High-density Electronic Excitation Research Articles

Femtosecond trimer quench in the unconventional charge-density-wave material 1T′−TaTe2

Ultrafast optical switching of materials properties promises future technological applications, enabled by fundamental insights about microscopic couplings and nonequilibrium phenomena. Transition-metal dichalcogenides (TMDCs) combine photosensitivity with strong correlations, furthering rich phase diagrams and enhanced tunability. The compound 1T′−TaTe2 exhibits an electronically and structurally unique set of charge density waves (CDWs), featuring an unusual increase in conductivity and amplitude modes of low prominence. Compared to other charge-ordered TMDCs, only very few studies addressed the ultrafast response of this material to optical excitation. In particular, the question whether such unconventional properties translate to unusual quench dynamics remains largely unresolved. Here, we investigate the structural dynamics in 1T′−TaTe2 by means of ultrafast nanobeam electron diffraction at an unprecedented repetition rate of 2MHz. We reveal a strongly directional cooperative atomic motion during the one-dimensional quench of the low-temperature trimer lattice. These dynamics are completed within less than 500fs, substantially faster than reported previously. In striking contrast, the periodic lattice distortion of the room-temperature phase is unusually robust against high-density electronic excitation. In conjunction with the known sensitivity of 1T′−TaTe2 to chemical doping, we thus expect the material to serve as a versatile platform for tunable structural control by optical stimuli. Published by the American Physical Society 2024

Wavelength-Resolved Photoluminescence and Cathodoluminescence Decay Times of LSO:Ce Scintillator CO-Doped with Lithium and Scandium

We study the luminescence decay times of Ce3+: Lu2SiO5 scintillator, co-doped with Sc3+ and Li+ ions, under the direct photoexcitation of Ce3+ ions at 337 nm and excitation by a pulsed electron beam with energies of 100 – 300 keV. The cathodoluminescence (CL) and photoluminescence (PL) decay times of studied sample are equal within the experimental error. Two emission bands are found: in the range of 370 – 480 nm (“high-energy band”) and 500 – 570 nm (“low-energy band”). The highenergy emission band has the decay times ∼40 ns. The low-energy emission band has the maximum decay time ∼60 ns at 545 nm. The observed results show that the transport stage of Li:Sc:Ce:Lu2SiO5 scintillation mechanism is not affected by a high negative space charge and high density of electronic excitations provided by the high-power electron beam.

Structure of ion tracks in ceria irradiated with high energy xenon ions

Structure and accumulation behavior of ion tracks in CeO2 irradiated with 200 MeV Xe ions were examined by transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to obtain fundamental knowledge on the microstructure evolution induced by fission fragments in nuclear fuels and transmutation targets, which is of importance for the development of advanced fuel/target materials at high burn-up conditions. Bright-field (BF) TEM images of ion tracks from an inclined direction showed Fresnel contrast along penetrating path of incident ions. The signal intensity of high-angle annular dark-field (HAADF) STEM images was decreased at the core damage region of ion tracks along the path of ions, revealing the reduction of atomic density inside the ion track. Preferential formation of smaller and larger ion tracks was observed at a high ion fluence of 1 × 1014 cm−2 compared to a low ion fluence of 1 × 1011 cm−2. Results were discussed due to the coalescences and incomplete recovery of the core damage regions during the overlap of high density electronic excitation damage, which is induced during the repetition of the formation and recovery of ion tracks within an influence region.

Radiation damage caused by swift heavy ions in CaF2 single crystals

The radiation-induced optical absorption in a wide spectral region of 1.5–10 eV have been investigated in nominally pure CaF2 single crystals irradiated at 295 K with 0.23 GeV Xe132 or 2.38 GeV Bi209 ions (fluences of 1011−1014 ions/cm2) providing an extremely high density of electronic excitations. The stepwise annealing of the radiation-induced optical absorption have been analysed at the heating of irradiated crystals up to 1023 K and accompanying thermally stimulated luminescence has been detected at 300–750 K. The origin of several absorption bands (incl. vacuum ultraviolet region), the possible creation mechanisms of the corresponding radiation defects as well as the difference in radiation damage caused by Xe132 or Bi209 ions have been considered.

