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Open AccessCCS ChemistryRESEARCH ARTICLE24 May 2022Pressure-Induced Intermetallic Charge Transfer and Semiconductor-Metal Transition in Two-Dimensional AgRuO3 Chuanhui Zhu, Jinjin Yang, Pengfei Shan, Mei-Huan Zhao, Shuang Zhao, Cuiying Pei, Bowen Zhang, Zheng Deng, Mark Croft, Yanpeng Qi, Lihong Yang, Yonggang Wang, Xiaojun Kuang, Long Jiang, Dao-Xin Yao, Jin-Guang Cheng and Man-Rong Li Chuanhui Zhu Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Jinjin Yang Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Pengfei Shan Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 , Mei-Huan Zhao Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Shuang Zhao Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Cuiying Pei School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210 , Bowen Zhang College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004 , Zheng Deng Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 , Mark Croft Department of Physics and Astronomy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854 , Yanpeng Qi School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210 ShanghaiTech Laboratory for Topological Physics, ShanghaiTech University, Shanghai 201210 Shanghai Key Laboratory of High-Resolution Electron Microscopy, ShanghaiTech University, Shanghai 201210 , Lihong Yang Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 , Yonggang Wang Center for High Pressure Science and Technology Advanced Research (HPSTAR), Beijing 100094 , Xiaojun Kuang College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004 , Long Jiang Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 , Dao-Xin Yao Guangdong Key Laboratory of Magnetoelectric Physics and Devices, School of Physics, Sun Yat-sen University, Guangzhou 510275 , Jin-Guang Cheng *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190 and Man-Rong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-sen University, Guangzhou 510275 https://doi.org/10.31635/ccschem.022.202201989 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The intricate correlation between charge degrees of freedom and physical properties is a fascinating area of research in solid state chemistry and condensed matter physics. Herein, we report on the pressure-induced successive charge transfer and accompanied resistive evolution in honeycomb layered ruthenate AgRuO3. Structural revisiting and spectroscopic analyses affirm the ilmenite type R-3 structure with mixed valence cations as Ag+1/+2Ru+4/+5O3 at ambient pressure. In-situ pressure- and temperature-dependent resistance variation reveals a successive insulator-metal-insulator transition upon pressing, accompanied by unprecedented charge transfer between Ag and Ru under applied pressure, and a further structural phase transition in the insulator region at higher pressure. These phenomena are also corroborated by in-situ pressure-dependent Raman spectra, synchrotron X-ray diffraction, bond valence sums, and electronic structure calculations, emphasizing the dominated rare Ag2+, and near zero thermal expansion in the ab-plane in the metallic zone mostly due to the Jahn-Teller effect of d9-Ag2+. The multiple electronic instabilities in AgRuO3 may offer new possibilities toward novel and unconventionally physical and chemical behaviors in strongly correlated honeycomb lattices. Download figure Download PowerPoint Introduction Pressure-induced crystal and electronic structural transition of solids have proven a treasure trove for solid-state chemistry, condensed matter physics, and material science.1–3 In particular, for transition metal oxides with strong interplays between lattice, charge, spin, and orbital, pressure has demonstrated significant influence on the exotic phenomena in correlated systems, including magnetoelectric multiferroics, colossal magnetoresistance, and high temperature superconductivity.4–6 The perovskite BiNiO3 and ilmenite FeTiO3 are two typical compounds that exhibit pressure-induced intermetallic charge transfer.7,8 The electronic