Weak Localization Research Articles

Strain control in graphene on GaN nanowires: Towards pseudomagnetic field engineering

Gallium nitride nanowire and nanorod substrates are prospective platforms allowing to control the local strain distribution in graphene films on top of them, resulting in an induction of pseudomagnetic fields. AFM measurements performed in a HybriD mode complemented by SEM allow for a detailed visualization of the strain distribution on graphene surface. Graphene in direct contact with supporting regions is tensile strained, while graphene located in-between is characterized by lower strain. Characteristic tensile strained wrinkles also appear in the areas between the supporting regions. A positive correlation between strain gradient and distances between borders of supporting regions is observed. These results are confirmed by analysis of the Raman D’ band intensity, which is affected by an enhancement of intravalley scattering. Furthermore, scanning tunneling spectroscopy shows a local modification of the density of states near the graphene wrinkle and weak localization measurements indicate the enhancement of pseudomagnetic field-induced scattering. Therefore, we show that nanowire and nanorod substrates provide strain engineering and induction of pseudomagnetic fields in graphene. The control of graphene morphology by modification of distances between supporting regions is promising for both further fundamental research and the exploration of innovative ways to fabricate pseudomagnetic field-based devices like sensors or filters.

Influence of annealing temperature and capping layer on the structural, magnetic and transport properties of ion beam sputtered Co2FeAl thin films on Si (1 0 0)

The impact of different annealing temperatures (Ta) and presence of capping layer on the structural, magnetic and transport properties of sputtered Co2FeAl (CFA (10 nm)) Heusler alloy thin films grown on Si (100) substrate is studied. The structural and topographical study reveal a direct impact of annealing on the crystallinity and surface roughness of the films. The Ferromagnetic resonance study shows that the Gilbert damping constant (α) is influenced by both crystallinity and roughness of the films and has a minimum value of 9.35(±0.15) × 10-3 for the 300 °C annealed (5.51(±0.27) × 10-3 for Al capped film) film having the lowest surface roughness. Also, the effect of capping layer on the spin dynamic properties of CFA layer is studied which reveals that a protective layer of low spin–orbit coupling material prevents the spin pumping to the formed oxide in case of uncapped CFA. The temperature-dependent resistivity depends on the roughness and crystallinity of the films (which are also a function of Ta) and revealed the dominance of weak localization at lower temperatures and electron–phonon scattering at higher temperatures. Temperature-dependent anomalous Hall effect measurements suggest the supremacy of the combined intrinsic and side jump contributions over skew scattering contribution.

Impurity states of electrons on the surface of a nanotube in a magnetic field

The localization of electrons in the field of isolated nonmagnetic impurity atoms on the surface of a nanotube in a magnetic field is considered. A model of a Gaussian separable impurity potential capable of localizing an electron at any intensity is used. The positions of the local level are found in the regime of strong and weak localizations. The positions of the resonances are found, their widths are estimated. They experience Aharonov-Bohm oscillations when the magnetic flux in the tube changes. It is shown that the positions of the resonances experience oscillations similar to the de Haas-van Alphen oscillations, which are not associated with a magnetic field.

Magnetoresistance of graphite nanoplatelets with different structure

The magnetoresistance of bulk specimens of graphite nanoplatelets obtained by different methods is studied in magnetic fields up to 2.2 T. It has been established that magnetoresistance is negative for graphite nanoplatelets prepared by chemical treatment of source graphite with a solution of potassium permanganate in sulfuric acid. This negative magnetoresistance can be explained in terms of the model of charge carrier's weak localization in a system with imperfect structure. It has been established that the magnetoresistance is positive and independent of temperature for graphite nanoplatelets produced by sonication method. Moreover, magnetoresistance is linear relative to a magnetic field in fields above ∼0.7 T. It is shown that linear magnetoresistance can be explained in the terms of the Abrikosov's model of quantum linear magnetoresistance.

Weak Localization and Weak Anti-Localization in Ultra Thin Sb2Te3 Nanoplates

Weak Localization and Weak Anti-Localization in Ultra Thin Sb2Te3 Nanoplates

Designing multifunctional single-molecule devices by mononuclear or binuclear manganese phthalocyanines

