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Physics Condensed Matter Mesoscale and Nanoscale Physics

cond-mat.mes-hall Watch Mesoscale and Nanoscale Physics

Monday 12 October

Mesoscale and Nanoscale Physics

Symmetry Condition and Mechanism for the Anomalous Hall Effect in Odd-Parity Magnets

Xun-Jiang Luo, Su-Qi Zhang, Yi-Dong Liu et al. (+3 more)

The anomalous Hall effect (AHE) in unconventional magnets has attracted considerable attention. A giant AHE has recently been observed in the candidate $p$-wave magnet Gd$_3$(Ru$_{1-\delta}$Rh$_{\delta}$)$_4$Al$_{12}$ [Nature 646, 837(2025)], while its theoretical interpretation invokes a spontaneous magnetization to break time-reversal symmetry ($\mathcal{T}$), although the magnetic order itself already breaks $\mathcal{T}$. Consequently, the microscopic origin of the AHE in odd-parity magnets (OPMs) remains an open question. Here we establish the symmetry condition and microscopic mechanism for the AHE in OPMs. Since the OPM order is classified by the spin group whereas the AHE is governed by the magnetic group, the AHE is allowed only when the magnetic point group is ferromagnetic; conversely, it is forbidden whenever the effective time reversal $\mathcal{T}\tau$ (with $\tau$ a fractional translation), which is essential to the realization of the OPM, is preserved. Microscopically, OPMs generically host a hidden Zeeman field (HZF) rooted in this intrinsic $\mathcal{T}$ breaking, and its interplay with spin-orbit coupling (SOC) generates the AHE without any external field. We demonstrate this picture with models on the square, honeycomb, and kagome lattices. In particular, a square-lattice helimagnet with sublattice-resolved SOC maps exactly onto an anisotropic Rashba–Zeeman model, whose Zeeman term is nothing but the HZF and directly yields the AHE. Our results establish the symmetry condition for the AHE in OPMs and identify the HZF–SOC interplay as a general route to realize it.
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All authors Xun-Jiang Luo, Su-Qi Zhang, Yi-Dong Liu, Yong-Ting Shi, Ding-Fu Shao, Mingliang Tian

Mesoscale and Nanoscale Physics

Wilson-loop formulations of the spin polarization and the spin Chern number

Kiminori Hattori and Ryuki Uchida

In this paper, we derive the spin polarization and the spin Chern number in momentum and real spaces in terms of the Wilson loop in each spin sector. For these derivations, the spin matrix in the subspace of occupied or unoccupied states is generalized by the unitary rotation to prevent the spectral gap from closing. In the presence of a finite spin gap, the spin eigenvectors recompose the eigenstates of the Hamiltonian in two spin sectors. A dimensional reduction leads to the charge polarization in each spin sector in terms of the Wilson loop along the effective Brillouin zone (BZ) in one dimension (1D). A linear combination of the charge polarizations defines the spin polarization, which naturally quantifies the spin Chern number in momentum space. It is also shown that the spin Chern number is directly derived from the local Wilson loops on a discrete 2D BZ. The theoretical treatment is further generalized for real space in a supercell framework, which can deal with ordered and disordered systems in a unified manner. In this framework, the spin Chern number in real space is formulated in terms of the Wilson loop around the BZ boundary. These formulations are used to numerically evaluate topological phases in the Bernevig-Hughes-Zhang model. Numerical results show an equivalence of the spin Chern number and the ${\mathbb{Z}_2}$ index.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.soft

Defining Thixotropy

Yogesh M. Joshi and Ronald G. Larson

Originally coined 100 years ago to represent a shear-reversible sol-gel transition, the range of the term "thixotropy" has gradually expanded to encompass time-dependent changes in viscosity. Since this risks confusing "thixotropy" with ordinary nonlinear viscoelastic behavior, in this work we seek simple, historically faithful, quantifiable, and experimentally testable definitions of "thixotropy" that avoid this confusion. By analysing both historical tradition and published experimental data, we arrive at three complementary definitions. Definition 0 provides an idealized, purely inelastic reference useful primarily for recognizing thixotropy in a constitutive equation. Definitions 1 and 2, which are more practical, require, respectively, unusually large strains to reach steady state, and unbounded growth of the material relaxation time during rest. Definition 1 can apply in either shear-rate or stress-controlled modes, with the latter encompassing the so-called "viscosity bifurcation." These criteria are tested across diverse materials that we identify as thixotropic based on their responses clearly exceeding their viscoelastic reference scales. The framework is deliberately phenomenological and does not require specific microscopic mechanisms, allowing diverse materials to be described within a common rheological language, as is typical for rheological terminology. We believe that these definitions provide a practical framework for assigning thixotropy a more definite place in rheological taxonomy.
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Mesoscale and Nanoscale Physics

Chirally pinned flat bands, helical minibands, and bias-controlled topology of Jackiw-Rebbi states in a periodically modulated Dirac mass texture

