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Physics Condensed Matter

cond-mat Watch Condensed Matter

Sunday 19 April

Materials Science

Spin State versus Potential of Zero Charge as Predictors of Density-Dependent Oxygen Reduction in M-N-C Electrocatalysts

Di Zhang, Zixun Yu, Fangzhou Liu et al. (+6 more)

Metal-site density strongly influences oxygen reduction activity and selectivity in M-N-C electrocatalysts, but the descriptors that predict these trends remain under debate. Here, we compare spin state and the potential of zero charge as predictors of density-dependent oxygen reduction behavior in Fe-N-C and Co-N-C catalysts. Using constrained-magnetization calculations combined with Landau analysis, we find that the ground-state magnetic moments vary only weakly across a broad range of metal-site densities, suggesting that magnetic descriptors alone cannot account for the pronounced performance changes. In contrast, explicit-solvent simulations reveal systematic density-dependent shifts in PZC, which alter the interfacial electric field and thereby modulate field-sensitive adsorption energetics of ORR intermediates. Incorporating these PZC shifts into a pH-field-coupled microkinetic model captures the density-dependent activity trends and reproduces the experimentally observed increase in two-electron selectivity at lower site densities under acidic conditions. Experimental PZC measurements further support the predicted trend. Together, these results show that PZC is a more effective predictor than spin state for density-dependent oxygen reduction activity and selectivity in M-N-C electrocatalysts.
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All authors Di Zhang, Zixun Yu, Fangzhou Liu, Yumeng Li, Jiaxiang Chen, Xun Geng, Yuan Chen, Li Wei, Hao Li

Strongly Correlated Electrons

Static and Dynamic Electronic Properties and the Possible Magnetic Structure of the $4f^3$-$\Gamma_6$ System NdCo2Zn$_{18}$Ga$_2$ Investigated Using $^{59}$Co Nuclear Quadrupole Resonance

Tetsuro Kubo, Atsushi Sasaki, Keita Murooka et al. (+4 more)

We report $^{59}$Co nuclear quadrupole resonance (NQR) measurements on the Nd-based compound NdCo$_2$Zn$_{18}$Ga$_2$, which undergoes an antiferromagnetic transition at $T_{\rm N} = 1.5$ K. Although the NQR spectra show no detectable change across $T_{\rm N}$, the nuclear spin-lattice relaxation rate, $1/T_1$, exhibits a clear anomaly at $T_{\rm N}$. An analysis based on the alignment of the Nd moments demonstrates that the internal magnetic fields generated by these moments cancel each other at the Co sites. If the nearest-neighbor Nd moments align antiferromagnetically, this finding suggests that Ga substitution removes magnetic frustration, thereby increasing $T_{\rm N}$.
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All authors Tetsuro Kubo, Atsushi Sasaki, Keita Murooka, Hisashi Kotegawa, Rikako Yamamoto, Takahiro Onimaru, Hideki Tou

Materials Science

Localized Exciton Emission with Spontaneous Circular Polarization in NiPS3/WSe2 Heterostructures

Adi Harchol, Shahar Zuri, Rajesh Kumar Yadav et al. (+7 more)

Two-dimensional (2D) van der Waals (vdW) heterostructures (HSs) provide a versatile platform for tailoring electronic, optical, and magnetic properties via proximity effects at their interfaces. In this work, we explore the optical response of few-layer NiPS3/WSe2 HSs using low-temperature micro-photoluminescence ({\mu}-PL) and magneto-PL spectroscopy. The HSs exhibit multiple sharp excitonic peaks that do not appear in the individual constituent materials, indicating the emergence of localized intralayer WSe2 excitons confined by interface-induced potentials. Notably, these excitons exhibit spontaneous circular polarization even in the absence of an external magnetic field, suggesting a magnetic proximity effect induced by uncompensated spins at the NiPS3 interface. Magneto-PL measurements further reveal nonlinear Zeeman splitting, consistent with the presence of an interfacial exchange field that alters the valley exciton dynamics. Density functional theory (DFT) calculations confirm the intralayer origin of the PL and reveal interfacial hybridization and spin texture modifications, supporting the experimental findings. These results highlight how combining a 2D semiconductor with a layered antiferromagnet enables control over valley polarization and spin degrees of freedom, offering new opportunities for chiral light sources and magnetically tunable optoelectronic devices.
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All authors Adi Harchol, Shahar Zuri, Rajesh Kumar Yadav, Nirman Chakraborty, Idan Cohen, Tomasz Woźniak, Thomas Brumme, Thomas Heine, Doron Naveh, Efrat Lifshitz

Soft Condensed Matter

Observation of Compressional Acoustic Wave Responses in Cell Culture Media Using a Quartz Crystal Microbalance

Hansa Kannan, Ram Prakash Babu, Trisha Ghosh et al. (+3 more)

