Weak Localization Research Articles

Geometrically induced localization of flexural waves on thin warped physical membranes.

We consider the propagation of flexural waves across a nearly flat, thin membrane, whose stress-free state is curved. The stress-free configuration is specified by a quenched height field, whose Fourier components are drawn from a Gaussian distribution with power-law variance. Gaussian curvature couples the in-plane stretching to out-of-plane bending. Integrating out the faster stretching modes yields a wave equation for undulations in the presence of an effective random potential, determined purely by geometry. We show that at long times and lengths, the undulation intensity obeys a diffusion equation. The diffusion coefficient is found to be frequency dependent and sensitive to the quenched height field distribution. Finally, we consider the effect of coherent backscattering corrections, yielding a weak localization correction that decreases the diffusion coefficient proportional to the logarithm of the system size, and induces a localization transition at large amplitude of the quenched height field. The localization transition is confirmed via a self-consistent extension to the strong disorder regime.

Suppression of the superconducting transition temperature and magnetic-field-induced quantum critical behavior in three-dimensional polycrystalline niobium films

We have investigated the superconducting transition temperature T c depression and magnetic-field-induced quantum critical behavior in three-dimensional (3D) polycrystalline niobium (Nb) films. For superconducting films with 26.8 nm 72.9 nm, the T c value increases from ∼4.94 to ∼8.74 K. Through comparing our experimental data with proximity effect and weak localization effect, the T c depression can be explained by the proximity effect predicted by Cooper. Pure size effect could not induce T c depression because the average energy level spacing of grains is much smaller than the bulk Nb superconducting energy gap. The reason for the proximity effect is the metallic niobium monoxide NbO surrounding Nb grains. The superconductor-insulator transition (SIT) induced by magnetic field has been observed in T s 433 and 523 K films. The critical field B c in SIT satisfies a scaling relationship, similar to that caused by quantum phase transition in a two-dimensional (2D) cleaning system. The reason for that is the average polycrystalline grain size is smaller than the bulk Nb coherence length. The quantum effect is probably related to the Nb oxides.

Visualization of Directional Beaming of Weakly Localized Raman from a Random Network of Silicon Nanowires.

Disordered optical media are an emerging class of materials that can strongly scatter light. These materials are useful to investigate light transport phenomena and for applications in imaging, sensing and energy storage. While coherent light can be generated using such materials, its directional emission is typically hampered by their strong scattering nature. Here, the authors directly image Rayleigh scattering, photoluminescence and weakly localized Raman light from a random network of silicon nanowires via real‐space microscopy and Fourier imaging. Direct imaging enables us to gain insight on the light transport mechanisms in the random material, to visualize its weak localization length and to demonstrate out‐of‐plane beaming of the scattered coherent Raman light. The direct visualization of coherent light beaming in such random networks of silicon nanowires offers novel opportunities for fundamental studies of light propagation in disordered media. It also opens venues for the development of next generation optical devices based on disordered structures, such as sensors, light sources, and optical switches.

Tunable Infrared Plasmonic Properties of Epitaxial Ti1-xMgxN(001) Layers.

Optical transmission and reflection spectra in combination with ellipsometry and transport measurements on epitaxial rocksalt structure Ti1-xMgxN(001) layers with 0.00 ≤ x ≤ 0.49 are employed to explore their potential as refractory infrared plasmonic materials. A red shift in the reflection edge ℏωe from 2.0 to 0.8 eV and the corresponding unscreened plasma energy ℏωpu from 7.6 to 4.7 eV indicate a linear reduction in the free carrier density N with increasing x. However, nitrogen vacancies in Mg-rich samples act as donors, resulting in a minimum N = 1.6 × 1022 cm-3 for x = 0.49. Photoelectron valence band spectra confirm the diminishing conduction band density of states and indicate a 0.9 eV decrease in the Fermi level as x increases from 0 to 0.49. The dielectric function ε = ε1 + iε2 can be divided into a low-energy spectral region where intraband transitions result in large negative and positive ε1 and ε2, respectively, and a higher energy interband transition region with both ε1 and ε2 > 0. The screened plasma energy Eps that separates these two regions red-shifts from 2.6 to 1.3 eV for x = 0-0.39, indicating a tunable plasmonic activity that extends from the visible to the infrared (470-930 nm). Electron transport measurements indicate a metallic temperature coefficient of resistivity (TCR) for TiN-rich alloys with x ≤ 0.26 but weak carrier localization and a negative TCR <60 K for x = 0.39 and <300 K for x = 0.49, attributed to Mg alloying-induced disorder. The plasmonic quality factor Q is approximately an order of magnitude larger than what was previously reported for polycrystalline Ti1-xMgxN, making Ti1-xMgxN(001) layers competitive with Ti1-xScxN(001).

