Statistical Mechanics
Irreversibility from Self-Reference: Gradient Flow and an H-Theorem for a Self-Referential Statistical Operator Framework
Lucio Marassi
This paper is a direct companion to arXiv:2605.06705, where the self-referential operator Omega was introduced and the Tsallis index q = alpha + beta was derived as a fixed-point condition within the local kernel approximation (LKA). Here we address four aspects deferred from the previous work. First, we carry out the first-order perturbative expansion of Omega beyond the LKA and demonstrate structural stability of q = alpha + beta at leading order in (xi/L)^2. Second, we define the iterative dynamical scheme Psi_(n+1) = Omega[Psi_n] and analyze convergence via Frechet spectral radius. Third, and centrally, we establish an H-
theorem rigorously within the LKA for both the discrete iteration and the continuous gradient flow: we compute dF/dtau explicitly along the flow, identify the negative semi-definite dissipation term, establish the result rigorously in the LKA using strict convexity of F proved in the companion paper, and provide numerical evidence showing monotone decrease of F[Psi_n] across 53 iterations on an N = 80 discrete system. Fourth, we characterize the non-perturbative role of the self-coupling parameter kappa, identifying a re-entrant disordered phase at kappa > kappa* approximately 0.50 +/- 0.05. The paper is explicit about what is proved, what is established numerically, and what open problems remain for a complete analytical proof beyond the LKA.
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Mesoscale and Nanoscale Physics
Long Spin Relaxation Times in CVD-Grown Nanodiamonds
Jeroen Prooth, Michael Petrov, Alevtina Shmakova et al. (+5 more)
Currently, the primary applications of fluorescent nanodiamonds (FNDs) are in the area of biosensing, by using photoluminescence or
spin properties of colour centres, mainly represented by the Nitrogen Vacancy (NV) point defect. The sensitivity of NV-FNDs to external fields is, however, limited by crystallographic defects, which influence their key
quantum state characteristics - the
spin longitudinal (\textit{T$_1$}) and
spin transversal (\textit{T$_2$}) relaxation and coherence times, respectively. We report on utilising an advanced FND growth technique consisting of heterogeneous nucleation on pre-engineered sites to create FNDs averaging around 60 nm in size, with mean longitudinal coherence times of 800 $\mu$s and a maximum over 1.8 ms, close to bulk theoretical values. This is a major, nearly ten-fold improvement over commercially available nanodiamonds for the same size range of 50 to 150 nm. Heavy-N doped nanodiamond shells, important for sensing events in nm proximity to the diamond surface, are fabricated and discussed in terms of re-nucleation and twinning on \{111\}
crystal facets. We also discuss scalability issues in order to enable the production of FND volumes matching the needs of sensing applications.
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All authors Jeroen Prooth, Michael Petrov, Alevtina Shmakova, Michal Gulka, Petr Cigler, Jan D'Haen, Hans-Gerd Boyen, Milos Nesladek
Statistical Mechanics · Cross-listed from quant-ph
Purification of a monitored qubit: exact path-integral solution
Matheus M. R. Poltronieri Martins and Henrique Santos Lima
We investigate the purification dynamics of a single
qubit under continuous in time monitoring. By employing a collisional model framework where the system interacts sequentially with ancillary
qubits, we describe the conditioned
evolution of the density
matrix through a
stochastic master equation. We show that for initial mixed states, the dynamics reduce to a multiplicative Langevin equation for a single scalar parameter representing the state's purity. This
stochastic process is solved exactly using the Onsager-Machlup path integral formalism, allowing us to derive the full probability distribution for the
qubit's trajectories. Our analytical results reveal that purification is characterized by a dynamical crossover from a
diffusion dominated regime to a measurement dominated regime, visible in the emergence of a bimodal state distribution. The analytical solutions are in strong agreement with numerical simulations, providing a robust theoretical
benchmark for the study of information extraction in monitored
quantum systems.
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Strongly Correlated Electrons
Imaging Interacting Two-Dimensional Anisotropic Electrons
Ziyu Xiang, Jianghan Xiao, Hongyuan Li et al. (+9 more)
We directly visualize a two-dimensional anisotropic Wigner
crystal and its
quantum melting in monolayer 1T-ReSe2 using non-invasive scanning tunnelling microscopy. In
crystals with anisotropic effective mass, an electron's
quantum wavefunction becomes elongated along the light-mass direction to reduce kinetic energy. At low electron density, such anisotropic electrons are predicted to form an oblique Wigner
crystal rather than the familiar triangular
lattice of isotropic systems. Despite longstanding theoretical interest, this physics has been little explored experimentally. Here we first image the anisotropic shape of individual electrons in gated monolayer ReSe2, whose wavefunctions are strongly elongated along the light-mass direction. At low density, these electrons crystallize into an oblique Wigner
lattice. As the density increases,
quantum fluctuations grow more rapidly along the light-mass direction than along the heavy-mass direction, driving a one-dimensional melting of the
crystal. The resulting state retains order along one direction but melts along the other, consistent with a smectic electron
liquid crystal between the electron solid and
Fermi liquid phases. Our work establishes monolayer ReSe2 as a platform for studying anisotropic correlated electrons,
quantum melting, and coupled one-dimensional electron chains.
