Mesoscale and Nanoscale Physics · Cross-listed from hep-th
Edge physics and the Casimir interaction in Maxwell-Chern-Simons theory on a strip
Nicola Maggiore
We study how boundaries affect Maxwell-Chern-Simons theory, a three-dimensional gauge theory that combines ordinary electromagnetic propagation with a topological Chern-Simons term. On a strip, the two boundaries can support edge excitations while the single massive bulk mode mediates a Casimir interaction between them. We use Symanzik's local boundary-field-theory framework, deriving the boundary conditions from the most general quadratic local boundary action considered here rather than imposing them by hand. Requiring these conditions to act consistently on the unique physical bulk mode selects a continuous family of admissible boundaries, characterized by an impedance and an edge velocity. The associated conserved currents form two boundary current
algebras with opposite levels, and for a symmetric strip the edge modes propagate in opposite directions. The bulk and residual edge sectors factorize, leaving a single physical scattering channel for the Casimir problem. We derive its reflection amplitude, identify a stable pole-free domain, and show that the force is attractive there. In the Maxwell limit the usual long-range one-channel Casimir interaction is recovered, whereas the topological mass produces exponential screening at large separation. Outside the pole-free domain, localized surface modes can appear and must be included separately. The analysis provides a unified description of edge dynamics, boundary conditions and
vacuum forces in a topologically massive gauge theory.
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Mesoscale and Nanoscale Physics
Thermal Hall tomography of chiral superconductivity in rhombohedral graphene
Kumar Ghosh
A chiral
superconductor carries chiral Majorana modes along its edges, and a single integer, the Bogoliubov–de Gennes Chern number, counts them. Thirty years of candidate materials have not yielded a measurement of that integer, because the
magnetic signatures usually invoked are not topologically protected. Rhombohedral
graphene makes the question both urgent and answerable:
magnetic imaging resolves rewritable time-reversal-breaking domains inside the superconducting phase, while
quantum oscillations reveal a normal state too intricate to reconstruct pocket by pocket. We show that the low-temperature thermal Hall conductance returns the integer directly, with no such reconstruction. For band-projected pairing it equals the pairing-
vortex winding enclosed by the occupied regions of momentum space. Splitting the intravalley
Hamiltonian into symmetric and antisymmetric parts isolates the trigonal warping and finite Cooper pair momentum of the real material: the antisymmetric part is topologically inert, direct Chern calculations across $525$ parameter points show the invariant preserved, and one inequality marks where a Bogoliubov Fermi surface removes quantization. The plateau $\kappa_{xy}/T=(\pi^2k_B^2/6h)\,C_{\rm BdG}$ then reads out the integer, its sign reverses with the imaged domain, a written domain wall should carry $2|C_{\rm BdG}|$ Majorana channels, and the thermometry required already resolves single thermal quanta in encapsulated
graphene at millikelvin temperatures.
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Mesoscale and Nanoscale Physics
Spin Vector Control for Heisenberg-Inspired Probabilistic Computing
Yuanqiu Tan, Rahul Tripathi, Saleh Bunaiyan et al. (+5 more)
Probabilistic bits (p-bits) have emerged as a cornerstone of probabilistic computing, enabling energy-efficient hardware implementation for probabilistic inference and combinatorial optimization. A critical challenge in advancing this field beyond binary p-bits lies in realizing and manipulating vector
spin information, essential for mapping complex energy-based models such as the Heisenberg
Hamiltonian.Here, we demonstrate a spintronic platform capable of real-space vector summation by using dual ferromagnetic
spin injections into a monolayer
graphene channel. By electrically tuning the
spin polarization through independently controlled injection currents, we achieve continuous control over the magnitude and direction of the resulting
spin accumulation vector. Experimental observations, supported by theoretical vector summation models and
spin-
circuit simulations, reveal coherent vector interactions and angular tunability of the
spin state. This approach enables direct implementation of vector-based
spin logic and lays the groundwork for mapping classical Heisenberg models using
stochastic low-barrier magnets. Our results establish a scalable pathway for realizing probabilistic
spin circuits based on two-dimensional materials, offering new opportunities for low-power, non-Boolean computing architectures.
