arXiv subjects

Christoph Schlueter

Publications and source records attributed to Christoph Schlueter.

At least 19 recordsLinked to original sources

arXiv2026

Charge transfer and competing symmetry breaking drive orbital reconstruction and emergent ferromagnetism in insulating oxide superlattices

Electron correlation, hopping, and ligand-to-metal charge transfer collectively lead to diverse electronic and magnetic phenomena in 3$d$ transition-metal oxides, where directional d orbitals make hopping highly sensitive to symmetry-dependent orbital overlap. Heterostructure engineering with atomically flat interfaces adds symmetry-breaking charge transfer as a further route to emergent behavior, yet whether interfacial mismatch between constituent oxides of a superlattice shapes ground states independent of epitaxial strain remains unresolved. Here we examine superlattices combining NdNiO$_3$ with Mott-insulating NdMnO$_3$. Varying layer thickness and combining transport with X-ray spectroscopy, we show that electron transfer from NdMnO$_3$ to NdNiO$_3$ drives a room-temperature insulating state with a distinct electronic structure, accompanied by a reversal in orbital symmetry beyond simple strain considerations, underscoring the interface's central role. These reconstructions stabilize an emergent ferromagnetic insulating state arising from interfacial Ni$^{2+}$-O-Mn$^{4+}$ superexchange. Our results establish a pathway to interface-engineered ferromagnetic insulating phases via competing interactions, with potential for spin-insulatronic applications.

cond-mat.mtrl-sci↗

arXiv2026

Lifetime effects and satellites in the photoelectron spectrum of platinum metal

This work presents a comprehensive investigation of the electronic structure and many-body photoemission effects in metallic platinum using reflection high-energy electron energy-loss spec- troscopy (RHEELS), soft X-ray photoelectron spectroscopy (SXPS), and hard X-ray photoelectron spectroscopy (HAXPES), supported by ab initio calculations. Shallow and deep core state spectra enable the systematic characterisation of intrinsic line-shape asymmetries and satellite structures. Correlation of photoelectron satellites with RHEELS loss features allows the assignment of inter- band transitions, surface and bulk plasmons, plasmonic overtones, and semi-core ionisation losses across the Pt spectrum. Several previously unresolved satellite features and spin-orbit splittings are identified and discussed. Comparison of experimental valence band spectra with orbital-projected densities of states calculated using ab initio density functional theory (DFT) and G0W0 approaches, with and without spin-orbit coupling, demonstrates the critical role of relativistic effects in reproducing the Pt valence electronic structure. Together, these results establish a unified, internally consistent spectroscopic reference for metallic platinum, providing a robust framework for interpreting photoelectron spectra of Pt-containing catalysts, electronic materials, and related 5d transition metal systems.

cond-mat.mtrl-sci↗

arXiv2026

From Film to Flakes: Electronic Properties and Magnetization Variations in Yttrium Iron Garnet

Yttrium iron garnet (YIG) is a ferrimagnetic insulator valued for its high Curie temperature, very low magnetic damping, and ability to support long-range spin-wave transport. These qualities have established it as a cornerstone material in the field of spintronics and magnonics. Most studies on YIG so far have been focused on bulk crystals, thin films, and nanoparticles, including variants with substitutions at the yttrium or iron site. New morphologies such as sub-micron flakes have drawn interest recently as their geometry and mechanical flexibility might enable different device architectures. However, detailed investigations combining their electronic structure and magnetic behavior remain scarce. In this work, we present a comparative study of the electronic and magnetic properties of a bulk-like YIG film and a sub-micron-sized YIG flake. Our results highlight the distinct behavior that emerges in sub-micron dimensions and point toward future uses for flake-based YIG in compact spintronics devices.

cond-mat.mtrl-sci↗

arXiv2025

Generic Rolling Access to Synchrotron Radiation Facilities

A generic model for rolling access to synchrotron radiation experiments is presented, which has the capacity to replace call-based access models. Proposal submission, evaluation and scheduling are all executed in a rolling fashion. This significantly reduces the waiting times between proposal submission and experiment execution than that of the call-based access model. The generic rolling access model is in principle applicable to any beamlines, regardless of the number of experimental methods or setups it provides to users. This access model is flexible and could provide faster access compared to call-based access model as well as accommodate experiments and projects requiring extended preparation times.

