From Mott insulator to ferromagnetic metal: a pressure study of Ca2_{2}RuO4_{4}

We show that the pressure-temperature phase diagram of the Mott insulator Ca$_{2}$RuO$_{4}$ features a metal-insulator transition at 0.5GPa: at 300K from paramagnetic insulator to paramagnetic quasi-two-dimensional metal; at $T \leq$ 12K from antiferromagnetic insulator to ferromagnetic, highly anisotropic, three-dimensional metal. % We compare the metallic state to that of the structurally related p-wave superconductor Sr$_{2}$RuO$_{4}$, and discuss the importance of structural distortions, which are expected to couple strongly to pressure.Comment: 4 pages, 4figure

The magnetic structure of the zigzagzigzag chain family Nax_{x}Ca1−x_{1-x}V2_2O4_4 determined by muon-spin rotation

We present muon-spin rotation measurements on polycrystalline samples of the complete family of the antiferromagnetic (AF) $zigzag$ chain compounds, Na$_x$Ca$_{1-x}$V$_2$O$_4$. In this family, we explore the magnetic properties from the metallic NaV$_2$O$_4$ to the insulating CaV$_2$O$_4$. We find a critical $x_c(\sim0.833)$ which separates the low and high Na-concentration dependent transition temperature and its magnetic ground state. In the $x<x_c$ compounds, the magnetic ordered phase is characterized by a single homogenous phase and the formation of incommensurate spin-density-wave order. Whereas in the $x>x_c$ compounds, multiple sub-phases appear with temperature and $x$. Based on the muon data obtained in zero external magnetic field, a careful dipolar field simulation was able to reproduce the muon behavior and indicates a modulated helical incommensurate spin structure of the metallic AF phase. The incommensurate modulation period obtained by the simulation agrees with that determined by neutron diffraction.Comment: 7 pages, 7 figures, accepted for publication in PR

Restoration of quantum critical behavior by disorder in pressure-tuned (Mn,Fe)Si

In second-order quantum phase transitions from magnetically ordered to paramagnetic states at T = 0, tuned by pressure or chemical substitution, a quantum critical point is expected to appear with critical behavior manifesting in the slowing down of spin fluctuations in the paramagnetic state and a continuous development of the order parameter in the ordered state. Quantum criticality is discussed widely as a possible driving force for unconventional superconductivity and other exotic phenomena in correlated electron systems. In the real world, however, quantum critical points and quantum criticality are often masked by a preceding first-order transition and/or the development of competing states. Pressure tuning of the itinerant-electron helical magnet MnSi is a well-known example of the suppression of a quantum critical point due to a first-order phase transition and resulting destruction of the ordered state. Utilizing muon spin relaxation experiments, here we report that 15% Fe-substituted (Mn,Fe)Si exhibits completely different behavior with pressure tuning, including the restoration of second-order quantum critical behavior and a quantum critical point at p QPC  ~ 21–23 kbar, which coincides with the T = 0 crossing point of the extrapolated phase boundary line of pure MnSi. This result is quantitatively consistent with the recent theory of itinerant-electron ferromagnets by Sang, Belitz, and Kirkpatrick, who argued that disorder would restore a quantum critical point which is otherwise hidden by a first-order transition

Role of two-dimensional electronic state in superconductivity in La22xSrxCuO4

We have measured out-of-plane resistivity ρc for La2-xSrxCuO4 under anisotropic pressure, c-axis compression, which decreases ρc, reduces Tc drastically, whereas c-axis extension, which increases ρc, enhances Tc from 38 K at ambient pressure to 51.6 K at 8 GPa. We find that the variation of Tc scales as a function of ρc, and that the c-axis pressure coefficient is much stronger than the ab-axis one. These findings imply that Tc depends primarily on the interlayer, rather than the in-plane, lattice parameter

Pressure-induced antiferromagnetism in UPt

UPt orders ferromagnetically at ambient pressure at 28 K. Upon increasing pressure, an additional magnetic phase transition appears around 17 K. While the upper transition behaves with field as a typical paramagnetic-ferromagnetic transition, the lower one exhibits antiferromagnetic behavior. With pressure both transitions shift towards lower temperatures. Around 1.5 GPa the upper magnetic transition completely disappears and only the lower persists up to ∼4 GPa. The electrical resistivity measured up to 8.0 GPa shows that structural transformation occurs in UPt under pressure

On the Robustness of the MnSi Magnetic Structure Determined by Muon Spin Rotation

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