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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Thursday 17 September

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

Fractional quantum spin Hall crystals from hidden density-wave order

Matthew Shammami and Sudip Chakravarty

Broken symmetry and topology are distinct descriptions for how matter organizes itself. Here, we propose one gives rise to the other. A mixed singlet-triplet $d$-density wave, which is a particle-hole condensate, has a hidden bond order that breaks translation symmetry and generates a time-reversed pair of Chern bands, without producing conventional charge or spin density order. At fractional fillings with odd denominators, finite size exact diagonalization studies show how repulsive interactions can transform this symmetry-broken topological state into a fractional quantum spin Hall liquid. Its opposite chiralities produce a crossed response in which charge flux transports spin, and spin flux transports charge. The same setting also reveals what borders such a phase. Strong interspin coupling and increasingly dilute fillings favor charge order and make residual band dispersion decisive, while at half filling, the spin-decoupled limit gives a homogeneous correlated liquid with signatures consistent with the physics of a composite Fermi liquid.
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Mesoscale and Nanoscale Physics

Quantum transport across normal-superlattice-normal graphene junctions: Fabry-Pérot interference, Hofstadter butterfly, and supersnake states

Che-Pin Hsu, Alina Mreńca-Kolasińska, Aitor Garcia-Ruiz et al. (+4 more)

Electrostatic modulation of graphene provides a tunable route to engineering miniband structures. We perform quantum transport simulations on a gate-defined graphene superlattice junction, formed by confining a two-dimensional superlattice graphene (SGr) region between two normal graphene (NGr) regions. In the low-field regime at low carrier densities, robust Fabry-Pérot interference fringes emerge even in the unipolar regime due to Fermi-velocity renormalization in the SGr region. At stronger magnetic fields but only up to 3 T, the conductance map clearly reveals the Hofstadter butterfly spectrum. At intermediate fields, our finite-width transport simulations reveal a new type of snake state, the supersnake state, composed of alternating anomalous cyclotron arcs on the SGr side and conventional semicircular arcs on the NGr side, forming a weaving trajectory along the junction. The resulting conductance oscillations agree well with geometrical conditions derived from semiclassical cyclotron orbits. Our results demonstrate that gate-defined NGr-SGr-NGr junctions provide a versatile platform hosting multiple transport regimes within a single device architecture and can be generalized to other types of superlattices not restricted to graphene.
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All authors Che-Pin Hsu, Alina Mreńca-Kolasińska, Aitor Garcia-Ruiz, Szu-Chao Chen, Denis Kochan, Klaus Richter, Ming-Hao Liu

Wednesday 16 September

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

Engineering and Probing a One-Dimensional Dipolar Spin Ensemble in Diamond

Lingjie Chen, Shreyas Parthasarathy, Simon A. Meynell et al. (+11 more)

Dimensionality plays a central role in determining the collective behavior of interacting quantum systems. Engineering strongly interacting ensembles of solid-state spin defects in reduced dimensions remains a significant challenge at the interface between the applied and fundamental sciences. Here, we create and characterize a positionally disordered, quasi-one-dimensional spin chain in diamond, consisting of optically dark substitutional nitrogen defects (P1 centers) and optically addressable probe nitrogen-vacancy (NV) centers. Our approach exploits the preferential incorporation of nitrogen along step bunches formed during chemical vapor deposition to achieve both lateral and vertical confinement. Combining spatially resolved materials characterization with nanoscale quantum sensing, we establish the one-dimensional character of the optically dark, unpolarized P1 spin ensemble. We then use correlation spectroscopy to probe local spin autocorrelations and investigate infinite-temperature dipolar spin transport. Our results establish a materials-based route for engineering low-dimensional quantum spin systems.
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All authors Lingjie Chen, Shreyas Parthasarathy, Simon A. Meynell, Lillian B. Hughes Wyatt, Eveline Postelnicu, Haopu Yang, Zilin Wang, Weijie Wu, Winston V. Peloso, Casey K. Kim, Chris R. Laumann, Kunal Mukherjee, Norman Y. Yao, Ania C. Bleszynski Jayich

Mesoscale and Nanoscale Physics

Multiferroic Quantum Dot in an Artificial van der Waals Heterostructure

Antti Karjasilta, Mohammad Amini, Liwei Jing et al. (+5 more)

Quantum dots (QDs) provide a versatile platform for engineering quantum-confined electronic states with functionalities relevant for optoelectronics, spintronics, and quantum technologies. While substantial progress has been achieved in coupling confined states to spin, valley, topological, or ferroelectric degrees of freedom, the realization of a multiferroic QD in which quantum confinement simultaneously intertwines with magnetism and ferroelectricity remains elusive. Here, we engineer a multiferroic QD in an artificial van der Waals heterostructure grown by molecular beam epitaxy under ultra-high-vacuum conditions. The heterostructure consists of ferroelectric SnTe nanoislands deposited on the layered magnet CrBr$_2$ supported on highly oriented pyrolytic graphite. Combining scanning tunneling microscopy and spectroscopy with ab initio calculations and low-energy tight-binding models, we demonstrate the emergence of spin-polarized discretized electronic states confined within the SnTe islands. Remarkably, the spectroscopic response of the QD strongly depends on the ferroelectric domain configuration of the SnTe nanoislands, demonstrating an interplay between quantum confinement, magnetic exchange, and ferroelectric order at the atomic scale. Our results establish engineered van der Waals heterostructures as a platform for multiferroic quantum confinement and open new routes toward electrically tunable quantum spintronic devices.
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All authors Antti Karjasilta, Mohammad Amini, Liwei Jing, Robert Drost, Jose Lado, Shawulienu Kezilebieke, Peter Liljeroth, Adolfo O. Fumega

