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At least 451 records · Page 25

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DOE OSTI2015

Lithographically defined few-electron silicon quantum dots based on a silicon-on-insulator substrate

Silicon quantum dot (QD) devices with a proximal single-electron transistor (SET) charge sensor have been fabricated in a metal-oxide-semiconductor structure based on a silicon-on-insulator substrate. The charge state of the QDs was clearly read out using the charge sensor via the SET current. The lithographically defined small QDs enabled clear observation of the few-electron regime of a single QD and a double QD by charge sensing. Tunnel coupling on tunnel barriers of the QDs can be controlled by tuning the top-gate voltages, which can be used for manipulation of the spin quantum bit via exchange interaction between tunnel-coupled QDs. The lithographically defined silicon QD device reported here is technologically simple and does not require electrical gates to create QD confinement potentials, which is advantageous for the integration of complicated constructs such as multiple QD structures with SET charge sensors for the purpose of spin-based quantum computing.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2022

Clean quantum point contacts in an InAs quantum well grown on a lattice-mismatched InP substrate

Strong spin-orbit coupling, the resulting large g factor, and small effective mass make InAs an attractive material platform for inducing topological superconductivity. The surface Fermi level pinning in the conduction band enables highly transparent ohmic contact without excessive doping. In this study, we investigate electrostatically defined quantum point contacts (QPCs) in a deep-well InAs two-dimensional electron gas. Despite the 3.3% lattice mismatch between the InAs quantum well and the InP substrate, we report clean QPCs with up to eight pronounced quantized conductance plateaus at zero magnetic field. Source-drain dc bias spectroscopy reveals a harmonic confinement potential with a nearly 5 meV subband spacing. We find a many-body exchange interaction enhancement for the out-of-plane g factor |$g^{*}_{⊥}$|=27±1 , whereas the in-plane g factor is isotropic |$g^{*}_{x}$|=|$g^{*}_{y}$|=12±2 , close to the bulk value for InAs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2017

Effect of temperature and magnetic field on disorder in semiconductor structures

We present the results of consistent theoretical analysis of various factors that may lead to influence of temperature and external magnetic field on disorder in semiconductor structures. Main attention is paid to quantum well (QW) structures in which only QWs or both QW and barriers are doped (the doping level is assumed to be close to the value corresponding to the metal–insulator transition). The above factors include (i) ionization of localized states to the region of delocalized states above the mobility edge, which is presumed to exist in the impurity band; (ii) the coexistence in the upper and lower Hubbard bands (upon doping of QWs as well as barriers); in this case, in particular, the external magnetic field determines the relative contribution of the upper Hubbard band due to spin correlations at doubly filled sites; and (iii) the contribution of the exchange interaction at pairs of sites, in which the external magnetic field can affect the relation between ferromagnetic and antiferromagnetic configurations. All these factors, which affect the structure and degree of disorder, lead to specific features in the temperature dependence of resistivity and determine specific features of the magnetoresistance. Our conclusions are compared with available experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Ferromagnetic Resonance Modes in the Exchange-Dominated Limit in Cylinders of Finite Length

We analyze the magnetic mode structure of axially magnetized finite-length nanoscopic cylinders in a regime where the exchange interaction dominates, along with simulations of the mode frequencies of the ferrimagnet yttrium iron garnet. For the bulk modes, we find that the frequencies can be represented by an expression given by Herring and Kittel by using wavevector components obtained by fitting the mode patterns emerging from these simulations. In addition to the axial, radial, and azimuthal modes that are present in an infinite cylinder, we find localized “cap modes” that are “trapped” at the top and bottom cylinder faces by the inhomogeneous dipole field emerging from the ends. Semiquantitative explanations are given for some of the modes, in terms of a one-dimensional Schrodinger equation, which is valid in the exchange-dominant case. The assignment of the azimuthal-mode number is carefully discussed, and the frequency splitting of a few pairs of nearly degenerate modes is determined through the beat pattern emerging from them.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

An approach to reduce the antiferromagnetic coupling of antiphase boundaries in half-metallic magnetite films

Highly conductive (∼10{sup 5} μΩ cm) Mn doped epitaxial Fe{sub 3}O{sub 4} films were fabricated by reactive sputtering. The larger size of magnetic domains compared to grain size with the increasing Mn content indicates that the partial antiferromagnetic coupling across the antiphase boundaries has been weakened, which was further demonstrated by the smaller exchange bias, faster saturated magnetization, and decreasing exchange interaction J{sub AF}. The decrease of antiferromagnetic strength originates from the larger Mn-O bond length than that of Fe-O bond. The first-principle calculation shows that the half-metallic feature (100% spin polarization) of Fe{sub 3}O{sub 4} was unchanged with the incorporation of Mn atoms.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Magnetic and magnetodielectric coupling anomalies in the Haldane spin-chain system Nd{sub 2}BaNiO{sub 5}