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All authors Yuanqiu Tan, Rahul Tripathi, Saleh Bunaiyan, Ryan Wagner, Neil Dilley, Kerem Camsari, Joerg Appenzeller, Zhihong Chen
Mesoscale and Nanoscale Physics
Resonant Far-Infrared Spectroscopy of Flat-Band Fermions in Magic Angle Graphene
Ayelet J. Uzan-Narovlansky, Ipsita Das, Juan F. Mendez Valderrama et al. (+10 more)
Moiré engineering in twisted two-dimensional (2D) materials radically alters low-energy bands, interactions and topological
quantum states. Despite extensive studies, optical spectroscopy of interacting moiré bands in the characteristic far-infrared (FIR) regime has remained largely unexplored due to extreme experimental challenges. Using a newly developed millikelvin FIR platform, we report the observation of the long-sought-after characteristic FIR resonances of flat-band electrons in magic-angle twisted bilayer
graphene (MATBG). We observe highly tunable spectroscopic signatures of interacting light and heavy
fermions that constitute the flat bands in MATBG. Using the topological heavy-
fermion model (THF), we show that itinerant topological electrons act as an "
antenna" that couples strongly to the optical field, with resonant frequencies renormalized by the hybridization with localized heavy electrons. We establish optical selection rules of MATBG which uncovers the key
symmetry governing light-heavy
fermion hybridization. At charge neutrality, we observe pronounced resonances at energies below the on-site Coulomb energy, implying the emergence of new many-body modes. Our experiments and modeling provide a fundamental understanding of light-matter interactions in MATBG and enable resonant optical spectroscopy of moiré bands down to millikelvin temperatures.
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All authors Ayelet J. Uzan-Narovlansky, Ipsita Das, Juan F. Mendez Valderrama, Yue Tang, Jonah Herzog-Arbeitman, Haoyu Hu, Pengjie Wang, Zhaoyi Joy Zheng, Haosen Guan, Kenji Watanabe, Takashi Taniguchi, B. Andrei Bernevig, Sanfeng Wu
Mesoscale and Nanoscale Physics
Emergent trans-moiré orbitals and topology in rhombohedral graphene
Yuqin Wang, Jian Xie, Yi-Jie Wang et al. (+13 more)
The fractional
quantum anomalous Hall effect (FQAHE) exhibited in fractional Chern
insulators has recently been demonstrated in twisted MoTe2 and rhombohedral
graphene/hBN moiré superlattices, promising new routes toward topological
quantum computation. Central to realizing this promise is the understanding of the underlying microscopic mechanism. This, however, remains elusive in the case of rhombohedral
graphene, with the crux being its two seemingly paradoxical conditions: a pronounced small-twist-angle ({\theta}) moiré interface, yet only when electrons are kept distant from it. Here, by scanning tunnelling microscopic imaging with both conditions fulfilled, we capture dramatic electronic structure reshaping in rhombohedral hexalayer
graphene by unforeseen 'trans-moiré orbitals', which emerge on the other, distant side of the moiré interface but nevertheless enforce the moiré periodicity at all measured fillings. We visualize a hierarchy of spatially and energetically distinct trans-moiré orbitals which doped electrons must sequentially occupy–the lowest-energy orbital, expectedly responsible for the FQAHE at small fillings, carries a hollow-cage-like shape. Remarkably, these trans-moiré orbitals vanish at {\theta} {\gtrsim} 1{\deg}, and so do QAHE plateaus in similar devices. Simulations reveal an interaction-driven charge-redistribution mechanism which shapes the trans-moiré orbitals and corresponding Chern minibands. With our findings providing the missing microscopic link, the paradoxical conditions find a natural explanation: electrons are not simply kept distant from a small-{\theta} moiré interface; they are forced into topological trans-moiré orbitals, forged precisely under such conditions. Our microscopic diagnostics unlocks a wide range of possible 'synthetic' FQAHE platforms.
