Mesoscale and Nanoscale Physics
Magnetoresistance ratio of a point-like contact with a 1 nm wide domain wall at different MFP asymmetries
Mudasar Bashir, Andrew Sanchez, Pranaba Muduli et al. (+1 more)
This work presents a unified theoretical framework for
spin-resolved electron transport in
magnetic point contacts (PCs) in nanoscale dimensions. This work advances existing research by presenting a model which seamlessly transitions between Sharvin ballistic and Maxwell-Holm diffusive limits across the wide range of relevant contact sizes without incorporating empirical fitting factors. We analyzed the magnetoresistance (MR) of
magnetic PCs formed with two
ferromagnetic monodomains that may have parallel and antiparallel magnetization
alignment, forming a constrained domain wall approximately 1.0 nm wide. The calculated MR exhibits strong dependence on scaling parameter (normalized contact radius), ratios of
spin-dependent mean free paths, and
Fermi wave-vectors. Furthermore, the calculated MR exhibits physically meaningful behavior over a wide range of
spin-asymmetry parameters. In most regimes, the MR decreases with increasing normalized point-contact radius, becoming negative at some conditions. These results demonstrate that nanoscaled
magnetic PCs have great efficiency in terms of magnetoresistnace change and promising for application due to their simplicity.
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All authors Mudasar Bashir, Andrew Sanchez, Pranaba Muduli, Artur Useinov
Statistical Mechanics
Quantum Brownian Motion: proving that the Schmid transition belongs to the Berezinskii-Kosterlitz-Thouless universality class
Francesco G. Capone, Antonio de Candia, Vittorio Cataudella et al. (+4 more)
We investigate the
equilibrium properties of a
quantum Brownian particle moving in a periodic potential, specifically addressing the nature of the dissipation-driven Schmid transition in the Ohmic regime. By employing World-Line Monte Carlo in the path-integral formalism and introducing a specific binary order parameter, we demonstrate that the transition belongs to the Berezinskii-Kosterlitz-Thouless universality class. This classification is substantiated through finite-size scaling analysis that reveals the characteristic logarithmic decay of the correlation functions associated with the order parameter at the critical point.
Quantum phase transition turns out to be extremely fragile: it disappears in both over- and sub-Ohmic dissipation regimes. Crucially, we find that the presence of the periodic potential does not alter the localization properties in the sub-Ohmic and super-Ohmic regimes, where the system exhibits the same qualitative behavior as the free
quantum Brownian particle. These findings highlight that the emergence of critical behavior is strictly governed by the low-frequency form of the environmental spectral function, which determines the long-range temporal decay of the dissipative kernel.
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All authors Francesco G. Capone, Antonio de Candia, Vittorio Cataudella, Rosario Fazio, Naoto Nagaosa, Carmine Antonio Perroni, Giulio De Filippis
Disordered Systems and Neural Networks · Cross-listed from physics.flu-dyn
Flow of yield stress fluid in a percolating network
Nathan Abitbol, Alex Hansen, Alberto Rosso et al. (+1 more)
We study the flow of a Bingham yield-stress fluid in a pore network model where the throats have radii drawn from a uniform distribution. We consider the case in which a fraction of the largest radii is blocked. The fluid can flow only through the percolating cluster that exists when the fraction is above the percolation threshold. Two distinct flow regimes are identified: above the percolation threshold the flow curve can be characterized by deterministic values of the critical pressure drop, permeability, and other observables, with subleading fluctuations that we quantify. At the percolation threshold these quantities become non-self-averaging, and their scaling is governed exclusively by the critical percolation backbone, independent of the specific realization of the radii.
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All authors Nathan Abitbol, Alex Hansen, Alberto Rosso, Laurent Talon
Strongly Correlated Electrons
Fractionalized anyons in counterflowing Quantum Hall Liquids
Jun-Xiao Hui, T.H. Hansson and Egor Babaev
A key property of topologically ordered systems, such as
Quantum Hall states, is the existence of excitations obeying fractional
quantum statistics - anyons. We develop a theory for multicomponent counterflow states where an ordinary Laughlin quasiparticle can split into fractional
vortices carrying fractions of its charge and statistical angle. There are two phases, separated by a
quantum phase transition, where in the first, although observable, the fractionalized charges are asymptotically confined. In the second phase, they are unconfined anyons and the topological order is different from that of the Laughlin state.
