Structure and electronic properties of BEDT-TTF and it's charge transfer salts

We report a detailed computational study, of the organic superconducting charge transfer BEDT-TTF salts, whose electronic properties depend strongly on the packing of the donor radical cations. Electronic structure calculations have been performed at the molecular and periodic level using density functional theory (DFT). These calculations have been used to investigate the magnetic ordering in the BEDT-TTF[FeBr₄] salt, where we obtained an antiferromagnetic solution in agreement with experiment. Detailed geometry optimisations have been carried out for both BEDT-TTF[FeBr₄] salt and the neutral BEDT-TTF crystal.     

Charge fluctuation,charge ordering,and zero-gap state in organic conductors

We have carried out a series of measurements of angular dependence of solid-state NMR spectrum using single crystal samples on various organic molecular conductors, in order to investigate the natures of the electronic states at low temperatures. We confirmed a charge ordered insulating state in α-(BEDT-TTF)₂I₃ and large charge disproportionation in the metallic state of this salt. In another charge ordered system, θ-(BEDT-TTF)₂RbZn(SCN)₄, we observed unusual NMR line broadening, proportional to resonance shift, in the metallic state above the transition. We found that this broadening is due to charge disproportionation, or more correctly, due to the inhomogeneity of local susceptibility at nuclear sites and analyzed its dynamics. We observed similar broadening in various organic molecular conductors as well, such as θ-(BEDT-TTF)₂CsZn(SCN)₄, an exotic Bechgaad salt, (TMTSF)₂FSO₃, and λ-type BETS salts, λ-(BETS)₂(Fe,Ga)Cl₄. We found the mechanism of CD in each system is different, respectively.     

Effect of deuteration on the electronic and electron-vibrational properties of the organic conductor k-(D8-BEDT-TTF)2[Hg(SCN)2Br]

We present here for the first time polarized reflection spectra and optical conductivity spectra of single crystals of the newly deuterated organic conductor k-(D₈-BEDT-TTF)₂[Hg(SCN)₂Br] at room temperature. The spectral region investigated is 700–40 000 cm⁻¹. We examined the effect of deuteration on electronic and electron-vibrational transitions observed in the spectra. The observed shift of the electron “dimer” transition in the infrared toward lower frequencies upon deuteration is linked with an increase in the interaction between neighboring, mutually perpendicular dimers in the structure of the deuterated crystal. A lowering of the symmetry of the BEDT-TTF molecule is demonstrated in crystals similar to k-(BEDT-TTF)₂[Hg(SCN)₂Br], relative to the symmetry D _2h of the free molecule. We refine the assignment of the spectral features determined by the interaction of electrons with the fully-symmetric intramolecular vibrations of the C=C, C-S, and C-C-H bonds of the BEDT-TTF molecule. Fiz. Tverd. Tela (St. Petersburg) 40, 1595–1598 (September 1998)     

Interpretation of scanning tunneling microscopy and atomic force microscopy images of 1T-TiS2

The surface of 1T-TiS₂ was examined by scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The STM and AFM images of this compound were interpreted on the basis of the partial electron density ρ(r,E_F) and total electron density ρ(r) of a slab which consists of six (001) 1T-TiS₂ layers. Electronic structure calculations were performed using the ab-initio Hartree–Fock program crystal. It was found that the bright spots in experimental STM images correspond to sulfur atoms at both positive and negative bias voltages. The AFM image showed a periodicity which can be explained by the atomic corrugation at the surface. Structural defects on the surface were also investigated, and their interpretation constitutes experimental proof that only sulfur atoms were detected by scanning probe microscopies.     

One-dimensional electronic order in underdoped surface of YBa2Cu3Oy studied by STM

We have performed low-temperature scanning tunneling microscopy (STM) experiments on the cold-cleaved surface of YBa₂Cu₃O_y single crystals to study the nanoscale electronic order in high-T_c superconductors. STM images measured at low-bias voltage below ∼50 meV show the one-dimensional (1D) electronic modulation along the Cu-O bonds (parallel to the b-axis). The 1D electronic modulation does not have long-range order and the periodicity along the a-axis varies within the range ∼2a-4a depending on the position on the surface, indicating the glassy electronic order in the underdoped CuO₂ plane.     

Determination of C60/C70 ratios in fullerene mixtures and film characterization by scanning tunneling microscopy