Quartz Crystal Microbalance (QCM) sensors are widely used to study biological and soft-matter interfaces due to their exceptional sensitivity to mass loading and interfacial mechanical properties. While classical QCM theory assumes predominantly shear-wave coupling into a semi-infinite Newtonian liquid, finite liquid thickness and acoustic reflections give rise to pronounced compressional (longitudinal) wave effects that strongly modulate both resonance frequency and motional resistance. Such compressional acoustic-wave responses should be properly accounted for when sensing in the liquid phase, for instance when working with cell suspensions. In this work, we systematically investigate compressional-wave responses in cell culture media including DMEM and RPMI-1640 across varying droplet volumes using a 5 MHz AT-cut QCM. Time-resolved measurements are analyzed using four parameters: the time period of compressional acoustic waves (Tca), the time associated with a phase shift between resonance frequency and resistance oscillations (Tp), the peak-to-peak shifts in frequency ({\Delta}fpp) and resistance ({\Delta}Rpp). DMEM and RPMI-1640 both exhibit strong volume-dependent periodic oscillations. At lower volumes, they exhibit low-frequency oscillations with a time period of approximately 40 minutes. However, as volume increases, the oscillations gradually evolve into high-frequency oscillations with a time period Tca of approximately 5 minutes. The peak-to-peak shifts ({\Delta}fpp) and ({\Delta}Rpp) are approximately 100-150 Hz and 40-60 {\Omega}, respectively. The resonance frequency and resistance oscillations also exhibit a phase shift Tp of approximately 10 minutes. These results highlight that compressional-wave artifacts occur even in simple cell culture media, necessitating their explicit consideration when interpreting QCM data in the presence of cells.
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All authors Hansa Kannan, Ram Prakash Babu, Trisha Ghosh, Arpita Mohapatra, Mainak Dutta, Adarsh Ganesan

Materials Science

Designer metal-free altermagnetism in honeycomb two-dimensional frameworks

Hongde Yu, Thomas Brumme and Thomas Heine

Altermagnetism combines momentum-dependent spin splitting of opposite-spin channels with zero net magnetization, enabling electric-field control of spin transport that is robust against external magnetic fields. Although widely explored in inorganic systems, metal-free altermagnets with pi-spin splitting, particularly in two-dimensional organic frameworks, have remained elusive. Here, we introduce a molecular design strategy that achieves designer metal-free altermagnetism in honeycomb 2D crystals. By reducing the monomer point-group symmetry from D3h to C2v in triangulene-derived radicals, inversion symmetry is selectively broken while the bipartite lattice is preserved. Spin-polarized density-functional-theory calculations reveal strong antiferromagnetic couplings of -130 meV, d-wave spin splitting of 17 meV at the M point, and Mott-Hubbard gaps of 1.26 eV, all fully consistent with Lieb's theorem. A minimal tight-binding model shows that anisotropic nearest-neighbor hopping arising from direction-dependent pi-orbital overlap is the microscopic origin of spin splitting and altermagnetism. Biaxial compressive strain further enhances the spin splitting to 27 meV. These results establish a general approach to room-temperature organic altermagnets and open a pathway toward carbon-based altermagnetism via engineered inversion-symmetry breaking.
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Materials Science

Modern Solid Electrolytes for All-Solid-State Batteries: Materials Chemistry, Structure, and Transport

Denys Butenko, Mustafa Khan, Liusuo Wu et al. (+1 more)

In this review, from crystallographic symmetry to amorphous local polyhedra arrangement and combinations, we examine inorganic solid state electrolytes through the lens of structure property relationships, with oxides, sulfides, and halides representing three major framework chemistries. Halide solid electrolytes and their derivatives, including mixed anion halides and antiperovskite related materials, have expanded this landscape further by introducing new ways to regulate local coordination chemistry, defect populations, and transport active frameworks. Across these families, fast ion conduction depends not simply on composition or crystallographic diffusion pathways, but on the coupled effects of framework topology, site energy distribution, defect chemistry, bottleneck response, and local anion flexibility. Oxides illustrate transport within chemically robust but geometrically constrained frameworks. Sulfides demonstrate that a soft, easily polarizable lattice can broaden the array of low energy migration pathways. Halides occupy an intermediate state, in which the closely packed anion sublattices, an approximately degenerate lithium environment, and mixed anion coordination enable effective transport while simultaneously enhancing oxidation stability and compatibility with cathodes. Building on these comparisons, we argue that long range ion transport is increasingly understood not as motion along a single idealized pathway, but as the macroscopic outcome of statistically connected low barrier local migration events distributed across the structure. We further discuss the experimental and computational approaches required to establish such multiscale structure property relationships and outline future strategies for designing transport active frameworks in which conductivity, stability, and processability are optimized together.
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All authors Denys Butenko, Mustafa Khan, Liusuo Wu, Jinlong Zhu

Materials Science

G-type antiferromagnetic structure in Rb1-xV2Te2O

Wu Xie, Changchao Liu, Fayuan Zhang et al. (+12 more)

