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1. Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet

   https://doi.org/10.1126/sciadv.abf1467  

   Asaba, T.; Ivanov, V.; Thomas, S. M.; Savrasov, S. Y.; Thompson, J. D.; Bauer, E. D.; Ronning, F. (March 2021, Science Advances)

   null (Ed.)

   The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCo 0.8 Ru 0.2 Al, reaching 23 microvolts per kelvin. Uranium’s 5 f electrons provide strong electronic correlations that lead to narrow bands, a known route to producing a large thermoelectric response. In addition, uranium’s strong spin-orbit coupling produces an intrinsic transverse response in this material due to the Berry curvature associated with the relativistic electronic structure. Theoretical calculations show that in UCo 0.8 Ru 0.2 Al at least 148 Weyl nodes, and two nodal lines, exist within 60 millielectron volt of the Fermi level. This work demonstrates that magnetic actinide materials can host strong Nernst and Hall responses due to their combined correlated and topological nature. 

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2. Anisotropic Phonon Response of Few‐Layer PdSe ₂ under Uniaxial Strain

   https://doi.org/10.1002/adfm.202003215  

   Luo, Weijun; Oyedele, Akinola D.; Gu, Yiyi; Li, Tianshu; Wang, Xingzhi; Haglund, Amanda V.; Mandrus, David; Puretzky, Alexander A.; Xiao, Kai; Liang, Liangbo; et al (July 2020, Advanced Functional Materials)

   Abstract PdSe₂, an emerging 2D material with a novel anisotropic puckered pentagonal structure, has attracted growing interest due to its layer‐dependent electronic bandgap, high carrier mobility, and good air stability. Herein, a detailed Raman spectroscopic study of few‐layer PdSe₂(two to five layers) under the in‐plane uniaxial tensile strain up to 3.33% is performed. Two of the prominent PdSe₂Raman peaks are influenced differently depending on the direction of strain application. The mode redshifts more than the mode when the strain is applied along thea‐axis of the crystal, while the mode redshifts more than the mode when the strain is applied along theb‐axis. Such an anisotropic phonon response to strain indicates directionally dependent mechanical and thermal properties of PdSe₂and also allows the identification of the crystal axes. The results are further supported using first‐principles density‐functional theory. Interestingly, the near‐zero Poisson’s ratios for few‐layer PdSe₂are found, suggesting that the uniaxial tensile strain can easily be applied to few‐layer PdSe₂without significantly altering their dimensions at the perpendicular directions, which is a major contributing factor to the observed distinct phonon behavior. The findings pave the way for further development of 2D PdSe₂‐based flexible electronics. 

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3. Origin of the anisotropic Beer–Lambert law from dichroism and birefringence in β -Ga2O3

   https://doi.org/10.1063/5.0226902  

   Adnan, Md_Mohsinur Rahman; Schubert, Mathias; Myers, Roberto C (August 2024, Applied Physics Letters)

   The anisotropic optical absorption edge of β-Ga2O3 follows a modified Beer–Lambert law having two effective absorption coefficients. The absorption coefficient of linearly polarized light reduces to the least absorbing direction beyond a critical penetration depth, which itself depends on polarization and wavelength. To understand this behavior, a Stokes vector analysis is performed to track the polarization state as a function of depth. The weakening of the absorption coefficient is associated with a gradual shift of linear polarization to the least absorbing crystallographic direction in the plane, which is along the a-exciton within the (010) plane or along the b-exciton in the (001) plane. We show that strong linear dichroism near the optical absorption edge causes this shift in β-Ga2O3, which arises from the anisotropy and spectral splitting of the physical absorbers, i.e., excitons. The linear polarization shift is accompanied by a variation in the ellipticity due to the birefringence of β-Ga2O3. Analysis of the phase relationship between the incoming electric field to that at a certain depth reveals the phase speed as an effective refractive index, which varies along different crystallographic directions. The critical penetration depth is shown to be correlated with the depth at which the ellipticity is maximal. Thus, the anisotropic Beer–Lambert law arises from the interplay of both the dichroic and birefringent properties of β-Ga2O3. 

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4. Second order Zeeman interaction and ferroquadrupolar order in TmVO4

   https://doi.org/10.1038/s41535-022-00475-1  

   Vinograd, I.; Shirer, K. R.; Massat, P.; Wang, Z.; Kissikov, T.; Garcia, D.; Bachmann, M. D.; Horvatić, M.; Fisher, I. R.; Curro, N. J. (June 2022, npj Quantum Materials)

   Abstract TmVO₄exhibits ferroquadrupolar order of the Tm 4f electronic orbitals at low temperatures, and is a model system for Ising nematicity. A magnetic field oriented along thec-axis constitutes a transverse effective field for the quadrupolar order parameter, continuously tuning the system to a quantum phase transition as the field is increased from zero. In contrast, in-plane magnetic fields couple to the order parameter only at second order, such that orienting along the primary axes of the quadrupole order results in an effective longitudinal field, whereas orienting at 45 degrees results in a second effective transverse field. Not only do in-plane fields engender a marked in-plane anisotropy of the critical magnetic and quadrupole fluctuations above the ferroquadrupolar ordering temperature, but in-plane transverse fields initially enhance the ferroquadrupolar order, before eventually suppressing it, an effect that we attribute to admixing of the higher crystalline electric field levels. 

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5. Orienting and shaping organic semiconductor single crystals through selective nanoconfinement

   https://doi.org/10.1039/D0SM01928C  

   Alaei, Aida; Zong, Kai; Asawa, Kaustubh; Chou, Tseng-Ming; Briseño, Alejandro L.; Choi, Chang-Hwan; Lee, Stephanie S. (January 2021, Soft Matter)

   null (Ed.)

   For organic semiconductor crystals exhibiting anisotropic charge transport along different crystallographic directions, nanoconfinement is a powerful strategy to control crystal orientation by aligning the fast crystallographic growth direction(s) with the unconfined axis(es) of nanoconfining scaffolds. Here, design rules are presented to relate crystal morphology, scaffold geometry, and orientation control in solution-processed small-molecule crystals. Specifically, organic semiconductor triisopropylsilylethynyl pyranthrene needle-like crystals with a dimensionality of n = 1 and perylene platelike crystals with n = 2 were grown from solution within nanoconfining scaffolds comprising cylindrical nanopores with a dimensionality of m = 1, representing one unconfined dimension along the cylinder axis, and those comprising nanopillar arrays with a dimensionality of m = 2. For m = n systems, native crystal growth habits were preserved while the crystal orientation in n = m direction(s) was dictated by the geometry of the scaffold. For n ≠ m systems, on the other hand, orientation control was restricted within a single plane, either parallel or perpendicular to the substrate surface. Intriguingly, control over crystal shape was also observed for perylene crystals grown in cylindrical nanopores ( n > m ). Within the nanopores, crystal growth was restricted along a single direction to form a needle-like morphology. Once growth proceeded above the scaffold surface, the crystals adopted their native growth habit to form asymmetric T-shaped single crystals with concave corners. These findings suggest that nanoporous scaffolds with spatially-varying dimensionalities can be used to grow single crystals of complex shapes. 

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