Current-controlled magnetization using interfacial localizations in heterostructures

A systematic study is conducted on an electrically controlled magnetization in tri-layers containing two layers of ferromagnetic (FM) SrRuO3, with antiferromagnetic (AFM) insulating manganites sandwiched in between. Magneto-resistance curves exhibit that a huge weak anti-localization or localization shapes around zero magnetic field and the localization shapes and coercive fields change simultaneously by currents. This results propose that interface-driven magnetic inhomogeneity such as the FM clusters embedded in the AFM matrix, and corresponding localizations play a role as a bridge between the current and magnetism. The results suggest that interfacial magnetism can cause new functions in a heterostructure, leading to an expansion of research subjects of current-controlled magnetism.

Genetic variants of the type-3 metabotropic glutamate receptor gene associated with human spatial localization ability

People present and transform spatial information through spatial cognitive abilities to learn and adapt to the environment. Accumulating evidence exhibits individual difference in spatial cognition, which is ascribed to environmental and genetic factors. The detail knowledge remains to be investigated. In this study, 941 healthy college students were recruited, and their spatial localization ability, an essential symbol of spatial cognitive ability, was measured. We analyzed the individual difference in spatial localization and relative influence factors. We selected the type-3 metabotropic glutamate receptor (GRM3) gene to dissect the genetic role in spatial cognition. In this study, both individual general cognitive ability and spatial localization were influenced by age, birthplace, growing environment, occupation and educational level of parent, and genetic variants of GRM3. The variants of GRM3 were significantly associated with spatial localization, and not associated with general cognitive ability. The carriers of the AA of rs6465084 or the TT of rs1468412 exhibited a weak spatial localization ability, and even the relative non-genetic factors above-mentioned were well controlled. Conclusion: Our results suggest that genetic variants of GRM3 are significantly associated with the spatial localization ability in the healthy people.

Transverse Electric Lasing at a Record Short Wavelength 244.63 nm from GaN Quantum Wells with Weak Exciton Localization

We have demonstrated a record short wavelength lasing at 244.63 nm with TE dominant polarization from GaN quantum wells (QWs) at room temperature (RT). The optical threshold of 310 kW/cm2 is comparable to state-of-the-art AlGaN QW lasers at similar wavelengths. The sample was grown on the AlN/sapphire template pesudomorphically. X-ray diffraction (XRD) shows unambiguous higher-order satellite peaks indicating a sharp interface amid the active region. The excitonic localization was revealed and studied by the photoluminescence (PL) and time-resolved PL (TRPL) spectroscopy at temperatures ranging from 15 K to RT. At 15 K, the multiple-component PL decay curves with the decay time varying from 62.6 to 2.77 ns at different energies confirmed the localized excitons. The peak energy of the temperature-dependent PL spectra exhibited the “S-shape” behavior; and the weak exciton localization with a small localization energy of 14.3 meV was observed. Therefore, even in the low temperature region, the escape possibility of excitons increased as the temperature rose. As a result, the fwhm of the emission spectra changed significantly when the temperature was below 150 K. Above 150 K, the PL decay shape changed from the two-component exponential decay to the single exponential decay, indicating complete delocalization of excitons. The work demonstrates the weak localization and thus smooth interface in the GaN/AlN active region, which are desirable for DUV lasers operating at RT.

