arXiv subjects

Shuyang Wang

Publications and source records attributed to Shuyang Wang.

At least 19 recordsLinked to original sources

arXiv2026

Evolution from an acoustic-plasmon-mediated superconductivity to an acoustic-phonon-mediated superconductivity in bilayers

Motivated by recent developments in van der Waals heterostructures, we revisit the acoustic plasmon mechanism of superconductivity in bilayer systems composed of a light layer (LL) and heavy layer (HL) by employing Eliashberg theory. The exchange of virtual plasmons in the HL can lead to a retarded in time attractive interaction between electrons of LL that we model through the screened interaction in the bilayer system within the random phase approximation. We explore the evolution from acoustic plasmon mediated superconductivity to phonon mediated superconductivity by studying the evolution of $T_c$ as the HL mass is increased by a few orders of magnitude compared with the electronic mass in LL. The lower HL mass corresponds to the bilayer acoustic plasmon, while the latter regime is closer to the Born-Oppenheimer regime of acoustic phonon mediated strongly retarded pairing. The heavy HL mass limit is known to obey Migdal's theorem by virtue of the small ratio of the two individual layer masses. We study the nonadiabatic effects for the arbitrary mass ratio with no small parameter systematically by using a frequency cut off in the Eliashberg theory, providing $T_c$ as a function of this cut off.

cond-mat.supr-con↗

arXiv2026

Pressure-Induced Structural and Magnetic Evolution in Layered Antiferromagnet YbMn$_2$Sb$_2$

Electronic states under pressure exhibit unconventional spin and charge dynamics that provide a powerful route to uncover exotic phases in quantum materials. Here, we present the structural, magnetic, and electronic evolution of YbMn$_2$Sb$_2$ under pressure. Single-crystal X-ray diffraction reveals a pressure-induced structural transition from the space group trigonal $P\bar{3}m1$ to the monoclinic $P2_1$/$m$ phase near 3.5 GPa, which remains stable up to 10 GPa. Magnetization measurements display an anomalously weak net magnetic moment and the absence of Curie-Weiss behavior up to 400 K, suggesting the formation of short-range Mn moment pairs that cancel macroscopically and subsequently evolve into long-range order upon cooling. Temperature-dependent resistivity shows semiconducting behavior with a transition at ~119 K at ambient pressure, while pressure induces a dramatic suppression of resistance and the emergence of metallic-like temperature dependence, stabilized beyond 5 GPa. This pressure-driven semiconductor-metal transition is consistent with our density functional theory calculations, confirming the closing of the band gap under compression. Neutron diffraction under pressure identifies an incommensurate magnetic structure with antiparallel correlations between paired spins. Together, these results demonstrate how pressure-driven structural tuning and competing exchange interactions stabilize unconventional magnetic states in this low-dimensional magnetic semiconductor.

cond-mat.mtrl-sci↗

arXiv2025

Conventional and practical metallic superconductivity arising from repulsive Coulomb coupling

A concrete question is discussed: Can there be conventional s-wave superconductivity in regular 3D (or 2D) metals, i.e., electrons in a jellium background, interacting via the standard Coulomb coupling? We are interested in 'practical' SC that can in principle be observed in experiments, so the $T=0$ ground state being SC is not of interest, or for that matter a $T_c$ which is exponentially small and therefore 'impractical' is also not of interest in the current work. We discuss both 2D and 3D cases, focusing mostly on the 3D case. We find that almost any theory based on the BCS-Migdal-Eliashberg paradigm, with some form of screened Coulomb coupling replacing the electron-phonon coupling in the BCS or Eliashberg theory, would uncritically predict absurdly high $T_c\sim100$ K for s-wave SC in all metals (including the alkali metals, which are well-described by the jellium model) arising from the unavoidable fact that the Fermi, plasmon, and Coulomb potential energy scales are all $>10^4$ K. Therefore, we conclude, based on reduction ad absurdum, that the violation of the venerable Migdal theorem in this problem is sufficiently disruptive that no significance can be attached to numerous existing theoretical publications in the literature claiming plasmon-induced (or other similar Coulomb coupling-induced) practical SC. Using a careful analysis of the Eliashberg gap equations we find that the $T_c$ of the 3D (or 2D) electron gas can be reduced well below $\sim1$ K depending on choices of frequency cut-off parameters that are introduced to satisfy Migdall's theorem but are apriori unknown. The only believable result is the one discovered 60 years ago by Kohn and Luttinger predicting non-s-wave SC arising from Friedel oscillations with exponentially low $T_c$. We provide several theoretical approaches using both BCS and Eliashberg theories and different screening models to make our point.

