Impurity Modes Research Articles

Symmetry adapted impurity modes in as grown n-type GaP:X and GaSb:X (X = S, Se and Te)

A comprehensive average-t-matrix Green’s function (ATM-GF) theory is used, in the framework of a realistic rigid-ion-model, to simulate the symmetry induced vibrational modes of different defect centers in GaP:X and GaSb:X (X = S, Se and Te) crystals. Explicit calculations are performed for isolated 32SP+ (34SP+) defects, nearest-neighbor 32SP+-CuGa2- (34SP+-CuGa2-) pairs in GaP, and next-nearest-neighbor complex 32SSb+-GaSb2-(34SSb+-GaSb2-) center in GaSb by using apposite perturbation (P↔) and Green’s function (G↔o) matrix elements. For the isolated closest mass isoelectronic and charged (donor and acceptor) defects, the study has provided a convincing empirical relationship associating the increase or decrease of force constant change between impurity-host atoms to the increase or decrease of covalency of impurity-host bond. The rule has accurately predicted the observed isotopic shifts of local vibrational and/or gap modes of isolated impurities and offered modes for different “donor–acceptor” pairs of reduced symmetry. We feel that the ATM-GF method will play a significant role identifying the site selectivity of defects for estimating their mode frequencies in the technologically important semiconductor materials.

Photon echo in germanium with shallow donors

A theoretical study was made of the conditions for observing the photon echo effect in a germanium crystal doped with shallow donors. A numerical calculation of the medium polarization excited by a sequence of two optical pulses at a frequency close to the impurity transition frequency has been made. The effect of excitation pulse parameters, such as the pulse duration, the inhomogeneous broadening of impurity transitions and the relaxation rate of population and coherence in the system on the echo is considered. The key aspect in the experimental implementation of the effect is the control of the crystal temperature under photoexcitation, since the rate of coherence relaxation in the system strongly depends on the temperature of the crystal lattice. Keywords: germanium, shallow donors, coherent effects, photon echo.

Critical Impurity and the Race for Critical Phenomenology

Informed by María Lugones’s understanding of the “logic of purity,” this essay analyzes the race for critical phenomenology. It suggests how Lugones’s analysis of such a logic may guide us in developing phenomenological analyses of complex social identities such as race. It also shows how traces of the logic of purity remain even in critical phenomenological analyses of race. Specifically, the essay analyzes the methodological call for a reduction of quasi-transcendental structures. Ultimately an attitude and practice of critical criticality and a mode of critical impurity are suggested so as to avoid problematic traces of the logic of purity. Ultimately, the discussion reveals how Latina feminist theorizing stands to enhance critical phenomenology.

Фотонное эхо в германии с мелкими донорами

A theoretical study was made of the conditions for observing the photon echo effect in a germanium crystal doped with shallow donors. A numerical calculation of the medium polarization excited by a sequence of two optical pulses at a frequency close to the impurity transition frequency has been made. The effect of excitation pulse parameters, such as the pulse duration, the inhomogeneous broadening of impurity transitions and the relaxation rate of population and coherence in the system on the echo is considered. The key aspect in the experimental implementation of the effect is the control of the crystal temperature under photoexcitation, since the rate of coherence relaxation in the system strongly depends on the temperature of the crystal lattice.

Fractional nonlinear surface impurity in a 2D lattice

We study the formation of localized modes around a generalized nonlinear impurity which is located at the boundary of a semi-infinite square lattice, and where we replace the standard discrete Laplacian by a fractional one, characterized by a fractional exponent 0<α<1 where α=1 marks the standard, non-fractional case. We specialize to two impurity cases: impurity at an “edge” and impurity at a “corner” and use the formalism of lattice Green functions to obtain in closed form the bound state energy and its mode amplitude. It is found that, for any fractional exponent and for impurity strengths above a certain threshold, there is always a single bound state for the linear impurity, while for the nonlinear (cubic) case, up to two bound states are possible. At small fractional exponents, the energy of the impurity mode becomes directly proportional to the impurity strength.

