Other Condensed Matter · Cross-listed from quant-ph
Basis-update and Galerkin time integration in canonical matrix-product-state form
Maximilian Fröhlich, Richard M. Milbradt, Martin Eigel et al. (+3 more)
Matrix product state
algorithms must enlarge their bond spaces as
entanglement grows and compress them to control cost. We formulate basis-update and Galerkin (BUG) time integration as a sequence of canonical MPS sweeps for
Hamiltonians represented as
matrix product operators. We show when two natural basis updates produce the same trial space and when transporting coefficients between successive bases preserves the represented state. Under these conditions, the existing first-order error bound for uncompressed tree-tensor-network BUG also applies to the alternating-endpoint MPS schedule. We verify the uncompressed implementation against an independent six-site calculation. We then compare BUG with two-site TDVP for 16-site transverse-field Ising and Haldane-Shastry dynamics. At matched timestep and truncation settings, BUG performs fewer local exponential actions and has lower runtime. These settings do not produce equal accuracy. The runtime versus accuracy curves cross for the Ising model and are close for the Haldane-Shastry model. The comparison therefore identifies model-dependent trade-offs rather than a general advantage for either method.
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All authors Maximilian Fröhlich, Richard M. Milbradt, Martin Eigel, Aaron Sander, Robert Wille, Christian B. Mendl
Statistical Mechanics · Cross-listed from hep-th
Bulk Criticality and Boundary Spectra in AdS from Matrix Product States
Faizan Bhat
We study interacting scalar
quantum field theories in anti-de Sitter (AdS) space using a tensor-network approach based on
matrix product states. We develop methods for extracting low-lying boundary spectra from global-AdS energy levels, probing bulk criticality through finite-size scaling in the AdS radius, and identifying conformal boundary conditions. Applying these methods to scalar $\phi^4$ theory in $\mathrm{AdS}_2$, we compute the lowest non-trivial $\mathbb{Z}_2$-odd and even boundary scaling dimensions non-perturbatively. We then locate the bulk $\mathbb{Z}_2$
symmetry-breaking transition and extract critical exponents and central charge consistent with the 2D Ising universality class. At the critical point, we determine the low-lying boundary spectrum and use it to identify the $\mathbb{Z}_2$-preserving free/ordinary Ising conformal boundary condition, thereby characterising both the bulk and boundary universality classes.
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Statistical Mechanics · Cross-listed from hep-th
A geodesic distance interpretation of Lanczos coefficients
Le-Chen Qu
We study the spread complexity of the infinite-temperature thermofield double state in large-$N$ random
matrix theory and investigate the physical meaning of its Lanczos coefficients $b_n$. The leading density of states defines a family of orthogonal
polynomials whose recursion coefficients coincide with the Lanczos coefficients of the associated Krylov dynamics. Starting from the Coulomb-gas equation, we derive a pair of nonlinear relations for these coefficients, which we call the Lanczos equations. For normalizable one-cut ensembles, the Lanczos coefficients approach constants at large Krylov index, rendering the asymptotic Krylov chain translationally invariant and the late-time growth of spread complexity linear, with the growth rate given by the spectral average of the group velocity obtained from a Bloch-wave analysis. For an even potential, we further show that the difference between consecutive squared Lanczos coefficients, $b_{n+1}^2-b_n^2$, is the generating function for two-legged maps whose marked legs are separated by the exact geodesic distance $n$. Consequently, the spread complexity acceleration operator equals twice the geodesic generating operator. We illustrate these results in the quartic
matrix model, where the expansion in the quartic coupling counts tetravalent planar maps, and in the double-scaled Sachdev–Ye–Kitaev model, where the auxiliary
Hilbert space can be identified with the physical chord
Hilbert space.
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Quantum Gases
Hydrodynamization in 1D Bose gases at nonzero temperature
Jeff Leiberton and Marcos Rigol
Hydrodynamization refers to the remarkably rapid process in relativistic heavy-ion collisions by which hydrodynamic descriptions become applicable. Following the observation of analogous behavior in ultracold one-dimensional (1D) Bose gases, hydrodynamization has been conjectured to be a universal dynamical phenomenon in
quantum systems following high-energy quenches. Theoretical studies in this cold-atom setting have so far been restricted to quenches from ground states. Here we study how nonzero temperatures affect hydrodynamization. Specifically, using a homogeneous 1D gas of hard-core
bosons, we explore how the initial temperature affects the timescales associated with hydrodynamization and prethermalization following a Bragg-pulse quench. We find that while the hydrodynamization coherence time remains unchanged, increasing temperature shortens both the damping time of the hydrodynamization oscillations and the prethermalization time. We argue that this is mainly the result of the broadening of the initial rapidity distribution, and introduce a nonzero-temperature dephasing time defined in terms of the extent of the rapidity distribution.
