Bi-2212 Single Crystals Research Articles

Microwave Hall measurements using a circularly polarized dielectric cavity.

We have developed a circularly polarized dielectric rutile (TiO2) cavity with a high quality-factor that can generate circularly polarized microwaves from two orthogonal linearly polarized microwaves with a phase difference of ±π/2 using a hybrid coupler. Using this cavity, we have established a new methodology to measure the microwave Hall conductivity of a small single crystal of metal in the skin-depth region. Based on the cavity perturbation technique, we have shown that all components of the surface impedance tensor can be extracted under the application of a magnetic field by comparing the right- and left-handed circularly polarized modes. To verify the validity of the developed method, we performed test measurements on tiny Bi single crystals at low temperatures. As a result, we have successfully obtained the surface impedance tensor components and confirmed that the characteristic field dependence of the ac Hall angle in the microwave region is consistent with the expectation from the dc transport measurements. These results demonstrate a significant improvement in sensitivity compared to previous methods. Thus, our developed technique allows for more accurate microwave Hall measurements, opening the way for new approaches to explore novel topological quantum phenomena, such as time-reversal symmetry breaking in superconductors.

Wavelength dependence of linear birefringence and linear dichroism of Bi2−xPbxSr2CaCu2O8+δ single crystals

Copper oxide high-temperature superconductors, such as Bi2Sr2CaCu2O8+δ (Bi2212), have garnered extensive research interest due to their high critical temperatures (Tc) surpassing the Bardeen–Cooper–Schrieffer (BCS) limit. The two-dimensional CuO₂ plane is widely regarded as the most crucial element of high-Tc cuprate superconductors. The anisotropy of this CuO₂ layer remains a topic of ongoing interest. Although a few experimental results have reported strong optical anisotropy in both ab and ac-planes of Bi2212 through optical “reflectivity” measurements, there is a lack of studies focusing on the optical anisotropy of these materials using optical “transmittance” measurements by utilizing ultraviolet and visible light. Using a generalized high-accuracy universal polarimeter, we observed significant linear birefringence and linear dichroism peaks in the UV region at room temperature. To investigate the origin of the significant optical anisotropy, single crystals of Bi2−xPbxSr2CaCu2O8+δ with different lead contents (x = 0, 0.4, and 0.6) were grown using the floating zone method, and the wavelength dependencies of linear birefringence and linear dichroism along the c axis were measured. The insights gained into the optical anisotropy of Bi2−xPbxSr2CaCu2O8+δ from this study are significant for discussing its origin of the mechanism of high-Tc superconductivity.

Insensitivity of Tc to the residual resistivity in high-Tc cuprates and the tale of two domes

One of the few undisputed facts about hole-doped high-Tc cuprates is that their superconducting gap Δ has d-wave symmetry. According to ‘dirty’ d-wave BCS theory, even structural (non-magnetic) disorder can suppress Δ, the transition temperature Tc and the superfluid density ρs. The degree to which the latter is affected by disorder depends on the nature of the scattering. By contrast, Tc is only sensitive to the total elastic scattering rate (as estimated from the residual resistivity ρ0) and should follow the Abrikosov-Gor’kov pair-breaking formula. Here, we report a remarkable robustness of Tc in a set of Bi2201 single crystals to large variations in ρ0. We also survey an extended body of data, both recent and historical, on the LSCO family which challenge key predictions from dirty d-wave theory. We discuss the possible causes of these discrepancies, and argue that either we do not understand the nature of disorder in cuprates, or that the dirty d-wave scenario is not an appropriate framework. Finally, we present an alternative (non-BCS) scenario that may account for the fact that the superconducting dome in Tl2201 extends beyond that seen in Bi2201 and LSCO and suggest ways to test the validity of such a scenario.

