We propose the Bose-Einstein condensation (BEC) and superfluidity of quasi-two-dimensional (2D) spatially indirect magnetobiexcitons in a slab of superlattice with alternating electron and hole layers consisting from the semiconducting quantum wells (QWs) and graphene superlattice in high magnetic field. The two different Hamiltonians of a dilute gas of magnetoexcitons with a dipole-dipole repulsion in superlattices consisting of both QWs and graphene layers in the limit of high magnetic field have been reduced to one effective Hamiltonian a dilute gas of two-dimensional excitons without magnetic field. Moreover, for $N$ excitons we have reduced the problem of $2N\times 2$ dimensional space onto the problem of $N\times 2$ dimensional space by integrating over the coordinates of the relative motion of an electron (e) and a hole (h). The instability of the ground state of the system of interacting two-dimensional indirect magnetoexcitons in a slab of superlattice with alternating electron and hole layers in high magnetic field is established. The stable system of indirect quasi-two-dimensional magnetobiexcitons, consisting of pair of indirect excitons with opposite dipole moments is considered. The density of superfluid component $n_{s}(T)$ and the temperature of the Kosterlitz-Thouless phase transition to the superfluid state in the system of two-dimensional indirect magnetobiexcitons, interacting as electrical quadrupoles, are obtained for both QW and graphene realizations.
Collective plasma excitations in moiré flat bands display unique properties reflecting strong electron-electron interactions and unusual carrier dynamics in these systems. Unlike the conventional two-dimensional plasmon modes, dispersing as $\sqrt{k}$ at low frequencies and plunging into particle-hole continuum at higher frequencies, the moiré plasmons pierce through the flat-band continuum and acquire a strong over-the-band character. Due to the complex structure of the moiré superlattice unit cell, the over-the-band plasmons feature several distinct branches connected through zone folding in…
When quantized, traces of classically chaotic single particle systems include eigenvalue statistics and scars in eigenfuntions. Since 2001, many theoretical and experimental works have argued that classically chaotic single electron dynamics influences and controls collective electron transport. For transport in semiconductor superlattices under tilted magnetic and electric fields, these theories rely on a reduction to a one-dimensional self-consistent drift model. A two-dimensional theory based on self-consistent Boltzmann transport does not support that single electron chaos influences colle…
Room Temperature Collective Blinking and Photon Bunching from CsPbBr3 Quantum Dot Superlattice
Institute of Science Tokyo（东京科学大学）；Katholieke Universiteit Leuven（荷语鲁汶大学）；Lingnan Normal University（岭南师范学院）；Max Planck Institute for Polymer Research（马克斯·普朗克聚合物研究所）
Plasmon collective excitations are studied in a planar graphene superlattice formed by periodically alternating regions of gapless graphene and of its gapped modification. The plasmon dispersion law is determined both for the quasi-one-dimensional case (the Fermi level is located within the minigap) and for the quasi-two-dimensional case (the Fermi level is located within the miniband). The problem concerning the absorption of modulated electromagnetic radiation at the excitation of plasmons is also considered.
