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R J Cava

Publications and source records attributed to R J Cava.

At least 19 recordsLinked to original sources
PubMed2005

Orbital dependence of the fermi liquid state in Sr2RuO4.

We have used angle-resolved photoemission spectroscopy to determine the bulk electronic structure of Sr(2)RuO(4) above and below the Fermi liquid crossover near 25 K. Our measurements indicate that the properties of the system are highly orbital dependent. The quasi-2D gamma band displays Fermi liquid behavior while the remaining low energy bands show exotic properties consistent with quasi-1D behavior. In the Fermi liquid state below 25 K, the gamma band dominates the electronic properties, while at higher temperatures the quasi-1D beta and alpha bands become more important.

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PubMed2004

Competing magnetic phases on a kagomé staircase.

We present thermodynamic and neutron data on Ni3V2O8, a spin-1 system on a kagomé staircase. The extreme degeneracy of the kagomé antiferromagnet is lifted to produce two incommensurate phases at finite T--one amplitude modulated, the other helical--plus a commensurate canted antiferromagnet for T-->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni3V2O8 thus provides an elegant example of order from subleading interactions in a highly frustrated system.

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PubMed2004

Anomalous hall heat current and nernst effect in the CuCr2Se4-xBrx ferromagnet.

In a ferromagnet, an anomalous Hall heat current, given by the off-diagonal Peltier term alpha(xy), accompanies the anomalous Hall current. By combining Nernst, thermopower, and Hall experiments, we have measured how alpha(xy) varies with hole density and lifetime tau in CuCr2Se4-xBrx. At low temperatures T, we find that alpha(xy) is independent of tau, consistent with anomalous-velocity theories. Its magnitude is fixed by a microscopic geometric area A approximately 34 A(2). Our results are incompatible with some models of the Nernst effect in ferromagnets.

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PubMed2004

Schizophrenic electrons in ruthenium-based oxides.

Ruthenium and oxygen form many ternary compounds in which the Ru 4d states and O 2p states are strongly hybridized. In some such compounds, this hybridization leads to the loss of local magnetic moment character for the Ru, and in others it does not. The factors that determine which is the case are often subtle and poorly understood. The richness of structure types seen in ternary Ru oxides leads to a corresponding richness in electronic and magnetic properties. This article briefly describes the author's current views on these materials.

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PubMed2004

Fermi surface and quasiparticle dynamics of Na0.7CoO2 investigated by angle-resolved photoemission spectroscopy.

We present the first angle-resolved photoemission study of Na0.7CoO2, the host material of the superconducting NaxCoO2.nH(2)O series. Our results show a hole-type Fermi surface, a strongly renormalized quasiparticle band, a small Fermi velocity, and a large Hubbard U. The quasiparticle band crosses the Fermi level from M toward Gamma suggesting a negative sign of effective single-particle hopping t(eff) (about 10 meV) which is on the order of magnetic exchange coupling J in this system. Quasiparticles are well defined only in the T-linear resistivity (non-Fermi-liquid) regime. Unusually small single-particle hopping and unconventional quasiparticle dynamics may have implications for understanding the phase of matter realized in this new class of a strongly interacting quantum system.

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PubMed2004

Charge ordering, commensurability, and metallicity in the phase diagram of the layered NaxCoO2.

The phase diagram of nonhydrated NaxCoO2 has been determined by changing the Na content x using a series of chemical reactions. As x increases from 0.3, the ground state goes from a paramagnetic metal to a charge-ordered insulator (at x=1/2), then to a "Curie-Weiss metal" (around 0.70), and finally to a weak-moment magnetically ordered state (x>0.75). The unusual properties of the state at 1/2 (including particle-hole symmetry at low T and enhanced thermal conductivity) are described. The strong coupling between the Na ions and the holes is emphasized.

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PubMed2004

Dissipationless anomalous Hall current in the ferromagnetic spinel CuCr2Se4-xBrx.

