Exotic Quantum States Research Articles

Strain-Driven Higher-Order Topological Dirac Semimetal in Noncentrosymmetric γ-GeSe.

Strain-engineered topological phases in noncentrosymmetric materials offer fertile ground for realizing exotic quantum states, yet their experimental realization remains elusive. Here, using first-principles calculations, we demonstrate that the van der Waals layered material γ-GeSe undergoes a sequence of strain-induced topological phase transitions, including the emergence of a higher-order topological Dirac semimetal phase. Under in-plane biaxial tensile strain, we uncover a sequential evolution of topological phases, including topological nodal-line semimetals, Dirac semimetals, and a higher-order topological Dirac semimetal phase. Notably, the noncentrosymmetric higher-order topological Dirac semimetal phase is characterized by Dirac points coexisting with higher-order topological insulating phases on the kz = 0 plane, enabled by quantization of the mirror-resolved Zak phase. These findings position γ-GeSe as an experimentally viable platform for investigating strain-engineered topological phenomena unique to noncentrosymmetric systems.

Filling-Dependent Intertwined Electronic and Atomic Orders in the Flat-Band State of 1T-TaS2.

The delicate interplay among the complex intra/inter-layer electron-electron and electron-lattice interactions is the fundamental prerequisite of these exotic quantum states, such as superconductivity, nematic order, and checkerboard charge order. Here, we explore the filling-dependent multiple stable intertwined electronic and atomic orders of the flat-band state of 1T-TaS2 encompassing hole order, phase orders, coexisting left- and right-chiral orders, and mixed phase/chiral orders via scanning tunneling microscopy (STM). Combining first-principles calculations, the emergent electronic/atomic orders can be attributed to the weakening of electron-electron correlations and stacking-dependent interlayer interactions. Moreover, achiral intermediate ring-like clusters and nematic charge density wave (CDW) states are successfully realized in intralayer chiral domain wall and interlayer heterochiral stacking regions through chiral overlap configurations. Our study not only deepens the understanding of filling-dependent electronic/atomic orders in flat-band systems but also offers perspectives for exploring exotic quantum states in correlated electronic systems.

High-throughput discovery of kagome materials in transition metal oxide monolayers

Abstract Kagome materials are known for hosting exotic quantum states, including quantum spin liquids, charge density waves, and unconventional superconductivity. The search for kagome monolayers is driven by their ability to exhibit neat and well-defined kagome bands near the Fermi level, which are more easily realized in the absence of interlayer interactions. However, this absence also destabilizes the monolayer forms of many bulk kagome materials, posing significant challenges to their discovery. In this work, we propose a strategy to address this challenge by utilizing oxygen vacancies in transition metal oxides within a "1+3" design framework. Through high-throughput computational screening of 349 candidate materials, we identified 12 thermodynamically stable kagome monolayers with diverse electronic and magnetic properties. These materials were classified into three categories based on their lattice geometry, symmetry, band gaps, and magnetic configurations. Detailed analysis of three representative monolayers revealed kagome band features near their Fermi levels, with orbital contributions varying between oxygen 2p and transition metal d states. This study demonstrates the feasibility of the "1+3" strategy, offering a promising approach to uncovering low-dimensional kagome materials and advancing the exploration of their quantum phenomena.

Direct probing of energy gaps and bandwidth in gate-tunable flat band graphene systems

Moiré systems featuring flat electronic bands exhibit a vast landscape of emergent exotic quantum states, making them one of the resourceful platforms in condensed matter physics. Tuning these systems via twist angle and the electric field greatly enhances our comprehension of their strongly correlated ground states. Here, we report a technique to investigate the nuanced intricacies of band structures in dual-gated multilayer graphene systems. We utilize the Landau levels of a decoupled monolayer graphene to extract the electric field-dependent bilayer graphene charge neutrality point gap. Then, we extend this method to analyze the evolution of the band gap and the flat bandwidth in twisted mono-bilayer graphene. The band gap maximizes at the same displacement field where the flat bandwidth minimizes, concomitant with the emergence of a strongly correlated phase. Moreover, we extract integer and fractional quantum Hall gaps to further demonstrate the strength of this method. Our technique paves the way for improving the understanding of electronic band structures in versatile flat band systems.

