Hyperfine Interaction-Dominated Dynamics of Nuclear Spins in Self-Assembled InGaAs Quantum Dots

Article · October 2011with13 Reads
DOI: 10.1103/PhysRevLett.107.167401 · Source: PubMed
Abstract
We measure the dynamics of nuclear spins in a single-electron charged self-assembled InGaAs quantum dot with negligible nuclear spin diffusion due to dipole-dipole interaction and identify two distinct mechanisms responsible for the decay of the Overhauser field. We attribute a temperature-independent decay lasting ∼100 sec at 5 T to intradot diffusion induced by hyperfine-mediated indirect nuclear spin interaction. By repeated polarization of the nuclear spins, this diffusion induced partial decay can be suppressed. We also observe a gate voltage and temperature-dependent decay stemming from cotunneling mediated nuclear spin flips that can be prolonged to ∼30 h by adjusting the gate voltage and lowering the temperature to ∼200 mK. Our measurements indicate possibilities for exploring quantum dynamics of the central spin model.
    • Atomic-species dependent nuclear Zeeman splitting of g N µ N ∼ 10MHz T −1 is supplemented by the quadrupolar coupling g N µ N B Q ∼ 1-10 MHz [25]. The effect of other dynamics, such as dipolar interaction between nuclear spins [15], electronmediated nuclear flip flops [16] or the precession of nuclei due to hyperfine coupling with the electron [26] [27] are neglected in this regime due to their weaker coupling strengths (see supplementary). For reference, we include the Rabi frequency of our spin control at ∼ 500 GHz, up to two orders of magnitude faster than any other process.
    [Show abstract] [Hide abstract] ABSTRACT: The Zeeman-split spin-states of a single electron confined in a self-assembled quantum dot provide an optically-accessible spin qubit. For III-V materials the nuclear spins of the solid-state host provide an intrinsic noise source, resulting in electron-spin dephasing times of few nanoseconds. While a comprehensive study of electron-spin dynamics at low magnetic field has recently been carried out, what limits the electron coherence in these systems remains unclear, in part due to the dominant effect of measurement-induced dynamic polarisation of the nuclear bath. We develop an all-optical method to access the quantum dot spin-state without perturbing the nuclear environment. We use this method to implement Hahn-echo decoupling and reach the intrinsic limit to coherence set by inhomogeneous strain fields coupling to quadrupolar moments of the nuclear bath. These results indicate that the extension of electron spin coherence beyond this few-microsecond limit necessitates the reduction of strain-induced quadrupolar broadening in these materials.
    Full-text · Article · Dec 2015 · Journal of Applied Physics
  • [Show abstract] [Hide abstract] ABSTRACT: The generalized Master equation of the Nakajima-Zwanzig (NZ) type has been used extensively to investigate the coherence dynamics of the central spin model with the nuclear bath in a narrowed state characterized by a well defined value of the Overhauser field. We revisit the perturbative NZ approach and apply it to the exactly solvable case of a system with uniform hyperfine couplings. This is motivated by the fact that the effective Hamiltonian-based theory suggests that the dynamics of the realistic system at low magnetic fields and short times can be mapped onto the uniform coupling model. We show that the standard NZ approach fails to reproduce the exact solution of this model beyond very short times, while the effective Hamiltonian calculation agrees very well with the exact result on timescales during which most of the coherence is lost. Our key finding is that in order to extend the timescale of applicability of the NZ approach in this case, instead of using a single projection operator one has to use a set of correlated projection operators which properly reflect the symmetries of the problem and greatly improve the convergence of the theory. This suggests that the correlated projection operators are crucial for a proper description of narrowed state free induction decay at short times and low magnetic fields. Our results thus provide important insights toward the development of a more complete theory of central spin decoherence applicable in a broader regime of timescales and magnetic fields.
