Confinement Potential Research Articles

Direct Visualization and Manipulation of Tunable Quantum Well State in Semiconducting Nb2SiTe4.

Quantum well states (QWSs) can form at the surface or interfaces of materials with confinement potential. They have broad applications in electronic and optical devices such as high mobility electron transistor, photodetector, and quantum well laser. The properties of the QWSs are usually the key factors for the performance of the devices. However, direct visualization and manipulation of such states are, in general, challenging. In this work, by using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy/spectroscopy (STM/STS), we directly probe the QWSs generated on the vacuum interface of a narrow band gap semiconductor Nb2SiTe4. Interestingly, the position and splitting of QWSs could be easily manipulated via potassium (K) dosage onto the sample surface. Our results suggest Nb2SiTe4 to be an intriguing semiconductor system to study and engineer the QWSs, which has great potential in device applications.

Single-electron pump with highly controllable plateaus

Future quantum based electronic systems will demand robust and highly accurate on-demand sources of current. The ultimate limit of quantized current sources is a highly controllable device that manipulates individual electrons. We present a GaAs single-electron pump, where electrons are pumped through a one-dimensional split-gate saddle point confinement potential, which show quantized plateaus with length and width that can be independently tuned with the application of a source-drain bias and RF amplitude. The plateaus can be over two orders of magnitude longer than conventional pumps, and flatness improves with the application of a source-drain bias.

Taming Faraday waves in binary fermionic clouds: The effect of Zeeman interaction

This work presents a study of the Faraday instability in a parametrically forced Fermi-Fermi mixture. The condensate is confined in the transversal spatial dimension with a strong parametric confinement potential and in the longitudinal spatial dimension with a weaker potential. The theoretical description is done using the mean-field theory with two amplitude equations that represent each spin state. In order to stabilize the Faraday patterns, a phenomenological damping term is introduced. The influence of the Zeeman interaction is analyzed in detail. In particular, phase diagrams of the existence and stability of the Faraday waves are calculated as a function of the Zeeman interaction, the coupling parameter, and the forcing amplitude. The degree of segregation of the two fields and their synchronization level is also calculated as a function of the Zeeman parameter. In addition, we examine how the pattern wavelength varies as a function of the Zeeman parameter and the forcing frequency.

Effects of laser field and electric field on exciton states in InAs/InxAl1-xAs quantum wells

Based on the effective mass approximation and envelope function theory, the exciton binding energy in a single InAs / InxAl1 − xAs quantum well is calculated by the variational method under the applied electric field and laser field. The effects of material composition, well width, laser parameters, and electric field intensity on the exciton binding energy are calculated. The results show that the confinement potential is distorted by the laser field and the barrier is tilted by the electric field. The exciton binding energy is reduced by the presence of laser field or electric field. It is also shown that the exciton binding energy increases with the decrease of the indium component within a certain range. In addition, the effect of the well width on the exciton binding energy is not linear, and the exciton binding energy increases to a maximum first and then decreases with the decrease of the well width. The binding energy of excitons depends on the interaction of several parameters.

Magnetocaloric effect, magnetic susceptibility and specific heat of tuned quantum dot/ring systems

In this paper, we have considered a GaAs quantum dot/quantum ring with a parabolic-inverse square confinement potential in addition to modified Gaussian potential in the presence of a perpendicular magnetic field. The energy levels were first determined and then the behavior of specific heat, magnetic susceptibility and magnetic entropy change of the system were investigated. We have presented the effect of magnetic field and the system parameters on temperature dependence of specific heat and magnetic susceptibility over wide temperature ranges. It is found that the magnetic susceptibility shows both positive and negative values with using different parameters. Also, the temperature dependence of the magnetic entropy change ( Δ S ) (magnetocaloric effect) shows a pronounced minimum at low temperatures. • A GaAs quantum dot/quantum ring with a parabolic-inverse square confinement potential in the presence of a magnetic field. • Investigating the behavior of specific heat, magnetic susceptibility and magnetic entropy change. • The magnetic susceptibility shows both positive and negative values with using different parameters.

