6,688 research outputs found
Charge ordering and phase separation in the infinite dimensional extended Hubbard model
We study the extended Hubbard model with both on-site (U) and nearest
neighbor (V) Coulomb repulsion using the exact diagonalization method within
the dynamical mean field theory. For a fixed U (U=2.0), the T-n phase-diagrams
are obtained for V=1.4 and V=1.2, at which the ground states of n=1/2 system is
charge-ordered and charge-disordered, respectively. In both cases, robust
charge order is found at finite temperature and in an extended filling regime
around n=1/2. The order parameter changes non-monotonously with temperature.
For V=1.4, phase separation between charge-ordered and charge-disordered phases
is observed in the low temperature and n < 0.5 regime. It is described by an
"S"-shaped structure of the n-/mu curve. For V=1.2, the ground state is
charge-disordered, and a reentrant charge-ordering transition is observed for
0.42 < n < 0.68. Relevance of our results to experiments for doped manganites
is discussed.Comment: 9 pages, 7 figures, submitted to Phys. Rev.
Cost-Effective Cache Deployment in Mobile Heterogeneous Networks
This paper investigates one of the fundamental issues in cache-enabled
heterogeneous networks (HetNets): how many cache instances should be deployed
at different base stations, in order to provide guaranteed service in a
cost-effective manner. Specifically, we consider two-tier HetNets with
hierarchical caching, where the most popular files are cached at small cell
base stations (SBSs) while the less popular ones are cached at macro base
stations (MBSs). For a given network cache deployment budget, the cache sizes
for MBSs and SBSs are optimized to maximize network capacity while satisfying
the file transmission rate requirements. As cache sizes of MBSs and SBSs affect
the traffic load distribution, inter-tier traffic steering is also employed for
load balancing. Based on stochastic geometry analysis, the optimal cache sizes
for MBSs and SBSs are obtained, which are threshold-based with respect to cache
budget in the networks constrained by SBS backhauls. Simulation results are
provided to evaluate the proposed schemes and demonstrate the applications in
cost-effective network deployment
Mott-Hubbard transition in infinite dimensions
We analyze the unanalytical structure of metal-insulator transition (MIT) in
infinite dimensions. By introducing a simple transformation into the dynamical
mean-field equation of Hubbard model, a multiple-valued structure in Green's
function and other thermodynamical quantities with respect to the interaction
strength are found at low temperatures. A unified description of stable,
metastable and unstable phases is obtained in the regime
, and the Maxwell construction is performed to evaluate
the MIT line . We show how the first-order MIT at
for evolves into second-order one at for . The phase
diagram near MIT is presented.Comment: 5 pages with 3 figures, text and figures revise
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Constitutions, not commitment: international law, diffusion, and the construction of universal constitutional principles
Constitutional design processes increasingly take on a transnational dimension. Whether through diffusion of human rights and liberal democratic norms into national texts or through the involvement of international organizations, non-governmental organizations, and expert advisors in a small but growing number of cases of drafting constitutions themselves, the conceptualization of national constitutional design as a largely domestic process can no longer be taken for granted.
What does international law say about constitutionalism and how do these legal arguments and political pressures change the incentive structures surrounding the drafters of national constitutions? Further, does the involvement of international law and international politics have any effect on the implementation of the text after adoption?
This dissertation endeavors to answer both questions by joining two stories of constitutional design. First, it describes top-down arguments of international legal compliance and the effects of international law in national constitutional design.
Second, exploring the role of political factions and pluralist responses to international influence in Cambodia and Vietnam, it shows how the bottom-up experience of domestic political actors and how responses to different the types and scales of intervention contributed to different experiences for these constitutions.
Together, these two stories show how the study of comparative constitutionalism can be enriched by further accounting for international factors as not only sources of normative diffusion, but also as politically disruptive factors in the political negotiations that determine both the text itself and the text’s implementation after adoption.Governmen
Deep Learning Topological Invariants of Band Insulators
In this work we design and train deep neural networks to predict topological
invariants for one-dimensional four-band insulators in AIII class whose
topological invariant is the winding number, and two-dimensional two-band
insulators in A class whose topological invariant is the Chern number. Given
Hamiltonians in the momentum space as the input, neural networks can predict
topological invariants for both classes with accuracy close to or higher than
90%, even for Hamiltonians whose invariants are beyond the training data set.
Despite the complexity of the neural network, we find that the output of
certain intermediate hidden layers resembles either the winding angle for
models in AIII class or the solid angle (Berry curvature) for models in A
class, indicating that neural networks essentially capture the mathematical
formula of topological invariants. Our work demonstrates the ability of neural
networks to predict topological invariants for complicated models with local
Hamiltonians as the only input, and offers an example that even a deep neural
network is understandable.Comment: 8 pages, 5 figure
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