Quantum Frontiers最新文献

Topological semimetals, possessing topologically non-trivial band structures, serve as excellent platforms for realizing topological superconductivity through hard point-contact experiments. In this study, we successfully induce superconductivity in the three-dimensional Dirac semimetal, cubic PtBi₂, using ferromagnetic and paramagnetic tips in hard point contact experiments. The induced superconductivity is proven to be insensitive to ferromagnetism and exhibits unconventional features in the point-contact spectra. The highest superconducting transition temperature ((T_{mathrm{c}})) reaches approximately 5.1 K, and the (T_{mathrm{c}}) values are proven to have a positive correlation with the coupling between the tip and the sample. Furthermore, we extend our point-contact experiments to trigonal PtBi₂, a material possessing a type-I Weyl semimetal band structure and triply degenerate points proximate to the Fermi level. Utilizing both ferromagnetic Ni tips and paramagnetic Ag tips, we successfully enhance superconductivity with a (T_{mathrm{c}}) of up to 3.0 K in this material. The findings from point-contact measurements reveal that the enhanced superconductivity is compatible with ferromagnetism and the magnetism of the tip can affect the symmetry of the enhanced superconducting state. Given that the lattice structure remains stable under pressure up to 51.2 GPa for cubic PtBi₂ and 12.9 GPa for trigonal PtBi₂, the emergent superconducting states observed in these two PtBi₂ materials could inherit their topological nontrivial nature and be promising candidates for topological superconductor.

Moiré superlattices have emerged as an excellent platform for investigating a plethora of exotic quantum states in condensed matter physics. Recent advancements have unveiled abundant discoveries in two-dimensional moiré superlattices. In this paper, we will present a review of the recent progresses in superconductivity and topological physics within graphene and transition metal dichalcogenides-based moiré superlattices. Additionally, we outline future potential challenges and desirable efforts for discovering, understanding, and controlling these novel states in two-dimensional moiré superlattices.

Bi₄Br₄ is a material rich in intriguing topological properties. Monolayer Bi₄Br₄ film exhibits helical edge states characteristic of a quantum spin Hall insulator, while bulk Bi₄Br₄ represents a higher-order topological insulator with hinge states. However, direct exfoliation from single crystal can only obtain thin nanowires due to the weak van der Waals forces between Bi₄Br₄ chains, which limits its optical analysis and application, while the growth of Bi₄Br₄ thin films is also full of challenges due to the extremely narrow growth temperature range and the accurate control of the BiBr₃ flux. Here, we reported the controlled growth of α-Bi₄Br₄ thin films on intrinsic silicon substrates using molecular beam epitaxy. The growth temperature, BiBr₃ flux, and the flux ratio of Bi and BiBr₃ were accurately controlled. Then, the morphology, composition, and bonding of the prepared films were investigated using atomic force microscopy, X-ray photoelectron spectroscopy and Raman spectroscopy. The growth of large, uniform thin films provides an ideal material platform for studying the physical properties of Bi₄Br₄. Additionally, we utilized Fourier-transform infrared spectroscopy to explore the film’s infrared characteristics, revealing strong absorption in the low frequency range due to the high proportion of one-dimensional topological edge states and laying the groundwork for further exploration of its potential applications in the optoelectronic field.

This review aims to provide a comprehensive overview of the development and current understanding of GaAs and InAs heterostructures, with a special emphasis on achieving high material quality and high-mobility two-dimensional electron gases (2DEGs). The review discusses the evolution of structural designs that have significantly contributed to the enhancement of electron mobility, highlighting the critical considerations of scattering mechanisms of the 2DEGs. In addition, this review examines the substantial contributions of Molecular Beam Epitaxy (MBE) to these developments, particularly through advancements in vacuum technology, source material purification, and precision control of growth conditions. The intent of this review is to serve as a useful reference for researchers and practitioners in the field, offering insights into the historical progression and technical details of these semiconductor systems.

The Nernst effect is a fundamental thermoelectric conversion phenomenon that was deemed to be possible only in systems with magnetic field or magnetization. In this work, we propose a novel dynamical chiral Nernst effect that can appear in two-dimensional van der Waals materials with chiral structural symmetry in the absence of any magnetic degree of freedom. This unconventional effect is triggered by time variation of an out-of-plane electric field, and has an intrinsic quantum geometric origin linked to not only the intralayer center-of-mass motion but also the interlayer coherence of electronic states. We demonstrate the effect in twisted homobilayer and homotrilayer transition metal dichalcogenides, where the strong twisted interlayer coupling leads to sizable intrinsic Nernst conductivities well within the experimental capacity. This work suggests a new route for electric control of thermoelectric conversion.

