Publications (202)468.85 Total impact
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ABSTRACT: Sr2IrO4 is a spinorbit coupled insulator with an antiferromagnetic (AFM) transition at TN=240 K. We report results of a comprehensive study of singlecrystal Sr2Ir1xTbxO4. This study found that mere 3% (x=0.03) tetravalent Tb4+(4f7) substituting for Ir4+ (rather than Sr2+) completely suppresses the longrange collinear AFM transition but retains the insulating state, leading to a phase diagram featuring a decoupling of magnetic interactions and charge gap. The insulating state at x=0.03 is characterized by an unusually large specific heat at low temperatures and an incommensurate magnetic state having magnetic peaks at (0.95, 0, 0) and (0, 0.95, 0) in the neutron diffraction, suggesting a spiral or spin density wave order. It is apparent that Tb doping effectively changes the relative strength of the SOI and the tetragonal CEF and enhances the Hund's rule coupling that competes with the SOI, and destabilizes the AFM state. However, the disappearance of the AFM accompanies no metallic state chiefly because an energy level mismatch for the Ir and Tb sites weakens charge carrier hopping and renders a persistent insulating state. This work highlights an unconventional correlation between the AFM and insulating states in which the magnetic transition plays no critical role in the formation of the charge gap in the iridate.  [Show abstract] [Hide abstract]
ABSTRACT: BaIrO 3 is a magnetic insulator driven by the spinorbit interaction (SOI), whereas BaRuO 3 is a paramagnet and exhibits a crossover from a metallic to an insulating regime. Our investigation of structural, magnetic, transport and thermal properties reveals that substitution of Ru 4+ (4d 4) ions for Ir 5+ (5d 5) ions in BaIrO 3 reduces the magnitudes of the SOI and a monoclinic structural distortion, and rebalances the competition between the SOC and the lattice degrees freedom to generate a rich phase diagram for BaIr 1x Ru x O 3 (0 x 1). There are two major effects of Ru additions: (1) Light Ru doping (0 < í µí±¥ ≤ 0.15) prompts simultaneous, precipitous drops in both the magnetic ordering temperature T N and the electrical resistivity, which exhibits a crossover behavior from a metallic to an insulating state near T N. (2) Heavier Ru doping (0.41 ≤ í µí±¥ ≤ 0.9) induces a robust metallic state with a strong spin frustration generated by competing antiferromagnetic and ferromagnetic interactions.  [Show abstract] [Hide abstract]
ABSTRACT: BaIrO3 is a magnetic insulator driven by the spinorbit interaction (SOI), whereas BaRuO3 is a paramagnet and exhibits a crossover from a metallic to an insulating regime. Our investigation of structural, magnetic, transport and thermal properties reveals that substitution of Ru4+ (4d4) ions for Ir5+ (5d5) ions in BaIrO3 reduces the magnitudes of the SOI and a monoclinic structural distortion, and rebalances the competition between the SOC and the lattice degrees freedom to generate a rich phase diagram for BaIr1xRuxO3 (0< x <1). There are two major effects of Ru additions: (1) Light Ru doping (0 < x < 0.15) prompts simultaneous, precipitous drops in both the magnetic ordering temperature TN and the electrical resistivity, which exhibits a crossover behavior from a metallic to an insulating state near TN. (2) Heavier Ru doping (0.41< x < 0.9) induces a robust metallic state with a strong spin frustration generated by competing antiferromagnetic and ferromagnetic interactions.  [Show abstract] [Hide abstract]
ABSTRACT: Sr 2 IrO 4 is a magnetic insulator assisted by strong spinorbit coupling (SOC) whereas the Sr 2 RuO 4 is a pwave superconductor. The contrasting ground states have been shown to result from the critical role of the strong SOC in the iridate. Our investigation of structural, transport, and magnetic properties reveals that substituting 4d Ru 4+ (4d 4) ions for 5d Ir 4+ (5d 5) ions in Sr 2 IrO 4 directly adds holes to the t 2g bands, reduces the SOC and thus rebalances the competing energies in singlecrystal Sr 2 Ir 1x Ru x O 4. A profound effect of Ru doping driving a rich phase diagram is a structural phase transition from a distorted I4 1 /acd to a more ideal I4/mmm tetragonal structure near x=0.50 that accompanies a phase transition from an antiferromagneticinsulating state to a paramagneticmetal state. We also make a comparison drawn with Rh doped Sr 2 IrO 4 , highlighting important similarities and differences. 
Article: From Jeff=1/2 insulator to pwave superconductor in singlecrystal Sr2Ir1xRuxO4 (0 < x< 1)
