Mesoscale and Nanoscale Physics
A Unified Description of Dirac-Cone Anisotropies in Two Dimensions
L. C. T. Brito, Cleverson Filgueiras, D. M. Lopes et al. (+2 more)
Anisotropies in two-dimensional materials are responsible for a variety of effects that significantly modify their physical properties. In this paper, we present a covariant modification of the Dirac equation that incorporates anisotropies into the effective low-energy description around the Dirac point. The model is constructed by analogy with a Lorentz-violating extension of the Standard Model of elementary particles and yields a (2+1)-dimensional framework describing physical effects such as shifted, tilted, and distorted Dirac cones through its free parameters. The proposed model thus provides a general and unified framework in which distinct anisotropy-induced modifications of the Dirac spectrum and their combinations can be systematically characterized. We further apply the model to strained
graphene and show that its effective parameters can be quantitatively extracted from first-principles electronic band structures while retaining a direct geometrical interpretation. These results establish a connection between the microscopic electronic structure and a material-independent effective description of anisotropic two-dimensional Dirac systems.
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All authors L. C. T. Brito, Cleverson Filgueiras, D. M. Lopes, A. G. Martins, Igor S. S. de Oliveira
Mesoscale and Nanoscale Physics
Magnons in Metallic Altermagnetic $\text{KV}_2\text{Se}_2\text{O}$
Daniel Lourenço R. Santos and António T. Costa
We investigate the
spin excitation in altermagnetic vanadium oxyselenides, focusing on the metallic quasi-two-dimensional compound $\text{KV}_2\text{Se}_2\text{O}$. Using a fermionic
Hamiltonian derived from ab initio calculations, we compute the transverse
spin susceptibilities within the random phase approximation and extract the
magnon dispersion relation. We show that the altermagnetic
symmetry leads to characteristic degeneracies and directional splittings in both the electronic bands and the
magnon spectra along high-
symmetry paths of the Brillouin zone. The metallic $\text{KV}_2\text{Se}_2\text{O}$ exhibits finite
magnon linewidths arising from the coupling to the particle-hole continuum. Furthermore, by fixing the magnitude of the wave vector and varying its in-plane direction, we uncover a pronounced angular dependence of the
magnon energies and lifetimes, with complementary damping behavior between the two
magnon branches. Upon including
spin-orbit coupling, the system exhibits an out-of-plane easy c-axis anisotropy, which opens a
magnon gap of approximately 6 meV at the $\Gamma$-point, possibly rendering the
magnetic order stable at room temperature. Our results demonstrate that altermagnetism controls not only the dispersion but also the anisotropic decay of
spin excitations, highlighting altermagnetic metals as promising platforms for directionally selective
magnon transport.
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Mesoscale and Nanoscale Physics
Tuning Boron-Vacancy Qubit Coherence through Layer Number in hBN
Juan M. Florez and Eric Suárez Morell
The negatively charged boron vacancy ($\VB$) in hexagonal boron nitride (\hBN{}) was the first optically addressable
spin qubit identified inside a van der Waals
crystal, allowing atomically defined placement relative to a target. Its coherence in bulk \hBN{} is limited by the boron
nuclei of the layers flanking the defect plane, which a thin flake removes. Here, we use a generalized cluster-correlation expansion with an extended central-
spin block to calculate the Hahn-echo coherence time of $\VB$ in h$^{11}$B$^{15}$N as a function of layer number. Our results show that $T_2$ rises from $199\ns$ in the bulk limit to $653\ns$ in a monolayer, a factor of $3.3$ that is already saturated at three layers and that a sublattice decomposition attributes entirely to boron. The enhancement is confined to low field, is insensitive to stacking registry and twist angle, and requires
alignment to within about two degrees, an onset we reproduce with no free parameters. Under dynamical decoupling, a $1/e$ threshold returns $206\ns$ for every thickness, but this is a zero of the first-shell modulation rather than a decay: measured without a threshold, the layer contrast survives and reaches a factor of $170$. Layer number thus emerges as a design parameter for $\VB$-based sensing.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Reciprocal asymmetric transmission in self-shadowed metallized gratings
Can Yesilyurt
A single oblique metallization step gives conventional gratings one-sided silver decoration, established by electron microscopy and elemental mapping. Opposite-face reversal measurements yield finite-aperture red/green/blue transmission ratios up to 1.74 for a 1-$\mu$m grating and 1.5-1.8 for a 30-$\mu$m grating, with reference-normalized transmittances for the latter. Flat-side coating and reduced silver dose give ratios near unity. Maxwell calculations on self-shadowed growth profiles reproduce the directional sign qualitatively and attribute the contrast to diffraction-channel redistribution. The structure remains reciprocal, including its normal-incidence zeroth-order transmission. These results establish directional metallization as a route to direction-selective optical transmission without multilayer
alignment or overlay lithography.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con
Singular Weak-Field Thermodynamics of 2D Superconductors
Guopeng Xu and Chunli Huang
In a bulk 3D type-II
superconductor, the lower critical field at which an isolated
