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Physics Condensed Matter Mesoscale and Nanoscale Physics

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Monday 7 September

Mesoscale and Nanoscale Physics

Trion-mediated quantum operations on a double-dot charge qubit

Alexander Tsukanov

A scheme for controlling a charge qubit on a semiconductor asymmetric double quantum dot with suppressed tunnel coupling between individual quantum dots is proposed. Laser pulses convert the logical single-electron states of the qubit into auxiliary trion states, for which the resonant tunneling condition is restored due to the compensation of structural asymmetry by the Coulomb interaction of the particles. The conditions for performing orthogonal single-qubit rotations are formulated. For a two-dimensional structure, the parameters of the qubit are calculated, and the dependence of the fidelity of the inversion operation on the parameters of the laser field and the rates of dissipative processes is obtained. It is shown that the considered algorithm is characterized by high speed and reliability.
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Mesoscale and Nanoscale Physics

Unveiling the Scaling Potential of Drain Merge through Active (DMtA) in CFETs: Breaking the Super-Via Bottlenecks and Unlocking New PPA Boosters

Jingru Jiang, Haoran Lu, Kairong Guo et al. (+11 more)

Drain merge (DM), a super via vertically connecting the common S/D terminals of stacked n/pFETs in Complementary FETs (CFETs), blocks further parasitic optimization and cell scaling. For the first time, this work systematically investigates the state-of-the-art Drain Merge through Active (DMtA), a revolutionary technology reported recently with the DM embedded in the active region, through a comprehensive DTCO framework spanning process integration, contact-configuration-dependent (CTCD) compact modeling, standardcell design, RO evaluation and block-level PPA benchmark on a 32-bit RISC-V Ibex core. By reducing DM parasitics and enabling DM-width optimization, DMtA improves RO frequency by 11.7% over its conventional Drain Merge through field (DMtF) counterpart. Active widening and Area Borrowing, the latter first reported in [8] and exploiting spatial slack in adjacent cells to further enlarge the nanosheet width (WNS), increase the maximum Ibex-core frequency by up to 34.8%. More importantly, DMtA also enables the once GAA-exclusive Hyper-cells on CFETs by merging the active regions across adjacent cell rows, providing a further 8.7% frequency gain. A post-routing floating-output-pin-aware optimization further removes redundant S/D contacts (CTs) and reduces power by 5.3%. Finally, DMtA facilitates more area-efficient 2.5T cell scaling by preserving single-row cell compatibility, reducing post-PR core area by 25.7%.
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All authors Jingru Jiang, Haoran Lu, Kairong Guo, Yibo Zhang, Yifei Chen, Wanyue Peng, Yu Liu, Jiacheng Sun, Xiaoyan Xu, Ming Li, Yibo Lin, Runsheng Wang, Ru Huang, Heng Wu

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

Metallic-Phase-Fe$_3$GaTe$_2$ Enabled Interface Engineering for Self-Powered and High-Gain WS$_2$ Photodetectors

Wajid Ali, Ming Huang, Juan Li et al. (+7 more)

Two-dimensional transition-metal dichalcogenides offer strong light-matter interaction but suffer from inefficient carrier separation and contact-related losses in photodetectors. Here, we demonstrate a high-gain WS$_2$/Fe$_3$GaTe$_2$ van der Waals heterostructure photodetector, where metallic Fe$_3$GaTe$_2$ serves as an active interfacial contact. The work-function mismatch, together with interfacial charge redistribution and asymmetric contact geometry, contributes to a built-in field that supports self-powered photodetection at zero bias. Under 450 nm illumination, the device delivers a zero-bias responsivity of 23.5 A/W and an apparent external quantum efficiency of 6.4 x 10$^3$%. At -1 V biasing, the heterostructure exhibits photoresponse at 450, 520 and 633 nm, achieving a responsivity of 9.7 x 10$^3$ A/W and a noise derived specific detectivity of 2.3 x 10$^13$ Jones at 100 Hz under 450 nm illumination. The high photoresponse is attributed to interfacial carrier separation, efficient extraction, and a likely contribution from trap-assisted photogating in multilayer WS$_2$. These results establish Fe$_3$GaTe$_2$-enabled interface engineering as an effective route for self-powered, highly sensitive 2D photodetectors.
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All authors Wajid Ali, Ming Huang, Juan Li, Jianhua Huang, Liuli Yang, Sajid Ur Rehman, Chinmay K. Mohanty, Zahir Muhammad, Ziwei Li, Maciej R. Molas

Mesoscale and Nanoscale Physics · Cross-listed from physics.chem-ph

Mixed quantum-classical evolution in open molecular systems

Michael Galperin and Abraham Nitzan

We consider a mixed quantum-classical formulation for open nonequilibrium molecular systems. We employ the pseudoparticle nonequilibrium Green's function (PP-NEGF) method as a fully quantum description and a starting point for introducing classical nuclear dynamics for an open system. We use this formulation to derive a density-matrix equation of motion describing the quantum-classical evolution. We compare our results with previous formulations and highlight important differences related to the nonequilibrium and open character of the molecular system. We also analyze the approximations needed to reduce full quantum-classical dynamics to the fewest-switches surface hopping (FSSH) method and show that these approximations are unreasonable when adiabatic surfaces come close to each other.
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Mesoscale and Nanoscale Physics

