Mesoscale and Nanoscale Physics
Influence of nonlocal excitonic optical response on in-plane exciton polaritons supported by monolayer transition-metal dichalcogenides
Kuniyuki Miwa
Achieving nanoscale light confinement and long-range propagation is a central challenge in nanophotonics, with important implications for controlling molecular photophysical processes and energy transfer. Monolayer transition-metal dichalcogenides (TMDs) support in-plane
exciton-polaritons (IPEPs), deeply subwavelength optical modes in the visible range. In this regime, electromagnetic fields vary on length scales comparable to the
exciton size, making spatial nonlocality in the excitonic optical response essential. Here, we develop a microscopic theory of the nonlocal excitonic optical response and investigate the near-field properties of IPEPs in hexagonal boron nitride (hBN)-encapsulated monolayer WS$_2$. Combining a two-band massive Dirac
fermion description with a generalized Mott-Wannier treatment, we derive a nonlocal electric susceptibility that accounts for both the center-of-mass and internal relative motion of
excitons. Spatial nonlocality in the excitonic optical response shifts the polariton dispersion toward lower in-plane wavevectors compared with the results based on the local-response approximation. This reduces high-wavevector contributions and produces smoother near-field distributions that extend over larger lateral distances while retaining nanoscale confinement. Furthermore, at higher excitation energies, multiple
exciton-polariton modes contribute at distinct wavevectors, and their relative contributions can be tuned by varying the thickness of the capping hBN layer. These insights elucidate the fundamental role of spatial nonlocality in the polaritonic properties of van der Waals
heterostructures.
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Mesoscale and Nanoscale Physics
Topological Frenkel excitons in 2D molecular crystals
Quentin Stein, Sidhartha Nayak, Song Jiang et al. (+7 more)
Collective molecular excitations govern energy transport in light-harvesting and optoelectronic materials. Theory predicts that excitonic band
topology can generate edge-confined states, offering a route to direct energy flow in molecular systems, yet experimental evidence is lacking. Here we use scanning tunnelling microscopy-induced luminescence to image Frenkel
excitons in two-dimensional zinc phthalocyanine molecular
crystals with nanometer resolution. By varying cluster size, we observe the transition from discrete
quantum-confined excitations to collective excitonic bands and directly resolve their real-space wavefunctions. We identify an excitonic state localized at the
crystal perimeter and show, through comparison with microscopic theory, that it originates from the momentum-resolved non-trivial
topology of the bulk excitonic bands. Our results establish molecular
crystals as a platform for engineering topological excitonic states and provide a strategy for controlling energy transport through excitonic band engineering.
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All authors Quentin Stein, Sidhartha Nayak, Song Jiang, Jakob Kuhlke, Michelangelo Romeo, Thomas Frederiksen, Fabrice Scheurer, Katharina Kaiser, Tomas Neuman, Guillaume Schull
Mesoscale and Nanoscale Physics
Momentum-Locked Edge Excitons in 2D Molecular Crystals
Sidhartha Nayak, Alex Boeglin, Quentin Stein et al. (+4 more)
In molecular
crystals, molecular excitations can hybridize into collective Frenkel
excitons. Here we show that doubly degenerate excitations in planar two-dimensional molecular
crystals generate excitonic bands with nontrivial momentum-resolved topological characteristics and edge states appearing only for selected crystallographic terminations. Using a model inspired by aggregates of zinc phthalocyanine molecules deposited on insulating surfaces, we demonstrate that the existence of edge
excitons depends sensitively on the crystallographic orientation of the boundary and is governed by the Zak phase of the underlying bulk bands. The resulting edge states exhibit momentum-locked elliptical polarization textures originating from the internal structure of the molecular excitations, potentially enabling directional optical coupling and excitation transport. Our results establish a route toward topological and polarization-textured excitonic states in two-dimensional molecular materials.
