Littlehales, M. T.; Bauer, A.; Bartkowiak, M.; Zolliker, M.; White, J. S.; Pfleiderer, C. In-situ azimuthal sample rotation within a horizontal-field cryomagnet for small-angle neutron scattering Unpublished (2026), arXiv:2611.05159. @unpublished{littlehalesInsituAzimuthalSample2026,
title = {In-situ azimuthal sample rotation within a horizontal-field cryomagnet for small-angle neutron scattering},
author = {M. T. Littlehales and A. Bauer and M. Bartkowiak and M. Zolliker and J. S. White and C. Pfleiderer},
url = {https://arxiv.org/abs/2611.05159},
doi = {10.48550/arXiv.2611.05159},
year = {2026},
date = {2026-11-03},
urldate = {2026-11-01},
publisher = {arXiv},
abstract = {We present an in-situ azimuthal rotation stage for small-angle neutron scattering measurements. The stage provides an azimuthal rotation range of 315∘ about the stage normal. At the reference sample-stick orientation, the stage normal is parallel to the incident neutron beam, so that the sample rotates in the plane perpendicular to the beam. Combined with a sample stick rotation about a second, orthogonal axis, the stage provides control of the magnetic-field direction relative to the crystal axes, while the field remains fixed in the laboratory frame. For a 5 mm diameter incident beam, sample-stick rotations of approximately ±315∘ are accessible with negligible additional attenuation from the holder. The commercial rotator is specified for operation between 1 and 373 K and in magnetic fields up to 35 T, making the stage nominally compatible with the full temperature (1.8 to 320 K) and field (0 to 6.8 T) ranges of the used cryomagnet. As a working example, we characterize its performance on a spherical single crystal of MnSi between 20 and 35 K and in fields up to 183 mT, observing local sample heating during operation of ∼20 mK per 5∘ step. We illustrate the design considerations behind the holder construction, with specific notes for mounting spherical samples, and discuss the resulting performance and limitations.},
note = {arXiv:2611.05159},
keywords = {},
pubstate = {published},
tppubtype = {unpublished}
}
We present an in-situ azimuthal rotation stage for small-angle neutron scattering measurements. The stage provides an azimuthal rotation range of 315∘ about the stage normal. At the reference sample-stick orientation, the stage normal is parallel to the incident neutron beam, so that the sample rotates in the plane perpendicular to the beam. Combined with a sample stick rotation about a second, orthogonal axis, the stage provides control of the magnetic-field direction relative to the crystal axes, while the field remains fixed in the laboratory frame. For a 5 mm diameter incident beam, sample-stick rotations of approximately ±315∘ are accessible with negligible additional attenuation from the holder. The commercial rotator is specified for operation between 1 and 373 K and in magnetic fields up to 35 T, making the stage nominally compatible with the full temperature (1.8 to 320 K) and field (0 to 6.8 T) ranges of the used cryomagnet. As a working example, we characterize its performance on a spherical single crystal of MnSi between 20 and 35 K and in fields up to 183 mT, observing local sample heating during operation of ∼20 mK per 5∘ step. We illustrate the design considerations behind the holder construction, with specific notes for mounting spherical samples, and discuss the resulting performance and limitations. |
Wiedemann, B.; Causer, G. L.; Chacon, A.; Zhang, S. L.; Khaydukov, Y.; Hesjedal, T.; Soltwedel, O.; Keller, T.; Mühlbauer, S.; Adams, T.; Halder, M.; Pfleiderer, C.; Böni, P. Reciprocal space mapping of magnetic order in thick epitaxial MnSi films Journal Article Phys. Rev. B 114, 254401 (2026). @article{wiedemannReciprocalSpaceMapping2026,
title = {Reciprocal space mapping of magnetic order in thick epitaxial MnSi films},
author = {B. Wiedemann and G. L. Causer and A. Chacon and S. L. Zhang and Y. Khaydukov and T. Hesjedal and O. Soltwedel and T. Keller and S. Mühlbauer and T. Adams and M. Halder and C. Pfleiderer and P. Böni},
url = {http://dx.doi.org/10.1103/pss4-vlyr},
doi = {10.1103/pss4-vlyr},
issn = {2469-9969},
year = {2026},
date = {2026-11-02},
urldate = {2026-11-01},
journal = {Phys. Rev. B},
volume = {114},
number = {25},
pages = {254401},
publisher = {American Physical Society (APS)},
