arXiv subjects

A. Loidl

Publications and source records attributed to A. Loidl.

At least 217 records · Page 12Linked to original sources

arXiv2006

Spin dynamics in the low-dimensional magnet TiOCl

We present detailed ESR investigations on single crystals of the low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates a stable orbital configuration below room temperature, and allows to estimate the energy of the first excited state as 0.3(1) eV ruling out a possible degeneracy of the orbital ground state. Moreover, we discuss the possible spin relaxation mechanisms in TiOCl and analyze the angular and temperature dependence of the linewidth up to 250 K in terms of anisotropic exchange interactions. Towards higher temperatures an exponential increase of the linewidth is observed, indicating an additional relaxation mechanism.

cond-mat.str-el↗

arXiv2006

Possible evidence for electromagnons in multiferroic manganites

Magnetodielectric materials are characterized by a strong coupling of magnetic and dielectric properties and in rare cases simultaneously exhibit both, magnetic and polar order. Among other multiferroics, TbMnO3 and GdMnO3 reveal a strong magneto-dielectric (ME) coupling and as a consequence fundamentally new spin excitations exist: Electro-active magnons, or electromagnons, i. e. spin waves which can be excited by ac electric fields. Here we show that these excitations appear in the phase with an incommensurate (IC) magnetic structure of the manganese spins. In external magnetic fields this IC structure can be suppressed and the electromagnons are wiped out, thereby inducing considerable changes in the index of refraction from dc up to THz frequencies. Hence, besides adding a new creature to the zoo of fundamental excitations, the refraction index can be tuned by moderate magnetic fields, which allows the design of a new generation of optical switches and optoelectronic devices.

cond-mat.str-el↗

arXiv2006

Colossal magnetocapacitance and colossal magnetoresistance in HgCr2S4

We present a detailed study of the dielectric and charge transport properties of the antiferromagnetic cubic spinel HgCr2S4. Similar to the findings in ferromagnetic CdCr2S4, the dielectric constant of HgCr2S4 becomes strongly enhanced in the region below 60 - 80 K, which can be ascribed to polar relaxational dynamics triggered by the onset of ferromagnetic correlations. In addition, the observation of polarization hysteresis curves indicates the development of ferroelectric order below about 70 K. Moreover, our investigations in external magnetic fields up to 5 T reveal the simultaneous occurrence of magnetocapacitance and magnetoresistance of truly colossal magnitudes in this material.

cond-mat.str-el↗

arXiv2006

31P NMR study of Na2CuP2O7: a S=1/2 two-dimensional Heisenberg antiferromagnetic system

The magnetic properties of Na2CuP2O7 were investigated by means of 31P nuclear magnetic resonance (NMR), magnetic susceptibility, and heat capacity measurements. We report the 31P NMR shift, the spin-lattice 1/T1, and spin-spin 1/T2 relaxation-rate data as a function of temperature T. The temperature dependence of the NMR shift K(T) is well described by the S=1/2 square lattice Heisenberg antiferromagnetic (HAF) model with an intraplanar exchange of J/k_B \simeq 18\pm2 K and a hyperfine coupling A = (3533\pm185) Oe/mu_B. The 31P NMR spectrum was found to broaden abruptly below T \sim 10 K signifying some kind of transition. However, no anomaly was noticed in the bulk susceptibility data down to 1.8 K. The heat capacity appears to have a weak maximum around 10 K. With decrease in temperatures, the spin-lattice relaxation rate 1/T1 decreases monotonically and appears to agree well with the high temperature series expansion expression for a S = 1/2 2D square lattice.

cond-mat.str-el↗

arXiv2006

$α$- and $β$- Relaxation Dynamics of a fragile plastic crystal

We present a thorough dielectric investigation of the relaxation dynamics of plastic crystalline Freon112, which exhibits freezing of the orientational degrees of freedom into a glassy crystal below 90 K. Among other plastic crystals, Freon112 stands out by being relatively fragile within Angell's classification scheme and by showing an unusually strong $β$-relaxation. Comparing the results to those on Freon112a, having only a single molecular conformation, points to the importance of the presence of two molecular conformations in Freon112 for the explanation of its unusual properties.