vortex becomes thermodynamically favorable is a size-independent material property. We show that the situation is different in 2D
superconductors: the larger the
superconductor, the weaker the field needed to create its first
vortex. The lower critical field in 2D is always size-dependent. For a disk of area $\mathcal A$, the lower critical field $B_v(\mathcal A)$ scales as $\mathcal A^{-1}\ln(\mathcal A/\mathcal A_0)$ in the weak-screening regime and as $\mathcal A^{-1/2}$ in the strong-screening regime. We derive these results from an analytically tractable microscopic model that admits many-body wavefunctions for both the uniform and singly quantized
vortex states in a
magnetic field, and incorporate screening by coupling their long-distance 2D supercurrents to 3D Maxwell equations. These results motivate organizing the weak-field ground-state of a 2D
superconductor in the $(1/\mathcal A,B)$ plane. The origin represents the zero-field thermodynamic limit and it is singular. Approaching the origin along the $B$ axis leads to an increasingly dilute
vortex lattice, whereas approaching along the $1/\mathcal A$ axis yields the uniform
vortex-free state. Our theory shows that every trajectory carrying fixed finite flux ultimately approaches the
vortex-free state in the thermodynamic limit and provides a firm microscopic foundation for the weak-field thermodynamics of 2D
superconductors.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con
Understanding the superconducting proximity effect in semiconductors through quantum oscillations
Milo Coombs, Teun A. J. van Schijndel, Yu Wu et al. (+5 more)
Superconductor-
semiconductor hybrids host emergent states of matter and offer a platform for new
qubits, but the superconducting metal shunts electrical transport, which rules out conventional
semiconductor characterization and leaves the hybrid parameters to speculation. Here we determine density, mass, $g$-factor, mobility and subband occupation beneath the
superconductor, from Shubnikov-de Haas oscillations of a buried InAs
quantum well under Al, Sn, V, Nb, Ta and Re films, with a Dingle analysis that accounts for the shunt. Every metal adds an interface subband whose occupation falls into one of two classes, whereas the mass and $g$-factor of the buried well are unchanged to within 10%. Within the uncertainty set by the transport mobility, no film shortens the
quantum lifetime of the buried well, and Al and Sn lengthen it.
Quantum lifetimes bound the hybridization of the interface subband to 2-4~meV. These measurements supply the normal-state parameters that tunnelling spectroscopy renormalizes but cannot measure.
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All authors Milo Coombs, Teun A. J. van Schijndel, Yu Wu, Jason T. Dong, Yilmaz Gul, Julian Choi, Christopher J. Palmstrøm, Greg P. Mazur
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Polar nanoregions and reentrant-like ferroelectric behavior in SrTiO$_3$
Yuan-Jie Sun, Fei Yang and Long-Qing Chen
Recent real-space imaging in
quantum paraelectric SrTiO$_3$ [Nature 656, 54 (2026)] reveals that local polar textures do not continuously grow upon cooling, but reach a maximum intensity at intermediate temperatures around 60-65 K and weaken again toward the
quantum paraelectric ground state. Such a reentrant-like weakening of local polar textures challenges the conventional paradigm in which ordering tendencies generally continue to strengthen as temperature decreases. Here we employ a self-consistent phase-field theory showing that this anomalous behavior naturally arises from the coupling between intrinsic polar and antiferrodistortiv (AFD) fluctuations. We demonstrate that the flexoelectric-like coupling strongly hybridizes the polar and AFD modes. As the uncoupled polar and AFD modes cross near 52 K, their hybridization is maximized, driving the lower hybridized branch to develop a minimum on a finite-wave-vector shell. This finite-$q$ softening triggers a Brazovskii-type instability, strongly enhancing polarization correlations and producing nanoscale polar textures. Away from the crossing temperature, the two modes become increasingly detuned, weakening their hybridization and the associated finite-$q$ softening. These results reaveal the origin of the formation of polar nanoregions in SrTiO$_3$, naturally explaining the unexpected confinement to an intermediate-temperature window and providing a mechanism beyond the
quantum-fluctuation-based interpretation suggested by experiment. Furthermore, we predict an unconventional reentrant-like sequence in weakly strained SrTiO$_3$, evolving from ferroelectric to paraelectric, polar-nanoregion, and eventually paraelectric regimes upon heating from zero temperature.
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Mesoscale and Nanoscale Physics
Antiresonances of Wannier-Stark ladders in Su-Schrieffer-Heeger lattices
Yonatan Betancur-Ocampo and Guillermo Monsivais
We demonstrate that Wannier-Stark ladders (WSLs) in one-dimensional Su-Schrieffer-Heeger (SSH) chains manifest as antiresonance signatures in the electron transmission. We study the
quantum transport in the SSH chain in the presence of a periodic potential profile plus a uniform electric field. By employing the global
matrix method with non-trivial matching conditions, we find that these WSL antiresonances correspond to states that are exponentially localized at the potential's incident boundary. These findings provide a robust, unified framework for identifying WSLs across diverse scales, ranging from electronic transport in trans-polyacetylene to classical-wave analogs in phononic
crystals and surface water waves.
