Mesoscale and Nanoscale Physics
Olga Arroyo-Gascón and Manuel Pino
We numerically analyze chaos and
diffusion in experimentally-relevant models of twisted bilayer
graphene. Our results indicate that finite systems at both commensurate and incommensurate rotation angles exhibit chaos. We compute the Thouless energy of the system at intermediate twist angles to further understand the diffusive processes associated with the chaotic nature of the spectrum. We find that the mean free path of the non-interacting electrons scales with the system size, which is consistent with diffusive processes caused by border-induced scattering. In summary, our results show the role of borders as a experimentally relevant single-particle scattering mechanism, which is much stronger than in single-layer
graphene and induces chaos and
diffusion regardless of the rotation angle and the
lattice commensurability.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Compound symmetries and double antisymmetry groups in linear time-invariant photonic systems
Yunrui Wang, Shiyu Li and Cheng Guo
Symmetry is fundamental to photonic systems. External (spatial)
symmetries and internal
symmetries—Lorentz reciprocity, energy conservation, and time-reversal
symmetry—constrain the electromagnetic response. Photonic systems can also possess compound
symmetries that combine external and internal transformations, exemplified by parity-time (PT)
symmetry. However, a unified framework for general compound
symmetries involving reciprocity, energy conservation, and time reversal remains lacking, leaving their classification and physical implications unexplored. In this paper, we present such a framework for linear photonic systems. We define compound transformations and
symmetries, and derive their constraints on electromagnetic fields and scattering
matrices. We show that internal, external, and compound
symmetries are naturally described by the theory of double antisymmetry groups. This theory classifies linear time-invariant photonic systems into twelve
symmetry categories, each imposing characteristic constraints on the electromagnetic response. We illustrate two representative categories with numerical examples of photonic
crystal slabs and apply the theory to examine Kirchhoff's law of thermal radiation for a gyrotropic sphere. Our work provides a systematic foundation for analyzing and engineering
symmetry in photonic systems.
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Mesoscale and Nanoscale Physics
Lifshitz transitions and isospin polarization in twist-decoupled monolayer-bilayer graphene
Alex Boschi, Leonardo Sabattini, Sergey Slizovskiy et al. (+10 more)
Bernal-stacked bilayer
graphene (BLG) hosts correlated electronic phases tied to low-energy Lifshitz transitions at saddle points in its valence band. To access this regime, ultralow charge disorder and control over a vertical electric field are simultaneously required. Here, we employ a twist-decoupled monolayer (MLG) to bias a proximal BLG in the absence of an external displacement field (D). We thereby reveal three-fold degenerate
quantum Hall states at D = 0, with multiple transitions driven by doping,
magnetic and electric field. Spontaneous broken
symmetry in the vicinity of the valence band edge is signaled by the emergence of
quantum oscillations with anomalous frequencies and large quasiparticle mass. These results indicate that electronic interactions in BLG are preserved in presence of an atomically close MLG, while showcasing the potential of CVD-grown
graphene multilayers for the exploration of correlated phases of matter.
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All authors Alex Boschi, Leonardo Sabattini, Sergey Slizovskiy, Vaidotas Mišeikis, Zewdu M. Gebeyehu, Stiven Forti, Antonio Rossi, Kenji Watanabe, Takashi Taniguchi, Fabio Beltram, Vladimir I. Fal'ko, Camilla Coletti, Sergio Pezzini
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
Tunable Skyrmions in a Topological Wigner Crystal
David A. Dahlbom, Daniel Kaplan, Premala Chandra et al. (+1 more)
In low density systems with strong interactions electrons are expected to crystallize into a Wigner solid. Recently, advances in two-dimensional systems where electrons carry Berry
curvature have added a topological dimension to Wigner crystallization. Using a model for pseudospin interactions in a topological Wigner
crystal, here we show that the competition between ferromagnetic Heisenberg exchange $J$ and the chiral interaction $\gamma$ on the triangular
lattice stabilizes a zero-field skyrmion
crystal whose density is continuously tunable through the ratio $\gamma/J$. The chiral interaction originates from finite Berry
curvature in an underlying time-reversal broken Wigner
crystal. In the continuum limit, the chiral interaction acts as a chemical potential for skyrmions, while higher-order gradient terms beyond the nonlinear sigma model select the skyrmion density. At large chiral coupling, we uncover a 24-site tetra-skyrmion
crystal carrying four units of topological charge per
magnetic unit
cell, which becomes degenerate with four-sublattice tetrahedral order as $J \to 0$. We characterize the
magnon structure for these phases and discuss the transition between the tunable skyrmion
crystal to these states at large $\gamma/J$. Our results establish the phase diagram of ferromagnetic topological Wigner
crystals.
