Strongly Correlated Electrons
Does a Fractional Quantum Hall Edge Have a Protected Intrinsic Dipole Moment?
Domagoj Perković, Konstantinos Vasiliou, S.A. Parameswaran et al. (+1 more)
We investigate the claims by Park and Haldane [Phys. Rev. B 90, 045123 (2014)] of an intrinsic protected value of the electric dipole moment at the physical edge of fractional
quantum Hall (FQH) systems. Contrary to prevailing expectations, we find that the edge dipole takes the expected intrinsic value only in certain very special cases. We identify key limitations in earlier numerical studies and employ density
matrix renormalization group (DMRG) methods to accurately compute the ground-state dipole. We focus on three representative systems: the $\nu=1/3$-
vacuum edge, the $\nu=2/3$-
vacuum edge, and the interface between Pfaffian and anti-Pfaffian phases. We find that the expected intrinsic dipole value occurs only at $\nu=1/3$, whereas the other systems do not exhibit the claimed intrinsic value. We give arguments based on composite
fermions as to why hierarchy states should generally not have protected intrinsic dipoles. These results have important implications for the energetics and edge structure of FQH states.
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All authors Domagoj Perković, Konstantinos Vasiliou, S.A. Parameswaran, Steven H. Simon
Strongly Correlated Electrons
Disentangling bulk and surface electronic structure using targeted cleave planes in RuO$_2$
Maria H. Visscher, Sebastian Buchberger, Bruno Saika et al. (+6 more)
Rutile RuO$_2$ has attracted significant interest due to its putative unconventional electronic and
magnetic properties and its proximity to superconductivity. However, the measurement and interpretation of its electronic structure has been complicated by a strongly three-dimensional
crystal structure. Here, we demonstrate how the preparation of targeted $(110)$ and $(100)$ surfaces via focused ion beam (FIB)-engineered cleaving allows the acquisition of high-quality measurements of the electronic structure using angle-resolved photoemission spectroscopy. Our results demonstrate that ARPES spectra of RuO$_2$ are, in fact, largely dominated by signatures of distinct surface electronic states. From comparison with density-functional theory, we resolve a surface termination-dependent variation of these, and disentangle them from highly-three-dimensional bulk states and surface resonances. Moreover, we find a marked role of the substantial
spin-orbit coupling of the Ru 4$d$ orbitals in the surface region, where a breaking of spatial inversion
symmetry leads to significant Rashba-type
spin splittings of the surface bands.
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All authors Maria H. Visscher, Sebastian Buchberger, Bruno Saika, Shu Mo, Lea Richter, Mats Leandersson, Craig Polley, Andrew P. Mackenzie, Phil D. C. King
Statistical Mechanics
Bootstrapping ground state properties of classical frustrated magnets
Nisarga Paul and Gil Refael
We introduce a method based on semidefinite programming that produces rigorous two-sided bounds on ground state energy densities and correlation functions of translation-invariant classical
spin models on infinite
lattices. In this method, the challenge of non-convex optimization on an infinite
lattice is replaced with a hierarchy of finite-size convex optimizations arising from positivity conditions that any probability distribution over
spin configurations must satisfy. This adapts the Lasserre hierarchy in the theory of
polynomial optimization to the context of frustrated magnetism, and we prove convergence of this hierarchy in the thermodynamic limit. Our method subsumes the Luttinger–Tisza method and further applies to non-quadratic
Hamiltonians and non-Bravais
lattices, thus addressing limitations of prior analytical methods. We apply the method to various two-dimensional frustrated
spin models, where it brackets the energy densities and observables accurately across large parameter ranges with typical run times of seconds per parameter point.
