Superconductivity In Compounds Research Articles

Yu–Shiba–Rusinov states induced by single Fe atoms on reconstructed compound superconductor V[formula omitted]Si

Reconstructed surfaces of the A15-compound superconductor V3Si(100) that are possibly induced by the segregation of bulk impurities serve as platforms to study the dependence of Yu–Shiba–Rusinov states induced by a single Fe atom on the adsorption site. Their number, energy and electron–hole asymmetry vary strongly with the atomic environment of the Fe atom. These variations are indicative of different Fe d-orbitals being active in the site-dependent exchange coupling with the substrate Cooper pairs. Spatially resolved spectroscopy gives rise to a short decay length of the Yu–Shiba–Rusinov states and thereby suggests the three-dimensional character of the scattering process underlying the bound states.

Effect of Fermi surface topology change on the Kagome superconductor CeRu2 under pressure

The cubic Laves phase compound CeRu2 with a Kagome substructure of Ru has been investigated to understand myriad fascinating phenomena resulting from competition among its various physical and geometric features. Such phenomena include flat bands, van Hove singularities, Dirac cones, reentrant superconductivity, magnetism, the Fulde–Ferrell–Larkin–Ovchinnikov state, valence fluctuations, time-irreversible anisotropic s-state superconductivity, etc. Extensive studies have thus been carried out since 1958 when the highly unusual coexistence of superconductivity and ferromagnetism was proposed for the mixed compounds (Ce,Gd)Ru2. Activity has accelerated in recent years due to increasing interest in topological states in superconductors. However, there has been little investigation of the mutual influence of these fascinating states. Therefore, we systematically investigated the superconductivity and possible Fermi surface topological change in CeRu2via magnetic, resistivity, and structural measurements under pressure up to ∼168 GPa. An unusual phase diagram that suggests an intriguing interplay between the compound's superconducting order and Fermi surface topological order has been constructed. A resurgence in its superconducting transition temperature was observed above 28 GPa. Our experiments have identified a structural transition above 76 GPa and a few tantalizing phase transitions driven by high pressure. Our high-pressure results further suggest that superconductivity and Fermi surface topology in CeRu2 are strongly intertwined.

Development of Super Fine Strand Nb3Al Cable for SuperKEKB Superconducting Sextupole Magnet System

Along the beam lines of the SuperKEKB main rings, 218 conventional sextupole magnets are installed, and they are used for tuning the chromaticity of the accelerator rings. Especially, 16 sextupole magnets in the beam straight lines in the Tsukuba area are quite essential for the Crab Waist beam colliding. To make the beam tuning process finer, we are studying the superconducting sextupole magnet system with three types of the corrector magnets. From the operation temperature of the system, A15 compound superconductor is chosen as the cable material, and the development of the react and wind coil production has been tried with the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al cable. The Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al super fine strand superconducting cable for the corrector magnets was manufactured, and its superconducting characteristics are being studied. In this paper, as the first test result of the development, the critical current characteristics of the straight cable as variables of temperature and magnetic field are reported. As the results, the critical current of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al cable was higher than the design values for the corrector magnets, and this level of cable performance also indicates the potential for the cable being used for the main sextupole magnet.

Room temperature organic superconductor compound prediction based on fractals in mesoscopic-scale regime

Based on paraffin-wax include, volcano-based antimony-bearing, phospholipid, antimony found in gold deposit and TIPSb/triisopropylantimony ever predicted as the room temperature organic superconductor compound. Many flourishing explanations have been declared, from variational method to inductive deduction, with all indicating the presence of C9H21O3Sb.PBr6. We consider that aplications in mesoscale regime, at least when using multiscale fractal of critical parameter that affects physical &amp; chemical properties and to adopt “mesoscopic scale” ever stated as “superconductors” by Holmvall (2017). Then the mathematical induction of variational method of exploitation on integers &amp; natural numbers herewith paraffin-wax etc can be resembled the ever built antimony containing compounds to keep the realm of predictions.

