Rafael Jaramillo

Harvard University, Cambridge, Massachusetts, United States

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Publications (45)158.83 Total impact

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    ABSTRACT: For most metals, increasing temperature (T) or disorder will quicken electron scattering. This hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals this predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Here we introduce the rare-earth nickelates (RNiO_3, R = rare earth) as a class of bad metals. We study SmNiO_3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conduction and the insulator-metal transition in the nickelates by shifting spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low \sigma while preserving the Drude model and without violating the uncertainty principle.
    09/2013; 10(4).
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    ABSTRACT: The rare-earth nickelates (LnNiO3, Ln = lanthanide) are interesting from both fundamental and applied perspectives, but synthesis remains a bottleneck to research due to their thermodynamic instability. Here we report the synthesis of SmNiO3 thin films on oxidized silicon wafers by physical vapor deposition followed by high pressure oxygen annealing at intermediate temperatures. The high pressure annealed films show an insulator–metal transition characteristic of bulk samples. Our experimental observations then allow us to estimate bounds on the phase stability regime, which are particularly useful given the dearth of direct thermodynamic data available for LnNiO3. We examine the limitations of these thermodynamic analyses applied to ultra-thin films. The stabilization of SmNiO3 on a canonical semiconductor template creates opportunities to study the utility of the above room temperature insulator–metal transition (at TIM = 400 K) in electronic devices.
    J. Mater. Chem. C. 03/2013; 1(13):2455-2462.
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    ABSTRACT: The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable insulator-metal transition that are subjects of active research. Here we present temperature-dependent transport measurements of the resistivity, magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial SmNiO3 thin films with varying oxygen stoichiometry. We find that from room temperature through the high temperature insulator-metal transition, the Hall coefficient is hole-like and the Seebeck coefficient is electron-like. At low temperature the N\'eel transition induces a crossover in the sign of RH to electron-like, similar to the effects of spin density wave formation in metallic systems but here arising in an insulating phase ~200 K below the insulator-metal transition. We propose that antiferromagnetism can be stabilized by bandstructure even in insulating phases of correlated oxides, such as RNiO3, that fall between the limits of strong and weak electron correlation.
    Physical review. B, Condensed matter 01/2013; 87(12). · 3.77 Impact Factor
  • R Jaramillo, Yejun Feng, T F Rosenbaum
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    ABSTRACT: We describe a technique for making electrical transport measurements in a diamond anvil cell using an alcohol pressure medium, permitting acute sensitivity while preserving sample fidelity. The sample is suspended in the liquid medium by four gold leads that are electrically isolated by a composite gasket made of stainless steel and an alumina-loaded epoxy. We demonstrate the technique with four-probe resistivity measurements of chromium single crystals at temperatures down to 4 K and pressures above 10 GPa. Our assembly is optimized for making high precision measurements of the magnetic phase diagram and quantum critical regime of chromium, which require repeated temperature sweeps and fine pressure steps while maintaining high sample quality. The high sample quality enabled by the quasi-hydrostatic pressure medium is evidenced by the residual resistivity below 0.1 μΩ cm and the relative resistivity ratio ρ(120 K)∕ρ(5 K) = 15.9 at 11.4 GPa. By studying the quality of Cr's antiferromagnetic transition over a range of pressures, we show that the pressure inhomogeneity experienced by the sample is always below 5%. Finally, we solve for the Debye temperature of Cr up to 11.4 GPa using the Bloch-Gruneisen formula and find it to be independent of pressure.
    The Review of scientific instruments 10/2012; 83(10):103902. · 1.52 Impact Factor
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    ABSTRACT: We use high-pressure magnetic x-ray diffraction and numerical simulation to determine the low-temperature magnetic phase diagram of stoichiometric CeFe2. Near 1.5 GPa we find a transition from ferromagnetism to antiferromagnetism, accompanied by a rhombohedral distortion of the cubic Laves crystal lattice. By comparing pressure and chemical substitution we find that the phase transition is controlled by a shift of magnetic frustration from the Ce-Ce to the Fe-Fe sublattice. Notably the dominant Ce-Fe magnetic interaction, which sets the temperature scale for the onset of long-range order, remains satisfied throughout the phase diagram but does not determine the magnetic ground state. Our results illustrate the complexity of a system with multiple competing magnetic energy scales and lead to a general model for magnetism in cubic Laves phase intermetallic compounds.
    Physical Review B 07/2012; 86(1):014422. · 3.77 Impact Factor
