Showing papers by "Alois Loidl published in 2001"

Nature and properties of the Johari–Goldstein β-relaxation in the equilibrium liquid state of a class of glass-formers

Abstract: Previous dielectric relaxation measurements of glycerol and propylene carbonate and new results on propylene glycol performed below the conventional glass transition temperatures T_(g) after long periods of aging all show that the excess wing (a second power law at higher frequencies) in the isothermal dielectric loss spectrum, develops into a shoulder. These results suggest that the excess wing, a characteristic feature of a variety of glass-formers, is the high frequency flank of a Johari–Goldstein β-relaxation loss peak submerged under the α-relaxation loss peak. With this interpretation of the excess wing assured, the dielectric spectra of all three glass-formers measured at temperatures above T_(g) are analyzed as a sum of a α-relaxation modeled by the Fourier transform of a Kohlrausch–Williams–Watts function and a β-relaxation modeled by a Cole–Cole function. Good fits to the experimental data have been achieved. In addition to the newly resolved β-relaxation on propylene glycol, the important results of this work are the properties of the β-relaxation in this class of glass-formers in the equilibrium liquid state obtained over broad frequency and temperature ranges.

Wide range dielectric spectroscopy on glass-forming materials: An experimental overview

Abstract: Dielectric spectroscopy is one of the most commonly used techniques for the investigation of the dynamic response of glass-forming materials. The tremendous slow-down of the particle motions when approaching the glass transition and especially the fast processes in glass-forming materials, which have come into the focus of scientific interest recently, make the investigation in a wide frequency range highly desirable. Recently, results from broadband dielectric spectroscopy on glass-forming materials in their liquid and supercooled-liquid state, covering more than 18 decades of frequency have been reported by our group. In the present paper, we give an overview of the various experimental setups and techniques used to collect these spectra. As an example, spectra of the prototypical glass former glycerol are presented.

Ac susceptibility studies of ferrimagnetic FeCr2S4 single crystals

Abstract: Ac linear and nonlinear susceptibilities, χ0 and χ2, of ferrimagnetic FeCr2S4 single crystals were measured in the temperature range from 4.2 to 300 K for different driving ac and applied dc magnetic fields in the frequency range of 10−1–103 Hz. For high driving ac fields the real part of χ0 exhibits a cusp at around Tm≈60 K correlated with the onset of dc magnetization irreversibilities. The imaginary part of χ0 shows a strong increase below 100 K and nonmonotonic temperature dependence with a maximum shifted toward low temperatures with an increase in the driving field. Both real and imaginary parts of the linear susceptibility, χ0′ and χ0″, show a pronounced frequency dependence between 90 and 20 K with a maximal difference at around 60 K. Below the Curie temperature the real part of the nonlinear susceptibility, χ2, exhibits a broad negative peak which is strongly shifted towards low temperatures with an increase in the driving field. No clearly divergent behavior of χ2′ around Tm is observed. The fie...

Optical conductivity and penetration depth in MgB(2).

Abstract: The complex conductivity of a MgB{sub 2} film has been investigated in the frequency range 4<{nu}<30 cm{sup -1} and for temperatures 2.7

Ferrimagnetic behavior of Nd 0.67 Sr 0.33 CoO 3

Abstract: The orthorhombic rare-earth cobaltate ${\mathrm{Nd}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{CoO}}_{3}$ is studied by neutron powder diffraction. The Co sublattice orders ferromagnetically below ${T}_{c}\ensuremath{\approx}200$ K with the magnetic moments aligned along the c axis. On cooling, ${\mathrm{Nd}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{CoO}}_{3}$ revealed an induced ferromagnetic order of the Nd sublattice coupled antiparallel to the Co lattice. Within a simple spin-only picture, steric considerations give evidence for a mixed-valent mixed-spin configuration of the Co ions. This is compared to an intermediate valence-state scenario that shows the necessity of detailed electronic-structure calculations.