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All authors Ziyu Xiang, Jianghan Xiao, Hongyuan Li, Woochang Kim, Tianle Wang, Zhihuan Dong, Takashi Taniguchi, Kenji Watanabe, Michael P. Zaletel, Steven G. Louie, Michael F. Crommie, Feng Wang
Superconductivity
Yu-Shiba-Rusinov States in Ising Superconductors
Michael Hein, Juan Carlos Cuevas and Wolfgang Belzig
The nature of the superconducting state in two-dimensional transition-metal dichalcogenides remains under active debate. A widely used description invokes so-called Ising superconductivity. In this work, we investigate theoretically this pairing state by employing single
magnetic impurities as local probes of the superconducting condensate. We analyze the formation of Yu-Shiba-Rusinov bound states in the presence of Ising
spin-orbit coupling and an in-plane
magnetic field to study how their spectral properties encode the underlying pairing structure. We identify distinct features in the bound-state spectrum and tunneling response that differentiate this system from conventional
superconductors. Our results demonstrate that
magnetic impurities provide a sensitive probe of the structure of the superconducting state and yield experimentally accessible signatures of unconventional aspects of Ising superconductivity.
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Soft Condensed Matter
Activity enhances transport while competing interactions preserve structure in colloidal microphase formers
Horacio Serna, José Martín-Roca, Ariel G. Meyra et al. (+1 more)
Colloidal models with short-range attraction and long range repulsion (SALR) have been extensively studied using theoretical and simulations methods due to their rich and universal
equilibrium phase behavior. Using Brownian Dynamics simulations, we study the dynamical phase behavior of active suspensions in which colloidal particles interact with each other via a SALR potential. Upon increasing the self-propulsion force of the particles, we observed that the structural transitions the active suspension undergoes resemble those observed in its passive counterpart by increasing the temperature of the thermal bath. However, when looking at the transport properties of active and passive suspensions with similar structure, we observed a clear mismatch. We demonstrated that increasing the activity enhances the particles mobility within the SALR fluid when simultaneously preserves the structure. This leads to a structure-dynamics decoupling induced by the activity whereas at the same time highlights the structural
memory of SALR potentials under non-
equilibrium conditions.
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All authors Horacio Serna, José Martín-Roca, Ariel G. Meyra, Eva G. Noya
Mesoscale and Nanoscale Physics
Negative Differential Resistance and Ultra-High TMR in Altermagnetic Tunnel Junctions
Sajjan Sheoran, Luke Keenan, Declan Nell et al. (+1 more)
Altermagnets can replace ferromagnets in tunnel junctions, yielding large tunneling magnetoresistance, ultrafast switching, and low-power functionality. While most studies explore the linear-response regime, interesting features emerge at finite bias, where the peculiar electronic structure of altermagnets gives rise to complex non-linear behaviour. Using non-
equilibrium Green's functions implemented with density functional theory, we predict that a large low-bias negative differential resistance can be observed in an altermagnetic tunnel junction. Our proposed junction incorporates the orbital-ordered altermagnet KV2Se2O, whose quasi-2D
Fermi surface plays a crucial role in realizing the negative differential resistance. Upon the application of a finite bias voltage, the current in the parallel configuration first increases sharply and then decreases, to be almost completely suppressed at around 0.14 V. At the same time, the antiparallel configuration displays a monotonic current-voltage curve. This behaviour, in addition to the negative differential resistance, supports a large tunneling magnetoresistance with sign inversion at 0.13 V. Our results suggest that altermagnetic tunnel junctions can be used as components in applications requiring strong non-linear response at low bias.
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All authors Sajjan Sheoran, Luke Keenan, Declan Nell, Stefano Sanvito
Mesoscale and Nanoscale Physics
Using a spin-triplet encoding to enhance shuttling fidelities in Si/SiGe quantum wells
Merritt P. R. Losert, S. N. Coppersmith and Mark Friesen
Spatial variations of the valley splitting in a
quantum well present a key challenge for conveyor-mode shuttling of electron
spins in Si/SiGe, giving rise to Landau-Zener-like excitations that cause leakage outside the
qubit subspace. Here, we propose an unconventional two-electron
qubit encoding, based on valley-singlet states, that is largely immune to Landau-Zener leakage processes. In contrast to single-electron
spins, the shuttling fidelity actually improves for small valley splittings, in this case. We show that high fidelities can be achieved without applying any special procedures, such as fine-tuning of the shuttling path.
