Results 1 - 10 of 1302
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[en] The Kitaev-Heisenberg model is source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. The material -RuCl is composed of weakly van der Waals bound honeycomb layers of edge sharing RuCl octahedra which has recently emerged as a prime candidate for realising such physics. We studied -RuCl by means of thermal transport measurements, a valuable tool to probe elementary excitations of systems with low dimensional spin structure. While the in-plane, longitudinal heat transport is governed by heat conduction of phonons that strongly scatter off the magnetic excitations present in the system, studying the thermal Hall effect (Rhighi-Leduc effect) opens up a new path towards detecting a direct contribution of unconventional magnetic excitations to entropy transport. We have observed a sizeable transversal heat conductivity , the agreement of which with the theoretical predictions for the pure Kitaev model being suggestive of heat transport by fractionalised quasiparticles in -RuCl.
[en] To have a better understanding of the thermal behaviors of lithium-ion batteries (LIBs) under discharge and overcharge conditions, some tests were conducted by a cone calorimeter. Several parameters were measured such as the battery surface temperature, voltage, the time to thermal runaway, the time to maximum temperature, heat release rate and total heat released. The results indicate that the lithium-ion battery will have obvious warming up during discharge owing to the reversible heat and irreversible heat. It was observed that the current treatment (discharge) has a significant influence on the thermal behaviors of LIBs. It can accelerate the warming up, result in earlier thermal runaway, and reduce the heat released. In addition, overcharge will make LIBs more unstable and easier to attain the thermal runaway.
[en] We study the effects of a scalar condensate on a class of 2+1 dimensional non-Fermi liquids by introducing fermionic probes in the corresponding asymptotically AdS4 black hole backgrounds. For the range of parameters and the type of couplings we consider, the system does not develop a gap and resembles a conventional (Landau) Fermi liquid. The detailed properties of the system depend strongly on the parameters, in a model-dependent way. (author)
[en] This paper proposed an analytical model which can calculate the effective thermal conductivity (ETC) of a spiral-wound Lithium-ion battery (Li-ion battery). It bases on a two-dimensional energy balance with both radial and spiral heat transfer, as well as internal thermal contact resistance (TCR) considered simultaneously and studies the influence of winding layers and winding tension on the ETC. Results show that the analytical data are in good agreement with the numerical results. With the winding layers decreased and the winding tension enhanced, the ETC of Li-ion battery increases gradually. The radial temperature in Li-ion battery is also investigated which demonstrates a relatively higher temperature when considering the internal TCR.
[en] This paper describes the design and experimental test of a passive magnetic bearing system composed by a superconductor magnetic bearing (SMB) and a permanent magnet bearing (PMB). This bearing setup is part of a flywheel energy storage system. The advantage of using a passive bearing system is that it offers low friction without the need of a magnetic bearing controller, increasing the reliability and decreasing the energy consumption. The first set of tests were quasi-static radial and axial force measurements of the PMB operating alone and together with the SMB. As the PMB is intrinsically unstable in one degree of freedom, the operation of the SMB together with the PMB is necessary to stabilize the system. After that, dynamic measurements were made for the SMB operating alone and together with the PMB. The resonant speeds were identified and the bearing radial and axial forces were also measured for the SMB and SMB + PMB operation. These results indicate that the studied bearing set is technologically feasible to be used in flywheel energy storage systems. (paper)
[en] Highlights: • The thermoelectric quantities perform the sensitivity to the inter-dot coupling strength. • Double Fano resonances can be created to largely enhance the thermoelectric effect at low-temperature. • Thermoelectric figure of merit can be improved due to the coexistence of local bipolar effect and Fano resonance. • Thermoelectric figure of merit can be optimized by adjusting the dot-lead coupling strengths. - Abstract: The thermoelectric transport properties of a parallel-coupled double quantum dot (PCDQD) system with side-coupled quantum dots (QDs) is investigated by using the Keldysh non-equilibrium Green's function technique. The thermoelectric quantities, including the thermal conductance, thermopower, and thermoelectric figure of merit denoted by ZT, are sensitive to the inter-dot coupling strength. With the help of side-coupled QD, unusual double Fano resonances are created in the conductance spectra to largely enhance the thermoelectric effect at low-temperature. Benefited from the coexistence of local bipolar effect and Fano resonance, the ZT can be improved by one-fold higher than that of original PCDQD system. Moreover, when the asymmetry parameter α, which indicates the geometric arrangement of coupled QDs with a given lead, takes appropriate value, the optimization of ZT can be achieved at high temperature. Our work suggests that the side-coupled QDs scheme holds promise for the designing of high-efficiency thermoelectric conversion devices.
