arXiv subjects

Hongyi Yu

Publications and source records attributed to Hongyi Yu.

At least 37 records · Page 2Linked to original sources

arXiv2024

Dipolar interactions enhanced by two-dimensional dielectric screening in few-layer van der Waals structures

We theoretically examined how the dielectric screening of two-dimensional layered materials affects the dipolar interaction between interlayer excitons in few-layer van der Waals structures. Our analysis indicates that the dipolar interaction is largely enhanced by two-dimensional dielectric screening at an inter-exciton separation of several nanometers or larger. The underlying mechanism can be attributed to the induced-charge densities in layered materials, which give rise to induced-dipole densities at large distances with directions parallel to that of the interlayer exciton. The interaction between quadrupolar excitons in trilayer structures are found to be enhanced even larger, with a magnitude one to two orders stronger than that without 2D dielectric screening. The strengths of these dipolar and quadrupolar interactions can be further tuned by engineering the dielectric environment.

cond-mat.mes-hall↗

arXiv2023

Melting of electronic and excitonic crystals in 2D semiconductor moiré patterns: a perspective from the Lindemann criterion

Using the Lindemann criterion, we analyzed the quantum and thermal melting of electronic/excitonic crystals recently discovered in two-dimensional (2D) semiconductor moiré patterns. We show that the finite 2D screening of the atomically thin material can suppress (enhance) the inter-site Coulomb (dipolar) interaction strength, thus inhibits (facilitates) the formation of the electronic (excitonic) crystal. Meanwhile, a strong enough moiré confinement is found to be essential for realizing the crystal phase with a wavelength near 10 nm or shorter. From the calculated Lindemann ratio which quantifies the fluctuation of the site displacement, we estimate that the crystal will melt into a liquid above a critical temperature ranging from several tens Kelvin to above 100 K (depending on the system parameters).

cond-mat.mes-hall↗

arXiv2023

Distinct moiré textures of in-plane electric polarizations for distinguishing moiré origins in homobilayers

In binary compound 2D insulators/semiconductors such as hexagonal boron nitride (hBN), the different electron affinities of atoms can give rise to out-of-plane electric polarizations across inversion asymmetric van der Waals interface of near 0-degree twist angles. Here we show that at a general stacking order where sliding breaks the in-plane C3 rotational symmetry, the interfacial charge redistribution also leads to an in-plane electric polarization, with a comparable magnitude to that of the out-of-plane ones. The effect is demonstrated in hBN bilayers, as well as in biased graphene bilayers with the gate-controlled interlayer charge redistribution. In long wavelength moiré patterns, the in-plane electric polarizations determined by the local interlayer stacking registries constitute topologically nontrivial spatial textures. We show that these textures can distinguish moiré patterns of different origins from twisting, biaxial- and uniaxial-heterostrain, where vector fields of the electric polarizations feature Bloch-type merons, Néel-type merons, and anti-merons, respectively. Combinations of twisting and heterostrain can further be exploited for engineering various electric polarization textures including 1D quasiperiodic lattices.

cond-mat.mes-hall↗

arXiv2023

Phonons of electronic crystals in two-dimensional semiconductor moiré patterns

We theoretically studied the phonon properties of the triangular-, stripe- and honeycomb-type electronic crystals recently found in two-dimensional semiconductor moiré patterns. By analyzing the phonon dispersions, we found the interaction induced lattice deformation in the zigzag-stripe crystal results in a much higher dynamical stability than in the linear-stripe crystal. Moreover, chiral phonons with finite magnetizations and large Berry curvatures can emerge in triangular and honeycomb crystals under time-reversal or inversion symmetry breaking. The small effective mass of the electrons allows the selective and efficient generation of chiral phonons from the optical activity of zone-center phonons combined with the anharmonicity, facilitating the realization of the phonon Hall effect. These findings point to an exciting new platform for exploring chiral phonons and the related topological phononic devices.

cond-mat.mes-hall↗

arXiv2021

HAD-GAN: A Human-perception Auxiliary Defense GAN to Defend Adversarial Examples

Adversarial examples reveal the vulnerability and unexplained nature of neural networks. Studying the defense of adversarial examples is of considerable practical importance. Most adversarial examples that misclassify networks are often undetectable by humans. In this paper, we propose a defense model to train the classifier into a human-perception classification model with shape preference. The proposed model comprising a texture transfer network (TTN) and an auxiliary defense generative adversarial networks (GAN) is called Human-perception Auxiliary Defense GAN (HAD-GAN). The TTN is used to extend the texture samples of a clean image and helps classifiers focus on its shape. GAN is utilized to form a training framework for the model and generate the necessary images. A series of experiments conducted on MNIST, Fashion-MNIST and CIFAR10 show that the proposed model outperforms the state-of-the-art defense methods for network robustness. The model also demonstrates a significant improvement on defense capability of adversarial examples.