In a ferromagnet, an applied electric field E invariably produces an anomalous Hall current JH that flows perpendicular to the plane defined by E and M (the magnetization). For decades, the question of whether JH is dissipationless (independent of the scattering rate) has been debated without experimental resolution. In the ferromagnetic spinel CuCr2Se4-xBrx, the resistivity rho (at low temperature) may be increased by several decades by varying x (Br) without degrading M. We show that JH/E (normalized per carrier, at 5 kelvin) remains unchanged throughout. In addition to confirming the dissipationless nature of JH, our finding has implications for the generation and study of spin-Hall currents in bulk samples.

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PubMed2003

Potential signature of a Kosterlitz-Thouless transition in BaNi2V2O8.

ESR measurements are reported for the quasi-two-dimensional honeycomb antiferromagnet BaNi2V2O8. Planar anisotropic properties are confirmed by angular dependent investigations of resonance field and linewidth. The divergence of the temperature-dependent linewidth on approaching T(N) from above is described in terms of the Kosterlitz-Thouless transition, the critical behavior close to long-range magnetic order, and the 2D Heisenberg antiferromagnet. We provide arguments that the Kosterlitz-Thouless scenario is compatible with the observed critical exponent and suggest BaNi2V2O8 is a weakly anisotropic 2D Heisenberg antiferromagnet.

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PubMed2003

Quantum-classical reentrant relaxation crossover in Dy2Ti2O7 spin ice.

We have studied spin relaxation in the spin ice compound Dy2Ti2O7 through measurements of the ac magnetic susceptibility. While the characteristic spin-relaxation time (tau) is thermally activated at high temperatures, it becomes almost temperature independent below T(cross) approximately 13 K. This behavior, combined with nonmonotonic magnetic field dependence of tau, indicates that quantum tunneling dominates the relaxational process below that temperature. As the low-entropy spin ice state develops below T(ice) approximately 4 K, tau increases sharply with decreasing temperature, suggesting the emergence of a collective degree of freedom for which thermal relaxation processes again become important as the spins become strongly correlated.

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PubMed2003

Superconductivity phase diagram of Na(x)CoO2*1.3H2O.

The microscopic origin of superconductivity in the high-transition-temperature (high-T(c)) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting T(c) on the degree of electronic band filling. The recent report of superconductivity near 4 K in the layered sodium cobalt oxyhydrate, Na(0.35)CoO2*1.3H2O, is of interest owing to both its triangular cobalt-oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting T(c) of this compound displays the same kind of behaviour on chemical doping that is observed in the high-T(c) copper oxides. Specifically, the optimal superconducting T(c) occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that Na(x)CoO2*1.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.

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PubMed2003

Ba2LnSbO6 and Sr2LnSbO6 (Ln = Dy, Ho, Gd) double perovskites: lanthanides in the geometrically frustrating fcc lattice.

Magnetic ground states in solids often arise as a result of a delicate balance between competing factors. One currently active area of research in magnetic materials involves compounds in which long-range magnetic ordering at low temperatures is frustrated by the geometry of the crystalline lattice, a situation known as geometrical magnetic frustration. The number of systems known to display the effects of such frustration is growing, but those that are sufficiently simple from theoretical, chemical, and physical perspectives to allow for detailed understanding remain very few. A search for model compounds in this family has led us to the double perovskites Ba2LnSbO6 and Sr2LnSbO6 (Ln = Dy, Ho, and Gd) reported here. Ba2DySbO6,Ba2HoSbO6,Sr2DySbO6, and Sr2HoSbO6 are structurally characterized by powder neutron diffraction at ambient temperature. The trivalent lanthanides and pentavalent antimony are found to be fully ordered in the double-perovskite arrangement of alternating octahedra sharing corner oxygens. In such a structure, the lanthanide sublattice displays a classical fcc arrangement, an edge-shared network of tetrahedra known to result in geometric magnetic frustration. No magnetic ordering is observed in any of these compounds down to temperatures of 2 K, and in the case of the Dy-based compounds in particular, frustration of the magnetic ordering is clearly present. Lanthanide-based double perovskites are proposed to be excellent model systems for the detailed study of geometric magnetic frustration.

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PubMed2003

Spin entropy as the likely source of enhanced thermopower in Na(x)Co2O4.