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All authors David A. Dahlbom, Daniel Kaplan, Premala Chandra, Cristian D. Batista
Mesoscale and Nanoscale Physics
Magneto-oscillations, nonlinearity, and nonreciprocity of Coulomb drag in quantum circuits
Alex Levchenko, Mingyang Zheng and Dominique Laroche
We consider the problem of Coulomb drag in interactively coupled
quantum circuits built of adiabatic constrictions:
quantum point contacts and short
quantum-wire channels. The interplay of spatial confinement and
magnetic field leads to a rich oscillatory response of the drag current as a function of gate voltage and
magnetic field: drag peaks track the depopulation of magnetoelectric subbands, are asymptotically periodic in inverse field, and their visibility is controlled by the competition of temperature with the field-sharpened tunneling width of the constriction. We derive a closed expression for the linear drag conductance whose interaction kernel simplifies dramatically in the experimentally relevant limit of a long thermal length compared with the range of the interwire coupling, investigate the drag in the nonlinear regime, where the drag current measures the transconductance of the drive channel at any field, and discuss physically motivated models of dissipation-induced nonreciprocity of the drag signal. Extensions accounting for Zeeman splitting, interaction renormalization of the barrier transmission, backscattering at high field, and the frequency structure of the
circuit coupling delineate how each mechanism imprints itself on the temperature dependence and lineshapes of the drag oscillations.
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Mesoscale and Nanoscale Physics
Broken site symmetry of Fe adatoms on Bi$_2$Te$_3$
Duy Nguyen, Hari Paudyal, Joseph R. Sink et al. (+2 more)
We report a combined scanning tunneling microscopy and atomistic theoretical study of Fe adatoms on the Bi$_2$Te$_3$(111) surface. Topographic imaging at $4.5$~K shows that Fe adatoms in fcc and hcp hollow sites exhibit a threefold-symmetric contrast, consistent with the $C_{3v}$
symmetry of the adsorption site. However, simultaneously acquired differential conductance ($dI/dV$) maps reveal a pronounced reduction in
symmetry, evidenced by differential contrast observed at nearest-neighbor Te sites. Density functional theory calculations show that the Fe/Bi$_2$Te$_3$ system undergoes a static Jahn–Teller distortion, reducing the adsorption
symmetry from $C_{3v}$ to $C_{1v}$, with the distorted configuration favored by $72.5$~meV. Orbital-projected density of states calculations show that the occupied states near the Fermi level are dominated by $d_{xz}$ and $d_{yz}$ orbitals, whereas the unoccupied states are primarily of $d_{z^2}$, $d_{x^2-y^2}$, and $d_{xy}$ character. The local density of states from these orbitals is in good qualitative agreement with experimental $dI/dV$ spectra. Furthermore, simulated local-density-of-states maps using a tight-binding Green's function approach are consistent with experimental $dI/dV$ maps, confirming that the reduced
symmetry originates from the $C_{1v}$ structural distortion.
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All authors Duy Nguyen, Hari Paudyal, Joseph R. Sink, Michael E. Flatte, Jay A. Gupta
Mesoscale and Nanoscale Physics
Nonlocality-induced critical-length hierarchy from non-Hermitian competition
Mengjie Yang, Alexander N. Poddubny and Ching Hua Lee
Spectral transitions in non-Hermitian
lattices often arise from the competition between non-reciprocal skin accumulation and inter-component hybridization. In short-range systems formed by two coupled chains, this competition conventionally leads to the logarithmic critical-length law $N_c\sim\ln D$, where $D$ is the transverse separation between the chains. Here we show that long-range hoppings fundamentally reorganizes this critical behavior, producing a hierarchy of distinct scaling laws. When only the hybridization couplings are power-law decaying with exponent $\alpha$, the onset becomes algebraic, $N_c\sim D^{\alpha/3}$. When the hoppings within each chain are themselves also power-law decaying, in addition to the hybridization couplings, the system enters a scale-covariant regime for $\alpha<2$, in which the criticality threshold equation depends only on the system aspect ratio $N_c/D$. At $\alpha=2$ and beyond, this regime is followed by a marginal logarithmically corrected and algebraically corrected regimes, respectively. We identify two new non-local mechanisms that enable this unconventional critical hierarchy: a nonanalytic band-edge dispersion from long-range intra-chain hoppings, and parity-mixing hybridization induced by non-reciprocity. Our results show that nonlocality systematically removes the physical length scales i.e. skin depth underlying conventional critical non-Hermitian skin behavior, offering a platform-independent framework testable in programmable topoelectrical
circuits, photonic
lattices and digital
quantum simulators.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
Spin-charge separation in the triangular-lattice Hofstadter-Hubbard model
Yuntian Gu, Hui Yang, Zhehao Dai et al. (+1 more)
Recent experiments in moiré materials have enabled the realization of a variety of exotic
quantum phases. In this context, the Hofstadter-Hubbard model has been proposed as a possible setting for hosting chiral
spin liquid. Concurrently, significant progress has been recently made in the computational methods for two-dimensional many-body
fermion systems, which makes numerically studying this challenging model a real possibility in genuine 2D geometry. Motivated by these advances, we investigate the putative chiral
spin liquid phase in the triangular-
lattice Hofstadter-Hubbard model using variational Monte Carlo with neural
quantum states (NQS) and projected entangled pair states (PEPS). We observe
spin-charge separation directly in real space through numerical
spin-pumping simulation and real-time
spin and charge motion. In addition, in the context of anyonic superconductivity conjectured in this model, we find a positive two-electron binding energy on small systems, but it decreases below our numerical resolution as the system size increases. Our work demonstrates NQS and PEPS as powerful tools, capable of cross-checking each other, for diagnosing topological order and fractionalized excitations in strongly correlated electronic systems.