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Mesoscale and Nanoscale Physics
Tunneling characteristics of twisted double bilayer graphene heterostructures
Alexey A. Sokolik, Azat F. Aminov, Evgenii E. Vdovin et al. (+5 more)
Electron tunneling between sheets of bilayer Bernal
graphene twisted at different small angles was studied experimentally and theoretically. The current-voltage characteristics exhibit resonant peaks, steps, and regions of negative differential resistance, the origin of which is explained by the intersections of energy- and momentum-shifted electron dispersions of adjacent layers. A theoretical analysis of tunneling transport demonstrated that the key to understanding this phenomenon lies in the competition between two contributions: between like (conductivity-conductivity or valence-valence) and unlike (conductivity-valence) bands of parallel bilayer
graphene sheets. A systematic
evolution of the tunneling current patterns with increase of the twist angle is investigated. Polarization of electron wave function across
graphene sublayers caused by displacement field within bilayer
graphene is shown to strongly affect the tunneling probability, thus enhancing negative differential resistance due to Van Hove singularities at the band edges.
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All authors Alexey A. Sokolik, Azat F. Aminov, Evgenii E. Vdovin, Yurii N. Khanin, Mikhail A. Kashchenko, Denis A. Bandurin, Sergey V. Morozov, Kostya S. Novoselov
Mesoscale and Nanoscale Physics
Alejandro S. Gómez, Rafael A. Molina, Pablo Burset et al. (+1 more)
Achieving robust topological superconductivity with multiple Majorana channels is a key step for scalable topological
quantum computing. To this end, we investigate magnetically doped three-dimensional topological
insulator heterostructures proximitized by an s-wave
superconductor within a fully three-dimensional extended Bernevig-Hughes-Zhang framework that explicitly accounts for vertical confinement. We show that
magnetic exchange coupling, orbital mixing, and bulk band inversion cooperate to generate effective equal-
spin p-wave pairing channels supporting multiple chiral Majorana modes. The number of Majorana channels is determined by the confined modes in the vertical direction and the high-Chern-number phases of the normal state, which allows us to derive an analytical criterion for the emergence of the multiple-Majorana topological superconducting phases. The chiral Majorana modes remain robust against strong disorder and moderate
symmetry-breaking perturbations. We also demonstrate that a superconducting phase difference defining a vertical Josephson junction acts as a tunable parameter that controls the hybridization, minigap, and effective multiplicity of the low energy Majorana channels. Our results establish
magnetic three-dimensional topological-
insulator heterostructures as a promising platform for engineering multiple chiral Majorana modes.
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All authors Alejandro S. Gómez, Rafael A. Molina, Pablo Burset, Yuriko Baba
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Exceptional Points in Photonics: From Non-Hermitian Physics to Applications
Fan Zhang, Nikolay Solodovchenko, Dmitrii N. Maksimov et al. (+5 more)
Open photonic systems provide a versatile platform for non-Hermitian physics, enabling control over complex spectra, transport, and light-matter interactions. Exceptional points (EPs), at which
eigenvalues and eigenvectors coalesce and the governing operator becomes defective, play a central role because they combine branch-point spectral
topology, nonanalytic perturbative response, and controllable eigenstate conversion. This Review provides a unified framework for EP photonics by systematically distinguishing exceptional degeneracies according to the underlying operator, spectral variable, boundary conditions, and experimentally accessible observables. We discuss
Hamiltonian EPs, absorbing EPs associated with scattering zeros, real-frequency scattering-
matrix and Jones-
matrix EPs, Bloch and Floquet EPs, and Liouvillian EPs in open
quantum systems. We review their spectral
topology, static and dynamical encircling, higher-order exceptional structures, and coexistence with bound states in the continuum, together with applications in sensing, lasing, coherent absorption, directional scattering, polarization and wavefront control, nonlinear optics, optical storage, nonreciprocal photonics, and
quantum photonics. We also critically assess the current limitations, practical challenges, and future perspectives of EP-based photonic technologies, with particular
attention to robustness, noise, scalability, and experimentally measurable performance.
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All authors Fan Zhang, Nikolay Solodovchenko, Dmitrii N. Maksimov, Xuchen Wang, Mingzhao Song, Filippo Capolino, C. T. Chan, Andrey Bogdanov
Mesoscale and Nanoscale Physics
Interferometry reveals spin-singlet fractional quantum Hall edges in graphene
R. Ayache, K. Kim, M. Kuiri et al. (+7 more)
The edge modes of
spin-singlet fractional
quantum Hall (FQH) phases are manifestations of multicomponent topological order and SU(2)
spin symmetry. An archetype is the
spin-unpolarized state at $\nu$=2/3, whose edge is expected to host spatially coexisting yet counter-propagating charge and neutral
spin modes. Despite efforts, its edge properties, including
spin coherence and
spin-charge separation, have remained elusive owing to the difficulty of resolving
spins in FQH edge transport. Here, we develop a
spin-sensitive probe of
graphene FQH edges by using a p-n junction to interface a target FQH state with a probe integer
quantum Hall (QH) state of opposite polarity. An Aharonov-Bohm (AB) interferometer forms along the interface, when the target and probe edge channels carry the same
spin. We identify the
spin-unpolarized and polarized edges of the $\nu$=2/3 states at lower and higher
magnetic fields, respectively, through
spin-dependent interference. A novel multiparticle AB interference between two electrons of opposite
spin emerges from
spin-charge separation and recombination on the unpolarized edge, featuring a
spin swap. Our results establish edge transport as a probe of
spin-singlet topological orders, with implications for parafermion platforms in
graphene-
superconductor hybrids.
