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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Friday 18 September

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.stat-mech

Universal Eigenvector Statistics of Non-Hermitian Random Matrices

Ze Chen, Zhenyu Xiao and Shinsei Ryu

Eigenvector overlaps quantify nonorthogonality and govern the response and dynamics of non-Hermitian systems. We extend the universality of non-Hermitian random matrices to the statistics of these overlaps. We obtain analytical expressions for eigenvector overlaps in the spectral bulk and near the origin, covering ten symmetry classes in the limit of large matrix size. Using fermionic replica nonlinear $\sigma$ models, we relate these overlaps to Hermitian level statistics, symmetry class by symmetry class and topological sector by topological sector. Numerical calculations in various physical models support the universality of the normalized overlaps in the regimes studied. Our work establishes a duality between Hermitian level statistics and non-Hermitian eigenvector overlaps.
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Mesoscale and Nanoscale Physics

Non-Hermitian impurity scattering in graphene: Boltzmann transport and thermoelectric response

Juan A. Cañas, Daniel A. Bonilla and A. Martín-Ruiz

We investigate charge and thermoelectric transport in monolayer graphene containing a dilute distribution of finite-range non-Hermitian scattering centers. The impurities are modeled as circular complex potentials, whose imaginary component describes local carrier loss or gain. By solving the Dirac scattering problem exactly within a partial-wave approach, we obtain the nonunitary scattering matrix and derive the transport and absorption cross sections, which separately characterize momentum relaxation and net carrier exchange with the environment. To connect the microscopic scattering problem with stationary transport, we formulate a semiclassical Boltzmann description in which an external reservoir compensates the equilibrium particle loss or gain. This leads to an effective relaxation time governed by both elastic momentum scattering and non-Hermitian flux exchange. Using the full energy-dependent relaxation time, we evaluate the Onsager coefficients and the resulting electrical conductivity, electronic thermal conductivity, Seebeck coefficient, Lorenz ratio, and electronic thermoelectric figure of merit. We find that weak gain increases the effective carrier lifetime and enhances both charge and heat conductivities, while absorption produces the opposite behavior. More importantly, gain enhances the magnitude of the thermopower and the electronic figure of merit, whereas loss suppresses them. The Lorenz ratio remains close to the Sommerfeld value, with non-Hermiticity mainly modifying its finite-temperature corrections. Our results show that non-Hermitian scattering provides an additional mechanism for controlling the energy dependence of carrier relaxation and, consequently, the thermoelectric response of graphene.
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Mesoscale and Nanoscale Physics

Photogalvanic effect in monolayers of transition metal dichalcogenides

G.V. Budkin and E.L. Ivchenko

We present a theoretical study of photocurrents generated by direct optical transitions induced by linearly polarized radiation in monolayers of transition metal dichalcogenides. A microscopic theory of both the ballistic and shift contributions to the photocurrent is developed that accounts for Coulomb attraction between photoexcited electrons and holes. It is demonstrated that, in typical 2D semiconductors, the Coulomb correlation causes the ballistic contribution to dominate over the shift photocurrent.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Quantized Transport of Gap Solitons in the Harper-Hofstadter Model

Oliver Ashfield, Henry Davenport, Tom Sheppard et al. (+6 more)

We investigate the dynamics of gap solitons found in the Harper-Hofstadter model with an additional cubic nonlinearity, such as that describing weakly-interacting cold atoms or a Kerr nonlinearity in an optical platform. We find that certain solitons with frequencies deep in the band gap travel with a robust, quantized velocity perpendicular to the applied force and maintain their spatial profile over long timescales. Crucially, this velocity is linearly proportional to the applied force, allowing for arbitrary steering of the nonlinear wavepacket, without breakdown, using only a tunable external force. This quantized transport persists despite a highly non-uniform occupation of the linear Bloch states, and contrasts with solitons near the band edge, which deform and delocalize under the same force. These results point to a robust and highly controllable mechanism for steering nonlinear solitons in experimentally accessible platforms.
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All authors Oliver Ashfield, Henry Davenport, Tom Sheppard, David G. Reid, Holly A. J. Middleton-Spencer, A. C. Berceanu, Frank Schindler, Iacopo Carusotto, Hannah M. Price

Mesoscale and Nanoscale Physics

Impact of exciton-phonon and exciton-magnon interaction on transport in anisotropic 2D magnetic semiconductors

Daniel Erkensten, Katarzyna Sadecka, Marie-Christin Heißenbüttel et al. (+2 more)