A-site Pb doping effect on structural, microstructural and magnetotransport properties of La0.5Sm0.2Ca0.3-xPbxMnO3 (x=0, 0.05, 0.10) manganite

Structural, microstructural and magneto-transport properties of polycrystalline La0.5Sm0.2Ca0.3-x PbxMnO3 (x = 0, 0.05 and 0.10) samples, prepared by conventional solid state reaction, were studied. Refinement of the Powder X-ray diffractograms revealed that all the samples crystallize into orthorhombic structure with Pnma space group and the lattice parameters increase with increasing Pb content. The scanning electron microscope (SEM) micrographs show a granular character and the estimated grain sizes ranges between 10 μm and 20 μm. The experimental electrical resistivity and the magnetoresistance in the temperature range 20 K–250 K under magnetic field of zero and 1 Ta, have been recorded using four probe technique. The resistivity curves without magnetic field, exhibit a ferromagnetic-metallic (FM) to paramagnetic-insulating (PI) transition at TMI = 125 K, 91 K and 37 K for x = 0, 0.05 and 0.10, respectively. The magnetoresistance reaches 58% for undoped sample and decrease with Pb doping under 1T. The resistivity values increases drastically with Pb doping. These two results were explained mainly by the increase of the disorder σ2 and <rA>. In the low temperature range (T < TMI) and for x = 0 and 0.05 samples, the resistivity curves is well fitted by a combination of the residual resistivity, weak localization and electron-electron scatterings; in addition to small polaron contribution or electron-phonon interaction respectively. In the high temperature regime T > TMI, all our resistivity curves has been analyzed using adiabatic small polaron hopping model (ASPH) above θD/2 and variable range hopping model (3D-VRH) (TMI < T < θD/2). An attempt to use the percolation model to describe the resistivity data in the entire temperature range for both x = 0 and 0.05 samples, was established. Finally, density of state, mean hopping distance Rh and mean hopping energy Eh were calculated and discussed.

Improved Semantic Segmentation of Tuberculosis-Consistent Findings in Chest X-rays Using Augmented Training of Modality-Specific U-Net Models with Weak Localizations.

Deep learning (DL) has drawn tremendous attention for object localization and recognition in both natural and medical images. U-Net segmentation models have demonstrated superior performance compared to conventional hand-crafted feature-based methods. Medical image modality-specific DL models are better at transferring domain knowledge to a relevant target task than those pretrained on stock photography images. This character helps improve model adaptation, generalization, and class-specific region of interest (ROI) localization. In this study, we train chest X-ray (CXR) modality-specific U-Nets and other state-of-the-art U-Net models for semantic segmentation of tuberculosis (TB)-consistent findings. Automated segmentation of such manifestations could help radiologists reduce errors and supplement decision-making while improving patient care and productivity. Our approach uses the publicly available TBX11K CXR dataset with weak TB annotations, typically provided as bounding boxes, to train a set of U-Net models. Next, we improve the results by augmenting the training data with weak localization, postprocessed into an ROI mask, from a DL classifier trained to classify CXRs as showing normal lungs or suspected TB manifestations. Test data are individually derived from the TBX11K CXR training distribution and other cross-institutional collections, including the Shenzhen TB and Montgomery TB CXR datasets. We observe that our augmented training strategy helped the CXR modality-specific U-Net models achieve superior performance with test data derived from the TBX11K CXR training distribution and cross-institutional collections (p < 0.05). We believe that this is the first study to i) use CXR modality-specific U-Nets for semantic segmentation of TB-consistent ROIs and ii) evaluate the segmentation performance while augmenting the training data with weak TB-consistent localizations.

From weak antilocalization to Kondo scattering in a magnetic complex oxide interface

Quantum corrections to electrical resistance can serve as sensitive probes of the magnetic landscape of a material. For example, interference between time-reversed electron paths gives rise to weak localization effects, which can provide information about the coupling between spins and orbital motion, while the Kondo effect is sensitive to the presence of spin impurities. Here we use low-temperature magnetotransport measurements to reveal a transition from weak antilocalization (WAL) to Kondo scattering in the quasi-two-dimensional electron gas formed at the interface between SrTiO$_3$ and the Mott insulator NdTiO$_3$. This transition occurs as the thickness of the NdTiO$_3$ layer is increased. Analysis of the Kondo scattering and WAL points to the presence of atomic-scale magnetic impurities coexisting with extended magnetic regions that affect transport via a strong magnetic exchange interaction. This leads to distinct magnetoresistance behaviors that can serve as a sensitive probe of magnetic properties in two dimensions.

