Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A41: Superconductivity: Multilayers and Josephson Effects |
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Sponsoring Units: DCMP Chair: John Clarke, University of California, Berkeley Room: F152 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A41.00001: THz wave emission from intrinsic Josephson junctions in in-plane magnetic fields controlled by surface impedance Yoshihiko Nonomura Since THz wave emission from intrinsic Josephson junctions without external magnetic fields was observed experimentally,[1] possible emission states have been discussed theoretically. My recent numerical simulations [2] revealed that both in-phase McCumber-like state and symmetry-breaking $\pi$-phase-kink state can be stable as the surface impedance $Z$ is varied. Quire recently effects of in-plane magnetic fields are about to be observed in the same samples used in experiments without external fields, and preliminary results still look controversial. In the present study such systems are investigated numerically, and $Z$ is also found out to be essential. When in-plane field is applied to the $\pi$-phase-kink state, two-step dynamical phase transitions to the incommensurate-phase-kink state (artifact due to small number of layers used in simulations) and in-phase state (independent of number of layers) occurs. Up to $Z \approx 40 \sim 50$ the onset field of the in-phase state is small and a characteristic peak in emission intensity is obvious, while for larger $Z$ this onset field shifts to larger values and emission intensity decreases monotonically as the field increases, which resolves controversy. \par \noindent [1] L.~Ozyuzer {\it et al.}, Science {\bf 318}, 1291 (2007); K.~Kadowaki {\it et al.}, Physica C {\bf 468}, 634 (2008). [2] Y.~N., PRB {\bf 80}, 140506(R) (2009). [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A41.00002: Tuning superconductivity by carrier injection Paul M\"uller, Yuri Koval, Xiaoyue Jin, Christoph Bergmann, Yilmaz Simsek, L\"utfi \"Ozy\"uzer, Huabing Wang, G\"unter Behr, Bernd B\"uchner In layered high-temperature superconductors, like Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, superconductivity is controlled by carrier doping of the conducting planes. Usually this is achieved by a non-stoichiometric composition. The current transport perpendicular to the planes of these layered materials shows intrinsic Josephson effects. Normally, current flow inside superconductors is never expected to be able to change the material itself. However, we were able to show that by extensive current injection along the c-axis the superconducting properties of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ can be changed effectively and reversibly. By injecting current perpendicular to the planes, we show that critical temperature, c-axis resistivity and critical current of intrinsic Josephson junctions can be tuned in a large range from underdoping to extreme overdoping. This effect is reversible and room-temperature stable. Apparently, the insulating layers are charged by injected carriers, and work as a floating gate. The result is hole doping of the conducting layers. This flash memory concept seems to be a general property of layered materials where the insulating charge reservoir layers are separated from the conducting planes. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A41.00003: The current carrying effects on the interplay between the superconductivity and the anti-ferromagnetism (AFM) in a multi-layered high $T_c$ cuprates Haitao Quan, Jian-Xin Zhu We study the current carrying effects on the interplay between the superconductivity and the anti-ferromagnetism (AFM) in a multi-layered high $T_c$ cuprates with various doping. Both the superconducting and AFM ordering are calculated by solving the BdG equation numerically. The current is calculated as a response to the phase gradient across the system. It is found that the AFM and SC can coexist in the presence of a current. In particular, we investigate how the AFM and the current will influence each other. