Electric-field tunable Type-I to Type-II band alignment transition in MoSe/WS heterobilayers.

Semiconductor heterojunctions are ubiquitous components of modern electronics. Their properties depend crucially on the band alignment at the interface, which may exhibit straddling gap (type-I), staggered gap (type-II) or broken gap (type-III). The distinct characteristics and applications associated with each alignment make it highly desirable to switch between them within a single material.

Mapping the intrinsic photocurrent streamlines through micromagnetic heterostructure devices.

Photocurrent in quantum materials is often collected at global contacts far away from the initial photoexcitation. This collection process is highly nonlocal. It involves an intricate spatial pattern of photocurrent flow (streamlines) away from its primary photoexcitation that depends sensitively on the configuration of current collecting contacts as well as the spatial nonuniformity and tensor structure of conductivity.

Vibronic Exciton-Phonon States in Stack-Engineered van der Waals Heterojunction Photodiodes.

Stack engineering, an atomic-scale metamaterial strategy, enables the design of optical and electronic properties in van der Waals heterostructure devices. Here we reveal the optoelectronic effects of stacking-induced strong coupling between atomic motion and interlayer excitons in WSe/MoSe heterojunction photodiodes. To do so, we introduce the photocurrent spectroscopy of a stack-engineered photodiode as a sensitive technique for probing interlayer excitons, enabling access to vibronic states typically found only in molecule-like systems.

Quieting a noisy antenna reproduces photosynthetic light-harvesting spectra.

Photosynthesis achieves near unity light-harvesting quantum efficiency yet it remains unknown whether there exists a fundamental organizing principle giving rise to robust light harvesting in the presence of dynamic light conditions and noisy physiological environments. Here, we present a noise-canceling network model that relates noisy physiological conditions, power conversion efficiency, and the resulting absorption spectra of photosynthetic organisms. Using light conditions in full solar exposure, light filtered by oxygenic phototrophs, and light filtered under seawater, we derived optimal absorption characteristics for efficient solar power conversion.

Spin Seebeck Effect from Antiferromagnetic Magnons and Critical Spin Fluctuations in Epitaxial FeF_{2} Films.

We report a longitudinal spin Seebeck effect (SSE) study in epitaxially grown FeF_{2}(110) antiferromagnetic (AFM) thin films with strong uniaxial anisotropy over the temperature range of 3.8-250 K. Both the magnetic-field and temperature-dependent SSE signals below the Néel temperature (T_{N}=70  K) of the FeF_{2} films are consistent with a theoretical model based on the excitations of AFM magnons without any net induced static magnetic moment.

Hot carrier-enhanced interlayer electron-hole pair multiplication in 2D semiconductor heterostructure photocells.

Strong electronic interactions can result in novel particle-antiparticle (electron-hole, e-h) pair generation effects, which may be exploited to enhance the photoresponse of nanoscale optoelectronic devices. Highly efficient e-h pair multiplication has been demonstrated in several important nanoscale systems, including nanocrystal quantum dots, carbon nanotubes and graphene. The small Fermi velocity and nonlocal nature of the effective dielectric screening in ultrathin layers of transition-metal dichalcogenides (TMDs) indicates that e-h interactions are very strong, so high-efficiency generation of e-h pairs from hot electrons is expected.

Natural Regulation of Energy Flow in a Green Quantum Photocell.

Manipulating the flow of energy in nanoscale and molecular photonic devices is of both fundamental interest and central importance for applications in light energy harvesting optoelectronics. Under erratic solar irradiance conditions, unregulated power fluctuations in a light-harvesting photocell lead to inefficient energy storage in conventional solar cells and potentially fatal oxidative damage in photosynthesis. Here, we compare the theoretical minimum energy fluctuations in nanoscale quantum heat engine photocells that incorporate one or two photon-absorbing channels and show that fluctuations are naturally suppressed in the two-channel photocell.

Topological Winding Number Change and Broken Inversion Symmetry in a Hofstadter's Butterfly.

Graphene's quantum Hall features are associated with a π Berry's phase due to its odd topological pseudospin winding number. In nearly aligned graphene-hexagonal BN heterostructures, the lattice and orientation mismatch produce a superlattice potential, yielding secondary Dirac points in graphene's electronic spectrum, and under a magnetic field, a Hofstadter butterfly-like energy spectrum. Here we report an additional π Berry's phase shift when tuning the Fermi level past the secondary Dirac points, originating from a change in topological winding number from odd to even when the Fermi-surface electron orbit begins to enclose the secondary Dirac points.

Excitonic phases from Weyl semimetals.

Systems with strong spin-orbit coupling, which competes with other interactions and energy scales, offer a fertile playground to explore new correlated phases of matter. Weyl semimetals are an example where the phenomenon leads to a low-energy effective theory in terms of massless linearly dispersing fermions in three dimensions. In the absence of interactions chirality is a conserved quantum number, protecting the semimetallic physics against perturbations that are translationally invariant.

Screening of point charge impurities in highly anisotropic metals: application to mu+-spin relaxation in underdoped cuprate superconductors.

We calculate the screening charge density distribution due to a point charge, such as that of a positive muon (mu+), placed between the planes of a highly anisotropic layered metal. In underdoped hole cuprates the screening charge converts the charge density in the metallic-plane unit cells in the vicinity of the mu+ to nearly its value in the insulating state. The current-loop-ordered state observed by polarized neutron diffraction then vanishes in such cells, and also in nearby cells over a distance of order the intrinsic correlation length of the loop-ordered state.

Universality of single-particle spectra of cuprate superconductors.

All the available data for the dispersion and linewidth of the single-particle spectra above the superconducting gap and the pseudogap in metallic cuprates for any doping have universal features. The linewidth is linear in energy below a scale omega(c) and constant above. The cusp in the linewidth at omega(c) mandates, due to causality, a waterfall, i.

Theory of the quantum critical fluctuations in cuprate superconductors.

The statistical mechanics of the time-reversal and inversion symmetry breaking order parameter, possibly observed in the pseudogap region of the phase diagram of the cuprates, can be represented by the Ashkin-Teller model. We add kinetic energy and dissipation to the model for a quantum generalization and show that the spectrum of the quantum-critical fluctuations is of the form postulated in 1989 to give the marginal Fermi-liquid properties. The model solved and the methods devised are likely to be of interest also to other quantum phase transitions.