physics.acc-ph↗

arXiv2025

Ge$_{1-x}$Si$_{x}$ single crystals for Ge hole spin qubit integration

Spin qubits are fundamental building blocks of modern quantum computing devices. The path of Ge-based hole-spin qubits has several advantages over Si-based electron-spin systems, such as the absence of valley band degeneracy, the possibility of efficient field control due to large spin-orbit coupling, and smaller effective masses. Among the possible Ge qubit devices, Ge/GeSi planar heterostructures have proven to be favourable for upscaling and fabrication. The Si concentration of the straining GeSi buffer serves as an important tuning parameter for the electronic structure of Ge/GeSi qubits. A particularly low Si concentration of x = 0.15 of the Ge$_{0.85}$Si$_{0.15}$ crystal should enable minimal lattice strain for spin qubit heterostructures, which is difficult to stabilize as a random alloy. We present a synchrotron-based study to investigate the chemical composition, valence band electronic structure and local atomic structure of a Ge$_{0.85}$Si$_{0.15}$ single crystal using the advanced combination of hard X-ray photoelectron spectroscopy (HAXPES), hard X-ray momentum microscopy (HarMoMic) and X-ray photoelectron diffraction (XPD). We found that the Ge$_{0.85}$Si$_{0.15}$ crystal has an individual, uniform valence band structure, with no signs of phase separation. The shapes of the valence bands resemble those of pure Ge, as do the low effective masses. XPD experiments and Bloch wave calculations, show the Si atoms located at Ge lattice sites within the crystal, forming a random alloy. This high chemical, electronic and structural quality of Ge$_{0.85}$Si$_{0.15}$ single-crystal substrates is of crucial importance for their implementation to enable long spin lifetimes in Ge-based hole-spin qubits. The results emphasise the power of combined X-ray spectromicroscopy techniques, which provide key insights into the qubit building blocks that form the basis of quantum technologies.

cond-mat.mtrl-sci↗

arXiv2025

Beyond being free: glassy dynamics of SrTiO$_3$-based two-dimensional electron gas

Electron glasses offer a convenient laboratory platform to study glassy dynamics. Traditionally, the interplay between long-range Coulomb interactions and disorder is deemed instrumental in stabilizing the electron glass phase. Existing experimental studies on electron glass have focused on doped semiconductors, strongly correlated systems, granular systems, etc., all of which are far from the well-delocalized limit. In this work, we expand the study of electron glasses to a well-known quantum paraelectric SrTiO$_3$ (STO) and unveil a new scenario: how naturally occurring ferroelastic twin walls of STO could result in glassy electrons, even in a metallic state. We show that the emergent two-dimensional electron gas at the $\gamma$-Al$_2$O$_3$/STO interface exhibits long-lasting temporal relaxations in resistance and memory effects at low temperatures, which are hallmarks of glassiness. We also demonstrate that the glass-like relaxations could be further tuned by application of an electric field. This implies that the observed glassy dynamics is connected with the development of polarity near the structural twin walls of STO and the complex interactions among them, arising from the coupling between ferroelastic and ferroelectric orders. The observation of this glassy metal phase not only extends the concept of electron glasses to metallic systems with multiple order parameters but also contributes to the growing understanding of the fascinating and diverse physical phenomena that emerge near the quantum critical point.

cond-mat.str-el↗

arXiv2024

Long-term stability and oxidation of ferroelectric AlScN devices: An operando HAXPES study

Aluminum scandium nitride (Al$_{1-x}$Sc$_x$N) is a promising material for ferroelectric devices due to its large remanent polarization, scalability, and compatibility with semiconductor technology. By doping AlN with Sc, the bonds in the polar AlN structure are weakened, which enables ferroelectric switching below the dielectric breakdown field. However, one disadvantage of Sc doping is that it increases the material's tendency towards oxidation. In the present study, the oxidation process of tungsten-capped and uncapped Al$_{0.83}$Sc$_{0.17}$N thin films is investigated by hard X-ray photoelectron spectroscopy (HAXPES). The samples had been exposed to air for either two weeks or 6 months. HAXPES spectra indicate the replacement of nitrogen by oxygen, and the tendency of oxygen to favor oxidation with Sc rather than Al. The appearance of an N$_2$ spectral feature thus can be directly related to the oxidation process. We present an oxidation model that mimics these spectroscopic results of the element-specific oxidation processes within Al$_{1-x}$Sc$_x$N. Finally, in operando HAXPES data of uncapped and capped AlScN-capacitor stacks are interpreted using the proposed model.

