Single Crystalline System Research Articles

Coexistence of giant Rashba splitting, multiple band inversion, and multiple Dirac surface states in the three-dimensional topological insulator XSnBi (X=Rb,Cs)

Composite quantum compounds (CQCs) have emerged as a key tool for examining the interactions between two different physical phenomena. Some of these CQCs that have lately received a lot of community attention are topological superconductors and axion insulators, among others. Despite being two distinct quantum phenomena, Rashba spin physics and topological nontriviality can coexist in a CQC platform. In this work, we have performed ab initio calculations to discover materials that inherit both giant Rashba splitting and topological nontrivial states in a single crystalline system, coined as intrinsic CQCs. In this regard, we have investigated two materials, RbSnBi and CsSnBi, that are found to be strong topological insulators along with giant Rashba splitting energy in valence bands (133 and $228\phantom{\rule{0.16em}{0ex}}\text{meV}$, respectively) with Rashba coefficient ${\ensuremath{\alpha}}_{\mathrm{R}}$ to be 4.26 and $6.4\phantom{\rule{0.16em}{0ex}}\phantom{\rule{4.pt}{0ex}}\text{eV}\phantom{\rule{4.pt}{0ex}}\AA{}$, respectively. These values of ${\ensuremath{\alpha}}_{\mathrm{R}}$ are quite large, and the Rashba coefficient of CsSnBi is the largest, corresponding to valance bands, among previously reported topological materials. Coexisting characteristics of Rashba spin physics and topological nontriviality have the potential to unveil new physical phenomena. For both RbSnBi and CsSnBi, the topological insulating state is found to be associated with coexisting multiple band inversion and multiple Dirac surface states.

Intertwined nontrivial band topology and giant Rashba spin splitting

Composite quantum compounds (CQCs) have become an important avenue for the investigation of intercorrelation between two distinct phenomena in physics. Topological superconductors, axion insulators, etc., are a few such CQCs which have recently drawn tremendous attention in the community. Topological nontriviality and Rashba spin physics are two different quantum phenomena but can be intertwined within a CQC platform. In this paper, we present a general symmetry-based mechanism, supported by ab initio calculations, to achieve intertwined giant Rashba splitting and topological nontrivial states simultaneously in a single crystalline system. Such coexistent properties can further be tuned to achieve other rich phenomena. We have achieved Rashba splitting energy ($\mathrm{\ensuremath{\Delta}}E$) and Rashba coefficient ($\ensuremath{\alpha}$) values as large as 161 meV and 4.87 eV \AA{}, respectively, in conjunction with the Weyl semimetal phase in ${\mathrm{KSnSb}}_{0.625}{\mathrm{Bi}}_{0.375}$. Interestingly, these values are even larger than the values reported for the widely studied topologically trivial Rashba semiconductor BiTeI. The advantage of our present analysis is that one can achieve various topological phases without compromising the Rashba parameters, within this CQC platform.

Coexistence of topological nontrivial and spin-gapless semiconducting behavior in MnPO4 : A composite quantum compound

Composite quantum compounds (CQC) are classic example of quantum materials which host more than one apparently distinct quantum phenomenon in physics. Magnetism, topological superconductivity, Rashba physics etc. are few such quantum phenomenon which are ubiquitously observed in several functional materials and can co-exist in CQCs. In this letter, we use {\it ab-initio} calculations to predict the co-existence of two incompatible phenomena, namely topologically non-trivial Weyl semimetal and spin gapless semiconducting (SGS) behavior, in a single crystalline system. SGS belong to a special class of spintronics material which exhibit a unique band structure involving a semiconducting state for one spin channel and a gapless state for the other. We report such a SGS behavior in conjunction with the topologically non-trivial multi-Weyl Fermions in MnPO$_4$. Interestingly, these Weyl nodes are located very close to the Fermi level with the minimal trivial band density. A drumhead like surface state originating from a nodal loop around Y-point in the Brillouin zone is observed. A large value of the simulated anomalous Hall conductivity (1265 $\Omega^{-1} cm^{-1}$) indirectly reflects the topological non-trivial behavior of this compound. Such co-existent quantum phenomena are not common in condensed matter systems and hence it opens up a fertile ground to explore and achieve newer functional materials.

