Materials Science
Electrostatic Effects of Self Trapped Holes in Gallium Oxide Devices
Nathan Wriedt, Joe McGlone, Davide Orlandini et al. (+1 more)
Gallium
oxide is an ultra-wide bandgap
semiconductor with exceptional properties for power electronics and UV-C optoelectronics, but its behavior under illumination remains poorly understood. In this work, we investigate how optically generated self-trapped holes influence electrostatics and current conduction in gallium
oxide devices. Using a vertical Schottky photodiode with a semi-transparent Ni anode, we performed capacitance-voltage, current-voltage, and temperature-dependent I-V measurements under dark and above-bandgap illumination. Analysis of photocurrent gain reveals that conventional image-force barrier-lowering models require unrealistically high interfacial electric fields, suggesting the presence of an alternative mechanism. By applying Fowler-Nordheim tunneling theory, we reconcile measured photocurrents and photo-capacitance results with physically plausible fields and quantify the two-dimensional concentration of self-trapped holes. Our findings demonstrate that illumination-induced charge significantly alters device electrostatics. Understanding this tunneling-based photocurrent gain mechanism is critical for designing gallium
oxide devices for UV-C detectors and power electronics.
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All authors Nathan Wriedt, Joe McGlone, Davide Orlandini, Siddharth Rajan
Mesoscale and Nanoscale Physics
Frustrated out-of-plane Dzyaloshinskii-Moriya interaction and the onset of atomic-scale 3$q$ magnetic textures in 2D Fe$_{3}$GeXTe (X = Te, Se, S) monolayers
Rabia Caglayan, Louise Desplat, Sergey Nikolaev et al. (+5 more)
We theoretically study the effect of in- and out-of-plane Dzyaloshinskii-Moriya interaction (DMI) on the
magnetic ground states of two-dimensional (2D) Fe$_3$GeXTe (X=Te, Se, S) monolayers, where X=Se, S correspond to antisymmetric Janus structures with nonvanishing in-plane DMI. We perform atomistic
spin simulations with the extended Heisenberg
Hamiltonian parametrized by first principles calculations. While we find that the base DMI in all systems is too weak to stabilize noncollinear states, we show how the frustrated out-of-plane DMI tends to favor atomic-scale $3q$
magnetic textures at the edge of the Brillouin zone. Owing to the ability to tune the DMI in 2D magnets via applied strain or electric field, we study the
evolution of the systems' ground state with increasing DMI amplitude. We find that nonplanar $3q$ states are favored under scaling factors as low as 3, while larger DMI tends to stabilize states reminiscent of nanoskyrmion
lattices at the atomic-scale.
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All authors Rabia Caglayan, Louise Desplat, Sergey Nikolaev, Fatima Ibrahim, Jing Li, Yesim Mogulkoc, Aybey Mogulkoc, Mairbek Chshiev
Disordered Systems and Neural Networks · Cross-listed from quant-ph
Monitoring of quantum walks with weak measurements
Klaus Ziegler, Tim Heine and Sabine Tornow
Measurements can be used to monitor the
evolution of
quantum systems and can give rise to quantized return statistics. It is known that the mean return time is quantized for strong monitoring through the winding number of the monitored
quantum state. We discuss that under coherent weak monitoring, implemented via ancilla coupling, the mean return time of a
quantum walk obeys a scaling relation with respect to the measurement strength. An analog scaling relation was previously found for random-time monitoring, indicating that weak and random-time monitoring have similar effects. We discuss how weak monitoring via ancilla coupling is linked to the
unitary evolution, and how this connection can be controlled by a convergent perturbation theory.
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Other Condensed Matter
Chemical tuning of electronic and transport properties of the Bi-Se-Te family of topological insulators
Maxwell Doyle, Benjamin Schrunk, D. L. Schlagel et al. (+2 more)
We use
laser-based Angle-Resolved Photoemission Spectroscopy (ARPES) to study how chemical substitution modifies the electronic properties of the Bi2(Se{1-x}Tex)3 (BiSeTe) family of topological
insulators. We find that increasing the Te content lowers the chemical potential, leading to a decrease in the binding energy of the Dirac point and a reduction in the density of states originating from the bulk band. This reduction leads to a transition from metallic to semiconducting temperature dependence of the resistivity. For the highest Te concentration, the resistivity nearly saturates at the lowest temperatures. The presence of this plateau indicates that metallic topological surface states dominate the conductance, opening the possibility of studying their transport properties.
