2,392 research outputs found
Orbital Magnetization under an Electric Field and Orbital Magnetoelectric Polarizabilty for a Bilayer Chern System
In the the real space formalism of orbital magnetization (OM) for a Chern
insulator without an external electric field, it is correct to average the
local OM either over the bulk region or over the whole sample. However for a
layered Chern insulator in an external electric field, which is directly
related to the nontrivial nature of orbital magnetoelectric coupling, the role
of boundaries remains ambiguous in this formalism. Based on a bilayer model
with an adjustable Chern number at half filling, we numerically investigate the
OM with the above two different average types under a nonzero perpendicular
electric field. The result shows that in this case, the nonzero Chern number
gives rise to a gauge shift of the OM with the bulk region average, while this
gauge shift is absent for the OM with the whole sample average. This indicates
that only the whole sample average is reliable to calculate the OM under a
nonzero electric field for Chern insulators. On this basis, the orbital
magnetoelectric polarizablity (OMP) and the Chern-Simons orbital
magnetoelectric polarizablity (CSOMP) with the whole sample average are
studied. We also present the relationship between the OMP (CSOMP) and the
response of Berry curvature to the electric field. The stronger the response of
Berry curvature to electric field, the stronger is the OMP (CSOMP). Besides
clarify the calculation methods, our result also provides an effective method
to enhance OMP and CSOMP of materials.Comment: 11 pages, 11 figure
Quantum Transports in Two-Dimensions with Long Range Hopping: Shielding, Localization and the Extended Isolated State
We investigate the effects of disorder and shielding on quantum transports in
a two dimensional system with all-to-all long range hopping. In the weak
disorder, cooperative shielding manifests itself as perfect conducting channels
identical to those of the short range model, as if the long range hopping does
not exist. With increasing disorder, the average and fluctuation of conductance
are larger than those in the short range model, since the shielding is
effectively broken and therefore long range hopping starts to take effect. Over
several orders of disorder strength (until times of nearest
hopping), although the wavefunctions are not fully extended, they are also
robustly prevented from being completely localized into a single site. Each
wavefunction has several localization centers around the whole sample, thus
leading to a fractal dimension remarkably smaller than 2 and also remarkably
larger than 0, exhibiting a hybrid feature of localization and delocalization.
The size scaling shows that for sufficiently large size and disorder strength,
the conductance tends to saturate to a fixed value with the scaling function
, which is also a marginal phase between the typical metal
() and insulating phase (). The all-to-all coupling expels
one isolated but extended state far out of the band, whose transport is
extremely robust against disorder due to absence of backscattering. The bond
current picture of this isolated state shows a quantum version of short circuit
through long hopping.Comment: 15 pages, 8 figure
An Improved Electrical Switching and Phase-Transition Model for Scanning Probe Phase-Change Memory
Scanning probe phase-change memory (SPPCM) has been widely considered as one of the most promising candidates for next-generation data storage devices due to its fast switching time, low power consumption, and potential for ultra-high density. Development of a comprehensive model able to accurately describe all the physical processes involved in SPPCM operations is therefore vital to provide researchers with an effective route for device optimization. In this paper, we introduce a pseudo-three-dimensional model to simulate the electrothermal and phase-transition phenomena observed during the SPPCM writing process by simultaneously solving Laplace’s equation to model the electrical process, the classical heat transfer equation, and a rate equation to model phase transitions. The crystalline bit region of a typical probe system and the resulting current-voltage curve obtained from simulations of the writing process showed good agreement with experimental results obtained under an equivalent configuration, demonstrating the validity of the proposed model
HexaaquaÂzinc(II) bisÂ(2,4,5-tricarboxybenzoate) 4,5-diazaÂfluoren-9-one disolvate dihydrate
The asymmetric unit of the title complex, [Zn(H2O)6](C10H5O8)2·2C11H6N2O·2H2O, contains one half of the complex cation with the ZnII ion located on an inversion center, a monovalent 2,4,5-tricarboxybenzoate (1,2,4,5-BTC) counter-anion, a 4,5-diazaÂfluoren-9-one (DAFO) molÂecule and an uncoordinated water molÂecule. In the crystal structure, O—H⋯O and O—H⋯N hydrogen bonds link the cations, anions and water molÂecules into a three-dimensional network
One Size Cannot Fit All: a Self-Adaptive Dispatcher for Skewed Hash Join in Shared-nothing RDBMSs
Shared-nothing architecture has been widely adopted in various commercial
distributed RDBMSs. Thanks to the architecture, query can be processed in
parallel and accelerated by scaling up the cluster horizontally on demand. In
spite of that, load balancing has been a challenging issue in all distributed
RDBMSs, including shared-nothing ones, which suffers much from skewed data
distribution. In this work, we focus on one of the representative operator,
namely Hash Join, and investigate how skewness among the nodes of a cluster
will affect the load balance and eventual efficiency of an arbitrary query in
shared-nothing RDBMSs. We found that existing Distributed Hash Join (Dist-HJ)
solutions may not provide satisfactory performance when a value is skewed in
both the probe and build tables. To address that, we propose a novel Dist-HJ
solution, namely Partition and Replication (PnR). Although PnR provide the best
efficiency in some skewness scenario, our exhaustive experiments over a group
of shared-nothing RDBMSs show that there is not a single Dist-HJ solution that
wins in all (data skew) scenarios. To this end, we further propose a
self-adaptive Dist-HJ solution with a builtin sub-operator cost model that
dynamically select the best Dist-HJ implementation strategy at runtime
according to the data skew of the target query. We implement the solution in
our commercial shared-nothing RDBMSs, namely KaiwuDB (former name ZNBase) and
empirical study justifies that the self-adaptive model achieves the best
performance comparing to a series of solution adopted in many existing RDBMSs
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