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All authors Sidhartha Nayak, Alex Boeglin, Quentin Stein, Katharina Kaiser, Guillaume Schull, Thomas Frederiksen, Tomáš Neuman
Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Gerónimo J. Caselli, Luis O. Manuel, Liliana Arrachea et al. (+1 more)
Geometric heat pumping is the transfer of energy between reservoirs at equal temperature where the heat exchanged in an adiabatic cycle depends only on the driving contour in parameter space. We implement this mechanism on a superconducting
quantum processor using a collision model, where a driven
qubit interacts repeatedly with two ancilla
qubits that are prepared in thermal states and emulate the reservoirs. Energy measurements before and after each interaction provide a calorimetric record of the heat exchanged with each reservoir. By reversing the driving contour, we extract a heat contribution associated with pumping. Its dependence on the period and on the contour follows exact simulations of the collision model, without adjustable parameters, and approaches in the slow-driving regime the
geometric limit of the corresponding Lindblad dynamics.
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All authors Gerónimo J. Caselli, Luis O. Manuel, Liliana Arrachea, Pablo S. Cornaglia
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
Ambipolar metal-insulator transition in bilayer MoSe2
Ha-Leem Kim, Hyungbin Lim, Terry Wu et al. (+7 more)
When Coulomb interactions dominate kinetic energy, dilute two-dimensional carriers form a Wigner solid. Both its melting and disorder-driven localization can drive density-tuned metal-
insulator transitions, but their relative roles and interplay have been debated for decades. Atomically thin
semiconductors offer a new test because transport and scanning tunneling microscopy can probe the same material, yet poor contacts impede transport where correlations are strongest. Here we introduce contact-free
radio-frequency reflectometry that senses sheet resistance through capacitive gate coupling. It resolves the electron and previously inaccessible hole transitions in the same bilayer MoSe2 device, whose carriers share disorder but not mass. Their critical densities differ fourfold yet correspond to comparable Coulomb-to-kinetic-energy ratios, where tunneling microscopy finds solid-
liquid coexistence. Magnetoresistance peaks sharply at each transition, reflecting a Pomeranchuk-like, field-driven expansion of the Wigner solid against the correlated
liquid. Our results demonstrate interaction-driven transitions in high-quality two-dimensional
semiconductors, opening quantitative transport in contact-limited systems.
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All authors Ha-Leem Kim, Hyungbin Lim, Terry Wu, Amogh Y. Waghmare, Garima Gupta, Takashi Taniguchi, Kenji Watanabe, Archana Raja, Eric Y. Ma, Feng Wang
Mesoscale and Nanoscale Physics
Valley transitions during spin shuttling in Si/SiGe heterostructures
Jonas R F Lima and Guido Burkard
Valley excitations are considered one of the biggest challenges for a coherent
spin shuttling in Si/SiGe
heterostructures. In this work we theoretically investigate the valley dynamics during
spin shuttling in a Si/SiGe
heterostructure by considering a two-level system given by the two low-lying valley states of the mobile
quantum dot. We assume that the electron is initially in the valley ground state and analyze the
evolution of this state during shuttling. We find that, depending on the shuttling velocity, the system can be in three distinct regimes. In one of these regimes, the
quantum information is preserved even in the presence of the valley excitations, allowing for coherent
spin shuttling independently of the shuttling distance. This regime is obtained for a shuttling velocity of tens of m/s, a desired regime where dephasing mechanisms, such as
spin-valley hotspots and spatially varying
spin splitting, can be suppressed. This result is in accordance with recent experimental work that obtained a high
spin shuttling fidelity after a fast and long shuttling. We also propose strategies based on the Partial Landau-Zener model to enhance the valley shuttling fidelity, which was found to be much more efficient than previously proposed strategies based on the standard Landau-Zener model.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
An efficient variational polaron master equation for non-Markovian spin-boson dynamics: Transformed initial states and general observables
