Disordered Systems and Neural Networks
Quantum and classical entropic complexity of the thermal state: coherence, decoherence, and the ergodic-to-localized crossover in random-matrix and many-body models
Imre Varga and Onur Aktan
Does thermal averaging preserve signatures of eigenstate complexity? We study the entropic complexity C = S_1 - S_2 (Shannon minus second-order Renyi
entropy) across the ergodic-to-localized crossover of three disordered models: the Rosenzweig-Porter (RP) ensemble, the power-law banded random
matrix (PLBRM) ensemble, and the random-field Heisenberg chain. We compare a wavefunction-level quantity C_eig to the complexity of the thermal (Gibbs) state before and after pointer-basis dephasing, via the trace (C_tr) and diagonal (C_diag) complexities. The answer is mostly no: thermal averaging strongly suppresses, but does not eliminate, eigenstate-complexity signatures. C_eig develops a pronounced mid-phase maximum in RP and, at matching fractal dimension, in the structurally independent PLBRM model; a high-statistics scan resolves a weak (about 10%) but reproducible thermal shadow of this peak in the thermal diagonal complexity. This feature is confined to the two random-
matrix models; in the Heisenberg chain the eigenstate complexity instead peaks at the many-body localization transition. A second, genuinely thermal feature, an edge just inside the ergodic phase, has no eigenstate counterpart and, in PLBRM, recedes with system size. Both features are tracked by scale-invariant crossover indicators: the log-ratio of the trace and diagonal crossover temperatures, which vanishes upon localization, and the relative
entropy of coherence. Entropic complexity thus cleanly separates thermal-state and eigenstate physics: a sharp wavefunction-level feature, reproducible across unrelated random-
matrix constructions, leaves only a faint, structurally distinct imprint on thermal observables.
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Mesoscale and Nanoscale Physics
Fermion parity of an Andreev molecule probed by nonlocal Josephson effect
S. Annabi, H. Riechert, K. Watanabe et al. (+5 more)
Fermion parity is a fundamental property of superconducting many-body states. Here, we show that the global
fermion parity of a delocalized superconducting state can be detected locally by exploiting the nonlocal Josephson effect. Using a carbon nanotube-based Andreev molecule formed by two coupled
quantum-dot Josephson junctions, we observe a pronounced nonlocal Josephson response and demonstrate the formation of delocalized Andreev molecular states extending across both junctions. We further show that changes in the molecular ground-state parity manifest as characteristic $\pi$-phase shifts in the nonlocal response. Supported by a minimal theoretical model, these results identify global
fermion parity as an experimentally accessible degree of freedom in hybrid superconducting
circuits that can be readily revealed through the nonlocal Josephson effect.
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All authors S. Annabi, H. Riechert, K. Watanabe, T. Taniguchi, J. Griesmar, E. Arrighi, L. Bretheau, J.-D. Pillet
Disordered Systems and Neural Networks
A Flickering Resonance Degeneracy on Entropy-Ruled Charge Transport in the Extended Hopping Sites for Biological Systems: Role of Static-to-Dynamic Disorder Transition
K. Navamani and M. Pavalamuthu
Charge transport (CT) in biological systems is of great interest due to its role in various functional activities like photobiology, bioenergetics, redox catalytic and other metabolic activities, etc. It has been observed by various studies that the presence of a dynamic disorder in biomolecules facilitates charge dynamics in the intermediate transport regime, i.e., far from hopping and towards a bank-like mechanism. Hitherto, the dynamic disorder (in a time scale, coupling between electronic and
nuclear dynamics) weightage on CT in molecules is not well-established for the measurement of molecular conductivity from small to long-range ordering. With this motivation, we propose the flickering resonance-coupled
entropy-ruled charge transport theory for transfer-rate and
diffusion-based mobility (D/{\mu}) calculations. The proposed analytical formalism incorporates the impact of dynamic disorder-correlated degeneracy (in a flickering-resonance manner) on the
entropy-ruled electron transfer rate and
diffusion-mobility, which are valid for localized hopping, delocalized band transport, and regimes in between. By this approach, it has been observed that the dynamics-driven electronic site matching probability enhances the degeneracy, which is quantified by the differential
entropy. The analysis clearly shows that for ultra-fast dynamical and degenerate cases, the
entropy-ruled mobility is transformed as band mobility, rather than the thermally activated hopping or
diffusion process.
