Zero-field superconducting diode effect induced by magnetic flux in the van der Waals trigonal superconductor <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub> <mml:mi>PtBi</mml:mi> <mml:mn>2</mml:mn> </mml:msub> </mml:math>

The superconducting diode effect is a nonreciprocal transport phenomenon in which the critical current depends on the current direction. This effect is typically realized in superconductors with broken spatial-inversion and time-reversal symmetry. To break the time-reversal symmetry, external magnetic fields are commonly used. Here, we demonstrate a sign-controllable superconducting diode effect under zero external magnetic field in a van der Waals superconductor, trigonal ${\mathrm{PtBi}}_{2}$. The sign of the zero-field superconducting diode effect is controlled by the poling magnetic field, which is a large magnetic field applied prior to measurements. This result indicates that trapped magnetic flux is responsible for breaking the time-reversal symmetry. Our findings highlight the crucial role of trapped magnetic flux in generating the superconducting diode effect and provide a general pathway for realizing a zero-field superconducting diode effect.