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A41.00004: Spin-triplet superconductivity induced by PdNi alloy in Co-based Josephson junctions Trupti Khaire, Mazin Khasawneh, William Pratt, Norman Birge The conventional proximity effect in superconducting/ferromagnetic (S/F) hybrid systems decays over a short length scale in the ferromagnet, as the two electrons from the spin-singlet Cooper pair enter different spin bands and rapidly lose phase coherence. Spin-triplet Cooper pairs are not subject to this constraint, hence a spin-triplet proximity effect should extend much further into the ferromagnet [1,2]. We report observation of a spin-triplet supercurrent in S/F/S Josephson junctions containing Co(d)/Ru(0.6 nm)/Co(d) trilayers as thick as 2d = 28 nm. The spin-triplet pair correlations are induced by thin layers of Pd$_{0.88}$Ni$_{0.12}$ alloy placed between the central Co and outer Nb layers. The spin-triplet critical supercurrent in our junctions exhibits no discernable decay with increasing Co thickness, whereas the supercurrent decays exponentially with a decay length of 2.4 nm in similarly-prepared junctions without PdNi [3]. When 2d = 20 nm, junctions containing PdNi exhibit a supercurrent more than 100 times greater than that of junctions without PdNi. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A41.00005: Systematic exploration of spin-triplet superconductivity in Co-based Josephson junctions Mazin Khasawneh, Trupti Khaire, William Pratt, Norman Birge The discovery that a few nm of PdNi alloy near a Nb superconductor induces spin-triplet superconducting correlations [1] begs the question: what other materials produce a similar effect? In this talk we will discuss several other materials we have tried in place of PdNi. So far, only Cu$_{0.48}$Ni$_{0.52}$ alloy has been successful, producing a factor of 20 enhancement of the supercurrent for Josephson junctions containing 20 nm of Co. We speculate that the key ingredients in the success of PdNi and CuNi alloys are small magnetic domain size and out-of-plane magnetocrystalline anisotropy. The latter, when combined with in-plane shape anisotropy of thin magnetic films, can lead to canting of the domain magnetization and hence the non-collinear domain magnetizations crucial to the appearance of the spin-triplet correlations [2,3]. \\[4pt] [1] T.S. Khaire, M.A. Khasawneh, W.P. Pratt, Jr., and N.O. Birge, previous abstract. \\[0pt] [2] F.S. Bergeret, A.F. Volkov, {\&} K.B. Efetov, \textit{Phys. Rev. Lett.} \textbf{86}, 4096 (2001). \\[0pt] [3] A. Kadigrobov, R.I. Shekhter, {\&} M. Jonson, \textit{Europhys. Lett.} \textbf{54} (3), 394 (2001). [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A41.00006: Output of a Josephson stimulated terahertz amplified radiation emitter Richard Klemm, Kazuo Kadowaki The angular dependence of the radiation-zone output power and electric polarization of the stimulated terahertz amplified radiation (STAR) emitted from a $dc$ voltage applied across cylindrical and rectangular stacks of intrinsic Josephson junctions is calculated. During coherent emission, a spatially uniform $ac$ Josephson current density in the stack acts as a surface electric current density antenna source, leading to an harmonic radiation frequency spectrum, as in experiment, but absent in all cavity models of cylindrical mesas. Spatial fluctuations of the $ac$ Josephson current cause its fundamental mode to lock onto the lowest finite energy cylindrical cavity mode, causing it to resonate, leading to a non-uniform magnetic surface current density radiation source, and a non-trivial combined fundamental frequency output power with linear polarization for general radiation directions. Existing Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ crystals atop perfect electric conductors could have STAR emitter power in excess of 5 mW, acceptable for many device applications. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A41.00007: Characterizing aluminum nanobridge Josephson junctions for molecular magnetometry R. Vijay, E.M. Levenson-Falk, I. Siddiqi NanoSQUIDs, which consist of a superconducting loop interrupted by two sub-micron weak link Josephson junctions,are commonly used for measuring nanomagnets. The weak links enhance flux coupling, and also reduce the junction area, thereby permitting the application of large in-plane magnetic fields to tune the energy level structure of the nanomagnet. However, these type of SQUIDs have lower flux sensitivity than conventional tunnel junction based devices, mainly on account of reduced nonlinearity in their current-phase relation. We explore different weak link junction geometries to optimize flux sensitivity. In particular, we have fabricated 30 nm wide and 50-400 nm long aluminum nanobridges contacted with 2D and 3D banks. We present I-V characteristics and critical current modulation data for both designs. The 3D nanoSQUIDs show enhanced flux modulation of the critical current. With 3D banks the phase drop across the junction is mainly confined to the nanobridge, thus enhancing nonlinearity. When combined with dispersive microwave techniques, these nanoSQUIDs can be used for high speed, low backaction measurements. