Materials Science
Origin of the Temperature-Induced Gap Bowing of Formamidinium-Methylammonium Lead Iodide Perovskites: Role of Cationic Rattlers
Kai Xu, Adrián Francisco-López, Bethan L. Charles et al. (+4 more)
A thorough understanding of the temperature dependence of
semiconductor band gaps is essential for optimizing optoelectronic devices. In this respect, the origin of the pronounced temperature-induced gap bowing observed in low-temperature phases of formamidinium-methylammonium (FA-MA) lead iodide perovskites has remained elusive until now. By combining temperature and pressure-dependent photoluminescence measurements on a series of FA$_x$MA$_{1-x}$PbI$_3$ mixed-cation single
crystals with $x\in[0,1]$, we unravel the origin of this bowing. Both thermal expansion as well as electron-
phonon interaction effects are responsible. However, the latter is the leading term, driven by the activation of an anomalous electron-
phonon coupling mechanism linked to mixed vibrational modes, which combine inorganic-cage
phonons involving octahedral tilting with low-frequency FA librations, i.e., FA rattler modes. This occurs in the orthorhombic and (pseudo)tetragonal low-temperature phases, presumably featuring stripe domains with alternating octahedral tilt-axis patterns for FA concentrations between 20% and 90%. In this way, we have shed light on an intriguing behavior of lead halide perovskites that directly affects their optoelectronic properties.
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All authors Kai Xu, Adrián Francisco-López, Bethan L. Charles, M. Isabel Alonso, Miquel Garriga, Mark T. Weller, Alejandro R. Goñi
Other Condensed Matter
Antisymmetric spontaneous resistivity anisotropy due to hard-axis collapse in polycrystalline Co thin films
Y. Fernandes, J. Geshev, A. M. H. de Andrade et al. (+1 more)
We investigate magnetoresistance phenomena associated with the magnetization hard-axis collapse in polycrystalline Co thin films. Transport measurements reveal that, for specific orientations of the applied
magnetic field, the system exhibits distinct remanent resistance levels in both the in-plane longitudinal and transverse voltage responses. In particular, the planar
Hall resistance shows multiple stable and reproducible levels at room temperature, enabling the identification of at least three remanent states that can be distinguished and used for information storage. These resistance levels originate from non-uniform
magnetic configurations stabilized after the application and removal of the external
magnetic field in the hard-axis region. Since this phenomenon remains largely unexplored, we present an incipient study addressing its potential implications from an applied-physics perspective. The observation of such behavior in polycrystalline Co thin films grown on Si substrates suggests a simple and low-cost platform for spintronic
memory and sensing devices based on the remanent planar
Hall effect.
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All authors Y. Fernandes, J. Geshev, A. M. H. de Andrade, A. D. C. Viegas
Materials Science
Electron Polaron at Neutral 180$^\circ$ Domain Wall in PbTiO$_3$: Stability, Trapping Energies, and Transverse Polarization
Mohammad Amirabbasi, Jochen Rohrer and Karsten Albe
We use density-functional theory with a Hubbard correction to investigate Ti-centered electron polarons at neutral PbO-centered $180^\circ$ domain walls in tetragonal PbTiO$_{3}$. The Hubbard parameter for Ti $3d$ states is determined using the finite-size-corrected polaronic energy-level
alignment procedure, yielding stable electron-polaron formation in bulk PbTiO$_3$ with a trapping energy of $-$0.06 eV. In the domain-wall supercell, the excess electron localizes on Ti and forms a Ti$^{3+}$ center with an occupied $d_{xy}$ orbital in-gap state. Comparison of bulk-like and near-wall Ti sites shows that their trapping energies differ by only about 0.01 eV, indicating that this neutral domain wall does not provide a significant thermodynamic driving force for electron-polaron segregation. While the Ising-like reversal of the out-of-plane polarization is preserved, the localized electron induces a finite transverse polarization component normal to the wall, enhancing a local Néel-like distortion that is strongest when the polaron is located at the wall. These results show that neutral $180^\circ$ domain walls in PbTiO$_3$ do not substantially alter the stability of Ti-centered electron polarons, but they can couple to the polaron-induced
lattice distortion through a localized transverse polarization response.
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Materials Science
Molecular Dynamics Study of Defect Evolution Mechanisms in 3C-SiC for Quantum Technologies
Irslan Ullah Ashraf, Gaetano Calogero, Ioannis Deretzis et al. (+4 more)
The migration of point defects and formation of
spin defects in 3C-SiC were investigated using molecular dynamics simulations, with migration barriers obtained from Nudged Elastic Band (NEB) calculations and finite temperature diffusivities evaluated using both mean square displacement (MSD) and jump frequency approaches. While both methods reproduce Arrhenius behavior, the jump frequency formulation exhibits improved statistical stability. Activation energies of 2.12~eV for carbon vacancies and 0.88~eV for carbon interstitials are obtained, consistent with literature. The resulting mobility hierarchy governs defect
evolution and complex formations. Interstitial vacancy recombination competes with vacancy aggregation into divacancies, influencing the stabilization of
spin active defect centers. The study also provides a consistent framework for
diffusion analysis in atomistic simulations.
