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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Wednesday 14 October · still arriving

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

Nodal nematic superconductivity in moiré graphene

Shuwen Sun, Grgur Palle, Kenji Watanabe et al. (+4 more)

Unconventional superconductivity can emerge in various forms in strongly correlated systems. Historically, the most extensively studied examples include nodal and triplet pairing, which were experimentally detected in cuprates and superfluid helium-3, respectively. Beyond these canonical cases, there are also nematic superconductors, in which the rotational symmetry of the lattice is broken in the superconducting state. While there are several examples of nematic superconductors whose pairing emerges inside a preexisting nematic normal state, experimental evidence for superconducting states that intrinsically break rotational symmetry on their own has been exceedingly scarce. Here, we report the experimental realization and identification of an intrinsic multi-component nodal nematic superconducting state in magic-angle twisted trilayer graphene (MATTG). By integrating tunneling spectroscopy with quantum transport measurements, we directly probe the symmetry of low-energy superconducting quasiparticles. Deep inside the superconducting state, we observe a pronounced two-fold anisotropy in the zero-bias tunneling conductance under an in-plane magnetic field, which is strongly suppressed together with superconductivity as the temperature or perpendicular magnetic field is increased. Remarkably, the major axis of anisotropy rotates by up to $90^{\circ}$ across the superconducting dome, indicating an intrinsic, field- and density-tunable symmetry of the low-energy excitations. These behaviors imply a nodal superconducting state in MATTG that spontaneously breaks the six-fold rotational symmetry of the moiré superlattice. Our results establish moiré graphene as a versatile platform for realizing and interrogating nodal nematic superconductivity, opening a new avenue for exploring unconventional symmetry-breaking superconducting states in two-dimensional materials.
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All authors Shuwen Sun, Grgur Palle, Kenji Watanabe, Takashi Taniguchi, Rafael M. Fernandes, Pablo Jarillo-Herrero, Jeong Min Park

Mesoscale and Nanoscale Physics

Landau quantization of $C_3$ high-fold fermions in honeycomb lattices

Luisa Madail and Joaquín Fernández-Rossier

We present the theory of the Landau levels of $C_3$ high-fold fermions in honeycomb lattices. These fermions arise as a generalization of the tight-binding model in a honeycomb lattice with one orbital per site, known to describe graphene, to a family of models with several orbitals per site transforming according to representations of $C_3$, relevant for triangulene two-dimensional crystals. The models are defined by the number of orbitals $n_a$ and $n_b$ at the $a$ and $b$ sites of the unit cell. They feature energy bands that combine flat bands and Dirac cones, that are isomorphic to those of graphene, the Lieb, $p_{xy}$ honeycomb, and the Kagome tight-binding models. Combining tight-binding and $k\cdot p$ methods, here we compute the Landau levels of the $(1,1)$, $(1,2)$, $(2,2)$ and $(3,3)$ models, we compare them to those of the reference models, and we explore the similarities and differences with the Landau levels in graphene. Our findings reveal that the nature of $C_3$ high-fold fermions combine Dirac-type Landau quantization, originating from the linear crossings and associated relativistic band dispersion, with anomalous Landau quantization rooted in the quantum geometry of the singular flat bands.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Anyons at impurities: Fractional charge measurement and "wrong sign" binding

Haydn S. Adlong, Arthur Christianen, Atac Imamoglu et al. (+1 more)

Recent photoluminescence experiments in twisted MoTe$_2$ have reported signatures of anyons binding to trapped trions. The measured binding energies near the fractional Chern insulators at fillings of $\nu=-2/3$ and $-3/5$ have a ratio close to the fractional-charge ratio $5/3$, even though such scaling is not generally guaranteed for the expected short-range trion-anyon interaction. We model the trapped trion as a fixed impurity, and investigate anyon-impurity binding using a lowest-Landau-level model with exact diagonalization and Jain wavefunctions. Surprisingly, we find that the anyon-impurity binding energy ratio between $\nu=-2/3$ and $-3/5$ is close to $5/3$ for either short- or long-ranged impurity-electron interactions. However, the ratio departs strongly from the charge ratio at intermediate ranges or different Jain states. Remarkably, we also uncover "wrong-sign" binding, whereby both quasiholes and quasiparticles can bind to either sign of a short-range impurity potential by selecting orbital channels with favorable local density oscillations. Our findings show that localized excitons or trions can be used to probe the internal density profile of anyons.
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All authors Haydn S. Adlong, Arthur Christianen, Atac Imamoglu, Glenn Wagner

Mesoscale and Nanoscale Physics

Orbitronics in Zero-dimensional Graphene Nanoflakes via Electrode Coupling

Washington F. dos Santos, Eudes Gomes da Silva, Denis R. Candido et al. (+1 more)

We show that graphene nanoflakes realize a previously unexplored form of orbitronics in which orbital currents emerge in the absence of intrinsic angular momentum of the constituent atoms. Starting with a hexagonal graphene nanoflake, we demonstrate the presence of energy-degenerate eigenstates with opposite non-zero expectation values of the angular momentum. Placing electrodes in an appropriate configuration on the flake couples the reservoirs more strongly to one of the two opposite-angular-momentum states, yielding a chiral current that carries a net orbital angular momentum. The resulting circulating orbital currents generate magnetic fringe fields that are experimentally accessible with spin centers or superconducting quantum interference magnetometers, providing avenues for their experimental realization. Finally, our work establishes a new platform for exploring and studying orbitronics in zero-dimensional systems.
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All authors Washington F. dos Santos, Eudes Gomes da Silva, Denis R. Candido, Alexandre Reily Rocha