Tao Zhou

Flat bands amplify interactions into correlated phases, and topological bound states are robust by principle; combining the two in a model that is both exactly analyzable and experimentally accessible is therefore especially desirable. Here we show that a square-lattice model with a slowly modulated Dirac mass texture, $m(x)=m+m_0\cos(2\pi x/L)$ with $L$ up to hundreds of sites, provides such a setting, without engineered geometries, lattice-scale textures, or long-range hoppings. Exact nesting reduces the problem, independently at each transverse momentum $k_y$, to a $1+1$ dimensional Dirac Hamiltonian with a position-dependent mass: every simple zero of $m(x)$ binds Jackiw-Rebbi zero modes, and the two walls of a period form an effective Su-Schrieffer-Heeger ring. The model interpolates between a chirally pinned flat band at large $L$, a helical miniband whose nodes are enforced by a boundary phase of the ring, and a bias-dimerized regime in which the sign of the bias sets a winding number. A central result is that the miniband bandwidth is not a single scale: a max-min measurement conflates three widths of different origin, the helical height $W_{\rm JR}$ exponential in the period, the zone-edge upturn $W_{\rm edge}\propto L^{-2/3}$ dominating at large $L$, and a bias bump exponential in $L$, so the flat band is robust against bias in the thermodynamic limit. We analyze two lattice realizations, a two-orbital model with modulated inter-orbital hybridization and a single-orbital bond-order texture sharing the nodal physics while leaving the van Hove points gapless, and propose experiments on cold-atom superlattices, Kekule-ordered graphene, STM designer lattices, and microwave arrays.
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Mesoscale and Nanoscale Physics

High-mobility spin transport mediated by indirect excitons in a van der Waals heterostructure

Zhiwen Zhou, W. J. Brunner, Luc M. Nguyen et al. (+4 more)

Efficient spin transport with high spin mobility is of both fundamental and practical significance for spintronics. Spin can be transferred by moving particles. However, particle scattering limits spin transfer and prevents high spin mobilities. In this work, we study the kinetics of spin transport mediated by a quantum system of indirect excitons (IXs) in a van der Waals heterostructure composed of atomically thin transition-metal dichalcogenides (TMDs). This spin transfer mechanism allows for the realization of efficient spin transport characterized by a high spin mobility reaching 10$^5$ cm$^2$/(eV$\cdot$s), which is orders of magnitude higher than that achieved via standard diffusive spin transport in TMD materials. The suppression of IX scattering enables high-mobility spin transport.
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All authors Zhiwen Zhou, W. J. Brunner, Luc M. Nguyen, C. A. Lozano, G. Mercado, L. H. Fowler-Gerace, L. V. Butov

Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Measuring higher-order bulk topology in a programmable quantum simulator

Pratik Sathe, Niclas Heinsdorf, Majid Kheirkhah et al. (+13 more)

Topological insulators lie outside the standard Landau paradigm: Instead of being characterized by local order parameters and spontaneous symmetry breaking, their defining properties are encoded in the global structure of their quantum states and manifested as robust boundary states. Crystalline higher-order topological insulators can extend this principle by hosting localized corner modes in two and higher dimensions. Although these boundary modes can be detected spectroscopically through the local density of states, determining the underlying bulk topology, for example through nested Wilson loops, remains an outstanding experimental challenge. Here we use up to 728 qubits in a superconducting analogue-digital quantum simulator to realize and probe a hierarchy of one-, two-, and three-dimensional topological insulators governed by effective tight-binding Hamiltonians. By reconstructing complex propagators from real-space measurements, we obtain complementary boundary and bulk characterizations: local densities of states reveal localized boundary modes, whereas experimentally reconstructed band projectors yield bulk topological invariants, culminating in a three-dimensional octupole moment extracted through third nested Wilson loops. These results establish a route to realizing, characterizing, and manipulating topologically protected states in programmable three-dimensional quantum matter.
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All authors Pratik Sathe, Niclas Heinsdorf, Majid Kheirkhah, Rahul Deshpande, Fabio Altomare, Andrew J. Berkley, Martin Dehn, Richard Harris, Emile Hoskinson, Stefan Paul, Gabriel Poulin-Lamarre, Jack Raymond, Chris Rich, Mohammad H. Amin, Marcel Franz, Andrew D. King

Sunday 11 October

Mesoscale and Nanoscale Physics

Probing the wave-vector gap in nanoscale magnonics

Ondřej Wojewoda, Samer Kurdi, Toeno van der Sar et al. (+3 more)

