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Jie Ma

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math.COcond-mat.mtrl-scicond-mat.str-elquant-phcond-mat.quant-gascond-mat.supr-conComputation and LanguageComputer Visioncond-mat.otherMachine Learningphysics.opticsArtificial Intelligencecond-mat.mes-hallphysics.app-phphysics.atom-phphysics.comp-ph

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56 publications

preprint2026arXiv

Evading Visual Aphasia: Contrastive Adaptive Semantic Token Pruning for Vision-Language Models

Are low-attention visual tokens truly redundant in vision-language reasoning? Existing pruning methods often assume so, ranking visual tokens by shallow text-to-image attention and discarding low-scoring patches to accelerate LVLM inference. We show that this scalar criterion is unreliable for compositional reasoning: tokens ignored in early layers can later become essential for resolving secondary objects, spatial relations, and contextual cues. Premature pruning can therefore induce Visual Aphasia, a failure mode in which the model loses visual grounding and falls back on language priors. We introduce COAST (COntrastive Adaptive Semantic Token Pruning), a training-free pruning framework that casts compression as adaptive semantic routing. COAST uses native cross-modal attention to identify query-specific anchors and estimate contextual dispersion via attention entropy, then adapts the retention trade-off between semantic evidence and spatial context. It further uses a contrastive routing score to preserve both anchor-aligned evidence and complementary spatial context. Across seven benchmarks, COAST reduces visual tokens by 77.8% and achieves a 2.15x latency speedup while retaining 98.64% of the original average performance. Beyond a single backbone or compression setting, COAST consistently outperforms strong pruning baselines across token budgets and generalizes across multiple LVLM families, showing that adaptive semantic routing is a robust alternative to one-shot scalar pruning

Open access
preprint2023arXiv

Bipartite-ness under smooth conditions

Given a family F\mathcal{F} of bipartite graphs, the {\it Zarankiewicz number} z(m,n,F)z(m,n,\mathcal{F}) is the maximum number of edges in an mm by nn bipartite graph GG that does not contain any member of F\mathcal{F} as a subgraph (such GG is called {\it F\mathcal{F}-free}). For 1≤β<α<21\leq β<α<2, a family F\mathcal{F} of bipartite graphs is (α,β)(α,β)-{\it smooth} if for some ρ>0ρ>0 and every m≤nm\leq n, z(m,n,F)=ρmnα−1+O(nβ)z(m,n,\mathcal{F})=ρm n^{α-1}+O(n^β). Motivated by their work on a conjecture of Erdős and Simonovits on compactness and a classic result of Andrásfai, Erdős and Sós, in \cite{AKSV} Allen, Keevash, Sudakov and Verstraëte proved that for any (α,β)(α,β)-smooth family F\mathcal{F}, there exists k0k_0 such that for all odd k≥k0k\geq k_0 and sufficiently large nn, any nn-vertex F∪{Ck}\mathcal{F}\cup\{C_k\}-free graph with minimum degree at least ρ(2n5+o(n))α−1ρ(\frac{2n}{5}+o(n))^{α-1} is bipartite. In this paper, we strengthen their result by showing that for every real δ>0δ>0, there exists k0k_0 such that for all odd k≥k0k\geq k_0 and sufficiently large nn, any nn-vertex F∪{Ck}\mathcal{F}\cup\{C_k\}-free graph with minimum degree at least δnα−1δn^{α-1} is bipartite. Furthermore, our result holds under a more relaxed notion of smoothness, which include the families F\mathcal{F} consisting of the single graph Ks,tK_{s,t} when t≫st\gg s. We also prove an analogous result for C2ℓC_{2\ell}-free graphs for every ℓ≥2\ell\geq 2, which complements a result of Keevash, Sudakov and Verstraëte in \cite{KSV}.

Open access
preprint2023arXiv

On the maximum number of edges in k-critical graphs

A graph is called kk-critical if its chromatic number is kk but any proper subgraph has chromatic number less than kk. An old and important problem in graph theory asks to determine the maximum number of edges in an nn-vertex kk-critical graph. This is widely open for any integer k≥4k\geq 4. Using a structural characterization of Greenwell and Lovász and an extremal result of Simonovits, Stiebitz proved in 1987 that for k≥4k\geq 4 and sufficiently large nn, this maximum number is less than the number of edges in the nn-vertex balanced complete (k−2)(k-2)-partite graph. In this paper we obtain the first improvement on the above result in the past 35 years. Our proofs combine arguments from extremal graph theory as well as some structural analysis. A key lemma we use indicates a partial structure in dense kk-critical graphs, which may be of independent interest.

Open access
preprint2022arXiv

A non-uniform extension of Baranyai's Theorem

A celebrated theorem of Baranyai states that when kk divides nn, the family KnkK_n^k of all kk-subsets of an nn-element set can be partitioned into perfect matchings. In other words, KnkK_n^k is 11-factorable. In this paper, we determine all n,kn, k, such that the family Kn≤kK_n^{\le k} consisting of subsets of [n][n] of size up to kk is 11-factorable, and thus extend Baranyai's Theorem to the non-uniform setting. In particular, our result implies that for fixed kk and sufficiently large nn, Kn≤kK_n^{\le k} is 11-factorable if and only if n≡0n \equiv 0 or −1(modk)-1 \pmod k.

Open access
preprint2022arXiv

Antiferromagnetic Kitaev interaction in $J_\rm{eff}=1/2$ cobalt honeycomb materials Na3_3Co2_2SbO6_6 and Na2_2Co2_2TeO6_6

Finding new materials with antiferromagnetic (AFM) Kitaev interaction is an urgent issue to broaden and enrich the quantum magnetism research significantly. By carrying out inelastic neutron scattering experiments and subsequent analysis, we conclude that Na3_3Co2_2SbO6_6 and Na2_2Co2_2TeO6_6 are new honeycomb cobalt-based AFM Kitaev systems. The spin-orbit excitons at 20-28~meV in both compounds strongly supports the idea that Co2+^{2+} ions of both compounds have a spin-orbital entangled $J_\rm{eff}=1/2$ state. Furthermore, we found that a generalized Kitaev-Heisenberg Hamiltonian can well describe the spin-wave excitations of both compounds with additional 3rd nearest-neighbor interaction. Our best-fit parameters show large AFM Kitaev terms and off-diagonal symmetric anisotropy terms of a similar magnitude in both compounds. We should stress that our parameters' optimized magnetic structures are consistent with the magnetic structures reported from neutron diffraction studies. Moreover, there is also the magnon-damping effect at the higher energy part of the spin waves, as usually observed in other Kitaev magnets. We demonstrate that Na3_3Co2_2SbO6_6 and Na2_2Co2_2TeO6_6 are the first experimental realization of AFM Kitaev magnets based on the systematic studies of the spin waves and analysis.

Open access
preprint2022arXiv

Disorder-induced linear magnetoresistance in Sr-doped Bi2Se3 thin films

Sr-doped Bi2Se3 thin films was known as a potential candidate of topological superconductor. The magnetoresistance (MR) of SrxBi2Se3 films with various doping concentrations x were found to be dominated by weak antilocalization (WAL) at low magnetic fields, whereas the classical MR, which originally dominated the MR, was almost completely suppressed. In contrast, the MR of all samples has been observed to be dominated by linear magnetoresistance (LMR) at high magnetic fields. The LMR, having the linear dependence on carrier mobility, can be successfully explained by the Parish-Littlewood model. This indicates that LMR originates from mobility fluctuation induced by Sr dopant atoms in doped Bi2Se3 films.