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Soft Condensed Matter
Energy-Efficient Control of Interacting Microscopic Systems: When Longer Paths Save Energy
Samuel Monter, Lars T. Stutzer, Sarah A.M. Loos et al. (+1 more)
We experimentally and theoretically study the thermodynamically optimal control of interacting multiple-particle systems, focusing on collections of colloidal particles individually confined in optical traps. We investigate protocols that transport the system between prescribed trap configurations within a fixed time in the most energy efficient way. For Markovian systems with conservative pairwise interactions, we establish a general result in the low-noise limit: optimal particle trajectories are linear in space and time, corresponding to steady straight-line motion, irrespective of the specific interaction potential, even for nonlinear forces. Thus, conservative interactions do not modify the geometry of the optimal paths. This property breaks down in the presence of strong noise or nonconservative interactions. For the paradigmatic case of hydrodynamic coupling, we demonstrate experimentally that optimal control can involve curved trajectories that significantly reduce the energetic cost by exploiting collectively generated fluid flows. The emergence of curved paths as optimal solutions highlights a fundamental distinction between non-interacting and interacting systems and reveals a cooperative mechanism for energy-efficient control.
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All authors Samuel Monter, Lars T. Stutzer, Sarah A.M. Loos, Clemens Bechinger
Statistical Mechanics
Percolation and Criticality in Hyperuniform Networks
Yongyi Wang, Jaeuk Kim, Yang Jiao et al. (+3 more)
Disordered hyperuniform many-particle systems exhibit an anomalous suppression of density fluctuations on macroscopic length scales relative to those of conventional disordered systems. Here we investigate percolation on Delaunay triangulation networks constructed from disordered stealthy hyperuniform (SHU) point configurations, a subclass of hyperuniform systems whose structure factor vanishes over a finite range of wavevectors near the origin and whose degree of stealthiness is controlled by $\chi$, with Poisson point configurations included for comparison. We consider a non-uniform bond percolation process in which bond occupation probabilities decrease with Euclidean edge length, and determine the percolation thresholds and critical exponents using finite-size scaling. We find that SHU networks exhibit lower percolation thresholds than Poisson networks. Notably, the percolation threshold of SHU networks decreases with the stealthiness parameter $\chi$, indicating that system-spanning connectivity emerges more readily as short-range order increases. All systems considered are consistent with the standard 2D percolation universality class, although the Poisson and low-$\chi$ SHU networks exhibit stronger corrections to scaling. Our work extends previous studies on transport properties of SHU systems from continuum two-phase media to networks. These results open new avenues for optimizing the resilience of statistically homogeneous disordered networks.
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All authors Yongyi Wang, Jaeuk Kim, Yang Jiao, Izabella Stuhl, Salvatore Torquato, Réka Albert
Materials Science
Optimizing Density Functional Theory for Strain-Dependent Magnetic Properties of Monolayer MnBi$_2$Te$_4$ with Diffusion Monte Carlo
Jeonghwan Ahn, Swarnava Ghosh, Seoung-Hun Kang et al. (+6 more)
Monolayer MnBi$_{2}$Te$_{4}$ (MBT) is an intrinsically
magnetic topological
insulator whose
magnetic response is strongly affected by strain and electron correlation. In density functional theory with an on-site Hubbard correction (DFT+$U$), however, predictions vary substantially with the choice of Hubbard $U$, making it difficult to establish a reliable strain-dependent picture of magnetism in this system. Here we use
diffusion Monte Carlo (DMC) to
benchmark DFT+$U$ for monolayer MBT and to determine an effective $U$ as a function of strain. We find that the predicted
magnetic phase diagram depends strongly on $U$, indicating that a single fixed value is not sufficient across the strain range considered. DMC nodal optimization further shows that the optimal $U$ increases with strain magnitude and is well captured by a simple quadratic form. When this DMC-informed strain-dependent $U$ is used in PBE+$U$, the calculated Mn local moments are brought into close agreement with DMC and are improved relative to commonly used fixed-$U$ choices. These results show that, for monolayer MBT, correlation strength itself should be treated as strain dependent, and they provide a practical many-body-guided strategy for improving strain-dependent DFT+$U$ descriptions of
magnetic van der Waals materials.
