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Physics Condensed Matter

cond-mat Watch Condensed Matter

Saturday 27 June

Mesoscale and Nanoscale Physics

Interplay of Electrode Coupling Engineering, Quasiperiodicity, and Magnetic Flux in Quantum Transport through a Su-Schrieffer-Heeger Ring

Sridhar, Souvik Roy and Malay Bandyopadhyay

We reveal that engineering electrode-coupling configurations can fundamentally reshape coherent transport phenomena in quasiperiodic quantum systems. Leveraging nonequilibrium Green's function theory, we systematically analyze charge and heat transport, as well as current fluctuations, in a magnetic-flux-threaded quasiperiodic Su-Schrieffer-Heeger ring with both symmetric and asymmetric multi-site reservoir couplings. Contrary to the conventional expectation that optimal transport is achieved near the homogeneous-hopping limit, our results reveal that multi-site lead coupling fundamentally reshapes the transport landscape, extending the regime of enhanced transport deep into the topological phase. Strikingly, asymmetric source-drain coupling induces a disorder-assisted conducting phase where quasiperiodic modulation enhances, rather than suppresses, charge and energy transport. Magnetic flux exerts a dual influence: it activates additional interference-mediated transmission channels that amplify transport while simultaneously suppressing the disorder-induced re-entrant conducting regime. Furthermore, we uncover a flux-driven migration of the optimal transport window with increasing disorder strength, shifting from the topological regime toward the trivial-hopping regime. This behavior highlights the intricate interplay among quasiperiodicity, dimerization, magnetic-flux-induced quantum interference, and the geometry of the system-reservoir coupling. Collectively, our findings position coupling engineering as a powerful paradigm for the rational control of nonequilibrium transport in quasiperiodic materials and chart a route toward quantum device configurations in which transport characteristics can be precisely tuned via the interplay of disorder, topology, and quantum interference.
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Strongly Correlated Electrons

Survival of the metallic state in a single-hole multiband $p$-orbital molecular system

Keisuke Matsui, Ryan A. Klein, Naoya Yoshikane et al. (+14 more)

Strong correlations and ferromagnetic Hund's coupling lead to diverse electronic phenomena in transition-metal oxides that sensitively depend on the $d$-orbital electron filling. Fullerides, their $p$-electron counterparts, exhibit effective antiferromagnetic Hund's coupling in a different energy range. At half-filling ($n=3$, three electrons in triply degenerate orbitals), both $d-$ and $p$-electron systems are Mott insulators due to strong correlations and Hund's coupling. Away from half-filling, in single-electron/hole ($n=1,5$) $d$-orbital systems, Hund's coupling opposes the correlations, reducing the Mott gap and allowing survival of metallicity. Here we report a single-hole multiorbital correlated $p$-electron system, orthorhombic-structured Yb$_2$CsC$_{60}$ comprising pentavalent C$_{60}^{5-}$ anions, which also exhibits a robust metallic state with no Mott transition, just like in the metastable single-electron cubic-structured CsC$_{60}$. We assert that particle-hole symmetry holds well in ($n=1,5$) fullerides and that their $p$-electron-derived states are analogous to those in $d$-orbital solids, providing impetus for further study of these correlated systems.
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All authors Keisuke Matsui, Ryan A. Klein, Naoya Yoshikane, John Arvanitidis, Matjaž Gomilšek, Urh Klopčič, Shogo Kawaguchi, Hitoshi Yamaoka, Nozomu Hiraoka, Hirofumi Ishii, Qiang Zhang, Shigeo Mori, Hiroki Ishibashi, Yoshiki Kubota, Craig M. Brown, Denis Arčon, Kosmas Prassides

Mesoscale and Nanoscale Physics

Topological phase and its effective tuning in a ladder lattice

Qi-Bo Zeng

We study a two-leg ladder model consists of a one-dimensional (1D) Su-Schrieffer-Heeger (SSH) lattice with staggered nearest-neighboring hopping amplitudes and a normal 1D tight-binding lattice with uniform hopping. By varying the strength of inter-leg coupling, we find that topologically nontrivial phase with zero-energy edge modes will emerge, even when the SSH leg is in the trivial regime. Compared with the single SSH model, the nontrivial region in the parameter space is significantly expanded in the ladder. The topological phase is characterized by quantized Berry phase, and the phase boundaries are determined analytically. We also analyze the distributions of topological zero modes in the ladder, and find that the nontrivial regime can be further divided into two regions, which are separated by a gap closing point in the energy spectrum and correspond to the cases with edge modes residing in different legs. These results indicate that the topological phase and edge modes can be effectively tuned through the manipulations in the trivial lattice. Our work unveils the emergence of nontrivial topology in the ladder lattices and provides a new platform for studying topological phases.
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Mesoscale and Nanoscale Physics

Inter-band coherence effects in disordered crystals: beyond the non-crossing approximation

Zhanning Wang, James H. Cullen, Roberto Raimondi et al. (+1 more)

