Superconductivity
Strain-induced structural change and nearly-commensurate diffuse scattering in the model high-temperature superconductor HgBa$_2$CuO$_{4+\delta}$
Mai Ye, Wenshan Hong, Tom Lacmann et al. (+9 more)
We investigate the strain response of underdoped HgBa$_2$CuO$_{4+\delta}$ (Hg1201), by synchrotron X-ray diffraction and corresponding simulations of thermal diffuse scattering. The compression in the crystallographic $a$ direction leads to relatively small expansion in the $b$ and $c$ directions, with Poisson ratios $\nu_{ba}$=0.16 and $\nu_{ca}$=0.11, respectively. However, the Cu-O distance in the $c$ direction exhibits a notable 0.9% increase at 1.1% $a$-axis compression. We further find strain-induced diffuse scattering which corresponds to a new type of two-dimensional charge correlation. Interestingly, this signal is insensitive to the onset of superconductivity and instead corresponds to a short-range, nearly commensurate modulation with a wave vector close to (0.5, 0, 0) and a correlation length of approximately four unit
cells. It closely resembles the charge order theoretically predicted in the phase diagram of the
spin-
liquid model with resonating valence bonds on a square
lattice.
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All authors Mai Ye, Wenshan Hong, Tom Lacmann, Mehdi Frachet, Igor Vinograd, Gaston Garbarino, Sofia-Michaela Souliou, Michael Merz, Rolf Heid, Amir-Abbas Haghighirad, Yuan Li, Matthieu Le Tacon
Statistical Mechanics
Koopman Mode Decomposition of Thermodynamic Dissipation in Nonlinear Langevin Dynamics
Daiki Sekizawa, Sosuke Ito and Masafumi Oizumi
Nonlinear oscillations are commonly observed in complex systems far from
equilibrium, such as living organisms. These oscillations are essential for sustaining vital processes, like neuronal firing, circadian rhythms, and heartbeats. In such systems, thermodynamic dissipation is necessary to maintain oscillations against noise. However, due to their nonlinear dynamics, it has been challenging to determine how the characteristics of oscillations, such as frequency, amplitude, and coherent patterns across elements, influence dissipation. To resolve this issue, we employ Koopman mode decomposition, which recasts nonlinear dynamics as a linear
evolution in a function space. This linearization allows the dynamics to be decomposed into temporal oscillatory modes coherent across elements, with the Koopman
eigenvalues determining their frequencies. Using this method, we decompose thermodynamic dissipation caused by nonconservative forces into contributions from oscillatory modes in overdamped nonlinear Langevin dynamics. We show that the dissipation from each mode is proportional to its frequency squared and its intensity, providing an interpretable, mode-by-mode picture. In the noisy FitzHugh–Nagumo model, we demonstrate the effectiveness of this framework in quantifying the impact of oscillatory modes on dissipation during nonlinear phenomena like coherent resonance and bifurcation. For instance, our analysis of coherent resonance reveals that the greatest dissipation at the optimal noise intensity is supported by a broad spectrum of frequencies, whereas at non-optimal noise levels, dissipation is dominated by specific frequency modes. Our work offers a general approach to connecting oscillations to dissipation in noisy environments and improves our understanding of diverse oscillation phenomena from a nonequilibrium thermodynamic perspective.
