high-Tc Superconductor Bi2Sr2CaCu2O8 Research Articles

Layer-Dependent Superconductivity in Iron-Based Superconductors CsCa2Fe4As4F2 and CaKFe4As4.

In the quasi-two-dimensional superconductor NbSe2, the superconducting transition temperature (Tc) is layer-dependent, decreasing by about 60% in the monolayer limit. However, for the extremely anisotropic copper-based high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212), the Tc of the monolayer is almost identical with that of its bulk counterpart. To clarify the effect of dimensionality on superconductivity, here, we successfully fabricate ultrathin flakes of iron-based high-Tc superconductors CsCa2Fe4As4F2 and CaKFe4As4. It is found that the Tc of monolayer CsCa2Fe4As4F2 (after tuning to the optimal doping by ionic liquid gating) is about 20% lower than that of the bulk crystal, while the Tc of three-layer CaKFe4As4 decreases by 46%, showing a more pronounced dimensional effect than that of CsCa2Fe4As4F2. By carefully examining their anisotropy and the c-axis coherence length, we reveal the general trend and empirical law of the layer-dependent superconductivity in these quasi-two-dimensional superconductors.

Periodic Atomic Displacements and Visualization of the Electron-Lattice Interaction in the Cuprate

Traditionally, X-ray scattering techniques have been used to detect the breaking of the structural symmetry of the lattice, which accompanies a periodic displacement of the atoms associated with charge density wave (CDW) formation in the cuprate pseudogap states. Similarly, the Spectroscopic Imaging Scanning Tunneling Microscopy (SI-STM) has visualized the short-range CDW. However, local coupling of electrons to the lattice in the form of a short-range CDW has been a challenge to visualize, thus a link between these measurements has been missing. Here, we introduce a novel STM-based technique to visualize the local bond length variations obtained from topographic imaging with picometer precision. Application of this technique to the high-Tc cuprate superconductor Bi2Sr2CaCu2O8+{\delta} revealed a high-fidelity local lattice distortion of the BiO lattice as large as 2%. In addition, analysis of local breaking of rotational symmetry associated with the bond lengths reveals modulations around four-unit-cell periodicity in both B1 and E representations in the C4v group of the lattice, which coincides with the uni-directional d-symmetry CDW (dCDW) previously identified within the CuO2 planes, thus providing direct evidence of electron-lattice coupling in the pseudogap state and a link between the X-ray scattering and STM measurements. Overall, our results suggest that the periodic lattice displacements in E representations correspond to a locally-frozen version of the soft phonons identified by the X-ray scattering measurements, and a fluctuation of the bond length is reflected by the fluctuation of the dCDW formation near the quantum critical point.

Restoring the surface layer of Bi2Sr2CaCu2O8+δ single crystals by current injection

In 2010, we reported a new method to electronically tailor the hole concentration of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi-2212). We discovered that current injection along the c-axis of Bi-2212 provides a convenient tool to reversibly increase the carrier concentration. In this study, we have investigated the current injection effect on the surface junction of Bi-2212 by a series of current injection experiments carried out in samples of slightly overdoped and of extremely underdoped Bi-2212 crystals. The c-axis transport characteristics of bulk and surface intrinsic Josephson junctions are examined in a wide range of doping by current injection. Before injection treatment, typically, our samples demonstrate non-superconducting characteristics across their surface junctions, because standard preparation procedures cause the topmost CuO2 layers to be heavily degraded by oxygen loss. The electronic doping enables to substantially restore the hole concentration of the topmost CuO2 layers. The contact resistance is significantly reduced, and the intrinsic junction at the surface gets back its superconducting tunneling characteristic. We examine systematically the doping results of surface and bulk junction stacks and discuss the enhancement of their superconducting properties.

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.

