Two-dimensional Quantum Materials Research Articles

Experimental scheme for determining the Berry phase in two-dimensional quantum materials with a flat band

Experimental scheme for determining the Berry phase in two-dimensional quantum materials with a flat band

Bias-Free Access to Orbital Angular Momentum in Two-Dimensional Quantum Materials.

The demonstration of a topological band inversion constitutes the most elementary proof of a quantum spin Hall insulator (QSHI). On a fundamental level, such an inverted band gap is intrinsically related to the bulk Berry curvature, a gauge-invariant fingerprint of the wave function's quantum geometric properties in Hilbert space. Intimately tied to orbital angular momentum (OAM), the Berry curvature can be, in principle, extracted from circular dichroism in angle-resolved photoemission spectroscopy (CD-ARPES), were it not for interfering final state photoelectron emission channels that obscure the initial state OAM signature. Here, we outline a full-experimental strategy to avoid such interference artifacts and isolate the clean OAM from the CD-ARPES response. Bench-marking this strategy for the recently discovered atomic monolayer system indenene, we demonstrate its distinct QSHI character and establish CD-ARPES as a scalable bulk probe to experimentally classify the topology of two-dimensional quantum materials with time reversal symmetry.

Properties and applications of two-dimensional quantum materials beyond carbon

Two-dimensional quantum materials are currently a hot research area in the field of materials. These unique materials allow electrons to move freely in only two dimensions and the other dimension is limited within the atom scale. The isolation of Graphene in 2004 showed the advantages of two-dimensional (2D) materials and led a rapid development in this field. Meanwhile, it stimulates the synthesis and research of the materials beyond carbon. 2D materials beyond carbon have different components and structures, showing a broader range of remarkable properties and applications than traditional graphene. This article will pay attention to the phenomenon and mechanism of these exceptional properties, including superconductivity, ferromagnetism, antiferromagnetism, and quantum spin liquid phase. Additionally, potential applications and future prospects of 2D materials beyond carbon will be explored. With the progress of technology, 2D materials beyond carbon are expected to have exciting developments in various fields, leading to significant changes in human life and production

Antisite-Defects Control of Magnetic Properties in MnSb2Te4.

The intrinsic magnetic topological materials Mn(Sb/Bi)2n+2Te3n+4 have attracted extensive attention due to their topological quantum properties. Although, the Mn-Sb/Bi antisite defects have been frequently reported to exert significant influences on both magnetism and band topology, their formation mechanism and the methods to manipulate their distribution and concentration remain elusive. Here, we present MnSb2Te4 as a typical example and demonstrate that Mn-Sb antisite defects and magnetism can be tuned by controlling the crystal growth conditions. The cooling rate is identified as the primary key parameter. Magnetization and chemical analysis demonstrate that a slower cooling rate would lead to a higher Mn concentration, a higher magnetic transition temperature, and a higher saturation moment. Further analysis indicates that the Mn content at the original Mn site (MnMn, 3a site) varies more significantly with the cooling rate than the Mn content at the Sb site (MnSb, 6c site). Based on experimental observations, magnetic phase diagrams regarding MnMn and MnSb concentrations are constructed. With the assistance of first-principles calculations, it is demonstrated that the Mn-Sb mixing states primarily result from the mixing entropy and the growth kinetics. The present findings offer valuable insights into defects engineering for preparation of two-dimensional quantum materials.

Holographic hydrodynamics of tilted Dirac materials

We present a gravity dual to a quantum material with tilted Dirac cone in 2+1 dimensional spacetime. In this many-body system the electronics degrees of freedom are strongly-coupled, constitute a Dirac fluid and admit an effective hydrodynamic description. The holographic techniques are applied to compute the thermodynamic variables and hydrodynamic transports of a fluid on the boundary of an asymptotically anti de Sitter spacetime with a boosted black hole in the bulk. We find that these materials exhibit deviations from the normal Dirac fluid which rely on the tilt of the Dirac cone. In particular, the shear viscosity to entropy density ratio is reduced and the KSS bound is violated in this system. This prediction can be experimentally verified in two-dimensional quantum materials (e.g. organic α-(BEDT-TTF)2I3 and 8Pmmn borophene) with tilted Dirac cone.

Electronic structure and transport properties of novel arsenene and its analogs from first-principles calculations

The electronic structure, dynamical stability, and carrier transport characteristics of monolayer P2/m-As (novel arsenene) of the third novel crystalline Pnm21-As, and its analogs P2/m-P and P2/m-Sb (novel phosphorene and antimonene) have been systematically studied by first-principles calculations based on density functional theory (DFT). The research results indicate that all three materials are dynamical stable and all are semiconductors with indirect band gaps. Significantly, we find that strain and electric fields can effectively adjust the band gaps and enable the materials to convert from indirect gap to direct gap. This indicates that the materials can be used for optoelectronic applications. Furthermore, we use the deformation potential method to study the differences in carrier mobility and find that their mobility reaches up to the order of 103 cm2 V−1 s−1 with high directional anisotropy and asymmetry between electrons and holes. Our research will provide new insights and ideas for developing the electronic properties of two-dimensional quantum materials.

