The Molecular Photo-Cell: Quantum Transport and Energy Conversion at Strong Non-Equilibrium

Non-equilibrium transport properties and energy conversion performance of a molecular photo-voltaic cell are analyzed using the Lindblad master equation within the open quantum systems approach. The method allows us to calculate the dynamics of a system driven by several non-equilibrium sources (a situation we call "strong non-equilibrium"), which is the natural operating condition of photovoltaic cells. We include both coherent and incoherent processes and treat electrons, photon, and phonons on an equal footing. We find that decoherence plays a crucial role in determining both the overall efficiency of the photovoltaic conversion and the optimal energy configuration of the system. Specifically, decoherence leads to better performance, due to a faster relaxation of the excited electrons to the electrodes. We also examine the effect of coherent interference on the efficiency. The approach we propose in this letter is suitable for studying transport and energy conversion in other nanoscale systems at non-equilibrium, where both coherent and incoherent processes take place.Comment: 5+ pages, 4 figures, Sci. Rep. In press (with additional supplementary information there

Nonequilibrium Quantum Phase Transitions in the XY model: comparison of unitary time evolution and reduced density matrix approaches

We study nonequilibrium quantum phase transitions in XY spin 1/2 chain using the $C^*$ algebra. We show that the well-known quantum phase transition at magnetic field $h = 1$ persists also in the nonequilibrium setting as long as one of the reservoirs is set to absolute zero temperature. In addition, we find nonequilibrium phase transitions associated to imaginary part of the correlation matrix for any two different temperatures of the reservoirs at $h = 1$ and $h = h_{\rm c} \equiv|1-\gamma^2|$, where $\gamma$ is the anisotropy and $h$ the magnetic field strength. In particular, two nonequilibrium quantum phase transitions coexist at $h=1$. In addition we also study the quantum mutual information in all regimes and find a logarithmic correction of the area law in the nonequilibrium steady state independent of the system parameters. We use these nonequilibrium phase transitions to test the utility of two models of reduced density operator, namely Lindblad mesoreservoir and modified Redfield equation. We show that the nonequilibrium quantum phase transition at $h = 1$ related to the divergence of magnetic susceptibility is recovered in the mesoreservoir approach, whereas it is not recovered using the Redfield master equation formalism. However none of the reduced density operator approaches could recover all the transitions observed by the $C^*$ algebra. We also study thermalization properties of the mesoreservoir approach.Comment: 25 pages, 10 figure

Reconnection of Stable/Unstable Manifolds of the Harper Map

The Harper map is one of the simplest chaotic systems exhibiting reconnection of invariant manifolds. The method of asymptotics beyond all orders (ABAO) is used to construct stable/unstable manifolds of the Harper map. When the parameter changes to the reconnection threshold, the stable/unstable manifolds are shown to acquire new oscillatory portion corresponding to the heteroclinic tangle after the reconnection.Comment: 24 pages, 11 figure

Nonequilibrium Peierls Transition

Nonequlibrium phase transition of an open Takayama-Lin Liu-Maki chain coupled with two reservoirs is investigated by combining a mean-field approximation and a formula characterizing nonequlibrium steady states, which is obtained from the algebraic field approach to nonequlibrium statistical mechanics. When the bias voltage is chosen to be a control parameter, the phase transition between ordered and normal phases is found to be first or second order. Then, the voltage-current characteristics is S-shaped in some parameter region. In contrast, when the current is chosen to be a control parameter, all the non-trivial solutions of the self-consistent equation are found to become stable. In this case, the phase transition between the ordered and normal phases is always second order and negative differential conductivity appears at low temperature.Comment: 30 pages, 21 frigure

Current in coherent quantum systems connected to mesoscopic Fermi reservoirs

We study particle current in a recently proposed model for coherent quantum transport. In this model, a system connected to mesoscopic Fermi reservoirs (meso-reservoir) is driven out of equilibrium by the action of super-reservoirs thermalized to prescribed temperatures and chemical potentials by a simple dissipative mechanism described by the Lindblad equation. We compare exact (numerical) results with theoretical expectations based on the Landauer formula

Reconnection of Stable/Unstable Manifolds of the Harper Map(6) Approaches from mathematical science and quantum information, Chaos and Nonlinear Dynamics in Quantum-Mechanical and Macroscopic Systems)

この論文は国立情報学研究所の電子図書館事業により電子化されました。Harper方程式は、パラメーターの変化によりセパラトリックスのつなぎ変えを起こす最も簡単なモデルのひとつである(リコネクション)。Harper方程式の時間を離散化したHarper写像において安定/不安定多様体の公式を解析的に導いた。これにより、両多様体はリコネクションが起こるパラメーター付近でリコネクションの前駆となる振動を始めることを示した