Lindbladian Phase Geometry and Hall Transport in Open Bloch Systems

Zhihao Jiang, Longjun Xiang and Jian Wang

Quantum geometry underlies a wide range of transport phenomena in Bloch systems. How quantum-geometric transport is modified when Bloch electrons are coupled to an environment, however, remains largely unexplored, even though the environment can alter both the electronic state and the physical current operator. Here we formulate dc linear response within a trace-preserving Lindblad kinetic theory by defining the physical velocity as the Liouvillian time derivative of the position operator. This construction reveals an environment-induced contribution to the current vertex whose momentum-space curl generates new Hall responses governed by the gauge-invariant phase geometry encoded by Lindblad jump amplitudes, which characterize electron-environment coupling. To leading order in the dissipative coupling, this geometry gives rise to interband shift-vector and diagonal-vorticity Hall responses arising from off-diagonal and diagonal jump amplitudes, respectively. Remarkably, both mechanisms can produce a finite Hall conductivity even when the conventional Berry-curvature anomalous Hall effect vanishes identically. Our work establishes Lindbladian phase geometry as an independent geometric origin of transverse transport in open quantum matter.
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Mesoscale and Nanoscale Physics

Theory of staggered damping-like spin-orbit torque on synthetic antiferromagnetic domain walls

Yuta Yamane and Takeshi Seki

The Néel, or staggered, spin-orbit torque (SOT) enables an efficient electrical manipulation of antiferromagnetic order. However, it has been tied to special crystal structures, and only its field-like component is established. We develop a theory of staggered damping-like SOT, which arises generically in synthetic antiferromagnets sandwiched by the same heavy metal on both sides. Deriving a Néel-vector Lagrangian, we obtain the domain-wall (DW) velocity in closed form: bilinear in current and in-plane field, opposite in sign for Néel and Bloch walls, and tunable through the interlayer exchange coupling. DW structure thus becomes a control knob for DW motion, while the DW motion, in turn, offers an electrical readout of its structure. Micromagnetic simulations confirm the theory. The predicted sign reversals and scaling laws of the DW velocity, together with the thresholds for the Néel-Bloch transition, give immediate experimental targets.
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Mesoscale and Nanoscale Physics

Self-Trapping Enabled Highly Bright Momentum-Indirect Interlayer Excitons

Dong Yang, Zisheng Gong, Hao Wen et al. (+6 more)

Interlayer excitons in two dimensional material heterostructures exhibit large exciton binding energies and long lifetimes, making them ideal platforms for studying excitonic devices and many body quantum phenomena. However, the spatially separated electron and hole nature of IXs reduces their oscillator strength by two orders of magnitude compared to intralayer excitons. Achieving high efficiency IX emission remains challenging and requires optimal material selection with appropriate momentum matching and meticulous device fabrication. Here we demonstrate a highly bright momentum indirect IX emission within heterostructures formed between 2D perovskites and monolayer transition metal dichalcogenides. The quantum yield of IX emission reaches 35.2% on average, over 50 times higher than that of the corresponding constituent TMD monolayer, with the highest value exceeding 60%. Notably, the radiative recombination efficiency of this momentum indirect IX exceeds that of momentum direct IXs in monolayer TMD-based heterostructures by two orders of magnitude. We suggest that the remarkably bright IX emission in our heterostructure originates from IX self trapping, induced by strong exciton phonon coupling arising from the soft lattice nature of the 2D perovskite. Our findings provide new insights into achieving high IX emission efficiency and open new avenues for exploring long lifetime excitonic devices.
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All authors Dong Yang, Zisheng Gong, Hao Wen, Yue Hu, Kaichen Jiang, Baixu Xiang, Weibo Gao, Qihua Xiong, Dehui Li

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

Restoring the Surface Magnetic Gap in MnBi$_2$Te$_4$

Ce Bian, Hengxin Tan and Wenhui Duan

A widespread experimental realization of quantized anomalous transport in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ is hindered by its elusive surface magnetic gap. Uncovering the origin of the gapless states is essential for accessing its topological properties. Here we show that surface defects lower the electrostatic potential, drive topological surface states into subsurface layers, suppress exchange interactions, thereby closing the gap. Tuning the surface electrostatic potential via external electric fields or interfacial fields in van der Waals heterostructures restores the expected gap and enables control of its topology. This is confirmed by model calculations and validated in defective MnBi$_2$Te$_4$ films interfaced with polar insulators, explaining the enhanced quantum anomalous Hall effect under AlO$_x$ capping observed in recent experiments. Our theory identifies electrostatically driven surface-state delocalization as a competitive origin of gap suppression and proposes displacement-field engineering for robust quantized transport.
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Sunday 6 September

Mesoscale and Nanoscale Physics

Evidence of Haldane-Chain Physics in an Fe-Dehydroindigo Coordination Polymer

Ritam Chakraborty and T. V. Ramakrishnan

We investigate whether an Fe–dehydroindigo coordination polymer can support Haldane-chain physics in its intrinsic, substrate-free limit. Spin-orbit-coupled density-functional-theory calculations and magnetic energy mapping yield a nearly isotropic antiferromagnetic coupling, together with much weaker single-ion anisotropy. Using these parameters, exact diagonalization and density-matrix renormalization group calculations give a nondegenerate ground state separated from the lowest triplet-derived excitations by a gap of about $13$–$14$~meV, on the scale expected for the spin-1 Haldane gap. The anisotropy produces only a small splitting of the low-energy modes. The dynamical spin structure factor places the lowest spectral weight near the antiferromagnetic wave vector $q=\pi$. These results identify the isolated cis-dehydroindigo chain as a material-specific coordination-polymer platform whose microscopic interactions place it in a gapped regime consistent with Haldane-chain physics.
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Mesoscale and Nanoscale Physics

