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At least 1,387 records · Page 77
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Structural properties of Bi{sub 2}Te{sub 3} topological insulator thin films grown by molecular beam epitaxy on (111) BaF{sub 2} substrates

Structural properties of topological insulator bismuth telluride films grown epitaxially on (111) BaF{sub 2} with a fixed Bi{sub 2}Te{sub 3} beam flux were systematically investigated as a function of substrate temperature and additional Te flux. A layer-by-layer growth mode is observed since the early stages of epitaxy and remains throughout the whole deposition. Composition of the epitaxial films produced here stays between Bi{sub 2}Te{sub 3} and Bi{sub 4}Te{sub 5}, as determined from the comparison of the measured x-ray diffraction curves with calculations. The substrate temperature region, where the growth rate remains constant, is found to be the most appropriate to obtain ordered Bi{sub 2}Te{sub 3} films. Line width of the L = 18 Bi{sub 2}Te{sub 3} diffraction peaks as low as 140 arcsec was obtained, indicating high crystalline quality. Twinning domains density rises with increasing growth temperature and reducing Te extra flux. X-ray reflectivity curves of pure Bi{sub 2}Te{sub 3} films with thickness from 165 to 8 nm exhibited well defined interference fringes, evidencing homogeneous layers with smooth surface. Our results demonstrate that Bi{sub 2}Te{sub 3} films with very well controlled structural parameters can be obtained. High structural quality Bi{sub 2}Te{sub 3} films as thin as only eight quintuple layers grown here are promising candidates for intrinsic topological insulator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Role of laser energy density on growth of highly oriented topological insulator Bi{sub 2}Se{sub 3} thin films

Topological insulators (TIs) are very promising in the field of nanoelectronics due to their exotic properties. Bismuth Selenide, a 3D Topological insulator is considered as reference TI owing to its simple band structure and large bandgap. However, the presence of unintentional doping, which masks the metallic surface states, is still a major concern. In this work, we report the effect of laser energy density on the growth of highly oriented and stoichiometric thin films of Bi{sub 2}Se{sub 3} by pulsed laser deposition (PLD). Structural characterizations by X-ray diffraction (XRD) and Raman Spectroscopy confirms the c-axis orientation and good crystallinity of films. Atomic force microscopy (AFM) study shows the increase in average grain size and rms roughness (from 3.1 nm to 5.1 nm) with the decrease in laser energy density. Compositional study by X-Ray Reflectivity (XRR) measurement is found to be in agreement with AFM results. Energy dispersive x-ray spectroscopy (EDS) measurements confirm the desired stoichiometry of the samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Growth of high quality single crystals of Bi{sub 2}Se{sub 3} topological insulator via solid state reaction method

Recently discovered, Topological Insulators (TIs) have garnered enormous amount of attention owing to its unique surface properties which has potential applications in the field of spintronics and other modern technologies. For all this, it should require a very good quality samples. There are a number of techniques suggested by people for the growth of good quality TIs. Here, we are reporting the growth of high quality single crystals of Bi{sub 2}Se{sub 3} (a TI) by slow cooling solid-state reaction method. X-ray diffraction measurements performed on a cleaved flake of single crystal Bi{sub 2}Se{sub 3} showed up with proper orientations of the crystal planes. High energy X-ray diffraction has been performed to confirm the stoichiometry of the compound and also recorded Laue patterns prove the single crystalline nature of Bi{sub 2}Se{sub 3}. Moreover, angle resolved photo-emission spectroscopy (ARPES) carried out on a flat crystal flake shows distinct Dirac dispersion of surface bands at the gamma point clarifying it as a 3D topological insulator.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Linear magneto-resistance in Bi{sub 2}SeTe{sub 2} topological insulator

Magnetic field and temperature dependent electronic transport measurements have been carried out on Bi{sub 2}SeTe{sub 2} topological insulator single crystals. The measurements reveal an insulating behavior and the carriers were found to be electrons (n-type) from Hall measurement. Magneto-resistance (MR) measurements in the field range (B) of 15 T to -15 T carried out at 4.2 K showed a cusp like weak anti-localization behavior for lower fields (-5 T 5 T. Upon increasing temperature, MR transforms to linear dependence of B at 40, 50 and 100 K. On further increasing temperatures (> 200 K), a parabolic MR is observed. Temperature dependent Hall data also showed a transition from a nonlinear to linear behavior upon increasing temperatures. Disorder induced changes in the electronic transport characteristics of bulk and surface electrons are believed to cause such changes in the magneto-transport behavior of this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Thermal radiative near field transport between vanadium dioxide and silicon oxide across the metal insulator transition