We develop a quantum kinetic theory for Bloch electrons driven by a uniform dc electric field, extending the nonequilibrium density-matrix formalism beyond the non-crossing approximation. This extension is required to capture steady-state terms that are nominally zeroth order in disorder strength and compete with intrinsic band-geometric responses, as in anomalous Hall and related spin, orbital, and valley transport. Working in the length gauge with Gaussian white-noise disorder, we include impurity scattering to fourth order in the disorder potential. An iterative solution for the impurity-induced density-matrix fluctuations yields a connected $V^4$ collision integral after subtracting disconnected impurity pairings, thereby avoiding double counting. The resulting terms separate into self-energy corrections, ladder-type vertex renormalization, and crossed quantum-interference contributions. We clarify the correspondence between this density-matrix kinetic equation and the Keldysh formalism, and decompose the response into Fermi-surface and Fermi-sea components. As an application, we study the two-dimensional massive Dirac fermion model. We obtain analytical expressions for the single-particle lifetime, transport relaxation time, and longitudinal conductivity at the Born level, and then evaluate the anomalous Hall conductivity including crossed impurity processes. These processes generate an extrinsic contribution of order $\tau^0$ that coexists with the intrinsic Berry-curvature term; for Gaussian white-noise disorder in this model, the $\Psi$-type contribution cancels while the $X$-type term remains finite. The formalism provides a consistent route for incorporating band geometry and crossed-disorder corrections into multiband transport, with applications to spin, pseudospin, orbital, and valley phenomena.
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All authors Zhanning Wang, James H. Cullen, Roberto Raimondi, Dimitrie Culcer

Superconductivity

Hydrostatic pressure effect on the pseudogap in Y0.77Pr0.23Ba2Cu3O7-\delta single crystals

A. L. Solovjov, L. V. Bludova, A. S. Kolesnik et al. (+6 more)

We studied the changes of resistivity $\rho(T)$, superconducting transition temperature $T_c$, fluctuation conductivity $\sigma'(T)$, and pseudogap $\Delta^*(T)$ of $Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta}$ (YPrBCO) ($x = 0.23$) single crystal under hydrostatic pressure (HP) up to $\sim 1.1$ GPa. Defects created by magnetic inclusions of $PrBa_2Cu_3O_{7-\delta}$ (PrBCO) were found to play a significant role in the behavior of the sample. The largest decrease in $\rho(T)$ was found depending on HP at a rate of $d\ln\rho(100~K)/dP = -(29 \pm 0.2) \% \cdot GPa^{-1}$. This indicates that the mechanisms of the influence of HP on the $\rho(T)$ and $T_c$ of YPrBCO and $YBa_2Cu_3O_{7-\delta}$ single crystals are different. From the analysis of the pseudogap, it was revealed that the mechanism of the interaction of charge carriers with magnetic PrBCO impurities changes three times with increasing HP. Relatively low HP, up to $\sim 0.5$ GPa, promotes the formation of defects caused by PrBCO magnetic inclusions. Starting from 0.6 GPa, the HP neutralizes the influence of magnetic impurities, and the magnetic maximum on $\Delta^*(T)$ disappears. Above $\sim 1.0$ GPa, HP aligns the magnetic moments of PrBCO, and the magnetic maximum reappears on $\Delta^*(T)$, more pronounced than at $P = 0$.
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All authors A. L. Solovjov, L. V. Bludova, A. S. Kolesnik, E. V. Petrenko, M. V. Shytov, A. Sedda, E. Lähderanta, D. Sergeyev, R. V. Vovk

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

A unified framework for determining transition dipole polarization in solid-state spin defects

Gaia Da Prato, Yong Yu, Wolfgang Tittel et al. (+1 more)

Spin-photon interfaces based on solid-state defects are key building blocks for scalable quantum networks and hybrid quantum platforms. Optimizing light-matter coupling in these systems requires precise knowledge of the optical transition dipole polarization, yet for many promising quantum emitters this quantity is hard to determine and therefore remains poorly characterized. Here, we develop a framework for reconstructing electric transition dipole polarization in spin-1/2 solid-state defects directly from ensemble spectroscopy. The approach combines the response of photoluminescence spectra to magnetic field, optical polarization, and strain. Applied to erbium ions in silicon, a particularly challenging system containing multiple crystallographic subsites, the framework identifies strain-induced shifts as the origin of asymmetric ensemble spectra and enables simultaneous determination of the optical dipole polarization and strain-orbital coupling tensor. The resulting model predicts how cavity-ion coupling depends on crystallographic orientation and magnetic-field direction, which we verify using single erbium ions coupled to a nanophotonic cavity. Together, these results establish a broadly applicable route for extracting microscopic properties of solid-state quantum emitters from ensemble spectroscopy and for engineering optimized spin-photon and spin-phonon interfaces.
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All authors Gaia Da Prato, Yong Yu, Wolfgang Tittel, Simon Gröblacher

Materials Science · Cross-listed from quant-ph

Properties of 2D Electron or Hole Gases at Tailored s-Si/SiGe Interfaces: A First-Principles Investigation

Garima Ahuja, Farha S, Tanuja Shridhar Joshi et al. (+2 more)

We have performed first-principles hybrid density functional theory calculations to study the formation and properties of two-dimensional electron or hole gases (2DEG or 2DHG) at s-Si/SiGe interfaces. For small Ge concentrations $x < 0.25$, we find a novel type of band alignment with no offset in the conduction bands, implying that a 2DEG cannot be formed, though a 2DHG can. In contrast, for $x > 0.25$ the band alignment suggests that either a 2DEG or 2DHG can be formed. The electronic band structure features two nearly degenerate 2DEG states at the bottom of the conduction bands, and two 2DHG states at the top of the valence band. These states can be accessed by appropriate doping and gating. Charge density plots of these states show that they feature carriers confined to the near vicinity (2–3 atomic layers) of the interface. Calculated effective masses are anisotropic, being markedly so for the 2DHG states, and in excellent agreement with experiment. This property can be exploited to create a 1D carrier gas. Our results are especially important for s-Si/SiGe-based semiconducting spin qubits for quantum computing applications.
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All authors Garima Ahuja, Farha S, Tanuja Shridhar Joshi, Anil Shaji, Shobhana Narasimhan