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All authors R. Ayache, K. Kim, M. Kuiri, Q. Benichou, H. Chakraborti, L. Pugliese, K. Watanabe, T. Taniguchi, H.-S. Sim, P. Roulleau
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Geometry-Controlled Magnetic and Electronic Landscapes in Anisotropic van der Waals Materials
Maciej Śmiertka, Ewelina Cybula, Oliwia Janikowska et al. (+10 more)
Electronic structure in van der Waals materials is commonly engineered through composition, strain, electrostatic gating and
heterostructure assembly. Here we introduce geometronics, a concept in which substrate geometry locally reorients an anisotropic
crystal, transforming homogeneous external perturbation into programmable
magnetic and electronic landscapes. We demonstrate this concept using a bilayer of the antiferromagnetic
semiconductor CrSBr transferred onto an inverted pyramidal nanoindentation, where the local
crystal orientation with respect to the external
magnetic field drives the coexistence of antiferromagnetic and ferromagnetic phases within a single continuous
crystal. The resulting
magnetic landscape creates a switchable excitonic potential well of up to 10–12 meV, directly visualised by spatially resolved spectroscopy. More generally, geometronics provides a universal route for deterministically programmed electronic and
magnetic landscapes without modifying the material itself. It therefore establishes substrate topography as a new design principle that exploits the intrinsic anisotropy of layered van der Waals materials.
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All authors Maciej Śmiertka, Ewelina Cybula, Oliwia Janikowska, Bartosz Hołyński, Gayatri, Grzegorz Krasucki, Mariusz Hasiak, Kseniia Mosina, Zdenek Sofer, Adam Babiński, Maciej R Molas, Paulina Plochocka, Michał Baranowski
Mesoscale and Nanoscale Physics
Layer-selective and magnetic-field-enhanced transport of topological kink states in rhombohedral multilayer graphene
Chengyu Shen and Zhe Hou
Topological valley kink states (VKSs), which are
quantum valley Hall states emerging at the interfaces between adjacent domains with opposite valley Chern numbers, have attracted considerable interest in
graphene-based systems. In this work, we investigate the
quantum transport of VKSs in ABC-stacked rhombohedral multilayer
graphene in the presence of Anderson disorder and a perpendicular
magnetic field. Two prominent transport characteristics are revealed. First, in the absence of a
magnetic field, the kink states exhibit strong layer polarization, with their wave functions predominantly localized and equally distributed on the outermost top and bottom layers. As a result, their transport properties are highly sensitive to the layer-selective disorder distribution. Second, under a perpendicular
magnetic field, the layer-symmetric spatial distribution of VKSs is broken, leading to a significant reduction in the wave-function overlap between counter-propagating VKSs from opposite valleys. Consequently, intervalley scattering is suppressed, and the transmission of VKSs through disordered regions is substantially enhanced. Our results provide new insights into multichannel topological valley transport in rhombohedral multilayer
graphene and demonstrates disorder-engineering and
magnetic fields as effective approaches for manipulating the propagation of VKSs.
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Mesoscale and Nanoscale Physics
Probing Nonlinear Interactions of Dipolar Interlayer Excitons in MoSe$_2$/WSe$_2$ Heterobilayers
Sai Shradha, Luc F. Oswald, Md Tarik Hossain et al. (+10 more)
Interlayer
excitons in transition-metal dichalcogenide heterobilayers possess intrinsic out-of-plane dipole moments, providing a platform for investigating
exciton-
exciton interactions at high densities. Here, we use excitation-energy-dependent photoluminescence excitation (PLE) spectroscopy to probe the nonlinear response of dipolar interlayer
excitons in chemical vapor deposition-grown MoSe$_2$/WSe$_2$ heterobilayers. By tuning the excitation energy across intralayer
exciton resonances at fixed excitation power, we selectively vary the population injected into the interlayer
exciton states. Resonant excitation drives the system into a nonlinear regime, leading to saturation of the interlayer
exciton photoluminescence and an apparent broadening of the intralayer $1s$ resonances in the PLE spectra. At the same time, the interlayer
exciton emission exhibits a pronounced blueshift, reaching approximately 2.5 meV at 4 K and 1 meV at 75 K. The blueshift increases systematically with the interlayer
exciton population and is consistent with a net repulsive
exciton-
exciton interaction, with contributions from dipole-dipole repulsion in the density regime investigated. Our results establish PLE as a sensitive approach for accessing the nonlinear, high-density regime of interlayer
excitons and probing their interactions in van der Waals
heterostructures.