cs.CV↗

arXiv2021

Waveguiding valley excitons in monolayer transition metal dicalcogenides by dielectric interfaces in the substrate

In monolayers of the semiconducting transition metal dichalcogenides, the electron-hole exchange interaction splits the exciton dispersion into a massive transverse branch, and a longitudinal branch that has very light or even zero mass depending on the form of screened Coulomb interaction. The group velocity of the longitudinal branch is sensitive to the strength of electron-hole exchange, which can be engineered through the dielectric environment. Here we show that dielectric patterning of the substrate can be exploited to realize waveguide of the exciton in the longitudinal branch in a homogeneous monolayer, leaving the massive transverse branch unaffected. At a lateral interface of different dielectric constant in the substrate, the transmission and reflection of exciton in the longitudinal branch obey the Snell-Descartes law of optical system, and total reflection can be exploited to realize excitonic waveguide using two parallel interfaces. The same dielectric pattern of the substrate appears to be completely transparent for the massive transverse branch exciton, which has no interface scattering. When the monolayer is placed on a one-dimensional dielectric superlattice, the dispersion of the longitudinal branch is strongly renormalized, and the wavefunctions exhibit one-dimensional features, confined to either the low-dielectric or high-dielectric regions. In contrast, the massive transverse branch excitons are not affected by the substrate dielectric pattern, exhibiting pristine properties as in a freestanding monolayer.

cond-mat.mes-hall↗

arXiv2021

Chiral excitonics in monolayer semiconductors on patterned dielectric

Monolayer transition metal dichalcogenides feature tightly bound bright excitons at the degenerate valleys, where electron-hole Coulomb exchange interaction strongly couples the valley pseudospin to the momentum of exciton. Placed on periodically structured dielectric substrate, the spatial modulation of the Coulomb interaction leads to the formation of exciton Bloch states with real-space valley pseudospin texture displayed in a mesoscopic supercell. We find this spatial valley texture in the exciton Bloch function is pattern-locked to the propagation direction, enabling nano-optical excitation of directional exciton flow through the valley selection rule. The left-right directionality of the injected exciton current is controlled by the circular polarization of excitation, while the angular directionality is controlled by the excitation location, exhibiting a vortex pattern in a supercell. The phenomenon is reminiscent of the chiral light-matter interaction in nano-photonics structures, with the role of the guided electromagnetic wave now replaced by the valley-orbit coupled exciton Bloch wave in a uniform monolayer, which points to new excitonic devices with non-reciprocal functionalities.

cond-mat.mes-hall↗

arXiv2021

Nanoscale trapping of interlayer excitons in a 2D semiconductor heterostructure

For quantum technologies based on single excitons and spins, the deterministic placement and control of a single exciton is a long-standing goal. MoSe2-WSe2 heterostructures host spatially indirect interlayer excitons (IXs) which exhibit highly tunable energies and unique spin-valley physics, making them promising candidates for quantum information processing. Previous IX trapping approaches involving moiré superlattices and nanopillars do not meet the quantum technology requirements of deterministic placement and energy tunability. Here, we use a nanopatterned graphene gate to create a sharply varying electric field in close proximity to a MoSe2-WSe2 heterostructure. The dipole interaction between the IX and the electric field creates an ~20 nm trap. The trapped IXs show the predicted electric field dependent energy, saturation at low excitation power, and increased lifetime, all signatures of strong spatial confinement. The demonstrated architecture is a crucial step towards deterministic trapping of single IXs, which has broad applications to scalable quantum technologies.

cond-mat.mes-hall↗

arXiv2021

Temperature dependent moiré trapping of interlayer excitons in MoSe2-WSe2 heterostructures

MoSe2-WSe2 heterostructures host strongly bound interlayer excitons (IXs) which exhibit bright photoluminescence (PL) when the twist-angle is near 0° or 60°. Over the past several years, there have been numerous reports on the optical response of these heterostructures but no unifying model to understand the dynamics of IXs and their temperature dependence. Here, we perform a comprehensive study of the temperature, excitation power, and time-dependent PL of IXs. We observe a significant decrease in PL intensity above a transition temperature that we attribute to a transition from localized to delocalized IXs. Astoundingly, we find a simple inverse relationship between the IX PL energy and the transition temperature, which exhibits opposite power dependent behaviors for near 0° and 60° samples. We conclude that this temperature dependence is a result of IX-IX exchange interactions, whose effect is suppressed by the moiré potential trapping IXs at low temperature.