Two-dimensional magnetic semiconductors provide a unique platform for exploring the interplay between charge, lattice, spin, and light. The strong Coulomb interaction combined with long-range magnetic order enables efficient coupling between excitons and both lattice vibrations and spin excitations, thereby strongly influencing exciton dynamics and transport in these materials. Here, we develop a microscopic many-body theory of exciton–phonon and exciton–magnon interactions in mono- and bilayer CrSBr. We demonstrate that the quasi-one-dimensional character of excitons in CrSBr gives rise to strongly anisotropic exciton–phonon and exciton–magnon scattering, resulting in direction-dependent diffusion coefficients. Furthermore, we identify a distinct temperature dependence originating from one-phonon and two-magnon scattering processes, providing characteristic signatures of the underlying microscopic interactions. Our work establishes a microscopic framework for understanding exciton transport in magnetic van der Waals semiconductors and provides guidance for future optical spectroscopy and exciton transport experiments.
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All authors Daniel Erkensten, Katarzyna Sadecka, Marie-Christin Heißenbüttel, Thorsten Deilmann, Ermin Malic

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

Floquet physics from quantized light-matter interaction: geometric phases, gauge consistency, and entanglement

Beatriz Pérez-González, Sigmund Kohler and Mónica Benito

Floquet engineering and cavity quantum electrodynamics represent two complementary descriptions of light-matter interaction, yet their precise connection beyond the small-coupling regime remains subtle. Here, by representing the cavity field in a large-photon-number phase basis, we show that the time-dependent Schrödinger equation of a classically driven system emerges directly from the fully quantized problem, without replacing the field by a classical trajectory or assuming a mean-field decoupling. This establishes a one-to-one correspondence between quasienergies and photon-sector-shifted quantum energies, while providing a direct quantum interpretation of the Floquet mean energy and Anandan phase. Because gauge invariance is exactly preserved at the level of the quantum Hamiltonian, the quantum-to-classical correspondence remains valid beyond perturbative and small-coupling regimes. We further show that light–matter entanglement is encoded in matter–harmonic correlations of the corresponding Floquet mode, thus identifying a semiclassical limit in which Floquet physics emerges without requiring the quantum field itself to approach a coherent classical state.
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Mesoscale and Nanoscale Physics

Field-free and Sub-ns Magnetization Reversal in Synthetic Anti-ferromagnets using Sequential Spin-Orbit Torque and Spin-Transfer Torque Pulses

Subham Das and Naveen Sisodia

Single free-layer Magnetic Tunnel Junctions (MTJs) with a perpendicular easy axis driven by spin currents have demonstrated fast, deterministic switching, non-volatility, and low-power memory capabilities when integrated with modern nanoelectronic devices. In this article, we numerically investigate sub-ns and field-free switching characteristics of a p-MTJ with a Synthetic Antiferromagnet (SAF) based free layer structure using sequential sub-ns current pulses of Spin Orbit Torque (SOT) and Spin Transfer Torque (STT). While the SOT pulse applied to one layer decreases the transition time by bringing the coupled layers into an in-plane configuration, the subsequent STT pulse on the system determines the switching direction. Various pulse protocols have been investigated for both SOT and STT current amplitude and pulse widths, while also considering the effect of the Reference Layer (RL). While significant regions of deterministic switching events are observed, limitations in SOT, STT parameters are further investigated in presence of thermal fluctuations.
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Mesoscale and Nanoscale Physics

Spatially resolved quantum magnetometry and stray-field reconstruction of permalloy microdisks using boron-vacancy centers in hexagonal boron nitride

Peiting Wen, Shuyu Wen, Jiang Qu et al. (+13 more)

Transferable hexagonal boron nitride (hBN) hosting negatively charged boron-vacancy (VB$^{-}$) spin defects offers a versatile platform for integrated quantum magnetometry, yet quantitative imaging of magnetic microstructures remains challenging. Here, we integrate a transferred hBN flake with a 4 $\mu$m-diameter permalloy (Py = Ni${81}$Fe${19}$) microdisk and perform spatially resolved optically detected magnetic resonance measurements at room temperature. An applied in-plane magnetic field distorts the vortex-state magnetization, generating edge-localized magnetic surface charges and pronounced stray-field signatures at opposite disk edges. By referencing each pixel to its local zero-field splitting and correcting for a residual out-of-plane bias field, we quantitatively reconstruct the out-of-plane stray-field distribution, revealing peak fields of approximately 11.2 mT. An edge-charge model reproduces the spatial distribution and amplitude of the reconstructed field, linking the ODMR response to the field-driven evolution of the vortex state. These results establish transferred hBN VB$^{-}$ sensors for quantitative magnetometry of magnetic microstructures.
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All authors Peiting Wen, Shuyu Wen, Jiang Qu, Zeling Xiong, Katrin Schultheiss, Slawomir Prucnal, Artur Erbe, Kenji Watanabe, Takashi Taniguchi, Jürgen Lindner, Jürgen Fassbender, Manfred Helm, Shengqiang Zhou, Ruslan Salikhov, Helmut Schultheiss, Yonder Berencén

Mesoscale and Nanoscale Physics

Transport statistics and thermoelectric performance of Hofstadters butterfly

Friedemann Queißer and Gernot Schaller

We investigate electrons subject to a magnetic field that may tunnel along a square lattice and additionally to two electronic leads. Using non-equilibrium Greens functions we evaluate the transmission and from that particle and energy currents and zero-frequency noise of the setup. We identify sweet spots for which the transmission reaches unity over broad frequency windows – carried by topologically protected boundary modes that are quite robust against local fluctuations of on-site energies. These transmission plateaus can be exploited for the implementation robust current standards, clocks and thermoelectric devices.
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Mesoscale and Nanoscale Physics

