60,447 research outputs found
Thermal Entanglement between Alternate Qubits of a Four-qubit Heisenberg XX Chain in a Magnetic Field
The concurrence of two alternate qubits in a four-qubit Heisenberg XX chain
is investigated when a uniform magnetic field B is included. It is found that
there is no thermal entanglement between alternate qubits if B is close to
zero. Magnetic field can induce entanglement in a certain range both for the
antiferromagnetic and ferromagnetic cases. Near zero temperature, the
entanglement undergoes two sudden changes with increasing value of the magnetic
field B. This is due to the changes in the ground state. This novel property
may be used as quantum entanglement switch. The anisotropy in the system can
also induce the entanglement between two alternate qubits.Comment: 10 pages, 3 figure
A new conducting polymer with exceptional visible-light photocatalytic activity derived from varbituric acid polycondensation
Abstract A novel covalent, metal-free, photocatalytic material is prepared by thermal polymerization of barbituric acid (BA). The structure of the photocatalyst is analyzed by using scanning electron microscopy, X-ray diffraction, and infrared, UV?visible, and 1H solution and 13C solid-state NMR spectroscopy. The photodegradation efficiency of BA thermally polymerized at different temperatures is tested by photocatalytic degradation of aquatic rhodamine B (RhB) dye under visible-light irradiation. It is shown that heating BA at an optimized temperature of 300 °C, that is, still in the range that polymer-like polycondensation takes place, results in a photocatalyst that can remove RhB with 96% photodegradation efficiency after 70 min exposure to visible light. The polycondensation reaction of BA is identified to process through precipitation of trimer units as primary building blocks. Reference experiments such as addition of scavengers and saturation with oxygen are studied to understand the photodegradation process. It is shown that the presence of triethanolamine, and excess of oxygen and p-benzoquinone in the solution of RhB and photocatalyst (BA300) is not beneficial, but decreases the photodegradation efficiency
Using Different Approaches to Evaluate Individual Social Equity in Transport
Inequalities not only exist in the field of economics in relation to income and wealth, but also in other areas, such as the transport sector, where access to and use of different transport modes varies markedly across population groups, and which provides the means to access everyday living activities. A key concern within the transport sector is that inequality has extended beyond the traditional measures of travel, and now covers a wide range of effects relating to social exclusion, freedom, well-being and being able to access reasonable opportunities and resources. In order to address the aforementioned issues, an important question to resolve is what type of methods can be used to measure inequalities in transport most effectively. Therefore, this study aims to apply different approaches, including the Capabilities Approach (CA) and a further six inequality indices, namely the Gini coefficient, the Atkinson index, the Palma ratio, the Pietra ratio, the Schutz coefficient and the Theil index, to the case study using the relatively migrant-rich lower-income neighbourhood of Tuqiao, in Beijing, in order to assess individual transport-related social inequity issues. The findings suggest that the CA is useful in assessing transport-related inequalities where there are significant barriers to the take up of accessibility, for example where there are high levels of disadvantaged groups and disaggregated analysis can be undertaken. The Palma ratio appears to have a larger effect than the Gini coefficient and the other inequality indices when measuring transport-related social inequity. In addition, we also found that most income inequality methods adapted from econometrics may be better suited to measuring transport-related social inequity between different regions, cities or countries, or within the same area, but at different points in time, rather than to measuring a single neighbourhood as a whole. Finally, we argue that to what extent politicians or transport planners can use appropriate management tools to measure transport-related social inequalities may be significant in terms of the progress that can be made in the fight against social inequity in the transport field
A hybrid stochastic hierarchy equations of motion approach to treat the low temperature dynamics of non-Markovian open quantum systems
The hierarchical equations of motion technique has found widespread success
as a tool to generate the numerically exact dynamics of non-Markovian open
quantum systems. However, its application to low temperature environments
remains a serious challenge due to the need for a deep hierarchy that arises
from the Matsubara expansion of the bath correlation function. Here we present
a hybrid stochastic hierarchical equation of motion (sHEOM) approach that
alleviates this bottleneck and leads to a numerical cost that is nearly
independent of temperature. Additionally, the sHEOM method generally converges
with fewer hierarchy tiers allowing for the treatment of larger systems.
Benchmark calculations are presented on the dynamics of two level systems at
both high and low temperatures to demonstrate the efficacy of the approach.