Open access
preprint2022arXiv

Label Semantics for Few Shot Named Entity Recognition

We study the problem of few shot learning for named entity recognition. Specifically, we leverage the semantic information in the names of the labels as a way of giving the model additional signal and enriched priors. We propose a neural architecture that consists of two BERT encoders, one to encode the document and its tokens and another one to encode each of the labels in natural language format. Our model learns to match the representations of named entities computed by the first encoder with label representations computed by the second encoder. The label semantics signal is shown to support improved state-of-the-art results in multiple few shot NER benchmarks and on-par performance in standard benchmarks. Our model is especially effective in low resource settings.

Open access
preprint2022arXiv

Low-energy Spin Dynamics of Quantum Spin Liquid Candidate NaYbSe2NaYbSe_{2}

The family of rare earth chalcogenides ARECh2ARECh_{2} (A = alkali or monovalent ions, RE = rare earth, and Ch = O, S, Se, and Te) appears as an inspiring playground for studying quantum spin liquids (QSL). The crucial low-energy spin dynamics remain to be uncovered. By employing muon spin relaxation (μμSR) and zero-field (ZF) AC susceptibility down to 50 mK, we are able to identify the gapless QSL in NaYbSe2NaYbSe_{2}, a representative member with an effective spin-1/2, and explore its unusual spin dynamics. The ZF μμSR experiments unambiguously rule out spin ordering or freezing in NaYbSe2NaYbSe_{2} down to 50 mK, two orders of magnitude smaller than the exchange coupling energies. The spin relaxation rate, λλ, approaches a constant below 0.3 K, indicating finite spin excitations featured by a gapless QSL ground state. This is consistently supported by our AC susceptibility measurements. The careful analysis of the longitudinal field (LF) μμSR spectra reveals a strong spatial correlation and a temporal correlation in the spin-disordered ground state, highlighting the unique feature of spin entanglement in the QSL state. The observations allow us to establish an experimental H-T phase diagram. The study offers insight into the rich and exotic magnetism of the rare earth family.

Open access
preprint2022arXiv

Nontrivial topological states in BaSn5 superconductor probed by de Haas-van Alphen quantum oscillations

We report herein the nontrivial topological states in an intrinsic type-II superconductor BaSn5 (Tc ~ 4.4 K) probed via measuring the magnetizations, specific heat, de Haas-van Alphen (dHvA) effect and performing first principles calculations. The first principles calculations reveal a topological nodal ring structure centering at the H point in the kz = π plane of the Brillouin zone (BZ), which could be gapped by spin-orbit coupling (SOC), yielding rather small gaps below and above the Fermi level about 0.04 eV and 0.14 eV, respectively. The SOC also results in a pair of Dirac points along the Γ-A direction and located ~ 0.2 eV above the Fermi level. The analysis of the dHvA quantum oscillations supports the calculations by revealing nontrivial Berry phase originated from three hole and one electron pockets related to the bands forming the Dirac cones. Our study thus provides an excellent avenue for investigating the interplay between superconductivity and nontrivial topological states.

Open access
preprint2022arXiv

Observation of interaction-induced mobility edge in a disordered atomic wire

Mobility edge, a critical energy separating localized and extended excitations, is a key concept for understanding quantum localization. Aubry-André (AA) model, a paradigm for exploring quantum localization, does not naturally allow mobility edges due to self-duality. Using the momentum-state lattice of quantum gas of Cs atoms to synthesize a nonlinear AA model, we provide experimental evidence for mobility edge induced by interactions. By identifying the extended-to-localized transition of different energy eigenstates, we construct a mobility-edge phase diagram. The location of mobility edge in the low- or high-energy region is tunable via repulsive or attractive interactions. Our observation is in good agreement with the theory, and supports an interpretation of such interaction-induced mobility edge via a generalized AA model. Our work also offers new possibilities to engineer quantum transport and phase transitions in disordered systems.

Open access
preprint2022arXiv

On the Convergence of Clustered Federated Learning

Knowledge sharing and model personalization are essential components to tackle the non-IID challenge in federated learning (FL). Most existing FL methods focus on two extremes: 1) to learn a shared model to serve all clients with non-IID data, and 2) to learn personalized models for each client, namely personalized FL. There is a trade-off solution, namely clustered FL or cluster-wise personalized FL, which aims to cluster similar clients into one cluster, and then learn a shared model for all clients within a cluster. This paper is to revisit the research of clustered FL by formulating them into a bi-level optimization framework that could unify existing methods. We propose a new theoretical analysis framework to prove the convergence by considering the clusterability among clients. In addition, we embody this framework in an algorithm, named Weighted Clustered Federated Learning (WeCFL). Empirical analysis verifies the theoretical results and demonstrates the effectiveness of the proposed WeCFL under the proposed cluster-wise non-IID settings.

Open access
preprint2022arXiv

Regulate the direct-indirect electronic band gap transition by electron-phonon interaction in BaSnO3

The neutron powder diffraction, specific heat, thermal conductivity, and Raman scattering measurements were presented to study the interplays of lattice, phonons and electrons of the Sr-doping Ba1-xSrxSnO3 (x was less than or equal to 0.1). Although Ba1-xSrxSnO3 kept the cubic lattice, the Raman spectra suggested a dynamic distortion at low temperature. The density functional theory was applied to analyze the electronic structures and phonon dispersions of Ba1-xSrxSnO3(x = 0, 0.0125), and the behaviors of electron bands around Fermi levels were discussed. According to the experimental and theoretical results, the Sr-doping played a significant role in tuning the indirect band gap of BaSnO3 and influenced the electron-phonon interaction.

Open access
preprint2022arXiv

Significant thermal Hall effect in the 3dd cobalt Kitaev system Na2Co2TeO6Na_2Co_2TeO_6

Kitaev physics has recently attracted attention in condensed matter for its anticipated novel quantum spin liquid state. The thermal transport measurement is crucial for probing the novel features of charge-neutral quasiparticles. In this letter, we report a significant thermal Hall effect in Na2Co2TeO6Na_2Co_2TeO_6 (NCTO), a Kitaev quantum spin liquid candidate, when the magnetic field is applied along the out-of-plane direction of the honeycomb plane. The thermal conductivity (κxxκ_{xx}) and thermal Hall conductivity (κxyκ_{xy}) in NCTO reveal distinct magnetic field dependences below and above the Neel temperature (TNT_N) of 27 K. For T>TNT>T_N, κxxκ_{xx} has a monotonic decrease in the field dependence, while κxyκ_{xy} persists up to T∗T^* = 150 K. On the other hand, both κxxκ_{xx} and κxyκ_{xy} exhibit complex field dependence for T<TNT<T_N.