configurations between two metallic ions can be altered simultaneously via intermetallic charge transfer, giving rise to drastic changes in structure and physical properties.9–11 For instance, RCu3Fe4O12 (R = rare earth or Bi) displays an intermetallic charge transfer accompanied with paramagnetism-to-antiferromagnetism transitions, negative thermal expansion (NTE), and above room-temperature metal-to-insulator transition (MIT) as well.12–14 Mixed valence arising from intermetallic charge transfer has also been observed in 4d/5d oxides, such as in pressure-induced hexagonal perovskite Ba3BiM2O9 (M = Ru, Ir) and delafossite Cu2IrO3, reflecting the spatial dispersion of d orbitals.15,16 As one of the most appealing quantum materials, low-dimensional ruthenates have received growing attention due to their unique physical properties and quantum critical phenomena as reported in quasi-two-dimensional (2D) Ruddlesden-Popper phase, PbSb2O6-type analog, delafossite derivative, and halogenide.17,18 Particularly, honeycomb-layered ruthenates, which resemble the so-called Kitaev model with strong geometric frustration, are extremely interesting.19 The PbSb2O6-type SrRu2O6 shows high antiferromagnetic (AFM) ordering up to 565 K originated from the strong exchange interactions and large magnon gap.20,21 In delafossite derivatives, dimerized Li2RuO3 is a valence bond liquid with spin-singlet ground state and Cu3LiRu2O6 demonstrates pressure-induced nonmetallic-metal to bad-metal transition.22–24 α-RuCl3 is a hotspot in 2D ruthenates, because it demonstrates very strong exchange anisotropy and is a promising candidate for Kitaev quantum spin liquid, which is usually expected in a frustrated 2D system with S = 1/2 ions, such as d5-Ru3+ and d9-Ni+, Cu2+, and Ag2+.25–29 Recently, AgRuO3, which adopts an exotic perovskite-related ilmenite structure (R-3c), was reported to show an AFM transition around 342 K with semiconducting behavior.30,31 The honeycomb-layered RuO6 sheets are effectively separated by the larger size Ag ions, which, when compared with other conventional ilmenites, increases the interlayer distance, and renders strengthened 2D features in AgRuO3. However, the structure and physical properties of AgRuO3 under high-pressure (HP) remain unknown and worthy of further exploration. In this work, we revisited the crystal structure of AgRuO3 over a wide temperature range, and plotted the temperature- and pressure-dependent resistance phase diagram to understand this emerging compound in more depth. By combining various diffraction and spectroscopic techniques, we show that the ambient pressure (AP) AgRuO3 belongs to the R-3 space group with the charge formula as Ag+1/+2Ru+4/+5O3, instead of the reported R-3c. Upon pressurization, a successive insulator-metal-insulator and structural transition is ignited in the honeycomb lattice. Comprehensive experimental and theoretical structure investigations reveal a simultaneous charge transfer between Ag and Ru under applied pressure, accompanied with insulator-metal transition (IMT) at the Ag2+-dominated area. Further structural transitions to P2/m at higher pressure are also predicted. Experimental Methods Synthesis Samples of AgRuO3 were prepared via hydrothermal reaction in a Teflon-lined stainless steel autoclave following a literature report.31 A mixture of Ag2O (99.9%, Aladdin) and excess KRuO4 (98%, Strem) at a molar ratio of 1∶2.1 was added into distilled water (8 mL) and sonicated for 20 min. Subsequently, the mixture was sealed in a 25 mL Teflon-lined steel autoclave and heated at 423 K (heating rate of 5 K/min) for 24–72 h, and then cooled (cooling rate between 10 K/day and natural cooling) to room temperature. Optimal conditions were found by heating 72 h before natural cooling. The obtained samples were washed with deionized water and then ethanol before being dried at 323 K for 30 min. Finally, black crystals were obtained. Chemical and crystal structure characterizations The sample was initially characterized by single-crystal X-ray diffraction (SCXRD, SuperNova, MoKα radiation, λ = 0.71073 Å) at 150 K and 293 K. Further in-situ variable-temperature SCXRD data were collected from 100 K to 293 K (Bruker D8 Adventure Photon III (Guangzhou, Guangdong, China), GaInKα radiation, λ = 1.34138 Å). Powder XRD (PXRD) patterns were also recorded using a Rigaku (DMAX 2200 VPC) (Guangzhou, Guangdong, China) instrument equipped with CuKα radiation (λ = 1.5418 Å) for phase determination. In-situ variable-temperature PXRD (VT-PXRD) data were collected from 4 to 293 K (SmartLab, CuKα, λ = 1.5418 Å). Room-temperature synchrotron powder XRD (SPXRD) data were collected at ambient and high pressures on beamline BL14B (λ = 0.68840 Å) and BL15U1 (λ = 0.6199 Å), respectively, at the Shanghai Synchrotron Radiation Facility (SSRF). A symmetric diamond anvil cell (DAC) with culet sizes of 250 μm and a Re gasket was used. Silicon oil was used as pressure transmitting media (PTM), and the pressure was determined by the ruby luminescence method. 2D diffraction images were analyzed using FIT2D software. Refinements of the SPXRD and single-crystal diffraction data were performed using programs of Topas Academic V6 and Olex2 (1.2), respectively.32,33 Thermogravimetric analysis (TGA) was carried out using an NETZSCH TG 209F1 Libra analyzer (Guangzhou, Guangdong, China). Two copies of crystals with a mass of 4.89 mg were placed into two alumina crucibles and heated at a rate of 10 K min−1 to 1173 K in Ar atmosphere. Physical properties and spectroscopic measurements The magnetic properties were measured on a Magnetic Property Measurement System (MPMS3) over a temperature range of 3 and 380 K. The susceptibility was evaluated in both zero-field-cooled (ZFC) and field-cooled (FC) modes under applied fields of 0.1, 1, and 7 T. The specific heat was measured with a two-tau relaxation method in a Physical Property Measurement System (PPMS-9T). The temperature-dependent resistance of AgRuO3 under different pressures was measured using a DAC made of BeCu alloy ranging from 2.6 to 36.3 GPa. The diameter of the diamond culet was 300 μm. The composite gasket consists of rhenium and insulating c-BN, where KBr used as a PTM was loaded into the sample chamber (∼100 μm), in which a AgRuO3 sample with dimensions of ∼90 μm × 30 μm × 20 μm and a ruby chip were placed. Four pieces of thin platinum were utilized as electrical contacts. The pressure was calibrated at room temperature using the ruby fluorescent method. Temperature dependent resistance measurements in DAC were performed in a liquid-helium cryostat. The X-ray absorption near edge spectroscopy (XANES) data were collected in the total electron yield mode at the Brookhaven National Synchrotron Light Source (NSLS-II) on beamline 7-ID SSA-2 using a Si (111) double crystal monochromators. In situ variable pressure Raman experiments were performed on crystals using a Linkam THMS350V with an excitation wavelength of 532 nm. High pressure was generated using a DAC ranging from 1.07 to 20.20 GPa. Theoretical calculations We have explored the crystal structures of AgRuO3 by employing a particle swarm optimization algorithm as implemented in the CALYPSO code in conjunction with first-principles density functional theory (DFT) total-energy calculations.34–36 Based on the projector augmented-wave method, DFT was implemented in the Vienna Ab Initio Simulation Package.37–39 The plane wave cut-off energy and k-point were 700 eV and 9 × 9 × 9 for geometry optimization and static electronic calculations, respectively. The convergence factor was set as the difference in total energy within 1.0 × 10−7 eV per atom. All atomic positions were fully relaxed until the remaining force on each atom was less than 1.0 × 10−2 eV·Å−1. The exchange correlation energy was treated by the generalized gradient approximation Perdew–Burke–Ernzerhof (GGA-PBE).40 Electron-electron Coulomb repulsion interactions (U) for Ru 4d orbitals were considered in the rotationally invariant form (GGA+U) with Ueff = 2.0 eV as used to predict semiconducting behavior of AgRuO3.31,41 Results and Discussion