Recent progress on quantum Hall effect in unconventional material systems

The quantum Hall effect (QHE) is an elegant macroscopic manifestation of quantum mechanical behavior on the microscopic scale, and its discovery is a major triumph in condensed matter physics. While QHE has been predominantly observed in two-dimensional electron gas (2DEG) systems, recently, many efforts have been devoted to searching for the QHE in unconventional materials platforms beyond the classical framework to extend the horizon of the QHE. In this Perspective, we highlight recent important experimental discoveries and progress on the QHE material platforms beyond 2DEG platforms, such as three-dimensional QHE, Weyl-orbit-based QHE, and QHE in two-dimensional insulators. In addition, novel phenomena arising from incorporating QHE with other exotic quantum states, such as topological band structures and superconductivity, will be discussed. We also present the emerging field-free version of QHE–quantum anomalous Hall effect on its transport characteristics, working principles as well as potential applications in quantum metrology and quantum computing. With the exploration of these unconventional QHE hosts and the development of the understanding of new physics arising from the interplay between QHE and other physical systems, QHE will continue to play a critical role in both advancing fundamental physics and developing next-generation quantum technologies.

Nonzero Berry curvature dipole, magnetic gapped edge states and persistent spin texture in a rotational symmetry preserved van der Waals magnetic topological insulator.

Berry curvature dipole and persistent spin textures are unconventional quantum marvels with paramount relevance towards nonlinear transport phenomena and futuristic spintronic applications. Here, we demonstrate that a magnetic van der Waals (vdW) heterobilayer, comprised of semiconducting monolayers 1H-MoSe2 (nonmagnetic) and ferromagnet 1H-VSe2, exhibits a finite Berry curvature dipole (DBcd) in the absence of strain or twist perturbation, where each constituent layer displays no DBcd owing to three-fold (C3v) rotational symmetry. A change in sign of DBcd is observed over an energy range, suggesting its oscillatory nature. Further, persistent spin texture (PST) with small spin canting and a prominent magnetic gap of 190 meV have emerged in the edge state spectrum of the vdW system. We find the emerged topological properties are crystal facet dependent. Moreover, an exotic quantum state of intrinsic spin-valley locking at two inequivalent K valleys with distinct spin identity is observed in the dispersion relation. We explore the subtle role of spin-valley locking in the generation of a finite DBcd, which has not been previously discussed in this context. Such quantum states promote right and left handed circular polarization and are perceived as a binary catalogue for information encoding and energy. Further, the role of symmetry breaking in the observed phenomena of the AB-stacked vdW heterobilayer is discussed. We propose that both time reversal symmetry breaking and spin-valley locking are essential for inducing nonzero DBcd in an AB stacked vdW heterostructure, where the three-fold (C3) rotational symmetry is preserved along the z-axis only.

Chiral spin-liquid-like state in pyrochlore iridate thin films

The pyrochlore iridates have become ideal platforms to unravel fascinating correlated and topological phenomena that stem from the intricate interplay among strong spin-orbit coupling, electronic correlations, lattice with geometric frustration, and itinerancy of the 5d electrons. The all-in-all-out antiferromagnetic state, commonly considered as the magnetic ground state, can be dramatically altered in reduced dimensionality, leading to exotic or hidden quantum states inaccessible in bulk. Here, by means of magnetotransport, resonant elastic and inelastic x-ray scattering experiments, we discover an emergent quantum disordered state in (111) Y2Ir2O7 thin films (thickness ≤30 nm) persisting down to 5 K, characterized by dispersionless magnetic excitations. The anomalous Hall effect observed below an onset temperature near 125 K corroborates the presence of chiral short-range spin configurations expressed in non-zero scalar spin chirality, breaking the macroscopic time-reversal symmetry. The origin of this chiral state is ascribed to the restoration of magnetic frustration on the pyrochlore lattice in lower dimensionality, where the competing exchange interactions together with enhanced quantum fluctuations suppress any long-range order and trigger spin-liquid-like behavior with degenerate ground-state manifold.

Emerging Chirality and Moiré Dynamics in Twisted Layered Material Heterostructures.

