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In spintronic terahertz (THz) emitters, THz radiation is generated by exciting an ultrafast spin current through femtosecond laser excitation of a ferromagnetic-nonmagnetic metallic heterostructure. Although an extensive phenomenological knowledge has been built up during the last decade, a solid theoretical modeling that connects the generated THz signal to the laser induced-spin current is still incomplete. Here, starting from general solutions to Maxwell's equations, we model the electric field generated by a superdiffusive spin current in spintronic emitters, taking Co/Pt as a typical example. We explicitly include the detector shape, which is shown to significantly influence the detected THz radiation. Additionally, the electron energy dependence of the spin Hall effect is taken into account, as well as the duration of the exciting laser pulse and the thickness of the detector crystal. Our modeling leads to realistic emission profiles and highlights the role of the detection method for distinguishing key features of the spintronic THz emission.

We report the observation of a light-induced subpicosecond phase transition in the antiferromagnetic Dirac semimetal EuAgAs, achieved through ultrafast optical excitation. Using ultrafast optical spectroscopy, we probe the nonequilibrium carrier dynamics, discovering distinct fluence-dependent responses in the antiferromagnetic and paramagnetic states, and revealing a possible magnetic order-driven transition between different topological states. Our results demonstrate that EuAgAs, with its highly tunable magnetic structure, possibly offers a unique platform for exploring topological phase transitions. These results underscore the potential of ultrashort optical pulses as powerful tools for the real-time control of topological phases, opening pathways for advances in spintronics, quantum computing, and energy-efficient information technologies.

Altermagnets are magnetic materials with antiferromagnetic spin ordering but exhibit ferromagnetic properties. Understanding the microscopic origin of the latter is a central problem. Ferromagnetlike properties such as the anomalous Hall effect are linked with weak ferromagnetism, whose microscopic origin in altermagnets remains unclear however. We show theoretically that the alternating g-tensor anisotropy in altermagnets can induce weak orbital ferromagnetism even when the Dzyaloshinskii-Moriya interaction is forbidden. We demonstrate this mechanism to explain weak ferromagnetism for both collinear and noncollinear spin altermagnets. Our findings provide new insights into the origin of weak ferromagnetism and suggest orbital-based ways for manipulating magnetic configurations in altermagnets.

The electronic, mechanical, optical and conductive properties of MgPbN₂ and ZnPbN₂ are investigated using hybrid-functional first-principles calculations. Our calculations show that these two compounds preferentially stabilize in the tetragonal chalcopyrite structure. The calculated phonon dispersions show that both compounds are thermodynamically stable. From the calculated elastic constants we infer that the compound are also mechanically stable. As for the electronic properties, MgPbN₂ is computed to be a semiconductor with a direct band gap of 1.071 eV at the Γ point, while ZnPbN₂ is semi-metallic with the valence band and conduction band crossing at the Γ point. Both MgPbN₂ and ZnPbN₂ show an enhanced optical absorption and electric conductivity compared to those of the related semiconductors MgSnN₂ and ZnSnN₂. ZnPbN₂ gives a much stronger electric conductivity and a slightly larger light absorption than MgPbN₂. In addition the influence of alloying is investigated for Mg_1-xZn_xPbN₂ at different doping amount. When Mg was doped at 25% level, a band gap of 0.22 eV opens up and turns the system to semiconducting. When Sn was doped at Pb site, a similar phenomena was observed. On the other hand, the electric conductivity will increase with a small amount of Zn doping in MgPbN₂. With large amount of doping, a conductivity higher than that of pristine ZnPbN₂ is achieved. Our investigation widens the knowledge of II-IV-N₂ family of ternary nitrides, and may hence help to boost their applications.

We theoretically investigate the optically induced demagnetization of ferromagnetic FePt using the timedependent density functional theory (TDDFT). We compare the demagnetization mechanism in the perturbative and nonperturbative limits of light -matter interaction and show how the underlying mechanism of the ultrafast demagnetization depends on the driving laser intensity. Our calculations show that the femtosecond demagnetization in TDDFT is a longitudinal magnetization reduction and results from a nonlinear optomagnetic effect, akin to the inverse Faraday effect. The demagnetization scales quadratically with the electric field E in the perturbative limit, i.e., A M z proportional to E 2 . Moreover, the magnetization dynamics happens dominantly at even multiples n co 0 , ( n = 0 , 2 , ... ) of the pump -laser frequency co 0 , whereas odd multiples of co 0 do not contribute. We further investigate the demagnetization in conjunction to the optically induced change of electron occupations and electron correlations. Dynamical correlations within the Kohn -Sham local -density framework are shown to have an appreciable yet distinct effect on the amount of demagnetization depending on the laser intensity. Comparing the ab initio computed demagnetizations with those calculated from spin occupations, we show that electronic coherence plays a dominant role in the demagnetization process, whereas interpretations based on the time -dependent occupation numbers poorly describe the ultrafast demagnetization.

