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Physics Condensed Matter Mesoscale and Nanoscale Physics

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

Sunday 9 August

Mesoscale and Nanoscale Physics

Layer-Locked Chiral Topological Superconductivity

Yijie Mo and Zhongbo Yan

We uncover a universal mechanism for realizing layer-locked topological phases. Guided by it, we investigate the realization of layer-locked chiral topological superconductivity-the superconducting analogue of the quantum anomalous layer Hall effect-in a nonsymmorphic bilayer antiferromagnetic system with s-wave pairing. We identify three distinct gate-tunable topological phases and establish a direct correspondence between the nearly quantized layer-resolved Chern numbers and the layer-locking behavior of chiral Majorana edge states, vortex-core Majorana zero modes, and nearly quantized thermal Hall responses.
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Mesoscale and Nanoscale Physics

Electronic bistability, discontinuous switching and stochasticity in a two-dimensional semiconductor

S. Jana, M. Kravtsov, A. Ermakov et al. (+12 more)

Bistability - two stable electronic states under the same bias - underlies switching and memory, but is usually absent in transistors and must be engineered through material means: doped tunnel junctions, filaments in memristors, or phase transitions. Here we demonstrate a transistor with intrinsic electronic bistability in a single chemically homogeneous crystal. In dual-gated black phosphorus, whose band gap narrows under a perpendicular electric field due to a giant Stark effect, the gates not only modulate carrier density but also reshape the band profile, forming interband tunnel junctions in the channel. Transport across the two-gate parameter space reveals competing conduction regimes - diffusive, two tunnelling channels and Zener breakdown - whose interplay produces negative differential conductance and transconductance, discontinuous switching, and hysteresis with the state set by gate history. Moreover, the switching remains intrinsically stochastic, yet statistically stable within a narrow range of gate voltages, providing an electrically programmable source of randomness. Devices based on this principle should be realisable in other two-dimensional semiconductors, opening a route to next-generation computing architectures in which nonlinearity, switching, memory and stochasticity are integrated within a single electrostatically programmable element.
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All authors S. Jana, M. Kravtsov, A. Ermakov, X. Zhou, A. Kudriashov, L. Elesin, X. Zhou, A. L. Shilov, D. A. Svintsov, T. Taniguchi, K. Watanabe, K. S. Novoselov, A. Avsar, A. Principi, D. A. Bandurin

Mesoscale and Nanoscale Physics

Time-Reversal-Invariant Altermagnetic Acoustic Crystals

Tianzhi Xia, Han-Rong Xia, Jinglin Liu et al. (+3 more)

Altermagnets have emerged as a new class of magnetic materials that combine spin-split electronic bands with zero net magnetization. Extending this paradigm to classical-wave systems has, however, been fundamentally challenging because conventional realizations require broken time-reversal symmetry (TRS). Here, we overcome this limitation by introducing two pseudospin degrees of freedom and constructing a pseudo-time-reversal operator that faithfully reproduces the action of its physical counterpart while preserving actual TRS. Building on this framework, we theoretically propose and experimentally realize the first time-reversal-invariant altermagnetic acoustic crystal. Acoustic measurements directly reveal pseudospin-dependent band splitting–a defining hallmark of altermagnetism–under strictly TRS-preserving conditions. Moreover, the altermagnetic acoustic crystal exhibits sublattice-pseudospin locking, enabling flexible control over acoustic pseudospin splitting and filtering. Our work establishes acoustic crystals as a versatile platform for exploring altermagnetic physics and opens new avenues for spin-inspired wave manipulation in nonmagnetic devices.
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All authors Tianzhi Xia, Han-Rong Xia, Jinglin Liu, Xiying Fan, Zebin Zhu, Zhen Gao

Mesoscale and Nanoscale Physics

Interaction between Rydberg Excitons in Cuprous Oxide Revealed through Resonant Second Harmonic Generation

Andreas Farenbruch, Henje Stolz, Peter Grünwald et al. (+5 more)

We report experimental and theoretical investigations of interacting excitons of the yellow series in cuprous oxide (Cu$_2$O) with principal quantum numbers up to by means of second harmonic generation (SHG). Using picosecond pulsed laser excitation up to 10 GW/cm$^2$ peak intensity we observe a pronounced change of the spectra with increasing pump laser intensity: an energetic shift to lower absolute energies and a spectral broadening. The absolute intensities of the spectral lines scale for low powers with the square of the pump power, but saturates at higher powers. At still higher powers the SHG intensity is actually reduced. To explain these results quantitively, we developed a semi-classical theory of resonant SHG where the process of SHG is fully coherent. The excitons are assumed to be bosons interacting by a distance dependent potential giving rise to both the changes in spectral line shape and the saturation by a $\it{Rydberg}$ blockade. The concomitant measurement of two-photon absorption allows to derive quantitative values for the exciton-exciton interaction. While the results agree in order of magnitude with those calculated by state-of-the art atomic-like van der Waals interaction theory, the scaling with principle quantum number is quite different. As a possible screening by an electron-hole plasma created by three-photon absorption into blue and violet band states could be ruled out, our results point toward fundamental differences between excitons and atoms.
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All authors Andreas Farenbruch, Henje Stolz, Peter Grünwald, Dirk Semkat, Nikita Siverin, Dmitri R. Yakovlev, Dietmar Fröhlich, Manfred Bayer

Mesoscale and Nanoscale Physics

Thermodynamic evidence for interaction-driven first-order topological quantum phase transitions

