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1. Electrically and magnetically switchable nonlinear photocurrent in РТ-symmetric magnetic topological quantum materials

   https://doi.org/10.1038/s41524-020-00462-9  

   Wang, Hua ; Qian, Xiaofeng ( December 2020 , npj Computational Materials)

   Nonlinear photocurrent in time-reversal invariant noncentrosymmetric systems such as ferroelectric semimetals sparked tremendous interest of utilizing nonlinear optics to characterize condensed matter with exotic phases. Here we provide a microscopic theory of two types of second-order nonlinear direct photocurrents, magnetic shift photocurrent (MSC) and magnetic injection photocurrent (MIC), as the counterparts of normal shift current (NSC) and normal injection current (NIC) in time-reversal symmetry and inversion symmetry broken systems. We show that MSC is mainly governed by shift vector and interband Berry curvature, and MIC is dominated by absorption strength and asymmetry of the group velocity difference at time-reversed ±kpoints. Taking$${\cal{P}}{\cal{T}}$$$PT$-symmetric magnetic topological quantum material bilayer antiferromagnetic (AFM) MnBi₂Te₄as an example, we predict the presence of large MIC in the terahertz (THz) frequency regime which can be switched between two AFM states with time-reversed spin orderings upon magnetic transition. In addition, external electric field breaks$${\cal{P}}{\cal{T}}$$$PT$symmetry and enables large NSC response in bilayer AFM MnBi₂Te₄, which can be switched by external electric field. Remarkably, both MIC and NSC are highly tunable under varying electric field due to the field-induced large Rashba and Zeeman splitting, resulting in large nonlinear photocurrent response down to a few THz regime, suggesting bilayer AFM-zMnBi₂Te₄as amore »tunable platform with rich THz and magneto-optoelectronic applications. Our results reveal that nonlinear photocurrent responses governed by NSC, NIC, MSC, and MIC provide a powerful tool for deciphering magnetic structures and interactions which could be particularly fruitful for probing and understanding magnetic topological quantum materials.

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2. Phase diagram of YbZnGaO4 in applied magnetic field

   https://doi.org/10.1038/s41535-021-00380-z  

   Steinhardt, William ; Maksimov, P. A. ; Dissanayake, Sachith ; Shi, Zhenzhong ; Butch, Nicholas P. ; Graf, David ; Podlesnyak, Andrey ; Liu, Yaohua ; Zhao, Yang ; Xu, Guangyong ; et al ( September 2021 , npj Quantum Materials)

   Recently, Yb-based triangular-lattice antiferromagnets have garnered significant interest as possible quantum spin-liquid candidates. One example is YbMgGaO₄, which showed many promising spin-liquid features, but also possesses a high degree of disorder owing to site-mixing between the non-magnetic cations. To further elucidate the role of chemical disorder and to explore the phase diagram of these materials in applied field, we present neutron scattering and sensitive magnetometry measurements of the closely related compound, YbZnGaO₄. Our results suggest a difference in magnetic anisotropy between the two compounds, and we use key observations of the magnetic phase crossover to motivate an exploration of the field- and exchange parameter-dependent phase diagram, providing an expanded view of the available magnetic states in applied field. This enriched map of the phase space serves as a basis to restrict the values of parameters describing the magnetic Hamiltonian with broad application to recently discovered related materials.
3. Magnetic field-temperature phase diagram of multiferroic (NH4)2FeCl5·H2O

   https://doi.org/10.1038/s41535-019-0180-1  

   Clune, Amanda J. ; Nam, Jisoo ; Lee, Minseong ; Hughey, Kendall D. ; Tian, Wei ; Fernandez-Baca, Jaime A. ; Fishman, Randy S. ; Singleton, John ; Lee, Jun Hee ; Musfeldt, Janice L. ( August 2019 , npj Quantum Materials)

   Owing to their overall low energy scales, flexible molecular architectures, and ease of chemical substitution, molecule-based multiferroics are extraordinarily responsive to external stimuli and exhibit remarkably rich phase diagrams. Even so, the stability and microscopic properties of various magnetic states in close proximity to quantum critical points are highly under-explored in these materials. Inspired by these opportunities, we combined pulsed-field magnetization, first-principles calculations, and numerical simulations to reveal the magnetic field–temperature (B–T) phase diagram of multiferroic (NH₄)₂FeCl₅⋅H₂O. In this system, a network of intermolecular hydrogen and halogen bonds creates a competing set of exchange interactions that generates additional structure in the phase diagram—both in the vicinity of the spin flop and near the 30 T transition to the fully saturated state. Consequently, the phase diagrams of (NH₄)₂FeCl₅⋅H₂O and its deuterated analog are much more complex than those of other molecule-based multiferroics. The entire series of coupled electric and magnetic transitions can be accessed with a powered magnet, opening the door to exploration and control of properties in this and related materials.
4. Evidence for unconventional superconductivity and nontrivial topology in PdTe

   https://doi.org/10.1038/s41598-023-33237-5  

   Chapai, Ramakanta ; Reddy, P. V. Sreenivasa ; Xing, Lingyi ; Graf, David E. ; Karki, Amar B. ; Chang, Tay-Rong ; Jin, Rongying ( April 2023 , Scientific Reports)

   PdTe is a superconductor withT_c ~ 4.25 K. Recently, evidence for bulk-nodal and surface-nodeless gap features has been reported in PdTe. Here, we investigate the physical properties of PdTe in both the normal and superconducting states via specific heat and magnetic torque measurements and first-principles calculations. BelowT_c, the electronic specific heat initially decreases inT³behavior (1.5 K < T < T_c) then exponentially decays. Using the two-band model, the superconducting specific heat can be well described with two energy gaps: one is 0.372 meV and another 1.93 meV. The calculated bulk band structure consists of two electron bands (α and β) and two hole bands (γ and η) at the Fermi level. Experimental detection of the de Haas-van Alphen (dHvA) oscillations allows us to identify four frequencies (F_α = 65 T,F_β = 658 T,F_γ = 1154 T, andF_η = 1867 T forH//a), consistent with theoretical predictions. Nontrivial α and β bands are further identified via both calculations and the angle dependence of the dHvA oscillations. Our results suggest that PdTe is a candidate for unconventional superconductivity.
5. Anisotropic magnetic and transport properties of orthorhombic o-Pr ₂ Co ₃ Ge ₅

   https://doi.org/10.1088/2515-7639/ac9ad9  

   Kyrk, Trent M. ; Kennedy, Ellis R. ; Galeano-Cabral, Jorge ; Wei, Kaya ; McCandless, Gregory T. ; Scott, Mary C. ; Baumbach, Ryan E. ; Chan, Julia Y. ( October 2022 , Journal of Physics: Materials)

   The crystal structure, electron energy-loss spectroscopy (EELS), heat capacity, and anisotropic magnetic and resistivity measurements are reported for Sn flux grown single crystals of orthorhombic Pr₂Co₃Ge₅(U₂Co₃Si₅-type,Ibam). Our findings show thato-Pr₂Co₃Ge₅hosts nearly trivalent Pr ions, as evidenced by EELS and fits to temperature dependent magnetic susceptibility measurements. Complex magnetic ordering with a partially spin-polarized state emerges nearT_sp= 32 K, with a spin reconfiguration transition nearT_M= 15 K. Heat capacity measurements show that the phase transitions appear as broad peaks in the vicinity ofT_spandT_M. The magnetic entropy further reveals that crystal electric field splitting lifts the Hund’s rule degeneracy at low temperatures. Taken together, these measurements show that Pr₂Co₃Ge₅is an environment for complexfstate magnetism with potential strongly correlated electron states.