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All authors Maxwell Doyle, Benjamin Schrunk, D. L. Schlagel, Thomas A. Lograsso, Adam Kaminski
Mesoscale and Nanoscale Physics
The role of polarization field terms in a model for a cavity quantum material
Arwen Lloyd, Adam Stokes, Alessandro Principi et al. (+1 more)
Constructing models for cavity
quantum materials requires a careful treatment of the light-matter coupling. In general, one must specify
matrix elements constructed from the material wavefunctions, which are often unknown in a tight-binding framework. The Peierls substitution is frequently used to avoid introducing these additional parameters in the multi-center dipole (or Peierls) gauge, under the assumption that contributions from intraband and interband dipole moments can be neglected. We present the derivation of the Peierls gauge description, including these dipole moment terms, in the passive view of canonical transformations. We construct a toy model for a multi-band system with two sites, which we couple to a uniform field in the Coulomb, dipole, and Peierls gauges. We find that all polarization field terms are required to describe multi-band coupling in the Peierls gauge. The Peierls substitution can only be justified under restriction to a single band in one dimension, provided one also ignores self-polarization corrections arising from bands outside the retained subspace. However, these corrections are frequently non-negligible. More generally, the Coulomb, dipole, and Peierls gauges define distinct partitions of the composite system into the light and matter subsystems. We illustrate the implications of this subsystem relativity for observables such as the
photon number and on the performance of orbital truncations in each gauge.
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All authors Arwen Lloyd, Adam Stokes, Alessandro Principi, Ahsan Nazir
Strongly Correlated Electrons
Nonequilibrium from Equilibrium: Chiral Current-Carrying States in the Spin-1 Babujian-Takhtajan Chain
Bahar Jafari-Zadeh, Chenan Wei, Hrachya M. Babujian et al. (+1 more)
We study the
spin-$1$ Babujian-Takhtajan chain deformed by its third conserved charge $Q_3$. We derive $Q_3$ and show that it is a dimensionless energy current and that its local density is a dressed scalar-chirality operator rather than bare chirality alone, as is the case for the
spin-$1/2$ Heisenberg chain. The deformation $H_\alpha=H+\alpha Q_3$ therefore provides a local, exactly solvable current bias: it leaves the eigenstates of the original
Hamiltonian unchanged, but reorders them so that selected high-energy current-carrying states become ground states of the tilted problem. Using the thermodynamic Bethe ansatz and confirming the analytical calculations with DMRG, we find a
quantum phase transition at $\alpha_c={J}/(8\pi)$. For $\alpha<\alpha_c$, the ground-state remains the undeformed Babujian-Takhtajan phase whose low-energy effective field theory is described by the $SU(2)$ Wess-Zumino-Witten (WZW) model at level $k=2$ representing a critical phase characterized by a central charge $c=3/2$ and $\langle Q_3\rangle=0$. For $\alpha>\alpha_c$, a finite rapidity interval forms, and the system enters a gapless
chiral current-carrying sector described by a $c=3/2$ CFT. Near the threshold, the free energy starts quadratically as a function of $\alpha-\alpha_c$, while the energy current turn on linearly. The scalar chirality turns on at the same threshold, showing that the postcritical sector is simultaneously current-carrying and
chiral. The most immediate experimental routes are composite
spin-1
bosons in optical
lattices, and programmable qutrit simulators based on trapped ions or superconducting
circuits.
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All authors Bahar Jafari-Zadeh, Chenan Wei, Hrachya M. Babujian, Tigran A. Sedrakyan
Mesoscale and Nanoscale Physics
Tunable anharmonicity in Sn-InAs nanowire transmons beyond the short junction limit
Amrita Purkayastha, Amritesh Sharma, Param J. Patel et al. (+11 more)
The anharmonicity of a transmon
qubit, defined as the difference in energy level spacing, is a key design parameter. In transmons built from hybrid
superconductor-
semiconductor Josephson elements, the anharmonicity is tunable with gate voltages that control both the Josephson energy and the weak link transparency. In Sn-InAs nanowire transmons, we use two-tone microwave spectroscopy to extract anharmonicity ranging in absolute value from the transmon charging energy $E_c$ to values smaller than $E_c/10$. This behavior contrasts with the predictions of the multi-channel short-junction model, which sets a lower limit on anharmonicity at $E_c/4$. Coherent operation of the
qubit is still possible at the point of the lowest anharmonicity. These findings demonstrate the potential of
quantum circuits that benefit from widely electrically tunable anharmonicity.
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All authors Amrita Purkayastha, Amritesh Sharma, Param J. Patel, An-Hsi Chen, Connor P. Dempsey, Shreyas Asodekar, Subhayan Sinha, Maxime Tomasian, Mihir Pendharkar, Christopher J. Palmstrøm, Moïra Hocevar, Kun Zuo, Michael Hatridge, Sergey M. Frolov
Materials Science · Cross-listed from cs.LG
Property-Guided Molecular Generation and Optimization via Latent Flows
Alexander Arjun Lobo, Urvi Awasthi and Leonid Zhukov
Molecular discovery is increasingly framed as an inverse design problem: identifying molecular structures that satisfy desired property profiles under feasibility constraints. While recent generative models provide continuous latent representations of chemical space, targeted optimization within these representations often leads to degraded validity, loss of structural fidelity, or unstable behavior. We introduce MoltenFlow, a modular framework that combines property-organized latent representations with flow-matching generative priors and gradient-based guidance. This formulation supports both conditioned generation and local optimization within a single latent-space framework. We show that guided latent flows enable efficient multi-objective molecular optimization under fixed oracle budgets with controllable trade-offs, while a learned flow prior improves unconditional generation quality.
