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Register a new userPressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn$_2$Bi$_2$
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In-situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn$_2$Bi$_2$ undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. CaMn2Bi2 displays a new structure type above 2.3 GPa, with the puckered Mn honeycomb lattice of the trigonal ambient-pressure structure converting to one-dimensional (1D) zigzag chains in the high-pressure monoclinic structure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic two order of magnitude drop of resistivity; although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as, surprisingly, are two resistivity anomalies with opposite pressure dependences. Based on the electronic structure calculations, we hypothesized that the newly emerged electronic state under high pressure is associated with a Fermi surface instability of the quasi-1D Mn chains, while we infer that the other is a magnetic transition. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn$_2$Bi$_2$ indicates that ferrimagnetism is thermodynamically favored.
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We have studied the effect of pressure on the pyrochlore iridate Eu$_2$Ir$_2$O$_7$, which at ambient pressure has a thermally driven insulator to metal transition at $T_{MI}sim120$,K. As a function of pressure the insulating gap closes, apparently continuously, near $P sim 6$,GPa. However, rather than $T_{MI}$ going to zero as expected, the insulating ground state crosses over to a metallic state with a negative temperature coefficient of resistivity, calling into question the true nature of both ground states. The high temperature state also crosses over near 6 GPa, from an incoherent to a conventional metal, suggesting a connection between the high and the low temperature states.
We utilize near-infrared pump and mid-infrared probe spectroscopy to investigate the ultrafast electronic response of pressurized VO$_2$. Distinct pump-probe signals and a pumping threshold behavior are observed even in the pressure-induced metallic state showing a noticeable amount of localized electronic states. Our results are consistent with a scenario of a bandwidth-controlled Mott-Hubbard transition.
How to realize applicably appreciated functionalities based on the coupling between charge and spin degrees of freedom is still a challenge in the field of spintronics. For example, anisotropic magnetoresistance (AMR) effect is utilized to read out the information stored by various magnetic structures, which usually originates from atomic spin-orbit coupling (SOC). However, the application of AMR in antiferromagnet-based spintronics is still hindered by rather small AMR value. Here, we discover a colossal AMR effect during the field-induced metal-to-insulator transition (MIT) in a nearly Dirac material EuMnSb$_2$ with an antiferromagnetic order of Eu$^{2+}$ moments. The colossal AMR reaches to an unprecedented value of 1.84$times$10$^6$% at 2 K, which is four orders of magnitude larger than previously reported values in antiferromagnets. Based on density functional theory calculations, a Dirac-like band structure, which is strongly dependent on SOC, is confirmed around Y point and dominates the overall transport properties in the present sample with predominant electron-type carriers. Moreover, it is also revealed that the indirect band gap around Fermi level is dependent on the magnetic structure of Eu$^{2+}$ moments, which leads to the field-induced MIT and plays a key role on the colossal AMR effect. Finally, our present work suggests that the similar antiferromagnetic topological materials as EuMnSb$_2$, in which Dirac-like fermions is strongly modulated by SOC and antiferromagnetism, would be a fertile ground to explore applicably appreciated AMR effect.
Recent experiments [arXiv: 1808.07865] on twisted bilayer graphene (TBLG) show that under hydrostatic pressure, an insulating state at quarter-filling of the moire superlattice (i.e., one charge per supercell) emerges, in sharp contrast with the previous ambient pressure measurements of Cao et al. where the quarter--filling state (QFS) is a metal [Nature 556, 43 & 80 (2018)]. In fact, the insulating state at the other commensurate fillings of two and three charges per supercell is also enhanced under applied pressure. Based on realistic computations of the band structure for TBLG which show that the bandwidth first shrinks and then expands with increasing hydrostatic pressure, we compute the ratio of the potential to the kinetic energy, $r_s$. We find an experimentally relevant window of pressure for which $r_s$ crosses the threshold for a triangular Wigner crystal, thereby corroborating our previous work [Nano Lett. (2018)] that the insulating states in TBLG are due to Wigner rather than Mott physics. A key prediction of this work is that the window for the onset of the hierarchy of Wigner states that obtains at commensurate fillings is dome-shaped as a function of the applied pressure, which can be probed experimentally. Theoretically, we find a peak for crystallization around $1.5$ GPa relative to the experimental optimal pressure of $1.33$ GPa for the observation of the insulating states. Consequently, TBLG provides a new platform for the exploration of Wigner physics and its relationship with superconductivity.
The electrical, magnetic, and structural properties of Sr$_3$(Ru$_{1-x}$Mn$_x$)$_2$O$_7$ (0 $leq x leq$ 0.2) are investigated. The parent compound Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal, critically close to magnetic order. We have found that, with a Ru-site doping by only a few percent of Mn, the ground state is switched from a paramagnetic metal to an antiferromagnetic insulator. Optical conductivity measurements show the opening of a gap as large as 0.1 eV, indicating that the metal-to-insulator transition is driven by the electron correlation. The complex low-temperature antiferromagnetic spin arrangement, reminiscent of those observed in some nickelates and manganites, suggests a long range orbital order.
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