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Correlations and electronic order in a two-orbital honeycomb lattice model for twisted bilayer graphene
摘要： The recent observation of superconductivity in proximity to an insulating phase in twisted bilayer graphene (TBG) at small “magic” twist angles has been linked to the existence of nearly ?at bands, which make TBG a fresh playground to investigate the interplay between correlations and superconductivity. The low-energy narrow bands were shown to be well described by an effective tight-binding model on the honeycomb lattice (the dual of the triangular Moiré superlattice) with a local orbital degree of freedom. In this paper, we perform a strong-coupling analysis of the proposed (px, py ) two-orbital extended Hubbard model on the honeycomb lattice. By decomposing the interacting terms in the particle-particle and particle-hole channels, we classify the different possible superconducting, magnetic, and charge instabilities of the system. In the pairing case, we pay particular attention to the two-component (d-wave) pairing channels, which admit vestigial phases with nematic or chiral orders, and study their phenomenology. Furthermore, we explore the strong-coupling regime by obtaining a simpli?ed spin-orbital exchange model which may describe a putative Mott-type insulating state at quarter-?lling. Our mean-?eld solution reveals a rich intertwinement between ferromagnetic and antiferromagnetic orders with different types of nematic and magnetic orbital orders. Overall, our work provides a solid framework for further investigations of the phase diagram of the two-orbital extended Hubbard model in both strong- and weak-coupling regimes.
关键词： spin-orbital exchange model,Hubbard model,twisted bilayer graphene,superconductivity,honeycomb lattice,correlations
更新于2025-09-09 09:28:46
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Response in the violation of the Bell inequality to imperfect photon addition and subtraction in noisy squeezed states of light
摘要： Violation of Bell inequality is a prominent detection method for quantum correlations present in composite quantum systems, both in finite and infinite dimensions. We investigate the consequence of the violation of local realism based on pseudospin operators when photons are added or subtracted in a single mode or in both the modes of the two-mode squeezed states of light in the presence of noise. In the noiseless situation, we show that for addition (subtraction) of photons in a single mode, there is an overall enhancement in the maximal violation, although we observe an interplay between monotonicity and nonmonotonicity in the violation of Bell inequality depending on the squeezing strength. Moreover, we report that for low squeezing or low number of photons added or subtracted, subtraction in both the modes can lead to higher violation of local realism than that in the case of addition. For any choice of parameters, such ordering is not seen if one compares their entanglement contents. In the event of a faulty twin-beam generator, we obtain a lower than expected squeezing in the state. In such a case, or in imperfect photon addition (subtraction) or under local noise, we find that the violation of local realism by the noise-affected two-mode squeezed states always decreases. Interestingly, however, we notice that photon addition (subtraction) can in general help to conquer the ill effects of noise by enhancing the violation of local realism or by transforming nonviolating states to violating ones, thereby acting as an activating agent.
关键词： two-mode squeezed states,photon subtraction,local realism,photon addition,quantum correlations,Bell inequality,noise
更新于2025-09-09 09:28:46
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Self-testing using only marginal information
摘要： The partial states of a multipartite quantum state may carry a lot of information: in some cases, they determine the global state uniquely. This result is known for tomographic information, that is for fully characterized measurements. We extend it to the device-independent framework by exhibiting sets of two-party correlations that self-test pure three-qubit states.
关键词： multipartite,correlations,quantum state,device-independent,self-testing
更新于2025-09-09 09:28:46
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Entanglement as the Symmetric Portion of Correlated Coherence
摘要： We show that the symmetric portion of correlated coherence is always a valid quantifier of entanglement, and that this property is independent of the particular choice of coherence measure. This leads to an infinitely large class of coherence based entanglement monotones, which is always computable for pure states if the coherence measure is also computable. It is already known that every entanglement measure can be constructed as a coherence measure. The results presented here show that the converse is also true. The constructions that are presented can also be extended to include more general notions of nonclassical correlations, leading to quantifiers that are related to quantum discord, thus providing an avenue for unifying all such notions of quantum correlations under a single framework.
关键词： coherence,entanglement,quantum correlations,quantum discord
更新于2025-09-09 09:28:46
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Long-lived non-classical correlations towards quantum communication at room temperature
摘要： Heralded single-photon sources with on-demand readout are a key enabling technology for distributed photonic networks. Such sources have been demonstrated in both cryogenic solid-state and cold-atoms systems. Practical long-distance quantum communication may benefit from using technologically simple systems such as room-temperature atomic vapours. However, atomic motion has so far limited the single-excitation lifetime in such systems to the microsecond range. Here we demonstrate efficient heralding and readout of single collective excitations created in warm caesium vapour. Using the principle of motional averaging we achieve a collective excitation lifetime of 0.27 ± 0.04 ms, two orders of magnitude larger than previously achieved for single excitations in room-temperature sources. We experimentally verify non-classicality of the light-matter correlations by observing a violation of the Cauchy-Schwarz inequality with R = 1.4 ± 0.1 > 1. Through spectral and temporal analysis we investigate the readout noise that limits single-photon operation of the source.
