Bi-2212 Intrinsic Josephson Junctions Research Articles

Enhancement of macroscopic quantum tunneling in the higher-order phase switches of Bi2212 intrinsic Josephson junctions

The macroscopic quantum tunneling (MQT) in the current-biased intrinsic Josephson junctions (IJJs) of high-Tc cuprates has attracted much attention for decades. Although the MQT for the phase switches from the zero to the first voltage state (1st SW) in the multiple-branched I-V curves is well explained by the conventional theory, the occurrence of MQT for the higher order switches such as the switch from the 1st to 2nd voltage state (2nd SW) has been still debated. Here, we present an experimental study on the phase switches of small IJJs fabricated from underdoped Bi2Sr2(Ca,Y)Cu2Oy. We observed the single photon transition between quantized energy levels in the 3rd phase switches at 59.15 GHz and 2 K. The comparison with the previous studies on the nearly optimal-doped Bi2Sr2CaCu2Oy clearly suggests a possibility that the MQT rate for the higher-order phase switches is commonly enhanced by the effective suppression of the energy barrier for the higher-order phase escape due to the phase-running state after the 1st SW, in spite of the large difference in a critical current density and Tc.

Transmission electron microscopy study of focused ion beam damage in small intrinsic Josephson junctions of single crystalline Bi2Sr2CaCu2Oy

We report a transmission electron microscopy (TEM) study on the damage produced by the focused ion beam (FIB) etching for small Bi2Sr2CaCu2Oy (Bi2212) intrinsic Josephson junctions (IJJs). The selected area diffraction patterns of TEM images demonstrate that the FIB damage causes the formation of an amorphous layer. The thickness of FIB damage is at least 30 nm for the Ga+ ion beam emitted at 50 pA and 30 kV, independent of the incident direction of the Ga+ ion beam. We also confirmed that the damage or the redeposition due to the FIB etching was effectively removed by the additional irradiation of Ar ions after the FIB etching. This suggests the advantage of the combinatorial method of the FIB and Ar-ion etchings in the successful fabrication of small and high-quality IJJs.

Efficient Fabrication of Intrinsic-Josephson-Junction Terahertz Oscillators with Greatly Reduced Self-Heating Effects

The hunt is on for sources of photons to fill the so-called ``terahertz gap'' in the electromagnetic spectrum, for applications spanning medical diagnosis, imaging, security, ultrahigh-speed communication, and biotechnology. The authors suppress the inevitable self-heating of intrinsic Josephson junctions in Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$, realizing an emitter with strongly expanded tunability and a range from 0.3 to 1.63 THz---and potentially extending to 15 THz, which would cover almost the entire gap with one device.

Oscillatory behavior of vortex-lattice melting transition line in mesoscopic Bi_{2}Sr_{2}CaCu_{2}O_{8+y} superconductors.

The vortex-lattice melting transition of a limited number of vortices confined in mesoscopic square superconductors was studied by c-axis resistance measurements using stacks of intrinsic Josephson junctions in Bi_{2}Sr_{2}CaCu_{2}O_{8+y}. In contrast to the melting transition in bulk crystals, we have first found a clear oscillatory behavior in the field dependence of the melting temperature in small samples of 5-10 μm square. The periods of the oscillations roughly obey the regularity of the matching conditions of square vortex lattices surrounded by a square boundary and the melting temperatures are enhanced around the vortex number of i^{2} (where i is an integer). The results suggest that a confinement effect by the square boundary stabilizes square lattice structures which are realized around i^{2} vortex number even in competition with the favorable Abrikosov triangular lattice in the bulk.

Hydrochloric acid modification process for fabricating Bi2Sr2CaCu2O8+x THz oscillator stack on-chip coupled to THz detector

We fabricated Bi2Sr2CaCu2O8+δ (Bi-2212) intrinsic Josephson junctions (IJJ) integrating of an oscillator and a detector by double side photolithography and material modification with a dilutehydrochloric acid solution (pH = 1.65) within a monolithic Bi-2212 crystal. The dilute hydrochloric solution modifies Bi-2212 to insulating BiOCl. Various lateral dimensions of the oscillator IJJ from 45 × 8 to 95 × 30 µm2 were formed; all of the detectors were about 15 × 10 to 15 × 30 µm2 in lateral dimensions. These stacks have 180–416 junctions. Zero voltage current for the detector stack was measured at 77 K while sweeping a bias voltage in the oscillator stack. The zero voltage current of the detector stack was strongly suppressed when a kink structure in the current–voltage curve of the oscillator stacks appeared. This indicates that the oscillator stack emits radiation at this voltage. From the Josephson voltage-frequency relation, it is found that the voltage corresponds to about 0.5–1 THz.