Then the hybrid method is used to generate the exact dynamics of systems that
are nearly impossible to treat by the standard hierarchy. First, exact energy
transfer rates are calculated across a broad range of temperatures revealing
the deviations from the Forster rates. This is followed by computations of the
entanglement dynamics in a system of two qubits at low temperature spanning the
weak to strong system-bath coupling regimes.Comment: 20 pages, 6 figure
Destruction of the Mott Insulating Ground State of Ca_2RuO_4 by a Structural Transition
We report a first-order phase transition at T_M=357 K in single crystal
Ca_2RuO_4, an isomorph to the superconductor Sr_2RuO_4. The discontinuous
decrease in electrical resistivity signals the near destruction of the Mott
insulating phase and is triggered by a structural transition from the low
temperature orthorhombic to a high temperature tetragonal phase. The magnetic
susceptibility, which is temperature dependent but not Curie-like decreases
abruptly at TM and becomes less temperature dependent. Unlike most insulator to
metal transitions, the system is not magnetically ordered in either phase,
though the Mott insulator phase is antiferromagnetic below T_N=110 K.Comment: Accepted for publication in Phys. Rev. B (Rapid Communications
Control of coherent backscattering by breaking optical reciprocity
Reciprocity is a universal principle that has a profound impact on many areas
of physics. A fundamental phenomenon in condensed-matter physics, optical
physics and acoustics, arising from reciprocity, is the constructive
interference of quantum or classical waves which propagate along time-reversed
paths in disordered media, leading to, for example, weak localization and
metal-insulator transition. Previous studies have shown that such coherent
effects are suppressed when reciprocity is broken. Here we show that by
breaking reciprocity in a controlled manner, we can tune, rather than simply
suppress, these phenomena. In particular, we manipulate coherent backscattering
of light, also known as weak localization. By utilizing a non-reciprocal
magneto-optical effect, we control the interference between time-reversed paths
inside a multimode fiber with strong mode mixing, and realize a continuous
transition from the well-known peak to a dip in the backscattered intensity.
Our results may open new possibilities for coherent control of classical and
quantum waves in complex systemsComment: Comments are welcom
Editorial Introduction: Shifting frontiers of the new spatial planning paradigm from a theoretical and methodological development perspective
There is currently an emerging discussion about the shifting frontiers of research in the field of urban and regional planning. This special section focuses on the new spatial planning paradigm from a theoretical and methodological development perspective. The six papers published in this special section contribute to and extend the spatial planning literature, specifically in relation to land use, ecology and urban geography. The main themes addressed by the papers are as follows: 1) advancing the theoretical development of the concept of new ruralism and its application as a spatial planning principle for sustainable development in rural areas of Korea; 2) the significance of spatial adaptation and the resilience of healthcare systems in responding to the unprecedented hazards and health risks resulting from the COVID-19 pandemic in Abu Dhabi City, in the United Arab Emirates; 3) transit-oriented development (TOD) and its spatial associations with the land use of low-density areas in the Keihanshin conurbation in Japan; 4) estimating the land surface temperature to enhance understanding of changes in the landscape for residents of the Kolkata Metropolitan area in India, using a radiative transfer equation algorithm; 5) how the ecological knowledge of local residents in Shiheung City in Korea, accessed via a spatial text mining approach, can be used to inform policy making; 6) applying a Q methodology to investigate individuals’ subjective views and perceptions of nature/natural features when visiting urban parks in Seoul, Korea. Bringing different types of theoretical and methodological perspectives together, this editorial concludes with a summary, critical discussion and suggestions for future research with regard to the new spatial planning paradigm
Pinned Low Energy Electronic Excitation in Metal Exchanged Vanadium Oxide Nanoscrolls
We measured the optical properties of mixed valent vanadium oxide nanoscrolls
and their metal exchanged derivatives in order to investigate the charge
dynamics in these compounds. In contrast to the prediction of a metallic state
for the metal exchanged derivatives within a rigid band model, we find that the
injected charges in Mn exchanged vanadium oxide nanoscrolls are pinned.
A low-energy electronic excitation associated with the pinned carriers appears
in the far infrared and persists at low temperature, suggesting that the
nanoscrolls are weak metals in their bulk form, dominated by inhomogeneous
charge disproportionation and Madelung energy effects.Comment: 4 figure
Competing Ground States in Triple-layered Sr4Ru3O10: Verging on Itinerant Ferromagnetism with Critical Fluctuations
Sr4Ru3O10 is characterized by a sharp metamagnetic transition and
ferromagnetic behavior occurring within the basal plane and along the c-axis,
respectively. Resistivity at magnetic field, B, exhibits low-frequency quantum
oscillations when B||c-axis and large magnetoresistivity accompanied by
critical fluctuations driven by the metamagnetism when B^c-axis. The complex
behavior evidenced in resistivity, magnetization and specific heat presented is
not characteristic of any obvious ground states, and points to an exotic state
that shows a delicate balance between fluctuations and order.Comment: 18 pages, 4 figure
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