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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Wednesday 30 September

Mesoscale and Nanoscale Physics

Resonances control when multiterminal Josephson currents reduce to two-terminal couplings

A. Bar{\i}ş Özgüler

Multiterminal Josephson junctions, in which one weak link couples three or more superconductors, are studied as hosts of topological Andreev bands, multi-pair supercurrents and tunable circuit elements. Their currents are modeled both as pairwise networks of two-terminal couplings and through genuinely multiterminal processes such as quartets. We ask how accurate the pairwise description is and what controls its error. In a microscopic scattering model of planar junctions, we compare exact currents with the best pairwise approximation, which allows arbitrary nonsinusoidal couplings. In ensembles of disordered three- and four-terminal junctions, the pairwise terms capture nearly all of the energy variation, yet the median current error ranges from 8.0 to 21.2 percent, about twice the energy error, because currents weigh the multiterminal harmonics more strongly. Designed devices extend the comparison to sixteen terminals. We identify a mechanism that controls this error. A normal-region mode near the Fermi level that couples to three or more terminals produces large nonpairwise currents, and detuning it with a gate suppresses them. Shifting only this mode, selected from normal-state properties, predicts the gate dependence of the error in eight three- and four-terminal devices without fitting currents. Finite-gap calculations in two clean devices confirm a drop from tens of percent near resonance to below one percent. An analytic single-level model gives a sufficient detuning for pairwise accuracy at any terminal count. Planar junctions thus inherit the resonant and cotunneling regimes known from quantum dots, and normal-state properties identify the mode that selects between them.
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Mesoscale and Nanoscale Physics

Quantization through dissipation: impurity states and the optical quantum Hall effect

Zhenisbek Tagay, Ahmed Abouelkomsan, Yugo Onishi et al. (+4 more)

The integer quantum Hall effect remains the most paradigmatic and remarkable example of exact quantization in condensed matter physics. Its Hall conductivity $\sigma_{xy}$ is fixed to $e^2/h$ times an integer to a precision limited only by measurement and independent of disorder or interactions. This robustness has several explanations, each capturing a strikingly different piece of the physics. In Laughlin's gauge argument, flux insertion pumps an integer charge between edges, so quantization follows from gauge invariance alone. The edge-channel picture instead attributes transport to chiral, ballistic 1D channels at the sample boundary, one per filled Landau level. Bulk arguments tie $\sigma_{xy}$ to a topological invariant of the disordered system, with extended states compensating exactly for the current not carried by localized ones. Here we demonstrate an additional route to understanding quantization, rooted in the finite-frequency dissipative electrodynamics of the bulk. Using high-precision terahertz Faraday rotation and numerics, we show that the cyclotron resonance alone does not give quantized plateaus under Kramers-Kronig transformation. Quantization is recovered only once a faint, low-frequency, topologically enforced dissipative contribution from impurity states is included. This dynamical mechanism, hiding in plain sight within the dissipative response, ties the DC value to the finite-frequency optical response and offers a new route to quantization through the optical Hall effect, complementary to previous pictures.
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All authors Zhenisbek Tagay, Ahmed Abouelkomsan, Yugo Onishi, Adbhut Gupta, Loren Pfeiffer, Liang Fu, N. P. Armitage

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

Phase-Field Modeling of Liquid Phases with Ordering

Yanzhou Ji and Chengyin Wu

A strong negative enthalpy of mixing can lead to ordering in liquid solutions. To describe ordering in liquids, different thermodynamic descriptions of liquid phases have been developed in the CALPHAD approach, including the associate model and the modified quasichemical model. In this study, we show that the key to directly incorporate these liquid phase models and their corresponding CALPHAD-type free energy formulations in phase-field simulations, is to identify the internal processes and to properly define the internal thermodynamic variables. The free energy of the liquid phase can then be converted to a function of component compositions and internal variables, and the evolution of the internal variables will be determined via Allen-Cahn-type relaxation equations. We demonstrate this modeling strategy using the formation of stoichiometric phases from liquid phases in Na-Sb and Na-Sn systems. This study will pave the way for predicting microstructure evolution in liquids with ordering using phase-field simulations.
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Mesoscale and Nanoscale Physics

Long-lived divergence from equilibrium of electrons, nuclear spins and lattice for a solid state ion trap at low temperature

Guy Matmon, Manuel Grimm, Markus Mueller et al. (+3 more)

A fundamental problem in physics as well as engineering is equilibration. For example, the regulation of thermal and quantum fluctuations enables thermal and quantum annealing of complex systems. A key question is how different subsystems, such as electrons, nuclear spins and phonons equilibrate on their own as well as with each other. Level crossings play a special role in the dynamics of coupled degrees of freedom, for it is here that entanglement can be maximized to speed up relaxation. Here we use optical methods to establish the electronuclear level scheme, including avoided and unavoided crossings, and to examine equilibration of a rare earth ion (Ho$^{3+}$) in a salt (LiYF$_4$), a model system with quantum fluctuations which can be tuned via an external magnetic field transverse to the crystallographic long axis of the tetragonal host. We track the state of the system by monitoring the populations of the levels as a function of swept longitudinal fields, and discover that at low temperatures, depending on the experimental protocol, vastly different non-equilibrium states arise. We find evidence that nuclear spin excitations diffuse and equilibrate without the assistance of phonons, and tend to acquire higher effective temperatures than the electronic spins and the yet cooler lattice. A theory of thermally assisted tunneling rules out the standard scenario of phonon -assisted tunneling, and instead suggests that fast dynamics at level anti-crossings are facilitated entirely by nuclear spin diffusion. This provides a new understanding of thermalization and related slow relaxation phenomena in dense, multi-component interacting quantum systems.
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All authors Guy Matmon, Manuel Grimm, Markus Mueller, Byron J. Villis, Andrew J. Fisher, Gabriel Aeppli