Floating Zone Growth of Pure and Pb-Doped Bi-2201 Crystals

In this review, we summarize recent progress in crystal growth and understanding of the influence of crystal structure on superconductivity in pure and Pb-doped Bi2Sr2CuOy (Bi-2201) materials belonging to the overdoped region of high-temperature cuprate superconductors. The crystal growth of Bi-2201 superconductors faces challenges due to intricate materials chemistry and the lack of knowledge of corresponding phase diagrams. Historically, a crucible-free floating zone method emerged as the most promising growth approach for these materials, resulting in high-quality single crystals. This review outlines the described methods in the literature and the authors’ synthesis endeavors encompassing Pb-doped Bi-2201 crystals, provides a detailed structural characterization of as-grown and post-growth annealed samples, and highlights optimal growth conditions that yield large-size, single-phase, and compositionally homogeneous Bi-2201 single crystals.

Structural Analysis, Characterization, and First-Principles Calculations of Bismuth Tellurium Oxides, Bi6Te2O15

A single crystal of Bi6Te2O15 was obtained from the melt of the solid-state reaction of Bi2O3 and TeO3. Bi6Te2O15 crystallizes in the Pnma space group (No. 62) and exhibits a three-dimensional network structure with a =10.5831(12) Å, b = 22.694(3) Å, c = 5.3843(6) Å, α = β = γ = 90°, V = 1293.2(3) Å3, and Z = 4. The structure was determined using single-crystal X-ray diffraction. An asymmetric unit in the unit cell, Bi3Te1O7.5, uniquely composed of four Bi3+ sites, one Te6+ site, and nine O2− sites, was solved and refined. As a bulk phase, Bi6Te2O15 was also synthesized and characterized using powder X-ray diffraction (XRD), infrared (FT-IR) spectrometry, and the thermogravimetric analysis (TGA) method. Through bond valence sum (BVS) calculations from the single crystal structure, Bi and Te cations have +3 and +6 oxidation numbers, respectively. Each Bi3+ cation forms a square pyramidal structure with five O2− anions, and a single Te6+ cation forms a six-coordinated octahedral structure with O2− anions. Since the lone-pair electron (Lp) of the square pyramidal structure, [BiO5]7−, where the Bi+ cation occupies the center of the square base plane, exists in the opposite direction of the square plane, the asymmetric environments of all four Bi3+ cations were analyzed and explored by determining the local dipole moments. In addition, to determine the extent of bond strain and distortion in the unit cell, which is attributed to the asymmetric environments of the Bi3+ and Te6+ cations in Bi6Te2O15, bond strain index (BSI) and global instability index (GII) were also calculated. We also investigated the structural, electronic, and optical properties of the structure of Bi6Te2O15 using the full potential linear augmented plane wave (FP-LAPW) method and the density functional theory (DFT) with WIEN2k code. In order to study the ground state properties of Bi6Te2O15, the theoretical total energies were calculated as a function of reduced volumes and then fitted with the Birch–Murnaghan equation of state (EOS). The band gap energy within the modified Becke–Johnson potential with Tran–Blaha parameterization (TB-mBJ) revealed a value of 3.36 eV, which was higher than the experimental value of 3.29 eV. To explore the optical properties of Bi6Te2O15, the real and imaginary parts of the dielectric function, refraction index, optical absorption coefficient, reflectivity, the real part of the optical conductivity extinction function, and the energy loss function were also calculated.

Fragile electronic superconductivity in a Bi single crystal

Fragile electronic superconductivity in a Bi single crystal

Electrosynthesis of Quasi-Epitaxial Crystals on Liquid Metals.

Electrosynthesis of single-crystalline metallic and intermetallic particles with a preferred orientation onto liquid metal electrodes has been performed. Liquid gallium electrodes immersed in aqueous alkaline electrolytes without any molecular additive or external solid seeding substrates were used to electroreduce separately Pb2+, Bi3+, Pd2+, and Mn2+. The crystallinity, composition, and orientation of the electrodeposition products were characterized by using scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, grazing incidence X-ray diffraction, and energy-dispersive X-ray spectroscopy. Electrodeposition of Pb and Bi results in the incipient formation of two-dimensional (2D) nuclei that subsequently direct the growth of Pb and Bi single crystals along the most close-packed [111] and [0001] directions, respectively. The absence of any intervening surface oxides and a low electroreduction flux are necessary to avoid polycrystalline dendrite formation. Under comparable conditions, the electrodeposition of Pd and Mn results in single-crystalline intermetallic particles at the interface. Each crystal exhibits a preferred orientation consistent with the unique atomic packing of the near-surface region of the liquid Ga. The presented study suggests a new concept in electrodeposition processes where the liquid metal structure imparts quasi-epitaxial growth in a system in which the electrode material specifically has no crystallinity or long-range order. This study is thus the first demonstration of highly oriented electrodeposition at a liquid/liquid interface under ambient conditions, highlighting the unique solvation environment of liquid metal interfaces for forming thin metallic and intermetallic films.