Mesoscale and Nanoscale Physics

Multi-level {\pi}-junction in a proximitized Ge/SiGe quantum dot probed by an on-chip superconducting microwave resonator

Luigi Ruggiero, Vera Jo Weibel, Pauline Drexler et al. (+5 more)

Using on-chip microwave measurements, we investigate multilevel $\pi$-junctions formed by proximitized quantum dot (QD) in a germanium (Ge)/silicon-germanium (SiGe) heterostructure. In the multilevel regime, where several QD orbitals contribute simultaneously to superconducting transport, the Josephson ground state is no longer determined solely by the occupation of a single orbital. By combining DC transport and microwave techniques, we identify the qualitative signatures of multilevel $\pi$-junctions in both their gate-voltage dependence and microwave response. In particular, we observe combinations phase transitions that are sharp or smooth in gate voltage and which exhibit distinct inductive and dissipative signatures. Such multilevel Josephson transport has previously been observed primarily in exceptionally clean systems such as carbon nanotubes. Our results establish proximitized Ge as a platform for investigating hybrid superconductor/semiconductor physics and demonstrate the integration of gate-defined superconducting quantum devices with high-quality on-chip microwave resonators.
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All authors Luigi Ruggiero, Vera Jo Weibel, Pauline Drexler, Carlo Ciaccia, Christian Olsen, Dominique Bougeard, Christian Schönenberger, Andrea Hofmann

Statistical Mechanics · Cross-listed from cs.LG

A Path-Space Formulation of Prediction in World Models: From a Single Action to Prediction, Planning, and Irreversibility

Gunn Kim

We propose a path-space formulation of prediction in AI world models. Rather than sequences of one-step conditional distributions, we argue that a world model implicitly defines a probability measure over future trajectories. In the local regime where latent dynamics admit an effective Markovian description, this path measure takes the Onsager-Machlup form. Within this framework, prediction (most probable trajectory), planning (constrained optimization), and uncertainty (fluctuations) emerge as operations on a single action functional. We decompose the latent dynamics into reversible and irreversible components and introduce operational measures of entropy production from model rollouts. In controlled small-scale attention-based models, we find that attention asymmetry is acquired during training in proportion to the irreversibility of the data. Symmetrizing the learned attention suppresses entropy production and selectively degrades long-horizon prediction of irreversible dynamics while preserving relaxational prediction. These results suggest that irreversibility may serve as a computational resource for predictive world models. More generally, the fundamental predictive object is a distribution over future paths rather than states.
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Strongly Correlated Electrons · Cross-listed from quant-ph

Dissipative phase decision without ground-state preparation

Hao-En Li, Yilun Yang and Lin Lin

We propose a dynamical approach to identifying ground-state quantum phases through short-time dissipative cooling. Rather than determining the phase by preparing highly accurate approximations to ground states, we prepare a representative state of a candidate phase and monitor the early-time response of phase-sensitive observables under cooling dynamics tailored to the target Hamiltonian. For a class of phase-decision problems in which the relevant observables can be inferred from the low-energy manifold, and with jump operators implementable using only short-time Hamiltonian simulation, the dissipative evolution rapidly suppresses high-energy components and drives the system into a low-energy manifold whose observables already reveal the underlying ground-state phase, well before mixing to the steady state. We demonstrate this strategy for the frustrated $J_1$–$J_2$ Heisenberg chain, the Kitaev honeycomb model, and the XXZ chain, including Berezinskii–Kosterlitz–Thouless and topological phase transitions. In particular, coarse filter resolutions and short evolution times suffice to recover phase-sensitive quantities such as the Luttinger parameter and topological diagnostics. We further provide theoretical justification that cooling dynamics with such jump operators can rigorously prepare low-energy manifolds for free-fermionic and free-bosonic systems, and investigate this mechanism for interacting fermionic systems. Our results suggest that phase decision is a plausible target for future utility-scale studies on early fault-tolerant quantum devices.
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Superconductivity

Anomalous Behavior of the Ni$^{1+}$ moment in bi-infinite-layered La$_3$Ni$_2$O$_5$F

Young-Joon Song, W. E. Pickett and K.-W. Lee

The discovery of superconductivity in hole-doped Ni$^{1+}$ systems with “infinite layer" NiO$_2$ square-lattices analogous to the Cu$^{2+}$ CaCuO$_2$ cuprate has renewed conflicting pictures of the Cu$^{2+}$$-$Ni$^{1+}$ similarities and distinctions. Recent synthesis of formal Ni$^{1+}$ La$_3$Ni$_{2}$O$_{5}$F with two infinite NiO$_{2}$ layers per cell provides a novel member of this class. We find that Ni$^{1+}$ in this material is unusually flexible. First principles density functional theory studies reveal a single partially occupied electron band derived from density in three interstitial layers that provides self-doping to a Ni$^{1.09+}$ charge state. The blocking La(O/F)La layer provides isolation of the NiO$_2$ bilayer to strictly two-dimensional electronic and magnetic systems. Fixed spin moment calculations of magnetic tendencies reveal behavior unlike previous nickelates, including an essentially vanishing differential susceptibility up to a large magnetic field and a metastable ferromagnetic (FM) state. Antiferromagnetic (AFM) order is favored energetically as expected; surprisingly both FM and AFM states are obtained without correlation (Hubbard $U$) corrections. Doping away from half-filling by an interstitial density derived band and likely two-dimensional fluctuations account for the lack of observation of any magnetic transition.
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Superconductivity

