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 was due to chiral magnons or Majorana fermions, but a group of scientists experimentally confirmed that the phenomenon arises from thermal phonons scattered by 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, resulting in the classic Hall effect. If a thermal gradient is created in the sample, directed perpendicular to the magnetic flux, an electric current can be detected in the material—this is the thermal Hall effect. It would seem that perpendicularity of the plane in which the heat flow propagates to the magnetic field is a necessary condition, but recently, scientists discovered the thermal Hall effect in a planar configuration, meaning that both the thermal and magnetic fluxes are directed in the same plane.
It's logical to assume that the cause of this behavior in the samples should be thermal excitations of the crystal lattice—phonons. However, physicists typically blame chiral magnons or Majorana fermions for this anomaly. This is because the formation of thermal phonons is prevented in the systems under study 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 the superconducting cuprates YBa2Cu3Oy, Nd2-xCexCuO4, and La2-y-xEuySrxCuO4 as experimental samples. To measure the thermal Hall conductivity, the scientists generated a thermal current along the long side of the crystals, and placed a magnetic field first perpendicular and then parallel to the current to compare the values for the conventional thermal Hall effect and the planar one. The scientists created a thermal gradient along the sample using a resistive heater connected to one end of the single crystal, with a heat sink made of silver-plated resin attached to the opposite end.
The results of measurements with a planar magnetic field showed that electrons made no contribution to the Hall effect, as they were not subject to the Lorentz force, which is expected for this configuration. Magnons did not cause the observed phenomenon, as they were trapped within the CuO2 planes. The scientists also varied the phonon concentration in the experiment by doping the samples (introducing positively or negatively charged impurities into the material). The temperature dependences in the samples demonstrated similarity with theoretical predictions for thermal phonons in single crystals.
The physicists explained the contradiction with the hypothesis that thermal phonons cannot arise in such structures due to their high crystalline symmetry by the fact that the studied samples contained various impurities, defects, and antiferromagnetic domains, which scattered the phonons. The authors thus cast doubt on the theory that chiral magnons or Majorana fermions are responsible for the planar thermal Hall effect.
There are many variations of the Hall effect, and even more contradictions and paradoxes associated with this phenomenon. For example, we previously wrote about how vacuum fluctuations disrupted the mechanism of the quantum Hall effect.
