Fluctuation Dominated Josephson Tunneling with a Scanning Tunneling Microscope

Abstract

We demonstrate Josephson tunneling in vacuum tunnel junctions formed between a superconducting scanning tunneling microscope tip and a Pb film, for junction resistances in the range 50-300 kΩ. We show that the superconducting phase dynamics is dominated by thermal fluctuations, and that the Josephson current appears as a peak centered at small finite voltage. In the presence of microwave fields (f = 15.0 GHz) the peak decreases in magnitude and shifts to higher voltages with increasing rf power, in agreement with theory. 07.79. Cz, 74.40.+k, 74.50.+r Scanning tunneling microscopy (STM) has been extensively used in the study of high-T c superconductors (HTSC), providing a spectroscopic tool with unparalleled energy and spatial resolution. Yet, while superconducting tips have been demonstrated in the past [1] all STM studies so far have been performed using normal-metal tips, thus probing only the single-particle excitation spectrum, the gap structure which is a consequence of superconductivity, but not the superconducting (SC) ground state itself. Results from STM measurements of HTSC show excitation gaps in situations where superconductivity is believed to be absent (pseudo-gap), such as in vortex cores [2] and above T c in underdoped samples [3], as well as inhomogenieties in the gap structure in reportedly high quality BiSrCaCuO crystals [4]. These results, due to the nature of the measurements, do not remove the ambiguity with respect to the existence of a finite SC pair amplitude in the situations studied.

Scanning Josephson spectroscopy on the atomic scale

“…Although spectroscopic mapping with a scanning tunneling microscope (STM) can provide evidence for variations in the local density of states (LDOS) through quasi-particle tunneling, such measurements probe the superconducting order parameter only indirectly. If the Josephson effect can be measured and mapped on the atomic scale, then it would allow for direct characterization of the local pairing order parameter and high-resolution studies of novel superconducting phases [11].This goal has motivated previous efforts in the use of superconducting tips in STM [12] and has led to the local observation of thermal phase fluctuating Josephson supercurrent close to the point contact regime [13][14][15]. Subsequent measurements have mapped the Josephson effect on the nanometer scale, applying this technique to vortices [16,17] and high-T c cuprates [18,19].…”

“…This goal has motivated previous efforts in the use of superconducting tips in STM [12] and has led to the local observation of thermal phase fluctuating Josephson supercurrent close to the point contact regime [13][14][15]. Subsequent measurements have mapped the Josephson effect on the nanometer scale, applying this technique to vortices [16,17] and high-T c cuprates [18,19].…”

Subharmonic gap structure in superconducting scanning tunneling microscope junctions

“…The splitting in the spectrum of scattered waves into sidebands is induced by the time varying phase across the junction with the Josephson fre- quency ω = 2eV / . Our experiments show that in the presence of thermal phase fluctuations there is a considerable broadening in the time evolution of the phase [22], where the linewidth Γ around the Josephson frequency depends on the effective noise temperature and the impedance of the junction's environment. In the RSJ model this broadening can be estimated [28] as Γ ≈ (2e/ ) 2 Z env k B T n = 2eV p / .…”

“…2 shows selected I−V curves from such a sequence, for junction resistances ranging from 70 kΩ to 8 kΩ. New features in the tunneling characteristics emerge as the junction resistance is lowered -a current peak near zero bias that is the signature of pair tunneling [22], and the subharmonic gap structure at voltages 2∆/2e and 2∆/3e. When the data are numerically differentiated (Fig.…”

“…For the data shown here V p = 0.11 ± 0.01 mV, resulting in Γ ∼ 300 GHz. We expect that the effects of phase fluctuations would be to broaden the subgap features [11,18] similarly to the broadening of microwave induced Shapiro steps [22]. It is therefore important to determine whether the broadening of the SGS features in our junctions (Fig.…”