Thermal Activation Measurements Research Articles

Stabilizing the Inverted Phase of a WSe2/BLG/WSe2 Heterostructure via Hydrostatic Pressure.

Bilayer graphene (BLG) was recently shown to host a band-inverted phase with unconventional topology emerging from the Ising-type spin-orbit interaction (SOI) induced by the proximity of transition metal dichalcogenides with large intrinsic SOI. Here, we report the stabilization of this band-inverted phase in BLG symmetrically encapsulated in tungsten diselenide (WSe2) via hydrostatic pressure. Our observations from low temperature transport measurements are consistent with a single particle model with induced Ising SOI of opposite sign on the two graphene layers. To confirm the strengthening of the inverted phase, we present thermal activation measurements and show that the SOI-induced band gap increases by more than 100% due to the applied pressure. Finally, the investigation of Landau level spectra reveals the dependence of the level-crossings on the applied magnetic field, which further confirms the enhancement of SOI with pressure.

Quantum Hall effect in a two-dimensional semiconductor with large spin-orbit coupling

We perform magnetotransport studies of atomically thin InSe field-effect transistors with large Rashba spin-orbit coupling (SOC), and extract the Landau level (LL) gaps via thermal activation measurements. Surprisingly, the Landau level gaps at even and odd filling factors are extrapolated to have positive and negative intercepts at $B=0$, respectively, which result from the spin-split Landau spectrum. We also show that for a material with large spin-orbit coupling, its LL gaps may vary nonlinearly and nonmonotonically as a function of magnetic field. Thus, its effective mass and $g$-factor cannot be reliably extracted using conventional expressions, but depend rather sensitively on the SOC's strength and tunability.

Accurate Measurement of the Gap of Graphene/h-BN Moiré Superlattice through Photocurrent Spectroscopy.

Monolayer graphene aligned with hexagonal boron nitride (h-BN) develops a gap at the charge neutrality point (CNP). This gap has previously been extensively studied by electrical transport through thermal activation measurements. Here, we report the determination of the gap size at the CNP of graphene/h-BN superlattice through photocurrent spectroscopy study. We demonstrate two distinct measurement approaches to extract the gap size. A maximum of ∼14 meV gap is observed for devices with a twist angle of less than 1°. This value is significantly smaller than that obtained from thermal activation measurements, yet larger than the theoretically predicted single-particle gap. Our results suggest that lattice relaxation and moderate electron-electron interaction effects may enhance the CNP gap in graphene/h-BN superlattice.

Probing Proximity-Induced Superconductivity in InAs Nanowires Using Built-In Barriers

Bound states in superconductor-nanowire hybrid devices play a central role, carrying informationon the ground states properties (Shiba or Andreev states) or on the topological properties of thesystem (Majorana states). The spectroscopy of such bound states relies on the formation of well-defined tunnel barriers, usually defined by gate electrodes, which results in smooth tunnel barriers.Here we used thin InP segments embedded into InAs nanowire during the growth process to forma sharp built-in tunnel barrier. Gate dependence and thermal activation measurements have beenused to confirm the presence and estimate the height of this barrier. By coupling these wires tosuperconducting electrodes we have investigated the gate voltage dependence of the induced gap inthe nanowire segment, which we could understand using a simple model based on Andreev boundstates. Our results show that these built-in barriers are promising as future spectroscopic tools

Magnetoelastic effects and random magnetic anisotropy in highly strained ultrathin Ni nanowires epitaxied in a SrTiO3 matrix