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Statistical Mechanics
Scaling regimes of the Kuramoto-Sivashinsky equation from the functional renormalization group
Liubov Gosteva, Nicolás Wschebor and Léonie Canet
We revisit the renormalization group (RG) approach to the one-dimensional
stochastic Kuramoto-Sivashinsky (KS) equation and show that previous approaches based on perturbative Wilsonian RG with a sharp cutoff are not valid, even though they yield a qualitatively correct picture. The reason is that taking momentum derivatives while using the sharp cutoff is not well-defined in some cases and leads to intrinsic divergencies. This is a well-known problem of Wilsonian RG, which can be simply cured by using a smooth cutoff, and employing the functional renormalization group (FRG) framework. We establish the flow equations for the KS model within the FRG, and demonstrate that it flows to the Kardar-Parisi-Zhang (KPZ) fixed point at large scales. We then calculate the full two-point correlation function over a wide range of momenta and frequencies. We show that it exhibits three universal scaling regimes that we characterize: the KPZ regime (with dynamical exponent $z=3/2$), the Edwards-Wilkinson regime (with $z=2$) and the recently discovered inviscid regime (with $z=1$). The latter develops over an extended range of large wavenumbers and originates from the vanishing of the effective viscosity. Lastly, we investigate the large-scale behavior of the deterministic KS equation by studying the limit of vanishing noise and we determine the scales where the KPZ regime can emerge in the deterministic case.
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Other Condensed Matter
Reversible and irreversible dynamical topological transitions of magnetic Hopfions
S. Y. Lu, H. M. Dong, D. X. Yu et al. (+1 more)
Magnetic Hopfions are three-dimensional (3D) topological
solitons characterized by a nonzero
Hopf invariant and offer a promising platform for 3D spintronics. While their static stabilization has been widely studied, their nonlinear dynamics under alternating
magnetic (AM) fields remain largely unexplored. We show, using 3D micromagnetic simulations and analytical mode analysis, that an AM field drives two qualitatively distinct dynamical regimes of a confined
magnetic Hopfion. In the weak-field regime, resonant excitation of intrinsic Hopfion modes induces a nonlinear instability and an irreversible topological reconfiguration from a Hopfion to a toron. In contrast, in the strong-field regime, the system undergoes reversible field-locked topological switching at GHz frequencies, with the magnetization periodically alternating between a topologically trivial
ferromagnetic configuration and a Hopfion state. The switching pathway is selected by the driving frequency: a 2 GHz field drives a breathing pathway associated with the low-frequency collective response, whereas a 40 GHz field produces a nonresonant rotational pathway governed by strong Zeeman-torque-driven precession and field locking. These results identify field amplitude and frequency as independent control knobs and reveal that reversible Hopfion switching can arise either from nonlinear continuation of low-frequency collective motion or from nonresonant high-frequency field locking.
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All authors S. Y. Lu, H. M. Dong, D. X. Yu, K. Chang
Quantum Gases
Controlling charge and spin currents through nonreciprocal dissipative processes
Catalin-Mihai Halati and Jean-Sébastien Bernier
We investigate the generation and control of both charge and
spin currents via nonreciprocal dissipative mechanisms in a two-dimensional spinful fermionic atom
quantum system with broken inversion and time-reversal
symmetries. Within the Gorini-Kossakowski-Sudarshan-Lindblad master equation formalism and using an approach based on the time-dependent generalized Gibbs ensemble, we identify in the weak dissipative coupling regime the minimal set of nonreciprocal jump operators required to induce charge and
spin currents and to control both their direction and magnitude. We find that in the presence of finite tunneling, Rashba coupling and
magnetic field, the combine application of two jump operators nonreciprocally coupling each
spin species to a different spatial direction of motion is sufficient to generate both types of current. Furthermore, by tuning the degree of nonreciprocity of the jump operators we modify the dominant transport mechanism from
spin to charge. Finally, we checked that this nonreciprocal current generation mechanism is robust to the application of dephasing noise as even in the presence of this additional dissipative process the steady-state occupation distributions for the quasiparticle modes of the
Hamiltonian remains non-trivial, an essential requirement to obtain non-zero currents.
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Strongly Correlated Electrons
Machine-learning test of the single-ion model for $dd$ excitations in cuprates
Maryia Zinouyeva, Leonardo Martinelli, Riccardo Arpaia et al. (+12 more)
We investigate $dd$ excitations in Resonant Inelastic X-ray Scattering spectra of YBa$_2$Cu$_3$O$_6$ and La$_2$CuO$_4$ using the local single-ion model. The data are analyzed by conventional global fitting and by a convolutional neural network trained within the same theoretical framework. For YBa$_2$Cu$_3$O$_6$, the excited state energies obtained with the two methods coincide, leading to the $xy$, $3z^2-r^2$, $xz/yz$ sequence for increasing energy. This result validates the use of machine learning tools for the analysis of RIXS spectra dominated by $dd$ excitations. By contrast, for La$_2$CuO$_4$, the two methods do not converge to a single solution, revealing the limitations of the single-ion model in describing $dd$ excitations in cuprates and pointing to the role of additional contributions beyond a purely local picture in shaping high-energy excitations.
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All authors Maryia Zinouyeva, Leonardo Martinelli, Riccardo Arpaia, Nicholas B. Brookes, Daniele Di Castro, Kurt Kummer, Floriana Lombardi, Giacomo Merzoni, Francesco Rosa, Alessandro Tarasio, Enrico Tassi, Flora Yakhou-Harris, Ezio Puppin, Marco Moretti Sala, Giacomo Ghiringhelli
Materials Science · Cross-listed from gr-qc
Einstein crystals in Snyder and Snyder-de Sitter noncommutative backgrounds
Anna Pachoł and Aneta Wojnar
We investigate the behavior of Einstein
crystals in noncommutative backgrounds described by the Snyder and Snyder-de Sitter models. Possible novel effects, which may arise in realistic systems such as diamond
crystals, are analyzed within a thermodynamical framework. We show that noncommutativity influences the key thermodynamic quantities, including internal energy and specific heat. These corrections can be directly related to modifications of the underlying uncertainty relations, of the generalized uncertainty principle (GUP) and generalized extended uncertainty principle (GEUP) types.
