Faculty of Physics: Research Data
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- Research DataSingle crystal growth and properties of Au- and Ge-substituted EuPd2Si20026-02-12We report on the single crystal growth of Eu(Pd1−xAux)2Si2, 0 < x ≤ 0.2, from a levitating Eu-rich melt using the Czochralski method. Our structural analysis of the samples confirms the ThCr2Si2-type structure as well as an increase of the room temperature a and c lattice parameters with increasing x. Chemical analysis reveals that, depending on the Au concentration, only about 25-35% of the amount of Au available in the initial melt is incorporated into the crystal structure, resulting in a decreasing substitution level for increasing x. Through Au substitution, chemical pressure is applied and large changes in valence crossover temperatures are already observed for low substitution levels x. In contrast to previous studies, we do not find any signs of a first-order transition in samples with xnom = 0.1 or AFM order for higher x. Furthermore, we observe the formation of quarternary side phases for a higher amount of Au in the melt. In addition, cubic-mm-sized single crystals of EuPd2(Si1−xGex)2 with xnom = 0.2 were grown. The analysis of the X-ray fluorescence revealed that the crystals exhibit a slight variation in the Ge content. Such tiny compositional changes can cause changes in the sample properties concerning variations of the crossover temperature or changes of the type of the transition from crossover to magnetic order. Furthermore, we report on a new orthorhombic phase EuPd1.42Si1.27Ge0.31 that orders antiferromagnetically below 17 K.
18 11 - Research DataNonlinear Spectroscopy as a Magnon Breakdown Diagnosis and its Efficient Simulation2026-01-29Identifying quantum spin liquids, magnon breakdown, or fractionalized excitations in quantum magnets is an ongoing challenge due to the ambiguity in interpreting excitation continua occurring in linear response probes. Recently, it was proposed that techniques measuring higher-order response, such as two-dimensional coherent spectroscopy (2DCS), could resolve such ambiguities. Numerically simulating nonlinear response functions can, however, be computationally very demanding. We present an efficient Lanczos-based method to compute second-order susceptibilities $\chi^{2}(\omega_t,\omega_\tau)$ directly in the frequency domain. Applying this to extended Kitaev models describing α-RuCl3, we find qualitatively different nonlinear responses between intermediate magnetic field strengths and the high-field regime. To put these results into context, we derive the general 2DCS response of partially-polarized magnets within the linear spin-wave approximation, establishing that \chi^2(\omega_t,\omega_\tau)$ is restricted to a distinct universal form if the excitations are conventional magnons. Deviations from this form, as predicted in our Lanczos-based simulations for α-RuCl3, can hence serve in 2DCS experiments as direct criteria to determine whether an observed excitation continuum is of conventional two-magnon type or of different nature.
35 13 - Research DataFunctional renormalization group for extremely correlated electrons2026-01-27At strong on-site repulsion U, the fermionic Hubbard model realizes an extremely correlated electron system. In this regime, it is natural to derive the low-energy physics with the help of non-canonical operators acting on a projected Hilbert space without double occupancies. Using a strong-coupling functional renormalization group technique, we study the physics of such extreme correlations in the strict U=∞ limit, where only kinematic interactions due to the Hilbert space projection remain. For nearest-neighbor hopping on a square lattice, we find that the electronic spectrum is significantly renormalized, with bandwidth and quasi-particle residue strongly decreasing with increasing electron density. On the other hand, damping and particle-hole asymmetry increase, while a polaronic continuum forms in the hole sector, below the single-particle band. Fermi liquid phenomenology applies only at low densities, where the system remains paramagnetic. At higher densities, we find a bad metal with strong magnetic correlations, indicating that the ground state is the Nagaoka ferromagnet at high densities and a stripe antiferromagnet at intermediate densities. Both in the paramagnetic and the ferromagnetic regimes, we observe a violation of Luttinger's theorem.