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    ABSTRACT: Samarium nickelate (SmNiO3) is a correlated oxide that exhibits a metal–insulator transition (MIT) above room temperature and is of interest for advanced electronics and optoelectronics. However, studies on SmNiO3 thin films have been limited to date, in part due to well-known difficulties in stabilizing the Ni3+ valence state during growth, which are manifested in non-reproducible electrical characteristics. In this work, we show that stable epitaxial SmNiO3 thin films can be grown by rf magnetron sputtering without extreme post-deposition annealing conditions using relatively high growth pressure (>200 mTorr). At low growth pressure, SmNiO3 is insulating and undergoes an irreversible MIT at ∼430 K. As pressure is increased, films become metallic across a large temperature range from 100 to 420 K. At high pressure, films are insulating again but with a reversible and stable MIT at ∼400 K. Phase transition properties can be continuously tuned by control of the sputtering pressure.
    Journal of Solid State Chemistry 06/2012; 190:233–237. · 2.04 Impact Factor
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    ABSTRACT: Quantum criticality is a central concept in condensed matter physics, but the direct observation of quantum critical fluctuations has remained elusive. Here we present an X-ray diffraction study of the charge density wave (CDW) in 2H-NbSe(2) at high pressure and low temperature, where we observe a broad regime of order parameter fluctuations that are controlled by proximity to a quantum critical point. X-rays can track the CDW despite the fact that the quantum critical regime is shrouded inside a superconducting phase; and in contrast to transport probes, allow direct measurement of the critical fluctuations of the charge order. Concurrent measurements of the crystal lattice point to a critical transition that is continuous in nature. Our results confirm the long-standing expectations of enhanced quantum fluctuations in low-dimensional systems, and may help to constrain theories of the quantum critical Fermi surface.
    Proceedings of the National Academy of Sciences 04/2012; 109(19):7224-9. · 9.74 Impact Factor
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    ABSTRACT: CeFe2 is a ferromagnet that exhibits antiferromagnetic fluctuations in its ground state. We use x-ray diffraction and diamond-anvil-cell techniques to directly measure the transition to antiferromagnetism in pure CeFe2 at high pressure which couples to the change in the lattice symmetry. Numerical simulations are adopted to identify the magnetic structure of the ground states and to quantitatively illustrate effects of competing magnetic energy scales and geometrical frustration on the magnetic phase diagram. Comparison of phase transitions under both chemical substitution and applied pressure suggests a general solution to the physics of Laves phase magnets.
    02/2012;
  • Rafael Jaramillo, Shriram Ramanathan
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    ABSTRACT: Better understanding and control of band alignment in oxide-semiconductor heterostructures is essential for improving the performance of devices such as sensitized solar cells and quantum dot based light emitting devices. We will present studies of Schottky junctions formed between Al-doped ZnO (AZO) conducting oxide thin films and lightly doped silicon. AZO films with varying oxygen content have been synthesized by control of oxygen pressure during growth. Transport measurements (I-V and C-V) on devices are used to illustrate the degree to which the oxide stoichiometry can be used to engineer the junction characteristics.
    02/2012;
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    ABSTRACT: We present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor.
    Applied Physics Letters 10/2011; 99(15):151910-151910-3. · 3.79 Impact Factor
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    ABSTRACT: CeFe$_2$ is a geometrically frustrated ferromagnet that lies close to an instability at which a subtle change in the lattice symmetry couples to a transition to antiferromagnetism. We use x-ray diffraction, diamond-anvil-cell techniques, and numerical simulation to identify the ground states and to quantitatively illustrate effects of competing magnetic energy scales and geometrical frustration on the magnetic phase diagram. Comparison of phase transitions under both chemical substitution and applied pressure suggests a general solution to the physics of pyrochlore rare earth inter-metallic magnets.
    09/2011;
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    Advanced Materials 09/2011; 23(39):4521-5. · 14.83 Impact Factor
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    ABSTRACT: CeFe2 is a ferromagnet that exhibits antiferromagnetic fluctuations in its ground state at low temperature. We use x-ray diffraction to measure directly the emergence of antiferromagnetic order in pure CeFe2 at high pressure. We present an analysis of both the magnetic and lattice symmetries in the newly discovered high pressure phase, and compare our results to those from doped CeFe2 systems. This comparison provides insights into the roles of pressure and chemical doping in driving the magnetic quantum phase transition.
    03/2011;
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    ABSTRACT: 2H-NbSe2 is the archetypical two-dimensional charge-density-wave system. Using x-ray diffraction in a diamond anvil cell, we track the evolution of the CDW order towards the buried quantum critical point inside the superconducting phase. We observe a pressure-dependent nesting vector as well as fluctuation broadening, and compare these results to the behavior of the three-dimensional spin-density-wave system, Chromium, at its quantum critical point.
