Mesoscale and Nanoscale Physics
Synergy of fivefold boost SOT efficiency and field-free magnetization switching with broken inversion symmetry: Toward neuromorphic computing
Badsha Sekh, Hasibur Rahaman, Subhakanta Das et al. (+4 more)
Non-volatile Neuromorphic Computing (NC) elements utilizing
Spin Orbit Torque (SOT) provide a viable solution to alleviate the
memory wall bottleneck in contemporary computing systems. However, the two challenges, low SOT efficiency and the need for in plane
symmetry breaking field for perpendicular magnetization switching, greatly limit its practical implementation. In this work, the enhanced SOT efficiency of Platinum (Pt) SOT layer and field free perpendicular magnetization switching are achieved by integrating thin Ruthenium
Oxide (RuO2) layer in our material stack. The optimal RuO2 thickness (0.5 nm) enhances 5.2 times Damping Like (DL) SOT efficiency compared with pure SOT layer (Pt), as determined by hysteresis loop shift measurements, with a relatively low resistivity (90 micro-Ohm-cm). Moreover, we achieve 3 times reduction of critical magnetization switching current density compared to reference sample. Our experimental findings also demonstrate Rashba-induced substantial field-free magnetization switching in the presence of an emergent built-in interfacial field. Notably, reliable multi resistance synaptic states are achieved by tailoring the synergistic effects of enhanced SOT and interfacial magnetism. The functionality of synaptic states has been further evaluated by implementing an artificial neural network and achieved image recognition accuracies of approximately 95% and 87% on the MNIST and Fashion-MNIST
datasets, respectively. This systematic study paves the way to energy-efficient, field-free SOT synapses for practical NC applications.
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All authors Badsha Sekh, Hasibur Rahaman, Subhakanta Das, Mitali, Ramu Maddu, Kesavan Jawahar, S.N. Piramanayagam
Mesoscale and Nanoscale Physics
A comprehensive semi-automated fabrication system for quartz tuning fork AFM probe with real-time resonance frequency monitoring and Q-factor control
Hankyul Koh, Joon-Hyuk Ko and Wonho Jhe
Quartz tuning fork-based atomic force microscopy (QTF-AFM) has become a powerful tool for high-resolution imaging of both conductive and insulating samples, including
semiconductor structures and metal-coated surfaces as well as soft matter under ambient conditions, while also enabling measurements in more demanding environments including ultrahigh
vacuum and cryogenic conditions where conventional cantilever-based AFM often encounters limitations. However, the broader adoption of QTF-AFM has been constrained by the difficulty of attaching a cantilever tip to a quartz tuning fork (QTF) with the positional and angular precision required for repeatable and reproducible probe fabrication. For stable operation, the tip must be placed precisely at the midline of a single tine, aligned parallel to the prong axis, and rigidly secured. Even slight lateral offsets or angular deviations disrupt the intrinsic antisymmetric flexural mode, induce torsional coupling, and ultimately lead to systematic image distortions and reduced measurement integrity. In this work, we present a comprehensive, semi-automated QTF-tip fabrication system that integrates precision
alignment, real-time frequency-sweep monitoring, and controlled Q-factor tuning within a single workflow. Experimental characterization demonstrates consistent probe preparation across multiple trials, preservation of sharp and well-defined resonance responses with deliberately adjustable damping, and high-fidelity, high-resolution imaging in practical scanning tests. This integrated approach provides a reproducible framework to QTF-based probe fabrication, lowering the technical barrier to QTF-AFM implementation and broadening its applicability across diverse sample types and operating environments.
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Materials Science · Cross-listed from eess.SP
Differentiating through binarized topology changes: Second-order subpixel-smoothed projection
Giuseppe Romano, Rodrigo Arrieta and Steven G. Johnson
A key challenge in
topology optimization (TopOpt) is that manufacturable structures, being inherently binary, are non-differentiable, creating a fundamental tension with gradient-based optimization. The subpixel-smoothed projection (SSP) method addresses this issue by smoothing sharp interfaces at the subpixel level through a first-order expansion of the filtered field. However, SSP does not guarantee differentiability under
topology changes, such as the merging of two interfaces, and therefore violates the convergence guarantees of many popular gradient-based optimization
algorithms. We overcome this limitation by regularizing SSP with the Hessian of the filtered field, resulting in a twice-differentiable projected density during such transitions, while still guaranteeing an almost-everywhere binary structure. We demonstrate the effectiveness of our second-order SSP (SSP2) methodology on both thermal and photonic problems, showing that SSP2 has faster convergence than SSP for connectivity-dominant cases – where frequent
topology changes occur – while exhibiting comparable performance otherwise. Beyond improving convergence guarantees for CCSA optimizers, SSP2 enables the use of a broader class of optimization
algorithms with stronger theoretical guarantees, such as interior-point methods. Since SSP2 adds minimal complexity relative to SSP or traditional projection schemes, it can be used as a drop-in replacement in existing TopOpt codes.