Magnetic, electronic, dielectric and optical properties of Pr(Ca:Sr)MnO3

Abstract: The charge-ordered perovskite Pr0.65Ca0.28Sr0.07MnO3 was investigated by means of magnetic susceptibility, specific heat, dielectric and optical spectroscopy and electron-spin resonance techniques. Under moderate magnetic fields, the charge order melts yielding colossal magnetoresistance effects with changes of the resistivity over eleven orders of magnitude. The optical conductivity is studied from audio frequencies far into the visible spectral regime. Below the phonon modes hopping conductivity is detected. Beyond the phonon modes the optical conductivity is explained by polaronic excitations out of a bound state. ESR techniques yield detailed informations on the (H,T ) phase diagram and reveal a broadening of the linewidth which can be modeled in terms of activated polaron hopping.

Magnetic ordering and spin-state transition in R 0.67 Sr 0.33 CoO 3

Abstract: A series of ceramic ${R}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{CoO}}_{3}$ samples with $R=\mathrm{La},$ Pr, ${\mathrm{Pr}}_{0.5}{\mathrm{Nd}}_{0.5},$ Nd, ${\mathrm{Nd}}_{0.5}{\mathrm{Sm}}_{0.5},$ and Sm have been prepared. We report on detailed structural, electrical, magnetic, and thermodynamic studies. All samples reveal magnetic order, spanning the range from a ferromagnetic metal to a ferrimagnet close to a metal-to-insulator transition. We found a clear correlation between structural properties and resistivity as well as between structure and magnetic exchange interactions. In this context the importance of double exchange is discussed. Heat- capacity experiments show a significant anomaly at the magnetic transition temperature of the La compound only. The samples with magnetic rare-earth ions reveal significant Schottky anomalies at low temperatures and allow for a rough estimate of the crystal-field splittings.

Magnetic susceptibility and ESR study of the covalent-chain antiferromagnets TlFeS 2 and TlFeSe 2

Abstract: We report on magnetic susceptibility, magnetization, electric resistivity, and ESR experiments on single crystals of the covalent-chain antiferromagnetic compounds $\mathrm{TlFe}{X}_{2}$ $(X=\mathrm{S},$ Se). Collinear magnetic order with strongly reduced moments sets in at ${T}_{\mathrm{N}}=196 \mathrm{K}$ for ${\mathrm{TlFeS}}_{2}$ and at ${T}_{\mathrm{N}}=290 \mathrm{K}$ for ${\mathrm{TlFeSe}}_{2},$ respectively. The magnetic moments are oriented perpendicular to the chain direction. The temperature dependence of the electric resistivity reveals semiconducting behavior for both compounds. However, high-temperature susceptibility and ESR measurements strongly suggest a one-dimensional metallic character.

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

Abstract: 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.

Monitoring the sign reversal of the valence band exchange integral in (Ga,Mn)As

Abstract: We report the successful growth of magnetic Ga 1− x Mn x As layers on (1 0 0) GaAs substrates by metal-organic vapour-phase epitaxy. Depending on the growth parameters, two different magnetic phases of Ga 1− x Mn x As can be grown. (i) At low Mn-concentrations, Ga 1− x Mn x As alloys are obtained. These alloys exhibit a paramagnetic behaviour with a strong exchange interaction between the localised magnetic moments of the Mn 2+ ions and the extended excitonic states. (ii) At high Mn-concentrations, Mn(Ga)As clusters are formed within a Ga 1− x Mn x As host. The samples are ferromagnetic even above room temperature. The ferromagnetism has been investigated by SQUID and ESR measurements. The s–d and p–d exchange integrals have been determined independently by combining photoluminescence excitation and spin-flip Raman spectroscopy. A reversal of sign of the valence band exchange integral has been detected along with the transition from the paramagnetic to the ferromagnetic phase.

Heavy fermions in LiV2O4: Kondo compensation versus geometric frustration

Abstract: 7Li nuclear magnetic resonance and heat capacity measurements were performed on the metallic spinel LiV2O4 down to 30 mK. The temperature dependencies of the linewidth, the Knight shift and the spin-lattice relaxation rate were investigated in the temperature range 30 mK≤T≤280 K and at applied magnetic fields of 4.6, 10, 44 and 83 kOe. The longitudinal nuclear magnetization was found to relax following a stretched-exponential form with a stretching exponent in the range 0.5<β<1. For temperatures T<1 K and at the lowest applied magnetic field of 4.6 kOe, we observe a spin-lattice relaxation rate which slows down exponentially, exhibiting a hindering barrier of the order of 1 K. This excitation energy separates different configurations of a highly degenerate ground state of a completely frustrated magnet.