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Superconductivity
High Pressure Superconducting transition in Dihydride BiH$_2$ with Bismuth Open-Channel Framework
Liang Ma, Xin Yang, Mei Li et al. (+13 more)
Metal hydrides MHx with low hydrogen content are not expected to show high-Tc superconductivity owing to the low hydrogen-derived electronic density of states at
Fermi level and the limited hydrogen contribution to electron-
phonon coupling strength. In this work, we report on the successful synthesis of a novel bismuth dihydride
superconductor, Cmcm-BiH$_2$, at approximately 150 GPa, and the discovery of superconductivity with Tc about 62 K at 163 GPa, marking the first instance of
superconductor among the MH$_2$-type metal dihydrides. Cmcm-BiH$_2$ adopts a unique host-guest type structure, in which the Bi atoms via weak Bi-Bi covalent bonds form a three-dimensional open-channel framework that encapsulates H$_2$-like molecules as guests, thereby broadening the structural diversity of hydrides under high pressures. The occurrence of superconductivity is evidenced by a sharp drop of resistivity to zero and the characteristic downward shift of Tc under applied
magnetic fields. Notably, Cmcm-BiH$_2$ remains stable down to at least 97 GPa during decompression, with the calculated lowest pressure for dynamic stability of 10 GPa. In-depth analysis reveals that the covalent bismuth open-channel structure forms metallic conduction channels, dominates the electronic states near the
Fermi level, and contributes approximately 51% of the total $lambda$ in Cmcm-BiH$_2$, distinguishing it from known high-pressure hydride
superconductors. These findings highlight the critical role of non-hydrogen elements in producing superconductivity and open new avenues for the design and optimization of high-Tc hydride
superconductors.
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All authors Liang Ma, Xin Yang, Mei Li, Pengfei Shan, Ziyi Liu, Jun Hou, Sheng Jiang, Lili Zhang, Chuanlong Lin, Pengtao Yang, Bosen Wang, Jianping Sun, Yang Ding, Huiyang Gou, Haizhong Guo, Jinguang Cheng
Statistical Mechanics
Faradaic and capacitive charging of an electrolyte-filled pore in response to a small applied potential
Timur Aslyamov, Massimiliano Esposito and Mathijs Janssen
Electrochemical devices often charge both through Faradaic reactions and electric double layer formation. Here, we study these coupled processes in a model system of a long electrolyte-filled pore subject to a small suddenly-applied potential, close to the
equilibrium potential $\Psi^\text{eq}$ at which there is no net Faradaic charge transfer. Specifically, we solve the coupled Poisson-Nernst-Planck and Frumkin-Butler-Volmer equations by asymptotic approximations, using the pore's small inverse aspect ratio as the small parameter. In the early-time limit, the reaction-
diffusion equations yield an extended Faradaic transmission line model that includes a voltage source, $\Psi_\text{eq}$, biasing the Faradaic reactions, captured by the resistance $R_F$. In the long-time limit, the model exhibits a nontrivial potential of zero charge, $\Psi_\text{pzc} = \Psi_\text{eq}[1 - \hat{Z}(0)/R_F]$, where $\hat{Z}(0)$ is the experimentally accessible zero-frequency impedance of the system. This expression provides a new means to experimentally measure the Faradaic contribution to $\Psi_\text{pzc}$.
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Soft Condensed Matter · Cross-listed from physics.bio-ph
Cell Competition Driven by Secreted Ligands: Modeling Liver Metastasis of Colorectal Cancer
Hossein Nemati, Saskia Jacoba Elisabeth Suijkerbuijk and Joost de Graaf
Cell competition in multicellular organisms has been shown to play a critical role during the development of organisms, cancer progression, and in the establishment and maintenance of tissue homeostasis. Various mechanisms of
cell competition have been identified, including active elimination via mechanical forces or induced apoptosis, as well as competition for nutrients and other beneficial factors. A recent experiment demonstrated hallmarks of
cell competition, associated with
cell cycle dynamics, between liver progenitor
cells and colorectal cancer
cells [Krotenberg Garcia et al., iScience 27, 109718 (2024)]. However, a mechanistic explanation for this form of competition remains lacking. Here, we present a mean-field model of competition for signaling ligands, coupled with
cell cycle dynamics, to provide such an understanding. Our model captures the salient features of the experiment, including population dynamics and
cell cycle variations. We demonstrate that secretion of a beneficial factor by
cells, coupled with the enhanced uptake efficiency of cancer
cells, suffices to reproduce the experimental outcome. Our model, reminiscent of competition for secreted growth factors, provides insight into the minimal level of complexity required to achieve the observed competitive outcome as well as its link to
cell cycle dynamics. It can also serve as a general framework for studying biological populations with growth-stage-dependent competition over consumer-produced products.