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Superconductivity
Contactless terahertz mapping of wafer-scale superconducting NbTiN thin films
Yayi Lin, Marc Neis, Marcello Pio Guardascione et al. (+9 more)
For large-scale superconducting
quantum technology, e.g.
quantum computing, the homogeneity of wafer-scale superconducting thin films is vital for consistent performance of the fabricated devices. Terahertz (THz) spectroscopy as a contactless and non-destructive measurement technique is a powerful tool to characterize the superconducting films. In this work, a set of niobium titanium nitride (NbTiN) thin films on 4-inch and 6-inch silicon wafers, grown via
plasma-enhanced magnetron sputtering, are investigated via THz spectroscopy: full wafers are mapped at room temperatures and exemplary segments are characterized at cryogenic temperatures. The deviations in observed sheet resistance depend on the used deposition device and the film thickness. While the deviations in superconducting sheet kinetic inductance match those of the normal-state sheet resistance, the critical temperature and energy gap exhibit little variation. This THz mapping technique demonstrates the feasibility of evaluating wafer-scale superconducting thin films before lithography, facilitating preparation of the thin films for reproducible device fabrication.
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All authors Yayi Lin, Marc Neis, Marcello Pio Guardascione, Janine Lorenz, Thomas J. Smart, F. Stefan Tautz, Felix Lüpke, Frederik Bolle, Martin Dressel, Rami Barends, Pavel A. Bushev, Marc Scheffler
Materials Science
Two-step growth of (In,Ga)N pseudo-substrates on GaN templates by plasma-assisted molecular beam epitaxy
Huaide Zhang, Jingxuan Kang, Aidan F. Campbell et al. (+3 more)
(In,Ga)N layers are grown by
plasma-assisted molecular beam epitaxy on GaN templates. We introduce a two-step protocol that involves switching the growth conditions from initially N-stable to metal-stable. Reflection high-energy electron diffraction as well as scanning electron and atomic force microscopy reveal that the first step results in a rough intermediate surface with open pits, whereas the final surface is smooth. The narrow linewidth of the photoluminescence band indicates an excellent compositional homogeneity of the upper layer. Its in-plane
lattice constant is determined to be $\approx$3.26 Åfrom X-ray diffraction measurements. This combination of favorable properties makes these layers attractive as pseudo-substrates for the growth of red-emitting (In,Ga)N light-emitting diodes. In particular, the approach presented here does not require any complex external processing and is, thus, scalable and economical.
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All authors Huaide Zhang, Jingxuan Kang, Aidan F. Campbell, Jonas Lähnemann, Oliver Brandt, Lutz Geelhaar
Materials Science
Interface-Engineered Giant Multistate Resistance Switching in Altermagnetic CrSb Multiferroic Tunnel Junctions
Zhi Yan, Yuwen Hua, Yueting Li et al. (+3 more)
Altermagnets enable
spin-split transport without stray
magnetic fields, yet converting their momentum-dependent
spin splitting into a strong tunnel-junction response requires interface-selected tunneling channels. Here, using density functional theory combined with nonequilibrium Green's function calculations, we demonstrate
giant multistate resistance switching in CrSb/$\alpha$-In$_2$Se$_3$ altermagnetic multiferroic tunnel junctions. The response is governed not by the bulk
spin splitting of CrSb alone, but by a
symmetry-selected interfacial mechanism in which Cr/Sb terminations and h-BN or
graphene insertion layers determine
spin-channel matching, while ferroelectric polarization reshapes the electrostatic barrier. Symmetric and asymmetric terminations reverse the correspondence between parallel/antiparallel Néel-vector configurations and high-/low-resistance states, showing that the actual
alignment of interfacial Cr moments selects the dominant tunneling channels. Monolayer-In$_2$Se$_3$ junctions exhibit four nonvolatile resistance states, with tunneling magnetoresistance (TMR) and tunneling electroresistance (TER) reaching 1626% and 2206%, respectively, and increasing to 9576% and 4144% upon
Fermi-level shifting. Finite-bias calculations further reveal robust
spin filtering and tunable
spin-polarized currents. Extending the barrier to
bilayer In$_2$Se$_3$ introduces interlayer polarization coupling, enabling eight resistance states with maximum TMR and TER values of $3.77\times10^{4}\%$ and $4.18\times10^{5}\%$, respectively. These results establish interface
symmetry,
spin-channel matching, and ferroelectric barrier reconstruction as design principles for stray-field-free multistate spintronic tunnel devices.