Floquet-spin-orbit compensation and flat- and quadratic-band contact in the $\alpha$-$T_3$ lattice

Imtiaz Khan, Muzamil Shah, Reza Asgari et al. (+1 more)

We investigate Floquet–spin-orbit compensation in the three-band $\alpha$-$T_3$ system, which interpolates between graphene at $\alpha=0$ and the dice lattice at $\alpha=1$. By tuning the interplay between an off-resonant circularly polarized optical field and intrinsic spin–orbit coupling, we identify three distinct pairwise band-degeneracy conditions in the quasienergy spectrum: (i) valley–spin degeneracy, (ii) spin–band degeneracy, and (iii) flat–dispersive band degeneracy. The flat band is not an artifact of a two-band reduction but rather an exact property of the continuum Hamiltonian at this compensation point, where the complete three-band Hamiltonian supports a momentum-independent dark state. A winding-two low-energy Hamiltonian and a finite-momentum Berry-curvature maximum, whose radius scales as the square root of the detuning, emerge from detuning this contact. We further show that the intrinsic transverse thermoelectric response encodes these spectrum patterns. The flat- and quadratic-band degeneracy can be predicted for $0<\alpha<1/2$ because the two response scales linked to the linear contacts have a monotonic ratio independent of the common spin–orbit energy scale. This allows an inverse determination of $\alpha$ and of the common spectral scale. The analytical results are confirmed by full three-band Kubo calculations, which also define the regime in which this inverse characterization is still observable. These results connect an experimentally accessible, Berry-curvature-sensitive thermoelectric response to tunable Floquet–spin–orbit band geometry.
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All authors Imtiaz Khan, Muzamil Shah, Reza Asgari, Gao Xianlong

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

Orbital angular momentum accumulation in SrVO3 thin films

Julien Brehin, Montserrat X. Aguilar-Pujol, Dongwook Go et al. (+3 more)

Orbital transport in light transition metals has emerged as a promising route toward angular-momentum electronics, with Hanle magnetoresistance (HMR) providing a direct electrical probe of the orbital Hall effect (OHE) in non-magnetic conductors. Here we report magnetoresistance signatures consistent with orbital HMR in epitaxial SrVO$*3$ (SVO), a narrow-band $d^1$ oxide grown on (001)-oriented (LaAlO$*3$)$*{0.3}$(Sr$*2$TaAlO$*6$)$*{0.7}$ substrates. In films of different thickness and longitudinal resistivity, field-dependent measurements reveal a reproducible positive, even-in-field, and approximately quadratic $\Delta\rho*{x-y}$ response, as expected for HMR. Angular measurements characterize the corresponding magnetoresistance anisotropy and reveal an additional non-Hanle contribution. Density functional theory (DFT) calculations yield an intrinsic orbital Hall conductivity $\sigma*{\mathrm{OH}}^{\mathrm{DFT}} = 390,(\hbar/e),\Omega^{-1}\mathrm{cm}^{-1}$ and a spin Hall conductivity $\sigma_{\mathrm{SH}}^{\mathrm{DFT}} = -12,(\hbar/e),\Omega^{-1}\mathrm{cm}^{-1}$ at the Fermi level, corresponding to $|\sigma_{\mathrm{OH}}^{\mathrm{DFT}}/\sigma_{\mathrm{SH}}^{\mathrm{DFT}}| \approx 33$, thus indicating a predominantly orbital response. Using the diffusive HMR framework with $\lambda_{\mathrm{OD}} = 2$ nm as a reference value, the 20.8 nm film gives conservative saturation-limit lower-bound (LB) estimates $\theta_{\mathrm{OH,LB}} = 0.0144 \pm 0.0004$ and $\sigma_{\mathrm{OH,LB}} = (460 \pm 11),(\hbar/e),\Omega^{-1}\mathrm{cm}^{-1}$, comparable to the DFT value. Across the series, lower-bound estimates obtained with the same $\lambda_{\mathrm{OD}}$ increase overall with longitudinal conductivity. These results suggest that narrow-band $d^1$ metallic oxides are a promising platform for orbital transport.
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All authors Julien Brehin, Montserrat X. Aguilar-Pujol, Dongwook Go, F. Casanova, J. Fontcuberta, E. Longo

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.stat-mech

The charge dependent hard-sphere model: Polycrystals as low-energy configurations

