Bi-2212 Single Crystals Research Articles

Effect of weak and high magnetic fields in longitudinal and transverse configurations on magneto-thermoelectric properties of quantum Bi wires

We report on the magneto-thermopower of single-crystal Bi nanowires with diameters 75 and 250 nm in a longitudinal and transverse magnetic fields of 0–14 T. The temperature range is 1.5–300 K. Bi nanowires in a glass capillary have been prepared by the high frequency liquid phase casting. The temperature dependence R(T) shows the transition from metallic to semiconducting behavior due to quantum size effect, where the Bi-wire diameter is reduced to less than 80 nm. It is for the first time that the effect of the negative magneto-resistance in a transverse magnetic field, due to the quantum size effect on 75 nm Bi wires, has been observed. The thermopower is very sensitive to the wire diameter, up to a change in the sign from negative to positive at low temperatures, and to a significant extent in a weak longitudinal magnetic field. The field dependences of longitudinal and transverse magneto-resistance have features characteristic of the occurrence of the quantum size effect and galvanomagnetic size effect, and provide information on the parameters of the energy spectrum and charge carrier mobility. The enhancement of the thermoelectric figure of merit for Bi nanowires is discussed. We also discuss the power factor α2σ and its dependence on the diameter, magnetic field and temperature.

Influence of the electrode potential and in situ STM scanning conditions on the phase boundary structure of the single crystal Bi(1 1 1)|1-butyl-4-methylpyridinium tetrafluoroborate interface

Interfacial structure of the Bi(111)|1-butyl-4-methylpyridinium tetrafluoroborate (BMPyBF4) interface has been studied applying the in situ scanning tunnelling microscopy (STM), cyclic voltammetry, impedance spectroscopy (EIS), and also computational chemistry methods. Influence of electrode potential and in situ STM scanning conditions on the interfacial superstructure formation process has been demonstrated. Slow electrical double layer formation limited by the mass transfer step of BMPyBF4 adsorption onto the Bi(111) surface, established by using EIS and verified also by STM data, has been explained by the BMPyBF4 multilayered superstructure formation. Possible reasons for the low values of series capacitance for the Bi(111)|BMPyBF4 interface have been analyzed on the basis of modern theoretical models and explained by the semimetallic nature of bismuth, i.e., by the potential drop inside the Bi(111) electrode surface layer, and by the strong van der Waals type attractive forces between the adsorbed BMPy+ cations, generating compact adsorbed structures with the low effective dielectric constant.It was found that at the fixed Bi(111) potential, the observed multilayered structure depends on the STM image scanning rate, the tip potential (bias) and on the tunnelling current. Multilayered formations of BMPyBF4 at the Bi(111) electrode were observed within potential range from −1.2V to −0.6V (vs. Ag|AgCl in BMPyBF4), for the positively charged tungsten STM tip and at the high scanning rates. At more negative Bi(111) potentials (and the slower scanning rates), the atomic resolution was observed similarly to the Bi(111)|Na2SO4+H2SO4 aqueous solutions interface.

Origin of coherent phonons in Bi2Te3excited by ultrafast laser pulses

Femtosecond laser pulses are used to excite coherent optical phonons in single crystal Bi${}_{2}$Te${}_{3}$ thin films. Oscillations from low- and high-frequency A${}_{1\mathrm{g}}$ phonon modes are observed. A perturbation model based on molecular dynamics reveals various possibilities of phonon generation due to complex interactions among different phonon modes. In order to elucidate the process of phonon generation, measurements on thin films with thicknesses below the optical absorption depth are carried out, showing that a gradient force is necessary to excite the observed A${}_{1\mathrm{g}}$ phonon modes, which provides a refined picture of displacive excitation of coherent phonon.

Effect of a Highly Metallic Surface State on the Magneto-Transport Properties of Single Crystal Bi Films

The magneto-transport properties of thin single crystal Bi films epitaxial grown on Si (111)-7 × 7 surfaces are investigated systematically as functions of film thickness (5–55 nm) and temperature. Under a perpendicular magnetic field, the positive magnetoresistance (PMR) effect is normally found, and its curve shapes are evolved systematically with film thickness. In contrast, under parallel magnetic fields the PMR effect observed for thinner Bi films develops into the negative magnetoresistance effect with the increasing magnetic field for the thicker Bi film. Our analysis indicates that there exists strong competition between the weak anti-localization effect in the surface states and the weak-localization effect in the bulk states of the Bi film, which induces the anomalous changes in the parallel magneto-resistance curves. The temperature-dependent experiments further demonstrate that the surface state plays an important role in the magneto-transport process of Bi films.