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ABSTRACT: Sr2IrO4 is a magnetic insulator assisted by strong spinorbit coupling (SOC) whereas the Sr2RuO4 is a pwave superconductor. The contrasting ground states have been shown to result from the critical role of the strong SOC in the iridate. Our investigation of structural, transport, and magnetic properties reveals that substituting 4d Ru4+ (4d4) ions for 5d Ir4+(5d5) ions in Sr2IrO4 directly adds holes to the t2g bands, reduces the SOC and thus rebalances the competing energies in singlecrystal Sr2Ir1xRuxO4. A profound effect of Ru doping driving a rich phase diagram is a structural phase transition from a distorted I41/acd to a more ideal I4/mmm tetragonal structure near x=0.50 that accompanies a phase transition from an antiferromagneticinsulating state to a paramagneticmetal state. We also make a comparison drawn with Rh doped Sr2IrO4, highlighting important similarities and differences.  [Show abstract] [Hide abstract]
ABSTRACT: We consider the onedimensional gas of fermions with spin S interacting via an attractive δfunction potential using the Bethe Ansatz solution. In zero magnetic field the atoms form bound states of fermions, i.e. generalized Cooper states with each atom having a different spin component. For low energy excitations the system is a Luttinger liquid and is properly described by a conformal field theory with conformal charge . The linear dispersion of a Luttinger liquid is asymptotically exact in the lowenergy limit where the band curvature terms in the dispersion are irrelevant. For higher energy excitations, however, the spectral function displays deviations in the neighborhood of the singleparticle (hole) energy, which can be described by an effective xray edge type model. Using the Bethe Ansatz solution we obtain expressions for the critical exponents for the singleparticle (hole) Green's function. This model can be relevant in the context of ultracold atoms with effective total spin S confined to an elongated optical trap.Nuclear Physics B 01/2015; 892. DOI:10.1016/j.nuclphysb.2015.01.009 · 3.93 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Here, we unveil evidence for a quantum phasetransition in CeCu_2Ge_2 which displays both an incommensurate spindensity wave (SDW) groundstate, and a strong renormalization of the quasiparticle effective masses (mu) due to the Kondoeffect. For all angles theta between an external magnetic field (H) and the crystallographic caxis, the application of H leads to the suppression of the SDWstate through a 2^ndorder phasetransition at a thetadependent criticalfield H_p(theta) leading to the observation of small Fermi surfaces (FSs) in the paramagnetic (PM) state. For H  caxis, these FSs are characterized by light mu's pointing also to the suppression of the Kondoeffect at H_p with surprisingly, no experimental evidence for quantumcriticality (QC). But as $H$ is rotated towards the aaxis, these mu's increase considerably becoming undetectable for \theta > 56^0 between H and the caxis. Around H_p^a~ 30 T the resistivity becomes proportional T which, coupled to the divergence of mu, indicates the existence of a fieldinduced QCpoint at H_p^a(T=0 K). This observation, suggesting FS hotspots associated with the SDW nestingvector, is at odds with current QC scenarios for which the continuous suppression of all relevant energy scales at H_p(theta,T) should lead to a line of quantumcritical points in the Htheta plane. Finally, we show that the complexity of its magnetic phasediagram(s) makes CeCu_2Ge_2 an ideal system to explore fieldinduced quantum tricritical and QC endpoints.Physical Review B 09/2014; 90(15). DOI:10.1103/PhysRevB.90.155101 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In the context of ultracold fermionic atoms with effective spin S confined to an elongated trap we study the onedimensional gas interacting via an attractive δfunction potential using the Bethe ansatz solution. There are N = 2S + 1 fundamental states: The particles can either be unpaired or clustered in bound states of 2, 3, ..., 2S and 2S + 1 atoms. In a magnetic field, the rich ground state phase diagram consists of these N states and various mixed phases in which combinations of the fundamental states coexist. The phase diagram simplifies considerably in zerofield, where only bound states of N atoms can exist. Due to the harmonic confinement and within the local density approximation, the density profile of bound states decreases along the tube from the center of the trap to its boundaries. In an array of tubes with weak Josephson tunneling superfluid order may arise. In zerofield the response functions determining the superfluid and density wave order are calculated using conformal field theory and the exact Bethe ansatz solution. The response function for superfluidity