Condensation of preformed charge density waves in kagome metals.

Charge density wave (CDW) is a spontaneous spatial modulation of charges in solids whose general microscopic descriptions are yet to be completed. Kagome metals of AV3Sb5 (A = K, Rb, Cs) provide a chance to realize CDW intertwined with dimensional effects as well as their special lattice. Here, based on a state-of-the-art molecular dynamics simulation, we propose that their phase transition to CDW is a condensation process of incoherently preformed charge orders. Owing to unavoidable degeneracy in stacking charge orders, phases of preformed orders on each layer are shown to fluctuate between a limited number of states with quite slower frequencies than typical phonon vibrations until reaching their freezing temperature. As the size of interfacial alkali atom increases, the fluctuations are shown to counterbalance the condensation of orderings, resulting in a maximized transition temperature for RbV3Sb5. Our results resolve controversial observations on their CDWs, highlighting a crucial role of their interlayer interactions.

Rapid Suppression of Quantum Many-Body Magnetic Exciton in Doped van der Waals Antiferromagnet (Ni,Cd)PS3.

The unique discovery of the magnetic exciton in van der Waals antiferromagnet NiPS3 arises between two quantum many-body states of a Zhang-Rice singlet excited state and a Zhang-Rice triplet ground state. Simultaneously, the spectral width of photoluminescence originating from this exciton is exceedingly narrow as 0.4 meV. These extraordinary properties, including the extreme coherence of the magnetic exciton in NiPS3, beg many questions. We studied doping effects using Ni1-xCdxPS3 using two experimental techniques and theoretical studies. Our experimental results show that the magnetic exciton is drastically suppressed upon a few % Cd doping. All this happens while the width of the exciton only gradually increases and the antiferromagnetic ground state is robust. These results highlight the lattice uniformity's hidden importance as a prerequisite for coherent magnetic exciton. Finally, an exciting scenario emerges: the broken charge transfer forbids the otherwise uniform formation of the coherent magnetic exciton in (Ni,Cd)PS3.

Ultrafast van der Waals diode using graphene quantum capacitance and Fermi-level depinning.

Graphene, with superior electrical tunabilities, has arisen as a multifunctional insertion layer in vertically stacked devices. Although the role of graphene inserted in metal-semiconductor junctions has been well investigated in quasi-static charge transport regime, the implication of graphene insertion at ultrahigh frequencies has rarely been considered. Here, we demonstrate the diode operation of vertical Pt/n-MoSe2/graphene/Au assemblies at ~200-GHz cutoff frequency (fC). The electric charge modulation by the inserted graphene becomes essentially frozen above a few GHz frequencies due to graphene quantum capacitance-induced delay, so that the Ohmic graphene/MoSe2 junction may be transformed to a pinning-free Schottky junction. Our diodes exhibit much lower total capacitance than devices without graphene insertion, deriving an order of magnitude higher fC, which clearly demonstrates the merit of graphene at high frequencies.

In-plane anisotropy of graphene by strong interlayer interactions with van der Waals epitaxially grown MoO3.

van der Waals (vdW) epitaxy can be used to grow epilayers with different symmetries on graphene, thereby imparting unprecedented properties in graphene owing to formation of anisotropic superlattices and strong interlayer interactions. Here, we report in-plane anisotropy in graphene by vdW epitaxially grown molybdenum trioxide layers with an elongated superlattice. The grown molybdenum trioxide layers led to high p-doping of the underlying graphene up to p = 1.94 × 1013 cm-2 regardless of the thickness of molybdenum trioxide, maintaining a high carrier mobility of 8155 cm2 V-1 s-1. Molybdenum trioxide-induced compressive strain in graphene increased up to -0.6% with increasing molybdenum trioxide thickness. The asymmetrical band distortion of molybdenum trioxide-deposited graphene at the Fermi level led to in-plane electrical anisotropy with a high conductance ratio of 1.43 owing to the strong interlayer interaction of molybdenum trioxide-graphene. Our study presents a symmetry engineering method to induce anisotropy in symmetric two-dimensional (2D) materials via the formation of asymmetric superlattices with epitaxially grown 2D layers.

Intersite Coulomb Interactions in Charge-Ordered Systems.

Using ab initio approaches for extended Hubbard interactions coupled to phonons, we reveal that the intersite Coulomb interaction plays important roles in determining various distinctive phases of the paradigmatic charge-ordered materials of Ba_{1-x}K_{x}AO_{3} (A=Bi and Sb). We demonstrated that all their salient doping dependent experiment features such as breathing instabilities, anomalous phonon dispersions, and transition between charge-density wave and superconducting states can be accounted for very well if self-consistently obtained nearest neighbor Hubbard interactions are included, thus establishing a minimal criterion for reliable descriptions of spontaneous charge orders in solids.

