Topological Electromagnetic Effects and Higher Second Chern Numbers in Four-Dimensional Gapped Phases.

Higher-dimensional topological phases play a key role in understanding the lower-dimensional topological phases and the related topological responses through a dimensional reduction procedure. In this work, we present a Dirac-type model of four-dimensional Z_{2} topological insulator (TI) protected by CP symmetry, whose 3D boundary supports an odd number of Dirac cones. A specific perturbation splits each bulk massive Dirac cone into two valleys separated in energy-momentum space with opposite second Chern numbers, in which the 3D boundary modes become a nodal sphere or a Weyl semimetallic phase. By introducing the electromagnetic (EM) and pseudo-EM fields, exotic topological responses of our 4D system are revealed, which are found to be described by the (4+1)D mixed Chern-Simons theories in the low-energy regime. Notably, several topological phase transitions occur from a CP-broken Z_{2} TI to a Z TI when the bulk gap closes by giving rise to exotic double-nodal-line or nodal-hyper-torus gapless phases. Finally, we propose to probe experimentally these topological effects in cold atoms.

A synthetic monopole source of Kalb-Ramond field in diamond.

Magnetic monopoles play a central role in areas of physics that range from electromagnetism to topological matter. String theory promotes conventional vector gauge fields of electrodynamics to tensor gauge fields and predicts the existence of more exotic tensor monopoles. Here, we report the synthesis of a tensor monopole in a four-dimensional parameter space defined by the spin degrees of freedom of a single solid-state defect in diamond. Using two complementary methods, we characterized the tensor monopole by measuring its quantized topological charge and its emanating Kalb-Ramond field. By introducing a fictitious external field that breaks chiral symmetry, we further observed an intriguing spectral transition, characterized by spectral rings protected by mirror symmetries. Our work demonstrates the possibility of emulating exotic topological structures inspired by string theory.

Erratum: Experimental Measurement of the Quantum Metric Tensor and Related Topological Phase Transition with a Superconducting Qubit [Phys. Rev. Lett. 122, 210401 (2019)].

This corrects the article DOI: 10.1103/PhysRevLett.122.210401.

Experimental Measurement of the Quantum Metric Tensor and Related Topological Phase Transition with a Superconducting Qubit.

A Berry curvature is an imaginary component of the quantum geometric tensor (QGT) and is well studied in many branches of modern physics; however, the quantum metric as a real component of the QGT is less explored. Here, by using tunable superconducting circuits, we experimentally demonstrate two methods to directly measure the quantum metric tensor for characterizing the geometry and topology of underlying quantum states in parameter space. The first method is to probe the transition probability after a sudden quench, and the second one is to detect the excitation rate under weak periodic driving. Furthermore, based on quantum metric and Berry-curvature measurements, we explore a topological phase transition in a simulated time-reversal-symmetric system. The work opens up a unique approach to explore the topology of quantum states with the QGT.

Topological Maxwell Metal Bands in a Superconducting Qutrit.

We experimentally explore the topological Maxwell metal bands by mapping the momentum space of condensed-matter models to the tunable parameter space of superconducting quantum circuits. An exotic band structure that is effectively described by the spin-1 Maxwell equations is imaged. Threefold degenerate points dubbed Maxwell points are observed in the Maxwell metal bands. Moreover, we engineer and observe the topological phase transition from the topological Maxwell metal to a trivial insulator, and report the first experiment to measure the Chern numbers that are higher than one.

Temperature Effects on the Wind Direction Measurement of 2D Solid Thermal Wind Sensors.

For a two-dimensional solid silicon thermal wind sensor with symmetrical structure, the wind speed and direction information can be derived from the output voltages in two orthogonal directions, i.e., the north-south and east-west. However, the output voltages in these two directions will vary linearly with the ambient temperature. Therefore, in this paper, a temperature model to study the temperature effect on the wind direction measurement has been developed. A theoretical analysis has been presented first, and then Finite Element Method (FEM) simulations have been performed. It is found that due to symmetrical structure of the thermal wind sensor, the temperature effects on the output signals in the north-south and east-west directions are highly similar. As a result, the wind direction measurement of the thermal wind sensor is approximately independent of the ambient temperature. The experimental results fit the theoretical analysis and simulation results very well.

Synthesis of ZnGa(2)O(4) Hierarchical Nanostructure by Au Catalysts Induced Thermal Evaporation.

In this paper, ZnGa(2)O(4) hierarchical nanostructures with comb-like morphology are fabricated by a simple two-step chemical vapor deposition (CVD) method: first, the Ga(2)O(3) nanowires were synthesized and employed as templates for the growth of ZnGa(2)O(4) nanocombs; then, the as-prepared Ga(2)O(3) nanowires were reacted with ZnO vapor to form ZnGa(2)O(4) nanocombs. Before the reaction, the Au nanoparticles were deposited on the surfaces of Ga(2)O(3) nanowires and used as catalysts to control the teeth growth of ZnGa(2)O(4) nanocombs. The as-prepared ZnGa(2)O(4) nanocombs were highly crystallized with cubic spinel structure. From the photoluminescence (PL) spectrum, a broad band emission in the visible light region was observed of as-prepared ZnGa(2)O(4) nanocombs, which make it promising application as an optical material.