Surajit Dutta, Nadav Auerbach, Chiho Yoon et al. (+12 more)

Topological quantum phase transitions in non interacting systems occur through continuous gap closing and reopening. In strongly interacting systems, however, competing ordered states have long been predicted to drive first order transitions, although this possibility has remained experimentally unresolved. Recent transport studies of correlated phases in charge neutral rhombohedral graphene were interpreted as evidence for continuous topological transitions. Here, using nanoSQUID on tip magnetometry, we directly image the local orbital magnetization of a spin orbit proximitized rhombohedral graphene quantum anomalous Hall (QAH) state. We provide the first real space visualization of a QAH phase with a record Chern number, reconstruct its local thermodynamic gap, and track the evolution of its magnetization across competing correlated states. Combined with self consistent Hartree Fock calculations, these measurements show that the sequential transitions between the layer antiferromagnetic, QAH, and layer polarized insulating states are first order, accompanied by discontinuous changes in orbital magnetization. Near the phase boundaries, we observe fluctuating magnetic domains, providing direct microscopic evidence of phase coexistence between nearly degenerate competing ordered states. Together, these observations provide the first direct thermodynamic evidence for first order topological quantum phase transitions and establish a microscopic framework for understanding interaction driven topological quantum phase transitions through phase competition and coexistence.
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All authors Surajit Dutta, Nadav Auerbach, Chiho Yoon, Tonghang Han, Matan Uzan, Zhengguang Lu, Niladri-Sekhar Kander, Yaozhang Zhou, Yuri Myasoedov, Martin E. Huber, Kenji Watanabe, Takashi Taniguchi, Long Ju, Fan Zhang, Eli Zeldov

Mesoscale and Nanoscale Physics

High second Chern number induced by long-range hopping in a four-dimensional Dirac model

Zheng-Rong Liu, Xiang Liu, Rui Chen et al. (+1 more)

Four-dimensional (4D) topological systems provide a promising platform for exploring topological phenomena beyond three dimensions. So far, extensive recent studies on 4D topological insulators have focused on the 4D Dirac model, while its second Chern number is restricted to a limited set of values. In this work, we demonstrate that introducing long-range hopping into the 4D Dirac model induces topological phases with high second Chern numbers. Furthermore, we show that the long-range hopping can transform a trivial insulator into a topological insulator with a nonzero second Chern number. Our work establishes long-range hopping as a powerful route for engineering 4D topological states and reveals new possibilities for realizing unconventional topological phases beyond minimal models.
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All authors Zheng-Rong Liu, Xiang Liu, Rui Chen, Bin Zhou

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

High Thermal Conductivity of Back-End-of-Line Compatible Diamond Films

Jinwen Liu, Chufei Cheng, Feifei Tan et al. (+6 more)

Back-end-of-line (BEOL) thermal management requires electrically insulating heat-spreading dielectric that can be integrated within thermal budgets below 400 C. Here, we report polycrystalline diamond films grown directly on Si at a substrate temperature below 400C. Two films with average thickness of 760 and 1000 nm were characterized by Raman spectroscopy, scanning electron microscopy (SEM), and time-domain thermoreflectance (TDTR). Raman spectra show a sharp diamond peak with minor signatures of non-diamond carbon, while SEM reveals lateral growth and large grain size. Temperature dependent TDTR measurements were performed from room temperature to 100C. Sensitivity analysis indicates that the sensitivity of cross-plane thermal conductivity is comparative to the in-plane thermal conductivity. Accordingly, the films were analyzed using an isotropic thermal model by considering the nearly-isotropic grain structure, yielding room temperature effective thermal conductivity of 73 and 86 W m-1 K-1, respectively. These values are about two orders of magnitude higher than those of conventional dielectric materials and demonstrate the potential of diamond films grown at low temperatures as dielectric heat-spreading layers.
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All authors Jinwen Liu, Chufei Cheng, Feifei Tan, Jinquan Zhang, Di Lu, Bing Dai, Jiaqi Zhu, Runsheng Wang, Zhe Cheng

Mesoscale and Nanoscale Physics

Spin-Orbital Hall Nano-Oscillators using PtCr/NiFe

Utkarsh Shashank, Akash Kumar, Daegeun Jo et al. (+11 more)

The orbital Hall effect provides a promising route for generating angular-momentum currents beyond conventional spin Hall physics. PtCr alloys exhibit unusually large current-induced torques, but the contribution of orbital transport and the ability of these torques to sustain coherent nonlinear magnetization dynamics remain unresolved. Here we demonstrate spin-orbital Hall nano-oscillators by exploiting a homogeneous heavy-metal/light-metal alloy in which orbital Hall currents generated by Cr are converted by Pt into spin currents, producing giant spin-orbit torques. Using PtCr/NiFe heterostructures, the effective torque efficiency increases from ~0.14 in Pt/NiFe to ~0.40 in Pt0.38Cr0.62/NiFe despite substantial Pt dilution, enabling coherent auto-oscillations with the threshold current density reduced from ~ 1.07 x 10^12 to ~ 4.4 x 10^11 A m^-2. First-principles calculations show that Cr alloying suppresses the intrinsic spin Hall conductivity while enhancing the orbital Hall conductivity, and reproduce the observed torque enhancement only when orbital transport is included. Our combined experimental and first-principles results show that alloy engineering enables giant spin-orbit torques through an intrinsic orbital-mediated contribution, enabling coherent auto-oscillations without engineered multilayers and establishing a scalable materials platform for low-power nonlinear spintronic and orbitronic devices.
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All authors Utkarsh Shashank, Akash Kumar, Daegeun Jo, Thi Ngoc Anh Nguyen, Jong-Guk Choi, Sambit Ghosh, Michal Strach, Lunjie Zeng, Andrew B. Yankovich, Roman Khymyn, Ahmad A. Awad, Eva Olsson, Peter M. Oppeneer, Johan Åkerman

