Ground State Of Impurity Research Articles

Synthetic dimension-induced pseudo Jahn-Teller effect in one-dimensional confined fermions

We demonstrate the failure of the adiabatic Born-Oppenheimer approximation to describe the ground state of a quantum impurity within an ultracold Fermi gas despite substantial mass differences between the bath and impurity species. Increasing repulsion leads to the appearance of nonadiabatic couplings between the fast bath and slow impurity degrees of freedom, which reduce the parity symmetry of the latter according to the pseudo Jahn-Teller effect. The presence of this mechanism is associated to a conical intersection involving the impurity position and the inverse of the interaction strength, which acts as a synthetic dimension. We elucidate the presence of these effects via a detailed ground-state analysis involving the comparison of fully correlated simulations with effective models. Our study suggests ultracold atomic ensembles as potent emulators of complex molecular phenomena. Published by the American Physical Society 2024

Analyzing photoionization cross-sectional modulation in CdSe/CdS core–shell nanodots: Impact of strain and applied electric field

The role of strain in material properties is well-established, serving as a tool for altering atomic positions and defect formation, adjusting electronic structures and lattice vibrations, and influencing phase transitions, physical characteristics, and chemical properties. In this study, we conducted theoretical calculations of the binding energy and photoionization cross section (PCS) within a spherical core/shell quantum dot (CSQD) for the different transitions between the ground state of a donor impurity and the four low-lying conduction band states. During our study, we employed the finite element method to determine the energy levels and wave functions of the system within the effective mass approximation. Subsequently, we investigated the changes in PCS and binding energy while varying shell width under the influence of an applied electric field, considering both cases with and without the effect of strain. The strain effect was incorporated based on Hooke's law, and we developed specific expressions and utilized the continuum linear elasticity mechanical model for a single spherically symmetric shell. The results demonstrate that the strain correction enhances the binding energy of the four low-lying energy levels, leading to a shift of the PCS peaks toward higher energies. Conversely, the application of an external electric field has varying effects depending on the specific transition being considered. We compared our theoretical results with available experimental data and found them to be in good agreement. The pronounced blue-shift and substantial enhancement in magnitude of PCS spectra concerning shell width, electric field, and strain make CSQDs highly promising candidates for applications in adjustable nano-optoelectronic devices.

Model of DC Tunneling Conductivity via Hydrogen‐Like Impurities in Heavily Doped Compensated Semiconductors

Herein, a theoretical model is proposed for the weakly temperature‐dependent electrical conductivity of compensated crystalline semiconductors with hydrogen‐like impurities near the insulator–metal concentration phase transition (Mott transition). The model uses a simple non‐stoichiometric cubic “impurity lattice” formed by the doping and compensating impurities in crystal matrix. A shift of the c‐band bottom (v‐band top) into the bandgap due to overlap of the excited states of neighboring impurities is considered. The distribution of electron (and hole) density of states in the band of ground (unexcited) states of impurities is assumed to be Gaussian. Tunneling transitions of electrons between nearest donors in the charge states () and (), and tunneling transitions of holes between acceptors in the charge states () and () are considered. It is shown that, at low temperatures, transitions of electrons (holes) near the Fermi level in the impurity band lead to electrical conductivity that weakly depends on temperature (in the form of a characteristic plateau). The results of calculating electrical resistivity in the zero‐temperature limit for the plateau region agree with the known experimental data for moderately compensated n‐ and p‐type Ge, Dia, Si, ZnSe, GaAs, InSb, and InP crystals.

Hydrogen-like donor impurity states in strongly prolate ellipsoidal quantum dot

In the current article, the binding energy and impurity-related photoionization cross-section are investigated for the case of an infinite confinement potential well model using a variational approach for hydrogen-like impurity states in strongly prolate ellipsoidal quantum dot. The binding energy is calculated as a function of ellipsoidal small and large semiaxes and as a function of impurity displacement. The behavior of the oscillator strength for the quantum transitions between the impurity ground state and the first two non-impurity states are considered for different values of incident light angles and geometrical parameters. In the work, we illustrate the results of photoionization cross-section computations depending on the incident photon energy for a range of incident angles and different values of impurity shift. It is revealed that the threshold frequency decreases with the increase of impurity displacement.