Revisiting the crystal and electronic structures and magnetism at AP Thermal stability of the as-made AgRuO3 was examined by TGA analysis ( Supporting Information Figure S1), which shows that AgRuO3 is stable up to around 400 °C in Ar before decomposing into a mixture of Ag and RuO2 ( Supporting Information Figure S1b). The observed weight loss of 6.31% is in reasonable agreement with the expected value of 6.23% due to oxygen release. The crystal structure of AgRuO3 at room temperature was determined by SCXRD, which yielded a rhombohedral R-3 rather than the previously reported R-3c ( Supporting Information Table S1) at the experimental conditions.31 At 100 K, the unit-cell volume (381.70(3) Å3) is almost identical to that at room temperature (381.75(4) Å3) as shown in Supporting Information Table S1, owing to the slight expansion (0.005 Å) in the ab-plane and contraction (0.031 Å) along the c-axis upon cooling. The unit-cell parameters extracted from in situ variable temperature SCXRD reveal that AgRuO3 undergoes a steep NTE in the ab-plane and a soft positive thermal expansion along the c-axis, rendering a slight overall cell contraction between 100 K and 293 K. This phenomenon can also be observed in Supporting Information Figure S2, where no additional structural transition appears down to 4 K. The anomalous thermal expansion could be responsible for the weak phase transition or the crossover between 125 and 200 K reported in literature.30 The R-3 AgRuO3 adopts the typical ilmenite structure (Figures 1a and 1b, Supporting Information Figure S3, and Tables S1 and S2) with alternative RuO6 and AgO6 honeycomb layers in the ab-plane, where the face-shared AgO6 and RuO6 octahedral pair along the c-axis are obstructed by octahedral vacancies. The refined cell dimension (Figure 1b, R-3, a = 5.2251(6) Å) in the ab-plane is similar to that in the literature (Figure 1c, R-3c, a = 5.2261(6) Å), while the unit cell along the c-axis (c = 16.0580(1) Å) is about half of the reported value (c = 32.358(5) Å). The obtained contractible structure is reproducible on single crystals from different batches, which were measured on different diffractometers with either Ga-In or Mo X-ray targets ( Supporting Information Tables S1 and S2), so the possibility of defect-induced uncertainty of structural determination can be ruled out by both SCXRD and PXRD measurements ( Supporting Information Figure S4). High resolution SPXRD data fittings (Figure 1d, R-3, a = 5.2412(2), c = 16.0700(6), V = 382.31(3), Rp/Rwp = 6.37/9.18%), which, to a great extent, are more reliable than the single-crystal diffraction way, further corroborating the validity of our R-3 structure. First-principles calculations are expected to distinguish between the more energetically stable structure between R-3 and R-3c. According to the ground state of AgRuO3 with AFM1 and semiconducting behavior, we further calculated the corresponding energy of different ground states ( Supporting Information Table S3). Electron-electron Coulomb repulsion interactions (U) for Ru 4d orbitals were considered in the rotationally invariant form (GGA+U) with Ueff = 2.0 eV as used to predict the semiconducting behavior of AgRuO3.31 Based on the AFM1 ground state, R-3 demonstrated lower energy than that of R-3c (35 meV), indicating that the R-3 of AgRuO3 seems more stable. Combined with the experimental results, we can further conclude that AgRuO3 belongs to the R-3 space group at AP. Figure 1 | Crystal structure determination of AgRuO3. (a) The edge-sharing RuO6-honeycomb layer motif in the ab-plane projected along the [001] direction. (b) The polyhedral unit cell structure in R-3 symmetry in this work. (c) The reported supercell polyhedral unit cell structure in R-3c for comparison. (d) Le Bail fit of the SPXRD data in R-3 symmetry in this work. Download figure Download PowerPoint Monovalent Ag+ with a simple closed shell (d10) is the dominant oxidation state of silver. Compounds containing higher oxidation states of Ag are interesting but rare. Ag2+ (d9) in the fluoride was