Effect of Sintering Temperatures on the Physical Properties of Bi-2223 System Prepared by Co-Precipitation Method

The challenge in this research is the producing of nanoparticle superconductor compound. The benefit of nanoparticle is making the improving of the superconductor compound. The Bi2Sr2Ca2Cu3O10 (Bi-2223) compound prepared by Co-precipitation method at different sintering temperatures (600, 700, 850 °C) (20 hrs) with pH value (7). The electric resistivity of samples measured under liquid helium closed cycle cryogenic system. The TEM estimation of sample prepared by Co-precipitation recognized the range of particles size is about (22 - 48nm) at sintering temperature (600 °C). The range of nanoparticles size is in about (65-112 nm) at sintering temperature (700 °C) and (80-124) nm at sintering temperatures (850°C). The critical temperature of sample at sintering temperatures (700, 850 °C) was about (109 ,112) K respectively The crystal structure confirmed by using X-ray diffraction, these peaks were found to be well indexed by the tetragonal phase of Bi-2223. It was defined the successful of this method is a function to presence of full properties for superconductor compound like Bi-2223 system. .

Study of the Properties of YBCO Superconductor Compound in Various Preparation Methods: A Short Review

Superconductors have entered into many applications and advanced technological fields, due to their excellent properties identified by zero resistance and expelling the magnetic field applied to them. Superconductivity is a viable technology to prevent energy losses contributed by electrical resistivity. Also, the magnetic flux is repelled entirely out of the body of superconducting material which makes the Meissner Effect. High-Temperature Superconductors (HTS) have become the focus of researchers and scientists. This is because it uses liquid nitrogen "LN" in cooling, which gives it significant critical temperatures compared to traditional materials based on liquid helium "LHe" in cooling. From this point of view, began to employ these materials in most disciplines and modern technologies. In this article, the phenomenon of Superconductivity will define with explain its most prominent characteristics and focus on the preparation of the HTS (Yttrium-Barium-Copper-Oxide) compound (Abbreviated as YBCO) in different methods "The Sol-Gel and Citrate Pyrolysis Methods", to creating ultrafine superconducting (Y-123) powders. Generally known that by adopting any preparation technique, the superconducting transition temperature (Tc) value of ≈ 92 K could be achieved in the bulk samples. The Citrate Pyrolysis method is a unique route to prepare reactive precursor mixtures through an ignition process of a concentrated aqueous solution including metallic ions of stoichiometric composition. This procedure enables to synthesize of highly homogeneous and fine powders for functional materials, in comparison to the Sol-gel technique.

Aromatic superconductors: Electron-phonon coupling and electronic correlations

&lt;sec&gt;Aromatic superconductors are a new type of high-temperature superconductor discovered in recent years. The superconducting transition temperature (&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt;) increases with the size of aromatic molecule increasing, which has attracted widespread attention of experimental and theoretical researchers. The driving mechanism for such a superconductivity, whether it is dominated by electron-phonon coupling or electronic correlation effects, has aroused great interest of many research groups. This paper briefly introduces the rich superconducting phenomena of metal doped aromatic compounds. From the perspectives of electron-phonon coupling or electronic correlations, the superconductivity of aromatic compounds is discussed, which is helpful in exploring aromatic superconductors with higher &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt;. The challenges currently faced in the field are also introduced.&lt;/sec&gt;&lt;sec&gt;The rest of this paper is organized as follows. We first introduce the existence of abundant superconducting phases in the experiment of metal doped aromatic compounds. Different doping concentrations of metal cause superconducting phases with different &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; values, especially the highest &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; value of the superconducting phase increases with the size of aromatic molecule increasing. Theoretical prediction shows that all aromatic hydrocarbon superconductors have a low-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; superconducting phase in a range of 5–7 K, which is a common feature. For systems with few benzene rings (such as benzene, naphthalene, and phenanthrene crystals), only low-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phase of 5–7 K exists, while in systems with multiple benzene rings (such as picene, dibenzopentacene, and others with the number of benzene rings more than 5), there are multiple superconducting phases; the highest &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; in long-benzene-ring system depends not only on the number of benzene rings, but also on the chain size of organic molecule. Further research indicates that low-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phase is induced by doping about 2 electrons and has good stability, while high-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phase results from doping 3 electrons and has slightly poorer stability.&lt;/sec&gt;&lt;sec&gt;Then, the electron-phonon coupling characteristics and electron-electron exchange correlation effects in aromatic compound superconductors are discussed. For low-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phases, the values of electronic density of states at the Fermi level are comparable to each other and relatively low, resulting in weak electron-phonon interactions. However, the &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; value predicted by this electron-phonon coupling mechanism is in good agreement with experimental value, indicating that the electron-phonon coupling is sufficient to describe the superconductivity of low-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phases. For high-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phases, the big values of electron density of states at the Fermi level imply strong electron-phonon interactions, and this electron-phonon coupling increases with the size of organic molecule increasing. However, the &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; value predicted only by the electron-phonon mechanism is lower than the experimental value. The study of electron-electron exchange correlation effect of aromatic compounds shows that the electronic correlation effect increases with the size of aromatic molecule increasing, which is consistent with the increase of &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; maximum value with the size of aromatic molecule increasing in a long-benzene-ring system. This indicates that the superconductivity of high&lt;i&gt;-T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; phase is driven by both the electron-phonon mechanism and the electronic correlation effect. This understanding of superconductivity is significant for exploring and discovering aromatic superconductors with higher transition temperatures.&lt;/sec&gt;&lt;sec&gt;Finally, comprehensive physical models and methods are required in this paper in order to gain a thorough understanding of the superconductivity of aromatic compound.&lt;/sec&gt;