Open access
preprint2022arXiv

The minimum number of clique-saturating edges

Let GG be a KpK_p-free graph. We say ee is a KpK_p-saturating edge of GG if e∉E(G)e\notin E(G) and G+eG+e contains a copy of KpK_p. Denote by fp(n,e)f_p(n, e) the minimum number of KpK_p-saturating edges that an nn-vertex KpK_p-free graph with ee edges can have. Erdős and Tuza conjectured that f4(n,⌊n2/4⌋+1)=(1+o(1))n216.f_4(n,\lfloor n^2/4\rfloor+1)=\left(1 + o(1)\right)\frac{n^2}{16}. Balogh and Liu disproved this by showing f4(n,⌊n2/4⌋+1)=(1+o(1))2n233f_4(n,\lfloor n^2/4\rfloor+1)=(1+o(1))\frac{2n^2}{33}. They believed that a natural generalization of their construction for KpK_p-free graph should also be optimal and made a conjecture that fp+1(n,ex(n,Kp)+1)=(2(p−2)2p(4p2−11p+8)+o(1))n2f_{p+1}(n,ex(n,K_p)+1)=\left(\frac{2(p-2)^2}{p(4p^2-11p+8)}+o(1)\right)n^2 for all integers p≥3p\ge 3. The main result of this paper is to confirm the above conjecture of Balogh and Liu.

Open access
preprint2022arXiv

Visualizing and Understanding Patch Interactions in Vision Transformer

Vision Transformer (ViT) has become a leading tool in various computer vision tasks, owing to its unique self-attention mechanism that learns visual representations explicitly through cross-patch information interactions. Despite having good success, the literature seldom explores the explainability of vision transformer, and there is no clear picture of how the attention mechanism with respect to the correlation across comprehensive patches will impact the performance and what is the further potential. In this work, we propose a novel explainable visualization approach to analyze and interpret the crucial attention interactions among patches for vision transformer. Specifically, we first introduce a quantification indicator to measure the impact of patch interaction and verify such quantification on attention window design and indiscriminative patches removal. Then, we exploit the effective responsive field of each patch in ViT and devise a window-free transformer architecture accordingly. Extensive experiments on ImageNet demonstrate that the exquisitely designed quantitative method is shown able to facilitate ViT model learning, leading the top-1 accuracy by 4.28% at most. Moreover, the results on downstream fine-grained recognition tasks further validate the generalization of our proposal.

Open access
preprint2021arXiv

A unified proof of conjectures on cycle lengths in graphs

In this paper, we prove a tight minimum degree condition in general graphs for the existence of paths between two given endpoints, whose lengths form a long arithmetic progression with common difference one or two. This allows us to obtain a number of exact and optimal results on cycle lengths in graphs of given minimum degree, connectivity or chromatic number. More precisely, we prove the following statements by a unified approach. (1) Every graph GG with minimum degree at least k+1k+1 contains cycles of all even lengths modulo kk; in addition, if GG is 2-connected and non-bipartite, then it contains cycles of all lengths modulo kk. (2) For all k≥3k\geq 3, every kk-connected graph contains a cycle of length zero modulo kk. (3) Every 3-connected non-bipartite graph with minimum degree at least k+1k+1 contains kk cycles of consecutive lengths. (4) Every graph with chromatic number at least k+2k+2 contains kk cycles of consecutive lengths. The first statement is a conjecture of Thomassen, the second is a conjecture of Dean, the third is a tight answer to a question of Bondy and Vince, and the fourth is a conjecture of Sudakov and Verstraëte. All of the above results are best possible.

Open access
preprint2021arXiv

Extremal problems of Erdős, Faudree, Schelp and Simonovits on paths and cycles

For positive integers n>d≥kn>d\geq k, let ϕ(n,d,k)ϕ(n,d,k) denote the least integer ϕϕ such that every nn-vertex graph with at least ϕϕ vertices of degree at least dd contains a path on k+1k+1 vertices. Many years ago, Erdős, Faudree, Schelp and Simonovits proposed the study of the function ϕ(n,d,k)ϕ(n,d,k), and conjectured that for any positive integers n>d≥kn>d\geq k, it holds that ϕ(n,d,k)≤⌊k−12⌋⌊nd+1⌋+εϕ(n,d,k)\leq \lfloor\frac{k-1}{2}\rfloor\lfloor\frac{n}{d+1}\rfloor+ε, where ε=1ε=1 if kk is odd and ε=2ε=2 otherwise. In this paper we determine the values of the function ϕ(n,d,k)ϕ(n,d,k) exactly. This confirms the above conjecture of Erdős et al. for all positive integers k≠4k\neq 4 and in a corrected form for the case k=4k=4. Our proof utilizes, among others, a lemma of Erdős et al. \cite{EFSS89}, a theorem of Jackson \cite{J81}, and a (slight) extension of a very recent theorem of Kostochka, Luo and Zirlin \cite{KLZ}, where the latter two results concern maximum cycles in bipartite graphs. Moreover, we construct examples to provide answers to two closely related questions raised by Erdős et al.

Open access
preprint2021arXiv

Improvements on induced subgraphs of given sizes

Given integers mm and ff, let Sn(m,f)S_n(m,f) consist of all integers ee such that every nn-vertex graph with ee edges contains an mm-vertex induced subgraph with ff edges, and let σ(m,f)=lim sup⁡n→∞∣Sn(m,f)∣/(n2)σ(m,f)=\limsup_{n\rightarrow\infty} |S_n(m,f)|/\binom{n}{2}. As a natural extension of an extremal problem of Erdős, this was investigated by Erdős, Füredi, Rothschild and Sós twenty years ago. Their main result indicates that integers in Sn(m,f)S_n(m,f) are rare for most pairs (m,f)(m,f), though they also found infinitely many pairs (m,f)(m,f) whose σ(m,f)σ(m,f) is a fixed positive constant. Here we aim to provide some improvements on this study. Our first result shows that σ(m,f)≤12σ(m,f)\leq \frac12 holds for all but finitely many pairs (m,f)(m,f) and the constant 12\frac12 cannot be improved. This answers a question of Erdős et. al. Our second result considers infinitely many pairs (m,f)(m,f) of special forms, whose exact values of σ(m,f)σ(m,f) were conjectured by Erdős et. al. We partially solve this conjecture (only leaving two open cases) by making progress on some constructions which are related to number theory. Our proofs are based on the research of Erdős et. al and involve different arguments in number theory. We also discuss some related problems.

Open access
preprint2020arXiv

Crystalline Electric-Field Excitations in Quantum Spin Liquids Candidate NaYbSe2NaYbSe_{2}

Very recently we revealed a large family of triangular lattice quantum spin liquid candidates named rare-earth chalcogenides, which features a high-symmetry structure without structural/charge disorders and spin impurities, and may serve as an ideal platform exploring spin liquid physics. The knowledge of crystalline electric-field (CEF) excitations is an essential step to explore the fundamental magnetism of rare-earth spin systems. Here we employed inelastic neutron scattering (INS) and Raman scattering (RS) to carry out a comprehensive CFE investigation on NaYbSe2NaYbSe_{2}, a promising representative of the family. By comparison with its nonmagnetic compound NaLuSe2NaLuSe_{2}, we are able to identify the CEF excitations at 15.8, 24.3 and 30.5 meV at 5K. The selected cuts of the INS spectra are well re-produced with a large anisotropy of gg factors (gab:gc∼3:1g_{ab}:g_{c}\sim3:1). Further, the CEF excitations are explained well by our calculations based on the point charge model. Interestingly, NaYbSe2NaYbSe_{2} exhibits an unusual CEF shift to higher energies with increasing temperatures, and the Raman mode close to the first CEF excitation shows an anomalously large softening with decreasing temperatures. The absence of the anomalies in NaLuSe2NaLuSe_{2} clearly demonstrates a CEF-phonon coupling not reported in the family. It can be understood in term of the weaker electronegativity of Se. The fact that the smallest first CEF excitation in the sub-family of NaYbCh2NaYbCh_{2} is ∼\sim 180K (Ch=O, S, Se), guarantees that the sub-family can be strictly described with an effective S=1/2 picture at sufficiently low temperatures. Interestingly the CEF-phonon coupling revealed here may present alternative possibilities to manipulate the spin systems.