20 1 - Research DataTowards a critical endpoint in the valence fluctuating Eu(Rh1-xCox)2Si2 system2025-10-15We report on the successful single crystal growth of pure EuRh${_2}$Si${_2}$ and of Eu(Rh$_{1-x}$Co$_{x}$)$_2$Si$_2$ with $x\leq0.23$ by the flux method. Through Co substitution, EuRh$_2$Si$_2$ can be tuned from stable antiferromagnetism via a valence-transition state towards the valence-crossover regime. From magnetization measurements, we constructed a $B - T$ phase diagram for EuRh${_2}$Si${_2}$ comprising multiple magnetic phases and showing a sizable magnetic anisotropy within the basal plane of the tetragonal unit cell. This indicates a complex antiferromagnetic ground state for $x=0$. By applying positive chemical pressure through the substitution series Eu(Rh$_{1-x}$Co$_{x}$)$_2$Si$_2$, a sharp temperature-induced first-order phase transition is observed in magnetization, resistivity and heat capacity for \textcolor{blue}{0.081 $\leq$ $x$ $\leq$ 0.126}. The critical end point of this valence transition is located in the phase diagram in the vicinity of \textcolor{blue}{0.126 $
18 6 - Research DataGrowth-Controlled Twinning and Magnetic Anisotropy in CeSb22025-11-14Cerium diantimonide (CeSb2) is a layered heavy-fermion Kondo lattice material that hosts complex magnetism and pressure-induced superconductivity. The interpretation of its in-plane anisotropy has remained unsettled due to structural twinning, which superimposes orthogonal magnetic responses. Here we combine controlled crystal growth with magnetization and rotational magnetometry to disentangle the effects of twinning. Nearly untwinned high-quality single crystals reveal the intrinsic in-plane anisotropy: the in-plane easy axis saturates at M_easy(4 T) ≈ 1.8 µ_B/Ce, while the in-plane hard axis magnetization is strongly suppressed, nearly linear, and comparable to the out-of-plane response. These results resolve long-standing discrepancies in reported magnetic measurements, in which in-plane metamagnetic transition fields and saturation magnetization varied significantly across previous studies. Growth experiments demonstrate that avoiding the proposed α-β structural transition - through Sb-rich flux and slower cooling - systematically reduces twinning. However, powder X-ray diffraction and differential thermal analysis measurements show no clear evidence of a distinct β phase. Our results establish a consistent magnetic phase diagram and provide essential constraints for crystal-electric field models, enabling a clearer understanding of the interplay between anisotropic magnetism and unconventional superconductivity in CeSb2.
5 30 - Research DataDispersion kinks from electronic correlations in an unconventional iron-based superconductor2024-11-17The attractive interaction in conventional BCS superconductors is provided by a bosonic mode. However, the pairing glue of most unconventional superconductors is unknown. The effect of electron-boson coupling is therefore extensively studied in these materials. A key signature is dispersion kinks that can be observed in the spectral function as abrupt changes in velocity and lifetime of quasiparticles. Here, we show the existence of two kinks in the unconventional iron-based superconductor RbFe2As2 using angle-resolved photoemission spectroscopy (ARPES) and dynamical mean field theory (DMFT). In addition, we observe the formation of a Hubbard band multiplet due to the combination of Coulomb interaction and Hund’s rule coupling in this multiorbital system. We demonstrate that the two dispersion kinks are a consequence of these strong many-body interactions. This interpretation is in line with a growing number of theoretical predictions for kinks in various general models of correlated materials. Our results provide a unifying link between iron-based superconductors and different classes of correlated, unconventional superconductors such as cuprates and heavy-fermion materials.
7 - Research DataObservation of Two Cascading Screening Processes in an Iron-based Superconductor2025-07-22Understanding how renormalized quasiparticles emerge in strongly correlated electron materials provides a challenge for both experiment and theory. It has been predicted that distinctive spin and orbital screening mechanisms drive this process in multiorbital materials with strong Coulomb and Hund’s interactions. Here, we provide the experimental evidence of both mechanisms from angle-resolved photoemission spectroscopy on RbFe2As2. We observe that the emergence of low-energy Fe 3dxy quasiparticles below 90K coincides with spin screening. A second process changes the spectral weight at high energies up to room temperature. Supported by theoretical calculations we attribute it to orbital screening of Fe 3d atomic excitations. These two cascading screening processes drive the temperature evolution from a bad metal to a correlated Fermi liquid.
