arXiv subjects

Qian Song

Publications and source records attributed to Qian Song.

At least 19 recordsLinked to original sources

arXiv2026

Foundry CMOS platform for multimodal quantum materials characterization

Quantum materials experiments increasingly rely on microwave, electrical, thermal, optical, and structural probes, but these capabilities are typically assembled from custom hardware that limits reproducibility and scalability. Here we show that a commercial 65-nm CMOS process can be repurposed as a passive, foundry-manufacturable characterization platform by functionally partitioning its metal stack into microwave, thermal, and electrical subsystems within a 1 mm2 footprint. The integrated RF architecture enables cryogenic magnetic susceptibility measurements of Fe3GeTe2 heterostructures at 1.75 K without sample-specific fabrication. We further demonstrate NV-center optically detected magnetic resonance (ODMR) with >20% contrast at 4-9 dBm microwave power, reducing power requirements by 20-25 dB relative to conventional antenna-based approaches while maintaining sensitivities of 2-3 uT/sqrt(Hz). We additionally confirm compatibility with in-situ electron-beam imaging, showing no measurable degradation in image quality upon device operation. These results establish a scalable, foundry-manufacturable platform for multimodal quantum sensing and materials characterization.

cond-mat.mtrl-sci↗

arXiv2026

Compact Syzygies for Feynman Integrals from Landau Singularities

We discuss a recent proposal for constructing ``syzygy solutions'', which play a crucial role in integration-by-parts reductions for multi-loop scattering amplitudes. We highlight a relationship between syzygies and the leading Landau singularities of Feynman diagrams and discuss how this implies that integral relations are controlled by infrared singularities. We use this insight to systematically construct highly compact, determinantal syzygy solutions, providing an explicit, simple example at one loop. We demonstrate the power of this approach by applying it to a two-loop pentabox family relevant for the NNLO corrections to $pp \rightarrow t\overline{t}H$ at the LHC.

hep-th↗

arXiv2026

Taming Symbolic IBP Reduction with Intermediate Bases

Despite many years of development in integration-by-parts reduction, reconstructing all reduction coefficients, which are rational polynomials of kinematic variables and the space-time dimension, remains a non-trivial problem. The main difficulty comes from the large number of unknowns in a general ansatz, which can lead to a linear system that is too large to solve. In this paper, we present an algorithm for reconstructing reduction coefficients through a sequence of intermediate bases. The resulting analytic reduction coefficients are products of a few analytic matrices, whose non-zero entries are simple rational polynomials. We demonstrate the efficiency of this algorithm with two cutting edge examples: a three-point massive box-triangle and a four-point massive pentagon-triangle. Reconstructing all reduction coefficients for the box-triangle (pentagon-triangle) requires 3289 (13013) numerical samplings, significantly fewer than the number of unknowns in the general ansatz, 1407406 (21638331).

hep-ph↗

arXiv2026

Anisotropic magnon transport in an antiferromagnetic trilayer heterostructure: is BiFeO$_3$ an altermagnet?

Magnons provide a route to ultra-fast transport and non-destructive readout of spin-based information transfer. Here, we report magnon transport and its emergent anisotropic nature in BiFeO$_3$ layers confined between ultrathin layers of the antiferromagnet LaFeO$_3$. Due to the confined state, BiFeO$_3$ serves as an efficient magnon transmission channel as well as a magnetoelectric knob by which to control the stack by means of an electric field. We discuss the mechanism of the anisotropic spin transport based on the interaction between the antiferromagnetic order and the electric field. This allows us to manipulate and amplify the spin transport in such a confined geometry. Furthermore, lower crystal symmetric and suppression of the spin cycloid in ultrathin BiFeO$_3$ stabilizes a non-trivial antiferromagnetic state exhibiting symmetry-protected spin-split bands that provide the non-trivial sign inversion of the spin current, which is a characteristic of an altermagnet. This work provides an understanding of the anisotropic spin transport in complex antiferromagnetic heterostructures where ferroelectricity and altermagnetism coexist, paving the way for a new route to realize electric-field control of a novel state of magnetism.