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Statistical Mechanics
Giant Thermal Amplification via Engineered Dissipation in a Sierpinski-Gasket Aharonov-Bohm Interferometer
Shubhra Shubhadarshini Mallick, Salil Bedkihal, Mattias Fitzpatrick et al. (+1 more)
We propose a three-terminal thermal amplifier based on a Sierpinski-gasket Aharonov-Bohm interferometer, where the third (base) terminal is realized as a floating Buttiker probe that acts as an engineered dissipative reservoir, exchanging energy with the conductor while carrying no net charge current. Using the nonequilibrium Green's function formalism, we demonstrate that the interplay of
quantum coherence and engineered dissipation gives rise to
giant,
magnetic-flux-controlled thermal amplification, whereas purely coherent transport exhibits little or no amplification. We show that the amplification originates from a flux-induced cancellation of the energy-resolved thermal response of the base terminal, causing its differential heat current to vanish while finite heat currents continue to flow through the emitter and collector terminals. As a result, the thermal gain diverges without requiring resonant transmission. This interference-driven cancellation gives rise to an emergent thermal transparency, closely analogous to electromagnetically induced transparency in optical systems, where destructive
quantum interference suppresses the thermal response of the base reservoir while maintaining finite heat transport through the remaining terminals. Our results establish the interplay of engineered dissipation and
quantum interference as a powerful mechanism for controlling heat flow and realizing high-performance thermal amplifiers in mesoscopic conductors.
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All authors Shubhra Shubhadarshini Mallick, Salil Bedkihal, Mattias Fitzpatrick, Malay Bandyopadhyay
Soft Condensed Matter
Size matters more than packing in bimodal colloidal gel compositions
Robert A. Campbell, Ziye Zhuang, Ali Mohraz et al. (+1 more)
Colloidal gels are frequently modeled as monodisperse particle networks, although practical formulations commonly contain particles with multiple characteristic sizes. Here, we use large-scale, hydrodynamically resolved simulations of colloidal depletion gels to isolate the effects of particle size and local packing in bimodal systems with a small-to-large size ratio of 1:2. Increasing the large-particle fraction introduces new heterotypic angular motifs and substantially increases the fraction of bonds participating in tetrahedral structures, with a maximum at intermediate composition. However, these additional rigid motifs do not reorganize into larger or more highly connected tetrahedral aggregates. The mean coordination and characteristic aggregate size remain nearly composition independent. By contrast, the void and cluster-size distributions coarsen systematically as the large-particle fraction increases. These mesoscale distributions largely collapse when normalized by a composition-dependent particle length scale, indicating that changes in composition primarily rescale gel architecture rather than producing distinct rigid-network
topologies. An elastic modulus estimated using Cauchy-Born theory similarly follows this effective length scale more closely than the abundance of local tetrahedral motifs. These results show that, for moderate size disparity, particle size controls the structural scale and predicted mechanical response of bimodal colloidal gels more strongly than enhanced local packing.
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All authors Robert A. Campbell, Ziye Zhuang, Ali Mohraz, Safa Jamali
Disordered Systems and Neural Networks
Spectral Edge Rigidity of Quantum Chaotic States
Joaquim Telles de Miranda and Tobias Micklitz
We determine the distribution of fidelity susceptibility for chaotic eigenstates at the spectral edge of Gaussian random-
matrix ensembles. Previous work showed that, in the
unitary class, the characteristic susceptibility scale of edge states grows as $D^{1/3}$, rather than proportionally to $D$ as in the spectral bulk, reflecting Airy-edge level
rigidity. Extending a determinant-based framework introduced for bulk states, we derive the universal edge distributions for both the orthogonal and
unitary ensembles. The two
symmetry classes share the scaling variable $g/D^{1/3}$ and exhibit a
symmetry-dependent cubic suppression of small susceptibilities, while their algebraic large-$g$ tails reflect the corresponding
symmetry-dependent level repulsion. Although eigenvector statistics retain their random-
matrix form throughout the spectrum, edge
rigidity makes low-lying chaotic states parametrically less sensitive to generic perturbations than bulk states. Our results establish universal,
symmetry-dependent spectral-edge fidelity-susceptibility statistics in systems whose chaotic dynamics extends down to the ground state.
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Mesoscale and Nanoscale Physics
Tuning single-molecule fluorescence by atomic-scale control of the local environment
Thiago G. L. Brito, Daniel Arribas, Sofia Canola et al. (+3 more)
Molecules that absorb and emit light play a central role in microscopy, light-emitting devices and photosynthesis. Their fluorescence arises from well-defined radiative transitions that are governed by the electronic states and their coupling to the
nuclear motion that are influenced by the local environment. Yet the effect of controlled atomic-scale variations in the emitter surroundings remains unexplored. Here, we use scanning tunneling microscopy combined with optical spectroscopy to investigate the optical response of a single phthalocyanine to the change in the position of a nearby molecule, controlled with precision better than 100 pm. Upon decreasing the intermolecular distance, the molecular emission energy
redshifts and its line profile evolves. Supported by theoretical calculations, we disentangle the electronic and
nuclear contributions to the changes in fluorescence. We find that the
redshift originates from the interaction between the excitations of the two molecules, while the lineshape changes reflect modifications of the molecular rotational degree of freedom and non-
equilibrium dynamics. We extend this control to larger assemblies, where one molecule tunes the energies of two chromophores, mimicking the environmental tuning in photosynthetic systems. Our study provides atomic-scale insight into how the local environment affects the optical properties of molecular systems.
