Quasiparticle Branches Research Articles

Spin polarized and degenerate tunneling spectra in intrinsic Josephson junctions of Bi2212

AbstractTunneling characteristics of HgBr2 intercalated superconducting Bi2212 single crystals have been obtained by using 10 x 10 mm2 intrinsic Josephson junction stacks, so called mesa structures. The spin degenerate current is driven along the c‐axis with Au layer using point contact tunneling assembly at 4.2 K. The spin polarized current is also driven along the c‐axis of crystals with Au/Co/Au multilayer. In order to understand the role of ferromagnetic layer (Co), quasiparticle branches are examined with and without magnetic field. The magnetic field evolution of switching currents are obtained for gaining further insight about the spin injection through the stack. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low hysteretic behavior of Al∕AlOx∕Al Josephson junctions

All-aluminum Josephson junctions with high-transparency barriers were fabricated using the shadow-evaporation technique and measured at low temperatures, T≈25mK. Due to the high junction transparency, the IV characteristics showed only small hysteresis with a retrapping-to-switching current ratio of up to 80%. The observed critical currents were as large as 80%-100% of the Ambegaokar-Baratoff values. High barrier quality was confirmed by the low subgap leakage currents in the quasiparticle branches, which makes the low hysteretic Al junctions promising for application in integrated rapid single-flux quantum - qubit circuitry.

Vortex structure from BCS to BEC superfluid

In atomic Fermi gases, the pairing character changes from BCS-like to BEC-like when decreasing the threshold energy of the Feshbach resonance. With this crossover, the system enters a strong-coupling regime through the population enhancement of diatom molecules, i.e., tightly bound Cooper pair, and the vortex structure shows much different features in contrast to the well-known core structure in BCS superfluid. In this paper, we study the structure of a single quantized vortex by numerically solving the generalized Bogoliubov–de Gennes equation derived from the boson–fermion model and clarify how the vortex structure changes with the crossover from BCS to BEC. As a result of numerical calculations, we find that the presence of the diatom molecular condensate enhances the matter density depletion inside the vortex core in the crossover regime and the Caroli–de Gennes–Matricon (CdGM) quasi-particle branch almost diminishes in BEC regime.

Neutrino collective excitations in the standard model at high temperature

Neutrino collective excitations are studied in the Standard Model at high temperatures below the symmetry breaking scale. Two parameters determine the properties of the collective excitations: a mass scale $m_\nu=gT/4$ which determines the \emph{chirally symmetric} gaps in the spectrum and $\Delta=M^2_W(T)/2m_\nu T$. The spectrum consists of left handed negative helicity quasiparticles, left handed positive helicity quasiholes and their respective antiparticles. For $\Delta < \Delta_c = 1.275...$ there are two gapped quasiparticle branches and one gapless and two gapped quasihole branches, all but the higher gapped quasiparticle branches terminate at end points. For $\Delta_c < \Delta < \pi/2$ the quasiparticle spectrum features a pitchfork bifurcation and for $\Delta >\pi/2$ the collective modes are gapless quasiparticles with dispersion relation below the light cone for $k\ll m_\nu$ approaching the free field limit for $k\gg m_\nu$ with a rapid crossover between the soft non-perturbative to the hard perturbative regimes for $k\sim m_\nu$.The \emph{decay} of the vector bosons leads to a \emph{width} of the collective excitations of order $g^2$ which is explicitly obtained in the limits $k =0$ and $k\gg m_\nu \Delta$. At high temperature this damping rate is shown to be competitive with or larger than the collisional damping rate of order $G^2_F$ for a wide range of neutrino energy.