found to be stabilized by the most electronegative F element. In contrast, cuprate-resembling Ag2+ oxides are very rare beyond binary systems. AgBO3 (B = Nb, Ta, Sb, Bi), AgNbO3 (Cmcm, Pbcm, and Pm-3m), and AgTaO3 (R3cH and R-3mH) adopt perovskite-related or LiNbO3-type structures and span a range of different coordination environments from 6 to 12, while AgSbO3 (R-3H) and AgBiO3 (R-3H) take analogous structures with AgRuO3.42–45 There are three shorter and three longer Ag–O bond lengths within the AgO6 units in AgRuO3 (Figure 2a). A comparison of the Ag–O bond length in R-3 AgBO3 is shown in Figure 2b, where the Ag–O distance in AgRuO3 is obviously shorter than those in AgBiO3 and AgSbO3. Generally, shorter Ag–O bond lengths mean higher oxidation states of Ag considering its ionic radius (1.15, 0.79, and 0.75 Å for Ag+, Ag2+, and Ag3+, respectively, in octahedral coordination) at different valence states.46 The bond valence sum (BVS) calculations of Ag give somewhat larger values in AgRuO3 than those in AgSbO3 and AgBiO3 (Figure 2b). Considering the shorter distance between Ag-Ru (3.174(1) Å) within the face-shared octahedral pairs, the higher oxidation state Ag ion could be ascribed to non-negligible covalent bonding nature as the case in fluorides, or charge disproportion between Ag and Ru, which, together with the long distance (5.406 Å) between the honeycomb RuO6 layers, endows remarkable 2D features of AgRuO3. Figure 2 | Local environment and electronic structure. (a) The distorted AgO6 octahedron in AgRuO3 at AP. (b) Comparison of the Ag–O bond length and corresponding BVS of Ag in AgBO3 (B = Ru, Bi, Sb) at AP and room temperature; XANES of AgRuO3 for (c) Ag-L3 edge and (d) Ru-L3 edge. Download figure Download PowerPoint XANES data were further analyzed to confirm the oxidation state of cations. As displayed in Figure 2c, Ag L3-edge XANES clearly reflects an emerged peak located at higher energy in AgRuO3 compared with Ag+ in Ag2O, which is also distinct with Ag+ and Ag3+ in AgO (Ag2O2). Since the oxidation state of Ru in AgRuO3 is between and to and (Figure the formula of the compound can be as The case could also be found in the of such as the ions in The electronic structure and valence state are also by the magnetic which are strongly to the electronic and exchange Ag2O, and AgO are which is expected for Ag+ (d10) and Ag3+ the could interesting magnetic such as a of Ag2+ ions with strong AFM in and a 2D AFM of Ag2+ ions in = with the in the of crystal and electronic structure to the magnetism of the as-made The temperature-dependent magnetic susceptibility of AgRuO3 is identical to that reported in as shown in Supporting Information Figure A AFM transition is observed around K in the at T. The of and appears upon at the AFM transition The magnetic susceptibility value is less magnetic above 1 and upon to K, which is then by in the At 7 the and The of magnetic Ag2+ ions and interactions between Ag2+ and could be responsible for the around K and transition around K in the in Supporting Information Figure magnetic transitions in the are strongly dependent on the applied and at higher magnetic magnetic phase The dependent ( Supporting Information Figure were measured on a at 3 using the The data can be by = in of Supporting Information in = = and = which are very to those reported by the electronic structure of Ag and Ru is mixed and respectively, in AgRuO3. and pressure-dependent resistance Pressure is a unique variable to the phase transitions and novel physical phenomena at ambient The resistance of AgRuO3 was measured by a method in a DAC between 3 and 300 K up to ( Supporting Information Figure As shown in Figure the temperature-dependent at 2.6 a semiconducting behavior, similar to that at AP in of as the pressure from 2.6 to and then to GPa. the at and a semiconducting behavior, it almost and to the appears from to above K, where initially upon and a between and 30 K, by an to semiconducting behavior at lower (Figure transitions were at but lower values and less temperature At is and an crossover