Three-Dimensional Fermiology and Thickness-Tuned Magnetotransport in Single-Crystalline Antimony Flakes

Mikhail Gaponov, Jicheng Wang, Liang Zha et al. (+1 more)

The extreme magnetoresistance of compensated semimetals is governed by both the Fermi-surface geometry and carrier relaxation, but these contributions are difficult to disentangle in finite-size structures. Here, we combine longitudinal and Hall magnetotransport measurements with temperature- and angle-dependent Shubnikov–de Haas oscillations in single-crystalline Sb flakes grown by chemical vapor deposition (CVD), with thicknesses ranging from 110 to 783 nm. As thickness increases, the non-saturating MR at 2 K and 14 T rises nearly 30-fold, reaching $7.13\times10^{5}\%$, while the primary frequency $F_{\alpha}$ remains approximately 99 T without any systematic shift. A joint three-channel analysis of $\rho_{xx}(B)$ and $\rho_{xy}(B)$ reveals that this evolution is driven by an increase in the mobility of a nearly compensated electron–hole pair, rather than by a reconstruction of the primary pockets. Angle-dependent measurements confirm the existence of a closed three-dimensional $\alpha$ pocket, and a reproducible high-frequency sector (335-377 T) is consistent with the electron $\beta$ orbit of the $L$-point pockets in bulk Sb. Together, the transport and quantum-oscillation results show that thickness tunes extreme MR through dimension-dependent scattering while preserving bulk-like fermiology.
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All authors Mikhail Gaponov, Jicheng Wang, Liang Zha, Rui Wu

Mesoscale and Nanoscale Physics

Emergent Chiral Metal Phase in Compressible Quantum Hall Fluids

Suparna Sahoo, Suvankar Purkait, Pooja Agarwal et al. (+4 more)

We report transmitted conductance measurements between a source and reflection-less contacts connected to a compressible quantum Hall fluid with filling fraction $\nu$. We observe that total sum of transmitted conductances universally approaches Hall conductance $\nu(e^2/h)$. The universality of this sum rule is established experimentally across integer and fractional quantum Hall regimes, remaining invariant under variation in temperatures, sample geometries, material qualities and quasi-particle interactions. Chiral transport in compressible quantum Hall fluids, characterized by suppressed dissipative transport with a distinct handedness, is confirmed by floating contact measurements. Consequently, this sum rule emerges as a conduction law of the chiral metal phase. Theoretically, we argue that time-reversal-symmetry breaking under a strong magnetic field within this chiral metal gives rise unidirectional trajectories of carriers in a nearly flat potential landscape with point-like disorder potentials, a regime realized when the screening length is smaller than the magnetic length. Within this chiral framework, longitudinal resistance does not originate from bulk dissipation but primarily from the equilibration of electrochemical potentials at the contacts. Our study introduces a new paradigm of chiral transport across a broad class of gapless two-dimensional systems characterized by short-range screening and broken time-reversal symmetry.
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All authors Suparna Sahoo, Suvankar Purkait, Pooja Agarwal, Tanmay Maiti, Sourin Das, Vladimir Umansky, Biswajit Karmakar

Mesoscale and Nanoscale Physics

Near-Field Spin Seebeck Effect

Gaomin Tang

The conventional spin Seebeck effect generates spin currents through interfacial thermal conduction. In this Letter, we establish a photon-mediated spin Seebeck effect driven by near-field thermal radiation. Using a monolayer transition metal dichalcogenide separated from a thermal emitter by a vacuum gap, we demonstrate Rashba spin-orbit coupling can convert optical orbital excitations into an electron spin polarization. This occurs via two complementary mechanisms: the direct transfer of angular momentum from chiral thermal photons, and the rectification of unpolarized thermal fluctuations by a magnetized two-dimensional electron gas. These findings unveil a radiative pathway for nanoscale electron spin manipulation.
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Mesoscale and Nanoscale Physics

Continuum Limit of Nonlocal Electrostatics in Random Media

Prashant K. Jha and Kaushik Dayal

We derive a two-scale continuum limit for the electrostatic energy of random charge density fields that are stationary and ergodic under lattice translations. Each microscopic cell is charge neutral in every realization, while its dipole moment may fluctuate and have a nonzero mean. Under assumptions on the stationary microscopic potential and the finite-volume fields, the local and nonlocal energies converge almost surely to deterministic limits. The local limit consists of the ensemble-averaged microscopic Coulomb energy and a cell-depolarization term, whereas the effective polarization determines the nonlocal limit. The cell-depolarization terms cancel in the total energy, which consists of the ensemble-averaged microscopic cell energy and the macroscopic electrostatic field energy. The deterministic specialization recovers the periodic two-scale limit. For an independent-cell random-displacement model, we verify the assumptions of the continuum-limit theorem and show that centered microscopic fluctuations can change the local energy without changing the effective polarization or the nonlocal energy.
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Saturday 5 September

