A wafer-thin flake of bismuth telluride can act a little like a one-way street for electricity, even when the push comes from an alternating signal. But the direction of that “street” is not fixed.

Bluetooth trackers, the small, battery-powered devices, help you find lost/misplaced everyday items such as keys, wallets, bags or passports. If the item is nearby, you open an app and trigger a loud ...