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Mesoscale and Nanoscale Physics
Controlling bubble and skyrmion lattice order and dynamics via stripe domain engineering in ferrimagnetic Fe/Gd multilayers
Tim Titze, Sabri Koraltan, Timo Schmidt et al. (+6 more)
Ferrimagnetic Fe/Gd multilayers host maze-like stripe domains that transform into a disordered bubble/skyrmion
lattice under out-of-plane
magnetic fields at ambient temperature. Femtosecond magneto-optics distinguishes these textures via their distinct coherent breathing dynamics. Crucially, applying a brief in-plane “set”
magnetic field to the stripe state enhances both frequency and amplitude of the bubble/skyrmion
lattice breathing mode. Lorentz transmission electron microscopy,
magnetic force microscopy, and micromagnetic simulations reveal that this enhancement arises from field-aligned stripes nucleating a dense, near-hexagonal bubble/skyrmion
lattice upon out-of-plane field application, with strong indications for a pure skyrmion
lattice. Thus, modifying the initial domain configuration by in-plane fields enables precise control of coherent magnetization dynamics on picosecond to nanosecond timescales and potentially even of
topology.
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All authors Tim Titze, Sabri Koraltan, Timo Schmidt, Mailin Matthies, Amalio Fernández-Pacheco, Dieter Suess, Manfred Albrecht, Stefan Mathias, Daniel Steil
Materials Science · Cross-listed from physics.app-ph
Combining metal dewetting and lateral etching for the scalable top-down fabrication of GaN nanowire arrays with independently tunable diameter and spacing
Jingxuan Kang, Rose-Mary Jose, Oliver Brandt et al. (+1 more)
The top-down fabrication of nanowires based on patterning via metal dewetting is a cost-effective and scalable approach that is particularly suited for applications requiring large arrays of nanowires. Advantageously, the nanowire diameter can be tailored by the initial metal film thickness. However, we show here that metal dewetting inherently leads to a coupling between the nanowire diameter and spacing. To overcome this limitation, we introduce two strategies that are exemplified for GaN nanowires: (i) modification of the surface and interface energies within the dewetting system, and (ii) thinning of the nanowires by lateral etching. In the first strategy, GaN(0001), SiOx, and SiNx substrate surfaces are combined with Au, Pt, and Pt-Au
alloy dewetting metals to tune the dewetting behavior. The differences in interface energies affect the relation between nanowire diameter and spacing, albeit within a limited range. The second strategy adds a lateral etching step to the conventional top-down nanowire fabrication process. This step at the same time reduces the nanowire diameter and increases the spacing, thus enabling combinations beyond the constraints of metal dewetting alone. When in addition different initial nanowire diameters are employed, it is possible to independently control diameter and spacing over a substantially extended range. Therefore, the inherent limitation of conventional dewetting-based patterning approaches for the top-down fabrication of nanowires is overcome.
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All authors Jingxuan Kang, Rose-Mary Jose, Oliver Brandt, Lutz Geelhaar
Strongly Correlated Electrons
Altermagnetism in an interacting model of Kagome materials
Alejandro Blanco Peces and Jaime Merino
The Hubbard model on the Kagome
lattice is a widely used interacting model for describing the electronic properties of various transition metal-based Kagome materials. We find altermagnetism driven by Coulomb interaction in the Kagome Hubbard model at Dirac filling with no
spin-orbit coupling nor explicit spatial
symmetry breaking present. We show how this insulating altermagnet is relevant to other
lattices with larger unit
cells such as the Lieb-Kagome
lattice. The ALM found displays a characteristic
magnon splitting which can be detected in inelastic
neutron scattering experiments on interacting Kagome materials.