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All authors Zhi Yan, Yuwen Hua, Yueting Li, Xujin Zhang, Jianhua Xiao, Xiaohong Xu
Mesoscale and Nanoscale Physics
Rajveer Fandan and Jorge Pedrós
Achieving optical isolation requires breaking
symmetry between forward- and backward-propagating light, a long-standing challenge at the nanoscale in the absence of
magnetic fields. Here we theoretically demonstrate electrically tunable nonreciprocal
phonon-plasmon polaritons in a
graphene/$\alpha$-MoO$_3$/SiC
heterostructure operating in the mid-infrared. A dc current in
graphene induces a wavevector-dependent Doppler shift that breaks reciprocity and generates strong directional asymmetry in hybrid plasmon-
phonon propagation. In the reciprocal regime, hybridization between
graphene plasmons and
hyperbolic phonon polaritons in $\alpha$-MoO$_3$, further shaped by the SiC substrate, enables gate-controlled transitions of isofrequency contours, including canalization along orthogonal
crystal axes. At drift velocities of 5% of the
Fermi velocity, the system exhibits pronounced momentum-dependent nonreciprocity with contrast reaching $\sim$ 0.3, while directions orthogonal to the drift remain unaffected due to
symmetry imposed constraints. Real-space calculations confirm that this momentum-space asymmetry translates into directional near-field intensity modulation. These results establish current-biased van der Waals
heterostructures as a platform for electrically tunable, magnet-free nonreciprocal nanophotonics in the mid-infrared.
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Superconductivity
First-Order Topological FFLO Transition and Superconducting Diode Sign Reversal in Altermagnetic Nanowires
Bo Fu, Kaizhi Bai, Chang-An Li et al. (+1 more)
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state conventionally emerges via a second-order phase transition driven by finite magnetization. Here we show that a
spin-orbit-coupled nanowire proximitized to $d$-wave altermagnets – with zero net magnetization – can realize topological FFLO states through a first-order transition, marked by a sharp sign-reversing superconducting diode effect. The altermagnetic field generates band-resolved competing pairing channels, giving rise to a double-valley free energy landscape whose global minimum switches discontinuously. It consequently leads to a first-order topological FFLO transition with simultaneous jumps in the Cooper pairing amplitude and finite center-of-mass momentum. Remarkably, this discontinuous topological reconfiguration substantially enhances the diode efficiency and drives a characteristic sharp sign reversal across the transition. The mechanism of such exotic phenomena is captured by Ginzburg–Landau theory. Our results provide a field-free altermagnetic route to topological FFLO states and identify their direct transport fingerprint.
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All authors Bo Fu, Kaizhi Bai, Chang-An Li, Shun-Qing Shen
Soft Condensed Matter
Statistical equivalence of reduced gravity and enhanced friction in granular packings
Haiyang Lu, Zhikun Zeng, Houfei Yuan et al. (+5 more)
Using X-ray
tomography, we compare granular packings prepared under buoyancy-reduced effective
gravity with normal
gravity packings of particles with systematically varied friction. We show that reducing
gravity lowers the random loose packing limit in a manner analogous to increasing friction. Granular packings under reduced
gravity and with enhanced friction exhibit identical volume distributions, compactivity, and
entropy, indicating that both routes sample statistically equivalent Edwards volume ensembles of mechanically stable states. This equivalence originates from a common relaxation of the mechanical stability constraint: under both conditions, fewer particles are required to participate in the underlying load-bearing bridge structures, leading to a lower contact-number requirement and a higher density of mechanically stable states. Nevertheless, reduced
gravity retains a distinct contact-scale signature through more isotropic contact orientations. These findings identify
gravity as a physical control governing the statistical accessibility of mechanically stable states within the Edwards framework and provide a unified statistical description of granular packings formed through different physical routes.
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All authors Haiyang Lu, Zhikun Zeng, Houfei Yuan, Chengjie Xia, Hanyu Li, Chijin Zhou, Zihang Xu, Yujie Wang
Soft Condensed Matter
Bifurcation reordering programs snap-through symmetry in folded elastic ribbons
Weicheng Huang, Qun Zhang, Bohan Zhang et al. (+1 more)
Snap-through in slender elastic structures is often viewed as a sudden transition between stable configurations, yet the pathway taken during this transition can differ fundamentally. A structure may snap while preserving
symmetry, or first lose
symmetry and pass through an asymmetric state before reaching its final configuration. What selects between these pathways remains less well understood, especially when the geometry and loading are themselves symmetric. Here, we show that snap-through
symmetry can be programmed by reordering competing bifurcations in folded elastic ribbons. We study an elastic ribbon with two localized folds placed symmetrically about the midpoint and show that varying the fold position changes the relative order of two instabilities: a
symmetry-breaking pitchfork bifurcation and a saddle-node bifurcation on the
symmetry-preserving branch. When the pitchfork bifurcation occurs first, the ribbon loses
symmetry before snapping and follows an asymmetric pathway. Conversely, when the saddle-node bifurcation occurs first, the ribbon loses stability while remaining on the symmetric branch, resulting in a
symmetry-preserving transition. Combining experiments, discrete differential geometry simulations and numerical continuation, we map this exchange in bifurcation ordering and construct a phase diagram that predicts the switch between asymmetric and symmetric snap-through regimes. A reduced-order double-mass von Mises truss model captures the same mechanism as a generic competition between
symmetry-breaking and
symmetry-preserving instabilities. These results establish bifurcation reordering as a
geometric mechanism for programming snap-through pathways in slender elastic structures, offering a design principle for multistable systems, morphing structures and instability-based mechanical devices.
