Materials Science
Intrinsic Negative-U Centers in Freestanding LaAlO3/SrTiO3 Micro-membranes
Giulia Meucci, Pinelopi O. Konstantinopoulou, Thies Jansen et al. (+15 more)
The LaAlO3/SrTiO3 (LAO/STO) interface hosts a rich range of electronic phenomena, including unconventional electron pairing that in
quantum dots gives rise to a negative effective charging energy U. Here, we show freestanding LAO/STO micro-membranes naturally hosting negative-U centers, where lateral confinement arises intrinsically, rather than from engineered nanostructures. These centers coexist with gate-tunable superconductivity and can remain stable upon thermal cycling from millikelvin temperatures to room temperature. Transport is in excellent agreement with calculations based on a negative-U Anderson model, and electrostatic simulations indicate characteristic center sizes of 20-80 nm. Our findings suggest that negative-U centers may arise from the intrinsic interfacial inhomogeneity typical of LAO/STO, and should therefore be considered a general feature of the LAO/STO interface. This could have important consequences for the microwave response of interfacial superconducting devices.
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All authors Giulia Meucci, Pinelopi O. Konstantinopoulou, Thies Jansen, Gunjan Nagda, Damon J. Carrad, Emiliano Di Gennaro, Yu Chen, Nicola Manca, Nicolas Bergeal, Manuel Bibes, Alexei Kalaboukhov, Marco Salluzzo, Roberta Citro, Felix Trier, Nini Pryds, Fabio Miletto Granozio, Alessia Sambri, Thomas S.Jespersen
Quantum Gases
Inferring rotations using a bosonic Josephson junction
Rhombik Roy and Ofir E. Alon
Rotation and
quantum tunneling are fundamental concepts in physics, and their interplay in the ultracold atomic systems is of particular interest. In this theoretical work, we explore how tunneling dynamics in a bosonic Josephson junction are modified when the system is placed in a rotating, non-inertial frame. We show that the tunneling dynamics of ultracold
bosons in a two-dimensional double-well potential offer an alternative pathway for inferring the rotation frequency. Using the mean-field and many-body analyses, we demonstrate that rotation strongly modifies the tunneling time period as well as the momentum and angular momentum dynamics. When the rotation axis passes through the center of the double well, the observables show distinct dynamical responses with increasing rotation frequency, enabling the rotation frequency to be assessed from changes in the tunneling dynamics. When the potential is displaced from the rotation axis, the rotation induces asymmetric tunneling and partial self-trapping, allowing both the rotation frequency and the displacement to be inferred. We further show that for an off-centered double well, the tunneling dynamics exhibit a pronounced orientation dependence, enabling the orientation of the double well to be inferred from the observed dynamics. The many-body analysis further shows that the depletion dynamics are strongly influenced by rotation, providing an additional tool for assessing the rotation frequency. Finally, we study the effect of time-dependent rotation in which the double well is gradually set into motion in the laboratory frame and identify distinct dynamical signatures that depend sensitively on the switching time. Together, these results establish a comprehensive framework for inferring the rotation frequency, radial displacement, and orientation directly from the tunneling dynamics.
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Materials Science
Colossal low-field negative magnetoresistance in CaAl$_{2}$Si$_{2}$-type diluted magnetic semiconductors (Ba,K)(Cd,Mn)$_{2}$As$_{2}$
Bijuan Chen, Zheng Deng and Changqing Jin
We report the
magnetic and magnetotransport properties of the layered CaAl$_2$Si$_2$-type diluted
magnetic semiconductor (Ba$_{1-x}$K$_x$)(Cd$_{1-y}$Mn$_y$)$_2$As$_2$ over a broad Mn (
spin) substitution range of $0.05 \le y \le 0.5$. K substitution introduces hole carriers, whereas Mn provides local moments, resulting in bulk ferromagnetism with Curie temperatures up to $\sim 17$ K. Intrinsic
magnetic ordering is further supported by an anomalous
Hall contribution and a specific-heat anomaly near $T_{\mathrm{C}}$. A key performance feature is a colossal negative magnetoresistance: for heavily Mn-doped compositions ($y \ge 0.3$), $\mathrm{MR}=[\rho(H)-\rho(0)]/\rho(0)$ reaches approximately $-100\%$ at 2 K and nearly saturates at a relatively low
magnetic field of $\sim 0.35\,\mathrm{T}$. The combination of soft ferromagnetism, strong
spin-charge coupling, and low-field MR saturation highlights (Ba,K)(Cd,Mn)$_2$As$_2$ as a promising bulk platform for low-temperature magnetoresistive functionalities.