6 1 - Research DataInfluence of graphene on the electronic and magnetic properties of an iron(III) porphyrin chloride complex2024-09-24Although iron-based single atom catalysts are regarded as a promising alternative to precious metal catalysts, their precise electronic structures during catalysis still pose challenges for computational descriptions. A particularly urgent question is the influence of the environment on the electronic structure, and how to describe this properly with computational methods. Here, we study an iron porphyrin chloride complex adsorbed on a graphene sheet using density functional theory calculations to probe how much the electronic structure is influenced by the presence of a graphene layer. Our results indicate that weak interactions due to van der Waals forces dominate between the porphyrin complex and graphene, and only a small amount of charge is transferred between the two entities. Furthermore, the interplay of the ligand field environment, strong p-d hybridization, and correlation effects within the complex are strongly involved in determining the spin state of the iron ion. By bridging molecular chemistry and solid state physics, this study provides first steps towards a joint analysis of the properties of iron-based catalysts from first principles.
1 7 - Research DataAnisotropic antiferromagnetic order in EuPd3Si22025-10-07Single crystals of EuPd$_3$Si$_2$ were grown using a high-temperature EuPd-flux method. The material was structurally and chemically characterized by single-crystal x-ray diffraction, powder x-ray diffraction, Laue method and energy-dispersive x-ray spectroscopy. The structural analysis confirmed the orthorhombic crystal structure (space group $Imma$) but revealed differences in the lattice parameters and bond distances in comparison to previous work by Sharma et al.. \\ The composition is close to the ideal 1:3:2 stoichiometry with an occupation of 7 \% of the Si sites by Pd. The heat capacity, electrical resistivity, and magnetic susceptibility show two magnetic transitions indicating antiferromagnetic ordering below $T_{\rm N1}=$ 61\,K and a spin reorientation at $T_{\rm N2}=$ 40\,K. The orthorhombic material shows magnetic anisotropy with field applied along the three main symmetry axes, which is summarized in the temperature-field phase diagrams. The susceptibility data hint to an alignment of the magnetic moments along $[100]$ between $T_{\rm N1}$ and $T_{\rm N2}$. Below $T_{\rm N2}$ the magnetic structure changes to an arrangement with moments canted away from $[100]$. In contrast to published workby Sharma et al., the single crystals investigated in this study show AFM order below $T_{\rm N1}$ instead of ferromagnetism that sets in at higher $T_{\rm C1}=78\,\rm K$ which might originate from certain differences in the structure, composition or defects that have an impact on the dominant coupling constants of the RKKY interaction.
2 9 - Research DataIn situ GJ complex in C. elegans2025-09-12Tilt series, alignment and mdoc files as well as reconstructed volumes obtained from cryo electron tomography of C. elegans primary cell culture.
56 3 - Research DataSingle crystal growth and physical characterization to fine tune YbIn1−xTxCu4 (T = Au, Ag) towards the critical endpoint of the valence transition2025-05-05Pure as well as Ag- and Au-substituted YbInCu$_4$ single crystals were structurally and chemically characterized and investigated by means of heat capacity, magnetization, resistivity and ultrasonic measurements. We studied the influence of different compositions of the initial melt as well as of Au and Ag substitutions on the valence change and investigated whether this change occurs via a first-order phase transition or via crossover. We constructed a phase diagram of YbInCu$_4$ as a function of various substitutions and show that the position of the critical endpoint of the valence transition depends on the substituent and on the conditions under which the samples were grown. Multiple thermal cycles through the first-order transition lead to a significant modification of the physical properties which clearly demonstrated the influence of defects in substituted YbInCu$_4$.