Structure of Defects and Microstructure Evolution in Oxide Ceramics – Role of Electronic Excitation and Selective Displacement Damage

Fluoriteand spinel-type oxide ceramics, such as yttria-stabilized zirconia, ceria, magnesium aliminate spinel, are known to be exceptionally resistant to irradiation damage with energetic particles. Those oxide compounds, thereofore, have potential applications to advanced nuclear fuels and innert matrices of transmutaion targets. Fundamental understanding of radiation damage is apparently essential to assess the microstructure stability of fuel/target materials under the hostile environment. In this presentation, structure and stability of radiation-induced defects in the oxide ceramics is reported. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) is utilized to examine the following topics focucing on role of electronic excitation and selective displacement damage: (1) effects of selective displacement damage of oxygen sublattice in fluorite-type oxides, such as stabilided cubic ZrO2, CeO2 [1-3], (2) atomic structure of ion tracks in CeO2 and MgAl2O4 induced by swift heavy ions, and microstructure evolution at high fluences under overlapping irradiation of high density electronic excitation damage [4-6], and (3) production and stability of defects in MgAl2O4 and alumina (-Al2O3) under synergistic irradiation with dispacement damage and electronic excitation [7].

Study of valence band tailing effect induced by electronic excitations in nanocrystalline cadmium oxide thin films

Nanocrystalline Cadmium oxide (nc-CdO) thin films were deposited on indium tin oxide (ITO) coated glass substrates by the sol–gel spin coating technique. The prepared films were irradiated with energetic ions for two different fluences for inducing electronic excitations and/or ionizations. Films are investigated for structural studies using grazing incidence X-ray diffraction (GIXRD) and for optical studies by UV–vis absorption spectroscopy. The structural study reveals that the films are polycrystalline in nature with cubic phase and the average crystallite size is in the range 24–27nm, which exhibit insignificant change upon irradiation. However, films show an increase in the band gap from 2.58±0.02 to 2.72±0.02eV with increasing in the ion fluence. The initial high value of the band gap is attributed to Burstein–Moss (BM) shift, while the increase in band gap upon irradiation is explained by disorder induced valence band tailing states created by high density of electronic excitations in the lattice.

Influence of complex impurity centres on radiation damage in wide-gap metal oxides

Different mechanisms of radiation damage of wide-gap metal oxides as well as a dual influence of impurity ions on the efficiency of radiation damage have been considered on the example of binary ionic MgO and complex ionic–covalent Lu3Al5O12 single crystals. Particular emphasis has been placed on irradiation with ∼2GeV heavy ions (197Au, 209Bi, 238U, fluence of 1012ions/cm2) providing extremely high density of electronic excitations within ion tracks. Besides knock-out mechanism for Frenkel pair formation, the additional mechanism through the collapse of mobile discrete breathers at certain lattice places (e.g., complex impurity centres) leads to the creation of complex defects that involve a large number of host atoms. The experimental manifestations of the radiation creation of intrinsic and impurity antisite defects (Lu|Al or Ce|Al – a heavy ion in a wrong cation site) have been detected in LuAG and LuAG:Ce3+ single crystals. Light doping of LuAG causes a small enhancement of radiation resistance, while pair impurity centres (for instance, Ce|Lu–Ce|Al or Cr3+–Cr3+ in MgO) are formed with a rise of impurity concentration. These complex impurity centres as well as radiation-induced intrinsic antisite defects (Lu|Al strongly interacting with Lu in a regular site) tentatively serve as the places for breathers collapse, thus decreasing the material resistance against dense irradiation.