We report the magnetic, heat-capacity, dielectric and magnetodielectric (MDE) behaviour of a Haldane spin-chain compound containing light rare-earth ion, Nd{sub 2}BaNiO{sub 5}, in detail, as a function of temperature (T) and magnetic field (H) down to 2 K. In addition to the well-known long range antiferromagnetic order setting in at (T{sub N} = ) 48 K as indicated in dc magnetization (M), we have observed another magnetic transition near 10 K; this transition appears to be of a glassy-type which vanishes with a marginal application of external magnetic field (even H = 100 Oe). There are corresponding anomalies in dielectric constant (ε′) as well with variation of T. The isothermal M(H) curves at 2 and 5 K reveal the existence of a magnetic-field induced transition around 90 kOe; the isothermal ε′(H) also tracks such a metamagnetic transition. These results illustrate the MDE coupling in this compound. Additionally, we observe a strong frequency dependence of a step in ε′(T) with this feature appearing around 25-30 K for the lowest frequency of 1 kHz, far below T{sub N}. This is attributed to interplay between crystal-field effect and exchange interaction between Nd and Ni, which establishes the sensitivity of dielectric measurements to detect such effects. Interestingly enough, the observed dispersions of the ε′(T) curves is essentially H-independent in the entire T-range of measurement, despite the existence of MDE coupling, which is in sharp contrast with other heavy rare-earth members in this series.

36 MATERIALS SCIENCE↗

DOE OSTI2013

Simulations of magnetic field gradients due to micro-magnets on a triple quantum dot circuit

To quantify the effects of local magnetic fields on triple quantum dots, the Heisenberg Hamiltonian has been diagonalized for three electrons coupled via the exchange interaction. In particular, we have investigated different geometries of micro-magnets located on top of the triple dot in order to optimize the field gradient characteristics. In this paper, we focus on two geometries which are candidates for an addressable EDSR triple quantum dot device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Magnetic properties of spin frustrated spinel ZnFe{sub 2}O{sub 4}/ZnCr{sub 2}O{sub 4} superlattices

Superlattice films composed of frustrated spin systems, [ZnFe{sub 2}O{sub 4}]{sub m}/[ZnCr{sub 2}O{sub 4}]{sub m}, are grown on MgAl{sub 2}O{sub 4} (111) substrates by pulsed laser deposition, and their frustrated magnetism is studied. The superlattice films show spin-glass behavior and the spin-glass transition temperature (T{sub g}) varies nonmonotonically as a function of the layer thickness m. T{sub g} increases with decreasing m due to an increase in the inhomogeneous exchange interaction between Cr{sup 3+} and Fe{sup 3+} at the interface. However, below m = 4, a decrease in T{sub g} is observed. Compared with the solid solution Zn(CrFe)O{sub 4} film and the other superlattices, the decrease in T{sub g} is considered to be a unique property of the periodically modulated short period superlattices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2014

Electron spin resonance study of Er-concentration effect in GaAs;Er,O containing charge carriers

Er-concentration effect in GaAs;Er,O containing charge carriers (n-type, high resistance, p-type) has been studied by X-band Electron spin resonance (ESR) at low temperature (4.7 K < T < 18 K). Observed A, B, and C types of ESR signals were identical to those observed previously in GaAs:Er,O without carrier. The local structure around Er-2O centers is not affected by carriers because similar angular dependence of g-values was observed in both cases (with/without carrier). For temperature dependence, linewidth and lineshape analysis suggested the existence of Er dimers with antiferromagnetic exchange interaction of about 7 K. Moreover, drastic decrease of ESR intensity for C signal in p-type sample was observed and it correlates with the decrease of photoluminescence (PL) intensity. Possible model for the Er-2O trap level in GaAs:Er,O is discussed from the ESR and PL experimental results.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

Dual oxidation state induced oxygen vacancies in Pr substituted BiFeO3 compounds: An effective material activation strategy to enhance the magnetic and visible light-driven photocatalytic properties

