Signatures of Valley Kondo Effect in Si/SiGe Quantum Dots

We report measurements consistent with the valley Kondo effect in Si/SiGe quantum dots, evidenced by peaks in the conductance versus source-drain voltage that show strong temperature dependence. The Kondo peaks show unusual behavior in a magnetic field that we interpret as arising from the valley degree of freedom. The interplay of valley and Zeeman splittings is suggested by the presence of side peaks, revealing a zero-field valley splitting between 0.28 to 0.34 meV. A zero-bias conductance peak for non-zero magnetic field, a phenomenon consistent with valley non- conservation in tunneling, is observed in two samples.Comment: 16 pages, 7 figure

Suppression of Decoherence and Disentanglement by the Exchange Interaction

Entangled qubit pairs can serve as a quantum memory or as a resource for quantum communication. The utility of such pairs is measured by how long they take to disentangle or decohere. To answer the question of whether qubit-qubit interactions can prolong entanglement, we calculate the dissipative dynamics of a pair of qubits coupled via the exchange interaction in the presence of random telegraph noise and $1/f$ noise. We show that for maximally entangled (Bell) states, the exchange interaction generally suppresses decoherence and disentanglement. This suppression is more apparent for random telegraph noise if the noise is non-Markovian, whereas for $1/f$ noise the exchange interaction should be comparable in magnitude to strongest noise source. The entangled singlet-triplet superposition state of 2 qubits ($\psi_{\pm}$ Bell state) can be protected by the interaction, while for the triplet-triplet state ($\phi_{\pm}$ Bell state), it is less effective. Thus the former is more suitable for encoding quantum information

Theory of Optical Orientation in n-Type Semiconductors

Time resolved measurements of magnetization in n-GaAs have revealed a rich array of spin decoherence processes, and have shown that fairly long lifetimes (\sim 100 ns) can be achieved under certain circumstances. In time-resolved Faraday rotation and time-resolved Kerr rotation the evolution of the magnetization can be followed as a function of temperature, applied field, doping level and excitation level. We present a theory for the spin relaxation in n-GaAs based on a set of rate equations for two interacting thermalized subsystems of spins: localized states on donor sites and itinerant states in the conduction band. The conduction band spins relax by scattering from defects or phonons through the D'yakonov-Perel' mechanism, while the localized spins relax by interacting with phonons (when in an applied field) or through the Dzyaloshinskii-Moriya interaction. In this model, numerous features of the data, including puzzling temperature and doping dependences of the relaxation time, find an explanation.Comment: 4 pages, 2 figures; revised version has a more complete discussion of the Elliott-Yafet and spin-phonon decay mechanism

Phase diagram of UPt3_3 in the E1gE_{1g} model

The phase diagram of the unconventional superconductor UPt$_3$ is explained under the long-standing hypothesis that the pair wavefunction belongs to the $E_{1g}$ representation of the point group. The main objection to this theory has been that it disagrees with the experimental phase diagram when a field is applied along the c-axis. By a careful analysis of the free energy this objection is shown to be incorrect. This singlet theory also explains the unusual anisotropy in the upper critical field curves, often thought to indicate a triplet pair function.Comment: 11 pages, Revtex, 2 figures (uuencoded, gzip'ed Postscript

Theory of Electron Spin Relaxation in ZnO

Doped ZnO is a promising material for spintronics applications. For such applications, it is important to understand the spin dynamics and particularly the spin coherence of this II-VI semiconductor. The spin lifetime $\tau_{s}$ has been measured by optical orientation experiments, and it shows a surprising non-monotonic behavior with temperature. We explain this behavior by invoking spin exchange between localized and extended states. Interestingly, the effects of spin-orbit coupling are by no means negligible, in spite of the relatively small valence band splitting. This is due to the wurtzite crystal structure of ZnO. Detailed analysis allows us to characterize the impurity binding energies and densities, showing that optical orientation experiments can be used as a characterization tool for semiconductor samples.Comment: 7 pages, 1 figure: minor changes Accepted by Phys. Rev.

Single-shot measurement of triplet-singlet relaxation in a Si/SiGe double quantum dot

We investigate the lifetime of two-electron spin states in a few-electron Si/SiGe double dot. At the transition between the (1,1) and (0,2) charge occupations, Pauli spin blockade provides a readout mechanism for the spin state. We use the statistics of repeated single-shot measurements to extract the lifetimes of multiple states simultaneously. At zero magnetic field, we find that all three triplet states have equal lifetimes, as expected, and this time is ~10 ms. At non-zero field, the T0 lifetime is unchanged, whereas the T- lifetime increases monotonically with field, reaching 3 seconds at 1 T.Comment: 4 pages, 3 figures, supplemental information. Typos fixed; updated to submitted versio