A comprehensive study of second harmonic generation on thermally oxidized MoS 2 flakes with the thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for nontreated and oxidized MoS 2 reveals that oxidation causes significant changes in the second harmonic response for all investigated structures. Excitation-power-dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of 6 h under the considered oxidation conditions. Here, polarization-dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the topmost S layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two-dimensional MoS 2 can be monitored with nonresonant and noninvasive SH microscopy by following the distinct fingerprints of structural modification in the nonlinear response.

A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.

Alloying and doping are crucial for enhancing the electronic and optical properties of semiconductors while simultaneously introducing disorder. This report explores the effects of alloying and Si (0.5 at.%) doping on In_0.10Ga_0.90N thin films that were grown by metal-organic vapor phase epitaxy. Post-growth X-ray diffraction measurements indicate that Si doping does not affect the lattice parameters and screw dislocations but significantly increases the edge dislocation density. Temperature-dependent time-resolved photoluminescence spectroscopy shows that Si-doped In_0.10Ga_0.90N exhibits higher photoluminescence intensity, blue-shifted peaks, narrower emission linewidths, and quenching of lower energy sidebands when compared to pristine In_0.10Ga_0.90N. The peak energies of the most dominant feature, the donor-bound exciton, for both samples show an S-shape behavior indicating the presence of disorder. Although doping improves luminescence, it also introduces deeper localized states. This suggests that impurity-induced disorder outweighs compositional fluctuations, as confirmed by higher disorder parameters and Stokes shifts. Thus, the Si doping leads to increased localization, reducing nonradiative recombination channels while enhancing radiative processes. The deeper states in the doped sample confirm improved carrier confinement, and their saturation leads to early thermalization, thereby lowering the red-blue shift transition from 165 K to about 50 K. Even though the high doping level makes Si-doped In_0.10Ga_0.90N a degenerate system, it exhibits enhanced luminescence properties. These findings shed light on the impact of silicon doping on charge transport in InGaN alloys for optoelectronic applications.

High magnetic field and low temperature transport measurements are carried out in order to gain insight into the properties of the charge carriers of PtSe 2 . In particular, the Shubnikov–de Haas oscillations arising at applied magnetic field strengths ≳ 4.5 T are found to occur exclusively in plane. An analysis of the oscillations via the Lifshitz-Kosevich formalism allows determining the charge carrier's cyclotron mass, quantum transport time, Berry phase, Fermi surface cross section, and Dingle temperature. The oscillations emerge at a layer thickness of ≈ 18 nm and decrease in frequency for thinner PtSe 2 flakes. Further, weak antilocalization (WAL) is observed despite the presence of magnetic moments from Pt vacancies, which typically inhibit such effects. An explanation is provided on how WAL and the Kondo effect can be observed within the same material.

High magnetic field and low temperature transport is carried out in order to characterize the charge carriers of PtSe_2. In particular, the Shubnikov-de Haas oscillations arising at applied magnetic field strengths ≳ 4.5 T are found to occur exclusively in plane and emerge at a layer thickness of ≈ 18 nm, increasing in amplitude and decreasing in frequency for thinner PtSe_2 flakes. Moreover, the quantum transport time, Berry phase, Dingle temperature and cyclotron mass of the charge carriers are ascertained. The emergence of weak antilocalization (WAL) lies in contrast to the presence of magnetic moments from Pt vacancies. An explanation is provided on how WAL and the Kondo effect can be observed within the same material. Detailed information about the charge carriers and transport phenomena in PtSe_2 is obtained, which is relevant for the design of prospective spintronic and orbitronic devices and for the realization of orbital Hall effect-based architectures.

The emergence of negative longitudinal magnetoresistance in the topologically nontrivial transition-metal dichalcogenide PtSe2 is studied. Low-T, high-mu 0H transport is performed for arbitrary field directions, and an analytical framework is established. The source of the negative longitudinal magnetoresistance is identified as the Kondo effect stemming from Pt vacancies contributing an uncompensated spin exclusively at the sample surface. The concentration of vacancies and the sample thickness are identified as tuning parameters. The findings are substantiated by density functional theory, which corroborates the proposed Pt-vacancy model.