Fullerene powder mixtures with different C₆₀/C₇₀ ratios have been analyzed by a variety of techniques, and results have been compared. The fullerence mixtures have been characterized as solutions in n-hexane by high-pressure liquid chromatography (HPLC) and UV-VIS spectroscopy. Thin films of fullerenes on Au(111) have been prepared from the mixtures by sublimation. The sublimation process has been studied by simultaneous thermogravimetric and differential thermal analyses. Thin fullerene films on Au(111) have been investigated by scanning tunneling microscopy (STM). The STM images show primarily two types of ballshaped molecules arranged in a lattice with hexagonal symmetry (fcc(111) face, nearest neighbour distance: 1 nm). The two species differ in diameter. STM images of films made of mixtures of different C₆₀/C₇₀ ratios show that C₇₀ molecules display a larger apparent diameter (0.8 nm) and corrugation than C₆₀ molecules (0.7 nm). The C₆₀/C₇₀ ratios obtained by counting the corresponding molecular species in the STM images of the thin films are compared to the C₆₀/C₇₀ ratios determined by HPLC on hexane solutions of the mixtures. The observed differences might be explained by different rates of sublimation for the two species. The STM images reveal film defects (vacancies and boundaries) and dynamic processes (displacement of C₇₀ molecules and vacancies). In films prepared to have a C₆₀ coverage of less than one monolayer, stable structural units of the C₆₀(111) surface consisting of three or seven C₆₀ molecules are revealed by STM. Occasionally, substructure within individual fullerene molecules is observed.     

Structure of AuSi nanoparticles on Si(111) from reflection high-energy electron diffraction and scanning tunneling microscopy

Gold-rich Au _x Si_1−x particles grown on Si(111)7 × 7 are studied by reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The diffraction patterns reveal that (1) at least two different crystal structures coexist on the substrate; (2) the most prominent data correspond to a rhombohedral or quasi closed-packed structure; and (3) the particles show formation of an unusual contact facet to the substrate. Complete crystal alignment of the particles to the substrate lattice is found with no hints of random orientation. The findings are compared to STM images in terms of their structure, orientation, and morphology.     

Spin dynamics of charge carriers in the process of their localization in α’-(BEDT-TTF)<Subscript>2</Subscript>IBr<Subscript>2</Subscript> single crystals

Sharp changes in the integral intensity and linewidth of the ESR spectrum that accompany the localization of the charge carriers have been revealed in α’-(BEDT-TTF)₂IBr₂ crystals. It has been found that the types of localization in two compounds under investigation are different: charge carriers in β″-(BEDT-TTF)₄NH₄[Cr(C₂O₄)₃] are localized on irregular defects of the crystal lattice, whereas charge carriers in α’-(BEDT-TTF)₂IBr₂ are localized at the regular positions of the unit cell. The exchange narrowing of the ESR line and a sharp decrease in the dc and ac magnetic susceptibilities are observed in α’-(BEDT-TTF)₂IBr₂ at low temperatures T < 50 K. The dc and ac magnetic susceptibilities observed in α’-(BEDT-TTF)₂IBr₂ at high temperatures T > 50 K differ from each other, because the thermally activated hopping frequency of the charge carriers is higher than the frequency of the measuring UHF field of an ESR spectrometer.     

Resonant Raman scattering from superconducting single crystals of (BEDT-TTF)2I3

A study of low energetic resonant Raman scattering of (BEDT-TTF)₂I₃ superconducors was performed. The vanishing of phonon bands accompanied by a decrease of the electronic background was observed belowT _c . We propose a theoretical explanation for this novel effect in terms of the Balseiro-Falicov model of phonon-superconducting amplitude mode interaction.     

Structural and electronic properties of a new molecular conductor, α-(BEDT-TTF)2CsCd(SCN)4

Structural and electronic properties of a new BEDT-TTF based radical cation salt, α-(BEDT-TTF)₂CsCd(SCN)₄, are presented. In the measurements of the electrical resistivity and the magnetic susceptibility, this new α-type salt shows metallic behavior down to low temperature. Measurements of the resistivity under in-plane c-axial strain reveal a newly observed insulating phase, suggesting that α-(BEDT-TTF)₂CsCd(SCN)₄ is placed near an insulating phase which might have close relationships with a superconducting phase realized in α-type salts.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Sr/Si界面沉积SrTiO3初始生长阶段的扫描遂道显微术研究

本文工作利用脉冲激光沉积术(PLD)和超高真空扫描隧道显微术(UHV-STM),研究了在Sr/Si(001)-(2×1)衬底表面上真空室温沉积几个单层SrTiO₃薄膜的初始生长过程.经660 ℃退火处理后,Sr/Si衬底表面上形成了纳米岛状结构.经分析,这些纳米小岛为C49-TiSi₂和 C54-TiSi₂.实验结果表明,在没有氧气的情况下退火,Sr/Si界面无法有效阻止SrTiO₃薄膜与Si衬底之间的相互作用. 关键词： 脉冲激光沉积术(PLD) 扫描隧道显微镜(STM) 3')" href="#">SrTiO₃ 2')" href="#">C54-TiSi₂     

Mismatch dislocations caused by preferential sputtering of a platinum-nickel alloy surface

Preferential sputtering and recoil mixing of a Pt₂₅Ni₇₅(111) single crystal surface leads to platinum enrichment in the upper monolayers, thereby increasing the lattice constant in these layers. This results in subsurface lattice mismatch dislocations, which have been studied by scanning tunneling microscopy. While the subsurface dislocations are only visible as shallow ditches in STM topographs, the Burgers vectors of the dislocation system can be determined by means of atomically resolved images of dislocations reaching the surface. A comparison with simulations of lattice relaxation using embedded-atom potentials shows good agreement with STM data and further allows the determination of the thickness of the Pt enrichment. We have estimated the Pt concentration in these layers from the dislocation density and studied the annealing behaviour of the surface.     

