arXiv subjects

Martin Wolf

Publications and source records attributed to Martin Wolf.

At least 19 recordsLinked to original sources

arXiv2026

Visualizing the Hidden Architecture of Molecular Films with Phase-Resolved Rotational SFG Microscopy

The highly variable physico-chemical properties of thin molecular films play an essential role in numerous research fields ranging from biophysics to the fabrication of functional devices such as molecular sensors. The properties of molecular films are largely governed by their three-dimensional molecular structure which often exhibits important spatial heterogeneity, either naturally, or introduced deliberately. In order to understand and control these properties microscopic insight into structural parameters such as composition, molecular orientation and conformation, as well as molecular order is required, which, so far, represents a mostly unachieved experimental target. In this contribution we present a powerful experimental approach that can overcome this limitation. Using phase-resolved rotational sum-frequency generation (SFG) microscopy all of these structural parameters can be obtained with sub-monolayer sensitivity and at sub-micron resolution. In measurements of monolayer assemblies of mixed phospholipids, we uncover the molecular packing structure in previously-unattained detail and demonstrate the large potential of the technique for the elucidation of the complex architecture inside molecular films. The structural insight provided by this nonlinear microscopy approach spans all the way from the molecular to the macroscopic scale opening the door to a completely new type of interfacial studies.

physics.chem-ph↗

arXiv2026

Classification of Killing Horizons in D=11 Supergravity

We initiate the classification of supersymmetric degenerate Killing horizons, with closed spatial cross section, away from the near-horizon limit in D=11 supergravity. We prove that all such solutions fall into two distinct classes, depending on lightcone chirality with respect to a Gaussian null coordinate system. For the first class of solutions, the negative lightcone chirality part of the Killing spinor is non-zero on the Killing horizon, and we prove that all such solutions are isometric to supersymmetric near-horizon geometries. In the second class, the negative lightcone chirality part of the Killing spinor vanishes on the Killing horizon. In this case, we prove that the spinorial Lie derivative of the Killing spinor with respect to the Killing vector which generates the Killing horizon vanishes, and that all such solutions with more than 13 supersymmetries are pp-waves.

hep-th↗

arXiv2026

In-plane vector-field imaging of propagating surface phonon polaritons

Polariton interferometry through optical near-field microscopy has become a powerful tool in nanophotonics, enabling direct spatial access to the propagation characteristics of strongly confined, evanescent polariton modes. Scattering-type near-field optical microscopy has matured as the prime tool for such studies, yet mostly the out-of-plane components of the optical near fields are probed, owing to the elongated geometry of the nanotip. Here, we demonstrate a complementary far-field nonlinear microscopy approach which allows to selectively probe in-plane polariton field components. Accessing the full vector field is interesting when studying complex mode patterns such as hyperbolic polaritons or skyrmions, where the in-plane field components are typically only inferred from the out-of-plane component but not measured directly. To this end, we use nonlinear infrared-visible wide-field sum-frequency generation microscopy, where the short visible wavelength of the nonlinear signal provides the high spatial resolution to access evanescent modes in the infrared. The symmetry selection rules of the nonlinear process further enable polarization-selective imaging of both in-plane polariton field components through spatial interferometry. The concept is demonstrated experimentally using surface phonon polaritons at the AlN-air interface launched by a gold antenna. A simple, semi-analytical model reproduces the peculiar propagation patterns. Hyperspectral imaging with a tunable narrowband laser further gives access to the polariton dispersion. The wide-field methodology holds high promise for in-depth and high-throughput studies of infrared nanophotonic structures.