Elastic properties of few unit cell thick superconducting crystals of Bi2Sr2CaCu2O8+δ

We present systematic measurements of the mechanical properties of few unit cell (UC) thick exfoliated crystals of a high-Tc cuprate superconductor Bi2Sr2CaCu2O8+δ. We determine the elastic properties of these crystals by deformation using an atomic force microscope at room temperature. With the spatial measurements of local compliance and their detailed modeling, we independently determine the Young's modulus of rigidity and the prestress. The Young's modulus of rigidity is found to be in the range of 22 GPa–30 GPa for flakes with a thickness from ∼5 UC to 18 UC. The prestress spreads over the range of 5 MPa–46 MPa, indicating a run-to-run variation during the exfoliation process. The determination of Young's modulus of rigidity for thin flakes is further verified from the recently reported buckling technique.

Local heating effects on the radiation intensity of high-Tc superconducting terahertz emitters

The terahertz electromagnetic waves can be generated by the mesa device made of a piece of high-Tc superconductor Bi2Sr2CaCu2O8+δ single crystal, which is known to be comprised of multi-stacks of intrinsic Josephson junctions. In the case of a thick mesa containing the large number of junctions, large amount of Joule heat is generated, resulting in the formation of the inhomogeneous temperature distribution T(r) in the mesa. It is known from previous studies that T(r) strongly affects the emission intensity and line width. When the mesa is irradiated partly by a focused laser beam, the terahertz radiation intensity grows almost two times as compared with the standard biases without laser irradiation as was found previously. It is also found that the enhanced emission remains without losing the intensity even after switching off the laser power. This result suggests that the normal current distribution in the mesa is modified irreversibly when the device is biased under laser irradiation, according to the thermoreflectance measurement.

Power enhancement of the high-Tc superconducting terahertz emitter with a modified device structure

Continuous and monochromatic terahertz electromagnetic waves can be generated with sizable power (∼30 μW) by the mesa-shaped device made of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, by synchronizing the phase of the Josephson currents between multi-stacked intrinsic Josephson junctions. From the previous experimental results on the line width, spatial distribution of the emission, etc., we think that the relatively weaker power may originate from the partial failure of the phase synchronization of the Josephson currents in the mesa device. This may be improved by modifying the device structure to enhance the Josephson current density inside the mesa. In the present study, an array of four rectangle-shaped mesas with the dimensions of 80±5 μm×350 μm×3.2 μm was fabricated on a thin Bi2Sr2CaCu2O8+δ single crystal, where several slots were made around the mesas by photolithography and chemical etching techniques in order to weaken electrical connection between mesa and superconductor base. The best result of the emission power for the array so far obtained is ~80 μW at 0.42 THz, which is about 2.7 times bigger than the previous champion data in our group.

High-Tc superconducting THz emitters fabricated by wet etching

We studied the etching of small crystals of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) with various dilute compositions of hydrochloric and nitric acids. A particular mixture of those acids was chosen to simultaneously fabricate multiple rectangular stand-alone Bi2212 mesa structures from a large, doubly-cleaved and doubly metallic-coated single crystal. The radiation characteristics of these devices were found to be very similar to stand-alone devices fabricated previously using dry-etching techniques. The greatly reduced time and cost of fabrication of stand-alone Bi2212 devices using our wet-etching technique should facilitate the mass production of a large number of identical stand-alone devices from a large single-crystalline Bi2212 substrate.

Improved excitation mode selectivity of high-Tc superconducting terahertz emitters

Using our recent design of thermally managed sandwich device structures, we studied the radiation frequency characteristics of three such devices of the same rectangular dimensions made from the same single crystal of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, and all three devices exhibit similar characteristics. Their observed radiation intensities appear to be enhanced at many transverse magnetic TMn,m cavity mode frequencies, possibly including some higher TM0,m modes with waves solely along the rectangular length, none of which have previously been reported. In addition, the temperature dependences of the radiation frequencies correspond strongly to the temperature dependences of the maximum bias voltages applied to the devices. The excitations of many cavity modes higher in frequency than that of the usually observed TM1,0 mode and the high reproducibility of the radiation frequency characteristics both appear to originate from the reduction in the Joule self-heating of the thermally managed sandwich structures. The information provided here should aid in the design of future devices to obtain the desired emission frequency ranges.

