Strongly Correlated Electrons
Comparative Raman study of Ruddlesden-Popper nickelates and the monolayer-trilayer polymorph
Vignesh Sundaramurthy, Abhi Suthar, Pascal Puphal et al. (+12 more)
Ruddlesden-Popper (RP) nickelates have attracted intense interest following the discovery of superconductivity in several members of the series, including
bilayer (BL) La$_3$Ni$_2$O$_7$, trilayer (TL) La$_4$Ni$_3$O$_{10}$, and structural polymorphs composed of monolayer-
bilayer or monolayer-trilayer (ML-TL) units. However, an inherent propensity of the RP series to form intergrown phases during single-
crystal synthesis, together with spatial variations in oxygen stoichiometry, has complicated the determination of their intrinsic material properties. As a consequence, conflicting reports have emerged on both their electronic phase transitions and
lattice dynamics. In this work, we perform a comparative study of the phononic and electronic Raman responses of high-quality ML-TL single
crystals and contrast them with those of other RP nickelates, using samples with optimized oxygen content. We establish several Raman spectral features that enable unambiguous phase identification across the series. Moreover, we uncover characteristics in the phononic and electronic Raman response of ML-TL that are not reflected in the pure ML and TL compounds. We attribute these differences to a distinctive electronic structure arising from self-doping and confinement effects induced by the ML unit within the ML-TL
lattice architecture.
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All authors Vignesh Sundaramurthy, Abhi Suthar, Pascal Puphal, Congcong Le, Yuhao Gu, Hasan Yilmaz, Pablo Sosa-Lizama, Peter A. van Aken, Y. Eren Suyolcu, Masahiko Isobe, Andreas P. Schnyder, Xianxin Wu, Matteo Minola, Bernhard Keimer, Matthias Hepting
Quantum Gases
Scattering Problem in Bose-Einstein Condensates with Magnetic Domain Wall
Mei Zhao, Lijia Jiang, Tao Yang et al. (+1 more)
We present a comprehensive theoretical study of linear wave scattering from
magnetic domain walls with varied twist angles $\Theta$ in
spin-$1/2$ Bose-Einstein condensates (BECs). Using a gauge transformation, we show that scattering observables depend solely on the total twist $\Theta$, independent of chirality. Within the Bogoliubov-de Gennes (BdG) framework, we develop a transfer-
matrix method to compute reflection and transmission coefficients for incident
phonons and free particles. Our results reveal a scattering threshold at the Zeeman energy $E = \hbar\Omega_0$, separating a pure
phonon regime from multi-channel scattering involving both collective and single-particle excitations above threshold. For large twist angles, competition between kinetic and Zeeman energies reduces the effective
spin rotation, leading to comb-like density modulations and Fano-like resonances below threshold. The transition probability between
phonon and particle channels is strongly tunable with $\Theta$, enhanced for odd multiples of $\pi$ but suppressed for even multiples. These findings establish twist-engineered domain walls as a versatile platform for controlling
quantum transport, with implications for atomtronic devices and
quantum simulation.
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All authors Mei Zhao, Lijia Jiang, Tao Yang, Jun-Hui Zheng
Materials Science
Experimental evidence of dominant ultrafast diffusive energy transport by hot electrons in Cu
Jasmin Jarecki, Lisa Mehner, Maximilian Mattern et al. (+15 more)
When the dimensions of structures shrink to the order of the inelastic mean free path of the energy-carrying quasi-particles, the character of energy transport changes from diffusive to ballistic. However, the point of transition remains a matter of debate. Here, we determine the dominant channel of energy transport through a nanoscale Cu layer as a function of its thickness. The energy rapidly transferred across Cu via hot electrons from a photo-excited Pt layer into a buried Ni detection layer translates into a rapid expansion of the Ni layer probed via ultrafast x-ray diffraction. The non-linear dependence of the Ni strain amplitude on the absorbed
laser fluence indicates that the transport through Cu becomes more efficient with increasing fluence. This fluence-dependent transport efficiency is reproduced by a diffusive energy transport model and serves as a generally applicable experimental approach to distinguish
diffusion from ballistic transport. Following this approach, we identify diffusive electronic energy transport to govern the spatial energy distribution for Cu layer thicknesses larger than twice the electronic inelastic mean free path.
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All authors Jasmin Jarecki, Lisa Mehner, Maximilian Mattern, Andrius Jurgilaitis, Steffen Peer Zeuschner, Byungnam Ahn, Florian Baltrusch, J. Carl Ekström, David Kroon, Marc Herzog, Constantin Walz, Fried-Conrad Weber, Jörgen Larsson, Michel Hehn, Jan-Etienne Pudell, Daniel Schick, Alexander von Reppert, Matias Bargheer
Quantum Gases
Observation of Fano-suppression in scattering resonances of bosonic erbium atoms
L. Lafforgue, N. P. Mehta, J. J. A. Houwman et al. (+4 more)
The collisional properties of lanthanides exhibit remarkable complexity due to their many valence electrons, leading to an extraordinarily dense Feshbach spectrum showing signs of
quantum chaos. Here we explore the situation of bosonic
spin mixtures of erbium, adding the additional
spin degree of freedom to the problem. We detect several inter- and intra-
spin scattering resonances, exhibiting a peculiar asymmetric shape with a pronounced loss minimum. By developing a simplified multi-channel model we are able to recreate this characteristic behavior and to trace its origin to destructive interference between multiple pathways as predicted by Fano. We additionally observe a series of Fano-Feshbach resonances across multiple
spin channels connected to the same molecular state, again confirmed by our theory. Our work opens the door for a detailed investigation to study multi-
spin strongly-coupled scattering phenomena.
