Quantum Gases
Superfluid response of bosonic fluids in composite optical potentials: angular dependence and Leggett's bounds
Daniel Pérez-Cruz, Grigori E. Astrakharchik and Pietro Massignan
We study the superfluid response of a dilute bosonic fluid in the presence of two-dimensional composite potentials (such as triangular, Kagomé and quasiperiodic potentials, or superlattices), which may be obtained for example by superposing multiple
laser beams. We first find a sufficient condition for the external potential to yield a fully isotropic superfluid response. Then, we derive analytical expressions for Leggett's upper and lower bounds to the superfluid fraction (valid in the perturbative regime) that allow us to find the optimal direction along which each bound should be measured. Finally, we solve the problem numerically, and we confirm our analytical findings.
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Strongly Correlated Electrons
Kondo scaling of $4f$-electron states and the Kondo singlet breakdown in heavy fermions
B. Tegomo Chiogo, M. Tagliavini, D. Wong et al. (+10 more)
The low-energy
spin- and charge-sensitive thermodynamic properties of a broad range of strongly correlated 4f-electron systems follow Kondo scaling, with a characteristic Kondo temperature, $T_K$. While the theory is known for thermodynamic properties and high-energy spectroscopies of Kondo materials, the surface sensitivity of electron spectroscopy limits the extent to which Kondo scaling can be quantitatively verified. In this study, bulk-sensitive
photon-in
photon-out temperature-dependent resonant inelastic X-ray scattering (RIXS), in combination with single-
impurity Anderson model (SIAM) calculations, is used to provide quantitative evidence of low- and high-energy Kondo scaling in CeSi$_2$. RIXS Ce M$_5$-edge spectra show a clear decrease in the occupancy of the $f^0$ state as temperature increases accompanied by an increase of the spectral weight of the $f^1\underline L^1$ state, in good agreement with the SIAM calculations. The results demonstrate the breakdown of the Kondo singlet state, coupled with thermal occupation of the low-lying first-excited
magnetic states. The RIXS data reveal a temperature
evolution of the $f^n$ spectral weights, which is in stark contrast to that extracted from photoemission and inverse photoemission spectroscopies. This study provides an accurate spectroscopic method to determine the Kondo energy $k_B$$T_K$ that is consistent with thermodynamic measurements, and highlights soft X-ray RIXS as a quantitative bulk probe of low- and high-energy-scale hybridization effects in strongly correlated materials.
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All authors B. Tegomo Chiogo, M. Tagliavini, D. Wong, C. Schulz, 1 V. Porée, A. Nicolaou, R. Feyerherm, T. Schweitzer, T. Mazet, M. W. Haverkort, A. Chainani, D. Malterre, K. Habicht
Quantum Gases
Double-weak-link interferometer of hard-core bosons in one dimension
A. Takacs, J. Dubail and P. Calabrese
We study the dynamics of a
lattice hard-core
boson gas released from a domain wall initial state in the presence of two weak links (defects). When the two defects are separated by a finite distance, the resulting density profile exhibits interferences, while when we take the large-distance limit between the two defects, those interferences are averaged out and one recovers a profile that is described by the standard Euler-scale hydrodynamic description of the gas. We analyze this in detail by computing the exact fermionic propagators and their repeated reflections at the defects. We derive a closed analytic expression for the density profile during the expansion, explicitly highlighting the role played by these interference processes.
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Quantum Gases
Phase-space microscopes for quantum gases: Imaging conjugate variables and momentum-weighted densities
N. R. Cooper, Y. Yang and C. Weitenberg
Quantum gas microscopes offer unprecedented insights into
quantum many-body states of cold atomic gases. Here we introduce concrete protocols for extending
quantum gas microscopes to measure in phase space, by mapping momentum onto auxiliary degrees of freedom and using positive operator-valued measures. We distinguish between two distinct operational modes. In the Husimi-Q phase space microscope, position and momentum are jointly measured; in this mode the fundamental
quantum noise is distributed between position and momentum. Conversely, the averaged-mode phase space microscope extracts the spatial dependence of averages of the momentum density (and its moments); these averages can be retrieved with arbitrary spatial resolution. We illustrate the utility of these techniques in diverse physical settings.
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Mesoscale and Nanoscale Physics
Gate-Tunable Mid-Infrared Electroluminescence from Te/MoS2 p-n Heterojunctions
Shiyu Wang, Delang Liang, Zhi Zheng et al. (+9 more)
Mid-infrared (MIR) emitters are critical components in advanced photonic systems, driving progress in fields such as chemical sensing, environmental monitoring, medical diagnostics, thermal imaging and free-space communications. Conventional MIR emitters based on III-V
heterostructures rely on complex epitaxial growth on rigid
lattice-matched substrates and suffer from limited integration compatibility with CMOS or flexible platforms. The recent development of novel MIR emitters based on two-dimensional (2D) materials such as black phosphorus (BP) is more suitable for on-chip applications but faces challenges related to stability and emission efficiency. Based on the recently discovered highly efficient photoluminescence of Te, we demonstrate a gate-tunable midinfrared light-emitting diode based on a van der Waals heterojunction formed by multilayer transition metal dichalcogenide (TMD) MoS2 and tellurium (Te). The device emits polarized electroluminescence (EL) centered at 3.5 $\mu$m under forward bias at 25 K, and the EL persists up to 80 K with reduced intensity. Gate control of the MoS2
Fermi level modulates the band
alignment and injection efficiency, enabling dynamic tuning of the EL intensity. The emission remains spectrally stable under varying bias and gating, indicating robust band-edge recombination. These results establish the Te/TMD
heterostructure as a promising platform for integrated polarized mid-infrared optoelectronics.