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Strongly Correlated Electrons · Cross-listed from quant-ph
Polynomial-time certification of fidelity for many-body mixed states and mixed-state universality classes
Yuhan Liu and Yijian Zou
Computation of Uhlmann fidelity between many-body mixed states generally involves full diagonalization of exponentially large
matrices. In this work, we introduce a
polynomial-time
algorithm to compute certified lower and upper bounds for the fidelity between
matrix product density operators (MPDOs). Our method maps the fidelity estimation problem to a variational optimization of sequential
quantum circuits, allowing for systematic improvement of the lower bounds by increasing the
circuit depth. Complementarily, we obtain certified upper bounds on fidelity by variational lower bounds on the trace distance through the same framework. We demonstrate the power of this approach with two examples: fidelity correlators in critical mixed states, and codeword distinguishability in an approximate
quantum error-correcting code. Remarkably, the variational lower bound accurately track the universal scaling behavior of the fidelity with a size-consistent relative error, allowing for the extraction of previously unknown critical exponents. Our results offer an exponential improvement in precision over known moment-based bounds and establish a scalable framework for the verification of many-body
quantum systems.
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Strongly Correlated Electrons
Stabilizer-Rényi Microscopy of Critical Correlations in Interacting Fermions
Jun Qi Fang, Fo-Hong Wang and Xiao Yan Xu
Quantum magic—the resource that separates universal
quantum computation from efficiently simulable Clifford
circuits—has emerged as a diagnostic of many-body
quantum states, yet the stabilizer Rényi
entropy (SRE) that quantifies it remains largely inaccessible in interacting
fermion systems. Estimating the global SRE requires a sum over exponentially many Majorana strings, which in determinant
quantum Monte Carlo can be sampled without a sign problem only for the interacting density
matrix obtained after averaging the auxiliary fields: sampling strings and fields simultaneously incurs a sign problem even for sign-problem-free models, so that the known sign-free alternative is a nested Monte Carlo that does not scale. We propose instead the two-point SRE—a practical stabilizer-Rényi correlator built from rank-2 SREs of one- and two-site reduced density
matrices, which are fixed exactly by single-particle Green functions and density correlations already measured in standard simulations—and show that it obeys universal finite-size scaling at fermionic
quantum critical points. In the one-dimensional half-filled spinless $t$-$V$ chain, the correlator distinguishes algebraic and exponential decay regimes and tracks the inverse-logarithmic finite-size drift characteristic of the Berezinskii–Kosterlitz–Thouless transition. On the honeycomb
lattice, sign-problem-free
quantum Monte Carlo yields Gross–Neveu–Ising scaling with finite anomalous dimension at zero temperature and two-dimensional Ising collapse at the thermal transition. Our results suggest stabilizer-Rényi microscopy as a spatially resolved, quantitatively universal probe of critical correlations in interacting fermionic matter, on par with conventional order-parameter correlators and accessible to
quantum-simulator measurements.
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Superconductivity
Towards reliable electrical measurements of superconducting devices inside a transmission electron microscope
Joachim Dahl Thomsen, Michael I. Faley, Joseph Vimal Vas et al. (+6 more)
Correlating structure with electronic functionality is central to the engineering of
quantum materials and devices whose properties depend sensitively on disorder. Transmission electron microscopy (TEM) offers high spatial resolution together with access to structural, electronic, and
magnetic degrees of freedom. However, operando electrical transport measurements on functional
quantum devices remain rare, particularly at
liquid helium temperature. Here, we demonstrate electrical transport measurements of niobium nitride (NbN) devices inside a TEM using a continuous-flow
liquid-helium-cooled sample holder. By optimizing a thermal radiation shield to limit radiation from the nearby pole pieces of the objective lens, we achieve an estimated base sample temperature of 8-9 K, as inferred from the superconducting transition temperatures of our devices. We find that both electron beam illumination and objective lens excitation perturb the superconducting state. In addition, we evaluate the imaging capabilities and stability of the sample holder at low temperature by imaging the
magnetic domain structure of the van der Waals ferromagnet CrBr$_3$. Finally, we perform calculations that underscore the importance of cryo-shielding for minimizing thermal radiation onto the device. This capability enables correlative low-temperature TEM studies, in which structural, spectroscopic, and electrical transport data can be obtained from the same device, thereby providing a platform for probing the microscopic origins of
quantum phenomena.
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All authors Joachim Dahl Thomsen, Michael I. Faley, Joseph Vimal Vas, Alexander Clausen, Thibaud Denneulin, Dominik Biscette, Denys Sutter, Peng-Han Lu, Rafal E. Dunin-Borkowski
Superconductivity
Symmetry-broken superconducting configurations from density functional theory for bcc and hcp metals and Nb3Sn
Shun-Li Shang and Zi-Kui Liu
We recently proposed a unified theoretical framework for superconductivity that broadens the applicability of Bardeen-Cooper-Schrieffer (BCS) theory to both conventional and unconventional
superconductors. Within this framework, superconductivity arises from the formation of a
symmetry-broken superconducting configuration (SCC) generated by atomic perturbations of the normal conducting configuration (NCC). The SCC emerges through electron-
phonon interaction and gives rise to distinct straight one-dimensional tunnels (SODTs) in the charge density difference of electrons and/or holes. These SODTs originate from regular and systematic atomic displacements between the SCC and NCC, a phenomenon revealed by density functional theory (DFT) calculations. To further verify this framework, we performed DFT-based calculations for 12 hexagonal close-packed (hcp) elements (Be, Mg, Sc, Y, Ti, Zr, Hf, Tc, Re, Ru, Os, and Zn), 5 body-centered cubic (bcc) elements (V, Nb, Ta, Mo, and W), and the compound Nb3Sn. Our results indicate that all these materials exhibit superconductivity at 0 K and 0 GPa, as evidenced by the predicted SODTs. Notably, Mg, Sc, and Y are predicted to be superconducting under ambient pressure, a finding that awaits experimental confirmation.
