arXiv subjects

Shang-Shun Zhang

Publications and source records attributed to Shang-Shun Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Neural Polaron: Learning Quasiparticle Operators in Quantum Many-Body Systems

Understanding dynamical properties of quantum many-body systems remains a central challenge because excitations generally require information beyond a ground-state wave function. Here we introduce a neural polaron ansatz that represents quasiparticle excitations by neural many-body operators acting on a correlated ground state. Instead of learning an independent excited-state wave function, the method parameterizes a local dressing operator through a compact neural head defined on the feature map of a pretrained ground-state network. This operator-based construction builds in translation symmetry, momentum resolution, and quasiparticle locality, while separating ground-state correlations from excitation-specific dressing. We benchmark the method on the square-lattice $J_1$-$J_2$ Heisenberg model, where it accurately reproduces magnon dispersions and spectral weights over a broad range of frustration. In particular, it captures nontrivial many-body features including the $(π,0)$ anomaly and its progressive softening with increasing $J_2/J_1$. These results establish neural operators as a physically transparent route for extending neural quantum states from ground-state properties to dynamical response.

cond-mat.str-el↗

arXiv2026

Double-$Q$ chiral stripe order in the anomalous Hall antiferromagnet CoNb$_3$S$_6$

We present fine momentum space resolution resonant elastic x-ray scattering measurements of the magnetic structure of the metallic antiferromagnet CoNb$_3$S$_6$. Using circular dichroism and full linear polarization analysis of the magnetic scattering, we reveal a non-coplanar double-$Q$ ($2Q$) order, with a non-collinear commensurate component and a long-wavelength incommensurate helical component. This $2Q$ structure exhibits a staggered scalar spin chirality that forms a modulated stripe-like pattern with no uniform component. This novel magnetic order is naturally explained by the presence of four-spin exchange interactions and exhibits a complex domain structure that suggests a lowering of the structural symmetry. A symmetry analysis indicates that the $2Q$ order enables a finite anomalous Hall effect in CoNb$_3$S$_6$. In addition to identifying a novel type of magnetic ordering and its origin, our results provide insight into the mechanism of the unconventional magnetotransport phenomena in CoNb$_3$S$_6$ and thus identifies potential routes for realizing novel electronic phenomena in metallic antiferromagnets.

cond-mat.str-el↗

arXiv2026

Superconductivity and fractionalized magnetic excitations in CeCoIn5

Recent experiments on CeCoIn5 -- a prototypical d-wave superconductor -- indicate that its normal state lies near an unconventional quantum critical point (QCP). One intriguing hypothesis is that quantum-critical fluctuations promote fractionalization of localized 4f moments into fermionic spinons. This fractionalized Fermi liquid (FL*) scenario provides a comprehensive framework for the unconventional QCP and superconductivity, and can reconcile a "missing" Fermi-surface volume relative to the Luttinger count in the normal state of CeCoIn5. To test this possibility, we performed inelastic neutron scattering (INS) measurements on CeCoIn5 across the superconducting transition and corresponding theoretical analysis. Our high-precision spectra reveal detailed momentum and temperature dependence of the spin resonance and a structured spin excitation continuum persisting even in the normal state, placing stringent constraints on the physical picture of pairing in a d-wave superconductor. We show that a Kondo-lattice framework incorporating proximity to FL* physics and d-wave pairing reproduces key features of the data. The model suggests that both the quasi-localized nature of the f-moments above Tc and the resonance below Tc arise from common underlying gauge dynamics, implying a unifying organizing principle linking spin fractionalization and unconventional superconductivity in strongly correlated metals.

cond-mat.str-el↗

arXiv2026

Spin dynamics of the spin-1 triangular lattice Heisenberg antiferromagnet K$_2$Ni(SeO$_3$)$_2$