The thermal radiative near field transport between vanadium dioxide and silicon oxide at submicron distances is expected to exhibit a strong dependence on the state of vanadium dioxide which undergoes a metal-insulator transition near room temperature. We report the measurement of near field thermal transport between a heated silicon oxide micro-sphere and a vanadium dioxide thin film on a titanium oxide (rutile) substrate. The temperatures of the 15 nm vanadium dioxide thin film varied to be below and above the metal-insulator-transition, and the sphere temperatures were varied in a range between 100 and 200 °C. The measurements were performed using a vacuum-based scanning thermal microscope with a cantilevered resistive thermal sensor. We observe a thermal conductivity per unit area between the sphere and the film with a distance dependence following a power law trend and a conductance contrast larger than 2 for the two different phase states of the film.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Skyrmion-induced bound states on the surface of three-dimensional topological insulators

The interaction between the surface of a 3D topological insulator and a skyrmion/anti-skyrmion structure is studied in order to investigate the possibility of electron confinement due to the skyrmion presence. Both hedgehog (Néel) and vortex (Bloch) skyrmions are considered. For the hedgehog skyrmion, the in-plane components cannot be disregarded and their interaction with the surface state of the topological insulator (TI) has to be taken into account. A semi-classical description of the skyrmion chiral angle is obtained using the variational principle. It is shown that both the hedgehog and the vortex skyrmion can induce bound states on the surface of the TI. However, the number and the properties of these states depend strongly on the skyrmion type and the skyrmion topological number N{sub Sk}. The probability densities of the bound electrons are also derived where it is shown that they are localized within the skyrmion region.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Development of the surface-sensitive soft x-ray absorption fine structure measurement technique for the bulk insulator

We have succeeded in measuring X-ray absorption fine structure (TEY-XAFS) spectra of insulating plate samples by total electron yield. The biggest problem is how to suppress the charge-up. We have attempted to deposit a gold stripe electrode on the surface and obtained a TEY-XAFS spectrum. This indicates that the metal stripe electrode is very useful in the TEY-XAFS measurement of the insulating plate samples. In the detailed analysis, we have found that the effective area for suppressing charge-up was approximately 120 μm from the edge of the electrode.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

H-terminated diamond field effect transistor with ferroelectric gate insulator

An H-terminated diamond field-effect-transistor (FET) with a ferroelectric vinylidene fluoride (VDF)-trifluoroethylene (TrFE) copolymer gate insulator was fabricated. The VDF-TrFE film was deposited on the H-terminated diamond by the spin-coating method and low-temperature annealing was performed to suppress processing damage to the H-terminated diamond surface channel layer. The fabricated FET structure showed the typical properties of depletion-type p-channel FET and showed clear saturation of the drain current with a maximum value of 50 mA/mm. The drain current versus gate voltage curves of the proposed FET showed clockwise hysteresis loops due to the ferroelectricity of the VDF-TrFE gate insulator, and the memory window width was 19 V, when the gate voltage was swept from 20 to −20 V. The maximum on/off current ratio and the linear mobility were 10{sup 8} and 398 cm{sup 2}/V s, respectively. In addition, we modulated the drain current of the fabricated FET structure via the remnant polarization of the VDF-TrFE gate and obtained an on/off current ratio of 10{sup 3} without applying a DC gate voltage.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Fabrication of Ohmic contact on semi-insulating 4H-SiC substrate by laser thermal annealing

The Ni contact layer was deposited on semi-insulating 4H-SiC substrate by magnetron sputtering. The as-deposited samples were treated by rapid thermal annealing (RTA) and KrF excimer laser thermal annealing (LTA), respectively. The RTA annealed sample is rectifying while the LTA sample is Ohmic. The specific contact resistance (ρ{sub c}) is 1.97 × 10{sup −3} Ω·cm{sup 2}, which was determined by the circular transmission line model. High resolution transmission electron microscopy morphologies and selected area electron diffraction patterns demonstrate that the 3C-SiC transition zone is formed in the near-interface region of the SiC after the as-deposited sample is treated by LTA, which is responsible for the Ohmic contact formation in the semi-insulating 4H-SiC.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Pulse number controlled laser annealing for GeSn on insulator structure with high substitutional Sn concentration