Mesoscale and Nanoscale Physics

Purely Electric-Field Control of Topological Magnetism in Two-Dimensional Magnets

Zhonglin He, Naafis Ahnaf Shahed, Kai Huang et al. (+2 more)

Electrical control of topological magnetism is central to realizing energy-efficient topological spintronics. Yet most electric-field approaches modify the competing magnetic interactions through a nonselective rearrangement of low-energy electronic states, yielding coarse magnetic phase control that often requires external magnetic fields to stabilize topological quasiparticles, while few schemes solely based on electric fields are restricted to specific conducting materials. Here, we establish a general approach for purely electric-field control of topological magnetism, in which an applied electric field electrostatically dopes a selected orbital-angular-momentum-polarized band edge of a two-dimensional (2D) van der Waals (vdW) magnetic semiconductor via proximity to an adjacent nonmagnetic vdW metal. We show that the resulting electrostatic doping predominantly tunes magnetic anisotropy of the 2D magnet, while leaving exchange interaction and Dzyaloshinskii-Moriya interaction nearly unchanged, thereby reversibly driving the system through ferromagnetic, skyrmion, spiral, and bimeron phases. We demonstrate this mechanism for CrBr3/graphene and Cr2Ge2Te6/TaS2 vdW heterostructures hosting electron and hole pockets of different orbital characters. These results establish a broadly applicable strategy for purely electric-field control in topological spintronics.
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All authors Zhonglin He, Naafis Ahnaf Shahed, Kai Huang, Himanshu Mavani, Evgeny Y. Tsymbal

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

Altermagnetism from the viewpoint of chemistry

Nayana Devaraj, Anumita Bose, Md Afsar Reja et al. (+2 more)

Magnetism has been a central theme of research in chemistry, physics, and materials science, with chemical composition and bonding playing key roles in determining magnetic behavior. Altermagnets are a newly identified class of magnetic materials that combine features of conventional ferromagnets and antiferromagnets, arising from specific symmetry and electronic structure motifs. In this review, we present a chemistry-driven viewpoint on altermagnetism, highlighting how crystal chemistry, bonding, and electronic structure enable this unconventional magnetic order. We begin by introducing the fundamental concepts required to understand altermagnets, with an emphasis on symmetry considerations, orbital character, and electronic structure signatures. We then survey the diverse material families in which altermagnetism has been identified, drawing attention to coordination environments and structure-property relationships that favor altermagnetic order. We subsequently present experimental approaches which are useful for the characterization of altermagnetic materials. We examine ab initio materials discovery as a promising strategy for identifying new altermagnets, emphasizing how chemical constraints, such as symmetry and bonding, can guide computational searches. Other than their intrinsic importance, altermagnets provide interesting possibilities for technology. For this reason, we highlight possible applications that may be enabled through altermagnetic materials, along with their coupling with existing orders such as ferroelectricity and superconductivity. In conclusion, we point out some challenges and prospects, where chemically-based design guidelines can play an important role towards advancing altermagnetism research. In summary, this review offers an account of recent developments in altermagnetism, from basic concepts to the current state-of-the-art.
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All authors Nayana Devaraj, Anumita Bose, Md Afsar Reja, Arka Bandyopadhyay, Awadhesh Narayan

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

Nanothermodynamics: stable thermal equilibrium and nanoscale fluctuations

Ralph V. Chamberlin

Nanothermodynamics describes the process where large systems subdivide into equilibrium distributions of small subsystems. A key ingredient is Hill's subdivision potential (E) that ensures adherence to the 1st and 2nd laws of thermodynamics in systems of any size. In this review and reassessment, it is emphasized that nanothermodynamics gives new insight into many measurements, theories, and simulations. Measurements establishing the need for nanothermodynamics show thermodynamic heterogeneity from multiple effective temperatures (T_i) inside most types of materials. One theoretical result that requires E=0 is the stable solution of Ising's original model for finite chains of interacting spins, a solution Ising could not have found 40 years before Hill's work. Another result is a novel solution to Gibbs' paradox that makes the entropy of the semiclassical ideal gas exactly extensive. Molecular dynamics simulations reveal how a standard fluctuation relation is modified when local degrees of freedom fluctuate faster than their coupling to the heat bath, consistent with the measured thermodynamic heterogeneity. Simulations of a Creutz-like model, comprised of Ising spins coupled to an explicit heat bath of Einstein oscillators, are used to study the 2nd law. It is found that maximizing the total entropy (S_t) requires an intrinsically irreversible step, providing a counterexample to the usual claim that statistical mechanics emerges from reversible dynamics. Furthermore, fluctuations of this model are best described by Einstein's reversal of Boltzmann's relation and the 2nd-law, not by recent fluctuation theorems.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics

Helicity-engineered nonlinear optical responses in photo-excited topological semimetals

Roshan Kumar Thakur, Prachi Venkat, Amar Bharti et al. (+1 more)