Mesoscale and Nanoscale Physics

Displacement field stabilizes even-denominator and partonic fractional quantum Hall states in the $\mathcal{N}{=}2$ Landau levels of Bernal-stacked bilayer graphene

Rakesh K. Dora, Udit Khanna and Ajit C. Balram

Bernal-stacked bilayer graphene (BLG), in which a graphene layer is stacked atop another and laterally shifted by a lattice constant, offers remarkable tunability in its single-particle states under applied magnetic and displacement fields. Owing to this tunability, recent transport and scanning tunneling microscopy experiments in the presence of a perpendicular magnetic field and finite interlayer displacement fields have observed even-denominator fractional quantum Hall (FQH) states at half-filling in the first excited, namely, $\mathcal{N}{=}2$, Landau level (LL) of BLG. In contrast, at zero displacement field, a gapless composite fermion Fermi liquid (CFFL) is realized at half-filling of the $\mathcal{N}{=}2$ LL. Motivated by these experiments, we compute the phase diagram as a function of the displacement field in the half-filled $\mathcal{N}{=}2$ LL of BLG by studying the competition between the CFFL and the Moore-Read state—a candidate even-denominator FQH state—by calculating their thermodynamic energies in this setting. We find that the modified effective Coulomb interaction, induced by changes in the single-particle states with increasing displacement field, softens the inter-electronic repulsion at short distances, thereby stabilizing the Moore-Read state over the CFFL in the $\mathcal{N}{=}2$ LL of BLG. We also study the nature of FQH states at fillings $2/5$, $3/7$, $4/9$, and $6/13$ in the $\mathcal{N}{=}2$ LL of BLG. Our results suggest that, with increasing displacement field, the Jain composite-fermion states at $3/7$, $4/9$, and $6/13$ transition into states with distinct topological order that are well-captured by parton wave functions.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Valley-Helical Superconductivity Driven by Repulsion and Quantum Geometry: Applications to Time-Reversal Symmetric Rhombohedral Graphene

Julian May-Mann, Trithep Devakul and Gal Shavit

Motivated by the recent experimental signatures of chiral superconductivity in valley-polarized graphene- and transition-metal-dichalcogenide-based systems, we investigate the possibility of chiral pairing in the absence of valley polarization or time-reversal symmetry breaking. For systems with opposite Berry curvature in the two valleys, we find that overscreened repulsion (i.e., the Kohn-Luttinger mechanism) can indeed induce pairing of opposite chiralities in each valley, producing a valley-helical state. Moreover, the valley-helical state can outcompete more conventional intervalley paired states at intermediate coupling, despite only the latter having a weak-coupling instability. Using a realistic model of time-reversal-symmetric rhombohedral graphene, we find extended ranges of fillings and displacement fields with dominant valley-helical superconductivity. This occurs for all layer numbers considered here, $4\leq N\leq 8$. We show that the valley-helical states can display topologically projected edge modes, unusual magnetic responses, and intertwined Kekulé-like bond-order; features that can be used to distinguish them from more conventional superconductors.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Liouvillian exceptional points in the emergence of quantum synchronization

Carlos Ortega Taberner, Rosalind E. Cotton and Paul R. Eastham

The coupling between self-sustained oscillators, such as lasers or polariton condensates, leads to different steady-states in different regimes. Weak coupling allows the oscillators to behave independently, with different frequencies and uncorrelated phases, while stronger coupling can produce synchronization. In classical or semiclassical theories transitions between these steady-states correspond to changes in the topology of the phase space attractors and occur at generalized exceptional points. We investigate the corresponding changes in a quantum theory of coupled lasers or condensates. We show how the different steady-state regimes of the classical limit give rise to distinct forms for the spectra and eigenmatrices of the slow modes of the Liouvillian. By following the spectral flow between the different regimes we show that the synchronization transition is controlled by cascades of exceptional points in these slow modes, protected by a generalized PT symmetry. Our results show how singularities of the quantum Liouvillian control the different dynamical regimes, and give rise to experimental signatures of the synchronization transition which remain well-defined in the few-particle regime dominated by quantum effects.
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Mesoscale and Nanoscale Physics

Layer-Dependent Vibrational and Optical Properties of $\mathrm{Mo}{0.58}\mathrm{W}{0.42}\mathrm{Se}_2$ Alloy

Szymon Socha, Tomasz Wozniak, Elena Blundo et al. (+7 more)

