C. J. Palmstrøm

University of California, Santa Barbara, Santa Barbara, California, United States

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Publications (258)478.23 Total impact

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    ABSTRACT: We demonstrate fast universal electrical spin manipulation with inhomogeneous magnetic fields. With fast Rabi frequency up to 127 MHz, we leave the conventional regime of strong nuclear-spin influence and observe a spin-flip fidelity > 96%, a distinct chevron Rabi pattern in the spectral-time domain, and spin resonance linewidth limited by the Rabi frequency, not by the dephasing rate. In addition, we establish fast z-rotations up to 54 MHz by directly controlling the spin phase. Our findings will significantly facilitate tomography and error correction with electron spins in quantum dots.
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    ABSTRACT: The recent rapid progress in the field of spintronics requires extensive studies of carrier and spin relaxation dynamics in semiconductors. In this work, we employed time and spin resolved differential transmission measurements in order to probe carrier and spin relaxation times in several InAsP ternary alloys. In addition, the dynamics of the excitonic radiative transitions of InAs0.13P0.87 epitaxial layer were studied through the time-resolved photoluminescence spectroscopy.
    SPIE NanoScience + Engineering; 08/2014
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    ABSTRACT: We observe a dc voltage peak at ferromagnetic resonance (FMR) in samples consisting of a single ferromagnetic (FM) layer grown epitaxially on the $\mathit{n-}$GaAs (001) surface. The FMR peak is detected as an interfacial voltage with a symmetric line shape and is present in samples based on various FM/$n$-GaAs hetrostructures, including Co$_{2}$MnSi/$n$-GaAs, Co$_{2}$FeSi/$n$-GaAs and Fe/$n$-GaAs. We show that the interface bias voltage dependence of the FMR signal is identical to that of the tunneling anisotropic magnetoresistance (TAMR) over most of the bias range. Furthermore, we show how the precessing magnetization yields a dc FMR signal through the TAMR effect and how the TAMR phenomenon can be used to predict the angular dependence of the FMR signal. This TAMR-induced FMR peak can be observed under conditions where no spin accumulation is present and no spin-polarized current flows in the semiconductor.
    Applied Physics Letters 08/2014; 105(21). DOI:10.1063/1.4902088 · 3.52 Impact Factor
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    J. Shabani, A. P. McFadden, B. Shojaei, C. J. Palmstrøm
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    ABSTRACT: We investigate the performance of gate-defined devices fabricated on high mobility InAs heterostructures. The magnetotransport properties of these structures strongly depend on the surface states. Heterostructures terminated in In$_{0.75}$Ga$_{0.25}$As are found to host a conductive sheet of electrons depending on details of epitaxial growth or deposition of the dielectric material and limit the performance of gate-defined devices. However, In$_{0.75}$Al$_{0.25}$As capped heterostructures show a robust gating behavior suitable for fabrication of gate-defined mesoscopic devices.
    Applied Physics Letters 08/2014; 105(26). DOI:10.1063/1.4905370 · 3.52 Impact Factor
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    ABSTRACT: Spin-orbit coupling in solids describes an interaction between an electron's spin, an internal quantum-mechanical degree of freedom, with its linear momentum, an external property. Spin-orbit interaction, due to its relativistic nature, is typically small in solids, and is often taken into account perturbatively. It has been recently realized, however, that materials with strong spin-orbit coupling can lead to novel states of matter such as topological insulators and superconductors. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing. In particular, theory predicts that narrow band gap semiconductors with strong spin-obit coupling are a suitable platform for the realization of Majorana zero-energy modes, predicted to obey exotic non-Abelian braiding statistics. The pursuit for realizing Majorana modes in condensed matter systems and investigating their exotic properties has been a subject of intensive experimental research recently. Here, we demonstrate the first realization of gate-defined wires where one-dimensional confinement is created using electrostatic potentials, on large area InAs two dimensional electron systems (2DESs). The electronic properties of the parent 2DES are fully characterized in the region that wires are formed. The strength of the spin-orbit interaction has been measured and tuned while the high mobility of the 2DES is maintained in the wire. We show that this scheme could provide new prospective solutions for scalable and complex wire networks.