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All authors Shiyu Wang, Delang Liang, Zhi Zheng, Mingyang Qin, Yuchun Chen, Jie Sheng, Shula Chen, Lin Li, Changgan Zeng, Anlian Pan, Jinluo Cheng, Dong Sun
Quantum Gases · Cross-listed from math-ph
Ground state energy of the Bose–Hubbard model with large coordination number with a polaron-type quantum de Finetti theorem
Shahnaz Farhat, Denis Périce and Sören Petrat
We consider the ground state energy of the Bose–Hubbard model on a graph with large and homogeneous coordination number. In the limit of infinite coordination number, we prove convergence of the ground state energy to the minimizer of a mean-field energy functional. This functional is obtained by averaging the hopping term over the large number of connected sites, while the interaction energy is not averaged. Hence, the resulting mean-field description is in the strong coupling regime, and is expected to provide a qualitatively correct picture of the phase diagram of the Bose–Hubbard model for large enough coordination number. For our proof, we develop a new version of a de Finetti-type
theorem, which we call the polaron-type
quantum de Finetti
theorem, and which we expect to be a more broadly useful extension of existing
quantum de Finetti results. Our
theorem covers the case where the
Hilbert space is a tensor product of some
Hilbert space with a bosonic Fock space. This
theorem is applied to the convergence of the ground state energy of the Bose–Hubbard model after reducing it to a polaron-type model.
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Strongly Correlated Electrons
Ground State Properties of the Doped Kitaev-Heisenberg Chain: Topological Superconducting and Mott Insulating Phases Driven by Magnetic Frustration
Cliò Efthimia Agrapidis and Satoshi Nishimoto
We study the hole-doped Kitaev-Heisenberg chain using the density-
matrix renormalization group. In the Kitaev-only limit, the bond-directional exchange itself promotes pairing, favoring
spin-singlet and
spin-triplet superconducting tendencies for
antiferromagnetic and
ferromagnetic Kitaev couplings, respectively, together with finite-size Majorana edge correlations suggestive of topological superconductivity. In the full Kitaev-Heisenberg chain, cooperative $J$ and $K$ exchanges broadly stabilize superconductivity, while competition between $J$ and $K$ induces a strong filling dependence and enables superconductivity even when both $J$ and $K$ are weak. At quarter filling, this competition produces a Mott
insulator with spontaneous hopping dimerization. These results identify
magnetic frustration as a common mechanism underlying superconducting and interaction-driven insulating phases in doped Kitaev systems.
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Materials Science
On the flash temperature in sliding contacts
M. H. Müser and B. N. J. Persson
The temperature increase in the contact regions between solids in sliding contact can easily reach several hundred Kelvin and thereby dramatically affect friction and wear. The classical theories by Jaeger, Archard, and Greenwood, commonly used to estimate flash temperature, ignore the multiscale nature of real surfaces and instead approximate the frictional heat sources with circular or square shapes. Here, we present an analytical theory for the flash temperature valid for randomly rough surfaces with roughness across arbitrarily many decades in length scale. The theory extends established methods for stress correlation functions and peak stresses to temperature. Numerical results for rubber sliding on concrete, and granite on granite, are presented as illustrations. We show that classical theories for flash temperature fail severely for surfaces with multiscale roughness.
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Superconductivity
Pressure-enhanced superconductivity and its correlation with suppressed resistance dip in (La,Pr)3Ni2O7 films
Jinyu Zhao, Guangdi Zhou, Shu Cai et al. (+11 more)
The discovery of superconductivity with a transition temperature (Tc) exceeding 40 K in La3Ni2O7 and (La,Pr)3Ni2O7 thin films at ambient pressure provides a viable platform for the experiments that can only be conducted under ambient-pressure conditions, and for the theoretical investigations aimed at understanding the commonalities and peculiarities of the behaviors related to the superconductivity between the film and the compressed bulk systems - including the effects of oxygen vacancies and strain. Consequently, it is crucial to determine whether Tc can be further enhanced and to uncover the underlying physics that controls the Tc value in these ambient-pressure superconducting thin films. Here, we report a systematic study of hydrostatic pressure effects on the superconducting properties of (La,Pr)3Ni2O7 thin films. We find that external pressure universally enhances Tc of the film samples regardless of their initial Tc value. The onset Tc of 68.5 K at 2.0 GPa demonstrates a notable increase from 62 K at 0.3 GPa. Furthermore, we observe that the samples without zero resistance show a resistance dip just above the superconducting transition, whereas the samples that exhibit zero resistance do not display this dip. Applying pressure can suppress the dips and drive the system toward zero resistance. Based on our results, we propose that this feature is associated with oxygen vacancies and that the depth of the dip can serve as an indicator of the concentration of the vacancies. It is plausible that the dip is caused by the localization of mobile electrons at the vacancy sites. Applying pressure can delocalize these electrons, which in turn may contribute to the increase in Tc.
