Polarized tip-enhanced Raman spectroscopy at liquid He temperature in ultrahigh vacuum using an off-axis parabolic mirror.

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering well below the diffraction limit down to the nanometer range and scanning probe microscopy and, possibly, spectroscopy. In this way, topographic and spectroscopic as well as single- and two-particle information may simultaneously be collected. While single molecules can now be studied successfully, bulk solids are still not meaningfully accessible. It is the purpose of the work presented here to outline approaches toward this objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum TERS. The setup is based on a scanning tunneling microscope and, as an innovation, an off-axis parabolic mirror having a high numerical aperture of ∼0.85 and a large working distance. The system is equipped with a fast load-lock chamber, a chamber for the in situ preparation of tips, substrates, and samples, and a TERS chamber. Base pressure and temperature in the TERS chamber were ∼3 × 10-11 mbar and 15 K, respectively. Polarization dependent tip-enhanced Raman spectra of the vibration modes of carbon nanotubes were successfully acquired at cryogenic temperature. The new features described here including very low pressure and temperature and the external access to the light polarizations, thus the selection rules, may pave the way toward the investigation of bulk and surface materials.

Calculation of an Enhanced A_{1g} Symmetry Mode Induced by Higgs Oscillations in the Raman Spectrum of High-Temperature Cuprate Superconductors.

In superconductors the Anderson-Higgs mechanism allows for the existence of a collective amplitude (Higgs) mode which can couple to eV light mainly in a nonlinear Raman-like process. The experimental nonequilibrium results on isotropic superconductors have been explained going beyond the BCS theory including the Higgs mode. Furthermore, in anisotropic d-wave superconductors strong interaction effects with other modes are expected. Here we calculate the Raman contribution of the Higgs mode from a new perspective, including many-body Higgs oscillations effects and their consequences in conventional, spontaneous Raman spectroscopy. Our results suggest a significant contribution to the intensity of the A_{1g} symmetry Raman spectrum in d-wave superconductors. In order to test our theory, we predict the presence of measurable characteristic oscillations in THz quench-optical probe time-dependent reflectivity experiments.

Raman Study of Cooper Pairing Instabilities in (Li_{1-x}Fe_{x})OHFeSe.

We studied the electronic Raman spectra of (Li_{1-x}Fe_{x})OHFeSe as a function of light polarization and temperature. In the B_{1g} spectra alone we observe the redistribution of spectral weight expected for a superconductor and two well-resolved peaks below T_{c}. The nearly resolution-limited peak at 110 cm^{-1} (13.6 meV) is identified as a collective mode. The peak at 190 cm^{-1} (23.6 meV) is presumably another collective mode since the line is symmetric and its energy is significantly below the gap energy observed by single-particle spectroscopies. Given the experimental band structure of (Li_{1-x}Fe_{x})OHFeSe, the most plausible explanations include conventional spin-fluctuation pairing between the electron bands and the incipient hole band and pairing between the hybridized electron bands. The absence of gap features in A_{1g} and B_{2g} symmetry favors the second case. Thus, in spite of various differences between the pnictides and chalcogenides, this Letter demonstrates the proximity of pairing states and the importance of band structure effects in the Fe-based compounds.

Fluctuations and pairing in Fe-based superconductors: light scattering experiments.

Inelastic scattering of visible light (Raman effect) offers a window into properties of correlated metals such as spin, electron and lattice dynamics as well as their mutual interactions. In this review we focus on electronic and spin excitations in Fe-based pnictides and chalcogenides, in particular but not exclusively superconductors. After a general introduction to the basic theory including the selection rules for the various scattering processes we provide an overview over the major experimental results. In the superconducting state below the transition temperature T c the pair-breaking effect can be observed, and the gap energies may be derived and associated with the gaps on the electron and hole bands. In spite of the similarities of the overall band structures the results are strongly dependent on the family and may even change qualitatively within one family. In some of the compounds strong collective modes appear below T c. In Ba1-x K x Fe2As2, which has the most isotropic gap of all Fe-based superconductors, there are indications that these modes are exciton-like states appearing in the presence of a hierarchy of pairing tendencies. The strong in-gap modes observed in Co-doped NaFeAs are interpreted in terms of quadrupolar orbital excitations which become undamped in the superconducting state. The doping dependence of the scattering intensity in Ba[Formula: see text] is associated with a nematic resonance above a quantum critical point and interpreted in terms of a critical enhancement at the maximal T c. In the normal state the response from particle-hole excitations reflects the resistivity. In addition, there are strongly temperature-dependent contributions from presumably critical fluctuations in the energy range of k B T which can be compared to the elastic properties. Currently it is not settled whether the fluctuations observed by light scattering are related to spin or charge. Another controversy relates to putative two-magnon excitations, typically in the energy range below 0.5 eV. Whereas this response presumably originates from charge excitations in most of the Fe-based compounds theory and experiment suggest that the excitations in the 60 meV range in FeSe stem from localized spins in a nearly frustrated system.