Mesoscale and Nanoscale Physics

Crystalline Germanium Josephson Junctions

Frederik H. Knudsen, Axel Leblanc, Yiliu Li et al. (+8 more)

Conventional superconducting quantum electronics rely on well-established Josephson junctions made of Al/AlO$_{x}$ where the weak link AlO$_{x}$ is amorphous and is believed to host two-level systems that limit coherence. Crystalline Josephson junctions exhibit atomically ordered interface quality but remain constrained by complex fabrication and intrinsic asymmetry of epitaxial growth. Here, we demonstrate a fully epitaxial approach based on superconductivity in gallium-doped germanium, enabling the realization of Josephson junctions entirely grown in situ by molecular beam epitaxy. These devices feature atomically sharp interfaces and crystalline weak links, resulting in strong Josephson coupling in the ultra-short regime. We observe an unconventional enhancement of the switching current under applied magnetic field, which we attribute to quasiparticle-assisted thermalization processes from the Al contacts. This platform combines structural coherence, fabrication simplicity, and scalability, offering a promising route toward low-disorder, CMOS-compatible superconducting qubits in a merged element transmon architecture.
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All authors Frederik H. Knudsen, Axel Leblanc, Yiliu Li, Patrick J. Strohbeen, Jechiel van Dijk, Logan Kusher, Arunav Bordoloi, Alisa Danilenko, Xiangchao Ma, Salva Salmani-Rezaie, Javad Shabani

Mesoscale and Nanoscale Physics

Quantum-Device Simulation of Optical Decoherence of Hole-Spin Qubits in Self-Assembled Quantum Dots

Jyun-Jie Jiang, Pericles Philippopoulos, Félix Beaudoin et al. (+1 more)

Spin-photon interfaces are essential for communications between distant spin qubits in quantum technologies, but the interband optical excitation can also damp electrically driven hole-spin Rabi oscillations in semiconductor self-assembled quantum dots (SAQDs). We report a device-level modeling workflow that integrates realistic SAQD geometry and multiband electronic-structure analysis with models of electrically driven spin control, interband optical transitions, and open-system dynamics. This workflow enables device-level estimation of Rabi-oscillation damping arising from repeated interband absorption-emission cycles. As an example, for a gated GaAs SAQD subjected to a uniform magnetic field $B_0$ along the growth direction of the SAQD, we predict the Rabi frequency of the hole spin qubit and its damping under external illumination. At $B_0=2$ T, the calculations yield a hole-spin Rabi frequency of 37.3 MHz. When the electrically driven SAQD is illuminated by a broadband LED centered at a wavelength of 790 nm, increasing the optical power from 0.3 to 1.5 mW shortens the Rabi-oscillation decay time from 90.3 to 17.5 ns. Increasing the SAQD height reduces the electron-hole overlap and thus the emission rate, but the resulting redshift moves the interband transitions into stronger spectral overlap with the LED spectrum, thereby increasing the rate of repeated absorption-emission cycles and enhancing photon-induced Rabi-oscillation damping. The results show that geometry, spin-control conditions, and illumination spectrum should be co-optimized in semiconductor spin-photon devices.
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All authors Jyun-Jie Jiang, Pericles Philippopoulos, Félix Beaudoin, Hong Guo

Saturday 8 August

Mesoscale and Nanoscale Physics

Magnetoelastic coupling in stripe-domain states of yttrium iron garnet

Nimisha Arora, Daniel Prestwood, Takashi Kikkawa et al. (+4 more)

We study magnetoelastic coupling in stripe-domain magnetic states of $3\,\mathrm{\mu m}$-thick YIG thin films grown on a GGG substrate. Broadband ferromagnetic resonance reveals low-frequency stripe-domain magnon branches modulated by a field-independent phonon comb with a frequency spacing of $3.5\,\mathrm{MHz}$, matching the value predicted for confined thickness-shear modes of the GGG substrate. Analytical fitting yields coupling rates that vary between $0.33$–$0.54\,\mathrm{MHz}$ and cooperativities of order $10^{-1}$, indicating that the system is in the weak-coupling regime without resolvable avoided-crossing gaps. Magnon-phonon mode-overlap calculations using finite-element simulations show that the weak coupling arises from phase and domain-sign cancellation: the local magnetoelastic coupling is sizable, but more than $99\%$ of the coherent overlap cancels across the stripe texture. Fully coupled simulations further demonstrate phonon-mediated excitation of a remote YIG layer and show that efficient propagating-phonon generation requires spatially asymmetric magnon modes, establishing magnetic texture as a control parameter for magnon–phonon coupling.
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All authors Nimisha Arora, Daniel Prestwood, Takashi Kikkawa, Eiji Saitoh, Jack Gartside, Will Branford, Hidekazu Kurebayashi