Effect of laser polarization and crystalline orientation on ZnO surface nanostructuring in the regime of high-density electronic excitation

The effect of laser polarization and crystalline orientation on nanoscale morphology of ZnO monocrystal has been studied in the regime of high-density electronic excitation related to the saturation of the exciton fluorescence and appearance of the electron-hole plasma continuum. The irradiation with femtosecond KrF laser (248 nm, 450 fs) was realized in the sub-melting regime of fluences about a half of the ablation threshold. Two types of nanostructures were observed: holes of 10 nm diameter arranged in quasi-periodic zigzag and straight lines, which propagate along the crystalline planes a, c, and m. The nanostructuring sensitively depends on the crystalline plane’s orientation to laser electric field polarization (p¯). In crystalline planes c and m, the zigzag and straight lines propagate along the crystalline cell axes with respectively small (≤30°) and large (90°) inclination to p¯. The nanostructuring in the crystalline plane a is rare and of low intensity. This behavior can be related to a stress field due to an accumulation of space charges in the crystalline structure.

Modification of ZnO thin films induced by high-density electronic excitation of femtosecond KrF laser

The effect of laser irradiation on nanoscale morphology of ZnO thin films has been studied in the regime of high-density electronic excitation. A femtosecond KrF laser (248 nm, 450 fs) was used to irradiate the films prepared by solgel (SG) and pulsed laser deposition (PLD) methods with fluences below ablation threshold and above that of Mott density. This regime corresponds to electron-hole plasma (EHP) creation, resulting in a specific nanostructuring of monocrystalline ZnO surfaces. A strong nanoscale modification of thin films in the domain of laser fluences 105  mJ/cm2<E<180  mJ/cm2 has been evidenced. While PLD films maintain a smooth surface with nanoholes arranged in straight and zig–zag lines (similar to ZnO monocrystals) surrounded by domains of ∼100  nm size, SG films show the flake-like structure with an open porosity. This behavior can be related to the density of the prepared films.

Defect formation and accumulation in CeO2 irradiated with swift heavy ions

We have investigated microstructure evolution in CeO2 irradiated with 210MeV Xe ions by using transmission electron microscopy to gain the fundamental knowledge on radiation damage induced by fission fragments in nuclear fuel and transmutation target. Analysis on the accumulation of ion tracks has revealed an influence region to recover pre-existing core damage regions of ion tracks to be 8.4nm in radius. Cross section observations showed that high-density electronic excitation induces both ion tracks and dislocation loops. At high fluences of 1.5×1019 and 1×1020ionsm−2, depth-dependent microstructure was developed with radiation-induced defects of ion tracks, dislocation loops (dot-contrast) and line dislocations. Formation of sub-divided small grains was found at shallow depth at a fluence of 1×1020ionsm−2. The microstructure evolution was discussed in terms of the accumulation of interstitials due to significant overlap of high density electronic excitation.

Development of dual-beam system using an electrostatic accelerator for in-situ observation of swift heavy ion irradiation effects on materials

We have developed the dual beam system which accelerates two kinds of ion beams simultaneously especially for real-time ion beam analysis. We have also developed the alternating beam system which can efficiently change beam species in a short time in order to realize efficient ion beam analysis in a limited beam time. The acceleration of the dual beam is performed by the 20UR Pelletron™ tandem accelerator in which an ECR ion source is mounted at the high voltage terminal [1,2]. The multi-charged ions of two or more elements can be simultaneously generated from the ECR ion source, so dual-beam irradiation is achieved by accelerating ions with the same charge to mass ratio (for example, 132Xe11+ and 12C+). It enables us to make a real-time beam analysis such as Rutherford Back Scattering (RBS) method, while a target is irradiated with swift heavy ions. For the quick change of the accelerating ion beam, the program of automatic setting of the optical parameter of the accelerator has been developed. The switchover time for changing the ion beam is about 5min. These developments have been applied to the study on the ion beam mixing caused by high-density electronic excitation induced by swift heavy ions.

