Surface soft phonon and the sqrt[3] x sqrt[3] <--> 3x3 phase transition in Sn/Ge(111) and Sn/Si(111)

Density functional theory calculations show that the reversible Sn/Ge(111) sqrt[3]xsqrt[3]<-->3x3 phase transition can be described in terms of a surface soft phonon. The isovalent Sn/Si(111) case does not display this transition since the sqrt[3]xsqrt[3] phase is the stable structure at low temperature, although it presents a partial softening of the 3x3 surface phonon. The rather flat energy surfaces for the atomic motion associated with this phonon mode in both cases explain the experimental similarities found at room temperature between these systems. The driving force underlying the sqrt[3]xsqrt[3]<-->3x3 phase transition is shown to be associated with the electronic energy gain due to the Sn dangling bond rehybridization.     

Surface phase transitions induced by electron mediated adatom-adatom interaction

We propose that the indirect adatom-adatom interaction mediated by the conduction electrons of a metallic surface is responsible for the sqrt[3]xsqrt[3]<==>3x3 structural phase transitions observed in Sn/Ge (111) and Pb/Ge (111). When the indirect interaction overwhelms the local stress field imposed by the substrate registry, the system suffers a phonon instability, resulting in a structural phase transition in the adlayer. Our theory is capable of explaining all the salient features of the sqrt[3]xsqrt[3]<==>3x3 transitions observed in Sn/Ge (111) and Pb/Ge (111), and is in principle applicable to a wide class of systems whose surfaces are metallic before the transition.     

Ba_(1-x)Sr_xTiO_3相结构的拉曼光谱研究

本文测定了不同Sr含量的钛酸锶钡(Ba1-xSrxTiO3)纳米晶的拉曼光谱,发现了随着Sr含量的增加,517cm-1[E(TO)与A1(TO)]光学声子模劈裂为双峰和相对低频声子模向低频漂移,而相对高频声子模向高频漂移现象。当x=0.4时,软模解冻,Ba1-xSrxTiO3铁电相不能维持而呈现顺电相结构。     

Disproportionation phenomena on free and strained Sn/Ge(111) and Sn/Si(111) surfaces

Distortions of the sqrt[3]x sqrt[3] Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, Q, related to the surface band occupancy. A novel understanding of the (3 x 3)-1U ("1 up, 2 down") and 2U ("2 up, 1 down") distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distortions. Negative strain attracts pseudocharge from the valence band causing first a (3 x 3)-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a (sqrt[3] x sqrt[3])-3U ("all up") state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.     

Direct observation of sn adatoms dynamical fluctuations at the Sn/Ge(111) surface

The well-known low-temperature phase transition sqrt[3]xsqrt[3] to 3x3 for the 1/3 monolayer of Sn adatoms on the Ge(111) surface has been studied by scanning tunneling microscopy. The STM tip was used as a probe to record the tunneling current as a function of time on top of the Sn adatoms. The presence of steps on the current-time curves allowed the detection of fluctuating Sn atoms along the direction vertical to the substrate. We discuss the effect of temperature and surface defects on the frequency of the motion, finding consistency with the dynamical fluctuations model.     

119Sn and151Eu Mössbauer study of EuBa2(Cu1-xSnx)3O7-y perovskites

Eu and Sn Mössbauer spectroscopy was used to study EuBa₂(Cu_1-xSn_x)₃O_7-y metallic oxides. The spectra are characteristic for Eu^III and Sn^IV states in all cases. The existence of at least two different Sn states was determined by decomposition of the Sn Mössbauer spectra. These Sn sites can be associated with nonequivalent Cu sites replaced by Sn atoms in the perovskite type lattice. Anomalous changes were observed in the isomer shifts and area fractions in the spectra measured at room temperature and at 77 K. It can be interpreted assuming low temperature phase transformation and phonon softening. Time dependent changes were found in the Sn spectra recorded at 77 K in the case of highest Sn concentration. The observed changes are consistent with a transitional stage of the low temperature phase transformation.     

Neutron diffraction study of the structural phase transition in SnS and SnSe

The structural phase transitions in SnS and SnSe have been investigated by neutron diffraction in the temperature range 295–1000 K using a high temperature furnace. Accurate positional and thermal parameters have been obtained as a function of temperature both in the α- (GeS-type, B16) and β-phase (TlI-type, B33). These investigations indicate that the α → β phase transitions in SnS and SnSe are of second order displacive type, and consist mainly of the continuous movement of Sn and S/Se atoms almost entirely along the [100] direction, and suggest the soft-mode behaviour of a zone-boundary phonon of the β-phase.     