Chien-Chang Chen and Hsi-Sheng Goan
We investigate the effects of initial states on the dynamics of a
quantum spin-
boson model using an efficient variational polaron master equation approach. The transformation of a factorized system-environment initial state into the variational polaron frame introduces inhomogeneous terms in the master equation, essential for accurately describing non-Markovian dynamics beyond the weak coupling regime. To efficiently account for these effects, we formulate the dynamics in an extended auxiliary Liouville space, converting the resulting integro-differential equations into coupled ordinary differential equations. We further introduce a simplified variational polaron technique for computing the time
evolution of expectation values of general observables, including those that do not commute with the polaron transformation, overcoming a major limitation of previous approaches and enabling the unified treatment of coherence and population dynamics. The effects of transformed initial states generated from arbitrary factorized initial states are systematically characterized through expectation values and trace-distance analysis, providing insight into the associated bath-relaxation dynamics over a broad range of tunneling amplitudes, temperatures, and coupling strengths. Comparison with the weak-coupling master equation reveals a delay phenomenon associated with multi-bath-quanta processes, pronounced at intermediate coupling strengths and tunneling amplitudes. We further show that coherent superposition initial states induce stronger nonequilibrium effects than localized states, with inhomogeneous terms playing a crucial role in the short-time dynamics. The proposed formalism applies to both undriven and resonantly driven
spin-
boson models, providing a computationally efficient framework for open
quantum-system dynamics with arbitrary factorized initial states and general physical observables.
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Mesoscale and Nanoscale Physics
Superconducting edge contact to a buried Ge quantum well
Dina Sokolova, Davide Costa, Davide Degli Esposti et al. (+4 more)
Semiconductor-
superconductor hybrid systems form the basis for a broad range of
quantum phenomena, such as Andreev bound states, topological superconductivity, and hybrid
quantum Hall states. The performance of these hybrid systems can strongly benefit from the exceptional electronic properties of
heterostructures with deeply buried
quantum wells. However, establishing high-quality superconducting contacts to such
quantum wells remains challenging. Here, we address this problem with an edge-contact approach that combines anisotropic dry etching to access the
quantum well followed by wet etching for interface preparation. Josephson junctions fabricated using this approach consistently exhibit near-unity interface transparency, while
quantum point contacts show strong Andreev enhancement, with the first conductance plateau approaching $4e^2/h$, the limit of perfect Andreev reflection. In superconducting
quantum point contacts, the high contact quality is further manifested by quantized supercurrent. These results establish the combination of dry and wet etching as a robust strategy for integrating deeply buried
quantum wells into superconducting hybrid devices.
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All authors Dina Sokolova, Davide Costa, Davide Degli Esposti, Tommaso Costanzo, Giordano Scappucci, Georgios Katsaros, Anton V. Bubis
Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el
Zero Mode Operators and Charge-Conjugation Defects in Non-Abelian Fractional Quantum Hall-Superconductor Heterostructures
Junyi Cao and Eduardo Fradkin
We study zero modes bound to charge-conjugation defects in fractional
quantum Hall-
superconductor (FQH-SC)
heterostructures built from Abelian and non-Abelian FQH states. Although the associated defects are well described by $G$-crossed braided tensor categories, a local operator description of their zero modes is not generally known. We generalize the bulk-to-boundary construction of localized zero mode operators to FQH-SCs with non-Abelian parent topological orders, mapping bulk Wilson operators in the folded theory to localized zero mode operators in the one-dimensional FQH-SC theory. This construction distinguishes zero mode anyons from the local zero mode operators that act on the defect