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Materials Science
Epitaxial thin film growth in the U-Ge binary system
Syed Akbar Hussain, Ali A. M. H. Jasem, Lottie M. Harding et al. (+2 more)
We explore the U-Ge phase diagram using thin film growth by co-deposition of U and Ge via d.c. magnetron sputtering. Using three different single
crystal substrates - MgO, CaF$_2$ and SrTiO$_3$ - we have stabilised mixed phase films of mostly UGe$_3$ and UGe, with evidence of UGe$_2$ as well. At higher temperatures UO$_2$ forms as a consequence of gettering of oxygen from several types of substrate. Several UGe$_3$ dominated samples grown on MgO substrates have also been characterised electrically, showing residual resistivity ratios up to six.
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All authors Syed Akbar Hussain, Ali A. M. H. Jasem, Lottie M. Harding, Ross S. Springell, Christopher Bell
Superconductivity
Resistivity anomalies and intrinsic spin-orbit coupling in superconducting thin film solid solutions of Nb$_{1-x}$U$_x$ for $0.15 < x < 0.40$
Syed Akbar Hussain, Katie Brewer, Livina Onuegbu et al. (+4 more)
Polycrystalline thin films of $\mathrm{Nb}_{1-x}\mathrm{U}_{x}$ solid solutions with $0.15\leq x \leq 0.40$ were prepared via d.c. magnetron sputtering at ambient conditions. X-ray characterisation of the samples revealed a systematic shift of the (110) Nb Bragg reflection with U concentration, consistent with substitutional replacement of the Nb by U. Superconductivity was observed in all samples below $2$ K. Analysis of the superconducting critical fields revealed a direct scaling of the
spin-orbit scattering and transport scattering times, indicating Elliott-Yafet-type
spin relaxation in the system. Magnetoresistivity measurements showed a feature in the range $4$ K $\leq T \leq30$ K suggesting a possible a complex interplay between electron-electron interaction and localisation physics.
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All authors Syed Akbar Hussain, Katie Brewer, Livina Onuegbu, Lottie M. Harding, Sean Giblin, Ross S. Springell, Christopher Bell
Mesoscale and Nanoscale Physics
Surface Phonon Hall Viscosity Induced Phonon Chirality and Nonreciprocity in Magnetic Topological Insulator Films
Abhinava Chatterjee and Chao-Xing Liu
The surface half-
quantum Hall effect, a hallmark consequence of
axion electrodynamics, can be induced by gapping out the surface states of topological
insulators through surface magnetization, and has led to a variety of topological response phenomena observed in experiments. In this work, we investigate
phonon dynamics originating from an acoustic analog - the surface
phonon Hall viscosity - that can also occur at the surface of
magnetic topological
insulators. This surface
phonon Hall viscosity stems from the Nieh-Yan action in the strain response of topological
insulators, where strain acts as the effective vierbein field for the bulk low-energy massive Dirac
fermions. Crucially, this viscosity term entangles
phonon dynamics with surface magnetization. In
magnetic topological
insulator films, we find that this interaction causes acoustic
phonons to become
chiral when the magnetization at the top and bottom surfaces is parallel, and nonreciprocal when it is anti-parallel. We further discuss potential experimental signatures of
phonon dynamics induced by surface
phonon Hall viscosity, specifically the
phonon thermal
Hall effect and
magnon-polarons.
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Mesoscale and Nanoscale Physics · Cross-listed from quant-ph
Quantum eigenvalues and eigenfunctions of an electron confined between conducting planes
Don MacMillen
Two of the most iconic systems of
quantum physics are the particle in a box and the Coulomb potential (the third is, of course, the harmonic oscillator). In this expository paper, we consider the
quantum solution to the problem of an electron confined between the grounded planes of an infinite capacitor. The potential arises from the image charges that form in the grounded planes, along with the added condition that at x = 0, L, where L is the distance between the planes, the wavefunction must be zero. This effectively couples a hydrogen like system to a particle-in-a-box (PIB) based on L, the distance between the planes. The problem of finding the electrostatic potential of this infinite series of image charges is an old one, going back to at least 1929. Here, we give a short derivation for one of the limiting cases that yields a compact expression and show how the Kellogg infinite summation formula converges to that value. We note here that this potential is a symmetric double well potential, so there will be many familiar properties of its solutions. Then using that potential, we solve Schrödinger's equation using a spectral technique. The limiting forms of a particle in a box for small L (and high E), and that of a (degenerate) bound image charge at large L and small energy are recovered. We also discuss the tunneling level splitting that occurs in the transition from the large L to the small L regime.
