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Physics Condensed Matter Mesoscale and Nanoscale Physics

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Thursday 27 August

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

Fast On-Chip Thermometry with TiN Kinetic Inductance Resonators in 22 nm Process Technology

Johann Drayne, George Ridgard, Lorenzo Peri et al. (+9 more)

Understanding the temporal and spatial dependence of temperature is critical in high-performance cryogenic devices. Typical thermometry techniques struggle to simultaneously combine high bandwidth, sensitivity and on-chip integration, limiting their ability to measure fast intra-device thermal fluctuations. Here, we demonstrate a time-resolved thermometry platform based on TiN kinetic inductance resonators integrated in a 22 nm FDSOI chip. By tracking temperature-dependent shifts in the resonant frequency, we achieve sub-millikelvin temperature sensitivity down to temperatures of 100 mK. Time-resolved on-chip pulsed heating experiments as a function of distance reveal an onset delay, consistent with a quasi-ballistic heat propagation velocity of 3.9 ${\pm}$ 0.1 mm ${\mu}$s${}^{-1}$. We also show that elevated temperatures increase net thermal conductance, shortening thermal relaxation times across all spatial separations. This behaviour manifests in two distinct regimes: a substrate-limited regime at 100 mK, where cooling rates vary with heater distance, and a Kapitza boundary-limited regime at 400 mK, where thermal relaxation becomes more spatially uniform. These measurements demonstrate kinetic inductance thermometry's ability to rapidly probe non-equilibrium temperature dynamics in cryogenic devices such as quantum processors.
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All authors Johann Drayne, George Ridgard, Lorenzo Peri, Frederic Schlattner, Fabio Olivieri, Mathieu de Kruijf, Isaac Harris, Grayson Noah, James Kirkman, Alberto Gomez-Saiz, M. Fernando Gonzalez-Zalba, Thomas Swift

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

A Superconducting Phase Transition Single-Electron Transistor

Gorka Aizpurua-Iraola, Thomas H. Swift, Felix-Ekkehard von Horstig et al. (+6 more)

Quantum computers require fast and accurate methods for qubit state detection. Phase-transition sensors exploit the abrupt change between two physical states of a material to achieve enhanced sensitivity and have enabled advanced detectors for quantum technologies, such as superconducting nanowire single-photon detectors. However, this sensing principle has not yet been applied to semiconductor spin qubits. Here, we demonstrate a superconducting phase-transition radio-frequency single-electron transistor (PTSET), a charge sensor for semiconductor spin qubits whose response is enhanced by a superconducting-to-normal phase transition. The transition is engineered by linking the sensor current to a low-critical-current, high-kinetic-inductance inductor integrated into the radio-frequency matching network. We demonstrate improvements in sensitivity of one and two orders of magnitude over conventional rfSETs in the large- and small-signal regimes, respectively. Our results establish phase-transition sensing as a route towards ultrasensitive, integrated charge sensors for semiconductor quantum computing and point to broader applications, including cryogenic photon detection for radio astronomy.
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All authors Gorka Aizpurua-Iraola, Thomas H. Swift, Felix-Ekkehard von Horstig, Domenic Prete, James Kirkman, Grayson M. Noah, Fabio Olivieri, Alberto Gomez-Saiz, M. Fernando Gonzalez-Zalba

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.quant-gas

Crossing the Rotational Sound Barrier in a Quantum Solvent

Baptiste Coquinot, Giacomo Bighin, Mikhail Lemeshko et al. (+1 more)

Molecules embedded in superfluids provide an experimentally controllable platform for investigating impurity physics. Here, we investigate a driven molecule rotating in a superfluid environment, including helium and Bose–Einstein condensates, at rotation frequencies similar to the bath dynamics. Within the experimentally relevant platform of an optical centrifuge, we show that the rotor remains localized up to a characteristic harmonic frequency that can realistically exceed the excitation energies of the bath, enabling access to ultrafast rotating impurities. In the co-rotating frame, the bath excitations experience a rotational Doppler shift, generating angular-momentum-resolved resonances absent in equilibrium angulon theory. We identify a dissipative rotational sound barrier at which the molecule resonantly emits bath excitations and undergoes strong angular momentum exchange with the surrounding medium. Overall, we establish the dynamical phase diagram of the driven rotor in a quantum solvent and introduce a generic platform for investigating fast driven rotating impurities in quantum many-body systems.
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All authors Baptiste Coquinot, Giacomo Bighin, Mikhail Lemeshko, Ragheed Alhyder

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

Intrinsic anomalous Hall response in the bilayer kagome ferromagnet Co$_3$Sn

Yuqi Qin, Soumya Sankar, Xingkai Cheng et al. (+4 more)