Mesoscale and Nanoscale Physics

Designing Hamiltonians with Exactly Flat Higher-Vortexable Bands

Daniel Spasic-Mlacak and Nigel R. Cooper

The zero modes of the Aharonov-Casher Hamiltonian form an exactly flat, vortexable Chern band in any nonuniform magnetic field, analogous to the lowest Landau level. We ask which local Hamiltonians host exactly flat higher-vortexable bands. For a scalar Hamiltonian, an exactly flat first-vortexable band requires a mass proportional to the local field; the Dirac analogue has the same structure. For periodic Bernal bilayer graphene with nonzero mean flux, the exact zero-mode pair is genuinely first-vortexable if and only if the interlayer hopping has nonzero mean; zero-mean hopping instead gives a decomposable pair. Multiorbital models yield exact higher-vortexable chains to arbitrary order, while remaining first order in momentum.
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Mesoscale and Nanoscale Physics

Coherent and dissipative coupling of multiple photon-magnon hybrids in a chiral microwave resonator

Supanat Chantarattana, Charlie W. F. Freeman, Daniel Prestwood et al. (+3 more)

We demonstrate control between level repulsion and level attraction of photon-magnon coupling spectra in a chiral microwave-photon resonator loaded with a 2 mm YIG sphere. By systematically varying the coupling mode volume via the YIG position and the dc-field orientation, we tune the ratio between coherent and dissipative coupling strengths, $J/\Gamma$, and thereby switch between level repulsion and level attraction in the hybrid spectra. Full-wave electromagnetic and magnetization-dynamics simulations reveal multiple photon-magnon hybrid modes with distinct spatial overlap factors, which quantitatively account for gap sizes and their angular dependence. Strong nonreciprocity appears near $J/\Gamma \approx 1$, yielding 28.6 dB isolation ratio in agreement with a unified single-bath input-output model and full-wave simulations. Angular dependence of spectral characterization confirms a picture of the chirality-selection rule and our method of geometric tuning might be useful for considering reconfigurable on-chip isolators.
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All authors Supanat Chantarattana, Charlie W. F. Freeman, Daniel Prestwood, Nimisha Arora, Vincent Castel, Hidekazu Kurebayashi

Mesoscale and Nanoscale Physics

Making Weak Interlayer Excitons Absorb: Manipulating the Effective Oscillator Strength via Purcell Engineering of Critical Coupling

Saroj Poudyal, Anabel Atash Kahlon, Michael Klein et al. (+7 more)

Interlayer excitons (IXs) in transition metal dichalcogenide bilayers possess unique dipolar properties, providing a path for exploring correlated many-body phases, electrically tunable dipole-dipole interactions, and long-range exciton transport. However, the large electron-hole separation underlying their dipolar character also results in a significantly reduced oscillator strength, limiting their direct optical interaction with light, and thus the direct probing of their dynamics and many-body interactions. Here, we show that by using Purcell engineering of critical coupling, a weak interlayer transition can absorb as strongly as the dominant intralayer transition, despite its much smaller intrinsic oscillator strength. Using this approach, in an open cavity architecture we demonstrate an order of magnitude IX absorption modulation in bilayer and trilayer MoS2, with doubling of the IX to A-exciton interaction strength weight from ~23% to ~53%. Furthermore, we demonstrate that in a tailored closed cavity architecture, the IX enhancement can reach beyond an order of magnitude, from ~5% to ~83% excitonic absorption, enabling IX absorption similar to intralayer excitons. An analytical description of the interaction is obtained via a closed-form transmission-line model which separates the excitonic and photonic contributions to the optical response, revealing how cavity architecture can be designed towards critical coupling. Our findings establish Purcell engineering of critical coupling as a general route to controlling and enhancing weak excitonic transitions.
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All authors Saroj Poudyal, Anabel Atash Kahlon, Michael Klein, Matan Meshulam, Yonatan Gershuni, Thomas Poirier, James H. Edgar, Mukesh Kumar, Seth Ariel Tongay, Itai Epstein

Mesoscale and Nanoscale Physics

Observation of correlations beyond the single-particle picture in fermionic counting statistics

Johann Zöllner, Hendrik Mannel, Daniel Oppers et al. (+3 more)

The analysis of charge fluctuations in a mesoscopic device can shed light on the fundamental question of whether or not a single-particle picture of electron transport is appropriate, even in the presence of Coulomb interaction. It has been shown on general grounds that mapping the interacting system onto a fermionic single-particle description is subject to strict constraints on the charge-transfer statistics. These constraints can be formulated in terms of a sign rule for the factorial cumulants of the full counting statistics. We report here on the experimental observation of a sign-rule violation in a most elementary model system, i.e. a single-level quantum dot, and provide theoretical modeling that can quantitatively reproduce the experimental data. This reveals the presence of correlations introduced by electron-electron interaction.
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All authors Johann Zöllner, Hendrik Mannel, Daniel Oppers, Martin Geller, Axel Lorke, Jürgen König

