Mesoscopic spin clusters, phase separation, and induced order in spin-gap magnets: A review

In many low-dimensional systems with antiferromagnetic interactions, a magnetic order of the Néel type is absent. These systems remain in a quantum-disordered (spin-liquid) state down to zero temperature. The disordered state appears to be stable to weak perturbations when magnetic excitations are separated from the ground state by an energy gap. The stability of the spin-liquid ground state is destroyed upon introduction of impurities or in a sufficiently strong magnetic field. This paper presents a review of the main results of the experimental investigations performed in order to reveal and identify mesoscopic spin clusters formed in the vicinity of impurity ions, to determine the spatial structure of an impurity-induced magnetic order in spin-gap systems, and to examine the low-frequency excitation spectra of field-induced antiferromagnetic phases.     

Influence of impurity-induced spin-orbit scattering on persistent spin helix

The persistent spin helix discovered in intrinsic spin-orbit coupled systems previously is reexamined using the motion equations of Green's functions by considering the effect of extrinsic impurity-induced spin-orbit coupling. We find both the intrinsic and extrinsic spin-orbit couplings can increase the excitation energy of spin helix. They together can reduce drastically the lifetime of the spin helix, making it severely departure from the ideal infinite value. The effect of impurity density on spin helix is also analyzed. The results may be helpful to understand experimental measurements on spin helix.     

Observation of a correlation between the dependences of the radiative decay time and intensity of the impurity core-valence luminescence on excitation energy in K1−x CsxCl and Rb1−x CsxCl mixed crystals

A study of the spectral and kinetic parameters of impurity core-valence luminescence (CVL) excited by synchrotron radiation in K_1−x Cs_xCl and Rb_1−x Cs_xCl mixed crystals is reported. The dependences of the intensity and radiative decay time τ of impurity CVL on exciting photon energy hν have been found to be correlated. It is concluded that the observed τ = f(hv) relation originates from nonradiative decay of core excitations near the surface, which govern also the excitation spectral features in this spectral region. Fiz. Tverd. Tela (St. Petersburg) 41, 1973–1975 (November 1999)     

Electronic energy band parameters of CsCl evaluated on core Bloch states and plane waves

Electronic energy bands of CsCl crystal have been calculated within the mixed basis approach with using the core Bloch states and plane waves. The calculated energy parameters of the crystal are in the satisfactory agreement with the experimental data obtained from the analysis of the core–valence luminescence spectra. The obtained results form a base for calculation of CVL spectra parameters.     

Impurity-induced tuning of quantum-well States in spin-dependent resonant tunneling

We report exact model calculations of the spin-dependent tunneling in double magnetic tunnel junctions in the presence of impurities in the well. We show that the impurity can tune selectively the spin channels giving rise to a wide variety of interesting and novel transport phenomena. The tunneling magnetoresistance, the spin polarization, and the local current can be dramatically enhanced or suppressed by impurities. The underlying mechanism is the impurity-induced shift of the quantum well states (QWSs), which depends on the impurity potential, impurity position, and the symmetry of the QWS.     

Selective laser spectroscopy of molecules in matrices. Model calculations

A review is presented of investigations of the mechanism that eliminates the inhomogeneous broadening in optical spectra of impurity centers in solids upon monochromatic excitation of luminescence and hole burning in the absorption spectra.Translated from Preprint No. 164 of the Lebedev Physics Institute, Academy of Sciences of the USSR, Moscow, 1988.     

Spectroscopy of mixed Ar-Xe clusters: Formation of a xenon core

The spectra of vacuum ultraviolet luminescence of heteroatomic Ar-Xe clusters condensed in supersonic jets of a mixture of these gases at a xenon impurity concentrations of 0.001–2% are measured. A new group of molecular luminescence bands of argon-xenon centers is recorded, which allows a highly sensitive detection of xenon in the argon matrix of a cluster. The phenomenon of phase separation into pure components with formation of a sharp interface between the xenon core and argon shell of the cluster is found.     

The charge density wave and the impurity in a linear conductor

Two kinds of functions of the impurity are studied for the charge density wave in a linear conductor. It is pointed out that the impurity-induced Friedel oscillation of electron density as well as the symmetry breaking effect of impurity govern the process of the onset and the pinning of the charge density wave.     

