One novel arena for designing superconductors with high
T
C
is the flat band system. A basic idea is that flat bands, arising from quantum mechanical interference, give unique opportunities for ...enhancing
T
C
with (i) many pair-scattering channels between the dispersive and flat bands, and (ii) an even more interesting situation when the flat band is topological and highly entangled. Here, we compare two routes, which comprise a multi-band system with a flat band coexisting with dispersive ones, and a one-band case with a portion of the band being flat. Superconductivity can be induced in both cases when the flat band or portion is “incipient” (close to, but away from, the Fermi energy). Differences are, for the multi-band case, we can exploit large entanglement associated with topological states, while for the one-band case a transition between different (d and p) wave pairings can arise. These hint at some future directions.
A review essay covering articles by 1) J Ramseyer, On the invention of identity politics: the Buraku outcastes in Japan, Review & Law Economics (2019) and 2) J Ramseyer and E Rasmusen, Outcaste ...politics and organized crime in Japan: the effect of terminating ethnic subsidies, Journal of Empirical Legal Studies (2018).
After the Meiji Restoration in 1868, Japan rapidly industrialized, greatly raising its level of economic productivity. However, the peasants were kept in a state of hunger under a semifeudal ...agricultural system. How should this semi-feudality be understood? About this question arose a debate among Japanese Marxists in prewar Japan: the Debate on Japanese Capitalism. This article examines the methodologies of three analysists of Japanese capitalism focusing on the level of abstraction of the analysis of capitalism, whose ideas were derived from Hegel’s Logic and Marx’s methodology of downward analysis and upward development: Moritarō Yamada of Kōzaha, Itsurō Sakisaka of Rōnōha, and Kōzō Uno, who distanced himself from both sides. Uno criticized Yamada and Sakisaka for directly analyzing a particular Japanese capitalism with a highly general theory such as Capital, and proposed the Three-Stage Theory: the Pure Theory, which is based on the assumption of a pure capitalism, such as Marx’s Capital; the Stage Theory, which clarifies the historical developmental stage of capitalism, such as Lenin’s Imperialism (1917); and the Empirical Analysis, which analyzes capitalism in each country at a given time. However, Uno’s main concern was to analyze Japanese capitalism in the Stage Theory, doing little to further advance it in the Empirical Analysis. Therefore, this article divided the Empirical Analysis into two levels of abstraction: the domain of theoretical construction of Japanese Capitalism, such as Lenin’s Development of Capitalism in Russia (1899), and of data analysis of specific conditions of Japanese capitalism, such as Engels’ The Condition of the Working Class in England (1844), and thus proposed the Four-Stage Theory. It is a hypothesis for complementing Uno’s Three-Stage Theory, which should be further developed by data. Finally, such methodological consideration for analyzing capitalism is applicable to non-Japanese capitalist societies.
We theoretically predict a nonequilibrium phase transition in quantum spin systems induced by a laser, which provides a purely quantum-mechanical way of coherently controlling magnetization. Namely, ...when a circularly polarized laser is applied to a spin system, the magnetic component of a laser is shown to induce a magnetization normal to the plane of polarization, leading to an ultrafast phase transition. We first demonstrate this phenomenon numerically for an S = 1 antiferromagnetic Heisenberg spin chain, where a new state emerges with magnetization perpendicular to the polarization plane of the laser in place of the topologically ordered Haldane state. We then elucidate its physical mechanism by mapping the system to an effective static model. The theory also indicates that the phenomenon should occur in general quantum spin systems with a magnetic anisotropy. The required laser frequency is in the terahertz range, with the required intensity being within a prospective experimental feasibility.
Scalar spin chirality, a three-body spin correlation that breaks time-reversal symmetry, is revealed to couple directly to circularly polarized laser. This is shown by the Floquet formalism for the ...periodically driven repulsive Hubbard model with a strong-coupling expansion. A systematic derivation of the effective low-energy Hamiltonian for a spin degree of freedom reveals that the coupling constant for scalar spin chirality can become significant for a situation in which the driving frequency and the on-site interaction are comparable. This implies that the scalar chirality can be induced by circularly polarized lights, or that it can be used conversely for probing the chirality in Mott insulators as a circular dichroism.
Abstract
Non-equilibrium engineering is becoming a seminal way for realising novel quantum phases that are unimaginable in equilibrium. In particular, Floquet theory applied to quantum mechanics ...revealed that we can even control the band topology in semimetallic/insulating systems, while the straightforward application to topological superconductivity fails for typical superconductors because the supercondicting gap function does not couple to the electromagnetic field in a direct manner. Here we show that we can overcome this difficulty by taking account of correlation effects. Namely, we study how a
d
-wave superconductivity is changed when illuminated by circularly-polarised light (CPL) in the repulsive Hubbard model in the strong-coupling regime. We adopt the Floquet formalism for the Gutzwiller-projected effective Hamiltonian with the time-periodic Schrieffer-Wolff transformation. We find that CPL induces a topological superconductivity with a
d
+
i
d
pairing, which arises from the chiral spin coupling and the three-site term generated by the CPL. The latter term remains significant even for low frequencies and low intensities of the CPL. This is clearly reflected in the obtained phase diagram against the laser intensity and temperature for various frequencies red-detuned from the Hubbard
U
, with the transient dynamics also examined. The phenomenon revealed here can open a novel, dynamical way to induce a topological superconductivity.
We propose a new class of tight-binding models where a flat band exists either gapped from or crossing right through a dispersive band on two-band (i.e., two sites/unit cell) tetragonal and honeycomb ...lattices. By imposing a condition on the hopping parameters for generic models with up to third-neighbor hoppings, we first obtain models having a rigorously flat band isolated from a dispersive band with a gap, which accommodate both rank reducing and non-rank reducing of the Hamiltonian. The models include Tasaki's flat-band models, but the present model generally has a nonzero flat-band energy whose gap from the dispersive band is controllable as well. We then modify the models by appropriately changing the second- or third-neighbor hoppings, leading to a new class of two-dimensional lattices where a (slightly warped) flat band pierces a dispersive one. As with the known flat-band models, the connectivity condition is satisfied in the present models, so that we have unusual, unorthogonalizable Wannier orbitals. We have also shown that the present flat-band model can be extended to three (or higher) dimensions. Implications on possible high-TC superconductivity are discussed when a repulsive electron-electron interaction is introduced, where the flat band is envisaged to be utilized as intermediate states in pair scattering processes.
We present a framework to determine nonequilibrium steady states in strongly correlated electron systems in the presence of dissipation. This is demonstrated for a correlated electron ...(Falicov-Kimball) model attached to a heat bath and irradiated by an intense pump light, for which an exact solution is obtained with the Floquet method combined with the nonequilibrium dynamical mean-field theory. On top of a Drude-like peak indicative of photometallization as observed in recent pump-probe experiments, new nonequilibrium phenomena are predicted to emerge, where the optical conductivity exhibits dip and kink structures around the frequency of the pump light, a midgap absorption arising from photoinduced Floquet subbands, and a negative attenuation (gain) due to a population inversion.
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