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Induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures.

Wan Z, Kazakov A, Manfra MJ, Pfeiffer LN, West KW, Rokhinson LP - Nat Commun (2015)

Bottom Line: Search for Majorana fermions renewed interest in semiconductor-superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime.Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor-superconductor ohmic contacts.High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA.

ABSTRACT
Search for Majorana fermions renewed interest in semiconductor-superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor-superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

No MeSH data available.


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Magnetic field dependence of induced superconductivity.(a,b) Differential resistance is measured as a function of B and Id.c. for two samples at 40 mK. Induced superconductivity (black region) is observed up to 0.2 T in both the samples. (c) 3-terminal resistance for a sample with all normal contacts (red) and between normal and superconducting contacts in sample B (I (2–4) and V (4−1) in Fig. 1) is measured at 70 and 40 mK, respectively. B<0 (B>0) induces clockwise (counterclockwise) chiral edge channels, note resistance scales difference for two field directions.
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f4: Magnetic field dependence of induced superconductivity.(a,b) Differential resistance is measured as a function of B and Id.c. for two samples at 40 mK. Induced superconductivity (black region) is observed up to 0.2 T in both the samples. (c) 3-terminal resistance for a sample with all normal contacts (red) and between normal and superconducting contacts in sample B (I (2–4) and V (4−1) in Fig. 1) is measured at 70 and 40 mK, respectively. B<0 (B>0) induces clockwise (counterclockwise) chiral edge channels, note resistance scales difference for two field directions.

Mentions: In one-dimensional junctions, the induced gap depends on the broadening of Andreev levels within the semiconductor38, where we introduce contacts transparencies D1 and D2. We assume for simplicity that D1=D2=1/(1+Z2), where 0<Z<∞ is a interface barrier strength introduced in ref. 39, and Bagwell's effective channel length Leff=L+2ξ0. Using NbN superconducting gap, (NbN is a strong-coupling superconductor, ) and Tc=0.3 K for we obtain Z=0.2. This value is consistent with the fit of the Ic versus T-dependence with D as a free parameter (Supplementary Fig. 2; Supplementary Note 2). Similar values of Z can be estimated from the analysis of the shape of dI/dV(V) characteristics at elevated temperatures, as shown in Fig. 3. At , Andreev reflection at S–2DEG interfaces results in an excess current flowing through the junction for voltage biases within the superconducting gap Δ0/e and corresponding reduction of a differential resistance dV/dI by a factor of 2. In the presence of a tunnelling barrier, normal reflection competes with Andreev reflection and reduced excess current near zero bias, resulting in a peak in differential resistance. Within the Blonder–Tinkham–Klapwijk theory39, a flat dV/dI(V) within Δ0/e, observed in our experiments, is expected only for contacts with very high transparency Z<0.2. For larger Z>0.2, a peak at low biases is expected (Supplementary Fig. 3, Supplementary Note 3). Several features of the experimental I(V) need to be mentioned. First, we observe several sharp peaks in the resistance at high biases (around 2 and 4 mV for T=4 K). Similar sharp resonances has been observed previously40, where authors attributed their appearance to the formation of Fabry–Pérot resonances between superconducting contacts. In our devices, the superconducting region is shunted by a low resistance (<100 Ω) 2DEG, thus appearance of >10 kΩ resonances cannot be explained by resonant electron trapping between contacts. These resonances are also observed in I(V) characteristics of a single S–2DEG interface (measured in the S–2DEG–N configuration between contacts 3 and 6, see Supplementary Fig. 3). Differential resistance does not change substantially across resonances, ruling out transport through a localized state. We speculate that in the contacts where these resonances are observed superconductivity is carried out by quasi-one-dimensional channels, and jumps in I/V characteristics are due to flux trapping at high currents. This scenario is consistent with the observation that peaks shift to lower currents at higher fields, see Fig. 4. The second notable feature of our data is reduction of the zero-bias resistance by ≈2.6 times at low temperatures, while Andreev reflection limits the reduction to the factor of 2. We attribute this reduction to the multiple Andreev reflection between two closely spaced contacts, for contacts with larger separation (20 μm) multiple Andreev reflection is suppressed and the reduction of resistance by a factor of 2 is observed (Supplementary Fig. 3).


Induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures.

