Electron Quantum Research Articles

Ring Polymer Molecular Dynamics with Electronic Transitions.

Full quantum dynamics of molecules and materials is of fundamental importance, which requires a faithful description of simultaneous quantum motions of the electron and nuclei. A new scheme is developed for nonadiabatic simulations of coupled electron-nuclear quantum dynamics with electronic transitions based on the Ehrenfest theorem and ring polymer molecular dynamics. Built upon the isomorphic ring polymer Hamiltonian, time-dependent multistate electronic Schrödinger equations are solved self-consistently with approximate equation of motions for nuclei. Each bead bears a distinct electronic configuration and thus moves on a specific effective potential. This independent-bead approach provides an accurate description of the real-time electronic population and quantum nuclear trajectory, maintaining a good agreement with the exact quantum solution. Implementation of first-principles calculations enables us to simulate photoinduced proton transfer in H_{2}O-H_{2}O^{+} where we find a good agreement with experiment.

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs.

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches.

Spin-Liquid Insulators Can Be Landau's Fermi Liquids.

The long search for insulating materials that possess low-energy quasiparticles carrying electron's quantum numbers except charge-inspired by the neutral spin-1/2 excitations, the so-called spinons, exhibited by Anderson's resonating-valence-bond state-seems to have reached a turning point after the discovery of several Mott insulators displaying the same thermal and magnetic properties as metals, including quantum oscillations in a magnetic field. Here, we show that such anomalous behavior is not inconsistent with Landau's Fermi liquid theory of quasiparticles at a Luttinger surface. That is the manifold of zeros within the Brillouin zone of the single-particle Green's function at zero frequency, and which thus defines the spinon Fermi surface conjectured by Anderson.

Study on Electron Tunneling Lifetime by Projected Green’s Function Approach in Single Quantum Well Semiconductor

The tunnelling lifetime of an electron in a single quantum well subject to an externally applied electric field is calculated using the Projected Green’s Function (PGF) approach. The lifetime in the biased single well decreases as a function of the electric field in good agreement with the previous result. The tunnelling lifetime is revealed with the behavior of the ground-state shift and the wave amplitude in the first well. As the electric field increases, the ground state energy shifts, leading to lessened constructive interference between the initial and reflected electron waves. Consequently, the study is essential to characterize the performance of high-speed quantum devices.

Topological thermoelectrics: New opportunities and challenges

Due to the great application potential of the efficient interconversion between heat and electricity, thermoelectric materials have long been the research focal points in condensed matter physics and material science. Moreover, since the discovery of topological properties in solid materials, numerous research studies have recently expanded into the role of the topological state in thermoelectrics. Topological insulators and semimetals/metals are characterized by their exotic electronic band structures and quantum surface states, which can provide a new degree of freedom to manipulate thermoelectric properties. Herein, we present a summary on the advantageous combination of topological properties in thermoelectric materials for enhancing the thermoelectric energy conversion efficiency. We demonstrated this concept through some examples on the applications of novel topological materials as a practical strategy towards high thermoelectric performance. Furthermore, the role of different topological states and the corresponding working mechanism is explained, and the related challenges for further development are also discussed. This review article presents an advanced understanding of topological states in a thermoelectric transport mechanism, providing a new strategy to search for outstanding thermoelectric materials.

Molecular Bridge Engineering for Tuning Quantum Electronic Transport and Anisotropy in Nanoporous Graphene

Recent advances on surface-assisted synthesis have demonstrated that arrays of nanometer wide graphene nanoribbons can be laterally coupled with atomic precision to give rise to a highly anisotropic nanoporous graphene structure. Electronically, this graphene nanoarchitecture can be conceived as a set of weakly coupled semiconducting 1D nanochannels with electron propagation characterized by substantial interchannel quantum interferences. Here, we report the synthesis of a new nanoporous graphene structure where the interribbon electronic coupling can be controlled by the different degrees of freedom provided by phenylene bridges that couple the conducting channels. This versatility arises from the multiplicity of phenylene cross-coupling configurations, which provides a robust chemical knob, and from the interphenyl twist angle that acts as a fine-tunable knob. The twist angle is significantly altered by the interaction with the substrate, as confirmed by a combined bond-resolved scanning tunneling microscopy (STM) and ab initio analysis, and should accordingly be addressable by other external stimuli. Electron propagation simulations demonstrate the capability of either switching on/off or modulating the interribbon coupling by the corresponding use of the chemical or the conformational knob. Molecular bridges therefore emerge as efficient tools to engineer quantum transport and anisotropy in carbon-based 2D nanoarchitectures.