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Materials Science
On-demand steering of hyperbolic chiral polaritons
Andrea S. Dai, Fuyang Tay, Ding Xu et al. (+11 more)
Control of light polarization and propagation in sub-wavelength architectures is foundational to nanophotonic technologies. A frontier direction is to leverage strong optical
spin-orbit interactions to realize polarization-selective light steering, known as the photonic
spin Hall effect. In this context,
hyperbolic plasmon polaritons (HPPs) are of particular interest as they offer large optical
spin-orbit coupling from strong confinement and dielectric anisotropy, as well as ray-like propagation. Despite theoretical predictions, however, the
hyperbolic spin Hall effect in natural materials has remained elusive. Here, we demonstrate the
hyperbolic spin Hall effect in the visible and near-infrared range in the natural
hyperbolic van der Waals metal MoOCl2. Enabling this discovery is a novel far-field pump-probe microscope that facilitates the launching and imaging of HPPs with exceptional sensitivity through interference with a high-momentum reference field. This approach preserves excellent control over light polarization, overcoming a key barrier to polarization-selective interrogation of
hyperbolic materials. We show that both
hyperbolic and surface plasmons in MoOCl2 display
chiral fields, and that their propagation direction can be completely switched upon light helicity reversal. Our results demonstrate on-demand steering of
chiral plasmons, firmly establishing natural
hyperbolic materials as ideal components for reconfigurable nanophotonics and
chiral light-matter coupling.
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All authors Andrea S. Dai, Fuyang Tay, Ding Xu, Inki Lee, Noah Bussell, Daria Balatsky, Francesco L. Ruta, Emma Lian, Colin Nuckolls, Xavier Roy, James G. Analytis, Andrew J. Millis, D. N. Basov, Milan Delor
Mesoscale and Nanoscale Physics
Direct-write electrochemical nanofabrication of ultrasmall graphene devices
Xiao Liu and Colm Durkan
Graphene nano-ribbons, GNRs, are promising channel materials for next-generation ultra-miniaturised devices due to their exceptional electrical and thermal properties which arise from their atomic thickness, as well as their ability to have a size-dependent band-gap [1-9]. However, despite extensive efforts to reliably fabricate narrow GNR-based field-effect transistors [10-12], their integration into conventional transistor technologies remains hindered by challenges such as high fabrication costs and complex processing requirements [13, 14]. In this study, we present a direct-write, relatively low-cost and robust approach for fabricating sub-10 nm GNR-based FETs using electrochemical atomic force microscopy lithography with an alternating current (AC) bias, obviating the need for electrodes. We also explain the underlying electrochemical process and provide a model which can be used to describe it. Leveraging the high-precision positioning capability of AFM, this method enables precise nanoscale
graphene patterning with feature sizes below 10nm. Compared with conventional lithographic techniques, photo- and electron-beam lithography i.e., PL & EBL, respectively [2, 15-20], it offers higher resolution, lower defect density, contamination-free processing, and the capability for in situ nanoscale device modification and characterisation. This work provides an efficient strategy for advancing GNR-based nanoelectronics.
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Mesoscale and Nanoscale Physics
Coupled Topological Interface States and Phonon Molecules in GaAs/AlAs Superlattices
S. Sandeep, O. Colmegna, C. Xiang et al. (+5 more)
Topological interface states in one-dimensional superlattices provide spatially localized
phonon modes protected by the
topology of the underlying band structure. In GaAs/AlAs distributed Bragg reflectors (DBRs), such states can be engineered through band inversion between superlattices with opposite Zak phases within the Su-Schrieffer-Heeger (SSH) framework. Here, we demonstrate topological
phonon molecules and extended chains formed by coupled nanophononic interface states. By concatenating three superlattices with alternating
topology, we realize two coupled interface states that hybridize into symmetric and antisymmetric modes, whose splitting can be tuned over tens of gigahertz by varying the reflectivity of the central DBR. Extending this concept, we engineer chains of up to N=6 coupled interface states that form narrow topological minibands while remaining strongly localized at the interfaces. We experimentally observe these coupled states in molecular-beam-epitaxy-grown GaAs/AlAs
heterostructures using time-domain pump-probe transient reflectivity measurements, and reproduce their behavior using transfer-
matrix calculations and a simple analytical model for the mode splitting. These results establish topological interface states as a robust platform for engineering coupled phononic systems and tunable nanophononic architectures in the GHz regime.
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All authors S. Sandeep, O. Colmegna, C. Xiang, E. R. Cardozo de Oliveira, K.Papatryfonos, M. Morassi, A. Lemaitre, N. D. Lanzillotti-Kimura
Materials Science
Bridging perturbation and variational approaches in brittle fracture
Serafim Egorov, Antoine Sanner, Jean Sulem et al. (+2 more)
We present a variational reduced-order model for three-dimensional coplanar propagation of sharp cracks in heterogeneous perfectly brittle solids under mixed-mode I+II+III loading. The approach connects the variational fracture formulation of Francfort and Marigo (1998) and the perturbation theory of Rice (1985) by computing
equilibrium crack-front configurations through minimization of the total energy defined as the sum of (i) the elastic potential energy, evaluated asymptotically from front deformations, and (ii) the dissipated energy, set by the fracture energy field. The potential energy and its derivatives are evaluated efficiently using the Fast
Fourier Transform. The resulting nonconvex box-constrained minimization problem is solved with a
matrix-free Newton conjugate gradient
algorithm with a trust region and physics-based preconditioning, enforcing irreversibility while resolving energy barriers and long-range elastic interactions. We validate our implementation against newly derived analytical solutions. We then perform 116,000 large-scale simulations of tensile and shear crack propagation in disordered media to quantify the impact of finite-size effects, disorder intensity, and mode mixity. The simulations reproduce the transition from smooth to intermittent crack growth, and show that mode mixity has limited influence on the onset of intermittency but induces quasi-
elliptic fronts in mixed II+III loading. They reveal a size-dependent crossover from disorder-induced weakening to toughening controlled by the emergence of depinning instabilities.
