Showing papers by "John B Ketterson published in 2002"
TL;DR: In this paper, the lattice constant increases linearly with Mn concentration due to the larger Mn atomic radius compared with Ge, strongly indicating that Mn ions are being incorporated into the host Ge lattice.
Abstract: We have successfully fabricated highly (up to 6%) Mn-doped bulk Ge single crystals. The lattice constant increases linearly with Mn concentration due to the larger Mn atomic radius compared with Ge, strongly indicating that Mn ions are being incorporated into the host Ge lattice. Alloys with lower Mn concentrations showed paramagnetism due to localized magnetic ions. ${\mathrm{Ge}}_{0.94}{\mathrm{Mn}}_{0.06}$ showed ferromagnetic ordering at \ensuremath{\sim}285 K, as determined from temperature-dependent magnetization and resistance measurements. The coersive field was 1260 Oe at 250 K.
253 citations
TL;DR: In this article, the discovery of a room-temperature ferromagnetic semiconductor in chalcopyrite was reported, which is consistent with the predictions of full-potential linearized augmented plane wave total energy calculations.
Abstract: We report on the discovery of a room-temperature ferromagnetic semiconductor in chalcopyrite $({\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}){\mathrm{GeP}}_{2}$ with ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}312\mathrm{K}$. We have also observed that, at temperatures below 47 K, samples for $x\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.056$ and 0.2 show a transition to the antiferromagnetic (AFM) state, so that ferromagnetism is well defined to be present between 47 and 312 K. The observation that the AFM phase is most stable at low temperatures is consistent with the predictions of full-potential linearized augmented plane wave total energy calculations and has consequences for other chalcopyrite materials.
141 citations
TL;DR: In this paper, the authors have successfully grown Mn-doped chalcopyrite ZnSnAs 2 single crystals from the melt using the vertical-temperature-gradient Bridgman method.
119 citations
TL;DR: In this article, structural and transport properties of Sb2Te3 thin films prepared by molecular beam epitaxy as a function of the Te/Sb flux ratio during deposition were studied.
Abstract: We have studied the structural and transport properties of Sb2Te3 thin films prepared by molecular beam epitaxy as a function of the Te/Sb flux ratio during deposition Both the crystallinity and the transport properties are found to be strongly affected by nonstoichiometry The most stoichiometric sample (prepared with a Te/Sb ratio of 36) had a high degree of crystallinity, high thermopower, and high carrier mobility However, Sb2Te3 films with excess Sb or Te had poorer crystallinity, reduced magnitude of the thermopower, and reduced mobility as a result of the formation of antisite defects These antisite defects were able to be reduced by controlling the relative flow rate ratio of Te to Sb during growth
101 citations
TL;DR: In this article, the measurements of the ferromagnetic resonance spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography are reported.
Abstract: We report on the measurements of the ferromagnetic resonance spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography. in contrast to plane ferromagnetic films, the spectra of the arrays show a number of additional resonance peaks, whose position depends strongly on the orientation of the external magnetic field and the interparticle interaction. Time-dependent micromagnetic simulations of the ac response show that these peaks are associated with coupled exchange and dipolar spin-wave modes.
100 citations
TL;DR: In this paper, the second-order susceptibilities in ZnO films have been studied systemically and it was observed that the second order susceptibilities for ZnOs depend on the thickness but not on the film growth technique used (reactive sputtering or plasma enhanced chemical vapor deposition).
Abstract: The second-order susceptibilities in the ZnO films have been studied systemically. In very thin films, we observe an enhancement of second-order susceptibilities, which is larger than that of single-crystal ZnO. It was also observed that the second-order susceptibilities for ZnO films depend on the thickness but not on the film growth technique used (reactive sputtering or plasma-enhanced chemical vapor deposition). We suggest a mechanism to explain the reduction in the second-order susceptibilities of the thicker films.
83 citations
TL;DR: In this article, the authors show that the primary contribution to the energy of the spin-wave modes with absorption peaks at magnetic fields smaller than the uniform mode peak is from exchange interactions, whereas the secondary contribution to energy of spinwave modes above the uniform resonance mode arises from dipolar interactions, although all spin wave modes have some contributions from both types of interactions.
Abstract: Micromagnetic simulations of the ferromagnetic resonance (FMR) spectrum of submicron circular ferromagnetic particles show a number of well-defined absorption peaks as a function of the applied dc magnetic field. In addition to a single large peak due to the uniform mode of precession, smaller peaks are observed on both sides of the uniform mode peak, corresponding to nonuniform or spin-wave modes of precession. The positions of these peaks are a function of particle size. Simulations of the FMR spectrum with and without exchange interactions show that the primary contribution to the energy of the spin-wave modes with absorption peaks at magnetic fields smaller than the uniform mode peak is from exchange interactions, whereas the primary contribution to the energy of the spin-wave modes above the uniform resonance mode arises from dipolar interactions, although all spin-wave modes have some contributions from both types of interactions.