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All authors Sai Shradha, Luc F. Oswald, Md Tarik Hossain, Lukas Krelle, Nicole Engel, Axel Printschler, Julian Führer, Honey Jayeshkumar Shah, Daria I. Markina, Kenji Watanabe, Takashi Taniguchi, Andrey Turchanin, Bernhard Urbaszek
Mesoscale and Nanoscale Physics · Cross-listed from physics.ins-det
Helium-Gas-Cooled Cryogenic Current Comparator Integrated with a Quantum Resistance Standard
Yuma Okazaki, Takehiko Oe and Nobu-Hisa Kaneko
We report a cryogen-free cryogenic current comparator (CCC) system for precision resistance measurements. A helium-gas chamber was developed and installed on the 4 K stage of a cryogen-free dilution refrigerator equipped with a pulse-tube cryocooler. The CCC probe was housed in this chamber and was cooled through helium gas serving as a heat exchange medium. The metrological performance of this helium-gas-cooled CCC was evaluated through precision resistance-ratio measurements and found to be comparable to that obtained under
liquid-helium cooling. A
quantum Hall resistance (QHR) device was also integrated into the same refrigerator, enabling QHR/100~$\Omega$ resistance-ratio measurements. The type-A uncertainty reached the 1~n$\Omega$/$\Omega$ level within an averaging time of 100~s, and the resulting resistance-ratio measurements agreed well with conventional
liquid-helium-based measurements at the level of a few n$\Omega$/$\Omega$. The system provides this level of measurement performance while consuming less than 1~L of helium gas per thermal cycle.
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Mesoscale and Nanoscale Physics
Interfacial orbital torques excite nanoscale terahertz magnons
Harshita Devda, Peter M. Oppeneer and Ulrich Nowak
Exchange-dominated
magnons in nanometer-thick ferromagnets extend to the terahertz regime through thickness quantization of perpendicular standing
spin-wave (PSSW) modes. While interfacial
spin-orbit torques (SOTs) have been shown to enable the excitation of such modes, the microscopic origin of the interfacial driving torque remains unclear. In particular, the coexistence of
spin and orbital currents complicates the understanding. Here, we develop and use an atomistic framework that explicitly resolves interfacial
symmetries and separates
spin and orbital torque contributions. Exploiting a trilayer geometry for a thin ferromagnet sandwiched between non-
magnetic layers, where the
symmetry-controlled polarity of the interfacial torque produces mode-selective
magnon excitation as observed in the recent experiment of Salikhov et al. Nature Phys. 19, 529 (2023), we disentangle the different interfacial torque contributions. By decomposing the torque into magnetization-even (field-like) and magnetization-odd components, we identify the field-like torque as the dominant contribution responsible for the excitation. Crucially, isolating orbital and
spin contributions reveals that the interfacial orbital torque provides the primary channel for the efficient excitation of exchange-dominated THz
magnons in thin ferromagnets. Our results establish a microscopic basis for
symmetry-engineered control of confined terahertz
spin dynamics in
magnetic multilayers.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Exchange striction determines how fast antiferromagnetic insulators demagnetize
Aleksandr Buzdakov, Ravi Kaushik, Nikolai Khokhlov et al. (+2 more)
Antiferromagnets combine terahertz
spin dynamics with insensitivity to stray fields, and how quickly their order can be manipulated sets the speed limit on device operation. Femtosecond optical pulses demagnetize antiferromagnetic
insulators on timescales that span picoseconds to nanoseconds across compounds, and no material parameter is known that accounts for the spread or predicts where a new compound will fall. In a compensated antiferromagnet, no angular momentum needs to leave the
spin system, so the rate is set by energy flow from the
lattice into the
spins. Time-resolved second-harmonic generation experiments show that Cr2O3 demagnetizes within 2 ps once the
lattice is driven above the Neel temperature, two orders of magnitude faster than the structurally similar FeBO3. First-principles calculations trace the disparity to exchange striction: short Cr-Cr contacts make the exchange coupling tenfold more sensitive to atomic displacements and widen the phase space for
phonon decay into
magnon pairs.
Spin-
lattice simulations with ab initio parameters reproduce the order of magnitude of the measured ratio. The derivative of the exchange coupling with respect to the ionic displacement thus emerges as a computable parameter that predicts how fast an insulating antiferromagnet can be demagnetized. The results advance our understanding of ultrafast control in insulating antiferromagnets, and suggest a practical pathway to screen candidate materials for thermally assisted antiferromagnetic
memory before synthesis.
