Gapless dispersive continuum in a modulated quantum kagome antiferromagnet

The pursuit of quantum spin liquid (QSL) states in condensed matter physics has drawn attention to kagome antiferromagnets (AFM) where a two-dimensional corner-sharing network of triangles frustrates conventional magnetic orders. While quantum kagome AFMs based on Cu²⁺ (3d⁹, s = ½) ions have been extensively studied, there is so far little work beyond copper-based systems. Here we present our bulk magnetization, specific heat and neutron scattering studies on single crystals of a new titanium fluorides Cs₈RbK₃Ti₁₂F₄₈ where Ti³⁺ (3d¹, s = ½) ions form a modulated quantum kagome antiferromagnet that does not order magnetically down to 1.5 K. Our comprehensive map of the dynamic response function <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>S</mml:mi> <mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mi>Q</mml:mi> <mml:mo>,</mml:mo> <mml:mi>ℏ</mml:mi> <mml:mi>ω</mml:mi></mml:mrow> <mml:mo>)</mml:mo></mml:mrow> </mml:math> acquired at 1.5 K where the heat capacity is T-linear reveals a dispersive continuum emanating from soft lines that extend along (100). The data indicate fractionalized spinon-like excitations with quasi-one-dimensional dispersion within a quasi-two-dimensional spin system.