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Statistical Mechanics
Direct Experimental Test of Conformal Invariance via Grazing Scattering: A Proposal for X-ray and Neutron Experiments
Alessandro Podo and Slava Rychkov
We propose a test of conformal invariance in critical phenomena based on the study of a two-point correlation function in the presence of a boundary. This two-point function can be studied using X-ray or
neutron scattering in the conditions of total reflection (so-called grazing scattering). The conformal Ward identity in momentum space is here expressed as a differential constraint on the scattering cross-section, as a function of the momentum transfer and the scattering angle. Experimental verification using X-rays and binary
alloys appears well within the existing techniques, while feasibility for
neutron scattering requires further study. This would be the first direct experimental test of conformal invariance in critical phenomena, a
symmetry widely assumed but never directly verified.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Giant-atom-enabled quantum optics with valley-polarized photons
Marcel A. Pinto, Giovanni Luca Sferrazza, Silvia Casulleras et al. (+3 more)
Valleytronics and valley photonics exploit the valley degree of freedom to encode and manipulate information. Here we show that photonic valleys can be selectively addressed in
quantum optics using a simple two-level emitter, provided it is coupled nonlocally to the field, thereby realizing a so-called
giant atom. Specifically, we consider a
qubit coupled at multiple points to an engineered honeycomb
lattice of resonators with detuned sublattice frequencies. By tailoring the geometry of the coupling points, the
giant atom can be made to emit selectively into a single valley. The emitted
photons thereby acquire a well-defined valley character and inherit the associated Berry
curvature. By placing the
qubit near a domain wall between regions of opposite sublattice detuning, whose interface supports valley-polarized edge modes, emission becomes
chiral along the domain wall. This provides a promising route toward implementation of single-
photon disorder-robust
chiral emission without breaking time-reversal
symmetry of the electromagnetic medium in platforms such as
circuit QED.
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All authors Marcel A. Pinto, Giovanni Luca Sferrazza, Silvia Casulleras, Alejandro Gonzalez-Tudela, Daniele De Bernardis, Francesco Ciccarello
Strongly Correlated Electrons
Quantum spin liquid on a 3D bipartite lattice of spin trimers stabilized by enhanced effective anisotropy
M. Gomilšek, L. Mangin-Thro, T. Arh et al. (+22 more)
Quantum spin liquids (QSLs) represent highly entangled states of matter in which frustration-induced
quantum fluctuations suppress any
symmetry-breaking phase transition down to absolute zero, giving rise to fractionalized excitations and emergent gauge fields. Theoretically, bond anisotropy can stabilize QSLs even on bipartite
lattices, as exemplified by the Kitaev honeycomb model; however, no material has so far been established to realize such a state as its true ground state. Here we identify the three-dimensional
spin-trimer magnet KBa$_3$Ca$_4$Cu$_3$V$_7$O$_{28}$ as a promising candidate for a bipartite
quantum spin liquid persisting to the lowest temperatures. Strongly coupled Cu$^{2+}$ trimers form effective pseudospin-1/2 degrees of freedom upon cooling, which in turn constitute a three-dimensional bipartite network. Bulk thermodynamic measurements,
neutron scattering, $\mu$SR, and NMR detect no
spin freezing or
symmetry-breaking phase transition down to 20 mK, but instead reveal a gapless dynamical ground state with algebraic
spin autocorrelations. Complementary Monte Carlo and exact-diagonalization calculations show that this state is stabilized by a strong enhancement of effective anisotropy: a weak microscopic Cu-Cu exchange anisotropy of approximately 15 percent is generically amplified at the trimer level, producing effective pseudospin-pseudospin interaction anisotropies of 60 to 100 percent. Our results establish trimer-based networks as a promising platform for realizing anisotropy-stabilized
quantum entangled states, even in three-dimensional bipartite systems with only weak microscopic anisotropy.
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All authors M. Gomilšek, L. Mangin-Thro, T. Arh, S. Petit, B. Grenier, V. Simonet, M. Pregelj, A. Zorko, B. Koteswararao, B.-G. Jeon, B. Sana, Y. Furukawa, Y. Inagaki, T. Asano, C. Repellin, B. Fåk, J. Ollivier, F. Fauth, C. V. Colin, E. Pachoud, V. Pomjakushin, J. S. Lord, H. Luetkens, K.-H. Kim, P. Khuntia
Quantum Gases
The Kubo-Thermalization Correspondence
Songtao Huang, Xingyu Li, Jianyi Chen et al. (+5 more)
Quantum thermalization describes how interacting
quantum systems relax toward thermal
equilibrium, a central problem in modern physics. Yet most experimental information on many-body systems comes from short-time transition spectroscopy, typically interpreted within Kubo's linear-response framework. These perspectives-long-time equilibration versus short-time response-seem fundamentally disconnected. Here we establish an exact link between them: the Kubo-Thermalization correspondence, which connects long-time thermalized magnetization under weak driving to short-time linear-response spectra for a
spin coupled to a thermal bath. The correspondence holds even when the steady state differs substantially from the initial state and when each regime is individually difficult to describe theoretically. We experimentally confirm the correspondence using effective
spin-$1/2$
impurities realized with ultracold
fermions in two internal states coupled to a
Fermi sea. Our results provide a rare exact statement about
quantum thermalization and offer a novel route to infer thermalization dynamics from
equilibrium response measurements in strongly interacting
quantum systems, independent of microscopic details of the system–bath coupling.