Dynamical Quantum Cherenkov Transition of Fast Impurities in Quantum Liquids.

The challenge of understanding the dynamics of a mobile impurity in an interacting quantum many-body medium comes from the necessity of including entanglement between the impurity and excited states of the environment in a wide range of energy scales. In this Letter, we investigate the motion of a finite mass impurity injected into a three-dimensional quantum Bose fluid as it starts shedding Bogoliubov excitations. We uncover a transition in the dynamics as the impurity's velocity crosses a critical value that depends on the strength of the interaction between the impurity and bosons as well as the impurity's recoil energy. We find that in injection experiments, the two regimes differ not only in the character of the impurity velocity abatement but also exhibit qualitative differences in the Loschmidt echo, density ripples excited in the Bose-Einstein condensate, and momentum distribution of scattered bosonic particles. The transition is a manifestation of a dynamical quantum Cherenkov effect and should be experimentally observable with ultracold atoms using Ramsey interferometry, rf spectroscopy, absorption imaging, and time-of-flight imaging.

Resonant boron acceptor states in semiconducting diamond

High-excited discrete states of boron hydrogenlike acceptors in crystalline diamond have been measured by temperature-dependent infrared absorption spectroscopy. We distinguish the boron resonant states in diamond crystals from the localized boron states in the band gap by differentiating the impurity transitions within state continua of the valence light- and heavy-hole subbands on the basis of their relative oscillator strengths, specific selection rules, and temperature dependences. Constraints on the boron binding energy and spin-orbit splitting of the valence band have been derived by analysis of the structure of the infrared absorption spectra of boron-doped diamond, in particular by a comparison with boron spectra in other semiconductors with a diamond lattice.

The fractional nonlinear impurity: A Green function approach

We use a lattice Green function approach to study the stationary modes of a linear/nonlinear (Kerr) impurity embedded in a periodic one-dimensional lattice where we replace the standard discrete Laplacian by a fractional one. The energies and the mode profiles are computed in closed form, for different fractional exponents and different impurity strengths. The energies of the impurity mode lie outside the linear band whose bandwidth decreases steadily as the fractional exponent decreases. For any fractional exponent values, there is always a single bound state for the linear impurity while for the nonlinear (Kerr) case, up to two bound states are possible, for impurity strengths above certain threshold. The energy of the linear mode (or that of the upper energy nonlinear one), becomes directly proportional to the impurity strength at large impurity strengths. The transmission of plane waves is also computed in closed form for several fractional exponents, and various impurity strengths. We observe that fractionality tends to increase the overall transmission. The selftrapping transition for the nonlinear impurity shifts to lower nonlinearity values as the fractional exponent is decreased. In both cases, linear and nonlinear, we observe a form of trapping at zero impurity strength, which can be explained by the near-degeneracy of the spectrum in the limit of a small fractional exponent.

Statistical analysis of the distribution of impurities during copper electrorefining

Electrolytic copper refining makes it possible to obtain high purity metal, so the analysis of the main ways of impurity transition into electrolysis products is an actual problem. If it is solved, the process can be controlled when changing the composition of raw materials and, as a result, the content of impurities in the anodes. This paper uses the comprehensive analysis and synchronization of a large array of data on impurities concentrations in various process media (anodes, electrolyte, slime, and cathode metal) obtained on the series of commercial cells to identify the directions of impurity flows and relationship between their content in these media. It is shown that the transition of impurities from one process medium (source) to another (receiver) is implemented according to four main patterns: linear increase, no visible dependence, the presence of a limit concentration in the receiver and the presence of a threshold concentration in the source. The paper provides the results obtained in the statistical analysis of the distribution of six impurities (bismuth, arsenic, lead, sulfur, nickel and silver) belonging to different groups in four main pairs of the impurity source – receiver: anode – solution, anode – slime, slime – cathode, solution – cathode. The coefficients of linear regression equations are determined and their significance is estimated for all dependencies of the impurity concentration in the source on the content in the receiver. The coefficients obtained make it possible to explain the impurity transition paths observed in the commercial cells and predict the quality of cathode copper and the composition of slimes when the anode composition changes. The calculations showed that impurities are accumulated in cathodes due to the occlusion of slime particles and incomplete solution removal from the surface of commercial cathodes rather than due to electrochemical reactions. The copper electrorefining technology should be improved and developed so as to find surface-active additives that would prevent the adsorption of suspended slime particles on the cathode surface, as well as better wash them from the electrolyte.