We analyze the magnetic anisotropy of Ni nanowires with diameters smaller than 5 nm. The nanowires are vertically epitaxied in a SrTiO3(001) matrix which generates huge tensile strains up to 3.6% along the nanowire axis. This leads to an unusual anisotropy, characterized by an easy magnetization plane perpendicular to the nanowire axis. Hysteresis cycles M(H) unveil an overall in-plane isotropy, while an opening of the M(H) cycles and thermal activation measurements indicate the presence of local energy barriers inside the nanowires. Surprisingly, the coercive field Hc(T) decays exponentially with increasing temperature, for both the easy plane and the hard axis. Based on these findings, we provide an analysis of magnetoelastic effects in the nanowires. By considering global averaging over the anisotropy distribution and local averaging according to the Random Magnetic Anisotropy model, we find that the global anisotropy, with its hard axis and isotropic easy plane, is related to the mean strain, while coercivity arises from local strain variations. We evidence that a thermally activated anisotropy softening occurs in the nanowires, in addition to Sharrock’s law of thermal reduction of coercivity. Possible mechanisms responsible for this thermal softening of anisotropy are proposed and discussed. Our study eventually allows to identify two major competing effects at play in the present system: an increasing magnetic anisotropy with increasing strain and a reduction of the anisotropy with increasing local strain fluctuations.

Phase diagram of quantum Hall breakdown and nonlinear phenomena for InGaAs/InP quantum wells

We investigate non-linear magneto-transport in a Hall bar device made from a strained InGaAs/InP quantum well: a material system with attractive spintronic properties. From extensive maps of the longitudinal differential resistance (r_xx) as a function of current and magnetic (B-) field phase diagrams are generated for quantum Hall breakdown in the strong quantum Hall regime reaching filling factor $\nu$=1. By careful illumination the electron sheet density (n) is incremented in small steps and this provides insight into how the transport characteristics evolve with n. We explore in depth the energetics of integer quantum Hall breakdown and provide a simple picture for the principal features in the r_xx maps. A simple tunneling model that captures a number of the characteristic features is introduced. Parameters such as critical Hall electric fields and the exchange-enhanced g-factors for odd-filling factors including nu=1 are extracted. A detailed examination is made of the B-field dependence of the critical current as determined by two different methods and compiled for different values of n. A simple rescaling procedure that allows the critical current data points obtained from r_xx maxima for even-filling to collapse on to a single curve is demonstrated. Exchange-enhanced g-factors for odd-filling are extracted from the compiled data and are compared to those determined by conventional thermal activation measurements. The exchange-enhanced g-factor is found to increase with decreasing n.

Coulomb-bound four- and five-particle intervalley states in an atomically-thin semiconductor

As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides (TMDCs) provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are still elusive and controversial despite recent efforts. Here, we present experimental evidence for four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tungsten diselenide. Through charge doping, thermal activation, and magnetic-field tuning measurements, we determine that the biexciton and the exciton-trion are bound with respect to the bright exciton and the trion, respectively. Further, both the biexciton and the exciton-trion are intervalley complexes involving dark excitons, giving rise to emissions with large, negative valley polarization in contrast to that of the two-particle excitons. Our studies provide opportunities for building valleytronic quantum devices harnessing high-order TMDC excitations.

Hydrogen-induced intergranular failure of iron

The hydrogen embrittlement of a commercial-grade pure iron was examined by using repeated stress-relaxation tests under simultaneous cathodic hydrogen charging. The hydrogen-charged iron, containing an estimated 25.8appm H, fractured after repeated transients, with a total strain of ∼5%. The fracture mode was intergranular. Thermal activation measurements show a decrease in activation volume and free energy, which is consistent with hydrogen enhancing the dislocation velocity. The microstructure beneath the intergranular facets displays a dislocation cell structure more complex than expected for intergranular fracture and this strain-to-failure. It is proposed that hydrogen accelerates the evolution of the dislocation microstructure through the hydrogen-enhanced plasticity mechanism and this work-hardening of the matrix along with the attendant hydrogen concentration at the grain boundaries are crucial steps in causing the observed hydrogen-induced intergranular failure.