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Soft Condensed Matter
Structure and Memory Control Self-Diffusion in Active Matter
Akinlade Akintunde and Stewart A. Mallory
Despite extensive progress in characterizing the emergent behavior of active matter, the microscopic origins of self-
diffusion in interacting active systems remain poorly understood. Here, we develop a framework that quantitatively links self-
diffusion to collisional forces and their temporal correlations in active fluids. We show that transport is governed by two contributions: an equal-time suppression of motion arising from anisotropic collisional forces, and a
memory correction associated with the temporal persistence of these forces. Together, these effects yield an exact expression for the self-diffusivity in terms of measurable force statistics and correlation times. We apply this framework to purely repulsive active Brownian particles and find that self-
diffusion is always reduced, with collisional
memory acting as a strictly dissipative correction. Our results establish a direct connection between microscopic force correlations and macroscopic transport, providing a general mechanical perspective for interpreting self-
diffusion in active matter.
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Strongly Correlated Electrons
Resonant level model from a Krylov perspective: Lanczos coefficients in a quadratic model
Merlin Füllgraf, Jiaozi Wang, Jochen Gemmer et al. (+1 more)
We study the Lanczos coefficients in a quadratic model given by an
impurity interacting with a multi-mode field of
fermions, also known as resonant level model. We analytically derive closed expressions for the Lanczos coefficients of Majorana
fermion operators of the
impurity for different structures of the coupling to the hybridization band at zero temperature. While the model remains quadratic, we find that the growth of the Lanczos coefficients structurally depends strongly on the chosen coupling. Concretely, we find $(i)$ approximately constant, $(ii)$ exactly constant, $(iii)$ square root-like as well as $(iv)$ linear growth in the same model. We further argue that in fact through suitably chosen couplings, essentially arbitrary Lanczos coefficients can be obtained in this model. These altogether evince the inadequacy of the Lanczos coefficients as a reliable criterion for classifying the integrability or chaoticity of the systems. Eventually, in the wide-band limit, we find exponential decay of autocorrelation functions in all the settings $(i)-(iv)$, which demonstrates the different structures of the Lanczos coefficients not being indicative of different physical behavior.
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All authors Merlin Füllgraf, Jiaozi Wang, Jochen Gemmer, Stefan Kehrein
Mesoscale and Nanoscale Physics
Zian Xia, Ruoban Ma, Chang Shu et al. (+2 more)
This study explores how
entanglement and
quantum teleportation of
magnons can be achieved in coupled
spin chain systems. By utilizing different
magnetic configurations, we show that parallel
spin chains function like magnonic beam splitters, whereas anti-parallel chains produce two-
magnon squeezing and strong
entanglement. Combining these components, we design magnonic
circuits capable of continuous-variable
quantum entanglement and teleportation, supported by
quantum Langevin simulations.
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All authors Zian Xia, Ruoban Ma, Chang Shu, Huaiyang Yuan, Jiang Xiao
Materials Science
Rotating Magnetocaloric Effect in First-order Phase Transition Material Gd5Si2Ge2
Rafael Almeida, Rodrigo Kiefe, Ricardo Moura Costa Pinto et al. (+5 more)
The rotating magnetocaloric effect (RMCE) induced by self-demagnetization has been investigated in the
giant magnetocaloric effect (GMCE) material Gd$_5$Si$_2$Ge$_2$. This shape-dependent effect had thus far only been reported in pure Gd, marking this as the first analysis of the effect in a sample with a magnetostructural first-order phase transition. By rotating the applied
magnetic field vector while keeping its intensity constant, the demagnetizing field within a high-aspect ratio sample changes significantly, resulting in a RMCE. We characterize RMCE by determining the adiabatic temperature change ($\Delta T_{ad}^{rot}$) directly through temperature measurements, and the isothermal
entropy change ($\Delta S_M^{rot}$) via magnetometry and magnetostatic simulations. We obtain a remarkable maximum $\Delta T_{ad}^{rot}$ of 1.77 K for a constant external field of 0.8 T, higher than that obtained under 1.0 T. The magnetostatic simulations not only corroborate the highly non-monotonous field-dependence of $|\Delta S_{M}^{rot}|$, which reaches 95% of its maximum value at 0.8 T, 6.12 J K$^{-1}$ kg$^{-1}$ for the experimentally measured shape, but also estimate a 35% increase in the maximum $|\Delta S_{M}^{rot}|$ up to 8.67 J K$^{-1}$ kg$^{-1}$ in a simulated shape with higher aspect ratio.