Altermagnetism, known for its non-relativistic spin-split band structures with yet compensated moments, is being intensively investigated. Discovering new altermagnetic materials with characteristics suitable for practical use remains an important ongoing task. Recently a metallic room-temperature altermagnet candidate Rb1-xV2Te2O with a layered structure and d-wave spin symmetry has been reported based on experimental results from the spin-resolved photoemission spectroscopy and scanning tunnelling microscopy/spectroscopy (STM/STS) measurements. Here we report neutron powder diffraction (NPD) investigations on the magnetic structure of Rb1-xV2Te2O, which shows a G-type antiferromagnetic structure below the transition temperature of 337 K. The result is different from the original theoretical expectation, which might lead to new insights on the physics of this altermagnet candidate.
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All authors Wu Xie, Changchao Liu, Fayuan Zhang, Zhenhong Tan, Wenhai Ji, Nan Zhao, Lingxiang Bao, Dong Zhang, Feiran Shen, Lunhua He, Hao Wang, Rong Du, Guanghan Cao, Chaoyu Chen, Ping Miao

Soft Condensed Matter

From Flow to Form: Emergence of the Cytokinetic Ring via Active Cortical Dynamics

Sabyasachi Mukherjee and Anirban Sain

During cell division active flows occur in the cortex, a thin layer of gel like network of acto myosin filaments, beneath the cell surface. The cortical flow and the associated stresses bring about change in the cell shape, in particular a sharp invagination at the mid cell. Using 3D phase field simulation of an active deformable shell, which captures coupled dynamics of cortical velocity and nematic order, we show how a nematic like actomyosin ring spontaneously emerge at the equator and drive sharp invagination. We further demonstrate how different cortical flow patterns, including counter rotating flows emerge near the division furrow. We show that these flow patterns, often attributed to intrinsic chirality of actomyosin filaments can instead arise from bias in the initial nematic alignment, revealing a memory effect in the system. By analyzing a simpler model of activity gradient driven compressive flow on a flat interface we decipher the main ingredients for surface instability leading to invagination and counter moving flows.
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Materials Science

Influence of Ni substitution on the phase transitions and magnetocaloric effect of NdCo2 at cryogenic temperatures

Vilde G. S. Lunde, Øystein S. Fjellvåg, Allan M. Döring et al. (+9 more)

We have investigated NdCo2-xNix cubic Laves compounds with 0 <= x <= 1 using neutron diffraction and bulk magnetization measurements to study the influence of partial Ni substitutions of Co on the phase transitions and the magnetocaloric effect. Upon cooling, NdCo2 undergoes a cubic to tetragonal transition at 100 K, and a tetragonal to orthorhombic transition at 42 K. The transitions are associated with long-range ferromagnetic ordering of the magnetic moments along the c axis and spin reorientation into the ab plane, respectively. Both transitions shift to lower temperatures as the Ni content x increases. For x >= 0.5, the orthorhombic phase is suppressed. Additionally, there was a reduction in the magnetic moment upon increasing the Ni substitution of Co. The magnetocaloric effect was determined both indirectly and directly, with good agreement between the methods. NdCo2 exhibits an adiabatic temperature change of 6.3 K for a field of 20 T, which is decreased to 4.9 K for NdCoNi for the same field strength due to the reduced magnetic moment upon Ni substitution.
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All authors Vilde G. S. Lunde, Øystein S. Fjellvåg, Allan M. Döring, Marc Straßheim, Vladimir Pomjakushin, Konstantin P. Skokov, Oliver Gutfleisch, Tino Gottschall, Joachim Wosnitza, Anja O. Sjåstad, Bjørn C. Hauback, Christoph Frommen

Superconductivity

Relaxation Dynamics of Superconducting/Normal Interfaces in the Intermediate State of a type-I superconductor

Mengju Yuan, Yugang Zhang, Ying Zhu et al. (+5 more)

The dynamics of phase boundaries, such as superconducting/normal (S/N) interfaces in type-I superconductors, are typically obscured in conventional magnetic measurements, which are strongly influenced by surface barriers and global flux processes. Here, we employ the complex ac magnetostrictive coefficient as a sensitive probe of the bulk dynamics of S/N interfaces in the intermediate state of lead. The ac magnetostrictive coefficient exhibits an unusual complex response, with a negative real component and a positive imaginary component. Frequency-dependent measurements show that the magnitude of the real component decreases monotonically with increasing frequency, whereas the imaginary component first increases, reaches a broad maximum, and then decreases. In contrast, ac susceptibility displays a conventional Debye-type magnetic relaxation with a different phase relation between its real and imaginary components. We propose that the magnetostrictive response arises from ac-field-driven relaxation and rearrangement of metastable S/N domain configurations toward states of lower free energy. These results reveal a distinct interfacial relaxation channel in the intermediate state and establish the complex ac magnetostrictive coefficient as a sensitive probe of nonequilibrium S/N interface dynamics.
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All authors Mengju Yuan, Yugang Zhang, Ying Zhu, Jingchun Gao, Aifeng Wang, Mingquan He, Jun-Yi Ge, Yisheng Chai

Mesoscale and Nanoscale Physics

Polarized light Raman scattering by an atom near an ultrathin periodically aligned carbon nanotube film

