Biomedical subjects

A Loidl

Publications and source records attributed to A Loidl.

At least 19 recordsLinked to original sources

PubMed2005

Observation of a griffiths phase in paramagnetic La1-xSrxMnO3.

We report on the discovery of a novel triangular phase regime in the system La1-xSrxMnO3 by means of electron spin resonance and magnetic susceptibility measurements. This phase is characterized by the coexistence of ferromagnetic entities within the globally paramagnetic phase far above the magnetic ordering temperature. The nature of this phase can be understood in terms of Griffiths singularities arising due to the presence of correlated quenched disorder in the orthorhombic phase.

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PubMed2005

Negative refraction in ferromagnet-superconductor superlattices.

Negative refraction, which reverses many fundamental aspects of classical optics, can be obtained in systems with negative magnetic permeability and negative dielectric permittivity. This Letter documents an experimental realization of negative refraction at millimeter waves, finite magnetic fields, and cryogenic temperatures utilizing a multilayer stack of ferromagnetic and superconducting thin films. In the present case the superconducting YBa2Cu3O7 layers provide negative permittivity while negative permeability is achieved via ferromagnetic (La:Sr)MnO3 layers for frequencies and magnetic fields close to the ferromagnetic resonance. In these superlattices the refractive index can be switched between positive and negative regions using external magnetic field as tuning parameter.

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PubMed2005

Glassy aging dynamics.

We present time-dependent dielectric loss data for various glass formers below the glass temperature. The observed aging dynamics is described using a modified Kohlrausch-Williams-Watts law taking into account the variation of the relaxation time during aging. It leads to values for the aging relaxation time and stretching exponent, fully consistent with the results from equilibrium measurements performed at higher temperatures. Irrespective of the dynamic process prevailing in the investigated frequency region, the aging dynamics is always determined by the structural relaxation process.

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PubMed2005

Vibronic and magnetic excitations in the spin-orbital liquid state of FeSc2S4.

Inelastic neutron scattering results on the spin-orbital liquid in FeSc2S4 are presented. This sulfospinel reveals strong geometric frustration in the spin and in the orbital sector. In the present experiments the orbital liquid is evidenced by a clear spectroscopic signature of a dynamic Jahn-Teller effect with a vibronic splitting 3Gamma approximately 2 meV in agreement with theoretical estimates. The excitations of the spin liquid reveal strong dispersion and can be characterized as cooperative spin excitations in a supercooled paramagnet with a spin gap of Delta approximately 0.2 meV.

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PubMed2005

Relaxor ferroelectricity and colossal magnetocapacitive coupling in ferromagnetic CdCr2S4.

Materials in which magnetic and electric order coexist--termed 'multiferroics' or 'magnetoelectrics'--have recently become the focus of much research. In particular, the simultaneous occurrence of ferromagnetism and ferroelectricity, combined with an intimate coupling of magnetization and polarization via magnetocapacitive effects, holds promise for new generations of electronic devices. Here we present measurements on a simple cubic spinel compound with unusual, and potentially useful, magnetic and electric properties: it shows ferromagnetic order coexisting with relaxor ferroelectricity (a ferroelectric cluster state with a smeared-out phase transition), both having sizable ordering temperatures and moments. Close to the ferromagnetic ordering temperature, the magnetocapacitive coupling (characterized by a variation of the dielectric constant in an external magnetic field) reaches colossal values, approaching 500 per cent. We attribute the relaxor properties to geometric frustration, which is well known for magnetic moments but here is found to impede long-range order of the structural degrees of freedom that drive the formation of the ferroelectric state.

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PubMed2005

Field-controlled phase separation at the impurity-induced magnetic ordering in the spin-Peierls Magnet CuGeO3.

The paramagnetic fraction surviving at the impurity-induced antiferromagnetic phase transition in the spin-Peierls magnet CuGeO3 is found to increase with an external magnetic field. This effect is explained by the competition of the Zeeman interaction and of the exchange interaction of local antiferromagnetic clusters formed on the spin-gap background near impurities.

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PubMed2005

Orbital freezing and orbital glass state in FeCr2S4.