Leonard Kreutz and Timo Ziereis

We investigate the emergence of rigid polycrystalline structures in atomistic ionic particle systems as low-energy configurations. The interaction between particles of opposite charge is modeled by hard spheres that interact when they are tangential. The interaction between particles of same charge is modeled as a hard repulsion that forces a minimal distance between them. The atomistic energy is frame invariant, and no underlying reference lattice is assumed on the ionic configurations. The asymptotic behavior of configurations with finite surface energy scaling is identified by means of $\Gamma$-convergence. The related continuum theory is described by piecewise constant fields that encode the local orientation of the configuration. The limiting energy is local and concentrates at grain boundaries, which correspond to the boundaries of the regions where the underlying configuration has a constant orientation. The limiting energy density is anisotropic and depends on the relative misorientation of the two grains, their translation misfit, and the normal to their interface. Furthermore, we perform a fine analysis of surface energies for solid-solid and solid-vacuum phase transitions and determine energetically favorable orientation mismatches. This relies on a structure result for our grain boundaries, which shows that, due to the rigid setup, interpolating layers near the grain interface are energetically not favorable.
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Mesoscale and Nanoscale Physics

Rydberg excitons in core-shell nanostructures I. Optical properties

Sylwia Zielińska-Raczyńska, Gerard Czajkowski and David Ziemkiewicz

Optical properties of Cu$_2$O-metal core-shell nanostructures containing Rydberg excitons are examined. Calculations of electron and hole eigenfunctions and eigenenergies are used to calculate exciton energy and its dependence on system geometry and dimensionality. The limits of very small and very large structures are discussed, making the results applicable to low principal quantum number excitons as well as to Rydberg states. The Real Density Matrix Approach method is used to calculate the optical properties of the studied systems.
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Mesoscale and Nanoscale Physics

Projective representation theory of spin space groups

Zheng Zhang, Peiyuan Wang and Y. X. Zhao

Spin space groups provide the natural symmetry framework for magnetic crystals with weak spin-orbit coupling, but extracting their physical consequences requires a general theory of irreducible representations. The central difficulty is that spin space groups are represented projectively: their factor systems can render lattice translations noncommuting, induce nonsymmorphic actions in momentum space, and modify the projective structure of little cogroups. These effects lie beyond conventional space-group and double-group representation theory. Here, using Mackey's theory of group extensions, we develop a unified constructive framework for all collinear, coplanar, and noncoplanar spin space groups, including antiunitary symmetries and general factor systems. A central technical result is a canonical decomposition of the relevant factor system $\nu$ into a translational factor $\sigma$, a mixed factor $\gamma$ coupling translations to point-group operations, and a point-group factor $\alpha$. This decomposition makes transparent how ordinary representation theory is modified: $\sigma$ determines the projective translation algebra and the appropriate Brillouin zone, $\gamma$ controls the momentum-space group action and can make it nonsymmorphic, and $\alpha$ contributes to the factor systems of little cogroups. On this basis, we construct all projective irreducible corepresentations by induction over momentum-space orbits. The framework identifies which nonsymmorphic momentum-space symmetries can be realized by spin space groups and reveals Brillouin spaces that are compact flat manifolds rather than tori, symmetry-enforced Zak phases, reconstructed high-symmetry momenta and band degeneracies, and new types of quasiparticles. Our results establish the representation-theoretic foundation for systematic studies of weak-spin-orbit-coupled magnetic materials.
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Thursday 17 September

Mesoscale and Nanoscale Physics

Defect-mediated orientational ordering in stripe-forming magnetic systems

Premarun Barik, Johan van Lierop and Robert L. Stamps

We investigate the nonequilibrium development of orientational order in a two-dimensional stripe-forming magnetic system with competing interactions. Following a quench into the stripe-ordered regime, horizontal and vertical stripes organize into orientational regions(super-domains) separated by characteristic defects. We show that the evolution towards a globally oriented stripe state is mediated by the motion and elimination of these defects, with T-junctions providing a direct microscopic measure of super-domain reorganization. The T-junction population reveals distinct dynamical regimes associated with super-domain formation, growth, and equilibration. Temperature, system size, and equilibrium stripe width modify the characteristic time scales of this evolution, while thermal fluctuations produce a finite defect population at higher temperatures. We further show that the defect dynamics observed in real space are reflected in the structure factor, providing a reciprocal-space signature of defect elimination. These results establish a microscopic defect-based description of orientational ordering in stripe-forming magnetic systems.
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Mesoscale and Nanoscale Physics

Bend Contour Electron Tomography (BCET): Quantitative strain and topography mapping

Henry G. Bell, Chenhang Xu, Arthur McCray et al. (+5 more)

Freestanding thin films of quantum materials naturally develop sub-micrometer, nonuniform strain fields that strongly affect their electronic, magnetic, and structural properties in both equilibrium and nonequilibrium conditions. However, current methods to quantitatively resolve these mesoscopic features are primarily restricted to scanning probes, making it challenging for dynamical measurements such as single-shot imaging and femtosecond microscopy. Here, we present a new computational framework which we denote as bend contour electron tomography (BCET), which efficiently converts bend contours in transmission electron microscope images into quantitative two-dimensional maps of strain and topography. By iteratively minimizing a designed loss function between experimental and simulated bend contour images, BCET retrieves both surface morphology and in-plane strain tensor fields without requiring scanning or diffraction mapping. We applied BCET to freestanding SrTiO$_3$ thin films, demonstrating the successful reconstruction of the local strain distribution and curvature field with high fidelity. Our approach provides a quantitative framework for characterizing mesoscale structures in freestanding films using wide-field imaging, opening new avenues to investigate how spatial inhomogeneity governs phase transitions and nonequilibrium dynamics in two-dimensional quantum materials.
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All authors Henry G. Bell, Chenhang Xu, Arthur McCray, Minyong Han, Yicheng Zhuang, Harold Y. Hwang, Colin Ophus, Alfred Zong