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All authors Jinyu Zhao, Guangdi Zhou, Shu Cai, Shuaihang Sun, Yaqi Chen, Jing Guo, Yazhou Zhou, Haoliang Huang, Jin-Feng Jia, Yang Ding, Qi Wu, Zhuoyu Chen, Qi-Kun Xue, Liling Sun
Disordered Systems and Neural Networks
Sampling at intermediate temperatures is optimal for training large language models in protein structure prediction
L. Ghiringhelli, A. Zambon and G. Tiana
Using a statistical mechanics framework, we investigate the parameter space of
transformer models trained on
protein sequence data. We sample the loss landscape at varying temperatures using Langevin dynamics to characterize the low-loss
manifold, and to understand the mechanisms underlying
transformers' superior performance in
protein structure prediction. We find that, at variance with networks not based on the
attention mechanism, the lack of a first–order–like transition in the loss of the
transformer produces a range of intermediate temperatures with good learning properties; this is true both for synthetic and natural
protein sequences. We also show that the parameters of most layers are highly conserved at these temperatures if the dimension of the
embedding is optimal, and we provide an operative way to find this dimension. Additionally, we show that the
attention matrix is more predictive of the contact maps of the
protein at higher temperatures and for higher dimensions of the
embedding than those optimal for learning. Finally, we showed that the models sampled at intermediate temperatures can predict the free-energy variation upon mutation, better than models obtained through standard optimization techniques.
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Statistical Mechanics · Cross-listed from math-ph
Derivative relations for determinants, Pfaffians and characteristic polynomials in random matrix theory
Gernot Akemann, Georg Angermann, Mario Kieburg et al. (+1 more)
Explicit expressions are proven for derivatives of the ratio of a determinant or Pfaffian determinant and a Vandermonde determinant. Such ratios appear for example in general group integrals of Harish-Chandra–Itzykson–Zuber type and in expectation values of products of characteristic
polynomials in random
matrix theory. In the latter case we start from known results for general non-Hermitian and Hermitian ensembles for expectation values without derivatives, at finite
matrix size. They are given in terms of the determinant or Pfaffian of the corresponding kernel, for
unitary or orthogonal and
symplectic ensembles, respectively. Several equivalent expressions are proven for general ratios of determinants, starting from first order derivatives containing the Borel transform of the corresponding
matrix or kernel. Higher order derivatives are expressed as sums over partitions containing determinants of derivatives of these, with coefficients given in terms of combinatorial expressions. Our most general result is valid for mixed higher order derivatives of ratios of determinants in several variables. This generalises previous findings, e.g. for mixed moments in specific ensembles of random
matrices, relevant in applications to the Riemann $\zeta$-function. Applications of our results to several examples are presented, including the complex Ginibre ensemble and the circular
unitary ensemble.
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All authors Gernot Akemann, Georg Angermann, Mario Kieburg, Adrian Padellaro
Materials Science
Phase-Singularity Control of Topological Acoustic Spin Textures
Huaijin Ma, Te Liu, Jiachen Sheng et al. (+6 more)
Acoustic
spin textures provide a degree of freedom for programmable topological field states, yet a unified framework for constructing and classifying higher-order textures in open-air acoustics remains lacking. Here we introduce polygonal standing-wave interference as a framework connecting discrete
symmetry, angular phase encoding, phase singularities,
chiral time-averaged energy flux, and acoustic
spin. By controlling the number and relative phases of the standing-wave channels, we experimentally realize D4 meron/anti-meron
lattice, D6 double-skyrmion superlattice, and D8 acoustic-
spin quasicrystal composed of four meron/anti-meron sublattices. Finite polygonal fields are normalized angular discretizations of continuous standing-wave spectra. Under the m=2 phase encoding, the D6 and D8 fields reproduce the local q=+2 core of a second-order Bessel
vortex while retaining distinct long-range order through their discrete interference channels. At fixed m increasing N under normalized angular sampling yields the continuous Bessel limit. These results establish phase encoding, pairwise interference, reciprocal-space composition, and angular discretization as a unified design principle for programmable topological acoustic
spin textures.