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Materials Science · Cross-listed from physics.chem-ph
Theoretical investigation of non-Förster exciton transfer mechanisms in perylene diimide donor, phenylene bridge, and terrylene diimide acceptor systems
Lei Yang and Seogjoo J. Jang
The rates of
exciton transfer within dyads of perylene diimide and terrylene diimide connected by oligophenylene bridge units have been shown to deviate significantly from those of Förster's resonance energy transfer theory, according to single molecule spectroscopy experiments. The present work provides a detailed computational and theoretical study investigating the source of such discrepancy. Electronic spectroscopy data are calculated by time-dependent density function theory and then compared with experimental results. Electronic couplings between
exciton donor and acceptor are estimated based on both transition density cube method and transition dipole approximation. These results confirm that the delocalization of
exciton to the bridge parts contribute to significant enhancement of donor-acceptor electronic coupling. Mechanistic details of
exciton transfer are examined by estimating the contributions of the bridge electronic states, vibrational modes of the dyads commonly coupled to both donor and acceptor, inelastic resonance energy transfer mechanism, and dark
exciton states. These analyses suggest that the contribution of common vibrational modes serves as the main source of deviation from Förster's spectral overlap expression.
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Mesoscale and Nanoscale Physics
Multi-resonant non-dispersive infrared gas sensing: breaking the selectivity and sensitivity tradeoff
Emma R. Bartelsen, J. Ryan Nolen, Christopher R. Gubbin et al. (+8 more)
In applications such as atmospheric monitoring of greenhouse gases and pollutants, the detection and identification of trace concentrations of harmful gases is commonly achieved using non-dispersive infrared (NDIR) sensors. These devices employ a broadband infrared emitter, thermopile detector, and a spectrally selective bandpass filter tuned to the vibrational resonance of the target analyte. However, the fabrication of these filters is costly and limited to a single frequency. This limitation introduces a fundamental tradeoff, as broadening the optical passband width enhances sensitivity but compromises selectivity, whereas narrowing improves selectivity at the expense of sensitivity. In this work, we validate a filterless NDIR approach using a multi-peak thermal emitter developed through inverse design. This emitter enhances detection sensitivity by targeting multiple absorption bands, demonstrated through the creation of a sensor designed for the C-H vibrational modes of propane. Additionally, a set of single-peak emitters were developed to showcase the capability of designing highly selective sensors operating within close spectral proximity. These emitters, targeting the stretching modes of carbon monoxide and carbon dioxide, exhibit Q-factors above 50 and minimal crosstalk, enabling accurate detection of the target gas without interference from gases with spectrally adjacent absorption bands. This is enabled by the implementation of an aperiodic distributed Bragg reflectors, which allows for higher Q-factors with fewer layers than a periodic Bragg reflector using the same materials and number of layers, thereby reducing fabrication complexity and cost. Experimental results validate that this approach breaks the tradeoff between sensitivity and selectivity. This work highlights the potential of optimized thermal emitters for more efficient and compact gas sensing applications.
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All authors Emma R. Bartelsen, J. Ryan Nolen, Christopher R. Gubbin, Mingze He, Ryan W. Spangler, Joshua Nordlander, Katja Diaz-Granados, Simone De Liberato, Jon-Paul Maria, James R. McBride, Joshua D. Caldwell
Mesoscale and Nanoscale Physics
Magnetic switching of exciton lifetime in CrSBr
Ina V. Kalitukha, Ilya A. Akimov, Mikhail O. Nestoklon et al. (+13 more)
Exciton dynamics in layered
magnetic semiconductors provide a sensitive probe of the interplay between
spin order and light-matter interaction. Here, we study thin CrSBr layers using time-resolved photoluminescence spectroscopy in an external
magnetic field, revealing a step-like reduction in the
exciton lifetime from 11 to 7 ps, during the magnetization flip from the
antiferromagnetic to the
ferromagnetic phase. The reduction of the
exciton lifetime in the
ferromagnetic phase persists below the Néel temperature, as evidenced by its strong
magnetic-field dependence that disappears in the paramagnetic phase. Ab initio calculations reveal a one-dimensional nature of free
excitons accompanied by a pronounced change in the oscillator strength across the
magnetic phase transition predicting a shorter radiative lifetime of free
excitons in the
antiferromagnetic phase of CrSBr contradicting the experimental observations. This discrepancy is explained by strong localization of
excitons at low tempature. We show both experimentally and theoretically that the observed
magnetic switching of the
exciton lifetime is attributed to a larger
exciton localization volume leading to a larger oscillator strength in the
ferromagnetic phase. The results show that disorder-induced localization effects play a key role in
exciton dynamics in CrSBr.