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All authors Weicheng Huang, Qun Zhang, Bohan Zhang, Mingchao Liu
Soft Condensed Matter
How Topology Shapes the Phase Behavior of Polyelectrolytes
David Beyer, Pierre J. Walker, Lena Tarrach et al. (+2 more)
We develop a
topology-specific theory of polyelectrolyte coacervation using the random phase approximation and apply it to both simple and complex coacervation. Our results for
stars and dendrimers show that more compact chain
topologies display a greater propensity for
liquid-
liquid phase separation, as a function of both Bjerrum length and salt concentration. For mixtures of different
topologies, we demonstrate that differences in
polymer topology alone are sufficient to drive multiphase coacervation of polyelectrolytes, which we rationalize in terms of an effective $\chi$ parameter. Analysis of a simplified global phase diagram reveals that the propensity for such
topology-driven phase separation is largest at a finite molecular weight. Overall, our results establish
polymer topology as a powerful design lever for tuning the phase diagram of charged macromolecules independently of molecular weight, net charge, and monomer chemistry, since changes in
topology enable fine-tuning of the effective charge density without altering these molecular characteristics.
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All authors David Beyer, Pierre J. Walker, Lena Tarrach, Zhen-Gang Wang, Christian Holm
Quantum Gases · Cross-listed from nucl-th
Growth of quartet correlations in neutron-rich tellurium isotopes within quartet Bardeen-Cooper-Schrieffer theory
Yixin Guo, Hiroyuki Sagawa and Masaaki Kimura
Quartet correlations in
neutron-rich Te isotopes are investigated within the quartet Bardeen-Cooper-Schrieffer (BCS) framework. Taking $^{100}$Sn as an inert core, we consider two valence
protons and valence
neutrons occupying the $2d_{5/2} \oplus 1g_{7/2}$ model space, and solve the quartet BCS variational equations with a charge-independent isovector pairing interaction. The effective pairing strength is constrained from empirical
neutron pairing gaps in the Te isotopic chain. We find that the valence quartet number increases as the valence
neutron number is enlarged from $N_{\rm val}=2$ to $14$. The same increasing behavior is also found for the condensed quartet component. The
proton occupation of the $1g_{7/2}$ orbit is strongly enhanced relative to the conventional like-particle BCS reference and is driven close to the degeneracy-weighted limit. These results suggest that additional valence
neutrons enhance the quartet admixture in the correlated quartet BCS state, while redistributing the fixed
proton weight from pair-like configurations to quartet configurations.
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Soft Condensed Matter
Kinetic Theory for the Shear Viscosity of Dense Binary Dipolar Fluid Mixtures
Christopher Devik Fjeldstad, Roberto Troncoso and Astrid S. de Wijn
We construct a kinetic theory for the shear viscosity of dense binary fluid mixtures of strongly-interacting dipolar hard spheres. We derive an expression for the pairwise correlations in the binary mixtures that is accurate up to packing fractions around 0.35. The approach is based on Enskog-Thorne theory, and inspired by the theory for dense pure fluids developed by Pousaneh and de Wijn. It relies on effective coupling parameters obtained from the pure fluids combined with mixing rules and a heuristic expression for the collision integral. We compare our results to viscosities obtained numerically from molecular-dynamics simulations of dipolar hard-sphere fluids. Our expression for the shear viscosity of the binary mixtures captures the density and composition dependent behavior of the binary dipolar fluids up to packing fraction of $\xi \lesssim 0.3$ without any mixture-derived fit parameters.
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Statistical Mechanics · Cross-listed from cs.LG
Neural Non-Equilibrium Hamiltonian Monte Carlo for Corrected Boltzmann Sampling
Moxian Qian
Learned dynamical proposals can generate configurations without providing a tractable endpoint density. Nonequilibrium path probabilities offer a way to correct such proposals, but correction alone does not determine their ability to connect separated regions. We introduce Neural Non-
Equilibrium Hamiltonian Monte Carlo (NHMC), which combines conditional momentum distributions with reversible, volume preserving dynamics. The forward–reverse path ratio gives the work used for training, importance weighting, normalizer estimation, and Metropolis correction. We then construct a configuration space round-trip kernel whose reverse and forward paths share an intermediate configuration. Conditional on that configuration, its path record update is independence Metropolis–Hastings. We compare its stationary inter-region flow with the flow obtained under exact conditional matching and bound their difference by the conditional path mismatch. This separates errors in the conditional proposal from dependence between the endpoint region and the intermediate configuration. Many well calculations test normalizers and mode probabilities; a controlled
lattice $\phi^4$ calculation compares ideal and accepted inter-sector flows and measures sector relaxation. In two dimensional $U(1)$, $\mathrm{SU}(2)$, and $\mathrm{SU}(3)$ gauge theories, corrected path weight estimates reproduce finite volume Wilson loops; $U(1)$ topological susceptibilities are also compared with analytic predictions across several
lattice volumes.
