Session T38: Electronic Phase Transitions in Correlated Systems

Transport Measurements at the metal-insulator transition in pure NiS2

Ni(S,Se)2 is a one of the few Mott-Hubbard systems where a structural phase transition does not preclude quantitative study of the localization of charge at the T = 0 metal-insulator transition. By application of hydrostatic pressures at milli-Kelvin temperatures using diamond anvil cell techniques, we study the corresponding behavior of pure NiS2 at its quantum critical point. Comparison between pure and doped NiS2 of the closing of the transport gap in the insulator and the power law temperature dependence of the conductivity in the metal should reveal the role of disorder at this quantum phase transition.   

Structural and magnetic characteristics of pure NiS$_{2}$ in the pressure induced Mott insulator-metal transition

NiS$_{2}$ is a Mott insulator with a half-filled $e_{g}$ band split by Coulomb repulsion of Ni $d$ electrons. The low temperature insulator exhibits two types of coexisting antiferromagnetic order (M1, M2), but the connection between the magnetism and electron correlations is not well understood. Using high-resolution x-ray diffraction in a diamond anvil cell, we probe both the lattice and M2 magnetic structures of pure NiS$_{2}$ across the pressure induced insulator-metal transition at T = 3.5 K. Unlike most Mott systems, which have reduced symmetry in the insulating phase, we find that the high pressure metallic state of NiS$_{2}$ is the phase with reduced symmetry. The M2 antiferromagnetism disappears along with the high symmetry phase at high pressure. Our observations suggest that the M2 antiferromagnetism arises from a super-exchange interaction between correlated electrons in the Mott insulating phase.   

Electric Field Induced Resistive Switch in Transition Metal Oxides: A ``Model'' for Future Non-Volatile Memory Devices

The kinetics of resistive switching in metal (Ag)--Pr$_{0.7}$Ca $_{0.3}$MnO$_{3}$ interfaces has been investigated. The resistance hysteresis $\Delta R $varies with the pulse amplitude $V_{0}$ roughly as a step function with existence of a threshold voltage $V_{t}$. On the other hand, the $\Delta R $varies with the pulse width ($T_{w})$ as a two-stage sequence: an initial exponential rise with a time constant $\tau _{S}\approx $ 2 X10$^{7}$ s and a slow linearly increasing tail. The slow linear part is dominant only in the quasi-dc switch (pulse width $\sim $ a few seconds) below $V_{t}$. The retentions of the $\Delta R $corresponding to the two stages are also extremely different, indicating that different underlying processes are involved. The relaxation time ($\tau _{R})$ is 10$^{8}$ s ($\sim $year) or higher for the sub-$\mu $s switching, in strong contrast with the total disappearance of the $\Delta R $after a few days for sub-threshold ($V_{0}<< \quad V_{t})$ switch. More results obtained suggest that defect creation/annihilation is likely the mechanism for the sub-$\mu $s switching and that a slow accumulative process (like diffusion) of defects may be responsible for the quasi-dc switch.   

Mapping the Ferromagnetic, Charge Ordered, and Antiferromagnetic phases in La$_{1-x-y}$Pr$_y$Ca$_x$MnO$_3$

Manganite compounds in the La$_{1-x-y}$Pr$_y$Ca$_x$MnO$_3$ series are known for exhibiting extremely high colossal magnetoresistance (CMR). We combined the x-ray photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (REXS) techniques to study the interplay between the ferromagnetic and charge-ordered/antiferromagnetic phases, respectively, in La$_{1-x-y}$Pr$_y$Ca$_x$MnO$_3$. In both measurements, we find a strong temperature dependence of the magnetic domains around the transition temperature. We will show images of magnetic domains, which provide clear evidence for phase separation on the submicron scale. We will also use these images, paired with the information on the charge-ordered/antiferromagnetic phase from the REXS data, to explain the temperature dependence of the CMR effect in this material. The research, SSRL, and the ALS are supported by U.S. Department of Energy, Office of Basic Energy Science.   

Competing Magnetic Interactions in La$_{0.7}$ Sr$_{0.3}$ Co$_{x}$Mn$_{1-x}$O$_{3}$(x=0.6, 0.7, 0.8, 0.9)

DC magnetization measurements and neutron diffraction (ND) techniques have been used to study the magnetic properties of La$_{0.7}$ Sr$_{0.3}$ Co$_{x}$Mn$_{1-x}$O$_{3}$(x=0.6, 0.7, 0.8, 0.9) compounds. The field cooled and zero field cooled dc magnetization data in 500 Oe applied field indicates that T$_{c}$ is suppressed with increasing Co content reaching 80K for the x = 0.9 composition. ND data at 300K and 16K confirms that increasing Co in the lattice results in the reduction of ferromagnetic exchange interaction thereby lowering T$_{c}$. This indicates that an antiferromagnetic (AFM) superexchange interaction between Mn and Co ions is responsible for the diminished ferromagnetism in the system. This fact also suggests that the double exchange interaction in manganites is weak and can be easily destroyed. The magnetization vs field data obtained at 5 K indicates linear field dependence as expected for an antiferromagnet and an interesting field-induced transition in low applied fields confirming the presence of mixed magnetic phases. Preliminary analysis of ND data supports the notion of competing FM and AFM interactions in these systems.   

