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1. Valence transition theory of the pressure-induced dimensionality crossover in superconducting ${\mathrm{Sr}}_{14-x}{\mathrm{Ca}}_x{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$

   https://doi.org/10.1103/PhysRevB.108.134510  

   Song, Jeong-Pil; Clay, R. Torsten; Mazumdar, Sumit (October 2023, Physical Review B)

   Stephen E. Nagler (Ed.)

   One of the strongest justifications for the continued search for superconductivity within the single-band Hubbard Hamiltonian originates from the apparent success of single-band ladder-based theories in predicting the occurrence of superconductivity in the cuprate coupled-ladder compound Sr{14−x}Ca{x}Cu{24}O{41}. Recent theoretical works have, however, shown the complete absence of quasi-long-range superconducting correlations within the hole-doped multiband ladder Hamiltonian including realistic Coulomb repulsion between holes on oxygen sites and oxygen-oxygen hole hopping. Experimentally, superconductivity in Sr{14−x}Ca{x}Cu{24}O{41} occurs only under pressure and is preceded by dramatic transition from one to two dimensions that remains not understood. We show that understanding the dimensional crossover requires adopting a valence transition model within which there occurs transition in Cu-ion ionicity from +2 to +1 , with transfer of holes from Cu to O ions [S. Mazumdar, Phys. Rev. B 98, 205153 (2018)]. The driving force behind the valence transition is the closed-shell electron configuration of Cu^{1+} , a feature shared by cations of all oxides with a negative charge-transfer gap. We make a falsifiable experimental prediction for Sr{14−x}Ca{x}Cu{24}O{41} and discuss the implications of our results for layered two-dimensional cuprates. 

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2. Large magnetic anisotropy and magnetostriction in thin films of ${\mathrm{CoV}}_2{\mathrm{O}}_4$

   https://doi.org/10.1103/PhysRevMaterials.7.014408  

   Kim, Sangsoo; Thompson, Christie J.; Xin, Yan; Beekman, Christianne (January 2023, Physical Review Materials)

   Spinel cobalt vanadate CoV2O4 has been grown on (001) SrTiO3 substrates. Using torque magnetometry experiments, we find that the previously observed temperature-induced anisotropy change, where the easy axis changes from the out-of-plane [001] direction to a biaxial anisotropy with planar <100> easy axes, occurs in a gradual second-order structural phase transition. This paper characterizes this transition and the magnetic anisotropies in the (001), (100), and (-110) rotation planes, and explores their field dependence up to 30 T. Below 80 K, hysteretic features appear around the hard axes, i.e., the out-of-plane direction in (-110) and (010) rotations and the planar <110> directions in (001) rotations. This is due to a Zeeman energy that originates from the lag of the magnetization with respect to the applied magnetic field as the sample is rotated. The appearance of the hysteresis, which persists up to very high fields, shows that the anisotropy at low temperature is rather strong. Additionally, field-dependent distortions to the symmetry of the torque response in increasing applied fields shows that magnetostriction plays a large role in determining the direction and magnitude of the anisotropy. 

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3. Hidden anomalies in topological $t\text{−}{\mathrm{PtBi}}_{2-x}$ probed by second harmonic generation

   https://doi.org/10.1103/PhysRevB.108.224104  

   Chavez, Bryan L; Kharal, Govinda; Xing, Lingyi; Crittenden, Scott; Crawford, Thomas M; Wu, Yanwen; Jin, Rongying (December 2023, Physical Review B)

   The noncentrosymmetric Weyl semimetal PtBi2−x (t-PtBi2−x) exhibits various interesting technologically important physical properties. We report the experimental investigation of PtBi1.6 via second harmonic generation (SHG), single-crystal x-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. While bulk structural, magnetic, and electrical properties show no phase transitions below room temperature, the temperature dependence of the SHG intensity reveals two anomalies: one at T ∗ ∼ 60 K and another at Tx ∼ 200 K. Quantitative analysis indicates that the SHG signal results from both the buckled Bi1 surface termination with the 3m symmetry and flat Bi2 surface termination with the m symmetry. However, the anomalies are mainly driven by Bi1 on the surface: (1) T ∗ marks the onset of surface states which is also manifested in the c-axis resistivity drop and (2) Tx corresponds to the lowest thermal contraction of the structure and enhanced magnetic susceptibility. This study demonstrates that SHG is a powerful technique for probing surface properties even for noncentrosymmetric materials. 

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4. Nonlinear Arrhenius behavior of self-diffusion in $\beta \text{−}\mathrm{Ti}$ and $\mathrm{Mo}$

   https://doi.org/10.1103/PhysRevMaterials.6.063402  

   Wang, Yaxian; Chen, Zhangqi; Windl, Wolfgang; Zhao, Ji-Cheng (June 2022, Physical Review Materials)

   While anomalous diffusion coefficients with non-Arrhenius-like temperature dependence are observed in a number of metals, a conclusive comprehensive framework of explanation has not been brought forward to date. Here, we use first-principles calculations based on density functional theory to calculate self-diffusion coefficients in the bcc metals Mo and β-Ti by coupling quasiharmonic transition state theory and large-displacement phonon calculations and show that anharmonicity from thermal expansion is the major reason for the anomalous temperature dependence. We use a modified Debye approach to quantify the thermal expansion over the entire temperature range and introduce a method to relax the vacancy structure in a mechanically unstable crystal such as β-Ti. The effect of thermal expansion is found to be crucial for the nonlinear, non-Arrhenius “anomalous” self-diffusion in both bcc systems, with β-Ti showing a 60% larger relative nonlinearity parameter than Mo. Our results point to temperature dependence in the diffusion prefactor from thermal expansion as the major origin of anomalous self-diffusion. The methodology proposed for β-Ti is general and simple enough to be applicable to other mechanically unstable crystals. 

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5. 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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