Short-wavelength spin waves are central to miniaturizing magnonic devices and to studying exchange-dominated magnetization dynamics. Yet the wave-vector range between roughly 30 and 300 radians per micrometre has remained difficult to access experimentally, posing challenges for both excitation and detection. This Perspective reviews recent advances in emerging measurement techniques and discusses prospects for their future development. Optical and microwave techniques are restricted to the vicinity of the Brillouin-zone centre. Inelastic neutron, electron and resonant hard x-ray scattering cover the full Brillouin zone but lack the energy resolution needed for low-frequency magnons at intermediate wave vectors. This Perspective examines five emerging approaches that are beginning to close this gap: Mie-enhanced Brillouin light scattering, colour-centre magnetometry, magnon momentum microscopy, extreme-ultraviolet transient-grating spectroscopy, and time-resolved scanning transmission x-ray microscopy. For each, we discuss the physical principle, demonstrated capabilities, current limitations and prospects for development. Compared side by side, the techniques give complementary access to magnon frequencies, local stray fields, real-space dynamics and momentum-space distributions, and they differ in their sensitivity to coherent and incoherent excitations. We therefore discuss the main advantages and use cases of each technique and consider the practical requirements for addressing different scientific questions. We highlight opportunities for further development, including higher sensitivity, improved spatial and temporal resolution, and broader frequency and wave-vector coverage. Together, these advances could enable, for example, systematic studies of nonlinear magnon interactions at nanoscale wavelengths and investigations of emerging magnetic materials such as altermagnets.
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All authors Ondřej Wojewoda, Samer Kurdi, Toeno van der Sar, Daniel Schick, Sebastian Wintz, Alexei Maznev

Mesoscale and Nanoscale Physics · Cross-listed from math.PR

A clarification of Fermi's formula for the maximum of partial sums

Alessandro Lodi

In Fermi's 1926 note, the expression assigned to the probability that all partial sums remain below a threshold is the complementary crossing probability. We obtain the correct probability from the same diffusion equation and locate the inconsistency in the final integration. We then state the limit for independent increments under the Lindeberg condition. A counterexample with smooth densities shows that small individual variances do not suffice to justify this limit. The probabilistic results are classical; the purpose of this note is to reconcile the stated event with the printed calculation and to make the sufficient hypotheses for the discrete limit explicit.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.dis-nn

Eigenstate transitions and critical transport in 3D Aubry-André model

Pragna Das, Lev Vidmar and Miroslav Hopjan

We numerically study spectral and wave-function properties, as well as time evolution, in finite-size commensurate approximations of the 3D Aubry-André model. We explicitly show, through scaling analysis, the existence of two critical points. One of the critical points is a localization-delocalization transition in real-space, similar to the critical point in the 3D Anderson model. The other is a localization-delocalization transition in a dual momentum space, which does not have any analog in the 3D Anderson model. We estimate their values from spectral measures, such as the gap ratio and the spectral form factor, as well as from wave-function measures, such as the inverse participation ratio and the participation entropy. Additionally, we demonstrate that both critical points exhibit hallmarks of critical points between quantum ergodicity and localization. First, the eigenstates and the eigenspectrum at the critical point fulfill the mutual universal relation between the eigenspectrum compressibility $\chi$ and the eigenstate fractal dimension $d^{(1)}$ [S. Jiricek et al., Proc. Nat. Acad. Sci. 123, e2518027123 (2026)]. Second, the dynamics of the wave packet at the critical point is governed by the critical dynamical exponent $z$, characterized by the spectral fractal dimension and the dimensionality of the lattice [M. Hopjan and L. Vidmar, Phys. Rev. Lett. 135, 060401 (2025)]. Thus, similarly to the 3D Anderson model, the critical dynamical exponent is sub-diffusive with the value of $z=3$.
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Mesoscale and Nanoscale Physics

Approaching Irrational Quantization

Zhihong Wu, Zhimou Chen, Siyuan Zhang et al. (+5 more)

The integer and fractional quantum Hall effects reveal that quantized physical responses can take integer and fractional values, respectively. This naturally raises the question of whether quantization can extend beyond rational numbers to an irrational value. Here we show that such a quantized response can emerge from the limiting behavior of a finite, experimentally realizable geometric network. We develop a method based on infinite geometric structures, in which a sequence of finite networks rapidly converges toward an infinite limit. Following this approach, we identify a planar network whose normalized resistance converges to the irrational value $\sqrt{3}$, and realize a finite approximation with high-precision geometric control. The measured normalized resistance is $1.735 \pm 0.001\, h/e^2$ for a fully occupied Landau level in the designed pattern. Our experimental results extend the concept of quantization beyond rational values and demonstrate that an irrational quantized response can be approached through a finite physical system.
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All authors Zhihong Wu, Zhimou Chen, Siyuan Zhang, Jinyang Chen, L. N. Pfeiffer, K. W. West, Yang Liu, Xi Lin

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Nonperturbative optomechanics with thin, weakly reflective intracavity dielectrics

Andrew Laeuger and Nancy Aggarwal

Levitated dielectric sensors are often treated as perturbative probes of a pre-existing optical field. We show that this picture fails for a novel sensor geometry: a dielectric plate whose transverse extent captures the full optical mode. Even an optically thin sensor with modest refractive index can heavily restructure the intracavity fields, effectively partitioning a cavity into two subcavities with substantial cross-coupling and producing new optical resonances and trapping sites with properties which deviate from the standard intuition. Conventional perturbative expressions for the cavity frequency shift and optomechanical coupling depart from the exact solution at thicknesses as small as $\sim\lambda/(10n)$. Furthermore, cavity locking decreases the coupling and increases the trap stiffness by orders of magnitude, and the coupled subcavity structure permits appreciable optical cooling at zero detuning, a distinct behavior unique to this nonperturbative regime. These effects, sharing a common origin, establish a distinct regime of cavity optomechanics in which the sensor is no longer merely a probe of the cavity field, but an element that helps define it.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Field Response of the Orbital Moment Probed by Integrated Magnetic Circular Dichroism