cond-mat.supr-con↗

arXiv2025

A Sequential Optimal Learning Approach to Automated Prompt Engineering in Large Language Models

Designing effective prompts is essential to guiding large language models (LLMs) toward desired responses. Automated prompt engineering aims to reduce reliance on manual effort by streamlining the design, refinement, and optimization of natural language prompts. This paper proposes an optimal learning framework for automated prompt engineering, designed to sequentially identify effective prompt features while efficiently allocating a limited evaluation budget. We introduce a feature-based method to express prompts, which significantly broadens the search space. Bayesian regression is employed to utilize correlations among similar prompts, accelerating the learning process. To efficiently explore the large space of prompt features for a high quality prompt, we adopt the forward-looking Knowledge-Gradient (KG) policy for sequential optimal learning. The KG policy is computed efficiently by solving mixed-integer second-order cone optimization problems, making it scalable and capable of accommodating prompts characterized only through constraints. We demonstrate that our method significantly outperforms a set of benchmark strategies assessed on instruction induction tasks. The results highlight the advantages of using the KG policy for prompt learning given a limited evaluation budget. Our framework provides a solution to deploying automated prompt engineering in a wider range applications where prompt evaluation is costly.

cs.CL↗

arXiv2024

Unveiling the multiband metallic nature of the normal state in nickelate La3Ni2O7

The discovery of unconventional superconductivity around 80 K in perovskite nickelates under high pressure has furnished a new platform to explore high-temperature unconventional superconductivity in addition to cuprates. Understanding the normal state of nickelate superconductors is crucial to uncovering the origin of this unconventional superconductivity and gaining further insight into its underlying mechanism. In this study, we systemically studied the transport properties of La3Ni2O7 by tuning the pressure under high magnetic fields. Magnetoresistance (MR) consistently exhibits a quasi-quadratic dependence on the magnetic field across all measured pressures and temperatures. Increased pressure enhances the metallicity of the system and leads to a monotonic increase in MR, which follows the extended Kohler's rule. These results suggest that the normal state of La3Ni2O7 to be a multiband metallic nature.

cond-mat.supr-con↗

arXiv2024

Theory of anomalous Hall effect from screened vortex charge in a phase disordered superconductor

Motivated by recent experiments showing evidence for chiral superconductivity in an anomalous Hall phase of tetralayer graphene, we study the relation between the normal state anomalous Hall conductivity and that in the phase disordered state above the critical temperature of the superconductor. By a numerical calculation of superconductivity in an anomalous Hall metal, we find that a difference in vortex and antivortex charge is determined by the Fermi surface Berry phase. Combining this with the vortex dynamics in a back-ground supercurrent leads to a Hall response in the phase disordered state of the superconductor that is close to the normal state anomalous Hall response. However, using a gauge-invariant superconducting response framework, we find that while vortex charge is screened by interactions, the screening charge, after a time-delay, reappears in the longitudinal current. Thus, the dc Hall conductivity in this phase, instead of matching the screened vortex charge, matches the ac Hall conductance in the superconducting and normal phase, which are similar.

cond-mat.supr-con↗

arXiv2024

A Mirror Descent Perspective of Smoothed Sign Descent

Recent work by Woodworth et al. (2020) shows that the optimization dynamics of gradient descent for overparameterized problems can be viewed as low-dimensional dual dynamics induced by a mirror map, explaining the implicit regularization phenomenon from the mirror descent perspective. However, the methodology does not apply to algorithms where update directions deviate from true gradients, such as ADAM. We use the mirror descent framework to study the dynamics of smoothed sign descent with a stability constant $\varepsilon$ for regression problems. We propose a mirror map that establishes equivalence to dual dynamics under some assumptions. By studying dual dynamics, we characterize the convergent solution as an approximate KKT point of minimizing a Bregman divergence style function, and show the benefit of tuning the stability constant $\varepsilon$ to reduce the KKT error.