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All authors Yuntian Gu, Hui Yang, Zhehao Dai, Yantao Wu
Mesoscale and Nanoscale Physics
Magnetic Skyrmion Interacting with Optical Skyrmion
Lan Bo, Jian Chen, Xichao Zhang et al. (+3 more)
Magnetic skyrmions (MSks) and optical skyrmions (OSks) embody
topology in matter and in light, respectively. Here we investigate the interaction between a single MSk and an OSk beam. Three distinct nonlinear dynamical modes are identified: rotation, skipping, and trochoidal motion. By decomposing the optical driving force into gradient, orbital-angular-momentum, and
spin-angular-momentum contributions, we clarify their respective roles of radial confinement, azimuthal drift, and precessional modulation. The skipping motion arises from the azimuthal asymmetry of the OSk beam and exhibits spatial selectivity originating from the magnetization-polarization coupling between the MSk and OSk. In three dimensions, the coupling acquires a propagation-dependent phase dominated by the differential Gouy phase, which yields $z$-asymmetric skipping trajectories. These results bridge topological particles and topological fields within a unified framework, offering helicity-selective and phase-programmable routes to optomagnonic control.
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All authors Lan Bo, Jian Chen, Xichao Zhang, Yan Zhou, Chengwei Qiu, Masahito Mochizuki
Mesoscale and Nanoscale Physics
Thermal Stability of Two-Dimensional Crystals with Extended OH Hydrogen-Bonded Chains
Alexander V. Savin
Numerical simulations of the dynamics of monolayer structures of molecules deposited on a sheet of hexagonal boron nitride (h-BN) have been performed. It is shown that molecules containing benzene rings and hydroxyl groups in their structure can form stable two-dimensional
crystals with linear chains of hydrogen bonds OH$\cdots$OH$\cdots$OH$\cdots$ Such structures are formed by the following molecules: phenol (C$_6$H$_5$OH), hydroquinone (C$_6$H$_4$(OH)$_2$), 4-phenylphenol (C$_6$H$_5$–C$_6$H$_4$OH), 4-(4-phenylphenyl)phenol (C$_6$H$_5$–C$_6$H$_4$–C$_6$H$_4$OH), paracetamol (CH$_3$C(O)NHC$_6$H$_4$OH), 4-hydroxybenzanilide (C$_6$H$_5$C(O)NHC$_6$H$_4$OH) and 4,4-dihydroxybenzanilide (C$_6$H$_4$OHC(O)NHC$_6$H$_4$OH). On the one hand, the benzene rings in these molecules ensure their strong interaction with the flat substrate; on the other hand, they do not hinder the formation of extended hydrogen-bonded chains. The monolayer structures of these molecules exhibit high thermal stability: the onset melting temperatures of their 2D
crystals are 47, 187, 127, 247, 167, 307, and 377 $^\circ$C, respectively. The simulations allow us to conclude that multilayer structures composed of h-BN sheets and molecules of hydroquinone, paracetamol, and 4-hydroxybenzanilide can be used for the development of novel
proton-exchange membranes capable of operating at elevated temperatures.
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Mesoscale and Nanoscale Physics
Strain engineering of transient exciton diffusion in WSe2 monolayers at cryogenic temperatures
Roberto Rosati, Mohammed Adel Aly Nouh, Robert Schmidt et al. (+2 more)
Tungsten-based transition metal dichalcogenides exhibit dark
excitons as the energetically lowest states. These are crucial for
exciton thermalization and propagation and they dominate low-temperature photoluminescence via the emergence of pronounced
phonon sidebands. After a resonant excitation, highly mobile hot dark
excitons are formed, which quickly thermalize into an
equilibrium distribution. The application of strain modifies the
exciton energy landscape and, in particular, the relative energy separation between bright and dark
exciton states. The impact of strain on the transient photoluminescence and
diffusion of non-
equilibrium excitons has remained largely unexplored so far. In this work, we investigate the spatiotemporal
exciton dynamics in strained hBN-encapsulated WSe2 monolayers at cryogenic temperatures. We demonstrate that tensile strain abruptly increases the excess energy of hot
excitons, thereby accelerating their transient
diffusion. We trace this back to suppressed
phonon-mediated scattering from bright to dark
excitons. Furthermore, we predict a periodic modulation of the transient
exciton diffusion in the presence of a compressive strain resulting from strain-driven emission of M
phonons. The gained microscopic insights illustrate how strain can be used to engineer transient photoluminescence and
exciton diffusion in technologically promising 2D
semiconductors.