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    J. Shabani, S. Das Sarma, C. J. Palmstrøm
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    ABSTRACT: We report on the first experimental observation of an apparent metal insulator transition in a 2D electron gas confined in an InAs quantum well. At high densities we find that the carrier mobility is limited by background charged impurities and the temperature dependence of the resistivity shows a metallic behavior with resistivity increasing with increasing temperature. At low densities we find an insulating behavior below a critical density of $n_{c} = 3.5 \times 10^{10}$ cm$^{-2}$ with the resistivity decreasing with increasing temperature. We analyze this transition using a percolation model arising from the failure of screening in random background charged impurities. We also examine the percolation transition experimentally by introducing remote ionized impurities at the surface. Using a bias during cool-down, we modify the screening charge at the surface which strongly affects the critical density. Our study shows that transition from a metallic to an insulating phase in our system is due to percolation transition.
    Physical Review B 07/2014; 90(16). DOI:10.1103/PhysRevB.90.161303 · 3.66 Impact Factor
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    ABSTRACT: We investigate the dynamicaly polarized nuclear-spin system in Fe/n-GaAs heterostructures via the response of the electron-spin system to nuclear magnetic resonance (NMR) in lateral spin-valve devices. The hyperfine interaction is known to act more strongly for donor-bound electrons than for those in the conduction band. We provide a quantitative model of the temperature dependence of the occupation of these donors. With this model we calculate the ratios of the hyperfine and quadrupolar nuclear relaxation rates of each isotope. For all temperatures measured, quadrupolar relaxation limits the spatial extent of nuclear spin-polarization to within a Bohr radius of the donor sites, and is directly responsible for the isotope dependence of the measured NMR signal size. The hyperfine interaction is also responsible for the 2 kHz Knight shift of the nuclear resonance frequency that is measured as a function of the electron spin accumulation. The Knight shift is shown to be an absolute measure of the electron spin-polarization that agrees with standard spin transport measurements.
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    ABSTRACT: The excitonic radiative transitions of InAsxP1−x (x = 0.13 and x = 0.40) alloy epitaxial layers were studied through magnetic field and temperature dependent photoluminescence and time-resolved photoluminescence spectroscopy. While the linewidth and lineshape of the exciton transition for x = 0.40 indicate the presence of alloy broadening due to random anion distribution and the existence of localized exciton states, those of x = 0.13 suggest that this type of compositional disorder is absent in x = 0.13. This localization is further supported by the behavior of the exciton transitions at low temperature and high magnetic fields. InAs0.4P0.6 exhibits anomalous “S-shaped” temperature dependence of the excition emission peak below 100 K as well as linewidth broadening at high magnetic fields due to the compression of the excitonic volume amid compositional fluctuations. Finally, photoluminescence decay patterns suggest that the excitons radiatively relax through two channels, a fast and a slow decay. While the lifetime of the fast decay is comparable for both compositions (∼30 ps), that of the slow decay increases from 206 ps to 427 ps as x increases from 0.13 to 0.40, attributable to carrier migration between the localization states of InAs0.4P0.6.
    Journal of Applied Physics 05/2014; 115(19):193503-193503-8. DOI:10.1063/1.4876121 · 2.19 Impact Factor
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    ABSTRACT: The surface and electronic structure of single crystal thin films of PtLuSb (001) grown by molecular beam epitaxy were studied. Scanning tunneling spectroscopy (STS), photoemission spectroscopy, and temperature dependent Hall measurements of PtLuSb thin films are consistent with a zero-gap semiconductor or semi-metal. STS and photoemission measurements show a decrease in density of states approaching the Fermi level for both valence and conduction bands as well as a slight shift of the Fermi level position into the valence band. Temperature dependent Hall measurements also corroborate the Fermi level position by measurement of p-type carriers.