Semiconducting Mo$_x$W$_{1-x}$Se$_2$ alloys provide a versatile platform for tailoring the optical properties of two-dimensional materials through both composition and layer thickness. Here, we systematically investigate mechanically exfoliated Mo$_{0.58}$W$_{0.42}$Se$_2$ flakes ranging from monolayer (1L) to nine layers by combining Raman scattering (RS), photoluminescence (PL), reflectance contrast (RC) spectroscopy, and first-principles phonon calculations. Thirteen RS peaks are identified, including the low-frequency interlayer shear mode, whose thickness dependence is well described by a linear-chain model, yielding an interlayer force constant of $K_s=(2.996\pm0.015)\times10^{19}$ N m$^{-3}$. PL measurements reveal a crossover from the direct-bandgap 1L to indirect-bandgap multilayers. The thickness evolution of the indirect optical transition is quantitatively reproduced using a quantum-confinement model, yielding an out-of-plane reduced effective mass of $\mu_\perp=0.75 m_0$. RC spectroscopy reveals four excitonic resonances. While the A and B excitons associated with the $K^\pm$ valleys remain nearly independent of layer thickness, the higher-energy C and D resonances originating from the band-nesting regions exhibit pronounced redshifts, reflecting substantial thickness-induced modifications of the electronic band structure. These results establish comprehensive spectroscopic fingerprints of flake thickness, interlayer coupling, and electronic structure in Mo$_x$W$_{1-x}$Se$_2$ alloys and provide a reliable, non-destructive framework for their optical characterization.
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All authors Szymon Socha, Tomasz Wozniak, Elena Blundo, Malgorzata Brzoska, Grzegorz Krasucki, Piotr Wrobel, Antonio Polimeni, Adam Babinski, Maciej R. Molas, Katarzyna Olkowska-Pucko

Mesoscale and Nanoscale Physics

Chiral classical and quantum acoustics with hole-spin qubits

Zhanning Wang, Yongtao Li, Nelson E. Rivas et al. (+5 more)

Gate-defined hole spins combine strong spin-orbit coupling with exceptional strain sensitivity, making surface acoustic waves a natural route to remote, phase-coherent control. We show that counterpropagating surface acoustic waves can differ in coupling strength and in whether they predominantly drive spin rotations or modulate the qubit frequency. We call the transverse transition-strength imbalance spin-acoustic chirality and show that it is tunable through gate-controlled reshaping of the dot confinement. Combining a multiband Luttinger-Kohn Hamiltonian with the Bir-Pikus description of strain and 3D piezoelectric finite-element simulations, we obtain the acoustic g-matrix modulation governing coherent SAW driving. Quantizing the same strain-mediated interaction yields the corresponding single-phonon coupling, and a generalized anisotropic Rabi interaction with co-rotating, counter-rotating, and longitudinal components, linking classical chirality to quantum anisotropy and directional one-phonon emission. Magnetic field orientation and dot shape tune the acoustic chirality and operator selectivity. We illustrate this framework through bichromatic coherence protection and phonon-mediated heralded Bell-state initialization.
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All authors Zhanning Wang, Yongtao Li, Nelson E. Rivas, Gonzalo García, Irene Castro, Rubén Seoane Souto, Daniel Ramos, José C. Abadillo-Uriel

Mesoscale and Nanoscale Physics

Fröhlich Bipolarons in Two-Dimensional Materials

A. Kudlis, V. Shahnazaryan, I. Iorsh et al. (+2 more)

Motivated by the progress in the physics of two-dimensional materials and the recent two-dimensional generalization of the Fröhlich model, we study the formation of bipolarons in polar monolayers. Because of very special nonlocal dielectric screening in two dimensions, this setting differs qualitatively from the conventional Fröhlich model. In monolayers, (i) long wavelength LO phonons acquire a nontrivial dispersion; (ii) the Fröhlich electron-phonon vertex becomes momentum-dependent and regular at small momenta; and (iii) direct repulsion between charge carriers takes the Keldysh-Rytova form. Using the Feynman path-integral variational approach, we show that the region in the parameter space where stable bipolarons exist is strongly modified compared to the usual quasi-two-dimensional model with dispersionless phonons. Specifically, in the most favorable limit, when the ratio $\sigma_0$ of the static polarizability to the high-frequency polarizability tends to infinity, the lower critical coupling, sufficient for the formation of bipolarons, can be made arbitrarily small. More surprisingly, we demonstrate that no stable bipolarons can exist in the strong coupling limit: in the isolated monolayer the stability region is always confined to a finite range of coupling constants. In general, the stability region is strongly shifted toward large values of the polarizability ratio $\sigma_0$, well beyond the parameter regimes in representative crystals, which indicates that polar monolayers do not favor bipolarons.
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All authors A. Kudlis, V. Shahnazaryan, I. Iorsh, I. A. Shelykh, I. V. Tokatly

Mesoscale and Nanoscale Physics

Altermagnetic Magnons in Dipolar Nanomagnet Arrays

Rhea Hoyer, Ephraim Spindler, Lukas Körber et al. (+3 more)

