Quasiparticle lifetime in a two-dimensional semimetal

The quasiparticle lifetime in a two-dimensional direct band semimetal limited by inelastic Coulomb scattering has been calculated in the case where two types of charge carriers are present. It has been shown that the electron lifetime at low and high concentrations of holes is mainly determined by the electron-hole and electron-electron scattering, respectively.     

Phonons in graphite studied by eels

We have studied the phonon spectrum of Graphite using Reflection Electron Energy Loss Spectroscopy (REELS), measuring high frequency dispersion curves in regions of the Brillouin Zone which have not been accessed in previous experiments. The results are compared with calculated (bulk) phonon dispersion curves.     

Many-body interactions in hole-doped high-Tc cuprates studied by high-resolution ARPES

We have performed high-resolution angle-resolved photoemission spectroscopy on hole-doped high-T_c cuprate superconductors (HTSCs) to study many-body interaction and the universality of low-energy excitation gap. In Bi₂Sr₂CaCu₂O₈ (Bi2212), we observed a kink in the dispersion in the off-nodal region in the superconducting state, which remarkably weakens on impurity substitution. We also find that the appearance of the kink in the off-nodal region is a common feature of Bi2212 and La_1.85Sr_0.15CuO₄ (LSCO), while the energy scale is remarkably different between two compounds (70 and 20 meV). We discuss universality of the kink in dispersion in the hole-doped HTSCs in terms of the coupling of electrons with spin fluctuations.     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.     

铝纳米晶的正电子湮没研究

采用自悬浮定向流-真空热压法,在不同压强下制得铝纳米晶材料,并利用X射线衍射(XRD)和正电子湮没寿命谱(PALS)分析手段对铝纳米晶的结构和微观缺陷进行表征.XRD分析表明:所制备的铝纳米晶的晶粒度为48 nm.PALS分析表明:铝纳米晶的微观缺陷主要为类空位以及空位团,而微孔洞很少;短寿命τ1,中间寿命τ2以及其对应的强度I1,I2随压强变化而呈现阶段性变化;压制压强(P)低于0.39 GPa时制得的纳米晶空位团随压强的增加而逐渐转变为类空位;0.39 GPa P 0.72 GPa时,各类缺陷发生消除;P 0.72 GPa时,各类缺陷进一步发生消除.随压强的提高,铝纳米晶的密度增加,其显微硬度也明显增高.     

Intercalation of indium chloride in graphite studied by TDPAC

TDPAC studies of graphite indium chloride reveal the presence of three sites for intercalated In Cl₃. Two sites correspond to static efg interactions, for which the nuclear quadrupole interaction parameters were determined. It is suggested that the third site corresponds to a time-dependent inter-action as a result of after-effect following the electron decay of¹¹¹In.     

Defects in electron irradiated GaP studied by positron lifetime spectroscopy

Defects induced by electron irradiation were investigated in GaP. The irradiation was performed at 15 K with an incident electron energy of 2 MeV and a fluence of 10¹⁸cm^–2. Annealing experiments were carried out in the temperature range between 100 and 1000 K.Ga and P vacancies were detected after electron irradiation and the different annealing behaviour of the two types of vacancies was observed. The recovery stage between 100 and 400 K was attributed to the annealing of Ga vacancies and the recovery at temperatures above 900 K to the annealing of P vacancies.We also performed Hall measurements to determine the location of the Fermi level in the bandgap during the annealing. Two different ionization levels of the P vacancy were found which can be attributed to the transitions V _P ⁺ /V _P ^o and V _P ^o /V _P ^– .Temperature-dependent measurements were performed to study the effect of shallow positron traps.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

Intercalation of potassium in graphite studied by thermal desorption spectroscopy

Thermal desorption spectra obtained after multilayer potassium depositions on graphite at T_s = 160 K reveal information about the formation and decomposition of potassium/graphite intercalation structures. Up to six desorption peaks were observed, of which three could be attributed to decomposition of three different intercalation-like surface structures. These structures are formed as the deposited potassium diffuses into the graphite substrate during the recording of the thermal desorption spectra. At submonolayer coverages, potassium desorbs in a sharp peak around 500 K, attributed to decomposition of a single intercalated potassium layer positioned between the two uppermost graphite layers.     

Radiation defect clustering in graphite studied by synchrotron radiation scattering

The formation of radiation defect clusters as interstitial dislocation loops in pyrographite irradiated by 20-MeV carbon ions in a cyclotron has been studied by means of synchrotron radiation scattering. The experimental data are interpreted in terms of the Krivoglaz model and the model of deformation domain formation by dislocation loops. Parameters of the pyrographite crystal lattice in the deformed state upon ion irradiation have been determined using experimental data on the shift and broadening of the Bragg peaks. The results of experiments and their theoretical analysis indicate that the phenomena of the crystal lattice densification (shrinkage) and the radiation-induced swelling (increase in volume) observed for graphite irradiated by fast neutrons in a nuclear reactor can be studied in model experiments on charged particle accelerators using irradiation of the samples with carbon ions at much lower doses and significantly shorter exposure times.     

