Abstract To date, it is unclear whether chemical order (or disorder) is in any way connected to double exchange, electronic phase separation, or charge ordering (CO) in manganites. In this work, we carry out an atomic resolution study of the colossal magnetoresistant manganite La2−2x Sr1+2x Mn2O7 (LSMO). We combine aberration-corrected electron microscopy and spectroscopy with spectroscopic image simulations, to analyze cation ordering at the atomic scale in real space in a number of LSMO single crystals. We compare three different compositions within the phase diagram: a ferromagnetic metallic material (x=0.36), an insulating, antiferromagnetic charge ordered (AF-CO) compound (x=0.5), which also exhibits orbital ordering, and an additional AF sample (x=0.56). Detailed image simulations are essential to accurately quantify the degree of chemical ordering of these samples. We find a significant degree of long-range chemical ordering in all cases, which increases in the AF-CO range. However, the degree of ordering is never complete nor can it explain the strongly correlated underlying ordering phenomena. Our results show that chemical ordering over distinct crystallographic sites is not needed for electronic ordering phenomena to appear in manganites, and cannot by itself explain the complex electronic behavior of LSMO.

On theoretical grounds, we show that orbital domain walls (ODWs), which are known to exist in the charge and orbital ordered layered manganite LaSr2Mn2O7, should be partially charged as a result of competition between orbital-induced strain and Coulomb repulsion. This unexpected result provides the necessary condition for the known thermodynamic stability of these ODWs, which are unlike the more typical domain walls that arise only from an external field. We offer experimental data consistent with this theoretical framework through a combined transport and x-ray-diffraction study. In particular, our transport data on this charge and orbital ordered manganite exhibit abrupt transformations to higher conductance at a threshold electric field. As transport phenomena closely resemble effects found for sliding charge-density waves (SCDWs) in pseudo-one-dimensional (1D) materials, a SCDW along such pseudo-1D ODWs provides a natural explanation of our data. Importantly, x-ray-diffraction data eliminate heating and melting of charge order as tenable alternative explanations of our data.

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of "medium purity" Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range 800 - 1400 C was done in a newly designed vacuum induction furnace. Q0 values of the order of 2x1010 at 2.0 K and peak surface magnetic field (Bp) of 90 mT were achieved reproducibly. A Q0-value of (5+-1)1010 at 2.0 K and Bp = 90 mT was obtained after heat treatment at 1400 C. This is the highest value ever reported at this temperature, frequency and field. Samples heat treated with the cavity at 1400 C were analyzed by secondary ion mass spectrometry, secondary electron microscopy, energy dispersive X-ray, point contact tunneling and X-ray diffraction and revealed a complex surface composition which includes titanium oxide, increased carbon and nitrogen content but reduced hydrogen concentration compared to a non heat-treated sample.

Stacks of intrinsic Josephson junctions in high-temperature superconductors enable the fabrication of compact sources of coherent terahertz radiation. Here, we demonstrate that multiple stacks patterned on the same Bi2Sr2CaCu2O8+δ crystal can—under optimized conditions—be synchronized to emit high-power THz-radiation. For three synchronized stacks, we achieved 610 μW of continuous-wave coherent radiation power at 0.51 THz. We suggest that synchronization is promoted by THz-waves in the base crystal. We note that synchronization cannot be achieved in all samples. However even in these cases, powers on the 100-μW scale can be generated.

We report a soft x-ray nanodiffraction study of antiferromagnetic domains in the strongly correlated bilayer manganite La${}_{0.96}$Sr${}_{2.04}$Mn${}_{2}$O${}_{7}$. We find that the antiferromagnetic domains form a unique domain pattern with each domain having an intrinsic memory of its spin direction. This can be explained by the presence of crystallographic or magnetic imperfections locked in during the crystal growth process which pin the antiferromagnetic domains. The antiferromagnetic domain pattern shows two distinct types of domain. One of the domain types was observed to contain a periodic ripple in the manganese spin direction with a period of approximately 4 $\ensuremath{\mu}$$\mathrm{m}$. We propose that the loss of inversion symmetry within a bilayer is responsible for this ripple structure through a Dzyaloshinskii-Moriya-type interaction.

New distorted variants of the cubic BaHg11 structure type have been synthesized in Ga flux. Multiple phases of CePd3+xGa8-x, which include an orthorhombic Pmmn structure (x = 3.21(2)), a rhombohedral R3m structure (x = 3.13(4)), and a cubic Fm3m superstructure (x = 2.69(6)), form preferentially depending on reaction cooling rate and isolation temperature. Differential thermal analysis and in situ temperature-dependent powder X-ray diffraction patterns show a reversible phase transition at approximately 640 °C between the low temperature orthorhombic and rhombohedral structures and the high temperature cubic superstructure. Single crystal X-ray diffraction experiments indicate that the general structure of BaHg11, including the intersecting planes of a kagomé-type arrangement of Ce atoms, is only slightly distorted in the low temperature phases. A combination of Kondo, crystal electric field, and magnetic frustration effects may be present, resulting in low temperature anomalies in magnetic susceptibility, electrical resistivity, and heat capacity measurements. In addition to CePd3+xGa8-x, the rare earth analogues REPd3+xGa8-x, RE = La, Nd, Sm, Tm, and Yb, were successfully synthesized and also crystallize in one of the lower symmetry space groups.