SK Firoz Islam, Michael Dean Pugh and Igor V. Bondarev

We present a systematic theoretical study of the Raman scattering effect for a two-level atomic system in near proximity of an ultrathin dielectric film with an embedded parallel array of periodically aligned single-wall semiconducting carbon nanotubes. More generally, our model provides a unified description of the quantum near-field medium-assisted enhancement effects for in-plane anisotropic metasurfaces, of which ultrathin periodically aligned carbon nanotube films are the representative example. Particular attention is given to incoming photon parameters of the external light radiation such as polarization and incidence plane orientation relative to the main anisotropy axis (nanotube alignment axis). By explicitly deriving the Raman scattering cross-section, we establish that for the two-level atomic system in the near-field zone of the carbon nanotube metasurface the effect can be enhanced by a factor of up to 10^4, not only for p-polarized but for s-polarized light as well.
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Quantum Gases · Cross-listed from hep-th

Bubble dynamics and vortex formation in holographic first-order superfluid phase transitions

Zhen-han Jin, Yu-ping An and Li Li

We investigate bubble dynamics in a holographic superfluid undergoing a first-order phase transition with spontaneous $U(1)$ symmetry breaking. Near the nucleation threshold, the system exhibits universal critical behavior governed by a single unstable mode, leading to logarithmic scaling of the time spent near the critical solution. The terminal bubble wall velocity increases with charge density but remains small due to strong dissipation. In multi-bubble collisions, vortex formation depends sensitively on the initial phases and deviates significantly from the geodesic rule. Notably, we identify a regime where three-bubble collisions produce a vortex-antivortex pair that subsequently annihilates, a phenomenon not predicted by the geodesic rule. The lifetime of this pair scales logarithmically with the distance to the critical collision radius. Our results underscore the crucial role of non-equilibrium dynamics in strongly coupled superfluids and provide new insights into topological defect formation during first-order phase transitions.
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Materials Science

Unraveling the significance of Raman modes, Gruneisen parameters and phonon lifetimes in the hexagonal allotropes of Silicon and Germanium compounds

Lekshmi S M, Upasana Agrawal, Akarsh Jain et al. (+2 more)

Advancement in quantum information and quantum technologies has ushered in a new era of technological revolution in large scale atomistic simulation and efficient system on a chip device fabrication. This has led to innovative ways of harnessing rigorous search algorithms for functional quantum materials and steered scientists to dig deeper into the world of quantum phenomenon and applications. In this work, we delineate the advanced electronic structure and vibrational properties utilizing the popular meta-GGA functionals, spectral signatures of the Raman active phonon modes, explored their average mean free paths, and whether they conserve helicity, by leveraging first principles density functional theory and density functional perturbation theory. A systematic analysis of the role of phonon lifetimes, consequences of phonon-phonon and three phonon scattering rates and phonon linewidths have been presented. Further, a study of the the frequency and temperature dependent Gruneisen parameter has been employed in conjecture with the temperature dependent thermal expansion and thermal conductivity to portray the effect of anharmonicity in the phonon spectra of these two materials. Finally, we provide strategies for tuning the properties of these materials in an effort to improve their efficacy for advanced thermoelectric, photovoltaic and optoelectronic device applications.
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All authors Lekshmi S M, Upasana Agrawal, Akarsh Jain, Siddharth Sastri, Suvadip Das

Superconductivity

Pairing Mechanism in Bilayer Nickelate La$_3$Ni$_2$O$_7$ Superconductors

Xianxin Wu, Tao Xiang and Jiangping Hu

The recent discovery of superconductivity with $T_c \approx 80$~K in bilayer nickelate La$_3$Ni$_2$O$_7$ provides a new setting in which to test the organizing principles of unconventional high-temperature superconductivity. We show that the gene principle and the collaborative Fermi-surface rule which were previously proposed to unify unconventional high temperature superconductors, extend naturally to this bilayer, multi-orbital system. We identify that there are two antiferromagnetic exchange channels that can provide the dominant pairing force: an interlayer intra-orbital nearest-neighbour exchange $J_\perp$ between $d_{z^2}$ orbitals mediated by the inner apical oxygen, and an intralayer inter-orbital nearest-neighbour exchange $J_{xz}$ between $d_{z^2}$ and $d_{x^2-y^2}$ orbitals mediated by the in-plane oxygen. Owing to the bilayer bonding–antibonding splitting and the $B_{1g}$ symmetry of the $d_{x^2-y^2}$ orbital, these two channels cooperate to produce a robust $s^\pm$ superconducting state with an internal sign reversal between mirror-even and mirror-odd Fermi-surface pockets in momentum space. Both pairing channels maximize the superconducting gap on the $\beta$ pocket with a form factor $(cosk_x-cosk_y)^2$ in momentum space. The result places La$_3$Ni$_2$O$_7$ within a unified framework for unconventional superconductivity while revealing a distinct electronic environment for high-$T_c$ pairing.
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Saturday 18 April

Materials Science · Cross-listed from physics.app-ph

Understanding Energy Flow and Inefficiency of a Thermomagnetic Generator by Transient Multi-Physics Modelling