The exchange bias phenomenon, inherent in exchange-coupled ferromagnetic and antiferromagnetic systems, has intrigued researchers for decades. Van der Waals materials, with their layered structures, offer an ideal platform for exploring exchange bias. However, effectively manipulating exchange bias in van der Waals heterostructures remains challenging. This study investigates the origin of exchange bias in MnPS3/Fe3GeTe2 van der Waals heterostructures, demonstrating a method to modulate nearly 1000% variation in magnitude through simple thermal cycling. Despite the compensated interfacial spin configuration of MnPS3, a substantial 170 mT exchange bias is observed at 5 K, one of the largest observed in van der Waals heterostructures. This significant exchange bias is linked to anomalous weak ferromagnetic ordering in MnPS3 below 40 K. The tunability of exchange bias during thermal cycling is attributed to the amorphization and changes in the van der Waals gap during field cooling. The findings highlight a robust and adjustable exchange bias in van der Waals heterostructures, presenting a straightforward method to enhance other interface-related spintronic phenomena for practical applications. Detailed interface analysis reveals atom migration between layers, forming amorphous regions on either side of the van der Waals gap, emphasizing the importance of precise interface characterization in these heterostructures.

We report on time-resolved optical and terahertz ultrafast spectroscopy of charge-carrier dynamics in the room-temperature antiferromagnetic semiconductor α-MnTe. By optically pumping the system with 1.55 eV photons at room temperature, we excite charge carriers in the conduction band through the indirect band gap and investigate the dynamical response of nonequilibrium states using optical as well as terahertz transmission probes. Three relaxation processes are revealed by their characteristic relaxation times of the order of 1, 10, and 100 ps, whose exact values are functions of the pump fluence. For high pump fluences nonlinear dependence on the pump fluence is observed both in the optical and terahertz probes.

In this study, we investigate the local structure of aluminum (Al) in a comprehensive series of AlxGa1−xN epilayers, where the Al concentration spans from the dilute limit to 100%. We analyze grazing incidence Al K-edge tender x-ray absorption spectroscopy data using both linear combination fitting based on reference limit spectra and full quantitative analysis. The results indicate deviations from random cation distribution with varying signs within the explored concentration range. Additionally, we observe a reduction in cationic interatomic distances over a wide concentration range, which contrasts with some previous studies conducted at hard x-ray absorption edges.

Charge states and lattice sites of Fe ions in virgin and Mn-doped Al x Ga1−x N samples were investigated using 57Fe emission Mössbauer spectroscopy following radioactive 57Mn+ ion implantation at ISOLDE, CERN. In the undoped Al x Ga1−x N, Fe2+ on Al/Ga sites associated with nitrogen vacancies and Fe3+ on substitutional Al/Ga sites are identified. With Mn doping, the contribution of Fe3+ is considerably reduced and replaced instead by a corresponding emergence of a single-line-like component consistent with Fe4+ on Al/Ga sites. Density functional theory calculations confirm the Fe4+ charge state as stabilised by the presence of substitutional Mn2+ in its vicinity. The completely filled spin up orbitals in Mn2+ (3d5) are expected to enhance magnetic exchange interactions. The population of the Fe4+ state is less pronounced at high Al concentration in Al x Ga1−x N:Mn, a behaviour attributable to hybridisation effects of 3d states to the semiconductor bands which weakens with increasing (decreasing) Al (Ga) content. Our results demonstrate that co-doping promotes the co-existence of unusual charge states of Fe4+ and Mn2+, whereas their trivalent charge states prevail with either transition metal incorporated independently in III-nitrides. Co-doping thus opens up a new avenue for tailoring novel magnetic properties in doped semiconductors.

A reentrant temperature dependence of the thermoresistivity ρxx(T) between an onset local superconducting ordering temperature Tloconset and a global superconducting transition at T=Tglooffset has been reported in disordered conventional 3-dimensional (3D) superconductors. The disorder of these superconductors is a result of either an extrinsic granularity due to grain boundaries, or of an intrinsic granularity ascribable to the electronic disorder originating from impurity dopants. Here, the effects of Fe doping on the electronic properties of sputtered NbN layers with a nominal thickness of 100 nm are studied by means of low-T/high-μ0H magnetotransport measurements. The doping of NbN is achieved via implantation of 35 keV Fe ions. In the as-grown NbN films, a local onset of superconductivity at Tloconset=15.72K is found, while the global superconducting ordering is achieved at Tglooffset=15.05K, with a normal state resistivity ρxx=22μΩ·cm. Moreover, upon Fe doping of NbN, ρxx=40μΩ·cm is estimated, while Tloconset and Tglooffset are measured to be 15.1 K and 13.5 K, respectively. In Fe:NbN, the intrinsic granularity leads to the emergence of a bosonic insulator state and the normal-metal-to-superconductor transition is accompanied by six different electronic phases characterized by a N-shaped T dependence of ρxx(T). The bosonic insulator state in a s-wave conventional superconductor doped with dilute magnetic impurities is predicted to represent a workbench for emergent phenomena, such as gapless superconductivity, triplet Cooper pairings and topological odd frequency superconductivity.