Hilbert space and yields a generalized zero mode
algebra. It also determines the fusion-channel dependence of tunneling: tunneling of zero mode operators probes bulk monodromy, while direct zero mode anyon transfer probes $G$-crossed associativity data. Applying this framework, we recover the $\mathbb{Z}_{2m}$ parafermion zero mode structure of fermionic Laughlin FQH-SCs at filling $\nu=1/m$, obtain a $\mathbb{Z}_2^{(n)}\times\mathbb{Z}_N$ structure for generalized Moore-Read states at filling $\nu=1/m$, with $N=2m$ for fermionic states and $N=m$ for bosonic states, and find a $\mathrm{Fib}\times\mathbb{Z}_N$ structure for the $k=3$ Read-Rezayi state at filling $\nu=3/(3M+2)$, with $N=2(3M+2)$ for fermionic states and $N=(3M+2)/2$ for bosonic states. In the Read-Rezayi case, zero mode operator tunneling and direct zero mode anyon transfer yield distinct universal amplitude ratios between the neutral fusion channels, providing a characteristic signature of the underlying Fibonacci defect structure.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci
Thermal Diffusivity in Metallic Nanowire Aerogels
Madeline A. Maben, WLNC Namila Liyanage, Cameron S. Jorgensen et al. (+4 more)
Metallic nanowire aerogels combine the functional properties of metals with the high porosity of aerogels. Here, Ag and Cu nanowire aerogels with densities of 20-500 mg cm-3 were synthesized by freeze casting and investigated using lock-in thermography during sequential mechanical compaction. Thermal transport depended strongly on nanowire diameter, with 30 nm Ag nanowires exhibiting substantially lower thermal diffusivity than 60 nm wires, likely due to enhanced surface scattering as the wire diameter approaches the electron mean free path in Ag. Surprisingly, increasing the aerogel density through mechanical compaction reduced the calculated thermal diffusivity by nearly 300x, opposite the conventional expectation that denser materials transport heat more efficiently. Further analysis showed that this decrease is dominated by the thickness term in the thermal diffusivity formalism, while the fitted transport coefficient changes comparatively little. The characteristic thermal
diffusion timescale decreased only modestly during compaction, indicating that densification primarily collapses pore volume without fundamentally restructuring the conductive nanowire network. This behavior resembles
topology-preserving densification, where large
geometric compression produces comparatively small changes in the effective percolative transport pathway. These results show that nanowire aerogels cannot always be treated as homogeneous bulk media, because their macroscopic geometry and conductive network evolve differently during compaction.
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All authors Madeline A. Maben, WLNC Namila Liyanage, Cameron S. Jorgensen, Erin Marlowe, Kai Liu, Gerd Duscher, Dustin A. Gilbert
Mesoscale and Nanoscale Physics
Quantum twisting microscopy as a momentum-resolved probe of quantum geometry and sublattice symmetries
Lucas V. Pupim and Mathias S. Scheurer
In two-dimensional (2D) flat-band systems, as engineered, for instance, by stacking and twisting van der Waals materials, the momentum dependence of the Bloch states is a key factor in determining the interaction-induced emergence of different phases. Hence, methods of probing this form of ”
quantum geometry” are crucial to understanding these systems. Yet, extracting the momentum dependence of the underlying
quantum geometric tensor from measurements is challenging in 2D. Here, we propose employing a
quantum twisting microscope (QTM) with a sublattice-polarized tip to fill this gap. We show how expectation values of sublattice Pauli
matrices can be probed across the scanned momentum path. This allows for the direct measurement of crucial
symmetries characterizing the different candidate instabilities in
graphene-based moiré systems. We furthermore show that the
quantum metric component along the scan direction can also be extracted; the full
quantum geometric tensor is also accessible, given that more than one sublattice polarization can be used. Our work illustrates the potential and extends the versatility of the QTM approach. If implemented in future experiments, this would provide unprecedented access to the
symmetry signatures of the correlated phases and to the underlying
quantum geometry in twisted bilayer
graphene and related systems.