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Strongly Correlated Electrons
Reflections on time-reversal in the Symmetry Topological Field Theory
Lea E. Bottini and Nick G. Jones
Symmetry under time-reversal appears in the microscopic description of many physical systems. In a
quantum mechanical setting it acts as an anti-
unitary operator, so does not fall under general analyses based on
unitary symmetries. In classifying zero temperature phases of matter in (1+1)d
lattice models, the role of anti-
unitary symmetries is, however, well-understood. In recent years, the
Symmetry Topological Field Theory (SymTFT) approach to this classification has given a general framework to understand
symmetries as topological defects, but does not naturally include anti-
unitary symmetries. Following recent proposals in the literature, we adopt a
symmetry-enriched SymTFT for a theory with both internal and time-reversal
symmetry. In particular, we take a standard SymTFT associated with an internal
unitary symmetry that is then enriched by a background time-reversal
symmetry. A detailed analysis of the topological boundary conditions of this enriched SymTFT allows us to characterize the corresponding (1+1)d gapped phases that preserve the enriching
symmetry (i.e. those that do not spontaneously break this
symmetry in the ground state). Line operators in the SymTFT approach are related to non-local string-order parameters (with charged end-point operators) for SPT phases. These are subtle in the anti-
unitary case and we explore them both on the
lattice and in the continuum. We include an analysis of
unitary string order parameters that reveal the Klein bottle SPT invariant. On the
lattice, we show that the correct end-point charge coincides with the time-reversal-charge only when the end-point operator is hermitian.
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Strongly Correlated Electrons
Ergodicity breaking in matrix-product-state effective Hamiltonians
Andrew Hallam, Jared Jeyaretnam and Zlatko Papić
Thermalization and its breakdown in interacting
quantum many-body systems are governed by mid-spectrum eigenstates, which are typically accessible only in small system sizes amenable to exact diagonalization. Here we demonstrate that the density-
matrix renormalization group (DMRG) effective
Hamiltonian, an object routinely used to variationally approximate ground states, encodes detailed information about the dynamics far from
equilibrium. In the random-field XXZ
spin chain, the spectrum of the effective
Hamiltonian is shown to capture the transition from thermal to many-body localized regimes, including spatially resolved probes of ergodic bubbles. Furthermore, the same approach also captures weak ergodicity breaking associated with
quantum many-body scars. Our results establish the DMRG effective
Hamiltonian as a versatile spectral probe of
quantum thermalization and its breakdown in large systems beyond exact diagonalization.
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Materials Science
Giant Magnetostriction by Design: A First-Principles Screening of Co-based Heusler Alloys
Pengju Wu, Jie Du, Liang Yao et al. (+4 more)
The pursuit of high-performance, rare-earth-free magnetostrictive materials is crucial for advancing technologies in sensing, actuation, and microelectromechanical systems. Heusler
alloys represent a promising, yet underexplored, class of materials for this purpose. In this work, we perform a systematic first-principles investigation of the magnetostrictive properties of 25 Co-based full Heusler
alloys, Co$_2$YZ (Y = V, Cr, Mn, Fe, Co; Z = Al, Ga, Si, Ge, Sn). Our screening identifies 10 compounds with large predicted magnetostriction ($|\lambda_{001}| > 100$~ppm), highlighted by Co$_3$Si with a
giant value of -966~ppm. Furthermore, we demonstrate two effective strategies for engineering magnetostriction: (i) tuning the
Fermi level, which enhances the magnetostriction of Co$_3$Sn to -905~ppm via Sb doping, and (ii) amplifying the
spin-orbit coupling, which boosts the magnetostriction of Co$_2$CrGa to a colossal -1008~ppm through Re substitution. Our analysis reveals a general predictive rule, uncovering a linear relationship between the magnetostriction and the choice of the Y-site transition metal. This work not only identifies novel candidates for magnetostrictive applications but also establishes clear, physically-grounded design principles to accelerate the discovery of new functional
magnetic materials.
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All authors Pengju Wu, Jie Du, Liang Yao, Hang Li, Xiaodong Zhou, Tao Zhu, Wenhong Wang
Soft Condensed Matter
Jamming and Flow in Granular Matter: A Physics Lab Course Experiment
Thomas Blochowicz, Emina Ismajli and Jan Philipp Gabriel
We describe a dynamic light scattering setup that uses diffusing wave spectroscopy (DWS) to investigate the dynamics in sand grains subjected to periodic vertical shaking by a loudspeaker. Along with the setup that is used in the undergraduate physics lab course at TU Darmstadt, the necessary DWS theory is introduced, including the proper treatment of the oscillatory excitation. Some exemplary results are presented that demonstrate the similarity of jamming in an athermal granular medium with the glass transition in thermally driven molecular systems, a relation that has frequently been pointed out but still is poorly understood. Similar, albeit more sophisticated experiments are currently conducted in microgravity environments such as the international space station ISS and the experiment may serve as an introduction to an exciting field of current research.