Open access
preprint2020arXiv

Establishing the carrier scattering phase diagram for ZrNiSn-based half-Heusler thermoelectric materials

Chemical doping is one of the most important strategies for tuning electrical properties of semiconductors, particularly thermoelectric materials. Generally, the main role of chemical doping lies in optimizing the carrier concentration, but there can potentially be other important effects. Here, we show that chemical doping plays multiple roles for both electron and phonon transport properties in half-Heusler thermoelectric materials. With ZrNiSn-based half-Heusler materials as an example, we use high-quality single and polycrystalline crystals, various probes, including electrical transport measurements, inelastic neutron scattering measurement, and first-principles calculations, to investigate the underlying electron-phonon interaction. We find that chemical doping brings strong screening effects to ionized impurities, grain boundary, and polar optical phonon scattering, but has negligible influence on lattice thermal conductivity. Furthermore, it is possible to establish a carrier scattering phase diagram, which can be used to select reasonable strategies for optimization of the thermoelectric performance.

Open access
preprint2020arXiv

Linear cycles of consecutive lengths

A well-known result of Verstraëte \cite{V00} shows that for each integer k≥2k\geq 2 every graph GG with average degree at least 8k8k contains cycles of kk consecutive even lengths, the shortest of which is at most twice the radius of GG. We establish two extensions of Verstraëte's result for linear cycles in linear rr-uniform hypergraphs. We show that for any fixed integers r≥3,k≥2r\geq 3,k\geq 2, there exist constants c1=c1(r)c_1=c_1(r) and c2=c2(r,k)c_2=c_2(r,k), such that every linear rr-uniform hypergraph GG with average degree d(G)≥c1kd(G)\geq c_1 k contains linear cycles of kk consecutive even lengths, the shortest of which is at most 2⌈log⁡nlog⁡(d(G)/k)−c2⌉2\lceil \frac{ \log n}{\log (d(G)/k)-c_2}\rceil. In particular, as an immediate corollary, we retrieve the current best known upper bound on the linear Turán number of C2krC^r_{2k} with improved coefficients. Furthermore, we show that for any fixed integers r≥3,k≥2r\geq 3,k\geq 2, there exist constants c3=c3(r)c_3=c_3(r) and c4=c4(r)c_4=c_4(r) such that every nn-vertex linear rr-uniform graph with average degree d(G)≥c3kd(G)\geq c_3k, contains linear cycles of kk consecutive lengths, the shortest of which has length at most 6⌈log⁡nlog⁡(d(G)/k)−c4⌉+66\lceil \frac{\log n}{\log (d(G)/k)-c_4} \rceil +6. Both the degree condition and the shortest length among the cycles guaranteed are best possible up to a constant factor.

Open access
preprint2020arXiv

Non-repeated cycle lengths and Sidon sequences

We prove a conjecture of Boros, Caro, Füredi and Yuster on the maximum number of edges in a 2-connected graph without repeated cycle lengths, which is a restricted version of a longstanding problem of Erdős. Our proof together with the matched lower bound construction of Boros, Caro, Füredi and Yuster show that this problem can be conceptually reduced to the seminal problem of finding the maximum Sidon sequences in number theory.

Open access
preprint2020arXiv

Observation of the Topologically Originated Edge States in large-gap Quasi-One-Dimensional a-Bi4_4Br4_4

Two-dimensional topological insulator features time-reversal-invariant spin-momentum-locked one-dimensional (1D) edge states with a linear energy dispersion. However, experimental access to 1D edge states is still of great challenge and only limited to few techniques to date. Here, by using infrared absorption spectroscopy, we observed robust topologically originated edge states in a-Bi4Br4 belts with definitive signature of strong infrared absorption at belt sides and distinct anisotropy with respect to light polarizations, which is further supported by first-principles calculations. Our work demonstrates for the first time that the infrared spectroscopy can offer a power-efficient approach in experimentally probing 1D edge states of topological materials.

Open access
preprint2020arXiv

Observation of Weyl fermions in a magnetic non-centrosymmetric crystal

Characterized by the absence of inversion symmetry, non-centrosymmetric materials are of great interest because they exhibit ferroelectricity, second harmonic generation, emergent Weyl fermions, and other fascinating phenomena. It is expected that if time-reversal symmetry is also broken, additional magneto-electric effects can emerge from the interplay between magnetism and electronic order. Here we report topological conducting properties in the non-centrosymmetric magnet PrAlGe. By photoemission spectroscopy, we observe an arc parametrizing surface-localized states---a topological arc. Using the bulk-boundary correspondence, we conclude that these arcs correspond to projected topological charges of ±1\pm{1} in the surface Brillouin zone, demonstrating the presence of magnetic Weyl quasiparticles in bulk. We further observe a large anomalous Hall response, arising from diverging bulk Berry curvature fields associated with the magnetic Weyl band structure. Our results demonstrate a topological phase with robust electronic surface states and anomalous transport in a non-centrosymmetric magnet for the first time, providing a novel material platform to study the interplay between magnetic order, band topology and transport.

Open access
preprint2020arXiv

Severing the Edge Between Before and After: Neural Architectures for Temporal Ordering of Events

In this paper, we propose a neural architecture and a set of training methods for ordering events by predicting temporal relations. Our proposed models receive a pair of events within a span of text as input and they identify temporal relations (Before, After, Equal, Vague) between them. Given that a key challenge with this task is the scarcity of annotated data, our models rely on either pretrained representations (i.e. RoBERTa, BERT or ELMo), transfer and multi-task learning (by leveraging complementary datasets), and self-training techniques. Experiments on the MATRES dataset of English documents establish a new state-of-the-art on this task.

Open access
preprint2020arXiv

Stochastic Batch Augmentation with An Effective Distilled Dynamic Soft Label Regularizer

Data augmentation have been intensively used in training deep neural network to improve the generalization, whether in original space (e.g., image space) or representation space. Although being successful, the connection between the synthesized data and the original data is largely ignored in training, without considering the distribution information that the synthesized samples are surrounding the original sample in training. Hence, the behavior of the network is not optimized for this. However, that behavior is crucially important for generalization, even in the adversarial setting, for the safety of the deep learning system. In this work, we propose a framework called Stochastic Batch Augmentation (SBA) to address these problems. SBA stochastically decides whether to augment at iterations controlled by the batch scheduler and in which a ''distilled'' dynamic soft label regularization is introduced by incorporating the similarity in the vicinity distribution respect to raw samples. The proposed regularization provides direct supervision by the KL-Divergence between the output soft-max distributions of original and virtual data. Our experiments on CIFAR-10, CIFAR-100, and ImageNet show that SBA can improve the generalization of the neural networks and speed up the convergence of network training.