(Invited) Catalysts for Fuel Cells

The progress in platinum-based catalysts with the goal of improving their activity and durability in a proton exchange membrane fuel cell will be presented. The studies of the oxygen reduction reaction on well-defined surfaces have guided the approach to design of the real world nanostructured catalysts. Insights from single crystalline systems and thin film surfaces have yielded atomically precise information related to surface structure and alloying with other metals in the quest to improve their intrinsic catalytic activity. These experimental efforts are also producing information necessary for the design of catalysts with enhancing stability. Atomic level control of the positioning of elements within catalyst structure enables the design of shapes and surfaces that mimic the key descriptors discovered through research on well-defined surfaces. The meticulous characterization of nanoscale materials using extremely high-purity conditions and cutting-edge techniques provides greater understanding of their performance and degradation pathways. This knowledge enables implementation of these materials in membrane electrode assemblies (MEA) for demonstration of targeted performance in a fuel cell vehicle. The catalyst, its support, water, ionomer, and gaseous reactants create challenging environment to control and engineer at the level that would be capable to address all technical targets that are necessary for wide deployment of this technology. The transition from high-purity liquid electrolytes into MEA still needs to be addressed before stepping into mass production of fuel cell stacks. The overall research strategy presented here exhibits the merits of a continuous pipeline from fundamental to applied research in the development of fuel cell technology.

(Invited) Electrocatalysts for Fuel Cell Cathodes

The studies of the oxygen reduction reaction on well-defined surfaces have guided the approach to design of the real world nanostructured catalysts. Insights from single crystalline systems and thin film surfaces have yielded atomically precise information about surface structure and alloying with other metals in the quest to improve their intrinsic catalytic activity. These experimental efforts are also producing information necessary for the design of catalysts with enhancing stability. These accomplishments were achieved through electrochemical in situ ICP-MS detection of parts per trillion levels of metal dissolution. Atomic level insight, has guided the chemical synthesis of well-defined nanoscale surfaces and creation of novel architectures that are associated with advanced catalytic performance. The progress in platinum-transition metal alloy nanocatalysts will be in focus of this lecture, with the goal of improving the activity and durability of the oxygen reduction electrocatalyst in a proton exchange membrane fuel cell. Atomic level control of the positioning of elements within alloy nanocrystals enables the design of shapes and surfaces that mimic the key descriptors discovered through research on well-defined surfaces. The meticulous characterization of nanocatalysts using extremely high-purity conditions and cutting-edge techniques provides greater understanding of their performance and degradation pathways. This knowledge allows better implementation of electrocatalysts in membrane electrode assemblies (MEA) for demonstration of expected performance in a fuel cell vehicle. An MEA is still viewed as a complex system with multiple interfaces. The catalyst, its support, water, ionomer, and gaseous reactants create challenging environment to control and engineer at the level that would be capable to address all technical targets that are necessary for wide deployment of this technology. The transition from high-purity liquid electrolytes into MEA still needs to be fully addressed in order to boost fuel cell stack performance. More recent research directions, such as platinum group metal (PGM) free catalysts will also be discussed. The main focus is being placed on replacement of platinum by more abundant cost effective constituents that could significantly lower the total cost of fuel cell. Typical PGM-free catalysts are consisted of transition metal sites coordinated by nitrogen dopants within a carbon matrix. While their intrinsic activity requires improvement, stability is equally critical in order to implement this class of materials in real fuel cells. Once again, well-defined, high-precision characterization and synthesis are proving vital to development of new generation of cathode catalysts. The overall research strategy presented here exhibits the merits of a continuous pipeline from fundamental to applied research in the development of fuel cell technology.

Behaviour of Nanostructured Si-Based Anode Materials Studied By Atom Probe Tomography and Hard X-Ray Photoelectron Spectrocopy

