Full momentum- and energy-resolved spectral function of a 2D electronic system

Probing Electronic Band Structure of Monolayer MoS2 in Gate-Controlled Resonant Tunneling Diodes

Experimental determination of band structures of monolayer transition metal dichalcogenides (TMDCs) is crucially important in the design and tailoring of the properties of TMDCs. Resonant tunneling spectroscopy (RTS) is an effective technique to probe the band structures of low-dimensional systems by measuring the density of states (DOS) and energy dispersions. Here, we report the investigation of the band structure of monolayer MoS2 (ML-MoS2) in a gate-controlled resonant tunneling diode. Three distinct resonant tunneling kinks are observed in the characteristic current-voltage curves at 0.47, 0.70, and 0.81 V, respectively, which correspond to the conduction band local minimum of ML-MoS2 at K, Q1, and Q2 points. When applying a large positive gate voltage to enhance ML-MoS2 conductivity, the three resonant kinks shift to lower bias at 0.10, 0.32, and 0.39 V, respectively, which is in excellent agreement with the theoretical calculations. Our work offers an effective and more precise way to explore the electronic band structures of TMDCs using RTS.

Anisotropic Resonant Tunneling in Twist-Stacked van der Waals Heterostructure

Resonant tunneling, with energy and momentum conservation, has been extensively studied in two-dimensional van der Waals heterostructures and has potential applications in band structure probing, multivalued logic, and oscillators. Lattice alignment is crucial in resonant tunneling transistors (RTTs) for achieving negative differential resistance (NDR) with a high peak-to-valley ratio (PVR) because twist-angle-induced momentum mismatch can break the resonant tunneling condition. Here, we report anisotropic resonant tunneling in twist-stacked ReSe2/h-BN/ReSe2 RTTs, where the PVR exhibits a strong dependence on the twist angle between the two ReSe2 layers, reaching a maximum at the twist angle of 102°. Theoretical calculations suggest that the twist angle modulates the joint density of states of the two anisotropic bands in ReSe2 layers during the tunneling process, significantly suppressing the valley current and thereby enhancing the PVR. Double NDR peaks were observed in twist-stacked RTTs, which are attributed to interband resonant tunneling. Moreover, our twist-stacked RTTs are utilized in multibit inverters and adjustable self-powered photodetectors, providing potentials for the design of high-performance RTTs and photodetectors via twist-stacked engineering.

Phonon-mediated ultrafast energy- and momentum-resolved hole dynamics in monolayer black phosphorus

The electron-phonon scattering plays a crucial role in determining the electronic, transport, optical, and thermal properties of materials. Here, we employ a non-Markovian stochastic Schrödinger equation (NMSSE) in momentum space, together with ab initio calculations for energy bands and electron-phonon interactions, to reveal the phonon-mediated ultrafast hole relaxation dynamics in the valence bands of monolayer black phosphorus. Our numerical simulations show that the hole can initially remain in the high-energy valence bands for more than 100 fs due to the weak interband scatterings, and its energy relaxation follows single-exponential decay toward the valence band maximum after scattering into low-energy valence bands. The total relaxation time of holes is much longer than that of electrons in the conduction band. This suggests that harnessing the excess energy of holes may be more effective than that of electrons. Compared to the semiclassical Boltzmann equation based on a hopping model, the NMSSE highlights the persistence of quantum coherence for a long time, which significantly impacts the relaxation dynamics. These findings complement the understanding of hot carrier relaxation dynamics in two-dimensional materials and may offer novel insights into harnessing hole energy in photocatalysis.

Time, momentum, and energy resolved pump-probe tunneling spectroscopy of two-dimensional electron systems

Real-time probing of electrons can uncover intricate relaxation mechanisms and many-body interactions in strongly correlated materials. Here, we introduce time, momentum, and energy resolved pump-probe tunneling spectroscopy (Tr-MERTS). The method allows the injection of electrons at a particular energy and observation of their subsequent decay in energy-momentum space. Using Tr-MERTS, we visualize electronic decay processes, with lifetimes from tens of nanoseconds to tens of microseconds, in Landau levels formed in a GaAs quantum well. Although most observed features agree with simple energy-relaxation, we discovered a splitting in the nonequilibrium energy spectrum in the vicinity of a ferromagnetic state. An exact diagonalization study suggests that the splitting arises from a maximally spin-polarized state with higher energy than a conventional equilibrium skyrmion. Furthermore, we observe time-dependent relaxation of the splitting, which we attribute to single-flipped spins forming skyrmions. These results establish Tr-MERTS as a powerful tool for studying the properties of a 2DES beyond equilibrium.

Phonon-Mediated Ultrafast Electron Relaxation Dynamics in Monolayer Black Phosphorus: Instantaneous Coherent Delocalization

Understanding carrier relaxation processes in semiconductors is crucial for designing high-performance optoelectronic and photocatalytic devices. Recent transient spectroscopic experiments on two-dimensional materials have revealed ultrafast optical responses within several tens of femtoseconds, which are usually ascribed to electron-electron scattering. Here, by conducting quantum dynamics simulations for monolayer black phosphorus, we show that electron-phonon scattering also profoundly influences the early stage of carrier dynamics. The photogenerated electron generally undergoes phonon-mediated instantaneous coherent delocalization in reciprocal space, accompanied by an entropy-driven sharp change in electronic energy. The distribution of the density of states controls the energy exchange between the electron and lattice vibrations. The phonon-induced quantum coherence significantly suspends the energy relaxation time, which is very beneficial for harvesting electron excess energy. These findings offer novel insights into the ultrafast carrier dynamics and energy flow in two-dimensional materials and may prompt new opportunities for regulation of carrier dynamic behaviors.