    Applied Physics Letters 05/2014; 104(20):201603-201603-4. DOI:10.1063/1.4879475 · 3.52 Impact Factor
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    ABSTRACT: We report on a method of quantifying spin accumulation in Co$_{2}$MnSi/$n$-GaAs and Fe/$n$-GaAs heterostructures using a non-magnetic probe. In the presence of a large non-equilibrium spin polarization, the combination of a non-constant density of states and energy-dependent conductivity generates an electromotive force (EMF). We demonstrate that this signal dephases in the presence of applied and hyperfine fields, scales quadratically with the polarization, and is comparable in magnitude to the spin-splitting. Since this spin-generated EMF depends only on experimentally accessible parameters of the bulk material, its magnitude may be used to quantify the injected spin polarization in absolute terms.
  • A. J. Young, B. D. Schultz, C. J. Palmstrøm
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    ABSTRACT: Epitaxial single crystal nanocomposites comprised of rare-earth arsenide nanoparticles embedded in GaAs (001) layers produce a larger change in lattice parameter than expected from the lattice parameters of relaxed films. Despite similar cubic structures and lattice parameters, elongation of the interfacial bond length between the two materials induces additional strain causing an expansion in the nanocomposite lattice. The interface bond length is material dependent with an average atomic layer spacing at the ErAs:GaAs interface of 1.9 Å while the spacing at the ScAs:GaAs interface is only 1.4 Å. Implications for lattice matching various single crystal epitaxial nanostructures in semiconductors are discussed.
    Applied Physics Letters 01/2014; 104(7). DOI:10.1063/1.4865905 · 3.52 Impact Factor
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    ABSTRACT: We demonstrate the integration of the lattice matched single crystal epitaxial Half Heusler compound CoTiSb with In0.52Al0.48As/InP(001) heterostructures using molecular beam epitaxy. CoTiSb belongs to the subset of Half Heusler compounds that is expected to be semiconducting, despite being composed entirely of metallic constituents. The lattice matching and epitaxial alignment of the CoTiSb films were confirmed by reflection high energy electron diffraction and X-ray diffraction. Temperature dependent transport measurements indicate semiconducting-like behavior, with a room temperature Hall mobility of 530 cm2/Vs and background Hall carrier density of 9.0 × 1017 cm−3, which is comparable to n-Si with similar carrier density. Below 100 K, the films show a large negative magnetoresistance, and possible origins of this negative magnetoresistance are discussed.
    Applied Physics Letters 01/2014; 104(2):022109-022109-5. DOI:10.1063/1.4862191 · 3.52 Impact Factor
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    ABSTRACT: We present a systematic study of the properties of room temperature deposited TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti1-xNx (x = 0.5) without substrate heating, the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive in-plane strain changes to weak tensile in-plane strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target-substrate distance set to 88 mm the contaminant levels increase from ˜0.1% to ˜10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target-substrate distance is increased from 88 to 266 mm. These contaminants are found to strongly influence the properties of TiN thin films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results strongly suggest that the energy of the sputtered TiN particles plays a crucial role in determining the TiN film properties, and that it is important to precisely control the energy of these particles to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 to 7 mTorr show a substantial increase in intrinsic quality factor from ˜104 to ˜106 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. Surprisingly, the films with a higher oxygen content exhibit lower loss, but care must be taken when depositing at room temperature to avoid nonuniform oxygen incorporation, which presents as a radially dependent resistivity and becomes a radially dependent surface inductance in the superconductor.
    Superconductor Science and Technology 01/2014; 27(1):5009-. DOI:10.1088/0953-2048/27/1/015009 · 2.80 Impact Factor
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    ABSTRACT: The Half Heuslers are currently an attractive family of compounds for high temperature thermoelectrics research, and recently, there has been renewed interest since some of these compounds are proposed to be topological insulators. NiTiSn belongs to the family of 18 valence electron Half Heuslers that are predicted to be semiconducting, despite being composed entirely of metallic elements. The growth of the Half Heusler compound NiTiSn by molecular beam epitaxy is demonstrated. The NiTiSn films are epitaxial and single crystalline as observed by reflection high-energy electron diffraction and x-ray diffraction. Temperature dependent transport measurements suggest the films may be semiconducting, but with a high background carrier density indicative of a high density of electrically active defect states. Methods of protecting the sample surface for synchrotron-based photoemission measurements are explored. These methods may be applied to the study of surface electronic structure in unconventional materials.
    Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 07/2013; 31(4-4):04D106. DOI:10.1116/1.4807715 · 1.36 Impact Factor
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    ABSTRACT: We present a systematic study of the properties of TiN films by varying the deposition conditions in an ultra-high-vacuum reactive magnetron sputtering chamber. By increasing the deposition pressure from 2 to 9 mTorr while keeping a nearly stoichiometric composition of Ti(1-x)N(x) (x=0.5), the film resistivity increases, the dominant crystal orientation changes from (100) to (111), grain boundaries become clearer, and the strong compressive strain changes to weak tensile strain. The TiN films absorb a high concentration of contaminants including hydrogen, carbon, and oxygen when they are exposed to air after deposition. With the target-substrate distance set to 88 mm the contaminant levels increase from ~0.1% to ~10% as the pressure is increased from 2 to 9 mTorr. The contaminant concentrations also correlate with in-plane distance from the center of the substrate and increase by roughly two orders of magnitude as the target-substrate distance is increased from 88 mm to 266 mm. These contaminants are found to strongly influence the properties of TiN films. For instance, the resistivity of stoichiometric films increases by around a factor of 5 as the oxygen content increases from 0.1% to 11%. These results suggest that the sputtered TiN particle energy is critical in determining the TiN film properties, and that it is important to control this energy to obtain high-quality TiN films. Superconducting coplanar waveguide resonators made from a series of nearly stoichiometric films grown at pressures from 2 mTorr to 7 mTorr show an increase in intrinsic quality factor from ~10^4 to ~10^6 as the magnitude of the compressive strain decreases from nearly 3800 MPa to approximately 150 MPa and the oxygen content increases from 0.1% to 8%. The films with a higher oxygen content exhibit lower loss, but the nonuniformity of the oxygen incorporation hinders the use of sputtered TiN in larger circuits.
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    ABSTRACT: The recent rapid progress in the field of spintronics involves extensive measurements of carrier and spin relaxation dynamics in III-V semiconductors. In addition, as the switching rates in devices are pushed to higher frequencies, it is important to understand carrier dynamic phenomena in semiconductors on femtosecond time-scales. In this work, we employed time and spin resolved differential transmission measurements; to probe carrier and spin relaxation times in several InAsP ternary alloys. Our results demonstrate the sensitivity of the spin and carrier dynamics in this material system to the excitation wavelengths, the As concentrations, and temperature.
    Applied Physics Letters 06/2013; 102(22). DOI:10.1063/1.4808346 · 3.52 Impact Factor
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    ABSTRACT: Arrays of metallic nanostructures embedded within a semiconducting matrix are of great interest for applications in plasmonics, photonic crystals, thermoelectrics, and nanoscale ohmic contacts. We report a method for growing single crystal arrays of semimetallic vertical and horizontal ErSb nanorods, nanotrees, and nanosheets embedded within a semiconducting GaSb matrix. The nanostructures form simultaneously with the matrix and have epitaxial, coherent interfaces with no evidence of stacking faults or dislocations as observed by high-resolution transmission electron microscopy. By combining molecular beam epitaxy growth and in situ scanning tunneling microscopy, we image the growth surface one atomic layer at a time and show that the nanostructured composites form via a surface-mediated self-assembly mechanism that is controlled entirely at the growth front and is not a product of bulk diffusion or bulk segregation. These highly tunable nanocomposites show promise for direct integration into epitaxial semiconductor device structures and also provide a unique system in which to study the atomic scale mechanisms for nucleation and growth.
