Spin-resolved counting statistics as a sensitive probe of spin correlation in transport through a quantum dot spin valve.

We investigate the noise in spin transport through a single quantum dot (QD) tunnel coupled to ferromagnetic (FM) electrodes with noncollinear magnetizations. Based on a spin-resolved quantum master equation, auto- and cross-correlations of spin-resolved currents are analyzed to reveal the underlying spin transport dynamics and characteristics for various polarizations. We find the currents of majority and minority spins could be strongly autocorrelated despite uncorrelated charge transfer. The interplay between tunnel coupling and the Coulomb interaction gives rise to an exchange magnetic field, leading to the precession of the accumulated spin in the QD. It strongly suppresses the bunching of spin tunneling events and results in a unique double-peak structure in the noise of the net spin current. The spin autocorrelation is found to be susceptible to magnetization alignments, which may serve as a sensitive tool to measure the magnetization directions between the FM electrodes.

Spin-selective thermoelectric transport along a vibratingα-helical protein molecule.

Motivated by the development of bio-thermoelectricity and spin caloritronics, we studied the nonlinear spin-selective transport along a vibratingα-helical protein molecule in the presence of thermal bias by using the standard nonequilibrium Green's function formalism. Our results demonstrate that the thermal bias induces the oscillation of spin-polarization between positive and negative values accompanied by spin current with increasing the chain length. Moreover, even for the very short preparable peptide chains, external electron-phonon interaction can give rise to the spin-selectivity, whereas characteristic electron-phonon interaction can not, but in conjunction with thermal bias, it has an important impact on the total current's direction and the spin-polarized current intensity. Finally, the spin-polarization induced by thermal bias can be modulated by gate-bias much more easily as compared to that induced by electric bias. We conclude that by applying thermal bias and gate-bias,α-helical protein molecules are conducive to the storage of binary digits.

Diversity and distribution of CO2-fixing microbial community along elevation gradients in meadow soils on the Tibetan Plateau.

Soil CO2-fixing microbes play a significant role in CO2-fixation in the terrestrial ecosystems, particularly in the Tibetan Plateau. To understand carbon sequestration by soil CO2-fixing microbes and the carbon cycling in alpine meadow soils, microbial diversity and their driving environmental factors were explored along an elevation gradient from 3900 to 5100 m, on both east and west slopes of Mila Mountain region on the Tibetan Plateau. The CO2-fixing microbial communities were characterized by high-throughput sequencing targeting the cbbL gene, encoding the large subunit for the CO2-fixing protein ribulose 1, 5-bisphosphate carboxylase/oxygenase. The overall OTU (Operational Taxonomic Unit) abundance is concentrated at an altitude between 4300 and 4900 m. The diversity of CO2-fixing microbes is the highest in the middle altitude area, and on the east slope is higher than those on the west slope. In terms of microbial community composition, Proteobacteria is dominant, and the most abundant genera are Cupriavidus, Rhodobacter, Sulfurifustis and Thiobacillus. Altitude has the greatest influence on the structural characteristics of CO2-fixing microbes, and other environmental factors are significantly correlated with altitude. Therefore, altitude influences the structural characteristics of CO2-fixing microbes by driving environmental factors. Our results are helpful to understand the variation in soil microbial community and its role in soil carbon cycling along elevation gradients.

Memory-effect-preserving quantum master equation approach to noise spectrum of transport current.

Within the time-nonlocal quantum master equation description, we develop an efficient method for calculating the noise spectrum of transport current through interacting mesoscopic systems. By introducing proper current-related density operators, we propose a practical and very efficient time-local equation of motion implementation to compute the noise spectrum, which contains the full information of emission and absorption. We obtain an analytical expression to characterize the nonequilibrium transport including Coulomb interaction and memory effect. We demonstrate the proposed method with double quantum dots systems and find good agreement with the exact results, whenever the system-reservoir coupling is smaller than the temperature.

Nonequilibrium Kondo regime current noise spectrum of quantum dot systems with the single impurity Anderson model.

We investigate the nonequilibrium current noise spectrum of single impurity Anderson model quantum dot systems on the basis of the accurate dissipation equation of motion evaluations. By comparing between the equilibrium and nonequilibrium cases and between the non-Kondo and Kondo regimes, we identify the current noise spectrum of the nonequilibrium Kondo features that actually appear in the entire region of ω ∈ [-eV, eV]. It is well known that the primary Kondo characteristics at ω = ±eV = ±(µL - µR) display asymmetrical upturns and remarkable peaks in S(ω) and dS(ω)/dω, respectively. These features are originated from the Rabi interference of the transport current dynamics, with the Kondo oscillation frequency of |eV|. Moreover, we also identify the minor but very distinguishable inflections, crossing over from ω = -eV to ω = +eV. This uncovered feature would be related to the interference between two Kondo resonance channels.

Current noise spectra and mechanisms with dissipaton equation of motion theory.

Based on the Yan's dissipaton equation of motion (DEOM) theory [J. Chem. Phys. 140, 054105 (2014)], we investigate the characteristic features of current noise spectrum in several typical transport regimes of a single-impurity Anderson model. Many well-known features such as Kondo features are correctly recovered by our DEOM calculations. More importantly, it is revealed that the intrinsic electron cotunneling process is responsible for the characteristic signature of current noise at anti-Stokes frequency. We also identify completely destructive interference in the noise spectra of noninteracting systems with two degenerate transport channels.

Improving the efficiency of hierarchical equations of motion approach and application to coherent dynamics in Aharonov-Bohm interferometers.

Several recent advancements for the hierarchical equations of motion (HEOM) approach are reported. First, we propose an a priori estimate for the optimal number of basis functions for the reservoir memory decomposition. Second, we make use of the sparsity of auxiliary density operators (ADOs) and propose two ansatzs to screen out all the intrinsic zero ADO elements. Third, we propose a new truncation scheme by utilizing the time derivatives of higher-tier ADOs. These novel techniques greatly reduce the memory cost of the HEOM approach, and thus enhance its efficiency and applicability. The improved HEOM approach is applied to simulate the coherent dynamics of Aharonov-Bohm double quantum dot interferometers. Quantitatively accurate dynamics is obtained for both noninteracting and interacting quantum dots. The crucial role of the quantum phase for the magnitude of quantum coherence and quantum entanglement is revealed.

Derivation of exact master equation with stochastic description: dissipative harmonic oscillator.

A systematic procedure for deriving the master equation of a dissipative system is reported in the framework of stochastic description. For the Caldeira-Leggett model of the harmonic-oscillator bath, a detailed and elementary derivation of the bath-induced stochastic field is presented. The dynamics of the system is thereby fully described by a stochastic differential equation, and the desired master equation would be acquired with statistical averaging. It is shown that the existence of a closed-form master equation depends on the specificity of the system as well as the feature of the dissipation characterized by the spectral density function. For a dissipative harmonic oscillator it is observed that the correlation between the stochastic field due to the bath and the system can be decoupled, and the master equation naturally results. Such an equation possesses the Lindblad form in which time-dependent coefficients are determined by a set of integral equations. It is proved that the obtained master equation is equivalent to the well-known Hu-Paz-Zhang equation based on the path-integral technique. The procedure is also used to obtain the master equation of a dissipative harmonic oscillator in time-dependent fields.