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

Fluctuation-induced ionic friction at solid-electrolyte interfaces

Damien Toquer, Baptiste Coquinot, Nikita Kavokine et al. (+1 more)

In this article, we explore how transport at a solid–electrolyte interface is affected by the internal fluctuations of the solid and quantify the role of solid excitations on ionic friction. Combining molecular dynamics simulations with theoretical modeling, we show that the presence of ions enhances the dynamical response of the electrolyte compared to pure water. This leads to an ionic contribution to the fluctuation-induced interfacial friction. We show that collective ionic modes dominate the dissipation, such that ionic friction arises from many-body effects rather than from individual drag forces. Furthermore, high-frequency molecular modes are found to be ion-specific, suggesting possible routes toward spectral separation of ions, with implications for desalination, filtration, and energy conversion.
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All authors Damien Toquer, Baptiste Coquinot, Nikita Kavokine, Lydéric Bocquet

Mesoscale and Nanoscale Physics

Topological Transverse Transport without a Gap in Critical Topological Flat Bands

Chang-geun Oh, Tomoki Ozawa and Jun-Won Rhim

Quantized Hall transport is traditionally anchored to a bulk spectral gap, which isolates the occupied subspace and exponentially suppresses thermal deviations. Recently discovered critical topological flat bands (CTFBs) challenge this paradigm: an exactly flat band touches a dispersive continuum while retaining a well-defined integer Chern number. However, their finite-temperature transverse transport properties remain entirely unexplored. Here, we develop a low-temperature theory of the intrinsic electrical, thermoelectric, and thermal Hall responses in CTFBs. At fixed particle number, the macroscopic flat-band degeneracy forces a singular Lambert-$W$ drift of the chemical potential, generating an algebraic-logarithmic hierarchy of low-temperature corrections: $T\ln(1/T)$ for electrical Hall, $T[\ln(1/T)]^2$ for thermoelectric Hall, and $T[\ln(1/T)]^3$ for thermal Hall conductivity, replacing the activated thermal protection of a gapped Chern insulator. By contrast, externally pinning the chemical potential to the flat-band energy locks the flat band to half occupation at any nonzero temperature, obstructing the recovery of the fully filled topological ground state as $T\to0^+$. Our results establish that a bulk spectral gap is unnecessary for zero-temperature Hall quantization, but indispensable for its exponential thermal protection.
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Mesoscale and Nanoscale Physics

Fingerprints of unconventional pairing in superconductor-hole-gas heterostructures

Serafim S. Babkin, Karsten Flensberg, Jeroen Danon et al. (+1 more)

Superconductor and two-dimensional hole gas heterostructures are promising platforms for quantum devices, but their intrinsic properties, e.g., unconventional pairings, are hard to access experimentally. Here, we show that the in-plane-field dependence of the superfluid stiffness and spin susceptibility exhibits nonanalyticities encoding the induced gap structure in the hole gas. Additionally, Fourier harmonics of the stiffness tensor's field-angle dependence, accessible via kinetic-inductance measurements, provide fingerprints of unconventional pairing and cubic Rashba coupling.
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All authors Serafim S. Babkin, Karsten Flensberg, Jeroen Danon, Maksym Serbyn

Mesoscale and Nanoscale Physics

Atomic-Scale Imaging of Lattice Relaxation and Topological Flat Bands in Helical Trilayer Graphene

Shudan Jiang, Zonglin Li, Yu Gu et al. (+15 more)

Helical trilayer graphene (HTG) has emerged as a highly tunable moire quantum material that hosts strong electronic correlations and nontrivial band topology. However, the atomic-scale lattice structure and local electronic properties have remained largely unexplored. Here we present a comprehensive real-space study of HTG using a combination of scanning near-field optical microscopy and low-temperature scanning tunneling microscopy. We directly image supermoire lattice relaxation, revealing large triangular domains separated by sharp domain walls, as well as stripe domains connected by smoothly varying boundaries. Atomic-scale spectroscopy uncovers flat bands with a honeycomb electronic texture and one-dimensional boundary states confined to domain walls. By systematically varying the twist angle, we identify a magic angle of approximately 1.9{\deg}, substantially larger than the 1.6 degree predicted by theory. Our results establish a direct microscopic link between lattice relaxation and flat bands in HTG.
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All authors Shudan Jiang, Zonglin Li, Yu Gu, Liang Liu, Dandan Guan, Yaoyi Li, Hao Zheng, Canhua Liu, Kenji Watanabe, Takashi Taniguchi, Shengwei Jiang, Xiaoxue Liu, Zhiwen Shi, Guorui Chen, Jinfeng Jia, Tingxin Li, Can Li, Shiyong Wang