physics.optics↗

arXiv2026

A two-color dual-oscillator infrared free-electron laser

We report on the design and performance of a two-color dual-oscillator infrared free-electron laser (FEL). The mid-infrared (MIR) FEL at the Fritz Haber Institute (FHI FEL) has been upgraded to include a second oscillator FEL beamline that permits lasing in the far-infrared (FIR) regime from 4.5 {\mu}m to 175 {\mu}m. In addition, a 500 MHz kicker cavity has been installed downstream of the electron accelerator. It allows to deflect electron bunches of up to 50 MeV energy alternately left and right by an angle of {\pm}2{\deg}. It can, thus, split the high-repetition-rate (1 GHz) electron bunch train from the accelerator into two bunch trains of 500 MHz repetition rate each; one is steered to the MIR FEL and the other one to the new FIR FEL. In this two-color mode of simultaneous, synchronized operation the wavelengths in both FELs can be tuned independently over wide ranges of up to a factor of four each by undulator-gap variation. In addition, two-color operation is also available at reduced repetition rates (e.g. 55.6 MHz of both MIR and FIR pulses), as needed for some applications. This unique two-color mode opens up a wealth of novel user applications such as, MIR-FIR pump-probe experiments.

physics.acc-ph↗

arXiv2026

Ultrafast nonlinear Hall effect in black phosphorus

The nonlinear Hall effect (NHE) is a recently discovered member of the Hall effect family in which the Hall voltage shows a nonlinear behavior when a transverse electric field is applied. While the NHE does not require broken time-reversal symmetry, such as that induced by a magnetic field, it requires broken inversion symmetry, which limits the range of suitable systems and potential applications. Here, we demonstrate an ultrafast NHE in centrosymmetric black phosphorus through dynamical symmetry breaking using femtosecond light pulses. We provide a detailed microscopic picture of excited carrier dynamics and induced fields using momentum-resolved photoemission spectroscopy combined with \textit{ab-initio} calculations. The ultrafast NHE is observed exclusively for the light polarization aligned with the armchair high-symmetry direction and persists over 300 fs, which opens new possibilities for selective and ultrafast light-to-current conversions.

cond-mat.mtrl-sci↗

arXiv2026

Ultrafast Formation and Annihilation of Strongly Bound, Anisotropic Excitons

Van der Waals (vdW) layered materials with long-range magnetic order have the potential to enable novel optoelectronic and spintronic applications. Among these, CrSBr is an air-stable, direct band gap semiconductor that hosts interlayer antiferromagnetic order, a highly anisotropic electronic structure, and strongly bound excitons. In particular, excitons in CrSBr have been shown to inherit the quasi-one-dimensional nature of the material and also couple to the underlying spinorder. However, mechanisms of exciton formation, dissociation, and interaction with free carriers remain largely unexplored, despite being crucial for spintronic and optoelectronic applications. Here, we employ time- and angle-resolved photoemission spectroscopy to map the electronic structure and excited state dynamics in CrSBr. We directly resolve an exceptionally large exciton binding energy (~800 meV) and a highly anisotropic momentum space distribution of the exciton, revealing its quasi-1D real-space character. We observe an excitation-density-dependent interconversion between bound excitons and quasi-free carriers on sub- to few-picosecond timescales, indicating that many-body effects govern the excited-state dynamics and optical properties during the initial stages of relaxation. Our work highlights the strongly bound, anisotropic character of excitons in CrSBr, as well as the microscopic interactions steering relaxation pathways after photoexcitation in elevated density regimes relevant for future device applications.

cond-mat.mtrl-sci↗

arXiv2026

Integrability from Homotopy Algebras

Homotopy algebraic methods have become increasingly influential in studying field theories. We consider semi-holomorphic Chern-Simons theory and its relation with the principal chiral model. In particular, we establish an explicit quasi-isomorphism between the cyclic $L_\infty$-algebras governing both theories which directly gives the Lax connection. This provides a concrete example for studying integrability of a two-dimensional system through the homotopy algebraic lens.

hep-th↗

arXiv2026

Probing topological Floquet states in graphene with ultrafast terahertz scanning tunneling microscopy

