theories. Here, we consider the difference from the view point of the limitation of the resolution of measurement devices.
In this study, we propose a quantum two-point measurement statistics with limited resolution. Our method is equivalent to the ordinary full counting statistics in an ideal condition, but can consider resolution of the device in the other cases. Using this method, we analyze resolution effects on the distribution of current flowing a resonant level coupled to two reservoirs. It is found that the poor resolution gives no influence to the averaged current but gives the excess noise. Since the excess noise is always positive, the measured noise is always larger than the intrinsic one. Furthermore, the thermal noise and the shot noise are investigated in more details.   We describe a new large-N approach, which is referred to as 1/(N-1) expansion, to an N-fold degenerate Anderson impurity model with a finite Coulomb interaction U [1,2]. This approach is different from the usual 1/N expansion [3], non-crossing approximation [4], and recent developments [5,6] along the conventional large-N theory which is based on a perturbation expansion with respect to the tunneling matrix element V between the impurity and conduction electrons with a scaling that takes NV 2 as a finite constant independent of N. In contrast, our formulation starts with the perturbation expansion in U, and then reorganizes the perturbation series according to the powers of 1/(N-1), using the scaling that takes u=(N-1) U as an independent variable. The factor N-1 represents the number of interacting orbitals, excluding the one prohibited by the Pauli principle. Our expansion scheme provides the Hartree-Fock (HF) approximation at zero order, where the limit N → ∞ is taken keeping u finite.
Then, to leading order in 1/(N-1) it describes the Hartree-Fock random phase approximation (HF-RPA). The higherorder corrections, starting from order 1/(N-1) 2 terms, capture systematically fluctuations beyond the HF-RPA. The results of the renormalized local-Fermi-liquid parameters, obtained up to terms of order 1/(N-1) 2 , agree closely with the exact numerical-renormalization-group results at N=4, where the degeneracy is still not really large as N=2 corresponds to the standard Anderson model with the spin degeneracy. This ensures the reliability of our approach for N ≥ 4. We also apply this approach to nonequilibrium transport through quantum dots in the Kondo regime.  These results have been obtained by cooperation with S. Hoshino at University of Tokyo.