Local compressibility measurement of theνtot=1quantum Hall state in a bilayer electron system

The filling ${\ensuremath{\nu}}_{tot}=1$ quantum Hall state under charge imbalance is investigated through both transport and thermodynamic measurements on a high-mobility low-density GaAs bilayer sample with negligible single particle tunneling. The ${\ensuremath{\nu}}_{tot}=1$ state demonstrates its robustness against imbalance by evolving continuously from the single layer regime (${\ensuremath{\nu}}_{upper}=1$, ${\ensuremath{\nu}}_{lower}=0$) to the bilayer regime with fillings ${\ensuremath{\nu}}_{upper}=1/3$ and ${\ensuremath{\nu}}_{lower}=2/3$ for the separate layers. The energy gap of the ${\ensuremath{\nu}}_{tot}=1$ state obtained from compressibility measurements using single electron transistors depends on position, i.e., the local disorder potential. Nevertheless, compressibility and thermal activation measurements yield comparable values for the energy gap under imbalance.

Electrical properties of pSi/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunctions: Experimental and theoretical evaluation of diode operation

In this work, we analyze electrically the Al/p-Si/[6,6] phenyl-C61 butyric acid methyl ester/Al hybrid heterojunction. The barrier height at the p-Si/PCBM interface corresponding to the difference between Si valence band edge and the lowest unoccupied molecular orbital energy level of PCBM is studied with current-voltage (J-V) and capacitance-voltage (C-V) methods and determined to be ≃0.55 eV. This value is in agreement with the onset energy of spectrally resolved photocurrent measurements presented in a previous publication [Matt et al., Adv. Mater. 22, 647 (2010)]. For the J-V characteristics, a thorough model based on an interface generation-recombination current is proposed. All relevant energy levels for this model are obtained experimentally. As origin of the large reverse current, the thermal generation of charge carriers throughout the Si depletion region is identified by the thermal activation measurements.

Effect of Mn Interface Doping in Polycrystalline Exchange Bias Thin Films

We report on the effect of 0-5 monoatomic layers of Mn deposited at the interface of a CoFe/IrMn polycrystalline thin film. All samples were produced via sputtering. Thermal activation measurements were carried out using the well-established York Protocols. In conjunction with grain size analysis performed on each sample the value of the antiferromagnet (AF) anisotropy KAF was found. An increase in exchange bias Hex was found in the case of 1-2 atomic layers, however a sharp decrease was seen with the addition of further layers. There was no change in the median grain diameter DM , or in the median blocking temperature (TB ), and consequently KAF, with the addition of Mn. However in the samples with the Mn interfacial layers Hex was found to increase by up to 100 Oe when the setting field Hset, was varied from 5 to 20 kOe.

Compositional dependence of antiferromagnetic anisotropy in IrMn/CoFe exchange bias systems

We report on a study of the effect of Ir content on the loop shift (HEX) and anisotropy constant (KAF) in the CoFe/IrMn system. The sample structure investigated was Si/NiCr(5 nm)/Ru(5 nm)/IrxMn1-x/CoFe(2 nm)/Ta(3 nm). All samples were produced by sputtering and the Ir and Mn levels were varied using a specially made composite target and deposited at ∼120 °C. The composition of the samples was analyzed using energy dispersive x-ray analysis. KAF was calculated from thermal activation measurements using the York Protocols. A plateau in HEX was found for Ir levels between 16– 20.5 at. %. HEX was found to decrease by 50% on either side of this window. This result is consistent with previous studies where the enhancement of HEX was attributed to an increase in the atomic ordering of the IrMn alloy. However, KAF decreases linearly with increasing Ir concentration and does not appear to correlate with the change in HEX.

Bulk and Interfacial Effects in Co–Cr<SUB>2</SUB>O<SUB>3</SUB> Nanocomposites

The exchange bias field has been measured in a set of Co-Cr2O3 nanocomposites in order to distinguish between the bulk and interfacial contributions to H(ex). The studies were carried out on a set of samples prepared by the sol gel technique in which the Co concentration was varied between 30 and 80 wt%. The particle sizes in all samples were carefully controlled so as to enable a comparison of their magnetic properties. Using thermal activation measurements we are able to distinguish between contributions to H(ex) arising from the thermal stability of the antiferromagnetic particles (bulk behaviour) and that due to changing interface density with increasing Co concentration. We have interpreted our results in terms of the independent particle volume model.