Floquet control of band topology is a central theme in ultrafast quantum materials science. Established experimental probes of light-induced topological states include ultrafast transport and time- and angle-resolved photoemission spectroscopy, each with important strengths but also well-known limitations. Here we propose ultrafast terahertz scanning tunneling microscopy (THz-STM) as a real space energy-resolved probe of Floquet physics. We show that THz-STM enables direct local detection of bulk Floquet gaps and distinct Floquet edge state signatures. We derive a nonequilibrium Green's-function formalism for time-dependent tunneling that directly extends standard STM theory and provides an intuitive interpretation of rectified ultrafast tunneling currents. We apply the approach to bulk graphene and graphene nanoribbons of variable width. For the bulk, we show that THz-STM provides direct spectroscopic access to Floquet-induced gap openings, and we contrast pulsed pump-probe protocols with the continuous-wave Floquet steady-state limit. For finite ribbons, we demonstrate time- and space-resolved imaging of Floquet-induced topological edge states and identify the ribbon-width scale below which edge state protection breaks down. We further show how band structures of graphene nanoribbons and Floquet chiral edge modes can be reconstructed via Floquet quasiparticle interference. Finally we demonstrate that chiral impurities that break time-reversal symmetry induce characteristic spatial THz-STM signatures that can be used as a direct probe of Floquet edge state chirality.

cond-mat.mes-hall↗

arXiv2026

Tailoring phonon-driven responses in {\alpha}-MoO3 through isotopic enrichment

The implementation of polaritonic materials into nanoscale devices requires selective tuning of parameters to realize desired spectral or thermal responses. One robust material is {\alpha}-MoO3, which as an orthorhombic crystal boasts three distinct phonon dispersions, providing three polaritonic dispersions of hyperbolic phonon polaritons (HPhPs) across the mid-infrared (MIR). Here, the tunability of both optical and thermal responses in isotopically enriched {\alpha}-MoO3 (98MoO3, Mo18O3 and 98Mo18O3) are explored. A uniform ~5 % spectral redshift from 18O enrichment is observed in both Raman- and IR-active TO phonons. Both the in- and out-of-plane thermal conductivities for the isotopic variations are reported. Ab initio calculations both replicate experimental findings and analyze the select-mode three-phonon scattering contributions. The HPhPs from each isotopic variation are probed with s-SNOM and their Q- factors are reported. A Q-factor maxima increase of ~50 % along the [100] in the RB2 and ~100 % along the [001] in the RB3 are reported for HPhPs supported in 98Mo18O3. Observations in both real and Fourier space of higher-order HPhP modes propagating in single slabs of isotopically enriched {\alpha}-MoO3 without the use of a subdiffractional surface scatterer are presented here. This work illustrates the tunability of {\alpha}-MoO3 for thermal and nanophotonic applications.

cond-mat.mtrl-sci↗

arXiv2025

Broadband Dielectric and THz Spectroscopy on Bio-Related Matter: Water, Amino Acids, Proteins, and Blood

In the present work, we examine the relevance and proper interpretation of broadband-dielectric and THz-spectroscopy data for the investigation of various types of biological matter. We provide an overview of the rich variety of different dynamic processes that can be detected by these experimental methods. Several experimental examples are discussed in detail, helping to understand the information that can be drawn from such studies. This includes dielectric spectra, extending well into the GHz region, for pure water, which can be considered as a simple but highly important biological molecule. We also discuss results for a prototypical aqueous solution of a protein, belonging to one of the most important classes of biological macromolecules. Moreover, we examine broadband dielectric spectra on blood as an example of functional biological matter in organisms. To demonstrate the relevance of THz spectroscopy for the investigation of biological molecules, we finally treat such experiments applied to different amino acids.

cond-mat.soft↗

arXiv2025

Non-equilibrium state during proton-deuteron exchange at a liquid-liquid interface

Proton-deuteron exchange is a very fast process, even across macroscopic length scales. Here we directly and quantitatively measure the formation of HDO within the first 100 microseconds of the reaction at the liquid-liquid interface between D$_2$O and H$_2$O, using a fast-flowing liquid flat jet combined with infrared spectroscopic imaging. We demonstrate that, at early stages HDO formation is reaction-limited, set by the low concentration of the hydroxide and hydronium ions that mediate the exchange. As the ion concentration rises, the rate rapidly approaches the diffusion limit. The reaction rate constant we extract is consistent with the picosecond timescale of the elementary proton-deuteron exchange. Access to these microsecond kinetics reveals a non-equilibrium state in the early H$_2$O/D$_2$O interface: the two liquids are fully mixed by diffusion, yet the HDO concentration remains well below equilibrium. Quantitative imaging of reactant and product concentrations at well-defined liquid-liquid interfaces, as introduced here, will enable the study of fast kinetics across a wide range of chemical reactions.