abstract = {We report near-surface small-angle neutron scattering (NS-SANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film grown on Si(111) in the thick-film limit. Providing a means of direct reciprocal space mapping, NS-SANS and OSR reveal one-dimensional magnetic modulations perpendicular to the film surface under magnetic fields parallel and perpendicular to the film surface, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of the field orientation, our data do not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct contrast to bulk MnSi thinned to similar nanoscale thicknesses, in which a skyrmion lattice dominates the phase diagram under perpendicular fields. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell of the film control the magnetic state, representing the main structural difference between epitaxial and thinned bulk MnSi.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We report near-surface small-angle neutron scattering (NS-SANS) and complementary off-specular neutron reflectometry (OSR) of the magnetic order in a single-crystalline epitaxial MnSi film grown on Si(111) in the thick-film limit. Providing a means of direct reciprocal space mapping, NS-SANS and OSR reveal one-dimensional magnetic modulations perpendicular to the film surface under magnetic fields parallel and perpendicular to the film surface, where additional polarized neutron reflectometry (PNR) and magnetization measurements are in excellent agreement with the literature. Regardless of the field orientation, our data do not suggest the presence of more complex spin textures, notably the formation of skyrmions. This observation establishes a distinct contrast to bulk MnSi thinned to similar nanoscale thicknesses, in which a skyrmion lattice dominates the phase diagram under perpendicular fields. Extended x-ray absorption fine structure measurements suggest that small shifts of the Si positions within the unstrained unit cell of the film control the magnetic state, representing the main structural difference between epitaxial and thinned bulk MnSi. |
Dutta, A.; Kollar, M. A universal gauge structure in the photon content of light-matter systems Unpublished (2026), arXiv: 2610.40446. @unpublished{dutta2026universalgaugestructurephoton,
title = {A universal gauge structure in the photon content of light-matter systems},
author = {A. Dutta and M. Kollar},
url = {https://arxiv.org/abs/2610.40446},
doi = {10.48550/arXiv.2610.40446},
year = {2026},
date = {2026-10-29},
urldate = {2026-10-01},
abstract = {After truncating the matter Hilbert space of a single-mode light-matter model, gauge transformations leave the spectrum invariant whereas the ground-state photon number is gauge dependent, hence, its meaning remains disputed. We show that this gauge dependence obeys an exact algebraic law whenever the gauge transformation acts as a displacement of the field coupled to a matter operator G. Namely, the ground-state photon number is a quadratic polynomial in the gauge parameter λ, ⟨a†a⟩λ = ⟨a†a⟩0 + Cλ + Qλ2. Here the coefficients are the correlator C=⟨Gp⟩ of G and the field momentum p, and Q=⟨G2⟩. Its minimum over all gauges is naturally gauge-invariant, and for the quantum Rabi model it is determined by the ground-state energy alone. We show that it is strictly positive for the quantum Rabi and Dicke models with level splitting, so that a residual photon content survives in every gauge. At weak coupling the gauge of minimal photon number is the Jaynes-Cummings gauge, and for the Dicke model it remains so at all couplings in the thermodynamic limit.},
note = {arXiv: 2610.40446},
keywords = {},
pubstate = {published},
tppubtype = {unpublished}
}
After truncating the matter Hilbert space of a single-mode light-matter model, gauge transformations leave the spectrum invariant whereas the ground-state photon number is gauge dependent, hence, its meaning remains disputed. We show that this gauge dependence obeys an exact algebraic law whenever the gauge transformation acts as a displacement of the field coupled to a matter operator G. Namely, the ground-state photon number is a quadratic polynomial in the gauge parameter λ, ⟨a†a⟩λ = ⟨a†a⟩0 + Cλ + Qλ2. Here the coefficients are the correlator C=⟨Gp⟩ of G and the field momentum p, and Q=⟨G2⟩. Its minimum over all gauges is naturally gauge-invariant, and for the quantum Rabi model it is determined by the ground-state energy alone. We show that it is strictly positive for the quantum Rabi and Dicke models with level splitting, so that a residual photon content survives in every gauge. At weak coupling the gauge of minimal photon number is the Jaynes-Cummings gauge, and for the Dicke model it remains so at all couplings in the thermodynamic limit. |
Koller, E.; Leeb, V.; Perkins, N. B.; Knolle, J. Raman Circular Dichroism and Quantum Geometry of Chiral Quantum Spin Liquids Journal Article Phys. Rev. B 114, (2026), datadoi: 10.5281/zenodo.14946046. @article{kollerRamanCircularDichroism2026,
title = {Raman Circular Dichroism and Quantum Geometry of Chiral Quantum Spin Liquids},
author = {E. Koller and V. Leeb and N. B. Perkins and J. Knolle},
url = {DOI: https://doi.org/10.1103/n6zn-xvsw},
doi = {10.1103/n6zn-xvsw},
year = {2026},
date = {2026-10-26},
urldate = {2026-10-01},
journal = {Phys. Rev. B},
volume = {114},
abstract = {We show that the quantum geometry of fractionalized spin excitations in Mott-insulating quantum spin liquids (QSLs) gives rise to a finite Raman circular dichroism (RCD) signal. We identify conditions under which the Loudon-Fleury framework, where light modulates the underlying exchange interactions, and the light-matter-coupling approach, where the effective vector potential couples directly to the spinon bands, yield equivalent Raman vertices for the fractionalized quasiparticles. Using the latter, we derive an analytical decomposition of the RCD into different quantum geometric contributions. This reveals the sensitivity of the RCD to the underlying structure of the wave functions and the handedness of the excitations, rather than a nonzero Chern number of spinon excitations. To illustrate this, we apply our approach to two examples, the Kitaev honeycomb model in a magnetic field and a parton mean-field state of the chiral QSL on the triangular lattice.},