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All authors Huaijin Ma, Te Liu, Jiachen Sheng, Xiaochang Pan, Wenwei Qian, Xiangyu Chen, Kaiyuan Cao, Jinpeng Yang, Jian Wang
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Zhenhai Sun, David Feldstein-Bofill, Ksenia Shagalov et al. (+10 more)
Superconducting transmon
qubits based on hybrid
superconductor-
semiconductor Josephson junctions (gatemons) offer gate tunability, but their relaxation times remain well below those of state-of-the-art transmons, and the origin of this discrepancy is not fully understood. Here, we co-fabricate gatemons and SIS-junction transmons with nominally identical
circuit layouts, gate dielectrics, and control lines, so that the Josephson element is the only intentional distinction. Across multiple chips, transmons in this architecture reach relaxation times in the tens of microseconds, whereas gatemons saturate in the few-microsecond range. Using the transmons as on-chip references, we construct a loss budget including Purcell decay, spontaneous emission through the control line, and internal dielectric loss, and find that the corresponding T1 limits exceed all measured gatemon values by more than an order of magnitude. Temperature-dependent T1 measurements follow a common quasiparticle-activation model and yield similar superconducting gaps for S-Sm-S and SIS junctions, indicating that the reduced gatemon coherence is dominated by additional temperature-independent, junction-intrinsic dissipation.
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All authors Zhenhai Sun, David Feldstein-Bofill, Ksenia Shagalov, Amalie T. J. Paulsen, Casper Wied, Shikhar Singh, Brian D. Isakov, Jacob Hastrup, Christopher W. Warren, Svend Krøjer, Anders Kringhøj, András Gyenis, Morten Kjaergaard
Materials Science
Thermal Conductivity and Temperature-Induced Band Gap Renormalization in Crystalline and Amorphous Ga$_2$O$_3$
Rustam Arabov, Jiaxuan Li, Xiaotong Chen et al. (+2 more)
The
lattice thermal conductivity (LTC) and electron-
phonon interactions in crystalline and amorphous gallium
oxide are herein determined by coupling a machine-learned interatomic potential, namely the moment tensor potential (MTP) model, to first-principles calculations. Crystalline $\beta$-Ga$_2$O$_3$ exhibits a substantial band gap renormalization (BGR) of $\sim$0.45 eV at 700 K, with $\sim$0.2 eV caused by zero-point BGR. The computed temperature dependence of BGR induced by classical
nuclear motion in $\beta$-Ga$_2$O$_3$ is stronger than that in amorphous Ga$_2$O$_3$, with the difference in BGR reaching $\sim$0.18 eV at 900 K. Thermal transport calculations reveal that the LTC of amorphous Ga$_2$O$_3$ remains near $0.9$ W$\cdot$ m$^{-1}$$\cdot$K$^{-1}$ for temperatures between 300 K and 700 K, which is approximately an order of magnitude lower than that of crystalline $\beta$-Ga$_2$O$_3$. Overall, the presented framework provides a computationally tractable and reliable route for predicting properties of
semiconductors (both crystalline and amorphous) under operating conditions relevant to microelectronics and optoelectronics.
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All authors Rustam Arabov, Jiaxuan Li, Xiaotong Chen, Nikita Rybin, Alexander Shapeev
Statistical Mechanics
Fundamental problems in Statistical Physics XIV: Lecture on Correlation and response functions in statistical physics
Thomas Franosch
In the first part of these short lecture notes, we will present an introduction on (auto-)correlation functions and linear-response functions in the
language of a physicist. In particular, the fluctuation-dissipation
theorem in classical physics is presented underlining the central role of correlation functions. The fundamental importance of (auto-)correlation functions raises the natural question on how they are characterized in general without referring to the concrete underlying dynamical laws. Perhaps unexpectedly – despite being elegant and long established in the mathematical literature (Bochner's
theorem for correlations; Herglotz-Nevanlinna representations for response) – this answer is not widely appreciated in physics, partly because the requisite tools lie outside the standard curriculum. In the second part we adopt a more rigorous viewpoint: we state the key structural properties of correlation functions and provide selected derivations of these results. Finally, we return to linear response and discuss general characterization results for response functions.
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Materials Science
Fe-site-resolved anisotropy energies in Nd$_2$Fe$_{14}$B for atomistic spin dynamics
Veronica T. C. Lai and Christopher E. Patrick
Nd-Fe-B magnets are the most widely used high performance magnets in the world today, and remain the subject of both experimental and computational research aimed at understanding and optimizing them. Atomistic
spin dynamics (ASD) is one technique which has been used in recent years to provide insight into
magnetic properties relevant to coercivity, such as domain wall width. Although it is relatively clear how to model magnetocrystalline anisotropy arising from rare-earth atoms in these simulations, the contribution from the transition metal Fe is less obvious, due to the itinerant nature of the magnetism. Here, we examine previous treatments of Fe anisotropy in ASD simulations and identify a discrepancy with previously-published first-principles studies. We derive two models which correct this discrepancy, one based on single-ion theory and the other on anisotropic exchange, and test their performance by comparing to first-principles torque calculations on Y$_2$Fe$_{14}$B. The torque calculations show a contribution which cannot be explained by the single-ion model but arises naturally from (antisymmetric) anisotropic exchange. We propose practical strategies to model Fe anisotropy in future ASD simulations, including a simplified (mean-field) description of anisotropic exchange, which may have applications beyond R$_2$Fe$_{14}$B to the wider class of itinerant
magnetic materials.