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Materials Science
Bubble-induced versus thermodynamic voltage losses during pressurized alkaline water electrolysis
Hannes Rox, Feng Liang, Robert Baumann et al. (+6 more)
Understanding how bubbles influence the efficiency of water electrolysis is crucial to achieve economically competitive hydrogen, generated by renewable energy sources, such as wind and
solar power. Water electrolysis is typically performed at high pressures to reduce the cost of energy-intensive mechanical compression of the produced H2. Thus, a better understanding of how the absolute pressure affects electrochemical performance and bubble size is necessary. In general, bubble sizes decrease as the pressure increases. Using different-sized pillar-patterned Ni electrodes generated by Direct
Laser Writing, the detached bubble sizes can be modified even at elevated pressures. As the pillar size increases, the bubbles become larger at all pressures investigated from 1 to 6 bar. At a current density of -25 mA/cm2, the cathodic potential increases with pressure according to the thermodynamic voltage losses given by the Nernst equation (~ 23 mV at p = 6 bar). Surprisingly, increasing the current density to 100 mA/cm2 leads to a reduction of the overpotential by up to ~ 60 mV. Reduced bubble sizes at increased pressures minimize the losses caused by the bubbles, thereby compensating for the thermodynamic voltage penalty. Applying the Buckingham {\Pi}-
theorem enables the derivation of dimensionless numbers to characterize the ratio of bubble-induced and thermodynamic voltage losses
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All authors Hannes Rox, Feng Liang, Robert Baumann, Mateusz M. Marzec, Krystian Sokołowski, Xuegeng Yang, Andrés F. Lasagni, Roel van de Krol, Kerstin Eckert
Statistical Mechanics
Topologically quantized macroscopic attractor states in hydrated DNA
Mariusz Pietruszka
Driven dissipative systems at ambient conditions typically exhibit continuous responses shaped by fluctuations and relaxation, with discrete macroscopic states arising only under specific dynamical constraints. Here, we report the emergence of discrete attractor states in a quasi-two-dimensional hydrated DNA sample under
magnetic excitation. The transverse polarization voltage $V_{xy}$ displays telegraph switching between well-defined levels, indicating
stochastic transitions between metastable macroscopic states. Statistical analysis of the voltage time series reveals bimodal distributions and strong
Bayesian model selection in favor of multiple coexisting states. A phenomenological phase-field framework describes the observed metastability and switching in terms of a proposed collective $U(1)$ polarization phase. Within this interpretation, phase-slip dynamics and integer-labeled winding sectors provide a possible organizing principle for the discrete macroscopic response. The results suggest that macroscopic quantization can emerge in a classical system at ambient conditions as a consequence of dissipative dynamics constrained by phase
topology.
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Materials Science
Anomalous phonon dispersion near yielding in athermal crystals
Fumiaki Nakai, Michio Otsuki, Kuniyasu Saitoh et al. (+1 more)
Vibrational properties of ordered athermal solids near yielding remain poorly understood. We show that yielding in a sheared
crystal is governed not by a single localized instability but by directionally extended multimode softening that forms a cross-shaped low-frequency region in wave number space. Near yielding, the acoustic dispersion $\omega\sim k$ is replaced by $\omega\sim k^2$ along the soft direction, and the vibrational density of states crosses over from Debye to non-Debye scaling, with a diverging length scale. We analytically derive these scaling laws.
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All authors Fumiaki Nakai, Michio Otsuki, Kuniyasu Saitoh, Hiroaki Katsuragi
Mesoscale and Nanoscale Physics
Gigahertz-clocked Generation of Highly Indistinguishable Photons at C-band Wavelengths
Robert Behrends, Lucas Rickert, Nils D. Kewitz et al. (+9 more)
High-performance single-
photon sources at telecom C-band wavelentghs are key building blocks for applications in long-distance
quantum communication. Here, we report the generation of highly indistinguishable, single
photons at a clock-rate of 2.5 GHz. This is achieved by coherently driving the biexciton transition ($T_1^\mathrm{XX}=64(1)\,$ps) of a
semiconductor quantum dot embedded in a microcavity with strong asymmetric Purcell enhancement. Employing pulsed two-
photon resonant excitation, strong multiphoton suppression with $g^{(2)}(0) < 4\%$ and high two-
photon-interference visibility of $V_\mathrm{raw}> 85\%$ is observed. The observed
photon indistinguishability is close to the theoretical limit expected for the photonically engineered radiative cascade and matches values obtained at lower repetition rates. Our results show a substantial advancement towards interference-based
quantum information protocols at unprecedented data rates in the telecom C-Band.