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

Quantum Formulation of Chiral Vortical Effect in Weyl Semi-metals

B. Q. Song

The chiral vortical effect (CVE) is the generation of an axial current in a rotating Weyl fermion; its description is presently based on semiclassical frameworks. In this work, we develop a fully quantum formulation for CVE, solving the exact evolution of microscopic spinful wavefunctions, which enables a bottom-up quantitative test of semi-classical theories and postulated distributions $f_{\text{CVE}}$ in different reference frames. Notably, it shows that $f_{\text{CVE}}$ is over a ground-state-free Floquet spectrum, qualitatively distinct from a thermal equilibrium distribution (i.e., fermi form $f_F$), underscoring CVE as a non-equilibrium phenomenon, distinguished from other chiral transports. The $f_F$ only approximately holds when three conditions are simultaneously fulfilled: (1) slow rotation $\omega R/v_F\ll 1$, (2) high chemical potential $\mu/(\hbar v_F R)\gg 1$, (3) isotropic symmetry, where $R$ is the size, $v_F$ is fermi velocity. In these conditions, the theory recovers established semiclassical results, including the current-response coefficients and the magnetization contribution; otherwise, it uncovers quantum phenomena such as “void states", deviation from the semiclassical formula $j_{\text{CVE}} \sim \mu^2$, a $v_F$-independent charge pumping. The theory is based on semimetals, providing more experimentally accessible detection than fundamental Weyl particles.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.str-el

Plasmon modes in quadratic and cubic nodal line semimetals

Wei Li and Jing-Rong Wang

Nodal line semimetals (NLSMs) host distinctive topological properties and unconventional collective excitations. While plasmons in linear NLSMs are well studied, those of higher-order dispersive NLSMs remain poorly understood. We investigate plasmon modes in 3D quadratic and cubic NLSMs within the random phase approximation (RPA), valid for $r_s\ll1$ but breaking down in the small-doping cubic regime where the density of states diverges. Deriving the one-loop polarizability and evaluating its coefficients numerically from the full 3D Lindhard integral, we find distinct carrier-density scaling laws: quadratic NLSMs exhibit $\omega_p\sim n^{1/2}$, while cubic NLSMs show $\omega_p\sim n^{2/3}$ at large doping crossing over to $\omega_p\sim n^{3/4}$ at small doping. The $n^{3/4}$ law is not quantitatively reliable: the diverging density of states drives $r_s>1$ already at $n\sim10^{18}$ cm$^{-3}$, limiting the observable window to at most $10^{18}\lesssim n\lesssim10^{19}$ cm$^{-3}$; below this, beyond-RPA correlations are essential. These scalings originate from the power-law density of states and intraband (Drude) response, and are analogous in phase-space power counting to bilayer and trilayer graphene, respectively. The long-wavelength intraband polarization dominates the plasmon frequency, while interband contributions are subleading (two to three orders of magnitude smaller for $\Omega\ll\mu$). Both systems share a plasmon anisotropy $\Omega_p^z/\Omega_p^\perp\to\sqrt{2}$ in the thin-ring limit, a generic consequence of the torus Fermi-surface geometry. This $\sqrt{2}$ doublet, though observable by HREELS, cannot distinguish quadratic from cubic dispersion; the density scaling exponent is the true distinguishing signature. Our results provide a framework for the collective dynamics of higher-order dispersive NLSMs and suggest experimentally testable signatures accessible by HREELS.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.supr-con

Quantum Metric Induced Critical Current Anomaly in Flat Band Josephson Junctions

Zhong C.F. Li, Yuxuan Deng, Dmitri K. Efetov et al. (+1 more)

In well-established theories of Josephson junctions, the superconducting critical current \( I_\mathrm{c} \) increases as the normal state conductance \( \mathcal{G} \) increases. However, in a recent experiment in twisted bilayer graphene (TBG) based Josephson junctions, unexpectedly, it was observed that the increase of the critical current is accompanied by a decrease of the normal state conductance. We call this phenomenon the critical current anomaly. In this work, we point out that in the TBG-based Josephson junction, due to the suppression of the conventional Josephson current by the flatness of the band and the quantum metric enabled Josephson current (QMJC), the critical current anomaly can occur. The QMJC appears if the quantum metric length is comparable or longer than the junction length. We show that both \( \mathcal{G} \) and \( I_\mathrm{c} \) have the conventional and the quantum metric contributions, and there are parameter regimes in which \( I_\mathrm{c} \) increases even when \( \mathcal{G} \) decreases. We first demonstrate the critical current anomaly by a simple modified Lieb-lattice model both analytically and numerically. The incredible consistency with the experimental results is demonstrated using a realistic six-band model of twisted bilayer graphene. Therefore, we suggest that the critical current anomaly observed in the experiment provide strong evidence of QMJC which were ignored in well-established theories of Josephson junctions.
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All authors Zhong C.F. Li, Yuxuan Deng, Dmitri K. Efetov, K. T. Law

Mesoscale and Nanoscale Physics

Multiphysics Modeling of Thermo-Viscoelastic Damage in Functionally Graded Abradable Coatings with Probabilistic Geometric Tolerance Analysis

Amjad El-Mellouhi, Yassine Adjal, Khaled Dhibi et al. (+1 more)

