The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals

Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin-lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.

Ultrafast Spin Density Wave Transition in Chromium Governed by Thermalized Electron Gas

The energy and momentum selectivity of time- and angle-resolved photoemission spectroscopy is exploited to address the ultrafast dynamics of the antiferromagnetic spin density wave (SDW) transition photoexcited in epitaxial thin films of chromium. We are able to quantitatively extract the evolution of the SDW order parameter Δ through the ultrafast phase transition and show that Δ is governed by the transient temperature of the thermalized electron gas, in a mean field description. The complete destruction of SDW order on a sub-100 fs time scale is observed, much faster than for conventional charge density wave materials. Our results reveal that equilibrium concepts for phase transitions such as the order parameter may be utilized even in the strongly nonadiabatic regime of ultrafast photoexcitation.

Disparate ultrafast dynamics of itinerant and localized magnetic moments in gadolinium metal

The Heisenberg–Dirac intra-atomic exchange coupling is responsible for the formation of the atomic spin moment and thus the strongest interaction in magnetism. Therefore, it is generally assumed that intra-atomic exchange leads to a quasi-instantaneous aligning process in the magnetic moment dynamics of spins in separate, on-site atomic orbitals. Following ultrashort optical excitation of gadolinium metal, we concurrently record in photoemission the 4f magnetic linear dichroism and 5d exchange splitting. Their dynamics differ by one order of magnitude, with decay constants of 14 versus 0.8 ps, respectively. Spin dynamics simulations based on an orbital-resolved Heisenberg Hamiltonian combined with first-principles calculations explain the particular dynamics of 5d and 4f spin moments well, and corroborate that the 5d exchange splitting traces closely the 5d spin-moment dynamics. Thus gadolinium shows disparate dynamics of the localized 4f and the itinerant 5d spin moments, demonstrating a breakdown of their intra-atomic exchange alignment on a picosecond timescale.

Due the strength of the intra-atomic exchange interaction, it is generally assumed that alignment of spin moments in intra-atomic orbitals is quasi-instantaneous. Here, the authors demonstrate the breakdown of this relation between the 4f and 5d electrons in gadolinium following ultrashort optical excitation.

A high-order harmonic generation apparatus for time- and angle-resolved photoelectron spectroscopy

We present a table top setup for time- and angle-resolved photoelectron spectroscopy to investigate band structure dynamics of correlated materials driven far from equilibrium by femtosecond laser pulse excitation. With the electron-phonon equilibration time being in the order of 1-2 ps it is necessary to achieve sub-picosecond time resolution. Few techniques provide both the necessary time and energy resolution to map non-equilibrium states of the band structure. Laser-driven high-order harmonic generation is such a technique. In our experiment, a grating monochromator delivers tunable photon energies up to 40 eV. A photon energy bandwidth of 150 meV and a pulse duration of 100 fs FWHM allow us to cover the k-space necessary to map valence bands at different kz and detect outer core states.

Low-dose prednisolone has a CD4-stabilizing effect in pre-treated HIV-patients during structured therapy interruptions (STI)

BACKGROUND

A favorable development of CD4+ T cells was firstly noticed in therapy-naive HIV-patients without antiretroviral therapy (ART) taking 5 mg prednisolone daily. This observation led to the prescription of prednisolone during structured therapy interruptions (STI).

OBJECTIVE

To evaluate the effect of low dose prednisolone on pre-treated patients during STI.

METHODS

A retrospective analysis including all pre-treated patients with prednisolone therapy for > or =6 months during STI has been conducted. The patients with prednisolone onset right at the beginning of STI (n = 95) were compared with all patients without prednisolone therapy during their first 6 months of STI (n = 49). Patients with prednisolone were divided into two subgroups: the ongoing STI-group and the patients with ART-restart. Additionally, the development of all 33 patients from the control group having started prednisolone later during STI was documented. Irrespective of the time of initiation of prednisolone therapy during STI, the development of CD4+ T cells in all patients with prednisolone for >12 months during STI was analyzed (n = 108).

RESULTS

The mean daily CD4+ T cell decrease during STI was significantly less pronounced in the prednisolone-group (-0.50 vs. -0.74 cells/day; p = 0.0361). The daily CD4+ T cell decline of the 33 patients from the control subgroup including patients with a later onset of prednisolone therapy was only -0.11 during a mean time of 715 days under prednisolone. The CD4+ T cell count of the STI-patients treated with prednisolone for >12 months (n = 108; mean: 837 days +/- 64.6 (366-1,756 days)) decreased from 677/microl to 504/microl. - 51 of 81 patients (63%) included in 2-year-analysis showed stable CD4+ T cell counts (mean daily CD4+ T cell decrease: 0.08) and continued ART interruption.

CONCLUSION

This retrospective evaluation provides evidence that low dose corticosteroids are associated with less decrease of CD4+ T cell count in pre-treated HIV patients resulting in prolongation of the potential time of structured treatment interruptions for many HIV patients.

Low dose prednisolone reduces CD4+ T cell loss in therapy-naive HIV-patients without antiretroviral therapy

BACKGROUND

A favorable development of CD4+ T cells was noticed in therapy-naive HIV-patients without antiretroviral therapy (ART) taking 5 mg prednisolone daily. Based on these encouraging observations, prednisolone therapy in further HIV-patients without antiretroviral therapy was initiated.

OBJECTIVE

To evaluate the effect of low dose prednisolone on therapy-naive HIV patients without antiretroviral therapy.

METHODS

A retrospective analysis has been conducted comparing the development of CD4+ T cells, viral load and clinical outcome in all therapy-naive HIV-patients with (n = 65; CD4 > or =300/microl) or without (n = 136; CD4 > or =300/microl) prednisolone treatment for > or =6 months.

RESULTS

After 3 years, therapy-naive patients on prednisolone therapy showed a CD4+ T cell increase of +50.1/microl whereas in the untreated group a decrease of -186.1/microl (p = 0.0021) was noted. After 12 months, nearly twice as much untreated patients experienced a first-time CD4+ T cell loss of >100/microl or initiation of HAART due to clinical development compared to prednisolone-treated patients (64.1% vs. 35.0%). CD4+ T cell increase was associated with viral load at baseline: Patients with lower viral loads at baseline (<30,000 copies/ml) showed a favorable development with statistically significant less drop-outs (defined as HAART-onset and/or prednisolone discontinuation for the prednisolone group) than patients with higher viral loads at baseline in the first 3 years in the prednisolone group. -

CONCLUSION

Low dose prednisolone seems to be associated with a stabilization of CD4+ T cell count in therapy-naive HIV patients resulting in a pronounced prolongation of the potential time without HAART for many HIV patients.