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All authors Thiago G. L. Brito, Daniel Arribas, Sofia Canola, Klaus Kuhnke, Tomáš Neuman, Anna Rosławska
Mesoscale and Nanoscale Physics
Surface weak ferromagnetism
Vladimir A. Zyuzin
This paper proposes a model of a Néel-ordered antiferromagnet exhibiting a weak
ferromagnetic moment restricted to the
crystal surface, generated by
spin-orbit coupling induced by broken surface
symmetries. Depending on
crystal termination, the surface
magnetic moment can align with identical or opposing signs on the top and bottom surfaces, resulting in a zero Faraday effect paired with a finite Kerr effect upon reflection for the latter configuration.
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Soft Condensed Matter
Approximation of anisotropic pairwise interactions for charged objects using multivariate polynomials and a multipole expansion
Mohammadreza Fakhraei, Dylan McElheny, Chris A. Kieslich et al. (+1 more)
We formulate a physics-informed data-driven method for modeling anisotropic pairwise interactions in the presence of long-ranged electrostatics. The method separates the total interaction into a long-ranged electrostatic interaction that is approximated using a multipole expansion truncated at the dipole level and a short-ranged residual interaction that is approximated using multivariate Chebyshev
polynomials fit to measurements from a limited number of configurations. We assess the approach on a sequence of aromatic molecules (benzene, benzonitrile, and phenoxide), finding that it produces satisfactory results using a modest cutoff distance for the short-ranged interaction. This method has applications for modeling complex interactions for, and conducting dynamic simulations of, synthetic and biological materials with charge.
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All authors Mohammadreza Fakhraei, Dylan McElheny, Chris A. Kieslich, Michael P. Howard
Materials Science
Automated Burgers Vector Identification for Individual Dislocations in Bulk Crystals
Abderrahmane Benhadjira, Carsten Detlefs, Vincent Favre-Nicolin et al. (+4 more)
Weak-beam imaging in dark-field X-ray microscopy (DFXM) can resolve individual dislocations in bulk
crystals, but assigning Burgers vectors from the resulting contrast typically requires manual comparison with forward simulations. Here, we train a physics-informed convolutional neural network (CNN) on geometrical optics simulations of isolated dislocations in face-centred cubic (FCC) aluminium, incorporating crystallographic constraints into the learning pro- cess. The model identifies Burgers vectors from weak-beam integrated rocking-curve images. On synthetic test data, the model achieves an accuracy of approximately 93%. In an experimental cross-slip case, the constrained model as- signs 72.7% of the layer-wise predictions to the reference Burgers vector. These results show that simulation-trained, physics-informed CNNs represent a step toward automated dislocation identification in DFXM.
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All authors Abderrahmane Benhadjira, Carsten Detlefs, Vincent Favre-Nicolin, Henning Friis Poulsen, Grethe Winther, Can Yildirim, Sina Borgi
Mesoscale and Nanoscale Physics
Classical Mechanics Exactly Yields the Full Bound-State Spectrum of the Two-Dimensional Coulomb Problem
Gang Zheng, Wenqi Xue, Mengli Wang et al. (+2 more)
High-lying Rydberg
excitons in two-dimensional
semiconductors universally exhibit a characteristic odd-integer energy scaling distinct from three-dimensional systems. While this hallmark of two-dimensional Coulomb interaction is well known from
quantum mechanical solutions, its deeper classical
geometric origin remains unclarified. Here we show that the complete bound-state spectral structure of the two-dimensional Coulomb problem—a central model for two-dimensional
exciton physics—follows as an exact
theorem from classical mechanics augmented by a single phase-scale parameter $\alpha$ with dimensions of action. We derive an amplitude-closure criterion as a necessary and sufficient condition for a classical propagator kernel to satisfy a linear
evolution equation, and demonstrate that the singular Coulomb potential can be mapped shell-by-shell via Levi-Civita regularization into the class of quadratic
Hamiltonians that obey this criterion exactly. The resulting spectrum bears odd-integer modal numbers, $1/N^2$ energy ratios and $N$-fold degeneracies, all independent of $\alpha$ and consistent with experimental observations of high-lying Rydberg
excitons. This work provides a pure classical-geometry
benchmark for two-dimensional
exciton spectral analysis, allowing quantitative disentanglement of universal Coulomb effects from material-specific screening effects. No semiclassical, short-wavelength or $\hbar \to 0$ approximation is invoked at any stage. Our results invert the usual logical hierarchy for this integrable system: the wave equation emerges as a representation of the underlying classical geometry, rather than as an independent first principle.