Moiré superstructures arising at twisted 2D interfaces have recently attracted the attention of the scientific community due to exotic quantum states and unique mechanical and tribological behaviors that they exhibit. Here, we predict the emergence of chiral distortions in twisted layered interfaces of finite dimensions. This phenomenon originates in intricate interplay between interfacial interactions and contact boundary constraints. A metric termed the fractional chiral area is introduced to quantify the overall chirality of the moiré superstructure and to characterize its spatial distribution. Despite the equilibrium nature of the discovered energetic and structural chirality effects, they are shown to be manifested in the twisting dynamics of layered interfaces, which demonstrates a continuous transition from stick-slip to smooth rotation with no external trigger.

On the feasibility of detecting quantum delocalization effects on relativistic time dilation in optical clocks

Abstract We derive the predicted time dilation of delocalized atomic clocks in an optical lattice setup in the presence of a gravitational field to leading order in quantum relativistic corrections. We investigate exotic quantum states of motion whose relativistic time dilation is outside of the realm of classical general relativity, finding a regime where 24 Mg optical lattice clocks currently in development would comfortably be able to detect the special-relativistic contribution to the quantum effect (if the technical challenge of generating the necessary states can be met and the expected accuracy of such clocks can be attained). We find that the gravitational contribution, on the other hand, is negligible in this setup. We provide a detailed experimental protocol and analyse the effects of noise on our predictions. We also show that the magnitude of our predicted quantum time dilation effect remains just out of detectable reach for the current generation of 87 Sr optical lattice clocks. Our calculations agree with the predicted time dilation of classical general relativity when restricting to Gaussian states.

Nematic Ising superconductivity with hidden magnetism in few-layer 6R-TaS2

In van der Waals heterostructures (vdWHs), the manipulation of interlayer stacking/coupling allows for the construction of customizable quantum systems exhibiting exotic physics. An illustrative example is the diverse range of states of matter achieved through varying the proximity coupling between two-dimensional (2D) quantum spin liquid (QSL) and superconductors within the TaS2 family. This study presents a demonstration of the intertwined physics of spontaneous rotational symmetry breaking, hidden magnetism, and Ising superconductivity (SC) in the three-fold rotationally symmetric, non-magnetic natural vdWHs 6R-TaS2. A distinctive phase emerges in 6R-TaS2 below a characteristic temperature (T*) of approximately 30 K, which is characterized by a remarkable set of features, including a giant extrinsic anomalous Hall effect (AHE), Kondo screening, magnetic field-tunable thermal hysteresis, and nematic magneto-resistance. At lower temperatures, a coexistence of nematicity and Kondo screening with Ising superconductivity is observed, providing compelling evidence of hidden magnetism within a superconductor. This research not only sheds light on unexpected emergent physics resulting from the coupling of itinerant electrons and localized/correlated electrons in natural vdWHs but also emphasizes the potential for tailoring exotic quantum states through the manipulation of interlayer interactions.

Superconductivity and topological quantum states in two-dimensional moiré superlattices

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.

Real-Space Imaging of Intrinsic Symmetry-Breaking Spin Textures in a Kagome Lattice.

The electronic orders in kagome materials have emerged as a fertile platform for studying exotic quantum states, and their intertwining with the unique kagome lattice geometry remains elusive. While various unconventional charge orders with broken symmetry is observed, the influence of kagome symmetry on magnetic order has so far not been directly observed. Here, using a high-resolution magnetic force microscopy, it is, for the first time, observed a new lattice form of noncollinear spin textures in the kagome ferromagnet in zero magnetic field. Under the influence of the sixfold rotational symmetry of the kagome lattice, the spin textures are hexagonal in shape and can further form a honeycomb lattice structure. Subsequent thermal cycling measurements reveal that these spin textures transform into a non-uniform in-plane ferromagnetic ground state at low temperatures and can fully rebuild at elevated temperatures, showing a strong second-order phase transition feature. Moreover, some out-of-plane magnetic moments persist at low temperatures, supporting the Kane-Mele scenario in explaining the emergence of the Dirac gap. The observations establish that the electronic properties, including both charge and spin orders, are strongly coupled with the kagome lattices.

Robust Edge States of Quasi-1D Material Ta2NiSe7 and Applications in Saturable Absorbers.