consists of a power law with distance, while the correlation function for density waves is a power law of distance times a sinusoidal factor oscillating with distance with a period given by two times the Fermi momentum. For S = 1/2 superfluidity is a possibility for all densities and density waves can be excluded. For S ≥ 3/2 superfluidity may occur at low densities but at high densities it gives way to density waves. We discuss the scenario of phase separation where for S ≥ 3/2 the system has superfluid longrange order toward the trap boundaries and density waves at its center.Journal of Physics Conference Series 08/2014; 529(1):012013. DOI:10.1088/17426596/529/1/012013  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by recent experiments on ultracold fermions we study the effect of the spinorbit interaction in an exactly solvable onedimensional model with strong local attractive correlations between the fermions. We show that the asymptotes of correlation functions, calculated in the framework of the conformal field theory and finite size corrections of the Bethe ansatz exact solution, are strongly affected by the spinorbit coupling. In the mixed phase, the correlation functions consist of terms that are the product of a power law of the distance and an oscillating function of the distance. We obtain the critical exponents for superfluidity and density waves. The leading exponents decrease as a function of spinorbit coupling. In particular, the exponent for superfluidity is the smallest one signaling an instability to a phase where a weak interchain coupling can produce superfluidity of the FuldeFerrellLarkinOvchinnikov (FFLO) type. The spatial oscillation of the order parameter is also modified by the spinorbit parameter, i.e., its modulation is not just given by the spin imbalance.Physical Review B 11/2013; 88(20). DOI:10.1103/PhysRevB.88.205127 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have observed a massive reconstruction of the Fermi surface of single crystal chromium as a function of high pressure and high magnetic fields caused by the spinflip transition, with multiple new orbits appearing above 0.93 GPa. Additionally, some orbits have fieldinduced effective masses of ~0.060.07 me, seen only at high magnetic fields. Based on the temperature insensitivity displayed by the oscillation amplitudes at these frequencies, we attribute the orbits to quantum interference rather than to Landau quantization.Physical review. B, Condensed matter 06/2013; 88(12). DOI:10.1103/PhysRevB.88.125119 · 3.66 Impact Factor 
Article: Observation of superconductivity and anomalous electrical resistivity in singlecrystal Ir3Te8
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ABSTRACT: We observe an unusual combination of normal and superconducting state properties without any signature of strong spin fluctuations in singlecrystal Ir3Te8. The electrical resistivity does not saturate by 700 K but exhibits a lowresistivity ratio, and it exhibits two extended linear regimes (approximately 20–330 and 370–700 K) with the same slope, separated by a small hysteretic interval marking a strong firstorder phase transition from cubic to rhombohedral lattice symmetry at TS = 350 K. The electronic heatcapacity coefficient (11 mJ mol−1 K−2) is consistent with a net diamagnetic, rather than a Pauli paramagnetic, normal state that yields to superconductivity below a critical temperature TC = 1.8 K. The size of the heatcapacity jump near TC indicates bulk superconductivity.Physical review. B, Condensed matter 05/2013; 87(17). DOI:10.1103/PhysRevB.87.174510 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Sr3Ir2O7 exhibits a novel Jeff=1/2 insulating state that features a splitting between Jeff=1/2 and 3/2 bands due to spinorbit interaction. We report a metalinsulator transition in Sr3Ir2O7 via either dilute electron doping (La3+ for Sr2+) or application of high pressure up to 35 GPa. Our study of singlecrystal Sr3Ir2O7 and (Sr1xLax)3Ir2O7 reveals that application of high hydrostatic pressure P leads to a drastic reduction in the electrical resistivity by as much as six orders of magnitude at a critical pressure, PC = 13.2 GPa, manifesting a closing of the gap; but further increasing P up to 35 GPa produces no fully metallic state at low temperatures, possibly as a consequence of localization due to a narrow distribution of bonding angles {\theta}. In contrast, slight doping of La3+ ions for Sr2+ ions in Sr3Ir2O7 readily induces a robust metallic state in the resistivity at low temperatures; the magnetic ordering temperature is significantly suppressed but remains finite for (Sr0.95La0.05)3Ir2O7 where the metallic state occurs. The results are discussed