physics.chem-ph↗

arXiv2025

Extremal Black Holes from Homotopy Algebras

The uniqueness and rigidity of black holes remain central themes in gravitational research. In this work, we investigate the construction of all extremal black hole solutions to the Einstein equation for a given near-horizon geometry, employing the homotopy algebraic perspective, a powerful and increasingly influential framework in both classical and quantum field theory. Utilising Gau{\ss}ian null coordinates, we recast the deformation problem as an analysis of the homotopy Maurer-Cartan equation associated with an $L_\infty$-algebra. Through homological perturbation theory, we systematically solve this equation order by order in directions transverse to the near-horizon geometry. As a concrete application of this formalism, we examine the deformations of the extremal Kerr horizon. Notably, this homotopy-theoretic approach enables us to characterise the moduli space of deformations by studying only the lowest-order solutions, offering a systematic way to understand the landscape of extremal black hole geometries.

hep-th↗

arXiv2025

Anisotropic Water Structure at Charged Interfaces Studied by Depth Resolved Vibrational SFG/DFG Spectroscopy

The molecular water structure at charged aqueous interfaces is shaped by interfacial electric fields, which can induce significant anisotropy in the molecular orientations extending over nanometer-scale distances. Despite great relevance, very little is known about the details of this depth-dependent anisotropic water structure, mainly due to the lack of appropriate experimental techniques. Here, we present a depth-resolved study of the water anisotropy at the interface to insoluble charged surfactants using a newly developed technique which allows for directly correlating nonlinear vibrational spectra with depth information on the nanometer scale. We demonstrate that the obtained data allows for a full reconstruction of the nonlinear vibrational responses as function of depth. The results for the case of low salinity solutions show the presence of two pronounced regions within the interfacial anisotropy with largely deviating degrees of preferential molecular orientations. A spectral analysis of the depth-dependent vibrational responses furthermore reveals that the natural local hydrogen-bond structure of bulk water remains largely unperturbed throughout the interfacial region, including water in direct proximity of the surface charges. These findings significantly refine our understanding of the anisotropic water structure at the interface to hydrophilic charged surfactants and showcase the large potential of our depth-resolved spectroscopic technique.

physics.chem-ph↗

arXiv2025

Spectral tuning of hyperbolic shear polaritons in monoclinic gallium oxide via isotopic substitution

Hyperbolic phonon polaritons - hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals - enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic $\beta$-Ga$_2$O$_3$ (bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near-field optical microscopy to image HShPs in $^{18}$O bGO films homo-epitaxially grown on a $^{16}$O bGO substrate, we demonstrate a spectral redshift of $\sim~40~$cm$^{-1}$ for the $^{18}$O bGO, compared to $^{16}$O bGO. The technique allows for direct observation and a model-free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far-field measurements and ab initio calculations - in good agreement with the near-field data - confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons.

physics.optics↗

arXiv2025

Orbital mixing and strong Hund's coupling stabilize spin order in van der Waals ferromagnet CrI3

Recent years have seen a vast increase in research into van der Waals magnetic materials. In many of these systems, magnetism is introduced via light 3d-transition metal elements, combined with chalcogenides or halogens. Despite the high technological promise in the field of spintronics, the connection between the d-orbital configuration and the occurrence of low-dimensional magnetic order is currently unclear. Here we address the prototypical two-dimensional ferromagnet CrI3, via complementary spectroscopies and density functional theory calculations. We reveal the electronic structure and orbital character of bulk CrI3 in the paramagnetic and ferromagnetic phases, describing the couplings underpinning its energy diagram, and providing a robust experimental demonstration that the stabilization of ferromagnetism is attributable to orbital mixing between I p and Cr eg states, and to the presence of strong Hund's coupling. These findings reveal the microscopic connection between orbital and spin degrees of freedom, providing fundamental insights into the behavior of low-dimensional magnetic materials.