Optical properties of a quantum well with Razavy confinement potential: Role of applied external fields

In this present work, the effect of applied external fields and well width on the optical absorption and refractive index change coefficient of a GaAs quantum well with Razavy confinement potential is theoretically presented. For this, firstly, the confined subband energy levels of the structure and the envelope wave functions corresponding to these states have been computed using the diagonalization method within the framework of effective mass and parabolic band approximation. Then, by using this information, the coefficients of total – linear plus nonlinear – intersubband optical absorption (TOAC) and relative refractive index changes (RRIC) of the structure are evaluated. The obtained numerical results show that the increase in the magnitude of the applied external fields creates a blue shift in TOAC and RRIC. Also, contrarily to the applied external fields, an increment of the quantum well width have induced a red shift in the TOAC and RRIC. We discuss in detail the variation of the diagonal matrix elements and the energy difference (E2–E1) which are responsible for the change of amplitudes of the calculated coefficients. We believe that the obtained numerical results can be useful in the design and application of the next generation devices used in the optoelectronics field. • The magnitude of the TOACs peak decrease with the static electric field. • The magnitude of the TOACs peak increase with the ILF. • The magnitude of the RRICs decrease with the ILF. • The increase of ILF leads to important effects on the optical properties of Razavy QW.

Interactions and non-magnetic fractional quantization in one-dimension.

In this Perspective article, we present recent developments on interaction effects on the carrier transport properties of one-dimensional (1D) semiconductor quantum wires fabricated using the GaAs/AlGaAs system, particularly the emergence of the long predicted fractional quantization of conductance in the absence of a magnetic field. Over three decades ago, it was shown that transport through a 1D system leads to integer quantized conductance given by N·2e2/h, where N is the number of allowed energy levels (N = 1, 2, 3, …). Recent experiments have shown that a weaker confinement potential and low carrier concentration provide a testbed for electrons strongly interacting. The consequence leads to a reconfiguration of the electron distribution into a zigzag assembly which, unexpectedly, was found to exhibit quantization of conductance predominantly at 1/6, 2/5, 1/4, and 1/2 in units of e2/h. These fractional states may appear similar to the fractional states seen in the Fractional Quantum Hall Effect; however, the system does not possess a filling factor and they differ in the nature of their physical causes. The states may have promise for the emergent topological quantum computing schemes as they are controllable by gate voltages with a distinct identity.

Adiabatic Switching Among Quantum Dot Eigenstates: Role of Anharmonicity and Gaussian White Noise

Current study inquires the time‐dependent quantum adiabatic switching (TDQAS) among the quantum dot (QD) eigenstates under the aegis of Gaussian white noise (GWN) with special emphasis on anharmonicity that may be present in the QD potential. The initial and the final eigenstates are distinguished by different values of magnetic field strength, confinement potential, anisotropy, and anharmonicity. The QAS has been conducted using four different switching functions (SFs), e.g., square, exponential, sinusoidal, and logarithmic. The time development of switching has been monitored with the help of overlap function and quantum information entropy (QIE). The SFs appear to recover the eigenstates of QD corresponding to the final Hamiltonian. The switching paths comprise features such as enhancement, depletion, maximization, minimization, and crossover of the overlap function and entropy. These characteristics of switching paths depend on presence/absence of noise, its pathway of application (additive/multiplicative), and symmetry (odd/even) of the anharmonic potential. The dependence on the type of SF used, however, has been found to be not much significant unless the exclusive role of noise strength on switching path is observed. The study merits importance in view of immense technological applications of QD‐based systems where the noise‐anharmonicity interplay may be exploited to a great extent.

Effects of applied external fields on the nonlinear optical rectification, second, and third harmonic generation in a quantum well with exponentially confinement potential

Effects of applied external fields on the nonlinear optical rectification, second, and third harmonic generation in a quantum well with exponentially confinement potential

Molecular Confinement Effects by Self-Assembled Coordination Cages

Abstract When substrates are confined in an isolated cavity, they experience circumstances that are distinctly different from those in a bulk solution. Molecular self-assembly has widened the potential of molecular confinement by offering synthetic cavities on the nanometer-scale and allowing chemists to treat molecular aggregates and larger molecules in the cavities. In this account, we introduce the molecular confinement effects of self-assembled cages as a strategy to discover new or hidden properties and reactivities from the confined substrates in the cages. By confining molecules, the cavity can gather, arrange, fold, compress, and twist the molecules. The molecular confinement thus becomes a powerful strategy to draw new aspects of molecules.