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Statistical Mechanics
Tensor Renormalization-Group study of the surface critical behavior of a frustrated two-layer Ising model
Christophe Chatelain
Two replicas of a 2D Ising model are coupled by frustrated
spin-
spin interactions. It is known that this inter-layer coupling is marginal and that the bulk critical behavior belongs to the Ashkin-Teller (AT) universality class, as the $J_1-J_2$ Ising model. In this work, the surface critical behavior is studied numerically by Tensor Renormalization-Group calculations. The Bond-Weight Tensor Renormalization Group
algorithm is extended to tackle systems with boundaries. It is observed that the two-fold degeneracy of the surface
magnetic scaling dimension of the AT model is lifted in the frustrated two-layer Ising model (F2LIM). The splitting is explained by the breaking of the ${\mathbb Z}_2$-
symmetry under
spin reversal of a single Ising replica in the F2LIM. The two distinct surface
magnetic scaling dimensions $x_1^s$ and $x_2^s$ of the F2LIM satisfies a simple duality relation $x_1^s=1/4x_2^s$.
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Quantum Gases · Cross-listed from math-ph
Ground state energy of a dilute inhomogeneous Fermi gas
Thomas Gamet
We study the ground state energy of a system of N
fermions with two
spin states in the large N limit. The particles are placed in an inhomogeneous trapping potential and interact via scaled interactions. We study a dilute limit where the range of the interaction potential is much smaller than the typical inter-particle distance. We show that the energy per particle converges to the Thomas-
Fermi energy of the system, with a perturbative term corresponding tot he interaction and exhibiting the scattering length of the potential. The proof is decomposed into two bounds. First, we construct an appropriate test-state to prove the upper bound. Then, we prove the lower bound by the Dyson lemma, which allows us to regularize the interaction potential, and several semi-classical tools.
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Mesoscale and Nanoscale Physics
Relativistic spin-Hall effect in an external magnetic field in Al and Pt
Yu.N. Chiang and M.O. Dzyuba
The relativistic Rashba contribution to the
spin-
Hall effect in external electric and
magnetic fields in Al and Pt was investigated. Schemes of edge accumulation of
spins are proposed that take into account the flip of
spins when they do not coincide with the direction of the
magnetic field. Based on the obtained experimental data on the
spin-
Hall effect, an assessment of the
spin-orbit interaction in the studied paramagnets was made.
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Soft Condensed Matter
Tracer Diffusion in Granular Suspensions: Testing the Enskog Kinetic Theory with DSMC and Molecular Dynamics
Antonio M. Puertas and Rubén Gómez González
We investigate the
diffusion of an intruder in a granular gas, with both components modeled as smooth hard spheres, both immersed in a low viscosity carrier fluid to form a particle-laden suspension. In this system, dissipative particle collisions coexist with the action of a solvent. The latter is modeled via a viscous drag force and a
stochastic Langevin-like force proportional to the background fluid temperature. Building on previous kinetic theory and random-walk results of the tracer
diffusion coefficient [R. Gómez González, E. Abad, S. Bravo Yuste, and V. Garzó, Phys. Rev. E 108, 024903 (2023)], where random-walk predictions were compared with Chapman–Enskog results up to the second Sonine approximation, we assess the robustness of the Enskog framework by incorporating molecular dynamics (MD) simulations, using direct simulation Monte Carlo (DSMC) results as an intermediate reference. In particular, we focus on the intruder velocity autocorrelation function, considering intruders different masses (from 0.01 to 100 times the mass of the granular particles), and analyse the behavior of the intruder temperature and
diffusion coefficient. Our results clarify the influence of the friction parameter and the conditions under which Enskog kinetic theory reliably describes intruder
diffusion in granular suspensions.