Defect-induced electric fields in ZnS

The optical absorption edge in cubic ZnS shows both phonon- and impurity-induced effects which determine its precise shape. Applying external electric fields and measuring the shift of the absorption edge makes it also possible to compute the impurity excitonic mass which was found to be ? 0.02m₀. Comparing the edge shifts due to the impurity-induced electric fields with those due to the external fields, one finds the approximate value for the average impurity-induced field as being about 3 × 10⁴ V/cm.     

Observation of Molybdenum Emission from Impurity-Induced Long-Lived m=1 Mode on the Experimental Advanced Superconducting Tokamak

We observe the spectra of molybdenum for the first time since the first wall of our experimental advanced superconducting tokamak(EAST) was changed mainly to molybdenum tiles.A large amount of molybdenum accumulated in the central plasma where the Jong-Jived m = 1 mode instability bursts is shown.Molybdenum is proved to be the main impurity species observed during the formation and lifetime of impurity-induced long-lived m = 1 mode.This may indicate that a close relationship exists between the high-Z impurity accumulation and the occurrence of long-lived m = 1 mode in EAST plasmas.     

Use of differential impurity induced spectra for the investigation of phonon dispersion relations

The use of differential impurity induced spectra for the investigation of elementary excitations in solids is discussed. It is shown theoretically that the differential spectra are expected to have peaks at energies corresponding to energies of the excitation at critical points of the pure crystal independent of the nature of the impurity used. As an example we present experimental results of the derivative of the intensity of Raman scattered light with respect to wavelength from Tl doped KBr crystals.     

Impurity-induced states on the surface of a three-dimensional Weyl semimetal

We study theoretically the features of impurity-induced states on the surface of a three-dimensional Weyl semimetal in this work. For calculating the impurity-induced local density of states based on T-matrix formulation, we found that for different Weyl semimetal phases the behaviors of a local impurity exhibit distinguishable prominent features for the surface Fermi arc states. Due to two opposite-directional and -chirality surface currents for a surface, a bound state appears at the unitary limit of scattering intensity near the impurity site. Then the resonance condition for different Weyl semimetal phases and scattering intensity is investigated. Our results can be used to identify distinctive topological phases of Weyl semimetal. Furthermore, the relevance of topological nodal-point and -line systems is discussed. Some relation between our theoretical results and current experimental scheme are also discussed.     

Investigations of EPR parameters and impurity structure for Co<Superscript>2+</Superscript> in Ca(OH)<Subscript>2</Subscript> crystal

The electron paramagnetic resonance (EPR) parameters (g _‖ andg _⊥ factors and hyperfine structure constantsA _‖,A _⊥) for Co²⁺ in Ca(OH)₂ are studied from the second-order perturbation formulas on the basis of the cluster approach. In these formulas, the contributions to EPR parameters from the state interactions and covalency effects are considered and the parameters related to both effects are obtained from the optical spectra and impurity structure of the studied system. From the study, it is found that the β angle between the metal-ligand bond and the C₃ axis changes from 61° in a pure crystal to 53.68(26)° in the impurity center of a Co²⁺-doped Ca(OH)₂ crystal because of the impurity-induced local lattice relaxation. The reduction of the angle β in the impurity center is also supported by the result obtained by analyzing the EPR zero-field splitting for Mn²⁺ in the same Ca(OH)₂ crystal. The EPR parameters of Ca(OH)₂:Co²⁺ are also reasonably explained by considering the suitable local lattice relaxation.     

New high-energy luminescence bands from Co2+ in ZnSe

Three sharp absorption features in the energy range 2.36–2.55 eV have been detected in the transmission spectrum of Co-diffused ZnSe, and a number of luminescence transitions originating from the lowest of these states at 2.361 eV have been observed. Photoluminescence excitation spectra prove that these are high energy excited states of the Co²⁺_Zn impurity, a conclusion confirmed by comparison of measured and predicted luminescence energies. This represents the first identification of luminescence branching from a higher excited state of a transition metal ion in any semiconductor. The sharp, weakly phonon-coupled transitions involve either intra-impurity excitation or transitions from the impurity to localised states split off from a minimum in the conduction band. The implications of these observations for the mechanism of host-impurity energy transfer and for the nature of the excited state wavefunctions are discussed.     