Qian-Sheng Shi, Ziqiang Wang, Jian Kang et al. (+1 more)

Magnetic circular dichroism (MCD) is an optical probe of orbital time reversal symmetry breaking. Although frequency integrated MCD is related to the self-rotation contribution to orbital magnetization at zero field, its magnetic response generally differs from the thermodynamic orbital susceptibility. Here, we define the MCD susceptibility and derive general expressions for multiband systems using an electromagnetic $U(1)$ gauge invariant approach. For insulators, the response contains contributions from the non-Abelian quantum metric, Berry curvature, and field induced interband mixing. For metals, a finite temperature Green function formulation separates the smooth response into Fermi sea and Fermi surface contributions. An independent Landau level calculation confirms the expressions for the zero-field MCD moment and its linear magnetic response. In a continuum model for twisted bilayer graphene, the MCD susceptibility vanishes within the charge neutrality gap, while the thermodynamic orbital susceptibility remains markedly diamagnetic. Near the gap edges, the orbital moment term dominates the response. Features near the van Hove energies are associated with density of states enhancement. These results clarify how the two susceptibilities reflect different orbital processes even for the same electronic structure and thus establish the MCD susceptibility as a distinct probe of orbital magnetic response.
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All authors Qian-Sheng Shi, Ziqiang Wang, Jian Kang, Yi Zhang

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Intrinsic Orbital Nernst Effect in Real Photonic Bands

Tao Zhu

We predict an intrinsic orbital Nernst effect in reciprocal photonic lattices with real Bloch modes. In the collisionless regime, a thermal gradient generates a transverse angular-displacement response although band angular moments and ordinary Berry curvatures vanish. The response is carried by angular-density dipoles formed through virtual mixing of distinct spatial modes. Their correlation with band velocity survives thermal averaging and is independently recovered from the angular-density readout of unitarily evolved wave packets. Two controls expose the mechanism: a fixed-generator two-level system has zero antisymmetric intrinsic response, and flattening the dispersion suppresses transport without changing the eigenstates. A canonical scalar Maxwell calculation realizes the effect through symmetry-selected virtual channels, even when one band dominates the thermal population. These findings establish an orbital-dipole mechanism for intrinsic thermal angular transport in real photonic bands.
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Mesoscale and Nanoscale Physics

A Solvable Model in the Half-filled Landau Level

Anders Karlhede

A solvable microscopic model of interacting electrons in a half-filled Landau level is presented. The Hamiltonian is a well-defined limit of the electron-electron interaction. The low energy sector consists of a set of independent, spatially interlaced, one-dimensional systems of non-interacting fermionic dipoles, each system with a Fermi sea ground state. The total number of dipoles in the set of Fermi seas forming the two-dimensional ground state equals the number of electrons in the system and the density of states at the Fermi level is that of a two-dimensional Fermi gas. The model is obtained by generalizing, or rather adopting, the existing exact solution on a thin, but not too thin, torus to the two-dimensional bulk case.
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Mesoscale and Nanoscale Physics

Microscopic foundation for orbitronics: orbital exchange interactions, orbital-dependent transport, and orbital switching

Hantao Zhang and Ran Cheng

Recent experiments highlighted the essential role of the orbital degrees of freedom underlying current-induced magnetic dynamics. However, a microscopic picture of how orbital angular momenta are transferred between itinerant and localized electrons, and the way it feeds back into transport, remains elusive. Here we develop a theoretical framework for the orbital exchange interactions by generalizing the Anderson-Kondo model to incorporate the orbitals of conduction bands. Illustrating with the $p$-orbitals, we determine the way electrons acquire orbital polarization and its influence on the transport properties, uncovering the orbital counterparts of phenomena analogous to spin filtering and tunnel magnetoresistance. Reciprocally, we demonstrate that orbital-polarized currents can exert torques on localized magnetic moments without having to convert into spin currents, which enables electrical switching of magnetization even when spin polarization becomes inactive. These striking discoveries lay a microscopic foundation for orbitronics and reveal critical mechanisms for orbital manipulations of magnetic order.
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Mesoscale and Nanoscale Physics

