Job 38:4
"In the physical world,
---phase transitions occur when a material such as a liquid, gas or solid changes from one state or form to another.
Superconductivity occurs when electrons pair up and flow in unison without resistance and without dissipating energy.
Fluctuations are temporary random changes in the thermodynamic state of a material that is on the verge of undergoing a phase
transition. A familiar example of a phase transition is the melting of ice to water. The Princeton experiment investigated fluctuations that occur in a superconductor at temperatures close to absolute zero.
"What we found, by directly looking at quantum fluctuations near the transition, was clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field," said Wu.
Indeed, for a long time, it was believed that superconductivity was impossible in a two-dimensional world.
"This came about because, as you go to lower dimensions, fluctuations become so strong that they 'kill' any possibility of superconductivity," said N. Phuan Ong.
The main way fluctuations destroy two-dimensional superconductivity is by the spontaneous emergence of what is called a quantum vortex (plural: vortices).
Each vortex resembles a tiny whirlpool composed of a microscopic strand of magnetic field trapped inside a swirling electron current. When the sample is raised above a certain temperature, vortices spontaneously appear in pairs: vortices and anti-vortices.
---But phase transitions occur on the quantum level as well.
These occur at temperatures approaching absolute zero (-273.15°
Celsius), and involve the continuous tuning of some external parameter, such as pressure or magnetic field, without raising the temperature.
Researchers are particularly interested in how quantum phase transitions occur in superconductors, materials that conduct electricity without resistance.
Researchers are particularly interested in how quantum phase transitions occur in superconductors, materials that conduct electricity without resistance.
Superconductivity occurs when electrons pair up and flow in unison without resistance and without dissipating energy.
---Normally, electrons travel through circuits and wires in an erratic manner, jostling each other in a manner that is ultimately inefficient and wastes energy.
---But in the superconducting state, electrons act in concert in a way that is energy efficient.
Princeton physicists have discovered an abrupt change in quantum
behavior while experimenting with a three-atom-thin insulator that can be easily switched into a superconductor.
The results were published in the journal Nature Physics in a paper titled "Unconventional Superconducting Quantum Criticality in Monolayer WTe2."
The results were published in the journal Nature Physics in a paper titled "Unconventional Superconducting Quantum Criticality in Monolayer WTe2."
Fluctuations are temporary random changes in the thermodynamic state of a material that is on the verge of undergoing a phase
"What we found, by directly looking at quantum fluctuations near the transition, was clear evidence of a new quantum phase transition that disobeys the standard theoretical descriptions known in the field," said Wu.
Indeed, for a long time, it was believed that superconductivity was impossible in a two-dimensional world.
"This came about because, as you go to lower dimensions, fluctuations become so strong that they 'kill' any possibility of superconductivity," said N. Phuan Ong.
The main way fluctuations destroy two-dimensional superconductivity is by the spontaneous emergence of what is called a quantum vortex (plural: vortices).
Their rapid motion destroys the superconducting state.
"A vortex is like a whirlpool," said Ong. "They are quantum versions of the eddy seen when you drain a bathtub."
Physicists now know that superconductivity in ultrathin films does exist below a certain critical temperature known as the BKT transition.
"A vortex is like a whirlpool," said Ong. "They are quantum versions of the eddy seen when you drain a bathtub."
Physicists now know that superconductivity in ultrathin films does exist below a certain critical temperature known as the BKT transition.
At temperatures close to absolute zero, a quantum transition is.jpeg)
induced by quantum fluctuations. In this scenario, the transition is distinct from the temperature-driven BKT transition.
The researchers began with a bulk crystal of tungsten ditelluride (WTe2), which is classified as a layered semi-metal. The researchers began by converting the tungsten ditelluride into a two-dimensional material by increasingly exfoliating, or peeling, the material down to a single, atom-thin layer.
---At this level of thinness, the material behaves as a very strong insulator, which means its electrons have limited motion and hence cannot conduct electricity.
.jpeg)
The researchers began with a bulk crystal of tungsten ditelluride (WTe2), which is classified as a layered semi-metal. The researchers began by converting the tungsten ditelluride into a two-dimensional material by increasingly exfoliating, or peeling, the material down to a single, atom-thin layer.
---At this level of thinness, the material behaves as a very strong insulator, which means its electrons have limited motion and hence cannot conduct electricity.
---Amazingly, the researchers found that the material exhibits a host of novel quantum behaviors, such as switching between insulating and superconducting phases. They were able to control this switching behavior by building a device that functions just like an "on and off" switch.
The researchers found that they could precisely control the properties of superconductivity by adjusting the density of electrons in the material via the gate voltage.
The researchers found that they could precisely control the properties of superconductivity by adjusting the density of electrons in the material via the gate voltage.
At a critical electron density, the quantum vortices rapidly proliferate and destroy the superconductivity, prompting the quantum phase transition to occur.
A second major surprise is that the vortex signal abruptly disappeared when the electron density was tuned just below the critical value at which the quantum phase transition of the superconducting state occurs.
A second major surprise is that the vortex signal abruptly disappeared when the electron density was tuned just below the critical value at which the quantum phase transition of the superconducting state occurs.
At this critical value of electron density, which the researchers call the quantum critical point (QCP) that represents a point at zero temperature in a phase diagram, quantum fluctuations drive the phase transition.
"We expected to see strong fluctuations persist below the critical
electron density on the non-superconducting side, just like the strong fluctuations seen well above the BKT transition temperature," said Wu.
"Yet, what we found was that the vortex signals 'suddenly' vanish the moment the critical electron density is crossed. And this was a shock. We can't explain at all this observation—the 'sudden death' of the fluctuations."
Ong added, "In other words, we've discovered a new type of quantum critical point, but we don't understand it."
In the field of condensed matter physics, there are currently two established theories that explain phase transitions of a superconductor, the Ginzburg-Landau theory and the BKT theory. However, the researchers found that neither of these theories explain the observed phenomena."
"We expected to see strong fluctuations persist below the critical
"Yet, what we found was that the vortex signals 'suddenly' vanish the moment the critical electron density is crossed. And this was a shock. We can't explain at all this observation—the 'sudden death' of the fluctuations."
Ong added, "In other words, we've discovered a new type of quantum critical point, but we don't understand it."
In the field of condensed matter physics, there are currently two established theories that explain phase transitions of a superconductor, the Ginzburg-Landau theory and the BKT theory. However, the researchers found that neither of these theories explain the observed phenomena."
Phys.org