Strong quantum fluctuations and unconventional spin dynamics are well established in the spin-1/2 triangular lattice Heisenberg antiferromagnet. However, their survival in the spin-1 case remains an open question. We investigate the spin dynamics of K$_2$Ni(SeO$_3$)$_2$, a nearly ideal spin-1 triangular lattice Heisenberg antiferromagnet, using inelastic neutron scattering. Below the ordering temperature $T_{\rm N}$, we observe coherent one-magnon excitations coexisting with a broad high-energy continuum. Two complementary approaches, a spectrally consistent $1/S$-corrected spin wave theory and a beyond-mean-field Schwinger boson theory, reproduce different facets of the continuum. Neither alone is complete, demonstrating substantial quantum fluctuations survive for $S\!=\!1$ and are reflected primarily in the spectral distribution of the continuum. Above $T_{\rm N}$, the continuum bandwidth is conserved while spectral weight is redistributed as magnons lose spatial coherence. Our results establish K$_2$Ni(SeO$_3$)$_2$ as a model triangular antiferromagnet, identifying bandwidth conservation and the distribution of spectral weight within the continuum as organizing principles to understand the spin dynamics of ordered quantum magnets beyond spin-1/2. Our results highlight the need for controlled calculations of the interacting multi-magnon sector of 2D antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2026

Magnon-Mediated Superconductivity in the Infinite-$U$ Triangular Lattice

We demonstrate that the infinite-$U$ triangular-lattice Hubbard model supports a superconducting state built from tightly bound Cooper pairs composed of two holes and one magnon ($2h1m$). Building on the seminal prediction of repulsively bound $2h1m$ states, we show that next-nearest-neighbor hopping $t_{2}$ coherently mixes symmetry-related configurations, stabilizing an $s$-wave bound state with substantial binding energy and a light effective mass. Large-scale DMRG calculations at finite doping identify a magnetization plateau corresponding to a gas of such bound states and quasi--long--range superconducting order with power-law $2h1m$ pair correlations. Our results establish a magnon-mediated superconducting mechanism driven by kinetic frustration, with immediate detectable signatures for moiré Hubbard materials and ultracold-atom simulators.

cond-mat.supr-con↗

arXiv2025

Intrinsic quantum disorder in Yb2Ti2O7 and the quantum S=1/2 pyrochlore phase diagram

We present an experimental and theoretical study of the anisotropic pyrochlore phase diagram. Inelastic field-dependent neutron scattering on Yb$_2$Ti$_2$O$_7$ shows intrinsic broadening and a flat low-energy magnon mode which is partially captured by interacting magnon models. Exact diagonalization reveals the existence of an emergent quantum phase between ferromagnetism and antiferromagnetism, in which Yb$_2$Ti$_2$O$_7$ Hamiltonian potentially resides. This behavior matches the phenomenology of quantum criticality in heavy fermion systems, and shows Yb$_2$Ti$_2$O$_7$ is a clean system which can be field-tuned from well-defined magnons to a nontrivial quantum ground state. This suggests that quantum criticality is a generic feature of the dipolar phase diagram.

cond-mat.str-el↗

arXiv2025

Spin dynamics of triple-Q magnetic orderings in a triangular lattice: Implications for multi-Q orderings in general two-dimensional lattices

Multi-Q magnetic structures on two-dimensional (2D) lattices provide a key route to realizing topological physics in 2D magnetism. A major experimental challenge is to unambiguously confirm their formation by excluding the possibility of topologically trivial multi-domain single- or double-Q magnetic orders, which cannot be distinguished using conventional diffraction techniques. Here, we propose that long-wavelength spin dynamics offers a universal diagnostic for triangular lattices: triple-Q orders that preserve rotational symmetry and single- or double-Q orders that break it exhibit qualitatively distinct anisotropies in their Goldstone mode velocities, stemming from fundamental differences in their underlying spin configurations. We validate this concept using the metallic triangular lattice antiferromagnet Co$_{0.325}$TaS$_{2}$, which hosts both a stripe-type single-Q state and a triple-Q tetrahedral ordering at different temperatures. Using inelastic neutron scattering (INS) and spin dynamics simulations, we first refine the spin Hamiltonian by fitting the paramagnetic excitation spectra, allowing us to develop an unbiased model independent of magnetic ordering. We then show that the observed velocity profiles of the Goldstone modes agree with the high-temperature model's predictions: markedly anisotropic for the single-Q phase and near isotropic for the triple-Q phase. Importantly, this contrast persists across various exchange parameters, highlighting its model-independent nature and suggesting potential applicability to other 2D lattice systems. This work provides universal insight into the dynamical properties of topological multi-Q magnetic orderings in 2D lattice structures, offering a broadly applicable diagnostic to distinguishing them from topologically trivial single- or double-Q counterparts. (For the full abstract, please refer to the manuscript)