Electron-spin-resonance studies of the ferrimagnetic semiconductor FeCr 2 S 4

Abstract: Electron-spin-resonance (ESR) measurements have been performed on ${\mathrm{FeCr}}_{2}{\mathrm{S}}_{4}$ single crystals in the temperature range 4.2--200 K. On decreasing temperature, the resonance lines strongly shift to low fields for the easy magnetization direction, 〈100〉, and to high fields for the hard directions, 〈110〉 and 〈111〉. In the hard directions additional resonance modes appear below 125 K. The nonmonotonic behaviors of the ESR linewidth and intensity indicate substantial changes of the relaxation processes below 125 K, which correlates with the macroscopic dc magnetization and ac susceptibility data.

Magnetic resonance on LiCuVO4

Abstract: EPR and 7Li NMR measurements were performed in the distorted inverse spinel V(LiCu)O4 down to 1.5 K. Anisotropy effects on magnetic resonance spectra due to the Jahn-Teller distortion of the oxygen octahedra surrounding the copper ions are discussed. The estimation of the spin-spin interactions deduced from the EPR-relaxation rate ΔH reveals a situation comparable to the prototypical one-dimensional S = 1/2 Heisenberg antiferromagnet CuGeO3. Approaching three-dimensional antiferromagnetic order ( TN≈ 2 K) from above, both magnetic relaxation rates, ΔHEPR and 7(1/T1), respectively, exhibit nearly the same critical divergence reminding to the onset of three-dimensional order in two-dimensional layered systems.

Phonon anomalies and electron-phonon interaction in the RuSr 2 GdCu 2 O 8 ferromagnetic superconductor: Evidence from infrared conductivity

Abstract: Critical behavior of the infrared reflectivity of RuSr_2GdCu_2O_8 ceramics is observed near the superconducting T_{SC} = 45 K and magnetic T_M = 133 K transition temperatures. The optical conductivity reveals the typical features of the c-axis optical conductivity of strongly underdoped multilayer superconducting cuprates. The transformation of the Cu-O bending mode at 288 cm^{-1} to a broad absorption peak at the temperatures between T^* = 90 K and T_{SC} is clearly observed, and is accompanied by the suppression of spectral weight at low frequencies. The correlated shifts to lower frequencies of the Ru-related phonon mode at 190 cm^{-1} and the mid-IR band at 4800 cm^{-1} on decreasing temperature below T_M are observed. It provides experimental evidence in favor of strong electron-phonon coupling of the charge carriers in the Ru-O layers which critically depends on the Ru core spin alignment. The underdoped character of the superconductor is explained by strong hole depletion of the CuO_2 planes caused by the charge carrier self-trapping at the Ru moments.

Metal-to-insulator transition in La 1-x Ba x TiO 3

Abstract: The ferroelectric insulator ${\mathrm{BaTiO}}_{3}$ can be electron-doped by substitution of divalent ${\mathrm{Ba}}^{2+}$ with trivalent ${\mathrm{La}}^{3+},$ creating d states at the transition-metal sites. The successive filling of the $3d$ Ti band initiates a sequence of electronic transitions. First, the system undergoes an Anderson-type insulator-to-metal-transition into a paramagnetic and metallic state, and second, a correlation-induced antiferromagnetic and insulating phase is established for La-rich concentrations. Ceramic ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{TiO}}_{3}$ samples covering the whole concentration range were systematically examined utilizing measurements of resistivity, specific heat, and magnetic susceptibility in the temperature range $1.8lTl300\mathrm{K}.$ The concentration dependence of the Sommerfeld coefficient $\ensuremath{\gamma}$ and the Pauli spin susceptibility was systematically investigated. The temperature dependence of the resistivity was studied in detail. In addition a $(T,x)$ phase diagram of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ba}}_{x}{\mathrm{TiO}}_{3}$ was constructed.