Lattice dynamical properties of antiferromagnetic oxides calculated using self-consistent extended Hubbard functional method.

We study the lattice dynamics of antiferromagnetic transition-metal oxides by using self-consistent Hubbard functionals. We calculate the ground states of the oxides with the on-site and intersite Hubbard interactions determined self-consistently within the framework of density functional theory. The on-site and intersite Hubbard terms fix the errors associated with the electron self-interaction in the local and semilocal functionals. Inclusion of the intersite Hubbard terms in addition to the on-site Hubbard terms produces accurate phonon dispersion of the transition-metal oxides. Calculated Born effective charges and high-frequency dielectric constants are in good agreement with experiment. Our study provides a computationally inexpensive and accurate set of first-principles calculations for strongly-correlated materials and related phenomena.

Anomalous optical excitations from arrays of whirlpooled lattice distortions in moiré superlattices.

Moiré superlattices formed by stacking two-dimensional crystals have reinvigorated the pursuit for emergent functionalities of engineered superlattices. Unique optical characteristics can be realized from the interplay between the electronic excitations and the atomic rearrangements owing to their intrinsic softness. Although large-scale reconstructions have been identified at small twist angles, they have been treated as being rigid at large twist angles. Here, we report that moiré superlattices made from single layers of MoS2 and WSe2 exhibit a pair of torsional strains with opposite chirality irrespective of the twist angle. The whirlpool-shaped periodic lattice distortions introduce fuzziness in the Raman spectra and universal redshifts to the intralayer excitons for all twist angles. We show that both of these modulations become weaker as the twist angle increases but do not disappear, whereas they are turned off when the constituent layers are not tightly coupled, thus establishing an essential structure-property relationship for moiré superlattices.

Magnetic Ordering, Anomalous Lifshitz Transition, and Topological Grain Boundaries in Two-Dimensional Biphenylene Network.

We study the electronic properties of a new planar carbon crystal formed through networking biphenylene molecules. Novel electronic features among carbon materials such as zone-center saddle point and peculiar type-II Dirac fermionic states are shown to exist in the low-energy electronic spectrum. The type-II state here has a nearly flat branch and is close to a transition to type I. Possible magnetic instabilities related to low-energy bands are discussed. Furthermore, with a moderate uniaxial strain, a pair of Dirac points merge with the zone-center saddle point, realizing concurrent Lifshitz transitions of van Hove singularity as well as pair annihilation of the Dirac fermions. A new effective Hamiltonian encompassing all distinctive low-energy states is constructed, revealing a finite winding number of the pseudospin texture around the Dirac point, quantized Zak phases, and topological grain boundary states.

Multiferroic-Enabled Magnetic-Excitons in 2D Quantum-Entangled Van der Waals Antiferromagnet NiI2.

Matter-light interaction is at the center of diverse research fields from quantum optics to condensed matter physics, opening new fields like laser physics. A magnetic exciton is one such rare example found in magnetic insulators. However, it is relatively rare to observe that external variables control matter-light interaction. Here, it is reported that the broken inversion symmetry of multiferroicity can act as an external knob enabling magnetic excitons in the van der Waals antiferromagnet NiI2 . It is further discovered that this magnetic exciton arises from a transition between Zhang-Rice-triplet and Zhang-Rice-singlet fundamentally quantum-entangled states. This quantum entanglement produces an ultrasharp optical exciton peak at 1.384 eV with a 5 meV linewidth. The work demonstrates that NiI2 is 2D magnetically ordered with an intrinsically quantum-entangled ground state.

Robust Quantum Oscillation of Dirac Fermions in a Single-Defect Resonant Transistor.

The massless nature of Dirac Fermions produces large energy gaps between Landau levels (LLs), which is promising for topological devices. While the energy gap between the zeroth and first LLs reaches 36 meV in a magnetic field of 1 T in graphene, exploiting the quantum Hall effect at room temperature requires large magnetic fields (∼30 T) to overcome the energy level broadening induced by charge inhomogeneities in the device. Here, we report a way to use the robust quantum oscillations of Dirac Fermions in a single-defect resonant transistor, which is based on local tunneling through a thin (∼1.4 nm) hexagonal boron nitride (h-BN) between lattice-orientation-aligned graphene layers. A single point defect in the h-BN, selected by the orientation-tuned graphene layers, probes local LLs in its proximity, minimizing the energy broadening of the LLs by charge inhomogeneity at a moderate magnetic field and ambient conditions. Thus, the resonant tunneling between lattice-orientation-aligned graphene layers highlights the potential to spectroscopically locate the atomic defects in the h-BN, which contributes to the study on electrically tunable single photon source via defect states in h-BN.