Open access
preprint2020arXiv

Ultralong carrier lifetime of topological edge states in a-Bi4Br4

The rising of quantum spin Hall insulators (QSHI) in two-dimensional (2D) systems has been attracting significant interest in current research, for which the 1D helical edge states, a hallmark of QSHI, are widely expected to be a promising platform for next-generation optoelectronics. However, the dynamics of the 1D edge states has not yet been experimentally addressed. Here, we report the observation of optical response of the topological helical edge states in a-Bi4Br4, using the infrared-pump infrared-probe microscopic spectroscopy. Remarkably, we observe that the carrier lifetime of the helical edge states reaches nanosecond-scale at room temperature, which is about 2 - 3 orders longer than that of most 2D topological surface states and is even comparable with that of the well developed optoelectronics semiconductors used in modern industry. The ultralong carrier lifetime of the topological edge states may be attributed to their helical and 1D nature. Our findings not only provide an ideal material for further investigations of the carrier dynamics of 1D helical edge states but also pave the way for its application in optoelectronics.

Open access
preprint2019arXiv

NbSeTe -A New Layered Transition Metal Dichalcogenide Superconductor

Transition metal dichalcogenides (TMDCs) usually exhibit layered polytypic structures due to the weak interlayer coupling. 2H-NbSe2 is one of the most widely studied in the pristine TMDC family due to its high superconducting transition temperature (Tc = 7.3K) and the occurrence of a charge-density wave (CDW) order below 33 K. The coexistence of CDW with superconductivity poses an intriguing open question about the relationship between Fermi surface nesting and Cooper pairing. Past studies of this issue have mostly been focused on doping 2H-NbSe2 by 3d transition metals without significantly changing its crystal structure. Here we replaced the Se by Te in 2H-NbSe2 in order to design a new 1T polytype layered TMDC NbSeTe, which adopts a trigonal structure with space group P-3m1. We successfully grew large size and high-quality single crystals of 1T-NbSeTe via the vapor transport method using I2 as the transport agent. Temperature-dependent resistivity and specific heat data revealed a bulk Tc at 1.3 K, which is the first observation of superconductivity in pure 1T-NbSeTe phase. This compound enlarged the family of superconducting TMDCs and provides an opportunity to study the interplay between CDW and superconductivity in the trigonal structure.

Open access
preprint2019arXiv

Quantum superhet based on microwave-dressed Rydberg atoms

The highly sensitive, phase- and frequency-resolved detection of microwave electric fields is of central importance for diverse fields ranging from astronomy, remote sensing, communication and microwave quantum technology. However, present quantum sensing of microwave electric fields primarily relies on atom-based electrometers only enabling amplitude measurement. Moreover, the best sensitivity of atom-based electrometers is limited by photon shot noise to few μμVcm−1^{-1}Hz−1/2^{-1/2}: While going beyond is in principle possible by using squeezed light or Schrödinger-cat state, the former is very challenging for atomic experiments while the latter is feasible in all but very small atomic systems. Here we report a novel microwave electric field quantum sensor termed as quantum superhet, which, for the first time, enables experimental measurement of phase and frequency, and makes a sensitivity few tens of nVcm−1^{-1}Hz−1/2^{-1/2} readily accessible for current experiments. This sensor is based on microwave-dressed Rydberg atoms and tailored optical spectrum, with very favorable scalings on sensitivity gains. We can experimentally achieve a sensitivity of 5555 nVcm−1^{-1}Hz−1/2^{-1/2}, with the minimum detectable field being three orders of magnitude smaller than existing quantum electrometers. We also measure phase and frequency, being able to reach a frequency accuracy of few tens of μμHz for microwave field of just few tens of nVcm−1^{-1}. Our technique can be also applied to sense electric fields at terahertz or radio frequency. This work is a first step towards realizing the long sought-after electromagnetic-wave quantum sensors with quantum projection noise limited sensitivity, promising broad applications such as in radio telescope, terahertz communication and quantum control.

Open access
preprint2019arXiv

Stability results on the circumference of a graph

In this paper, we extend and refine previous Turán-type results on graphs with a given circumference. Let Wn,k,cW_{n,k,c} be the graph obtained from a clique Kc−k+1K_{c-k+1} by adding n−(c−k+1)n-(c-k+1) isolated vertices each joined to the same kk vertices of the clique, and let f(n,k,c)=e(Wn,k,c)f(n,k,c)=e(W_{n,k,c}). Improving a celebrated theorem of Erdős and Gallai, Kopylov proved that for c<nc<n, any 2-connected graph GG on nn vertices with circumference cc has at most max⁡f(n,2,c),f(n,⌊c2⌋,c)\max{f(n,2,c),f(n,\lfloor\frac{c}{2}\rfloor,c)} edges. Recently, Füredi et al. proved a stability version of Kopylov's theorem. Their main result states that if GG is a 2-connected graph on nn vertices with circumference cc such that 10≤c<n10\leq c<n and e(G)>max⁡f(n,3,c),f(n,⌊c2⌋−1,c)e(G)>\max{f(n,3,c),f(n,\lfloor\frac{c}{2}\rfloor-1,c)}, then either GG is a subgraph of Wn,2,cW_{n,2,c} or Wn,⌊c2⌋,cW_{n,\lfloor\frac{c}{2}\rfloor,c}, or cc is odd and GG is a subgraph of a member of two well-characterized families which we define as Xn,c\mathcal{X}_{n,c} and Yn,c\mathcal{Y}_{n,c}. We prove that if GG is a 2-connected graph on nn vertices with minimum degree at least kk and circumference cc such that 10≤c<n10\leq c<n and e(G)>max⁡f(n,k+1,c),f(n,⌊c2⌋−1,c)e(G)>\max{f(n,k+1,c),f(n,\lfloor\frac{c}{2}\rfloor-1,c)}, then one of the following holds: (i) GG is a subgraph of Wn,k,cW_{n,k,c} or Wn,⌊c2⌋,cW_{n,\lfloor\frac{c}{2}\rfloor,c}, (ii) k=2k=2, cc is odd, and GG is a subgraph of a member of Xn,c∪Yn,c\mathcal{X}_{n,c}\cup \mathcal{Y}_{n,c}, or (iii) k≥3k\geq 3 and GG is a subgraph of the union of a clique Kc−k+1K_{c-k+1} and some cliques Kk+1K_{k+1}'s, where any two cliques share the same two vertices. This provides a unified generalization of the above result of Füredi et al. as well as a recent result of Li et al. and independently, of Füredi et al. on non-Hamiltonian graphs. Moreover, we prove a stability result on a classical theorem of Bondy on the circumference.