Probing the Pairing Interaction and Multiple Bardasis-Schrieffer Modes Using Raman Spectroscopy.

In unconventional superconductors, understanding the form of the pairing interaction is the primary goal. In this regard, Raman spectroscopy is a very useful tool, as it identifies the ground state and also the subleading pairing channels by probing collective modes. Here, we propose a general theory for a multiband Raman response and identify new features in the spectrum that can provide a robust test for a pairing theory. We identify multiple Bardasis-Schrieffer type collective modes and connect the weights of these modes to the subleading gap structures within a microscopic pairing theory. While our conclusions are completely general, we apply our approach to interpret the specific case of B_{1g} Raman scattering in hole-doped BaFe_{2}As_{2}.

Raman-scattering detection of nearly degenerate s-wave and d-wave pairing channels in iron-based Ba0.6K0.4Fe2As2 and Rb0.8Fe1.6Se2 superconductors.

We show that electronic Raman scattering affords a window into the essential properties of the pairing potential V(k,k') of iron-based superconductors. In Ba0.6K0.4Fe2As2 we observe band dependent energy gaps along with excitonic Bardasis-Schrieffer modes characterizing, respectively, the dominant and subdominant pairing channel. The d(x(2)-y(2)) symmetry of all excitons allows us to identify the subdominant channel to originate from the interaction between the electron bands. Consequently, the dominant channel driving superconductivity results from the interaction between the electron and hole bands and has the full lattice symmetry. The results in Rb(0.8)Fe(1.6)Se(2) along with earlier ones in Ba(Fe(0.939)Co(0.061))(2)As(2) highlight the influence of the Fermi surface topology on the pairing interactions.

Feedback effect on high-energy magnetic fluctuations in the model high-temperature superconductor HgBa2CuO(4+δ) observed by electronic Raman scattering.

We use electronic Raman scattering to study the model single-layer cuprate superconductor HgBa(2)CuO(4+δ). In an overdoped sample, we observe a pronounced amplitude enhancement of a high-energy peak related to two-magnon excitations in insulating cuprates upon cooling below the critical temperature T(c). This effect is accompanied by the appearance of the superconducting gap and a pairing peak above the gap in the Raman spectrum, and it can be understood as a hitherto-undetected feedback effect on the high-energy magnetic fluctuations due to the Cooper pairing interaction. This implies a direct involvement of the high-energy magnetic fluctuations in the pairing mechanism. All of these effects occur already above T(c) in two underdoped samples, demonstrating a related feedback mechanism associated with the pseudogap.

Femtosecond response of quasiparticles and phonons in superconducting YBa(2)Cu(3)O(7-δ) studied by wideband terahertz spectroscopy.

We measure the anisotropic midinfrared response of electrons and phonons in bulk YBa(2)Cu(3)O(7-δ) after femtosecond photoexcitation. A line shape analysis of specific lattice modes reveals their transient occupation and coupling to the superconducting condensate. The apex oxygen vibration is strongly excited within 150 fs, demonstrating that the lattice absorbs a major portion of the pump energy before the quasiparticles are thermalized. Our results attest to substantial electron-phonon scattering and introduce a powerful concept probing electron-lattice interactions in a variety of complex materials.

An electron-boson glue function derived from electronic Raman scattering.