Xenon-plasma light ultrahigh-resolution ARPES study of low-energy single-particle excitation gap in (Bi,Pb)2Sr2CuO6

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy of (Bi,Pb)₂Sr₂CuO₆ by using a newly developed xenon-plasma light source to clarify the origin of the pseudogap (PG). We determined the comprehensive momentum and temperature dependences of the superconducting (SC) gap and the PG, and revealed a smooth evolution of the PG from the SC gap. We also found a linear scaling behavior of the characteristic PG temperature with the SC gap size regardless of the momentum location. These experimental results strongly suggest that the observed PG is caused by the precursor pairing.     

On the problem of reactor graphite lifetime

Fine-grained dense graphites of different manufacturers were tested and studied for the purpose of predicting their lifetimes at high (∼2000°C) temperatures. The tests consisted in sample heating by electric current to a destruction temperature. The study included a set of electron microscopy, X-ray diffraction, and electrical methods. X-ray diffraction data for initial samples were compared with data for domestic MPG graphite. Scanning electron microscopy was applied to both initial and heated samples. The lifetime prediction was based on the Zhurkov classical formula. It was shown that CGD, Carbone Lorraine, and MPG graphite composites can differ appreciably in both structural characteristics and high-temperature lifetimes.     

Anomalous quasiparticle lifetime and strong electron-phonon coupling in graphite

We have performed ultrahigh-resolution angle-resolved photoemission spectroscopy on high-quality single crystals of graphite to elucidate the character of low-energy excitations. We found evidence for a well-defined quasiparticle (QP) peak in the close vicinity of the Fermi level comparable to the nodal QP in high-T(c) cuprates, together with the mass renormalization of the band at an extremely narrow momentum region around the K(H) point. Analysis of the QP lifetime demonstrates the presence of strong electron-phonon coupling and linear energy dependence of the QP scattering rate indicative of a marked deviation from the conventional Fermi-liquid theory.     

Electronic superstructures on the graphite surface studied by first-principles calculations

We present results of a first-principles study of the graphite surface in the presence of defects. Our calculations, based on density functional theory, show superstructures of periodicity ( ) in the electronic structure of the surface. In good agreement with STM experiments, these superstructures show a variety of patterns with their intensity decaying away from the defect. Two kind of defects were considered: metallic adatoms absorbed on graphite, and vacancies in the surface lattice. Similar results were found in both cases. Our results give strong support to the idea that these superstructures are due to purely electronic effects, and do not correspond to any atomic reconstruction of the graphite surface.     

Quasiparticle relaxation in Heavy Fermions studied using Inverse Fourier Transform of optical conductivity

Inverse Fourier Transform of optical conductivity is used for studies of quasiparticle relaxation in Heavy Fermions in time domain. We demonstrate the usefulness of the procedure on model spectra and then use it to study quasiparticle relaxation in two Heavy Fermions YbFe₄Sb₁₂ and CeRu₄Sb₁₂. Optical conductivity in time domain reveals details of quasiparticle relaxation close to the Fermi level, not readily accessible from the spectra in the frequency domain. In particular, we find that the relaxation of heavy quasiparticles does not start instantaneously, but typically after a few hundred femto-seconds.     

Helium implanted CuHf as studied by TDPAC and positron lifetime measurements

¹⁸¹Ta time differential perturbed angular correlation (TDPAC) and positron lifetime measurements were carried out on homogeneously α-implanted CuHf samples. TDPAC measurements indicate the trapping of vacancy clusters and helium associated defect complexes by Hf atoms. The presence of helium-vacancy complexes and helium stabilised voids has been identified by positron lifetime measurements. Further the nucleation and growth stages of helium bubbles have been identified. TDPAC and positron lifetime measurements indicate that Hf atoms act as heterogeneous nucleating centers for helium bubbles. Hf atoms are found to suppress the bubble growth in CuHf as indicated by the results of positron lifetime measurements.     

Phase transition in SbCl5 - graphite studied by spin resonance experiments

Conduction Carrier Spin Resonance (CCSR) experiments in stage 3 and 4 acceptor SbCl₅ - Graphite Intercalated Compounds (GICs) reveal an almost stage independent phase transition at T_c ≋ 225K. The CCSR linewidth shows an anomalous broadening below T_c, which is observable only when the sample is kept in a region close to T_c (190–225K); the broadening was not observed for rapid cooling. This broadening has a much faster kinetics for heating than for cooling rum. These thermal hysteresis and kinetics are quite similar to those found in AlCl₃-GICs. We attribute this phase transition to the quasi-2D “solid liquid” transformation experienced by the minority molecules of SbCl₃ present among the intercalated species in SbCl₅-GICs, proposed by Homma and Clarke. For stage 2 data does not show evidences of this phase transition.