Magneto-spectroscopy of ordered arrays of magnetic semiconductor quantum wires

Abstract: Hexagonally ordered arrays of quantum wires of the diluted magnetic semiconductor Cd 1−x Mn x S (0 have been obtained by incorporating the semiconductor alloy into the ordered wire-like pores of mesoporous silica hosts (MCM-41). Electron paramagnetic resonance measurements indicate that the crystalline structure of the Cd 1− x Mn x S quantum wires changes from zincblende to wurtzite with increasing x . Photoluminescence and photoluminescence excitation spectroscopy have been performed at liquid helium temperatures in external magnetic fields up to 7.5 T . The photoluminescence studies of the transitions within the half-filled Mn 3d-shell reveal the existence of at least two kinds of Mn-centres with different crystal fields. A blue shift of more than 200 meV of the absorption edge of the Cd 1− x Mn x S wires compared to that of bulk material is found due to the lateral quantum confinement in the nanowires. In addition, the p–d exchange-induced bowing of the band gap with x is enhanced in the wire structures due to the confinement-induced shift of the lowest valence band state towards the Mn 3d-states.

NMR, EPR, and bulk susceptibility measurements of one-dimensional SrNbO 3.41

Abstract: NMR, EPR, and bulk susceptibility measurements were performed in the perovskite-related layered compound ${\mathrm{SrNbO}}_{3.41}.$ We determined ${}^{93}\mathrm{Nb}$ NMR spectra, spin-spin relaxation rates ${1/T}_{2}$ and spin-lattice relaxation rates ${1/T}_{1}$ down to 600 mK. From the spin-lattice relaxation ${1/T}_{1}$ we identified an activated behavior ${1/T}_{1}\ensuremath{\propto}\mathrm{exp}(\ensuremath{-}{\ensuremath{\Delta}}_{\mathrm{NMR}}{/k}_{B}T)$ with a gap ${\ensuremath{\Delta}}_{\mathrm{NMR}}=6.5\mathrm{meV}$ for temperatures $Tg20\mathrm{K}.$ Below 20 K, the line width $\ensuremath{\Delta}H(T)$ from acceptor-EPR and ${}^{93}{(1/T}_{1})(T)$ both exhibit a temperature dependence propotional to $\mathrm{exp}[\ensuremath{-}{(T}_{0}{/T)}^{1/2}]$ which is indicative for a variable range hopping process. We discuss the possibility of charge-density-wave formation at low temperatures.

Heavy-fermion formation at the metal-to-insulator transition in Gd 1 − x Sr x TiO 3

Abstract: The perovskitelike transition-metal oxide ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{TiO}}_{3}$ is investigated by measurements of resistivity, specific-heat, and electron paramagnetic resonance (EPR). Approaching the metal-to-insulator transition from the metallic regime $(xg~0.2),$ the Sommerfeld coefficient $\ensuremath{\gamma}$ of the specific heat becomes strongly enhanced and the resistivity increases quadratically at low temperatures, which both are fingerprints of strong electronic correlations. The temperature dependence of the dynamic susceptibility, as determined from the ${\mathrm{Gd}}^{3+}\ensuremath{-}\mathrm{EPR}$ linewidth, signals the importance of strong spin fluctuations, as observed in heavy-fermion compounds.

Radio-frequency response of semiconducting CdF 2 :In crystals with Schottky barriers

Abstract: The dielectric response of semiconducting ${\mathrm{CdF}}_{2}$ crystals with bistable In centers in the range of ${10}^{1}\ensuremath{-}{10}^{6}\mathrm{Hz}$ reveals a quasi-Debye relaxation due to Schottky barriers at the Au/Ag contacts. These spectra can be modeled with a two- or tri-layer capacitor, the characteristics of which are determined by the conductivity and capacity of the crystal volume and the depletion layers at the contacts (the Maxwell-Wagner capacitance). Analyses of the temperature dependence of these parameters show that the volume conductivity is due to free-electron motion, whereas the depletion-layer conductivity probably is caused by electron jumps over the deep In centers. Illumination of the crystals in the photoionization absorption band of the deep centers has the same effect upon the dielectric response as an increase of the temperature since both factors increase the free-electron concentration.