Distinct Roles of Hydrogen in Superconducting and Ferromagnetic Phases of CoZr$_{2}$H$_{x}$

Yuto Watanabe, Takayoshi Katase, Daisuke Takegami et al. (+1 more)

Hydrogenation offers a versatile route to tuning the physical properties of intermetallic compounds. In this study, we synthesized CoZr$_{2}$H$_{x}$ with different hydrogen contents and found that hydrogen is incorporated in two distinct concentration regimes separated by a wide composition gap: a low-concentration hydrogenated superconducting phase ($x$ = 0-0.054) and a high-concentration hydrogenated ferromagnetic phase ($x$ = 2.786). Hydrogen plays fundamentally different roles in the two concentration regimes. In the high hydrogen concentration phase, the Zr-H interactions substantially modify the metallic bands crossing the Fermi level, leading to the emergence of ferromagnetism. In contrast, in the low hydrogen concentration phase, hydrogen behaves as a nonmagnetic impurity without altering the electronic band structure. Despite the nearly identical Debye temperatures across the low-concentration series, the superconducting transition temperature ($T_{\mathrm{c}}$) is progressively suppressed with increasing hydrogen content.The observed $T_{\mathrm{c}}$ suppression is quantitatively described by the Abrikosov-Gor'kov pair-breaking theory, indicating that the superconducting gap of CoZr$_{2}$ is anisotropic or multigap rather than a fully isotropic $s$-wave symmetry.
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All authors Yuto Watanabe, Takayoshi Katase, Daisuke Takegami, Yoshikazu Mizuguchi

Materials Science

Emergence of half-semiconductor behavior and tunable magnetism via carrier doping in Janus VXSe (X=Cl, Br, I) monolayers

Zhixiang Wang, Aining Wang, Zikui Ye et al. (+5 more)

Two-dimensional ferromagnetic semiconductors with high Curie temperature, large magnetic anisotropy, and electrically tunable properties are highly sought for nanoscale spintronics. Here, using first-principles calculations, we predict a new class of Janus VXSe (X=Cl, Br, I) monolayers. All three compounds are intrinsic ferromagnetic semiconductors with indirect band gaps of 1.66-2.33 eV, Curie temperatures up to 128 K, and magnetic anisotropy energies up to 910 ueV per V, leading to easy-plane magnetization for VClSe and VBrSe and an out-of-plane easy axis for VISe. The robust ferromagnetic order originates from the competition between superexchange and itinerant magnetism. Remarkably, VISe is a pristine half-semiconductor, whereas carrier doping unlocks fully spin-polarized states in the initially non-ideal VClSe and VBrSe. We also find that hole doping can switch the magnetic easy axis of VBrSe from in-plane to out-of-plane, enabling a spin-field-effect transistor with giant magnetoresistance. Our findings highlight carrier doping as a key to unlock hidden half-semiconducting behavior and establish Janus VXSe as a promising platform for 2D spintronics and magnetoelectric devices.
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All authors Zhixiang Wang, Aining Wang, Zikui Ye, Yuqiao Qu, Qian Zheng, Yiding Liu, Qiang Fan, Gang Yao

Statistical Mechanics

Forward-backward correspondence between stationary structure and splitting probabilities in active matter

Derek Frydel

Active particles confined by hard walls accumulate at boundaries and may become dynamically adsorbed due to directional persistence. In this work, we show that the same persistence mechanism also gives rise to a finite wall splitting probability, meaning that a particle initialized at a wall can reach the opposite boundary before returning to its starting point. By comparing forward and backward evolution equations directly in position–velocity phase space, we derive exact relations linking stationary distributions and splitting probabilities for run-and-tumble, active Brownian, and active Ornstein–Uhlenbeck particles. In particular, we show that the stationary density is generated by the spatial derivative of the splitting probability, while the distribution of dynamically adsorbed particles at the walls is encoded in wall splitting probabilities. The correspondence is valid in arbitrary spatial dimension and establishes an exact bridge between stationary and first-passage descriptions of confined active matter, revealing them as complementary representations of the same persistence-driven dynamics.
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Materials Science · Cross-listed from quant-ph

High-Fidelity Hole Spin Qubits Reveal Quadrupolar Nuclear-Bath Dynamics in Isotopically Purified Planar Germanium

Jian Zeng, Xiangjun Tan, Hongzhang Wang et al. (+10 more)