Zeeman effects on Yu-Shiba-Rusinov states

When the exchange interaction between the impurity spin and the spins of itinerant quasiparticles is strong or weak enough, the ground states for a magnetic impurity in a superconductor are the screened or free spins, respectively. In both cases, the lowest excited state is a bound state within the superconducting gap, known as the Yu-Shiba-Rusinov (YSR) state. The YSR state is spatially localized, energetically isolated, and fully spin polarized, leading to applications such as functional scanning probes. While any application demands identifying whether the impurity spin is screened or free, a suitable experimental technique has been elusive. Here we demonstrate an unambiguous way to determine the impurity ground state using the Zeeman effect. We performed ultralow-temperature scanning tunneling spectroscopy of junctions formed between a Cu(111) surface and superconducting Nb tips decorated by single magnetic Fe atoms. Depending on the condition of the Fe adsorbate, the YSR peak in the spectrum either splits or shifts in a magnetic field, signifying that the Fe spin is screened or free, respectively. Our observations provide renewed insights into the competition between magnetism and superconductivity and constitute a basis for the applications of the YSR state.

Kondo effect in a spin-3/2 Fermi gas

We investigate the Kondo effect of a spin-3/2 Fermi gas and give a detailed calculation of the impurity resistance and ground state energy based on the s-d exchange model. It is found that the impurity resistance increases logarithmically with the decrease of temperature in the case of antiferromagnetic coupling similar to the spin-1/2 system but has a larger resistance minimum value due to the increase of spin scattering channels. In the case of antiferromagnetic interaction, the ground state is still the Kondo singlet state while the septuplet state has the lowest energy for ferromagnetic coupling. And with the same antiferromagnetic s-d coupling parameter, the energy of the Kondo singlet state is lower than spin-1/2, which indicates that the larger spin, the easier it is to enter the Kondo-screened phase. This provides some theoretical support for the realization of the Kondo effect with ultra-cold atoms.

Thermal ionization energy of hydrogen-like impurities in semiconductor materials

In the work the dependence of the thermal ionization energy of hydrogen-like donors and acceptors on their concentration in n- and p-type semiconductors is analyzed analytically and numerically. The impurity concentrations and temperatures at which the semiconductors are on the insulator side of the concentration insulator – metal phase transition (Mott transition) are considered. It is assumed that impurities in the crystal are distributed randomly (according to Poisson), and their energy levels are distributed normally (according to Gauss). In the quasi-classical approximation, it is shown, for the first time, that the decrease in the ionization energy of impurities mainly occurs due to the joint manifestation of two reasons. Firstly, from the excited states of electrically neutral impurities, a quasicontinuous band of allowed energy values is formed for c-band electrons in an n-type crystal (or for v-band holes in a p-type crystal). This reduces the energy required for the thermally activated transition of electron from the donor to the c-band (for the transition of the hole from the acceptor to the v-band). Secondly, from the ground (unexcited) states of impurities a classical impurity band is formed, the width of which at low temperatures is determined only by the concentration of impurity ions. In moderately compensated semiconductors (when the ratio of the concentration of minority impurities to the concentration of majority impurities is less than 50 %) the Fermi level is located closer to the edge of the band of allowed energy values than the middle of the impurity band, that issue reduces thermal ionization energy of impurities from states in the vicinity of the Fermi level (transition of electron from a donor to the c-band, or hole from an acceptor to the v-band). Previously, these two causes of decrease in the thermal ionization energy due to increase in the concentration of impurities were considered separately. The results of calculations according to the proposed formulas are quantitatively agree with the known experimental data for a number of semiconductor materials (germanium, silicon, diamond, gallium arsenide and phosphide, silicon carbide, zinc selenide) with a moderate compensation ratio.