Molecular-scale magnetic devices have attracted great attention of research in recent years. In this work, by using the non-equilibrium Green’s function (NEGF) method in combination with density functional theory (DFT), we investigated the spin transport properties of manganese phthalocyanine based spintronic devices constructed by a mononuclear manganese phthalocyanine (MnPc) or binuclear manganese phthalocyanine (Mn2Pc2) sandwiched between two zigzag-edge graphene nanoribbon (zGNR) electrodes. The calculation results show that both MnPc and Mn2Pc2 devices are good spin filters manifesting excellent spin filtering effect, which is originated from the distinct difference in the energy alignments of the spin-up and spin-down molecular electronic states with respect to the Fermi energy (EF) of zGNR electrodes. More interestingly, the spin polarization of the tunneling electrons through the devices is intimately opposite to that of Mn atom(s). Specifically, for a spin-up polarization of Mn in the devices, almost only the spin-down electrons are allowed to go through the device, and vice versa. In addition, spin filtering efficiency (SFE) of the devices is evidently enhanced in Mn2Pc2 devices in comparison with that of the MnPc devices, of which the value of SFE can reach almost 100%. This enhancement is attributed to the fact that the spin electronic states of Mn2Pc2 devices are closer to EF and there is a weaker localization in their spatial distributions induced by the external bias voltage. Furthermore, both MnPc and Mn2Pc2 devices can act as NOT logic gates by taking the spin polarization characteristics of Mn atom(s) and current of devices as input and output signals, respectively. This work demonstrates that mononuclear MnPc and binuclear Mn2Pc2 molecules have potential applications in designing multifunctional spintronic single-molecule devices.

The electrical- and magneto-transport properties of Rb-, Sn-, and Co-doped BiCuSeO crystals

Doped BiCuSeO is one of the promising thermoelectric oxide candidates. However, the research on doping effects on the electrical transport properties of BiCuSeO, especially in crystalline samples, is still limited. Here, we studied the transport properties of doped BiCuSeO crystals, including three types of doping species (Rb, Sn, and Co) with varying concentrations. In the case of Rb-doped BiCuSeO crystals, few percentage (≤1%) Rb-doping make BiCuSeO display metallic behavior, while high one (≥2%) displays bad-metallic behavior. Both Sn- and Co-doped BiCuSeO crystals have similar electrical evolution as Rb-doped ones. The charge carriers of all these doped BiCuSeO crystals are holes, and the increased dopant concentration decreases the hole concentrations regardless of the type of dopant species. There is negative magnetoresistance (MR) in Rb- and Sn-doped BiCuSeO at low temperature (<15 K), which is due to the breakdown of weak localization by magnetic field B, but the MR behaviors in Co-doped BiCuSeO crystals are strongly correlated with their magnetic properties. The analysis of the temperature-dependent mobility of these doped BiCuSeO crystals substantiates that at low temperatures (<50 K), electron-impurity scattering dominates, while electron–phonon scattering dominates at high temperatures (>50 K). The evolution of the above-mentioned electrical/magneto-transport properties of doped BiCuSeO can be understood as follows: the dopant compensates the Bi-deficiency in pristine BiCuSeO crystals and decreases the hole concentration and leads to the metal–Anderson-insulator transition. These results may be valuable to optimize the electrical properties of layered compounds similar to BiCuSeO.

Boosting proximity spin\u2013orbit coupling in graphene/WSe2 heterostructures via hydrostatic pressure

Van der Waals heterostructures composed of multiple few layer crystals allow the engineering of novel materials with predefined properties. As an example, coupling graphene weakly to materials with large spin–orbit coupling (SOC) allows to engineer a sizeable SOC in graphene via proximity effects. The strength of the proximity effect depends on the overlap of the atomic orbitals, therefore, changing the interlayer distance via hydrostatic pressure can be utilized to enhance the interlayer coupling between the layers. In this work, we report measurements on a graphene/WSe2 heterostructure exposed to increasing hydrostatic pressure. A clear transition from weak localization to weak antilocalization is visible as the pressure increases, demonstrating the increase of induced SOC in graphene.

Enhancement of spin-orbit coupling and magnetic scattering in hydrogenated graphene

Spin-orbit coupling (SOC) can provide essential tools to manipulate electron spins in two-dimensional materials like graphene, which is of great interest for both fundamental physics and spintronics application. In this paper, we report the low-field magnetotransport of in situ hydrogenated graphene where hydrogen atoms are attached to the graphene surface in continuous low temperature and vacuum environment. Transition from weak localization to weak antilocalization with increasing hydrogen adatom density is observed, indicating enhancing Bychkov-Rashba-type SOC in a mirror symmetry broken system. From the low-temperature saturation of phase breaking scattering rate, the existence of spin-flip scattering is identified, which corroborates the existence of magnetic moments in hydrogenated graphene.