cond-mat.mes-hall↗

arXiv2025

Feynman Integral Reduction and Landau Singularities

We propose that Feynman integral reduction is controlled by solutions of the Landau equations. We study integral relations with prescribed propagator powers using syzygy methods and discuss how syzygies can be expressed as a sum over components of the Landau singularity locus. This leads to a determinantal approach to solving the syzygy problem, giving rise to highly compact and physically transparent solutions. We demonstrate the method in applications to planar two-loop five-point integrals relevant for the $pp \rightarrow t\overline{t}H$ process. Our results suggest an efficient method of Feynman integral reduction and provide a novel physical perspective on the problem.

hep-th↗

arXiv2025

Robust Orbital-Selective Flat Bands in Transition-Metal Oxychlorides

Flat electronic bands, which amplify electron correlations by quenching kinetic energy, provide an ideal foundation for exotic quantum phases. However, prevailing strategies -- including geometrically frustrated lattices, moire superlattices and heavy-fermion physics -- suffer from inherent trade-offs among robustness, tunability and orbital selectivity, limiting their broad applicability. Here, we unveil an intrinsic orbital-selective flat-band mechanism in the van der Waals materials NbOCl2 and TaOCl2, directly observed by angle-resolved photoemission spectroscopy (ARPES) and understood through density functional theory (DFT) and Wannier analysis. Crucially, we experimentally demonstrate that this momentum-independent flat band exhibits remarkable robustness, surviving from the bulk crystal down to the few-layer limit at room temperature. Our theoretical analysis traces its origin to the hybridization between Nb-dz2 orbital chains and the Lieb-like dx2-y2 sublattice, which is further reinforced by Peierls dimerization. Our findings not only establish transition-metal oxychlorides as a robust and tunable platform for flat-band-driven correlated phases under ambient conditions, but also uncover a new orbital-selective design principle for realizing flat bands in quantum materials.

cond-mat.str-el↗

arXiv2025

Critical Points and Syzygies for Feynman Integrals

We investigate a novel theoretical structure underlying the computation of integration-by-parts relations between Feynman integrals via syzygy-based methods. Building on insights from intersection theory, we analyze the large-$\epsilon$ limit of dimensional regularization on the maximal cut, showing that total derivatives vanish on the critical locus of the logarithm of the Baikov polynomial--the locus known to govern the number of master integrals. We introduce "critical syzygies" as a distinguished subset of syzygies that captures this behavior. We show that, when the critical locus is isolated, critical syzygies generate a sufficient set of total derivatives in the large-$\epsilon$ limit. We study their structure analytically at one loop and develop a numerical approach for their construction at two loops. Our results demonstrate that critical syzygies are a valuable tool for integral reduction in cutting-edge two-loop examples, offering a novel geometric perspective on integration-by-parts relations.

hep-th↗

arXiv2025

Electrical switching of an unconventional odd parity magnet

Magnetic states with zero magnetization but non-relativistic spin splitting are outstanding candidates for the next generation of spintronic devices. Their electron-volt (eV) scale spin splitting, ultrafast spin dynamics and nearly vanishing stray fields make them particularly promising for several applications. A variety of such magnetic states with nontrivial spin textures have been identified recently, including even-parity d, g, or i-wave altermagnets and odd-parity p-wave magnets. Achieving voltage-based control of the nonuniform spin polarization of these magnetic states is of great interest for realizing energyefficient and compact devices for information storage and processing. Spin-spiral type-II multiferroics are optimal candidates for such voltage-based control, as they exhibit an inversion-symmetry-breaking magnetic order which directly induces ferroelectric polarization, allowing for symmetry protected cross-control between spin chirality and polar order. Here we combine photocurrent measurements, first-principle calculations and group-theory analysis to provide direct evidence that the spin polarization of the spin-spiral type-II multiferroic NiI2 exhibits odd-parity character connected to the spiral chirality. The symmetry-protected coupling between chirality and polar order enables electrical control of a primarily non-relativistic spin polarization. Our findings represent the first direct observation of unconventional odd-parity magnetism in a spin-spiral type-II multiferroic, and open a new frontier of voltage-based switching of non-relativistic spin polarization in compensated magnets.

