Of spin and charge in the cuprates

Centraal in dit proefschrift staan de elektlonische eigenschappen van de cupraten, waarbij we zowel spinexcitaties als ladingsexcitaties bestudcrcn. De cupraten zijn voornamelijk bekend vanwege hun metallische fase met buitengewoon hoge ovelgangstemperaturen naar de supergeleinde toestand, maar de magnetische eigensxappen van de isolerende fase zijn niet minder interessant. Onze voorrraamste interesse is het verband tussen magnetisme en supergeleiding. Eerst geven we een uitgebreid overzicht van de stand van zaken in de wetenschappelijke discussie over de cupraten. Dit overzicht vormt de achtergrond waarin we onze eigen resultaten plaatsen. ... Zie: Samenvatting

Magnetic-field-induced critical dynamics in magnetoelectric TbPO4_4

We report on the magnetoelectric dynamics in the linear magnetoelectric antiferromagnet TbPO$_4$ studied by broadband dielectric spectroscopy. For the phase transition into the magnetoelectric antiferromagnetic phase at $T_{N} \approx 2.3$ K, a finite magnetic field $H$ induces critical behavior in the quasi-static permittivity $\varepsilon'$. Plotting the corresponding anomaly as function of $T/T_N(H)$, we observe the scaling behavior $\Delta \varepsilon' \propto H^2$, a clear fingerprint of linear magnetoelectric antiferromagnets. Above the phase transition, we find a critical slowing down of the ferroic fluctuations in finite magnetic field. This behaviour can be understood via a magnetic-field-induced relaxational response that resembles the soft-mode behaviour in canonical ferroelectrics and multiferroics

Hubbard exciton revealed by time-domain optical spectroscopy

We use broadband ultra-fast pump-probe spectroscopy in the visible range to study the lowest excitations across the Mott-Hubbard gap in the orbitally ordered insulator YVO3. Separating thermal and non-thermal contributions to the optical transients, we show that the total spectral weight of the two lowest peaks is conserved, demonstrating that both excitations correspond to the same multiplet. The pump-induced transfer of spectral weight between the two peaks reveals that the low-energy one is a Hubbard exciton, i.e. a resonance or bound state between a doublon and a holon. Finally, we speculate that the pump-driven spin-disorder can be used to quantify the kinetic energy gain of the excitons in the ferromagnetic phase.Comment: 5 pages and 6 figures, 9 pages and 12 figures with additional material

Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2 \times 2)$ or a $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 \times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.Comment: 18 pages with 14 figures, including Supplemental Materia

Phase separation in the non-equilibrium Verwey transition in magnetite

We present equilibrium and out-of-equilibrium studies of the Verwey transition in magnetite. In the equilibrium optical conductivity, we find a step-like change at the phase transition for photon energies below about 2 eV. The possibility of triggering a non-equilibrium transient metallic state in insulating magnetite by photo excitation was recently demonstrated by an x-ray study. Here we report a full characterization of the optical properties in the visible frequency range across the non-equilibrium phase transition. Our analysis of the spectral features is based on a detailed description of the equilibrium properties. The out-of-equilibrium optical data bear the initial electronic response associated to localized photo-excitation, the occurrence of phase separation, and the transition to a transient metallic phase for excitation density larger than a critical value. This allows us to identify the electronic nature of the transient state, to unveil the phase transition dynamics, and to study the consequences of phase separation on the reflectivity, suggesting a spectroscopic feature that may be generally linked to out-of-equilibrium phase separation

Of spin and charge in the cuprates

Centraal in dit proefschrift staan de elektlonische eigenschappen van de cupraten, waarbij we zowel spinexcitaties als ladingsexcitaties bestudcrcn. De cupraten zijn voornamelijk bekend vanwege hun metallische fase met buitengewoon hoge ovelgangstemperaturen naar de supergeleinde toestand, maar de magnetische eigensxappen van de isolerende fase zijn niet minder interessant. Onze voorrraamste interesse is het verband tussen magnetisme en supergeleiding. Eerst geven we een uitgebreid overzicht van de stand van zaken in de wetenschappelijke discussie over de cupraten. Dit overzicht vormt de achtergrond waarin we onze eigen resultaten plaatsen. ... Zie: Samenvatting.