Phase Transitions of a (La$_{0.5}$Pr$_{0.5})_{0.67}$Ca$_{0.33}$MnO$_{3}$ Thin Film Repeated with Increasing Voltage as Probed by Low Temperature Magnetic Force Microscopy

Low-temperature magnetic force microscopy was used to study the phase diagram of a (La$_{0.5}$Pr$_{0.5})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin film grown on a (110) NdGaO$_{3}$ (NGO) substrate by pulsed laser deposition. Traditionally, one can observe the phase change at the nanoscale level as the sample is cooled through the transition temperature, but in this case a fixed voltage was applied before each cooling cycle. From in-situ transport measurements, it is observed that the temperature of the peak of the transition increases with applied field; however, the MFM images show that the magnetic transition begins at a lower temperature with the same increase in field.~ The MFM data will be presented to show the complete phase change as it is cooled through each transition at a fixed voltage.   

Pressure induced metallization of ferromagnetic insulating Manganite

Hole doped Manganites show a ferromagnetic insulating state (FMI) at low hole doping. The FMI state occurs at low temperatures and is distinct from the high temperature paramagnetic polaronic insulating state (PMI). The FMI state is also not a Charge ordered insulating state and shows absence of any colossal magnetoresistance. We report complete 2 stage metallization of the FMI state in the system hole doped manganite La$_{0.79}$Ca$_{0.21}$MnO$_{3}$ under hydrostatic pressure to a FMM phase with a paramagnetic metallic state. Beyond a pressure of 6GPa even the high temperature polaronic insulating state collapses to a metallic state. In the process of pressure induced metallization, in a certain pressure range, the material shows a coherence temperature where a highly resistive incoherent metal crosses over to a less resistive band type metal. The pressure induced metallic phase so created under has no appreciable magnetoresistance unlike colossal magnetoresistance seen in systems at higher hole concentration.   

Colossal Electroconductance

An abrupt, colossal jump in conductance is observed at a critical electric field, E$_{c}$, in bilayer manganite, La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$, crystals exhibiting charge order below T$_{CO}$. The four-terminal conductance measured on an ab-plane facet jumps well over three orders-of-magnitude at 135 K for x=0.6 and a smaller amount for x=0.5. The very large conductance anisotropy isolates four-terminal measurements on opposite faces of our crystals, so the temperature rise due to the dissipation at E$_{c}$ can be quantitatively determined and ruled out as the cause. Detailed data for x=0.5 and t=1-T/T$_{CO}<$0.15 show that E$_{c}$ extrapolates linearly to zero at T$_{CO}$ ($\sim $221 K) with E$_{c}$/t$\sim $13500 V/m. Possible mechanisms are being explored.   

Anisotropic Transport in Manganite Films: Tuning Emergent Electronic Phase Separation

Emergent electronic phase separation has been linked to many exciting behaviors such as the metal-insulator transition, colossal magnetoresistance, and high Tc superconductivity. The intricate energy overlaps of spin-charge-lattice-orbital interactions in complex materials that lead to electronic phase separation have made the phenomena very difficult to study. By selectively tuning the elastic energy in manganite films, we have uncovered never before seen anisotropic transport properties that answer fundamental questions on the role of electronic phase separation in manganites. Using La[5/8-x]Pr[x]Ca[3/8]MnO[3] (x = 0.3) (LPCMO) as a model system, we have found that we can selectively induce anisotropic electronic domain formation along one axis of a pseudocubic perovskite single crystal thin film manganite by epitaxially locking it to an orthorhombic substrate. Simultaneous temperature-dependent resistivity measurements along the two perpendicular in-plane axes show significant differences in the metal-insulator transition temperatures and extraordinarily high anisotropic resistive behaviors on macroscales. These findings show that emergent electronic phase domain formation can be selectively tuned over long distances which gives us a fuller understanding of the balanced energetics that drive emergent behaviors in complex materials.   

Structurally-driven Mott Transition in Single-Crystal Ca$_{2}$Ru$_{1-x}$Cr$_{x}$O$_{4}$

We report results of a structural, magnetic and transport study of single crystal Ca$_{2}$Ru$_{1-x}$Cr$_{x}$O$_{4}$. Slight substitution of Cr for Ru drastically suppresses the structural distortion and reduces the Mott transition, but causes no parallel changes in the magnetic properties. These phenomena suggest an unusual relationship between Mott transition and magnetic ordering. The results will be presented and discussed along with comparison drawn with other related systems.   

