Single-Electron Transport in Electrically Tunable Nanomagnets

Abstract

We study a single-electron transistor (SET) based upon a II-VI semiconductor quantum dot doped with a single-Mn ion. We present evidence that this system behaves like a quantum nanomagnet whose total spin and magnetic anisotropy depend dramatically both on the number of carriers and their orbital nature. Thereby, the magnetic properties of the nanomagnet can be controlled electrically. Conversely, the electrical properties of this SET depend on the quantum state of the Mn spin, giving rise to spindependent charging energies and hysteresis in the Coulomb blockade oscillations of the linear conductance. DOI: 10.1103/PhysRevLett.98.106805 PACS numbers: 73.23.Hk, 78.55.Et, 78.67.Hc, 85.75.ÿd Nanomagnets attract interest both because of their intriguing behavior as relatively macroscopic quantum objects and their potential technological applications as magnetic bits [1] and qbits [2]. The two fundamental properties of a nanomagnet are the net spin of its ground state, S and its magnetic energy anisotropy tensor, K that governs the stability of the magnetization with respect to quantum and thermal fluctuations. Although recent experiments show that single-molecule magnets like Mn 12 [3,4] or metallic Co [5] nanoparticles can be probed in singlemolecule transistor measurements, their properties can hardly be tuned once they are fabricated. Here we show that a single-electron transistor (SET) consisting of a II-VI quantum dot doped with a single-Mn atom behaves like a tunable nanomagnet whose magnetization and anisotropy axis can be reversibly manipulated electrically. Conversely, the conductance and charging energy of the tunable nanomagnet depend on the quantum state of the Mn spin and are not uniquely determined by the gate and the bias voltage.Our proposal is based on two independent progress in nanofabrication. On one side, the fabrication and optical probing of single CdTe quantum dots doped with a singleMn atom [6 -8]. In the absence of carriers, the spin S 5=2 of the Mn is free. Optical excitation of electron-hole pairs into the dot shows that the Mn spin is exchange coupled to both the electron and the hole [6 -9]. On the other side, the control of the charge state of II-VI semiconductor quantum dots with single-electron accuracy has been experimentally demonstrated [10,11] as well as in the case of single-Mn-doped quantum dots [12] and Mn-doped GaAs islands [13].Hamiltonian.-We consider a CdTe quantum dot (QD) doped with a single-Mn, weakly coupled to two metallic and nonmagnetic electrodes. The dot can be gated so that either the valence band or the conduction band is in resonance with the metallic reservoir and the number of either electrons or holes is varied at will. The total Hamiltonian readsHere H QD is the Hamiltonian for the diluted magnetic semiconductor (DMS) quantum dot. In analogy with the standard model [14] for bulk DMS, H QD describes confined conduction-band electrons and valence holes interacting with a localized Mn spin S 5 2 , denoted asM, via a local exchange interaction. QD carriers o...

Single-exciton spectroscopy of single Mn doped InAs quantum dots

“…Quantum dots doped with a single magnetic atom are a model system for nanospintronics. [9][10][11] These systems can also be used as a reference to interpret the experiments on Mn doped nanocrystals. 12,13 The focus of this work is the single exciton spectroscopy of a single Mn atom in a InAs quantum dot ͑QD͒, motivated by recent experimental results on InAs QD ͑Ref.…”

“…7 This has been explained in terms of the different effective spin Hamiltonian for the Mn as a single additional carrier is added into the dot. 7,11,16 Since Mn is an acceptor in InAs quantum dot, the single Mn exciton PL is expected to be different from CdTe, where Mn acts as an isoelectronic impurity. Even before photoexcitation, charge neutrality implies that the Mn acceptor in InAs binds a hole.…”

Limit cycles and chaos in the current through a quantum dot

“…Several recent works have considered the influence of such an impurity on the transport properties through the dot [25][26][27][28] . In this context, our model can be viewed as the largespin counterpart of the previously studied models and in particular the spin-1/2 impurity model of Refs.…”

Coherent manipulation of a single magnetic atom using polarized single electron transport in a double quantum dot

“…The other QD is localized in a homogeneous magnetic field and connected to a normal conductance, playing a role of a spin filter. In previous study, 2,6,7,9 electrons transport through the magnetic atom without any qualification to their spins, as a result, interaction between the magnetic atom and electrons are very weak. In contrast, the QD spin filter induces spin dependent tunneling, which makes sure each electron would be completely flipped by the magnetic atom before it passes through the filter.…”

“…In a QD doped with a single Mn atom, charge and conductance of the single electron tunneling can be related to the spin state of the Mn atom. 9 Whereas, it is hard to exactly connect a quantum state of the magnetic atom to a single electron in the above QD system, which should be a problem required to be solved for future quantum information processing.…”