Controllable growth of two-dimensional quantum materials

Controllable growth of two-dimensional quantum materials

Electrically switchable anisotropic polariton propagation in a ferroelectric van der Waals semiconductor.

Tailoring of the propagation dynamics of exciton-polaritons in two-dimensional quantum materials has shown extraordinary promise to enable nanoscale control of electromagnetic fields. Varying permittivities along crystal directions within layers of material systems, can lead to an in-plane anisotropic dispersion of polaritons. Exploiting this physics as a control strategy for manipulating the directional propagation of the polaritons is desired and remains elusive. Here we explore the in-plane anisotropic exciton-polariton propagation in SnSe, a group-IV monochalcogenide semiconductor that forms ferroelectric domains and shows room-temperature excitonic behaviour. Exciton-polaritons are launched in SnSe multilayer plates, and their propagation dynamics and dispersion are studied. This propagation of exciton-polaritons allows for nanoscale imaging of the in-plane ferroelectric domains. Finally, we demonstrate the electric switching of the exciton-polaritons in the ferroelectric domains of this complex van der Waals system. The study suggests that systems such as group-IV monochalcogenides could serve as excellent ferroic platforms for actively reconfigurable polaritonic optical devices.

Substrate engineering of plasmonic nanocavity antenna modes.

Plasmonic nanocavities have emerged as a promising platform for next-generation spectroscopy, sensing and photonic quantum information processing technologies, benefiting from a unique confluence of nanoscale compactness and integrability, ultrafast functionality and room-temperature viability. Harnessing their unprecedented optical field confinement and enhancement properties for such diverse application domains, however, demands continued innovation in cavity design and robust strategies for engineering their plasmonic mode characteristics, with the aim of optimizing spatial and spectral matching conditions for strong light-matter interaction involving embedded quantum emitters. Adopting the canonical gold bowtie nanoantenna, we show that the complex refractive index, n + ik, of the substrate material provides additional design flexibility in tailoring the properties of plasmonic nanocavity modes, including their resonance wavelengths, hotspot locations, intracavity field polarization and radiative decay rates. In particular, we predict that highly refractive (n ≥ 4) or highly absorptive (k ≥ 4) substrates provide two complementary approaches to engineering nanocavity modes that are especially desirable for coupling two-dimensional quantum materials, featuring namely an elevated hotspot with a dominantly in-plane polarized near-field, as well as a strongly radiative character. Our study elucidates the benefits and intricacies of a largely unexplored facet of nanocavity mode manipulation, beyond the widely practiced synthetic control over the cavity topology or physical dimensions, and paves the way for plasmonic cavity quantum electrodynamics with two-dimensional excitonic matter.

Holes and magnetic polarons in a triangular lattice antiferromagnet

The intricate interplay between charge motion and magnetic order in geometrically frustrated lattices is central for the properties of many two-dimensional quantum materials. The triangular lattice antiferromagnet is a canonical example of a frustrated system, and here we analyze the dynamics of a hole in such a lattice focusing on observables that have become accessible in a new generation of experiments. Using the $t\ensuremath{-}J$ model, we solve the problem exactly within linear spin wave theory in the limit of strong magnetic interactions, showing that the ground state is described by a coherent state of spin waves. The derivation highlights the crucial role played by the interaction between a static hole and the neighboring spins, which originates in the geometric frustration and has often been omitted in earlier works. Furthermore, we show that the nonequilibrium dynamics after a hole has abruptly been inserted at a lattice site is given by a coherent state with time-dependent oscillatory coefficients. Physically, this describes a burst of magnetic frustration propagating through only two-thirds of the lattice sites, since a destructive interference of spin waves leaves spins parallel to that removed by the hole unperturbed. After the wave has propagated through the lattice, the magnetization relaxes to that of the ground state. We then use our analytical solution to benchmark the widely used self-consistent Born approximation (SCBA) in the limit of strong magnetic interactions, showing that it is very accurate also for a triangular lattice. The magnetic polaron spectrum is analyzed for general magnetic interactions using the SCBA, and we compare our results with those for a square lattice.