Topological materials provide a transformative arena for light-driven control of electronic motion; yet, direct manipulation of electron dynamics on sub-cycle timescale remains a significant challenge. We demonstrate that the strong-field-driven high-harmonic generation of a Weyl semimetal can be controlled and manipulated through bicircular pump-probe driving fields. While a lone probe pulse generates exclusively odd-order harmonics, the addition of a pump pulse triggers a series of sidebands arising from nonlinear frequency mixing of pump and probe photons. Our results reveal that the sideband intensities are highly sensitive to the relative helicity of the pulses and the orientation of the polarization plane. This sensitivity stems from the chiral nature of the Weyl nodes, which couples efficiently to the light's helicity only when the node-separation axis is perpendicular to the polarization plane. Furthermore, the significant suppression of sideband intensity with increasing pump-probe delay identifies these features as a potential clock for electron-hole decoherence. These findings establish frequency-mixed high-harmonic generation as a sensitive probe of chiral quantum dynamics and suggest a robust framework for manipulating topological currents via structured light, with implications for lightwave electronics and ultrafast quantum information processing.
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All authors Roshan Kumar Thakur, Prachi Venkat, Amar Bharti, Gopal Dixit

Mesoscale and Nanoscale Physics

One-dimensional quasicrystals with tensor-network finite-state automata

Milla Kolehmainen, Jose L. Lado and Anouar Moustaj

Quasicrystals occupy a distinctive position between the translational order of crystals and the disordered amorphous matter. Simulating this physics has remained challenging, since quasiperiodic structures lack the translational symmetry exploited for crystals and generally require costly diagonalization of large finite approximants. Quasicrystalline order admits two equivalent descriptions, a cut-and-project scheme from a higher-dimensional periodic crystal, and a discrete set of substitution rules acting on a finite alphabet. We show that the latter, written in a numeration system adapted to the substitution, defines a deterministic finite automaton with output, the digits of a site index are fed, and the automaton returns the letter occupying that site. We further exploit another equivalence to a different construction, the transition matrices are exactly the tensors of a matrix product state, whose bond dimension is the number of automaton states and is independent of system size. This allows efficient representation of extremely large tight-binding Hamiltonians in the tensor-train language, thereby yielding an exact matrix product operator for the quasicrystal Hamiltonian at any system size. We show how this framework works for two families of one-dimensional quasicrystals, the metallic-mean and $k$-bonacci families and we explicitly construct the Fibonacci, silver-mean, and Tribonacci quasicrystals. By leveraging efficient tensor-network compression and the kernel polynomial method, we compute spectral densities for chains with more than $10^9$ sites and directly resolve the hierarchical structure of the spectrum.
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Mesoscale and Nanoscale Physics

Static and Ensemble-Dependent Thermodynamics of the Strain-Induced Parity Anomaly in Gapped Graphene

Ara Sedrakyan and Klaus Ziegler

A static deformation of graphene can act on its Dirac electrons as a valley-odd magnetic field. In sublattice-gapped graphene this field makes the two valleys add in the parity-odd response rather than cancel. We derive the equilibrium thermodynamics of this effect and separate it from the finite-frequency transport response. At fixed electrochemical potential, reversing the pseudomagnetic field removes every nonzero pseudo-Landau level in the continuum theory. The remaining grand-potential difference is fixed by the spectrally asymmetric zeroth level. The static charge response is a thermally broadened plateau confined to the gap and has no metallic $m/|\mu|$ tail. Near a band edge, pseudofield reversal transfers $\mathrm{B}\ln2$ of entropy per unsplit zero-mode state in the low-temperature window. The fixed-$\mu$ heat capacity has two side lobes per edge and a universal peak $0.439229\,D_B k_{\mathrm B}$. We then formulate a definite constant-gate-voltage circuit and show that the measured sheet heat capacity depends on the electrical boundary condition. The full massive-Dirac density of states and an exact finite-field pseudo-Landau-level calculation give the same gate crossover in their common limit. At fixed carrier number, the low-temperature edge value is $-2(\ln2)^2 D_B k_{\mathrm B}$, rather than a node. A finite geometric capacitance gives a continuous and experimentally tunable interpolation. Finally, we give a trace-free triaxial strain geometry, a disorder–interaction window, and realistic calorimetric and quantum-capacitance scales. The field-reversal protocol isolates an equilibrium electromechanical anomaly without a real magnetic field.
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Mesoscale and Nanoscale Physics

Large Nernst effect in chemically derived multilayer graphene at millitesla magnetic fields

Valentin Semkin, Denis Borisenko, Yana Litun et al. (+4 more)

Simple synthesis of graphitic compounds, their high conductivity, and integrability with other materials motivate the effort toward graphite-based thermoelectric generators. At the same time, semimetallic nature of graphite and graphene results in nearly-zero thermopower due to electron-hole compensation. Here, we observe large transverse thermopower in chemically derived multilayer graphene films with strong fluctuations of thickness and carrier density at low magnetic fields $B$. Using the scanning laser-induced heating of macroscopic film, we find that transverse (Nernst) thermoelectric voltage becomes comparable to the longitudinal thermoelectric voltage at the metal-doped graphene contact at $B^*\approx4$ mT and ambient conditions. Estimates of transverse thermopower $S_{xy}$ based on the known laser-induced temperature show that it is as large as $\sim 10$ $\mu$V/K at $B^*$, and raises in a sub-linear fashion to 250 $\mu$V/K at $B\approx315$ mT, the maximum field we reach with centimeter-sized permanent magnet. Extra increase in thermoelectric signal is achieved upon voltage measurement at Hall probes when the dc field lines are co-directional with local Nernst current. Our results show the promise of large-scale multilayer graphene for thermoelectricity generation.
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All authors Valentin Semkin, Denis Borisenko, Yana Litun, Oleg Kononenko, Dmitry Mylnikov, Alexey Bocharov, Dmitry Svintsov

