Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F36: Superconductivity: Josephson Effect |
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Sponsoring Units: DCMP Chair: Carmen Almasan, Kent State University Room: 344 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F36.00001: High Temperature Superconducting Terahertz Emitters with Various Mesa Structures Kaveh Delfanazari, M. Tsujimoto, T. Kashiwagi, H. Asai, T. Kitamura, T. Yamamoto, M. Sawamura, K. Ishida, C. Watanabe, S. Sekimoto, H. Minami, M. Tachiki, T. Hattori, R. A. Klemm, K. Kadowaki In 2007, the first observation of the coherent terahertz (THz) electromagnetic (EM) waves from a mesa structures of intrinsic Josephson junctions (IJJs) in high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (Bi-2212) is reported [1]. The ac-Josephson effect as well as the cavity resonance conditions is considered as the principle mechanism of the THz radiation [1, 2]. In order to understand the cavity effect in THz radiation from IJJ mesas more clearly, we studied mesas with various geometries; various kinds of triangles [3], and pentagonal mesas with various sizes and thicknesses. The focused ion beam (FIB) milling technique is used in all mesa fabrications. In this talk, we discuss our recent progress in THz emission observation in pentagonal mesas.\\[4pt] [1] L. Ozyuzer \textit{et al}., Science \textbf{318} (2007) 1291.\\[0pt] [2] M. Tsujimoto \textit{et al}., Phys. Rev. Lett. \textbf{108}, (2012) 107006.\\[0pt] [3] K. Delfanazari \textit{et al}., Submitted (2012) [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F36.00002: Tunable THz radiation from intrinsic Josephson junctions in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ in a localized phase rotating mode Manabu Tsujimoto, Kaveh Delfanazari, Takeo Kitamura, Masashi Sawamura, Kazuya Ishida, Shunsuke Sekimoto, Chiharu Watanabe, Takashi Yamamoto, Takanari Kashiwagi, Hidetoshi Minami, Kazuo Kadowaki After the first report of intense continuous THz electromagnetic wave radiation from high-$T_{\mathrm{c}}$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ [L. Ozyuzer \textit{et al.}, Science \textbf{318}, 1291 (2007)] with remarkably higher intensity, a great deal of interest has been drawn not only to the physical mechanism of the radiation but also to the possible variety of applications in the vast fields of THz science and technology. Recently, the authors pointed out that the contributions to the output power from the Josephson current source was found to be comparable in magnitude [K. Kadowaki \textit{et al.}, J. Phys. Soc. Jpn. \textbf{79}, 023703 (2010); M. Tsujimoto \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 107006 (2012)]. As R. Kleiner \textit{et al.} observed in 1992 [R. Kleiner \textit{et al.}, Phys. Rev. Lett. \textbf{68}, 2394 (1992)], the intrinsic junctions in the phase rotating mode produce an equal number of $I$-$V$ characteristic branches. Here we show clear evidence that the mesas can emit radiation at many frequencies in various localized phase rotating modes, and that the resulting radiation is tunable over a broad range of frequencies, allowing us to construct a powerful THz source device that could fill the THz gap. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F36.00003: Direct imaging of hot spot in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ mesa terahertz sources Timothy Benseman, Ken Gray, Alexei Koshelev, Wai-Kwong Kwok, Ulrich Welp, Vitalii Vlasko-Vlasov, Kazuo Kadowaki, Hidetoshi Minami Stacks of intrinsic Josephson junctions (IJJs) made from high-temperature superconductors such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212) are a promising source of coherent continuous-wave terahertz radiation. When electrical power is applied to these devices, it is thought that hot spots may form due to resistive self-heating, and that these spots may be highly beneficial for the generation of high levels of THz power from Bi-2212 stacks. In order to better understand these hot spots, we have performed a thermal imaging study of BSCCO stacks which generate approximately 50 microwatts of radiation power at 0.59 THz. Utilizing the temperature-dependent 612nm fluorescence line of Eu$^{3+}$, we are able to directly measure the temperature distribution at the top surface of these stacks with a resolution of $+$/- 1K. The images reveal a highly non-uniform temperature distribution in which the temperature in the middle of the stack can exceed the superconducting transition temperature by tens of Kelvin under biasing conditions typical for THz-emission. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F36.00004: Magnetic field effects on THz radiation from Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesa structures Takeo Kitamura, Takanari Kashiwagi, Manabu Tsujimoto, Kaveh Delfanazari, Masashi Sawamura, Kazuya Ishida, Shunsuke Sekimoto, Chiharu Watanabe, Takashi Yamamoto, Hidetoshi Minami, Masashi Tachiki, Kazuo Kadowaki In a previous our study, coherent and continuous electromagnetic radiation phenomena in mesa structures of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystal have been investigated precisely in magnetic fields up to only 150 Oe [1]. This experimental result showed that the emission intensity decreases sharply for the field parallel to the $c$-axis, while it decreases gradually as increasing magnetic field for the in-plane field. In order to improve the measurement, we developed a new system with a better angular resolution and much wider magnetic field range up to 6 T, and a mesa having much stronger THz emission power. The mesa structure is also changed to the stand-alone type of mesa, which produces higher power THz radiation with ideal distribution of radiation [2]. In this presentation, the recent detailed results will be shown in magnetic fields both parallel and perpendicular to the \textit{ab}-plane of Bi2212, where the Josephson and pancake vortices are playing an important role for THz radiation.\\[4pt] [1] K. Yamaki \textit{et al}., physica C \textbf{470} (2010) S804.\\[0pt] [2] T. Kashiwagi \textit{et al}., Jpn. J. Appl. Phys. \textbf{51} (2012) 010113. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F36.00005: Simultaneous observation of temperature distribution and THz emitting spectra of Bi2212 THz devices Chiharu Watanabe, Hidetoshi Minami, Takashi Yamamoto, Takanari Kashiwagi, Kazuo Kadowaki When the intrinsic Josephson junctions in high-$T_{\mathrm{c}}$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ are fabricated to a mesa structure and biased by a dc-voltage, it is known to emit coherent, stable and continuous THz electromagnetic waves [1] whose frequency lies between 0.3-1 THz with the line width of 0.5 GHz and the radiation maximum power of 30 $\mu $W. Recently, we have succeeded in measuring the temperature distribution of the mesa directly while the mesa is emitting THz radiation and found an extreme temperature inhomogeneity (hot-spot) in the mesa [2]. By this way, we could determine the temperature of a mesa, as a result we could observe inhomogeneous temperature distribution, and we found that relation between THz emitting characteristics and temperature distribution. The simultaneous observation of the emission frequency by FTIR strongly indicates that the THz emission phenomenon is not influenced by the formation of the hot-spot at all. In this meeting, we would like to discuss the relations between THz emission phenomena hot-spot formation.\\[4pt] [1] L. Ozyuzer, \textit{et al}., Science \textbf{318}, 1291 (2007).\\[0pt] [2] H. Minami, \textit{et} \textit{al}., in preparation. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F36.00006: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F36.00007: Control of Spin-Triplet Josephson Junctions with Perpendicular Anisotropy Eric Gingrich, Simon Diesch, William Pratt, Norman Birge We present recent work on S/F'/F/F''/S Josephson Junctions with the magnetic multilayer Ni/[Co/Ni]$_n$ as the central F layer [1]. This multilayer possesses strong intrinsic perpendicular (out-of-plane) anisotropy at Co and Ni thicknesses of a few monolayers. If a hard ferromagnet is used for F', and a softer ferromagnet is used for F'', both with magnetizations in plane, the direction of the F'' layer's magnetization is predicted to control the state of the junction [2]. We are fabricating such junctions with the goal of controllably switching the junction between the $0$ and $\pi$ states. By integrating these junctions in a SQUID device, a measurement of the $0$and $\pi$ state of the junction can be performed. We will report on our progress.\\ \\ $[$1$]$ E.C. Gingrich, P. Quarterman, Y. Wang, R. Loloee, W.P. Pratt, and N.O. Birge, arXiv:1208.3118v1.