In aircraft engines, functionally graded abradable coatings are used to control blade-tip clearance, but their durability is governed by effects that are often treated separately in existing models, including temperature-dependent viscoelastic softening, progressive damage, deposition-induced microstructural modulation, and geometric tolerances. This study integrates these effects within a unified multiphysics–probabilistic framework. The solved domain is a local through-thickness coating column driven by prescribed strain and temperature histories that include thermal eigenstrain and coating–substrate expansion mismatch. The results show that periodic property modulation increases end-of-cycle damage relative to the monotonic gradient and concentrates the maximum stress within a localized modulation crest. Propagating symmetric geometric tolerances through the coupled solver produces a strongly right-skewed damage distribution and a higher classification-level exceedance probability for the modulated gradient than for the monotonic gradient. This exceedance behavior cannot be obtained from a deterministic analysis performed at nominal geometry. The implementation is verified through spatial and temporal refinement, closed-form unit tests, nested Monte Carlo convergence with Wilson confidence intervals, and a hold-out-validated polynomial response surface. Numerical verification is clearly distinguished from experimental validation, and the simulated case is presented as a generic benchmark rather than an identified material model. The framework links deposition parameters and tolerance bands to damage-exceedance risk, supporting reliability-oriented screening of graded and multilayer coating systems.
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All authors Amjad El-Mellouhi, Yassine Adjal, Khaled Dhibi, Fedwa El-Mellouhi

Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.dis-nn

Composite-State Localization Beyond the External Landscape in Non-Hermitian Quasicrystals

Tian Zhou, Xue-Bin Wang, Zhongmin Yang et al. (+1 more)

A composite excitation need not inherit the localization behavior of its constituents. We show that an interacting non-Hermitian quasiperiodic ladder realizes a controllable and reversible localization inversion between composite and unbound excitations, where internal configuration, rather than only the external potential, becomes a control parameter for localization. Opposite complex potentials on the two legs cancel at first order for a same-rung pair but act directly on separated particles, allowing extended composite states to persist while the unpaired sector becomes localized. A strong-coupling theory identifies the composite state as an emergent weakly modulated non-Hermitian quasicrystal generated by virtual unpaired configurations. Breaking the potential antisymmetry restores a direct modulation of the composite band and reverses the localization hierarchy. Engineering configuration-space pathways further stabilizes an extended composite band embedded within a localized continuum, the inverse of the conventional bound-state-in-the-continuum scenario. Our results establish internal configuration as a reversible control parameter for localization.
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All authors Tian Zhou, Xue-Bin Wang, Zhongmin Yang, Tao Liu

Mesoscale and Nanoscale Physics

All-electrical Coherent Control of a Single Rare-earth Spin Qubit

Yaowu Liu, Dasom Choi, Stefano Reale et al. (+8 more)

Electrical control of single spin qubits is a major frontier for nanoscale, high-speed, and scalable quantum devices. Yet, extending it to highly shielded rare-earth 4f electrons remains an experimental challenge across solid-state platforms. Here we demonstrate all-electrical coherent control of a single Er electron spin, which is exchange-coupled to a nearby Ti atom. Scanning tunneling microscopy-based electron spin resonance with three-dimensional magnetic-field control enables comprehensive mapping of the resonance and Rabi frequencies, revealing pronounced anisotropies in both the Er g-tensor and the Er-Ti exchange interaction. The electrical modulation of the anisotropic Er-Ti coupling results in an efficient drive of the Er spin, allowing us to achieve near-gigahertz Rabi frequencies - a ten-fold improvement over the present record for rare-earth spin qubits. By establishing anisotropic exchange as a general resource for electrically accessing shielded rare-earth spins, our results open a new route to ultrafast and local control of rare-earth spins in solid-state quantum devices.
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All authors Yaowu Liu, Dasom Choi, Stefano Reale, Jeongmin Oh, Seorhin Choi, Lei Fang, We-hyo Soe, Arzhang Ardavan, Andreas J. Heinrich, Soo-hyon Phark, Fabio Donati

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

Quantum tribology: acceleration-induced Stokes friction and Magnus force in correlated Bose fluids

V. M. Kovalev, A. N. Osipov, O. V. Kibis et al. (+1 more)

The Landau criterion, a cornerstone of quantum fluid dynamics, dictates that dissipation is forbidden for uniform motion below a critical velocity. Yet, the fundamental question of how acceleration reshapes the principles of quantum friction has remained open since Landau and Pitaevskii's seminal works. Here, we establish a theoretical framework for the quantum tribology of non-inertial motion, describing a probe particle undergoing composite translation and rotation within a weakly interacting Bose condensate. Using the nonlinear Gross-Pitaevskii equation, we show that centripetal acceleration fundamentally modifies the energy-momentum constraints on elementary excitations. This leads to a finite drag force in the subsonic regime of the probe particle motion, and a characteristic quantum stick-slip behaviour in the deeply supersonic regime – a direct generalization of the classical Landau-Pitaevskii picture. Beyond this dissipative response, we uncover a fundamentally distinct mechanism: the nonlinearity of the quantum fluid, combined with the broken symmetry of the trajectory, gives rise to a non-dissipative anomalous transverse force. This quantum Magnus-like response, emerging from the second-order density perturbation, performs no work and is rooted in the geometric asymmetry of the dynamically induced flow. Our findings lay the foundation for a universal program in quantum tribology of accelerated motion, establishing a direct and experimentally testable connection among non-inertial dynamics, nonlinear response, and topological symmetry breaking across platforms ranging from ultracold atoms and exciton-polariton condensates to cosmological analog systems.
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All authors V. M. Kovalev, A. N. Osipov, O. V. Kibis, I. G. Savenko