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All authors Ina V. Kalitukha, Ilya A. Akimov, Mikhail O. Nestoklon, Torsten Geirsson, Alejandro Molina-Sánchez, Eyüp Yalcin, Claudia Ruppert, Daniel A. Mayoh, Geetha Balakrishnan, Muthumalai Karuppasamy, Zdeněk Sofer, Yadong Wang, Daniel J. Gillard, Xuerong Hu, Alexander I. Tartakovskii, Manfred Bayer
Soft Condensed Matter
A finite viscoelastic constitutive model for low to high strain rate response of elastomers with application of strain rate-induced glass transition
Bibekananda Datta, Sushan Nakarmi and Nitin P. Daphalapurkar
Amorphous elastomers exhibit significant rate-stiffening and unique viscous flow characteristics across a wide range of strain rates, often undergoing glass transition above a strain rate threshold. We have developed a thermodynamically-consistent and micromechanically-inspired constitutive model for soft elastomers to capture the rate-dependent stress-strain behavior and hysteresis when subjected to low to high strain rates. Our proposed constitutive model encapsulates the viscous flow of materials through molecular motion at low strain rates and intermolecular rearrangement and
alignment of the molecules at high strain rates, essentially covering the glass transition. We applied our constitutive model to uniaxial compression experiments performed at low and high strain rates for polyborosiloxane (PBS) to identify the material parameters, and subsequently, performed numerical simulations of single and multi-cycle compression, stress relaxation, and small amplitude oscillatory tension-compression. Our analyses indicate that the model predicts higher total energy dissipation with increasing strain rate; however, dissipation associated with molecular relaxation decreases because, beyond a crossover strain rate, intermolecular rearrangement and
alignment become dominant, which is consistent with the onset of the glass transition. For cyclic loading-unloading, we observed that dissipation over a cycle remains constant at low strain rates but decreases non-monotonically at high strain rates before becoming constant, with the peak stress over the cycle becoming higher, which can be interpreted as more loading being carried elastically by the
polymer network as the intermolecular rearrangement process occurs. Additionally, our model was able to qualitatively predict the storage modulus and loss modulus in the limit of small strain over a wide range of frequency sweeps.
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Materials Science · Cross-listed from math-ph
Modeling phononic band gap in microstructured solids using the Riemann-Cartan geometric framework
Ilya Peshkov and Loïc Le Marrec
This paper discusses the modeling of acoustic wave fields in microstructured elastic solids within the framework of Riemann-Cartan geometry. We consider a scenario in which microstructural deformations occur significantly faster than those of the bulk material. This time-scale separation creates apparent
geometric incompatibilities at the macroscopic level, even in the absence of permanent inelastic deformation or damage. We formalize this phenomenon by using a non-holonomic frame field to represent macroscopic elastic deformations and an associated torsion field to characterize the resulting
geometric incompatibilities. The spatial components of the torsion tensor quantify the instantaneous
geometric incompatibility of the macroscopic deformations, while its temporal components capture the inertial effects arising from the reversible energy exchange between the micro- and macro-scales. A key finding is that the model's dispersion relation predicts a complete frequency band gap. Furthermore, the governing equations exhibit a mathematical analogy to Maxwell's equations, potentially bridging the modeling of phononic and photonic metamaterials.
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Quantum Gases
Emission Dynamics of Rydberg Excitons in $\mathbf{\mathrm{Cu_2O}}$: Distinguishing Second Harmonic Generation from Secondary Emission
Kerwan Morin, Poulab Chakrabarti, Delphine Lagarde et al. (+5 more)
Rydberg
excitons in $\mathrm{Cu_2O}$ simultaneously give rise to two very different optical responses under resonant two-
photon excitation: a coherent second-harmonic signal mediated by the excitonic second order susceptibility tensor $\chi^{(2)}$, and a secondary emission originating from the radiative decay of real
exciton populations. Distinguishing these two channels is essential for interpreting nonlinear and
quantum-optical experiments based on high-$n$ states, yet their temporal, spectral, and power-dependent signatures often overlap. Here we use time-resolved resonant two-
photon excitation to cleanly separate SHG and SE and to map how each depends on $n$, temperature, excitation power, and
crystal quality. This approach reveals the markedly different sensitivities of the two processes to
phonons, defects, and many-body effects, and establishes practical criteria for identifying SE and SHG in a wide range of experimental conditions. Our results provide a unified framework for interpreting emission from Rydberg
excitons and offer guidelines for future studies aiming to exploit their nonlinear response and long-range interactions.