cond-mat.mtrl-sci↗

arXiv2025

Adjusting Higher Chern-Simons Theory

A fundamental problem in formulating higher Chern-Simons theories is the construction of a consistent higher gauge theory that circumvents the fake-flatness constraint. Here, we propose a solution to this problem using adjusted higher connections. Concretely, we shall demonstrate that there is an obstruction to the direct construction of such action functionals since, generically, adjusted higher gauge algebras do not admit an inner product. To overcome this obstruction, we introduce half-adjusted higher Chern-Simons theories. These theories have both well-defined underlying kinematic data as well as the expected properties of a higher generalisation of Chern-Simons theory. We develop the general construction of these theories in arbitrary dimensions and provide explicit details for the four-dimensional case. We also present the complete differential cohomological framework for principal 2-bundles with half-adjusted connections. Finally, we discuss an alternative approach introducing additional trivial symmetries.

hep-th↗

arXiv2025

Terahertz time-domain signatures of the inverse Edelstein effect in topological-insulator|ferromagnet heterostructures

Three-dimensional topological insulators possess topologically protected surface states with spin-momentum locking, which enable spin-charge-current interconversion (SCI) by the inverse Edelstein effect (IEE). However, it remains experimentally challenging to separate the surface-related IEE from the bulk-type inverse spin Hall effect (ISHE). Here, we search for distinct time-domain signatures of the two SCI phenomena in a $\mathcal{F}$|TI model stack of a ferromagnetic-metal layer $\mathcal{F}$ (Co and Fe) and a topological-insulator layer TI (Bi$_2$Te$_3$, SnBi$_2$Te$_4$ and Bi$_{1-x}$Sb$_x$ with $x$ = 0.15 and 0.3), where the focus is on Bi$_2$Te$_3$. A femtosecond laser pulse serves to induce a transient spin voltage $\mu_s^{\mathcal{F}}$ in $\mathcal{F}$ and, thus, drive an ultrafast spin current out of $\mathcal{F}$. SCI results in a transverse charge current with a sheet density $I_c$ that is detected by sampling the emitted terahertz electric field. Analysis of the dynamics of $I_c(t)$ vs time $t$ relative to $\mu_s^{\mathcal{F}}(t)$ reveals two components with distinct time scales: (i) a quasi-instantaneous response and (ii) a longer-lived response with a relaxation time of 270 fs, which is independent of the chosen $\mathcal{F}$ material. Component (i) is consistently ascribed to the ISHE. In contrast, we interpret component (ii) as a signature of interfacial spin accumulation and the IEE at the $\mathcal{F}$/Bi$_2$Te$_3$ interface, with a fraction of $< 10^{-2}$ of the incident spins participating. This assignment is fully consistent with respect to its dynamics and magnitude. We rate other possible signal contributions, such as spin trapping in intermediate states, as less likely. Our results show that the femtosecond dynamics of photocurrents provide important insights into the mechanisms of spin transport and SCI in $\mathcal{F}$|TI stacks.

cond-mat.mes-hall↗

arXiv2025

Ultrafast dynamics of local charge order in a THz-induced metastable quantum state

Controlling quantum materials with ultrafast light pulses enables access to transient and metastable states that are inaccessible under equilibrium conditions. Yet their local dynamics remain poorly understood due to the challenge of resolving ultrafast processes with angstrom-scale spatial resolution. Here, we use terahertz scanning tunnelling microscopy (THz-STM) to probe coherent collective dynamics within a THz-induced metastable state in the layered charge density wave (CDW) material 1T-TaS2. Following ultrafast photoexcitation, we locally resolve coherent oscillations of the CDW amplitude mode at 2.5 THz together with two previously unreported modes at 1.3 THz and 0.7 THz. Comparison with phonon calculations identifies these as interlayer breathing and shear vibrations that are sensitive to the stacking configuration. These coherent dynamics are observed within a THz-induced metastable state that exhibits long-lived and spatially inhomogeneous modifications of the local density of states within the insulating gap, while higher THz fields drive a local redistribution and disordering of Star-of-David clusters near defects and domain boundaries. Our results suggest that the THz-induced metastable state involves a modification of the local interlayer stacking configuration, and demonstrate the role of interlayer degrees of freedom in the ultrafast dynamics of light-induced phases in layered quantum materials.

cond-mat.mes-hall↗