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All authors Irslan Ullah Ashraf, Gaetano Calogero, Ioannis Deretzis, Giorgio Lo Presti, Damiano Ricciarelli, Elisabetta Paladino, Antonino La Magna
Mesoscale and Nanoscale Physics
Gaussian-Bond Hall Viscosity and Nonlocal Hall Response of a Sampled Landau Level
Danyu Shu, Ryotaro Sano, Ai Yamakage et al. (+2 more)
Hall viscosity on a
lattice depends on how strain changes microscopic hopping amplitudes. For a specified Gaussian envelope, bond-length derivatives close on the undeformed hopping, expressing the stress as an embedded momentum Hessian and relating the viscosity to higher-order band geometry. In the Kapit–Mueller parent family at rational flux, we show that the stress response equals the strain
curvature of the sampled-LLL Gram
matrix. The density-normalized viscosity approaches the continuum value 1/4, with an exponentially small sampling correction at finite flux. By contrast, for a straight-bond Peierls source, a low-frequency mass, and a fixed-envelope susceptibility, the viscosity–conductivity comparison exhibits an algebraic deviation, whose leading coefficient follows from a weak-field expansion.
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All authors Danyu Shu, Ryotaro Sano, Ai Yamakage, Hiroshi Funaki, Mamoru Matsuo
Materials Science
Individual Characterization of Fast-Responding Trap States at the NO-Annealed SiO$_2$/4H-SiC Interface
Takahiro Ono, Mizuki Ohashi, Tomohiro Shigeno et al. (+4 more)
Fast-responding trap states introduced by NO-annealing are suspected to limit the channel mobility of 4H-SiC MOSFETs, yet their microscopic characterization remains challenging because conventional electrical methods are spatially averaged and do not readily isolate such fast processes. Here, we visualize and analyze individual fast-responding trap states at the NO-annealed SiO$_2$/4H-SiC interface using the energy dissipation signal in frequency-modulation atomic force microscopy (FM-AFM), which selectively probes charge-exchange dynamics on sub-$\mu$s time scales. Ring-shaped dissipation patterns were observed in the NO-annealed sample but not in the control sample without NO-annealing, indicating that the detected states are associated with nitridation. Spectroscopic measurements were also performed to determine the dependence of energy dissipation on the tip bias and the tip-sample distance. Combined with finite-element electrostatic calculations, this analysis allowed us to determine trap energies relative to the
Fermi level, $E_t - E_f$, and revealed that the trap-energy distribution extends toward the interfacial conduction-band edge. These results provide microscopic evidence that NO-annealing generates fast-responding trap states near the SiO$_2$/4H-SiC interface.
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All authors Takahiro Ono, Mizuki Ohashi, Tomohiro Shigeno, Yutaro Uchida, Yuuki Yasui, Koji Kita, Yoshiaki Sugimoto
Soft Condensed Matter
Designing Multivalent Copolymers for Selective Targeting of Multicomponent Surfaces
Vid Ravnik, Urban Bren and Tine Curk
Selective targeting of membranes with a specific receptor profile is an ongoing challenge in targeted drug delivery. We investigate the adsorption of copolymers on a multicomponent receptor-covered surface using grand-canonical Monte Carlo simulations and demonstrate that
polymers can be designed to target a particular receptor density profile. To achieve this, the ligand profile on the
polymers should match the targeted receptor profile, and the ligand–receptor affinity should be inversely proportional to the ligand profile. While the same can be obtained using multivalent nanoparticles, the entropic effects due to
polymer conformations significantly enhance the binding selectivity of multivalent
polymers compared to nanoparticles. Surprisingly, the ligand distribution on the
polymer plays a crucial role, whereas the persistence length does not. The optimal selectivity to the overall receptor concentration is obtained by the Poisson distribution of ligands (random copolymer), whereas the maximal selectivity to a specific receptor profile is obtained by a defined sequence of grouped alternating ligands (regular copolymer). Interestingly, the regular copolymer can become anti-selective when ligands of the same type are in homogenous blocks, showing that specific ligand distribution qualitatively affects the targeting ability. These findings suggest that sequence control is necessary to selectively target a specific density profile of membrane receptors using linear copolymers.
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Materials Science · Cross-listed from physics.optics
Probing the Effects of Heat Treatment Atmosphere on the Structural and Electrical Properties of NBT via Eu Photoluminescence
Zongxue Wang, Duanting Yan and Hancheng Zhu
The effect of oxygen partial pressure during the pre-calcination step (high
vacuum, air, nitrogen, and oxygen) on the
crystal structure, microstructure, and electrical properties of Na0.5Bi0.465Sr0.02Eu0.005TiO3
oxide-ion-conducting ceramics was systematically investigated. Dense ceramic samples were prepared by a conventional solid-state reaction route under different atmospheres. The results show that the oxygen partial pressure strongly affects Bi volatilization, grain growth, and oxygen-vacancy concentration. The largest average grain size was obtained for the nitrogentreated sample, whereas the oxygen-treated sample exhibited the finest grains but the highest grain-boundary conductivity. X-ray diffraction and Raman spectroscopy indicate that low oxygen partial pressure enhances structural disorder, while high oxygen partial pressure stabilizes the
lattice and promotes charge-transfer transitions. Eu3+ photoluminescence further reveals the correlation between local structural
evolution and oxygen-vacancy concentration. These findings clarify how oxygen partial pressure regulates grain-boundary behavior and ion-transport mechanisms in NBT-based
oxide-ion conductors, providing guidance for optimizing the total conductivity of polycrystalline electrolytes.