Sn/Ge(111) surface charge-density-wave phase transition

Angle-resolved photoemission has been utilized to study the surface electronic structure of 1 / 3 monolayer of Sn on Ge(111) in both the room-temperature (sqrt[3]xsqrt[3] )R30 degrees phase and the low-temperature ( 3x3) charge-density-wave phase. The results reveal a gap opening around the ( 3x3) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting. A highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the ( 3x3) symmetry.     

Neutron spin-echo investigation of slow spin dynamics in kagomé-bilayer frustrated magnets as evidence for phonon assisted relaxation in SrCr9xGa12-9xO19

A neutron spin-echo investigation of the low temperature spin dynamics in two well-characterized kagomé bilayer compounds SrCr9xGa12-9xO19 (x=0.95, SCGO) and Ba2Sn2ZnCr7xGa10-7xO22 (x=0.97, BSZCGO) reveals two novel features. One is the slowing down of the relaxation rate without critical behavior at Tg, where a macroscopic spin-glass-like freezing occurs. The second is, in SCGO at 4 K (approximately Tg)相似文献   

Mg-Sn-Si系合金的热力学基础及合金相演变过程分析

研究了Mg-Sn-Si系合金的热力学基础及合金相的演变过程. 结果表明: 对于Mg-Sn-Si系合金, 合金相的比热容随着温度增加而增加, 在低温下变化迅速, 而在高温下变化平缓, 其热膨胀系数在低温范围内随温度升高呈指数形式增加, 而在高温范围内呈线性增大. 在Mg₂ (Si_x, Sn_1-x)、Mg₂ (Sn_x, Si_1-x)相结构中, Sn(Si)原子的取代位置不固定, 可以是面心, 也可以是顶点. 常规凝固过程中, 由于处于非平衡状态, x的取值范围有所波动, 对于Mg₂ (Si_x, Sn_1-x)和Mg₂ (Sn_x, Si_1-x) 两种结构, x的取值范围在0.25或0.75附近. Mg₂ (Si, Sn)的生成温度较高, 可从液相中直接析出, 也可由Mg₂Si转化而来, 而Mg₂ (Sn, Si)的生成温度较低, 只能从基体中析出, 随着Sn含量的增加, 开始析出Mg₂ (Sn, Si)相的温度升高.     

Evolution of the 251 cm^-1 infrared phonon mode with temperature in Ba_0.6K_0.4Fe₂As₂

The far-infrared optical reflectivity of an optimally doped Ba1-xKxFe2As2(x =0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm-1 . This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Ba0.6K0.4Fe2As2 compound.     

Low temperature disordered phase of alpha-Pb/Ge(111)

A new structural phase transition has been observed at low temperature for the one-third of a monolayer (alpha phase) of Pb on Ge(111) using a variable-temperature scanning tunneling microscope. The well-known (sqrt[3] x sqrt[3])R30 degrees to (3 x 3) transition is accompanied by a new structural phase transition from (3 x 3) to a disordered phase at approximately 76 K. The formation of this "glasslike" phase is a consequence of competing interactions on different length scales.     

Determination of the Sn 4d line shape of the Sn/Ge(111) radical3 x radical3 and 3x3 surfaces

The Sn 4d line shapes of the Sn/Ge(111) sqrt[3]xsqrt[3] and 3x3 surfaces are currently under debate. By employing LEED, core-level, and valence band spectroscopy we have been able to determine the correct Sn 4d line shapes for these surfaces. Contrary to a recent study we conclude that the majority of the earlier reports present line shapes close to the correct ones. At 70 K we identify three 4d components in the 3x3 spectrum, two of which are identified with the two types of Sn atoms in the 3x3 cell. The third component is attributed to Sn atoms surrounding Ge substitutional defects.     

Theoretical prediction of the structural,elastic, electronic and thermodynamic properties of V3M (M = Si,Ge and Sn) compounds

Density functional theory (DFT), is used in our calculations to study the V₃M (M = Si, Ge and Sn) compounds, we are found that V₃Sn compound is mechanically unstable because of a negative C₄₄ = −19.41 GPa. For each of these compounds considered, the lowest energy structure is found to have the lowest N(E_f) value. Also there is a strong interaction between V and V, the interaction between M (M = Si, Ge, Sn) and V (M and M) is negative, not including Si [Sn]. In phonon density of states PDOS, the element contributions varies from lighter (high frequency) to heaviest (low frequency).     