Valley-resolved high harmonics probe symmetry breaking beyond the Dirac cone

Pablo Pais, Alfredo Iorio and Marcelo F. Ciappina

Low-energy electrons in graphene obey an emergent, rotationally invariant Dirac equation, while the honeycomb lattice enters only through higher-order, symmetry-breaking corrections, the leading one being trigonal warping. This provides a unique solid-state arena to address a cornerstone challenge in quantum gravity: how to detect ultraviolet symmetry-breaking granular corrections, suppressed by a fundamental scale, atop an emergent infrared relativistic continuum. We show that intense counter-rotating $\omega$-$2\omega$ fields and valley-resolved high-harmonic generation (HHG) provide such a probe. Within a family of models that interpolates continuously between the isotropic Dirac cone and nearest-neighbour graphene at fixed Fermi velocity, the valley contrast of the harmonic yield vanishes identically in the symmetric theory, grows linearly with the warping coupling, and its valley-even counterpart appears only at second order, as expected from effective-field-theory power counting. The stronger the laser, the farther it drives electrons from the Dirac points, so the laser intensity plays the role of the collision energy. For the fourth harmonic, the leading warping correction alone accounts for the valley contrast of real graphene up to fundamental intensities of about $10^{11}$ W/cm$^2$, below the damage threshold of monolayer graphene, whereas the fifth harmonic already reveals higher-order lattice corrections at intensities around $10^{10}$ W/cm$^2$. The two harmonics, which rotate in opposite directions, are emitted by opposite valleys. Valley-resolved harmonics are far more sensitive than the residual valley populations, making them a practical detector of lattice-scale physics hidden beneath an emergent Dirac theory.
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Saturday 10 October

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Transition-Metal Chalcogenide Nanowires: A First-Principles Study of Stability, Electronic Structure, and Mechanical Response

Danilo de Araújo Moura, Flavio Bento de Oliveira and Andreia Luisa da Rosa

Subnanometer transition-metal chalcogenide nanowires provide a one-dimensional platform in which electronic functionality and mechanical response can be tuned through chemical composition. Here, density functional theory calculations are combined with machine-learning force-field (MLFF) molecular dynamics to investigate MoS, MoSe, MoTe, WS, WSe, and WTe nanowires. The optimized structures are dynamically stable within the harmonic approximation, and their electronic character evolves systematically across the chalcogen series: MoS, MoSe, WS, and WSe are metallic, whereas MoTe and WTe are semiconducting, with HSE06 band gaps of 0.52 and 0.20 eV, respectively. The effective axial stiffness decreases from sulfides to selenides and tellurides, consistent with the increase in metal–chalcogen bond length; using a circular effective cross section, the corresponding Young moduli range from 392 to 718 GPa. All six nanowires have negative formation energies relative to the selected elemental reference phases. MLFF trajectories extending to 2.5 ns at 300 K preserve the one-dimensional morphology and local atomic order, providing evidence of finite-temperature dynamical stability on the simulated time scale. The corresponding MLFF molecular-dynamics energy traces are shown in. We further examine the response of the nanowires to axial strain and curvature, including strain-induced band-gap modulation in the tellurides. These results establish composition and mechanical deformation as complementary routes for tuning the electronic and mechanical properties of ultrathin transition-metal chalcogenide nanowires and support their further investigation as components of flexible nanoelectronic systems.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Superlattice-Stabilized Fractional Chern Insulators in a "Berry Basin" Model

Yongxin Zeng and Andrew J. Millis

The discovery of integer and fractional Chern insulators in rhombohedral graphene, following their discovery in the isolated topological flat band in twisted MoTe$_2$, motivates a search for the conditions under which an electronic band can host a topological correlated state. In this Letter we introduce a “Berry basin" model which captures the essential features of rhombohedral graphene: a dispersive band with a flat bottom, momentum-space winding of a “layer" degree of freedom, and Coulomb interactions, while also including periodic potentials, enabling controlled studies of the interplay of quantum geometry, superlattice modulation, and electron-electron interactions in producing correlated topological states beyond the isolated topological band paradigm. While integer Chern insulators emerge at strong coupling without a superlattice potential, stabilizing the fractional state requires nonzero potential for all interaction strengths we access. We provide insight into the greater robustness of integer and fractional Chern insulators in rhombohedral graphene when electrons are polarized away from the moiré interface. Our results establish a new class of systems that host robust fractional Chern insulators and provide a controlled numerical platform to test competing theories of interacting topological phases.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Investigation on Thermal Stability of Epi-GeSn Directly on Si (001)

Tharundev V V, Dushyant Singh, Vigneshswari SR et al. (+3 more)

In this paper we investigate the structural properties and thermal stability of metastable Ge1-xSnx epilayers (~4.5 at.% Sn) grown directly on Si(001) substrates by high-resolution X-ray diffraction (HRXRD) and Raman spectroscopy. Our results demonstrate that both 70 and 140 nm films are fully relaxed, with the thicker film exhibiting a small tensile strain (+0.09%). The thermal stability of the heterostructures was evaluated up to 813 K using temperature-dependent Raman spectroscopy and scanning electron microscopy. The surface-weighted Raman response (532 nm excitation) and surface morphology of the 70 nm film remain highly stable. In contrast, the 140 nm film undergoes severe degradation, characterized by significant attenuation and broadening of the Ge-Ge longitudinal optical phonon mode, alongside enhanced surface Sn segregation. These findings demonstrate that the thermal stability of direct GeSn/Si heterostructures is governed by the initial defect structure established during epitaxial growth, providing key design rules for understanding the fabrication of GeSn-based optoelectronic devices.
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All authors Tharundev V V, Dushyant Singh, Vigneshswari SR, Subha Maity, Prince Rathor, Krista R Khiangte

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Crossed Electric Spin Generation in Multilayers