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Mesoscale and Nanoscale Physics
Geometric Photon-drag Effect in Unconventional Magnets
Bristi Ghosh, Vivek Pandey, Malay Bandyopadhyay et al. (+2 more)
The
photon-drag effect is a nonlinear optical phenomenon in which the finite momentum carried by incident
photons is transferred to charge carriers, thereby generating a dc photocurrent even in systems with inversion
symmetry. Within the density-
matrix formalism in the velocity gauge, and accounting for nonvertical optical transitions driven by finite
photon momentum, we derive the nonlinear injection and shift conductivities that characterize the
photon-drag photogalvanic response. We investigate these
photon-drag responses in unconventional $p_x$-wave and $d_{x^2-y^2}$-wave magnets in the presence of Rashba
spin-orbit coupling (RSOC), highlighting the distinct roles of
symmetry and
quantum geometry in shaping their nonlinear optical behavior. The key finding of our work is that, despite the broken inversion
symmetry, the conventional photogalvanic response vanishes identically in the $q=0$ limit after Brillouin-zone integration in both unconventional
magnetic phases, resulting in a nonlinear dc photocurrent entirely
photon-drag driven. The two unconventional magnets exhibit qualitatively distinct responses governed by their
symmetry properties: the $p$-wave magnet supports only linear injection and circular shift conductivities, whereas the $d$-wave altermagnet, which simultaneously breaks time-reversal and inversion
symmetries, admits all linear and circular components. Our findings establish altermagnets as a promising platform for controlling
symmetry-selective nonlinear photocurrents and may enable applications in polarization-sensitive photodetection, nonlinear optoelectronics, and
quantum geometric photogalvanics.
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All authors Bristi Ghosh, Vivek Pandey, Malay Bandyopadhyay, Pankaj Bhalla, Snehasish Nandy
Mesoscale and Nanoscale Physics
Quasiparticle-resolved variational theory of Andreev spin qubits
Teodor Iličin and Rok Žitko
An Andreev
spin qubit stores a single unpaired
spin, yet its coupling to the superconducting phase can be dominated by coherent virtual quasiparticle pairs. We develop a continuum variational description of the odd-parity doublet in a
quantum-dot Josephson junction with Coulomb interaction and
spin-dependent background tunneling. Retaining configurations with up to two Bogoliubov quasiparticles provides compact analytical expressions for energies and wavefunctions. We
benchmark the results against numerical renormalization group calculations and identify sources of error. We obtain closed-form expressions for the conventional and
spin-dependent Josephson couplings. Despite their small probability, two-quasiparticle configurations, through coherence with the zero-quasiparticle component, supply exactly half of the leading
spin-dependent Josephson coupling even without Coulomb repulsion and dominate it as the repulsion grows.
Spin-orbit-induced
spin transfer between the dot and the leads produces a Josephson-current contribution that is common to both
qubit states and finite at zero phase bias when the dot and lead Zeeman energies differ. We also map the localization of the virtual quasiparticles and the
spin density in the leads. The results connect strongly correlated
quantum-dot models to effective
Hamiltonians used for Andreev-
spin-
qubit circuits and clarify how the wavefunction structure determines their couplings.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.soft
Networks of triblock ellipses: from kagome order to disordered porous networks
Susanne Wagner, Gerhard Kahl and Carina Karner
In colloids, anisotropy in either shape or particle interactions can steer assemblies away from close-packed monolayers and towards more intricate structures. In this study we utilize both shape and interaction anisotropy to stabilize porous surface assemblies of triblock elliptical particles, whose surfaces are decorated with two attractive patches at opposite poles. The resulting structures display a tunable degree of crystalline kagome order, controlled by quenching the temperature and/or changing the chemical potential. In particular, we find evidence that nucleation into the kagome
lattice proceeds via a one-step pathway, in contrast to the two-step route widely reported in spherical triblock systems, making triblock ellipses an interesting model system for studying how self-assembly pathways change with particle shape. Beyond ordered crystalline structures, we find disordered porous networks with complex internal geometry. To characterize this range of assemblies, we introduce a particle-patch graph that explicitly captures the localization of bonds at distinct patches. Bonding motifs follow directly from this representation, while the loops, that we identify as the pore boundaries, can be determined from the graph's planar
embedding. We detect all loops as the face boundaries in the planar
embedding by means of a face-walking
algorithm, distinct from shortest-path approaches used in ring detection. Together with a new local and global order parameter measuring the loop network's regularity with respect to a selected loop size, this framework connects microscopic bonding motifs to mesoscopic pore properties and quantifies the tunable order of these porous assemblies.