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All authors L. Lafforgue, N. P. Mehta, J. J. A. Houwman, F. Claude, S. T. Rittenhouse, F. Ferlaino, M. J. Mark
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Enhanced and directional light emission from two-dimensional excitons using Mie voids
Avishek Sarbajna, Ganesh Ghimire, Ilia Breev et al. (+5 more)
Controlling light emission at the nanoscale has important applications in solid-state lighting, displays, and
quantum light sources. Achieving this control requires both enhanced local electromagnetic fields to boost emission intensity and engineered radiation patterns to direct
photons efficiently. Mie voids, consisting of an air cavity surrounded by a high-index
semiconductor, are particularly suited for this purpose because they expose their strongest fields in an accessible region for nearby emitters while supporting resonances that shape directional emission through interference. Here, we demonstrate an all-van der Waals nanophotonic platform that couples
excitons in atomically thin WS$_2$ to Mie void resonators formed in WSe$_2$. Guided by electromagnetic simulations, we identify void geometries that maximize photoluminescence through synergistic enhancement of excitation and emission processes. We also develop a two-step fabrication strategy that enables independent control of void diameter and depth, providing a route to systematically tune the optical response. Experimentally, we observe up to a 600-fold increase in photoluminescence intensity from monolayer WS$_2$ placed on individual voids compared to on an unstructured WSe$_2$, along with pronounced out-of-plane beaming of light that yields a forward-to-off-axis enhancement of 2.6 dB. Our results establish Mie voids in van der Waals
semiconductors as a new platform for controlling light-matter interactions and realizing compact, directional, and efficient nanoscale light sources.
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All authors Avishek Sarbajna, Ganesh Ghimire, Ilia Breev, Xavier Zambrana-Puyalto, Cheng Xiang, Alexander Huck, Timothy J. Booth, Søren Raza
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Refrigeration of a 1D gas of microwave photons
Lukas Schamriß, Louis Garbe and Peter Rabl
We discuss a conceptually simple scheme for cooling a one dimensional gas of microwave
photons in a superconducting transmission line. By shunting one end of the transmission line by a nonlinear Josephson element, we show how a cooling mechanism can be engineered that transfers
photons from high- into low-frequency modes, while preserving their total number. We evaluate the resulting nonequilibrium steady state of the
photon gas, which arises from a competition between this engineered cooling process and the natural, number non-conserving thermalization with the surrounding bath. Our analysis predicts that for realistic experimental parameters, this mechanism can be used to prepare photonic gases at sub-millikelvin temperatures, considerably below the typical base temperature of a dilution refrigerator. In addition, the system exhibits a new type of condensation transition that does not occur in the corresponding
equilibrium scenario. As an outlook, we discuss potential applications of this cooling approach for
quantum simulation schemes with interacting microwave
photons.
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Materials Science
Structure and Magnetic Properties of Vacuum-Annealed CoFeB Thin Films: From Amorphous Alloy to Metastable (Co,Fe)23B6 tau-Boride
P. Shvets, G. Kirichuk, V. Salnikov et al. (+4 more)
Controlled crystallization of amorphous
alloys offers a powerful route to tailor
magnetic and structural properties at the nanoscale. Thin films of CoFeB
alloy are essential for the development of various spintronic devices. The crystallization mechanisms of CoFeB during the annealing process have been thoroughly investigated in earlier studies, demonstrating that boron diffuses from the amorphous film, allowing the remaining CoFe to form a body-centered cubic
lattice. Here, a distinct transformation pathway in pulsed-
laser-deposited amorphous Co40Fe40B20 films is revealed, where
vacuum annealing drives the formation of a metastable tau-boride phase, (Co,Fe)23B6. Comprehensive structural characterization - combining X-ray diffraction, transmission electron microscopy, and compositional analysis - proves that tau-boride forms with high crystalline quality and minimal boron loss. Following the transition from amorphous CoFeB films to crystalline (Co,Fe)23B6, an improvement of
magnetic properties is observed, with corresponding increases in such values as saturation magnetization, coercivity, loop squareness, and average
magnetic moment. The reproducible stabilization of a boron-rich metastable phase in CoFeB thin films expands the known crystallization landscape of this technologically important
alloy system. These findings provide new insight into phase engineering in transition-metal borides and open opportunities for designing nanostructured
magnetic materials with tunable functionality for next-generation spintronic and nanoelectronic devices.