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

Andreev-reflection spectroscopy of SrPt3P single crystals

Valeriy A. Stepanov, Nikolai D. Zhigadlo, Erik Piatti et al. (+1 more)

The interplay between inversion symmetry breaking and superconducting pairing mechanisms is a long-standing puzzle of solid-state physics. Ternary platinum phosphide SrPt$_3$P is an excellent case study in this regard, since it shares the distorted anti-perovskite crystal structure of non-centrosymmetric superconductor CePt$_3$Si but preserves the inversion symmetry center, enabling a direct comparison between the two compounds. Despite intense scrutiny, the superconducting pairing strength, gap symmetry, and magnitude of SrPt$_3$P remain an open question. Here, we study the gap structure of high-quality SrPt$_3$P single crystals with $T_c = 8.6$ K by using point-contact Andreev-reflection spectroscopy. The spectra systematically showed symmetric conductance maxima and a zero-bias minimum, as is typical of fully gapped superconductors, and dips at finite voltage $V_\mathrm{dip}$ due to the local transition of the contact region to the resistive state. In contacts with high resistance, where the dips fall sufficiently far from the gap features, the conductance curves can be well fitted with a single isotropic $s$-wave gap. The temperature dependence $\Delta(T)$ of the gap amplitude extracted from the fit is in good agreement with the one predicted by the BCS theory, with a zero-temperature mean value $\Delta(0)=1.88\pm0.08$ meV, and a mean gap ratio $2\Delta/k_\mathrm{B}T_c = 5.2\pm0.2$.
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All authors Valeriy A. Stepanov, Nikolai D. Zhigadlo, Erik Piatti, Dario Daghero

Mesoscale and Nanoscale Physics

Magnetic field induced modulation of heat transport in CrPS$_4$ probed by TaIrTe$_4$ transverse Seebeck thermometry

Krishnaraajan Sundararajan, Sergio Alvarruiz, Cédric A. Cordero-Silis et al. (+5 more)

Understanding and controlling heat flow in low-dimensional magnetic materials is central to thermoelectrics and spin caloritronics. One of the challenges is the absence of sensitive probes of small, field-dependent modulations in thermal transport. Here we demonstrate transverse Seebeck thermometry using exfoliated TaIrTe$_4$ as a probe of thermal conductivity modulated by an external magnetic field on a van der Waals magnet, CrPS$_4$. With a temperature gradient oriented at $45^\circ$ to its principal crystallographic axes, TaIrTe$_4$ exhibits a transverse thermopower exceeding $200~\mathrm{\mu V\,K^{-1}}$ for 900 nm wide and 30 $\mu$m long electrodes, enabling precise detection of variations in the local temperature gradients. Exploiting this with CrPS$_4$ in a nonlocal geometry, we resolve distinct field dependent thermal conductivity changes associated with both the spin-flip and spin-flop magnetic transitions which we attribute to variations in the magnon$-$phonon coupling in CrPS$_4$. At 5 K, the transverse Seebeck response is modulated by more than 8$\%$ by an external magnetic field, demonstrating strong magnetic control of heat transport in CrPS$_4$. This all van der Waals platform offers high sensitivity to modulations of thermal conductivity and a broad adaptability to a wide range of exfoliated magnetic systems.
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All authors Krishnaraajan Sundararajan, Sergio Alvarruiz, Cédric A. Cordero-Silis, Majid Ahmadi, Marcos H.D. Guimarães, Jagoda S\lawińska, Joseph Barker, Bart J. van Wees

Mesoscale and Nanoscale Physics

Nonadiabatic dynamics under slow parametric modulation in nonlinear graphene resonators

Chris F. D. Wattjes, Haemin Lim, Pierpaolo Belardinelli et al. (+3 more)

Resonantly driven nonlinear systems, from superconducting circuits to nanomechanical resonators, are often described in a rotating frame, where amplitude and phase evolve slowly within a fixed landscape of stable and unstable states. How such systems behave when this landscape moves on their intrinsic timescale remains to be understood. Here, we exploit the strong tension sensitivity of a graphene drum's resonance frequency to reshape the rotating-frame phase space of a bistable mechanical mode in real time, combining an MHz-frequency resonant drive with kHz-frequency tension modulation. When the modulation rate becomes comparable to the mechanical relaxation rate, adiabatic tracking of the fixed points breaks down. Instead, parametric instabilities emerge in the slow dynamics at frequencies set by libration around these points rather than by the mechanical resonance; stronger modulation drives period doubling and chaos. A slow-variable theory predicts where the instabilities occur, and Melnikov analysis captures the onset of chaos. These results show that the rotating frame itself can be parametrically driven, offering a general route to controlling nonadiabatic dynamics in systems ranging from Josephson bifurcation amplifiers to cold atoms.
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All authors Chris F. D. Wattjes, Haemin Lim, Pierpaolo Belardinelli, John E. Sader, Peter G. Steeneken, Farbod Alijani

Mesoscale and Nanoscale Physics

Orientation-Dependent Universality of Spin-Transport Fluctuations in Two-Dimensional Altermagnets

Yihan Zhang, Zheng Qin, Donghui Xu et al. (+3 more)

