Observation of Unprecedented Fractional Magnetization Plateaus in a New Shastry-Sutherland Ising Compound

Geometrically frustrated magnetic systems, such as those based on the Shastry-Sutherland lattice (SSL), offer a rich playground for exploring unconventional magnetic states. The delicate balance between competing interactions in these systems leads to the emergence of novel phases. We present the characterization of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:msub> <a:mrow> <a:mi>Er</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mrow> <a:msub> <a:mrow> <a:mi>Be</a:mi> </a:mrow> <a:mrow> <a:mn>2</a:mn> </a:mrow> </a:msub> </a:mrow> <a:mrow> <a:msub> <a:mrow> <a:mi>GeO</a:mi> </a:mrow> <a:mrow> <a:mn>7</a:mn> </a:mrow> </a:msub> </a:mrow> </a:math> , an SSL compound with <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:msup> <c:mrow> <c:mi>Er</c:mi> </c:mrow> <c:mrow> <c:mn>3</c:mn> <c:mo>+</c:mo> </c:mrow> </c:msup> </c:mrow> </c:math> ions forming orthogonal dimers separated by nonmagnetic layers whose structure is invariant under the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mi>P</e:mi> <e:mover accent="true"> <e:mn>4</e:mn> <e:mo stretchy="false">¯</e:mo> </e:mover> <e:msub> <e:mn>2</e:mn> <e:mn>1</e:mn> </e:msub> <e:mi>m</e:mi> </e:math> space group. Neutron scattering reveals an antiferromagnetic dimer structure at zero field, typical of Ising spins on that lattice and consistent with the anisotropic magnetization observed. However, magnetization measurements exhibit fractional plateaus at <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"> <i:mrow> <i:mn>1</i:mn> <i:mo>/</i:mo> <i:mn>4</i:mn> </i:mrow> </i:math> and <k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"> <k:mrow> <k:mn>1</k:mn> <k:mo>/</k:mo> <k:mn>2</k:mn> </k:mrow> </k:math> of saturation, in contrast to the expected <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"> <m:mrow> <m:mn>1</m:mn> <m:mo>/</m:mo> <m:mn>3</m:mn> </m:mrow> </m:math> plateau of the SSL Ising model. By comparing the energy of candidate states with ground-state lower bounds we show that this behavior requires spatially anisotropic interactions, leading to an anisotropic Shastry-Sutherland Ising model symmetric under the <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"> <o:mi>C</o:mi> <o:mi>m</o:mi> <o:mi>m</o:mi> <o:mn>2</o:mn> </o:math> space group. This anisotropy is consistent with the small orthorhombic distortion observed with single-crystal neutron diffraction. The other properties, including thermodynamics, which have been investigated theoretically using tensor networks, point to small residual interactions, potentially due to further couplings and quantum fluctuations. This study highlights <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"> <q:mrow> <q:msub> <q:mrow> <q:mi>Er</q:mi> </q:mrow> <q:mrow> <q:mn>2</q:mn> </q:mrow> </q:msub> </q:mrow> <q:mrow> <q:msub> <q:mrow> <q:mi>Be</q:mi> </q:mrow> <q:mrow> <q:mn>2</q:mn> </q:mrow> </q:msub> </q:mrow> <q:mrow> <q:msub> <q:mrow> <q:mi>GeO</q:mi> </q:mrow> <q:mrow> <q:mn>7</q:mn> </q:mrow> </q:msub> </q:mrow> </q:math> as a promising platform for investigating exotic magnetic phenomena.