    03/2011;
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    ABSTRACT: Among materials displaying charge density wave order, NbSe2 stands out because its ordering vector does not correspond to any obvious nesting properties of its Fermi surface or band structure. The well known Peierls mechanism is thus less effective in singling out an ordering vector for NbSe2, and the transition is driven instead by an increase of the susceptibility over a wide range of wave numbers. As the CDW transition is suppressed towards zero temperature, such a broad susceptibility gives rise to quantum fluctuations with an equally broad span in wavelengths. Here, we examine the role of these quantum fluctuations as the critical point is approached. We compare our theoretical findings to recent measurements of the ordering wave vector of NbSe2 under pressure and show that its properties can be understood as arising from the combined effect of the presence of quantum fluctuations and the coupling of the CDW order parameter to the lattice.
    03/2011;
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    ABSTRACT: The electrical properties of grain boundaries in technologically relevant oxide thin films are the subject of both applied and fundamental research. Here we present an investigation of the local density of states (LDOS) in sputtered Al-doped ZnO using a scanning tunneling microscope. We observe a pronounced difference in the tunneling conductivity recorded on- and off-grain, with the grain boundary LDOS peaked ~600 meV below the Fermi level. This provides a direct measurement of the distribution of charge traps that is of relevance in advancing understanding of carrier conduction in this transparent conducting oxide.
    Physical review. B, Condensed matter 02/2011; · 3.77 Impact Factor
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    ABSTRACT: We use synchrotron x-ray diffraction and electrical transport under pressure to probe both the magnetism and the structure of single crystal NiS2 across its Mott-Hubbard transition. In the insulator, the low-temperature antiferromagnetic order results from superexchange among correlated electrons and couples to a (1/2, 1/2, 1/2) superlattice distortion. Applying pressure suppresses the insulating state, but enhances the magnetism as the superexchange increases with decreasing lattice constant. By comparing our results under pressure to previous studies of doped crystals we show that this dependence of the magnetism on the lattice constant is consistent for both band broadening and band filling. In the high pressure metallic phase the lattice symmetry is reduced from cubic to monoclinic, pointing to the primary influence of charge correlations at the transition. There exists a wide regime of phase separation that may be a general characteristic of correlated quantum matter.
    Physical Review B 01/2011; 83:035106. · 3.77 Impact Factor
  • Rafael Jaramillo, Shriram Ramanathan
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    ABSTRACT: The ability to controllably tune the work function of transparent conductors is essential for optimizing the efficiency of thin film solar photovoltaics. Here we use Kelvin force microscopy on lithographically patterned ZnO:Al thin films to measure the dependence of the work function on oxygen content during synthesis. Films were synthesized by low temperature reactive sputtering from a Zn:Al 1.2 wt% target. At optimal oxygen content the resistivity is 3×10−4 Ω cm with nearly 90% optical transmission in the 400–1100 nm wavelength range relevant to solar photovoltaics. We find that the expected relationship between band filling and work function breaks down in the vicinity of optimal oxygen stoichiometry. For oxygen-rich ZnO:Al films we measure large work functions close to 5 eV. Our results suggest a method for fabricating transparent oxide electron conductors with large, tunable work functions that could be of relevance in designing electrodes for solid state energy conversion technologies.
    Solar Energy Materials and Solar Cells. 01/2011;
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    ABSTRACT: The elemental antiferromagnet Cr at high pressure presents a new type of naked quantum critical point that is free of disorder and symmetry-breaking fields. Here we measure magnetotransport in fine detail around the critical pressure, Pc approximately 10 GPa, in a diamond anvil cell and reveal the role of quantum critical fluctuations at the phase transition. As the magnetism disappears and T-->0, the magnetotransport scaling converges to a non-mean-field form that illustrates the reconstruction of the magnetic Fermi surface, and is distinct from the critical scaling measured in chemically disordered CrV under pressure. The breakdown of itinerant antiferromagnetism only comes clearly into view in the clean limit, establishing disorder as a relevant variable at a quantum phase transition.
    Proceedings of the National Academy of Sciences 08/2010; 107(31):13631-5. · 9.74 Impact Factor

Publication Stats

135 Citations
158.83 Total Impact Points

Institutions

  • 2010–2013
    • Harvard University
      • School of Engineering and Applied Sciences
      Cambridge, Massachusetts, United States
  • 2012
    • Argonne National Laboratory
      Lemont, Illinois, United States
  • 2005–2009
    • University of Chicago
      • James Franck Institute
      Chicago, IL, United States