note = {datadoi: 10.5281/zenodo.14946046},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We show that the quantum geometry of fractionalized spin excitations in Mott-insulating quantum spin liquids (QSLs) gives rise to a finite Raman circular dichroism (RCD) signal. We identify conditions under which the Loudon-Fleury framework, where light modulates the underlying exchange interactions, and the light-matter-coupling approach, where the effective vector potential couples directly to the spinon bands, yield equivalent Raman vertices for the fractionalized quasiparticles. Using the latter, we derive an analytical decomposition of the RCD into different quantum geometric contributions. This reveals the sensitivity of the RCD to the underlying structure of the wave functions and the handedness of the excitations, rather than a nonzero Chern number of spinon excitations. To illustrate this, we apply our approach to two examples, the Kitaev honeycomb model in a magnetic field and a parton mean-field state of the chiral QSL on the triangular lattice. |
Meier, T. N. G.; Back, C. Thermal stability of skyrmions Journal Article J. Phys.: Condens. Matter 38, 423004 (2026), datadoi: 10.14459/2026mp1859939. @article{meierThermalStabilitySkyrmions2026,
title = {Thermal stability of skyrmions},
author = {T. N. G. Meier and C. Back},
url = {http://dx.doi.org/10.1088/1361-648X/aeb085},
doi = {10.1088/1361-648x/aeb085},
issn = {1361-648X},
year = {2026},
date = {2026-10-23},
urldate = {2026-10-01},
journal = {J. Phys.: Condens. Matter},
volume = {38},
number = {42},
pages = {423004},
publisher = {IOP Publishing},
abstract = {Magnetic skyrmions are vortex-like magnetic textures with a topologically nontrivial spin structure making a continuous transformation from the skyrmion to the uniform state impossible. Hence skyrmions are regarded as topologically protected attracting large interest concerning applications in data storage, neuromorphic computing, and digital logic. For such kind of applications an extraordinary stability of the skyrmion state against thermal fluctuations is required. In real matter, due to its discrete nature, topological protection translates into the existence of an energy barrier between the skyrmion and the uniform state leading to a finite skyrmion lifetime. Therefore the experimental determination of the skyrmion lifetime becomes important. Within this work we focus on experimental studies of the skyrmion lifetime in a variety of magnetic systems ranging from monolayers to bulk samples. We discuss previous experimental studies on the thermal stability of magnetic skyrmions in systems based on Fe/Ir(111) and in B20-materials. New experimental results on skyrmion nucleation and decay are obtained for thin Ni/Fe-films on Cu(001) using threshold photoemission magnetic circular dichroism in combination with photoemission electron microscopy (TP-MCD-PEEM). The results are interpreted using an Arrhenius-like activation behavior and evaluated regarding energy barriers and pre-exponential factors of the skyrmion dynamics.},
note = {datadoi: 10.14459/2026mp1859939},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Magnetic skyrmions are vortex-like magnetic textures with a topologically nontrivial spin structure making a continuous transformation from the skyrmion to the uniform state impossible. Hence skyrmions are regarded as topologically protected attracting large interest concerning applications in data storage, neuromorphic computing, and digital logic. For such kind of applications an extraordinary stability of the skyrmion state against thermal fluctuations is required. In real matter, due to its discrete nature, topological protection translates into the existence of an energy barrier between the skyrmion and the uniform state leading to a finite skyrmion lifetime. Therefore the experimental determination of the skyrmion lifetime becomes important. Within this work we focus on experimental studies of the skyrmion lifetime in a variety of magnetic systems ranging from monolayers to bulk samples. We discuss previous experimental studies on the thermal stability of magnetic skyrmions in systems based on Fe/Ir(111) and in B20-materials. New experimental results on skyrmion nucleation and decay are obtained for thin Ni/Fe-films on Cu(001) using threshold photoemission magnetic circular dichroism in combination with photoemission electron microscopy (TP-MCD-PEEM). The results are interpreted using an Arrhenius-like activation behavior and evaluated regarding energy barriers and pre-exponential factors of the skyrmion dynamics. |