Van der Waals α-MoO3 samples offer a wide range of attractive catalytic, electronic, and optical properties. We present herein an emission Mössbauer spectroscopy (eMS) study of the electric-field gradient (EFG) anisotropy in crystalline free-standing α-MoO3 samples. Although α-MoO3 is a two-dimensional (2D) material, scanning electron microscopy shows that the crystals are 0.5–5-µm thick. The combination of X-ray diffraction and micro-Raman spectroscopy, performed after sample preparation, provided evidence of the phase purity and crystal quality of the samples. The eMS measurements were conducted following the implantation of 57Mn (t1/2 = 1.5 min), which decays to the 57Fe, 14.4 keV Mössbauer state. The eMS spectra of the samples are dominated by a paramagnetic doublet (D1) with an angular dependence, pointing to the Fe2+ probe ions being in a crystalline environment. It is attributed to an asymmetric EFG at the eMS probe site originating from strong in-plane covalent bonds and weak out-of-plane van der Waals interactions in the 2D material. Moreover, a second broad component, D2, can be assigned to Fe3+ defects that are dynamically generated during the online measurements. The results are compared to ab initio simulations and are discussed in terms of the in-plane and out-of-plane interactions in the system.

A coherent THz optical lattice mode is triggered by femtosecond laser pulses in the antiferromagnetic van der Waals semiconductor FePS$_3$. The 380 nm thick exfoliated flake was placed on a substrate and laser-driven lattice and spin dynamics were investigated as a function of the excitation photon energy and sample temperature. The pump-probe spectroscopic measurements reveal that the photo-induced phonon is generated by a displacive mechanism. The amplitude of the phononic signal decreases as the sample is heated up to the N\'eel temperature and vanishes as the phase transition to the paramagnetic phase occurs. This evidence confirms that the excited lattice mode is intimately connected to the long-range magnetic order. Therefore our work discloses a pathway towards a femtosecond coherent manipulation of the magneto-crystalline anisotropy in a van der Waals antiferromagnet. In fact, it is reported that by applying a magnetic field the induced phonon mode hybridizes via the Kittel-mechanism with zone-centre magnons.

In the Fe-doped GaN phase-separated magnetic semiconductor Ga δ FeN, the presence of embedded γ ' - Ga_y Fe_4-y N nanocrystals determines the magnetic properties of the system. Here, through a combination of anomalous X-ray diffraction and diffraction anomalous fine structure, the local structure of Ga in self-assembled face-centered cubic (fcc) γ ' - Ga_y Fe_4-y N nanocrystals embedded in wurtzite GaN thin layers is investigated in order to shed light onto the correlation between fabrication parameters, local structural arrangement and overall magnetic properties of the material system. It is found, that by adjusting the growth parameters and thus, the crystallographic surroundings, the Ga atoms can be induced to incorporate into 3 c positions at the faces of the fcc crystal lattice, reaching a maximum occupancy of 30

In the Fe-doped GaN phase-separated magnetic semiconductor Ga $$\delta $$ FeN, the presence of embedded $$\gamma '$$ - $$\hbox {Ga}_y \hbox {Fe}_{4-y}$$ N nanocrystals determines the magnetic properties of the system. Here, through a combination of anomalous X-ray diffraction and diffraction anomalous fine structure, the local structure of Ga in self-assembled face-centered cubic (fcc) $$\gamma '$$ - $$\hbox {Ga}_y \hbox {Fe}_{4-y}$$ N nanocrystals embedded in wurtzite GaN thin layers is investigated in order to shed light onto the correlation between fabrication parameters, local structural arrangement and overall magnetic properties of the material system. It is found, that by adjusting the growth parameters and thus, the crystallographic surroundings, the Ga atoms can be induced to incorporate into 3c positions at the faces of the fcc crystal lattice, reaching a maximum occupancy of 30%. The magnetic response of the embedded nanocrystals is ferromagnetic with Curie temperature increasing from 450 to 500 K with the Ga occupation. These results demonstrate the outstanding potential of the employed experimental protocol for unravelling the local structure of magnetic multi-phase systems, even when embedded in a matrix containing the same element under investigation.