of The in the temperature region upon at pressures ranging between and 36.3 where the with are observed K (Figure The overall a at the crossover with the K, where the values of is At 36.3 increases more upon 250 K. The evolution can be more by the heat in Supporting Information Figure Figure 3 | Pressure dependent resistance variation between 3 and 300 K. Pressure dependent variation of AgRuO3 up to 36.3 GPa. (d) and temperature-dependent heat of evolution of AgRuO3. Download figure Download PowerPoint The overall pressure- and temperature-dependent phase diagram of is shown in Figure AgRuO3 undergoes a and transition when up to and respectively. is that the metallic state is at high where the pressure-induced in AgRuO3 can be to from pressure-induced has also been in quantum materials, structural can the electron and to in an insulating The cell dimension at pressure, so it is to the factor and for the of the the at lower further The pressure-dependent are displayed in Supporting Information Figure At high with pressure, while at to a around and then to rise until by less pressure-dependent at higher pressure. the pressure-dependent evolution and structural in AgRuO3, in-situ pressure-dependent SPXRD and Raman were at room temperature with pressure Pressure-induced intermetallic charge transfer the of the successive in AgRuO3, in situ pressure-dependent SPXRD patterns were collected at room temperature under various pressures to the structural (Figure The SPXRD at can be by the rhombohedral R-3 structure of the due to as displayed in Figure peak is observed in the pressure which is distinct to the at lower to in the indicating the of pressure-induced structural phase structural changes are with the successive in the AgRuO3. and 4d the pressure of parameters from Le Bail fit of the SPXRD in AgRuO3. As the c-axis with pressure, indicating a of the In contrast, the ab-plane with changes and an negative is observed in the pressure region where metallic behavior is indicating that the structural has a on the electrical Figure 4 | Pressure-induced structural (a) In-situ variable pressure SPXRD patterns collected at various pressures for AgRuO3 upon pressing, the the peak evolution upon (b) SPXRD patterns at pressure to the peak and the at the new (c and Pressure of the Download figure Download PowerPoint to and from the DAC the be refined from the SPXRD Combined with the CALYPSO reasonable crystal for the pressure range of the Figure displays the calculated pressure-dependent structural evolution of the face-shared octahedral pair in the R-3 where the Ag-Ru distance with along the c-axis, from Å at AP to Å at which is of strong between Ru and The shorter bond length endows between the the octahedral a The Ag atom and in the octahedral while Ru from the octahedral drastic of the AgO6 The of Ag charge transfer between Ag and Ru the of the this the charge in AgRuO3 under pressure was using BVS determined from calculated structural ( Supporting Information Tables and atomic

In this study we report the vortex-glass behavior of superconducting ternary germanide SrPd2Ge2 single crystals with a ThCr2Si2-type structure. We observed flux trapping and its nonexponential decay with time after the magnetic field was turned off at T = 2 K. In addition, we found that the diamagnetism in the zero field cooling (ZFC) mode below Tc was irreversible, depending on the temperature and field history, whereas the diamagnetism in the field-cooled warming (FCW) mode was reversible if the applied magnetic field was parallel to the c-axis. An irreversibility line Tr(H) was determined by the ZFC and FCW measurements at various magnetic fields, and the temperature dependence of Tr(H) was found to agree with the de Almeida–Thouless relation, H = H0[1−Tr(H)/Tc(0)]γ, where γ = 3/2. Including these vortex-glass behaviors, we discuss the critical current density, Jc(T), determined from isothermal magnetization measurements at various temperatures, and the pinning potential, determined from the slope of an Arrhenius plot, lnR(T,B) versus 1/T.