Altermagnetism is conventionally understood in a spin-conserving framework, where symmetry-enforced momentum-dependent spin splitting emerges in collinear magnets with vanishing net magnetization. Here, we show that its defining signatures persist in nanomagnet arrays coupled exclusively by dipolar interactions, despite the intrinsic breaking of spin conservation. Using a macrospin theory of dipolar-coupled ferromagnetic nanoislands, corroborated by micromagnetic simulations, we demonstrate that arrays engineered with altermagnetic symmetries exhibit spin-split magnon bands whose eigenstates can partially remain strongly spin polarized. The resulting spin expectation value displays the characteristic $d$-wave pattern throughout the Brillouin zone, establishing a mesoscopic realization of altermagnetic magnons beyond the conventional spin-conserving paradigm. As a consequence, spin-wave propagation becomes strongly direction dependent, providing a highly tunable platform for anisotropic magnon transport and synthetic altermagnetic functionality.
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All authors Rhea Hoyer, Ephraim Spindler, Lukas Körber, Tobias Wagner, Mathias Weiler, Alexander Mook

Mesoscale and Nanoscale Physics

Tuneable terahertz transitions in zigzag graphene nanoribbons

R. R. Hartmann and M. E. Portnoi

We show that a transverse electric field applied to a zigzag graphene nanoribbon opens a band gap whose energy can be tuned through the terahertz (THz) range. The field breaks the reflection symmetry of the ribbon, allowing optical transitions that are forbidden in its absence. The relative strengths of transitions polarized parallel and transverse to the ribbon depend strongly on excitation frequency and become equal at a particular frequency. At this frequency, each helicity selectively excites carriers in only one of the two field-induced edge-state valleys, with the selected valley reversed for the opposite helicity. Since this excitation occurs away from the valley minima, the photoexcited carriers have finite group velocity, giving rise to a circular photogalvanic response. Van Hove singularities enhance radiative recombination at the band edge, providing a route towards compact, tunable THz emitters.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.other

Chern Insulators on a Twisted Klein Bottle

Rong Xiao and Y. X. Zhao

Recently, momentum-space nonsymmorphic symmetries have attracted considerable attention. However, their realizations typically rely on fine-tuning hopping phases to satisfy the required projective symmetry algebras. Here, we show that such symmetry algebras arise naturally on bipartite lattices through the interplay of crystalline and sublattice symmetries. Unlike previously studied cases, these symmetry operators can anticommute with the Hamiltonian and are therefore referred to as momentum-space nonsymmorphic chiral symmetries. Although the free actions of these symmetries reduce the Brillouin torus to more elementary manifolds, the resulting manifolds are twisted in the Atiyah–Segal formalism. In particular, chiral glide reflection (screw rotation) gives rise to a twisted Klein bottle (dicosm) in two (three) dimensions, and this twisting dramatically alters the topological classification. Unlike the untwisted Klein bottle, whose nonorientability forces the Chern number to vanish, the twisted Klein bottle can host nonzero Chern numbers, corresponding to even Chern numbers on the torus. At open boundaries, the corresponding topological invariant manifests itself through chiral edge states exhibiting a glide-reflection structure in energy–momentum space. Our results establish momentum-space nonsymmorphic chiral symmetry as a new organizing principle for discovering and engineering topological phases beyond conventional crystalline classifications.
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Mesoscale and Nanoscale Physics

Landscape geometry of Majorana zero modes in inhomogeneous superconductors

Guo-Jian Qiao, Zhi-Lei Zhang, Kang Xu et al. (+1 more)

Spatial inhomogeneity breaks translational symmetry and prevents conventional Bloch-band topological invariants from directly answering a practical question: do Majorana zero modes survive in a given inhomogeneous superconducting device? In this Letter, we develop a real-space landscape approach to address this question. Rather than solving the zero-energy equation as a boundary-value problem, we treat the spatial coordinate as an evolution parameter and recast the equation as a first-order dynamical system. A Majorana zero mode is then identified with a stable trajectory that satisfies the physical boundary condition and approaches the origin at large distance. We show that the landscape geometry fixes the dimensions of the stable and unstable subspaces, while the intersection of the stable subspace with the physical boundary subspace determines the number of Majorana zero modes. In the homogeneous limit, the topological phase transition is manifested as a geometric transition of the landscape. Applied to one-dimensional spinless $p$-wave superconductors and nanowire–superconductor systems, the approach yields sufficient bounds on both the amplitude and spatial gradient of the inhomogeneity, providing quantitative criteria for the design of Majorana devices.
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All authors Guo-Jian Qiao, Zhi-Lei Zhang, Kang Xu, C. P. Sun

Mesoscale and Nanoscale Physics

Microscopic theory of spin-torque ferromagnetic resonance in nonmagnetic-metal/ferromagnetic-metal heterostructures

Takumi Funato and Takeo Kato

We develop a microscopic theory of spin-torque ferromagnetic resonance (ST-FMR) in nonmagnetic-metal/ferromagnetic-metal heterostructures. Tracing out the conduction-electron degrees of freedom in the FM, we derive an effective interfacial exchange coupling between the localized spins and the conduction electron spins in the adjacent electron system. Based on this interaction, we calculate the current-induced driving torque, resonance-frequency shift, damping modulation, and resulting dc voltage. As a concrete example, we apply the formulation to a disordered Rashba two-dimensional electron gas and demonstrate that the ST-FMR spectrum reflects the dynamical spin responses of the adjacent electron system. Our formulation applies to a broad class of heterostructures and establishes a unified microscopic framework connecting ST-FMR spectra directly to the electronic spin responses of adjacent systems.
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Mesoscale and Nanoscale Physics

Unconventional linear transverse exciton transport in valley-layer coupling two-dimensional materials