Observation of Josephson-vortex-flow submodes in Bi 2Sr 2CaCu 2O 8+ x intrinsic Josephson junctions

Stacked intrinsic Josephson junctions form in highly anisotropic high- T c superconductors such as Bi 2Sr 2CaCu 2O 8+ x . Josephson-vortex dynamics in such systems attracts much research interests, both academic and application points of view. In a dense Josephson-vortex state (in the range of 4–5 T external magnetic field applied in parallel with the junction planes) of Bi 2Sr 2CaCu 2O 8+ x stacked intrinsic Josephson junctions vortex-flow motion is in resonance with the collective plasma oscillation modes. With increasing magnetic fields beyond ∼2 T hysteretic quasiparticle branches keep shrinking, while the collective Josephson-vortex-flow branches start appearing and become clearer. The Josephson-vortex-flow branches split into multiple sub-branches corresponding to the number of coupled junctions in the stack. The low-bias characteristics of the Josephson-vortex-flow sub-branches fit well to the inductive-capacitive hybrid coupling model.

Fabrication of DC SQUIDs Based on&amp;lt;tex&amp;gt;$rm Bi_2rm Sr_2rm CaCu_2rm O_y$&amp;lt;/tex&amp;gt;Intrinsic Josephson Junctions

We report on the first observation of clear voltage modulation in dc SQUID based on intrinsic Josephson junctions of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/. Stacked series array of intrinsic Josephson junctions and dc SQUID based on it have been made from Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ single crystals using a double-side etching process. The number of junctions in the stack is rather controllable by monitoring the junction resistance at room temperature. The current-voltage characteristics of the SQUIDs with stacks of 20-50 IJJs at 4.2 K exhibit additional closely spaced branches together with typical multiple quasi-particle branches with voltage spacing of /spl sim/20 mV. On applying magnetic field clear periodic modulation of the lowest critical current and voltage of the SQUID are observed and the flux period obtained from the voltage-flux characteristics agrees with one flux quantum. The maximum voltage modulation and transfer function are about 80 /spl mu/V and 0.75 mV//spl Phi//sub 0/ at 26 K, respectively. The property of dc SQUID with shunt resistances is also discussed.

MODULATION OF SUPERCONDUCTIVITY BY Si+ IMPLANTATION IN Bi2212 SINGLE CRYSTALS

The multiple quasiparticle tunnelling branches with large hysteresis were measured on the mesas of Si + implanted Bi 2 Sr 2 CaCu 2 O 8+x single crystals. For given Si + dosage the tunnelling critical current I c, decreased for junctions located closer to the mesa surface; meanwhile the voltage interval between the neighboring quasiparticle branches decreased with increasing the Si + dosage.

Depression of the interlayer Josephson coupling in Bi 2Sr 2CaCu 2O 8+ x single crystals by spin-polarized current injection along the c-axis

We studied the effect of spin-injection on tunneling conduction properties of intrinsic Josephson junctions (IJJs) formed in Bi 2Sr 2CaCu 2O 8+ x single crystals. Properties of an identical stack (10 × 5 × 0.030 μm 3) of IJJs were compared for the bias current injection through Au and Co electrodes. Clear quasiparticle branches in tunneling current–voltage ( I– V) curves from the IJJs in the stack were observed for an unpolarized bias current through the Au electrode. Spin-injection through the Co electrode caused pair breaking in the CuO 2 double layers, which led to reduction of the Josephson critical current and the superconducting gap revealed in the tunneling I– V curves.

Fabrication and characteristics of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+x single crystals

Mesa-structured intrinsic Josephson junctions are fabricated in Bi2Sr2CaCu2O8+x single crystals. Typical current-voltage characteristics of intrinsic Josephson junctions are observed, which include multiple quasi-particle branches, surface junction with critical current lower than those of inner junctions, and subgap structures on quasiparticle branches. The corresponding physical explanations are also given. The energy gap voltage of the intrinsic Josephson junctions at 30 K is about 20 mV. Besides, the measuredI c -T relationship agrees quite well with the theoretical computations based on\(d_{x^2 - y^2 } - wave\) superconductor. Our measureddI/dV−V relationship shows theV-shaped gap structure, obviously differing from the U-shaped gap structure of the s-wave superconductor.

