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 Physics , 2011, DOI: 10.1103/PhysRevB.86.144532 Abstract: We performed measurements of switching current distribution in a submicron La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) intrinsic Josephson junction (IJJ) stack in a wide temperature range. The escape rate saturates below approximately 2\,K, indicating that the escape event is dominated by a macroscopic quantum tunneling (MQT) process with a crossover temperature $T^{*}\approx2\,$K. We applied the theory of MQT for IJJ stacks, taking into account dissipation and the phase re-trapping effect in the LSCO IJJ stack. The theory is in good agreement with the experiment both in the MQT and in the thermal activation regimes.
 Physics , 2009, DOI: 10.1038/nphys1697 Abstract: We investigate experimentally the physics of quantum phase slips in one-dimensional Josephson Junction chains. These quantum phase-slips are induced by quantum phase fluctuations occurring on single junctions of the chain. In our experiment we can tune the strength of these fluctuations as each chain junction is realized in form of a SQUID leading to tunable Josephson coupling. We determine the ground state of the chain via switching current measurements of the chain shunted by a large Josephson junction. Our results can be well fitted with a tight binding Hamiltonian taking into account quantum phase-slips.
 Physics , 2007, DOI: 10.1103/PhysRevB.77.024506 Abstract: We investigate the magnetic response of a superconducting Nb ring containing a ferromagnetic PdNi Josephson junction and a tunnel junction in parallel. A doubling of the switching frequency is observed within certain intervals of the external magnetic field. Assuming sinusoidal current-phase relations of both junctions our model of a dc-SQUID embedded within a superconducting ring explains this feature by a sequence of current reversals in the ferromagnetic section of the junction in these field intervals. The switching anomalies are induced by the coupling between the magnetic fluxes in the two superconducting loops.
 Physics , 2002, DOI: 10.1063/1.1588752 Abstract: We have developed a scheme for a high resolution measurement of the switching current distribution of a current biased Josephson tunnel junction using a timing technique. The measurement setup is implemented such that the digital control and read-out electronics are optically decoupled from the analog bias electronics attached to the sample. We have successfully used this technique to measure the thermal activation and the macroscopic quantum tunneling of the phase in a small Josephson tunnel junction with a high experimental resolution. This technique may be employed to characterize current-biased Josephson tunnel junctions for applications in quantum information processing.
 Physics , 2010, Abstract: An experimental investigation of the critical current noise in underdamped niobium based Josephson junctions by a technique based on the switching current measurements is reported. By sweeping the junction with a current ramp we measure the critical current switching using the standard time of flight technique and analyze the data to extract the current noise. The experimental results show a linear behavior of the current white noise from both the junction area and the temperature. These measurement provide very useful information about the intrinsic noise of Josephson devices involving SQUIDs and qubits.
 Physics , 2001, DOI: 10.1143/JPSJ.71.538 Abstract: Transport and Cu-NQR studies have been carried out for the T*-type high temperature superconducting Cu oxide Nd1.6-xCexSr0.4CuO4 with single layered CuO2 planes formed by CuO5 pyramids. No anomaly related to the static or quasi static "stripe" order has been observed in the temperature T and x dependence of the thermoelectric powers S. We have not found such kind of anomaly in the x dependence of the superconducting transition temperature Tc, either. Although the decrease of the Cu NQR intensity with decreasing T (wipeout) has been observed in this system, it can be understood by considering the loss of the itinerant nature of the electrons, which takes place in the proximity region of the metal-insulator phase boundary, and cannot be connected with the static or quasi static "stripe" order.
 Physics , 2006, DOI: 10.1103/PhysRevLett.96.177003 Abstract: We have investigated macroscopic quantum tunneling in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic Josephson junctions at millikelvin temperatures using microwave irradiation. Measurements show that the escape rate for uniformly switching stacks of N junctions is about $N^2$ times higher than that of a single junction having the same plasma frequency. We argue that this gigantic enhancement of macroscopic quantum tunneling rate in stacks is boosted by current fluctuations which occur in the series array of junctions loaded by the impedance of the environment.
 Physics , 2011, DOI: 10.1063/1.3544037 Abstract: Intrinsic Josephson junction (IJJ) stacks of cuprate superconductors have potential to be implemented as intrinsic phase qubits working at relatively high temperatures. We report success in fabricating submicron La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) IJJ stacks carved out of single crystals. We also show a new fabrication method in which argon ion etching is performed after focused ion beam etching. As a result, we obtained an LSCO IJJ stack in which resistive multi-branches appeared. It may be possible to control the number of stacked IJJs with an accuracy of a single IJJ by developing this method.
 Physics , 1998, DOI: 10.1103/PhysRevB.59.14712 Abstract: We present the results of neutron-scattering studies on various aspects of crystalline and magnetic structure in single crystals of La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12 and 0.15. In particular, we have reexamined the degree of stripe order in an x=0.12 sample. Measurements of the width for an elastic magnetic peak show that it saturates at a finite value below 30 K, corresponding to a spin-spin correlation length of 200 A. A model calculation indicates that the differing widths of magnetic and (previously reported) charge-order peaks, together with the lack of commensurability, can be consistently explained by disorder in the stripe spacing. Above 30 K, the width of the nominally elastic signal begins to increase. Interpreting the signal as critical scattering from slowly fluctuating spins, the temperature dependence of the width is consistent with renormalized classical behavior of a 2-dimensional anisotropic Heisenberg antiferromagnet. Inelastic scattering measurements show that incommensurate spin excitations survive at and above 50 K, where the elastic signal is neglible. We also report several results related to the LTO-to-LTT transition.
 Physics , 2005, DOI: 10.1103/PhysRevB.72.012512 Abstract: We study thermal fluctuation phenomena in small Bi-2212 intrinsic Josephson junctions. Being able to measure switching currents of a {\it single} intrinsic junction, we observe that it's statistics can be very well described by thermal activation from a periodic Josephson potential with the sinusoidal current-phase relation. This is a direct evidence for the dc-intrinsic Josephson effect and the first unambiguous confirmation of the tunnelling nature of interlayer transport in strongly anisotropic high temperature superconductors. Furthermore, the fluctuation-free critical current, extracted from the analysis of switching current statistics, exhibits a temperature dependence typical for superconductor- insulator- superconductor tunnel junctions.
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