Mesoscale and Nanoscale Physics

Thermal magnon transport in FM/AFM bilayers

Moumita Kundu and Ulrich Nowak

Progress in information processing relies on spintronics, where magnetic states serve as efficient carriers for data storage and transfer. In this work, we theoretically study magnon propagation in a bilayer composed of a ferro-magnet and an antiferromagnet. For this purpose, we probe the spin Seebeck effect by introducing a spatially varying temperature profile. This generates a local magnon excitation and a continuous magnon flux from hot to cold regions which we quantify through the resulting non-equilibrium magnon accumulation. Based on the chirality of these modes, we identify specific constraints for magnon modes traveling either from the ferromagnet into the antiferromagnet or vice versa. A key finding is the observation of a thermally triggered spin current in the antiferromagnet, a phenomenon typically absent in bulk antiferromagnets that obey time-reversal symmetry. These results provide important insights into the design of heterostructures for magnonic chirality-selective spin transport.
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Mesoscale and Nanoscale Physics

Multi-Branch Transport in a Back-gated WS$_2$ Transistor at Deep-Cryogenic Temperature

Megan Powell, Vilas Patil, Hazel Neill et al. (+5 more)

Two-dimensional materials are promising candidates for electronic applications beyond the operating limits of conventional semiconductor technologies. Within this class, transition-metal dichalcogenides offer attractive properties for field-effect transistor operation, with tungsten disulphide (WS$_2$) emerging as a particularly promising material for operation at cryogenic temperatures. Here, we investigate the electrical performance of a back-gated multilayer WS$_2$ transistor at deep cryogenic temperature, down to 20 mK. The device remains strongly gate-tunable throughout the cryogenic regime, with an effective on/off current ratio exceeding $10^{5}$. Most notably, the low-temperature turn-on characteristics exhibit reproducible shoulder-like features, which we describe using a phenomenological model comprising multiple effective conduction branches operating in parallel.
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All authors Megan Powell, Vilas Patil, Hazel Neill, Stephen O'Sullivan, Paul K. Hurley, Lida Ansari, Farzan Gity, Alessandro Rossi

Mesoscale and Nanoscale Physics

Magnetization relaxation of interacting chains of nanomagnets

Denis Ledue, Renaud Patte, Francois Vernay et al. (+1 more)

We investigate the magnetization dynamics crossover from single-particle to collective behavior in a one-dimensional chain of dipolar-coupled nanomagnets with uniaxial anisotropy. Using both an intermediate-to-high damping (IHD) analytical approach based on Langer's theory and time-quantified Monte Carlo (TQMC) simulations, we derive and validate semi-analytical expressions for the relaxation rate and the magnetization relaxation curves. Our main results include: (i) a closed-form expression for the relaxation rate accounting for (weak) dipolar interactions, (ii) a two-exponential semi-analytical formula for the magnetization dynamics $m(t)$ of an interacting chain, and (iii) a systematic comparison with TQMC simulations, showing good agreement for a wide range of parameters. The analysis reveals a field-controlled crossover from uniform (macrospin-like) reversal to edge-nucleation propagation, driven by the spatial inhomogeneity of dipolar stabilization. The derived expressions provide a computationally efficient framework for predicting the relaxation behavior of dipolar-coupled nanomagnetic assemblies.
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All authors Denis Ledue, Renaud Patte, Francois Vernay, Hamid Kachkachi

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

Topological Fermi-polaron Polaritons with Electrically Invertible Topology

Xin Xie, Chenxi Liu, Yuze Liu et al. (+8 more)

Topological polaritons provide a route to engineer optical Chern bands, but realizing large gaps or efficient electrical tunability remains challenging because time-reversal-symmetry breaking in semiconductor polariton systems is typically limited by weak Zeeman splittings. Here, we demonstrate a topological Bose-Fermi mixture — Fermi-polaron polaritons —where large gaps and electrically invertible topology emerge from the interplay of Fermi-polaron many-body interactions, photonic symmetry, and strong light–matter coupling. Using a gate-controlled monolayer transition-metal dichalcogenide integrated with a symmetry-engineered photonic crystal, we create Fermi-polaron polaritons with topological band structures. The direct coupling between band topology and gate-tunable many-body interactions provides a mechanism for amplified time-reversal-symmetry breaking beyond bare Zeeman splitting and enables electrical inversion of topological gaps, Berry curvature, and Chern numbers. These results establish an electrically reconfigurable platform for interaction-driven topological states and programmable topological photonics.
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All authors Xin Xie, Chenxi Liu, Yuze Liu, Lingxiao Zhou, Chulwon Lee, Yuhan Zhang, Nathanial Lydick, Kenji Watanabe, Takashi Taniguchi, Kai Sun, Hui Deng

Mesoscale and Nanoscale Physics

Anisotropic wavevector-dependent damping of thickness-quantized magnons

Tamara Azevedo, Rostyslav O. Serha, Yannik Kunz et al. (+7 more)