Ci Li

Valley-layer coupling (VLC) two-dimensional (2D) materials define a distinct class of quantum systems in which valleys are related by crystal, rather than time-reversal ($\mathcal{T}$) symmetry, enabling gate-controlled valley-contrasted layer polarization. Here we extend this concept to excitons. Using TiSiCo as a prototype VLC material, we show that a perpendicular electric field $E_{\perp}$ controls exciton- dispersion anisotropy and thereby generates unconventional linear transverse exciton transport in both monolayer and twisted bilayer structures. In the monolayer, this response is characterized by anisotropy-induced transverse conductivities, arising from the antisymmetric combination of diagonal elements in the conductivity tensor and strongly tunable by $E_{\perp}$. In the twisted bilayer, symmetry additionally permits transverse responses from the symmetric part of the conductivity tensor. Since intralayer excitons in different layers are connected by the Förster coupling, these symmetric and antisymmetric responses coexist and compete with the recently proposed $\mathcal{T}$-even layer Hall and Nernst exciton counterflow. This interplay is highly tunable by twisted angle, temperature, and the direction of the in-plane driving force, providing a route to disentangle distinct transverse exciton transport signals experimentally. Our results establish $E_{\perp}$ as a powerful knob for controlling exciton transport via VLC and identify VLC 2D materials as a promising platform for engineered excitonic phenomena.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.mtrl-sci

High-performance orbital-torque magnetic memory on the 300-mm platform

Dinggui Zeng, Yang Gao, Jinyu Duan et al. (+24 more)

Contemporary memory technologies are increasingly constrained by the fundamental trilemma of storage capacity, access latency, and power consumption. Among the emerging technologies, spin-orbit torque magnetic random-access memory (SOT-MRAM) shows promise to circumvent these challenges, owing to its fast switching dynamics and high endurance. However, the application of SOT-MRAM is hindered by the relatively low write and read efficiencies, resulting in a large bitcell area and an insufficient sensing margin. Meanwhile, the involvement of an ultrathin spin-source channel, typically within a few nanometers, imposes technological challenges for mass production. Here, we resolve these issues on a 300-mm wafer platform by exploiting the emerging orbital degree of freedom and the resultant orbital torque (OT) from the relatively thick Ti/W bilayer. In particular, OT memory nanodevices exhibit a giant tunnel magnetoresistance (TMR) of 182%, nanosecond-scale response, 1012 endurance, together with an enhanced switching efficiency (E_b/I_c), which consequently enables an ultra-low write energy of less than 0.1 pJ/bit. Our findings demonstrate that orbital angular momentum can be implemented for building energy-efficient MRAM devices, offering a practical pathway towards low-latency memory that is demanded for high-performance computing and AI applications.
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All authors Dinggui Zeng, Yang Gao, Jinyu Duan, Lei Zhao, Yuhao An, Xing He, Jintao Ke, Yonglong Ga, Shasha Wang, Zhenghui Ji, Muyuan Chen, Hengan Zhou, Xuejie Xie, Enlong Liu, Junlu Gong, Qijun Guo, Yihui Sun, Zejie Zheng, Weiming He, Xiaolei Yang, Fantao Meng, Yaohua Wang, Hongxin Yang, Delin Zhang, Yong Jiang, Wanjun Jiang, Shikun He

Mesoscale and Nanoscale Physics

Experimental assessment of the Wiedemann-Franz law in thin metal films using thermoreflectance and electrical measurements

Zhiwei Deng, Jinlong Ma and Puqing Jiang

Accurate thermal properties of metal thin films are essential for microelectronic thermal modeling and for interpreting thermoreflectance measurements when signals are sensitive to film thermal transport. In practice, film thermal conductivity is commonly inferred from electrical resistivity via the Wiedemann-Franz (WF) law with the Sommerfeld-Lorenz number L_0, neglecting phonon contributions and microstructure-dependent scattering. Here, we combine square-pulsed source (SPS) thermoreflectance with van der Pauw measurements to characterize thermal and electrical transport in Al, Ti, and Ta thin films prepared by thermal evaporation, e-beam evaporation, and magnetron sputtering. SPS measurements from 80 to 300 K yield the film thermal conductivity k_m and volumetric heat capacity C_m, with C_m agreeing with bulk values within +/-8%. Compared with WF-based estimates using L_0 and a modeled resistivity, h̊o}_model (T)=h̊o}_bulk (T)+h̊o}_0, k_m deviates by 5-20% for Al and Ti and up to 40% for sputtered Ta at 300 K. While the smaller deviations are comparable to experimental uncertainty, the large mismatch for Ta demonstrates the limitation of applying the bulk L_0 to highly resistive metal films. Apparent Lorenz numbers L_app=k_m h̊o}_meas/T from 150 to 300 K further reveal that deposition-induced disorder, grain-boundary scattering, and possible phase-related effects can modify the correlation between heat and charge transport in thin metal films. These results clarify the applicability of the WF law to deposited metal thin films and offer practical guidance for thermal modeling and thermoreflectance analysis.
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Tuesday 15 September