Optical manipulation of the magnetization of thin films opens up exciting possibilities for ever-faster magnetic storage applications. In this context, 𝐿⁢1₀ chemically ordered Fe⁢Pt thin films are of particular interest due to their high perpendicular magnetic anisotropy and their use as a storage material for heat-assisted magnetic recording devices. However, these materials are difficult to manipulate with external fields due to their high coercivity field. Thus, we want to explore the possibility of tailoring the properties of these materials to enable switching using all-optical techniques. While stochastic all-optical switching between partially magnetized states has been reported for undoped Fe⁢Pt thin films, we have investigated to what extent doping with third elements can influence the switching behavior. Reducing the saturation magnetization may be one way to facilitate all-optical switching. While this is expected with the introduction of additional elements, we also want to highlight the role of the inverse Faraday effect and the magnetic circular dichroism in stochastic all-optical switching. In this study, Cr was found to be a promising dopant, which can almost double the relative magnetization change of the partially magnetized states compared to pure Fe⁢Pt.

We explore the impact of optical excitation using two interfering ultrashort optical pulses on ultrafast magnetization dynamics. Our investigation focuses on Pt/Co/Pt multilayers and TbCo alloy samples, employing a dual pump approach. We observe significant variations in the dynamics of magnetization suppression and subsequent recovery when triggered with two optical pulses of the same polarization-essentially meeting conditions for interference. Conversely, dynamics triggered with cross-polarized pump beams exhibit expected similarity to that triggered with a single pulse. Delving into the underlying physical processes contributing to laser-induced demagnetization and recovery dynamics, we find that our current understanding cannot elucidate the observed trends. Consequently, we propose that optical excitation with interfering light possesses the capacity to induce long-lasting alterations in the dynamics of angular momentum.

We study two aspects of the superconductivity in a cuprate model system, its doping dependence and the influence of competing pairing mediators. We first include electron-phonon interactions beyond Migdal's approximation and solve self-consistently, as a function of doping and for an isotropic electron-phonon coupling, the full-bandwidth, anisotropic vertex-corrected Eliashberg equations under a non-interacting state approximation for the vertex correction. Our results show that such pairing interaction supports the experimentally observed d_x²-y² -wave symmetry of the superconducting gap, but only in a narrow doping interval of the hole-doped system. Depending on the coupling strength, we obtain realistic values for the gap magnitude and superconducting critical temperature T_c close to optimal doping, rendering the electron-phonon mechanism an important candidate for mediating superconductivity in this model system. Second, for a doping near optimal hole doping, we study multichannel superconductivity, by including both vertex-corrected electron-phonon interaction and spin and charge fluctuations as pairing mechanisms. We find that both mechanisms cooperate to support an unconventional d-wave symmetry of the order parameter, yet the electron-phonon interaction is mainly responsible for the Cooper pairing and high critical temperature T_c. Spin fluctuations are found to have a suppressing effect on the gap magnitude and critical temperature due to their repulsive interaction at small coupling wave vectors.

Light–matter interaction at the nanoscale in magnetic alloys and heterostructures is a topic of intense research in view of potential applications in high-density magnetic recording. While the element-specific dynamics of electron spins is directly accessible to resonant x-ray pulses with femtosecond time structure, the possible element-specific atomic motion remains largely unexplored. We use ultrafast electron diffraction (UED) to probe the temporal evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. The diffraction interference between Fe and Pt sublattices enables us to demonstrate that the Fe mean square vibration amplitudes are significantly larger that those of Pt as expected from their different atomic mass. Both are found to increase as energy is transferred from the laser-excited electrons to the lattice. Contrary to this intuitive behavior, we observe a laser-induced lattice expansion that is larger for Pt than for Fe atoms during the first picosecond after laser excitation. This effect points to the strain-wave driven lattice expansion with the longitudinal acoustic Pt motion dominating that of Fe.

The position operator in a Bloch representation acquires a gauge correction in the momentum space on top of the canonical position, which is called the anomalous position. We show that the anomalous position is generally orbital dependent and thus plays a crucial role in the description of the intrinsic orbital Hall effect in terms of Wannier basis. We demonstrate this from the first-principles calculation of orbital Hall conductivities of transition metals by Wannier interpolation. Our results show that consistent treatment of the velocity operator by including the correction term originating from the anomalous position predicts the orbital Hall conductivities different from those obtained by considering only the group velocity. We find the difference is crucial in several metals. For example, we predict negative signs of the orbital Hall conductivities for elements in groups X and XI such as Cu, Ag, Au, and Pd, for which the previous studies predicted positive signs. In this paper, we suggest the importance of consistently describing the spatial dependence of basis functions by first-principles methods, as it is fundamentally missing in the tight-binding approximation.