Magnon damping is a key factor governing spin-wave transport and nonlinear dynamics of multimode magnon systems. However, many descriptions rely on the assumption of an effective mode-independent parameter, which can mask wavelength-dependent relaxation processes that depend on propagation geometry and mode profile. Here, we employ high-resolution parametric-instability spectroscopy to probe thickness-quantized spin waves with wavelengths down to about a hundred nanometers in micrometer-thick yttrium iron garnet films. The instability threshold exhibits a regular sawtooth dependence on the magnetic field, arising from switching between discrete thickness modes. Comparison with dipole–exchange theory reveals anisotropic wavevector-dependent damping that increases with mode number and depends differently on the in-plane and out-of-plane wavevector components.
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All authors Tamara Azevedo, Rostyslav O. Serha, Yannik Kunz, Matthias R. Schweizer, Vitaliy I. Vasyuchka, Mathias Weiler, Andrii V. Chumak, Burkard Hillebrands, Mikhail Kostylev, Alexander A. Serga

Mesoscale and Nanoscale Physics

Emergent Quantum Geometric Phases in Holey Graphene

Pablo Canteli, Yuriko Baba, Juan Salvador-Sánchez et al. (+11 more)

In graphene and other two-dimensional materials, periodic modulations of the electron density can significantly alter the energy spectrum and transport properties. Here, we report magnetotransport measurements in encapsulated monolayer graphene with ultra-high-quality patterned periodic antidot lattices that preserve the intrinsic electronic properties of the material. This lithographically defined platform enables controlled access to commensurability and superlattice phenomena at length scales otherwise difficult to achieve. By systematically tuning the lattice dimensions, we reveal a hierarchy of classical commensurability features arising from cyclotron orbits with comparable radii that follow multiple classical trajectories, resulting in broadened resistance peaks beyond the conventional single-orbit picture. Superimposed on these features, we observe pronounced Brown-Zak oscillations arising from the quantum commensurability between the magnetic flux quantum and the unit cell of the engineered Bravais lattices. We demonstrate that the intrinsic geometric phase of our system is directly measurable and show a precise matching of the magnetic field periodicity to the lithographic periodic patterning, where moiré-like electronic spectra can be geometrically generated in single-layer graphene without the need for twist, lattice mismatch, or multilayer stacking. Our results establish nanopatterned graphene as a clean, tunable, and scalable platform for realizing and exploring moiré physics through on-demand real-space design
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All authors Pablo Canteli, Yuriko Baba, Juan Salvador-Sánchez, Jorge Estrada-Álvarez, Ana Pérez-Rodríguez, Carlos Sánchez-Sánchez, Vito Clericò, Takashi Taniguchi, Kenji Watanabe, Francisco Domínguez-Adame, Rafael A Molina, Enrique Diez, Elena Díaz, Mario Amado

Mesoscale and Nanoscale Physics

Giant PhotoMagnetoDiode Effect

V. L. Korenev and S. A. Tarasenko

We report the observation of a photomagnetodiode effect with a giant rectification ratio of 10 at a magnetic field of 0.1 T. The effect consists of a diode-like dependence of the photocurrent on the applied voltage that emerges in an external magnetic field. Such a pronounced nonreciprocity is observed in high-quality VPE-grown GaAs samples at low temperatures at interband photoexcitation. The photoconductivity is invariant upon reversing the polarity of both electric and magnetic fields. Similar behavior with even higher asymmetry ratio is observed in photoluminescence. The findings are well described by the theory of ambipolar drift of carriers in electric and magnetic fields together with fast surface recombination.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Quantum Well Resonant Tunneling Diode Probe of Correlated States in Twisted Bilayer MoS$_2$

Chengjie Zhou, Jin-Xin Hu, Sihong Xu et al. (+4 more)

Moiré superlattices formed in transition metal dichalcogenides (TMDs) offer a versatile platform for exploring emergent quantum phases arising from strong electronic correlations. In this work, we develop a new experimental platform, the quantum well resonant tunneling diode (QWRTD), to probe the electronic landscape of $\approx 57^\circ$ twisted bilayer MoS$_2$ (tMoS$_2$). By measuring the differential conductance ($dI/dV_{\text{Probe}}$) as a function of filling factor $\nu$ and displacement field $D$, we observe a robust integer correlated insulating state at $\nu = 1$ that persists across the measured displacement field range. Furthermore, we identify a displacement-field-induced fractional insulating state at $\nu = 3/4$ for $D < -75$~mV/nm. Temperature- and magnetic-field-dependent measurements characterize this $\nu = 3/4$ state as a potential generalized Wigner crystal. These correlated states are also observed in another device with a similar twist angle ($\approx 56.5^\circ$). Our results provide direct evidence of correlated states in near-AB-stacked tMoS$_2$ and establish QWRTD as a powerful experimental tool for investigating strong correlations and topology in van der Waals heterostructures.
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All authors Chengjie Zhou, Jin-Xin Hu, Sihong Xu, Zijing Jin, Hui Li, Kam Tuen Law, Jiannong Wang

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

Reservoir- and Measurement-free Microwave Initialization of Semiconductor Spin Qubits

Leon C. Camenzind, Ik Kyeong Jin, Akito Noiri et al. (+4 more)