Molecular dynamics simulations of swift heavy ion induced defect recovery in SiC

Swift heavy ions induce a high density of electronic excitations that can cause the formation of amorphous ion tracks in insulators. No ion tracks have been observed in the semiconductor SiC, but recent experimental work suggests that irradiation damaged SiC can undergo defect recovery under swift heavy ion irradiation. It is believed that local heating of the lattice due to the electronic energy deposition can anneal, and thereby recover, some of the disordered structure. We simulate the local heating due to the ions by the inelastic thermal spike model and perform molecular dynamics simulations of different model damage states to study the defect recovery on an atomistic level. We find significant recovery of point defects and a disordered layer, as well as recrystallization at the amorphous-to-crystalline interface of an amorphous layer. The simulation results support the swift heavy ion annealing hypothesis.

Disorder induced semiconductor to metal transition and modifications of grain boundaries in nanocrystalline zinc oxide thin film

This paper report on the disorder induced semiconductor to metal transition (SMT) and modifications of grain boundaries in nanocrystalline zinc oxide thin film. Disorder is induced using energetic ion irradiation. It eliminates the possibility of impurities induced transition. However, it is revealed that some critical concentration of defects is needed for inducing such kind of SMT at certain critical temperature. Above room temperature, the current-voltage characteristics in reverse bias attributes some interesting phenomenon, such as electric field induced charge transfer, charge trapping, and diffusion of defects. The transition is explained by the defects induced disorder and strain in ZnO crystallites created by high density of electronic excitations.

Creation and clustering of Frenkel defects at high density of electronic excitations in wide-gap materials

A complex nature of the dependence of the intensity of intrinsic or impurity emission on the excitation density by single electron pulses is determined by the existence or absence of self-trapped holes and/or excitons in ZnS, BaF2, MgO, BeO and Al2O3. A powerful electron (300keV) or ion (Au, U, ∼2GeV) irradiation of pure and doped LiF, MgO and Al2O3 crystals induces the optical absorption, certain high-temperature annealing stages of which appear only under high LET conditions. Swift-ion-irradiation causes drastic changes in the spectrum of fundamental reflection of LiF, especially in the region of the exciton resonance. The irradiation providing high density of electronic excitations (LET>20keV/nm) leads not only to the creation of stable Frenkel defects but also to the excitation of a whole group of crystal ions, thus, causing the creation of bivacancies, lithium and fluorine interstitials as well as their associations/clusters.

Surface modification of monocrystalline zinc oxide induced by high-density electronic excitation

Strong modifications of semiconductors can be provoked by high-density electronic excitation. We report on surface structuring of monocrystalline wurtzite O-face (0001) ZnO excited by UV femtosecond laser pulses (248 nm) below the ablation threshold. At fluences above 11 mJ/cm2, nanoholes of D=10 nm diameter appear quasi-periodically separated by a distance ∼30 nm (=3 D). Dual-pulse (pump-pump) experiments permit estimation of the electronic excitation lifetime responsible for this nanostructuring, which is in agreement with the electron-hole plasma lifetime 220 ps. The nanostructuring results in a smaller monocrystalline domain of ∼0.1 μm size and increases the crystalline interplane c-distance by 0.11%. The excitonic luminescence of the irradiated sample is found to increase by about 10 times. The nanostructuring remains stable in a limited range of laser fluences: above 40 mJ/cm2 the surface melts, which accelerates the photoinduced bonds breaking leading to surface erosion. We tentatively ascribe the related mechanism to the nucleation-growth of cluster vacancies at crystal dislocations accelerated by the non-thermal (electronic) melting of the surface layer. At fluences lower than 11 mJ/cm2, larger volcano-like features of 60-nm diameter were observed. The characteristic crater shape and irregular surface repartition permit their assignment to thermal explosion of impurities due to multiple exciton condensation.