cond-mat.mes-hall↗

arXiv2025

Higgs-mode electromagnon in the spin-spiral multiferroic CuBr${}_2$

Below a continuous symmetry breaking phase transition, the relevant collective excitations are due to longitudinal and transverse fluctuations of the order parameter, which are referred to as Higgs and Goldstone modes, respectively. In solids, these modes may take on a different character than the equivalent excitations in particle physics due to the diverse vacuum states accessible in condensed matter. However, the Higgs mode in particular is quite difficult to observe experimentally as it decays quickly into the lower-energy Goldstone bosons and thus has a negligible lifetime in most systems. In this work, we report evidence for a novel Higgs mode in the multiferroic material CuBr${}_2$, which shows up as a coherent oscillation in the time-resolved second harmonic generation signal upon excitation with a femtosecond light pulse. Since the spiral spin order in CuBr${}_2$ induces a nonzero electric dipole moment in equilibrium, the Higgs mode--which is due to fluctuations in the amplitude of the on-site spin expectation value--is an electromagnon, and thus acquires an inversion quantum number of -1. This is in stark contrast to the Higgs boson of particle physics, which has even parity. Moreover, the excitation described here represents an entirely new type of electromagnon, distinct from the traditional electromagnon in linear spin wave theory which is due to the Goldstone mode.

cond-mat.str-el↗

arXiv2025

Spin-forbidden excitations in the magneto-optical spectra of CrI$_3$ tuned by covalency

Spin-forbidden ($\Delta S \neq 0$) multiplet excitations and their coupling to magnetic properties are of increasing importance for magneto-optical studies of correlated materials. Nonetheless, the mechanisms for optically brightening these transitions and their generality remain poorly understood. Here, we report magnetic circular dichroism (MCD) spectroscopy on the van der Waals (vdW) ferromagnet (FM) CrI$_3$. Previously unreported spin-forbidden ($\Delta S = 1$) ${}^4A_{2\mathrm{g}} \to{}^2E_\mathrm{g}/{}^2T_{1\mathrm{g}}$ Cr${}^{3+}$ $dd$ excitations are observed near the ligand-to-metal charge transfer (LMCT) excitation threshold. The assignment of these excitations and their Cr$^{3+}$ multiplet character is established through complementary Cr $L_3$-edge resonant inelastic X-ray scattering (RIXS) measurements along with charge transfer multiplet (CTM) calculations and chemical trends in the chromium trihalide series (CrX$_3$, X = Cl, Br, I). We utilize the high sensitivity of MCD spectroscopy to study the thickness dependent optical response. The spin-forbidden excitations remain robust down to the monolayer limit and exhibit a significant magnetic field tunability across the antiferromagnetic to FM transition in few-layer samples. This behavior is associated to changes in the metal-ligand covalency with magnetic state, as supported by our CTM analysis. Our results clarify the magneto-optical response of CrI$_3$ and identify covalency as a central mechanism for the brightening and field-tunability of spin-forbidden multiplet excitations.

cond-mat.str-el↗

arXiv2024

How Certain are Uncertainty Estimates? Three Novel Earth Observation Datasets for Benchmarking Uncertainty Quantification in Machine Learning

Uncertainty quantification (UQ) is essential for assessing the reliability of Earth observation (EO) products. However, the extensive use of machine learning models in EO introduces an additional layer of complexity, as those models themselves are inherently uncertain. While various UQ methods do exist for machine learning models, their performance on EO datasets remains largely unevaluated. A key challenge in the community is the absence of the ground truth for uncertainty, i.e. how certain the uncertainty estimates are, apart from the labels for the image/signal. This article fills this gap by introducing three benchmark datasets specifically designed for UQ in EO machine learning models. These datasets address three common problem types in EO: regression, image segmentation, and scene classification. They enable a transparent comparison of different UQ methods for EO machine learning models. We describe the creation and characteristics of each dataset, including data sources, preprocessing steps, and label generation, with a particular focus on calculating the reference uncertainty. We also showcase baseline performance of several machine learning models on each dataset, highlighting the utility of these benchmarks for model development and comparison. Overall, this article offers a valuable resource for researchers and practitioners working in artificial intelligence for EO, promoting a more accurate and reliable quality measure of the outputs of machine learning models. The dataset and code are accessible via https://gitlab.lrz.de/ai4eo/WG_Uncertainty.

