Precise Characterization of a Waveguide Fiber Interface in Silicon Carbide.

Spin-active optical emitters in silicon carbide are excellent candidates toward the development of scalable quantum technologies. However, efficient photon collection is challenged by undirected emission patterns from optical dipoles, as well as low total internal reflection angles due to the high refractive index of silicon carbide. Based on recent advances with emitters in silicon carbide waveguides, we now demonstrate a comprehensive study of nanophotonic waveguide-to-fiber interfaces in silicon carbide.

Nonlinear Landau Fan Diagram for Graphene Electrons Exposed to a Moiré Potential.

Due to Landau quantization, the conductance of two-dimensional electrons exposed to a perpendicular magnetic field exhibits oscillations that generate a fan of linear trajectories when plotted in the parameter space spanned by density and field. This fan looks identical, irrespective of the dispersion and field dependence of the Landau level energy. This is no surprise because the position of conductance minima depends solely on the level degeneracy that is linear in flux.

Ambipolar Superconductivity with Strong Pairing Interaction in Monolayer 1T'-MoTe.

Gate tunable two-dimensional (2D) superconductors offer significant advantages in studying superconducting phase transitions. Here, we address superconductivity in exfoliated 1T'-MoTe monolayers with an intrinsic band gap of ∼7.3 meV using field effect doping.

Orbitally Controlled Quantum Hall States in Decoupled Two-Bilayer Graphene Sheets.

The authors report on integer and fractional quantum Hall states in a stack of two twisted Bernal bilayer graphene sheets. By exploiting the momentum mismatch in reciprocal space, the single-particle tunneling between both bilayers is suppressed. Since the bilayers are spatially separated by only 0.

Highly Polarized Single Photons from Strain-Induced Quasi-1D Localized Excitons in WSe.

Single photon emission from localized excitons in two-dimensional (2D) materials has been extensively investigated because of its relevance for quantum information applications. Prerequisites are the availability of photons with high purity polarization and controllable polarization orientation that can be integrated with optical cavities. Here, deformation strain along edges of prepatterned square-shaped substrate protrusions is exploited to induce quasi-one-dimensional (1D) localized excitons in WSe monolayers as an elegant way to get photons that fulfill these requirements.

Two-Dimensional Quantum Hall Effect and Zero Energy State in Few-Layer ZrTe.

Topological matter plays a central role in today's condensed matter research. Zirconium pentatelluride (ZrTe) has attracted attention as a Dirac semimetal at the boundary of weak and strong topological insulators (TI). Few-layer ZrTe is anticipated to exhibit the quantum spin Hall effect due to topological states inside the band gap, but sample degradation inflicted by ambient conditions and processing has so far hampered the fabrication of high quality devices.

Emerging perovskite monolayers.

The library of two-dimensional (2D) materials has been enriched over recent years with novel crystal architectures endowed with diverse exciting functionalities. Bulk perovskites, including metal-halide and oxide systems, provide access to a myriad of properties through molecular engineering. Their tunable electronic structure offers remarkable features from long carrier-diffusion lengths and high absorption coefficients in metal-halide perovskites to high-temperature superconductivity, magnetoresistance and ferroelectricity in oxide perovskites.

Single-spin resonance in a van der Waals embedded paramagnetic defect.

A plethora of single-photon emitters have been identified in the atomic layers of two-dimensional van der Waals materials. Here, we report on a set of isolated optical emitters embedded in hexagonal boron nitride that exhibit optically detected magnetic resonance. The defect spins show an isotropic g-factor of ~2 and zero-field splitting below 10 MHz.

Odd Integer Quantum Hall States with Interlayer Coherence in Twisted Bilayer Graphene.

We report on the quantum Hall effect in two stacked graphene layers rotated by 2°. The tunneling strength among the layers can be varied from very weak to strong via the mechanism of magnetic breakdown when tuning the density. Odd-integer quantum Hall physics is not anticipated in the regime of suppressed tunneling for balanced layer densities, yet it is observed.

Optoelectronic Properties of a van der Waals WS Monolayer/2D Perovskite Vertical Heterostructure.

Two-dimensional (2D) Ruddlesden-Popper perovskites have been demonstrated to possess great potential for optical and optoelectronic devices. Because they exhibit better ambient stability than three-dimensional (3D) perovskites, they have been considered as potential substitutes for 3D perovskites as light absorbing layers to improve the photoresponsivity of monolayer transition metal dichalcogenide (TMDC)-based photodetectors. Investigation of the optoelectronic properties of TMDC monolayer/2D perovskite vertical heterostructures is however at an early stage.

Quasiparticle Tunneling across an Exciton Condensate.

The bulk properties of the bilayer quantum Hall state at total filling factor one have been intensively studied in experiment. Correlation induced phenomena such as Josephson-like tunneling and zero Hall resistance have been reported. In contrast, the edge of this bilayer state remains largely unexplored.

Type-II Ising pairing in few-layer stanene.

Spin-orbit coupling has proven indispensable in the realization of topological materials and, more recently, Ising pairing in two-dimensional superconductors. This pairing mechanism relies on inversion symmetry-breaking and sustains anomalously large in-plane polarizing magnetic fields whose upper limit is predicted to diverge at low temperatures. Here, we show that the recently discovered superconductor few-layer stanene, epitaxially strained gray tin (α-Sn), exhibits a distinct type of Ising pairing between carriers residing in bands with different orbital indices near the Γ-point.

Reliable Postprocessing Improvement of van der Waals Heterostructures.

