111 research outputs found
Exact Numerical Solution of the BCS Pairing Problem
We propose a new simulation computational method to solve the reduced BCS
Hamiltonian based on spin analogy and submatrix diagonalization. Then we
further apply this method to solve superconducting energy gap and the results
are well consistent with those obtained by Bogoliubov transformation method.
The exponential problem of 2^{N}-dimension matrix is reduced to the polynomial
problem of N-dimension matrix. It is essential to validate this method on a
real quantumComment: 7 pages, 3 figure
Dynamically stabilized decoherence-free states in non-Markovian open fermionic systems
Decoherence-free subspaces (DFSs) provide a strategy for protecting the
dynamics of an open system from decoherence induced by the system-environment
interaction. So far, DFSs have been primarily studied in the framework of
Markovian master equations. In this work, we study decoherence-free (DF) states
in the general setting of a non-Markovian fermionic environment. We identify
the DF states by diagonalizing the non-unitary evolution operator for a
two-level fermionic system attached to an electron reservoir. By solving the
exact master equation, we show that DF states can be stabilized dynamically.Comment: 11 pages, 3 figures. Any comments are welcom
Hump Structure below Tc in the thermal conductivity of MgB2 superconductor
A reasonable cause of absence of hump structure in thermal conductivity of
MgB2 below the superconducting transition temperature (Tc) lies in the
appearance of multigap structure. The gaps of lower magnitude can be suppressed
by defects so that this system becomes effectively a single gap superconductor.
When such a situation is created, it is hoped that thermal conductivity will
show hump below Tc. Proceeding along these lines, a sample of MgB2 with a
relatively higher residual resistivity (33.3 mili-Ohm-cm)has been found to show
a hump structure below Tc. The actual electronic thermal conductivity kel of
this sample is less than that expected from the Wiedeman- Franz law by more
than a factor of 2.6 in the considered temperature range. Modifying the
Wiedeman- Franz law for the electronic contribution by replacing the Lorenz
number by an effective Lorenz number Leff (L0) we have obtained two sets of
kel, namely those with Leff = 0.1L0 and 0.2L0. Corresponding to these two sets
of kel, two sets of the phonon thermal conductivity kph are obtained. kph has
been analyzed in terms of an extended Bardeen- Rickayzen- Tewordt theory. The
main result of this analysis is that the hump structure corresponds to a gap
ratio of 3.5, and that large electron-point defect scattering is the main
source of drastic reduction of the electronic thermal conductivity from that
given by the usual Wiedeman- Franz law.Comment: 18 pages of Text + Figs: comments welcome
([email protected]
Staircase Quantum Dots Configuration in Nanowires for Optimized Thermoelectric Power
The performance of thermoelectric energy harvesters can be improved by nanostructures that exploit inelastic transport processes. One prototype is the three-terminal hopping thermoelectric device where electron hopping between quantum-dots are driven by hot phonons. Such three-terminal hopping thermoelectric devices have potential in achieving high efficiency or power via inelastic transport and without relying on heavy-elements or toxic compounds. We show in this work how output power of the device can be optimized via tuning the number and energy configuration of the quantum-dots embedded in parallel nanowires. We find that the staircase energy configuration with constant energy-step can improve the power factor over a serial connection of a single pair of quantum-dots. Moreover, for a fixed energy-step, there is an optimal length for the nanowire. Similarly for a fixed number of quantum-dots there is an optimal energy-step for the output power. Our results are important for future developments of high-performance nanostructured thermoelectric devices
Acoustic spin pumping: Direct generation of spin currents from sound waves in Pt/Y3Fe5O12 hybrid structures
Using a Pt/Y3Fe5O12 (YIG) hybrid structure attached to a piezoelectric
actuator, we demonstrate the generation of spin currents from sound waves. This
"acoustic spin pumping" (ASP) is caused by the sound wave generated by the
piezoelectric actuator, which then modulates the distribution function of
magnons in the YIG layer and results in a pure-spin-current injection into the
Pt layer across the Pt/YIG interface. In the Pt layer, this injected spin
current is converted into an electric voltage due to the inverse spin-Hall
effect (ISHE). The ISHE voltage induced by the ASP is detected by measuring
voltage in the Pt layer at the piezoelectric resonance frequency of the
actuator coupled with the Pt/YIG system. The frequency-dependent measurements
enable us to separate the ASP-induced signals from extrinsic heating effects.