    Nano Letters 05/2013; 13(6). DOI:10.1021/nl4012563 · 12.94 Impact Factor
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    ABSTRACT: The Heusler alloy Co2MnSi is predicted to be a half-metal, a material that is spin-polarized at the Fermi energy. We have demonstrated growth by molecular beam epitaxy of Co2MnSi, Cr/Co2MnSi, and a complete Co2MnSi/MgO/Co2MnSi(001) magnetic tunnel junction on epitaxial GaAs(001) surfaces without air exposure. Epitaxial Cr layers have been used to exchange bias Co2MnSi. In-situ electron diffraction and scanning tunneling microscopy, and ex-situ X-ray diffraction techniques are used to characterize the crystal quality. The magnetic properties are investigated using vibrating sample and superconducting quantum interference device magnetometry. We present these results and will compare them with temperature dependent magnetotransport and tunneling spectroscopy measurements, with emphasis on the influence of Co2MnSi surface termination.
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    ABSTRACT: We present an investigation of the magnetic properties of Co2MnSi films grown by molecular beam epitaxy on lattice matched Sc0.3Er0.7As films grown on GaAs (001) substrates with various capping layers (Cr, Al, Au). Co2MnSi thickness varied from 3 to 21.4 nm. X-ray diffraction analysis confirmed the single crystal nature and crystallographic orientations of the films. Magnetization measurements reveal square loops with low in-plane saturation fields and very narrow (few Oe) coercive fields. An interesting feature of the loops in several of the samples is the presence of a small (<10 Oe) exchange-bias field observed at room temperature. Room temperature ferromagnetic resonance (FMR) measurements were carried out at 35 GHz as a function of in-plane angle to quantify the anisotropy in these structures. Resonances follow the typical derivative lineshape with relatively narrow line widths ranging from 30 to 140 Oe, consistent with high quality Heusler alloy film formation. Four-fold anisotropy is clearly observed in all samples confirming the high quality single-crystal nature of the films. A small unidirectional anisotropy associated with the exchange bias mentioned above is also observed. We will also present results on preliminary MJT structures.
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    ABSTRACT: Superconducting coplanar waveguide (CPW) resonators are widely used structures in the fields of photon detection and quantum information processing. Recently, there has been a growing interest in titanium nitride (TiN) thin films due to their widely tunable critical temperature, large surface inductance, and ability to produce high intrinsic quality factor (Qi) resonators. We have deposited nearly stoichiometric TiN films on Si substrates by reactive magnetron sputtering. By increasing the deposition pressure and adjusting the N2 flow rate to maintain stoichiometry, the film stress was changed from ˜100 MPa to >3000 MPa and the Qi of CPW resonators made from these films increased from ˜10^4 to ˜10^6 for single photon excitations measured at ˜100 mK. In this talk, we discuss relationships between deposition parameters, film properties, and microwave electrodynamic responses in these resonators.

Publication Stats

3k Citations
478.23 Total Impact Points

Institutions

  • 2008–2014
    • University of California, Santa Barbara
      • Department of Electrical and Computer Engineering
      Santa Barbara, California, United States
  • 1992–2011
    • University of Minnesota Duluth
      • Department of Mechanical and Industrial Engineering
      Duluth, Minnesota, United States
  • 2009
    • University of the Pacific (California - USA)
      Stockton, California, United States
  • 2006
    • National High Magnetic Field Laboratory
      Tallahassee, Florida, United States
  • 2003
    • Technische Universität Berlin
      Berlín, Berlin, Germany
  • 1996
    • Texas A&M University
      • Department of Electrical and Computer Engineering
      College Station, TX, United States
    • Arizona State University
      Phoenix, Arizona, United States
  • 1995
    • Office of Naval Research
      Arlington, Virginia, United States
  • 1993
    • Princeton University
      • Department of Electrical Engineering
      Princeton, NJ, United States
  • 1983–1990
    • Cornell University
      • Department of Materials Science and Engineering
      Итак, New York, United States