Study of device characteristics of intrinsic Josephson junction terahertz emitters related to annealing conditions of the crystals

We fabricated terahertz (THz) wave emitters from high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystals annealed under oxygen gas (O2) flow and nitrogen gas (N2) flow conditions. To better understand the annealing effects of the crystal for the device, we evaluated both device properties and a c-axis lattice constant using x-ray diffraction. Compared to the N2-annealed sample, the O2-annealed sample shows higher critical current in the current–voltage characteristics and no clear emission. In addition, multiple hysteresis loops were observed above 75 K. Based on the x-ray diffraction measurements, it is suggested that the presence of multiple hysteresis loops observed in the I–V characteristics of the O2-annealed sample is caused by the existence of layers that have varying levels of oxygen content along the c-axis direction of the crystal. The formation of these layers is attributed to the deposition process of metallic thin films during the device fabrication procedure. This result indicates that the Bi2212 crystal surface of the O2-annealed sample is more sensitive than that of the N2-annealed one. The information is useful for preparing the Bi2212 crystals for THz-wave emitting devices.

Enhancement of Carrier Mobility and Bandgap in Plastically Deformed Bi Single Crystal

Enhancement of Carrier Mobility and Bandgap in Plastically Deformed Bi Single Crystal

Characterization of single crystal Bi:YIG for microwave and optical communication

Characterization of single crystal Bi:YIG for microwave and optical communication

Intrinsic Josephson effect in biaxially-textured Bi-2212 thin films

Bi2Sr2CaCu2O8+x (Bi-2212) thin film with a thickness of micrometers was prepared by a sol-gel method using metal acetate as starting material. The crystal orientation and the superconducting properties of the thin film were studied. Besides, a pattern with a diameter of 300 µm and a thickness of 1.05 µm was prepared. The I-V curve indicates that the biaxially-textured Bi-2212 thin film has a zero-voltage overcurrent and shows a voltage jump under the zero-voltage current. These characteristics are similar to those of a single crystal and exhibit the unique Josephson effect of a intrinsic junction. In the superconducting state, the zero-voltage current increases linearly with the temperature decresing and the hysteresis area of the curve increases gradually. Further analysis indicates that its I-V characteristics are similar to those of a terahertz source device made from Bi-2212 single crystal. Based on these findings, it is predicted that it may generate THz waves with a frequency range of 0.144 THz-1.29 THz. The radiation frequency can be further regulated by controlling the thickness and bias voltage of the biaxially textured film as well as the envirmental temperature.

Revised phase diagram of the high- Tc cuprate superconductor Pb-doped Bi2Sr2CaCu2O8+δ revealed by anisotropic transport measurements

Although phase diagrams can be leveraged to investigate high transition temperature (high-${T}_{c}$) superconductivity, the issue has not been discussed thoroughly. In this study, we elucidate the phase diagram of the overdoped side of high-${T}_{c}$ cuprates via systematic anisotropic transport measurements for Pb-doped Bi-2212 single crystals. We demonstrate that the characteristic temperatures of the ``weak'' pseudogap opening and electronic coherence cross each other at a critical doping level, while those of the ``strong'' pseudogap merges into that of superconducting fluctuations above the critical doping level. Our results indicate the importance of Mottness in high-${T}_{c}$ superconductivity.