Tuning of carrier concentration and superconductivity in high-entropy-alloy-type metal telluride (AgSnPbBi)(1-)/4In Te

High-entropy-alloy-type (HEA-type) compound superconductors have been drawing much attention as a new class of exotic superconductors with local structural inhomogeneity. NaCl-type (Ag,In,Sn,Pb,Bi)Te is a typical HEA-type superconductor, but the carrier doping mechanism had been unclear. In this study, we synthesized (Ag,In,Sn,Pb,Bi)Te with various In concentration using high-pressure synthesis: the studied system is (AgSnPbBi)(1-x)/4InxTe (x = 0-0.4). Single-phase samples were obtained for x = 0-0.3. A semiconductor-like temperature dependence of resistivity was observed for x = 0, while superconductivity appeared for the In-doped samples. The highest transition temperature (Tc) was 3.0 K for x = 0.3. The Seebeck coefficient decreases with increase of x, which suggests that In3+ generates electron carriers in (AgSnPbBi)(1-x)/4InxTe. Tuning of carrier concentration and superconducting properties of (Ag,In,Sn,Pb,Bi)Te would be useful for further investigation of exotic superconductivity in the HEA-type compound.

Ab initio study of Li-Mg-B superconductors

LiB, a predicted layered compound analogous to the ${\mathrm{MgB}}_{2}$ superconductor, has been recently synthesized via cold compression and quenched to ambient pressure, yet its superconducting properties have not been measured. According to prior isotropic superconductivity calculations, the critical temperature (${T}_{\mathrm{c}}$) was expected to be only 10--15 K. Using the anisotropic Migdal-Eliashberg formalism, we show that the ${T}_{\mathrm{c}}$ may actually exceed 32 K. Our analysis of the contribution from different electronic states helps explain the detrimental effect of pressure and doping on the compound's superconducting properties. In the search for related superconductors, we screened Li-Mg-B binary and ternary layered materials and found metastable phases with ${T}_{\mathrm{c}}$ close to or even 10--20% above the record 39 K value in ${\mathrm{MgB}}_{2}$. Our reexamination of the Li-B binary phase stability reveals a possible route to synthesize the LiB superconductor at lower pressures readily achievable in multianvil cells.