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Mesoscale and Nanoscale Physics
Anomaly-Induced Hybrid Bulk Electromagnetic Mode in Weyl Semimetals
Subrahmanyam D, Suhas Gangadharaiah and E. G. Mishchenko
Collective modes provide direct fingerprints of
quantum matter. We predict a previously unidentified hybrid bulk electromagnetic mode in Weyl semimetals arising from the interplay between the
chiral anomaly and the orientation of its associated
chiral magnetic response relative to the direction of the wave-vector. When the anomaly-induced
chiral magnetic current has a component along the propagation direction, oscillations of valley imbalance hybridize with plasmonic charge oscillations, producing a linearly dispersing mode that undergoes avoided crossing with the bulk plasmon, producing a hybrid bulk excitation absent in ordinary metals. The hybrid mode provides a direct signature of Weyl semimetals and a probe of the
chiral anomaly and its associated
chiral magnetic effect, with observable features in electron energy-loss spectroscopy. Studying this interplay can uncover various optical and electronic properties of Weyl semimetals.
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Soft Condensed Matter
Kinetic Superselectivity in Multivalent Binding
Vid Ravnik, Baptiste Chabaud, Urban Bren et al. (+2 more)
Multivalent binding employs multiple simultaneous supramolecular interactions, increasing avidity and selectivity compared with monovalent binding. While
equilibrium aspects of multivalency are well characterized, non-
equilibrium behavior remains poorly understood. By combining experiments on hyaluronic acid
polymers with kinetic modeling based on
stochastic chemical kinetics and molecular dynamics simulations, we systematically investigate the kinetics of multivalent binding. Notably, we find that both association and dissociation kinetics can be more selective than
equilibrium binding. We explain this behavior using a two-step binding model featuring a combination of fast, weak and slow, strong interactions. These findings demonstrate a new approach: superselective targeting based on the association rate instead of the
equilibrium state. The kinetic theory and experiments presented here provide a fundamental understanding of multivalent kinetics and establish design rules for superselective targeting in out-of-
equilibrium systems.
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All authors Vid Ravnik, Baptiste Chabaud, Urban Bren, Galina V. Dubacheva, Tine Curk
Other Condensed Matter · Cross-listed from hep-ph
Looking for Condensed Gluons: A Cross-Scale Journey from the Deep Structure of Protons to High-Energy Cosmic Rays – A Mini-Review
Wei Zhu, Yu-Chen Tang, Ye-Yin Zhao et al. (+2 more)
Quark-
gluon dynamics within
protons and high-energy radiation phenomena in the universe are typically regarded as two entirely distinct fields. This paper aims to demonstrate that
gluon condensation (GC) may serve as a direct bridge between these two fields. We review three key aspects of GC research: first, the Zhu-Shen-Ruan (ZSR) equation, as a nonlinear
evolution equation based on structural
symmetry, exhibits self-consistent connections with the DGLAP, BFKL and GLR-MQ-ZRS equations, providing a theoretical foundation for the generation of GC; second, the chaotic solutions and the shadowing-antishadowing synergy inherent in this equation can drive
gluons to aggregate near the critical momentum, thereby forming a novel type of high-density, strongly interacting matter; third, these changes in microstructure manifest themselves as a broken-power-law feature in high-energy
cosmic gamma-ray spectra, thereby offering new insights into the hadronic scenarios underlying certain astrophysical sources. Consequently, GC not only concerns the novel behaviour of
quantum chromodynamics under extreme conditions but may also serve as a vital window for probing the deep structure of
protons using
cosmic-ray signals. With the advancement of higher-precision gamma-ray observations,
hadron collision experiments and related theoretical research, the physical picture of GC and its observational criteria are expected to undergo more rigorous testing. Should this picture be confirmed, certain features in the high-energy gamma-ray spectrum will need to be re-examined within the deeper context of hadronic dynamics; simultaneously, GC may also provide a new entry point for research into pion condensation in
nuclear physics and even condensed matter physics. Consequently, the significance of the search for GC extends beyond the model itself, reaching into multiple fields of natural science.