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

The main criticism raised in the preceding Comment concerns our suggestion that sharp conduction peaks in Bi2Sr2CaCu2O8+delta mesas, along with absent dip-hump features, may, in general, be a result of self-heating. The author points to the variety of experimental configurations, matrix-element effects, and doping dependencies that might allow a diversity of conductance spectra. We argue that numerous mesa studies (with fixed matrix elements) firmly establish the systematic development of sharp conductance peaks with increased self-heating, and thus, the issue of nonuniversality of tunneling characteristics is not relevant. The author mentions a number of studies that show that the mesa is superconducting near the conductance peak voltage. This is not in dispute and indicates a misinterpretation of our analysis that is clarified here. To address further comments on the technical details of our heating model, we reiterate that our conclusions are independent of our model but rather are based solely on experimental data that are not in dispute.

The main criticism raised in the preceding Comment concerns our suggestion that sharp conduction peaks in Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+}$${}_{\ensuremath{\delta}}$ mesas, along with absent dip-hump features, may, in general, be a result of self-heating. The author points to the variety of experimental configurations, matrix-element effects, and doping dependencies that might allow a diversity of conductance spectra. We argue that numerous mesa studies (with fixed matrix elements) firmly establish the systematic development of sharp conductance peaks with increased self-heating, and thus, the issue of nonuniversality of tunneling characteristics is not relevant. The author mentions a number of studies that show that the mesa is superconducting near the conductance peak voltage. This is not in dispute and indicates a misinterpretation of our analysis that is clarified here. To address further comments on the technical details of our heating model, we reiterate that our conclusions are independent of our model but rather are based solely on experimental data that are not in dispute.

C. Kurter,1,2 L. Ozyuzer,1,3 T. Proslier,1,2 J. F. Zasadzinski,2 D. G. Hinks,1 and K. E. Gray1,* 1Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA 2Department of BCPS, Physics Division, Illinois Institute of Technology, Chicago, Illinois 60616, USA 3Department of Physics, Izmir Institute of Technology, TR-35430 Izmir, Turkey (Received 3 June 2011; revised manuscript received 7 September 2011; published 5 October 2011)

The c -axis current-voltage I ( V ) characteristics have been obtained on a set of mesas of varying height sculpted on Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi2212) crystals intercalated with HgB 2 . The intercalation, along with the small number of junctions in the mesa, N =6–30, minimizes the degree of self-heating, leading to a consistent Josephson critical current, I C , among junctions in the mesa. The Bi2212 crystals with a bulk T C =74 K are overdoped and display negligible pseudogap effects allowing an accurate measure of the normal state resistance, R N . These properties make the mesas nearly ideal for the determination of the Josephson I C R N product. We find I C R N values consistently ∼30% of the quasiparticle gap parameter, Δ/ e , which was measured independently using a mechanical contact, break junction technique. The latter was necessitated by higher bias heating effects in the mesas which prevented direct measurements of the superconducting gap. These values are among the highest reported and may represent the maximum intrinsic value for I C R N . The results indicate that the c-axis transport is a mixture of coherent and incoherent tunneling.

We have measured coherent terahertz emission spectra from Bi2Sr2CaCu2O8+\delta\ mesa devices, as a function of temperature and mesa bias voltage. The emission frequency is found to be tunable by up to 12% by varying the temperature and bias voltage. We attribute the appearance of tunability to asymmetric boundaries at the top and bottom, and the non-rectangular cross-section of the mesas. This interpretation is consistent with numerical simulations of the dynamics of intrinsic Josephson junctions in the mesa. Easily tunable emission frequency may have important implications for the design of terahertz devices based on stacked intrinsic Josephson junctions.

We have measured coherent terahertz emission spectra from Bi2Sr2CaCu2O8+delta mesa devices as a function of temperature and mesa bias voltage. The emission frequency is found to be tunable by up to 12% by varying the temperature and bias voltage. We attribute the appearance of tunability to asymmetric boundaries at the top and bottom and the nonrectangular cross section of the mesas. This interpretation is consistent with numerical simulations of the dynamics of intrinsic Josephson junctions in the mesa. Easily tunable emission frequency may have important implications for the design of terahertz devices based on stacked intrinsic Josephson junctions.