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Featured researches published by J. W. Brill.


Solid State Communications | 2004

Hydrostatic pressure effects on the magnetic susceptibility of ruthenium oxide Sr3Ru2O7: evidence for pressure-enhanced antiferromagnetic instability

Yuri V. Sushko; B.A. deHarak; G. Cao; Gargi Shaw; D.K. Powell; J. W. Brill

Abstract Hydrostatic pressure effects on the temperature- and magnetic field dependencies of the in-plane and out-of-plane magnetization of the bi-layered perovskite Sr 3 Ru 2 O 7 have been studied by SQUID magnetometer measurements under a hydrostatic helium-gas pressure. The anomalously enhanced low-temperature value of the paramagnetic susceptibility has been found to systematically decrease with increasing pressure. The effect is accompanied by an increase of the temperature T max of a pronounced peak of susceptibility. Thus, magnetization measurements under hydrostatic pressure reveal that the lattice contraction in the structure of Sr 3 Ru 2 O 7 promotes antiferromagnetism and not ferromagnetism. The effects can be explained by the enhancement of the inter-bi-layer antiferromagnetic spin coupling, driven by the shortening of the superexchange path, and suppression, due to the band-broadening effect, of competing itinerant ferromagnetic correlations.


Physical Review B | 2004

Field-tuned collapse of an orbitally ordered and spin-polarized state: Colossal magnetoresistance in the bilayered ruthenate Ca3Ru2O7

G. Cao; L. Balicas; X.N. Lin; S. Chikara; E. Elhami; V. Duairaj; J. W. Brill; R. C. Rai; J. E. Crow

Ca3Ru2O7 with a Mott-like transition at 48 K and a Neel temperature at 56 K features different in-plane anisotropies of the magnetization and magnetoresistance. Applying a magnetic field along the magnetic easy- axis precipitates a spin-polarized state via a first-order metamagnetic transition, but does not lead to a full suppression of the Mott state, whereas applying a magnetic field along the magnetic hard axis does, causing a resistivity reduction of three orders of magnitude. The colossal magnetoresistivity is attributed to the collapse of an orbitally ordered and spin-polarized state. This phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detrimental to the colossal magnetoresistence in this 4 d-based electron system. Evidence of a density wave is also presented. DOI: 10.1103/PhysRevB.69.014404 PACS number~s!: 75.47.Gk, 71.70.Ej The physics of magnetoresistance has generated enor- mous interest in recent years. While this quantum mechani- cal phenomenon is in general associated with the spin scat- tering process of conduction electrons, the origins of various kinds of magnetoresistance are vastly different. The giant magnetoresistance observed in magnetic metallic multiplayer structures can be qualitatively explained using the two cur- rent model, corresponding to up-spin and down-spin electrons. 1 Tunneling magnetoresistance, often seen in mag- netic tunnel junctions separated by an insulating spacer layer, is a consequence of spin-polarization. On the other hand, colossal magnetoresistance ~CMR!, seen only in the mixed- valence manganites so far, originates from a metal-insulator transition in the vicinity of the Curie temperature driven pri- marily by double exchange due to the hopping of eg elec- trons of Mn 31 ions and the Jahn-Teller effect. 2 The novelty of the bilayered Ca3Ru2O7 , as presented in this paper, is that the colossal magnetoresistivity is a result of the collapse of the orbitally ordered state that is realized by demolishing the spin-polarized state . This phenomenon is striking in that the spin polarization, which is a fundamental driving force for all other magnetoresistive systems, is detri- mental to the colossal magnetoresistence in this 4 d-based electron system. Indeed, the electron kinetic energy hinges on the spin-orbital-lattice coupling in such a way that apply- ing magnetic field, B, along the magnetic easy axis ~a axis! precipitates a spin-polarized state via a first-order metamag- netic transition, but does not lead to a full suppression of the Mott state, whereas applying B along the magnetic hard axis ~b axis! does, giving rise to a resistivity reduction of three orders of magnitude. Our previous work indicated the puz- zling anisotropic behavior observed in the field dependence


Physical Review B | 2006

Dynamics of the Electro-Optic Response of Blue Bronze

L. Ladino; J. W. Brill; Mario Freamat; M. Uddin; D. Dominko

We have studied the charge density wave (CDW) repolarization dynamics in blue bronze by applying square-wave voltages of different frequencies to the sample and measuring the changes in infrared transmittance, proportional to CDW strain. The frequency dependence of the electro-transmittance was fit to a modified harmonic oscillator response and the evolution of the parameters as functions of voltage, position, and temperature are discussed. Resonant frequencies decrease with distance from the current contacts, indicating that the resulting delays are intrinsic to the CDW with the strain effectively flowing from the contact. For a fixed position, the average relaxation time has a voltage dependence given by tau_0~V^-p, with 1<p<2. The temperature dependence of the fitting parameters shows that the dynamics are governed by both the force on the CDW and the CDW current: for a given force and position, both the relaxation and delay times are inversely proportional to the CDW current as temperature is varied. The long relaxation and delay times (~ 1 ms) suggest that the strain response involves the motion of macroscopic objects, presumably CDW phase dislocation lines.