The helical edge states (ESs) protected by underlying Z2 topology in two-dimensional topological insulators (TIs) arouse upsurges in saturable absorptions thanks to the strong photon-electron coupling in ESs. However, limited TIs demonstrate clear signatures of topological ESs at liquid nitrogen temperatures, hindering the applications of such exotic quantum states. Here, we demonstrate the existence of one-dimensional (1D) ESs at the step edge of the quasi-1D material Ta2NiSe7 at 78 K by scanning tunneling microscopy. Such ESs are rather robust against the irregularity of the edges, suggesting a possible topological origin. The exfoliated Ta2NiSe7 flakes were used as saturable absorbers (SAs) in an Er-doped fiber laser, hosting a mode-locked pulse with a modulation depth of up to 52.6% and a short pulse duration of 225 fs, far outstripping existing TI-based SAs. This work demonstrates the existence of robust 1D ESs and the superior SA performance of Ta2NiSe7.

The Growth of V5S8 Single Crystals by Chemical Vapour Transport

Single crystals of the d-electron antiferromagnetic metal V5S8 can be prepared by chemical vapour transport with gaseous iodine as a transport agent. We present the outcomes of an endeavour to synthesise high-purity single crystals of V5S8 with reduced crystalline disorder, important to the formation of novel quantum orders. We report results on the residual resistivity ratio of the single crystals as growth parameters are varied including growth temperature, temperature gradient and pre-growth processing of the initial apparatus and reagents. We demonstrate that single crystals of at least a few mm in size can be successfully grown at relatively low temperatures in the range 550–600 °C. The optimisation of this method may imply a better crystallographic organisation, reducing sulphur vacancies and increasing vanadium positional order. The resulting longer electron mean free paths may enhance the probability of finding exotic quantum states of matter at low temperatures. The results presented here may also be of relevance to the development of vanadium sulphide-based energy storage and spintronic devices.

Discovery of a long-ranged charge order with 1/4 Ge1-dimerization in an antiferromagnetic Kagome metal

Exotic quantum states arise from the interplay of various degrees of freedom such as charge, spin, orbital, and lattice. Recently, a short-ranged charge order (CO) was discovered deep inside the antiferromagnetic phase of Kagome magnet FeGe, exhibiting close relationships with magnetism. Despite extensive investigations, the CO mechanism remains controversial, mainly because the short-ranged behavior hinders precise identification of CO superstructure. Here, combining multiple experimental techniques, we report the observation of a long-ranged CO in high-quality FeGe samples, which is accompanied with a first-order structural transition. With these high-quality samples, the distorted 2 × 2 × 2 CO superstructure is characterized by a strong dimerization along the c-axis of 1/4 of Ge1-sites in Fe3Ge layers, and in response to that, the 2 × 2 in-plane charge modulations are induced. Moreover, we show that the previously reported short-ranged CO might be related to large occupational disorders at Ge1-site, which upsets the equilibrium of the CO state and the ideal 1 × 1 × 1 structure with very close energies, inducing nanoscale coexistence of these two phases. Our study provides important clues for further understanding the CO properties in FeGe and helps to identify the CO mechanism.

Layer-Dependent Electromechanical Response in Twisted Graphene Moiré Superlattices.

The coupling of mechanical deformation and electrical stimuli at the nanoscale has been the subject of intense investigation in the realm of materials science. Recently, twisted van der Waals (vdW) materials have emerged as a platform for exploring exotic quantum states. These states are intimately tied to the formation of moiré superlattices, which can be visualized by directly exploiting the electromechanical response. However, the origin of the response, even in twisted bilayer graphene (tBLG), remains unsettled. Here, employing lateral piezoresponse force microscopy (LPFM), we investigate the electromechanical responses of marginally twisted graphene moiré superlattices with different layer thicknesses. We observe distinct LPFM amplitudes and spatial profiles in tBLG and twisted monolayer-bilayer graphene (tMBG), exhibiting effective in-plane piezoelectric coefficients of 0.05 and 0.35 pm/V, respectively. Force tuning experiments further underscored a marked divergence in their responses. The contrasting behaviors suggest different electromechanical couplings in tBLG and tMBG. In tBLG, the response near the domain walls is attributed to the flexoelectric effect, while in tMBG, the behaviors can be comprehended within the context of the piezoelectric effect. Our results not only provide insights into electromechanical and corporative effects in twisted vdW materials with different stacking symmetries but may also offer a way to engineer them at the nanoscale.

