Large oscillating nonlocal voltage in multiterminal single-wall carbon nanotube devices

Abstract

We report on the observation of a non-local voltage in a ballistic (quasi) one-dimensional conductor, realized by a single-wall carbon nanotube with four contacts. The contacts divide the tube into three quantum dots which we control by the back-gate voltage Vg. We measure a large oscillating non-local voltage V nl as a function of Vg. Though a classical resistor model can account for a non-local voltage including change of sign, it fails to describe the magnitude properly. The large amplitude of V nl is due to quantum interference effects and can be understood within the scattering-approach of electron transport.PACS numbers: 73.23.Ad,73.63.Fg,73.63.Nm,73.63.Kv,72.80.Rj The recent realization of the spin field-effect transistor in carbon nanotube (CNT) devices [1] demonstrated the ability to control spin transport in a quantum dot (QD) [2]. However, additional effects, such as the anomalous magnetoresistance, can contribute to the observed signal in spin-valves [3,4,5,6]. It seems clear, that despite a number of large responses seen in CNT-based devices [1,7,8,9], one needs to go beyond two terminal structures by realizing multi-terminal devices where non-local measurements are feasible [10]. The non-local measurement in spin-valve devices has been pioneered by Johnson and Silsbee [11] in metallic spin-valves and was further applied to various other systems [12,13,14]. This technique separates spin from charge effects. Recent application of the non-local spin technique in CNTs [10] showed the feasibility and yet tremendous challenge of performing such measurements in low dimensional mesoscopic systems. The hallmark of these measurements is that a positive voltage is measured when the magnetization of the injector and detector electrodes are parallel and a negative only when they are antiparallel. However, it has been reported recently that the four-probe resistance with non-magnetic probes in CNTs can be negative due to interference effects [15]. This suggests that the measurement of the non-local spin transport in mesoscopic systems like CNTs with ferromagnetic contacts should be strongly influenced by quantum interference effects.We report here on measurements of a large non-local voltage V nl in multi-terminal CNT devices (Fig. 1a) in the quantum-dot (QD) regime which changes sign and magnitude as the back-gate voltage is swept. We show that V nl cannot be explained by a classical resistor model. Instead, a quantum approach is required. We also show that in these devices, which have relative transparent contacts with resistances in the range of 10 − 100 kΩ, the magnitude of the oscillating V nl greatly exceeds any nonlocal spin signal.Our devices consist of single-wall CNTs grown by chemical vapor deposition (CVD) and contacted with four probes as shown in Fig 1a. Two middle electrodes are ferromagnetic (F) made of PdNi(20nm)/ Co(25nm)/ Pd(10nm) tri-layer, whereas the two outer probes are normal (N) Pd(40nm) electrodes. A PdNi alloy with 30% Pd is used, because it makes stable contacts to the CNT [1], whi...

Spin–Orbit Interaction in Single Wall Carbon Nanotubes: Symmetry Adapted Tight-Binding Calculation and Effective Model Analysis

“…Spin-dependent current injected from ferro-magnetic lead demonstrates that spin scattering length is longer, at least, than the nanotube length which is typically sub-micron. [8][9][10][11][12][13][14][15][16][17][18] Moreover, spin states in carbon nanotube quantum dot seems to be independent from the orbital as shown the four-electron shell structure. [19][20][21][22][23][24][25][26][27][28] These experiments seem to be consistent with an expectation of few spin scattering events in carbon materials, because of relatively small spin-orbit interaction in a carbon atom.…”

Four-probe measurements of carbon nanotubes with narrow metal contacts

“…A similar four-probe study of carbon nanotubes with narrow metal contacts was reported in a recent preprint. 15 …”

Conductivity of multiwall carbon nanotubes: Role of multiple shells and defects

“…Another approach for these fluctuations is given by Makarovski et al [127] and Gunnarsson et al [128]. The authors found significant voltages in four-point measurements of SWCNTs in non-local geometry and traced it back to different coupling of the probes to the two orbital modes of the tube.…”

“…First, the straight way passing the second contact in the outermost shell directly below the electrode. Although Ke et al [126] calculated that the transmission straight through the contact is drastically decreasing even for narrow contacts, Makarovski et al [127] and Gunnarsson et al [128] both found a non-constant non-local potential in SWCNTs indicating a non-vanishing transmission through the contact. For the MWCNT used in our experiment, an additional pathway that does not cross the contact is possible.…”