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All authors Robert Behrends, Lucas Rickert, Nils D. Kewitz, Martin v. Helversen, Pratim K. Saha, Mareike Lach, Jochen Kaupp, Yorick Reum, Tobias-Huber-Loyola, Sven Höfling, Andreas Pfenning, Tobias Heindel
Mesoscale and Nanoscale Physics
Beyond the Quantum Picture: The Electrodynamic Origin of Chiral Nanoplasmonics
Vasil Saroka, Lorenzo Cupellini, Nicolò Maccaferri et al. (+2 more)
Chiral plasmonic nanostructures are rapidly emerging as ideal substrates for enantioselective sensing,
chiral near-field engineering, and plasmon-assisted catalysis, owing to their exceptional sensitivity to structural handedness. However, the physical origin of plasmonic chirality, whether intrinsically
quantum or primarily governed by collective electrodynamics, remains an open question, limiting the development of predictive theoretical methods for the design of novel
chiral plasmonic architectures. Here, we show that a fully atomistic classical electrodynamic model, coupling intraband charge transport and interband polarization, quantitatively reproduces state-of-the-art ab initio and experimental chiroptical spectra across the
quantum-to-classical regime, from atomistically defined
chiral Ag and Au nanostructures to DNA-origami-assembled Au nanorods containing up to $\sim 10^5$ atoms. Our results support a unified electrodynamic origin of plasmonic chirality, providing the missing foundation to connect local structural motifs to chiroptical response and local
chiral near fields, and paving the way for the atomistically defined, rational design of
chiral plasmonic nanostructures optimized for targeted applications.
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All authors Vasil Saroka, Lorenzo Cupellini, Nicolò Maccaferri, Alessandro Fortunelli, Tommaso Giovannini
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Massless Dirac Fermions in curved surfaces with localized curvature
A. R. N. Lima, D. F. S. Veras and J. E. G. Silva
We investigate how a localized
curvature affects the dynamics of massless Dirac
fermions in a curved surface. We consider a smooth bump with axial
symmetry, adopting two specific
geometric models, namely a Gaussian and a volcano-like bumps. By considering a minimal coupling between the spinor and the surface geometry, described by the vielbeins and the
spin connection, we study the behavior of the wave function over the surface. By using appropriate numerical methods, we find a linear discrete energy spectrum for the Dirac
fermions and its corresponding wavefunctions when the
Fermi velocity is considered. It turns out that, since the
curvature vanishes asymptotically, the electron states are free waves far from the bumps, but around the curved points, the wave function increases its probability density.
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Materials Science
Refining hydrogen positions in {\alpha}-FeOOH through combined neutron diffraction and computational techniques
Yusuke Nambu, Akihide Kuwabara, Masahiro Kawamata et al. (+3 more)
The hydrogen positions and
magnetic structure of goethite $\alpha$-FeOOH, a key component of iron rust, were examined through
neutron diffraction. All
symmetry-allowed
magnetic structures under the space group $Pnma$ with the
magnetic wavevector $\vec{q}_{\rm m} = (0, 0, 0)$ r.l.u. were analysed using irreducible representation and
magnetic space group approaches. The
magnetic moments aligned along the $b$-axis form
antiferromagnetic spin arrangements, as reproduced by first-principles calculations. Accurately determining the hydrogen positions is crucial for understanding the mechanism of catalytic reduction of CO$_2$ in $\alpha$-FeOOH. These positions were precisely identified through diffraction and calculations, highlighting the effectiveness of using both methods for undeuterated compounds.
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All authors Yusuke Nambu, Akihide Kuwabara, Masahiro Kawamata, Seira Mori, Megumi Okazaki, Kazuhiko Maeda
Statistical Mechanics
Non-equilibrium Green's function formalism for radiative heat transfer
Yahan Liu and Tao Zhu
Radiative heat transfer (RHT) at the nanoscale can vastly exceed the far-field blackbody limit due to the tunneling of evanescent waves, a phenomenon traditionally described by fluctuational electrodynamics (FE). While FE has been exceptionally successful for systems in local thermal
equilibrium, its foundational assumptions break down in the growing number of scenarios involving genuine non-
equilibrium conditions, such as in active devices or driven materials. This review introduces the non-
equilibrium Green's function (NEGF) formalism as a powerful and versatile framework to study RHT beyond these classical limits. Rooted in
quantum many-body theory, NEGF provides a unified
language to describe energy transport by
photons, electrons, and
phonons on an equal footing. We first outline the theoretical foundations of the NEGF approach for RHT, demonstrating how it recovers the canonical results of FE in the local
equilibrium limit. We then survey recent breakthroughs enabled by NEGF, including: (i) providing a
quantum-accurate description of
equilibrium RHT that naturally incorporates non-local and finite-size effects, resolving unphysical divergences predicted by local models; (ii) unifying heat transfer channels to reveal the non-additive synergy between radiation, electron tunneling, and
phonon conduction at sub-nanometer gaps; (iii) enabling the
quantum design of materials and metamaterials with tailored thermal properties through band structure and topological engineering; and (iv) describing active control of heat flow in driven systems, which allows for phenomena like isothermal heat transfer and pumping heat against a temperature gradient.
