Axionic Tunneling Confirmed Via Scanning Tunneling Microscopy Reveals Magnetization at 30 meV

The search for materials that mimic the exotic behaviour of fundamental particles has led scientists to explore topological states of matter, revealing properties reminiscent of relativistic physics, but a key prediction, the axionic term in a material’s electromagnetic response, has remained elusive. Now, Saikat Banerjee from Rutgers University and the University of Greifswald, alongside Anuva Aishwarya and Fei Liu from Sun Yat-sen University and Zhejiang University, and colleagues including Lin Jiao and Vidya Madhavan, demonstrate a direct signature of this axionic physics using a scanning tunneling microscope. Their work reveals that previously observed spin polarization in topological materials does not arise from static magnetism, but instead originates from an axionic electrodynamic effect, where extremely small voltages induce measurable magnetic moments at the nanoscale. This discovery not only explains existing experimental observations, but also establishes millivolt-level control of spin polarization, opening exciting possibilities for both fundamental studies of axionic electrodynamics and the development of novel spintronic devices.

Correlations and Topology in Ta3S2

Scientists are investigating the interplay between strong electronic interactions and topological properties in Ta3S2, a material with potential as a Weyl semimetal. This research aims to understand how these interactions influence the material’s electronic structure and, consequently, its physical characteristics, such as how it conducts electricity and responds to light. The team employs sophisticated theoretical modelling, combining density functional theory and dynamical mean-field theory, alongside experimental techniques like angle-resolved photoemission spectroscopy and transport measurements. This detailed investigation of Ta3S2’s electronic structure reveals a complex relationship between how electron orbitals combine and how electron interactions affect them.

The results demonstrate that strong electronic correlations significantly alter the material’s band structure, reducing the effective mass of charge carriers and creating a unique topological surface state. Furthermore, the team elucidates how electron-electron interactions enhance conductivity and suppress scattering, leading to high carrier mobility. This research advances understanding of correlated topological materials and provides insights into designing novel electronic devices with improved performance.

Spin-Polarized Tunneling with Varied Tips

This research investigates spin-polarized tunneling currents observed using scanning tunneling microscopy on an antiferromagnetic material, Fe1+xTe, employing both SmB6 and chromium tips. The goal is to determine the origin of the observed spin contrast and whether it arises from the magnetic properties of the tip or the sample itself. The results suggest that the spin contrast is not simply due to the topological surface states of SmB6, but a more complex phenomenon linked to the tip’s magnetic characteristics. The team quantified the strength of the magnetic moment from the tunneling current measurements, finding that the ratio of currents at different positions is proportional to the magnetic moment.

Theoretical calculations demonstrate that summing over all momenta washes out any spin-momentum locking expected from topological surface states, suggesting an alternative origin for the observed contrast. Experimental analysis of the tunneling current, considering the spin-dependent density of states of both tip and sample, supports this conclusion. Comparisons of spin contrast using SmB6 and chromium tips reveal distinct patterns, indicating the tip’s magnetic properties play a crucial role. These findings demonstrate that the observed spin polarization arises from a complex interplay between the tip’s magnetic properties and the antiferromagnetic order of the sample.

Voltage Controls Magnetization in SmB6 Nanowires

Scientists have directly observed axionic physics, a fundamental property predicted to arise from the topology of certain materials, using scanning tunneling microscopy. Experiments demonstrate that extremely small voltages, around 30 millivolts, generate a measurable magnetization at the tip of the microscope when scanning SmB6 nanowires. Critically, this induced magnetization reverses direction with the applied voltage, a signature consistent with axionic coupling. The magnitude of this signal definitively rules out static magnetism as the source, establishing a new mechanism for controlling spin polarization.

Measurements reveal a normalized spin contrast ratio of approximately 0. 13 at a bias voltage of 35 millivolts when comparing SmB6 and chromium tips, indicating a tip magnetization of order 0. 4μB per Sm atom. Crucially, the voltage reversal of this spin contrast confirms that the SmB6 tip exhibits multiferroic behavior, responding directly to the applied electric field. Temperature-dependent measurements of the voltage-reversed spin contrast show behavior consistent with an order parameter, scaling with temperature, demonstrating that this magnetization originates within the SmB6 material itself.

Further analysis indicates a magnetization per Sm atom consistent with calculations based on the electric field generated during tunneling. The team extracted the magnetic spectrum of the SmB6 tunneling tip, finding that the tunnel current is dominated by a component directly proportional to the applied voltage, confirming the axionic character of the nanowire. These results establish a new route for probing axionic electrodynamics and open avenues for future spintronics applications based on millivolt-scale control of spin polarization.