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All authors Jeonghwan Ahn, Swarnava Ghosh, Seoung-Hun Kang, Dameul Jeong, Markus Eisenbach, Young-Kyun Kwon, Fernando A. Reboredo, Jaron T. Krogel, Mina Yoon
Soft Condensed Matter
Mechanical anisotropy of 3D-printed digital materials at large strains
Seunghwan Lee, Gisoo Lee, Seounghee Yun et al. (+3 more)
3D-printed digital materials whose mechanical behavior travels between those from thermoplastic to rubbery
polymers have become increasingly important. However, their mechanical functionalities have not been fully exploited due to intrinsic mechanical anisotropy resulting from microstructural heterogeneity. Here, we combine mechanical testing, microscopy analysis and micromechanical modeling for a comprehensive understanding of complex deformation mechanisms responsible for the printing-orientation-dependent nonlinear mechanical behavior of digital materials at small to large strains. Towards this end, we construct representative volume elements that account for highly anisotropic microstructural features resulting from the printing-orientation-dependent
diffusion and mixing between photocurable base resins. We then demonstrate, through micromechanical analysis, that stable compressive deformation of well-aligned elliptical hard thermoplastic inclusions embedded within the surrounding soft rubbery
matrix gives rise to initial elastic anisotropy. Our experimental and micromechanical modeling results also show that the interplay between buckling instability and plastic deformation of the high-aspect-ratio hard domains governs mechanical anisotropy at large strains as well as the printing-orientation-dependent resilience and energy dissipation capabilities in these digital materials.
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All authors Seunghwan Lee, Gisoo Lee, Seounghee Yun, Sumin Lee, Jeonyoon Lee, Hansohl Cho
Materials Science
Pressure-driven vibrational and structural peculiarities in the honeycomb layered magnetoelectrics Mn4(B)2O9 (B= Nb, Ta)
Rajesh Jana, Afsal S Shajahan, Boby Joseph et al. (+6 more)
The high-pressure behavior of two Mn-based honeycomb-structured magnetoelectric materials, Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO), was investigated using Raman spectroscopy, synchrotron x-ray diffraction, and density functional theory (DFT) calculations. In MTO, the application of a small pressure of only 0.5 GPa induces an isostructural transition driven by local
symmetry breaking. With further increase in pressure, three additional isostructural transitions are observed at about 3.2, 6, and 10 GPa, followed by the onset of a long-range structural transition near 14 GPa, where the ambient P-3c1 phase begins to transform into a P2/c phase. These two phases coexist up to 27 GPa. The Nb analogue, MNO, also exhibits similar isostructural transitions at about 2, 6.6, and 10 GPa. However, the onset of the mixed P2/c and P-3c1 phases occurs at a slightly lower pressure of 12.5 GPa, with phase coexistence extending up to 26.5 GPa. These long-range transitions are supported by pressure-dependent enthalpy changes obtained from DFT calculations. Rietveld refinement reveals pronounced anisotropic
lattice compression, with a 42 to 49 percent difference between the c and a axes, leading to a notable reduction in the c/a ratio. This anisotropy may strengthen interlayer coupling and promote
magnetic ordering under compression, consistent with the appearance of Raman modes similar to those reported at low temperatures, together with anomalous changes in Raman mode linewidth and intensity. The marked changes in Raman self-energy parameters, anomalies in the reduced pressure-Eulerian strain profile, and the onset of local
symmetry breaking at much lower pressures in MTO than in MNO highlight the important role of differences in
spin-orbit coupling strength and orbital hybridization associated with Nb5+ and Ta5+ cations.
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All authors Rajesh Jana, Afsal S Shajahan, Boby Joseph, Brahmananda Chakraborty, Irshad K A, Anuj Upadhyay, Alka Garg, Rekha Rao, Thomas Meier
Materials Science
Nonmagnetic Ground State of Rutile RuO$_2$ from Diffusion Quantum Monte Carlo
Jeonghwan Ahn, Seoung-Hun Kang, Panchapakesan Ganesh et al. (+1 more)
Rutile RuO$_2$ has been proposed as an altermagnet, but its bulk
magnetic ground state is still under debate because density-functional calculations give conflicting predictions. Using fixed-node
diffusion quantum Monte Carlo, we find that stoichiometric bulk RuO$_2$ is nonmagnetic in the pristine structure, lying 23(9) meV per formula unit below the lowest
antiferromagnetic state considered. A 3$\%$ compressive strain instead stabilizes antiferromagnetism, placing RuO$_2$ near a strain-tunable
magnetic instability and helping reconcile apparently conflicting experimental reports.