Transition-metal kagome magnets provide a rich platform for investigating the interplay between layer stacking, magnetic order, and band topology. Here, we report the molecular beam epitaxy and experimental investigation of high-quality thin films of the kagome metal Co$_3$Sn, which has not been synthesized in bulk form yet. Structural and chemical analyses confirm a hexagonal lattice structure ($P6_3/mmc$) composed of direct A-B stacked Co$_3$Sn kagome bilayers. Magnetometry reveals robust easy-plane ferromagnetism with a Curie temperature exceeding $300\,\text{K}$. Magneto-transport measurements demonstrate metallic behavior (carrier density $n\approx5.01\times10^{22}\,\text{cm}^{-3}$) alongside a temperature-independent anomalous Hall conductivity of $\sigma_{\rm AHE}\approx90\,\Omega^{-1}\cdot{\rm cm}^{-1}$, extending from $2\,\text{K}$ up to room temperature. Results from first-principles density functional theory calculations attribute this anomalous Hall response to intrinsic Berry curvature hotspots near the Fermi level in the spin-split band structure. Our results establish Co$_3$Sn as a room-temperature kagome ferromagnet and highlight the impact of the layer stacking sequence on the material properties of kagome metals from the CoSn family.
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All authors Yuqi Qin, Soumya Sankar, Xingkai Cheng, Yifan Jiang, Shiming Lei, Junwei Liu, Berthold Jäck

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

Atomic-scale composition of the ternary III-V semiconductor (Al,Ga)Sb visualized by cross-sectional scanning tunneling microscopy

Hitesh Kumar, Vladimir Kaganer, Zijin Lei et al. (+3 more)

Cross-sectional scanning tunneling microscopy at 5 K is used to investigate cation mixing in (Al,Ga)Sb layers grown by molecular beam epitaxy, via direct atom counting at the (1-10) and (110) cleavage planes. Electronic contrast between Al and Ga surface cations enables statistical analysis of the metal sublattice along the non-equivalent <110> directions within the zincblende (001) surface and along the [001] growth direction. The cation distribution is found to be random both along the growth direction and within the growth plane, with no evidence of long-range order or anisotropic growth kinetics; notably, the mean numbers of consecutive cations of the same type along the two in-plane directions are equal, possibly due to statistical averaging over randomly distributed subsurface cations. The results are compatible with either strain-mediated interactions during cation incorporation at the growth front or ideal, uncorrelated cation mixing. Overall, the examined (Al,Ga)Sb alloy shows an exceptionally high degree of atomic-level homogeneity.
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All authors Hitesh Kumar, Vladimir Kaganer, Zijin Lei, Rüdiger Schott, Werner Wegscheider, Stefan Fölsch

Mesoscale and Nanoscale Physics

Magnon-induced phononic Chern insulator

Rui-Chang Shen, Yihao Yang and Haoran Xue

High-frequency artificial phononic crystals offer a low-loss platform compatible with on-chip integration, yet realizing Chern phononic phases at GHz frequencies remains challenging. Here, we propose a magnon-induced phononic Chern insulator in a honeycomb phononic crystal hybridized with ferromagnetic islands at the hexagon centers. A circularly polarized Kittel mode couples to the surrounding phonons with a phase winding, which breaks time-reversal symmetry and opens a full Chern gap. In the large-detuning regime, this mechanism leads to an effective Haldane-type phononic model with magnon-induced complex hopping. By tuning the magnon-phonon interaction, the full hybrid system accesses Chern phases with tunable Chern numbers |C|=1 and |C|=2. The predicted gaps can exceed realistic phonon and magnon linewidths, enabling their observation in GHz acoustic devices. Our work establishes chiral magnon–phonon hybridization as a route to magnetically reconfigurable topological phononics.
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Mesoscale and Nanoscale Physics

Spin preservation in screw-symmetric molecules

Jonas Bloch, Fedor Baranov and Maxim Breitkreiz

In electronic transport through long molecules, spin is expected to be preserved only when the dwell time is much shorter than the characteristic spin-mixing timescale $\hbar/\Delta$, where $\Delta$ is the magnitude of a spin-dependent potential, such as spin-orbit coupling. We show that, in molecules featuring discrete screw symmetry, spin preservation can be enhanced far beyond this timescale owing to strong spin separation in quasi-momentum. This spin fidelity in long molecules is consistent with chirality-induced spin selectivity (CISS), suggesting spin-dependent transport in long, chiral molecules with amplified spin-splitting mechanisms. We provide analytical derivation of the enhanced spin preservation and test it on tight-binding models, which confirm that the effect gradually weakens when the screw symmetry is broken or changes from discrete to continuous. Furthermore, we perform transport simulations to show that a strong magnetoresistance trace of symmetry-protected spin fidelity emerges in a spin-valve setup with two magnetic leads, which we propose as an experimentally accessible signature.
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Mesoscale and Nanoscale Physics

Hall effect in viscous flows of two-dimensional electrons in samples with edges of arbitrary roughness