Pseusogap in Cuprates by Electronic Raman Scattering

We present Raman experiments on underdoped and overdoped Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals. We reveal the pseudogap in the electronic Raman spectra in the B1g and B2g geometries. In these geometries we probe respectively, the antinodal (AN) and nodal (N) regions corresponding to the principal axes and the diagonal of the Brillouin zone. The pseudogap appears in underdoped regime and manifests itself in the B1g spectra by a strong depletion of the low energy electronic continuum as the temperature decreases. We define a temperature T* below which the depletion appears and the pseudogap energy, ωPG the energy at which the depeletion closes.The pseudogap is also present in the B2g spectra but the depletion opens at higher energy than in the B1g spectra. We observe the creation of new electronic states inside the depletion as we enter the superconducting phase. This leads us to conclude (as proposed by S. Sakai et al. [1]) that the pseudogap has a different structure than the superconducting gap and competes with it. We show that the nodal quasiparticle dynamic is very robust and almost insensitive to the pseudogap phase contrary to the antinodal quasiparticle dynamic. We finally reveal, in contrast to what it is usually admitted, an increase of the nodal quasiparticle spectral weight with underdoping. We interpret this result as the consequence of a possible Fermi surface disturbances in the doping range p = 0.1 − 0.2.

Emission of the THz waves from large area mesas of superconducting Bi2Sr2CaCu2O8+δ by the injection of spin polarized current

Rectangular Au/Co/Au/Bi2Sr2CaCu2O8+δ (Bi2212) mesa structures with large areas and high thicknesses were fabricated on as-grown Bi2212 single crystals in order to obtain small critical current from as-grown mesas by the injection of spin polarized current and so eliminate the adjustment of doping level for successful THz emission. We have performed c-axis resistance versus temperature (R–T), current–voltage (I–V) characteristics and bolometer measurements. It is the first time that THz emission has been observed from as-grown mesas due to injection of spin polarized current.

Quantum Interference of Surface States in Bismuth Nanowires in Transverse Magnetic Fields

We report the results of studies of the magnetoresistance (MR) and electric field effect (EFE) of single-crystal Bi nanowires with diameter d<80 nm at low temperatures. Single-crystal nanowire samples were prepared by the Taylor-Ulitovsky technique; they were cylindrical single crystals with the \((10\bar{1}1)\) orientation along the wire axis where the C 3 axis was inclined at an angle of 70∘ to the wire axis. According to theory of S. Murakami, bismuth bilayers can exhibit the quantum spin Hall effect. A Bi crystal can be viewed as a stacking of bilayers with a honeycomblike lattice structure along the [111] direction. An interpretation of transverse MR oscillations with using this theory is presented.

Formation of Bi, Pb-2223 and Microstructural Evolution in Pb-Ca-Cu Deposited Bi-2212(001) Single Crystal by Heat Treatment

In order to study the mechanism of phase transformation from Bi-2212 to Bi-2223, we fabricated Pb-Ca-Cu deposited Bi-2212 (001) single crystals as precursors of the Bi,Pb-2223 phase. These single crystals were heated in a sintered Bi-Pb-Sr-Ca-Cu-O pellet, and microstructure was observed by analytical electron microscopy. As a result of the microstructural observation around the Bi,Pb- 2223 phase, two formation paths of Bi,Pb-2223 were recognized. One is the formation from a liquid phase which is formed by Bi- 2212 and melting of the secondary phases such as (Ca,Sr)2PbO4 and CuO. The other is the growth of Bi,Pb-2223 into Bi-2212 phase by diffusion of Pb, Ca and Cu atoms along the <hk0> directions of Bi-2212.

Phonon dynamics in CuxBi2Se3 (x = 0, 0.1, 0.125) and Bi2Se2 crystals studied using femtosecond spectroscopy

Phonon dynamics in Bi, Bi2Se2, and CuxBi2Se3 (x = 0, 0.1, 0.125) single crystals was investigated using femtosecond pump-probe spectroscopy. Two damped oscillations in measured transient reflectivity changes were observed, respectively, attributed to coherent optical and acoustic phonons. The frequency shift of the phonon modes in Bi-rich Bi2Se2 crystals indicates the intercalation of the additional Bi atoms into the Bi2Se3 matrix and formation of the Bi2 layer between the quintuple layers (QLs). The deformation of QLs in CuxBi2Se3 crystals was also found. These phonon dynamics results suggest that Cu atoms in CuxBi2Se3 crystals are predominantly intercalated between pair of QLs.