Friday 7 August

Mesoscale and Nanoscale Physics

Probing intermittent polariton vortex dynamics with two-point correlations

R. Cherbunin, A. Liubomirov, P. Kozhevin et al. (+5 more)

Superconducting single-photon detectors with time resolution of 80 ps have been used to study the spatiotemporal dynamics of a trapped bosonic condensate of exciton-polaritons. Both second- and first-order correlation functions are found to exhibit pronounced oscillations in time governed by the dynamics of the polariton condensate in the trap. We have identified the intermittent regime of stochastic transitions between stationary and limit-cycle regimes near the Andronov-Hopf bifurcation manifested in asymmetric distortions of the correlation function. This rich interplay of deterministic and stochastic condensate dynamics is explained theoretically as a manifestation of mutually compensating self-repulsion and reservoir-mediated attraction.
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All authors R. Cherbunin, A. Liubomirov, P. Kozhevin, M. Chukeev, M. Petrov, S. Kavokina, A. Nalitov, A. Kavokin

Mesoscale and Nanoscale Physics

Dynamical quantum phase transitions in a hybrid quantum dot system with superconducting and ferromagnetic leads

Antoni Jankiewicz, Kacper Wrześniewski and Ireneusz Weymann

We theoretically explore the non-equilibrium dynamics of a single quantum dot system coupled to both ferromagnetic and superconducting electrodes. To investigate its time evolution, we utilize the time-dependent numerical renormalization group technique, which captures the system's response to abrupt parameter changes in a fully non-perturbative manner. Our analysis focuses on dynamics following a sudden modification in the couplings to the leads or a shift of the orbital level. In particular, we calculate the time evolution of the induced local superconducting pairing correlations and magnetization. In this context, the relevant energy spectra are examined. Moreover, we study the behavior of the Loschmidt echo and the return function to shed light on the signatures of dynamical quantum phase transitions. The determined dependencies reveal non-trivial competition between relevant correlations, involving superconducting pairing and ferromagnetic-contacted induced exchange field, and deepen our understanding of nanoscale hybrid systems' dynamical behavior.
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Mesoscale and Nanoscale Physics

Ferroelectric-controllable spin-orbit torque in two-dimensional multiferroic heterostructure

Weiyi Pan, Gusthavo M.S. Brizolla and Jaroslav Fabian

Spin-orbit torque (SOT), which enables electrical control of magnetization, plays a crucial role in the development of next-generation spintronic devices. Realizing SOT in two-dimensional van der Waals systems, together with achieving efficient nonvolatile manipulation via ferroelectricity, would be highly beneficial for the implementation of tunable logic devices with enhanced storage density. In this work, based on first-principles calculation and using a multiferroic Fe$_{3}$GeTe$_{2}$/In$_{2}$Se$_{3}$ heterostructure as a representative example, we demonstrate that switching the ferroelectric polarization of the In$_{2}$Se$_{3}$ layer induces a pronounced modification in the magnetization-dependent distribution of torkance within the heterostructure. Specifically, when the magnetization is in the plane, where the torque is maximal, reversing the polarization of In$_{2}$Se$_{3}$ from upward to downward enhances the total torkance to more than 150% of its original value. This substantial variation primarily originates from the polarization-induced modulation of the $z$ component of the time-reversal-odd torkance, which is mainly associated with an approximately 233% change in the atomic-resolved torque contributed from the middle Fe layer in Fe$_{3}$GeTe$_{2}$ layer. Further analysis reveals that the electronic states near $\Gamma$ on the Fermi surface undergo significant reconstruction upon polarization switching, which is responsible for the observed variation in the time-reversal-odd torque. Our results not only provide new insights into the functional potential of van der Waals multiferroic heterostructures, but also offer a viable strategy for achieving electrically tunable SOT, paving the way for future programmable spintronic devices.
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Mesoscale and Nanoscale Physics

Optimally embedded tight binding for reproducing geometry dependent observables

Jonas J Telle and Gunnar F Lange

In tight-binding models, the position operator is reduced to intra-cell orbital positions (embeddings). While accurately reproducing band structures, such models often fail for geometry dependent responses depending on the position operator. To address this, we investigate the role of these embeddings and introduce the general framework of optimally embedded tight binding. Treating the embeddings as geometric tuning parameters to be fixed against a reference response (obtained from ab-initio computation or experiment), we obtain tight-binding models of GaAs and CdS which quantitatively reproduce non-linear optical responses at no cost to band structure accuracy. The optimal embeddings are determined efficiently using position derivatives obtained from decomposing tight-binding observables into a geometry independent and dependent part, and explicit derivatives are provided for key quantities such as the quantum geometric tensor. The decomposition reveals where geometric effects dominate, and we show in both toy models and in the Chern insulator V2O3 how geometry can dramatically alter the local metric trace. Our results highlight that orbital embeddings should be treated as a genuine model parameter which should be explicitly fixed against physical data to get accurate minimal models.
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Mesoscale and Nanoscale Physics

Type-II Mirror Chern Insulator in Altermagnets

Amrita Mukherjee, Pritesh Srivastava, Rahul Verma et al. (+1 more)