A.V. Gert and P.S. Alekseev

In ultra-clean conductors, fast inter-particle collisions can lead to the formation of a viscous electron fluid and realization of the hydrodynamic transport regime. Here we develop a theory of hydrodynamic magnetotransport of two-dimensional (2D) electrons in samples with low densities of defects and edges of arbitrary roughness. Within our model the roughness is described by a single parameter with the dimension of speed in the boundary conditions on sample edges. The electron-fluid flow profiles in long samples, as well as the corresponding longitudinal and Hall resistances, are calculated. The contribution to the Hall resistance associated with the relaxation processes exhibits a saturation in the limit of high magnetic field and a minimum as a function of the magnetic field for sufficiently rough edges. The minimum disappears as the edge roughness decreases or the sample width and bulk scattering by defects increase. These properties of the Hall resistance can serve as the signs to identify the hydrodynamic regime of electron transport in experiments and can be used to determine its parameters, in particular, the degree of edge roughness.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Isotropic Nanoscale Quantum Sensor at Room-Temperature

Daniel T. Möller, Baha Sakar, Ekrem T. Güldeste et al. (+9 more)

Color-center based quantum sensors provide nanoscale resolution under ambient conditions, yet their applicability remains limited. Because the quantization axes are locked to the host lattice, conventional color centers suffer severe signal loss in off-axis magnetic fields. To address this, we report an isotropic magnetometer enabled by the neutrally charged nitrogen-vacancy center (NV0) in diamond. Here, spin-to-charge dynamics yield an NV0-dark spin pair whose quantization axis dynamically aligns with the external field. Read out through NV charge-state-selective fluorescence, this system exhibits microsecond room-temperature coherence and nanotesla sensitivity for arbitrary field directions. We demonstrate alignment-free mapping of steep field gradients and single paramagnetic micro-targets, alongside isotropic readout from randomly oriented nanodiamonds. Resolving longstanding orientation constraints, this platform unlocks unrestricted nanoscale magnetometry across life sciences and quantum materials.
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All authors Daniel T. Möller, Baha Sakar, Ekrem T. Güldeste, Dhruv Wadhwani, Olt Gashi, Daniel A. García Vaca, Mokessh K. Ciwan, Simone P. Crozier, Christian Laube, Sirswa K. Shree Ram, Matteo Slaviero, Nabeel Aslam

Mesoscale and Nanoscale Physics

Exceptional point induced by hyperbolicity in an electrostatic spherical shell

Álvaro Buendía, Marcelo S. Barreiro and Nuno M.R. Peres

Exceptional points (EPs) are non–Hermitian degeneracies at which both eigenvalues and eigenvectors coalesce, usually engineered through balanced gain and loss or non–reciprocity. In this work, we show that the radial electrostatic problem of an anisotropic core–shell nanoparticle realizes an EP without any of the alluded conditons. Written in the logarithmic radial coordinate, the problem is a Cauchy–Euler equation that maps onto a damped oscillator and, equivalently, onto a two-site Hatano–Nelson Hamiltonian, so that the non-Hermiticity is emulated in space rather than in time and the sole control parameter is the degree of dielectric anisotropy, $\varepsilon_t/\varepsilon_r$. The shell is passive, reciprocal, and lossless, yet it hosts an EP and a non–Hermitian phase transition governed by hyperbolicity alone. Beyond a hyperbolicity threshold the electric field inside the shell turns from monotonic decay into spatial oscillation, producing hotspots at intermediate radii which can be exploited to tailor fluorescence, strong coupling, sensing, or cloaking. More broadly, this establishes hyperbolic media as a platform to simulate non–Hermitian physics without gain, loss, or non–reciprocity.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

The Effect of Geometry on Thermodynamic Response

Bojana Bokic, Sébastien R. Mouchet, Biljana Stankov et al. (+9 more)

At the nano- and microscale, various patterns influence and shape thermal and optical response, making them essential for the survival of various biological species. In addition, controlling thermal radiation is vital for a broad range of applications, such as thermal management, spectroscopy, optoelectronics, and energy conversion technologies. For this reason, there is strong pressure to elucidate the physics of thermal radiation at the nanoscale. In this article, we provide evidence that complex nanoscale geometries affect thermal management, leading to an unusual thermal response in heat-capacity measurements as a function of temperature. Beyond identifying the structural constraints associated with this unusual thermodynamic response, the current study introduces the possibility of shaping the apparent heat-capacity response through geometry without necessarily altering the system's chemistry.
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All authors Bojana Bokic, Sébastien R. Mouchet, Biljana Stankov, Sanja Ostojic, Nicolas Roy, Darko Vasiljevic, Yin Chang, Marija Radmilovic-Radjenovic, Branislav Radjenovic, Thierry Verbiest, Mohamed Hatifi, Branko Kolaric

Mesoscale and Nanoscale Physics

Switchable giant room-temperature nonlinear Hall effect in Bilayer Graphene

Margherita Melegari, Ignacio Gutiérrez-Lezama, Justin C. W. Song et al. (+1 more)