Realizing high doping efficiency and thermoelectric performance in n-type SnSe polycrystals via bandgap engineering and vacancy compensation

SnSe, a wide-bandgap semiconductor, has attracted significant attention from the thermoelectric (TE) community due to its outstanding TE performance deriving from the ultralow thermal conductivity and advantageous electronic structures. Here, we promoted the TE performance of n-type SnSe polycrystals through bandgap engineering and vacancy compensation. We found that PbTe can significantly reduce the wide bandgap of SnSe to reduce the impurity transition energy, largely enhancing the carrier concentration. Also, PbTe-induced crystal symmetry promotion increases the carrier mobility, preserving large Seebeck coefficient. Consequently, a maximum ZT of ∼1.4 at 793 K is obtained in Br doped SnSe–13%PbTe. Furthermore, we found that extra Sn in n-type SnSe can compensate for the intrinsic Sn vacancies and form electron donor-like metallic Sn nanophases. The Sn nanophases near the grain boundary could also reduce the intergrain energy barrier which largely enhances the carrier mobility. As a result, a maximum ZT value of ∼1.7 at 793 K and an average ZT (ZTave) of ∼0.58 in 300–793 K are achieved in Br doped Sn1.08Se–13%PbTe. Our findings provide a novel strategy to promote the TE performance in wide-bandgap semiconductors.

Detecting Trace Gases at PPM Levels with Low Temperature Plasma Optical Emission Spectroscopy

Detecting Trace Gases at PPM Levels with Low Temperature Plasma Optical Emission Spectroscopy

Assessment of intrinsic and doped defects in Bridgman grown Cd1-xZnxTe alloys

A comprehensive experimental and theoretical study is presented to empathize the dynamical properties of Bridgman grown Cd1-xZnxTe samples. Low temperature Raman scattering study has offered an excellent testimony to the lattice phonons of the perfect alloys with a strong evidence of Te anti-site defect (TeCd) modes. An average transfer matrix (ATM) Green’s function (GF) method is employed to examine the isotopic shifts of localized vibrational modes (LVMs) of isolated oxygen OTe defects in CdTe and the low frequency A1 and E phonon modes of TeCd. For the nearest-neighbor (NN) OTe – VCd pair, while the ATM-GF’s study revealed quite different values of LVMs from the existing infrared (IR) absorption data – the results, however, provided good justification to a simple perturbation relationship of 16OTe isotopic mode. An excellent agreement between theory and IR data of impurity modes has validated the formation of NN donor–acceptor pairs and next-nearest-neighbor complex centers involving intrinsic defects. The study is important not only for empathizing the growth process of technologically important Cd1-xZnxTe alloys but also identifying the role of carrier compensation mechanism by various donors and acceptors.

Antiferromagnetic spin dynamics in exchanged-coupled Fe/GdFeO3 heterostructure* *Project supported by the National Key Research Program of China (Grant Nos. 2018YFF01010303, 2017YFB0702702, and 2016YFA0300701), the National Natural Sciences Foundation of China (Grant Nos. 52031015, 1187411, 51427801, and 51871235), and the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant Nos. QYZDJ-SSW-JSC023, KJZD-SW-M01, and ZDYZ2012-2).

