AAPPS Bulletin最新文献

Gravitational waves induced by large primordial curvature fluctuations may result in a sizable stochastic gravitational wave background. Interestingly, curvature fluctuations are gradually generated by initial isocurvature fluctuations, which in turn induce gravitational waves. Initial isocurvature fluctuations commonly appear in multi-field models of inflation as well as in the formation of scattered compact objects in the very early universe, such as primordial black holes and solitons like oscillons and cosmic strings. Here, we provide a review on isocurvature induced gravitational waves and its applications to dark matter and the primordial black hole dominated early universe.

Flat bands arise in periodic media when symmetries or fine-tuning result in perfect wavepacket localisation. Flat band localisation is fragile and exhibits remarkably sharp sensitivity to perturbations including interactions and disorder, leading to a variety of interesting quantum and classical phenomena. Originally a theoretical curiosity, advances in fabrication methods have allowed flat band physics to be observed down to the nanoscale. This article briefly reviews progress in the study of flat bands and disorder over the past decade and provides an outlook on where this exciting field is headed.

In this paper, molecular dynamics simulations have been employed to investigate the phonon thermal transport in bilayer polycrystalline graphene nanoribbon (pGNR/pGNR), compared with bilayer graphene nanoribbon (GNR/GNR) and pGNR/GNR heterostructure. The interfacial thermal resistance (ITR) of bilayer structures was also calculated using the heat dissipation method. The effects of interlayer interaction, grain size, and vacancy defects on ITR and in-plane phonon thermal conductivity of bilayer structures were investigated. It was found that the ITR as well as in-plane phonon thermal conductivity of pGNR/pGNR was less than that of pGNR/GNR and much less than that of GNR/GNR, for the same size. For the studied bilayer structures, both the ITR and in-plane phonon thermal conductivity decrease with increasing interlayer interactions. Moreover, ITR increases with increasing grain area size whereas decreases with increasing vacancy defects in pGNR-based bilayers. The introduction of pGNR interface roughness and vacancy defects results in an enhanced phonon coupling in pGNR-based bilayers compared to pure GNR/GNR bilayers. Presented simulation investigations will help to understand the interlayer thermal transport properties of polycrystalline graphene and provide essential guidance for experimentally regulating phonon thermal transport between layers of polycrystalline graphene.

The real atomic scale details of molecular junctions would be of much complexity and can yield a plethora of “counterintuitive” results. Here, we provide an overview of four unconventional intentional or unintentional transport phenomena in molecular junctions, in particular, unconventional tunneling length-dependent transport behavior, deviation from Kirchhoff’s superposition law, dual roles of imperfect engineering, and masked quantum interference. These abnormal phenomena are not engaged in a dead end. On the contrary, it offers plenty of research opportunities in molecular electronics.

A growing cohort of experimental linear photonic networks implementing Gaussian boson sampling (GBS) have now claimed quantum advantage. However, many open questions remain on how to effectively verify these experimental results, as scalable methods are needed that fully capture the rich array of quantum correlations generated by these photonic quantum computers. In this paper, we briefly review recent theoretical methods to simulate experimental GBS networks. We focus mostly on methods that use phase-space representations of quantum mechanics, as these methods are highly scalable and can be used to validate experimental outputs and claims of quantum advantage for a variety of input states, ranging from the ideal pure squeezed vacuum state to more realistic thermalized squeezed states. A brief overview of the theory of GBS, recent experiments, and other types of methods are also presented. Although this is not an exhaustive review, we aim to provide a brief introduction to phase-space methods applied to linear photonic networks to encourage further theoretical investigations.

Controllably fabricating low-dimensional systems and unraveling their exotic states at the atomic scale is a pivotal step for the construction of quantum functional materials with emergent states. Here, by utilizing the elaborated molecular beam epitaxy growth, we obtain various Fe_xSe_y phases beyond the single-layer FeSe/SrTiO₃ films. A synthetic strategy of lowering substrate temperature with superfluous Se annealing is implemented to achieve various stoichiometric FeSe-derived phases, ranging from 1:1 to 5:8. The phase transitions and electronic structure of these Fe_xSe_y phases are systematically characterized by atomic resolution scanning tunneling microscopy measurements. We observe the long-ranged antiferromagnetic order of the Fe₄Se₅ phase by spin-polarized signals with striped patterns, which is also verified by their magnetic response of phase shift between adjacent domains. The electronic doping effect in insulating Fe₄Se₅ and the kagome effect in metallic Fe₅Se₈ are also discussed, where the kagome lattice is a promising structure to manifest both spin frustration of d electrons in a quantum-spin-liquid phase and correlated topological states with flat-band physics. Our study provides promising opportunities for constructing artificial superstructures with tunable building blocks, which is helpful for understanding the emergent quantum states and their correlation with competing orders in the FeSe-based family.

In this article we outline the historical development and key results obtained to date for free parafermionic spin chains. The concept of free parafermions provides a natural N-state generalization of free fermions, which have long underpinned the exact solution and application of widely studied quantum spin chains and their classical counterparts. In particular, we discuss the Baxter-Fendley free parafermionic Z(N) spin chain, which is a relatively simple non-Hermitian generalization of the Ising model.

Recent progress of advanced operation modes in tokamaks is addressed focusing upon internal transport barrier (ITB) discharges. These ITB discharges are being considered as one of candidate operation modes in fusion reactors. Here, “internal” means core region of a fusion plasma, and “transport barrier” implies bifurcation of transport phenomena due to suppressing plasma turbulence. Although ITB discharges have been developed since the mid-1990, they have been suffering from harmful plasma instabilities, impurity accumulation, difficulty of feedback control of kinetic plasma profiles such as pressure or current density, and so on. Sustainment of these discharges in long-pulse operations above wall saturation time is another huddle. Recent advances in ITB experiments to overcome the difficulties of ITB discharges are addressed for high β_p plasmas in DIII-D, broad ITB without internal kink mode in HL-2A, F-ATB (fast ion-induced anomalous transport barrier) in ASDEX upgrade, ion and electron ITB in LHD, and FIRE (fast ion regulated enhancement) mode in KSTAR. The core-edge integration is discussed in the ITB discharges. The DIII-D high β_p plasmas facilitate divertor detachment which weakens the edge transport barrier (ETB) but extends the ITB radius resulting in a net gain in energy confinement. Double transport barriers were observed in KSTAR without edge localized mode (ELM). FIRE modes in KSTAR are equipped with the I-mode-like edge which prevents the ELM burst and raise the fusion performance together with ITB. Finally, long sustainment of ITBs is discussed. EAST established electron ITB mode in long-pulse operations. JET achieved quasi-stationary ITB with active control of the pressure profile. JT-60U obtained 28 s of high β_p hybrid mode, and KSTAR sustained stable ITB in conventional ITB mode as well as FIRE mode. These recent outstanding achievements can promise ITB scenarios as a strong candidate for fusion reactors.

We theoretically investigate the polaron physics of an impurity immersed in a two-dimensional Fermi sea, interacting via a p-wave interaction at finite temperature. In the unitary limit with a divergent scattering area, we find a well-defined repulsive Fermi polaron at short interaction range, which shows a remarkable thermal stability with increasing temperature. The appearance of such a stable repulsive Fermi polaron in the resonantly interacting limit can be attributed to the existence of a quasi-bound dressed molecule state hidden in the two-particle continuum, although there is no bound state in the two-particle limit. We show that the repulsive Fermi polaron disappears when the interaction range increases or when the scattering area is tuned to the weakly interacting regime. The large interaction range and small scattering area instead stabilize attractive Fermi polarons.