Raman scattering cross sections depend on photon polarization. In the cuprates, nodal and antinodal directions are weighted more strongly in B(2g) and B(1g) symmetries, respectively. On the other hand, in angle-resolved photoemission spectroscopy (ARPES), electronic properties are measured along well-defined directions in momentum space rather than their weighted averages being taken. In contrast, the optical conductivity involves a momentum average over the entire Brillouin zone. Newly measured Raman response data on high-quality Bi(2)Sr(2)CaCu(2)O(8 + δ) single crystals up to high energies have been inverted using a modified maximum entropy inversion technique to extract from B(1g) and B(2g) Raman data corresponding electron-boson spectral densities (glue), and these are compared to the results obtained with known ARPES and optical inversions. We find that the B(2g) spectrum agrees qualitatively with nodal direction ARPES while the B(1g) results look more like the optical spectrum. A large peak around 30-40 meV in B(1g) and a much less prominent one in B(2g) are taken as support for the importance of (π, π) scattering at this frequency.

Vibrational spectra of C60.C8H8 and C70.C8H8 in the rotor-stator and polymer phases.

C(60).C(8)H(8) and C(70).C(8)H(8) are prototypes of rotor-stator cocrystals. We present infrared and Raman spectra of these materials and show how the rotor-stator nature is reflected in their vibrational properties. We measured the vibrational spectra of the polymer phases poly(C(60)C(8)H(8)) and poly(C(70)C(8)H(8)) resulting from a solid-state reaction occurring on heating. On the basis of the spectra, we propose a connection pattern for the fullerene in poly(C(60)C(8)H(8)), where the symmetry of the C(60) molecule is D(2h). On illuminating the C(60).C(8)H(8) cocrystal with green or blue light, a photochemical reaction was observed leading to a product similar to that of the thermal polymerization.

Interlaboratory comparison of ability to detect nucleic acid of Mycoplasma gallisepticum and Mycoplasma synoviae by polymerase chain reaction.

In recent years polymerase chain reaction (PCR) assays have become widely used as methods to confirm the presence of Mycoplasma gallisepticum and Mycoplasma synoviae in poultry flocks, but there has been limited standardization of the protocols used. Thirteen laboratories from five different countries participated in an interlaboratory comparison of detection of M. gallisepticum and M. synoviae DNA by PCR in samples that contained 10-fold dilutions of these bacteria. The concentration of bacteria ranged from 10(5) to 10(2) genome copies/100 microl sample, as quantified by real-time PCR, and the samples were supplied on dry cotton swabs. Each laboratory was asked to use its standard method for PCR testing of these pathogens. A questionnaire was supplied with the samples to obtain details of the methods that were used in testing. One-half of the laboratories used a commercially available test kit, while the others used an in-house protocol. The protocols used for DNA extraction varied greatly, even among those using commercially available test kits. Two laboratories had developed the primers for nucleic acid amplification themselves, and one of these used real-time PCR for amplification. While the majority of the laboratories detected M. synoviae down to the 100 copy limit of the comparison, the detection limit for M. gallisepticum was somewhat higher. Furthermore, different results were obtained from laboratories that used the same commercial test kit. To the best of our knowledge this is the first investigation of its kind in the field of avian diseases.

Cryptosporidium infection in domestic geese (Anser anser f. domestica) detected by in-situ hybridization.

An in-situ hybridization (ISH) procedure was developed for the detection of Cryptosporidium sp. in paraffin wax-embedded tissues with a digoxigenin-labelled probe targeting the 18S rRNA. This technique was used in addition to traditional methods, such as haematoxylin and eosin staining, periodic acid-Schiff reaction, transmission electron microscopy and the polymerase chain reaction, to examine the bursa of Fabricius (BF), conjunctiva and other tissues from 20 domestic geese aged 16-36 days for the presence of cryptosporidia. Positive signals were found to a moderate or marked extent in both conjunctival samples (89%) and BF samples (88%) but not in other tissues. Sequencing of the PCR amplification product revealed identity with Cryptosporidium baileyi. The infected geese showed no clinical signs and only scanty histological lesions. These results confirm reports showing that young waterfowl are especially vulnerable to cryptosporidium infection and indicate that the BF and conjunctiva are the preferred sites for the presence of the protozoon. ISH proved a good method for detecting and identifying even small numbers of cryptosporidia in tissue sections.