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All authors Serafim Egorov, Antoine Sanner, Jean Sulem, Lars Pastewka, Mathias Lebihain
Materials Science · Cross-listed from physics.optics
Visible-NIR-Frequency Hyperbolic Response in Nodal-Line Semimetal PbTaSe$_2$
Vivian J. Santamaria-Garcia, Morgan G. Blevins, Simo Pajovic et al. (+2 more)
Natural
hyperbolic materials offer a powerful platform for light-matter interactions by supporting highly anisotropic electromagnetic modes without the need for artificial patterning. In this work, we experimentally demonstrate that the nodal-line semimetal PbTaSe$_2$ exhibits robust
hyperbolic optical behavior in the visible to near-infrared spectral range, which arises intrinsically from its anisotropic electronic structure and layered
crystal symmetry. By combining first-principles calculations, ellipsometry, Drude-Lorentz modeling, and reflectance measurements, we establish a consistent experimental and theoretical picture of bulk hyperbolicity in this material. This hyperbolicity is of plasmonic origin and is characterized by a competitive quality factor ($Q_\mathrm{max} \approx 2.8$) and a very large anisotropy parameter ($|R| \approx 231$) at 0.78 eV.
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All authors Vivian J. Santamaria-Garcia, Morgan G. Blevins, Simo Pajovic, Carolina Orona-Navar, Svetlana V. Boriskina
Strongly Correlated Electrons
Lattice Gauging Interfaces and Noninvertible Defects in Higher Dimensions
David Hofmeier, Giovanna Pimenta and Weiguang Cao
We study gauging interfaces and their defect descendants in
lattice models with generalized
symmetries in higher dimensions. We construct explicit interface
Hamiltonians for gauging a $\mathbb Z_2^{(0)}$
symmetry in $(2+1)d$ and a $\mathbb Z_2^{(1)}$
symmetry in $(3+1)d$. In higher dimensions, and especially in the presence of higher-form
symmetries, the topological nature of gauging interfaces is obscured by the fact that the constrained
Hilbert space depends on the location of the interface. We resolve this by introducing movement operators acting on a common unconstrained
Hilbert space, which transport both the interface
Hamiltonians and the associated constraints. As applications, we analyze condensation defects obtained from finite-region gauging and reconstruct the gauging map from movement operators. Finally, we apply the same framework to subgroup gauging, focusing on the example of gauging $\mathbb Z_2\subset \mathbb Z_4$. This produces a dual
symmetry carrying a mixed anomaly, which we diagnose on the
lattice through
symmetry fractionalization on condensation defects. Our results provide an explicit
lattice framework for studying topological interfaces, condensation defects, and the associated anomalies arising from gauging in higher-dimensional systems.
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Disordered Systems and Neural Networks
The critical slowing down in training diffusion models
Luca Maria Del Bono, Giulio Biroli, Patrick Charbonneau et al. (+1 more)
Computational sampling has been central to the sciences since the mid-20th century. While machine-learning-based approaches have recently enabled major advances, their behavior remains poorly understood, with limited theoretical control over when and why they succeed. Here we provide such insight for
diffusion models—a class of generative schemes highly effective in practice—by analyzing their application to the $O(n)$ model of statistical field theory in the Gaussian limit $n \to \infty$. In this analytically tractable setting, we show that training a score model with a one-layer network architecture matching the exact solution exhibits a form of critical slowing down in parameter learning. This slowing down also impacts the generation process, indicating that the well-known difficulties of sampling near criticality persist even for learned generative models. To overcome this bottleneck, we consider the power of architectural depth. We find that using a two-layer architecture drastically reduces the critical slowing down, with the training time scaling logarithmically rather than quadratically with system size. Using a
Fourier implementation of the architecture, we further show that this acceleration in training time can be achieved without drastically increasing operational complexity. Taken together, these results demonstrate that
diffusion models can overcome the critical slowing down through appropriate architectural design, and establish a controlled framework for understanding and improving learned sampling methods in statistical physics and beyond.