Screened shallow impurity properties of quantum well heterosystems with high-κ dielectric barrier environment

The effect of high-κ dielectric mismatch on the impurity states screened by free electron gas is investigated theoretically in high-κ dielectric barrier/quantum well/high-κ dielectric barrier (hh-κ) and low-κ dielectric barrier/quantum well/high-κ dielectric barrier (lh-κ) types of quasi two-dimensional (Q2D) heterostructures. A restriction of the characteristic long-range nature of the impurity Q2D screened interaction due to the high-κ dielectric mismatch has been revealed here - atypical of the Q2D dielectrically homogeneous systems. Accordingly, for the hh-κ type heterostructure, an impurity screened interaction potential at moderately large (relative to the quantum well width values) in-plane distances behaves as the Q2D Debye-Hückel potential, while at rather large distances falls off on the background of the high-κ dielectric constant of the barrier medium. The screened impurity ground state energy in the framework of the Q2D Debye-Hückel potential has been investigated variationally for the first time. A role of high-κ dielectric mismatch is exposed when comparing the results for the discussed heterostructures based on the realistic InSb/HfO2 interface. The characteristic ranges where the quantum confinement effect, enhancing the Coulomb interaction, balances the joint weakening influence of the impurity screening and high-κ dielectric mismatch effects are revealed. The received admissible values of the in-plane density/temperature ratio parameter for the hh-κ type heterostructure typically refer to non-degenerate electron gas while for the lh-κ type both electron gas statistics are significant. Due to the high-κ dielectric mismatch a binding energy sufficient decline with reducing of the quantum well width has been established. In the hh-κ type heterostructure an impurity binding is quite sizable at the wide quantum wells (d ∼8 ÷ 10 nm). Meanwhile, for the lh-κ-type heterostructure, an impurity binding energy is enough large (∼10 meV) in both wide and narrow quantum well cases.

Energy of Hydrogenic Impurities in Spherical Quantum Dot

The present study has provided a new method for calculating the ground state and binding energy. The ground state and binding energy of a hydrogenic impurity in spherical quantum dot calculate by using the Feynman path integral. Hydrogenic impurity located at the center of quantum dot. The framework is center of the effective-mass theory.

Effect of External Hydrostatic Pressure and Temperature on the Impurity States and Diamagnetic Susceptibility in Strongly Oblate Ellipsoidal Quantum Dot

In this work, the hydrogen-like impurity states in a strongly oblate ellipsoidal quantum dot have been investigated in the presence of the external hydrostatic pressure and temperature. Numerical computations were performed within the framework of the variational method. The dependences of the binding energy on the location of the donor impurity were obtained for various values of external pressure and temperature. At various values of external pressure, the dependence of the diamagnetic susceptibility on the impurity coordinate has been revealed. The dependence of the oscillator strength for the first two transitions from the impurity ground state to the conduction band has been computed. The three-dimensional dependence of the photoionization cross section on the frequency of the incident light and hydrostatic pressure is given.

Absorption Spectra of Hydrogen-Like Donor Impurity in GaAs Quantum Well with Modified Pöschl-Teller Potential

Hydrogen-like donor impurity states in a quantum well with modified Poschl-Teller confinement potential have been considered. The dependence of binding energy on hydrogen-like donor impurity position and confinement potential parameters has been revealed. It has been shown that the dependence of binding energy on hydrogen-like donor impurity position has the bell-shaped character. The existence of binding energy maximum is explained by the concurrency of the size-quantization effect and the Coulomb interaction between electron and hydrogen-like donor impurity. The coefficient of an optical absorption in the presence of hydrogen-like donor impurity center has been computed. The absorption spectra for the quantum transitions between the hydrogen-like donor impurity ground state and conduction band ground state have been obtained.