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Strongly Correlated Electrons
Contrasting Spin Excitations in Octahedral and Square-Planar n=8 Ruddlesden-Popper Nickelates
K. Scott, H. LaBollita, G. A. Pan et al. (+15 more)
The discovery of superconductivity in reduced square-planar nickelates marked a major advance in identifying structural and electronic analogs to the high-$T_c$ cuprates. The more recent observation of superconductivity in parent Ruddlesden-Popper (RP) octahedral nickelates with a clear difference in electron count with respect to cuprates raises new questions about the nature of superconductivity across these related but distinct nickelate families. Here, we use Ni $L_3$-edge resonant inelastic x-ray scattering (RIXS) to probe the low-energy excitations in a representative compound of both families: the parent octahedral $n=8$ RP phase Nd$_9$Ni$_8$O$_{25}$ (p-RP), which is non-superconducting, and its reduced square-planar counterpart Nd$_9$Ni$_8$O$_{18}$ (r-RP), which exhibits superconducting correlations with a $T_c \approx 5$ K. The $n=8$ p-RP develops a
spin-density-wave (SDW) ground state with ordering wave vector $q_{\mathrm{SDW}} = (1/4,\, 1/4)$, analogous to the
bilayer RP, while the $n=8$ r-RP shows an elastic peak at $q^\star = (1/3,\, 0)$. Polarimetric RIXS shows that the p-RP exhibits low-energy spectra dominated by weakly dispersive paramagnons along the 0$\rightarrow\pi$ and $\pi \!\rightarrow\! \pi$ directions, whereas the r-RP with superconducting correlations displays dispersionless
magnetic excitations. Our results comprehensively map out the
spin excitations and reveal fundamental differences in the ground state between these two distinct structural families.
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All authors K. Scott, H. LaBollita, G. A. Pan, X. Yang, A. Kar, C. Lim, A. Thorshov, D. Ferenc Segedin, C. M. Brooks, F. Yakhou-Harris, K. Kummer, N. B. Brookes, F. Boschini, A. Frano, J. A. Mundy, E. H. da Silva Neto, A. S Botana, S. Blanco-Canosa
Soft Condensed Matter
Transition from contact to hydrodynamic lubrication over rough surfaces
Vincent Bertin and Olivier Pouliquen
We present a theoretical analysis of frictional transitions along the Stribeck curve for rough elastic contacts lubricated by a Newtonian fluid. Building on the mean-field framework of Persson and Scaraggi (J. Phys.: Condens. Matter 21 (2009) 185002), we formulate a minimal elastohydrodynamic model that couples contact mechanics and lubrication through a homogenized pressure decomposition. Dimensional analysis reveals three independent dimensionless parameters governing the frictional response, which correspond to a dimensionless speed, normal load, and surface roughness. Using asymptotic expansions, we first characterize the boundary and hydrodynamic lubrication regimes, which arise naturally as the quasistatic and smooth-surface limits of the model. In both limits, the contact morphology converges toward Hertzian contact in the regime of large elastic deformation, with boundary layers regularizing the separation profile at the edge of the contact zone. We then analyze the mixed lubrication regime and derive asymptotic expressions for the friction coefficient in the low- and high-speed limits. At high speeds, friction decomposes into a viscous contribution and a residual contact term, leading to a roughness- and load-dependent criterion for the transition to hydrodynamic lubrication that departs from constant-{\Lambda} ratio theories. At low speeds, friction reduction results from the progressive redistribution of the applied load between asperity contact and hydrodynamic pressure, yielding a characteristic transition speed from boundary to mixed lubrication. These results are summarized in a phase diagram that generalizes the classical Stribeck curve to a multidimensional parameter space.