THz emission from a triangular mesa structure of Bi-2212 intrinsic Josephson junctions

Coherent, continuous and monochromatic terahertz electromagnetic waves were observed from the isosceles triangular mesa structures made of single crystalline high temperature superconductor Bi2Sr2CaCu2O8+δ (Bi-2212) intrinsic Josephson junctions (IJJs), fabricated by the focus ion beam (FIB) technique. We studied experimentally the current-voltage (I–V) characteristics and the emission spectra by the FT-IR spectrometer in order to elucidate the radiation mechanism. The wide range of radiation frequency observed in the case of isosceles triangular mesa implies that the cavity resonance is not working properly but the ac-Josephson effect is working as a primary mechanism of THz radiation.

Size-dependent transformation from triangular to rectangular fluxon lattice in Bi-2212 mesa structures

We present a systematic study of the field and size dependencies of the static fluxon lattice configuration in Bi-2212 intrinsic Josephson junctions and investigate conditions needed for the formation of a rectangular fluxon lattice required for a high power flux-flow oscillator. We fabricate junctions of different sizes from Bi2Sr2CaCu2O8+x and Bi1.75Pb0.25Sr2CaCu2O8+x single crystals using the mesa technique and study the Fraunhofer-like modulation of the critical current with magnetic field. The modulation can be divided into three regions depending on the formed fluxon lattice. At low field, no periodic modulation and no ordered fluxon lattice is found. At intermediate fields, modulation with half-flux quantum periodicity due to a triangular lattice is seen. At high fields, the rectangular lattice gives integer flux quantum periodicity. We present these fields in dependence on the sample size and conclude that the transitions between the regions depend only on λJ(Jc) and occur at about 0.4 and 1.3 fluxons per λJ, respectively. These numbers are universal for the measured samples and are consistent with performed numerical simulations.

Magnetic field effects and dynamic control of terahertz electromagnetic wave emission from high-Tc superconducting Bi2Sr2CaCu2O8+δ mesa structures

Terahertz electromagnetic waves are very useful for a number of security and medical applications. Recently, intense and coherent terahertz emission from high-Tc superconducting Bi2Sr2CaCu2O8+δ (Bi2212) intrinsic Josephson junctions (IJJs) has been intensively investigated. In this paper, we report magnetic field effects and dynamic control of terahertz electromagnetic wave emission generated by the rectangular mesa structure of Bi2212. The magnetic field affects anisotropically the emission intensity at T = 25 K Terahertz emission is strongly suppressed by an external magnetic field (less than 200 Oe) parallel to the c axis. In contrast, a 20% enhancement of radiated power is observed when an appropriate magnetic field (H = φ0) is applied parallel to the ab plane. This anisotropy seems to result from the anisotropic vortex formation inside Bi2212. We also demonstrate that the intensity of terahertz emission can be controlled dynamically by applying a weak magnetic field, suggesting that we can modulate the continuous power of terahertz emission by applying weak magnetic field pulses.

Evolution of interlayer tunneling spectra and superfluid density with doping in Bi2Sr2CaCu2O8+δ

Interlayer tunneling spectra, the Josephson critical current density ${J}_{\mathrm{c}}$, and the normal tunneling resistance have been measured simultaneously for intrinsic Josephson junctions in Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+\ensuremath{\delta}}$ over a wide range of doping using a small mesa structure and a short-pulse technique. It is found that ${J}_{\mathrm{c}}$ decreases by nearly two orders of magnitude in the underdoped region. Experimental ${J}_{\mathrm{c}}$ is found to show a significant deviation from the theoretical estimates. In order to interpret the result, we propose a model in which the superconducting order parameter is inhomogeneous in $\mathbf{k}$ space rather than in real space. Based on this model, quasiparticle current-voltage characteristics are calculated, giving a qualitative agreement with the experiment.