Three-dimensional hidden phase probed by in-plane magnetotransport in kagome metal CsV3Sb5 thin flakes

Transition metal compounds with kagome structure have been found to exhibit a variety of exotic structural, electronic, and magnetic orders. These orders are competing with energies very close to each other, resulting in complex phase transitions. Some of the phases are easily observable, such as the charge density wave (CDW) and the superconducting phase, while others are more challenging to identify and characterize. Here we present magneto-transport evidence of a new phase below ~ 35 K in the kagome topological metal CsV3Sb5 (CVS) thin flakes between the CDW and the superconducting transition temperatures. This phase is characterized by six-fold rotational symmetry in the in-plane magnetoresistance (MR) and is connected to the orbital current order in CVS. Furthermore, the phase is characterized by a large in-plane negative magnetoresistance, which suggests the existence of a three-dimensional, magnetic field-tunable orbital current ordered phase. Our results highlight the potential of magneto-transport to reveal the interactions between exotic quantum states of matter and to uncover the symmetry of such hidden phases.

Biorthogonal Majorana zero modes, ELC waves and soliton-fermion duality in non-Hermitian sl(2) affine Toda coupled to fermions

We study a non-Hermitian (NH) sl(2) affine Toda model coupled to fermions through soliton theory techniques and the realizations of the pseudo-chiral and pseudo- Hermitian symmetries. The interplay of non-Hermiticity, integrability, nonlinearity, and topology significantly influence the formation and behavior of a continuum of bound state modes (CBM) and extended waves in the localized continuum (ELC). The non-Hermitian soliton-fermion duality, the complex scalar field topological charges and winding numbers in the spectral topology are uncovered. The biorthogonal Majorana zero modes, dual to the NH Toda solitons with topological charges 2πargz=±i=±1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\frac{2}{\\pi}\\arg \\left(z=\\pm i\\right)=\\pm 1 $$\\end{document}, appear at the complex-energy point gap and are pinned at zero energy. The zero eigenvalue λ(z = ± i) = 0, besides being a zero mode, plays the role of exceptional points (EPs), and each EP separates a real eigenvalue A\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{A} $$\\end{document}-symmetric and A\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{A} $$\\end{document}-symmetry broken regimes for an antilinear symmetry A∈PTγ5PT\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{A}\\in \\left\\{\\mathcal{PT},{\\gamma}_5\\mathcal{PT}\\right\\} $$\\end{document}. Our findings improve the understanding of exotic quantum states, but also paves the way for future research in harnessing non-Hermitian phenomena for topological quantum computation, as well as the exploration of integrability and NH solitons in the theory of topological phases of matter.

Emergent Spin-Gapped Magnetization Plateaus in a Spin-1/2 Perfect Kagome Antiferromagnet.

The two-dimensional spin-1/2 kagome Heisenberg antiferromagnet is believed to host quantum spin liquid (QSL) states with no magnetic order, but its ground state remains largely elusive. An important outstanding question concerns the presence or absence of the 1/9 magnetization plateau, where exotic quantum states, including topological ones, are expected to emerge. Here we report the magnetization of a recently discovered kagome QSL candidate YCu_{3}(OH)_{6.5}Br_{2.5} up to 57T. Above 50T, a clear magnetization plateau at 1/3 of the saturation moment of Cu^{2+} ions is observed, supporting that this material provides an ideal platform for the kagome Heisenberg antiferromagnet. Remarkably, we found another magnetization plateau around 20T, which is attributed to the 1/9 plateau. The temperature dependence of this plateau reveals the presence of the spin gap. The observation of 1/9 and 1/3 plateaus highlights the emergence of novel states in quantum spin systems.

Quantum Slush State in Rydberg Atom Arrays.

In this Letter, we propose an exotic quantum state that does not order at zero temperature in a Rydberg atom array with antiblockade mechanism. By performing an unbiased large-scale quantum MonteCarlo simulation, we investigate a minimal model with facilitated excitation in a disorder-free system. At zero temperature, this model exhibits a heterogeneous structure of liquid and glass mixture. This state, dubbed quantum slush state, features a quasi-long-range order with an algebraic decay for its correlation function, and is different from most well-established quantum phases of matter.