cond-mat.str-el↗

arXiv2025

Large-$N$ SU(4) Schwinger boson theory for coupled-dimer antiferromagnets

We develop a systematic large-$N$ expansion based on the Schwinger boson representation of SU(4) coherent states of dimers for the paradigmatic spin-$1/2$ bilayer square lattice Heisenberg antiferromagnet. This system exhibits a quantum phase transition between a quantum paramagnetic state and a Néel order state, driven by the coupling constant $g = J'/J$, which is defined as the ratio between the inter-dimer $J'$ and intra-dimer $J$ exchange interactions. We demonstrate that this approach accurately describes static and dynamic properties on both sides of the quantum phase transition. The critical coupling constant $g_c \approx 0.42$ and the dynamic spin structure factor reproduce quantum Monte Carlo results with high precision. Notably, the $1/N$ corrections reveal the longitudinal mode of the magnetically ordered phase along with the overdamping caused by its decay into the two-magnon continuum. The present large-$N$ $SU(N)$ Schwinger boson theory can be extended to more general cases of quantum paramagnets that undergo a quantum phase transition into magnetically ordered states.

cond-mat.str-el↗

arXiv2025

Universal dynamics of a pair condensate

Pair condensates appear in multiple branches of physics, always introducing exotic phenomena. The pair condensate in quantum magnetism is the spin nematic, whose static (quadrupolar) order is difficult to access, favoring dynamical probes. Here, we perform high-resolution neutron spectroscopy to obtain direct evidence for the presence of two spin-nematic phases induced in the triangular-lattice antiferromagnet Na2BaNi(PO4)2 by controlling the applied magnetic field. By combining precise experiments with quantitative theoretical and numerical analysis, we identify universal dynamics arising from the pair condensate. We show explicitly how the gapless Goldstone mode influences the dispersion and induces Cherenkov-like velocity-selective decay of the gapped single-quasiparticle band. These common spectral features shed new light on spin-nematic dynamics and underline the universal phenomenology shared by pair condensates across different physical systems.

cond-mat.str-el↗

arXiv2024

Non-BCS behavior of the pairing susceptibility near the onset of superconductivity in a quantum-critical metal

We analyze the dynamical pairing susceptibility $χ_{pp} (ω_m)$ at $T=0$ in a quantum-critical metal, where superconductivity emerges out of a non-Fermi liquid ground state once the pairing interaction exceeds a certain threshold. We obtain $χ_{pp} (ω_m)$ as the ratio of the fully dressed dynamical pairing vertex $Φ(ω_m)$ and the bare $Φ_0 (ω_m)$ (both infinitesimally small). For superconductivity out of a Fermi liquid, the pairing susceptibility is positive above $T_c$, diverges at $T_c$, and becomes negative below it. For superconductivity out of a non-Fermi liquid, the behavior of $χ_{pp} (ω_m)$ is different in two aspects: (i) it diverges at the onset of pairing at $T=0$ only for a certain subclass of bare $Φ_0 (ω_m)$ and remains non-singular for other $Φ_0 (ω_m)$, and (ii) below the instability, it becomes a non-unique function of a continuous parameter $ϕ$ for an arbitrary $Φ_0 (ω_m)$. The susceptibility is negative in some range of $ϕ$ and diverges at the boundary of this range. We argue that this behavior of the susceptibility reflects a multi-critical nature of a superconducting transition in a quantum-critical metal when immediately below the instability an infinite number of superconducting states emerges simultaneously with different amplitudes of the order parameter down to an infinitesimally small one.