cond-mat.dis-nn↗

arXiv2006

Incommensurate structure of the spin-Peierls compound TiOCl

We report on a detailed single crystal x-ray diffraction study of the unconventional spin-Peierls compound TiOCl. The intermediate phase of TiOCl is characterized by an incommensurate modulation which is virtually identical to that recently found in the homologue compound TiOBr. The first order phase transition between the spin-Peierls ground state and the incommensurate phase reveals the same kind of thermal hysteresis in both, its crystal structure and magnetic susceptibility. A weak, but significant magnetic field effect is found for this phase transition with a field induced shift of the transition temperature of Delta Tc1= -0.13 K for an applied field of B=10 T along the chain direction. The field induced changes of the incommensurate crystal structure are compatible with a scenario of competing intra- and inter-chain interactions.

cond-mat.str-el↗

arXiv2005

Unconventional anisotropic superexchange in NaV2O5

The strong line broadening observed in electron spin resonance on NaV$_2$O$_5$ is found to originate from an unusual type of the symmetric anisotropic exchange interaction with simultaneous spin-orbit coupling on both sites. The microscopically derived anisotropic exchange constant is almost two orders of magnitude larger than the one obtained from conventional estimations. Based on this result we systematically evaluate the anisotropy of the ESR linewidth in terms of the symmetric anisotropic exchange, only, and we find microscopic evidence for precursor effects of the charge ordering already below 150 K.

cond-mat.str-el↗

arXiv2005

Dielectric spectroscopy on aging glasses

In the present work, we provide further evidence for the applicability of a modified stretched-exponential behavior, proposed recently for the description of aging-time dependent data below the glass temperature [P. Lunkenheimer et al., Phys. Rev. Lett. 95 (2005) 055702]. We analyze time-dependent dielectric loss data in a variety of aging glasses, including new data on Salol and propylene carbonate, using a conventional stretched exponential and the newly proposed approach. Also the scaling of aging data obtained at different measuring frequencies, which was predicted on the basis of the new approach, is checked for its validity.

cond-mat.dis-nn↗

arXiv2005

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.

cond-mat.str-el↗

arXiv2005

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 YBa_2Cu_3O_7 layers provide negative permittivity while negative permeability is achieved via ferromagnetic (La:Sr)MnO_3 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.

cond-mat.str-el↗

arXiv2005

Dynamic conductivity of semiconducting manganites approaching the metal-insulator transition

We report the frequency-dependent conductivity of the manganite system La1-xSrxMnO3 (x <= 0.2) when approaching the metal-insulator transition from the insulating side. Results from low-frequency dielectric measurements are combined with spectra in the infrared region. For low doping levels the behavior is dominated by hopping transport of localized charge carriers at low frequencies and by phononic and electronic excitations in the infrared region. For the higher Sr contents the approach of the metallic state is accompanied by the successive suppression of the hopping contribution at low frequencies and by the development of polaronic excitations in the infrared region, which finally become superimposed by a strong Drude contribution in the fully metallic state.

cond-mat.str-el↗

arXiv2005

Slowing down of the relaxational dynamics at the ferroelectric phase transition in one-dimensional (TMTTF)2AsF6

We present measurements of the dielectric response of quasi one-dimensional (TMTTF)2AsF6 in a wide temperature and frequency range. We provide a thorough characterization of the relaxational dynamics observed close to the ferroelectric-like transition at Tc = 100 K. Our measurements, extending up to 100 MHz, reveal a continuous slowing down of the mean relaxation time when approaching Tc from high as well as from low temperatures. The simultaneous critical rise of dielectric constant and relaxation time point to an explanation of the transition in terms of a classic ferroelectric scenario.

