Abstract
We show that the most generic form of spin-singlet superconducting order parameter for chiral fermions in systems with broken time reversal symmetry and inversion symmetry is of the
$$\Delta ..._s+i\gamma ^5\Delta _5$$
Δ
s
+
i
γ
5
Δ
5
where
$$\Delta _s$$
Δ
s
is the usual order parameter and
$$\Delta _5$$
Δ
5
is the pseudo-scalar order parameter. After factoring out the
U
(1) phase
$$e^{i\phi }$$
e
i
ϕ
, this form of superconductivity admits yet additional complex structure in the plane of
$$(\Delta _s,\Delta _5)$$
(
Δ
s
,
Δ
5
)
. The polar angle
$$\chi $$
χ
in this plane, which we call the chiral angle, can be controlled by the external flux bias. We present a synthetic setup based on stacking of topological insulators (TIs) and superconductors (SCs). Alternatively flux biasing the superconductors with a fluxes
$$\pm \Phi $$
±
Φ
leads to
$$\Delta _5=\Delta _0 \sin (\chi )$$
Δ
5
=
Δ
0
sin
(
χ
)
, where
$$\Delta _0$$
Δ
0
is the superconducting order parameter of the SC layers, and the chiral angle
$$\chi ={2\pi }\Phi /\Phi _0$$
χ
=
2
π
Φ
/
Φ
0
is directly given by the flux
$$\Phi $$
Φ
in units of the flux quantum
$$\Phi _0=h/(2e)$$
Φ
0
=
h
/
(
2
e
)
. This can be used as a building block to construct a two-dimensional Josephson array. In this setup
$$\chi $$
χ
will be a background field defining a pseudoscalar
$$\Delta _5$$
Δ
5
that can be tuned to desired configuration. While in a uniform background field
$$\Delta _5$$
Δ
5
the dynamics of
$$\phi $$
ϕ
is given by standard XY model and its associated vortices, a
staggered
background
$$\pm \Delta _5$$
±
Δ
5
(or equivalently
$$\chi $$
χ
and
$$\chi +\pi $$
χ
+
π
in alternating lattice sites) creates a new set of minima for the
$$\phi $$
ϕ
field that will support half-vortex excitations. An isolated single engineered “half-vortex” in the
$$\chi $$
χ
field in an otherwise uniform background will bind a
$$\phi $$
ϕ
-half-vortex. This is similar to the way a p-wave superconducting vortex core binds a Majorana fermion.
Narrative Economics Shiller, Robert J.
The American economic review,
04/2017, Letnik:
107, Številka:
4
Journal Article
Recenzirano
This address considers the epidemiology of narratives relevant to economic fluctuations. The human brain has always been highly tuned toward narratives, whether factual or not, to justify ongoing ...actions, even such basic actions as spending and investing. Stories motivate and connect activities to deeply felt values and needs. Narratives "go viral" and spread far, even worldwide, with economic impact. The 1920-1921 Depression, the Great Depression of the 1930s, the so-called Great Recession of 2007-2009, and the contentious political-economic situation of today are considered as the results of the popular narratives of their respective times. Though these narratives are deeply human phenomena that are difficult to study in a scientific manner, quantitative analysis may help us gain a better understanding of these epidemics in the future.
CREDIT-MARKET SENTIMENT AND THE BUSINESS CYCLE López-Salido, David; Stein, Jeremy C.; Zakrajšek, Egon
The Quarterly journal of economics,
08/2017, Letnik:
132, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Using U.S. data from 1929 to 2015, we show that elevated credit-market sentiment in year t − 2 is associated with a decline in economic activity in years t and t + 1. Underlying this result is the ...existence of predictable mean reversion in credit-market conditions. When credit risk is aggressively priced, spreads subsequently widen. The timing of this widening is, in turn, closely tied to the onset of a contraction in economic activity. Exploring the mechanism, we find that buoyant credit-market sentiment in year t − 2 also forecasts a change in the composition of external finance: net debt issuance falls in year t, while net equity issuance increases, consistent with the reversal in credit-market conditions leading to an inward shift in credit supply. Unlike much of the current literature on the role of financial frictions in macroeconomics, this article suggests that investor sentiment in credit markets can be an important driver of economic fluctuations.