Friday 4 September

Mesoscale and Nanoscale Physics

Spectroscopy With Intelligent Feature Tracking for automated characterization of excited-state structure in semiconductor quantum dot devices

A. R. Huffman, Daniel Schug, Merritt Losert et al. (+6 more)

The characterization of excited-state structure in semiconductor quantum dot (QD) devices is an important component of tuning them for spin-qubit operation. We present Spectroscopy With Intelligent Feature Tracking (SWIFT), a framework that combines machine-learning (ML)-assisted feature identification with physics-informed geometric processing to extract energy-level splittings from pulsed-gate spectroscopy data. SWIFT isolates the relevant spectral features and exploits their characteristic geometry to reduce the two-dimensional spectroscopy analysis to a one-dimensional peak-detection problem. It further combines an ensemble-based confidence metric with sequential accumulation of rapid, low-SNR scans, allowing the inferred spectrum to be reevaluated as experimental evidence accumulates. Using Si/SiGe QD devices, we demonstrate SWIFT both offline and in real time, including automated tracking of QD excited states and lead resonances. Benchmarking on 255 manually labeled scans shows that SWIFT reduces the median splitting error to 0.05 mV from 0.11 mV for a classical baseline, with the largest improvement on lower-quality measurements. These results provide a path toward incorporating excited-state spectroscopy into autonomous QD characterization, tuning, and optimization, which will be essential in large-scale quantum dot devices.
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All authors A. R. Huffman, Daniel Schug, Merritt Losert, Sanghyeok Park, Daniel King, Yuna Chun, Giordano Scappucci, M. A. Eriksson, Justyna P. Zwolak

Mesoscale and Nanoscale Physics

A magneto-mechanical gyroscope with spintronic readout

Andrea Meo, Francesca Garesci, Pedro Bossi Nunez et al. (+10 more)

Gyroscopes are essential elements in navigation, consumer electronics, robotics, and aerospace applications. Most micro electro-mechanical systems (MEMS) implementations rely on capacitive sensing mechanisms, which limit the dimensional scaling to the micrometer scale. In this work, we introduce a MEMS-like two-degree-of-freedom (2-DOF) gyroscope that exploits the rectification functionality of magnetic tunnel junctions (MTJs) as its readout mechanism and as the transducer of the mechanical dynamics. Experimentally characterized MTJs have been used to calibrate and perform an experiment-informed design of the magneto-mechanical model combining micromagnetic theory with 2-DOF mechanical equations. We demonstrated that the output is linear with angular rate, and that the proposed device is able to extract the angular rate in dynamic cases exploiting a homodyne demodulation approach. The results open a path towards a compact, complementary metal-oxide semiconductor (CMOS)-compatible readout pathway that relaxes reliance on tight capacitive gaps and motivates multi-physics designs of the device.
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All authors Andrea Meo, Francesca Garesci, Pedro Bossi Nunez, Maddalena Fiorentino, Teresa Natale, Ludovico Dindelli, Victor Lopez-Dominguez, Banibrato Sinha, Riccardo Tomasello, Francesco Dell'Olio, Pedram Khalili Amiri, Mario Carpentieri, Giovanni Finocchio

Mesoscale and Nanoscale Physics

Gauge-covariant magnetic Bloch sums for general multiorbital Hofstadter models

Hao Shi, Tianyu Qiao, Wangqian Miao et al. (+3 more)

We formulate a unified treatment of the Hofstadter problem for general two-dimensional multiorbital Peierls tight-binding Hamiltonians using a gauge-covariant magnetic Bloch-sum basis. The construction retains the full Bravais geometry, arbitrary intracell orbital positions, and the original hopping table, so lattice geometry, orbital embedding, hopping range, and orbital content can all be handled within the same framework. At rational flux $\Phi/\Phi_0=p/q$, commuting magnetic translations reduce the Peierls Hamiltonian to minimal $qN_{\rm orb}\times qN_{\rm orb}$ blocks. Their dimension depends only on the flux through the primitive cell, even when the fractional orbital coordinates are irrational. Electromagnetic gauge transformations act by unitary conjugation within the construction and do not alter the required magnetic supercell. We derive an explicit sparse matrix in an oblique Landau gauge and establish the associated band counting, spectral redundancy, Chern-number formulation, magnetic spatial constraints, and flux periodicity. Numerical examples include elementary lattices, topological and flat-band models, and a spinful 22-band Wannier Hamiltonian of monolayer $\mathrm{MoS}_2$, demonstrating a direct interface with first-principles electronic-structure calculations. As a complementary representation, we also derive exact generalized Harper equations from the same hopping data and relate them to the finite magnetic-Bloch blocks.
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All authors Hao Shi, Tianyu Qiao, Wangqian Miao, Jin-Tao Jin, Quansheng Wu, Xi Dai