The successful assembly of heterostructures consisting of several layers of different 2D materials in arbitrary order by exploiting van der Waals forces has truly been a game changer in the field of low-dimensional physics. For instance, the encapsulation of graphene or MoS between atomically flat hexagonal boron nitride (hBN) layers with strong affinity and graphitic gates that screen charge impurity disorder provided access to a plethora of interesting physical phenomena by drastically boosting the device quality. The encapsulation is accompanied by a self-cleansing effect at the interfaces.

Spin-Split Band Hybridization in Graphene Proximitized with α-RuCl Nanosheets.

Proximity effects induced in the two-dimensional Dirac material graphene potentially open access to novel and intriguing physical phenomena. Thus far, the coupling between graphene and ferromagnetic insulators has been experimentally established. However, only very little is known about graphene's interaction with antiferromagnetic insulators.

Gate-Tunable Tunneling Transistor Based on a Thin Black Phosphorus-SnSe Heterostructure.

Tunneling field-effect transistors (TFETs) are of considerable interest owing to their capability of low-power operation. Here, we demonstrate a novel type of TFET which is composed of a thin black phosphorus-tin diselenide (BP-SnSe) heterostructure. This combination of 2D semiconductor thin sheets enables device operation either as an Esaki diode featuring negative differential resistance (NDR) in the negative gate voltage regime or as a backward diode in the positive gate bias regime.

Probing Exfoliated Graphene Layers and Their Lithiation with Microfocused X-rays.

X-ray diffraction is measured on individual bilayer and multilayer graphene single-crystals and combined with electrochemically induced lithium intercalation. In-plane Bragg peaks are observed by grazing incidence diffraction. Focusing the incident beam down to an area of about 10 μm × 10 μm, individual flakes are probed by specular X-ray reflectivity.

Reversible superdense ordering of lithium between two graphene sheets.

Many carbon allotropes can act as host materials for reversible lithium uptake, thereby laying the foundations for existing and future electrochemical energy storage. However, insight into how lithium is arranged within these hosts is difficult to obtain from a working system. For example, the use of in situ transmission electron microscopy to probe light elements (especially lithium) is severely hampered by their low scattering cross-section for impinging electrons and their susceptibility to knock-on damage.

A cascade of phase transitions in an orbitally mixed half-filled Landau level.

Half-filled Landau levels host an emergent Fermi liquid that displays instability toward pairing, culminating in a gapped even-denominator fractional quantum Hall ground state. While this pairing may be probed by tuning the polarization of carriers in competing orbital and spin degrees of freedom, sufficiently high quality platforms offering such tunability remain few. We explore the ground states at filling factor ν = 5/2 in ZnO-based two-dimensional electron systems through a forced intersection of opposing spin branches of Landau levels taking quantum numbers = 1 and 0.

Quantum Light in Curved Low Dimensional Hexagonal Boron Nitride Systems.

Low-dimensional wide bandgap semiconductors open a new playing field in quantum optics using sub-bandgap excitation. In this field, hexagonal boron nitride (h-BN) has been reported to host single quantum emitters (QEs), linking QE density to perimeters. Furthermore, curvature/perimeters in transition metal dichalcogenides (TMDCs) have demonstrated a key role in QE formation.

Ultrafast lithium diffusion in bilayer graphene.

Solids that simultaneously conduct electrons and ions are key elements for the mass transfer and storage required in battery electrodes. Single-phase materials with a high electronic and high ionic conductivity at room temperature are hard to come by, and therefore multiphase systems with separate ion and electron channels have been put forward instead. Here we report on bilayer graphene as a single-phase mixed conductor that demonstrates Li diffusion faster than in graphite and even surpassing the diffusion of sodium chloride in liquid water.

Vibrational Properties of a Two-Dimensional Silica Kagome Lattice.

Kagome lattices are structures possessing fascinating magnetic and vibrational properties, but in spite of a large body of theoretical work, experimental realizations and investigations of their dynamics are scarce. Using a combination of Raman spectroscopy and density functional theory calculations, we study the vibrational properties of two-dimensional silica (2D-SiO), which has a kagome lattice structure. We identify the signatures of crystalline and amorphous 2D-SiO structures in Raman spectra and show that, at finite temperatures, the stability of 2D-SiO lattice is strongly influenced by phonon-phonon interaction.

Structural Attributes and Photodynamics of Visible Spectrum Quantum Emitters in Hexagonal Boron Nitride.

Newly discovered van der Waals materials like MoS, WSe, hexagonal boron nitride (h-BN), and recently CN have sparked intensive research to unveil the quantum behavior associated with their 2D structure. Of great interest are 2D materials that host single quantum emitters. h-BN, with a band gap of 5.

Charge Inversion and Topological Phase Transition at a Twist Angle Induced van Hove Singularity of Bilayer Graphene.

van Hove singularities (VHS's) in the density of states play an outstanding and diverse role for the electronic and thermodynamic properties of crystalline solids. At the critical point the Fermi surface connectivity changes, and topological properties undergo a transition. Opportunities to systematically pass a VHS at the turn of a voltage knob and study its diverse impact are however rare.

MgZnO/ZnO heterostructures with electron mobility exceeding 1 × 10(6) cm(2)/Vs.

The inherently complex chemical and crystallographic nature of oxide materials has suppressed the purities achievable in laboratory environments, obscuring the rich physical degrees of freedom these systems host. In this manuscript we provide a systematic approach to defect identification and management in oxide molecular beam epitaxy grown MgZnO/ZnO heterostructures which host two-dimensional electron systems. We achieve samples displaying electron mobilities in excess of 1 × 10(6) cm(2)/Vs.

Erratum: Signatures for Wigner Crystal Formation in the Chemical Potential of a Two-Dimensional Electron System [Phys. Rev. Lett. 113, 076804 (2014)].

This corrects the article DOI: 10.1103/PhysRevLett.113.