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All authors Kerwan Morin, Poulab Chakrabarti, Delphine Lagarde, Maxime Mauguet, Sylwia Zielińska - Raczyńska, David Ziemkiewicz, Xavier Marie, Thomas Boulier
Materials Science
Layered CrGe1-xSe3+y with Cr Kagome Lattice and Antiferromagnetic Ordering
Jeremy G. Philbrick, Chaoguo Wang, Xin Gui et al. (+1 more)
We report the synthesis and properties of a new layered material, CrGe1-xSe3+y. The
crystal structure was determined by using single
crystal x-ray diffraction and transmission electron microscopy. CrGe1-xSe3+y crystallizes with a space group R-3m, featuring a double Kagome layer of chromium atoms, sandwiched between disordered Ge-Se layers. This compound displays
antiferromagnetic order below 50 K, unlike the
ferromagnetic behavior in CrGeTe3. Chromium displays an effective moment of ~3.9 {\mu}B/Cr, consistent with Cr3+ oxidation. The presence of this phase limits the possibility of a
ferromagnetic CrGeSe3 analogous to CrGeTe3.
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All authors Jeremy G. Philbrick, Chaoguo Wang, Xin Gui, Tai Kong
Superconductivity
Oxygen distribution and segregation at grain boundaries in Nb and Ta-encapsulated Nb thin films for superconducting qubits
Jaeyel Lee, Dieter Isheim, Zuhawn Sung et al. (+14 more)
We report on atomic-scale analyses of oxygen distribution and segregation at grain boundaries (GBs) of Nb and Ta-encapsulated Nb (Ta/Nb) thin films for superconducting
qubits using atom-probe
tomography (APT) and transmission electron microscopy (TEM). We observe oxygen segregation at grain boundaries (GBs) relative to the oxygen concentration within the grains for both Nb and Ta-capped Nb thin films for superconducting
qubits and find that higher oxygen concentration in the interior of Nb grains lead to greater oxygen segregation levels at GBs. This finding emphasizes that controlling oxygen
impurities in Nb during film deposition and fabrication processing is important to reduce the level of oxygen segregation at GBs in Nb. The enrichment factor (Cgb/Cgrain) for oxygen segregation at GBs in Nb is 2.7 (error bar: 0.3) for Nb films, and Ta-capped Nb thin films exhibit slightly reduced Nb GB enrichment factors of 2.3 (error bar: 0.3) while GBs in the Ta capping layer itself possess higher enrichment factors of 3.0 (error bar: 0.3). We hypothesize that the Ta capping layer can trap oxygen and thereby affect oxygen in-
diffusion and segregation at GBs in the underlying Nb thin films. Finally, we find that increases in the oxygen concentration in both Nb grains and GBs correlate with a suppression in the critical temperature for superconductivity (Tc). Together, our comparative chemical and charge transport property analyses provide atomic-scale insights into a potential mechanism contributing to decoherence in superconducting
qubits.
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All authors Jaeyel Lee, Dieter Isheim, Zuhawn Sung, Francesco Crisa, Sabrina Garattoni, Mustafa Bal, Cameron J. Kopas, Josh Y. Mutus, Hilal Cansizoglu, Jayss Marshall, Kameshwar Yadavalli, Dominic P. Goronzy, Mark C. Hersam, David N. Seidman, Alex Romanenko, Anna Grassellino, Akshay A. Murthy
Strongly Correlated Electrons
Spin-triplet paired Wigner crystal stabilized by quantum geometry
Dmitry Zverevich, Alex Levchenko and Ilya Esterlis
We have used variational states to analyze the effects of band geometry on the two-dimensional Wigner
crystal with one and two electrons per unit
cell. At sufficiently low electron densities, we find that increasing Berry
curvature drives a transition into a crystalline state composed of
spin-triplet pairs carrying relative orbital angular momentum $m=-1$. The essential features of this transition are captured by an effective two-electron
quantum dot problem in the presence of Berry
curvature. Our results point to a purely electronic, strong-coupling mechanism for local
spin-triplet pairing in correlated two-dimensional electron systems with
quantum geometry.