Utilizing quantum second-order nonlinear transport for practical junction-free devices require materials with large and tunable nonlinearites at room temperature – a current materials platform challenge. Here, we report the nonlinear Hall effect (NLHE) in double-ionic gated bilayer graphene devices that enable unusually strong inversion breaking. We observe NLHE that are readily switchable (on, off, and sign reversed) with second order nonlinear susceptibilities $\chi^{(2)}_{yxx}$ that reaches giant room-temperature values of $3\,10^{-3}\,\mu\mathrm{m}\,\mathrm{S/V}$, comparable to values commonly observed at low temperature in WTe$_2$ or in graphene-based moiré superlattices, and three-to-four orders of magnitude larger than values reported in material systems recently employed in search of a room-temperature NLHE. Our devices produce corresponding THz voltage responsivities $\simeq 4\,10^{4}\,\mathrm{V/W}$, comparable to commercially available Schottky diodes. These are orders of magnitude better than for previously reported room-temperature NLHE devices rendering double-ionic gated bilayer graphene a choice platform for junction-free nonlinear technology.
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All authors Margherita Melegari, Ignacio Gutiérrez-Lezama, Justin C. W. Song, Alberto F. Morpurgo

Mesoscale and Nanoscale Physics

Observation of dodecagonal replica bands in 30$^{\circ}$-twisted bilayer WSe$_2$

Christian Valentiner-Branth Fokdal, Chakradhar Sahoo, Thomas S. Nielsen et al. (+6 more)

Twisted bilayers of two-dimensional (2D) transition metal dichalcogenides are promising systems for achieving tunable quasicrystalline orders with emergent properties. The underpinning electronic structure and scattering processes in 2D quasicrystals with multiorbital dodecagonal replica bands have so far not been determined. Here, we utilize angle-resolved photoemission spectroscopy (ARPES) with micrometer spatial resolution to directly observe replica bands in 30$^{\circ}$-twisted bilayer WSe$_2$. The symmetry and intensity distribution of the observed replicas are explained by interlayer Umklapp scattering from bottom to top WSe$_2$ layers. Our spectral function measurements are consistent with the presence of a van Hove singularity adjacent to the $\mathrm{K}$-valleys, which underlines the possibility of inducing electronic reconstructions in bilayers with a large interlayer twist angle.
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All authors Christian Valentiner-Branth Fokdal, Chakradhar Sahoo, Thomas S. Nielsen, Alfred J. H. Jones, Zhihao Jiang, Kenji Watanabe, Takashi Taniguchi, Marcin Mucha-Kruczyński, Søren Ulstrup

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

Self-selective growth of GaAs1-xBix on GaAs zinc blende/wurtzite nanowire heterostructures

Rohit Yadav, Sebastian Lehmann, Vidar Flodgren et al. (+5 more)

Site-selective nanostructure growth and material incorporation at the atomic scale offer a promising pathway for engineering quantum materials and nanodevices. Here, GaAs nanowires (NWs) with an axial heterostructure of alternating zinc blende (Zb) and wurtzite (Wz) crystal phases are employed as templates for site-selective Ga and Bi overgrowth. Using X-ray photoemission electron microscopy (XPEEM) with nanoscale spatial resolution, we map elemental distribution and local chemical bonding to reveal the incorporation behavior of Bi atoms in {110} Zb and {11-20} Wz facets. Bi incorporation proceeds through an anion-exchange process, where Bi atoms replace As, forming local Ga-Bi bonds and producing a thin GaAs1-xBix shell. We observe crystal-phase-dependent Bi incorporation, with higher Bi concentration in the Zb segments than in the neighboring Wz segments within the same NW. Furthermore, the Zb segment with higher Bi content exhibits reduced susceptibility to oxidation compared with the Wz segment, resulting in increased Ga-oxide in the Wz surfaces. This study highlights GaAs NW Zb/Wz heterostructures as a template for controlled growth of GaBi and GaAs1-xBix nanostructures with tailored functionalities for quantum applications
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All authors Rohit Yadav, Sebastian Lehmann, Vidar Flodgren, Evangelos Golias, Alexei Zakharov, Kimberly A. Dick, Anders Mikkelsen, Rainer Timm

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

Stabilization of Interband Phase Solitons in Two-Band Noncentrosymmetric Superconducting Rings

Yuriy Yerin, Boris Malomed, Stefan-Ludwig Drechsler et al. (+1 more)

Two-band superconductors maintain a relative interband phase which can carry winding soliton excitations in a superconducting ring, supported by independent winding numbers in the two bands. In rings of superconductors obeying the inversion symmetry the interband phase solitons are metastable states, separated from the uniform ground state by the energy of screening currents. In this work we find that, breaking the inversion symmetry strongly enough, one can make the soliton a true ground state. In that case, a magneto-electric coupling, absent in centrosymmetric materials, contributes critically above a certain threshold, a relevant, free-energy term, odd with respect to the winding number, which biases the energy balance in favor of a particular winding sign. Once the bias outweighs the energy cost that originally made the soliton metastable, a phase soliton with a finite winding number becomes the ground state, with chirality set by the applied field. In current–flux measurements performed in equilibrium states, the effect is demonstrated by field-odd soliton branches, that replace the metastable ones existing in mesoscopic rings, built by two-component superconductors, realizing a magneto-electric analog of the Little–Parks fluxoid-branch physics in the interband relative-phase sector.
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All authors Yuriy Yerin, Boris Malomed, Stefan-Ludwig Drechsler, F. Sebastian Bergeret