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Materials Science
Continuous three-dimensional imaging of nanoscale dynamics by in situ electron tomography
Timothy M. Craig, Adrien Moncomble, Ajinkya A. Kadu et al. (+4 more)
Visualizing nanoscale transformations in three dimensions (3D) is essential for understanding materials
evolution under operating conditions, yet dynamic electron
tomography remains limited by slow tilt series acquisition and by reconstruction methods that assume static specimens. These constraints prevent continuous 3D imaging of evolving structures and require electron doses that can alter the specimens and their dynamics. Here, we demonstrate a framework for dynamic electron
tomography that combines continuous tilting with a self-supervised deep-learning reconstruction strategy. Our approach incorporates the temporal aspect into the electron
tomography reconstruction process to recover 3D volumes at arbitrary time points from a single tilt series. We validate the method using simulations and demonstrate its merit in experimental studies of heat-induced transformations, including morphological
evolution of Au nanostars and alloying in Au@Ag nanocubes. Our results establish a practical framework for dynamic, dose-efficient electron
tomography, enabling in situ 3D investigation of nanomaterial transformations as well as the characterization of beam-sensitive structures.
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All authors Timothy M. Craig, Adrien Moncomble, Ajinkya A. Kadu, Gail A. Vinnacombe-Willson, Luis M. Liz-Marzán, Robin Girod, Sara Bals
Strongly Correlated Electrons · Cross-listed from hep-lat
Lattice Field Theory Analysis of the Chiral Heisenberg Model
Simon Hands and Johann Ostmeyer
Motivated by ongoing interest in the universal behaviour of the Hubbard model of spinning electrons on honeycomb and $\pi$-flux
lattices at the semi-metal – Mott
insulator phase transition, we formulate the 3D
chiral Heisenberg model, a theory of relativistic
fermions in three spacetime dimensions, as a
lattice field theory using domain wall
fermions. The contact interaction term preserves an SU(2) global
symmetry. We perform numerical simulations using the Rational Hybrid Monte Carlo
algorithm on system sizes $L^3\times L_s$ with $L\in\{8,\ldots,24\}$ and domain wall separation $L_s\in\{8,16,24\}$. We locate the phase transition corresponding to spontaneous SU(2)$\to$U(1) breaking, yielding critical exponent estimates $\nu^{-1}=0.63(3)$, $\eta_\Phi=1.42(8)$. These values are considerably removed from estimates obtained from simulations performed in (2+1)D, ie. with the time and spatial directions treated differently, but align more closely with analytic estimates obtained using 3D covariant field theory. We also present first results for the
fermion correlator, ultimately needed for the determination of the exponent $\eta_\Psi$, highlighting the need to rotate the
fermion source to a common reference direction in isospace in order to obtain a signal.
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Mesoscale and Nanoscale Physics
Quantum transport reveals spin glass correlations in a 2D network of TbPc$_{2}$ single-molecule magnets grafted on graphene
Nianjheng Wu, Jules Lefeuvre, Andrew Mayne et al. (+6 more)
The low temperature magnetoresistance of
graphene functionalized by an array of
magnetic Terbium Phthalocyanines molecules is found to exhibit a
magnetic field-dependent 1/f noise, along with universal conductance fluctuations (UCFs) typical of a mesoscopic phase-coherent sample. A thorough analysis of the
magnetic field, temperature and chemical potential dependence of this 1/f noise and UCFs reveals that long range, 2D Ising
spin-glass like,
magnetic correlations are induced in
graphene through exchange interactions between the
magnetic molecules and charge carriers in
graphene. These experiments show that
graphene functionalized with organic molecules constitutes a versatile platform for the investigation of
magnetic phase transitions in two dimensions.
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All authors Nianjheng Wu, Jules Lefeuvre, Andrew Mayne, Stéphane Campidelli, Jérôme Lagoute, Cyril Chacon, Sophie Guéron, Richard Deblock, Hélène Bouchiat
Materials Science
Unquenched orbital angular momentum as the origin of spin inertia
Tarek Moussa, Darpa Narayan Basu, Ritwik Mondal et al. (+1 more)
The recent proposal and observation of
spin inertia, and the consequent high-frequency
spin nutation mode, have raised key questions for our understanding of magnetization dynamics, especially considering its high relevance for
magnetic memories and ultrafast switching. Notwithstanding recent progress, a clear identification of
spin inertia's physical origin thereby offering predictive power remains to be accomplished. Here, discussing general principles for identifying this physical origin, we examine unquenched orbital angular momentum (OAM) finding it to be a key candidate, despite its typically small value. Treating OAM and
spin within a two-sublattice model, we derive the equivalent single-sublattice framework for magnetization dynamics making appropriate approximations. The latter naturally manifests the
spin inertia term and parameter, which are otherwise introduced phenomenologically. The inertia parameter evaluated within our model is found to be in good agreement with its experimentally observed value in cobalt. We further delineate key experimental signatures that could verify or rule out the unquenched OAM as the origin of the observed high-frequency mode, and avoid a spurious optical mode in a two-sublattice ferromagnet from being identified as nutation. Our analysis offers a potential link between the recently emerged fields of orbitronics and
spin inertia, thereby motivating investigations at their intersection.