In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This is the Peltier effect, and it underlies all thermoelectric refrigerators. The increased interest in thermoelectric cooling applications has led to a search for more efficient Peltier materials and to renewed theoretical investigation into how electron-electron interaction may enhance the thermopower of materials such as the transition-metal oxides. An important factor in this enhancement is the electronic spin entropy, which is predicted to dominate the entropy current. However, the crucial evidence for the spin-entropy term, namely its complete suppression in a longitudinal magnetic field, has not been reported until now. Here we report evidence for such suppression in the layered oxide Na(x)Co2O4, from thermopower and magnetization measurements in both longitudinal and transverse magnetic fields. The strong dependence of thermopower on magnetic field provides a rare, unambiguous example of how strong electron-electron interaction effects can qualitatively alter electronic behaviour in a solid. We discuss the implications of our finding--that spin-entropy dominates the enhancement of thermopower in transition-metal oxides--for the search for better Peltier materials.

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PubMed2003

Quantum phase transition in quasi-one-dimensional BaRu6O12.

We report the first systematic study of the electrical transport and magnetic properties of BaRu6O12, which has a quasi-one-dimensional (quasi-1D) hollandite structure. We show that BaRu6O12 is quasi-1D electronically as well. Its physical properties were found to be extremely sensitive to disorder. Furthermore, a transition from being metallic with a resistance drop around 2 K to being weakly insulating as the applied magnetic field was increased was also found. We propose that these two features are related to the possible presence of a quantum phase transition in this material system.

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PubMed2003

Two Ti-doped distrontium ruthenium tetraoxides: Sr2Ru0.93Ti0.07O4 and Sr2Ru0.81Ti0.19O4.

Crystals of titanium-doped distrontium ruthenium tetraoxide, Sr(2)Ru(1-x)Ti(x)O(4), with x = 0.07 and 0.19, were grown by floating-zone melting, and their structures were solved using single-crystal X-ray diffraction. Increasing Ti content leads to a distinctive systematic variation of cell parameters and interatomic distances with respect to the undoped material.

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PubMed2002

Orbital ordering transition in La4Ru2O10.

We report experimental evidence for a full orbital ordering transition in the two-dimensional lanthanum ruthenate La4Ru2O10. The observable consequences of this orbital ordering include the loss of the Ru local moment, a structural distortion which partitions Ru-O bonds into axially oriented short and long sets, a sharp jump in electrical resistivity, and the opening of a spin gap that is visible in neutron scattering experiments. This is a rare example of a discrete orbital ordering transition in a 4d transition metal oxide and demonstrates that orbital effects can have an influence on the properties of layered ruthenates, a family of compounds that notably includes the p-wave superconductor Sr2RuO4 and the field-tuned quantum critical metamagnet Sr3Ru2O7.

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PubMed2002

Multiple field-induced phase transitions in the geometrically frustrated dipolar magnet: Gd(2) Ti(2)O(7).

Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet Gd(2)Ti(2)O(7), where crystal-field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real space and spin space, so the transitions in Gd(2)Ti(2)O(7) arise from field-induced "cooperative anisotropy," reflecting the broken spatial symmetries of the pyrochlore lattice.

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PubMed2002

Coherence incoherence and dimensional crossover in layered strongly correlated metals.

The properties of an interacting electron system depend on the electron correlations and the effective dimensionality. For example, Coulomb repulsion between electrons may inhibit, or completely block, conduction by intersite electron hopping, thereby determining whether a material is a metal or an insulator. Furthermore, correlation effects increase as the number of effective dimensions decreases; in three-dimensional systems, the low-energy electronic states behave as quasiparticles, whereas in one-dimensional systems, even weak interactions break the quasiparticles into collective excitations. Dimensionality is particularly important for exotic low-dimensional materials where one- or two-dimensional building blocks are loosely connected into a three-dimensional whole. Here we examine two such layered metallic systems with angle-resolved photoemission spectroscopy and electronic transport measurements, and we find a crossover in the number of effective dimensions from two to three with decreasing temperature. This is apparent from the observation that, in the direction perpendicular to the layers, the materials have an insulating character at high temperatures but become metal-like at low temperatures, whereas transport within the layers remains metallic over the whole temperature range. We propose that this change in effective dimensionality correlates with the presence of coherent quasiparticles within the layers.

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