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All authors Wei Zhu, Yu-Chen Tang, Ye-Yin Zhao, Bo Yang, Yu-Chen Xiong
Disordered Systems and Neural Networks · Cross-listed from cs.LG
Sampling Data with Chains of Forward-Backward Diffusion Steps
Hyunmo Kang, Noam Itzhak Levi, Corinna Elena Wegner et al. (+2 more)
Sampling from learned high-dimensional distributions is a foundational computational problem. We introduce U-turn chains: Markov chains obtained by iterating short forward-backward steps of a
diffusion model, in which each step proposes a move that remains on the learned data
manifold and, paired with a Metropolis-Hastings correction, samples from energy-modified targets. For synthetic
languages, we show that minimal U-turn dynamics undergoes an ergodicity-breaking phase transition driven by fragmentation of the data
manifold; ergodicity is restored at larger U-turn magnitude. In the non-ergodic regime, low-level features relax faster than high-level ones, an ordering that inverts only at sufficiently large U-turn magnitude. We test these predictions on natural
language and natural images. In both modalities, minimal U-turns relax slowly, especially for high-level features approximated by deep representations in CNNs or LLMs. The layer-ordering inversion appears only at large noise when mixing is efficient – signatures consistent with strongly constrained, weakly mixing local dynamics. We discuss the implications of these results for sampling with
diffusion models.
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All authors Hyunmo Kang, Noam Itzhak Levi, Corinna Elena Wegner, Daniel J. Korchinski, Matthieu Wyart
Materials Science
Temperature dependence of the magnon-phonon coupling in yttrium iron garnet/gadolinium gallium garnet high overtone bulk acoustic resonators
Johannes Weber, Manuel Müller, Mikhail Cherkasskii et al. (+7 more)
Weexperimentally study the temperature dependence of the
magnon-
phonon coupling in a yttrium iron garnet (YIG)/gadolinium gallium garnet (GGG)
heterostructure. More specifically, we use broadband
ferromagnetic resonance to investigate the magneto-elastic coupling between the Kittel mode of a YIG thin film and the transverse acoustic
phonon modes of the YIG/GGG high overtone bulk acoustic wave acoustic resonator for in and out-of-plane field directions in the temperature range between T = 5K and 300K. We find that for a
magnetic field applied normal to the film surface, magneto-elastic coupling decreases with decreasing temperature, whereas it increases for the in-plane
magnetic field configuration. The observed temperature dependence differs from earlier observations on bulk YIG samples, which might be due to the temperature dependent stress imposed by the GGG substrate.
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All authors Johannes Weber, Manuel Müller, Mikhail Cherkasskii, Stephan Geprägs, Rudolf Gross, Christian H. Back, Sebastian T. B. Goennenwein, Silvia Viola Kusminskiy, Matthias Althammer, Hans Huebl
Disordered Systems and Neural Networks · Cross-listed from stat.ML
Signal-to-Noise Ratio and Sample Size Govern Representational Alignment in Neural Networks
Ali Hussaini Umar and Alessandro Laio
Neural networks are known to develop latent representations that are $aligned$, namely structurally similar across networks trained with different architectures, training protocols, or training
datasets. We study this phenomenon in a controlled setting, where we train an ensemble of networks on regression and classification tasks using training sets perturbed by independent realizations of a noise process. We show that the signal-to-noise ratio (SNR) and the training sample size influence the
alignment in qualitatively similar ways in networks trained on real-world
datasets and in an extremely simple $linear$ network with a single hidden layer, for which the
alignment can be estimated analytically. Across linear and nonlinear networks, regression and classification tasks, and both synthetic and real-world data, we consistently observe that
alignment varies monotonically with SNR but non-monotonically with training sample size. In particular, the
alignment is minimized near the interpolation threshold, and a stronger
alignment does not necessarily correspond to better generalization error. These findings reveal a non-trivial dependence of
alignment on data quality and quantity, decoupled from generalization performance.
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Strongly Correlated Electrons
Inter-Landau-Level Pair Tunneling Rigidifies the Long-Wavelength Structure Factor of the {\nu}=1/3 Laughlin State
Hongyi Liu
A textbook chain of
reasoning, originating with Girvin, MacDonald, and Platzman, predicts that Landau-level mixing (LLM) at nu=1/3 should soften the long-wavelength structure factor S(L): virtual excitations suppress V_1, the magnetoroton gap closes, and the single-mode approximation then forces S(L) to increase. We test this chain on the Haldane sphere at nu=1/3, N=6, 2Q=15. (i) Perturbatively, via first-order Schrieffer-Wolff dressing of the LLL Laughlin state, fed Lowdin-renormalised pseudopotentials: the dressed state gives Delta S(1) at kappa=10 of approximately +21%, consistent with the GMP prediction (~19% softening of V_1^eff). (ii) Non-perturbatively, via rigorous two-Landau-level (LLL+SLL) exact diagonalisation using a repaired DiagHam 2LL module that retains, for the first time on the sphere, the full inter-LL scattering channels. The response reverses sign: S(1) rigidifies monotonically from 0.118 at kappa->0 to 0.080 at kappa=10, a 32% suppression. Perturbative and non-perturbative results thus disagree in sign, with a gap of ~53 percentage points between +21% (SW) and -32% (full ED). Channel-mask ablation localises the reversal to the DD<->UU pair-tunneling channel (98% of the shift). A four-way decomposition shows the three-body coherent content overpowers the perturbative admixture by ~2.5x. Repaired code: https://github.com/Cannan-1/DiagHamOnSphereOpenSource (v2.0).