Ali Izadi, Bruno Neumann and Sebastian Fähler

Waste heat recovery improves energy efficiency and reduces greenhouse gas emissions; however, much industrial and environmental heat is wasted at low temperature. Thermomagnetic recovery of waste heat has a high potential for sustainable production of electric energy, especially for low-grade waste heat where conventional technology is inefficient or infeasible. Of particular interest are thermomagnetic generators (TMG) as they require almost no mechanically moving parts, which is beneficial for high reliability. However, all existing prototypes have two remaining challenges: low efficiency and low cycle frequency. In this work, we develop a digital twin of a recent TMG with genus 3 by using multi-physics simulations. We identify shortcomings of previous simulation approaches, and describe why simulations in three dimensions are necessary, which consider coupling between magnetic, thermal, fluid flow, and electrical physics domains. We validate our model, which only uses known geometry and material parameters, by experimental data of the TMG with highest power density today, and attain 96% accuracy in open-circuit voltage and 95% accuracy in power output. This high accuracy allows us to identify the origin of both challenges for TMGs, which are not accessible by experiments. First, we uncover inefficiencies by analyzing the energy flow within a Sankey diagram. Second, we trace the transient heat flow through the generator, which identifies the factors limiting frequency. This paves the way for more efficient and faster TMGs, and their development will be accelerated by our validated digital twin.
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Disordered Systems and Neural Networks · Cross-listed from math-ph

On the asymptotic duality of spectral variances in random matrix theory and the "1/6" formula

Peng Tian, Roman Riser and Eugene Kanzieper

A "mysterious" relation between the number variance and the variance of the $L$-th ordered eigenvalue, first suggested by French et al. [Ann. Phys. 113, 277 (1978)], is revisited and proven to be asymptotically exact for the $\beta=2$ Dyson symmetry class. Central to the proof is a previously unknown sum rule for the level spacing auto-covariances. Its derivation hinges on our previous work on the power spectrum description of eigenvalue fluctuations in random matrix theory. Analytical results for $\beta=2$ are complemented by conjectural extensions to the $\beta=1$ and $\beta=4$ symmetry classes. Our findings are corroborated by a comprehensive numerical analysis.
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Disordered Systems and Neural Networks · Cross-listed from cs.ET

A fully parallel densely connected probabilistic Ising machine with inertia for real-time applications

Ruomin Zhu, Abhishek Kumar Singh, Jérémie Laydevant et al. (+5 more)

Ising machines—special-purpose hardware for heuristically solving Ising optimization problems—based on probabilistic bits (p-bits) have been established as a promising alternative to heuristic optimization algorithms run on conventional computers. However, it has—until now—been thought that Ising spins that are connected in probabilistic Ising machines (PIMs) cannot be updated in parallel without ruining the machine's solving ability. This has presented a major challenge to realizing the potential for probabilistic Ising machines to act as fast solvers for densely connected Ising problems. In this paper, we show that it is possible to circumvent this conventional wisdom. We introduce a modified form of Ising spin dynamics for PIMs, adding an inertia term, and verify in algorithm simulations, field-programmable gate array (FPGA) emulation, and in FPGA experiments that the modified dynamics enables fully parallel, synchronous updates and at the same time improves the achieved success probability. Our evaluations were performed with various types of abstract (Max-Cut and Sherrington-Kirkpatrick model) and application-derived (multiple-input and multiple-output, MIMO detection) dense Ising benchmark instances. Performing fully parallel updates results in a speed advantage that grows superlinearly with the number of spins, giving rise to large time-to-solution reductions for practical problem sizes. For both MC and the SK model at a problem size of 200, our approach achieved an average speedup of ~34x, with the best single-instance speedup reaching 150x. As an example of the practical utility of our approach in an application where speed is critical, we co-design the algorithm dynamics and hardware implementation for MIMO detection, achieving improved detection accuracy relative to the standard linear detector and higher throughput than the conventional sequential-update PIM.
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All authors Ruomin Zhu, Abhishek Kumar Singh, Jérémie Laydevant, Fan O. Wu, Ari Kapelyan, Davide Venturelli, Kyle Jamieson, Peter L. McMahon

Strongly Correlated Electrons

Experimental Signatures of Topological Transport in Polycrystalline FeSi Thin Films

R. Mantovan, A. Bozhko, V. Zhurkin et al. (+5 more)

Iron monosilicide $\epsilon$-FeSi has recently been predicted to host nontrivial topological states, yet experimental evidence remains scarce. Here we report transport signatures of Weyl semimetal behavior in polycrystalline $\epsilon$-FeSi films (65 nm thick) grown by solid-state reaction of Fe on Si (100). Below 200 K, the anomalous Hall conductivity $\sigma_{xy}^{AHE}$ becomes temperature-independent ($\sigma_{xy}^{AHE}$$\approx$14 S/sq.$\sim$ const($\sigma_{xx})$), unequivocally demonstrating an intrinsic anomalous Hall effect driven by Berry curvature. By comparing our data with the large set of data available in the literature for FeSi single crystals and polycrystalline/amorphous/crystalline FeSi thin films of thicknesses between 10 nm and 1 mm, we discover a universal scaling of $\rho_{xy}^{AHE}$$\sim$$\rho_{xx}^{2}$, which confirms the existence of surface-dominated electron transport. In this regime, the chiral anomaly manifests in both anisotropic longitudinal magnetoresistance and the planar Hall effect as well. The above observations establish $\epsilon$-FeSi as a Weyl semimetal and allow to relate $\sigma_{xy}^{AHE}$ to a "quantized" Hall response estimating the effective Weyl-point separation as $(k_{+}^W$$-$$k_{-}^{W})/(2\pi)$$\approx$0.36. Our findings confirm the topological origin of electron transport in $\epsilon$-FeSi and discover its potential as a new high temperature and noble metal-free Weyl semimetal.
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All authors R. Mantovan, A. Bozhko, V. Zhurkin, A. Bogach, A. Khanas, S. Zarubin, A. Zenkevich, V. Glushkov

