161 research outputs found
Measurement of Absorption Cross Section of a Lossy Object in Reverberation Chamber Without the Need for Calibration
A reliable and simple procedure is proposed to measure the averaged absorption cross section (ACS) of a lossy object in a reverberation chamber (RC). This procedure is based on the time-domain measurement of the ACS in an RC. In the time-domain, to obtain the ACS, the chamber decay time needs to be known. Conventionally, the ACS is normally measured in the frequency domain, and a full two-port calibration must be carried out before collecting the S-parameters, which is tedious and time-consuming. In reality, the chamber decay time depends on the diffused loss of the RC, not the insertion loss of the cables. In this paper, by making use of this fact, the ACS can be measured accurately without calibration, which will simplify the measurement process and shorten the measurement time at the same time
Quasi-Floquet prethermalization in a disordered dipolar spin ensemble in diamond
Floquet (periodic) driving has recently emerged as a powerful technique for
engineering quantum systems and realizing non-equilibrium phases of matter. A
central challenge to stabilizing quantum phenomena in such systems is the need
to prevent energy absorption from the driving field. Fortunately, when the
frequency of the drive is significantly larger than the local energy scales of
the many-body system, energy absorption is suppressed. The existence of this
so-called prethermal regime depends sensitively on the range of interactions
and the presence of multiple driving frequencies. Here, we report the
observation of Floquet prethermalization in a strongly interacting dipolar spin
ensemble in diamond, where the angular dependence of the dipolar coupling helps
to mitigate the long-ranged nature of the interaction. Moreover, we extend our
experimental observation to quasi-Floquet drives with multiple incommensurate
frequencies. In contrast to a single-frequency drive, we find that the
existence of prethermalization is extremely sensitive to the smoothness of the
applied field. Our results open the door to stabilizing and characterizing
non-equilibrium phenomena in quasi-periodically driven systems.Comment: 7+13 pages, 3+8 figure
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Revealing of the Activation Pathway and Cathode Electrolyte Interphase Evolution of Li-Rich 0.5Li2MnO3·0.5LiNi0.3Co0.3Mn0.4O2 Cathode by in Situ Electrochemical Quartz Crystal Microbalance.
The first-cycle behavior of layered Li-rich oxides, including Li2MnO3 activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the Li2MnO3 activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLi2MnO3·(1- x) LiNi0.3Co0.3Mn0.4O2 ( x = 0, 0.5, 1) cathode materials were studied with an in situ electrochemical quartz crystal microbalance (EQCM). The results demonstrate that a synergistic effect between the layered Li2MnO3 and LiNi0.3Co0.3Mn0.4O2 structures can significantly affect the activation pathway of Li1.2Ni0.12Co0.12Mn0.56O2, leading to an extra-high capacity. It is demonstrated that Li2MnO3 activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure Li2MnO3 governed by chemical decomposition (Li2O evolution). CEI evolution is closely related to Li+ extraction/insertion. The valence state variation of the metal ions (Ni, Co, Mn) in Li-rich materials can promote CEI formation. This study is of significance for understanding and designing Li-rich cathode-based batteries
Computational Simulation of Team Creativity: The Benefit of Member Flow
This study simulates the team cognition model through NetLogo 6.0.2 to view a dynamic changing of team creativity during knowledge sharing when the team members perform problem-solving tasks. A hypothesis is proposed: (a) when people possess various characteristics, members who own high-level normal knowledge and have high communication frequency are suited to perform problem construction process and members who own high-level creative knowledge and have less communication frequency are suited to perform divergent exploration process; (b) member flow that old-timer is replaced by a new member, can improve the team creativity and keep it more stable. The team cognition model is based on the social network of the team, where members are assigned cognition tasks. Also, the simulation experiments are conducted in 6 conditions and each condition has one situation including “MemberFlow” procedure, and one excluding “MemberFlow” procedure. Each experiment contains 500 repetitive experiments and in each repetition, there are 100 steps of “GO” procedure are performed. The results show that the team creativity is maximal and stable in the condition of hypothesis (a), and member flow can optimize the team creativity
Isotope engineering for spin defects in van der Waals materials
Spin defects in van der Waals materials offer a promising platform for
advancing quantum technologies. Here, we propose and demonstrate a powerful
technique based on isotope engineering of host materials to significantly
enhance the coherence properties of embedded spin defects. Focusing on the
recently-discovered negatively charged boron vacancy center
() in hexagonal boron nitride (hBN), we grow
isotopically purified crystals
for the first time. Compared to in hBN with the
natural distribution of isotopes, we observe substantially narrower and less
crowded spin transitions as well as extended
coherence time and relaxation time . For quantum sensing,
centers in our
samples exhibit a factor of