Growth and Characterization of High-Quality Single Crystals of Ni- and Zn-Doped Bi2Sr2Ca(Cu2−xMx)O8+δ (M = Ni or Zn) High-Temperature Superconductors

High-quality large single crystals of Ni- and Zn-doped Bi2Sr2Ca(Cu2−xMx)O8+δ (M = Ni or Zn) have been successfully grown by the traveling solvent floating zone technique. The single crystals are characterized by compositional and structural analyses and their physical properties are investigated by magnetic susceptibility and resistivity measurements. A record high critical temperature with a Tc = 97.5 K has been achieved in the annealed pristine Bi2212 single crystal. Substitution of Cu by Ni or Zn reduces the superconducting transition temperature when compared with pristine Bi2Sr2CaCu2O8+δ (Bi2212) grown under similar conditions. The successful growth of such pristine Ni- and Zn-doped Bi2212 single crystals will facilitate studies of the relationship between the magnetism and superconductivity in high-temperature cuprate superconductors.

Magnetic phase diagram of the layered superconductor Bi2+xSr2−xCuO6+δ (Bi2201) with Tc ≈ 7 K

We report on magnetization measurements performed on a single crystal of Bi2+xSr2−xCuO6+δ with Tc ≈ 7 K for H ∥ c-axis. The isofield M(T) curves show a large reversible region, with a pronounced rounding effect as M approaches zero which prevents the determination of Tc(H). Deviations from the linear behavior of magnetization near Tc(H) are studied through the asymptotic behavior of , where m is an exponent and Ta an apparent temperature transition. Values of m deviate from the expected mean field value, 1/2, suggesting the importance of phase fluctuations. Resulting values of Ta, interpreted as the onset of phase correlations, decrease as field increases, showing an upward curvature. Values of Tc(H) are obtained through a two-dimensional critical scaling analysis obeyed by many M(T) curves. The resulting phase diagrams do not show upward curvature and lie below the Ta(H) line. The value of Hc2(0) estimated from the initial slope dHc2/dT is twice the value suggested by the phase diagram. Amplitude fluctuations above Ta(H) are explained in terms of a Ginzburg–Landau approach extended to high reduced temperatures and magnetic fields by the introduction of a total-energy cutoff in the fluctuation spectrum.

Thermodynamic growth model of single crystal Bi-2212 whisker

In this study the growth mechanism of superconducting Bi-2212 whiskers is investigated from a thermodynamic point of view. The evolution of the free energy of the growth region in the main matrix is evaluated by estimating that of its surface energy. It is shown that the local free energy change for partial melting is the same as that of the whole structure of the sample. It is pointed out that the formation of whiskers requires a higher energy than that of plate like grains.

Investigation of Electroreduction Kinetics of the Hexaamminecobalt(III) Cations on the Electrochemically Polished Bi Planes by the Electrochemical Impedance Spectroscopy Method

Electrochemical impedance spectroscopy has been used for investigation of electroreduction kinetics of hexaamminecobalt(III) cations on the electrochemically polished Bi planes. The total charge transfer resistance obtained depends noticeably on the electrode potential and [Co(NH3)6]3+ concentration. Systematic analysis of results show that the equivalent circuit taking into account the adsorption of one reacting or intermediate particle is the more probable and simplest physical model of the complicated [Co(NH3)6]3+ electroreduction reaction under discussion. The charge transfer resistance, adsorption capacitance and adsorption resistance values depend on the electrode potential as well as on the reactant concentration. In the base electrolyte solution with addition of [Co(NH3)6]3+ the adsorption and charge transfer resistance value was maximal and the adsorption capacitance value was minimal in the region of zero charge potential for Bi single crystal (111) and (011-) planes.

Improved characteristics of mesa-type intrinsic Josephson junctions by vacuum cleavage process for Bi2Sr2CaCu2O8+δ/Au contacts

Improvement in the quality of stacked intrinsic Josephson junctions (IJJs) was attempted using Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals. The introduction of a clean Bi-2212/Au interface reduced contact resistivity (ρc), decreased low frequency noise, and suppressed the drop in the maximum Josephson current (Ic), which are very important factors for applications such as a superconducting quantum interference device. The ρc of samples produced by crystal cleavage in air or vacuum were, respectively, 8.2 × 10−5 and 7.0 × 10−6 Ω cm2 at 300 K and 8.2 × 10−5 and 2.1 × 10−6 Ω cm2 at 77 K. Random-telegraph voltage noise observed with the air-cleaved samples did not occur for the samples cleaved in a vacuum. The degradation of Ic was suppressed for IJJs produced by the vacuum cleavage process.