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All authors Rafael Almeida, Rodrigo Kiefe, Ricardo Moura Costa Pinto, João Sequeira Amaral, Kyle Dixon-Anderson, Yaroslav Mudryk, João Pedro Araújo, João Horta Belo
Materials Science
High-Resolution Capacitance Dilatometry of Microscopically Thin Samples Using a Miniature Dilatometer
R. Küchler, S. Panja, S. Wirth et al. (+1 more)
We present a novel application of our high-resolution capacitance dilatometer, specifically engineered for the precise characterization of
quantum materials. These materials, which often appear as ultrathin, platelet-shaped
crystals, are known for exotic phenomena such as superconductivity, topological order and
quantum spin liquid. However, these
crystals seldom reach macroscopic dimensions, making them unsuitable for conventional dilatometry techniques. By introducing a modified sample-mounting configuration, our design enables high-resolution measurements of thermal expansion and magnetostriction along in-plane crystallographic directions in samples with thicknesses well below 500 $\mu$m. Validation measurements using a
Quantum Design PPMS system confirm reliable performance for a 300 $\mu$m-thick silver platelet, relatively hard
ferromagnetic EuB$_6$ single
crystals down to 50 $\mu$m, and a 40 $\mu$m-thin, soft AgCrS$_2$ single
crystal. This advancement significantly broadens the applicability of capacitance dilatometry, providing a powerful platform for investigating emergent phenomena in reduced-dimensional
quantum systems.
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All authors R. Küchler, S. Panja, S. Wirth, P. Gegenwart
Mesoscale and Nanoscale Physics
A functionally reversible probabilistic computing architecture enabled by interactions of current-controlled magnetic devices
Shreyes Nallan and Jian-Gang Zhu
Probabilistic computers replace logic gates with networks of interacting random variables, creating bidirectional systems that can back-derive inputs from outputs. Such architectures enable efficient generation of random samples, implementations of novel
algorithms, and natural solutions to classically hard problems such as
prime factorization. We present a new physical implementation for these networks:
ferromagnetic disks whose magnetization switching process is triggered by current pulses, skewed by external
magnetic fields, and randomized by ambient thermal noise. We show that geometry-dependent magnetostatic interactions between these
magnetic cells lead to system behavior that emulates deterministic logic gates. Furthermore, by chaining multiple "gates," we achieve a highly accurate bidirectional one-bit full-adder, a proof of concept for complex multi-gate logic functions with reversible information flow. This analog
magnetic probabilistic computer methodology improves on other implementations in speed, tunability, and energy efficiency, thereby enabling a powerful new pathway towards practical solution of classically hard problems.
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Quantum Gases
Anomalous diffusion and localization in a disorder-free atomic mixture
Stefano Finelli, Beatrice Restivo, Alessio Ciamei et al. (+5 more)
The concept of random walk, in which particles or waves undergo multiple collisions with the microscopic constituents of a surrounding medium, is central to understanding diffusive transport across many research areas. However, this paradigm may break down in complex systems, where
quantum interference and
memory effects render the particle propagation anomalous, often fostering localization. Here we report on the observation of such anomalous dynamics in a minimal setting: an ultracold mass-imbalanced mixture of two fermionic gases in three dimensions. We release light
impurities into a gas of heavier atoms and follow their
evolution across different collisional regimes. Under strong interspecies interactions, by lowering the temperature we unveil a crossover from normal
diffusion to subdiffusion. Simultaneously, a localized fraction of the light gas emerges, displaying no discernible dynamics over hundreds of collisions. Our findings, incompatible with the conventional
Fermi-
liquid picture, are instead captured by a model of an atom propagating through a (quasi-)static disordered landscape of point-like scatterers. These results highlight the key role of
quantum interference in our mixture, which emerges as a versatile platform for exploring disorder-free localization phenomena.
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All authors Stefano Finelli, Beatrice Restivo, Alessio Ciamei, Andreas Trenkwalder, Massimo Inguscio, Dmitry S. Petrov, Sergey E. Skipetrov, Matteo Zaccanti
Mesoscale and Nanoscale Physics
Patrick J. Wong, Zackary White and Alexander V. Balatsky
The interplay between the curved spacetimes of general relativity and
quantum mechanical systems is an active field of research. However, analysis of relativistic gravitation on extended
quantum systems remains understudied. To this end, we study here the effects of a general relativistic curved spacetime on the topological phases of the Su-Schrieffer-Heeger model and Kitaev superconducting wire. We find that the topological states remain robust and well localized. In the topological
insulator we find that the energy level of the topological state becomes shifted away from zero according to the gravitational
redshift, breaking the system's
chiral symmetry. In contrast, the Majorana zero mode of the topological
superconductor remains at zero energy. Furthermore, within the topological
superconductor, we identify the possibility of a gravitationally induced topological phase transition leading to the formation of a domain wall, shifting one of the boundary Majorana zero modes into the bulk.