Gate-tunable superconductivity and charge-density wave in monolayer 1T'-MoTe2 and 1T'-WTe2.

Using first-principles calculation methods, we reveal a series of phase transitions as a function of gating or electron doping in monolayered quantum spin Hall (QSH) insulators, 1T'-MoTe2 and 1T'-WTe2. With increasing electron doping, we show that a phonon mediated superconducting phase is realized first and is followed by a charge density wave (CDW) phase with a nonsymmorphic lattice symmetry. The newly found CDW phase exhibits Weyl energy bands with spin-orbit coupling with fractional band filling, and reforms into a topological nontrivial phase with fully filled bands. The robust resurgence of the QSH state coexisting with the CDW phase is shown to originate from band inversions induced by the nonsymmorphic lattice distortion through the strong electron-phonon interaction, thus suggesting the realization of various interfacial states between superconducting states, and various CDW and topological states on a two-dimensional crystal by inhomogeneous gating or doping.

Numerical Analysis on Natural Convection Heat Transfer in a Single Circular Fin-Tube Heat Exchanger (Part 2): Correlations for Limiting Cases.

This research focused on the correlations associated with the physics of natural convection in circular fin-tube models. The limiting conditions are defined by two conditions. The lower limit ( D o / D → 1, s/D = finite value) corresponds to a horizontal circular tube, while the upper limit ( D o / D → ∞, s/D << 1) corresponds to vertical flat plates. In this paper, we proposed a corrected correlation based on empirical result. The circular fin-tube heat exchanger was divided into the A and B types, the categorizing criteria being D o / D = 1.2 , where D and D o refer to the diameter of the circular tube and the diameter of the circular fin, respectively. Moreover, with the computational fluid dynamics technique used to investigate the limiting conditions, the parametric range was extended substantially in this research for type B, namely 1.2 < D o / D ≤ 10. The complex correlation was also simplified to the form Nu L = C Ra s n , where C and n are the functions of the diameter ratio D o / D .

Coherent many-body exciton in van der Waals antiferromagnet NiPS3.

An exciton is the bosonic quasiparticle of electron-hole pairs bound by the Coulomb interaction1. Bose-Einstein condensation of this exciton state has long been the subject of speculation in various model systems2,3, and examples have been found more recently in optical lattices and two-dimensional materials4-9. Unlike these conventional excitons formed from extended Bloch states4-9, excitonic bound states from intrinsically many-body localized states are rare. Here we show that a spin-orbit-entangled exciton state appears below the Néel temperature of 150 kelvin in NiPS3, an antiferromagnetic van der Waals material. It arises intrinsically from the archetypal many-body states of the Zhang-Rice singlet10,11, and reaches a coherent state assisted by the antiferromagnetic order. Using configuration-interaction theory, we determine the origin of the coherent excitonic excitation to be a transition from a Zhang-Rice triplet to a Zhang-Rice singlet. We combine three spectroscopic tools-resonant inelastic X-ray scattering, photoluminescence and optical absorption-to characterize the exciton and to demonstrate an extremely narrow excitonic linewidth below 50 kelvin. The discovery of the spin-orbit-entangled exciton in antiferromagnetic NiPS3 introduces van der Waals magnets as a platform to study coherent many-body excitons.

Promising photovoltaic efficiency of a layered silicon oxide crystal Si3O.

Computational searching and screening of new functional materials exploiting Earth abundant elements can accelerate the development of their energy applications. Based on the state-of-the-art material search algorithm and ab initio calculations, we demonstrate a recently suggested stable silicon oxide with a layered structure (Si3O) as an ideal photovoltaic material. With many-body first-principles approaches, the monolayer and layered bulk of Si3O show direct quasiparticle gaps of 1.85 eV and 1.25 eV, respectively, while an optical gap of about 1.2 eV is nearly independent of the number of layers. Spectroscopic limited maximum efficiency (SLME) is estimated to be 27% for a thickness of 0.5 µm, making it a promising candidate for solar energy applications.

Symmetry Dictated Grain Boundary State in a Two-Dimensional Topological Insulator.