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Circular Dichroism Spectroscopy of Single Objects: Problems, Artifacts, and Corrections

Stefan Goppelt, Lisa M. Günther and Jürgen Köhler

Molecular aggregates are at the basis of a broad range of functional systems, including photosynthetic light-harvesting complexes, organic photovoltaics, optoelectronic devices, and molecular sensors. Their structural organization, however, is often difficult to resolve because intrinsic heterogeneity. In this context, circular dichroism (CD) spectroscopy is particularly powerful because of its sensitivity to the three-dimensional organization of molecular building blocks. Extending CD measurements from ensembles to individual objects therefore offers a unique approach to probing supramolecular architecture in heterogeneous, non-crystallizable systems. However, without spatial and orientational averaging, measurements are highly sensitive to imbalances between left- and right-circularly polarized excitation, deviations from ideal circular polarization, and polarization imperfections coupled to linear dichroism, which creates spurious CD signals. Here, we present a systematic theoretical and experimental framework for understanding, quantifying, and controlling artifacts in single-object CD spectroscopy. We develop a mathematical description of the principal artifact mechanisms, identify their origins in practical implementations, including Pockels-cell-based polarization modulation, and establish strategies for their minimization. We validate this framework using an experimental implementation capable of acquiring CD spectra from individual molecular aggregates over 690-785 nm. Our results demonstrate that reliable single-object CD measurements are achievable when polarization-related artifacts are rigorously characterized and controlled. This Tutorial Review provides a unified theoretical framework and practical guide for single-object CD spectroscopy and its application to single-particle chirality, supramolecular photonics, and biophotonics.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.other

Non-Hermitian Skin Effect from Radiative Coupling in a Reciprocal Chiral Medium

Kin Hung Fung, Changhao Meng, Yixin Xiao et al. (+1 more)

We show that long-range radiative coupling in a passive chiral medium produces an unusual non-Hermitian skin effect in a chain of dipoles. The skin-mode profile consists of a long-range light-line branch-cut contribution, a non-exponential factor $1/[n(\ln n)^2]$ with $n$ being the site number. Finite-chain calculations show that part of the open-boundary spectrum contracts toward the Bloch spectrum, whereas the number of boundary-localized modes remains extensive. The retarded radiative coupling makes both the real-space profile and the finite-size spectral signatures of boundary localization depart from standard finite-range and pure exponential coupling theory.
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All authors Kin Hung Fung, Changhao Meng, Yixin Xiao, C. T. Chan

Mesoscale and Nanoscale Physics

Topologically protected perfect crossed Andreev reflection in flux-engineered quantum wire junctions

Abhiram Soori

Generating non-locally entangled electron pairs via Cooper-pair splitting is vital for solid-state quantum information processing. However, isolating the underlying crossed Andreev reflection (CAR) is challenging due to competing transport processes like electron tunneling (ET) and local Andreev reflection (AR). Here, we propose a flux-tunable four-terminal normal metal-superconductor junction that achieves deterministic, 100% efficient CAR. We demonstrate that at exactly half a magnetic flux quantum ($\phi=\pi$), exact destructive Aharonov-Bohm and Peierls interferences structurally forbid ET and AR respectively. By tuning the central junction hopping, electron reflection is also suppressed to zero. Using the Cauchy argument principle, we prove that this suppression manifests as a quantized topological winding number, guaranteeing a topologically protected unit CAR probability. We establish that this regime is characterized by a strictly positive cross-correlation shot noise, providing an unambiguous experimental signature of Cooper-pair splitting. Furthermore, this perfect CAR is nearly broadband within the superconducting gap and remarkably robust against structural disorder, offering a highly resilient architecture for deterministic nonlocal entanglement generation.
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Mesoscale and Nanoscale Physics

Optical spectroscopy of composite fermion edge states in the fractional quantum Hall effect