Open access
preprint2016arXiv

Cycles of given lengths in hypergraphs

In this paper, we develop a method for studying cycle lengths in hypergraphs. Our method is built on earlier ones used in [21,22,18]. However, instead of utilizing the well-known lemma of Bondy and Simonovits [4] that most existing methods do, we develop a new and very simple lemma in its place. One useful feature of the new lemma is its adaptiveness for the hypergraph setting. Using this new method, we prove a conjecture of Verstraëte [37] that for r≥3r\ge 3, every rr-uniform hypergraph with average degree Ω(kr−1)Ω(k^{r-1}) contains Berge cycles of kk consecutive lengths. This is sharp up to the constant factor. As a key step and a result of independent interest, we prove that every rr-uniform linear hypergraph with average degree at least 7r(k+1)7r(k+1) contains Berge cycles of kk consecutive lengths. In both of these results, we have additional control on the lengths of the cycles, which therefore also gives us bounds on the Turán numbers of Berge cycles (for even and odd cycles simultaneously). In relation to our main results, we obtain further improvements on the Turán numbers of Berge cycles and the Zarankiewicz numbers of even cycles. We will also discuss some potential further applications of our method.

Open access
preprint2016arXiv

Cycles with two blocks in kk-chromatic digraphs

Let kk and ℓ\ell be positive integers. A cycle with two blocks c(k,ℓ)c(k,\ell) is an oriented cycle which consists of two internally (vertex) disjoint directed paths of lengths at least kk and ℓ\ell, respectively, from a vertex to another one. A problem of Addario-Berry, Havet and Thomassé (2007) asked if, given positive integers kk and ℓ\ell such that k+ℓ≥4k+\ell\ge 4, any strongly connected digraph DD containing no c(k,ℓ)c(k,\ell) has chromatic number at most k+ℓ−1k+\ell-1. In this paper, we show that such digraph DD has chromatic number at most O((k+ℓ)2)O((k+\ell)^2), improving the previous upper bound O((k+ℓ)4)O((k+\ell)^4) obtained by Cohen, Havet, Lochet and Nisse (2016). In fact, we are able to find a digraph which shows that the answer to the above problem is no. We also show that if in addition DD is Hamiltonian, then its underlying simple graph is (k+ℓ−1)(k+\ell-1)-degenerate and thus the chromatic number of DD is at most k+ℓk+\ell, which is tight.

Open access
preprint2016arXiv

On edges not in monochromatic copies of a fixed bipartite graph

Let HH be a fixed graph. Denote f(n,H)f(n,H) to be the maximum number of edges not contained in any monochromatic copy of HH in a 2-edge-coloring of the complete graph KnK_n, and ex(n,H)ex(n,H) to be the {\it Turán number} of HH. An easy lower bound shows f(n,H)≥ex(n,H)f(n,H)\ge ex(n,H) for any HH and nn. In \cite{KS2}, Keevash and Sudakov proved that if HH is an edge-color-critical graph or C4C_4, then f(n,H)=ex(n,H)f(n,H)= ex(n,H) holds for large nn, and they asked if this equality holds for any graph HH when nn is sufficiently large. In this paper, we provide an affirmative answer to this problem for an abundant infinite family of bipartite graphs HH, including all even cycles and complete bipartite graphs Ks,tK_{s,t} for t>s2−3s+3t>s^2-3s+3 or (s,t)∈{(3,3),(4,7)}(s,t)\in\{(3,3),(4,7)\}. In addition, our proof shows that for all such HH, the 2-edge-coloring cc of KnK_n achieves the maximum number f(n,H)f(n,H) if and only if one of the color classes in cc induces an extremal graph for ex(n,H)ex(n,H). We also obtain a multi-coloring generalization for bipartite graphs. Some related problems are discussed in the final section.

Open access
preprint2016arXiv

Some extremal results on complete degenerate hypergraphs

Let Ks1,s2,⋯ ,sr(r)K^{(r)}_{s_1,s_2,\cdots,s_r} be the complete rr-partite rr-uniform hypergraph and ex(n,Ks1,s2,⋯ ,sr(r))ex(n,K^{(r)}_{s_1,s_2,\cdots,s_r}) be the maximum number of edges in any nn-vertex Ks1,s2,⋯ ,sr(r)K^{(r)}_{s_1,s_2,\cdots,s_r}-free rr-uniform hypergraph. It is well-known in the graph case that ex(n,Ks,t)=Θ(n2−1/s)ex(n,K_{s,t})=Θ(n^{2-1/s}) when tt is sufficiently larger than ss. In this note, we generalize the above to hypergraphs by showing that if srs_r is sufficiently larger than s1,s2,⋯ ,sr−1s_1,s_2,\cdots,s_{r-1} then ex(n,Ks1,s2,⋯ ,sr(r))=Θ(nr−1s1s2⋯sr−1).ex(n, K^{(r)}_{s_1,s_2,\cdots,s_r})=Θ\left(n^{r-\frac{1}{s_1s_2\cdots s_{r-1}}}\right). This follows from a more general Turán type result we establish in hypergraphs, which also improves and generalizes some recent results of Alon and Shikhelman. The lower bounds of our results are obtained by the powerful random algebraic method of Bukh. Another new, perhaps unsurprising insight which we provide here is that one can also use the random algebraic method to construct non-degenerate (hyper-)graphs for various Turán type problems. The asymptotics for ex(n,Ks1,s2,⋯ ,sr(r))ex(n, K^{(r)}_{s_1,s_2,\cdots,s_r}) is also proved by Verstraëte independently with a different approach.

Open access
preprint2016arXiv

Structural and magnetic phase transitions in Ca0.73_{0.73}La0.27_{0.27}FeAs2_2 with electron overdoped FeAs layers

We report a study of the Ca0.73_{0.73}La0.27_{0.27}FeAs2_2 single crystals. We unravel a monoclinic to triclinic phase transition at 58 K, and a paramagnetic to stripe antiferromagnetic (AFM) phase transition at 54 K, below which spins order 45∘^\circ away from the stripe direction. Furthermore, we demonstrate this material is substantially structurally untwinned at ambient pressure with the formation of spin rotation walls (S-walls). Finally, in addition to the central-hole and corner-electron Fermi pockets usually appearing in Fe pnictide superconductors, angle-resolved photoemission (ARPES) measurements resolve a Fermiology where an extra electron pocket of mainly As chain character exists at the Brillouin zone edge.

Open access
preprint2015arXiv

Band gap corrections for molecules and solids using Koopmans theorem and Wannier functions

We have proposed a method for correcting the Kohn-Sham eigen energies in the density functional theory (DFT) based on the Koopmans theorem using Wannier functions. The method provides a general approach applicable for molecules and solids for electronic structure calculations. It does not have any adjustable parameters and the computational cost is at the DFT level. For solids, the calculated eigen energies agree well with the experiments for not only the band gaps, but also the energies of other valence and conduction bands. For molecules, the calculated eigen energies agree well with the experimental ionization potentials and electron affinities, and show better trends than the traditional Delta-self-consistent-field (?SCF) results.