Fabrication and transport properties of two dimensional Bi2Sr2Ca2Cu3O10+δ micro-bridge

Ultra-thin high-temperature superconducting films have attracted continuous interest due to their potential electronic applications, which also provide a unique platform of novel physics and properties in the two-dimensional limit. We, here, realized fabrication of two-unit-cell-thick micro-bridges from mechanically exfoliated ultra-thin Bi2Sr2Ca2Cu3O10+δ (Bi2223) single crystals and systematically investigated their transport properties. The two-dimensional superconducting nature is verified by the existence of the Berezinskii–Kosterlitz–Thouless transition, which is simultaneously revealed by current-voltage properties and the zero-field temperature dependence of resistance. Comparing with Bi2223 bulk crystal, a Bi2223 micro-bridge shows a slight lower upper critical field but pronounced improvement in the critical current density. Our findings indicate that the ultra-thin Bi2223 single crystal is highly prospective for both scientific investigations of unconventional superconductivity and applications of high Tc superconducting devices.

Investigation of wet etching solutions and method for thicker stand alone type of mesa structures of Bi2212 single crystals

In order to develop terahertz emitting devices based on high temperature superconductor Bi2212, we explored a simple and easy fabrication process of the stand alone type of mesa structures (SAMs) by using a wet etching method. We underscored a fact that a thicker mesa structure up to about 10 μm can be made by finding an appropriate etching process comparing four different solutions in the wet etching processes. In addition, we have also improved a method to produce free standing SAM chips which are not fixed on any substrates by glues. We measured the characteristics of the device fabricated using this wet etching method and observed clear emission from a chip with the dimensions of 170 × 170 × 8.3 μm3 without the local heating signature of back bending behavior in the current–voltage characteristics. This mesa chip is several times thicker than those used previously. Thus, the method shown here enables us to make many thicker SAMs at a time and would be useful to improve the emission power of superconducting terahertz emitters.

Mapping of Structural Changes Induced by X-ray Nanopatterning via Nano-X-ray Diffraction and Corresponding Electrical Effects

We have investigated the modifications induced in high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212) single crystals by irradiation with an X-ray nanobeam 250 × 250 nm2 in size with E = 14.85 keV and a time-averaged photon flux Φ0 = 7.53 × 108 ph s–1. Crystals were mounted onto amorphous substrates with electrical contacts to allow for both X-ray diffraction (XRD) and electrical measurements on the same sample. Changes in the domain structure were monitored via space-resolved nano-XRD mapping, whereas electrical properties were measured off-line at different levels of irradiation dose. The results show that irradiation induces both an increase of the mosaicity spread and a localized release of the mechanical stress of the crystals. This implies a transient local change of mechanical properties that could be due to thermal fatigue or nonthermal melting. Analysis of the electrical properties of the irradiated regions indicates a strong underdoping effect by the nanobeam and an exponential increase of the normal-state resistivity with the irradiation dose, according to the average rate α = (1.48 ± 0.12) × 10–11 Gy–1. These results represent an important step toward the full development of the X-ray nanopatterning technique.

Microwave Hall effect measurement for materials in the skin depth region

We developed a new microwave Hall effect measurement method for materials in the skin depth region at low temperatures using a cross-shaped bimodal cavity. We analytically calculated electromagnetic fields in the cross-shaped cavity and the response of the cavity including the sample, whose property is represented by the surface impedance tensor; furthermore, we constructed the method to obtain the Hall component of the surface impedance tensor in terms of the change in resonance characteristics. To confirm the validity of the new method, we applied our method to measure the Hall effect in metallic Bi single crystals at low temperatures, and we confirmed that the microwave Hall angles coincide with the DC Hall angle. Thus, it becomes clear that the Hall angle measurement under cryogenic conditions becomes possible without any complicated tuning mechanisms, and our bimodal cavity method can be used to measure the microwave Hall effect on materials in the skin depth region. The result opens a new approach to discuss the Hall effect in condensed matter physics such as the microwave flux-flow Hall effect in superconductors.

Structural Transformations in Crystals Induced by Radiation and Pressure. Part 10. The Crystallographic Picture of Photochemical Behaviour of bi(anthracene-9,10-dimethylene) under High Pressure

The results of the monitoring of the [4 + 4] photocycloaddition reaction path in single crystals of bi(anthracene-9,10-dimethylene) at high pressure are presented. The crystal structures for several steps of the phototransformation at 0.3 GPa and 1.0 GPa were determined and analysed. The applied high pressure did not halt the photochemical reaction and almost 100% of the product molecules were obtained, although the reaction was very slowly reversible similarly to that of ambient conditions. During the crystal phototransformation the intramolecular geometry, molecular orientation and intermolecular interactions of the reactant changed more and more towards the values observed for the product. The initial increase in the unit cell volume brought about by the photochemical reaction was diminished by high pressure. High pressure itself did not significantly influence the intramolecular geometry of the reactant and product molecules, but it influenced the intermolecular interactions.