cond-mat.str-el↗

arXiv2005

Polaronic excitations in CMR manganite films

In the colossal magnetoresistance manganites polarons have been proposed as the charge carrier state which localizes across the metal-insulator transition. The character of the polarons is still under debate. We present an assessment of measurements which identify polarons in the metallic state of La{2/3}Sr{1/3}MnO{3} (LSMO) and La{2/3}Ca{1/3}MnO{3} (LCMO) thin films. We focus on optical spectroscopy in these films which displays a pronounced resonance in the mid-infrared. The temperature dependent resonance has been previously assigned to polaron excitations. These polaronic resonances are qualitatively distinct in LSMO and LCMO and we discuss large and small polaron scenarios which have been proposed so far. There is evidence for a large polaron excitation in LSMO and small polarons in LCMO. These scenarios are examined with respect to further experimental probes, specifically charge carrier mobility (Hall-effect measurements) and high-temperature dc-resistivity.

cond-mat.str-el↗

arXiv2005

Multiferroic behavior in CdCr2X4 (X = S, Se)

The recently discovered multiferroic material CdCr2S4 shows a coexistence of ferromagnetism and relaxor ferroelectricity together with a colossal magnetocapacitive effect. The complex dielectric permittivity of this compound and of the structurally related CdCr2Se4 was studied by means of broadband dielectric spectroscopy using different electrode materials. The observed magnetocapacitive coupling at the magnetic transition is driven by enormous changes of the relaxation dynamics induced by the development of magnetic order.

cond-mat.str-el↗

arXiv2005

Single-ion anisotropy in the gadolinium pyrochlores studied by an electron paramagnetic resonance

The electron paramagnetic resonance is used to measure the single-ion anisotropy of Gd3+ ions in the pyrochlore structure of (Y{1-x}Gdx)2Ti2O7. A rather strong easy-plane type anisotropy is found. The anisotropy constant D is comparable to the exchange integral J in the prototype Gd2Ti2O7, D~0.75J and exceeds the dipolar energy scale. Physical implications of an easy-plane anisotropy for a pyrochlore antiferromagnet are considered. We calculate the magnetization curves at T=0 and discuss phase transitions in magnetic field.

cond-mat.str-el↗

arXiv2005

Phonon anomalies and charge dynamics in Fe_{1-x}Cu_{x}Cr_{2}S_{4} single crystals

A detailed investigation of phonon excitations and charge carrier dynamics in single crystals of Fe_{1-x}Cu_{x}Cr_{2}S_{4} (x = 0, 0.2, 0.4, 0.5) has been performed by using infrared spectroscopy. In FeCr_{2}S_{4} the phonon eigenmodes are strongly affected by the onset of magnetic order. Despite enhanced screening effects, a continuous evolution of the phonon excitations can be observed in the doped compounds with x = 0.2 (metallic) and x = 0.4, 0.5 (bad metals), but the effect of magnetic ordering on the phonons is strongly reduced compared to x = 0. The Drude-like charge-carrier contribution to the optical conductivity in the doped samples indicates that the colossal magneto-resistance effect results from the suppression of spin-disorder scattering.

cond-mat.str-el↗

arXiv2005

Relaxation dynamics and colossal magnetocapacitive effect in CdCr2S4

A thorough investigation of the relaxational dynamics in the recently discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has been performed. Broadband dielectric measurements without and with external magnetic fields up to 10 T provide clear evidence that the observed magnetocapacitive effect stems from enormous changes of the relaxation dynamics induced by the development of magnetic order.

cond-mat.str-el↗

arXiv2005

Charge-Density-Wave like Behavior in the One-Dimensional Charge-Ordered Semiconductor (NbSe4)3I

We report on broadband dielectric spectroscopy on the one-dimensional semiconductor (NbSe4)3I. Below the structural phase transition close to 270 K we observe colossal dielectric constants with a frequency and temperature dependence very similar to what is observed in canonical charge-density wave systems. Guided by structural details we interpret this structural phase transition as driven by complex charge-order processes.

cond-mat.str-el↗