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Strongly Correlated Electrons
Unveiling Hidden Magnons with Anomalous Rotational Symmetry
Dirk Wulferding, Francesco Gabriele, Wojciech Brzezicki et al. (+7 more)
Correlated materials with competing
spin-orbit and
crystal-field interactions can host composite
spin-orbital
magnons that are highly susceptible to structural and electronic perturbations, enabling the control of
magnetic dynamics beyond
spin-only physics. Using Raman spectroscopy on Ca$_2$RuO$_4$, we show that the partial substitution of Ru with Mn reconstructs the
magnon spectrum and reveals one-
magnon modes that are hidden in the undoped state. We demonstrate that the transition-metal substitution activates otherwise
symmetry-forbidden
magnon modes through mirror-
symmetry breaking of the underlying
spin-orbital configuration. This effect can be theoretically explained by the local structural distortions induced in the RuO$_6$ octahedra near the dopant, that enable the observation of mixed-parity one-
magnon modes. These excitations display a distinctive polarization dependence, with a lowering from fourfold to twofold rotational
symmetry arising from the mixed-parity character of the coupled
magnons and interference between resonant and nonresonant scattering channels. Our results show that
spin-orbit-
lattice entanglement provides a route to tailoring collective
magnetic excitations and their polarization response in
spin-orbit-coupled correlated systems.
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All authors Dirk Wulferding, Francesco Gabriele, Wojciech Brzezicki, Mario Cuoco, Changyoung Kim, Mariateresa Lettieri, Anita Guarino, Antonio Vecchione, Rosalba Fittipaldi, Filomena Forte
Materials Science
Beta-AlGaO/Ga2O3 Tri-Gate MOSHEMT with 70GHz fT and 55GHz fmax
Noor Jahan Nipu, Chinmoy Nath Saha and Uttam Singisetti
We report Beta-AlGaO/Ga2O3 tri-gate
heterostructure MOSHEMTs incorporating a thin 5 nm Al2O3 gate
oxide layer for improved gate control and reduced leakage. The devices were fabricated on AlGaO/GaO
heterostructures grown by ozone MBE on Fe-doped Ga2O3 (010) substrates. The tri-gate MOSHEMTs, with 1 micron-wide fins and Lg=155 nm, exhibit a peak current-gain cut-off frequency fT=70 GHz and a power-gain cut-off frequency fMAX=55 GHz.The fT.L product of 10.85 GHz-micron is the highest among reported Ga2O3 FETs to date. The devices show Vth =-0.5 V, an on/off ratio 10^6 I=80 mA/mm, a peak gm=60 mS/mm, and a low gate leakage current of 10^(-10) mA/mm at Vgs=0.5 V. Passivation with a 100 nm ALD Al2O3 layer effectively removes DC/RF dispersion and maintains stable operation under pulsed IV and repeated RF measurements. These results demonstrate the potential of tri-gate AlGaO/GaO MOSHEMTs for next-generation high-frequency and high-power applications.
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Mesoscale and Nanoscale Physics
Electronic phonon-induced magnetism in moiré Mott-Wigner crystals
Weide Liang, Xiaoying Du, Na Zhang et al. (+1 more)
We show that magnetism in moiré Mott-Wigner
crystals can be induced by the collective vibration of electrons around their
equilibrium positions (i.e., electronic
phonons), even without
spin interactions between electrons. Due to a
geometric valley-orbit coupling from the Berry phase effect, the zero-point energy of electronic
phonons reaches minimum when electrons are fully valley polarized. This leads to a spontaneous magnetization when below a critical temperature. We also propose to engineer the magnetism through the photoexcitation of
chiral electronic
phonons.
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All authors Weide Liang, Xiaoying Du, Na Zhang, Hongyi Yu
Mesoscale and Nanoscale Physics
Abelian and non-Abelian fractionalized states in twisted MoTe$_2$: A generalized Landau-level theory
Bohao Li, Yunze Ouyang and Fengcheng Wu
Fractional Chern
insulators are
lattice analogs of fractional
quantum Hall states that realize fractionalized quasiparticles without an external
magnetic field. A key strategy to understand and design these phases is to map Chern bands onto Landau levels (LLs). Here, we introduce a universal framework that variationally decomposes Bloch bands into generalized LLs, providing a controlled and quantitative characterization of their effective LL nature. Applying this approach to twisted
bilayer MoTe$_2$ modeled by first-principles-derived moiré
Hamiltonians, we find that the first moiré valence band is dominated by the generalized zeroth LL across a broad range of twist angles, facilitating the formation of Abelian fractional Chern
insulators in the Jain sequences. The second moiré band, renormalized via Hartree-Fock calculations at hole filling $\nu_h = 2$, is dominated by the generalized first LL at twist angles $\theta = 2.45^\circ$ and $2.13^\circ$. At $\theta = 2.45^\circ$, we find numerical evidence for a non-Abelian Moore–Read (MR) state at $\nu_h = 5/2$, with consistent signatures in both the energy spectrum and the particle
entanglement spectrum. Interpolation studies further demonstrate an adiabatic connection between this state and the MR state in the conventional first LL. In contrast, at $\theta = 2.13^\circ$, a charge-density-wave state prevails in the competition with the MR state due to the larger bandwidth. Our variational mapping provides a theoretical framework for exploring exotic fractionalized phases, including non-Abelian states, in realistic systems.