Organic conductors and superconductors: A comparative survey

The nature of the electronic and structural ground states observed in the two-stack TCNQ charge transfer salts, the [TMTS(T)F]₂X and (BEDT-TTF)₂X single-stack series successively synthesized during the last 15 years, is reviewed. Special attention is devoted to physical parameters, such as dimensionality of the electron gas, electron-electron interactions and coupling between electronic and structural degrees of freedom, which seem to control the physics of these organic conductors.     

Molecular designing analysis of a new superconducting metal dithiolene complex

The map of the intermolecular overlap integrals(S) of the lowest unoccupied molecular orbital(LUMO) of Ni(dmit)₂ was calculated by varying the intermolecular configuration parameters in order to examine the dimensionality of the electronic band structure of the molecular conductors based on Ni(dmit)₂. Despite the similarity of the molecular structure between Ni(dmit)₂ and BEDT-TTF, the intermolecular transverse interaction of Ni(dmit)₂ becomes very weak. Although the crystal structure of a new molecular superconductor (TTF)[Ni(dmit)₂]₂ exhibits the three-dimensional (3-D) sulfur network, its tight-binding band calculation gives a strongly 1-D band.     

Surface potential images of polycrystalline organic semiconductors obtained by Kelvin probe force microscopy

P-type copper phthalocyanine (CuPc) and n-type hexadecafluorophthalocyanina-tocopper (F₁₆CuPc) polycrystalline films were investigated by Kelvin probe force microscopy (KPFM). Topographic and corresponding surface potential images are obtained simultaneously. Surface potential images are related with the local work function of crystalline facets and potential barriers at the grain boundaries (GBs) in organic semiconductors. Based on the spatial distribution of surface potential at GBs, donor- and acceptor-like trapping states in the grain boundaries (GBs) of p-CuPc and n-F₁₆CuPc films are confirmed respectively. In view of spatial energy spectrum in micro-scale provided by KPFM, it is going to be a powerful tool to characterize the local electronic properties of organic semiconductors.     

Scanning tunneling microscopy studies of diazo dye monolayers on HOPG

We report on scanning tunneling microscopy (STM) studies of monolayers of the diazo dye 4-[4-(N,N-dimethylamino)phenylazo]azobenzene (D2, summation formula C₂₀H₁₉N₅) on the basal plane of highly oriented pyrolytic graphite (HOPG). Monolayers of the dye were prepared by vapour deposition or by dissolving the molecules in the liquid crystal octylcyanobiphenyl (8CB). The STM images show a double-row structure exhibiting different types of lattice defects and various domains. High-resolution images allow the identification of individual molecules and the observation of intramolecular contrast. The different orientations of the rows can be explained by a commensurate registry of the molecules with the substrate. A model for the unit cell is proposed.     

Tm:Y2SiO5晶体的生长和光谱性质研究

采用中频感应提拉法生长了高质量的Tm:Y₂SiO₅(Tm:YSO)晶体,测定了晶体的晶格常数和分凝系数.运用劳厄照相法确定了单斜晶系Tm:YSO晶体的三个偏振轴〈010〉,D₁和D₂,在室温下测量了三个偏振轴方向的吸收光谱、荧光光谱和荧光寿命,计算了晶体吸收峰的吸收线宽和吸收截面.研究发现,相对于其他两个偏振轴方向,D₁方向在790nm处出现较强的吸收峰, 关键词： 2SiO₅')" href="#">Tm:Y₂SiO₅ 单斜晶系 吸收光谱 荧光光谱     

Highly efficient transparent Zn2SiO4:Mn phosphor film on quartz glass

Highly efficient transparent Zn₂SiO₄:Mn²⁺ film phosphors on quartz substrates were deposited by the thermal diffusion of sputtered ZnO:Mn film. They show a textured structure with some preferred orientations. Our film phosphor shows, for the best photoluminescence (PL) brightness, a green PL brightness of about 20% of a commercial Zn₂SiO₄:Mn²⁺ powder phosphor screen. The film shows a high transmittance of more than 10% at the red-color region. The excellence in PL brightness and transmittance can be explained in terms of the textured crystal growth with a continuous gradient of Zn₂SiO₄: Mn²⁺ crystals.     

Structural systematics in the family of (BEDT-TTF)2X salts

A crystallographic family tree can be developed for the (BEDT-TTF)₂X salts which aids in the systematization and rationalization of the structural and electrical properties of this novel series of materials. Importantly, it is recognized that the ambient-pressure superconductor α-(BEDT-TTF)₂I₃ has a crystal structure coincident with the parent subcell of this crystallographic family tree. Moreover, all first-generation supercells (doubling of any one subcell vector) are exclusively populated by salts with non-centrosymmetric anions of tetrahedral symmetry, and apparently, second-generation supercells (doubling of any two subcell vectors) are populated by salts with centrosymmetric anions of octahedral symmetry.