Lithium-ion batteries (LIB) are considered for electric vehicles (EV) as well as for home or off-grid energy storage for example in combination with photovoltaics. These specific applications drive the research on high energy density, cost-effective, safe and environmental friendly battery materials. To obtain deeper insight into the reaction mechanisms advanced characterization such as in-situ TEM or in-situ XRD1 are often used in combination with simplified model systems. For LIB anodes Si has the highest specific capacity among all anode materials. However, the mechanical fatigue (lack of microstructural integrity) associated with large volume expansion of pure Si during the lithiation process has hindered their wider use for commercial applications so far. Also, there are a series of side reactions which happen at the surface forming the solid electrolyte interface (SEI). In this work we investigate the role of mechanically confined nanocrystalline Si particles being embedded in a SiTiNi matrix2. Samples are produced by melt-spinning from a rapidly solidified SiTiNi alloy. The ribbon exhibits a nanocomposite microstructure with nano-Si particles (~50-200 nm) as potential sites for Li-storage embedded in an amorphous matrix of SiTiNi (STN). Such alloys have shown potential for anode materials in high capacity Li-ion batteries. Systematic lithiation and de-lithiation cycles were performed and selected samples after different cycles were investigated using Atom Probe Tomography (APT) to understand the Li diffusion mechanism in and out of the Si particles. We consider Atom Probe Tomography as a powerful tool to understand the phase changes at an atomic level3. Aided by Focus Ion Beam (FIB), we can address the nm-scale element distribution in any local structure. On the other hand, Hard X-Ray Photoelectron Spectroscopy (HAXPES) is adopted for a detailed SEI study. The employed high energy X-ray beams can probe deeper information of core level photoemission [1] Bach, P.; Valencia-Jaime, I.; Ruett, U.; Gutowski, O.; Romero, A.H.; Renner, F.U. Chemistry of Materials (2016), doi: 10.1021/acs.chemmater.5b04719; Renner, F.U.; Kageyama, H.; Siroma, Z.; Shikano, M.; Schröder, S.; Gründer, Y.; Sakata, O. Gold Model Anodes for Li-Ion Batteries: Single Crystalline Systems Studied by In Situ X-ray Diffraction, Electrochim. Acta 2008, 53, 6064-6069. [2] Son et al. “A Highly Reversible Nano Si Anode Enabled by Mechanical Confinement in an Electrochemically Activated LiXTi4Ni4Si7 Matrix.” Advanced Energy Materials 2 (2012), 1226. [3] Diercks et al., Microscopy of Chemical and Mechanical Heterogeneities in Lithium Cobalt Oxide, Microscopy and Microanalysis 21 (2015), 523.

Structural Evolution during Lithiation Cycles of the Li-Au System Observed By in-Situ High-Energy x-Ray Diffraction

Recent new Li-Ion Battery (LIB) developments include alloy anodes such as Si-Li, and a general trend towards nanostructured components. With technological development LIB were proposed to store energy for stationary applications as well as electromobility. Batteries need to become more robust achieving longer cycle lifetime in use to achieve this future goal. Therefore fundamental insight into the acting atomic-scale processes and structural evolution within the active materials during the charging, i.e. lithiation and delithiation cycles is required. Crystalline phases are often formed during cycling as well as amorphous phases which prominently form in alloy anodes based on Si or Ge. Due to its inert nature Au has been often used as a model material in electrochemical research including Li ion storage (1,2). Au was as well employed in composite LIB anodes for example as seed layers to further grow Si nanowires or as coatings. For gold model electrodes as well as gold contained in a LIB anode several studies have pointed to many intermediate metastable crystalline Li-Au phases. X-ray diffraction is a classical non-destructive method to characterize crystalline phases and synchrotron light has more recently facilitated the analysis of surfaces and thin films as well as in-situ or in-operando characterization. Using high-energy x-ray sources a complete reciprocal plain can be imaged on short time scales necessary to follow also quick structural changes on the time-scale of a few seconds. The huge benefit of high-energy x-rays has been demonstrated in-operando for detailed studies from catalytic surface reactions and molecular interface structures and layering to battery processes. We describe an in-situ high-energy x-ray diffraction study (3) following the alloying of Li (charging) and dealloying (de-charging) with a Au thin film and discuss the observations with the result of a theoretical screening of potential metastable phases. During the entire charging and discharging cycles we observed in total 6 different crystalline phases which are mainly so far unknown metastable intermediate phases leading in the here chosen end state to the known stable Li3Au phase. The detailed evolution of the different phases have been followed during cycling. Using a thin Au film grown on single crystalline Cu substrates we also observe the final crystalline cubic Li3Au to emerge in oriented relation to the below Cu substrate thus indicating this phase nucleates at the interface. 1. P. Bach, M. Stratmann, I. Valencia-Jaime, A. H. Romero, F. U. Renner, Lithiation and Delithiation Mechanisms of Gold Thin Film Model Anodes for Lithium Ion Batteries: Electrochemical Characterization, Electrochimica Acta (2015), 164, 81–89. 2. F.U. Renner, H. Kageyama, Z. Siroma, M. Shikano, S. Schöder, Y. Gründer, O. Sakata, Gold model anodes for Li-ion batteries: Single crystalline systems studied by in situ X-ray diffraction, Electrochim. Acta 53 (2008), 6064-6069. 3. P. Bach, I. Valencia-Jaime, U. Rütt, O. Gutowski, I. Valencia-Jaime, A. H. Romero, F. U. Renner, to be published. Figure 1

Epitaxially Grown PtxIr100-X Alloys with Enhanced Properties for Ammonia Electro-Oxidation