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All authors Tarek Moussa, Darpa Narayan Basu, Ritwik Mondal, Akashdeep Kamra
Statistical Mechanics
Force Geometry and Irreversibility in Nonequilibrium Overdamped Dynamics
Erez Aghion and Swetamber Das
Recent experiments have revealed heterogeneous dissipation in optically trapped systems, often anticorrelated with local positional fluctuations, exposing a structural gap in the scalar
stochastic thermodynamic description. While the scalar framework successfully quantifies dissipation through currents and
entropy production rates, it does not reveal the underlying vectorial force geometry that shapes spatial dissipation patterns. Here, we bridge this gap by identifying force geometry as an organizing principle for nonequilibrium thermodynamics, introducing force
alignment as a
geometric determinant of irreversibility. We show that
entropy production depends not only on force magnitudes but also on the relative orientation between deterministic driving forces and entropic gradients, vanishing only under exact anti-
alignment with matched magnitudes. We formalize this
geometric alignment through a time-dependent force-correlation coefficient, quantifying the relative orientation between the forces. This yields an instantaneous
geometric lower bound on
entropy production that remains valid even when force magnitudes are matched. For overdamped dynamics, perfect anti-
alignment defines a thermodynamic stall where net transport vanishes and the lower bound on
entropy production is saturated. This force-level perspective provides a structural explanation for the experimentally observed fluctuation-dissipation anticorrelation and nonuniform dissipation. We construct
geometric control charts for both constant dragging and sinusoidal driving protocols, explicitly locating experimental operating points within this force-space representation. Together, these results position force geometry as a unifying structural perspective on irreversibility, spanning active biological systems, microrheology, and naturally extending to underdamped dynamics.
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Soft Condensed Matter
Concentration-Dependent Restructuring of Ionic Liquid Micelles Induced by an Anionic Surfactant
Devansh Kaushik, Sajal K. Ghosh and Syed M. Kamil
The self-assembling behaviour of ionic
liquids in aqueous solution is important for understanding their physicochemical properties and for their industrial applications. While the influence of ionic
liquids on surfactant micellization has been widely studied, much less
attention has been given to how surfactants affect the aggregation of ionic
liquids, particularly when the surfactant concentration is below its critical micelle concentration (CMC). In this work, we examine the effect of the anionic surfactant sodium dodecyl sulfate (SDS), introduced at sub-CMC concentration, on the micellization of 1-methyl-3-octylimidazolium chloride in aqueous solution maintained above the IL CMC, using surface tension measurements, theoretical analysis, and coarse-grained molecular dynamics simulations. We find that at low SDS concentrations (approx 2 mM), SDS inserts smoothly into the pre-existing IL micelles, producing stable mixed micelles with favourable IL-SDS interactions. When the SDS concentration approaches (approx 4 mM), the micelles exhibit distinct changes in their internal dynamics, reflected in deviations in the thermodynamic parameters. Beyond this point, as more SDS is added, the system reorganizes and forms stable mixed micelles again, now containing a higher fraction of SDS but still enriched in IL. The synergistic behaviour is quantified using Clint and Rubingh's models, and simulations supports the structural transitions, showing variations in micelle size, aggregation number, and radial distribution functions. This work demonstrates that SDS acts as an effective modulator of IL aggregation, providing mechanistic insight into IL-surfactant co-assembly.
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Materials Science
Plasmon Engineering in Intercalated 2H-TaS$_2$
Luigi Camerano, Laura Martella, Lorenzo Battaglia et al. (+8 more)
Plasmons in low dimensional materials provide a powerful platform for nanoscale control of light matter interactions, yet strategies to tailor their coherence and dissipation remain limited. Here, we demonstrate that transition metal intercalation offers a fundamentally distinct route to engineer plasmonic response in layered materials. By combining high-resolution core-level photoemission spectroscopy with first-principles calculations, we show that Fe and Co intercalation in 2H-TaS2 does not act as conventional electron doping, but instead reshapes the low energy electronic structure through orbital hybridization and structural reconstruction. This process introduces a dense continuum of low energy excitations that efficiently damp and ultimately suppress the plasmon mode. First principle calculations of the energy loss function reveal a transition from a well defined collective excitation to an overdamped response, signaling the breakdown of coherent charge dynamics. Our results establish intercalation as a chemically controlled pathway to tune plasmon losses and dielectric response in
quantum van der Waals materials, providing a new design principle for plasmonic and optoelectronic functionalities at the nanoscale.