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Materials Science
Toward more performant eye safe lasers: effect of increasing sensitizer amount in Yb3+,Er3+:YAG transparent ceramic on its spectral characteristics
Agnieszka Szysiak, Robert Tomala, Helena Węglarz et al. (+3 more)
Developing efficient Er3+,Yb3+:YAG eye-safe
lasers is a priority of modern
laser technology. This paper focuses on the influence of the concentration of Yb3+ ions on the spectroscopic properties of Er3+,Yb3+:YAG transparent ceramics. Four samples with different concentrations of Yb3+ ions were prepared by solid-state reaction sintering. The study revealed the influence of Yb3+ ions on the microstructure and the sintering process. A high concentration of Yb3+ ions leads to the formation of Y3+-rich
impurity phases and causes segregation of Er3+ and Yb3+ and Si4+ into these phases. The influence of Yb3+ ions on the shape of emission spectra and the lifetimes of both Er3+ and Yb3+ ions was shown. Changes in the spectroscopic properties were ascribed to increase in neat transfer between Yb3+ and Er3+ ions. IQE of Er3+ and Yb3+ luminescence were calculated, and optimal Er3+/Yb3+ ions ratio were proposed
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All authors Agnieszka Szysiak, Robert Tomala, Helena Węglarz, Juraj Kajan, Mirosław Słobodzian, Mykhailo Chaika
Mesoscale and Nanoscale Physics
Optimal spin-charge interconversion in graphene through spin-pseudospin entanglement control
Joaquín Medina Dueñas, Santiago Giménez de Castro, Jose H. Garcia et al. (+1 more)
The electrical generation of
spin signals is of central interest for spintronics, where
graphene stands as a relevant platform as its
spin-orbit coupling (SOC) is tuned by proximity effects. Here, we propose an enhancement of
spin-charge interconversion in
graphene by controlling the intraparticle
entanglement between the
spin and pseudospin degrees of freedom. We demonstrate that, although the
spin alone is not conserved in Rashba-Dirac systems, a combined
spin-pseudospin operator is conserved. This conserved quantity represents the interconversion between pure
spin and pseudospin textures to a
spin-pseudospin entangled structure, where Kane-Mele SOC tunes this balance. By these means, we achieve
spin-charge interconversion of 100% efficiency via the Rashba-Edelstein effect.
Quantum transport simulations in disordered micron-size systems demonstrate the robustness of this effect, and also reveal a disorder resilient
spin Hall effect generated by the interplay between Rashba and Kane-Mele SOC. Our findings propose a platform for maximally efficient
spin-charge interconversion, and establish
spin-pseudospin correlations as a mechanism to tailor spintronic devices.
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All authors Joaquín Medina Dueñas, Santiago Giménez de Castro, Jose H. Garcia, Stephan Roche
Materials Science
Temperature-Dependent Spectroscopy of Cr3+:YGG Nanophosphors with Multisite Emission
Mykhailo Chaika
An important feature of Cr3+ is the ability to tune the absorption and emission spectra by changing the host. However, in some cases, different emission spectra can be detected in samples with similar Racah parameters. The present paper reports changes in the luminescence properties of Cr3+:YGG nanocrystals. Cr3+:YGG nanocrystals were synthesized by a modified Pechini method to obtain nanocrystals containing two fractions of Cr3+:YGG nanocrystals with different
lattice parameters and crystalline sizes. The increase in the concentration of Cr3+ ions leads to a
redshift of the 4T2g emission from 750 nm to 850 nm for luminescence spectra measured at 80K. In contrast, room temperature luminescence spectra showed similarity in the shape of the emission spectra. The calculated
crystal field strength was found to increase with the concentration of Cr3+ ions, so the detected patterns in low-temperature luminescence spectra were explained by energy transfer chain processes.
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Statistical Mechanics · Cross-listed from quant-ph
Scrambling Without Chaos in Random Free-Fermionic Systems
Ali Mollabashi and Mohammad-Javad Vasli
We study the role of randomness in the scrambling of
quantum information within integrable free-fermionic systems. Considering quadratic
Hamiltonians with varying degrees of randomness, we analyze
entanglement-based measures to characterize the scrambling structure. We show that the
memory effect in the
entanglement of disjoint subsystems of Gaussian states vanishes when the local couplings are random, indicating information delocalization. The tripartite mutual information exhibits negative saturation values similar to those in chaotic systems, albeit with a smaller magnitude, revealing weaker scrambling under integrable quadratic dynamics. Despite integrability, spectral analyses reveal that local random models display a spectral-form-factor ramp and a partial crossover in the single-particle level-spacing ratio from Poisson-like to Wigner–Dyson-like behavior within a certain range of random couplings. These results demonstrate that randomness can act as a minimal ingredient for inducing information scrambling in integrable quadratic fermionic models.