Lattice Dynamics in the NASICON NaZr2(PO4)3 Solid Electrolyte from Temperature-Dependent Neutron Diffraction, NMR, and Ab Initio Computational Studies

Natrium super ionic conductor (NASICON) compounds form a rich and highly chemically tunable family of crystalline materials that are of widespread interest because they include exemplars with high ionic conductivity, low thermal expansion, and redox tunability. This makes them suitable candidates for applications ranging from solid-state batteries to nuclear waste storage materials. The key to an understanding of these properties, including the origins of effective cation transport and low, anisotropic (and sometimes negative) thermal expansion, lies in the lattice dynamics associated with specific details of the crystal structure. Here we closely examine the prototypical NASICON compound, NaZr2(PO4)3, and obtain detailed insights into such behavior via variable-temperature neutron diffraction and 23Na and 31P solid-state NMR studies, coupled with comprehensive density functional theory-based calculations of NMR parameters. Temperature-dependent NMR studies yield some surprising trends in the chemical shifts and the quadrupolar coupling constants that are not captured by computation unless the underlying vibrational modes of the crystal are explicitly taken into account. Furthermore, the trajectories of the sodium, zirconium, and oxygen atoms in our dynamical simulations show good qualitative agreement with the anisotropic thermal parameters obtained at higher temperatures by neutron diffraction. The work presented here widens the utility of NMR crystallography to include thermal effects as a unique probe of interesting lattice dynamics in functional materials.

Hybrid Symmetry Epitaxy of the Superconducting Fe(Te,Se) Film on a Topological Insulator.

It is challenging to grow an epitaxial 4-fold compound superconductor (SC) on a 6-fold topological insulator (TI) platform due to the stringent lattice-matching requirement. Here, we demonstrate that Fe(Te,Se) can grow epitaxially on a TI (Bi2Te3) layer due to accidental, uniaxial lattice match, which is dubbed as "hybrid symmetry epitaxy". This new growth mode is critical to stabilizing robust superconductivity with TC as high as 13 K. Furthermore, the superconductivity in this FeTe1-xSex/Bi2Te3 system survives in the Te-rich phase with Se content as low as x = 0.03 but vanishes at Se content above x = 0.56, exhibiting a phase diagram that is quite different from that of the conventional Fe(Te,Se) systems. This unique heterostructure platform that can be formed in both TI-on-SC and SC-on-TI sequences opens a route to unprecedented topological heterostructures.

Progress on hydrogen-rich superconductors under high pressure

In recent years, hydrogen-rich compounds under extremely high pressure have become the hot target materials for high-temperature superconductors. At present, two landmark progresses have been made in this field. Covalent H&lt;sub&gt;3&lt;/sub&gt;S hydrogen-rich superconductors (&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; = 200 K) and ionic hydrogen-rich superconductors with hydrogen-cage structure, such as LaH&lt;sub&gt;10&lt;/sub&gt; (&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; = 260 K, –13 ℃), YH&lt;sub&gt;6&lt;/sub&gt; and YH&lt;sub&gt;9&lt;/sub&gt;, have been successively synthesized, setting a new record of superconducting temperature. These studies have given rise to the hope of discovering room-temperature superconductors in hydrogen-rich compounds under high pressure. This paper focuses on the progress of hydrogen-rich superconductors with high critical temperature under high pressure, discusses the physical mechanism of high-temperature superconductivity in hydrogen-rich compounds, provide an outlook on the possibility of discovering room-temperature superconductors in hydrogen-rich compounds in the future, and offer the candidate system for high superconductivity in multiple hydrogen-rich compounds.

Investigation of the physical properties of (Cu0.5Tl0.5)Ba2Ca2Cu3O10-δ impregnated with mono cobalt(II)-substituted Undecatungstosilicate Nanoparticles