Universal conductance fluctuations are conventionally organized by symmetry class, but crystalline anisotropy introduces additional structure in spin transport. Here we demonstrate orientation-dependent universal spin-transport fluctuations in disordered two-dimensional $d$-wave altermagnets without spin-orbit coupling. Rotating the altermagnetic orientation changes both longitudinal spin-conductance and transverse spin-Hall conductance fluctuations, while each conserved-spin sector remains in class AI. Altermagnetic orientation controls both the fluctuation amplitudes and the development of disorder-insensitive plateaus. These plateaus become flatter and less sensitive to the altermagnetic strength over the studied parameter ranges as the orientation approaches $45^\circ$, with representative fluctuation amplitudes of approximately $0.60e/(4\pi)$ for the longitudinal spin conductance and $0.40\,e/(4\pi)$ for the spin-Hall conductance. The longitudinal fluctuation amplitude correlates strongly with anisotropy of the clean-band Fermi-velocity tensor. Spin-sector covariances explain how the conserved channels combine into total fluctuations. Our results extend anisotropy-sensitive universal-fluctuation physics to conserved-spin altermagnetic transport and establish crystalline orientation as a control parameter within a fixed symmetry class.
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All authors Yihan Zhang, Zheng Qin, Donghui Xu, Junjie Zeng, Zhen Ning, Rui Wang

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

Dynamical Decoupling of Nonclassical States in a Biased-Noise Spin Qudit

Arjen Vaartjes, Namit Anand, Rocky Y. Su et al. (+16 more)

Encoding quantum information in $d$-dimensional systems (qudits) with $d>2$ offers appealing new directions for resource-efficient quantum computing. However, protecting the coherence of nonclassical qudit resource states remains an open challenge. Dynamical decoupling (DD) is a well-established technique for extending coherence, but its application to qudits is hindered by the quadratic scaling of the decoupling sequence overhead with the dimension $d$. As a result, demonstrations of qudit DD have so far been limited to low-dimensional qudits. Here, we show that noise-biased qudits enable a nontrivial reduction of this overhead, for example, from quadratic to linear. We perform qudit DD on a model $d=8$ qudit, the nuclear spin of an antimony donor in silicon. We benchmark DD performance on highly nonclassical states such as uniform superpositions, cat states, and Haar random states and measure coherence improvements up to a factor $5$ to $10$. For a spin-cat error correction code, continuous DD extends the coherence time of the logical superposition $|+\rangle_L$ to $225$~ms, compared to $19$~ms under free evolution and $85$~ms with a Hahn echo. This improvement shows that DD sequences tailored to a system's noise bias can significantly enhance the lifetime of encoded logical qubits.
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All authors Arjen Vaartjes, Namit Anand, Rocky Y. Su, Akram Youssry, Ritik Sareen, Jeffrey Marshall, Mark R. van Blankenstein, Danielle Holmes, Benjamin Wilhelm, Bibek Pokharel, Xi Yu, Laura A. O'Neill, Martin Nurizzo, Alexander M. Jakob, Fay E. Hudson, Kohei M. Itoh, Andrew S. Dzurak, David N. Jamieson, Andrea Morello

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

Magnetic proximity beyond the interface: Layer-sequence-dependent exchange in WSe$_2$/CrI$_3$ heterostructures

Natalie Kuhn, Marc Schütte, Joachim Emming et al. (+8 more)

Magnetic proximity effects in transition metal dichalcogenide (TMD)/2D-magnet heterostructures are often described in terms of the magnetization of the interfacial magnetic layer. Here, we show that this picture is sufficient to describe the neutral-exciton valley splitting of the TMD, but incomplete for its hole-doped electronic structure. We study a gate-tunable van der Waals heterostructure composed of monolayer WSe$_2$ in direct contact with trilayer CrI$_3$. By combining spatially and helicity-resolved reflection-contrast spectroscopy with magnetic circular dichroism, we identify two distinct magnetic domains in the trilayer CrI$_3$: one with purely antiferromagnetic interlayer coupling and one with a mixed antiferromagnetic (AFM)/ferromagnetic~(FM) coupling. In both domains, the sign of the neutral-exciton valley splitting of WSe$_2$ switches with the magnetization reversal of the adjacent CrI$_3$ layer. In contrast, helicity-resolved positive-trion spectroscopy of WSe$_2$ reveals a qualitatively different response. While the AFM domain shows a strongly helicity-selective trion resonance, the mixed AFM/FM domain exhibits trion resonances in both helicities. This behavior disappears when all CrI$_3$ layers are aligned in a parallel magnetization direction. Our results demonstrate that charged excitons are sensitive to the full layer-resolved magnetic sequence of the adjacent 2D-magnet, revealing magnetic proximity beyond the interface.
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All authors Natalie Kuhn, Marc Schütte, Joachim Emming, Felix Hellenkamp, Leo Dreyer, Frank Volmer, Kenji Watanabe, Takashi Taniguchi, Lutz Waldecker, Christoph Stampfer, Bernd Beschoten

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

Pressure engineering of the excitonic moiré-potential landscape and exciton-charge interplay in semiconductor moiré superlattices

Wenbing Li, Tianhao Yue, Wenjian Su et al. (+10 more)