Nanyang Technological University（南洋理工大学）；Sun Yat-Sen University (Zhuhai Campus)（中山大学（珠海校区））；Nanjing University of Aeronautics and Astronautics（南京航空航天大学）；École Polytechnique Fédérale de Lausanne (EPFL)（洛桑联邦理工学院）；National Institute for Materials Science（物质材料研究机构）；University of Science and Technology of China（中国科学技术大学）；Sun Yat-Sen University (Guangzhou Campus)（中山大学（广州校区））
Novel SuperLattice Plasmon Mode in a Grating of 2D Electron Strips
Institute of Solid State Physics, RAS（俄罗斯科学院固体物理研究所）；Kotelnikov Institute of Radio-engineering and Electronics of the RAS（俄罗斯科学院科捷利尼科夫无线电电子学研究所）；Institute of Solid State Physics, Vienna University of Technology（维也纳工业大学固体物理研究所）
We investigate a hybrid quantum system where an ensemble of nitrogen-vacancy (NV) centers in diamond is interfaced with a piezomagnetic superlattice that supports surface phonon polaritons (SPhPs). We show that the strong magnetic coupling between the collective spin waves in the NV spin ensemble and the quantized SPhPs can be realized, thanks to the subwavelength nature of the SPhPs and relatively long spin coherence times. The magnon-polariton coupling allows different modes of the SPhPs to be mapped and orthogonally stored in different spatial modes of excitation in the solid medium. Becaus…
Starting from a continuum-model description, we develop a microscopic weak-coupling theory for superconductivity in twisted double-bilayer graphene. We study both electron-phonon and entirely electronic pairing mechanisms. In each case, the leading superconducting instability transforms under the trivial representation, $A$, of the point group $C_3$ of the system, while the subleading pairing phases belong to the $E$ channel. We explicitly compute the momentum dependence of the associated order parameters and find that the leading state has no nodal points for electron-phonon pairing but exhib…
The electronic properties of twisted bilayer graphene (TBG) can be dramatically different from those of a single graphene layer, in particular when the two layers are rotated relative to each other by a small angle. TBG has recently attracted a great deal of interest, sparked by the discovery of correlated insulating and superconducting states, for twist angle $θ$ close to a so-called 'magic angle' $\approx 1.1°$. In this work, we unveil, via near-field optical microscopy, a collective plasmon mode in charge-neutral TBG near the magic angle, which is dramatically different from the ordinary si…
Propagating Collective Spin-valley Modes in Twisted WSe2
University of California at Santa Barbara（加州大学圣塔芭芭拉分校）；University of California at Berkeley（加州大学伯克利分校）；Harvard University（哈佛大学）；Massachusetts Institute of Technology（麻省理工学院）；Arizona State University（亚利桑那州立大学）；National Institute for Materials Science（国立材料研究所）；Okinawa Institute of Science and Technology Graduate University（冲绳科学技术大学院大学）
Electron Delocalization versus Emission Coherence of Quantum Dot Superlattices
National Gravitation Laboratory, MOE Key Laboratory of Fundamental Physical Quantities Measurement, and School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院，引力教育部重点实验室，国家引力实验室）；Wuhan National Laboratory for Optoelectronics and School of Physics, Huazhong University of Science and Technology（华中科技大学物理学院，武汉光电国家研究中心）；Yunnan Key Laboratory of Ⅲ-Ⅴ Infrared Detectors, Kunming Institute of Physics（昆明物理研究所，云南省Ⅲ-Ⅴ族红外探测器重点实验室）
Collective electron transport causes a weakly coupled semiconductor superlattice under dc voltage bias to be an excitable system with $2N+2$ degrees of freedom: electron densities and fields at $N$ superlattice periods plus the total current and the field at the injector. External noise of sufficient amplitude induces regular current self-oscillations (coherence resonance) in states that are stationary in the absence of noise. Numerical simulations show that these oscillations are due to the repeated nucleation and motion of charge dipole waves that form at the emitter when the current falls b…
Moiré Fractional Chern Insulators III: Hartree-Fock Phase Diagram, Magic Angle Regime for Chern Insulator States, the Role of the Moiré Potential and Goldstone Gaps in Rhombohedral Graphene Superlattices
The combined effect of a lateral square superlattice potential and the Coulomb interaction on the ground state of a two-dimensional electron gas in a perpendicular magnetic field is studied for different rational values of $Γ$, the inverse of the number of flux quanta per unit cell of the external potential, at filling factor $ν=1$ in Landau level $N=0.$ When Landau level mixing and disorder effects are neglected, increasing the strength $W_{0}$ of the potential induces a transition, at a critical strength $W_{0}^{\left( c\right) },$ from a uniform and fully spin polarized state to a two-dimen…
Collective motions of electrons in solids are often conveniently described as the movements of quasiparticles. Here we show that these quasiparticles can be hierarchical. Examples are valley electrons, which move in hyperorbits within a honeycomb lattice and forms a valley pseudospin, or the self-rotation of the wave-packet. We demonstrate that twist can induce higher level motions of valley electrons around the moire superlattice of bilayer systems. Such larger scale collective movement of the valley electron, can be regarded as the self-rotation (spin) of a higher-level quasiparticle, or wha…