cond-mat.mtrl-sci↗

arXiv2024

Enabling two-dimensional electron gas with high room-temperature electron mobility exceeding 100 cm$^2$/Vs at a perovskite oxide interface

In perovskite oxide heterostructures, bulk functional properties coexist with emergent physical phenomena at epitaxial interfaces. Notably, charge transfer at the interface between two insulating oxide layers can lead to the formation of a two-dimensional electron gas (2DEG) with possible applications in, e.g., high-electronmobility transistors and ferroelectric field-effect transistors. So far, the realization of oxide 2DEGs is, however, largely limited to the interface between the single-crystal substrate and epitaxial film, preventing their deliberate placement inside a larger device architecture. Additionally, the substrate-limited quality of perovskite oxide interfaces hampers room-temperature 2DEG performance due to notoriously low electron mobility. In this work, we demonstrate the controlled creation of an interfacial 2DEG at the epitaxial interface between perovskite oxides BaSnO$_3$ and LaInO$_3$ with enhanced room-temperature electron mobilities up to 119 cm$^2$/Vs - the highest room-temperature value reported so far for a perovskite oxide 2DEG. Using a combination of state-of-the-art deposition modes during oxide molecular beam epitaxy, our approach opens up another degree of freedom in optimization and $in$-$situ$ control of the interface between two epitaxial oxide layers away from the substrate interface. We thus expect our approach to apply to the general class of perovskite oxide 2DEG systems and to enable their improved compatibility with novel device concepts and integration across materials platforms.

cond-mat.mes-hall↗

arXiv2024

2D synthetic ferrimagnets by magnetic proximity coupling

Proximity effects allow for the adjustment of magnetic properties in a physically elegant way. If two thin ferromagnetic (FM) films are brought into contact, electronic coupling alters their magnetic exchange interaction at their interface. For a low-TC rare-earth FM coupled to a 3d transition metal FM, even room temperature magnetism is within reach. In addition, magnetic proximity coupling is particularly promising for increasing the magnetic order of metastable materials such as europium monoxide (EuO) beyond their bulk TC, since neither the stoichiometry nor the insulating properties are modified. We investigate the magnetic proximity effect at Fe/EuO and Co/EuO interfaces using hard X-ray photoelectron spectroscopy. By exciting the FM layers with circularly polarized light, magnetic dichroism is observed in angular dependence on the photoemission geometry. In this way, the depth-dependence of the magnetic signal is determined element-specifically for the EuO and 3d FM parts of the bilayers. In connection with atomistic spin dynamics simulations, the thickness of EuO layer is found to be crucial, indicating that the observed antiferromagnetic proximity coupling is a short-ranged and genuine interface phenomenon. This fact turns the bilayer into a strong synthetic ferrimagnet. The increase in magnetic order in EuO occurs in a finite spatial range and is therefore particularly strong in the 2D limit-a counterintuitive but very useful phenomenon for spin-based device applications.

cond-mat.mtrl-sci↗

arXiv2023

Active Sites of Te-hyperdoped Silicon by Hard X-ray Photoelectron Spectroscopy

Multiple dopant configurations of Te impurities in close vicinity in silicon are investigated using photoelectron spectroscopy, photoelectron diffraction, and Bloch wave calculations. The samples are prepared by ion implantation followed by pulsed laser annealing. The dopant concentration is variable and high above the solubility limit of Te in silicon. The configurations in question are distinguished from isolated Te impurities by a strong chemical core level shift. While Te clusters are found to form only in very small concentrations, multi-Te configurations of type dimer or up to four Te ions surrounding a vacancy are clearly identified. For these configurations a substitutional site location of Te is found to match the data best in all cases. For isolated Te ions this matches the expectations. For multi-Te configurations the results contribute to understanding the exceptional activation of free charge carriers in hyperdoping of chalcogens in silicon.

cond-mat.mtrl-sci↗

arXiv2023

Revealing the bonding nature and electronic structure of early transition metal dihydrides