Binding energy and photoionization cross-section of hydrogen-like donor impurity in strongly oblate ellipsoidal quantum dot

Hydrogen-like donor impurity states in strongly oblate ellipsoidal quantum dot have been studied. The hydrogen-like donor impurity states are investigated within the framework of variational method. The trial wave function constructed on the base of wave functions of the system without impurity. The dependence of the energy and binding energy for the ground and first excited states on the geometrical parameters of the ellipsoidal quantum dot and on the impurity position have been calculated. The behavior of the oscillator strength for different angles of incident light and geometrical parameters have been revealed. Photoionization cross-section of the electron transitions from the impurity ground state to the size-quantized ground and first excited states have been studied. The effects of impurity position and the geometrical parameters of the ellipsoidal quantum dot on the photoionization cross section dependence on the photon energy have been considered.

The effects of polaronic mass and conduction band non-parabolicity on a donor binding energy under the simultaneous effect of pressure and temperature basing on the numerical FEM in a spherical quantum dot

Basing on the numerical Finite Element Method (FEM), we have investigated the influences of polaronic mass and conduction band non-parabolicity on the binding energy of the ground state of an on-center hydrogenic donor impurity in a spherical GaAs/Ga1−xAlxAs quantum dot structure. The calculations have been made with a realistic potential barrier height in the framework of the effective mass approximation including the combined effect of hydrostatic pressure and temperature. The donor binding energy is computed as a function of dot size, Al concentration x, hydrostatic pressure and temperature both in the absence and presence of polaronic mass and conduction band non-parabolicity effects. We have taken into account the electronic effective mass, dielectric constant, and conduction band offset between the dot and barriers varying with pressure and temperature. It has been found that the binding energy is strongly affected by the effect of polaronic mass and conduction band non-parabolicity for narrow quantum dot and large Al concentration x. The results show again that the donor binding energy increases linearly with the pressure in direct gap regime and its variation is larger for narrower dots only and drops slightly with the temperature. A good agreement is obtained with the existing literature values.

Photoionization and electrostatic multipoles properties of spherical core/shell/shell quantum nanolayer with off-center impurity

The electronic, optical and electrostatic properties of the spherical core/shell/shell quantum nanolayer with an off-centered impurity have been studied. Spherical nanolayers of both “small” and “large” radii have been considered in the framework of perturbation theory and the variational method. Photoionization cross-section that corresponds to the electron transitions from the impurity ground state to the size-quantized levels have been studied. The dependence of the photoionization cross section on the photon energy, impurity position and the geometrical parameters of the spherical nanolayer have been obtained. The electrostatic multipoles of the considered system have been investigated.

Electronic states in low-dimensional nano-structures: Comparison between the variational and plane wave basis method

A comparison is made between the plane wave basis and variational method. Within the framework of effective-mass approximation theory, the variational and plane wave basis method are used to calculate ground state energy and ground state binding energy in low-dimensional nano-structures under the external electric field. Comparing calculation results, the donor binding energies of ground state display the consistent trend, both of them are strongly dependent on the quantum size, impurity position and external electric field. However, the impurity ground state energy calculated using variational method may be larger than the real value and it results in the smaller binding energy for variational method. In addition, the binding energy is more sensitive to the external electric field for the variational method, which can be seen more clearly from Stark shift.

Intense laser field and conduction band-edge nonparabolicity effects on hydrogenic impurity states of InGaN QW

In this paper, hydrogenic impurity ground-state binding energy in unstrained wurtzite (In, Ga)N symmetric quantum well is investigated. The heterostructure is considered under the action of an intense laser field (ILF) incorporating an additional internal probe as well as the conduction band-edge nonparabolicity effect (CBENP). The variational approach is used within the framework of single band effective-mass approximation with two-parametric 1S-hydrogenic trial wavefunction. The competition effect between internal and external perturbations is also shown. Our results reveal that the binding energy is the largest for the well width around the effective Bohr radius and is strongly influenced by both parameters. Moreover, the principle effect of ILF (CBENP) is to reduce (enhance) the binding energy. It is found that the lift of the conduction band-edge can be easily eliminated by adjusting the ILF-parameter.