Emergent phenomena arising from the collective behavior of electrons is generally expected when Coulomb interactions dominate over the kinetic energy, as in delocalized quasiparticles in highly degenerate flat bands. Bernal-stacked bilayer graphene intrinsically supports a pair of flat bands predicted to host a variety of spontaneous broken-symmetry states arising from van Hove singularities and a four-fold spin-valley (isospin) degeneracy. Here, we show that ultra-clean samples of bilayer graphene display a cascade of symmetry-broken states with spontaneous and spin and valley ordering at zer…
We present theoretical calculations for the cyclotron resonance and various magnetoplasmon modes of a Coulomb interacting two-dimensional GaAs electron gas (2DEG) modulated as a lateral superlattice of quantum dots subjected to an external perpendicular constant magnetic field. We use a real-time excitation approach based on the Liouville-von Neumann equation for the density operator, that can go beyond linear response delivering information of all longitudinal and transverse collective modes of interest to the same order. We perform an extensive analysis of the coexisting collective modes due…
In condensed matter systems, higher temperatures typically disfavors ordered phases leading to an upper critical temperature for magnetism, superconductivity, and other phenomena. A notable exception is the Pomeranchuk effect in 3He, in which the liquid ground state freezes upon increasing the temperature due to the large entropy of the paramagnetic solid phase. Here we show that a similar mechanism describes the finite temperature dynamics of spin and valley-isospins in magic-angle twisted bilayer graphene. Most strikingly a resistivity peak appears at high temperatures near superlattice fill…
The collective dynamics of topological structures have been of great interest from both fundamental and applied perspectives. For example, the studies of dynamical properties of magnetic vortices and skyrmions not only deepened the understanding of many-body physics but also led to potential applications in data processing and storage. Topological structures constructed from electrical polarization rather than spin have recently been realized in ferroelectric superlattices, promising for ultrafast electric-field control of topological orders. However, little is known about the dynamics of such…
A two-dimensional electron gas in a static external magnetic field exhibits two distinct collective excitation modes. The lower frequency mode propagates along the periphery of the domain almost freely with an extended lifetime, which is referred to as edge magnetoplasmons. Peculiar phenomena caused by a capacitive interaction between nearest neighbor domains are known, such as the emergence of Tomonaga-Luttinger liquid and charge density fractionalization. Meanwhile, the number of coupled domains investigated in the past has been limited to a small number. Here, we performed calculations usin…
Realization and manipulation of spiral charge density waves in a two-dimensional metal
Beijing Institute of Technology（北京理工大学）；Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences（中国科学院上海微系统与信息技术研究所）；Beihang University（北京航空航天大学）
Two-dimensional (2D) heterostructures (HS) formed by transition-metal dichalcogenide (TMDC) monolayers offer a unique platform for the study of intralayer and interlayer excitons as well as moiré-pattern-induced features. Particularly, the dipolar charge-transfer exciton comprising an electron and a hole, which are confined to separate layers of 2D semiconductors and Coulomb-bound across the heterojunction interface, has drawn considerable attention in the research community. On the one hand, it bears significance for optoelectronic devices, e.g. in terms of charge carrier extraction from phot…
Signatures of broken symmetries in the excitations of a periodic 2DEG coupled to a cylindrical photon cavity
University of Iceland（冰岛大学）；Reykjavik University（雷克雅未克大学）；Yerevan State University（埃里温国立大学）；National Taiwan University（台湾大学）；National Center for Theoretical Sciences（国家理论科学中心）；University of Sulaimani（苏莱曼尼大学）；National United University（国立联合大学）
Surfaces are at the frontier of every known solid. They provide versatile supports for functional nanostructures and mediate essential physicochemical processes. Being intimately related with 2D materials, interfaces and atomically thin films often feature distinct electronic states with respect to the bulk, which are key for many relevant properties, such as catalytic activity, interfacial charge-transfer, or crystal growth mechanisms. Of particular interest is reducing the surface electrons' dimensionality and spread with atomic precision, to induce novel quantum properties via lateral scatt…
Structural engineering of van der Waals heterostructures via stacking and twisting has recently been used to create moiré superlattices, enabling the realization of new optical and electronic properties in solid-state systems. In particular, moiré lattices in twisted bilayers of transition metal dichalcogenides (TMDs) have been shown to lead to exciton trapping, host Mott insulating and superconducting states, and act as unique Hubbard systems whose correlated electronic states can be detected and manipulated optically. Structurally, these twisted heterostructures also feature atomic reconstru…