47 citations
TL;DR: In this article, a Cr-doped bulk Ge single crystal using the vertical gradient solidification method was fabricated and the material showed ferromagnetic ordering at 126 K, as determined from temperature-dependent magnetization and resistance measurements.
Abstract: We have fabricated Cr-doped bulk Ge single crystal using the vertical gradient solidification method. The material shows ferromagnetic ordering at 126 K, as determined from temperature-dependent magnetization and resistance measurements. A sample with x=0.01 was p type with nh=3×1017 cm−3 at 350 K. The measured magnetic moment per Cr was 0.83μB at 5 K.
45 citations
TL;DR: In this paper, the optical properties of epitaxial orthorhomic cuprous oxide films grown on MgO@110# were investigated and the observed splitting of the energy levels was used to calculate the coherency strain based on an earlier-parametrized theory for the level splitting by Waters et al.
Abstract: We investigated the optical properties of epitaxial orthorhomic cuprous oxide films grown on MgO@110#. Absorption measurements show clear excitonic peaks up to n55p. Photoluminescence at 2 K shows several sharp emission peaks in the vicinity of 610 nm associated with a splitting of various 1 s orthoexciton energy levels. The evolution of the peaks with temperature indicates that three peaks at higher energy are due to direct recombination of 1s yellow orthoexcitons and three corresponding peaks at lower energy are their phonon replicas. The symmetry of each level is identified by the polarization properties of their photoluminescence emissions. The observed splitting of the energy levels is used to calculate the coherency strain based on an earlier-parametrized theory for the level splitting by Waters et al. based on the known symmetry of the electronic states. This predicted strain is compared with that determined by x-ray diffraction measurements of the measured lattice parameters.
37 citations
01 Mar 2002
TL;DR: In this article, the measurements of the ferromagnetic resonance spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography are reported.
Abstract: We report on the measurements of the ferromagnetic resonance spectra of arrays of submicron size periodic particle arrays of permalloy produced by electron-beam lithography. In contrast to plane ferromagnetic films, the spectra of the arrays show a number of additional resonance peaks, whose position depends strongly on the orientation of the external magnetic field and the interparticle interaction. Time-dependent micromagnetic simulations of the ac response show that these peaks are associated with coupled exchange and dipolar spin-wave modes.
21 citations
TL;DR: In this paper, a qubit system exploiting manipulations of the Andreev bound state (ABS) levels in the SINIS junction by applying appropriate bias voltages and transport currents is suggested.
TL;DR: Fits to the data provide a simple relation between Delta(T) and the collective mode frequency, and Resonant absorption into a collective mode, with energy approximately proportional to the superconducting gap, is proposed as a possible explanation.
Abstract: We present results of a microwave surface impedance study of the heavy fermion superconductor UBe(13). We clearly observe an absorption peak whose frequency and temperature dependence scales with the BCS gap function Delta(T). Resonant absorption into a collective mode, with energy approximately proportional to the superconducting gap, is proposed as a possible explanation. Fits to the data provide a simple relation between Delta(T) and the collective mode frequency.
TL;DR: In this article, a large increase in the magnetoresistance of polycrystalline and epitaxial thin Bi films, which were subjected to a post-annealing procedure at 3°C below the Bi melting point, was observed.
TL;DR: In this paper, the authors demonstrate that an anomalous field enhancement occurs in the vicinity of contact points between two plasmon-resonant surfaces, and that the enhancement is caused by the localization of surface plasm polaritons in vicinity of the contact point brought on by a decrease of the wavelength and an enhancement of the field of plasmons propagating toward the tip.
Abstract: We demonstrate, both theoretically and experimentally, that an anomalous field enhancement occurs in the vicinity of contact points between two plasmon-resonant surfaces. The theoretical model involves two spheres in contact, and the enhancement is shown to result from the localization of surface plasmon polaritons in the vicinity of the contact point brought on by a decrease of the wavelength and an enhancement of the field of plasmons propagating toward the tip. This model is applied to experiments involving silver films having mesoscopic surface protrusions which are irradiated by intense laser pulses: films in close contact (where the protrusions touch) suffer laser-induced damage which is absent in separately irradiated films. This is interpreted as arising from the additional field enhancement, relative to the well-known enhancement occurring at isolated silver surface protrusions, brought on by the contacts.
TL;DR: In this paper, a diffraction grating is created by the interference of two coherent infrared beams from a pulsed yttritium-aluminum-garnet laser.
Abstract: Thin films consisting of self-assembled chromophoric superlattices exhibit very large second-order nonlinear responses [χ(2)]. Using such films, a “static” diffraction grating is created by the interference of two coherent infrared beams from a pulsed yttritium–aluminum–garnet laser. This grating is used to switch the second-harmonic and third-harmonic “signal” beams (generated from the fundamental “pump” beam or mixed within the chromophoric superlattice) into different channels (directions). Ultrafast switching response as a function of the time overlap of the pumping beams is demonstrated. It is suggested that such devices can be used to spatially and temporally separate signal trains consisting of pulses having different frequencies and arrival times.
TL;DR: In this paper, the authors have observed different Bi surface structures on polar InSb(111)A and B faces as well as thickness dependent surface structure on the A face.