Limited pathogenicity of usutu virus for the domestic goose (Anser anser f. domestica) following experimental inoculation.

Usutu virus (USUV) is a mosquito-borne flavivirus of the Japanese encephalitis virus group, which has been associated with avian mortality in Austria since 2001. The affected birds are predominantly Eurasian blackbirds (Turdus merula). In the present study, the pathogenicity of USUV for domestic geese (Anser anser f. domestica) was investigated. Eleven 2-week-old geese were inoculated intramuscularly with 5 x 10(4) 50% tissue culture infectious dose of USUV strain Vienna-2001 blackbird. No clinical signs were seen during the observation period. Four inoculated and one in-contact geese died without preceding clinical signs. Two of the deaths could be attributed to bacterial septicaemia and strangulation, respectively. The cause of death of two experimental and one in-contact animals remained unclear, but lack of evidence for viral lesions and viral antigen in their tissues argued against association with the USUV infection. Although in organs of the majority of inoculated geese (9/11) USUV was detected by reverse transcriptase-polymerase chain reaction, immunohistochemistry for USUV antigen was negative in all tissues of all geese. Evidence of plasma viraemia or viral excretion was found in one goose only. Seroconversion was detected in three inoculated geese 10 days post-inoculation. Geese placed in contact with inoculated geese and control animals did not exhibit USUV in their internal organs or plasma and lacked USUV-specific antibodies. This experiment shows that USUV is able to replicate in geese, but does not induce clinical disease, is unlikely to induce mortality, and only infrequently leads to viraemia or virus shedding.

Dynamical properties of charged stripes in La2-xSrxCuO4.

Inelastic light-scattering spectra of underdoped La2-xSrxCuO4 single crystals are presented which provide direct evidence of the formation of quasi-one-dimensional charged structures in the two-dimensional CuO2 planes. The stripes manifest themselves in a Drude-like peak at low energies and temperatures. The selection rules allow us to determine the orientation to be along the diagonals at x=0.02 and along the principal axes at x=0.10. The electron-lattice interaction determines the correlation length which turns out to be larger in compound classes with lower superconducting transition temperatures. Temperature is the only scale of the response at different doping levels demonstrating the importance of quantum critical behavior.

Limited pathogenicity of Usutu virus for the domestic chicken (Gallus domesticus).

In summer 2001, Usutu virus (USUV), a mosquito-borne flavivirus, was isolated for the first time in Europe during a mortality incident among Eurasian blackbirds (Turdus merula) in Austria. Chickens are frequently used as sentinel animals for arbovirus surveillance systems. In the present study, the pathogenicity of USUV for specific pathogen free chickens was investigated. Ten 2-week-old chickens were inoculated intravenously with 0.1 ml inoculum containing 10(3) median (50%) tissue culture infectious dose of USUV strain Vienna 2001-blackbird (939/01). Clinical signs, viraemia, gross and microscopic lesions, contact transmission and immunological response were evaluated. No clinical signs were observed in the USUV-inoculated animals during the experimental period. Pathological examination showed moderate splenomegaly and follicular infiltrates in the liver of several inoculated animals. Mild non-suppurative encephalitis was observed in the brain tissue of one virus-inoculated chicken examined 7 days post inoculation (d.p.i.). USUV nucleic acid was detected by reverse-transcriptase polymerase chain reaction in the organs of six inoculated chickens, although immunohistochemistry for flavivirus antigen was negative in all tissues from all chickens. Virus shedding was shown in three inoculated birds by detecting USUV RNA in cloacal swabs of two chickens at 5 d.p.i., and in the cloacal and pharyngeal swabs of one chicken at 7 d.p.i. Based on detection of viral RNA in peripheral blood mononuclear cells, viraemia was detected only in two chickens (at 7 d.p.i.). Only one of the inoculated chickens developed an antibody response. There was no evidence of virus transmission to chickens kept in contact with inoculated birds. No USUV was isolated from in-contact birds and all in-contact and control animals lacked USUV-specific antibodies. The present data suggest that domestic chickens are not at risk of developing clinical disease following USUV infection and that chickens are unlikely to be useful for sentinel purposes in USUV surveillance programmes.

