I would start with a higher power of x, and then fiddle with that to get what you want.
Something like (x^6)/1000 looks close - change the 6 to a higher number if you want a wider gap, and increase the 1000 to a higher number if you want a less steep curve.
Something like (x^6)/1000 looks close - change the 6 to a higher number if you want a wider gap, and increase the 1000 to a higher number if you want a less steep curve.