Mesoscale and Nanoscale Physics

Singular high-harmonic transport above a quantum threshold

Yugo Onishi, Su-Yang Xu and Liang Fu

We show that Landau-Zener tunneling in a small-gap insulator produces a singular DC current–voltage relation, and correspondingly, strong high-harmonic generation in AC transport. Remarkably, high-harmonic responses decrease subexponentially with harmonic order, which is parametrically slower than in ordinary conductors and PN diodes. We further derive a scaling law for the current amplitude at frequency $n\omega$ produced by the applied electric field $E$ at fundamental frequency $\omega$. Our work offers a promising route to THz generation based on frequency multiplication in solids.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Shape Deformation and Braid Statistics of Fractional Quantum Hall Quasiparticles

Mytraya Gattu and J. K. Jain

For “ideal anyons,” the Berry phase associated with a closed loop of an anyon around another is robust, i.e., independent of the size and shape of the loop, and directly yields the braid statistics. This is not the case for fractional quantum Hall (FQH) quasiparticles (QPs), which are charged and have finite size. The Berry phase $\Theta$ associated with a loop of a fractionally charged QP around another contains two contributions: $\Theta=\Theta_{\rm AB}+\Theta_{\rm braid}$, where $\Theta_{\rm AB}$, the Aharonov-Bohm phase, is proportional to the enclosed area, and $\Theta_{\rm braid}$ is the order-one contribution from braid statistics. The latter is obtained by determining $\Theta$ for a closed loop twice, once with and once without the other QP inside, and taking the difference. It is known that even small uncertainties in the enclosed areas can produce corrections to $\Theta_{\rm AB}$ larger than $\Theta_{\rm braid}$. We consider how the Berry phase depends on the shape of the QP, which, unlike its charge, is not topological and varies along the path in response to the local potential. Our calculations show that practically imperceptible differences in the shape of the outer QP in the two experiments can produce a non-universal value for the braid statistics, revealing an additional challenge for a precise determination of the braid statistics. We discuss these results in the context of interference experiments. We also note that the fractional phase jumps in these experiments can be understood without assuming QPs with sharply quantized fractional charges at the edges of the FQH system; instead, they directly measure the fractionally quantized vorticity of bulk QPs.
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Mesoscale and Nanoscale Physics

Pressure and Proximity Tuned Twisted Bilayer Graphene

David T. S. Perkins and Joseph J. Betouras

The coupling between layered atomically-thin materials mediated by van der Waals forces allows strong electronic correlations, unique topological signatures, and non-trivial spin textures, all of which play an important role in the creation of spin-, valley-, and orbitronic devices. Here, we demonstrate that twisted bilayer graphene encapsulated by transition metal dichalcogenides exhibits a generically non-radial spin texture yet hosts a purely collinear Edelstein effect despite this lack of radial symmetry. Moreover, we show how uniaxial pressure can be used to further tune the band structure and change the number of active Fermi surfaces without compromising the collinear response. Lastly, we illustrate how the quantum geometry changes in the encapsulated twisted graphene bilayer with larger pressures spreading the Berry curvature over large regions of the moiré Brillouin zone. These results illustrate how encapsulation and pressure can be used to drastically alter the topology and spin-charge interconversion processes of moiré heterostructures.
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Mesoscale and Nanoscale Physics

Coherent and ultra-low-power EDSR with a flopping-mode spin qubit in germanium

Alexei Orekhov, Wonjin Jang, Pan Zhang et al. (+13 more)

Hole spin qubits in semiconductor quantum dots (QDs) enable high-fidelity all-electric control, but conventional electric dipole spin resonance (EDSR) can require substantial rf drive power at the low magnetic fields that are favorable for qubit coherence and readout. In planar Ge hole spin qubits, this can reach -27 dBm at the device, posing challenges for scalable architectures due to heating and crosstalk. Here, we demonstrate a flopping-mode (FM) qubit in Ge, where a single spin is delocalized in a double QD, combining first-order protection against charge noise with exceptionally efficient electric driving. By mapping out coherence sweet-spots as a function of magnetic field orientation we achieve $T_2^*= 1.4\mu\mathrm{s}$, $T_2^{\mathrm{Hahn}}= 11.5 \mu\mathrm{s}$, $T^{\phi, \mathrm{CPMG32}}_2= 130 \mu\mathrm{s}$, and $T_1= 226 \mu \mathrm{s}$, and a single-qubit gate fidelity of up to 99.76$\%$ for a gate time $t_{X\pi} = 88$ ns. Importantly, these results are obtained at a nearly in-plane magnetic field of 5 mT using only -52 dBm drive power at the device. We further find that qubit relaxation in this regime is consistent with a two-photon Orbach process, providing a route for further optimization. Our results demonstrate that FM-EDSR supports ultra-low-power, high-fidelity single-qubit operations, improvements that could benefit scalable hole-spin-based architectures and hybrid spin-photon interfaces.
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All authors Alexei Orekhov, Wonjin Jang, Pan Zhang, Konstantinos Tsoukalas, Fabian Oppliger, Franco De Palma, Elena Acinapura, Younghun Ryu, Inga Seidler, Lisa Sommer, Leonardo Massai, Felix J. Schupp, Matthias Mergenthaler, Stefano Bosco, Patrick Harvey-Collard, Pasquale Scarlino