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Soft Condensed Matter
Non-Additive Ion Effects on the Coil-Globule Equilibrium of a Generic Polymer in Aqueous Salt Solutions
Kushagra Goel, Monika Choudhary and Swaminath Bharadwaj
Mixtures of weakly and strongly hydrated anions induce non-additive changes in the LCST of thermoresponsive
polymers such as PNIPAM and PEO. Large-scale atomistic simulations of PNIPAM–NaI–Na$_{2}$SO$_{4}$ mixtures have shown that these effects arise from the interplay between favorable PNIPAM–iodide interactions and depletion of strongly hydrated sulfate ions. Here, we investigate whether chemically specific
polymer–anion interactions are necessary to reproduce such behavior. To this end, we study the coil–globule transition of a generic uncharged linear
polymer with non-specific
polymer–water and
polymer–ion van der Waals interactions in atomistic aqueous solutions of single and mixed salts. Simulations are performed at fixed concentrations of the strongly hydrated salt, Na$_{2}$SO$_{4}$, and increasing concentrations of the weakly hydrated salts, NaSCN and NaI. The generic
polymer qualitatively reproduces experimentally observed trends in pure NaSCN and Na$_{2}$SO$_{4}$ solutions, as well as the non-additive behavior in mixed salt solutions. In particular, the model captures the mutually reinforcing preferential accumulation of the weakly hydrated SCN$^{-}$ ions and depletion of the strongly hydrated SO$_{4}^{2-}$ ions near the
polymer that underlies the non-additive behavior. This mutual enhancement correlates with partitioning of sodium ions from the counterion cloud of SCN$^{-}$ ions to that of SO$_{4}^{2-}$ ions and is consistent with atomistic simulations of PNIPAM solutions. The model also reproduces the effects of background salt concentration and weakly hydrated anion identity on the non-additive behavior. These results demonstrate that non-specific
polymer–ion and
polymer-water interactions are sufficient to reproduce non-additive salt effects, suggesting a dominant role of bulk ion–ion and ion–water interactions.
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Mesoscale and Nanoscale Physics
Electron- and Lattice-Temperature Dependence of the Optical Response of Gold Nanoparticles
Nour E. H. Chetoui, Jonas Grumm, Robert Lemke et al. (+2 more)
Transient absorption spectroscopy is routinely used to study the electron dynamics in plasmonic gold nanoparticles. Typically, the transient absorption bleach is analyzed as measure for the electron temperature. However, the implicitly assumed linear dependence between bleach intensity and temperature has not been systematically studied. Similarly, the influence of
lattice heating also lacks a detailed analysis. Here, we solve momentum-resolved metal Boltzmann-Bloch equations for a semi-analytic access to the temperature-dependent gold nanoparticle absorption. We confirm the theory with steady state and transient absorption experiments, define regions of linear correlation between transient absorption bleach intensity and electron temperature and reveal a strong impact of the
lattice temperature on the TA bleach intensity.
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All authors Nour E. H. Chetoui, Jonas Grumm, Robert Lemke, Andreas Knorr, Holger Lange
Statistical Mechanics · Cross-listed from quant-ph
Symmetry-resolved properties of the trace distance in thermalizing SU(2) systems
Haojie Shen, Jie Chen and Xiaoqun Wang
We study diagnostics of thermalization in
quantum many-body systems with global SU(2)
symmetry, where the standard eigenstate thermalization hypothesis (ETH) is generalized to its non-Abelian form. As an eigenstate-level probe, we introduce a
symmetry-resolved trace distance constructed from the block structure of the reduced density
matrix. This block structure separates
spin-sector probabilities from configurational fluctuations within each sector, naturally leading to a decomposition into a probability trace distance and a configurational trace distance. The microcanonical average of the former is bounded by fluctuations of the corresponding
spin-sector probabilities within a microcanonical energy window, whereas the latter captures finer intra-sector fluctuations. In non-Abelian thermalizing systems, these
spin-sector-probability fluctuations are constrained by the non-Abelian ETH and therefore become exponentially suppressed with system size. Numerical studies of the one-dimensional \(J_1\)–\(J_2\) Heisenberg chain are consistent with this picture and suggest that, in the thermal regime, the trace distance is asymptotically dominated by the configurational trace distance.
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Mesoscale and Nanoscale Physics
Numerical calculation of the $k$-space second Chern number in four dimensions
Xiang Liu, Xiao-Xia Yi, Zheng-Rong Liu et al. (+2 more)
We propose an efficient numerical method to compute the $k$-space second Chern number in four-dimensional (4D) topological systems. Our approach employs an adaptive mesh refinement scheme to evaluate the Brillouin-zone integral, which automatically increases the grid density in regions where the Berry
curvature is sharply peaked. We compare our method with the 4D
lattice-gauge extension of the Fukui-Hatsugai-Suzuki method and a direct uniform grid integration scheme. Compared with these approaches, our method (i) achieves the same accuracy with substantially fewer diagonalizations, and thus runs faster; (ii) requires minimal
memory to execute, enabling calculations for larger systems; and (iii) remains accurate even near topological phase transitions where conventional methods often face challenges. These results demonstrate that the adaptive subdivision strategy is a practical and powerful tool for calculating the $k$-space second Chern number.