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All authors Jeonghwan Ahn, Seoung-Hun Kang, Panchapakesan Ganesh, Jaron T. Krogel
Statistical Mechanics
Conditional Ergodicity and Universal Fluctuations in Weak Ergodicity Breaking
Dan Shafir and Stanislav Burov
Time averages extracted from single-particle trajectories in complex media often vary strongly from one trajectory to another, even for long measurement times. Such persistent trajectory-to trajectory scatter is commonly observed in anomalous
diffusion and signals weak ergodicity breaking driven by scale-free trapping. Here we identify conditional ergodicity: conditioning on a natural internal clock restores self-averaging of time-averaged observables. Combining conditional ergodicity with the
stochastic mapping between the internal clock and physical time implies a universal law: once rescaled by their mean, time-averaged transport coefficients in systems exhibiting weak ergodicity breaking follow the Mittag-Leffler distribution. We demonstrate this universality across multiple models of disordered media displaying anomalous
diffusion.
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Superconductivity
Zi-Jie Yan, Zihao Wang, Bing Xia et al. (+13 more)
Iron-based
superconductors are a fascinating family of materials in which multiple electronic bands and strong
antiferromagnetic (AFM) correlations are key ingredients for competing ground states, including antiferromagnetism, electronic nematicity, and unconventional superconductivity. FeTe, unlike its superconducting isostructural counterpart FeSe, has long been regarded as an AFM metal sans superconductivity. In this work, we employ molecular beam epitaxy to grow FeTe films and perform post-growth annealing under a Te flux. By performing
spin-polarized scanning tunneling microscopy and spectroscopy, we demonstrate that the AFM order in as-grown FeTe films is induced by interstitial Fe atoms that disrupt the ideal 1:1 stoichiometry. Remarkably, the removal of these interstitial Fe atoms through Te annealing yields stoichiometric FeTe films that show no AFM order and instead exhibit robust superconductivity with a critical temperature of ~13.5K. This superconducting state is further confirmed by the observation of Cooper pair tunneling, zero electrical resistance, and the Meissner effect. Therefore, our results demonstrate that stoichiometric FeTe is inherently a
superconductor, overturning a long-held view that it is an AFM metal. This work clarifies the origin of superconductivity in FeTe-based
heterostructures and demonstrates the importance of stoichiometry control in understanding the competition between AFM and superconductivity in iron-based
superconductors.
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All authors Zi-Jie Yan, Zihao Wang, Bing Xia, Stephen Paolini, Ying-Ting Chan, Nikalabh Dihingia, Hongtao Rong, Pu Xiao, Kalana D. Halanayake, Jiatao Song, Veer Gowda, Danielle Reifsnyder Hickey, Weida Wu, Jiabin Yu, Peter J. Hirschfeld, Cui-Zu Chang
Strongly Correlated Electrons
SU(N) Quantum Spin Model with Weak and Strong First-Order Néel to Valence-Bond Solid Transitions
Ryan Flynn and Anders W. Sandvik
We introduce an SU($N$) symmetric two-dimensional
quantum spin model, the X-Q model, which hosts a ground state transition between Néel
antiferromagnetic and spontaneously dimerized states. The Q terms are products of two adjacent singlet projectors on first-neighbor sites, as in the often studied J-Q model (where J is the Heisenberg exchange), while the X terms are products of two permutation operators on second-neighbor sites.
Quantum Monte Carlo simulations reveal close proximity to a deconfined
quantum critical point for $N=2$, as in the J-Q model. However, for $N>2$ the transformation becomes strongly first order, contrary to conventional expectations that increasing $N$ should weaken discontinuities. We attribute this behavior to the inability of the X term, which dominates at the transition for large $N$, to induce significant U(1) fluctuations of the dimer pattern. These results provide insights into the microscopic interactions that support deconfined criticality.
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Quantum Gases
Efimovian Phonon Production for an Analog Coasting Universe in Bose-Einstein Condensates
Yunfei Xue, Jiabin Wang, Li Chen et al. (+2 more)
Efimov effects arise from scale invariance, a fundamental
symmetry with universal implications. While spatial Efimov physics has been extensively studied, realizing its temporal counterpart remains challenging, as it requires a dynamical system that breaks time-translation
symmetry yet preserves the essential time-scaling
symmetry. Analog cosmology offers a powerful platform to address this challenge, bridging the domains of Efimov physics and cosmology. Here, we predict a temporal Efimov effect in an analog linearly expanding universe realized with a quasi-two-dimensional Bose-Einstein condensate. The invariance of
phonon mode equations under time rescaling leads to particle production with two distinct dynamics: power-law growth and log-periodic oscillations, with the latter being the hallmark signature of the Efimov effect. Furthermore, these dynamics map directly onto sub- and super-horizon cosmological modes. Our predictions can be directly verified through time-averaged measurements of the density-fluctuation spectrum $S_{k}(t)$ in current experiments.