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All authors Yuqin Wang, Jian Xie, Yi-Jie Wang, Jiajun Zhang, Yiting Gao, Zaizhe Zhang, Da Yi, Yan Xie, Jingjing Shi, Guanqin Zhao, Chengyu Xiong, Kenji Watanabe, Takashi Taniguchi, Zhi-Da Song, Xiaobo Lu, Yi Chen
Mesoscale and Nanoscale Physics
Exceptional activated mode theory for generalized real-complex transitions
Mengjie Yang, Alexander N. Poddubny and Ching Hua Lee
Real-to-complex spectral transitions mark the onset of amplification in non-Hermitian systems, but their thresholds are often treated as model-specific quantities. Here we develop a general, non-perturbative activated-mode principle that governs the real-to-complex threshold across broad classes of non-Hermitian systems. A central insight is that only a small
Hilbert subspace is “activated" at the transition onset, which can be variationally determined through the competition between spectral detuning and mode-level projected non-Hermitian couplings. The result is a closed-form exceptional-activation condition for arbitrarily large “disturbances", rather than a perturbative estimate. We apply our framework to three contrasting illustrative problems, establishing (i) a closed-form threshold for critical non-Hermitian skin amplification at all system sizes; (ii) a new link between impurity tunneling threshold and exceptional point switching; and (iii) activation channel switching without underlying topological phase transition. Overall, our findings recast real-to-complex transitions as generic mode-selection problems independent of any specific
symmetry.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con
Floquet Quasiparticle Poisoning of Frozonium
Haoyu Guo and Debanjan Chowdhury
Periodic driving can suppress the Josephson nonlinearity of a fluxonium superconducting
circuit, producing a nearly harmonic Floquet spectrum at isolated freezing points [K. Lewellen et al., Newton 2, 100434 (2026)]. Here we show that this dynamically frozen behavior does not generically suppress quasiparticle-induced dissipation in the resulting frozonium
circuit. We formulate quasiparticle processes in the frozonium using a Floquet framework and analyze both drive-assisted Cooper-pair breaking and tunneling of pre-existing quasiparticles. Pair generation is controlled by gap-breaking thresholds at high drive frequencies, while multiphoton resonances produce pronounced rate enhancements at lower frequencies. Quasiparticle tunneling exhibits connected resonance structures organized by the harmonic Floquet-Magnus spectrum near the freezing point, with resonant hybridization generating characteristic avoided crossings. Our results show that suitable operating regimes must balance dynamical freezing against quasiparticle loss and provide a framework for identifying experimental drive parameters away from harmful resonances.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
The "Moiré Capacitor Effect" and Stabilization of Fractional Chern Insulators in Rhombohedral Graphene Superlattices
Nicolas Regnault, Heqiu Li, Yves H. Kwan et al. (+2 more)
While the necessity of a moiré potential for fractional Chern
insulators (FCIs) in rhombohedral
graphene-hBN superlattices, first predicted in Ref. 1, is now grounded in experiments, a theory of its origin and importance remains at large. We present a mechanism—the moiré capacitor effect—that enhances the moiré potential by electrostatically imprinting the valence charge density onto the conduction bands. We derive the analytical form of this term and reveal its crucial role in stabilizing a parent state with Chern number $C=1$ at filling $\nu=1$. We propose a parent state theory which posits that stability of the Chern
insulator and flatness of its hole excitations are necessary for obtaining FCIs upon doping. We then perform multi-band exact diagonalization calculations to confirm the emergence of FCIs at $\nu = 2/3$ in the presence of the moiré capacitor effect. Our FCI state is stabilized by inter-band fluctuations, unlike in the moire-free case which collapses with band-mixing. We provide the first consistent theory for this state in aligned samples and explain its absence in unaligned ones.
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All authors Nicolas Regnault, Heqiu Li, Yves H. Kwan, B. Andrei Bernevig, Jonah Herzog-Arbeitman
Mesoscale and Nanoscale Physics
Is the Aharonov-Casher phase geometrical or dynamical?