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

Engineering Weak Universality with Quantum Dots

Warre Missiaen, Michael Wimmer and Natalia Chepiga

Quantum critical theories with continuously varying critical exponents remain challenging to access experimentally. Here, we propose quantum-dot architectures for realizing the quantum Ashkin-Teller and XYZ/eight-vertex models using resonator-mediated and direct Coulomb interactions, respectively. We focus on the Ashkin-Teller and eight-vertex critical lines, which are connected by a non-local duality relating local order parameters to topological string operators. The resulting platforms provide microscopic control over the parameters explicitly controlling critical exponents, with the Coulomb-based implementation enabling stronger interaction regimes. We demonstrate that continuously varying critical behavior can already be resolved in short chains. For experimentally realistic parameters, the accessible interaction range is expected to produce changes in the critical exponents of order $10\%$ for the Ashkin-Teller model and substantially larger changes for the eight-vertex model. In both cases, the predicted variations lie well above the estimated measurement uncertainties.
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Mesoscale and Nanoscale Physics

Disorder-induced modulation of the nonlinear Hall effect in Weyl semimetals

Juan A. Cañas, Daniel A. Bonilla and A. Martín-Ruiz

We study the effects of impurity scattering on the nonlinear Hall response of Weyl semimetals within the semiclassical Boltzmann approach. We derive the rectified and second-harmonic conductivity tensors for a general momentum-dependent transport relaxation time and evaluate this quantity microscopically for short-range, Gaussian, screened Coulomb, and magnetic impurities. For scalar disorder, the relaxation time is isotropic and chirality independent. The nonlinear Hall response is then determined by the anomalous velocity associated with the Berry curvature, and a finite net response requires energetically inequivalent Weyl nodes to avoid cancellation between opposite chiralities. Polarized magnetic impurities lead to a qualitatively different behavior. We find that interference between the first- and second-order Born amplitudes generates a helicity-dependent anisotropic correction to the relaxation time. This anisotropy changes the tensor structure of the nonlinear Hall conductivity and gives rise to a finite second-harmonic current for suitable orientations of the electric field relative to the impurity polarization. For representative parameters, however, the anisotropic contribution is several orders of magnitude smaller than the dominant isotropic response. These results establish how the microscopic form of impurity scattering enters the nonlinear Hall response of Weyl semimetals through the transport relaxation time.
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Mesoscale and Nanoscale Physics

Coplanar Lateral Gating MoS2 on SrTiO3: A Unified Platform for Classical and Quantum Devices

Prasad Muragesh, Manav Murali, Venkatesha Modur Ramachandra et al. (+1 more)

Atomically thin transition-metal dichalcogenides, such as MoS2, offer a promising route to surpass the scaling limits of silicon owing to their excellent electrostatic control and resilience to short-channel effects. Realizing this potential depends critically on gate-stack engineering, where strong gate coupling must be achieved without compromising the pristine two-dimensional interface. Here, we demonstrate a coplanar lateral-gating architecture for MoS2 field-effect transistors fabricated directly on single-crystal SrTiO3. The exceptionally high permittivity of SrTiO3 enhances gate-channel coupling. The coplanar geometry eliminates the need for a separate gate insulator, reducing interface disorder. Owing to the quantum paraelectric nature, the SrTiO3 dielectric constant increases upon cooling, enhancing the gate coupling, leading to a decrease in threshold voltage and subthreshold swing an effect that contrasts with conventional FET architectures. The dielectric-free MoS2 surface, combined with enhanced electrostatic control, makes this architecture a promising platform for low-power two-dimensional electronics and cryogenic quantum devices.
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All authors Prasad Muragesh, Manav Murali, Venkatesha Modur Ramachandra, Madhu Thalakulam

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

Ultralow-Tensile Strain Enables Exciton Funneling and Energy Transfer to Boost MoSe2 Photoluminescence Quantum Yield

Gayatri, Mehdi Arfaoui, Debashish Das et al. (+14 more)

