Physicists have questioned the reasons why the thermal Hall effect occurs in a planar configuration of a temperature gradient and a magnetic field. Previously, it was believed that this phenomenon occurs due to chiral magnons or Majorana fermions, but a group of scientists experimentally confirmed that the phenomenon occurs due to thermal phonons scattering on local symmetry violations in single crystals. The article was published in Physical Review X.
When physicists place a conductor in a transverse magnetic field and pass a current through it perpendicular to the field, a potential difference arises at the edges of the sample and the classic Hall effect is obtained. If a thermal gradient is created in the sample, which is directed perpendicular to the magnetic flux, then it is possible to register the occurrence of an electric current in the material - this is already the thermal Hall effect. It would seem that the perpendicularity of the plane in which the heat flow is distributed to the magnetic field is a mandatory condition, however, not long ago, scientists discovered the thermal Hall effect in a planar configuration, that is, both the thermal and magnetic flux were directed in the same plane.
It is logical to assume that the reason for such behavior of the samples should be thermal excitations of the crystal lattice - phonons, however, physicists usually name chiral magnons or Majorana fermions as the culprits of such anomaly. This is because in the studied systems the occurrence of thermal phonons is not allowed due to the high symmetry of the crystal structure.
Lu Chen of the University of Sherbrooke, together with colleagues from Germany, Canada, France, the USA and Japan, measured the thermal Hall effect in three planar samples and found a significant contribution to the result from thermal phonons in the crystals, with no contribution from magnons.
The physicists used very thin (37 to 168 micrometers thick) single crystals of superconducting cuprates YBa2Cu3Oy, Nd2-xCexCuO4 and La2-y-xEuySrxCuO4 as experimental samples. To measure the thermal Hall conductivity, the scientists obtained a thermal current along the long side of the crystals, and the magnetic field was first placed perpendicular and then parallel to the current to compare the values for the conventional thermal Hall effect and the planar one. The scientists provided a thermal gradient along the sample using a resistive heater connected to one end of the single crystal, attaching a heat sink made of silver-plated resin to the opposite end.
The results of the measurements with a planar applied magnetic field showed that the electrons did not contribute to the Hall effect, since they were not affected by the Lorentz force, which is quite expected for such a configuration, and the magnons did not cause the observed phenomenon, because they were trapped inside the CuO2 planes. The scientists also changed the phonon concentration in the experiment by doping the samples (introducing positively or negatively charged impurities into the material), and the temperature dependences in the samples demonstrated similarity with theoretical predictions for thermal phonons in single crystals.
The contradiction with the hypothesis that thermal phonons cannot arise in such structures due to high crystalline symmetry was explained by the physicists by the fact that the studied samples contained various impurities, defects and antiferromagnetic domains on which the phonons were scattered. The authors of the work thus cast doubt on the theory that chiral magnons or Majorana fermions are the culprits of the planar thermal Hall effect.
There are many varieties of the Hall effect, and even more contradictions and paradoxes associated with this phenomenon. For example, we wrote before about how vacuum fluctuations disrupted the mechanism of the quantum Hall effect.