Crystalline GeSn-on-insulator structures with high Sn concentration (>8%), which exceeds thermal equilibrium solid-solubility (∼2%) of Sn in Ge, are essential to achieve high-speed thin film transistors and high-efficiency optical devices. We investigate non-thermal equilibrium growth of Ge{sub 1−x}Sn{sub x} (0 ≤ x ≤ 0.2) on quartz substrates by using pulsed laser annealing (PLA). The window of laser fluence enabling complete crystallization without film ablation is drastically expanded (∼5 times) by Sn doping above 5% into Ge. Substitutional Sn concentration in grown layers is found to be increased with decreasing irradiation pulse number. This phenomenon can be explained on the basis of significant thermal non-equilibrium growth achieved by higher cooling rate after PLA with a lower pulse number. As a result, GeSn crystals with substitutional Sn concentration of ∼12% are realized at pulse irradiation of single shot for the samples with the initial Sn concentration of 15%. Raman spectroscopy and electron microscopy measurements reveal the high quality of the grown layer. This technique will be useful to fabricate high-speed thin film transistors and high-efficiency optical devices on insulating substrates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Bandgap modulation in photoexcited topological insulator Bi{sub 2}Te{sub 3} via atomic displacements

The atomic and electronic dynamics in the topological insulator (TI) Bi{sub 2}Te{sub 3} under strong photoexcitation were characterized with time-resolved electron diffraction and time-resolved mid-infrared spectroscopy. Three-dimensional TIs characterized as bulk insulators with an electronic conduction surface band have shown a variety of exotic responses in terms of electronic transport when observed under conditions of applied pressure, magnetic field, or circularly polarized light. However, the atomic motions and their correlation between electronic systems in TIs under strong photoexcitation have not been explored. The artificial and transient modification of the electronic structures in TIs via photoinduced atomic motions represents a novel mechanism for providing a comparable level of bandgap control. The results of time-domain crystallography indicate that photoexcitation induces two-step atomic motions: first bismuth and then tellurium center-symmetric displacements. These atomic motions in Bi{sub 2}Te{sub 3} trigger 10% bulk bandgap narrowing, which is consistent with the time-resolved mid-infrared spectroscopy results.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Insulator-protected mechanically controlled break junctions for measuring single-molecule conductance in aqueous environments

We present a method to fabricate insulated gold mechanically controlled break junctions (MCBJ) by coating the metal with a thin layer of aluminum oxide using plasma enhanced atomic layer deposition. The Al{sub 2}O{sub 3} thickness deposited on the MCBJ devices was varied from 2 to 15 nm to test the suppression of leakage currents in deionized water and phosphate buffered saline. Junctions coated with a 15 nm thick oxide layer yielded atomically sharp electrodes and negligible conductance counts in the range of 1 to 10{sup −4} G{sub 0} (1 G{sub 0} = 77 μS), where single-molecule conductances are commonly observed. The insulated devices were used to measure the conductance of an amphiphilic oligophenylene ethynylene derivative in deionized water.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Observation of fast expansion velocity with insulating tungsten wires on ∼80 kA facility

This paper presents experimental results on the effects of insulating coatings on tungsten planar wire array Z-pinches on an 80 kA, 100 ns current facility. Expansion velocity is obviously increased from ∼0.25 km/s to ∼3.5 km/s by using the insulating coatings. It can be inferred that the wire cores are in gaseous state with this fast expansion velocity. An optical framing camera and laser probing images show that the standard wire arrays have typical ablation process which is similar to their behaviors on mega-ampere facilities. The ablation process and precursor plasma are suppressed for dielectric tungsten wires. The wire array implosion might be improved if these phenomena can be reproduced on Mega-ampere facilities.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

Gate-tunable supercurrent and multiple Andreev reflections in a superconductor-topological insulator nanoribbon-superconductor hybrid device