Low-temperature specific heat measurements and dielectric spectroscopy have been performed on polycrystalline and single-crystalline FeCr2S4, the single crystals showing a transition into a low-temperature orbital glass phase. The freezing of the orbital moments is revealed by a glasslike specific heat anomaly and by a clear relaxational behavior of the dielectric permittivity, exhibiting several hallmark features of glassy dynamics. The orbital relaxation dynamics continuously slows down over six decades in time, before at the lowest temperatures the glass transition becomes suppressed by quantum tunneling.

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PubMed2004

Spin dynamics and charge order in beta-Na1/3V2O5.

We present detailed electron-spin resonance investigations on single crystals of the one-dimensional vanadium-oxide bronze beta-Na(1/3)V2O5. From the angular dependence of the g value it can be concluded that the electrons are primarily located on the V1 zigzag chains. The anisotropy of the linewidth, which is determined by the intrachain symmetric anisotropic exchange interaction, favors statistic electron distribution in the metallic and blockwise charge-order in the insulating phase. The temperature dependence of the linewidth indicates the opening of a charge gap at the metal-to-insulator transition at T(MI)=132 K.

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PubMed2004

Spin and orbital frustration in MnSc2S4 and FeSc2S4.

Crystal structure, magnetic susceptibility, and specific heat were measured in the normal cubic spinel compounds MnSc2S4 and FeSc2S4. Down to the lowest temperatures, both compounds remain cubic and reveal strong magnetic frustration. Specifically the Fe compound is characterized by a Curie-Weiss (CW) temperature ThetaCW = -45 K and does not show any indications of order down to 50 mK. In addition, the Jahn-Teller ion Fe2+ is orbitally frustrated. Hence, FeSc2S4 belongs to the rare class of spin-orbital liquids. MnSc2S4 is a spin liquid for temperatures T>TN approximately 2 K.

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PubMed2004

Antiferromagnetic resonances and magnetization of a canted antiferromagnet.

A two-sublattice model has been applied to a canted antiferromagnet including both Dzyaloshinsky-Moria interaction and double-exchange. Static magnetization and frequencies of the antiferromagnetic resonances (AFMR) were calculated for static magnetic fields along the principal crystallographic axes of an orthorhombic crystal. The experimental data for La(0.95)Sr(0.05)MnO3, which include magnetization, positions and excitation conditions of the AFMR modes can be well accounted for using the model calculations.

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PubMed2003

Response of disordered matter to electromagnetic fields.

We have studied a variety of different disordered materials, including molecular and ionic liquids, supercooled liquids, glasses, ionic conductors, and doped semiconductors, in ac electromagnetic fields over an exceptional broad dynamic range, including the rarely investigated GHz to THz region. All classes of disordered matter exhibit an astonishingly similar response: In addition to Jonscher's time-honored "universal dielectric response," a superlinear power-law increase of the frequency-dependent conductivity shows up bridging the gap between the classical dielectric and the infrared region. Thus the universal dielectric behavior of disordered matter extends up to much higher frequencies than thought until now.

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PubMed2003

Potential signature of a Kosterlitz-Thouless transition in BaNi2V2O8.

ESR measurements are reported for the quasi-two-dimensional honeycomb antiferromagnet BaNi2V2O8. Planar anisotropic properties are confirmed by angular dependent investigations of resonance field and linewidth. The divergence of the temperature-dependent linewidth on approaching T(N) from above is described in terms of the Kosterlitz-Thouless transition, the critical behavior close to long-range magnetic order, and the 2D Heisenberg antiferromagnet. We provide arguments that the Kosterlitz-Thouless scenario is compatible with the observed critical exponent and suggest BaNi2V2O8 is a weakly anisotropic 2D Heisenberg antiferromagnet.

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PubMed2003

Evidence for Jahn-Teller distortions at the antiferromagnetic transition in LaTiO3.

LaTiO3 is known as a Mott insulator which orders antiferromagnetically at T(N)=146 K. We report on results of thermal expansion and temperature dependent x-ray diffraction together with measurements of the heat capacity, electrical transport measurements, and optical spectroscopy in untwinned single crystals. At T(N) significant structural changes appear, which are volume conserving. Concomitant anomalies are also observed in the dc resistivity, in bulk modulus, and optical reflectivity spectra. We interpret these experimental observations as evidence of orbital order.