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All authors Yunfei Xue, Jiabin Wang, Li Chen, Chenwei Lv, Ren Zhang
Quantum Gases
Tuning Topological Charge and Gauge Field Anisotropy in a Spin-1 Synthetic Monopole
Nicholas Milson, Arina Tashchilina, Kathleen Tamura et al. (+2 more)
Higher-dimensional
Hilbert spaces in
quantum simulation, as in all
quantum science, expand the range of accessible phenomena. In this work, we experimentally realize a synthetic monopole using an ultracold
spin-1 ensemble, where the monopole charge is quantified by the topologically invariant first Chern number and sources a synthetic
magnetic field quantified by the Berry
curvature. By using a three-level system with tunable
spin-tensor coupling, we introduce anisotropy to the field, directly measure the Chern number, and observe a topological phase transition. We verify the robustness of the monopole's topological charge under deformation, and observe signatures of the topological phases using
spin-texture and Majorana-
star measurements. This work demonstrates
spin-tensor coupling as a tuning parameter for engineering both
geometric anisotropy and a rich topological phase space.
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All authors Nicholas Milson, Arina Tashchilina, Kathleen Tamura, Douglas Florizone, Lindsay J. LeBlanc
Strongly Correlated Electrons
Theory of Magnetoacoustic Resonance to Probe Multipole Effects Due to a Crystal Field Quartet
Mikito Koga and Masashige Matsumoto
We present a new method of acoustically driven resonance that probes octupole degrees of freedom as well as a quadrupole usually hidden by the
magnetic properties of a
crystal field quartet. A characteristic of the quadrupole is reflected in the anisotropic resonance transition rate, which depends on the propagation direction of a surface acoustic wave under an external
magnetic field parallel to a typical crystallographic axis. The transition rate is modulated by the anisotropic Zeeman splitting associated with octupoles. We demonstrate how to obtain information about the quartet quadrupole-strain coupling and evaluate the anisotropic octupole effect quantitatively. We also discuss the applicability of our method to identifying a quadrupole order parameter using a multipole-multipole interaction model. For large excitation energy gaps under strong
magnetic fields, we propose a
photon-assisted magnetoacoustic resonance formulated on the basis of the Floquet theory.
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Materials Science
Pressure and strain tuning of the alternating bilayer-trilayer Ruddlesden-Popper nickelate: crystal and electronic structure
Huan Wu, Yi-Feng Zhao and Antia S. Botana
We use first-principles calculations to investigate the
crystal and electronic structure of the hybrid
bilayer-trilayer Ruddlesden-Popper (RP) nickelate La$_7$Ni$_5$O$_{17}$ under hydrostatic pressure and biaxial compressive strain. By analyzing the irreducible representations of the dynamically unstable
phonon modes in the high-
symmetry $P4/mmm$ structure, we identify a dynamically stable lower-
symmetry $C2/c$ structure containing octahedral tilts. The application of both pressure and compressive strain tends to suppress the octahedral tilts, effectively tetragonalizing the structure, in analogy with the conventional RPs. The electronic structure under hydrostatic pressure and strain has similarities, but it differs in the position of the $d_{z^2}$ bonding band from the trilayer block. This band crosses the
Fermi level at a pressure of 30 GPa, but it remains below it for any level of compressive strain. This strain-induced modification mirrors the electronic structure changes observed in the conventional
bilayer nickelate.