A series of (Cu0.5Tl0.5)Ba2Ca2Cu3O10-δ superconducting samples impregnated with different percentage additions of potassium salt {CoSiW11}x nanoparticles (x = 0.00, 0.01, 0.02, 0.04, 0.06, 0.08 and 0.12 wt.%) were prepared using a single-step solid-state reaction technique at ambient pressure. These samples were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to study their structures, construction of the synthesized samples and elemental composition, respectively. XRD studies indicate that the tetragonal structure of (Cu0.5Tl0.5)-1223 phase has not changed by the addition of {CoSiW11} nanoparticles the same as for the values of lattice parameters a and c, where they show a non-remarkable change. The relative volume fraction of (CuTl)-1223 phase record a maximum value for x = 0.02 wt.%. The formation of (CuTl)-1223 phase matches the presence of rectangular-shaped plates seen in SEM images. The enhancement of these plates after the addition of {CoSiW11} nanoparticles up to x = 0.02 wt.% agreed with the measurements of relative volume fraction. The mass percentage of all elements was measured using the EDX technique, which shows the distribution composition of both superconductor and nanoparticle compounds. Vibrational mode analyses of oxygen were accomplished via Fourier transform infrared (FTIR) spectroscopy in (CuTl)-1223 phase impregnated with {CoSiW11} nanoparticles. The Oδ oxygen modes observed are almost unaltered while the other oxygen modes of planar CuO2 are slightly softened and hardened. Superconducting transition temperature (Tc) and critical current density (Jc) were determined from the electrical resistivity and I–V measurements, respectively. The results indicate an increase in the Tc values from 125 K for the pure sample to 132 K for x = 0.02 wt.% impregnated samples, while a decrease in the Jc values is observed upon increasing ‘x’ value from x = 0.00 up to 0.12 wt.%. The suppression of these electrical superconducting parameters for x > 0.02 wt.% may be due to an increase of weak links' connectivity among the grain boundaries and growth of impurity phases that affect the formation of (CuTl)-1223 superconductor phase. The fluctuation-induced conductivity (FIC) analysis has been done in the light of the Aslamasov–Larkin (AL) theory to study the fluctuation conductivity Δσ above Tc as a function of reduced temperature for (CoSiW11)x/(CuTl)-1223 superconductor. The results show four non-identical fluctuation regions that are short-wave (Sw), two-dimensional (2D), three-dimensional (3D) and critical (Cr). The zero-temperature coherence length ( $${\xi }_{c}(0)$$ ), the effective layer thickness of the two-dimensional system (d), inter-layer coupling (I), Fermi velocity (vF) and Fermi energy (EF) were evaluated as a function of ‘x’ addition from FIC analysis of the samples and then correlated to the superconductivity order parameters. The superconducting parameters were increased upon the addition of {CoSiW11} nanoparticles.

Synthesis and Characterization of New Superconductors Materials

In the last few decades, the persisting scientific interest in high temperature superconductor (HTS) cuprates has been accompanied by the search for new families of superconducting compounds (SPCs) [...]

Critical Temperature Prediction for a Superconductor: A Variational Bayesian Neural Network Approach

Much research in recent years has focused on using empirical machine learning approaches to extract useful insights on the structure-property relationships of superconductor material. Notably, these approaches are bringing extreme benefits when superconductivity data often come from costly and arduously experimental work. However, this assessment cannot be based solely on an open black-box machine learning, which is not fully interpretable, because it can be counter-intuitive to understand why the model may give an appropriate response to a set of input data for superconductivity characteristic analyses, e.g., critical temperature. The purpose of this study is to describe and examine an alternative approach for predicting the superconducting transition temperature $T_c$ from SuperCon database obtained by Japan's National Institute for Materials Science. We address a generative machine-learning framework called Variational Bayesian Neural Network using superconductors chemical elements and formula to predict $T_c$ . In such a context, the importance of the paper in focus is twofold. First, to improve the interpretability, we adopt a variational inference to approximate the distribution in latent parameter space for the generative model. It statistically captures the mutual correlation of superconductor compounds and; then, gives the estimation for the $T_c$ . Second, a stochastic optimization algorithm, which embraces a statistical inference named Monte Carlo sampler, is utilized to optimally approximate the proposed inference model, ultimately determine and evaluate the predictive performance. As a result, in comparison with the standard evaluation metrics, the results are promising and also agree with the existing models prevalent in the field. The $R^2$ value obtained is very close to the best model (0.94), whereas a considerable improvement is seen in the RMSE value (3.83 K). Notably, the proposed model is known as the first of its kind for predicting a superconductor's $T_c$ .