\\ $[$2$]$ A.F. Volkov, F.S. Bergeret, and K.B. Efetov, Phys. Rev. Lett. \textbf{90}, 117006 (2003). [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F36.00008: Search for $\pi $/2 state in large-area spin-triplet ferromagnetic Josephson junctions Yixing Wang, Norman O. Birge, William P. Pratt, Jr. The current-phase relationship of ferromagnetic spin-triplet Josephson junctions was predicted to be tuned by the magnetization orientations of different magnetic layers [1]. Given the random domain structure in large-area ferromagnetic junctions, the possibility of a random distribution of 0 or $\pi $ sub-junctions across the area could lead to a global $\pi $/2 junction [2]. Critical current measurements as a function of area provide indirect evidence for $\pi $/2 coupling [3], but they do not provide phase-sensitive information. Unfortunately there are technical difficulties faced by a direct current-phase measurement, especially for large area junctions. We are currently working toward a SQUID-based experiment that should be able to distinguish the $\pi $/2 state from either the 0 or $\pi $ states. In this talk we will report our progress toward this goal. [1] A.F. Volkov, F.S. Bergeret, and K.B. Efetov, Phys. Rev. Lett. 90, 117006 (2003). [2] A. Zyuzin, B. Spivak, Phys. Rev. B 61 5902 (2000). [3] Y Wang, W. P. Pratt Jr., N. O. Birge, Phys. Rev. B 85 214522 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F36.00009: Odd-Frequency Triplet Josephson Current Through an Exchange Spring Andreas Bill, Thomas E. Baker, Adam Richie-Halford, Adam K. Moke The existence of an odd-frequency long range triplet component in the order parameter of a proximity system with singlet superconductors is a recent prediction that has garnered great interest. The experimental fingerprint of this phenomenon is difficult to establish. We investigate a hybrid structure in which the emergence of the long range triplet component may be measured and identified. We consider a superconductor - exchange spring - superconductor Josephson junction as a function of increasing twist of the magnetic domain wall in the exchange spring. We show that as the domain wall is generated the long range triplet component emerges and modifies the current flowing through the Josephson junction. The critical temperature is also affected by the increased twist of the domain wall. The calculations lead us to propose an experiment where the long range triplet component can unequivocally be identified. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F36.00010: Spin-triplet supercurrent in planar geometry ferromagnetic Josephson junctions William M. Martinez, W.P. Pratt, Jr., Norman O. Birge The spin-triplet supercurrent in ferromagnetic Josephson junctions is obtained by surrounding the central ferromagnet with noncollinear ferromagnetic layers, F' [1]. In metallic ferromagnets, the long-range nature of the spin-triplet supercurrent has only been tested to lengths of a few tens of nm [2]. In this work, we are fabricating and measuring S/F'/F/F'/S junctions where the central F layer has a lateral geometry with lengths up to a few hundred nm. We will report on our recent progress. \\[4pt] [1] A.F. Volkov, F.S. Bergeret and K.B. Efetov, Phys. Rev. Lett., \textbf{90}, 117006 (2003).\\[0pt] [2] M.A. Khasawneh, T.S. Khaire, C. Klose, W.P. Pratt, Jr., and N.O. Birge, Supercond. Sci. Technol., \textbf{24}, 024005 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F36.00011: Skewness and Kurtosis of the Switching Current Distribution in Superconductor-Graphene-Superconductor Junctions and Superconductor-Nanowire-Superconductor Devices Andrew Murphy, Thomas Aref, Ulas Coskun, Phillip Weinberg, Alex Levchenko, Victor Vakaryuk, Alexey Bezryadin We study statistical properties of the switching current in superconductor-graphene-superconductor proximity junctions and superconductor-nanowire-superconductor devices. The fluctuations of the switching current are related to Little's phase slips, generated by thermal and quantum fluctuations of the superconducting order parameter. The study focuses on higher moments of the statistical probability distributions of the switching current. Namely we study the skewness, which defines the asymmetry of the distribution, and kurtosis, which is a measure of the ``peakedness.'' The skewness