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Soft Condensed Matter
Counting unlabelled multigraphs with three nodes
Andrea Bonato
Unlabeled multigraphs have diverse applications across scientific fields, from transportation and social networks to
polymer physics. In particular, multigraphs are essential for studying the relationship between the spatial organization and biological function of chromatin, which is often folded into complex
polymer networks whose structure is closely tied to patterns of gene expression. A fundamental yet challenging aspect in applying graph theory to these areas is the enumeration of multigraphs, especially under structural constraints For example, when coupled with the statistical mechanics of
polymer networks, the ability to identify traversable and connected multigraphs provides powerful tools for predicting statistically favored motifs that may arise within chromatin networks. In this work, by counting the adjacency
matrices, we derive
polynomial expressions that enumerate all connected, undirected, and unlabeled multigraphs with three nodes and fixed degree, and provide a method to efficiently generate them.
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Materials Science
Machine Learning Guided Polymorph Selection in Molecular Beam Epitaxy of In2Se3
Ryan Trice, Mingyu Yu, Eric Welp et al. (+3 more)
Indium selenide (In2Se3), a layered chalcogenide with multiple polymorphs, is a promising material for optoelectronic and ferroelectric applications. However, achieving polymorph-pure thin films remains a major challenge due to the complex growth space. In this work,
Bayesian optimization (BO) is successfully leveraged to guide the molecular beam epitaxy growth of In2Se3 on Al2O3 substrates. By training a predictive Gaussian process regressor with sequential learning, we efficiently explored substrate temperature, indium flux, selenium flux, and cracker temperature, reducing experimental trials required for successful synthesis. A {\gamma}-In2Se3 film with 91% phase purity was achieved in fewer than 10 BO run samples. Attempts to isolate {\alpha}-In2Se3 were limited by amorphous film formation at low temperatures, indicating that single-step codeposition is unsuitable for crystalline {\alpha}-In2Se3 on Al2O3. Overall, this study validates BO as a powerful approach for phase-selective growth in complex material systems.
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All authors Ryan Trice, Mingyu Yu, Eric Welp, Morgan Applegate, Wesley Reinhart, Stephanie Law
Materials Science
Magnetism and 3D Electron Diffraction Solution of Hydrated Rubidium-Ruthenium Oxide Rb$_2$Ru$_2$O$_7$.H$_2$O
Krystof Chrappova, Jeremiah P. Tidey, Christopher Bell et al. (+1 more)
The
crystal structure of Rb$_2$Ru$_2$O$_7$.H$_2$O was determined by three-dimensional electron diffraction from the individual crystallites of a solid-state powder product. Rb$_2$Ru$_2$O$_7$.H$_2$O crystallizes in space group C2/c ($a=7.841(3)$ Å, $b=12.500(3)$ Å, $c=8.392(2)$ Å, $\beta=93.57(4)^\circ$, Z=4). The structure contains infinite chains that run normal to the (101) plane and consist of alternating RuO$_6$ octahedra and square-pyramidal RuO$_5$ units connected via shared O-O edges.
Magnetic properties were measured on the bulk powder, showing a diamagnetic baseline from 300 to 60 K with a small Curie tail below 55 K. The
magnetic moment, calculated from the 1.8 K isotherm, saturates at $M=4.4\times10^{-3}\,\mu_\mathrm{B}\,\mathrm{Ru}^{-1}$, much less than would be expected for $S=1$ ruthenium. Bond-valence-sum analysis indicates high-valent Ru, and the near-diamagnetic response is consistent with the edge-sharing Ru-Ru motif, where weak direct Ru-Ru overlap yields a local singlet.
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All authors Krystof Chrappova, Jeremiah P. Tidey, Christopher Bell, Simon R. Hall
Materials Science
Disentangling the Discrepancy Between Theoretical and Experimental Curie Temperatures in Ferroelectric PbTiO$_3$
Denan Li, Christian S. Ahart and Shi Liu
Accurately predicting the Curie temperature ($T_c$) of ferroelectrics from first principles remains a major challenge, as theoretical estimates often fall significantly below experimental values. In this work, we investigate the origin of these discrepancies in the prototypical ferroelectric PbTiO$_3$ by performing extensive constant-pressure ab initio molecular dynamics (AIMD) simulations and benchmarking them against classical molecular dynamics (MD) using machine learning force fields (MLFFs) derived from first-principles data. Our results show that the underestimation of $T_c$ primarily stems from the limitations of the exchange-correlation functional, rather than inaccuracies in the MLFF fitting. We uncover a critical interplay between finite-size effects and the range of interatomic interactions: although short-range MLFFs appear to yield better agreement with experimental $T_c$, this improvement results from a fortuitous cancellation of errors. Incorporating explicit long-range interactions improves accuracy for larger supercells but ultimately leads to lower predicted $T_c$ values. These findings highlight that accurate finite-temperature predictions require not only high-quality training data and sufficiently large simulation
cells, but also the explicit treatment of long-range interactions and improved exchange-correlation functionals.