Cavity Resonance Effects on Phase-Locking of the Flux-Flow in Bi-2212 Intrinsic Josephson Junctions

We report on the appearance of high-speed flux-flow branches characterized by velocity-matching the maximum mode velocities in quasi-one-dimensional Bi-2212 intrinsic Josephson junctions. As the external magnetic field lower than 0.1 T parallel to the superconducting CuO2 layers decreased, rapid progress of flux-flow velocity toward the maximum mode velocity was observed below a certain field. We show that the progressions of flux-flow velocities are depicted by a universal dependence on the number of vortices in the junctions. This strongly suggests that such a high-Q cavity resonance in Bi-2212 IJJ as the Fiske resonance governs the collective vortex motion interacting with the in-phase electromagnetic mode excited in the junction.

Counterintuitive consequence of heating in strongly-driven intrinsic junctions ofBi2Sr2CaCu2O8+δmesas

Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi2212) mesas have been universally interpreted as superconducting energy gaps, but here we show they are a result of heating. This interpretation follows from a direct comparison to the equilibrium gap, $\mathit \Delta$, measured in break junctions on similar Bi2212 crystals. As the dissipated power increases with a greater number of junctions in the mesa, the conductance peak abruptly sharpens and its voltage decreases to well below 2$\mathit \Delta$. This sharpening, found in our experimental data, defies conventional intuition of heating effects on tunneling spectra, but it can be understood as an instability into a nonequilibrium two-phase coexistent state. The measured peak positions occur accurately within the voltage range that an S-shaped backbending is found in the {\it calculated} current-voltage curves for spatially {\it uniform} self-heating and that S-shape implies the potential for the uniform state to be unstable.

Investigation of I– V characteristics of Bi-2212 intrinsic Josephson junctions for THz-wave generation

Voltage jumps have been observed in the quasiparticle branch I– V characteristics of Bi 2Sr 2CaCu 2O 8+ x intrinsic Josephson junctions fabricated as 60 μm × 300 μm × 0.5 μm mesas. The Josephson frequency at the highest jump voltage was estimated to be 0.65 THz, close to the fundamental cavity resonance frequency of the transverse Josephson plasma mode. Numerical simulations using coupled sine-Gordon equations demonstrated voltage jumps similar to the above experimental results, which appeared in the I– V characteristics, and the electric field distribution at the highest jump voltage exhibited resonance in the junctions. Moreover, the maximum radiation power, which was obtained in the vicinity of the highest voltage, was proportional to the square of the number of junctions. This can be explained by reduction of the impedance mismatch between the junctions and free space with increasing the number of junctions.

Emission of terahertz electromagnetic wave in intrinsic Josephson junction stacks

We overview our recent theoretical works on the emission of terahertz electromagnetic waves recently observed by Ozyuzer et al in the current-biased Bi-2212 intrinsic Josephson junctions. In the voltage state the Josephson current oscillates with a frequency ω = (2e/ħ)(V/N), N being the number of voltage junctions in a stack of intrinsic Josephson junctions. When the frequency ω coincides with an eigen mode frequency of the stacks, strong emission of the electromagnetic wave takes place. We performed numerical simulations for two 2D models composed of intrinsic Josephson junctions and the outside vacuum and clarified the pattern of emitted electromagnetic waves.

Small-number arrays of intrinsic Josephson junctions

Arrays of nanometre-thick Bi2212-intrinsic Josephson junctions (IJJ’s) are studied in various geometries. The samples with only a few IJJ’s allow for the intrinsic-tunnelling spectroscopy with minimum of Joule heating. The reproducible low-voltage peaks of the spectra probably stem from a superconducting gap which is half the usual size. We estimate the internal temperature in the IJJ stacks and analyze the importance of the self-heating for the macroscopic-quantum-tunnelling experiments involving IJJ’s.