Materials Science · Cross-listed from quant-ph

Time evolution of quantum gates and the necessity of complex numbers

M.P. Vaughan

As physical systems, qubits must evolve from input to output state. We describe a simple scheme in which the effect of a quantum gate is described by the action of an effective Hamiltonian acting for some characteristic time. This model shows that the action of common unary gates is to induce Bloch sphere trajectories along lines of latitude relative to an eigenvector of the gate. Such trajectories would immediately move a `rebit', initially confined to a line of latitude, off this line and acquire a complex phase. The role of the complex phase in bringing about the entanglement of two qubits is also highlighted. It is then asked whether such dynamics could be modelled using real QM. It is shown that the continuous evolution required for such dynamics can only be provided by members of the special orthogonal group of the vector space. Since the matrices representing many quantum gates of interest have determinant -1, no real special orthogonal operators can model their evolution if the dimension of the real vector space is the same as that of the complex space. Next we look at the mapping from a complex vector space of dimension $N$ to a real space of dimension $2N$ that is often used to construct `real' QM. It is shown that this is just an isomorphic mapping from the scalar representation of complex numbers to their $2\times2$ matrix equivalents, so that the resulting matrices are actually represent complex matrices. Finally, we investigate the endomorphism of real vector spaces of dimension $N = 2^{n}$, where $n \in \mathbb{Z}^{+}$, suitable for the modelling of $n-1$ qubits. We confirm that the mapping from $\mathbb{C}^{2^{n-1}} \to \mathbb{R}^{2^{n}}$ only maps elements of $\mathrm{End}(\mathbb{C}^{2^{n-1}})$ to a restricted subspace of $\mathrm{End}(\mathbb{R}^{2^{n}})$ that reproduces the `real' representation of complex matrices.
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Materials Science

Medium-Throughput Evaluation of Quantum Geometry-Driven Topological Transports in Altermagnets

Fu Li, Bo Zhao, Vikrant Chaudhary et al. (+4 more)

Altermagnets provide a promising platform for a wide spectrum of applications integrating advantages of conventional ferromagnets and antiferromagnets. In this work, we implement a medium-throughput first-principles workflow and evaluate topological transport properties driven by quantum geometry for 135 altermagnets in the MAGNDATA database. Based on automated Wannier construction, both linear and nonlinear responses, including the anomalous Hall effect, magneto-optical Kerr effect, and bulk photovoltaic effect, are evaluated with further symmetry verifications. Detailed analysis is done on representative cases like metallic VNb3S6 with enhanced anomalous Hall conductivity, CaIrO3 with giant MOKE, and CuFeS2 with large shift current in non-centrosymmetric. These results establish a symmetry-guided computational route for identifying experimentally accessible fingerprints and functional transport properties in altermagnets.
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All authors Fu Li, Bo Zhao, Vikrant Chaudhary, Shengqiao Wang, Chen Shen, Hao Wang, Hongbin Zhang

Statistical Mechanics · Cross-listed from quant-ph

Operator-Valued Hardy Spaces and Kramers–Kronig Relations for Non-Markovian Quantum Memory Kernels

Kejun Liu

Retarded support, upper-half-plane holomorphy, and Hardy boundary control are distinct properties of a memory kernel. We give sufficient conditions linking them for the Nakajima–Zwanzig kernel of an open system with finite-dimensional system Liouville space. If the projected kernel has an absolutely continuous real-axis representation with density w in L1 intersect Lp0, no singular part, and p0 > 1, its transform lies in the operator-valued Hardy class Hp(B) for 1 < p <= p0. A finite-dimensional componentwise argument yields the corresponding principal-value and once-subtracted Kramers–Kronig (KK) boundary formulas; the H1 endpoint is separate. Microscopic unitary evolution gives holomorphy and a trace-norm bound for reduced-state transforms for arbitrary trace-class initial states, without that spectral hypothesis. On a common analytic domain, a perturbative force-fit equation has a first-order quotient pole at a simple baseline state-transform zero zeta only when the first-order inhomogeneous numerator I_tilde^(1)(zeta) is nonzero; at finite perturbation the actual zero and numerator must be checked, while matrix reconstructions additionally require adjugate non-cancellation. For rational reconstructions, the printed local or global clearing and no-cancellation hypotheses convert a rational kernel pole into a genuine upper-half-plane propagator pole. Such a pole is incompatible with a uniformly bounded CPTP family when the transforms agree on an open set, with Vieta's formula giving the imaginary-part budget. We formalize these paper-specific implications in Lean 4 from named classical inputs. A finite Jaynes–Cummings calculation provides a shifted, distributional KK check with a 0.024 percent fixed-truncation residual.
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Statistical Mechanics