() enhancement in DC (AC) magnetic field sensitivity. For quantum registers,
the individual addressability of the hyperfine
levels enables the dynamical polarization and coherent control of the three
nearest-neighbor nuclear spins. Our results demonstrate the
power of isotope engineering for enhancing the properties of quantum spin
defects in hBN, and can be readily extended to improving spin qubits in a broad
family of van der Waals materials.Comment: 8+4+8 pages, 4+4+6 figure
Probing many-body noise in a strongly interacting two-dimensional dipolar spin system
The most direct approach for characterizing the quantum dynamics of a
strongly-interacting system is to measure the time-evolution of its full
many-body state. Despite the conceptual simplicity of this approach, it quickly
becomes intractable as the system size grows. An alternate framework is to
think of the many-body dynamics as generating noise, which can be measured by
the decoherence of a probe qubit. Our work centers on the following question:
What can the decoherence dynamics of such a probe tell us about the many-body
system? In particular, we utilize optically addressable probe spins to
experimentally characterize both static and dynamical properties of
strongly-interacting magnetic dipoles. Our experimental platform consists of
two types of spin defects in diamond: nitrogen-vacancy (NV) color centers
(probe spins) and substitutional nitrogen impurities (many-body system). We
demonstrate that signatures of the many-body system's dimensionality, dynamics,
and disorder are naturally encoded in the functional form of the NV's
decoherence profile. Leveraging these insights, we directly characterize the
two-dimensional nature of a nitrogen delta-doped diamond sample. In addition,
we explore two distinct facets of the many-body dynamics: First, we address a
persistent debate about the microscopic nature of spin dynamics in
strongly-interacting dipolar systems. Second, we demonstrate direct control
over the spectral properties of the many-body system, including its correlation
time. Our work opens the door to new directions in both quantum sensing and
simulation.Comment: 10 + 8 + 5 pages; 3 + 5 figure
Observation of Fractionally Quantized Anomalous Hall Effect
The integer quantum anomalous Hall (QAH) effect is a lattice analog of the
quantum Hall effect at zero magnetic field. This striking transport phenomenon
occurs in electronic systems with topologically nontrivial bands and
spontaneous time-reversal symmetry breaking. Discovery of its putative
fractional counterpart in the presence of strong electron correlations, i.e.,
the fractional quantum anomalous Hall (FQAH) effect, would open a new chapter
in condensed matter physics. Here, we report the direct observation of both
integer and fractional QAH effects in electrical measurements on twisted
bilayer MoTe. At zero magnetic field, near filling factor (one
hole per moir\'e unit cell) we see an extended integer QAH plateau in the Hall
resistance that is quantized to while the
longitudinal resistance vanishes. Remarkably, at and
we see plateau features in at and
, respectively, while remains small. All these
features shift linearly in an applied magnetic field with slopes matching the
corresponding Chern numbers , , and , precisely as expected for
integer and fractional QAH states. In addition, at zero magnetic field,
is approximately near half filling () and
varies linearly as is tuned. This behavior resembles that of the
composite Fermi liquid in the half-filled lowest Landau level of a
two-dimensional electron gas at high magnetic field. Direct observation of the
FQAH and associated effects paves the way for researching charge
fractionalization and anyonic statistics at zero magnetic field.Comment: 15 pages, 4 figures for main text. 8 extended data figure
FRA2A is a CGG repeat expansion associated with silencing of AFF3
Folate-sensitive fragile sites (FSFS) are a rare cytogenetically visible subset of dynamic mutations. Of the eight molecularly characterized FSFS, four are associated with intellectual disability (ID). Cytogenetic expression results from CGG tri-nucleotide-repeat expansion mutation associated with local CpG hypermethylation and transcriptional silencing. The best studied is the FRAXA site in the FMR1 gene, where large expansions cause fragile X syndrome, the most common inherited ID syndrome. Here we studied three families with FRA2A expression at 2q11 associated with a wide spectrum of neurodevelopmental phenotypes. We identified a polymorphic CGG repeat in a conserved, brain-active alternative promoter of the AFF3 gene, an autosomal homolog of the X-linked AFF2/FMR2 gene: Expansion of the AFF2 CGG repeat causes FRAXE ID. We found that FRA2A-expressing individuals have mosaic expansions of the AFF3 CGG repeat in the range of several hundred repeat units. Moreover, bisulfite sequencing and pyrosequencing both suggest AFF3 promoter hypermethylation. cSNP-analysis demonstrates monoallelic expression of the AFF3 gene in FRA2A carriers thus predicting that FRA2A expression results in functional haploinsufficiency for AFF3 at least in a subset of tissues. By whole-mount in situ hybridization the mouse AFF3 ortholog shows strong regional expression in the developing brain, somites and limb buds in 9.5-12.5dpc mouse embryos. Our data suggest that there may be an association between FRA2A and a delay in the acquisition of motor and language skills in the families studied here. However, additional cases are required to firmly establish a causal relationship
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