Magnetic-field dependence of energy levels in ultrasmall metal grains

We present a theory of mesoscopic fluctuations of g tensors and avoided crossing energies in a small metal grain. The model, based on random matrix theory, contains both the orbital and spin contributions to the g tensor. The two contributions can be experimentally separated for weak spin-orbit coupling while they merge in the strong coupling limit. For intermediate coupling, substantial correlations are found between g factors of neighboring levels.Comment: 9 pages, 5 figure

Relaxation, dephasing, and quantum control of electron spins in double quantum dots

Recent experiments have demonstrated quantum manipulation of two-electron spin states in double quantum dots using electrically controlled exchange interactions. Here, we present a detailed theory for electron spin dynamics in two-electron double dot systems that was used to guide these experiments and analyze experimental results. The theory treats both charge and spin degrees of freedom on an equal basis. Specifically, we analyze the relaxation and dephasing mechanisms that are relevant to experiments and discuss practical approaches for quantum control of two-electron system. We show that both charge and spin dephasing play important roles in the dynamics of the two-spin system, but neither represents a fundamental limit for electrical control of spin degrees of freedom in semiconductor quantum bits.Comment: 18 pages, 10 figures (reduced in length from V1, removed extraneous content, added references

Charge Relaxation in a Single Electron Si/SiGe Double Quantum Dot

We measure the interdot charge relaxation time T_1 of a single electron trapped in an accumulation mode Si/SiGe double quantum dot. The energy level structure of the charge qubit is determined using photon assisted tunneling, which reveals the presence of a low lying excited state. We systematically measure T_1 as a function of detuning and interdot tunnel coupling and show that it is tunable over four orders of magnitude, with a maximum of 45 microseconds for our device configuration. Measured relaxation times are consistent with a phonon mediated energy relaxation process and indicate that low lying excited states may have important implications in the development of silicon spin qubits.Comment: Related papers at http://pettagroup.princeton.ed

Studies of spin-orbit scattering in noble-metal nanoparticles using energy level tunneling spectroscopy

The effects of spin-orbit scattering on discrete electronic energy levels are studied in copper, silver, and gold nanoparticles. Level-to-level fluctuations of the effective $g$-factor for Zeeman splitting are characterized, and the statistics are found to be well-described by random matrix theory predictions. The strength of spin-orbit scattering increases with atomic number and also varies between nanoparticles made of the same metal. The spin-orbit scattering rates in the nanoparticles are in order-of-magnitude agreement with bulk measurements on disordered samples.Comment: 4 pages, 3 figures, 1 in colo

Singlet-triplet spin blockade and charge sensing in a few-electron double quantum dot

Singlet-triplet spin blockade in a few-electron lateral double quantum dot is investigated using simultaneous transport and charge-sensing measurements. Transport from the (1,1) to the (0,2) electron occupancy states is strongly suppressed relative to the opposite bias [(0,2) to (1,1)]. At large bias, spin blockade ceases as the (0,2) triplet state enters the transport window, giving a direct measure of exchange splitting as a function of magnetic field. A simple model for current and steady-state charge distribution in spin-blockade conditions is developed and found to be in excellent agreement with experiment.Comment: related papers available at http://marcuslab.harvard.ed

Circuit Quantum Electrodynamics with a Spin Qubit

Circuit quantum electrodynamics allows spatially separated superconducting qubits to interact via a "quantum bus", enabling two-qubit entanglement and the implementation of simple quantum algorithms. We combine the circuit quantum electrodynamics architecture with spin qubits by coupling an InAs nanowire double quantum dot to a superconducting cavity. We drive single spin rotations using electric dipole spin resonance and demonstrate that photons trapped in the cavity are sensitive to single spin dynamics. The hybrid quantum system allows measurements of the spin lifetime and the observation of coherent spin rotations. Our results demonstrate that a spin-cavity coupling strength of 1 MHz is feasible.Comment: Related papers at http://pettagroup.princeton.edu

Kondo Effect in Electromigrated Gold Break Junctions

We present gate-dependent transport measurements of Kondo impurities in bare gold break junctions, generated with high yield using an electromigration process that is actively controlled. Thirty percent of measured devices show zero-bias conductance peaks. Temperature dependence suggests Kondo temperatures \~7K. The peak splitting in magnetic field is consistent with theoretical predictions for g=2, though in many devices the splitting is offset from 2guB by a fixed energy. The Kondo resonances observed here may be due to atomic-scale metallic grains formed during electromigration.Comment: 5 pages, 3 figure

Electronic and Magnetic Properties of Partially-Open Carbon Nanotubes

On the basis of the spin-polarized density functional theory calculations, we demonstrate that partially-open carbon nanotubes (CNTs) observed in recent experiments have rich electronic and magnetic properties which depend on the degree of the opening. A partially-open armchair CNT is converted from a metal to a semiconductor, and then to a spin-polarized semiconductor by increasing the length of the opening on the wall. Spin-polarized states become increasingly more stable than nonmagnetic states as the length of the opening is further increased. In addition, external electric fields or chemical modifications are usable to control the electronic and magnetic properties of the system. We show that half-metallicity may be achieved and the spin current may be controlled by external electric fields or by asymmetric functionalization of the edges of the opening. Our findings suggest that partially-open CNTs may offer unique opportunities for the future development of nanoscale electronics and spintronics.Comment: 6 figures, to appear in J. Am. Chem. So