Transport Spectroscopy of an Impurity Spin in a Carbon Nanotube Double Quantum Dot

Abstract

We make use of spin selection rules to investigate the electron spin system of a carbon nanotube double quantum dot. Measurements of the electron transport as a function of the magnetic field and energy detuning between the quantum dots reveal an intricate pattern of the spin state evolution. We demonstrate that the complete set of measurements can be understood by taking into account the interplay between spin-orbit interaction and a single impurity spin coupled to the double dot. The detection and tunability of this coupling are important for quantum manipulation in carbon nanotubes. DOI: 10.1103/PhysRevLett.106.206801 PACS numbers: 73.63.Kv, 71.70.Ej, 73.23.Hk, 73.63.Fg Spin qubits defined in carbon nanotube quantum dots are of considerable interest for encoding and manipulating quantum information. Because of the absence of hyperfine interaction in the dominant 12 C isotope, spin coherence times are expected to be exceptionally long, while the presence of spin-orbit interaction [1-4] may allow for electrical or even optical [5] control of the spin states. However, before carbon nanotubes can find applications in quantum information processing schemes, we need to understand and control the coupling between individual electron spins and the interaction of the electron spins with their environment.A powerful method to probe the spin system of quantum dots is to measure the electron transport in a double quantum dot device in the spin blockade regime [6]. In this transport regime, the tunneling of an electron between the quantum dots is forbidden by spin selection rules; hence, the current is suppressed. However, the spin blockade can be lifted by the interaction of the electron spins with their environment, and a measurement of the (leakage) current thus directly probes these interactions. The main spin relaxation and decoherence mechanisms in carbon nanotubes that have been considered so far are spin-orbit coupling and hyperfine interaction in 12 C-enriched nanotubes [7,8]. However, a further important consideration in any realistic nanotube device is the presence of bends, impurities, or defects and their coupling to the electron spins. In this work we make use of the excellent spin sensitivity in the spin blockade regime to investigate the spin system coupling of a carbon nanotube double quantum dot and spin impurities in its environment. In a series of detailed measurements, we show how the interplay of a single impurity spin and the spin-orbit interaction affects the spin states of the nanotube double quantum dot and demonstrate that the coupling to spin impurities can be tuned by gate electrodes.The device that we consider is a single-walled carbon nanotube grown with natural isotope ratios contacted by Au electrodes. Side and top barrier gates are used to define and control the double quantum dot; see Fig. 1(a). A typical charge stability diagram of the device is shown in Figs. 1(c) and 1(d) in which the ordered pairs ðn; mÞ indicate the effective electron occupancies of the many-electron double quan...

Measuring the Complex Admittance of a Carbon Nanotube Double Quantum Dot

“…Nevertheless, spin blockade has been observed previously in carbon nanotubes [10,11,13]. In the spin blockade regime, transitions between a (1,1) and (0,2) charge state directly depend on whether the (1,1) state is a singlet or triplet.…”

“…Our measurements thus present a first quantitative analysis of the complex admittance of a double quantum dot. The demonstrated technique also provides the basis for a simple and fast detection scheme for charge and spin state readout in carbon nanotubes -a material with considerable potential for spin-based quantum information processing [8][9][10][11][12][13] -without the need for a separate charge detector [14].…”

Electronic structure of (1e,1h) states of carbon nanotube quantum dots

“…The spin transitions in quantum dots (QDs) are monitored experimentally via the spin-valley blockade [16][17][18][19][20] of the current flow across the double dot system. The current blockade is lifted by the electric dipole spin resonance (EDSR) driven by ac external voltages [19][20][21][22][23][24][25] for resonant frequencies.…”

Shape-sensitive Pauli blockade in a bent carbon nanotube

“…This case is relevant for, e.g., III-V semiconductors [33,34,37], and CNTs with strong disorder [36,38]. In the absence of singlet-triplet mixing, the (1,1) → (0,2) transition is forbidden for the triplet states by Pauli's exclusion principle, hence the current is zero.…”