Planar Germanium has emerged as a promising platform to build spin-based large scale quantum computers. By exploiting the anisotropic hyperfine interaction of holes in Ge, qubits with long T2* have been recently realized. While the performance of single qubits is still more or less limited by 73Ge nuclear spin fluctuations, the site-to-site variation of qubit sweet spot becomes obstacles to maintaining high fidelity of each qubit across the whole wafer. To achieve high performance Ge-based quantum circuit, it is therefore essential to eliminate the origin source of hyperfine noise. In its Silicon counterparts, reduction of 29Si abundance enables exceptional high-fidelity operation. In contrast, hole qubits based on isotopically purified Ge have not been demonstrated. Here, we report the synthesis of high quality 2-dimensional hole gas (2DHG) with enriched 70GeH4 precursor. Due to the suppression of nonzero spin nucleus, the qubits' T2* on the sweet spot is moderately extended beyond 20 us, surpassing the previous best reported Ge hole qubits. More importantly, the qubits' T2* off the sweet spot is enhanced to above 3 us, enabling single qubit gate fidelity exceeding 99.9% in both operating regimes. Hahn-echo spectroscopy further resolves a finite-frequency nuclear-noise channel that is distinct from the conventional Larmor-linked hyperfine response. We associate this channel with quadrupole-modified dynamics of residual 73Ge nuclei sampling local electric-field gradients near the Ge/SiGe interface. Its field scaling and angle-dependent visibility are consistent with a qubit-visible quadrupolar nuclear-noise component transduced through the anisotropic hyperfine interaction of Ge holes. These results establish isotopically purified planar Ge as a high-coherence scalable platform for hole spin qubits and provide a spectroscopic probe of interfacial quadrupolar nuclear dynamics.
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All authors Jian Zeng, Xiangjun Tan, Hongzhang Wang, Yulei Zhang, Wendong Bian, Lingting Wu, Chenggang Yang, Zhengshan Guo, Jiankun Li, Yongfeng Wang, Jun Lu, Jun-Wei Luo, Tian Pei

Mesoscale and Nanoscale Physics

Direct observation of interfacial exchange coupling in a magnetic tunnel junction through spin-polarized quasiparticle interference

Xu Wang, Chenxi Wang, Ying Yang et al. (+5 more)

Interfacial exchange coupling plays a critical role in enabling novel phenomena in magnetic heterostructures, such as spin triplet superconductivity, quantum anomalous Hall effect (QAHE), and advanced spintronic functionalities. While microscopic characterization of this coupling is essential for elucidating the underlying mechanism, it remains technically challenging. Here, using spin-polarized scanning tunneling microscopy (SP-STM) and quasiparticle interference, we directly observed interfacial exchange coupling in a magnetic tunnel junction formed by an Fe coated tip and a Cr(001) surface. We found the ferromagnetic tip induces significant energy shift (up to 10 meV) in the spin-polarized surface state of Cr(001). This shift is highly sensitive to the tip-surface distance and the spin-alignment between Fe tip and Cr surface, which can be switched by external magnetic field. Our results demonstrate that extended 2D surface states can mediate strong exchange coupling across a heterojunction, enabling local control of interfacial exchange interaction induced phenomena.
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All authors Xu Wang, Chenxi Wang, Ying Yang, Yining Hu, Qingle Zhang, Chen Chen, Donglai Feng, Tong Zhang

Strongly Correlated Electrons

Sixteen-Fold Way for Fermionic Topological Orders

Ryohei Kobayashi, Abhinav Prem and Matthew Yu

Fermionic topological orders can host 't Hooft anomalies with no bosonic counterpart. We identify a new sixteen-fold family of (2+1)D fermionic topological orders, forming a fermionic analogue of Kitaev's sixteen-fold way. This family is distinguished by the mod 16 't Hooft anomaly of a $\mathbb{Z}_2$ one-form symmetry, generated in each theory by a single nontrivial $\mathbb{Z}_2$ anyon. This intrinsically fermionic anomaly permits anyon spins that are forbidden in bosonic phases; the simplest new example is an Abelian fermionic topological order containing a single $\mathbb{Z}_2$ Abelian anyon of spin 1/8. Each theory can be realized as the gapped boundary of a (3+1)D fermionic symmetry-protected topological (SPT) phase protected by the $\mathbb{Z}_2$ one-form symmetry, which acquires a $\mathbb{Z}_{16}$ classification once the spacetime spin structure is twisted by the one-form symmetry. We realize these phases microscopically via lattice models built from Walker-Wang models coupled to local fermions.
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Materials Science

Strain engineering of ultrafast magnetism in the room-temperature vdW ferromagnet Fe3GaTe2

Fan Fei, Yangchen He, Wuzhang Fang et al. (+6 more)

Controlling ultrafast magnetic dynamics is critical to understanding nonequilibrium spin interactions and advancing high-speed spintronics. However, a lack of efficient in situ tuning strategies leaves most ultrafast studies largely dependent on the intrinsic properties of the individual materials. Here we demonstrate continuous strain tuning of both the equilibrium magnetic response and ultrafast demagnetization dynamics in the room-temperature van der Waals ferromagnet Fe3GaTe2. Applying up to 4.2% uniaxial tensile strain increases the coercive field from nearly zero to 100 Oe, consistent with an enhancement of the effective perpendicular magnetic anisotropy. Time-resolved magneto-optical Kerr effect measurements further reveal strain-accelerated ultrafast demagnetization, with 1.2% tensile strain reducing the characteristic demagnetization time by approximately 20%. Remarkably, strain accesses an accelerated demagnetization regime that cannot be reached simply by increasing pump fluence in the unstrained sample. Combined with first-principles calculations, our results resolve that the applied strain modifies the spin-lattice energy transfer, leading to the observed accelerated demagnetization. These findings establish mechanical strain as an effective route for on-demand control of ultrafast magnetic dynamics while reducing the required optical energy by reconfiguring the magnetic energy landscape and associated spin-relaxation pathways.
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All authors Fan Fei, Yangchen He, Wuzhang Fang, Jessica Kienbaum, Jacopo Simoni, Robert Boyd, Yuan Ping, Daniel A. Rhodes, Jun Xiao