Wan Z, Kazakov A, Manfra MJ, Pfeiffer LN, West KW, Rokhinson LP - Nat Commun (2015)

Magnetic field dependence of induced superconductivity.(a,b) Differential resistance is measured as a function of B and Id.c. for two samples at 40 mK. Induced superconductivity (black region) is observed up to 0.2 T in both the samples. (c) 3-terminal resistance for a sample with all normal contacts (red) and between normal and superconducting contacts in sample B (I (2–4) and V (4−1) in Fig. 1) is measured at 70 and 40 mK, respectively. B<0 (B>0) induces clockwise (counterclockwise) chiral edge channels, note resistance scales difference for two field directions.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4490403&req=5

f4: Magnetic field dependence of induced superconductivity.(a,b) Differential resistance is measured as a function of B and Id.c. for two samples at 40 mK. Induced superconductivity (black region) is observed up to 0.2 T in both the samples. (c) 3-terminal resistance for a sample with all normal contacts (red) and between normal and superconducting contacts in sample B (I (2–4) and V (4−1) in Fig. 1) is measured at 70 and 40 mK, respectively. B<0 (B>0) induces clockwise (counterclockwise) chiral edge channels, note resistance scales difference for two field directions.
Mentions: In one-dimensional junctions, the induced gap depends on the broadening of Andreev levels within the semiconductor38, where we introduce contacts transparencies D1 and D2. We assume for simplicity that D1=D2=1/(1+Z2), where 0<Z<∞ is a interface barrier strength introduced in ref. 39, and Bagwell's effective channel length Leff=L+2ξ0. Using NbN superconducting gap, (NbN is a strong-coupling superconductor, ) and Tc=0.3 K for we obtain Z=0.2. This value is consistent with the fit of the Ic versus T-dependence with D as a free parameter (Supplementary Fig. 2; Supplementary Note 2). Similar values of Z can be estimated from the analysis of the shape of dI/dV(V) characteristics at elevated temperatures, as shown in Fig. 3. At , Andreev reflection at S–2DEG interfaces results in an excess current flowing through the junction for voltage biases within the superconducting gap Δ0/e and corresponding reduction of a differential resistance dV/dI by a factor of 2. In the presence of a tunnelling barrier, normal reflection competes with Andreev reflection and reduced excess current near zero bias, resulting in a peak in differential resistance. Within the Blonder–Tinkham–Klapwijk theory39, a flat dV/dI(V) within Δ0/e, observed in our experiments, is expected only for contacts with very high transparency Z<0.2. For larger Z>0.2, a peak at low biases is expected (Supplementary Fig. 3, Supplementary Note 3). Several features of the experimental I(V) need to be mentioned. First, we observe several sharp peaks in the resistance at high biases (around 2 and 4 mV for T=4 K). Similar sharp resonances has been observed previously40, where authors attributed their appearance to the formation of Fabry–Pérot resonances between superconducting contacts. In our devices, the superconducting region is shunted by a low resistance (<100 Ω) 2DEG, thus appearance of >10 kΩ resonances cannot be explained by resonant electron trapping between contacts. These resonances are also observed in I(V) characteristics of a single S–2DEG interface (measured in the S–2DEG–N configuration between contacts 3 and 6, see Supplementary Fig. 3). Differential resistance does not change substantially across resonances, ruling out transport through a localized state. We speculate that in the contacts where these resonances are observed superconductivity is carried out by quasi-one-dimensional channels, and jumps in I/V characteristics are due to flux trapping at high currents. This scenario is consistent with the observation that peaks shift to lower currents at higher fields, see Fig. 4. The second notable feature of our data is reduction of the zero-bias resistance by ≈2.6 times at low temperatures, while Andreev reflection limits the reduction to the factor of 2. We attribute this reduction to the multiple Andreev reflection between two closely spaced contacts, for contacts with larger separation (20 μm) multiple Andreev reflection is suppressed and the reduction of resistance by a factor of 2 is observed (Supplementary Fig. 3).

Bottom Line: Search for Majorana fermions renewed interest in semiconductor-superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime.Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor-superconductor ohmic contacts.High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

View Article: PubMed Central - PubMed

Affiliation: Department of Physics and Astronomy, Purdue University, West Lafayette, Indiana 47907, USA.

ABSTRACT
Search for Majorana fermions renewed interest in semiconductor-superconductor interfaces, while a quest for higher-order non-Abelian excitations demands formation of superconducting contacts to materials with fractionalized excitations, such as a two-dimensional electron gas in a fractional quantum Hall regime. Here we report induced superconductivity in high-mobility two-dimensional electron gas in gallium arsenide heterostructures and development of highly transparent semiconductor-superconductor ohmic contacts. Supercurrent with characteristic temperature dependence of a ballistic junction has been observed across 0.6 μm, a regime previously achieved only in point contacts but essential to the formation of well separated non-Abelian states. High critical fields (>16 T) in NbN contacts enables investigation of an interplay between superconductivity and strongly correlated states in a two-dimensional electron gas at high magnetic fields.

No MeSH data available.


Related in: MedlinePlus