Physicists have theoretically studied the cooling of a chain of magnetically interacting atoms towards absolute zero. The very slow dynamic behavior of the system leads to the persistence of defects, expressing physics resembling certain scenarios of the primordial universe expansion.
One of the most intriguing phenomena in physics is probably the spontaneous generation of phase transitions in complex systems: aggregations of particles interacting at short distances (few molecular distances) can macroscopically self-organize when the temperature is lowered below a certain distance through distances spontaneously breaking symmetries.
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The best-known example is undoubtedly the solidification of liquids, a transition after which the crystalline solid, which consists of an ordered arrangement of superimposed molecular planes, no longer exhibits the isotropy (isotropy characterizes the invariance of physical properties of a medium in …), which prevailed in the liquid. Another example of the transition, called “continuous”, is provided by magnets: the magnetization results from an orientation (In the literal sense, orientation denotes or materializes the direction of the East (sunrise…)) concerted action of the small magnets possessed by certain atoms (like iron). These microscopic magnets (called spins) tend to align their magnetization to produce macroscopic magnetization (the one we experience at our scale), a phenomenon thwarted by agitation (agitation is the act of making it happen). to mix one or more thermal phases…) (Thermal is the science concerned with the production of energy, the use of…). At a precise threshold (770 degrees for iron) the magnetization disappears because the dynamic disorder (The word dynamic is often used to denote or qualify what is related to motion. It…) is so strong that it the spins orient in all directions on average, and no magnetism macroscopic ion no longer exits.
From a dynamic point of view, the continuous phase transitions are always accompanied by complex competition phenomena between the different orientations that arise in the differently ordered zones that grow at the four corners of the macroscopic sample (the crystallization of a liquid differs from this scenario). Furthermore, this competition slows down as the temperature approaches the phase change, which has consequences in any real-world experiment. In fact, temperature drop always occurs at a certain rate, and however slow it may be, it eventually “crashes” the system, which then tends to freeze up in a defect-rich configuration (grains with disordered magnetic orientation for a magnet). .
The form factor (product of domain wall density and magnetic susceptibility) is a new tool to measure the distance to equilibrium.
Red dot: thermal equilibrium. Green dot: growth after immediate deterrence.
Blue interval: range of variation for infinitely slow attenuations.
Photo credits: C. Godrèche and J.-M. lucky
An important question then is how the density of these defects depends on the cooling rate. Unexpectedly, it was first placed in a very different context (The context of an event includes the circumstances and conditions surrounding it; the…) by the British physicist Th. Kibble in 1976 than the condensed matter that cosmology (Cosmology is the Branch of astrophysics that studies the universe as a system…) of the early universe: after particle physics (particle physics is the branch of physics that studies the constituent parts…), the cooling following the expansion of the universe caused it went through several continuous phase transitions (of quantum origin), which must have left behind so-called topological defects, which probably change in their neighborhood (The notion of neighborhood corresponds to an axiomatic approach similar to that of … ) the usual laws of physics. We therefore understand Kibble’s question that the density of such defects is an important parameter (A parameter is in the broadest sense an item of information to be considered…) important for a complete description of the current Universe.
Kibble’s ideas were applied to condensed matter by the physicist W. Zurek a few years later, since the basic mechanism is the same in both cases: critical deceleration stops near a phase transition (In physics, a phase transition is a transformation of the system under study…). the system away from equilibrium during the passage of this transition. These ideas gave rise to the Kibble-Zurek theory, which established laws that describe, among other things, the behavior of the defect density as a function of the cooling rate.
This theory has just been revisited by researchers from the Institute for Theoretical Physics (IPhT, CNRS/CEA) in a recently published article. They consider an exactly solvable one-dimensional model, the classical ferromagnetic chain of Ising spins subjected to random dynamics involving the interaction (An interaction is an exchange of information, affect, or energy between two agents within…) with a thermostat (A thermostat is a system that ensures a constant temperature. It can be a device…) with adjustable temperature, the change in time of which is a priori arbitrary. Due to its one-dimensional structure, this model has a continuous phase transition at absolute zero, which makes it possible to approach it “from above” and study the generally common properties around critical points: critical slowdown with persistence (persistence (stats) persistence (calc ) in painting: The…) of defects, emergence of ever larger magnetic domains…
The authors examine in detail the different possible scenarios of cooling towards absolute zero and introduce a new parameter, the “shape factor”, which synthetically takes into account the system’s distance from equilibrium induced by the cooling protocol. These results were published in the Journal of Physics A.
References:
The Glauber-Ising chain under low-temperature protocols, C. Godrèche, J.-M. Luck, Journal of Physics A: Mathematical and Theoretical, published December 16, 2021.
DOI: 10.1088/1751-8121/aca84c
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