physics.optics↗

arXiv2021

Luminescence anomaly of dipolar valley excitons in homobilayer semiconductor moiré superlattices

In twisted homobilayer transition metal dichalcogenides, intra- and inter-layer valley excitons hybridize with the layer configurations spatially varying in the moiré. The ground state valley excitons are trapped at two high-symmetry points with opposite electric dipoles in a moiré supercell, forming a honeycomb superlattice of nearest-neighbor dipolar attraction. We find that the spatial texture of layer configuration results in a luminescence anomaly of the moiré trapped excitons, where a tiny displacement by interactions dramatically increases the brightness and changes polarization from circular to linear. At full filling, radiative recombination predominantly occurs at edges and vacancies of the exciton superlattice. The anomaly also manifests in the cascaded emission of small clusters, producing chains of polarization entangled photons. An interlayer bias can switch the superlattice into a single-orbital triangular lattice with repulsive interactions only, where the luminescence anomaly can be exploited to distinguish ordered states and domain boundaries at fractional filling.

cond-mat.mes-hall↗

arXiv2021

Intrinsic Donor-Bound Excitons in Ultraclean Monolayer Semiconductors

The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe2. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (> 6 $μ$s) and polarization lifetimes (> 100 ns). Resonant excitation of the free inter- and intra-valley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellites photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors.

cond-mat.mes-hall↗

arXiv2020

Interferences of electrostatic moiré potentials and bichromatic superlattices of electrons and excitons in transition metal dichalcogenides

Recent experimental progresses have demonstrated the great potential of electronic and excitonic moiré superlattices in transition metal dichalcogenides (TMDs) for quantum many-body simulations and quantum optics applications. Here we reveal that the moiré potential landscapes in the TMDs heterostructures have an electrostatic origin from the spontaneous charge transfer across the heterointerfaces dependent on the atomic registry. This allows engineering tunable multi-chromatic superlattices through the interference of moiré potentials from independently configurable heterointerfaces in multilayers. We show examples of bichromatic moiré potentials for valley electrons, holes, and interlayer trions in MX2/M'X'2/MX2 trilayers, which can be strain switched from multi-orbital periodic superlattices to quasi-periodic disordered landscape. The trilayer moiré also hosts two independently configurable triangular superlattices of neutral excitons with opposite electric dipoles. These findings greatly enrich the versatility and controllability of TMDs moiré as a quantum simulation platform.

cond-mat.mes-hall↗

arXiv2020

Electrically tunable topological transport of moiré polaritons

Moiré interlayer exciton in transition metal dichalcogenide heterobilayer features a permanent electric dipole that enables the electrostatic control of its flow, and optical dipole that is spatially varying in the moiré landscape. We show the hybridization of moiré interlayer exciton with photons in a planar 2D cavity leads to two types of moiré polaritons that exhibit distinct forms of topological transport phenomena including the spin/valley Hall and polarization Hall effects, which feature remarkable electrical tunability through the control of exciton-cavity detuning by the interlayer bias.

cond-mat.mes-hall↗

arXiv2020

Monolayer Semiconductor Auger Detector

Auger recombination in semiconductors is a many-body phenomenon in which recombination of electrons and holes is accompanied by excitation of other charge carriers. Being nonradiative, it is detrimental to light emission. The excess energy of the excited carriers is normally rapidly converted to heat, making Auger processes difficult to probe directly. Here, we employ a technique in which the Auger-excited carriers are detected by their ability to tunnel out of the semiconductor through a thin barrier, generating a current. We employ vertical van der Waals (vdW) heterostructures with monolayer WSe2 as the semiconductor and the wide band gap hexagonal boron nitride (hBN) as the tunnel barrier to preferentially transmit high-energy Auger-excited carriers to a graphite electrode. The unambiguous signatures of Auger processes are a rise in the photocurrent when excitons are created by resonant excitation, and negative differential photoconductance resulting from the shifts of the exciton resonances with voltage. We detect holes Auger-excited by both neutral and charged excitons, and find that the Auger scattering is surprisingly strong under weak excitation. The selective extraction of Auger carriers at low, controlled carrier densities that is enabled by vdW heterostructures illustrates an important addition to the techniques available for probing relaxation processes in 2D materials.