Abstract: Bi thin films have been grown on InSb(111)A/B substrates using molecular beam epitaxy. We have observed different Bi atomic surface structures on polar InSb(111)A and B faces as well as thickness dependent surface structures on the A face. Bi deposited on InSb(111)B shows a (1×1) surface structure, i.e., no surface reconstruction. However, Bi on InSb(111)A shows a (2×2) structure for thicknesses less than 16 A, thereafter changing to a (1×1) structure. On both surfaces, Bi grows in a layer-by-layer growth mode with high crystallinity and sharp interfaces.
TL;DR: In this article, the ultrasonic properties of a URu 2 Si 2 single crystal were measured in pulsed (25-ms) magnetic fields up to 50 T. In the frequency range from 20 to 200 MHz, both the attenuation and velocity of sound change markedly in the vicinity of the metamagnetic transitions (near 40 T).
Abstract: The ultrasonic properties of a URu 2 Si 2 single crystal were measured in pulsed (25 ms) magnetic fields up to 50 T. In the frequency range from 20 to 200 MHz, both the attenuation and velocity of sound change markedly in the vicinity of the metamagnetic transitions (near 40 T). The data for URu 2 Si 2 show three transitions, in agreement with earlier magnetization results of other authors. A unique fast data-acquisition ultrasonic spectrometer was assembled and utilized for these studies.
TL;DR: In this paper, the velocity and attenuation of longitudinal ultrasound were measured on two single crystals of UPt3 in magnetic fields up to 33 T and temperatures down to 40 mK.
Abstract: The velocity and attenuation of longitudinal ultrasound were measured on two single crystals of UPt3 in magnetic fields up to 33 T and temperatures down to 40 mK. For fields applied in the basal plane, a strong dip in the velocity is seen at a field of 20 T in both samples and is accompanied by a simultaneous increase in the dc magnetization. At lower temperatures the dip gets more pronounced and sharper. At a temperature around 0.6 K the velocity dip develops a second feature at a slightly higher field of ~21.6 T.
TL;DR: In this paper, the authors presented Nb/Al/AlOx/AlNb/NbN junctions with critical current densities up to 38 kA/cm2 and small hysteresis at 4.2 K.
Abstract: Nb/Al/AlOx/Al/AlOx/Al/Nb junctions with critical current densities, jc, up to 38 kA/cm2 were fabricated. A critical voltage of Vc=1.25 mV and small hysteresis (about 6% of the critical current) at 4.2 K were obtained for jc=21 kA/cm2. Also, Nb/Al/AlOx/Al/Nb/Al/NbN junctions with jc=33 kA/cm2 and Vc=0.76 mV were fabricated, where it is suggested that a second tunnel barrier at the Al/NbN interface is present as a result of Al nitridation during NbN deposition.
01 Jan 2002
TL;DR: An overview of magnetic, electrical transport, and ferromagnetic resonance studies of thin films of superconducting and magnetic materials with patterned features that are laterally confined on the scale of 10 nm to 1 μm is presented in this paper.
Abstract: An overview is presented of magnetic, electrical transport, and ferromagnetic resonance studies of thin films of superconducting and magnetic materials with patterned features that are laterally confined on the scale of 10 nm to 1 μm. Emphasis is given to finite-size and correlation effects found at the limiting size scale of stability of long-range magnetic order, and the power of superconductivity as a probe of nanoscale magnetism. A summary is given of state-of-art methods of fabricating highly ordered arrays of ferromagnetic features precisely patterned by electron beam and laser interference techniques. Certain directions for future research in the theoretical description of small magnetic features, and the crucial role of mesoscale phenomena in nanotechnology are discussed.
TL;DR: In this paper, the authors performed ultrasonic velocity measurements on a single crystal of URu2Si2 at various temperatures in the 20 ms 50T pulsed magnets at the NHMFL, Los Alamos, NM.
Abstract: Ultrasonic velocity measurements were performed on a single crystal of URu2Si2 at various temperatures in the 20 ms 50T pulsed magnets at the NHMFL, Los Alamos, NM. At 4 K a three-fold splitting of the metamagnetic transition is seen; corresponding features were observed earlier in magnetization studies. As the temperature is raised, the three-fold splitting evolves first into two features, then into a single broad dip, after which it disappears entirely.
TL;DR: Three ultrasonic spectrometers have been constructed for use with the continuous and pulsed magnetic field facilities of the NHMFL at the Tallahassee and Los Alamos sites as mentioned in this paper.
Abstract: Three ultrasonic spectrometers have been constructed for use with the continuous and pulsed magnetic field facilities of the NHMFL at the Tallahassee and Los Alamos sites. The first of these, designed for use with the dc magnets, is a heterodyne spectrometer with the capabilities of high velocity resolution and near-simultaneous measurements at several frequencies (typically near transducer harmonics). The other two spectrometers are designed for the short-pulsed magnets, with the capability of fast data acquisition. These spectrometers have been used in the study of metamagnetism in the heavy fermion materials.