Quantum-quasiclassical analysis of center-of-mass nonseparability in hydrogen atom stimulated by strong laser fields *

We have developed a quantum-quasiclassical computational scheme for quantitative treating of the nonseparable quantum–classical dynamics of the 6D hydrogen atom in a strong laser pulse. In this approach, the electron is treated quantum mechanically and the center-of-mass (CM) motion classically. Thus, the Schrödinger equation for the electron and the classical Hamilton equations for the CM variables, nonseparable due to relativistic effects stimulated by strong laser fields, are integrated simultaneously. In this approach, it is natural to investigate the idea of using the CM- velocity spectroscopy as a classical ‘build-up’ set up for detecting the internal electron quantum dynamics. We have performed such an analysis using the hydrogen atom in linearly polarized laser fields as an example and found a strong correlation between the CM kinetic energy distribution after a laser pulse and the spectral density of electron kinetic energy. This shows that it is possible to detect the quantum dynamics of an electron by measuring the distribution of the CM kinetic energy.

Ultra-low-current driven InGaN blue micro light-emitting diodes for electrically efficient and self-heating relaxed microdisplay

InGaN-based micro-light-emitting diodes have a strong potential as a crucial building block for next-generation displays. However, small-size pixels suffer from efficiency degradations, which increase the power consumption of the display. We demonstrate strategies for epitaxial structure engineering carefully considering the quantum barrier layer and electron blocking layer to alleviate efficiency degradations in low current injection regime by reducing the lateral diffusion of injected carriers via reducing the tunneling rate of electrons through the barrier layer and balanced carrier injection. As a result, the fabricated micro-light-emitting diodes show a high external quantum efficiency of 3.00% at 0.1 A/cm2 for the pixel size of 10 × 10 μm2 and a negligible Jmax EQE shift during size reduction, which is challenging due to the non-radiative recombination at the sidewall. Furthermore, we verify that our epitaxy strategies can result in the relaxation of self-heating of the micro-light-emitting diodes, where the average pixel temperature was effectively reduced.

Electron-Beam Source with a Superconducting Niobium Tip

Electron-beam sources are foundational in high-resolution electron microscopy and spectroscopy, but applications have been limited due to their relatively large energy spread. The authors fabricate a monocrystalline niobium nanotip electron field emitter and characterize it in superconducting and normal-conducting regimes. This bright, stable, coherent electron beam source features an exceptionally narrow energy spread. The authors also study the role of xenon adsorption and two-electron correlations. This work may improve aberration-corrected microscopy and electron energy-loss spectroscopy and enable high-resolution vibrational spectroscopy or quantum electron microscopy.

Lateral quantum confinement regulates charge carrier transfer and biexciton interaction in CdSe/CdSeS core/crown nanoplatelets

Charge carrier dynamics essentially determine the performance of various optoelectronic applications of colloidal semiconductor nanocrystals. Among them, two-dimensional nanoplatelets provide new adjustment freedom for their unique core/crown heterostructure. Herein, we demonstrate that by fine-tuning the core size and the lateral quantum confinement, the charge carrier transfer rate from the crown to the core can be varied by one order of magnitude in CdSe/CdSeS core/alloy-crown nanoplatelets. In addition, the transfer can be affected by a carrier blocking mechanism, i.e., the filled carriers hinder further possible transfer. Furthermore, we found that the biexciton interaction is oppositely affected by quantum confinement and electron delocalization, resulting in a non-monotonic variation of the biexciton binding energy with the emission wavelength. This work provides new observations and insights into the charge carrier transfer dynamics and exciton interactions in colloidal nanoplatelets and will promote their further applications in lasing, display, sensing, etc.

Compact localized boundary states in a quasi-1D electronic diamond-necklace chain

Zero-energy modes localized at the ends of one-dimensional (1D) wires hold great potential as qubits for fault-tolerant quantum computing. However, all the candidates known to date exhibit a wave function that decays exponentially into the bulk and hybridizes with other nearby zero-modes, thus hampering their use for braiding operations. Here, we show that a quasi-1D diamond-necklace chain exhibits an unforeseen type of robust boundary state, namely compact localized zero-energy modes that do not decay into the bulk. We find that this state emerges due to the presence of a latent symmetry in the system. We experimentally realize the diamond-necklace chain in an electronic quantum simulator setup.