Non-Fermi-Liquid Behavior from Cavity Electromagnetic Vacuum Fluctuations at the Superradiant Transition

We study two-dimensional materials where electrons are coupled to the vacuum electromagnetic field of a cavity. We show that, at the onset of the superradiant phase transition towards a macroscopic photon occupation of the cavity, the critical electromagnetic fluctuations, consisting of photons strongly overdamped by their interaction with electrons, can in turn lead to the absence of electronic quasiparticles. Since transverse photons couple to the electronic current, the appearance of non-Fermi-Liquid behavior strongly depends on the lattice. In particular, we find that in a square lattice the phase space for electron-photon scattering is reduced in such a way to preserve the quasiparticles, while in a honeycomb lattice the latter are removed due to a nonanalytical frequency dependence of the damping ∝|ω|^{2/3}. Standard cavity probes could allow us to measure the characteristic frequency spectrum of the overdamped critical electromagnetic modes responsible for the non-Fermi-liquid behavior.

The quantum twisting microscope

The invention of scanning probe microscopy revolutionized the way electronic phenomena are visualized¹. Whereas present-day probes can access a variety of electronic properties at a single location in space², a scanning microscope that can directly probe the quantum mechanical existence of an electron at several locations would provide direct access to key quantum properties of electronic systems, so far unreachable. Here, we demonstrate a conceptually new type of scanning probe microscope-the quantum twisting microscope (QTM)-capable of performing local interference experiments at its tip. The QTM is based on a unique van der Waals tip, allowing the creation of pristine two-dimensional junctions, which provide a multitude of coherently interfering paths for an electron to tunnel into a sample. With the addition of a continuously scanned twist angle between the tip and sample, this microscope probes electrons along a line in momentum space similar to how a scanning tunnelling microscope probes electrons along a line in real space. Through a series of experiments, we demonstrate room-temperature quantum coherence at the tip, study the twist angle evolution of twisted bilayer graphene, directly image the energy bands of monolayer and twisted bilayer graphene and, finally, apply large local pressures while visualizing the gradual flattening of the low-energy band of twisted bilayer graphene. The QTM opens the way for new classes of experiments on quantum materials.

Resonant Tunneling between Quantized Subbands in van der Waals Double Quantum Well Structure Based on Few-Layer WSe2

We demonstrate van der Waals double quantum well (vDQW) devices based on few-layer WSe₂ quantum wells and a few-layer h-BN tunnel barrier. Due to the strong out-of-plane confinement, an exfoliated WSe₂ exhibits quantized subband states at the Γ point in its valence band. Here, we report resonant tunneling and negative differential resistance in vDQW at room temperature owing to momentum- and energy-conserved tunneling between the quantized subbands in each well. Compared to single quantum well (QW) devices with only one QW layer possessing quantized subbands, superior current peak-to-valley ratios were obtained for the DQWs. Our findings suggest a new direction for utilizing few-layer-thick transition metal dichalcogenides in subband QW devices, bridging the gap between two-dimensional materials and state-of-the-art semiconductor QW electronics.

Berry Curvature Effects on Quasiparticle Dynamics in Superconductors

We construct a theory for the semiclassical dynamics of superconducting quasiparticles by following their wave packet motion and reveal rich contents of Berry curvature effects in the phase space spanned by position and momentum. These Berry curvatures are traced back to the characteristics of superconductivity, including the nontrivial momentum-space geometry of superconducting pairing, the real-space supercurrent, and the charge dipole of quasiparticles. The Berry-curvature effects strongly influence the spectroscopic and transport properties of superconductors, such as the local density of states and the thermal Hall conductivity. As a model illustration, we apply the theory to study the twisted bilayer graphene with a d_{x^{2}+y^{2}}+id_{xy} superconducting gap function and demonstrate Berry-curvature induced effects.

Room temperature strain-induced Landau levels in graphene on a wafer-scale platform

Graphene is a powerful playground for studying a plethora of quantum phenomena. One of the remarkable properties of graphene arises when it is strained in particular geometries and the electrons behave as if they were under the influence of a magnetic field. Previously, these strain-induced pseudomagnetic fields have been explored on the nano- and micrometer-scale using scanning probe and transport measurements. Heteroepitaxial strain, in contrast, is a wafer-scale engineering method. Here, we show that pseudomagnetic fields can be generated in graphene through wafer-scale epitaxial growth. Shallow triangular nanoprisms in the SiC substrate generate strain-induced uniform fields of 41 T, enabling the observation of strain-induced Landau levels at room temperature, as detected by angle-resolved photoemission spectroscopy, and confirmed by model calculations and scanning tunneling microscopy measurements. Our work demonstrates the feasibility of exploiting strain-induced quantum phases in two-dimensional Dirac materials on a wafer-scale platform, opening the field to new applications.