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Quantum Gases
Unveiling the BEC-droplet transition with Rayleigh superradiant scattering
Mithilesh K. Parit, Mingchen Huang, Ziting Chen et al. (+3 more)
Light scattering plays an essential role in uncovering the properties of
quantum states through light-matter interactions. Here, we explore the crossover from Bose-Einstein condensate (BEC) to droplets in a dipolar $^{166}$Er gas by employing superradiant light scattering as both a probing and controlling tool. We observe that the efficiency of superradiant scattering exhibits a non-monotonic behavior akin to the rate of sample expansion during the transition, signaling its sensitivity to the initial
quantum state, and in turn, revealing the BEC-droplet crossover. Through controlled atom depletion via superradiance, we analyze the sample's expansion dynamics and aspect ratio to identify the BEC-droplet phases distinctly, supported by Gaussian variational ansatz calculations. Finally, using these two approaches, we track how the BEC-droplet crossover points shift under varying
magnetic field orientations. Our results show that superradiance can provide a state-sensitive probe and implement rapid atom-number quenches, opening new avenues for studying
quantum states in atomic matter.
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All authors Mithilesh K. Parit, Mingchen Huang, Ziting Chen, Yifei He, Haoting Zhen, Gyu-Boong Jo
Mesoscale and Nanoscale Physics
Versatile tunable optical injection of chiral polarized Weyl fermions in a magnetic Weyl semimetal Co3Sn2S2
Zipu Fan, Junchao Ma, Jinying Yang et al. (+12 more)
Precise probe and control of various
quantum degrees of freedom in novel
quantum matter are central to understanding fundamental
quantum physics and hold promise for innovative routes to encode and process information. Chirality is one such degree of freedom that has recently attracted intense research interest, especially for Weyl
fermions in topological Weyl semimetals. The coupling of
chiral degrees of freedom through light-matter interactions and the versatile control of these couplings through external fields can lead to precise
quantum control of Weyl
fermions. In this work, we demonstrate the observation of light chirality-dependent photocurrent in the mid-infrared regime. Excitation wavelength-dependent measurements reveal that the photocurrent originates from the injection of
chiral polarized Weyl
fermions by
chiral polarized mid-infrared
photons. The optical process that generates unbalanced
chiral polarized Weyl
fermions is determined to be a third-order nonlinear photocurrent process. Compared with nonmagnetic Weyl semimetals, such coupling is versatilely tunable in
magnetic Weyl semimetals with the magnetization direction and external electric field in addition to the chirality of light. Our results are not only directly applicable to tunable circular-polarization-sensitive photodetection in the mid-infrared regime, but also pave the way toward functional
quantum devices that utilize the
chiral quantum degrees of freedom of Weyl
fermions.
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All authors Zipu Fan, Junchao Ma, Jinying Yang, Yan Sun, Zhuocheng Lu, Shuxia Chen, Delang Liang, Dehong Yang, Chang Xu, Qinsheng Wang, Anlian Pan, Ji Feng, Enke Liu, JinLuo Cheng, Dong Sun
Mesoscale and Nanoscale Physics
One-dimensional moiré engineering in zigzag graphene nanoribbons on hBN
Ryosuke Okumura, Naoto Nakatsuji, Takuto Kawakami et al. (+1 more)
We study the structural relaxation and electronic properties of a one-dimensional (1D) moiré system composed of a zigzag
graphene nanoribbon (GNR) placed on a hexagonal boron nitride (hBN) substrate. Using an effective grid model derived from continuum elasticity theory, we calculate the relaxed atomic structure of the GNR/hBN system for various twist angles and ribbon widths. The relaxation gives rise to a characteristic 1D domain structure consisting of alternating commensurate AB$'$ regions and two distinct types of domain boundaries. At finite twist angles, the ribbon adopts a wavy shape, locally tracing the hBN zigzag direction but occasionally sliding to adjacent atomic rows. The resulting moiré potential strongly modulates the electronic structure: the zero-energy zigzag edge states are modulated by the local stacking, leading to densely packed subbands in the AB$'$ domains and sharply localized domain-wall states in the energy gaps between domain plateaus, which together realize gate-tunable one-dimensional arrays of
quantum-confined electronic states. Our results demonstrate that moiré modulation in GNR/hBN
heterostructures provides a versatile platform for electronic structure engineering and the design of 1D moiré nanodevices.
