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1. Absence of $E_{2g}$ Nematic Instability and Dominant $A_{1g}$ Response in the Kagome Metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$

   https://doi.org/10.1103/PhysRevX.14.031015  

   Liu, Zhaoyu; Shi, Yue; Jiang, Qianni; Rosenberg, Elliott W; DeStefano, Jonathan M; Liu, Jinjin; Hu, Chaowei; Zhao, Yuzhou; Wang, Zhiwei; Yao, Yugui; et al (July 2024, Physical Review X)

   Ever since the discovery of the charge density wave (CDW) transition in the kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$, the nature of its symmetry breaking has been under intense debate. While evidence suggests that the rotational symmetry is already broken at the CDW transition temperature ( $T_{\mathrm{CDW}}$), an additional electronic nematic instability well below $T_{\mathrm{CDW}}$has been reported based on the diverging elastoresistivity coefficient in the anisotropic channel ( $m_{E_{2g}}$). Verifying the existence of a nematic transition below $T_{\mathrm{CDW}}$is not only critical for establishing the correct description of the CDW order parameter, but also important for understanding low-temperature superconductivity. Here, we report elastoresistivity measurements of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$using three different techniques probing both isotropic and anisotropic symmetry channels. Contrary to previous reports, we find the anisotropic elastoresistivity coefficient $m_{E_{2g}}$is temperature independent, except for a step jump at $T_{\mathrm{CDW}}$. The absence of nematic fluctuations is further substantiated by measurements of the elastocaloric effect, which show no enhancement associated with nematic susceptibility. On the other hand, the symmetric elastoresistivity coefficient $m_{A_{1g}}$increases below $T_{\mathrm{CDW}}$, reaching a peak value of 90 at $T^{*}=20\mathrm{K}$. Our results strongly indicate that the phase transition at $T^{*}$is not nematic in nature and the previously reported diverging elastoresistivity is due to the contamination from the $A_{1g}$channel. Published by the American Physical Society2024 

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2. Observation of Medium-Induced Yield Enhancement and Acoplanarity Broadening of Low- $p_{\mathrm{T}}$ Jets from Measurements in $pp$ and Central Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.133.022301  

   Acharya, S; Adamová, D; Aglieri_Rinella, G; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; Akindinov, A; et al (July 2024, Physical Review Letters)

   The ALICE Collaboration reports the measurement of semi-inclusive distributions of charged-particle jets recoiling from a high transverse momentum (high $p_{\mathrm{T}}$) hadron trigger in proton-proton and central Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. A data-driven statistical method is used to mitigate the large uncorrelated background in central Pb-Pb collisions. Recoil jet distributions are reported for jet resolution parameter $R=0.2$, 0.4, and 0.5 in the range $7<p_{\mathrm{T},\text{jet}}<140\mathrm{GeV}/c$and trigger-recoil jet azimuthal separation $\pi /2<\mathrm{\Delta }\phi <\pi$. The measurements exhibit a marked medium-induced jet yield enhancement at low $p_{\mathrm{T}}$and at large azimuthal deviation from $\mathrm{\Delta }\phi \sim \pi$. The enhancement is characterized by its dependence on $\mathrm{\Delta }\phi$, which has a slope that differs from zero by $4.7\sigma$. Comparisons to model calculations incorporating different formulations of jet quenching are reported. These comparisons indicate that the observed yield enhancement arises from the response of the QGP medium to jet propagation. © 2024 CERN, for the ALICE Collaboration2024CERN 

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3. Measurements of Chemical Potentials in Pb-Pb Collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$

   https://doi.org/10.1103/PhysRevLett.133.092301  

   Acharya, S; Adamová, D; Aglieri_Rinella, G; Aglietta, L; Agnello, M; Agrawal, N; Ahammed, Z; Ahmad, S; Ahn, S U; Ahuja, I; et al (August 2024, Physical Review Letters)