Xukun Feng, Wei Du, Jin Cao et al. (+3 more)

We propose a crossed electric spin generation effect in multilayer structures. This effect combines the advantages of in-plane and gate electric fields for manipulating spin, and unveils a nice quantum geometric structure in terms of the underlying hybrid Berry curvatures that are unique to quasi-2D systems and manifest inherent spin-layer-orbital coupling. Combining our theory with first-principles calculations, we predict sizable spin generation in 2D ferromagnetic Fe3GeTe2 and altermagnetic V2Te2O. Our theory opens a new Berry-curvature route for all-electric spin manipulation in 2D materials
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All authors Xukun Feng, Wei Du, Jin Cao, Qian Niu, Shengyuan A. Yang, Cong Xiao

Mesoscale and Nanoscale Physics

Magnon Klein paradox

D. Wang

By considering the energy transport of spin wave, we show that, like the electron Klein paradox, there is no magnon Klein paradox for reflectivity. Heuristically, this can be understood according to the fact that, even if the ground state is unstable, any energy-carrying wave cannot have greater-than-one reflectivity off a passive interface, which will violate the conservation of energy. The magnon Klein paradox discussed in the literature is derived from assigning a wrong sign to the transmitted spin wave.
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Mesoscale and Nanoscale Physics

Transport kinetics in excitonic Mott insulator

W. J. Brunner, Zhiwen Zhou, Luc M. Nguyen et al. (+4 more)

Indirect excitons (IXs) in a periodic moiré potential in a van der Waals heterostructure form an experimental platform for exploring Bose-Hubbard physics, including the superfluid state and the Mott insulator state. The realization of these collective states requires low-disorder materials. In particular, the localization due to the Mott insulator must be distinguished from disorder-induced localization. We studied the transport kinetics of IXs in a MoSe$_2$/WSe$_2$ van der Waals heterostructure with an anomalously high IX mobility, consistent with superfluidity. This high-mobility transport demonstrates that disorder-induced localization is negligible. We found a remarkable suppression of this efficient IX transport, directly indicating the Mott insulator. As the IX density increases during the excitation pulse, the IX cloud initially expands and then contracts upon entering the Mott insulator phase. The emergence of IX cloud contraction is accompanied by a peak in the IX spatial distribution at the origin. Our simulations reproduce the observed non-monotonic IX transport kinetics, including the temporal expansion and contraction of the IX cloud, as well as the peak in its spatial distribution.
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All authors W. J. Brunner, Zhiwen Zhou, Luc M. Nguyen, C. A. Lozano, G. Mercado, L. H. Fowler-Gerace, L. V. Butov

Friday 9 October

Mesoscale and Nanoscale Physics

Direct Observation of Chirality-Dependent Berry Curvature Dipole via Cross-Axial Nonlinear Hall Effect in Trigonal Te

Jaehyun Lee, Junhee Shin, Daryll J. C. Dalayoan et al. (+8 more)

Quantum geometric quantities, such as Berry curvature (BC) and quantum metric, are known to govern the physical and topological properties of materials, making access to these quantities critically important for the comprehensive understanding of the materials. Chiral crystals offer a natural platform in which structural handedness can control quantum geometry and nonlinear transport, but direct experimental access to the quantum geometric quantities has remained elusive. Here, we show that trigonal tellurium (Te) hosts an enantiomer-dependent Berry curvature dipole (BCD) that can be accessed through the measurement of cross-axial nonlinear Hall effect (NHE), unlike previously studied in-plane configurations where the BCD contribution vanishes. Angle-resolved measurements on left- and right-handed Te devices reveal a second-order transverse response whose sign reverses with crystal chirality. The extracted cross-axial nonlinear susceptibility, $\chi_{yxz}$, exceeds the in-plane susceptibility, $\chi_{xxx}$, by more than two-folds, indicating a strong BCD-driven contribution. First-principles calculations attribute this behavior to a topologically nontrivial hedgehog-like BC texture near the H points and predict a BCD that reverses sign between opposite enantiomers, consistent with experiment. These results establish cross-axial nonlinear Hall transport as a symmetry-selective probe of chirality-controlled quantum geometry in tellurium and related chiral materials.
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All authors Jaehyun Lee, Junhee Shin, Daryll J. C. Dalayoan, Miju Park, Changhee Sohn, Gang Qiu, Noejung Park, Binghai Yan, Tony Low, Hosub Jin, Seon Namgung

Mesoscale and Nanoscale Physics

Establishing the role of thickness in LaOBr; an alternative gate dielectric for vdW heterostructures

Isaac Antoine Lagaud, Justin Boddison-Chouinard, Antoine Labbé et al. (+9 more)