Open access
preprint2015arXiv

Cycle lengths and minimum degree of graphs

There has been extensive research on cycle lengths in graphs with large minimum degree. In this paper, we obtain several new and tight results in this area. Let GG be a graph with minimum degree at least k+1k+1. We prove that if GG is bipartite, then there are kk cycles in GG whose lengths form an arithmetic progression with common difference two. For general graph GG, we show that GG contains ⌊k/2⌋\lfloor k/2\rfloor cycles with consecutive even lengths and k−3k-3 cycles whose lengths form an arithmetic progression with common difference one or two. In addition, if GG is 2-connected and non-bipartite, then GG contains ⌊k/2⌋\lfloor k/2\rfloor cycles with consecutive odd lengths. Thomassen (1983) made two conjectures on cycle lengths modulo a fixed integer kk: (1) every graph with minimum degree at least k+1k+1 contains cycles of all even lengths modulo kk; (2) every 2-connected non-bipartite graph with minimum degree at least k+1k+1 contains cycles of all lengths modulo kk. These two conjectures, if true, are best possible. Our results confirm both conjectures when kk is even. And when kk is odd, we show that minimum degree at least k+4k+4 suffices. This improves all previous results in this direction. Moreover, our results derive new upper bounds of the chromatic number in terms of the longest sequence of cycles with consecutive (even or odd) lengths.

Open access
preprint2015arXiv

Generation of 30-fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser

Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode locked 8.43 optical-cycle pulses have a spectral bandwidth of ~ 50 nm and a pulse repetition frequency of ~ 113.5 MHz. To our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique to generate few-cycle optical pulses directly from a laser oscillator.

Open access
preprint2015arXiv

Photon Devil's staircase: photon long-range repulsive interaction in lattices of coupled resonators with Rydberg atoms

The realization of strong coherent interactions between individual photons is a long-standing goal in science and engineering. In this report, based on recent experimental setups, we derive a strong photon long-range repulsive interaction, by controlling the van der Waals repulsive force between Cesium Rydberg atoms located inside different cavities in extended Jaynes-Cummings-Hubbard Lattices. We also find novel quantum phases induced by this photon long-range repulsive interaction. For example, without photon hopping, a photon Devil's staircase, induced by the breaking of long-range translation symmetry, can emerge. If photon hopping occurs, we predict a photon-floating solid phase, due to the motion of particle-and hole-like defects. More importantly, for a large chemical potential in the resonant case, the photon hopping can be frozen even if the hopping term exists. We call this new phase the photon-frozen solid phase. In experiments, these predicted phases could be detedted by measuring the number of polaritons via resonance fluorescence.

Open access
preprint2015arXiv

Temperature and composition phase diagram in the iron-based ladder compounds Ba1-xCsxFe2Se3

We investigated the iron-based ladder compounds (Ba,Cs)Fe2_2Se3_3. Their parent compounds, BaFe2_2Se3_3 and CsFe2_2Se3_3, have different space groups, formal valences of Fe and magnetic structures. Electrical resistivity, specific heat, magnetic susceptibility, X-ray diffraction and powder neutron diffraction measurements were conducted to obtain temperature and composition phase diagram of this system. Block magnetism observed in BaFe2_2Se3_3 is drastically suppressed with Cs doping. In contrast, stripe magnetism observed in CsFe2_2Se3_3 is not so fragile against Ba doping. New type of magnetic structure appears in intermediate compositions, which is similar to stripe magnetism of CsFe2_2Se3_3, but inter-ladder spin configuration is different. Intermediate compounds show insulating behavior, nevertheless finite TT-linear contribution in specific heat was obtained at low temperatures.

Open access
preprint2014arXiv

Coloring Digraphs with Forbidden Cycles

Let kk and rr be two integers with k≥2k \ge 2 and k≥r≥1k\ge r \ge 1. In this paper we show that (1) if a strongly connected digraph DD contains no directed cycle of length 11 modulo kk, then DD is kk-colorable; and (2) if a digraph DD contains no directed cycle of length rr modulo kk, then DD can be vertex-colored with kk colors so that each color class induces an acyclic subdigraph in DD. The first result gives an affirmative answer to a question posed by Tuza in 1992, and the second implies the following strong form of a conjecture of Diwan, Kenkre and Vishwanathan: If an undirected graph GG contains no cycle of length rr modulo kk, then GG is kk-colorable if r≠2r\ne 2 and (k+1)(k+1)-colorable otherwise. Our results also strengthen several classical theorems on graph coloring proved by Bondy, Erdős and Hajnal, Gallai and Roy, Gyárfás, etc.

Open access
preprint2014arXiv

Cycles with consecutive odd lengths

It is proved that there exists an absolute constant c > 0 such that for every natural number k, every non-bipartite 2-connected graph with average degree at least ck contains k cycles with consecutive odd lengths. This implies the existence of the absolute constant d > 0 that every non-bipartite 2-connected graph with minimum degree at least dk contains cycles of all lengths modulo k, thus providing an answer (in a strong form) to a question of Thomassen. Both results are sharp up to the constant factors.

Open access
preprint2014arXiv

Diameter critical graphs

A graph is called diameter-kk-critical if its diameter is kk, and the removal of any edge strictly increases the diameter. In this paper, we prove several results related to a conjecture often attributed to Murty and Simon, regarding the maximum number of edges that any diameter-kk-critical graph can have. In particular, we disprove a longstanding conjecture of Caccetta and Häggkvist (that in every diameter-2-critical graph, the average edge-degree is at most the number of vertices), which promised to completely solve the extremal problem for diameter-2-critical graphs. On the other hand, we prove that the same claim holds for all higher diameters, and is asymptotically tight, resolving the average edge-degree question in all cases except diameter-2. We also apply our techniques to prove several bounds for the original extremal question, including the correct asymptotic bound for diameter-kk-critical graphs, and an upper bound of (16+o(1))n2(\frac{1}{6} + o(1))n^2 for the number of edges in a diameter-3-critical graph.

Open access
preprint2014arXiv

Maximizing proper colorings on graphs

The number of proper qq-colorings of a graph GG, denoted by PG(q)P_G(q), is an important graph parameter that plays fundamental role in graph theory, computational complexity theory and other related fields. We study an old problem of Linial and Wilf to find the graphs with nn vertices and mm edges which maximize this parameter. This problem has attracted much research interest in recent years, however little is known for general m,n,qm,n,q. Using analytic and combinatorial methods, we characterize the asymptotic structure of extremal graphs for fixed edge density and qq. Moreover, we disprove a conjecture of Lazebnik, which states that the Turán graph Ts(n)T_s(n) has more qq-colorings than any other graph with the same number of vertices and edges. Indeed, we show that there are infinite many counterexamples in the range q=O(s2/log⁡s)q = O({s^2}/{\log s}). On the other hand, when qq is larger than some constant times s2/log⁡s{s^2}/{\log s}, we confirm that the Turán graph Ts(n)T_s(n) asymptotically is the extremal graph achieving the maximum number of qq-colorings. Furthermore, other (new and old) results on various instances of the Linial-Wilf problem are also established.

Open access
preprint2012arXiv

A problem of Erdős on the minimum number of kk-cliques

Fifty years ago Erdős asked to determine the minimum number of kk-cliques in a graph on nn vertices with independence number less than l. He conjectured that this minimum is achieved by the disjoint union of l−1l-1 complete graphs of size nl−1\frac{n}{l-1}. This conjecture was disproved by Nikiforov who showed that the balanced blow-up of a 5-cycle has fewer 4-cliques than the union of 2 complete graphs of size n2\frac{n}{2}. In this paper we solve Erdős' problem for (k,l)=(3,4)(k,l)=(3,4) and (k,l)=(4,3)(k,l)=(4,3). Using stability arguments we also characterize the precise structure of extremal examples, confirming Erdős' conjecture for (k,l)=(3,4)(k,l)=(3,4) and showing that a blow-up of a 5-cycle gives the minimum for (k,l)=(4,3)(k,l)=(4,3).