Scalable quantum processors require repeated qubit initialization throughout large arrays. In semiconductor spin qubits, fast initialization commonly relies on local reservoir access or measurement-based feedback, requiring dedicated infrastructure that becomes increasingly difficult to distribute as processors scale. Here, we demonstrate reservoir- and measurement-free initialization of a silicon spin-qubit pair in an industrially fabricated Si/SiGe quantum-dot device using a fixed sequence of microwave and baseband pulses. Odd spin-parity states relax to the singlet charge state, whereas blocked even spin-parity states are microwave-driven through the triplet manifold and subsequently converted to the singlet by singlet-triplet mixing and charge hybridization. Repeated cycles produce the singlet-associated charge outcome with a median probability of 99.4% across the sampled preparation states, while exchange spectroscopy independently verifies mapping to the target $|\uparrow\downarrow\rangle$ operational state. Microwave spectroscopy and time-domain measurements identify the dark-state-limited single-cycle transfer and the subsequent blockade-lifting dynamics that set the initialization time scale. The demonstrated pumping sequence uses approximately $12\,\mu\mathrm{s}$ of microwave bursts and mixing dwells, while we project sub-microsecond initialization under improved device conditions. These results establish fixed-sequence microwave initialization as a scalable control primitive for semiconductor spin-qubit processors, based on singlet-triplet physics that can be adapted to platforms with suitable Pauli-blockade transitions.
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All authors Leon C. Camenzind, Ik Kyeong Jin, Akito Noiri, Kenta Takeda, Takashi Nakajima, Takashi Kobayashi, Seigo Tarucha

Mesoscale and Nanoscale Physics

Spin Birefringence and Spin Polarized Transmission across p-wave Altermagnetic Heterostructure

Yong-Mei Zhang and Zhi-Ping Niu

This paper investigates electron transport properties across N/AM interface or N/AM/N heterojunction. Based on Hamiltonians in the three regions, general wave functions are proposed and combined linear equations are constructed. Reflection and transmission coefficients as well as spin polarizatioin of the altermagnet heterostructure are obtained. When electron incident from normal metal to altermagnet, it is scattered and split into + branch and _ branch transmitting into altermagnet zone with separate refractive angles. Total internal reflection might happen to + branch. Critical angle of total reflection is determined by incident conditions and properties of AM material. In the heterostructure case, spin-up and spin-down electrons transmitted with same propagation directions but different transmission probabilities. Transmission and polarization periodically change with length of the junction. Exchange field and spin-splitting strength have pronounced effect on spin-polarized transmittance while the effect of spin-orbit coupling is quite small. Optimized system can be found by tuning parameters of the AM material. This study will lay the theoretical groundwork for experimental research on spin-selective outputs and spintronic devices.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Surface quantum response of plasmonic heterodimers: revealing the hidden nonlocality

Ida Juliane Bundgaard, Christos Mystilidis, Frederik Wulff Christensen et al. (+7 more)

We probe the quantum response of plasmonic heterodimers with extended (plane waves) and localized (point dipoles and electron beams) sources. Implementing the surface-response formalism based on Feibelman parameters, we explore nanosphere heterodimers where one particle is characterized by electron spill-in, while the other by spill-out. We identify situations where the two quantum surface corrections that act in opposite spectral-shift directions can cancel out, producing mesoscopic spectra that do not differ from the predictions of the local-response approximation. This picture changes dramatically once localized sources are implemented, allowing one to selectively probe the quantum response of the particles independently. Such probing occurs in terms of higher-order modes in the case of electron beams, or for the complete modal landscape in the case of point dipole sources. This description offers a new insight into the mechanisms governing light–matter interactions in extreme plasmonic architectures, and opens new perspectives for the understanding and tailoring of plasmonic–quantum emitter architectures.
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All authors Ida Juliane Bundgaard, Christos Mystilidis, Frederik Wulff Christensen, Lorenz Huber, P. Elli Stamatopoulou, Carsten Rockstuhl, P. A. D. Gonçalves, Ulrich Hohenester, N. Asger Mortensen, Christos Tserkezis

Mesoscale and Nanoscale Physics

Optical observation of interlayer spin correlation

Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin et al. (+16 more)

Spin correlations govern numerous collective behaviors of quantum materials, underpinning exotic phenomena such as unconventional superconductivity and topological magnetism. In layered materials, the interlayer spin correlation is important because it characterizes the magnetic ground state and determines spin transport across the interface. Yet interlayer spin correlations have remained hard to measure directly, leaving one of the most basic quantities of two-dimensional magnetism out of experimental reach. Here we provide the first direct optical probe of interlayer spin correlations, using two-magnon Raman scattering in the van der Waals antiferromagnet (AFM) CrSBr, in remarkable agreement with a microscopic spin-wave model without any fitting parameter. Moreover, the two-magnon channel switches on only in the AFM state and vanishes when a magnetic field takes the crystal to a ferromagnetic state. We furthermore establish an exciton-mediated variant of the technique, where tuning the laser near the exciton resonance enhances the signal roughly tenfold due to the exciton's large oscillator strength. Magnon-pair Raman spectroscopy thus opens a direct optical window into interlayer spin correlations in van der Waals magnets, extendable to twisted bilayers and proximity-coupled heterostructures.
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All authors Akiyoshi Park, Pranshoo Upadhyay, Andrey Grankin, Emil Viñas Boström, Mahdi Ghafariasl, Hassan Alnatah, Masoud Mohammadi-Arzanagh, Gautam Nambiar, Beini Gao, Alireza Alvandi, Sakthi Rajmano Madhan Kumar, Ghadah Alshalan, Isaac Sherwood, Mahmoud Jalali Mehrabad, You Zhou, Arun Ramanathan, Xavier Roy, Angel Rubio, Mohammad Hafezi