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All authors Roberto Rosati, Mohammed Adel Aly Nouh, Robert Schmidt, Rudolf Bratschitsch, Ermin Malic
Mesoscale and Nanoscale Physics
Comparison of Lindblad and circuit approaches for quantum heat transport
Bayan Karimi
We compare two popular models applicable to analyzing heat transport by thermal microwave
photons in
quantum circuits. The first model is derived from a weak-coupling Lindblad master equation, with transition rates determined by Fermi's golden rule induced by thermal dissipation sources. The second approach employs a
circuit model, where thermal Johnson-Nyquist noise generated by dissipative elements introduces currents, and consequently Joule power, in other parts of the
circuit. This leads to a Landauer type expression of heat transport where the transmission coefficient is proportional to the transconductance in the
circuit. We find that the two models yield identical results in a linear
circuit in the weak coupling limit with an analytic expression of power in an archetypal
circuit of a cavity mediating heat between two baths. Our analysis yields a quantitative assessment of the range of validity of the weak coupling assumption in a
circuit. Due to the correspondence of the two results, we feel confident in applying the weak coupling Lindblad model also for analyzing heat transport in
quantum circuits consisting, e.g. of
qubits and/or non-linear resonators.
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Mesoscale and Nanoscale Physics
Discrete states and ballistic interference in quantum wires approaching macroscopic lengths
Henok Weldeyesus, Taras Patlatiuk, Christian P. Scheller et al. (+4 more)
Increasing the size of a system showing
quantum effects is a difficult task limited by decoherence, a diminishing
quantum level spacing, and the effects of disorder spoiling the
quantum behavior when growing in size. Systems in 1D offer very strong confinement in the transverse directions, thus generally enhancing
quantum effects, but are notoriously sensitive to disorder. In this work, we present a system of 1D electrons exhibiting discrete
quantum levels and fully ballistic coherent
quantum interference with lengths of up to 18\,$\mu$m. Tunneling spectroscopy between two parallel
quantum wires with a central gated segment shows intricate interference patterns exhibiting several different periods in
magnetic field and density. An analysis over three different wire lengths and a comparison with single particle numerical simulations without any free parameters remarkably explains the full pattern including the observed periods. Therefore, these wires are essentially ideal 1D systems with aspect ratios approaching 1'000. In addition, at low bias, we also observe not only the Coulomb charging energies but can clearly resolve the discrete orbital and
spin states in up to 10\,$\mu$m long wires when filling 100 electrons with the center gate. This is made visible by a state-of-the-art low temperature and low noise measurement system. The
spin filling sequence is completely regular, strictly alternating
spin up and down, avoiding high
spin states, while the peak conductance is modulated in accordance with the previously discussed interference patterns. These striking results show that single particle Schrödinger
quantum mechanics such as ballistic
quantum interference and discrete
quantum states may be observed, under the right conditions, in systems of up to 18\,$\mu$m length, thus approaching macroscopic sizes.
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All authors Henok Weldeyesus, Taras Patlatiuk, Christian P. Scheller, Amir Yacoby, Loren N. Pfeiffer, Ken W. West, Dominik M. Zumbühl
Mesoscale and Nanoscale Physics
Floquet spin-group theory for light-tailored spin splitting in collinear magnets
Tongshuai Zhu, Zixuan Li, Huaiqiang Wang et al. (+2 more)
Spin groups have emerged as a fundamental
symmetry framework for
spin-splitting antiferromagnets, including altermagnets. For periodically driven magnets, where new
spin-splitting electronic structures can emerge beyond
equilibrium, an analogous
symmetry theory is still lacking. Here, we develop a Floquet
spin-group theory for optically driven collinear magnets by combining the static
spin group of the
magnetic crystal with the dynamical
symmetry group of the light field. This theory determines the
symmetry-allowed parity, momentum dependence, and nodal structure of light-induced
spin splitting under general periodic driving. We show that odd-, even-, and mixed-parity
spin splittings can be selectively switched within the same material by tailoring the drive
symmetry, accompanied by distinct
symmetry-allowed transport responses. We further uncover higher-order $h$- and $k$-wave Floquet
spin splittings unique to three-dimensional systems. Our work extends
spin-group theory from static magnets to periodically driven systems, providing a general
symmetry principle for engineering light-controlled nonequilibrium
spin-resolved electronic structures.
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All authors Tongshuai Zhu, Zixuan Li, Huaiqiang Wang, Su-Huai Wei, Jiawei Ruan
Mesoscale and Nanoscale Physics
Second-harmonic generation in twisted double bilayer graphene: Double-resonant enhancement from moiré flat bands
Takaaki V. Joya, Takuto Kawakami and Mikito Koshino
We theoretically investigate second-harmonic generation (SHG) in twisted double bilayer
graphene (TDBG) with AB–AB and AB–BA stacking configurations using a perturbative approach based on an effective continuum
Hamiltonian. We present a systematic analysis of the SHG response as a function of twist angle, vertical bias voltage, Fermi energy, and stacking configuration. We find that the SHG signal is strongly enhanced at small twist angles due to the emergence of moiré flat bands and the associated increase in the joint density of states. Beyond this conventional enhancement mechanism, we demonstrate that the reduced bandwidth enables a pronounced double-resonant process, in which optical transitions at both $\omega$ and $2\omega$ are simultaneously satisfied over extended regions of the moiré Brillouin zone. This mechanism leads to a substantial amplification of the SHG response, analogous to the enhancement observed in systems with discrete energy levels, but realised here in a tuneable moiré band structure. Furthermore, we show that the AB–AB and AB–BA configurations exhibit a systematic $\pi$ phase shift in the SHG response at large bias voltages, reflecting their distinct
symmetry and electronic structure. Our results identify double-resonance effects as a generic mechanism for enhancing SHG in moiré systems and provide a unified framework for understanding and controlling nonlinear optical responses in tuneable flat-band materials.