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Mesoscale and Nanoscale Physics
Ultra-small-angle graphene twistronics with monolayer spacers: From interlayer hybridisation and moiré effects to high-temperature magnetotransport oscillations
Benjamin Fuller, Haowei Fan, Agustin Montemurro et al. (+4 more)
Based on the analysis of moiré superlattice characteristics and interlayer hybridisation in small-angle twisted
graphene multilayers with a monolayer hexagonal boron nitride (hBN) spacer, we predict spectral features and
quantum transport effects in these systems. Here, we consider both a twisted bilayer and double bilayers with a spacer, discuss double bilayers in two stacking orders - with and without inversion
symmetry - and also compare the results obtained with self-consistent Hartree calculations with and without in-plane periodic charge distribution, offering computationally efficient models for these
graphene/hBN/
graphene moiré systems. We note parametric intervals where the appearance of minibands can be enhanced by tuning with an out-of-plane displacement field, and identify the miniband edges and van Hove singularities across a broad range of densities and displacement fields. These system host Lifshitz transitions at which the Fermi lines form a network across moiré Brillouin zones, and to which Brown-Zak oscillations converge at low
magnetic field and elevated temperatures. We analyse interlayer hybridisation across the same parameter space, discuss how hybridisation affects in-plane magneto-transport in ultra-high-mobility structures, and predict that semiclassical magneto-oscillations of interlayer hybridisation can lead to 1/B-periodic
quantum oscillations of resistivity persistent to high temperature.
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All authors Benjamin Fuller, Haowei Fan, Agustin Montemurro, Yuyuan Pan, Alexey I. Berdyugin, Vladimir I. Falko, Xiao Li
Mesoscale and Nanoscale Physics
Interfacial Charge Transfer and Morphology Govern Magnetism in Ultrathin VOx/MoSe2 Heterostructures
Yasinthara Wadumesthri, Nalaka Kapuruge, Raphael B. de Oliveira et al. (+8 more)
Engineering magnetism in two-dimensional
semiconductors through interfacial interactions offers an attractive alternative to substitutional doping and provides opportunities for integrating
magnetic and optoelectronic functionality within the same
heterostructure. Here, we investigate ultrathin VOx films deposited on monolayer MoSe2 and demonstrate a pronounced enhancement of the
magnetic response upon formation of the VOx/MoSe2 interface. Combined structural, spectroscopic,
magnetic, and first-principles analyses support a consistent picture in which finite VOx clusters possess intrinsic
magnetic moments that are further enhanced by electron transfer from MoSe2 to the oxide. The predicted charge transfer is also consistent with the increased trion contribution observed in the photoluminescence response of the
heterostructure. Beyond this interfacial enhancement, the
magnetic behavior is strongly governed by the morphology of the ultrathin oxide. A crossover from discontinuous clusters to more continuous films produces qualitatively different temperature dependences: the continuous-film regime exhibits conventional thermal demagnetization and coercive softening, whereas the clustered regime displays an unusual increase in magnetization and nonmonotonic coercivity with increasing temperature. These contrasting responses are captured by a phenomenological model in which morphology determines the collective
magnetic dynamics through a distribution of effective reversal barriers and their thermally activated accessibility. Together, the results establish a unified picture in which interfacial charge transfer enhances the local
magnetic moments of VOx, while nanoscale morphology governs their collective
magnetic dynamics, providing complementary routes for controlling magnetism in oxide/transition-metal dichalcogenide
heterostructures.