along with comparisons drawn with Sr2IrO4, a prototype of the Jeff = 1/2 insulator.Physical review. B, Condensed matter 04/2013; 87(23). DOI:10.1103/PhysRevB.87.235127 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We observe superconductivity below a critical temperature TC = 1.8 K in singlecrystal Ir3Te8, which also exhibits normalstate diamagnetism and a linear temperature dependence of electrical resistivity for a wide temperature interval, 20 K < T < 700 K. Singlecrystal Ir3Te8 also undergoes a structural phase transition at TS = 350 K from a cubic (above TS) to a rhombohedral lattice below TS. Our firstprinciples electronic structure calculations reveal two bands crossing the Fermi level; despite the threedimensional lattice, one band is quasitwodimensional, and is responsible for the observed diamagnetism and structure transition. The strong nonFermiliquid behavior characterized by the observed linearity in resistivity in such a nonmagnetic state suggests novel physics in this newly discovered superconductor.  [Show abstract] [Hide abstract]
ABSTRACT: We report the magnetic and electric properties of Ba$_3$NiNb$_2$O$_9$, which is a quasitwodimensional spin1 triangular lattice antiferromagnet (TLAF) with trigonal structure. At low $T$ and with increasing magnetic field, the system evolves from a 120 degree magnetic ordering phase (A phase) to an upupdown ($uud$) phase (B phase) with a change of slope at 1/3 of the saturation magnetization, and then to an "oblique" phase (C phase). Accordingly, the ferroelectricity switches on at each phase boundary with appearance of spontaneous polarization. Therefore, Ba$_3$NiNb$_2$O$_9$ is a unique TLAF exhibiting both $uud$ phase and multiferroicity.Physical Review Letters 12/2012; 109(25):257205. DOI:10.1103/PhysRevLett.109.257205 · 7.51 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Sr2IrO4 is a magnetic insulator driven by spinorbit interaction (SOI) whereas the isoelectronic and isostructural Sr2RhO4 is a paramagnetic metal. The contrasting ground states have been shown to result from the critical role of the strong SOI in the iridate. Our investigation of structural, transport, magnetic and thermal properties reveals that substituting 4d Rh4+ (4d5) ions for 5d Ir4+(5d5) ions in Sr2IrO4 directly reduces the SOI and rebalances the competing energies so profoundly that it generates a rich phase diagram for Sr2Ir1xRhxO4 featuring two major effects: (1) Light Rh doping (0\leqx\leq0.16) prompts a simultaneous and precipitous drop in both the electrical resistivity and the magnetic ordering temperature TC, which is suppressed to zero at x = 0.16 from 240 K at x=0. (2) However, with heavier Rh doping (0.24< x<0.85 (\pm0.05)) disorder scattering leads to localized states and a return to an insulating state with spin frustration and exotic magnetic behavior that only disappears near x=1. The intricacy of Sr2Ir1xRhxO4 is further highlighted by comparison with Sr2Ir1xRuxO4 where Ru4+(4d4) drives a direct crossover from the insulating to metallic states.Physical Review B 09/2012; 86(12):125105. DOI:10.1103/PhysRevB.86.125105 · 3.74 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We report an experimental/theoretical study of singlecrystal Bi(2)Ir(2)O(7) that possesses a metallic state with strongly exchangeenhanced paramagnetism. The ground state of Bi(2)Ir(2)O(7) is characterized by the following features: (1) a divergent lowtemperature magnetic susceptibility that indicates no longrange order down to 50 mK; (2) strongly fielddependent coefficients of the lowtemperature T and T(3) terms of the specific heat; (3) a conspicuously large Wilson ratio R(W) ≈ 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi(2)Ir(2)O(7).Journal of Physics Condensed Matter 07/2012; 24(34):345601. DOI:10.1088/09538984/24/34/345601 · 2.35 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In the context of ultracold atoms with effective spin S = 5/2 confined to an elongated trap we study the onedimensional Fermi gas interacting via an attractive δfunction potential using the Bethe ansatz solution. There are N = 2S + 1 = 6 fundamental states: The particles can either be unpaired or clustered in bound states of 2, 3, …, 2S and 2S + 1 fermions. The rich ground state phase diagram consists of these six states and various mixed phases in which combinations of the fundamental states coexist. Possible scenarios for phase separation due to the harmonic confinement along the tube are explored within the local density approximation. In an array of tubes with weak Josephson tunneling superfluid order may arise. The response functions determining the type of superfluid order are calculated using conformal field theory