cs.LG↗

arXiv2024

Nature of excitons and their ligand-mediated delocalization in nickel dihalide charge-transfer insulators

The fundamental optical excitations of correlated transition-metal compounds are typically identified with multielectronic transitions localized at the transition-metal site, such as $dd$ transitions. In this vein, intense interest has surrounded the appearance of sharp, below band-gap optical transitions, i.e. excitons, within the magnetic phase of correlated Ni$^{2+}$ van der Waals magnets. The interplay of magnetic and charge-transfer insulating ground states in Ni$^{2+}$ systems raises intriguing questions on the roles of long-range magnetic order and of metal-ligand charge transfer in the exciton nature, which inspired microscopic descriptions beyond typical $dd$ excitations. Here we study the impact of charge-transfer and magnetic order on the excitation spectrum of the nickel dihalides (NiX$_2$, X $=$ Cl, Br, and I) using Ni-$L_3$ resonant inelastic x-ray scattering (RIXS). In all compounds, we detect sharp excitations, analogous to the recently reported excitons, and assign them to spin-singlet multiplets of octahedrally-coordinated Ni$^{2+}$ stabilized by intra-atomic Hund's exchange. Additionally, we demonstrate that these excitons are dispersive using momentum resolved RIXS. Our data evidence a ligand-mediated multiplet dispersion, which is tuned by the charge-transfer gap and independent of the presence of long-range magnetic order. This reveals the mechanisms governing non-local interactions of on-site $dd$ excitations with the surrounding crystal/magnetic structure, in analogy to ground state superexchange. These measurements thus establish the roles of magnetic order, self-doped ligand holes, and intersite coupling mechanisms for the properties of $dd$ excitations in charge-transfer insulators.

cond-mat.str-el↗

arXiv2023

Probing dark sector fermions in Higgs precision studies and direct searches

In this paper, we investigate the discovery prospect of simplified fermionic dark sectors models through Higgs precision measurements at $e^+e^-$ colliders and direct searches at hadron colliders. These models extend the Standard Model with two Majorana or Dirac fermions that are singlets, doublets or triplets under the weak SU(2) group. For all models, we consider two scenarios where the lightest new fermion is either stable, or where it decays into other visible final states. For the Higgs precision observables we primarily focus on $\sigma(e^+e^-\to ZH)$, which can deviate from the Standard Model through one-loop corrections involving the new fermions. Deviations of 0.5\% or more, which could be observable at future $e^+e^-$ colliders, are found for TeV-scale dark sector masses. By combining the constraints from the oblique parameters, $\text{Br}(H\to\gamma\gamma)$, and direct production of the new fermions at the LHC, a comprehensive understanding of the discovery potential of these models can be achieved. In both scenarios, there exist some parameter regions where the Higgs precision measurements can provide complementary information to direct LHC searches.

hep-ph↗

arXiv2023

Spontaneous orbital polarization in the nematic phase of FeSe

The origin of nematicity in FeSe remains a critical outstanding question towards understanding unconventional superconductivity in proximity to nematic order. To understand what drives the nematicity, it is essential to determine which electronic degree of freedom admits a spontaneous order parameter independent from the structural distortion. Here, we use X-ray linear dichroism at the Fe K pre-edge to measure the anisotropy of the 3d orbital occupation as a function of in situ applied stress and temperature across the nematic transition. Along with X-ray diffraction to precisely quantify the strain state, we reveal a lattice-independent, spontaneously-ordered orbital polarization within the nematic phase, as well as an orbital polarizability that diverges as the transition is approached from above. These results provide strong evidence that spontaneous orbital polarization serves as the primary order parameter of the nematic phase.

cond-mat.supr-con↗

arXiv2023

Signatures of pressure-enhanced helimagnetic order in van der Waals multiferroic NiI$_2$