In electronic devices, the quality of the dielectric material is determined by figures of merit such as the band gap, leakage onset field, breakdown field and the number of built-in defects, which can all limit device performance. An additional and important metric to consider is the strength of the dielectric constant. Hexagonal boron nitride (hBN) is the benchmark gate dielectric for two-dimensional van der Waals heterostructures, owing to its wide bandgap, and atomically flat interface. At the same time, its dielectric constant (2–4) highlights the opportunity to explore alternative higher-permittivity van der Waals dielectrics that provide stronger electrostatic control. Previous work established lanthanum oxybromide (LaOBr), a layered dielectric with a predicted dielectric constant of 12.5, as a potential dielectric for van der Waals heterostructures. Using graphene field-effect devices with LaOBr, we show that the leakage-onset and breakdown fields decrease systematically with thickness. Dual-gate capacitance measurements combined with a dead-layer model reveal a bulk dielectric constant of $\varepsilon_{\mathrm{bulk}} = 12.2$, in agreement with theory, while interfacial dead-layers cause the effective dielectric constant to vary from 6 to 11 across multiple thicknesses. Benchmarked against established gate dielectrics, LaOBr combines breakdown fields comparable to hBN with dielectric constants 2 to 3 times larger. By establishing the operating regime of devices with various thicknesses, we further confirm the advantage of LaOBr over hBN for high-capacitance gate-dielectric operation. These results validate LaOBr as a promising dielectric for integration into two-dimensional heterostructures, and establish thickness-dependent benchmarking as essential for evaluating emerging layered dielectric materials.
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All authors Isaac Antoine Lagaud, Justin Boddison-Chouinard, Antoine Labbé, Ayse Melis Aygar, Alex Bogan, Li-Lin Tay, Takashi Taniguchi, Kenji Watanabe, Aljoscha Söll, Zdeněk Sofer, Adina Luican-Mayer, Louis Gaudreau

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Emergence of quantum transport in SnTe films upon structural refinement

Shrinkhala Sharma, Rabindra Basnet, Thomas Parker et al. (+4 more)

Topological crystalline insulators are distinguished from the conventional Z2 topological insulators by crystalline symmetry protection of the surface states. Quantitative characterization of these states requires magneto-transport measurements from which the phase coherence length and number of independent coherent channels can be extracted. Here we report such measurements on textured thin films of SnTe grown on Bi2Te3 buffered c-plane sapphire (Al2O3) by pulsed laser ablation. A post deposition annealing protocol was implemented to control the crystallographic texture, which reveals improved crystallinity and enhanced epitaxial alignment along the [111] direction in films annealed at elevated temperatures. The higher carrier density and lower resistivity seen in films annealed at 100 oC are suggestive of Sn vacancies which act as native acceptor defects, while films annealed at 200 oC approach the 1:1 stoichiometry. The magnetoresistance of all samples at T < 10 K characterized by a cusp centered around zero-field is suggestive of weak antilocalization and interpreted in the framework of the Hikami-Larkin-Nagaoka theory in the strong spin-orbit coupling limit. At T > 20 K, the MR is small and positive, and deviates from conventional Kohler scaling. An approximate collapse of the MR curves between 50 and 250 K is obtained using an empirical temperature-dependent scaling factor, indicating that changes in the Hall-derived carrier density alone cannot account for the observed scaling behavior. These results demonstrate that post growth annealing strongly influences the interplay between stoichiometry, crystallinity, and coherent transport in SnTe thin films.
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All authors Shrinkhala Sharma, Rabindra Basnet, Thomas Parker, Afolabi Akinfaderin, Alexander Samokhvalov, Rajeswari Kolagani, Ramesh C. Budhani

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Anisotropic pressure response of thermal conductivity in crystalline polytetrafluoroethylene

Maissa Adi, Jamal Abou Haibeh and Samuel Huberman

Compression can change thermal conductivity differently along polymer chains and across them by altering phonon velocities and lifetimes. We examine this directional response in orthorhombic Form III polytetrafluoroethylene (PTFE) at calculated pressures of 1.503, 3.452, and 5.056 GPa, using first-principles harmonic and cubic force constants within the single-mode relaxation-time approximation (SMRTA). At 100, 200, and 300 K, the chain-axis conductivity decreases at every pressure step. Between 3.452 and 5.056 GPa, both transverse components recover: at 300 K they increase by 23.7% and 13.7%, while the longitudinal conductivity decreases by 14.5%. An exact factorization separates the conductivity into a heat-capacity–velocity factor and a weighted effective lifetime. The transverse recovery follows from larger heat-capacity-weighted squared velocities, consistent with stiffer interchain restoring forces. Along the chains, shorter effective lifetimes dominate, and both conductivity-weighted Normal and Umklapp rates increase at 300 K. Comparing the same cubic force-constant blocks at all three pressures, we find that interchain coupling is already substantial at 1.503 GPa and increases under compression.
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Mesoscale and Nanoscale Physics

Electric-dipole-induced nuclear electric resonance in silicon

Jiyun Su, Shihang Zhang and Peihao Huang

The nuclear spin of a phosphorus atom in silicon is considered as a promising quantum information carrier for quantum computing due to its long coherence time. Coherent control of the nuclear spin is typically realized using nuclear magnetic resonance (NMR). In this work, we theoretically propose that with the aid of hyperfine interactions and magnetic field gradients, fast electric-dipole-induced nuclear electric resonance (EDNER) can be achieved. The stronger EDNER (compared with NMR) arises from the combined effects of hyperfine interaction between the nuclear and electron spins and coupling between the electron spin and charge degrees of freedom mediated by magnetic field gradients in this scheme. By operating at specific working points, the nuclear-spin splitting becomes less sensitive to electrical noise, enabling long coherence times and high-fidelity quantum operations. All elctrical, addressable control of nuclear spins paves the way for scalable silicon-based quantum computing.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Hierarchical Multi-Agent Reasoning Systems for Knowledge-Grounded Material Substitution