Tuning of Anisotropy in IrMn/CoFe Exchange Bias Systems

In this paper, we report an enhancement of the anisotropy constant <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> of IrMn/CoFe exchange bias systems with the variation of the NiCr seed layer thickness. This is related to an increase in the (111) texture of IrMn parallel to the interface. Silang100rang /NiCr (Xnm)/IrMn (7 nm)/CoFe (3 nm)/Ta (10 nm) with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> = 2, 4, 6, 10, 15, 20 nm were prepared by direct current (dc) sputtering. X-ray diffraction studies revealed a peak in the (111) texture of the IrMn parallel to the interface for the samples with a 6-nm NiCr seed layer which reduced with increasing NiCr thickness. NiCr seed layer thicknesses of 2, 4, and 20 nm had negligible (111) texture. Measurement of the grain size distribution and thermal activation measurements were used to calculate the anisotropy constant of the IrMn. This was found to increase from (0.4 plusmn 0.1) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ergs/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a 20-nm NiCr seed layer to (4.5 plusmn 2) times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> ergs/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a 6-nm NiCr seed layer. TEM studies of the seed layers showed the 6-nm NiCr layer was amorphous/nanocrystalline, and the grain size increased with NiCr thickness. This change in the morphology of the seed layer results in a lattice mismatch with the IrMn reducing the (111) texture of the IrMn and hence <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> . This allows tuning of the anisotropy by controlling the lattice mismatch between the seed layer and the IrMn.

Effect of Cu Impurities on ${\rm K}_{\rm AF}$ in IrMn$_{1 - {x}}$Cu$_{x}$/CoFe Exchange Bias System

We report on the effect of introducing non-magnetic Cu as an impurity in IrMn in IrMn/CoFe exchange bias systems. Films were prepared by sputtering from a specially designed target. Thermal activation measurements and determination of the grain size distribution from TEM analysis of these samples allowed the anisotropy constant (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> ) of the IrMn to be determined. The level of Cu impurity in the IrMn was varied from 0.0% to 17.7% at.% Cu. The anisotropy constant initially rose from (10 plusmn 1) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> erg/cc at 0.0 at.% Cu to (13 plusmn 2) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> erg/cc at 1.8 at.% Cu and reduced with increasing Cu concentration to (4.7 plusmn 1) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> erg/cc at 17.7 at.% Cu. The change in K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> is explained by the effect of the Cu on the stoichiometry of the IrMn which in turn affected the ordering of the L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> phase causing the variation in K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">AF</sub> . The exchange bias (H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex</sub> ) of these samples was reduced ~40% with only 1 at.% Cu in the IrMn, and continued to reduce exponentially down to 5 Oe with 17.7 at.% Cu.

Texture Effects in IrMn/CoFe Exchange Bias Systems

We report an enhancement of the average blocking temperature <T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> > in IrMn/CoFe exchange bias systems due to an increase in the anisotropy constant of the IrMn. This is related to a (111) texture of IrMn parallel to the interface. Si<100>/Seed (5 nm)/IrMn (10 nm)/CoFe (3 nm)/Ta (10 nm) were prepared by dc sputtering. Cu, Ru, and NiCr seed layers were used. X-ray diffraction studies involving both thetas/2thetas and grazing incidence scans revealed a strong (111) texture of the IrMn parallel to the interface for the samples with a NiCr seed layer. Cu and Ru seed layers did not cause a marked texture. Thermal activation measurements found an enhancement of the average blocking temperature from 367 K for the sample with Cu seed layer to 477 K for the sample with a NiCr seed. The anisotropy constant was found to increase from (2.8plusmn0.2) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a Cu seed layer to (3.3 plusmn 0.4) times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> erg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> for the sample with a NiCr seed layer. We find that the increase in the anisotropy constant leads to an enhanced blocking temperature.