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Disordered Systems and Neural Networks
Absence of Spin-Glass Order on Migdal–Kadanoff Hierarchical Lattices near Three Dimensions
Manaka Okuyama and Masayuki Ohzeki
We derive a rigorous sufficient condition for the absence of
spin-glass order in the Ising
spin glass with symmetric binary couplings on Migdal–Kadanoff (MK) hierarchical
lattices with even branching number. The key observation is that a single exact renormalization-group step creates zero effective bonds with positive probability, thereby reducing the problem to bond percolation on the corresponding hierarchical
lattice. When the induced dilution exceeds the percolation threshold, both
spin-glass order and stiffness are absent at all temperatures, including zero temperature. As a consequence, our criterion gives a rigorous proof that the Ising
spin glass on the square
lattice does not exhibit a
spin-glass phase within the MK approximation. More unexpectedly, by choosing sufficiently large scale factors and branching numbers, we construct MK hierarchical
lattices whose fractal dimensions are arbitrarily close to three from below but still exhibit no
spin-glass order. This sharply contrasts with numerical estimates obtained for MK hierarchical
lattices with relatively small scale factors and branching numbers, which placed the lower critical dimension near \(2.52\).
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Mesoscale and Nanoscale Physics
Strong intervalley mixing between copropagating quantum Hall edge channels in a silicon MOSFET
Gento Yamahata and Takase Shimizu
Copropagating
quantum Hall edge channels provide a promising platform for compact electron interferometry and flying
quantum states. In silicon, the valley degree of freedom offers a natural alternative to
spin-resolved edge channels because
spin-flip scattering is strongly suppressed by the weak
spin-orbit interaction. Here, we investigate interchannel transitions between copropagating valley edge channels in a double-layer-gated silicon metal-
oxide-
semiconductor field-effect transistor. With the bulk filling factor set to $\nu=2$, two
spin-polarized valley edge channels are brought into close proximity near a depleted side gate. We observe strong intervalley mixing, with a transition probability close to 1/2, indicating nearly complete equilibration between the two valley edge channels. In contrast, interchannel transport between edge channels with different
spin orientations shows negligible transition probability, consistent with suppressed
spin-flip scattering in silicon. These results demonstrate that intervalley coupling at a Si/SiO$_2$ interface can provide a beam-splitter-like operation for copropagating valley edge channels, establishing a key building block toward compact silicon
quantum Hall interferometers.
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Mesoscale and Nanoscale Physics
Long range spatial correlations in the periodically driven transverse field Ising model
Lazar Paroski and Ivan Iorsh
We consider a periodically driven transverse field Ising model and study the long-time behavior of the correlations between the excitations in the
spin chain. We show that the longest correlation length is defined by the interference between the topological defects induced by the analog of the Kibble-Zurek mechanism and those induced by the Floquet resonance. We show that although the correlations always have a finite correlation length since the system is integrable, the correlation length can be made arbitrarily large by tuning the drive period.
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Statistical Mechanics · Cross-listed from quant-ph
Fate of dynamical quantum phase transitions from sudden quench to slow quench limit
Xiang Zhang and Fuxiang Li
Previous investigations of dynamical
quantum phase transition (DQPT) have predominantly focused on sudden quench protocols. In this work, we systematically explore the fate of DQPT from the sudden to the slow quench regime. We establish that DQPT remains robust under slow quenching when the protocol crosses an
equilibrium quantum phase transition. Conversely, we show that accidental DQPTs,which occur when the pre and post quench
Hamiltonians reside in the same
equilibrium phase, can be filtered out in the slow quench limit. This demonstrates that slow quenches can reveal the intrinsic connection between DQPT and
equilibrium quantum phase transitions, thereby enabling DQPT to serve as a dynamical probe for identifying
equilibrium quantum critical points. We illustrate these findings with examples from the XY chain, the Aubry-André model, and the trimer Su-Schrieffer-Heeger model.
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Mesoscale and Nanoscale Physics · Cross-listed from physics.optics
Near-field Dressing of Thermal Emission
Victor Guillemot, Raphael Mol, Riccardo Messina et al. (+4 more)
Radiative heat transfer at subwavelength distances is generally understood as enhanced energy exchange mediated by
photon tunnelling between neighboring bodies. While near-field interactions can dramatically increase mutual heat transfer, whether they also modify the thermal radiation emitted by the bodies themselves remains an open question. Here we experimentally show that near-field electromagnetic coupling reshapes far-field thermal emission through a distance-dependent dressed emissivity. Using a dual-probe calorimetric platform, we independently monitor the radiative balance of two borosilicate microspheres over separations ranging from 120 micrometers to a few hundred nanometers, spanning the transition from the far field to the near field. Nanowatt-resolved differential radiometry reveals asymmetric heat fluxes and a non-monotonic response of the hotter sphere, demonstrating that thermal radiation is governed not only by emitter-bath interactions but also by coupling to the surrounding photonic environment. By analyzing the total power exchanged between the coupled system and the external thermal bath, we directly extract a dressed emissivity and show that near-field interactions renormalize the far-field thermal emission of the pair through a redistribution of the electromagnetic modes available to thermal fluctuations. These observations provide direct experimental evidence that thermal emitters are dressed by their electromagnetic environment, establishing a thermal analogue of the Purcell effect.