Igor Kuzmenko, Y. B. Band and Yshai Avishai
We consider two two-dimensional (2D) electronic systems in the presence of a perpendicular homogeneous electric field that generates a Rashba
spin-orbit interaction (RSOI): a system of non-interacting electrons in a 2D conductor, modeled using the 2D Schrödinger equation (SE), and a single-layer
graphene system, modeled using a 2D Dirac equation (DE) for massless
fermions. In both cases the RSOI is expressed via an $SU(2)$ Rashba vector potential ${\bf A}_{R}$. We demonstrate that ${\bf A}_{R}$ cannot be eliminated from either the 2D SE or the 2D DE via a gauge transformation. Nevertheless, for a plane wave solution, an $SU(2)$
matrix exists that eliminates ${\bf A}_{R}$ from the resulting 1D SE. This
unitary matrix is an Aharonov-Casher (AC) phase factor, and facilitates the calculation of the AC phase in the Schrödinger scheme. The plane wave solution for the DE contains two components of ${\bf A}_{R}$: $A_{R, k}$ in the direction of the wave vector ${\bf k}$, and $A_{R, n}$ normal to ${\bf k}$. The latter generates an effective electron mass that cannot be eliminated from the DE. The former generates an AC phase that can be eliminated by a time-dependent
unitary transformation. Thus, the Dirac AC phase is time-dependent, i.e., it is a dynamical phase. This is in contradistinction to the Schrödinger AC phase which is geometrical.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
Probing Impurity Quantum Criticality with Entanglement Witnesses
Mateo Cárdenes Wuttig and Andrew J. Millis
Entanglement is a defining feature of
quantum mechanics, and its relation to
quantum criticality is of considerable current interest. Here we show that measurable
spin and charge fluctuations provide an
entanglement witness of
quantum criticality in the two-impurity Kondo model, which has experimental realizations in terms of coupled
quantum dots and
magnetic impurities added to surfaces. We use density-
matrix renormalization group and numerical renormalization group calculations to resolve the non-Fermi-
liquid critical point separating two independently Kondo-screened impurities from an inter-impurity singlet and interpret it as a change in the dominant
entanglement partner of each local moment: from
entanglement of the local moment with an extended set of conduction-electron degrees of freedom to
entanglement with the other impurity. This reorganization is accompanied by a singular response of the impurity-bath
entanglement and the inter-impurity susceptibility. We show how the same structure is encoded in the
quantum Fisher information of collective
spin and charge operators at zero and finite temperatures, connecting the
entanglement picture to experimentally accessible dynamical response functions. Our results establish impurity systems as controlled settings in which
quantum-critical
entanglement can be detected through measurable correlations, and provide further insight into the possibility of understanding heavy-
fermion physics in terms of
entanglement.
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Mesoscale and Nanoscale Physics
Dušan \DJ or\dj ević, Fabián Molina and Vladimir Juričić
Strong coupling to a
magnetic texture makes an electron propagate through an emergent curved space. We show that an elementary
vortex realizes the exterior spatial geometry of an Ellis wormhole: an ultrastatic throat with radius fixed by the topological charge and Hund exchange, cut off at short distances by the microscopic core. Two separable signatures follow directly: the electron deflection collapses onto a single Ellis curve governed by the
vortex winding and exchange coupling, while the
spin Berry phase produces a half-flux Aharonov–Bohm response switched on and off by winding parity. The same metric can be emulated in a designer honeycomb
lattice, where the valley-symmetrized wave-packet response follows the predicted exterior geodesic. These signatures are accessible through real-space electron deflection and scattering, providing experimentally distinct probes of the emergent geometry and Berry flux. The
magnetic vortex thus turns a topological defect into a tunable curved-space lens for electrons in
quantum materials and designer
lattices.
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Mesoscale and Nanoscale Physics
Exact solution for stationary states of a closed memristor with mobile charged vacancies
Irina V. Boylo and Konstantin L. Metlov
A nonlinear model of a memristor based on charged mobile vacancies is considered taking into account the electrostatic interaction between them. This interaction significantly affects the stationary (limiting) vacancy distributions formed under the action of the electric current flowing through the memristor, for which analytical expressions are obtained in this work. Between the regions with reduced and increased vacancy concentrations, an intermediate electrically neutral region is formed due to the electrostatic interaction. Interestingly, the limiting resistances in the “on” and “off” states of such a memristor do not depend on the strength of the electrostatic interaction, at least in the leading first order in the vacancy concentration.
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Mesoscale and Nanoscale Physics
Second-Chern Bounds in Non-Abelian Quantum Geometry
Junwen Zhao, Zhiming Pan, Kang Yang et al. (+1 more)
We study the
quantum geometry of doubly degenerate energy levels in a four-dimensional parameter space. We find that the scalar
quantum metric $g$ and the Berry
curvature $F$ obey $(\operatorname{tr} g)^2/16\geq\sqrt{\det g}\geq |2\Tr(F\wedge F)-\Tr F\wedge \Tr F|/24$. The first inequality characterizes the anisotropy in the metric. The second determinant inequality measures the self-duality of the traceless $SU(2)$ part of the
curvature under Hodge
star operation and the algebraic closedness of the inter-level polarization amplitudes under $SU(2)$ rotations in the doubly degenerate levels. The saturation of the determinant bound imposes a quaternionic Cauchy-Riemann equation, analogous to the complex analyticity imposed by ideal-band conditions in two-dimensional Chern
insulators. As examples, four-band Dirac
Hamiltonians automatically saturate the determinant bound and possess a topological zero in $\operatorname{tr}(F\wedge F)$. We compare the differences between Kramers degeneracy and ordinary $U(2)$ degeneracy. In addition to the non-Abelian
geometric bound, the latter also obeys an independent first-Chern bound.
