F. Mayr

Magnetic properties and the phase diagram of La1-xSrxMnO3 for x ≤ 0.2

We report on susceptibility, magnetization, magnetoresistance, and ultrasonic studies of La1-xSrxMnO3 for x≤0.2. The magnetic susceptibilities in the different high-temperature paramagnetic phases are studied in detail. We provide experimental evidence for the occurrence of short-range ferromagnetic order within the paramagnetic phase. Furthermore, we can exclude the possibility of electronic phase separation for x<0.1. Variable-range-hopping conductivity dominates in the paramagnetic and magnetically ordered phases. From the anomalies at the structural and magnetic phase transitions, we arrive at a phase diagram which shows a ferromagnetic and insulating ground state for 0.1≤x≤0.15, followed by a canted antiferromagnetic phase at higher temperatures. The ferromagnetic transition is strongly coupled to a structural transition from a Jahn-Teller-distorted phase to a pseudocubic orthorhombic phase and is accompanied by the appearance of large positive magnetoresistance effects. We interpret the rich phase diagram close to x = 1/8 in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn-Teller distortions.

Hydrogen-bond equilibria and lifetimes in a monohydroxy alcohol.

Dielectric loss spectra covering 13 decades in frequency were collected for 2-ethyl-1-hexanol, a monohydroxy alcohol that exhibits a prominent Debye-like relaxation, typical for several classes of hydrogen-bonded liquids. The thermal variation of the dielectric absorption amplitude agrees well with that of the hydrogen-bond equilibrium population, experimentally mapped out using near infrared (NIR) and nuclear magnetic resonance (NMR) measurements. Despite this agreement, temperature-jump NIR spectroscopy reveals that the hydrogen-bond switching rate does not define the frequency position of the prominent absorption peak. This contrasts with widespread notions and models based thereon, but is consistent with a recent approach.

Spin-driven phonon splitting in bond-frustrated ZnCr2S4.

Magnetic susceptibility, specific heat, thermal expansion, and IR spectroscopy provide experimental evidence that the two subsequent antiferromagnetic transitions in ZnCr2S4 at TN1 = 15 K and TN2 = 8 K are accompanied by significant thermal and phonon anomalies. The anomaly at TN2 reveals a temperature hysteresis typical for a first-order transformation. Because of strong spin-phonon coupling, both magnetic transitions at TN1 and TN2 induce a splitting of phonon modes. The anomalies and phonon splitting observed at TN2 are suppressed by strong magnetic field. Regarding the small positive Curie-Weiss temperature Theta approximately 8 K, we argue that this scenario of two different magnetic phases with different magnetoelastic couplings results from the strong competition of ferromagnetic and antiferromagnetic exchange.

Extremely small energy gap in the quasi-one-dimensional conducting chain compound SrNbO3.41.

Resistivity, optical, and angle-resolved photoemission experiments reveal unusual one-dimensional electronic properties of highly anisotropic SrNbO3.41. Along the conducting chain direction, we find an extremely small energy gap of only a few meV at the Fermi level. A discussion in terms of typical 1D instabilities (Peierls, Mott-Hubbard) shows that neither seems to provide a satisfactory explanation for the unique properties of SrNbO3.41.

Optical evidence for symmetry changes above the Néel temperature of KCuF3.

We report on optical measurements of the 1D Heisenberg antiferromagnet KCuF3. The crystal-field excitations of the Cu2+ ions have been observed and their temperature dependence can be understood in terms of magnetic and exchange-induced dipole mechanisms and vibronic interactions. Above TN we observe a new temperature scale TS characterized by the emergence of narrow absorption features that correlate with changes of the orbital ordering as observed by Paolasini et al. [Phys. Rev. Lett. 88, 106403 (2002)]. The appearance of these optical transitions provides evidence for a symmetry change above the Néel temperature that affects the orbital ordering and paves the way for the antiferromagnetic ordering.

Coupling of phonons and electromagnons in GdMnO3

The infrared and Terahertz properties of GdMnO_3 have been investigated as function of temperature and magnetic field, with special emphasis on the phase boundary between the incommensurate and the canted antiferromagnetic structures. The heterogeneous incommensurate phase reveals strong magnetodielectric effects, characterized by significant magnetoelectric contributions to the static dielectric permittivity and by the existence of electrically excited magnons (electromagnons). In the commensurate canted antiferromagnetic phase the magnetoelectric contributions to the dielectric constant and electromagnons are suppressed. The corresponding spectral weight is transferred to the lowest lattice vibration demonstrating the strong coupling of phonons with electromagnons.

The phase diagram and optical properties of La1−xSrxMnO3 for x⩽0.2

Abstract We report on detailed susceptibility, magnetization and magnetoresistance studies in La1−xSrxMnO3 for x⩽0.2. We arrive at a phase diagram which shows a ferromagnetic and insulating ground state for 0.1⩽x⩽0.15, followed by a canted antiferromagnetic phase at higher temperatures. The ferromagnetic transition is strongly coupled to a structural transition from a Jahn–Teller distorted phase to a pseudocubic orthorhombic phase, and is accompanied by the appearance of large positive magnetoresistance effects. We interpret the rich phase diagram close to x= 1 8 in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn–Teller distortions. In addition, we report on the temperature dependence of phonon spectra and on optical conductivities. The temperature dependence of bending and stretching modes clearly indicate the structural and magnetic-phase transitions. The optical conductivities are explained in terms of small polaron absorption.

Transport, magnetic, thermodynamic and optical properties in Ti-doped Sr2RuO4

We report on electrical resistivity, magnetic susceptibility, and magnetization on heat-capacity and optical experiments in single crystals of Sr 2 Ru 1 - x Ti x O 4 . Samples with x = 0.1 and 0.2 reveal purely semiconductingresistivity behavior along c and the charge transport is close to localization within the ab plane. A strong anisotropy in the magnetic susceptibility appears at temperatures below 100 K. Moreover magnetic ordering in the c direction with a moment of order 0.01 μ B /f.u. occurs at low temperatures. On doping the low-temperature linear term of the heat capacity becomes significantly reduced and probably is dominated by spin fluctuations. Finally, the optical conductivity reveals the anisotropic character of the dc resistance, with the in-plane conductance roughly following a Drude-type behavior and an insulating response along c.

Cooperative dynamics in doped manganite films: Phonon anomalies in the ferromagnetic state

We present optical measurements of phononic excitations in La

On the nature of superconductivity in the anisotropic dichalcogenide NbSe2{CoCp2}(x)

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