Thermal activation of bulk and interfacial order in exchange biased systems

In this work, we report on thermal activation measurements of both the interfacial spins and the bulk of the antiferromagnet (AF) in exchange bias trilayers consisting of two ferromagnetic (F) layers of different thicknesses separated by an AF layer. Systems with two different AF thicknesses have been measured. Thermal activation of the interfacial spins was achieved by heating in a negative field with only the thicker F layer in negative saturation, while thermal activation of the bulk was achieved by heating the AF with both F layers reversed. By following a detailed measurement procedure where all measurements are made at a temperature at which the AF is free of thermal activation, the hysteresis loop of the thicker layer could be shifted along the field axis, while the loop corresponding to the thinner F layer did not move. From the thermal activation measurements, it is clear that the order at the interface accounts for approximately ∼50% of the exchange bias, while the remaining 50% is due to the order in the bulk. For both samples, the median blocking temperature was found to be greater for the interfacial spins by ∼25K. The energy barriers to reversal for both bulk and interfacial spins have been calculated from the magnetic data and we find that the distributions are of similar form but with that for the interfacial spins occurring at higher values.

Electrical characteristics and simulations of self-switching-diodes in SOI technology

Self-switching devices (SSDs) are new nano-scale field effect active components. In the present work, these devices are made in silicon-on-insulator (SOI) technology and operate at room temperature. We investigate their current–voltage ( I– V) characteristics which show a diode-like behaviour due to electrostatic effects. Thermal activation measurements are presented and discussed. We also present simulations to gain better understanding of device physics and also to optimize the critical parameters of the fabrication process.

Landau level anticrossing manifestations in the phase-diagram topology of a two-subband system

In a two-subband $\mathrm{Ga}\mathrm{As}∕\mathrm{Al}\mathrm{Ga}\mathrm{As}$ two-dimensional electron system, the phase diagram of longitudinal resistivity ${\ensuremath{\rho}}_{xx}$ in density and magnetic field plane exhibits an intriguing structure centered at filling factor $\ensuremath{\nu}=4$ which is strikingly different from the ringlike structures at lower magnetic fields. Thermal activation measurements reveal an anticrossing gap on each boundary of the structure where intersubband Landau levels with parallel or antiparallel spin are brought into degeneracy. While the physics of the anticrossing can be ascribed to the pseudospin quantum Hall ferromagnetism, as reported earlier by Muraki et al., the mapping and modeling of the phase-diagram topology allow us to establish a more complete picture of the consequences of real spin/pseudospin interactions for the two-subband system.

Changes in Atomic Bomb Survivors' Dosimetry with the New Dosimetry System DS02

From 1987 through the late 1990s, thermal neutron activation measurements of materials exposed to the atomic bomb in Hiroshima appeared to show a different distance trend than estimates from dosimetry system DS86, calling into question the accuracy of DS86 and ultimately resulting in the formation of a joint US-Japan scientific working group to develop an improved system, DS02. DS02 recalculates both the source term and radiation transport using the newest methods and input data, with new estimates of explosive yield (15kt→16kt) and height of detonation (580m→600m) of the Hiroshima bomb, but no similar change in Nagasaki. DS02 developers evaluated new measurements in all previous categories and two new categories, made designed measurement inter-comparisons on split samples, and achieved a resolution of the apparent neutron discrepancy. DS02 free-in-air gamma kerma is about 10%>DS86 at distances>1km where almost all survivors were located; neutron kerma ranges from about 10%>DS86 at 1km to 15% 2km in Nagasaki. Average changes in shielded kerma and representative organ doses vs. distance are reviewed, concluding with a discussion of plans to improve dosimetry implementation at the Radiation Effects Research Foundation (RERF) by using tools such as the Geographical Information System (GIS).