A branch of physics that studies the inherent quantum properties of materials that can be altered but not changed is the focus of new research conducted by the physicists at the University. By repeating an experiment first conducted by researchers at Kyoto University, the team has clarified key aspects of the original experiment, as well as reached novel and diverging conclusions.
The first example of a magnetic insulator that exhibits the thermal Hall effect arising from quantum edge modes of bosons was demonstrated by the researchers.
There's a background to the experiment.
The experiment's beginnings are in the work of Phil Anderson, who won the 1977 Nobel Prize in physics. Physicists call these types of magnetic materials magnetic phase transitions. This describes an abrupt transition to a state in which the spin at each lattice site is either aligned in a perfectly parallel pattern or alternates in an orderly fashion between up and down. This phase transition is experienced by ninety-nine percent of magnetic materials. The term geometric frustration was suggested by Anderson.
The senior author of the paper said to imagine trying to seat couples around a dinner table under the rule that every woman is to be seated between two men. This arrangement is not possible if we have a guest who arrives alone.
A Russian physicist proposed in 2006 that Anderson's spin liquid state could be achieved without using Anderson's idea of geometric frustration. He predicted the existence of Majoranas and visons in a series of equations. The Majorana particle is a strange and elusive particle that was first created in 1937. It is a type of fermion and the only one that is similar to its own antiparticle.
A lot of research was done to find materials that could realize Kitaev's calculations. Two physicists predicted two years ago that ruthenium chloride would be the closest proximate. This material is an excellent conductor.
It has become one of the most investigated candidates for quantum spin liquids in the past ten years. Physicist Yuji Matsuda and his colleagues at Kyoto University reported the observation of the half-quantized thermal Hall effect, which was predicted in Kitaev's calculations.
The thermal Hall effect is similar to the electrical Hall effect. If the direction of the magnetic field is reversed, the temperature difference between the two edges of the sample will change. The thermal Hall effect is found in metals such as copper and gallium, but it is very rare in insulator. The heat current is conveyed by lattice vibrations that are indifferent to the magnetic field.
Matsuda reported that the thermal Hall Conductivity was half quantized. The magnitude is dependent upon the Boltzmann constant and the Planck constant. The community was interested in the experiment that implied the observation of Majorana particles.
There was something amiss with Matsuda's conclusion according to the research team. "I was unable to put my finger on it," he said.
There is an experiment.
The experiment was repeated by the group of people. They wanted to conduct the experiment at a higher resolution and at a larger temperature interval.
Peter Czajka is a graduate student in physics and the lead author of the paper. Our experiment is a great example of something that is conceptually simple but difficult to implement. It's easy to measure the electrical resistance of something, but it's not as easy to measure the thermal conductivity of a sample.
The first part of the experiment required the researchers to pick a sample of ruthenium chloride that had several different characteristics. The temperature was measured with the attached sensitive thermometers.
The only thing we're doing is measuring the temperature on a crystal. We need a resolution between a thousand and a millionth of a degree to do this.
The sample was subjected to a strong magnetic field after the researchers cooled the material down. They used an electrical heat source to warm up the edge of the crystal. The experiment required a lot of time.
Czajka said that the sample was cold for six months. Most researchers don't want to do a single experiment for six months.
The thermal Hall Effect was the first thing the researchers noticed. The researchers realized this when they noticed that the flow of the heat current was either one side or the other.
The analogy of a raft going down a river with a packet of heat in it was used to explain this. The left bank says that your raft is being pushed to one side of the river. The rafts are being pushed to the left bank. The left bank's temperature goes up by a small amount.
The signal is also affected by the magnetic field. You can find all the rafts if you repeat the experiment with the magnetic field reversed.
This effect doesn't happen in most of the insulators. The rafts won't accumulate on either side of the river.
The effect in these new materials is very striking. There is a phenomenon called the Berry curve.
Michael Berry is a mathematical physicist at the University of Bristol. The wave functions are described in the Berry Curvature. The Berry curve is finite. It acts like a magnetic field on charged particles.
The Berry curvature has been missing for the last sixty years, but has come to the forefront in the last five years or so. The cause of Matsuda's observation is due to the Berry Curvature that we proved in this paper.
Matsuda's experiment predicted the presence of the Majorana fermion, but it was not confirmed by the researchers. The thermal Hall effect was traced back to a particle called a boson.
Particles in nature are either fermions or bolos. A fermion is a particle that is a boson. The wave-like collectives of the magnetic moments are what cause bois. The thermal Hall effect can be caused by both types of particles if they are made of the same material.
We show that the observed particles are not fermions. If the particles were identified as fermions, the signal would be different. The signal's temperature dependence is very similar to a quantitative model for topological bosons.
The first example of a bosonic material showing quantum edge transport is what we are doing.
Future research and implications.
Their research has strong implications for fundamental physics research.
By clarifying the presence of bosons rather than fermions, we were able to open the door to use the thermal Hall Effect in the same way that the quantum Hall Effect has been used.
It is likely that the particles discovered in the experiment will have practical applications for such things as quantum computing or quantum devices, though it is not certain at this time. The members of the research laboratory intend to continue their research by looking for similar Hall effects in related materials and studying the quantum possibilities of ruthenium chloride. Scientists collaborated with Oak Ridge National Laboratories, the University of Tennessee, Tokyo University and Purdue University.
The Planar thermal Hall effect of topological bosons in the Kitaev magnet was reported by Peter Czajka and his team.
Journal information: Nature Materials
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