Strain engineering is a powerful route for controlling the exciton dynamics in van der Waals (vdW) heterostructures (HSs). The interlayer energy transfer (ET) process is another key factor in controlling the photocarrier relaxation pathways in vdW HSs. In this work, we combine these two processes to achieve an 8-fold enhancement to the relative photoluminescence (PL) quantum yield (QY) in a HS formed from monolayers of ReS2 and MoSe2, separated by a thin hBN interlayer, placed onto an hBN bubble. We achieve this enhancement by applying only 0.1% biaxial tensile strain, which results in efficient exciton funneling and an increased transition dipole moment. Our experimental data are supported by first-principles density-functional theory and coherent transfer-matrix method calculations, ruling out optical interference as the dominant origin of the enhancement. This work provides an innovative route for enhancing the PL QY of vdW materials via interplay between the tensile strain and the ET process.
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All authors Gayatri, Mehdi Arfaoui, Debashish Das, Mateusz Raczyński, Marta Bilska, Piotr Tatarczak, Aleksandra Krystyna Dąbrowska, Tomasz Kazimierczuk, Takashi Taniguchi, Kenji Watanabe, Piotr Kossacki, Andrzej Wysmołek, Saroj Kumar Nayak, Adam Babiński, Johannes Binder, Maciej R. Molas, Arka Karmakar

Mesoscale and Nanoscale Physics

Predictive Structure to Thermal Conductivity Modeling Framework for BEOL Interconnect Stacks in Advanced Technology Nodes Enabled by Extensive Layer Resolved Thermal Measurements

Zifeng Huang, Yiyang Sun, Tianyu Jia et al. (+2 more)

The increasing structural complexity of BEOL interconnect stacks in advanced integrated circuits demands a structure-aware thermal conductivity (ąppa}) modeling framework. However, generalizable models derived from layer-resolved thermal measurements that quantitatively capture the dependence of ąppa} on interconnect structures remain lacking, limiting predictive thermal analysis. Here, we establish an experimentally derived, structure-aware ąppa} modeling framework enabled by time-domain thermoreflectance measurements with ~100 nm depth resolution. Statistical analysis of a compiled dataset comprising over 40 experimentally measured layer-resolved ąppa} values across diverse BEOL layers reveals a generalizable empirical structure-to-ąppa} relationship, enabling predictive modeling based on interconnect structure. An intra-layer three-dimensional ąppa} distribution model based on effective medium theory and realistic layouts further resolves spatial ąppa} variations within practical interconnect layers. Together, these models establish an experimentally derived, structure-aware ąppa} modeling framework for predictive and generalizable thermal analysis of advanced interconnect stacks and 3D ICs.
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All authors Zifeng Huang, Yiyang Sun, Tianyu Jia, Runsheng Wang, Zhe Cheng

Mesoscale and Nanoscale Physics

Plasmons in twisted bilayer graphene across dispersive and flat bands

Antonio Palamara, Michele Pisarra and Antonello Sindona

The dynamical dielectric response of twisted bilayer graphene is explored in large-angle dispersive-band and small-angle quasi-flat-band regimes using time-dependent density-functional theory within the random-phase approximation. At the reference bilayer-graphene interlayer distance, weak coupling in the largest-angle structures preserves Dirac dispersions and the intrinsic $\pi$ plasmon. Electron doping activates a two-dimensional Dirac plasmon with energies obeying approximate geometric twist-angle scaling, while acoustic-like branches remain embedded in the single-particle continuum. The prohibitively large first-magic-angle supercell is represented by a tractable cell with its interlayer separation reduced to the angle-dependent magic distance, where four quasi-flat bands emerge around the Fermi level. Their partial occupation produces a dispersive low-energy plasmon-like excitation without a clear dielectric zero at resonance. A distinct interband plasmon is instead identified, supported by transitions involving the quasi-flat manifold and neighboring high-density-of-states regions. Band-energy rescaling places its characteristic energy in the mid-infrared range of interband collective excitations measured near the magic angle.
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Mesoscale and Nanoscale Physics

Highly uniform first-electron position in qubit arrays fabricated on dedicated QSOI(R) 300mm commercial platform

Johan Pelloux-Prayer, Elise Prin, Giselle A. Elbaz et al. (+11 more)

We report progress toward the development of a quantum silicon-on-insulator (QSOI(R)) technology compatible with 300mm CMOS fabrication and adapted from the 28nm Fully-Depleted SOI (28nm FD-SOI) platform for scalable quantum computing. We compare quantum devices fabricated with standard 28nm FD-SOI and QSOI(R) technologies, and show striking improvements of room temperature electrostatic properties of the individual device. Moreover, the QSOI(R) technology has significantly reduced the device variability and the dispersion of device properties at the wafer level. Transistor metrics are reproduced by TCAD simulations showing that the electrostatics of the devices behave as expected for QSOI(R) technology. Wafer-scale measurements at sub-2K show reproducible quantum dots down to the few-electron regime with 69% yield for successful charge detection of the first electron and a dispersion of the first electron position of $\mathrm{\pm 35 mV}$ over 377 quantum dots. These results establish QSOI(R) as a promising platform for CMOS-compatible quantum device co-integration.
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All authors Johan Pelloux-Prayer, Elise Prin, Giselle A. Elbaz, Pierre-Louis Julliard, Amaryllis Comiti, Clément Nguyen, Sylvain Martin, Patrick Torresani, Renan Lethiecq, Carlos Augusto Suarez Segovia, Franck Arnaud, Etienne Nowak, Tristan Meunier, Bruna Cardoso Paz