Semiconductor moiré superlattices, owing to strongly bound excitons and pronounced electronic correlation, have emerged as an intriguing solid platform for exploring correlated bosonic phenomena, where the moiré potential landscape plays a central role. Hydrostatic pressure allows continuous, in-situ tuning of this landscape, but remains underexplored because of experimental challenges. Here we integrate a diamond anvil pressure cell with high quality dual gated WSe2/WS2 twisted bilayers and perform cryogenic optical spectroscopy. We observe distinct pressure evolutions of moiré intralayer excitons, which reveal drastic modulation of the excitonic moiré-potential landscape captured by a continuum model and suggest the capability of engineering the exciton-band topology. Moreover, we demonstrate efficient pressure control over the interplay between charges and both intralayer and interlayer excitons: the intralayer charge-transfer exciton becomes less sensitive to doping, likely due to reduced charge-transfer character; in contrast, the interlayer exciton exhibits enhanced exciton-charge repulsion. Our results establish hydrostatic pressure as a powerful tuning knob for engineering bosonic quasiparticles and Bose-Fermi interactions in moiré materials.
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All authors Wenbing Li, Tianhao Yue, Wenjian Su, Zhexu Shan, Chen Xu, Jichen Zhou, Kenji Watanabe, Takashi Taniguchi, Liguo Ma, Yuanfeng Xu, Shiyao Zhu, Yang Gao, Yanhao Tang

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

Odd-to-Even-Layer Switch of Spontaneous Excitonic State in $\rm{TiSe_{2}}$ Thin Films

Kaili Wang, Zhuowei Liu, Yuyang Mu et al. (+8 more)

$\rm{TiSe_{2}}$ is a candidate of high temperature excitonic condensates with spontaneous exciton formation in addition to its charge density wave (CDW). However, direct observation and manipulation of this electron-hole paring state formation is still lacking. Here, using varying-temperature angle-resolved photoemission spectroscopy (VT-ARPES), we observe a distinct odd-to-even-layer switch of spontaneous exciton state formation with the evident correlated band structure in $\rm{TiSe_{2}}$ thin films. While the odd-layer $\rm{TiSe_{2}}$ films show a sharp band structure in single-particle picture superimposed by CDW renormalization, in the even-layer $\rm{TiSe_{2}}$ films, the full-filling valence band (VB) opens a distinct notch at the top, and the conductive band (CB) shows a significant extra signal extending above Fermi level with spectra broadening. These features indicate that the extra holes (notched VB) and electrons (broadening CB above Fermi level) form spontaneous excitons in even-layer $\rm{TiSe_{2}}$. Based on a correlated electron-hole model extracted from the tight-binding description, we reproduce the characterized band structures with spontaneous exciton formation, where the interlayer inter-band coupling in $\rm{TiSe_{2}}$ plays a key role, which leads to an odd-to-even-layer switch of the excitonic state formation. Our results provide direct evidences of the spontaneous exciton formation, promising an excitonic condensate as versatile platform for realizing Bose-Einstein condensate (BEC) and investigating novel correlated phenomena.
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All authors Kaili Wang, Zhuowei Liu, Yuyang Mu, Zhujuan Li, Zhenjie Fan, Yu Gui, Xiaodong Qiu, Baigeng Wang, Rui Wang, Yi Zhang, Qi-Kun Xue

Tuesday 13 October · still arriving

Mesoscale and Nanoscale Physics

Chern insulator optics

Jason G. Kattan and J. E. Sipe

Chern insulators have optical properties that are strikingly different than ordinary chiral media. Unlike the natural optical activity of the latter, which is a finite-wavelength effect arising from spatial dispersion, the optical activity of a Chern insulator is already present in the long-wavelength limit, and exhibits a striking directional asymmetry in the propagation of circularly polarized light. We study this unnatural optical activity using a recently developed field theory for Chern insulators. For bulk Chern insulators, we derive the normal-incidence transmission and reflection coefficients of multilayer structures, identify the associated Faraday and Kerr angles, and show that these optical rotations provide a direct optical signature of topology at low frequencies. We also show that an interface between bulk Chern insulators can act as a kind of mirror, completely reflecting light at normal incidence when the Chern invariants have opposite signs. Moving to a more realistic model of a two-dimensional current sheet, we study the Faraday and Kerr effects in a thin film of $\mathrm{Mn}\mathrm{Bi}_2\mathrm{Te}_4$ in its Chern insulator phase. The predicted rotations are many orders of magnitude larger than those of optically active media of comparable thickness that are topologically trivial.
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Mesoscale and Nanoscale Physics · Cross-listed from math-ph

On optimal Wannier decay in dimension three

Simon Becker, Zhongkai Tao and Mengxuan Yang

For a Chern nontrivial Bloch bundle over the three torus, we construct an orthonormal Wannier basis with the optimal pointwise decay $\mathcal{O}(|x|^{-5/2})$. All but one of the generators decay exponentially. This resolves the question left open in arXiv:2505.01999.