Mesoscale and Nanoscale Physics

Fast universal parametric spin control in an acoustically modulated quantum dot

Mateusz Kuniej and Michał Gawełczyk

Quantum communication, distributed computing, and hybrid architectures rely on nodes enabling coherent control of qubits and coupling to propagating quantum modes. While semiconductor quantum-dot (QD) spins couple to microwave and optical photons, weak interaction with mechanical waves has limited the integration of single-QD spin qubits into on-chip, acoustically coupled hybrid systems. The existing theory of acoustic QD spin control suffers from a limited range of rotation-axis angles, enforcing complex realizations of gate primitives and long gate times, leaving little margin against decoherence from trion decay and quasi-static nuclear-spin noise. We propose parametric control that overcomes these problems. We use far-detuned optical coupling to a trion state to dress and thus mix spin states, combined with acoustic modulation of the optical transition energy. Instead of relying on direct acoustic resonance with the spin splitting that leads to significant bottlenecks, we induce spin rotations parametrically via resonance with the dressed-spin splitting. We thus develop a spin analog of the “swing-up” charge-state excitation. Our scheme provides fast universal qubit control with nearly arbitrary rotation axes. Our ${\sim}$155 ps Pauli-$X$ gate duration is ${\sim}290\times$ faster than in the previous acousto-optical formulation and ${\sim}14\times$ faster than optical Faraday-geometry spin rotation. The parametric scheme naturally enables higher-harmonic processes. Numerical simulations for ${\sim}44$ GHz acoustic driving show average gate fidelity $\ge99.9\%$ even for uncooled nuclear-spin environments of GaAs and InAs QDs for trion lifetime $\gtrsim1.25$ ns. These metrics suggest practically usable control and may introduce a spin-phonon interface with high interaction rates, versatility, and multi-phonon processes, essential for future acoustically coupled hybrid architectures.
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Mesoscale and Nanoscale Physics

First principles calculations of electric-field-driven topological phase transitions in silicene, germanene and stanene

Julián Antonio Villarreal Murúa, Pablo Roura-Bas, Javier Daniel Fuhr et al. (+1 more)

The emergence of two-dimensional topological materials, particularly the group-14 monolayers known as silicene, germanene, and stanene has opened promising pathways for next-generation nanoelectronics and spintronics. Their buckled honeycomb structure and strong spin-orbit coupling allow for bandgap engineering via a perpendicular electric field, leading to topological phase transitions (TPTs) from non-trivial to trivial insulating states. However, precise determination of the critical electric field $E_z^{\text{cr}}$ at which these transitions occur remains challenging, with tight-binding models often underestimating these values. Here, we present a first-principles framework that combines density-functional theory (DFT), maximally localized Wannier functions, and evolution of the Wannier charge centers (WCC) to accurately characterize TPTs in silicene, germanene, and stanene through the $\mathbb{Z}_2$ topological invariant. In contrast to earlier work, at each electric-field strength we run fully self-consistent ab initio simulations to obtain the screened electronic structure, accounting for the material's dielectric response from both electrons and ions. From these converged results we construct a Wannier tight-binding Hamiltonian at each electric field strength, which then enables a gauge-invariant calculation of the $\mathbb{Z}_2$ topological invariant. This methodology yields significantly more accurate numerical predictions of $E_z^{\text{cr}}$, $0.020$ and $0.250$ V/Å for silicene and germanene, respectively. Compared to previous approaches, our framework delivers a marked quantitative improvement for predicting topological phase boundaries, essential for guiding the design of topological field-effect transistors and electrostatically controlled quantum devices based on two-dimensional materials.
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All authors Julián Antonio Villarreal Murúa, Pablo Roura-Bas, Javier Daniel Fuhr, Ricardo Faccio

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

Hawking Radiation in non-Hermitian Microscopic Analogues

Diego F. Munoz-Arboleda, Cristiane Morais Smith and Marcus Stålhammar

Non-Hermitian systems have recently emerged as a convenient platform to realize black-hole physics. Here, we propose a microscopic open quantum system, namely a fermionic tight-binding chain coupled to Markovian reservoirs, which emulates black-hole properties. The resulting quadratic Lindbladian admits an effective non-Hermitian limit that produces a gain and loss lattice model with a non-reciprocal next-nearest-neighbor hopping. The rapidity spectrum of the full Lindbladian forms tilted exceptional cones that define an effective Painlevé-Gullstrand geometry and separates into black-hole and white-hole sectors, while steady-state particle densities and currents retain clear signatures at the horizon positions. We then analyze the effective non-Hermitian Hamiltonian that arise from the microscopic quantum system. The biorthogonal flux identifies the outgoing exterior and interior channels. Finally, we introduce a fermionic Gaussian Nambu extension and formulate Hawking-pair witnesses through frequency-resolved scattering, nonlocal density-density correlations, and anomalous Hawking-partner amplitudes. The resulting correlations display analogue Hawking radiation signatures, while the frequency-resolved Hawking-partner correlations satisfy the fermionic covariance-positivity constraint. These results establish a connection between microscopic open-system dynamics, emergent non-Hermitian geometry, and fermionic many-body probes of analogue Hawking radiation.
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Mesoscale and Nanoscale Physics

Variable Charge State, Magnetic Excitations, and Kondo Effect of Sm/g/Ir(111)

Shixuan Shan, Tamara de Ara, Lina Liu et al. (+7 more)