Ammonia is a highly toxic gas, a major environmental pollutant, and a potential fuel for fuel cells. Thus the electro-oxidation of NH3 has important applications in NH3 safety sensors, NH3 decontamination in wastewater, and power production. However, on most electrode surfaces, the electro-oxidation of NH3 is plagued with low surface reactivity and surface contamination by reaction intermediates. The vast majority of studies on the NH3 oxidation reaction have been performed on Pt electrocatalysts, and Pt remains the electrocatalyst which shows the highest current density at the lowest overpotential for catalyzing the oxidation of NH3 to N2. On Pt, the oxidation of ammonia occurs predominantly on (100) terraces. However, to the best of our knowledge, the effect of strain in the outmost metal layers and hybridization of the Pt d-states with Ir atoms (ligand effect) on the electro-oxidation of NH3 have never been studied. Accordingly, we have investigated the epitaxial growth PtxIr100-x alloys model electrocatalysts that mimic single crystalline systems. This was achieved by deposition on MgO(100) via pulsed laser deposition. It will be shown that Pt and Ir form a kinetically stable alloy and that the PtxIr100-x alloys grew with the desired PtIr(100)[010]//[010](100)MgO configuration over the whole composition range. Also, the XPS data show that a surface alloy is formed over the whole composition range. Electrochemical measurements were performed in 0.5 M H2SO4 to highlight the preferential (100) surface orientation. Cyclic-voltametry and chrono-amperometry experiments in alkaline media were then carried out in presence of ammonia. They demonstrate the importance of the (100) surface structure for the electro-oxidation of ammonia compared to randomly oriented surface (polycrystalline sample). The presence of iridium decreases the surface poisoning at PtIr (100) alloys. Finally, it will be shown that surface poisoning can be further reduced using a simple cyclic-voltammetry procedure.

Magnetocaloric Effect and Refrigerant Capacity of Non-Stoichiometric ${\rm Nd}_{0.5}{\rm Sr}_{0.5}{\rm MnO}_{3}$ Single Crystalline

Non-stoichiometric Nd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> MnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (NSMO) single crystalline has been grown by the floating zone method. Their magnetic properties, magnetocaloric effect (MCE) and refrigerant capacity ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> ) at near both first- and second-order phase transitions were investigated. The NSMO system goes through an antiferomagnetic charge-ordered transition at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CO</sub> ~ 152 K followed by a ferromagnetic to paramagnetic transition at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> ~ 272 K. The transition region spread over a broader temperature range at the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> but it had a narrower temperature range at the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CO</sub> . The maximum magnetic entropy change Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> gives about 1.65 J · kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at the first order magnetic transition and -1.13 J · kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> · K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> at the second order magnetic transition, respectively. <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> is obtained and it shows much higher value about 34.85 J/kg at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</sub> than the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> 19.06 J/kg at <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CO</sub> . The slight non-stoichiometric NSMO single crystalline system raises the Curie temperature around 30 K towards room temperature without degradation of the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> and temperature span corresponding to the full width at half maximum of Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> (Δ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FWHM</sub> ) compared to pure single crystalline NSMO system.

Influence of strain on anisotropic thermoelectric transport in Bi2Te3and Sb2Te3

On the basis of detailed first-principles calculations and semi-classical Boltzmann transport, the anisotropic thermoelectric transport properties of Bi$_2$Te$_3$ and Sb$_2$Te$_3$ under strain were investigated. It was found that due to compensation effects of the strain dependent thermopower and electrical conductivity, the related powerfactor will decrease under applied in-plane strain for Bi$_2$Te$_3, while being stable for Sb$_2$Te$_3$. A clear preference for thermoelectric transport under hole-doping, as well as for the in-plane transport direction was found for both tellurides. In contrast to the electrical conductivity anisotropy, the anisotropy of the thermopower was almost robust under applied strain. The assumption of an anisotropic relaxation time for Bi$_2$Te$_3$ suggests, that already in the single crystalline system strong anisotropic scattering effects should play a role.

Ultrafast IR spectroscopic study of coherent phonons and dynamic spin–lattice coupling in multiferroic LuMnO3

The concurrent existence of ferroelectricity and magnetism within a single crystalline system characterizes the multiferroic materials discovered in recent years. To understand and develop the multiferroic phenomenon, we need to investigate the unusual coupling between spin and lattice degrees of freedom. Spins in multiferroics are expected to be elastically coupled to phonons. Therefore, the time-dependent study can be a crucial factor in understanding the coupled dynamics. Here, we report the observations of strong dynamic spin–lattice coupling in multiferroic LuMnO3. A coherent optical phonon of 3.6 THz and its temperature dependence is measured for the first time from our femtosecond IR pump and probe spectroscopy. Also, we observed a coherent acoustic phonon of 47 GHz similar to a previous report (Lim et al 2003 Appl. Phys. Lett. 83 4800). Temperature-dependent measurements show that both optical and acoustic phonons become significantly underdamped as temperature decreases to TN, and they disappear below TN. These observations reveal that phonons are coupled to spins by magneto-elastic coupling, and the disappearance of phonon modes at TN is consistent with the isostructural coupling scheme suggested by Lee et al (2008 Nature 451 805).