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Mesoscale and Nanoscale Physics
Optical switching of magnetic order in few-layer CrSBr
Lukas Husel, Julian Trapp, Moritz Würf et al. (+7 more)
Manipulating magnetism with light is crucial for both fundamental understanding and technological advancements of information storage devices. The layered antiferromagnet CrSBr, part of the recently emerging class of two-dimensional van der Waals magnets, offers a unique path towards optical control of magnetism via magneto-
excitons, which allow optical readout of the
spin alignment and also provide a strong absorption channel. Here, we use
exciton absorption and photoluminescence to demonstrate optical switching of the
magnetic order in bi- and trilayer CrSBr. Using a continuous-wave
laser with a power as low as a few microwatts at near-critical external
magnetic fields, we demonstrate both local and remote switching of the
magnetic configuration in extended lateral domains and further employ this mechanism to deterministically prepare the zero-field
magnetic configuration. Our results establish optical switching as a useful means of controlling magnetism in CrSBr, with potential applications in magneto-optoelectronic devices.
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All authors Lukas Husel, Julian Trapp, Moritz Würf, Anna Rupp, Tim Wedl, Kenji Watanabe, Takashi Taniguchi, Iva Plutnarova, Zdenek Sofer, Alexander Högele
Mesoscale and Nanoscale Physics
Valley-controlled chiral magnetism in transition metal dichalcogenide monolayers
Igor S. Krivenko, Vladimir N. Mantsevich and Dmitry S. Smirnov
We put forward the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in transition metal dichalcogenide monolayers as a tool to create and control a chiral
magnetic texture. We show that in the
spin-valley locking regime, the RKKY interaction acts as a Dzyaloshinskii-Moriya coupling with an effective
spin rotation period exactly equal to the tripled
lattice constant. Using mean field theory and classical Monte Carlo simulations, we demonstrate that this interaction qualitatively reshapes the phase diagram of atomically thin antiferromagnets. It destroys the chirality-related phase transition by selecting a single chirality value even when the RKKY interaction is small. At the same time, it shifts the Berezinskii-Kosterlitz-Thouless transition associated with
spins orientation to higher temperatures. We argue that the valley degree of freedom of electrons mediating the RKKY interaction provides a powerful control knob for exploring non-universal phase transitions and
quantum spin liquid states in two-dimensional van der Waals
heterostructures.
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Mesoscale and Nanoscale Physics
Proximity effects of chiral magnetization on transition metal dichalcogenides monolayers
Vladimir N. Mantsevich, Igor S. Krivenko and Dmitry S. Smirnov
We consider the proximity effects of a commensurate chiral $120^{\circ}$ Néel
magnetic structure on the transport and optical properties of a transition metal dichalcogenide monolayer (TMD ML). The enlarged
magnetic unit
cell leads to the folding of the Brillouin zone and enables efficient intervalley
spin-flip scattering. Starting from a six band tight binding model, we develop an effective $k\cdot p$ model, which describes the coupling between charge carriers in the TMD ML and the chiral magnetization. We predict the anomalous antiferromagnetic Hall effect (AHE) for conduction electrons and describe it accounting for the interplay between anomalous velocity, side jump, and skew scattering contributions. The proximity of the chiral magnetization also leads to the mixing and splitting of the
exciton resonances in the two valleys which are related by the time reversal
symmetry despite zero net magnetization. Finally, we demonstrate the possibility to measure the distribution of in-plane orientation of the chiral magnetization through Faraday rotation and the ellipticity of incident linearly polarized light.
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Mesoscale and Nanoscale Physics
Charge Tunable Optical Nonlinearity of Moiré Exciton-Polaritons
Zhijie Li, Kok Wee Song, Jens-Christian Drawer et al. (+16 more)
Transition metal dichalcogenides represent a versatile platform to study strong light-matter interactions based on
excitons and electrons in ordered
lattices. Twist-engineering of moiré structures further enables the manipulation of the polaritonic nonlinearities via engineering the
exciton landscape on the nanoscale. In this work, we demonstrate in-situ control of the optical saturation-based nonlinearity of moiré
exciton-polaritons by phase space restriction via charge doping. Strong
exciton-
photon coupling is established in a gate-controllable MoTe$_2$-MoSe$_2$ heterobilayer, embedded in a spectrally-tunable open cavity. A small gate voltage can effectively lower the necessary polariton density by one order of magnitude to achieve a similar nonlinear saturation effect as in the charge-neutral case. Our microscopic description successfully explains the observed phenomena in the framework of Pauli blocking for the moiré superlattices with charge preoccupation.