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All authors Yasinthara Wadumesthri, Nalaka Kapuruge, Raphael B. de Oliveira, Bruno Ipaves, Guilherme S. L. Fabris, Kinga Lasek, Florence A. Nugera, Jianjun Pan, Xiaomei Jiang, Douglas S. Galvão, Humberto Rodríguez Gutiérrez
Mesoscale and Nanoscale Physics
Calculation of structural symmetry and applicability of small AkBlCm nanoclusters (A=Rb, Cs; B=Pb, Sn, Bi; C=Cl, Br, I) as building blocks for synthesis of larger-size nanoclusters
Roman Makarevych, Igor Dmytruk, Yevhen Hrabovskyi et al. (+3 more)
In this report we analyze charasteristics of structural
symmetry of small AkBlCm nanoclusters (A - Rb, Cs; B - Pb, Sn, Bi; C - Cl, Br, I), binding energies, stability, and, ultimately, applicability as building blocks for construction of larger-size nanoclusters. Preliminary spatial modeling of ternary compound nanoclusters has been carried out on the basics of geometrical approach, namely interatomic distances taken as a sums of corresponding ionic radii. The geometry optimization and binding energies of the modeled structures have been determined through
quantum-chemical calculations. Geometry optimization has been performed with Gaussian09 software package using Density Functional Theory (DFT) within the generalized gradient approximation of the Perdew-Burke-Ernzerhof (PBE) and hybrid approximation of the Becke-3-Lee-Yang-Parr (B3LYP) exchange-correlation functionals. The Stuttgart/Dresden (SDD) and Los Alamos National Laboratory 2 Double Zeta (LANL2DZ) scalar-relativistic effective basic potentials were employed throughout the calculations. As a result of these calculations, a clear relationship between the atomic radii, their ratios within the nanoclusters and their stability has been obtained and analyzed.
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All authors Roman Makarevych, Igor Dmytruk, Yevhen Hrabovskyi, Iryna Pundyk, Nataliia Bashmakova, Nataliya Berezovska
Mesoscale and Nanoscale Physics
Negative charging energy in LaAlO$_3$/SrTiO$_3$ quantum dots induced by SrTiO$_3$ polarization
J. Czarnecki, A. Sierant, B. Szafran et al. (+11 more)
We propose a microscopic mechanism for the negative charging energy in LaAlO$_3$/SrTiO$_3$
quantum dots, arising from the electrostatic image charge associated with the paraelectric response of SrTiO$_3$. Using a self-consistent configuration-interaction approach that accounts for both Coulomb repulsion and image-charge interactions, we find a robust regime of electron pairing whose strength is tunable through the confinement potential and the effective image-charge distance, set by the thickness of the interfacial two-dimensional electron gas. Our results establish dielectric image-charge effects as a route to controllable electron pairing in oxide
quantum dots and other materials with a strong dielectric response.
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All authors J. Czarnecki, A. Sierant, B. Szafran, P. Szumniak, T. S. Jespersen, G. Meucci, T. Jansen, R. Citro, A. Maiellaro, N. Bergeal, M. Bibes, M. Salluzzo, A. Kalaboukhov, P. Wójcik
Mesoscale and Nanoscale Physics
Ferroaxial Electronic Response to Local Lattice Rotation
Ken Uchino, Yuuki Ogawa and Satoru Hayami
We theoretically investigate electronic responses to
lattice deformation in a ferroaxial system using a tight-binding model on a two-dimensional square
lattice and linear response theory. While conventional elastic responses are commonly discussed in terms of symmetric strain, the displacement gradient also contains an antisymmetric component describing a local
lattice rotation. Focusing on this rotational degree of freedom, we treat the symmetric strain and antisymmetric local rotation as independent perturbations and incorporate their effects microscopically through deformation-induced modulations of the electronic hopping amplitudes. We find that the local rotation induces characteristic diagonal electric quadrupoles that are distinct from those generated by the symmetric shear strain. The rotation-induced response is dominated by the intraband contribution and increases approximately linearly with the ferroaxial
crystal field, vanishing in the absence of ferroaxial order. In contrast, the symmetric-shear-induced response remains finite without ferroaxial order and contains both intraband and interband contributions. These results demonstrate that the antisymmetric local rotation provides a distinct electronic response channel characteristic of the ferroaxial state.