Highlights: • BiFeO{sub 3} with tunable oxygen vacancy is achieved through Pr substitution. • Dual oxidation of Pr ions is found to be the origin of oxygen vacancies in BiFeO{sub 3}. • Mechanism of the observed photocatalytic activity could be due to the oxygen vacancy. - Abstract: Bi{sub 1-x}P{sub x}FeO{sub 3} (BP{sub x}FO) nanoparticulates with tunable oxygen vacancy have been synthesized via sol-gel process. Substitution induced oxygen vacancy defects in BP{sub x}FO compositions were evidenced from their X-ray photoelectron spectra, where the mechanism behind the formation of oxygen vacancy was essentially found to be the dual oxidation states of Pr (+3/+4) ions. The origin of the observed enhanced magnetic properties in BP{sub x}FO nanoparticulates is attributed to be the oxygen vacancy mediated alterations in FeOFe bond angle and the alignment of Pr-3d magnetic moments with Fe-3d magnetic moments, which is resulted the ferromagnetic exchange interactions in the BP{sub x}FO system. The observed enhanced photocatalytic activity of BP{sub x}FO compositions is attributed to the formation of defective states in the valence band due to the oxygen vacancies that favored the dispersion of band edge position with required photocatalytic redox potential as well as the acceleration of the charge separation process in these defective BP{sub x}FO systems.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Conventional magnetic superconductors

We discuss several classes of conventional magnetic superconductors including the ternary rhodium borides and molybdenum chalcogenides (or Chevrel phases), and the quaternary nickel-borocarbides. These materials exhibit some exotic phenomena related to the interplay between superconductivity and long-range magnetic order including: the coexistence of superconductivity and antiferromagnetic order; reentrant and double reentrant superconductivity, magnetic field induced superconductivity, and the formation of a sinusoidally-modulated magnetic state that coexists with superconductivity. We introduce the article with a discussion of the binary and pseudobinary superconducting materials containing magnetic impurities which at best exhibit short-range “glassy” magnetic order. Early experiments on these materials led to the idea of a magnetic exchange interaction between the localized spins of magnetic impurity ions and the spins of the conduction electrons which plays an important role in understanding conventional magnetic superconductors. Furthermore, these advances provide a natural foundation for investigating unconventional superconductivity in heavy-fermion compounds, cuprates, and other classes of materials in which superconductivity coexists with, or is in proximity to, a magnetically-ordered phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Exchange enhancement of the electron g factor in strained InGaAs/InP heterostructures

The exchange enhancement of the electron g factor in strained InGaAs/InP heterostructures with a two-dimensional electron gas is studied. Analysis of the temperature dependence of the resistance in the minima of the Shubnikov-de Haas oscillations in perpendicular magnetic fields up to 12 T in the vicinity of the odd filling factors of the Landau levels yields the values of the effective electron Lande factor g* from −8.6 to −10.1. The experimental values are compared with the results of theoretical calculations of the g factor of quasiparticles. The calculations are performed using an eight-band k · p Hamiltonian and take into account exchange interaction in the two-dimensional electron gas. It is shown that, under the conditions of a large overlap between the spin-split Landau levels, the maximum value of the quasiparticle g factor can be attained in the vicinity of even filling factors. This is caused by the nonparabolicity of the electron dispersion relation.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Sub-millitesla magnetic field effects on the recombination reaction of flavin and ascorbic acid radicals

Even though the interaction of a <1 mT magnetic field with an electron spin is less than a millionth of the thermal energy at room temperature (k{sub B}T), it still can have a profound effect on the quantum yields of radical pair reactions. We present a study of the effects of sub-millitesla magnetic fields on the photoreaction of flavin mononucleotide with ascorbic acid. Direct control of the reaction pathway is achieved by varying the rate of electron transfer from ascorbic acid to the photo-excited flavin. At pH 7.0, we verify the theoretical prediction that, apart from a sign change, the form of the magnetic field effect is independent of the initial spin configuration of the radical pair. The data agree well with model calculations based on a Green’s function approach that allows multinuclear spin systems to be treated including the diffusive motion of the radicals, their spin-selective recombination reactions, and the effects of the inter-radical exchange interaction. The protonation states of the radicals are uniquely determined from the form of the magnetic field-dependence. At pH 3.0, the effects of two chemically distinct radical pair complexes combine to produce a pronounced response to ∼500 μT magnetic fields. These findings are relevant to the magnetic responses of cryptochromes (flavin-containing proteins proposed as magnetoreceptors in birds) and may aid the evaluation of effects of weak magnetic fields on other biologically relevant electron transfer processes.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2019

Investigation on Electronic Structures and Magnetic Properties of (Mn, Ga) Co-doped SnO{sub 2}