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All authors Ryosuke Okumura, Naoto Nakatsuji, Takuto Kawakami, Mikito Koshino
Strongly Correlated Electrons
Takafumi Kitazawa, Yasuyuki Shimura, Takahiro Onimaru et al. (+7 more)
We investigated electron-
nuclear spin entanglement in the paramagnetic ground state of the Ho-based cubic compound HoCo2Zn20. From analyses of magnetization and specific heat data, we determined the cubic crystalline electric field (CEF) parameters, the
magnetic exchange constant, and the hyperfine coupling constant between the 4f
magnetic moment and the 165Ho
nuclear spin. Our results show that the Gamma5 CEF ground state is split by the hyperfine coupling, with an energy width of 1.3 K at 0 T, and that the true paramagnetic ground state is a quasi-sextet arising primarily from
entanglement between the f-electron effective
spin S = 1 and the 165Ho
nuclear spin I = 7/2. We further demonstrate that, depending on the CEF parameters, the paramagnetic ground state can switch to an electron-
nuclear coupled dectet. These findings underscore the importance of accurately identifying the electron-
nuclear level scheme for understanding the low-temperature properties of rare-earth compounds containing
spin-active
nuclei.
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All authors Takafumi Kitazawa, Yasuyuki Shimura, Takahiro Onimaru, Shun Tsuchida, Katsunori Kubo, Yoshinori Haga, Hironori Sakai, Yoshifumi Tokiwa, Shinsaku Kambe, Yo Tokunaga
Materials Science
Multistability of interstitial magnesium and its carrier recombined migration in gallium nitride
Yuansheng Zhao, Kenji Shiraishi, Tetsuo Narita et al. (+1 more)
We present density-functional-theory calculations which provide a microscopic picture of the recombination-enhanced migration of interstitial Mg in GaN. We determine stable structures and migration pathways with accurate HSE approximation to the exchange-correlation energy, and also computed recombination rates using the obtained energy spectrum and wavefunctions. It is found that the migration between the most stable octahedral sites (Mg$_{\textrm{O}}$) via newly found interstitial complex structure shows the lowest migration energy in which one or two electrons are captured during the migration, that the most stable charge state of 2+ changes to 1+ or neutral, and that by this recombination of carriers the migration barrier is significantly reduced. Starting from Mg$_{\textrm{O}}^{2+}$, Mg captures an electron becoming the 1+ charge state and overcomes the barrier of 1.65 eV, much reduced from 2.23 eV in case of the migration with the 2+ charge state kept. Moreover, further electron capture is realized accompanied by substantial structural relaxation, thus Mg becoming neutral. Detailed HSE calculations for this second capture show that the migration barrier is 1.55 eV, thus clarifying the important role of the carrier recombination for Mg migration in GaN. These findings are corroborated by the present quantitative calculations of recombination rates based on electronic
Hamiltonian constructed from our DFT-obtained energy spectrum. The timescale of the recombination is clarified to be in or under the timescale of the migration with typical electron density and the enhancement is expected to be significant.