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Materials Science
PFP/MM: A Hybrid Approach Combining a Universal Neural Network Potential with Classical Force Fields for Large-Scale Reactive Simulations
Yu Miyazaki, Atsuhiro Tomita, Akihide Hayashi et al. (+3 more)
Universal machine-learning interatomic potentials (uMLIPs) enable reactive molecular simulations with near-DFT accuracy, yet applying them efficiently to large, realistic condensed-phase systems remains computationally demanding. Here we present PFP/MM, a hybrid approach that combines a uMLIP, PreFerred Potential (PFP), with molecular mechanics (MM) to enable both large-scale and long-time simulations that are challenging for uMLIP-only calculations. Using an alanine dipeptide in explicit water, we achieve multi-ns/day enhanced sampling and obtain a Ramachandran plot consistent with established basins. For an intramolecular nucleophilic addition reaction in a polar solvent environment, we reproduce the expected solvent-induced stabilization in the free-energy profile. We further apply the approach to a cytochrome P450 Compound I hydroxylation reaction and obtain a free-energy landscape consistent with the accepted reaction mechanism. These results demonstrate that uMLIP-based reactive simulations can be applied to diverse condensed-phase processes in large, realistic environments.
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All authors Yu Miyazaki, Atsuhiro Tomita, Akihide Hayashi, So Takamoto, Mizuki Takemoto, Hodaka Mori
Statistical Mechanics
Population Annealing as a Discrete-Time Schrödinger Bridge
Masayuki Ohzeki
We present a theoretical framework that reinterprets Population Annealing (PA) through the lens of the discrete-time Schrödinger Bridge (SB) problem. We demonstrate that the heuristic reweighting step in PA is derived by analytically solving the Schrödinger system without iterative computation via instantaneous projection. In addition, we identify the thermodynamic work as the optimal control potential that solves the global variational problem on path space. This perspective unifies non-
equilibrium thermodynamics with the
geometric framework of optimal transport, interpreting the Jarzynski equality as a consistency condition within the Donsker-Varadhan variational principle, and elucidates the thermodynamic optimality of PA.
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Statistical Mechanics · Cross-listed from quant-ph
Qudit Implementation of the Rodeo Algorithm for Quantum Spectral Filtering
Julio Cesar Siqueira Rocha and Rodrigo Alves Dias
Qudits, the multi-level generalization of
qubits, provide a natural extension of the binary paradigm in
quantum computation and offer new opportunities to enhance algorithmic performance. Beyond their direct applicability to the simulation of multi-level
quantum systems, higher-dimensional ancillae can improve sampling efficiency in
quantum algorithms by enabling the simultaneous implementation of multiple control operations, thereby reducing
circuit complexity. In this work, we pursue three main objectives. First, we present a formulation of the Rodeo
algorithm employing a general $d$-level ancilla qudit. Second, we introduce the concept of the Rodeo kernel, defined as a two-frequency interferometer, which acts as a spectral filter in the energy domain. Finally, we propose a microcanonical protocol for the Rodeo
algorithm. This protocol enables the estimation of entropic quantities through a single energy sweep and admits a natural interpretation as a Gaussian convolution of the density of states. To support the theoretical analysis, we perform numerical evaluations of the corresponding
quantum circuit using ancilla qudits of dimensions three, four, and five. The simulations are performed for the one-dimensional Ising model, considering both
spin-$\sfrac{1}{2}$ and
spin-$1$ particles. The ancilla qutrit implementation exhibits an $18\%$ reduction in fluctuations compared to the
qubit implementation. Our results show that qudits provide a framework for spectral analysis and thermodynamic characterization of multi-level
quantum systems.
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Mesoscale and Nanoscale Physics
Summary overview of present state of basic electrostatic field electron emission theory
Richard G. Forbes
This technical note provides a high-level overview of the present state of basic field electron emission (FE) theory, as suitable for use in the context of technological applications of FE theory. At present there is much theoretical confusion in FE literature, and a partial breakdown of the peer review system. Even in sensitive technological contexts, many papers have stated and used out-of-date theory that makes current-density predictions that are several hundred times less than those of modern FE theory. A primary aim of this note is to help reduce the confusion and error in future published FE literature. It is not intended as a detailed review of FE theory.
