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Microwave spectroscopy evidence of superconducting pairing in the magnetic- field-induced metallic state of InOx films at zero temperature

Event Details

Event Dates: 

Thursday, April 17, 2014 - 12:00pm

Seminar Location: 

  • JILA X317

Speaker Name(s): 

N. Peter Armitage

Speaker Affiliation(s): 

Johns Hopkins University
Seminar Type/Subject

Scientific Seminar Type: 

  • Condensed Matter Seminar

Event Details & Abstract: 

The conventional wisdom is that metallic states don’t exist in two spatial dimensions.  Counter to this “wisdom”, is the observation that - in a wide variety of instances - a quantum phase transition to a dissipative state is observed.  In this work, we have investigated one such case in the field-tuned quantum phase transition in a 2D low-disorder amorphous InOx film using microwave spectroscopy. In the zero-temperature limit, the ac data are consistent with a scenario where this transition is from a superconductor to a metal instead of a direct transition to an insulator. The intervening metallic phase is unusual with a small but finite resistance that is much smaller than the normal state sheet resistance at the lowest measured temperatures. Moreover, it exhibits a superconducting response on short length and time scales while global superconductivity is destroyed.  Such a metallic state with superconducting correlations that persist to the lowest temperatures, cannot be a conventional metal.  We present evidence that the true quantum critical point of this 2D superconductor metal transition is located at a field Bsm far below the conventionally defined critical field Bcross where different isotherms of magnetoresistance cross each other. The superfluid stiffness in the low- frequency limit and the superconducting fluctuation frequency from opposite sides of the transition both vanish at B ~ Bsm. The lack of evidence for finite-frequency superfluid stiffness surviving Bcross signifies that Bcross is a crossover above which superconducting fluctuations make a vanishing contribution to dc and ac measurements.