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All authors P. Shvets, G. Kirichuk, V. Salnikov, J. O'Connell, V. Rodionova, A. Goikhman, K. Maksimova
Materials Science · Cross-listed from physics.ins-det
Discretized Halbach spheres: Icosahedral symmetry for optimal field homogeneity
Ingo Rehberg and Peter Blümler
Halbach spheres provide a theoretically elegant means of generating highly homogeneous
magnetic fields, but practical implementation is hindered by challenging fabrication and restricted interior access. This study examines discrete spherical Halbach configurations assembled from permanent magnets placed at the vertices of Platonic and Archimedean solids. Analytical calculations, numerical field simulations, and experimental measurements indicate that polyhedra with icosahedral
symmetry achieve the most favorable balance among field strength, homogeneity, and interior accessibility. They produce exceptionally flat fourth-order central saddle points, resulting in a usable homogeneous field volume up to a factor of 260 larger than that of traditional Halbach disk or cylindrical arrays. Several magnet assemblies composed of cubical NdFeB magnets are fabricated and their three dimensional field distributions characterized, demonstrating homogeneous regions of up to several cubic centimeters with deviations below 1%. The findings establish discrete icosahedrally symmetric magnet arrays as practical, scalable building blocks for compact, highly homogeneous
magnetic field sources suited to mobile
magnetic resonance, and magnetophoretic applications.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Probing an electron spin ensemble with squeezed microwave signals
P. Oehrl, F. Fesquet, K. E. Honasoge et al. (+5 more)
The efficient transfer of
quantum states into a long-lived storage unit such as solid-state
spin ensembles is widely recognized as a critical challenge with significant implications for
quantum communication, sensing and computing applications. Here, we experimentally investigate the interaction of propagating squeezed microwaves with an electron
spin resonance transition in order to evaluate the use of
spin ensembles as
quantum memories for GHz signals. We generate continuous variable microwave states with a squeezing of up to 5dB below the
vacuum level and let this signal interrogate a
spin ensemble, which is inductively coupled to a lumped element superconducting microwave resonator with a cooperativity of C=0.3. Analyzing this signal using Wigner
tomography, we observe a transfer efficiency of around 61% between the squeezed microwaves and the
spin excitation. We successfully model our experimental results with a dedicated steady-state model based on the
quantum input-output formalism and provide guidance for design parameters required to enable
spin-based
quantum memories.
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All authors P. Oehrl, F. Fesquet, K. E. Honasoge, M. Handschuh, A. Marx, R. Gross, K. G. Fedorov, H. Huebl
Disordered Systems and Neural Networks · Cross-listed from nlin.PS
Nonlinear skin breathing modes in one-dimensional nonreciprocal mechanical lattices
Bertin Many Manda
We investigate the interplay of nonreciprocity and nonlinearity in a one-dimensional nonlinear Klein-Gordon chain of classical oscillators coupled by asymmetric springs, akin to a mechanical analogue of the Hatano-Nelson model with onsite nonlinearity. Using multiple-scale analysis, we show that families of nonlinear skin breathing modes – time-periodic, boundary-localized oscillations – emerge from their linear counterparts at any nonreciprocal strength. We derive an explicit nonlinear frequency shift for these families of nonlinear breathing modes, showing its dependence on amplitude, nonlinearity type,
lattice size, and nonreciprocity, and we predict the emergence of genuine skin end breathers at the boundary once their oscillation frequency and higher harmonics enter the spectral gaps of the linear spectrum. Numerical pseudo-arclength continuation confirms full families of solutions for both hardening and softening nonlinearities. Furthermore, the Floquet analysis shows that these modes can be either linearly stable or unstable, with Floquet eigenvectors exhibiting skin localization inherited from the asymmetric couplings. Our results extend the nonlinear non-Hermitian skin effect from stationary modes to intrinsically time-periodic excitations, providing a pathway to engineer and control breathing modes in nonreciprocal mechanical metamaterials.
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Materials Science
Bandgap Engineering for Efficient Perovskite Solar Cells Under Multiple Color Temperature Indoor Lighting
Miqad S. Albishi, Faisal I. Alabdulkarem, George Perrakis et al. (+20 more)
Perovskite indoor photovoltaics (PIPVs) are emerging as a transformative technology for low-light intensity energy harvesting, owing to their high power conversion efficiencies (PCEs), low-cost fabrication, solution-processability, and compositionally tunable band gaps. In this work, methylammonium-free perovskite absorbers were compositionally engineered to achieve band gaps of 1.55, 1.72, and 1.88 eV, enabling matching the spectral photoresponse with the indoor lighting. Devices based on a scalable mesoscopic n-i-p architecture were systematically evaluated under white LED illumination across correlated color temperatures (3000-5500 K) and light intensities from 250 to 1000 lux with active area of 1 cm2. The 1.72 eV composition exhibited the most promising performance across different light intensities and colors, achieving PCEs of 35.04 % at 1000 lux and 36.6 % at 250 lux, with a stable device operation of over 2000 hours. On the other hand, the 1.88 eV band-gap variant reached a peak PCE of 37.4 % under 250 lux (5500 K), however performance trade-offs were observed across the different color lights LEDs. Our combined experimental and theoretical optical-electrical simulations suggest that decreasing trap-assisted recombination in wide-bandgap compositions may further improve PIPV performance across the different illumination conditions. In contrast, devices with 1.55 eV band gap underperformed in such conditions due to suboptimal spectral overlap and utilization. These findings establish bandgap optimization and device architecture as key design principles for high-efficiency, stable PIPVs, advancing their integration into self-powered electronic systems and innovative indoor environments.