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Strongly Correlated Electrons
Charge Order in the half-filled bond-Holstein Model
Charles Jordan, George Issa, Ehsan Khatami et al. (+3 more)
We use determinant
quantum Monte Carlo to study the half-filled `bond-Holstein' model on a square
lattice. We find that the model exhibits a charge-density-wave (CDW) phase transition with a critical temperature $T_\mathrm{cdw}$ considerably higher than that of the canonical `site-Holstein' model. Using a finite-size scaling analysis of the charge structure factor $S_{\rm cdw}$, we obtain $T_\mathrm{cdw}$ to greater than one percent accuracy. At the same time, local observables also show clear signatures consistent with the transition temperatures inferred from our scaling analysis. We attribute the enhanced CDW tendencies to a
phonon-mediated nearest-neighbor electron repulsion that is directly proportional to the dimensionless electron-
phonon coupling $\lambda$ in the atomic ($t\rightarrow 0$) limit. This behavior contrasts with the site-Holstein case, where the same limit yields only an on-site attraction. We supplement our analysis with results from several unsupervised machine learning methods, which not only confirm our estimates of $T_\mathrm{cdw}$ but also provide insight into the high-temperature crossover between a metallic and bipolaron
liquid regime.
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All authors Charles Jordan, George Issa, Ehsan Khatami, Richard Scalettar, Benjamin Cohen-Stead, Steven Johnston
Materials Science
Robust phonon engineering and symmetry-selective lattice dynamics in CrSBr$_{1-x}$Cl$_{x}$
Satyam Sahu, Arsalan Hashemi, Mahdi Ghorbani-Asl et al. (+9 more)
Atomic substitution provides a controlled route to engineer
lattice dynamics in low-
symmetry two-dimensional materials. Here, by combining polarization-resolved Raman spectroscopy and first-principles calculations, we investigate the
evolution of
phonon characteristics in CrSBr$_{1-x}$Cl$_{x}$ ($0 \leq x \leq \sim 0.5$) upon partial substitution of Br with Cl atoms. Progressive Cl substitution of Br induces systematic shifts of parent CrSBr out-of-plane $A_\textrm{g}$
phonon modes and activates additional Raman features. These features persist across different polarization configurations and excitation energies, reflecting substitution-induced
symmetry lowering and local
lattice perturbations. Explicit supercell
phonon calculations combined with Raman $\Gamma$-density-of-states simulations identify these features as
symmetry-lowered descendants of parent modes arising from
alloy disorder. Complementary strain-dependent calculations reveal that anisotropic
lattice compression plays a key role in renormalizing Cr-S dominated
phonons. Under near-resonant excitation, stimulated Raman scattering-like amplification remains observable with increasing Cl content, highlighting the resilience of anisotropic electron-
phonon coupling in this system.
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All authors Satyam Sahu, Arsalan Hashemi, Mahdi Ghorbani-Asl, János Koltai, Jan Maňák, Bing Wu, Aljoscha Söll, Zdeněk Sofer, Mikko Karttunen, Arkady V. Krasheninnikov, Matěj Velický, Otakar Frank
Strongly Correlated Electrons
Superconductivity in doped symmetric mass generation insulator: a quantum Monte-Carlo study
Sibo Guo, Wei-Xuan Chang, Yi-Zhuang You et al. (+1 more)
Understanding unconventional superconductivity (SC) driven by strong electronic correlations is a central challenge in condensed matter physics. In this work, we employ sign-problem-free
quantum Monte Carlo (QMC) simulations to systematically investigate a
bilayer fermionic model featuring strong interlayer
antiferromagnetic (AFM) exchange and on-site repulsive Hubbard interactions. This system serves as a prototypical model for realizing a symmetric mass generation (SMG)
insulator. Our numerically exact results unambiguously demonstrate that robust superconducting pairing emerges upon doping the SMG phase. Remarkably, we find that the SC order is significantly enhanced by the repulsive Hubbard interaction. Given its potential relevance to the essential features of the high-$T_c$
superconductor $\mathrm{La}_{3}\mathrm{Ni}_{2}\mathrm{O}_{7}$ under pressure, our study establishes a new paradigm for superconductivity arising from a doped SMG parent state and provides key theoretical guidance for future experimental investigations.
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All authors Sibo Guo, Wei-Xuan Chang, Yi-Zhuang You, Zi-Xiang Li