Frequency dependence of dielectric nonlinearity in PMN relaxor system

Abstract: The linear e 1 ∗ and the third harmonic complex dielectric constant e 3 ∗ in PbMg 1 3 Nb 2 3 O3 (PMN) single crystal were measured as a function of frequency and temperature in a small electric field. The dielectric dispersion of e 3 ∗ was studied and the characteristic relaxation time together with the static dielectric nonlinearity were determined as a function of the temperature. The static dielectric nonlinearity βs is increasing with decreasing temperature in the temperature range 266–242 K, while the relaxation time follows the Vogel–Fulcher law.

Magnetic field dependence of the transverse plasmon in SmLa0.8Sr0.2CuO4-delta

Abstract: 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.

Dynamic Processes at the Glass Transition

Abstract: We will give an overview over the rich variety of dynamic processes in glass-forming materials Dielectric spectra extending over more than 18 decades of frequency are presented and the time and temperature evolution of the basic spectral features associated with various dynamic processes are discussed in detail Among them are the structural α-relaxation, the excess wing, the fast process in the GHz — THz region and the boson peak The relevance of these features for glass formation is discussed

Lattice vibrations in the mixed crystals (NH4I)0.3(KI)0.7, (ND4I)0.3(KI)0.7 and (NH4Br)0.3(KBr)0.7

Abstract: Transverse optical phonons have been studied in (NH4I)0.3(KI)0.7, (ND4I)0.3(KI)0.7 and (NH4Br)0.3(KBr)0.7 using Fourier-transform infrared techniques. In all materials three phonon modes have been observed in the far-infrared regime and were followed as a function of temperature in detail. While the first low-frequency mode characterizes a sublattice motion of the alkali ions against the halogenides K+ ↔ I− and K+ ↔ Br−, the second mode involves vibrations of the ammonium molecules against the halogenide ions NH4+ ↔ I−, ND4+ ↔ I− and HH4+ ↔ Br−. Despite the fact that the average symmetry of these crystals is cubic, the transitional and orientational disorder locally breaks the cubic symmetry, the infra-red selection rules are relaxed and, hence, a third phonon contribution appears. Deviations of the temperature dependence of the eigenfrequencies, as expected for anharmonic modes, are interpreted in terms of a rotation–translation coupling where the phonon modes interact with the orientational degrees of freedom.

Molecular dynamics of C70S48: Dielectric function and NMR study.

Abstract: Molecular dynamics of C70S48 have been studied by dielectric relaxation spectroscopy and NMR In accordance with the NMR data, the rotation of C70 molecules is not hindered by sulfur, and even at r

Molecular Dynamics of C70S48: Dielectric Function and NMR Study

Abstract: Molecular dynamics of C70S48 have been studied by dielectric relaxation spectroscopy and NMR. In accordance with the NMR data, the rotation of C70 molecules is not hindered by sulfur, and even at r...

Magnetic ordering and spin-state of cobalt in R0.67Sr0.33CoO3

Abstract: We report on detailed magnetization and specific heat studies in R0.67Sr0.33CoO3. All samples reveal ferromagnetic order with increasing Tc for increasing average ionic radius 〈rA〉. The magnetic measurements in R0.67Sr0.33CoO3 clearly indicate a spin-state transition as a function of 〈rA〉. For some samples a further magnetic transition accompanied with a reduction in magnetization has been observed at low temperatures. Whether a temperature induced spin-state transition for the cobalt ions or crystal field effects for the rare earth ions hold for the reduced magnetization is still open.

Broadband infrared conductivity in an YBa2Cu3O6.7 thin film

Abstract: Infrared reflectivity of an oxygen reduced YBa2Cu3O6.7 thin film was measured in the frequency range 30 cm-1 < v < 7000 cm-1 and for temperatures 5 K < T < 300 K. At lower frequencies, 3 cm-1 < v < 40 cm-1, the complex conductivity σ*=σ1+iσ2 was measured directly using a transmission technique. Both data sets were combined to obtain the reflectivity in a broad frequency range and thus to improve substantially the quality of the Kramers-Kronig analysis at low frequencies. The analysis of the conductivity spectra reveals that the low-frequency spectral weight strongly depends on temperature directly above TC but saturates for T ≥ 150 K.