Electronic excitations and defect creation in wide-gap MgO and Lu3Al5O12 crystals irradiated with swift heavy ions

A comparative study of radiation effects in two groups of single crystals with an energy gap of about 8eV possessing drastically different lattice and electron energy structures − fcc MgO and Lu3Al5O12 with 160 atoms per a unit cell − has been performed using crystal irradiation with vacuum ultraviolet radiation, electrons, fast fission neutrons and, in particular, ∼2.2GeV uranium ions. In MgO with the absence of self-trapping for valence holes, the localization of holes near impurity ions or bivacancies (both as-grown or induced by a plastic stress) has been detected. In LuAG, the peculiarities of the motion of hole polarons and excitons, the radius of which is smaller than the size of a unit cell, have been revealed and analysed. The irradiation of MgO and LuAG with swift heavy ions providing an extremely high density of electronic excitations causes also the nonimpact creation of long-lived Frenkel defects in an oxygen sublattice.

Radiation damages in CeO 2 thin films irradiated with ions having the same nuclear stopping and different electronic stopping powers

In order to characterize a possible modification due to high-density electronic excitation, thin films of CeO 2 have been irradiated with 10-MeV Ni and 120-MeV Xe ions, both having the same nuclear stopping powers, while having different electronic stopping powers. The comparison of the fluence dependence of X-ray diffraction (XRD) peaks suggests that the defect production cross section for 120-MeV Xe is nearly one order of magnitude higher than that for 10-MeV Ni. The comparative study demonstrates the prominent damage observed for 120-MeV Xe irradiation is due to high electronic energy deposition.

Clarification of high density electronic excitation effects on the microstructural evolution in UO 2

In order to understand the properties of ion tracks and the microstructural evolution under accumulation of ion tracks in UO 2, 100 MeV Zr 10+ and 210 MeV Xe 14+ ions irradiation examinations have been done at a tandem accelerator facility of JAEA-Tokai, and it has been observed the microstructure by means of a transmission electron microscope (TEM) and a scanning electron microscope (SEM) in CRIEPI. Comparison of the diameter of ion tracks between UO 2 and CeO 2 under irradiation with 100 MeV Zr 10+ and 210 MeV Xe 14+ ions at room temperature clarify that the sensitivity on high density electronic excitation of UO 2 is much less than that of CeO 2. By the cross-sectional observation of UO 2 under irradiation with 210 MeV Xe 14+ ions at 300 °C, elliptical changes of fabricated pores that exist till ∼6 μm depth and the formation of dislocations have been observed in the ion fluence over 5 × 10 14 ions/cm 2. The drastic changes of surface morphology and inner structure in UO 2 indicate that the overlapping of ion tracks will cause the point defects, enhance the diffusion of point defects and dislocations, and form the sub-grains at relatively low temperature.

高密度電子励起により誘起されるスピネル化合物中の不規則化領域の評価

高密度電子励起により誘起されるスピネル化合物中の不規則化領域の評価

Luminescence of modified nonbridging oxygen hole centers in silica and alkali silicate glasses

This paper reports on the results of the investigation of the cathodoluminescence spectra of silica and alkali silicate glasses upon excitation with a pulsed electron beam (energy, 180 keV; current density, 700 A/cm2; pulse duration, 2 ns). The luminescence band observed in the energy range 2.4–2.6 eV is assigned to modified structural defects of the ≡Si-O·/Me+ type. These defects are revealed under high-density electronic excitation and, unlike the known L centers in alkali silicate glasses, are interpreted as a variety of nonbridging oxygen hole centers (defects of the dangling bond type) subjected to a disturbing action of the nearest neighbor alkali metal cations. The cathodoluminescence of similar centers is observed in neutron-irradiated silica glasses with lithium impurities; alkali silicate glasses with Li, Na, and K cations; and glasses in the two-alkali Na-K systems. It is established that the energy of the radiative transition of a modified nonbridging oxygen hole center, namely, ≡Si-O·/Me+, depends on the alkali cation type.