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All authors Miqad S. Albishi, Faisal I. Alabdulkarem, George Perrakis, Tariq F. Alhuwaymel, Ala H. Sabeeh, Abdullah S. Alharbi, Naif R. Alshamrani, Ibrahim H. Khawaji, Nikolaos Tzoganakis, Majed M. Aljomah, Dimitris Tsikritzis, Sami A. Alhusaini, Abdullah Aljalalah, Kadi S. AlShebl, Ali Alanzi, Abrar Bin Ajaj, Fay M. Alotaibi, Hamad Albrithen, Konstantinos Petridis, Maria Kafesaki, Emmanuel Kymakis, George Kakavelakis, Essa A. Alharbi
Materials Science · Cross-listed from cs.LG
Deep Learning-Based Surrogate Creep Modelling in Inconel 625: A High-Temperature Alloy Study
Shubham Das, Kaushal Singhania, Amit Sadhu et al. (+2 more)
Time-dependent deformation, particularly creep, in high-temperature
alloys such as Inconel 625 is a key factor in the long-term reliability of components used in aerospace and energy systems. Although Inconel 625 shows excellent creep resistance, finite-element creep simulations in tools such as ANSYS remain computationally expensive, often requiring tens of minutes for a single 10,000-hour run. This work proposes deep learning based surrogate models to provide fast and accurate replacements for such simulations. Creep strain data was generated in ANSYS using the Norton law under uniaxial stresses of 50 to 150 MPa and temperatures of 700 to 1000 $^\circ$C, and this temporal
dataset was used to train two architectures: a BiLSTM Variational Autoencoder for uncertainty-aware and generative predictions, and a BiLSTM
Transformer hybrid that employs self-
attention to capture long-range temporal behavior. Both models act as surrogate predictors, with the BiLSTM-VAE offering probabilistic output and the BiLSTM-
Transformer delivering high deterministic accuracy. Performance is evaluated using RMSE, MAE, and $R^2$. Results show that the BiLSTM-VAE provides stable and reliable creep strain forecasts, while the BiLSTM-
Transformer achieves strong accuracy across the full time range. Latency tests indicate substantial speedup: while each ANSYS simulation requires 30 to 40 minutes for a given stress-temperature condition, the surrogate models produce predictions within seconds. The proposed framework enables rapid creep assessment for design optimization and structural health monitoring, and provides a scalable solution for high-temperature
alloy applications.
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All authors Shubham Das, Kaushal Singhania, Amit Sadhu, Suprabhat Das, Arghya Nandi
Soft Condensed Matter
Collective Hard Core Interactions Leave Multiscale Signatures in Number Fluctuation Spectra
Eleanor K. R. Mackay, Anna Drummond Young, Adam Carter et al. (+2 more)
A full understanding of transport in dense, interacting suspensions requires analysis frameworks sensitive to self and collective dynamics across all relevant spatial and temporal scales. Here we introduce a trajectory-free approach to address this problem based on the power spectral density of particle number fluctuations (N-PSD). By combining colloidal experiments and theory we show that the N-PSD naturally probes behaviour across multiple important dynamic regimes and we fully uncover the mechanistic origins of characteristic spectral scalings and timescales. In particular, we demonstrate that while high-frequency scalings link to self-
diffusion, low-frequency scalings sensitively capture long-lived correlations and collective dynamics. In this regime, interactions lead to non-trivial spectral signatures, governed by pairwise particle exchange at small length scales and collective rearrangements over large scales. Our findings thus provide important insight into the effect of interactions on microscopic dynamics and fluctuation phenomena and establish a powerful new tool with which to probe dynamics in complex systems.
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All authors Eleanor K. R. Mackay, Anna Drummond Young, Adam Carter, Sophie Marbach, Alice L. Thorneywork
Mesoscale and Nanoscale Physics
Quantum geometry, localization, and topological bounds of spin fluctuations
Carlos Saji and Roberto E. Troncoso
We study how topological crystalline defects–dislocations–reshape the real-space
quantum geometric tensor and act as tunable sources of
quantum geometry. We show that dislocations strongly enhance the
quantum metric, establishing a direct link between
lattice topology and the
Hilbert-space geometry of states. We characterize the
quantum geometry of topological
magnons in ordered arrays of dislocations, demonstrating that defect-induced
geometric enhancement controls their localization and topological protection. In disordered arrays, dislocation-driven geometry expands the accessible topological phase space and enables transitions to disorder-induced topological phases. Our results identify the
quantum metric as a tunable bridge between crystalline
topology, magnonic excitations, and emergent topological matter in aperiodic solid-state and synthetic systems.