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All authors Luca Maria Del Bono, Giulio Biroli, Patrick Charbonneau, Marylou Gabrié
Strongly Correlated Electrons
Incommensurate Spin-Density Waves in a Frustrated Maple-Leaf Lattice Ferromagnet
Paul L. Ebert, Yasir Iqbal and Alexander Wietek
We study how ferromagnetism breaks down in the
spin-$\tfrac12$ nearest-neighbor Heisenberg model on the maple-leaf
lattice with
ferromagnetic $J_t,J_d$ and
antiferromagnetic $J_h$, motivated by the mixed ferro-
antiferromagnetic interactions in Na$_2$Mn$_3$O$_7$. Exact diagonalization shows that the
ferromagnetic boundary does not feature a zero-field
spin-nematic phase on the clusters studied here, but an extended regime of incommensurate
spin-density-wave correlations with continuously evolving ordering vector. The phase diagram also contains collinear Néel, canted $120^\circ$, and hexagonal-singlet regimes, separated by regions that remain difficult to classify from exact diagonalization alone. Variational tests of fully symmetric Gutzwiller-projected Abrikosov-
fermion U(1) and $\mathbb{Z}_2$ states find no competitive
spin-
liquid description of the interior unresolved regions. By contrast, on the ruby-
lattice boundary we identify a point between the collinear Néel and hexagonal-singlet phases where a projected $\mathbb{Z}_2$ Ansatz reproduces the finite-size energy and
spin correlations with good accuracy.
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Mesoscale and Nanoscale Physics
Coherent control of spinmons
Johanne Bratland Tjernshaugen, Florinda Viñas Boström, Jeroen Danon et al. (+3 more)
The protection of superconducting
qubits from certain noise sources often comes at the cost of increased sensitivity to other decoherence channels. Here, we explore a route to avoid this tradeoff by encoding
quantum information in
quantum states of a transmon entangled with the
spin of a trapped Andreev quasiparticle. We term such devices spinmons. We lift the spinmon Kramers degeneracy by introducing a Zeeman field and develop two routes for full
qubit control via electrostatic gates and an AC flux drive, providing multiple directions for experimental implementations. Finally, we compute coherence times and verify the
qubit robustness against flux and charge noise sources.
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All authors Johanne Bratland Tjernshaugen, Florinda Viñas Boström, Jeroen Danon, Jacob Linder, Karsten Flensberg, Antonio L. R. Manesco
Strongly Correlated Electrons · Cross-listed from quant-ph
Scalable Measurement-Based Quantum Simulation Patterns for Benchmarking
V. W. Scarola
Measurement-based
quantum computing uses measurement patterns on predefined
quantum resource states to execute
quantum logic.
Quantum simulation offers an important use case on near-term devices. However, pattern optimization depends on the multivariable interplay between hardware and software constraints and is therefore use-dependent and highly non-trivial. Optimization of large-scale patterns under realistic assumptions remains a barrier. We announce the release of the
quantum measurement pattern library QPatLib, a
dataset that, in v1.0, presents patterns for use in measurement-based
quantum simulation. We present the workflow for generating patterns that execute Pauli-string
unitaries needed for many
quantum algorithms. We provide
benchmark patterns for measurement-based
quantum unitary evolution. The measurement patterns are defined with different conventions for commuting Pauli-string subsets to allow scaling of pattern size and complexity. The purpose of the library is to (i) serve as a standardized testbed for pattern-optimization protocols for measurement-based
quantum simulation routines, (ii) offer a suite of patterns for direct use on hardware, (iii) provide data to empirically justify pattern design principles, and (iv) provide a flexible resource for future storage and use of measurement-based patterns beyond
quantum simulation.
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Soft Condensed Matter
Designing Coulombic Contact Interactions between Polarizable Particles through Asymmetry
Yanyu Duan and Zecheng Gan
Polarizable particle systems, including charged colloids, polarizable ions, biomolecular assemblies, and soft nanomaterials, can exhibit contact electrostatic interactions that depart strongly from Coulomb behavior when dielectric mismatch and
geometric singularities amplify polarization effects. Here we use charged dielectric spheres as a model system and show that these polarization contributions can be canceled by jointly tuning size, charge, and dielectric asymmetries. By extending a recently developed image-charge formula to contacting dielectric spheres, we derive analytical conditions under which the contact interaction reduces to the bare Coulomb form. Accurate two-sphere calculations validate the resulting contact design rules with relative errors below $3\%$. Strikingly, many-body molecular dynamics simulations reveal that systems satisfying these two-body rules self-assemble into structures that closely match their pure Coulomb references. These results establish asymmetry as a route for turning electrostatic complexity into Coulombic simplicity at contact, with implications for controlled self-assembly and materials design.
