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M Grüninger

Publications and source records attributed to M Grüninger.

14 recordsLinked to original sources
PubMed2006

Raman scattering in the Mott insulators LaTiO3 and YTiO3: evidence for orbital excitations.

Raman scattering is used to observe pronounced electronic excitations around 230 meV--well above the two-phonon range--in the Mott insulators LaTiO3 and YTiO3. Based on the temperature, polarization, and photon energy dependence, the modes are identified as orbital excitations. The observed profiles bear a striking resemblance to magnetic Raman modes in the insulating parent compounds of the superconducting cuprates, indicating an unanticipated universality of the electronic excitations in transition metal oxides.

Journal Article↗
PubMed2005

Zero-field incommensurate spin-Peierls phase with interchain frustration in TiOCl.

We report on the magnetic, thermodynamic, and optical properties of the quasi-one-dimensional quantum antiferromagnets TiOCl and TiOBr, which have been discussed as spin-Peierls compounds. The observed deviations from canonical spin-Peierls behavior, e.g., the existence of two distinct phase transitions, have been attributed previously to strong orbital fluctuations. This can be ruled out by our optical data of the orbital excitations. We show that the frustration of the interchain interactions in the bilayer structure gives rise to incommensurate order with a subsequent lock-in transition to a commensurate dimerized state. In this way, a single driving force, the spin-Peierls mechanism, induces two separate transitions.

Journal Article↗
PubMed2004

Unifying magnons and triplons in stripe-ordered cuprate superconductors.

Based on a two-dimensional model of coupled two-leg spin ladders, we derive a unified picture of recent neutron scattering data of stripe-ordered La15/8Ba1/8CuO4, namely, of the low-energy magnons around the superstructure satellites and of the triplon excitations at higher energies. The resonance peak at the antiferromagnetic wave vector Q(AF) in the stripe-ordered phase corresponds to a saddle point in the dispersion of the magnetic excitations. Quantitative agreement with the neutron data is obtained for J=130-160 meV and Jcyc/J=0.2-0.25.

Journal Article↗
PubMed2003

c-Axis optical sum rule and a possible new collective mode in La2-xSrxCuO4.

We present the c-axis optical conductivity sigma(1c)(omega,T) of underdoped (x=0.12) and optimally doped (x=0.15) La2-xSrxCuO4 from 4 meV to 1.8 eV obtained by a combination of reflectivity and transmission spectra. In addition to the opening of the superconducting gap, we observe an increase of conductivity above the gap up to 270 meV with a maximal effect at about 120 meV. This may indicate a new collective mode at a surprisingly large energy scale. The Ferrell-Glover-Tinkham sum rule is violated for both doping levels. Although the relative value of the violation is much larger for the under-doped sample, the absolute increase of the low-frequency spectral weight, including that of the condensate, is higher in the optimally doped regime. Our results resemble in many respects the observations in YBa(2)Cu(3)O(7-delta).

Journal Article↗
PubMed2003

Charge-order-induced sharp Raman peak in Sr14Cu24O41.

In the two-leg S=1/2 ladders of Sr14Cu24O41, a modulation of the exchange coupling arises from the charge order within the other structural element, the CuO2 chains. In general, breaking transla-tional invariance by modulation causes gaps within the dispersion of elementary excitations. We show that the gap induced by the charge order can drastically change the magnetic Raman spectrum, leading to the sharp peak observed in Sr14Cu24O41. This sharp Raman line gives insight into the charge-order periodicity and hence into the distribution of carriers. The much broader spectrum of La6Ca8Cu24O41 reflects the response of an undoped ladder in the absence of charge order.

Journal Article↗
PubMed2002

Evidence for spin-charge separation in quasi-one-dimensional organic conductors.

Interacting conduction electrons are usually described within Fermi-liquid theory, which states that, in spite of strong interactions, the low-energy excitations are electron-like quasiparticles with charge and spin. In recent years there has been tremendous interest in conducting systems that are not Fermi liquids, motivated by the observation of highly anomalous metallic states in various materials, most notably the copper oxide superconductors. Non-Fermi-liquid behaviour is generic to one-dimensional interacting electron systems, which are predicted to be Luttinger liquids. One of their key properties is spin-charge separation: instead of quasiparticles, collective excitations of charge (with no spin) and spin (with no charge) are formed, which move independently and at different velocities. However, experimental confirmation of spin-charge separation remains a challenge. Here we report experiments probing the charge and heat current in quasi-one-dimensional conductors--the organic Bechgaard salts. It was found that the charge and spin excitations have distinctly different thermal conductivities, which gives strong evidence for spin-charge separation. The spin excitations have a much larger thermal conductivity than the charge excitations, which indicates that the coupling of the charge excitations to the lattice is important.

Journal Article↗
PubMed2002

Experimental quest for orbital waves.

One challenge in condensed-matter physics is the experimental confirmation of a new kind of elementary excitation orbital waves, or orbitons, which are predicted to exist in an orbitally ordered state. Saitoh et al. have observed three peaks at 160, 144 and 126 meV in the Raman scattering of orbitally ordered lanthanum manganate (LaMnO(3)), and interpret these as evidence of orbitons. However, we find similar peaks in the optical conductivity, sigma(omega), of LaMnO(3) and point out that the direct observation of orbitons in sigma(omega) is prohibited by a selection rule. This suggests that the Raman peaks observed by Saitoh et al. arise from multiphonons, and that the existence of orbitons has yet to be experimentally confirmed.

