Vortex excitations in the Insulating State of an Oxide Interface

  Michael Mograbi [1]  ,  E. Maniv [1]  ,  P. K. Rout [1]  ,  D. Graf [2]  ,  J. -H Park [2]  ,  Y. Dagan [1]  
[1] Tel-Aviv University
[2] National High Magnetic Field Laboratory, Tallahassee

In two-dimensional (2D) superconductors an insulating state can be induced either by applying

a magnetic field, H, or by increasing disorder. Many scenarios have been put forth to explain the

superconductor to insulator transition (SIT): dominating fermionic physics after the breaking of

Cooper pairs, loss of phase coherence between superconducting islands and localization of Cooper

pairs with concomitant condensation of vortex-type excitations. The difficulty in characterizing the

insulating state and its origin stems from the lack of a continuous mapping of the superconducting

to insulating phase diagram in a single sample. Here we use the 2D electron liquid formed at the

interface between (111) SrTiO3 and LaAlO3 to study the SIT as a function of electrostatic gate

and magnetic field. This crystalline interface surprisingly exhibits very strong features observed

previously only in amorphous systems. These features persist deep into the insulating state. We

identify a new magnetic field scale, Hpairing, where superconducting fuctuations are muted and

find a lengthscale interpreted as the size of the vortex fluctuation in the insulating state. Our

findings suggest that vortex fluctuation excitations and Cooper pair localization are responsible for

the observed SIT and that these excitations surprisingly persist deep into the insulating state.