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1. Five-dimensional SCFTs and gauge theory phases: an M-theory/type IIA perspective

   https://doi.org/10.21468/SciPostPhys.6.5.052  

   Closset, Cyril; Del Zotto, Michele; Saxena, Vivek (January 2019, SciPost Physics)

   We revisit the correspondence between Calabi-Yau (CY) threefoldisolated singularities \mathbf{X} 𝐗 and five-dimensional superconformal field theories (SCFTs), which ariseat low energy in M-theory on the space-time transverse to \mathbf{X} 𝐗 .Focussing on the case of toric CY singularities, we analyze the“gauge-theory phases” of the SCFT by exploiting fiberwise M-theory/typeIIA duality. In this setup, the low-energy gauge group simply arises onstacks of coincident D6-branes wrapping 2-cycles in some ALE space oftype A_{M-1} A M − 1 fibered over a real line, and the map between the Kähler parameters of \mathbf{X} 𝐗 and the Coulomb branch parameters of the field theory (masses and VEVs)can be read off systematically. Different type IIA “reductions” giverise to different gauge theory phases, whose existence depends on theparticular (partial) resolutions of the isolated singularity \mathbf{X} 𝐗 .We also comment on the case of non-isolated toric singularities.Incidentally, we propose a slightly modified expression for theCoulomb-branch prepotential of 5d \mathcal{N}=1 𝒩 = 1 gauge theories. 

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2. Crossover of high-energy spin fluctuations from collective triplons to localized magnetic excitations in Sr14−xCaxCu24O41 ladders

   https://doi.org/10.1038/s41535-022-00502-1  

   Tseng, Y.; Thomas, J.; Zhang, W.; Paris, E.; Puphal, P.; Bag, R.; Deng, G.; Asmara, T. C.; Strocov, V. N.; Singh, S.; et al (December 2022, npj Quantum Materials)

   Abstract We studied the magnetic excitations in the quasi-one-dimensional (q-1D) ladder subsystem of Sr 14−x Ca x Cu 24 O 41 (SCCO) using Cu L 3 -edge resonant inelastic X-ray scattering (RIXS). By comparing momentum-resolved RIXS spectra with high ( x  = 12.2) and without ( x  = 0) Ca content, we track the evolution of the magnetic excitations from collective two-triplon (2 T) excitations ( x  = 0) to weakly-dispersive gapped modes at an energy of 280 meV ( x  = 12.2). Density matrix renormalization group (DMRG) calculations of the RIXS response in the doped ladders suggest that the flat magnetic dispersion and damped excitation profile observed at x  = 12.2 originates from enhanced hole localization. This interpretation is supported by polarization-dependent RIXS measurements, where we disentangle the spin-conserving Δ S  = 0 scattering from the predominant Δ S  = 1 spin-flip signal in the RIXS spectra. The results show that the low-energy weight in the Δ S  = 0 channel is depleted when Sr is replaced by Ca, consistent with a reduced carrier mobility. Our results demonstrate that off-ladder impurities can affect both the low-energy magnetic excitations and superconducting correlations in the CuO 4 plaquettes. Finally, our study characterizes the magnetic and charge fluctuations in the phase from which superconductivity emerges in SCCO at elevated pressures. 

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3. Finite dimensionality of Besov spaces and potential-theoretic decomposition of metric spaces

   https://doi.org/10.54330/afm.163110  

   Kumagai, Takashi; Shanmugalingam, Nageswari; Shimizu, Ryosuke (January 2025, Annales Fennici Mathematici)

   In the context of a metric measure space \((X,d,\mu)\), we explore the potential-theoretic implications of having a finite-dimensional Besov space. We prove that if the dimension of the Besov space \(B^\theta_{p,p}(X)\) is \(k>1\), then \(X\) can be decomposed into \(k\) number of irreducible components (Theorem 1.1). Note that \(\theta\) may be bigger than \(1\), as our framework includes fractals. We also provide sufficient conditions under which the dimension of the Besov space is 1. We introduce critical exponents \(\theta_p(X)\) and \(\theta_p^{\ast}(X)\) for the Besov spaces. As examples illustrating Theorem 1.1, we compute these critical exponents for spaces \(X\) formed by glueing copies of \(n\)-dimensional cubes, the Sierpiński gaskets, and of the Sierpiński carpet. 