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All authors Songtao Huang, Xingyu Li, Jianyi Chen, Alan Tsidilkovski, Gabriel G. T. Assumpção, Pengfei Zhang, Hui Zhai, Nir Navon
Strongly Correlated Electrons
Pro-Tensor Network
Gen Yue, Ansi Bai, Linqian Wu et al. (+1 more)
We introduce the pro-tensor network, a categorification of the tensor network, as a fully rigorous yet graphically transparent framework for studying the collection of many many-body theories, which we dub many-many-body theory. We provide a comprehensive toolbox for the graphical calculations using pro-tensor networks. As applications, we recover the Levin-Wen model as a "uniform" pro-tensor network and generalize a result of Kitaev and Kong by characterizing particles as modules over promonads. One can also interpret the string-net pro-tensor network as the space of symmetric tensor networks, thus our framework also applies to the study of generalized
symmetry and topological holography. Notably, our generalization dispenses with the assumptions of semisimplicity, finiteness, and
rigidity, potentially facilitating the exploration of many-body physics beyond these constraints.
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All authors Gen Yue, Ansi Bai, Linqian Wu, Tian Lan
Mesoscale and Nanoscale Physics
Colossal Magnetoresistance and Phonon Driven Exchange Dynamics in Eu$_5$Sn$_2$As$_6$
Luke Pritchard Cairns, Kohtaro Yamakawa, Shengzhi Zhang et al. (+9 more)
The emergence of colossal magnetoresistance in a new generation of Eu$^{2+}$-based antiferromagnets is intriguing given stark contrasts to the archetypal perovskite manganites and doped Eu-chalcogenides. In this study the thermal conductivity and magnetostriction of Eu$_5$Sn$_2$As$_6$ – one such representative – have been measured to better understand the role of the
crystal lattice. Both properties are strongly field-dependent and mirror the magnetization, saturating once the Eu$^{2+}$ moments are polarized. The field-enhancement of the
phonon-dominated thermal conductivity is interpreted through the lifting of a degeneracy of
spin configurations, and the subsequent saturation due to quenched magnetostrain in high field. Comparison with
spin-glass
insulators suggests that this phenomenon is not a byproduct but rather the driver of electron delocalization due to the suppression of strong
phonon scattering arising from exchange frustration.
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All authors Luke Pritchard Cairns, Kohtaro Yamakawa, Shengzhi Zhang, Youzhe Chen, Bernard Field, Rainer Reczek, Ryan P. Day, Joel E. Moore, Marcelo Jaime, Sinead M. Griffin, Robert J. Birgeneau, James G. Analytis
Mesoscale and Nanoscale Physics
Electrically controlled Heat Assisted Magnetic Recording in Intercalated 2D Magnets
Josue Rodriguez, Ruishi Qi, Catherine Xu et al. (+3 more)
The ever-increasing demand for fast, reliable, and energy-efficient information storage continues to push
magnetic memory technologies toward their fundamental limits. Conventional scaling strategies, which rely on reducing bit size, inevitably run into the "
magnetic recording trilemma," where signal-to-noise ratio, thermal stability, and writability cannot all be optimized simultaneously. Heat-assisted
magnetic recording (HAMR) has emerged as the leading solution, enabling high-density storage by transiently heating the medium during the write cycle. However, the reliance on
laser optics and plasmonic transducers restricts HAMR primarily to hard-disk drives, limiting its integration with on-chip or embedded architectures. Here, we demonstrate an electronic variant of HAMR in which Joule heating from low-current density current pulses facilitates data writing, while the anomalous
Hall effect provides electronic readout. Employing intercalated 2D magnet Ni$_{1/4}$TaSe$_2$, we show direct evidence that current pulses heat the material above its Curie temperature, during which a small
magnetic field of ~2mT (100 times smaller than the coercive field) enables efficient data writing. The all-electronic approach combined with the 2D
magnetic medium creates timely opportunities to revisit the energy-assisted magnetization recording, enabling new recording schemes that combine fundamental novelty with technological impact.