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Materials Science
Automated multiphase identification and refinement in powder diffraction using mismatch-tolerant machine learning
Lalit Yadav, Yongqiang Cheng and Mathieu Doucet
Powder diffraction is a primary structural characterization tool in materials science, yet automated phase identification remains a major bottleneck for autonomous discovery. Existing workflows rely heavily on search–match heuristics and manual Rietveld refinement, and broadly usable end-to-end automation is especially limited for
neutron powder diffraction, where comparable tools are largely absent. Here we introduce RADAR-PD, a modality-aware machine learning framework for phase identification and quantification across both X-ray and
neutron powder diffraction. RADAR-PD couples a mismatch-tolerant neural network operating on coarse momentum-transfer fingerprints with automated
lattice nudging and physics-constrained Rietveld verification, enabling dominant-phase hypotheses to be generated from elemental constraints and secondary phases to be isolated recursively. On an experimental RRUFF PXRD
benchmark, RADAR-PD outperforms DARA in recovering the reference phase. RADAR-PD further provides robust multiphase analysis on complex time-of-flight and constant-wavelength
neutron datasets, addressing an important unmet need in automated
neutron diffraction analysis. These results establish RADAR-PD as an auditable, instrument-agnostic framework for autonomous structural discovery.
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Quantum Gases
Programmable Superradiance in an Interacting Qubit Array
Botao Du, Qihao Guo and Ruichao Ma
When multiple
quantum emitters couple to a common electromagnetic environment, interference in their collective radiative dynamics gives rise to superradiance and subradiance. In regimes where coherent interactions and collective dissipation compete, the microscopic many-body dynamics and
quantum correlations among the emitters that underlie superradiance and subradiance are theoretically challenging and remain experimentally elusive, even though collective emission has been observed in many physical systems. Here, we realize a superconducting
qubit array coupled to a common microwave waveguide that mediates collective dissipation, with simultaneous access to coherent interactions and microscopic measurements of many-body dynamics. Engineered
qubit-waveguide couplings with tunable amplitude and phase enable control of collective interference and the resulting super- and subradiant states. Leveraging site-resolved control and readout, we directly observe the microscopic decay dynamics of multi-
qubit states across different excitation
manifolds and track the
evolution of populations and tunable
quantum correlations. We reveal collective decay in regimes beyond the ideal Dicke model, where strong
qubit-
qubit interactions stabilize superradiance and subradiance against local dephasing and reshape decay pathways through spatially and spectrally structured many-body eigenstates. Our results establish a flexible platform for exploring collective phenomena in many-body
quantum optics and driven-dissipative approaches to robust
quantum information processing.
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Materials Science
Equivariant Space Group and Hamiltonian for Collinear Magnetic Systems
Chaoxi Cui, Zhi-Ming Yu, Yilin Han et al. (+3 more)
Condensed matter physics increasingly focuses on exploiting the
magnetic order parameter orientation n as a tuning knob for properties of collinear
magnetic materials, but a general method for constructing effective
Hamiltonians with explicit n-dependence has been lacking. Here, we develop a
symmetry-based framework, built on the equivariant space group, for constructing such
Hamiltonians, termed equivariant
magnetic Hamiltonians (EMHs). The resulting EMH lives in a higher-dimensional k-n space and exhibits unconventional
symmetry actions and topological features. Using a 1D
ferromagnetic chain and a 3D antiferromagnet as examples, we demonstrate that explicit n-dependence in EMHs enables the study of
magnetic-dynamics-driven topological pumping, including even-integer charge pumping and a second-Chern-number-induced quantized pumping of surface anomalous
Hall conductivity. Beyond model systems, we incorporate the framework into first-principles calculations to construct ab-initio EMHs that accurately capture the n-dependent band structures of real materials. The approach can also be generalized to non-collinear
magnetic systems. Our work establishes a general framework for constructing EMHs and for exploring the rich physics arising from
magnetic anisotropy and
magnetic dynamics.
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All authors Chaoxi Cui, Zhi-Ming Yu, Yilin Han, Run-Wu Zhang, Shengyuan A. Yang, Yugui Yao
Soft Condensed Matter
Fluctuation spectra of embryonic cell-cell interfaces reveal inverse-square scaling
Brian Huynh, Shinuo Weng and José Alvarado
Tissue-scale shape changes are driven by ensembles of intracellular forces. However measuring force in these contexts remains a difficult challenge. Here we perform spectral analysis of transverse fluctuations of
cell-
cell junctions in Xenopus embryonic tissue explants undergoing convergent extension. We developed an image analysis pipeline to extract fluctuation amplitude profiles $u(x,t)$ from time-lapse confocal movies and computed two-dimensional spatiotemporal power spectra. We observe power-law scaling of mean-squared fluctuation power spectra consistent with $\langle u_q^2 \rangle \sim q^{-2}$ and $\langle u_f^2 \rangle \sim f^{-2}$. The spatial scaling agrees with predictions from the Helfrich
Hamiltonian, and the temporal scaling agrees with overdamped dynamics of a fluctuating membrane, both in the tension-dominated regime. Pharmacological reduction of actomyosin contractility (via low-dose blebbistatin or latrunculin B) did not significantly alter either scaling exponent. Our results provide an early empirical characterization of junction fluctuation spectra in an actively shape-changing tissue. Simple tension-dominated membrane models appear sufficient to describe transverse junction dynamics despite their active and coupled nature. This work establishes a quantitative baseline for future studies of tension-bearing tissues and motivates the development of physical models specific to multicellular systems.