Mesoscale and Nanoscale Physics

A semiconductor photon-pair source based on a polariton cascade

Karol Kawa

We present a theoretical study of a semiconductor photon-pair source based on a radiative cascade enabled by a permanent exciton dipole. The source consists of a GaAs quantum dot placed between a metal nanoparticle and a mirror. The dot exciton and the localized electromagnetic mode mix to form upper and lower polaritons. Without a permanent dipole, a symmetry separating states with even and odd excitation numbers forbids photon emission between the polaritons. Separation of the mean electron and hole positions gives the exciton a permanent dipole and breaks this symmetry. The upper polariton can then emit an idler photon as it decays to the lower polariton. The lower polariton can emit a signal photon as the system returns to its ground state. Using an effective model, we calculate the probability that a prepared upper polariton emits both photons through the metal antenna. We also analyse fluctuations in the emitted photon counts and correlations between the two emission channels. The calculation establishes an operating principle for assumed emitter properties and optical loss rates. Whether the device can be built and its photon correlations measured remains open.
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Mesoscale and Nanoscale Physics

Entropy spectroscopy of a tunable two-site Hubbard molecule

Uhjin Kim, Seokyeong Lee, Gibum Yun et al. (+9 more)

Determining which microscopic states remain thermally active when tunneling and interactions compete is a central question in Hubbard physics. Here we develop an entropy measurement protocol for a double quantum dot, extending the charge-based approach established for a single dot, and measure the total entropy of a tunable two-site Hubbard molecule realized in a GaAs double dot. By shifting both dot levels together at fixed detuning, the entropy is correctly probed with only a single charge sensor. As interdot tunneling increases, the system evolves from two atomic-like dots through hybridized molecular states to a merged single dot. Across this evolution, the entropy decreases as tunnel-induced energy splitting exceeds the thermal energy and suppresses the occupation of higher-energy states. The measurements resolve the diminishing contribution of antibonding states and the changing thermal contributions of hybridized singlet and triplet states, in quantitative agreement with a two-site Hubbard model. By distinguishing states with the same charge configuration but different orbital and spin content, entropy reveals how tunneling and interactions determine the thermally active states of the minimal Hubbard system.
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All authors Uhjin Kim, Seokyeong Lee, Gibum Yun, Dongsung T. Park, Soobeom Choi, Sangwoo Jeong, Donghoon Kim, V. Umansky, Yunchul Chung, Hyoungsoon Choi, H.-S. Sim, Hyung Kook Choi

Mesoscale and Nanoscale Physics

Visualizing Berry curvature in a Floquet-Chern insulator

Younsik Kim, Rishi Acharya, Hannah E. Aguirre et al. (+3 more)

Circularly polarized light can open a topological gap in a gapless Dirac system, creating a Floquet-Chern insulator. However, its Berry curvature has not been directly resolved. Here we use circular-dichroism time- and angle-resolved photoemission spectroscopy (Tr-ARPES) to map the light-induced Berry curvature of Floquet-Bloch states on the surface of Bi$_2$Se$_3$. By comparing spectra for opposite helicities of the mid-infrared pump, we isolate the induced geometric response. We observe alternating-sign Berry-curvature features at Floquet gaps that repeat at intervals set by the pump photon energy and redistribute with pump field strength in agreement with calculations for a periodically driven Dirac Hamiltonian. Both the induced Berry-curvature and the Floquet Dirac gap decay faster than the replica intensity, showing that spectral replicas can persist even after coherent Floquet hybridization weakens. These results establish momentum-resolved Berry-curvature mapping as a direct probe of light-induced topology and coherent Floquet band formation.
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All authors Younsik Kim, Rishi Acharya, Hannah E. Aguirre, Lucca Figari, Daniel P. Shoemaker, Fahad Mahmood

Mesoscale and Nanoscale Physics

Exact solutions of nonreciprocal Su-Schrieffer-Heeger model with domain walls

Tong Wang, Mengjie Yang and Flore K. Kunst

We investigate domain-wall physics in a non-Hermitian Su-Schrieffer-Heeger model featuring the non-Hermitian skin effect and its higher-dimensional extensions, focusing on analytical eigenstate solutions under open boundary conditions. By gluing two Su-Schrieffer-Heeger chains with inverted coupling ratios together, we identify two distinct interface geometries, and derive closed-form quantization conditions for the complex wave number and wave functions using symmetry-based ansatzes. Besides conventional skin modes and topological zero modes, the domain walls generate additional localized states at finite energy that detach from the bulk open-boundary-condition continuum. We validate the analytical results by exact diagonalization, and further generalize the interface construction to a two-dimensional Lieb lattice, where competing nonreciprocities produce tunable funneling regimes towards codimension-one and codimension-two interfaces.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph

Exceptional Topology Survives Strong Hermitian Fields in Radiative Atomic Arrays

Tian-Shu Gou, Yi-Cheng Wang, Ya-Tang Yu et al. (+3 more)

