1,067 research outputs found
Coherent state path integral approach to correlated electron systems with deformed Hubbard operators: from Fermi liquid to Mott insulator
In strongly correlated electron systems the constraint which prohibits the
double electron occupation at local sites can be realized by either the
infinite Coulomb interaction or the correlated hopping interaction described by
the Hubbard operators, but they both render the conventional field theory
inapplicable. Relaxing such the constraint leads to a class of correlated
hopping models based on the deformed Hubbard operators which smoothly
interpolate the locally free and strong coupling limits by a tunable
interaction parameter . Here we propose a coherent state
path integral approach appropriate to the deformed Hubbard operators for {\it
arbitrary} . It is shown that this model system exhibits the
correlated Fermi liquid behavior characterized by the enhanced Wilson ratio for
all . It is further found that in the presence of on-site Coulomb
interaction a finite Mott gap appears between the upper and lower Hubbard
bands, with the upper band spectral weight being heavily reduced by .
Our approach stands in general spatial dimensions and reveals an unexpected
interplay between the correlated hopping and the Coulomb repulsion.Comment: 9 pages, 5 figures (including several appendices
Local Kondo entanglement and its breakdown in an effective two-impurity Kondo model
Competition between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida
interaction in the two-impurity Kondo problem can be phenomenologically
described by the Rasul-Schlottmann spin model. We revisit this model from the
quantum entanglement perspective by calculating both the inter-impurity
entanglement and the local Kondo entanglement, the latter being the
entanglement between a local magnetic impurity and its spatially nearby
conduction electron. A groundstate phase diagram is derived and a discontinuous
breakdown of the local Kondo entanglement is found at the singular point,
associated concomitantly with a jump in the inter-impurity entanglement. An
entanglement monogamy holds in the whole phase diagram. Our results identify
the important role of the frustrated cross-coupling and demonstrate the local
characteristic of the quantum phase transition in the two-impurity Kondo
problem. The implications of these results for Kondo lattices and quantum
information processing are also briefly discussed.Comment: 10 pages + 6 figure
Estimating the Probability of Wind Ramping Events: A Data-driven Approach
This letter proposes a data-driven method for estimating the probability of
wind ramping events without exploiting the exact probability distribution
function (PDF) of wind power. Actual wind data validates the proposed method
Robust Unit Commitment Considering Strategic Wind Generation Curtailment
Wind generation is traditionally treated as a non-dispatchable resource and
is fully absorbed unless there are security issues. To tackle the operational
reliability issues caused by the volatile and non-dispatchable wind generation,
many dispatch frameworks have been proposed, including robust unit commitment
(RUC) considering wind variation. One of the drawbacks that commonly exist in
those dispatch frameworks is increased demand on flexibility resources and
associated costs. To improve wind dispatchability and reduce flexibility
resource costs, in this paper, we propose a novel RUC model considering
strategic wind generation curtailment (WGC). Strategic WGC can reduce wind
uncertainty and variability and increase the visibility of wind generation
capacity. As a result, the ramping requirement for wind generation will be
reduced and ramp-up capability of wind generation can be increased, leading to
reduced day-ahead operational cost with guaranteed operational reliability
requirement of power systems. The economic benefits also include profits gained
by wind farm by providing ramping-up capacities other auxiliary services. We
also propose a solution algorithm based on the column and constraint generation
(C&CG). Simulations on the IEEE 39-bus system and two larger test systems
demonstrate the effectiveness of the proposed RUC model and efficiency of the
proposed computational methodology.Comment: arXiv admin note: substantial text overlap with arXiv:1510.0331
Dirac-Kondo semimetals and topological Kondo insulators in the dilute carrier limit
Heavy fermion systems contain not only strong electron correlations, which
promote a rich set of quantum phases, but also a large spin-orbit coupling,
which tends to endow the electronic states a topological character. Kondo
insulators are understood in terms of a lattice of local moments coupled to
conduction electrons in a half-filled band, i.e., with a dense population of
about one electron per unit cell. Here, we propose that a new class of Kondo
insulator arises when the conduction-electron band is nearly empty ( or,
equivalently, full ) . We demonstrated the effect through a honeycomb Anderson
lattice model. In the empty carrier limit, spin-orbit coupling produces a gap
in the hybridized heavy fermion band, thereby generating a topological Kondo
insulator. This state can be understood in terms of a nearby phase in the
overall phase diagram, a Dirac-Kondo semimetal whose quasiparticle excitations
exhibit a non-trivial Berry phase. Our results point to the dilute carrier
limit of the heavy-fermion systems as a new setting to study strongly
correlated insulating and topological states.Comment: 7 pages + 4 figures + supplementa