cs.LG↗

arXiv2024

Raman Spectroscopic Study on Bi2Rh3Se2: Two-dimensional-Ising Charge Density Wave and Quantum Fluctuations

The ternary chalcogenide Bi2Rh3Se2 was found to be a charge density wave (CDW) superconductor with a 2*2 periodicity. The key questions regarding the underlying mechanism of CDW state and its interplay with lattice and electronic properties remains to be explored. Here, based on the systematic Raman scattering investigations on single crystalline Bi2Rh3Se2, we observed the fingerprinting feature of CDW state, a collective amplitude mode at 39 cm-1. The temperature evolution of Raman shift and line width for this amplitude mode can be well described by the critical behavior of two-dimensional (2D) Ising model, suggesting the interlayer interactions of Bi2Rh3Se2 is negligible when CDW state is formed, as a consequence, the quantum fluctuations play an important role at low temperature. Moreover, temperature dependence of Raman shift for Ag9 mode deviates significantly from the expected anharmonic behavior when approaching the CDW transition temperature 240 K, demonstrated that strong electron-phonon coupling plays a key role in the formation of CDW. Our results reveal that Bi2Rh3Se2 is an intriguing quasi-2D system to explore electronic quantum phase transition and modulate the correlations between CDW and superconductivity.

cond-mat.supr-con↗

arXiv2024

Pressure induced nonmonotonic evolution of superconductivity in 6R-TaS2 with a natural bulk van der Waals heterostructure

The natural bulk van der Waals heterostructures compound 6R-TaS2 consists of alternate stacking 1T- and 1H-TaS2 monolayers, creating a unique system that incorporates charge-density-wave (CDW) order and superconductivity (SC) in distinct monolayers. Here, after confirming that the 2D nature of the lattice is preserved up to 8 GPa in 6R-TaS2, we documented an unusual evolution of CDW and SC by conducting highpressure electronic transport measurements. Upon compression, we observe a gradual suppression of CDW within the 1T-layers, while the SC exhibits a dome-shaped behavior that terminates at a critical pressure Pc around 2.9 GPa. By taking account of the fact that the substantial suppression of SC is concomitant with the complete collapse of CDW order at Pc, we argue that the 6R-TaS2 behaves like a stack of Josephson junctions and thus the suppressed superconductivity can be attributed to the weakening of Josephson coupling associated with the presence of CDW fluctuations in the 1T-layers. Furthermore, the SC reversely enhances above Pc, implying the development of emergent superconductivity in the 1T-layers after the melting of T-layer CDW orders. These results show that the 6R-TaS2 not only provides a promising platform to explore emergent phenomena but also serves as a model system to study the complex interactions between competing electronic states.

cond-mat.supr-con↗

arXiv2024

Interaction robustness of the chiral anomaly in Weyl semimetals and Luttinger liquids from a mixed anomaly approach

The chiral anomaly is one of the robust quantum effects in relativistic field theories with a chiral symmetry where charges in chiral sectors appear to be separately conserved. The chiral anomaly, which is often associated with a renormalization-invariant topological term, is a violation of this conservation law due to quantum effects. Such anomalies manifest in Weyl materials as an electromagnetic field-induced transfer of charge between Fermi pockets. However, the emergent nature of the conservation of chiral charge leads to manifestations of the chiral anomaly response that depend on the details of the system such as the strength of interactions. In this paper, we apply an approach where the chiral symmetry in solid materials is replaced by the combination of charge $U(1)$ gauge and spatial translation symmetry. The chiral anomaly in this case is replaced by a mixed anomaly between the two symmetries and the chiral charge can be defined as being proportional to the total momentum. We show that the chiral anomaly associated with this chiral charge is unrenormalized by interactions in contrast to other chiral charges in $(1+1)D$ whose renormalization is regularization dependent. In $(3+1)$D Weyl systems, this chiral anomaly is equivalent to the charge transferred between Fermi surfaces which can be measured through changes in Fermi-surface-enclosed volume. We propose a pump-probe technique to measure this.

cond-mat.str-el↗

arXiv2023

Two Distinct Charge Density Wave Orders and Emergent Superconductivity in Pressurized CuTe