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Statistical Mechanics
A tridiagonal matrix-valued process with stochastic resetting for arbitrary Dyson index $\beta>0$
Gernot Akemann, Satya N. Majumdar and Patricia Päßler
We introduce a symmetric tridiagonal
matrix-valued process ($\beta$-TMP) $H(t)$ whose diagonal entries $H_{k,k}(t)$ evolve independently via an Ornstein-Uhlenbeck process starting at the origin and the off-diagonal entries $H_{k,k+1}(t)$ evolve independently via the Cox-Ingersoll-Ross process, starting at the origin and with parameters that depend on the row index $k$. We show that the joint distribution of the entries of the
matrix can be computed exactly at all times and moreover, the joint distribution of its $N$ real
eigenvalues can be computed exactly at all times too. We then subject this time-evolving
matrix-valued process to
stochastic resetting with rate $r$ in two different settings: (i) simultaneous resetting of the
matrix entries to the origin with rate $r$ ($\beta$-SRTMP process) and (ii) independent resetting of the
matrix entries to the origin with rate $r$ ($\beta$-IRTMP process). We show that the joint distribution of the
eigenvalues of the $\beta$-SRTMP process at long times can be computed analytically and it coincides with the joint distribution of the positions of the resetting Dyson Brownian motion in its stationary state for arbitrary $\beta>0$. For the $\beta$-IRTMP stationary ensemble, computing analytically the joint distribution of
eigenvalues or even the average density of
eigenvalues is difficult. However, generating the stationary $\beta$-IRTMP ensemble numerically is relatively straightforward and we compare its numerical average
eigenvalue density to the corresponding analytical results for the $\beta$-SRTMP stationary ensemble with same parameter values, showing that they are quite different from each other. Finally, we provide a simple and concrete application of this tridiagonal
matrix-valued process in computing the annealed partition function of a disordered
quantum tight-binding
Hamiltonian on a one-dimensional
lattice.
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Statistical Mechanics
Long-range deformations in Gaussian States
Francisco Pereira, Nandagopal Manoj and Sara Murciano
Imaginary-time
evolution by a local
Hamiltonian cannot induce a phase transition in one dimension, but longer-range interactions may subvert such constraints. Starting from the ground state of the Kitaev Majorana chain, we modify the wave function by an imaginary-time
evolution generated by a quadratic
Hamiltonian with power-law couplings that enhance pairing correlations, typically of the form $1/r^{\alpha}$, where $r$ is the distance between two sites. As the state remains Gaussian,
entanglement and correlation functions can be computed analytically. We find that the decay exponent $\alpha$ controls three distinct infrared regimes: for $\alpha>1$, the deformation produces only smooth crossovers at finite deformation strength, while the topological regime is reached only asymptotically as the deformation strength tends to infinity. At $\alpha=1$, the deformation induces an immediate flow to the topological phase: an infinitesimal deformation strength drives the system to a topological regime, and in a particular case, an emergent Kramers-Wannier
symmetry enforces Ising-like scaling at long distances. For $\alpha<1$, the deformed state shows the same critical-like behavior for all non-zero deformation strength. We observe that even with arbitrarily long-range interactions, these models do not display a sharp phase transition at non-zero deformation strength.
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Soft Condensed Matter
Structure and energetics of grain boundaries in self-assembled double-gyroid block copolymer networks
Jing Chen, Aiping Zhu, Dan Wei et al. (+2 more)
Grain boundaries (GBs) are ubiquitous defects in crystalline materials, but remain less explored in block copolymer ordered phases. Here, we develop a self-consistent field theory framework to investigate GB structure and energetics in double-gyroid (DG) diblock copolymer networks. Two energy landscapes are obtained for tilt and twist GBs as functions of GB orientations revealing multiple local minima. The lowest-energy GB among the sampled GBs is an asymmetric-tilt network-switching GB ($[\bar{1}\bar{1}1]$ ATNS) rather than the experimentally observed $(422)$ twin boundary (TB) among the sampled GBs. Comparative analyses of tilt ($[\bar{1}\bar{1}1]$, $[100]$ ATNS) and twist ($(422)$ TB, $[0\bar{1}\bar{1}]$ TNS) GBs reveal that, unlike enthalpy-dominated hard matter, GB stability in DG networks is
entropy-driven. Twist-type GBs generate new nodes and disrupt nodal coplanarity, causing chain-packing frustration and
entropy penalties. Although the [100] ATNS does not generate new nodes, its disruption of nodal coplanarity still incurs large
entropy penalties. Conversely, the $[\bar{1}\bar{1}1]$ ATNS preserves favorable network connectivity and minimizes conformational constraints, making it the energetically preferred GB.
