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1. Phonon mixing in the charge density wave state of ScV6Sn6

   https://doi.org/10.1038/s41535-023-00590-7  

   Gu, Yanhong; Ritz, Ethan T.; Meier, William R.; Blockmon, Avery; Smith, Kevin; Madhogaria, Richa Pokharel; Mozaffari, Shirin; Mandrus, David; Birol, Turan; Musfeldt, Janice L. (December 2023, npj Quantum Materials)

   Abstract Kagomé metals are widely recognized, versatile platforms for exploring topological properties, unconventional electronic correlations, magnetic frustration, and superconductivity. In theRV₆Sn₆family of materials (R= Sc, Y, Lu), ScV₆Sn₆hosts an unusual charge density wave ground state as well as structural similarities with theAV₃Sb₅system (A= K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with first-principles lattice dynamics calculations to reveal phonon mixing processes in the charge density wave state of ScV₆Sn₆. In the low temperature phase, we find at least four new peaks in the vicinity of the V-containing totally symmetric mode near 240 cm⁻¹suggesting that the density wave acts to mix modes ofP6/mmmand$$R\bar{3}m$$ $R\overline{3}m$symmetry - a result that we quantify by projecting phonons of the high symmetry state onto those of the lower symmetry structure. We also test the stability of the short-range ordered density wave state under compression and propose that both physical and chemical pressure quench the effect. We discuss these findings in terms of symmetry and the structure-property trends that can be unraveled in this system. 

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2. Emergence of nodal Bogoliubov quasiparticles across the transition from the pseudogap metal to the d-wave superconductor

   https://doi.org/10.1038/s41535-023-00608-0  

   Christos, Maine; Sachdev, Subir (January 2024, npj Quantum Materials)

   Abstract We model the pseudogap state of the hole- and electron-doped cuprates as a metal with hole and/or electron pocket Fermi surfaces. In the absence of long-range antiferromagnetism, such Fermi surfaces violate the Luttinger requirement of enclosing the same area as free electrons at the same density. Using the Ancilla theory of such a pseudogap state, we describe the onset of conventionald-wave superconductivity by the condensation of a chargeeHiggs boson transforming as a fundamental under the emergent SU(2) gauge symmetry of a backgroundπ-flux spin liquid. In all cases, we find that thed-wave superconductor has gapless Bogoliubov quasiparticles at 4 nodal points on the Brillouin zone diagonals with significant velocity anisotropy, just as in the BCS state. This includes the case of the electron-doped pseudogap metal with only electron pockets centered at wavevectors (π, 0), (0, π), and an electronic gap along the zone diagonals. Remarkably, in this case, too, gapless nodal Bogoliubov quasiparticles emerge within the gap at 4 points along the zone diagonals upon the onset of superconductivity. 

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3. Quadrupolar charge dynamics in the nonmagnetic FeSe _1−x S _x superconductors

   https://doi.org/10.1073/pnas.2020585118  

   Zhang, Weilu; Wu, Shangfei; Kasahara, Shigeru; Shibauchi, Takasada; Matsuda, Yuji; Blumberg, Girsh (May 2021, Proceedings of the National Academy of Sciences)

   null (Ed.)

   We use polarization-resolved electronic Raman spectroscopy to study quadrupolar charge dynamics in a nonmagnetic F e S e 1 − x S x superconductor. We observe two types of long-wavelength X Y symmetry excitations: 1) a low-energy quasi-elastic scattering peak (QEP) and 2) a broad electronic continuum with a maximum at 55 meV. Below the tetragonal-to-orthorhombic structural transition at T S ( x ) , a pseudogap suppression with temperature dependence reminiscent of the nematic order parameter develops in the X Y symmetry spectra of the electronic excitation continuum. The QEP exhibits critical enhancement upon cooling toward T S ( x ) . The intensity of the QEP grows with increasing sulfur concentration x and maximizes near critical concentration x c r ≈ 0.16 , while the pseudogap size decreases with the suppression of T S ( x ) . We interpret the development of the pseudogap in the quadrupole scattering channel as a manifestation of transition from the non-Fermi liquid regime, dominated by strong Pomeranchuk-like fluctuations giving rise to intense electronic continuum of excitations in the fourfold symmetric high-temperature phase, to the Fermi liquid regime in the broken-symmetry nematic phase where the quadrupole fluctuations are suppressed. 

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4. Distinguishing Optical and Acoustic Phonon Temperatures and Their Energy Coupling Factor under Photon Excitation in nm 2D Materials

   https://doi.org/10.1002/advs.202000097  

   Wang, Ridong; Zobeiri, Hamidreza; Xie, Yangsu; Wang, Xinwei; Zhang, Xing; Yue, Yanan (May 2020, Advanced Science)

   Abstract Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long‐history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state‐resolved Raman spectroscopy. ΔT_{OP −AP}is measured to take more than 30% of the Raman‐probed temperature rise. A breakthrough is made on measuring the intrinsic in‐plane thermal conductivity of suspended nm MoS₂and MoSe₂by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman‐based thermal conductivity measurement of 2D materials.G_OP↔APfor MoS₂, MoSe₂, and graphene paper (GP) are characterized. For MoS₂and MoSe₂,G_OP↔APis in the order of 10¹⁵and 10¹⁴W m⁻³K⁻¹andG_ZO↔APis much smaller thanG_LO/TO↔AP. Under ns laser excitation,G_OP↔APis significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP,G_LO/TO↔APis 0.549 × 10¹⁶W m⁻³K⁻¹, agreeing well with the value of 0.41 × 10¹⁶W m⁻³K⁻¹by first‐principles modeling. 

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5. A model of d -wave superconductivity, antiferromagnetism, and charge order on the square lattice

   https://doi.org/10.1073/pnas.2302701120  

   Christos, Maine; Luo, Zhu-Xi; Shackleton, Henry; Zhang, Ya-Hui; Scheurer, Mathias_S; Sachdev, Subir (May 2023, Proceedings of the National Academy of Sciences)

   We describe the confining instabilities of a proposed quantum spin liquid underlying the pseudogap metal state of the hole-doped cuprates. The spin liquid can be described by a SU(2) gauge theory ofN_f= 2 massless Dirac fermions carrying fundamental gauge charges—this is the low-energy theory of a mean-field state of fermionic spinons moving on the square lattice withπ-flux per plaquette in the ℤ₂center of SU(2). This theory has an emergent SO(5)_fglobal symmetry and is presumed to confine at low energies to the Néel state. At nonzero doping (or smaller Hubbard repulsionUat half-filling), we argue that confinement occurs via the Higgs condensation of bosonic chargons carrying fundamental SU(2) gauge charges also moving inπℤ₂-flux. At half-filling, the low-energy theory of the Higgs sector hasN_b= 2 relativistic bosons with a possible emergent SO(5)_bglobal symmetry describing rotations between ad-wave superconductor, period-2 charge stripes, and the time-reversal breaking “d-density wave” state. We propose a conformal SU(2) gauge theory withN_f= 2 fundamental fermions,N_b= 2 fundamental bosons, and a SO(5)_f×SO(5)_bglobal symmetry, which describes a deconfined quantum critical point between a confining state which breaks SO(5)_fand a confining state which breaks SO(5)_b. The pattern of symmetry breaking within both SO(5)s is determined by terms likely irrelevant at the critical point, which can be chosen to obtain a transition between Néel order andd-wave superconductivity. A similar theory applies at nonzero doping and largeU, with longer-range couplings of the chargons leading to charge order with longer periods. 

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