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All authors Gang Zheng, Wenqi Xue, Mengli Wang, Peng Chen, Benniu Zhang
Statistical Mechanics · Cross-listed from math-ph
Asymptotic analysis of a Fredholm determinant occurring in the description of the dynamical correlation functions of the Lieb–Liniger Bose gas
Frank Göhmann, Karol K. Kozlowski and Mikhail D. Minin
We perform a large-$x$ asymptotic analysis of the Fredholm determinant of an integrable integral operator with generalized sine kernel, where $x$ controls the strength of the oscillations along the integration contour of the operator and will play the role of the distance variable in applications to the correlation functions of integrable
quantum systems. Our generalized sine kernel involves a number of functional parameters that will allow us to adapt it to the analysis of the dynamical correlation functions of the Lieb–Liniger model at finite temperature and for all positive values of the coupling constant. It will also allow us to consider a class of
equilibrium correlation functions that are governed by generalized Gibbs ensembles. Our work is based on the analysis of a
matrix Riemann–
Hilbert problem that is canonically connected with our integrable integral operator.
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Strongly Correlated Electrons
Resolving the Magnetic Ground State and Field-Induced Transitions in Magnetic Dirac Semimetal Candidate EuMnSb$_2$
Yiu-Fung Chiu, Jian-Rui Soh, Sanjay Sharma et al. (+9 more)
The
magnetic structure of a
magnetic topological semimetal EuMnSb$_2$ is investigated in fields up to 30 T using polarized and unpolarized
neutron diffraction, pulsed-field x-ray
magnetic circular dichroism and pulsed-field magnetometry. We determine the zero-field
magnetic structures of the Eu and Mn sublattices, and find that
magnetic transitions induced by applied fields below 2 T correspond to changes in the
magnetic order of the Eu
spins alone without detectable perturbation to the order of the Mn
spins. An additional
magnetic transition is observed at fields close to the saturation field for the Eu
spins. We present a mean-field model which describes key features of the
magnetic behavior and allows us to estimate the dominant Eu–Eu and Eu–Mn exchange interactions responsible for the coupling between magnetism and electronic
topology.
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All authors Yiu-Fung Chiu, Jian-Rui Soh, Sanjay Sharma, J. Alberto Rodríguez-Velamazán, John Singleton, Eugen Weschke, Oleksandr Prokhnenko, Oksana Zaharko, Dharmalingam Prabhakaran, Stephen J. Blundell, Paul A. Goddard, Andrew T. Boothroyd
Mesoscale and Nanoscale Physics
Reduced vortex descriptors linking polycrystallinity in magnetic nanoparticles with polarized magnetic small-angle neutron scattering
M.P. Adams, J. Leliaert, A. Michels et al. (+1 more)
Analytical
vortex models reduce polarized
magnetic small-angle
neutron scattering (SANS) from nanoparticle ensembles to a small set of texture descriptors. In this work, we apply this reduction to micromagnetic simulations of polycrystalline iron
oxide nanoflowers at a fixed particle size and examine how a controlled parametrization of multigrain disorder is reflected in the remanent descriptors. The particles are represented by explicit Voronoi microstructures, and intraparticle disorder is varied through the intergrain exchange coupling and anisotropy-axis coherence. Fitting each remanent magnetization state to a
hyperbolic vortex model reveals a predominantly two-channel organization: the intergrain exchange coupling is associated mainly with the radial
vortex profile, whereas the anisotropy-axis coherence is associated mainly with the orientational moment of the
vortex-axis distribution. The normalized
spin-flip SANS cross sections are accurately represented by independent fits of the analytical linear-
vortex SANS expression obtained from the first-order expansion of the
hyperbolic profile. The fitted orientational descriptor agrees closely with its independent real-space estimate, whereas the corresponding radial descriptors exhibit a strong global nonlinear relation. This separation identifies which information from the micromagnetic
vortex textures is robustly retained by the reduced analytical representation.
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All authors M.P. Adams, J. Leliaert, A. Michels, E.M. Jefremovas
Disordered Systems and Neural Networks
Exact mobility rings in non-Hermitian quasiperiodically decorated Lieb lattices
Ming-Jie Tao, Yi-Ting Wang, Jing Li et al. (+3 more)
The mobility ring (MR), a critical boundary in the complex energy plane separating extended and localized states, is fundamental to understanding the Anderson transition in non-Hermitian (NH) disordered systems. While MRs have been extensively studied in one-dimensional (1D) NH quasiperiodic models, rigorous analytical frameworks beyond 1D remain critically scarce. Here, we investigate a class of two-dimensional (2D) quasiperiodically decorated Lieb
lattices (QDLLs) featuring complex incommensurate potentials selectively applied to the
lattice vertices. By exactly mapping these 2D structures onto NH generalized Aubry-André-Harper (AAH) models and leveraging extended-localized transition point, we analytically derive the Lyapunov exponents and obtain exact expressions for the MRs. These exact theoretical boundaries are strongly corroborated by numerical computations of wavefunction fractal dimensions and real-space probability distributions. Furthermore, we reveal distinct evolutionary behaviors of the MRs driven by the quasiperiodic potential strength: systems characterized by $\kappa=2$ possess a single MR, whereas systems with $\kappa=3$ undergo a dynamic sequential
evolution from a single integrated ring into two independent rings. We hope that our exact results of MRs in 2D will benefit the study of Anderson localizations and MRs in high-dimensional NH systems.