Tuesday 29 September

Mesoscale and Nanoscale Physics

From Dirac surface response to Casimir forces in Bumblebee-CFJ electrodynamics

F. C. E. Lima

We examine the Casimir interaction between planar massive Dirac materials coupled to a Lorentz-violating Bumblebee-CFJ vector sector. The standard renormalized one-loop polarization tensor of the Dirac quasiparticles is used as the nonlocal surface response in the Lifshitz scattering formalism. For the normally oriented Lorentz-violating background, the surface response remains parity-even. Hence, the repulsive regimes do not originate from a Hall-like conductivity. Instead, they arise from the CFJ-induced mixing of TE and TM bulk modes, combined with the spatially dispersive response of the Dirac sheets. Furthermore, we show that the Dirac mass suppresses the interaction and changes its large-distance scaling, whereas the CFJ parameter drives transitions between attractive and repulsive regimes.
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Mesoscale and Nanoscale Physics

From Quantized Hall Plateaus to Topological Surfaces: Quantum Capacitance as a Unifying Probe

A. Siddiki

The quantum Hall effect, topological insulators and quantum capacitance are usually treated as distinct topics, yet they share a common thread. Bulk topology determines a protected boundary state, and quantum capacitance is one of the few electrostatic methods able to detect the density of states. This paper examines that connection. The integer quantum Hall effect showed that quantised boundary transport has a topological origin given by the Chern number, later extended to zero field in Z2 topological insulators with helical edge and Dirac surface states. Quantum capacitance, developed for two-dimensional electron gases, has since become a key technique for mapping the Landau level density of states underlying the quantum Hall effect and the linear Dirac density of states of topological surface states. This picture is complicated by self-consistent screening theory, which shows that the quantum Hall bulk splits into compressible and incompressible regions that shift with field. Local quantum capacitance measurements confirm that the bulk is not uniformly insulating across a quantised plateau, even though the plateau stays quantised. The paper argues that quantum capacitance is not a secondary feature of these phenomena but one of the clearest windows into the boundary density of states that topology imposes, and that it reveals the local, field-dependent nature of the insulating bulk assumption underlying the analogy between the quantum Hall effect and topological insulators.
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Mesoscale and Nanoscale Physics

Interior contacts in a narrow quantum Hall bar: a two-dimensional self-consistent screening calculation of the current distribution

A. Siddiki

We present fully two-dimensional self-consistent calculations of the electron density, local filling factor and current distribution in a six-terminal GaAs/AlGaAs Hall bar of width W = 3 micrometers with two additional square contacts, A and B, placed on the channel axis and connected to the outside by air bridges, following the interior-contact experiment of Kendirlik et al. [Nat. Commun. 8, 14082 (2017)]. Within the screening theory of the integer quantized Hall effect, the Thomas-Fermi-Poisson equations are solved on the full device plane, including the probe arms and the depletion regions around the interior contacts, and the current is obtained from a local Ohm's law with a filling-factor dependent conductivity tensor, for source-drain and interior-contact excitation separately. Across the nu = 2 plateau the interior contacts pass through three regimes as the field is lowered: galvanically connected to the current path in a compressible bulk, floating in the Hall potential of a fully incompressible bulk, and isolated from both the bulk and the edge strips by closed incompressible rings encircling each contact. In the floating and isolated regimes the two-terminal resistance between the interior contacts rises by orders of magnitude. The isolation interval lies within the Hall plateau and ends before its low-field edge; its high-field end is sharply set by the disappearance of the incompressible bulk, while its low-field end shrinks with temperature and Landau-level broadening. The resistance between an interior contact and a perimeter probe approaches one half of the quantized Hall resistance across the plateau. The topology of the incompressible regions and the boundaries of the isolation interval are insensitive to the regularization of the local conductivity, to the grid and to moderate disorder, whereas the magnitude of the isolation resistance is not.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Causality violations in cluster GW+DMFT: Exact Lehmann moments and the necessity of non-local vertex corrections