Maria Sebastian, Ashutosh Singh and Alexey Belyanin

We show that edge states in fractional quantum Hall effect samples can be selectively probed and excited with sub-terahertz optical spectroscopy. Using the composite fermion (CF) mean-field framework, which maps the strongly correlated fractional quantum Hall problem onto an effective integer quantum Hall problem, we calculate the absorbance spectrum for the Jain sequence of filling fractions including both bulk and edge states. The CF edge-state absorption peaks appear in the millimeter-wave to sub-terahertz range, e.g., 60-500 GHz at B = 10 T in GaAs, i.e. they are blueshifted with respect to the bulk CF cyclotron frequency but are well below the integer quantum Hall cyclotron frequency scale at the same magnetic fields. The number of resolved peaks in each series of the absorption spectrum counts the filled Lambda-levels and fingerprints the fraction. Inversion symmetry breaking near the edge activates optical transitions forbidden in the bulk and enables second-order nonlinear processes in electric-dipole approximation. The absolute frequency scale of the spectrum is set by the CF effective mass, which is generated entirely by electron-electron interactions, so the absorption spectrum provides a direct optical probe of this interaction-induced mass.
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Wednesday 26 August

Mesoscale and Nanoscale Physics

Statistical Disorder in MBE-Grown AlGaAs/GaAs Superlattices for Quantum Bragg Mirrors using Synchrotron X-ray Diffraction

Mateus T. Souza, Germano M. Penello, Guilherme A. Calligaris et al. (+1 more)

AlGaAs/GaAs superlattices grown by Molecular Beam Epitaxy (MBE) are foundational for advanced optoelectronic devices, including Quantum Bragg Mirror (QBM) infrared detectors. The performance of these devices critically depends on achieving near-perfect periodicity and abrupt interfaces, however, intrinsic statistical fluctuations during MBE growth introduce nanoscale structural disorder that can degrade device efficiency. In this study, we present a comprehensive methodology for quantifying this disorder in a 203-layer QBM device. High-resolution structural characterization was performed using high-energy (25 keV) synchrotron X-ray diffraction. By coupling a recursive dynamical diffraction formalism with an ensemble simulated annealing refinement, we extracted statistically robust, layer-by-layer thickness profiles. Our analysis reveals highly systematic, material-specific deviations from the nominal design: all AlGaAs barrier layers were consistently thinner than nominal by 0.3-0.6 nm. Furthermore, the sequential thickness profile successfully identified a significant 35 nm deficit in the final macroscopic top contact layer and a 40 nm deficit in the first GaAs layer. Achieving a statistical precision of about 0.2 to 0.6 nm (approximately 1-2 atomic monolayers), this non-destructive diagnostic approach provides directly actionable feedback for MBE flux calibration protocols.
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All authors Mateus T. Souza, Germano M. Penello, Guilherme A. Calligaris, Sergio L. Morelhao

Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Optically Induced Thermal Runaway in Phase-Change VO2 Nanostructures

Jiří Kabát, Peter Kepič, Andrea Konečná et al. (+1 more)

While thermal runaway occurs across diverse physical disciplines, its presence in subwavelength phase-change photonics remains unexplored because existing thermo-optical models break down near sharp optical transitions. Here, we introduce an iterative multiphysics framework coupling full-wave electrodynamics with heat transfer, and we discover optically induced thermal runaway in vanadium dioxide (VO2) nanostructures. The runaway switching threshold depends on the illumination wavelength, ambient temperature, and underlying substrate, with external thermal biasing significantly reducing the optical intensity required for runaway ignition. Our results push forward the modeling of sharp photothermal transitions and provide essential design principles for active metasurfaces, neuromorphic photonic devices, and nanoscale thermal management.
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All authors Jiří Kabát, Peter Kepič, Andrea Konečná, Filip Ligmajer

Mesoscale and Nanoscale Physics

Magnetic Communication with an Acoustically Actuated Magnetoelectric Resonator and a Quantum Diamond Magnetometer

Christopher T.-K. Lew, Andrew D. M. Charles, Brant C. Gibson et al. (+2 more)

Wireless communication via propagating magnetic fields is a communication modality that has recently garnered significant interest for short-to-medium range communication in conductive mediums, such as underwater and underground, where existing approaches utilizing electric fields are highly inefficient. Typical implementations of magnetic communication make use of loop antennas as both the transmitter and receiver, with the sensitivity and frequency response scaling with and inversely with the loop cross-sectional area, respectively. Here, we explore an alternative hybrid magnetic communication system consisting of an highly radiation efficient and compact acoustically actuated magnetoelectric resonator as the transmitter, and a highly sensitive micrometer scale quantum magnetometer based on nitrogen-vacancy centers in diamond as the receiver, with their core properties unconstrained by size. We demonstrate amplitude and phase-encoded transmission and reception of AC magnetic fields at $f_{\mathrm{AC}}$ = 20 kHz, achieving a sensitivity of 50 pT/$\sqrt{\mathrm{Hz}}$ and 1.2 mrad/$\sqrt{\mathrm{Hz}}$, respectively. This work establishes the use of hybrid magnetoelectric resonator and quantum diamond magnetometer communication system as a viable alternative to existing loop-based approaches.
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All authors Christopher T.-K. Lew, Andrew D. M. Charles, Brant C. Gibson, Jean-Philippe Tetienne, David A. Broadway

Mesoscale and Nanoscale Physics

Majorana signatures in an asymmetrically coupled quantum dot–topological superconducting nanowire junction