关键词： quantum communication,room-temperature atomic vapours,motional averaging,non-classical correlations,single-photon sources
更新于2025-09-09 09:28:46
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Strong entanglement criterion involving momentum weak values
摘要： In recent years weak values have been used to explore interesting quantum features in novel ways. In particular, the real part of the weak value of the momentum operator has been widely studied, mainly in connection with Bohmian trajectories. Here we focus on the imaginary part and its role in relation with the entanglement of a bipartite system. We establish an entanglement criterion based on weak momentum correlations, that allows to discern whether the entanglement is encoded in the amplitude and/or in the phase of the wave function. Our results throw light on the physical role of the real and imaginary parts of the weak values, and stress the relevance of the latter in the multi-particle scenario.
关键词： Diffusive velocity and quantum potential,Quantum correlations with weak values,Momentum weak values,Entanglement criteria
更新于2025-09-09 09:28:46
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The Correlation between the Energy Gap and the Pseudogap Temperature in Cuprates: The YCBCZO and LSHCO Case
摘要： The paper analyzes the influence of the hole density, the out-of-plane or in-plane disorder, and the isotopic oxygen mass on the zero temperature energy gap (2Δ(0)) Y1???Ca??Ba2(Cu1???Zn??)3O7??? (YCBCZO) and La1.96???Sr??Ho0.04CuO4 (LSHCO) superconductors. It has been found that the energy gap is visibly correlated with the value of the pseudogap temperature (???). On the other hand, no correlation between 2Δ(0) and the critical temperature (????) has been found. The above results mean that the value of the dimensionless ratio 2Δ(0)/???????? can vary very strongly together with the chemical composition, while the parameter 2Δ(0)/??????? does not change significantly. In the paper, the analytical formula which binds the zero temperature energy gap and the pseudogap temperature has been also presented.
关键词： energy gap,pseudogap temperature,superconductors,cuprates,electron-phonon interaction,electron-electron correlations
更新于2025-09-09 09:28:46
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Internal strain tunes electronic correlations on the nanoscale
摘要： In conventional metals, charge carriers basically move freely. In correlated electron materials, however, the electrons may become localized because of strong Coulomb interactions, resulting in an insulating state. Despite considerable progress in the last decades, elucidating the driving mechanisms that suppress metallic charge transport, the spatial evolution of this phase transition remains poorly understood on a microscopic scale. Here, we use cryogenic scanning near-field optical microscopy to study the metal-to-insulator transition in an electronically driven charge-ordered system with a 20-nm spatial resolution. In contrast to common mean-field considerations, we observe pronounced phase segregation with a sharp boundary between metallic and insulating regions evidencing its first-order nature. Considerable strain in the crystal spatially modulates the effective electronic correlations within a few micrometers, leading to an extended “zebra” pattern of metallic and insulating stripes. We can directly monitor the spatial strain distribution via a gradual enhancement of the optical conductivity as the energy gap is depressed. Our observations shed new light on previous analyses of correlation-driven metal-insulator transitions.
关键词： nanoscale,electronic correlations,metal-insulator transition,condensed matter physics,strain
更新于2025-09-04 15:30:14
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Pressure-Induced Reversible Phase Transitions in a New Metastable Phase of Vanadium Dioxide
摘要： Exploring the structural and physical properties of new vanadium dioxide (VO2) allotropes have attracted considerable interest because of the structure diversity and unique physical properties of VO2. Here we demonstrate a reversible pressure-induced structural transition and metallization of the novel metastable polymorph VO2(Mx'), and a thermally-driven structural transition from VO2(Mx') to the monoclinic phase VO2(M1) at relative low temperature based on X-ray diffraction (XRD), Raman and infrared spectroscopy. It is shown that the metastable phase VO2(Mx') undergo the structural transitions of VO2(Mx')-(12 GPa)VO2(Mx'')-(30-80 GPa)VO2(X) upon compression, obviously different with the pressure-induced amorphization observed in other metastable phases VO2(A) and VO2(B). Moreover, the IR data demonstrated that the pressure-induced metallization (PIM) occurs in the VO2(Mx'') phase at about 40 Gpa, which is mainly associated with electron-electron correlations. Further analysis suggests that all the sample transforming into the same high pressure VO2(X) phase with the stable M1 phase could mainly results from the VO6 octahedra and empty spaces between VO6 octahedra in their intermediate high pressure phases VO2(Mx'') and VO2(M1') following similar variations under pressure. These findings present new insight into the differences of structural transitions and physical properties between the stable and metastable phases of transition metal oxides under pressure.
关键词： electron-electron correlations,vanadium dioxide,metallization,pressure-induced structural transition,metastable phase
更新于2025-09-04 15:30:14
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Perturbation approach for computing frequency- and time-resolved photon correlation functions
摘要： We propose an alternative formulation of the sensor method presented in Phys. Rev. Lett. 109, 183601 (2012) for the calculation of frequency-?ltered and time-resolved photon correlations. Our approach is based on an algebraic expansion of the joint steady state of quantum emitter and sensors with respect to the emitter-sensor coupling parameter (cid:2). This allows us to express photon correlations in terms of the open quantum dynamics of the emitting system only and ensures that computation of correlations are independent on the choice of a small value of (cid:2). Moreover, using time-dependent perturbation theory, we are able to express the frequency- and time-resolved second-order photon correlation as the addition of three components, each of which gives insight into the physical processes dominating the correlation at different time scales. We consider a bioinspired vibronic dimer model to illustrate the agreement between the original formulation and our approach.
关键词： vibronic dimer model,time-resolved photon correlations,quantum emitter,photon correlation functions,sensor method
更新于2025-09-04 15:30:14