Tarjei Paule Hage, Subhadeep Pal, Wei Lu et al. (+4 more)

AI-assisted scientific discovery increasingly seeks to move beyond generating plausible ideas toward reproducible, constraint-aware workflows that can support practical engineering decisions. We present MARS, a hierarchical Multi-Agent Reasoning System for knowledge-grounded material substitution under scientific and manufacturing constraints. We apply MARS to replace per- and polyfluoroalkyl substances (PFAS), a demanding materials-design problem in which substitutes must satisfy functional performance, environmental, experimental, and processing requirements. MARS decomposes substitution into a closed-loop workflow comprising requirement analysis, grounded candidate generation, and manufacturability assessment with process design. The system integrates retrieval-augmented generation, knowledge-graph reasoning, a database of lab-available materials, persistent memory of intermediate results, and rejection-aware iterative feedback. In a PFAS-substitution case study, MARS generates structured material requirements, identifies an evidence-grounded substitute candidate, and develops a lab-scale manufacturing route. In a multi-seed ablation evaluation, MARS achieves the highest overall performance across all three subsystems, with gains that persist when the backbone model is held fixed. These results suggest that hierarchical decomposition, knowledge grounding, and manufacturability-aware refinement improve material-substitution reasoning beyond model capability alone.
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All authors Tarjei Paule Hage, Subhadeep Pal, Wei Lu, Yu-Chuan Hsu, Gayla Lyon, Jiezhu Jin, Markus J. Buehler

Mesoscale and Nanoscale Physics

Parametric Glassiness in Graphene-based Electronic Lévy Glass

Caue N. da Silva, Diego B. Fonseca, Adauto J. F. de Souza et al. (+3 more)

Complex disordered conductors can display highly structured and parameter-sensitive transport even in the absence of a thermodynamically stable glass phase. Overlap distributions originally developed for replica symmetry breaking (RSB) provide a useful diagnostic of this organization, provided that their interpretation is distinguished from the equilibrium Parisi construction. In this study, we report RSB-like overlap phenomenology and parametric mesoscopic glassiness in a graphene-based electronic Lévy glass, a device that mimics optical phenomena via electronic processes. By tuning the Fermi energy, we can control the number of propagating wave modes and observe reentrant changes from narrowly centered to broad or bimodal overlap distributions. The phenomenology is observed in both graphene-based and conventional two-dimensional electron-gas electronic Lévy glasses, indicating its generality. Sample-resolved overlap distributions show that the effect is intrinsic to each disorder landscape, whereas the normalized conductance variance shows that the growth of the absolute variance is mainly caused by the increasing mean conductance. We therefore interpret the overlap as a measure of correlation among hierarchically organized transport pathways and the reentrant behavior as evidence of pinning, parameter-induced reorganizations, and mesoscopic chaos in coherent transport.
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All authors Caue N. da Silva, Diego B. Fonseca, Adauto J. F. de Souza, Antônio M. S. Macêdo, Luiz Felipe C. Pereira, Anderson L. R. Barbosa

Mesoscale and Nanoscale Physics

Giant geometric nonlinear transport in inhomogeneously strained Bi$_2$Se$_3$ single crystalline slabs

Igor Veremchuk, Pavlo Makushko, Anita Guarino et al. (+13 more)

Electrical transport in non-centrosymmetric materials exhibits a nonlinear conductivity tensor component that generates second-harmonic voltages in response to applied electric fields. Harnessing this effect is essential for developing zero-threshold electromagnetic energy harvesters and broadband frequency upconverters, both of which require maximizing the nonlinear responsivity. Here, we demonstrate that inhomogeneously strained macroscopic slabs of the topological insulator Bi$_2$Se$_3$ achieve a record-high nonlinear responsivity of $10^4\,\mu$m/V: a six orders of magnitude boost compared to its thin-film counterparts. We attribute this giant enhancement to a classical geometric nonlinear transport effect, which is driven by complex strain gradients within the bulk material. Notably, a similar geometric mechanism known as geometric nonlinear Hall was previously observed only in microscale-patterned structures. Our findings establish macroscopic strain engineering as a powerful route to enhance nonlinear transport, with immediate relevance to a broad class of layered systems, including transition metal dichalcogenides and moiré heterostructures.
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All authors Igor Veremchuk, Pavlo Makushko, Anita Guarino, Muhammad Waqee Ur Rehman, Rosalba Tatiana Fittipaldi, Tobias Kosub, Alexej Pashkin, Fabian Ganss, Oleg A. Petrenko, Proloy T. Das, Rui Xu, Rhonald Burgos Atencia, Debottam Mandal, Antonio Vecchione, Carmine Ortix, Denys Makarov