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Mesoscale and Nanoscale Physics
Theory of Magic Angles in Twisted Bilayer Graphene: from Non-Abelian Gauge Fields to Flat Bands
Leonardo A. Navarro-Labastida and Gerardo G. Naumis
At a discrete set of “magic” twist angles, twisted bilayer
graphene develops two bands at charge neutrality that are almost perfectly flat. We present a unified theory of this phenomenon within the chiral continuum model. The central object is the square of the chiral
Hamiltonian, a $2\times2$ Schrödinger operator for an electron moving in a non-Abelian $SU(2)$ pseudo-
magnetic field generated by the interlayer tunneling. From it we derive an energy sum rule in which the interlayer coupling splits into a bounded overlap channel and an unbounded current channel; their different scaling with the coupling $\alpha$ explains why the first magic angle is qualitatively different from the rest. The asymptotic rule $\alpha_{m+1}-\alpha_m\to3/2$ follows from a rescaling of the moiré potential together with its three-
cell magnetic periodicity, with the two parity families of zero modes each repeating with period $3$ and interleaved with each other. Near the AA point the zero-mode problem becomes a lowest-Landau-level problem in an effective field $B_{\rm eff}=3\alpha$, so that the high-order zero modes are coherent Landau states of width $1/\sqrt{3\alpha}$ whose guiding centres converge to $\pm(\pi/3) q_\mu$, with kinetic and confinement energies in equipartition. Deforming the coupling into an Abelian one removes the magic-angle sequence, which shows that the non-Abelian structure is essential. Between consecutive magic angles, band inversions at $\Gamma$ and $M$ produce phases with Chern numbers $C=\pm2$, and the
quantum metric reaches a sharp maximum at $\theta\approx0.43^\circ$, identifying a promising regime for correlated and topological phases beyond the first magic angle.
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Mesoscale and Nanoscale Physics
Bichargon Hall Effect
Ping Tang
The conventional Hall effect arises from the transverse deflection of mobile electronic charge carriers in conductors. Here, we predict a bosonic charge Hall effect in charge-ordered
insulators mediated by bichargons, collective bosonic excitations of charge order carrying quantized electric charges $\pm 2e$. In contrast to conventional electronic carriers, a thermally excited bichargon gas does not support dc electrical conduction in response to an external electric field despite its constituents carrying finite electric charges. We show that a temperature gradient can instead drive a diffusive flow of thermal bichargons, while an out-of-plane
magnetic field deflects the oppositely charged bichargons in opposite transverse directions via the Lorentz force, leading to a finite Hall charge current and opposite charge accumulations at the sample edges. Our results broaden the paradigm of charge Hall transport beyond conventional electronic conductors, establishing that a charge Hall response can emerge in an electrically insulating system through charged bosonic quasiparticles.
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Mesoscale and Nanoscale Physics
Sparse qubit operation in a 6$\times$6 quantum dot array
Alexader S. Ivlev, Harold L. Co, Damien R. Crielaard et al. (+4 more)
Fault-tolerant
quantum computation requires increasing the
qubit count while maintaining high performance and connectivity.
Semiconductor qubits have demonstrated high-fidelity operation, but growing the system size at sufficient connectivity remains challenging, such that scaling mainly occurred in one spatial direction to preserve access for control lines in the other. Here, we realise an approach that relaxes the requirements on fabrication and demonstrate a 6$\times$6
quantum dot array, defined by a shared barrier gate and individual plunger gates. The
quantum dots are tuned through coherent
spin shuttling over distances up to 5.2um into a sparse configuration of 10 simultaneously operated
qubits, leaving space for shuttling connections. We reduce crosstalk by 42$\times$ in this separated regime, enabling high-fidelity simultaneous single-
qubit control, while control-Z operations can be implemented through shuttling and plunger-only control. These results promise high-connectivity
quantum circuits, enabled by scaling to two-dimensional geometries and exploiting shuttling.
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All authors Alexader S. Ivlev, Harold L. Co, Damien R. Crielaard, Alice Petrillo, Setareh Kazemzadeh, Giordano Scappucci, Menno Veldhorst