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Soft Condensed Matter
Variational approach to droplet motion on uneven solid surfaces, including contact line dynamics and evaporation
Gyula I Tóth, David N Sibley, Agnes J Bokányi-Tóth et al. (+2 more)
We show how dynamical equations for
liquid films and drops on uneven surfaces, including contact line dynamics and evaporation/condensation effects, may be formulated as a variational dynamics, generated via Onsager's variational principle. The theory applies in the isothermal overdamped-dynamics limit. We apply this general approach to obtain several well-known results on contact line dynamics and to study drops pinning and sliding on inclined corrugated surfaces. This approach constructs the dynamical equations starting from the free energy of the system and therefore has the advantage that it naturally incorporates the correct
equilibrium properties.
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All authors Gyula I Tóth, David N Sibley, Agnes J Bokányi-Tóth, Dmitri Tseluiko, Andrew J Archer
Superconductivity
Hadi H. Hassan, Santiago J. Carreira, M. Cabero et al. (+4 more)
Spin-pumping experiments in
superconductor/ferromagnet
heterostructures, which probe
spin-sinking by the
superconductor, have revealed a variety of complex behaviors. Most studies have focused on conventional s-wave
superconductors combined with metallic or insulating ferromagnets. Here, we study a d-wave
superconductor paired with a half-metallic ferromagnet, in epitaxial YBa2Cu3O7-d/La0.7Sr0.3MnO3
heterostructures with two crystalline orientations: one in which YBCO is c-axis oriented, and the other in which YBCO grows along the (103) direction. Using
ferromagnetic resonance (FMR), we probe the temperature-dependent Gilbert damping coefficient {\alpha}. For (103)
heterostructures, {\alpha}(T) initially decreases below Tc, but then increases at lower temperatures, exceeding normal-state levels. This behavior can be understood in terms of the opening of the superconducting gap and
spin transport via nodal quasiparticles, which dominate when the ab-plane of YBCO is exposed at the interface. In stark contrast, c-axis
heterostructures exhibit a pronounced enhancement of {\alpha}(T) below Tc, peaking at 0.65-0.7Tc before decaying. This anomaly suggests the dominance of interface-bound Andreev states, arising from a locally suppressed superconducting order parameter due to proximity effects with the half-metallic LSMO.
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All authors Hadi H. Hassan, Santiago J. Carreira, M. Cabero, F. Martinet, Alexander Buzdin, Jacobo Santamaria, Javier E. Villegas
Mesoscale and Nanoscale Physics
Optical Response of a screw dislocated GaAs Quantum Wire: Temperature and Pressure Effects
Vinod Kumar, Shweta Kumari and Surender Pratap
We investigate the influence of a screw dislocation, characterized by the dislocation parameter, on the optical response of a
parabolic GaAs cylindrical
quantum wire under the combined effects of temperature, hydrostatic pressure, and the axial
magnetic field. Using a torsion-modified metric together with pressure- and temperature-dependent material properties, namely the effective mass and dielectric permittivity, we obtain exact solutions of the Schrödinger equation in terms of Whittaker functions. The screw dislocation introduces a \(k_z\)-dependent coupling that breaks the
symmetry between the angular momentum states \(m\) and \(-m\) and modifies the centrifugal term in the effective potential. Based on the resulting eigenstates, we evaluate the linear and third-order nonlinear optical absorption coefficients, as well as the corresponding refractive index changes, for the dipole-allowed transitions \(m = 0 \to +1\) and \(m = 0 \to -1\). Our results show that increasing the dislocation parameter produces a pronounced
redshift and suppresses the resonance amplitude for the \(m = 0 \to +1\) transition, whereas the \(m = 0 \to -1\) transition exhibits a blueshift accompanied by peak enhancement. We further find that increasing temperature shifts the resonances toward higher
photon energies and enhances their amplitudes, while hydrostatic pressure causes a
redshift and reduces the peak intensity for both transitions. In addition, the
magnetic field strengthens the optical response and induces a blueshift for the \(m = 0 \to +1\) transition, whereas the opposite behavior is obtained for the \(m = 0 \to -1\) transition. We have also examined the behavior of the refractive index changes, which exhibit analogous asymmetric dependence on the dislocation parameter.
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Statistical Mechanics
Link length and energy fluctuations in extensible freely jointed chains
Michael R. Buche
The freely jointed chain is often applied to model the thermodynamics of single
polymer chains, but the traditional formulation of the model lacks internal energy changes due to bond stretching. For this reason, the extensible freely jointed chain model includes a potential energy function, typically harmonic, that governs the length of each link in the chain. Among the other quantities of interest that are subject to thermal fluctuations, these link lengths and energies too fluctuate about their ensemble average values. Since a plethora of models for
polymer chains and networks incorporate chain dissociation as a function of either link length or energy, these fluctuations are crucial to understand and quantify. Motivated by this fact, fluctuations in link length and energy are analyzed within a freely jointed chain under an applied force. These fluctuations are quantified through their average values, standard deviations, and probability distributions. Across all values, asymptotically correct analytic relations and their less ergonomic exact counterparts are introduced. The asymptotic relations are verified to be accurate through direct comparison and to be correct within transcendentally small terms through error analysis. In certain cases, the fluctuations are shown to be approximately normally distributed. Hereafter, model components predicated on link length or energy ought to account for these fluctuations.
