Spin dynamics in semiconductors

This article reviews the current status of spin dynamics in semiconductors which has achieved a lot of progress in the past years due to the fast growing field of semiconductor spintronics. The primary focus is the theoretical and experimental developments of spin relaxation and dephasing in both spin precession in time domain and spin diffusion and transport in spacial domain. A fully microscopic many-body investigation on spin dynamics based on the kinetic spin Bloch equation approach is reviewed comprehensively.Comment: a review article with 193 pages and 1103 references. To be published in Physics Reports

Control of electron spin decoherence caused by electron-nuclear spin dynamics in a quantum dot

Control of electron spin decoherence in contact with a mesoscopic bath of many interacting nuclear spins in an InAs quantum dot is studied by solving the coupled quantum dynamics. The nuclear spin bath, because of its bifurcated evolution predicated on the electron spin up or down state, measures the which-state information of the electron spin and hence diminishes its coherence. The many-body dynamics of nuclear spin bath is solved with a pair-correlation approximation. In the relevant timescale, nuclear pair-wise flip-flops, as elementary excitations in the mesoscopic bath, can be mapped into the precession of non-interacting pseudo-spins. Such mapping provides a geometrical picture for understanding the decoherence and for devising control schemes. A close examination of nuclear bath dynamics reveals a wealth of phenomena and new possibilities of controlling the electron spin decoherence. For example, when the electron spin is flipped by a $\pi$-pulse at $\tau$, its coherence will partially recover at $\sqrt{2}\tau$ as a consequence of quantum disentanglement from the mesoscopic bath. In contrast to the re-focusing of inhomogeneously broadened phases by conventional spin-echoes, the disentanglement is realized through shepherding quantum evolution of the bath state via control of the quantum object. A concatenated construction of pulse sequences can eliminate the decoherence with arbitrary accuracy, with the nuclear-nuclear spin interaction strength acting as the controlling small parameter

External Potential Modifies Friction of Molecular Solutes in Water

Stokes’s law for the friction of a sphere in water has been argued to work down to molecular scales, provided the effective hydrodynamic radius includes the hydration layer. In interpretations of experiments and in theoretical models, it is tacitly assumed that the solvent friction experienced by a solute does not depend on whether an external confinement potential acts on the solute. Using a novel method to extract the friction memory function from molecular dynamics simulations, we show that the solvent friction of a strongly harmonically confined methane molecule in water increases by 60% compared to its free-solution value, which is caused by an amplification of the slowest component of the memory function. The friction enhancement occurs for potential strengths typical of physical and chemical bonds and is accompanied by a significant slowing-down of the hydration water dynamics. Thus, the solvent friction acting on molecular solutes is not determined by solvent properties and solute-solvent interactions alone but results from the coupling between solute and solvent dynamics and thereby can be tuned by an external potential acting on the solute. This also explains why simulations of positionally constrained solutes do not reproduce free-solution diffusivities. Dynamic scaling arguments suggest similar effects also for macromolecular solutes provided the solution viscosity is sufficiently enhanced

Spins, disorder and interactions in GaAs and graphene

This thesis describes experiments on semiconductor spin physics under the influence of diverse disorder and carrier-carrier interaction. Motivated by recent observations of GaAs spin qubit coherence limited by hyperfine coupling to nuclear-spin en- semble fluctuations, we started out to find ways to study the electron-spin nuclear-spin coupling or to avoid the nuclear spin bath altogether. This can be done in several different ways and here we pursued two fairly different approaches. One is the investigation of the dynamics of nuclear spin polarization in GaAs and the other aims at spin-related effects in graphene na- nostructures which possibly have negligible nuclear spin contributions due to the natural abundance (about 99 %) of zero- spin isotopes. The experiments on GaAs are performed using a non-local spin injection device with Fe ferromagnetic contacts on a degene- rately n-doped epilayer. At low temperatures, where the injected spin polarization allows dynamic polarization of the nuclear spins via hyperfine interaction, distinct spin signals are used to study the dynamics of the nuclear spin system both in presen- ce and absence of net electron spin polarization. The nuclear spin-lattice relaxation in an unpolarized environment reveals an unexpected breakdown of the Korringa law of nuclear spin relaxation otherwise valid for metallic systems. This is manifested in the observed deviation from a linear tem- perature dependence of the nuclear T_1 time and is interpreted as a result of hyperfine coupling to conduction electrons which are influenced by the interplay of disorder and carrier-carrier interaction. This finding therefore gives important insight into the strong influence of intimate coupling between the electron and nuclear spin sub-systems. Transport experiments on lithographically defined graphene quantum dots are performed at low temperatures. Three graphe- ne quantum dots of different nanometer sizes fabricated on a single graphene flake allow a detailed investigation of the size dependence of the Coulomb interaction, the energy spectra, and the influence of disorder within the nanostructures. The onset of Landau quantization in perpendicular magnetic fields reveals signatures of the electron-hole crossover reflecting the bandstructure symmetry of graphene. Suppression of orbital effects by applying external magnetic fields parallel to the sam- ple plane allows to address spin effects of the charge transitions in the quantum dots. The observed field dependence of Cou- lomb blockade peak splittings is not inconsistent with the Zeeman splitting proportional to an expected g-factor of 2. The transport data evidence strong influence of disorder supposably induced by both charged impurities in the close vicinity of the quantum dots and by edge disorder as a result of the fabrication process lacking precise control of the edge structures

Noise suppression and long-range exchange coupling for gallium arsenide spin qubits

This thesis presents the results of the experimental study performed on spin qubits realized in gate-defined gallium arsenide quantum dots, with the focus on noise suppression and long-distance coupling.Comment: PhD thesis, supervised by Charles M. Marcus and Ferdinand Kuemmeth, submitted to the PhD School of the Faculty of Science, University of Copenhagen in June 2017, 223 pages, 92 figure