Comparison of the contribution of different types of interface-optical phonon modes to electron–phonon scattering in a quasi-two-dimensional system

Utilizing the operator projection technique, we theoretically study the contribution of different types of optical phonon modes to electron–phonon scattering in a quasi-two-dimensional system through calculating the expression for the magnetophonon resonance absorption power (MPR-AP) caused by scattering between the electrons with interface-optical phonons in parabolic quantum wells (PQWs). Electron interactions with different modes of interface-optical-phonons include the symmetric and antisymmetric modes in both the barrier and well layers together are considered in detail. The full width at half maximum (FWHM) of the resonant peaks due to different interface-optical-phonon modes was measured by using the profile method. Numerical results show that the strength of electron–interface-phonon interaction increases with confining frequency of parabolic confinement potential, temperature, and magnetic field under all antisymmetric- and symmetric-modes in both well and barrier layers. The electron-interaction with symmetric-interface-phonon has stronger strength compared to antisymmetric-interface-phonon in all well and barrier layers. The strongest-electron-scattering for symmetric phonon in well layer, meanwhile weakest that for antisymmetric phonon in barrier layer. At higher value of the photon frequency, the contributions of interface-optical-phonons is not significant. Our present calculations well accord with previous experimental and theoretical studies.

Ultrafast Plasmon Thermalization in Epitaxial Graphene Probed by Time‐Resolved THz Spectroscopy

AbstractThe control of carrier transport by electrical, chemical, or optical Fermi level tuning is central to graphene electronics. Here, an optical pump—terahertz (THz) probe spectroscopy—is applied to investigate ultrafast sheet conductivity dynamics in various epitaxially grown graphene layers representing a large variety of carbon allotropes, including H2 intercalated films. The graphene layers display a prominent plasmonic response connected with induced THz transparency spectra on ultrashort timescale. It is generally believed that the plasmonic excitations appear due to wrinkles, and substrate terraces that bring about natural confinement potentials. It is shown that these potentials act within micrometer‐sized domains with essentially isotropic character. The measured ultrafast dynamics are entirely controlled by the quasi‐Fermi level of laser‐excited carriers through their temperature. The photocarriers undergo a disorder‐enabled super‐collision cooling process with an initial picosecond transfer of the optically deposited heat to the lattice followed by a sub‐nanosecond relaxation governed by the lattice cooling. The transient spectra is described by a two‐temperature Drude‐Lorentz model revealing the ultrafast evolution of the carrier temperature and chemical potential and providing crucial material parameters such as Fermi energy, carrier mobility, carrier confinement length, and disorder mean free path.

Ultra-wide frequency tuning range for optical properties in coaxial quantum well driven by intense laser field

A new approach for controlling intersubband transitions and optical properties of coaxial cylindrical quantum well is proposed. Considering the correct dressing effect for the confinement potential, the new ultra-wide tuning mechanism is based on the variation of quasibound states with the intensity of long-wavelength laser field. By means of the finite difference method, the results provide a new explanation for the anticrossing of energy levels and exhibit great potential for ultra-wide frequency tunability (ranging from terahertz to the mid-infrared region) of the optical absorption coefficient and refractive index changes by modulating the laser parameters and barrier thickness.

Perspective on exchange-coupled quantum-dot spin chains

Electron spins in semiconductor quantum dots are a promising platform for quantum-information processing applications because their quantum phase coherence can persist for extremely long times. Nearest-neighbor electron spins naturally interact with each other via Heisenberg exchange coupling. Heisenberg exchange coupling results from the interplay of the electrostatic confinement potential together with the Pauli exclusion principle, which requires that no two electrons can have the same quantum number. Exchange coupling enables a host of useful capabilities, including the generation of different types of qubits, multi-qubit gates, ways to increase connectivity in systems of quantum-dot spin qubits, and routes to explore intriguing many-body phenomena.

Linear and nonlinear optical absorption coefficients and refractive index changes of zigzag quantum wells affected by terahertz laser field

This study introduces the influence of terahertz laser field on optical absorption coefficients (OACs) and refractive index changes (RICs) related to the quantum wells with zigzag confinement potential. The energy levels and wave functions of the system are obtained by solving the Schrödinger equation within the framework of the effective-mass. The OACs and RICs are inferred from the scheme of compact density-matrix formalism. The results show that the peak of OACs and RICs are modulated by terahertz laser field and the potential parameter of quantum wells.

Optical properties of a GaAs quantum well with new type of hyperbolic confinement potential: Effect of structure parameters and intense laser field

In this present study, we have investigated theoretically the effect of structure parameters (η and κ) and high-frequency intense laser field (ILF) on the linear and nonlinear optical properties of a GaAs quantum well (QW) with new type of hyperbolic confinement potential. Within the framework of the effective-mass and parabolic band approximations, we have computed the confined subband energy levels and their corresponding densities of probability by using the diagonalization method. Besides, we evaluated the total optical absorption coefficients (TOACs) and total refractive index changes (TRICs). The obtained numerical findings show that an increase of the η (κ) structure parameter causes an increase (decrease) in amplitude in TOACs and a blue (red) shift of the resonance energy spectrum. In addition, with the increase of the ILF, the resonance energy spectrum shifts to blue, while the amplitude of TOACs (TRICs) increased (decreased). We think that the obtained these results can be useful in the design of new device’s generation employed in optoelectronic domain.