Gold model anodes for Li-ion batteries: Single crystalline systems studied by in situ X-ray diffraction

We present the first results of experiments on the electrochemically driven alloying and de-alloying of Au substrates with Li-ions from organic electrolytes studied by in situ X-ray diffraction employing synchrotron radiation. Au(1 1 1) single crystals, 1 1 1-textured thin Au films and nanometer-sized Au particles on highly oriented pyrolytic graphite (HOPG) have been examined using a specially designed in situ X-ray cell. Starting from single crystalline Au systems we could follow the structural degradation and pulverization under different electrochemical conditions. Reciprocal lattice space maps show the occurrence of powder-like intensity associated with different Li–Au alloy phases. The diffraction data from the bulk Au crystals are compared with the Au(1 1 1) thin films and the behavior of the Au nano-particles on HOPG.

Oxygen doping effects on the magnetization of Bi 2Sr 2CaCu 2O 8+δ single crystalline system in the mixed state

We present a systematic study of the oxygen overdoping effects on the dc magnetization loops of Bi 2Sr 2CaCu 2O 8+δ single crystalline samples. The overdoping is produced by high oxygen pressure (from 1 to 190 bar) annealing at 500°C. The superconducting critical temperature and the c-axis lattice parameter are used to identify the oxygen content of the samples. The dc magnetization loop measurements are performed with the applied magnetic field parallel to the c-axis of the crystals. The characteristic second peak in the magnetization is present for all the investigated oxygen contents. We show that the position of this second peak and the irreversibility field increase monotonically as a function of oxygen doping.

Dopant Reduction in p-Type Oxide Films upon Oxygen Absorption

The lower valent cations were homogeneously doped in p-type oxide films. The prepared doped oxide systems were Mg-doped α-Cr2O3, Ni-doped α-Cr2O3, and Li-doped NiO. The Mg doping induced a homogeneous mixed valence state of Cr3+ and Cr6+ in the Cr2O3 film, reducing the Mg dopant to the metallic valence state in the Cr2O3 lattice, which is completely different from the powder or single crystalline system. The mixed valence state formation gave rise to a marked chromism. The mixed valence formation and perfect reduction of the lower valent dopants were evidenced by X-ray photoelectron spectroscopy, electrical conductivity change, and optical absorption. Ni-doped α-Cr2O3 and Li-doped NiO showed similar results. The reduction kinetics of the mixed valence state in the p-type oxide lattice with the aid of the electrical conductivity measurement suggested that the rate-determining step is not the diffusion of oxygen.

Low‐Temperature Anomalies in the CdI2Cd Crystalline System

AbstractFrom the measurements of elastic coefficients, piezooptical constants, and dilatometrical lengthening the existence of an improper ferroelastic phase transition of first order in the CdI2Cd single crystalline system within the temperature range 40 to 60 K is found. Different values of the temperature hysteresis are revealed for the piezooptical and elastic constants. From the data obtained an essential role of electron‐phonon interaction in the appearance of the mentioned phase transitions is concluded. The obtained results clearly show that in the CdI2Cd single crystalline system at lower temperatures long‐range forces appear between the impurity subsystems, which due to the specific electron‐phonon interactions tend to the appearance of the phase transitions. The behaviour of the elastic constants and the lengthening indicate the possible ferroelastic nature of these transitions. Simultaneously the existence of the temperature hysteresis reflects the first‐order nature of the mentioned transitions.

Terrylene in p-terphenyl: a novel single crystalline system for single molecule spectroscopy at low temperatures

Terrylene in p-terphenyl represents a promising novel single crystalline system for optical spectroscopy of single impurity molecules at low temperatures. The optical spectra of terrylene in p-terphenyl show four origins in absorption and emission, indicating four crystalline sites. In one of the sites saturated fluorescence count rates of up to 600000 s −1 were measured for single terrylene molecules. From a measurement of the fluorescence autocorrelation function for single terrylene molecules we could determine the populating and depopulating rates of the triplet state. Using the very small populating rate the triplet quantum yield was calculated to be only ~ 10 −5.