and the exact Bethe ansatz solution. They consist of a power law with distance times a sinusoidal term oscillating with distance. The wavelength of the oscillations is related to the periodicity of a generalized FuldeFerrellLarkinOvchinnikov (FFLO) state.Modern Physics Letters B 06/2012; 26(16):30009. DOI:10.1142/S0217984912300098 · 0.75 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigated the magnetic phase diagram of single crystals of SrNdFeO4 by measuring the magnetic properties, the specific heat and the dielectric permittivity. The system has two magnetically active ions, Fe3+ and Nd3+. The Fe3+ spins are antiferromagnetically ordered below 360 K with the moments lying in the ab plane, and undergo a reorientation transition at about 35–37 K to an antiferromagnetic order with the moments along the c axis. A shortrange, antiferromagnetic ordering of Nd3+ along the c axis was attributed to the reorientation of Fe3+ followed by a longrange ordering at lower temperature [ Oyama et al. J. Phys.: Condens. Matter. 16 1823 (2004)]. At low temperatures and magnetic fields above 8 T, the Nd3+ moments are completely spin polarized. The dielectric permittivity also shows anomalies associated with spin configuration changes, indicating that this compound has considerable coupling between spin and lattice. A possible magnetic structure is proposed to explain the results.Physical review. B, Condensed matter 06/2012; 85(22). DOI:10.1103/PhysRevB.85.224429 · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In the context of a gas of ultracold atoms with effective spin S=3/2 confined to an elongated trap, we study the onedimensional Fermi gas interacting via an attractive δfunction potential within the grandcanonical ensemble. The particles can be either unbound or clustered in bound states of two, three, and four fermions. The rich μ versus H groundstate phase diagram (μ is the chemical potential and H the external magnetic field) consists of the four basic states and the various possible mixed phases in which some these states coexist. Extending the analysis of K. Yang [ Phys. Rev. B 63 140511 (2001)] for S=1/2, we study the correlation functions of the generalized Cooper clusters of bound states of two, three, and four particles using conformal field theory and the exact Bethe Ansatz solution. The correlation functions consist of a power law with distance times a sinusoidal term oscillating with distance. In an array of tubes with weak Josephson tunneling, the type of superfluid order is determined by these correlation functions. The wavelength of the oscillations is related to the periodicity of a generalized FuldeFerrellLarkinOvchinnikov (FFLO) state for higher spin particles. All the relevant states are analyzed for S=3/2.Physical review. B, Condensed matter 05/2012; 85(20). DOI:10.1103/PhysRevB.85.205129 · 3.66 Impact Factor 
Article: Pressure dependence of the BaFe_ {2} As_ {2} Fermi surface within the spin density wave state
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ABSTRACT: Measuring surface conductivity we have observed the evolution of Shubnikov de Haas oscillations under quasihydrostatic pressure for the pnictide parent compound BaFe2As2. Prior results in the reconstructed state have observed small pockets which emerge from zone folding as a result of structural changes with cooling. For pressures below 20 kbar, both Fermi surface orbits grow in size. The effective masses increase with pressure suggesting enhanced correlation in the system, and a series of magnetic breakdown orbits are observed confirming that band structure calculations setting them in close proximity are correct.Physical review. B, Condensed matter 04/2012; 85(13). DOI:10.1103/PhysRevB.85.134503 · 3.66 Impact Factor
Publication Stats
2k  Citations  
468.85  Total Impact Points  
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Institutions

19912014

Florida State University
 Department of Physics
Tallahassee, Florida, United States


20052012

National High Magnetic Field Laboratory
Tallahassee, Florida, United States


20072011

University of Kentucky
 Department of Physics & Astronomy
Lexington, Kentucky, United States 
San Diego State University
San Diego, California, United States


2002

Northwest University
Ch’angan, Shaanxi, China


19871991

Temple University
 Department of Physics
Philadelphia, PA, United States


1988

Tufts University
 Department of Physics and Astronomy
Бостон, Georgia, United States


19791980

Freie Universität Berlin
 Institute of Theoretical Physics
Berlin, Land Berlin, Germany


19721975

Max Planck Institute for Astrophysics
Arching, Bavaria, Germany


1973

Max Planck Institute for Physics
München, Bavaria, Germany
