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- Title
The Andreev conductance in superconductor-insulator-normal metal structures.
- Authors
Seliverstov, A.; Tarasov, M.; Edel'man, V.
- Abstract
The Andreev subgap conductance at 0.08-0.2 K in thin-film superconductor (aluminum)-insulator-normal metal (copper, hafnium, or aluminum with iron-sublayer-suppressed superconductivity) structures is studied. The measurements are performed in a magnetic field oriented either along the normal or in the plane of the structure. The dc current-voltage ( I-U) characteristics of samples are described using a sum of the Andreev subgap current dominating in the absence of the field at bias voltages U < (0.2-0.4)Δ/ e (where Δ is the energy gap of the superconductor) and the single-carrier tunneling current that predominates at large voltages. To within the measurement accuracy of 1-2%, the Andreev current corresponds to the formula $${I_n} + {I_s} = {K_n}\tanh \left( {{{eU} \mathord{\left/ {\vphantom {{eU} {2k{T_{eff}}}}} \right. \kern-\nulldelimiterspace} {2k{T_{eff}}}}} \right) + {K_s}{{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}$$ following from a theory that takes into account mesoscopic phenomena with properly selected effective temperature T and the temperature- and fieldindependent parameters K and K (characterizing the diffusion of electrons in the normal metal and superconductor, respectively). The experimental value of K agrees in order of magnitude with the theoretical prediction, while K is several dozen times larger than the theoretical value. The values of T in the absence of the field for the structures with copper and hafnium are close to the sample temperature, while the value for aluminum with an iron sublayer is several times greater than this temperature. For the structure with copper at T = 0.08-0.1 K in the magnetic field B = 200-300 G oriented in the plane of the sample, the effective temperature T increases to 0.4 K, while that in the perpendicular (normal) field B ≈ 30 G increases to 0.17 K. In large fields, the Andreev conductance cannot be reliably recognized against the background of single- carrier tunneling current. In the structures with hafnium and in those with aluminum on an iron sublayer, the influence of the magnetic field is not observed.
- Subjects
ELECTRIC admittance; SUPERCONDUCTOR-insulator-superconductor devices; ELECTRIC potential measurement; SUPERCONDUCTIVITY; BAND gaps; VOLTAGE control
- Publication
Journal of Experimental & Theoretical Physics, 2017, Vol 124, Issue 4, p643
- ISSN
1063-7761
- Publication type
Article
- DOI
10.1134/S1063776117030153