Voltage fluctuations are common disturbances in power grids. Initially, it is necessary to selectively identify individual sources of voltage fluctuations to take actions to minimize the effects of ...voltage fluctuations. Selective identification of disturbing loads is possible using a signal chain consisting of demodulation, decomposition, and assessment of the propagation of component signals. The accuracy of such an approach is closely related to the applied decomposition method. The paper presents a new method for decomposition by approximation with pulse waves. The proposed method allows for a correct identification of selected parameters, that is, the frequency of changes in the operating state of individual sources of voltage fluctuations and the amplitude of voltage changes caused by them. The article presents results from numerical simulation studies and laboratory experimental studies, based on which the estimation errors of the indicated parameters were determined by the proposed decomposition method and other empirical decomposition methods available in the literature. The real states that occur in power grids were recreated in the research. The metrological interpretation of the results obtained from the numerical simulation and experimental research is discussed.
I construct a daily business cycle index based on quarterly GDP growth and textual information contained in a daily business newspaper. The newspaper data are decomposed into time series representing ...news topics, while the business cycle index is estimated using the topics and a time-varying dynamic factor model where dynamic sparsity is enforced upon the factor loadings using a latent threshold mechanism. The resulting index classifies the phases of the business cycle with almost perfect accuracy and provides broad-based high-frequency information about the type of news that drive or reflect economic fluctuations. In out-of-sample nowcasting experiments, the model is competitive with forecast combination systems and expert judgment, and produces forecasts with predictive power for future revisions in GDP. Thus, news reduces noise. Supplementary materials for this article are available online.
Modular multilevel converter (MMC) is a newly emerging multilevel topology for high-voltage applications during recent years. In this paper, a new MMC is proposed, and the structure and operating ...principle are analyzed. Owing to the cascaded basic cells without multiwinding transformer, the voltage balancing of floating capacitors must be considered. However, the voltage fluctuation also exists, and theoretical analysis indicates that the amplitude is inversely proportional to the fundamental frequency. This paper has proposed a voltage-fluctuation-suppression method which can reduce the amplitude of the voltage fluctuation in low-frequency region. It can also be used in motor driving with pump/blowerlike load at low frequency and improve the start-up performance significantly. A low-power three-phase five-level prototype is designed and built up to demonstrate the validity of this method.
WHAT'S NEWS IN BUSINESS CYCLES Schmitt-Grohé, Stephanie; Uribe, Martín
Econometrica,
November 2012, Letnik:
80, Številka:
6
Journal Article
Recenzirano
Odprti dostop
In the context of a dynamic, stochastic, general equilibrium model, we perform classical maximum likelihood and Bayesian estimations of the contribution of anticipated shocks to business cycles in ...the postwar United States. Our identification approach relies on the fact that forward-looking agents react to anticipated changes in exogenous fundamentals before such changes materialize. It further allows us to distinguish changes in fundamentals by their anticipation horizon. We find that anticipated shocks account for about half of predicted aggregate fluctuations in output, consumption, investment, and employment.
•We develop a model of commodity prices and the global economy that yields a factor structure for commodity prices.•The model yields identification conditions that can be used to provide a structural ...interpretation to the factor structure in commodity prices.•We apply this approach to a cross-section of commodity prices to decompose the historical sources of commodity price fluctuations.
Guided by a macroeconomic model with endogenous commodity prices, we apply a new factor-based identification strategy to decompose the historical sources of changes in commodity prices and global economic activity. The model yields a factor structure for commodity prices and identification conditions that provide an economic interpretation: one factor captures the combined contribution of shocks that affect commodity markets only through general-equilibrium forces. Applied to a cross-section of commodity prices since 1968, the theoretical restrictions are consistent with the data and yield structural interpretations of the common factors in commodity prices. Commodity-related shocks have contributed modestly to global economic fluctuations.
This study investigates the suitability of the annealed approximation in high-dimensional systems characterized by dense networks with quenched link disorder, employing models of coupled oscillators. ...We demonstrate that dynamic equations governing dense-network systems converge to those of the complete-graph version in the thermodynamic limit, where link disorder fluctuations vanish entirely. Consequently, the annealed-network systems, where fluctuations are attenuated, also exhibit the same dynamic behavior in the thermodynamic limit. However, a significant discrepancy arises in the incoherent (disordered) phase wherein the finite-size behavior becomes critical in determining the steady-state pattern. To explicitly elucidate this discrepancy, we focus on identical oscillators subject to competitive attractive and repulsive couplings. In the incoherent phase of dense networks, we observe the manifestation of random irregular states. In contrast, the annealed approximation yields a symmetric (regular) incoherent state where two oppositely coherent clusters of oscillators coexist, accompanied by the vanishing order parameter. Our findings imply that the annealed approximation should be employed with caution even in dense-network systems, particularly in the disordered phase.
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