Plasmonic quantum nonlinear Hall effect in noncentrosymmetric two-dimensional materials

We investigate an interplay between quantum geometrical effects and surface plasmons through surface plasmonic structures, based on an electron hydrodynamic theory. First we demonstrate that the quantum nonlinear Hall effect can be dramatically enhanced over a very broad range of frequency by utilizing plasmonic resonances and near-field effects of grating gates. Under the resonant condition, the enhancement becomes several orders of magnitude larger than the case without the nanostructures, while the peaks of high-harmonic plasmons expand broadly and emerge under the off-resonant condition, leading to a remarkably broad spectrum. Furthermore, we clarify a universal relation between the photocurrent induced by the Berry curvature dipole and the optical absorption, which is essential for computational material design of long-wavelength photodetectors. Next we discuss a novel mechanism of geometrical photocurrent, which originates from an anomalous force induced by oscillating magnetic fields and is described by the dipole moment of orbital magnetic moments of Bloch electrons in the momentum space. Our theory is relevant to 2D quantum materials such as layered WTe${}_2$ and twisted bilayer graphene, thereby providing a promising route toward a novel type of highly sensitive, broadband terahertz photodetectors.

Upconversion photoluminescence excitation reveals exciton–trion and exciton–biexciton coupling in hBN/WS_{2}/hBN van der Waals heterostructures

Monolayers of transition-metal dichalcogenides with direct band gap located at the binary K_{-}/K_{+} points of the Brillouin zone are promising materials for applications in opto- and spin-electronics due to strongly enhanced Coulomb interactions and specific spin-valley properties. They furthermore represent a unique platform to study electron–electron and electron–phonon interactions in diverse exciton complexes. Here, we demonstrate processes in which the neutral biexciton and two negative trions, namely the spin-triplet and spin-singlet trions, upconvert light into a bright intravalley exciton in an hBN-encapsulated WS_{2} monolayer. We propose that the energy gains required in the polarized upconversion photoluminescence originate from different interactions including resonant optical phonons, a cooling of resident electrons and a non-local and an anisotropic electron–hole exchange, respectively. The temperature dependence (7–120 K) of the excitonic upconversion intensity obtained at excitation energies corresponding to the biexciton and trions provides insight into an increasing phonon population as well as a thermally enhanced electron scattering. Our study sheds new light on the understanding of excitonic spin and valley properties of van der Waals heterostructures and improves the understanding of photonic upconversion mechanisms in two-dimensional quantum materials.

Physical properties of novel electronic states related to flat band in twisted two-dimensional quantum materials

Two-dimensional (2D) materials can exhibit novel quantum phenomena and be easily tuned by the external environment, which has made them one of the most attractive topics in condensed matter physics during the recent decades. The moiré superlattice induced by varied stacking geometry can further renormalize the material band structure, resulting in the electronic flat bands. With the help of external fields, one can tune the electron-electron correlated interaction in these flat bands, even control the overall physical properties. In this paper we review the recent researches of novel properties in twisted 2D materials (graphene and transition metal dichalcogenide heterostructure), involving strong correlation effect, unconventional superconductivity, quantum anomalous Hall effect, topological phase, and electronic crystals. We also discuss some open questions and give further prospects in this field.

Fast spectroscopic mapping of two-dimensional quantum materials

Spectroscopic mapping refers to the massive recording of spectra whilst varying an additional degree of freedom, such as: magnetic field, location, temperature, or charge carrier concentration. As this involves two serial tasks, spectroscopic mapping can become excruciatingly slow. We demonstrate exponentially faster mapping through our combination of sparse sampling and parallel spectroscopy. We exemplify our concept using quasiparticle interference imaging of Au(111) and Bi2Sr2CaCu2O8 (Bi2212), as two well-known model systems. Our method is accessible, straightforward to implement with existing scanning tunneling microscopes, and can be easily extended to enhance gate or field-mapping spectroscopy. In view of a possible four orders of magnitude speed advantage, it is setting the stage to fundamentally promote the discovery of novel quantum materials.

Revisiting Einstein's diffusion-mobility relation for universal quantum materials: A generalized paradigm

We have revisited the Einstein diffusion-mobility ratio, on the basis of the generalized Einstein relation, which quantifies both the quantum and classical levels of understanding on electron-hole dynamics in ordered and disordered materials. While the Einstein relation (diffusion-mobility ratio, D/) works only for classical systems under equilibrium cases, our generalized paradigm is quite general and works for systems ranging from classical to quantum systems in any dimensions. The proposed D/ formalism relies on the chemical potential which provides the cooperative nature between electronic and temperature counterparts on fundamental transport. Our generalized paradigm is applicable to both degenerate and non-degenerate cases, and explains the transition from linear to nonlinear relationship between the charge density and chemical potential. The symmetrical nature of electron-hole transport in highly degenerate two-dimensional quantum materials gives linear dispersion, preserving the time-reversal invariance property, while the symmetry is found to be broken in the nonlinear regime. Our generalized diffusion-mobility relation explains both the relativistic (Dirac particles) and non-relativistic regimes (Schrödinger particles) from low temperature to high temperature, and reproduces the classical Einstein relation in an appropriate limit.