Fundamental temperature in the superstatistical description of non-equilibrium steady states

Sergio Davis

Among the statistical mechanical frameworks able to describe systems in non-equilibrium steady states such as collisionless plasmas, self-gravitating systems and other complex systems, superstatistics have gained recent attention. Superstatistics postulates a superposition of canonical systems with inverse temperatures $\beta$ described by a probability distribution depending on the external conditions. Unfortunately, the uncertainty about $\beta$ cannot be attributed to fluctuations of a phase space function, and this suggests that the distribution of $\beta$ is purely of statistical nature and must be inferred rather than measured. This lack of direct observability of the superstatistical temperature then becomes a conceptual issue in need of resolution. In this work we address this issue, showing that a mapping exists between functions of the superstatistical temperature and functions of the recently proposed fundamental temperature, a model-dependent function of the energy, in such a way that their expectation values coincide. We illustrate the use of this mapping by computing the conditional distribution of inverse temperature given energy for the $q$-canonical ensemble, as well as the full inverse temperature distribution, without the use of Laplace inversion.
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Mesoscale and Nanoscale Physics

The origin and promise of transition metal dichalcogenide hosted single photon emitters for quantum technologies

Mayank Chhaperwal, Amartyaraj Kumar and Kausik Majumdar

Single photon emitters (SPEs) are integral parts of several quantum technology implementations. Over the past decade or so, monolayers of transition metal dichalcogenides (TMDCs) have emerged as one of the promising candidates for SPE platforms with attractive characteristics. To move ahead, it is necessary to understand the atomistic origin of SPEs in TMDCs - a topic which is highly debated with contradicting proposals. In this paper, we critically review these existing proposals to elucidate their origin. Further, we perform a critical trend analysis for different figures of merit of TMDC-based SPEs, and propose a characterization methodology to streamline the reporting process. Finally, we review several quantum technology implementations where solid state SPEs are being used, and identify the advancements required in TMDC-based SPEs for their successful adoption in these technologies.
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Materials Science · Cross-listed from physics.chem-ph

Effective theory of quantum phases in the dipolar planar rotor chain

Estêvão V.B. de Oliveira, Muhammad Shaeer Moeed and Pierre-Nicholas Roy

In this work, we develop a theoretical description of the collective behavior of interacting dipolar planar rotors by using time independent perturbation theory and a small angle quadratic approximation. The ground state properties for both the ordered and disordered quantum phases of the system are directly calculated and analyzed. Time-independent perturbation theory is shown to be appropriate for the disordered phase. For the ordered phase, we construct a quadratic approximation based on the stable equilibrium configurations of the dipolar ordering; we show that the inclusion of the quartic terms from the expansion of the potential energy are essential to correct the shift in the energy spectrum due to quantization ambiguities. Numerical techniques such as Exact Diagonalization and Density Matrix Renormalization Group are used for the benchmark the quality of both approximations.
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Soft Condensed Matter

Motility and interfacial instability of confined chemically active droplets

Pawan Kumar, Sobiya Ashraf, Naveen Tiwari et al. (+2 more)

Microorganisms navigating through narrow spaces encounter significant hydrodynamic challenges. To overcome these constraints and sustain efficient motion, they employ adaptive strategies, including adaptive oscillatory body deformations. While artificial microdroplets can traverse channels narrower than their diameter, studies of their locomotion have thus far been largely restricted to steady-shape regimes. In this work, we demonstrate a transition from steady shape to dynamic interfacial undulations in 5CB (4'-pentyl-4-cyanobiphenyl) droplets within aqueous trimethylammonium bromide (TTAB) solutions. We show that while droplets in dilute, additive-free solutions maintain a steady shape, the introduction of solutes or higher surfactant concentrations triggers pronounced interfacial undulations. Notably, both steady and undulating droplets exhibit a comparable velocity dependence on the confinement ratio, characterized by an initial deceleration followed by saturation, governed by the competition between hydrodynamic resistance and phoretic flow within the lubrication film. Furthermore, we find that increased surfactant concentration increases the capillary number, resulting in a thicker lubrication layer that facilitates a symmetry-breaking transition. Upon varying confinement, the droplet interface shifts from bilateral undulations to a mode localized on one side, forming a traveling-wave pattern strongly coupled to flow field fluctuations at the droplet's anterior. Linear stability analysis identifies the Yih-Marangoni instability as the underlying mechanism for these oscillations, revealing a previously unrecognized mode of adaptive locomotion in confined active matter.
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All authors Pawan Kumar, Sobiya Ashraf, Naveen Tiwari, Dipin Pillai, Rahul Mangal

Statistical Mechanics

Solution of the Ising model with Brascamp-Kunz boundary conditions by the transfer matrix method