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All authors Junwen Zhao, Zhiming Pan, Kang Yang, Congjun Wu
Mesoscale and Nanoscale Physics
Electron transport in a 1.6~nm-thick double-gated (100) silicon nanosheet: A theoretical study accounting for phonon confinement and remote-phonon scattering
Shoaib Mansoori, Bimin Cai, Edward Chen et al. (+2 more)
We study theoretically electron transport in an top-and bottom-gated (100) 1.6 nm-thin silicon nanosheet with SiO2/HfO2 gate stacks, focusing on the intrinsic physical processes that affect transport: the confinement of
phonons and the presence of interface hybrid plasmon-
phonon excitations (IPPs or `remote
phonons'). The band structure is calculated using local empirical pseudopotentials; an approximated elastic continuum model is used to consider the confinement of acoustic
phonons; the dielectric continuum limit is used to deal with the IPPs. We find that the electron mobility is affected significantly by the boundary conditions chosen to deal with
phonon confinement. The more realistic assumption of
phonons clamped at the SiO2/HfO2 interfaces and optical
phonons at the Si/SiO2 interfaces results in a room temperature mobility much smaller than what is obtained using the common assumption of bulk
phonons in the elastic, high-temperature approximation. We also find that, as a result of the complicated structure of the primed subbands, the high-field saturated velocity is significantly lower than its bulk value, as it had been measured in the past in the case of Si inversion layers but never explained theoretically. Finally, we find that IPP scattering does depress the low-field mobility but to a small extent, thanks to the presence of the interfacial SiO2 layers and to the proximity of the metal gates. Moreover, by keeping electrons `cooler', IPP scattering results in a higher saturated velocity. Therefore, the presence of high-kappa materials in the gate-
insulator stacks should not affect negatively the performance of field effect transistors based on Si nanosheets.
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All authors Shoaib Mansoori, Bimin Cai, Edward Chen, Dallin O. Nielsen, Massimo V. Fischetti
Mesoscale and Nanoscale Physics
Lectures on ultrathin film ferromagnetism
D. Pescia
In these Lecture Notes we review some of the fundamental principles that have emerged from research on the ferromagnetism of ultrathin films consisting of 3d transition-metal overlayers. Their growth is often layer-by-layer. This growth mode produces
quantum wells along the vertical direction that profoundly impact any physical property of the materials. In addition, the vertical confinement establishes
spin ensembles that extend to macroscopic distances along the in-plane directions and are finite along the vertical (perpendicular) direction, i.e. they are two-dimensional. Accordingly, they display ground state properties that originate from the two-dimensionality, such as “dead”
magnetic layers or “enhanced
magnetic moments”, an oscillatory interlayer
magnetic coupling and an anomalous perpendicular versus in-plane
magnetic anisotropy that produces, in some specific situations, a perpendicular collective orientation of the
spins. At finite temperatures, ferromagnetic order is observed to persist and an analysis of the
magnetic order of ultrathin films in terms of the renormalization group provides a suitable framework for explaining this observation. The ferromagnetic order is lost at a phase transition which follows closely the two-dimensional Ising universality class, as shown by an accurate analysis of data in the vicinity of the critical point. The perpendicular
spin orientation is often observed to turn in-plane by a reorientation phase transition which is also properly described by a renormalization group argument. Finally, the perpendicular
spin orientation introduces topological excitations of the ferromagnetic order, consisting of stripes of reversed perpendicular
spin direction. The stripe order undergoes a phase transition to the paramagnetic state that is not yet completely understood.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
The electrical transport of intrinsic two-dimensional ferroelectric metal PtBi2
Dan Li, Liu Yang and Lei Li
