<p>We consider <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathcal {O}(n)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi mathvariant="script">O</mi> <mo stretchy="false">(</mo> <mi>n</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>-invariant and reflection-positive quantum spin systems on the integer lattice in any dimension, and prove that at low temperatures, spin-spin correlations decay exponentially fast provided <i>n</i> is large enough. This answers a question of Ueltschi, who proved that for small <i>n</i> there is instead long-range order at low temperatures (for <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(d\ge 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>d</mi> <mo>≥</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation>).</p>
Exponential Decay in \(\mathcal {O}(n)\)-Invariant Quantum Spin Systems
We consider \(\mathcal {O}(n)\)-invariant and reflection-positive quantum spin systems on the integer lattice in any dimension, and prove that at low temperatures, spin-spin correlations decay exponentially fast provided n is large enough. This answers a question of Ueltschi, who proved that for small n there is instead long-range order at low temperatures (for \(d\ge 3\)).