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Soft Condensed Matter
Cooling mechanism controls motility-induced phase separation in inertial active liquids
Manuel Mayo, Lorenzo Caprini, María Isabel García de Soria et al. (+4 more)
Motility-induced phase separation (MIPS) is a central collective phenomenon in active matter, theoretically established in the overdamped regime. We discover that the dynamical origin of MIPS is fundamentally altered by inertia, which induces a cooling mechanism absent in overdamped active matter. This conclusion is supported by an active variant of the direct simulation Monte Carlo method and by a kinetic theory for inertial self-propelled hard spheres derived from the microscopic dynamics. In contrast to the overdamped case, both analyses demonstrate that inertial MIPS does not rely on volume exclusion but on a cooling mechanism involving density, polarization, and temperature fields. This mechanism emerges from the competition between activity and a density dependent collision rate, arising from spatial correlations between colliding particles. These findings open a pathway to fundamentally connect inertial active matter with granular physics.
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All authors Manuel Mayo, Lorenzo Caprini, María Isabel García de Soria, Umberto Marini Bettolo Marconi, Pablo Maynar, Luca Pizzoli, Andrea Puglisi
Strongly Correlated Electrons
Emergence of a hidden-order phase well below the charge density wave transition in a topological Weyl semimetal (TaSe$_4$)$_2$I
Sk Kalimuddin, Sudipta Chatterjee, Arnab Bera et al. (+7 more)
The emergence of a charge density wave (CDW) in a Weyl semimetal – a correlated topological phase, is exceptionally rare in condensed matter systems. In this context, the quasi-one-dimensional type-III Weyl semimetal (TaSe$_4$)$_2$I undergoes a CDW transition at $T_{\mathrm{CDW}} \approx 263$~K, providing an exceptional platform to investigate correlated topological CDW states. Here, we uncover an additional hidden-order phase transition at $T^* \sim 100$ K, well below the CDW onset, using low-frequency resistance noise spectroscopy, electrical transport, and thermoelectric measurements. This transition is characterized by a sharp enhancement in the noise exponent ($\alpha$) and variance of resistance fluctuations. Analysis of higher-order statistics of resistance fluctuations reveals the correlated dynamics underlying the transition. A pronounced anomaly in the Seebeck coefficient near $T^*$ further suggests a
Fermi surface reconstruction. First-principles calculations reveal a structural distortion from the high-
symmetry $I422$ phase to a low-
symmetry $C2$ phase, via an intermediate $I4$
symmetry. This leads to renormalization of the electronic structure near the
Fermi level and opening of a bandgap in the hidden-order phase. These findings demonstrate a previously unidentified correlated phase transition in the topological CDW-Weyl semimetal (TaSe$_4$)$_2$I, enriching the phase diagram of this material and establishing it as an ideal platform for studying intertwined electronic and structural orders.
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All authors Sk Kalimuddin, Sudipta Chatterjee, Arnab Bera, Satyabrata Bera, Deep Singha Roy, Soham Das, Tuhin Debnath, Ashis K. Nandy, Shishir K. Pandey, Mintu Mondal
Mesoscale and Nanoscale Physics
Direct demonstration of time-reversal-symmetry-breaking spin injection from a compensated magnet
Jone Mencos, Antonin Badura, Eoin Dolan et al. (+21 more)
The injection, propagation and detection of
spin currents are essential physical processes in spintronics. So far, the separation of charge and
spin currents was facilitated by the electrical
spin injection from a ferromagnet (FM) or the injection by a relativistic
spin Hall effect. The devices employed are lateral
spin valves comprising spatially separated injection and detection electrodes, connected by a
spin-propagation channel. The time-reversal
symmetry (TRS) breaking FM
spin injection is realized in a geometry with an electrical bias applied between the injection electrode and the channel and is modelled by a conserved
spin-polarized drift current. In contrast, the
spin injection by the T-symmetric relativistic
spin Hall mechanism is driven by an electrical bias applied across the injection electrode alone, and is modelled by a non-conserved
spin current transverse to the applied bias. In this work, we use a lateral
spin valve with a Mn5Si3 injection electrode to directly demonstrate a TRS-breaking
spin injection from a compensated magnet with a vanishing net magnetization. Specifically, the TRS-breaking is demonstrated by the fact that switching between time-reversed states of the compensated magnet changes the detected
spin signal. Moreover, the TRS-breaking nature of the
spin injection is observed in both experimental geometries with the different electrical biasing, while using the same detection electrode. We show that this unconventional
spin-injection is consistent with different magnitudes and propagation angles of electrical currents in the
spin-up and
spin-down channel in a d-wave altermagnet. Here our
symmetry analysis and first-principles calculations are based on the compensated collinear altermagnetic order which has provided a comprehensive microscopic interpretation of earlier structural,
magnetic, and anomalous
Hall and Nernst measurements in Mn5Si3 thin films.