cond-mat.mes-hall↗

arXiv2020

Non-adiabatic Hall effect at Berry curvature hot spot

Hot spot of Berry curvature is usually found at Bloch band anti-crossings, where the Hall effect due to the Berry phase can be most pronounced. With small gaps there, the adiabatic limit for the existing formulations of Hall current can be exceeded in a moderate electric field. Here we present a theory of non-adiabatic Hall effect, capturing non-perturbatively the across gap electron-hole excitations by the electric field. We find a general connection between the field induced electron-hole coherence and intrinsic Hall velocity. In coherent evolution, the electron-hole coherence can manifest as a sizeable ac Hall velocity. When environmental noise is taken into account, its joint action with the electric field favors a form of electron-hole coherence that is function of wavevector and field only, leading to a dc nonlinear Hall effect. The Hall current has all odd order terms in field, and still retains the intrinsic role of the Berry curvature. The quantitative demonstration uses the example of gapped Dirac cones, and our theory can be used to describe the bulk pseudospin Hall current in insulators with gapped edge such as graphene and 2D MnBi$_{2}$Te$_{4}$

cond-mat.mes-hall↗

arXiv2020

Valley Phonons and Exciton Complexes in a Monolayer Semiconductor

The coupling between spin, charge, and lattice degrees of freedom plays an important role in a wide range of fundamental phenomena. Monolayer semiconducting transitional metal dichalcogenides have emerged as an outstanding platform for studying these coupling effects because they possess unique spin-valley locking physics for hosting rich excitonic species and the reduced screening for strong Coulomb interactions. Here, we report the observation of multiple valley phonons, phonons with momentum vectors pointing to the corners of the hexagonal Brillouin zone, and the resulting exciton complexes in the monolayer semiconductor WSe2. From Lande g-factor and polarization analyses of photoluminescence peaks, we find that these valley phonons lead to efficient intervalley scattering of quasi particles in both exciton formation and relaxation. This leads to a series of photoluminescence peaks as valley phonon replicas of dark trions. Using identified valley phonons, we also uncovered an intervalley exciton near charge neutrality, and extract its short-range electron-hole exchange interaction to be about 10 meV. Our work not only identifies a number of previously unknown 2D excitonic species, but also shows that monolayer WSe2 is a prime candidate for studying interactions between spin, pseudospin, and zone-edge phonons.

cond-mat.mes-hall↗

arXiv2019

Giant magnetic field from moiré induced Berry phase in homobilayer semiconductors

When quasiparticles move in condensed matters, the texture of their internal quantum structure as a function of position and momentum can give rise to Berry phases that have profound effects on materials properties. Seminal examples include the anomalous Hall and spin Hall effects from the momentum-space Berry phases in homogeneous crystals. Here we explore a conjugate form of electron Berry phase arising from the moiré pattern, the texture of atomic configurations in real space. In homobilayer transition metal dichalcogenides, we show the real-space Berry phase from moiré manifests as a periodic magnetic field up to hundreds of Tesla. This quantity tells apart moiré patterns from different origins, which can have identical potential landscape but opposite quantized magnetic flux per supercell. For low energy carriers, the homobilayer moirés realize topological flux lattices for the quantum spin Hall effect. An interlayer bias can continuously tune the spatial profile of moiré magnetic field, whereas the flux per supercell is a topological quantity that can only have a quantized jump observable at moderate bias. We also reveal the important role of the non-Abelian Berry phase in shaping the energy landscape in small moiré. Our work points to new possibilities to access ultra-high magnetic field that can be tailored in the nanoscale by electrical and mechanical controls.

cond-mat.mes-hall↗

arXiv2019

Nonlinear optics in the electron-hole continuum in 2D semiconductors: two-photon transition, second harmonic generation and valley current injection

We investigate two-photon transitions to the electron-hole scattering continuum in monolayer transition-metal dichalcogenides, and identify two contributions to this nonlinear optical process with opposite circularly polarized valley selection rules. In the non-interacting limit, the competition between the two contributions leads to a crossover of the selection rule with the increase of the two-photon energy. With the strong Coulomb interaction between the electron and hole, the two contributions excite electron-hole scattering states in orthogonal angular momentum channels, while the strength of the transition can be substantially enhanced by the interaction. Based on this picture of the two-photon transition, the second harmonic generation (SHG) in the electron-hole continuum is analyzed, where the Coulomb interaction is shown to greatly alter the relative strength of different cross-circular polarized SHG processes. Valley current injection by the quantum interference of one-photon and two-photon transition is also investigated in the presence of the strong Coulomb interaction, which significantly enhances the injection rate.

cond-mat.mes-hall↗