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All authors Jing Chen, Aiping Zhu, Dan Wei, An-Chang Shi, Kai Jiang
Soft Condensed Matter
Dynamics of ring polymer melts: Memory function approach
Nail Fatkullin, Carlos Mattea, Kevin Lindt et al. (+2 more)
We investigated the static and dynamic properties of a Rouse ring
polymer modified by introducing an effective, spherically symmetric, attractive potential of entropic nature and a
memory function describing the effect of dynamic
entanglement. Renormalized Rouse formalism is used to approximate the time dependence of the
memory matrix. The results obtained are in good agreement with existing experimental data and the results of computer simulations of ring
polymer ring with , , where N_e is the number of Kuhn segments in linear
polymer melts between neighboring entanglements and , the number of Kuhn segments. For large molecular weights, a refined self-consistent approximation is proposed to describe the time dependence of the
memory function. It is shown that this approximation allows us to describe an exponential decrease in the self-
diffusion coefficient with molecular weight of the rings, i.e., the effect of dynamic localization.
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All authors Nail Fatkullin, Carlos Mattea, Kevin Lindt, Siegfried Stapf, Margarita Kruteva
Materials Science
Active learning of collinear magnetic Moment Tensor Potentials using the spin-MLIP package from soft-constrained spin-polarized DFT calculations: a case study of Fe-Pd
Arseniy Burov, Alexey S. Kotykhov, Dmitry A. Aksyonov et al. (+2 more)
Explicit incorporation of
magnetic degrees of freedom in machine-learning interatomic potentials (
magnetic MLIPs) plays a crucial role in the correct description of
magnetic materials and their properties. An important ingredient for fitting of
magnetic MLIPs is
spin-polarized density functional theory (DFT) calculations with non-
equilibrium magnetic moments, i.e. DFT calculations with constraints on
magnetic moments. In this study, we present a workflow for active learning of
magnetic Moment Tensor Potential (mMTP) during molecular dynamics (MD) simulations.
Magnetic MTP and its active learning
algorithm were implemented in the open-source
spin-MLIP code, DFT soft-constrained
spin-polarized calculations were performed with the VASP code, and MD simulations were conducted in the open-source LAMMPS code. We test our workflow on the Fe-Pd
crystal. The dependencies of magnetization and
phonon density of states (DOSs) on the volume of a supercell (or, pressure) are in good agreement with those calculated with DFT. Furthermore, the calculated DOSs correspond to the experimental ones.
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All authors Arseniy Burov, Alexey S. Kotykhov, Dmitry A. Aksyonov, Ivan S. Novikov, Vladimir V. Ladygin
Statistical Mechanics · Cross-listed from hep-th
Constrained Symplectic Quantization II: The Free Scalar Field
Francesco Scardino, Martina Giachello and Giacomo Gradenigo
Constrained
symplectic quantization is a functional formulation of
quantum field theory in which
quantum fluctuations are sampled through a deterministic
Hamiltonian flow in an auxiliary intrinsic time $\tau$. In this paper we extend the
quantum-mechanical framework introduced in [1] to a relativistic scalar
quantum field theory in Minkowski space-time. The construction is based on the analytic continuation of fields and action from $\mathbb{R}$ to $\mathbb{C}$ together with constraints that select stable intrinsic-time trajectories and, at the same time, define convergent integration cycles for the corresponding microcanonical functional. We show that, in the continuum limit, the microcanonical generating functional reproduces the Feynman generating functional. For the free scalar field in $1+1$ dimensions we derive the constrained equations of motion, implement the resulting dynamics numerically, and verify real-time two-point correlators, equal-time commutator relations, and Dyson–Schwinger equations including the expected contact terms.
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Mesoscale and Nanoscale Physics
Orbital Magnetization from Uniform and Periodic Magnetic Fields
Chunli Huang
Magnetization is thermodynamically defined as the derivative of the grand potential with respect to a uniform
magnetic field. However, a uniform
magnetic field makes the kinetic momentum operators noncommuting and Landau-quantizes the electron motion. This changes the zero-field momentum-space to Landau-levels and raises a fundamental question: how can the thermodynamic response to a uniform field be reproduced by a linear-response calculation carried out in the momentum space of the zero-field problem? We address this question analytically in a
quantum Hall ferromagnet that allows the orbital magnetization $M$ to be computed in a closed form. We first compute $M$ from the local Hartree–Fock projector response to a periodic
magnetic field with zero net flux. We then compute $M$ from the derivative of the grand potential with respect to a uniform
magnetic field along the Středa line. The two approaches give the same result, even though the first keeps the
Hilbert space fixed while the second changes the Landau-level degeneracy. Their agreement suggests that we should view orbital magnetization as the energy associated with the spectral flow that gives rise to the Středa formula. Our work provides a tutorial introduction to orbital magnetization and its relation to the Středa formula.