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All authors Ming-Jie Tao, Yi-Ting Wang, Jing Li, Hongsheng Hou, Xiang-Ping Jiang, Lei Pan
Mesoscale and Nanoscale Physics
Engineering two-qubit gates via anisotropic exchange in germanium spin qubits
Leonardo Massai, Bence Hetényi, Eoin G. Kelly et al. (+14 more)
Germanium hole
spin qubits are a promising and versatile platform for
quantum computation and simulation. In this system, strong
spin-orbit interaction (SOI) renders the single-
qubit $g$-tensor anisotropic and electrically tunable, enabling operational sweet spots with reduced noise sensitivity. SOI also transforms the isotropic two-
qubit exchange coupling into an anisotropic tensor whose geometry is inherited from the single-
qubit $g$-tensors and
spin-flip tunnelling. Here, using two hole
spin qubits in a strained-germanium
quantum well and full vector control of the
magnetic field, we map this exchange tensor, separate it into longitudinal and transverse components, and show that they govern controlled-phase and SWAP-like dynamics, respectively. We find that the longitudinal exchange can be tuned via the
magnetic field orientation from a conventional positive value, through zero, to an effectively negative one, as measured by inverted exchange-split
spin transitions. The
magnetic field direction thus provides continuous control over the interaction
Hamiltonian: at a point of purely transverse exchange, we engineer a single-pulse baseband iSWAP, unattainable under isotropic exchange. Linking $g$-tensor geometry to exchange anisotropy establishes native
Hamiltonian engineering, enabling
spin-based
quantum simulation and gate sets selected by the global field orientation alone.
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All authors Leonardo Massai, Bence Hetényi, Eoin G. Kelly, Inga Seidler, Konstantinos Tsoukalas, Michele Aldeghi, Alexei Orekhov, Lisa Sommer, Marta Pita-Vidal, Uwe von Lüpke, Stephan Paredes, Stephen W. Bedell, Felix J. Schupp, Matthias Mergenthaler, Gian Salis, Andreas Fuhrer, Patrick Harvey-Collard
Mesoscale and Nanoscale Physics
Proximity-induced superconductivity in a bilayer graphene quantum point contact
Clara Galante-Agero, Christoph Adam, Artem O. Denisov et al. (+9 more)
We report the realization of a gate-defined
quantum point contact (QPC) in
bilayer graphene proximitized by a single aluminum superconducting electrode. Superconducting correlations induced in the ballistic channel enhance the conductance plateaus beyond their normal-state values. In addition, we observe a pronounced above-gap conductance anomaly which serves as a spectroscopic signature of the loss of superconductivity and the associated collapse of the Andreev excess current. By reconstructing the nonlinear current-voltage characteristics, we find that the magnitude of the excess current increases as successive QPC modes are populated. Additionally, we find that the switching current associated with the loss of superconductivity follows the underlying mode structure of the QPC, exhibiting discrete levels consistent with a heat dissipation-driven transition. These results demonstrate that the one-dimensional transport modes of the QPC govern both the
equilibrium proximity effect and the non-
equilibrium dynamics of the hybrid system.
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All authors Clara Galante-Agero, Christoph Adam, Artem O. Denisov, Jonas D. Gerber, Markus Niese, Alexandra Mestre-Torà, Marta Perego, Jessica Richter, Takashi Taniguchi, Kenji Watanabe, Klaus Ensslin, Thomas Ihn
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Time-optimal quantum gates with bang-bang control in multilevel systems
Valentín Reparaz, Santiago Ferreyra, María José Sánchez et al. (+2 more)
We determine the optimal
quantum manipulation protocols for implementing high-fidelity, fast single-
qubit gates. We demonstrate that the time-optimal pulse sequence is a “bang-bang” sequence: discrete pulses of either positive or negative maximum amplitude or zero. The non-adiabatic bang-bang pulse sequence minimizes gate duration providing a speedup for low-frequency architectures. We first derive the protocol for a transversally driven two-level system, extracting exact analytic expressions for minimum gate times. We then extend this framework to general multilevel architectures, identifying conditions that enable the coherent suppression of leakage errors. Using the fluxonium
circuit as a representative case study, we optimize $X/2$ and $Y/2$ gates through a combination of discrete bang sequences and continuous waveform smoothing. This approach preserves near-optimal execution speeds while mitigating transitions outside the computational subspace. Open-system simulations demonstrate that these sequences outperform commensurate and resonant pulse schemes across different fluxonium regimes, achieving low-error manipulation significantly faster than standard resonant control, even in the presence of $1/f$ flux noise and dissipation.