Collective dynamics of Josephson fluxons in Bi 2Sr 2CaCu 2O 8+ δ intrinsic junctions

The dynamic characteristics of Josephson fluxons in naturally stacked intrinsic Josephson junctions in Bi 2Sr 2CaCu 2O 8+ δ single crystals were studied experimentally. The Josephson fluxons were generated either by irradiating microwaves or by applying dc magnetic fields in parallel with the junction planes. In the presence of a finite tunneling bias current the fluxons are driven along the junctions in resonance with the excitation modes of Josephson plasma and thus exhibit a variety of dynamic characteristics. Thus in the flux-flow state of fluxons in both cases above, for a relatively low vortex density, sub-branch splitting of the ‘supercurrent’ branch in the current–voltage ( I– V) characteristics was observed, which corresponded to different plasma excitation modes. For a higher vortex density, on the other hand, all the quasiparticle branches merged into a single non-hysteretic I– V curve which exhibited step-like kink features, very similar to the recently predicted ones corresponding to different moving fluxon patterns. The detailed agreement with the theoretical prediction, however, is yet to be confirmed.

The influence of microwave irradiation on the pronounced subgap structures in intrinsic Josephson junctions of (Bi 1− xPb x) 2Sr 2CaCu 2O y

We have studied the behavior of pronounced subgap structures in the current–voltage ( I– V) characteristics of intrinsic Josephson junctions in (Bi 1− x Pb x ) 2Sr 2CaCu 2O y (with x=∼0.15, ∼0.2) mesas at 4.2 K without and with 2–20 GHz microwave irradiation. Without microwave irradiation, the I– V characteristics exhibited a set of superconducting and resistive quasiparticle branches corresponding to the constituent intrinsic Josephson junctions, with accompanying small subgap structures on the quasiparticle branches. Under irradiation of relatively high power of microwaves, the pronounced hysteretic subgap structures reduced, and new fine structures appeared beside them on the quasiparticle branches. The appearance of both original and new fine subgap structures was interpreted with regard to the interference of Raman-optical phonons with ac Josephson current in the intrinsic Josephson junctions.

Characterization of intrinsic Josephson junctions for La 2− xSr xCuO 4 single crystals

We have fabricated c-axis micro-bridges of La 2− x Sr x CuO 4 (LSCO) single crystals in order to characterize the LSCO intrinsic Josephson junctions (IJJs). The current–voltage characteristics of the micro-bridges exhibited a large hysteresis with a voltage jump of the order 0.5–3 V and no multiple branching structures. A superconducting energy gap was clearly observed on the quasi-particle branch and showed BCS-like temperature dependence. In addition, the temperature dependence of the critical current of the IJJ was in good agreement with the theoretical curves for superconductor–insulator–superconductor (SIS) Josephson junctions. These results demonstrate that the IJJs of LSCO are characterized as stacked series SIS junctions.

Microwave oscillation from Bi2Sr2CaCu2Oy intrinsic Josephson junction arrays

We report the novel nonequilibrium microwave emission from current-injected Bi2Sr2CaCu2Oy (BSCCO)/I/Au intrinsic Josephson junctions and two-mesa array of intrinsic Josephson junctions. For the single intrinsic junctions, it appeared as three different modes of oscillations. Among them, two modes are Josephson plasma emissions which appeared at the quasiparticle branches and the gap-edges. The mutual phase interaction between two-mesa structures of BSCCO intrinsic Josephson junctions has been studied. The measured output power of the two-mesa structures of intrinsic Josephson junctions was almost four times larger than that of the single junction, indicating an exact phase-locking state between the two intrinsic Josephson junctions. The maximum detected power was about microwatt level at millimetre-wave range.

New features of subgap structures in intrinsic Josephson junctions of (Bi 1− xPb x) 2Sr 2CaCu 2O y

We have investigated the pronounced subgap structures in intrinsic Josephson junctions of (Bi 1− x Pb x ) 2Sr 2CaCu 2O y mesas with x≲0.2. The structures appear to be composed of multiple subbranches on quasiparticle branches in the current–voltage characteristics, independent of x, junction geometry, weak magnetic field and temperature T lower than the disappearance temperatures of subbranches T d. The excess current densities of the subbranches are approximately proportional to the critical current density of the intrinsic Josephson junctions at 4.2 K, and tend to increase, when T approaches T d. These features of the structures are discussed within the framework of the recently proposed coupling mechanisms between optical phonons and Josephson oscillations.

