Frictionless, directional propagation of particles at the boundary of topological materials

Amazing stuff! Somehow this reminds me of superconductivity!

"... But in certain exotic materials, electrons can appear to flow with single-minded purpose. In these materials, electrons may become locked to the material’s edge and flow in one direction, like ants marching single-file along a blanket’s boundary. In this rare “edge state,” electrons can flow without friction, gliding effortlessly around obstacles as they stick to their perimeter-focused flow. Unlike in a superconductor, where all electrons in a material flow without resistance, the current carried by edge modes occurs only at a material’s boundary.

Now MIT physicists have directly observed edge states in a cloud of ultracold atoms. For the first time, the team has captured images of atoms flowing along a boundary without resistance, even as obstacles are placed in their path. ...

In their new study, the team worked with a cloud of about 1 million sodium atoms, which they corralled in a laser-controlled trap, and cooled to nanokelvin temperatures. They then manipulated the trap to spin the atoms around, much like riders on an amusement park Gravitron.

“The trap is trying to pull the atoms inward, but there’s centrifugal force that tries to pull them outward,” Fletcher explains. “The two forces balance each other, so if you’re an atom, you think you’re living in a flat space, even though your world is spinning. There’s also a third force, the Coriolis effect, such that if they try to move in a line, they get deflected. So these massive atoms now behave as if they were electrons living in a magnetic field.”

Into this manufactured reality, the researchers then introduced an “edge,” in the form of a ring of laser light, which formed a circular wall around the spinning atoms. As the team took images of the system, they observed that when the atoms encountered the ring of light, they flowed along its edge, in just one direction. ..."

From the abstract:

"The frictionless directional propagation of particles at the boundary of topological materials is a striking transport phenomenon. These chiral edge modes lie at the heart of the integer and fractional quantum Hall effects, and their robustness against noise and disorder reflects the quantization of Hall conductivity in these systems. Despite their importance, the controllable injection of edge modes, and direct imaging of their propagation, structure and dynamics, remains challenging. Here we demonstrate the distillation of chiral edge modes in a rapidly rotating bosonic superfluid confined by an optical boundary. By tuning the wall sharpness, we reveal the smooth crossover between soft wall behaviour in which the propagation speed is proportional to wall steepness and the hard wall regime that exhibits chiral free particles. From the skipping motion of atoms along the boundary we infer the energy gap between the ground and first excited edge bands, and reveal its evolution from the bulk Landau level splitting for a soft boundary to the hard wall limit. Finally, we demonstrate the robustness of edge propagation against disorder by projecting an optical obstacle that is static in the rotating frame."

Atoms on the edge | MIT News | Massachusetts Institute of Technology

Observation of chiral edge transport in a rapidly-rotating quantum gas (pre-print, open access)

Observation of chiral edge transport in a rapidly rotating quantum gas (no public access)

New non-equilibrium states and new states of matter uncovered in flatland fractional quantum Hall effect electrons

Amazing stuff!

"... In a series of experiments at extremely cold conditions, close to -459°F (-273°C), and under a magnetic field nearly 100,000 times stronger than Earth’s, Mani, Wijewardena and colleagues went to work. They applied a supplementary current to high mobility semiconductor devices made from a sandwich structure of gallium arsenide (GaAs) and aluminum gallium arsenide (AlGaAs) materials, which helps to realize electrons in a flatland. They observed all the FQHE states splitting unexpectedly, followed by crossings of split branches, which allowed them to explore the new non-equilibrium states of these quantum systems and reveal entirely new states of matter. The study highlights the crucial role of high-quality crystals ..."

From the abstract:

"Studies of fractional quantum Hall effects (FQHE) across various two-dimensional electronic systems (2DES) have helped to establish the equilibrium FQHE many-body ground states with fractionally charged excitations and composite particles in condensed matter. Then, the question arises whether an FQHE system driven to non-equilibrium can approach a different stationary state from the known equilibrium FQHE states. To investigate this question, we examine FQHE over filling factors, ν, 2 ≥ ν ≥ 1 under non-equilibrium finite bias conditions realized with a supplementary dc-current bias, IDC, in high mobility GaAs/AlGaAs devices. Here, we show that all observable canonical equilibrium FQHE resistance minima at ∣IDC∣ = 0 undergo bimodal splitting vs. IDC, yielding branch-pairs and diamond shapes in color plots of the diagonal resistance, as canonical FQHE are replaced, with increasing IDC, by excited-state fractionally quantized Hall effects at branch intersections. A tunneling model serves to interpret the results."

“Unexplored levels” uncovered in flatland fractional quantum Hall effect electrons

Georgia State Discovery Provides Insight Into Behavior of Electrons (original news release)

Non-equilibrium excited-state fractionally quantized Hall effects observed via current bias spectroscopy (open access)

Fig. 3: Splitting of equilibrium FQHE states under current bias.

3D quantum spin liquid discovery gains theoretical support

Amazing stuff! Exotic physics or is there more to it?

"... Then, in 2019, researchers ... found experimental evidence that a pyrochlore magnet, Ce2Zr2O7, could be a long-sought-after example of a three-dimensional quantum spin liquid (3D QSL). This “experimental conjecture” has now received further support from physicists ... the researchers used a combination of theoretical techniques to conclude that Ce2Zr2O7 (CZO for short) is indeed a 3D QSL. ...
QSL will remain in a liquid state even at temperatures near absolute zero, where most materials freeze solid. ..."

From the abstract:

"The search for quantum spin liquids—topological magnets with fractionalized excitations—has been a central theme in condensed matter and materials physics. Despite numerous theoretical proposals, connecting experiment with detailed theory exhibiting a robust quantum spin liquid has remained a central challenge. Here, focusing on the strongly spin-orbit coupled effective S = 1/2 pyrochlore magnet Ce2Zr2O7, we analyze recent thermodynamic and neutron-scattering experiments, to identify a microscopic effective Hamiltonian through a combination of finite temperature Lanczos, Monte Carlo, and analytical spin dynamics calculations. Its parameter values suggest the existence of an exotic phase, a π-flux U(1) quantum spin liquid. Intriguingly, the octupolar nature of the moments makes them less prone to be affected by magnetic disorder, while also hiding some otherwise characteristic signatures from neutrons, making this spin liquid arguably more stable than its more conventional counterparts."

3D quantum spin liquid discovery gains theoretical support – Physics World

Sleuthing out exotic quantum spin liquidity in the pyrochlore magnet Ce2Zr2O7 (open access)

Fig. 1: Quantum spin liquid in a pyrochlore magnet and its experimental ramifications