cond-mat.supr-con↗

arXiv2024

Probing Chiral Kitaev Spin Liquids via Dangling Boundary Fermions

Identifying experimental probes capable of diagnosing extreme quantum behavior is widely regarded as one of the foremost challenges in modern condensed matter physics. Here, we propose a novel approach for detecting chiral Kitaev spin liquid states through measurements of the local dynamical spin structure factor on the boundary using scanning tunneling microscopy (STM). We specifically focus on unpaired ("dangling") Majorana fermions, which naturally emerge along boundaries of Kitaev spin liquids, and can serve as indicators of chiral boundary modes under broad conditions, thereby offering a clear signature of these exotic quantum states.

cond-mat.str-el↗

arXiv2024

Low-energy edge signatures of the Kitaev spin liquid

Recent experimental work indicates that the Kitaev spin liquid may be realizable close to zero magnetic field in exfoliated $α$-RuCl$_3$ flakes, thus providing a more versatile setting for studying non-Abelian anyons. Here, we propose a robust nanoscale signature of the Kitaev spin liquid that results from its edge states and manifests in the low-energy spin dynamics. In particular, we highlight a singular peak in the dynamical spin structure factor of a zigzag edge whose energy scales linearly with only one component of the magnetic field. This sharp feature in the local spin dynamics directly reflects the bond-directional Kitaev spin interactions and, more generally, the projective symmetries of the Kitaev spin liquid. We demonstrate that our proposed signature survives in the presence of edge disorder as well as non-Kitaev interactions, and provide detailed guidelines for experimentally observing it using inelastic electron tunneling spectroscopy and color-center relaxometry.

cond-mat.str-el↗

arXiv2024

Electronic Raman scattering of antiferromagnetic excitonic insulators

The excitonic insulator, a quantum mechanical state arising from exciton condensation, was proposed theoretically many years ago but has yet to be experimentally confirmed. The discovery of correlated transition metal oxides based on $4d$ and $5d$ elements, where the on-site Coulomb repulsion is comparable to the dominant hopping amplitude, presents a unique opportunity to study exciton condensation. By constructing an effective mean field Raman operator for the Hubbard model, we derive the low-energy electronic Raman scattering cross section, demonstrating Raman spectroscopy as a powerful tool for detecting exciton condensation. Here, we demonstrate that Raman scattering directly reveals exciton condensation in the bilayer iridate Sr$_3$Ir$_2$O$_7$ under pressure.

cond-mat.str-el↗

arXiv2024

Applicability of Eliashberg theory for systems with electron-phonon and electron-electron interaction: a comparative analysis

We present a comparative analysis of the validity of Eliashberg theory for the cases of fermions interacting with an Einstein phonon and with soft nematic fluctuations near an Ising-nematic/Ising-ferromagnetic quantum-critical point (QCP). In both cases, Eliashberg theory is obtained by neglecting vertex corrections. For the phonon case, the reasoning to neglect vertex corrections is the Migdal ``fast electron/slow boson'' argument because the phonon velocity is much smaller than the Fermi velocity, $v_F$. The same argument allows one to compute the fermionic self-energy within Eliashberg theory perturbatively rather than self-consistently. For the nematic case, the velocity of a collective boson is comparable to $v_F$ and this argument apparently does not work. Nonetheless, we argue that while two-loop vertex corrections near a nematic QCP are not small parametrically, they are small numerically. At the same time, perturbative calculation of the fermionic self-energy can be rigorously justified when the fermion-boson coupling is small compared to the Fermi energy. Furthermore, we argue that for the electron-phonon case Eliashberg theory breaks down at some distance from where the dressed Debye frequency would vanish, while for the nematic case it holds all the way to a QCP. From this perspective, Eliashberg theory for the nematic case actually works better than for the electron-phonon case.

