Spin liquids on the tetratrillium lattice

The tetratrillium lattice has recently been proposed as being responsible for the dynamical properties observed in the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mi>S</a:mi> <a:mo>=</a:mo> <a:mn>1</a:mn> </a:mrow> </a:math> langbeinite compound <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi mathvariant="normal">K</b:mi> <b:mn>2</b:mn> </b:msub> <b:msub> <b:mi>Ni</b:mi> <b:mn>2</b:mn> </b:msub> <b:msub> <b:mrow> <b:mo>(</b:mo> <b:msub> <b:mi>SO</b:mi> <b:mn>4</b:mn> </b:msub> <b:mo>)</b:mo> </b:mrow> <b:mn>3</b:mn> </b:msub> </b:mrow> </b:math> . Here, we study in detail the classical spin liquid properties of this lattice of tricoordinated tetrahedra using classical Monte Carlo and large- <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mi>N</d:mi> </d:math> theory calculations. In the large- <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mi>N</e:mi> </e:math> limit, we find that the system presents a gapped spectrum with flat bottom bands, giving rise to a fragile spin liquid with exponentially decaying correlations according to the classification of classical spin liquids. We confirm that this scenario also holds in the more realistic Ising and Heisenberg cases, for which the system does not exhibit any finite-temperature phase transition, and the low-temperature spin structure factors exhibit excellent quantitative agreement with the large- <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mi>N</f:mi> </f:math> theory. We also provide insight into the quantum <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"> <g:mrow> <g:mi>S</g:mi> <g:mo>=</g:mo> <g:mn>1</g:mn> <g:mo>/</g:mo> <g:mn>2</g:mn> </g:mrow> </g:math> limit by performing pseudo-Majorana functional renormalization-group calculations at finite temperatures, and we discuss the possible phases that can arise in the ground state due to quantum fluctuations.