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The conductivity and magnetoresistance of La0.5Pb0.5Mn1−xCrxO3 (0.0⩽x⩽0.45) measured at 0.0 and 1.5 T magnetic field have been reported. All the oxide samples except x=0.45, showed metal insulator transition (MIT) between 158–276 K, depending on x. In contrast to the behavior of a similar sample La0.7Ca0.3Mn1−xCrxO3 showing no (MIT) for x⩾0.3, the Pb doped samples showed MIT even with x=0.35. The MIT peak temperature (Tp) shifts towards lower temperature with increasing x while magnetic field shifts Tp to the high temperature regime. The metallic (ferromagnetic) part of the temperature dependent resistivity (ρ) curve (below Tp) is well fitted with ρ(T)=ρ0+ρ2.5T2.5 indicating the importance of electron–magnon interaction (second term). We have successfully fitted the high temperature (T>θD/2, θD is Debye temperature) conductivity data, both in presence and in absence of magnetic field, with small polaron hopping conduction mechanism. Adiabatic small polaron hopping conduction mechanism is followed by the samples showing MIT while nonadiabatic hopping conduction mechanism is obeyed by the samples showing no MIT. The lower temperature (between Tp and θD/2) conductivity data of all the samples can be well fitted to the variable range hopping (VRH) model similar to the case of many semiconducting transition metal oxides. Temperature dependent Seebeck coefficient data also support the small polaron hopping conduction mechanism above Tp.

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Magnetic properties of inhomogeneous nanoisland FeNi films were studied by SQUID magnetometry. The FeNi films with nominal thickness ranging from 0.6 to 2.0 nm were deposited by rf sputtering on Sitall glass substrates and covered by a protecting Al2O3 layer on the top. The SQUID data indicate pronounced irreversibility behavior for the out-of-plane temperature-dependent magnetization response (measured at H≃100 Oe) using zero-field cooling (ZFC) and field-cooled warming (FCW) after the applied dc magnetizing field Hm≃2 T for the FeNi samples with nominal thickness 1.1 nm ≲d≲1.8 nm, below the percolation threshold. The positive difference between the FCW and ZFC data identifies two irreversibility temperature scales, TB≈50 K and T⁎≈200 K, which can be associated with the superparamagnetic and superferromagnetic behavior in inhomogeneous nanoisland FeNi films, respectively. However, above the film percolation threshold, we observed a crossover from the out-of-plane to in-plane magnetization orientation. Here, the in-plane FCW-ZFC difference implies negative remanent magnetization response in the temperature range TB≲T≲T⁎. The observed magnetization properties can be associated with the presence of the superferromagnetic phase in self-assembled clusters of quasi-2D metallic magnetic FeNi nanoislands.

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At low temperatures, the c-axis transport properties of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub 8/ single crystals depend strongly on the magnetic field history for fields applied along the c-axis, indicating the effect of a critical state in the ab direction. In this study, we report zero field cooled (ZFC), field cooled warming (FCW), and field cooled cooling (FCC) c-axis transport properties. In addition, we also report magnetic relaxation measurements on the same samples to identify the c-axis dissipation mechanism. We observe a relaxation of the c-axis resistivity that follows the relaxation of the flux profile across the ab plane. By identifying the irreversible temperature vs magnetic field for H/spl par/c, we construct a phase diagram of vortex states. Finally, we will discuss the implication of the phase diagram and the features related to the transport properties of the Josephson-coupled system and the role of pancake vortices in highly two-dimensional superconductors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Magnetization vs. temperature measurements were carried out on superconducting YBa2Cu3Ox (YBCO) single crystals. Samples with oxygen concentrations corresponding to x = 6.90, 6.85, 6.78 and 6.72 were studied. Zero field cooling (ZFC), field cooled cooling (FCC) and field cooled warming (FCW) prescriptions were employed. Measurements were performed in the temperature interval between 20 K and 120 K in constant magnetic fields ranged from 0.05 T to 5 T. A negative slope was observed in the FCC and FCW curves below a field-dependent minimum located in temperatures less than the irreversibility point (Tirr). This negative slope is identified as the high-field paramagnetic effect (HFPE). With a fixed magnetic field, the magnitude of the observed HFPE increases as the temperature decreases. The field dependence of the HFPE at fixed temperatures is not trivial. The overall magnitude of the measured HFPE depends on the oxygen content and becomes smaller when x decreases. The results are discussed on the basis of the effects of flux compression and spin moment polarization inside the vortex cores.

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