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All authors Valentín Reparaz, Santiago Ferreyra, María José Sánchez, Daniel Domínguez, Leandro Tosi
Other Condensed Matter · Cross-listed from quant-ph
Crossover from Fast Scrambling to Operator Confinement Tuned by an Auxiliary Qubit
Ryan Buechele, Joseph Szabo and Nandini Trivedi
We demonstrate a static, disorder-free
spin chain
Hamiltonian which, by tuning the coupling to an auxiliary
qubit, realizes a crossover between super-ballistic, ancilla-accelerated scrambling and sub-ballistic operator confinement. Our minimal model is the mixed-field Ising chain with a
spin-1/2 ancilla coupled to its longitudinal magnetization. The ancilla mediates an effective all-to-all interaction which accelerates operator spreading and
entanglement growth when weakly coupled, but rapidly saturates its
entanglement and projects
spin chain operators into effectively frozen subspaces when the ancilla coupling is strong. We locate this crossover independently through both a divergent peak in the mutual-information saturation time near $\lambda_c N/h \approx 8$ and an exponential suppression of the late-time OTOC growth rate, $\log\alpha \propto 1/\lambda$. Through a Feshbach-Fano projection and Schrieffer-Wolff transformation, we reveal an effective hidden
symmetry on the chain which confines operators on the chain for a time exponential in the coupling strength. This reconciles the fast, $\log(N)$ scrambling reported for random-
unitary-
circuit realizations of the
star geometry with the confinement previously found in its time-independent
Hamiltonian analog, showing both emerge from a single
Hamiltonian family as a function of one dimensionless parameter.
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Mesoscale and Nanoscale Physics
Topological insulator realization induced by fermionic interaction through BF mediators
G. B. de Gracia, R. da Rocha, A. A. Nogueira et al. (+1 more)
This paper demonstrates that ordinary
fermions, interacting via a BF mediator, form a renormalized structure with non-trivial topological properties. We explore the analogy between the renormalized
fermion and a subset of real three-dimensional topological
insulators (TIs) in the vicinity of their single Dirac cones. Using the typical magnitude of TI
lattice and gaps, we constrain model parameters. The topological structure induced by radiative corrections enters a class of modified Dirac equations, ensuring the existence of helical gapless near-boundary modes. We derive an effective potential for inter-quasi-particle interactions that incorporates finite-size effects in the axial direction, revealing signatures of
spin-orbit coupling and time-reversal invariance. Remarkably, particles with different
spins do not interact via the characteristic
spin-orbit coupling, and a non-central
spin-dependent force arises. We also address the behavior of the system in the phase transition associated with the thin-film limit.
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All authors G. B. de Gracia, R. da Rocha, A. A. Nogueira, M. A. C. de Barcelos
Materials Science
A nuclear-quantum-corrected machine-learning potential reveals quantum-enhanced hydrogen segregation at general grain boundaries in alpha-iron
Kazuma Ito
Atomistic descriptions of hydrogen
diffusion and trapping at defects are essential for understanding hydrogen embrittlement. As the lightest solute in metals, hydrogen exhibits
nuclear quantum effects that alter these processes even at room temperature. Explicit treatment of such effects is computationally demanding, limiting large-scale simulations of complex environments. Here, we use an Fe-H machine-learning interatomic potential (MLIP) based on the performant implementation of the atomic cluster expansion (PACE), covering diverse Fe-H environments, and relabel the training configurations underpinning its transferability with
quantum mean forces from centroid-constrained path-integral molecular dynamics at 300 K. This yields a
nuclear-
quantum-corrected PACE (NQC-PACE) without additional density functional theory calculations. At parent PACE, NQC-PACE describes
nuclear quantum effects on hydrogen trapping at vacancies, dislocations, surfaces and general grain boundaries, H-H interactions, and
diffusion in alpha-Fe. Grand-canonical Monte Carlo/molecular dynamics simulations show
nuclear quantum effects markedly enhance hydrogen segregation at general grain boundaries and trapping behaviour in closer agreement with experimental trends. This enhancement arises from selective
quantum stabilisation of open, anisotropically soft local environments. Our framework uses finite-temperature
quantum mean forces to relabel the configurational space covered by an MLIP, enabling large-scale analysis of complex materials where light-element
quantum effects matter.
