Researchers Observe What Happens During a Quantum Phase Transition
When ice is heated, the water molecules that form the structure vibrate harder until they finally forces are no longer strong enough to keep them together - the ice melts and becomes liquid water. Quantum physics predicts that similar phenomena may occur if the quantum fluctuations of the particles in a material can be changed. These state changes caused by the purely quantum effects - known as quantum phase transitions - play an important role in many surprising phenomena in semiconductor systems, including high temperature superconductivity. Researchers from Switzerland, Britain, France and China specifically amended the magnetic structure of TlCuCl3 material by exposing it to a variable external pressure at different temperatures. How to neutron scattering measurements, we were able to observe what happens at quantum phase transition, and compare the "quantum melting" of the magnetic structure with the "thermal fuse" conventional phase transition.
If water is liquid or solid form of ice, which depends on two energy takes over. One is the binding energy of the water molecules, the kinetic energy of other molecular motion, which is getting stronger, the higher the temperature. If ice is heated above zero degrees centigrade, the movement of the molecules becomes so intense that the hydrogen bonds are no longer able to hold together and melting ice. The general physical condition is changed or, in the terminology of physics, a phase transition occurs. A similar phenomenon was observed in the magnets - if a magnet is heated, it becomes non-magnetic - and for a similar reason. We can imagine that the magnet is composed of many tiny bar magnets, physicists refer to as magnetic moments. If all these moments are aligned in parallel, all the magnetic material is controlled and behaves like a magnet. If the material is heated, the direction of the moments fluctuates more strongly until the forces alignment and magnetic order disappears exceed: fade effect.
Quantum-physical state
This "classical" fusion is triggered by changes in temperature, but a comparable and equally fundamental phenomenon is determined by the laws of quantum physics. Quantum mechanics tells us that certain properties of the particles of a material can not be known exactly. This uncertainty is often called quantum fluctuations: similar to conventional fluctuations described above, the position or alignment of the magnetic moments of particles fluctuates over time. Although the origin of two types of fluctuations is completely different, in some situations, they may have similar effects. A "merger" of the state of the network ordered system triggered by quantum fluctuations - quantum phase transition - is quantum physics equivalent of the transition from classical phase heat. Phase transitions in quantum mechanics are the key to many of the most exotic phenomena in solid state physics, including high-temperature superconductivity.
The challenge of quantum fluctuations
Researchers at the Paul Scherrer Institute (Villigen, Switzerland) have teamed up with colleagues from University College London, the Institut Laue-Langevin (Grenoble, France) and Renmin University (Beijing, China) to study the precise impact of quantum fluctuations and their interaction with classical fluctuations. The main experimental challenge was to find a system for direct control of quantum fluctuations, and it uses TlCuCl3 materials, which was produced at the University of Bern. Changing conventional fluctuations is simple - the material can be heated and cooled. However, to control the quantum fluctuations in a magnetic material forces alignment between the moments to be modified. Investigators have exploited the fact that TlCuCl3 is relatively soft, so that the inter-atomic distances, and therefore the interactive forces in the material can be modified by applying an external pressure. In the experiment, varying the pressure and temperature in a wide range and the material studied using neutron sources ILL and PSI. This allowed them to determine exactly how the state of matter has changed over the quantum and classical phase transitions.
Disorder is not necessarily disorder
The researchers studied the arrangement of the magnetic moments. In TlCuCl3 moments come in pairs, and the magnetic forces of low pressure between the pairs are weaker, giving a state without magnetic order. "However, this disorder is completely different from a conventional disordered magnet, where the directions of the magnetic moments are just random," says Christian Rüegg, a head of laboratory at the Paul Scherrer Institute and supervisor of the research project. "Here the other two adjacent magnetic moments form a pair, with the two moments pointing in diametrically opposite directions. Interaction between neighboring pairs, however, is not strong enough, so no order is made long range. "In this case the laws of quantum physics do not specify that one of those moments of pairs of points in any direction and it was the most complete uncertainty about the orientation of the individual moments corresponds to strong quantum fluctuations. If now the pressure is increased, the magnetic moments move together so that the moments of adjacent pairs feel each other with increasing force until the coupled state is replaced by a long-range magnetic order: a quantum phase transition occurs due to pressure.
Quantum dynamics of the magnetic moments
In their experiment, the researchers focused mainly on "magnetic excitations" into matter, which provide very accurate information about the quantum states of information now information. These excitations can be imagined as a time of coordinated common magnetic oscillation, like a wave of water, or the vibration of a guitar string. excitations are related to "disorder" in the magnetic material that other excitations, the stronger the magnetic moments fluctuate physics. quantum requires most of the magnetic excitation in TlCuCl3 minimum energy required to excite, and the ease with which they can be activated dependent on the interaction between the magnetic moments - in this experiment and the temperature controlled by the pressure in the sample . researchers have shown that some excitations both low and high pressures require relatively high levels of energy and rarely addressed. However, if the pressure is adjusted to the value where quantum phase transition occurs, the energy decreases and a minimum number of different excitations can be observed. These include the origin and mathematical description is exactly analogous to the Higgs boson in particle physics, so some researchers refer to the Higgs particle in solid materials. Rüegg explains: "We were very surprised to find that these excitations have played a key role, regardless of whether the order was destroyed by quantum mechanics or classical fluctuations - a fascinating feature of quantum phase transitions."
Neutrons reveal excitations
Researchers conducted experiments neutron spectroscopy neutron source at the Institute Paul Scherrer Institut Laue-and Langevin. In the measurement, a current passing through neutron sample TlCuCl3 and watch the path and the rate of change of neutron. This allowed the team to study both the order and magnetic excitations if the neutron emerges move slower than its entry, must have lost its energy by activating emotion. "These fluctuations can not be observed with neutrons and it is vital that you have the opportunity to study the sample at different levels of pressure and temperature," says Martin Boehm, who oversaw the measures at ILL. "In doing so, we benefit from the neutron essential characteristics: they can fly through the walls of the pressure cell where the sample is virtually unrestricted. "
Miracle Material
"This type of spectroscopy experiment can be done for the first time TlCuCl3 because the magnetic interactions are so sensitive to the applied pressure," says Rüegg. "In all other documents we know that a much larger pressure is needed, which means that you can only use very small samples -. Too small for spectroscopic experiments with neutrons Otherwise, you can try to produce many different samples vary slightly in structure, but it would take a long time and still would not give a complete picture of the behavior. "
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