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All authors Josue Rodriguez, Ruishi Qi, Catherine Xu, Feng Wang, James G. Analytis, Hossein Taghinejad
Materials Science
Moire based strain analysis in wurtzite GaAs – rock-salt (Pb,Sn)Te core-shell nanowires grown by molecular beam epitaxy
Maciej Wojcik, Sania Dad, Piotr Dziawa et al. (+3 more)
We investigate core/shell GaAs/(Pb,Sn)Te nanowire nanoheterostructures with wurtzite (wz) GaAs cores and (Pb,Sn)Te topological crystalline
insulator shells. The nanostructures have been grown by molecular beam epitaxy using two distinct MBE systems dedicated to III-V, and IV-VI
semiconductors. The interface structure of wz-GaAs/(Pb,Sn)Te nanowires is investigated using high resolution transmission electron microscopy, scanning transmission electron microscopy and
geometric phase analysis. Misfit dislocations and moiré fringes are observed as a direct result of the
lattice mismatch between the core and the shell materials, and used to estimate strain in crystalline topological
insulator shells. Our results point to a possibility of using moiré patterns analysis as an alternative, for estimating strain in the core-shell nanowire structures.
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All authors Maciej Wojcik, Sania Dad, Piotr Dziawa, Slawomir Kret, Wojciech Pacuski, Janusz Sadowski
Materials Science
Molecular dynamics simulation study of mechanical properties of 3C-SiC with extended defects
Serhii Shmahlii and Andrey Sarikov
In this study, large-scale molecular dynamics simulations with the Vashishta potential and the analytic bond-order potential (ABOP) were performed to investigate the effect of extended defects on the elastic properties of cubic silicon carbide (3C-SiC). Specifically, we focused on systems containing Shockley partial dislocations terminating stacking faults, along with double and triple dislocation complexes. The changes in the independent elastic stiffness constants C11, C12 and C44 upon varying the mentioned extended defects concentrations were quantified. Using the values of these constants, the effective bulk, shear, and Young's
moduli were calculated for different defect types and concentrations. The
moduli were calculated along particular crystallographic directions aligned with the mentioned defect configurations as well as evaluated using Voigt-Reuss-Hill averaging to provide overall orientation-independent characterization of the defect-altered
lattice. The obtained results reveal a general trend of diminishing the material's stiffness with increasing densities of Shockley partial dislocations and dislocation complexes. Depending on the defect configuration, the average elastic
moduli decrease by up to approximately 6 % with the Vashishta potential and up to about 4 % using the analytic bond-order potential. At this, triple dislocation complexes induce smaller perturbations. These findings demonstrate that extended defect networks can measurably modify the elastic response of 3C-SiC and should be considered in further scientific research and practical applications of this material.
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Mesoscale and Nanoscale Physics
Twisted Kagome Bilayers: High-Order Van Hove Singularities, Sublattice Interference, Magic Angles, and Possible Topology
David T. S. Perkins and Joseph J. Betouras
We develop a low-energy continuum model to describe the moiré physics of
heterostructures, which is a generalization of the celebrated Bistritzer-MacDonald (BM) method [R. Bistritzer and A. H. MacDonald, Proc. Natl. Acad. Sci. U.S.A. 108, 12233 (2011)]. We take as an example the moiré physics of electrons in twisted
bilayer kagome metals near 1/3 filling where monolayer Dirac cones lie. We demonstrate the emergence of magic angles where significant local band flattening occurs as a high-order Van Hove singularity appears and find a momentum-dependent anti-
unitary particle-hole
symmetry potentially enabling stable
topology. We, furthermore, show that while sublattice interference effects are present, their role is not as prominent as in monolayer kagome.
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Strongly Correlated Electrons
Emergence of a correlated insulating state in bulk 1T-NbSe$_2$ via metal intercalation
M. Tomlinson, AKM A. Rahman, S. Devi et al. (+6 more)
The 1T polymorph of NbSe$_2$, long confined to the monolayer limit, has remained inaccessible in bulk. Here, we report the realization of bulk 1T-NbSe$_2$ via electrochemical Sn intercalation. Transmission electron microscopy directly reveals the formation of the 1T structure induced by Sn intercalation. The intercalated samples exhibit insulating transport behavior, in stark contrast to metallic 2H-NbSe$_2$. Density functional theory calculations, however, predict a metallic band structure, highlighting the crucial role of emergent electronic correlations in the observed insulating state. Raman spectroscopy further reveals vibrational modes associated with Sn intercalation and possible charge density wave order. Our results establish electrochemical intercalation as an effective route to stabilize otherwise inaccessible bulk polytypes, positioning bulk 1T-NbSe$_2$ as a new platform for investigating correlated electronic states.