Levente Máthé, Ioan Grosu, Liviu P. Zârbo et al. (+1 more)

We present a theoretical study of the quantum transport through a nanoscale system in which a central quantum dot (QD) is coupled asymmetrically to normal leads and to two Majorana bound states (MBSs) localized at the ends of a topological superconducting nanowire threaded by a tunable magnetic flux. The effects of the leads–QD coupling asymmetry parameter $\alpha$ and the bias voltage asymmetry parameter $q$ on the system's linear conductance are considered for the case of unhybridized and hybridized MBSs. In the zero-temperature limit, for unhybridized MBSs the system's linear conductance is finite only when the magnetic flux phase $\phi = (2n+1)\pi$ ($n\in\mathbb{Z}$) and it scales as $\mathcal{G}=2q\alpha e^2/[h(\alpha+1)]$, while for hybridized MBSs it presents a complicated dependence on the system's parameters. At finite temperature, for unhybridized MBSs, the system's linear conductance oscillates as a function of the magnetic flux phase $\phi$ with a period of $2\pi$, and the position of the linear conductance maxima can be shifted from $\phi=2n\pi$ to $\phi=(2n+1)\pi$ by simply varying the value of the bias voltage asymmetry parameter $q$. For hybridized MBSs, the conductance exhibits a similar behavior when the energy level of the central QD, $\varepsilon_d$, is tuned at the leads' Fermi level ($\varepsilon_d=\varepsilon_F$), although when $\varepsilon_d\neq\varepsilon_F$ the oscillation period changes to $4\pi$, and the position of the linear conductance maxima depends on the actual value of $\varepsilon_d$ and other parameters in the system. Our results highlight the experimental importance of the leads-QD and bias voltage asymmetry parameters, which are often present in realistic experimental setups, and can strongly affect the identification and observation of MBSs transport signatures.
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All authors Levente Máthé, Ioan Grosu, Liviu P. Zârbo, Ionel Ţifrea

Mesoscale and Nanoscale Physics

How quantum is quantum geometry?

Ady Stern and Felix von Oppen

The quantum geometric tensor - the Berry curvature together with the quantum metric - now underlies a long list of observables, from the anomalous Hall effect to the superfluid weight of a flat band. We ask which of these observables actually require quantum mechanics. To answer this question, we study a purely classical system: a point particle carrying a classical magnetic moment $\boldsymbol{\ell}$ that precesses in a momentum-dependent magnetic field $\mathbf{B}(\mathbf{p})$. Within Hamiltonian classical mechanics, the component of $\boldsymbol{\ell}$ along the field reproduces the Berry-curvature phenomena, while its precessing transverse component reproduces the quantum-metric phenomena. The particle acquires a position spread whose second moment is the metric, an orbital magnetic moment, and - most strikingly - an inertial mass generated by a position-dependent force, and with it a nonzero Drude weight in a system that is nominally dispersionless.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Supercurrent detection and manipulation of topological phase transitions in Shiba-Majorana hybrid systems

Debika Debnath, Ioannis Ioannidis, Paramita Dutta et al. (+2 more)

The non-Abelian statistics of Majorana zero modes has inspired numerous proposals for their detection and manipulation in topological superconductors. Implementations based on magnetic adatoms deposited on superconductors draw particular attention due to their capabilities for precise atomic manipulation and the control over disorder. Here, we propose a scheme for detecting changes in the ground state parity of a topologically non-trivial adatom system by passing supercurrent through their low-energy modes. We unravel characteristic discontinuities in the critical current driven by zero-energy level crossings. We apply these findings to a setup where the Majorana coupling is mediated by a single control magnetic adatom hosting a Yu-Shiba-Rusinov state, and test the robustness of our results against finite temperatures and different tunneling regimes. Our findings introduce a non-invasive approach for reading out and controlling the ground state parity of Majorana states in Shiba-Majorana hybrid systems.
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All authors Debika Debnath, Ioannis Ioannidis, Paramita Dutta, Mircea Trif, Thore Posske

Mesoscale and Nanoscale Physics

Hidden Frustration in Collinear Altermagnets: Pairing Vortices and Equilibrium Spin Current Loops

Vincent P. Flynn and Benedetta Flebus

We show that a magnet can remain perfectly collinear while its quantum vacuum circulates. In a centrosymmetric altermagnet, a symmetry-allowed locally staggered Dzyaloshinskii-Moriya coupling imprints a gauge-irremovable vortex-antivortex pair into the anomalous magnon pair correlations at high-symmetry points in momentum space. In real space, these hidden vortices produce an antiferrochiral array of equilibrium spin currents circulating oppositely around neighboring plaquettes. Gauge-irremovable frustration therefore survives despite classical collinearity: it exists entirely in the quantum correlations. Our results show that complex pairing, gauge-invariant fluxes, and equilibrium loop currents - structures encountered across electronic flux phases, spin liquids, and frustrated quantum magnets - can be encoded in the squeezed vacuum of a collinear magnet.
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Mesoscale and Nanoscale Physics