Materials Science

Structural relaxation and stagnation of grain boundary during migration

Tingting Yu

Instability is a major bottleneck in nanomaterials due to grain boundary (GB) activities under thermal or mechanical stimuli. The relaxation of GB will stabilize the properties of materials by structure modification of GBs to lower energy states. However, lack of understanding of mechanisms limits the application of GB relaxation. In this study, we certify that GB can realize relaxation through defect emission including vacancy, twinning and dislocations, etc, which lower the average atomic energy of GB. In particular, we found stagnation and shear-coupling migration accompany the relaxation process, where a lower average atomic energy and lower average atomic volume of grain boundaries can be the reasons for stagnation. In this study, we found when simulated by ramped energy-conserving orientated driving force, under a specific driving force, the grain boundary suddenly stops migrating during the migration process, and as the driving force increases to a certain value, the grain boundary continues to migrate. Even at fixed driving forces, the grain boundary migration process can stall. This phenomenon has been found in many grain boundaries, and it has been found that the reason for the stagnation is the change of the average atomic energy of grain boundaries and the average atomic volume of grain boundaries. The discovery of this stagnation phenomenon is helpful to better understand the migration characteristics of grain boundaries and lays a foundation for improving the strength of materials by designing grain boundary microstructures.
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Friday 26 June

Statistical Mechanics

Brownian Bridge for Coherent State Path Integral Monte Carlo

Riccardo Fantoni

We propose a new Brownian bridge construction for our newly devised Coherent States Path Integral Monte Carlo algorithm. We apply it to the numerically exact calculation of the thermodynamic properties of the Helium fluid on a plane at low non zero temperature. We find very good agreement with the conventional plane waves path integral Monte Carlo results.
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Statistical Mechanics

Phase Time Crystals and Pairing in Binary Active Chiral Systems

C. Reichhardt and C.J.O. Reichhardt

We introduce a class of dynamic systems we call phase time crystals consisting of a binary assembly of particles with intermediate or long-range repulsive interactions that are subjected to a circular drive of uniform chirality in which each particle species is out of phase from the other by 180 degrees. As a function of the particle density and orbit radius, this system can organize into a rich variety of dynamical crystalline states, including one in which the out of phase particles form bound pairs that assemble into a triangular lattice. We also find stripe phases, overlapping packed crystals, disordered or phase glass states with no diffusion, mixed fluids, and different types of phase-separated states. We show that these states are robust against the addition of thermal fluctuations, and that the paired crystal can melt into a paired fluid. If the drive on each particle species is of opposite chirality, the system forms stripes and packed lattices, but no paired crystal is present. We demonstrate that by modifying the nature of the chiral driving, it is possible to realize numerous kinds of active molecular lattices, including dynamic square spin ice geometries and higher-order complex structures.
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Mesoscale and Nanoscale Physics

Graphene as a Tunable Nonradiative Bath for Moiré Excitons

Katsunori Wakabayashi

A minimal theory of nonradiative energy transfer from a two-dimensional (2D) moiré exciton to a nearby graphene layer is presented. From Fermi's golden rule the transfer rate is the overlap of the exciton near-field spectrum with the dissipative density response of graphene, weighted by an exciton form factor, and it reproduces the established $\GET\propto z^{-4}$ law in the point-dipole limit. A finite exciton size filters out the high-momentum near field once the spacer thickness approaches the transition-polarization radius $R_X$, so the distance dependence of the rate—and of the photoluminescence (PL) quenching—probes the exciton size. A low-momentum expansion shows that, relative to the calibrated point-dipole response of the same bath, this leading correction is set by $R_X$ alone. In the ideal coherent-envelope limit the accompanying giant oscillator strength makes the rate non-monotonic in the exciton size, with a peak near $\lX\approx z$. Treating graphene as a gate-tunable bath, Pauli blocking suppresses the interband channel once $2|\muF|$ approaches $\hbar\omega$, partially restoring PL, and a full random-phase-approximation benchmark confirms the normalized interband distance dependence to within a few percent away from the threshold. Mapping the PL observables across the transition-metal dichalcogenide/hexagonal boron nitride/graphene parameter space, we find that a graphene gate acts not as a passive electrostatic element but as a tunable 2D electronic reservoir probed through exciton PL quenching.
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Mesoscale and Nanoscale Physics

Voltage-tunable Josephson Junctions on Germanium Quantum Wells with in-situ Aluminum Contacts

Joshua P. Thompson, Jason T. Dong, Bernardo Langa Jr et al. (+12 more)

