Upper limit on magnetic field expulsion in optically driven <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi mathvariant="normal">K</mml:mi> <mml:mn>3</mml:mn> </mml:msub> <mml:msub> <mml:mi mathvariant="normal"

Photoexcited <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi mathvariant="normal">K</a:mi> <a:mn>3</a:mn> </a:msub> <a:msub> <a:mi mathvariant="normal">C</a:mi> <a:mn>60</a:mn> </a:msub> </a:mrow> </a:math> displays several properties reminiscent of equilibrium superconductivity, including transient optical spectra, pressure dependence, and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"> <d:mrow> <d:mi>I</d:mi> </d:mrow> </d:math> – <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"> <e:mrow> <e:mi>V</e:mi> </e:mrow> </e:math> characteristics. However, these observations do not decisively establish nonequilibrium superconductivity, which may be further evidenced by transient magnetic field expulsion measurements, as shown recently in driven <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:mi>YB</f:mi> <f:msub> <f:mi mathvariant="normal">a</f:mi> <f:mn>2</f:mn> </f:msub> <f:mi mathvariant="normal">C</f:mi> <f:msub> <f:mi mathvariant="normal">u</f:mi> <f:mn>3</f:mn> </f:msub> <f:msub> <f:mi mathvariant="normal">O</f:mi> <f:mrow> <f:mn>6.48</f:mn> </f:mrow> </f:msub> </f:mrow> </f:math> . Here, we search for transient magnetic field expulsion in <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"> <k:mrow> <k:msub> <k:mi mathvariant="normal">K</k:mi> <k:mn>3</k:mn> </k:msub> <k:msub> <k:mi mathvariant="normal">C</k:mi> <k:mn>60</k:mn> </k:msub> </k:mrow> </k:math> by measuring Faraday rotation in a magneto-optic material placed in its vicinity. Unlike in the case of <n:math xmlns:n="http://www.w3.org/1998/Math/MathML"> <n:mrow> <n:mi>YB</n:mi> <n:msub> <n:mi mathvariant="normal">a</n:mi> <n:mn>2</n:mn> </n:msub> <n:mi mathvariant="normal">C</n:mi> <n:msub> <n:mi mathvariant="normal">u</n:mi> <n:mn>3</n:mn> </n:msub> <n:msub> <n:mi mathvariant="normal">O</n:mi> <n:mrow> <n:mn>6.48</n:mn> </n:mrow> </n:msub> </n:mrow> </n:math> , inhomogeneous, metallic <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:mrow> <s:msub> <s:mi mathvariant="normal">K</s:mi> <s:mn>3</s:mn> </s:msub> <s:msub> <s:mi mathvariant="normal">C</s:mi> <s:mn>60</s:mn> </s:msub> </s:mrow> </s:math> powders reduce the size of the effect. With the ∼50 nT magnetic field resolution achieved in our experiments, we provide an upper limit for the photoinduced diamagnetic volume susceptibility ( <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"> <v:mrow> <v:msub> <v:mi>χ</v:mi> <v:mi>v</v:mi> </v:msub> <v:mo>&gt;</v:mo> <v:mo>−</v:mo> <v:mn>0</v:mn> <v:mrow> <v:mo>.</v:mo> <v:mn>1</v:mn> </v:mrow> </v:mrow> </v:math> ). On this basis, we conclude that the photoinduced phase has weaker magnetic susceptibility than superconducting <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"> <w:mrow> <w:msub> <w:mi mathvariant="normal">K</w:mi> <w:mn>3</w:mn> </w:msub> <w:msub> <w:mi mathvariant="normal">C</w:mi> <w:mn>60</w:mn> </w:msub> </w:mrow> </w:math> at zero temperature. Yet, from recent nonlinear transport measurements in this granular material, we expect a light-induced state similar to the equilibrium superconductor near <z:math xmlns:z="http://www.w3.org/1998/Math/MathML"> <z:mrow> <z:mn>0.8</z:mn> <z:mspace width="0.16em"/> <z:msub> <z:mi>T</z:mi> <z:mi>c</z:mi> </z:msub> </z:mrow> </z:math> , for which <bb:math xmlns:bb="http://www.w3.org/1998/Math/MathML"> <bb:mrow> <bb:msub> <bb:mi>χ</bb:mi> <bb:mi>v</bb:mi> </bb:msub> <bb:mo>&gt;</bb:mo> <bb:mo>−</bb:mo> <bb:mn>0</bb:mn> <bb:mrow> <bb:mo>.</bb:mo> <bb:mn>1</bb:mn> </bb:mrow> </bb:mrow> </bb:math> . A definitive conclusion on the presence or absence of Meissner diamagnetism cannot be made for <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML"> <cb:mrow> <cb:msub> <cb:mi mathvariant="normal">K</cb:mi> <cb:mn>3</cb:mn> </cb:msub> <cb:msub> <cb:mi mathvariant="normal">C</cb:mi> <cb:mn>60</cb:mn> </cb:msub> </cb:mrow> </cb:math> with the current resolution.