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Strongly Correlated Electrons
Competing Paramagnetic Phases in the Maple-Leaf Heisenberg Antiferromagnet
Paul L. Ebert, Yasir Iqbal and Alexander Wietek
We establish a remarkably rich ground state phase diagram in the maple-leaf
lattice spin-$1/2$ Heisenberg antiferromagnet as a function of the three
symmetry-inequivalent nearest-neighbor bonds using exact diagonalization and tower-of-states analysis on clusters up to $N=36$ sites. Besides a hexagonal plaquette state, a
star-shaped valence bond solid state is discovered in close vicinity to the (canted) $120^\circ$
magnetic phase, strongly reminiscent of a de-confined critical point or Dirac
spin liquid scenario on the triangular
lattice antiferromagnets. Moreover, an exact dimer product-state is observed next to a collinear Néel-state, similar to the Shastry-Sutherland model. All identified phases compete in a parameter regime close to the isotropic point, providing a promising region for
spin liquids to emerge. By analyzing Gutzwiller-projected wave-functions we identify a sliver of parameter regime where a gapped $\mathbb{Z}_{2}$
spin liquid Ansatz is in astonishing agreement with the exact $N=36$ ground state. This rich competition of paramagnetic phases demonstrates that the maple-leaf antiferromagnet is a promising platform for exotic states of matter and
quantum critical phenomena.
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Mesoscale and Nanoscale Physics
Interacting electrons in silicon quantum interconnects
Anantha S. Rao, Christopher David White, Sean R. Muleady et al. (+2 more)
Coherent interconnects between gate-defined silicon
quantum processing units are essential for scalable
quantum computation and long-range
entanglement. We argue that one-dimensional electron channels formed in the silicon
quantum well of a Si/SiGe
heterostructure exhibit strong Coulomb interactions and realize strongly interacting Luttinger
liquid physics. At low electron densities, the system enters a Wigner regime characterized by dominant 4kF correlations; increasing the electron density leads to a crossover from the Wigner regime to a Friedel regime with dominant 2kF correlations. We support these results through large-scale density
matrix renormalization group (DMRG) simulations of the interacting ground state under both screened and unscreened Coulomb potentials. We propose experimental signatures of the Wigner-Friedel crossover via charge transport and charge sensing in both zero- and high-
magnetic field limits. We also analyze the impact of short-range correlated disorder - including random
alloy fluctuations and valley splitting variations - and identify that the Wigner-Friedel crossover remains robust until disorder levels of about 400 micro eV. Finally, we show that the Wigner regime enables long-range capacitive coupling between
quantum dots across the interconnect, suggesting a route to create long-range
entanglement between solid-state
qubits. Our results position silicon interconnects as a platform for studying Luttinger
liquid physics and for enabling architectures supporting nonlocal
quantum error correction and
quantum simulation.
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All authors Anantha S. Rao, Christopher David White, Sean R. Muleady, Anthony Sigillito, Michael J. Gullans
Mesoscale and Nanoscale Physics
Giant and Oscillatory Junction Magnetoresistance via RKKY-like Spin Coupling in Spin-Gapless Mn$_2$CoAl/SiO$_2$/p-Si Heterostructures
Nilay Maji, Subham Mohanty and Pujarani Dehuri
Here, we report
spin-selective transport and exceptionally large positive junction magnetoresistance (JMR) in sputter-deposited Mn$_2$CoAl/native-SiO$_2$/p-Si
heterostructures. Highly ordered inverse-Heusler Mn$_2$CoAl thin films with near-ideal XA chemical ordering (S$\approx$0.97) and a Curie temperature of $\sim$590 K are realized using magnetron sputtering process. The
spin-gapless semiconducting nature of Mn$_2$CoAl is experimentally supported by a weakly temperature-dependent resistivity with a very small negative temperature coefficient of resistance ($\mathrm{TCR} \approx -4.2 \times 10^{-9}\,\Omega \cdot \mathrm{m} \cdot \mathrm{K}^{-1}$) and a nonsaturating linear magnetoresistance over a wide range of
magnetic fields and temperatures. A
giant positive JMR of $\sim$825% at 10 K and $\sim$134% at room temperature is observed despite the presence of only a single
ferromagnetic electrode. Systematic variation of the SiO$_2$ tunnel barrier thickness reveals a reproducible oscillatory sign reversal of the JMR accompanied by a monotonic decay in magnitude. This behavior reflects thickness-dependent modulation of
spin-selective tunneling mediated by phase-coherent interfacial carriers. It can be described phenomenologically by an RKKY-like functional form without invoking conventional metallic exchange interactions. These results identify Mn$_2$CoAl/native-SiO$_2$/p-Si
heterostructures as robust and scalable platforms for room-temperature
spin-selective transport, with potential applications in
semiconductor-compatible
spin filters,
magnetic field sensors, and reconfigurable spintronic logic elements.