Quantized Transport through a Supermoiré Chern Mosaic

Li-Qiao Xia, Aviram Uri, Zachary W. Gomez et al. (+8 more)

Magic-angle helical trilayer graphene—three graphene layers sequentially twisted in the same direction by $\sim1.8^\circ$—relaxes into a mosaic of domains that, at zero field, carry opposite valley-resolved Chern numbers, with boundaries hosting a network of gapless conducting modes. Charge transport through this network depends sensitively on how the modes connect and scatter, making well-quantized transport unlikely. Contrary to this expectation, we observe a field-induced Chern gap with Chern number $C=-6$ emanating from charge neutrality; in this gap, the Hall resistance is quantized to within $2\%$ of the expected value, $-h/6e^2$, at 4.6 K. We explain this behavior using both Hofstadter and orbital Zeeman calculations, which show that a moderate magnetic field drives a valley-selective topological transition. Above the transition, the total Chern number of the occupied states in each spin-valley flavor becomes identical across neighboring domains, and the domain-wall modes can become gapped. Though the central valence-band Chern numbers still differ between the two domain types, the observed quantized transport attests to a global gap.
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All authors Li-Qiao Xia, Aviram Uri, Zachary W. Gomez, Molly P. Andersen, Julian May-Mann, Kenji Watanabe, Takashi Taniguchi, Trithep Devakul, Yves H. Kwan, Pablo Jarillo-Herrero, Aaron Sharpe

Mesoscale and Nanoscale Physics

Multicomponent Magnetic Domain Walls in Rhombohedral Graphene

Mainak Das, Nemin Wei and Chunli Huang

Spatial textures of magnetic order, such as domain walls and skyrmions, are fundamental objects in magnetism. In rhombohedral multilayer graphene, magnetic order involves spin and valley degrees of freedom, opening the possibility of qualitatively new spatial textures. Here, we explore this possibility through a microscopic study of a one-dimensional domain wall in the valley-imbalanced quarter-metal phase of rhombohedral graphene. We uncover two different classes of domain walls. One resembles a conventional magnetic domain wall, locally rotating between the two bulk states, whereas the other is intrinsically multicomponent and explores states that are not occupied in either bulk domain. Which texture is realized is controlled by the competition between intervalley Hund's coupling and spin-orbit coupling, and we identify experimental signatures to distinguish them. For a superconducting junction formed across the wall, we further show that the superconducting phase difference couples directly to the intervalley-coherent phase of the texture, allowing precession of this internal phase to generate a voltage. This mechanism provides an explanation for the resistive domain walls recently observed in chiral superconductors [arXiv2605.13303] and predicts accompanying magnetization oscillations that could be detected using GHz-frequency AC probes. Our theory establish a microscopic connection between multicomponent magnetic textures, their internal dynamics, and superconducting transport.
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Mesoscale and Nanoscale Physics

Viscochiral Transport: Chiral Selection of Hydrodynamic Vortices by Berry Curvature

Archisman Panigrahi and Khachatur Nazaryan

We predict a new viscochiral regime of electronic transport in which spatially varying Hall viscosity selects vortical flow patterns. Although uniform Hall viscosity cannot alter incompressible bulk flow, its spatial gradient redistributes vorticity, amplifying vortices in one chamber while suppressing the vortex in the other. We further determine the underlying mechanism to be generic to recirculating flows and insensitive to the details of the device geometry. The paper maps the resulting phase diagram and shows that the regime is experimentally accessible in valley-polarized bilayer graphene.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.quant-gas

Exact quantum geometry from sublattice symmetry: Closed-form solution of the quarter-flux Harper-Hofstadter model

Isaac Tesfaye and André Eckardt

Quantum geometry has emerged as a guiding principle across atomic and condensed-matter physics, shaping the topological responses of Bloch bands and the stability of the correlated phases they host. Sublattice symmetry, though common among bipartite lattice models, has not yet been exploited to obtain closed-form quantum geometry in multiband systems. For this purpose, we derive a general expression for the QGT of sublattice-symmetric systems in terms of contributions from the individual sublattice sectors, and show that this symmetry renders the Bloch Hamiltonian of a paradigmatic four-band model, the quarter-flux Harper-Hofstadter model, anti-block-diagonal, analytically yielding the spectrum, eigenstates, and full quantum geometric tensor (QGT), including the Berry curvature and quantum metric, for all four bands. The model, describing charged particles on a two-dimensional square lattice subjected to a uniform magnetic field, has recently been realized experimentally with ultracold atoms, photons, and superconducting circuits. Finally, we evaluate fractional-Chern-insulator stability criteria analytically and quantify the lowest band of the quarter-flux Harper-Hofstadter model to be a nearly ideal Chern band. Our approach opens a route for studying also the quantum geometry of other sublattice-symmetric multiband systems.
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