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Strongly Correlated Electrons
Self-Consistent Spectral Quadrature Approach to Many-Body Green Functions
Stanislav Yu. Kruchinin
We develop a self-consistent spectral quadrature (sc-SQ) framework for the calculation of many-body Green functions. The method approximates the Källén–Lehmann spectral measure by Gauss–Christoffel (GC) quadrature, yielding a rational Green function representation with guaranteed spectral positivity that exactly reproduces the first $2N$ spectral moments at pole order $N$. A key component is an SVD-based rank-selection criterion on the Hankel
matrix, which identifies the numerically resolvable pole rank $N^*$ from the singular-value gap and acts as a precision-guided diagnostic of correlation complexity. The scheme is made self-consistent by requiring that the spectral function used to evaluate expectation values coincides with the spectral function generated by the quadrature reconstruction. This defines a fixed-point hierarchy that connects systematically to established approximations, including Hartree–Fock and Hubbard-I, and incorporates non-perturbative features such as multi-peak spectral structure. We
benchmark the approach for the Anderson
impurity model against numerical renormalization group (NRG) results and apply it within dynamical mean-field theory for the Hubbard model on the Bethe
lattice. The method captures the three-peak Anderson
impurity spectrum and the suppression of quasiparticle weight in the half-filled Hubbard model on the Bethe
lattice, including Mott-gap formation on the insulating branch for $N\geqslant 5$, in qualitative agreement with NRG references.
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Materials Science
Ultrafast signatures of Dirac / flat-band hybrid states from time-resolved ARPES
Maria-Elisabeth Federl, Johannes Gradl, Franziska Bergmeier et al. (+12 more)
Hybridization of highly itinerant Dirac electrons with localized flat-band states is predicted to yield emergent phenomena such as exotic heavy-
fermion behaviour. Epitaxial
graphene on two-dimensional adsorbate structures on SiC(0001), which host flat bands, offers a promising platform to explore these effects. However, direct experimental evidence of interlayer hybridization in such systems has so far been lacking. Here, we address this gap using time- and angle-resolved photoemission spectroscopy (trARPES) where interlayer hybridization manifests in three key observations: (1) accelerated Dirac-carrier relaxation arising from additional electronic and phononic decay channels provided by the flat-band subsystem, (2) transient charging of the Dirac cone enabled by direct optical excitation from the flat bands, and (3) ultrafast back-transfer of charge into the flat bands on timescales governed by the interlayer coupling strength. We further demonstrate that the degree of hybridization can be tuned via the atomic number of the atoms intercalated at the
graphene-SiC interface, establishing a controllable platform for investigating exotic correlated ground states.
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All authors Maria-Elisabeth Federl, Johannes Gradl, Franziska Bergmeier, Leonard Weigl, Lukas Bruckmeier, Theresa Glaser, Zamin Mamiyev, Christoph Tegenkamp, Niclas Tilgner, Thomas Seyller, Teresa Tschirner, Domenica Convertino, Stiven Forti, Camilla Coletti, Isabella Gierz
Materials Science
Electronic properties governing the phase stability and elastic anisotropy of C14 and C15 Cr-Hf-Nb Laves phases
M. Díaz-Choque, C. F. Schuch and L. T. F. Eleno
This study utilizes Density Functional Theory (DFT) to investigate the thermodynamic stability, elastic anisotropy, and electronic properties of C14 and C15 Laves phases within the Cr–Hf–Nb system. Both formation enthalpies and comprehensive elastic property analyses confirm the energetic and mechanical stability of the C14 (HfNb$_2$, HfCr$_2$, NbCr$_2$) and C15 (HfCr$_2$, NbCr$_2$) phases. Furthermore, the evaluation of elastic anisotropy reveals a descending order of HfCr$_2$ > NbCr$_2$ > HfNb$_2$ for the C14 phase, contrasting with NbCr$_2$ > HfNb$_2$ > HfCr$_2$ for the C15 phase. Finally, electronic structure and COHP analyses indicate that strong anti-bonding behavior near the
Fermi level within the XM$_2$ M–M bonds acts as a primary destabilization mechanism for both of these Laves phases.
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Materials Science
Radiative electronic bound states in the continuum from defects in semiconductors
Seong Yun Hong, Liang Z. Tan, Ki Hoon Lee et al. (+2 more)
Continuum-buried defect states in
semiconductors are generally expected to be optically inactive due to their strong coupling to continuum bands. Here, we show that such defects can instead host radiative electronic bound states in the continuum (BICs), using the silicon G-center as a prototypical example. Hybrid-functional first-principles calculations with a Hubbard $U$ correction reveal that a localized defect state, initially buried below the valence band maximum (VBM) in the ground state, undergoes exchange-driven energy-level reordering under optical excitation and shifts above the VBM. This exchange-induced transition suppresses nonradiative decay and enables robust radiative emission. By computing temperature-dependent nonradiative lifetimes and comparing them with experimental photoluminescence (PL) lifetimes, we quantitatively reproduce the observed temperature dependence of the emission. These results uncover a stabilization mechanism for continuum-embedded defect states and establish electronic BICs as a general paradigm for designing defect-based optical systems, including
quantum emitters and
qubits.