is defined as sk= $m_3/m_2^{3/2}$ where $m_2$ is the second moment of the distribution, called the variance, and $m_3$ is the third moment. Kurtosis is defined as kur= $m_4/m_2^2$, where $m_4$ is the fourth moment of the distribution. It is known that for Gaussian distributions sk=0 and kur=3. On our devices we find, in most cases, sk $\sim$ -1 and kur $\sim$ 5. These results are in agreement with numerical simulations as well as an analytic model. Finally we present preliminary experimental results for a two-nanowire device. We have found that the standard deviation, skewness and kurtosis of the switching current distributions in these devices vary periodically with magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F36.00012: Realization of short ballistic vertical graphene Josephson junction Gil-Ho Lee, Hu-Jong Lee We realized short ballistic vertical graphene Josephson junctions (vGJJs), where a monolayer graphene sheet is sandwiched between two superconducting electrodes along the c-axis of graphene. To enhance the transparency between electrodes and graphene layer we thermally deposited aluminum superconducting electrodes on both surfaces of the graphene sheet by using a ``flip-transfer'' scheme instead of transferring graphene onto the bottom electrode. With the highly transparent contacts and atomically short channel length, vGJJ shows a very large value of I$_c$R$_N$ product (2.2$\Delta_{Al}$). This value is in sharp contrast to much suppressed value of $I_cR_N$$ < \Delta_{Al}$, observed in planar graphene Josephson junctions. Surprisingly, $I_c$ decreases superlinearly with increasing temperature ($T$) from 50 mK up to the junction critical temperature, which is a typical character of a short ballistic Josephson junction. To our best knowledge, this feature has long been predicted but never been reported in proximity-coupled Josephson junctions. $I_c$-$T$ curve fits well to the short ballistic Josephson junction model (KO-2)\footnote[1]{K. K. Likharev, Rev. Mod. Phys. \textbf{51}, 101 (1979)} with the transparency of 0.94. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F36.00013: Direct measurements of the current-phase relation in graphene Josephson junctions Christopher English, David Hamilton, Dale Van Harlingen, Nadya Mason The current-phase relation (CPR) of a Josephson junction can provide key information about the microscopic processes and symmetries that control the supercurrent. In this talk, we present CPR measurements on Josephson junctions incorporating single-layer graphene as a weak link between Al superconducting electrodes with spacing \textless 100nm that are in the quasi-ballistic regime. We use a phase-sensitive SQUID technique to determine the supercurrent amplitude and phase as a function of temperature and electrostatic doping (gate voltage). As the critical current is varied, we observe a crossover from forward skewing in the CPR that arises from the low density of discrete electronic states in the junction to backward skewing induced by noise-rounding in the CPR measurement. We compare our results to theoretical models. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F36.00014: Josephson currents in semiconductor nanowire / s-wave superconductor nanostructures Li Mao, Sumanta Tewari, Chuanwei Zhang It has been theoretically predicted that a nanostructure composing of a semiconductor nanowire with strong spin-orbit coupling and an s-wave superconductor can support two Majorana fermions at the ends of the nanowire in the presence of a Zeeman field. Recently, following the theoretical proposals, some preliminary experimental signatures (e.g., zero-bias conductance peak) which may be related to the existence of Majorana fermions have been observed in the charge transport experiments. Here we investigate the Josephson currents with the zero-bias voltage in the topologically trivial region of a superconductor-insulator-superconductor junction in the presence of strong spin-orbit coupling and Zeeman field. This structure may be relevant to the Delft experiment by considering the possible proximity effect of the superconductor lead to the normal part of the nanowire. Our results indicate that the experimentally observed zero-bias conductance peak may not originate from Majorana fermions. [Preview Abstract] |
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