Observation of a Mott insulating ground state for Sn/Ge(111) at low temperature

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.     

Yb-C_(60)插入化合物的拉曼光谱研究

本文主要对 Ybx C6 0 插入化合物在室温下的拉曼散射的实验结果进行了分析和讨论。名义组分为 Yb3C6 0 的化合物是超导相 ,可观测到由电 -声子相互作用引起的拉曼谱线展宽。高Yb组分的 Ybx- C6 0 ( x≥ 4)的拉曼谱线的展宽则更为严重 ,这表明 C6 0 分子已发生了严重的畸变。当 Ybx C6 0 样品暴露于空气中时 ,虽然该样品已变成了非晶态 ,但 C6 0 分子的畸变已大大的减小了。     

Effect of substrate temperature on the morphological,structural,and optical properties of RF sputtered Ge_(1-x)Sn_x films on Si substrate

In this study, Ge_(1-x)Sn_x alloy films are co-sputtered on Si(100) substrates using RF magnetron sputtering at different substrate temperatures. Scanning electron micrographs, atomic force microscopy(AFM), Raman spectroscopy, and x-ray photoemission spectroscopy(XPS) are conducted to investigate the effect of substrate temperature on the structural and optical properties of grown Ge Sn alloy films. AFM results show that RMS surface roughness of the films increases from 1.02 to 2.30 nm when raising the substrate temperature. This increase could be due to Sn surface segregation that occurs when raising the substrate temperature. Raman spectra exhibits the lowest FWHM value and highest phonon intensity for a film sputtered at 140?C. The spectra show that decreasing the deposition temperature to 140?C improves the crystalline quality of the alloy films and increases nanocrystalline phase formation. The results of Raman spectra and XPS confirm Ge–Sn bond formation. The optoelectronic characteristics of fabricated metal-semiconductor-metal photodetectors on sputtered samples at room temperature(RT) and 140?C are studied in the dark and under illumination. The sample sputtered at 140?C performs better than the RT sputtered sample.     

Evolution of the 251 cm-1 infrared phonon mode with temperature in Ba0.6K0.4Fe2As2

The far-infrared optical reflectivity of an optimally doped Ba1-xKxFe2As2(x =0.4) single crystal is measured from room temperature down to 4 K. We study the temperature dependence of the in-plane infrared-active phonon at 251 cm-1 . This phonon exhibits a symmetric line shape in the optical conductivity, suggesting that the coupling between the phonon and the electronic background is weak. Upon cooling down, the frequency of this phonon continuously increases, following the conventional temperature dependence expected in the absence of a structural or magnetic transition. The intensity of this phonon is temperature independent within the measurement accuracy. These observations indicate that the structural and magnetic phase transition might be completely suppressed by chemical doping in the optimally doped Ba0.6K0.4Fe2As2 compound.     

Phonon anomalies and superconductivity in the hcp metals,Tc, Re,and Ru

The phonon dispersion curves have been measured for the hcp metals, Tc, Re, and Ru in the [001] direction at room temperature. A pronounced phonon anomaly has been observed for the LO mode in Tc and Re which is absent in Ru. A low temperature study of Tc revealed a strong temperature dependence of the anomaly. It is suggested that the magnitude of the phonon anomaly is correlated with the strength of the electron-phonon interaction in these metals.     

Electron spin resonance study of the LaIn(3-x)Sn(x) superconducting system

The LaIn(3-x)Sn(x) alloy system is composed of superconducting Pauli paramagnets. For LaIn3 the superconducting critical temperature T(c) is approximately 0.7 K and it shows an oscillatory dependence as a function of Sn substitution, presenting its highest value T(c) ≈ 6.4 K for the LaSn3 end member. The superconducting state of these materials was characterized as being of the conventional type. We report our results for Gd3+ electron spin resonance measurements in the LaIn(3-x)Sn(x) compounds as a function of x. We show that the effective exchange interaction parameter J(fs) between the Gd3+ 4f local moment and the s-like conduction electrons is almost unchanged by Sn substitution and observe microscopically that LaSn3 is a conventional superconductor.