Breaking the conventional stereotype that ferroelectrics are necessarily insulating, two-dimensional (2D) ferroelectric metals combine seemingly incompatible switchable electric polarization and metallic conductivity, providing a fertile ground for the discovery of novel electrical transport phenomena and the development of innovative electronic devices. Using the semiclassical Boltzmann equation and first-principles calculations, we systematically investigate the linear and nonlinear transport responses of the intrinsic 2D ferroelectric metal \ch{PtBi2} to an applied electric field. Our ab initio molecular dynamics simulations reveal that it possesses a high Curie temperature reaching $800~\text{K}$. We propose that the
crystal structure of its high-temperature paraelectric phase can be explicitly distinguished through simple measurements of the in-plane electrical conductivity. Quantitative calculations of the Edelstein effect and the intrinsic
spin Hall effect demonstrate a sizable charge-to-
spin conversion efficiency, highlighting its potential in spintronics. We also find that a Berry
curvature dipole-induced nonlinear Hall effect emerges in uniaxially strained \ch{PtBi2}. Furthermore, we highlight the unique advantages of 2D ferroelectric metals in gate-controlled transport applications. Based on the domain wall scattering mechanism, we conceptually design a novel ferroelectric metal field-effect transistor (FEM-FET) capable of nonvolatile switching between high-resistance and low-resistance states under a gate voltage. Our work not only unveils the rich transport physics in 2D ferroelectric metals but also provides valuable insights into the design of next-generation nonvolatile
memory and spintronic devices.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
Two routes to quantum anomalous Hall states in altermagnets
Makoto Naka, Shuntaro Sumita, Yukitoshi Motome et al. (+1 more)
We theoretically propose two possible routes to realizing
quantum anomalous Hall states in altermagnetic materials. We consider a minimal square-
lattice Hubbard model with antisymmetric
spin-orbit coupling associated with an orthorhombic
crystal structure, which supports a topologically trivial altermagnetic state. By incorporating Rashba-type
spin-orbit coupling and external perturbations, we demonstrate that this trivial state can be turned into topological altermagnetic phases in two distinct ways. The first route is driven by a staggered potential that breaks the
symmetry connecting crystallographically equivalent sublattices, leading to a topological altermagnetic ground state characterized by a quantized Hall conductivity $\left| \sigma_{xy} \right|=e^2/h$ and a Chern number $C=1$. The second route is realized by applying a
magnetic field perpendicular to the two-dimensional plane. The resulting topological state appears as a metastable state in the
magnetic hysteresis loop, exhibiting a quantized Hall conductivity $\left| \sigma_{xy} \right|=2e^2/h$ associated with a Chern number $C=2$. We show that these topological transitions are accompanied by characteristic gap closings at the Brillouin-zone boundary, with the number of gap-closing points determining the Chern number. Ribbon-geometry calculations reveal chiral edge states consistent with the bulk topological invariants and demonstrate distinct
spin polarizations between the $C=1$ and $C=2$ states. Our results establish experimentally accessible routes to quantized anomalous Hall responses in altermagnets.
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All authors Makoto Naka, Shuntaro Sumita, Yukitoshi Motome, Hitoshi Seo
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Beam Routing through Excitons in Transition Metal Dichalcogenide Monolayers
Yonas Lebsir, Jacob Terndrup Heiden, Jorge Barcia Rodríguez et al. (+6 more)
Routing light at the nanoscale typically relies on nanostructured surfaces to imprint directionality on the emission. Using low-temperature, angle-resolved cathodoluminescence spectroscopy, we show that the intrinsic excitonic transitions of a
semiconductor can themselves produce routed emission. We probe monolayers of WSe$_2$, MoSe$_2$, and MoTe$_2$ and resolve the excitonic species of monolayer WSe$_2$ – the bright
exciton, the trion, and the
spin-forbidden dark
exciton – through their distinct angular emission profiles. While the in-plane transition dipoles of the bright
exciton and trion radiate predominantly toward the surface normal, the out-of-plane dipole of the dark
exciton, inaccessible under normal-incidence optical excitation, produces a directional emission channel at large angles. We further tune the balance between neutral and charged
exciton emission through the local dielectric environment. Our results establish dark
excitons in TMD monolayers as a platform for directional light emission in compact photonic architectures without additional nanostructuring.