An Alternative Approach to the Extended Drude Model

The original Drude model, proposed over a hundred years ago, is still used today for the analysis of optical properties of solids. Within this model, both the plasma frequency and quasiparticle scattering rate are constant, which makes the model rather inflexible. In order to circumvent this problem, the so-called extended Drude model was proposed, which allowed for the frequency dependence of both the quasiparticle scattering rate and the effective mass. In this work we will explore an alternative approach to the extended Drude model. Here, one also assumes that the quasiparticle scattering rate is frequency dependent; however, instead of the effective mass, the plasma frequency becomes frequency-dependent. This alternative model is applied to the high Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) with Tc = 92 K, and the results are compared and contrasted with the ones obtained from the conventional extended Drude model. The results point to several advantages of this alternative approach to the extended Drude model.

Structural and functional modifications induced by X-ray nanopatterning in Bi-2212 single crystals

X-ray nanopatterning induces both mosaicity increase and oxygen depletion in the high-Tc superconductor Bi2Sr2CaCu2O8+δ.

The present status of high-Tc superconducting terahertz emitters

A terahertz (THz) wave emitter using the stack of intrinsic Josephson junctions present in the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) has been developed. By applying a dc voltage V across the stack, the ac-Josephson effect converts this to an ac-current that emits photons at the Josephson frequency proportional to V. The Bi2212 device also behaves as and electromagnetic (EM) cavity, so depending upon the shape of the Bi2212 crystal, when the Josephson frequency matches that of a cavity resonance, the emission power is enhanced. However, the EM radiation characteristics also strongly depend upon the effects of Joule self heating of the device. In order to alleviate this Joule heating problem, we fabricated three distinct stand-alone Bi2212 sandwich device shapes, each crystal being first covered with Au on its top and bottom, and then sandwiched between sapphire plates. From our comparative studies of the three devices, we obtained important clues that could help to increase the emission power up to ∼mW and the frequency range up to several THz, as necessary for many applications such as security screening, high speed communications, medical and biological sensing, and astronomical detection, etc.

Applications using high-Tc superconducting terahertz emitters.

Using recently-developed THz emitters constructed from single crystals of the high-Tc superconductor Bi2Sr2CaCu2O8+δ, we performed three prototype tests of the devices to demonstrate their unique characteristic properties for various practical applications. The first is a compact and simple transmission type of THz imaging system using a Stirling cryocooler. The second is a high-resolution Michelson interferometer used as a phase-sensitive reflection-type imaging system. The third is a system with precise temperature control to measure the liquid absorption coefficient. The detailed characteristics of these systems are discussed.

Solution of Sine-Gordon Equations with Weak and Moderate Inductive Couplings under Bias Current and Dissipations

It is found that coherent electromagnetic (EM) radiation in terahertz (THz) band takes place when a single crystal of cuprate high-Tc superconductor Bi2Sr2CaCu2O8+δ (BSCCO) is biased by a dc voltage. In this work we study the inductively coupled sine-Gordon equations which compose a good model for intrinsic Josephson junctions (IJJs) realized in BSCCO due to the strong layer structure. We derive a general solution for the coupled sine-Gordon equations valid for weak and moderate inductive couplings, which can enhance injection of dc energy into IJJs and convert it to THz EM radiation. This solution evolves into the π phase kink state known before for strong inductive coupling.