We investigate the ultrafast spin dynamics of an antiferromagnet in a ferromagnet/antiferromagnet heterostructure Fe/GdFeO3 via an all-optical method. After laser irradiation, the terahertz spin precession is hard to be excited in a bare GdFeO3 without spin reorientation phase but efficiently in Fe/GdFeO3. Both quasi-ferromagnetic and impurity modes, as well as a phonon mode, are observed. We attribute it to the optical modification of interfacial exchange coupling between Fe and GdFeO3. Moreover, the excitation efficiency of dynamics can be modified significantly via the pump laser influence. Our results elucidate that the interfacial exchange coupling is a feasible stimulation to efficiently excite terahertz spin dynamics in antiferromagnets. It will expand the exploration of terahertz spin dynamics for antiferromagnet-based opto-spintronic devices.

Impurity mode induced turbulent particle transport and its temperature screening effect

Turbulent transport of impurity ions with hollow density profiles (HDPs), which are widely observed in magnetically confined plasmas and desirable for fusion reactor, is self-consistently investigated. A full gyrokinetic description is employed for main and impurity ions. Instead of conventional ion temperature gradient (ITG, including impurity ITG) and trapped electron modes (TEMs), impurity modes (IMs), driven by impurity ion density gradient opposite to that of electrons, are considered. The impurity ion flux induced by IMs is shown to be approximately one order of magnitude higher than that induced by TEMs when both kinds of modes coexist. Main ITG and electron temperature gradient (ETG) are found to reduce influx of impurity ions significantly, resembling temperature screening effect of neoclassical transport of impurity ions. The simulation results such as peaking factor of the HDPs and the effects of main ITG are found in coincidence with the evidence observed in argon injection experiment on HL-2A tokamak. Thus, the IM turbulence is demonstrated to be a plausible mechanism for the transport of impurity ions with HDPs. A strong main ITG, ETG, and a low electron density gradient are expected to be beneficial for sustainment of HDPs of impurity ions and reduction of impurity accumulation in core plasma.

Междолинные процессы релаксации состояний мелких доноров в германии

The role of the intervalley processes of electron-phonon interaction in the relaxation of excited shallow arsenic donors in germanium is analyzed. The rates of intracenter inter-valley transitions with emission of TA phonons in ger-manium are calculated in dependence on the uniaxial compression stress along {111} crystallographic direction. It is shown that inter-valley transitions to the ground state of the donor with emission of phonons can play a significant role in the relaxation of excited impurities only upon uniaxial stress of the crystal, since at zero stress, there are no exact resonances between impurity transitions and intervalley phonons. There are also transitions from highly excited states lying in a narrow band of energies (~ 0.5 meV) under very bottom of the conduction band to the first excited state 1s(3)(Г5) (in stressed germanium crystal to 1s(3)(Г3) state). The average rate of these transitions is estimated at 0.3×109 s-1.

A fundamental assessment of the impacts of cation (Cd, Co, Fe) substitution on the molecular chemistry and surface reactivity of sphalerite

A series of cadmium-, cobalt-, and iron-substituted (0–4 wt%) sphalerite mineral samples were synthesised to test how the presence of these cation substituents alter the crystal structure, surface bonding environment and electronic properties of sphalerite and to illustrate how these alterations impact the electrokinetic behaviour of mineral surfaces under conditions relevant to sphalerite flotation. A combination of X-ray Diffraction (XRD), Raman and ultraviolet–visible spectroscopies, and zeta potential experiments were used to demonstrate these impacts. XRD results showed a positive correlation between the measured lattice distortions and the ionic size and concentration of the respective cation substituents. Raman results confirmed the presence of new metal-sulphur (M−S) bonds within the ZnS host cluster reflected by a new impurity mode which correlates to the concentration and chemical nature of the cation substituents. The presence of metal substituents further induced a change in the band gap of sphalerite which reduced according to Cd > Fe > Co. These measurements informed the effect of the molecular characteristics of impurity-bearing sphalerite on the electrokinetic response of sphalerite in the presence of CuSO4 and xanthate collector. Cobalt-bearing sphalerites which have the lowest band gap and greatest distortions to molecular and crystal structure had the most notable impact on zeta potential measurements. A small increase in Co concentration induced a larger shift in point of zero charge for collectorless activation but did not impact the collector interactions. Conversely, Cd which has a wider band gap is less electronegative than Co exhibited the reverse behaviour. This study thus details how the presence of cation substituents modifies sphalerite bulk structure, in turn impacting its surface bonding environment and electronic properties, and ultimately exerting a notable control on flotation chemistry.