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All authors Yonas Lebsir, Jacob Terndrup Heiden, Jorge Barcia Rodríguez, Maria Papadopoulou, Kenji Watanabe, Takashi Taniguchi, N. Asger Mortensen, Sergii Morozov, Nicolas Ubrig
Mesoscale and Nanoscale Physics
Mismatch between Raman shear modes and ferroelectric polarization in 3R-MoS$_{2}$
Johannes Schwandt-Krause, Jan-Niklas Heidkamp, Mohammed El Amine Miloudi et al. (+7 more)
Sliding ferroelectricity in parallel-stacked two-dimensional van der Waals materials enables a broad range of novel device concepts, but exploiting it requires reliable, non-destructive assignment of the underlying stacking order and polarization state. Here, we combine Kelvin-probe force microscopy (KPFM) with low-frequency Raman spectroscopy to probe the polarization domains and stacking configurations of a exfoliated trilayer 3R-MoS$_{2}$ flake on a hBN substrate. We find that ABA and BAB - both stackings with zero net polarization - are indistinguishable in KPFM, yet show drastically different low-frequency shear modes. This observation is reproduced across multiple flakes and is corroborated by low-temperature photoluminescence. Notably, the standard bond-polarizability model does not account for the difference in shear-mode activity between the ABA and BAB configurations, indicating that the interlayer Raman response of these stackings is governed by physics beyond a simple polarizability picture. Our results show that none of the here-used individual techniques alone is sufficient to assign sliding-ferroelectric stacking order and motivate a combined spectroscopic-scanning-probe approach.
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All authors Johannes Schwandt-Krause, Jan-Niklas Heidkamp, Mohammed El Amine Miloudi, Swarup Deb, Elisabeth Grune, Kenji Watanabe, Takashi Taniguchi, Rico Schwartz, Oliver Kühn, Tobias Korn
Mesoscale and Nanoscale Physics
Dimensional crossover and local strain induced deflection of the spin spiral state in multiferroic NiI2
Tianxing Jiang, Lianchuang Li, Haiyan Zhu et al. (+10 more)
Low-dimensional multiferroics hold great promise for integrated magnetoelectric devices.
Spin spiral state has recently been shown to induce ferroelectricity in single-layer van der Waals (vdW) material NiI2. However, how this state evolves and can be tuned towards the two-dimensional limit remain unclear. Here, we combine
spin-polarized scanning tunneling microscopy, layer-by-layer film growth, and multi-scale theoretical modeling to investigate the
spin spirals in NiI2 thin films. As the film thickness increases from 1 to 7 monolayers, we observed a continuous increase of
spin-spiral wavelength and a rotation of wavevector from near [110] to [1-10] direction, which evidences a dimensional crossover primarily driven by enhanced interlayer exchange energy. Moreover, we find that the film wrinkles can cause deflection of the
spin spiral wavevector, which is caused by local
curvature induced modification of exchange interactions. Our findings establish thickness and local strain as two tuning methods for engineering non-collinear helical magnetism and accompanied electric polarization in vdW multiferroics.
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All authors Tianxing Jiang, Lianchuang Li, Haiyan Zhu, Hongyu Wang, Junchao Tian, Wenzhao Wang, Weiyi Pan, Haitao Wang, Changlin Zheng, Hongjun Xiang, Changsong Xu, Donglai Feng, Tong Zhang
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con
Spin-polarized supercurrents and Josephson diode effect in altermagnets
Janus F. Niebuhr, Matthias Eschrig and Danilo Nikolić
We present a systematic theoretical study of the Josephson effect in junctions consisting of a d-wave altermagnet (AM) placed between two BCS
superconductors (SC). In general, the SC/AM interfaces are
spin-active and modeled by
spin-dependent $\delta$ potentials, allowing for an arbitrary direction of the local exchange field vector. The model is formulated within the fully
quantum (Gor'kov) and quasiclassical (Eilenberger) Green's function technique, applied to two distinct cases of (i) a weakly
spin-polarized AM (exchange field much smaller compared to the Fermi energy) and (ii) a strongly
spin-polarized AM (exchange field comparable to the Fermi energy). We apply our model to the SC/AM/SC geometry, accounting for the Josephson current-phase relation (CPR). In the weakly
spin-polarized regime, the CPR displays the normal Josephson effect. Irrespective of the orientation of the altermagnet, the junction undergoes the $0-\pi$ transition. Depending on the orientation, the system displays the features similar to those of a ferromagnetic or an antiferromagnetic junction. To investigate the
spin-polarized currents and nonreciprocal transport as the central results of the present work, we put the main focus on the strongly
spin-polarized regime. Within this regime, we distinguish two cases. A coplanar exchange field profile across the junctions displays the normal Josephson effect; however, with a pure and stable long-range second harmonic in the CPR. In contrast, a noncoplanar exchange field profile gives rise to the so-called
quantum geometric phases across the junction, leading to the absence of the phase-inversion center in the Josephson CPR. As a result, a Josephson diode effect emerges with a significant charge diode efficiency larger than 30% and a perfect
spin diode efficiency of 100%.