Altermagnets with momentum-dependent spin splitting despite zero net magnetization can support unique topological states under broken time-reversal symmetry. We predict a mirror-symmetry-protected topological crystalline insulator with momentum-separated edge modes in a two-dimensional altermagnet. Using a square-octagon lattice model, we show that altermagnetic order generates symmetry-related valley-polarized Dirac nodes, which are gapped by spin-orbit coupling to yield a mirror Chern insulator with $C_{\mathcal{M}}=2$. In contrast to conventional mirror Chern insulators, where the two mirror-protected edge modes cross at the same momentum to form a Dirac cone, altermagnetic spin splitting and valley-selective band inversion separate these edge modes in momentum. We refer to this phase as a type-II mirror Chern insulator. We further propose a PbSe/$\mathrm{V_2Se_2O}$ heterobilayer as a candidate material for realizing this phase through the altermagnetic proximity effect. Our results establish altermagnetism as a route to mirror-protected topological phases with momentum-separated edge modes.
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All authors Amrita Mukherjee, Pritesh Srivastava, Rahul Verma, Bahadur Singh

Mesoscale and Nanoscale Physics

Exciton Alchemy: Chern Excitons from Trivial Bands

Yoonseok Hwang, Henry Davenport and Frank Schindler

Exciton topology is commonly inherited from the topology of the underlying electronic bands. Recent theoretical work, however, has shown that the exciton Chern number can in general receive an additional contribution from the topology of the exciton envelope wave function, allowing, in principle, interaction-induced topological excitons even when the constituent electronic bands are topologically trivial. Here, we provide an explicit realization of this case by constructing a two-dimensional exciton model with topologically trivial conduction and valence bands that nevertheless hosts a Chern exciton diagnosed by inversion symmetry. Starting from a real-space limit of exponentially localized Wannier states for the conduction and valence bands, we identify the essential ingredients responsible for the emergent exciton topology and formulate a simple construction recipe. Our work demonstrates that interactions alone can generate nontrivial exciton topology, independent of the topology of the underlying electronic bands, and establishes a general framework for designing interaction-induced topological excitons.
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Mesoscale and Nanoscale Physics · Cross-listed from cond-mat.quant-gas

Crossing over universal scaling laws in two-dimensional driven dissipative condensates

Q. Fontaine, F. Helluin, M. Escalera et al. (+8 more)

In low dimensional systems, fluctuations are enhanced and prevent the spontaneous breaking of continuous symmetries. As a result, spatial and temporal correlation functions decay at large distances and long times. A well established example is given by two-dimensional bosonic condensates at equilibrium, which do not display long-range order of the coherence but algebraic decay belonging to the Berezinski-Kosterlitz-Thouless universality class. In contrast, the universal behaviors of non-equilibrium bosonic condensates are more diverse and many open questions remain. Here, we explore the spatio-temporal coherence properties of two-dimensional driven-dissipative polariton condensates in semiconductor optical microcavities. By tuning microscopic parameters, we observe a cross-over between two scaling laws that we attribute to the Edwards-Wilkinson (EW) and the Kardar-Parisi-Zhang (KPZ) universality classes. We demonstrate the collapse of the measured first-order correlations onto the EW and KPZ universal scaling functions and obtain critical exponents, well matching the values predicted theoretically. Our results highlight the intrinsic non-equilibrium nature of polariton condensates and establish them as a platform of choice for controlled exploration of the two-dimensional KPZ universality class.
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All authors Q. Fontaine, F. Helluin, M. Escalera, D. Pinto Dias, A. Lemaître, M. Morassi, M. Wouters, A. Minguzzi, L. Canet, S. Ravets, J. Bloch

Mesoscale and Nanoscale Physics

Perfect absorption by metal-contacted two-dimensional systems with ultra-proximate reflectors

Kirill Kapralov, Vladislav Atlasov, Alina Khisameeva et al. (+2 more)

Electromagnetic absorbance by most two-dimensional electron systems is typically well below unity, which hinders both practical applications in photodetection and fundamental studies of their optical properties. Here, we show that a periodic structure comprised of narrow two-dimensional sections connected with wide perfectly conducting metal sections enables large absorbance. It reaches 50 % provided the filling factor by the two-dimensional system $f$ equals its dimensionless conductivity $\eta=\sigma Z_0/2$, where $Z_0$ is the free-space impedance. The absorbance is further raised to 100 % if the periodic structure is placed above a perfectly conducting electromagnetic reflector, and provided $f=2\eta$. Surprisingly, the optimal distance between two-dimensional system and reflector may fall well below the quarter of incident wavelength $\lambda_0/4$, which was assumed as conventional absorption enhancement condition in optics. For low filling factors $f\ll1$, large dielectric constants of the substrate, and grating periods comparable with $\lambda_0$, the optimal distance to reflector tends to zero. Above the critical values of the grating geometrical parameters, the absorbance maximum ceases to exist. The critical behavior manifests as a large-amplitude resonance in 'dirty' two-dimensional system with purely real conductivity, while enhancement of carrier momentum relaxation time lowers the resonant peak. Such resonance mimics the plasmonic one, but does not rely on high electron mobility.
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All authors Kirill Kapralov, Vladislav Atlasov, Alina Khisameeva, Viacheslav Muravev, Dmitry Svintsov