Persistent electrical doping of Bi2Sr2CaCu2O8+xmesa structures

Application of a significantly large bias voltage to small Bi2Sr2CaCu2O8+x mesa structures leads to persistent doping of the mesas. Here we employ this effect for analysis of the doping dependence of the electronic spectra of Bi-2212 single crystals by means of intrinsic tunneling spectroscopy. We are able to controllably and reversibly change the doping state of the same single crystal from underdoped to overdoped state, without changing its chemical composition. It is observed that such physical doping is affecting superconductivity in Bi-2212 similar to chemical doping by oxygen impurities: with overdoping the critical temperature and the superconducting gap decrease, with underdoping the c-axis critical current rapidly decreases due to progressively more incoherent interlayer tunneling and the pseudogap rapidly increases, indicative for the presence of the critical doping point. We distinguish two main mechanisms of persistent electric doping: (i) even in voltage contribution, attributed to a charge transfer effect, and (ii) odd in voltage contribution, attributed to reordering of oxygen impurities.

Exfoliated Thin $\hbox{Bi}_{2}\hbox{Sr}_{2} \hbox{CaCu}_{2}\hbox{O}_{8 + \delta}$ Single Crystal and Its Intrinsic Josephson Junctions

Thin Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CaCu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+δ</sub> (Bi-2212) single crystals with the typical thicknesses of 20-200 nm are prepared with mechanical exfoliation, which can be firmly attached to a substrate by van der Waals and/or capillary force instead of using any glue. The thin Bi-2212 single crystals are interesting for superconduct- ing device fabrication and applications as well as the study of low-dimension superconductivity. A stack of intrinsic Josephson junctions including eight junctions is demonstrated.

Fabrication process of intrinsic Josephson junction stacks in Bi2Sr2CaCu2O8+x crystals by double-sided patterning process using dilute hydrochloric acid

We have developed a new double-sided patterning (DSP) process that employs dilute acid (pH=1.65) to fabricate Bi2Sr2CaCu2O8+x (Bi-2212) stacks of intrinsic Josephson junctions. These stacks, which were fabricated from a single crystal of Bi-2212, were surrounded by an acid-treated product. The critical aspect of this process is that the Bi-2212 surrounding the photoresist pattern was converted into a transparent material, BiOCl, which was connected with the Bi-2212 crystal. Consequently, this process provides a simple way to achieve DSP of the surfaces of Bi-2212 crystals. This new DSP process realizes a remarkably improved reproducibility in fabricating Bi-2212 stacks that exhibit good current–voltage characteristics with a large hysteresis and multiple branches at T=77K.

Quick, Controlled Synthesis of Ultrathin Bi2Se3 Nanodiscs and Nanosheets

Ultrathin (4-6 nm) single-crystal Bi(2)Se(3) nanodiscs and nanosheets were synthesized through a simple and quick solution process. The growth mechanism was investigated in detail. Crystal seeds grew via 2D self-attachment of small nanoparticles followed by epitaxial recrystallization into single crystals. The lateral dimension of the nanodiscs increased and their shape changed from circles to hexagons as the reaction temperature increased. Positively charged polymer surfactants greatly enlarged the lateral dimension to produce nanosheets with uniform thickness.

Bismuth nanowire growth under low deposition rate and its ohmic contact free of interface damage

High quality bismuth (Bi) nanowire and its ohmic contact free of interface damage are quite desired for its research and application. In this paper, we propose one new way to prepare high-quality single crystal Bi nanowires at a low deposition rate, by magnetron sputtering method without the assistance of template or catalyst. The slow deposition growth mechanism of Bi nanowire is successfully explained by an anisotropic corner crossing effect, which is very different from existing explanations. A novel approach free of interface damage to ohmic contact of Bi nanowire is proposed and its good electrical conductivity is confirmed by I-V characteristic measurement. Our method provides a quick and convenient way to produce high-quality Bi nanowires and construct ohmic contact for desirable devices.

Periodic DOS modulation in underdoped Bi2223

Abstract In this report, we show the results of the STM/STS study on the Bi2Sr2Ca2Cu3O10+ δ (Bi2223) single crystals whose doping levels were ranging from an optimally doped to an underdoped region. Bi2223 single crystals were grown by a TSFZ method and their doping levels were adjusted with annealing in an oxygen deficient atmosphere. Single crystals were cleaved in an UHV at around 77 K and STM/STS measurements were carried out in the same conditions. We successfully obtained the tunneling spectrum maps as well as topographic images. We found that the superconducting gap was much more homogeneous than in the case of the Bi2212 in optimal doping, but becomes inhomogeneous with decreasing a doping level. This suggests the decoupling of the three Cu-O layers in terms of the SC correlation. More importantly, we found a new periodic modulation in the LDOS map with periods about 2 a 0, which was almost dispersion-less and observed only in the underdoped TC=85 K sample. This modulation is possibly related to the charge/spin order in the inner plane, which is supposed to be highly undrerdoped.