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All authors M. Tomlinson, AKM A. Rahman, S. Devi, R. Tuchikawa, M. Ishigami, D. Le, Md Z. Mohayman, A. Kushima, Y. Nakajima
Statistical Mechanics
A Rayleigh criterion for mechanical instability: inducing activity by chemo-mechanical coupling
Aaron Beyen, Francesco Casini and Christian Maes
Instabilities in thermodynamic systems are often undesirable, as they can lead to loss of control or even catastrophic behavior. Yet, the same mechanisms can also generate rich nonequilibrium behavior and may play a constructive role in living systems. We introduce a theoretical framework, inspired by Rayleigh's analysis of thermoacoustic instabilities, to study the emergence of mechanical activity. In particular, we derive Rayleigh-like criteria governing the onset of activity and the generation of rotational motion in a slow Newtonian probe coupled to driven chemical processes, described by Markov jump processes. These criteria are expressed in terms of the phase relation between entropic and frenetic contributions, providing a transparent condition for when chemical driving results in sustained rotational or active mechanical motion.
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Materials Science
On Fano effect in IR spectra of hydrogenated nanodiamonds
Andrei A. Shiryaev and Evgeni A. Ekimov
Hydrogenated nanodiamonds may show a "transmission window" in infra-red spectra in the vicinity of diamond Raman frequency. This phenomenon is a manifestation of resonance coupling of incident
photons with continuum states (Fano resonance). Hpwever, precise mechanism of appearence of the resonance and of related conductivity - surface hydrogenation or specific type of surface reconstruction - remains debatable. We present detailed analysis of infra-red spectra of nanodiamonds of different sizes (2.6-30 nm) possessing the "transmission window" and show that the C-H stretch vibrations of adsorbed functional groups cannot be responsible the the Fano resonance. At the same time, it is suggested that a bending mode of monohydride termination on nanodiamond (111) face may couple with diamond optical
phonon, explaining the Fano resonance in some cases. The relative importance of the monohydride contribution and of the graphitic islets to the appearence of the "transmission window" and conductivity is likely dependent on dominating morphology and size distribution of nanodiamond grains.
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Soft Condensed Matter
Non-Local Particle Flows Become Local When Considering Dissipative Stress
Martin Trulsson
Dense granular and suspension flows under inhomogeneous shear exhibit persistent particle motion in regions where the local yield criterion is subcritical, an apparent breakdown of locality that has motivated the development of a generation of nonlocal rheological models. Using particle-resolved simulations of frictionless dense suspensions in two-dimensional Kolmogorov flow, we show that two independent considerations together account for this signature. First, replacing the conventional shear stress by a shear-rate-weighted dissipative stress $\tau_W=\langle \tau \dot \gamma \rangle/\langle \dot \gamma \rangle$, which isolates the component of stress that performs irreversible work, restores the homogeneous $\mu(J)$ law throughout the bulk of the flow, with the inferred friction remaining strictly above yield. Second, a simple
geometric mixing-length construction, applied with conventional stresses and requiring no fluctuation input, accounts for the residual sub-yielding within a sub-diameter layer at flow reversals. Each approach is based on a different philosophy and mechanism, and together they suggest that much of the apparent non-locality in this geometry and frictionless case is an artefact of how stress is measured and averaged rather than an intrinsic breakdown of local rheology.
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Superconductivity
Pair-Breaking and Dimensionality in Spin-Orbit Coupled Superconductors
Reiley Dorrian, Mizuki Ohno, Elena Williams et al. (+2 more)
The response of ultra-thin superconducting materials under parallel
magnetic fields is often leveraged to obtain insight into the nature of the condensate, including features attributable to unconventional forms of pairing. Despite there being multiple competing mechanisms responsible for suppressing superconductivity, it is common for these analyses to overlook certain depairing channels. Here we report an analysis of thickness dependent superconductivity in thin films of \ce{LaBi2} using the multi-mechanism Kharitonov-Feigel'man framework . By resolving field-enhanced superconductivity in the thin-limit, we obtain an estimate the role of
spin exchange scattering, in addition to paramagnetic and orbital effects. Our analyses offer insight into how fundamental quantities such as the critical temperature as well as Pauli limit are defined, recasting the landscape for how scattering times in two-dimensional
superconductors can be interpreted.