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All authors Seong Yun Hong, Liang Z. Tan, Ki Hoon Lee, Youngho Kang, Yeonghun Lee
Mesoscale and Nanoscale Physics
Terahertz spin-current transparency through rough interfaces
Jiří Jechumtál, Jakub Zázvorka, Ondřej Novák et al. (+8 more)
Spin transport across interfaces is critical for spintronic devices, yet remains difficult to probe on ultrafast timescales. We use terahertz emission spectroscopy on Co|Pt
heterostructures whose interface roughness is tuned through the thickness of an underlying Au buffer layer, while leaving other growth parameters unchanged. From the measured THz electric field, we extract the interface
spin-current transparency ts after correcting for the changes in sample impedance and optical absorption of the stack. Surprisingly, we find that ts decreases by only approximately 30% as the interface root-mean-square roughness and the lateral grain size both increase by a factor of three, with no measurable change in the THz spectrum. These results demonstrate that interfacial
spin transport is relatively robust against morphological variations on ultrafast timescales, establishing terahertz emission spectroscopy as a reliable probe of
spin dynamics across imperfect interfaces.
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All authors Jiří Jechumtál, Jakub Zázvorka, Ondřej Novák, Martin Rejhon, Peter Kubaščík, Lukáš Nowak, Petr Němec, Eva Schmoranzerová, Martin Veis, Lukáš Nádvorník, Zdeněk Kašpar
Quantum Gases
Influence of helicoidal spin-orbit coupling and Rabi coupling in dynamics of 2D Bose-Einstein Condensates
Sai Satyaprakash Biswal and S. Saravana Veni
This study explores the dynamics of Bose-Einstein condensates (BECs) with helicoidal
spin-orbit coupling (SOC) and Rabi coupling, confined under a two-dimensional harmonic potential. The relationship between helicoidal SOC, non-linear interactions along with Rabi coupling and their impact on the stability and non linear trapped modes of the condensate is analyzed using a coupled Gross-Pitaevski (GP) framework. A linearized GP equation is used to investigate modulation instability (MI), demonstrating the impact of strong coupling effects and anisotropic confinement on the instability dynamics. It has been shown that the modulation instability in the condensate is predominantly governed by the competition between the attractive and repulsive mean-field interactions. Additionally, stability regimes are altered by the harmonic confinement, enhancing their susceptibility to SOC-induced asymmetry and intra- and intercomponent interactions. These results shed light on the possibility for unique
quantum phases and emergent characteristics of helicoidal SOC-driven condensates.
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Statistical Mechanics · Cross-listed from quant-ph
Nonclassical energy-change distribution as a witness of non-Markovian quantum dynamics
Marco Pezzutto, Anton Corr, Gabriele De Chiara et al. (+2 more)
We address the problem of identifying non-Markovian
quantum time evolutions of an open
quantum system by only performing measurements of the system's energy. We demonstrate that violations of CP-divisibility are always witnessed by non-positive values of the energy-change Kirkwood-Dirac quasiprobability distribution associated with the system's
Hamiltonian, evaluated at consecutive times. The link between non CP-divisibility and non-positivity of the system's energy-change distribution is stronger when the system-environment interactions are energy-preserving. The witness works whenever anomalous energy fluxes, due to non-Markovianity, are realized. Anomalous fluxes are also detected by the non-Markovianity measure built over the
quantum mutual information between the states of the open system and of a
quantum correlated reference.
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All authors Marco Pezzutto, Anton Corr, Gabriele De Chiara, Salvatore Lorenzo, Stefano Gherardini
Materials Science
Defect engineering of ultrathin gallium nitride via electric fields for advanced electronic, magnetic, and gas sensing applications
Yujia Tian, Devesh R. Kripalani, Ming Xue et al. (+1 more)
Scaling wide-band-gap
semiconductors to the ultrathin limit offers a transformative pathway for power electronics, with gallium nitride (GaN) representing a cornerstone material in this class. However, the operational resilience and functional tunability of its two-dimensional form (g-GaN) remain underexplored. This work shifts the focus from idealized systems to the complex materials behavior under realistic conditions, investigating how the synergistic effects of point vacancy defects, strain, and external electric fields govern its electronic,
magnetic, and sensing landscapes. We demonstrate that these factors are not merely perturbations but are fundamental to modulating the material response. Our first-principles calculations suggest g-GaN maintains electronic stability under intense electric fields; notably, gallium vacancies are predicted to further extend the theoretical stability limit. While in-plane tension preserves the band gap
evolution under an electric field, in-plane compression facilitates low-field metallization. Using nitrogen monoxide (NO) adsorption as a prototype, we find that the interaction is defect-modulated and potentially tunable by electric fields. Analysis of adsorption energetics and
diffusion barriers suggests the gallium vacancy may act as a thermodynamic trap for NO. Targeted hybrid-functional (HSE06) validation confirms the reliability of observed adsorption trends and theoretical metallization thresholds, while revealing that precise electronic-exchange treatment is critical for capturing the
magnetic ground state of nitrogen vacancies. By systematically examining the geometry, energetics, band structure, density of states,
magnetic response, and charge transfer, this study clarifies the interplay between defects and external electric fields, providing insights into theoretical upper bounds for property tuning and
semiconductor device engineering.
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All authors Yujia Tian, Devesh R. Kripalani, Ming Xue, Kun Zhou