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Register a new userStrain Induced Slater Transition in Polar Metal LiOsO$_3$
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LiOsO3 is the first experimentally confirmed polar metal. Previous works suggested that the ground state of LiOsO$_3$ is just close to the critical point of metal-insulator transition. In this work the electronic state of LiOsO$_3$ is tuned by epitaxial biaxial strain, which undergoes the Slater-type metal-insulator transition under tensile strain, i.e., the G-type antiferromagnetism emerges. The underlying mechanism of bandwidth tuning can be extended to its sister compound NaOsO$_3$, which shows an opposite transition from a antiferromagnetic insulator to a nonmagnetic metal under hydrostatic pressure. Our work suggests a feasible route for the manipulation of magnetism and conductivity of polar metal LiOsO$_3$.
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LiOsO$_3$ undergoes a continuous transition from a centrosymmetric $Rbar{3}c$ structure to a polar $R3c$ structure at $T_s=140$~K. By combining transport measurements and first-principles calculations, we find that $T_s$ is enhanced by applied pressure, and it reaches a value of $sim$250~K at $sim$6.5~GPa. The enhancement is due to the fact that the polar $R3c$ structure of LiOsO$_3$ has a smaller volume than the centrosymmetric $Rbar{3}c$ structure. Pressure generically favors the structure with the smallest volume, and therefore further stabilizes the polar $R3c$ structure over the $Rbar{3}c$ structure, leading to the increase in $T_s$.
Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point---a four-fold-degenerate band-crossing point with cubic dispersion in a plane and a linear dispersion in the third direction---can be stabilized through the presence of a nonsymmorphic glide mirror symmetry in the space group of the crystal. Notably, the cubic Dirac point in our case appears on a threefold axis, even though it has been believed previously that such a point can only appear on a sixfold axis. We show that a cubic Dirac point involving a threefold axis can be realized close to the Fermi level in the non-ferroelectric phase of LiOsO$_3$. Upon lowering temperature, LiOsO$_3$ has been shown experimentally to undergo a structural phase transition from the non-ferroelectric phase to the ferroelectric phase with spontaneously broken inversion symmetry. Remarkably, we find that the broken symmetry transforms the cubic Dirac point into three mutually-crossed nodal rings. There also exist several linear Dirac points in the low-energy band structure of LiOsO$_3$, each of which is transformed into a single nodal ring across the phase transition.
As a member of the Ruddlesden-Popper Ln$_{n+1}$Ni$_n$O$_{3n+1}$ series rare-earth-nickelates, the Pr4Ni$_3$O$_{10}$ consists of infinite quasi-two-dimensional perovskite-like Ni-O based layers. Although a metal-to-metal phase transition at Tpt = 157 K has been revealed by previous studies, a comprehensive study of physical properties associated with this transition has not yet been performed. We have grown single crystals of Pr4Ni3O10 at high oxygen pressure, and report on the physical properties around that phase transition, such as heat-capacity, electric-transport and magnetization. We observe a distinctly anisotropic behavior between in-plane and out-of-plane properties: a metal-to-metal transition at Tpt within the a-b plane, and a metal-to-insulator-like transition along the c-axis with decreasing temperature. Moreover, an anisotropic and anomalous negative magneto-resistance is observed at Tpt that we attribute to a slight suppression of the first-order transition with magnetic field. The magnetic-susceptibility can be well described by a Curie-Weiss law, with different Curie-constants and Pauli-spin susceptibilities between the high-temperature and the low-temperature phases. The single crystal X-ray diffraction measurements show a shape variation of the different NiO6 octahedra from the high-temperature phase to the low-temperature phase. This subtle change of the environment of the Ni sites is likely responsible for the different physical properties at high and low temperatures.
Very recently, a new type of two-dimensional layered material MoSi2N4 has been fabricated, which is semiconducting with weak interlayer interaction, high strength, and excellent stability. We systematically investigate theoretically the effect of vertical strain on the electronic structure of MA2Z4 (M=Ti/Cr/Mo, A=Si, Z=N/P) bilayers. Taking bilayer MoSi2N4 as an example, our first principle calculations show that its indirect band gap decreases monotonically as the vertical compressive strain increases. Under a critical strain around 22%, it undergoes a transition from semiconductor to metal. We attribute this to the opposite energy shift of states in different layers, which originates from the built-in electric field induced by the asymmetric charge transfer between two inner sublayers near the interface. Similar semiconductor to metal transitions are observed in other strained MA2Z4 bilayers, and the estimated critical pressures to realize such transitions are within the same order as semiconducting transition metal dichalcogenides. The semiconductor to metal transitions observed in the family of MA2Z4 bilayers present interesting possibilities for strain-induced engineering of their electronic properties.
Recently discovered class of 2D materials based on transition metal phosphorous trichalcogenides exhibit antiferromagnetic ground state, with potential applications in spintronics. Amongst them, FePS$ _{3} $ is a Mott insulator with a band gap of $sim$ 1.5 eV. This study using Raman spectroscopy along with first-principles density functional theoretical analysis examines the stability of its structure and electronic properties under pressure. Raman spectroscopy reveals two phase transitions at 4.6 GPa and 12 GPa marked by the changes in pressure coefficients of the mode frequencies and the number of symmetry allowed modes. FePS$_3$ transforms from the ambient monoclinic C2/m phase with a band gap of 1.54 eV to another monoclinic C2/m (band gap of 0.1 eV) phase at 4.6 GPa, followed by another transition at 12 GPa to the metallic trigonal P-31m phase. Our work complements recently reported high pressure X-ray diffraction studies.
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