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All authors Reiley Dorrian, Mizuki Ohno, Elena Williams, Adrian Llanos, Joseph Falson
Soft Condensed Matter
Cooking crystalline candies and the ductile to brittle transition in concentrated suspensions
Andreia F. Silva, James A. Richards, Fiona Jeffrey et al. (+4 more)
The existence and origin of the ductile to brittle transition in non-Brownian suspensions and pastes is underexplored despite the ubiquity of such materials in practical applications. We demonstrate the phenomenon in candies of sugar
crystals in a water-
protein-fat
matrix prepared by boiling a sugar-cream-butter mixture (known as 'fudge' in some countries). As cooking time or final cooking temperature increases, we observe a transition from a fluid to a ductile solid, then to a brittle solid that abruptly fractures in compression. We propose that this is driven by rising solid sugar
crystal volume fraction, and indeed find the same sequence of behaviour in a suspension of non-Brownian calcite particles as the solid fraction moves from frictional jamming to random close packing. Particle-based simulations reveal the sensitivity of the observed phenomenon to boundary conditions.
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All authors Andreia F. Silva, James A. Richards, Fiona Jeffrey, Rory E. O'Neill, Daniel J. M. Hodgson, Christopher Ness, Wilson C. K. Poon
Mesoscale and Nanoscale Physics
Engineering a driven-dissipative bath of altermagnetic quantum magnons for controlling classical dynamics of spins hosting spin waves, domain walls, or skyrmions
Felipe Reyes-Osorio and Branislav K. Nikolic
Using Schwinger-Keldysh field theory (SKFT), we engineer a dissipative and driven (i.e., out of
equilibrium) bosonic bath acting on classical localized
spins within a
ferromagnetic insulator (FI) layer whose dynamics is governed by the Landau-Lifshitz-Gilbert equation, as is usually assumed in spintronics and magnonics. The bosonic bath is comprised of
quantum magnons within a layer of altermagnetic
insulator (AMI) that is attached to a conventional FI layer, often one of the key ingredients within spintronic and magnonic multilayers, so that interaction between slow classical (in the FI layer) and fast
quantum (in the AMI layer) localized
spins ensues. Such a bath, including its driving to produce a nonequilibrium distribution of altermagnetic
magnons, generates a rich structure of the SKFT-derived extended LLG equation for classical
spins within the FI layer. Our LLG equation contains two damping terms, both of which are spatially nonlocal and anisotropic, while one of them is also intrinsically non-Markovian, i.e., nonlocal in time. We demonstrate how to exploit these terms for tuning spintronic and magnonic effects within the FI layer of AMI/FI
bilayers that involve
spin wave or domain wall propagation, as well as skyrmion annihilation.
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Statistical Mechanics
Activation in Vesicle-Mediated Signaling Shaped by Batch Arrival Statistics
Jan Hauke, Julian B. Voits and Ulrich S. Schwarz
Vesicle-mediated secretion of ions or molecules is a central mechanism of cellular communication, for example in processes such as neurotransmission or hormone release. These events are inherently
stochastic: vesicle fusions lead to bursts of variable sizes, releasing discrete packets of transmitters that are subsequently cleared or degraded. The dynamics are intrinsically time-directed due to the interplay of spontaneous bursts and continuous degradation. Using generating functions and a recursion relation, we derive an exact solution for the full time-dependent probability distribution of a general batch arrival degradation model. This framework also enables a full analysis of first-passage times to a concentration threshold representing downstream activation. We show that activation kinetics are not determined by mean dynamics alone, but depend sensitively on the temporal statistics of arrival events, batch-size variability, and degradation. In particular, different arrival processes with identical mean rates can lead to qualitatively distinct first passage behavior, reflecting the role of time-asymmetric fluctuations. We also discuss extensions incorporating vesicle depletion. Our results provide a transparent link between
stochastic release dynamics and activation timing in vesicle-mediated signaling.
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Mesoscale and Nanoscale Physics
Electrical Spin Pumping in Exchange-coupled Molecules
Paul Greule, Wantong Huang, Kwan Ho Au-Yeung et al. (+5 more)
Electron
spins in single molecules are a promising platform for
quantum information processing. However, their practical implementation as
qubits requires reliable control at the single-entity level, including an efficient state initialization. Here, we demonstrate the remote, all-electrical initialization of the electron
spin in single molecules: Using electron
spin resonance scanning tunneling microscopy, we investigate coupled pairs of S=1/2 molecules (Fe-FePc), where one molecule serves as a readout and pumping unit for the neighboring one. We show that the exchange interaction between them enables angular momentum transfer, which allows for the control of the remote
spin state via the direction and magnitude of the
spin-polarized tunneling current and the exchange coupling strength. These results establish a general, all-electrical approach for remote
spin initialization that is readily transferable to a wide range of
spin-based
quantum architectures.