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All authors Aleksandr Buzdakov, Ravi Kaushik, Nikolai Khokhlov, Sergey Artyukhin, Alexey Kimel
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Tangent fermions can restore vacuum stability of discrete-time Dirac models
C.W.J. Beenakker
The Dirac
quantum walk (a 1+1 dimensional space-time discretization of the Dirac equation) has a $2\mu$ mass gap both at the center and at the corner of the quasi-energy-momentum Brillouin zone. Gupta and Short recently noted [
Quantum 9, 1845 (2025)] that the Dirac
vacuum can create a particle-hole pair at the zone corner with the release of an energy $2\mu$ to the environment. They removed this
vacuum instability at the expense of
fermion doubling, the appearance of a second low-energy Dirac cone. Here we show that an alternative discretization scheme, with a tangent rather than a sine dispersion relation, offers stability while retaining a single Dirac cone. The key step is the Cayley transformation from the unit circle of Floquet
eigenvalues $e^{-i\varepsilon}$ to the real line of unbounded energies $E=2\tan(\varepsilon/2)$. We compute the Schwinger effect (particle-hole pair creation in a uniform electric field) for tangent
fermions and show that the pair-production rate agrees with the continuum result for a single Dirac cone. The zone corner is exactly decoupled if the scalar potential is coupled through the Hermitian generator of the
quantum map. If it is coupled as a split operator, in order to preserve exact gauge invariance on the
lattice, the corner does contribute - with a weight that vanishes quadratically with the
lattice constants, in contrast to the Dirac
quantum walk where the zone-corner instability survives the continuum limit.
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Mesoscale and Nanoscale Physics
Topological charges and parity selection at Floquet quasienergy degeneracies
Sigmund Kohler and David Guéry-Odelin
The quasienergy spectrum of a strongly driven two-level system as a function of the driving parameters exhibits conical intersections, which are enabled by hidden time-nonlocal
symmetries. We show that each such crossing carries a quantized topological charge: the Floquet–Berry phase acquired along an adiabatic loop around a cone is equal to a $\mathbb{Z}_2$-valued charge. We further identify a second family of degeneracies that occurs at vanishing driving amplitude, when the level splitting matches $m$ energy quanta of the field. Along the Stark-shifted resonance line, the minimum quasienergy gap opens as $|A|^m$, and the charge is nontrivial only for odd $m$. We analytically derive both results from a perturbative reduction to a
spin-$1/2$ in an effective two-dimensional
magnetic field and confirm them numerically through the Bargmann invariant. Moreover, we propose a chirality-based protocol that cancels the dynamical phase to isolate the
geometric one, and an ancilla-based Ramsey readout that renders the topological charge directly observable.
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Mesoscale and Nanoscale Physics
Francesco Buonemani, Massimo Balmelli and Olesia Dmytruk
We theoretically study a hybrid
superconductor-
semiconductor platform hosting Majorana bound states coupled to a single mode photonic cavity. Starting with a one-dimensional wire coupled to a bulk $s$-wave
superconductor embedded in a photonic cavity, we derive an effective light-matter
Hamiltonian for such a platform. Assuming that the photonic vector potential is aligned along the tunneling between a wire and a
superconductor, we find that the cavity coupling enters only in the effective superconducting pairing term. By solving the coupled electron-
photon Hamiltonian using different approaches, such as exact diagonalization in case of zero or large number of
photons, high-frequency expansion, and mean-field decoupling, we find that the phase boundary between the topological trivial phases is shifted to smaller values of the Zeeman energy compared to the uncoupled case. Cavity
embedding has the strongest effect on the phase diagram in the semiclassical regime, corresponding to a large number of
photons. In all cases, we find that at large values of the light-matter coupling strength the effective superconducting pairing is suppressed, driving the system into the gapless phase. We demonstrate that even small light-matter coupling strength allows for entering the topological phase at values of the Zeeman energy compared to the uncoupled platform.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Large Exchange Magnetostriction in a Kagome Antiferromagnet at Room Temperature
Jie Du, Liang Yao, Hang Li et al. (+5 more)
The pursuit of high-performance magnetostrictive materials is crucial for advancing technologies in sensing, actuation, and microelectromechanical systems. Although pronounced magnetostrictive effects have been observed in a few ferromagnets, a systematic exploration of magnetostriction across a broader range of antiferromagnets remains limited. Here, we report the observation of large magnetostriction in the kagome antiferromagnet YMn6Sn6. Under a
magnetic field, the system undergoes a field-induced
evolution from a helical
magnetic ground state toward a collinear field-polarized state, accompanied by a large anisotropic
lattice strain and a substantial volume magnetostriction exceeding 400 ppm at room temperature. Remarkably, the magnetostrictive response remains nearly fully reversible up to 9 T with nearly hysteresis-free behavior, effectively minimizing the energy dissipation commonly associated with domain-wall pinning. Combined experimental measurements and theoretical calculations reveal that the large, nearly hysteresis-free magnetostriction originates from the competition between intralayer ferromagnetic and interlayer exchange interactions. This exchange-driven magnetoelastic coupling further gives rise to a strongly direction-dependent
lattice distortion pathway. This work establishes kagome helimagnets as a tunable platform for low-dissipation magnetoelastic functionalities and responsive magnetomechanical applications.