Our model calculation based on the linear response theory provides us with a
qualitative and quantitative understanding of the ASP in the Pt/YIG system.Comment: 8 pages, 6 figure
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Layered oxychalcogenides: structural chemistry and thermoelectric properties
Layered oxychalcogenides have recently emerged as promising thermoelectric materials. The alternation of ionic oxide and covalent chalcogenide layers found in these materials often results in interesting electronic properties, and also facilitates the tuning of their properties via chemical substitution at both types of layers. This review highlights some common structure types found for layered oxychalcogenides and their interrelationships. This review pays special attention to the potential of these materials for thermoelectric applications, and provides an overview of the thermoelectric properties of materials of current interest, including BiCuSeO
Thermal conductivity and thermal boundary resistance of nanostructures
International audienceWe present a fabrication process of low-cost superlattices and simulations related with the heat dissipation on them. The influence of the interfacial roughness on the thermal conductivity of semiconductor/semiconductor superlattices was studied by equilibrium and non-equilibrium molecular dynamics and on the Kapitza resistance of superlattice's interfaces by equilibrium molecular dynamics. The non-equilibrium method was the tool used for the prediction of the Kapitza resistance for a binary semiconductor/metal system. Physical explanations are provided for rationalizing the simulation results
Circadian Rhythm and Sleep Disruption: Causes, Metabolic Consequences and Countermeasures.
Circadian (βΌ 24 hour) timing systems pervade all kingdoms of life, and temporally optimize behaviour and physiology in humans. Relatively recent changes to our environments, such as the introduction of artificial lighting, can disorganize the circadian system, from the level of the molecular clocks that regulate the timing of cellular activities to the level of synchronization between our daily cycles of behaviour and the solar day. Sleep/wake cycles are intertwined with the circadian system, and global trends indicate that these too are increasingly subject to disruption. A large proportion of the world's population is at increased risk of environmentally-driven circadian rhythm and sleep disruption, and a minority of individuals are also genetically predisposed to circadian misalignment and sleep disorders. The consequences of disruption to the circadian system and sleep are profound and include myriad metabolic ramifications, some of which may be compounded by adverse effects on dietary choices. If not addressed, the deleterious effects of such disruption will continue to cause widespread health problems; therefore, implementation of the numerous behavioural and pharmaceutical interventions that can help restore circadian system alignment and enhance sleep will be important
Complete Genome Sequence of the N2-Fixing Broad Host Range Endophyte Klebsiella pneumoniae 342 and Virulence Predictions Verified in Mice
We report here the sequencing and analysis of the genome of the nitrogen-fixing endophyte, Klebsiella pneumoniae 342. Although K. pneumoniae 342 is a member of the enteric bacteria, it serves as a model for studies of endophytic, plant-bacterial associations due to its efficient colonization of plant tissues (including maize and wheat, two of the most important crops in the world), while maintaining a mutualistic relationship that encompasses supplying organic nitrogen to the host plant. Genomic analysis examined K. pneumoniae 342 for the presence of previously identified genes from other bacteria involved in colonization of, or growth in, plants. From this set, approximately one-third were identified in K. pneumoniae 342, suggesting additional factors most likely contribute to its endophytic lifestyle. Comparative genome analyses were used to provide new insights into this question. Results included the identification of metabolic pathways and other features devoted to processing plant-derived cellulosic and aromatic compounds, and a robust complement of transport genes (15.4%), one of the highest percentages in bacterial genomes sequenced. Although virulence and antibiotic resistance genes were predicted, experiments conducted using mouse models showed pathogenicity to be attenuated in this strain. Comparative genomic analyses with the presumed human pathogen K. pneumoniae MGH78578 revealed that MGH78578 apparently cannot fix nitrogen, and the distribution of genes essential to surface attachment, secretion, transport, and regulation and signaling varied between each genome, which may indicate critical divergences between the strains that influence their preferred host ranges and lifestyles (endophytic plant associations for K. pneumoniae 342 and presumably human pathogenesis for MGH78578). Little genome information is available concerning endophytic bacteria. The K. pneumoniae 342 genome will drive new research into this less-understood, but important category of bacterial-plant host relationships, which could ultimately enhance growth and nutrition of important agricultural crops and development of plant-derived products and biofuels
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