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

Vortex-mediated spin current injection into two-dimensional superconductor NbSe2

Meng Yang, Xiaolong Yin, Jingjing Liu et al. (+7 more)

Injection of pure spin current into superconductors remains a major challenge in superconducting spintronics. Previous studies have primarily focused on spin-polarized quasiparticles and spin-triplet supercurrents, while vortices, ubiquitous topological defects in type-2 superconductors, have been theoretically proposed as alternative carriers of spin angular momentum, yet direct experimental evidence is still lacking. Here, we report the vortex-mediated spin current injection in NbSe2/LiAl2Fe3O8(LAFO) bilayer, an Ising superconductor/ferrimagnetic insulator heterostructure. Under an out-of-plane temperature gradient and an in-plane magnetic field, the NbSe2/LAFO bilayer shows a pronounced thermoelectric peak near the upper critical magnetic field, which has the opposite sign to the conventional vortex Nernst signal observed in a single-layer NbSe2. The sign reversal suggests that the vortex flow induced by spin current injection is opposite to the flow driven by the temperature gradient, which is consistent with theoretical mechanisms including spin-vorticity transmutation and the inverse vortex spin Hall effect. By mapping the field temperature phase diagram, we reveal that the spin current injection occurs exclusively in the vortex liquid phase of NbSe2. Absence of the thermoelectric signal above the superconducting transition temperature further rules out the quasiparticle contribution. Our results establish vortices as efficient carriers of spin information in superconductors, opening a new route towards vortex-mediated superconducting spintronic devices.
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All authors Meng Yang, Xiaolong Yin, Jingjing Liu, Yifeng Chen, Qinwu Gao, Hongxing Zhu, Danni Huang, Lang Chen, Shuo-Ying Yang, Junxue Li

Mesoscale and Nanoscale Physics

Observation of Dyakonov-Perel-type magnon spin relaxation in uniaxial antiferromagnetic insulators

Qinwu Gao, Andi Cong, Bokai Liang et al. (+6 more)

Long-distance transport of magnon spin currents in antiferromagnetic (AFM) insulators has attracted tremendous attention recently, however, the AFM magnon spin relaxation mechanisms remain elusive. Here, we report that the Dyakonov-Perel-type magnon spin relaxation mechanism governs the spin current transport along the easy axis in two prototypical uniaxial AFM insulators, Cr2O3 and alpha-Fe2O3. First, an over 450% enhancement of the first-harmonic nonlocal signal induced by a magnetic field is observed prior to the spin-flop transition, which can be well-interpreted by our model incorporating Dyakonov-Perel-type magnon spin relaxation. Secondly, we find that the magnon spin diffusion length in both crystals increases with magnetic field and saturates at fields above 0.8 T, consistent with our model. Finally, the temperature dependence of the zero-field magnon spin diffusion length in both AFM insulators can be qualitatively explained through our model. These findings are valuable for the development of low-dissipation antiferromagnetic spintronic devices.
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All authors Qinwu Gao, Andi Cong, Bokai Liang, Meng Yang, Jingjing Liu, Lang Chen, Shiwei Wu, Ka Shen, Junxue Li

Mesoscale and Nanoscale Physics

Transport Relaxation Mechanisms in Bilayer Graphene: Effects of Pauli Blocking

Amit Varshney and SSZ Ashraf

We present a unified analytical treatment of carrier transport relaxation in bilayer graphene (BLG), including scattering by surface roughness, charged and neutral impurities, acoustic and optical phonons, and substrate polar phonons. The calculation is formulated within the low-energy two-band approximation using Fermi's golden rule and the semiclassical Boltzmann transport equation. Closed-form expressions for the transport relaxation rates are derived and verified by numerical evaluation of the corresponding scattering integrals. Particular attention is given to Pauli blocking, whose influence becomes important under degenerate carrier conditions owing to the approximately parabolic low-energy dispersion and nearly constant density of states of BLG. We find that Pauli blocking modifies the relaxation rates in a mechanism- and energy-dependent manner for both suspended and substrate-supported BLG. For the parameters considered, neutral-impurity scattering provides the dominant contribution over a substantial energy range, while acoustic-phonon and charged-impurity scattering remain important competing mechanisms. Optical-phonon scattering is suppressed below its emission threshold, whereas substrate polar phonons provide an additional relaxation channel in supported BLG. Comparison with monolayer graphene and a conventional two-dimensional electron gas highlights the role of band dispersion, density of states, screening, and chirality in determining the distinct scattering behavior of BLG. These results provide analytical insight into the relative importance of the principal momentum-relaxation mechanisms and the role of Pauli blocking in BLG transport.
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