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Disordered Systems and Neural Networks · Cross-listed from cs.LG
Robust Reasoning as a Symmetry-Protected Topological Phase
Ilmo Sung
Large
language models suffer from "
hallucinations"-logical inconsistencies induced by semantic noise. We propose that current architectures operate in a "Metric Phase," where causal order is vulnerable to spontaneous
symmetry breaking. Here, we identify robust inference as an effective
Symmetry-Protected Topological phase, where logical operations are formally isomorphic to non-Abelian anyon braiding, replacing fragile
geometric interpolation with robust topological invariants. Empirically, we demonstrate a sharp topological phase transition: while
Transformers and RNNs exhibit gapless decay, our Holonomic Network reveals a macroscopic "mass gap," maintaining invariant fidelity below a critical noise threshold. Furthermore, in a variable-binding task on $S_{10}$ ($3.6 \times 10^6$ states) representing symbolic manipulation, we demonstrate holonomic generalization: the topological model maintains perfect fidelity extrapolating $100\times$ beyond training ($L=50 \to 5000$), consistent with a theoretically indefinite causal horizon, whereas
Transformers lose logical coherence. Ablation studies indicate this protection emerges strictly from non-Abelian gauge
symmetry. This provides strong evidence for a new universality class for logical
reasoning, linking causal stability to the
topology of the semantic
manifold.
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Statistical Mechanics
How many-body chaos emerges in the presence of quasiparticles
Sibaram Ruidas, Sthitadhi Roy, Subhro Bhattacharjee et al. (+1 more)
Many-body chaos is a default property of many-body systems; at the same time, near-integrable behaviour due to weakly interacting quasiparticles is ubiquitous throughout condensed matter at low temperature. There must therefore be a, possibly generic, crossover between these very different regimes. Here, we develop a theory encapsulating the notion of a cascade of lightcones seeded by sequences of scattering of weakly interacting harmonic modes as witnessed by a suitably defined chaos diagnostic (classical decorrelator) that measures the spatiotemporal profile of many-body chaos. Our numerics deals with the concrete case of a classical Heisenberg chain, for either sign of the interaction, at low temperatures where the short-time dynamics are well captured in terms of non-interacting
spin waves. To model low-temperature dynamics, we use ensembles of initial states with randomly embedded point defects in an otherwise ordered background, which provides a controlled setting for studying the scattering events. The decorrelator exhibits a short-time integrable regime followed by an intermediate `scarred' regime of the cascade of lightcones in progress; these then overlap, leading to an avalanche of scattering events which finally yields the standard long-time signature of many-body chaos.
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All authors Sibaram Ruidas, Sthitadhi Roy, Subhro Bhattacharjee, Roderich Moessner
Strongly Correlated Electrons
Stability of the Local Ni$^{2+}$ Electronic Structure to $A$-site Disorder in the Pyrochlore Antiferromagnet NaCaNi$_2$F$_7$
M. F. DiScala, A. de la Torre, J. W. Krizan et al. (+5 more)
NaCaNi$_2$F$_7$ is a unique example of
spin-1 Heisenberg antiferromagnet on the pyrochlore
lattice, but the presence of Na$^{1+}$/Ca$^{2+}$ $A$-site disorder complicates the local electronic and
magnetic environment of the Ni$^{2+}$ $B$-site. We utilize resonant inelastic X-ray scattering (RIXS) to study the influence of $A$-site disorder on the $B$-site electronic structure of NaCaNi$_2$F$_7$. Ni L-edge RIXS measurements reveal a Ni$^{2+}$ electronic structure in nearly ideal octahedral coordination, with only a small trigonal compression ($\delta$ = -200$\;$meV) required to capture all spectral features. Within the $D_{3d}$
symmetry of the Ni local environment, we extract an anisotropic $g$-factor of $g_{\parallel} = 2.26$ and $g_{\perp} = 2.27$, and a corresponding paramagnetic moment of $\mu_{\rm{eff}}=3.2\;\mu_B$. To simulate disorder, RIXS spectra were calculated with realistic distributions of
crystal field parameters; however, these spectra are invariant relative to a disorder-free model, demonstrating the robustness of the Ni$^{2+}$ electronic environment to the $A$-site disorder, within the resolution of our measurement.