Voltage-tunable Josephson junctions (VT-JJs) are an emerging element in superconducting quantum electronics with potential to expand the functionality of conventional designs. While VT-JJs are largely compatible with wafer-scale semiconductor processing, their integration into quantum circuits remains a challenge due to unmitigated semiconductor microwave loss. Here, a deep mesa etch process, wherein the epitaxial material is removed except the VT-JJ device, will facilitate the integration of VT-JJs with low-microwave-loss circuit elements by allowing these circuit elements to be placed directly on a low-loss substrate. A Germanium quantum well is grown by Molecular Beam Epitaxy (MBE) on a float zone silicon substrate with in-situ deposited aluminum contacts. This combination allows the formation of an oxide-free superconductor-semiconductor interface. The deep mesa etch process is optimized to produce a sidewall taper sufficient for continuous metal deposition from the substrate to the top of the mesa for electrostatic gate electrodes and interconnects. The fabricated Josephson junctions demonstrate gate-tunable supercurrents with a maximum critical current over 100 nA and critical-current normal-resistance product of $8.63~\mu V$. These results demonstrate a pathway toward improved integration of voltage-tunable superconducting circuit elements with quantum electronic building blocks such as couplers and qubits.
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All authors Joshua P. Thompson, Jason T. Dong, Bernardo Langa Jr, Chomani K. Gaspe, Riss Card, Brycelynn Bailey, Shiva Davari, Bethany E. Matthews, Matthew J. Olszta, Silas Hoffman, Thomas M. Hazard, Kyle Serniak, Hugh O.H. Churchill, Kasra Sardashti, Christopher J.K. Richardson

Strongly Correlated Electrons

Magnetic symmetry implications of the zero- and applied-field Hall effect of UNi$_4$B

Z. W. Riedel, W. S. Simeth, S. M. Thomas et al. (+2 more)

The zero-field and applied-field Hall effects in noncollinear antiferromagnets provide evidence for topological states of matter and are tied to materials' magnetic symmetry. For UNi$_4$B, the antiferromagnetic state with $T_\mathrm{N}=20$ K at zero and low magnetic field is debated due to recent magnetoelectric measurements and theory work calling into question the proposed toroidal arrangement of magnetic dipole moments. For a magnetic field applied within the plane of uranium magnetic moments, the field-dependent Hall resistivity of UNi$_4$B shows a curved response for $\rho_{yz}$ ($H{\parallel}x$, $I{\parallel}z$) and $\rho_{zx}$ ($H{\parallel}y$, $I{\parallel}x$) up to $\sim$8 T at 2 K, while an out-of-plane field results in linear behavior of $\rho_{yx}$ ($H{\parallel}z$, $I{\parallel}x$) up to 16 T. Analysis using conventional empirical relationships for the Hall effect indicate that an intrinsic effect from momentum-space Berry curvature contributes significantly to the curved transverse resistivity. Moreover, a finite zero-field Hall effect emerges at the onset of magnetic order for $\rho_{yz}$ and $\rho_{zx}$, further supporting an intrinsic origin of the Hall response. Symmetry arguments for a finite Berry curvature, an observable magnetoelectric effect, and reported magnetic structures suggest that the previously proposed magnetic space groups for the zero-field magnetic structure cannot account for the observed finite zero-field effect for two Hall orientations. Instead, we propose that $Cm'$ or $Pm'$ magnetic symmetry, depending on the parent nonmagnetic space group, is consistent with Hall resistivity, neutron diffraction, and magnetoelectric effect measurements.
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All authors Z. W. Riedel, W. S. Simeth, S. M. Thomas, F. Ronning, E. D. Bauer

Materials Science

Surrogate-Gated Generation and Foundation-Model Embeddings for Bayesian Materials Design

Sk Md Ahnaf Akif Alvi, Jan Janssen, Danny Perez et al. (+2 more)

Closed-loop materials discovery iterates between proposing candidate structures and evaluating their properties, and property evaluation dominates the cost. In the generative variant, a learned prior proposes candidate crystals and a property oracle scores them; we ask whether a cheap probabilistic surrogate can triage the generator's output, and what such a surrogate must do well. Across three architecturally distinct pretrained diffusion priors (MatterGen, CrystalFlow, ADiT) and two targets (room-temperature heat capacity and bulk modulus), we insert a Gaussian process acquisition gate between structure generation and the oracle in an RL-steered generative workflow. The gate matches or exceeds ungated fine-tuning of the generative model while capping oracle calls at a fixed per-cycle budget. Budget-matched ablations isolate the mechanism. At an identical four-call budget, ranking-based selection outperforms arbitrary selection, confirming that the gain comes from the surrogate's choice; the gate comes within $\sim$9% of exhaustive oracle spending at roughly one-fifth of the calls. A density-functional-theory check of the bulk-modulus discoveries confirms the learned oracle to within 2.5% on average and the surrogate's ranking of the generated structures at Spearman $\rho = 0.94$. A cross-factorial benchmark of surrogate performance spanning mechanical, electronic, and vibrational properties identifies pretrained ORB embeddings with a Gaussian process as the most reliable combination, which we adopt as the building blocks of the proposed workflow. The complete pipeline is released as open-source software.
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All authors Sk Md Ahnaf Akif Alvi, Jan Janssen, Danny Perez, Douglas Allaire, Raymundo Arroyave

Mesoscale and Nanoscale Physics

TetMaG-Guided Design and Operando Electron Holography Validation of Current-Induced Domain-Wall Motion in 3D Curved and Cornered Fe Nanobridges

Sameh Okasha, Shriyar Tariq, Attila Kákay et al. (+4 more)