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All authors Jone Mencos, Antonin Badura, Eoin Dolan, Sebastian Beckert, Rafael Gonzalez-Hernandez, Nicolás Sigales, Tim Kokkeler, Ismaila Kounta, Matthieu Petit, Charles Guillemard, Anna Birk Hellenes, Warlley Campos, Javier Rial, Dominik Kriegner, Vincent Baltz, Luis E. Hueso, Jairo Sinova, F. Sebastian Bergeret, Olena Gomonay, Tomas Jungwirth, Libor Smejkal, Lisa Michez, Helena Reichlova, Fèlix Casanova
Disordered Systems and Neural Networks · Cross-listed from stat.ML
Perfect reconstruction of sparse signals using nonconvexity control and one-step RSB message passing
Xiaosi Gu, Ayaka Sakata and Tomoyuki Obuchi
We consider sparse signal reconstruction via minimization of the smoothly clipped absolute deviation (SCAD) penalty, and develop one-step replica-
symmetry-breaking (1RSB) extensions of approximate message passing (AMP), termed 1RSB-AMP. Starting from the 1RSB formulation of belief propagation, we derive explicit update rules of 1RSB-AMP together with the corresponding state
evolution (1RSB-SE) equations. A detailed comparison shows that 1RSB-AMP and 1RSB-SE agree remarkably well at the macroscopic level, even in parameter regions where replica-symmetric (RS) AMP, termed RS-AMP, diverges and where the 1RSB description itself is not expected to be thermodynamically exact. Fixed-point analysis of 1RSB-SE reveals a phase diagram consisting of success, failure, and diverging phases, as in the RS case. However, the diverging-region boundary now depends on the Parisi parameter due to the 1RSB ansatz, and we propose a new criterion—minimizing the size of the diverging region—rather than the conventional zero-complexity condition, to determine its value. Combining this criterion with the nonconvexity-control (NCC) protocol proposed in a previous RS study improves the algorithmic limit of perfect reconstruction compared with RS-AMP. Numerical solutions of 1RSB-SE and experiments with 1RSB-AMP confirm that this improved limit is achieved in practice, though the gain is modest and remains slightly inferior to the Bayes-optimal threshold. We also report the behavior of thermodynamic quantities—overlaps, free
entropy, complexity, and the non-self-averaging susceptibility—that characterize the 1RSB phase in this problem.
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Mesoscale and Nanoscale Physics
Theory of electric reactance emerging from spin Hall effect
Yasufumi Araki and Jun'ichi Ieda
The
spin Hall effect in a heavy metal intercorrelates an AC electric current to the magnetization dynamics in an adjacent ferromagnet, which manifests as an electric reactance in the system's current-voltage response. We present a comprehensive theoretical analysis for this emergent reactance contribution in the frequency regime relevant to transport measurements up to a few GHz. Our analysis reveals that the reactance becomes inductor-like at low frequency below the
ferromagnetic resonance. Crucially, we find that the sign of the reactance is directly governed by the
spin transfer mechanism at the interface, which depends on the competition between its damping-like and field-like components parametrized by the
spin mixing conductance. This characteristic behavior in the reactance offers a powerful transport observable in distinguishing the interfacial
spin transfer processes in spintronic materials.
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Strongly Correlated Electrons
Altermagnetism and its induced higher-order topology on the Lieb lattice
Xingmin Huo, Xingchuan Zhu, Chang-An Li et al. (+4 more)
Altermagnetism (AM) has brought renewed
attention to the Lieb
lattice. Here, we broaden the scope of altermagnetic models on the Lieb
lattice by using a general scheme based on
spin clusters. We design various altermagnetic models with d- and g-wave on the Lieb
lattice, and investigate its interplay with
spin-orbit coupling. While the altermagnetic unit
cell reconstructs the topological edge states in the strip geometry and leads to the emergence of Dirac points, the in-plane
magnetic moments of AM can induce gaps at these points. In an open square geometry, corner modes emerge within these gaps, realizing higher-order topological states. We further verify that the induction of higher-order
topology is applicable to all altermagnetic configurations constructed here on the Lieb
lattice, and is most pronounced for AM by comparing with the other types of magnetism such as ferromagnetism and ferrimagnetism. Our results highlight the exotic properties of AM, and suggest its potential applications in engineering topological
quantum states.
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All authors Xingmin Huo, Xingchuan Zhu, Chang-An Li, Shiping Feng, Song-Bo Zhang, Shengyuan A.Yang, Huaiming Guo
Strongly Correlated Electrons
One-dimensional physics of the frustrated quantum magnet PHCC
Alexander A. Tsirlin, Oleg Janson and Ioannis Rousochatzakis
We report a comprehensive microscopic study of the frustrated
quantum magnet PHCC, (C$_4$H$_{12}$N$_2$)Cu$_2$Cl$_6$, using density-functional band-structure calculations combined with numerical
quantum many-body simulations of the underlying
spin Hamiltonian. We show that the magnetism of PHCC is captured by a one-dimensional model of the frustrated
spin chain with alternating nearest-neighbor couplings ($J_1=23.1$ K, $J_1'=7.0$ K) and uniform next-nearest-neighbor couplings ($J_2=13.9$ K). This model, which can also be thought of as a zigzag ladder, provides a quantitative description of the
magnetic susceptibility and the magnetization process, and accounts for the observed dispersion of the single-triplet band and its merging into a continuum near the Brillouin zone center. We also make predictions for the existence of sharp bound (anti-bound) states of two triplets, below (above) the bottom (upper) edge of the two-particle scattering continuum.