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All authors Luigi Camerano, Laura Martella, Lorenzo Battaglia, Federico Giannessi, Filippo Camilli, Luca Lozzi, Polina M. Sheverdyaeva, Paolo Moras, Luca Ottaviano, Gianni Profeta, Federico Bisti
Mesoscale and Nanoscale Physics
Nonlinear hydrodynamic response of a quantum Hall system
Hiroki Isobe
The
quantum Hall effect realizes a quantized
Hall resistance $R_{xy} = h/(\nu e^2)$ whereas the longitudinal resistance vanishes. The quantized value consists of the fundamental physical quantities, the elementary charge $e$ and the Planck constant $h$, along with an integer or fractional constant $\nu$. High precision measurements of $R_{xy}$ allude to a linear relation between the applied current $I$ and the
Hall voltage $V_\mathrm{H}$. Here, we argue that a nonlinear relation between $I$ and $V_\mathrm{H}$ could arise when the electric field is spatially inhomogeneous. We first discuss that the linear $I$-$V_\mathrm{H}$ relation holds with Galilean invariance. Then we consider a hydrodynamic description of a
quantum Hall liquid to deal with an axially symmetric electric field. It reveals a nonlinear electronic response arising from the centrifugal force exerted on a curved flow and the density gradient invoked by vorticity.
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Soft Condensed Matter
Emergent charge crystallization and frustration in a particle anti-spin Ice
Renaud Baillou, Matthew Terkel, Cristiano Nisoli et al. (+1 more)
Artificial
spin ices have transcended their origins in frustrated rare-earth pyrochlores to become a versatile platform for engineering exotic states of matter. Across diverse implementations, from nanomagnets and superconducting
vortices to colloids,
quantum annealers,
liquid crystals, and metamaterials, they are unified by the ice rule, which often leads to degeneracy and constrained disorder by enforcing minimization of the local topological charge. Here, we report the first realization of an "anti-
spin ice" in which not only the ice rule does not hold, but its opposite is true as the system seeks to maximize, rather than minimize,
spin ice charges. Using fast-rotating, in-plane
magnetic fields to generate isotropic attraction between colloidal particles, we invert the conventional paradigm of repulsive interactions in colloidal
spin ices. Combining experiments and simulations across standard square and honeycomb
lattices as well as novel pentaheptite geometries, we establish rules for order and disorder in the anti-
spin ice. With the pentaheptite
lattice, we demonstrate that the anti-
spin ice system can also exhibit frustration, but of a new kind. This topological charge frustration arises from the
lattice connectivity, where networks of unequal, odd-sided polygons suppress charge crystallization at high interaction strength.
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All authors Renaud Baillou, Matthew Terkel, Cristiano Nisoli, Pietro Tierno
Superconductivity
Altermagnetic-doping interplay as a route to enhanced d-wave pairing in the Hubbard model
Ji Liu, Jianyu Li, Peng Zhang et al. (+2 more)
Altermagnets - collinear, zero-net-moment magnets with momentum-odd
spin splitting protected by crystalline
symmetries - offer a tunable route to suppress long-range antiferromagnetism while preserving strong short-range
spin fluctuations. We show that this environment robustly stabilizes unconventional superconductivity and naturally produces mixed-
symmetry pairing. Through a strong-coupling analysis of a
spin-anisotropic Hubbard model, we derive an anisotropic t-J model where exchange interactions cooperatively enhance singlet d-wave pairing and promote triplet p-wave pairing. Our mean-field analysis reveals a pairing
evolution driven by altermagnetic anisotropy: for small
spin anisotropy, the d-wave channel is enhanced, closely resembling the dominant pairing
symmetry in cuprate
superconductors, which suggests that weak
spin anisotropy may be an essential ingredient in realistic models of these materials. Constrained-path
quantum Monte Carlo simulations confirm this picture, showing a regime where dominant d-wave correlations coexist with an emergent p-wave component near optimal doping. As
spin anisotropy increases, strong C2 anisotropy and
spin splitting activate the triplet channel, leading to a stable d+p mixed-pairing state. This synergistic state exhibits significantly enhanced overall pairing strength, suggesting the possibility of a higher superconducting transition temperature.