Far- and near-infrared photoluminescence from n-GaAs/AlGaAs multiple quantum wells

The results of experimental investigations of impurity-assisted photoluminescence at low temperatures from n-GaAs/AlGaAs multiple quantum wells both in near- and far-infrared (terahertz) spectral ranges are presented. The optical electron transitions from impurity ground state to heavy hole subband in near-infrared spectral range are revealed. The depopulation of the donor ground state due to these transitions allowed us to observe photoluminescence in terahertz spectral range related to electron transitions from the first electron subband to donor ground state as well as to intracenter optical transitions. Experimental results in far- and near-infrared spectral ranges are in good agreement with the results on THz photoconductivity and energy spectrum calculation.

Hydrostatic pressure and magnetic field effects on donor states in pyramidal quantum dots

The simultaneous effects of the hydrostatic pressure and magnetic field on the ground state of impurity confined by a GaAs pyramid-shaped QD are investigated within the effective mass approximation. By using a variational method, the binding energy and the extent of the electron wavefunction in the structure are examined for different dot sizes and impurity positions. The influence of above mentioned parameters on the diamagnetic shift of the impurity energy is also discussed. We found that the diamagnetic coefficient could be appreciably tuned by a proper tailoring of the heterostructure parameters (dot size, impurity position) as well as by changing the hydrostatic pressure and/or the magnetic field strength.

Role of electron correlation effects in δ-Pu and “115”-Pu-based unconventional superconductors

Electronic structure calculations combining the local density approximation with an exact diagonalization of the Anderson impurity model show intermediate 5f5–5f6-valence ground state and delocalization of the 5f5 multiplet of the Pu atom 5f-shell in PuCoGa5, and δ-Pu. The 5f-local magnetic moment is compensated by a moment formed in the surrounding cloud of conduction electrons. For PuCoGa5 and δ-Pu the compensation is complete and the Anderson impurity ground state is a singlet. This can be important for analyses of the superconducting pairing mechanism in PuCoGa5. It is likely that in PuCoGa5 unconventional d-wave superconductivity is mediated by the 5f-states valence fluctuations, rather than antiferromagnetic fluctuations. Les méthodes de calcul des structures électroniques qui combinent l'approximation de densité locale avec une diagonalisation exacte du modèle d'impureté d'Anderson prévoyaient, pour δ-Pu et pour PuCoGa5, un état fondamental de valence intermédiaire 5f5–5f6, ainsi qu'une délocalisation de la couche électronique 5f du Pu. Le moment magnétique local 5f est compensé par un moment formé dans le milieu des électrons de conduction. Pour PuCoGa5 et δ-Pu, cette compensation est complète et l'état fondamental de l'impureté d'Anderson est un singulet non magnétique. Ce résultat peut avoir des conséquences importantes sur l'analyse des mécanismes de couplage de Cooper dans PuCoGa5. Il est possible que la supraconductivité non conventionnelle de symétrie d dans PuCoGa5 soit véhiculée par des fluctuations de valence des états 5f plutôt que par des fluctuations magnétiques.

Impurity absorption in ZnSe/InP/ZnS core/shell/shell spherical quantum dots

Impurity states in ZnSe/InP/ZnS core/shell/shell spherical quantum dot where electrons are localized in the InP shell are considered using variational method. It is assumed that the hydrogenlike impurity is located in the center of quantum dot core (ZnSe). The impurity ground state wave function and energy, as well as electron binding energy are obtained. Interband direct transitions from the ground valence state into the ground donor state are considered. Dependences of absorption edge on the inner and outer radii of the quantum layer are derived.