The discovery of multiple charge-density-wave (CDW) orders in superconducting cuprates and Kagome CsV3Sb5 has offered a unique milieu for studying the interplay of CDW and superconductivity and altered our perspective on their nature. Here, we report a high-pressure study of quasi-one-dimensional CDW material CuTe through ultralow-temperature (400 mK) electrical transport and temperature-dependent Raman spectroscopy measurements and first-principles calculations. We provide solid evidence that the pristine CDW order (CDW1) transforms into a distinct CDW order (CDW2) at ~6.5 GPa. Calculations show that the driving force of CDW1 is due to the nesting effect and that of CDW2 probably arises from the electronic correlated interaction. Strikingly, pressure-induced superconductivity is observed with a dome-like phase diagram and its transition displays an extraordinary broadening along with the crossover from CDW1 to CDW2. These results demonstrate that pressurized CuTe provides a promising playground for understanding the intricated interplay of multiple CDWs and superconductivity.

cond-mat.supr-con↗

arXiv2023

An Ensemble Method of Deep Reinforcement Learning for Automated Cryptocurrency Trading

We propose an ensemble method to improve the generalization performance of trading strategies trained by deep reinforcement learning algorithms in a highly stochastic environment of intraday cryptocurrency portfolio trading. We adopt a model selection method that evaluates on multiple validation periods, and propose a novel mixture distribution policy to effectively ensemble the selected models. We provide a distributional view of the out-of-sample performance on granular test periods to demonstrate the robustness of the strategies in evolving market conditions, and retrain the models periodically to address non-stationarity of financial data. Our proposed ensemble method improves the out-of-sample performance compared with the benchmarks of a deep reinforcement learning strategy and a passive investment strategy.

q-fin.TR↗

arXiv2023

HalluAudio: Hallucinating Frequency as Concepts for Few-Shot Audio Classification

Few-shot audio classification is an emerging topic that attracts more and more attention from the research community. Most existing work ignores the specificity of the form of the audio spectrogram and focuses largely on the embedding space borrowed from image tasks, while in this work, we aim to take advantage of this special audio format and propose a new method by hallucinating high-frequency and low-frequency parts as structured concepts. Extensive experiments on ESC-50 and our curated balanced Kaggle18 dataset show the proposed method outperforms the baseline by a notable margin. The way that our method hallucinates high-frequency and low-frequency parts also enables its interpretability and opens up new potentials for the few-shot audio classification.

cs.SD↗

arXiv2023

Pressure-induced nontrivial $Z_2$ band topology and superconductivity in transition metal chalcogenide $\mathrm{Ta}_2 \mathrm{Ni}_3 \mathrm{Te}_5$

The unique electronic and crystal structures driven by external pressure in transition metal chalcogenides (TMCs) can host emergent quantum states. Here we report pressure-induced metallization, nontrivial $Z_2$ band topology and superconductivity in TMC $\mathrm{Ta}_2 \mathrm{Ni}_3 \mathrm{Te}_5$. Our electrical transport measurements show that the metallization emerges at 3.3 GPa, followed by appearance of the superconductivity at $P_{\mathrm{c}}$ = 21.3 GPa with $T_{\mathrm{c}} \sim$ 0.4 K. Room-temperature synchrotron x-ray diffraction experiments demonstrate the stability of the pristine orthorhombic structure upon compression. Our first-principles calculations further reveal a topological phase transition (from $Z_2=0$ to $Z_2=1$), which occurs after $\mathrm{Ta}_2 \mathrm{Ni}_3 \mathrm{Te}_5$ is turned into an electron-hole compensated semimetal by pressure. The pressure-induced superconductivity at $P_{\mathrm{c}}$ could be attributed to the abruptly enhanced density of states at the Fermi level. These findings demonstrate that $\mathrm{Ta}_2 \mathrm{Ni}_3 \mathrm{Te}_5$ is a new platform for realizing exotic quantum phenomena in TMCs, as well as exploring the interplay between topological property and superconductivity.

cond-mat.supr-con↗

arXiv2023

Pressure-induced superconductivity in quasi-one-dimensional semimetal $\mathrm{Ta}_2 \mathrm{PdSe}_6$