Enhanced room-temperature positive magnetoresistance of graphene by decorating Co particles on the surface

Magneto-transport properties of graphene and graphene/Co have been investigated. Enhanced linear positive magnetoresistance (PMR) of graphene is obtained at room temperature by depositing Co particles on the surface, which introduces strong inhomogeneity. Graphene is still in weakly disordered regime after Co deposition as proved by weak localization effect and Raman spectra. And interfacial hybridization between graphene and Co further enhances spatial fluctuation of carrier density and mobility, inducing a higher PMR which is useful for magnetic sensing at room temperature.

Correlation between weak localization effects and resistivity saturation in dilute Ti-Al alloys

Resistivity saturation in Ti-Al alloys has been studied by fitting empirical functions of temperature, T, to 20 sets of literature data. By replacing the commonly used temperature-independent saturation term by a term in T−1/2, to model weak localization effects at high temperature, the quality of the fits improved significantly and physically acceptable values were obtained for the fitted parameters. A linear correlation was found between the fitted high-T weak localization parameters and experimental data for the corresponding low-T weak localization conduction terms. No such correlation could be detected when using constant saturation terms. The results suggest that weak localization is a possible cause for resistivity saturation in Ti-Al alloys.

Full spin-orbit coefficient in III-V semiconductor wires based on the anisotropy of weak localization under in-plane magnetic field

Because of the one-dimensional confinement of electron momentum in narrow semiconductor wires, spin relaxation is suppressed irrespective of the presence of spin-orbit (SO) interaction. In quantum transport, weak localization corrections to conductivity are reflected as suppressed spin relaxation, which makes quantification of the SO strength difficult because quantum correction theory requires weak antilocalization when evaluating SO coefficients. Narrow wires with strong SO interaction are potential platform for Majorana particles and parafermions for topological electronics and quantum computation, so revealing the SO strength in semiconductor wire structures is beneficial. Herein, we present quantification of the full SO coefficient under weak localization in InGaAs-based narrow wires. Using anisotropic weak localization observed under an in-plane external magnetic field with various orientations, one can ascertain the relative ratio between Rashba ($\ensuremath{\alpha}$) and linear Dresselhaus (${\ensuremath{\beta}}_{1}$) SO coefficients with no fitting. Furthermore, we find that widely tuning the potential profile of the quantum well through the top gate can expose a Rashba-predominant region in magnetoconductance, where the $\ensuremath{\alpha}$ value can be extracted reliably from two-dimensional quantum correction theory. Finally, we quantify the full SO coefficients including Rashba, linear Dresselhaus, and cubic Dresselhaus terms in the wire.

Electric-Field-Tunable Valley Zeeman Effect in Bilayer Graphene Heterostructures: Realization of the Spin-Orbit Valve Effect.

We report the discovery of electric-field-induced transition from a topologically trivial to a topologically nontrivial band structure in an atomically sharp heterostructure of bilayer graphene (BLG) and single-layer WSe_{2} per the theoretical predictions of Gmitra and Fabian [Phys. Rev. Lett. 119, 146401 (2017)PRLTAO0031-900710.1103/PhysRevLett.119.146401]. Through detailed studies of the quantum correction to the conductance in the BLG, we establish that the band-structure evolution arises from an interplay between proximity-induced strong spin-orbit interaction (SOI) and the layer polarizability in BLG. The low-energy carriers in the BLG experience an effective valley Zeeman SOI that is completely gate tunable to the extent that it can be switched on or off by applying a transverse displacement field or can be controllably transferred between the valence and the conduction band. We demonstrate that this results in the evolution from weak localization to weak antilocalization at a constant electronic density as the net displacement field is tuned from a positive to a negative value with a concomitant SOI-induced splitting of the low-energy bands of the BLG near the K(K^{'}) valley, which is a unique signature of the theoretically predicted spin-orbit valve effect. Our analysis shows that quantum correction to the Drude conductance in Dirac materials with strong induced SOI can only be explained satisfactorily by a theory that accounts for the SOI-induced spin splitting of the BLG low-energy bands. Our results demonstrate the potential for achieving highly tunable devices based on the valley Zeeman effect in dual-gated two-dimensional materials.