Corticosterone in chicken eggs.

Birds are discussed as models for prenatal stress. In this study, several experiments were conducted to gain basic knowledge of if, how, and when maternal adrenocortical activity is reflected by corticosterone concentrations in the egg. Radiolabeled corticosterone was administered to 10 laying hens to investigate the uptake into as well as the distribution within the eggs. The yolk was dissected in concentric layers and analyzed. Less than 1% of the administered radioactivity entered the egg but was, however, not evenly distributed. On the day after injection, highest radioactivity (Bq/g) was detected in the albumen and the outmost layer, whereas concentration peaked 4-7 days later in the inner layers. In two other experiments, increased plasma levels of corticosterone were induced by injection of adrenocorticotropic hormone (ACTH) or feeding of corticosterone. Again, yolk disks were cut in layers and analyzed with a corticosterone enzyme immunoassay. No effect of the ACTH administration was detected, whereas feeding of corticosterone resulted in increased immunoreactive corticosterone concentrations in the yolk. Straight-phase high-performance liquid chromatographic (HPLC) separations were also performed to characterize immunoreactive steroids in the yolk. Two close-eluting peaks at the approximate elution position of corticosterone could be observed after the feeding experiment, whereas in untreated control eggs they were absent. It was concluded that transfer from plasma to egg is low for corticosterone and that further investigations concerning the transport mechanisms and the exact nature of yolk steroids are necessary.

Measurement of corticosterone metabolites in chicken droppings.

(1) A non-invasive technique for stress assessment is needed. Therefore, an enzyme immunoassay (EIA) for measurement of glucocorticoid metabolites in chicken droppings was established and validated. (2) Radiolabelled corticosterone was administered intravenously to detect the time course of excreted metabolites. The metabolites were then characterised by chemical and immunological methods to find a suitable antibody. (3) Reversed-phase high-performance liquid chromatography (RP-HPLC) separations of the peak concentration samples revealed that corticosterone was extensively metabolised, mainly to more polar substances. (4) HPLC fractions were tested in several EIAs for glucocorticoid metabolites, where the highest quantities were detected by a newly established cortisone assay, measuring metabolites with a 3,11-dione structure. (5) The biological relevance of this cortisone EIA was confirmed by stimulation of adrenocortical activity by adrenocorticotropic hormone (ACTH). (6) With this newly developed EIA it should be possible to measure adrenocortical activity non-invasively in chickens and other galliformes, thus providing a tool for a variety of research fields, such as poultry production, ethology and behavioural ecology.

Distribution and origin of steroid hormones in the yolk of Japanese quail eggs (Coturnix coturnix japonica).

The yolk of avian eggs contains steroid hormones, which may influence the development and behaviour of hatched birds. The aim of the present study was to investigate the concentration as well as the distribution of various gonadal steroids in the yolk spheres of quail eggs. Steroid concentrations of dissected yolk layers were analysed after alcoholic extraction using enzyme immunoassays (EIAs) for progesterone, androstenedione and testosterone. To monitor the uptake of testosterone into the yolk, radioactive testosterone was injected i.m. into six female quails. The radioactivity of yolk layers of subsequently laid eggs was measured by liquid scintillation counting. Progesterone concentrations were highest in the outer layer (median: 2265 nmol/kg). Androstenedione (median: 453 nmol/kg), as the major androgen, and testosterone (median: 99 nmol/kg) reached their highest concentrations in interior layers, whereas in the centre the concentration of all three hormones was low. No significant variation of steroid levels in yolk layers of subsequently laid eggs was found. The highest radioactivity was detected in the outer yolk layer in those eggs laid 1 day after injection and in subsequently laid eggs was measured nearer to the centre. These results indicated local origin of the steroid hormones especially because of the result that only 0.1% of the radioactivity entered the yolk. We conclude that steroid concentrations in the yolk layers reflected progesterone and androgen production of the cells of the follicular wall at the time.