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

Cavity-assisted dynamic nuclear polarisation

Tristan Lorriaux, Yutian Wen, Patrice Bertet et al. (+2 more)

Dynamic nuclear polarisation transfers polarisation from electron to nuclear spins by relying on a dissipative channel. Here we introduce a cavity-based DNP mechanism which uses the radiative relaxation of the electronic spin through the Purcell effect to bring about the nuclear spin polarisation. We implement this protocol on an ensemble of bismuth donor spins (I=9/2, S=1/2) in isotopically purified silicon coupled to a superconducting resonator. A single cycle, combining adiabatic microwave and radio-frequency pulses with Purcell relaxation, increases a target ground state's population by a factor of $1.79\pm0.04$; a cumulative enhancement of $4.6\pm0.2$ is reached with additional cycles, to be compared with the theoretical limit of ninefold set by the degeneracy of the ground manifold. We explain this difference by the competition between Purcell and intrinsic relaxation on the one hand and a collective effect that slows down the electronic spin radiative decay as the nuclear spin population grows on the other hand. Our results place radiative DNP as an available mechanism at millikelvin temperatures where phonon-mediated relaxation is frozen.
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All authors Tristan Lorriaux, Yutian Wen, Patrice Bertet, Benjamin Huard, Audrey Bienfait

Mesoscale and Nanoscale Physics

Pseudo-relativistic fermions and topological states in zinc-blende semiconductors and their heterostructures (Habilitation thesis)

Sergey S. Krishtopenko

Tracing my scientific path from many-body physics in semiconductor nanostructures to topological quantum matter, this Habilitation à diriger des recherches (HDR) memoir presents only part of my research since defending two PhD theses in Toulouse and Nizhny Novgorod in 2011. Focusing on pseudo-relativistic fermions and topological states in zinc-blende semiconductors and their heterostructures, it highlights my independent research program: identifying physical problems, developing predictive theories, and initiating new directions with experimentalists. The first part addresses Kane fermions in bulk HgCdTe: temperature-driven band inversion, domain-wall states, the collapse of Landau levels in crossed electric and magnetic fields, and disorder-induced topological transitions. The second examines pressure-, temperature-, and disorder-driven phase transitions in single and double HgTe quantum wells (QWs), including bilayer-graphene-like semimetals and a two-dimensional higher-order topological insulator (2D HOTI) phase beyond the minimal BHZ model. The third develops three-layer InAs/Ga(In)Sb QWs as a III-V platform for massless Dirac fermions, large-gap quantum spin Hall insulators, 2D HOTI phases, and semimetals with quadratic band touching, and discusses InAs$_{x}$Sb$_{1-x}$/InAs$_{y}$Sb$_{1-y}$ heterostructures as a promising alternative. Several of these predictions are supported by experimental observations, including quantized helical edge transport up to 60 K and the unconventional quantum Hall effect. Defended on March 25, 2026, at the University of Montpellier. Jury: Michel GOIRAN (chair and rapporteur, LNCMI-CNRS, Toulouse); Alexey KAVOKIN (rapporteur, Westlake University, Hangzhou); Robson FERREIRA (rapporteur, LPENS-CNRS, Paris); Alexei BARANOV (member, IES-CNRS, Montpellier); Benoit JOUAULT and Frédéric TEPPE (members, L2C-CNRS, Montpellier).
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Mesoscale and Nanoscale Physics

Cooperative Boundary Switching of Mesoscopic Real Spectra in a Non-Hermitian Chain Family

Chengxi Li, Hao Zhu, Tie-Fu Zhang et al. (+2 more)

At fixed bulk criticality, every fixed nonsingular real single-end potential preserves a growing local population of real eigenvalues in an open analytic family of non-Hermitian chains. An open region of two-end real potentials removes this population. A boundary-amplitude criterion fixes the minimum number of controlled ends. The population grows as $\sqrt N$, where $N$ is chain length, although its normalized spectral weight vanishes and the local limiting bands remain unchanged. For a representative chain's four zero-detuning settings, an independent passive Hermitian double chain converts this distinction into separated resonances at one fixed local port under a common coupling scan at fixed probe carrier. Its absolute reflection contrast and stability margin both scale as $N^{-1}$, with explicit sufficient resource budgets. Boundary amplitudes thus fix a minimum control requirement that limiting bands and thermodynamic spectral density do not specify.
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All authors Chengxi Li, Hao Zhu, Tie-Fu Zhang, Wanzi Sun, Wuming Li

Mesoscale and Nanoscale Physics

Topological superconductivity in layered d-wave altermagnetic metals

Yijie Mo and Zhongbo Yan

We investigate multilayer d-wave altermagnetic metals with odd-parity helical p-wave pairing. In the minimal bilayer system, interlayer tunneling reconstructs the spin-split Fermi surfaces and generates two layer-hybridized sectors with distinct effective chemical potentials. Without Rashba spin-orbit coupling (RSOC), the emergent mirror symmetry and subsystem chiral symmetry stabilize phases with different numbers of helical Majorana edge-mode pairs and additional counterpropagating pairs, jointly characterized by the mirror Chern number and the winding numbers defined along high-symmetry lines. The combination of RSOC and the altermagnetic exchange field gaps out the helical Majorana pairs while, in a certain parameter regime, leaving another pair of counterpropagating Majorana edge modes gapless, giving rise to second-order topological phases with isolated or twofold Majorana corner modes, as well as a hybrid-order phase in which gapless Majorana edge modes coexist with Majorana corner modes. We identify a layer symmetry that elucidates the physical origin of these phases. For an arbitrary N-layer system, such a symmetry naturally arises from the interlayer-coupling matrix, allowing N-fold Majorana corner modes to emerge when all layer-hybridized sectors enter the nontrivial second-order regime. Our results establish the interplay among the layer degree of freedom, odd-parity pairing, and Fermi-surface configurations as a controllable route to engineering topological superconductivity in layered altermagnetic systems.
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Mesoscale and Nanoscale Physics