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All authors M. F. DiScala, A. de la Torre, J. W. Krizan, J. Wouters, V. Bisogni, J. Pelliciari, R. J. Cava, K. W. Plumb
Mesoscale and Nanoscale Physics
Why finite-size non-Hermitian eigenvalues can miss asymptotic eigenstates in topological physics
Lucien Jezequel, Loïc Herviou and Jens H. Bardarson
Non-Hermitian systems exhibit a fundamental spectral dichotomy absent in Hermitian physics: the
eigenvalue spectrum and the eigenstate spectrum can deviate significantly in the thermodynamic limit. We explain how extrapolating finite-size
eigenvalues can miss important asymptotic eigenstates, with the unidirectional Hatano-Nelson model serving as a minimal realization of this phenomenon. Through exact analytical solutions, we show that this model contains not only hidden asymptotic modes but also quasi-Jordan structures that become macroscopic in the thermodynamic limit and appear more generally in systems with nontrivial bulk winding. Our framework explains how the apparent failure of the bulk-edge correspondence in models such as the non-Hermitian SSH chain can reflect the failure of finite-size
eigenvalue extrapolation to detect asymptotic edge states in systems with a skin effect. These results clarify the limitations of using finite-size
eigenvalues alone to infer thermodynamic-limit
topology.
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Soft Condensed Matter
Mechanics of axis formation in $\textit{Hydra}$
Arthur Hernandez, Cuncheng Zhu and Luca Giomi
The emergence of a body axis is a fundamental step in the development of multicellular organisms. In simple systems such as $\textit{Hydra}$, growing evidence suggests that mechanical forces generated by collective cellular activity play a central role in this process. Here, we explore a physical mechanism for axis formation based on the coupling between active stresses and tissue elasticity. We analyse the elastic deformation induced by activity-generated stresses and show that, owing to the spherical
topology of the tissue, forces globally condense toward configurations in which both elastic strain and nematic defect localise at opposite poles. These mechanically selected states define either a polar or apolar head-food axis. To characterize the condensed regime, we introduce a compact parametrization of of the active force and flux distributions, enabling analytical predictions and direct comparison with experiments. Using this framework, we calculate experimentally relevant observables, including areal strain, lateral pressure, and normal displacements during muscular contraction, as well as the detailed structure of topological defect complexes in head and foot regions. Together, our results identify a mechanical route by which active tissues can spontaneously break
symmetry at the organismal scale, suggesting a general physical principle underlying body-axis specification during morphogenesis.
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Statistical Mechanics
Fluctuation-response relation for a nonequilibrium system with resolved Markovian embedding
Rémi Goerlich, Antoine Tartar, Yael Roichman et al. (+1 more)
Fluctuation-response relations must be modified to describe nonequilibrium systems with non-Markovian dynamics. Here, we experimentally demonstrate that such relation is quantitatively recovered when the appropriate Markovian
embedding of the dynamics is explicitly resolved. Using a colloidal particle optically trapped in a harmonic potential and driven out of
equilibrium by a controlled colored noise, we study the response to a perturbation of the stiffness of the confining potential. While the reduced dynamics violates
equilibrium fluctuation-response relations, we show that the dynamical response to the stiffness perturbation is fully determined by steady-state correlations involving the exact conjugate observable in the Markovian
embedding.
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All authors Rémi Goerlich, Antoine Tartar, Yael Roichman, Igor M Sokolov
Materials Science
Tuning the Fermi surface of MoTe$_2$ by Nb Doping
Andrew P. Weber, Iñigo Robredo, Philipp Rüssmann et al. (+8 more)
Ab initio calculations and angle-resolved photoemission experiments demonstrate that the bulk and surface electronic structure of Weyl semimetal candidate MoTe2 changes significantly by tuning the chemical potential by less than 0.4 eV. Calculations indicate that several Lifshitz transitions can occur among multiple electron and hole
Fermi pockets of differing orbital character. Experiments show that 18% Nb-Mo substitution reduces the occupation of bulk and (001) surface bands, effectively producing a quasi-rigid band shift of $\simeq$ 0.2 eV. Orbital character and dimensionality of the bulk bands is examined by soft X-ray angle resolved photoemission with control of the excitation light polarization. Deposition of alkali atoms on the surface produces electron binding energy shifts consistent with increased band filling. The results indicate that multiple regimes of electronic properties can be easily accessed in this versatile, layered material.