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A41.00008: Phase Dynamics of Ferromagnetic Josephson Junctions Ivana Petkovic, Marco Aprili We have investigated the classical phase dynamics of underdamped ferromagnetic Josephson junctions by measuring the switching probability in both the stationary and nonstationary regimes down to 350 mK. We found the escape temperature to be the bath temperature, with no evidence of additional spin noise. In the nonstationary regime, we have performed a pump-probe experiment on the Josephson phase by increasing the frequency of the junction current bias. We show that an incomplete energy relaxation leads to dynamical phase bifurcation. Bifurcation manifests itself as premature switching, resulting in a bimodal switching distribution. We directly measure the phase relaxation time by following the evolution of the bimodal switching distribution when varying the bias frequency. Numerical simulations account for the experimental values of the phase relaxation time. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A41.00009: Non-equilibrium effects in a Josephson junction coupled to a precessing spin Cecilia Holmqvist, Sofian Teber, Manuel Houzet, Denis Feinberg, Mikael Fogelstrom We present a theoretical study of a Josephson junction consisting of two s-wave superconducting leads coupled over a classical spin. When an external magnetic field is applied, the classical spin will precess with the Larmor frequency. This magnetically active interface results in a time-dependent boundary condition with different tunneling amplitudes for spin-up and spin-down quasiparticles and where the precession produces spin-flip scattering processes. We show that as a result, the Andreev states develop sidebands and a non-equilibrium population which depend on the precession frequency and the angle between the classical spin and the external magnetic field. The Andreev states lead to a steady-state Josephson current whose current-phase relation could be used for characterizing the precessing spin. In addition to the charge transport, a magnetization current is also generated. This spin current is time-dependent and its polarization axis rotates with the same precession frequency as the classical spin. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A41.00010: Coherent THz radiation from multiple I-V branching structures in intrinsic Josephson junctions of Bi-2212 Manabu Tsujimoto, Kazuhiro Yamaki, Takashi Yamamoto, Hidetoshi Minami, Takanari Kashiwagi, Kazuo Kadowaki, Masashi Tachiki Intense and coherent terahertz (THz) radiation was observed from the intrinsic Josephson junction (IJJ) system of the single crystalline high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212).\footnote{L. Ozyuzer et al., Science \textbf{318,} (2007) 1291.}$^,$\footnote{K. Kadowaki et al., Physica C \textbf{468,} (2008) 634.} In the present work, we demonstrate the importance of the THz radiation from inner branching structures of the $I-V$ characteristic curve of the IJJ system. The radiation frequency has previously been thought to be uniquely constrained to the mesa size, but it turns out in the inner branches that it varies very flexibly and widely perhaps according to the inductive and capacitive coupling strength existing in the mesa itself. Therefore, the radiation frequency does not follow the previously established cavity resonance condition. This new experimental feature may provide a unique opportunity to understand the dynamical nature of IJJ as well as the mechanism of high-$T_{c}$ superconductivity in this particular Bi-2212 compound. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A41.00011: Coherent THz radiatin and its manipulation in intrinsic Josephson junctions of Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\delta}}$ Kazuo Kadowaki, M. Tsujimoto, T. Koike, N. Orita, K. Deguchi, K. Yamaki, T. Yamamoto, Krsto Ivanovic, T. Kashiwagi, H. Minami, M. Tachiki, S. Fukuya, R.A. Klemm We have recently succeeded in generating THz emission from mesas fabricated in high $T_c$ superconductor single crystalline Bi$_2$Sr$_2$CaCu$_2$O$_{8+{\delta}}$ intrinsic Josephson junction systems. The emission frequency can be controlled by the geometrical cavity conditions, and ranges from about 200 GHz to about 1 THz as a fundamental frequency with up to the 3rd harmonics corresponding to 3 THz, depending on the cavity resonance conditions, as long as the $ac$- Josephson condition is fulfilled. The intensity is estimated to be $\sim$10 $\mu$W, but in some cases it reaches even upto 50 $\mu$W. Furthermore, the radiation is coherent and continuous with high stability. All these features are extremely beneficial for quantum mechanical namipulation of qubits. We demonstrate experimentally the tunability of the emission generated from two mesas showing resonance merging behavior as an example. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A41.00012: Coherent THz radiation with geometrical full wave length resonance in single crystalline Bi2212 T. Kashiwagi, K. Yamaki, M. Tsujimoto, K. Deguchi, T. Koike, N. Orita, R. Nakayama, H. Minami, T. Yamamoto, K. Kadowaki Recently, continuous terahertz emission has been observed from a mesa structure of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi2212) single crystals.\footnote{L. Ozyuzer et al., Science \textbf{318} (2007) 1291.} From previous experimental results, it has been established that the emission frequency is determined by both ac Josephson effect and geometrical cavity resonance condition. However, in the recent experimental results have shown that the above condition is often violated keeping the ac Josephson effect to be still valid. It seems that the half wave length resonance mode disappears and the full wave length resonance mode emerges resulting in the double frequency as a fundamental mode. This can be found not only in the rectangular mesas but also in square mesas. In the case of 80 $\mu $m width mesa, for example, it can generate 930 GHz as a fundamental mode with up to 3rd harmonics corresponding to 2.8 THz. Although the origin of this full wave length resonant fundamental mode is not well understood, it is worth obtaining higher frequencies for practical applications. [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A41.00013: Microwave Activation of a Current-Biased Josephson Junction~Near the Classical-Quantum Crossover Zechariah Thrailkill, Joseph Lambert, Steven Carabello, Thilanka Galwaduge, Roberto Ramos We examine the microwave activation of a current-biased Josephson junction near the crossover temperature $T_{cr} ={hf} \mathord{\left/ {\vphantom {{hf} {2\pi k_b }}} \right. \kern-\nulldelimiterspace} {2\pi k_b }$ in order to show how the device transitions from the classical regime to the quantum regime.~ We use microwaves to probe the quantum energy states that exist in the potential well.~ The quantum features, in the form of peaks in escape rate enhancements,~are visible until the junction is heated up to the crossover temperature, at which point the line widths of the energy levels overlap and become indistinguishable from one another. The result is a step-like structure that is characteristic of escape rate enhancements in the classical regime. Well above this temperature, the junction behaves classically when resonantly activated with microwaves. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A41.00014: Very Large Scale Integration of Nano-patterned YBa$_2$Cu$_3$O$_{7-\delta}$ Josephson Junctions in a Two-dimensional Array Shane Cybart, S.M. Anton, S.M. Wu, John Clarke, R.C. Dynes Very large scale integration of Josephson junctions in a two- dimensional series-parallel array has been achieved by ion irradiating a YBa$_2$Cu$_3$O$_{7-\delta}$ film through slits in a nano-fabricated mask created with electron beam lithography and reactive ion etching. The mask consisted of 15,820 high-aspect ratio (20:1), 35-nm wide slits that restricted the irradiation in the film below to form a 28 x 565 Josephson junction array. Characterizing each parallel segment, containing 28 junctions, with a single critical current we found a standard deviation of about 16\%. To study array configuration, the number of SQUIDs connected in parallel was decreased by etching the array with photolithography and ion milling. We compare voltage --- magnetic field characteristics for the different configurations and have found that the modulation depth of the voltage has a much stronger than expected dependence on the inductances of the SQUIDs. [Preview Abstract] |
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