The electronic structures and magnetic properties of (Mn, Ga) co-doped SnO{sub 2} are studied by the first-principles calculations in full-potential linearized augmented plane wave formalism within generalized gradient approximations. We found that the doped system favors the ferromagnetic state and room-temperature ferromagnetism can be expected in it. The origin of ferromagnetism can be attributed to p-d exchange interactions as Mn-O-Mn. Ga acts as acceptor in (Mn, Ga) co-doped SnO{sub 2} host and can donate one hole, increasing the carrier concentration and thus leading to a more stable ferromagnetic state. These results suggest a recipe obtaining promising dilute magnetic semiconductor by doping nonmagnetic elements in SnO{sub 2} matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Fe–Ni solid solutions in nano-size dimensions: Effect of hydrogen annealing

Highlights: • Fe–Ni solid solution with nano-size dimensions were prepared and characterized. • Both as prepared and hydrogenated solid solutions have FCC structure of Ni. • Paramagnetic and ferromagnetic domains coexist in these samples. - Abstract: Nanoparticles of Ni{sub 0.50}Fe{sub 0.50} and Ni{sub 0.75}Fe{sub 0.25} alloys were prepared by chemical reduction in ethylene glycol medium. XRD and {sup 57}Fe Mössbauer studies have confirmed the formation of Fe–Ni solid solution in nano-size dimensions with FCC structure. These samples consist of both ferromagnetic and paramagnetic domains which have been attributed to the coexistence of large and small particles as confirmed by atomic force microscopic (AFM) and {sup 57}Fe Mössbauer spectroscopic studies. Improved extent of Fe–Fe exchange interaction existing in Ni{sub 0.50}Fe{sub 0.50} alloy compared to Ni{sub 0.75}Fe{sub 0.25} alloy explains the observed increase in the relative extent of ferromagnetic domains compared to paramagnetic domains in the former sample. Increase in the relative extent of ferromagnetic domains for hydrogenated alloys is due to increase in particle size brought about by the high temperature activation prior to hydrogenation.

36 MATERIALS SCIENCE↗

DOE OSTI2014

Understanding ferromagnetism and optical absorption in 3d transition metal-doped cubic ZrO{sub 2} with the modified Becke-Johnson exchange-correlation functional

The electronic structure, magnetic, and optical properties in cubic crystalline phase of Zr{sub 1−x}TM{sub x}O{sub 2} (TM = V, Mn, Fe, and Co) at x = 6.25% are studied using density functional theory with the Generalized Gradient Approximation and the modified Becke-Johnson of the exchange-correlation energy and potential. In our calculations, the zirconia is a p-type semiconductor and has a large band gap. We evaluated the possibility of long-range magnetic order for transition metal ions substituting Zr. Our results show that ferromagnetism is the ground state in V, Mn, and Fe-doped ZrO{sub 2} and have a high value of energy in Mn-doped ZrO{sub 2}. However, in Co-doped ZrO{sub 2}, antiferromagnetic ordering is more stable than the ferromagnetic one. The exchange interaction mechanism has been discussed to explain the responsible of this stability. Moreover, it has been found that the V, Mn, and Fe transition metals provide half-metallic properties considered to be the leading cause, responsible for ferromagnetism. Furthermore, the optical absorption spectra in the TM -doped cubic ZrO{sub 2} are investigated.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

Semiconductor-to-metal phase transition in monolayer ZrS{sub 2}: GGA+U study

We report structural and electronic properties of ZrS{sub 2} monolayer within density functional theory (DFT) by inclusion of Hubbard on-site Coulomb and exchange interactions. The importance of on-site interactions for both ZrS{sub 2} bulk and monolayer has been highlighted that significantly improves the electronic band-gap. It is demonstrated that mechanical strain induces structural phase transition that results in semiconductor-to-metal transition in monolayer ZrS{sub 2}. This phenomenon has important implications in technological applications such as flexible, low power and transparent electronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2018

A Role of the 3d Electron Subsystem in the Evolution of Band Structure and Magnetic and Optical Properties of ErNi{sub 5 –} {sub x}Co{sub x} Compounds (x = 0–4)

The evolution of the electronic structure and optical and magnetic properties of ErNi{sub 5 –} {sub x}Co{sub x} compounds (x = 0–4), occurring upon substitution of nickel by cobalt atoms, has been investigated. Spin-polarization calculations of the band spectrum of these intermetallic compounds have been performed within the local spin density approximation with a correction for strong electron correlations in the 4f shell of a rare-earth ion (method LSDA+U). The values of magnetic moments for erbium, nickel, and cobalt ions (located at different crystallographic sites) are obtained. The exchange-interaction parameters for the 3d sublattice of transition metals are determined. The spectral properties of the compounds have been investigated in the wavelength range of 0.22–15 μm by optical ellipsometry. The experimental frequency dependences of the optical conductivity in the interband absorption region are compared with the corresponding characteristics calculated based on the density of electronic states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