Scientists have long thought that quantum oscillations are an indicator of the differences between metals and insulators. Electrons in metals are highly mobile and have very weak electrical resistivity. About a century ago, researchers observed that electrons shift from a "classical" state to a quantum state when exposed to magnetic fields and extremely low temperatures, causing quantum oscillations. In an insulator, by contrast, electrons can't move, and the material's resistivity is so high that no quantum oscillations occur no matter how much magnetic field strength is applied. 

But when a team at Princeton University studied a material called Tungsten Ditelluride (WTe2), they made a surprising discovery. 

Gradually scraping tungsten ditelluride down to a single atomic layer, the researchers found that the thick tungsten ditelluride material behaves like metal, but becomes a super-strong insulator when reduced to a single layer. The researchers then set out to measure the electrical resistivity of monolayer tungsten disulfide in the presence of a magnetic field. They found that the electrical resistivity of the insulator, while still large, began to oscillate as the magnetic field increased, exhibiting the most significant quantum property of metals, namely the transition into a quantum state.

The finding came as a surprise because there is no theory to explain the phenomenon.  Sanfeng Wu, an assistant professor of physics at Princeton University, has a bold hypothesis: that a new kind of quantum matter could be born out of this, and instead of electrons oscillating, a new type of particle called a "neutral fermion" could be born out of very strongly interacting electrons, with quite remarkable quantum effects.

In quantum materials, charged fermions can be negatively charged electrons or positively charged holes that conduct electricity. In other words, if the material is an electrical insulator, these charged fermions cannot move freely. But in theory, neutral particles with neither a negative nor a positive charge can exist and move around in an insulator. The results contradict all theories of charged fermions, and only neutral fermions can explain them.

If the experimental data are correct, more insulators with similar quantum properties may be discovered in the future, a new quantum world hidden in insulators. The team says more experiments are needed to see if neutral fermions exist, or to find other existing theories that could also explain them.

What is Tungsten telluride WTe2?

Tungsten telluride (WTe2) is an inorganic semi-metallic compound. 

In October 2014, tungsten ditelluride was found to exhibit extreme magnetoresistance: resistance increased by 13 million percent in a 0.5 Kelvin 60 Tesla magnetic field.  Resistance is proportional to the square of the magnetic field, without saturation. This may be because the material is the first example of compensating semi-metals, where the number of moving holes is the same as the number of electrons. Tungsten ditelluride has a layered structure similar to many other transition metal disulfides, but its layers are so twisted that the honeycomb lattices that many of them have in common are difficult to identify in WTe2. Instead, the tungsten atoms form zigzag chains and are thought to behave as one-dimensional conductors. Unlike electrons in other two-dimensional semiconductors, electrons in WTe2 can easily move between layers. 

When subjected to pressure, the reluctance effect in WTe2 decreases. Pressure reluctance above 10.5GPa disappears and the material becomes a superconductor. At 13.0GPa, the transition to superconductivity occurs below 6.5K. 

WTe2 is predicted to be a Weyl semimetal, especially in the case of type II Weyl semimetal, where Weyl nodes exist at the intersection of electrons and hole pockets. 

It has also been reported that light pulses at the terahertz frequency can switch the crystal structure of WTe2 between the rhombic and monoclinic systems by changing the atomic lattice of the material. 

Tungsten ditelluride can be stripped into thin sheets up to a single layer. It was initially predicted that monolayer WTe2 would remain a Wall semi-metal in the 1T' phase. Later transmission measurements show that below 50K, monolayer WTe2 acts like an insulator, but has an offset current independent of local electrostatic gate doping. When a contact geometry with short-circuit conduction along the edge of the device is used, this offset current disappears, suggesting that this almost quantized conduction is localized to the edge -- a behavior consistent with monolayer WTe2 as a two-dimensional topological insulator. The same measurements for two - and three-layer thick samples showed the expected semi-metallic response. Subsequent studies using other techniques were consistent with the transmission results, including studies using angle-resolved photoelectron spectroscopy and microwave impedance microscopy. It is also observed that monolayer WTe2 has superconductivity under medium doping and a critical temperature that can be adjusted by doping level.

It is also observed that two and three layers of WTe2 are polar metals with both metallic behavior and switchable electrical polarization. In theory, polarization results from vertical charge transfer between layers, which is switched by sliding between layers. 

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