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Materials Science
Magnetic, Structural, and Electronic Properties of CrOCl with the PBE Functional
Brahim Marfoua, Mohammad Amirabbasi and Marcus Ekholm
CrOCl is a van der Waals-layered
insulator with an
antiferromagnetic ground state, making it a promising platform for exfoliation and the exploration of low-dimensional magnetism. An accurate ab initio description is therefore essential. Previous density-functional studies have shown that DFT+$U$ calculations may erroneously favor
ferromagnetic order depending on the choice of parametrization, an issue that cannot be remedied by simply adjusting the value of $U$. Here, we demonstrate that an explicit Hubbard correction is unnecessary: the PBE functional correctly reproduces the AFM ground state while simultaneously improving the description of structural properties. Moreover, PBE provides a reliable account of the electronic structure. These findings clarify the role of correlation effects in CrOCl and identify PBE as a robust starting point for future ab initio studies of CrOCl-based materials.
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Materials Science
Eliminating the irregular surface layer of anodically-grown Ni-Ti-O nanopore arrays in a two-stage anodization
S.A. Mousavi, A. Moshfeghi, F. Davoodian et al. (+1 more)
Nanopores (NPs) grown by anodizing can be partially hidden beneath a relatively compact surface
oxide layer, which limits the volumetric surface area of these nanostructures. In this work, nitinol (NiTi)
alloy was anodized in an electrolyte containing ethylene glycol, water, and sodium chloride in static and stirred electrolyte stages with the aim of removing the irregular surface array while achieving a thick NP layer. Electron micrographs showed that anodization in the static electrolyte provides a controlled thickness of NP layers covered by an irregular surface layer. In contrast, anodizing in the stirred electrolyte reduced the thickness and the degree of irregularity, which were controlled by the different kinetics of dissolution at the tops, perimeters and bottoms of NPs. To benefit simultaneously from the thickness and regularity of the
oxide layers, two-stage anodizing under static and then stirred electrolyte conditions was found to be effective. Following a 30 min anodization in the static electrolyte, anodizing for 30 min under the stirred conditions provided the highest regularity in the
oxide array, resulting in NPs of almost 40 nm and 11 {\mu}m in diameter and layer thickness, respectively. Two-stage anodizing under static then stirred electrolyte conditions is proposed in order to promote NP structures for applications demanding higher surface areas.
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All authors S.A. Mousavi, A. Moshfeghi, F. Davoodian, E. Salahinejad
Soft Condensed Matter
Reentrant melting of scarred odd crystals by self-shear
Uttam Tiwari, Pragya Arora, A K Sood et al. (+3 more)
Spatial confinement can induce geometrical frustration in condensed phases, giving rise to topological defects that confer materials with new and exotic properties. Here, we experimentally uncover the remarkable effect of confinement-induced defect strings termed `grain boundary scars' on the behavior of dense two-dimensional assemblies of granular spinners, a canonical odd elastic solid. We show that the spatial arrangement of these scars fundamentally reshapes the flows triggered by
chiral activity. Specifically, they cause the topologically protected edge flows - a ubiquitous feature of confined spinner assemblies - to decouple from the bulk. Strikingly, increasing the net
chiral activity of the system by tuning the ratio of counterclockwise to clockwise spinners caused spontaneous self-shearing. The resulting odd radial stresses led to a
chiral activity-mediated reentrant melting transition at a fixed areal spinner density. Our findings open new avenues for exploiting geometrical frustration to elicit novel responses from odd elastic solids.
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All authors Uttam Tiwari, Pragya Arora, A K Sood, Sriram Ramaswamy, Rituparno Mandal, Rajesh Ganapathy
Materials Science
Comparative analysis of electrodeposited Pt, Ru and Pt-Ru overlays for high-temperature oxidation protection
Majid Hosseinzadeh and Erfan Salahinejad
Platinum (Pt) and ruthenium (Ru), both members of the platinum-group metals (PGMs), are renowned for their exceptional resistance to corrosion, oxidation, and high temperatures, making them promising candidates for advanced high-temperature applications. This study investigates the direct current (DC) electrodeposition of Pt, Ru, and a binary Pt-Ru
alloy onto NiCoCrAlYTa-coated single-
crystal superalloy CMSX-4, along with their
vacuum annealing and respective effects on the isothermal oxidation behavior of the system at 1100 {\deg}C. All the electrodeposited overlays demonstrated substantial enhancement in oxidation resistance. However, Pt exhibited the highest protection efficiency, Ru the least, and the Pt-Ru
alloy provided an intermediate level of performance. Microscopic and X-ray diffraction analyses revealed that the competitive formation of protective {\alpha}-Al2O3 and spinel NiAl2O4 phases on the coated surfaces played a crucial role in determining the oxidation resistance, driven by atomic interactions between the elements in the NiCoCrAlYTa bond coat and the overlay metals. Despite Ru's relatively lower oxidation resistance compared to Pt, its significantly lower cost offers potential advantages in cost-sensitive, high-temperature applications. These findings provide valuable insights into optimizing Pt-group metal coatings for durability in high-performance systems.