A systematic comparison of polar and semipolar Si-doped AlGaN alloys with high AlN content

With a view to supporting the development of ultra-violet light-emitting diodes and related devices, the compositional, emission and morphology properties of Si-doped n-type Al x Ga1-x N alloys are extensively compared. This study has been designed to determine how the different Al x Ga1-x N crystal orientations (polar (0001) and semipolar (11–22)) affect group-III composition and Si incorporation. Wavelength dispersive x-ray (WDX) spectroscopy was used to determine the AlN mole fraction (x ≈ 0.57–0.85) and dopant concentration (3 × 1018–1 × 1019 cm−3) in various series of Al x Ga1-x N layers grown on (0001) and (11–22) AlN/sapphire templates by metalorganic chemical vapor deposition. The polar samples exhibit hexagonal surface features with Ga-rich boundaries confirmed by WDX mapping. Surface morphology was examined by atomic force microscopy for samples grown with different disilane flow rates and the semipolar samples were shown to have smoother surfaces than their polar counterparts, with an approximate 15% reduction in roughness. Optical characterization using cathodoluminescence (CL) spectroscopy allowed analysis of near-band edge emission in the range 4.0–5.4 eV as well as various deep impurity transition peaks in the range 2.7–4.8 eV. The combination of spatially-resolved characterization techniques, including CL and WDX, has provided detailed information on how the crystal growth direction affects the alloy and dopant concentrations.

Terahertz transient stimulated emission from doped silicon

Transient-type stimulated emission in the terahertz (THz) frequency range has been achieved from phosphorus doped silicon crystals under optical excitation by a few-picosecond-long pulses generated by the infrared free electron lasers FELIX and CLIO. The analysis of the lasing threshold and emission spectra indicates that the stimulated emission occurs due to combined population inversion based lasing and stimulated Raman scattering. Giant gain has been obtained in the optically pumped silicon due to large THz cross sections of intracenter impurity transitions and resonant intracenter electronic scattering. The transient-type emission is formed under conditions when the pump pulse intervals exceed significantly the photon lifetime in the laser resonator.

Thermoelectric performance of p-type CaxFe1.3Co2.7Sb12 skutterudites from high pressure synthesis

P-type CaxFe1.3Co2.7Sb12 skutterudites were successfully synthesized with a high pressure synthesis method followed by spark plasma sintering. The structure, composition and thermoelectric properties were investigated. With fixed Fe substitution level, the hole concentration was tuned with Ca filling. Compared with the unfilled Co3.25Fe0.73Sb12, Ca-filled samples possessed a relatively large effective mass, which is beneficial for enhancements in Seebeck coefficient and power factor. Moreover, the joint impurity and rattling mode scattering further suppressed the lattice thermal conductivity. Consequently, the Ca0.6Fe1.3Co2.7Sb12 sample showed the optimal ZT of 0.70 at 820 K. Composition analysis and simple electron counting suggested 0.77 hole per molecular formula for this sample, close to the value (about 0.7) we proposed for optimal filling and substitution contents in Co-rich p-type skutterudites.

Impurity states in InGaAsP/InP core–shell quantum dot under external electric field

In the framework of effective mass envelope function approximation, the impurity states are calculated in InGaAsP/InP core–shell quantum dots by plane wave expansion method, and the effect of electric field is considered. The impurity binding energies in 1s and 2p ± states, and the impurity transition energy between 1s state and 2p ± state are calculated and analyzed in detail with the change of shell thickness, core radius, impurity position, and electric field strength. The results show that the impurity states in the core are stable when the shell thickness is more than 0.4a * . The binding energies and transition energies decrease with the increase of core radius. The applied electric field destroys the symmetrical distribution of binding energy and transition energy about the core center. The increase or decrease of the binding energy and transition energy depend upon the position of impurity and the direction of electric field.