34 3 - Research DataBound-state formation and thermalization within the Lindblad approach2025-03-11The Lindblad master equation is a frequently used Markovian approach to describe open quantum systems in terms of the temporal evolution of a reduced density matrix. Here, the thermal environ- ment is traced out to obtain an expression to describe the evolution of what is called a system: one particle or a chain of interacting particles, which is/are surrounded by a thermal heat bath. In this work, we investigate the formation of non-relativistic bound states, involving the P¨oschl- Teller potential, in order to discuss the formation time and the thermal equilibrium, applying scales from nuclear physics. This problem is borrowed from the field of heavy-ion collisions, where the deuteron is a probe which is measured at temperature regimes around the chemical freeze out temperature, while the deuteron itself has a binding energy which is much lower. This is known and often described as a “snowball in hell”. We use a reformulated Lindblad equation, in terms of a diffusion-advection equation with sources and therefore provide a hydrodynamical formulation of a dissipative quantum master equation
29 13 - Research DataUnderstanding the microscopic origin of the magnetic interactions in CoNb2O62025-01-18Motivated by the on-going discussion on the nature of magnetism in the quantum Ising chain CoNb2O6, we present a first-principles-based analysis of its exchange interactions with additional modeling, addressing drawbacks of a purely density functional theory ansatz. This method allows us to extract and understand the origin of the magnetic couplings—including all symmetry-allowed terms - and resolve conflicting model descriptions in CoNb2O6. We find that the twisted Kitaev chain and transverse-field ferromagnetic Ising chain views are mutually compatible, although additional off- diagonal exchanges are required for a complete picture. We show that the dominant exchange interaction is a ligand-centered process—involving eg electrons -, rendered anisotropic by low- symmetry crystal fields in CoNb2O6, resulting in dominant Ising exchange. Smaller bond-dependent anisotropies are found to originate from d − d kinetic exchange processes involving t2g electrons. We demonstrate the validity of our low-energy model by comparing its predictions to measured THz and INS spectra.
35 6 - Research DataMagnetic frustration and weak Mn magnetic ordering in EuMn2P22025-04-10We report on the electron spin resonance (ESR), heat capacity, magnetization, nuclear magnetic resonance (NMR), magnetic circular and linear dichroism (XMCD, XMLD), as well as the electrical resistivity of EuMn2P2 single crystals. Antiferromagnetic order of Eu was observed in several quanti- ties at T Eu N = 18 K. The temperature dependencies of ESR linewidth and resonance shift show, when approaching the Eu-ordered state, a divergence towards T Eu N , indicating the growing importance of magnetic correlations and the build-up of internal magnetic fields. An additional temperature scale of ≈ 47 K has considerable impact on linewidth, resonance field and intensity. This points to the presence of weak Mn-based ordering. The observed ESR line is interpreted as an Eu2+ resonance, which probes the weak magnetic background of the Mn subsystem. Such picture is suggested by the lineshape which keeps to be Lorentzian across the 47 K scale and by the ESR intensity which can be described by the same Curie-Weiss temperature above and below 47 K. In the same temperature range anomalies were observed at 48.5 K and 51 K in the heat capacity data as well as a pronounced broadening of the NMR signal of the EuMn2P2 samples. In XMCD and XMLD measurements, this weak magnetic order could not be detected in the same temperature range which might be due to the small magnetic moment, with a potential c-component or frustration.
65 2 - Research DataLindblad dynamics of deuteron motivated bound states2025-03-10The Lindblad master equation is a frequently used Markovian approach to describe open quantum systems in terms of the temporal evolution of a reduced density matrix. Here, the thermal environment is traced out to obtain an expression to describe the evolution of what is called a system: one particle or a chain of interacting particles, which is/are surrounded by a thermal heat bath. In this work, we investigate the formation of non-relativistic bound states, involving the P\"oschl-Teller potential, in order to discuss the formation time and the thermal equilibrium, applying scales from nuclear physics. This problem is borrowed from the field of heavy-ion collisions, where the deuteron is a probe which is measured at temperature regimes around the chemical freeze out temperature, while the deuteron itself has a binding energy which is much lower. This is known and often described as a ``snowball in hell". We use a reformulated Lindblad equation, in terms of a diffusion-advection equation with sources and therefore provide a hydrodynamical formulation of a dissipative quantum master equation.
24 3 - Research DataData for Figures 1 to 4 of Conference Proceeding2025-03-10The Lindblad master equation is a frequently used Markovian approach to describe open quantum systems in terms of the temporal evolution of a reduced density matrix. Here, the thermal environment is traced out to obtain an expression to describe the evolution of what is called a system: one particle or a chain of interacting particles, which is/are surrounded by a thermal heat bath. In this work, we investigate the formation of non-relativistic bound states, involving the Pöschl-Teller potential, in order to discuss the formation time and the thermal equilibrium, applying scales from nuclear physics. This problem is borrowed from the field of heavy-ion collisions, where the deuteron is a probe which is measured at temperature regimes around the chemical freeze out temperature, while the deuteron itself has a binding energy which is much lower. This is known and often described as a ``snowball in hell". We use a reformulated Lindblad equation, in terms of a diffusion-advection equation with sources and therefore provide a hydrodynamical formulation of a dissipative quantum master equation.