   This Letter presents the most precise measurement to date of the matter-antimatter imbalance at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair $\sqrt{s_{\mathrm{NN}}}=5.02\mathrm{TeV}$. Using the Statistical Hadronization framework, it is possible to obtain the value of the electric charge and baryon chemical potentials, ${\mu }_Q=-0.18\pm 0.90\mathrm{MeV}$and ${\mu }_B=0.71\pm 0.45\mathrm{MeV}$, with unprecedented precision. A centrality-differential study of the antiparticle-to-particle yield ratios of charged pions, protons, $\mathrm{\Omega }$baryons, and light (hyper)nuclei is performed. These results indicate that the system created in Pb-Pb collisions at the LHC is on average baryon-free and electrically neutral at midrapidity. © 2024 CERN, for the ALICE Collaboration2024CERN 

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4. Orbital Ingredients and Persistent Dirac Surface State for the Topological Band Structure in ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$

   https://doi.org/10.1103/PhysRevX.14.021043  

   Li, Y-F; Chen, S-D; García-Díez, M; Iraola, M I; Pfau, H; Zhu, Y-L; Mao, Z-Q; Chen, T; Yi, M; Dai, P-C; et al (June 2024, Physical Review X)

   ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$(FTS) occupies a special spot in modern condensed matter physics at the intersections of electron correlation, topology, and unconventional superconductivity. The bulk electronic structure of FTS is predicted to be topologically nontrivial due to the band inversion between the $d_{xz}$and $p_z$bands along $\mathrm{\Gamma }\text{−}Z$. However, there remain debates in both the authenticity of the Dirac surface states (DSSs) and the experimental deviations of band structure from the theoretical band inversion picture. Here we resolve these debates through a comprehensive angle-resolved photoemission spectroscopy investigation. We first observe a persistent DSS independent of $k_z$. Then, by comparing FTS with FeSe, which has no band inversion along $\mathrm{\Gamma }\text{−}Z$, we identify the spectral weight fingerprint of both the presence of the $p_z$band and the inversion between the $d_{xz}$and $p_z$bands. Furthermore, we propose a renormalization scheme for the band structure under the framework of a tight-binding model preserving crystal symmetry. Our results highlight the significant influence of correlation on modifying the band structure and make a strong case for the existence of topological band structure in this unconventional superconductor. Published by the American Physical Society2024 

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5. Robust Nodal Behavior in the Thermal Conductivity of Superconducting ${\mathrm{UTe}}_2$

   https://doi.org/10.1103/PhysRevX.15.021029  

   Hayes, Ian M; Metz, Tristin E; Frank, Corey E; Saha, Shanta R; Butch, Nicholas P; Mishra, Vivek; Hirschfeld, P J; Paglione, Johnpierre (April 2025, Physical Review X)

   The superconducting state of the heavy-fermion metal ${\mathrm{UTe}}_2$has attracted considerable interest because of evidence of spin-triplet Cooper pairing and nontrivial topology. Progress on these questions requires identifying the presence or absence of nodes in the superconducting gap function and their dimension. In this article, we report a comprehensive study of the influence of disorder on the thermal transport in the superconducting state of ${\mathrm{UTe}}_2$. Through detailed measurements of the magnetic-field dependence of the thermal conductivity in the zero-temperature limit, we obtain clear evidence of the presence of point nodes in the superconducting gap for all samples with transition temperatures ranging from 1.6 to 2.1 K obtained by different synthesis methods, including a refined self-flux method. This robustness implies the presence of symmetry-imposed nodes throughout the range studied, further confirmed via disorder-dependent calculations of the thermal transport in a model with a single pair of nodes. In addition to capturing the temperature dependence of the thermal conductivity up to $T_c$, this model provides some information about the locations of the nodes, suggesting a $B_{1u}$or $B_{2u}$symmetry for the superconducting order parameter. Additionally, comparing the new, ultrahigh conductivity samples to older samples reveals a crossover between a low-field and a high-field regime at a single value of the magnetic field in all samples. In the high-field regime, the thermal conductivity at different disorder levels differs from each other by a simple offset, suggesting that some simple principle determines the physics of the mixed state, a fact which may illuminate trends observed in other clean nodal superconductors. Published by the American Physical Society2025 

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