Michael O. Atambo

The $GW$ approximation is known to violate causality, yielding self-energies with unphysical poles in the upper half of the complex frequency plane. The combined $GW$+DMFT approach attempts to cure this by injecting the exact local self-energy. Using exact diagonalization on minimal Hubbard clusters, we rigorously benchmark the analytic structure of the $GW$ and $GW$+DMFT self-energies. We demonstrate a “dimer anomaly”: on the two-site dimer, inversion symmetry forces the non-local self-energy to be purely real, masking the causality violation. Moving to the non-bipartite three-site Hubbard ring, where hopping frustration yields a finite imaginary part in the non-local self-energy, we prove that both $GW$ and $GW$+DMFT violate the Matsubara causality condition. By analytically evaluating the exact Lehmann moments, we derive a closed-form expression for the high-frequency tail of the self-energy, $C = U^2\,\mathrm{Var}(d)\,I$, and show that standard methods fail to capture the exact equal-time two-particle correlators required to enforce it. The equal-time correlators that govern this tail are precisely those encoded in the high-frequency asymptotics of the fully irreducible vertex $\Lambda$; nevertheless, our intermediate-frequency analysis demonstrates that the local-only truncation of $\Lambda$ in $GW$+DMFT leaves non-local RPA resonances uncorrected. The two diagnostics are logically independent: $GW$+DMFT satisfies the necessary high-frequency condition $C\succeq0$ yet still violates the Matsubara sign condition at finite frequencies. Taking the interpolation-free real-axis evaluation of the exact self-energy as the primary causality certificate, with rational continuation as supporting evidence, we establish a rigorous benchmark demonstrating that local self-energy corrections are insufficient to guarantee a causal analytic structure on frustrated clusters.
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Mesoscale and Nanoscale Physics

Three-Dimensional Shankar Skyrmions in Frustrated Antiferromagnets

Vladyslav M. Kuchkin, Ricardo Rama-Eiroa, Carlos Saji et al. (+2 more)

We formulate a continuum theory for three-dimensional (3D) Shankar skyrmions in frustrated chiral antiferromagnets (AFs) and derive the conditions for metastable finite-size $\pi_3(SO(3))$ solitons. A Derrick–Hobart scaling analysis shows that exchange, Dzyaloshinskii–Moriya interaction (DMI), anisotropy, and frustration can balance to set a finite equilibrium size. We identify two microscopic routes to this common topological and stabilization framework: intrinsic rotation-frame order in noncollinear AFs and an $\mathbb{S}^3$ extension of an amplitude-softened Néel field. In the latter, the smooth four-component texture projects onto a Néel field containing a spatially separated pair of oppositely charged Bloch points. Numerical minimization yields metastable monopole textures, while the effective dynamics identifies coherent breathing oscillations as their characteristic finite-frequency collective mode. Our results provide a microscopic framework for the statics and dynamics of Shankar skyrmions and identify frustrated chiral AFs as promising hosts of 3D non-Abelian topological textures.
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All authors Vladyslav M. Kuchkin, Ricardo Rama-Eiroa, Carlos Saji, Alvaro S. Nunez, Roberto E. Troncoso

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

Single-particle detection of the Néel transition in LaCrO$_3$ microcrystals via widefield nitrogen-vacancy center magnetic imaging

Carson O. Patterson, Ethan Q. Williams, Eden Tzanetopoulos et al. (+6 more)

Rare-earth orthochromites exhibit rich, chemically tunable magnetic behavior, making them attractive for temperature sensing and temperature-dependent switching applications. Realizing this potential requires understanding how synthesis conditions and particle morphology influence the underlying Cr$^{3+}$ magnetic ordering. Orthochromite magnetism has traditionally been characterized by ensemble techniques such as neutron diffraction and SQUID magnetometry, which report only averaged magnetic properties and cannot access particle-level information. Here we use widefield nitrogen-vacancy (NV) magnetometry to image the stray magnetic fields of individual LaCrO$_3$ microcrystals produced via molten salt synthesis, resolving the antiferromagnetic Néel transition in single particles. Of the 18 particles measured between 278 and 298 K, five show a clear onset of magnetization near the ensemble Néel temperature, four show no measurable magnetic signal, and nine display temperature-independent signatures consistent with localized magnetic impurities undetected by ensemble measurements. Below the Néel temperature, particle magnetization remains fixed under a rotating in-plane field, consistent with canted antiferromagnetic ordering rather than paramagnetism, while repeated thermal cycling shows that the magnetic field pattern varies between cooldowns with certain features recurring at the same locations, consistent with micron-scale structurally pinned antiferromagnetic domains in which the sign of the weak ferromagnetic moment is set independently each time they pass through the Néel transition. These results establish single-particle NV magnetometry as a sensitive probe of magnetic ordering and domain structure in individual microcrystals, a key step toward correlating individual particle morphology and defect structure with magnetic ordering in orthochromites.
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All authors Carson O. Patterson, Ethan Q. Williams, Eden Tzanetopoulos, Benjamin C. Li, Rachel T. Smith, Matthew Chang, Hideyuki Watanabe, Daniel R. Gamelin, Kai-Mei C. Fu

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

Interferometric Readout of Momentum-Space Topology in a Programmable Dissipative Photonic Circuit

Andrea Cataldo, Emil J. Bergholtz, Daniel Leykam et al. (+2 more)

Topology under non-Hermitian dynamics is encoded in the phase of the bulk evolution, yet strong dissipation suppresses the amplitudes carrying it. We resolve this using a programmable photonic integrated circuit that implements such dynamics in synthetic momentum space through unitary dilation, with the phase recovered by phase-shifted interferometry. For the non-Hermitian Su-Schrieffer-Heeger model, the method distinguishes trivial and non-trivial Zak phases and yields a coherence winding $q=\pm 1$ induced by an exceptional point. Extending to a synthetic torus via a Rice-Mele pump gives the first Chern number $\mathrm{Ch}_1=0,1$ for trivial and non-trivial cycles, showing a programmable route to momentum-space topology under strongly dissipative non-Hermitian dynamics.
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All authors Andrea Cataldo, Emil J. Bergholtz, Daniel Leykam, Jun Gao, Ali W. Elshaari