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PubMed2002

High-precision fluorescence assay for sphingomyelinase activity of isolated enzymes and cell lysates.

Sphingomyelinases are important enzymes of signal transduction. They catalyze the hydrolysis of sphingomyelin, giving rise to the intracellular formation of biologically active ceramide and phosphatidylcholine. Here we report on a fluorescence method for the fast and accurate determination of this enzyme in biological samples. The assay is based on a fluorescent sphingomyelin analog carrying fluorescent 7-nitro-2-1,3-benzooxadiazolyl amino-dodecanoic acid instead of an aliphatic acyl chain at the nitrogen atom. The fluorescent substrate is hydrolysed by sphingomyelinases to form fluorescent ceramide, which can be separated from the remaining substrate using TLC on silica gel. The fluorescence intensity pattern obtained on the TLC plate can accurately be determined using a CCD camera. Typically, a large number of samples can be analyzed simultaneously. Examples for the quantitative analysis of sphingomyelinases from freshly prepared cellular homogenates as well as from commercial sources are given.

4-Chloro-7-nitrobenzofurazan↗

PubMed2001

An inhibitor-resistant histone deacetylase in the plant pathogenic fungus Cochliobolus carbonum.

We have partially purified and characterized histone deacetylases of the plant pathogenic fungus Cochliobolus carbonum. Depending on growth conditions, this fungus produces HC-toxin, a specific histone deacetylase inhibitor. Purified enzymes were analyzed by immunoblotting, by immunoprecipitation, and for toxin sensitivity. The results demonstrate the existence of at least two distinct histone deacetylase activities. A high molecular weight complex (430,000) is sensitive to HC-toxin and trichostatin A and shows immunoreactivity with an antibody against Cochliobolus HDC2, an enzyme homologous to yeast RPD3. The second activity, a 60,000 molecular weight protein, which is resistant even to high concentrations of well-known deacetylase inhibitors, such as HC-toxin and trichostatin A, is not recognized by antibodies against Cochliobolus HDC1 (homologous to yeast HOS2) or HDC2 and represents a different and/or modified histone deacetylase which is enzymatically active in its monomeric form. This enzyme activity is not present in the related filamentous fungus Aspergillus nidulans. Furthermore, in vivo treatment of Cochliobolus mycelia with trichostatin A and analysis of HDACs during the transition from non-toxin-producing to toxin-producing stages support an HC-toxin-dependent enzyme activity profile.

Ascomycota↗

PubMed2001

Magnetic field dependence of the transverse plasmon in SmLa(0.8)Sr(0.2)CuO(4-delta).

The magnetic field and temperature dependence of the transverse and longitudinal plasmons in SmLa(0.8)Sr(0.2)CuO(4-delta) have been investigated. A transition between a vortex-glass and a vortex-liquid regime, which revealed different field dependencies of the resonance frequencies, could be clearly observed. The positions and the spectral weights of the plasmons were successfully described using the multilayer model [Phys. Rev. B 64, 024530 (2001)], which takes the compressibility of the electronic liquid into account. The absolute value of the compressibility is close to that of a two-dimensional noninteracting electron gas.

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PubMed2001

Optical conductivity and penetration depth in MgB(2).

The complex conductivity of a MgB(2) film has been investigated in the frequency range 4<nu<30 cm(-1) and for temperatures 2.7<T<300 K. The overall temperature dependence of both components of the complex conductivity is reminiscent of BCS-type behavior, although a detailed analysis reveals a number of discrepancies. A peak in the temperature dependence of the real part of the conductivity is detected for frequencies below 9 cm(-1). The superconducting penetration depth follows a T(2) behavior at low temperatures.

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PubMed2001

Observation of the transverse optical plasmon in SmLa0.8Sr0.2CuO4-delta.

We present microwave and infrared measurements on SmLa0.8Sr0.2CuO4-delta, which are direct evidence for the existence of a transverse optical plasma mode, observed as a peak in the c-axis optical conductivity. This mode appears as a consequence of the existence of two different intrinsic Josephson couplings between the CuO2 layers, one with a Sm2O2 block layer, and the other one with a (La,Sr)2O2-delta block layer. From the frequencies and the intensities of the collective modes we determine the value of the compressibility of the two dimensional electron fluid in the copper oxygen planes.

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