Observation of 2D transport in Sn- and In-doped Bi2−xSbxTe3−ySey topological insulator

Here we report magnetotransport properties of Bi2−xSbxTe3−ySey (BSTS), In- and Sn-doped BSTS single crystals, grown through modified Bridgeman technique. In- and Sn-doped BSTS single crystals show bulk insulation in temperature dependency resistivity measurements and are confirmed from the observed impurity band mediated three dimensional variable-range hopping behavior at low temperatures over virgin BSTS with metallic bulk. Magnetotransport measurements for BSTS and Sn-doped BSTS reveal a zero field sharp positive cusp and is identified as two dimensional (2D) weak antilocalization (WAL) effect, which is the consequence of π Berry phase of the carriers. For In-doped BSTS single crystals, crossover is identified from WAL to weak localization with field variation at low temperatures and also with an increase in temperature from 2 K. For all the single crystals, phase coherence lengths (lϕ) are determined by fitting low field magnetotransport data with Hikami–Larkin–Nagaoka equation. Temperature dependency of phase coherence lengths is described with 2D electron–electron (e–e) and 2D electron–phonon (e–p) interactions for virgin and In-doped BSTS single crystals while for Sn-doped BSTS specimen lϕ(T) follows T−0.53 power law.

Weak localization and dimensional crossover in compositionally graded AlxGa1−xN

The interaction between the itinerant carriers, lattice dynamics, and defects is a problem of long-standing fundamental interest for developing quantum theory of transport. Here, we study this interaction in the compositionally and strain-graded AlGaN heterostructures grown on AlN substrates. The results provide direct evidence that a dimensional crossover (2D–3D) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. These heterostructures show a robust polarization-induced 3D electron gas and a metallic-like behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the evolution of the interaction between charge carriers, lattice dynamics, and defects as a function of temperature. A negative longitudinal magnetoresistance emerges at low temperatures, in line with the theory of weak localization. A weak localization fit to near zero-field magneto-conductance indicates a coherence length that is larger than the elastic mean free path and film thickness (lφ&amp;gt;t&amp;gt;lel), suggesting a 2D weak localization in a three-dimensional electron gas. Our observations allow for a clear and detailed picture of two distinct localization mechanisms that affect carrier transport at low temperature.

Effect of Rare Earth Elements at Amorphous ReAlO3/SrTiO3 (Re = La, Pr, Nd, Sm, Gd, and Tm) Heterointerfaces.

Although the amorphous two-dimensional electron gas (a-2DEG) of oxides provides new opportunities to explore nanoelectronic as well as quantum devices, the intrinsic effect of rare earth (Re = La, Pr, Nd, Sm, Gd, and Tm) elements at ReAlO3/SrTiO3 heterointerfaces is still largely unknown and needs to be addressed systematically. Herein, we first propose that the ionization potential of Re elements is a critical factor for the 2DEG fabricated by chemical spin coating. Furthermore, the photoresponsive properties of heterointerfaces are investigated comprehensively with the ionization potential ranging from 35.79 to 41.69 eV. The results show that the sheet resistances significantly increase with increasing the ionization potential, and a resistance upturn phenomenon is observed at TmAlO3/SrTiO3 heterointerfaces, which can be attributed to the weak localization effect theoretically. The most important observation is the dramatic transition from negative (-178.3%, Re = La) to positive (+89.9%, Re = Gd) photoresponse at ReAlO3/SrTiO3 heterointerfaces under the irradiation of 405 nm light at 50 K. More remarkably, a unique recovery behavior of transient-persistent photoconductivity coexistence at low temperatures is discovered at the TmAlO3/SrTiO3 heterointerface. This work reveals an effective approach to tune the transport and photoresponsive properties by changing Re elements and paves the way for the application of all-oxide devices.