Sensitivity of correlation effects and Shannon entropy in Be@C60 to the nature of confinement potentials

The multi-configuration Dirac-Fock (MCDF) method is used to study the correlation effects in Be@C60 by investigating the localization properties of the electronic probability density distribution. The confinement environment is simulated by two types of model potentials: (A) the commonly employed compact annular square well (ASW) and (B) the diffusive Gaussian annular square well (GASW). The present work aims to investigate the sensitivity of correlation effects and localization properties to the contrasting nature of the confinement potentials. To accomplish this, the variation of the correlation energy and Shannon’s correlation information entropy with the depth of both confinement potentials are presented to give an idea of the sensitivity of these parameters to the nature of the confining potential. The present work concludes that the correlation energy and the associated electronic structure attributes are extremely susceptible to the nature of the confining potential.

Chiral symmetry in the confinement phase of QCD

Based on the Pomeranchuk theorem, one constructs the [Formula: see text] parameter to measure the difference between experimental data for the particle–particle and particle–antiparticle total cross-section at same energy. The experimental data for the proton–proton and proton–antiproton total cross-section were used to show that, at the same energy, this parameter tends to zero as the collision energy grows. Furthermore, one assumes a classical description for the total cross-section, dividing it into a finite number of non-interacting disjoint cells, each one containing a quark–antiquark pair subject to the confinement potential. Near the minimum of the total cross-section, one associates [Formula: see text] with the entropy generated by these cells, analogously to the [Formula: see text]-model. Using both the Quigg–Rosner and Cornell confinement potentials and neglecting other energy contributions, one can calculate the internal energy of the hadron. One obtains that both the entropy and internal energy possess the same logarithmic dependence on the spatial separation between the pairs in the cell. The Helmholtz free energy is used to estimate the transition temperature, which is far from the temperature widely related to the Quark–Gluon Plasma.

$$\text {L}^2$$-Hypocoercivity and Large Time Asymptotics of the Linearized Vlasov–Poisson–Fokker–Planck System

This paper is devoted to the linearized Vlasov–Poisson–Fokker–Planck system in presence of an external potential of confinement. We investigate the large time behaviour of the solutions using hypocoercivity methods and a notion of scalar product adapted to the presence of a Poisson coupling. Our framework provides estimates which are uniform in the diffusion limit. As an application in a simple case, we study the one-dimensional case and prove the exponential convergence of the nonlinear Vlasov–Poisson–Fokker–Planck system without any small mass assumption.

Electronic Structure and Electron Delocalization in Bare and Dressed Boron Pentamer Clusters.

The electronic structures of the lowest energy spin-states of the cationic, neutral and anionic bare boron pentamer clusters have been investigated by means of high level multiconfigurational type calculations, in view of the large static and dynamical electron correlation effects for these species. We found that resembles a singlet spin-state perfect pentagon, which bears no intra-annular chemical bonding interactions, as shown by our analysis of the electron delocalization carried out in terms of the normalized Giambiagi ring-current index, and the total and adjacent atom-pair delocalization indices. However, its lowest-energy triplet and quintet spin-state isomers have C2v symmetry, with large intra-annular chemical bonding interactions. This geometrical feature extends to both the neutral and the anionic species. Namely, the lowest-energy isomers of boron pentamer neutral and anionic clusters have peripheral and intra-annular sizable bonding interactions reflected in the delocalization of both π- and σ-type valence natural orbitals over the whole molecular plane, which impart large structural stability. In accordance to our calculations, the lowest energy triplet spin-state isomer of the anionic boron pentamer cluster has C2 symmetry, and consequently, it should show optical activity. Finally, we have studied the change of the geometrical structure of the boron pentamer clusters from planar to compact three-dimensional structures caused by the bonding of ligands to the boron atoms. Our explicit all-electron calculations have been rationalized in terms of the shell-closure of the delocalized valence orbitals of the clusters as predicted by the jellium model extended to nonspherical confinement potentials, circumscribing the role of the ligand to modulate the total number of valence electrons assigned to the core cluster.