Exceptional points are topological defects of complex band structures that are stable against weak perturbations, yet in two-band systems with bounded non-Hermitian couplings they are ultimately removed by a sufficiently strong Hermitian field. Here we demonstrate a striking exception in a two-dimensional subwavelength atomic array. Along a continuous square-to-triangular deformation at fixed magnetic field, the bulk passes between line-gapped topological regions with \textcolor{black}{band Chern numbers \( (C_1,C_2)=(2,-2) \)} through a gapless exceptional region. Within a finite interval of lattice deformation, increasing the magnetic field merely drives the exceptional points toward the light cone because of the singular radiative dipolar couplings. We further show that skin localization toward open boundaries responds non-monotonically to the same field. An intermediate field drives a bipolar skin localization, with bulk modes from different spectral branches accumulating at opposite boundaries, whereas stronger fields suppress boundary localization. Our results establish the interplay of lattice geometry, light-cone singularity, and Hermitian magnetic field as a route to engineering Chern and exceptional topology beyond the conventional strong-field limit.
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All authors Tian-Shu Gou, Yi-Cheng Wang, Ya-Tang Yu, Guin-Dar Lin, Jhih-Shih You, H. H. Jen

Mesoscale and Nanoscale Physics

Self-balancing luminescence kinetics of charged excitons in WS$_2$ monolayer

A.I. Veretennikov, A.I. Galimov, M.M. Glazov et al. (+1 more)

Two-dimensional tungsten-based transition metal dichalcogenides exhibit a rich emission spectrum with distinct lines of exciton and biexciton states, neutral and charged, which are typically considered as independent entities. Here, we present a study of the emission kinetics of a WS$_2$ monolayer, demonstrating that these states together form a single interacting ensemble, with their population and depletion mutually coordinated with temperature. Dark trions make a long-lived contribution to the bright trion on a time scale of 200-300 picoseconds, and replenishment from dark states promotes intense luminescence of charged biexcitons. The developed self-consistent rate equation model reproduces the temperature dependence of the kinetics well, opening the way to controlling exciton states in such materials.
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All authors A.I. Veretennikov, A.I. Galimov, M.M. Glazov, T.V. Shubina

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

Preparation-protocol-dependent quantum Mpemba dynamics in a magnetically tunable graphene nanotorus qubit

J. Furtado

We investigate the quantum Mpemba effect in a graphene nanotorus qubit and show that its strength depends strongly on how the initial state is prepared. We compare two protocols that share the same final Hamiltonian, thermal bath, and Markovian Liouvillian: bare Gibbs preparation, in which the coherent drive is switched on only at the final quench, and driven steady-state preparation, in which the initial states are stationary states of the driven dynamics. Using the distance to the final stationary state and an integrated Mpemba parameter, we find broad regions of strong anomalous relaxation for bare Gibbs preparation, with $M_B$ approaching unity, whereas the driven steady-state protocol almost completely suppresses the effect. A Liouvillian-mode analysis reveals the mechanism: the bare Gibbs protocol typically yields a smaller hot-state weight in the slow active sector than the cold state, $R_s<1$, while the driven steady-state protocol predominantly gives $R_s>1$. Since the final Liouvillian spectrum is identical for both protocols at corresponding parameter points, the contrast originates from preparation-dependent modal weights rather than from changes in the decay-rate hierarchy. These results identify state preparation as a control parameter for anomalous quantum relaxation in a curved graphene qubit.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Interplay of dimerization and quasiperiodicity in the superconducting proximity effect of a one-dimensional hybrid ring

Sourav Karmakar and Santanu K. Maiti

Recent studies of the superconducting proximity effect in quasicrystalline and topological systems have opened up a new research direction for exploring how quasiperiodicity and topology influence proximity induced superconductivity. In this work, we investigate spatial variation of the proximity induced pairing amplitude in a hybrid ring composed of a spin singlet superconductor and a normal region described by three different lattice models using the self-consistent Bogoliubov-de Gennes formalism. We first consider the normal region described by the diagonal Aubry-André-Harper (AAH) model. Increasing the quasiperiodic potential enhances spatial fluctuations in the induced order parameter, while progressively suppressing its magnitude in the normal region. Beyond the localization transition, proximity-induced pairing is strongly diminished due to the localized nature of the underlying electronic states. The normal region is then modeled by the Su-Schrieffer-Heeger (SSH) chain to investigate the effect of hopping dimerization. Weak dimerization introduces oscillatory modulations in the induced pairing that extend deep into the normal region, whereas strong dimerization confines these oscillations and significantly reduces the penetration of superconducting correlations. Finally, we study the combined SSH-AAH model to explore the interplay between the dimerized hopping and quasiperiodicity. The results show that the SSH dimerization determines the oscillatory behavior and penetration of the induced pairing, while the AAH potential enhances spatial inhomogeneity and further reduces its magnitude. Together, these two effects provide a versatile means of controlling proximity-induced superconductivity in quasiperiodic hybrid systems.
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Mesoscale and Nanoscale Physics

Nonlinear Hall Effect in Altermagnetic Warped Topological Insulators

Debashree Chowdhury

Low-dimensional Dirac systems offer a versatile platform for quantum-geometric non-linear transport. Hexagonal warping in topological insulator (TI) surface states generates rich momentum-space geometric textures. However, mirror and time-reversal symmetries strictly constrain both the Berry curvature and quantum metric dipoles to zero. Here, we show that proximity-coupling a hexagonally warped TI to a d-wave altermagnet lifts spatial mirror symmetry via the interplay of C_{3v} warping and d-wave spin-splitting. This symmetry breaking enables finite Berry curvature dipole components, which, however, remain heavily suppressed near charge neutrality, establishing the intrinsic quantum metric dipole as the sole driver of the non-linear Hall response. This scattering-independent response vanishes at \mu = 0 and exhibits odd parity under chemical potential inversion. Importantly, rotating the altermagnetic orientation angle \phi continuously tunes quantum metric dipole and induces a sign reversal, peaking near \phi=0 and vanishing at \phi = \pi/4. Our findings highlight TI-altermagnet interfaces as ideal candidates for gate- and orientation-tunable quantum metric spintronics.
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Mesoscale and Nanoscale Physics