Mesoscale and Nanoscale Physics

Hidden Angular Momentum Loop Currents in Symmetric Crystals

Rule Yi, Vincent P. Flynn and Benedetta Flebus

Loop-current order has been invoked to explain unconventional electronic phases, with the crystal lattice usually regarded as a passive host for the underlying collective dynamics. Here we show that circulating currents can originate in the lattice itself, sustained by quantum and thermal fluctuations. We develop a microscopic description of angular momentum exchange that separates transfer between sites from crystalline restoring torques and reveals how circulation can emerge in a symmetry-preserving equilibrium state. In a two-dimensional elastic lattice preserving time-reversal, inversion, and fourfold rotational symmetries, we uncover opposite current loops on neighboring plaquettes while the mean angular momentum vanishes at every site. These currents are sustained by zero-point fluctuations in the phonon vacuum and persist at finite temperature. We trace their origin to directional elastic interactions in locally symmetry-lowered environments, with the higher crystal symmetry organizing the resulting currents into a compensated pattern of circulation. Our results extend the physics of equilibrium loop currents to lattice dynamics, revealing that elastic geometry can organize fluctuations into persistent angular momentum circulation without electronic or magnetic order, external driving, or spontaneous symmetry breaking.
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Mesoscale and Nanoscale Physics

Spatially dispersive photogalvanic effects as a probe of quantum geometric tensor

Yicong Chen, Bumseop Kim, Senin Ahammed Akkara Paramban et al. (+8 more)

Berry curvature and quantum metric of Bloch states govern a wide range of phenomena, yet unified experimental access to both remains limited. Here we show that light spatial-dispersion alters the selection rules of second-order photocurrents and responses forbidden by parity become allowed in centrosymmetric crystals without any external fields, magnetic order, interfaces, strain, or engineered symmetry breaking. The resulting response is intrinsic and its weight is set by the quantum geometry of the Bloch states. Conventional photogalvanic effects are constrained by crystal symmetry and entangle the quantum metric with shift vector contributions. The spatially-dispersive response instead isolates the quantum metric and Berry curvature in distinct polarization channels. Implementing this approach in 1T'-MoTe2 across its temperature-driven transition to Td-Weyl phase, we resolve helicity-even and helicity-odd photocurrents corresponding to quantum metric and Berry curvature contributions, respectively, from the same device. The metric-dominated response persists across both phases and exhibits a robust spectral structure reproduced by first-principles calculations and linked to momentum-resolved quantum metric hotspots. In contrast, the curvature-driven channel emerges only when inversion symmetry is broken and shows strong sensitivity to carrier doping through competing momentum-space contributions. Our results establish photogalvanic effects with spatially varying optical fields as a general route to accessing quantum geometry in materials where photon energy and electronic filling probe different parts of the excitation manifold.
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All authors Yicong Chen, Bumseop Kim, Senin Ahammed Akkara Paramban, Zhurun Ji, Utkarsh Khandelwal, Shupeng Xu, Jiachen Shi, Sergiy Krylyuk, Albert V. Davydov, Andrew M. Rappe, Ritesh Agarwal

Mesoscale and Nanoscale Physics

Interlayer Fermi Polarons in Bilayer MoTe$_2$

Ruihao Ni, Eugen Dizer, Maximilian Wolf et al. (+15 more)

Atomic bilayers of transition metal dichalcogenides (TMDs) host quantum phases governed by the layer degree of freedom, including bilayer Wigner crystals, fractional Chern insulators, and exciton condensates. These phases are probed primarily through exciton spectroscopy, yet it remains poorly understood how excitons and carriers interact to form Fermi polarons in bilayers, where both the impurity and the Fermi sea carry a layer pseudospin. Progress has been limited because most TMD bilayers have momentum-indirect optical bandgaps, in which non-radiative decay and inhomogeneous broadening obscure the intrinsic spectra. Here, we show that bilayer MoTe$_2$, unlike most TMD bilayers, retains a direct optical bandgap, providing a clean platform for studying bilayer Fermi-polaron physics. In a dual-gated device, an out-of-plane electric field continuously tunes the hybridization between intralayer and interlayer excitons, forming layer-coherent excitons. Upon electrostatic doping, the excitonic spectrum evolves into multiple polaron branches, controlled by both carrier doping and the out-of-plane electric field. Among these, we identify a polaron with no analog in monolayers, in which a layer-coherent exciton is dressed by carriers in the opposite layer, and is quantitatively captured by our field-theoretic model. Our results establish that pseudospin structure in both the impurity and the bath reshapes polaron formation, opening new avenues to many-body states such as Bose-Einstein condensates with interlayer coherence.
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All authors Ruihao Ni, Eugen Dizer, Maximilian Wolf, Son T. Le, Sharadh Jois, Jeffrey J. Schwartz, Liuxin Gu, Rundong Ma, Suji Park, Beini Gao, Lifu Zhang, Houk Jang, Takashi Taniguchi, Kenji Watanabe, Aubrey T. Hanbicki, Adam L. Friedman, Richard Schmidt, You Zhou