The van der Waals (vdW) type-II multiferroic NiI$_2$ has emerged as a candidate for exploring non-collinear magnetism and magnetoelectric effects in the 2D limit. Frustrated intralayer exchange interactions on a triangular lattice result in a helimagnetic ground state, with spin-induced improper ferroelectricity stabilized by the interlayer interactions. Here we investigate the magnetic and structural phase transitions in bulk NiI$_2$, using high-pressure Raman spectroscopy, optical linear dichroism, and x-ray diffraction. We obtain evidence for a significant pressure enhancement of the antiferromagnetic and helimagnetic transition temperatures, at rates of $\sim15.3/14.4$ K/GPa, respectively. These enhancements are attributed to a cooperative effect of pressure-enhanced interlayer and third-nearest-neighbor intralayer exchange. These results reveal a general path for obtaining high-temperature type-II multiferroicity via high pressures in vdW materials.

cond-mat.mtrl-sci↗

arXiv2023

Effects of Pressure on the Electronic and Magnetic Properties of Bulk NiI$_{2}$

Transition metal dihalides have recently garnered interest in the context of two-dimensional van der Waals magnets as their underlying geometrically frustrated triangular lattice leads to interesting competing exchange interactions. In particular, NiI$_{2}$ is a magnetic semiconductor that has been long known for its exotic helimagnetism in the bulk. Recent experiments have shown that the helimagnetic state survives down to the monolayer limit with a layer-dependent magnetic transition temperature that suggests a relevant role of the interlayer coupling. Here, we explore the effects of hydrostatic pressure as a means to enhance this interlayer exchange and ultimately tune the electronic and magnetic response of NiI$_{2}$. We study first the evolution of the structural parameters as a function of external pressure using first-principles calculations combined with x-ray diffraction measurements. We then examine the evolution of the electronic structure and magnetic exchange interactions via first-principles calculations and Monte Carlo simulations. We find that the leading interlayer coupling is an antiferromagnetic second-nearest neighbor interaction that increases monotonically with pressure. The ratio between isotropic third- and first-nearest neighbor intralayer exchanges, which controls the magnetic frustration and determines the magnetic propagation vector $\mathbf{q}$ of the helimagnetic ground state, is also enhanced by pressure. As a consequence, our Monte Carlo simulations show a monotonic increase in the magnetic transition temperature, indicating that pressure is an effective means to tune the magnetic response of NiI$_{2}$.

cond-mat.mtrl-sci↗

arXiv2023

Fermionic Electroweak NNLO Corrections to $e^+ e^- \to ZH$ with Polarized Beams and Different Renormalization Schemes

Recently, the next-to-next-to-leading order (NNLO) electroweak corrections with fermion loops to the Higgsstrahling process were computed. Here we present numerical results for polarized electron/positron beams, as well as for two input parameter schemes known as the $α(0)$ and $G_μ$ schemes. The size of the NNLO corrections strongly depends on the beam polarization, leading to an increase of the $ZH$ cross-section by 0.76% for $e^+_{\rm L} e^-_{\rm R}$ beams, and a decrease of 0.04% for $e^+_{\rm R} e^-_{\rm L}$ beams. Furthermore, inclusion of the NNLO corrections is found to significantly reduce the discrepancy between the results in the $α(0)$ and $G_μ$ schemes. Using the remaining difference, together with other methods, the theory uncertainty from missing bosonic electroweak corrections is estimated to be less than 0.3%.

hep-ph↗

arXiv2023

Biomass Estimation and Uncertainty Quantification from Tree Height

We propose a tree-level biomass estimation model approximating allometric equations by LiDAR data. Since tree crown diameters estimation is challenging from spaceborne LiDAR measurements, we develop a model to correlate tree height with biomass on the individual tree level employing a Gaussian process regressor. In order to validate the proposed model, a set of 8,342 samples on tree height, trunk diameter, and biomass has been assembled. It covers seven biomes globally present. We reference our model to four other models based on both, the Jucker data and our own dataset. Although our approach deviates from standard biomass-height-diameter models, we demonstrate the Gaussian process regression model as a viable alternative. In addition, we decompose the uncertainty of tree biomass estimates into the model- and fitting-based contributions. We verify the Gaussian process regressor has the capacity to reduce the fitting uncertainty down to below 5%. Exploiting airborne LiDAR measurements and a field inventory survey on the ground, a stand-level (or plot-level) study confirms a low relative error of below 1% for our model. The data used in this study are available at https://github.com/zhu-xlab/BiomassUQ .

stat.AP↗