Suppression of weak localization due to AlN interlayer in AlGaN/GaN 2DEG

We have conducted a comprehensive investigation of the magneto-transport properties of Al0.30Ga0.70N/GaN and Al0.30Ga0.70N/AlN/GaN based HEMT structure in terms of inelastic scattering time via weak localization (WL) measurement. Owing to 1 nm AlN interlayer, a small negative-magnetoresistance (MR) effect which appeared over the interval −0.10 T ≤ B⊥ ≤ 0.10 T is further suppressed to −0.040 T ≤ B⊥ ≤ 0.040 T. Using HRXRD measurement and fitting our experimental magnetoresistance (MR) data with theoretical model, it has been analyzed that WL suppression in 2DEG is mainly due to the interlayer while contribution due to crystalline disorder is negligible. Different weak localization parameters like elastic scattering time τe and inelastic scattering time τi are extracted by fitting temperature dependent negative MR data using Hikami-Larkin-Nagaoka (HLN) model. A linear dependence of inelastic scattering rate (τi−1∝ T) with temperature is also observed up to temperature 15 K. Our combined experimental and modeled results demonstrate a significant role of AIN interlayer in suppressing the WL in AlGaN/GaN 2DEG system. Quantitative analysis within the framework of HLN model leads to the conclusion that AlN interlayer break the electronic phase coherence, decrease τi and suppress the localization effect.

Evidence of weak Anderson localization revealed by the resistivity, transverse magnetoresistance and Hall effect measured on thin Cu films deposited on mica

We report the resistivity of 5 Cu films approximately 65 nm thick, measured between 5 and 290 K, and the transverse magnetoresistance and Hall effect measured at temperatures 5 K < T < 50 K. The mean grain diameters are D = (8.9, 9.8, 20.2, 31.5, 34.7) nm, respectively. The magnetoresistance signal is positive in samples where D > L/2 (where L = 39 nm is the electron mean free path in the bulk at room temperature), and negative in samples where D < L/2. The sample where D = 20.2 nm exhibits a negative magnetoresistance at B < 2 Tesla and a positive magnetoresistance at B > 3 Tesla. A negative magnetoresistance in Cu films has been considered evidence of charge transport involving weak Anderson localization. These experiments reveal that electron scattering by disordered grain boundaries found along L leads to weak Anderson localization, confirming the localization phenomenon predicted by the quantum theory of resistivity of nanometric metallic connectors. Anderson localization becomes a severe obstacle for the successful development of the circuit miniaturization effort pursued by the electronic industry, for it leads to a steep rise in the resistivity of nanometric metallic connectors with decreasing wire dimensions (D < L/2) employed in the design of Integrated Circuits.

Quantum percolation of monopole paths and the response of quantum spin ice

We consider quantum spin ice in a temperature regime in which its response is dominated by the coherent motion of a dilute gas of monopoles. The hopping amplitude of a monopole is sensitive to the configuration of its surrounding spins, taken to be quasi-static on the relevant timescales. This leads to well-known blocked directions in the monopole motion; we find that these are sufficient to reduce the coherent propagation of monopoles to quantum diffusion. This result is robust against disorder, as a direct consequence of the ground-state degeneracy, which disrupts the quantum interference processes needed for weak localization. Moreover, recent work [Tomasello et al., Phys. Rev. Lett. 123, 067204 (2019)] has shown that the monopole hopping amplitudes are roughly bimodal: for $\approx 1/3$ of the flippable spins surrounding a monopole, these amplitudes are extremely small. We exploit this structure to construct a theory of quantum monopole motion in spin ice. In the limit where the slow hopping terms are set to zero, the monopole wavefunctions appear to be fractal; we explain this observation via a mapping to quantum percolation on trees. The fractal, non-ergodic nature of monopole wavefunctions manifests itself in the low-frequency behavior of monopole spectral functions, and is consistent with experimental observations.

In-depth analysis of anisotropic magnetoconductance in Bi2Se3 thin films with electron–electron interaction corrections

A combination of out-of-plane (OOP) and in-plane (IP) magnetoconductance (MC) study in topological insulators (TI) is often used as an experimental technique to probe weak anti-localization (WAL) response of the topological surface states (TSSs). However, in addition to the above WAL response, weak localization (WL) contribution from conducting bulk states are also known to coexist and contribute to the overall MC; a study that has so far received limited attention. In this article, we accurately extract the above WL contribution by systematically analyzing the temperature and magnetic field dependency of conductivity in Bi2Se3 films. For accurate analysis, we quantify the contribution of electron–electron interactions to the measured MC which is often ignored in the WAL studies. Moreover, we show that the WAL effect arising from the TSSs with finite penetration depth, for OOP and IP magnetic field can together explain the anisotropic magnetoconductance (AMC) and, thus, the investigated AMC study can serve as a useful technique to probe the parameters like phase coherence length and penetration depth that characterise the TSSs in 3D TIs. We also demonstrate that increase in bulk-disorder, achieved by growing the films on amorphous SiO2 substrate rather than on crystalline Al2O3(0001), can lead to stronger decoupling between the top and bottom surface states of the film.

