Publications

Below is a selection of publications representing our research interests. 

 

Low-Dimensional Magnetism

  • Specific Heat of CsNiF3: Evidence for Spin Solitons, Phys. Rev. Lett. 49, 227 (1983): Discovered sine-Gordon soliton contribution to specific heat in the quasi-1D, spin-1 ferromagnet CsNiF­3.
  • Specific Heat of Defects in Haldane Systems Y2BaNiO5 and NENP - Absence of Free Spin-1/2 Excitations, Phys. Rev. Lett. 72, 3108 (1994): Demonstrated that the specific heat of uncompensated spins at the ends of s=1 antiferromagnetic chains in Y2BaNiO5  are not described by s=1/2 as expected from the Haldane effect, but rather by s=1 Schottky anomalies.

Frustrated Magnetism

  • Strongly Geometrically Frustrated Magnets, Annu. Rev. Mater. Sci. 24, 453 (1994): Introduced the criterion of strong frustration (θ­w/Tc > 10) to define a class of geometrically frustrated magnets.
  • Strong Frustration and Dilution-Enhanced Spin Glass Order in a Quasi-2D Antiferromagnet, Phys. Rev. Lett. 64, 2070 (1990): Identified a new liquid-like phase of the antiferromagnetic Heisenberg kagome system SrCr9Ga3O19.
  • Zero Point Entropy in Spin-Ice, Nature 399, 527 (1999): Discovered zero-point entropy in the pyrochlore Dy2Ti2O7, demonstrating the spin-ice ground state.
  • Phonon Density of States and Negative Thermal Expansion in ZrW2O8, Nature 396, 147 (1998) & Giant Dielectric Constant Response in a Copper-Titanate, Solid State Commun. 115, 217 (2000): Identified the role of geometrical frustration for understanding non-magnetic phenomena, specifically negative thermal expansion in ZrW2O8, and large dielectric response in CaCu­3Ti4O12.
  • Quasispin Glass in a Geometrically Frustrated Magnet, Phys. Rev. Lett. 110, 017203 (2013): Discovered, in the vacancy-doped spinel ZnCr2O4, evidence for freezing among quasi-spins with range-independent interactions.

Superconductivity

  • Electronic Properties of La2-xSrxCuO4 High-Tc Superconductors, Phys. Rev. B 35, 5347 (1987): Performed first measurement of the specific heat jump at Tc in cuprate superconductors, in La2-xSrxCuO4.
  • Superconductivity at 18K in Potassium-doped C60, Nature 350, 600 (1991): Co-discovered superconductivity in A3C60.
  • Isotope Effect in Superconducting Rb3 C60, Phys. Rev. Lett. 68, 1058 (1992): Determined the carbon isotope T­c-shift in A3C60, demonstrating a phonon-mediated process.
  • Specific Heat Jump at Tc and Normal State Susceptibility of A3C60, Phys. Rev. Lett. 69, 1687 (1992): Performed the only measurement to date of the specific heat jump at Tc in A3C60 (for A = K); showed consistency among ωph, λ, and μ for phonon-mediated pairing.
  • Fermi-Liquid Renormalization in the Superconducting State of UBe13, Phil. Mag. 79, 111 (1999): Found the Fermi-liquid effect, known for 3He, in a superconductor, the heavy fermion system UBe13.

Colossal Magnetoresistance

  • Low Temperature Magnetoresistance and the Magnetic Phase Diagram of La1-xCaxMnO3, Phys. Rev. Lett 75, 3336 (1995): Determined the first complete phase diagram for a CMR system, La1-xCaxMnO3.
  • Magnetic Field Dependent Specific Heat of V3Si -- Downward Curvature as a General Feature of Superconductivity, Physics Letters A 211, 59 (1996) & Thermal Conductivity of Manganite Perovskites, Phys. Rev. Lett. 78, 3947 (1997): Demonstrated coupling of the lattice to magnetism using sound velocity and thermal conductivity.
  • Large Enhancement of Magnetoresistance in Tl2Mn2O7 : Pyrochlore vs. Perovskite, Science 277, 546 (1997) & Colossal Magnetoresistance in Cr-Chalcogenide Spinels, Nature 386, 166 (1997): Demonstrated CMR in a Mn-containing non-perovskite and a non-Mn containing spinel.

2-Dimensional Hole Gas

  • Nonmonotonic Temperature-Dependent Resistance in Low Density 2D Hole Gases, Phys. Rev. Lett. 83, 2805 (1999): Demonstrated scaling of non-monotonic insulating-to-metallic crossover in a high mobility 2d hole-gas.

Multiferroics

Organic Semiconductors

  • Bias-Dependent Generation and Quenching of Defects in Pentacene, Phys. Rev. Lett. 93, 076601 (2004): Found that deep traps in pentacene are due to formation of meta-stable hydrapentacene impurities.
  • Dependence of Mobility on Density of Gap States in Organics by GAME(a)S-Gate Modulated Activation Energy Spectroscopy, J. Appl. Phys. 104, 054512 (2008): Developed simple spectroscopy, Gate Modulated Activation Energy Spectroscopy (GAMES) for study of deep and shallow trap states.
  • Mobility-independent doping in crystalline rubrene field-effect transistors, Solid State Commun. 142, 483 (2007): co-discovered a method for doping organic crystals – i.e. changing carrier density without changing mobility (patented).
  • Mechanism for Oxygen-Enhanced Photoconductivity in Rubrene: Electron Transfer Doping, Chem. Mat. 21, 5519 (2009): Co-discovered mechanism for oxygen-induced doping in rubrene.

Specific heat of 1D chains in CsNiF3. The temperature dependence of the peaks yields the soliton rest mass.

Negative thermal expansion in ZrW2O8 can be viewed as an effect of structural frustration

Field dependence of the specific heat of heavy-fermion superconductor UBe13. The upper inset shows the upper critical field phase boundary. The lower inset shows the normalized second derivative of the spin susceptibility in the superconducting phase of UBe13 as well as in the B-phase of He3.