21aPS-51 Bi2212固有ジョセフソン接合における高磁場でのジョセフソン磁束状態(領域8ポスターセッション(低温),領域8,強相関系:高温超伝導,強相関f電子系など)

21aPS-51 Bi2212固有ジョセフソン接合における高磁場でのジョセフソン磁束状態(領域8ポスターセッション(低温),領域8,強相関系:高温超伝導,強相関f電子系など)

Collective Josephson vortex dynamics in a finite number of stacked intrinsic Josephson junctions

We report the experimental confirmation of the collective transverse plasma modes excited by the Josephson vortex lattice in stacks of intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ single crystals. The excitation was confirmed by analyzing the temperature ($T$) and magnetic field ($H$) dependencies of the multiple sub-branches in the Josephson-vortex-flow region of the current-voltage characteristics of the system. In the near-static Josephson vortex state for a low tunneling bias current, pronounced magnetoresistance oscillations were observed, which represented a triangular-lattice vortex configuration along the c axis. In the dynamic vortex state in a sufficiently high magnetic field and for a high bias current, splitting of a single Josephson vortex-flow branch into multiple sub-branches was observed. Detailed examination of the sub-branches for varying $H$ field reveals that sub-branches represent the different modes of the Josephson-vortex lattice along the c axis, with varied configuration from a triangular to a rectangular lattices. These multiple sub-branches merge to a single curve at a characteristic temperature, above which no dynamical structural transitions of the Josephson vortex lattice is expected.

Spin Polarized Current Injection Through ${\rm HgBr}_{2}$ Intercalated Bi2212 Intrinsic Josephson Junctions

To investigate the effect of polarized current on tunneling characteristics of intrinsic Josephson junctions (IJJs), spin- polarized and spin-degenerate current have been injected through the c-axis of HgBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intercalated Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.1</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.5</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.4</sub> Cu <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+delta</sub> (Bi2212) single crystals on which 10 times 10 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> mesas have been fabricated. These two spin conditions are achieved by depositing either Au (15 nm)/Co (80 nm)/Au (156 nm) multilayers or single Au film on HgBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intercalated Bi2212 with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 74 K followed by photolithography and Ar ion beam etching. The I-V characteristics have been measured with and without a magnetic field parallel to c-axis at 4.2 K. A fine, soft Au wire is used to make a gentle mechanical contact on the top of a particular mesa in the array. Tunneling conductance characteristics were obtained and the magnetic field dependence of sumgap voltage peaks was investigated. These peaks do not change in position with increasing magnetic field for both contact configurations. In addition, the temperature dependence of tunneling characteristics of the IJJs are obtained and existence of pseudogap feature is observed above T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for HgBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intercalated Bi2212.

MQT observation in Bi2212 intrinsic Josephson junctions

The quantum dynamics of Bi 2Sr 2CaCu 2O 8+ δ intrinsic Josephson junctions (IJJ’s) is studied based on the escape rate measurements. The saturations observed in the escape temperature and the width of the switching current below 0.45 K (= T ∗) indicate the transition of the switching mechanism from the thermal activation to the macroscopic quantum tunneling at T ∗. It is shown that most of the switching properties are consistently explained in terms of the underdamped Josephson junction with quality factor of about 70 in spite of possible damping due to d-wave superconductivity. The present result gives the upper limit of the dissipation of IJJ’s.

Collective macroscopic tunneling in intrinsic Josephson junctions

A theory for photon-assisted macroscopic quantum tunneling (MQT) in a stack of capacitively-coupled intrinsic Josephson junctions is proposed. The theory predicts how the quantized energy-levels in the N-junction stacked system distribute. Furthermore, the theory reveals that the MQT escape rate from the single-plasmon states to the uniform voltage ones under microwave irradiation is proportional to the square of the junction number (N2). This result is consistent with the recent observation in Bi-2212 intrinsic Josephson junctions.

Experimental study of macroscopic quantum tunnelling in Bi2212 intrinsic Josephsonjunctions

The quantum dynamics of Bi2Sr2CaCu2O8+δ intrinsic Josephson junctions (IJJs) is studied based on escape rate measurements. The saturationsobserved in the escape temperature and in the width of the switching current below 0.5 K(= T*) indicate the transition of the switching mechanism from thermal activation to macroscopicquantum tunnelling. It is shown that the switching properties are consistentlyexplained in terms of the underdamped Josephson junction with a quality factor of70 ± 20 in spite of possible damping due to the nodal quasiparticles of d-wave superconductivity.The present result gives the upper limit of the damping of IJJs.