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All authors Paul Greule, Wantong Huang, Kwan Ho Au-Yeung, Máté Stark, Johannes Schwenk, Christoph Sürgers, Wolfgang Wernsdorfer, Philip Willke
Statistical Mechanics
Criticality around the Spinodal Point of First-Order Quantum Phase Transitions
Fan Zhang, Chiao Wang and H. T. Quan
Universality and scaling are hallmarks of second-order phase transitions but are generally unexpected in first-order
quantum phase transitions (FOQPTs). We present a microscopic theory showing that
quantum criticality can emerge around the
quantum spinodal point of FOQPTs where metastability disappears. We demonstrate that, at this instability, resonant local excitations dynamically decouple a
Hilbert subspace characterized by an emergent discrete translational
symmetry. Projecting the original
Hamiltonian onto this subspace yields an effective
Hamiltonian that exhibits a genuine second-order
quantum phase transition (SOQPT) and the Kibble-Zurek scaling. We validate this framework in the tilted Ising chain which breaks $\mathbb{Z}_2$
symmetry, and predict the absence of criticality in the staggered-field PXP model. This work indicates that the dynamics of FOQPTs is usually governed by an emergent critical point around the
quantum spinodal point. Our results uncover a hidden criticality in FOQPTs, reshaping the conventional understanding of FOQPTs beyond the mean-field theory.
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Statistical Mechanics
Comparative Study of Potts Machine Dynamics and Performance for Max-k-Cut
Bjarke Almer Frederiksen, Robbe De Prins and Peter Bienstman
Combinatorial optimization problems in logistics, finance, energy, and scheduling routinely involve multi-state decision variables. Ising machines (IMs) require binary expansions (e.g., one-hot encoding) to encode such variables, whereas Potts machines (PMs) represent them natively. By doing so, PMs are expected to outperform IMs on multi-state problems. To the best of our knowledge, no systematic study of PM models has yet assessed whether this expectation holds. We therefore
benchmark five representative PMs against a reference IM on Max-3-Cut and Max-4-Cut, using 800-vertex GSet graphs and random graphs of up to 50 vertices. Surprisingly, the reference IM still outperforms every PM, and the IM supremacy increases significantly in going from Max-3-Cut to Max-4-Cut. These results provide clear evidence that current PM dynamics underperform relative to binary approaches, even in regimes where they are presumed advantageous. We provide a way forward by quantifying the underperformance of current PMs, as well as by identifying three dynamical properties that correlate strongly with their performance ranking. Our work stresses the need for more systematic assessments of algorithmic performance in order to guide the design of more effective Potts machines.
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Statistical Mechanics
Finite-Time Optimal Control by Noisy Traps
Luca Cocconi, Henry Alston and Thibault Bertrand
The optimal control of passive systems in
equilibrium typically favours quasistatic (infinite-time) protocols. We show that a breakdown of quasistatic optimality occurs when the controller itself is dissipative. Concretely, we study a Brownian particle confined by a harmonic trap with stochastically fluctuating stiffness, driven by an external protocol. When these fluctuations violate detailed balance, the probe-controller coupling continuously exchanges work with the system, altering the optimisation landscape. In this regime, optimal protocols are characterised by a finite duration which vanishes above a critical fluctuation strength. This transition can be directly observed in a short-time expansion of the mean work functional. When imposing an endpoint constraint, the transition to zero duration disappears and finite duration protocols remain optimal for all values of the controller fluctuations. These results demonstrate that finite-time optimality can emerge in passive systems under nonequilibrium control.
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Strongly Correlated Electrons
Superconducting and correlated phases of an effective Hubbard model on the BCC lattice
Theja N. De Silva
We investigate the electronic phases of an effective Hubbard model on the body-centered-cubic
lattice, motivated by alkali-doped fulleride molecular solids. The model incorporates renormalized on-site interactions and an effective inverted Hund's coupling originating from electron-
phonon interactions. To access complementary interaction regimes, we employ two theoretical approaches. In the intermediate-coupling regime, the on-site repulsive interaction is approximated by a long-range interaction in momentum space, yielding an exactly solvable Hatsugai-Kohmoto model supplemented by a BCS-type pairing term. Within this framework, we analyze the superconducting instability and demonstrate a first-order normal-superconducting phase transition, characterized by a discontinuous jump of the order parameter. In the strong-coupling regime, where pairing fluctuations are suppressed, we apply the
spin rotationally invariant slave-
boson formalism to map out the temperature-interaction phase diagram. This analysis reveals first-order transitions between a
Fermi-
liquid phase, an
antiferromagnetic phase, and a Mott insulating phase, with a narrow intermediate region where all three phases compete. The resulting phase diagram captures the interplay of itinerancy,
magnetic order, and Mott localization in three dimensions and provides a unified perspective on superconducting and correlation-driven phenomena in fulleride-inspired
lattice systems.
