Magnetoresistance engineering and singlet/triplet switching in InAs nanowire quantum dots with ferromagnetic sidegates

G. Fábián, P. Makk, M. H. Madsen, J. Nygård, C. Schönenberger, and A. Baumgartner

Phys. Rev. B 94, 195415 – Published 10 November 2016

Abstract

We present magnetoresistance (MR) experiments on an InAs nanowire quantum dot device with two ferromagnetic sidegates (FSGs) in a split-gate geometry. The wire segment can be electrically tuned to a single dot or to a double dot regime using the FSGs and a backgate. In both regimes we find a strong MR and a sharp MR switching of up to 25% at the field at which the magnetizations of the FSGs are inverted by the external field. The sign and amplitude of the MR and the MR switching can both be tuned electrically by the FSGs. In a double dot regime close to pinch-off we find two sharp transitions in the conductance, reminiscent of tunneling MR (TMR) between two ferromagnetic contacts, with one transition near zero and one at the FSG switching fields. These surprisingly rich characteristics we explain in several simple resonant tunneling models. For example, the TMR-like MR can be understood as a stray-field controlled transitions between singlet and triplet double dot states. Such local magnetic fields are the key elements in various proposals to engineer novel states of matter and may be used for testing electron spin based Bell inequalities.

Received 25 August 2016

DOI:https://doi.org/10.1103/PhysRevB.94.195415

Physics Subject Headings (PhySH)

Andreev reflection Coulomb blockade Electrical conductivity Ferromagnetism Magnetization switching Magnetoresistance Magnetotransport Micromagnetism Semiconductors Spin current Spin filtering Surface states

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

G. Fábián¹, P. Makk¹, M. H. Madsen^2,*, J. Nygård², C. Schönenberger¹, and A. Baumgartner^1,†

¹Institute of Physics, University of Basel, Klingelbergstrasse 82, 4056 Basel, Switzerland
²Center for Quantum Devices and Nano-Science Center, Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark

^*Present address: Danish Fundamental Metrology, DK-2800 Kgs. Lyngby, Denmark.
^†andreas.baumgartner@unibas.ch

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Vol. 94, Iss. 19 — 15 November 2016

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Images

Figure 1
(a) Schematic of a NW with ferromagnetic sidegates (FSG1 and FSG2) that result in a local stray field $B_{st}$ . The magnetizations $\vec{M}$ of the FSGs can be inverted by an external field $B$ . (b) False-color SEM image of the actual device. (c) Magnetic force microscopy (MFM) image showing the magnetic field component perpendicular to the wafer surface of two Py strips outlined by dashed lines and oriented as the FSGs in (b).
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Figure 2
(a) Differential conductance $G$ as a function of the voltages $V_{SG 1}$ and $V_{SG 2}$ applied to the two FSGs. This plot exhibits two regions with characteristics of a single QD (top part) and a DQD (bottom part). (b) $G$ as a function of $V_{SG 1}$ and the external magnetic field $B$ for the gate voltages pointed out by a yellow square in (a). The top and the bottom panel show an up and a down sweep, respectively. (c) Maximum conductance $G_{\max}$ vs $B$ for the resonance in (b). (d) $G_{\max}$ vs $B$ for the resonance labeled by a green circle in (a).
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Figure 3
(a) $G$ as a function of $V_{SG 1}$ and $V_{SG 2}$ in the DQD regime R2. (b) and (c) Large-field MR on and off a DQD triple point, respectively. The latter gate configuration is pointed out by a yellow circle in (a). (d) Low-field MR of the resonance in (c), showing the MRS off a triple point.
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Figure 4
(a) Spin-split energy levels in the left and right QD. (b) and (c) Illustrations of the QD level energies for (b) aligned and (c) nonaligned zero-field QD states. The green arrows point out the magnetic fields of resonant tunneling through the DQD. (d) and (e) Calculated conductance $G$ as a function of $B$ and the gate voltage for resonances (d) on and (e) off a triple point. The overlayed curves show the maximum conductance $G_{\max}$ vs $B$ .
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Figure 5
(a) $G$ as a function of $V_{SG 1}$ and $V_{SG 2}$ in regime R3 at a backgate voltage $V_{BG} = - 0.9$ V. The dashed lines highlight the DQD charging diagram. (b) $G$ as a function of $B$ and $V_{SG 1}$ in an up sweep (upper panel) and a down sweep (lower panel) at the gate voltages indicated in (a) by a green circle. (c) Resonance maxima from (b) vs $B$ . (d) Resonance maximum of a different resonance in regime R3 vs $B$ .
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Figure 6
(a) Energy levels in a strongly coupled DQD for a homogeneous magnetic field and no FSG stray field. The triplet states are labeled by $T_{+}, T_{-}$ , and $T_{0}$ , respectively, and the singlet by $S$ . Where the $S$ and $T_{\pm}$ cross, the lowest energy state changes its character from triplet (T) to a singlet character (S). (b) Similar plot as in (a) with a stray field $B_{st}$ that switches orientation at $B_{sw}$ and a characteristic singlet/triplet transition at $B_{tr}$ . (c) Similar to (b) with nonparallel local magnetic fields $B_{1}$ and $B_{2}$ on the two QDs, with a $\pm 30^{\circ}$ angle with respect to the external field. (d) Calculated conductance $G$ as a function of $B$ obtained from the DQD model discussed in the text.
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