Mesoscale and Nanoscale Physics

Topological surface altermagnets in SSH-stacked magnetic layers

Rui Chen, Bin Zhou and Dong-Hui Xu

Surface altermagnetism opens new avenues in spintronics by unlocking altermagnetic spin-splitting at the boundaries of conventional antiferromagnets, bypassing the strict symmetry requirements of bulk altermagnets. In this work, we propose creating topological surface altermagnet by stacking magnetic layers in a Su-Schrieffer-Heeger pattern. We show that while the bulk of the system is a standard antiferromagnet with degenerate bands protected by $PT$ symmetry, breaking the local symmetry at the boundary gives rise to a topologically protected surface altermagnetic state residing within the topological gap. Furthermore, we propose that this effect can be experimentally detected by applying a perpendicular electric field. Besides, this approach can be readily generalized to surface altermagnetism of different types. Our work establishes topological boundaries as a natural platform for surface altermagnetism, offering a distinct route for realizing and manipulating topological surface altermagnets.
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Mesoscale and Nanoscale Physics

Magneto-optical signal from $\mathrm{Co_2Mn}$-based Heusler thin films in MOKE and BLS

Anna Maria Friedel, Nicolas Fermon, Tobias Böttcher et al. (+3 more)

$\mathrm{Co_2Mn}$-based Heusler compounds offer a versatile, composition-tunable platform for magnonics and spintronics. Among them, the half-metallic $\mathrm{Co_2MnSi}$ is of particular interest for magnonics owing to its ultralow Gilbert damping, yet its weak magneto-optical response in the visible challenges optical probing such as Brillouin light scattering (BLS). We study the magneto-optical response of epitaxial $\mathrm{Co_2Mn}X$ films ($X = \{\mathrm{Al}_x\mathrm{Si}_{1-x}, \mathrm{Ga}_x\mathrm{Ge}_{1-x}, \mathrm{Sn}\}$) by magneto-optical Kerr effect (MOKE) spectroscopy and BLS. Angle-resolved MOKE resolves a significant, wavelength-dependent quadratic MOKE (QMOKE) only for $\mathrm{Co_2MnSi}$, whereas $\mathrm{Co_2MnAl}$, $\mathrm{Co_2MnGa}$ and $\mathrm{Co_2MnSn}$ respond dominantly linearly. Comparing BLS intensities of thermal magnons at two wavelengths, $\mathrm{Co_2MnSi}$ gives the weakest signal at 532 nm yet among the strongest at 457 nm, tracking the spectral dependence of its Kerr angle. These results emphasise the relation between the two magneto-optical techniques, guiding the choice of probing wavelength for $\mathrm{Co_2Mn}$-based Heusler compounds.
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All authors Anna Maria Friedel, Nicolas Fermon, Tobias Böttcher, Sébastien Petit-Watelot, Stéphane Andrieu, Philipp Pirro

Mesoscale and Nanoscale Physics

Out-of-equilibrium spin-valley dynamics of ferromagnets in topological Chern bands

J. James, I. Krastilevskiy, F. Pichler et al. (+8 more)

Understanding quantum matter far from equilibrium is a central goal of modern physics. Twisted MoTe2 bilayers constitute a promising platform for exploring this frontier by combining strong Coulomb interactions, nontrivial band geometry, and optical control. Here, we exploit this setting to investigate the role of topology and many-body correlations in the out-of-equilibrium dynamics of ferromagnets in Chern bands. Using a focused circularly polarized light pulse, we create a local magnetic domain oriented opposite to an external magnetic field and directly image its subsequent spin-valley relaxation in spatially and time-resolved low-temperature experiments. We demonstrate that in the vicinity of both integer and fractional Chern insulating states, the dynamics is governed by qualitatively different mechanisms than in ferromagnetic metals. Whereas metallic domains collapse by shrinking, Chern domains melt via thermal activation, resulting in drastically different temporal spin evolution and orders-of-magnitude longer relaxation times. These findings demonstrate the influence of topology and strong correlations on far-from-equilibrium collective spin phases, opening new opportunities for dynamical control of ferromagnets in the quantum Hall regime.
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All authors J. James, I. Krastilevskiy, F. Pichler, L. Wang, A. Iafarova, F. Menzel, K. Watanabe, T. Taniguchi, C. Kuhlenkamp, M. Knap, T. Smoleński

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

Multimode phonon-mediated enhancement of entanglement and competing synchronization in cavity magnomechanics

Z.Imara, Jia-Xin Peng, E.K. Berinyuy et al. (+2 more)

The generation of quantum correlations in hybrid quantum systems remains a central challenge due to the intrinsic limitations of linear interactions. In cavity magnomechanical platforms, the cavity-magnon coupling gives rise to hybridized cavity-magnon polaritons (CMPs). However, as a beam-splitter-type interaction, it does not by itself generate entanglement between the polariton modes in the absence of additional nonlinear or parametric processes. Here, we propose a mechanism based on multimode phonon mediation, in which multiple vibrational modes act as parallel scattering channels that couple the polaritons through Stokes and anti-Stokes processes. We show that , in the parameter regime explored here, the presence of multiple phonon modes leads to a monotonic enhancement of steady-state entanglement, thereby going beyond the limitations of conventional single-mode schemes. Furthermore, we demonstrate that quantum synchronization between the polariton modes originates from the same underlying scattering processes responsible for entanglement generation, yet exhibits an opposite scaling behavior with increasing phonon number for the phase quadrature, while the amplitude synchronization reveals collective squeezing that grows with the number of phonon channels. Our results provide new insights into the role of multimode interactions in shaping quantum correlations and establish a viable pathway for controlling entanglement and collective dynamics in hybrid magnomechanical platforms.
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All authors Z.Imara, Jia-Xin Peng, E.K. Berinyuy, S.K. Singh, A. El Allati