Spatial correlations of entangled polymer dynamics.

The spatial correlations of entangled polymer dynamics are examined by molecular dynamics simulations and neutron spin-echo spectroscopy. Due to the soft nature of topological constraints, the initial spatial decays of intermediate scattering functions of entangled chains are, to the first approximation, surprisingly similar to those of an unentangled system in the functional forms. However, entanglements reveal themselves as a long tail in the reciprocal-space correlations, implying a weak but persistent dynamic localization in real space. Comparison with a number of existing theoretical models of entangled polymers suggests that they cannot fully describe the spatial correlations revealed by simulations and experiments. In particular, the strict one-dimensional diffusion idea of the original tube model is shown to be flawed. The dynamic spatial correlation analysis demonstrated in this work provides a useful tool for interrogating the dynamics of entangled polymers. Lastly, the failure of the investigated models to even qualitatively predict the spatial correlations of collective single-chain density fluctuations points to a possible critical role of incompressibility in polymer melt dynamics.

Crossover magnetoresistance in non-transferred synthesized graphdiyne film

Two-dimensional layered carbon materials such as graphene constructed by sp2 hybridization have raised considerable interest due to their large, non-saturating magnetoresistance. As a new sp-sp2 hybridization structure layered carbon material, the transport performance of graphdiyne has been founded comparable with graphene, while the magnetoresistance remains to be clarified. In this work, we developed a modified non-transferred graphdiyne film synthesized method and studied its magnetoresistance effect. We observed an unexpected temperature-dependent crossover between positive and negative magnetoresistance in graphdiyne film. The film shows negative magnetoresistance at 5–19 K, the maximum intensity of the negative magnetoresistance was about 150% at 5 K under 3 T magnetic field. It presents positive magnetoresistance at 19–21 K under 3 T magnetic field, the magnetoresistance value reached 150% at 20 K. The mechanism of crossover positive and negative magnetoresistance originates from a superposition of the magnetic polaron model and weak localization.

Observation of antiferromagnetic ordering from muon spin resonance study and the Kondo effect in a Dy-doped Bi2Se3 topological insulator

The transport and magnetic properties as well as the microscopic electronic properties of the Dy-doped topological insulator Bi2Se3 (Bi1.9Dy0.1Se3) are examined in detail. It is demonstrated that Dy doping induces antiferromagnetic (AFM) ordering in Bi2Se3. It is also observed that Dy doping opens a surface band gap of ∼52 meV at 6.5 K. The transition from AFM to ferromagnetic occurs at a lower temperature, ∼5 K in a magnetic field of ∼3 T. Furthermore, Dy doping in Bi2Se3 leads to the Kondo effect and weak localization to weak anti-localization crossover. Muon spin resonance measurements indicate that the signal measured with magnetization reflects the magnetic properties of 2% of the sample. The experimental findings are supported by theoretical calculations based on density functional theory.

Magnetoconductance in epitaxial bismuth quantum films: Beyond weak (anti)localization

The complex behavior of magnetoconductance of Bi films grown epitaxially on Si(111) with a thickness of 20--100 bilayers (BL) was measured at $T$= 9 K in magnetic fields up to $B=4\phantom{\rule{0.16em}{0ex}}\mathrm{T}$, oriented in-plane parallel and perpendicular to the electric dc current $I$. Contributions to magnetoconductance (MC) by diffuse scattering, by weak localization (WL) as well as by weak antilocalization (WAL) were identified. All these components to MC turned out to be isotropic in two dimensions, i.e., no dependence on angle between $B$ and $I$ within the surface plane was found. Only for $B\phantom{\rule{0.16em}{0ex}}\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}I$ an increase of MC was detected that is, to first approximation, $\ensuremath{\propto}{B}^{2}$. It is ascribed to ballistic scattering between the Rashba-split interfaces that allow Umklapp scattering without spin flip. While MC within the surface states, dominant at small thicknesses, $d$, shows negligible diffuse scattering under the chosen geometry, their quantum corrections are characterized by WAL with $\ensuremath{\alpha}=\ensuremath{-}0.3$ and a coupling strength that decays $\ensuremath{\propto}1/d$ with layer thickness. The admixing of quantized bulk states, which dominates MC above 50 BL, not only increases diffuse scattering, it introduces WL in combination with WAL. Presumably due to hybridization with the surface states, it also modifies strongly the WAL component for $d&gt;60$ BL. Thus our findings suggest an intriguing interplay in magnetotransport between 2D and quantized 3D states at the Fermi surface of ultrathin bismuth quantum films and provide further deep insight into the electronic transport in quantized and partly spin split bands.