Phase transition in epitaxial bismuth nanofilms

Raman and coherent phonon spectroscopies were used to investigate the thickness-dependent phononic properties of ultrathin single-crystal Bi films prepared by molecular beam epitaxy on Si(111) substrates. Both the A1g and Eg Raman peaks disappeared in the Raman spectra of a 4 nm Bi film, indicating a complete transition from the low-symmetry A7 structure to the high-symmetry A17 structure. Coherent phonon signals of the A1g mode also showed a strong dependence on the film thickness, where thin samples (≤15 nm) exhibited lower phonon frequency and shorter phonon lifetimes than the thick samples (≥30 nm). This difference is attributed to a shallower energy potential barrier caused by both a permanent phase transition, which is determined by the film thickness, and a temporary structural transition by photo-excited carriers. Our results not only provide evidence of a phase transition from the A7 to the A17 structure with the decreasing Bi film thickness but also reveal the influence of this phase transition on phonon dynamics. Understanding these material performance traits will facilitate modern application of Bi thin films in electronic devices.

Controlled growth of single-crystalline Bi.333(Bi6S9)Br nanorods under hydrothermal conditions for enhanced photocatalytic reduction of Cr (VI)

Abstract The rational design of composition and structure of the photocatalytic materials is especially critical for the practical application of photocatalytic technology. Herein, uniform single-crystal Bi.333(Bi6S9)Br nanorods were synthesized for the first time using a one-pot hydrothermal method. By prolonging the hydrothermal reaction time, the evolution process of Bi.333(Bi6S9)Br from nanoparticles to short nanorods, and finally to long nanorods was observed. Bi.333(Bi6S9)Br nanorods can effectively inhibit the recombination of photogenerated electron-hole pairs and effectively adsorb of Cr (VI) due to the presence of surface defects and unique structure, thereby promoting the photocatalytic activity. Bi.333(Bi6S9)Br nanorods exhibited excellent photocatalytic performance as well as promising recyclability in rapidly reducing aqueous Cr (VI) to Cr (III) under visible light. The degradation rate of Cr (VI) reached 90.96% within 60 min, and the structure of Bi.333(Bi6S9)Br did not change after six photocatalytic cycles. The photocatalytic reaction mechanism of nanorods was proposed based on the band structure of Bi.333(Bi6S9)Br and electron spin resonance radical scavenging assay.

Absence of μSR evidence for magnetic order in the pseudogap phase of Bi2+xSr2−xCaCu2O8+δ

We present an extended zero-field muon spin relaxation (ZF-$\mu$SR) study of overdoped Bi$_{2+x}$Sr$_{2-x}$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals, intended to elucidate the origin of weak quasistatic magnetism previously detected by $\mu$SR in the superconducting and normal states of optimally-doped and overdoped samples. New results on heavily-overdoped single crystals show a similar monotonically decreasing ZF-$\mu$SR relaxation rate with increasing temperature that persists above the pseudogap (PG) temperature $T^*$ and does not evolve with hole doping ($p$). Additional measurements using an ultra-low background apparatus confirm that this behavior is an intrinsic property of Bi2212, which cannot be due to magnetic order associated with the PG phase. Instead we show that the temperature-dependent relaxation rate is most likely caused by structural changes that modify the contribution of the nuclear dipole fields to the ZF-$\mu$SR signal. Our results for Bi2212 emphasize the importance of not assuming the nuclear-dipole field contribution is independent of temperature in ZF-$\mu$SR studies of high-temperature (high-$T_c$) cuprate superconductors, and do not support a recent $\mu$SR study of YBa$_2$Cu$_3$O$_{6+x}$ that claims to detect magnetic order in the PG phase.