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All authors Ji Liu, Jianyu Li, Peng Zhang, Xiaosen Yang, Ho-Kin Tang
Materials Science
Decoding Dopant-Induced Electronic Modulation in Graphene via Region-Resolved Machine Learning of XANES
Yinan Wang, Arpita Varadwaj, Teruyasu Mizoguchi et al. (+1 more)
Revealing how heteroatom doping alters the local electronic structure of
graphene is crucial for understanding and controlling its functional properties. In this study, we combine density functional theory (DFT) and machine learning (ML) to interpret how boron (B) and nitrogen (N) dopants influence the local electronic environments of
graphene. A
dataset of 415 DFT-simulated XANES spectra from 91 distinct configurations was analyzed using a region-specific approach by decomposing each spectrum into pi*, sigma*, and post-edge regions. Random forest models trained on these spectral segments identified the pi* region as the most informative for predicting key local electronic descriptors, particularly the Bader charge and mean dopant-carbon bond length. The Bader charge quantifies dopant-induced charge redistribution and local bonding polarity, directly reflecting the degree of electronic perturbation introduced by heteroatom substitution. The enhanced predictive power of the pi* region arises from its strong coupling to the perturbed pi-electron network, which captures these charge-transfer and hybridization effects more effectively than sigma* or post-edge regions. These findings establish Bader charge as a robust and physically meaningful descriptor for quantifying dopant-induced electronic modulation and demonstrate that region-resolved ML analysis of XANES spectra provides a powerful pathway to uncover structure-property relationships in doped
graphene and related materials.
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All authors Yinan Wang, Arpita Varadwaj, Teruyasu Mizoguchi, Masato Kotsugi
Mesoscale and Nanoscale Physics
Heat Conduction and Energy Relaxation in an InAs Nanowire Approaching the Clean One-Dimensional Limit
Subhomoy Haldar, Diego Subero, Mukesh Kumar et al. (+5 more)
We investigate heat conduction and energy relaxation in an InAs
semiconductor nanowire using a hybrid
semiconductor-
superconductor architecture. Local electronic temperatures are measured with an in-situ grown
quantum dot thermometer, while controlled Joule heating is applied at different locations along the wire to probe temperature gradients at sub-kelvin temperatures. With a onedimensional heat transport model, we calculate an electron-
phonon heat flow that scales as Q_{e-ph} \propto T^2.6, which is in close agreement with the T^3 dependence predicted for a clean one-dimensional electron gas coupled to a
phonon bath. We further estimate a characteristic length l_{eq} = 370 nm, beyond this length scale,
phonon-mediated heat transport dominates over heat conduction in our nanowire. Our results provide a quantitative measure of energy relaxation mechanisms in a onedimensional
semiconductor and provide a framework for studying heat flow in low-dimensional nanostructures.
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All authors Subhomoy Haldar, Diego Subero, Mukesh Kumar, Bayan Karimi, Adam Burke, Lars Samuelson, Jukka Pekola, Ville F. Maisi
Strongly Correlated Electrons
Parafermionic Truncated Wigner Approximation
Javad Vahedi and Martin Garttner
We introduce the parafermionic truncated Wigner approximation ($p$TWA), a semiclassical phase-space framework for simulating the nonequilibrium dynamics of
lattice systems with fractional exchange statistics. The method extends truncated Wigner approaches developed for bosonic and fermionic systems to $\mathbb{Z}_n$ Fock parafermions by expressing the
Hamiltonian in terms of local Hubbard operators that form a closed Lie
algebra. This representation leads to a Lie–Poisson phase-space formulation in which
quantum dynamics is approximated by
stochastic sampling of initial conditions followed by deterministic semiclassical
evolution. We
benchmark the approach in several settings, including single-site clock dynamics, the fully connected $\mathbb{Z}_n$ clock model, long-range $\mathbb{Z}_3$ clock chains, and disordered $\mathbb{Z}_3$ Fock parafermion chains. The method reproduces key features of the exact dynamics, including excitation spreading, disorder-induced suppression of transport, and the emergence of long-time imbalance plateaus. Our results demonstrate that $p$TWA provides a practical tool for exploring the dynamics of parafermionic systems in regimes where exact numerical methods are limited by
Hilbert-space growth.
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Mesoscale and Nanoscale Physics
Layer-selective hydrogenation and proton transport in twisted bilayer graphene
J. Tong, G. Chen, H. Li et al. (+13 more)
Recent work investigated
graphene's hydrogenation with independent control of the electric field, E, and charge density, n, in the
crystal and showed that the process is controlled by n. Here, we demonstrate layer-selective conductor-
insulator transitions in twisted
bilayer graphene, driven by hydrogenation at fixed n under strong E. This process is accompanied by
proton transport through the
bilayer, enabling several parallel and configurable logic gates in the devices. Selectivity arises because the large twist angle decouples the two layers' electronic systems, enabling independent control of their charge densities. Polarisation by the field then induces a charge imbalance at fixed total n, triggering hydrogenation when one of the layers' charge densities reaches the threshold for monolayer hydrogenation. Our results introduce a new type of electrode-electrolyte interface in which electrochemical processes are controlled with two decoupled 2D electron gases, opening new design opportunities for energy and information processing devices.
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All authors J. Tong, G. Chen, H. Li, E. Hoenig, M. Alhashmi, X. Zhang, D. Bahamon, G. R. Tainton, S. Sullivan-Allsop, Y. Mayamei, D. R. da Costa, L. F. Vega, S. J. Haigh, D. Domaretskiy, F. M. Peeters, M. Lozada-Hidalgo