Effect of Composition and Dielectric Properties for (YBCO) Superconductor Compound in Different Preparation Methods

The superconductor compound (YBa2Cu2.8Zn0.2O7+δ) is prepared by solid state reaction (SSR), Sol-gel (SG) and laser Pulse deposition (PLD) methods. We used the X-ray diffraction technique, which shows an orthorhombic crystalline system for all the samples, and increase in the high-phase (Y-123) and decrease in low-phase and vary in proportion according to the method of preparation with the emergence of some impurities. The behavior of the samples in terms of electrical resistance and critical temperature was investigated all samples showed superconducting behavior. The properties of the dielectric (real dielectric constant, imaginary dielectric constant, loss tangent, alternating electrical conductivity) were studied as a function of frequency and range (50 Hz-1MHz) at room temperature. The results are demonstrate the apparent change in the dielectric properties by using the method of preparation of laser pulse deposition PLD at the expense of other methods, Microscopic images of the samples were examined at very high magnitudes through the scanning electron microscopy (SEM). In addition to using the titration method to determine the proportion of oxygen.&#x0D;

Predicted high-temperature superconductivity in cerium hydrides at high pressures

A systematic structure search in the Ce-H system under pressure reveals novel stable phases with intriguing electronic properties. Several cerium hydrides, CeH4, CeH9, and CeH10, were found to be dynamically stable. Electron-phonon calculations coupled to Bardeen-Cooper-Schrieffer arguments indicate that they might be high-Tc superconductors. In particular, the maximum Tc values for F4¯3m-CeH9 and Fm3¯m-CeH10 are 142 K and 168 K at 94 GPa. These findings may pave the way for achieving room temperature superconductors in dense hydrogen-rich compounds.

Electronic band structure and superconducting properties of SnAs

We report comprehensive study of physical properties of the binary superconductor compound SnAs. The electronic band structure of SnAs was investigated using both angle-resolved photoemission spectroscopy (ARPES) in a wide binding energy range and density functional theory (DFT) within generalized gradient approximation (GGA). The DFT/GGA calculations were done including spin-orbit coupling for both bulk and (111) slab crystal structures. Comparison of the DFT/GGA band dispersions with ARPES data shows that (111) slab much better describes ARPES data than just bulk bands. Superconducting properties of SnAs were studied experimentally by specific heat, magnetic susceptibility, magnetotransport measurements and Andreev reflection spectroscopy. Temperature dependences of the superconducting gap and of the specific heat were found to be well consistent with those expected for the single band BCS superconductors with an isotropic s-wave order parameter. Despite spin-orbit coupling is present in SnAs, our data shows no signatures of a potential unconventional superconductivity, and the characteristic BCS ratio $2\Delta/T_c = 3.48 - 3.73$ is very close to the BCS value in the weak coupling limit.

Enhanced superconductivity in a misfit compound (PbSe)1.12 (TaSe2)2 with double TaSe2 layers

We report the discovery of superconductivity with a maximum Tc ≈ 1.85 K in a new misfit layered compound , which consists of alternating layers of a distorted rocksalt layer PbSe and a double-layer dichalcogenide . A large anisotropic parameter of resistivity and that of the upper critical field = are obtained. The estimated c-axis coherence length is comparable to the c-axis lattice constant. Moreover, the estimated upper critical field , significantly exceeding the Pauli paramagnetic limit. All these data indicate a quasi-two-dimensional nature of the superconducting properties in . Compared with the physical properties of the misfit compound superconductor with only the half-layer of the unit, our study suggests that the enhancement of Tc in with double TaSe2 layers is presumably related to the increase in hole carrier density. The reduced charge transfer from the PbSe layer to the conducting TaSe2 layer could account for the variation in the charge carrier density.

Type-I superconductivity in Al6Re

While the pure elements tend to exhibit Type-I rather than Type-II superconductivity, nearly all compound superconductors are Type-II, with only a few known exceptions. We report single crystal growth and physical characterization of the rhenium aluminide Al$_6$Re, which we conclude is a Type-I superconductor based on magnetization, ac-susceptibility, and specific-heat measurements. This detection of superconductivity, despite the strong similarity of Al$_6$Re to a family of W and Mo aluminides that do not superconduct, suggests that these aluminides are an ideal testbed for identifying the relative importance of valence electron count and inversion symmetry in determining whether a material will superconduct.