Nanoscale Mapping of Strain-engineered Excitons in Monolayer WSe$_2$

Anna S. Roche, Nicholas V. Proscia, Paul D. Cunningham et al. (+12 more)

Excitons in monolayer transition metal dichalcogenides (TMDs) are highly sensitive to external stimuli, allowing control of resonance energies and lifetimes through electric and magnetic fields, heterostructures, moire potentials and mechanical strain. Observations of single-photon emission (SPE) near sample edges, wrinkles, and strained nanobubbles have motivated strain-engineering aimed at realizing localized emitters in 2D materials. The origin of these emitters remains unclear and prior studies have relied on far-field spectroscopy with strain profiles inferred separately through topography, limiting direct correlation between local strain and excitonic response. We directly probe strain-modified excitonic spectra in monolayer WSe$_2$ using cryogenic scattering-type scanning near-field optical microscopy (s-SNOM). We compare two strain-engineering techniques, nano-indentation and patterned substrates, used to produce SPEs. At 9 K, we spatially resolve exciton resonance shifts with sub-diffraction limit resolution, simultaneously measuring near-field optical response and topography. We observe that strain transferred from nano-indentation into a deformable polymer substrate differs from that produced by draping TMDs over patterned dielectric pillars. Adhesion and tearing during plastic deformation drive a compressive exciton blueshift, while draping produces dominant tensile strain. Comparing the strain model results to the experimentally observed energy shifts shows that the nano-indentation strain is over an order of magnitude smaller than previously inferred, due to puncturing of the fragile monolayer, whereas pillar-strained devices remain intact under more modest strain. These differences suggest SPE arises from complex, strain-mediated interactions rather than simple confinement potentials, establishing s-SNOM as a powerful probe of quantum emission in strained 2D materials.
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All authors Anna S. Roche, Nicholas V. Proscia, Paul D. Cunningham, Kathleen M. McCreary, Dante J. O'Hara, Hsun Jen Chuang, Cory Cress, Collin Sanborn, Samuel W. LaGasse, Jose J. Fonseca, Jeremy T. Robinson, Takashi Taniguchi, Kenji Watanabe, John R. Schaibley, Brian J. LeRoy

Mesoscale and Nanoscale Physics

Transmission representation of the Josephson free energy beyond the short-junction limit

Anton R. Akhmerov

Beenakker's formula, $U(\varphi)-U(0)=\Delta\sum_n\bigl(1-\sqrt{1-\tau_n\sin^2(\varphi/2)}\bigr)$, expresses the zero-temperature energy-phase relation of a short Josephson junction through its normal-state transmission eigenvalues. We generalize this transmission representation beyond the short-junction limit and express the phase-dependent free energy of a time-reversal-symmetric $s$-wave junction as a Matsubara sum over channels with frequency-dependent transmissions. The representation constrains the small-phase expansion of the free energy. In particular, we prove that the normalized ratio of its quartic and quadratic coefficients satisfies $-1\le\alpha_r\le7/80$ at finite temperature. We also construct a four-site junction with $\alpha_r>0$, which disproves the conjectured upper bound $\alpha_r\le0$. So that these results can be trusted without repeating the derivations, every numbered equation is formally verified in the Lean proof assistant.
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Mesoscale and Nanoscale Physics

Non-Abelian quantum Hall states at strong Landau level mixing: finding anyons with first-principles AI

Pierre-Antoine Graham, Ahmed Abouelkomsan and Liang Fu

Non-Abelian anyons, strange particles forming the basis of fault-tolerant topological quantum computing, are predicted to emerge in fractional quantum Hall states at even-denominator filling factors. Identifying which topological order is realized has proven to be a challenging problem that requires an accurate, non-perturbative treatment of realistic Landau level mixing, which has so far been missing from theoretical studies. Here we present the first complete solution of non-Abelian quantum Hall problem that includes all electrons and all Landau levels, for spin-polarized electrons at filling factor $\nu = 3/2$. We develop a momentum-resolved neural-network variational wave function, optimized solely on the microscopic Hamiltonian, which gives direct access to the topologically degenerate ground states. Without any physics input about the expected outcome, it discovers a degenerate ground-state manifold whose entanglement reveals anyons with quantum dimension $\sqrt{2}$, the hallmark of Ising-type non-Abelian order. By directly computing the overlap with trial wave functions, we further show that Landau level mixing favors the anti-Pfaffian over other candidate states. Our results demonstrate the power of neural-network wave functions in finding and characterizing non-Abelian anyons from first principles, in regimes of strong Landau level mixing, relevant to many recent experiments.
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Mesoscale and Nanoscale Physics

Single anyons and anyon binding in a quantum Hall dot

Marco Valentini, Will Wang, Amir Shapour Mohammadi et al. (+10 more)