Quasiparticle current and phase locking of intrinsic Josephson junctions

On the base of our experiments on thin film Josephson junctions in mesa geometry we discuss the quasiparticle branches of the intrinsic arrays within a tunnelling model using d-wave superconductor density of states. We find temperature dependent current contributions and a zero bias anomaly. The coherent behaviour is studied for intrinsic arrays with an additional side-wall shunt. The existence of thresholds of phase locking at small as well as at large inductances is demonstrated. We discuss the problems with experimental realisation of the shunts as well as with an alternative concept to enhance phase locking in such arrays towards application as oscillators in the frequency range up to some THz.

Effect of thermal and quantum fluctuations on superfluid density in layered superconductors

We calculate suppression of inter- and intra-layer superconducting currents due to equilibrium phase fluctuations and find that, in contrast to a recent prediction, the effect of thermal fluctuations cannot account for linear temperature dependence of the superfluid density in high- T c superconductors at low temperatures. Quantum fluctuations are found to dominate over thermal fluctuations at low temperatures due to hardening of their spectrum caused by the Josephson plasma resonance. Near T c sizeable thermal fluctuations are found to suppress the critical current in the stack direction stronger than in the direction along the layers. Fluctuations of quasi-particle branch imbalance make the spectral density of voltage fluctuations at small frequencies non-zero, in contrast to what may be expected from a naive interpretation of Nyquist formula.

In-plane magnetic field dependence of intrinsic Josephson junctions in Tl-Ba-Ca-Cu-O thin films

We have fabricated intrinsic Josephson bridges using Tl-Ba-Ca-Cu-O thin films grown on vicinal lanthanum aluminate substrates. Measurements of the lock-in transition allow us align a magnetic field of up to 4 T with the copper oxide planes to better than 0.3/spl deg/. This allows us to study the dynamics of Josephson fluxons in the films. For fields above 1.4 T and at low currents we observe Josephson flux creep. Above a field-dependent threshold current we observe free flow of the fluxons with a flux-flow resistance which is proportional to field. The fluxons reach a field-independent velocity before the jump to the quasiparticle branch occurs. Our results suggest that the development of a sub-mm-wave oscillator using these bridges will require sub-micron patterning.

Josephson plasma oscillation of a Bi 2Sr 2CaCu 2O y intrinsic junction

We report the three different modes of microwave emissions from the quasiparticle(QP)-injected Bi 2Sr 2Ca 2Cu 2Oy (BSCCO) intrinsic Josephson junctions. Three different modes of microwave emissions corresponding to the different branches of hysteretic current-voltage ( I-V) characteristics of the intrinsic junction are observed by using a heterodyne technique. In addition to Josephson emission at low bias voltage, we found the two emissions due to nonequilibrium effects: an incoherent emission at QP branches and a sharp coherent emission at the gap edge.

Andreev bound states and Josephson coupling in YBa 2Cu 3O 7− δ/Pb tunnel junctions

We have made in situ [1 0 3] and [1 1 0] YBCO/Pb junctions, which display an hysteretic behavior with both a well-defined quasi-particle branch and a Josephson current. We have therefore been able to confirm the existence of a surface state which breaks time-reversal symmetry. However, we have not seen any evidence of a coupling between Josephson current and the zero-energy ABS, predicted by Tanaka and Kashiwaya (Phys. Rev. B 56 (1997) 892).

TUNNELING CHARACTERISTICS OF INTRINSIC JOSEPHSON JUNCTIONS IN (Bi1-xPbx)2Sr2CaCu2Oy SINGLE CRYSTALS

We have studied the current-voltage characteristics of intrinsic Josephson junctions in mesa-shaped Bi 2 Sr 2 CaCu 2 O y and ( Bi 1-x Pb x)2 Sr 2 CaCu 2 O y single crystals as a function of x. Pronounced Josephson tunneling characteristics, which are characterized by high critical current densities and multiple quasiparticle branches with clear subgap structures, have been observed for ( Bi ~0.8 Pb ~0.2)2 Sr 2 CaCu 2 O y. A strong correlation between the characteristics and structural modulation in this system has been found.