cond-mat.str-el↗

arXiv2023

Superfluid stiffness within Eliashberg theory: the role of vertex corrections

In this work we consider the superfluid stiffness of a generically non-Galilean invariant interacting system and investigate under what conditions the stiffness may nonetheless approach the Galilean-invariant value $n/m$. Within Eliashberg theory we find that the renormalized stiffness is approximately given by $n/m$ in the case when the $l=0$ and $l=1$ components of the effective Fermi-surface projected interaction are approximately equal over a range of frequencies. This holds, in particular, when the interaction is peaked at zero momentum transfer. We examine this result through three complementary lenses: the $δ(ω)$ term in the conductivity, the phase dependence of the Luttinger-Ward free energy, and the coupling of the amplitude and phase sectors in the Hubbard-Stratonovich collective mode action. From these considerations we show that the value of the stiffness is determined by the strength of renormalization of the current vertex and that the latter can be interpreted as the shift of the self-consistent solution due to flow of the condensate, or alternatively as coupling of the phase mode to $l=1$ fluctuations of the order parameter. We highlight that even though the superfluid stiffness in some non-Galilean systems approaches the Galilean value, this is not enforced by symmetry, and in general the stiffness may be strongly suppressed from its BCS value. As a corollary we obtain the generic form of the phase action within Eliashberg theory and charge and spin Ward identities for a superconductor with frequency dependent gap function.

cond-mat.supr-con↗

arXiv2023

Tetrahedral triple-Q magnetic ordering and large spontaneous Hall conductivity in the metallic triangular antiferromagnet Co1/3TaS2

The triangular lattice antiferromagnet (TLAF) has been the standard paradigm of frustrated magnetism for several decades. The most common magnetic ordering in insulating TLAFs is the 120 structure. However, a new triple-Q chiral ordering can emerge in metallic TLAFs, representing the short wavelength limit of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the first example of tetrahedral triple-Q magnetic ordering with the associated topological Hall effect (non-zero σ_{xy}(H=0)). We also present a theoretical framework that describes the emergence of this magnetic ground state, which is further supported by the electronic structure measured by angle-resolved photoemission spectroscopy. Additionally, our measurements of the inelastic neutron scattering cross section are consistent with the calculated dynamical structure factor of the tetrahedral triple-Q state.

cond-mat.str-el↗

arXiv2023

Instabilities of heavy magnons in an anisotropic magnet

The search for new elementary particles is one of the most basic pursuits in physics, spanning from subatomic physics to quantum materials. Magnons are the ubiquitous elementary quasiparticle to describe the excitations of fully-ordered magnetic systems. But other possibilities exist, including fractional and multipolar excitations. Here, we demonstrate that strong quantum interactions exist between three flavors of elementary quasiparticles in the uniaxial spin-one magnet FeI2. Using neutron scattering in an applied magnetic field, we observe spontaneous decay between conventional and heavy magnons and the recombination of these quasiparticles into a super-heavy bound-state. Akin to other contemporary problems in quantum materials, the microscopic origin for new physics in FeI2 is the quasi-flat nature of excitation bands and the presence of Kitaev anisotropic magnetic exchange interactions.

cond-mat.str-el↗

arXiv2023

Witnessing quantum criticality and entanglement in the triangular antiferromagnet KYbSe$_2$

The Heisenberg triangular lattice quantum spin liquid and the phase transitions to nearby magnetic orders have received much theoretical attention, but clear experimental manifestations of these states are rare. This work investigates a new spin-half Yb$^{3+}$ delafossite material, KYbSe$_2$, whose inelastic neutron scattering spectra reveal a diffuse continuum with a sharp lower bound. Applying entanglement witnesses to the data reveals significant multipartite entanglement spread between its neighbors, and analysis of its magnetic exchange couplings shows close proximity to the triangular lattice Heisenberg quantum spin liquid. Key features of the data are reproduced by Schwinger-boson theory and tensor network calculations with a significant second-neighbor coupling $J_2$. The strength of the dynamical structure factor at the $K$ point shows a scaling collapse in $\hbarω/k_\mathrm{B}T$ down to 0.3 K, indicating a second-order quantum phase transition. Comparing this to previous theoretical work suggests that the proximate phase at larger $J_2$ is a gapped $\mathbb{Z}_2$ spin liquid, resolving a long-debated issue. We thus show that KYbSe$_2$ is close to a spin liquid phase, which in turn sheds light on the theoretical phase diagram itself.

cond-mat.str-el↗