Propulsion of magnetic domain walls via elastic damping in magnetostrictive materials

Miguel Moreno, Rocio Yanes and Luis Lopez-Diaz

The interaction between magnetic and elastic degrees of freedom is increasingly central to advanced spintronics technology. In magnetostrictive materials, a moving domain wall carries a localized elastic deformation. Here, we investigate these elastic deformations and their back-action on field-driven domain wall dynamics using a fully coupled micromagnetic-elastodynamic framework and an analytical collective coordinates model. We show that mechanical damping acts as a non-magnetic dissipation channel, extracting energy from the magnetic texture and sustaining translational domain wall motion even in the complete absence of intrinsic Gilbert damping ($\alpha=0$). Specifically, elastic damping impacts domain wall dynamics in a manner equivalent to magnetic losses, allowing us to map it onto an effective magnetoelastic contribution to the Gilbert damping parameter, $\alpha_\text{me}$. We calculate the localized stress tensor profiles co-moving with the domain wall and show that mechanical dissipation breaks their spatial symmetry. Using our collective coordinates model, we demonstrate that these asymmetric stress profiles-specifically the localized stress gradients across the wall core-exert direct propelling forces that balance field-induced magnetic precession, enabling steady-state motion. Overall, these findings reveal a fundamental energy transfer and relaxation mechanism between magnetic and elastic subsystems, providing valuable insights for controlling magnetic texture dynamics via strain engineering and mechanical damping design in acoustic spintronic devices.
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Mesoscale and Nanoscale Physics

Generation of pure spin currents via nonadiabatic quantum pumping in an antiferromagnetic chain

Leila Eslami, Fatemeh Bourbour, Somaieh Ahmadi et al. (+1 more)

In this study, quantum spin pumping in an antiferromagnetic chain driven by time-dependent potential is investigated. The aim is to explore the possibility of generating and controlling spin currents in the absence of external bias and to examine the role of exchange field and periodic driving in the separation of spin-up and spin-down currents. The system is described using a tight-binding model, and spin-resolved currents are calculated employing the Keldysh non-equilibrium Green's function formalism. Two time-dependent potentials with a specific phase difference are applied to the two ends of the chain, while the chemical potentials of both electrodes are set equal. The results demonstrate that in the adiabatic regime (low frequencies), the response of the two spin channels is nearly identical. However, as the driving frequency increases and the system enters the nonadiabatic regime, absorption and emission processes of energy quanta become activated, leading to significant differences between spin-up and spin-down currents. The pumped current exhibits strong dependence on the chemical potential, allowing for the control of both magnitude and direction of the spin current through its adjustment. With increasing frequency, the spin current enhances, and parameters can be tuned such that the charge current nearly vanishes while a considerable spin current persists. This finding indicates the feasibility of achieving nearly pure spin pumping without net charge transfer in the antiferromagnetic chain. The results provide a promising perspective for designing spin-pumping devices based on antiferromagnetic systems.
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All authors Leila Eslami, Fatemeh Bourbour, Somaieh Ahmadi, Santanu K. Maiti

Mesoscale and Nanoscale Physics

Diffusive molecules share the 1/3 shot noise suppression of quantum conductors

Henri Vo Van Qui, Ignacio Madrid, Simon Grall et al. (+7 more)

Electrical noise measurement, especially shot noise, which arises from the discreteness of charge, is an indispensable tool in quantum transport for probing the nature of charge carriers beyond simple conductance measurements. Diffusive conductors share a universal shot noise reduction of 1/3, a result obtained independently from quantum scattering theory, semi-classical kinetics, or by classical exclusion processes8. Until now, however, experimental tests of this universality have been limited to cryogenic conditions, leaving its origin incompletely understood. Here we show that ferrocene redox cycling in a microfluidic gap is a room temperature realization of this universality. We derive the full counting statistics of diffusing single-electron molecular shuttles and verify the predicted current noise experimentally, showing that the universal 1/3 shot noise suppression is recovered in the diffusion-limited regime. Our result identifies diffusion and sequential charge transfer as sufficient ingredients for this universal noise reduction, rather than quantum coherence, fermionic statistics or cryogenic conditions. We anticipate that this study will establish electrochemical microfluidics as a room-temperature platform for mesoscopic counting statistics and bring noise-based probes to molecular transport and reaction kinetics. Furthermore, this liquid-based quantum-inspired study will provide a novel insight on the yet to be understood links between quantum and biology.
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All authors Henri Vo Van Qui, Ignacio Madrid, Simon Grall, Thibaut Jonckheere, Akira Fujiwara, Laurent Jalabert, Masayuki Hashisaka, Christophe Demaille, Soo Hyeon Kim, Nicolas Clément