Using low-temperature scanning tunneling microscopy we investigate the charge state, magnetic excitations, and Kondo features of individual Sm adatoms on graphene/Ir(111). Depending on the number and distance of their neighbors, Sm atoms can be in two discrete charge states. At certain distances, a reversible transition between these two states is induced by the electric field of the STM tip leading to concentric charge rings in the images. Only atoms in one of the two charge states exhibit magnetic excitations in d$I$/d$V$ spectra. Two such excitations are located at 35~meV and 54~meV and related to transitions from the $J = 1/2$ ground state doublet to the first crystal field split $J = 3/2$ multiplet. Together with the intra-atomic exchange excitations at higher energy, these observations indicate that Sm transfers one $6s$ electron to the substrate while it retains its gas-phase $4f$ filling. New for lanthanide adatoms, we observe a Kondo resonance. The Zeeman splitting of the Kondo peak reveals that Sm retains its large gas-phase $g$-factor. Comparison of d$I$/d$V$ spectra to cotunneling theory yields the crystal field acting on the $4f$ shell and, consequently, on the $J = 3/2$ quadruplet, confirms Sm$^+$ as the ground state, and identifies Sm$^{2+}$ as being energetically close, thereby rationalizing our observation of variable charge states.
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All authors Shixuan Shan, Tamara de Ara, Lina Liu, Zhipeng Wang, Marina Pivetta, François Patthey, Tadahiro Komeda, Daria Kývala, Jindřich Kolorenč, Harald Brune

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

A photonic source with half-a-GHz single-photon flux

P. Zahalka, S. Huijser, A. Pancaldi et al. (+10 more)

Advanced optical quantum technologies demands high quality quantum light generation at very high rates. Here, we report on a deterministic single-photon source that simultaneously combines high excitation rates with high system efficiency to reach over 500 MHz of in-fibre single-photon flux. The source delivers optical power of over 100 pW, as is measured with an off-the-shelf powermeter, enabling a simple and direct way of determining the single-photon source fiber efficiency.
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All authors P. Zahalka, S. Huijser, A. Pancaldi, S. Kruger, X. Zhao, Z. Liu, I. Suleiman, R. Jensen, L. Stefan, A. Ludwig, V. Remesh, J. C. Loredo, P. Lodahl

Mesoscale and Nanoscale Physics

Robustness of RKKY interactions across a Weyl node-annihilation transition

João V. F. Alves, Joelson F. Silva and Luis G. G. V. Dias da Silva

Weyl semimetals (WSMs), with their unique topological properties and distinct electronic structure, exhibit intriguing properties when either time-reversal or inversion symmetries are broken. In this work, we consider the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between magnetic impurities in time-reversal symmetry-breaking WSMs. We derive analytical expressions for the full RKKY exchange tensor in arbitrary two-band spinful lattice systems. Our approach reveals both Heisenberg, anisotropic Ising and Dzyaloshinsky-Moriya terms, which can be calculated by energy-integrating real-space Green's functions across the entire Brillouin zone, with the band edge acting as a natural energy cutoff. We apply this framework to study a two-band tight-binding model for a time-reversal symmetry-breaking WSM that interpolates between a Weyl phase with well-separated chiral nodes and a quadratic band-touching semimetal phase. Remarkably, the spatial profile, magnitude, and anisotropic tensor structure of the exchange couplings remain persistent across the node-annihilation transition. This topological robustness reveals that short- and intermediate-range RKKY interactions are mediated by the global, Brillouin-zone-integrated quantum metric of the full valence band rather than being strictly dictated by local low-energy Berry curvature monopoles. These findings demonstrate the necessity of full-band tight-binding formulations when predicting real-space magnetic interactions, providing key insights for electric-field tuning of magnetic anisotropy and constructing realistic models of heavy-fermion and Weyl-Kondo semimetals.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Dynamical Reduction of Two Series Josephson Junctions to a Synthetic High-Transparency Josephson Element

Claudio Guarcello, Sergio Pagano, Carlo Barone et al. (+3 more)

Two conventional Josephson junctions connected in series can reproduce, in the static limit in which the currents through the capacitive and resistive channels are negligible, the current-phase relation of a single effective weak link with tunable transparency. Therefore, the two-junction series can be treated as a single synthetic high-transparency element. Here, we investigate to what extent this mapping remains valid under finite-frequency drive and retaining the junctions' resistive and capacitive terms. The full resistively and capacitively shunted junction equations are compared with an effective synthetic element with tunable transparency that retains the synthetic tunable-transparency current-phase relation together with effective capacitive and dissipative terms, thus reducing the two second order degree of freedom system to a single second order degree of freedom. The resulting single-element dynamics is compared with the complete two-junction system under ac excitation. The agreement is quantified through a normalized root-mean-square error between the full and effective voltage waveforms. A broad low-error region is found at low drive frequency, while pronounced deviations emerge as the drive frequency approaches the relevant plasma-frequency scale and at larger drive amplitudes. The results provide a quantitative dynamical criterion for using the reduced single-element description of a synthetic high-transparency Josephson element in superconducting circuits.
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All authors Claudio Guarcello, Sergio Pagano, Carlo Barone, Alessandro Bruno, A. Mert Bozkurt, Giovanni Filatrella