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All authors Xiang Liu, Xiao-Xia Yi, Zheng-Rong Liu, Rui Chen, Bin Zhou
Other Condensed Matter · Cross-listed from hep-th
Electromagnetic radiation mediated by topological surface states
M. Ibarra-Meneses and A. Martín-Ruiz
We study electromagnetic radiation from classical sources near a planar interface separating a topological and a trivial
insulator, modeled within
axion electrodynamics. The system features a piecewise constant $\theta$-term that encodes the magnetoelectric response of topological surface states. Treating this coupling perturbatively, we derive analytical corrections to the standard Liénard-Wiechert potentials and obtain modified radiation fields in the far zone. As applications, we analyze the emission from linear
antennas and the bremsstrahlung radiation of accelerated charges near the interface. For
antennas, the surface
Hall response breaks axial
symmetry and produces azimuthal modulations that grow with the electrical length, leading to distinct scaling behaviors in the total and angular radiated power. {For accelerated charges, the emitted intensity is uniformly reduced by a factor $1 - (\sigma_{\mathrm{Hall}} / 2\epsilon v)^2$, which we interpret as a process-specific attenuation of the radiative strength due to interference with its image
magnetic monopole inside the topological medium.} These results reveal how topological surface states mediate measurable modifications to classical radiation, establishing a link between
axion electrodynamics, topological phases, and field theories with spatially varying couplings.
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Quantum Gases
Analysis of the singular band structure occurring in one-dimensional topological normal and superfluid fermionic systems: A pedagogical description
Marcello Calvanese Strinati and Giancarlo Calvanese Strinati
Topological properties of solid-state materials arise when crossings occur in their band-structure
eigenvalues, which give rise to discontinuities in the associated Bloch-function eigenvectors once these are mapped over the whole Brillouin zone. These nonanalytic properties have direct consequences on the spatial decay of the corresponding Wannier functions, leading to what is nowadays referred to as the "obstruction to finding symmetric Wannier functions" for a given set of bands, as well as on the need for shifting the Wannier functions to interstitial positions, related to what is nowadays known as the "bulk-boundary correspondence." The importance of nonanalytic points of Bloch eigenfunctions and their consequences for the spatial decay of Wannier functions were historically anticipated back in 1978 [G. Strinati, Phys. Rev. B 18, 4104-4119 (1978)], somewhat before the work of Berry on what came to be referred to as the "Berry phase" [M. V. Berry, Proc. R. Soc. London, Ser. A 392, 45 (1984)]. In particular, the former paper identified key precursors and physical insights that are now understood, in hindsight, to be closely related to the later developments mentioned above. Here, we recap the essential features of these key issues in a rather pedagogical way, by considering in full details two instructing examples for which the origin of the discontinuities in the eigenvectors can be readily traced and mapped out, and the rate of the spatial falloff of the associated Wannier functions can be fully determined. For this analysis to be as complete as possible, two cases, one for noninteracting and one for interacting
fermions, are considered on equal footing.
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Materials Science · Cross-listed from physics.optics
Role of a Quarter-Wave Plate in Confocal Microscopy: Signature of Spin-Orbit Interactions
Wenze Lan, Anton Lögl, Meryem Benelajla et al. (+3 more)
Spin-orbit interactions of light couple polarization and spatial degrees of freedom, underpinning phenomena such as the
spin Hall effect of light. Although widely explored at interfaces and in tightly focused beams, their impact in nominally paraxial confocal systems remains largely unexamined. Here we show that a single quarter-wave plate embedded in a simple confocal geometry between polarizers can strongly reshape the transverse structure of a Gaussian beam. We observe an enhancement of the polarization extinction ratio by more than two orders of magnitude, accompanied by a transformation of the Gaussian intensity profile into a first-order Hermite-Gaussian-like two-lobe mode. The orientation of this pattern is continuously tunable via rotation of the wave plate, evidencing polarization-controlled reorientation of the transverse field. To explain these observations, we introduce a minimal extension of Jones
matrix formalism incorporating complex parameters that quantitatively reproduces the measurements. Our results uncover a previously overlooked form of
spin-orbit-mediated mode control in standard confocal optics and establish a simple route to on-demand spatial mode engineering for applications in resonant spectroscopy, optical imaging and
quantum optics.
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All authors Wenze Lan, Anton Lögl, Meryem Benelajla, Clemens Schäfermeier, Khaled Karrai, Bernhard Urbaszek
Statistical Mechanics
Caloric Phenomena and Stirling-Cycle Performance in Heisenberg- Kitaev Magnon Systems
Bastian Castorene, Martin HvE Groves, Francisco J. Peña et al. (+5 more)
We investigate the Stirling-cycle performance of a Heisenberg–Kitaev magnonic medium with Dzyaloshinskii–Moriya (DM) interactions. Using linear
spin-wave theory, we show the DM interaction preserves spectral
symmetry, yielding even caloric responses and symmetric Stirling engine efficiency. In contrast, bond-dependent Kitaev exchange asymmetrically distorts the magnonic density of states, enabling distinct direct and inverse caloric effects. Consequently, Kitaev-driven cycles achieve significantly higher efficiencies than DM-driven protocols, approaching a high-performance saturation regime for negative couplings. This establishes exchange-anisotropic magnets as highly tunable platforms for nanoscale solid-state energy conversion.
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All authors Bastian Castorene, Martin HvE Groves, Francisco J. Peña, Nicolas Vidal-Silva, Miguel Letelier, Roberto E. Troncoso, Felipe Barra, Patricio Vargas