Spin-Twisted Optical Lattices: Tunable Flat Bands and Larkin-Ovchinnikov Superfluids.

Moiré superlattices in twisted bilayer graphene and transition-metal dichalcogenides have emerged as a powerful tool for engineering novel band structures and quantum phases of two-dimensional quantum materials. Here we investigate Moiré physics emerging from twisting two independent hexagonal optical lattices of atomic (pseudo-)spin states (instead of bilayers) that exhibit remarkably different physics from twisted bilayer graphene. We employ a momentum-space tight-binding calculation that includes all range real-space tunnelings and show that all twist angles θ≲6° can become magic and support gapped flat bands. Because of the greatly enhanced density of states near the flat bands, the system can be driven to superfluidity by weak attractive interaction. Strikingly, the superfluid phase corresponds to a Larkin-Ovchinnikov state with finite momentum pairing that results from the interplay between flat bands and interspin interactions in the unique single-layer spin-twisted lattice. Our work may pave the way for exploring novel quantum phases and twistronics in cold atomic systems.

Growth and stability of Pb intercalated phases under graphene on SiC

Graphene intercalation is a novel way to control graphene's band structure and generate two-dimensional quantum materials with unusual spintronic and electronic properties. Despite its importance, information about the intercalation mechanism is lacking, especially the role of low density domain boundaries between regions of graphene of different thickness. With high resolution surface diffraction we have systematically studied Pb intercalation on epi-graphene grown on SiC, with domain boundaries between buffer and single layer graphene. By examining the evolution of different diffraction spots as a function of tempertature, the location of Pb and stability of the intercalated phases underneath were determined.

Probing quantum materials

Spectroscopy Unraveling the functionalities of quantum materials such as spin-valley-electronic, topological, and many-body effects provides a route to exploiting these materials for applications. Borsch et al. introduce a spectroscopic technique based on the concept of crystal-momentum combs. By extending the ideas of frequency combs of metrology and superresolution imaging, they demonstrate the ability to directly map out the properties of quantum electronic structures under ambient conditions. Using this technique combined with accurate many-body computations, they were able to reveal tomographic images of two-dimensional quantum materials. Science , this issue p. [1204][1] [1]: /lookup/doi/10.1126/science.abe2112

Tensor network investigation of the double layer Kagome compound Ca10Cr7O28

Quantum spin liquids are exotic quantum phases of matter that do not order even at zero temperature. While there are several toy models and simple Hamiltonians that could host a quantum spin liquid as their ground state, it is very rare to find actual, realistic materials that exhibit their properties. At the same time, the classical simulation of such instances of strongly correlated systems is intricate and reliable methods are scarce. In this work, we investigate the quantum magnet Ca10Cr7O28 that has recently been discovered to exhibit properties of a quantum spin liquid in inelastic neutron scattering experiments. This compound has a distorted bilayer Kagome lattice crystal structure consisting of Cr5＋ ions with spin-1∕2 moments. Coincidentally, the lattice structure renders a tensor network algorithm in 2D applicable that can be seen as a new variant of a projected entangled simplex state algorithm in the thermodynamic limit. In this first numerical investigation of this material that takes into account genuine quantum correlations, good agreement with the experimental findings is found. Our study contributes to uplifting tensor networks from conceptual tools to methods to describe real two-dimensional quantum materials.

High-pressure synthesis, crystal structure, and magnetic properties of the Shastry-Sutherland-lattice oxides BaLn2ZnO5 (Ln = Pr, Sm, Eu)

BaPr2ZnO5, BaSm2ZnO5, and BaEu2ZnO5 – a series of 4f magnetic insulators comprising the Shastry-Sutherland lattice – were synthesized via a solid-state reaction under high-pressure and high-temperature conditions. The magnetic behaviors are well characterized by the localized 4f electrons of each Ln3+ (= Pr, Sm, Eu) under the influence of the crystal-electric field of the surrounding ions. The magnetic susceptibility and heat capacity measurements of BaPr2ZnO5 (Pr3+; 4f23H4) indicate the splitting of the ground state J-multiplet due to the crystal-electric field. BaSm2ZnO5 (Sm3+; 4f56H5/2) is a strong Van Vleck paramagnet, while BaEu2ZnO5 is non-magnetic due to the 7F0 ground state (Eu3+; 4f67F0). The series of new oxides enriches a group of the Shastry-Sutherland lattice materials and helps to develop a range of two-dimensional quantum materials.