Effects of spin-orbit interaction on the electronic structures of 5$d$ double perovskite $A_{2}$FeReO$_{6}$ ($A$=Ba and Ca)

Recently, the role of spin-orbit coupling (SOC) in 5$d$ transition metal oxides (TMOs) attracted a lot of attention. In 5$d$ TMOs, the energy scale of the SOC is larger than that of 4$d$ or 3$d$ TMOs and it can induce novel Mott insulating state through the cooperation with the electron correlation.[1]~We investigated the electronic structures of 5$d$ double perovskite $A_{2}$FeReO$_{6}$ ($A$=Ba and Ca) using optical and x-ray absorption spectroscopy. The experimental spectra showed clear changes from metallic to insulating states, when the Ba ions were substituted with Ca ions. The observations are consistent with the results of density functional theory calculations when both the on-site Coulomb interaction $U$ and the SOC are properly considered. Our study indicates that the subtle interplay of electron correlation, spin-orbit interaction, and lattice distortion can explain the electronic structures of $A_{2}$FeReO$_{6}$. [1] S.J. Moon \textit{et al}., Phys. Rev. Lett. \textbf{100}, 116404 (2008), B.J. Kim \textit{et al}., Phys. Rev. Lett. \textbf{101}, 076402 (2008)   

Magnetoelectric Phase Transition in a Triangular Lattice Antiferromagnet CuCrO$_{2}$

A focus of this study is a triangular lattice antiferromagnet CuCrO$_{2}$, a recently discovered magnetically-induced ferroelectric with a modulated 120-degree spiral spin structure [1-3]. We investigated magnetic and magnetoelectric properties of this material by using single crystal samples. Our magnetization and dielectric measurements clearly reveal that a magnetic field along a triangular lattice plane induces a meta-magnetic phase transition accompanied with drastic changes of electric polarization, i.e. a magnetoelectric phase transition [4]. Symmetry analysis based on these experimental results suggests that the magnetoelectric phase transition is characterized as a 90-degree polarization flop by a 90-degree spiral plane flop, which is further evidenced by our spin-polarized neutron diffraction measurements. [1] S. Seki et al., Phys. Rev. Lett. 101, 067204 (2008). [2] K. Kimura et al., Phys. Rev. B 78, 140401 (2008). [3] M. Poienar et al., Phys. Rev. B 79, 014412 (2009) [4] K. Kimura et al., Phys. Rev. Lett. 103, 107201 (2009).   

Fabrication and Characterization of PrBa$_{2}$[Cu$_{x}$M$_{1-x}$]$_{3}$O$_{7}$ (M=Ga, Al ,x=0.2) Epitaxial Thin Films

We have fabricated epitaxial thin films of highly resistive material PrBa$_{2}$(Cu$_{1-x}$M$_{x})_{3}$O$_{7}$ (M=Al, Ga, x = 0.2) by substituting Cu with Ga and Al in PrBa$_{2}$Cu$_{3}$O$_{7}$.The electrical resistivity in these materials are many orders higher than in PrBa$_{2}$Cu$_{3}$O$_{7}$ at 77K, which will provide an effective potential barrier to YBa$_{2}$Cu$_{3}$O$_{7}$ in high T$_{c}$ S-I-S Josephson junction. X-ray diffraction, atomic force microscopy, Raman and temperature dependent resistivity measurements were performed to characterize the thin films. We will discuss the results of Raman spectroscopy with regard to the site detection of incorporated dopants in PrBa$_{2}$(Cu$_{1-x}$M$_{x})_{3}$O$_{7}$ and transport studies with regard to the mechanism of hopping conductivity.   

AC specific heat and MCE measurements in 50T pulsed magnetic fields

Specific heat (Cp) and magnetocaloric effect (MCE) are useful tools in the experimental determination and understanding of the temperature-magnetic field phase diagram of materials. While previous studies are mostly limited to measurements in DC magnets, here we discuss a new AC calorimeter to measure these properties in 250 ms, 50T capacitor bank- and motor generator-driven pulsed magnetic fields. As a test sample we choose the spin S = 1/2 dimmer compound Sr3Cr2O8, which shows a field-induced phase transition between Hc1 = 30.4 T and Hc2 = 62 T. We compare our pulsed field data with the previous results measured in DC fields and analyze advantages and shortcomings of our new approach.   

Magnetization Dependent Hall Coefficient in EuB$_{6}$: a Signature of Electronic Phase Separation

The nonlinear Hall effect (HE) in EuB$_{6}$ has been shown to be a signature of magnetically-driven electronic phase separation [1]: A distinct switch in the slope of its Hall resistivity versus the \textit{perpendicular} applied magnetic field is observed in the paramagnetic phase, and it occurs at a \textit{single critical magnetization} over a wide temperature range. Here we report HE measurements on a EuB$_{6}$ crystal platelet with the sample plane oriented nearly \textit{parallel} to the applied magnetic field. The measurements in this configuration revealed a distinct change of the Hall coefficient as a function of magnetization induced by a parallel field, and the results are fully consistent with the perpendicular field HE measurements. They demonstrate unambiguously that the change in the Hall coefficient depends solely on the magnetization of the sample, regardless of the direction of the applied field. A two-component model based on carrier delocalization provides excellent scaling and quantitative description of our data. Work Supported in part by NSF DMR-0908625. [1] X. Zhang et al., Phys. Rev. Lett. \textbf{103}, 106602 (2009)