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All authors Victor Guillemot, Raphael Mol, Riccardo Messina, Valentina Krachmalnicoff, Philippe Ben-Abdallah, Wilfrid Poirier, Yannick De Wilde
Quantum Gases
Isothermal compression of a Fermi gas to deep quantum degeneracy
Kirill Karpov, Jonas Auch, Eduard Heidt et al. (+3 more)
The standard approach for generating deeply degenerate
quantum gases is evaporative or sympathetic cooling in a harmonic trap, after which the gas has reached its minimum
entropy. All subsequent state transformations rely on adiabatic changes of a closed system, and coupling to the environment or non-adiabatic processes monotonically increase the
entropy. Here, we demonstrate that this experimental paradigm can be bypassed by utilizing species-selective trapping with a low-dissipation optical tune-out trap in a dual-species mixture. We successfully reduce the
entropy of a two-component fermionic
quantum gas via isothermal compression within a bosonic bath, reaching deep
quantum degeneracy of $T/T_F = 0.024^{+0.007}$, with $T_F$ the
Fermi temperature. By characterizing the cross-dimensional relaxation and thermalization, we demonstrate that cooling light
fermions with heavy
bosons remains efficient and fast, even deep in the degenerate regime, where the thermalization time is found to be independent of $T/T_F$. Our results pave the way for direct cooling within optical
lattices, box traps, or other complex potentials, thereby eliminating the reliance on adiabatic state transformations to reach strongly interacting many-body regimes.
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All authors Kirill Karpov, Jonas Auch, Eduard Heidt, Florian Kiesel, Alexandre De Martino, Christian Groß
Strongly Correlated Electrons
Topology and compact molecular orbitals in twisted bilayer WSe$_2$
Chenyuan Li, Rwik Dutta, Fang Xie et al. (+4 more)
Recent observations of superconductivity in twisted
bilayer WSe$_2$ (tWSe$_2$) have motivated theoretical proposals for unconventional pairing mechanisms. A central question is whether band
topology plays an essential role in the system's correlation physics. In this letter, we develop a first-principles-based description of the top moiré valence bands in tWSe$_2$. Using density functional theory (DFT) calculations, we identify the bands in the relevant range of twist angles to be topologically non-trivial, with the top valence bands carrying Chern numbers $C=(+1,+1)$ for the $K$ valley. In order to treat the strong correlation physics, we construct compact molecular orbitals directly from the DFT wave functions through a partial Wannierization procedure and with the guidance of spinful $C_{3z}$
symmetry representations. This yields a localized $f$ orbital together with a complementary topological $c$ orbital, allowing us to extract hopping and hybridization amplitudes from first principles. The resulting parameters provide an ab initio
benchmark for the effective
Hamiltonian. Our work establishes a foundation for understanding superconductivity in moiré TMDs and highlights tWSe$_2$ as a promising platform for exploring topological superconductivity.
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All authors Chenyuan Li, Rwik Dutta, Fang Xie, James R. Chelikowsky, Jennifer Cano, Mit H. Naik, Qimiao Si
Materials Science
General and scalable vapor etching and transformation platform for two-dimensional materials
Zhiguo Du, Jikai Zhang, Jonas Björk et al. (+10 more)
Two-dimensional (2D) nanomaterials derived from non-van der Waals (non-vdW) solids offer exceptional physicochemical properties, yet their synthesis is impeded by intrinsic covalent/metallic bonding and high surface reactivity of the precursors. Here, we report a general vapor-phase etching and transformation platform for producing a library of 36 2D carbides, nitrides, and carbonitrides, exhibiting electrical conductivities spanning six orders of magnitude. Using reactive vapors like hydrogen chloride, we selectively remove A-layers from MAX phases to yield well-defined layers (MXenes), including previously inaccessible semiconducting Hf2CTx. By varying the reactive vapor environment, MXenes can be engineered at X-site and surface-termination site and even be transformed into non-vdW layers such as 2D MAX phases. This general and scalable vapor-phase platform reframes 2D material synthesis, opening new avenues for various applications.