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Materials Science
Data-driven discovery and rapid, direct synthesis of MXenes
Ali Saffar Shamshirgar, Guilherme Ribeiro Portugal, Soheil Ershadrad et al. (+9 more)
MXenes, two-dimensional transition-metal carbides and nitrides, are typically obtained from MAX phases, yet historical reports suggest a broader, largely unexplored chemical space. Here we combine machine-learning-assisted database mining with experiments to uncover overlooked multilayer (ml) MXenes. Screening of repositories reveals a "Treasure Chest" of 38 previously synthesized but unrecognized ml-MXene candidates. Guided by these findings, we rediscover five MXenes using a rapid, scalable self-propagating high-temperature synthesis that requires no sustained external heating and completes within minutes. Inspired by the identified chemistries, we further realize 11 previously unexplored rare-earth-based M2CT2 MXenes (M= Pr, Nd, Sm, Gd, Tb, Ho, and Tm). Experiments and theory reveal semiconducting behavior and diverse
magnetic states across this family. Together, these results expand the MXene family and demonstrate a data-driven strategy for accelerating materials discovery through sustainable methods.
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All authors Ali Saffar Shamshirgar, Guilherme Ribeiro Portugal, Soheil Ershadrad, Roman Ivanov, Martin Dahlqvist, Florian Chabanais, Sanjay Chakraborty, Rainer Traksmaa, Irina Hussainova, Fredrik Heintz, Per O.Å. Persson, Johanna Rosen
Materials Science
Strain control of mid-IR spectroscopic nonlinear photocurrents in PtSe$_2$
M. Gerlei, G. J. de Coster, J. Papp et al. (+4 more)
Semi-metallic noble-metal dichalcogenides are promising materials for infrared optoelectronics, yet the origin and tunability of their nonlinear optical responses remain poorly understood. Here, we demonstrate in-situ mechanical control of
photon-drag photocurrents in polycrystalline PtSe$_2$ thin films grown on flexible polyimide substrates. Using polarization-resolved mid-infrared photocurrent spectroscopy under uniaxial tensile strain, we observe pronounced strain-dependent changes in resistivity, photoconductivity, and helicity-dependent nonlinear photocurrents. The spectral response is consistent with optical excitations across the
spin-orbit-coupling-induced (SOC) band gap near the K point. We develop a theoretical framework attributing the photocurrent primarily to
photon-drag induced by
photon-momentum
symmetry breaking. Strain modifies its magnitude and polarization dependence through deformation potentials that tune the SOC-induced band gap. These results establish mechanical strain as a route toward reconfigurable infrared polarization detection.
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All authors M. Gerlei, G. J. de Coster, J. Papp, S. Heiserer, S. Schlosser, G. S. Duesberg, P. Seifert
Statistical Mechanics
Statistical Mechanics of a Quantum Harmonic Oscillator with Folded Gaussian Frequency
Liu Zhao
A self-contained statistical-mechanics treatment of a single
quantum harmonic oscillator is presented, whose frequency $\omega$ is drawn from a folded Gaussian distribution: $\omega=|\xi|$ with $\xi\sim\mathcal{N}(\mu,\sigma^2)$. The exact integral representations for the partition function, internal energy, free energy, heat capacity, and
entropy are derived, and analytic approximations are given in two complementary limits—small variance ($\sigma\ll\mu$) via a cumulant expansion, and the zero-center case ($\mu=0$) via low-frequency asymptotic analysis. The model is extended to $N$ independent oscillators, where the heat capacity is shown to be extensive with self-averaging fluctuations $\propto N^{-1/2}$, and finally to a disordered oscillator
lattice, where the folded-Gaussian kink at $\omega=0$ produces a soft-mode infrared tail. For a single isolated oscillator with $\mu=0$, both $C$ and $S$ vanish linearly at low $T$. In the
lattice case, the van Hove factor converts this to a $T^d$ power law. The oft-quoted “third-law violation” for disordered
phonons is here shown to be a spectral property—the absence of an energy gap and a power-law freeze-out—driven by the single-site distribution kink rather than by a genuine Lifshitz tail (which requires rare large-scale spatial fluctuations). The folded Gaussian thus serves as a minimal
benchmark for soft-mode disorder thermodynamics.
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Strongly Correlated Electrons
Scarred discrete time crystal in a periodically driven dimerized spin chain
Davood Marripour, Saeed S. Jahromi and Jahanfar Abouie
We investigate the emergence of a scarred discrete time
crystal (SDTC) phase in a periodically driven dimerized
spin chain. While generic interacting Floquet systems are expected to thermalize according to the eigenstate thermalization hypothesis (ETH), we demonstrate that this system hosts
quantum many-body scars (QMBS) that induce a regime of weak ergodicity breaking. Through an analysis of Floquet level statistics,
entanglement entropy, and eigenstate fidelity, we identify a
manifold of low-
entanglement states characterized by semi-Poisson statistics embedded within an otherwise thermal spectrum. These scarred states support robust subharmonic oscillations with period doubling, signaling the spontaneous breaking of discrete time-translation
symmetry. We show that the SDTC response is robust against a variety of initial state configurations, demonstrating its stability beyond fine-tuned conditions. A finite-size scaling analysis reveals that the time-crystalline lifetime grows with system size within the range accessible to our exact-diagonalization calculations. However, drawing on the general phenomenology of approximate many-body scars, we expect that hybridization between Floquet scars and the thermal continuum will eventually curtail this growth, causing the lifetime to saturate at system sizes beyond our current numerical reach. This characterizes the SDTC as a long-lived metastable dynamical regime rather than a strictly stable thermodynamic phase, providing a comprehensive framework for understanding the interplay between periodic driving and constrained many-body dynamics in disorder-free systems.