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4. Phase Transition across Anisotropic NbS ₃ and Direct Gap Semiconductor TiS ₃ at Nominal Titanium Alloying Limit

   https://doi.org/10.1002/adma.202000018  

   Wu, Kedi; Blei, Mark; Chen, Bin; Liu, Lei; Cai, Hui; Brayfield, Cassondra; Wright, David; Zhuang, Houlong; Tongay, Sefaattin (March 2020, Advanced Materials)

   Abstract Alloying selected layered transitional metal trichalcogenides (TMTCs) with unique chain‐like structures offers the opportunities for structural, optical, and electrical engineering thus expands the regime of this class of pseudo‐one‐dimensional materials. Here, the novel phase transition in anisotropic Nb_(1−x)Ti_xS₃alloys is demonstrated for the first time. Results show that Nb_(1−x)Ti_xS₃can be fully alloyed across the entire composition range from triclinic‐phase NbS₃to monoclinic‐phase TiS₃. Surprisingly, incorporation of a small concentration of Ti (x ≈0.05–0.18) into NbS₃host matrix is sufficient to induce triclinic to monoclinic transition. Theoretical studies suggest that Ti atoms effectively introduce hole doping, thus rapidly decreases the total energy of monoclinic phase and induces the phase transition. When alloyed, crystalline and optical anisotropy are largely preserved as evidenced by high resolution transmission electron microscopy and angle‐resolved Raman spectroscopy. Further Raman measurements identify Raman modes to determine crystalline anisotropy direction and offer insights into the degree of anisotropy. Overall results introduce Nb_(1−x)Ti_xS₃as a new and easy phase change material and mark the first phase engineering in anisotropic van der Waals (vdW) trichalcogenide systems for their potential applications in two‐dimensional superconductivity, electronics, photonics, and information technologies. 

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5. Two-dimensional Blue-AsP monolayers with tunable direct band gap and ultrahigh carrier mobility show promising high-performance photovoltaic properties

   https://doi.org/10.1039/c9nr01261c  

   Cai, Xinyong; Chen, Yuanzheng; Sun, Bai; Chen, Jiao; Wang, Hongyan; Ni, Yuxiang; Tao, Li; Wang, Hui; Zhu, Shouhui; Li, Xiumei; et al (April 2019, Nanoscale)

   The successful fabrication of black phosphorene (Black-P) in 2014 and subsequent synthesis of layered black As 1−x P x alloys have inspired research into two-dimensional (2D) binary As–P compounds. The very recent success in growing blue phosphorene (Blue-P) further motivated exploration of 2D Blue-AsP materials. Here, using ab initio swarm-intelligence global minimum structure-searching methods, we have obtained a series of novel and energetically favored 2D Blue-AsP (denoted x-AsP, x = I, II, III, IV, V) compounds with As : P = 1 : 1 stoichiometry. They display similar honeycomb structures to Blue-P. Remarkably, the lowest-energy AsP monolayer, namely I-AsP, not only possesses a quasi-direct band gap (2.41 eV), which can be tuned to a direct and optimal gap for photovoltaic applications by in-plane strain, but also has an ultrahigh electronic mobility up to ∼7.4 × 10 4 cm 2 V −1 s −1 , far surpassing that of Blue-P, and also exhibits high absorption coefficients (×10 5 cm −1 ). Our simulations also show that 30 nm-thick I-AsP sheet-based cells have photovoltaic efficiency as high as ∼12%, and the I-AsP/CdSe heterostructure solar cells possess a power conversion efficiency as high as ∼13%. All these outstanding characteristics suggest the I-AsP sheet as a promising material for high-efficiency solar cells. 

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