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All authors Jie Du, Liang Yao, Hang Li, Xiaodong Zhou, Yuan Yao, Xuekui Xi, Yong-Chang Lau, Wenhong Wang
Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Spatio-temporal coherent molding and retrieval of pulsed signals in optical waveguides
Nika Durishvili, Ramaz Khomeriki, Vakhtang Jandieri et al. (+3 more)
Optical waveguides are key elements for high fidelity, long distance optical communications. Coupled waveguide arrays allow for higher information density, steerting the propagation direction, and for encoding information. However, due to the mixing of relative phases for short pulses containing multiple waveguide-mode frequencies, a process for retrieving an encoded input state once these signals undergo coherent propagation remains elusive. A concept is presented to extract with high fidelity the phase-encrypted input signal from spatio-temporal propagated states. As a realization, an array of coupled waveguides is suggested with the
retrieval mechanism being realized by local phase shifts that comply with the identified
retrieval concept. Three dimensional full-wave electromagnetic simulations for broadband optical signals in coupled dielectric waveguides confirm the validity of the scheme and the high fidelity of information
retrieval pointing to potential applications, for instance in ultrafast coherent coding and decoding of information imprinted on pulse sequences.
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All authors Nika Durishvili, Ramaz Khomeriki, Vakhtang Jandieri, Douglas H. Werner, Daniel Erni, Jamal Berakdar
Mesoscale and Nanoscale Physics
Anti-higher-order Weyl semimetal
Cheng-Ming Miao, Yu-Hao Wan and Qing-Feng Sun
Higher-order
topology extends the bulk-boundary correspondence by enabling corner or hinge localized states. Here we identify an anti-higher-order Weyl semimetal, a three dimensional topological phase in which the conventional boundary hierarchy is reversed. Unlike conventional higher-order Weyl semimetals where bulk
topology enforces hinge Fermi arcs, this phase hosts anti-hinge states, meaning the bulk
topology forces states to vanish at specific hinge orientations. Using a minimal two-band model, we show how Weyl points separate the Brillouin zone into
quantum anomalous Hall and anti-higher-order topological insulating regions, with the latter characterized by a band-inversion surface enclosing two distinct high-
symmetry points carrying opposite topological charges. Analytical solutions for Weyl points, Berry
curvature monopoles, and slice Chern numbers are derived, with numerical simulations confirming the resulting anti-hinge behavior. Our work establishes anti-higher-order
topology as a dual counterpart to conventional higher-order phenomena, further extending the exploration of topological phases.
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Mesoscale and Nanoscale Physics
Anti-higher-order topological insulators
Cheng-Ming Miao, Yu-Hao Wan, Ying-Tao Zhang et al. (+1 more)
Duality is a fundamental concept in physics that connects complementary opposites like particles and holes. Similarly, while topological states in topological
insulators localize at boundaries, the existence and nature of their dual counterparts remain unexplored. Here, we introduce anti-topological states as the dual of topological states, exemplified by anti-higher-order topological
insulators. Unlike higher-order topological
insulators, where states localize at corners, anti-higher-order topological
insulators host states along edges but absent at corners, realizing an inverted distribution of states. We demonstrate this phenomenon in a bilayer Chern
insulator with opposite Chern numbers, where the band inversion surfaces enclose distinct high-
symmetry points. The topological invariant distinguishing these phases is given by the topological charges enclosed by band inversion surfaces. This work establishes anti-
topology as a new paradigm, opening a chapter in topological research.
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All authors Cheng-Ming Miao, Yu-Hao Wan, Ying-Tao Zhang, Qing-Feng Sun
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Superconducting Flux Memory for Cryogenic Applications
Gregory R. Boyd, Jeremy Clark, Mark Covington et al. (+15 more)
We report the development of flux
memory for use with superconducting
circuits. This technology stores persistent currents in superconducting loops on-chip to be used to provide flux biasing for superconducting
circuits, like
qubits. We developed three types of flux
memory and draw comparisons among them for
circuit design. We demonstrate the utility of flux
memory by using an in-situ flux detector and characterize each approach and further demonstrate that once flux is set in a
memory cell, benchtop DC control sources can be powered off, leaving the on-chip flux bias in place. We propose that flux
memory can be arranged in a two-dimensional configuration to multiplex control signals and reduce how line counts scale (N^2 devices -> 2N control lines), and our experimental results pave the path to the proposed scalability. We demonstrate the use of flux
memory to flux bias a transmon
qubit and show the tunability of the
qubit state to a target frequency which remained stable on-chip for 20 hours.
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All authors Gregory R. Boyd, Jeremy Clark, Mark Covington, David G. Ferguson, Aref Fouladi, Keith D. Hillaire, Moe Khalil, John McFarland, Aaron Pesetski, Anthony J. Przybysz, John X. Przybysz, Sambarta Rakshit, Aruna N. Ramanayaka, Brian Sears, Robert Smith, Colin Stack, Robert M. Young, Tony X. Zhou
Mesoscale and Nanoscale Physics
Classifying coherent peaks in nanoelectronic devices by the presence or absence of spin exchange
Jongbae Hong
Coherent peaks appearing in the differential conductance of
quantum-dot single-electron transistors (QDSETs) and
quantum point contact (QPC) devices are classified into two categories according to the scaling function onto which the temperature-scaled differential-conductance maxima collapse and the underlying
spin dynamics. The zero-bias peaks (ZBPs) observed in QPCs and in the triplet state of the even-particle sector of QDSETs belong to the same category, whereas the ZBP in the odd-particle sector of a QDSET belongs to a different category together with all finite-bias coherent peaks observed in QPCs and in the even-particle sector of QDSETs. The
spin dynamics of the former category involve
spin exchange, a hallmark of Kondo dynamics, whereas those of the latter category involve only cotunneling of an up–down
spin pair. Furthermore, for the former type of ZBP, the scaling temperature coincides with one-half of the full width at half maximum (FWHM), which corresponds to the Kondo temperature. In contrast, for the latter type, the scaling temperature does not coincide with the (1/2)FWHM-derived energy scale. To support these findings, the gate-voltage-dependent differential-conductance line shapes measured in the odd-particle sector of a QDSET are theoretically reproduced. The results demonstrate that the observed ZBP is a merging of two coherent side peaks generated solely by the cotunneling of up–down
spin pairs.