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All authors Zhiguo Du, Jikai Zhang, Jonas Björk, Zongju Cheng, Ningjun Chen, Qi Zhao, Hao Chen, Yuxuan Ye, Guang Yang, Haiyang Wang, Bin Li, Johanna Rosen, Shubin Yang
Materials Science
Symmetry-Enforced Dirac Fermions and Structural Metastability in Pentagonal Monolayers of Transition-Metal Ditellurides
Manoj Gadtoula, Clayton Conner, Avinash Sah et al. (+2 more)
The recent synthesis of pentagonal PdTe$_2$ monolayer motivates broader research interests in transition-metal ditellurides whose electronic phases are governed by
symmetry and structural reconstruction. The pentagonal phase of transition-metal ditellurides exhibits electronic properties that are dramatically different from those of its hexagonal counterpart due to its lower crystalline
symmetry. Using first-principles calculations, we study monolayer $X$Te$_2$ ($X=\mathrm{Pd},\mathrm{Pt},\mathrm{Ni}$) in both hexagonal and pentagonal polymorphs. By constructing a continuous structural interpolation between the hexagonal and pentagonal phases, we show that the semimetal (hex)-to-
semiconductor (penta) transition occurs only after an intermediate structural threshold rather than at the onset of
symmetry reduction. The gap opening coincides with the formation of Te–Te dimers, which drive the bonding–antibonding splitting of the Te $p$ states and reorganize the band edges. In addition, the nonsymmorphic
symmetry of the pentagonal phase enforces band degeneracies at the Brillouin-zone boundary, leading to
symmetry-protected two-dimensional (2D) Dirac states. These results establish pentagonal $X$Te$_2$ monolayers as a new class of 2D
semiconductors in which
symmetry constraints and local bonding collectively shape the unconventional semiconducting electronic structure.
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All authors Manoj Gadtoula, Clayton Conner, Avinash Sah, Ronghao Luo, Guang Bian
Mesoscale and Nanoscale Physics
Hot-Carrier Distribution Spectroscopy by Transconductance in Two-Dimensional Field-Effect Transistors
Katsunori Wakabayashi
The transconductance $g_m = dI_D/dV_G$ of a field-effect transistor (FET) is conventionally read as a proxy for carrier density. We show that it is instead a spectroscopic probe of the carrier distribution: because $g_m$ weights the spectral current $j(E)$ by the gate-voltage derivative $\partial f(E)/\partial V_G$ and integrates over energy, it is sensitive to the shape of $f(E)$, not merely its integrated weight $n$. We develop an energy-resolved transport framework for two-dimensional (2D) FETs and, within a gate-independent spectral-kernel approximation, derive the decomposition $g_m = g_m^{(n)} + g_m^{(\alpha)}$ into the conventional density-modulation term $g_m^{(n)}$ and a distribution-shape-driven term $g_m^{(\alpha)}$. The latter, obtained as the residual after subtracting the smooth density-modulation background from the measured $g_m$, exhibits a characteristic anomalous peak at a gate voltage $V_G^{\rm pk}$. This peak has no counterpart in
equilibrium transport and cannot be explained by carrier density modulation alone. With the spectral kernel calibrated, the peak position and height – extracted from standard DC/lock-in $g_m$ sweeps – constrain the hot-carrier energy $E_0$, spectral width $\sigma$, and generation threshold $n_c$, realizing a steady-state, all-electrical spectroscopy of the carrier distribution. An optional time-resolved extension further recovers the carrier relaxation time $\tau$ from the transient response following a pump excitation, establishing the 2D FET as a distribution-function spectrometer that requires no optical readout.
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Materials Science
Terahertz Time-Domain Spectroscopy as a Universal Defect Fingerprinting Tool for Organic Halide Perovskite Solar Cells
Inhee Maeng, Young Mi Lee, Jinwoo Park et al. (+2 more)
Organic-inorganic hybrid perovskites (OHPs) deliver certified single-junction power conversion efficiencies (PCEs) exceeding 26% and perovskite-silicon tandem values surpassing 34%, yet a substantial gap with the Shockley-Queisser (S-Q) limit persists-primarily due to grain-boundary (GB) defects that drive non-radiative recombination, ion migration, and degradation. Rational passivation demands a non-contact tool capable of identifying and quantifying specific defect species in device-relevant thin films, a capability absent from conventional probes. This short review demonstrates that terahertz time-domain spectroscopy (THz-TDS, 0.3-3.0 THz) fulfills this role. Across four OHP compositions fabricated by sequential
vacuum evaporation (SVE)-MAPbI3, MAPbBr3, FAPbI3, and CsPbI3-the THz spectral window captures both intrinsic
phonon modes and GB-localized molecular defect vibrations, enabling species-specific, quantitative characterization at room temperature. Notably, the oscillator strength of the SVE-specific 1.58 THz absorption in MAPbI3 scales linearly with XPS-quantified CH3NH2 defect concentration, establishing THz-TDS as a direct, non-destructive defect meter. Building on these findings, we propose a three-pillar framework for THz-guided defect engineering: (I) quantitative defect measurement via oscillator-strength analysis, (II) material-specific fingerprint identification from a systematically constructed THz library, and (III) fingerprint-guided defect elimination with real-time feedback-together defining a closed-loop quality-control cycle that connects spectroscopic diagnosis to passivation strategy and, ultimately, to enhanced
solar-
cell efficiency.
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All authors Inhee Maeng, Young Mi Lee, Jinwoo Park, Seung Jae Oh, Min-Cherl Jung