Grain boundaries (GBs) are ubiquitous in solids and have been of central importance in understanding the nature of polycrystals. In addition to their classical roles, topological insulators (TIs) offer a chance to realize GBs hosting distinct topological states that can be controlled by their crystal symmetries. However, such roles of crystalline symmetry in two-dimensional (2D) TIs have not been definitively measured yet. Here, we present the first direct evidence of a symmetry-enforced metallic state along a GB in 1T'-MoTe2, a prototypical 2D TI. Using scanning tunneling microscopy, we show a metallic state along a GB with nonsymmorphic lattice symmetry and its absence along another boundary with symmorphic symmetry. Our atomistic simulations demonstrate in-gap Weyl semimetallic states for the former, whereas they demonstrate gapped states for the latter, explaining our observation well. The observed metallic state, tightly linked to its crystal symmetry, can be used to create a stable conducting nanowire inside TIs.

A New Family of Two-Dimensional Crystals: Open-Framework T3 X ( T = C, Si, Ge, Sn; X = O, S, Se, Te) Compounds with Tetrahedral Bonding.

To accelerate development of innovative materials, their modelings and predictions with useful functionalities are of vital importance. Here, based on a recently developed crystal structure prediction method, we find a new family of stable two-dimensional crystals with an open-channel tetrahedral bonding network, rendering a potential prototype for electronic and energy applications. The proposed structural prototype with a space group of Cmme hosts at least 13 different freestanding T3 X compounds with group IV ( T = C, Si, Ge, Sn) and VI ( X = O, S, Se, Te) elements. Moreover, the proposed materials display diverse electronic properties ranging from direct band gap semiconductor to topological insulator at their pristine forms, which are further tunable by mechanical strain.

Enhanced Thermoelectric Properties in a New Silicon Crystal Si24 with Intrinsic Nanoscale Porous Structure.

Thermoelectric device is a promising next-generation energy solution owing to its capability to transform waste heat into useful electric energy, which can be realized in materials with high electric conductivities and low thermal conductivities. A recently synthesized silicon allotrope of Si24 features highly anisotropic crystal structure with nanometer-sized regular pores. Here, based on first-principles study without any empirical parameter we show that the slightly doped Si24 can provide an order-of-magnitude enhanced thermoelectric figure of merit at room temperature, compared with the cubic diamond phase of silicon. We ascribe the enhancement to the intrinsic nanostructure formed by the nanopore array, which effectively hinders heat conduction while electric conductivity is maintained. This can be a viable option to enhance the thermoelectric figure of merit without further forming an extrinsic nanostructure. In addition, we propose a practical strategy to further diminish the thermal conductivity without affecting electric conductivity by confining rattling guest atoms in the pores.

Dirac electrons in a dodecagonal graphene quasicrystal.

Quantum states of quasiparticles in solids are dictated by symmetry. We have experimentally demonstrated quantum states of Dirac electrons in a two-dimensional quasicrystal without translational symmetry. A dodecagonal quasicrystalline order was realized by epitaxial growth of twisted bilayer graphene rotated exactly 30°. We grew the graphene quasicrystal up to a millimeter scale on a silicon carbide surface while maintaining the single rotation angle over an entire sample and successfully isolated the quasicrystal from a substrate, demonstrating its structural and chemical stability under ambient conditions. Multiple Dirac cones replicated with the 12-fold rotational symmetry were observed in angle-resolved photoemission spectra, which revealed anomalous strong interlayer coupling with quasi-periodicity. Our study provides a way to explore physical properties of relativistic fermions with controllable quasicrystalline orders.

Strain-shear coupling in bilayer MoS2.

Layered materials such as graphite and transition metal dichalcogenides have extremely anisotropic mechanical properties owing to orders of magnitude difference between in-plane and out-of-plane interatomic interaction strengths. Although effects of mechanical perturbations on either intralayer or interlayer interactions have been extensively investigated, mutual correlations between them have rarely been addressed. Here, we show that layered materials have an inevitable coupling between in-plane uniaxial strain and interlayer shear. Because of this, the uniaxial in-plane strain induces an anomalous splitting of the degenerate interlayer shear phonon modes such that the split shear mode along the tensile strain is not softened but hardened contrary to the case of intralayer phonon modes. We confirm the effect by measuring Raman shifts of shear modes of bilayer MoS2 under strain. Moreover, by analyzing the splitting, we obtain an unexplored off-diagonal elastic constant, demonstrating that Raman spectroscopy can determine almost all mechanical constants of layered materials.