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Mesoscale and Nanoscale Physics
Process-Technology Co-optimization for 2D-FETs
Shao-Heng Yang, Jainil Dharmil Shah, Mayukh Das et al. (+15 more)
We present the first experimental machine learning (ML)-enabled Process-Technology Co-Optimization (PTCO) framework for optimizing 2D transition metal dichalcogenide (TMD) FET fabrication directly from statistically meaningful experimental data rather than pure simulation data. We first introduce a transition voltage metric, VTrans, to quantify the gate voltage required for off-to-on switching and reveal its direct correlation with subthreshold swing (SS), highlighting an overlooked switching characteristic that governs both off-state and on-state performance. By integrating automated metric extraction, multi-objective recipe ranking, and predictive modeling, our framework uncovers hidden process-performance correlations and predicts the performance of unexplored fabrication recipes from limited experimental data. Experimental validation shows close agreement with ML predictions, thus demonstrating the framework's ability to efficiently guide gate stack optimization through iterative experimental feedback.
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All authors Shao-Heng Yang, Jainil Dharmil Shah, Mayukh Das, Yuanqiu Tan, Hao-Yu Lan, Hsing-Chien Chien, Himani Jawa, Shalini Tripathi, Marco Antonio Villena, Xiangyu Wu, Daire Cott, Kaustav Banerjee, Pierre Morin, César Javier Lockhart de la Rosa, Gaurav Thareja, Dennis Lin, Joerg Appenzeller, Zhihong Chen
Mesoscale and Nanoscale Physics
Narrow-Sense Type-III Dirac Cones and Additional Flat Lines on a Honeycomb Lattice with Anisotropic Next-Nearest-Neighbor Hoppings
Keita Kishigi and Yasumasa Hasegawa
Critically tilted Dirac cones have attracted considerable
attention because of their unconventional electronic properties in two-dimensional massless Dirac-
fermion systems. Among them, the narrow-sense type-III Dirac cone is characterized by a flat dispersion along the direction connecting the two Dirac points. Using a tight-binding model on a single-orbital honeycomb
lattice, we demonstrate that narrow-sense type-III Dirac cones can be realized by tuning anisotropic next-nearest-neighbor hoppings. Type-I and type-II Dirac cones emerge on either side of the critical point, enabling a systematic investigation of the electronic properties across the type-I, narrow-sense type-III, and type-II regimes. We further find that the present model exhibits additional flat lines in momentum space whose energy coincides with the Dirac-point energy at the narrow-sense type-III critical point. This band structure produces a pronounced enhancement of the density of states at the Fermi energy and, consequently, a substantial enhancement of the electronic specific heat compared with that expected for an ideal narrow-sense type-III Dirac cone. Our results show that the honeycomb
lattice with anisotropic next-nearest-neighbor hoppings provides a simple platform for exploring unconventional thermodynamic properties arising from the energy coincidence of narrow-sense type-III Dirac cones and additional flat lines at the Fermi energy.
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Mesoscale and Nanoscale Physics
Floquet scattering and Fano resonances in nodal-ring and multi-Weyl semimetals: Role of propagating and evanescent modes
Sandip Bera and Ipsita Mandal
We develop a comprehensive Floquet scattering theory for
quantum transport in periodically driven nodal-ring semimetals (NRSs) and multi-Weyl semimetals (mWSMs), extending our earlier study reported in Annalen der Physik 535, 2200460 (2023), in which evanescent modes were neglected, to a complete formalism that includes both propagating and evanescent channels. By solving the full boundary-value problem, we obtain the complete set of scattering states and show that, although evanescent modes are indispensable for satisfying the matching conditions at the potential interfaces, they carry zero net probability current and do not contribute to any observable transport quantity. We identify a previously unexplored transport regime in NRSs in which two propagating channels coexist and participate in coherent scattering, producing multi-channel
quantum interference and Floquet-induced Fano resonances. The transmission, reflection, pumped shot noise, and the associated Fano resonance features are determined entirely by the propagating channels, and the resonance energies coincide with those of the corresponding quasi-bound states of the static potential well. Our results establish a unified framework for Floquet transport in anisotropic topological semimetals, and confirm that the approximation of neglecting evanescent modes in our earlier work is justified for all measurable transport properties.
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