Anyons are particles with quantized fractional statistics. In theory, a single anyon can be stabilized as a localized fractional charge within the bulk of a topologically ordered two-dimensional insulator. However, experiments to date have focused on probing anyons in structures in which many anyonic excitations coexist and interact. Here, we demonstrate the trapping of a single anyon in a device consisting of a graphene-based quantum point contact with an embedded tunable quantum Hall dot. Benchmarking our device at v = 1, we demonstrate backscattering between Fermi liquid edge states mediated by the whole-electron Coulomb blockade of the dot. The cleanliness and control available in our device allow depletion of the dot to the last quasihole, and we observe spin blockade for transitions to the opposite-spin Landau level. At v = 1/3 and 2/3, we observe backscattering resonances with three times the frequency, consistent with resonant transmission of e/3 charges via a single anyon transistor. We demonstrate controlled depletion to the last quasiparticle to form a single anyon quantum dot. Tracing the charge addition spectrum for quasiparticle number Nqp = 1-5, we observe highly irregular addition energies, consistent with DMRG modeling showing a complex orbital shell-filling behavior. At higher Nqp = 10-20, the addition energies show a striking pattern: quasiholes at 1/3 (and quasiparticles at 2/3) show a regular addition spectrum while quasiparticles at 1/3 (and quasiholes at 2/3) do not. We trace the origin of this irregularity to an attractive component of the inter-anyon interaction: by experimentally tuning the confining energy, we observe complete suppression of alternating charge sectors, after which the dot charges in increments of twice the fundamental charge. This observation indicates the emergence of an anyon binding energy as large as the charging energy.
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All authors Marco Valentini, Will Wang, Amir Shapour Mohammadi, Noah Samuelson, Chengxuan Li, Liam Cohen, Mark Zakharyan, Jiechao Feng, Taige Wang, Takashi Taniguchi, Kenji Watanabe, Michael P. Zaletel, Andrea F. Young

Mesoscale and Nanoscale Physics

Spin textures at the Fermi surface of ferromagnets and associated torque

T. Valet, J. Li, H. Jaffrès et al. (+3 more)

We establish theoretically that transverse spin textures are generically induced by the spin-orbit interaction at the Fermi surface of centrosymmetric ferromagnetic metals. We confirm this prediction with ab initio computations for Fe, Co and Ni. By symmetry, this cannot be associated with spin-orbit torques under a uniform current. However, a new current-gradient-induced torque emerges in nanoscale geometries. For a series of Ni-based bilayers, the computed Ni thickness dependence of the current-gradient-induced torque is in quantitative agreement with measurements previously attributed to orbital torques. We discuss how this opens a new avenue for the electrical control of magnetic orders, possibly also including the Néel vector of metallic altermagnets.
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All authors T. Valet, J. Li, H. Jaffrès, V. Cros, M. Chshiev, R. Raimondi

Mesoscale and Nanoscale Physics

Dark state preparation in a zero-field-split molecular triplet

Maria Pfahler and Andrea Donarini

We propose a protocol to prepare and probe dark states in a single-molecule electron spin resonance based on atomic force microscopy (ESR-AFM), thus extending existing techniques developed to address Rabi oscillations. To this end, we simulate in analogy to atomic physics the coherent manipulation of a three-level system in the V-configuration using a multifrequency magnetic field. More specifically, we consider a neutral molecule with a zero-field-split (ZFS) first excited triplet and analyze within a pump-probe scheme the dynamics of the total triplet population. In the triplet subspace, the dark state is a coherent superposition formed by two ZFS states which does not evolve in time despite a continuous resonant driving towards the third one. We characterize the properties of this state comparing it to its fully incoherent counterpart. Within a density-matrix formalism for open systems, we then compute the expected dark state signatures both for ideal scenarios and for the realistic parameterization of ESR-AFM on pentacene.
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Mesoscale and Nanoscale Physics

Plasmon modes in triple-point semimetal, its quadratic cousin, and Hopf semimetal

Ipsita Mandal

We study collective charge excitations in pseudospin-1 triple-point semimetals within the random-phase approximation, considering both the linearised model with a flat-band and its quadratic-corrected counterpart. We analyse their density response, Drude weights, screening, plasmon dispersion and damping, particle-hole continua, and Friedel oscillations from the long-wavelength to the arbitrary-momentum regime. The quadratic correction renders the flat-band dispersive and endows it with a Fermi surface, thereby affecting the overall response. Comparisons with Weyl and Hopf semimetals reveal how bandstructure and quantum geometry jointly govern collective charge dynamics. While the Drude weights are determined by the Fermi surface and band-velocities, interband screening, plasmon dispersion, and Friedel oscillations reflect the quantum geometry of the eigenstates.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

Quantitative Modelling of Solvation in 2D Materials via Alchemical Decoupling and Potential Energy Surfaces

Lindsey N. Ioos, Duncan A. Peterson and Scott C. Warren

Understanding why 2D materials remain suspended in solutions is essential to understanding the mechanisms of aggregation and exfoliation. These dynamics have been difficult to model because of the long timescales and large activation energies involved in these simulations. Here we describe an improved strategy for modelling the solvation and stability of 2D materials that uses the Gibbs free energy of solvation to accurately calibrate two disconnected potential energy surfaces for the fully desolvated and fully solvated 2D materials. By joining these surfaces using solvation energy, we obtain predictions for 2D material stability in various solvents that are qualitatively different than prior simulations and that have excellent agreement with experimental results. Our approach predicts a much higher barrier to full desolvation of the material, suggesting that materials that appear to be aggregated in fact still retain monolayers of solvent between adjacent layers of 2D materials. This work provides much needed understanding on the interactions between the solvent and 2D material that can guide future experimental work.
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