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All authors Zhijie Li, Kok Wee Song, Jens-Christian Drawer, Vita Solovyeva, Martin Esmann, Niklas Nilius, Muhammad Sufyan Ramzan, Caterina Cocchi, Falk Eilenberger, Takashi Taniguchi, Kenji Watanabe, Atanu Patra, Subhamoy Sahoo, Simon Betzold, Sven Höfling, Alexander Högele, Oleksandr Kyriienko, Christian Schneider, Bo Han
Mesoscale and Nanoscale Physics
Band topology and symmetry-driven magneto-optical response in two-dimensional d-wave altermagnets with staggered spin-orbit coupling
Meysam Bagheri Tagani Carmine Autieri and Wojciech Brzezicki
Altermagnets combine compensated collinear
magnetic order with momentum-dependent
spin splitting, providing a route to transverse electronic and optical responses without a net ferromagnetic moment. We develop a strictly periodic four-band tight-binding model for a two-dimensional d-wave altermagnet and distinguish the roles of three
spin orbit coupling (SOC) channels: uniform Rashba SOC, a sublattice-staggered Rashba interaction, and bond-staggered SOC. In the absence of SOC, the d-wave kinetic anisotropy produces
spin-polarized Dirac points on orthogonal Brillouin-zone boundaries, related by the altermagnetic fourfold
spin-group
symmetry. Uniform Rashba SOC mixes the
spin sectors and shifts these nodes but preserves the antiunitary
symmetry that forbids an integrated Hall response. The sublattice-staggered Rashba term breaks this
symmetry and activates transverse optical response, whereas the bond-staggered SOC provides the mass that gaps the boundary nodes. Their combined action generates strong Berry-
curvature hot spots and, within a narrow parameter window, an isolated lower two-band
manifold with Chern number C=-2. For the representative parameters considered here it is possible to stabilize a Chern
insulator phase. Using covariant-velocity Kubo calculations, we show that large optical Hall conductivity and circular dichroism extend well beyond the nonzero-Chern region and are controlled by SOC-induced avoided crossings and
symmetry breaking. We further find that carrier doping strongly modifies the resonant and dc Hall responses through Pauli blocking and the occupation of Berry-
curvature hot spots, enabling gate-controlled sign reversals. These results identify the complementary roles of distinct interfacial SOC mechanisms in producing
topology and tunable magneto-optical activity in compensated two-dimensional magnets.
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Mesoscale and Nanoscale Physics
Time reversal symmetry broken quantum spin hall effect in pseudospin-1 Dirac-Rashba system
Puspita Parui and Bheema Lingam Chittari
The
Quantum spin Hall (QSH) phase is conventionally understood to be protected by time-reversal
symmetry (TRS). Here, we theoretically investigated the fate of the QSH phase in a pseudospin-1 fermionic $\alpha-\mathcal{T}_3$ system in the presence of a TRS-breaking ferromagnetic exchange field and
spin-nonconserving Rashba
spin-orbit coupling. Despite broken TRS, the QSH phase survives over a finite parameter regime and is characterised by a non-zero projected
spin-Chern number $C_\sigma (\sigma = \uparrow, \downarrow)$, protected by a
spin-spectral gap. In the absence of Rashba coupling, the QSH phase remains robust up to an $\alpha$-dependent critical exchange field. Rashba SOC qualitatively reshapes the phase diagram by driving transitions into two distinct
quantum anomalous Hall (QAH) phases: a $C=2$ phase, irrespective of $\alpha$-values, and a $C=1$ phase for $\alpha \neq 0,1$, which is further identified as a valley-polarized QAH phase arising from a single valley. Rotating the magnetization to in-plane gaps out the first-order helical edge states and gives rise to second-order topological
insulator (SOTI) phases that host localized corner states in suitable finite geometry. We further identify a topological phase transition between two different SOTI phases, mediated by nanoribbon edge states at an exchange field equal to $\alpha$. These results establish
spin-resolved
topology in a higher pseudospin system as well as the $\alpha-\mathcal{T}_3$
lattice as a versatile platform for engineering and controlling multiple topological phases through
magnetic exchange and
spin-orbit coupling.