De-Zhang Li and Xin Wang

The square lattice Ising model under the Brascamp-Kunz boundary conditions is a well-known exactly solvable lattice model. The exact solution of this system has been derived within the framework of Pfaffian-type method. In this paper we provide a derivation for the solution by the Schultz-Mattis-Lieb method in the transfer matrix formalism. We set special interactions on the boundaries and take certain limit of these interactions, so that the system under the Brascamp-Kunz boundary conditions is transformed into another system under the toroidal boundary conditions. The Schultz-Mattis-Lieb method is applied to the mapping system and the partition function is exactly solved in the fermionic representation. The Fisher zeros are analytically calculated and the physical critical point is identified. We also discuss the difference between the transfer matrix approaches to the Brascamp-Kunz and to the toroidal boundary conditions. Our work introduces a member to the family of transfer-matrix-based studies for Ising model under various boundary conditions.
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Soft Condensed Matter · Cross-listed from physics.flu-dyn

Directed droplet motion – Its versatile nature and anticipated applications

Panagiotis E. Theodorakis and Andrey Milchev

Applications such as digital microfluidics and bio-diagnostics rely on droplet locomotion. A prominent example of such motion is durotaxis, a phenomenon that requires a stiffness gradient along a surface for the transport of liquids, cells, or other nano-objects. Using surfaces with varying properties in specific directions can be exploited as a universal concept for fluid transport with or without external energy supply. Changes in properties may refer to substrate patterns, Laplace pressure changes, wettability gradients, etc., leading to exciting phenomena, which can be employed in novel applications in various technologies. Here, we report on key results and progress in the area of directed droplet motion over the years, and we provide perspectives and implications for anticipated applications.
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Materials Science

Hierarchical spectral inhomogeneity in photoluminescence of a twisted MoSe2/WSe2 heterobilayer moiré superlattice revealed by hyperspectral mapping

Nurul Fariha Ahmad, Yuto Urano, Kenji Watanabe et al. (+3 more)

Low-temperature photoluminescence from MoSe2/WSe2 moire superlattice often consists of a broad interlayer emission background with dense, narrow peaks, making microscopic line-by-line assignment difficult. Here, we use hyperspectral photoluminescence mapping and peak-decomposition-free spectral analyses to determine how this spectral complexity is organized in space. A 20 x 20 map acquired with a 400 nm pitch reveals three dominant spectral families that form contiguous real-space domains. Feature-wise spatial correlation analysis and whole-spectrum similarity yield a characteristic micron-scale length of 1.27-2.05 um, all exceeding the 0.85 um optical spot size. At the same time, individual pixels retain a dense, multi-peak structure, implying an unresolved local spectral manifold below optical resolution. Correlations among centroid, dominant energy, asymmetry, width, entropy, sharp fraction, and roughness indicate that the micron-scale energy landscape and local manifold complexity can be statistically separated, while remaining correlated across the map, consistent with a hierarchical organization of the emission spectrum. These results establish hierarchical inhomogeneity as an organizing principle of MoSe2/WSe2 moire superlattice photoluminescence.
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All authors Nurul Fariha Ahmad, Yuto Urano, Kenji Watanabe, Takashi Taniguchi, Daichi Kozawa, Ryo Kitaura

Materials Science

On the Energy Dissipation in the Landau-Lifshitz-Gilbert Equation

Kutay Kulbak, Mohamed Iyad Boualem, Charlie Masse et al. (+2 more)

The dynamics of magnetization near a stable equilibrium in ferromagnetic nanomagnets are examined within the Landau–Lifshitz–Gilbert (LLG) framework. For a small angle precession, the dependence of ferromagnetic resonance (FMR) frequency, the damping constant and the resulting quality factor $Q$ of the resonance on the local curvature around the free-energy minimum is systematically analyzed. Special attention is devoted to the behavior of the FMR decay time in the vicinity of bifurcation points, where the number of metastable energy minima changes and the commonly used approximation for the quality factor $Q\simeq 1/2\alpha$ (where $\alpha$ denotes the Gilbert damping) fails.
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All authors Kutay Kulbak, Mohamed Iyad Boualem, Charlie Masse, Mariana Delalibera de Toledo, Vasily V. Temnov

Materials Science

Strain Correlated Linearly Polarized Photoluminescence in WS2/WSe2 Moiré Superlattices

Yuto Urano, Ryo Tamura, Yui Tamogami et al. (+6 more)

Reliable optical control of valley degrees of freedom in moiré excitons requires that the emitted polarization faithfully reflect the underlying valley state. Here, we show that linearly polarized photoluminescence from WSe2/WS2 moiré excitons is largely insensitive to the excitation polarization and therefore does not arise from valley coherence. Automated polarization-resolved photoluminescence and Raman mapping at cryogenic temperature reveals that the degree of linear polarization correlates strongly with local Raman shifts and moiré-exciton observables, identifying strain as the dominant experimental correlate. Linear-regression analysis further shows that strain-related descriptors provide the best prediction of the observed polarization. Guided by theory, we attribute this behavior to strain-amplified breaking of C3 symmetry in the moiré potential: weak uniaxial strain produces only partial cancellation of locally elliptical emission, yielding a finite far-field degree of linear polarization. These results establish strain as a key control parameter for reliable optical readout in TMD moiré superlattices.
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All authors Yuto Urano, Ryo Tamura, Yui Tamogami, Toshikaze Kariyado, Yasumitsu Miyata, Daichi Kozawa, Kenji Watanabe, Takashi Taniguchi, Ryo Kitaura