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Statistical Mechanics · Cross-listed from quant-ph
Information Thermodynamics in Generalized Probabilistic Theories
Koki Ono, Shun Umekawa and Hiroyasu Tajima
Generalized probabilistic theories (GPTs) provide a unified framework for describing probabilistic physical theories, encompassing classical and
quantum theories as well as hypothetical theories beyond
quantum mechanics. Since most GPTs are highly unrealistic and far removed from known physical theories, it is important to constrain them by physically reasonable principles. One of the most important such principles is consistency with thermodynamics, which has been extensively studied through toy models involving semipermeable membranes (SPMs) implementing measurements. On the other hand, information thermodynamics, which plays a central role in understanding the relationship between measurement and thermodynamics in classical and
quantum theory, has remained largely undeveloped in GPTs. In this work, we construct information thermodynamics in GPTs and provide a unified framework for analyzing the relationship between measurement, feedback, information erasure, and the second law of thermodynamics. We also formulate a general framework for SPM models and analyze the thermodynamic cost of measurements implemented by SPMs. As a result, we show that no work can be extracted in contradiction with the second law as long as the measurement processes are consistent with
entropy nondecrease, and derive sufficient conditions for this property for several
entropy definitions proposed in GPTs. Moreover, by considering measurement processes violating these conditions, we construct explicit GPT systems realizing isothermal SPM cycles from which positive work can be extracted. These results demonstrate that violations of the second law can arise from the lack of fundamental
entropy properties or discrepancies between
entropy definitions, and provide a unified and model-independent foundation for understanding the relationship between thermodynamics and measurement in GPTs.
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Materials Science
Graphene lattice recoil in hard X-ray photoemission: Experiment and Theory
Simone Ritarossi, Alice Apponi, Orlando Castellano et al. (+6 more)
Hard-x-ray C 1s photoemission from monolayer
graphene probes a regime in which
nuclear recoil and intrinsic electronic asymmetry contribute on comparable energy scales to the observed spectral line shape. Here we combine experiment and modeling over the
photon-energy range 0.8 keV–8 keV to resolve this interplay quantitatively. A
graphene-specific implementation of the Fujikawa–Takata cumulant formalism, based on an anisotropic vibrational density of states constrained by first-principles
phonon calculations, captures the expected
recoil scaling with
photon energy and emission geometry but fails to reproduce the pronounced asymmetric tails of the measured spectra. To overcome this limitation, we introduce an explicit electronic convolution model in which an intrinsic,
photon-energy-independent electronic line shape extracted from near-recoilless 0.8 keV data is convolved with a
phonon recoil kernel carrying the full dependence on
photon energy and emission angle. This approach reproduces both the measured line-shape
evolution and the observed centroid shifts across the explored energy range without refitting the spectra at higher
photon energies. The results show that
recoil in
graphene cannot be described by a baseline treatment in which the
phonon recoil kernel is combined only with symmetric lifetime broadening, but must be treated together with the intrinsic many-body electronic response of the C 1s line.
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All authors Simone Ritarossi, Alice Apponi, Orlando Castellano, José Lorenzana, Domenica Convertino, Camilla Coletti, Tien-Lin Lee, Francesco Offi, Alessandro Ruocco
Quantum Gases
The wave nature of a Mott insulator
Xudong Yu, Chengyang Wu, Wenhan Chen et al. (+10 more)
Quantum phases of matter are routinely identified by coherence features, with interference patterns being one of the most directly observable quantities. In
lattices, the superfluid-to-Mott-
insulator (SF-MI) transition is commonly viewed as a change from wave-like coherence to particle-like localization: interference peaks are taken as a hallmark of superfluidity, whereas their disappearance is used to diagnose insulating behavior. Here, we challenge this picture for one-dimensional (1D) strongly interacting gases subject to a
lattice potential. We realize a gapped Mott
insulator through pinning in a shallow
lattice and find that pronounced interference peaks persist deep in the insulating regime. Strikingly, the interference becomes stronger as the Mott fraction increases, demonstrating that a certain degree of coherence still exists in the
insulator state. Measurements of the one-body correlation function reveal an oscillatory, exponentially decaying coherence pattern across several
lattice sites, in quantitative agreement with
quantum Monte Carlo (QMC) simulations. Our work shows that interference does not uniquely diagnose superfluidity and it exposes the unexpected wave nature of a 1D Mott
insulator.
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All authors Xudong Yu, Chengyang Wu, Wenhan Chen, Igor Zhuravlev, Zekui Wang, Yi Zeng, Sudipta Dhar, Milena Horvath, Thierry Giamarchi, Manuele Landini, Hanns-Christoph Nägerl, Hepeng Yao, Yanliang Guo