Hydrogen as a fuel plays a crucial role in driving the transition to net zero greenhouse gas emissions. To realise its potential, obtaining a means of efficient storage is paramount. One solution is using metal hydrides, owing to their good thermodynamical absorption properties and effective hydrogen storage. Although metal hydrides appear simple compared to many other energy materials, understanding the electronic structure and chemical environment of hydrogen within them remains a key challenge. This work presents a new analytical pathway to explore these aspects in technologically relevant systems using Hard X-ray Photoelectron Spectroscopy (HAXPES) on thin films of two prototypical metal dihydrides: YH$_{2-δ}$ and TiH$_{2-δ}$. By taking advantage of the tunability of synchrotron radiation, a non-destructive depth profile of the chemical states is obtained using core level spectra. Combining experimental valence band spectra collected at varying photon energies with theoretical insights from density functional theory (DFT) calculations, a description of the bonding nature and the role of d versus sp contributions to states near the Fermi energy are provided. Moreover, a reliable determination of the enthalpy of formation is proposed by using experimental values of the energy position of metal s band features close to the Fermi energy in the HAXPES valence band spectra.

cond-mat.mtrl-sci↗

arXiv2023

Experimental benchmark data for Monte Carlo simulated radiation effects of gold nanoparticles. Part I: Experiment and raw data analysis

Electron emission spectra of gold nanoparticles (AuNPs) after photon interaction were measured over the energy range between 50 eV and 9500 eV to provide reference data for Monte Carlo radiation-transport simulations. Experiments were performed with the HAXPES spectrometer at the PETRA III high-brilliance beamline P22 at DESY (Hamburg, Germany) for photon energies below and above each of the gold L-edges, i.e., at 11.9 keV, 12.0 keV, 13.7 keV, 13.8 keV, 14.3 keV, and 14.4 keV. The study focused on a sample with gold nanoparticles with an average diameter of 11.0 nm on a thin carbon foil. Additional measurements were performed on a sample with 5.3 nm gold nanoparticles and on reference samples of gold and carbon foils. Further measurements were made to calibrate the photon flux monitor, to characterize the transmission function of the electron spectrometer and to determine the size of the photon beam. This allowed the determination of the absolute values of the spectral particle radiance of secondary electrons per incident photon flux. The paper presents the experimental and raw data analysis procedures, reviews the data obtained for the nanoparticle samples and discusses their limitations.

physics.med-ph↗

arXiv2023

Modulation-Doping a Correlated Electron Insulator

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard x-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5x10^21 cm^(-3) without inducing any measurable structural changes. We find that the MIT temperature (T_MIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions.

cond-mat.mtrl-sci↗

arXiv2022

Hard x-ray angle-resolved photoemission from a buried high-mobility electron system

Novel two-dimensional electron systems at the interfaces and surfaces of transition-metal oxides recently have attracted much attention as they display tunable, intriguing properties that can be exploited in future electronic devices. Here we show that a high-mobility quasi-two-dimensional electron system with strong spin-orbit coupling can be induced at the surface of a KTaO$_3$ (001) crystal by pulsed laser deposition of a disordered LaAlO$_3$ film. The momentum-resolved electronic structure of the buried electron system is mapped out by hard x-ray angle-resolved photoelectron spectroscopy. From a comparison to calculations it is found that the band structure deviates from that of electron-doped bulk KTaO$_3$ due to the confinement to the interface. Nevertheless, the Fermi surface appears to be clearly three-dimensional. From the $k$ broadening of the Fermi surface and core-level depth profiling we estimate the extension of the electron system to be at least 1 nm but not much larger than 2 nm, respectively.

cond-mat.str-el↗

arXiv2022

Tackling Disorder in $γ$-Ga$_2$O$_3$

Ga$_2$O$_3$ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga$_2$O$_3$ offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. $γ$-Ga$_2$O$_3$ presents a particular challenge across synthesis, characterisation, and theory due to its inherent disorder and resulting complex structure -- electronic structure relationship. Here, density functional theory is used in combination with a machine learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the $γ$-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of $γ$-Ga$_2$O$_3$. The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by PLE spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step towards exploring how their electronic structure can be understood in terms of local coordination and overall structure.