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All authors Andrew P. Weber, Iñigo Robredo, Philipp Rüssmann, Maxim Ilyn, Arnaud Magrez, Philippe Bugnon, Nan Xu, Vladimir Strocov, J. Hugo Dil, J. Enrique Ortega, Julen Ibañez-Azpiroz
Strongly Correlated Electrons
Chiral Graviton Modes in Fermionic Fractional Chern Insulators
Min Long, Zeno Bacciconi, Hongyu Lu et al. (+3 more)
Chiral graviton modes are hallmark collective excitations of Fractional
Quantum Hall (FQH)
liquids. However, their existence on the
lattice, where continuum
symmetries that protect them from decay are lost, is still an open and urgent question, especially considering the recent advances in the realization of Fractional Chern
Insulators (FCI) in transition metal dichalcogenides and rhombohedral pentalayer
graphene. Here we present a comprehensive theoretical and numerical study of graviton-modes in fermionic FCI, and thoroughly demonstrate their existence. We first derive a
lattice stress tensor operator in the context of the fermionic Harper-Hofstadter(HH) model which captures the graviton in the flat band limit. Importantly, we discover that such
lattice stress-tensor operators are deeply connected to
lattice quadrupolar density correlators, readily generalizable to generic Chern bands. We then explicitly show the adiabatic connection between FQH and FCI
chiral graviton modes by interpolating from a low flux HH model to a Checkerboard
lattice model that hosts a topological flat band. In particular, using state-of-the-art
matrix product state and exact diagonalization simulations, we provide strong evidence that
chiral graviton modes are long-lived excitations in FCIs despite the lack of continuous
symmetries and the scattering with a two-magnetoroton continuum. By means of a careful finite-size analysis, we show that the
lattice generates a finite but small intrinsic decay rate for the graviton mode. We discuss the relevance of our results for the exploration of graviton modes in FCI phases realized in solid state settings, as well as cold atom experiments.
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All authors Min Long, Zeno Bacciconi, Hongyu Lu, Hernan B. Xavier, Zi Yang Meng, Marcello Dalmonte
Statistical Mechanics · Cross-listed from q-bio.PE
Cell size control in bacteria is modulated through extrinsic noise, single-cell- and population-growth
Arthur Genthon and Philipp Thomas
Living
cells maintain size homeostasis by actively compensating for size fluctuations. Here, we present two
stochastic maps that unify phenomenological models by integrating fluctuating single-
cell growth rates and size-dependent noise mechanisms with
cell size control. One map is applicable to mother machine lineages and the other to lineage trees of exponentially-growing
cell populations, which reveals that population dynamics alter size control measured in mother machine experiments. For example, an adder can become more sizer-like or more timer-like at the population level depending on the noise statistics. Our analysis of bacterial data identifies extrinsic noise as the dominant mechanism of size variability, characterized by a quadratic conditional variance-mean relationship for division size across growth conditions. This finding contradicts the reported independence of added size relative to birth size but is consistent with the adder property in terms of the independence of the mean added size. Finally, we derive a trade-off between population-growth-rate gain and division-size noise. Correlations between size control quantifiers and single-
cell growth rates inferred from data indicate that bacteria prioritize a narrow division-size distribution over growth rate maximisation.
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Strongly Correlated Electrons
Surface chiral Abelian topological order on multilayer cluster Mott insulators
Xu-Ping Yao, Chao-Kai Li and Gang v. Chen
The surface states of a
symmetry protected topological state can have many possibilities. Here we propose a
chiral Abelian topological order on a distinct surface of a multilayer-stacked cluster Mott insulating system. The first-principle calculation and the slave-rotor mean-field theory are applied to study the surface states of the relevant material system. The angle-resolved photoemission spectroscopic measurement is further suggested to detect the anomalous surface fractionalization of the
chiral Abelian topological order on the surface. The connection with real materials is further discussed. We expect our results to inspire the interest in the emergent exotic and correlation physics among the cluster Mott insulating systems and in the interplay between the two different branches of topological phases.
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Soft Condensed Matter
Hydrodynamic interactions in a binary-mixture colloidal monolayer
M. Chamorro-Burgos and Alvaro Domínguez
A colloidal monolayer embedded in the bulk of a fluid experiences a "compressible", long-range hydrodynamic interaction which, far from boundaries, leads to a breakdown of Fick's law above a well defined length scale, showing up as anomalous collective
diffusion. We here extend the model to study the effect of the hydrodynamic interaction on a monolayer formed by two types of particles. The most interesting finding is a new regime, in the limit of very dissimilar kinds of particles, where the effective dynamics of the concentration of "big" (slow) particles appears to obey Fick's law at large scales, but the corresponding collective diffusivity is completely determined, through hydrodynamic coupling, by the diffusivity of the "small" (fast) particles.
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