We propose the superconducting van der Waals material 4Hb-TaS₂ to realize the (mathbb{Z}_{2}) topological order and interpret the recent discovery of the spontaneous vortex generation in 4Hb-TaS₂ as the vison-vortex nucleation. For the alternating stacking of metallic/superconducting and Mott insulating layers in 4Hb-TaS₂, we expect the local moments in the Mott insulating 1T-TaS₂ layer to form the (mathbb{Z}_{2}) topological order. The spontaneous vortex generation in 4Hb-TaS₂ is interpreted from the transition or nucleation between the superconducting vortex and the (mathbb{Z}_{2}) vison in different phase regimes. Differing from the single vison-vortex nucleation in the original Senthil–Fisher’s cuprate proposal, we consider such nucleation process between the superconducting vortex lattice and the vison crystal. We further propose experiments to distinguish this proposal with the (mathbb{Z}_{2}) topological order from the chiral spin liquid scenarios.

Entanglement constitutes one of the key concepts in quantum mechanics and serves as an indispensable tool in the understanding of quantum many-body systems. In this work, we perform extensive numerical investigations of extensive entanglement properties of coupled quantum spin chains. This setup has proven useful for e.g. extending the Lieb–Schultz–Mattis theorem to open systems, and contrasts the majority of previous research where the entanglement cut has one lower dimension than the system. We focus on the cases where the entanglement Hamiltonian is either gapless or exhibits spontaneous symmetry breaking behavior. We further employ conformal field theoretical formulae to identify the universal behavior in the former case. The results in our work can serve as a paradigmatic starting point for more systematic exploration of the largely uncharted physics of extensive entanglement, both analytical and numerical.

Neural network force fields have significantly advanced ab initio atomistic simulations across diverse fields. However, their application in the realm of magnetic materials is still in its early stage due to challenges posed by the subtle magnetic energy landscape and the difficulty of obtaining training data. Here we introduce a data-efficient neural network architecture to represent density functional theory total energy, atomic forces, and magnetic forces as functions of atomic and magnetic structures. Our approach incorporates the principle of equivariance under the three-dimensional Euclidean group into the neural network model. Through systematic experiments on various systems, including monolayer magnets, curved nanotube magnets, and moiré-twisted bilayer magnets of CrI₃, we showcase the method’s high efficiency and accuracy, as well as exceptional generalization ability. The work creates opportunities for exploring magnetic phenomena in large-scale materials systems.

In a quantum many-body system, autocorrelation functions can determine linear responses nearby equilibrium and quantum dynamics far from equilibrium. In this letter, we bring out the connection between the operator complexity and the autocorrelation function. In particular, we focus on a particular kind of operator complexity called the Krylov complexity. We find that a set of Lanczos coefficients ({b_{n}}) computed for determining the Krylov complexity can reveal the universal behaviors of autocorrelations, which are otherwise impossible. When the time axis is scaled by (b_{1}), different autocorrelation functions obey a universal function form at short time. We further propose a characteristic parameter deduced from ({b_{n}}) that can largely determine the behavior of autocorrelations at the intermediate time. This parameter can also largely determine whether the autocorrelation function oscillates or monotonically decays in time. We present numerical evidences and physical intuitions to support these universal hypotheses of autocorrelations. We emphasize that these universal behaviors are held across different operators and different physical systems.

Abstract

The superconducting tunneling effect in heterostructures, describing the process where single electrons or Cooper pairs tunnel through the barrier, can always play a significant role in understanding the phase coherence and pairing mechanisms in superconductors. Taking advantage of the easy cleavage to atomically-thin monolayer structure of layered superconductors and resulting quantum confinement of electrons or Cooper pairs at two-dimensional limit, van der Waals superconducting materials hosting superconducting order in monolayers or heterostructures can exhibit extensive emergent phenomena associated with quantum phase transitions of vortex and anti-vortex pairs. Examples of superconducting tunnel junctions (STJs) based on layered superconductors have been demonstrated to achieve novel phenomena, including Andreev bound states, Majorana bound states and 0/π-phase junctions. Since the characteristic parameters of quasiparticle tunneling through the barrier are directly associated with the energy gap values of superconductors, such critical parameter can be obtained within the STJ device geometry, which helps us understand and control the pairing states and emerging phenomena in superconductors. In this review, from the perspective of STJs with single electron tunneling and Cooper pair tunneling, we discuss Andreev reflection, Majorana bound states, photon-induced tunneling effects, non-reciprocal transport and superconducting diode phenomena, as well as prospects for layered-superconductor-based STJs.