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Statistical Mechanics
Casimir versus Helmholtz forces in the Gaussian model: exact results for Dirichlet–Dirichlet, Neumann–Dirichlet, Neumann–Neumann, and periodic boundary conditions
Daniel Dantchev and Joseph Rudnick
We present results and compare the behavior of two fluctuation-induced forces pertinent for their corresponding ensembles: the critical Casimir force in the grand canonical (fixed external field $h$) one and the critical Helmholtz force in the canonical (fixed average value of the order parameter $m$) one. We do so by deriving exact results for their behavior near the bulk critical point at $T=T_c$ in the three-dimensional Gaussian model. We consider Dirichlet-Dirichlet, Neumann-Dirichlet, Neumann-Neumann, and periodic boundary conditions. For every boundary condition examined, we confirm that both forces follow a finite-size scaling. We find that for Dirichlet-Dirichlet and Neumann-Dirichlet boundary conditions the Casimir and the Helmholtz force differ from each other. For Dirichlet-Dirichlet boundary conditions the Casimir force is always attractive, while the Helmholtz force can be both attractive and repulsive as a function of $T$ and $m$. For Neumann-Dirichlet boundary conditions the Casimir force changes sign from repulsive to attractive with increase of $h$, while the Helmholtz force stays always repulsive. Under periodic and Neumann-Neumann boundary conditions the Casimir force and the Helmholtz force coincide - the first does not depend on $h$, while the latter does not depend on $m$; they are always attractive.
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Soft Condensed Matter
Dynamics of particle lane formation in confined viscoelastic fluids under shear
Hiroto Yokoyama, Masanori Honda, Rinya Miyakawa et al. (+5 more)
Simple shear flow can induce flow-aligned chain formation of particles suspended in viscoelastic fluids. Although this phenomenon has been reported for decades, direct in situ measurements of the
alignment dynamics and particle trajectories during chain formation remain limited. Here, we develop an in situ observation platform based on parallel rotating disks separated by a gap comparable to the particle diameter, enabling simultaneous observation of particle
alignment under radially varying shear rates. The narrow gap strongly confines particle motion, thereby enhancing hydrodynamic interactions and collision events between particles. Using a viscoelastic fluid
embedding zircon particles as the sample, we find that
alignment occurs once the local particle Weissenberg number exceeds unity (Wi$_\mathrm{p} \geq 1$), defined using an effective shear rate based on the wall velocity and the available gap width. Particle tracking further reveals a back-and-forth shuttling motion that accompanies the
alignment process. Using the image brightness in a colored fluid as a proxy for out-of-plane position, we show that the shuttling originates from vertical displacement of the particles. We further construct a minimal
agent-based model in which the vertical particle position follows a Ginzburg-Landau-type double-well potential, and demonstrate that collision-driven accumulation emerges in numerical simulations. In the strongly confined geometry,
alignment occurs by an effective attraction due to collision, which is reminiscent of motility-induced clustering often observed in active matter.
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All authors Hiroto Yokoyama, Masanori Honda, Rinya Miyakawa, Yuki Shinohara, Kota Nakamura, Kojiro Otoguro, Kiwamu Yoshii, Yutaka Sumino
Mesoscale and Nanoscale Physics
3D tomography of exchange phase in a Si/SiGe quantum dot device
Dylan Albrecht, Sarah Thompson, N. Tobias Jacobson et al. (+1 more)
The exchange interaction is a foundational building block for the operation of
spin-based
quantum processors. Extracting the exchange interaction coefficient $J(\mathbf{V})$, as a function of gate electrode voltages, is important for understanding disorder, faithfully simulating device performance, and operating
spin qubits with high fidelity. Typical coherent measurements of exchange in
spin qubit devices yield a modulated cosine of an accumulated phase, which in turn is the time integral of exchange. As such, extracting $J(\mathbf{V})$ from experimental data is difficult due to the ambiguity of inverting a cosine, the sensitivity to noise when unwrapping phase, as well as the problem of inverting the integral. As a step toward obtaining $J(\mathbf{V})$, we tackle the first two challenges to reveal the accumulated phase, $\phi(\mathbf{V})$. We incorporate techniques from a wide range of fields to robustly extract and model a 3D phase volume for
spin qubit devices from a sequence of 2D measurements. In particular, we present a measurement technique to obtain the wrapped phase, as done in phase-shifting digital holography, and utilize the max-flow/min-cut phase unwrapping method (PUMA) to unwrap the phase in 3D voltage space. We show this method is robust to the minimal observed drift in the device, which we confirm by increasing scan resolution. Upon building a model of the extracted phase, we optimize over the model to locate a minimal-gradient $\pi$ exchange pulse point in voltage space. Our measurement protocol may provide detailed information useful for understanding the origins of device variability governing device yield, enable calibrating device models to specific devices during operation for more sophisticated error attribution, and enable a systematic optimization of
qubit control. We anticipate that the methods presented here may be applicable to other
qubit platforms.
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All authors Dylan Albrecht, Sarah Thompson, N. Tobias Jacobson, Ryan Jock