We report on the observation of gate-tunable proximity-induced superconductivity and multiple Andreev reflections (MARs) in a bulk-insulating BiSbTeSe 2 topological insulator nanoribbon (TINR) Josephson junction with superconducting Nb contacts. We observe a gate-tunable critical current (I C ) for gate voltages (V g ) above the charge neutrality point (V CNP ), with I C as large as 430 nA. We also observe MAR peaks in the differential conductance (dI/dV) versus DC voltage (V dc ) across the junction corresponding to sub-harmonic peaks (at V dc = V n = 2Δ Nb /en, where ΔNb is the superconducting gap of the Nb contacts and n is the sub-harmonic order). The sub-harmonic order, n, exhibits a V g -dependence and reaches n = 13 for V g = 40 V, indicating the high transparency of the Nb contacts to TINR. Our observations pave the way toward exploring the possibilities of using TINR in topologically protected devices that may host exotic physics such as Majorana fermions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Effects of transmission line geometry on traveling-wave metal-insulator-metal rectenna infrared detectors

The transmission line impedance of traveling-wave diodes can circumvent resistance-capacitance time constant limitations of metal-insulator-metal diodes in rectennas operating at optical frequencies. We performed three-dimensional simulations of a traveling-wave diode rectenna using a linear finite-element electromagnetic solver. We develop a method to analyze metal-insulator-metal traveling-wave rectennas by using the field profiles from the linear finite-element solver and accounting for the nonlinear current-voltage characteristics during postprocessing. The traveling-wave diode length produces resonance at half surface plasmon wavelength intervals. With optimized cross section and length parameters, we observe a peak system responsivity of 239μA/W and a detectivity of 5.7×10 4 Jones.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Direct observation of proximity-induced magnetism and spin reorientation in topological insulator on a ferrimagnetic oxide

The quantum anomalous Hall effect (QAHE) in a magnetic topological insulator system without the need of an external magnetic field becomes of great interest because of its dissipationless spin currents. The proximity effect via a magnetic adlayer is a promising strategy to bring magnetism into a topological insulator (TI) due to its boosted Curie temperature for a realistic application. In this work, a proximity-induced magnetism above 90 K in a TI, (Bi 0.25 Sb 0.75 ) 2 Te 3 , on a ferrimagnetic adlayer with perpendicular magnetic anisotropy, thulium-iron-garnet (TIG), was evidenced directly by using magnetic circular dichroism. A competition between exchange couplings with different sublattice-sites was further revealed element-specifically in a temperature-driven spin-reorientation of TI, bringing an issue on magnetic inhomogeneity at the interface, which might cause difficulties in obtaining QAHE in such a TI/TIG bilayer system. Our results prove directly the robust magnetism of TI above liquid nitrogen temperatures and also give a deep insight into the mechanism of interfacial coupling in the proximity effect for the bilayer of TI and magnetic oxide, which could be essential for the design of a system with QAHE.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Method for improved voltage determination for pulsed power systems utilizing a magnetically insulated transmission line

Because the voltage is difficult to measure in a magnetically insulated transmission line (MITL), measurement of the currents flowing in the cathode and anode are often used with MITL theory to estimate the voltage in a given experiment. However, that estimate contains a space charge correction term whose magnitude depends on what is referred to as the g factor that describes the distribution of charge and current in the electron flow layer in the MITL. Typically, g is on the order of unity but the accuracy of the voltage estimate depends on its actual value. While the space charge correction term is small when the MITL flow is strongly insulated, it is particularly important near self-limited MITL flow. System parameters that affect the distribution of electron flow are studied here at self-limited flow in order to illustrate this matter and develop a methodology to improve the voltage determination.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Spin Seebeck effect in insulating SrFeO 3–δ films

In SrFeO 3–δ , noncollinear antiferromagnetic spin textures have led to the observation of the topological Hall effect and may give rise to novel magnetic excitations. Here, we measured the spin Seebeck effect in epitaxial, oxygen reduced SrFeO 3–δ films, which are nonferromagnetic and insulating at low temperatures. Here, we detected a spin Seebeck signal below 300K and the signal is enhanced below approximate to 100K, near the ordering temperature of incommensurate antiferromagnetism found in bulk single crystals of SrFeO 3–δ . The spin Seebeck signal increases in magnitude and develops a nonlinear dependence on the magnetic field at lower temperatures. Control experiments were used to verify that the measured signal arises from a spin current and to rule out magnetic proximity effects. Our work shows a detectable spin Seebeck signal in nonferromagnetic insulator SrFeO 3–δ .

36 MATERIALS SCIENCE↗