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All authors Yuansheng Zhao, Kenji Shiraishi, Tetsuo Narita, Atsushi Oshiyama
Disordered Systems and Neural Networks
The geometry and dynamics of annealed optimization in the coherent Ising machine with hidden and planted solutions
Federico Ghimenti, Adithya Sriram, Atsushi Yamamura et al. (+2 more)
The coherent Ising machine (CIM) is a nonconventional hardware architecture for finding approximate solutions to large-scale combinatorial optimization problems. It operates by annealing a
laser gain parameter to adiabatically deform a high-dimensional energy landscape over a set of soft
spins, going from a simple convex landscape to the more complex optimization landscape of interest. We address how the evolving energy landscapes guides the optimization dynamics against problems with hidden planted solutions. We study the Sherrington-Kirkpatrick
spin-glass with
ferromagnetic couplings that favor a hidden configuration by combining the replica method, random
matrix theory, the Kac-Rice method and dynamical mean field theory. We characterize energy, number, location, and Hessian eigenspectra of global minima, local minima, and critical points as the landscape evolves. We find that low energy global minima develop soft-modes which the optimization dynamics can exploit to descend the energy landscape. Even when these global minima are aligned to the hidden configuration, there can be exponentially many higher energy local minima that are all unaligned with the hidden solution. Nevertheless, the annealed optimization dynamics can evade this cloud of unaligned high energy local minima and descend near to aligned lower energy global minima. Eventually, as the landscape is further annealed, these global minima become rigid, terminating any further optimization gains from annealing. We further consider a second optimization problem, the Wishart planted ensemble, which contains a hidden planted solution in a landscape with tunable ruggedness. We describe CIM phase transitions between recoverability and non-recoverability of the hidden solution. Overall, we find intriguing relations between high-dimensional geometry and dynamics in analog machines for combinatorial optimization.
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All authors Federico Ghimenti, Adithya Sriram, Atsushi Yamamura, Hideo Mabuchi, Surya Ganguli
Disordered Systems and Neural Networks · Cross-listed from physics.bio-ph
Kramers rate for Brownian particles in excitable media with deformable double-well substrates
Alain M. Dikande
We address the Kramers escape problem for Brownian particles in bistable substrates with deformable double-well shapes. The shape deformability is considered of three distinct forms: in one, the positions of the two degenerate minima can be shifted continuously without affecting the barrier height. In the second the minima positions are kept fix while the barrier height is continuously shifted. In the third the minima positions and barrier height can be tuned simultaneously by changing a single real parameter that we refer to as deformability parameter. We obtain the analytical expression of the Kramers escape rate for the three different double-well models, and identify a critical value of the deformability parameter above which non-Gaussian corrections become relevant. Remarkable enough in the latter region, statistical mechanics predicts a first-order transition in
quantum tunneling using the functional action associated with the family of deformable double-well potentials.
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Mesoscale and Nanoscale Physics
The spinterface mechanism for the chiral-induced spin selectivity effect: A Critical Perspective
Subhajit Sarkar, Amos Sharoni, Oliver L. A. Monti et al. (+1 more)
The
chiral-induced
spin selectivity (CISS) effect, whereby
chiral molecules preferentially transmit electrons of one
spin orientation, remains one of the most intriguing and debated phenomena at the interface of spintronics, molecular electronics, and
quantum materials. Despite extensive experimental observations across diverse platforms - including transport junctions, photoemission, and enantioselective chemistry - a comprehensive theoretical framework is still lacking. In this perspective, we critically examine the spinterface mechanism as a unifying explanation for the CISS effect. The spinterface model, which hypothesizes a feedback interaction between electron motion in
chiral molecules and fluctuating surface
magnetic moments, is shown to quantitatively reproduce experimental data across various systems and conditions. We contrast it with some existing theoretical models, highlighting key experimental features. Importantly, we also address open questions and criticisms of this model, including the nature of surface magnetism, the role of dissipation, and the applicability of the mechanism to non-helical or electrode-free systems. By offering falsifiable predictions and reconciling theory with experimental raw data, this work aims to sharpen the dialogue surrounding the microscopic origin of CISS and stimulate further experimental and theoretical progress.
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All authors Subhajit Sarkar, Amos Sharoni, Oliver L. A. Monti, Yonatan Dubi