Here we report the discovery of pressure-induced superconductivity in quasi-one-dimensional $\mathrm{Ta}_2 \mathrm{PdSe}_6$, through a combination of electrical transport, synchrotron x-ray diffraction, and theoretical calculations. Our transport measurements show that the superconductivity appears at a critical pressure $P_{\mathrm{c}} \sim 18.3$ GPa and is robust upon further compression up to $62.6$ GPa. The estimated upper critical field $μ_0 H_{\mathrm{c} 2}(0)$ in the pressurized $\mathrm{Ta}_2 \mathrm{PdSe}_6$ is much lower than the Pauli limiting field, in contrast to the case in its isostructural analogs $M_2 \mathrm{Pd}_{\mathrm{x}} X_5$ $(M=\mathrm{Nb}$, Ta; $X=\mathrm{S}, \mathrm{Se})$. Concomitant with the occurrence of superconductivity, anomalies in pressuredependent transport properties are observed, including sign reversal of Hall coefficient, abnormally enhanced resistance, and dramatically suppressed magnetoresistance. Meanwhile, room-temperature synchrotron x-ray diffraction experiments reveal the stability of the pristine monoclinic structure (space group $C 2 / m$ ) upon compression. Combined with the density functional theory calculations, we argue that a pressure-induced Lifshitz transition could be the electronic origin of the emergent superconductivity in $\mathrm{Ta}_2 \mathrm{PdSe}_6$.

cond-mat.supr-con↗

arXiv2022

Pressure-induced superconductivity in kagome single crystal Pd3P2S8

Kagome lattice offers unique opportunities for the exploration of unusual quantum states of correlated electrons. Here, we report on the observation of superconductivity in a kagome single crystal Pd3P2S8 when a semiconducting to metallic transition is driven by pressure. High-pressure resistance measurements show that the metallization and superconductivity are simultaneously observed at about 11 GPa. With increasing pressure, the superconducting critical temperature Tc is monotonously enhanced from 2.6 K to a maximum 7.7 K at ~52 GPa. Interestingly, superconductivity retains when the pressure is fully released. Synchrotron XRD and Raman experiments consistently evidence that the emergence of superconductivity is accompanied with an amorphization and the retainability of superconductivity upon decompression can be attributed to the irreversibility of the amorphization.

cond-mat.supr-con↗

arXiv2021

Pressure-induced Reemergence of Superconductivity in Topological Kagome Metal CsV3Sb5

Quasi-two-dimensional kagome metals AV3Sb5 (A = K, Rb, and Cs) have attracted much recent interest due to exotic quantum phenomena such as unconventional superconductivity, topological charge order and giant anomalous Hall effect. Here we report pressure-induced reemergent superconductivity in CsV3Sb5 by electrical transport measurements under high pressures up to 47.9 GPa. We show that the superconducting critical temperature Tc is first enhanced by pressure and reaches its first maximum ~ 8.9 K at 0.8 GPa, then the Tc is suppressed by pressure and cannot be detected above 7.5 GPa, forming a dome-shaped superconducting phase diagram. Remarkably, upon further compression above 16.5 GPa, a new superconducting state arises, of which Tc is enhanced by pressure to a second maximum ~ 5.0 K and the reemergent superconductivity keeps robust up to 47.9 GPa. Combined with high-pressure synchrotron x-ray diffraction measurements that demonstrate the stability of the pristine hexagonal phase up to 43.1 GPa, we suggest that the reemergence of superconductivity in the V-based superconductor could be attributed to a pressure-induced Lifshitz transition.

cond-mat.supr-con↗

arXiv2020

Minimal flag triangulations of lower-dimensional manifolds

We prove the following results on flag triangulations of 2- and 3-manifolds. In dimension 2, we prove that the vertex-minimal flag triangulations of $\mathbb{R} P^2$ and $\mathbb{S}^1\times \mathbb{S}^1$ have 11 and 12 vertices, respectively. In general, we show that $8+3k$ (resp. $8+4k$) vertices suffice to obtain a flag triangulation of the connected sum of $k$ copies of $\mathbb{R} P^2$ (resp. $\mathbb{S}^1\times \mathbb{S}^1$). In dimension 3, we describe an algorithm based on the Lutz-Nevo theorem which provides supporting computational evidence for the following generalization of the Charney-Davis conjecture: for any flag 3-manifold, $γ_2:=f_1-5f_0+16\geq 16 β_1$, where $f_i$ is the number of $i$-dimensional faces and $β_1$ is the first Betti number over a field. The conjecture is tight in the sense that for any value of $β_1$, there exists a flag 3-manifold for which the equality holds.

math.CO↗