    Article · Aug 2011
  • [Show abstract] [Hide abstract] ABSTRACT: Self-assembled quantum dots (QDs) are born out of lattice mismatched ingredients where strain plays an indispensable role. Through the electric quadrupolar coupling, strain affects the magnetic environment as seen by the nuclear spins. To guide prospective single-QD nuclear magnetic resonance (NMR), as well as dynamic nuclear spin polarization experiments, an atomistic insight to the strain and quadrupolar field distributions is presented. A number of implications of the structural and compositional profile of the QD have been identified. A high aspect ratio of the QD geometry enhances the quadrupolar interaction. The inclined interfaces introduce biaxiality and the tilting of the major quadrupolar principal axis away from the growth axis; the alloy mixing of gallium into the QD enhances both of these features while reducing the quadrupolar energy. Regarding the NMR spectra, both Faraday and Voigt geometries are investigated, unraveling in the first place the extend of inhomogeneous broadening and the appearance of the normally forbidden transitions. Moreover, it is shown that from the main extend of the NMR spectra the alloy mole fraction of a single QD can be inferred. By means of the element-resolved NMR intensities it is found that In nuclei has a factor of 5 dominance over those of As. In the presence of an external magnetic field, the borderlines between the quadrupolar and Zeeman regimes are extracted as 1.5 T for In and 1.1 T for As nuclei. At these values the nuclear spin depolarization rates of the respective nuclei get maximized due to the noncollinear secular hyperfine interaction with a resident electron in the QD.
    Article · Nov 2011
  • [Show abstract] [Hide abstract] ABSTRACT: The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a semiconductor quantum dot offers a unique setting for the study of many-body spin physics in the condensed matter. The dynamics of this system and its coupling to electron spins is fundamentally different from its bulk counter-part as well as that of atoms due to increased fluctuations that result from reduced dimensions. In recent years, the interest in studying quantum dot nuclear spin systems and their coupling to confined electron spins has been fueled by its direct implication for possible applications of such systems in quantum information processing as well as by the fascinating nonlinear (quantum-)dynamics of the coupled electron-nuclear spin system. In this article, we review experimental work performed over the last decades in studying this mesoscopic,coupled electron-nuclear spin system and discuss how optical addressing of electron spins can be exploited to manipulate and read-out quantum dot nuclei. We discuss how such techniques have been applied in quantum dots to efficiently establish a non-zero mean nuclear spin polarization and, most recently, were used to reduce fluctuations of the average quantum dot nuclear spin orientation. Both results in turn have important implications for the preservation of electron spin coherence in quantum dots, which we discuss. We conclude by speculating how this recently gained understanding of the quantum dot nuclear spin system could in the future enable experimental observation of quantum-mechanical signatures or possible collective behavior of mesoscopic nuclear spin ensembles.
    Full-text · Article · Feb 2012
  • [Show abstract] [Hide abstract] ABSTRACT: Resonant optical excitation of lowest-energy excitonic transitions in self-assembled quantum dots leads to nuclear spin polarization that is qualitatively different from the well-known optical orientation phenomena. By carrying out a comprehensive set of experiments, we demonstrate that nuclear spin polarization manifests itself in quantum dots subjected to finite external magnetic field as locking of the higher energy Zeeman transition to the driving laser field, as well as the avoidance of the resonance condition for the lower energy Zeeman branch. We interpret our findings on the basis of dynamic nuclear spin polarization originating from noncollinear hyperfine interaction and find excellent agreement between experiment and theory. Our results provide evidence for the significance of noncollinear hyperfine processes not only for nuclear spin diffusion and decay, but also for buildup dynamics of nuclear spin polarization in a coupled electron-nuclear spin system.
    Article · May 2012
    +1 more author...Martin Claassen
  • [Show abstract] [Hide abstract] ABSTRACT: Nuclear spin diffusion in single self-assembled InAlAs quantum dots was studied by using erase-pump-probe technique. As a measure of nuclear spin polarization, transients of the Overhauser shift (OHS) of positive trion were probed in the respective pulse region. The achieved stable OHS was relaxed with a long depolarization time as long as a few tens of seconds and a diffusion constant of ∼5 × 10−15 cm2/s was estimated. The slow transfer of the spin energy is considered to be due to the dipole-dipole interaction of the nuclei. In addition, through the magnetic field dependence of OHS, the stability of the nuclear spin polarization was briefly discussed, and the saturated maximum values of OHS were investigated experimentally.
    Article · May 2012
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