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Mesoscale and Nanoscale Physics
Complete Structural Determination of Mesostructural Dodecagonal Quasicrystalline Particles
Xueliang Zhang, Xi Wang, Nobuhisa Fujita et al. (+2 more)
Quasicrystals have revolutionized our understanding of order in solids by demonstrating exotic structural and physicochemical properties with diverse potential applications. Despite the development of various theoretical models and experimental techniques to describe quasicrystal structures, the precise determination of local three dimensional (3D) arrangements of constituent atoms, or of secondary building units such as clusters or micelles, remains elusive. This challenge is particularly acute in self assembled soft matter quasicrystalline systems, where the complex assembly of molecular groups introduces additional defects and structural modulations. Herein, we report the first complete structural determination of self-assembled mesostructural dodecagonal quasicrystalline particles. Employing advanced electron
tomography, combined with dedicated structural tracing and processing workflows, the 3D coordinates of all nodal sites were extracted. This approach reveals that the actual structure deviates from the conventionally assumed tetrahedral close packing geometry, exhibiting diverse coordination environments and displacive fluctuations. We identified and quantified rotational intergrowths arising from node exchange, as well as various defects and disorder, with these features discernible only through 3D analysis. Additionally, we propose a simplified two-layer stacking of isomorphic hexagonal model to form dodecagonal quasicrystal. This work advances our understanding of soft-matter dodecagonal quasicrystals and paves the way for detailed structural elucidation of self-assembled systems.
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All authors Xueliang Zhang, Xi Wang, Nobuhisa Fujita, Osamu Terasaki, Lu Han
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Adaptive Spectroscopy of Fast Two-Level-System Dynamics in Superconducting Qubits
Fabrizio Berritta, David Pahl, Lukas Pahl et al. (+15 more)
Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting
quantum processors. Our sub-second adaptive spectroscopy reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral
diffusion with diffusivity $D \approx 0.9~\mathrm{MHz}^2/\mathrm{s}$. These timescales are about $3 \times 10^2$ times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting
qubits. We observe similar defect dynamics across multiple
qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting
quantum processors.
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All authors Fabrizio Berritta, David Pahl, Lukas Pahl, William P. Banner, Gabriel Cutter, Jan A. Krzywda, Spencer Weeden, Shravan Patel, Paul Buttles, Stanislav Eilhart, Michael Gingras, Bethany M. Niedzielski, Robert McDermott, Mollie E. Schwartz, Kyle Serniak, Max Hays, Jeffrey A. Grover, William D. Oliver
Mesoscale and Nanoscale Physics
Resolving High-Energy States of Interlayer Excitons in MoSe$_2$/WSe$_2$ Heterostructures
Chirag Chandrakant Palekar, Paulo E. Faria Junior, Tobias Manthei et al. (+8 more)
High-energy states of interlayer
excitons (IXs) in van der Waals
heterostructures remain largely unexplored despite their importance for understanding many-body interactions and nonlinear optical phenomena. Here, we use photoluminescence excitation (PLE) spectroscopy to resolve a Rydberg-like series of excited IX states in MoSe$_2$/WSe$_2$
heterostructures. We observe multiple PLE resonances below the intralayer
exciton energies, which we assign to the 2s-, 3s-, and 4s-like states of the IXs. These resonances are consistently observed across
heterostructures with different twist angles, indicating that the high-energy state spectrum is only weakly affected by the twist angle. Wannier-
exciton calculations incorporating screened Coulomb interactions reproduce the overall energy scale and qualitative trends of the measured Rydberg-like series, supporting the assignment of the observed resonances. Our findings demonstrate that PLE provides direct experimental access to the previously unexplored high-energy IX states in van der Waals
heterostructures.
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All authors Chirag Chandrakant Palekar, Paulo E. Faria Junior, Tobias Manthei, Maximilian Nagel, Bhabani Sankar Sahoo, Shachi Machchhar, Imad Limame, Martin Podhorský, Jaroslav Fabian, Bárbara Rosa, Stephan Reitzenstein
Mesoscale and Nanoscale Physics
Emission dynamics in zincblende InAsxP1-x quantum dots in InP nanowires: influence of quantum dot size, composition and nanowire geometry
Tomasz Gzyl, Giada Bucci, Krzysztof Gawarecki et al. (+10 more)
Hereby, we present an experimental and theoretical investigation of emission dynamics in zincblende InAsxP1-x
quantum dots (QDs) embedded in InP nanowires (NWs) grown via vapour-
liquid-solid mechanism by chemical beam epitaxy, using Au nanoparticles as a nucleation catalyst. By measuring time-resolved photoluminescence from an ensemble of QD-NWs it was possible to determine the
exciton lifetime dependence on QD composition and height. Changes in the InP shell thickness surrounding the InP NW stem with a QD, brought additional insight into the influence of photonic environment on the carrier dynamics. High-resolution transmission electron microscopy, combined with energy-dispersive X-ray spectroscopy, provided actual structural parameters. The experimentally obtained lifetimes were interpreted in the light of results of 8 band kp calculations combined with configuration-interaction model to take into account the Coulomb interactions and finite-difference time domain photonic simulations to include the effect of optical confinement. The full understanding of the experimental results required considering both, the changes in the QD potential and the Purcell effect, the latter leading to spontaneous emission inhibition in the case of NWs with thin InP shell.