Quantum-Path-Resolved Attosecond High-Harmonic Spectroscopy.

Strong-field ionization of molecules releases electrons which can be accelerated and driven back to recombine with their parent ion, emitting high-order harmonics. This ionization also initiates attosecond electronic and vibrational dynamics in the ion, evolving during the electron travel in the continuum. Revealing this subcycle dynamics from the emitted radiation usually requires advanced theoretical modeling. We show that this can be avoided by resolving the emission from two families of electronic quantum paths in the generation process. The corresponding electrons have the same kinetic energy, and thus the same structural sensitivity, but differ by the travel time between ionization and recombination-the pump-probe delay in this attosecond self-probing scheme. We measure the harmonic amplitude and phase in aligned CO_{2} and N_{2} molecules and observe a strong influence of laser-induced dynamics on two characteristic spectroscopic features: a shape resonance and multichannel interference. This quantum-path-resolved spectroscopy thus opens wide prospects for the investigation of ultrafast ionic dynamics, such as charge migration.

Bridge-Mediated Metal-to-Metal Electron and Hole Transfer in a Supermolecular Dinuclear Complex: A Computational Study Using Quantum Electron-Nuclear Dynamics.

Bimetallic electron donor-acceptor complexes can facilitate electron and energy transfer with excellent structural control through synthetic design. In this work, we investigate the photochemical dynamics in a Ru-Cu bimetallic complex after photoexcitation of the Ru-centered charge transfer state. The physical underpinnings of the metal-to-metal directional charge transfer process are unraveled via analyses of the quantum electronic dynamics and electron-nuclear trajectories. The effects of molecular vibrations in the photoexcited state on the charge transfer processes are also analyzed.

Density-wave tendency from a topological nodal-line perspective

The understanding of density waves is a vital component of our insight into electronic quantum matters. Here, we propose an additional mosaic to the existing mechanisms such as Fermi-surface nesting, electron–phonon coupling, and exciton condensation. In particular, we find that certain two-dimensional (2D) spin density-wave systems are equivalent to three-dimensional (3D) Dirac nodal-line systems in the presence of a magnetic field, whose electronic structure takes the form of Dirac-fermion Landau levels and allows a straightforward analysis of its optimal filling. The subsequent minimum-energy wave vector varies over a continuous range and shows no direct connection to the original Fermi surfaces in 2D. Also, we carry out numerical calculations where the results on model examples support our theory. Our study points out that we have yet to attain a complete understanding of the emergent density wave formalism.

Interdependence of External Magnetic and Temperature Effect on Thermodynamic Properties of GaAs Two Electron Quantum Dot

The particular interest of this paper is to investigate the impact of various values of temperature exposed to weak and strong magnetic field strength. A thermodynamic property's oscillatory change as a function of magnetic field effect (B) intensifies the quantization of electron orbits in a constant magnetic field intensity and is the primary contributor to the de Haas-van Alphen effects due to cyclotron frequency and its impact on localizing electron at circular region imposed with the magnetic field that is in contrary to the result of the temperature effect. Thus the interdependent effects of external magnetic field and temperature on thermodynamic properties are studied with harmonic oscillator potentials considering material parameters of GaAs quantum dot. The finite energy state is analytically solved using Nikiforov-Uvarov mathematical formalism. Moreover, the direct impact of the external magnetic fields and temperature on thermodynamic properties of the system is analyzed, and numerically simulated using matlab R2017a version. The dominance of temperature over the external magnetic field and vice versa effect is investigated, thus the value specific heat capacity fluctuated, while the equiponderate impact of temperature and magnetic field shows similar steady values of the specific heat capacity. The study clearly shows the interdependence of magnetic field and temperature affect thermodynamic quantities: partition function, mean energy, entropy, and specific heat capacity.

Gate-controlled hysteresis curves and dual-channel conductivity in an undoped Si/SiGe 2DEG structure

Exploring the cryogenic transport properties of two-dimensional electron gas in semiconductor heterostructures is always a focus of fundamental research on Si-based gate-controlled quantum devices. In this work, based on the electrical and magnetic transport characteristics of Si/SiGe heterostructure Hall bar-shaped field effect transistors (FETs) at 10 K and 1.6 K, we study the effects of electron tunneling, which occurs in the heterostructure and populates the oxide/semiconductor interface, on its transport properties. The initial position of dual-channel conduction is determined by the gate-controlled electrical hysteresis curves. Furthermore, we discover that there exist different tunneling mechanisms of electrons in a Si quantum well under the action of gate voltage, and the electron tunneling can well explain the two drain current plateaus appearing in direct-current transfer characteristics. Combining the power-law exponent of electron mobility versus density curve and the gate-related Dingle ratio, we clarify the dominant scattering mechanism, and the result can be supported by different tunneling mechanisms. Our work demonstrates the gate-dependent electronic transport performance in undoped Si/SiGe heterostructure FETs, which has an implication for the development of Si/SiGe heterostructure gate-defined quantum dot quantum computation.