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Materials Science
Influence of Pt/Ru ratios on the oxidation mechanism of MCrAlYTa coatings modified with Pt-Ru overlays
Majid Hosseinzadeh and Erfan Salahinejad
This study investigates the influence of varying Pt/Ru ratios on the oxidation mechanism of NiCoCrAlYTa coatings with electrodeposited,
vacuum-annealed Ptsingle bondRu overlays. Weight change measurements, scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were used for high-temperature oxidation analyses, showing superior resistance with higher Pt contents. This was attributed to the creation of a denser, thinner, and more homogeneous layer of alumina (alpha-Al2O3) in the thermally-grown
oxide (TGO) layer. On the contrary, an increase in Ru contents led to the development of other
oxides and microcracks along with alumina in the TGO layer, undermining oxidation protection. The accommodation of Ti and Ta, in the minimally-deteriorative form of carbide, along with Y into the TGO layer with increasing Pt contents further enhanced oxidation resistance. In addition to the explored significant impact of the Pt/Ru ratio on
oxide scale characteristics and oxidation resistance, the lower cost of Ru compared to Pt suggests the potential for designing cost-effective systems through optimized Pt/Ru ratios and microstructural engineering.
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Superconductivity
Thermodynamic evidence for full-gap superconductivity in the dodecagonal quasicrystal Cu-doped Ta$_{1.6}$Te
N. Kabeya, Y. Tokumoto, K. Tomiyama et al. (+2 more)
We report the superconducting gap in the van der Waals layered quasicrystal Cu-doped Ta$_{1.6}$Te, using a fast relaxation technique that removes the large
nuclear contribution of $^{181}$Ta. The initial-slope method enabled detection of the electronic specific heat down to 60~mK, revealing a fully gapped state with $\Delta(0)/k_{\rm B} = 1.43$~K. Both the gap magnitude and specific-heat jump are smaller than BCS predictions, while quasiparticle excitations are strongly suppressed, consistent with a theoretical expectation for aperiodic systems. AC-susceptibility measurements show a large upper critical field and pronounced anisotropy, reflecting the quasi-two-dimensional structure. These results provide the first thermodynamic evidence for a full-gap superconducting state in a quasicrystal and highlight unconventional pairing mechanisms beyond periodic
lattices.
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All authors N. Kabeya, Y. Tokumoto, K. Tomiyama, N. Kimura, K. Edagawa
Mesoscale and Nanoscale Physics
Kamalesh Bera, Tanay Nag and Arijit Saha
We present a theoretical investigation of the emergence of
chiral topological superconductivity in small-angle twisted
bilayer graphene (tBLG) and twisted double
bilayer graphene (tDBLG). Using the low-energy continuum model and incorporating
spin-triplet $p_{x}+i p_{y}$ pairing in each
graphene layer, we construct the effective models for both tBLG and tDBLG with superconductivity. By varying the chemical potential, superconducting order parameter, and twist angle, we explore the emergence of topological superconducting phases via the calculation of Chern numbers. Our phase diagrams for tBLG and tDBLG (both AB-AB and AB-BA stackings) reveal distinct topological transitions, which are consistently marked by bulk gap-closing points. To gain further insight, we analyze the
evolution of Chern numbers by tracking the number and location of gap closings within the moiré Brillouin zone. Additionally, we illustrate representative squared amplitude of Bloch states corresponding to different topological phases. In the later part of our study, the effect of trigonal warping on the topological superconducting properties is also discussed. Beyond the quantitative results, our study highlights how the interplay between moiré band structure and unconventional pairing
symmetries enriches the landscape of possible superconducting states in twisted
graphene systems. The framework developed here may also be extended to other multilayer moiré materials, offering a route towards engineering exotic topological superconductivity with tunable parameters.
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Quantum Gases
The crossover from classical to quantum transport in a weakly-interacting Fermi gas
Hadrien Kurkjian
We present an exact solution of the
quantum kinetic equation of a weakly interacting
Fermi gas in the crossover from the degenerate
Fermi-
liquid regime to the classical Boltzmann gas. We construct families of orthogonal
polynomials tailored to each angular momentum channel, enabling a fast and systematically improvable decomposition of the phase-space distribution. This approach yields accurate, non-variational predictions for the shear viscosity, thermal diffusivity, and
spin diffusivity to leading order in the scattering length. We demonstrate that the commonly used relaxation-time approximation fails dramatically at low temperature–by up to 25%. Our method provides a numerically efficient framework for benchmarking transport in strongly correlated regimes and for simulating the kinetics of
quantum gases beyond hydrodynamics.
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Mesoscale and Nanoscale Physics
Kibble-Zurek mechanism in a polariton supersolid
Dmitry Solnyshkov, Rafal Mirek, Darius Urbonas et al. (+12 more)
We study the formation of topological defects via the Kibble-Zurek mechanism in a polariton supersolid in a
liquid crystal microcavity with tunable Rashba-Dresselhaus
spin-orbit coupling. We predict analytically two different scalings in the slow- and fast-quench regimes, and confirm these predictions numerically. We also present experimental results for the slow-quench regime, demonstrating an original Kibble-Zurek scaling exponent $\eta_{KZM}=1.0\pm 0.2$
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All authors Dmitry Solnyshkov, Rafal Mirek, Darius Urbonas, Etsuki Kobiyama, Pietro Tassan, Ioannis Georgakilas, Rainer F. Mahrt, Michael Forster, Ullrich Scherf, Marcin Muszynski, Wiktor Piecek, Piotr Kapuściński, Jacek Szczytko, Thilo Stoferle, Guillaume Malpuech