Three-dimensional (3D) magnetic nanostructures offer new opportunities for controlling domain-wall (DW) configurations beyond the limitations of planar systems, providing promising architectures. However, the realization of reliable 3D magnetic devices requires precise control of geometry-dependent DW behaviour and quantitative experimental validation of the resulting magnetic states. Here, we combine TetMaG micromagnetic simulations, focused electron beam induced deposition (FEBID), and off-axis electron holography to investigate the influence of curvature and corner geometries on DW behaviour in 3D magnetic nanobridges. TetMaG simulations predict fundamentally different magnetic properties for curved and cornered geometries. Cornered nanobridges act as preferential DW pinning sites, stabilizing localized magnetic configurations and enabling controlled switching between neighbouring pinning positions. While curved nanobridges promote gradual magnetization rotation, reduced pinning, and smoother DW motion. These optimized geometries were fabricated with high structural fidelity using FEBID and subsequently characterized by quantitative electron holography. Electron holography measurements revealed magnetic induction maps that matched the simulated magnetization configurations, providing direct experimental validation of the TetMaG predictions. Curved and cornered geometries exhibited distinct DW characteristics governed by their local structural features, demonstrating the critical role of geometry in tailoring magnetic behaviour in 3D systems. Operando current-biasing experiments further revealed current-induced DW motion, including the displacement of a tail-to-tail DW into a head-to-tail configuration within corner structures.
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All authors Sameh Okasha, Shriyar Tariq, Attila Kákay, Ryan Yang, Gregor Hlawacek, Akhil G. Nair, Rafal Dunin-Borkowski

Statistical Mechanics

Ultrafast directed transport via energy recuperation in non-Markovian systems

Mateusz WIśniewski and Jakub Spiechowicz

A recent pioneering experiment [Nat. Commun. 16, 10114 (2025)] demonstrated that a driven overdamped colloidal particle in a harmonic trap immersed in a viscoelastic fluid can recuperate energy dissipated into the surrounding bath and convert it into useful work. In this article we considerably extend the original predictions. In particular, we show that energy recuperation is a generic feature of non-Markovian systems both in and out of equilibrium, even as simple as a free Brownian particle. Moreover, we demonstrate that inertia alone, even in the strong damping regime, can lead to this effect despite the absence of any external forcing. These results suggest that energy recuperation can be ubiquitous in nature and it may be the modus operandi of various phenomena in setups with memory. We show that this novel mechanism of energy recovery is the source of memory-induced ultrafast directed transport of a particle in a periodic potential in which it almost attains its top speed corresponding to the system with no energy barriers. Our results may answer from the fundamental point of view the question why the cytosol, the intracellular fluid in biological cells, is viscoelastic.
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Superconductivity · Cross-listed from hep-th

Dynamically Generated Fermi Surface Mismatch and Relativistic Superfluidity in a Two-Component Massless Fermionic Theory

Susobhan Mandal and Sambuddha Sanyal

When fermions pair across mismatched Fermi surfaces, the mismatch reflects a built-in inequivalence between the species. We show it can instead arise dynamically by spontaneous symmetry breaking. In a massless two component Dirac theory with exact SU(2) flavor symmetry, a self-interacting vector boson condenses, splitting the Fermi surfaces while preserving time reversal. Pairing then yields a stable relativistic superfluid, promoting the Chandrasekhar-Clogston line to a surface in coupling space, the mismatch fixed self-consistently by the symmetry-breaking coupling.
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Mesoscale and Nanoscale Physics

The Position Space Chern Number: A Topological Index for Chiral Magnetic Systems

Zachariah Addison

This paper introduces an index that categorizes the topology of insulating chiral magnetic systems. The position space Chern number, $C_R$ is distinct from its momentum space counterpart, $C_K$. A nonzero index guarantees the existence of topologically protected in-gap states that localize on the edge of local potential barriers in momentum space. The Chern-Simons effective field theory describing position space Chern insulators reveals a topologically quantized correlation between transverse force operators that describe the flow of quanta in momentum space. We demonstrate the existence of nonzero $C_R$ in systems hosting skyrmion magnetic phases and show how the index generalizes the classical concept of a skyrmion winding number. Lastly we investigate the competition between momentum space and position space topologies, and highlight an apparent obstruction to having systems with both $C_R \neq 0$ and $C_K \neq 0$.
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Materials Science · Cross-listed from quant-ph

High yield creation of germanium vacancy centers in diamond by focused ion beam implantation and high temperature annealing

S. Dietel, E. Scattolo, J. Fuhrmann et al. (+5 more)

Negatively charged germanium vacancy centers (GeV) in diamond are a promising platform for quantum computing and quantum communication. However, these applications require the precise incorporation of GeV centers with good optical properties inside of nanophotonic structures. In this work, we demonstrate the highly efficient local creation of GeV centers in diamond via focused-ion-beam implantation, followed by high-temperature annealing. We report the successful creation of GeV centers over the depth range of 5.5 - 30 nm. Implantation at low fluence enables the creation of single GeV centers. The formation yield strongly depends on implantation energy and fluence, reaching up to 33% at energies of 35 and 70 keV. This method, therefore, enables the efficient creation of GeV centers within a small, well-defined local sample volume and offers a potential means of incorporating them into photonic structures.
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All authors S. Dietel, E. Scattolo, J. Fuhrmann, L. Kazak, A. Cian, E. Missale, D. Giubertoni, F. Jelezko