New method of transport measurements on van der Waals heterostructures under pressure

The interlayer coupling, which has a strong influence on the properties of van der Waals heterostructures, strongly depends on the interlayer distance. Although considerable theoretical interest has been demonstrated, experiments exploiting a variable interlayer coupling on nanocircuits are scarce due to the experimental difficulties. Here, we demonstrate a novel method to tune the interlayer coupling using hydrostatic pressure by incorporating van der Waals heterostructure based nanocircuits in piston-cylinder hydrostatic pressure cells with a dedicated sample holder design. This technique opens the way to conduct transport measurements on nanodevices under pressure using up to 12 contacts without constraints on the sample at the fabrication level. Using transport measurements, we demonstrate that a hexagonal boron nitride capping layer provides a good protection of van der Waals heterostructures from the influence of the pressure medium, and we show experimental evidence of the influence of pressure on the interlayer coupling using weak localization measurements on a transitional metal dichalcogenide/graphene heterostructure.

Impact of nitrogen doping on the band structure and the charge carrier scattering in monolayer graphene

The addition of nitrogen as a dopant in monolayer graphene is a flexible approach to tune the electronic properties of graphene as required for applications. Here, we investigate the impact of the doping process that adds N dopants and defects on the key electronic properties, such as the mobility, the effective mass, the Berry phase, and the scattering times of the charge carriers. Measurements at low temperatures and magnetic fields up to 9 T show a decrease of the mobility with increasing defect density due to elastic, short-range scattering. At low magnetic fields weak localization indicates an inelastic contribution depending on both defects and dopants. Analysis of the effective mass shows that the N dopants decrease the slope of the linear bands, which are characteristic for the band structure of graphene around the Dirac point. The Berry phase, however, remains unaffected by the modifications induced through defects and dopants, showing that the overall band structure of the samples is still exhibiting the key properties as expected for Dirac fermions in graphene.

Physical properties of face-centered cubic structured high-entropy alloys: Effects of NiCo, NiFe, and NiCoFe alloying with Mn, Cr, and Pd

This paper reports a comprehensive study of electrical and thermal transport properties of a series of face-centered cubic structured high-entropy alloys by alloying Mn, Cr, and Pd elements in NiCo, NiFe, and NiCoFe alloys. X-ray diffraction revealed a single-phase Cu-type cubic structure, and scanning electron microscopy displayed elongated grained microstructures in all alloys. Like NiCo, NiFe, and NiCoFe alloys, the alloys containing Cr/Mn/Pd exhibit metallic behavior; however, their electrical transport properties, such as residual resistivity, residual resistivity ratio, and temperature coefficient of resistivity, vary significantly due to the increase of chemical disorder and defects. The analysis of resistivity of these alloys further showed different scattering mechanisms at low temperatures. Interestingly, the electrical resistivity of NiCoCr, NiCoFeCr, and NiCoFeMn alloys is nearly linear at low temperatures, most likely related to the Mott-Ioffe-Regel limit. Additionally, the NiCoMnCr and NiCoFeMnCr alloys exhibit a minimum in resistivity at low temperatures, which can be explained by the weak localization effect. The Seebeck coefficient measurements reveal that the charge carrier for thermoelectric transport in NiCo, NiFe, and NiCoFe is changed from electrons to holes with Mn alloying. In contrast, a sign reversal of the charge carriers observed in the Cr-containing alloys is connected to the compensation of electron and hole carriers. Furthermore, the NiCoCr, NiCoFeCr, NiCoMnCr, and NiCoFeMnCr alloys show a negative phonon drag effect at low temperatures due to electron-phonon interaction. The measured thermal conductivity behaves similarly in all alloys, except for a considerable reduction in magnitude in Cr/Mn/Pd-containing alloys. This is attributed to a significant decrease of electronic thermal conductivity due to an increased electron scattering by disorders and lattice distortions and a substantial modification of band structure. There is almost an equal contribution of electronic and lattice to the total thermal conductivity in Cr/Mn/Pd-containing alloys, suggesting a semimetallic nature. The temperature dependence of lattice thermal conductivity of these alloys is described by different phonon scattering mechanisms.