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All authors Tomasz Gzyl, Giada Bucci, Krzysztof Gawarecki, Elisa García-Tabarés, Anna Musiał, Valentina Zannier, Ylea Vlamidis, Fabio Beltram, Julian V. Montero, Beatriz Galiana, Lucia Sorba, Wojciech Rudno-Rudziński, Grzegorz Sęk
Mesoscale and Nanoscale Physics
Extracting higher-order nonlinearities in nanomechanical resonators using the backbone relation
Maria Kallergi, Daniel K. J. Boneß, Maximilian Seitner et al. (+2 more)
Nanomechanical resonators are a powerful platform for studying nonlinear dynamics with high sensitivity and precision. We explore the nonlinear response of a high-Q nanomechanical string resonator in and beyond the Duffing regime and introduce a robust framework for accurately extracting its conservative nonlinearities. The method is based on the backbone curve obtained from ringdown measurements, making it inherently resilient to small frequency fluctuations while explicitly accounting for both
symmetry-breaking and non-
symmetry-breaking nonlinearities. To validate the approach, we perform complementary ringdown and frequency-response measurements on the nanostring resonator and
benchmark the backbone-based extraction against established frequency-response techniques. The comparison confirms the accuracy of the proposed framework and demonstrates its advantages over conventional methods for nonlinear characterization.
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All authors Maria Kallergi, Daniel K. J. Boneß, Maximilian Seitner, Wolfgang Belzig, Eva M. Weig
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con
Spin Josephson diode effect induced by higher-harmonic spin Josephson currents in a diffusive Josephson junction
Shin-ichi Hikino
We theoretically investigate the
spin Josephson diode effect (SJDE) in a diffusive Josephson junction with a Rashba metal layer under a ferromagnetic exchange field. Within the quasiclassical Green's function framework, we derive analytical expressions for the first- and second-harmonic
spin Josephson currents. The interplay between Rashba
spin-orbit interaction and the exchange field breaks inversion and time-reversal
symmetries, generating additional cosine terms in the
spin current-phase relations and a finite phi0 phase shift. This phase shift induces an intrinsic asymmetry between forward and backward
spin currents, leading to the SJDE without an external
magnetic field. Numerical results show that the efficiency decreases with increasing metal thickness due to suppression of the second-harmonic component, while its dependence on
spin-orbit interaction strength reflects competing effects between phase shift enhancement and harmonic suppression. These findings demonstrate that the interplay between harmonic components provides a mechanism for nonreciprocal
spin transport without requiring suppression of
spin-singlet correlations.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Resonantly-enhanced Raman response in graphene-capped bismuthene on SiC
Lukas Gehrig, Cedric Schmitt, Erica Fragomeni et al. (+15 more)
Two-dimensional
quantum spin Hall
insulators based on atomic monolayers offer a promising route toward dissipationless electronics, yet their practical use is often limited by environmental instability. Encapsulating the system with a
graphene capping layer has been shown to be a reliable method to prevent oxidation and degradation. However, the confirmation of a successful encapsulation still relies on ultra-high
vacuum techniques, that considerably slow the process. Here, we present an ex situ, rapid, nondestructive and spatially resolved Raman characterization of
graphene-capped bismuthene, a honeycomb monolayer of Bi on SiC. A pronounced Raman scattering peak at around 122 cm-1 is identified as the E2g
phonon of bismuthene, via a comparison with density functional perturbation theory calculations. We use excitation-energy and polarization-dependent Raman measurements to enable an unambiguous assignment of the spectral features. Tuning the excitation energy close to the excitonic transition in pristine bismuthene, we observe a strong enhancement of the Raman response and the emergence of additional scattering peaks. In this regime, higher-order
phonon features, as well as interfacial modes between bismuthene and the SiC substrate, become visible, suggesting the involvement of resonant scattering processes. Our results establish Raman micro-spectroscopy as a versatile tool for probing
graphene-protected
quantum materials, providing access to
lattice dynamics and interlayer coupling.
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All authors Lukas Gehrig, Cedric Schmitt, Erica Fragomeni, Simone Sotgiu, Stefan Enzner, Tommaso Venanzi, Bing Liu, Kilian Strauß, Jonas Erhardt, Martin Kamp, Elena Stellino, Paolo Postorino, Jörg Schäfer, Simon Moser, Christoph Stampfer, Giorgio Sangiovanni, Ralph Claessen, Leonetta Baldassarre
Mesoscale and Nanoscale Physics
Wannier-Stark localization, confinement and edge states
N. Aucar Boidi, A. Aharony, O. Entin-Wohlman et al. (+1 more)
The boundary between a system's bulk and the
vacuum can be modeled by a potential which confines the electrons to the bulk. Here we present the example of the Wannier-Stark linear potential, generated by an electric field along one axis of a two-dimensional
lattice. Along that axis, the potential generates electronic states which are localized around each
lattice site, with eigenenergies which form the Wannier-Stark ladder. In the transverse direction, the states are governed by a tight binding model with free Bloch states. In the ground state, filling these states under the Pauli principle generates an insulating bulk and an edge which can be metallic in the transverse direction. This paper explains the concepts of localization, localization length, filling and confinement. The paper also acquaints the readers with the use of Bessel functions in the solution of a simple physical model. These topics can be easily included in courses on solid state physics, and only require prior knowledge of
quantum mechanics.
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All authors N. Aucar Boidi, A. Aharony, O. Entin-Wohlman, C.Proetto