[en] Dynamics of luminescence spectra of photoexcited electron donor–acceptor complexes in a Debye solvent has been investigated within the Bixon–Jortner model. It has been shown that the main factors affecting the spectral dynamics in highly exergonic systems at times of up to several picoseconds are intramolecular vibrational relaxation in reactants and relaxation of solvent in the vicinity of an ion pair. Exact solutions of the mathematical model are obtained for two cases in which one relaxation channel dominates over another. The influence of the intramolecular reorganization energy on the luminescence-band profile at short times has been analyzed.
[en] The ground state geometry structures of the KBn(n = 1-9) clusters are studied by using the generalized gradient approximation (GGA) based on the density functional theory framework. We have systematically calculated the binding energy per atom (Eb), the second-order difference of energy (△2E), splitting energy (9D(n, n-1)), and the gap between the highest occupy orbit (HOMO) and the lowest unoccupy orbit (LUMO). The results indicate that with the change in size of KBn (n = 1-9) clusters, its stability will be gradually increased, where KB3 and KB5 are attributed to magic number clusters. The energy gap of KBn (n = 1-9) clusters presents the change of oscillation with the increasing of the total number of atom. Besides, the analysis of density of states suggests that the change of oscillation is due to the gap difference of clusters. (authors)
[en] -LiFeSiO is proposed as a promising candidate material for lithium ion batteries. The Fe atoms are tetrahedrally coordinated by Oxygen. Iron has the Fe (S=2) oxidation state and is displaced from the tetrahedron center resulting in a electric field gradient caused by the distorted tetrahedral crystal field. The Mössbauer spectrum of the powder sample shows one dominant site exhibiting magnetic order at 2.1 K and a considerable quadrupole splitting as observed at room temperature. The magnetic hyperfine field of B = 14.7(4) T is oriented orthogonal to the largest principle axis of the electrical field gradient V = -125(3) V/Å. The isomer shift of = 1.1(1) mm/s is consistent with the high spin Fe (S=2) state. We will discuss the implications of these findings on the actual magnetic structure in this system. The observed static order is in agreement with susceptibility measurements showing a transition to antiferromagnetic order below 17 K.
[en] Highlights: • Several Potfit-based methods are used to fit the potential energy surface of H3O2−. • These fits are used to compute its ground state via relaxation with (ML-)MCTDH. • Fitting methods are evaluated for accuracy and its impact on (ML)-MCTDH performance. • A Monte-Carlo-accelerated version of Multi-Layer Potfit (RS-MLPF) is presented. • RS-MLPF provides highly accurate PES fits that allow fast ML-MCTDH computations. - Abstract: Quantum molecular dynamics simulations with MCTDH or ML-MCTDH perform best if the potential energy surface (PES) has a sum-of-products (SOP) or multi-layer operator (MLOp) structure. Here we investigate four different POTFIT-based methods for representing a general PES as such a structure, among them the novel random-sampling multi-layer Potfit (RS-MLPF). We study how the format and accuracy of the PES representation influences the runtime of a benchmark (ML-)MCTDH calculation, namely the computation of the ground state of the H3O2− ion. Our results show that compared to the SOP format, the MLOp format leads to a much more favorable scaling of the (ML-)MCTDH runtime with the PES accuracy. At reasonably high PES accuracy, ML-MCTDH calculations thus become up to 20 times faster, and taken to the extreme, the RS-MLPF method yields extremely accurate PES representations (global root-mean-square error of ∼0.1 cm−1) which still lead to only moderate computational demands for ML-MCTDH.