Area dependence and influence of crystal inhomogeneity on superconducting properties of Bi2212 mesa structures

The rapid increase in applications of terahertz waves requires new techniques to obtain continuous wave terahertz sources. Mesa structures fabricated from high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystal have been observed as an intense, coherent, continuous electromagnetic wave source in the terahertz (THz) frequency region. However, in order to produce coherent radiation with high applicable power, we need large mesa structures that enter a collective electromagnetic state in which their oscillations are largely synchronized in phase. On the other hand, large mesa structures cause a heating problem. In this study, we report on the critical current density dependence of mesa area and the crystal inhomogeneity to understand heating problems in large area mesas for terahertz radiation. Since the doping dependence of Bi2212 is an important parameter, the as-grown Bi2212 crystals were heat-treated at various temperatures under vacuum conditions. We have fabricated triple mesa structures from Bi2212 single crystal using e-beam lithography and argon ion beam etching techniques with same area and with different area on the same chip. We investigated and compared characteristics of triple mesas which are on the same chip and next to each other. In this way, we searched the crystal inhomogeneity in triple mesa structures and studied the critical current density dependence of mesa area to obtain high emission power for the THz radiation. Our experimental results clearly show that the Josephson critical current density is decreasing when the area of mesa is increasing.

Computed tomography image using sub-terahertz waves generated from a high-Tc superconducting intrinsic Josephson junction oscillator

A computed tomography (CT) imaging system using monochromatic sub-terahertz coherent electromagnetic waves generated from a device constructed from the intrinsic Josephson junctions in a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ was developed and tested on three samples: Standing metallic rods supported by styrofoam, a dried plant (heart pea) containing seeds, and a plastic doll inside an egg shell. The images obtained strongly suggest that this CT imaging system may be useful for a variety of practical applications.

Doping dependence of the electron–phonon and electron–spin fluctuation interactions in the high-Tc superconductor Bi2Sr2CaCu2O8+δ

Using ultrafast optical techniques, we detect two types of bosons strongly coupled to electrons in the family of Bi2Sr2CaCu2O8+δ (Bi-2212) from the underdoped to overdoped regimes. The different doping dependences of the electron–boson coupling strengths enable us to identify them as phonons and spin fluctuations: electron–phonon coupling (λe−ph) peaks at optimal doping, while electron–spin fluctuation coupling (λe−sf) decreases monotonically with doping. This observation is consistent with two facts: (i) superconductivity is in close proximity with antiferromagnetism at low dopings and (ii) a pronounced lattice renormalization effect at larger dopings.

Method for Measuring Anisotropic Electrical Resistivity

A novel method is presented for measuring anisotropic electrical resistivity. In this method, a pedestal structure is fabricated on a surface of a crystal. Such a single sample enables measurement of anisotropic resistivity tensor elements simultaneously. The pedestal structure can be very small, in which case the measurement provides a more precise evaluation of anisotropic resistivity than ever. This method is exclusively advantageous in the case of laminar materials or thin films. An example is given of a measurement of the high-Tc superconductor Bi2Sr2CaCu2O8+δ.

Method for Measuring Anisotropic Electrical Resistivity

A novel method is presented for measuring anisotropic electrical resistivity. In this method, a pedestal structure is fabricated on a surface of a crystal. Such a single sample enables measurement of anisotropic resistivity tensor elements simultaneously. The pedestal structure can be very small, in which case the measurement provides a more precise evaluation of anisotropic resistivity than ever. This method is exclusively advantageous in the case of laminar materials or thin films. An example is given of a measurement of the high-Tc superconductor Bi2Sr2CaCu2O8+δ.

Terahertz Radiation Emitted from Intrinsic Josephson Junctions in High-Tc Superconductor Bi2Sr2CaCu2O8+δ

The present status of superconducting terahertz emitter using the intrinsic Josephson junctions in high-Tc superconductor Bi2Sr2CaCu2O8+δ is reviewed. Fabrication methods of the emitting device, electrical and optical characteristics of them, synchronizing operation of two emitters and an example of applications to the terahertz imaging will be discussed. After the description of fabrication techniques by an Argon ion milling with photolithography or metal masks and by a focused ion beam, optical properties of radiation spectra, the line width, polarization and the spatial distribution of emission are presented with some discussion on the operation mechanism. For electrical properties, reversible and irreversible operations at high and low electrical currents, respectively, and electrical modulation of the radiation intensity for terahertz imaging are presented.