The carbon state in dilute germanium carbides

Conduction and valence band states for the highly mismatched alloy (HMA) Ge:C are projected onto Ge crystal states, Ge vacancy states, and Ge/C atomic orbitals, revealing that substitutional carbon not only creates a direct bandgap but also the new conduction band is optically active. Overlap integrals of the new Ge:C conduction band state with states from unperturbed Ge show that the new band cannot be attributed to any single Ge band but is a mixture of multiple Ge states. The Ge Γ conduction band valley state plays the largest single role, but L and X valley states collectively contribute a larger share than Γ due to the multiplicity of degenerate states. C sites structurally resemble uncharged vacancies in the Ge lattice, similar to Hjalmarson's model for other HMAs. C also perturbs the entire Ge band structure even at the deepest crystal core energy levels, particularly if staggered supercells are used to mimic a disordered alloy. Projection onto atomic sites shows a relatively weak localization compared with other HMAs, but it does show a strong anisotropy in probability distribution. L-valley conduction band states in Ge contribute to the conduction band minimum in Ge:C, but the optical transition strength in Ge:C remains within a factor of 2 of the direct gap transition in Ge.

Structural, magneto transport and magnetic properties of Ruddlesden–Popper La2-2xSr1+2xMn2O7 (0.42≤x≤0.52) layered manganites

The magneto transport of Ruddlesden–Popper, La2-2xSr1+2xMn2O7 (0.42≤x≤0.52), 2-dimensional bilayered manganites have been investigated in a broad temperature range. The samples have been synthesized using the solid-state reaction method. Rietveld refinement of the X-ray diffraction data indicates the tetragonal structure formation with I4/mmm space group. The resistivity curves of the samples present a general characteristic of metal-insulator (MI) transition at certain temperature (TMI). Besides, for samples with x = 0.48, 0.50, and 0.52, at a certain temperature (TCO) well below TMI the charge ordering is also evident. Furthermore, the samples display a shallow upturn in resistivity below Tmin due to the Kondo like spin scattering effect, weak localization and electron-phonon interaction. The high temperature semiconducting (T&amp;gt;TMI) region, the resistivity curve follows the 3D Mott’s variable hopping transport mechanism. The overall suppression of resistivity accounts for a substantial magnetoresistance by applying a magnetic field and the characteristic change in TMI, TCO, and Tmin are discussed. Temperature-dependent magnetization demonstrates the suppression of ferromagnetism and evident of antiferromagnetic nature with an apparent charge ordering with increasing concentration of Sr2+.

Microstructure dependent magnetotransport properties in polycrystalline La0.7Sr0.3MnO3 ultrathin films on thermally oxidized Si substrates

The magnetotransport properties of polycrystalline La0.7Sr0.3MnO3 (LSMO) ultrathin films depend strongly on their thickness and microstructure. In particular, the resistivity and magnetoresistance can be tuned by varying the film thickness and grain size. Here, we have deposited LSMO films with thicknesses ranging from 100 nm down to 10 nm by pulsed laser deposition on thermally oxidized Si substrates, retaining the films’ metallic nature. To avoid reaction of the films with SiO2 at high temperatures, we have introduced a two-step deposition method (half the film thickness grown at 400 °C and the remaining half at 800 °C). X-ray diffraction (XRD) revealed the polycrystalline nature, while atomic force microscopy (AFM) revealed the granular structure of the films from which, surface roughness and grain size are found to decrease with decreasing film thickness. However, low temperature resistivity upturn increases with decreasing film thickness and grain size. The variations of low temperature resistivity minima and magnetoresistance (MR) with film thickness and grain size have been interpreted in the regime of quantum interference effects (viz. weak localization and e-e interaction), intergranular spin polarized transport (SPT) phenomena and their dominance. It is found that SPT was the dominating phenomena. The signature of SPT was also evidenced clearly from low field MR at low temperatures. These kinds of ultrathin films are suitable as ferromagnetic electrode for spintronic device applications.