Phonon and magnon dynamics across the antiferromagnetic transition in the two-dimensional layered van der Waals material CrSBr

We report temperature-dependent reflectivity spectra of the layered van der Waals magnet CrSBr in the far-infrared region. Polarization-dependent measurements resolve the vibrational modes along the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mi>E</a:mi><a:mo>∥</a:mo><a:mi>a</a:mi></a:mrow></a:math> and <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>b</b:mi></b:math> axes and reveal clear structural anisotropy. While the <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mi>a</c:mi></c:math>-axis phonons notably harden on cooling, the <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>b</d:mi></d:math>-axis phonon frequencies are almost temperature independent. A phonon splitting due to the antiferromagnetic phase transition is observed for the 180 <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msup><e:mi mathvariant="normal">cm</e:mi><e:mrow><e:mo>−</e:mo><e:mn>1</e:mn></e:mrow></e:msup><e:mo> </e:mo><e:mi>a</e:mi></e:math>-axis vibrational mode, accompanied by a phonon softening below <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:msub><g:mi>T</g:mi><g:mi>N</g:mi></g:msub></g:math>. Furthermore, an additional mode with strong magnetic characteristics at <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mrow><h:mo>∼</h:mo><h:mn>360</h:mn></h:mrow><h:mo> </h:mo><h:msup><h:mi mathvariant="normal">cm</h:mi><h:mrow><h:mo>−</h:mo><h:mn>1</h:mn></h:mrow></h:msup></h:math> is identified and attributed to the magnon excitation of CrSBr.