量子能量转移链的两个级别（自旋）的单位，在其两端连接到两个热库，分析在两个限制：（i）在非共振制度，当特征子系统激发能差距大于水库的频率，或浴温度低。(ii)在共振区，当链激发间隙匹配填充浴模式。在后一种情况下，该模型使用主方程的方法进行研究，表明动态是弹道的特定链模型探索。在前一种情况下，我们利用最近开发的能量转移Born-Oppenheimer形式[Phys. Rev. E { \bf 83}，051114（2011）]分析研究了系统动力学，证明了能量在超交换（桥辅助隧穿）机制中跨链转移，能量流随距离呈指数下降。此行为对链细节不敏感。由于在低温下，分子系统的激发光谱可以被截断，类似于自旋链模型，我们认为，这里得到的超交换行为应该在满足非共振条件的广泛系统中观察到。
英文摘要：
Quantum energy transfer in a chain of two-level (spin) units, connected at its ends to two thermal reservoirs, is analyzed in two limits: (i) In the off-resonance regime, when the characteristic subsystem excitation energy gaps are larger than the reservoirs frequencies, or the baths temperatures are low. (ii) In the resonance regime, when the chain excitation gaps match populated bath modes. In the latter case the model is studied using a master equation approach, showing that the dynamics is ballistic for the particular chain model explored. In the former case we analytically study the system dynamics utilizing the recently developed Energy-Transfer Born-Oppenheimer formalism [Phys. Rev. E {\bf 83}, 051114 (2011)], demonstrating that energy transfers across the chain in a superexchange (bridge assisted tunneling) mechanism, with the energy current decreasing exponentially with distance. This behavior is insensitive to the chain details. Since at low temperatures the excitation spectrum of molecular systems can be truncated to resemble a spin chain model, we argue that the superexchange behavior obtained here should be observed in widespread systems satisfying the off-resonance condition.
Because of their long coherence times and potential for scalability, semiconductor quantum-dot spin qubits hold great promise for quantum information processing. However, maintaining high connectivity between quantum-dot spin qubits, which favor linear arrays with nearest neighbor coupling, presents a challenge for large-scale quantum computing. In this work, we present evidence for long-distance spin-chain-mediated superexchange coupling between electron spin qubits in semiconductor quantum dots. We weakly couple two electron spins to the ends of a two-site spin chain. Depending on the spin s…
We study the ballistic-to-diffusive transition induced by the weak breaking of integrability in a boundary-driven XXZ spin-chain. Studying the evolution of the spin current density $\mathcal J^s$ as a function of the system size $L$, we show that, accounting for boundary effects, the transition has a non-trivial universal behavior close to the XX limit. It is controlled by the scattering length $L^*\propto V^{-2}$, where $V$ is the strength of the integrability breaking term. In the XXZ model, the interplay of interactions controls the emergence of a transient "quasi-ballistic" regime at lengt…
Quantum gates between spin qubits can be implemented leveraging the natural Heisenberg exchange interaction between two electrons in contact with each other. This interaction is controllable by electrically tailoring the overlap between electronic wavefunctions in quantum dot systems, as long as they occupy neighbouring dots. An alternative route is the exploration of superexchange - the coupling between remote spins mediated by a third idle electron that bridges the distance between quantum dots. We experimentally demonstrate direct exchange coupling and provide evidence for second neighbour …
We study an open Heisenberg XXZ spin chain with long-range Ising-type interaction, which is incoherently driven at its boundaries and therefore a far-from-equilibrium steady state current is induced. Quantum Monte Carlo techniques make a regime accessible, where the system size is large enough to examine the transition from a diffusive to a ballistic transport regime. We find that the chain lengths for this transition are increasing with decreasing range of the Ising-type interactions between distant spins, while the strength of the incoherent driving does not have a relevant effect on the tra…
The explicit expressions for a nonresonant tunneling current mediated by the bridging units of the molecular wire embedded between the metallic electrodes, are derived. The specific regimes of the charge transmission controlled by nonresonant and resonant participation of terminal's energy levels are studied, and the role of energy position of delocalized orbitals in formation of a distant superexchange electrode-electrode coupling is clarified. The criteria for reduction of the superexchange model of charge tunneling to the flat barrier model are formulated and the parameters of the barrier m…
We study conversion of thermal energy to mechanical energy and vice versa in $α$-Fermi-Pasta-Ulam-Tsingou~(FPUT) chain with spatially sinusoidal profile of initial temperature. We show analytically that coupling between macroscopic dynamics and quasiballistic heat transport gives rise to mechanical vibrations with growing amplitude. This new phenomenon is referred to as "ballistic resonance". At large times, these mechanical vibrations decay monotonically, and therefore the well-known FPUT recurrence paradox occurring at zero temperature is eliminated at finite temperatures.
Efficient DFT+U calculations of ballistic electron transport: Application to Au monatomic chains with a CO impurity
提出了一种在DFT+U中计算开放量子系统的Landauer-Buettiker电导的有效方法。在电子结构和输运计算中，Hubbard势作为非局域赝势系数的简单重正化，通过限制赝势截止球内的现场占位的积分。我们将该方法应用于Au单原子链上吸附CO分子的情况，结果表明，Hubbard U修正消除了Au单原子链在平衡间距处的虚假磁化，以及d电子对电导的非物理贡献，从而得到一个自旋简并的传输通道和更接近实际的电导1 G 0.我们发现，由于CO吸附的电导降低是大得多的顶部网站比桥网站，使电子输运在拉伸的Au链的局部密度近似下给出的一般图片仍然有效的平衡Au-Au间距内DFT+U。
We present a simple, first principles scheme for calculating mechanical properties of nonequilibrium bulk systems assuming an ideal ballistic distribution function for the electronic states described by the external voltage bias. This allows for fast calculations of estimates of the current-induced stresses inside bulk systems carrying a ballistic current. The stress is calculated using the Hellmann-Feynman theorem, and is in agreement with the derivative of the nonequilibrium free energy. We illustrate the theory and present results for one-dimensional (1D) metal chains. We find that the susc…
In the easy-plane regime of XXZ spin chains, spin transport is ballistic, with a Drude weight that has a discontinuous fractal dependence on the value of the anisotropy $Δ= \cos πλ$ at nonzero temperatures. We show that this structure necessarily implies the divergence of the low-frequency conductivity for generic irrational values of $λ$. Within the framework of generalized hydrodynamics, we show that in the high-temperature limit the low-frequency conductivity at a generic anisotropy scales as $σ(ω) \sim 1/\sqrtω$; anomalous response occurs because quasiparticles undergo Lévy flights. For ra…
Nonequilibrium Generalised Langevin Equation for the calculation of heat transport properties in model 1D atomic chains coupled to two 3D thermal baths
We present an exact solution for the heat conductance along a harmonic chain connecting two reservoirs at different temperatures. In this model, the end points correspond to Brownian particles with different damping coefficients. Such analytical expression for the heat conductance covers its behavior from mesoscopic to very long one-dimensional quantum chains, and validates the ballistic nature of the heat transport in the latter example. This implies the absence of the Fourier law for classical and quantum harmonic chains. We also provide a thorough analysis of the normal modes of system whic…