Heterostructures consisting of alternating GaN/AlN epitaxial layers represent the building-blocks of state-of-the-art devices employed for active cooling and energy-saving lightning. Insights into the heat conduction of these structures are essential in the perspective of improving the heat management for prospective applications. Here, the cross-plane (perpendicular to the sample's surface) thermal conductivity of GaN/AlN superlattices as a function of the layers' thickness is established by employing the 3ω-method. Moreover, the role of interdiffusion at the interfaces on the phonon scattering is taken into account in the modelling and data treatment. It is found, that the cross-plane thermal conductivity of the epitaxial heterostructures can be driven to values as low as 5.9 W/(m·K) comparable with those reported for amorphous films, thus opening wide perspectives for optimized heat management in III-nitride-based epitaxial multilayers.

Research on ultrathin quantum materials requires full control of the growth and surface quality of the specimens in order to perform experiments on their atomic structure and electron states leading to ultimate analysis of their intrinsic properties. We report results on epitaxial FeSe thin films grown by pulsed laser deposition (PLD) on CaF2 (001) substrates as obtained by exploiting the advantages of an all-in-situ ultra-high vacuum (UHV) laboratory allowing for direct high-resolution surface analysis by scanning tunnelling microscopy (STM), synchrotron radiation X-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES) on fresh surfaces. FeSe PLD growth protocols were fine-tuned by optimizing target-to-substrate distance d and ablation frequency, atomically flat terraces with unit-cell step heights are obtained, overcoming the spiral morphology often observed by others. In-situ ARPES with linearly polarized horizontal and vertical radiation shows hole-like and electron-like pockets at the Γ and M points of the Fermi surface, consistent with previous observations on cleaved single crystal surfaces. The control achieved in growing quantum materials with volatile elements such as Se by in-situ PLD makes it possible to address the fine analysis of the surfaces by in-situ ARPES and XPS. The study opens wide avenues for the PLD based heterostructures as work-bench for the understanding of proximity-driven effects and for the development of prospective devices based on combinations of quantum materials.

γ ′‐GayFe4−y N nanocrystals determines the magnetic properties of the system. Here, through a combination of anomalous X‐ray diffraction and diffraction anomalous fine structure, the local structure of Ga in self‐assembled face‐centered cubic (fcc) γ ′‐GayFe4−y N nanocrystals embedded in wurtzite GaN thin layers is investigated in order to shed light onto the correlation between fabrication parameters, local structural arrangement and overall magnetic properties of the material system. It is found, that by adjusting the growth parameters and thus, the crystallographic surroundings, the Ga atoms can be induced to incorporate into 3c positions at the faces of the fcc crystal lattice, reaching a maximum occupancy of 30%. The magnetic response of the embedded nanocrystals is ferromagnetic with Curie temperature increasing from 450 to 500 K with the Ga occupation. These results demonstrate the outstanding potential of the employed experimental protocol for unravelling the local structure of magnetic multi‐phase systems, even when embedded in a matrix containing the same element under investigation.

In the Fe-doped GaN phase-separated magnetic semiconductor Ga $$\delta $$ FeN, the presence of embedded $$\gamma '$$ - $$\hbox {Ga}_y \hbox {Fe}_{4-y}$$ N nanocrystals determines the magnetic properties of the system. Here, through a combination of anomalous X-ray diffraction and diffraction anomalous fine structure, the local structure of Ga in self-assembled face-centered cubic (fcc) $$\gamma '$$ - $$\hbox {Ga}_y \hbox {Fe}_{4-y}$$ N nanocrystals embedded in wurtzite GaN thin layers is investigated in order to shed light onto the correlation between fabrication parameters, local structural arrangement and overall magnetic properties of the material system. It is found, that by adjusting the growth parameters and thus, the crystallographic surroundings, the Ga atoms can be induced to incorporate into 3c positions at the faces of the fcc crystal lattice, reaching a maximum occupancy of 30%. The magnetic response of the embedded nanocrystals is ferromagnetic with Curie temperature increasing from 450 to 500 K with the Ga occupation. These results demonstrate the outstanding potential of the employed experimental protocol for unravelling the local structure of magnetic multi-phase systems, even when embedded in a matrix containing the same element under investigation.