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Register a new userSevere Fermi Surface Reconstruction at a Metamagnetic-Transition in Ca$_{2-x}$Sr$_x$RuO$_4$ (for $0.2 leq x leq 0.5$)
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Luis Molinuevo Balicas Dr
Publication date
2005
fields
Physics
and research's language is
English
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We report an electrical transport study in Ca$_{2-x}$Sr$_{x}$RuO$_4$ single crystals at high magnetic fields ($B$). For $x =0.2$, the Hall constant $R_{xy}$ decreases sharply at an anisotropic metamagnetic (MM) transition reaching its value for Sr$_2$RuO$_4$ at high fields. A sharp decrease in the $A$ coefficient of the resistivity $T^2$-term and a change in the structure of the angular magnetoresistance oscillations (AMRO) for $B$ rotating in the planes, confirms the reconstruction of the Fermi surface (FS). Our observations and LDA calculations indicate a strong dependence of the FS on the Ca concentration and suggest the coexistence of itinerant and localized electronic states in single layered ruthenates.
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The magnetoelastic coupling in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ and in Ca$_{1.5}$Sr$_{0.5}$RuO$_4$ has been studied combining high-resolution dilatometer and diffraction techniques. Both compounds exhibit strong anomalies in the thermal-expansion coefficient at zero and at high magnetic field as well as an exceptionally large magnetostriction. All these structural effects, which are strongest in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$, point to a redistribution of electrons between the different $t_{2g}$ orbitals tuned by temperature and magnetic field. The temperature and the field dependence of the thermal-expansion anomalies in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ yield evidence for a critical end-point lying close to the low-temperature metamagnetic transition; however, the expected scaling relations are not well fulfilled.
We report the effects of electron doping on the crystal structure and electrical resistivity of LaOBiS$_{2-x}$F$_x$ (0.05 $leq$ $x$ $leq$ 0.2). The $ab$ plane is found to be relatively insensitive to the amount of F, whereas the $c$ axis shrinks continuously with increasing $x$, suggesting that the doped F atoms substitute selectively into the apical sites in the BiS$_2$ layer. At $x$ = 0.10, as the temperature is decreased from room temperature, the electrical resistivity is temperature-independent from room temperature to 285 K, increases linearly with decreasing temperature from 285 K to 205 K and then shows obvious insulating behavior below 205 K, which may be due to strong spin-orbit coupling. LaOBiS$_{1.9}$F$_{0.1}$ shows the significantly weak and temperature-independent diamagnetism without any evident anomalies caused by a phase transition.
The alloy Ca$_{2-x}$Sr$_x$RuO$_4$ exhibits a complex phase diagram with peculiar magnetic metallic phases. In this paper some aspects of this alloy are discussed based on a mean field theory for an effective Kugel-Khomskii model of localized orbital and spin degrees of freedom. This model results from an orbital selective Mott transition which in the three-band system localized two orbitals while leaving the third one itinerant. Special attention is given to the region around a structure quantum phase transition at $ x approx 0.5 $ where the crystal lattice changes from tetragonal to orthorhombic symmetry while leaving the system metallic. This transition yields, a change from ferromagnetic to antiferromagnetic spin correlations. The complete mean field phase diagram for this transition is given including orbital and spin order. The anisotropy of spin susceptibility, a consequence of spin-orbit coupling and orbital correlation, is a tell-tale sign of one of these phases. In the predominantly antiferromagnetic phase we describe a metamagnetic transition in a magnetic field and show that coupling of the itinerant band to the localized degrees of freedom yields an anomalous longitudinal magnetoresistance transition. Both phenomena are connected with the evolution of the ferromagnetic and antiferromagnetic domains in the external magnetic field and agree qualitatively with the experimental findings.
We report the temperature and magnetic field dependence of transport properties in epitaxial films of the manganite La$_{1-x}$Ca$_{x}$MnO$_{3}$ in the overdoped region of the phase diagram for $x > 0.5$, where a charge--ordered (CO) and an antiferromagnetic (AF) phase are present. Resistivity, magnetoresistance and angular dependence of magnetoresistance were measured in the temperature interval $4.2 ~mathrm{K} < T < 300 ~mathrm{K}$, for three concentrations $x = 0.52, 0.58$ and $0.75$ and in magnetic fields up to 5 T. The semiconductor/insulator--like behavior in zero field was observed in the entire temperature range for all three concentrations textit{x} and the electric conduction, at lower temperatures, in the CO state obeys 3D Motts variable--range hopping model. A huge negative magnetoresistance for $x = 0.52$ and $x = 0.58$, a metal--insulator transition for $B > 3 ~mathrm{T}$ for $x = 0.52$ and the presence of anisotropy in magnetoresistance for $x = 0.52$ and $x = 0.58$ show the fingerprints of colossal magnetoresistance (CMR) behavior implying the existence of ferromagnetic (FM) clusters. The declining influence of the FM clusters in the CO/AF part of the phase diagram with increasing $x$ contributes to a possible explanation that a phase coexistence is the origin of the CMR phenomenon.
We present a comprehensive angle-resolved photoemission spectroscopy study of Ca$_{1.8}$Sr$_{0.2}$RuO$_4$. Four distinct bands are revealed and along the Ru-O bond direction their orbital characters are identified through a light polarization analysis and comparison to dynamical mean-field theory calculations. Bands assigned to $d_{xz}, d_{yz}$ orbitals display Fermi liquid behavior with fourfold quasiparticle mass renormalization. Extremely heavy fermions - associated with a predominantly $d_{xy}$ band character - are shown to display non-Fermi-liquid behavior. We thus demonstrate that Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ is a hybrid metal with an orbitally selective Fermi liquid quasiparticle breakdown.
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