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Statistical Mechanics · Cross-listed from math-ph
Dynamical cooling driven by self-similar fronts in the 2D nonlinear Schrödinger model
Jason Laurie, Simon Thalabard and Sergey Nazarenko
We analyze the dynamics towards partial thermalization and subsequent cooling in the defocusing two-dimensional nonlinear Schrödinger model, using direct simulations and insights from the wave-kinetic equations (WKE) and a fourth-order differential approximation model (DAM). We show that the evolving WKE spectrum exhibits two distinct similarity ranges–the quasi-thermal core and the ultraviolet tail–whereas in the DAM, an additional range of infrared self-similarity appears. By stretching the quasi-thermal region, the self-similar fronts drive an effective dynamical cooling process towards the formal but ill-defined
equilibrium state at vanishing temperature–analogous to an ultraviolet catastrophe in a system of classical waves.
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Disordered Systems and Neural Networks
Lecture Notes on Statistical Physics and Neural Networks
Olaf Hohm
These lecture notes introduce some topics of classical statistical physics, particularly those that are relevant for neural networks and deep learning. Statistical physics is treated as a branch of probability theory or statistics, with the goal of making concepts such as phase transitions and the renormalization group accessible to readers without prior knowledge of physics. We introduce the Boltzmann-Gibbs distribution and the thermodynamic potentials on a finite configuration space, notably for Ising
spins and
spin-glass models on a
lattice, and then define phase transitions as discontinuities that arise in the limit that the number of
lattice points goes to infinity. We further introduce Hopfield networks and Boltzmann machines, which are governed by the same energy function as
spin-glass models, and discuss the learning
algorithm for restricted Boltzmann machines. In this
algorithm hidden
neurons are integrated out as in the renormalization group. Finally, modern deep learning is introduced, whose early developments were in part motivated by restricted Boltzmann machines in that they carry many layers of hidden
neurons. A description of large
language models is given.
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Mesoscale and Nanoscale Physics
Genus-protected higher-order topological phases
Shahroze Shahab, Hui Liu, Daniel Varjas et al. (+1 more)
Higher-order topological phases (HOTPs) feature protected gapless modes on boundaries of higher codimension, such as the corners or hinges of a
crystal. They are understood as being protected by
lattice symmetries: If the latter are broken, it becomes possible to remove the boundary modes without closing the bulk gap. In this work, we present construction schemes for HOTPs protected solely by the bulk gap, by fundamental
symmetries, and by the global
topology of the system shape (its genus, or number of holes), independent of any crystalline
symmetries. As long as the fundamental local
symmetries are preserved, the resulting boundary states cannot be removed by any purely-surface perturbation.
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All authors Shahroze Shahab, Hui Liu, Daniel Varjas, Ion Cosma Fulga
Disordered Systems and Neural Networks · Cross-listed from stat.ML
The Interplay of Data Structure and Imbalance in the Learning Dynamics of Diffusion Models
Flavio Nicoletti, Chenxiao Ma, Enrico Ventura et al. (+2 more)
Real-world
datasets are inherently heterogeneous, yet how per-class structural differences and sampling imbalance shape the training dynamics of
diffusion models-and potentially exacerbate disparities-remains poorly understood. While models typically transition from an initial phase of generalization to memorizing the training set, existing theory assumes homogeneous data, leaving open how class imbalance and heterogeneity reshape these dynamics. In this work, we develop a high-dimensional analytical framework to study class-dependent learning in score-based
diffusion models. Analyzing a random-features model trained on Gaussian mixtures, we derive the feature-covariance spectrum to characterize per-class generalization and memorization times. We reveal the explicit hierarchy governing these dynamics: class variance is the primary determinant of learning order-consistently favoring higher-variance classes-while centroid geometry plays a secondary role. Sampling imbalance acts as a modulator that can reverse this ordering and, under strong imbalance, forces minority classes to acquire distinct, delayed speciation times during backward
diffusion. Together, these results suggest that
diffusion models can memorize some classes while others remain insufficiently learned. We validate our theoretical predictions empirically using U-Net models trained on Fashion MNIST.
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All authors Flavio Nicoletti, Chenxiao Ma, Enrico Ventura, Luca Saglietti, Stefano Sarao Mannelli