Dynamics of a locally distorted impurity in a host crystal with displacive phase transition

We study the dynamic behaviour of a soft displacive impurity in a host crystal undergoing a displacive phase transition. The impurity-induced localized mode and the dynamic autocorrelation function of the impurity below the local freezing temperature are investigated in mean-field approximation (MFA). Furthermore, we give a physical interpretation of the MFA result of the local freezing-out, and discuss the fluctuation behaviour of various types of impurities in relation to recent experiments.     

Luminescent properties of the oxygen impurity centres in A1N

Emission luminescence spectra of A1N with oxygen impurity are composed of several overlapping bands ranging from 2.37 eV to 3.51 eV. The intensity of these bands depends on the excitation conditions. The Stokes shift of corresponding excitation bands is ～0.93 eV; this shift is compatible with the experimental half-width of individual bandsΘ ～ 0.46 eV. The observed complex spectra are ascribed to the excitation and recombination processes in D-A complexes formed of substitutional oxygen donors O_N and aluminum vacancies V _Al . Tentative models of these centers are proposed.     

阻挡杂质带红外探测器中的界面势垒效应

通过变温暗电流和变偏压光电流谱实验对阻挡杂质带红外探测器的跃迁机理和输运特性进行了研究. 结合器件能带结构计算的结果, 证明了在阻挡杂质带红外探测器中主要由导带底下移效应引起的界面势垒的存在. 提出了阻挡杂质带红外探测器的双激发工作模型, 并从变偏压光电流谱中成功地分离出了与这两种物理过程所对应的光谱峰, 进一步证实了器件的能带结构. 研究了界面势垒效应对阻挡杂质带红外探测器的光电流谱、响应率和内量子效率的影响. 研究表明, 考虑进界面势垒效应, 计算得到的器件响应率与实验值符合得很好. 同时发现阻挡杂质带红外探测器中内建电场的存在等效降低了发生碰撞电离增益所需的临界电场强度.     

The exchange interaction effect in the scanning tunneling spectroscopy of a two-orbital Anderson impurity on metallic surface

We investigate the scanning tunneling spectroscopy (STS) of a two-orbital Anderson impurity adsorbed on a metallic surface by using the numerical renormalization group (NRG) method. The density of state of magnetic impurity and the local conduction electron are calculated. We obtain the Fano resonance line shape in the STM conductance at zero temperature. For the impurity atom with antiferromagnetic inter-orbital exchange interaction and a spin singlet ground state, we show that a dip in the STM spectra around zero bias voltage regime and side peaks of spin excitation can be observed. The spin excitation energy is proportional to the exchange interaction strength. As the exchange interaction is ferromagnetic, the underscreened Kondo effect dominates the low energy properties of this system, and it gives rise to drastically different STM spectra as compared with the spin singlet case.     

A multispectroscopic investigation of the electronic structure of luminescent,tetrahedrally coordinated vanadium-oxygen compounds: Case of YVO4

Photoelectron (XPS) and core ionization loss (CILS) spectra of YVO₄ have been measured and compared to our earlier appearance potential (APS) data on the same compound, literature data on X-ray emission of YVO₄, X-ray emission and absorption of Na₃VO₄, and the energy level diagram of molecular orbitals of the VO₄^3- cluster. Strong many-body effects, with participation of a 2p core hole on vanadium atoms, are found to lead to a drastic local perturbation of the outer molecular orbitals of the anionic cluster VO₄^3-, at the site of the excited vanadium atoms. Identification of the spectroscopic features in XPS on one hand and the edge spectroscopies CILS and APS on the other hand, enables determination of an empirical energy level diagram of excited configurations, which is different in different spectroscopies used. As a byproduct of our experiment, data on the excitation of oxygen (within the VO₄^3- cluster), yttrium (outside the VO₄^3- cluster), and carbon impurity are presented and discussed.     

VUV spectra of krypton-xenon mixtures in a cooled DC-excited discharge

An experimental study was made of VUV spectra of a gas-discharge krypton plasma with a xenon impurity. The mixture pressure, the xenon impurity concentration, and the current were varied in ranges of 40–1000 hPa, 0.001–1%, and 10–50 mA, respectively. The heteronuclear XeKr dimers were shown to play a crucial role in the quenching of excitation energy of krypton dimers. The mechanism of transformation of the energy of excited krypton dimers in a low-temperature plasma of a binary krypton-xenon mixture is discussed.