Photocurrent as a multiphysics diagnostic of quantum materials

The photoexcitation life cycle from incident photon (and creation of photoexcited electron–hole pair) to ultimate extraction of electrical current is a complex multiphysics process spanning across a range of spatiotemporal scales of quantum materials. Photocurrent is sensitive to a myriad of physical processes across these spatiotemporal scales, and over the past decade it has emerged as a versatile probe of electronic states, Bloch band quantum geometry, quantum kinetic processes and device characteristics of quantum materials. This Technical Review outlines the key multiphysics principles of photocurrent diagnostics, for resolving band structure and characterizing topological materials, for disentangling distinct types of carrier scattering that can range from femtosecond to nanosecond timescales and for enabling new types of remote-sensing protocols and photocurrent nanoscopy. These distinctive capabilities underscore photocurrent diagnostics as a novel multiphysics probe for a growing class of quantum materials with properties governed by physics spanning multiple spatiotemporal scales. Photocurrent can be used to reveal the out-of-equilibrium properties of quantum materials over a range of spatiotemporal scales. This Technical Review outlines the principles of photocurrent diagnostics and how it can be used to probe electronic states, quantum geometry and quantum kinetics of materials.

Magnetic and Electronic Properties of Edge-Modified Triangular WS2 and MoS2 Quantum Dots

The magnetic and electronic properties of zigzag-triangular WS2 and MoS2 quantum dots are investigated using density functional theory calculations. The pristine WS2 and MoS2 nanodots hold permanent spin on their edges which originates from the unpaired electrons of the transition metals at the edges. The ferromagnetic spin ordering in zigzag-triangular WS2 and MoS2 can be transformed to antiferromagnetic ordering with S = 0 and to nonmagnetic, respectively, by edge passivation with 2H. The calculations of the Curie Temperature indicate that these magnetic states are stable and withstand room temperature. The paramagnetic susceptibility of these structures significantly decreases by edge sulfuration. Moreover, it can be converted to diamagnetic susceptibility by edge passivation with 2H as found in WS2 nanodots. These structures are semiconductors with energy gaps of ~3.3 eV that decrease unexpectedly by edge passivation due to the existence of lone pairs from S atoms that give a high contribution to the low-energy molecular orbitals. With these preferable magnetic properties and controlled electronic ones, WS2 and MoS2 quantum dots are potential candidates for spintronic applications.

Role of various scattering mechanisms on the resistivity of Pr-doped high-Tc cuprate superconductor YBa2Cu3O7–δ

In this paper, we theoretically studied the electrical resistivity of the heavily Pr-doped YBa2Cu3O7−δ single crystal high-Tc superconductor cuprate. The proposed formulation expands the concept of relaxation times from frequency (energy) line widths for multiple scattering processes by leveraging electron quantum dynamics with the aid of the double-time electron Green function. A thorough analysis was carried out to investigate the effect of dopant-atom scattering as well as magnon scattering on the temperature and reduced frequency dependences of electrical resistivity in the temperature range of 20–300 K. An acceptable level of agreement between theory and experiment has been achieved.

Universal conductance fluctuations in nanoscale topological insulator devices

Topological materials are promising candidates in fault-tolerant quantum information processing architectures, making it essential to understand the dephasing mechanisms in these materials. Here, we investigate gated, nanoscale mesas fabricated on thin films of cadmium arsenide (Cd3As2), a three-dimensional Dirac semimetal that can be tuned into different topological phases. We observe two independent types of conductance oscillations, one as a function of the applied magnetic field and the other as a function of the gate voltage. Varying the dimensions of the nanostructures allows the discrimination of a variety of scenarios for similar oscillations previously reported in the literature. We conclude that the conductance oscillations are not a signature of topological boundary states per se, but rather are universal conductance fluctuations. These results broadly inform future interpretations of electronic quantum interference in mesoscopic devices made from topological materials.