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Mesoscale and Nanoscale Physics
Quantum Anomalous Hall Effect in $d^{10}$ Oxide Monolayers
Zeyu Li, Xudong Zhu, Yulei Han et al. (+1 more)
Quantum anomalous Hall effect (QAHE) arises from the interplay between
magnetic order and
spin-orbit coupling, which opens up a topologically nontrivial band gap to host chiral edge states in the absence of
magnetic field. So far,
magnetic order of QAHE usually originates from partially filled transition-metal $d$ orbitals or correlation-driven moiré bands. Here, we propose an experimentally accessible family of two-dimensional oxides, M$_2$DO$_6$ (M = Zn, Cd; D = Se, Te), that can realize QAHE from the half-filled O-$2p$ orbital induced spontaneous ferromagnetism. In M$_2$DO$_6$ monolayers,
spin-polarized Dirac points appear at K/K$^{\prime}$ valleys and along $\Gamma$-K/$\Gamma$-K$^{\prime}$ lines. $C_3$ rotational
symmetry then generates eight
symmetry-related crossings in the first Brillouin zone. Upon gap opening by
spin-orbit coupling, each massive Dirac point contributes half Chern number, resulting in a high-Chern-number QAHE phase with $\mathcal{C}=4$. We establish cation deintercalation as a general strategy to activate O-$2p$ ferromagnetism in oxides. Our finding provides a route to realize QAHE from O-$2p$ ferromagnetism and offers design principles applicable to oxygen-based
magnetic topology platforms beyond conventional $d$-electron systems.
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All authors Zeyu Li, Xudong Zhu, Yulei Han, Zhenhua Qiao
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Optically Tunable Threshold Switching and Thermally Activated Transport in Planar Ag/MAPbI$_3$ Thin Single-Crystal Devices
Ofelia Durante, Valeria Demontis, Sebastiano De Stefano et al. (+14 more)
Halide perovskites have enabled major advances in optoelectronics, extending well beyond photovoltaics. Their mixed ionic-electronic conduction, once regarded as detrimental to device stability, is increasingly viewed as a functional degree of freedom for
memory and neuromorphic-inspired devices, especially when coupled to external stimuli such as light. Specifically, single
crystals are attractive models because they suppress grain-boundary effects and microstructural disorder that can mask intrinsic transport and interfacial mechanisms in polycrystalline films. Here, we report the growth of thin methylammonium lead iodide (MAPbI$_3$) single
crystals by a space-confined method and their integration into planar two-terminal devices with directly deposited Ag contacts. At room temperature, the devices exhibit ultra-low dark currents ($10^{-13}-10^{-12}$ A) and negligible hysteresis in the dark. Under illumination, the current increases due to photogeneration and the I-V characteristics develop a pronounced polarity-dependent hysteresis and a threshold-like transition between two conductance states. Temperature-dependent dark measurements (300-400 K) show thermionically activated, contact-influenced transport and a weakly varying normalized hysteresis metric. Together with the back-to-back Schottky-diode analysis and control devices using more inert contact materials, these results support a transport model in which Ag/perovskite interfaces play a central role and the hysteretic response is influenced by coupled interfacial and ionic processes.
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All authors Ofelia Durante, Valeria Demontis, Sebastiano De Stefano, Selene Matta, Adolfo Mazzotti, Daniela Marongiu, Emanuele Meloni, Elisa Pili, Fang Liu, Nicola Sestu, Angelica Simbula, Mauro Carta, Michele Saba, Andrea Mura, Massimiliano Di Ventra, Giovanni Bongiovanni, Antonio Di Bartolomeo