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Register a new userPressure tuning of structural and magnetic transitions in EuAg$_4$As$_2$
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We report temperature dependent measurements of ambient pressure specific heat, magnetic susceptibility, anisotropic resistivity and thermal expansion as well as in-plane resistivity under pressure up to 20.8 kbar on single crystals of EuAg$_4$As$_2$. Based on thermal expansion and in-plane electrical transport measurements at ambient pressure this compound has two, first order, structural transitions in 80 - 120 K temperature range. Ambient pressure specific heat, magnetization and thermal expansion measurements show a cascade of up to seven transitions between 8 and 16 K associated with the ordering of the Eu$^{2+}$ moments. In-plane electrical transport is able to detect more prominent of these transitions: at 15.5, 9.9, and 8.7 K as well as a weak feature at 11.8 K at ambient pressure. Pressure dependent electrical transport data show that the magnetic transitions shift to higher temperatures under pressure, as does the upper structural transition, whereas the lower structural transition is suppressed and ultimately vanishes. A jump in resistivity, associated with the upper structural transition, decreases under pressure with an extrapolated disappearance (or a change of sign) by 30-35 kbar. In the 10 - 15 kbar range a kink in the pressure dependence of the upper structural transition temperature as well as the high and low temperature in-plane resistivities suggest that a change in the electronic structure may occur in this pressure range. The results are compared with the literature data for SrAg$_4$As$_2$.
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In this paper, the magnetic and transport properties were systematically studied for EuAg$_4$As$_2$ single crystals, crystallizing in a centrosymmetric trigonal CaCu$_4$P$_2$ type structure. It was confirmed that two magnetic transitions occur at $textit{T}$$_{N1}$ = 10 K and $textit{T}$$_{N2}$ = 15 K, respectively. With the increasing field, the two transitions are noticeably driven to lower temperature. At low temperatures, applying a magnetic field in the $textit{ab}$ plane induces two successive metamagnetic transitions. For both $textit{H}$ $parallel$ $textit{ab}$ and $textit{H}$ $parallel$ $textit{c}$, EuAg$_4$As$_2$ shows a positive, unexpected large magnetoresistance (up to 202%) at low fields below 10 K, and a large negative magnetoresistance (up to -78%) at high fields/intermediate temperatures. Such anomalous field dependence of magnetoresistance may have potential application in the future magnetic sensors. Finally, the magnetic phase diagrams of EuAg$_{4}$As$_{2}$ were constructed for both $textit{H}$ $parallel$ $textit{ab}$ and $textit{H}$ $parallel$ $textit{c}$.
Two charge density wave transition can be detected in LaAu$_x$Sb$_2$ at ~ 110 and ~ 90 K by careful electrical transport measurements. Whereas control of the Au site occupancy in LaAu$_x$Sb$_2$ (for 0.9 < x < 1.0) can suppress each of these transitions by ~ 80 K, the application of hydrostatic pressure can completely suppress the lower transition by ~ 10 kbar and the upper transition by ~ 17 kbar. Clear anomalies in the resistance as well as the magnetoresistance are observed to coincide with the pressures at which the charge density wave transitions are driven to zero.
Pb$_2$CoOsO$_6$ is a newly synthesized polar metal in which inversion symmetry is broken by the magnetic frustration in an antiferromagnetic ordering of Co and Os sublattices. The coupled magnetic and structural transition occurs at 45 K at ambient pressure. Here we perform transport measurements and first-principles calculations to study the pressure effects on the magnetic/structural coupled transition of Pb$_2$CoOsO$_6$. Experimentally we monitor the resistivity anomaly at $T_N$ under various pressures up to 11 GPa in a cubic anvil cell apparatus. We find that $T_N$ determined from the resistivity anomaly first increases quickly with pressure in a large slope of $dT_N/dP$ = +6.8(8) K/GPa for $P < 4$ GPa, and then increases with a much reduced slope of 1.8(4) K/GPa above 4 GPa. Our first-principles calculations suggest that the observed discontinuity of $dT_N/dP$ around 4 GPa may be attributed to the vanishing of Os magnetic moment under pressure. Pressure substantially reduces the Os moment and completely suppresses it above a critical value, which relieves the magnetic frustration in the antiferromagnetic ordering of Pb$_2$CoOsO$_6$. The Co and Os polar distortions decrease with the increasing pressure and simultaneously vanish at the critical pressure. Therefore above the critical pressure a new centrosymmetric antiferromagnetic state emerges in Pb$_2$CoOsO$_6$, distinct from the one under ambient pressure, thus showing a discontinuity in $dT_N/dP$.
La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals have been grown out of an NaAs flux in an alumina crucible and characterized by measuring magnetic susceptibility, electrical resistivity, specific heat, as well as single crystal x-ray and neutron diffraction. La$_{0.4}$Na$_{0.6}$Fe$_2$As$_2$ single crystals show a structural phase transition from a high temperature tetragonal phase to a low-temperature orthorhombic phase at T$_s$,=,125,K. This structural transition is accompanied by an anomaly in the temperature dependence of electrical resistivity, anisotropic magnetic susceptibility, and specific heat. Concomitant with the structural phase transition, the Fe moments order along the emph{a} direction with an ordered moment of 0.7(1),$mu_{textup{B}}$ at emph{T},=,5 K. The low temperature stripe antiferromagnetic structure is the same as that in other emph{A}Fe$_{2}$As$_{2}$ (emph{A},=,Ca, Sr, Ba) compounds. La$_{0.5-x}$Na$_{0.5+x}$Fe$_2$As$_2$ provides a new material platform for the study of iron-based superconductors where the electron-hole asymmetry could be studied by simply varying La/Na ratio.
A pressure-induced simultaneous metal-insulator transition (MIT) and structural-phase transformation in lithium hydride with about 1% volume collapse has been predicted by means of the local density approximation (LDA) in conjunction with an all-electron GW approximation method. The LDA wrongly predicts that the MIT occurs before the structural phase transition. As a byproduct, it is shown that only the use of the generalized-gradient approximation together with the zero-point vibration produces an equilibrium lattice parameter, bulk modulus, and an equation of state that are in excellent agreement with experimental results.
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