Topological insulators with perfect vacancy superstructure and possible implications for iron chalcogenide superconductors
Motivated by the newly-discovered intercalated iron chalcogenide
superconductors, we construct a single orbital tight-binding model for
topological insulators on the square lattice with a perfect vacancy
superstructure. We find that such lattice structure naturally accommodates a
non-vanishing geometry phase associated with the next-nearest-neighbor
spin-orbit interaction. By calculating the bulk band structures and the finite
stripe edge states, we show that the topological insulator phases can be tuned
at certain electron fillings in a wide range of the model parameters. The
possible implications of these results for the iron deficient compounds
have been discussed.Comment: 4 pages, 6 figure
Formation of Molecular-Orbital Bands in a Twisted Hubbard Tube: Implications for Unconventional Superconductivity in K2Cr3As3
We study a twisted Hubbard tube modeling the [CrAs] structure of
quasi-one-dimensional superconductors A2Cr3As3 (A = K, Rb, Cs). The
molecular-orbital bands emerging from the quasi-degenerate atomic orbitals are
exactly solved. An effective Hamiltonian is derived for a region where three
partially filled bands intersect the Fermi energy. The deduced local
interactions among these active bands show a significant reduction compared to
the original atomic interactions. The resulting three-channel Luttinger liquid
shows various interaction-induced instabilities including two kinds of
spin-triplet superconducting instabilities due to gapless spin excitations,
with one of them being superseded by the spin-density-wave phase in the
intermediate Hund's coupling regime. The implications of these results for the
alkali chromium arsenides are discussed.Comment: 5 pages + 3 figures + Supplemental Materia
Risk-Based Admissibility Assessment of Wind Generation Integrated into a Bulk Power System
The increasing integration of large-scale volatile and uncertain wind
generation has brought great challenges to power system operations. In this
paper, a risk-based admissibility assessment approach is proposed to
quantitatively evaluate how much wind generation can be accommodated by the
bulk power system under a given unit commitment (UC) strategy. Firstly, the
operational risk brought by the variation and uncertainty of wind generation is
developed as an admissibility measure of wind generation. Then its linear
approximation is derived for practical implementation. Furthermore, a
risk-minimization model is established to mathematically characterize the
admissible region of wind generation. This model can be solved effectively by a
modified column and constraint generation (C&CG) algorithm. Simulations on the
IEEE 9-bus system and the real Guangdong power grid demonstrate the
effectiveness and efficiency of the proposed methodology.Comment: 11 page
An Adjustable Chance-Constrained Approach for Flexible Ramping Capacity Allocation
With the fast growth of wind power penetration, power systems need additional
flexibility to cope with wind power ramping. Several electricity markets have
established requirements for flexible ramping capacity (FRC) reserves. This
paper addresses two crucial issues that have rarely been discussed in the
literature: 1) how to characterize wind power ramping under different forecast
values and 2) how to achieve a reasonable trade-off between operational risks
and FRC costs. Regarding the first issue, this paper proposes a concept of
conditional distributions of wind power ramping, which is empirically verified
by using simulation and real-world data. For the second issue, this paper
develops an adjustable chance-constrained approach to optimally allocate FRC
reserves. Equivalent tractable forms of the original problem are devised to
improve computational efficiency. Tests carried out on a modified IEEE 118-bus
system demonstrate the effectiveness and efficiency of the proposed method
Robust Estimation of Reactive Power for an Active Distribution System
Increasing distributed energy resources (DERs) may result in reactive power
imbalance in a transmission power system (TPS). An active distribution power
system (DPS) having DERs reportedly can work as a reactive power prosumer to
help balance the reactive power in the TPS. The reactive power potential (RPP)
of a DPS, which is the range between the maximal inductive and capacitive
reactive power the DPS can reliably provide, should be accurately estimated.
However, an accurate estimation is difficult because of the network
constraints, mixed discrete and continuous variables, and the nonnegligible
uncertainty in the DPS. To solve this problem, this paper proposes a robust RPP
estimation method based on two-stage robust optimization, where the uncertainty
in DERs and the boundary-bus voltage is considered. In this two-stage robust
model, the RPP is pre-estimated in the first stage and its robust feasibility
for any possible instance of the uncertainty is checked via a tractable problem
in the second stage. The column-and-constraint generation algorithm is adopted,
which solves this model in finite iterations. Case studies show that this
robust method excels in yielding a completely reliable RPP, and also that a
DPS, even under the uncertainty, is still an effective reactive power prosumer
for the TPS.Comment: 8 pages, 2 figures. This paper will be submitted to IEEE Transactions
on Power System
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