35 3 - Research DataInterconnected renormalization of Hubbard bands and Green's function zeros in Mott insulators induced by strong magnetic fluctuations2024-10-10We analyze the role of spatial electronic correlations and, in particular, of the magnetic fluctuations in Mott insulators. A half-filled Hubbard model is solved at large strength of the repulsion 𝑈 on a two-dimensional square lattice using an advanced diagrammatic approach capable of going beyond Hartree-Fock and single-site dynamical mean-field theories. We show that at high temperatures, when the magnetic fluctuations are weak, the electronic self-energy of the system is mainly local and is well reproduced by the atomic (Hubbard-I) approximation. Upon lowering the temperature toward the magnetically ordered phase, the nonlocality of the self-energy becomes crucial in determining the momentum dispersion of the Hubbard bands and the Green's function zeros. We therefore establish a precise link between Luttinger surface, nonlocal correlations and spectral properties of the Hubbard bands.
21 45 - Research Data8x8 Patch-Antenna-Coupled TeraFET Detector Array for Terahertz Quantum-Cascade-Laser Applications2024-07-31North, Nicholas K.Horbury, Michael D.Kondawar, SanchitKundu, ImonSalih, MohammedKrysl, AnastasiyaLi, LianheLinfield, Edmund H.Freeman, Joshua R.Valavanis, AlexanderLisauskas, AlvydasRoskos, Hartmut G.Monolithically integrated, antenna-coupled field-effect transistors (TeraFETs) are rapid and sensitive detectors for the terahertz range (0.3-10 THz) that can operate at room temperature. We conducted experimental characterizations of a single patch-antenna coupled TeraFET optimized for 3.4 THz operation and its integration into an 8×8 multi-element detector configuration. In this configuration, the entire TeraFET array operates as a unified detector element, combining the output signals of all detector elements. Both detectors were realized using a mature commercial Si-CMOS 65-nm process node. Our experimental characterization employed single-mode Quantum-Cascade Lasers (QCLs) emitting at 2.85 THz and 3.4 THz. The 8x8 multi-element detector yields two major improvements for sensitive power detection experiments. First, the larger detector area simplifies alignment and enhances signal stability. Second, the reduced detector impedance enabled the implementation of a TeraFET+QCL system capable of providing a -3 dB modulation bandwidth up to 21 MHz, which is currently limited primarily by the chosen readout circuitry. Finally, we validate the system’s performance by providing high resolution gas spectroscopy data for methanol vapor around 3.4 THz, where a detection limit of 1.6e-5 absorbance, or 2.6e11 molecules/cm3 was estimated under optimal coupling conditions.
101 8 - Research DataAb initio study of highly tunable charge transfer in β-RuCl3/graphene heterostructures2024-06-25Heterostructures of graphene in proximity to magnetic insulators open the possibility to investigate exotic states emerging from the interplay of magnetism, strain and charge transfer between the layers. Recent reports on the growth of self-integrated atomic wires of $\beta$-RuCl$_3$ on graphite suggest these materials as versatile candidates to investigate these effects. Here we present detailed first principles calculations on the charge transfer and electronic structure of $\beta$-RuCl$_3$/graphene heterostructures and provide a comparison with the work function analysis of the related honeycomb family members $\alpha$-RuX$_3$ (X = Cl,Br,I). We find that proximity of the two layers leads to a hole-doped graphene and electron-doped RuX$_3$ in all cases, which is sensitively dependent on the distance between the two layers. Furthermore, strain effects due to lattice mismatch control the magnetization which itself has a strong effect on the charge transfer. Charge accumulation in $\beta$-RuCl$_3$ strongly drops away from the chain making such heterostructures suitable candidates for sharp interfacial junctions in graphene-based devices.
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