cond-mat.mtrl-sci↗

arXiv2021

Time-resolved diffraction and photoelectron spectroscopy investigation of the reactive molecular beam epitaxy of $\mathrm{Fe_3O_4}$ ultrathin films

We present time-resolved high energy x-ray diffraction (tr-HEXRD), time-resolved hard x-ray photoelectron spectroscopy (tr-HAXPES) and time-resolved grazing incidence small angle x-ray scattering (tr-GISAXS) data of the reactive molecular beam epitaxy (RMBE) of $\mathrm{Fe_3O_4}$ ultrathin films on various substrates. Reciprocal space maps are recorded during the deposition of $\mathrm{Fe_3O_4}$ on $\mathrm{SrTiO_3(001)}$, MgO(001) and NiO/MgO(001) in order to observe the temporal evolution of Bragg reflections sensitive to the octahedral and tetrahedral sublattices of the inverse spinel structure of $\mathrm{Fe_3O_4}$. A time delay between the appearance of rock salt and spinel-exclusive reflections reveals that first, the iron oxide film grows with $\mathrm{Fe_{1-δ}O}$ rock salt structure with exclusive occupation of octahedral lattice sites. When this film is 1.1$\,$nm thick, the further growth of the iron oxide film proceeds in the inverse spinel structure, with both octahedral and tetrahedral lattice sites being occupied. In addition, iron oxide on $\mathrm{SrTiO_3(001)}$ initially grows with none of these structures. Here, the formation of the rock salt structure starts when the film is 1.5$\,$nm thick. This is confirmed by tr-HAXPES data obtained during growth of iron oxide on $\mathrm{SrTiO_3(001)}$, which demonstrate an excess of $\mathrm{Fe^{2+}}$ cations in growing films thinner than 3.2$\,$nm. This rock salt phase only appears during growth and vanishes after the supply of the Fe molecular beam is stopped. Thus, it can be concluded the rock salt structure of the interlayer is a property of the dynamic growth process. The tr-GISAXS data link these structural results to an island growth mode of the first 2-3$\,$nm on both MgO(001) and $\mathrm{SrTiO_3(001)}$ substrates.

cond-mat.mes-hall↗

arXiv2020

The Breakdown of Mott Physics at VO$_2$ Surfaces

Transition metal oxides such as vanadium dioxide (VO$_2$), niobium dioxide (NbO$_2$), and titanium sesquioxide (Ti$_2$O$_3$) are known to undergo a temperature-dependent metal-insulator transition (MIT) in conjunction with a structural transition within their bulk. However, it is not typically discussed how breaking crystal symmetry via surface termination affects the complicated MIT physics. Using synchrotron-based x-ray spectroscopy, low energy electron diffraction (LEED), low energy electron microscopy (LEEM), transmission electron microscopy (TEM), and several other experimental techniques, we show that suppression of the bulk structural transition is a common feature at VO$_2$ surfaces. Our density functional theory (DFT) calculations further suggest that this is due to inherent reconstructions necessary to stabilize the surface, which deviate the electronic structure away from the bulk d$^1$ configuration. Our findings have broader ramifications not only for the characterization of other "Mott-like" MITs, but also for any potential device applications of such materials.

cond-mat.str-el↗

arXiv2020

Impact of Fe substitution on the electronic structure of URu$_2$Si$_2$

The application of pressure as well as the successive substitution of Ru with Fe in the hidden order (HO) compound URu$_2$Si$_2$ leads to the formation of the large moment antiferromagnetic phase (LMAFM). Here we have investigated the substitution series URu$_{2-x}$Fe$_x$Si$_2$ with $x$ = 0.2 and 0.3 with non-resonant inelastic x-ray scattering (NIXS) and 4$f$ core-level photoelectron spectroscopy with hard x-rays (HAXPES). NIXS shows that the substitution of Fe has no impact on the symmetry of the ground-state wave function. In HAXPES we find no shift of spectral weight that would be indicative for a change of the 5$f$-electron count. Consequently, changes in the exchange interaction $\cal{J}$ due to substitution must be minor so that the conjecture of chemical pressure seems unlikely. An alternative scenario is discussed, namely the formation of long range magnetic order due the substitution induced local enhancement of the magnetization in the vicinity of the $f$-electron ions while the overall electronic structure remains unchanged.

cond-mat.str-el↗