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Materials Science
Machine Learning-Accelerated Band-Edge Engineering of Pnictogen Chalcohalide Solid Solutions for Solar Energy Technologies
Cibrán López, David Rovira, Edgardo Saucedo et al. (+1 more)
Pnictogen chalcohalide (MChX; M=Bi,Sb; Ch=S,Se; X=I,Br) solid solutions combine earth-abundant constituents, tunable band gaps ($1.2$-$2.1$ eV), and strong optical absorption, making them attractive for
solar energy conversion. Yet their vast compositional space has so far prevented a systematic assessment of how band-edge positions vary with stoichiometry and surface termination. Here, we combine first-principles density functional theory with machine learning to predict the valence and conduction band-edge positions of $\mathrm{Bi}_x\mathrm{Sb}_{1-x}\mathrm{S}_y\mathrm{Se}_{1-y}\mathrm{I}_z\mathrm{Br}_{1-z}$ solid solutions across their full compositional range on the two most stable surfaces, (010) and (011). We find that the valence-band maximum and conduction-band minimum can be tuned by more than $1$ eV through composition alone, and shift by up to $0.6$ eV between the two surface terminations for a same composition despite their nearly degenerate formation energies, establishing facet selection as a design parameter on par with chemical substitution. Guided by these results, we identify specific compositions capable of driving hydrogen, ammonia, methane, hydrogen peroxide, and oxygen (photo)electrochemical half-reactions, and show that several electron- and hole-transport contact materials commonly used in photovoltaic devices align with MChX solid solutions only as hole-selective contacts.
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All authors Cibrán López, David Rovira, Edgardo Saucedo, Claudio Cazorla
Mesoscale and Nanoscale Physics
Five-terminal quantized transconductance originating from symmetric quantum fluctuations
Klaiv Mertiri and Yuli V. Nazarov
We consider the renormalization of transport in a
quantum contact by the external electromagnetic environment and show that weak
quantum fluctuations from a symmetric environment can lead to a quantized transconductance in five-terminal contacts. This mechanism does not work for fewer terminals. We present an implementation of the environment and investigate several example
quantum contacts where the quantization takes place.
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Statistical Mechanics · Cross-listed from quant-ph
Evidence for cavity-induced metallic phase from entropic electron correlation effects
Jacob Horak and Dominik Sidler
Experiments have revealed that collective strong coupling in optical cavities can drastically alter the conductivity and dielectric properties of molecular ensembles, or even trigger abrupt phase transitions (in Rayleigh scattering or dispersion force driven conformational
equilibrium). Until now, the underlying physical mechanism has remained unknown. A recently proposed theory attributes these collective effects to cavity-induced electron correlations that share properties of the known spherical Sherrington-Kirkpatrick model of a
spin glass. As a consequence, entropic effects can trigger a phase transition in the inter-molecular electron correlations that could potentially explain the above experiments. In the following work, we derive analytic expressions for the correlation (free-)energy-induced polarizability changes, which reveal that under certain conditions collective strong coupling can even render our molecular ensemble metallic by entropically ionizing collectively degenerate molecular orbitals. Our results are a first major step towards a holistic theory of cavity-mediated electron correlation effects.
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Materials Science · Cross-listed from physics.chem-ph
Through the Looking-Glass: Efficient Parity-Complete Learning via Local $O(2)$ Frames
Zemin Xu and Wenbo Xie
Making all
symmetry-allowed tensor representations computationally accessible remains a central challenge in equivariant atomistic learning. Clebsch–Gordan tensor products (CGTPs) are computationally demanding, whereas efficient $SO(2)$-based alternatives lack a unified treatment of natural- and unnatural-parity representations. We first introduce a generalized Wigner-$6j$ convolution that exactly recouples interactions involving additional node representations, replacing the first edge-level tensor-product intermediate with reusable node features. This reduces edge computation and storage but leaves the $\mathcal O(L^5)$ angular scaling of the sparse CGTPs unchanged. To address this remaining bottleneck, we develop a local $O(2)$ framework whose convolutions scale as $\mathcal O(L^3)$. The framework explicitly accounts for both rotations and reflections. Its linear maps, tensor products, and gated nonlinearities, combined with frame transformations, ensure global $O(3)$ equivariance for both spatial parities. Incorporating time-reversal labels extends this construction to $O(3)\times\mathbb Z_2^{\mathcal T}$. We apply the framework to
magnetic TACE (mTACE), using distinct interactions for systems with and without
spin–orbit coupling. On collinear CrN and noncollinear Fe
benchmarks, mTACE achieves substantial reductions in atomic- and
magnetic-force errors. We also provide EquivariantX, a library for global and local $O(2)$-equivariant computation.
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