Physical Review B | 2009

Characterization of the torsional piezoelectriclike response of tantalum trisulfide associated with charge-density-wave depinning

J. Nichols; D. Dominko; L. Ladino; J. Zhou; J. W. Brill

We have studied the frequency and voltage dependence of voltage-induced torsional strains in orthorhombic TaS3 [V. Ya. Pokrovskii, et al, Phys. Rev. Lett. 98, 206404 (2007)] by measuring the modulation of the resonant frequency of an RF cavity containing the sample. The strain has an onset voltage below the charge-density-wave (CDW) threshold voltages associated with changes in shear compliance and resistance, suggesting that the strain is associated with polarization of the CDW rather than CDW current. Measurements with square-wave voltages show that the strain is very sluggish, not even reaching its dc value at a frequency of 0.1 Hz, but the dynamics appear to be very sample dependent. By applying oscillating torque while biasing the sample with a dc current, we have also looked for strain induced voltage in the sample; none is observed at the low biases where the voltage-induced strains first occur, but an induced voltage is observed at higher biases, probably associated with strain-dependent CDW conductance.


Physical Review B | 2007

Anomalous itinerant magnetism in single-crystal Sr4Ru3O10 : A thermodynamic and transport investigation

G. Cao; Shalinee Chikara; J. W. Brill; P. Schlottmann

A thermodynamic and transport study of Sr4Ru3O10 as a function of temperature and magnetic field is presented. The central results include a growing specific heat C with increasing field B, a magnetic contribution to C/T at low temperatures proportional to -log(T), an abrupt jump and a peak in C/T at 2.90 T and 7 T for B||ab-plane and B||c-axis, respectively, and corresponding changes in the low T power laws of the resistivity. The novelty of this work lies in the fact that this system is strongly anisotropic displaying spontaneous ferromagnetism along the c-axis and an intralayer metamagnetic transition with a possibility of a nearby quantum critical point. The exotic behavior reflects new physics that is yet to be understood.


Physical Review B | 2007

Electro-optic search for critical divergence of the charge-density-wave diffusion constant at the onset of depinning

L. Ladino; E. G. Bittle; M. Uddin; J. W. Brill

We have used electroreflectance measurements to study charge-density-wave (CDW) polarization dynamics at voltages near the CDW depinning onset


Physical Review B | 2005

Thermal and magnetic properties of the spin-chain material Pr 3 Ru O 7

Mario Freamat; X.N. Lin; V. Durairaj; S. Chikara; G. Cao; J. W. Brill

({V}_{mathrm{on}})


Solid State Sciences | 2000

Comparison of Young's modulus and specific heat anomalies at the magnetic transition in α′-NaV2O5

Emanuel Postolache; D.K. Powell; Guerman Popov; Ram C. Rai; Martha Greenblatt; J. W. Brill

in the quasi-one-dimensional conductor


Physical Review B | 2004

Probing Localization Effects in Li0.9Mo6O ;17 Purple Bronze: An Optical-Properties Investigation

J. Choi; J. L. Musfeldt; J. He; R. Jin; J. R. Thompson; D. Mandrus; X.N. Lin; V. A. Bondarenko; J. W. Brill

{mathrm{K}}_{0.3}mathrm{Mo}{mathrm{O}}_{3}


Physical Review B | 1998

Specific heat ofα′−NaV2O5at its spin-Peierls transition

D. K. Powell; J. W. Brill; Z. Zeng; Martha Greenblatt

(``blue bronze). For low voltages, where the phase-slip rate is low, it is expected that the polarization relaxation time should be inversely proportional to the CDW diffusion constant, which is expected to diverge at

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G. Cao

University of Kentucky

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X.N. Lin

University of Kentucky

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V. Durairaj

University of Kentucky

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S. Chikara

University of Kentucky

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Shalinee Chikara

Los Alamos National Laboratory

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L. Ladino

University of Kentucky

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P. Schlottmann

Florida State University

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D. Dominko

University of Kentucky

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E. Elhami

University of Kentucky

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Hao Zhang

University of Kentucky

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