Journal Article↗
PubMed2001

Fractional and integer excitations in quantum antiferromagnetic spin 1/2 ladders.

Spectral densities are computed in unprecedented detail for quantum antiferromagnetic spin 1/2 two-leg ladders. These results were obtained due to a major methodical advance achieved by optimally chosen unitary transformations. The approach is based on dressed integer excitations. Considerable weight is found at high energies in the two-particle sector. Precursors of fractional spinon physics occur supporting the conclusion that there is no necessity to resort to fractional excitations in order to describe features at higher energies.

Journal Article↗
PubMed2001

Observation of two-magnon bound states in the two-leg ladders of (Ca,La)(14)Cu(24)O(41).

Phonon-assisted two-magnon absorption is studied in the spin- 1/2 two-leg ladders of (Ca,La)(14)Cu(24)O(41) for E parallel c (legs) and E parallel a (rungs). We verify the theoretically predicted existence of two-magnon singlet bound states, which give rise to peaks at approximately equal to 2140 and 2800 cm(-1). The two-magnon continuum is observed at approximately equal to 4000 cm(-1). Two different theoretical approaches (Jordan-Wigner fermions and perturbation theory) describe the data very well for J parallel approximately equal to 1020-1100 cm(-1), J parallel/J perpendicular approximately equal to 1-1.2. At high energies, the magnetic contribution to sigma(omega) is strikingly similar in the ladders and in the undoped high-T(c) cuprates, which emphasizes the importance of strong quantum fluctuations in the latter.

Journal Article↗
PubMed2001

Strong anisotropy of superexchange in the copper-oxygen chains of La(14)-xCaxCu24O41.

Electron spin resonance data of Cu2+ ions in La(14)-xCaxCu24O41 single crystals (x = 9,11,12) reveal a very large width of the resonance line in the paramagnetic state. This signals an unusually strong anisotropy of approximately 10% of the isotropic Heisenberg superexchange in the Cu-O chains of this compound. The strong anisotropy can be explained by the specific geometry of two symmetrical 90 degrees Cu-O-Cu bonds, which boosts the importance of orbital degrees of freedom. Our data show the apparent limitations of the applicability of an isotropic Heisenberg model to the low-dimensional cuprates.

Journal Article↗
PubMed1996

Direct measurement of cytokines (IFN-gamma, IL-4, -5, and -6) from organs after antigenic challenge.

A mouse in vivo model has been developed in which cytokines produced within the regional lymph nodes and other organs after an antigenic challenge were measured directly without the need of an in vitro reculturing step. Mice were subcutaneously (s.c.) immunized with antigens such as keyhole limpet hemocyanin (KLH) in aluminium hydroxide (alum) at the base of the tail. After defined periods of time lymph nodes, spleen, liver, and lung were collected and frozen. The organs were then homogenized (or sonicated), centrifuged, and the cytokine levels, for example, IFN-gamma, IL-4, IL-5, and IL-6 were determined by ELISA. It was found that injection of alum alone caused measurable cytokine production in draining lymph nodes, but inoculation of an antigen-alum mixture, such as KLH and dust-mite antigen (DMA), significantly increased in vivo cytokine production over that of alum alone. In the dose range tested there appeared to be an inverse relationship between the dose of antigen and cytokine levels. Thus, 0.1-microgram KLH induced more IL-4 in vivo than 10-micrograms KLH. Kinetic studies showed that for IL-4 and IFN-gamma, peak production occurred around days 5 and 6, respectively. Route of immunization and the dose of antigen were found to be very critical, in that injection of 10-micrograms DMA intraperitoneally induced significant levels of cytokines in the lung, while the same dose of antigen given at the base of tail, s.c., did not induce appreciable levels of cytokines in any organ tested. Cyclosporin A inhibited in vivo production of IFN-gamma, IL-4, and IL-5 by approximately 80%, but not that of IL-6. Surprisingly, dexamethasone enhanced the production of IL-6 while inhibiting all the other cytokines. In conclusion, these results suggest that direct determination of cytokines in the organs may provide an easy readout to assess differential effects of immunoregulatory molecules on the production of Th1- and Th2-type cytokines in vivo.

Alum Compounds↗
PubMed1985

Serum amyloid P component and autoimmune parameters in the assessment of arthritis activity in MRL/lpr/lpr mice.

The concentration of immunoglobulins, anti-ssDNA, anti-dsDNA, anti-TNP antibodies, IgM rheumatoid factor, C3, immune complexes and serum amyloid P component (SAP), in the serum were measured in 68 male and female MRL/lpr/lpr mice, a strain affected with a systemic autoimmune disease. The degree of lymphoproliferation was assessed by the spleen weight. Spontaneous secretion of immunoglobulins and anti dsDNA antibodies were measured in spleen cell cultures. All mice presented age related increases or decreases (C3) in the level of measured parameters. Inflammatory lesions were detected, by light microscopy in the joints of all mice. There was a significant correlation, in both sexes between the serum level of SAP and the severity of the polyarthritis, as assessed by light microscopy. In female mice the levels of anti-dsDNA antibodies, immunoglobulins, either measured in serum or in the spleen compartment, and circulating immune complexes also showed correlation with the activity of the arthritis, but neither of these variables correlated as closely with arthritis scores as did serum SAP.

Age Factors↗