Open access
preprint2012arXiv

Anharmonic Phonons and Magnons in BiFeO3

The phonon density of states (DOS) and magnetic excitation spectrum of polycrystalline BiFeO3_3 were measured for temperatures 200≤T≤750 200 \leq T \leq 750\,K, using inelastic neutron scattering (INS). Our results indicate that the magnetic spectrum of BiFeO3_3 closely resembles that of similar Fe perovskites, such as LaFeO3_3, despite the cycloid modulation in BiFeO3_3. We do not find any evidence for a spin gap. A strong TT-dependence of the phonon DOS was found, with a marked broadening of the whole spectrum, providing evidence of strong anharmonicity. This anharmonicity is corroborated by large-amplitude motions of Bi and O ions observed with neutron diffraction. These results highlight the importance of spin-phonon coupling in this material.

Open access
preprint2012arXiv

Large feedback arc sets, high minimum degree subgraphs, and long cycles in Eulerian digraphs

A minimum feedback arc set of a directed graph GG is a smallest set of arcs whose removal makes GG acyclic. Its cardinality is denoted by β(G)β(G). We show that an Eulerian digraph with nn vertices and mm arcs has β(G)≥m2/2n2+m/2nβ(G) \ge m^2/2n^2+m/2n, and this bound is optimal for infinitely many m,nm, n. Using this result we prove that an Eulerian digraph contains a cycle of length at most 6n2/m6n^2/m, and has an Eulerian subgraph with minimum degree at least m2/24n3m^2/24n^3. Both estimates are tight up to a constant factor. Finally, motivated by a conjecture of Bollobás and Scott, we also show how to find long cycles in Eulerian digraphs.

Open access
preprint2012arXiv

Microscopic mechanism of low thermal conductivity in lead-telluride

The microscopic physics behind low lattice thermal conductivity of single crystal rocksalt lead telluride (PbTe) is investigated. Mode-dependent phonon (normal and umklapp) scattering rates and their impact on thermal conductivity were quantified by the first-principles-based anharmonic lattice dynamics calculations that accurately reproduce thermal conductivity in a wide temperature range. The low thermal conductivity of PbTe is attributed to the scattering of longitudinal acoustic phonons by transverse optical phonons with large anharmonicity, and small group velocity of the soft transverse acoustic phonons. This results in enhancing the relative contribution of optical phonons, which are usually minor heat carrier in bulk materials.

Open access
preprint2012arXiv

Properties of single crystalline AZn2Sb2 (A=Ca,Eu,Yb)

Single crystals of CaZn2Sb2, EuZn2Sb2, and YbZn2Sb2 were grown from melts of nominal composition AZn5Sb5 (A=Ca,Eu,Yb) with the excess melt being removed at 1073\,K. The electrical transport properties are consistent with those previously reported for polycrystalline samples. This confirms that the pp-type carrier concentrations ranging from 2x10^{19}cm^{-3} to ~1x10^{20}cm^{-3} are intrinsic to these materials. Also consistent with transport in polycrystalline materials, the carrier mobility is found to be lowest in CaZn2Sb2, suggesting the trends in mobility and thermoelectric efficiency within these compounds are inherent to the material systems and not due to inhomogeneity or impurities in polycrystalline samples. These results suggest CaZn2Sb2 has the strongest coupling between the doping/defects and the electronic framework. Magnetization measurements reveal an antiferromagnetic transition near 13\,K in EuZn2Sb2, and the observed magnetic anisotropy indicates the spins align parallel and anti-parallel to cc in the trigonal lattice. Powder neutron diffraction on polycrystalline samples of CaZn2Sb2 and YbZn2Sb2 reveals smooth lattice expansion to 1000\,K, with cc expanding faster than aa. The Debye temperatures calculated from specific heat capacity data and the isotropic displacement parameters are found to correlate with the carrier mobility, with the CaZn2Sb2 displaying the largest Debye temperature and smallest mobility.

Open access
preprint2012arXiv

Thermodynamics of spin-orbit-coupled Bose-Einstein condensates

In this paper we develop a quantum field approach to reveal the thermodynamic properties of the trapped BEC with the equal Rashba and Dresselhaus spin-orbit couplings. In the experimentally-feasible regime, the phase transition from the separate phase to the single minimum phase can be well driven by the tunable temperature. Moreover, the critical temperature, which is independent of the trapped potential, can be derived exactly. At the critical point, the specific heat has a large jump and can be thus regarded as a promising candidate to detect this temperature-driven phase transition. In addition, we obtain the analytical expressions for the specific heat and the entropy in the different phases. In the single minimum phase, the specific heat as well as the entropy are governed only by the Rabi frequency. However, in the separate phase with lower temperature, we find that they are determined only by the strength of spin-orbit coupling. Finally, the effect of the effective atom interaction is also addressed. In the separate phase, this effective atom interaction affects dramatically on the critical temperature and the corresponding thermodynamic properties.

Open access
preprint2011arXiv

Band Structure Engineering of Multinary Chalcogenide Topological Insulators

Topological insulators (TIs) have been found in strained binary HgTe and ternary I-III-VI2 chalcopyrite compounds such as CuTlSe2 which have inverted band structures. However, the non-trivial band gaps of these existing binary and ternary TIs are limited to small values, usually around 10 meV or less. In this work, we reveal that a large non-trivial band gap requires the material having a large negative crystal field splitting ΔCFΔ_{CF} at top of the valence band and a moderately large negative s−ps-p band gap Egs−pE_g^{s-p}. These parameters can be better tuned through chemical ordering in multinary compounds. Based on this understanding, we show that a series of quaternary I2-II-IV-VI4 compounds, including Cu2HgPbSe4, Cu2CdPbSe4, Ag2HgPbSe4 and Ag2CdPbTe4 are TIs, in which Ag2HgPbSe4 has the largest TI band gap of 47 meV because it combines the optimal values of ΔCFΔ_{CF} and Egs−pE_g^{s-p}.

Open access
preprint2011arXiv

Light-shift-induced quantum phase transitions of a Bose-Einstein condensate in the optical cavities

In this paper we reveal the rich ground-state properties induced by the strong nonlinear atom-photon interaction, which has been found in the recent experiment about a Bose-Einstein condensate coupling with a high-finesse cavity [Nature (London) 464, 1301 (2010)]. Two detuning-dependent phase diagrams are revealed by investigating the experimentally-measurable atomic population. In particular, two new quantum phase transitions from the superradiant phase or the normal phase to a dynamically-unstable phase are predicted in the blue detuning, and moreover, the three-phase coexistence points are found. It is also demonstrated that these predicted quantum phase transitions are the intrinsic transitions governed only by the second-order derivative of the ground-state energy. Finally, we also point out that the region involving coexistence of the superradiant phase and the normal phase predicted before can not happen in the ground state.

Open access

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